Download Review on "Complex organic molecules at metal surfaces: bonding

Surface Science Reports 50 (2003) 201–341
Complex organic molecules at metal surfaces:
bonding, organisation and chirality
S.M. Barlow, R. Raval*
Department of Chemistry, Surface Science Research Centre and Leverhulme Centre for
Innovative Catalysis, University of Liverpool, Liverpool L69 7ZD, UK
Received in final form 15 January 2003
Abstract
Surface science techniques have now reached a stage of maturity that has enabled their successful deployment in
the study of complex adsorption systems. A particular example of this success has been the understanding that has
been gained regarding the behaviour of multi-functional organic molecules at metal surfaces. These organic–metal
systems show enormous diversity, starting from their local description which can vary in terms of chemical structure,
orientation and bonding. Additionally, in many cases, these complex organic molecules self-organise into beautiful,
ordered superstructures held together by networks of intermolecular bonds. Both these aspects enable a single
organic molecule–metal system to exhibit a wide-ranging and flexible approach to its environment, leading to a
variety of adsorption phases, according to the prevailing temperature and coverage conditions. In this review we
have attempted to capture this complexity by constructing adsorption phase diagrams from the available literature
for complex carboxylic acids, amino acids, anhydrides and ring systems, all deposited under controlled conditions
onto defined metal surfaces. These provide an accessible, pictorial basis of the adsorption phases which are then
discussed further in the text of the review. Finally, interest has recently focused on the property of chirality that can
be bestowed at an achiral metal surface by the adsorption of these complex organic molecules. The creation of such
architectures offers the opportunity for ultimate stereocontrol of reactions and responses at surfaces. We have,
therefore, specifically examined the various ways in which chirality can be expressed at a surface and provide a
framework for classifying chiral hierarchies that are manifested at surfaces, with particular attention being paid to
the progression of chirality from a local to a global level.
# 2003 Elsevier B.V. All rights reserved.
Abbreviations: AES, Auger electron spectroscopy; DFT, density functional theory; EELS, electron energy loss
spectroscopy; FTIR, Fourier transform infrared spectroscopy; HREELS, high resolution electron energy loss spectroscopy;
LEED, low energy electron diffraction; NEXAFS, near-edge extended absorption fine structure spectroscopy; PhD,
photoelectron diffraction; RAIRS, reflection absorption infrared spectroscopy; RAS, reflection anisotropy spectroscopy; STM,
scanning tunnelling microscopy; ToF-SIMS, time of flight secondary ion mass spectroscopy; TPD, thermal desorption
spectroscopy; UHV, ultra-high vacuum; UPS, ultraviolet photoelectron spectroscopy; XPD, X-ray photoelectron diffraction;
XPS, X-ray photoelectron spectroscopy; ML, monolayer; Sat. ML, saturated monolayer; y, coverage; R, rectus; S, sinister
*
Corresponding author. Tel.: þ44-151-794-6891; fax: þ44-151-794-3896.
E-mail address: [email protected] (R. Raval).
0167-5729/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0167-5729(03)00015-3
202
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1. Complex adsorption phase diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. Manifestations of chirality at the organic/inorganic interface . . . . . . . . . . . . . . . . . . . . . . .
1.2.1. Surface chirality from adsorption of non-chiral molecules . . . . . . . . . . . . . . . . . . .
1.2.1.1. Point chirality: adsorption-induced chiral motifs . . . . . . . . . . . . . . . . . . .
1.2.1.2. Organisational chirality: adsorption-induced chirally ordered domains . . . .
1.2.2. Surface chirality from adsorption of chiral molecules . . . . . . . . . . . . . . . . . . . . . .
1.2.2.1. Point chirality: molecule-induced chiral motifs . . . . . . . . . . . . . . . . . . . .
1.2.2.2. Organisational chirality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3. Systems of study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Carboxylic acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Tartaric acid (L(+)-2,3-dihydroxysuccinic acid) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1. R,R-tartaric acid on Cu(1 1 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1.1. Low coverage (9 0, 1 2) phase: the bitartrate assembly . . . . . . . . . . . . . .
2.1.1.2. Medium coverage (4 0, 2 3) phase: the monotartrate assembly . . . . . . . . .
2.1.1.3. High coverage (4 1, 2 3) phase: the dimer–monomer assembly . . . . . . . . .
2.1.1.4. The emergence of global organisational chirality . . . . . . . . . . . . . . . . . . .
2.1.1.5. Creation of chiral spaces within the chiral surfaces . . . . . . . . . . . . . . . . .
2.1.1.6. Switching global organisational chirality. . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1.7. Sustaining a single rotational domain . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2. R,R-tartaric acid on Ni(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2.1. The room temperature phase: adsorption stresses and chiral reconstructions
2.1.2.2. The nature of electronic chiral communication from molecule to metal . . .
2.1.2.3. From local chiral reconstructions to global organisational chirality . . . . . .
2.2. Succinic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1. Succinic acid on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. Benzoic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1. Benzoic acid on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1.1. Room temperature substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1.2. Liquid nitrogen cooled substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2. Benzoic acid on p(2 1)O/Cu(1 1 0) and Cu(1 1 1). . . . . . . . . . . . . . . . . . . . . . .
2.3.3. Benzoic acid on Ni(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4. 3-Thiophene carboxylic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1. 3-Thiophene carboxylic acid on Cu(1 1 0) and p(2 1)O/Cu(1 1 0). . . . . . . . . . . .
2.5. 4-Trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.1. PVBA on Pd(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2. PVBA on Cu(1 1 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.3. PVBA on Ag(1 1 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6. p-Aminobenzoic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1. p-Aminobenzoic acid on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Glycine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1. Glycine on Cu(1 1 0) and Cu(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2. Glycine on Pt(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Alanine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.2.1. Alanine on Cu(1 1 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1.1. Adsorption at room temperature . . . . . . . . . . . . . . . .
3.2.1.2. High coverage at 420 K: the (2 2, 5 3) phase . . . . . .
3.2.1.3. High coverage at 470 K: the (3 2) phase . . . . . . . . .
3.3. S-proline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1. S-proline on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. S-(or L-)norvaline, methionine and serine . . . . . . . . . . . . . . . . . . . . . .
3.4.1. S-norvaline on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2. S-methionine on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3. S-serine on Cu(1 1 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5. R- and S-phenylglycine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6. Tripeptides: tri-L-alanine and tri-L-leucine. . . . . . . . . . . . . . . . . . . . . .
3.6.1. Tri-L-alanine and tri-L-leucine on Cu(1 1 0) . . . . . . . . . . . . . . .
4. Aromatic anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. Phthalic anhydride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1. Phthalic anhydride on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . .
4.1.2. Phthalic anhydride on p(2 1)O/Cu(1 1 0) . . . . . . . . . . . . . . .
4.2. Pyromellitic dianhydride (PMDA) . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1. PMDA on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2. PMDA on p(2 1)O/Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3. PMDA on Cu(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.4. PMDA on Pt(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Naphthalene-1,8-dicarboxylic anhydride (NDCA) . . . . . . . . . . . . . . . . .
4.3.1. NDCA on Ni(1 1 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2. NDCA on p(2 2)O/Ni(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . .
4.4. 1,4,5,8-Naphthalene-tetracarboxylic dianhydride (NTCDA) . . . . . . . . . .
4.4.1. NTCDA on Ag(1 1 1), Ag(1 0 0) and Ag(1 1 0). . . . . . . . . . . .
4.4.2. NTCDA on Cu(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.3. NTCDA on Ni(1 1 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5. Perylene-3,4,9,10-tetracarboxylic acid-3,4,9,10-dianhydride (PTCDA). . .
4.5.1. PTCDA on Ag(1 1 1) and Ag(1 1 0) . . . . . . . . . . . . . . . . . . . .
4.5.2. PTCDA on Cu(1 0 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.3. PTCDA on Ni(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6. Substituted derivatives of PTCDA . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.1. PTCDI on Ni(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.2. Me-PTCDI on Ag(1 1 1) and Ag(1 1 0). . . . . . . . . . . . . . . . . .
4.6.3. Me-PTCDI on Cu(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.4. DPP-PTCDI on Ag(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. Closed ring structures without additional functional groups . . . . . . . . . . . . . .
5.1. Planar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1. Perylene on Cu(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2. Coronene on Cu(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3. Pentacene, pentacenequinine or pentacenetetrone on Cu(1 0 0) . .
5.2. Helical molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1. M- and P-heptahelicene (M-[7]-helicene and P-[7]-helicene) . . .
5.2.1.1. M-[7]- and P-[7]-helicene on Cu(1 1 1) and Cu(3 3 2).
5.2.1.2. M-[7]- and P-[7]-helicene on Ni(1 0 0) and Ni(1 1 1) .
5.2.2. Thioheterohelicene on Au(1 1 1) . . . . . . . . . . . . . . . . . . . . . . .
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6. Closed ring structures with additional functional groups . .
6.1. 1-Nitronaphthalene (1-NN) . . . . . . . . . . . . . . . . . .
6.2. Benzotriazole and related molecules on Cu(1 0 0) . .
6.3. 2,5-Dimethyl-dicyanoquinonediimine (DMe-DCNQI)
6.4. Hexabutyloxytriphenylene (HBT) . . . . . . . . . . . . . .
7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Matrix notation for overlayer unit cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
Organic and biological molecules provide an important means of introducing complex reactive
functionalities and architectures at a metal surface. In most cases the organic layer serves to provide a
more sophisticated activity, passivation or selectivity function than would have been possible with a
bare metal surface. This is particularly true for the selectivity function, which requires the finesse of the
organic system to be amalgamated to the robust and, generally, reactive inorganic substrate. Such
organic functionalisation of a metal surface has important applications, e.g. in catalysis, sensors,
adhesion, corrosion inhibition, molecular recognition, optoelectronics and lithography. Within such a
technological context, it is clear that the development of future organic/inorganic interfaces is critically
dependent on establishing a fundamental understanding of the various bonding and lateral interactions
that govern the ultimate orientation, conformation and two-dimensional (2D) organisation of these
molecules at a metal surface. Only then, will the generic factors emerge with which to design complex
interfaces with intelligent and sophisticated capabilities. It is only in recent years that this issue has
been tackled with a rigorous surface science approach, involving controlled dosing of known molecules
on defined surfaces under ultra-clean conditions and using an armoury of techniques to probe the
interface. This report will concentrate on such research only and the vast area of self-assembled
monolayers (SAMs) created under liquid environments will not be addressed here. We also only review
complex, multifunctional organic molecules and do not report work on the simpler organic molecules
for which excellent reviews already exist [1–4].
An important point to note is that a number of developments have enabled the field of surface
science to progress from the study of simpler adsorbates to the interrogation of bigger and more
complicated molecules. An almost trivial advance was the demonstration that these compounds, which
are largely solid at room temperature, could be effectively sublimed intact into ultra-high vacuum
(UHV) chambers using Knudsen cells without compromising subsequent vacuum integrity. In
addition, more significant advances in surface spectroscopies have accompanied and underpinned this
progress. Notable amongst these are improvements in sensitivity, spectral resolution and spectral span,
which have enabled the requisite depth of data collection and analysis to be attempted. Finally,
advances in data simulation and theoretical calculations have allowed these large adsorbed species to
be modelled more effectively. What stands out from this literature review is that the complexity of the
organic/metal interface calls upon a number of complementary surface spectroscopies to be combined
in order for detailed molecular level models to be constructed. For example, local structural details
such as the chemical nature of the adsorbed species, its bonding and orientation are best obtained by
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
205
the techniques of reflection absorption infrared spectroscopy (RAIRS), electron energy loss
spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), photoelectron diffraction (PhD) and
near-edge extended absorption fine structure spectroscopy (NEXAFS). A surprising discovery of the
research in this area is the revelation that, unlike smaller organic molecules, these complex molecules
possess an extraordinary capability for self-organisation and produce remarkably ordered, crystallinelike structures at the metal surface. The details of these 2D assemblies have been best captured by low
energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) experiments. We
attribute this behaviour directly to the multiple functionalities possessed by these molecules which
enable strong lateral interactions to be expressed.
Throughout the text and figures, coverage, y, at the surface is given either in terms of fractional
monolayers (MLs), quoted with respect to the number density of surface metal atoms, or in terms of the
saturated monolayer (Sat. ML). The overlayer unit mesh is given in real space matrix notation as
follows and quoted in the text as (m11 m12, m21 m22)
ao
bo
¼
m11 m12
m21 m22
as
bs
where ao, bo are the overlayer net vectors and as, bs the underlying metal surface mesh vectors. It
should be noted that there is widespread confusion within the surface science community (including
on occasion ourselves), on the use of crystallographic conventions to define overlayer and substrate
vectors and crystal directions. Thus although overlayer unit cells are generally correctly determined,
the overlayer matrices are often not consistent with the recommended guidelines in International
Tables for Crystallography [5]. This is discussed more fully in the chapter on ‘‘Surface
Crystallography’’ by Unertl in [6] and in Appendix A here, where a comparison is made between
the overlayer matrix descriptions quoted in the text (from the original articles reviewed) and those
deduced using a more consistent approach. We would like to take this chance to urge authors to use
the criteria given in Appendix A in future work to enable comparisons between different systems to
be more readily made.
A number of phenomena emerge from our review of these systems. We wish to highlight two here:
complex adsorption phase diagrams,
manifestations of chirality at the interface.
Both of these aspects are discussed briefly below.
1.1. Complex adsorption phase diagrams
The work reviewed here clearly demonstrates that the adsorbed molecules are dynamic and evolving
entities, displaying different characteristics in response to conditions such as adsorption temperature
and adlayer coverage. This range of response is directly attributable to the multifunctionality of these
molecules which can lead to a number of surface–molecule and molecule–molecule interactions. Most
of these interactions are delicately balanced, leading to a rapid system response to varying conditions.
This complexity in behaviour is manifested at two levels: at the local level, variations of molecular
form, orientation and bonding are observed; while at the extended level, a variety of ordered assemblies
are observed. It can, therefore, be appreciated that even the combination of just one type of organic
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Fig. 1.1. Schematic diagram to show how the adsorption of one simple molecule can unleash a cascade of phases on a
particular surface. Modifiers can take up many different combinations of molecular structure, bonding, orientation and 2D
order at a surface, yielding many different phases.
molecule and one metal surface can unleash a cascade of phases, each possessing a different
combination of chemical, orientational and self-organisational behaviour (Fig. 1.1). This emphatically
highlights the sheer versatility of performance that can be generated. At present, only a few papers in
the literature attempt to summarise the multifaceted nature of these organic/inorganic interfaces. We
have, therefore, captured this information in terms of ‘adsorption phase diagrams’ that provide an
overall and succinct perspective on the behaviour of the systems. These are not thermodynamic phase
diagrams but, rather, have been constructed to give chemical and structural information on the various
adsorption phases created as a function of coverage and temperature. The data for each system have
been collated by us from the reviewed references, so varies in the degree of detail presented, according
to the nature of the original work. In some cases, data is only available at one temperature and coverage
so the term ‘phase diagram’ is not strictly appropriate. Nevertheless, we believe that this large pictorial
representation provides a valuable summary of the behaviour of each of the adsorption systems
reviewed. We have not attempted to give full accounts of the experiments reviewed but have selected
data which demonstrates particular aspects of the phase diagrams. We would like to point out that these
phase diagrams are, in fact, the essential backbone of this review, with the text providing supplementary
and supportive narrative.
1.2. Manifestations of chirality at the organic/inorganic interface
Of the various attributes that an organic molecule can bring to a metal surface, there is one that stands
out for special attention. This is the ultimate selectivity function of chirality. Chirality is simply a
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207
geometric property which dictates that the mirror transformation of an object is a non-identity
operation, i.e. the object and its mirror image are non-superimposable by any translation or rotation.
Clearly for this to hold, the object must not possess any inverse symmetry elements (i.e. centre of
inversion or reflection planes). As a result, a chiral object can exist in two distinguishable mirror, or
enantiomeric, forms. The property of chirality has profound effects in physics, chemistry and biology,
ranging from parity violations for weak forces, to the exclusive use of one mirror form of amino acids
by all life forms on earth. In the organic system, chirality generally emerges at the tetrahedral carbon,
provided sufficient complexity is present, e.g. that all the four attached substituent groups are different.
The absolute configuration of such chiral centres can be labelled R (for rectus) or S (for sinister) as
determined by the Cahn–Ingold–Prelog rules [7,8].
Chiral expression at surfaces has only attracted increasing attention in recent years, despite the fact
that it is actually easier to create chirality in a 2D system since a surface cannot possess a centre of
inversion and can only maintain reflection mirror symmetry planes normal to the surface. Although
intrinsically chiral metal surfaces can be created by cutting to expose step and kink sites that are chiral,
the interesting point for the organic/inorganic interface is how the adsorption of organic molecules
bestows chirality to a previously non-chiral surface. In fact, surface chirality can be manifested in a
number of ways and a hierarchy of surface chirality can be identified [9]. We suggest the following
classification of surface chiral systems that includes both the creation of local chiral motifs by single
adsorption events (i.e. point chirality) and the creation of chiral domains arising from the chiral
arrangements of the individual motifs (i.e. organisational chirality). We also differentiate between
molecule-induced chirality and adsorption-induced chirality and between expressions of local and
global chirality. A summary of the classification is shown in Fig. 1.2 and a description of how chirality
can be manifested at non-chiral surfaces is given below.
1.2.1. Surface chirality from adsorption of non-chiral molecules
The adsorption of non-chiral molecules at non-chiral metal surfaces has been shown to lead
under certain conditions to expressions of chirality at a metal surface. The chirality is essentially
adsorption-induced and two major classes of chirality are expressed, described below. In both cases,
the chirality is strictly only expressed at a local level, and disappears at the global level.
1.2.1.1. Point chirality: adsorption-induced chiral motifs. This is the most basic form of chirality,
arising because the adsorption site symmetry of the molecule locally destroys all surface mirror planes.
For example, this can arise simply by adsorption of the molecule so that the molecular reflection planes
do not align with the surface mirror planes. Therefore, any system with adsorption site (or point group)
symmetry C1, C2, C3, C4 or C6 qualifies for this class of chirality, e.g. even a CO molecule tilted along a
non-symmetry direction. What is very important to realise is that in such cases, energetically equivalent
reflectional configurations will always exist so that random adsorption will yield equal populations of
image and mirror image adsorption motifs. This means that the surface is a 50:50 racemic mixture and
possesses no overall chirality.
1.2.1.2. Organisational chirality: adsorption-induced chirally ordered domains. This type of chirality
arises when ordered adsorption structures are formed where the 2D organisation of molecules destroys
the reflection symmetry planes of the underlying surface. Such ordered domains belong to one of the five
possible chiral space groups (P1, P2, P3, P4 or P6) that can exist at a surface. The organisational chirality
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Fig. 1.2. Classification of chirality at a surface.
generally arises because local adsorption-induced chiral motifs of the type described above can lead to
asymmetries in lateral interactions, culminating in growth directions that lie along non-symmetry axes.
However, again, due to the inherent non-chirality of the initial molecule, there is equal probability of
nucleating reflectional chiral domains. As a result, these systems always consist of coexisting mirror
chiral domains, leading to an overall non-chiral, racemic surface.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
209
1.2.2. Surface chirality from adsorption of chiral molecules
When a chiral molecule is adsorbed at a non-chiral surface, its very presence inevitably introduces
chirality at the surface. However, there are different levels of chiral expressions, ranging from point
chirality (molecule-induced) to highly organised, extended forms of chirality (molecule- and
adsorption-induced). Crucially, the chirality of the adsorbed molecule enables chiral expression at
the surface to progress from a local to a global level.
1.2.2.1. Point chirality: molecule-induced chiral motifs. The adsorption of any chiral molecule at a
surface which leaves the molecular chiral centre intact will inevitably lead to a local chiral motif. Since
the inherent chirality of the molecule forbids the creation of its mirror image with all random adsorption
events, no mirror chiral motifs can be conceived. Therefore, an overall chiral system is always
produced.
1.2.2.2. Organisational chirality. The adsorption of chiral molecules on non-chiral surfaces can also
lead to a range of ordered structures. If one ignores the local chirality possessed by the molecule and,
instead, observes the organisation of the adsorbates with respect to the surface, it is found that both nonchiral and chiral arrangements can exist. Of the latter, two classes of chiral arrangements can exist, one in
which reflectional domains coexist, and the other in which they cannot.
Adsorption-induced chiral organisation (at the local level). In this class, the local chiral adsorption
motifs organise into a 2D chiral arrangement. However, we predict that in systems where lateral
interactions are mediated by groups that are non-chiral and sufficiently remote from the chiral centres,
reflectional domain arrangements may also be nucleated and will coexist at the surface. Therefore,
organisationally both the image and mirror image chiral domains can exist. Overall, however, the
system is still chiral, because if the inherent chirality of the molecule is taken into account, then the two
domains are only pseudo-reflections of each other. At present, no published work on such systems
exists and this remains a hypothetical classification.
Adsorption-induced chiral organisation (at the global level). This is the highest expression of
chirality at a surface, involving both the creation of a molecule-induced chiral motif and an adsorptioninduced chiral organisation, with each present in only one of its two possible mirror arrangements. As a
result, only one unique chiral domain is nucleated and sustained over the entire interface so that a chiral
surface is created possessing both global point and global organisational chirality. The expression of
such global organisational chirality is difficult to attain and the first few examples have only been
recently recorded. The constraints that govern the creation of a truly chiral organised array [10] can be
appreciated from Fig. 1.3 which shows that the number of allowed space groups rapidly dwindles when
going from three-dimensional (3D) space (230 space groups) to 2D chiral space groups (only five space
groups). More importantly, when a surface chiral space group is created, it can be expressed in either of
its mirror forms (Fig. 1.3). For a truly global organised chiral system, it is vitally important that the
manifestation of any of the five 2D chiral space groups is strictly restrained so that only one mirror
image of the unit mesh is allowed to exist at the surface.
Clearly, chirality can be manifested at a surface in a number of ways. However, we note that only those
systems possessing global chirality can be considered to be truly chiral surfaces. To aid the reader, Table 1
shows the various combinations of chiral manifestations that can occur at surfaces and their various
outcomes in terms of exhibiting local or global chirality. We also note that there are some other aspects of
chirality that are particularly well demonstrated by the tartaric acid systems, discussed in Section 2.1.
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Fig. 1.3. The five chiral space groups allowed in 2D space. Each of the space groups can yield two mirror motifs at the
surface. A surface with global organisational chirality can only tolerate the existence of one of these two possible mirror
motifs.
1.3. Systems of study
In this review, we have concentrated solely on the following classes of complex organic molecules:
(i)
(ii)
(iii)
(iv)
(v)
multifunctional carboxylic acids,
amino acids,
aromatic anhydrides,
closed and fused ring structures (without additional functional groups),
closed and fused ring structures (with additional functional groups).
Table 1
Various combinations of chiral manifestations at surfaces along with the type of chiral surface createda
a
Note that for a chiral molecule with a chiral adsorption motif with no mirror image and a chiral organisation with no
mirror image this leads to the special case of global organisational chirality.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
211
Our choice represents systems for which a critical mass of literature has only recently been
accumulated, with sufficient multi-technique information available to enable a more holistic understanding to be attempted.
2. Carboxylic acids
The simple carboxylic acids (functional group COOH) such as formic and acetic acid have been the
subject of much detailed earlier work and their properties at metal surfaces have been reviewed elsewhere
[1]. Here we concentrate on carboxylic acids that have more than one functionality that may be involved
in bonding to the metal surface—either additional rings or further COOH groups. As indicated in the text,
some of the carboxylic acids are truly chiral, others are achiral and some are prochiral, in the sense that
they can be adsorbed onto a surface with one of two possible faces uppermost, leading to a form of 2D
chirality. The varying interactions of these molecules are discussed for a range of metal surfaces.
2.1. Tartaric acid (L(þ)-2,3-dihydroxysuccinic acid)
Techniques used: RAIRS, LEED, TPD, STM, periodic DFT calculations.
Preparation: Sublimation of pure enantiomers of tartaric acid (99%).
Tartaric acid occupies a special place in the scientific history of chirality with Pasteur’s famous
discovery that the molecule existed in two mirror crystalline forms [11]. More recently, it has been used in
chiral technology where there is a strong driver to establish enantioselective catalytic methods whereby
pure enantiomeric forms of pharmaceuticals, flavours, agrochemicals, etc. can be produced. One
successful way of creating heterogeneous chiral catalysts [12–14] is to adsorb chiral organic ‘modifier’
molecules at metal surfaces in order to introduce asymmetry. The dicarboxylic acid, tartaric acid, which
possesses two chiral centres is one of the most successful chiral modifiers. Its presence on Ni, Cu and
Co surfaces [13,14] endows significant discrimination to the hydrogenation of methylacetoacetate
(MAA) to give methyl-3-hydroxybutrate (MHB) as shown in Fig. 2.1. For example, on a nickel surface
modified by R,R-tartaric acid, the reaction is stereodirected so that the R-product is produced in >90%
enantiomeric excess. Conversely, modification by S,S-tartaric acid favours the S-product. In such
modifications, the need to create a globally chiral surface is self-evident. In order to gain a fundamental
understanding of the nature of the organic/metal interface created in such systems, the behaviour of the
modifier R,R-tartaric acid on defined Cu(1 1 0) and Ni(1 1 0) surfaces has been recently investigated. As
we go to press, we also note that further adsorption studies have been carried out on the Ni(1 1 1) surface
[15,16].
2.1.1. R,R-tartaric acid on Cu(1 1 0)
The adsorption behaviour of R,R-tartaric acid on Cu(1 1 0) under varying coverage and temperature
conditions reveals that the molecule/metal system occupies a complex and varied phase space (Fig. 2.2)
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Fig. 2.1.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
The stereodirected hydrogenation of MAA to give MHB on Ni catalysts chirally modified by tartaric acid (TA).
[10,17–21]. Locally, the chemical, bonding and orientational structure adopted at the surface undergoes
dynamic changes with conditions, causing the chiral organic molecule to adopt the monotartrate, the
bitartrate or the dimer forms (Fig. 2.2). Furthermore, these local units self-organise into a range of
ordered, crystalline architectures at the surface, some of which possess the property of true global
organisational chirality. The adsorption of this molecule will be considered in some detail as this
system presents a particularly good example of the way one simple organic entity can lead to a
multitude of organic/metal interfaces, each possessing different characteristics. The phase diagram in
Fig. 2.2 shows that, overall, at least six different types of monolayer phases are fashioned in the course
of adsorbing R,R-tartaric acid on Cu(1 1 0) [18] depending on adsorption temperature, coverage and
holding time. Of these, only three will be discussed in detail: the (9 0, 1 2) phase which is the
energetically preferred low coverage phase, the (4 0, 2 3) phase which dominates at intermediate
coverages and the (4 1, 2 3) phase that is created at the highest coverages.
2.1.1.1. Low coverage (9 0, 1 2) phase: the bitartrate assembly. The (9 0, 1 2) phase is the
thermodynamically preferred phase on Cu(1 1 0) at low coverages. However, a significant activation
barrier of greater than 70 kJ mol1 is associated with its creation [19] and it only forms spontaneously at
temperatures in excess of 400 K. Detailed RAIR spectroscopic data show that this adsorbed layer consists
entirely of the doubly deprotonated bitartrate species in which the two oxygen atoms in each COO unit
are held almost equidistant from the surface. Both carboxylate ends of the molecule are involved in
bonding to the surface, leaving the C2–C3 bond almost parallel to the surface and yielding a fairly rigid
adsorption geometry. Recent periodic DFT calculations [22] have confirmed this general geometry and
have shown that the adsorption site of the bitartrate is one where the oxygens occupy on-top positions
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Fig. 2.2.
213
Phase diagram of R,R-tartaric acid on Cu(1 1 0).
across the two short bridge sites with the Cu–O bonds possessing, on average, a length of 1.97 Å
(Fig. 2.3c).
The 2D nature of this phase, shown in Fig. 2.3d has been constructed from a comprehensive analysis
of the LEED and STM data. LEED data (Fig. 2.3a) reveal that the long-range ordering of tartaric acid at
the surface yields a (9 0, 1 2) structure which is consistent with a large unit cell possessing the
dimensions 23:04 — 7:68 Å, a ¼ 19:47 . High resolution STM images, displayed in Fig. 2.3b, show
that there are three bitartrate molecules per unit cell, resulting in a fractional coverage of 1/6. The STM
images also reveal that rows of three bitartrate molecules assemble at the surface to form long chains,
which are aligned along the h1 1 4i surface direction. These growth directions are believed to be
dictated by the presence of the a-hydroxy groups attached to both chiral centres of the molecule
[10,22].
2.1.1.2. Medium coverage (4 0, 2 3) phase: the monotartrate assembly. Although a medium coverage
phase, this structure only presents a low activation barrier for formation and so is created directly as
adsorption is carried out on a clean Cu(1 1 0) surface at 300 K. STM data show that the nucleation of this
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Fig. 2.3. Details of the (9 0, 1 2) chiral phase created at low coverages and high temperatures showing: (a) the LEED pattern
obtained at 31 eV; (b) 150 — 200 Å STM image of the chiral surface showing ‘trimers’ of bitartrate molecules aligned in
columns directed along the chiral h1 1 4i direction; (c) the local bonding description of the bitartrate unit; (d) a schematic
model of the overlayer constructed from the STM, LEED and RAIRS.
phase occurs in the early stages of adsorption and is preferentially located at step edges. Increasing
coverage leads to the steady growth of these islands until the entire surface is covered in this phase.
Again, detailed information on the nature of this phase has been constructed using a combination of STM,
LEED and RAIRS data. First, LEED photographs of this phase (Fig. 2.4a) show sharp diffraction spots
indicating that the islands possess very ordered arrangements of the modifier molecules in a (4 0, 2 3)
structure, on the Cu(1 1 0) surface. STM data (Fig. 2.4b) reveal that there are two other molecules within
this unit cell, giving a much more packed structure with a local coverage of 0.25 ML. The reason for the
LEED inequivalence of these two extra molecules is not clear. However, there are a number of indications
that suggest that the details of the H-bonding network govern the overlayer symmetry and possibilities
are discussed in the original reference [18].
The chemical detail of the adsorbed entities is provided by RAIR spectra [18] obtained for this
phase (Fig. 2.4c). The presence of both the n(C=O) vibrations of the acid COOH functionality
at 1705 cm1 and the ns(COO) vibration of the carboxylate group at 1437 cm1, reveal that the
R,R-tartaric acid is adsorbed as a monotartrate species which is bound to the surface via the
deprotonated carboxylate group. In addition, the free and intact COOH acid group is held away from
the surface and the considerable downshift in frequency of the n(C=O) vibration of this group suggests
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
215
Fig. 2.4. The (4 0, 2 3) phase of R,R-tartaric acid on Cu(1 1 0): (a) LEED pattern obtained at 26 eV; (b) STM image
(80 — 75 Å; Vtip ¼ 1:52 V; It ¼ 1:25 nA) showing the position of individual adsorbates; (c) RAIRS data monitoring the
nature of the adsorbed species; (d) a structural model of the phase with the unit cell outlined.
that it is involved in intermolecular H-bonding interactions with the alcohol groups of neighbouring
monotartrate species, leading to a strong tendency for island growth in this phase. Combining all these
different pieces of information, a fairly complete description of this adsorbed phase is constructed
(Fig. 2.4d).
2.1.1.3. High coverage (4 1, 2 3) phase: the dimer–monomer assembly. Further adsorption beyond a
coverage of 0.25 ML leads to the creation of a high coverage ordered phase at 300 K, which produces a
new (4 1, 2 3) LEED pattern. The RAIRS data of this (4 1, 2 3) phase [18] retain the overall fingerprint
of the monotartrate species, but reveal perturbation of the n(C=O) band which splits into two
contributions at 1759 and 1674 cm1, the latter frequency typical of H-bonded cyclic acid dimers
and the former indicative of a monomer or open-chain acid group which involves less H-bonding. In
contrast, the 1437 cm1 vibrational band due to the carboxylate functionality anchored to the Cu(1 1 0)
remains almost unchanged, indicating that this part of the adlayer is essentially similar to that of the
(4 0, 2 3) phases.
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Fig. 2.5. STM images of the (4 1, 2 3) structure taken at different tunnelling conditions. (a) Molecular resolution showing
position of the adsorbed R,R-tartaric acid molecules (150 — 150 Å; Vtip ¼ 0:03 V; It ¼ 0:51 nA); (b) lines along the h1 1 2i
and h1 1 0i directions coinciding with the positions that would have to be adopted by OH groups in order to create a network of
H-bonds between molecules (150 — 150 Å; Vtip ¼ 1:11 V; It ¼ 0:46 nA).
STM images of this phase exhibit an ordered and dense adlayer, also with a (4 1, 2 3) repeat structure
(Fig. 2.5a). Using the chemical information and data from STM images taken at different tunnelling
conditions, a model of the (4 1, 2 3) adlayer has been constructed [18,21]. The STM image in Fig. 2.5a
shows that along the h0 0 1i direction, repeat units consisting of a bright two-lobed structure and a
smaller feature are seen. The RAIRS data for this structure show that cyclic carboxylic acid dimers and
monomer acid groups coexist in this structure, the former consistent with the two-lobed STM features,
and the latter associated with the single monotartrate unit (Fig. 2.5). Each cyclic dimer unit possesses
an OH group at each end and it has been proposed that lateral interactions (possibly H-bonding)
between the OH groups of adjacent dimer units naturally force the dimer chain along h1 1 2i direction.
The STM image in Fig. 2.5b also shows strong lines along h1 1 2i direction, which coincides with the
alignments that need to be adopted by the dimer OH groups in order to facilitate strong H-bonding
intermolecular interactions. This STM image also shows that similar direct lines are also observed
along the h1 1 0i direction, suggesting that the OH groups on the single monotartrate molecules are also
involved in H-bonding with the adjacent dimer units, tying the entire structure together by connecting
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217
Fig. 2.6. (a) Schematic diagram showing the unit cell and the position of the R,R-tartaric acid molecules in the (4 1, 2 3)
structure; (b) formation of cyclic dimers and hydrogen bonding interaction between dimers along the h1 1 2i direction; (c)
depiction of single monotartrate molecules involved in further H-bonding interactions along the h1 1 0i direction, thus weaving
dimer chains together. Note that in all structures the carboxylate groups bonding with the surface are placed in short-bridged
sites which places each oxygen above a Cu atom.
neighbouring dimer chains, as shown in Fig. 2.6c. In such a structure the acid group of the single
monotartrate molecules remains free, rationalising the emergence of the additional n(C=O) band at
1759 cm1. The overall pattern adopted by the dimer and monomer adsorbates on Cu(1 1 0) is shown in
Fig. 2.6a.
2.1.1.4. The emergence of global organisational chirality. For all the phases described above, the
inherent chirality of R,R-tartaric acid always leads to the establishment of point chirality and a local
chiral motif. However, in addition, a closer analysis reveals that for certain phases this local chirality
transfers to a truly global chiral organisation. Ignoring the local chirality possessed by the molecule and,
instead, observing the arrangement of the adsorbates with respect to the surface, it can be seen that the
(4 0, 2 3) pattern possesses two reflection symmetry elements but the (9 0, 1 2) and the (4 1, 2 3) templates
possess none (Fig. 2.7). Clearly, for the latter two cases, a global point and global organisational chiral
surface has been created, in which the arrangement of the molecules annihilates both reflection symmetry
planes of the underlying Cu(1 1 0). Importantly, for both phases, the same growth directions and
arrangements are maintained over the entire surface and as a result a perfect globally chiral surface is
created which is non-superimposable on its mirror image. A consideration of the (9 0, 1 2) and (4 1, 2 3)
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Fig. 2.7. Overall adsorbate templates created by the (4 0, 2 3), the (9 0, 1 2) and the (4 1, 2 3) structures. Whereas the (4 0, 2 3)
pattern retains both reflection symmetry planes of the Cu(1 1 0) surface, the (9 0, 1 2) and the (4 1, 2 3) patterns do not, thus
creating surfaces with global organisational chirality.
structures at the molecular level enables the following three conditions to be identified as central to the
expression and sustainment of true chirality over an entire interface:
chirality of the modifier molecule,
rigid and defined adsorption geometry,
directional and anisotropic lateral interactions.
It can be seen that the (9 0, 1 2) bitartrate and the (4 1, 2 3) dimer phase fulfil all these requirements.
The inherent chirality of these molecules and their two-point bonding at the surface uniquely defines
the ‘footprint’ the molecule casts at a surface and dictates the position of all its functional groups in
space. Once this is achieved, the intermolecular interactions between the modifiers control the
placement of neighbouring molecules. Here, the chirality of the adsorbates ensures that these lateral
interactions are anisotropic. For both cases, it is believed that the growth direction is dictated by the
spatial positioning of the a-hydroxy groups attached at the chiral centres of the adsorbed species
(Figs. 2.3 and 2.6), making the intermolecular interaction uniquely directional. Although, the detailed
nature of these interactions still remain a matter of debate [10,22] it is, nevertheless, clear that a better
description of these global organised chiral structures is that they are a supramolecular assembly [23]
of chiral modifiers. These lateral interactions essentially ensure that an energy difference exists
between one chiral unit mesh and that of its mirror twin, so that only the energetically favoured chiral
domain is created. For the (9 0, 1 2) phase, DFT calculations show that an energy difference of
10 kJ mol1 exists between R,R-tartaric acid arranged in the (9 0, 1 2) mesh and its mirror (9 0, 1 2)
arrangement [22]. Therefore, at 300 K, over 95% of the adsorption would occur with one preferred
chiral arrangement.
2.1.1.5. Creation of chiral spaces within the chiral surfaces. The models presented of the surfaces
with global organisational chirality created by R,R-tartaric acid on Cu(1 1 0) lead to another notable fact,
namely, that the structure is open enough to reveal empty, nanosized chiral channels and spaces in which
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the underlying metal is exposed. The genesis of these empty chiral spaces is still a matter of investigation,
with some indication that they are due to strain-breaks in the modifier overlayer arrangement and,
possibly, reconstruction of the underlying metal surface. These nanosized spaces combine the reactivity
of the underlying metal with the selectivity of the decorating chiral molecules and may be an important
route to create molecular recognition or transformation sites, in which stereospecific docking and
reaction processes can be controlled.
2.1.1.6. Switching global organisational chirality. An important aspect of the asymmetric hydrogenation of b-ketoesters on tartaric acid modified surfaces is that switching the chirality of the
tartaric acid switches enantioselectivity from the R- to the S-reaction product (Fig. 2.1). Therefore,
one would expect the mirror enantiomer of R,R-tartaric acid to create a mirror surface structure. This
was first tested out by the adsorption of the opposite enantiomer, S,S-tartaric acid, on Cu(1 1 0) and
creating the parallel structures [10]. Turning first to the bitartrate structure, a sharp LEED pattern is
obtained from the equivalent S,S-tartaric acid phase, but the positions of the diffraction spots are
switched to the mirror (9 0, 1 2) structure. This chiral switching is better illustrated by the STM images
(Fig. 2.8) where the (9 0, 1 2) phase of S,S-tartaric acid on Cu(1 1 0) is revealed to be a true mirror
Fig. 2.8. Switching global organisational chirality for the (9 0, 1 2) phase: R,R-tartaric acid versus S,S-tartaric acid. This
chiral switching is illustrated by the 108 — 108 Å STM images and the schematic models of mirror adlayers created
when R,R- and S,S-tartaric acid are adsorbed on Cu(1 1 0). Note the mirror positioning of the OH groups for the R,R- and the
S,S-tartaric acid adsorbates.
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Fig. 2.9. Switching global organisational chirality for the (4 1, 2 3) phase: R,R-tartaric acid versus S,S-tartaric acid. This
chiral switching is illustrated by the 110 — 100 Å STM images and the schematic models of mirror adlayers created when
R,R- and S,S-tartaric acid are adsorbed on Cu(1 1 0).
image of the (9 0, 1 2) phase obtained for R,R-tartaric acid, with every adsorbate position reflected in
space [10]. The preference for each enantiomer to describe a particular chiral domain is further
demonstrated when a mixture of R,R- and S,S-tartaric acid is adsorbed at the Cu(1 1 0) surface and
shows the 2D version of the famous Pasteur experiment, i.e. distinct and separate R,R- and S,S-mirror
chiral domains are formed. This chiral switching is also observed for the (4 1, 2 3) phase where LEED
and STM images (Fig. 2.9) show a reversal of all adsorbate positions for the S,S-tartaric acid system to
create the chiral twin surface.
Surface chiral switching has been explained in terms of how opposite enantiomers guide the
supramolecular assembly directions. For both global organisationally chiral phases created by
tartaric acid, the growth direction of the adlayer is dictated by the conformation of the a-hydroxy
groups attached to the chiral centres of the adsorbed species. In both structures, the R,R- and
S,S-adsorbed species possess defined and rigid adsorption geometries arising from their two-point
bonding with the metal surface. As a result they exhibit one major difference, namely the spatial
orientation of their OH groups, Figs. 2.8 and 2.9. Looking down at the surface, it can be seen that the
positions of the OH groups on R,R-tartaric acid in the (9 0, 1 2) structure are oriented in a mirror
configuration to those of S,S-tartaric acid. Thus, the enantiomers adopt mirror growth directions, with
intermolecular interactions leading to a molecular chain alignment along the h1 1 4i direction for
R,R-tartaric acid, but the mirror h1 1 4i direction for S,S-tartaric acid. The same is true for the (4 1,
2 3) phase, where the OH groups connecting the dimer and monomer units occupy mirror positions
for the R,R- and the S,S-tartaric acid systems, leading to the mirror chiral arrays being created.
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The central role played by the chiral OH groups in guiding a unique supramolecular assembly
direction is also nicely illustrated by comparing the (9 0, 1 2) and the (4 1, 2 3) structures. For the
(9 0, 1 2) structures the bitartrate species possesses OH groups along the bottom left to top right
diagonal, while for the cyclic dimer, these groups are now oriented along the bottom right to top
left diagonal. As a result, the assembly directions for the two structures also switch their alignment
sense accordingly.
2.1.1.7. Sustaining a single rotational domain. The importance of sustaining just one reflection, or
mirror, unit mesh in order to create a perfect global organised chiral array has been discussed. There is
another aspect of the (9 0, 1 2) and the (4 1, 2 3) structures that is also appealing, namely that a single
rotational domain is also sustained across the entire surface. Although rotational domains do not
threaten the chirality of a system and would not affect the heterogeneous enantioselective response,
some optical applications may require the stringent double-demand of a single reflection and rotational
domain to be maintained across an entire surface. However, adsorption is a random process and initial
nucleation events cannot be controlled to occur in only one of many energetically equivalent rotational
positions. Generally, rotational domains occur when the overlayer surface mesh possesses a lower
symmetry than that of the clean substrate. Therefore, to ensure that all rotational domains that are
created are equivalent, rotational symmetry matching of the unit mesh with the surface is needed so that
all equally probable adsorption geometries result in an identical nucleation point. For example, it can be
seen that this is exactly the case for the (9 0, 1 2) and the (4 1, 2 3) structures (Figs. 2.8 and 2.9). Both
possess unit meshes with C2 point group symmetry, which rotationally matches the twofold axis of the
underlying Cu(1 1 0) surface.
2.1.2. R,R-tartaric acid on Ni(1 1 0)
R,R-tartaric acid modified nickel surfaces are perhaps the most successful heterogeneous
enantioselective catalysts [13,14] and, therefore, the nature of chiral modification of Ni(1 1 0) by R,Rtartaric acid represents a particularly relevant system for study [24–26]. As for Cu(1 1 0), the adsorbed
molecule shows significant chemical versatility with the bi-acid, the monotartrate and the bitartrate forms
all being created at various points of the adsorption phase diagram (Fig. 2.10). However, there are two
main points of difference with respect to the Cu(1 1 0) surface. First, the barrier to the creation of the
bitartrate form is much lower so that it occurs spontaneously upon initial adsorption at room temperature.
Second, there is little evidence of the beautifully ordered supramolecular structures observed for
Cu(1 1 0). Here, the work on the room temperature phase only is reviewed in detail since it introduces new
aspects of chiral manifestation at the surface.
2.1.2.1. The room temperature phase: adsorption stresses and chiral reconstructions. Fig. 2.11 shows
the STM data obtained with increasing coverage of R,R-tartaric acid on Ni(1 1 0) at room temperature.
From these, it can be seen that no long-range ordered structures of the type observed on Cu(1 1 0) are
formed. However, despite the lack of 2D ordering, Fig. 2.11 shows that all the adsorbed molecules
exhibit a very preferred growth direction along the main h1 1 0i direction, which also coincides with a
mirror symmetry direction of the surface. Again, this behaviour is very different from that observed
for Cu(1 1 0) where the molecules assemble along non-symmetry directions, thus annihilating all
the mirror planes. By the application of surface infrared spectroscopy and density functional
theory (DFT) it has been possible to obtain a fundamental understanding of the local adsorption
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Fig. 2.10. Phase diagram of R,R-tartaric acid on Ni(1 1 0) from [26].
structures created on the Ni(1 1 0) surface and gain new and interesting insights on the nature of chiral
induction.
RAIRS data for low coverages of R,R-tartaric acid adsorbed on Ni(1 1 0) at 300 K is consistent with
the creation of the bitartrate species, adsorbed so that both oxygen atoms in each carboxylate group are
approximately equidistant from the surface and the OCO planes are largely inclined towards the surface
plane [24,26]. However, the most valuable insight into the adsorption of R,R-tartaric acid on Ni(1 1 0)
comes from consideration of its adsorption site using periodic DFT calculations [24]. For adsorption on
the bulk truncated Ni(1 1 0) surface, the most stable adsorption geometry is one in which two-point
bonding of the bitartrate occurs with the oxygen atoms of both carboxylate functionalities adsorbing on
top of adjacent Ni atoms, their positions in a plane parallel to the surface (Fig. 2.12a). This adsorption
site is similar to that found for the bitartrate species on a bulk truncated Cu(1 1 0) surface [22].
Interestingly, this adsorption geometry is preferred largely due to the minimisation of repulsive
interactions between the molecular OH groups and the Ni surface atoms. However, such computed
configurations involving adsorption on the bulk truncated surface constrain the resulting groundstate
severely, as they do not allow for any change of the lateral distances between the Ni surface atoms.
Therefore, the adsorption energetics have been investigated [24] starting with a fully relaxed bitartrate-Ni4
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Fig. 2.11. STM images obtained with increasing coverage from (a) to (c) of R,R-tartaric acid on Ni(1 1 0) at room
temperature. (a) 300 — 300 Å image with Vtip ¼ 2:115 V and It ¼ 1:16 nA; (b) 300 — 300 Å image with
Vtip ¼ 1:467 V and It ¼ 1 nA; (c) 300 — 300 Å image with Vtip ¼ 1:76 V and It ¼ 1 nA. In (a) areas of flat terraces
are shown alongside step edges. In (b) and (c) the areas imaged as long dark stripes are due to co-adsorbed H atoms, which are
produced during the chemisorption process via a double deprotonation of the R,R-tartaric acid molecule. The H-atoms
segregate in separate islands and are thought to be adsorbed in a local missing-row structure.
complex which is then free to optimise the positions of the Ni atoms comprising the adsorption site.
The most interesting outcomes of this are: (i) the interaction between the chiral OH groups of the
molecule and the Ni atoms leads to an acute distortion in the molecule whereby the C(2)–C(3) bond is
skewed by about 458 with respect to each of the other C–C bonds, and (ii) the pairs of Ni atoms
constituting the adsorption site are now placed a significant distance apart and describe an oblique unit
mesh at the surface for which all mirror planes are lost locally (Fig. 2.12b). As a result, the adsorbed
bitartrate conveys its chirality not just simply by its presence at the surface, but more profoundly via
the chiral footprint it places onto the surface. Since the Ni footprint of the adsorbed complex requires
the long-bridge distance between the pairs of Ni atoms to be about 7.47 Å, it can only be accommodated on a bulk truncated Ni(1 1 0) surface via local paired-row and missing-row reconstructions
(Fig. 2.12c).
STM images (Fig. 2.11) support these general conclusions. The molecular structures in the STM
images occupy, on average, a space of 6:8 — 4:6 Å, in good agreement with the calculated area of
7:04 — 4:98 Å encompassing the relaxed bitartrate-Ni4 complex (Fig. 2.12b). In addition, the small
scale structure of the molecular adsorption reveals severe distortions in the immediate vicinity
(Fig. 2.11a and b), extending, on average, over a 15:5 — 12:5 Å area for a single molecule. For the
bulk truncated surface this represents an approximate 6 3:5 surface cell, consistent with either a local
paired-row or missing-row reconstructions, with the resulting perturbation being propagated a number
of atomic distances away from the adsorption centre. Schematic models depicting such local
reconstructions (Fig. 2.12c) show that the perturbation caused by the adsorbed bitartrate extends over at
least a 5 3 unit cell. For both geometries, the OCO planes of carboxylate units are significantly
inclined towards the surface plane, consistent with the RAIRS data and the calculated value of 38.58 for
the adsorbed bitartrate-Ni4 complex. Finally, the driving force for this reconstruction is sufficient for it
to be manifest even for the single molecule adsorption event. This is supported by the calculations
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Fig. 2.12. (a) Adsorption configurations for the bitartrate species adsorbed on Ni(1 1 0) aligned along the [0 0 1] axis;
(b) progressive distortions calculated for the C1–C2–C3–C4 skeleton a bitartrate-Ni4 species adsorbed on a Ni(1 1 0) surface.
Note that the significant skewing of the molecular skeleton causes the four bonding nickel atoms to describe an oblique, chiral
footprint; (c) schematic models to demonstrate how the relaxed bitartrate-Ni4 footprint could be accommodated at the
Ni(1 1 0) surface by local reconstruction of the surface to (i) a paired-row structure and (ii) a missing-row structure.
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which show that the adsorbed bitartrate-Ni4 species possesses a very large binding energy of
656 kJ mol1 at a Ni(1 1 0) surface. This value is consistent with DFT calculations of individual
carboxylate groups bonding to the surface, reported to be 357 kJ mol1 for formate on a Cu10(1 1 0)
cluster and 318 kJ mol1 for acetate on a Cu10(1 1 0) cluster [27]. The strength of the bitartrate-surface
bond is also reflected in temperature programmed desorption (TPD) experiments [24] where an
explosive decomposition releasing the products, H2, H2O and CO2, is observed at 454 K, indicating that
the bitartrate–metal interaction is so strong that intramolecular bonds break prior to metal–molecule
bonds.
2.1.2.2. The nature of electronic chiral communication from molecule to metal. An intriguing aspect
of the R,R-tartaric acid/Ni(1 1 0) system is that the chiral carbon centres of the molecule are two positions
removed from bonding oxygen groups which are attached to the non-chiral carboxylate carbons.
Nevertheless, a chiral restructuring of the metal occurs. Therefore, the question arises: how is the
chirality of the molecule communicated from the chiral centres to the bonding groups and thence to the
metal surface itself? A detailed analysis of the charge distribution and electronic states of the bitartrate
species adsorbed on Ni(1 1 0) [25] reveals that adsorption involves a complex interplay of molecular
distortion, hybridisation of molecule and metal states, charge donation and back donation. First, the
charge distribution of all occupied states even in the gas phase bitartrate (Fig. 2.13a) shows that the chiral
properties of the molecule are actually manifest at the level of single electron states which show electron
distributions that connect the chiral OH groups directly to the carboxylate groups. Calculations of total
charge at each atomic position show that the carbons act as donors and oxygens as electron acceptors.
Crucially, the negative charge surplus at the four oxygens atoms turns out to be chiral in distribution,
ranging from 0.27 for O(2) and O(3) and 0.33 for O(1) and O(4). Upon attachment of the bitartrate
species to Ni atoms, this asymmetric charge distribution in the carboxylate oxygens vanishes and the
single electron states of the bitartarte-Ni4 entity are altered (Fig. 2.13b) due to hybridisation of
molecular states with metal states. In fact, it is the donation and backdonation of charge via
hybridisation of molecular orbitals with nickel orbitals which removes the charge asymmetry in the
carboxylate groups and instead, propagates this chiral signature to the hybridised electron states of the
bitartarte-Ni4 complex. This makes the four Ni–O bonds inequivalent, with the Ni–O(1) and Ni–O(4)
bonds being 2.04 Å long and Ni–O(2) and Ni–O(3) bonds being shorter at 1.94 Å. The main
consequence of this is that the adsorption site, constituting the four Ni atoms now attains a chiral
electronic structure!
2.1.2.3. From local chiral reconstructions to global organisational chirality. At present, two separate
approaches have been utilised to create metal surfaces with global organisational chirality. One,
demonstrated by the R,R-tartaric acid/Cu(1 1 0) system involves the assembly of chiral molecules
into organised chiral arrays at the surface. In such cases, the adlayer template possesses chiral growth
directions which serve to destroy the mirror planes of the surface and chirality is bestowed without
necessarily having to introduce chiral rearrangements of the metal surface atoms. The second approach
has been to utilise metal single crystals, cut to expose specific surface planes which are intrinsically chiral
exposing non-straight step edges with kink sites in which the metal atoms are arranged in a chiral
configuration [28–30]. The research on R,R-tartaric acid on Ni(1 1 0) demonstrates that these two
attributes can be fused together, with the adsorption of the chiral molecule causing an attendant chiral
restructuring of the underlying metal atoms. We note that the underlying chiral reconstruction of metal
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Fig. 2.13. Total charge distributions for single electron states of (a) gas phase bitartrate [25] and (b) bitartrate-Ni4 complex
[25].
surfaces by chiral adsorbates may well be a more widely occurring phenomenon and there is now clear
need to re-examine other systems with appropriate structural techniques.
Again, the most important aspect to appreciate is that to progress from a local chiral structure to a
truly global organisationally chiral surface requires the creation of mirror chiral motifs to be severely
curtailed. This is a stringent requirement and a number of systems which exhibit local chirality, created
at surfaces by adsorption of molecules [31,32] or by restructuring of the metal [33], are unable to
advance to a truly global chiral surface because the mirror adsorption or the mirror reconstruction are
equally allowed, leading to an overall racemic system. In order to establish whether R,R-tartaric acid
can also create a mirror footprint at the surface, the adsorption energies for the relaxed bitartrate-Ni4
complex in the twin chiral footprint geometries showed in Fig. 2.14 were computed. These spinpolarised calculations showed that the adsorption of R,R-tartaric acid in the mirror footprint geometry
(Fig. 2.14b) was less favoured energetically by about 6 kJ mol1. Although this is a small energy
difference, a simple consideration of the Boltzmann distribution law shows that, at 300 K, this difference
is sufficient to ensure that over 90% of the adsorbed bitartrate molecules adopt the preferred chiral
footprint (Fig. 2.14a) creating an overall highly chiral surface. As a comparison, it should be noted that in
homogeneous catalysis enantioselectivity is successfully achieved in systems operating with energy
differences of 10 kJ mol1 between two reaction pathways leading to opposite enantiomers [34].
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Fig. 2.14. Depiction of the relaxed bitartrate-Ni4 species adsorbed in twin mirror chiral footprints at the Ni(1 1 0) surface.
2.2. Succinic acid
Techniques used: RAIRS, TPD, LEED.
Preparation: Thermal sublimation of solid powder under vacuum.
Succinic acid is a very similar molecule to tartaric acid, with the only difference being that the two
alcohol groups present in tartaric acid are replaced by hydrogen atoms leading to the consequent loss of
chirality. The adsorption behaviour of succinic acid on Cu(1 1 0) surface has been studied [35] in the
same manner as tartaric acid, allowing for a comparison of the behaviour of both to be made, especially
regarding the importance of the hydroxyl groups in tartaric acid which enable global organisational
chirality to be achieved.
2.2.1. Succinic acid on Cu(1 1 0)
As for tartaric acid, succinic acid is capable of existing in at least three different forms: the neutral biacid form, the monosuccinate form, where one of the carboxylic groups has deprotonated, and the
bisuccinate form in which both acid groups have deprotonated. The adsorption behaviour is
summarised in the phase diagram (Fig. 2.15). The creation of the bi-acid form is confined to low
temperature adsorption, while at higher temperatures both the bisuccinate and the monosuccinate are
formed. Interestingly, the formation of bisuccinate is no longer kinetically hindered at room
temperature as seen for the bitartrate/Cu(1 1 0) system, and is formed spontaneously at low coverages.
It remains the preferred adsorption mode at low coverages, and only at high coverages is the
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Fig. 2.15. Phase diagram of succinic acid on Cu(1 1 0).
monosuccinate formed. Again, unlike tartaric acid, where the bitartrate form is converted to
the monotartrate form, here the bisuccinate is never transformed, but rather coexists with the newly
formed monosuccinate form. The monosuccinate form is largely unstable with respect to the bisuccinate
form and can be converted to the latter upon heating or with time, suggesting that its creation at high
coverages arises from kinetic factors. RAIRS data [35] show that the general adsorption geometry of the
bisuccinate is very similar to that of the bitartrate on Cu(1 1 0).
Information on the self-organisation of the monosuccinate and bisuccinate phases is provided by
LEED studies. The monosuccinate phase gives rise to a p(4 2) overlayer, whereas in the
bisuccinate phase, coexisting (9 0, 1 1) and (9 0, 1 1) chiral unit cells are observed (Fig. 2.16).
Whereas there is no chirality associated with the monosuccinate species, either at the local or the
organisational level, the bisuccinate does give rise to a local non-chiral motif organised in a chiral (9
0, 1 1) domain. However, the inherent symmetry of the molecule enables the reflection (9 0, 1 1)
domain to coexist leading to an overall racemic system. In conclusion, although the 2D organisation
of the bisuccinate adlayers can be chiral, two equivalent mirror image domains are always nucleated
and found at the surface (Fig. 2.16). This emphasises the importance of the alcohol groups of tartaric
acid in creating surfaces with global point and organisational chirality on the Cu(1 1 0). It is also
interesting to note from Fig. 2.17 that succinic acid molecules are adsorbed closer together than
tartaric acid molecules. This is again a direct consequence of the absence of the hydroxy groups in
succinic acid.
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Fig. 2.16. LEED pattern observed when succinic acid is adsorbed on Cu(1 1 0) at 300 K, showing a mixture of p(4 2),
(9 0, 1 1) and (9 0, 1 1) structures.
TPD from the bisuccinate adlayer shows an explosive desorption at around 600 K arising from very
strong bonding interaction between the succinic acid molecules in the bisuccinate phase and the metal
surface. Similar explosive desorption has been observed in the bitartrate phase [19].
2.3. Benzoic acid
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Fig. 2.17. Schematic diagram showing the alignment of the succinic and tartaric acid molecules when they are adsorbed on a
Cu(1 1 0) surface with the bidentate orientation.
Techniques used: RAIRS, TPD, LEED, STM, NEXAFS, XPS, HREELS.
Preparation: Solid powder sublimed under vacuum and exposed to crystal through leak valve.
Studies on benzoic acid at metal surfaces provided some of the first evidence that the adsorption of
multifunctional molecules is not just dependent on the strength of interaction between the adsorbing
molecules and the metal atoms. The structure of the underlying metal surface and, thus, the mobility of
the metal atoms lead to different effects for benzoic acid adsorbed on different surfaces of the same
metal, e.g. Cu(1 1 0) and Cu(1 1 1). The strength of the metal–adsorbate bond also leads to variations in
behaviour between different metals, e.g. Cu(1 1 0) and Ni(1 1 0). At all times there is an interplay
between preferred adsorption site, bonding functionality and mobility of both the underlying metal
atoms and the adsorbed species. In its acid state, benzoic acid is a prochiral molecule and thus capable
of adsorbing with a local chiral motif but incapable of sustaining global chirality. However, it is often
adsorbed in the deprotonated carboxylate form which has no inherent chirality. Benzoic acid is capable
of forming large ordered adsorbate structures and, for either chemical form, the possibility of creating
chirally organised domains exists in which both reflection domains are equally probable.
2.3.1. Benzoic acid on Cu(1 1 0)
Coverage and temperature dependent studies of this system have led to a very detailed understanding
of the way benzoic acid molecules interact with the Cu(1 1 0) surface [36–39]. Most measurements
have been conducted with the copper held at room temperature but more recent studies have been
undertaken with the copper surface cooled by liquid nitrogen. The various forms of benzoic acid seen
on Cu(1 1 0) are collated in the phase diagram shown in Fig. 2.18, constructed from the results
presented in the above references. This is a modified version, to include molecular sketches, of the
phase diagram provided by the authors of [38].
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Fig. 2.18. Phase diagram of benzoic acid on Cu(1 1 0).
2.3.1.1. Room temperature substrate. At room temperature, in an analogous manner to the simpler
carboxylic acids, both RAIRS and HREELS show no evidence for vibrations associated with the C=O or
OH groups but vibrations associated with the carboxylate COO ion are present, indicating that the
molecule adsorbs with the carboxylic acid group deprotonated. However, the presence of the aromatic
ring significantly modifies other aspects of the behaviour of the molecule compared to that of the simpler
carboxylic acid molecules such as formic acid and acetic acid. These latter molecules adopt an upright
stance on Cu(1 1 0) at all coverages with their carboxylate plane perpendicular to the surface. The
coverage dependent RAIR spectra (Fig. 2.19) of benzoic acid on this surface clearly show that at very
low coverage the only vibration observed is connected with the out-of-plane CH deformation, g(CH)
around 720–740 cm1 which indicates that the aromatic ring is aligned parallel to the surface. With
increasing coverage new additional features appear, most notably the symmetric carboxylate stretch
ns(COO) around 1440 cm1 which shows that at least some of the COO groups adopt a more upright
stance. This mode of vibration becomes increasingly strong and at saturation coverage all the molecules
appear to be aligned in an upright fashion. The Fourier transform infrared spectroscopy (FTIR) data
alone cannot determine whether or not the intermediate coverage spectra with vibrations due to both
g(CH) and ns(COO) arise as a result of the tilting of the original flat-lying molecules or the coadsorption of both flat-lying and perpendicular species. However, LEED and STM offer some clues
about the adsorption process.
At very low coverage no LEED pattern is seen and the system exhibits no ordered structure. With
increasing coverage a well-ordered structure appears, referred to as the at phase, which can be
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Fig. 2.19. Room temperature, coverage dependent RAIR spectra for exposure of clean Cu(1 1 0) to benzoic acid. No LEED
pattern is observed initially (A) but with increasing exposure (B–D), the (4 3, 1 5) periodicity of the a-phase appears. At
higher exposure (E), the (4 3, 1 9) LEED pattern of the b-phase is observed and finally, at saturation (F) the c(8 2)
periodicity occurs. Reproduced from Fig. 1 [39] by permission of World Scientific Publishing.
characterised by the matrix notation (4 3, 1 5). This phase continues to exist, even as the vibrations
associated with the upright molecules start to grow in, until the vibrations due to the flat-lying
molecules, mainly g(CH), reach a maximum (Fig. 2.19, spectrum D). With further exposure to benzoic
acid another structure, the b-phase, characterised by a (4 3, 1 9) LEED structure is observed. Here,
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Fig. 2.20. Small area STM image of an island of the a-phase of benzoic acid on Cu(1 1 0) (160 — 160 Å; 2 V sample
bias; 5 min exposure). Circular features of about 7 Å diameter are arranged in groups of four (dashed white diamond; not a
unit cell) along the dominant h4, 3i direction and are identified with flat-lying species by comparison with RAIRS data.
Occasional bright features, always occurring at the centre of a group of four, correlate to a second benzoate species oriented
perpendicular to the surface. The (4 3, 1 5) unit cell is superimposed, connecting the centres of groups of four circular
features. Reproduced from Fig. 7(A) [39] by permission of World Scientific Publishing.
flat-lying molecules are still present but the vibrations connected with the upright molecules start to
dominate the RAIR spectra. At saturation coverage, where all the molecules are upright, the LEED
pattern is that of a c(8 2) structure. The sizes of the structures formed in this system are surprisingly
large—a single unit cell for the a-phase is 23 times larger than the underlying copper unit cell and the
b-phase has a unit cell 39 times as large, covering an area of 359 Å2. The phases are also all stable on
heating, desorbing from the surface at 580 K.
A very detailed analysis involving STM has determined that the a-phase starts with all the molecules
lying flat to the surface in dimer arrangements with four molecules per unit cell but that as the coverage
increases, an extra upright species is incorporated into the structure. The STM image associated with
this phase is shown in Fig. 2.20 with the occasional bright features, at the centre of groups of four
atoms, arising due to the fifth upright species. As the coverage increases further, the b-phase grows in
with six flat-lying molecules per unit cell together with single and paired rows of upright molecules. A
small region of this phase is shown in the STM image of Fig. 2.21. This structure then converts into the
final c(8 2) structure associated with the saturated phase where there are four species per 16 metal
atoms. Fig. 2.22 shows the STM images associated with this phase. The phenyl ring is upright and,
along with the COO groups, aligned parallel to the close-packed rows, the ½1 1 0 azimuth, and form
zig zag rows in the [0 0 1] direction as modelled in Fig. 2.23.
Finally, considerable mobility allows very large single domain islands to form, covering the terraces.
In some areas two domains are observed but these are related by mirror planes reflected over the h1 1 0i
direction. At low coverage the molecule favours a large surface footprint but the interaction of the ring
with the surface is not very site-specific so the molecule is able to diffuse across the surface, enabling it
to establish a 2D crystallinity which then acts as a template for higher coverage structures. Thus the
more upright species grows in with the same large domain sizes even though it has a much stronger site
specificity and more limited diffusion ability than the flat-lying species. All the phases can be shown to
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Fig. 2.21. STM image of a small region of the b-phase of benzoic acid on Cu(1 1 0) (125 — 125 Å; 2 V; 13 min
exposure). The (4 3, 1 9) unit cell is outlined. Rows of paired and single upright species seen in h4, 3i direction. Reproduced
from Fig. 9(B) [39] by permission of World Scientific Publishing.
be directly related to each other, with the new phase developing from the old one. At low coverage there
is faceting at the step edges of the crystal leading to mass transport across the surface. The saturated
surface structure then grows in from the step edges with the c(8 2) structure retaining the preference
for the h4, 3i direction of the other structures. STM clearly shows regions of the surface where one
phase is growing into another, as in Fig. 2.24.
2.3.1.2. Liquid nitrogen cooled substrate. When benzoic acid is adsorbed on a Cu(1 1 0) surface cooled
by liquid nitrogen, molecules still in their acid form are seen to be present between 85 and 119 K [38].
Fig. 2.22. STM image of a small region of the saturated phase of benzoic acid on Cu(1 1 0) (40 — 40 Å; 0.5 V). This
shows typical elliptical features interpreted as upright phenyl rings aligned along the [1 1 0] direction. Reproduced from
Fig. 11(C) [39] by permission of World Scientific Publishing.
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Fig. 2.23. Model of c(8 2) saturated phase of benzoic acid on Cu(1 1 0). Upright benzoate species aligned parallel to
[1 1 0] with the carboxylate bonded in short bridge sites forming zig zag rows along [0 0 1]. Reproduced from Fig. 12 [39] by
permission of World Scientific Publishing.
At very low coverages these acid molecules diffuse and form small clusters and as the coverage increases
above 0.1 ML they form hydrogen bonded acid dimers. No long range periodicity is observed at these low
temperatures but the dimers grow into periodic one-dimensional (1D) double row structures aligned
along the h0 0 1i direction. Some upright benzoate species are also present and as the temperature is
increased all the benzoic acid species deprotonate and convert to benzoate species by 175 K. At the same
time, changes in the intermolecular bonding occur, leading to the formation of the ordered room
temperature a-phase. It is believed that this is accompanied by the activated incorporation of copper
atoms in the adlayer, giving rise to a very faceted underlying copper surface for this phase.
2.3.2. Benzoic acid on p(2 1)O/Cu(1 1 0) and Cu(1 1 1)
When benzoic acid is adsorbed on a different surface of copper, Cu(1 1 1), or on p(2 1)O/
Cu(1 1 0), i.e. Cu(1 1 0) reconstructed by the adsorption of oxygen, it is found that the molecule is
upright at all coverages at room temperature and no ordered surface structures are seen [40]. Fig. 2.25
shows the simple phase diagram for these systems which can be understood in terms of the mobility and
availability of the copper atoms at the surface. Calculations on the surface energies of copper show a
trend in the order ð1 1 0Þ > ð1 0 0Þ > ð1 1 1Þ, with adatom migration being the energetically favoured
mechanism for diffusion on the (1 1 0) and (1 1 1) faces. On the Cu(1 1 1) surface the copper atoms are,
therefore, less likely to diffuse than on the Cu(1 1 0) face and on the (1 1 0) surface pre-dosed with
oxygen, the number and mobility of copper atoms at the surface is likewise reduced. This inhibits the
creation of flat-lying dimers at the surface and the formation of large domains of such molecule pairs.
Without the existence of these domains the ordered higher coverage phases are unable to grow,
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Fig. 2.24. STM image of a-phase growing into b-phase of benzoic acid on Cu(1 1 0) (580 — 900 Å; 2 V; 11 min
exposure). The single features in a pseudohexagonal array correspond to upright benzoate species in a fully populated
a-structure. The broader features, also arranged along the dominant h4, 3i and h4, 3i directions, correspond to pairs of
upright species. Reproduced from Fig. 8(A) [39] by permission of World Scientific Publishing.
confirming the view that each of the phases on the Cu(1 1 0) surface grows directly from the previous
phase in a templated manner. The need for mobile metal atoms at the surface, in order to produce
dimers, has been shown by providing additional metal adatoms on the Cu(1 1 1) face, e.g. co-adsorption
of sodium or copper with the benzoic acid leads to flat lying species.
2.3.3. Benzoic acid on Ni(1 1 0)
When the metal is changed from copper to nickel the behaviour of benzoic acid is different again due
to stronger adsorbate–substrate interactions [41] leading to the phase diagram shown in Fig. 2.26. Initial
adsorption is again via p-bonding of the phenyl ring and, with increasing coverage, adsorption is via the
deprotonated O atoms of the carboxylate group. However, the ordered structures and phases seen differ
from those of the Cu(1 1 0) system. XPS and NEXAFS studies have identified that the molecule
changes orientation with coverage, going from an almost flat lying geometry to an upright stance,
whilst at intermediate coverages there are two molecular orientations present with mainly flat-lying
molecules and some upright species. Interestingly, these changes are accompanied by alterations in the
aromatic ring alignments. As the molecules start to adopt the upright stance, they are oriented with their
planes close to the ½1 1 0 surface azimuth but at approximately half a monolayer coverage, they turn
their planes closer to the [0 0 1] azimuth. By saturation coverage further reorientation occurs so that the
molecules are twisted with their planes 308 off the ½1 1 0 surface azimuth and tilted by 308 with
respect to the surface normal. These changes in adsorption geometry can be understood in terms of an
interplay between the adsorbate–substrate interactions and the lateral interactions occurring within the
adsorbate layer. It is proposed that the saturated room temperature surface is consistent with a unit cell
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Fig. 2.25. Phase diagram of benzoic acid on Cu(1 1 1) and p(2 1)O/Cu(1 1 0).
Fig. 2.26. Phase diagram of benzoic acid on Ni(1 1 0).
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described by a (1 1, 1 2) matrix giving a coverage of one benzoic acid molecule per three nickel
atoms. This spatial orientation is quite different from that observed on Cu(1 1 0), with the adsorbate–
substrate interaction much stronger on the Ni(1 1 0) surface.
2.4. 3-Thiophene carboxylic acid
Techniques used: HREELS, LEED, STM, RAS.
Preparation: Solid powder sublimed under vacuum and exposed to crystal through leak valve.
3-Thiophene carboxylic acid provides a useful progression with the replacement of the benzene ring
of benzoic acid with a sulphur-containing aromatic ring providing further information on the influence
of the ring system on the molecule–metal interactions. Again, as for benzoic acid, this system exhibits
no global chiral effects. However, local chirality can manifest itself exactly as discussed for benzoic
acid in Section 2.3.
Fig. 2.27. Phase diagram of 3-thiophene carboxylic acid on Cu(1 1 0).
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Fig. 2.28. HREEL spectra of 3-thiophene carboxylic acid on Cu(1 1 0) for increasing exposure. Measured on-specular at
Ep ¼ 2 eV, following exposure for: (A) 5 min, (B) 30 min and (C) 90 min. The LEED pattern corresponding to spectrum (B) had
a c(4 8) periodicity and that of spectrum (C) was p(2 1). Reprinted from Fig. 1 [43] with permission from Elsevier Science.
2.4.1. 3-Thiophene carboxylic acid on Cu(1 1 0) and p(2 1)O/Cu(1 1 0)
Detailed, coverage dependent studies of the adsorption of 3-thiophene carboxylic acid on Cu(1 1 0)
[42–44] show that this system also displays a number of different adsorption phases at room
temperature (Fig. 2.27). The HREEL spectra for these various phases are shown in Fig. 2.28 and
provide evidence that at all coverages the carboxylic acid group is ionised to give the carboxylate, with
no indication of either nC=O or nOH stretches typical of the acid form being present. At the lowest
coverage, the molecule is seen to be flat-lying, with the thiophene ring parallel to the surface and
pH-bonded to the metal. As coverage increases, dipole active vibrations associated with this parallel
ring orientation completely disappear, with a new set of modes making an appearance. These vibrations
include the C–O symmetric stretch around 1441 cm1 but not the C–O asymmetric stretch which would
be expected around 1560 cm1, indicating that the molecule adopts a more upright stance in which the
oxygen atoms of the carboxylate group are bonded to adjacent copper atoms across short bridge sites.
Slight variation in the HREELS intensities together with LEED and STM studies of the system,
further reveal that there are two high coverage phases, with different arrangements of the molecules on
the metal surface. Initially, a c(4 8) structure grows in with the molecules arranged so that the rings
are perfectly aligned along the [1 1 0] direction, with four adjacent molecules placed face to face in the
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unit cell. However, as the coverage increases a p(2 1) LEED structure grows which allows more
molecules to be packed onto the surface. In this structure the thiophene rings are tilted away from the
high symmetry direction and calculations show that the rings are twisted 308 from the plane of the
carboxylate group to minimise the steric repulsion between the hydrogens of adjacent rings. The STM
image of the c(4 8) structure and the ideal structural models of the two high coverage phases are
shown in Fig. 2.29.
Fig. 2.29. 3-Thiophene carboxylic acid on Cu(1 1 0). (A) STM image (80 — 100 Å; þ2 V) for c(4 8) structure of with
unit cell and high symmetry directions as indicated. (B) Ideal c(4 8) structural model. Upright thiophene carboxylate species
with thiophene rings aligned along [1 1 0] direction to minimise steric repulsions with neighbours on adjacent rows.
Antiparallel alignment of S atoms favoured by 40 meV. (C) Ideal p(2 1) structural model with carboxylates bound in short
bridge sites along [1 1 0] but thiophene rings rotated 308 out of plane to minimise steric repulsions. Parallel alignment of
thiophene rings. Reprinted from Fig. 4 [43] with permission from Elsevier Science.
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Fig. 2.30. Phase diagram of 3-thiophene carboxylic acid on p(2 1)O/Cu(1 1 0).
On the p(2 1)O/Cu(1 1 0) surface only one phase is observed with HREELS when 3-thiophene
carboxylic acid is adsorbed at room temperature [44], as shown in Fig. 2.30. The molecule is again
present in the carboxylate form and adopts an upright stance at all coverages. It does not, however,
show any ordered surface structures. Clearly, the reduced mobility of copper atoms at the reconstructed
surface restricts the opportunities for creating a range of ordered phases, in a similar manner to that
seen for benzoic acid on this surface.
2.5. 4-Trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA)
Techniques used: STM.
Preparation: Thermal evaporation from Knudsen cell under UHV conditions.
The molecule 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid, known as PVBA, is an asymmetric, planar,
linear molecule, consisting of a benzoic acid group joined via a C=C linkage to a pyridyl ring. The
controlled growth of films of this non-centrosymmetric molecule is of particular interest due to its potential
use in non-linear optical devices. PVBA is known to form strong intermolecular hydrogen bonds in a headto-tail fashion which are expected to yield organic films with increased thermal and mechanical stability.
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Fig. 2.31. Phase diagram of PVBA on Pd(1 1 0).
Fig. 2.32. PVBA molecules on Pd(1 1 0). (a) Low coverage (0.018 ML) STM image (190 — 180 Å; 1.04 V;1 nA) at
substrate temperature of 325 K and deposition rate of 3 105 ML/s. (b) Ball model for unrelaxed PVBA molecules on
Pd(1 1 0). Light (dark) circles: Pd atoms of the first (second) layer; small circles: individual PVBA atoms (omitting H atoms).
The molecular axis is oriented by 358 w.r.t. ½1 1 0 Pd rows. Reproduced from Fig. 1 [47] with permission from the author.
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Although surface studies of this molecule on metals have been limited to variable temperature STM,
sufficient information has been gained to provide a picture of the way different metal surfaces influence the
adsorption of PVBA. Strongly interacting metals lead to ‘‘hit and stick’’ type adsorption with very sitespecific adsorption features whereas on less strongly interacting metals beautifully ordered superstructures
are formed which reflect the influence of the interactions between the adsorbed molecules. In all cases the
adsorption behaviour is determined by the relative strengths of the attractive substrate–adsorbate
interactions versus the intermolecular interactions between the PVBA molecules themselves. Finally,
PVBA is an example of a prochiral molecule that can adsorb to give a surface which is locally chiral.
The molecule itself is not chiral, but being planar and asymmetric, adsorption can lead to the creation of
local chiral motifs, dependent on which face of the molecule is uppermost. In addition, under certain
conditions, these chiral motifs can self-organise into domains where the molecules are chirally arranged.
However, overall the surface remains racemic as mirror image motifs and domains are always allowed.
2.5.1. PVBA on Pd(1 1 0)
STM alone cannot determine the actual bonding mechanism of the PVBA molecule to the surface,
but it appears from the STM results obtained for the system [45–49] that the molecule bonds strongly
to the Pd(1 1 0) surface at room temperature, since it is impossible to manipulate the bonded
molecules using the STM tip. The adsorption phase diagram for the system is shown in Fig. 2.31. The
most likely bonding mechanism is via p-bonding of the pyridyl and benzyl rings which are held flat
to the surface. The STM pictures show molecules with a ‘dog-bone’ appearance, as in Fig. 2.32
which also tends to favour this adsorption mechanism. The PVBA molecules are adsorbed randomly
across the surface but they show a preferred local arrangement in which each molecule adsorbs across
three adjacent close-packed ½1 1 0 rows, at an angle of 358 to the rows, as shown in Fig. 2.32b. Four
equivalent adsorption configurations are theoretically possible but only two are observed, as the STM
images are not able to distinguish between the head and tail groups of the PVBA molecule. These
configurations correspond to the two chiral motifs that can be created on adsorption with one
molecule adsorbed as a ‘left-handed’ species with one face uppermost and the other adsorbed as a
‘right-handed’ species with the other face uppermost, i.e. as the mirror image of the ‘left-handed’
species. The two configurations are held 1108 apart rather than the 1808 which would be the ideal
angle for intermolecular H-bonding. This confirms that the strong bonding with the surface leads to
the PVBA molecules being arranged for optimal interaction with the Pd surface atoms rather than
each other. The random distribution of the molecules across the surface is maintained even when the
temperature or coverage is increased with no island formation observed but a parquet-like pattern of
flat-lying molecules evolving. No images are seen where a molecule changes from one adsorption
configuration to the other, which would require a ‘flip’ of the molecule, a process with a high
activation barrier. However, diffusion along the close-packed rows is observed on warming, with an
activation barrier of 0:83 0:03 eV, indicating that 1D hopping of the PVBA molecules can occur
along these rows.
2.5.2. PVBA on Cu(1 1 1)
On the Cu(1 1 1) surface it is possible to form more than one surface structure when depositing
PVBA at 160 K [45,49] as shown in the phase diagram of Fig. 2.33. Flat-lying dendritic islands are
created which coexist with isolated molecules, as shown in Fig. 2.34. The isolated molecules are
suspected to be in an upright orientation from their STM images but no chemical evidence of this
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Fig. 2.33. Phase diagram of PVBA on Cu(1 1 1).
Fig. 2.34. STM image of PVBA molecules on Cu(1 1 1). Flat molecules in dendritic islands coexist with isolated molecules
in an upright bonding configuration (single protrusions) after adsorption at 160 K. Reproduced from Fig. 1(c) [49] with
permission from Wiley/VCH Verlag GmbH.
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Fig. 2.35.
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Phase diagram of PVBA on Ag(1 1 1).
bonding geometry is available. Within the islands, irregularities observed in the dendritic arms indicate
that the PVBA molecules are orientated in several different ways with respect to the underlying surface.
Thus it can be concluded that, on the Cu(1 1 1) surface, there is no unique adsorption geometry. On
annealing, only flat-lying species are observed but long-range ordering of the molecules is obstructed
by strong lateral interactions with copper adatoms, evaporated from the substrate steps, which fix the
positions of the adsorbed molecules. This system thus exhibits fairly strong substrate–molecule
interactions but shows higher surface mobility and greater lateral interactions between the PVBA
molecules than for the Pd(1 1 0) system.
2.5.3. PVBA on Ag(1 1 1)
The Ag(1 1 1) surface encourages the formation of more ordered structures of PVBA molecules as the
intermolecular interactions are strong compared to those between the substrate and the adsorbate [45,49].
Fig. 2.35 summarises the various adsorption phases seen. Adsorption at 125 K leads to flat lying
complexes which aggregate into curved strings at the surface, shown in Fig. 2.36. The shape is indicative
of attractive interactions between the end groups of adjacent molecules and weak substrate corrugations.
On annealing to 300 K (or on adsorption at this higher temperature) beautifully ordered 1D
superstructures are created as seen in Fig. 2.37. These take the form of twin chains of molecules
across the surface forming stripes in the h1 1 2i direction. Individual chains consist of molecules
H-bonded together in a head-to-tail fashion, with the molecular axis oriented along the chain direction.
Each twin chain consists of two antiparallel chains with one chain slightly displaced with respect to the
other, allowing weak intermolecular interactions (CH O¼C) between adjacent chains. The twin chains
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Fig. 2.36. STM image of PVBA molecules on Ag(1 1 1) at 125 K. The complex aggregation of flat-lying molecules reflects
the surface mobility and attractive interactions between molecular end groups. Reproduced from Fig. 1(d) [49] with
permission from Wiley/VCH Verlag GmbH.
are then kept apart by long range repulsive dipole–dipole interactions. The chains are observed to undergo
spontaneous enantioresolution into areas of a similar handedness during the self-assembly process, with
only one enantiomer (i.e. one chiral adsorption motif) present in each twin chain. Although arrangements
of chains are theoretically feasible with a combination of different enantiomers, they lead to energetically
less favourable structures and are not seen. Thus the twin chain structures always consist of molecules
adsorbed in a single enantiomeric form with the direction of the displacement of one chain of the pair with
respect to the other dependent on the particular enantiomer. This effectively leads to a chirally organised
arrangement of the molecules across the surface, as shown in Fig. 2.35. As the coverage increases the twin
chains are forced together leading to an ordered, but non-chain-like structure.
2.6. p-Aminobenzoic acid
Techniques used: LEED, STM, HREELS, TPD.
Preparation: Thermal evaporation from Knudsen cell under UHV conditions.
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Fig. 2.37. STM images of the formation of 1D supramolecular PVBA super-structure by self-assembly mediated by H-bond
formation on Ag(1 1 1) after annealing to 300 K (measured at 77 K). (a) STM topograph of a single domain extending over
two terraces demonstrates ordering at the millimetre scale. (b) A close-up image of the self-assembled twin chains reveals that
they consist of coupled rows of PVBA molecules. Reproduced from Fig. 2 [49] with permission from Wiley/VCH Verlag
GmbH.
2.6.1. p-Aminobenzoic acid on Cu(1 1 0)
This molecule is essentially benzoic acid with an additional amino group and unlike the naturally
occurring a-amino acids, has a rigid planar structure. This means that it provides an ideal model
(i.e. planar but multifunctional) to study the adsorption characteristics of the more complex amino
acids which are discussed in the next section. Recent studies of the adsorption of p-aminobenzoic
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Fig. 2.38. Phase diagram of p-aminobenzoic acid on Cu(1 1 0).
acid on Cu(1 1 0) [50–52] have shown that the behaviour of this system shows many of the features
associated with the adsorption of benzoic acid on this metal surface. The phase diagram of the system
is given in Fig. 2.38. At room temperature, the molecule initially adsorbs in a flat-lying geometry
with the carboxylic acid group ionised to give a carboxylate group. An ordered surface structure is
formed with a (3 4)g periodicity and a glide plane along the h0 0 1i direction. As the coverage
increases, additional upright species appear, at the grain boundaries, which are aligned preferentially
along the (3, 4) directions. On warming this room temperature surface, partial dehydrogenation
occurs and, again, in a similar manner to benzoic acid, a number of dimer structures are formed. In
addition, the adsorption process causes faceting with evidence of significant mass transport on the
copper surface.
3. Amino acids
In the context of this review, amino acids can be considered as molecules that are essentially similar
to carboxylic acids (COOH) but with an additional amino (NH2) group. Thus they provide two different
functional groups, either or both of which could potentially be involved in bonding to the metal surface.
Some amino acids also have additional reactive side groups, e.g. sulphur, S, which can contribute to the
bonding.
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An important aspect of amino acids is that their chemical form varies from acidic (NH2RCOOH), to
anionic (NH2RCOO), to cationic (NH3 þ RCOOH), or to zwitterionic (NH3 þ RCOO ), according to
their environment. Thus a critical point to address when studying their behaviour at a metal surface is
‘‘What is their chemical form on adsorption?’’ In the gas phase, most amino acids are present in their
acidic form whereas in the solid phase they are usually zwitterionic. In many cases, organo-metallic
complexes of amino acids have formed a good starting point for considering the likely chemical forms
when adsorbed on a metal surface.
In the usual solid zwitterionic form of amino acids, strong intermolecular hydrogen bonding
(H-bonding) interactions influence the crystalline structure. Such H-bonding interactions can also be
expected to drive the formation of any ordered structures formed at a metal surface. In addition, all
naturally occurring amino acids, except the simplest, glycine, are chiral, existing in the L (or S)
enantiomeric form. Thus interest has also centred on the chirality of the adsorbate structures formed by
amino acids.
The ability of amino acids to form ordered surface structures at defined metal surfaces was first
recognised as long ago as 1978 by Atanasoska et al. [53] using LEED. These authors even showed that
the D- and L-forms of tryptophan yielded structures related by a mirror inversion. However, it is only
relatively recently, with the advent of STM that a true appreciation has been gained of the way some
amino acid/metal systems can form chiral surfaces with global point and organisational chirality. In
addition, highly sensitive techniques, such as RAIRS, have enabled the growth of amino acids to be
monitored in real time under carefully controlled conditions.
Within this review, we have focused on studies of amino acids deposited on defined metal surfaces,
rather than polycrystalline metal films, as these systems provide the requisite depth of information. We
have included both naturally occurring and synthesised amino acids and also relevant work on peptides,
which are essentially just groups of amino acids connected together by amide bonds.
3.1. Glycine
Techniques used: RAIRS, LEED, STM, TPD, XPS, NEXAFS, XPD.
Preparation: Thermal evaporation under UHV conditions.
3.1.1. Glycine on Cu(1 1 0) and Cu(1 0 0)
Glycine is the simplest and only non-chiral amino acid. The adsorption of glycine on the Cu(1 1 0)
surface has been extensively investigated by a number of groups with a wide range of techniques
[54–57]. This has resulted in a good understanding of the various different phases created and these are
summarised in the phase diagram of Fig. 3.1. A detailed RAIRS study [54] has identified that, at room
temperature, glycine adsorbs in its anionic form, NH2CH2COO, forming a Sat. ML. The bonding and
orientation of this room temperature glycinate species is dependent on coverage and further changes
occur on annealing to 420 K. The RAIR spectra of Fig. 3.2 show that the main features of the bonding
can be monitored by observation of the relative intensities of the symmetric and asymmetric
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Fig. 3.1. Phase diagram of glycine on Cu(1 1 0).
carboxylate stretches, ns(COO) and nas(COO), at 1417 and 1630 cm1 together with those of the
NH2 scissors vibration, d(NH2), at 1578 cm1 and the intermixed NH2 wag/CN stretch vibration,
rw(NH2)/n(CN), in the 1088–1105 cm1 region. At low coverage (phase I), no vibrations associated
with nas(COO) or d(NH2) are seen but the main peaks are due to ns(COO) and rw(NH2)/n(CN). From
this the authors deduce that the molecule initially bonds through equivalent oxygen atoms of the
carboxylate group and propose that the plane of this group is oriented broadly perpendicular to the
surface, similar to the bonding found in simple carboxylic acids. As the coverage increases (phase II),
vibrations associated with the nas(COO) also appear, indicating that the carboxylate group is bound in
a more asymmetric (unidentate) fashion with the oxygen atoms no longer equidistant from the surface.
In addition, vibrations connected with d(NH2) appear, as do vibrations associated with the CH2 scissors
motion, d(CH2), around 1441 cm1, and the RAIR spectra become very similar to the infrared spectra
of a copper glycinate complex. On warming the fully covered surface to 420 K (phase III), further
changes in the RAIR spectra occur, with the spectra becoming more similar to those obtained at low
coverage. The intensity associated with nas(COO) decreases but the d(NH2) and d(CH2) vibrations
remain, suggesting an orientation where O2–C–C–N backbone is essentially parallel to the surface, with
bonding through the amino group and both oxygen atoms of the carboxylate group, which are held
equidistant from the surface. This Sat. ML phase III is also created directly when glycine is adsorbed on
the Cu(1 1 0) surface held above 408 K. The proposed orientations of the glycine molecule in the
monolayer are illustrated in Fig. 3.3. We note that the bonding proposed for the lowest coverage,
phase I, only involves the oxygen atoms of the carboxylate group. As the thermodynamically favoured
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Fig. 3.2. RAIR spectra of glycine adsorbed on Cu(1 1 0) at room temperature. (a)–(d) represent increasing coverage to room
temperature saturation and (e) follows annealing of the saturated surface to 420 K. Reprinted from Fig. 2 [54] with permission
from Elsevier Science.
phase III bonds via the amino group as well, it is surprising that this group is not also involved in the
bonding at low coverage. It is possible that the intensity of the d(NH2) vibration is too weak to observe
at low coverage, although other differences between the low coverage spectra and those of the Sat. ML,
such as the appearance of the d(CH2) vibration at the higher coverage, have been taken to suggest an
alternative bonding mechanism between the two phases. We also note that both asymmetric and
symmetric carboxylate stretches are seen in phase II, which can be interpreted as due either to a
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Fig. 3.3. Proposed conformational models of glycine adsorbed on Cu(1 1 0). (I) Low coverage, (II) room temperature
saturation coverage and (III) after annealing to 420 K. Reprinted from Fig. 6 [54] with permission from Elsevier Science.
complete reorientation of the molecule over the entire surface, or to the coexistence of two adsorption
geometries, where some carboxylate groups are arranged with their oxygen atoms equidistant from the
surface and others are arranged so that bonding is in a more unidentate manner.
The saturated annealed monolayer gives rise to the major ordered phase associated with this system,
showing a (3 2) LEED pattern, with missing spots, characteristic of a glide line. Further X-ray
photoelectron diffraction (XPD) and NEXAFS studies, together with cluster calculations [55–57] have
shown that the most likely adsorption geometry is as shown in Fig. 3.4 with the molecular axis
predominantly parallel to the surface and bonding occurring across two close-packed rows of copper
atoms. The carboxylate oxygen atoms are bonded to adjacent copper atoms on one close-packed row
and the nitrogen atoms bonded to copper atoms on the next close-packed row, with both the oxygen and
nitrogen atoms slightly displaced from atop positions. Interestingly, the primitive unit cell for a (3 2)
monolayer with this adsorption geometry is very similar in both size and shape to the ac basal plane of
bulk glycine, as indicated in Fig. 3.5a. The adsorption geometry in this phase essentially yields a local
chiral motif which can exist in both mirror forms depending on the C–CN backbone twist. Overall, each
motif would exist in equal number leading to an achiral surface. This coexistence of two motifs also
results in two possible models for the glycine/Cu(1 1 0) superstructure, given in Fig. 3.5b and c, in
Fig. 3.4. Optimised adsorption geometry for glycine adsorbed on Cu(1 1 0). Reprinted from Fig. 7 [55] with permission from
Elsevier Science.
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Fig. 3.5. Proposed superstructure of the glycine/Cu(1 1 0) system. (a) Comparison of a ‘‘centred’’ (3 2) unit cell (heavy
full line) on Cu(1 1 0) with the basal plane ac unit cell of bulk glycine (heavy dashed line). Dimensions for the (3 2) unit cell
are shown to the left of the figure while dimensions of the bulk glycine unit cell are shown in italics to the right. The
underlying Cu(1 1 0) net is indicated by the light dashed lines. (b) Homochiral structure for the glycinate/Cu{1 1 0} surface
based on the molecular orientation suggested by the IR data, the similarity with the bulk glycine structure and on the observed
LEED pattern, although the required glide line is not strictly present. (c) Alternative heterochiral structure with true glide lines
shown as the solid lines. Reprinted from Fig. 7 [54] with permission from Elsevier Science.
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which either a homochiral domain, containing molecules adsorbed with the same chiral motif, is
formed, or a heterochiral domain, in which both chiral motifs are accommodated, is produced.
Although the homochiral structure in Fig. 3.5b is consistent with the molecular orientation suggested by
the RAIRS data, the bulk glycine structure and the main features of the observed LEED pattern, it does
not take due account of the glide plane symmetry, which arises naturally from the heterochiral domain
of Fig. 3.5c where the glide lines are indicated. This reflection of the glycine molecule, resulting in two
chiral adsorption motifs and leading to the possibility of hetero- and homochiral domains has been the
subject of some debate [58]. Recent analysis of the experimental data by Woodruff and co-workers [59]
strongly favours the formation of heterochiral domains. In any case, the proposed structures for the
(3 2) phase are thought to be held together by a network of intermolecular CH O and NH O
hydrogen bonds with the (3 2) structure being a compromise between optimal adsorbate site,
intermolecular hydrogen bonding and maximum adsorbate density.
Adsorption of glycine just below room temperature leads to the formation of multilayers where the
glycine is present in its zwitterionic form, with the second layer bound more strongly to the first layer
than subsequent layers. At liquid nitrogen temperatures (85 K), it can be seen that some of the glycine
adsorbs in its acid form, with evidence of the carbonyl C=O stretch in the RAIR spectra.
The literature of glycine adsorbed on Cu(1 0 0) is restricted to an early LEED study which showed
a (4 2) pattern [53] and recent STM [60,61] studies carried out at room temperature. Initial STM
measurements [60], shown in Fig. 3.6, show that a c(2 4) structure forms on the terraces of
Cu(0 0 1). However, later work by the same authors [61] suggests the coexistence of homochiral
c(2 4) and heterochiral p(2 4) superstructures. A very recent, more detailed analysis including
PhD [59], suggests that, given the absence of certain diffraction beams, the LEED pattern is better
described as p(4 2)pg which is consistent with either a coexisting p(4 2) and c(4 2) structure
or a single heterochiral p(4 2) structure, but not a single homochiral c(4 2) structure.
An important finding of this work is that the adsorption of glycine causes extensive faceting of
the surface and results in h3 1 0i faceted steps which bunch together to form h3 1 17i 1 1 glycine
facets.
Fig. 3.6. STM images of glycine on Cu(1 0 0) after being annealed to 430 K. (a) c(2 4) structure (43 — 43 Å; 50 mV;
1.0 nA). (b) Image of a (3 1 17) facet (91 — 91 Å; 1.2 V; 0.5 nA). Reprinted from Fig. 2 [60] with permission from
Elsevier Science.
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One final comment we wish to highlight here is that the heterochiral and homochiral domains would
be energetically inequivalent, so for a thermodynamically stable phase is unlikely that both would
coexist at a surface. Coexistence may only be expected in certain kinetically constrained systems or
when the energy difference is very small. Data on the crystalline phases of racemic mixtures of chiral
molecules would suggest that, on the whole, a clear energy preference for a homo- or a heterochiral
structure exists.
3.1.2. Glycine on Pt(1 1 1)
The reactivity of the Pt(1 1 1) surface results in completely different adsorption behaviour to that
observed on copper. Recent XPS and TPD studies [62] on the Pt(1 1 1) surface have concluded that
glycine only adsorbs intact when deposited below 250 K and that it is present in its zwitterionic,
rather than anionic form, even in the monolayer. The N 1s spectrum for the monolayer shows a large
peak at 401.3 eV, associated with the ionised amino (NH3 þ ) group and a much smaller peak around
399.9 eV, believed to be due to either the neutral amino (NH2) group or decomposed glycine
fragments. The O 1s peak at 531.7 eV and the C 1s peak around 289.2 eV are consistent with a
deprotonated carboxyl group. Angle resolved spectra for glycine in the monolayer region reveal no
preferential orientation and suggest that the molecular axis is broadly parallel to the surface with
bonding occurring through the ionised amino (NH3 þ ) and carboxylate (COO) groups. The phase
diagram constructed for the system is given in Fig. 3.7 where it can be seen that multilayers form,
with again, as for copper, the second and possibly third layers bound more strongly than subsequent
layers. TPD measurements have shown that in the monolayer, desorption occurs by 360 K with the
proportion of glycine molecules desorbing intact dependent on the coverage. For the Sat. ML,
Fig. 3.7. Phase diagram of glycine on Pt(1 1 1).
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approximately 50% of the glycine molecules desorb intact and the rest dissociate on desorption but
for coverages less than 0.25 of a monolayer all the glycine dissociates on desorption. These studies
did not look for ordered structures, although a (2 2) LEED pattern was reported for this system in
1989 [63]. However, there are significant differences between the results of these two studies,
particularly with respect to thermal desorption products, possibly as a result of different crystal
cleaning techniques, so it is not clear if this ordered structure is truly characteristic of the saturated
glycine monolayer.
3.2. Alanine
Techniques used: RAIRS, XPS, STM, LEED, TPD.
Preparation: Deposition via thermal sublimation in UHV.
Alanine is the simplest chiral amino acid and can exist in the naturally occurring S-configuration or
its mirror R-configuration. So far, extensive studies are only available for alanine adsorption on
Cu(1 1 0) [20,64–66] with some LEED information reported on the Cu(1 1 1) surface [53] and LEED
[53] and STM [67] information reported on the Cu(1 0 0) surface.
3.2.1. Alanine on Cu(1 1 0)
The adsorption phase diagram of S-alanine on Cu(1 1 0) is displayed in Fig. 3.8 and shows that at low
coverages and/or at low temperatures no real long-range structures are created. However, annealing to
420 and 470 K, results in very ordered adlayers giving rise to good diffraction patterns consistent with
the (2 2, 5 3) and (3 2) structures, respectively. These three different types of adsorption phases are
discussed below.
3.2.1.1. Adsorption at room temperature. Fig. 3.9 displays the RAIR spectra obtained for adsorption
carried out at the Cu(1 1 0) surface at 300 K up to a maximum first layer saturation coverage of just over
0.33 ML, representing one chiral molecule per three surface metal atoms. Although complex vibrational
spectra are observed, identification of the adsorbed species is readily made since the frequency of the
observed bands are almost identical to those reported for the metal–alanine complexes, Cu–(ala)2 and
Ni–(ala)2 [68] in which alanine exists in its anionic form. Direct evidence for the anionic species is
provided by the symmetric ns(COO) vibration at 1411 cm1 and the asymmetric nas(COO) vibration at
1626 cm1, attributed to the deprotonated carboxylate functionality. In addition XPS data for the
adsorbed phase [64] show a N 1s binding energy at between 399.5 and 399.9 eV, consistent with a NH2
unit rather than the protonated NH3 þ group which displays a N 1s binding energy that is almost 2 eV
higher [69].
By application of the strict RAIRS dipole selection rule, the orientation of the alanine molecule can
be pieced together by considering vibrations of the distinct functional groups on the molecule. A
careful analysis, reported in detail elsewhere [65], shows that at low coverages, the molecule straddles
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Fig. 3.8. Phase diagram of S-alanine on Cu(1 1 0).
the surface with the two carboxylate oxygens co-ordinated in a bidentate fashion and placed equidistant
from the surface, rendering the ns(COO) vibration active and the nas(COO) vibration inactive
(Fig. 3.9a–c). Similarly, it can be deduced that the NH2 plane is almost parallel to the surface and the
CH and CH3 units are held at an angle away from the surface normal. A structural model for this
adsorption geometry is depicted in Fig. 3.9, which is strikingly similar to the structural model suggested
for glycine on Cu(1 1 0) on the basis of scanned energy mode N 1s and O 1s PhD studies [57]. Thus, we
would suggest that the alanine is strongly bonded with the carboxylate group aligned along one closepacked row of Cu(1 1 0) atoms with the oxygen atoms located in on-top metals sites and the molecular
skeleton ‘bending’ over to bring the NH2 unit close to the adjacent row of close-packed copper atoms.
LEED data during this adsorption phase show no extra diffraction spots while STM images reveal that
initial adsorption at 300 K occurs preferentially at step edges.
As coverage is increased in the first layer, a second type of anionic species is stabilised alongside the
already adsorbed alanine, leading to the emergence of a strong nas(COO) vibration at 1626 cm1 in
Fig. 3.9d and e. A detailed analysis [65] of the changes observed reveals that a new, differently oriented
alanine species is now coadsorbed on the surface, with the carboxylate unit tilted with respect to the
surface (monodentate coordination) and the methyl group held almost vertical along the surface
normal. A schematic orientation is shown in Fig. 3.9, bottom right. STM data from this high coverage
phase reveals that, though there are no good ordered overlayers, there is considerable local ordering
leading to the formation of single rows and doublet rows on the terraces of the surface [64]. This final
phase is essentially a disordered precursor to the (2 2, 5 3) structure which forms upon annealing to a
higher temperature, as discussed below.
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Fig. 3.9. RAIR spectra recorded with increasing coverage in the first layer of S-alanine on Cu(1 1 0) at 300 K. The molecular
orientations adopted at low coverage (spectra a–c) and high coverage (spectra d, e) are depicted on the right.
3.2.1.2. High coverage at 420 K: the (2 2, 5 3) phase. The (2 2, 5 3) structure is formed at high
coverages and upon annealing the room temperature adlayer to the temperature range 420–460 K.
Fig. 3.10 shows the STM image and the schematic adlayer of the (2 2, 5 3) structure formed at 400 K.
STM data show that the 2D order of the S-alanine extends over long distances (>400 Å) across the
surface, in which the S-alanine molecules are arranged in regular groups of six or eight, which are aligned
in doublet rows set at a definite angle along the surface. This growth axis is not coincident with either of
the major directions of the metal surface, thus destroying the two mirror planes that exist at the face
centred cubic (f.c.c.) (1 1 0) surface. As for tartaric acid on Cu(1 1 0), the growth direction and
arrangement of the S-alanine enables global organisational chirality to be expressed at the surface.
This chiral arrangement is only ever expressed in one-handedness for this phase, with no mirror domains
being created. Thus the (2 2, 5 3) phase possesses the double quality of global point and global
organisational chirality.
RAIRS data indicate that this phase consists of both orientations of anionic alanine shown in Fig. 3.9.
In that respect, the local description of this phase is similar to the high coverage phase formed at 300 K.
Using the information provided by LEED, STM and RAIRS, a detailed model of this surface can be
constructed (Fig. 3.10c). From this model it has been proposed [66] that the molecule effectively
adsorbs in two orientations within a ‘hexamer’ unit that involves a network of H-bonding interactions
which occurs at two levels: close to the surface involving the NH2 group of one molecule and the
carboxylate group of the neighbouring molecule (lateral NH O bonds) and at a higher level
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Fig. 3.10. The (2 2, 5 3) structure of S-alanine on Cu(1 1 0). STM image: (a) 400 — 400 Å; (b) 100 — 100 Å; (c)
schematic diagram of the (2 2, 5 3) S-alanine structure on Cu(1 1 0); (d) H-bonds thought to interconnect in the (2 2, 5 3)
structure.
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Fig. 3.11. Switching chirality: (2 2, 5 3) S-alanine versus (5 3, 2 2) R-alanine on Cu(1 1 0). The need to keep the CH3 away from the surface causes the
C–C–N skeleton to take on a right- or left-handed kink which is enantio-specific as shown.
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involving the upwards tilted oxygen of the monodentate carboxylate and the CH3 groups of a molecule
in neighbouring row (transversal CH O bonds; Fig. 3.10d). Again, it is interesting to note that this
phase satisfies the three conditions followed by the (9 0, 1 2) R,R-tartaric acid phase on Cu(1 1 0),
namely intrinsic chirality of the molecule; a rigid and defined adsorption geometry and chiral lateral
interactions. All these factors enable a unique growth direction to be forged, so that no mirror
arrangements can be nucleated.
Experiments with the opposite enantiomer, R-alanine show that this global organisational chirality
can be switched to form the mirror (5 3, 2 2) structure (Fig. 3.11). This reversal in chirality can be
rationalised in terms of the fact that the ‘footprint’ of the alanine molecule is essentially governed by
the methyl group which, for steric reasons, must be kept pointing away from the surface [66]. As a
result, for S-alanine, the footprint which describes the COO–C–NH2 placement at the surface possesses
a right-hand kink (d), while that of R-alanine has a left-hand kink (e). These footprints then direct
supramolecular assembly along two different mirror directions.
3.2.1.3. High coverage at 470 K: the (3 2) phase. Upon heating a Sat. ML of alanine on Cu(1 1 0)
to T > 460 K, the local structure of all the alanine units reverts to the one in which the carboxylate has
a bidentate interaction. This geometry, initially adopted in the low coverage phase, turns out to be the
thermodynamically favoured one throughout the temperature and coverage range studied. For
example, heating the high coverage monolayer created at 300–470 K leads to the conversion of
all the tilted species to the adsorption geometry stabilised at low coverages. This configuration is then
maintained upon recooling to 300 K. This high temperature annealed phase yields a (3 2) structure.
STM data (Fig. 3.12) of this structure, however, show that there is another molecule placed within the
(3 2) mesh and the absence of half-order spots in the LEED images of this phase means that the
mirror plane must be transformed into a pseudo-glide plane [64]. Clearly, for the chiral amino acid
only a homochiral domain can exist and therefore, the mirror chiral adsorption required for a true glide
plane cannot be created. For this reason, we refer to a pseudo-glide plane. Although the scattering
centres that give rise to the diffraction spots are not known at present, we presume they are either
dominated by the bonding carboxylate groups or by reconstructed underlying copper atoms. Again,
intermolecular H-bonding interactions lead to a preferred growth direction, but the actual raw
arrangement is not chiral because molecules are placed at the corners of the primitive (3 2)
mesh in which the mirror plane along the [0 0 1] crystallographic direction is retained. We note that the
(3 2) LEED structure is quite ubiquitous in amino acid adsorption on Cu(1 1 0) surfaces and is also
created at saturation coverage and high temperatures for glycine and amino acids with longer side
chains, e.g. norvaline.
3.3. S-proline
Techniques used: RAIRS, LEED, TPD.
Preparation: Thermal evaporation under UHV conditions.
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Fig. 3.12. S-alanine on Cu(1 1 0): (3 2) structure. STM image and model of arrangement.
This chiral amino acid is unusual in that the nitrogen is fixed rigidly within a pyrrolidine ring and is
present as an imino (NH) group. This results in limited conformational mobility of the N–H bond with
respect to the carboxyl group, which is, itself, restricted in mobility by the pyrrolidine cycle. S-proline
and its derivatives are commonly used as chiral modifiers for hydrogenation reactions, steering the
direction that the hydrogen can attack an organic reactant at a metal surface and thus influencing the
enantiomeric nature of the reaction product. Thus the adsorption characteristics of S-proline at a metal
surface have attracted particular interest [70]. Although most hydrogenation reactions take place over
metals such as rhodium or ruthenium, the first detailed adsorption study has been performed on
Cu(1 1 0) as it was expected that the reactivity of this surface would be such as to not break up the
molecule on adsorption [70].
3.3.1. S-proline on Cu(1 1 0)
RAIR spectra of this system show that throughout the adsorption regime at 300 K, a single phase is
formed, as shown in the phase diagram of Fig. 3.13. A particularly detailed analysis of the RAIR spectra
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Fig. 3.13. Phase diagram of S-proline on Cu(1 1 0).
is possible as, unusually, vibrations from each of the CH2 or CH groups on the ring can be separately
identified, using data available from a normal co-ordinate calculation [71]. This allows the orientation of
the ring system to be more precisely determined than would normally be expected. S-proline is seen to
bond to the copper surface in an anionic form, via the oxygen atoms of the carboxylate (COO)
functionality and the nitrogen of the imino (NH) group. Both carboxylate oxygen atoms are found to be
equidistant from the surface while the pyrrolidine ring is held at a small angle to the surface plane.
Unlike other amino acids, the molecule does not show a range of different phases with varying coverage
or temperature conditions and does not significantly reorient at high coverage or on annealing. This
difference in behaviour is attributed to the structural rigidity of the molecular structure which severely
restricts the degrees of freedom generally associated with amino acid end and side groups. As a result,
the proline is forced to adopt the same footprint at the copper surface at all coverages, which also
requires slightly more physical space than other amino acids. This leads to the creation of an ordered
surface structure, at Sat. ML coverage, which possesses a (4 2) LEED pattern rather than the more
commonly observed (3 2) arrangement. Thus this chiral molecule has a very defined chiral adsorption
motif but organises with a non-chiral arrangement. The bonding of the proline layer to Cu(1 1 0) is
strong, creating a robust adlayer which is stable up to 450 K, after which the molecule dehydrogenates
and, subsequently, decomposes. This strong and defined mode of interaction with the metal surface must
play an important role in the use of this molecule as a chiral modifier in heterogeneous
diastereoselective catalysis, where its attachment to an organic molecule determines the adsorption
and orientation adopted by the latter at a surface, thus creating a strong inequality in the probability of
hydrogen addition at the two prochiral faces of the reactant.
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3.4. S-(or L-)norvaline, methionine and serine
Techniques used: RAIRS, LEED, TPD, XPS.
Preparation: Thermal evaporation under UHV conditions.
These three amino acids have been studied by our group and show the effects of increasing the side
chain length, or substituting other potentially reactive, functional groups in the side chain, on molecular
adsorption characteristics.
3.4.1. S-norvaline on Cu(1 1 0)
S-norvaline shows many similarities to S-alanine, differing in structure only by the presence of an
additional two CH2 groups in the side chain. On adsorption at 300 K, the molecule is present in its
anionic form and two coverage dependent adsorption phases are identified, shown in Fig. 3.14 [72]. At
low coverage, bonding is via the carboxylate (COO) and amino (NH2) groups. Both oxygen atoms are
equidistant from the surface and the carboxylate plane is tilted away from the surface normal with the
plane of the amino group largely parallel to the surface. The side chain is held away from the surface
with vibrations from both the symmetric and asymmetric CH3 deformations observed implying that the
Fig. 3.14. Phase diagram of S-norvaline on Cu(1 1 0).
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C–CH3 bond is held at an angle to the surface normal. At higher coverages, a differently oriented
norvaline species is also observed, with the COO group adopting a sideways tilted orientation where
the oxygen atoms have an inequivalent interaction with the surface, in a manner similar to that reported
for alanine and glycine. The symmetric CH3 deformation exhibits a larger dipole activity, indicating
that the C–CH3 bond is now oriented essentially normal to the surface. LEED studies show that the high
coverage phase gives the now familiar (3 2) structure that is also obtained on warming the saturated
S-alanine/Cu(1 1 0) or glycine/Cu(1 1 0) monolayers. The norvaline molecule is strongly bonded to the
surface, desorbing at 535 K, a slightly higher temperature to that observed for alanine, probably as a
result of increased alkyl side chain interactions occurring between neighbouring norvaline molecules.
3.4.2. S-methionine on Cu(1 1 0)
This amino acid differs from norvaline by the presence of a potentially more reactive sulphur atom in
the side chain, situated between the terminal CH3 group and the adjacent CH2 group. On adsorption at
300 K, RAIR spectra of S-methionine again show, like all other amino acids, except proline, the
existence of two coverage dependent adsorption phases (Fig. 3.15) [72] characterised by the differently
oriented carboxylate groups. At low coverages this functionality is strongly tilted away from the surface
normal and both oxygen atoms are equidistant from the surface. However, in contrast to norvaline and
alanine, the terminal NH2 group is no longer parallel to the surface and the CCN backbone section of the
molecule is tilted with respect to the surface rendering both the CN and CCN asymmetric stretches
dipole active. Direct bonds between the sulphur and the surface cannot be observed as the associated
vibrations lie at too low a wavenumber to be accessible by conventional RAIRS. Far-IR RAIRS
experiments at the Daresbury synchrotron carried out to detect Cu–S bonds reveal that in this region of
the spectrum there is much band overlap and mode mixing with other molecular vibrations making
Fig. 3.15.
Phase diagram of S-methionine on Cu(1 1 0).
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isolation of the Cu–S vibrations very difficult [72]. However, a mid-IR RAIRS examination of the dipole
active modes of the other side chain vibrations reveal that at least some of the S atoms must be bonding
to the surface at low coverage. This is further corroborated by XPS measurements of the sub-monolayer
[72] which show two sets of S 2p peaks. Species bonded through the S atoms have S 2p3/2 and 2p1/2
peaks at binding energies of 161.6 and 162.7 eV while species not bonded through the S atoms are
associated with another set of S 2p peaks at 163.7 and 164.8 eV. As the coverage increases, the RAIR
spectra show that there is a reorientation of the main anchoring groups, with the disappearance of the
bands associated with the CN and CCN groups, which then allows further adsorption of new species
adsorbed with the carboxylate group oriented in a unidentate fashion. Although at high coverage, in the
monolayer, there still appear to be some residual species bonded via the COO, NH2 and S
functionalities, it is suspected that the second species that grows in is only bonded via the COO and
NH2 groups. The presence of two differently bonded species is further confirmed by the TPD spectrum.
This shows a strong sharp desorption peak at 460 K, somewhat like the peaks seen for alanine and
norvaline, which is associated with the species that has no additional bonding through the S atom. At the
same time a broader shoulder is observed around 430 K, attributed to the dissociation and fragmentation
of the three-point bonded methionine, which completely desorbed by 530 K. Interestingly, no ordered
structures have been obtained with LEED for the methionine/Cu(1 1 0) system, presumably due to
disruption in self-assembly caused by the additional interacting S group in some of the species.
3.4.3. S-serine on Cu(1 1 0)
This molecule has a slightly shorter side chain than norvaline with the terminal methyl replaced by an
OH group. This offers more opportunity for intermolecular H-bonding interactions, with the anhydrous
crystalline form of S- (or L-)serine known to form a network of OH OH bonds [73]. On adsorption at
Fig. 3.16. Phase diagram of S-serine on Cu(1 1 0).
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room temperature it is again found that the RAIR spectra of the S-serine/Cu(1 1 0) system show the
same adsorption pattern with two coverage dependent phases, as in Fig. 3.16. We note that the
asymmetric carboxylate stretch dominates the spectrum at high coverage and is a broad feature,
possibly indicative of H-bonding interactions. However, vibrations associated with the OH group are
difficult to assign, being inherently weak and mixed with other vibrations of the molecule so the
orientation of this group cannot be deduced from the RAIRS. An ordered LEED pattern is only seen on
warming the higher coverage phase to around 400 K which is accompanied by a large reduction in the
intensity of the asymmetric carboxylate stretch and a return to a RAIR spectrum more like that obtained
at low coverage. This ordering into the low coverage, thermodynamically favoured phase is similar to
the behaviour of glycine and alanine. However, the final LEED pattern is not the commonly observed
(3 2) structure but is described by the matrix (1 2, 4 0). This organisation of S-serine molecules on
the copper surface is chiral, breaking the symmetry of the underlying (1 1 0) surface. Furthermore, only
one chiral organisation is seen across the entire surface, i.e. a surface with global organisational
chirality is created. This behaviour shows that the OH group profoundly affects the self-organisation at
the surface, with presumably intermolecular OH OH H-bonding occurring, as in the solid crystal.
3.5. R- and S-phenylglycine
Techniques used: LEED.
Preparation: Thermal evaporation from Knudsen cell under UHV conditions.
Both enantiomers of this chiral amino acid have been adsorbed on Cu(1 1 0), separately and together
in a well-defined mixture [74] with the phase diagrams for the various systems shown in Fig. 3.17. In an
analogous manner to glycine and alanine, the molecule is believed to adsorb in its anionic form via both
the carboxylate and amino groups. The phenyl group is held away from the surface normal. Ordered
surface structures are formed at saturation coverage, after adsorption at room temperature for each
enantiomer with the S-form producing a (5 3, 4 1) LEED pattern that is the mirror image of the
R-form (5 3, 4 1). These chiral structures, with a chiral unit cell, are believed to be driven by the
intermolecular H-bonding occurring in the saturated adsorbate layer. Thus the adsorption of R- and
S-phenylglycine on Cu(1 1 0) can create surfaces with global point and organisational chirality. When an
enantiomeric mixture is adsorbed on the surface, LEED patterns associated with both enantiomers can be
seen which, on warming to 420 K, merge to give a (3 2) pattern with missing half-order spots, typical
of a glide plane and similar to that seen for either enantiomer of alanine in the high coverage, high
temperature phase. However, as noted earlier, for chiral molecules such as phenylglycine, it is not
possible to reflect the molecule into its own enantiomer on the surface so any associated glide lines must
really be pseudo-glide lines. It is not clear from the literature whether or not the chiral unit cells
associated with just the single enantiomers of phenylglycine also rearrange to the (3 2) structure on
further warming but the p(3 2)g structure illustrated in [74] shows molecules of one enantiomer only.
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Fig. 3.17. Phase diagram of S- or R-phenylglycine on Cu(1 1 0).
3.6. Tripeptides: tri-L-alanine and tri-L-leucine
Techniques used: RAIRS, LEED.
Preparation: Thermal evaporation under UHV conditions.
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3.6.1. Tri-L-alanine and tri-L-leucine on Cu(1 1 0)
Tripeptides are essentially three amino acid molecules joined together with the elimination of water and
the formation of two amide (H–N–C=O) bonds. Thus, possible functionalities that can be involved in
bonding to a metal surface include not only the usual terminal groups associated with amino acids but also
the connecting amide (or peptide) groups as well as any reactive groups on the side chains. There is only
one detailed study available which attempts to analyse the complex adsorption behaviour of such
multifunctional molecules. In a recent RAIRS study [75], we have been able to determine that both triL-alanine and tri-L-leucine adsorb intact on the Cu(1 1 0) surface in their anionic form, with bonding to
the surface through the terminal carboxylate ions (COO), amino groups (NH2) and the C=O
functionality of the amide group. The detailed RAIR spectra obtained for tri-L-alanine adsorbed on
Cu(1 1 0) show that vibrations due to the asymmetric and symmetric COO stretches are seen around 1615
and 1398 cm1, the NH2 deformation around 1568 cm1, Amide I (mainly amide C=O stretch) around
1678/1640 cm1 and Amide II (mainly amide NH deformation) around 1522 cm1 as well as vibrations
associated with the CH3 groups. Similar values for the vibrations associated with tri-L-leucine adsorption
on Cu(1 1 0) are also observed but vibrational intensities for the two systems differ.
In order to further understand the behaviour of such complex molecular adsorption systems, it is
useful to understand the crystallographic structures that peptides can form. These are either based on
a-helices or b-sheets and can usually be distinguished by their infrared spectra in the Amide I region. In
the solid state, tri-L-alanine exists as a zwitterion and forms crystalline b-sheets, where molecules are
Fig. 3.18.
Parallel and antiparallel b-sheets of tri-L-alanine.
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Fig. 3.19.
Phase diagram of tri-L-alanine on Cu(1 1 0).
hydrogen bonded together in either parallel or antiparallel rows, as illustrated in Fig. 3.18. Tri-L-leucine
also forms b-sheet type structures in the solid state, although these are held in a more unusual twisted
arrangement. The two b-sheet forms of tri-L-alanine have been found to exhibit different infrared
spectra, owing to their differing hydrogen bonding arrangements, allowing them to be distinguished
spectroscopically. Although on adsorption the absolute values of infrared absorptions for any particular
group will be modified, due to coupling of the vibration to the surface, it has proved possible to
compare the general form of the RAIR spectra of tri-L-alanine to those of the solid state spectra,
allowing some structural information to be deduced. Tri-L-alanine shows a complex range of adsorption
phases, summarised in Fig. 3.19, which are particularly sensitive to growth conditions. At high flux and
with the substrate held at room temperature (300 K) three phases are identified, shown in the RAIR
spectra of Fig. 3.20. Phase I occurs at low coverages with tri-L-alanine molecules randomly adsorbed
and isolated from each other. As coverage increases, phase II is formed which represents a monolayer
with intermolecular hydrogen bonding occurring across the surface and resulting in broader, higher
intensity Amide I vibrations, in particular. At even higher coverages, a saturated bilayer, phase III, is
created with perpendicularly oriented C=O functionalities of the amide groups being involved in strong
interlayer H-bonding, as evidenced by the large Amide I band. By comparison with the shape of the
solid state infrared spectra, it appears that, locally, phase III has strong similarities to the antiparallel
b-sheet form of the solid crystal, although no long-range ordered surface structures are seen with
LEED. Multilayers are formed under high flux conditions when the Cu(1 1 0) surface is cooled to 83 K.
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Fig. 3.20.
RAIR spectra of tri-L-alanine on Cu(1 1 0) at 300 K under high flux conditions.
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Fig. 3.21. RAIR spectra of tri-L-leucine on Cu(1 1 0) at 300 K under high flux conditions.
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Interestingly, under low flux conditions, the molecule reorients after initial adsorption so that the amide
C=O functionalities are more flat lying, possibly chelating to the surface, making it difficult to grow
higher coverage phases. In this more flat-lying orientation, opportunities for hydrogen bonding between
layers are limited, making the surface essentially ‘‘non-stick’’.
Similar adsorption behaviour is observed for tri-L-leucine but the longer, bulkier side chains aligned
along the surface normal, appear to sterically inhibit the growth of the phase III bilayer, as seen in the
RAIR spectra of Fig. 3.21.
We note that both molecules adsorb with a chiral motif and that this chirality will also be manifested
in any local b-sheet aggregates. No ordered LEED patterns are observed for either tripeptide system,
presumably as a result of the strength of the molecule–substrate interactions which inhibit molecular
diffusion across the surface and the formation of long-range ordered surface structures.
4. Aromatic anhydrides
In this section of the review we focus on the interactions with metal surfaces of larger molecules
consisting of ring systems with anhydride groups (O=C–O–C=O). Such molecules have been the
subjects of a whole range of studies, as reviewed here, due to their technological importance. For
example, an interest in the surface properties of pyromellitic dianhydride (PMDA) was initially aroused
due to its role as a precursor for the production of the polymer, polyimide, used as a packaging material
in the microelectronics industry. Another technologically interesting aromatic anhydride which has been
studied is perylene-tetracarboxylic acid dianhydride (PTCDA) which has been successfully used for
device fabrication and shows interesting electrical properties. In order to fully understand the behaviour
of such relatively large molecules at ordered metal surfaces, a number of studies have also been
undertaken of other smaller anhydrides with aromatic ring systems. An underlying aim of much of this
work has been to elucidate the relative influence exerted by the anhydride groups compared to that of the
ring systems in driving the coupling of the adsorbate with the metal substrate. The anhydrides reviewed
here are highly symmetric planar molecules and are largely adsorbed with their ring systems parallel to
the surface. As a result, these molecules have a low propensity for creating chiral structures at a surface
and, in fact, this aspect has not been discussed explicitly in the associated literature. Overall, the inherent
achirality of these molecular structures makes them incapable of bestowing global chirality at a surface.
However, we note that some of the ordered structures do have chiral organisation and coexist with mirror
domains at the surface. This interesting point has not been picked up in the existing literature associated
with these molecules but could be an important aspect for certain technological applications. We predict
that many of the chiral manifestations will be similar to those seen for the prochiral carboxylic acid
systems which are discussed in Section 2 and we have not reiterated them here.
4.1. Phthalic anhydride
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Techniques used: RAIRS, HREELS, TPD.
Preparation: Solid powder sublimed under vacuum and exposed to crystal through leak valve.
Detailed coverage and temperature dependent studies have been performed for the phthalic anhydride
molecule adsorbed on both Cu(1 1 0) and oxygen pre-dosed Cu(1 1 0) surfaces [76] and a rich variety
in the surface chemistry has been observed.
4.1.1. Phthalic anhydride on Cu(1 1 0)
Utilising the published RAIRS and HREELS studies on phthalic anhydride exposed to a Cu(1 1 0)
surface, a detailed phase diagram can be constructed as shown in Fig. 4.1. When the surface is held at
room temperature, the molecule initially adsorbs with the phenyl ring structure broadly parallel to the
surface. The RAIR spectra associated with this initial phase (see Fig. 4.2a and b) show a single strong
band around 735 cm1 interpreted as vibrations associated with the out-of-plane CH deformation of the
ring. This flat-lying geometry maximises the opportunity for interactions between individual molecules
and the metal, by providing a large footprint on the surface. The actual molecular species has not been
determined unambiguously, but by comparison with the RAIR spectra of the benzoate species, it has
been suggested that cleavage of the anhydride ring occurs, with the loss of CO, to give a terminal COO
group. As the coverage increases this is certainly seen to be the case with the formation of a Sat. ML
(Fig. 4.2c and d) where the molecule generally adopts a more upright stance and is bound to the surface
via a bridging carboxylate species. No vibrations associated with either the C=O or anhydride ring COC
groups are observed in the RAIR spectra of this monolayer but a strong peak at 1407 cm1 also appears,
Fig. 4.1.
Phase diagram of phthalic anhydride on Cu(1 1 0).
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Fig. 4.2. RAIR spectra of phthalic anhydride adsorbed on Cu(1 1 0) at 300 K with increasing coverage. Spectra (a)–(c)
recorded during 3 109 mbar continual dose, spectrum (c) after total exposure of 6 Langmuir, spectrum (d) after single
4 107 mbar exposure. Reprinted with permission from Fig. 1(a) [76]. Copyright 1998 American Chemical Society.
characteristic of the symmetric carboxylate stretch, confirming the identification and orientation of the
adsorbed species. It is deduced that the plane of the COO group is tilted approximately 208 from the
vertical but that the ring is held more normal to the surface. In contrast to the benzoate species, there
seems to be a higher kinetic barrier to reorientation with some flat-lying molecules remaining even at
this high coverage. TPD shows that the upright species is stable on the surface up to around 510 K when
the molecule starts to decompose, forming a number of desorption species up to temperatures of 570 K.
When deposited at the lower temperature of 95 K phthalic anhydride remains largely intact although
there is some evidence for the presence of a small amount of a dissociated species at the higher
monolayer coverage. The RAIR spectra indicate that the molecule is not preferentially ordered in any
one direction but there is a tendency for the ring system to be held mainly parallel to the surface. When
the Sat. ML formed at low temperature is warmed a number of different features are observed in the
RAIR spectra as seen in Fig. 4.3. On warming between 150 and 250 K, cleavage of the anhydride ring
occurs and a species is observed that has both COO and C=O groups. On further warming of this
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Fig. 4.3. RAIR spectra of approximately one monolayer of phthalic anhydride adsorbed on Cu(1 1 0) at 95 K and the subsequent
changes with thermal evolution. Reprinted with permission from Fig. 4 [76]. Copyright 1998 American Chemical Society.
species to 300 K, the symmetrically bound carboxylate species associated with the saturated room
temperature monolayer is formed. This procedure leads to a higher coverage surface than that created by
direct deposition at room temperature. This is attributed to flat-lying molecules being able to surmount
the reorientation barrier in order to adopt the vertical position when the surface is warmed. Interestingly,
a different series of spectra are observed when the low coverage, low temperature species is warmed to
300 K. Initially, the phenyl ring remains broadly parallel to the surface with the anhydride ring being
cleaved to leave a carboxylate species. This latter group is bound asymmetrically to the copper surface
with the planarity of the molecule being lost so that the carboxylate group is twisted with respect to the
phenyl ring. Above 250 K the molecule reorients and the phenyl ring becomes upright with bonding
occurring symmetrically through the COO group, as in the higher coverage, room temperature species.
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Fig. 4.4. Phase diagram of phthalic anhydride adsorbed on p(2 1)O/Cu(1 1 0).
4.1.2. Phthalic anhydride on p(2 1)O/Cu(1 1 0)
When the Cu(1 1 0) surface is modified by the pre-adsorption of oxygen to form a p(2 1) oxygen
overlayer the bonding of the phthalic anhydride molecule is different. Furthermore, the chemistry of
this molecule at the surface becomes much simpler and the phase diagram given in Fig. 4.4 applies. At
low temperature, 95 K, the molecule is again intact and flat-lying. However, on warming this species to
300 K, or on direct adsorption at 300 K, only the phthalate species is formed. This has two carboxylate
groups with four oxygen atoms which all bond to the surface with the molecule held with its phenyl
ring largely parallel to the surface at all coverages. This species desorbs from the surface in a single
‘‘explosion’’ at 585 K with no intermediate species seen.
4.2. Pyromellitic dianhydride (PMDA)
Techniques used: RAIRS, HREELS, TPD, NEXAFS, XPS.
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Preparation: Solid powder sublimed by gentle heating under vacuum and exposed to crystal through
leak valve.
A number of studies have been performed on the adsorption of PMDA on a variety of metal surfaces,
including Cu(1 1 0) [77,78], Cu(1 1 1) [79], Ni(1 1 0) [77] and Pt(1 1 1) [80–82] in an effort to
understand the conditions under which monolayers or, more importantly for thin films applications,
crystalline multilayers are formed. During these investigations it became apparent that the chemical
form of the molecule and its orientation within the first layer is critically dependent not only on the
nature of the underlying metal surface, but also on the temperature and coverage conditions. The results
associated with the more detailed work on the adsorption in the monolayer regime are discussed here
and, as with the phthalic anhydride molecule, it can be seen that certain conditions cause one of the
anhydride rings to be broken, leading to a variety of surface species.
4.2.1. PMDA on Cu(1 1 0)
The phase diagram, constructed in Fig. 4.5, for this system shows that the PMDA molecule is not
adsorbed intact on the Cu(1 1 0) surface at any temperature [77,78]. Even at an adsorption temperature
of 95 K, RAIRS indicates that one of the anhydride groups is opened giving a carbonyl (C=O) and
carboxylate group (COO) with the molecule oriented with its ring system parallel to the surface. This
arrangement is also the case on initial adsorption of PMDA on a surface held at room temperature.
Fig. 4.5. Phase diagram of PMDA adsorbed on Cu(1 1 0).
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However, there is some difference between the subsequent behaviour with increasing coverage and
temperature for the two systems created at the two different temperatures. When the initial adsorption
temperature is 95 K, the adsorbed species loses CO above 330 K and reorients to give an upright
bidentate carboxylate species. In contrast, this species is not created directly by warming the low
coverage room temperature adsorption species but is reached by a series of intermediate states. As the
coverage at room temperature is increased, bonding to the surface occurs through the asymmetric
carboxylate group, with the plane of this group being twisted out of that of the ring system. At full
monolayer coverage the molecule adopts an upright stance and the carboxylate group is bound
symmetrically through both O atoms but the carbonyl group is still present. On warming this surface
species, CO is seen to be lost from above 370 K, a higher temperature than for the low temperature
system, and the bidentate carboxylate species is observed. This species subsequently desorbs from
above 470 K. Multilayers of PMDA form readily and are stable up to around 350 K, so care is needed to
create and observe the lower coverage conditions.
4.2.2. PMDA on p(2 1)O/Cu(1 1 0)
On the p(2 1)O/Cu(1 1 0) surface [78], the chemistry is again modified significantly to give a new
type of surface species which is similar to that observed for phthalic anhydride on p(2 1)O/Cu(1 1 0)
at room temperature. One of the anhydride rings is broken and a reaction with the adsorbed oxygen on
the surface occurs, leading to the formation of a dicarboxylate species. This species bonds to the
surface through four oxygen atoms in an upright fashion. No measurements are reported at other
temperatures and only this room temperature phase is shown in Fig. 4.6.
Fig. 4.6. Phase diagram of PMDA adsorbed on p(2 1)O/Cu(1 1 0).
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Fig. 4.7. Phase diagram of PMDA adsorbed on Cu(1 1 1).
4.2.3. PMDA on Cu(1 1 1)
Detailed coverage and temperature dependent studies on this surface are also not available as
PMDA films created at room temperature have all shown a single species on the Cu(1 1 1) surface
[79], as illustrated in Fig. 4.7. NEXAFS studies have indicated that again the anhydride ring is
broken and the molecule loses CO, leaving a carboxylate group. This is bound to the Cu(1 1 1)
surface in a bidentate manner through both O atoms with the molecule adopting a largely upright
orientation with respect to the surface. This species is the same as that reported at high temperature
on Cu(1 1 0).
4.2.4. PMDA on Pt(1 1 1)
Most of the studies on the Pt(1 1 1) surface [80–82] have been concerned with the formation of
ordered crystalline films and the system readily forms multilayers, even at room temperature if the flux
is sufficiently high. Above 340 K the molecule desorbs and decomposes, so there are relatively few
conditions under which a monolayer or sub-monolayer coverage can be created. However, with a low
flux, around 300 K, it has been possible to conclude, with careful NEXAFS experiments, that the
molecule remains intact and adopts a flat-lying orientation with respect to the surface. There is a
strong interaction between the p-system of the phenyl ring and the metal which keeps the ring system
parallel to the surface. Interestingly, the NEXAFS signal for the p resonance shows considerable
intensity with s-polarised light, although for flat-lying molecules, this is strictly not allowed by dipole
selection rules. This intensity was initially assumed to be a result of molecular disorder or tilting but it
has been shown that this anomalous weak linear dichroism is due to induced hybridisation which
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Fig. 4.8. Phase diagram of PMDA adsorbed on Pt(1 1 1).
creates an upward bend in the anhydride groups of 208. The phase diagram for the system is shown
in Fig. 4.8.
4.3. Naphthalene-1,8-dicarboxylic anhydride (NDCA)
Techniques used: TPD, XPS, UPS, NEXAFS, LEED, FTIR, DFT calculations (Xa-SW).
Preparation: Vacuum sublimation onto sample held at 100 K.
The adsorption characteristics of naphthalene-1,8-dicarboxylic anhydride (NDCA) are of interest as
it is effectively one-half of a PTCDA molecule. Thus the molecule offers the opportunity to study the
effects of a single anhydride group in the presence of a number of rings.
4.3.1. NDCA on Ni(1 1 1)
Nickel is an interesting choice of substrate because it is known that vertically oriented anhydride
species can be created on Ni(1 1 1) by heating and dehydration of two adjacent adsorbed formate
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Fig. 4.9. Phase diagram of NDCA adsorbed on Ni(1 1 1).
species [83]. Therefore, any tendency for NDCA to be flat-lying would be indicative of the bonding role
played by the aromatic ring. The phase diagram constructed for the adsorption of NDCA on this
Ni(1 1 1) surface from a range of different measurements [84–87] is shown in Fig. 4.9. NEXAFS
experiments do indeed show that at low coverages the p-bonding of the naphthalene core is sufficiently
strong for the NDCA molecule to be flat-lying. Fig. 4.10a shows the C K-edge NEXAFS spectra for
NDCA on Ni(1 1 1) at normal (polarisation vector parallel to the surface) and grazing (polarisation
vector perpendicular to the surface) incidence for s- and p-polarised light, respectively. It can be seen
that the three sharp p-resonances, which are normal to the molecular plane and occur between 285 and
290 eV, are not present at normal incidence so the polarisation vector must lie in the plane of the
molecule. NEXAFS and XPS measurements also confirm that the anhydride groups are involved to
some extent in the bonding but, unlike PMDA, there is no conversion to carboxylate groups. Further
details of the bonding process are provided by Xa-SW calculations (a form of DFT calculation using
the local density approximation) which can successfully replicate the NEXAFS spectra and show that
the anhydride bonding involves charge donation to the substrate surface (which already has a high
electron density). This sub-monolayer phase is ordered with the molecules lining up in densely packed
rows with spaces between the rows. It appears that the permanent dipole moment of the molecule acts
attractively along the rows and repulsively perpendicular to the rows, keeping the molecules apart and
preventing further adsorption of flat-lying molecules. As coverage increases, the gaps between the rows
are filled by molecules with a more upright orientation which are less strongly bound and this phase
shows no long-range ordering. With further coverage a bilayer is formed where the molecules are
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Fig. 4.10. NEXAFS data from the C K-edge of NDCA on Ni(1 1 1) for two different angles of incidence of the photon beam:
normal (s-polarisation, continuous lines) and grazing (p-polarisation, dashed lines). (a) NDCA monolayer on clean Ni(1 1 1);
(b) NDCA monolayer on p(2 2)O precovered Ni(1 1 1). Reproduced from Fig. 6 [86] with permission from Wiley/VCH
Verlag GmbH.
mainly held parallel to the surface and on further exposure a less well-ordered multilayer is formed.
Unusually, the monolayer is only formed by adsorption at low temperature (100 K) and cannot be
produced by warming the multilayer, as dissociation occurs. The sub-monolayer ordered phase also
dissociates on heating but the higher coverage phase with the more upright molecules is very weakly
bonded to the metal and desorbs intact at a lower temperature than the bilayer.
4.3.2. NDCA on p(2 2)O/Ni(1 1 1)
Pre-coverage of the nickel surface with oxygen again significantly alters the behaviour of the NDCA
molecules at the Ni(1 1 1) surface [84–87]. The phase diagram constructed in Fig. 4.11 shows that at
low coverage and temperature, two species coexist—a flat lying species and a more upright species held
at 708 to the surface and bonded through the anhydride group. On further exposure to NDCA, or on
heating this low coverage phase, a phase is created with a single upright species and the NEXAFS
spectra associated with this phase are shown in Fig. 4.10b. Here the polarisation dependence is almost
the opposite of that seen on the clean surface with the highest p-intensities observed for normal
incidence, i.e. the plane of the molecule is essentially normal to the surface. This is due to both a
reduction in the strength of the interaction between the p-electrons and the substrate and to an increase
in the strength of the interaction between the anhydride unit and the substrate. XPS, NEXAFS and
electronic structure studies of this adsorbed species show that all three O atoms of the anhydride group
are nearly equivalent and that the bond order of the C=O unit has been reduced. The pre-adsorbed
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Fig. 4.11. Phase diagram of NDCA adsorbed on p(2 2)O/Ni(1 1 1).
oxygen is charge accepting which reduces the electron density of the Ni(1 1 1) surface which in turn
can then accept charge more easily from the anhydride than for the clean surface. Thus, in the presence
of pre-adsorbed oxygen the reactivity of the nickel surface is effectively reduced.
4.4. 1,4,5,8-Naphthalene-tetracarboxylic dianhydride (NTCDA)
Techniques used: XPS, NEXAFS, LEED, UPS, STM, LEED, semi-empirical calculations.
Preparation: Vacuum sublimation.
The NTCDA molecule has two anhydride groups but a smaller aromatic core (naphthalene as
opposed to perylene) than PTCDA. An extensive comparative study on the adsorption of NTCDA on a
range of metal surfaces has been undertaken [88–92]. This has shown that the formation of ordered
surface structures is critically dependent on the strength of the interaction between the metal and the
adsorbate and that structures formed vary according to both the deposition conditions and the arrangement
of the metal atoms at the surface. Thus, for example, a range of different coverage dependent phases are
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formed for each of the silver surfaces Ag(1 1 1), (1 1 0) and (1 0 0) where, although the ‘global’ electronic
properties of all the silver surfaces are very similar, specific local bonding interactions can lead to
differences in behaviour. In addition, more strongly bonded ordered phases are created on Cu(1 0 0) than
on silver with even stronger bonding on Ni(1 1 1) preventing the formation of any ordered structures. The
XPS and O K-edge monolayer NEXAFS spectra confirm that on silver only the two terminal O atoms of
the anhydride groups are involved in the bonding, whereas on both nickel and copper all three O atoms of
the anhydride groups are involved. This is understood in terms of the relative positions of the d-bands
from the Fermi edge for each of the metals. In silver the d-bands are relatively far from the Fermi edge
(binding energy 4–8 eV), in copper they are closer (binding energy 2–6 eV) whereas in nickel there is a
significant d-band density of states at the Fermi edge.
4.4.1. NTCDA on Ag(1 1 1), Ag(1 0 0) and Ag(1 1 0)
On all the silver surfaces studied, NTCDA bonds to the surface through the molecular p-system of the
naphthalene core with only relatively weak interactions occurring through the C=O groups of the
anhydride unit. Thus, in the monolayer, molecules adsorb intact and are held parallel to the surface. The
relatively weak bonding also enables the molecules to diffuse on the surface and find their preferred
adsorption sites. The covalent bonding between the molecule and the surface is, however, strong enough
to modify the molecular orbitals of the naphthalene core, which are assigned in the NEXAFS spectra with
reference to the simpler NDCA molecule and semi-empirical CNDO calculations. The only situation
where molecules may not tend to adsorb parallel to the surface is in the multilayer. When multilayers are
Fig. 4.12.
Phase diagram of NTCDA adsorbed on Ag(1 1 1).
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Fig. 4.13. The relaxed monolayer structure A of NTCDA adsorbed on Ag(1 1 1). (a) STM image (Usample ¼ 0:2 V,
I ¼ 2:0 nA, image size: 2:8 nm 2:8 nm). The unit cell is indicated by the base vectors ~
a1 and ~
a2 . (b) Model for the real-space
arrangement of the molecules (the ‘‘envelope’’ is given by the van der Waals’ radii). The short lines in the upper left corners
represent one of the symmetry planes of the molecule, which was used for evaluation of the molecular orientation. Reprinted
from Fig. 3 [90] with permission from Elsevier Science.
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Fig. 4.14. The compressed monolayer structure B of NTCDA adsorbed on Ag(1 1 1). (a) STM image (Usample ¼ 0:3 V,
I ¼ 0:3 nA, image size: 5:5 nm 5:5 nm). (b) Model for the real-space arrangement of the molecules. The ‘‘incorrect’’ unit
cell indicated by dashed lines is discussed in the reference. The short lines in the left part of the images represent one of the
molecular symmetry planes. Reprinted from Fig. 4 [90] with permission from Elsevier Science.
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grown at 150 K, all the molecules will grow parallel to the surface but if grown at temperatures greater
than about 285 K they adopt a more upright stance. This latter orientation is more similar to that of the
bulk crystal where the molecular planes are oriented perpendicular to the natural cleavage plane.
However, at lower temperatures it appears that the intermolecular interactions between molecules are
strong enough to promote layer by layer growth.
The phase diagram constructed in Fig. 4.12 shows that two reversible ordered phases are formed on the
Ag(1 1 1) surface [88–92], characterised as a relaxed monolayer, A, and a compressed monolayer, B, with
Fig. 4.15. Coexistence of monolayers A and B of NTCDA adsorbed on Ag(1 1 1). (a) STM image for y ¼ 0:9 ML. Structures A
and B coexist in parallel stripes (image size: 44 nm 44 nm, Usample ¼ 1:0 V, I ¼ 0:4 nA). (b) Phase boundary between structure
A (darker area) and B (11 nm 11 nm, Usample ¼ 1:0 V, I ¼ 1:0 nA). The corresponding unit cells are indicated. (c) Realspace model of a phase boundary containing both unit cells and the corresponding base vectors. An ‘‘incorrect’’ unit cell for
structure B (dashed lines) is also given (see reference). Reprinted from Fig. 5 [90] with permission from Elsevier Science.
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molecules arranged in rows as in Figs. 4.13 and 4.14. Structure A, with a (4 0, 3 6) LEED pattern is
essentially a twisted brick wall arrangement with bright and dark rows seen in the STM images due to
inequivalent adsorption sites of the corner and centre molecules. Structure B which has a (6 1, 1 7)
LEED pattern is more like the herringbone structure seen for NTCDA on inert substrates such as graphite.
Below saturation monolayer coverage, domains of both A and B exist forming stripes with the average
width of the B stripes increasing with coverage, as shown in Fig. 4.15. Unlike some of the anhydride
systems, the relatively weak adsorption of NTCDA on Ag(1 1 1) allows the molecules to desorb intact and
monolayers can be formed by either slow dosing at lower temperatures or by heating the multilayer.
On Ag(1 0 0) [92] two ordered monolayer structures are again observed although these differ from the
structures seen on the Ag(1 1 1) surface due to the difference in the underlying arrangement of the silver
atoms. The phase diagram constructed for this system is given in Fig. 4.16. The relaxed monolayer has a
(3 3, 4 4) LEED pattern and the compressed monolayer has a non-commensurate (but commensurate to
second-order) LEED patterns of (6 1.5, 0 3) leading to the structural models shown in Fig. 4.17.
The phase diagram constructed for NTCDA adsorbed on the more corrugated Ag(1 1 0) surface [92] is
given in Fig. 4.18. Here, there is only one ordered monolayer structure with a (3 0, 1 3) LEED pattern.
Rows of molecules are aligned parallel to the silver rows in a relatively open structure, as shown in
Fig. 4.19. On this surface, where the silver atoms are arranged in regular ‘‘peaks and troughs’’, the local
bonding conditions are such that it is not possible to compress the monolayer and form additional higher
coverage monolayer structures.
Fig. 4.16.
Phase diagram of NTCDA adsorbed on Ag(1 0 0).
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Fig. 4.17. Possible real space configurations for (a) compressed and (b) relaxed monolayer structures of NTCDA adsorbed
on Ag(1 0 0). Reproduced from Fig. 4 [92].
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Fig. 4.18.
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Phase diagram of NTCDA adsorbed on Ag(1 1 0).
4.4.2. NTCDA on Cu(1 0 0)
NTCDA adsorbs on Cu(1 0 0) [89,91,92] intact with the molecules p-bonding through the
naphthalene core and all the oxygen atoms of the anhydride group also involved in the bonding process.
As for silver, the multilayer orientation depends on the surface temperature—at low deposition rates
and temperature the molecules adopt the ‘‘optimum’’ position for chemically bonding to the surface
and form parallel layers. However, at higher temperatures or high deposition rates the molecules adopt
a more perpendicular orientation, typical of the bulk crystal. In the monolayer, one ordered (2 3, 2 3)
phase is formed at higher coverage or on annealing as shown in Fig. 4.20 with the molecules arranged
as in Fig. 4.21. The greater strength of the bonding of NTCDA to copper as compared with silver leads
to the molecule dissociating on heating.
4.4.3. NTCDA on Ni(1 1 1)
On Ni(1 1 1) [91,92] NTCDA adsorbs intact with a broadly parallel orientation but there is evidence
of some loss of the planar symmetry of the molecule. p-Bonding occurs through the naphthalene core
but all the oxygen atoms of the anhydride group are also strongly involved in the bonding. The
molecule dissociates on heating the monolayer, indicating the strength of the chemical bond to
the surface. This is further illustrated by the absence of any ordered structures on this surface, unlike
the smaller anhydride molecule, NDCA. Essentially, the molecules ‘‘hit and stick’’ and are not able to
diffuse to preferred adsorption sites and create ordered superstructures. The phase diagram for this
system is shown in Fig. 4.22.
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Fig. 4.19. Monolayer structure of NTCDA adsorbed on Ag(1 1 0). (a) LEED pattern, (b) simulation of LEED pattern,
(c) suggested real space structure. Reproduced from Fig. 6 [92].
4.5. Perylene-3,4,9,10-tetracarboxylic acid-3,4,9,10-dianhydride (PTCDA)
Techniques used: NEXAFS, TPD, XPS, LEED, STM, FT-RAIRS, UPS, HREELS, DFT calculations.
Preparation: Vacuum sublimation.
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Fig. 4.20. NTCDA adsorbed on Cu(1 0 0).
As mentioned in the introduction to this section on anhydrides, PTCDA is of particular technological
interest and, together with its simpler derivatives, has been the target molecule for a range of studies. It
consists of a perylene core with two anhydride units and, despite its relative size, has been found to be
capable of forming ordered structures on certain surfaces. The adsorption of PTCDA has been studied
extensively on a range of single crystal metal surfaces and a very detailed understanding gained of the
way the molecule interacts, particularly on the surface of silver [85,86,93–102]. The main findings,
with respect to the bonding, orientation and surface structures formed, are summarised below but much
additional experimental and theoretical information is present in the original papers. It should also be
noted that studies on PTCDA have also been performed on (1 1 1) and reconstructed (1 1 1) faceted
gold surfaces (deposited on KBr crystals) [103,104]. However, these experiments are not reviewed here
as the surfaces are not true single crystals and some of the measurements were taken under atmospheric
conditions so are difficult to compare directly with results from systems studied under more controlled
conditions.
4.5.1. PTCDA on Ag(1 1 1) and Ag(1 1 0)
On both these silver surfaces, at room temperature, PTCDA adsorbs intact with the molecular plane
essentially parallel to the surface but with a possible slight upwards bending of the CH groups
observed. Detailed analysis of NEXAFS features and calculations on the Ag(1 1 1) surface [85,86,99]
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Fig. 4.21. Monolayer structure of NTCDA adsorbed on Cu(1 0 0). (a) LEED pattern, (b) simulation of LEED pattern,
(c) suggested real space structure. Reproduced from Fig. 7 [92].
show that both the lowest unoccupied p-orbital (LUMO) and the highest occupied molecular orbital
(HOMO) are strongly involved in the bonding and essentially localised on the perylene core. Thus a
strong chemisorptive bond is formed, via the p orbitals of the perylene ring and further enhanced
by the anhydride groups. Surprisingly, HREELS spectra of PTCDA sub-monolayers on Ag(1 1 1)
[100,102] show strong in-plane modes above 900 cm1 which would normally indicate that the
molecules are not flat-lying. However, this appears to be due to a coupling between the molecular
vibrations of the adsorbate and electronic excitations in which charge is redistributed perpendicular to
the substrate giving rise to a perpendicular dynamical dipole moment from these in-plane modes. This
interfacial dynamical charge transfer is due to the strength of bonding between the PTCDA and the
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Fig. 4.22. Phase diagram of NTCDA adsorbed on Ni(1 1 1).
Fig. 4.23.
Phase diagram of PTCDA adsorbed on Ag(1 1 1).
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Fig. 4.24. Phase diagram of PTCDA adsorbed on Ag(1 1 0).
Ag(1 1 1) surface which leads to the formation of hybrid orbitals from the PTCDA molecular orbitals
and the wave functions of the substrate. No such resonance coupling occurs on the Ag(1 1 0) surface
[100–102] indicating a difference in the interaction due to, presumably, the different bonding sites on
the two silver surfaces.
Fig. 4.25. STM image of PTCDA forming at step edges of Ag(1 1 1) (Utip ¼ 1:3 V, I ¼ 1 nA, sample 704 Å across).
Reprinted from Fig. 3 [96] by permission of Elsevier Science.
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Fig. 4.26. Monolayer structure for PTCDA adsorbed on Ag(1 1 1). (a) STM image with the unit cell of the superstructure
as indicated (tunnelling parameters: Utip ¼ 0:20 V; I ¼ 1:3 nA; scan area 45 — 45 Å). The white parts arise from
protrusions. (b) Real-space model of the adsorption geometry. Reprinted from Fig. 6(b) [96] by permission of Elsevier
Science.
The ordered structures formed on the two silver surfaces are also found not to be the same, as
illustrated in the phase diagrams constructed in Figs. 4.23 and 4.24. On Ag(1 1 1) [85,86,96,
99,100,102] an ordered (6 1, 3 5) superstructure is formed with six domains (three reflectional and
two rotational) containing two molecules per unit cell, with each molecule adsorbed at inequivalent
Fig. 4.27. STM image of PTCDA forming islands on the terraces of Ag(1 1 0) (tunnelling parameters: Utip ¼ 1:6 V;
I ¼ 1:4 nA). Reprinted from Fig. 4 [96] by permission of Elsevier Science.
Fig. 4.28. Monolayer structure for PTCDA adsorbed on Ag(1 1 0). (a) STM image with the unit cell of the superstructure as
indicated (tunnelling parameters: Utip ¼ 0:90 V; I ¼ 1:0 nA; scan area 48 — 48 Å). (b) Real-space model of the adsorption
geometry. Reprinted from Fig. 8(b) [96] by permission of Elsevier Science.
Fig. 4.29. Phase diagram of PTCDA adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
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sites. The molecules start to nucleate at the step edges, as seen in the STM image of Fig. 4.25 and grow
into very large mobile islands in a T-shaped herringbone-like structure (Fig. 4.26) characteristic of
PTCDA crystals in the solid state. This herringbone arrangement continues into the multilayer which
grows layer by layer. In contrast, on Ag(1 1 0) [96–102] initial nucleation is on the flat terraces between
the steps as depicted in the STM image of Fig. 4.27 and the molecules aggregate in smaller rhombic
islands to form a brick wall-like structure, seen in Fig. 4.28, with an ordered (2 3, 2 3) superstructure.
Only one domain is formed with no rotational or mirror domains and there is only one molecule per unit
cell. Individual PTCDA molecules are adsorbed with their centres located between close-packed rows of
silver atoms and their long axes parallel to the [0 0 1] direction. With further coverage, second and
subsequent layers of PTCDA grow on top of this brick wall structure and assume the herringbone
arrangement characteristic of the solid crystal. Both monolayer structures are, however, commensurate
with the underlying silver surfaces, indicating the strength of the covalent bonding of the PTCDA to
the metal surface, which favours adsorption at specific, high symmetry adsorption sites. On Ag(1 1 1), the
herringbone structures are believed to be initiated by the lateral interactions of the quadrupole moments
of the molecules. On the more open Ag(1 1 0) surface the lateral electrostatic interactions are
overcompensated by the site-specific bonding which leads to a slightly stronger bonding and reduces
the mobility of the PTCDA atoms, encouraging smaller islands and the formation of the brick wall
structures.
4.5.2. PTCDA on Cu(1 0 0)
Most of the PTCDA films prepared on Cu(1 0 0) [93–95] have been grown with the copper surface held
above room temperature, between 363 and 403 K, with ordered LEED patterns observed above 373 K.
XPS measurements show that the PTCDA molecules chemically react with the Cu(1 0 0) surface and
suggest that at least one oxygen atom is removed from each anhydride group—probably the bridging
O of the anhydride O=C–O–C=O group. LEED indicates that an ordered commensurate
p
psuperstructure
is formed with two domains rotated by 908 and a rectangular unit cell ð4 2 5 5Þ R ¼ 45
which contains a glide plane. Thermal desorption spectra show that after formation of the monolayer,
a bilayer is formed, followed by a strained molecular crystallographic version in layers 3–6 with
further modifications occurring for thicker layers. The phase diagram for this system is shown in
Fig. 4.29.
4.5.3. PTCDA on Ni(1 1 1)
PTCDA reacts strongly with the Ni(1 1 1) surface [85,86,99], bonding intact at room temperature
and being held broadly parallel to the surface. Initial NEXAFS measurements reported significant
tilting and bending of the molecules in the sub-monolayer region and the possibility of more than one
adsorption state [85]. However, more recent analysis of NEXAFS data [99] suggests that large
differences seen between sub-monolayer and multilayer spectra on Ni(1 1 1) are due to the strong
influence of the substrate bonding on the unoccupied orbitals. Such differences are also seen with
Ag(1 1 1) but are much greater for nickel where the bonding associated with the p-resonance of the
perylene core is particularly strong. Due to the strength of this bonding, ordered superstructures are not
formed in the monolayer, as seen in the phase diagram of Fig. 4.30. Slightly more order appears to
occur in the multilayer but again, this is not as great as for the more weakly bound Ag(1 1 1) interface
and after a few layers epitaxial order is lost. This illustrates an important point that has been found with
respect to the formation of ordered thin films of all these complex anhydrides—truly ordered epitaxial
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Fig. 4.30.
Phase diagram of PTCDA adsorbed on Ni(1 1 1).
films are only created if the underlying template (i.e. the monolayer film) is ordered and commensurate
with the metal surface.
4.6. Substituted derivatives of PTCDA
Techniques used: NEXAFS, LEED, STM, XPS, AFM.
Preparation: Vacuum sublimation.
A number of studies have been undertaken on derivatives of PTCDA where the central O of the
anhydride group has been substituted for another functional group. Naming of such compounds appears
to be somewhat ambiguous but the simplest derivative is obtained by substituting the bridging oxygen
atoms by NH groups to give a diimide.
Perylene tetracarboxylic-diimide (PTCDI)
A further derivative is obtained on substituting the H of the diimide by methyl (CH3) groups.
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Methyl-perylene-tetracarboxylic-diimide (Me-PTCDI or DMe-PTCDI)—also referred to as dimethylperylene-bis(dicarboximide) (DM-PBCDI or DMPI) or dimethyl-antra-diisoquinoline-tetrone
A non-planar derivative is further obtained by substituting 2,6-isopropylphenyl rings for the H of the
diimide.
Perylene-tetracarboxylic-diimide-di(2,6-isopropylphenyl) (DPP-PTCDI)
The degree of detail to which these molecules have been studied varies but the following summarises
the main findings and can be used to compare the effects of the substituted groups on the adsorption
behaviour with that of PTCDA on the relevant surfaces.
4.6.1. PTCDI on Ni(1 1 1)
At room temperature the molecule PTCDI adsorbs intact on the Ni(1 1 1) [85] surface and appears to
be oriented mainly parallel to the surface with weaker covalent bonding than for the PTCDA molecule,
i.e. the absence of the central oxygens of the anhydride groups reduces the strength of the molecule
surface interaction. No ordered surface structures are reported and the phase diagram of Fig. 4.31 has
been constructed.
4.6.2. Me-PTCDI on Ag(1 1 1) and Ag(1 1 0)
Adsorption of Me-PTCDI at room temperature gives intact flat-lying molecules on either surface
[96] with herringbone-like structures forming from the step edges on the Ag(1 1 1) surface and brick
wall-like surface structures forming on the terraces for Ag(1 1 0), which look very similar to the
packing displayed by PTCDA. However, noticeable differences from PTCDA exist. On the Ag(1 1 1)
surface (Fig. 4.32) the surface lattice is not commensurate with the underlying metal surface, showing
a (6.2 0.6, 2.1 6.2) superstructure. This is presumably a consequence of the larger footprint offered
by the Me-PTCDI molecule compared to the locations of the preferred binding sites and weak metal–
molecule bonds. On Ag(1 1 0) (Fig. 4.33) the ordered sub-monolayer created on initial exposure is
commensurate but has a (2 4, 2 3) superstructure with two reflectional domains observed. It should be
noted that other authors [105] describe this initial overlayer structure by the matrix (4 2, 3 2) which is
a specific example of where the choice of inconsistent substrate and overlayer unit cell vectors result in
a different matrix notation from the original work [96]—neither of which are entirely consistent with
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Fig. 4.31.
Fig. 4.32.
Phase diagram of PTCDI adsorbed on Ni(1 1 1).
Phase diagram of Me-PTCDI adsorbed on Ag(1 1 1).
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Fig. 4.33. Phase diagram of Me-PTCDI adsorbed on Ag(1 1 0).
the conventions defined in Appendix A. This more recent detailed work [105] on the room temperature
adsorption of Me-PTCDI on Ag(1 1 0) has shown that a range of compressed monolayer structures can
be formed on this surface following the creation of the initial commensurate superstructure by
increasing the exposure and annealing. These are not detailed here but illustrate the fact that varying
coverage and temperature conditions can lead to more than one adsorption phase being created by
these complex molecules and that these are not always observed in initial adsorption studies. Finally, if
the sub-monolayer is created by warming and desorbing the multilayer, the structure at the surface
shows strong similarities to that formed by PTCDA on Ag(1 1 0), leading to the possibility that either
the methyl groups are lost on heating the multilayer, giving PTCDI, or that a compressed monolayer of
Me-PTCDI is formed.
4.6.3. Me-PTCDI on Cu(1 0 0)
Me-PTCDI exposed at 403 K to Cu(1 0 0) [95] appears to adsorb dissociatively, probably (but this is
not confirmed) with the loss of the N–CH3 groups. It forms an ordered superstructure and is flat-lying to
the surface, as shown in Fig. 4.34. Interestingly, as the second layer starts to grow in, islands form and
no layer by layer growth is observed. Presumably this is related to the strong interaction of the molecule
with the surface which causes it to dissociate in the first layer.
4.6.4. DPP-PTCDI on Ag(1 1 0)
DPP-PTCDI is non-planar as the two isopropylphenyl groups are antisymmetrically arranged with
respect to the perylene core. The molecule appears to adsorb intact and when deposited at room
temperature the perylene core is held broadly parallel to the surface, i.e. the interaction between the
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Fig. 4.34. Phase diagram of Me-PTCDI adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
Fig. 4.35.
Phase diagram of DPP-PTCDI adsorbed on Ag(1 1 0).
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metal and the ring system dominates [106]. However, if a monolayer is formed by desorbing the
multilayer, a close-packed structure is formed and the end groups interact directly with the surface with
the perylene core held more perpendicular to the metal surface. A number of ordered superstructures
are observed according to the preparation conditions and the details are summarised in the phase
diagram of Fig. 4.35. At room temperature and low coverage DPP-PTCDI accumulates at the step
edges and forms an ordered p(6 3) structure, with rows of molecules parallel to the ½1 1 0 direction,
and individual molecules held parallel to the [0 0 1] direction. As the coverage increases, the structure
becomes more disordered but on annealing another ordered (5 2, 3 6) structure forms where the
individual molecules are still held parallel to the [0 0 1] direction but domains are created with rows
consisting of tilted pairs of antisymmetrically oriented molecules. An even closer packed herringbone
p(7 3) structure is formed when the monolayer is created by warming the multilayer with rows again
parallel to the ½1 1 0 direction but individual molecules stacked 358 to the [0 0 1] direction.
5. Closed ring structures without additional functional groups
Closed ring structures usually consist of aromatic molecules joined together in a planar arrangement
and most surface studies have been carried out on such systems. However, it is also possible to
synthesise helical aromatic molecules (helicenes) where the steric repulsion of the terminal aromatic
rings induces a helical formation. Such helicenes possess unusual and interesting physical properties
and are also chiral, with the twist of the helix being either left or right-handed. They have thus recently
been the subject of a number of surface studies, particularly with respect to their chiral properties. In
this review we will first concentrate on recent work on the non-chiral planar ring systems and then turn
our attention to the non-planar chiral helicenes.
5.1. Planar molecules
In order to understand the roles played by the perylene and naphthalene cores of molecules such as
PTCDA and NTCDA interest has focused on comparing the behaviour of such closed ring structures
with the corresponding anhydrides. Most of the information on such systems was gained some while
ago and is referenced in the 1992 review article of Netzer and Ramsey [1]. To summarise, naphthalene,
anthracene, tetracene, perylene and coronene all adsorb on Ag(1 1 1) with their molecular planes (i.e.
the rings) essentially parallel to the metal surface [107–109]. Unusually, for these planar molecules,
tetracene (four carbon ring systems in a row) is found to adsorb on Cu(1 0 0) with its molecular plane
perpendicular to the metal surface [110]. It is suggested that this may be the result of intermolecular
interactions and the close matching of the Cu(1 0 0) lattice with the slightly distorted a,b plane of bulk
tetracene. However, as far as we are aware, no other planar molecule has been observed to adsorb in the
monolayer region without the ring system being held parallel to the surface.
More recent work is only available for a number of polycyclic aromatics adsorbed on Cu(1 0 0) in
conjunction with experiments on PTCDA and Me-PTCDI and these are summarised here [93]. Also
included are results for pentacene with additional oxygen groups as these illustrate the way that the
presence of side groups change the superstructures formed. An interesting point to note is that in all
cases ordered surface structures are formed in the monolayer regime but that these are not the same
as for the bulk crystals. We again note that the literature does not discuss any chiral organisations
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associated with the adsorption of these molecules and in all cases reflectional domains are always
present.
Techniques used: LEED, TPD.
Preparation: Vacuum sublimation.
5.1.1. Perylene on Cu(1 0 0)
Fig. 5.1 shows the adsorption behaviour of perylene on the Cu(1 0 0) surface [93]. A monolayer is
formed if the substrate temperature is kept in the range 423–473 K during deposition with a multilayer
forming for deposition temperatures below 423 K. The molecule is believed to adsorb intact with its
molecular plane parallel to the metal surface. The monolayer shows an ordered LEED pattern with
pseudo sixfold symmetric packing and some asymmetry in the unit cell. Although the actual adsorption
sites of the molecule cannot be confirmed from the LEED data alone, it appears from the proposed
packing pattern [93], reproduced in the phase diagram, that there are two inequivalent adsorption sites
Fig. 5.1. Phase diagram of perylene adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
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on the Cu(1 0 0) surface and that the structure is not commensurate, indicating the weak site specificity
of this molecule.
5.1.2. Coronene on Cu(1 0 0)
The phase diagram constructed for the coronene/Cu(1 0 0) system [93] is shown in Fig. 5.2.
Coronene forms a monolayer when adsorbed onto the Cu(1 0 0) held above 373 K and appears to have
two sub-monolayer states. Below a coverage of 0.7 ML a disordered state is formed which desorbs
above 573 K. Above this coverage, but below a multilayer coverage, an ordered state is formed which
desorbs above 423 K. This ordered layer shows a close-packed sixfold symmetric LEED pattern
Fig. 5.2. Phase diagram of coronene adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
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with two domains, with a 2 1 superstructure imposed on the fourfold symmetric substrate. The
superstructure is only fully commensurate along one crystal axis, being commensurate in the other
direction for only every second coronene molecule, presumably again, an indication of the weak site
specificity of these larger ring systems. Although there is no direct evidence provided, the authors [93]
believe that the molecule prefers to interact with the surface by lying flat but at the same time tries to
remain isolated from the other coronenes, accounting for the different desorption temperatures
associated with the two sub-monolayer phases. It is hypothesised that as the coverage increases above
that necessary to produce a close-packed monolayer, some molecules may adsorb ‘‘edge-on’’ (i.e. more
upright as seen with tetracene) but again there is no direct experimental evidence provided.
5.1.3. Pentacene, pentacenequinine or pentacenetetrone on Cu(1 0 0)
Fig. 5.3 shows the phase diagram that has been constructed for pentacene adsorbed on Cu(1 0 0) [93]
where it can be seen that a monolayer is formed when the molecule is adsorbed at 373 K. The
pentacene is believed to adsorb flat with an ordered, non-commensurate superstructure showing
approximately hexagonal packing. When oxygens are substituted onto the ring structures, to give either
pentacenequinine (two oxygens) or pentacenetetrone (four oxygens), monolayers are formed at higher
temperatures, above 413 K, and commensurate superstructures are formed with more square-like
lattices and true glide planes, as outlined in the phase diagrams of Figs. 5.4 and 5.5. It is concluded that
the packing geometries of the ordered structures of pentacenequinine and pentacenetetrone are altered
from that of pentacene due to the presence of the highly polar oxygens which result in adsorption with a
higher site specificity than observed for the unsubstituted pentacene. Interestingly, although these
aspects are not discussed explicitly in the literature, the unit cells shown are chiral but with both mirror
domains equally allowed.
5.2. Helical molecules
The most extensive studies of helically shaped polyaromatic molecules have concentrated on
heptahelicene, a molecule consisting of seven benzene rings, in a spiral arrangement, adsorbed on
copper or nickel surfaces [111–114], but STM results have also been reported for thioheterohelicene, a
molecule with alternating benzene and sulphur-containing thiophene rings, adsorbed on gold [115]. In
both cases, interest has centred on the chiral nature of the adsorbate systems created.
Techniques used: LEED, NEXAFS, STM, XPD, ToF-SIMS.
Preparation: Vacuum sublimation.
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Fig. 5.3. Phase diagram of pentacene adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
Fig. 5.4. Phase diagram of pentacenequinine adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
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Fig. 5.5. Phase diagram of pentacenetetrone adsorbed on Cu(1 0 0) (superstructure shown adapted from [93]).
5.2.1. M- and P-heptahelicene (M-[7]-helicene and P-[7]-helicene)
The structures of the two enantiomers of heptahelicene, M-[7]- and P-[7]-helicene (C30H18) are
shown in Fig. 5.6.
5.2.1.1. M-[7]- and P-[7]-helicene on Cu(1 1 1) and Cu(3 3 2). An XPD study at sub-monolayer
coverage (1/3 saturated ML) of M-[7]-helicene adsorbed on copper surfaces [113] has shown that on both
Cu(1 1 1) and Cu(3 3 2) the molecule binds via its terminal phenanthrene group (i.e. the three terminal
C6 rings). This group is oriented parallel to the (1 1 1) surface (or (1 1 1) terrace in the case of the (3 3 2)
surface) with the rest of the molecule spiralling away from the surface. The XPD pattern on the Cu(1 1 1)
surface exhibits sixfold rotational symmetry which means that, owing to the underlying hexagonal
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Fig. 5.6. Top and side views of the structure of the enantiomers of heptahelicene. This helical aromatic hydrocarbon is built
up by seven C6 rings with adjacent rings sharing two carbon atoms. Hydrogen atoms are not shown. Reproduced from Fig. 1
[111] by permission of the author.
symmetry, at least six azimuthal orientations of the molecule are possible on this surface. Thus, although
the same local chirality is sustained across the surface, there is no occurrence of organisational chirality.
In contrast, on the steps of the Cu(3 3 2) surface the XPD pattern, given in Fig. 5.7, shows that the
molecules are arranged with one preferred azimuthal orientation. The authors suggest that at these steps
there is only room for one azimuthal orientation of the molecule, leading to a chiral surface with single
phase helical orientation. Thus a chirally organised surface is created under these very particular
conditions where the adsorption sites and mobility of the heptahelicene molecules are both limited.
An ordered (5 1, 1 4) LEED pattern with one domain is only observed for the Sat. ML of M-[7]helicene on Cu(1 1 1), formed by adsorption at 400 K and subsequent cooling to 150 K. If a racemic
mixture of heptahelicene is adsorbed on Cu(1 1 1) ‘‘split-spot’’ LEED structures are observed [114] at
Sat. ML coverage. These are characteristic of domains on the surface which are mirror images of each
other. Thus it appears that when the close-packed monolayer of the racemate is formed, there is
sufficient mobility of the atoms at the surface for them to separate into two different mirror image
domains, as modelled in Fig. 5.8. These domains are relatively large, consisting of approximately 250
atoms. The phase diagram for the heptahelicene/copper system is shown in Fig. 5.9.
5.2.1.2. M-[7]- and P-[7]-helicene on Ni(1 0 0) and Ni(1 1 1). The phase diagram constructed for
P-[7]-helicene adsorbed on Ni(1 0 0) is shown in Fig. 5.10. ToF-SIMS measurements confirm that the
molecule is adsorbed intact and NEXAFS experiments [112] on the Sat. ML prepared at room temperature
show that P-[7]-helicene adsorbs with its helical axis at an angle of 43 5 from the surface. This is larger
than would be expected if the molecule was essentially standing upright, with its first two or three rings
broadly parallel to the surface, when an angle of 158 would be subtended. It is believed that this non-axis
parallel adsorption geometry is favoured due to intermolecular p–p interactions in a close-packed layer.
No ordered superstructures are reported for this system as reliable data was not obtained.
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Fig. 5.7. XPD pattern and molecular orientation for M-heptahelicene adsorbed on Cu(3 3 2). (a) Angle-scanned XPD pattern
for C 1s emission. Handedness of molecule is directly reflected in diffraction pattern. (b) Schematic drawing of the molecular
orientation showing how the molecule-step interaction confines the molecule to the particular azimuthal orientation shown.
Reproduced from Fig. 5a and d [113] by permission of the author.
On Ni(1 1 1) [111], at low coverages, both M-[7]- and P-[7]-helicene molecules spiral away from
the surface in a more upright fashion than that seen for the close-packed monolayer on Ni(1 0 0).
However, it is suggested, but not shown in the published literature, that the adsorption geometry for
the heptahelicene molecules at higher coverage on Ni(1 1 1) will be similar to that of the Ni(1 0 0)
surface, due to the similar close-packed nature of both nickel surfaces. Each enantiomer shows no
lateral order at low coverage but at monolayer coverage and room temperature there is a suggestion
of some longer range ordering in the LEED by the appearance of extra rings around the main nickel
substrate diffraction spots. The STM images also show fuzzy quasi-4 4 hexagonal lattices on the
terraces. However, as the underlying nickel also has a hexagonal symmetry, it is difficult to
determine the extent of any longer-range ordering. Unfortunately, this problem is compounded by the
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Fig. 5.8. Model structure for Sat. ML of racemic heptahelicene on Cu(1 1 1). The enantiomers are separated into two
domains with the domain wall indicated by the dashed line. The unit cells for the substrate and adsorbate lattices are as
indicated with the angle between both unit cells 10.98. Reproduced from Fig. 4 [114] by permission of the author.
fact that the Ni(1 1 1) crystal used in these particular experiments was mis-cut leading to relatively
small terraces. Nevertheless, superstructure models have been constructed as shown in the phase
diagram in Fig. 5.11, where the C6 ring of each P-[7]-helicene molecule on the surface is arbitrarily
set on top of a nickel surface atom. However, each molecule can still be oriented so that the topmost
rings of the spiral terminate in one of six possible positions. Thus the uppermost rings vary in
distance from each other in the STM images, possibly giving rise to the deviations from perfect
hexagonal order that are seen. We note that the surfaces thus created are truly chiral in that the
handedness of the local motif is preserved throughout. However, no organisational chirality emerged
from this system.
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Fig. 5.9.
Phase diagram of M-[7]-helicene adsorbed on Cu(1 1 1) and Cu(3 3 2).
Fig. 5.10. Phase diagram of P-[7]-helicene adsorbed on Ni(1 0 0).
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Fig. 5.11. Phase diagram of heptahelicene adsorbed on Ni(1 1 1).
5.2.2. Thioheterohelicene on Au(1 1 1)
Another molecule with helical chirality that shows alignment effects according to adsorption site is
thioheterohelicene [11TH]. This molecule consists of five benzene rings interspersed with six thiophene
rings and has two enantiomers, each with their helical turns in opposite directions. Molecular resolution
STM images of a racemic mixture of [11TH] adsorbed at room temperature on Au(1 1 1) surfaces
(created by growing gold on mica terraces) have shown that on the flat terraces molecules are randomly
adsorbed with little interaction and no chiral separation or resolution [115]. However, in the stepped
regions of the Au(1 1 1) surface, molecules of the same chirality form tightly packed rows. It is not
possible to determine from the STM data whether or not the rows alternate in their chirality but enough
detail is seen to confirm that each row is homochiral.
6. Closed ring structures with additional functional groups
In this section of the review we focus on a number of larger organic molecules based on ring systems
but with additional functionalities that make their properties at surface interesting. The choice of
molecules is somewhat subjective but we have tried to concentrate on systems for which there are
relatively detailed studies available and for which the information obtained illustrates particular points
with respect to factors such as the chemical bonding and molecular orientation at the surface or the
manner in which ordered surface structures are formed.
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6.1. 1-Nitronaphthalene (1-NN)
Techniques used: NEXAFS, STM, DFT calculations.
Preparation: Deposited at RT through leak valve (gas handling line at 350 K) and cooled very slowly
(over 30 h) to <50 K.
1-NN is an example of a non-chiral molecule that is capable of exhibiting some aspects of chirality,
according to which face adsorbs on the metal surface. It also has the ability to form strong intermolecular
bonds which leads to a particularly rich variety of surface structures. A very detailed picture has been
gained of the way the molecule adsorbs on the reconstructed Au(1 1 1) surface, using STM and
calculations [116–123]. We have taken the various structures reported for different coverage conditions
and constructed the pictorial phase diagram of Fig. 6.1. At room temperature no ordered structures are
Fig. 6.1. Phase diagram of 1-NN adsorbed on Au(1 1 1).
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formed as the molecule is too mobile (i.e. it is weakly bonded). However, if the molecule is adsorbed on a
room temperature surface and cooled down very slowly, clear molecular resolution STM images can be
captured from 5 to 50 K. If adsorption takes place directly at 120 K, a multilayer forms immediately.
The molecule adsorbs with the rings parallel to the surface and, at all coverages, intermolecular
H-bonding drives the formation of supramolecular assemblies. 1-NN can be adsorbed on the metal
surface with either face of the ring uppermost so is an example of a molecule that can display a local
chiral motif on adsorption. The handedness of the motif can be determined from a careful visual
Fig. 6.2. STM images of 1-NN structures on Au(1 1 1) observed at very low coverages. (a) Preferential adsorption at step
edges (V ¼ 2 V; I ¼ 0:16 nA; T ¼ 5 K). (b) Second row starting to form along step edges on lower terraces (arrows), first in
f.c.c. domains (y ¼ 0:03 ML, V ¼ 0:7 V, I ¼ 10 pA, T ¼ 50 K). (c) Trimers nucleating on terraces (y ¼ 0:03 ML,
V ¼ 0:8 V, I ¼ 10 pA, T 60 K). Reprinted from Fig. 2 [120] by permission of Elsevier Science.
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Fig. 6.3. STM images of 1-NN decamers forming on Au(1 1 1). (a) In addition to equal amounts of L and R decamers,
tetramers (diamond) and undecamers (circle) form (y ¼ 0:1 ML, V ¼ 0:33 V, I ¼ 10 pA). (b) The pinwheel-shaped
decamers exist in two enantiomeric forms (y ¼ 0:1 ML, V ¼ 0:2 V, I ¼ 20 pA). Reproduced from Fig. 2 [118] by
permission of the author.
inspection of the STM data. What is interesting about this system is that at low coverages the overall
racemic system separates into a coexisting mixture of homochiral domains, while at high coverage,
heterochiral, racemic chains are formed.
Fig. 6.4. STM images of 1-NN double chain formation on Au(1 1 1). (a) Double chains forming in f.c.c. domains.
Remaining decamer indicated by arrow (y ¼ 0:3 ML, V ¼ 0:4 V, I ¼ 0:1 nA, T ¼ 50 K). (b) Double chains forming in
h.c.p. domain as well. Chains in f.c.c. domains starting to form zig zag structure (y ¼ 0:4 ML, V ¼ 1:5 V, I ¼ 10 pA,
T ¼ 50 K). Reprinted from Fig. 4 [120] by permission of Elsevier Science.
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Fig. 6.5. STM images of 1-NN on Au(1 1 1) at saturation coverage. (a) Most domain walls remain free of molecules
(V ¼ 1:7 V, I ¼ 0:1 nA, T ¼ 50 K). (b) Below straight substrate steps in ½1 1 0 direction double chains run parallel to steps, i.e.
perpendicular to uniaxial reconstruction domains. At larger distances from step, double chains guided by herringbone
reconstruction (V ¼ 1:6 V, I ¼ 0:1 nA, T ¼ 50 K). (c) Small fraction of surface covered with molecules in 2D periodic
structure (y ¼ 1 ML, V ¼ 54 mV, I ¼ 10 pA, T ¼ 50 K). Reprinted from Fig. 5 [120] with permission from Elsevier Science.
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The reconstructed gold surface has alternating f.c.c. and h.c.p. domains in a herringbone structure,
giving rise to inequivalent adsorption sites at the elbows. Initially, at very low coverages, small chains
of molecules form at the f.c.c. steps, driven by the step electrostatic potential and, as the coverage
increases, molecular clusters start to form on the open terraces (see Fig. 6.2). Eventually clusters are
formed with 85% present as decamers which self-assemble to include only molecules with the same
local chirality, as shown in Fig. 6.3. These decamers (or ‘‘magic clusters’’ as they are also known) show
chiral organisation and can be individually manipulated by the STM tip to give domains with the same
chirality, indicating that the decamers behave as supermolecules. As the coverage of 1-NN increases,
Fig. 6.6. Models showing intermolecular interactions for various saturation coverage structures (detailed in Fig. 6.5) of 1-NN
on Au(1 1 1). Arrows indicate H-bonds between neighbouring molecules. STM image and model of: (a) and (b) double chains,
(c) and (d) 2D periodic structure, (e) and (f) dense structure within f.c.c. domains. Reprinted from Fig. 8 [120] with permission
of Elsevier Science.
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double chains of molecules form, again made up of molecules with the same local chirality, forming
first in the f.c.c. and then in the h.c.p. domains. These double chains are shown in Fig. 6.4. With higher
coverage, zig zag rows form in the f.c.c. domains and the chains become racemic mixtures of
molecules. By monolayer coverage 2D periodic structures are densely packed in the f.c.c. domains,
coexisting with 1D chains in the h.c.p. domains, plus some areas that are completely covered with 2D
structures, as illustrated in Fig. 6.5. More detailed models of the saturated coverage structures are
shown in Fig. 6.6, indicating the way intermolecular bonding drives the formation of the superstructures
and can influence the creation of structures that exhibit local chirality.
6.2. Benzotriazole and related molecules on Cu(1 0 0)
Techniques used: NEXAFS.
Preparation: Vacuum sublimation.
These molecules are included in the review as they demonstrate how slight changes in molecular
functionalities can completely alter molecular adsorption at a surface and subsequent chemical
behaviour. Figs. 6.7–6.9 summarise the information available for the three molecules taken from a
NEXAFS study [124].
Benzotriazole is used as a corrosion inhibitor and has three nitrogen atoms contained in the ring
as shown. The molecule is found to adsorb intact, except for the loss of the imino hydrogen, and in
the monolayer region has an approximately upright orientation with respect to the Cu(1 0 0)
surface. The actual bonding site is not clear but it is believed to be either through two adjacent
N atoms or through all three N atoms, with the ‘‘central’’ N bonding to a copper atom in the second
underlying layer.
If the imino H is replaced by a methyl, CH3 group, to give 1-methyl benzotriazole, no preferred
orientation is found in the monolayer and the molecules are randomly oriented with respect to the
surface. Likewise, if the central N is replaced by a CH group to give benzimidazole, the monolayer
adsorbs with no preferred orientation. Neither 1-methyl benzotriazole nor benzimidazole are useful as
corrosion inhibitors and it suspected that this is due to the lack of the very specific binding observed in
the first layer for benzotriazole.
Fig. 6.7.
Fig. 6.8.
Phase diagram of benzotriazole adsorbed on Cu(1 0 0).
Phase diagram of 1-methyl benzotriazole adsorbed on Cu(1 0 0).
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Fig. 6.9. Phase diagram of benzimidazole adsorbed on Cu(1 0 0).
6.3. 2,5-Dimethyl-dicyanoquinonediimine (DMe-DCNQI)
Techniques used: NEXAFS, LEED, TDS, XPS, STM.
Preparation: Vacuum sublimation.
The behaviour of this molecule at a metal surface has attracted interest due to its ability to form
radical ion salts with unusual electronic and structural properties. Thus adsorption studies [125,126]
have been undertaken on Ag(1 1 0) and (1 1 1) surfaces, summarised in the phase diagrams of Figs. 6.10
and 6.11. We note that this molecule should also create local chiral motifs upon adsorption, the
handedness of which are determined by which face is closest to the surface. However, this aspect has
not been investigated for this system.
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Fig. 6.10. Phase diagram of DMe-DCNQI adsorbed on Ag(1 1 1).
Fig. 6.11. Phase diagram of DMe-DCNQI adsorbed on Ag(1 1 0).
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On Ag(1 1 1) the molecule adsorbs intact with the plane of the ring and the cyano (CBN) parallel to
the surface and forms an ordered commensurate structure which extends into the second layer. On the
more corrugated Ag(1 1 0) surface a range of structures are formed with increasing compression it
adsorbs. Initially, the molecule compresses along the rough (0 0 1) direction, i.e. across the rows but as
the coverage increases the molecule rotates and compresses along the smooth ½1 1 0 direction, i.e. along
the rows. All the transitions occur along the long axis of the DMe-DCNQI molecule, i.e. the axis
terminated by the CBN groups.
6.4. Hexabutyloxytriphenylene (HBT)
Techniques used: XAS, LEED, TDS, XPS, STM.
Preparation: Vacuum sublimation.
Fig. 6.12.
Phase diagram of HBT adsorbed on Au(1 1 1).
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Fig. 6.13. Phase diagram of HBT adsorbed on Cu(1 1 1).
HBT is a molecule that has the ability to form liquid crystalline phases and therefore its orientation and
growth mode in ultrathin films is of interest. XAS and XPS measurements have been performed on HBT
adsorbed on Au(1 1 1) [127] and show that the molecule adsorbs in the monolayer regime with its phenyl
plane almost parallel to the surface. The phase diagram of Fig. 6.12 shows that as the coverage increases
and a multilayer is formed there is a more tilted orientation with respect to the surface. On heating this
multilayer the molecules start to orient more normal to the surface, i.e. become ‘‘edge-on’’. This means
that, perhaps unexpectedly, there is no epitaxial growth of the outer layers on top of a well-ordered
monolayer. Further studies on Cu(1 1 1) [128] show that, on this surface, at substrate temperatures up to
400 K, the molecule again adsorbs intact and with the phenyl ring parallel to the surface. However, as
illustrated in the phase diagram of this system in Fig. 6.13, on warming to 600 K the alkyl (C4H9) chains
are removed and the hexaoxytriphenylene formed reacts with the surface and bonds strongly to the copper
via the O atoms. This new species then forms a stable ordered superstructure which is stable at 600 K.
7. Summary
In this review we have concentrated on the controlled adsorption of organic molecules on defined
metal surfaces, grown under UHV conditions, and sought to address the following questions for each
adsorption system:
How does the organic molecule bond to the metal surface?
What is the molecular orientation at the surface?
Do the molecules self-organise at the surface to form ordered structures?
Table 2
Summary of carboxylic acid adsorption systems
Cu(1 1 0)
p(2 1)O/Cu(1 1 0)
Range of ordered phases
dependent on coverage and
temperature. Monotartrate,
bitartrate and dimer species.
Adsorption of one enantiomer can
create surface with global point
and organisational chirality.
Chirality can be switched if
adsorb opposite enantiomer
Succinic acid
Range of ordered phases
dependent on coverage and
temperature. Non-chiral molecule
but can create chirally organised
domains. Mirror images of
domains also present so surface
overall racemic
Benzoic acid
Large range of ordered phases,
flat-lying and upright molecules,
dimer formations
Room temperature: from
flat-lying to upright
through two different
ordered phases as
coverage increases
3-Thiophene
carboxylic
acid
Room temperature: from flat-lying
to upright through two different
ordered phases as coverage
increases
Room temperature: from
flat-lying to upright
through two different
ordered phases as
coverage increases
PVBA
Ni(1 1 0)
Pd(1 1 0)
Ag(1 1 1)
Room temperature:
flat-lying molecules
strongly interacting
with substrate.
Preferred adsorption
geometry. Chiral
adsorption motif
but opposite motif
equally likely
Low temperature: flat-lying
aggregates in curved
strings. Room temperature: highly
ordered 1D supramolecular
superstructure. H-bonded twin
chains form with local chiral
motifs and organisational
chirality. Overall surface
racemic
Adsorbed as bi-acid, mono- or
bitartrate species dependent on
coverage and temperature.
No ordered structures.
Preferred growth directions.
Chiral footprint created due to
reconstruction of underlying
metal atoms
Room temperature:
upright at all
coverages, no
ordered structures
Low temperature:
flat-lying islands,
isolated upright
molecules. On
warming only flat
lying, no long-range
order
Room temperature: changes from
flat-lying to tilted upright with
increasing coverage. Ordered
saturated surface
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Tartaric acid
Cu(1 1 1)
p-Aminobenzoic Room temperature: low coverage:
acid
flat-lying anionic form, high
coverage: some upright molecules
as well. Ordered structure changes
on warming with dehydrogenation
and dimer formation occurring
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Does the molecular adsorption system show aspects of local or global chirality?
How do all the above adsorption characteristics vary with coverage or/and temperature?
We have captured the information in adsorption phase diagrams for each system and discussed the
main features and experimental evidence in the text. In this summary we provide a very brief overview
of the adsorption features of each of the main classes of molecules reviewed and present comparative
tables of all the systems to aid the reader further.
(i) Carboxylic acids (Table 2)
These generally prefer to bond through the ionised carboxylate group, held with its plane
broadly perpendicular to the surface unless functionalities are present which may override the
Table 3
Summary of amino acid adsorption systems
Cu(1 1 0)
Cu(0 0 1)
Pt(1 1 1)
Glycine
Room temperature: range of phases with
increasing coverage. Anionic bonding. Sat. ML
ordered. Forms multilayers at low temperatures
Room
temperature:
ordered structure
Zwitterionic bonding. Adsorbs
intact below 250 K, molecules
dissociate on desorption
Alanine
Room temperature: range of ordered phases, anionic
bonding. Ordered surface created with S-form
mirror image of R-form. Can create surface
with global point and organisational chirality
Proline
Room temperature: only one ordered phase, anionic
bonding, ring tilted at small angle to surface plane
Norvaline
Room temperature: anionic form. Low and high
coverage phases differing in orientation. Ordered
high coverage phase similar to alanine
Methionine
Room temperature: anionic form. Low and high
coverage phases. No ordered structures. Some,
but not all, molecules bound through S atom
Serine
Room temperature: anionic form. Low and high
coverage phases. Annealed high coverage phase shows
ordered LEED with molecules in a chiral arrangement
Phenylglycine
Room temperature: both enantiomers adsorb in anionic
form with phenyl ring away from surface normal.
Ordered surface with LEED of R-form mirror image
of S-form. Can create surface with global point
and organisational chirality
Tri-L-alanine
Room temperature: intact anionic form, bonding
through terminal groups and amide C=O. Coverage
and flux dependent phases. No long-range ordered
structures but localised H-bonding to give bilayers
with antiparallel b-sheets.
Tri-L-leucine
Similar to tri-L-alanine but longer side chains
sterically prevent bilayer formation
Table 4
Summary of anhydrides
Phthalic anhydride
Adsorbed molecule
p(2 1)O/Cu(1 1 0)
As coverage increases goes
from flat-lying intact molecule
to phthalate species
Room temperature:
dicarboxylate species bonded
through four O atoms
Ag(1 0 0)
Cu(1 1 1)
Pt(1 1 1)
Room temperature: bidentate
carboxylate species bonded
through two O atoms
Ag(1 1 0)
Room temperature: intact
flat-lying molecules. Low
temperature: forms multilayers
Cu(1 0 0)
NDCA
NTCDA
PTCDA
Low coverage: flat-lying,
ordered. Higher coverage:
coexistence of flat and more
upright weaker bonded
molecules. No ordering. On O
covered surface flat and
upright at low coverage,
upright at higher coverage
Molecular plane parallel to surface.
C=O of anhydride involved in bonding
Grows from relaxed ML (brick wall)
to compressed ML (herringbone)
Molecular plane parallel to surface.
Intact molecules
Large mobile islands nucleate from
step edges giving herringbone structure
Grows from relaxed ML to
compressed, incommensurate
ML with different structures
from Ag(1 1 1) surface
Single ML structure:
open rows
Brick wall structures
nucleate from terraces
Molecules parallel to surface.
All O atoms involved
One ordered ML structure
Molecules mostly parallel to
surface. All O atoms involved
No ordered structure.
Molecules ‘‘hit and stick’’
parallel orientation but at least
one O atom removed
One ordered structure
on annealing
Broadly parallel, some
tilting and bending
Broadly parallel. No ordering
PTCDI
Me-PTCDI
DPP-PTCDI
Ni(1 1 1)
Intact parallel to surface.
No ordered structure
Flat lying, intact. Incommensurate,
herringbone structure. Nucleation at steps
Flat-lying, intact.
Commensurate, brick wall
structure on terraces
Dissociative adsorption.
Flat lying, ordered
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PMDA
Cu(1 1 0)
Large range of coverage and temperature
dependent phases and species. As coverage
and temperature increases, goes from
flat-lying intact species to
upright carboxylate species
Large range of coverage and temperature
dependent species. Molecule not intact
at any temperature
Ag(1 1 1)
Deposition at room
temperature: two flat-lying
structures. If ML created by
desorption of multilayer
perylene core more upright
and structure different
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dominance of this interaction, e.g. ring systems. A wide variety of coverage and temperature
dependent phases are generally created, with differently oriented species and organised structures.
Some non-chiral carboxylic acids can show local chiral adsorption motifs and chiral organisation.
Chiral carboxylic acids can create chiral surfaces showing global point and organisational chirality
under certain conditions.
(ii) Amino acids (Table 3)
Amino acids bond in their anionic form, through the ionised carboxylate group and the amino
group. Other functionalities present in the side chains may also be involved in the bonding. For all
amino acids, except the rigid proline, the carboxylate group can show two orientations, one where
the oxygen atoms are equidistant from the surface and the other where one oxygen atom is held
further from the surface. The former orientation is thermodynamically preferred as reorientation of
the latter arrangement occurs on warming. On Cu(1 1 0) the (3 2) surface structure is ubiquitous
for the high coverage, annealed phase except when the inherent footprint is different (e.g. rigid
proline) or when self-assembly is disrupted or affected by side chains (e.g. methionine, serine).
Table 5
Summary of closed ring structure without additional functional groups
Cu(1 0 0)
Perylene
Room temperature:
flat-lying ordered
structure
Coronene
Room temperature:
low coverage disordered.
Higher coverage:
flat-lying ordered
structure
Pentacene (and
derivatives)
Room temperature:
flat-lying ordered
structures, different
for each molecule
Heptahelicene
Thioheterohelicene
Room temperature:
adsorbs on flat terraces
with little interaction or
chiral discrimination.
On steps molecules of
same chirality grow
Cu(1 1 1)/(3 3 2)
Ni(1 0 0)
Ni(1 1 1)
Room temperature
deposition: intact
molecules spiral away
from surface and form
close-packed ordered ML.
Six azimuthal orientations
on Cu(1 1 1) but
single-phase helical
orientation on Cu(3 3 2)
Room temperature:
intact adsorption,
helical axis tilted
at angle to surface
Room temperature:
intact molecules
spiral away from
surface in upright
fashion. Some
ordering on terraces
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Table 6
Summary of closed ring structure with additional functional groups
Au(1 1 1)
1-NN
Cu(1 0 0)
Cu(1 1 1)
Benzotriazoleþ
related molecules
Ag(1 1 1)
Room temperature:
adsorbs flat with
range of ordered
phases varying as
compression
increases with
coverage
Deposited on
liquid nitrogen
cooled
substrate:
adsorbs flat
with ordered
commensurate
structure
Room temperature:
benzotriazole adsorbs
upright with loss of imino
H. Other similar
molecules have no
preferred orientation
w.r.t. surface
Dme-DCNQI
HBT
Ag(1 1 0)
Deposited at room
temperature, cooled
to 5–50 K—forms
large range of
ordered structures.
2D locally chiral
adsorption, some
structures show
organisational
chirality
Adsorbs intact
with rings parallel
to surface
Room temperature:
adsorbs intact but
weakly bound. On
heating molecule
dissociates and
desorbs and then
bonds strongly through
terminal O atoms
Under very particular adsorption conditions, surfaces with global point and global organisational
chirality can be created.
(iii) Anhydrides (Table 4)
Smaller anhydrides can be dissociated and bond through the carboxylate species, resulting in a
variety of coverage and temperature dependent phases. Anhydrides with larger ring systems adsorb
intact with the plane of the rings parallel to the surface and substrate dependent ordered
superstructures.
(iv) Closed ring structures without additional functionalities (Table 5)
Planar molecules adsorb with rings parallel to the surface and a range of substrate dependent
ordered superstructures. Chiral helical molecules spiral away from surface and can form ordered
superstructures which may be chirally organised under specific adsorption conditions.
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(v) Closed ring structures with additional functionalities (Table 6)
These are generally adsorbed with the plane of their rings parallel to surface unless the reactivity
of any side group is stronger. Molecules may adsorb with a local chiral motif and organised
superstructures are often formed. In some cases, under particular adsorption conditions, these are
chirally organised.
The application of rigorous surface science tools to study organic molecules at surfaces is rapidly
increasing and even as we conclude this review, new and interesting papers are appearing in the literature.
What is clear from our review is that the organic/metal systems possess a great diversity and manifest
many different phases upon adsorption. Thus, the robust organic surface is endowed with much of the
flexibility of the organic system, leading to many interfaces which all possess different functionalities.
What has also become clear from reviewing the literature is that the multifaceted nature of these systems
can only be captured by a multi-technique approach in which both the local and the global nature of the
interface is probed, the former providing information on the adsorbed entity, its orientation and bonding,
and the latter providing views on the beautiful self-organisation capacity of these units. At present only a
very few studies have attempted this and we believe that this approach must be more widely adopted so
that we may lay down the generic principles that govern the controlled creation of these interfaces,
tailored to the needs of areas such as nanotechnology, catalysis, optoelectronics and biotechnology.
Acknowledgements
We would like to specifically thank one of the referees of this report, Dr. Brian Frederick, for his
extremely thorough reading of the draft.
Appendix A. Matrix notation for overlayer unit cells
In this review all the metals studied have a f.c.c. crystal structure. Therefore, following the
International Tables for Crystallography Conventions [5] described for surfaces by Unertl [6], the real
space unit cell and important directions for each of the most common surfaces studied (1 0 0), (1 1 0)
and (1 1 1) of the metal can be defined by the vectors shown in Figs. A.1–A.3.
Important points to note are:
(i) The co-ordinate system is right-handed, i.e. the defining axis system is such that x goes to y, y goes
to z and z goes to x in an anticlockwise direction. Likewise, the substrate vectors as and bs are such
that as goes to bs in an anticlockwise direction.
(ii) as points downwards and bs points to the right.
(iii) jas j jbs j.
(iv) The angle between the substrate vectors g 90 .
Note we have chosen to name the metal substrate vectors as and bs to correspond to the crystallographic
a and b vectors of the surface (which are parallel to the x- and y-axis for a cubic crystal) with the c-axis
parallel to the z-axis (which is above and normal to the surface). Some authors prefer to name these
vectors a1 and a2.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Fig. A.1.
f.c.c. (1 0 0) surface. Atoms are at intersections of lines.
The overlayer vectors ao and bo are then chosen such that:
(i) The co-ordinate system is right-handed, i.e. ao goes to bo in an anticlockwise direction.
(ii) ao points downwards and where possible bo points to the right.
(iii) The angle between the overlayer vectors g 90 .
Fig. A.2.
f.c.c. (1 1 0) surface.
333
334
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Fig. A.3.
f.c.c. (1 1 1) surface.
Note that the overlayer vectors are named ao and bo here whilst others prefer to use b1 and b2. The
relationship between the overlayer and substrate vectors is given by:
m11 m12
as
ao
¼
bo
m21 m22
bs
where the matrix M ¼ ðm11 m12 ; m21 m22 Þ describes the overlayer arrangement and, for a right-handed
system, the determinant jMj 0.
Common problems encountered in the literature with describing overlayers include the use of lefthanded co-ordinate axis systems or overlayer vectors where the included angle is 908. The substrate
vectors for a (1 1 1) surface are sometimes shown with an included angle of 908 which leads to a nonprimitive smaller unit cell and one of the substrate directions quite different to the correct one. There also
appears to be widespread confusion over the use of crystallographic directions, especially for the (1 1 0)
surface, where the ½1
10 direction is frequently shown using a left-handed co-ordinate system with respect
to the [0 0 1] direction. However, the real space unit cells deduced for the overlayers in the articles
reviewed are generally believed to be correct and many of the matrices quoted in the texts are correct with
respect to the substrate vectors actually shown, even if these are not defined in the above manner.
Interestingly, the overlayer vectors for ‘‘mirror image’’ unit cells associated with the adsorption of
molecules with opposite chirality on the (1 1 0) surface often create particular problems. The need to keep
the angle between the overlayer vectors greater than or equal to 908 and maintain a right-handed axis
system forces the vectors not to be mirror images of each other even if this is the case for the unit cells.
In this appendix we have attempted to use the real space data presented in the articles that we have
reviewed to compare the matrix notations used by the authors for the commensurate overlayer structures to
this more consistent approach. As can be seen there are many variations for all the reasons outlined above.
We would urge authors to follow this more rigorous crystallographic approach in future as it then becomes
easier to compare overlayer structures resulting from the various different phases created on adsorption.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Adsorbate
Metal surface
and preferred
substrate vectors
Overlayer unit cell
Overlayer matrix Overlayer matrix
with preferred
description
description with
vectors (not to scale) used in text
preferred vectors
R,R-tartaric acid Cu(1 1 0)
S,S-tartaric acid
Succinic acid
Cu(1 1 0)
Benzoic acid
335
Cu(1 1 0)
9
1
0
2
4
2
0
3
4
2
1
3
9
1
0
2
4
2
1
3
9
1
0
1
9
1
0
1
4
1
3
5
4
1
3
9
1
9
2
0
2
4
3
0
2
4
3
1
9
1
0
2
4
2
1
3
1
9
1
0
9
1
0
1
4
1
3
5
4
1
3
9
336
Adsorbate
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Metal surface
and preferred
substrate vectors
Overlayer unit cell
Overlayer matrix Overlayer matrix
with preferred
description
description with
vectors (not to scale) used in text
preferred vectors
Benzoic acid
Ni(1 1 0)
S-alanine
Cu(1 1 0)
R-alanine
S-serine
Cu(1 1 0)
S-phenylglycine
Cu(1 1 0)
R-phenylglycine
NTCDA
Ag(1 1 1)
Ag(1 0 0)
1 1
1 2
2 2
5 3
5 3
2 2
1
4
2
0
5 3
4 1
5 3
4 1
4 0
3 6
6 1
1 7
3 3
4 4
1
2
1
1
2
5
2
3
5
2
3
2
1
4
2
0
5
4
3
1
4
5
1
3
4
3
0
6
6
1
1
7
3
4
3
4
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Adsorbate
Metal surface
and preferred
substrate vectors
Overlayer unit cell
Overlayer matrix Overlayer matrix
with preferred
description
description with
vectors (not to scale) used in text
preferred vectors
Ag(1 0 0)
Cu(1 0 0)
PTCDA
Ag(1 1 1)
Ag(1 1 0)
DM-PBCDI
Ag(1 1 0)
DPP-PTCDI
Ag(1 1 0)
Heptahelicene
Ag(1 1 1)
337
6
0
1:5
3
3
1
0
3
2
2
3
3
6
3
1
5
2
2
3
3
2
2
4
3
4
3
2
2
5
3
2
6
5
1
1
4
or
6
0
3
1
1:5
3
0
3
2
2
3
3
3
6
2
7
3
3
2
2
3
4
2
2
5
3
2
6
5
1
1
4
338
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
Adsorbate
Metal surface
and preferred
substrate vectors
Overlayer unit cell
Overlayer matrix Overlayer matrix
with preferred
description
description with
vectors (not to scale) used in text
preferred vectors
DMe-DCNQI
Ag(1 1 1)
Ag(1 1 0)
4 2
6 4
3
2
3 0
1 2
1
2
6
4
10
2
3
2
1
2
1
3
2
0
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
F.P. Netzer, M.G. Ramsey, Crit. Rev. Solid State Mater. Sci. 17 (1992) 397.
N. Sheppard, C. de la Cruz, Adv. Catal. 42 (1998) 181.
N. Sheppard, C. de la Cruz, Adv. Catal. 41 (1996) 1.
N. Sheppard, Ann. Rev. Phys. Chem. 39 (1988) 589.
J.S. Kasper, K. Lonsdale (Eds.), International Tables for Crystallography, Kynoch Press, Birmingham, 1959.
W.N. Unertl, Physical structure, in: S. Holloway, N.V. Richardson (Eds.), Surface Crystallography, Handbook of
Surface Science, vol. 1, Elsevier, Amsterdam, 1996, Chapter 1.
R.S. Cahn, C.K. Ingold, V. Prelog, Angew. Chem. 78 (1966) 413.
R.S. Cahn, C.K. Ingold, V. Prelog, Angew. Chem. Int. Edit. 5 (1966) 385.
R. Raval, Current opinion in solid state and materials science, 2003, in press.
M. Ortega Lorenzo, C.J. Baddeley, C. Muryn, R. Raval, Nature 404 (2000) 376.
L. Pasteur, C.R. Hebd, Seances Acad. Sci. 26 (1848) 535.
A. Baiker, H.U. Blaser, Handbook of Heterogeneous Catalysis, vol. 5, VCH, New York, 1997.
Y. Izumi, Adv. Catal. 32 (1983) 215.
A. Tai, T. Harada, Tailored Metal Catalysts, Reidel, Dordrecht, 1986.
T.E. Jones, C.J. Baddeley, Surf. Sci. 519 (2002) 237.
T.E. Jones, C.J. Baddeley, Surf. Sci. 513 (2002) 453.
R. Raval, C.J. Baddeley, S. Haq, S. Louafi, P. Murray, C. Muryn, M.O. Lorenzo, J. Williams, Stud. Surf. Sci. Catal. 122
(1999) 11.
M. Ortega Lorenzo, S. Haq, P. Murray, R. Raval, C.J. Baddeley, J. Phys. Chem. B 103 (1999) 10661.
M. Ortega Lorenzo, V. Humblot, P. Murray, C.J. Baddeley, S. Haq, R. Raval, J. Catal. 205 (2002) 123.
R. Raval, CATTECH 5 (2001) 12.
R. Raval, J. Phys. 14 (2002) 4119.
L.A.M.M. Barbosa, P. Sautet, J. Am. Chem. Soc. 123 (2001) 6639.
J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995.
V. Humblot, S. Haq, C. Muryn, W.A. Hofer, R. Raval, J. Am. Chem. Soc. 124 (2002) 503.
W.A. Hofer, V. Humblot, R. Raval, in preparation.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
339
[26] V. Humblot, S. Haq, R. Raval, in preparation.
[27] O. Karis, J. Hasselstrom, N. Wassdahl, M. Weinelt, A. Nilsson, M. Nyberg, L.G.M. Pettersson, J. Stohr, M.G. Samant,
J. Chem. Phys. 112 (2000) 8146.
[28] G.A. Attard, J. Phys. Chem. B 105 (2001) 3158.
[29] D.S. Sholl, A. Asthagiri, T.D. Power, J. Phys. Chem. B 105 (2001) 4771.
[30] C.F. McFadden, P.S. Cremer, A.J. Gellman, Langmuir 12 (1996) 2483.
[31] G.P. Lopinski, D.J. Moffat, D.D. Wayner, R.A. Wolkow, Nature 392 (1998) 909.
[32] R. Viswanathn, J.A. Zasadzinski, D.K. Schwartz, Nature 368 (1994) 440.
[33] M. Schunack, E. Laegsgaard, I. Stensgaard, I. Johannsen, F. Besenbacher, Angew. Chem. Int. Edit. 40 (2001) 2623.
[34] J. Halpern, Science 217 (1982) 401.
[35] M. Ortega Lorenzo, V. Humblot, C.J. Baddeley, S. Haq, R. Raval, in preparation.
[36] B.G. Frederick, Q. Chen, F.M. Leibsle, M.B. Lee, K.J. Kitching, N.V. Richardson, Surf. Sci. 394 (1997) 1.
[37] B.G. Frederick, Q. Chen, F.M. Leibsle, S.S. Dhesi, N.V. Richardson, Surf. Sci. 394 (1997) 26.
[38] Q. Chen, C.C. Perry, B.G. Frederick, P.W. Murray, S. Haq, N.V. Richardson, Surf. Sci. 446 (2000) 63.
[39] B.G. Frederick, F.M. Leibsle, S. Haq, N.V. Richardson, Surf. Rev. Lett. 3 (1996) 1523.
[40] C.C. Perry, S. Haq, B.G. Frederick, N.V. Richardson, Surf. Sci. 409 (1998) 512.
[41] M. Neuber, M. Zharnikov, J. Walz, M. Grunze, Surf. Rev. Lett. 6 (1999) 53.
[42] B.G. Frederick, N.V. Richardson, J. Electr. Spectrosc. Rel. Phenom. 73 (1995) 149.
[43] B.G. Frederick, Q. Chen, S.M. Barlow, N.G. Condon, F.M. Leibsle, N.V. Richardson, Surf. Sci. 352–354 (1996) 238.
[44] B.G. Frederick, R.J. Cole, J.R. Power, C.C. Perry, Q. Chen, N.V. Richardson, P. Weightman, C. Verdozzi, D.R.
Jennison, P.A. Schultz, M.P. Sears, Phys. Rev. B 58 (1998) 10883.
[45] J. Weckesser, J.V. Barth, L. Burgi, O. Jewandupeux, B. Muller, P. Gunter, K. Kern, Poster Presented at Fundamental
Aspects of Surface Science: Structure and Dynamics of Organic and Biological Molecules at Interfaces, Castelvecchio
Pascoli, Italy, September 3–8, 1999.
[46] J. Weckesser, J.V. Barth, C. Cai, B. Muller, K. Kern, Surf. Sci. 431 (1999) 168.
[47] J. Weckesser, J.V. Barth, K. Kern, J. Chem. Phys. 110 (1999) 5351.
[48] B. Muller, C.Z. Cai, M. Bosch, M. Jager, C. Bosshard, P. Gunter, J.V. Barth, J. Weckesser, K. Kern, Thin Solid Films
344 (1999) 171.
[49] J.V. Barth, J. Weckesser, C.Z. Cai, P. Gunter, L. Burgi, O. Jeandupeux, K. Kern, Angew. Chem. Int. Edit. 39 (2000)
1230.
[50] Q. Chen, D.J. Frankel, N.V. Richardson, Langmuir 17 (2001) 8276.
[51] Q. Chen, D.J. Frankel, N.V. Richardson, J. Chem. Phys. 116 (2002) 460.
[52] Q. Chen, D.J. Frankel, N.V. Richardson, Surf. Interf. Anal. 32 (2001) 43.
[53] L.L. Atanasoska, J.C. Buchholz, G.A. Somorjai, Surf. Sci. 72 (1978) 189.
[54] S.M. Barlow, K.J. Kitching, S. Haq, N.V. Richardson, Surf. Sci. 401 (1998) 322.
[55] J. Hasselstrom, O. Karis, M. Weinelt, N. Wassdahl, A. Nilsson, M. Nyberg, L.G.M. Pettersson, M.G. Samant, J. Stohr,
Surf. Sci. 407 (1998) 221.
[56] M. Nyberg, J. Hasselstrom, O. Karis, N. Wassdahl, M. Weinelt, A. Nilsson, L.G.M. Pettersson, J. Chem. Phys. 112
(2000) 5420.
[57] N.A. Booth, D.P. Woodruff, O. Schaff, T. Giessel, R. Lindsay, P. Baumgartel, A.M. Bradshaw, Surf. Sci. 397 (1998)
258.
[58] Q. Chen, D. Frankel, N. Richardson, Surf. Sci. 497 (2002) 37.
[59] R.L. Toomes, J.H. Kang, D.P. Woodruff, M. Polcik, M. Kittel, J.T. Hoeft, Surf. Sci. 522 (2003) L9.
[60] X.Y. Zhao, Z. Gai, R.G. Zhao, W.S. Yang, T. Sakurai, Surf. Sci. 424 (1999) L347.
[61] X.Y. Zhao, H. Wang, R.G. Zhao, W.S. Yang, Mater. Sci. Eng. C 16 (2001) 41.
[62] P. Lofgren, A. Krozer, J. Lausmaa, B. Kasemo, Surf. Sci. 370 (1997) 277.
[63] K.H. Ernst, K. Christmann, Surf. Sci. 224 (1989) 277.
[64] D. Le Roux, S.M. Barlow, S. Louafi, J. Williams, C. Muryn, S. Haq, R. Raval, in preparation.
[65] J. Williams, S. Haq, R. Raval, Surf. Sci. 368 (1996) 303.
[66] S.M. Barlow, S. Louafi, D. Le Roux, C. Muryn, S. Haq, R. Raval, in preparation.
[67] X.Y. Zhao, R.G. Zhao, W.S. Yang, Surf. Sci. 442 (1999) L995.
340
[68]
[69]
[70]
[71]
[72]
[73]
[74]
[75]
[76]
[77]
[78]
[79]
[80]
[81]
[82]
[83]
[84]
[85]
[86]
[87]
[88]
[89]
[90]
[91]
[92]
[93]
[94]
[95]
[96]
[97]
[98]
[99]
[100]
[101]
[102]
[103]
[104]
[105]
[106]
[107]
[108]
[109]
[110]
[111]
[112]
[113]
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
G.C. Percy, H. Stretton, J. Chem. Soc., Dalton Trans. (1976) 2429.
D.T. Clark, J. Peeling, L. Colling, Biochim. Biophys. Acta 453 (1976) 533.
E. Mateo Marti, S.M. Barlow, S. Haq, R. Raval, Surf. Sci. 501 (2002) 191.
I.D. Reva, S.G. Stepanian, A.M. Plokhotnichenko, E.D. Radchenko, G.G. Sheina, Y.P. Blagoi, J. Mol. Struct. 318
(1994) 1.
J. Williams, Adsorption and characterisation of chiral amino acids on Cu(1 1 0) single crystal surfaces, Ph.D. Thesis,
University of Liverpool, 1998.
T.J. Kristenmacher, G.A. Rand, R.E. Marsh, Acta Crystallogr. B 30 (1974) 2573.
Q. Chen, C.W. Lee, D.J. Frankel, N.V. Richardson, Phys. Chem. Commun. 9 (1999).
S.M. Barlow, S. Haq, R. Raval, Langmuir 17 (2001) 3292.
S. Haq, R.C. Bainbridge, B.G. Frederick, N.V. Richardson, J. Phys. Chem. B 102 (1998) 8807.
N.V. Richardson, B.G. Frederick, W.N. Unertl, A. El Farrash, Surf. Sci. 309 (1994) 124.
S. Haq, N.V. Richardson, J. Phys. Chem. B 103 (1999) 5256.
A. Schertel, C. Woll, M. Grunze, J. Phys. IV 7 (1997) 537.
C. Hahn, T. Strunskus, M. Grunze, J. Phys. Chem. 98 (1994) 3851.
C. Mainka, P.S. Bagus, A. Schertel, T. Strunskus, M. Grunze, C. Woll, Surf. Sci. 341 (1995) L1055.
C. Mainka, H. Wegner, A. Schertel, C. Woll, M. Grunze, Z. Phys. Chem. 198 (1997) 221.
W. Erley, D. Sander, J. Vacuum Sci. Technol. A 7 (1989) 2238.
J. Taborski, V. Wustenhagen, P. Vaterlein, E. Umbach, Chem. Phys. Lett. 239 (1995) 380.
J. Taborski, P. Vaterlein, H. Dietz, U. Zimmerman, E. Umbach, J. Electr. Spectrosc. Rel. Phenom. 75 (1995) 129.
E. Umbach, C. Seidel, J. Taborski, R. Li, A. Soukopp, Phys. Status Solidi B 192 (1995) 389.
U. Baston, M. Jung, E. Umbach, J. Electr. Spectrosc. Rel. Phenom. 77 (1996) 75.
D. Gador, C. Buchberger, R. Fink, E. Umbach, Europhys. Lett. 41 (1998) 231.
D. Gador, C. Buchberger, R. Fink, E. Umbach, J. Electr. Spectrosc. Rel. Phenom. 96 (1998) 11.
U. Stahl, D. Gador, A. Soukopp, R. Fink, E. Umbach, Surf. Sci. 414 (1998) 423.
D. Gador, Y. Zou, C. Buchberger, M. Bertram, R. Fink, E. Umbach, J. Electr. Spectrosc. Rel. Phenom. 103 (1999) 523.
R. Fink, D. Gador, U. Stahl, Y. Zou, E. Umbach, Phys. Rev. B 60 (1999) 2818.
T.J. Schuerlein, A. Schmidt, P.A. Lee, K.W. Nebesny, Jpn. J. Appl. Phys., Part 1 34 (1995) 3837.
T.J. Schuerlein, N.R. Armstrong, J. Vacuum Sci. Technol. A 12 (1994) 1992.
A. Schmidt, T.J. Schuerlein, G.E. Collins, N.R. Armstrong, J. Phys. Chem. 99 (1995) 11770.
K. Glockler, C. Seidel, A. Soukopp, M. Sokolowski, E. Umbach, M. Bohringer, R. Berndt, W.D. Schneider, Surf. Sci.
405 (1998) 1.
M. Bohringer, W.D. Schneider, R. Berndt, K. Glockler, M. Sokolowski, E. Umbach, Phys. Rev. B 57 (1998) 4081.
M. Bohringer, W.D. Schneider, K. Glockler, E. Umbach, R. Berndt, Surf. Sci. 419 (1998) L95.
E. Umbach, K. Glockler, M. Sokolowski, Surf. Sci. 404 (1998) 20.
F.S. Tautz, S. Sloboshanin, V. Shklover, R. Scholz, M. Sokolowski, J.A. Schaefer, E. Umbach, Appl. Surf. Sci. 166
(2000) 363.
F.S. Tautz, S. Sloboshanin, J.A. Schaefer, R. Scholz, V. Shklover, M. Sokolowski, E. Umbach, Phys. Rev. B 61 (2000)
16933.
V. Shklover, F.S. Tautz, R. Scholz, S. Sloboshanin, M. Sokolowski, J.A. Schaefer, E. Umbach, Surf. Sci. 454 (2000) 60.
T. Schmitz-Hubsch, T. Fritz, F. Sellam, R. Staub, K. Leo, Phys. Rev. B 55 (1997) 7972.
T. Schmitz-Hubsch, T. Fritz, R. Staub, A. Back, N.R. Armstrong, K. Leo, Surf. Sci. 437 (1999) 163.
C. Seidel, A.H. Schafer, H. Fuchs, Surf. Sci. 459 (2000) 310.
R. Nowakowski, C. Seidel, H. Fuchs, Phys. Rev. B 63 (2001) 195418.
K.H. Frank, P. Yannoulis, R. Dudde, E.E. Koch, J. Chem. Phys. 89 (1988) 7569.
P. Yannoulis, K.H. Frank, E.E. Koch, Surf. Sci. 241 (1991) 325.
P. Yannoulis, R. Dudde, K.H. Frank, E.E. Koch, Surf. Sci. 189/190 (1987) 519.
P. Yannoulis, E.E. Koch, M. Lahdeniemi, Surf. Sci. 192 (1987) 299.
K.H. Ernst, M. Bohringer, C.F. McFadden, P. Hug, U. Muller, U. Ellerbeck, Nanotechnology 10 (1999) 355.
K.H. Ernst, M. Neuber, M. Grunze, U. Ellerbeck, J. Am. Chem. Soc. 123 (2001) 493.
R. Fasel, A. Cossy, K.H. Ernst, F. Baumberger, T. Greber, J. Osterwalder, J. Chem. Phys. 115 (2001) 1020.
S.M. Barlow, R. Raval / Surface Science Reports 50 (2003) 201–341
341
[114] K.H. Ernst, Y. Kuster, R. Fasel, M. Muller, U. Ellerbeck, Chirality 13 (2001) 675.
[115] M. Taniguchi, H. Nakagawa, A. Yamagishi, K. Yamada, Surf. Sci. 454 (2000) 1005.
[116] M. Bohringer, K. Morgenstern, W.D. Schneider, R. Berndt, F. Mauri, A. De Vita, R. Car, Phys. Rev. Lett. 83 (1999)
324.
[117] M. Bohringer, K. Morgenstern, W.D. Schneider, R. Berndt, Angew. Chem. Int. Edit. 38 (1999) 821.
[118] M. Bohringer, K. Morgenstern, W.D. Schneider, R. Berndt, J. Phys. 11 (1999) 9871.
[119] M. Bohringer, W.D. Schneider, R. Berndt, Angew. Chem. Int. Edit. 39 (2000) 792.
[120] M. Bohringer, K. Morgenstern, W.D. Schneider, M. Wuhn, C. Woll, R. Berndt, Surf. Sci. 444 (2000) 199.
[121] M. Bohringer, K. Morgenstern, W.D. Schneider, R. Berndt, Surf. Sci. 457 (2000) 37.
[122] M. Vladimirova, M. Stengel, A. De Vita, A. Baldereschi, M. Bohringer, K. Morgenstern, R. Berndt, W.D. Schneider,
Europhys. Lett. 56 (2001) 254.
[123] M. Bohringer, W.D. Schneider, R. Berndt, Surf. Rev. Lett. 7 (2000) 661.
[124] J.F. Walsh, H.S. Dhariwal, A. Gutierrrez-Sosa, P. Finetti, C.A. Muryn, N.B. Brookes, R.J. Oldman, G. Thornton, Surf.
Sci. 415 (1998) 423.
[125] C. Seidel, H. Kopf, H. Fuchs, Phys. Rev. B 60 (1999) 14341.
[126] M. Bassler, R. Fink, C. Heske, J. Muller, P. Vaterlein, J.U. von Schutz, E. Umbach, Thin Solid Films 285 (1996) 234.
[127] H. Wegner, K. Weiss, M. Grunze, C. Woll, Appl. Phys. A 65 (1997) 231.
[128] H. Wegner, K. Weiss, C. Woll, Surf. Rev. Lett. 6 (1999) 183.