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ATOMISTIC INTERACTIONS IN STM ATOM MANIPULATION
A dissertation presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Doctor of Philosophy
Aparna Deshpande
March 2007
This dissertation entitled
ATOMISTIC INTERACTIONS IN STM ATOM MANIPULATION
by
APARNA DESHPANDE
has been approved for
the Department of Physics and Astronomy
and the College of Arts and Sciences by
Saw-Wai Hla
Associate Professor of Physics and Astronomy
Benjamin M. Ogles
Dean, College of Arts and Sciences
Abstract
DESHPANDE, APARNA, Ph.D., March 2007, Physics
ATOMISTIC INTERACTIONS IN STM ATOM MANIPULATION (125 pp.)
Director of Dissertation: Saw-Wai Hla
This thesis describes the study of two diverse systems, a cluster of silver atoms, and
individual silver and bromine atoms, deposited on a metallic single crystal Ag(111)
substrate, in the domain of atomic and molecular manipulation techniques, using a
custom-built ultrahigh vacuum low temperature scanning tunneling microscope.
A cluster of silver atoms was created by a controlled tip-sample contact. Single atoms
were extracted from the cluster by using STM tip induced lateral manipulation. To
investigate the mechanism of extraction in detail, atom extraction was carried out for
different values of manipulation voltage and current. The threshold distance to pull an
atom out from the cluster was determined. The tip-cluster distance proved to be the
governing factor for the atom extraction mechanism.
Lateral manipulation of a metal atom, silver, and a halogen atom, bromine, was
carried out on a silver substrate with a silver coated tip apex. Silver atoms were extracted
from a cluster of atoms, and bromine atoms were extracted from a cobalt porphyrin
molecule using tunneling electron-induced bond dissociation technique. The threshold
distance necessary to manipulate the silver atom and the bromine atom was determined.
The lateral manipulation signals provided a value of the angle made by the tip with the
surface at the first jump of the atom during manipulation. The interaction energy curves
for these atoms were calculated using density functional theory. From a combination of
all these results, a numerical value of force was obtained. This force corresponds to the
threshold force necessary to move a silver atom and a bromine atom on the surface. The
values of force provide an insight into the ionic and metallic interactions on the surface at
the single atom level.
The manipulation capability of the scanning tunneling microscope to build
nanostructures was demonstrated by constructing a parabolic corral using locally
extracted atoms. Since the surface vacancies and defects created during construction can
be sealed off with the atoms and clusters after construction, this procedure resembles an
atomic scale analog of a macroscale construction site.
Approved:
Saw-Wai Hla
Associate Professor of Physics and Astronomy
Acknowledgments
I am grateful to my advisor for his mentorship. His crucial input at every stage of my
projects, his wisdom, scientific and otherwise, and his pedagogy have helped me all
along. Thank you for your patience and for the opportunity to be involved in the
construction of a sophisticated system.
My thesis committee members for devoting a share of their precious time to read my
thesis. A special note of thanks to Prof. Elizabeth who was willing to be on the committee
at a late notice.
Dr. Kai-Felix Braun for his help with the STM instrumentation, standing wave pattern
calculation program, and insightful discussions.
For all the physics classes and the professors who gave them, thank you.
Prof. Sergio for letting me participate in the theory group meetings and the chance to
broaden my physics and world outlook.
Prof. Nancy for being a friend and a great listener.
Ennice, Ruth, Tracy, Chris and Roger for all their help over the years.
Violeta for the enriching friendship.
To all my team mates in the Hla group for the camaraderie.
Swati for the good times, Deepshikha for the fun get togethers and the fantastic Indian
food, Anand for sharing the passion for movies and music.
Collins for the love, friendship and the stoic support that I have always counted on.
My Kaku and Kaka for making me a part of their family and for their continued love and
affection.
My Baba for always being there. Thank you for being a great parent, for instilling the
value of education and for letting me follow my dreams.
My Tai for the love and emotional support, Tejas for the wonderful growing up years.
Suhita for being my pillar of strength.
Dedicated to Suhita
8
Table of Contents
Page
Abstract……………………………………………………………………………………3
Acknowledgments…………………………………………………………………………5
Dedication…………………………………………………………………………………7
List of Table..................................................................................................................….11
List of Figures……………………………………………………………………………12
Chapter 1. Introduction ..................................................................................................... 19
Chapter 2. Instrumentation ............................................................................................... 24
2.1 Operating Principle of STM ...................................................................... 24
2.2 UHV-LT-STM System ...............................................................................31
2.2.1 UHV System ......................................................................................32
2.2.2 STM Unit ...........................................................................................34
2.2.3 STM Hardware and Software ............................................................36
Chapter 3. Background ..................................................................................................... 38
3.1 STM Manipulation Techniques ................................................................. 38
3.1.1 Lateral Manipulation..........................................................................39
3.1.2 Vertical Manipulation ........................................................................43
3.1.3 Inelastic Tunneling Electron Induced Excitations .............................46
3.2 Atomistic Interactions................................................................................ 48
3.2.1 Types of Interactions..........................................................................48
3.2.2 Interactions in STM Manipulation.....................................................51
9
3.3 Materials ......................................................................................................52
3.3.1 Substrate..............................................................................................53
3.3.2 Molecule .............................................................................................55
Chapter 4. Atom Extraction by Controlled Tip-Cluster Interaction ................................. 57
4.1 Introduction.................................................................................................57
4.2 Experimental Details...................................................................................60
4.3 Atom Extraction..........................................................................................60
4.3.1 Lateral Manipulation for Extraction ................................................. 61
4.3.2 Threshold Resistance .........................................................................64
4.4 Modeling ......................................................................................................67
4.5 Discussion ....................................................................................................70
Chapter 5. Atomistic Constructions.................................................................................. 72
5.1 Construction Scheme .................................................................................. 72
5.2 Reconstruction Scheme............................................................................... 78
5.3 Discussion ....................................................................................................79
Chapter 6. Atom Selective Force Measurement ............................................................... 81
6.1 Introduction……………………………………………. …………………81
6.2 Force Measurement Strategy .......................................................................82
6.3 Experimental Details....................................................................................84
6.4 Br Atoms......................................................................................................87
6.4.1 Extraction of Br Atoms.......................................................................87
6.4.2 Threshold Resistance Measurement of Br Atoms ...............................90
6.4.3 Lateral Manipulation for Angle Measurement ....................................94
10
6.4.4 Height Correction for Br Atoms ..........................................................96
6.4.5 Numerical Value of Force....................................................................97
6.5 Ag Atoms ...................................................................................................100
6.5.1 Extraction of Ag Atoms .....................................................................100
6.5.2 Threshold Resistance Measurements for Ag Atoms..........................101
6.5.3 Lateral Manipulation for Angle Measurement ..................................104
6.5.4 Numerical Value of Force..................................................................105
6.6 Discussion ..................................................................................................108
Chapter 7. Conclusions and Outlook .............................................................................. 111
Bibliography……………………………………………………….…………………....115
Appendix A List of Publications………………………………………………………..122
Appendix B List of Contributed Talks and Posters in Conferences………………....…124
11
List of Tables
Page
Table 1. Values of energy and distance from the Ag-Br interaction energy curve……...99
Table 2. Values of total force and lateral force calculated using equation (1) and values
from Table 1……………………………………………………………………………...99
Table 3. Values of energy and distance from the Ag-Ag interaction energy curve…….108
Table 4. Values of total force and lateral force calculated using equation (1) and values
from Table 3…………………………………………………………………………….108
Table 5. Values of threshold lateral force, total force and threshold resistance for Ag and
Br atoms………………………………………………………………………………...109
12
List of Figures
Page
Figure 2.1 Illustration of a scanning tunneling microscope. (a) The STM tip is attached to
a piezo tube. By applying an appropriate voltage to the piezo tube, the tip can be
moved in x, y, and z directions on the sample. (b) Illustration of electron
tunneling at the tip-sample junction at an applied sample bias ‘V’. Here φ is the
work function of the material, Ef is the Fermi level and E vac is the vacuum
energy. ………………………...............................................................................25
Figure 2.2 Schematic representation of STM operation in constant current mode (a) and
constant height mode (b)…………………………………………………………26
Figure 2.3 Schematic of direction of electron flow based on polarity of applied bias…..28
Figure 2.4 Energy diagram for elastic and inelastic tunneling mechanisms through an
adsorbate on the surface along with the corresponding form of I-V, dI/dV and
d2I/dV2 spectra that represent the processes. A change in I-V curve appears as a
step-like change in dI/dV signal, which in turn produces a peak in d2I/dV2 signal.
Figure from [17] …………………………………………………………..……..29
Figure 2.5 Tunneling resistance as a function of tip-sample distance acquired from an I-Z
spectroscopy at a fixed bias. A reduction in resistance leads to an increase in the
tunneling current. A red line is drawn for the eye guidance of the ohmic
contact……………………………………………………………………………31
Figure 2.6 Homebuilt UHV-LT-STM system…………………………………………...33
13
Figure 2.7 A close up of the STM unit attached to the base of the
cryostat………………………………………………...........................................34
Figure 2.8 Schematic of cryostat, radiation shields, and STM unit. Picture from [22]….35
Figure 3.1 Illustration of lateral manipulation. Picture from [22]…………………….... 39
Figure 3.2. Competing forces during pulling mode of atom manipulation on the periodic
surface potential………………………………………………………………… 41
Figure 3.3 Pushing mode of lateral manipulation………………………………………. 41
Figure 3.4 Characteristic lateral manipulation signals………………………………….. 42
Figure 3.5 Illustration of vertical manipulation. Picture from [22]……………………...43
Figure 3.6 Illustration of an atomic switch with the double well potential for Xe atom and
vibrational energy levels. Picture from [36]……………………………………..44
Figure 3.7 Vertical manipulation. (a) The double potential well becomes a single
potential well by reducing the tip-atom distance to a contact point (b). By
retracting the tip, the manipulated atom moves vertically……………………….45
Figure 3.8 Illustration of adsorbate-induced resonance state for an adsorbate on a metal
substrate. Picture adapted from [30]……………………………………………..46
Figure 3.9 Bond dissociation using tunneling electrons. Picture from [38]......................47
Figure 3.10 (a) Dipole formation for bonding. (b) Lennard-Jones potential. (c) Transfer of
electrons for an ionic bond formation between two atoms. (d) Energy E vs.
distance r plot for ionic interaction. (e) Sharing of electrons in a covalent bond. (f)
Metallic bonding…………………………………………………………………49
14
Figure 3.11 STM image showing atomic resolution of Ag(111) surface. The surface
directions used for manipulation are marked. The three-fold hollow site favored
by adatoms Ag and Br for adsorption is highlighted by circles…………………53
Figure 3.12 (a) Ag(111) surface state bear the fermi level in the bulk band dispersion
diagram. Picture from [45], (b) dI/dV spectrum showing the onset of the surface
state at -65 mV…………………………………………………………………...54
Figure 3.13 Standing waves on Ag(111) surface created by scattering of surface state
electrons off a step edge and Ag atoms that act as point scatterers……………...55
Figure 3.14 Chemical structure of cobalt porphyrin molecule…………………………..56
Figure 3.15 STM image of cobalt porphyrin molecule showing the central porphyrin ring
lifted by the 4 lobes. Size of the molecule is 15 Å x 15 Å………………………56
Figure 4.1 Illustration showing the tip-sample distance of operation of different types of
scanning probe microscopes using the Lennard – Jones potential………………58
Figure 4.2 Creation of a cluster of Ag atoms by tip-sample contact. Image (a) shows a
bare Ag(111) surface. Image (b) shows an Ag nanocluster created after contact.
Image parameters: V= 50 mV, I= 1 nA, image size 400 Å x 180 Å, and cluster
size 70 Å x 24 Å………………………………………………………………….59
Figure 4.3 STM images of atom extraction before and after the extraction process. A ball
model of the tip marks the site of extraction. Image parameters: V= 30 mV, I= 1
nA, dimensions of the cluster: height 6 Å, width 30 Å, length 100 Å…………...61
Figure 4.4 Lateral manipulation signal recorded during the atom extraction process…...62
Figure 4.5 Signature of atom extraction from a cluster. Ball models explain the peaks in
the signals with reference to the jump height of the tip………………………….63
15
Figure 4.6 Probability of atom extraction as a function of current at fixed bias
voltage....................................................................................................................64
Figure 4.7 Threshold resistance Rth to extract an atom from the cluster………………...65
Figure 4.8 Threshold distance corresponding to the threshold resistance for atom
extraction is obtained from the curve. A model shows the correlation of this
distance with the tip-cluster framework………………………………………….66
Figure 4.9 Effect of the tip on the energy barriers at adatom sites. Picture from
[50]……………………………………………………………………………….67
Figure 4.10 Model system for atom extraction along with the path taken by the atom
down the slope of the cluster. Picture from [51]………………………………....68
Figure 4.11 Lowering of the tip-cluster energy barrier due to the presence of the tip......69
Figure 5.1 An illustration of the nanoscale construction scheme. A controlled tip crash
creates holes and clusters on the surface (1-2), the atoms are extracted from the
cluster by using lateral manipulation (3), the atoms are arranged in desired
geometry (4), the holes are sealed with the unused clusters (5), and the tip is
retracted to imaging height (6). Picture from [51] ………………........................74
Figure 5.2 Constant current image of a corral constructed on Ag(111) surface. Image
parameters: (a) V= 30 mV, I= 1nA; image size 220 Å x 240 Å; (b) V= -30 mV, I
= 1 nA; image size 220 Å x 240 Å………………………………………………75
Figure 5.3 Constant current image of a parabola constructed by an assembly of 15 Ag
atoms. Image parameters: (a) V= 40 mV, I= 1nA; image size 170 Å x 270 Å; (b)
V= -60 mV, I= 1 nA; image size 170 Å x 270 Å………………………………...76
16
Figure 5.4 dI/dV maps at (a) 30 mV and (b)-30 mV generated using scattering
theory………………………………………………………….............................78
Figure 5.5 Reconstruction of a surface at an atomic level by filling the holes on the
surface using atoms and clusters. Picture from [57]……………………………..79
Figure 6.1 Atomistic force measurement strategy. An STM tip-height signal
(manipulation signal) shows single atomic site hopping of the atom on Ag(111)
surface when the atom is manipulated along a close-packed surface direction. The
drawings indicate the changes of vertical and lateral force components during
manipulation. Initially only a vertical force component exists but when the tip
traces the atom contour, the lateral force component increases. When the lateral
force component overcomes the surface force, the atom hops to the next surface
site. Here, ‘φ’ is the force vector angle and ‘r’ is the tip-atom distance.
……………………………………………………………………………………83
Figure 6.2 A series of images before tip crash (a) and after tip crash (b), showing a
sharper image resolution after the tip crash. The crash site is marked by a circle.86
Figure 6.3 CoTBrPP molecules on Ag(111) surface seen in domains (a) and in a cluster
arrangement (b)…………………………………………………………………..86
Figure 6.4 STM image of CoTBrPP single molecule taken at V= 1000 mV, I= 1 nA
(Image size 38 Å x 28 Å) along with a ball model based on the chemical structure
of the molecule…………………………………………………………………...88
Figure 6.5 Dissociation of phenyl in CoTBrPP molecule by I-V spectroscopy showing an
abrupt drop in the current the breaking of the C-Br bond………………………..89
17
Figure 6.6 Dissociation of CoTBrPP molecules on Ag(111). The dissociated Br atom is
seen in the image. Image parameters are: V= 1000 mV, I= 1.2 nA, size 52 Å x 32
Å………………………………………………………………………………….90
Figure 6.7 A series of images showing dissociation of Br atom and its subsequent lateral
manipulation. Image parameters: V= 1000 mV, I = 0.44 nA, size 90Å x 40 Å…91
Figure
6.8
Probability
plots
for
Br
atom
manipulation
at
positive
voltages…………………………………………………………………………..92
Figure 6.9 Probability plots for Br atom manipulation at negative voltages…………….93
Figure 6.10 Linear relationship between current and voltage during Br atom
manipulation……………………………………………………………………..94
Figure 6.11 LM curve for the Br atom during manipulation at positive
voltage……………………………………………………………………………95
Figure 6.12 LM curve for the Br atom during manipulation at negative voltage………..95
Figure 6.13 Height correction scheme for atoms………………………………….……..96
Figure 6.14 Interaction energy between Ag-Br atoms as a function of the separation
between the atoms is shown in (a). The plot is rescaled to Ag-Br bond energy and
to the distance between outer Van der Waal radii of the two atoms as shown in
(b).………………………………………………………………………………..97
Figure 6.15 Measurement of angle φ as a function of total force between the tip and the
Br atom…………………………………………………………………………...98
Figure 6.16 Extraction of Ag atoms from an Ag cluster. (Image parameters: V= 50 mV,
I= 1 nA, size 90 Å x 30 Å)……………………………………………………...101
Figure 6.17 Ag atom manipulation at positive voltages………………………………..102
18
Figure
6.18
Ag
atom
manipulation
at
negative
voltages…………………………………………………………………………103
Figure 6.19 Linear relationship between the current and voltage during Ag atom
manipulation……………………………………………………………………104
Figure 6.20 LM curves to measure the angle made by the tip and the Ag atom during
lateral manipulation…………………………………………………………….105
Figure 6.21 Interaction energy between Ag-Ag atoms as a function of the separation
between the atoms (a). The plot is rescaled to Ag-Ag bond energy and to the
distance between outer Van der Waal radii of the two atoms, shown in (b)…...106
Figure 6.22 Measurement of angle φ as a function of total force between the tip and the
Ag atom…………………………………………………………………………107
19
Chapter 1
Introduction
The field of surface science has seen consistent path breaking developments since the
1960s. Every new technological breakthrough in areas as diverse as vacuum technology
and semiconductor microelectronics contributed to a better understanding of surfaces and
interfaces. The invention of the scanning tunneling microscope (STM) in 1982 attracted a
great deal of attention and appreciation as it was the first instrument with the fine lateral
resolution and vertical resolution necessary to image single atoms in real space [1] .
However the operation of STM was not limited to imaging. The technique of scanning
tunneling spectroscopy explored the capability of an STM to resolve the energy states of
a surface at a fixed position [2].
The operation of an STM in ultrahigh vacuum and at low temperatures (UHV-LTSTM) broadened the scope of its applications. One remarkable achievement was the
technique of atom manipulation where a precise control of interactions between the tip
and the adsorbed atom on the surface could move the atoms to different locations on the
surface [3]. A bidirectional vertical transfer of the atom between the tip and the surface
demonstrated the function of the atom as a switch [4]. It introduced the concept of using
atoms as active elements in an electronic circuit, one of the key features of bottom-up
nanotechnology. Construction of corrals using atoms made it possible to visualize the
20
standing wave patterns within the corral created by an interference of the confined
electrons. A fascinating corral experiment exploited the geometry of the corral and the
physics of the Kondo effect to create a quantum mirage from the localized spectroscopic
response of a magnetic atom [5].
Manipulation techniques have made a significant contribution to a detailed
understanding of molecules by probing chemistry at length scales comparable to bond
lengths in a molecule. Individual bonds in the molecule can be dissociated using energy
provided by tunneling electrons from the tip [6]. Vibrational spectroscopy using STM can
`fingerprint’ the molecule by identifying the chemical bonds in the molecule [7].
Chemical reactions performed in the confines of a laboratory can be actualized on a metal
substrate. The Ullman reaction is a spectacular example of this demonstration [8]. Lateral
manipulation of molecules can probe the mechanical stability of a molecule and record
the intramolecular dynamics in response to the tip [9]. The switching behavior of
molecules can be studied for possible applications in molecular electronic devices [10].
Thus, the extent of STM applications is interdisciplinary in nature.
Lateral manipulation of an atom involves different types of interactions and forces
that come into play when the atom encounters energy barriers of different magnitude and
character. The surface on which the atom is adsorbed acts as the host providing a
potential energy landscape for the atom to bind to the surface either by physisorption or
chemisorption. The corrugation of the surface presents a barrier for diffusion of the atom.
The tip acts as a highly localized perturbation that modifies the potential energy
landscape and affects the state of the adsorbed atoms. Under the influence of the tip, the
atoms experience either attractive or repulsive interactions. The lateral movement of the
21
atoms on the surface is guided by the nature of this interaction. When the atom
acquires enough energy from the tip-induced interaction to overcome the diffusion
barrier, it jumps to the next energetically favorable site. The dynamics of all these
processes are controlled by tuning the distance between the tip and the atom.
The interactions between the tip and the surface can be controlled to carry out local
surface modifications. A controlled tip crash can create single atoms and reshape the tip
apex making the tip sharper for a better resolution of images [11]. The STM tip can be
used as a construction tool to dig out clusters from a metal surface. The manipulation
technique used to move a single atom on the surface can be employed to extract an atom
from a three dimensional cluster.
The complexity of such phenomena and the precision with which an STM can
attempt to explore and understand them makes the field of STM manipulation a very
challenging enterprise and an equally intriguing pursuit. The wide spectrum of
applications of STM: imaging, manipulation and spectroscopy, makes it an ideal tool to
communicate with the nanoworld.
A part of this dissertation explores the rich physics of interactions between the STM
tip and a metallic cluster using a homebuilt UHV-LT-STM. A single atom can be
extracted from a cluster of silver atoms using the STM lateral manipulation technique.
The extracted atom is initially bound to the other atoms inside the cluster. The presence
of the tip enhances the interaction between the tip and the cluster. The atom is pulled out
from the cluster by breaking the bonds within the cluster. What are the driving forces
behind this mechanism? At what critical tip-cluster distance is this mechanism feasible?
22
How does the potential energy landscape of the cluster change due to the presence of
the tip? These are some of the questions explored in this study.
Another interesting area probed in this dissertation is the extensive manipulation
study of two different types of atoms – a metal atom (silver) and a halogen atom
(bromine) on a metallic substrate (Ag(111)). Under identical conditions of manipulation,
using the same tip and the substrate, how does the chemical nature of the atom manifest
itself? What is the threshold force necessary for lateral manipulation of these atoms? An
attempt is made to answer these questions with a systematic study using STM
manipulation. With this objective in mind, the organization of this dissertation is as
follows:
Chapter two describes the instrumentation used for the projects and the basic
functions of an STM. The UHV-LT-STM is a custom-built system. The experience of
being involved in the construction of this machine helped to understand the intricacies of
such a sophisticated and powerful tool. The operating principle of STM is explained and
different spectroscopic techniques are introduced.
Chapter three gives an account of the manipulation techniques since the projects
carried out here are based extensively on atomic and molecular manipulation. Different
types of interactions between the tip and the adsorbate are discussed to help focus the
analysis and interpretation of the experimental results. The substrate and the molecule
used for the experiments are introduced.
Chapter four is a detailed study of the process of extracting a single atom from a
metallic cluster using the technique of lateral manipulation. The cluster is created from
the substrate material by a controlled tip sample contact. The interactions between the
23
STM tip and the cluster are investigated by determining the threshold tip-cluster
distance essential for a successful atom extraction.
Chapter five is an account of creating nanostructures on the surface by an atom-byatom assembly. The atoms are extracted from a metallic cluster. The holes created on the
surface during a tip-sample contact can be sealed with the extracted atoms. The ability of
the scanning tunneling microscope to build structures at the nanoscale, analogous to a
macroscopic construction project, is thereby demonstrated.
Chapter six is an exhaustive investigation of two different types of atoms, Ag and Br,
on a metal substrate. The Ag atoms are obtained from the substrate and a vapor deposited
porphyrin molecule is the source of Br atoms. Lateral manipulation trials of Ag and Br
give a value of threshold resistance necessary for manipulation. The threshold force
required to move these atoms is also estimated.
Chapter seven summarizes the significant conclusions of this study and provides a
broad outlook of the projects carried out in this dissertation.
24
Chapter 2
Instrumentation
The construction of a low temperature ultrahigh vacuum scanning tunneling microscope
(UHV-LT-STM) was undertaken in the first year of my research. This chapter describes
the principle of operation of an STM and the spectroscopic techniques followed by the
UHV-LT-STM construction.
2.1 Operating Principle of STM
The scanning tunneling microscope, invented by Binnig and Rohrer in early 1980s,
marked a pivotal contribution to the study of surfaces and was recognised by the award of
a Nobel Prize in 1986. The realization of a scanning tunneling microscope (STM) was the
result of a classic combination of surface science and engineering[1]. The field of surface
science was revolutionized by STM due to its unprecedented resolution. The high
resolution of STM is the result of the quantum mechanical phenomenon of tunneling.
When a sharp metallic tip is placed a few angstroms away from a conducting sample and
a bias voltage is applied between the tip and the sample, electrons can tunnel at this
junction (Figure. 2.1), thereby tunneling current flows between the tip and the sample. If
25
the applied bias is positive, (for positive bias the sample is positive with respect to the
tip, negative bias implies that the sample is negative with respect to the tip), then the
electrons tunnel from the tip to the sample, while a negative bias directs the flow of
electrons from the sample to the tip. The current varies exponentially with the distance
between them and is given by:
I ∝ V ⋅ ρs ⋅ e−z
ϕ
V, ρs and φ represent the bias voltage, density of states of the sample at the Fermi level
and the workfunction of the sample. The distance between the tip and the sample is
denoted by z. The basic set up of STM is represented in Figure 2.1.
(a)
(b)
Figure 2.1 Illustration of a scanning tunneling microscope. (a) The STM tip is attached to
a piezo tube. By applying an appropriate voltage to the piezo tube, the tip can be moved
in x, y, and z directions on the sample. (b) Illustration of electron tunneling at the tipsample junction at an applied sample bias ‘V’. Here φ is the work function of the
material, Ef is the Fermi level and E vac is the vacuum energy.
26
By keeping the current constant using a feedback loop, a topographic STM image
is generated when the tip scans the surface of the sample; this image reflects the constant
local density of states of the sample surface. Alternatively, it is also possible to obtain an
STM image by keeping the tip at a constant height with the feedback loop turned off; the
image thus recorded is a snapshot of the variation in the tunneling current at each point
on the sample [12]. Figure 2.2 illustrates these two operational modes.
Figure 2.2 Schematic representation of STM operation in constant current mode (a) and
constant height mode (b).
STM imaging provides information about the electronic states of the sample over the
spatial extent of the image. It is also possible to study the local electronic structure of the
sample at a specific x,y location by keeping the tunneling gap constant and applying a
voltage ramp on the tunneling junction. This technique is known as scanning tunneling
spectroscopy (STS). Most commonly used spectroscopy techniques are current-voltage
spectroscopy [I-V], conductance spectroscopy [dI/dV - V] and vibrational spectroscopy
[d2I/dV2 – V]. I-V spectroscopy provides information about the transport of electrons
between the tip and the surface as a function of varying voltage. dI/dV probes the local
2
2
27
electronic energy states of the sample whereas d I/dV signal probes the vibrational
energy states.
The tunneling current measured by STM is an integral of the density of states of the
tip ρT and density of states of the sample ρs close to the Fermi level within an energy
range eV. The expression for tunneling current is given by
eV
I∝
∫ ρ (E
s
f
− eV + ε ) ρT ( E f + ε )dε
0
When the tip density of states is constant, then the differential tunneling conductance
(dI/dV) is proportional to the sample density of states [13, 14]
dI
∝ ρs ( E f − eV )
dV
The dI/dV spectrum records the peaks at energies where the electrons tunnel through
the specific electronic states between the tip and the sample for different values of
applied bias voltage. At a negative bias voltage the electrons tunnel from the sample to
the tip, providing information about the occupied electronic states in the sample. For a
positive bias voltage, the electrons tunnel from the tip to the sample, providing
information about the unoccupied electronic states of the sample. This is illustrated in
Figure 2.3.
28
Figure 2.3 Schematic of direction of electron flow based on the polarity of applied bias.
To record the tunneling spectrum the feedback loop is interrupted. An ac bias
modulation of few mV is added to the dc bias using a lock-in amplifier. The first and
second harmonics of the tunneling current are detected by the lock-in amplifier as the dc
bias is swept over the selected energy range. The differential conductance dI/dV is
proportional to the first harmonic while the d2I/dV2 is proportional to the second
harmonic of the tunneling current.
Depending on the energy of the tunneling electrons, tunneling spectroscopy is
categorized into two types: elastic tunneling spectroscopy and inelastic tunneling
spectroscopy. In case of elastic tunneling spectroscopy the electrons tunnel without any
loss of energy; this is generally seen in spectroscopic studies of semiconductors and bare
metal surfaces [2, 15, 16] .When the electron tunnels into a vibrational energy state of the
adsorbate, there is an induced excitation in the adsorbate with a concomitant energy loss
by the electron. Such a process is inelastic in nature and is known as inelastic electron
29
tunneling spectroscopy (IETS) [17]. A schematic of this description is shown in Figure
2.4.
Electronic spectra of adsorbates contain features related to energy levels of the
substrate and adsorbate whereas vibrational spectra are characteristics of chemical bonds
in the molecules [7]. Scanning tunneling spectroscopy is hence an indispensable tool for
exploring physics and chemistry at the atomic and nanoscale.
Figure 2.4 Energy diagram for elastic and inelastic tunneling mechanisms through an
adsorbate on the surface along with the corresponding form of I-V, dI/dV and d2I/dV2
spectra that represent the processes. A change in I-V curve will appear as a step-like
change in dI/dV signal, which in turn produces a peak in d2I/dV2 signal. Figure from [17].
The variation of tunneling current with the width of the tunneling junction between the
tip and the sample is exponential in nature. However, there is a transition from the
30
tunneling regime to a point contact marked by a sharp increase in the current at a
specific distance between the tip and the sample. The STM can be used to measure this
change in the tunneling current as a function of the tip-sample distance. This technique is
known as I-z spectroscopy. For I-z measurement, the tip height is set by a small value of
voltage. The feedback is turned off and the tip is approached towards the sample by
varying the distance between the tip and the sample in small steps. As the tip approaches
the surface, it is attracted towards the surface whereas the atoms of the surface
underneath the tip are pulled towards the tip. Strong adhesive forces between these two
metals, i.e the tip and the surface at distances of the order of bond lengths, are responsible
for this phenomenon. At a specific distance the tip jumps to contact with the surface
followed by an abrupt increase in the tunneling current (Figure 2.5). The relationship
between the current and voltage in this regime is ohmic in nature [18-20]. The current at
each point of the tip trajectory can be converted to resistance, thus each value of junction
resistance can be assigned to the corresponding tip-sample distance. I-z spectroscopy can
thus be used for calibration of the tip height in STM measurements.
31
Figure 2.5 Tunneling resistance as a function of tip-sample distance acquired from an I-z
spectroscopy at a fixed bias. A reduction in resistance leads to an increase in the
tunneling current. A red line is drawn for the eye guidance of the ohmic contact.
2.2 UHV-LT-STM System
Manipulation of single atoms on a surface requires an atomically clean surface and
low surface temperatures to prevent diffusion of the adsorbates due to thermal energy.
Operation of an STM in an ultrahigh vacuum (UHV) condition keeps the sample clean
for a longer period of time whereas cooling the STM to low temperatures (LT) greatly
reduces the drift. Our UHV-LT-STM consists of 3 main units: the UHV system, the
STM, and the hardware and software that operate the STM. These units are described in
brief below.
32
2.2.1 UHV System
The vacuum chamber comprises of three main parts- a sample preparation chamber, a
load lock chamber, and a liquid helium bath cryostat. The sample preparation chamber
bears ports that can be utilized for different functionalities like an ion gun port for
sputtering facility, view ports for optical access, and a residual gas analyzer to check the
gas components in ultrahigh vacuum. The crucial component of this chamber is the
manipulator arm that holds the sample during cleaning and deposition of molecules, and
transfers the sample into the STM chamber. The manipulator includes facilities for the x,
y, z travel, rotation, as well as cooling (20 K) and heating (1100 K) of the sample. The
load lock chamber is connected to the sample preparation chamber via a gate-valve and it
contains a Knudsen cell for vapor deposition of molecules. The cryostat consists of a tall
cylindrical unit with electrical feedthroughs at the top. The STM is attached to the base of
the cryostat which holds the coolants- liquid nitrogen and liquid helium.
33
Figure 2.6 Custom-built UHV-LT-STM system.
All the constituents of the UHV-LT-STM, preparation chamber, cryostat and STM
were assembled. as seen in Figure. 2.6. Vacuum is achieved by pumping the chamber
using a set of pumps in ascending order of pumping capabilities- rotary pump, turbo
pump, titanium sublimation pump and ion pump. A pressure below 10-10 torr can be
attained with these pumps. The filling of the cryostat with coolants further lowers the
base pressure to 10-12 torr (normal atmospheric pressure is 760 torr or 760 mmHg). To
reach ultrahigh vacuum pressure, the chamber needs to be `baked’, i.e. heated to about
150ºC.
34
2.2.2 STM Unit
The design of the STM scanner unit is based on the Besocke-beetle type [21]. It
consists of a base plate that holds three coarse piezo tubes and one main piezo tube in the
center to which the tip is attached at its base with a magnetic tip holder. The tip is made
of electrochemically etched tungsten wire. A Cu disc machined with three equidistant
ramps rests on the coarse piezo tubes, supported by sapphire balls. See Figure 2.7 for a
photo of the STM unit.
Figure 2.7 A close up of the STM unit attached to the base of the cryostat.
By applying a voltage to the piezo tubes, movement in the x,y direction is achieved
by displacement of the piezos in the same direction. For z-movement all three piezos are
35
moved in a sequential order to execute a rotation of the ramp. A finer displacement in z
direction is achieved by applying a voltage to the center piezo tube that holds the tip. The
tip wire is a stainless steel wire with shielding. The wiring between the scanner and
electrical feedthroughs is done with specialized Kapton insulated fine NiCu wires for low
thermal conductivity. For mechanical vibration isolation, the STM scanner unit is
suspended from the base of the cryostat with stainless steel springs. The STM is then
surrounded by two concentric radiation shields (75 K and 4.2 K temperatures,
respectively) to ensure effective thermal isolation (Figure 2.8). The base plate of the
scanner has three magnets for eddy current damping. The entire system frame rests on
four Newport vibration isolators for vibration isolation from ambient disturbances.
Figure 2.8 Schematic of cryostat, radiation shields and STM unit. Picture from [22].
The sample and sample holder geometry is designed to match with the STM scanner.
The sample, circular in shape, 8mm in diameter and 1mm high, is mounted on top of a
36
button heater using a tantalum cap. The sample holder is made of oxygen-free copper.
It has two electrical ports that line up with sample contacts for electrical connection when
mounted in the STM system and align with the manipulator arm during sputtering and
annealing of the sample.
For temperature measurements, two silicon diodes are used; one at the base plate of
the scanner reads sample temperature. The other diode wired at the base of the cryostat
records the cryostat temperature that serves as a guide to refill the coolants.
2.2.3 STM Hardware and Software
STM software contains all the commands for data acquisition, ranging from imaging
to manipulation and spectroscopy. Image processing and 3D image rendering can also be
done using the same program. The STM feedback system is digitally controlled by the
program with appropriate settings selected by the operator.
The real-time electronic control system of the STM includes a PC-32 digital signal
processing (DSP) board for analog to digital and digital to analog transfer (AD and DA)
operations, and low noise high-voltage amplifiers with variable gain setting for the x, y,
and z piezo tubes, that controls the movement of the tip in all three dimensions. The
output channels of the DSP board control piezo voltages and sample bias and the input
channels register tunneling current and dI/dV signal from an external lockin amplifier. A
Femto preamplifier is used to amplify the tunneling current signal which in effect adjusts
the feedback loop to maintain a constant current during imaging and manipulation.
37
Alternatively, this loop can be disabled during dI/dV and I-z spectroscopy. The
preamplifier gain can be varied over a wide range to utilize the manipulation capabilities
of the system.
38
Chapter 3
Background
This chapter gives an introduction to the manipulation techniques used in atomic and
molecular manipulation followed by an overview of basic types on interactions between
atoms. The substrate and the molecule used for the experiments are introduced in the
final section.
3.1 STM Manipulation Techniques
One of the striking features of the design of this custom-built UHV-LT-STM is its
ability to manipulate atoms and molecules. Atom manipulation, a precursor for bottomup nanotechnology, focuses on the use of atoms for construction of nanostructures with
specific functionality and for exploration of properties of matter at nanometer scale [2326]. The pioneers of this field, Eigler and Schweizer, wrote the letters `IBM’ by precise
positioning of xenon atoms on a Ni(110) surface [3]. This technique has adapted itself
well to molecules of different sizes to exploit their potential for applications in molecular
electronics [10, 27-29].
39
Manipulation techniques can be broadly classified into three types: lateral
manipulation, vertical manipulation, and inelastic tunneling electron-induced excitations.
3.1.1 Lateral Manipulation
When the adsorbate moves in a direction parallel to the substrate, the technique is
referred to as lateral manipulation. The key element of lateral manipulation is the tuning
of interactions between the tip and the adsorbate by reducing the tunneling barrier
between the two. This is actualized by increasing the tunneling current. The tip is thus
approached closer to the adsorbate. In the constant current mode of manipulation, the
current is maintained at a fixed value. The tip is then assigned a path from the adsorbate
to the substrate. Based on the nature of tip-adsorbate interaction, the adsorbate moves
under the influence of the tip. Once the trajectory assigned to the tip is covered, the
adsorbate is relocated to a new position on the substrate. The tip is then retracted to
imaging height. The subsequent image reveals the new location of the adsorbate (Figure
3.1).
Figure 3.1 Illustration of lateral manipulation. Picture from [22]
40
The adsorbate is bound to the substrate either by physisorption or chemisorption. In
the case of physisorption, the adsorbate is weakly bound to the surface. If the adsorbate
binds to the substrate by forming a chemical bond, then it is considered to be
chemisorbed and the binding is much stronger. The depth of the physisorption potential
energy well is small (~ 25meV) compared to chemical bond energies (~1 eV) [30]. It is
therefore logical that a stronger interaction is necessary to move a chemisorbed species as
compared to a physisorbed species. A sign of stronger interaction is that the tip needs to
be in close proximity to the adsorbate, which effectively implies that manipulation is
successful at a higher current or lower junction resistance. A physisorbed atom can be
moved at a lower current or a higher junction resistance. This has been observed in case
of lateral manipulation of different types of atoms. A threshold resistance of 5 MΩ was
necessary to move Xe atoms physisorbed on Ni(110) surface, Pt atoms chemisorbed on
Pt(111) could only be manipulated with a threshold resistance of 20 kΩ [31].
An extensive study of lateral manipulation of Cu and Pb atoms and CO molecules on
a Cu(211) surface was crucial in categorizing the types of lateral manipulation based on
an attractive or repulsive interaction between the tip and the adsorbate [32]. Three
manipulation modes, pulling, sliding and pushing modes were established. The
interaction between the tip and the atom is attractive in the pulling mode. To move the
atom to the next surface site, it must overcome the diffusion barrier encountered by the
atom. During the manipulation process in the pulling mode, the tip first follows a part of
the manipulated atom contour. By doing so, the vertical force component F┴ gradually
decreases while the lateral force component F// increases. When F// overcomes the
41
diffusion barrier, the atom hops to the next surface site under the tip. This action
“alerts” the STM feed-back system and the tip retracts in order to maintain the constant
current, thus producing an abrupt increase in the tip height. Repeating this movement of
atom along the manipulation path produces a saw-tooth like signal (Figure 3.2).
(a)
(b)
(c)
Figure 3.2 Competing forces during pulling mode of atom manipulation on the periodic
surface potential.
In the pushing mode, however, the repulsive interaction between the atom and tip
drives the atom movement. In this case, the atom moves to the next surface site before the
tip reaches to the maximum contour height, so the tip needs to be moved forward toward
the surface. This produces a rapid decrease in tip height. The atom still moves in a
discontinuous manner on the surface but the manipulation signal in this case produces a
reverse saw-tooth pattern of the pulling (Figure 3.3).
(a)
Figure 3.3 Pushing mode of lateral manipulation.
(b)
42
When the atom follows the tip by being bound to the substrate during the entire
time of the trajectory, the movement is continuous and the atom is located under the tip.
This produces a smooth tip-height signal unlike the previous manipulation cases. This
can be understood in terms of the formation of a van der Waals trap between the tip and
the atom. If the magnitude of this trap is less than the physisorption well, the atom stays
its course on the surface [33]. The atom is thus equally attracted to the tip and the surface.
This mode of manipulation is known as the sliding mode.
Lateral manipulation is generally carried out by maintaining a constant current at the
tunneling junction. As the adsorbate moves under the influence of the tip, the current
changes due to varying interactions between the tip and the adsorbate. The feedback loop
in its effort to maintain a constant current adjusts the height of the tip accordingly. The
feedback loop signature is recorded as the variation of tip height versus the lateral
distance traversed by the tip. This lateral manipulation signal is characteristic to each of
the three modes of manipulation, pushing, pulling and sliding, irrespective of the
adsorbate in most cases (Figure 3.4).
Figure 3.4 Characteristic lateral manipulation signals.
43
The nature of the tip movement provides information on the type of interactions
between the tip and the adsorbate. The lateral distance between the consecutive jumps of
the atom reveals the direction along the atomic rows on the substrate traversed by the
atom. Based on the direction assigned for manipulation of a single atom, diverse signals
can be recorded which convey the intricate details of the manipulation process [34, 35].
3.1.2 Vertical Manipulation
When there is a vertical transfer of an adsorbate between the tip and the substrate, the
manipulation technique is known as vertical manipulation (Figure 3.5).
Figure 3.5 Illustration of vertical manipulation. Picture from [22].
This transfer can be bidirectional; the adsorbate can be transferred to the tip and vice
versa. The energy barrier encountered by the adsorbate for the transfer depends on the
separation between the tip and the surface. The adsorption energy of the adsorbate has to
be overcome to ensure its transfer to the tip. There are various ways to tune this energy
barrier. By applying a voltage pulse the atom can be transferred to the tip. Reversal of the
bias polarity transfers the atom back to the surface. This technique was first demonstrated
44
by Eigler by repeated transfer of a Xe atom between the tip and the Ni(110) surface
[4]. Such a process can be understood by considering the Xe in a double well potential;
one well represents the bonding with the substrate and the other well represents the
bonding with the tip (Figure 3.6).
Figure 3.6 Illustration of an atomic switch with the double well potential for Xe atom and
vibrational energy levels. Figure from [36].
The Xe atom transfer from Ni(110) surface to the tip is marked by an inelastic
electron tunneling process. The adsorbate-substrate bond is broken by gaining energy
from tunneling electrons. This energy input brings about an excitation of vibrational
energy levels of the adsorbate-substrate bond. The rate of atom transfer is given by
~
R ≈ nΓ↑ exp(−
~
VB
k B Γυ
)
where VB is (n-1)ħΩ, close to the actual barrier height VB,
Γ ↑ is
the excitation rate
between the vibrational ground state and first excited state, and Tυ is the characteristic
45
temperature that justifies the atom transfer mechanism to be a vibrational heating
mechanism.
Other examples of vertical manipulation include the transfer of covalently-bonded Si
atoms using a combination of high voltage pulses typical to field evaporation and tipsample interactions [37, 38]. CO molecules can be transferred to the tip by using 2.4 V
voltage pulses. The molecules can also relocate to adjacent sites on the adsorbate [39].
Another vertical atom transfer mechanism is based on the direct tip-atom contact. By
reducing the tip-height, the distance between the double potential wells representing the
manipulated atom and the tip-apex atom reduces. At sufficiently close tip-atom distance,
this double potential well becomes a single well (Figure 3.7). Note that the vertical tipmanipulated atom distance here needs to reach the contact point, thus it is much less than
the vertical tip-atom distance used in lateral manipulation. The manipulated atom is then
bound to the tip. By retracting the tip back to an image height the manipulated atom,
which is now adsorbed to the tip is moved vertically away from the surface.
(a)
(b)
(c)
Figure 3.7 Vertical manipulation. (a) The double potential well becomes a single
potential well by reducing the tip-atom distance to a contact point (b). By retracting the
tip, the manipulated atom moves vertically.
46
3.1.3 Inelastic Tunneling Electron-Induced Excitations
Inelastic tunneling electron-induced excitations can be used to manipulate atoms and
molecules as well. In particular, a rich variety of manipulations, such as rotation,
conformational changes, and dissociation can be performed on the molecules. Bond
dissociation, the breaking of a bond between two atoms in a molecule, can be carried out
by injecting low-energy tunneling electrons into the molecule by an adsorbate-induced
resonance state. The presence of an adsorbate on a substrate modifies the energy levels of
both the adsorbate and the substrate. The atomic states in the adsorbate are broadened
into a resonance state that is accessible to the electrons tunneling into the metal from the
STM tip. This is illustrated in Figure 3.8.
Figure 3.8 Illustration of the concept of an adsorbate-induced resonance state for an
adsorbate on a metal substrate. Picture adapted from [30].
The tunneling electron energy can be transferred to the molecule through the
adsorbate induced resonance state by means of a temporary electron attachment to the
47
molecule. The incident electrons occupy an available molecular orbital during the
lifetime of this resonance state and lead to changes in the molecule, depending on energy
provided by the electrons [40, 41]. This is done by positioning the STM tip above the
molecular bond to be dissociated by applying low bias voltage pulses. The electrons can
be injected from the tip or the substrate depending on the polarity of the bias (Figure 3.9).
The changes in tunneling current provide information about the event of dissociation.
This technique is known as Inelastic electron tunneling spectroscopy (IETS). The rate of
dissociation can be determined from the relation
R α IN
where R is the dissociation rate, I is the tunneling current, and N is the number of
electrons involved in the IET dissociation process [6, 42].
Figure 3.9 Bond dissociation using tunneling electrons. Picture from [38].
Tunneling electrons can also provide the energy necessary for a chemical association
of two species resulting in the formation of a chemical bond between them. A classic
demonstration of this process was shown in the formation of Fe(CO) and Fe(CO)2
complexes [43] and in C-C bond formation in the case of two phenyl groups [8].
48
3.2 Atomistic Interactions
The interaction between the two atoms, one at the tip-apex and the other on the
surface, is central to STM manipulation of atoms. The interaction between the tip-apex
atom and manipulated atom is exploited in a controlled manner for both lateral and
vertical manipulation processes. This part of the chapter presents a brief overview of
different types of interactions that govern the bonding between atoms and molecules. The
interactions are then discussed in the context of STM manipulation.
3.2.1 Types of Interactions
Depending on the chemical nature of atoms, the binding between two atoms can be
based on four different types of interactions [44]: van der Waals interaction, ionic
interaction, covalent interaction, and metallic interaction. In case of inert gases/closed
shell atoms, there is an interaction between atoms due to an induced dipole moment that
is attractive in nature and characterized by an overlap of individual charge distributions.
When the interatomic separation is lower than the bonding distance between the atoms,
electronic repulsion sets in as a result of the Pauli exclusion principle. Refer to Figure
3.10 for an illustration of the concepts underlying each interaction.
49
Figure 3.10 (a) Dipole formation for bonding. (b) Lennard-Jones potential. (c) Transfer of
electrons for an ionic bond formation between two atoms. (d) Energy E vs. distance r plot
for ionic interaction. (e) Sharing of electrons in a covalent bond. (f) Metallic bonding.
The effective interaction in a covalent bond is modeled by Lennard Jones Potential. It
represents the change in the interaction between two atoms as a function of their distance,
given by
50
⎡⎛ σ ⎞
V (r ) = 4ε ⎢ ⎜ ⎟
⎣⎝ r ⎠
12
⎛σ⎞ ⎤
−⎜ ⎟ ⎥
⎝ r⎠ ⎦
6
where r is the separation between two atoms, σ and ε are parameters that can be fit to
experimental data related to gas phase ionization potential. Here, the term
12
6
⎛ 1⎞
⎛ 1⎞
⎜ ⎟ describes the repulsive part of the interaction while ⎜ ⎟ represents the attractive
⎝ r⎠
⎝ r⎠
part of the interaction (Fig. 3.10b).
Ionic interaction arises from the electrostatic attraction of positively and negatively
charged ions. Two atoms participate in the interaction, one being the electron donor and
the other being the acceptor, leading to the formation of an ionic bond. The classic
example of an ionic bond is a sodium chloride (NaCl) molecule (Figure 3.10c). The
interaction energy between the ions can be expressed as a sum of a repulsive potential
λe
−r/ρ
q2
, (λ and ρ are empirical parameters) and a coulomb potential ±
.A
r
graphical representation is drawn in Figure 3.10d.
Covalent bonds are formed by sharing of electrons between two atoms. They are
known for their strength and directionality and are ubiquitous in organic compounds.
Sigma (σ) bonds and pi (π) bonds are the types of covalent bonds that play an active role
in hybridization of orbitals and bond formation, and are highly directional. In the case of
hydrogen molecule, the covalent bond between two hydrogen atoms is formed by an
overlap of wavefunctions of the electron in each atom. The bond formation depends on
the spin orientation of the electrons, as dictated by the Pauli exclusion principle.
51
A bond, specific to a hydrogen atom, is the hydrogen bond which involves bonding
between hydrogen, and oxygen, fluorine, or nitrogen. It is seen in water molecules and in
proteins and nucleic acids. It is weaker than an ionic bond or a covalent bond.
In the case of metals, the valence electrons are delocalized from the respective atoms
and the positive ions are immersed in a sea of electrons (Figure 3.10f). The high electrical
conductivity of metals is an outcome of this uniform distribution of electrons. A metallic
bond can also be classified as a non-directional and non-polar covalent bond. In the order
of bond strength covalent bonds are the strongest (metallic<ionic<covalent).
3.2.2 Interactions in STM Manipulation
STM provides access to surfaces of metals and semiconductors at atomic length
scales abundant with interplay of different short range forces. The force between the tip
and the sample originates from the distance-dependent interactions between them. Based
on this distance the force can be categorized as van der Waals force, resonance force or
the repulsive force [14]. Van der Waals force follows a power law and is always present
in normal STM operation. But the dominant force is the attractive force between the tip
and the sample that arises from an overlap of the tip and the sample wavefunctions, also
known as resonance force.
Lateral manipulation of atoms using an STM is a direct technique to explore this
regime of attractive interactions. For an identical tip and substrate, manipulation of atoms
of different types can provide insight about the nature of attractive interactions.
52
To move an atom adsorbed on a surface there are various energy considerations.
The adsorbed atom binds to the surface either by physisorption, eg. Xe on Pt(111) [31],
or chemisorption as is the case for Cu on Cu(111), and Ag on Ag(111) [32, 34]. During
manipulation, when the interaction between the tip and the atom is strong enough, the
atom overcomes the binding with the substrate and follows the tip till the end of the
trajectory of the tip. The atom then localizes its position to the closest available energy
minimum on the substrate. It is possible to estimate the threshold distance necessary for
manipulation. In case of Ag atom manipulation on an Ag(111) surface the critical tipatom distance is experimentally determined to be 1.3 Å between the van der Waals radii
of the two atoms. This is the distance where the electronic wavefunctions between the
two atoms overlap and the attraction between the two atoms originates from temporary
chemical bonding. Since both the manipulated atom and tip-apex atom are silver atoms,
this bonding should be metallic in nature. One could speculate, based on this result, that
the extensive manipulation of halogen atoms would have a strong contribution from ionic
interactions. Thus lateral manipulation can be seen as a selective technique to probe
interactions at an atomic level.
3.3 Materials
For the projects carried out in this dissertation a single crystal Ag(111) surface was
used as the main substrate. The characteristic features of the sample are discussed below.
The molecule used for one of the projects is also introduced.
53
3.3.1 Substrate
The Ag(111) surface is composed of a hexagonal arrangement of atoms with a nearest
neighbor atom distance of 2.89 Å along the close-packed row directions, i.e. [110]
surface directions. The atoms used for the manipulation in this dissertation, Ag and Br,
are normally adsorbed on the surface at three-fold hollow sites. Refer to Figure 3.12.
Figure 3.11 STM image showing atomic resolution of Ag(111) surface. The surface
directions used for manipulation are marked. The three-fold hollow site favored by
adatoms Ag and Br for adsorption is highlighted by circles.
The surface of a crystal represents a planar termination of the bulk. Electronic
structure calculations reveal the existence of surface states in the projected bulk band gap
of metals [30, 45]. Figure 3.12 shows the location of these states and their characteristic
free-electron parabolic energy dispersion curve. Scanning tunneling spectroscopy (STS)
measurements on a bare Ag(111) surface show the onset of the surface state at -65 mV
with respect to EF.
54
Figure 3.12 (a) Ag(111) surface state near the Fermi level in the bulk band dispersion
diagram, picture from [45], (b) dI/dV spectrum showing the onset of the surface state at
-65 mV.
These states disperse parallel to the surface and contribute to the local density of
states (LDOS) at the surface. The surface state electrons bounce off the defects like step
55
edges or other adsorbates on the surface and give rise to standing waves created by
oscillations in the local density of states of Ag(111) surface (Figure 3.13). At the ground
state the distance between the two peaks of the waves, 39 Å, corresponds to half of the
Fermi wavelength of Ag(111) surface state.
Figure 3.13 Standing waves on Ag(111) surface created by scattering of surface state
electrons off a step edge and Ag atoms that act as point scatterers.
3.3.2 Molecule
For manipulation of Br atoms, instead of depositing Br atoms on the surface, Br
atoms were extracted using the technique of bond dissociation from a cobalt porphyrin
molecule.
The
molecule
5,10,15,20-Tetrakis-(4-bromophenyl)-porphyrin-Co(II),
(C44H24Br4CoN4) has been extensively investigated in our research group [29]. It consists
of a central porphyrin ring with the magnetic cobalt atom at the center of the ring. Four
bromophenyl groups are attached to the molecule. Figure 3.14 shows the chemical
structure of the molecule.
56
Figure 3.14 Chemical structure of cobalt porphyrin molecule.
The molecules are vapor deposited on to the Ag(111) surface. STM image of the
molecule shows four lobes that correspond to the phenyl groups. The central porphyrin
ring is elevated from the surface by these lobes. The C-Br bond is located at the edge of
the lobes. This bond is dissociated using tunneling electrons to extract the Br atom. An
STM image of the molecule is shown in Figure 3.15.
Figure 3.15 STM image of cobalt porphyrin molecule showing the central porphyrin ring
lifted by the 4 lobes. Size of the molecule is 15 Å x 15 Å.
57
Chapter 4
Atom Extraction by Controlled TipCluster Interaction
This chapter focuses on the interactions between the STM tip and a bare Ag(111)
surface at 4 K. An indentation of the tip on the surface coats the tip apex with Ag atoms.
When such indentations penetrate a few atomic layers on the surface, clusters of Ag
atoms are created. By tuning the tip-cluster interaction single atoms can be extracted from
the cluster. A modeling study carried out using interaction potentials from Embedded
Atom Method (EAM) shows that the proximity of the tip to the cluster leads to a
lowering of the potential barrier between the tip-cluster system and aids the extraction
mechanism.
4.1 Introduction
Scanning probe microscopy explores the interaction between a probe tip and a sample
surface at different distance regimes. The parameter that controls this interaction is
58
specific to the type of probe microscopy. For example, in case of atomic force
microscopy (AFM), the deflections of a cantilever record the force between the cantilever
and the sample surface. For a scanning tunneling microscope the tunneling current
between the tip and the sample controls the interactions. The different parts of the
Lennard-Jones potential energy curve are relevant to the operation of STM and AFM
(Figure 4.1).
Figure 4.1 Illustration showing the tip-sample distance of operation of different types of
scanning probe microscopes using Lennard – Jones potential.
The interaction between an Ir tip and an Ir sample in an STM set up probed as a
function of the tip-sample distance was found to be attractive, originating from shortrange metallic adhesion forces [46]. The tip-sample interaction between a semiconductor
surface, Si(111) and a tungsten tip was used to transfer silicon atoms and clusters
between the tip and the surface by tuning electrostatic and chemical forces using voltage
pulses [37]. An interesting theoretical study of single Al atom extraction from Al(111)
sample surface attributed the mechanism to a chemical interaction between the tip and
59
substrate as a function of applied bias [47]. All these studies focus on a vertical tip
approach towards the sample and are unique and insightful.
In the present work, a controlled tip-sample contact is indeed employed. But the
semblance to previous studies ends at this point. The tip-sample contact is highly
localized. The tip penetrates a few atomic layers of the sample and creates a nanometer
size cluster of Ag atoms upon retraction (Figure 4.2).
Figure 4.2 Creation of a cluster of Ag atoms by tip-sample contact. Image (a) shows a
bare Ag(111) surface. Image (b) shows an Ag nanocluster created after contact. Image
parameters: V= 50 mV, I= 1 nA, image size 400 Å x 180 Å, and cluster size 70 Å x 24 Å.
Single atoms are then extracted from this cluster using the technique of lateral
manipulation. The extraction of atoms from a metallic cluster using lateral manipulation
is a unique and novel application of LM as this technique has been generally used for
single atoms and molecules [8, 24, 31]. Here the tip can interact with the 3D contour of
the cluster. Prior to extraction, the atom is bound to the other atoms inside the cluster.
60
The tip-cluster interaction during manipulation needs to be strong enough to break the
interatomic bonds within the cluster. Once the atom is pulled off the cluster, it moves
along the uneven terrain of the cluster following the tip till it reaches its final destination
on the terrace away from the cluster.
4.2 Experimental Details
For this experiment a bare Ag(111) sample is cleaned in situ by sputtering and
annealing. It is then cooled to 4 K. The tungsten tip is coated with Ag by gently crashing
the tip on the surface [11]. This restructures the tip apex for better imaging and the
chemical identity of the apex, Ag in this case, is established.
4.3 Atom Extraction
With a well-defined tip apex a silver cluster is deposited on the surface. A suitable
voltage is applied to the z piezo tube. A controlled tip-sample contact is thereby achieved.
This results in the deposition of a cluster. The topography of the cluster is analyzed in
3-D and any protrusions on the cluster are preferred for extraction. The cluster serves as a
repository of single atoms.
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4.3.1 Lateral Manipulation for Extraction
Once the Ag cluster is imaged, the manipulation mode of STM operation is invoked.
Manipulation is carried out at constant current, which implies that the tip is maintained at
a constant height by setting a suitable current. The height of the tip is lowered from that
in the imaging mode to the parameters set in the manipulation mode. The tip is then
assigned a specific path from the cluster to the surface. The increase in tip-cluster
interaction leads to the breaking of the bonds within the cluster effective enough to `free’
a single atom. This atom then continues to follow the tip to the final destination on the
surface. The extracted atom can be seen in the image recorded after the extraction trial is
completed. Refer to Figure 4.3 for the STM images.
Figure 4.3 STM images of atom extraction before and after the extraction process. A ball
model of the tip marks the site of extraction. Image parameters: V=30 mV, I =1 nA;
dimensions of the cluster: height 6 Å, width 30 Å, and length 100 Å.
The dynamics of this atom extraction process can be inferred from the STM
manipulation signal shown in Figure 4.4. To comprehend the atomistic behavior during
62
this process, every detail of the manipulation signal needs to be analyzed. The
manipulation signal shows a peak at the topmost location of the cluster caused by sudden
changes of vertical tip position. Over hundreds of repeated atom extraction trials confirm
that such peaks can be associated with a successful atom extraction. Since the extracted
atom is a constituent of the cluster, removing it from the original position involves
severing the bonds with neighboring atoms within the cluster. Thus the appearance of
such peaks in the manipulation signal can be caused by a sudden removal of the atom
from its original location. This event leads to a re-adsorption of the atom at the next
favorable site at the downward slope of the cluster. Several subsequent atom removals
occur along the lateral tip movement providing more peaks in the manipulation signal. At
the flat terrace, the tip height suddenly increases followed by a smooth manipulation
curve. This event is associated with the positioning of the extracted atom under the tip
apex. The atom then moves together with the tip as in the case of sliding mode [32]. In
the sliding mode, the manipulated atom is temporarily bound to or trapped under the tip,
and both the tip and the atom move together smoothly across the surface.
Figure 4.4 Lateral manipulation signal recorded during the atom extraction process.
63
The pattern of sharp peaks along the contour of the cluster followed by a relatively
smooth trace is seen repeatedly in the extraction trials. Figure 4.5 shows some lateral
manipulation signals along with a ball model explaining the peaks with reference to the
tip height. The tip jump height is dictated by the cluster geometry in general. Extraction
can be carried out from clusters of various sizes and shapes. No constraint is posed by the
dimensions of the cluster.
Figure 4.5 Signature of atom extraction from a cluster, ball models explain the peaks in
the signals with reference to the jump height of the tip.
A range of bias voltages from 10mV to 35 mV was used during the manipulation.
Since the tip apex itself is composed of silver atoms due to the in situ tip-preparation
64
process the possibility of atom extraction from the tip cannot be completely ruled out
[11].
4.3.2 Threshold Resistance
To get a quantitative feel for atom extraction as a function of the current and
voltage used in manipulation the value of the threshold tunneling resistance (Rth) is
determined from the probability of atom extraction. The probability here is defined as the
ratio between the distance traveled by the atom (Latom) and the lateral tip movement
distance (Ltip). The values of Latom/Ltip for atom extraction events are determined by
varying the tunneling current at several fixed tunneling biases.
Figure 4.6 Probability of atom extraction as a function of current at fixed bias voltage.
65
The probability plots can be interpreted as follows: consider the plot for 18 mV in
Figure 4.6, the ratio Latom/Ltip is zero below 400 nA tunneling current. This means that no
atom has been extracted below that value. Above 400 nA, however, the Latom/Ltip value is
~1 and implies that the atom can be successfully extracted. In a few cases, Latom/Ltip
remains zero even at current values higher than 400 nA, indicating that not all extraction
events are successful. We find a success rate of ~85% from all the trials in the
experiment. All the plots shown in Figure 4.6 show a characteristic step-function with a
definitive current threshold for the voltages used.
The procedure of determining threshold current is carried out for different bias
voltages and the results are displayed in Figure 4.7 as a plot of threshold current versus
bias voltage. Here, each data point is determined by plotting the probability plot for a
specific bias voltage that gives a specific threshold current as shown in Figure 4.6. The
value of threshold resistance, Rth comes out to be 47 ± 2 kΩ from the inverse of the slope
of this curve.
Figure 4.7 Threshold resistance Rth to extract an atom from the cluster.
66
Tunneling resistance is a measure of both the tip-cluster distance and the tip-cluster
interaction strength. A lowering of the tunneling resistance causes a reduced tip-cluster
distance and consequently, an increased tip-cluster interaction. The Rth value can be
related to the threshold tip-cluster distance necessary to extract an atom via a tip-height
versus tunneling resistance curve, shown in Figure 4.8. The threshold tip-cluster distance
corresponding to Rth = 47 ± 2 kΩ is determined to be 0.6 Å. This analysis shows that a
distance of 0.6 Å or less between the edges of van der Waals radii of the tip-apex atom
and the cluster atom is necessary for a successful atom extraction; a model is shown in
Figure 4.8 for clarity.
Figure 4.8 Threshold distance corresponding to the threshold resistance for atom
extraction is obtained from the curve. A model shows the correlation of this distance with
the tip-cluster framework.
Within the bias range explored here, the threshold tunneling current varies linearly
with the bias (Figure 4.7). This linear relationship is indicative of the lack of contribution
from tunneling electron-induced excitation or local heating effects. Furthermore, since
low voltages (< 35 mV) are used in the atom extraction scheme, the contribution of
67
electric field in this process is negligible [11, 19]. Therefore, the tip-cluster interaction
should be the governing mechanism in this case.
At the length scales of the cluster, typically few tens of Å, the tip apex is more
localized with respect to the cluster geometry in general. So the tip proximity influences
the extraction process.
4.4 Modeling
The STM tip plays a crucial role in manipulation experiments. It acts as an active
element that brings about a dynamical change in the location of adatoms by modifying
the energy barriers responsible for manipulation. This phenomenon is illustrated in Figure
4.9. Such a scenario has been explored in case of lateral and vertical manipulation of
single atoms on metal surfaces using total energy calculations with interaction potentials
from the embedded atom method [48-50].
Figure 4.9 Effect of the tip on the energy barriers at adatom sites. Picture from [50].
68
The extraction of a single atom from the cluster using the tip was exposed to a
similar theoretical treatment [51]. The changes in the potential energy landscape due to
the tip-cluster interaction were explored using embedded atom method and molecular
dynamics simulations. The prototypic system used for the total energy calculations
consists of a silver tip composed of 35 silver atoms, and a 28-atom silver cluster on
Ag(111) as shown in Figure 4.10. The Ag(111) substrate under the cluster is formed by
six atomic layers with 80 silver atoms in each layer. Initially, the entire system is allowed
to relax to its minimum energy configuration using the conjugate gradient method. Since
only small biases are used in the experiment, the electric field effect is not included in the
calculation. The total energy calculations provide the potential energy landscape of the
system while the molecular dynamics simulation introduces the motion of the atom and
the minimum atom movement path.
Figure 4.10 Model system for atom extraction along with the path taken by the atom
down the slope of the cluster. Picture from [51].
The energy required to extract the atom from the cluster can be defined as the energy
barrier to move the atom from its original location, A, to the next site, B, within the
69
cluster as shown in Figure 4.8. The barriers are altered by the presence of the tip as the
tip modifies the energy landscape of the system drastically [50].
The lateral position of the tip apex atom and the adatom is equally significant for the
extraction process in the simulations. The adatom moves towards the tip only at a lateral
distance of 2.7 Å. The energy barrier for the atom to jump from site A to site B (Figure
3.9) is reduced from 0.29 eV to 0.03 eV as the tip approaches the adatom from a height of
3.23 Å to 2.43 Å (Figure 4.11).
Figure 4.11 Lowering of the tip-cluster energy barrier due to the presence of the tip.
Picture from [51].
Thus the modeling study predicts that the lowering of the energy barrier due to the
tip-cluster proximity is the key factor in the extraction of a single atom from the cluster.
The results from the simulations are in qualitative agreement with experimental findings.
70
4.5 Discussion
The interaction between the STM tip and a metallic cluster was investigated. The
metallic cluster was created in situ by a controlled tip-sample contact. Single atoms were
extracted from the cluster using the technique of lateral manipulation. The ability of the
tip to break a single atom from the cluster, and direct it from the three dimensional
uneven terrain of the cluster onto the substrate marks novelty of the experiment in
comparison with lateral manipulation of atoms on a flat terrace. The variation of tip
height with the lateral distance covered during manipulation gives insight into the
atomistic mechanism of the process. The interaction induced by the proximity of the tip
to the cluster breaks the bond within the atoms in the cluster to extract a single atom. The
extracted atom continues to readsorb on different sites of the cluster while following the
tip. Once it is totally free from the cluster, the atom slides onto the flat terrace keeping its
position underneath the tip constant until it reaches the final destination on the terrace.
Quantitative parameters, like the threshold resistance and threshold tip-cluster distance
for extraction, were determined. A linear relationship between the current and voltage
used for extraction rules out the contribution of tunneling electron-induced excitations or
local heating effect to the extraction mechanism. A critical distance of 0.6 Å between the
tip apex and the cluster is essential for successful atom extraction. Thus the proximity of
the tip to the cluster gives rise to a series of attractive and repulsive interactions that
guide the extraction process. A modeling study of atom extraction attributes the
extraction mechanism to the tip-induced changes in potential energy landscape of the tip-
71
cluster system. In conclusion, the detailed investigation of atom extraction from a
cluster provides a fundamental understanding of the process and is a reliable way to
produce single atoms for atomistic constructions.
72
Chapter 5
Atomistic Constructions
The role played by the STM tip in atom manipulation on metal surfaces can be
compared to a construction tool in the real world. With a high degree of precision the tip
can create a metallic cluster from the substrate, extract single atoms from the cluster, and
build nanostructures by positioning the atoms in predetermined locations on the surface.
The holes created during tip-surface contact to extract debris from the substrate for
construction can be sealed with unused atoms or clusters. This chapter describes the
construction and reconstruction schemes that demonstrate the potential of STM
manipulation for nanoscale engineering of surfaces.
5.1 Construction Scheme
For a demonstration of atomic level constructions and reconstructions, the choice of
substrate continues to be a single crystal Ag(111) metal surface. Surface states are present
in the projected bulk band gap for noble metal (111) surfaces [52]. The electrons in these
surface states form a two dimensional electron gas. The interaction of step edges or
73
defects on the surface with the surface state electrons gives rise to oscillations in the
charge density on the surface, also known as Friedel oscillations. STM images of metal
surfaces, Au(111) and Cu(111), provided the first real space image of the wave-like
interference patterns of these oscillations due to the scattering of electrons [53, 54]. The
wave patterns are isotropic and waves around the adsorbates on the surface scatter
uniformly around them. This is a reflection of the free electron-like behavior of surface
state electrons. The oscillations are characterized by a period λF/2 at the ground state,
where λF is the Fermi wavelength. These waves decay in the order of 1/x1/2 as the distance
x from the step edge decreases [53, 54].
Manipulation of atoms using an STM allows precise positioning of atoms on the
surface. The atoms can be arranged in specific geometries that dictate the scattering of
the electron waves. Such an arrangement of atoms, commonly known as a quantum
corral, gives rise to spectacular standing wave patterns in electron density caused by the
constructive or destructive interference of the electron waves. These patterns can be
interpreted as eigenstates of the structure using a particle-in-a-box model [55]. By
changing the applied bias voltage various energy eigen states of the structure can be
probed.
The geometry of the corral makes a significant contribution while exploring different
phenomena on metal surfaces. An elliptical quantum corral with a magnetic cobalt atom
located at one focus could project the Kondo signature at the other empty focus, creating
a quantum mirage [5]. Calculations for parabolic corrals of different geometries suggest
their role as possible quantum beacons to transmit information in quantum structures
[56].
74
In the present construction scheme a corral of Ag atoms was constructed using Ag
atoms. Single Ag atoms were extracted from clusters created in situ by tip-sample
contact. The concept of atomistic construction is described in Figure 5.1.
Figure 5.1 An illustration of the nanoscale construction scheme. A controlled tip crash
creates holes and clusters on the surface (1-2), the atoms are extracted from the cluster by
using lateral manipulation (3), the atoms are arranged in desired geometry (4), the holes
are sealed with the unused clusters (5), the tip is retracted to imaging height (6). Picture
from [57].
This scheme involves the following systematic steps. A construction site is chosen on
the surface by taking STM images. By dipping the STM-tip into the substrate, debris and
holes are formed. The debris normally is composed of substrate materials. By controlling
the tip-cluster interaction, individual atoms are extracted from the clusters. The extracted
atoms are then repositioned to the desired locations on the surface with atomic precision
to construct various nanostructures. The remaining holes are filled back with the
75
atoms/clusters to ‘reconstruct’ the surface. The tip is retracted back to the normal
imaging height.
To position the atoms precisely at predetermined locations on the substrate, the
technique of lateral manipulation is used [22]. 17 Ag atoms extracted from a Ag cluster
were arranged in the form of a parabolic corral. The surface state electrons are scattered
by Ag atoms resulting in characteristic standing wave patterns at the set bias voltage.
Figure 5.2 Constant current image of a corral constructed on Ag(111) surface. Image
parameters: (a) V= 30 mV, I= 1nA, image size 240Å x 220 Å; (b) V= -30 mV, I= 1 nA;
image size 240 Å x 220 Å).
Another example of such a construction is a single parabola of 15 atoms shown in
Figure 5.3.
76
Figure 5.3 Constant current image of a parabola constructed by an assembly of 15 Ag
atoms. Image parameters: (a) V= 40 mV, I= 1nA, image size 270 Å x 170 Å; (b) V= -60
mV, I= 1 nA; image size 270 Å x 170 Å.
The STM images shown in Figure 5.2 and Figure 5.3 are taken in constant current
mode. So they represent an integrated local density of states from 0 mV to 30 mV above
Ef in 5.2(a), 5.2(b) shows all energy states 30 mV below Ef. Figure 5.3(a) contains energy
states from 0 mV to 40 mV above Ef and 5.3(b) represents all energy states 40 mV below
Ef.
The standing wave patterns within the corral can be explained using multiple
scattering theory [23, 26]. The atoms can be considered as point scatterers and the wave
patterns can be calculated using a program based on multiple scattering theory [26]. The
concepts involved in the calculations are explained in the following description.
The tip is modeled as a point source emanating a circular wave. The atoms are
modeled as point scatterers. The electron wave from the tip experiences a phase shift δi
due to the scatterers, and the amplitude of the wave decreases, represented by absorption
αi . These parameters are used in the scattering T matrix:
77
(α i e 2iδ i − 1)
ti =
2
A silver atom emits an s wave that travels to the other scatterers or to the tip. Interference
of the `tip’ wave and the `atom’ wave is responsible for the intensity seen in the dI/dV
maps generated by the program. Using the T matrix in the Schrodinger equation, the
solution is given by:
→T
→
~
LDOS α Re[ a T • (1 − A) −1 • a ]
→
→
where aT and a refer to vectors of dimension N , N is the number of Ag adatoms,
→
→
a T ( kri ) and ai ( kri ) are values of the vectors aT and a and represent the value of the
~
circular wave with wave vector k and tip-adatom distance ri, A is an N X N matrix , aij =
ai(krij) are the off diagonal elements of the matrix and the diagonal elements are zero.
The dI/dV maps corresponding to the corral shown in Figure 5.2 were calculated for
same voltages used in the constant current mode of STM imaging. These maps are shown
in Figure 5.4. These maps reveal the density of states within the corral at a specific
energy of 30 mV above the Fermi level, 5.4 (a) and at 30 mV below the Fermi level,
5.4(b). Hence no identical match between the STM images and calculated dI/dV map is
expected here.
78
Figure 5.4 dI/dV maps at (a) 30 mV, (b) -30 mV generated using scattering theory.
5.2 Reconstruction Scheme
The area on the surface perturbed by the creation of the cluster and subsequent
atomistic constructions can be `reconstructed’ using the remnants of the cluster or atoms,
thereby demonstrating a remarkable nanoscale analog of a real world construction
scheme. Using lateral manipulation procedure the tip is approached close enough to the
cluster to ensure an interaction strong enough to direct the cluster towards the hole or a
crater. The tip is then assigned a path that terminates at the center of the hole to be filled.
This is repeated in steps till the hole is sealed and the surface `reconstruction’ leaves no
mark of the indentations executed at the site.
The craters or holes are the defects on the surface and usually have a stronger binding
to the adsorbate than a normal terrace. When the cluster reaches at the periphery of the
crater during the manipulation process, it is adsorbed by the crater. The sequence of
images A-F in Figure 5.5 shows the step-by-step filling of the crater using the debris,
which originated from the same area during the tip-crash. After completing the process,
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the final area has no recognizable imprint of the crater [57]. This renders the surface
area defect free for further measurements if required.
Figure 5.5 Reconstruction of a surface at an atomic level by filling the holes on the
surface using atoms and clusters. Figure from [57].
5.3 Discussion
The surface state electrons of noble metal surfaces provide an ideal platform to study
the scattering mechanisms on the surface perpetrated by the charge density of these
electrons. Scattering off step edges and defects gives information about the oscillatory
nature of the charge density fluctuations. The scattering is isotropic in nature which is a
80
characteristic of the surface being probed. Lateral manipulation of atoms with an STM
takes this study further by creating artificial nanostructures (quantum corrals) using
single atoms. Characteristic standing wave patterns are generated within these structures
due to the constructive and destructive interference of the electrons waves. The
constituent atoms of the corrals act as point scatterers with equal participation from the
tip waves. Multiple scattering theory based on these concepts can generate wave patterns
within these structures.
The atoms and clusters created in situ for the nanoscale constructions can leave
craters on the surface as a result of trials that involve tip-surface contact. These craters
can be filled after the construction using unused atoms or clusters and the surface
imperfections can be erased for aesthetics and measurements demanding a defect-free
area.
In conclusion, manipulation techniques carried out using a UHV-LT-STM bring out
the versatility of STM and broaden the scope of exploration of metal surfaces.
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Chapter 6
Atom Selective Force Measurement
This chapter takes the study of single atom manipulation to a quantitative force
measurement. Two types of atoms, a metal atom, Ag and a halogen atom, Br are chosen
for this experiment. Extensive lateral manipulation measurements are carried out to
determine the minimum threshold distance between the Ag-Ag and Ag-Br necessary for
manipulation. From the lateral manipulation signals the specific angle at which the atom
moves is measured. The interaction energy between these atoms is obtained from density
functional theory calculations. A combination of these results yields a numerical value of
threshold lateral force required to move these atoms. The experiment is described in
detail followed by a discussion of the results.
6.1 Introduction
Lateral manipulation (LM) of the atoms controls the interaction between the tip and
the atom to bring about a change in the spatial location of the atom. Atoms adopt
different modes of manipulation in their attempt to balance the competing interactions
presented by the proximity of the tip and the substrate corrugation [32]. The presence of
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the tip modifies the potential energy landscape of the surface in such a way that the
atom overcomes the barriers presented by the surface and moves on to the next favorable
site [49]. Intricate pathways traced by atoms when manipulated at different directions on
an fcc(111) surface prove to be insightful in understanding the mechanisms involved in
manipulation [34]. A comprehensive investigation of LM for metal atoms like Ag and Au
reveals a linear relationship between threshold tunneling current and tunneling voltage
for small values of bias [19, 34]. In the present work, a metal atom, Ag and a halogen
atom Br, is each subjected to several LM trials to understand how the chemical nature of
the atom is reflected under identical manipulation conditions, with same tip and substrate.
The lateral force necessary for manipulation is then determined.
6.2 Force Measurement Strategy
Lateral manipulation of an atom is recorded as the variation of tip height with the
lateral distance traversed by the atom on the surface when the manipulation is carried out
with a closed feedback loop [32, 34]. Under competing influences of the substrate and the
tip, the atom jumps under the influence of the tip and follows the trajectory assigned to
the tip. The contour of the first jump of the atom makes a characteristic angle φ with the
surface (Figure 6.1).
83
Figure 6.1 Atomistic force measurement strategy. An STM tip-height signal
(manipulation signal) shows single atomic site hopping of the atom on Ag(111) surface
when the atom is manipulated along a close-packed surface direction. The drawings
indicate the changes of vertical and lateral force components during manipulation.
Initially only a vertical force component exists but when the tip traces the atom contour,
the lateral force component increases. When the lateral force component overcomes the
surface force, the atom hops to the next surface site. Here, ‘φ’ is the force vector angle
and ‘r’ is the tip-atom distance.
84
The cosine of this angle is a crucial factor for determination of lateral force needed
for manipulation. The numerical value of the lateral force is obtained by using equation
(1) and the total force is given by equation (2):
FLat =
dU (r )
× cos ϕ
dr
--------- (1),
FTot =
dU (r )
dr
--------- (2)
where U(r) is the interaction energy between the atoms
r is the separation between the atoms
φ is the angle between the tip and the atom
U(r) is obtained from density functional theory (DFT) calculations. The distance between
the atoms is obtained from threshold resistance measurements of Ag and Br atoms.
Extensive lateral manipulation trials (925 trials for Ag, 1150 trials for Br) are carried out
to determine the threshold resistance. The angle φ is measured from LM signals. Using
equation (1) and (2) a numerical value of the lateral force and total force is calculated
respectively.
6.3 Experimental Details
The present experiment involves the study of Br and Ag atoms on an Ag(111)
surface. Distinction between two different types of adsorbed atoms in an STM image is
difficult without the assistance of an additional investigative technique. Cu and Co atoms
on Cu(111) could be distinguished by detecting Kondo signal from Co atom using
85
tunnelling spectroscopy. Single Ag atoms have been imaged on Ag(111) surface at 4K
and they appear as protrusions on the surface [20, 26, 34]. There is no known
characteristic of a Br atom to identify and distinguish it from an Ag atom. Also single Br
atoms have not been investigated on Ag(111) to date. Hence codeposition of these atoms
would present itself with the complexity of identification of these atoms from the
topography in the STM image. To circumvent this issue these atoms were extracted from
obtained by bond dissociation of a cobalt porphyrin molecule.
The experiment was performed using our homebuilt ultrahigh vacuum scanning
tunneling microscope. A single crystal Ag(111) sample was cleaned by several cycles of
sputtering and annealing. This was followed by vapor deposition of cobalt porphyrin
molecules, (5,10,15,20-Tetrakis-(4-bromophenyl)-porphyrin-Co(II)), referred to as
CoTBrPP henceforth. The sample was then transferred to the STM chamber and cooled
to 4K for the manipulation experiment.
In an STM experiment it is important to have a reliable tip apex for unambiguous data
and reproducibility of results. Standard methods of tip preparation involve
electrochemical etching but once the tip is in imaging environment the control over its
apex structure diminishes rapidly. In our experimental setup we can modify the tip apex
by tip-sample indentations prior to the experiment and even in the midst of it [11]. This
coats the tip apex with silver atoms and improves the resolution of the image as shown in
Figure 6.2.
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Figure 6.2 A series of images before tip crash (a) and after tip crash (b) showing a sharper
image resolution after the tip crash. The crash site is marked.
Cobalt porphyrin molecules selfassemble on Ag(111) in various patterns forming
domains with characteristic ordering on the terrace and along the step edges as well. They
can also be seen in random clusters (Figure 6.3). They are chemisorbed on the surface.
Figure 6.3 CoTBrPP molecules on Ag(111) surface seen in domains (a) and in a
cluster arrangement (b).
87
6.4 Br Atoms
Br atoms are extracted from CoTBrPP molecules. Lateral manipulation of Br atoms is
carried out on the Ag(111) surface along the closepacked row direction [110] for
different values of manipulation voltage and current. The threshold resistance to move a
Br atom is obtained from this data. The distance between the tip and Br atom
corresponding to this resistance is read from Iz spectroscopy curve. A height correction is
made to this distance r. Then by using the Ag-Br interaction energy curve, the energy
U(r) corresponding to r is traced. The angle φ is determined from a LM curve. Using all
these values in equation (1) a numerical value of force is obtained. All these steps are
described in detail in the following sections.
6.4.1 Extraction of Br Atoms
The individual CoTBrPP molecule is seen as four lobes. In accordance with the
chemical structure of the molecule, the 4 lobes correspond to the phenyl groups that are
attached to the central porphyrin ring. The region of interest in this molecule is the C-Br
bond which is a constituent of the phenyl lobes. So in principle each molecule can
provide four Br atoms (Figure 6.4).
88
Figure 6.4 STM image of CoTBPP single molecule taken at V=1000 mV, I = 1nA (Image
size 38 Å x 28 Å) along with a ball model based on the chemical structure of the
molecule.
The Br atom can be thereby obtained by scission of this bond. Dissociation of
molecules using tunneling electrons is a powerful technique and can be carried out with a
great degree of precision [6, 58]. Br atom is attached to the phenyl ring. The bonds in this
region of interest include C-C, C-H and C-Br. The C-Br bond has the lowest strength out
of all 3 bonds [58]. To estimate the bond dissociation energy for the C-Br bond
dissociation was performed by positioning the tip above one of the lobe of the molecule.
The voltage was ramped from -1.5 V to 2 V. The corresponding tunnelling current
showed an abrupt drop at 1.8 V.
89
Thus it was established that the C-Br bond
dissociation needs a minimum energy of 1.8V (Figure 6.5).
Figure 6.5 Dissociation of phenyl in CoTBrPP molecule by I-V spectroscopy showing an
abrupt drop in the current the breaking of the C-Br bond.
To extract the Br atom, the C-Br bond was dissociated using a voltage pulse of 2 V.
The variation of tunnelling current with time showed the characteristic change in the
current. The images of CoTBPP before and after dissociation show the change in the
molecule at the dissociation site along with the extracted bromine atom. The images and
the trace showing a drop in the tunnelling current is shown in Figure 6.6.
90
Figure 6.6 Dissociation of CoTBPP molecules on Ag(111). The dissociated Br atom is
seen in the image. Image parameters are: V= 1000mV, I= 1.2nA, size 52 Ǻ x 32 Ǻ.
6.4.2 Threshold Resistance Measurement of Br Atoms
The Br atom obtained from bond dissociation is used for extensive lateral
manipulation measurements. The atom is moved along the closepacked row direction
[110] of Ag(111) surface (Figure 6.7).
91
Figure 6.7 A series of images showing dissociation of Br atom and its subsequent lateral
manipulation. Image parameters: V=1000 mV, I = 0.44nA, size 90 Å x 40 Å.
To get the current-voltage relationship during manipulation, a value of threshold
resistance Rth is determined from the probability of Br atom manipulation. The
probability of Br atom manipulation is obtained from the ratio between the distance
traveled by the atom (Latom) and the lateral tip movement distance (Ltip). The values of
Latom/Ltip are plotted as a function of tunneling current. The plots show a characteristic
step function form. There is a specific value of current for each value of voltage marking
the onset of manipulation. This is repeated for different values of manipulation voltage.
92
A set of the probability plots for manipulation at positive voltage are shown in
Figure 6.8 plots for manipulation at negative voltage are shown in Figure 6.9.
Figure 6.8 Probability plots for Br atom manipulation at positive voltages.
93
Figure 6.9 Probability plots for Br atom manipulation at negative voltages.
The threshold current values recorded at each bias voltage are plotted as a function of
the bias voltage. The plot is linear and an inverse of the slope gives the value of threshold
resistance. This value is 499±11 kΩ. The linear nature of the plot implies an ohmic
relationship between the current and voltage during manipulation (Figure 6.10).
94
Figure 6.10 Linear relationship between current and voltage during Br atom
manipulation.
6.4.3 Lateral Manipulation for Angle Measurement
Lateral manipulation signals for Br atom manipulation are used to obtain the value of
the angle φ. Figure 6.11 shows the signals at positive voltage; Figure 6.12 shows the
signals at negative voltage.
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Figure 6.11 LM signals during manipulation at positive voltage.
Figure 6.12 LM signals during manipulation at negative voltage.
96
6.4.4 Height Correction for Br Atoms
The junction resistance in an STM experiment is a measure of the distance between
the tip and the sample. The calibration is done by Iz spectroscopy as explained in Chapter
2. Based on this distance calibration a threshold resistance of 499 kΩ corresponds to a
distance of 1.7 Å. However as the tip-sample distance calibration is done for Ag atoms,
the difference in the height of Br atoms a need to be accounted for as the tip approaches
Br atoms. The difference in height of Ag and Br atoms from STM images is 0.6 Å. This
implies that the tip needs to approach the Br atom at a closer distance to get the set
tunneling current. The same scenario prevails for lateral manipulation in a constant
current mode. Hence the real distance for manipulation is corrected to 1.27 Å (Figure
6.13).
Figure 6.13 Height correction scheme for Br atoms.
97
6.4.5 Numerical Value of Force
The potential energy curve for Ag-Br is calculated using GAMESS package [64]. For
the present calculation B3LYP hybrid density functional theory was used with SBKJC
basis set which uses pseudopotentials for core level electrons [59]. A 4-atom-ag-cluster
was used to represent the Ag(111) surface with a Br atom on the three-fold hollow site.
The total energy of the system was calculated as a function of interatomic seperation
(Figure 6.14).
Figure 6.14 Interaction energy between Ag-Br atoms as a function of the separation
between the atoms is shown in (a). The plot is rescaled to Ag-Br bond energy and to the
distance between outer Van der Waal radii of the two atoms, shown in (b).
The numerical value of the lateral force is obtained using equation (1) from Section 6.2
FLat =
dU (r )
× cos ϕ ----------------------- (1)
dr
98
The total force is given by
FTot =
dU (r )
dr ---------------------------------- (2)
The values of r are read from the R-z calibration curve, Figure 4.8. The values of U(r)
corresponding to r are read from the rescaled energy curve, Figure 6.14 (b). For different
LM curves, the angle φ is determined. The total force is calculated using equation (2).
The angle φ is plotted as a function of the total force as shown in Figure 6.15.
Figure 6.15 Measurement of angle φ as a function of total force between the tip and the
Br atom.
The angle φ is almost constant for the small tip-atom distance variation range, mean
φ=84º, cos φ =0.1045. The lateral force Flat is obtained using equation (1), the average
99
lateral force amounts to 50 pN for Br atoms. The total force Ftot is obtained using
equation (2), the average value is 478 pN.
Refer to Table 1 and Table 2 for the values of U(r), r, Ftot and Flat. The description of the
tables is as follows:
R kΩ is the threshold resistance measured from the experiment.
r Å is the distance between the outer Van der Waal radii of the atoms Ag and Br.
ΔE eV is the energy in eV corresponding to r obtained from the rescaled interaction
energy curve in Figure 6.14(b).
U(r) = - ΔE
Ftot = U(r)/r in pN
Flat = Ftot cos φ, mean φ=84º, cos φ =0.1045
Table 1. Values of energy and distance from the Ag-Br interaction energy curve.
R kΩ
510
499
488
rǺ
ΔE eV
1.274
-0.3758
1.265
-0.3779
1.258
-0.3805
U(r) eV
0.3758
0.3779
0.3805
Table 2. Values of total force and lateral force calculated using equation (1) and equation
(2) and values from Table 1.
R kΩ
510
499
488
Ftotal =[U(r)/r] pN
472
478
484
Flat = [Ftot cos φ] pN
49.3
49.9
50.6
Thus, to conclude, the total force between the Ag atom at the tip and Br atom under
manipulation is 478 pN. To overcome the surface forces in order to move the Br atom
from one atomic site to the other, a lateral force of 50 pN is necessary. Manipulation
100
carried out at negative or positive bias does not influence the force required for
manipulation. This is also reflected in the threshold resistance measurement results, the
plot is linear for both positive and negative manipulation voltages.
6.5 Ag Atoms
The study of Ag atoms on Ag(111) surface follows the same steps as those for Br
atoms. The following sections describe the Ag atom manipulation in detail.
6.5.1 Extraction of Ag Atoms
The technique for extraction of Ag atoms is similar to the one described in Chapter 4.
A cluster of Ag atoms is created from the substrate by a controlled tip-surface indentation
[51]. Then the interaction between the tip and the cluster is increased using lateral
manipulation technique. The tip is moved along a protrusion on the cluster to the
substrate. The strong tip-cluster interaction causes local bond breaking to remove a single
atom from the cluster. The extracted atom is seen in the next image (Figure 6.16).
101
Figure 6.16 Extraction of Ag atoms from an Ag cluster. (Image parameters: V= 50 mV,
I= 1 nA, size 90 Å x 30 Å).
6.5.2 Threshold Resistance Measurements for Ag Atoms
Manipulation of Ag atoms is carried out at positive and negative bias voltages. For
each value of voltage the threshold current at which the atom moves is determined. The
probability of atom manipulation defined by the ratio Latom/Ltip is plotted versus the
current. Figure 6.17 shows some plots for manipulation at positive voltages and Figure
6.18 shows some plots for manipulation at negative voltages.
102
Figure 6.17 Ag atom manipulation at positive voltages.
103
Figure 6.18 Ag atom manipulation at negative voltages.
The threshold current values for each manipulation voltage are plotted as a function of
the applied voltage (Figure 6.19).
104
Figure 6.19 Linear relationship between current and voltage during Ag atom
manipulation.
The threshold resistance is obtained from the inverse of the slope of this plot. The
value is 255±3 kΩ. This corresponds to a distance of 1.3 Å from Iz spectroscopy curve.
6.5.3 Lateral Manipulation for Angle Measurement
The angle for lateral manipulation is read from the lateral manipulation curves. Figure
6.20 shows two curves used to determine the corresponding angle.
105
Figure 6.20 LM curves to measure the angle made by the tip and the Ag atom during
lateral manipulation.
6.5.4 Numerical Value of Force
The potential energy curve for Ag-Ag is calculated using GAMESS package [64]. A
4-atom-ag-cluster was used to represent the Ag(111) surface with an Ag atom on the
three-fold hollow site. The total energy of the system was calculated as a function of
interatomic seperation (Figure 6.21).
106
Figure 6.21 Interaction energy between Ag-Ag atoms as a function of the separation
between the atoms (a). The plot is rescaled to Ag-Ag bond energy and to the distance
between outer Van der Waal radii of the two atoms, shown in (b).
The numerical value of force is obtained using equation (1) and equation (2) as discussed
in Section 6.2
FLat =
dU (r )
× cos ϕ --------------------------- (1)
dr
The total force is given by
FTot =
dU (r )
dr -------------------------------------- (2)
The values of r are read from the R-z calibration curve, Figure 4.8. The values of U(r)
corresponding to r are read from the rescaled energy curve, Figure 6.21 (b). For different
LM curves, the angle φ is determined. The total force is calculated using equation (2).
The angle φ is plotted as a function of the total force as shown in Figure 6.22.
107
Figure 6.22 Measurement of angle φ as a function of total force between the tip and the
Ag atom.
Refer to Table 3 and Table 4 for the values of U(r), r Ftot and Flat . The description of the
tables is as follows:
R kΩ is the threshold resistance measured from the experiment
r Å is the distance between the outer Van der Waal radii of the atoms Ag and Ag.
ΔE eV is the energy in eV corresponding to r obtained from the rescaled interaction
energy curve.
U(r) = - ΔE
Ftot = U(r)/r in pN
Flat = Ftot cos φ, mean φ=80.5º, cos φ =0.1650
108
Table 3. Values of energy and distance from the Ag-Ag interaction energy curve.
R kΩ
rǺ
ΔE eV
258
256
253
1.298 -0.2661
1.294 -0.2725
1.290 -0.2797
U(r) eV
0.2661
0.2725
0.2797
Table 4. Values of total force and lateral force calculated using equation (1) and equation
(2) and values from Table 3.
R kΩ
258
256
253
Ftotal =U(r)/r pN
328
337
347
Flat = [Ftot cos φ] pN
54.1
55.6
57.2
From Table 4, a lateral force of 55 pN is necessary to move an Ag atom on an
Ag(111) surface. The interaction between Ag-Ag is metallic in nature [44]. It is
delocalized and weaker than an ionic or covalent bond. In comparison with the total force
required for Br atom manipulation, Ag atoms need less force for manipulation on an
Ag(111) substrate.
6.6 Discussion
Interactions between two types of atoms, Ag and Br, on an Ag(111) surface were
investigated using the homebuilt UHV-LT-STM. The Ag and Br atoms were extracted
from two different sources in situ. Lateral manipulation trials of these atoms were carried
out for different values of applied bias voltage. The threshold resistance and the
109
necessary for manipulation were determined. The minimum lateral force required for
manipulation on Ag(111) surface was calculated. Table 5 summarizes these results.
Table 5. Values of threshold lateral force, total force and threshold resistance for lateral
manipulation of Ag and Br atoms.
Parameter
Ag atoms
Br atoms
Flat pN
55
50
Ftot pN
337
478
Rth kΩ
255
499
The interaction between Ag and Br is ionic in nature and more localized [60-63].
Hence a stronger lateral force is essential to overcome the barrier posed by this
interaction. The metallic Ag-Ag interaction is weaker in comparison and delocalized. The
Ag atom can be manipulated with a smaller force. The most important finding of this
study is that the interaction between the tip and atom carries an imprint of the inherent
bonding nature of the atom.
The structure of the tip apex can influence the manipulation of single atoms. Also the
manipulation trials are repeated for different tip shapes and for a large number of Br and
Ag atoms to minimize the error induced by this entity.
Although force and interactions between the tip and the sample have been
investigated using STM, our results present the first systematic enquiry of these short
range forces scaling down to a single atom. The threshold lateral force necessary to move
the atom is quantitatively estimated. This study can be extended to a combination of
110
different types of atoms thereby paving the way for probing interactions at an atomic
level. It boosts the potential of single atom manipulation to a new frontier where tuning
and control of interactions for applications using the bottom up approach of
nanotechnology can be developed further.
111
Chapter 7
Conclusions and Outlook
In this dissertation we have explored two diverse systems in the purview of atomic
and molecular manipulation techniques. These systems, a cluster of metal atoms and
single atoms, were subjected to a comprehensive investigation by using STM tip induced
manipulations. The entire study was carried out on a metallic silver substrate using our
homebuilt ultrahigh vacuum low temperature scanning tunneling microscope.
For an inquiry into tip-cluster interactions, a cluster of silver atoms was created by a
controlled tip-sample contact. The technique of lateral manipulation was used to extract
single silver atoms from the cluster. The proximity of the tip to the cluster was changed
by tuning the manipulation parameters. The lateral manipulation signals provided an
insight into the dynamics of the event. The relationship between the current and voltage
parameters used for extraction was found to be linear. The threshold distance necessary to
pull an atom out from the cluster was determined. This distance was found to be much
smaller than the threshold distance required to move a single atom on the surface. This
proves that atom extraction from the cluster demands a much stronger interaction than
that required for the relocation of an atom on the surface. The sole mechanism guiding
the extraction was the tip-cluster distance. The geometrical size of the cluster did not
pose any constraints for the extraction process. A modeling study was carried out for
112
such a tip-cluster system. The presence of the tip was responsible for a modification
in the potential energy surface of the cluster. This led to a lowering of the energy barrier
between the tip-cluster system that facilitated the extraction of the atom from the cluster.
The results from the modeling study are thus in agreement with the experimental results.
This study of atom extraction from a metallic cluster presents a novel application of the
lateral manipulation technique. The precise control over the process at the nanoscale can
be seen as one of the ways to produce single atoms for a manipulation experiment.
We have used lateral manipulation to explore the interactions between the STM tip
and two atoms of different chemical identity adsorbed on a metal substrate. With a silver
atom at the tip apex, a silver atom and a bromine atom adsorbed on Ag(111) surface were
investigated. The silver atom was extracted from a cluster of silver atoms. The bromine
atom was dissociated from a cobalt porphyrin molecule using the vertical manipulation
technique. Lateral manipulation of these atoms was carried out for different values of
voltage and current. The threshold distance necessary to move these atoms was
determined from the manipulation trials. The angle marking the first jump of the atom in
response to the tip was measured from the lateral manipulation curves. The interaction
energy curves for Ag-Ag and Ag-Br atoms were calculated using density functional
theory. The interaction energy corresponding to the threshold manipulation distance for
Ag and Br was measured from these curves. A numerical value of force was obtained
using a combination of all these parameters. This force represents the threshold force
necessary to move a silver atom and a bromine atom on the surface. The values of the
force obtained carry a signature of the intrinsic bonding nature of the atom. The
interaction between the tip apex silver atom and the silver atom on the surface is metallic
113
in nature. The interaction between the silver atom at the tip apex and the bromine
atom is a combination of ionic and metallic character. We find that a stronger force is
necessary to move the bromine atom as compared to the silver atom. The interaction of
the atoms with the substrate plays a significant role in directing their response to the atom
at the tip apex. The choice of same substrate and same tip for manipulation of both atoms
is crucial to sense the details of the inherent nature of the atom. Our study presents the
first quantitative force measurement at an atomic level the STM lateral manipulation on a
metal surface.
We have also exploited the potential of the STM manipulation techniques to construct
nanostructures on the surface. Single atoms were assembled on predetermined locations
on the surface using lateral manipulation; the source of atoms was a cluster of silver
atoms extracted from the native substrate. The holes or imperfections on the surface
created during the construction were sealed by using these atoms. This demonstrates the
role of the STM tip as a construction tool at the nanoscale.
In future the strategy of exploring interactions at the atomic level demonstrated in this
work could be extended to magnetic systems. For example, using a magnetic atom at the
tip apex for the manipulation of a magnetic atom on a metallic substrate, the spin-spin
interaction between these atoms could be probed at a single atom level. This would
further the current understanding of magnetic interactions on a metal surface. Another
interesting application would be to determine the force necessary to manipulate
molecules on a metallic substrate. New experiments are being initiated in this direction.
Lateral manipulation of molecules induces conformational changes in the molecule [28].
The ability of a molecule to switch between the conformations in response to STM tip
114
induced manipulation provides a new research direction to explore the functional
aspect of molecules from the viewpoint of device applications [10]. The technique of
force measurement could be applied for various conformations of the molecule.
Depending on the adsorption of the molecule on the substrate and the related
conformations, a suitable manipulation technique could be selected to change the
conformation of the molecule. The threshold force necessary for a conformational change
could provide relevant information about interaction between the molecule-substrate
system.
115
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Appendix A
List of Publications
ƒ
Aparna Deshpande, Kai-Felix Braun, Saw-W. Hla, “Piconewton force
measurement in single atom manipulation”, submitted
ƒ
Aparna Deshpande, Dandapani Acharya, Joel Vaughn, Saw-W. Hla, Handan.
Yildirim, Abdelkader Kara, Talat Rahman, “Atom-by-atom extraction using
scanning tunneling microscope tip-cluster interaction”, Phys. Rev. Lett. 98 (2007)
028304.
ƒ
Aparna Deshpande, Kendal Clark, Dandapani Acharya, Joel Vaughn, Kai-F.
Braun, Saw-W.Hla, “Atomistic Constructions by using Scanning Tunneling
Microscope Tip”, IEEE-NANO 2006, 6th IEEE Conference on Nanotechnology,
Conference proceedings.
ƒ
Violeta Iancu, Aparna Deshpande, Saw-W. Hla, “Manipulating Kondo
Resonance in Two-Dimensional Molecular Self-Assembly”, Phys. Rev. Lett. 97
(2006) 266603.
ƒ
Violeta Iancu, Aparna Deshpande, Saw-W. Hla, “Manipulating Kondo
Temperature via Single Molecule Switching”, Nano Lett. 6 (2006) 820-823.
ƒ
Violeta Iancu, Aparna Deshpande, Saw-W. Hla, “Controlling formation of
nanodots and nanocavities using scanning tunneling microscope”, IEEE-NANO
2006, 6th IEEE Conference on Nanotechnology, Conference proceedings.
123
ƒ
Saw-W. Hla, Kai-Felix. Braun, Violeta Iancu, Aparna Deshpande, “Single
atom extraction by scanning tunneling microscope tip-crash and nanoscale surface
engineering”, Nano Lett. 4 (2004) 1997-2001. Appeared in ‘Science News’,
‘Smashing the Microscope: Tiny crashes harnessed for nanoconstruction’,
11/06/2004.
124
Appendix B
List of Contributed Talks and Posters in
Conferences
ƒ
“Atomistic constructions using Scanning tunneling microscope tip”, Oral
Presentation at the 6th IEEE Conference on Nanotechnology, July 2006
ƒ
“Controlling Formation of Nanodots and Nanocavities Using Scanning
Tunneling Microscope”, Oral Presentation at the 6th IEEE Conference on
Nanotechnology, July 2006
ƒ
“Force measurement with STM”, Poster Presentation Ohio Center for
Technology and Science (OCTS) summit, April 2006
ƒ
“Atom selective force measurement with STM”, Oral Presentation at APS
March meeting, March 2006
ƒ
“Atom-by-atom extraction by controlling a scanning tunneling microscope tipcluster interaction”, Oral Presentation at APS March meeting, March 2005
ƒ
“Atom-by-Atom Extraction using Scanning Tunneling Microscope Tip-Cluster
Interaction”, Poster Presentation Ohio Center for Technology and Science
(OCTS) summit, March 2005
125
ƒ
“In-situ tip preparation and nanoscale surface modification using STM
manipulation”, Poster Presentation AVS 50th International Symposium and
Exhibition, November 2003
ƒ
“Atomic-scale surface modification and in-situ tip preparation using STM
manipulation”, Poster Presentation APS March Meeting, March 2003