Download EEW508 II. Structure of Surfaces

EEW508
II. Structure of Surfaces
Surface structure
Rice terrace
EEW508
II. Structure of Surfaces
Surface structure revealed by SEM and STM
Surface Chemistry
and Catalysis, second edition
G. A. Somorjai and Y. Li (2010)
Using STM (Scanning tunneling microscopy) or other techniques such as field ion microscopy (FIM) or LEED
(low energy electron diffraction), atomic model of surface structure can be determined.
EEW508
II. Structure of Surfaces
Terrace-step-kink model
Steps and kinks are line defects to distinguish them from atomic vacancies or adatoms,
which are called point defects.
Relative concentration of atoms in terraces, in line defects, or in point defects can be
altered, depending the methods of sample preparation.
EEW508
II. Structure of Surfaces
Terrace – flat surface
Stepped surface
Kinked surface
EEW508
II. Structure of Surfaces
Dislocations creat surface defects such as steps and kinks
Surface Chemistry
and Catalysis,
second edition
G. A. Somorjai and
Y. Li (2010)
On heterogeneous solid surface, atoms in terraces are surrounded by the largest number
of nearest neighbors. Atoms in steps have fewer, and atoms in kinks have even fewer.
In a rough surface, 10-20% of atoms are often step sites, with about 5% of kink sites.
EEW508
II. Structure of Surfaces
Limitation of Terrace-step-kink model
Terrace-step-kink model has the assumption of a rigid lattice where every surface atom is located in its
bulk-like equilibrium position and can be located by the projection of the bulk structure to that surface.
The vertical position of surface atoms is shifted from the atomic positions in the bulk– exhibiting a
significant contraction or ‘relaxation’ of the interlayer distance between the first and the second layer.
As the surface structure with less packing density, the contraction perpendicular to the surface becomes
larger.
Not only the vertical direction, but the relocation of surface atoms along the surface takes place. Also, the
adsorption of molecules or atoms lead to relocation of surface atoms to optimize the strength of the
adsorption-substrate bond.
EEW508
II. Structure of Surfaces
Determination of surface structure
– Low energy electron diffraction (LEED)
LEED produce the quantitative data on bond distance and angles as well as on location of
surface atoms and of adsorbed molecules.
EEW508
II. Structure of Surfaces
Surface Diffraction – LEED, X-ray diffraction, and atom diffraction
The de Broglie wavelength of a particle is given by
h
h
 
p
2mE
Where h is Planck’s constant, m is the mass of the particle, and E is the kinetic
energy of the particle
For electron, and He atoms
o
e ( A) 
150
E (eV )
o
He ( A) 
0.02
E (eV )
For X-ray
E  h 
hc

1.24 104
 photon( A) 
E (eV )
o
EEW508
II. Structure of Surfaces
Surface Diffraction – LEED, X-ray diffraction, and atom diffraction
Electrons with energies in the range of 10-200 eV and
helium atoms with thermal energy (~0.026 eV at
300K) has the atomic diffraction condition ( < 1A)
Glazing angle X-ray diffraction is used for surface and
interface structure studies
X-ray bombardment induced emission of electron 
photoelectron diffraction
EEW508
II. Structure of Surfaces
Principle of Low energy electron diffraction (LEED)
The single crystal surfaces are used in LEED studies. After chemical or ion-bombardment cleaning in UHV, the
crystal is heated to permit the ordering of surface atoms by diffusion to their equilibrium positions.
The electron beam (in the range of 10-200 eV) is backscattered. The elastic electrons that retain their incident kinetic
energy are separated from the inelastically scattered electron by applying the reverse potential to the retarding
grids. These elastic electrons are accelerated to strike a fluorescent screen and LEED pattern can be obtained.
Types of LEED
Video LEED : LEED patterns can be visualized on a fluorescent screen.
Dynamic LEED or called I-V curve: the intensity I of the diffracted beam is measured as a function of the kinetic
energy.
EEW508
II. Structure of Surfaces
LEED pattern of a Si(100) reconstructed surface.
The underlying lattice is a square lattice while the
surface reconstruction has a 2x1 periodicity. The
diffraction spots are generated by acceleration of
elastically scattered electrons onto a
hemispherical fluorescent screen. Also seen is
the electron gun which generates the primary
electron beam. It covers up parts of the screen.
EEW508
II. Structure of Surfaces
Example – Si(111)- (7x7)
DAS structure: dimer, adatom, and stacking fault
EEW508
II. Structure of Surfaces
Scanning Tunneling Microscopy – brief description
EEW508
II. Structure of Surfaces
Example – Si(111)- (7x7)
Gerd Binnig
and Heinrich
Rohrer
Nobel prize in
Physics (1986)
EEW508
II. Structure of Surfaces


If the surface unit-cell vector a ' and b ' that are different

from a and b obtained from the bulk projection, then
the surface unit vector can be related to the bulk unit
vectors



a '  m11a  m12b



b '  m21a  m22b
mij defines a matrix
 m11 m12 
M 

m
m
22 
 21
On unreconstructed surface
1 0
M 

0 1
EEW508
II. Structure of Surfaces
Unreconstructed surface of the face-centered crystal structure
EEW508
II. Structure of Surfaces
Unreconstructed surface of the body-centered crystal structure
EEW508
II. Structure of Surfaces
Unreconstructed surface of the diamond crystal structure
EEW508
II. Structure of Surfaces
For example, fcc (100) – (2x2)
EEW508
II. Structure of Surfaces
For example, fcc (111) – (2x2)
2 0
M 

0
2


EEW508
II. Structure of Surfaces
For example, fcc (110) – (2x2)
2 0
M 

0
2


EEW508
II. Structure of Surfaces
Abbreviated and Matrix Notation for a variety of superlattices
EEW508
II. Structure of Surfaces
Abbreviated and Matrix Notation for a variety of superlattices
EEW508
II. Structure of Surfaces
Notation of High-Miller-Index Stepped Surface
EEW508
II. Structure of Surfaces
Notation of High-Miller-Index Stepped Surface
EEW508
II. Structure of Surfaces
Notation of High-Miller-Index Stepped Surface
stepped
surface
6(111) x (100)
4(111) x (100)
kinked
surface
EEW508
II. Structure of Surfaces
Bond-Length Contraction or Relaxation
close-packed
less close-packed
EEW508
II. Structure of Surfaces
Chemical bonds and
surface reconstruction
EEW508
II. Structure of Surfaces
Strong chemical bonds
Ionic bonds: Na+ (cation) - Cl-(anion)
These oppositely charged cations and anions are attracted to one
another because of their opposite charges. That attraction is called
an ionic bond.
EEW508
II. Structure of Surfaces
Strong chemical bonds
Covalent bonds: H –F
both atoms are trying to attract electrons that are shared tightly
between the atoms. The force of attraction that each atom exerts on
the shared electrons is what holds the two atoms together.
EEW508
II. Structure of Surfaces
Strong chemical bonds
Metallic bonds :
Metal consists of metal ions floating in a sea of electrons. The
mutual attraction between all these positive and negative charges
bonds them all together.
the sharing of "free" electrons among a lattice of positivelycharged ions (cations),
EEW508
II. Structure of Surfaces
Dangling bonds
EEW508
II. Structure of Surfaces
Reconstruction – (2x1) Reconstruction of Si(100)
The (2x1) reconstruction of Si (100) crystal structure as obtained by LEED crystallography.
Note that the surface relaxation extends to three atomic layer into the bulk
EEW508
II. Structure of Surfaces
(7x7) Reconstruction of Si (111)
DAS structure: dimer, adatom, and stacking fault
LEED and STM image of (7x7) reconstructed structure of Si (111)
The total number of dangling bonds is reduced from 49 to 19
through this reconstruction.
EEW508
II. Structure of Surfaces
(7x7) Reconstruction of Si (111)
19 dangling bonds of (7x7) reconstructed surface
(12 adatom, 6 rest atom, 1 corner hole)
EEW508
II. Structure of Surfaces
Reconstruction on metallic surface – Ir(100)
(5x1) reconstruction
Bulk structure:the square lattice
Surface structure: hexagonally close packed layer
EEW508
II. Structure of Surfaces
Reconstruction on metallic surface –Ir (110) missing dimer row
(2x1) reconstruction structure
EEW508
II. Structure of Surfaces
Reconstruction – Ionic crystal
Ionic crystal consists of charged spheres stacked in a lattice.
EEW508
II. Structure of Surfaces
Surfaces with strong chemical bonds exhibits more drastic
rearrangement of surface atoms
Generally speaking, surfaces with weak chemical bonds
(van der Waals, hydrogen, dipole-dipole and ion-dipole)
exhibits less pronounced reconstructed structure
-- for example, Graphite (0001) surface
EEW508
II. Structure of Surfaces
Reconstruction of high-Miller-index surfaces
Reconstruction at
Cu(410) stepped
surface. Atoms in the
first row at the each
step become adatoms
which are pointed out
in the side view of the
reconstructed surface.
Roughening transition: If the surface is heated near the melting
temperature, the steps become curved and break up into small
islands
III. Molecular and Atomic
Process on Surfaces
EEW508
III. Molecular and Atomic Process on Surfaces
Structure of ordered monolayer
When atoms or molecules adsorb on ordered crystal surface, they usually
form ordered surface structure over a wide range of temperature and surface
coverages.
Two factors which decide the surface ordering of adsorbates are
Adsorbate-adsorbate(AA) interaction and adsorbate-substrate(AS) interaction
Chemisorption – adsorbate-substrate interaction is stronger than adsorbateadsorbate interaction, so the adsorbate locations are determined by the
optimum adsorbate-substrate bonding, while adsorbate-adsorbate interaction
decides the long-range ordering of the overlayer.
Physisorption or physical adsorption – AA interaction dominates the AS
interaction –the surface could exhibit incommensurate structures.
EEW508
III. Molecular and Atomic Process on Surfaces
Coverage of adsorbate molecules
Definition of coverage: one monolayer corresponds to one adsorbate atom or
molecules for each unit cell of the clean, unreconstructed substrate surface.
For example, the surface coverage of atom on fcc(100) is one-half a monolayer.
EEW508
III. Molecular and Atomic Process on Surfaces
Ordering of adsorbate molecules
Atomic oxygen on Ni (100)
Up to one quarter of the coverage: Ni(100)-(2x2)-O
Between one quarter and one half Ni(100)-c(2x2)-O
EEW508
III. Molecular and Atomic Process on Surfaces
Epitaxial Growth
With metallic adsorbates, very close packed overlayers can
form because of attractive force among adsorbed metal atoms.
When the atomic sizes of the overlayer and substrate metals
are nearly the same, we can observe a one-monolayer (1x1)
surface. This is called epitaxial growth.
EEW508
III. Molecular and Atomic Process on Surfaces
Adsorbate induced restructuring – Ni (100) – c(2x2) - C
Carbon chemisorption induced restructuring of the Ni (100) surface.
Four Ni atoms surrounding each carbon atom rotate to form reconstructed substrate.
EEW508
III. Molecular and Atomic Process on Surfaces
Adsorbate induced restructuring – Fe (110) – (2x2)-S
S-Fe (110), Sulfur-chemisorption-induced restructuring of the Fe(110) surface.
EEW508
III. Molecular and Atomic Process on Surfaces
Adsorbate induced restructuring of steps to multiple-height step
– terrace configuration
73 nm × 70 nm
90 nm × 78 nm
77 nm × 74 nm
Hydrogen: 1.7 atm.
“nested” missing-row
reconstructions
Oxygen: 1 atm.
fcc (111) microfacets
Carbon Monoxide: 1 atm.
Unreconstructed (111)
terraces separated by
multiple height steps
EEW508
III. Molecular and Atomic Process on Surfaces
Sulfur-chemisorption-induced restructuring of the Ir (110) surface
Open and rough surfaces reconstruct more readily upon chemisorption.
For example, fcc(111) surface restructure more frequently upon chemisorption than
do the closer-packed crystal faces.
EEW508
III. Molecular and Atomic Process on Surfaces
Penetration of atoms through or below the first layer
EEW508
III. Molecular and Atomic Process on Surfaces
Surface structure of alloy, AlCu
Cu84Al16 alloy (111) structure exhibiting 3 x  3 R30o
The surface composition is 50%
EEW508
III. Molecular and Atomic Process on Surfaces
Growth modes of metal surfaces
Auger signal of adsorbate