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Rehearsal questions in Surface Physics 2006
1.
In order to study clean surfaces one claims that ultra high vacuum is required. Explain
this and give an estimate of how low a pressure is needed if it shall take a few hours for
a monolayer of contaminants to develop on a freshly prepared clean surface.
2.
Describe four different methods for preparing clean surfaces in ultra high vacuum.
Explain what is meant by relaxation- and reconstruction-effects on surfaces.
3.
Electron spectroscopy is often used for chemical analysis of surfaces. Explain why and
how a high surface sensitivity can be obtained. If it is assumed that the material can be
described as built up by atom layers with a layer distance = d, show that the intensity
ratio can be expressed as;
Isurf/I
= exp{d/cos} – 1 (when refraction at the surface can be neglected).
bulk
4.
Describe the principles of photoelectron spectroscopy (XPS or ESCA) for chemical
analysis of surfaces. Also describe the information that typically is obtained as well as
the advantages and disadvantages when using this experimental technique.
5.
Describe the factors that come into play when one wishes to perform a careful
quantitative analysis using photoelectron spectroscopy (XPS or ESCA). Also describe
the methods typically used when performing a quantitative analysis.
6.
Describe the principles of Auger electron spectroscopy (AES) for chemical analysis of
surfaces. Also describe the information that one typically can obtain as well as the
advantages and disadvantages when using this experimental technique.
7.
Describe the factors that come into play when one wishes to perform a careful
quantitative analysis using Auger electron spectroscopy (AES). Also describe the
methods typically used when performing a quantitative analysis.
8.
Ion scattering experiments can be utilized for determining the chemical composition at
surfaces. Describe the principles of such experiments and why a scattering angle of 90o
or 180o often is selected.
9.
Woods notation is often used in order to specify the structure of the surface layer or an
overlayer. Upon adsorption on the (100) surface of Ag metal (an fcc metal) two ordered
overlayer structures were observed,
Ag(100) - (2 x2)R45o and Ag(100) - (3x2).
Draw pictures showing the atoms in the first substrate layer and the arrangement of
atoms for these two overlayers.
10.
Describe the principles of LEED. Specify the diffraction condition (equation) and
illustrate it using Ewalds construction. Based on this show that the lattice parameter, a,
at normal incidence of the electron beam, can be obtained as:
a = (h2+k2) /sin(2)
Explain the parameters involved in this relation. For simplicity you may assume
diffraction from a material having a simple cubic structure.
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11.
Give an account of the diffraction condition (equation) for scattering against a two
dimensional crystal structure and illustrate this using Ewalds construction.
Assume that the scattering against a three dimensional crystal is totally dominated by
the scattering from the surface layer atoms. Draw pictures of the atomic structure of the
(100) and (110) surfaces for a FCC metal and also of the diffraction patterns to be
expected from these surfaces under these assumptions
12.
In order to determine bond lengths and atomic positions, within the ”unit cell”, using
LEED requires a special type of measurement and for the interpretation a dynamic
theory has to be used. Explain how such measurements are done and explain why and ,
in short, how a dynamic theory is used.
13.
Describe the principles of RHEED. Illustrate the diffraction condition using Ewalds
construction. Based on this show that the lattice parameter, a, at close to glancing
incidence of the electron beam, can be obtained as:
a = L(h2+k2) /t
Explain the parameters involved in this relation. For simplicity you may assume
diffraction from a material having a simple cubic structure.
14.
Describe the two effects that often are used and how they are used for structure analysis
in ion scattering experiments.
15.
Describe the principles of a ”Field Ion Microscope”. Illustrate the experimental
geometry and explain how a picture is generated.
16.
Describe "Scanning Tunneling Microscopy", the principles of the method, how this
typically is realized and the type of information that can be obtained.
17.
For calculating the surface and bulk electronic structure of an ordered solid the "density
functional method" and the "local density approximation” are often used. Explain what
this means and what simplifications they introduce in such calculations.
18.
One contribution to the work function originate from the so called dipole layer at the
surface. Explain the origin of this dipole layer and the effect it has on the potential
energy of the electrons in the material, i.e. on the work function. Also explain why the
work function for a close packed metal surface in general is larger than for an open
(less densely packed) surface.
19.
Different types of surface states can arise in the electron structure of a solid. Explain
using the nearly free electron model and a two plane wave approximation the origin
of a surface state of the Shockley-type.
20.
Different types of surface states can arise in the electron structure of a solid. Describe
using a tight binding model the conditions to be fulfilled in order for a surface state of
the Tamm-type to appear.
21.
Describe how the band structure of a single crystal solid are mapped out using angle
resolved photoemission. Specify the entities that are conserved in the photoexcitation
event, the principles of how the energy bands Ei(ki) are mapped out and how one can
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distinguish between contribution of surface- and bulk-states. Also specify advantages of
using synchrotron radiation as the light source.
22. The bulk band structure and surface states can also be investigated using momentum
resolved inverse photoemission (KRIPES). Describe the principles of this experimental
method and how such studies often are carried out. Also state why inverse
photoemission is used and not conventional photoemission..
23.
24.
Below the band structure calculated along the <100> direction of two metals are shown.
Assume that you are about to do photoemission studies of the (100) surface of these two
metals and that you collect spectra at normal emission (i.e. e=0o). Where (in energy)
do you expect that surface states may appear in the spectra you collect. Also explain
why you expect surface states to appear at these energy locations.
The two figures below show 4f photoemission spectra recorded from gold and
samarium metal. Explain why the 4f spectrum of Au and Sm are so different and also
what the difference between the UPS and XPS spectrum of Sm indicates.
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25.
Surface shifted core levels can be detected in photoemission spectra from, for example,
the transition metals (d-band metals). You are planning to study the metals Hf and Ir,
whose ground state outer electron configurations are respectively 5s25p65d26s2(Hf)
and 5s25p65d76s2(Ir). Explain what sign of the surface shift you expect in these two
cases, i.e. if the surface core level shifts to larger or smaller binding energy.
26.
Describe, in short, how surface shifts can be utilized in surface segregation studies of
binary alloys, in particular when the elements are neighbors in the periodic table.
27.
Explain why surfaces of pure semiconductors, like Si and Ge, are particularly inclined
to reconstruct.
28.
The figure below shows how the measured work function of Si(111) varies with the
doping concentration. This is not what was expected since the location of the Fermi
level in the band gap is expected to show large variations with doping concentration. In
view of this give an explanation why the work function is almost independent of the
doping concentration.
29.
With STM one can obtain information about the local electron density of states.
Explain, in short, how one for example can locate different surface states on a
reconstructed Si surface.
30.
The diffracted intensity in a LEED experiment is temperature dependent. Applying the
Debye model one can show that:
I hk (T) = I hk (0)exp(const (
sin 2 T
) 2)
D
Describe how this expression is derived and state what typically is found concerning
the vibration amplitudes and the Debye temperature for the surface compared to the
values for the bulk.
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31.
In a metal a wave like perturbation in the charge density that is localized to the surface
can be obtained, so called surface plasmons. Derive an expression for the surface
plasmon frequency when the dielectric function of the metal is given by:
()= 1- p2/2
32.
Give an explanation for physisorption. Describe the origin of the attractive and
repulsive forces, and illustrate using a potential energy diagram
33.
Give an explanation for chemisorption. Illustrate using a potential energy diagram.
Describe how the outer energy level of the adatom typically is affected and how charge
transfer may occur (you can select Li and Cl as adatoms).
34.
Describe the differences in the type and strength of bonding between physisorption and
chemisorption.
35.
Explain how adsorbed atoms/molecules typically affects the work function of a metal.
You can use Li and Cl adsorption on a metal to illustrate the effects.
36.
Molecular or dissociative adsorption can result when molecules adsorb on a solid
surface. Describe how one, by photoemission studies, can distinguish between
molecular and dissociative adsorption. To exemplify this you may use CO adsorption
on a metal surface and O2 adsorption on GaAs.
37.
What is meant by a Schottky-barrier. Illustrate by drawing a figure and explain the
effect it has on the voltage-current characteristics of a metal- semiconductor junction.
38.
Oxygen adsorption on semiconductor surfaces can give completely different effects
dependent on the material. Describe if and how the Fermi level is pinned upon oxygen
adsorption n-doped Si(111) and GaAs(110) respectively. Also give a short
explanation to why so different results are obtained for these two surfaces.
39.
Electron Energy Loss Spectroscopy (EELS) and Infrared Reflection Absorption
Spectroscopy (IRAS) are often used for characterization of atoms and molecules
adsorbed on a surface. Describe the principles of these two methods and the information
that typically can be extracted using them.
40.
Explain how Surface Extended X-ray Absorption FineStructure (SEXAFS) studies
typically are carried out, the principles of the method, and the type of information that
can be extracted using it. Assume that you investigate a K-edge of an adsorbate atom on
a single crystal surface.
41.
What does NEXAFS stand for and of what use is this method? You may use a "COexample" for the explanation and illustration.
42.
Pronounced diffraction effects are observed also in photoemission investigations.
Describe some different ways of utilizing these and the type of information that
typically can be extracted.
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43.
The curves below show how the work function of some different surfaces of W are
affected upon adsorption of Cs. Explain why this special form of the curves is obtained.
44.
The local electron density of states calculated for an ideal Al/GaAs(110) interface is
shown in the figure below. Explain what these results indicate concerning Schottky
barrier, Fermi level-pinning, metal induced gap states etc.