Surface science (Maarten Vos Ph 54985, Office: RSPE Cockcroft 4.04
email: maarten.vos@anu.edu.au)
slides at wattle (powerpoint or pdf)
Goals of the course
Get an appreciation of surface science and the wide range of problems that are
investigated under the banner of surface science.
Assessment:
Homework 20%
Student Presentation 20%
Exam 60%
Student Presentation:
Using a compilation of review papers of various topics studied in surface science,
each student will be asked to summarize one application in a 10-15 minutes
presentation. Emphasize will be on motivation of the area studied, rough explanation
of the techniques used, and type of results obtained, your opinion of this type of
research (a complete understanding of the review paper is not required).
3 student presentations per session
thus 10-15 minute, talk 5 minute discussion
Course structure:
17 sessions:
11 traditional lectures
4 sessions of student presentations
1 question session/lab tour
1 exam
Two homework assignments: to be handed in at Monday April 29 lecture
and Monday May 13 lecture.
Contents of student talk will not be part of the exam, but I will try to make
exam questions of topics presented in the traditional lectures, but illustrated in
the student presentation and following discussion.
We will use a compilation of review papers, published as the 500th issue of the journal
“Surface Science” for the student presentation.
I (will) put the table of contents on the web, plus some indication of articles that are
suitable. Pick a topic that is of interest to you, but do not waste too much time doing so.
I will give two “example presentations” as part of the traditional lectures. Do not use
these chapters marked in blue in the table of contents.
You can download the chapter of your choice from the web, if you do it at the ANU, or go
through the proxy of the library.
http://www.sciencedirect.com/science?_ob=PublicationURL&_cdi=5546&_pubType=J&_
acct=C000028338&_version=1&_urlVersion=0&_userid=554534&md5=13128e764d923
2dad4ee66cc53a24c9d&jchunk=500#500
or:
http://people.physics.anu.edu.au/~vos107/surfsci/contents_vol500.pdf
Tell me by email before May 1 which chapter you plan to do. If two people want to do
the same chapter, then it will go to the student that send me an email first.
If you have problems preparing your talk, come and see me.
This part of the course is a bit an experiment.
Good resources on the web:
[1] An Introduction to Surface Chemistry
http://www.chem.qmul.ac.uk/surfaces/scc/
Roger M. Nix. School of Biological & Chemical Sciences
Queen Mary, University of London
[2] Introduction to Surface Analysis
(http://www.cem.msu.edu/~cem924sg/LectureNotes.html)
Simon J. Garrett, Michigan State University
[3] Lecture notes on Surface Science
(http://www.philiphofmann.net/surflec/surflec.html)
Philip Hofmann Institute for Storage Ring Facilities (ISA) and
Interdisciplinary Nanoscience Center (iNANO), Arhus Denmark
[4] Atoms and Molecules at Surfaces
(www.nottingham.ac.uk/~ppzpjm/amshome.htm )
P. Moriarty, Nottingham U., UK
[5] XPS graduate course University Western Ontario
now http://mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf)
Surface Science Western, Canada (Roger Smart, Stewart McIntyre, Mike
Bancroft, Igor Bello & Friends)
[6] Bio surface science (various authors)
(MITopencourseware
http://ocw.mit.edu/courses/materials-science-and-engineering/3-051j-materials-for-biomedical-applications-spring-2006/lecturenotes/ bio-surface science
[7] surface and interface science Rutgers University (Bartynski + company)
http://www.physics.rutgers.edu/grad/627/ (quite detailed))
Why is surface/interface science important:
If we want to make things we rarely have a pure single material,
For example a transistor:
Three metal wires connected to Si piece with different levels of doping.
How does the current flow from the metal to the semiconductor and in between the
different doped layers?
Crystal growth: What happens at the
surface of a crystal if we want to try to grow
a new layer?
Islands
layer-by Layer
islands-on-layers
Gas-solid interaction:
Why does Aluminium not rust in the way iron does?
Why does the catalytic converter of your car makes the exhaust gas clean?
petrochemical industry example
octane rating: resilience against self-ignition
One process uses a platinum catalyst on a
zeolite base at a temperature of about
250°C and a pressure of 13 - 30
atmospheres. It is used particularly to
change straight chains containing 5 or 6
carbon atoms into their branched isomers.
Biological:
How do we extract oxygen when we
breath?
How do the membranes of cells work?
Surface science became possible due to the development in vacuum
technology.
Why is this?
Two reasons:
-Interpretation is easier if molecules above a surface travel along straight
lines i.e. do not collide with each other. Large Mean free path of molecules
above surface.
-Surfaces can change due to the interaction with gases surrounding it.
So what gas density can we allow above a surface and be confident that the
surface does not change during the experiment due to reactions with this
gas?
-Mean free path l for atom-atom
Collisions:
l
1
 1  kT 

 
2n  2  p 
With p the pressure and  the
cross section of an atom ( =p d2
wit d the radius of the atom) and n
the particle density.
(Note the
This is due to the
2
fact that all atoms are moving.)
see link for a derivation
Impingement rate:
See ref[4] section 3.2 for full
derivation
nv
Za 
4
Outline derivation of:
Density of atoms moving in direction dw:
n
dw
4p
The speed distribution of the molecules is given by
f(v).
Number of particles N with velocity v and moving
along direction dw hitting surface area dS per unit
time with speed between v and v+dv is then
proportional to volume of the oblique cylinder (v dt
cosq dS), the speed distribution function and
density of atoms moving along dw:
The total rate Z is then obtained by
N
dw
 nf (v )dv
v cos(q )ds
(1)
integrating (1) over dw and v.
dt
4p
nv
 p /( 2pmkT) 0.5
This can be written in a different form:
4
(homework question 1)
Pressure units
Official Pascal (Pa): Newton/meter2
Ambient pressure is 1 atmosphere  100 000 Pa
1 torr = 1mm Hg  133 Pa
The torr unit is still most frequently used in day-to-day
pressure measurements.
Also used ‘mbar’=0.001 bar = 0.1 kPa = 1 hPa
(hectopascal) = 1,000 dyn/cm2
One monolayer
About 1015 atoms/cm2
(actual value depends on
Crystal surface)
There are roughly 1015 atoms/cm2,
hence if all impinging atoms react
with a surface the top layer can change
very quickly, in 10-9 seconds at ambient pressure.
Hence the requirement of good vacuum
In surface science.
At 10-9 torr ( 112 less than ambient pressure)
it takes 1000 seconds to change top layer.
Note the tension
We want to explain day-today phenomena we observe at ambient pressure,
but we need ultra high vacuum (UHV) to do reproducible measurements.
This is often referred to as the ‘pressure gap’, but somehow things we learn
under UHV can be used to explain day-to-day phenomena.
brief history of surface science
start could be: Benjamin Franklin
(1706-1790) oil on water
“I fetched out a cruet of oil and dropped a little of it on the water. I saw it spread itself
with surprising swiftness upon the surface… Thoughnot more than a teaspoonful,
produced an instant calm over a space several yards square which spread
amazingly and extended itself gradually till it reached the lee side, making all that
quarter of the pond, perhaps half an acre, as smooth as a looking glass.”
Had Franklin made some simple quantitative calculations he would have found
out that if a teaspoonful (2 ml) of oil is spread over an area of half an acre, the
thickness of the film on the surface of water must be less than 2 nm.
1950’s
-ultra-high vacuum systems become available
transistor invented so people wanted to know how does it really work
1960’s
-surface analytical techniques developed
(e.g. electron spectroscopy, low energy electron diffraction)
-study of single crystal surfaces
1980’s
-invention of scanning probe microscope
this makes it possible to study inhomogeneous surfaces
more recent days:
surface science applied to biological systems, and nano-sized systems
(next few slides based on: Pure Appl. Chem., Vol. 83, No. 1, pp. 243–252, 2011.
Integration of surface science, nanoscience,
and catalysis Cun Wen, Yi Liu, and Franklin (Feng) Tao, a nice, recent (chemical oriented)
overview of the subject)
surface science versus nano-science
In nano-science the property of a
system depends on the size of the
system. In surface science this is
not the case
e.g. a 1cm2 single crystal surface has
the same colour as a 2 cm2 single
crystal surface.
In practise the distinction is not always
clear.
fraction of atoms at interface
How to make a good vacuum
“ first roughing”
in
(1)
out
(2)
b
a
b
(3)
a
a
(4)
b
a
b
from wikipedia
relies on collisions between molecules, i.e. mean free path smaller than pump
opening (low vacuum) (above 10-3 torr)
turbo pump
in
moves faster than the velocity of molecules
rotating blades transfer momentum to molecules
So works in the low pressure region where mean
free path < pump dimensions
(high vacuum, 10-3 to 10-10 torr)
out
from wikipedia
getter pump
e.g Titanium sublimation pump
-Evaporate a reactive material on walls of chamber
molecules colliding with walls will “stick”
 reduced pressure
(works only in ultra-high vacuum)
evaporate at
inside water cooled
surface
(from Lesker.com)