Surface science (Maarten Vos Ph 54985, Office: RSPE Cockcroft 4.04email: maarten.vos@anu.edu.au)slides at wattle (powerpoint or pdf)

Goals of the courseGet 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 lectures4 sessions of student presentations1 question session/lab tour1 exam

Two homework assignments: to be handed in at Monday April 29 lectureand Monday May 13 lecture.

Contents of student talk will not be part of the exam, but I will try to makeexam 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=13128e764d9232dad4ee66cc53a24c9d&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 Chemistryhttp://www.chem.qmul.ac.uk/surfaces/scc/

Roger M. Nix. School of Biological & Chemical SciencesQueen 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)

(MITopencoursewarehttp://ocw.mit.edu/courses/materials-science-and-engineering/3-051j-materials-for-biomedical-applications-spring-2006/lecture-notes/ 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 for atom-atom Collisions:

p

kT

n

2

1

2

1

With p the pressure and thecross section of an atom ( =d2 wit d the radius of the atom) and n the particle density.

(Note the This is due to the fact that all atoms are moving.)

2

see link for a derivation

Outline derivation of:

See ref[4] section 3.2 for full derivation

Density of atoms moving in direction d4

dn

The speed distribution of the molecules is given by f(v).

Number of particles N with velocity v and moving along direction dhitting surface area dS per unit time with speed between v and v+dv is then proportional to volume of the oblique cylinder (v dt cosdS), the speed distribution function and density of atoms moving along d

dsvd

dvvnfdt

N cos

4

The total rate Z is then obtained by integrating (1) over d and v.

5.0)2/(4

mkTpvn This can be written in a different form:

(homework question 1)

Impingement rate:

4

vnZa

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-daypressure measurements.

Also used ‘mbar’=0.001 bar = 0.1 kPa = 1 hPa (hectopascal) = 1,000 dyn/cm2

One monolayerAbout 1015 atoms/cm2

(actual value depends onCrystal surface)

There are roughly 1015 atoms/cm2,hence if all impinging atoms reactwith a surface the top layer can changevery quickly, in 10-9 seconds at ambient pressure.

Hence the requirement of good vacuumIn surface science.

At 10-9 torr (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 learnunder 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 work1960’s-surface analytical techniques developed(e.g. electron spectroscopy, low energy electron diffraction)-study of single crystal surfaces1980’s-invention of scanning probe microscopethis 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)

fraction of atoms at interface

surface science versus nano-science

In nano-science the property of asystem depends on the size of the system. In surface science this isnot the casee.g. a 1cm2 single crystal surface has the same colour as a 2 cm2 single crystal surface.

In practise the distinction is not alwaysclear.

How to make a good vacuum

“ first roughing”

relies on collisions between molecules, i.e. mean free path smaller than pumpopening (low vacuum) (above 10-3 torr)

in out a

a

a

a

b

b

b

b

(1) (2)

(3) (4)

from wikipedia

turbo pump

moves faster than the velocity of moleculesrotating blades transfer momentum to molecules

So works in the low pressure region where mean free path < pump dimensions

in

outfrom wikipedia

(high vacuum, 10-3 to 10-10 torr)

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)

(from Lesker.com)

evaporate atinside water cooledsurface