Surface science (Maarten Vos Ph 54985, Office: RSPE Cockcroft 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 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 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. acct=C &_version=1&_urlVersion=0&_userid=554534&md5=13128e764d923 2dad4ee66cc53a24c9d&jchunk=500#500 or: Tell me by 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 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 ChemistryAn Introduction to Surface Chemistry Roger M. Nix. School of Biological & Chemical Sciences Queen Mary, University of London [2] Introduction to Surface AnalysisIntroduction to Surface Analysis ( Simon J. Garrett, Michigan State University [3] Lecture notes on Surface ScienceLecture notes on Surface Science ( Philip Hofmann Institute for Storage Ring Facilities (ISA) and Interdisciplinary Nanoscience Center (iNANO), Arhus Denmark [4] Atoms and Molecules at SurfacesAtoms and Molecules at Surfaces ( ) P. Moriarty, Nottingham U., UK [5] XPS graduate course University Western OntarioXPS graduate course University Western Ontario now Surface Science Western, Canada (Roger Smart, Stewart McIntyre, Mike Bancroft, Igor Bello & Friends) [6] Bio surface science (various authors) ( MITopencourseware notes/ notes/ bio-surface science [7] surface and interface science Rutgers University (Bartynski + company) (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 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: With p the pressure and  the cross section of an atom (  =  d 2 wit d the radius of the atom) and n the particle density. (Note theThis is due to the fact that all atoms are moving.) see link for a derivationlink

Outline derivation of: See ref[4] section 3.2 for full derivationref[4] Density of atoms moving in direction d  The speed distribution of the molecules is given by f(v). Number of particles N with velocity v and moving along direction d  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 cos  dS), the speed distribution function and density of atoms moving along d   The total rate Z is then obtained by integrating (1) over d  and v. This can be written in a different form: (homework question 1) Impingement rate:

Pressure units Official Pascal (Pa): Newton/meter 2 Ambient pressure is 1 atmosphere  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/cm 2 One monolayer About atoms/cm 2 (actual value depends on Crystal surface)

There are roughly atoms/cm 2, hence if all impinging atoms react with a surface the top layer can change very quickly, in seconds at ambient pressure. Hence the requirement of good vacuum In surface science. At 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 learn under UHV can be used to explain day-to-day phenomena.

brief history of surface science start could be: Benjamin Franklin ( ) 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, 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 a system depends on the size of the system. In surface science this is not the case e.g. a 1cm 2 single crystal surface has the same colour as a 2 cm 2 single crystal surface. In practise the distinction is not always clear.

How to make a good vacuum “ first roughing” relies on collisions between molecules, i.e. mean free path smaller than pump opening (low vacuum) (above 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 molecules rotating blades transfer momentum to molecules So works in the low pressure region where mean free path < pump dimensions in out from wikipedia (high vacuum, to 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)from Lesker.com evaporate at inside water cooled surface