INTRODUCTION TO SURFACE SCIENCE We will start the course ...leung.uwaterloo.ca/CHEM/750/Lectures 2007/SSNT-2... · will first give an overview of some basic definitions. 1. ... Middle

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INTRODUCTION TO SURFACE SCIENCE General Reference: http://www.chem.qmul.ac.uk/surfaces/scc/ We will start the course with a general description of surfacescience, and then the three main topics: physical structure,electronic structure, and surface interactions and reactions. Wewill first give an overview of some basic definitions. 1. Why study surfaces? Nature itself is full of entities with large AREA/Volume or

surface − Biology is really surface science of living things! Heterogeneous catalysis − Chemistry that involves a solid

phase as the support Nanoscale science and technology − The “surface” region

plays a pivotal role in controlling properties of nanoscalematter.

2. What is surface science? Webster's Encyclopedic Dictionary: Surface the twodimensional boundary of a material body,having zero or constant or variable curvature;

http://www.chem.qmul.ac.uk/surfaces/scc/

http://www.m-w.com/



12 entries found for surface. The first 10 are listed below.To select an entry, click on it. For more results, click here.surface[1,noun]surface[2,adjective]surface[3,verb]control surfaceruled surfacescratch[1,verb]surfaceactive

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Main Entry: 1sur·face Pronunciation: 's&rf&sFunction: nounEtymology: French, from sur + face face, from Old French more at FACEDate: circa 16041 : the exterior or upper boundary of an object or body2 : a plane or curved twodimensional locus of points (as the boundary of a threedimensionalregion) <plane surface> <surface of a sphere>3 a : the external or superficial aspect of something b : an external part or layer on the surface : to all outward appearances

Science knowledge acquired by careful observation, bydeduction of the laws which govern changes and conditions,and by testing these deductions by experiment. 22 entries found for science. The first 10 are listed below.To select an entry, click on it. For more results, click here.sciencebehavioral scienceChristian Sciencecognitive sciencecreation sciencedomestic scienceearth science

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Main Entry: sci·ence Pronunciation: 'sI&n(t)sFunction: nounEtymology: Middle English, from Middle French, from Latin scientia, from scient, scienshaving knowledge, from present participle of scire to know; probably akin to Sanskrit chyatihe cuts off, Latin scindere to split more at SHEDDate: 14th century1 : the state of knowing : knowledge as distinguished from ignorance or misunderstanding2 a : a department of systematized knowledge as an object of study <the science oftheology> b : something (as a sport or technique) that may be studied or learned likesystematized knowledge <have it down to a science>

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=face

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=shed



3 a : knowledge or a system of knowledge covering general truths or the operation of generallaws especially as obtained and tested through scientific method b : such knowledge or sucha system of knowledge concerned with the physical world and its phenomena : NATURALSCIENCE

4 : a system or method reconciling practical ends with scientific laws <culinary science> 5 capitalized : CHRISTIAN SCIENCE

Surface science is a branch of study that is concerned with thefacts, principles and methods of surfaces. But the term meansdifferent things to different people. What does “surfacescience” mean to us? A solid surface or an interface (not liquid, not soft or slimy

membrane) Mesoscopic (“quantum”) phenomena − Scale range: 51000

nm Surface chemical bonds and surface reactions New materials and new technologies Expensive and equipment intensive field UltraHigh Vacuum (UHV) − Conventionally, though not

any more Highly employable fields such as semiconductor, chemical

and pharmaceutical industries Fun and interesting things to do in life Nanotechnology − One entry found for nanotechnology.Main Entry: nano·tech·nol·o·gy Pronunciation: "nanOtek'näl&jE

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=scientific

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=natural+science

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=scientific

http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=christian+science



Function: noun Date: 1987

: the art of manipulating materials on an atomic or molecular scale especially to buildmicroscopic devices (as robots)

The science of fabricating, characterizing and manipulating

structures from the atomic scale up to about 100 nm(usually involving the bottomup approach).

It’s all about the development of “control” in both the 3Dstructure and their associated properties

3. What is a surface? We can define a surface as “a twodimensional boundary ortermination of a solid” or a medium (such as liquid). An oftenused jargon is the word interface, which we can define as theboundary between two media (e.g. gasliquid, gassolid, liquidsolid, etc.). For the majority of this course, we will be dealing with a solidsurface, as well as a gassolid interface and a solidsolidinterface. Gassolid interfaces and the corresponding gassolidinterfacial reactions are important because they are involved inmany chemical processing steps in semiconductor industry, incatalytic conversion in automobiles, etc. The products of thesegassolid reactions often give rise to solidsolid interfaces (e.g.polymers on silicon, or metal on silicon or silicides, etc.)



EXAMPLE: Si(111) Surface [http://nanowiz.tripod.com/sisteps/si111.htm ] EXERCISE: Si has a unit cell dimension of 5.29 Å while Gehas a unit cell dimension of 5.65 Å. Use Surface Explorerwebbased tool to generate the (123) plane of Ge.

http://nanowiz.tripod.com/sisteps/si111.htm

http://surfexp.fhi-berlin.mpg.de/



Twodimensional symmetry A surface is an interesting

phase or medium because strictly speaking it has only twodimensions. As such, only 2D related properties exist for a



surface. In other words, the properties of a surface shouldbe quite different from those of a solid, because a solid is a3D medium. So, we can only use solidstate physics orsolidstate chemistry as a guide to explore these unique 2Dproperties. In other words, there are abrupt changes in thesystem properties with distance (the depth dimension). Schematically, we may picture an interface as:

adsorbates

top layer surfaceproperties second surface region third (reconstructionstarts) lower layers of substrate bulk properties

See the collection of structures at the Gallary of BALSAC pictures EXAMPLE: Different surface structures of Si(100)

http://www.fhi-berlin.mpg.de/~hermann/Balsac/pictures.html



EXAMPLE: Dehydrogenation of ethylene on Ni(111) by A.Hetey

Heterogeneous A surface contains many different parts:domains − slabs with the same crystallographic plane;boundaries − lines that separate domains;defects − deviations from the ideal surface with the perfect

periodicity.



For each of these, there are different bonding sites, each withdifferent activation energy and other thermodynamicproperties.



Mobile The activation energy for a surface atom required

to “move” from one site to another is usually less than halfof that is required for desorption (taking off from thesurface) or for diffusion (sinking into the surface). Theresidence time of a surface atom is about 1 second.

Positive surface free energy This is an important

thermodynamics property and is responsible for variouswetting, adhesion or lubrication and nucleation processes.

Usually not easy to study This is due to low surface

density, ~1014 to 1015/cm2 or 109 mole (or nanomole nmole).

Usually dirty I.e. covered with impurities. 4. How to study a surface? Define the properties of interest:

Relative atom positions or physical structure Electronic structure Composition − spatially (2D) or in the depth direction Reactions



Need to appreciate the notions of “probe or informationvolume” and “effects of the measurement (damage ormodification, induced reactions)”.

Surface preparation

For most singlecrystals, we follow



OR CleaveOR Deposition or epitaxy or other insitu growth techniques

under HV or UHV conditions [Ref: R.G. Musket et al., Applications in Surf. Sci. 10 (1982)

143207.]

Surface analysis techniques

There are many techniques. We focus here on the techniquesused to study the four areas of interest in this course (seeabove). A brief tour of some of the more common techniquesfollows… Physical structure SEM, TEM LEED EXAFS, SEXAFS Photoelectron Holography STM, AFM IRS, EELS Electronic structure ARPES Composition AES, XPS SIMS Reactions ESIADS, TPD



Fabrication Electrochemistry PLD PVD CVD, MBE Patterning Optical lithography EBL Singleatom control



5. An Aside: Why is UltraHigh Vacuum (UHV) necessary? It is necessary because of the techniques used to

interrogate the properties of the surface. In mosttechniques that we will discuss, these techniques involvefree particles such as electrons and ions which can tolerateno more than 104 Torr vacuum, otherwise the mean freepath of these particles will be too small.

Duration of the experiment vs the rate of contamination



Let n = average number of particles striking a 1 cm2 surface,then from kinetic gas theory we can show

where C = number of gas molecules per cm3; v = meanthermal velocity

where R is the gas constant; T is the temperature; P is thepressure (in mbar; 1 mbar = 0.75 Torr); M is the molecularweight. EXAMPLE : Take N2, so M = 28 (amu), and T = 300 K. Also, assume that a monolayer capacity is 3 x 1014

particles/cm2 (= n), then we have

n 106 P (monolayers/s) If P = 106 Torr, then we can build up one monolayer in 1second. Define

Langmuir = 1 x 106 Torrsecond



The time of coverage c then depends on the stickingcoefficient S (i.e. the probability that an impinging particlebecomes adsorbed).

c = ( n S )1 = ( 106 P S )1 For clean surface, S is very close to unity. In order to have anhour or so working time for an experiment, we must get thepressure down to 1010 Torr, or UHV. (Note: By convention106 Torr is known as High Vacuum). In order to study the properties of “clean” surface and not“dirty” surface, we must therefore maintain UHV condition. Note: The sticking coefficient depends on the nature of thesurface, and changes with the condition (coverage) of thesurface itself. This is not a simple number. Rather, it is acomplex function of the coverage and other surfaceparameters. In effect, the time of coverage becomes longer andlonger once adsorption starts. In other words, the impuritylayer makes the surface inert. The actual working time for theexperiment at 1010 Torr is therefore usually longer than thesimple relation used above. Pressure gap problems − Can we really trust what we

learn under UHV conditions?



How reliable can we extrapolate such properties to “real”conditions such as those used in heterogeneous catalysisand other chemical processing?

“Dust” or “dirt” problems in nanoscience − This is a

specially difficult problem for making nanoscale devices,and often clean room is required.

Back to surface analysis techniques (nanoanalytics)... A brief tour of some of the surface analysis techniques: 1. There are many different ways to interrogate surface

properties. Each involves a totally different set up andunderlying principle.

2. None of them is perfect. All of these methods have theirown inadequacy. We must know which one to use once weknow what properties we want to study.

In general, we may characterize a technique according towhich type of information it provides.



Overview of techniques by Somorjai [ Download PDF ] Summary of techniques by Charles Evans & Associates [Download PDF ]

http://leung.uwaterloo.ca/CHEM/750/Lectures%202007/SSNT-2-Overview%20of%20Surface%20Science%202_files/020199.pdf

http://www.eag.com/

http://leung.uwaterloo.ca/CHEM/750/Lectures%202007/SSNT-2-Overview%20of%20Surface%20Science%202_files/Wallchart_0203.pdf



[ Download PDF ]

Physical structure Scanning Electron Microscopy A standard microscopytool which one uses a high energy (keV) electron beam tobombard a surface and look at the resulting tranmission of thiselectron beam. Micrometer to nanometer spatial resolution. See movie at https://www.youtube.com/watch?v=VWxYsZPtTsI | EducationalMicroscopy Links

http://leung.uwaterloo.ca/CHEM/750/Lectures%202007/SSNT-2-Overview%20of%20Surface%20Science%202_files/BubbleChart.pdf

https://www.youtube.com/watch?v=VWxYsZPtTsI

http://www.ou.edu/research/electron/www-vl/educatio.shtml



Low Energy Electron Diffraction We will talk a lot moreabout this later when we discuss surface structure. It is by farthe most popular surface science technique. Here, we use thefact that at 100 eV kinetic energy, the de Broglie wavelength of



this electron “matter wave” is of the order of 1Å, i.e. theatomic size. This is a diffraction phenomenon.

Surface Extended Xray Absorption Fine Structure Another diffraction based technique. It makes use of thediffraction of outgoing photoelectron waves and backscatteringeffect from the surrounding ligands.



Scanning Tunneling Microscopy Tunneling effect. Overlapof wavefunctions between the tip and the surface atoms to giverise to current. A very intensely developed field. New way tomove single atom to make quantum wire. More later.

[ http://nanowiz.tripod.com/stmbasic/stmbasic.htm ]

http://nanowiz.tripod.com/stmbasic/stmbasic.htm



Quantum Corrals [ http://www.americanscientist.org/issues/pub/capturingquantumcorrals ]

Scientists discovered a new method for confining electrons to artificial structures at the nanometer lengthscale. Surface stateelectrons on Cu(111) were confined to closed structures (corrals) defined by barriers built from Fe adatoms. The barriers wereassembled by individually positioning Fe adatoms using the tip of a low temperature scanning tunneling microscope (STM). Acircular corral of radius 71.3 Angstrom was constructed in this way out of 48 Fe adatoms.

This STM image shows the direct observation of standingwave patterns in the local density of states of the Cu(111) surface.These spatial oscillations are quantummechanical interference patterns caused by scattering of the twodimensional electron

gas off the Fe adatoms and point defects.

Infrared Reflective Spectroscopy IR on surfaces. HighResolution Electron Energy Loss Spectroscopy Vibrational spectroscopy without photons! See most surfacevibrations and other surface species.

http://www.americanscientist.org/issues/pub/capturing-quantum-corrals



Electronic structure AngleResolved PhotoElectron Spectroscopy Fermi level,surface states, band structure. Much more later.

A series of angle resolved photoemissionspectra of a Mo(001) single crystalsurface. The Mo(001) surface is knownto show surface reconstruction from a(1x1) to an incommensurate c(2.2x2.2)phase upon cooling below Tc=230K. Byanalyzing the photoelectron spectra,changes in the electronic structure,which are related to the surfacereconstruction, could be measured. Thephotoelectron spectra in the figure werecollected at T=52K and with KII alongthe surface S axis. The spectra consist ofsurface states(S1), surface resonancestates(SR1, SR2) and bulk valence bandstates(B1). Twodimensional Fermi



surface contours can be obtained bydetermining the Fermi wave vectorkII(EF) at which S1 crosses the Fermilevel.

[ http://pfwww.kek.jp/outline/instr/angles.html ]

[ photon.physics.ucf.edu/XRAY/Courses/SoftXrayPhysics/ SoftXrayPhysicsPart2.PDF ]

http://pfwww.kek.jp/outline/instr/angles.html

http://leung.uwaterloo.ca/CHEM/750/Lectures%202007/SSNT-2-Overview%20of%20Surface%20Science%202_files/SoftXrayPhysicsPart2.pdf



Composition

Secondary Ions Mass Spectrometry Depth profiling. Filmgrowth.



Auger Electron Spectroscopy



Surface Reactions and Kinetics

Electron Stimulated Desorption Adsorption kinetics



Thermal Desorption



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INTRODUCTION TO SURFACE SCIENCE We will start the course ...leung.uwaterloo.ca/CHEM/750/Lectures 2007/SSNT-2... · will first give an overview of some basic definitions. 1. ... Middle