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Surface Chemistry of Materials, Chem 5610 Spring 2013

Surface Chemistry of Materials, Chem 5610 Spring 2013. Lecture I: Introduction to Surfaces Why are surfaces different from the bulk? Why we need a vacuum (no Hoover jokes, please) Methods for probing surfaces Reading: Somorjai , Chapt . 1. Why Surface Science?.

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Surface Chemistry of Materials, Chem 5610 Spring 2013

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  1. Surface Chemistry of Materials, Chem 5610 Spring 2013 • Lecture I: Introduction to Surfaces • Why are surfaces different from the bulk? • Why we need a vacuum (no Hoover jokes, please) • Methods for probing surfaces • Reading: Somorjai, Chapt. 1

  2. Why Surface Science? • Many important chemical reactions occur at outermost atomic layers of materials(typically, outermost 1-50 Å) • Langmuir, H2 reactions at a W surface (1913) • Haber, N2 + 3 H2 2 NH3 over iron catalyst (about the same time) • The main drivers of surface science today: • Catalysis • Micro/nanoelectronics • Energy (photovoltaics, fuel cells…) • And Tomorrow(?) • Biological issues (tissue/prosethic compatibility, membrane chemistries…) • Neuronetworks, biological and not

  3. Atoms at a surface are low-coordinate relative to the bulk Surface atom, 5 bonds to nearest neighbors vacuum Surface Bulk Bulk atom, 6 bonds to nearest neighbors

  4. Unused surface bonds can interact, causing change in surface structure Surface dimerization

  5. Reconstruction of Si(100) From http://www.chem.qmul.ac.uk/surfaces/scc/scat1_6a.htm

  6. Surface is different electronically: Distribution of Surface Charge • Electron density trails off exponentially away from the surface into the vacuum • This partially depletes negative charge just below the surface Ion cores partially unshielded, net + charge Charge neutrality in the bulk Region above surface negatively charged (several angstroms) Bulk Surface Lang and Kohn, PRB 1 (1970) 4555

  7. Redistribution of Charge near surface sets up the Surface Dipole - - - - - + + + + + Bulk

  8. Work function is the extra energy needed to promote an electron from the HOMO (Fermi level) into the vacuum different for different surfaces e.g~ 4.3 eV, W ~ 5.3 eV, Pt EVacuum Work Function E EFermi

  9. Why do we need a vacuum? H2O CO2 hydrocarbons O2 • Atoms at the surface directly interact with gases in the environment • Rxns occur at the surface that don’t occur in the bulk • We need to control this

  10. Typical Atom Surface Density: ~ 1015 atoms/cm2 Flux of atoms of mass M to this surface from the gas phase (F) is given by (at gas temperature T) : F (atoms/cm2-sec) = 3.51 x 1022 P(Torr) x [M(g/mole) T]-1/2 (Somorjai) Note: At P = 3 x 10-5 Torr, M = 28 gr/mole; T = 300 K F ~ 1015 atoms/cm2-sec. Thus, assuming a “sticking coefficient” of 1, the surface is covered by a fresh monolayer every second under a mild vacuum

  11. Sticking Coefficient = probability/collision that an atom coming from the vacuum and colliding with the surface will stick! • Sticking coefficients are often small (e.g., N2 on Au) but can approach 1 for , e.g., N2 on clean W. • We need to keep surface contaminant concentrations low over the course of an experiment (~ 1 hour, say). Therefore, pressures ~ 10-9 or lower are required. • This is known as ultra-high vacuum (UHV). • Important: in measuring surface concentrations of adsorbed atoms, it is NOT pressure, but Pressure x Time [Exposure] that is important. • 1 Langmuir = 10-6 Torr-sec is the standard unit of exposure

  12. Methods for maintaining and measuring ultrahigh vacuum (see standard texts, such as Briggs and Seah, Practical Surface Analysis: Chamber Materials: 304 Stainless steel now almost universal Pumps: (1) Turbomolecular pump with mechanical pump backing  can go to ~ 10-10 Torr if careful. Typically ~ 10-9 Torr – 5 x 10-10 Torr  advantage, can pump many different types of gases, rapid pump down from relatively high gas loadings back to UHV  disadvantage, expensive, can malfunction during power outages, etc. (2) Ion pump with Ti Sublimator  can maintain vacuums better than 5 x 10-11 Torr  Fussy about what it will pump (O2, H2O good, CO bad)  Relatively cheap, long lasting, restarts after power outages  low pumping speeds, needs turbo to rough down from high gas loadings Other pumps include oil diffusion pumps, but not much used anymore.

  13. Measuring a vacuum: The “nude” (Bayard-Alpert) ion gauges A+ e- + A  A+ + 2 e- e- Filament emits electrons accelerated by grid Electrons ionize gas phase molecules Ions collected a grid. Grid current proportional to pressure.

  14. Ion gauges: Practical upper limit ~ 10-3 Torr Lower limit ~ 10-11 Torr Very reliable: They only fail during important experiments

  15. Methods for Probing Surfaces

  16. How do we investigate surfaces? Low energy electrons(Ekin < 1000 eV) are surface sensitive: penetration/escape depths < 100 Å hvin hvout e- Photons in/electrons out: Are surface sensitive e-

  17. How do we investigate surfaces? Photon penetration and escape depths, typically > 0.1 microns, not surface sensitive hvin hvout Photons in/photons out: Not surface sensitive

  18. Since we are interested in the structures of (typically) the outermost 1-20 atomic layers, we want surface probes with sampling depths of ~ 50 Å or less What determines sampling depth Typically, it is the escape depth of the detected photon/ion/electron Photon escape depths typically ~ 10 nm or more (not good) Ions, can be as little as one monolayer, but may present other problems Electrons ~ escape depth determined by inelastic mean free path (IMFP = λ) Typically, λ = λ(KE, electron density of medium)

  19. From surf. Sci. Western (Univ. of W. Ontario)

  20. Surface, no electrons attenuated hv e- e- e- Some e- from bulk or near/surface suffer inelastic collisions, change kinetic energy, lose chemical information

  21. λ and the continuum model for attenuation of electron intensity Electron intensity out (Ix) Electron intensity in I0 dI = -(dx/λ) I dI/I = -dx/λ I(x) = I0 exp (-x/λ) dx λ-1 is the probability per unit length for the electron to undergo inelastic collision An overlayer thickness of λ will attenuate signal intensity by a factor of 1/e At a thickness of 3λ, signal attenuated by 1/e3 ~ 98%

  22. Since we want escape/sampling depths < 100 Å, we want to detect low energy electrons coming from surfaces (EK < 1000 eV) From surf. Sci. Western (Univ. of W. Ontario)

  23. Surface Probes using low energy electrons TechniqueInOut XPS hv ~ 1250 ev-1500ev e-, Ek < 1000 eV AES e- ~ 3000 eV e-, Ek < 1000 eV LEED e- ~ 50-300 eV e-, Ek = Ein UPS hv ~ 21 – 40 ev e-, EK < hv

  24. Development of low energy electron-based surface probes: • LEED (low energy electron diffraction) –since 1927 (Davisson and Germer and the birth of modern quantum mechanics) • AES (Auger electron spectroscopy) –1960’s, Palmberg, et al. • XPS (x-ray photoelectron spectroscopy )-1960’s Kai Sigbahn and others at Uppsala University All the above linked to technological developments: LEED: Glass-based vacuum systems, fluorescent screens AES, XPS: Development of accurant electron energy analyzers All the above: improved vacuum technology

  25. Improvements: *Angle-Resolved photoemission, (band structure) *spin polarized LEED (magnetic systems) *spin-polarized photoemission (magnetic systems) *time –resolved measurements (has not caught on) *synchrotron-based photoemission, very popular for tuning sampling depths.

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