Chapter 9 A (Very) Short Introduction to Surface Physics Surface Physics from fundamental… Scanning probes and surface states Carbon nanotubes …to applied Microelectronics / Nanotechnology Heterogeneous Catalysis References Physics at surfaces: A. Zangwill Surface Chemistry and Catalysis: G.A. Somorjai Principles Of Adsorption And Reaction On Solid Surfaces: R.A. Masel Theoretical Surface Science: A. Groß Lecture 1: 9.1 Geometric structure of surfaces 9.2 Adsorption at Surfaces 9.3 Diffusion at Surfaces 9.1 Geometric structure of surfaces “Ideal” surfaces [001] n Cutting close-packed materials (fcc/hcp/bcc) α vicinal surfaces steps low-index surfaces (fcc) (111) (110) (100) kinks (11 13 19) (335) Relaxation and reconstruction: metals Layer relaxation General Interpretation: Smoluchowski smoothing (Finnis & Heine) +++ -- -- Relaxation and reconstruction: metals 280 Å fcc(110)-(1x2) missing row reconstruction Au(111)-22x√3 herringbone structure Relaxation and reconstruction: semiconductors Dimerization at (001) surface of group IV elements “Dangling bond minimization” 9.2 Adsorption at surfaces Adsorption at surfaces top A surface exhibits many different ‘sites’: - simple overlayers (hollow, bridge, top) bridge - adsorbate induced reconstructions (missing row, oxide formation etc.) hollows - step/kink/defect decoration fcc(111) O(2x2) O(2x1) Lateral interactions between adsorbates - multiple sites as f(θ ), bond strength = f(θ ) - ordered adlayers ((in)commensurable) - islands, domains - competitive coadsorption… 3O(2x2) O(1x1) On an atomic scale adsorption is not just “attachment” Adsorption at surfaces Complex Surface Oxide Reconstructures are also common: eg O/Ag(111) 4x4 4x4 5x5 5x3 Adsorption at surfaces General statements in this area are dangerous, but: Well converged DFT GGA calculations mostly predict the correct adsorption site …many examples STM2 1 Eg. H2O Monomer Adsorption : 1 Michaelides, et al. Phys. Rev. Lett. 90, 216102 (2003) 2Mitsui et al. Science 297, 1850 (2002) Top view N adsorption on a W{100} surface (PW91) Side view N W 0.44 (0.41±0.05) 2.19 (2.13±0.05) W 0.26 (0.27±0.06) • Calculations on adsorbate structues often exhibit the same level of accuracy as calculations on molecular species. Michaelides, et al. Phys. Rev. Lett. 90, 246103 (2003) Adsorption at surfaces •But sometimes DFT gets it wrong…especially when the potential energy surface for adsorption is smooth: •Famous example CO/Pt(111)1 and CO on other transition metal surfaces •“Standard” DFT GGA predicts 3-fold site; Experiment says hollow CO sits at the hollow •site. •There are lots of explanations (and solutions, good or otherwise!) for this error. Perhaps the best is from Kresse who says it is because “standard” GGAs get the CO HOMO-LUMO gap wrong2 top bridge hollows fcc(111) 1P.J. Feibelman, et al. J. Phys. Chem. B 105, 4018-4025 (2001). 2 Key quantities in Surface Science Adsorption: binding energy at T = 0K tot tot tot Ebind = [ E ( ) - E ( ) - E ( ) ] Physisorption (0.1-0.3 eV/atom) - Chemisorption (>0.5 eV/atom) eg Xe, Ar eg O, N DFT adsorption energies can differ from experiment substantially: Hammer et al. Physical Review B, Vol. 59, 7413 (1999)) MolecularMolecular AdsorptionAdsorption Adsorbate Structures are modified upon adsorption Eg: ice on metals: In bulk ice (Ih) – the naturally occurring form of ice every water is involved in four H bonds (remember the cohesion chapter) MolecularMolecular AdsorptionAdsorption1 When ice is adsorbed on Pd(111) most waters in the interface layer are involved in three H bonds. On other surfaces other ice structures may form…people are a lot about this these days! 1J. Cerda, et el. Phys. Rev. Lett. 93, 116101 (2004). Molecular Adsorption In general for molecular adsorption: adsorbate bonds can be modified a little (typically lengthened by up to ~0.03 A) Internal vibrations altered (typically softened by up to several hundred cm-1; this can depend on the adsorption site: CO; hydrocarbons) Dipole moment, internal electronic structure will be modified 9.3 Diffusion at surfaces Diffusion on Metal Surfaces CLICK ON MOVIE Diffusion on Metal Surfaces CLICK ON MOVIE Atoms and molecules follow low energy pathways. So do Mr Men! Diffusion at metal surfaces: surprises… Hopping mechanism Ag(100) ∆E = 0.45 eV Au(100) ∆E = 0.83 eV Exchange mechanism Ag(100) ∆E = 0.73 eV Au(100) ∆E = 0.65 eV B.D. Yu and M. Scheffler, Phys. Rev. B 56, R15569 (1997) Diffusion on Metal Surfaces1 Molecular diffusion can be more complex than atomic diffusion. The orientation of the molecule can be important and the transition state for diffusion need not necessarily be a high symmetry adsorption site. 1Michaelides et al. PRB 69, 075409 (2004) Diffusion on Metal Surfaces •STM Experiments indicate that H2O dimers diffuse about 1000 times faster than H2O monomers across a Pd(111) surface1. •DFT calculations indicate that this might be because H2O dimers can waltz2. a b c d e f CLICK ON MOVIE 1 Mitsui et al. Science 297, 1850 (2002) 2Ranea, et al. Phys. Rev. Lett. 92 136104 (2003).