Surface Structure, Chemisorption and Reactions
Eckhard Pehlke, Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany.
Topics: (i) interplay between the geometric and electronic structure of solid surfaces, (ii) physical properties of surfaces: surface energy, surface stress and their relevance for surface morphology (iii) adsorption and desorption energy barriers, chemical reactivity of surfaces, heterogeneous catalysis (iv) chemisorption dynamics and energy dissipation: electronically non-adiabatic processes Technological Importance of Surfaces
Solid surfaces are intriguing objects for basic research, and they are also of high technological utility:
substrates for homo- or hetero-epitaxial growth of semiconductor thin films used in device technology
surfaces can act as heterogeneous catalysts, used to induce and steer the desired chemical reactions Sect. I:
The Geometric and the Electronic Structure of Crystal Surfaces Surface Crystallography
2D 3D number of space groups: 17 230 number of point groups: 10 32 number of Bravais lattices: 5 14
2D- symbol lattice 2D Bravais space point crystal system parameters lattice group groups
m mp 1 oblique (mono- γ b 2 a, b, γ 2 clin) a o op b (ortho- a, b a m rectangular rhom- o 7 γ = 90 2mm bic) oc b a
t (tetra- a = b 4 o tp a 3 square gonal) γ = 90 a 4mm
a h hp 3 o a hexagonal (hexa- a = b 120 6 o gonal) γ = 120 5 3m 6mm Bulk Terminated fcc Crystal Surfaces z z fcc a (010) x y c a=c/ √ 2 x square lattice (tp) z z fcc (110) a c _ [110] y x rectangular lattice (op) z _ (111) fcc [011] _ [110] a y hexagonal lattice (hp) x Surface Atomic Geometry
Examples: a
reduced inter-layer H/Si(111) separation
normal relaxation
2a
Si(111) (7x7) (2x1) reconstruction
K. Brommer et al., Phys. Rev. Lett. 68, 1355 (1992) Electronic Structure of Surfaces: Shockley States in the Projected Band Structure wave function matching at the surface κ z ψ virtual induced gap states ~ e bridge the band gap ε cusp no surface state > v VG 0 gap 2|VG| ψ κ surface state k π/a
real energy,complex Bloch vector VG <0 κ v ei(k+i )z The Al(100) Surface State
BZ of fcc lattice
bulk band structure for wave-vector ARUPS spectra _ Al(100) surface state in the projected perpendicular to the surface exit angle in (011) band structure D. Spanjaard et al., Phys. Rev. B G.V. Hansson, S.A. Flodström, Phys. Rev. B 18, 1562 (1978). 19, 642 (1979). Figures taken from: M.C. Desjonqueres, D. Spanjaard, "Concepts in Surface Physics", Springer (Berlin, 1993). Electronic Structure of Semiconductor Surfaces: Dangling Bonds on Si (001)
surface dimer empty + conduction σ εβ+ * o | | band (CB) - empty (CB) ε Si p 3 db (down) ε band gap empty o db state E ε F o sp3 db (up) hybrid- fully (occupied) occupied orbital σ ε − β o | | valence 1 ε band (VB) s 2 x 4 el occupied Si (VB)
atommolecular fictitious surface after reconstruction sp3 sp2+ p orbital with almost and relaxation picture non-interacting dbs Interplay of the Atomic and Electronic Structure of Si(001)
0.0 p(2x2) Si(001)(1x2)Si(001) decoup. dimer
-0.1
Energie pro Dimer [eV] -0.2
energy per dimer [eV] -20 -10 0 10 20 Verkippungswinkeldimer buckling des angle Si-Dimers [°] [o] lowest unoccupied surface state (LUMO)
0.5 π∗ H 0.0 Si H π -0.5 Si band center [eV] Bandschwerpunkt [eV]
-1.0 -20 -10 0 10 20 o Verkippungswinkeldimer buckling des angle Si-Dimers [°] [] highest occupied symmetric dimers buckled dimers surface state (HOMO) Mechanisms for Lowering the Surface Energy reduce density of dangling bonds -> by dimerization (Si(100), ~1 eV/db) -> ad-atoms (Si(111), rebonded steps on Si(100) vicinals) formation of π bonds between dangling bonds -> Pandey’s model of Si(111) (2x1)
Jahn-Teller-like distortions: relaxation and re-hybridization -> dimer buckling on Si(100) minimization of elastic strain unusual atomic configurations -> subsurface interstitial on Si(113) and other mechanisms (e.g. for compound semiconductors) Sect. II:
Material Properties of Crystal Surfaces:
Surface Energy
Surface Stress Tensor Surface Energy and the Thermo- (3) equilibrium crystal shape (ECS)
Î
ÓÒ×Ø Î
Ò
µ Ò ´ ÑÒ dynamic Stability of Facets µÑÒ ´ Ö
µ Ò ´ ¡ Ò
Á polar plot of the n surface energy h (1) Definition: γ = excess free energy of a surface per surface area γ (n) r(h)= γ (n)/n .h
(2) Calculation: total-energy DFT calculations for slab geometries equilibrium crystal shape (ECS) (i) slab with equivalent surfaces: Wulff-