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The Theoretical Toolbox to Describe the Electronic Structure of Surfaces
Patrick RinkeFritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, D-14195 [email protected]
Acknowledgements: Jutta Rogal, Philipp Eggert and Karsten Reuter
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 1
Surfaces of Solids
STM image of “atomic” scriptAFM image of magnetic hard drive(25 µm x 25 µm). Wires are about2000 atoms wide
STM image of electron standing waves ata Ag step
General: - surface is the skin of the solid
Applications: - Microelectronics and semiconductor devicesControlled atom manipulation at surfaces (Nano…)Surface electronic structure and transport at surfacesCrystal growth and epitaxy
- Heterogeneous catalysisChemical bonds at surfaces
- Corrosion / mechanical failureSegregation of minority ingredients Fracture of engineering materialsPassivation, coating layers
Fundamental: Symmetry break (3D → 2D)New localized electronic and vibrational states (surface states & surface phonons)Continuum of states vs. discrete gas particle states
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 2
Controlled surface studies: Surface Science
Real vs. single crystal surfaces
- Real surfaces are very complex and often ill defined: polycrystalline materials, disorder, grain boundaries, defects and other irregularities
- Highly dependent on the environment (gas adsorption)- Segregation of impurities depends on sample treatment
⇒ Normal surface experiments often not reproducible (sometimes not even qualitatively!)
⇒ One Solution: the Surface Science Ansatz- Study low-index surfaces of single crystals. - Understand these “idealized” surfaces first, then introduce defects/irregularities in a controlled manner. - Gradually make systems more complex and hope that such systems provide good models to real problems.
(100)
(111) (110)
SEM image of polycrystalline Cu
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 3
STM image of GaSb screw dislocations(10 µm x 10 µm)
Experiment and Theory
Experiment
• theory development• first principles simulations
Quantum Mechanics
Physics
• geometric & vibrationalstructure at surfaces
• surface composition• surface electronic structure
Theory
IN OUT Prominent techniques
electrons electrons LEED, RHEED, AES, HREELSphotons photons SXRD, IRAS/RAIRSphotons electrons XPS, UPSelectrons photons IPESions ions ISS/LEIS, SIMS
Special: STM/STS, AFM, TPD
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 4
Electronic Structure Methods
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 5
quantitative description(as accurately as possible)
∑ ∑ ∑∑∑≠ ≠== −
+−
−−
++=JI Ii ji jiIi
I
JI
JIN
i
iL
I I
I
rre
RreZ
RReZZ
mp
MPH
,
222
1
2
1
2
||21
||||21
22
Free-electron model Indept. el. approximationJellium model Fermi-EnergyDrude/Sommerfeld th. Transport
Band theory Brillouin zoneKronig-Penney Band structure, DOSNearly-free el. model Band gaps, metal/insulator
LCAO Bandwidth↔overlapTight binding s,p,d-bands(Extended) Hückel
Homogeneous electron gas Exchange/correlationThomas-Fermi Theory ScreeningRandom Phase Approx. Quasi-particle concept
(Fermi liquid theory)
Quantum chemistry Hartree-Fock theory- Single reference
Møller-Plesset (MP)Conf. interaction (CI)Coupled cluster (CC) Density-Functional Theory
- Multi reference - LDAMulticonf. SCF (MCSCF) - GGAs (PBE, BLYP)Complete active space - Meta-GGAsSCF (CASSCF) - OEP/EXX (B3LYP)
Quantum Monte-Carlo
Many-Body Perturbation Theory- GW- BSE
Scattering Theory- KKR in LDA, GGA, GW
Tight Binding
Interatomic Potentials- Pair potentials, force fields- Cluster potentials (Stillinger-
Weber, Keating, (M)EAM, BOP…)
Born-Oppenheimer Approximation:
H = Tel + Vnucl-el + Vel-el
Many-body Schrödinger Equation:
qualitative description(conceptual aspects)
somewhere inbetween
Representation of Surface
• Quantum Chemistry
• Quantum Monte Carlo
• Hartree-Fock
• Density-Functional Theory
• GW, BSE
• Tight-Binding
• Interatomic Potentials
• Scattering Theory
• Density-Functional Theory
• GW
• Tight-Binding
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 6
DFT - Groundstate
Hohenberg-Kohn Theorem:
• Ground state energy is unique functional of the density n(r)
• universal functional:
• variational:
• Exchange-correlation:
• Hartee Energy:
• exact unknow fl suitable approximations:- local density approximation (LDA), gradient corrected (GGA)
[ ] [ ] ( ) ( ) rrr dnvnFnE exttot ∫+=
[ ] 00ˆˆ Ψ+Ψ= UTnF
[ ] 0=nnE
δδ
[ ] [ ] [ ] [ ]nEnETnEnE Hexttotxc −−−=
[ ]nExc
[ ] ( ) ( ) ( )∫∫ −= r'rr'rr'r ddvnnnEH 21
: kinetic energy
: electron-electron interaction
T
U
extvn ⇔
minimum at exact density
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 7
DFT – Kohn-Sham Scheme
Kohn-Sham:
map system of interacting electrons onto fictitious system of non-interacting electrons that reproduce the exact density
• Kohn-Sham equation:
• Density:
• Hartee potential:
• Exchange-correlation potential:
• in practice: start with trial density and then iterate to self-consistency
( ) ( ) ( ) ( ) ( )rrrrr iiixcHext vvv φεφ =⎥⎦
⎤⎢⎣
⎡+−+
∇−
2
2
( ) ( )∑=occ
iin 2rr φ
( ) ( ) ( )∫ −= rr'rrr dvnvH
( ) [ ]( )r
rn
nEv xcxc δ
δ=
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 8
Surfaces in DFT – Repeated Slab Approach – Vacuum Convergence
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 9
Density and Vxc in LDA/GGA decay exponentially outside the surface
slabs decoupledonly very short ranged interactions in LDA/GGA z-direction
Ele
ctro
n de
nsity
SlabVacuum
hydrogen passivated Si(001) film
Surfaces in DFT – Repeated Slab Approach – Slab Convergence
hydrogen passivated Si(001) film
finite size effectsslab convergence canbe slow
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 10
DFT Total Eneregies - Potential Energy Surfaces
Schrödinger Equation:
• Potential Energy Surface (PES) or also Born-Oppenheimer Surface:
( ) ( ) ( ) ( )}{}{}{}{ˆ RRRR Ψ=Ψ totEH
( )}{RtotE
GaAs(001) ζ(4×2)As Ga
Potential-energy surface for the adsorption of As (left panel) and Ga (right panel) on the Ga-rich GaAs(001)(4×2) surface. The contour spacing is 0.15 eV. Light regions indicate low-energy adsorption positions.
• As prefers site with 3-fold Ga coordination
• Ga prefers the trenchTop and side view of the relaxed GaAs(001) ζ(4×2) surface. Light (dark) balls represent Ga (As) atoms.
K. Seino et. al., Surf. Sci. 507, 406 (2002)
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 11
Forces in DFT
Schrödinger Equation:
• Potential Energy Surface (PES) or also Born-Oppenheimer Surface:
• Assume motion of nuclei as classic:
• Hellmann-Feynman force:
• Forces in DFT:
( ) ( ) ( ) ( )}{}{}{}{ˆ RRRR Ψ=Ψ totEH
( )}{RtotE
ii FR =dtdM i
( ) ( ) 00 }{ˆ}{ Ψ∂∂
Ψ−=∂∂
−= RR
RR
Fii
i HEtot
( ) ( ) ( ) ( )444 3444 21444 3444 21
part electronicpartnuclear
21}{ 3
23
2
rRrRrrRR
RRR
R ii
ji
jii
d--neZ-
-
eZZE i
ij
jitot ∫∑ −−=
∂∂
≠
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 12
Reconstruction at Si(001) surface
surface cuts two bonds per atom• lone pairs (dangling bonds)• metallic surface• high surface energy
dangling bonds
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 13
DFT force minimisation• surface atoms pair up• dimers form• semiconduction state • surface energy lowered
dimers
Phase diagram – ab initio thermodynamics
surface free energy γ:
• Gibbs free energy G:
• Helmholtz free energy F:
• For solids pV and Fvib are typically small:
γ =1A
G T, p,{Ni},{Ri}( )− Niµii
∑⎡
⎣ ⎢
⎤
⎦ ⎥
A : surface area Ni : number of species i µi : chemical potential of species i
G T, p,{Ni},{Ri}( )= F T,V ,{Ni},{Ri}( )+ pV T, p,{Ni},{Ri}( )
F T,V ,{Ni},{Ri}( )= E V ,{Ni},{Ri}( )+ Fvib T,V ,{Ni},{Ri}( )
γ ≈1A
E V ,{Ni},{Ri}( )− Niµii
∑⎡
⎣ ⎢
⎤
⎦ ⎥
DFT
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 14
First-principles atomistic thermodynamics applied to oxidation of Pd(100)
µO2(T, p)
G(T, p) = Etot + Fvib – TSconf + pV
DFTµΟ (T, p) = ½ µΟ (T, p0) + ½ kT ln(p/p0)
2
FP-(L)APW+loGGASupercell-Approach
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 15
C.M. Weinert and M. Scheffler, Mater. Sci. Forum 10-12, 25 (1986);
E. Kaxiras et al., Phys. Rev. B 35, 9625 (1987)
K. Reuter and M. Scheffler, Phys. Rev. B 65, 035406 (2002);
Phys. Rev. Lett. 90, 046103 (2003)
Stability of different phases on Pd(100)
∆µO (eV)
pO (atm)
(√5 × √5)R27°
p(2 × 2)
c(2 × 2) clean Pd(100)
T=300 K
T=600 K
∆G
(meV
/Å2 )
metal adla
yer
surf
. oxi
de
bulk oxide
∆G ∆µ0( ) ≈ −1A
EO@Mtot − EM
tot − NO12
EO2
tot + ∆µ0
⎛ ⎝ ⎜
⎞ ⎠ ⎟
⎡
⎣ ⎢ ⎤
⎦ ⎥ = −1A
NO Eb + NO∆µ0[ ]
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 16
Stability of different phases on Pd(100)
Experiment Theory
E. Lundgren et al., Phys. Rev. Lett. 92, 046101 (2004).
µΟ (T, p) = ½ µΟ (T, p0) +½ kT ln(p/p0)
2
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 17
Introducing CO
µO2(T, p) µCO(T, p)X
equilibrium(“constrained”)
G(T, p) = Etot + Fvib – TSconf + pV
DFTFP-(L)APW+loGGASupercell-Approach
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 18
Introducing CO - Structures
hollow bridge on-top
• (√5 × √5)R27° surface oxide:
• 4 top, 2 bridge, 6 hollow, 2 hollow-substitutional sites • adsorption of O, CO, vacancies, mixed phases ...
⇒ close to 200 structures considered !!10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 19
3D-Theoretical “phase diagram”
00
-0.5
-1.0
-1.5
-1.0
-0.5
-1.5-2.0
-2.5200
100
0
-100
-200
∆µ O (eV)∆µ
CO (eV)
∆G(m
eV/Å
2 )
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 20
Theoretical “phase diagram” in an O2 and CO environment (constrained equilibrium)
∆µO (eV)
∆µ C
O(e
V)
p CO
(atm
)
pO (atm)surface oxide + 2CO bridge
300 K600 K
surface oxide + CO bridge
PdO bulksurface oxide(√5 × √5)R27°p(2 × 2)-O/Pd(100)
clean Pd(100)
c(2√2 × √2)R45°CO/Pd(100)
(1 × 1)-CO bridge/Pd(100)
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 21
Wavefunctions and Density - Insights into the Spatial Distribution of the Electrons
( ) ( ) ( ) ( ) ( )rrrrr iiixcHext vvv φεφ =⎥⎦
⎤⎢⎣
⎡+−+
∇−
2
2
Kohn-Sham equation:
• Electron density:
• Density difference:(adsorption, desorption, adlayers, defects)
• Difference density:(adsorption)
wavefunctions
single particle energies(atomic/molecular levels, bandstructure)
( ) ( )∑=occ
iin 2rr φ
∆n r( )= n r( )− nref
surface r( )
n∆ r( )= n r( )− nref
surface r( )− nrefadsorbate r( )
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 22
CO Adsorption at Transition Metal Surfaces - A Model System
Wavefunctions and energies: three outer valence orbitals of molecular CO
Electron density of the valence molecular orbitals of a free CO molecule and their DFT-GGA Kohn-Sham eigenvalues (far left) with respect to the vacuum level. The lower and upper small black dots represent the positions of the C and O atoms, respectively. The first contour lines are at 8 x 10-3 bohr-3, except for the 2π∗ orbital where it is 15 x 10-3 bohr-3, and the highest- valued contour lines are at 0.5, 0.3, 0.2, 0.15, and 0.15 bohr-3 for the 3σ, 4σ, 1π, 5σ, and 2π∗ orbitals, respectively.
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 23
C. Stampfl and M. Scheffler in Handbook of Surface Science Vol. 2
CO on Ru(0001)
C. Stampfl and M. Scheffler in Handbook of Surface Science Vol. 2
Electron density distribution of the CO-derived states for CO adsorbed on the on-top site of Ru(0001) and their DFT-GGA Kohn-Sham eigenvalues (far left) with respect to the vacuum level. The lower and upper small black dots represent the positions of the C, O and Ru atoms, respectively.
n r( )
n∆ r( )= nCO @ Ru(0001) r( ) − nRu(0001) r( )− nCO r( )
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 24
Co-adsorption - CO and O on Ru(0001)
C. Stampfl and M. Scheffler in Handbook of Surface Science Vol. 2
Perspective and side views of the various phases of O and CO on Ru(0001). Large and small (green and red) circles represent Ru, O and C atoms, respectively. The lower panel shows the electron density of the valence states. The contour lines are in bohr-3 and the distance in Angström.
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 25
Bandstructure - Electronic Structure of Bulk Silicon
Kohn-Sham equation: −
∇2
2+ vext r( )− vH r( )+ vxc r( )
⎡
⎣ ⎢
⎤
⎦ ⎥ φnk r( )= εnkφnk r( )
Brillouin zone
fcc crystal structure
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 26
Projected Bandstructure - Si -> Si(001)
Broken translational symmetry at surface -> k no longer a good quantum number, but k||
E = En (k|| ,{k⊥}) := E PBS (k|| )
Projected Bandstructure:
Bulk SiBulk Si in Si(001) p1x1 surface Brillouin zone
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 27
Surface Bandstructure - Si(001)
dangling bonds
Bulk terminated Si(001) surface:• 2 lone pairs (dangling bonds)• metallic surface
from Schmeidts et al., Phys. Rev. B 27, 5012 (1983)
bridge bond state
dangling bond state
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 28
Si(001) - Reconstructions
p1x1
(bulk terminated)
p2x1
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 29
c4x2p2x2
up
down
∆Etot /per dimer : p2x1 0,057meV⎯ → ⎯ ⎯ ⎯ p2x2 0,003meV⎯ → ⎯ ⎯ ⎯ c4x2
surface unit cells
Si(001) - ARPES
DFT total energy calculations predict c4x2 as ground state, but p2x2 is only 3 meV/dimer higher in energy -> alternative criterium
ARPES
c4x2 2x1
ARPES : Angle Resolved PhotoEmission Spectroscopy from Enta et al., Phys. Rev. Lett. 65, 2704 (1990)
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 30
Si(001) - ARPES: from Spectrum to Bandstructure
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 31
from Enta et al., Phys. Rev. Lett. 65, 2704 (1990)
c4x2 2x1
ARPES
Si(001) 2x1 experiment <-> theory
Theory: Rohlfing et al., PRB 52, 1905 (1995)Exp: Uhrbert et al., PRB 24, 4684 (1981)
Johansson et al., PRB 42, 1305 (1990)
c4x2 : open symbols2x1 : solid symbols
Si(001) 2x1 - Surface Bandstructure
Projection onto atomic orbitals of dimer:
φnk (r) ≈ cnkµµ∑ χµ (r) → Nnk (M) = cnkµχµ (r)
µ ∈M∑
2
= cnkµ
* cnkµ χµ χνµ,ν ∈M∑
from DFT code
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 32
Si(001) 2x1 - Surface Bandstructure at Γ
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 33
black diamonds : projection onto dimersred squares : projection onto surface layer
nP (r) = fnkwk φnk (r) 2
nk ∈P∑ → OP = nP (z)dz
surface state
b
c
∫ nP (z)dza
c
∫
surface resonance
fnk : occupation factor, wk : k-point weight
Si(001) 2x1 - Projected Density of States
Density of states: Projected density of states:
N DOS (ε) = wnkδ(nk∑ ε −εnk ) Nν
PDOS (ε) = wnk χν φnk2δ(
nk∑ ε −εnk )
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 34
Si(001) 4x2 - Surface Bandstructure at Γ
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 35
Further Reading
• Handbook of Surface Science, ed. S. Holloway and N. V. Richardson, Elsevier Science (Amsterdam, 2000)
• Theoretical Surface Science – a Microscopic Perspective, A. Gross, Springer (Berlin, 2002)
• Principles of Surface Physics, F. Bechstedt, Springer (Berlin Heidelberg 2003)
• Modern Techniques of Surface Science, D.P. Woodruff and T.A. Delchar,Cambridge Univ. Press (Cambridge, 1994)
• Physics at Surfaces, A. Zangwill, Cambridge Univ. Press (Cambridge, 1988)
• Principles of Adsorption and Reaction on Solid Surfaces, R. Masel, Wiley (New York, 1996)
• Solid State Physics, N.W. Ashcroft and N.D. Mermin, Saunders College (Philadelphia, 1976)
10/10/2005 The Theoretical Toolbox to Describe the Electronic Structure of Surfaces 36