Eckhard Pehlke, Institut für Laser- und Plasmaphysik, Universität Essen, 45117 Essen, Germany. Surface Structure and Chemisorption (ii) physical properties of surfaces: surface energy, surface stress and their relevance for surface (i) interplay between the geometric and electronic structure of solid surfaces, Topics: morphology reactivity of surfaces -> heterogeneous catalysis (iii) adsorption and desorption energy barriers, chemical
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Surface Structure and Chemisorption - Theory Department · Surface Structure and Chemisorption (ii) ... A1 n A2 2 nA3 3 (a) (b) n2 A A2 A ... (model-potential) locate transition state
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Eckhard Pehlke, Institut für Laser- und Plasmaphysik, Universität Essen,45117 Essen, Germany.
Surface Structure and Chemisorption
(ii) physical properties of surfaces: surface energy, surface stress and their relevance for surface
(i) interplay between the geometric and electronic structure of solid surfaces,
Topics:
morphology
reactivity of surfaces -> heterogeneous catalysis(iii) adsorption and desorption energy barriers, chemical
substrates for homo- or hetero-epitaxial growth of semiconductorthin films used in device technology
and steer the desired chemical reactionssurfaces can act as heterogeneous catalysts, used to induce
Solid surfaces are intriguing objects for basic research,
and they are also of high technological utility:
Technological Importance of Surfaces
The Geometric and
Sect. I:
the Electronic Structure
of Crystal Surfaces
number of space groups:number of point groups:number of Bravais lattices:
Influence of the Reconstruction on Surface Stress Anisotropy
Stress anisotropy calculated from total energy differences:
J. Dabrowski, E. P., M. Scheffler, Phys. Rev. B 49, 4790 (1994).T.W. Poon, S. Yip, P.S. Ho, F.F. Abraham, Phys. Rev. B 45, 3521 (1992).M.B. Webb, F.K. Men, B.S. Swartzentruber, R. Kariotis, M.G. Lagally, Surf. Sci. 242, 23 (1991).
slab geometry with up to 12 Si layers, H-termination on both sidesPW91 GGA for the XC functionalHamann pseudopotentials for Si, 1/r Coulomb potential for Hkinetic-energy cut-off for plane-wave basis-set: 50 Ry9 special k-points in the irreducible part of the Brillouin zone
DFT total-energy calculations with fhi96md
5.24 5.34 5.44 5.54 5.64c|| [Angstroem]
−31.436
−31.416
−31.396
2E(N
)−E
(2N
) [
eV] σ = − 0.01 eV/Angstroem
2
5.43 5.44 5.45 5.46 5.47 5.48c|| [Angstroem]
−0.05
−0.03
−0.01
0.01
0.03
0.05
dE/d
A [
eV/A
ngst
roem
2 ]
−0.007 eV/Angstroem2
zus.
relaxiert
The "Reference Surface" Si(111) (1x1) - H
Method
withadditionalrelaxation
Calculation of Surface Stress from Total-Energy Differences:
Sect. III:
on/from a Si(100) Surfaceof Hydrogen Molecules
and Recombinative DesorptionDissociative Adsorption
Model System:
Chemisorption on Semiconductor Surfaces
Langmuir-
mechanismHinshelwood
Surface Reactivity and Heterogeneous Catalysis
The rate of chemical reactions depends on the reaction energy-barriersalong the reaction path:
Dissociative adsorption and recombinative desorption of moleculeson a solid surface are an essential step of heterogeneous catalysis:
HSi
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pote
ntia
l ene
rgy
energy
barrier
adsorptionbarrier
desorption
chemisorption
reaction pathcoordinate
Dissociative Adsorption of a Molecule on a Solid Surface
hydrogen molecule / H-precovered Si(100)(2x2)
potential energyalong the reaction path
electronic mechanism of bond formation and breaking
-0.4-0.2
00.2
0.40.6
0.81-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-0.5
0
0.5
z [Å]d [Å]
V
The (most intensely studied) intra-dimer reaction path:
small sticking coefficient -> large adsorptionenergy barrier
desorbing hydrogen molecules do not havelarge (> thermal) kinetic energies
of freedom in the adsorption/desorption dynamics.It is essential to include the "mechanical" surface degree
separation of hydrogenz molecule from surface
d surface degree offreedom
K.W. Kolasinski et al., Phys. Rev. Lett. 72, 1356 (1994).
Microreversibility?
2
SiSi
What makes H /Si a fascinating system?
HH
W. Brenig, A. Groß, R. Russ (see e.g. Phys. Rev. B 54, 5978 (1996)).
(model-potential)
locate transition state(automated search for special saddle points in the potential energy surface)
chemisorption geometries and energies(equilibrium geometries, reaction energy)
strain energy of substrate at thetransition geometry, etc.("computer experiments")
reaction path(steepest descent from transition state)
PES, vibrational frequencies
adsorption and desorptionenergy barrier
d [Å]
[Å]
dz
z
D
A
TS
PES (schematic)
analyse electronic structure(learn about bond breakingand forming mechanism)
(high-dimensional PES!)
molecular dynamics,quantum-mech. sticking calc.
A. Groß, Surf. Sci. Rep. 32, 291 (1998).
DFT for Chemisorption: Reaction Path, PES, ...
P. Kratzer, B. Hammer, J.K. Norskov,Phys. Rev. B 51, 13432 (1995).
2.0 1.5 1.0 0.5 0.0Abstand zur Monohydridkonfiguration [Å]
0
1
2
3E
nerg
ie r
elat
iv M
onoh
ydrid
konf
. [e
V]
Hydrogen Molecules on Partially H-Precovered Si(001)
Existence of small adsorption energy barriers for H3 and H2 sites (inter-dimer paths).Comparative study of adsorption sites with fewer pre-adsobed H atoms:
Reaction path without adsorption energy-barrier for H4 site.
2.41 eV
2.06 eV
2.49 eVen
ergy
rel
ativ
e m
ono-
hydr
ide
[eV
]
separation from mono-hydride conf. [A]
H
Si
HOMO
LUMO
+
+
[Å]dHH
xz
Si
H
MH
H20.53
0.03.850.53
3.5 eV
0.0 eV
z [Å]
Highly Reactive Sites for the Dissociative Adsorption of
Si clusters with >= 3 surface dimers should beused.
accurate for H /Si reaction barriers.2
PW91 calculations are nevertheless usefull tocompare between various reaction paths!
E.g., decrease reaction energy by 0.2 eV (Steckel et al.).
(4) Be aware of different reaction paths.Different reaction paths for dissociative adsorption of hydrogen molecules on Si surfaces.Reaction barriers influenced by electronic and geometric effects! Dramatic increase of reactivity at steps and on partially H-precovered surfaces.
E. Penev, P. Kratzer, M. Scheffler, J. Chem. Phys. 110, 3986 (1999).
J.A. Steckel, T. Phung, K.D. Jordan, P. Nachtigall, J. Phys. Chem. B 105, 4031 (2001).H /Si(001)intra-dimer path