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Atomic-scale Reconstructions on
Metal and Semiconductor Surfaces
Andrew Wee
Surface Science Laboratory
Department of Physics, NUS
IMS Workshop, 27 Nov 04
Surface Science Lab NUS
VT-STM/XPS/LEED system +
growth chamber with molecular
beam & reactive atom sources [+
cryogenic STM]
Soft X-ray synchrotron end
station on SINS beamline [+
growth chamber + STM/AFM]
Cameca IMS 6f Magnetic sector
SIMS
VG ESCA MkII/SIMSLAB
[EXAFS endstation]
Grand Challenge:
Self assembly of single molecule devices
W Ho et al., Science, published
online Sept. 4, 2003.
H Park et al., Nature, 417 (2002) 722
Scope of Presentation
1. Structure of Surfaces
A rule for structures of open (high index) metal surfaces
A high index surface: Cu(210)
SiC(0001)-63x63 “honeycomb” reconstruction
2. Adsorbate-induced Reconstructions
SiC(0001)-O
Cu(210)-O; Cu(210)-Br
3. Surface as Template
Monodispersed Co nanoparticles on SiC(0001) honeycomb
template
Co ring clusters on Si(111)-(7×7)
1. Structure of Surfaces
A rule for structures of open (high index) metal
surfaces
A high index surface: Cu(210)
Adsorbate-induced reconstructions: Cu(210)-O;
Cu(210)-Br
SiC(0001)-63x63 “honeycomb” reconstruction
A rule for structures of open metal surfaces
Ref: YY Sun, YP Feng, CHA Huan, ATS Wee, Phys. Rev. Lett. 93 (2004) 136102.
Open metal surfaces: The coordination of the atoms in
at least two layers is reduced when creating the surface;
hence, more than one atomic layer is “exposed” to the
vacuum.
Rule: “At bulk-truncated configuration, define a surface
slab in which the nearest neighbors of all atoms are
fewer than those in the bulk; in the process of relaxation,
the interlayer spacing between each pair of atomic layers
within this slab contracts, while the spacing between this
slab and the substrate expands.”
Surface Contracts
Slab Expands
Bulk
Density Functional Theory (DFT)
Kohn-Sham equation:
where the last term (the exchange-correlation) is not known exactly.
Various approximations are available. Among others, the LDA and
GGA are most widely used.
LDA GGA
Methodology
Plane Wave Expansion: Advantages:
Simple mathematical formulism
Independency of basis set on ion
positions
Availability of fast Fourier transform
(FFT) between direct and reciprocal
spaces
Pseudopotentials:
Keep the eigenvalues and scattering properties unchanged compared
with those of the real potential.
Softer in the core regions, hence fewer PW’s are needed for the expansion
above.
Vienna Ab-initio Simulation Package (VASP) is a very efficient
implementation of the pseudopotential plane-wave package.
A rule for structures of open metal surfaces
First-principles
calculations:
Based on density
functional theory with
either LDA or GGA
approximation for the
exchange-correlation
functional
Ref: Sun YY, Phys. Rev.
Lett. 93 (2004) 136102.
A rule for structures of open metal surfaces
Physical picture: For more open surfaces, electrons from the deeper
layers contribute to the smoothing, hence more layers relax.
Further evaluation of the rule
Ni Cu Rh Pd Ag Ir Pt
(311) (- + …) (- + …) (- + …) (- + …) (- + …) (- + …) (- + …)
(331) (- - + …) (- - + …) (- - + …) (- - + …) (- - + …) (- - + …) (- - + …)
(210) (- - + …) (- - + …) (- - + …) (- - + …) (- - + …) (- - + …) (- - + …)
All fcc(311) surfaces have relaxation sequence (- + …)
All fcc(331) and fcc(210) surfaces have relaxation sequence (- - + …)
All these surfaces obey the rule.
Reference: Sun YY, Xu H, Feng YP, Huan ACH, Wee ATS, Surf. Sci. 548, 309 (2004).
Low Energy Electron Diffraction (LEED):
Quantitative Determination of Surface Structure
LEED diffraction pattern
I-V data Q-LEED
collection analysis
Guess a
structure
Multiple scattering
structure
Adjust
calculations
Reliability
factor Bad
Good
Stop
A high index surface: Cu(210)
Clean Cu(210):
I-V LEED
• Studied by layer-
doubling LEED
analysis and
pseudopotential DFT
calculations.
• Excellent
agreement between
the calculated and
measured I-V curves
as judged by small
Pendry R factor of
0.12.
Sun YY, Xu H, Zheng JC, Zhou JY, Feng YP, Huan ACH, Wee ATS, Phys. Rev. B 68 (2003) 115420
A high index surface: Cu(210)
Multilayer relaxation of Cu(210) surface: IV-LEED vs DFT
LEED DFT
Δd12 (%) -11.1 -16.7
Δd23 (%) -5.0 -4.3
Δd34 (%) +3.7 +6.8
Δr12 (%) -1.9 -1.0
Δr23 (%) -1.9 -0.6
Δr34 (%) +0.6 +1.9
Sun YY, Xu H, Zheng JC, Zhou JY, Feng YP, Huan ACH, Wee ATS, Phys. Rev. B 68 (2003) 115420
cf. A rule for structures of open metal surfaces
Structure of 6H-SiC
Wide band gap semiconductor, very hard, good
thermal conductor, chemical inert.
Structure: Si-C sp3 configuration, different Si-C
bilayer stacking sequence and orientation, ≥200
polytypes, determine the physical property.
C atom
Si atom
A A
A
B B
B
15.11Å
A C
C
C A
A
A C
3C 4H 6H
7.55Å
B
B
10.05Å
B
C
A A
A
C B Eg (eV) 2.3 3.2 3.0
B
A C
C
B A
A
3C-SiC(111) 4H-SiC(0001) 6H-SiC(0001)
Monodispersed Co nanoparticles on
SiC(0001) honeycomb template
6H-SiC(0001) surface reconstruction
30 nm x 20 nm
(1x1) (3x3) (√3x√3) R30 (6√3 x 6√3 )R30
1170K 1230K 1250K
Photoelectron spectroscopy data of
SiC(0001) surface reconstructions
C 1s
o
h eV 284.4 eV
o
(e) graphite (1300 C)
Counts (a. u.)
285.1 eV
o
(d) nanomesh + graphite (1200 C)
o
(c) nanomesh (1100 C)
o
(b) Root 3 + nanomesh (1050 C)
o
(a) Root 3 (950 C) 282.9 eV
280 282 284 286 288
Binding energy (eV)
The C1s binding energy of carbon nanomesh is at 285.1 ev; graphite (HOPG) is at 284.4 eV.
The well-developed carbon nanomesh surface is formed before the graphitization of the SiC surface.
Therefore, the carbon nanomesh surface is not due one monolayer graphite.
C 1s of the carbon nanomesh surface
C 1s for the carbon nanomesh S1
h eV S1 (285.1 eV)
S2 (283.8 eV)
B (282.9 eV)
Counts (a. u.)
S2
o
(a) B
S1
B S2
o
(b)
278 280 282 284 286 288 290
Binding energy (eV)
The carbon nanomesh is a honeycomb superstructure formed by the self-assembly of
carbon atoms at high temperature.
Two surface-related components for the carbon nanomesh surface have been identified
with a binding energy of 283.8 eV and 285.1 eV, respectively.
Building the SiC(0001) honeycomb model
one-layer thick nanomesh; identical honeycomb cells
topmost Si atoms desorb
all the outermost surface atoms are C atoms
C atoms collapse, can substitute Si atoms below
Building model
Class III
III-12
Building model
Class III
III-13b
DFT-LDA Calculation results
Structure Optimization: force on ion < 10 meV/Å
C atom
Si atom
H atom
fixed
Unit cell parameters: a=b= 18.450Å, c= 20.0Å.
~ 300 atoms; CPU time ~ 3 weeks
DFT-LDA Calculation results
STM images calculated to compare with
experimental images
Partial charge density calculated
Smoothing techniques
STM images
DFT-LDA Calculation results
Model III-12 Model III-13b
DFT-LDA Calculation results
Relaxed structure of Relaxed structure of
model III-12, 2x2x1 cell model III-13b, 2x2x1 cell
Simulated STM images
Model III-12, V=1.6 eV Model III-13b, V=1.6 eV
(a) VT = 1.5V (b) VT = 1.8V
Explanation of PES Peaks
Relaxed nanomesh structure consists of graphene-like
superstructure bonded to Si atoms below.
C1s spectrum can be C 1s for the carbon nanomesh S1
understood h eV S1 (285.1 eV)
S2 (283.8 eV)
accordingly by: B (282.9 eV)
• a graphite-like C-C
Counts (a. u.)
peak (S1) S2
• an asymmetric low o
(a) B
energy tail due to the
boundary C atoms S1
which have both C-C
bonds and C-Si
bonds (S2) B S2
• bulk SiC substrate (b)
o
with Si-C bonds (B)
278 280 282 284 286 288 290
Binding energy (eV)
2. Adsorbate-induced
Reconstructions
SiC(0001)-O
Cu(210)-O; Cu(210)-Br
6H-SiC(0001)-3×3
LEED, E=70eV
60 × 60 nm2 and detailed 9 ×7 nm2 (insert).
(I = 0.30 nA VT = 2.2 V)
6H-SiC (0001) 3×3 twisted reconstructed model
U. Starke et.al, PRL, 80, 758 (1998); PRB, 62, 10335 (2000).
T
T2 30º
60º 60º
T0 T
T3
1 T
T1
3
T
3 4
4
T2 T
T2
2
2 6
6
T
T0 T
T3
A
A B
B T
T0 T
T3
Top view
5
5 1
T
T1
3
1 T1
T
3 4
4 T
T2 2
2
T0
T
T
T3
T1
T
Tetra-
1st-layer
cluster T0 T0
2nd-layer T0
T1 T2 T3
32 1 4A
T1 T2
6
T3
3
B2 1 4
T1 T2 T3 Side
5
Si adlayer view
Bulk Si
Bulk layer
Initial oxidation mechanism
F. Amy, et. al., Phys. Rev. Lett. 86, 4342 (2001)
O2
O2 reacts with the
third Si-layers.
Dangling bond
O2 Si-adatom is
much more active.
Si-adatom sites or
the third Si-layers?
Initial oxidation mechanism
Clean Surface 0.2 L O2 1.0 L O2 2.0 L O2
*
* *
*
** *
2 nm I = 0.10 nA, VT = 2.2 V
In-situ oxidation with low tunneling current to minimize the
inelastic tunneling electron scattering induced reactions.
Dark sites appear initially, saturated after 1.0L O2 exposure.
Bright sites appear after O2 exposure, and keep increasing.
Explanation
Or
O2, initial Si
Si Si
Si adatom+trimer O2 attach on the dangling bond of Si adatom.
Dark sites, O2 depletes the DOS of Si atom
More O2
Si
O2 inserts into the back bonds of Si
adatom. Bright sites, Si atom is lifted
by 0.5 Å. Thermal stable sites.
DFT simulations (Using CASTEP codes)
Models where O2 reacts with the third Si-layer
O Top view
1 1 T1
T1
2 6 2 6
A T3
A
T3
O O O
5 5
3 4 3 4
T2 T2
Side view
A-O-6
2-O-A
2-O-1 2-O-A =120.8o
=119.5o
=121.2o =128.4o
T0 T0 T0
T1 T2 T3 T1 T2 T3 T1 T2 T3
32 1 4A 6 5 32 1 4A 6 5
A1 A2
Models where O2 reacts with the third Si-layer
T0 Top view T0
T3 T3
O
T1 T1
3 3 4
4 T1 T1
O O
6 6
T0 B T0
B
5 5
1 T3 1 T3
O
T2 T2
2 2
T0 T0
T3 T3
Side view
B-O-3 B-O-3
3-O-4 =117.7o B-O-1
=120.1o
=118.5o =119.4o
T0 T0 T0 T0
T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3
6 5 B32 1 4 6 5 B32 1 4
A3 A4
Models where O2 reacts with Si-adatoms
O2
+ Tetra-
cluster
T0 T0
T2 T3 T2 T3
T1 T1
5 2 5 2
4 1 63 4 1 63 T0 T0
C1
T2 T3
T2 T3
T1 T1
+ 5 2 5 2
4 1 63 4 1 6
T0 T0
C3 C4
T2 T3 T2 T3
T1 T1
5 2 5 2
4 1 63 4 1 63
C2
The model where O2 insets into the back bonds of
the Si-adatoms is thermally most stable!
Oxygen Chemisorption
Surface
coverage: C energy:
models
(ML) ∆E (eV/unit cell)
C1 -4.10
C2 -4.32
C3 -5.61
x=1, C = 2/9
C4 -6.93
3×3:2O
A1 -3.52
surface
A2 -3.48
A3 -3.50
A4 -3.51
Chen W, Xie XN, Xu H, Wee ATS, Loh KP
Atomic scale oxidation of silicon nanoclusters on silicon carbide surfaces
J PHYS CHEM B 107 (42): 11597-11603 OCT 23 2003
Adsorbate-induced Surface Reconstructions
O-Cu(210) adsorbate induced reconstructions
(a)
FHS-BR FHS-MR LBS-BR
d0
3
d02 d01 d1
[120] 2
L0
2 [120]
L0
1
L0
(2x1) 3
Definition of parameters for LBS-MR
[001]
Superstructure formation in the LBS-MR
Cu(210)-O system LBS-MR (oxygen at long bridge site with missing row), LBS-
1000 x 1000 Å2 image of (2x1) BR (long bridge site with inward buckled row), FHS-MR
reconstruction (four-fold hollow site with missing row) and FHS-BR (four-
fold hollow site with inward buckled row)
Wee ATS, Foord JS, Egdell RG,
Pethica JB, Phys. Rev. B 58 (1998) Tan K. C., Guo Y. P., Wee A. T. S. and Huan C. H. A., Surf.
R7548. Rev. Lett. 6 (1999) pp. 859-863
Adsorbate-induced Surface Reconstructions
1st Cu-O row (side view) 2nd Cu-O row (side view)
d01=0.12Å d01’=-0.17Å
d12 d12 ’
0.99Å (+22.6%) 1.03Å (+27.6%)
D22=0.17Å D22=0.17Å
d23’
d23 0.78Å (-3.4%)
0.70Å (- D33
13.3%) D33
d34 d34’
D44 D44
LEED study of oxygen-
induced
reconstructions on L01’=0.54Å
Cu(210) L01=0.25Å
Buckled (3x1)
reconstruction – 2/3
ML L00=4.84Å(+19.9%)
Guo YP, Tan KC, Top view
Wang HQ, Huan CHA, [120]
Wee ATS, Phys. Rev.
B 66 (2002) 165410.
[001]
Adsorbate-induced Surface Reconstructions
Cu(210)-O superstructures
(a) (b)
A
B
C
D
[121]
[12 1] [001]
[001]
(a) (a) 2000 x 2000 Å2 (VB = -1.0 V, IT = 2.5 nA),
(b) (b) 300 x 300 Å2 (VB = -1.0 V, IT = 0.30 nA) images after 500 L RT oxygen exposure and
subsequent annealing to 620 K for a few minutes. Analysis of corrugation profiles shows
that A and C are at the same height, whereas B is one unit cell below and D one above.
Adsorbate-induced Surface Reconstructions
[121]
[001]
Cu(210)-Br system Cu(100)-Br system
200 x 200 Å2 images of the triangular checkerboard T.W. Fishlock, J.B. Pethica and R.G. Egdell,
recorded at VB = -1.0 V, IT = 0.1 nA, showing an inversion of Surf. Sci. 445, L47 (2000)
the triangles during different scans but using the same tunnel
current and sample bias.
Wee ATS, Fishlock TW, Dixon RA, Foord JS, Egdell RG,
Pethica JB, Chem. Phys. Lett. 298, 146 (1998)
Adsorbate-induced Surface Reconstructions
Cu(100)-N
system
Adsorbate
induced
nanostructures
also observed in
Cu(110),
Cu(111)-N
systems
F. M. Leibsle,
Surf. Sci. 514, 33
(2002)
3. Surface as Template
Monodispersed Co nanoparticles
on SiC(0001) honeycomb template
Co ring clusters on Si(111)-(7×7)
Self-assembly in a Honeycomb template?
Monodispersed Co nanoparticles on
SiC(0001) honeycomb template
(a) (b)
4nm
(c)
1.7Å 0.1ÅCo 16×16nm2 STM filled state images for
(1) the carbon nanomesh with:
(a) 0.1Å Co coverage
nanomesh (2) (b) 0.2Å Co coverage
(c) line profile 1 for (a) and line 2 for
clean surface. VT=2.5V
Monodispersed Co nanoparticles on
SiC(0001) honeycomb template
At the lower coverage (0.1Å Co), the
clusters will adsorb on these active sites, with
a diameter of 1.4±0.2nm and a height of
1.7±0.1Å.
At the higher coverage (2.0Å Co),
neighbouring Co clusters will coalesce to
form big clusters, 3.4±0.2 nm in diameter
and 3.3±0.1Å in height.
• Monodisperse Co nanoclusters can be
fabricated on SiC honeycomb template under
submonolayer condition.
• Boundaries of honeycomb structures serve
8nm×8nm STM image: blue circles as active sites for Co cluster growth.
highlight the Co cluster adsorption sites.
References:
W Chen, KP Loh, H Xu, ATS Wee, Appl. Phys. Lett. 84 (2004) 281
W Chen, KP Loh, H Xu, ATS Wee, to appear in Langmuir.
cf. Boron Nitride Nanomesh
M. Corso et al., Science, 303 217 (2004)
The BN nanomesh was formed by deposition of B3N3H6 on
Rh(111).
Hole formation is likely driven by the lattice mismatch of the film and
the rhodium substrate.
This regular nanostructure is thermally very stable and can serve as
a template to organize molecules, e.g. C60 molecules.
Co ring clusters on Si(111)-(7×7)
C
C′
U
B F
A
Co ring clusters on Si(111)-(7×7)
M.A.K. Zilani, Y.Y. Sun et al.,
in preparation
Empty state: 1.9 V , 0.1 nA
STM simulation
Published work on other nanotemplates
In nanocluster array formed on 3nm
Si(111)-7×7 surface.
J. L. Li, PRL, 88, 066101 (2202) The hexagonal networks were formed by
co-deposition of PTCDI and melamine
molecules on Ag/Si(111). J.
A. Theobald, Nature, 424, 1029 (2203)
Acknowledgements
Current students: Research Fellows:
Md. Abdul Kader Zilani Dr Xu Hai
Qi Dongchen Dr Liu Lei
Dr Guo Yong Ping
Past students: Dr Xie Xianning
Ong Wei Jie Dr Gao Xingyu
Tan Kian Chuan
Wang Huiqiong Collaborators:
Dr Zheng Jincheng Dr Loh Kian Ping
Dr Sun Yiyang* Dr Tok Eng Soon
Dr Chen Wei* Dr Wang Xuesen
A/P Alfred Huan
* Currently Research Fellow A/P Feng Yuan Ping
