ON THE INTERPRETATION OF GRAPHITE IMAGES OBTAINED BY STM
Constantinos Zeinalipour-Yazdi1, Jose Gonzalez2, Karen I. Peterson2, and David P. Pullman2.
(1)Department of Chemistry, UC San Diego & San Diego State University, San Diego, CA 92182-1030, czeinali@chem.ucsd.edu.
(2) Department of Chemistry, San Diego State University.
It
0
130
260
390
520
distance in a.u.
We also simulated the STM image of these
structures using Tersoff-Hamann3 treatment for
the tunneling current. The tunneling current at
small bias voltage under the spherical tip
approximation is:
3
2
4
6
0.110
0.078
2
0.1
74
0.0 0.04
78 6
0.174
0.142
6
0.04 8
0
0.07
0.11
0.14
10
0.1
0.174
2
-4
0.174
46
0.0
-6
0.142
-5
0.078
0.110
0
0.11
-4
0.078
0.174
0.0
46
0.142
0.1
74
0.14
0.174
0.110
0.078
0.046
4
0.17
0.110.078
0
0.078
0.110
-3
0.1
74
0.046
0.078
-2
0.046
78
0.0
0
0.11
0.174
2
0.14
46
0.0 8
0
0.07
0.11
0.142
-1
0.142
0.174
0.142
10
0.1
0.1
74
1
-0
0.14
2
0.17
4
0.046
2
0.14
2
0.17
4
0.110
-2
-0
2
4
6
x axis in bohr units
Discussion and conclusion:
We believe that the anomalous corrugation
seen in STM images of HOPG may not be
associated with any interaction of subsurface
graphitic layers. Our argument is based on STM
images of monolayer Graphite islands formed
from thermal decomposition of Silicon carbide5
or heteroepitaxially grown from ethylene on
Pt(111)6 have shown features identical to those
of HOPG. The large interlayer distance of
3.354 A estimated from single crystal X-ray
diffraction and the weak Van der Waals
interaction makes this argument even stronger.
Our simulated STM images show that it is
possible to have a pi-localized molecular
orbitals as a result of H atoms chemically
bonded at the periphery of graphitic layers.
The simulated STM images suggests that
this localization would in fact produce the
features we observe in experimental STM images
of HOPG.
Calculations at higher level and
on larger systems are under way to better
mimic HOPG STM images.
(2)GAMESS, M.W. Schmidt,J.Comput.Chem.,1992,
14,1347-1363.
C48H20
C48H18
Y direction
H
H
2
0.14
(1)Park S.S.,J.Phys.Chem.,1998,102,6020-4.
C30H14
H
H
H
H
0.110
0.142
Literature cited:
C30H16
X direction
3
4
C16H10
H
H
4
0.17
H
H
Observation:
The elliptical shape
of the bright spots that
we and other groups have
seen in our experimental
STM images of HOPG
suggests the possibility
of alternative
explanations.
-0
2
x axis in bohr units
74
0.1
C16H12
2nd layer
0.174
5
0.110
0.078
0.046
layer
6
In order to test this hypothesis we
did ab-initio electronic structure
calculations2 using Density Functional
Theory of several polycyclic aromatic
and antiaromatic hydrocarbons.
A
-2
4
B
st
3
C-C # Average XRD B3LYP/6-31G
bond length bond length
1
1.341 A
1.365 A
2
1.428 A
1.431 A
3
1.423 A
1.434 A
4
1.399 A
1.407 A
5
1.438 A
1.440 A
6
1.383 A
1.397 A
Only MO’s that belong to B2g B3g Au & B1u
irreducible representations and had significant
contribution in the center of the molecule were
included to avoid edge effects.
Model:
A
-4
0.17
B
-6
74
0.1
1
A
-5
4
0.17
Interpretation given by Park et al1 :
B
-4
0.046
We used the B3LYP functional and the 6-31G
basis set which gave very good agreement with the
structure of pyrene obtained from low temperature
X-ray diffraction4.
4
A
-3
6
Results:
2
B
1
-2
0.04
We did not evaluate the first term of
eqn.1, because it effects only the contrast of
the STM image. Only the second term (eqn.2)
which is the LDOS near the fermi level affects
the shape of the spots in the simulated STM
image.
5
A
2
-1
Hypothesis:
1
B
3
3
-0
v
Maybe the protrusion that appear in
HOPG STM images do not correspond to
Carbon positions but to enhanced
Introduction:
electron density near the Fermi level
that is a result of pi-localized
Atomic resolution STM Image of Graphite based molecular orbitals. This pi-localization
on the crystal
that would correspond to a structure of
images of Graphite
structure
higher energy might be a result of
hydrogen atoms chemisorbed at the
periphery of graphitic layers in such a
manner that promotes pi-localization
close packed hexagonal
honeycomb like
rather than delocalization.
structure
structure
2 kind of Carbon
atoms,A and B,B has a
neighbor carbon in the 2nd
layer whereas A does not.
A and B sites of Graphite
have a different LDOS near
the Fermi Level.
Protrusions in STM image
correspond to B sites.
4
8  e    V  Dt E F   R  

2R

It =
 exp
 2  me      ro , E F  eqn.1
 

me 2
2


eqn.2
 ro , E F  =  v  ro     E v  E F 
2
Greater number of spots in Y
direction than in X direction.
5
y axis in bohr units
Highly Oriented Pyrolytic Graphite
(HOPG) has been used as a standard for STM
calibration for over a decade because of
the relative ease of imaging in air and
vacuum coupled with the known carbon-carbon
distances. Most images show only three of
the six carbon atoms in a given sixmembered ring. This observation has been
rationalized in several ways, although no
entirely satisfactory explanation exists
yet. In this work, a new interpretation of
the graphite STM image is proposed.
y axis in bohr units
Abstract:
H
H
We simulated the constant height STM images
of both C48H20 and C48H18. The tip surface
distance of the former was 1.3 A. The constant
current STM simulation is still running
!#$%*)?.
(3)Tersoff, D.R.Hamann,Phys.Rev.B,1985,31,805.
(4)C.S.Frampton,J.Molec.struc.,2000,520,29-32.
(5)A. Charrier,J.Appl.Phys.,2002,92(5),2479.
(6)T.A. Land,J.Phys.Chem.,1992,97(9),6774.