PHYSICAL REVIE%' B
VOLUME 39, NUMBER 15
15 MAY 1989-II
Surface structure of donor graphite intercalation compounds
by scanning tunneling microscopy
D. Anselmetti, R. %'iesendanger, and H. J. Guntherodt
Department
of Physics,
University
of Basel, Klingelbergstrasse 82, CH 4056-Basel, Switzerland
(Received 3 February 1989)
Scanning tunneling microscopy (STM) has been used to study the surface of C6Li as a stage-1
donor graphite intercalation compound from a submicrometer down to the atomic scale. Ordered
superlattices commensurate as well as incommensurate with the graphite lattice have been observed.
The measured STM corrugation at small bias voltage (&200 mV) has been found to be similar to
graphite whereas at larger bias voltage a strong decrease of the corrugation was observed. This experimental result is compared with recent theoretical predictions.
Graphite intercalation compounds (GIC's)' have been
the object of numerous experimental and theoretical investigations in recent years due to their interesting electronic, optical, and transport properties. Their microscopic bulk structure has been studied in detail by highresolution transmission electron microscopy.
On the
other hand, only a little is known about the surface structure of GIC's on a submicrometer scale. Pioneering work
in this field has been reported by Levi-Setti et aI. ' They
developed a high-resolution
scanning-ion
microprobe
capable of 20 nm lateral resolution for studying the surfaces of GIC's. Since the invention of the scanning tunneling microscope (STM) by Binnig et al. , it has become
possible to study the structure of surfaces directly in real
space even on an atomic scale. Nowadays graphite is
commonly used as a test sample for STM in various environments due to its relatively inert surface, although the
unusual electronic and elastic properties of graphite have
caused problems in the interpretation
of STM im' The extension of STM investigations to GIC's
ages.
is currently of interest for two reasons. First, the
different electronic structure such as the shape of the Fermi surface of GIC's and graphite
should lead to
differences in the STM results. A strong reduction of the
measured STM corrugation on C6Li is theoretically predicted in comparison with graphite.
Second, one
should be able to study the unknown surface structure of
GIC's on an atomic scale by using STM. The extreme
sensitivity to surface contamination particularly of donor
GIC's' has prevented detailed STM studies of GIC's so
far. A first report of STM applied mainly to acceptor
GEC's has recently been given by Gauthier et al. ' '
They found a graphitic surface structure for two acceptor
GIC's with the same range of corrugation as on graphite.
The experimental results were interpreted by a surfaceacceptor depletion. ' In this paper we will present the
first atomic-resolution STM studies on C6Li as a stage-1
donor GIC. Ordered superlattices commensurate as well
as incommensurate
with the graphite lattice have been
observed with different STM tips on different positions of
the sample surface. The measured corrugation in the
STM images on C6Li are nearly the same as on graphite
',
39
for a sample bias up to 200 mV. At larger bias a strong
reduction of the corrugation is observed.
The small and rigid scanning tunneling microscope
used in this study was fully machined out of glass ceramic
(Macor) with a thermal expansion coefficient similar to
the piezoelectric material used for the scanning unit.
Therefore a low thermal-drift rate during the STM measurements could be achieved ( &0.01 nm/s). The approach of the sample towards the tip is based on a
differential lever system driven by two screws. The STM
itself is mounted on a stack of five stainless-steel plates
separated by Viton rubber elements for vibration isolation. The STM tips were prepared by electrochemical
etching a 0.2-mm-diam tungsten wire in a KOH solution
and finally cleaned in hot distilled water. The C6Li samples were obtained by a liquid-phase reaction of highly
oriented pyrolytic graphite (HOPG) with molten lithium
(reaction temperature: 250 C, exposure times between 24
and 60 h). The stage of the samples were determined by
x-ray diffraction. The bulk structure of C6M (M denotes
an alkali metal) compounds at room temperature
is
shown in Fig. 1(a). The alkali metal is located between
the graphite layers and forms a &3X&3 superlattice.
The 2X2 superlattice shown in Fig. 1(b) corresponds to
the bulk structure of C8M compounds.
The surface
structure
of GEC's is almost unknown.
To our
detailed
knowledge,
low-energy
electron diffraction
(LEED) studies have only been reported for evaporated
alkali-metal overlayers on HOPG,
where both or-
~3 x~g
X
FIG. 1. Bulk structure of (a) C6M
and (b)
(I denotes
an alkali metal)
C,M graphite intercalation compounds at room temper-
ature.
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1989
The American Physical Society
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BRIEF REPORTS
39
'10-
lI
0.3
(J/
l
/( ll
0.4
I I (& I I
I(
I
0.5 ( nrn)
FIG. 4. Histogram showing the distribution
in-plane lattice constants on the C6Li surface.
FIG. 2. STM overview image (250X250 nm ) of the C6Li
surface obtained in the constant-current mode (I = 1 nA, sam200 mV).
ple bias voltage U = —
dered and disordered overlayers were observed depending
on the temperature range. It is the aim of this paper to
contribute to the understanding of the surface structure
of GIC's. The STM measurements at room temperature
on the C6Li samples had to be performed in a high-purity
ThereAr atmosphere to avoid surface contamination.
fore the STM was operated in a dedicated stainless-steel
and the samples have been cleaved in situ
glove box,
We want to
prior to each series of STM measurements.
point out that cleaving stage-1 GIC s like C6Li will obviously leave part of the intercalant on top of the surface.
This fact will be important for the interpretation of the
STM results.
First we imaged the surface of C6Li samples on a submicrometer scale to determine the surface step density
(a)
(b)
of the observed
and the size of atomically Hat regions preferable for
high-resolution STM studies. A typical constant-current
STM image of a 250X250-nm surface area of C6Li is
presented in Fig. 2.
Terraces as large as (100 nm) without any steps can
easily be found. After locating the STM tip above the
defect-free terraces, we observed well-ordered superlattices on an atomic scale with either positive or negative
sample bias voltage many times (more than 80) with
different STM tips. In Figs. 3(a)—3(c) we present STM
current images
of some superlattices observed on the
C6Li surface. The in-plane lattice constants seen in these
images are 0.35+0.02 nm [Fig. 3(a)], 0. 42+0. 02 nm [Fig.
3(b)], and 0. 49+0. 02 nm [Fig. 3(c)]. The first-mentioned
in-plane lattice constant would correspond to an incommensurate lattice with nearly close-packed lithium. (The
nearest-neighbor
distance in hexagonal lithium is 0.311
nm. However, if there is a charge transfer from the lithium to the graphite, electrostatic repulsion would be likely
to increase this spacing. ) The other two lattice constants
are commensurate with the graphite lattice constant corresponding to &3X&3 and 2X2 superlattices. The inplane lattice constants observed in many other STM images of the C6Li surface are grouped around the three
values mentioned above as shown in the histogram of Fig.
4. The spread around the lattice constant of 0.35 nm is
somewhat larger than for the other two lattice constants
which can be explained by the incommensurability
of this
The measured variation of the tunneling
superlattice.
current at small bias voltage ( (200 mV) is about 15%%uo of
(c)
FIG. 3. STM current images (1.8 X 1. 8 nm ) obtained on defect-free terraces of the C6Li surface showing different kinds of superlattices with in-plane lattice constants of (a) 0.35+0.02 nm, (b) 0.42+0. 02 nm, and (c) 0.49+0. 02 nm. The lateral length scale was
calibrated by comparison with atomic resolution STM images on graphite (lattice constant 0.246 nm) obtained with the same STM
= 1 nA, sample bias voltage U = —100 mV for all three images. )
tip. (Mean tunneling current
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11 137
the mean value (1 nA) in the STM image with the smallest in-plane lattice constant of 0.35 nm. This is
significantly smaller than the observed variation of the
tunneling current of about 30% for the STM images with
in-plane lattice constants of 0.42 and 0.49 nm. A variation of the tunneling current of about 30%%uo corresponds
to a STM corrugation of about 0. 10—0. 15 nm as measured
in the constant-current
mode. This is comparable with
the corrugation observed on HOPG. A strong decrease
of the corrugation on C6Li below the noise level of about
0.01 nm is observed for a sample bias voltage above 300
mV. On the other hand, the observed corrugation on
HOPG remains far above the noise level up to a sample
bias voltage of at least 800 mV. Finally we note that an
increase of the noise level was observed with time which
we attribute to surface contamination, e.g. , lithium reacted with residual 02 or H20. We believe that such reactions start at surface regions containing many defects like
the one in the lower right-hand corner of Fig. 2 before
they take place on the defect-free, atomically Aat terraces. This would explain the surprising good quality of
the atomic-resolution
STM images on the terraces obtained directly after cleaving the sample.
Any explanation of the experimental results has to take
into account the electronic structure of the C6Li surface
because STM images of graphite and its intercalation
compounds are known to be dominated by the surface
electronic rather than the atomic structure.
Recently,
Selloni et a/. ' have calculated the surface electronic
structure of C6Li and derived the expected STM corrugation. Their model consisted of a slab of four carbon and
three lithium layers with the lithium forming a &3 X &3
superlattice.
They predicted an STM corrugation of
about 0.02—0.04 nm at small bias voltage ((300—500
mV), whereas for a larger bias voltage, the corrugation
should rapidly fall to zero due to tunneling into smooth
interlayer states. Although the latter prediction is in full
agreement with our experimental
results, we observe
significantly larger corrugations of about 0. 1 nm for a
bias voltage of less than 200 mV. The most likely explanation for this obvious discrepancy is that the top layer of the C6Li sample is not a carbon layer as assumed in
the theoretical model but a lithium layer. It is important
to note that the experimental procedure used to prepare
clean C6Li surfaces by cleaving the sample will leave part
of the intercalant, i.e., lithium, on top of the surface. The
occurence of the observed in-plane lattice constants
which are not consistent with the bulk &3 X &3 structure, in particular the incommensurate superlattices, can
also be better understood by a lithium layer being on top
of the surface. It is unlikely that a possible enhancement
of the observed corrugation due to an elastic response of
the C6Li surface, similar to the case of graphite, ' ' can
fully account for the difference between the observed and
predicted corrugation, particularly in a bias voltage range
of 50—200 mV and a mean-tunneling current of 1 nA.
The first real-space observation of ordered superlattices
on the surface of C6Li compounds with mainly three
different in-plane lattice constants as presented in this article may be compared with LEED studies of evaporated
lithium overlayers on top of HOPG substrates,
where
overlayers without long-range order as well as overlayers
strucforming hexagonal close-packed incommensurate
tures have been identified, depending on the temperature
and coverage range. For the C6Li compounds studied in
this work it seems likely that due to the high mobility of
lithium on top of the surface, different kinds of superlattices either commensurate or incommensurate with the
graphite lattice can locally be realized. Under this assumption, different values of in-plane lattice constants at
room temperature and even transitions from one in-plane
lattice constant to another as observed during the STM
measurements can be explained. It is one of the great advantages of STM that the real-space atomic-scale structure of small surface regions can be studied. Therefore
detailed structural information on surfaces exhibiting
only local order can be obtained in contrast to LEED experiments. Finally, we want to point out that STM applied to donor GIC's will be extremely useful to study the
metallic
physical properties of quasi-two-dimensional
monolayers in detail.
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ACKNOWLEDGMENTS
We would like to thank V. Geiser for preparing and
characterizing the GIC samples and Dr. A. W. Moore
(Union Carbide Corporation)
for kindly providing
HOPG. We also thank M. Baur, H. R. Hidber, P.
Reimann, and L. Rosenthaler for technical help. We are
also thankful to the whole STM/AFM group at our University for many stimulating
discussions.
Finally, we
thank Professor Dr. K. Liiders for his help in the early
stage of this work. Financial support from the Swiss National Science Foundation is gratefully acknowledged.
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