Ultramicroscopy 73 (1998) 185—189

Giant superstructures formed on graphite surface treated withNaOH solutions studied by scanning tunneling microscopy
K. Miyake!,*, K. Akutsu!, T. Yamada!, K. Hata!, R. Morita",#,M. Yamashita",#, H. Shigekawa!,#,1!Institute of Materials Science, and Center for Tsukuba Advanced Research Alliance (TARA), University of Tsukuba, Tsukuba 305-8573, Japan"Department of Applied Physics, Hakkaido University, Sapporo 062-8626, Japan#CREST, Japan Science and Technology Corporation (JST), Japan
Received 7 July 1997; received in revised form 11 September 1997AbstractGiant superstructures with 1.71 and 9.1 nm lattice constants were observed by scanning tunneling microscopy (STM)on the surface of highly oriented pyrolytic graphite (HOPG) treated with NaOH solution. These structures were analyzed
by the moiré pattern hypothesis. For both the structures, the orientation angle mismatch of the giant lattices relative tothe original atomic lattice of graphite was observed between the theoretical and experimental values. In addition, for thelatter structure, the observed boundaries between the superstructural domain and the normal areas were straight, which
is different from the theoretically predicted zigzag shape boundary. These discrepancies suggest the existence of someother mechanism responsible for the giant superstructures appearing in graphite intercalation compound surfaces. ( 1998 Elsevier Science B.V. All rights reserved.

The graphite (0 0 0 1) surface has been used asa standard sample for scanning tunneling microscopy (STM) study, because of the stability of its
surface obtained by cleaving even in air. However,
it is also well known that the graphite surface
shows various apparent STM images; superstructures appearing near defects [1,2], structures similar to that of macro molecules [3], and so on.
More recently, anomalously large scale periodic* Corresponding author. E-mail: miyake@mat.ims.tsukuba.ac.jp
and hidemi@mat.ims.tsukuba.ac.jp; Home page: http://w.w.w.
ims.tsukuba.ac.jp/lab/shigekawa.
1Corresponding author.

superstructures have been observed on fresh graphite surfaces [4,5]. The anomalous superstructures
have been interpreted by the moiré pattern hypothesis suggested by Kuwabara et al. [4]; Moiré
patterns originate from the lattice misorientation
introduced by the overlapping of a misoriented top
layer of graphite to the underlying graphite single
crystal.
On the other hand, graphite is known to form
intercalation compounds which have many interesting properties differing from those of the hostmaterials, e.g., superconductivity and low-dimensional conductivity. Among them, alkali-metal
graphite intercalation compounds (AM-GIC) have0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved
PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 5 3 - 8

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K. Miyake et al. / Ultramicroscopy 73 (1998) 185—189

been studied widely because of their simplicity inanalysis [6,7]. According to STM observations ofsurface structures of binary and ternary AM-GICs[8—10], in addition to superstructures such as 2]2
and J3]J3, anomalous large scale periodic
superstructures, which were similar to the moiré
patterns described above, were also observed on
the stage-1 AM-GIC (M" Li, K, Rb, Cs) surfaces.
Attempts were made to interpret these large scale
superstructures as charge density wave (CDW)
structures instead of the analysis by the moiré pattern hypothesis, but the origin of the superstructures has not been clarified yet. In order tounderstand the mechanism of the giant superstructures formed on GIC, it is important to clarify theeffect of the intercalants on the host graphite inmore detail.
Recently, we found highly oriented pyrolytic
graphite (HOPG) treated with NaOH solutions tohave an X-ray diffraction pattern characteristic ofa stage-8 structure [11]. On this surface, we also
observed giant superstructures with 1.71 and
9.1 nm lattice constants in addition to the superstructures of 2]2 and J3]J3 by STM. These
giant superstructures appear similar to those previously observed on clean graphite surfaces [4,5].
Since guest materials are separated further in thehigher stage GICs compared to the structure of theAM-GICs described above, it must be very usefulto analyze the giant superstructures formed on thestage-8 GIC in detail.
In this paper, we present the results of the analysis
performed on the giant superstructures appearing
on a graphite surface treated with NaOH solution,on the basis of the moiré pattern hypothesis.
Samples were prepared by pouring NaOH aqueous solutions onto freshly cleaved highly oriented
pyrolytic graphite (HOPG) surfaces or dipping
HOPG crystals into NaOH aqueous solutions.
STM was performed in air at room temperature byusing Pt/Ir tip. All STM images in this paper are
taken in the constant height mode.
Fig. 1 shows STM images of giant superstructures with (a) 1.71 nm and (b) 9.1 nm lattice constants. These giant superstructures have three-foldsymmetry like graphite. Similar superstructures
were observed on clean graphite surface, as described above, which could be well interpreted asFig. 1. STM images of giant superstructures with (a) 1.71 nm(sample bias; » "!20 mV, and tunneling current I "1.9 nA)
45and (b) 9.1 nm lattice constants (» "30 mV, I "0.7 nA).
45moiré patterns [4,5]. Therefore, we apply the moiré
pattern hypothesis to the STM images obtained in
our work in order to clarify the origin of thesesuperstructures.

K. Miyake et al. / Ultramicroscopy 73 (1998) 185—189

187

By the moiré pattern hypothesis, the period D of
the produced Moiré pattern is given by [4]D"d/[2 sin(h/2)] ,(1)

where d is the lattice constant of graphite and h isa rotation angle between the topmost and second
graphite layers. Then, the orientation of the giant
lattice relative to the atomic lattice of graphite H
can be given by [5]H"30°!h/2 .

(2)Let us discuss the obtained giant superstructures inFig. 1 by this method. Magnifications of Fig. 1aand b are shown in Fig. 2a and b, respectively.
In addition to the giant superstructure, thegraphite lattice could be clearly seen as shown in
the figures.
In the case of the giant superstructure shown inFig. 2a, the period of giant lattice D is 1.71 nm.
Substituting the value and d"0.245 nm into
Eq. (1), the value of h becomes &8.38°. Therefore,
the theoretical value of the orientation H of thegiant lattice relative to the atomic lattice of graphite can be obtained as 25.81°. On the other hand,the angle obtained experimentally from the STM
image in Fig. 2a is 26.6°, which is slightly different
from the theoretical value. The observed value inthe difference between theory and experiment issmall (&0.8°), but is within the experimental resolution. Therefore, some other mechanism for thisstructure is expected supposed to exist beyond themoiré pattern hypothesis.
Next we discuss the structure shown in Fig. 2b.
Substituting D"9.1 nm and d"0.245 nm into
Eq. (1), we obtain the value of h as 1.57°, andH becomes 29.22° from Eq. (2). However, the observed value of H is 27.9°. The angle mismatch(&1.3°) is clearly recognized as shown in Fig. 2b,which also suggests the existence of some othermechanism for this structure rather than the moiré
pattern. In addition, there are doubts bout thestructure of the boundary.
The observed boundary between the giant superstructure and the normal graphite lattice has
a straight shape, and both domain boundaries run
parallel as shown in Fig. 3a. As is described above,
moiré patterns originate from the misorientation

Fig. 2. Magnified STM images of (a) Fig. 1(a) (» "20 mV,
4I "1.6 nA), and (b) Fig. 1(b) (» "!20 mV, I "0.9 nA).
54
5

introduced by the overlapping of a misoriented toplayer of graphite to the underlying layers, the structure of which is shown schematically in Fig. 3b. In
Fig. 3b, a misoriented top layer with a rectangular

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K. Miyake et al. / Ultramicroscopy 73 (1998) 185—189

shape indicated by dashed boundaries is overlapped to the underlying graphite surface indicatedby the large square in the figure. The graphitelattices are not drawn here, and the open circles
indicate the giant superstructures which are expected to be observed by STM. The lattice of thesuperstructure extended to the normal areas at theboundaries are indicated by solid circles in Fig. 3b.
When the top layer has a straight boundary, theobserved superstructure induced by the moiré
pattern must have a zigzag shape at the boundary
as indicated by the solid lines in Fig. 3b. In
fact, according to the previous report on theSTM image of a moiré pattern, the observed
boundary between a superstructure and the normalgraphite structure had a zigzag shape similar tothat in Fig. 3b [4]. Therefore, from the results of
the angle mismatch and the difference in theboundary structures between the theory and experiment, the moiré pattern hypothesis is not suitable for the analysis of the giant superstructure inthis case, too.
The obtained results in our work suggest theexistence of some other mechanism for the formation of the observed giant superstructures on GIC;
some interaction between the intercalants, and thehost graphite may result in the structures. In orderto understand the relationship between the characteristic properties of GIC and the giant superstructures, further experiments and theoretical studies
are necessary.
In conclusion, giant superstructures with 1.71
and 9.1 nm lattice constants were observed by
STM on the surface of HOPG treated with
NaOH solution. Analysis by the Moiré pattern
hypothesis could not explain the structurescomprehensively. Some other mechanism must beintroduced to understand the giant superstructures
on GIC.

Fig. 3. STM image (» "30 mV, I "0.9 nA) of the boundary
45
between giant superstructure with 1.71 nm lattice constant andnormal graphite lattice (a) and its structural model (b).

This work was supported by the Shigekawa Project of TARA, University of Tsukuba. Support ofa Grant-in-Aid for Scientific Research from theMinistry of Education, Science and Culture ofJapan is also acknowledged. One of the authors(K.M.) was financially supported by the Japan
Society for the Promotion of Science (JSPS) Fellowships for Japanese Junior Scientists.

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