Atomic force microscopy and scanning tunneling microscopy studies of AlCl3-graphite intercalation compound

Letters to the Editor

Atomic force microscopy and scanning tunneling microscopy AlCl,-graphite intercalation compound

277

studies of

N. I-YA, E. SHIMAZU and s. HARA Department of Materials Science and Processing, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita 565, Japan H. SHIOYAMA and Y. SAWADA Osaka National Research Institute, AlST, Midorigaoka 1-8-31, Ikeda, 563, Japan Key Words - Atomic force microscopy; scanning tunneling microscopy; AlC13-graphite intercalation

compound; surface structure

A major topic in intercalation chemistry is elucidating the relationship between the graphite intercalation compounds’ (GICs) properties of interest and their atomic structures. Metal chloride-GICs(MC-GICs) have generated much interest as highly conducting materials, solid lubricant materials, and so on. The microscopic bulk structures of MC-GICs have been extensively investigated using x-ray and electron diffraction methods [l]. In spite of the importance of the knowledge about the surface structures, only a few studies have been performed. For example, Levi-Setti et al. [2] have reported on the surface microstructure of SbCls-GIC using a high-resolution scanning ion microscope. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been introduced as direct probes for elucidating the surface electronic structures as well as surface atomic arrangements. Although surface structures of MC-GICs have been investigated by STM in the past few years [3], the intercalants at the top most layers have not been imaged. We have already reported atomic-resolution AFM studies on surface structures of several kinds of acceptor-type GICs [4,5]. In this paper, we report on atomic-resolution AFM and STM studies on the AlC13GIG surface. The AlCls-GIC samples were prepared by the standard two zone method. In this method, the highly oriented pyrolytic graphite(HOPG, Union Carbide, type ZYA) and AQ(reagent grade) were sealed under a presence of 360mmHg chlorine gas and heated for 8 days at 573K. The stage structure of samples, which is the stacking sequence along the c-axis, was verified by the x-ray diffraction. All of the AFM and STM measurements were carried out in air at room temperature using a Nanoscope III (Digital Instruments Inc.). The experimental details of AFM observations have been described elsewhere [4,5]. Atomic-resolution STM images were acquired by the constant height mode(tunneling current mapping). A Pt-Ir wire was used as the STM tunneling tip. The set-point current (It) and the sample bias voltage were typically l-2nA and +lO-lOOmV, respectively. All of the samples were freshly cleaved just before the observations. For reference purposes, the atomic-resolution AFM and

STM images of the graphite were recorded before and after the experiments of GIC. From x-ray diffraction , the repeat distances along the c-axis, I,, of stage-l and -2 AlCls are 0.959nm and 1.287nm, respectively, which agree with those reported [6]. A small amount of unreacted graphite was recognized. Although the microscopic bulk structure was observed by transmission electron microscopy (TEM) at 200kV accelerating voltage, the bright field images changed with time due to the irradiation damage by the electron beam or to the decomposition of the sample under a vacuum condition. Therefore, the arrangements of the AlCls molecules intercalated sandwiched between graphite layers were not determined by TEM. Our AFM and STM results on the graphite surface are in good agreement with those reported in earlier papers [4,7]. As for the CuClz- and FeC13-GIC surface [5], the atomically flat terrace-step structure could be exposed easily by cleaving for AlCls-GIC. On an atomic level, AFM and STM experiments yielded completely different images. Figure la is an AFM image showing a hexagonal lattice with spacings of 0.36nm. On the other area, a graphitic structure with spacings of 0.25nm was also observed. As reported previously [5], we consider that the intercalated molecules are not divisible and the AlCl3 molecules remain on either the upper or the lower graphite layer on cleaving. This is attributed to the weak binding force between the graphite layers and the intercalated molecules. Therefore, we interpret that the bright spots in Fig. 1a as corresponding to Cl atoms (hatched atoms in Fig. lb) above the plane of Al atoms. By means of STM, besides the graphite lattice, a longer-range hexagonal lattice with spacings of about lnm forming a moire pattern appeared as discerned in Figure 2a-c. Upon altering the sample bias in the range between -1OOmV and lOOmV, no changes in this atomic structure were recognized. The direction of the longerrange lattice is rotated by about 22 * 2’ from the graphite lattice with spacings of 0.25nm. Most probably, the tunneling maxima in the moire pattern correspond to a higher electron density caused by stronger interaction between the Cl (in the AlCl3 molecules) and C atoms, although the hexagonal lattice

Letters to the Ediior

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-3.00

-2.00

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0.959nm

I

e graphite

0

Cl

:

1 aye*

l

Al

Figure l(a). Atomic resolution AFM image (3.5 mn x 3.5 nm) of AlCl+XC surface. (b) The structural model of cross section of AIClr-GIG. The large and small circles correspond to Cl and Al atoms, respectively. The cleavage occurs at me two arrows. The hatched Cl atoms are visible in the image (a).

-10.0

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5:o

7.5

10.0

-0 12’.5.5.00

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Figure 2(a) Atomic resolution STM image (12.5 nm x 12.5 nm) of AICII-GIC surface showing me moire pattern (V, = -lOmV, I, = 2nA). (b) Details (5 nm x 5 nm) of (a) (V, = -lOmV, I, = 2nA). (c) Filtered image of (b). (d) Structural model of the Cl atoms on the graphite lattice. The large and small circles correspond to the Cl and p-site C atoms, respectively.

Letters to the Editor of the Cl atoms with spacings of 0.36nm was never seen It is generally accepted that the bright spots in STM images of graphite correspond to the C atoms on the p-sites, which are located above the center of the sixfold C rings in the second layer [7]. So, we interpret that the tunneling maxima are associated with the Cl atoms bound in atop (or near-atop) binding sites above or below the p-site C atoms. The moire pattern presumably arises from periodic variations in the binding site of Cl atoms necessitated by the significant mismatch between the Cl and C atoms. Therefore, a possible structural model is constructed on geometric grounds, as shown in Fig.2d. In this model, the large and small circles correspond to the Cl atoms determined by AFM (Fig.la) and the p-site C atoms, respectively. A main feature of the model is that the longer-range hexagonal lattice with spacings of l.O8nm, close to the observed values, is rotated by 23.4” from the graphite lattice. Indeed, such higher tunneling maxima for atop coordination are very noticeable in the halide adsorbate-noble metal [8] and metal adsorbates-HOPG [9] systems as discerned by STM. Therefore, it was found that the intercalated AlCls molecules were rotated by 23.4” from the graphite lattice on the stage-l AlCls-GIC surface. On the other areas, the moire pattern was not evident, but only the graphitic structure was observed because the samples contain the stage-2 AK&-GIC and unreacted graphite. Although we have reported on the incommensurate superlattice structure on the stage- 1 CuC12- and FeCls.GIC by AFM[5], the arrangement of the intercalated molecules with respect to the graphite lattice could not be determined. It was found that comparative AFM and STM studies yielded

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information about the precise positions of the intercalated molecules on the MC-GICs surface. AFM and STM studies on the other MC-GICs surfaces are now under investigation. In conclusion, we have observed the surface structures of AlCla-GIC in air by AFM and STM. According to the AFM results, we have found that the Cl atoms form a hexagonal superlattice structure with spacings of 0.36nm. On the other hand, STM results yielded a completely different atomic image showing a moire pattern. Comparative AFM and STM experiments demonstrate that the intercalated AlC13 molecules are rotated by 23.4” from the graphite lattice on the stage- 1 AlCls-GIC surface. Acknowledgment

- This work was supported by the Ministry of Education, Science and Culture, Grant-in-Aid for Encouragement of Young Scientists No.06750678.

REFERENCES 1_

E.L. Evans and J.M. Thomas, J. Solid State Chem., 14,

99 (1975).

2. R. Levi-Setti, G. Crow, Y.L. Wang, N.W. Parker, R. Mittleman and D.M. Hwang, Phys. Rev. Lett., 54, 2615 (1985). 3. P. Biensan, J.-C. Roux, H. Saadaoui and S. Flandrois, Microsc. Microanal. Microstruct., 1, 103 (1990). 4. N. Ikemiya, S. Hara, K. Ogino and T. Nakajima, Surf. Sci., 274, L524 (1992). 5. N. Ikemiya, Y. Okazaki, S. Hara and T. Nakajima, Carbon, 32, 119 1 (1994). E. Stwnpp, Mater. Sci. Eng., 31,53 (1977). 76: S. GWOand C.K. Sbih, Phys. Rev, B47, 13059 (1993). 8, For example, W. Haiss, J.K. Sass, X. Gao and M.J. Weaver, Surf: Sci., 274, L593. 9. V. Maurice and P. Marcus, Surf: Sci., 275,65 (1992).

of carbon nanotubes

K. ESUMI,M. ISHIGAMI, A. NAKAJIMA, K. S AWADA and H. HONDA Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Sbinjuku-ku, Tokyo 162, Japan (Received

12 October

1995;

accepted

Key Words- Carbon nanotubes;

The carbon nanotubes discovered by Iijima [l] exhibit many interesting properties such as high mechanical strength, [2] and a remarkable electronic structure [3-S] which promise a wide range of potential uses. There are two methods for preparation of carbon nanotubes, i.e., arc-discharge [l] and catalytic methods.[6] The main limitation of the arc-discharge process is that the size of carbon nanotubes is relatively small (< 1 mm) and the low yield of the process makes the product expensive. On the other hand, the catalytic production method overcomes these difficulties. The carbon nanotubes produced by this method can have lengths up to 15 mm. In addition, the process is simpler and has a higher productivity than the arc-discharge process.

in revised form I7 December

1995)

oxidation treatment; zeta potential

However, the carbon nanotubes manufactured by the catalytic process are usually thicker than those by the arc-discharge process and often consist of large aggregates up to a few tens of mm. So, it is vital to render those aggregates into a microscopic scale for many interesting applications. The objective of this study was to elucidate the effects of a chemical treatment of carbon nanotubes in order to obtain a better dispersion in the individual fibrils. The carbon nanotubes used were produced by the catalytic process by passing mixtures of some gases over the catalyst bed of A1203 and Fe. Concentrated nitric acid and sulfuric acid were used for chemical treatment. Samples (lg) of the carbon nanotubes were