Scanning tunneling microscopy study of ternary alkali-metal graphite intercalation compounds

UItramicroscopy 42 44 (1992) 624-629

North-Holland

u//ram/ero,seopy

Scanning tunneling microscopy study of ternary alkali-metal graphite intercalation compounds H . P . L a n g , V. T h o m m e n - G e i s e r

a n d R. W i e s e n d a n g e r

lnstitut fiir Physik, Uniuersith't Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland

Received 12 August 1991

Scanning tunneling microscopy (STM) has been applied to investigate the submicrometer and atomic structure of ternary alkali-metal graphite intercalation compounds (GIC's) in a high-purity inert gas environment. Here we focus on ternary stage-1 GIC's as MM'C s (M, M' = K, Cs, Rb). On a submicrometer scale, we observe islands of decreased or increased apparent topographic height, suggesting an inhomogeneous alkali-metal distribution on a lateral scale of a few tens of nanometers. Variation of the local work-function as a possible explanation is discussed. On the atomic scale, both hexagonal 2×2 superlanices and one-dimensional linear structures similar to stage-1 CsCs and RbCs are observed. In addition, a novel non-hexagonal superlattice rotated at an angle of 18° respective to the graphite host lattice with a periodicity of about 1.9 nm has been observed.

1. Introduction T h e s c a n n i n g t u n n e l i n g microscope (STM) developed by Binnig and R o h r e r [1] is a very appropriate tool to characterize clean surfaces in real space. Impressive results have b e e n achieved with layered materials, which can easily be cleaved to give " f r e s h " surfaces for S T M investigations. Graphite intercalation compounds (GIC's) provide model systems for q u a s i - t w o - d i m e n s i o n a l materials, which can be i n f l u e n c e d to a large extent in their electronic and m a g n e t i c properties according to the chosen i n t e r c a l a n t species. Surface studies by m e a n s of S T M may clarify the effects related to intercalation. We have acquired S T M data on the whole series of b i n a r y and t e r n a r y alkali-metal graphite i n t e r c a l a t i o n c o m p o u n d s including Li-, K-, Rband C s - G I C ' s of stage 1. In addition we have studied t e r n a r y alkali-metal G I C ' s such as KRb-, KCs- and R b C s - G I C ' s . Here we only focus on the t e r n a r y systems KCs and RbCs. T h e other results will be p r e s e n t e d elsewhere [2]. T h e S T M images o b t a i n e d on a s u b m i c r o n scale indicate an inho-

m o g e n e o u s distribution of the intercalant. A t o m ic-resolution S T M studies show a variety of hexagonal and non-hexagonal superlattices. T e r n a r y G I C ' s exhibit o n e - d i m e n s i o n a l superlattices similar to those observed in binary G I C ' s [3,4]. In addition, novel o r t h o r h o m b i c superlattices are f o u n d on the surface of binary and t e r n a r y alkali-metal GIC's. Recently, a similar o r t h o r h o m b i c superlattice was observed in a K H g C 4 - G I C [5]. We suggest that the p r e s e n c e of a surface-driven charge density wave ( C D W ) is the most likely explanation for this type of superlattice structure.

2. Experiment T e r n a r y G I C ' s investigated in this study were p r e p a r e d by a liquid-phase reaction of highly o r i e n t a t e d pyrolytic graphite ( H O P G ) with m o l t e n alkali-metal KsoCss 0 a n d C s x R b 1 x (0.1 < x < 0.33) alloys. R e a c t i o n t e m p e r a t u r e s d u r i n g synthesis r a n g e d from 100 to 180°C. Exposure times of the H O P G to the liquid alloy were b e t w e e n 12

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H.P. Lang et al. / Ternary alkali-metal graphite intercalation compounds

h and 14 days in a high-purity argon environment. The stage of all samples was determined by X-ray diffraction. The STM used in this study is a commercially available instrument [6] that was operated at room t e m p e r a t u r e in a stainless-steel glove box filled with 1 bar high-purity Ar. Contamination originating from oxygen, nitrogen and water vapour is kept below our detection limit of 1 p p m by means of a two-stage gas purification system. The G I C samples were introduced to the glove box in sealed glass tubes through a fast-entry air lock. Thus the G I C samples were never exposed to air. Prior to each STM m e a s u r e m e n t series, the glass tubes were broken and the samples were cleaved. Different mechanically prepared Pts0Ir20 tips were used on the same kind of sample to exclude tip-geometry-induced artifacts. All large-scale images were taken in the constant-current mode to give correct corrugation amplitudes. Most of the atomic-scale images were also recorded using the constant-current mode.

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Fig. l. Constant-current STM survey (400 nm X 400 nm) with islands of apparently increased topographic height on a stage-I KCs-GIC (tunneling current I t = 1 nA, sample bias voltage U~ -- 100 mY).

3. Results and discussion

As mentioned in the introduction, this contribution is dedicated only to the ternary alkali-metal GIC's. Compared to binary alkali metal GIC's, similar features are observed in the ternary systems. In fig. 1 we present a large-scale image of the surface of a stage-1 KCs-GIC. Besides monatomic steps, we observe islands that range, from 50 to 200 nm in size, which show an apparently increased topographic height. We attribute the observation of such islands to an inhomogeneous distribution of the intercalant. For ternary GIC's, this may be evidence of the partial lack of one intercalant species, as was found by Hwang et al. in a ternary stage-4 SbC15-GIC by scanning transmission electron microscopy [7]. The lack of both intercalant species is unlikely. On the atomic scale, KCs-GIC's exhibit various superlattice structures. Fig. 2 shows a commensurate 2 x 2 superlattice similar to stage-l, Rb- and Cs-GIC's. One-dimensional superlattices, comparable to those also found in stage,1 RbC 8 and CsCs [3,4], are also present in stage-1

Fig. 2. Constant-current atomic-resolution STM image (10 nm x 10 nm) showing a c omme ns ura t e 2 x 2 superlanice on a stage-1 KCs-GIC ( I t = 2.2 nA, U ~ - - 1 0 0 mV). Corrugation amplitude (peak to peak) is about 0.2 nm.

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Fig. 3. Constant-current STM image (50 nm x 5 0 nm) with a non-hexagonal, one-dimensional, irregularly spaced super|atrice on a staged KCs-GIC. Typical distances between the individual line-features are 2 and 2.9 nm (I~ = 0.6 hA, U , - 100 mV). Corrugation amplitude is about 0.15 nm.

KCs-GIC's. This is demonstrated in fig. 3. Moreover, we observe also non-hexagonal, irregularly spaced, one-dimensional superlattice structures. Finally, we have discovered a novel non-hexagonal orthorhombic superlattice with a periodicity of about 1.9 _+ 0.1 nm. This superlattice is similar to the structure observed in a KC 8 stage-1 sample [2]. A detailed Fourier analysis shows that there are, in fact, two different superimposed superlattices present in addition to the underlying graphitic host lattice. This is demonstrated in fig. 4 which shows, besides our measured data (a), its two-dimensional Fourier transform (b). Three clearly distinguishable groups of spots can be detected in the reciprocal space. The two groups of spots with hexagonal symmetry shown in the two outermost rings correspond to the graphitic host lattice (c) and to the 2 x 2 superlattice (d). The innermost group of spots is related to the novel superlattice structure with orthorhombic symmetry (e). This simultaneous observation of two superlattices of different symmetries has important consequences, which will be discussed later.

We also investigated RbCs-GIC's, which exhibit similar superlattices as KCs-GIC's. Fig. 5 shows a 2 x 2 superlattice, whereas equally spaced one-dimensional chain-like structures are presented in fig. 6. In the following, we discuss possible origins of the observed superlattices. Since the samples were cleaved prior to the STM measurements, it is expected that cleavage might lead to two equally intercalant-covered surfaces. The alkali metal at the surface is likely to evaporate off in the argon gas environment. Therefore, we assume a graphitic surface layer followed by the first intercalated layer. The existence of 2 x 2 superlattices of the intercalated layers is well known from bulk diffraction experiments. Explanations for this 2 x 2 superlattice could either be based on electronic states induced by the ordered arrangement of the alkali metal atoms in the first gallery of H O P G or buckling of the graphitic surface layer due to the intercalant [8,9]. We have up to now no experimental evidence for a two-component intercalant layer, which would lead to locally varying corrugation amplitudes over the 2 x 2 superlattice. A random distribution of the two intercalant species is deduced from X-ray diffraction in ternary K R b - G I C [10]. Further investigations are necessary to clarify this issue. The non-hexagonal, one-dimensional superlattice structures may be interpreted either by a quasi-one-dimensional chain-like ordering of the alkali metal intercalant as observed also in the bulk of high-stage Cs-GIC's by scanning transmission electron microscopy [11] leading to electronic contributions of the intercalant layer in addition to the graphitic host layer or surfacedriven charge density waves (as found by means of angle-resolved photoemission spectroscopy [12]). This question could be clarified by a comparative STM and A F M study, since only STM is highly sensitive to C D W [13,14]. We also found a novel orthorhombic superlattice, which was simultaneously observed with the 2 x 2 superlattice (figs. 4a-4e) and the graphitic host lattice in a stage-1 KCs-GIC. Since the 2 x 2 superlattice is most likely due to a 2 x 2 ordering of the intercalant, the non-hexagonal orthorhombic superlattice cannot be explained by a simple

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Fig. 4. (a) Constant-height atomic-resolution STM image (10 n m x 10 nm) showing the novel orthorhombic superlattice with a periodicity of about 1.9 nm on a stage-I KCs-GIC ( I t = 1 nA, U~ = - 6 5 mV). (b) Two-dimensional fast Fourier transform of image (a). Details are described in the text. (c) Graphite host lattice extracted from (a). (d) 2 x 2 guest superlattice extracted from (a). (e) Orthorhombic superlattice extracted from (a).

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in a surface-driven CDW. Similar orthorhombic superlattices have been found in stage-1 K - G I C [2] and stage-1 K H g - G I C [5]. More experimental and theoretical work is required to obtain a better understanding of these observations.

4. Conclusion

Fig. 5. Constant-current atomic-resolution STM-image (10 nm × 10 nm) showing a commensurate 2 × 2 superlanice on a stage-1 CsRb-G1C ( I t - 1 nA, U~ = 8 mV). Corrugation amplitude is about 0.6 nm.

electronic structure contribution of the intercalation layer. Therefore, it is most likely that the observed orthorhombic superlattice has its origin

Scanning tunneling microscopy (STM) has been applied to ternary stage-1 alkali-metal G1C's from a submicrometer down to the atomic scale. Evidence for an inhomogeneous distribution of the intercalant was found by the observation of islands showing enhanced topography at a lateral extent of 50 to 200 nm. On the atomic scale, various superlattices such as hexagonal 2 × 2, non-hexagonal one-dimensional, and non-hexagonal orthorhombic superlattices were observed. The possible origin of these superlattices has been discussed. In ternary KCs-GIC's, we have found indications for a surface-driven C D W by the simultaneous observation of two superlattices, one with hexagonal symmetry and another with an orthorhombic symmetry in addition to the graphitic host lattice. However, further experiments have to be performed in order to prove the existence of a surface C D W in stage-1 alkali-metal GIC's.

Acknowledgements We would like to thank D. Anselmetti and G. Overney for valuable discussions. We also thank A.W. Moore (Union Carbide) for generously providing us with the H O P G samples. Furthermore, we are grateful to the Kommission zur F6rderung der Wissenschaftlichen Forschung and to The Swiss National Science Foundation for their financial support.

References Fig. 6. Constant-height atomic-resolution STM image (10 nm X 10 nm) of non-hexagonal, one-dimensional features separated by about 1.25 nm (l~ - 5.6 nA, U~ = - 8 mV).

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