Using a scanning transmission electron microscope with an electron beam size of ∼800 Å, we have found that unsaturated cesium intercalated graphite at 98±2 K exhibits multiple structural phases with a typical domain size of ∼1 μm. Electron diffraction patterns from individual structural islands were studied, and the p(2×2), p(3√×2), and p(3√×13−−√) in-plane superlattices were identified

Crewe, A. V.

Hwang, D. M.

Parker, N. W.

Utlaut, Mark

English

University of Portland

University of PortlandPilot ScholarsPhysics Faculty Publications and Presentations Physics1-15-1983Observation of nonhexagonal superlattices in high-stage cesium intercalated graphiteD. M. HwangN. W. ParkerMark UtlautUniversity of Portland, utlaut@up.eduA. V. CreweFollow this and additional works at: http://pilotscholars.up.edu/phy_facpubsPart of the Plasma and Beam Physics CommonsThis Journal Article is brought to you for free and open access by the Physics at Pilot Scholars. It has been accepted for inclusion in Physics FacultyPublications and Presentations by an authorized administrator of Pilot Scholars. For more information, please contact library@up.edu.Citation: Pilot Scholars Version (Modified MLA Style)Hwang, D. M.; Parker, N. W.; Utlaut, Mark; and Crewe, A. V., "Observation of nonhexagonal superlattices in high-stage cesiumintercalated graphite" (1983). Physics Faculty Publications and Presentations. 38.http://pilotscholars.up.edu/phy_facpubs/38PHYSICAL REVIEW B VOLUME 27, NUMBER 2 15 JANUARY 1983 Observation of nonhexagonal superlattices in high-stage cesium intercalated graphite D. M. Hwang Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60680 N. W. Parker,* M. Utlaut,t and A. V. Crewe Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (Received 6 August 1982) Using a scanning transmission electron microscope with an electron beam size of - 800 A, we have found that unsaturated cesium intercalated graphite at 98 ± 2 K exhibits multiple structural phases with a typical domain size of - 1 µm. Electron diffraction patterns from individual structural islands were studied, and the p (2 x 2), p ( .J3 x 2), and p ( .J3 x .Jii) in-plane super-lattices were identified. The in-plane structures of intercalated molecules or atoms in graphite intercalation compounds (GIC's) have been extensively studied for many years. 1 In several GIC's, the x-ray or electron diffraction pat-terns exhibit distinct diffraction spots, indicating that the intercalated layers are of ordered structures, but the patterns cannot be interpreted by simple in-plane structures.1 It has been proposed1•2 that, in those GIC's, several structural phases may coexist even in well-staged samples. The traditional diffraction probe beam size is - 1 mm for x rays and - 1 µ.m for elec-trons from conventional transmission electron micro-scopes. Several studies2-s have suggested that the typical size of the structural islands is smaller than the size of their probe beams, and their observed pat-terns consist of diffractions from many domains of various structures. The stage-1 heavy alkali-metal intercalated graphite compounds are among the few GIC's whose in-plane structures have been well established. However, their high-stage compounds exhibit many interesting structural phases which are not yet fully character-ized.1 Consider the high-stage Cs-graphite compound at liquid-nitrogen temperature as an example. An early x-ray study2 suggested the coexistence of (2 x 2), (3 x .J7), ( .J39 x ../39), and ( .J7 x .J7) su-perlattices. The (3 x .J7) and ( .J39 x ../39) phases were "characterized by diffraction patterns having an abnormally large number of unobservable reflec-tions. " 2 A recent study3•4 proposed a structure of a modulated incommensurate triangular lattice orienta-tionally locked to the graphite lattice in coexistence with the ( 2 x 2) commensurate phase. Electron dif-fraction studies5•6 with -1 µ.m beam size yield very complicated patterns. Unlike the cases of Rb-graph-ite 7 and K-graphite, 8 no detailed analysis has been published, but it is said6 that the structure is incom-mensurate. A scanning transmission electron microscope (STEM) is used in this study to probe the diffraction patterns from individual structural domains in high-stage Cs intercalated New York natural single-crystal graphite. The electron beam size on the sample was estimated to be - 800 A. The STEM was operated with an electron energy of 18 ke V and at - 3 x 10-10 Torr vacuum. The sample holder was in thermal contact with a liquid-nitrogen cold finger and was maintained at 98 ± 2 K. Details of the experi-mental setup will be presented elsewhere.9 The sample suitable for STEM study must be only a few hundred A thick. Such a thin alkali-metal-graphite sample could be intercalated or deintercalat-ed within a few seconds by the conventional two-temperature method. The sample is too thin to be characterized by the x-ray, neutron, or optical methods and is easily contaminated. A vacuum sample transfer technique has been developed to handle air-sensitive GIC's. 10 A "near-saturated" Cs-graphite sample was transferred at lQ-9 Torr vacuum into the STEM and quenched to 98 ± 2 K. A homogenous area of the sample of thickness - 500 A was found. The recorded electron diffraction pattern is similar to that shown in Fig. 1 (b), indicating a ( 2 x 2) superlattice. This was the only pattern observed, and it remained unchanged for one week while the sample was maintained at 98 ± 2 K and 3 x 10-10 Torr vacuum. Therefore it is estab-lished that this near-saturated sample is essentially the. stage-1 CsCs. The sample was then deintercalated at room tem-perature and lQ-9 Torr vacuum for one week and cooled to 98 ±2 K again. A typical bright-field image from this partially vacuum-desorbed sample is shown in Fig. 1 (a). It clearly indicates the formation of is-lands with a characteristic dimension of -1 µ.m. Several different types of diffraction patterns were observed, including the patterns shown in Figs. l(b), 2 (a), and 3 (a), as well as patterns similar to Figs. 2 (a) or 3 (a), but rotated 60° clockwise or counter-clockwise. By moving the electron beam a few µ.m 1458 ©1983 The American Physical Society OBSERVATION OF NONHEXAGONAL SUPERLATTICES IN HIGH ... 1459 (a) (b) FIG. 1. Scanning transmission electron microscopic study of the unsaturated Cs-graphite sample at 98 ±2 K. (a) The bright field image. The black area at the upper-right-hand corner is a part of the support grid. The white area at the lower-right-hand corner is a hole on the sample. (b) One of the diffraction patterns observed. This is the well-known (2 x 2) superlattice pattern. across the sample, we were able to obtain any of the above patterns, or a superposition of two or three of them. At present, we are not able to correlate the observed pattern with the position in the direct image. The pattern shown in Fig. 1 (b) corresponds to the well-known (2 x 2) superlattice observed in stage-1 heavy alkali-metal graphite MC8 (M = K, Rb, and Cs). It is also established2--4 as one of the structural phases for high-stage Cs-graphite at liquid-nitrogen temperature. The simple rectangular pattern shown in Fig. 2 (a) can be matched by the (.JS x 2) superlattice shown in Fig. 2(b). Figure 2(c) is the diffraction pattern con-structed from Fig. 2(b). Three different orientations • • (e) (c) (b) FIG. 2. (a) One of the diffraction patterns observed from the unsaturated Cs-graphite sample at 98 ± 2 K. (b) The p ( .J3 x 2) in-plane super lattice on the graphite in-plane net-work. (c} The reciprocal lattice of (b). The large dots are due to the graphite lattice. The small dots result from the Cs superlattice. of this pattern, corresponding to the three equivalent orientations of the (.JS x 2) superlattice, have been observed. Actually, a set of faint spots can be ob-served in Fig. 2(a), which corresponds to the strong set rotated 60° counterclockwise. The in-plane C:Cs ratio for the p (.JS x 2) super-lattice is 8:1. As can be seen in Fig. 2(b), moving every other column of Cs atoms in the p( .JS x 2) lat-tice by a lattice constant up or down results in the • • • • • • (c) • • • • • • • (b) (d) FIG. 3. (a) One of the diffraction patterns observed from the unsaturated Cs-graphite sample at 98 ± 2 K. (b) The p ( .J3 x ..Jfj) in-plane superlattice on the graphite in-plane network. (c) The reciprocal lattice of (b). (d) The diffraction pat-tern expected from a sample of a pe.rfect a{J stacking sequence. (e} The diffraction pattern expected from a mixture of a{J and a[J' stacking sequences. 1460 D. M. HWANG, N. W. PARKER, M. UTLAUT, AND A. V. CREWE p(2 x 2) lattice. The superlattice associated with the pattern shown in Fig. 3(a) is less obvious. Since all diffraction spots fall on the vertical lines connecting the graphite spots, one of the translation vectors in real space must be -J3. The diffraction spots are elongated along the vertical direction, indicating that the translation symmetry along this direction is poorly ordered. We find the pattern is best fitted by the ( .J3 x .JIT) superlattice. A simple ( .J3 x .JIT) su-perlattice is shown in Fig. 3(b), and its reciprocallat-tice is shown in Fig. 3 (c). The ( ± 1, k) spots in Fig. 3 (c) are consistent with the observation. However, the (0, ±1) and (0, ±3) spots are missing from Fig. 3 (a). This can be inter-preted as being the result of the nearest-Cs-layer stacking correlation. In contrast to the p (2 x 2) su-per lattice, where an a layer can be followed by any one of the {3, y, and 8 layers with equal probability, the p ( .J3 x .JIT) has preferred nearest-layer sites. If a Cs layer occupies the a sites as indicated in Fig. 3 (b), then the next Cs layer has a tendency to locate near the centers of the a lattice, denoted as {3 or (3' sites in Fig. 3 (b). The Cs atoms should still be at the hexagon centers of the adjacent carbon layers and their exact positions depend on the graphite-layer stacking arrangement and the stage number. This kind of nearest-layer correlation always exists wheth-er the long-range stacking sequence is ordered or not. For the sake of simplicity, we assume a perfect a{3 stacking sequence. The diffraction pattern becomes as shown in Fig. 3 (d)-all ( h, k) spots with odd h + k disappear. A mixture of the a{3 and the af3' phases results in the diffraction pattern shown in Fig. 3 (e) which is consistent with Fig. 3 (a). The in-plane C:Cs ratio is 14: 1 for the p ( .J3 x .JIT) superlattice. It is interesting to note that a mixture of one-third p ( .J3 x 2) with two-thirds p( .J3 x .JIT) results in an in-plane C:Cs ratio of 12:1. A nonprimitive in-plane unit cell of ( .J3 x 2) or ( .J3 x .JIT) will result in a too-high in-plane density. Our unsaturated sample is prepared by vacuum desorption. Desorption under ultrahigh vacuum should be equivalent to deintercalation by the stan-dard two-temperature method with the charge end kept near 0 K. Both should result in higher-stage compounds, possibly mixtures 'of several stages. A recent low-energy-electron diffraction (LEED) study showed that no surface adsorption layer results from vacuum desorption. 11 The x-ray4 and other studies1 indicate that, for n :;i!l 2, the in-plane structures are not stage sensitive except some shifts in transition temperatures. Nevertheless, the structures we found are not consistent with the x-ray results.2-4 Similar conflicting observations exist between the x-ray and the electron diffraction studies for high-stage Rb-graphite7 and K-graphite.8 It seems that the in-plane structures for ultrathin GIC samples are different from that of bulk samples. More progress in this field is needed to understand the structural differ-ences of GIC's of various sizes, origins, and prepara-tion conditions. Both the nonhexagonal in-plane superlattice phases we observed, p ( .J3 x 2) and p ( .J3 x .JIT), have an unusually small Cs nearest-neighbor distance of .J3a0 =4.27 A. This distance is less than that in the p(2 x2) saturated phase (2a0 =4.93 A), and that in Cs metal (5.24 A), 12 but still greater than the Cs ion diameter (3.34 A) .12 Our observations suggest that the Cs atoms in the less-saturated sample have a ten-dency to form chainlike structures. The nearest-neighbor distance is actually reduced with a reduction of the number of nearest neighbors. This phenom-enon has not been predicted for graphite intercalation compounds. However, it is not new in two-dimen-sional physics. For example, a monolayer of gold atoms adsorbed on a silicon (111) surface between 400 and 700 °C exhibits a ( .J3 x .J3) superlattice structure. While at a coverage between 0.2 to 0.5 monolayer, LEED study yields a star like pattern which can be decomposed into three (1 x 5) superlat-tice patterns of equivalent orientations. 13- 15 Various models have been proposed to explain this result. 13• 14 The in-plane structures for intercalation com-pounds are further complicated by the formation of staging and the interlayer correlation. The inter-inter-calation-layer correlation certainly plays an essential role in the formation of in-plane structures, as evi-denced by our observation that the same in-plane structure is preserved over more than 100 layers along the c axis. Models more elaborate than that for the surface adsorption system 13• 14 are required to describe the GIC structures. A highly anisotropic superlattice will yield a diffrac-tion pattern with spots closely packed on a few paral-lel lines. The superposition of these linelike patterns of various equivalent orientations results in a starlike pattern. Attempts to interpret such a pattern with a single superlattice will result in a very large unit cell and many missing diffraction spots. 2• 7 For example, the CvXpattern of Kambe et al.7 has been interpret-ed as a superposition of ( .Jf2 x M) and ( ,/39 x ,/39) with many missing and extra diffraction spots. We found that the CvXpattern can be matched by a superposition of ( .J3 x .J7), ( .J3 x 3), and ( .J3 x 4), missing only one equivalent set of dif-fraction spots. Finally, we point out that there may be other struc-tural phases, especially those with diffuse scattering, which have escaped our observation. We are current-ly improving the performance of our STEM, and will then study the detailed temperature dependence of the island sizes, the intercalation density distribution, and the in-plane structures, ~s well as the correlation between various in-plane structures and their in-plane densities. OBSERVATION OF NONHEXAGONAL SUPERLATTICES IN HIGH ... 1461 ACKNOWLEDGMENTS We would like to thank Dr. M. Ohtsuki for many valuable discussions and technical assistance, and to Profes-sor J. Curray for assistance in smoothing the English. This work is supported by the Research Board of the University of Illinois at Chicago, the National Science Foundation under Grant No. DMR NSF8014176, and the Materials Research Laboratories at the University of Chicago. · *Present address: Hughes Research Laboratory, Malibu, Cal. 90265. tPresent address: Varian Associates, Gloucester, Maine 01930. lFor general background, consult the following two recent review articles: M. S. Dresselhaus and G. Dresselhaus, Adv. Phys. 30, 139 (1981); S. A. Solin, Adv. Chem. Phys. 12., 455 (1982). 2G. S. Parry, Mater. Sci. Eng. 31, 99 (1977). 3R. Clarke, N. Caswell, S. A. Solin, and P. M. Horn, Phys. Rev. Lett. 43, 2018 (1979). 4R. Clarke, N:Caswell, S. A. Solin, and P. M. Horn, Physica (Utrecht) 99B, 457 (1980). so. D. L. Chung, G. Dresselhaus, and M. S. Dresselhaus, Mater. Sci. Eng. 31, 107 (1977). 6M. S. Dresselhaus, N. Kambe, A. N. Berker, and G. Dresselhaus, Synth. Met. b 121 (1980). 7N. Kambe, G. Dresselhaus, and M. S. Dresselhaus, Phys. Rev. B 21, 3491 (1980). 8A. N. Berker, N. Kambe, G. Dresselhaus, and M. S. Dresselhaus, Phys. Rev. Lett. 45, 1452 (1980). 9N. W. Parker, M. Utlaut, A. V. Crewe, and D. M. Hwang (unpublished). 100. M. Hwang, M. Utlaut, M. Issacson, and S. A. Solin, Phys. Rev. Lett. il, 882 (1979). llN. J. Wu and A. Ignatiev, Bull. Am. Phys. Soc. 27, 310 (1982). 12C. Kittel, Introduction to Solid State Physics, 5th ed. (Wiley, New York, 1976), pp. 32, 100. 13W. Haidinger and S. C. Barnes, Surf. Sci. 20, 313 (1970). 14H. Lipson and K. W. Singer, J. Phys. C 7, 12 (1974). 15G. Le Lay, G. Quentel, J.P. Faurie, and-A. Masson, Thin Solid Films 35, 289 (1976). 16D. M. Hwang, Phys. Rev. B (in press). (a) (b) FIG. I. Scanning transmission electron microscopic study of the unsaturated Cs-graphite sample at 98 ±2 K. (a) The bright field image. The black area at the upper-right-hand corner is a part of the support grid. The white area at the lower-right-hand corner is a hole on the sample. (b) One of the diffraction patterns observed. This is the well-known (2 x 2) superlattice pattern. (c) (b) FIG. 2. (a) One of the diffraction pauerns observed from the unsaturated Cs-graphite sample at 98 ± 2 K. (b) The p ( .J3 x 2) in-plane superlauice on the graphite in-plane net-work. (c) The reciprocal Janice of (b). The large dots are due to the graphite lauice. The small dots result from the Cs superlauice. • • • b*i--: . a• . • • • (c) • • • • • • • • • (e) (b) (d) FIG. 3. (a) One of the diffractio n patterns observed from the unsaturated Cs-graphite sample at 98 ± 2 K. (b) The p (.Ji x .Ji3) in-plane superlattice on the graphite in-plane network. (c) The reciprocal la ttice of (b) . (d) The diffraction pal· tern expected from a sample of a perfect a/3 stacking sequence. (e) The diffraction pattern expected from a mixture of af3 and a/3' stacking sequences. 

Observation of nonhexagonal superlattices in high-stage cesium intercalated graphite

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Observation of nonhexagonal superlattices in high-stage cesium intercalated graphite

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