Scanning Tunneling Microscopy Observations of Mercury Droplets on

distance between the small lines varies with atomic density. Mercury observed with one edge having a triangular shape smoothened with each scan. Durin...
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Scanning Tunneling Microscopy Observations of Mercury Droplets on Graphite P. J. Ouseph* and T. Poothackanal Physics Department, University of Louisville, Louisville, Kentucky 40294 Received June 14, 1995. In Final Form: December 21, 1995X Scanning tunneling microscopy surface pictures of micrometer size mercury accumulated around dislocations of graphite are presented. Mercury accumulated between linear dislocations appears to have an ordered structure; cross sections of vertical layers are seen as small lines with atomic resolution. The distance between the small lines varies with atomic density. Mercury observed with one edge having a triangular shape smoothened with each scan. During the fifth scan a smooth edge, without any indication of the triangular edge, is seen. One and two atom thick mercury layers, due to strong interaction with the supporting graphite, are in the solid phase.

Introduction Since the development of scanning tunneling microscopy (STM) by Binnig and Rohrer,1,2 a wide variety of interesting studies have been conducted on solid surfaces and nanometer size structures supported on solid surfaces. These studies include observation of surface reconstruction,3,4 nanometer-scale surface modifications,5 writing using single atoms,6 atomic corrals and standing electronic waves around them,7,8 superlattices and their modifications,9-11 and charge density waves.12 The most important advantage of STM is its ability, under ideal conditions, to obtain pictures of solid surfaces with subatomic resolution. Liquid surfaces are rarely studied because the random motion of atoms do not allow us to take advantage of the high-resolution capabilities of STM. The surface pictures of liquids are, therefore, expected to be smooth. Bruckner-Lea et al. obtained STM surface pictures of mercury drops of diameters in the millimeter range showing smooth surfaces with superimposed 3-5 Hz waves of large amplitude.13 Studies of surfaces of 2 to 30 atom thick mercury, reported in this paper, show unexpected and interesting results along with expected results. For this study micrometer and smaller size mercury drops on graphite substrate were prepared by an evaporation method. The size, shape, and in some cases the structure of the mercury drops are found to be influenced by dislocations in graphite. We have observed triangular, circular, and linear edges for mercury deposited over graphite. Further, mercury accumulated between parallel dislocation lines appears to have a partially ordered structure closely resembling the structure of fluids * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Binnig, G.; Rohrer, H.; Gerber, Gh.; Weibel, E. Appl. Phys. Lett. 1982, 40, 178. (2) Binnig, G.; Rohrer, H. Helv. Phys. Acta 1982, 55, 726. (3) Becker, R. S.; Glovchenko, J. A.; McRae, E. G.; Swartzentruber, B. S. Phys. Rev. Lett. 1985, 55, 2028. (4) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. Rev. B 1990, 42, 9307. (5) Majumdar, A.; Oden, P. I.; Carrep, J. P.; Nagahara, L. A.; Graham, J. J.; Alexander, J. Appl. Phys. Lett. 1992, 61, 6951. (6) Stroscio, J. A.; Eigler, D. M. Science 1991, 254, 1319. (7) Crommie, M. F.; Lutz, C. P.; Eigler, D. M. Science 1993, 262, 218. (8) Heller, E. J.; Crommie, M. F.; Lutz, C. P.; Eigler, D. M. Nature 1994, 369, 464. (9) Xhie, J.; Sattler, K.; Ge, M.; Venakteswaran, N. Phys. Rev. B 1993, 47, 15835. (10) Rong, Z. Y.; Kuiper, P. Phys. Rev. B 1993, 48, 17427. (11) Ouseph, P. J. Phys. Rev. B 1996, 53, 9610. (12) Slough, S. G.; Giambattista, B.; McNairy, W. W.; Coleman, R. V. Phys. Rev. B 1989, 39, 5496. (13) Bruckner-Lea, C.; Janata, J.; Conroy, J.; Pungor, A.; Caldwell, K. Langmuir 1993, 9, 3612.

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in the first stages of layering.14 None of the pictures showed any effect of mechanical or electrical noise. The purpose of the previous mercury study13 was to understand the behavior of organic molecules clamped on to the mercury surface. A motivation for this study is the recent interest in few atom thick liquids for their layering and oscillatory frictional properties.14-17 Behavior of fluid layers in contact with solid surfaces have been widely studied by computer modeling18,19 and experimentally with the help of colloidal fluids. These studies have shown that the atomic layers immediately next to the surface have crystalline structure and the layers above them have layered fluid structure. The aim of this study is to learn about the structure of few atoms thick mercury layers supported on graphite and to investigate the effects of dislocations on the boundaries of mercury drops over graphite. Experiment In ref 13 mercury drops were prepared using two different methods. In the first method the drop is placed in a conical dimple on a metal sample stage whereas in the second method the drop is held at the end of a syringe. The latter method had the advantage that the diameter and the flatness of the drop could be adjusted by controlling the piston of the syringe but it also had the disadvantage of the mercury surface vibrating at a low frequency of 3-5 Hz. In our experiments mercury drops are deposited on graphite using cluster preparation equipment, consisting of a chamber in which mercury in a tungsten boat is evaporated by ohmic heating. The evaporation method is best suited for making micrometer size drops. Details of the evaporation chamber and the STM used are discussed elsewhere.20 The top of the evaporation chamber has a small nozzle at its center and half of the STM containing the sample holder with the graphite substrate glued to it is kept above the chamber. The distance between the nozzle hole and the graphite substrate is about 10 cm. Before mercury is evapoarated, the chamber along with the half of the scanning tunneling microscope fixed above the chamber is evacuated and filled with argon gas at 1.3 × 105 Pa of pressure. The tungsten boat containing the mercury is then heated to 450 °C. The mercury vapor in the chamber is swept through the nozzle by a burst of argon lasting about 1 s and deposited on the graphite substrate located above the nozzle. (14) Murray, C. A.; Grier, D. G. Am. Sci. 1995, 83, 238. (15) Grier, D. G.; Murray, C. A. J. Chem. Phys. 1994, 100, 9088. (16) Bhushan, B.; Israelachvilli, J. N.; Landman, U. Nature 1995, 374, 607. (17) Israelchavilli, J. N.; McGuiggan, P. M. Science 1988, 241, 795. (18) Ma, W.-J.; Banavar, J. R.; Koplik, J. J. Chem. Phys. 1992, 97, 485. (19) Sokol, P. E.; Ma, W.-J.; Herwig, K. W.; Snow, W. M.; Wang, Y.; Koplik, J.; Banavar, J. R. Appl. Phys. Lett. 1992, 61, 777. (20) Poothackanal, T.; Ouseph, P. J.; Mathur, G. Rev. Sci. Instrum. 1994, 65, 400.

© 1996 American Chemical Society

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Figure 1. Two sets of dislocation lines merge forming a single set of lines. The brightness variations seen along the lines are due to mercury accumulating along the lines. (a, top left) Across the white line the distance between the centers of the two lines is about 2500 Å and the height variation is 40 Å for the left side and 50 Å for the right side mercury accumulation (see Figure 2). (b, top right) Distance between the dislocation lines is continuously decreasing. There is a gap of 3200 Å between parts a and b. (c, bottom) At the junction, where the dislocation lines meet, mercury has a triangular shape and a height of 32 Å. Below the junction mercury is mainly along one dislocation line with the height varying between 25 and 30 Å. The area of each picture is 3215 Å × 3215 Å. The bias voltage and tunneling current are set at 0.15 V and 1.1 nA, respectively. The half of the scanning tunneling microscope with the sample is then removed from the evaporation chamber and joined to the other half of the scanning tunneling microscope with the piezoelectric tube and the tungsten probe. During this process, the sample remains in an argon atmosphere. Before each evaporation, a fresh basal plane graphite surface is prepared by peeling off the top layers by cellophane tape. The graphite surface is then scanned to make sure the surface is smooth within the scanning range. Whenever normal hexagonal pictures are not obtained, the surface is scanned several times; repeated scanning is known to correct misalignments between layers that produce anomalous surface pictures.21 The STM probes are prepared by electrochemical etching of 20 mil tungsten wire in 1 to 2 N potassium hydroxide solution. The probe is then washed in distilled water and deoxidized by dipping in hydrofluoric acid. The STM pictures are obtained at room temperature in the argon atmosphere. The scanning tunneling microscope is operated in a constant current mode and the bias voltage and (21) Ouseph, P. J.; Poothackanal, T.; Mathew, G. Phys. Lett. A 1995, 205, 65.

scanning speed are selected to reduce the possibility of the tip making contact with the mercury structures of up to 60 Å height.

Results and Discussion Mercury is found to accumulate along dislocations having a variety of shapes. It is known that dislocations attract and hold added atoms and surface impurities. This phenomenon is the basis of the coloration technique used to visualize dislocations.22 Figure 1 shows a region of graphite containing two families of dislocation lines, about 2500 Å apart at the top (Figure 1a), which merge to form a single set of lines (Figure 1c). Interactions between dislocation lines including their merger are discussed in several articles by Amelinckx and co-workers.22,23 The merger of dislocation lines in Figure 1c is consistent with the results of their studies. The dislocation lines in Figure (22) Amelinckx, S. Direct Observation of Dislocations, Solid State Physics, Supplement 6, Academic Press: New York, 1964. (23) Delavignette, P.; Amelinckx, S. J. Nucl. Mater. 1962, 5, 17.

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Figure 2. Variation of ∆Z, change in the distance between the tip and the sample, along the line in Figure 1a. The peak of mercury is about 40 Å above the graphite surface on the lefthand side.

1 are longer than the y-range of the scanning tunneling microscope (about 15 000 Å). Pictures in Figure 1 show the presence of mercury along dislocation lines and in the region between the lines. The amount of mercury decreases in the negative y-direction, that is, toward the bottom of the picture. Sudden changes in the amount of mercury are also evident from Figure 1a. It appears that the mercury is spreading toward the bottom. The cursor profile, variation of ∆Z with distance, along the line in figure 1a is shown in Figure 2. The peak of mercury accumulation along the line drawn is 40 Å above the graphite background on the left hand side, and the peak for mercury at the top of the left dislocation lines is 50 Å. The accumulation of mercury shows a sudden change where the dislocation lines merge (Figure 1c). The shape of the accumulated mercury at the junction is clearly influenced by the merging lines. The peak height of mercury immediately below the junction is about 25 Å. Mercury continuously decreases along dislocation lines below the point of merger, and the surface of the mercury along the lines clearly shows visible striations, small wiggly lines, as the amount of mercury decreases below 20 Å. The striations observed in a different region are discussed below. Figure 3 shows the top view of a mercury drop of 1700 Å diameter and 37 Å height over another area, far removed from the area of Figure 1, where two families of dislocations meet. Since there is more mercury at this junction than at the junction in Figure 1c the influence of dislocation lines on the shape of the mercury drop is not that drastic but still perceptible. The shape of the contact line of the top half of the drop is circular, while the shape of the contact line of the bottom half is distorted, partly due to the direction change of the merging dislocation lines. The right bottom corner of the drop is distorted because this corner is at the end of the scanning range of the scanning tunneling microscope and consequently the x- and ytranslations are not proportional to the applied scan voltages. The surface of the drop is smooth showing very little electrical or mechanical noise. As expected, scans of the smaller areas of liquid surface did not yield pictures with atomic resolution. The fact that no features such as mechanical waves are seen gives us confidence about the results discussed below, especially about the striations, the small lines, seen in mercury accumulated around dislocation lines. Figure 4a shows the edges of two mercury layers with an average distance of 170 Å between the edges. An

Ouseph and Poothackanal

Figure 3. A mercury drop of approximately 1700 Å radius and 37 Å height at the intersection of two sets of dislocation lines. The conditions of scanning are as follows: bias voltage, 0.14 V; tunneling current, 1.3 nA; scan speed, 3 × 104 Å/s.

interesting difference between the two layers is that the right layer is smooth while the left layer has sharp variations in height parallel to its edge. The left layer of mercury is formed between dislocation lines and therefore its edge is straight, whereas the right layer is not constrained by dislocation lines and its edge is slightly curved. The shape of the edges is the reason for the increasing separation between the edges. The pictures obtained during the left to right and right to left scans, clearly indicating that the height variations are not due to oscillations of the system following a change in the scan direction. The reason for the height variations in the left layer is that more mercury atoms accumulate along the dislocation lines and less in between them. The scanning tunneling microscope tip is moved to the left to scan the left layer in detial and the result is shown in Figure 4b. Note that since the tip is moved to the left and the scanning range is kept constant, a larger area of the left layer can be seen in Figure 4b. In addition to more dislocation lines, closely spaced, smaller, wiggly lines, almost parallel to each other, are seen in the central region of the layer. Figure 4c shows the region, right of the center of Figure 4b, expanded. The small lines contain up to ten bright circular spots strung together. As can be seen in Figure 4c, the bright spots are sufficiently resolved to determine the separation between them. The measured distance of 3.2 ( 1 Å is in the range of distance between atoms in liquid mercury, and therefore, the bright circular spots possibly are mercury atoms. The distance between the smaller lines varies from 5 to 7 Å; they are closer together in the brighter regions containing more mercury, that is, along dislocation lines, and farther apart in between dislocation lines where the amount of mercury is less. Pictures taken with different scanning speeds and bias voltages did not change the appearance of the picture (except for low bias voltages) indicating that the parallel lines are not due to mechanical waves or electronic modulations. Figure 4d shows a picture typical for bias voltages of 0.05 V and below. The picture is of poor resolution, and therefore, the smaller lines are not seen clearly anymore. Since the experiments are done in a constant current mode, as the bias voltage decreases, the tip to sample distance decreases, increasing the interaction between tip and sample. This increased interaction is the reason for the decrease in resolution for small bias voltages.

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Figure 4. (a, top left) Two mercury layers with an average distance of 170 Å between them. The right layer is smooth and the left layer has variations in height parallel to the edge. The scan area is 400 Å × 400 Å and the maximum value of ∆Z is 14 Å. The bias voltage is 0.14 V and the scanning speed is 5581 Å/s. (b, top right) Result of scanning the left layer of Figure 4a. Scan area is 400 Å × 400 Å. In addition to the intensity variations parallel to the edge, small, wiggly lines can be seen. (c, bottom left) Result of scanning 160 Å × 160 Å area of Figure 4b. Smaller lines are clearly visible in this picture and the circular bright spots, possibly mercury atoms, in some of these lines are well resolved. Pictures b and c are taken for the same scan conditions: bias voltage, 0.15 V; tunneling current 1.0 nA; and scan speed, 450 Å/s. (d, bottom right) Same area as in part b but the bias voltage is reduced to 0.05 V.

The layer of mercury in Figure 4b has a width of 500 Å and a length of 2600 Å, and the dislocation lines constraining the mercury are estimated to be several hundreds of micrometers long. To verify that the denser accumulation of mercury is along the dislocation lines, we followed them and located a region without any mercury accumulation and a section of these lines without mercury was studied as a function of bias voltage (Figure 5). The lines are seen only for bias voltages below 0.10 V. The brightness of the lines decreases with increasing bias voltage; the lines are almost invisible for bias voltages above 0.15 V. Results of scanning a smaller section of Figure 5 containing only one dislocation line for two bias voltages are shown in Figure 6. The line obtained for bias voltage of 40 mV (Figure 6a) has atomic resolution and it is brighter than the line obtained for 70 mV (Figure 6b). The line disappeared for bias voltages above 150 mV. The height of the dislocation line above the graphite background is 2 Å for bias voltage of 40 mV and 1.5 Å for 60 mV. (For comparison, the average corrugation of the

carbon atoms in the same picture is only 0.3 Å.) The brightness variation of dislocation lines with bias voltages is explained in terms of the local density of states near the Fermi level.24 Along dislocation lines there is an increase in the local density of states very close to the Fermi level. At low bias voltages the contribution to the tunneling current is from levels close to the Fermi level. As the bias voltage increases, contribution from lower levels increases and, therefore, the contrast of the lines decreases with increasing bias voltages. The variation of intensity with increasing bias voltage is a typical behavior of dislocation lines. However, the presence of impurities, mercury in this case, along dislocation lines makes the lines visible even at higher bias voltages and reduces the brightness dependence on bias voltage. The distance between the bright spots in the small lines is 3.2 ( 0.1 Å, equal to interatomic distance in liquid (24) Garbarz, J.; Lacaze, E.; Faivre, G.; Gauthier, S.; Schott, M. Philos. Mag. 1992, 65, 853.

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Figure 5. A section of dislocation lines without mercury. Distance between this region and the region with mercury (Figure 4) is about 5000 Å. The scan conditions are as follows: area of scan, 400 Å × 400 Å; bias voltage, 0.10 V; and scan speed, 4494 Å/s.

mercury, but the distance between the lines, as mentioned earlier, varies from 5 to 7 Å. Figure 7 shows the height variation, ∆S, along the white line drawn in Figure 4c. The value of ∆Z varies with a maximum change of 7 Å. Therefore, these lines appear to be cross sections of vertical mercury liquid layers of at least three-atom thickness. The resemblance of these small lines to layered colloidal fluids enclosed in channels defined by two smooth surfaces is strong.14,15 There is also indirect experimental evidence for formation of such layers in simple fluids.17 Since the small lines are almost parallel to the dislocation lines, the dense mercury regions formed along the dislocation lines may be providing the surfaces of the channels. However, it is worth noting that these high-density mercury regions do not provide smooth solid surfaces. The graphite at the bottom of the mercury layer also provides a confining smooth surface, but since the scanning tunneling microscope can see only the surface, we cannot obtain any information about the effect of graphite on the mercury layer. A dramatic surface picture of mercury with a triangular edge is shown in Figure 8a. One edge of the mercury layer is pinned along what appears to be boundaries of triangular dislocations. The direction of scanning did not have any effect on the features in Figure 8, removing any doubts about the possibility that they are artifacts of scanning. Electron microscope22 and STM25 studies have revealed triangular dislocations in interaction of two different types of dislocations lying in the same plane and the dimensions of the triangular dislocations depend on the width of interacting dislocation ribbons and the angle between them. There is also evidence that triangular dislocations can form when a graphite layer is rotated with respect to the others through an angle of 0.5° or less. The observation of triangular dislocations resulting from misorientation of layers in the same graphite sample is reported elsewhere.11 However, it should be noted that the distance between the apex points of the dislocations previously reported are in the range of 90 nm25 to 30 µm,23 whereas in Figure 8a it is much smaller, only 15.7 ( 0.2 Å. The thickness of the accumulated mercury, ∆Z, is 14.4 ( 0.2 Å, about four atoms thick. Figure 8b is obtained (25) Snyder, S. R.; Gerberich, W. W.; White, H. S. Phys. Rev. B 1993, 47, 10823.

Figure 6. (a, top) A close-up view of one of the lines in Figure 5. Bias voltage is 40 mV. (b, bottom) Same line for a bias voltage of 70 mV. The line becomes indistinguishable from the graphite background for bias voltages above 100 mV. Area of scan is 90 Å × 90 Å and scan speed is 1286 Å/s.

Figure 7. Variation of the tip height, ∆Z, along the line drawn in Figure 4c. Distance between the small lines varies between 5 and 7 Å.

during the third scanning of the same area of Figure 8a and Figure 8c during the fifth scanning. In five scans the triangular edge has changed to a smooth edge showing no

STM Observations of Mercury Droplets on Graphite

Figure 8. (a, top) A layer of liquid mercury, one edge is triangular in shape. This edge may be along the boundaries of triangular dislocations in the supporting graphite. Continued scanning slowly smoothened the triangular edge. (b, middle) Picture obtained during third scan. (c, bottom) Picture obtained during fifth scan. The pictures, 220 Å × 220 Å, are taken for the same conditions of scan: bias voltage, 0.13 V; tunneling current, 0.121 nA; and scan speed 4337 Å/s.

evidence of the triangular shape. It may be that the probe tip is forcing the mercury atoms to move into the gaps between the triangular edge, smoothing the edge. Forces

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between the positively biased tip and the sample atoms have been used to move atoms around, as is done in writing with atoms and in building atomic corrals.6-8 Distortions of contact lines of spreading liquids have been observed in the presence of single and multiple defects on the surface.26,27 Ondarc¸ uhu and Veyssie´ studied periodic distortions of contact lines and their relaxations.27 In their experiment distortion was produced by placing equally spaced liquid drops ahead of an advancing liquid. As the advancing liquid merged with the drops, they obtained a contact line with a continuous quasiharmonic distortion having a predominant wavelength equal to the distance between the drops. They also observed the relaxation of the contact line, between their results and results in Figure 8. However, from the shape of the contact line in Figure 8, especially the straight triangular edges, it is clear that the distortion is produced not by point defects but by triangular defects. The smoothening of the edge in Figure 8 is similar to the relaxation of the contact line observed by Ondarc¸ uhu and Veyssie´. In their case the liquid front was forced to advance, contrary to our experiment. We could postulate two reasons for the mercury front to advance: (1) The strong attraction between mercury and the dislocations, as demonstrated by the increased accumulation of mercury along dislocation lines in Figures 1 and 4, may force mercury to move toward the dislocation lines. (2) The force between the tip and the mercury, as explained above, may be partially responsible for the movement of the front. In any case the smoothening of the contact line decreases the surface energy and increases the stability of the liquid. The very high surface energy of mercury, 484 mN/m, raises another question. Is it possible for liquid mercury to have a perfect triangular edge? Since the accumulated mercury is only a few atoms thick, it may not have “liquid-like” properties. From the results of tribometric measurements of ultrathin liquid films Van Alsten and Granick have concluded that a few molecule thick liquid films lose “liquid-like” properties and attain “solid-like” properties.28 A 100-Å wide strip of mercury with triangular edges on both sides has also been observed. In this case, the distance between the apex points is about 50 Å. As mentioned earlier, since STM can see only the surface, the effect of graphite surface on mercury layers immediately above the surface cannot be observed if the mercury is more than a few atoms thick. On the other hand, influence of the substrate could be seen if the layer is one or two atoms thick. A two atom thick mercury layer (620 Å × 620 Å), seen on a region of graphite surface without any dislocations (Figure 9), clearly shows the effect of the graphite substrate. The two atom thick mercury layer has atomic resolution indicating that mercury in this case is in the solid phase. The mercury atoms in the first layer are on the top of the graphite β-site atoms. A region with less mercury where atomic organization is not complete may be seen on the left side of the mercury layer. Previously reported experimental observations lend indirect evidence for simple liquids to form solid layers on strongly interacting surfaces.17 Such layers have also been observed in colloidal fluids.14,15 A possible reason for the strong influence of graphite on mercury atoms is the structure of graphite which is of the Bernal type with D6h4 symmetry.29,30 Graphite is built (26) Brochard-Wyart, F.; di Meglio, J.-M.; Que´re´, D.; de Gennes, P.G. Langmuir 1991, 7, 335. (27) Ondarc¸ uhu, T.; Veyssie´, M. Nature 1991, 352, 418. (28) Van Alsten, J.; Granick, S. Phys. Rev. Lett. 1988, 61, 2570. (29) Bernal, J. D. Proc. R. Soc. London, Ser. A 1924, 106, 749. (30) Holzwarth, N. A. W.; Loui, S. G.; Rabii, S. Phys. Rev. B 1982, 26, 5382.

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in the adjacent planes. The other half of the carbon atoms, β-site atoms, have holes (centers of carbon hexagons) in adjacent planes. The pz orbitals of the β-site atoms on the surface are, therefore, not involved in the binding between the planes making them more chemically active than the R-site atoms. This explains the observation of mercury atoms over β-site atoms in Figure 9. Mercury atoms in smaller structures, such as dimers (Hg2) and trimers (Hg3), are found to be located above graphite β-site atoms (unpublished data). Studies of palladium and silver clusters deposited over graphite also show that their individual atoms are bound to the graphite β-site atoms.31,32

Figure 9. One and two atom thick mercury layers on graphite. The layers are in the solid phase. The scan area is 620 Å × 620 Å and the bias voltage is 0.103 V. The maximum variation in height is 1.6 Å.

in layers, each layer having atoms arranged in hexagonal lattices, with lattice constant a ) 2.46 Å and with layer separation C/2 ) 3.35 Å. The atoms in the graphitic planes are covalently bound by the sp2 (s-px-py) hybridized orbitals. The weak binding between the planes is due to the nonhybridized pz orbitals. In the most common type of graphite, the hexagonal type, every other plane is shifted in the horizontal direction giving the well-known ABAB stacking. Because of this shift, half of the carbon atoms, R-site atoms, have carbon atoms directly above and below

Conclusion In conclusion we have observed micrometer sized mercury on graphite, some of them accumulating along dislocations. In one case mercury seems to accumulate along triangular dislocations. In this case the triangular edge disappeared in five scans. Mercury between dislocation lines seems to have a structure very close to that of layered fluids. One and two atom thick mercury layers on graphite are in the solid phase. The latter observations are similar to the experimental results of Murray and Grier14,15 using colloidal liquids. Acknowledgment. The authors are grateful to Dr. C. L. Davis, Physics Department, University of Louisville, for his helpful suggestions in preparing this article. This work is partially supported by NSF Grant EHR-9018764 and a grant from Research Committee, College of Arts and Sciences, University of Louisville. LA950468T (31) Mu¨ller, U.; Sattler, K.; Xhie, J.; Venketeswaran, N.; Raina, G. J. Vac. Sci. Technol. 1992, B9, 829. (32) Ganz, E.; Sattler, K.; Clarke, J. Phys. Rev. Lett. 1988, 60, 1856.