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STM study of single-crystal graphite
Ultramicroscopy 45 (1992) 337-343 North-Holland
~nH-~,nn,J~nclt~wlhl~m
STM study of single-crystal graphite Z h o u h a n g Wang, Martin Moskovits Department of Chemistry, Unitrersity of Toronto, and The Ontario Laser and LightwaL'e Research Centre, Toronto, Canada M5S IA 1
and Paul Rowntree Department of Nuclear Medicine and Radiobiology, Uniuersity of Sherbrooke, Sherbrooke, Quebec, Canada J1H 5N4 Received 21 May 1992
Freshly cleaved surfaces of single-crystal graphite (SCG) are shown to have very large expanses of step-free surface; however, the tunnelling signal is found to be considerably noisier than with H O P G . We speculate that the noise is due to carbon adatoms hopping onto the STM tip. Carbon adatoms are normally immobilized at steps. The lower step density on SCG results in an increased average surface concentration of carbon adatoms as compared with H O P G . Atomically resolved topographic images of the (0001) surface of SCG were also obtained. They show the same trigonal symmetry that characterizes the analogous images of H O P G , but with a two- to three-fold increase in corrugation depth. In addition to the low density of steps, SCG is relatively devoid of graphitic artifacts and pseudo-periodic features that have been mistaken for D N A and other genuine molecular structures, implying that SCG may be a useful substrate for STM studies of biological samples.
I. Introduction
The application of scanning tunnelling microscopy (STM) and related technologies such as atomic force microscopy and scanning tunnelling spectroscopy now extend into disciplines where the solid surface of the experiment is of secondary importance, and the object of the studies lies in the characterization of atoms [1], clusters [2], molecules [3] or macromolecules [4] deposited on the surface. In this class of study, the substrate must be clean, stable, and relatively inert to the adsorbed material. Graphite has often been the surface/substrate of choice for adsorbate characterization studies, both by traditional techniques and, more recently, by STM. This permits, in principle, the direct correlation between STM "real space" ad-
sorbate characterizations with the "reciprocal space" determinations made by diffraction-based measurements, and allows the STM experiment to draw upon the vast quantities of thermodynamic adsorption data available using C(0001) as a substrate. The resistance of the C(0001) surface to chemical or physical modifications during adsorption suggests that the differences in the STM images obtained before and after adsorption can be assigned exclusively to the presence of the adsorbate. This is in sharp contrast to many relatively inert metal surfaces (such as Au) which have long-range surface reconstructions and highly mobile surface defects [5], making the atomically resolved STM experiment extremely sensitive to the sample preparation and experimental conditions. The graphite C(0001) surface is air-stable, does
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
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Z. Wang et aL / An STM study of single-crystal graphite
not react with normal atmospheric constituents under ambient conditions, and fresh surfaces can be easily prepared by mechanical "peeling" of the upper layer(s) away from the bulk. In theory, therefore, the stability and ease of surface preparation should make C(0001) an ideal substrate for adsorption studies by STM. To date, the vast majority of STM studies of C(0001) have been performed using a synthetic form, highly oriented pyrolytic graphite (HOPG). This polycrystalline form preserves the familiar laminar structure of crystalline graphite, with the " c " crystallographic axis of the many crystal grains approximately collinear, and collectively perpendicular to the surface of the sample; there is no long-range azimuthal order between the crystal domains in H O P G . H O P G has been used in traditional adsorption studies because its extremely high surface area ( ~ 20 m : / g ) allows for rather large quantities of adsorbates to be deposited, even at submonolayer coverages. This facilitates measurements of small pressure changes during depositions, and allows the use of neutron-diffraction for structural determinations, despite the low neutron-scattering cross-sections per molecule. Unfortunately, the lack of long-range azimuthal order of the H O P G eliminates the possibility for direct determinations of the orientation of the adsorbate structure with respect to the substrate. In addition, the ideal characteristics of an adsorption surface are not fully realized with this form of graphite, because of its polycrystalline nature and the inevitable abundance of pits, steps, and defects at the surface and in the bulk. It is well known that H O P G can produce anomalous STM images with spurious high-order periodicity extending over tens-to-hundreds of C(0001) unit cells [6], and unphysical "surface corrugations" as well as "moirE" patterns resulting from the incommensurate rotation of a top layer of graphite with respect to its underlayer [7,8]. Although the physical basis for many of these observations remains uncertain, it is apparent that poorly characterized surfaces do not provide the optimal platform for adsorption studies. In addition, the steps and defects are usually the preferred sites for physisorption processes on
most surfaces, including graphite; the structural and dynamical characteristics of adsorbed species can therefore be fundamentally altered in the presence of surface imperfections [9]. In the case of an extremely short-range probe such as the STM, the capability for high spatial resolution can be completely offset by the need to survey large regions of the sample, in order to ensure that the investigated portions are representative of the whole. Beyond the ambiguities associated with the actual physical system under consideration, the presence of high defect densities on H O P G can lead to significant ambiguities in the interpretation of the STM images. Even in the absence of obvious steps or mechanical defects of the surface, it is possible that the actual surface does not have a uniform electronic structure across large regions of the surface; Kuwabara et al. [6] have reported STM images of H O P G showing moire patterns which could be the result of poorly aligned layers of the graphite substrate. Although the effect of this sub-surface misalignment on adsorbate properties is unknown, the large spatial variations of the surface density of electronic states, on otherwise flat C(0001) planes, can lead to considerable confusion in the analysis of the data. Clemmer et al. [10], for example, have shown that DNA-like "strands" can be identified on freshly cleaved H O P G surfaces, even in the absence of sample deposition. The observed "pseudo-adsorbates" had similar structural characteristics as macro-molecules (such as chain length, chain width, and helix pitch) and were found lying across steps of the H O P G surface, thereby reinforcing the mistaken impression that they were extrinsic to the C(0001) surface. Myrick et al. [11] have recently used an STM operated in the tunnelling spectroscopy mode (i.e. measuring the variation in the tunnelling current as a function of the probe-sample potential, dI/dV) to show that defects on H O P G can have a significantly lower surface density of states than that measured on defect-free portions of the surface, and that the magnitude of the reduction (approximately 30% change in dl/dV) is similar to the difference between a clean and an adsorbatecovered substrate. It can therefore be difficult or
Z. Wang et al. / An STM study of single-crystal graphite
impossible to distinguish between defects and adsorbates, either by simple topographs or by the more site-sensitive tunnelling spectroscopy. It is not surprising that several groups [10-12] have suggested that graphite is not a suitable substrate for imaging adsorbates via STM, because of the cumulative experience with H O P G . In the present communication, we will use STM to investigate naturally occurring singlecrystal graphite (SCG) as a possible substrate. Although high-quality SCG is more difficult to obtain than the commercially available H O P G , the experimental advantages of a relatively defect-free crystalline surface suggests that SCG would be a superior substrate in STM studies. There is a growing body of electron [13], X-ray [14] and atomic [15] diffraction results using SCG as a substrate which would simply not be possible on the defect-riddled (and azimuthally disordered) H O P G ; it is probable that as the probes of surface science become increasingly sensitive to only the surface, SCG will become the preferred form of graphite substrate. One of us (P.R.) has observed, using helium-atom diffraction, that naturally occurring SCG has a higher degree of long-range azimuthal order than synthetic "SCG", which has occasionally been used for adsorbate phonon measurements [16], as well as for one STM investigation [17] that we are aware of. With this in mind, we have chosen to study and use naturally occurring SCG via STM.
2. Experiment 2.1. Sample preparation The graphite samples used in these studies were extracted from naturally occurring marble rock purchased from Ward's Natural Science Establishment. The graphite flakes were liberated from the surrounding quartz matrix by gentle dissolution in - 1 M HCl/distilled water; this process was performed with a sufficiently dilute acid mixture to avoid excessive agitation of the acid solution (and the suspended fragile flakes) by the too-rapid evolution of CO 2 gas. Those crystals which were visually judged to be of acceptable
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quality were then removed by hand from the residual debris of the acid bath, and rinsed copiously with fresh distilled water. The crystals were selected on the basis of size (2-4 mm diameter), flatness, and the absence of visible striations across the surface which would indicate possible grain boundaries. This extraction procedure has been proven to consistently produce graphite crystals of extremely high quality with a minimum of surface irregularities and azimuthal disorder, as judged by helium atom diffraction [15]. The graphite flakes were electrically and mechanically mounted to the STM platform using colloidal silver paint. After allowing the silver paint to air-dry, the uppermost layers of the graphite flake were peeled from the bulk using Scotch T M tape; this peeling process was repeated until bright surfaces were obtained. Single-crystal graphite was found to cleave easily over large areas often exceeding 2 mm x 2 mm with greatly reduced numbers of steps, flakes or other visible debris. The surface also looked flatter and shinier than that of H O P G . Electrical contact was first established with conducting silver glue, and subsequently by means of a spring clamp on a corner of the flake after cleaving.
2.2. Instruments The STM used is a home-built instrument using a Burleigh inchworm motor for tip approach and Z-range adjustment. The inchworm is driven with a controller obtained from R H K Technology Inc. A single tube scanner is directly mounted in the inchworm's hollow shaft. The total scan range is approximately 6 /xm in X and Y with the high-voltage setting and 1.4 /zm in Z. The electronics were adapted from a Nanoscope I (Digital Instruments) interfaced to an 80386 based computer using a DT2801A I / O board (Data Translation). The data acquisition and processing software was written in Turbo Pascal 6.0. Some of the images presented were displayed using software obtained from R H K Technology Inc. Tips were made by electro-chemically etching tungsten wire in 2M N a O H solution held by surface tension within a gold ring surrounding the wire. Approximately 3 V DC are applied between
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z. Wang et a L / An STM study of single-crystal graphite
the W wire and the gold ring. T h e tip was washed in distilled water before use. Each sample is freshly cleaved before examination. A n optically flat, shiny area of the surface is located u n d e r the tip by manually guiding the sample while observing through the optical microscope. After initial approach, the tip is brought to its middle Z - r a n g e and a wide scan (500 nm x 500 nm or 1 ~ m x 1 ~tm at 1 Hz X-frequency) is carried out in o r d e r to examine for surface features and to assess flatness. A 6 /~m x 6 p,m region is examined in 1 p,m x 1 /.tm sections by using a 1.5 /~m offset in the X and Y directions. If features are found, m o r e detailed images are recorded. Likewise the scan frequency is c h a n g e d in order to examine surface features m o r e effectively. In situ tip i m p r o v e m e n t (by pulsing the tunnelling voltage or decreasing the integrator constants to make the feedback oscillate for a while) is carried out when n e e d e d to correct a p o o r tip. Images with atomic resolution were recorded for some samples and tips.
3. Results and discussion Tunnelling to single-crystal graphite seemed considerably noisier than to H O P G when using the same tip. A f t e r in situ tip improvement or tip
Fig. 1. Topographic image of single-crystal graphite surface showing a few parallel steps. Scan area is 500 nmx500 nm. Tunnelling parameters: Ut = - 20 mV, I t = 1.0 nA.
Fig. 2. Topographic image of another location on the same single-crystal graphite surface showing a double step and some single steps. Parameters: Ut = - 20 mV, I t = 1.0 nA. change, the tunnelling signal usually improved, but quickly deteriorated resulting in a jumpy image. T e n samples and between 20 and 30 cleaved fresh surfaces were studied. O n each surface, more than 3 different locations were examined, and more than 40 tips were used in this experiment. All showed substantially the same behavior, convincing us that the noisiness was a characteristic of the S C G and not of the tip or of the electrical contact. Despite the excess noise, images of sufficient quality were obtained to indicate that the surface of S C G is characterized by very large featureless flat regions with elevation variations ranging from 0.3 to 1 nm over square regions 500 nm to 1 ~ m on the side. No pseudoperiodic features like those seen on H O P G graphite were found on these surfaces. I n d e e d even steps were only found at fewer than 10 locations. A typical example of these is shown in fig. 1 where a few single, uniform, parallel steps were e n c o u n t e r e d with terrace widths of approximately 80 nm. However, they did not cover a very large area. Close by, the step pattern c h a n g e d to a less orderly a r r a n g e m e n t of double and single steps with varying terrace widths. T h e r e is also some evidence in this region for steps u n d e r n e a t h the top layer (fig. 2). At a n o t h e r location, two parallel grooves approximately 250-300 nm apart were observed. They continue out of the range of
Z. Wang et al. / An STM study of single-crystal graphite
the scan, i.e. for more than 6 /~m (fig. 3). The depth of the grooves may exceed several layers. Images of SCG recorded with atomic resolution are characterized by large corrugation depths. Nevertheless, clear atomic images are relatively hard to obtain because of the aforementioned noise problem, especially in the constant-current mode. Constant-height atomically resolved images of SCG are shown in figs. 4 and 5. The observed atomic arrangement has the familiar trigonal pattern with lattice constant of approximately 0.25 nm that is normally seen with H O P G . This structure only shows every alternate atom of
341
each of the carbon hexagons characterizing the (0001) surface, resulting from the interaction with the second graphitic layer. Clearly this is also the case with SCG so that the noisiness of the signal is not due to exfoliation of single graphite sheets. Moreover, when imaging H O P G with the same tip, the corrugation amplitude observed is considerably lower than for SCG under comparable experimental conditions. In fig. 4, a corrugation of approximately 0.1 nA or 10% of the tunneling is observed, while the corrugation in fig. 5 is approximately 0.14 nA or 14% of the average tunnelling current. For a typical H O P G sample
Fig. 3. T o p o g r a p h i c i m a g e s of single-crystal g r a p h i t e surface showing some d e e p grooves a n d a few o t h e r features. T h e grooves traverse all t h r e e i m a g e s a n d c o n t i n u e b e y o n d the r a n g e of the scan. T h e t h r e e f r a m e s are n e i g h b o r i n g 1 ~ m × 1 ~zm area. P a r a m e t e r s : Ut = - 20 mV, I t = 1.0 nA.
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Z. Wang et al. / An STM study of single-crystal graphite
Fig. 4. Constant height image of single-crystal graphite surface showing atomic resolution of the surface. Scan area: 3 nm x 3 nm. Tunnelling parameters: Ut = - 10 mV, I t = 1.0 nA.
Fig. 6. Topographic image of single-crystal graphite surface showing a single 12 nmx25 nm hole approximately 50 ,~ deep. Scan area: 500 nm x 500 nm. Parameters: Ut = - 2 0 mV, I t = 1.0 nA.
c o r r u g a t i o n d e p t h s rarely exceed 5 % of t h e tunnelling c u r r e n t u n d e r the c o n d i t i o n s that we use. O n l y two u n u s u a l surface d e f e c t s w e r e observed in t h e e n t i r e a r e a e x a m i n e d . O n e is a hole, a p p r o x i m a t e l y 5 nm d e e p a n d 1 0 - 1 5 nm in d i a m eter, s i t u a t e d on a large t e r r a c e (fig. 6) which was f o u n d in the vicinity o f the a r e a shown in figs. 1 a n d 2. T h e region a r o u n d t h e hole is relatively f e a t u r e l e s s , implying t h a t it was m o s t likely
Fig. 5. Constant height image of single-crystal graphite surface showing more detailed atomic resolution. Scan area: 5 nm x5 nm. Tunnelling parameters: Ut = - 10 mV, I t = 1.0 nA.
f o r m e d by an electrical s p a r k b e t w e e n tip a n d s a m p l e d u r i n g t u n n e l l i n g a n d scanning. T h e seco n d o n e is a large f e a t u r e (fig. 3b) consisting o f a roughly circular " h e a d " a p p r o x i m a t e l y 70 n m in d i a m e t e r from which 6 " t a i l s " e m a n a t e , each a p p r o x i m a t e l y 100 n m long a n d 20 n m wide. This is the only f e a t u r e r e s e m b l i n g the m a n y that a r e often seen on H O P G . C o l l o i d a l gold p a r t i c l e s w e r e d e p o s i t e d on o n e o f these S C G surfaces in o r d e r to see if the p a r t i c l e s can b e i m a g e d . Two d r o p s of t h e aqueous gold colloid w e r e a p p l i e d to the surface, the s e c o n d a f t e r the first was dry. S o m e a g g r e g a t e d gold p a r t i c l e s c o u l d be s e e n t h r o u g h t h e optical microscope. Four promising locations were s e a r c h e d for gold particles, w i t h o u t success. (Actually, the two grooves a n d the f e a t u r e s shown in fig. 3 w e r e f o u n d on this s a m p l e . ) This m a y be d u e to t h e fact t h a t gold p a r t i c l e s are r a t h e r m o b i l e on SC g r a p h i t e a n d h e n c e move a b o u t u n d e r t h e influence o f the tip. O n H O P G they a g g r e g a t e n e a r steps a n d o t h e r surface features. H e n c e the inability to d e t e c t gold p a r t i c l e s on S C G surfaces m a y b e d u e to the d r a m a t i c r e d u c tion in the n u m b e r of steps a n d e d g e s on t h e s e samples. This p r o b l e m m a y r e d u c e the usefulness o f S C G as a s u b s t r a t e for m e t a l particles. Biological samples, on t h e o t h e r b a n d , b i n d m o r e strongly
z. Wang et al. / An STM study of single-crystal graphite
to graphite surfaces; h e n c e for t h e m S C G will probably be a serviceable substrate.
4. Conclusions F r e s h l y cleaved surfaces of single-crystal graphite are shown to have very large expanses of step-free surface; however, the t u n n e l l i n g signal is f o u n d to be c o n s i d e r a b l y noisier t h a n with H O P G . This is n o t due to poor contact or to low sample conductivity; rather, it is due to surface c o n t a m i n a t i o n . Since m u c h less noise was experie n c e d with H O P G using the same tips a n d u n d e r the same e x p e r i m e n t a l conditions, the excess noise on S C G may be due to c a r b o n a d a t o m s h o p p i n g o n t o the S T M tip. O n H O P G c a r b o n a d a t o m s are immobilized at steps after surface diffusion. Since the step density o n S C G surfaces is considerably r e d u c e d as c o m p a r e d with H O P G , the c a r b o n a d a t o m c o n c e n t r a t i o n may be greater o n S C G t h a n on H O P G , resulting in a larger n u m b e r of c a r b o n a t o m j u m p s o n t o the tip, each p r o d u c i n g a spike in the t u n n e l l i n g signal. A t o m i c a l l y resolved t o p o g r a p h i c images of the (0001) surface of S C G were also o b t a i n e d . They showed the same trigonal symmetry that characterizes the a n a l o g o u s images of H O P G b u t with two- to three-fold larger c o r r u g a t i o n depth. In a d d i t i o n to the low density of steps, S C G is relatively devoid of graphitic artifacts a n d p s e u d o - p e r i o d i c f e a t u r e s that have b e e n m i s t a k e n for D N A a n d o t h e r g e n u i n e m o l e c u l a r features, implying that S C G may be a useful substrate for S T M studies of biological samples.
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References [1] D.M. Eigler, P.S. Weiss, E.K. Schweizer and N.D. Lang, Phys. Rev. Lett. 66 (1991) 1189. [2] J.F. Womelsdorf, W.C. Ermler and C.J. Sandroff, J. Phys. Chem 95 (1991) 503. [3] P.H. Lippel, R.J. Wilson, M.D. Miller, C. W611 and S. Chiang, Phys. Rev. Lett. 62 (1989) 171. [4] (a) T.P. Beebe, Jr., T.E. Wilson, D.F. Ogletree, J.E. Katz, R. Balhorn, M.B. Salmeron and W.J. Siekhaus, Science 243 (1989) 370; (b) R.J. Driscoll, M.G. Youngquist and I.D. Baldeschwieler, Nature 346 (1990) 294. [5] E. Holland-Moritz, J. Gordon II, G. Borges and R. Sonnenfeld, Langmuir 7 (1991) 301. [6] M. Kuwabara, D.R. Clarke and D.A. Smith, Appl. Phys. Lett. 56 (1990) 2396. [7] T. Tiedje, J. Varon, H. Deckman and J. Stokes, J. Vac. Sci. Technol. A 6 (1988) 372. [8] R.V. Coleman, B. Drake, P.K. Hansma and G. Slough, Phys. Rev. Lett. 55 (1985) 394. [9] K. Kern, P. Zeppenfeld, R. David, R.L. Palmer and G. Comsa, Phys. Rev. Lett. 57 (1986) 3187. [10] C.R. Clemmer and T.P. Beebe, Jr., Science 251 (1991) 640. [11] M.L. Myrick, N.V. Hud, S.M. Angel and D.G. Garvis, Chem. Phys. Lett. 180 (1991) 156. [12] R.J. Colton, S.M. Baker, R.J. Driscoll, M.G. Youngquist, J.D. Baldeschwieler and W.J. Kaiser, J. Vac. Sci. Technol. A 6 (1988) 349. [13] M.F. Toney and S.C. Fain, Jr., Phys. Rev. B 36 (1987) 1248. [14] K.L. D'Amico and D.E. Moncton, J. Vac. Sci. Technol. A 4 (1986) 1455. [15] P.A. Rowntree, G. Scoles and J.C. Ruiz-Sufirez,J. Phys. Chem. 94 (1990) 8511. [16] J.P. Toennies and R. Vollmer, Phys. Rev. B 40 (1989) 3495. [17] S. Morita, S. Tsukada and N. Mikoshiba, J. Vac. Sci. Technol. A 6 (1988) 354.
~nH-~,nn,J~nclt~wlhl~m
STM study of single-crystal graphite Z h o u h a n g Wang, Martin Moskovits Department of Chemistry, Unitrersity of Toronto, and The Ontario Laser and LightwaL'e Research Centre, Toronto, Canada M5S IA 1
and Paul Rowntree Department of Nuclear Medicine and Radiobiology, Uniuersity of Sherbrooke, Sherbrooke, Quebec, Canada J1H 5N4 Received 21 May 1992
Freshly cleaved surfaces of single-crystal graphite (SCG) are shown to have very large expanses of step-free surface; however, the tunnelling signal is found to be considerably noisier than with H O P G . We speculate that the noise is due to carbon adatoms hopping onto the STM tip. Carbon adatoms are normally immobilized at steps. The lower step density on SCG results in an increased average surface concentration of carbon adatoms as compared with H O P G . Atomically resolved topographic images of the (0001) surface of SCG were also obtained. They show the same trigonal symmetry that characterizes the analogous images of H O P G , but with a two- to three-fold increase in corrugation depth. In addition to the low density of steps, SCG is relatively devoid of graphitic artifacts and pseudo-periodic features that have been mistaken for D N A and other genuine molecular structures, implying that SCG may be a useful substrate for STM studies of biological samples.
I. Introduction
The application of scanning tunnelling microscopy (STM) and related technologies such as atomic force microscopy and scanning tunnelling spectroscopy now extend into disciplines where the solid surface of the experiment is of secondary importance, and the object of the studies lies in the characterization of atoms [1], clusters [2], molecules [3] or macromolecules [4] deposited on the surface. In this class of study, the substrate must be clean, stable, and relatively inert to the adsorbed material. Graphite has often been the surface/substrate of choice for adsorbate characterization studies, both by traditional techniques and, more recently, by STM. This permits, in principle, the direct correlation between STM "real space" ad-
sorbate characterizations with the "reciprocal space" determinations made by diffraction-based measurements, and allows the STM experiment to draw upon the vast quantities of thermodynamic adsorption data available using C(0001) as a substrate. The resistance of the C(0001) surface to chemical or physical modifications during adsorption suggests that the differences in the STM images obtained before and after adsorption can be assigned exclusively to the presence of the adsorbate. This is in sharp contrast to many relatively inert metal surfaces (such as Au) which have long-range surface reconstructions and highly mobile surface defects [5], making the atomically resolved STM experiment extremely sensitive to the sample preparation and experimental conditions. The graphite C(0001) surface is air-stable, does
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
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Z. Wang et aL / An STM study of single-crystal graphite
not react with normal atmospheric constituents under ambient conditions, and fresh surfaces can be easily prepared by mechanical "peeling" of the upper layer(s) away from the bulk. In theory, therefore, the stability and ease of surface preparation should make C(0001) an ideal substrate for adsorption studies by STM. To date, the vast majority of STM studies of C(0001) have been performed using a synthetic form, highly oriented pyrolytic graphite (HOPG). This polycrystalline form preserves the familiar laminar structure of crystalline graphite, with the " c " crystallographic axis of the many crystal grains approximately collinear, and collectively perpendicular to the surface of the sample; there is no long-range azimuthal order between the crystal domains in H O P G . H O P G has been used in traditional adsorption studies because its extremely high surface area ( ~ 20 m : / g ) allows for rather large quantities of adsorbates to be deposited, even at submonolayer coverages. This facilitates measurements of small pressure changes during depositions, and allows the use of neutron-diffraction for structural determinations, despite the low neutron-scattering cross-sections per molecule. Unfortunately, the lack of long-range azimuthal order of the H O P G eliminates the possibility for direct determinations of the orientation of the adsorbate structure with respect to the substrate. In addition, the ideal characteristics of an adsorption surface are not fully realized with this form of graphite, because of its polycrystalline nature and the inevitable abundance of pits, steps, and defects at the surface and in the bulk. It is well known that H O P G can produce anomalous STM images with spurious high-order periodicity extending over tens-to-hundreds of C(0001) unit cells [6], and unphysical "surface corrugations" as well as "moirE" patterns resulting from the incommensurate rotation of a top layer of graphite with respect to its underlayer [7,8]. Although the physical basis for many of these observations remains uncertain, it is apparent that poorly characterized surfaces do not provide the optimal platform for adsorption studies. In addition, the steps and defects are usually the preferred sites for physisorption processes on
most surfaces, including graphite; the structural and dynamical characteristics of adsorbed species can therefore be fundamentally altered in the presence of surface imperfections [9]. In the case of an extremely short-range probe such as the STM, the capability for high spatial resolution can be completely offset by the need to survey large regions of the sample, in order to ensure that the investigated portions are representative of the whole. Beyond the ambiguities associated with the actual physical system under consideration, the presence of high defect densities on H O P G can lead to significant ambiguities in the interpretation of the STM images. Even in the absence of obvious steps or mechanical defects of the surface, it is possible that the actual surface does not have a uniform electronic structure across large regions of the surface; Kuwabara et al. [6] have reported STM images of H O P G showing moire patterns which could be the result of poorly aligned layers of the graphite substrate. Although the effect of this sub-surface misalignment on adsorbate properties is unknown, the large spatial variations of the surface density of electronic states, on otherwise flat C(0001) planes, can lead to considerable confusion in the analysis of the data. Clemmer et al. [10], for example, have shown that DNA-like "strands" can be identified on freshly cleaved H O P G surfaces, even in the absence of sample deposition. The observed "pseudo-adsorbates" had similar structural characteristics as macro-molecules (such as chain length, chain width, and helix pitch) and were found lying across steps of the H O P G surface, thereby reinforcing the mistaken impression that they were extrinsic to the C(0001) surface. Myrick et al. [11] have recently used an STM operated in the tunnelling spectroscopy mode (i.e. measuring the variation in the tunnelling current as a function of the probe-sample potential, dI/dV) to show that defects on H O P G can have a significantly lower surface density of states than that measured on defect-free portions of the surface, and that the magnitude of the reduction (approximately 30% change in dl/dV) is similar to the difference between a clean and an adsorbatecovered substrate. It can therefore be difficult or
Z. Wang et al. / An STM study of single-crystal graphite
impossible to distinguish between defects and adsorbates, either by simple topographs or by the more site-sensitive tunnelling spectroscopy. It is not surprising that several groups [10-12] have suggested that graphite is not a suitable substrate for imaging adsorbates via STM, because of the cumulative experience with H O P G . In the present communication, we will use STM to investigate naturally occurring singlecrystal graphite (SCG) as a possible substrate. Although high-quality SCG is more difficult to obtain than the commercially available H O P G , the experimental advantages of a relatively defect-free crystalline surface suggests that SCG would be a superior substrate in STM studies. There is a growing body of electron [13], X-ray [14] and atomic [15] diffraction results using SCG as a substrate which would simply not be possible on the defect-riddled (and azimuthally disordered) H O P G ; it is probable that as the probes of surface science become increasingly sensitive to only the surface, SCG will become the preferred form of graphite substrate. One of us (P.R.) has observed, using helium-atom diffraction, that naturally occurring SCG has a higher degree of long-range azimuthal order than synthetic "SCG", which has occasionally been used for adsorbate phonon measurements [16], as well as for one STM investigation [17] that we are aware of. With this in mind, we have chosen to study and use naturally occurring SCG via STM.
2. Experiment 2.1. Sample preparation The graphite samples used in these studies were extracted from naturally occurring marble rock purchased from Ward's Natural Science Establishment. The graphite flakes were liberated from the surrounding quartz matrix by gentle dissolution in - 1 M HCl/distilled water; this process was performed with a sufficiently dilute acid mixture to avoid excessive agitation of the acid solution (and the suspended fragile flakes) by the too-rapid evolution of CO 2 gas. Those crystals which were visually judged to be of acceptable
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quality were then removed by hand from the residual debris of the acid bath, and rinsed copiously with fresh distilled water. The crystals were selected on the basis of size (2-4 mm diameter), flatness, and the absence of visible striations across the surface which would indicate possible grain boundaries. This extraction procedure has been proven to consistently produce graphite crystals of extremely high quality with a minimum of surface irregularities and azimuthal disorder, as judged by helium atom diffraction [15]. The graphite flakes were electrically and mechanically mounted to the STM platform using colloidal silver paint. After allowing the silver paint to air-dry, the uppermost layers of the graphite flake were peeled from the bulk using Scotch T M tape; this peeling process was repeated until bright surfaces were obtained. Single-crystal graphite was found to cleave easily over large areas often exceeding 2 mm x 2 mm with greatly reduced numbers of steps, flakes or other visible debris. The surface also looked flatter and shinier than that of H O P G . Electrical contact was first established with conducting silver glue, and subsequently by means of a spring clamp on a corner of the flake after cleaving.
2.2. Instruments The STM used is a home-built instrument using a Burleigh inchworm motor for tip approach and Z-range adjustment. The inchworm is driven with a controller obtained from R H K Technology Inc. A single tube scanner is directly mounted in the inchworm's hollow shaft. The total scan range is approximately 6 /xm in X and Y with the high-voltage setting and 1.4 /zm in Z. The electronics were adapted from a Nanoscope I (Digital Instruments) interfaced to an 80386 based computer using a DT2801A I / O board (Data Translation). The data acquisition and processing software was written in Turbo Pascal 6.0. Some of the images presented were displayed using software obtained from R H K Technology Inc. Tips were made by electro-chemically etching tungsten wire in 2M N a O H solution held by surface tension within a gold ring surrounding the wire. Approximately 3 V DC are applied between
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z. Wang et a L / An STM study of single-crystal graphite
the W wire and the gold ring. T h e tip was washed in distilled water before use. Each sample is freshly cleaved before examination. A n optically flat, shiny area of the surface is located u n d e r the tip by manually guiding the sample while observing through the optical microscope. After initial approach, the tip is brought to its middle Z - r a n g e and a wide scan (500 nm x 500 nm or 1 ~ m x 1 ~tm at 1 Hz X-frequency) is carried out in o r d e r to examine for surface features and to assess flatness. A 6 /~m x 6 p,m region is examined in 1 p,m x 1 /.tm sections by using a 1.5 /~m offset in the X and Y directions. If features are found, m o r e detailed images are recorded. Likewise the scan frequency is c h a n g e d in order to examine surface features m o r e effectively. In situ tip i m p r o v e m e n t (by pulsing the tunnelling voltage or decreasing the integrator constants to make the feedback oscillate for a while) is carried out when n e e d e d to correct a p o o r tip. Images with atomic resolution were recorded for some samples and tips.
3. Results and discussion Tunnelling to single-crystal graphite seemed considerably noisier than to H O P G when using the same tip. A f t e r in situ tip improvement or tip
Fig. 1. Topographic image of single-crystal graphite surface showing a few parallel steps. Scan area is 500 nmx500 nm. Tunnelling parameters: Ut = - 20 mV, I t = 1.0 nA.
Fig. 2. Topographic image of another location on the same single-crystal graphite surface showing a double step and some single steps. Parameters: Ut = - 20 mV, I t = 1.0 nA. change, the tunnelling signal usually improved, but quickly deteriorated resulting in a jumpy image. T e n samples and between 20 and 30 cleaved fresh surfaces were studied. O n each surface, more than 3 different locations were examined, and more than 40 tips were used in this experiment. All showed substantially the same behavior, convincing us that the noisiness was a characteristic of the S C G and not of the tip or of the electrical contact. Despite the excess noise, images of sufficient quality were obtained to indicate that the surface of S C G is characterized by very large featureless flat regions with elevation variations ranging from 0.3 to 1 nm over square regions 500 nm to 1 ~ m on the side. No pseudoperiodic features like those seen on H O P G graphite were found on these surfaces. I n d e e d even steps were only found at fewer than 10 locations. A typical example of these is shown in fig. 1 where a few single, uniform, parallel steps were e n c o u n t e r e d with terrace widths of approximately 80 nm. However, they did not cover a very large area. Close by, the step pattern c h a n g e d to a less orderly a r r a n g e m e n t of double and single steps with varying terrace widths. T h e r e is also some evidence in this region for steps u n d e r n e a t h the top layer (fig. 2). At a n o t h e r location, two parallel grooves approximately 250-300 nm apart were observed. They continue out of the range of
Z. Wang et al. / An STM study of single-crystal graphite
the scan, i.e. for more than 6 /~m (fig. 3). The depth of the grooves may exceed several layers. Images of SCG recorded with atomic resolution are characterized by large corrugation depths. Nevertheless, clear atomic images are relatively hard to obtain because of the aforementioned noise problem, especially in the constant-current mode. Constant-height atomically resolved images of SCG are shown in figs. 4 and 5. The observed atomic arrangement has the familiar trigonal pattern with lattice constant of approximately 0.25 nm that is normally seen with H O P G . This structure only shows every alternate atom of
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each of the carbon hexagons characterizing the (0001) surface, resulting from the interaction with the second graphitic layer. Clearly this is also the case with SCG so that the noisiness of the signal is not due to exfoliation of single graphite sheets. Moreover, when imaging H O P G with the same tip, the corrugation amplitude observed is considerably lower than for SCG under comparable experimental conditions. In fig. 4, a corrugation of approximately 0.1 nA or 10% of the tunneling is observed, while the corrugation in fig. 5 is approximately 0.14 nA or 14% of the average tunnelling current. For a typical H O P G sample
Fig. 3. T o p o g r a p h i c i m a g e s of single-crystal g r a p h i t e surface showing some d e e p grooves a n d a few o t h e r features. T h e grooves traverse all t h r e e i m a g e s a n d c o n t i n u e b e y o n d the r a n g e of the scan. T h e t h r e e f r a m e s are n e i g h b o r i n g 1 ~ m × 1 ~zm area. P a r a m e t e r s : Ut = - 20 mV, I t = 1.0 nA.
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Z. Wang et al. / An STM study of single-crystal graphite
Fig. 4. Constant height image of single-crystal graphite surface showing atomic resolution of the surface. Scan area: 3 nm x 3 nm. Tunnelling parameters: Ut = - 10 mV, I t = 1.0 nA.
Fig. 6. Topographic image of single-crystal graphite surface showing a single 12 nmx25 nm hole approximately 50 ,~ deep. Scan area: 500 nm x 500 nm. Parameters: Ut = - 2 0 mV, I t = 1.0 nA.
c o r r u g a t i o n d e p t h s rarely exceed 5 % of t h e tunnelling c u r r e n t u n d e r the c o n d i t i o n s that we use. O n l y two u n u s u a l surface d e f e c t s w e r e observed in t h e e n t i r e a r e a e x a m i n e d . O n e is a hole, a p p r o x i m a t e l y 5 nm d e e p a n d 1 0 - 1 5 nm in d i a m eter, s i t u a t e d on a large t e r r a c e (fig. 6) which was f o u n d in the vicinity o f the a r e a shown in figs. 1 a n d 2. T h e region a r o u n d t h e hole is relatively f e a t u r e l e s s , implying t h a t it was m o s t likely
Fig. 5. Constant height image of single-crystal graphite surface showing more detailed atomic resolution. Scan area: 5 nm x5 nm. Tunnelling parameters: Ut = - 10 mV, I t = 1.0 nA.
f o r m e d by an electrical s p a r k b e t w e e n tip a n d s a m p l e d u r i n g t u n n e l l i n g a n d scanning. T h e seco n d o n e is a large f e a t u r e (fig. 3b) consisting o f a roughly circular " h e a d " a p p r o x i m a t e l y 70 n m in d i a m e t e r from which 6 " t a i l s " e m a n a t e , each a p p r o x i m a t e l y 100 n m long a n d 20 n m wide. This is the only f e a t u r e r e s e m b l i n g the m a n y that a r e often seen on H O P G . C o l l o i d a l gold p a r t i c l e s w e r e d e p o s i t e d on o n e o f these S C G surfaces in o r d e r to see if the p a r t i c l e s can b e i m a g e d . Two d r o p s of t h e aqueous gold colloid w e r e a p p l i e d to the surface, the s e c o n d a f t e r the first was dry. S o m e a g g r e g a t e d gold p a r t i c l e s c o u l d be s e e n t h r o u g h t h e optical microscope. Four promising locations were s e a r c h e d for gold particles, w i t h o u t success. (Actually, the two grooves a n d the f e a t u r e s shown in fig. 3 w e r e f o u n d on this s a m p l e . ) This m a y be d u e to t h e fact t h a t gold p a r t i c l e s are r a t h e r m o b i l e on SC g r a p h i t e a n d h e n c e move a b o u t u n d e r t h e influence o f the tip. O n H O P G they a g g r e g a t e n e a r steps a n d o t h e r surface features. H e n c e the inability to d e t e c t gold p a r t i c l e s on S C G surfaces m a y b e d u e to the d r a m a t i c r e d u c tion in the n u m b e r of steps a n d e d g e s on t h e s e samples. This p r o b l e m m a y r e d u c e the usefulness o f S C G as a s u b s t r a t e for m e t a l particles. Biological samples, on t h e o t h e r b a n d , b i n d m o r e strongly
z. Wang et al. / An STM study of single-crystal graphite
to graphite surfaces; h e n c e for t h e m S C G will probably be a serviceable substrate.
4. Conclusions F r e s h l y cleaved surfaces of single-crystal graphite are shown to have very large expanses of step-free surface; however, the t u n n e l l i n g signal is f o u n d to be c o n s i d e r a b l y noisier t h a n with H O P G . This is n o t due to poor contact or to low sample conductivity; rather, it is due to surface c o n t a m i n a t i o n . Since m u c h less noise was experie n c e d with H O P G using the same tips a n d u n d e r the same e x p e r i m e n t a l conditions, the excess noise on S C G may be due to c a r b o n a d a t o m s h o p p i n g o n t o the S T M tip. O n H O P G c a r b o n a d a t o m s are immobilized at steps after surface diffusion. Since the step density o n S C G surfaces is considerably r e d u c e d as c o m p a r e d with H O P G , the c a r b o n a d a t o m c o n c e n t r a t i o n may be greater o n S C G t h a n on H O P G , resulting in a larger n u m b e r of c a r b o n a t o m j u m p s o n t o the tip, each p r o d u c i n g a spike in the t u n n e l l i n g signal. A t o m i c a l l y resolved t o p o g r a p h i c images of the (0001) surface of S C G were also o b t a i n e d . They showed the same trigonal symmetry that characterizes the a n a l o g o u s images of H O P G b u t with two- to three-fold larger c o r r u g a t i o n depth. In a d d i t i o n to the low density of steps, S C G is relatively devoid of graphitic artifacts a n d p s e u d o - p e r i o d i c f e a t u r e s that have b e e n m i s t a k e n for D N A a n d o t h e r g e n u i n e m o l e c u l a r features, implying that S C G may be a useful substrate for S T M studies of biological samples.
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References [1] D.M. Eigler, P.S. Weiss, E.K. Schweizer and N.D. Lang, Phys. Rev. Lett. 66 (1991) 1189. [2] J.F. Womelsdorf, W.C. Ermler and C.J. Sandroff, J. Phys. Chem 95 (1991) 503. [3] P.H. Lippel, R.J. Wilson, M.D. Miller, C. W611 and S. Chiang, Phys. Rev. Lett. 62 (1989) 171. [4] (a) T.P. Beebe, Jr., T.E. Wilson, D.F. Ogletree, J.E. Katz, R. Balhorn, M.B. Salmeron and W.J. Siekhaus, Science 243 (1989) 370; (b) R.J. Driscoll, M.G. Youngquist and I.D. Baldeschwieler, Nature 346 (1990) 294. [5] E. Holland-Moritz, J. Gordon II, G. Borges and R. Sonnenfeld, Langmuir 7 (1991) 301. [6] M. Kuwabara, D.R. Clarke and D.A. Smith, Appl. Phys. Lett. 56 (1990) 2396. [7] T. Tiedje, J. Varon, H. Deckman and J. Stokes, J. Vac. Sci. Technol. A 6 (1988) 372. [8] R.V. Coleman, B. Drake, P.K. Hansma and G. Slough, Phys. Rev. Lett. 55 (1985) 394. [9] K. Kern, P. Zeppenfeld, R. David, R.L. Palmer and G. Comsa, Phys. Rev. Lett. 57 (1986) 3187. [10] C.R. Clemmer and T.P. Beebe, Jr., Science 251 (1991) 640. [11] M.L. Myrick, N.V. Hud, S.M. Angel and D.G. Garvis, Chem. Phys. Lett. 180 (1991) 156. [12] R.J. Colton, S.M. Baker, R.J. Driscoll, M.G. Youngquist, J.D. Baldeschwieler and W.J. Kaiser, J. Vac. Sci. Technol. A 6 (1988) 349. [13] M.F. Toney and S.C. Fain, Jr., Phys. Rev. B 36 (1987) 1248. [14] K.L. D'Amico and D.E. Moncton, J. Vac. Sci. Technol. A 4 (1986) 1455. [15] P.A. Rowntree, G. Scoles and J.C. Ruiz-Sufirez,J. Phys. Chem. 94 (1990) 8511. [16] J.P. Toennies and R. Vollmer, Phys. Rev. B 40 (1989) 3495. [17] S. Morita, S. Tsukada and N. Mikoshiba, J. Vac. Sci. Technol. A 6 (1988) 354.