J. MATER. CHEM., 1991,1(3), 469-472 Surface Structure of a Glassy Carbon Scanning Tunnelling Microscopy Study Norman M. D. Brown* and Hong Xing You Surface Science Laborators Department of Applied Physical Sciences, University of Ulster, Coleraine, Co. Londonderry BT52 ISA, UK Scanning tunnelling microscopy (STM) has been used to study the topographical details of the surface structure of a glassy carbon at ultra-high resolution. In addition to small, localised graphite-like domains <50 A x 50 B( in area, the STM images of the glassy carbon studied, taken in the constant-tunnel-current mode in air, are in general characterized by two topographical features: (i) a granular structure with grain sizes typically in the range 80.0-250.0A in some surface locations and (ii) straight or curved fibrillar structures lying side by side, in other surface locations.The latter are considered to reflect the residual structure of the polymer chains initially present during processing of the material. Keywords: Scanning tunnelling microscopy; Glassy carbon; Surface structure; topography 1. Introduction Glassy carbons, which exhibit uniqueness and diversity in their physical, chemical and mechanical properties, such as high mechanical strength, thermal stability, inertness to strong oxidising agents, high biocompatibility, good electrical con- ductivity etc., have started to find wide application in industry, engineering and scientific The physical, chemical and mechanical properties of interest are heavily dependent on the microstructure of the glassy carbon.So, for a better understanding of the relationship between these properties and structure, and to broaden the areas of application, glassy carbons have been studied widely by X-ray diffra~tion,'.~ small-angle X-ray scattering,' vs low-energy electron diffrac- tion: X-ray photoelectron ~pectroscopy,~ electron microscopy [SEM (scanning) and TEM (transmi~sion)]'~~~~ and Raman ~pectroscopy.~The focus of such structural studies is on establishing whether glassy carbons have a random disordered structure or are more graphite-like. However, the fundamental understanding of the structures of glassy carbons has been hampered by the lack of techniques for revealing local struc- tural details at ultra-high resolution, namely on the iO-9-iO-'0 m scale.With the invention of the scanning tunnelling microscope,1o it became possible to study the structure of such surfaces directly in real space, even down to atomic resolution. Even so, little is known about the surface structure of glassy carbons at scales below a nanometre. So far, the sensitivity of the STM tip to loose particular carbon on the surface appears to have prevented topographical details from being obtained at ultra-high resolution, especially when operating in air."*" The first report of the use of STM techniques to a commercial pitch-based glassy carbon has been published recently." The STM images obtained with the STM tip and the sample under paraffin oil showed ordered regions extending over no more than ca.20A. These regions consist of atomic rows spaced at either cu. 1.5 A or ca. 2.2 A; no explanation was given for the occurrence of the two different periodicities on this particular glassy carbon surface. Atomic resolution STM image, i.e. periodic rows, has also been observed from a further commercial glassy carbon surface, again in an oil environment, but in this case no structural details were presented.'* Furthermore, even though small graphitic domains in a pyrolysed polyimide matrix have been observed by STM, no other detailed surface structures were revealed in that case.13 In this paper, we present topographical details observed by STM in air of the surface of a commercial glassy carbon.First, it is shown that small, graphite-like domains are present on the surface of the glassy carbon sampled. Secondly, evi- dence for the existence of a residual polymeric structure in the form of straight or curved fibrils in some surface locations is presented. Finally, a more granular structure, frequently observed at other surface locations, is also shown. 2. Experimental The glassy carbon samples used (Cardio Carbon, Swansea) were prepared by heating a newly developed phenolic resin in shaped pieces to temperatures in excess of 1000 "C in an inert atm0~phere.l~ These were then ground and polished using a 4 pm diamond particle-impregnated lead pad to give a uniform thickness and surface finish, as required. As- received plates of this glassy carbon were cut into 10 mm x 8 mm x 1.0 mm pieces prior to mounting on the sample holder of the scanning tunnelling microscope (W.A. Technology, Cambridge). This consists of a scanning head suitable for operation in either air or vacuum, an electronic control unit, a Tandon 386 microcomputer with VGA colour monitor and frame store facility, and a monochrome monitor for image display. The STM tips were prepared by the electrochemical etching of a 0.5 mm diameter tungsten wire in an aqueous 1 mol dm-3 KOH ~olution.'~These were then rinsed in distilled water and finally in redistilled acetone before use. The STM images, composed of 256x256 pixels, were obtained in air in the constant-tunnel-current mode with tip transit speeds between 1 and 100 A s-'. Typical tunnelling conditions were 1 nA for the tunnel current and 800-1500 mV for the sample bias (with the sample positive).3. Results In the first instance, much effort was devoted to the search for graphite-like structures on the surface of the glassy carbon examined. Thorough and careful imaging in various surface locations showed that local graphite-like topography could be detected reproducibly by the STM tip. Fig. 1 illustrates the two kinds of topographical features repeatedly observed on the surface of the glassy carbon sampled. One of these is characterized by a mixture of incomplete hexagonal arrange- ments and periodic rows; the other is characterized by atomic Fig. 1 Typical STM images of the glassy carbon taken in the constant- current mode (1.0 nA) showing (a)a mixture of incom lete hexagonal arrangements and periodic rows (image dimension 20 1x 20 A x 5 A; sample bias, 1006 mV) and (b) the zigzag atomic rows (image dimen- sion 21 x 21 A x 7 A;sample bias, 1006mV) rows running in a zigzag manner.Typically, both graphite- like topographies appear, as shown in Fig. 1, on small scan areas, i.e. 20 A x 20 A and 21 A x 21 A. The spacings of the atomic rows in Fig. l(b) are measured to be 4.2f0.1 A, the same as the distance between two adjacent atomic corru- gations of the incomplete hexagonal arrangements in Fig. l(a).’ Fig. 2 shows further graphite-like surface details over a larger scan area, i.e. 39 A x 39 A.Upon close inspection of Fig. 2, it can be seen that there exists a small-scale hexagonal ordering in the right front part of the image with the distance between two adjacent atomic corrugations of 4.2 & 0.1 A. The atomic rows running in a zigzag manner characterise the rest of the image, with the same row spacing (4.2 f0.1 A). The second STM topographical feature frequently observed J. MATER. CHEM., 1991, VOL. 1 on the surface of the glassy carbon studied here, when scanning over areas of some hundreds of square Angstroms, is that of a straight or curved fibrillar structure. Fig. 3 and 4 show images typical of those observed. In Fig. 3(a), there are several aligned fibrils in an uneven form, with a typical fibrillar separation of ca.44.0 A.In Fig. 3(b), the aligned fibrils are rather smoother in outline with a different fibrillar separation of ca.34.0 A. In addition, somewhat curved fibrils, sitting side by side, could occasionally be observed on the surface of the glassy carbon, as shown in Fig. 4. Fig. 4(a) and 4(b),obtained from different positions of the sample surface, show two kinds of curved fibril. In Fig. 4(a), they have an uneven form and different corrugation heights, whereas in Fig. 4(b) they are markedly smooth with nearly the same corrugation height. The fibrillar separations are measured to be ca.32.0 A for Fig. 4(a) and ca. 46.0A for Fig.4(b). By changing the tip scanning directions, the alignment of the fibrillar structure is not changed in any one location.Correspondingly, the fibrillar structure was found to have different alignments on moving the tip to another location on the sample surface. Thus, the possibility of the fibrillar structure arising from the tip-induced artefacts can be discounted. A third STM topographical feature frequently observed on the surface of the glassy carbon examined is the granular structure found over rather larger scan areas. As shown in Fig. 5, the granules with different sizes, typically ca. 168.0 A and ca.230.0 8, [Fig. 5(a)], and with nearly uniform size, typically ca. 165.0 A or 180.0 A [Fig. 5(b)],are distributed on either side of a boundary. Similarly, Fig. 5(c) shows an island on the surface of the glassy carbon composed of granules of various shapes and sizes, in the range 80.0-170.0 A.The STM images showing the above topographical features Fig. 3 Typical STM images of the glassy carbon taken in the constant- current mode showing the straight fibrillar structure (a) with an uneven form (image dimension, 380 A x 380 A.x 49 A;sample bias, Fig. 2 An STM image of the glassy carbon with a somewhat larger 1050 mV; tunnel current, 0.8 nA) and (b) with a more smooth form graphite-like structure (dimension, 39 8,x 39 A x 8 A;sample bias, (image dimension, 219 A x 219 A x 78 A; sample bias, lo00 mV; tunnel 1000 mV; tunnel current, 1.0 nA) current, 1.0 nA) J. MATER. CHEM., 1991, VOL. I Fig. 4 Typical STM images of the glassy carbon taken in the constant- tunnel-current mode showing the curved fibrillar structure (a) with an uneven form (image dimension, 500 8,x 500 8, x 19 8,;sample bias, 1045 mV; tunnel current, 1.0 nA) and (b)with a more smooth form (image dimension, 282 8, x 3 14 8, x 35 8,;sample bias, 1034 mV; tunnel current, 0.8 nA) were routinely checked for reproducibility by scanning in either the x or the y direction with different sample biases, in the range 800-1500mV in both bias directions, and with different tunnel currents.Thus, all the topographies presented here were found to be reproducible. In addition, while it was found that the fibrillar structure referred to was usually not tangled in appearance, some STM images did show regions probably with tangled fibrils, existing where individual fibrils could not be distinguished clearly.Furthermore, the individual straight fibrils identified were found to extend over several hundreds of Angstroms without a change in their alignment. The curved fibrils were often observed in more isolated local areas. Occasionally, some curved fibrils were found with one end running into a straighter fibrillar region. Finally, it should be noted that the fibrillar structure and the granular structure identified were seldom found side by side on the sample surface. In the above survey of the samples, some of the STM images obtained did not show any resolvable topographical features. These might arise from the contaminated locations on the sample surface, or from surface regions of poor conductivity, or more likely from high local disorder.4. Discussion Now the topography of graphite has been studied extensively and in depth by the scanning tunnelling microscope in under water16 and in UUCUO.~~The occurrence of so-called giant corrugations within the unit cell, often close to or exceeding the lattice constant in the graphite (0001) plane (2.46 A), particularly when operating in air and water, is generally attributed to elastic deformations induced by the interatomic forces between tip and ~urface.'~ In the work 47 1 Fig. 5 Typical STM images of the glassy carbon taken in the constant- current mode (1.0 nA) showing the granular structure. (a)The granules with two different sizes sitting on either side of a boundary (image dimension, 2000 8, x 2000 8,x 200 8,;sample bias, 1000 mV); (b) the granules with nearly the same size on either side of a boundary (image dimension, 2000 8, x 2000 8, x 196 8,; sample bias, 1200 mV) and (c) an island containing the granules described with various sizes (image dimension, 1000 8,x 1000 A x 104 8,;sample bias, 1 100 mV) described here, the giant corrugations of 2.0-5.0 8, are in the same range as those of an earlier graphite study.I6 It is found that the STM tip sometimes cannot resolve the individual atoms of the graphite unit cell, particularly in the case of more amorphous carbon materials.11*12*20 Here, the zigzag rows of poorly resolved atoms appear in the locally ordered regions imaged, as shown in Fig.l(b) and Fig.2. The periodic phenomena shown in the various figures might be considered to arise from multiple tunnelling as a consequence of the condition of the tip used, e.g. the tip may have extraneous molecules adsorbed on it or have closely adjacent high spots. On the other hand, the periodicities of 4.2 0.1 8, found are larger than the lattice constant of the normal graphite unit cell (2.468,), but they are very close to J3 x2.46 A (4.26A), the lattice constant of the unit cell of the J3 x J3 graphite structure. Since multiple-tip effects, which sometimes occur in STM graphite studie~,'~,'~ seem unlikely to match these dimensions, they are discounted here, particularly since such image details were reproducible with different tips. In addition to the above, high-resolution electron micro~copy'*~-~has revealed that glassy carbons consist of a random arrangement of labyrinth-like carbon chains or rib- bons of near polymeric chain dimensions, i.e.a few tens of square Angstroms in cross-section with several tens or hun- dreds of Angstroms between cross-links. Here, the straight and curved fibrillar structures (shown in Fig. 3 and 4)lying side by side on the surface of the glassy carbon are assumed to be derived from a single polymer chain or possibly from small bundles of a few polymer chains.21i22 The uneven or smooth forms of the fibrils might be associated with pro- cessing-dependent changes of the local surface of the glassy carbon studied and/or be related to the structure of the polymer precursor used. From an earlier electrical study of a glassy ~arbon,~ the mean free path of conducting electrons has been estimated to be ca.15 A, again on the scale of the width of a polymer chain. Multiples of this width are matched reasonably well by the fibrillar separations or widths shown in Fig. 3 and 4. Hence, the significance of the mean free path as a regular fraction of the fibrillar features is stressed. Depending on the processing temperatures and the polymer resins used, the yields of glassy carbons from polymer pyrolysis are usually 40-60%.',23 Accordingly, some of the polymer resins do not given an amorphous glassy carbon but retain their own general structure, as suggested by Fig. 3 and 4. Other starting materials generally transform to a disordered non-graphitized carbon with glass-like structural properties.The crystallite sizes of such glassy carbons measured by X-ray diffracti~nl-~~~are generally no more than 100 A. Therefore, the granular sizes in the range 80.0-250.08, found in this work (Fig. 5) are somewhat larger than those indicated by X-ray diffraction. In this regard, it should be noted that the scanning tunnelling microscope can measure the distribution of granular sizes concerned, whereas X-ray diffraction can only give a mean value, because the X-ray beam samples a significantly larger sample cross-section. 5. Conclusion The STM images of glassy carbon show the following topo- graphical features: (1) the surface of this glassy carbon is generally characterised by a non-graphitic str~cture.~*~*' How-ever, in very local positions, small graphite-like domains of <50 A x 50 A are observed.The 4.2k0.1 A periodicity of these domains is commensurate with the lattice constant of the 43xJ3 graphite structure unit cell (4.26 8,). (2) The aligned fibrillar structures with a straight or curved shape are observed in some locations of this glassy carbon surface. This fibrillar structure is believed to reflect the structure of the J. MATER. CHEM., 1991, VOL. 1 precursor polymer chains. (3) The granular structure found at other surface locations suggests a crystallite size generally larger than the mean derived by X-ray diffraction for such materials.This work provides impetus for further STM studies of the relationship between the surface structures of glassy carbons, the precursor materials and their processing. Likewise, STM offers much in the investigation of. the surface-modification work on such systems. H-X. Y. thanks the University of Ulster for the postgraduate research studentship. Thanks are also due to the IRCSS at the University of Liverpool and to the International Fund for Ireland for support. The author thanks Dr. W. D. Unsworth of Cardio Carbon Company Ltd. for the supply of samples. References 1 G. M. Jenkins and K. Kawamura, in Polymeric Carbon-Carbon Fibre, Glass and Char, Cambridge University Press, Cambridge, 1976. 2 J. Rautavuori and P. Tormala, J. Muter.Sci., 1979, 14, 2020. 3 P. Delhaes and F. Carmona, in Chemistry and Physics of Carbon, ed. 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