Band gaps in diamond-graphite hybrids
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Band gaps in diamond-graphite hybrids Rahul Sen, R. Sumathy, and C. N. R. Raoa) Materials Research Centre and CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore 560012, India, and Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India (Received 23 August 1995; accepted 30 July 1996)
The HOMO-LUMO gaps have been estimated in a graphite-like sp2 carbon network with a progressive increase in the fraction of sp3 carbons, taking into account several possible structural alternatives for each composition. The gap is shown to increase exponentially with the fraction of sp3 carbons. Accordingly, the gap in a diamond-like sp3 network decreases with the increase in the fraction of sp2 carbons.
There is renewed interest in the different forms of carbon in the last few years. Besides structures involving mixtures of sp2 and sp3 carbons such as hydrogenated amorphous carbon (a-C : H) and diamond-like carbon,1–3 there are other forms of carbon proposed on the basis of theoretical considerations. Thus, on the basis of topological considerations, Schwarzites involving 5, 6, 7, 8, and 9-membered rings have been proposed by Townsend et al.,4 while Hoffmann et al.5 have suggested a novel form of conducting carbon. We have studied the stability of structures of diamond-graphite hybrids with different sp2ysp3 atom ratios by employing the force field method.3 It is of considerable importance to explore whether such hybrids exhibit a range of band gaps between those of graphite and diamond. Balaban et al.6 have theoretically examined cubic or hexagonal diamond layers connected by graphitic strips and have predicted that the band gap should decrease with the increase in the strip width. Band gap estimates for the diamondgraphite hybrid structures with different fractions of sp3 carbons, which take into account several possible structural alternatives, are, however, not available. Since the band gap in such hybrids would depend not only on the sp2ysp3 atom ratio but also on how the sp3 (or sp2 ) carbon atoms are distributed in a sp2 (or sp3 ) network, we have carried out a systematic study of the HOMO-LUMO gaps in diamond-graphite hybrids, with varying sp2ysp3 ratios, starting with a pure sp2 layer at the graphite end and a three-dimensional sp3 network at the diamond end. In order to study the effect of incorporating sp3 (or sp2 ) carbons in a sp2 (or sp3 ) network, on the band (HOMO-LUMO) gap, we have started with large hydrocarbons containing either only sp2 or sp3 carbons and estimated the gaps by extended Huckel theory.7 It may be noted that for a series of related systems, the EHT method provides estimates of the HOMO-LUMO a)
Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 11, No. 12, Dec 1996
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gaps which follow the trend in experimental values.7,8 The atomic coordinates for the structures were obtained from force field minimization by using the molec
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