Simulation of the Structure of Screw Dislocations in the Inter-layer of Graphite

Introduction Researches on dislocations in graphite have been carried out from the 1950s. For example, edge dislocations [1] and partial dislocations in the basal plane [2] have been observed by transmission electron microscopy (TEM). The spiral growth patterns which originate from the screw dislocation have been observed [3, 4]. Moreover, it was observed by atomic force microscope (AFM) that growth spirals had the height of 6.7 Å (length of double-layer of graphite) or 3.3 A (single-layer) [5]. Although these results indicate that the screw dislocations are formed in the graphites, the direct evidence of screw dislocations has not been obtained. Recently, it has been reported that the screw dislocations were observed in the inter-layer of the multi-walled carbon nanotubes by TEM observation [6]. The screw dislocations in the multi-walled nanotubes were highly mobile as characterized by its glide, climb, and the glide-climb interactions, at temperatures of about 2000 C. However, dynamical process and detailed structure do not yet become clear. The stable structure of the screw dislocation dipole in the graphite is studied by Suaretz et al. with the simulation using local density approximation (LDA) [7]. The LDA calculation is good accuracy, but need a high computational resource. Therefore it is difficult to deal with the dynamical process for the dislocations, the properties in finite temperature, and large scale simulation. Tight-binding method is one of the efficient methods to overcome these difficulties. In the present study, as the first step for the tight-binding calculation, the stable structure of the screw dislocation in the graphite was investigated with the tight-binding method. The detail of the core structure is studied, and compare to the result of LDA calculation. The formation energy by the tight-binding method is lower than that of LDA calculation, but the optimal structure quite similar with each other. It is shown that the optimized structure do not depend on the initial position of the screw dislocation. Fig. 1 The stable structure of the screw dislocation dipole in inter-layer graphite. The atoms of the dislocation core are denoted by red. The unit cell for the structural optimization had 6 layers of graphene as shown in this figure.