Density functional investigations of defect induced mid-gap states in graphane

We have carried out ab initio electronic structure calculations on graphane (hydrogenated graphene) with single and double vacancy defects. Our analysis of the density of states reveal that such vacancies induce the mid gap states and modify the band gap. The induced states are due to the unpaired electrons on carbon atoms. Interestingly the placement and the number of such states is found to be sensitive to the distance between the vacancies. Furthermore we also found that in most of the cases the vacancies induce a local magnetic moment. Introduction Graphane1 i.e. the hydrogenated graphene, is a recent addition to the family of novel carbon materials known for their exotic properties.1,2,3 Recently graphane is experimentally realized by Elias et al4 who demonstrated that the process of hydrogenation is reversible. This observation makes graphane a suitable candidate for the hydrogen-storage material. Equally interesting is the possibility of a direct observation of metal-insulator transition in two dimensional systems.5 as a function of hydrogen coverage. Graphane is an insulator with a reported theoretical band gap of ∼ 3.5 eV using density functional theory (DFT).1 It is well known that a single sheet of graphene is susceptible to a variety of disorders like topological defects, impurity states, ripples, cracks etc. Pereira et al6,7 have studied different models of local disorders in graphene and have investigated their electronic structure within tight binding method. They have observed a significant changes in the low energy spectrum of graphene viz., localized zero modes, strong resonances, gap and pseudogap behavior etc depending upon the type of disorder. Their results also indicate that by and large disorder significantly modifies the states near the Fermi level. Yazyev and Helm8 have studied defect induced magnetism in graphene using DFT. Their work shows that the adsorption of hydrogen or the creation of defect on graphene sheet lead to the local magnetic moment. For an extensive survey of studies of disorders 1In order to avoid the confusion with the word ‘graphene’, the word ‘graphane’ is written in italics throughout this paper.