Nanomachining Graphene with Ion Irradiation
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1259-S18-02
Nanomachining Graphene with Ion Irradiation Jani Kotakoski1 and Ossi Lehtinen1 1
Div. of Materials Physics, University of Helsinki, P.O. Box 42, 00014 Helsinki, Finland.
ABSTRACT We present molecular dynamics simulations using both empirical potentials (EP) and density functional theory (DFT) on ion irradiation of graphene. The comparison between the two methods shows that EP gives not only qualitatively but also quantitatively reasonable estimates of defect production during ion irradiation in carbon nanosystems. Ion irradiation is shown to give rise to a range of interesting defects e.g. single, double and triple vacancies, bond rotations, close-by Frenkel pairs and more complex defect structures. We show that the creation of these defects is related to the atomic processes upon the ion impact, and define the critical irradiation angles both for maximum damage and no penetration as a function of the ion mass. INTRODUCTION Graphene is the ultimately thin membrane – a chicken wire structure made from sp2hybridized carbon atoms – which has attracted enormous attention since its discovery [1], not least because of its unique electronic properties [2]. An important aspect in this regard is the effect of disorder, especially since graphene grown with the chemical vapor deposition (CVD) method is known to contain a large proportion of dislocations and disordered areas, although some of the seen defects may be a result from the processes in preparing and imaging the structure [3]. Ion irradiation is a good method for a systematic study of the role of the disorder on the properties of graphene, because the irradiation doses and hence the amount of disorder can be easily controlled. To estimate the ion-irradiation–induced disorder in graphene, Raman spectroscopy [4, 5], atomic force microscopy [5, 6], local mobility measurements [7] and atomic resolution scanning tunneling microscopy [8] have been used. However, the underlaying atomic scale mechanisms are not known. In fact, even the separation between the directly created damage on graphene and the damage caused to the substrate or secondary damage to graphene via backscattering from the substrate has not been studied. In this study, we simulate impacts of energetic ions onto a suspended graphene sheet to describe the atomic scale events during ion irradiation of this material. THEORY In order to evaluate the characteristic quantities from first principles, we employed density functional theory (DFT) molecular dynamics (MD) simulations with the plane-wave basis set code VASP [9] with projector augmented wave (PAW) potentials [10] to describe the core electrons, and the generalized gradient approximation (GGA) [11] for exchange and correlation. In order to validate the use of PAW in high energy impact simulations where atoms appear at close separations, we compared PAW calculations with all-electron calculations carried
out with the simulation code DMOL [12]. Comparing the energy of a Ar-C dimer with both methods as a function of the dimer length showed tha
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