SIMULATION OF NANOSCALE ETCHING FOR NANOTUBE AND GRAPHENE DEVICES
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SIMULATION OF NANOSCALE ETCHING FOR NANOTUBE AND GRAPHENE DEVICES Koichi Kusakabe1 1 Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka, 560-8531, Japan ABSTRACT In order to find an efficient method to etch nano-carbon materials by hydrogenation in a controlled manner, we have studied hydrogen-atom adsorption on various deformed nanotubes using computer simulations based on the density-functional theory. The nanotube with an atomic lack is compared to a deformed tube with the Stone-Wales defect and a twisted tube wall. Similar to the known experimental etching condition for graphene, an atomic lack is effective to accumulate hydrogen atoms around the defect. Compared to the flat graphene, however, nanotube walls with curvature allow on-top adsorption of a hydrogen atom and selectivity in the hydrogenated site becomes worse. To achieve a controlled etching process, usage of a tungsten tip which realizes focused hydrogenation is proposed for natotubes and curved graphene. INTRODUCTION Finding of fabrication techniques of nano-meter-scale carbon structures is demanded for realization of nano-carbon devices designed in the atomic scale. Etching of graphitic honeycomb structures is an inevitable technique for this purpose. Hydrogenation is known to be a method to etch the graphitic structures including carbon nanotubes [1-3]. Recently, specific hole structures were selectively found in a hydrogenation-annealing process of graphite surfaces [4,5]. Motivated by these experimental findings, we performed computer simulations to explore hydrogenation of various deformed nanotubes. Based on the obtained results, we propose a possible controlled etching technique for nanotubes and curved graphene. THEORY We perform the structural and electronic-structure optimization using methods by the density functional theory (DFT) [6,7]. Using a generalized gradient approximation (DFT-GGA) [8], we obtain consistent results for metallic atom adsorption and hydrogenation of nanotubes. The simulation is done using the Quantum Espresso package [9], where the ultra-soft pseudo potentials are adopted to describe ionic potentials [10]. Major parameters for hydrogenated nanotubes are the energy cut off of 25Ry for the plane-wave expansion of the Kohn-Sham orbitals and that of 150Ry for the charge density. A super-cell method is adopted with 8 k-points sampled along the tube axis. The convergence criterion for the structural optimization is that the total absolute value of the inter-atomic force vector becomes less than 1 × 10 −4 Ry/a.u. DISCUSSION
To gain characteristics of hydrogenation, we obtained hydrogen-adsorbed nanotubes.
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Hydrogen adsorption on defective and deformed nanotubes
It is known that several late-transition elements may break carbon-carbon bonds in thin nanotubes [11], when the nanotubes are deformed before the metal atoms are adsorbed. The mechanical distortion can enhance the reactivity. We can see even creation of a nano-scale cleavage in the tube wall by the deformation. To
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