Hardness of Pillared-Graphene Nanostructures via Indentation Simulation
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Hardness of Pillared-Graphene Nanostructures via Indentation Simulation R. Sasaki and K. Shintani Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan ABSTRACT Pillared-graphene is one of nanocarbon hybrids. It consists of graphene sheets and carbon nanotubes (CNTs); the latter are bonded vertically to the former. In order to investigate the hardness of pillared-graphene, indentation simulations are performed using a molecular dynamics method. It is revealed that the hardness of pillared-graphene increases with increasing the diameter of the CNTs, whereas it decreases with increasing the distance between CNTs or temperature. Such tendencies can be understood by considering the deformations of graphene and CNTs individually. INTRODUCTION The electronic and mechanical properties of graphene and CNTs are extraordinarily superior, and they are applicable to electronic devices and nano-electro-mechanical-systems (NEMS). To make the most of the advantages of graphene and CNTs, the hybrid structures of them are devised. A pillared-graphene nanostructure is one of such hybrids. It consists of parallel graphene sheets and CNTs; CNTs are atomically bonded to the graphene sheets and connect the sheets as pillars. Pillared-graphene nanostructures were synthesized on a Co/TiN catalyst film on a Si substrate [1] or a Cu film on a SiO2/Si substrate [2] by chemical vapor deposition (CVD). Applications of pillared-graphene spread over a wide range of nanotechnology, e. g., storage of hydrogen [3] or methane [4], separation of gases H2S/CH4 [5], noble gases [6], N2/O2 and CO2/CH4/H2 [7], supercapacitors [8,9], thermal treatment [10,11], etc. To apply pillaredgraphene to these nanodevices, the knowledge of its mechanical properties is essential. In this paper, a mechanical property, hardness, of pillared-graphene is investigated via indentation simulations using a molecular dynamics method. Although the indentation simulations for pillared-graphene were already performed by Wang et al. [12], more kinds of CNT and interpillar distance are prepared in our models than in their models, which will enable us to obtain more accurate data for the hardness of pillared graphene. COMPUTAIONAL MODEL AND METHOD A pyramidal indenter of the dihedral angle 136 deg is used in our indentation simulations. The side view of the indenter is shown in figure 1(a). The indenter consists of 1440 carbon atoms. On the other hand, a model of pillared-graphene consists of two graphene sheets of the square 20 nm 20 nm and four single-walled CNTs of the length 10 nm. The CNTs are bonded perpendicularly to the graphene sheets by arranging seven- and five-membered rings of carbon atoms between six-membered rings at the junctions properly. The single-walled CNTs of armchair type are selected so that their diameter jumps are as small as possible and their diameter
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