Thermal Rectification Characteristics of Graphene Nanoribbons of Asymmetric Geometries
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Thermal Rectification Characteristics of Graphene Nanoribbons of Asymmetric Geometries T. Iwata and K. Shintani Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan ABSTRACT The rectification of heat in graphene nanoribbons (GNRs) of asymmetric geometries is investigated by means of nonequilibrium molecular dynamics (NEMD). Two kinds of geometries of GNRs are addressed; a trapezoidal or T-shaped step is inserted halfway through a GNR in its longitudinal direction. The thermal conductivities (TCs) of the GNRs in the two longitudinal directions, forward and backward, are calculated making their width and temperature change. It is revealed that the thermal rectification ratio (TRR) of T-shaped GNRs are larger than those of trapezoidal GNRs and that the characteristics of heat transport in such asymmetric GNRs can be understood by considering the local phonon density of states (DOSs). INTRODUCTION Graphene, a single atomic layer consisting of honeycomb lattices of carbon atoms, possesses many superior mechanical, electronic, and thermal properties that make its application to nanodevices possible. Especially, the thermal properties of graphene attract researchers’ attentions nowadays because its TC within its plane is of the same order of magnitude as the TCs of carbon nanotubes (CNTs), and one order of magnitude higher than the TCs of metals. In addition, the potentiality of controlling heat flows in electronic and energy devices by thermal rectification (TR) using some kinds of graphene devices stimulates researchers’ interest in its thermal properties. Wang et al. [1] suggested one of the methods of realizing TR is to change the width of a GNR at its intermediate position. They confirmed TR in such GNRs of asymmetric geometries although the TR diminishes with the increase of the width of the GNRs. However, the straight parts of their GNRs outside the trapezoidal or T-shaped steps are very short. In this paper, TR in GNRs having a trapezoidal or T-shaped step and long straight parts is addressed, and its mechanism is discussed by examining the local phonon DOSs. SIMULATION METHOD AND MODELS All simulations in this work are performed using an open source code LAMMPS (Largescale Atomic/Molecular Massively Parallel Simulator) [2]. The 2nd generation REBO (Reactive Empirical Bond Order) potential [3] is employed for calculating interactions between carbon atoms. The velocity Verlet algorithm is used to integrate equations of motions, and the NoséHoover thermostat is adopted to control temperature. The TCs of models are calculated with the help of the nonequilibrium molecular dynamics (NEMD) method. In this method, heat is added at one end of a model whereas the same amount of heat is subtracted until a steady temperature gradient is constructed in the model, and its TC can be calculated from the Fourier’s law.
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