Evaluation of thermal conductivity and its structural dependence of a single nanodiamond using molecular dynamics simulation

In the present study, we investigated thermal conductivity and its structural dependence of a spherical nanodiamond with 2.5 nm in diameter using molecular dynamics simulation. We briefly discussed the difficulty of computing the thermal conductivity of a free nanoparticle using conventional methods and here we derived it from the non-equilibrium molecular dynamics simulation of a composite system where a nanodiamond is sandwiched between two solid blocks. The structural dependence was examined by applying this method based on a composite system to the 2.5 nm nanodiamonds having different ratios of 3- and 4-coordinate carbons (termed sp(2)-like and sp(3)-like carbons, respectively), which were obtained from annealing at different temperatures. The thermal conductivity of the nanodiamond decreased from 28 to 10 W/(m.K) with decreasing ratio of spa-like carbons until the number of sp(2)-like bonds exceeded that of sp(3)-like bonds. When sp(2)-like bond became richer than spa-like bond, the thermal conductivity was less sensitive to further increase of the ratio of sp(2)-like carbons. Based on the consideration of the heat transfer associated with a single C-C bond, we interpreted that this structural dependence reflects the heat transfer characteristics of sp(3)- or sp(2)-like bond, whichever is more abundant. This interpretation, as well as the methodology, is helpful for understanding thermal conductivity of nanodiamonds and other carbon nanomaterials.