In order to study the effects of alkyl chain on the thermal properties of fullerene derivatives, we perform molecular dynamics (MD) simulations to predict the thermal conductivity of fullerene (C 60) and its derivative phenyl-C61-butyric acid methyl ester (PCBM). The results of non-equilibrium MD simulations show a length-dependent thermal conductivity for C 60 but not for PCBM. The thermal conductivity of C 60, obtained from the linear extrapolation of inverse conductivity vs. inverse length curve, is 0.2Wm-1 K-1 at room temperature, while the thermal conductivity of PCBM saturates at ∼0.075Wm-1 K-1 around 20nm. The different length-dependence behavior of thermal conductivity indicates that the long-wavelength and low-frequency phonons have large contribution to the thermal conduction in C 60. The decrease in thermal conductivity of fullerene derivatives can be attributed to the reduction in group velocities, the decrease of the frequency range of acoustic phonons, and the strong scattering of low-frequency phonons with the alkyl chains due to the significant mismatch of vibrational density of states in low frequency regime between buckyball and alkyl chains in PCBM.
Bibliographical noteFunding Information:
L.C. and S.K. acknowledge support from National Science Foundation Grant CBET-1236416. X.W. thanks the startup funding support from the University of Minnesota.