Abstract
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 (C60) and its derivative phenyl-C61-butyric acid methyl ester (PCBM). The results of non-equilibrium MD simulations show a length-dependent thermal conductivity for C60 but not for PCBM. The thermal conductivity of C60, obtained from the linear extrapolation of inverse conductivity vs. inverse length curve, is 0.2 W m−1 K−1 at room temperature, while the thermal conductivity of PCBM saturates at ~0.075 W m−1 K−1 around 20 nm. 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 C60. 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.
Highlights
There have been a few experimental studies on the thermal conductivity of fullerene and its derivatives
Considering two extremes of pure fullerene[11,14] and pure organic alkyl-chain systems[16,17], both have higher thermal conductivities than the fullerene derivatives. Fullerene derivatives such as phenyl-C61-butyric acid methyl ester (PCBM) and phenyl-C61-butyric acid n-buylester (PCBNB) cannot be treated as a mixture of fullerene and alkyl chains in the study of their thermal conductivity, and their thermal transport mechanisms are complicated by adding the alkyl chains to buckyballs via chemical bonding
We study the thermal transport in crystalline C60 and its derivative PCBM using molecular dynamics (MD) simulations
Summary
There have been a few experimental studies on the thermal conductivity of fullerene and its derivatives. Wang et al.[14] conducted TDTR measurements to study the thermal conductivity of fullerene C60 and its derivatives including PCBM and phenyl-C61-butyric acid n-buylester (PCBNB). They observed low thermal conductivity of PCBM and PCBNB around 0.05 ~ 0.06 W m−1 K−1. Based on the different length dependence of the thermal conductivities of C60 and PCBM and the reduction in the speed of sound of PCBM observed by the aforementioned works, we attribute the decrease in the thermal conductivity of PCBM to the scattering of low frequency phonons imposed by alkyl chains
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