Spectral Contributions to the Thermal Conductivity of C60 and the Fullerene Derivative PCBM

We investigate the heat transport mechanisms responsible in driving the characteristic temperature-dependent thermal conductivities of C-60 and PCBM crystals via molecular dynamics simulations. We find that the thermal conductivity of PCBM is "ultralow" across the temperature range studied in this work. In contrast, the temperature-dependent thermal conductivity of C-60 crystals exhibits two regimes: "crystal-like" behavior at low temperatures where thermal conductivity increases rapidly with decreasing temperature and temperature-independent thermal conductivities at higher temperatures. The spectral contributions to thermal conductivity for C-60 suggest that the majority of heat is carried by modes in the low-frequency regime (<2 THz), which is a consequence of intermolecular interactions. Unlike for C-60, these modes are not responsible for heat conduction in PCBM due to the mismatch in density of states introduced by the addition of low-frequency modes from the alkyl chains that are attached to the fullerene moieties.