Ultralow thermal conductivity of fullerene derivatives

Recently, Duda et al. [J. C. Duda, P. E. Hopkins, Y. Shen, and M. C. Gupta, Phys. Rev. Lett. 110, 015902 (2013)] reported that the fullerene derivative [6,6]-phenyl-C${}_{61}$-butyric acid methyl ester (PCBM) has the lowest thermal conductivity \ensuremath{\Lambda} ever observed in a fully dense solid, \ensuremath{\Lambda} \ensuremath{\approx} 0.03 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$. We have investigated a variety of phases and microstructures of PCBM and the closely related compound [6,6]-phenyl-C${}_{61}$-butyric acid n-butyl ester (PCBNB) and find that the thermal conductivities of PCBM and PCBNB films are mostly limited to the range 0.05 \ensuremath{\Lambda} 0.06 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ with a few samples having slightly higher \ensuremath{\Lambda}. The conductivities we observe are \ensuremath{\approx}70$%$ larger than reported by Duda et al. but are still ``ultralow'' in the sense that the thermal conductivity is a factor of \ensuremath{\approx}3 below the conductivity predicted by the minimum thermal conductivity model using an estimate of the thermally excited modes per molecule.