Comparing the phonons and vibrations in carbon allotropes with neutron spectroscopy: Microdiamond, few-layer graphene, and amorphous fibers

Powder-averaged inelastic neutron spectroscopy was performed in order to compare the phonons of microdiamond, few-layer graphene, and amorphous carbon fibers at room temperature. Both acoustic and optical phonons were observed in the crystalline allotropes. We present average group velocities, phonon densities of states, and heat capacities relevant to intrinsic thermal transport. High-temperature measurements were performed to evaluate whether quasiparticle broadening from phonon–phonon scattering could be detected. The glassy carbon form in the fibers exhibits pronounced spectral broadening and an enhanced low-energy density of states beyond the Debye prediction. Methods for estimating thermal conductivity using inelastic neutron data as input to the phonon-gas model are discussed. We highlight opportunities and challenges for employing higher-resolution methods to directly determine phonon lifetimes.