Effect of Fluorination on The Thermal Conductivity of Graphite Fluoride (CF)

For thermal management applications, there are surging demands for materials with high thermal conductivity and electrically insulating properties. Graphite fluoride (CF), one of the novel graphene-based layered materials, has emerged as a promising thermal management material owing to its similar crystal structure to that of graphite yet electrically insulating properties, i.e., electronic band gap of over 3 eV. CF is expected to show high thermal conductivity due to its structural similarity to graphite. However, the degree of which fluorination alters the thermal conductivity is largely unexplored, and even the thermal conductivity is not experimentally established. Here, we report the through- and in-plane thermal conductivity of mechanically exfoliated CF flakes for the first time by using time-domain thermoreflectance (TDTR). At room temperature, a 70-nm-thick CF flake shows (1700±300) W m^-1 K^-1 for in-plane direction, which is about 90 % of that of graphite, and (4.0±0.8) W m^-1 K^-1 for through-plane direction. The through-plane thermal conductivity of CF flakes shows quasi-ballistic behavior for thicknesses < 200 nm, which is similarly observed in graphite but shows two times higher through-plane thermal conductivity than that of graphite. We calculate the phonon properties of monolayer graphene and graphene fluoride to reveal the intrinsic phonon transport mechanisms by solving the phonon Boltzmann transport equation (BTE). BTE estimates the thermal conductivity of graphene and graphene fluoride as 3000 W m^-1 K^-1 and 270 W m^-1 K^-1, respectively. Phonon mode thermal conductivity indicates that the fluorination opens an out-of-plane acoustic (ZA) phonon scattering channel and the ZA phonon contribution to the thermal conductivity of CF is greatly suppressed, compared to the case of graphene. To understand the discrepancy in thermal conductivity between the experimental and theoretical results, we characterize the chemical and optical properties of exfoliated CF flakes to correlate those properties with thermal conductivity results. We believe this work reveals the role of functional groups on thermal conductivity in graphene/graphite-derivative materials.