Disorder-Induced Extremely Low Thermal Conductivity of Graphite Fluoride

Graphite fluoride represents an important class of functionalized graphene-based materials. While theoretical studies have extensively examined the functionalization effects on graphene's thermal conductivity, experimental establishment of fluorination effects on thermal transport remains limited, especially regarding the through-plane thermal conductivity of the layered structures. We determine the through-plane and in-plane thermal conductivity of mechanically exfoliated graphite fluorides using co-aligned and beam-offset time-domain thermoreflectance, cross-validated with frequency-domain thermoreflectance. Graphite fluorides exhibit the exceptionally low through-plane thermal conductivity of < 0.030 W m^-1 K^-1 for thicknesses of 53-243 nm at room temperature - the lowest among fully dense solids. The in-plane thermal conductivity is measured as 4.2-5.6 W m^-1 K^-1 for a 178 nm-thick flake, resulting in an anisotropy ratio of thermal conductivity exceeding 100. The record-low through-plane thermal conductivity stems from widely distributed interlayer spacing with configurational and stacking disorders as well as mass and rotational disorders, while reduced in-plane thermal conductivity results from nm-sized fluorinated grains with configurational disorder. We further investigate the effect of fluorine incorporation on surfaces, revealing that surface fluorination minimally impacts interfacial thermal conductance. Our results demonstrate how structural disorder creates unprecedented lower bounds for thermal conductivity in layered materials modified by functional groups.