Grain boundary-limited thermal transport in suspended thin graphite across an unexplored thickness regime

We present systematic thermal conductivity measurements of suspended thin graphite ribbons, 234-527 nm thick, using a four-probe 3-omega method. Unlike recent reports of phonon hydrodynamics and exceptionally high thermal conductivity in micrometer-thick graphite (Science, 2020),we observe significantly lower thermal conductivity and no signatures of collective phonon flow in this intermediate thickness regime. Instead, our measured thermal conductivity lies between few-layer graphene and bulk graphite.These results agree with a first-principles-informed Peierls-Boltzmann transport model with spatially resolved Monte Carlo sampling. Additionally, the temperature for the peak thermal conductivity shifts lower with increasing thickness, due to the interplay of phonon-boundary and phonon-isotope scattering. Incorporating grain boundary scattering into simulations is necessary to replicate the experimental trends. These findings delineate the boundary between ballistic, hydrodynamic, and diffusive transport regimes in graphite, and underscore the dominant role of disorder and geometry in phonon transport in quasi-two-dimensional materials, offering insights for nanoscale thermal management.