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Thermal conductivity of thin graphitic materials has attracted significantinterests due to their applications in thermal management such as fillers inpolymer composites for thermal interface materials and heat exchangers.Recently graphite nanoribbons have also been under investigations forintriguing phonon hydrodynamic transport. However, graphite ribbons withthickness in the range of hundreds of nanometers have not been measured despitetheir importance as both thermal conductivity fillers and as a testbed forphonon hydrodynamics. In this work, we developed a process to exfoliategraphite nanoribbons within this thickness range and with length up to 1 mm andtransferred them to a suspended platform to measure their thermal conductivity.We obtained thermal conductivity around 800 W/m-K at room temperature. Withdecreasing temperature, the thermal conductivity initially increased and peakedat around 150 K, with peak values ranging from 1000 to 1600 W/m-K for ribbonthickness of 352 to 680 nm. This temperature dependence suggests the strongphonon Umklapp scattering, which is consistent with the absence of the phononhydrodynamics observed in our experiments, as shown by nearly identical thermalconductivity in two samples with similar thickness (525 and 560 nm) anddifferent lengths (0.5 and 1 mm). This is contrary to the previous experimentalresults suggesting phonon hydrodynamic at room temperature on even thickerribbons. Finally, the lower than bulk value of thermal conductivity revealsstrong phonon boundary scattering, suggesting long phonon mean free path, onthe other of 100s nm, in the cross-plane direction of graphite, confirmingcertain previous studies.

Thermal Conductivity and Role of Phonon Scatterings in Suspended Thin Graphite Nanoribbons