Molecular Dynamics Study of Thermal Energy Transport in Graphene Based Materials
Self heating is a critical issue in micro and nano electronics, and efficient heat removal is crucial for the long term reliability and performance of micro and nano electronic devices. In many semiconductor devices, the power dissipation is non uniform, which results in hot spots in the device and makes the heat removal very complicated. When two dimensional materials are used for the electronic applications, heat dissipation problem can be aggregated because even a small amount of Joule heat can lead to a dramatic temperature rise in the ultrathin materials. In industrial practice, high performance site specific heat spreader and thermoelectric material based solid cooling component are two potential tools for the hot spot mitigation. Graphene, which has ultrahigh thermal conductivity, electrical conductivity and thermoelectric power factors, can potentially be used as the core material for these two applications. In my Ph.D. study, the thermal transport properties of graphene and its derivatives were investigated using classical molecular dynamics simulations. The research goal was to understand how the chemical functionalization, isotopic effects and structural modification to influence the phonon transport in graphene and across graphene metal interfaces, which could provide important insights to the thermal transport physics in graphene, and are of practical significance for graphene based devices in nano electronics and thermal management applications. Four projects have been finished in my Ph.D. study. In the first project, the phonon transport physics of pristine graphene, hydrogenated graphene and graphene oxide was investigated using large scale molecular dynamics simulations. For the pristine graphene, highly ballistic thermal transport was observed. As for the hydrogenated graphene and graphene oxide, the thermal conductivity was significantly reduced when the hydrogen and oxygen coverage increased. For example, an oxygen coverage of 5% could reduce the graphene's thermal conductivity by ~90% and a coverage of 20% oxygen could lower it to ~8.8 W/mK. This value is even smaller than the calculated thermal conductivity of graphene in amorphous limit ~11.6 W/mK) which is usually regarded as the lower boundary of graphene's thermal conductivity. The vibrational power spectral analyses showed that this large reduction in thermal conductivity was due to the significantly enhanced phonon scattering induced by the oxygen and hydrogen defects. In the second project, the thermal conductivity of oxidized polycrystalline graphene was studied. Grain boundaries and the spontaneous oxidization around grain boundary regions are the inherent features of graphene. Our study found the thermal conductivity of oxidized polycrystalline graphene decreased as oxygen coverage increased, which was due to the phonon defect scattering. However, the relative thermal conductivity reduction of oxidized polycrystalline graphene when the oxidization was localized at the grain boundary regions was much smaller compared.