Thermal Transport Properties Of Vacancy-defective Graphene Ribbons

The growing energy crisis and the lower energy use efficiency make people to improve the energy utilization rate as high as possible. Thermoelectric materials can realize power and heat energy conversion, which is expected to convert waste heat into electrical energy again for people to use. In recent years, Graphene has become one of the hot spots in condensed matter physics because of its excellent physical properties. In the preparation process of graphene, it is inevitable to introduce defects in graphene ribbons, making graphene ribbons become the defective one. Many studies show that vacancy defective graphene ribbons have a ultra lower thermal conductivity compared to the pristine ones, and therefore, the vacancy defective graphene ribbons are expected to be the low dimensional thermoelectric materials with excellent properties.In this thesis, by using Klemens' perturbation theory combined with the mechanism of bond-order-length-strength correlation, the thermal transport properties of vacancy-defective graphene ribbons was investigated. On the impact of phonon scattering by vacancy, the phonon scattering of under-coordinated atoms around vacancies was investigated. We find that in bulk crystal, the phonon scattering rate due to change of force constant-1A? is about three orders of magnitude lower than that due to missing mass and missing linkage-1V?, therefore-1A? would be negligible;in two-dimensional materials,-1A? can be 3-10 folds lager than-1V?,-1A? can not be negligible. Based on this new model, we theoretically investigate the effects of size and edge roughness on thermal conductivity of single vacancy-defective graphene ribbons. The results show that due to the severe suppression of high-frequency phonons by phonon-vacancy scattering, the low-frequency ballistic phonons have a higher contribution to the thermal conductivity, which results in the stronger length,weaker width and weaker edge roughness dependence on thermal conductivity of vacancy-defective graphene ribbons than that of pristine ones. These results indicate that vacancy defects and ribbons' size can tune thermal transport properties of graphene ribbons efficiently.