Thermal Transport Properties In Graphene And Boron Nitride Nanoribbons

Nanostructual materials are served as the important components for building novel electronic devices in the future. The study of their physical properties has become a hot spot in modern condensed matter physics. In this thesis, we mainly investigate the thermal transport properties of graphene and boron nitride nanoribobns (GNRs and BNNRs) by using Green's function method.In chapter one, we first briefly introduce the fabrication of graphene and boron nitride, then we introduce their thermal and electronic properties. Finally, we outline the research contents of this thesis.In chapter two, we introduce the Green's function method. Through introducing the force constant model and applying the Green's function method, we calculate the phononic transmission coefficient, thermal conductivity, thermoelectric figure of merit and other parameters.In the third thapter, we study the phononic transport properties in periodic T-shaped GNRs. Interesting resonant phenomenon is found in the phonon transmission of out-of-plane mode. When the T-shaped GNR includes n constrictions, there are (n-1)-fold resonant splitting peaks in the low frequency region of the transmission spectrum. The peaks are induced by low quasibound states in which phonons are intensively localized in the stubs. While (n-2)-fold resonant splitting rule occurs at frequency slightly higher than the first threshold frequency. These resonant peaks are originated from high quasibound states in which phonons are mainly localized in the constrictions. To the high quasibound states the constriction acts as a potential well rather than a potential barrier, which is the inverse of the case of the low quasibound states. These resonant splitting peaks in the spectrum of out-of-plane mode can also be found in the total transmission.In the fourth thapter, we investigate the thermal transport properties of BNNRs with a triangle vacancy. The effect of triangle vacancy on the phonon transmission of zigzag-edged BNNRs (Z-BNNRs) is different from that of armchair-edged BNNRs (A-BNNRs). The triangle vacancy induces antiresonant dips in the spectrum of Z-BNNRs. Moreover, the boron-terminated triangle vacancy causes antiresonant zero-transmission dip and the number of the zero-transmission dip increases with the geometrical size of the triangle vacancy. For the A-BNNRs with triangle vacancy, except some antiresonant dips, resonant peak is found in the transmission. The antiresonant and resonant phenomena are explained by analyzing local density of states and local thermal currents. Although the antiresonant dip and the resonant peak are both originated from quasibound states, their distributions of local thermal currents are distinct, which leads to the transport discrepancy. In addition, the thermal conductance of BNNRs decreases linearly with increasing the vacancy size.In the fifth chapter, we investigate the thermoelectric transport properties of hybrid GNRs and BNNRs. It is found that thermoelectric figure of merit can be effectively enhanced by periodic embeding boron nitride. As for the armchair-edged nanoribbons, the figure of merit can be enhanced about 20 times in contrast to the perfect GNRs. While for the zigzag-edged nanoribbons, the figure of merit can be enhanced about 3 times. These phenomena are explained by analyzing electrical conductance, Seebeck coefficient and thermal conductivity.In the sixth chapter, we make a summary for all the work, and the future investigation of graphene and boron nitride is described.