Thermal Transport In Graphene Nano-junctions

As typical two-dimensional nano-materials, graphene has attracted considerable attentions. A large number of work shows that these strange materials possess the unique electrical, mechanical, thermal, magnetism and optical properties. These novel performances would be used in many fields in the future. For instance, composite-materials, flat displays and heat dissipation et al. It is an unavoidable that excellent thermal dissipation management is necessary in the design and performance of graphene-based materials which have extremely small size. This is primary importance for the thermal transport properties of the nano-devices based on the graphene. In this paper,?we have studied the relation between thermal properties and geometrical structures of the grapheme nano-jucntion by using nonequilibrium Green's function method. We illustrate the mechanisms of thermal transport for the graphene nano-junction. The results provide significant physical models and theoretical validity in designing the thermal devices based on the graphene nano-junction.The paper has been organized as follow. In the introduction, the background and objectives of the study are introduced.In chapter two, the Green's function is introduced,which includes nonequilibrium Green's function method and the relevance of physical quantity calculated methods.In chapter three, we have studied the thermal transport properties of L-shaped graphene nano-junctions which consist of a semi-infinite armchair-edged nanoribbon and a semi-infinite zigzag-edged nanoribbon. It is shown that the thermal conductance of the L-shaped graphene nano-junctions depends on the included angles and the widths of the graphene nanoribbons. As the angle of L-shaped graphene nano-junctions increases from 30°to 90°and further to 150°, the thermal conductance obviously increases. For the right-angle L-shape graphene nano-junction, the thermal conductance undergoes a transition with the increasing of the widths of the armchair nanoribbons. The thermal conductance decreases at low temperature region and increases at high temperature region. While the thermal conductance of L-shape graphene nano-junction with included angle 150°decreases by increasing the widths of zigzag-edged nanoribbons at both low and high temperature region. These thermal transport phenomena can be reasonably explained by analyzing the phonon transmission coefficient. We illustrate the mechanisms of thermal transport for the different L-shaped graphene nano-junctions. ? In chapter four, the thermal transport properties of four terminal graphene nano-junction which consist of a semi-infinite armchair-edged nanoribbon and a semi-infinite zigzag-edged nanoribbon have been calculated. The thermal transport in the four terminal graphene nano-junction is sensitive its geometric shape. It is shown that the thermal conductance of the four terminal graphene nano-junction depends on the widths of the semi-infinite graphene nanoribbons and center region. These thermal transport phenomena can be reasonably explained by analyzing the phonon transmission coefficient.In summary, a final conclusion is drawn for all the results obtained and the prospect of future investigation is explicitly described.