Thermal Transport,Electron Transport And Thermal Energy Conversion Mechanisms In Quasi-one-dimensional Nanostructures

With the deepening of scientific researches and the developing of processing technologies,novel nano-devices that are more functional,more environmental-friendly,and with higher efficiency and higher integrations than traditional devices,are becoming available.However,during the working time of these nano-devices,heat will always be produced around the devices and influences greatly the efficiency,stability,and even the life of these nano-devices.Controlling the heat transfer in nanostructures and investigating the heat-influenced electron transport in nano-devices are becoming more and more important and urgent.On the other hand,it will be perfect if we can transform the produced heat directly into usable electricity with some energy conversion mechanisms.In fact,the aggravation of pollution and energy shortage is also demanding mechanisms that can convert the abundant heat energy in our environment into usable electricity.Moreover,designing thermal functional devices by regarding heat as a signal is also showing exciting application potential.In this thesis,we conduct researches on heat related topics including thermal transport,heat energy conversion and phonon-electron interaction by utilizing nanostructures.By doing this,we manage to design excellent nanoscale heat cables,thermal rectifiers and nanogenerators.We also manage to gain an insight into the effect of phonons,which are usually inevitable in experiments,on the electron transport properties of nanostructures,and thus learn better the electron transport in practical environments.In detail,we conduct the following researches:First of all,to analyze the thermal transport in core-shell nanowires,we calculate systematically the distributions of heat flux in InAs/GaAs and GaAs/InAs core-shell nanowires by using nonequilibrium molecular dynamics simulations.The results show that for InAs/GaAs core-shell nanowires,the heat current tends to transport in the shell,while for GaAs/InAs core-shell nanowires the heat current tends to transport through the core.The interface between the InAs and GaAs divides the core-shell nanowire into two heat transfer channels.Moreover,a simple equation is presented to describe the relationship of the thermal conductance among the core,the tubular shell,and core-shell nanowire.It is suggested that the core-shell nanowires can be served as heat cables.Secondly,we investigate the distinctive thermal transport properties of graded nanowires.in the quest for the origin of the different thermal rectifying behavior of two graded nanowires,we reveal the important role that standing waves play in the thermal transport properties of such graded structures.Evidence for the existence of standing waves is given from two angles,and one possible scenario of the origin of the standing wave is presented.The key point is that the formation of the standing wave,which greatly hinders the propagation of phonon waves,occurs only when the narrow end of the nanowire is at a higher temperature than the wide end,making the heat current flow preferably from the wide end to the narrow end.Then,following the second work,we investigate systematically the standing wave and the accompanying resonance process in asymmetric nanowires to understand the standing wave itself and its great effect on thermal rectification.Results show that the standing wave is sensitive to both the structural and thermal properties of the material,and its great effect on enhancing the thermal rectification is realized not only by the energy-localization nature of the standing wave,but also by the resonance-caused large amplitude and high energy of the standing wave.Next,by utilizing lattice dynamics theories and molecular dynamics simulations,,we demonstrate that the piezoelectricity in free-standing non-centrosymmetric nanowires can be triggered directly by heat to produce electricity.The feasibility of the idea is first analyzed by the dynamic theory of crystal lattices and then confirmed by molecular dynamics simulations.The most salient point is that the heat-induced voltage drop across the cross section of the free-standing nanowires alternates periodically with the vibration of the nanowire.Moreover,the electric potential induced by heat here(as large as 0.34V)is proved to be comparable with the previously reported potentials induced by mechanical energy,and the maximum value can be tuned by controlling the size of the nanowire and the applied heat.Finally,we manage to propose a combined method to investigate intuitively the phonon-influenced electron transport in zigzag graphene nanoribbons.Phonons are introduced by optimizing the graphene nanoribbon with molecular dynamics so that all phonons and their interactions are included,and then the electron transport properties are studied by using non-equilibrium Greens functions in combination with the density functional theory.Results show that the electron transport fluctuates greatly due to the incessant lattice vibration of the nanoribbons.More interestingly,the phonons behave like a double-edged sword that it boosts the conductance of symmetric zigzag nanoribbons(containing even number of zigzag chains along the width direction)while weakens the conductance of asymmetric nanoribbons.As a result,the spin filter effect that exists in perfect symmetric zigzag graphene nanoribbon is significantly weakened by phonons.The research is very meaningful for guiding the realization of theoretical phenomena in experiments or explaining the mismatches between theoretical predictions and experimental results.