Thermal Transport Properties Of Low-Dimensional Nanostructures

The research on the thermal transport in low-dimensional nanomaterials is a newsubject for both theoretical significance and potential applications. The relevant studycan help us to solve the serious heat dissipation problems in ever-smaller integratedcircuits, improve the effect of thermal insulation in high power engines, fabracatethermoelectric materials high efficiency, and design novel thermal managementdevices. In this thesis, by using the Nonequilibrium Green’s function (NEGF) method,the quantum thermal transport in some typical low-dimensional nanostructures isstudied systematically. The results could provide useful guidelines for the applicationsand performance improvements. The primary coverage of this thesis is as follows:1. Thermal transport properties of isotopic-superlattice graphene nanoribbons withzigzag edge (IS-ZGNRs) are investigated. We find that by isotopic superlatticemodulation the thermal conductivity of graphene nanoribbon can be reducedsignificantly. The thermal transport property of the IS-ZGNRs strongly dependson the superlattice period length and the isotopic mass. As the superlattice periodlength decreases, the thermal conductivity undergoes a transition from decreasingto increasing. This unique phenomenon is explained by analyzing the phonontransmission coefficient. While the effect of isotopic mass on the conductivity ismonotonic, namely larger mass difference induces smaller thermal conductivity.In addition, the influence of geometry size is also discussed.2. Graphene nanojunctions (GNJs) are important components of future nanodevicesand nanocircuits. Using the NEGF method, we investigate the phononic propertiesof three-terminal GNJs (TGNJs). The results show that the heat flux runspreferentially along the direction from narrow to wide terminals, presenting anevident ballistic thermal rectification effect in the asymmetric TGNJs. Therectification efficiency is strongly dependent on the asymmetry of thenanojunctions, which increases rapidly with the width discrepancy between theleft and right terminals. Meanwhile, the corner form of the TGNJs also plays animportant role in the rectification effect. The mechanism of this thermalrectification is explained by a qualitative analysis.3. Based on folded graphene nanoribbons we report a thermal conductancemodulator which performs analogous operations as the rheostat in electroniccircuits. This fundamental device can controllably and reversibly modulate thethermal conductance by varying the geometric structures and its tuning range canbe up to40%of the conductance of unfolded nanoribbons (~1nm wide and7~15nm long). Under this modulation, the conductance shows a linearly dependence onthe folded angle, while undergoes a transition with the variation of the inter-layerdistance. This primary thermal device may have great potential applications forphononic circuits and nanoscale thermal management.4. The thermal transport properties of hexagonal boron nitride nanoribbons (BNNRs)are investigated. By calculating the phonon spectrum and thermal conductance, it is found that the BNNRs possess excellent thermal transport properties. Thethermal conductance of BNNRs can be comparable to that of graphenenanoribbons (GNRs) and even exceed the later below the room temperature. Afitting formula is obtained to describe the features of thermal conductance inBNNRs, which reveals a critical role of theT1.5dependence in determining thethermal transport. In addition, an obviously anisotropic thermal transportphenomenon is observed in the nanoribbons. The thermal conductivity ofzigzag-edged BNNRs is shown to have about20%larger than that ofarmchair-edged nanoribbons at room temperature.5. Graphyne, a new allotrope of carbon, is a hot spot in present nanomaterialresearch community. Using the nonequilibrium Green’s function method, weinvestigate the thermal transport property in graphyne nanoribbons (GYNRs). It isfound that the thermal conductance of GYNRs is about40%of that of graphenenanoribbons (GNRs). Distinct width dependence of thermal property is observedin GYNRs as well. The conductance for armchair-edged GYNRs (A-GYNRs)presents linear width dependence, while step-like width dependence is displayedin zigzag-edged GYNRs (Z-GYNRs). Meanwhile, the A-GYNR is shown to havelarger thermal conductance than that of Z-GYNR, indicating an obviousanisotropic thermal transport in graphyne (as twice as that in graphene). Theorigination of this anisotropic behavior is explained by analyzing the phononspectra of two edged GYNRs. Additionally, we also explore the thermal transportin graphyne family nanoribbons. The results show that the conductance ofgraphyne family nanoribbons is insensitive to the acetylenic linkages, but dependson the benzene rings.