Phonon Transport Properties Of Hybrid Quasi-One-Dimensional Nanostructures: A Molecular Dynamics Study

In recent years, with the rapid development of nanoscience and nanotechnology, the new nanomaterials and nanodevices have attracted widespread attention. Quasi one-dimensional nanomaterials, such as carbon nanotubes, graphite and silicon nanowires have gradually become a hot topic due to their excellent optical, electrical and magnetic properties, and are also considered to be the ideal material for next-generation nanoelectronic devices. However, as the feature size of electronic device decrease to nanometer level, electronic circuit will be highly integrated and heat dissipation problem is becoming one of the main bottlenecks, which restricts the development of the field. So, it is important to understand the laws of heat conduction and the underlying mechanisms in quasi-one-dimensional nanostructures for control of heat transfer in the nanodevice. In the present thesis, we investigate phonon transport properties of hybrid quasi-one-dimensional nanostructures by appling the molecular dynamics method. Some interesting and meaningful results have been obtained.First of all, thermal transport properties in hybridised graphene and boron nitride ribbons(HGBNRs) under different strains are studied by using reverse nonequilibrium molecular dynamics simulations. It is found that the effect of strains on the thermal conductivity is different for different types of strains. When the tensile and shear strains are applied, the thermal conductivity can be modulated at least up to 50% at room temperature as the strain ε ranges from 0 to 0.2. However, when the compressive and flexural strains are respectively applied, the thermal conductivity is insensitive to the variation of the strain. In addition, it is also found that the thermal conductivity of HGBNRs depends sensitively on t he dimension of the hybridised ribbon and the relative amount of h-BN to graphene. A brief analysis of these results is given.Secondly, thermal transport across graphene/hexagonal boron nitride(h-BN) nanoribbon interface is investigated using nonequilibrium molecular dynamics method. It is found that the heat current runs preferentially from the h-BN to graphene domain, which demonstrates pronounced thermal rectification behavior in this heterostructure. The observed phenomena can be attributed to the resonance effect between out-of-plane phonon modes of the graphene and h-BN domains in the low frequency region. In addition, we demonstrate that the optimum conditions for thermal rectification include low temperature, large temperature bias, short sample length and small interface densities. More interestingly, an unexpected negative differential thermal resistance(NDTR) behavior is also found at graphene/h-BN(CBN) nanoribbon interface with special edge geometry. Phonon spectra analysis reveals that the transverse acoustic wave plays an important role for the heat transfer at such interface.And then, the thermal transport properties of the graphene and boron nitride superlattice(CBNSL) are investigated via nonequilibrium molecular dynamics(NEMD) simulations. The simulation results show that a minimum lattice thermal conductivity can be achieved by changing the period length of the superlattice, which implies a crossover from coherent to incoherent phonon transport. Additionally, it is found that the period length at the minimum thermal conductivity shifts to lower values at higher temperatures, and that the depth of the minimum increases with decreasing temperature. In particular, at 200 K, the thermal conductivities of CBNSLs with certain specific period lengths are nearly equal to the corresponding values at 300 K. A detailed analysis of the phonon spectra shows that this anomalous thermal conductivity behavior is a result of strong phonon w ave interference. These observations indicate a potential new strategy for manipulation of thermal transport in superlattices.Finally, we examined the thermal conductivity of different types of nanotubes, including zigzag-edged graphyne nanotubes(ZGNTs), armchair-edged graphyne nanotubes(AGNTs) and CNTs by using equilibrium molecular dynamics simulations. Our numerical results demonstrate that the ZGNTs exhibits exceptionally low thermal conductivity with exciting applications for thermoelectric, which is only approximately 10% that of CNTs. Meanwhile, the AGNTs are shown to have higher thermal conductivity than that of the ZGNTs, implying evident chirality-dependence thermal thransport in GNTs. Through calculating the phonon spectrum and performing vibrational eigenmode analysis, the origin of this reduction is illustrated. It is found that the introduction of sp carbon atoms results in local resonances for low-frequency phonons, which products a s eries of flat local branches across the phonon spectrum. Furthermore, phonon relaxation time in GNTs is also reduced due to such the local resonances. According to the kinetic theory, the reduction of phonon group velocity and relaxation time would greatly suppress the thermal conducntivity of GNTs.