Thermal Transport And Thermoelectric Properties In Low-Dimensional Nanostructures

With the rapid development of the new material technology and nanotechnology. there is an inevitable trend for the dimension of devices coming into nanoscale. Due to the size effects of nanodevices. they exhibit many novel physical properties and very broad application prospects. In the present thesis, we investigate the thermal transport by phonons and thermoelectric properties in nanostructures (cylindrical nanowires. nanoribbons) by applying the Landauer transport theory combining with the scattering matrix method or the nonequilibrium Green’s function method, and get some meaningful results.First of all. in the elastic continuum model approximation, we investigate the phonon transmission and thermal conductance in a cylindrical nanowire modulated with a coupling nanocavity by developing the mode matching numerical technique. Results show that the phonon transmission coefficient exhibits a periodical transmission behavior in low frequency region. However, for large incoming frequency, the transmission coefficient becomes very complicated, since some higher order modes make their contributions to the phonon transmission. Importantly the periodic resonant transmission behaviors of zero mode at particular energies can be observed in the system. In the limit Tâ†'0. the thermal conductance approaches the universal quantum value Ï€2kB2T/3h. and such a quantum is robust against geometrical parameters. However, the total reduced thermal conductance exhibits nonmonotonical behaviors with increasing temperature due to the scattering from discontinuities, which sensitively depends on geometrical parameters of the nanocavity. It is suggested that adjusting geometrical parameters of the nanocavity may provide an available way to modulate the thermal conductance to match practical requirements in application of nanoscale devices.Then, we study the phonon thermal transport properties of zigzag graphene nanoribbons (GNRs) with central defects (vacancy and SW defect) by using the nonequilibrium Green’s function formalism. Results show that the combined effect between the edge and defect plays an important role in determining the thermal transport of GNRs with defects. The influence of a vacancy on thermal transport properties is more stronger than a SW defect in low frequency region, while the case is reverse in high frequency region. In the limitTâ†'0, the thermal conductance approaches the universal quantum value3κ0(κ0=Ï€2kB2T/3h) even when structural defects are introduced in GNRs. A transform from a quantum to aclassical feature occurs as the temperature goes up. It is suggested that the phonon transport and thermal conductance in GNRs can be tuned by modulating structural defects and the ribbon width.Based on atomistic simulation of electron and phonon transport, we also investigate the thermoelectric properties in T-shaped graphene nanoribbons by using the nonequilibrium Green’s function method. The results show that the phonon transport is dramatically suppressed due to the elastic scattering for phonons; while the thermopower S can be enhanced by a few times of magnitude. This leads to a strong enhancement of the figure of merit ZT. Moreover, it is found that the enhancement of ZT can be effectively tuned by modulating geometric parameters and edge shapes of T-shaped graphene nanoribbons, which offers an effective way to improve the thermoelectric performance of graphene nanoribbons.We also study nonlinear phonon properties in asymmetric graphene-based three terminal junctions (AGTTJs) by using the nonequilibrium Green’s function and the Landauer transport theory. Results show the temperature Tr from the central probe exhibits the asymmetric up-bending parabolic behavior related to the ballistic nature of phonon transport in AGTTJs. Based on the phonon nonlinearities, a rectifying behavior can be analytically and numerically obtained, of which the efficiency is much higher than that for other materials. Increasing the asymmetric degree can obviously enhance the rectifying efficiency. In addition, the central terminal not only acts a temperature probe, but also tunes the rectifying behavior efficiently. These results indicate that AGTTJs may be served as a good ballistic thermal rectifier.Finally, we study the ballistic thermoelectric properties in boron nitride nanoribbons (BNNRs). Results show that the electric conductance, the thermal conductance of electrons increases steplikely as the chemical potential increases in boron nitride nanoribbons; while the Seebeck coefficient presents a very sharp peak in the first conduction band edge, of which the value can be up to1200kB/e. Because of the wide band gap of BNNRs, the electronic thermal conductance of BNNRs is quite lower than that of the corresponding graphene nanoribbons (GNRs); while the power factor of BNNRs is several times than that of GNRs, which indicates the thermoelectric performance of BNNRs is far superior to that of GNRs. When the size of BNNRs increases, the quantized platforms of the electric conductance gradually smooth, and even disappear; while the peak values of the Seebeck coefficient reduce rapdily. These results suggest that the BNNRs with small sizes can be used to improve the efficiency of thermoelectric devices.