Thermoelectric Properties Study In Low-dimensional Quantum Structure

With the rapid development of the material science and nanotechnology, the newnanomaterials and nanodevices have attracted increasingly interest in the materialscience region. Due to the quantum size effect, the nanomaterials usually exhibit avarious of fascinating physical properties. Therefore, they have potential applicationin nanoelectronics, spintronics and nanodevices. However, the current electronicdevices and integrated circuites based on nanomaterials are becoming increasinglysmall and dense in practice，the―waste heat‖is inevitably produced in their runningprocess. It has become a bottleneck for the further developmence of the nanodevices.How to deal with the―waste heat‖has been an extremely important issue and facedwith serious challenge. The best way out of the obstacle is to utilize highthermoelectric properties materials which can directly and effectively realize theconversion between thermal and electrical energy. In the present thesis, by using theLandauer transport theory combining with the nonequilibrium Green’s functionmethod or the scacttering matrix method, we investigate the thermal transportproperties and thermoelectric properties in the nanostructures. Some interesting andmeaningful reaults have been obtained.First of all, by using the nonequilibrium Green’s function approach and theLandauer transport theory, ballistic thermoelectric properties in graphene-nanoribbon-based heterojunctions are investigated. The results show that thetransmission coeffecient is always lesss than3for these heterojunctions, even at verylow frequency. The non-integer quantum thermal conductance arises. The reason isthat the leads (left and right) have the different width and the long-wavelengthphonons can’t transport perfectly through the central scattering region. The phononthermal conductances increase monotonically with the temperature and have almostsimilar effects for the different heterojunctions. However, the electron transportproperties are highly sensitive to the geometry details of the heterojunctions. Thelarge fluctuations of electronic transmissions appear and strongly enhance thethermopower. By optimizing the thermopower, together with suppression of thephonon transport by mismatching interface structures, which can reduce signi ficantlythe phonon thermal conductance, we can obtain the high thermoelectric figure ofmerit ZT~0.6at room temperature T=300K and ZT~0.9at low temperature T=100K for a certain of heterojunctions. Secondly, we investigate the thermal conductivity in the armchair and zigzaggraphyne nanoribbons (AGYNRs/ZGYNRs) by using the nonequilibrium moleculardynamics method. We find that both the thermal conductivites AGNRsand ZGNRsdecrease with the increase of the temperature T, which is attributed to thephonon-phonon Umkapp scattering. Because the Umklapp scattering increases withthe temperature and then it reduces the phonon thermal transport. The strongorientation dependence is observed in the thermal conductivity. More than15%thermal conductivity for the AGYNR is larger than the ZGYNR with the identicalwidth (7:6nm) and length (17nm) at room temperature T=300K. Furthermore, theorientation dependence increases with the width of GYNRs decrease. To furtherdisclose the underlying mechanism for this intrinsic orientation dependence, we plotthe phonon dispersion for both the AGYNR and the ZGYNR. We find that AGYNRspossess more phonon transport channels and higher phonon velocity throughout theoverall frequency range. These are responsible for the high conductivity of AGYNRs.The results have important implications for the application of GYNRs in thenanoelectronics and thermoelectricity.Thirdly, the thermoelectric properties of InAs nanowires modulated with themultiple-stub structures are studied by using the scattering-matrix method. The resultsshow that both the phonon and electron transport depend sensitively on the geometricstructure due to the very large surface-to-volume ratio in such system. The phononthermal conductance is substantially reduced by the interface scatterings. Carrierenergy filterings and electron scatterings in such structrures enhance strongly thethermopowers by the local distortion of electronic density of states, but the highelectron conductances are still remained. Consequently, the optimized thermopower,together with the signifcant reduction of the phonon thermal conductance, yields thehigh thermoelectric figure of merit ZT to0.3~1.9. These results will be helpful fordesigning of high performance thermoelectric devices in the future.Then, we study the thermoelectric propeties in bended graphene nanoribbons. Theinfluences of the bended GNRs on the phonon and electron transport in ballisticregion have been systematically studied by using the nonequilibrium Green’s functionmethod. We find that, due to the mismatch of the phonon modes, phonontransmissions are suppressed greatly for both AA-GNRs and ZZ-GNRs. But the highconductances have been preserved by the electronic resonant tunneling. In addition,The electron-wave quantum interference effect can be tuned by such kind of structure,so that the magnitude of the Seebeck coefficient can be regulated by the structures and the enhanced thermopower can be obtained in the structures, combined with thedramatically reduction of the thermal conductance, the high values of ZT are thenachieved for a certain kind of bended GNRs. Moreover, the resulting thermoelectricproperties are highly sensitive to the edge shapes and sizes of the assembled GNRs insuch system. The figure of merit ZT exhibits very complex relations to the geometryparameters. So the geometry influences of the system have been investigatedsystematically. Such tunability will be helpful for designing and fabrication ofhigh-performance thermoelectric devices such as ultrasensitive nanothermocouples.Finially, combining the nonequilibrium Green’s function approach and theLandauer transport theory, ballistic thermoelectric properties in boron nitridequantum-dot (QD) are investigated. We find that the phonon transport is substantiallysuppressed in the QD. But, some fluctuations apprear in the conductance curves, thethermopower has been significantly enhanced. Consequently, the boron nitride QDpossesses the high thermoelectric figure of merit.