Controllable Preparation And Physical-Mechanical Properties Of Low-Dimensional Materials For Energy Conversion

Energy conversion materials such as thermoelectric and optoelectric materials are important to green energy systems, but at present their energy conversion efficiency is relatively low. Highly effective conversion of energy based on unique nanoscale multi-field coupling effect between local field and external field is a frontier field of current S & T development. In this thesis, some efficient approaches were investigated for improving energy conversion efficiency by developing new experimental methods, and combining experiment with theory. The following main results are achieved:(1) Synthesis, Thermoelectric Transport Properties and First Principles Calculations of PbTe NanowiresPbTe nanomaterials have important applications in thermoelectric devices, but the diameter of PbTe nanowires is over 60 nm at present. The theoretical studies show the thermoelectric properties can be remarkably enhanced by decreasing the nanowire diameter. Here, a novel two-step hydrothermal synthesis method has been developed and was used to successfully synthesize uniform single-crystalline PbTe nanowires, with average diameter of about 30 nm, below its average excitonic Bohr radius of 46 nm. The results show that smooth and straight PbTe nanowires can be obtained at 100 oC, while pearl-necklace-shaped PbTe nanowires can be achieved at 180 oC. The growth mechanisms for the two nanostructures were presented reasonably. To our knowledge, this is the first time to synthesize a single-crystal compound nanostructure by a nontopotactic transformation process between the crystal structures of the template nanostructure and product. Thermoelectric transport measurements indicate that the films composed of the obtained PbTe nanowires have a high Seebeck coefficient more than 137% exceeding that of the state-of-the-art bulk PbTe for the smooth nanowires, while more than 16% enhancement for the pearl-necklace-shaped PbTe nanowires.By quantum mechanism calculation based on the density functional theory, the electrical structures of different diameter PbTe nanowires were calculated for the first time. The obtained local density of states (LDOS) shows a strong increase with decreasing nanowire diameter, and when the LDOS of nanowire with diameter of 0.326 nm is two orders in magnitude higher than the corresponding bulk sample. This result shows that further decreasing nanowire diameter is beneficial to improving thermoelectric properties of PbTe nanomaterials.(2) Sonochemistry-Assisted Microwave Synthesis, Phase Transition and Optical Properties of CdS NanostructuresThe pure microwave and sonochemistry synthesis methods are widely used synthesis techniques, but they has advantages and disadvantages. Here, a sonochemistry-assisted microwave synthesis method has been developed and was used to successfully synthesize uniform single-crystalline CdS nanoflowers. Optical measurements of the nanoflowers show a large blue-shift up to 100 nm in comparing with simple low-dimensional CdS nanostructures, which shows that the obtained products have significant ultraviolet optical properties.Phase transition from the metastable zinc blende to the stable wurtzite phase for CdS nanoparticles was successfully realized for the first time by an inorganic salt-induced process in a sonochemistry-assisted microwave synthesis system. The phase transition of CdS nanoparticles was explained by the effect of the halide ions, which inhibit the formation of (111) faces and accelerate the growth of (100) and (101) faces. It is found that this phase transition can also be induced by the pure microwave synthesis method, but the resulting products using this approach are non-uniform. This research provides a novel phase controlled synthesis approach for semiconductor nanostructures under mild conditions.(3) Liquid Phase Synthesis, Optical and Mechanical Properties of ZnO NanowiresBased on the attractive application prospects and multifield coupling characteristics of nanodevices, ZnO nanowire arrays with diameter of 10~250 nm and length of 10μm have been successfully prepared via a hydrothermal process. It is reported for the first time that the annealing temperature has strong influence on the optical properties of ZnO nanowire arrays. The result shows that it is beneficial to improving optical properties of ZnO nanowire arrays by annealing the samples at 450℃. A characteristic peak at 328 cm-1 assigned to second-order Raman scattering was observed after the annealing treatment. This research provides a new approach to study the optical properties of semiconductor luminescent materials.Based on the combination of SPM and MEMS fabricatation techniques, the mechanical properties of a single ZnO nanowire above the tunnel of a silicon microstructure have been measured. The elastic modulus of single ZnO nanowire with diameter of 150 nm was measured and an experimental value of 172.5 GPa was obtained by a three-point bending test in vacuum, while a reduced modulus 21.7 GPa was obtained in air, 7 times lower than the actual value due to the effect of water film. In addition, electromechanical coupling properties of a single ZnO nanowire were also investigated. This research provides a basis for deeply electromechanical coupling studies of nanomaterials.