Thermoelectric Nanowire Arrays Modulation Growth

Providing a firm and green supply of energy would become a major problem for the 21^st century. The excellent thermoelectric materials can effectively scavenge the thermal energy and improve the refrigeration performance, which give a practical route to solve the energy crisis. Recently, the corresponding research is focused on the improvement of the figures of merit (ZT) of thermoelectric materials. It is indicated by theories that the thermoelectric performance of low dimensional systems can be enhanced remarkably due to the sharper density of states and enhanced phonon scattering. It has also been demonstrated by experimental results that the thermoelectric superlattice films, nanowires and the bulk materials with embedded nanocrystals reveal the impressive ZT compared to that of the bulk. However, due to the limitation of materials preparation and characterization, there are still many challenges in the study of morphology controllable synthesis of nanoscale thermoelectric materials with optimal composition and structure.In this dissertation, we have a deep investigation of the fabrication of thermoelectric nanomaterials by porous anodic alumina (PAA) template assisted electrodeposition method, and the conditions for the growth of Bi_xTe_1-x nanowire arrays with desirable composition and structure were stuied. Focusing on the hot topic of thermoelectric superlattice nanowires, we successfully developed a novel and convenient route for the spontaneous formation of multiple heterostructure nanowire arrays, and grew the Bi₂Te₃/Sb superlattice with modulated periods. In addition, the CdTe/Te multiple heterostructure nanowires were also synthesized, and the corresponding optical properties were studied systematically.In chapter 1, the progresses in the studies on the thermoelectric materials as well as the thermoelectric applications are reviewed. The main contents include: the dominating factors and the possible routes for improving ZT, the state-of-art thermoelectric materials and the novel nanostructured thermoelectric systems, and the synthesis approaches. Towards the thermoelectric device applications, some open questions are discussed at the end of the chapter.In chapter 2, due to the importance of size, composition and structure in dominating the thermoelectric properties, we systematically study the parameters for the deposition of Bi-Te alloy nanowire arrays with desirable composition. It is found that the stoichiometric Bi₂Te₃ nanowire arrays can be prepared in the electrolyte solution containing 42% Bi³⁺ ions, and the growth direction was determined by the applied voltage. Finally, the Bi₂Te₃ nanowire arrays with different diameters (40-250nm) can be fabricated by adjusting the pore size of the PAA templates.In chapter 3, a novel and convenient synthetic approach for the spontaneous formation of multiple heterostructure nanowire arrays was developed based on the precipitation reaction under confined system. The Bi₂Te₃/Te multiple heterostructure nanowire arrays were successfully synthesized through a simple thermal annealing process of the supersaturated Bi-Te alloy nanowire arrays. The as-prepared nanowires were dense and continuous, and the segments were single crystalline with a clear interface. The DSC and XRD results revealed that the crystallization process started at around 120℃and the Bi₂Te₃/Te multiple heterostructure nanowires decompose at 400℃. By the observation of the microstructure of nanowires annealed at different temperatures, it is considered that the precipitation confined by porous anodic alumina (PAA) membranes leads to the spontaneous formation of the block-by-block structure.In chapter 4, the Bi₂Te₃/Sb superlattice nanowire arrays were successfully synthesized by the template assisted pulsed electrodeposition process, based on the detailed studies of cyclic voltammograms (CVs) of the Bi-Sb-Te electrolyte solution. The microscope results showed that the nanowires were straight and uniform with high aspect ratios, and perfect periodicity in the spatial distribution of the as-prepared superlattice nanowires were confirmed by the elemental EDS mapping analyses. With manipulating the deposition time, the Bi₂Te₃/Sb nanowire arrays with tunable periods can be obtained, and the minimum period can be reached as 10 nm, which presents a good candidate for the further thermoelectric device applications.In chapter 5, high quality CdTe/Te multiple heterostructure nanowire arrays were successfully fabricated by a nano-confined precipitation method. It was demonstrated that the randomly selected neighboring segments are single crystalline CdTe and Te, respectively. Comparing with the absorption spectra of pure CdTe and Te nanowires, the quenching of the absorption peak strongly indicates the coupling of electronic bands in the CdTe/Te heterojunction.