Synthesis Of Chalcogenides Quantum Dots Thermoelectric Materials And Synergistically Optimize The Electrical And Thermal Transport Properties

This dissertation selected the chalcogenides compound quantum dots with special crystal structure and behavior as the research object to achieve the goal of synergistically optimization of electrical and thermal transport for high efficient thermoelectric materials. We realize the controllability and large quantity synthesis of chalcogenides quantum dots through simple colloid synthesis strategy, and systematically investigated the correlation between these special behavior and thermoelectric properties. The details are summarized briefly as follows:1. Nearly monodisperse silver chalcogenide quantum dots were successfully synthesized through a facile colloidal method, and their thermoelectric property across the semiconductor-superionic conductor phase transition was systematically investigated. We put forward for the first time the general law that the optimized thermoelectric property could be achieved around the phase transition temperature. Furthermore, we firstly synthesized the Ag4SeS quantum dots through the alloying of the Ag2Se and Ag2S quantum dots. Benefit from the alloy introduced point defects and atoms fluctuations, nearly full wavelength range of the phonons could be effectively scattered, which further reduced the thermal conductivity and improved the thermoelectric property.2. By the introduction of heavy atom Bi in the Ag2Se quantum dots, significant improvement of ZT value was achieved. We also firstly found that di-metal chalcogenides may show more sophisticated and unexpected properties:reversible p-n-p semiconducting switching during the phase transition. Our experimental results and theoretical calculations indicate that the Ag-Bi atoms exchange during the rhombohedral-cubic phase transition plays an important role in the p-n-p switching, while the high ZT(ZT=1.5at700K) value is benefit from high electrical conductivity and low thermal conductivity, which derive from the full disordering of Ag/Bi. The di-metal chalcogenides semiconductors not only may be a unique catalog of material both for temperature dependent p-n-p switching and high performance thermoelectric devices, and open a new avenue to design multifunctional materials and device. The discovery of bi-metal atoms exchange during the phase transition brings novel phenomena with unusual properties which definitely enrich solid-state chemistry and materials science. 3. In previous work, the high ZT value was obtained, but the working temperature range is relatively narrow. In order to overcome this shortfall, we synthesized the AgBi1-xSbxSe2solid solution by the substitution of Sb in the lattice frameworks and successfully stabilized the nonambient cubic AgBiSe2phase to room temperature, which leads to the improved thermoelectric performance in a wide working temperature range. Furthermore, in-situ formed homojuncitons on the surface of solid-solutioned nanoplates were also first achieved through a simple colloidal method, and the ZT value of the solid-solutioned homojunction nanoplate reached1.0at550K. We firstly put forward that the formation of solid solution coupled with homojunction in disordering compound allows synergistically much enhanced thermoelectric properties, which should be an effective strategy to achieve in line with the concept of "phonon glass electron crystal".4. We demonstrate a new concept of decoupled optimization of the thermoelectric parameters through magnetic ions doping in wide band-gap semiconductors quantum dots. The insights gained from the experimental results and theoretical calculations indicate that magnetic ions can create spin entropy, narrow band-gap and strengthen anharmonic phonons coupling to realize the coexistence of large Seebeck coefficient, high electrical conductivity and low thermal conductivity in one compound. As a result, a maximum ZT value of0.42at700K for Ni-doped Cu2ZnSnS4quantum dotss is readily and consistently achievable, which is increased by7.4times compared to that of pure Cu2ZnSnS4quantum dots. Considering the large number and rich types of the wide band gap semiconductors, it is reasonably believed that higher ZT value can be expected through the decoupled optimization of thermoelectric parameters in the systems with carefully selected parent compounds and magnetic dopants. In this sense, this current study opens a new cost-effective and nontoxic means to design and broaden the prospective thermoelectric materials.5. We extend the concept of decoupled optimization of the thermoelectric parameters through magnetic ions doping to fully substituted in wide band-gap semiconductors quantum dots. We found that the magnetic ions not only create the spin entropy but also enhance the electron density of states near the Fermi level and strengthen the harmonic phonons scattering, which finally realize the coexistence of large Seebeck coefficient, high electrical conductivity and low thermal conductivity in one compound. As a result, a maximum ZT value of0.31at700K for Cu2FeSnS4quantum dots is readily and consistently achievable, which is increased by5.2times compared to that of pure Cu2ZnSnS4quantum dots. Furthermore, our result clearly indicated that the further enhanced ZT value could be achieved in a compound containing magnetic ions with larger crystals field stabilization energy, smaller electronegativity difference and larger ions radii difference of constituent elements. For an example, the fully Co2+substituted Cu2CoSnS4quantum dots represents9.2times improvement in ZT value, which reaches0.51at700K.