Assessment of Lead Chalcogenide Nanostructures as Possible Thermoelectric Materials

The assembly of nanostructures into “multi”-dimensional materials is one of the main topics occurring in nanoscience today. It is now possible to produce high quality nanostructures reproducibly but for their further application larger structures that are easier to handle are required. Nevertheless during their assembly their nanometer size and accompanying properties must be maintained. This challenge was addressed in this work. Lead chalcogenides have been chosen as an example system because they are expected to offer great opportunities as thermoelectric materials. Three different ways to achieve assemblies of lead chalcogenide nanostructures were used and the resulting structures characterized with respect to their potential application as thermoelectric material. The first means by which a “multi”-dimensional assembly of lead chalcogenide quantum dots can be produced is the formation of porous structures such as aerogels and xerogels. A procedure, where the addition of an initiator such as oxidizers or incident radiation is unnecessary, is introduced and the formation process studied by absorption spectroscopy. The time-consuming aggregation step could be significantly reduced by employing a slightly elevated temperature during gelation that does not lead to any observable differences within the resulting gel structures. After either supercritical or subcritical drying, highly porous monolithic gel structures can be achieved. During the gel formation the size and the shape of the particles changed and they were directly linked together. Nevertheless the resulting porous structures remain crystalline and size dependent effects of the optical properties could be shown. Gels produced from a mixture of PbS and PbSe QDs show a homogenous distribution of both materials but it is not clear to what extent they form an alloy. Although the particles are directly linked together the resulting porous structures possess a very high resistivity and so it was not possible to characterize the semiconductor aerogels with regard to their thermoelectric properties. To achieve an enhanced conductivity porous structures containing PbS and Au nanoparticles have been produced. As has been seen for the pure semiconductor gels the size of the PbS quantum dots has increased and elongated particles were formed. In contrast to the PbS QDs the Au nanoparticles did not change their size and shape and are unevenly distributed within the PbS network. Through the use of the gold nanoparticles the conductivity could be increased and although the conductivity is still quite small, it was possible to determine Seebeck coefficients near room temperature for a mixed semiconductor-metal gel. The second means by which QD solids could be formed was by the compaction of the QD building blocks into a material that is still nanostructured. Therefore the synthesis of PbS was optimized to achieve sufficient amounts of PbS quantum dots. The ligands used in the synthesis of the QDs unfortunately act as an insulating layer resulting in QD solids with resistivities as high as 2 Gigaohm. For this reason different surface modification strategies were introduced to minimize the interparticle distance and to increase the coupling between the QDs so as to increase the conductivity of the resulting quantum dot solids. One very promising method was the exchange of the initial ligands by shorter ones that can be destroyed at lower temperatures. By such heat treatments the resistivity could be decreased by up to six orders of magnitude. For the pressing of the quantum dots two different compaction methods (SPS and hydraulic pressing) were compared. While the grain growth within the SPS pressed samples is significantly higher the same densification can be achieved by a cold hydraulic pressing as well as by SPS. The densification could be further increased through the use of preheated PbS QDs due to the destruction of the ligands. Samples which had been surface modified with MPA and subsequently thermally treated show the best results with respect to their thermopower and resistivities. Nevertheless the conductivity of the QD solids is still too high for them to be used as efficient thermoelectric materials. The final assembly method does not involve QDs but instead with one dimensional nanowires. Therefore a synthesis was developed that enables the formation of PbS nanowires of different diameters and one that is easy up-scalable. By the use of a less reactive sulfur precursor and an additional surfactant the formation of nuclei is significantly retarded and within an annealing time of two hours nanowires can be formed presumably by an oriented attachment mechanism. Single crystalline nanowires with a diameter of 65-105 nm could be achieved with the longest axes of the nanowires being parallel to [100]. The resulting nanowires were used as building blocks for film formation on glass substrates by an easily implemented method that requires no special equipment. To characterize the films with a view to their possible application as a thermoelectric material, surface modifications of the films were performed to improve the charge transfer in the films and the Seebeck coefficients of the resulting films measured. Therefore the previous approach of using MPA was applied and a subsequent thermal treatment demonstrated very promising results. In addition an crosslinking ligand was used for surface treatment that leads to similar results as was observed for the thermally treated MPA approach. Both approaches lead to an order of magnitude decrease in the resistivity and due to the fewer grain boundaries present in the films composed of nanowires as compared to the QD assemblies the conductivity is significantly higher. The Seebeck coefficient measurements show that the thermal treatment only slightly affects the Seebeck coefficients. Therefore a significantly higher power factor could be achieved for the nanowire films than for the QD solids