| Summary: | Application of nanocrytslas in transistors, photovoltaic and thermoelectric devices, and light-emitting diodes became an important topic of research in the past two decades. Important event to achieve that goal was development of colloidal synthesis of nanocrystals. The as-synthesized nanocrystals with narrow size distribution are capped with bulky insulating organic ligands, which hiders the use of nanocrystals for device application. One of the major breakthroughs in this field was introduction of ligand exchange from organic ligands to small inorganic chalcogenidometallates. From the experience with chalcogenidometllates the use of smaller and simpler ligands like chalcogenides, halides, etc. was also explored. Chalcogenide ligands showed a great ability to stabilize nanocrystals in variety of solvent and allowed direct use of more benign solvent in comparison to hydrazine (formamide, N-methylformamide and H2O). Even then, the properties of devices made with these nanocrystals were not as good as what was shown with nanocrystals capped with chalcogenidometallates (namely, In2Se42- and Cu7S 4-). To alleviate this drawback we suggest here to add metal cation solutions to solutions of chalcogenide-capped nanocrystals or dip films chalcogenide-capped nanocrystals into metal cation solutions. Metal cations bind to surface chalcogenide ligands reducing amount of dangling bonds and trap states. That simple technique allows drastic improvement of device characteristics. For example, charge-carrier mobilities in field-effect transistors made with CdSe/K2S after dipping in a solution of Cd2+ showed 25-fold increase.
Chalcogenidometallates and chalcogenides do not always have the same elemental composition as nanocrystal the stabilized in polar solvents. We propose in this thesis a general strategy to synthesize largely unexplored soluble chalcogenidometallates of cadmium. These compounds can be employed as "solders" for semiconductors that can be used in photovoltaics and transistors. The addition of "solder" helps to bond crystal surfaces and link nano- or mesoscale particles together. For example, CdSe nanocrystals with Na2Cd2Se3 solder can be used as a soluble precursor for CdSe films with electron mobilities around 450cm2/Vs. CdTe powder can be molded into various shapes in the presence of a small additive of composition-matched chalcogenidometallate or chalcogel, thus opening new design spaces for semiconductor technologies.
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