| Summary: | In this thesis, we focus on the emerging field of inorganic ligands of colloidal nanocrystals (NCs). Rapid progress has been made since the discovery of chalocogenidometallates as surfactants for NCs in 2009, including the developments of various classes of inorganic ligands, and the significant improvements in the performance of NC-based electronic, optical, and thermoelectric devices. However, the study of inorganic ligands is still at its early stage with many intriguing and under-explored questions.
In Chapters 2 and 3, we present the syntheses and design of two new classes of inorganic ligands. The halide-, pseudohalide-, and halometallate-based ligands enrich the current toolbox of inorganic ligands. These short ligands facilitate the electronic coupling in NC solids, as indicated by the high carrier mobility (> 30 cm2/Vs) of sintered CdSe thin films. These ligands also introduce additional functionalities to the core NCs. For example, the CdCl3- ligands act as the sintering promoters for CdTe NCs. The CdTe solar cell made from an ink of CdCl 3--capped CdTe NCs without CdCl2 treatment shows an efficiency over 10%, comparable to the record efficiency (12--13%) of the solution-processed devices in previous reports. On the other hand, we propose a general route to synthesize soluble lead- and bismuth chalcogenidometallates. Starting from these soluble compounds, a large variety of new molecules and materials can be obtained through chemical transformations, such as the cation exchange reaction and the formation of phase-pure chalcogenide nanograins. These molecular- and nanoscale materials are ideal solders for lead- and bismuth chalcogenides, which are widely investigated in the field of thermoelectrics. The versatility of solders in both compositions and scales offers an additional degree of control over the chemical and physical processes at the semiconductor interfaces, leading to improved transport properties of the soldered semiconductors.
In Chapter 4, we present a general route to achieve homogeneous NC colloids in unusual solvents like inorganic molten salts and liquid metals. NCs are intact in these media, both structurally and optically. The surface engineering and the interactions between NCs and the solvent molecules impose a vital role in the colloidal stability. For example, Pt NCs are well-dispersed in the Lewis-acidic NaCl/KCl/AlCl3 molten salts, but not in the Lewis-neutral ones. These NC colloids in unusual solvents serve as a model system to understand the interactions between the NC surface and the surrounding media through surface ligands/chemistry. Moreover, our observations bridge the traditional molten chemistry and metallurgy and the emerging colloidal nanoscience, and open up a completely new space for designing functional materials.
In Chapter 5, we show the colloidal synthesis of monodisperse sub-10 nm Bi1-xSbx alloy NCs with controllable size and compositions. Surface chemistry of Bi1-xSbx NCs was tailored with inorganic ligands to improve the interparticle charge transport as well as to control the carrier concentration and the grain size. The charge transport properties (carrier mobility, carrier concentration, electrical conductivity) are highly dependent on the grain size and surface chemistry of these NC-based devices, but rather independent on the temperatures. This behavior is in a stark contrast to the bulk Bi1-xSb x, whose transport behavior is largely determined by the temperatures. We propose a model explaining such behavior. Preliminary measurements of thermoelectric properties showed a ZT value comparable to those of bulk Bi1?xSbx alloys at 300 K, suggesting a potential of Bi1?xSbx NCs for low temperature thermoelectric applications.
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