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Abstract
Colloidal quantum dots (CQDs) are semiconductor nanocrystals with sizes typically ranging from 2 to 20 nm. Due to quantum confinement, their bandgap can be tuned by controlling particle size, so that materials with the same chemical composition (e.g., CdSe) exhibit size-dependent optical properties. For example, smaller CQDs emit in the blue or cyan region, whereas larger CQDs emit deep red light. Using colloidal synthesis techniques such as hot-injection and heat-up methods in high-boiling-point organic solvents (e.g., 1-octadecene, oleylamine), relatively monodisperse CQDs can be obtained. Further size-selective precipitation can yield nearly monodisperse CQDs. These methodologies have been generalized to most II–VI and IV–VI semiconductors, halide perovskites, and InP and InAs nanocrystals. With appropriate surface treatment and epitaxial shell growth, highly emissive CQDs can be synthesized, achieving photoluminescence quantum yields (PLQYs) as high as 100%. Despite the extensive development of II–VI and In–V nanomaterials, gallium pnictide nanomaterials have lagged behind. Gallium pnictides, such as GaAs and GaP, are among the most widely used semiconductors for electronic, optoelectronic, and photonic applications. However, solution-phase syntheses of gallium pnictide nanomaterials remain much less developed than those of many other colloidal semiconductors, including indium pnictides, II–VI and IV–VI compounds, and lead halide perovskites. In Chapter II, I demonstrate that the Wells dehalosilylation reaction can be carried out in molten inorganic salt solvents to synthesize colloidal GaAs, GaP, and GaP₁-ₓAsₓ nanocrystals. I show that discrete colloidal nanocrystals can be nucleated and grown in a molten salt medium with control over both size and composition. Additionally, I find that reaction temperatures above 400 °C are crucial for annealing structural defects in GaAs nanocrystals. We also highlight the utility of the as-synthesized GaP nanocrystals by demonstrating that GaP can be solution-processed into high-refractive-index coatings and patterned by direct optical lithography with micron-scale resolution. Finally, we show that dehalosilylation reactions in molten salts can be generalized to synthesize indium pnictide (Pn = As, P) and ternary (In₁–ₓGaₓAs and In₁–ₓGaₓP) quantum dots. Beyond isotropic nanomaterials, I also carried out extensive research on anisotropic nanomaterial synthesis in molten salts. Based on the Solution–Liquid–Solid (SLS) growth mechanism, I introduced gold nanoseeds into the molten salt reaction system and successfully produced GaAs nanowires. The as-synthesized GaAs nanowires exhibit band-edge photoluminescence. We further generalized this strategy to synthesize GaP and GaPₓAs₁-ₓ nanowires. These one-dimensional nanowire syntheses are discussed in detail in Chapter III. Redox chemistry in molten salts has proven effective for GaAs nanocrystal synthesis, but for indium-based systems, it is challenging to use In₂Br₄ to reduce pnictide halides. In Chapter IV, I demonstrate that AlBr3 can be used to activate InBr. The resulting InBr–AlBr3 adduct can reduce pnictide halides to form indium pnictides. Moreover, transmission electron microscopy (TEM) analysis reveals that the as-synthesized InP structures adopt a two-dimensional morphology. In summary, molten salts provide a versatile platform for semiconductor nanocrystal synthesis. By adapting strategies originally developed in organic solvents, one-dimensional nanostructures can be realized in molten salt media. Furthermore, employing Lewis acid adducts enhances the reducing power of indium halides (e.g., InI/InBr in InAlBr4), enabling the synthesis of indium pnictides with controlled morphology. Notably, the InP produced from InAlBr₄ exhibits a distinct 2D morphology, highlighting the potential of molten salt chemistry for accessing new classes of semiconductor nanostructures.