Colloidal synthesis of multinary metal chalcogenide nanocrystals with composition, shape and crystal phase control

This thesis describes novel colloidal approaches towards the synthesis of multicomponent semiconductor nanocrystals with tight control over morphology, composition and crystal phase. The thorough study of the complexity of the reaction mechanism and judicious selection of ligands and metal precursors along with temperature windows provide exquisite control over the sequential nucleation and growth events leading to the evolution of desired shapes and phase in synthesized nanocrystals. Preparation of highly monodisperse and homogenous CdSexS1-x nanorods with tunable aspect ratios is reported in Chapter 3. The optical properties are compositionally tunable with the relative molar ratios of the chalcogens (Se/S). Importantly, the combined influence of Se/S composition and ligand concentration on shape control is explored to hold the desired aspect ratios for each composition. The synthetic protocol is sufficiently robust to allow good control of nanorod aspect ratios, suited for their rational assembly either into vertical superstructures or into 1D rail-tracks depending on the net charge. Chapter 4 describes controlled engineering of shape and crystal phases in colloidal synthesis of complex quinary Cu2ZnSn(SSe)4 semiconductor nanocrystals. Combination of coordinating solvent and phosphonic acid entails the evolution of atypical polytypic heterostructure whose shape could be controlled by the nature of metal precursors. Furthermore, polytypism could be ruled out while retaining anisotropy allowing for single-phase wurtzite nanorods by using non-coordinating solvent with phosphonic acids. Chapter 5 explores the synthesis of copper based metal tellurides ranging from binary to quaternary compositions via ligand assisted colloidal approach. The gradual changes in the nanocrystal composition were observed to have dramatic effect on the shape, dispersity and crystal phase of nanocrystals. A solution based phase transformation of multicomponent semiconductor nanocrystal system is detailed in Chapter 6. Metastable wurtzite nanocrystals were transformed into highly stable zinc-blende and kesterite phases by solution based post treatment approach and thermal annealing method respectively. This phase transformation approach gives clear insight about the phase-to-phase and phase to ligand relationships in multinary nanocrystal systems.