Compound copper chalcogenide nanocrystals: synthetic approaches to fabricate multi-component semiconductor nanocrystals for technological applications

This thesis describes the development of novel synthetic routes to fabricate a range of multi-component, copper chalcogenide semiconductor nanocrystals using colloidal chemistry. Colloidal routes to ternary and quaternary semiconductor nanocrystals hold significant promise for technological applications, where solution processability combined with size dependent properties offers considerable advantages over bulk fabrication routes to these materials. The preparation of highly monodisperse, anisotropic CuInGaS2 (CIGS) colloidal nanocrystals is described in Chapter 3. Importantly, the results show that although the synthesis is a one-pot reaction, the formation of CIGS nanocrystals always occurs sequentially, starting with binary copper sulfide, transitioning through ternary copper indium sulfide before the final gallium insertion to yield the quaternary CIGS nanocrystals. Chapter 4 explores the effect of selenium incorporation to yield quinary CuInGa(SSe)2 (CIGSSe) nanocrystals. Interestingly, the formation of thin, hexagonal nanoplates, which display remarkably high diffraction contrast effects known as bending contours, was observed on the surface of each nanocrystal. Through control of the reaction conditions, routes were devised to selectively synthesize CIGSSe nanocrystals with complete phase control, allowing either the wurtzite or cubic crystal structure to be obtained. Chapter 5 describes the shape controlled synthesis of Cu2ZnSnS4 (CZTS) nanocrystals, with tight control in the size, shape and crystal phase. Subtle changes in the reaction chemistry were observed to have significant effects on the resultant nanocrystal morphology. An in-situ investigation of the nucleation and growth processes in colloidal wurtzite CZTS nanocrystals is discussed in Chapter 6. Through the use of time-resolved, quick X-ray absorption spectroscopy (QEXAFS), it was possible to successfully monitor the formation of stoichiometric CZTS nanocrystals, in-situ, in a custom built reaction flask. This represents the first report of its kind into studying the nucleation and growth processes of a complex quaternary, nanocrystal system.