Synthesis and characterisation of CuInS2 quantum dots

The interesting optical properties of quantum dots (QDs) may be important for many applications including bioimaging, LEDs and solar cells. Until recently, cadmium-based QDs have been by far the most developed, since the synthesis is straight-forward and their band gaps lie in the visible region of the spectrum, allowing for simple characterisation. However, cadmium is toxic and its commercial use in such applications has therefore been limited by legislation worldwide, motivating research into alternative, less ‘toxic’ QD materials such as CuInS2. The ternary nature of CuInS2 (CIS) results in an abundance of intrinsic point defects which introduce additional complexities into the excited state dynamics. Although the optoelectronic properties of cadmium-based QDs are very well understood, the apparent defect-related photoluminescence emission in CIS QDs is not understood completely. In addition, the synthesis of CIS QDs is not as well developed and can only currently be performed in high temperature solvothermal reactions, producing hydrophobic QDs. This thesis is concerned with the synthesis and characterisation of CIS and CIS/ZnS core/shell QDs and attempts to elucidate the excited state dynamics of these technologically important QDs as well as developing alternative synthesis and post-synthesis methods to produce water-soluble CIS QDs. Transient absorption spectroscopy and 'pump-dump-probe' spectroscopy were used to resolve excited state dynamics, showing that the emission does not originate from the conduction band, as hypothesised in the literature, but from a high-lying intra-gap donor state. An established synthesis procedure for CIS QDs was modified to provide alternative stabilising surface ligands such as hydroxyl and carboxylic acid terminated thiol ligands, which enabled QDs to be dispersed in methanol and water. A post-synthesis modification step, involving the encapsulation of hydrophobic CuInS2/ZnS QDs with amphiphilic polymers (amphipols) to produce hydrophilic QD/polymer hybrid nanoparticles that is stable in the aqueous phase, was also shown to be an effective method.