Design and synthesis of colloidal quantum dots and their optoelectronic properties.

Colloidal quantum dots (QDs) are semiconductor nanocrystals (several to tens nm), whose size is smaller than the exciton Bohr radius of the bulk material. QDs exhibit discrete energy levels resulting from quantum confinement effect in all three spatial dimensions, rendering them unique size/composition/shape-dependent optoelectronic properties. Compared to bulk materials, QDs have unique properties such as tunable band gaps, broad absorption spectra, multiple exciton generation and high extinction coefficients. The controlled synthesis of colloidal semiconductor QDs has defined a new way to use them as potential building blocks in multiple emerging technologies such as biomedical labels, thermal sensors, light-emitting diodes, QDs sensitized solar cells (QDSCs), solar-driven photoelectrochemical (PEC) cells for hydrogen (H₂) generation and photodetectors. In this thesis, colloidal bare, doped and core/shell QDs are designed and synthesized with tunable size and narrow size distribution. Subsequently, their structural and optoelectronic properties are investigated systematically. A facile approach was designed for the synthesis of colloidal PbS QDs using thiourea and lead acetate as precursors for sulfur and lead, respectively. The sizes of the PbS QDs could be systematically controlled by simply adjusting the reaction parameters. Then, a Cd post-treatment on the bare PbS QDs via a cation exchange method was performed for increasing the stability of QDs, which is favorable for the fabrication of device under ambient environment. As a proof of concept, as-synthesized PbS QDs were employed as light harvesters for both (i) solar-driven PEC cells for H₂ generation and (ii) QDSCs. A saturated photocurrent density of ~2 mA/cm² under one sun illumination (AM 1.5G, 100 mW/cm²) is observed for PEC device based on PbS QDs synthesized using thiourea, which is comparable with that of PEC device based on PbS QDs synthesized using air-sensitive and unstable bis(trimethylsilyl) sul