Colloidal quantum dots (QDs) are nanometer-sized semiconductors synthesized by wet chemical methods and stabilized by surface ligands in solvents. They are prized for the size-dependent electronic band structures, giving rise to tunable optical properties. Their solution form is also suitable for large-area and low-cost fabrication processes. These unique characteristics make this class of materials promising as building blocks for next-generation thin-film electronic and optoelectronic devices. However, the construction of QD based devices requires precise control of their material properties, including carrier mobility, lifetime, doping concentration and energy positions of the conduction and valence band edges. The large surface-to-volume ratio allows these properties to be manipulated through surface modification of QDs. In this thesis, we systematically study the effect of surface treatments, such as ligand exchange, surface passivation, remote doping, on the chemical and physical properties of QD dispersions and thin films. We design surface modified QDs with desirable characteristics and integrate them into QD based devices, including field-effect transistors (FETs), solar cells and photodetectors to enhance device performance. We design QD thin films with specific surface treatments to improve two important interfaces in PbS QD solar cells. By introducing a CdI2-treated CdSe QD buffer layer at the ZnO nanoparticle/PbS QD junction interface and improving the p-type doping of the ethanedithiol-PbS QD layer via sulfur enrichment at the back-contact interface, we aim at suppressing interface recombination and facilitating carrier extraction. The ionization of dopants added on the surface of nanostructures during remote doping is inefficient. Both experimentally and theoretically, we study the effect of dielectric confinement on the doping efficiency in PbSe nanowires. On the FET platform, we show improved doping efficiency by encapsulating the nanowires with high-dielectric media to reduce dielectric mismatch between them. We further study the synthesis and surface chemistry of III-V QDs. We develop a general route to prepare InP, InAs, InSb and InAsxSb1-x QDs based on the co-reduction of indium and pnictogen halide precursors. This simplifies the preparation and enhances the stability of V precursors compared to existing approaches. We develop ligand exchange and doping strategies for III-V QD thin films to fabricate high performance devices.