Colloidal quantum dots (CQDs) are attractive materials for lasers, displays and light-emitting applications due to their narrow and brighter spectral emission bandwidth, size-tunable bandgap and high-photoluminescence quantum yield (PLQY). However, these CQDs undergo inevitable degradation of their unique optical properties overt time due to their sensitive surface chemistry. To overcome these limitations, several approaches have recently been used such as over coating with an inorganic semiconductor shell of a wider band gap (core-shell hetrostructures), surface functionalization with new ligands or polymer coating and composites etc. In particular, core-shell heterostructured quantum dots with thick shell (so called “giant” quantum dots (g-QDs)) has shown higher PLQYs and improved photochemical stability than traditional thin-shelled or core only CQDs. The outstanding properties of g-QDs essentially depend on both the structure (defects, surface chemistry etc.) and the properties of interfacial layer (sharp or smooth core/shell interface). These structural and interfaces properties of g-QDs are strongly influenced and can be tailored by the synthetic parameters. Here, we will present our recent results on understanding the behavior of interfacial layer and structural defects on the photophysical properties of g-QDs.