The structures and properties of Si/SiO₂ core/shell quantum dots studied by density-functional tight-binding calculations

Si/SiO₂ core/shell quantum dots (QDs) have been shown with wavelength-tunable photoluminescence in addition to their inert, nontoxic, abundant, low-cost, biocompatible advantages. Due to their big size, here, we apply density-functional tight-binding (DFTB) method to perform calculations to study their structures and properties. We systematically investigate the effects of surface passivation, thickness of SiO₂ shell, and Si/O ratio on the structures and properties of Si/SiO₂ core/shell quantum dots. We find that hydroxyl passivated Si/SiO₂ core/shell quantum dots are able to stabilize the quantum dots compared with hydrogen passivated Si/SiO₂ core/shell quantum dots. By using DFTB method, we are able to study Si/SiO₂ core/shell quantum dots of big size (3 nm) and we find that, in Si/SiO₂ core/shell quantum dots, there are competing effects between quantum confinement (blueshift) and oxidation (redshift) with the decrease of the size of Si core. The transition point is when Si/SiO₂ ratio is around 1:1. The effect of the thickness of SiO₂ on energy gap is not as significant as the effect of the size of the Si core. Our study provides theoretical basis for designing Si quantum dots with tunable photoluminescence.