Bonding of Oxygen at the Oxide/Nanocrystal Interface of Oxidized Silicon Nanocrystals: An Ab Initio Study

Ab initio methods based on density functional theory are employed to investigate the bonding of oxygen (O) at the oxide/nanocrystal (NC) interface after hydrogen (H)-passivated silicon (Si) NCs are oxidized. Besides the well-known quantum confinement effect, the type of O bonding and the oxidation state of Si at the oxide/NC interface are found to significantly affect the optical properties of oxidized Si NCs. After oxidation, the excitation energies of Si-35 and Si-66 increase, while those of Si-87 and Si-123 decrease. We show that oxidation-induced redshifts in the light emission from Si NCs do not always result from defective O such as doubly bonded O at the oxide/NC interface. When Si atoms at the oxide/NC interface are mainly in low oxidation states, backbond O at the interface per se results in the redshifts in the light emission from Si NCs. When Si atoms at the oxide/NC interface are mainly in high oxidation states. Si3+=O at the interface leads to the redshifts in the light emission from Si NCs. It is found that for Si NCs with perfect oxide/NC interface (i.e., O at the interface is all backbond O) the seriously weakened next-nearest-neighboring Si Si of Si3+ readily breaks after excitation. At the oxide/NC interface. Si2+=O induced strong electronic localization and Si2+=O and Si3+=O induced reduction of interface polarization stabilize the geometry of oxidized Si NCs at the excited state. The electronic localization of severely stressed bridge O at the oxide/NC interface is relatively weak. This facilitates the breaking of nearest-neighboring Si Si at the oxide/NC interface as oxidized Si NCs are excited.