Probing the electronic and optical properties of silica-coated quantum dots with first-principles calculations

The electronic and optical natures of silica-coated semiconductor nanocrystals (Cd2Te2@(SiO2)24) have been investigated by density functional theory (DFT) and time-dependent DFT calculations. The calculated results of Cd2Te2@(SiO2)24 have revealed that the structural synergy effect between the Cd2Te2 quantum dots (QDs) and the silica coating shell plays a dominant role in the photoelectric properties. The binding of embedded Cd2Te2 to the outer silica coating shell leads to the distortion of the silica nanocage, indicating strong coupling between the QDs and silica shell. The optical features of Cd2Te2 clusters and Cd2Te2@(SiO2)24 complexes were evaluated using the time-dependent DFT method. It is determined that the maximal absorption peak of isolated Cd2Te2 in a UV-Vis absorption spectrum appears at 584 nm, which shifts to 534 nm when the Cd2Te2 QDs were encapsulated by silica, in close agreement with the experimental evidence. The excited process has a direct electronic transition character from the occupied Cd2Te2 states to the outer silica nanocage excited states (core → shell electronic transitions). A deep insight into silica-coated QD systems is beneficial for understanding their optical nature and the development of core/shell QDs.