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 (Cd ₂Te ₂at(SiO ₂) ₂₄) have been investigated by density functional theory (DFT) and time-dependent DFT calculations. The calculated results of Cd ₂Te ₂at(SiO ₂) ₂₄ have revealed that the structural synergy effect between the Cd ₂Te ₂ quantum dots (QDs) and the silica coating shell plays a dominant role in the photoelectric properties. The binding of embedded Cd ₂Te ₂ 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 Cd ₂Te ₂ clusters and Cd ₂Te ₂atSiO ₂) ₂₄ complexes were evaluated using the time-dependent DFT method. It is determined that the maximal absorption peak of isolated Cd ₂Te ₂ in a UV-Vis absorption spectrum appears at 584 nm, which shifts to 534 nm when the Cd ₂Te ₂ QDs were encapsulated by silica, in close agreement with the experimental evidence. The excited process has a direct electronic transition character from the occupied Cd ₂Te ₂ 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.