  We study the optical properties of hydrogen passivated silicon, germanium and
mixed Ge/Si core/shell quantum dots (QDs) using high accuracy Density
Functional Theory (DFT) and time-dependent DFT (TDFT). We employ the hybrid DFT
functional of Becke, Lee, Yang and Parr (B3LYP) in combination with good
quality basis sets. As we have shown in our previous work, this combination is
an accurate and computationally efficient way for such calculations. The mixed
quantum dots, as would be expected, are more versatile and offer more
possibilities for band gap engineering, with gap values (electronic and
optical) between those of the corresponding Si and Ge dots. Our results support
the quantum confinement theory for all three types of QDs.
