A systematic study of the optical absorption of small silicon nanocrystals (Si-NCs) embedded in silicon dioxide is performed using real-time time-dependent density-functional theory. The modeled Si-NCs contain up to 47 Si atoms with the surrounding oxide being described by a shell of ${\text{SiO}}_2$. The oxide-embedded Si-NCs exhibit absorption spectra that differ significantly from the spectra of the hydrogen-passivated Si-NCs. In particular, the minimum absorption energy is found to decrease when the Si-NCs are exposed to dioxide coating. Unexpectedly, the absorption energy of the oxide-embedded Si-NCs remains approximately constant for core sizes down to 17 atoms, whereas the absorption energy of the hydrogen-passivated Si-NCs increases with decreasing crystal size. This trend suggests a different mechanism for producing the lowest-energy excitations in these two cases.

• Received 5 March 2009

DOI:https://doi.org/10.1103/PhysRevB.79.235332