In this presentation, we will describe the PECVD synthesis of silicon quantum dots (Si QDs) that provides the ability to incorporate high concentrations (~1020 cm–3) of either p-type (B:Si) or n-type (P:Si) dopants. We also can use this synthetic technique to prepare co-doped QDs containing both P and B atoms in the same PB:Si QD. Whereas the optical absorption of singly P:Si and B:Si QDs exhibit plasmonic features characteristic of a high concentration of dopants, the co-doped PB:Si QDs instead exhibit photophysics that resemble those of intrinsic Si QDs, for example emission lifetimes of ~100 μs, but with size-dependent optical transitions that are tunable even below the energy of bulk Si (1.12 eV). Finally, we will describe new work on III–V semiconductors, in particular gallium nitride (GaN) QDs, that also exhibit tunable photophysics and serve as useful model systems for photochemically-driven small molecule conversion.