Band Gap Tuning in Transition Metal and Rare-Earth-Ion-Doped TiO(2), CeO(2), and SnO(2) Nanoparticles

The energy gap [Formula: see text] between the valence and conduction bands is a key characteristic of semiconductors. Semiconductors, such as TiO [Formula: see text] , SnO [Formula: see text] , and CeO [Formula: see text] have a relatively wide band gap [Formula: see text] that only allows the material to absorb UV light. Using the s-d microscopic model and the Green’s function method, we have shown two possibilities to reduce the band-gap energy [Formula: see text] —reducing the NP size and/or ion doping with transition metals (Co, Fe, Mn, and Cu) or rare earth (Sm, Tb, and Er) ions. Different strains appear that lead to changes in the exchange-interaction constants, and thus to a decrease in [Formula: see text]. Moreover, the importance of the s-d interaction, which causes room-temperature ferromagnetism and band-gap energy tuning in dilute magnetic semiconductors, is shown. We tried to clarify some discrepancies in the experimental data.