Tuning the electronic properties of naturally nanostructured compounds

Tuning of band gaps in materials by changing the ionic content of nanopores (in the case of 12CaO·7Al₂O₃) and nanochannels (in Ca-phosphate apatite) was demonstrated using low-temperature luminescence spectroscopy methods under vacuum ultraviolet excitation. Luminescence excitation spectra enabled us to conclude that the replacement of O²⁻ ions with F⁻ in the cages of 12CaO·7Al₂O₃ shifts the absorption edge from  ≈ 5.1 to  ≈ 5.8 eV. Non-homogeneously broadened 2p states of O²⁻ ions inside cages of 12CaO·7Al₂O₃ form the dominating trapping levels for valence band holes and their replacement with F⁻ ions facilitates efficient self-trapping of holes at the framework oxygen ions. In the case of Ca-phosphate apatite, the replacement of OH⁻ groups with F⁻ ions shifts the onset of host absorption from  ≈ 7.8 to  ≈ 8.4 eV. The intrinsic emissions of Ca-phosphate apatites revealed in the UV region were tentatively interpreted as radiative decay of Frenkel-type self-trapped excitons, where the hole component is localized on oxygen ions of PO₄ tetrahedra. (paper)