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 12CaOcenterdot7Al2O3) 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 12CaOcenterdot7Al2O3 shifts the absorption edge from ≈ 5.1 to ≈ 5.8 eV. Non-homogeneously broadened 2p states of O2− ions inside cages of 12CaOcenterdot7Al2O3 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