Radiation- and photo-induced precipitation was used for the synthesis of nanopowder scintillators based on ZnO:Ga. It is very efficient method published rarely outside of our research group. In principle, an aqueous solution of precursors (in the case of ZnO usually zinc nitrate or formate, a scavenger of OH radicals and/or hydrogen peroxide) is irradiated by non-ionizing (UV) or ionizing (γ, or accelerated electrons) radiation, causing the interaction with water radio- or photolysis products and leading to the precipitation of the solid nanocrystalline product - ZnO precursor (zinc carbonate hydrate, peroxide or directly zinc oxide). The following thermal decomposition leads to the formation of nanocrystals with particle size in the range of 20-100 nm, depending on the irradiation and heat treatment conditions.
The principle of band gap modulation consists in shifting the conduction band edge by affecting the energy levels of 2p orbital of Zn2+ by foreign ions and their energy levels. These foreign ions may occupy either the lattice position of Zn2+ ions or interstitial positions. If the concentration of foreign ions is sufficient, the bottom edge of the conduction band shifts. Doping by appropriate ions, such as Cd, Mg, Ca or Sr leads to the narrowing or broadening of the band gap. In summary, the changes in the band gap structure will be directly affected by the electron configuration of doping ion in the crystal lattice and with increasing concentration the shift of BG increases (Makino et al., 2001; Lange et al., 2012). The effect of doping ion can be observed as “red” or “blue” shift in emission spectra to lower or higher energies.
Cadmium and magnesium ions were chosen for the investigation of the ZnO band gap modulation, based on the previous promising results (Procházková 2013). It was shown that the concentrations up to 1mol% of Cd in ZnO structure have no effect on the intensity and position of exciton-related emission in UV range, but the increasing intensity of defect-related emission in visible range was observed. At higher Cd concentrations, modulation of the band gap and appropriate shift of emission maxima were observed. Significant blue or red shift of the UV emission maxima was obtained with the maximal admixture of 13 mol % of Cd or Mg ions, without any emission in the visible spectral range. It was manifested that the UV exciton-related emission can be shifted in the range of 376-425 nm at the cost of the decrease in total luminescence intensity.
T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda., H. Koinuma, “Bang gap engineering based on MgxZn1-xO and CdyZn1-yO ternary alloy films,” Appl. Phys. Lett. 78, 9 (2001)
M. Lange, C. P. Dietrich, K. Brachwitz, T. Böntgen, M. Lorenz, M. Grundmann, “(Zn,Cd)O thin films for the application in heterostructures: Structural and optical properties,” J. Appl. Phys. 112 103517 (2012)
L. Procházková, Master thesis (2013)