1. Institute of Applied Physics, TU Bergakademie Freiberg, Leipziger Str. 23, 09599 Freiberg, Germany
2. Helmholtz-Zentrum Dresden-Rossendorf, Helmholtz Institute Freiberg for Resource Technology, Chemnitzer Str. 40, 09599 Freiberg, Germany
3. Luminescent Materials Laboratory, Department of Biotechnology, University of Verona, and INSTM, UdR Verona, Strada Le Grazie 15, 371 34 Verona, Italy
4. Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom