[1] Univ Munich, Photon & Optoelect Grp, Dept Phys, D-80799 Munich, Germany
[2] Univ Munich, CeNS, D-80799 Munich, Germany
[3] Univ Jaume 1, Dept Fis, E-12071 Castellon de La Plana, Spain
[4] Helmholtz Zentrum Berlin Mat & Energie, D-14109 Berlin, Germany
[5] Delft Univ Technol, Dept Chem Engn, Optoelect Mat Sect, NL-2628 BL Delft, Netherlands
[6] City Univ Hong Kong, Dept Phys & Mat Sci, Kowloon, Hong Kong, Peoples R China