1. Department of Electronic Engineering; The Chinese University of Hong Kong; New Territories Hong Kong
2. Department of Physics; Chemistry and Biology (IFM); Linköping University; Linköping SE-58183 Sweden
3. Institute of Advanced Materials; Nanjing Tech University; 210009 Nanjing P. R. China
4. Photophysics & OptoElectronics; Zernike Institute for Advanced Materials; Nijenborgh 4 Groningen 9747 AG The Netherlands