1. School of Physics and Astronomy, Yunnan University, Kunming 650091, P. R. China
2. Kunming Institute of Physics, Kunming 650223, P. R. China
3. Yunnan Key Laboratory of Advanced Photoelectric Materials & Devices, Kunming 650223, P. R. China
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAP, P. R. China
5. School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia
6. Department of Electronic and Electrical Engineering, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, UK