1. School of Physics and Astronomy University of Nottingham Nottingham NG7 2RD UK
2. State Key Laboratory of Integrated Optoelectronics Key Laboratory of Automobile Materials of MOE College of Materials Science and Engineering Jilin University Changchun 130012 China
3. Department of Electrical and Computer Engineering University of California Los Angeles CA 90095 USA
4. School of Physics and Astronomy Cardiff University Cardiff Wales CF24 3AA UK
5. Institute for Problems of Materials Science The National Academy of Sciences of Ukraine Chernivtsi Branch Chernivtsi 58001 Ukraine