1. Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Hong Kong
2. Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA
3. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
4. School of Instrumentation Science and Optoelectronics Engineering, Beihang University, Beijing 100191, People's Republic of China