1. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
2. Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong 999077, China
3. School of Metallurgy and Environment, Central South University, Changsha 410083, China