1. College of Materials Science and Engineering Shenzhen University, and Shenzhen Key Laboratory of Special Functional Materials Shenzhen 518060 P. R. China
2. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province College of Optoelectronic Engineering Shenzhen University Shenzhen 518060 P. R. China
3. Department of Materials Science and Engineering National University of Singapore 7 Engineering Drive 1 Singapore 117575 Singapore
4. Department of Physics and Astronomy Clemson University Clemson SC 29634‐0978 USA