1. Department of Physics College of Physics and Energy Shenzhen University Shenzhen Guangdong 518060 China
2. Department of Physics The Hong Kong University of Science and Technology Clear Water Bay Kowloon 999077 Hong Kong
3. Department of Electronic Science and Technology College of Electronic Sciences and Technology Shenzhen University Shenzhen Guangdong 518060 China
4. Department of Materials Science and NanoEngineering Rice University Houston TX 77005 USA