1. School of Electronic Science and Applied Physics; Hefei University of Technology; Hefei 230009 P. R. China
2. Center of Super-Diamond and Advanced Films (COSDAF); Department of Physics and Materials Science; City University of Hong Kong; Hong Kong SAR P. R. China
3. Shenzhen Institutes of Advanced Technology; Chinese Academy of Sciences; Shenzhen P. R. China
4. City University of Hong Kong Shenzhen Research Institute; Shenzhen P. R. China