1. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China, Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China, Department of Applied Physics, Hefei University of Technology, Anhui Hefei, China, and Center of Super-Diamond and Advanced Films and Department of Physics and Material Science, City University of Hong Kong, Hong Kong Special Administrative Region, China