1. Department of Physics; The Chinese University of Hong Kong; Shatin Hong Kong SAR China
2. School of Material Science and Engineering; Shaanxi Normal University; Xi'an 710119 China
3. Key Laboratory for Thin Film and Microfabrication of Ministry of Education; Department of Micro/Nano Electronics; School of Electronic Information and Electrical Engineering; Shanghai Jiao Tong University; Shanghai 200240 China
4. Shenzhen Research Institute; The Chinese University of Hong Kong; Shenzhen 518057 China