1. Key Lab of Artificial Micro- and Nano- structures of Ministry of Education of China, Department of Electronic Science and Technology, School of Physics and Technology, Wuhan University, Wuhan 430072, People's Republic of China
2. School of Physical Science and Technology, Guangxi University, Nanning, Guangxi 530004, People's Republic of China
3. School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan 430073, People's Republic of China