1. School of Materials Science and EngineeringAnhui Provincial Key Laboratory of Advanced Functional Materials and DevicesHefei University of Technology Hefei Anhui 230009 China
2. School of Electronic Science and Applied Physics and Anhui Provincial Key Laboratory of Advanced Materials and DevicesHefei University of Technology Hefei Anhui 230009 China
3. Department of PhysicsNational Cheng Kung University Tainan 701 Taiwan