1. Department of Physics; Soochow University; Suzhou 215006 China
2. The Key Laboratory of Thin Films of Jiangsu; Soochow University; Suzhou 215006 China
3. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology; Chinese Academy of Sciences; 865 Changning Road Shanghai 200050 China
4. Analysis and Testing Center; Soochow University; Suzhou 215006 China