1. Key Laboratory of Applied Surface and Colloid Chemistry Ministry of Education Shaanxi Key Laboratory for Advanced Energy Devices School of Materials Science and Engineering Shaanxi Normal University Xi'an 710119 P. R. China
2. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors Chinese Academy of Sciences & Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100083 P. R. China
3. Department of Materials Science Fudan University Shanghai 200433 P. R. China
4. School of Physics and Information Technology Shaanxi Normal University Xi'an 710119 P. R. China
5. State Key Laboratory for Mechanical Behavior of Materials School of Materials Science and Engineering Xi'an Jiaotong University Xi'an 710049 P. R. China