1. Ningbo Institute of Materials Technology and EngineeringChinese Academy of Sciences Ningbo 315201 P. R. China
2. Tianjin Key Laboratory of Electronic Materials and DevicesSchool of Electronics and Information EngineeringHebei University of Technology Tianjin 300401 P. R. China
3. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China