1. School of Materials Science and EngineeringHanshan Normal University Chaozhou Guangdong 521041 P. R. China
2. SEU‐FEI Nano‐Pico CenterKey Laboratory of MEMS of Ministry of EducationSchool of Electrical Science and EngineeringSoutheast University Nanjing 210096 P. R. China
3. Department of Physics and Department of Materials Science and EngineeringCity University of Hong Kong Tat Chee Avenue Kowloon Hong Kong 999077 P. R. China