1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. Department of Applied Physics; Chongqing University; Chongqing 400044 China
3. State Key Laboratory of Silicon Materials; School of Materials Science & Engineering; Cyrus Tang Center for Sensor Materials and Applications; Zhejiang University; Hangzhou Zhejiang 310027 China
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 China