1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. Key Laboratory of Optoelectronic Technology and Systems (Ministry of Education); Department of Optoelectronic Engineering; Chongqing University; Chongqing 400044 P. R. China
3. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 P. R. China