1. School of Material Science and Engineering; Georgia Institute of Technology; Atlanta Georgia 30332-0245 USA
2. State Key Laboratory of Modern Optical Instrumentation; Department of Optical Engineering; Zhejiang University; Hangzhou 310027 China
3. School of Electrical and Computer Engineering; Georgia Institute of Technology; Atlanta Georgia GA 30332-0250 USA
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing China