1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices; Guangdong Engineering Technology Research Center of Optoelectronic Functional Materials and Devices; Institute of Optoelectronic Materials and Technology; South China Normal University; Guangzhou 510631 China
3. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 China