1. Department of Optoelectronic EngineeringKey Laboratory of Optoelectronic Technology and SystemsMinistry of EducationChongqing University Chongqing 400044 P. R. China
2. Department of Materials Science and EngineeringStanford University Stanford CA 94305 USA
3. School of Materials Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332 USA
4. College of Physics and Electronic EngineeringChongqing Normal University Chongqing 400044 P. R. China
5. College of Chemistry and Chemical EngineeringChongqing University Chongqing 400044 P. R. China
6. Beijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100083 P. R. China