1. National Demonstration Center for Experimental Physics and Electronics EducationSchool of Physics and ElectronicsHenan University Kaifeng 475004 P. R. China
2. Beijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100083 P. R. China
3. School of Materials Science and EngineeringGeorgia Institute of Technology Atlanta 30332‐0245 GA USA