Department of Applied Physics College of Physics and Electronics and College of Materials Science and Engineering and State Key Laboratory of Chemo/Biosensing and Chemometrics College of Chemistry and Chemical Engineering Hunan University Changsha 410082 People's Republic of China;
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center Tsinghua University Beijing 100084 People's Republic of China;
College of Physics Liaoning University Shenyang 110036 People's Republic of China;
