1. National Laboratory of Solid States Microstructures, School of Physics, Nanjing University, Nanjing 210093, P. R. China
2. Key Laboratory of Mesoscopic Chemistry of Ministry of Education, Institute of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China
3. College of Electronic Engineering, Nanjing Xiaozhuang University, Nanjing 211171, P. R. China