1. National Laboratory of Solid-State Microstructures; School of Electronic Science and Engineering; Collaborative Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 P. R. China
2. Key Laboratory of Mesoscopic Chemistry of MOE; School of Chemistry and Chemical Engineering; Nanjing University; Nanjing 210093 P. R. China
3. Department of Physics; Southeast University; Nanjing 211189 P. R. China