1. School of Electronic and Information Engineering (Department of Physics), Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, Shandong 250353, P.R. China
2. Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, Jinan 250358, China
3. Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
4. Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, Stockholm S-106 91, Sweden