1. School of Chemical Engineering & Technology Xi'an Jiaotong University Xi'an 710049 P. R. China
2. Beijing National Center for Condensed Matter Physics Beijing Key Laboratory for Nanomaterials and Nanodevices Institute of Physics Chinese Academy of Sciences Beijing 100190 P. R. China
3. College of Physical Science and Technology Xiamen University Xiamen 361005 P. R. China
4. Department of Chemistry School of Science Tianjin Key Laboratory of Molecular Optoelectronic Science Tianjin University Tianjin 300072 P. R. China
5. Institute of Electronics Chinese Academy of Sciences Beijing 100190 P. R. China
6. Division of Chemistry and Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University 21 Nanyang Link Singapore 637371 Singapore