1. School of Materials Science and Engineering Key Laboratory of Electronic Packaging and Advanced Functional Materials of Hunan Province Central South University Changsha 410083 P. R. China
2. School of Physics and Electronics Hunan University Changsha 410082 P. R. China
3. School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore
4. College of Chemistry and Chemical Engineering Jishou University Jishou Hunan 416000 P. R. China