1. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798
2. School of Electrical and Electronics
Engineering, Nanyang Technological University, Singapore 637371
3. State
Key Laboratory and Institute of Elemental-Organic Chemistry, Nankai University, Tianjin, P. R. China 300071
4. South University of Science and Technology, 1088 Xue-Yuan Road, Shenzhen,
Guangdong, China 518055