1. Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
2. Department of Chemistry, Purdue University, 560 Oval Drive,
P.O. Box 68, West Lafayette, Indiana 47907, United States
3. Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China
4. Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States