1. Laser Micro/Nano-Fabrication Laboratory; School of Mechanical Engineering; Beijing Institute of Technology; Beijing 100081 P. R. China
2. Key Laboratory of Cluster Science; Ministry of Education; School of Chemistry; Beijing Institute of Technology; Beijing 100081 P. R. China
3. Research Center of Materials Science; School of Materials Science & Engineering; Beijing Institute of Technology; Beijing 100081 P. R. China
4. Department of Electrical and Computer Engineering; University of Nebraska-Lincoln; Lincoln NE 68588-0511 USA