1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
2. Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA
3. School of Physics, Sun-Yat Sen University, Guangzhou 510275, China
4. Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom
5. Department of Physics, University of South Florida, Tampa, Florida 33620, USA
6. Federal University of Campina Grande, Campina Grande, Paraíba, Brazil
7. Center for Opto-Electronic Materials and Devices Research, Korea Institute of Science and Technology, Seoul 136-791, South Korea
8. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA