1. Graduate Program of Material Science and Engineering, University of California, San Diego, La Jolla, California 92037, USA
2. Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden
3. Department of Materials Science and Engineering, Gwangju institute of Science and Technology (GIST), Gwangju 500-712, South Korea
4. Department of Electronic Engineering, LED-IT Fusion Technology Research Center, Yeungnam University, Daegu 712-749, South Korea
5. Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92037, USA