1. Department of Physics, University of California, Santa Barbara, CA 93106–9530, USA.
2. California NanoSystems Institute, University of California, Santa Barbara, CA 93106–9530, USA.
3. Green Innovation Research Laboratories, NEC Corporation, Tsukuba, Ibaraki 305-8501, Japan.
4. Department of Electrical Engineering, University of California, Riverside, CA 92521, USA.