1. Department of Electrical Engineering, Device Research Laboratory, University of California, Los Angeles, California 90095, USA
2. Department of Material Science, University of California, Los Angeles, California 90095, USA
3. Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA
4. California NanoSystems Institute, University of California, Los Angeles, California 90095, USA
5. WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan