1. Department of Materials Science and Engineering, University of California 1 , Los Angeles, California 90095, USA
2. Department of Electrical and Computer Engineering, University of California 2 , Riverside, California 92521, USA
3. CENTERA Laboratories, Institute of High-Pressure Physics, Polish Academy of Sciences 3 , Warsaw 01-142, Poland
4. California NanoSystems Institute, University of California 4 , Los Angeles, California 90095, USA