1. Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093, USA
2. Department of Physics, Chemistry and Biology, Linköping University, Linköping 58183, Sweden
3. Graduate Program of Material Science and Engineering, University of California, San Diego, La Jolla, California 92093, USA
4. Department of NanoEngineering, University of California, San Diego, La Jolla, California 92093, USA