1. Materials Science and Engineering Program, University of California at San Diego, La Jolla, California 92093, USA
2. Chemical Engineering Program, University of California at San Diego, La Jolla, California 92093, USA
3. Department of NanoEngineering, University of California at San Diego, La Jolla, California 92093, USA
4. Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, California 92093, USA