1. Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089-0271, USA
2. Center for Energy Nanoscience, University of Southern California, Los Angeles, California 90089-0271, USA
3. Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-0271, USA
4. Department of Physics, University of Southern California, Los Angeles, California 90089-0271, USA