1. Graduate Program of Material Science and Engineering, University of California, San Diego, California 92093
2. Department of Electrical and Computer Engineering, University of California, San Diego, California 92093
3. Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
4. Graduate Program of Material Science and Engineering, University of California, San Diego, California 92093 and Department of Electrical and Computer Engineering, University of California, San Diego, California 92093