1. Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA.
2. Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India.
3. Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA.
4. Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA.
5. Materials Department, University of California, Santa Barbara, CA 93106, USA.
6. Department of Physics, University of California, Santa Barbara, CA 93106, USA.
7. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
8. School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK.
9. California NanoSystems Institute, University of California, Santa Barbara, CA 93106, USA.