1. Department of Materials Science and Engineering University of California Los Angeles CA 90095 USA
2. School of Physics and Electronics Hunan University Changsha 410082 China
3. Department of Chemistry and Biochemistry University of California Los Angeles CA 90095 USA
4. NG NEXT Northrop Grumman Aerospace Systems Redondo Beach CA 90278 USA
5. California Nanosystems Institute University of California Los Angeles CA 90095 USA