1. Materials Research Laboratory, University of California 1 , Santa Barbara, California 93106, USA
2. Mechanical and Aerospace Engineering, Arizona State University 2 , Tempe, Arizona 85287, USA
3. HEDPS, Center for Applied Physics and Technology, College of Engineering and School of Physics, Peking University 3 , Beijing 100871, People's Republic of China
4. Department of Materials Science and Engineering, University of Illinois Urbana-Champaign 4 , Urbana, Illinois 61801, USA
5. Department of Chemistry and Biochemistry, University of California 5 , Santa Barbara, California 93106, USA
6. Materials Science and Engineering, Arizona State University 6 , Tempe, Arizona 85287, USA
7. Department of Physics, Arizona State University 7 , Tempe, Arizona 85287, USA