1. J. Wang, C. Gao, I. Takeuchi, X.-D. Xiang, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
2. Y. Yoo, Department of Physics, University of California, Berkeley, CA 94720, USA.
3. X. Sun and H. Chang, Department of Chemistry, University of California, Berkeley, CA 94720, USA.
4. P. G. Schultz, Materials Sciences Division, Lawrence Berkeley National Laboratory, and Howard Hughes Medical Institute, Department of Chemistry, University of California, Berkeley, CA 94720, USA.