1. Department of Materials Science and Engineering Stanford University Stanford CA 94305 USA
2. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology Beijing 100190 P. R. China
3. Beijing Key Laboratory of Advanced Chemical Energy Storage Technologies and Materials Beijing 100191 P. R. China
4. Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory 2575 Sand Hill Road Menlo Park CA 94025 USA