1. Department of Materials Science and Engineering University of California, Berkeley Berkeley CA 94720 USA
2. Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST Campus UAB Bellaterra Barcelona 08193 Spain
3. Department of Physics University of California Berkeley CA 94720 USA
4. Department of Applied Physics Stanford University Stanford CA 94305 USA
5. Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory Menlo Park CA 94025 USA
6. Department of Nuclear Engineering University of California, Berkeley Berkeley CA 94720 USA
7. Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA