1. Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
3. Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.
4. Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA.