1. Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, USA
2. Microsoft, One Microsoft Way, Redmond, Washington 98052, USA
3. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA