1. Department of Physics University of California at Berkeley Berkeley CA 94720 USA
2. Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA
3. Department of Physics Yonsei University Seoul 03722 South Korea
4. Center for Nanomedicine Institute for Basic Science Seoul 03722 South Korea
5. National Center for Electron Microscopy, The Molecular Foundry Lawrence Berkeley National Laboratory Berkeley CA 94720 USA
6. Kavli Energy NanoSciences Institute University of California at Berkeley Berkeley CA 94720 USA