1. NSF Nanoscale Science and Engineering Center, University of California, 94720 Berkeley, California, USA
2. Institute for Theoretical Physics, Vienna University of Technology (TU Wien), A-1040 Vienna, Austria
3. Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, USA
4. Faculty of Science and Faculty of Engineering, University of Hong Kong, Hong Kong, China