1. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61820, USA
2. Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
3. Oxford Instruments, Asylum Research, Goleta, California 03117, USA
4. Center for Nanoscale Systems, Faculty of Arts and Sciences, Harvard University, Cambridge, Massachusetts 02138, USA