1. Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois, Urbana-Champaign, Urbana, Illinois 61801, USA
2. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
3. Department of Materials Science and Engineering, University of California-Berkeley, Berkeley, California 94720, USA
4. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA