1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
2. Department of Mechanical Engineering, Villanova University, Villanova, Pennsylvania 19085, USA
3. Department of Physics, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, USA
4. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
5. Materials Department, University of California, Santa Barbara, California 93106, USA