1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
2. Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, School of Physics and Electronic Science, Hunan University, Changsha, Hunan 410082, P. R. China
3. Department of Electrical and Computer Engineering, George Washington University 800 22nd St., Science & Engineering Hall, Washington, D.C. 20052, United States