1. Department of Electrical and Computer Engineering, Microelectronics Research Center, The University of Texas at Austin 1 , Texas 78758, USA
2. Department of Electrical and Computer Engineering, Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign 2 , Urbana, Illinois 61801, USA
3. Department of Electrical and Computer Engineering, The Ohio State University 3 , Columbus, Ohio 43210, USA
4. Department of Materials Science and Engineering, The Ohio State University 4 , Columbus, Ohio 43210, USA