1. Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78758, United States
2. Department of Physics, Birla Institute of Technology Mesra, Ranchi, Jharkhand 835215, India
3. Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States
4. Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India