1. Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Salt Lake City, Kolkata 700098, India
2. Deparment of Electronic Science, University of Calcutta, 92 A.P.C. Road, Kolkata 700009, India
3. Department of Electrical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India