1. Department of Physics, University of Calcutta, 92 A P C Road, Kolkata 700 009, India
2. Department of Physics, Raiganj University, Uttar Dinajpur 733 134, India
3. Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
4. Center for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-III, Saltlake City, Kolkata 700 098, India