1. Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India and Department of Applied Sciences, Haldia Institute of Technology, Haldia, 721657 Purba Medinipur, West Bengal, India
2. Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India