1. Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India
2. Department of Physics, Jadavpur University, Kolkata 700032, India
3. Department of Botany, Visva-Bharati University, Santiniketan 731235, India
4. Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India