1. Center for Nanotechnology; Indian Institute of Technology Guwahati; Guwahati 781039 India
2. Department of Physics; Indian Institute of Technology Guwahati; Guwahati 781039 India
3. Department of Chemistry; Indian Institute of Technology Guwahati; Guwahati 781039 India
4. Department of Physics; Indian Institute of Technology; Kharagpur 721302 India