1. Department of Physics, University of Kerala, Kariavattom, Thiruvananthapuram 695581, India
2. Department of Nanoscience and Nanotechnology, University of Kerala, Kariavattom, Thiruvananthapuram 695581, India
3. Materials Science and Technology Division, CSIR-National Institute of Interdisciplinary Sciences (CSIR-NIIST), Thiruvananthapuram, Kerala 695019, India