1. Department of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India
2. Advanced Materials & Device Metrology Division, National Physical Laboratory, New Delhi 110012, India
3. Polymer Research Centre and Centre for Advanced Functional Materials, Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, Nadia, West Bengal 741246, India