[1] Koneru Lakshmaiah Educ Fdn, Dept Phys, Vaddeswaram 522502, AP, India
[2] Andhra Loyola Coll, Dept Phys, Vijayawada 520008, AP, India
[3] Acharya Nagarjuna Univ, Dept Nanotechnol, Guntur, AP, India
[4] Delhi Technol Univ, Dept Appl Phys, Bawana Rd, New Delhi 110042, India