[1] Univ Nigeria, Dept Phys & Astron, Nsukka, Nigeria
[2] Natl Ctr Phys, Quaid I Azam Univ Campus, Islamabad 44000, Pakistan
[3] iThemba Labs Natl Res, Nanosci African Network NANOAFNET, Somerset West, South Africa
[4] Univ South Africa UNISA, Coll Grad Studies, Nanosci Nanotechnol, POB 392, Pretoria, South Africa
[5] COMSATS Univ Islamabad, Ctr Micro & Nano Devices, Dept Phys, Islamabad, Pakistan
[6] Univ Ibadan, Dept Phys, Ibadan, Nigeria
[7] Northwestern Polytech Univ, NPU NCP Joint Int Res Ctr Adv Nanomat & Defects E, Xian 710072, Peoples R China
[8] King Fahd Univ Petr & Minerals, Dhahran 31261, Saudi Arabia
[9] Northwestern Polytech Univ, Sch Mat Sci & Engn, Xian 710072, Peoples R China