1. Department of Physics, College of Science, Engineering and Technology (CSET), University of South Africa, Private Bag X6, Florida, 1710 Johannesburg, South Africa
2. School of Engineering, Ulster University, Shore Road, Newtownabbey, Belfast BT37 0QB, Northern Ireland, United Kingdom
3. Department of Physics, Tamkang University, Tamsui, 251, Taipei, Taiwan