1. Department of Physics, Sultan Qaboos University, Muscat 123, Oman
2. School of Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China
3. Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), School of Physics, Trinity College, Dublin 2, Ireland
4. Centre of Nano Science and Nano Technology, Panjab University, Sector-14, Chandigarh 160014, India