1. School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, P. R. China
2. Department of Physics, Federal Urdu University of Arts, Science and Technology, Islamabad 44000, Pakistan
3. Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan
4. Experimental Physics Laboratories, National Centre for Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan
5. Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, P. R. China