1. Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan
2. Department of Physics, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan
3. Advanced Nanofabrication, Imaging and Characterization Core Lab (ANIC), King Abdullah University of Science & Technology (KAUST), Thuwal, Makkah 23599-6900, Kingdom of Saudi Arabia
4. Center for Micro and Nano Devices, Department of Physics, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan
5. Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, USA