1. Department of Physics and Astronomy, Purdue University 1 , West Lafayette, Indiana 47907, USA
2. Birck Nanotechnology Center, Purdue University 2 , West Lafayette, Indiana 47907, USA
3. Department of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology 3 , Dhaka 1000, Bangladesh
4. Elmore Family School of Electrical and Computer Engineering, Purdue University 4 , West Lafayette, Indiana 47907, USA
5. Department of Electrical and Computer Engineering, University of California Santa Barbara 5 , Santa Barbara, California 93106, USA