1. Department of Electrical Engineering, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53211, USA
2. Guangxi Key Lab for Relativistic Astrophysics, Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, Guangxi 530004, China
3. Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA
4. Department of Electrical and Computer Engineering and Petersen Institute of Nano Science and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA