1. Department of Electrical and Computer Engineering, Micro and Nanotechnology Laboratory, Materials Research Laboratory, Institute of Carbon-Neutral Energy Research Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
2. Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, Ohio 45433, USA
3. Department of Electrical Engineering, University of Texas at Arlington, Arlington, Texas 76019, USA