1. School of Electronic Engineering, University of Bangor, Wales, United Kingdom LL57 1UT
2. Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, United States
3. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
4. Materials
Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States