1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245
2. School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245
3. Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332-0245
4. School of Materials Science & Engineering, Georgia Institute of Technology; Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, China e-mail: