1. Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia 23284, USA
2. Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, Illinois 60439, USA
3. Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, USA
4. Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA