1. Department of Physics and Astronomy, Louisiana State University 1 , Baton Rouge, Louisiana 70803, USA
2. NIST Physical Measurement Lab, 100 Bureau Drive 2 , Gaithersburg, Maryland 20899, USA
3. Department of Chemistry, Louisiana State University 3 , Baton Rouge, Louisiana 70803, USA
4. Center for Advanced Microstructures and Devices, Louisiana State University 4 , Baton Rouge, Louisiana 70806, USA