1. Department of Physics, Purdue University, West Lafayette, Indiana
47907, United States
2. School
of Electrical and Computer
Engineering, Purdue University, West Lafayette,
Indiana 47907, United States
3. Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United
States
4. Department
of Chemistry, Purdue University, West Lafayette, Indiana
47907, United States