1. Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA
2. Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, California 92093, USA
3. Nanoscience and Technology Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
4. Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3003, USA