1. Department of Physics and Astronomy and Smalley-Curl Institute, Rice University, Houston, Texas 77005, United States
2. Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States
3. Materials Science and Engineering Program, University of Colorado, Boulder, Colorado 80309, United States
4. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
5. Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77005, United States