1. Center for Nanoscale Science and Technology, Rice University, Houston, TX 77005, USA.
2. Carbon Nanotechnology Laboratory, Rice University, Houston, TX 77005, USA.
3. Department of Chemical Engineering, Rice University, Houston, TX 77005, USA.
4. Department of Chemistry, Rice University, Houston, TX 77005, USA.
5. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.