1. Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States
2. Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas 79409, United States
3. National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States
4. Applied Physics Program, Smalley-Curl Institute, Rice University, Houston, Texas 77005, United States
5. SIMS Laboratory, Shared Equipment Authority, Rice University, Houston, Texas 77005, United States
6. Department of Physics, Florida State University, Tallahassee, Florida 32306, United States