1. School of Electrical, Computer, and Energy Engineering, Arizona State University 1 , Tempe, Arizona 85281, USA
2. Department of Materials Science and Nanoengineering, Rice University 2 , Houston, Texas 77005, USA
3. Applied Physics Graduate Program, Smalley-Curl Institute, Rice University 3 , Houston, Texas 77005, USA
4. Department of Physics, Arizona State University 4 , Tempe, Arizona 85281, USA
5. SIMS laboratory, Shared Equipment Authority, Rice University 5 , 6100 Main Street, Houston, Texas 77005, USA
6. Department of Electrical and Computer Engineering, Rice University 6 , Houston, Texas 77005, USA