1. Departamento de Física, Universidade Federal de São Carlos, 13565-905 São Carlos, São Paulo, Brazil
2. Instituto de Física and International Center for Physics, Universidade de Brasília, 70919-970 Brasília, Distrito Federal, Brazil
3. Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States
4. Department of Electrical and Computer Engineering, California NanoSystems Institute, University of California─Los Angeles, Los Angeles, California 90095, United States