1. Department of Physics, University of California, Berkeley, CA 94720, USA.
2. Centro de Física de Materiales CSIC/UPV-EHU-Materials Physics Center, Paseo Manuel de Lardizabal 5, E-20018 San Sebastián, Spain.
3. Department of Chemistry, University of California, Berkeley, CA 94720, USA.
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. Donostia International Physics Center, Paseo Manuel de Lardizabal 4, E-20018 San Sebastián, Spain.
6. Nano-Bio Spectroscopy Group and ETSF Scientific Development Center, Dpto. de Física de Materiales, Universidad del País Vasco UPV/EHU, Avenida de Tolosa 72, E-20018 San Sebastián, Spain.