1. Chemistry and Physics of Materials Unit and DST Unit on Nanoscience, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India, School of Electrical Sciences, Vellore Institute of Technology, Vellore 632014, India, and NEST CNR-INFM and Scuola Normale Superiore, Piazza San Silvestro 12, 56127, Pisa, Italy