1. Chemistry and Physics of Materials Unit, DST Unit on Nanoscience, and Theoretical Sciences Unit, Jawaharlal Nehru Center for Advanced Scientific Research, Jakkur P. O, Bangalore 560064, India, Institute for Solid State Physics, University of Tokyo Kashiwa-shi, Chiba 277-8581, Japan, and Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700064, India