1. Department of Chemistry, Indian Institute of Technology, Madras, Chennai-600036, India, Laboratory of Photonics and Interfaces, ICMB-LPI, Swiss Federal Institute of Technology, Lausanne CH-1015, Switzerland, Group de Physique des aggregate en surface, FSB, Swiss Federal Institute of Technology, Lausanne, CH-1015, Switzerland, and Laboratoire de Metallurgie Chimique, Swiss Federal Institute of Technology, Lausanne, CH-1015, Switzerland