1. Department of Chemistry, Jadavpur University, Kolkata 700 032, India, Department of Physics, Sripat Singh College, Jiaganj, Murshidabad, West Bengal 742 123, Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India, and Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea