1. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan, Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan, Department of Chemistry, National Taiwan Normal University, Taipei 106, Taiwan, Department of Materials Science and Optoelectronic Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, and Institute of Physics, Academia Sinica, Taipei 115, Taiwan