1. Department of Materials Science; National Engineering Laboratory for TFT-LCD Materials and Technologies; Fudan University; Shanghai 200433 P.R. China
2. Department of Physics; National Sun Yat-Sen University; Kaohsiung 80424 Taiwan
3. Department of Photonics; National Sun Yat-Sen University; Kaohsiung 80424 Taiwan
4. Department of Material and Optoelectronic Science; National Sun Yat-Sen University; Kaohsiung 80424 Taiwan