1. Department of Chemical Engineering, Electrical and Computer Division, Pohang University of Science and Technology, San 31, Hyoja-Dong, Nam-Gu, Pohang, Kyungbuk 790-784, Korea, and Department of Materials Science and Engineering, Pohang University of Science and Technology, San31, Hyoja-Dong, Nam-gu, Pohang, Kyungbuk 790-784, Korea