1. Department of Materials Science & Engineering, Seoul National University, Seoul 151-742, Korea, Chemical and Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, School of Advanced Materials Engineering, Kookmin University, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Korea, and Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260