National Creative Research Initiative Center for Oxide Nanocrystalline Materials, School of Chemical and Biological Engineering, School of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea, Diagnostic Radiology, Seoul National University Hospital, the Institute of Radiation Medicine, Medical Research Center, Seoul National University, Seoul 110-744, Korea, and New Material Evaluation Center, Korea Research Institute of Standards and Science, Taejon 305-600, Korea (At 2008-11-14)