1. Institute of Nanoscience and Nanotechnology, Huazhong Normal University, Wuhan 430079, People’s Republic of China; Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 CNRS and University of Strasbourg, 23, rue du Loess, BP43, F-67034 Strasbourg cedex 2 France; and Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi-shi, Ishikawa 923-1292, Japan and CREST, Japan Science and Technology Agency, Sanban-cho, Tokyo 102-0075, Japan