1. Key Laboratory of Materials Physics, and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, HFIPS Chinese Academy of Sciences P. O. Box 1129 Hefei 230031 China
2. Department of Materials Science and Engineering University of Science and Technology of China Hefei 230026 China
3. Department of Electrical Engineering École de Technologie Supérieure (ÉTS) Montréal Québec H3C 1K3 Canada
4. Institut National de la Recherche Scientifique (INRS), Centre Énergie Matériaux Télécommunications Varennes Québec J3X 1P7 Canada