1. Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, P.R. China
2. Department of Physics, The Chinese University of Hong Kong, Shatin, New Territory, Hong Kong
3. Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia