1. Department of Physics The Chinese University of Hong Kong Shatin, Hong Kong SAR China
2. Henan Provincial Key Laboratory of Nanocomposites and Applications Institute of Nanostructured Functional Materials Huanghe Science and Technology College Zhengzhou 450006 China
3. Shaanxi Key Laboratory for Advanced Energy Devices Shaanxi Engineering Lab for Advanced Energy Technology School of Materials Science and Engineering Shaanxi Normal University Xi'an 710119 China
4. Department of Chemistry The Chinese University of Hong Kong Shatin, Hong Kong SAR China