1. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore.
2. Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104−6323, USA.
3. Jiangsu Key Laboratory of Thin Films and Department of Physics, Soochow University, Suzhou 215006, China.
4. Department of Physics, Shanghai University, Shanghai 200444, China.
5. Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603, Singapore.