1. School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China
2. Department of Chemical Engineering & Materials Science, Yuan Ze University, Taoyuan City 320, Taiwan, ROC
3. Elementary Educational College, Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Capital Normal University, Beijing 100048, P. R. China
4. Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China
5. Centre for Nano Science and Nano Technology, Panjab University, Chandigarh160014, India
6. Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and Bioengineering Research (AMBER), School Chemistry, Trinity College Dublin, Dublin 2, Ireland