This work was supported by China postdoctoral science foundation (No. 2015M580299), Fundamental Research Funds for the Central Universities (Nos. 1514011 and 222201718003), and National Natural Science Foundation of China (No. 21576082). FP7 Staff-exchange for K. K. Z. and Z. T. L. under the ELECTRONANOMAT program (No. PIRSES-GA-2012-318990) sponsored by the Marie-Curie Act program of EU is acknowledged. K. K. Z. and Z. T. L. acknowledge the Danish Agency for Science, Technology and Innovation (No. 5132-00053B) for supporting collaboration with DTU. Q. J. C. acknowledges the Danish Council for Independent Research DFF-FTP (No. 12-127447) and the Danish Agency for Science, Technology and Innovation (No. 5132-00053B) for the financial support. We thank Prof. Xinhua Zhong for helpful discussions.
Reprints and Permission requests may be sought directly from editorial office.