1. Key Laboratory of Fluid and Power Machinery, School of Material Science and Engineering, Xihua University, Chengdu 610039, P. R. China
2. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. Department of NanoEngineering and Program of Chemical Engineering, University of California San Diego, La Jolla, California 92093-0448, United States