1. State Key Laboratory of Electronic Thin Film and Integrated Devices Department of Electronic Science and Engineering University of Electronic Science and Technology of China Chengdu Sichuan 610054 China
2. Department of Materials Science and Engineering National University of Singapore Singapore 117576 Singapore
3. School of Chemical and Biomedical Engineering Nanyang Technological University Singapore 637459 Singapore
4. Department of Materials Science and Engineering and A. J. Drexel Nanomaterials Institute Drexel University Philadelphia PA 19104 USA