1. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, People's Republic of China
2. Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA
3. Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, USA