1. Department of Applied PhysicsThe Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong P. R. China
2. Institute of Textiles and ClothingThe Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong P. R. China
3. State Key Laboratory of Electronic Thin Films and Integrated DevicesUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China
4. Division of Energy and EnvironmentGraduate School at ShenzhenTsinghua University Shenzhen 518055 P. R. China