1. State Key Laboratory of Optoelectronic Materials and TechnologiesSchool of MaterialsSun Yat‐sen University Guangzhou 510275 P. R. China
2. Department of Applied PhysicsThe Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong P. R. China
3. The Hong Kong Polytechnic University Shenzhen Research Institute Shenzhen 518054 P. R. China