1. Department of Electronic EngineeringThe Chinese University of Hong Kong Shatin, New Territories Hong Kong 999077 P. R. China
2. School of Environment and EnergyState Key Laboratory of Luminescent Materials and DevicesNational Engineering Laboratory for VOCs Pollution Control Technology and EquipmentSouth China University of Technology Guangzhou 510006 P. R. China
3. State Key Laboratory of Silicon Materials and School of Materials Science & EngineeringZhejiang University Hangzhou 310027 P. R. China
4. Department of PhysicsThe Chinese University of Hong Kong Shatin, New Territories Hong Kong P. R. China
5. Department of PhysicsThe Hong Kong University of Science and Technology Clear Water Bay, Kowloon Hong Kong P. R. China