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. Department of PhysicsThe Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong 999077 P. R. China
4. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong ProvinceShenzhen University Shenzhen 518060 P. R. China
5. College of ScienceChina University of Petroleum No. 66, Changjiang West Road, Huangdao District Qingdao 266580 P. R. China
6. Department of PhysicsThe Chinese University of Hong Kong Shatin, New Territories Hong Kong 999077 P. R. China