1. State Key Laboratory of Chemical Resource Engineering, P.O. Box 98; Beijing University of Chemical Technology; Beijing 100029 P.R. China
2. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
3. SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology & Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province; College of Optoelectronic Engineering; Shenzhen University; Shenzhen 518060 P.R. China