1. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
2. Institut für Physik; Technische Universität Chemnitz; Chemnitz D-09126 Germany
3. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
4. College of Physics Science & Energy; Shenzhen University; Shenzhen 518060 China
5. State Key Laboratory of Luminescent Materials and Devices; Institute of Polymer Optoelectronic Materials and Devices; South China University of Technology; Guangzhou 510640 China