1. Department of Chemical Engineering University of Waterloo 200 University Avenue West Waterloo Ontario N2L 3G1 Canada
2. Waterloo Institute for Nanotechnology University of Waterloo 200 University Avenue West Waterloo Ontario N2L 3G1 Canada
3. Key Laboratory of Luminescence and Optical Information Ministry of Education Institute of Optoelectronic Technology Beijing Jiaotong University Beijing 100044 P. R. China