1. Key Laboratory of Special Functional Materials for Ecological Environment and Information Ministry of Education School of Material Science and Engineering Hebei University of Technology Tianjin 300130 P. R. China
2. Department of Physics and Astronomy University of California Irvine CA 92697 USA
3. School of Materials Science and Engineering Tianjin University Tianjin Haihe Education Park Tianjin 300072 P. R. China