1. College of Science, Institute of Materials Physics and ChemistryNanjing Forestry University Nanjing 210037 P.R. China
2. State Key Laboratory of Advanced Technology for Materials Synthesis and ProcessingWuhan University of Technology Wuhan 430070 P.R. China
3. Testing CenterYangzhou University Yangzhou 225009 P.R. China
4. Key Laboratory for Photonic and Electronic Band MaterialsMinistry of Education, School of Physics and Electronic EngineeringHarbin Normal University Harbin 150025 P.R. China