1. Key Laboratory of Applied Surface and Colloid ChemistryMinistry of EducationShaanxi Key Laboratory for Advanced Energy DevicesShaanxi Engineering Lab for Advanced Energy TechnologySchool of Materials Science & EngineeringShaanxi Normal University Xi'an 710119 P. R. China
2. School of Physical Science and Technology & Key Laboratory for Magnetism and Magnetic Materials of MoE & Key Laboratory of Special Function Materials and Structure DesignMoE & National & Local Joint Engineering Laboratory for Optical Conversion Materials and TechnologyLanzhou University Lanzhou 730000 P. R. China
3. Electron Microscopy CentreSchool of Physical Science and TechnologyLanzhou University Lanzhou 730000 P. R. China
4. Center for Excellence in Nanoscience (CAS)Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS)National Center for Nanoscience and Technology Beijing 100190 P. R. China