Imaging the node-linker coordination in the bulk and local structures of metal-organic frameworks

Porous metal-organic frameworks (MOFs) have shown wide applications in catalysis, gas storage and separation due to their highly tunable porosity, connectivity and local structures. However, the electron-beam sensitivity of MOFs makes it difficult to achieve the atomic imaging of their bulk and local structures under (scanning) transmission electron microscopy ((S)TEM) to study their structure-property relations. Here, we report the low-dose imaging of a beam-sensitive MOF, MIL-101, under a Cs-corrected STEM based on the integrated differential phase contrast (iDPC) technique. The images resolve the coordination of Cr nodes and organic linkers inside the frameworks with an information transfer of similar to 1.8 angstrom. The local structures in MIL-101 are also revealed under iDPC-STEM, including the surfaces, interfaces and defects. These results provide an extensible method to image various beam-sensitive materials with ultrahigh resolution, and unravel the whole framework architectures for further defect and surface engineering of MOFs towards tailored functions. Metal-organic frameworks (MOFs) are so sensitive to electron beams that it is challenging to atomically image their structures by electron microscopy. Here, the authors use the integrated differential phase contrast technique in STEM to achieve the low-dose imaging of a beam-sensitive MOF, revealing its detailed local structure.