Local structure evolvement in MOF single crystals unveiled by scanning transmission electron microscopy

The response of metal-organic frameworks (MOFs) toward external stimuli, such as light, heat, and electrons, usually involves a local change in their structures. Although diffraction methods allow for monitoring of the overall dynamics of the crystal structure, the local environment on each crystal and specific position within the crystal remains largely unknown. Here, the collective change in the pores of MOF single crystals was visualized by scanning transmission electron microscopy (STEM) with an interpretable resolution of 4.7 angstrom to give a clear observation of each individual pore. The local evolvement occurred before the deterioration of image resolution; therefore, the maintenance of overall image resolution does not guarantee the preservation of structure integrity. The local evolvement in response to electron irradiation was quantitatively analyzed and was found to be dependent on both the lattice plane and specific position in the crystal. The crystal shrunk gradually as the electron dose increased. This was likely attributed to the displacement of the molecular building blocks in the MOF backbone. The displacements were inhomogeneous across the individual crystal, corroborating with the local twisting and rotation of the crystal. This STEM method provided an example for the spatial and temporal study of the local structure evolvement of MOFs under stimuli.