Probing Local Structural Variations in Metal-Organic Framework Thin Films Using Nano-FTIR

Precise control over coordination structures and orientation is crucial for integrating metal-organic framework (MOF) thin films into functional devices. In this study, we combined atomic force microscopy with nano-Fourier transform infrared spectroscopy to definitively characterize local coordination bonding in a representative MOF thin film, NAFS-1, fabricated at the air/liquid interface. Our nanoscale analysis revealed structural inhomogeneities often obscured in macroscopic measurements. Additionally, we identified several characteristic regions in NAFS-1, including areas with uncoordinated carboxyl groups and with large, elevated features exhibiting effective coordination. Furthermore, omitting the water-rinsing step, usually conducted after transferring a film to the substrate to eliminate surface contaminants, increased structural inhomogeneity while promoting coordination bonding on average. These results highlight the importance of local characterization in understanding film crystallinity and uniformity and provide critical insights into the MOF thin-film formation mechanism at the air/liquid interface.