Mode-Selective Raman Imaging of Metal–Organic Frameworks Reveals Surface Heterogeneities of Single HKUST-1 Crystals

Metal organic frameworks (MOFs) are nanoporous materials with high surface-to-volume ratios that have potential applications as gas sorbents. Sample quality is, however, often compromised and it is unclear how defects and surface contaminants affect the spectral properties of single MOF crystals. Raman microspectroscopy is a powerful tool for characterizing MOFs, yet spatial heterogeneity distributions of single MOF crystals have not been reported so far. In this work, we use Raman microspectroscopy to characterize spatially isolated, single crystals of the MOF species HKUST-1. In the first step, we validate the HKUST-1's Raman spectrum based on DFT simulations, and we identify a previously unreported vibrational feature. In the second step, we acquire diffraction-limited, mode-selective Raman images of single HKUST-1 crystals that reveal how the spectral variations are distributed across the crystal surface. In the third step, we explore how multivariate data analysis can aid feature identification in Raman images of single MOF crystals. Finally, we statistically analyze the measured spectral peak positions and line widths for quantifying the variability occurring within the same crystal as well as between different crystals taken from the same batch. For enabling validation and reuse, we make the spectroscopic data and simulation code publicly available.