Vibrational Fingerprinting of Defects Sites in Thin Films of Zeolitic Imidazolate Frameworks

Surface-mounted metal-organic frameworks (SURMOFs) are crystalline films of MOFs and have garnered a great deal of attention in the past years. So far, thin-film MOF research has been mainly focused on the synthesis and the exploration of potential applications of these materials, while a detailed understanding of their growth is still lacking. In this report evidence is provided for the inter-grown nature of surface-mounted thin films of Zn-ZIF-8 (SURZIF-8; ZIF=zeolitic imidazolate framework). Two distinct SURZIF-8 thin films have been made through layer-by-layer (LBL) growth after applying 20 and 50 LBL cycles. They have been characterized with atomic force microscopy (AFM) and Raman micro-spectroscopy. A detailed analysis of the Raman mapping data, inter alia using principal component analysis (PCA), revealed the existence of phase boundaries within the 20-cycle thin film, while the 50-cycle thin film is chemically more homogeneous. To further analyze these chemical heterogeneities, density functional theory (DFT) calculations were performed of three theoretical models providing spectroscopic fingerprints of the molecular vibrations associated with the Zn-ZIF-8 thin films. Based on these calculations and the experimental data distinct vibrational markers indicative for the presence of defects sites were identified.