Influence of Deposition Temperature on Cu-BDC Surface-Anchored Metal-Organic Framework Formation

Surface-anchored metal-organic frameworks (surMOFs) are crystalline, nanoporous, supramolecular materials mounted to substrates that have the potential for integration within device architectures relevant for a variety of electronic, photonic, sensing, and gas storage applications. This research investigates the thin film formation of the Cu-BDC (copper benzene-1,4-dicarboxylate) MOF system on a carboxylic acid-terminated self-assembled monolayer by alternating deposition of solution-phase inorganic and organic precursors. X-ray diffraction (XRD) and atomic force microscopy (AFM) characterization demonstrate that crystalline Cu-BDC thin films are formed via Volmer-Weber growth. Changes in film morphology as the deposition temperature increases are seen by AFM with more isolated nanorod-like crystallites observed at lower temperatures, while vertical nanoplatelet-like structures form at higher temperatures. At 45 °C, the nanoplatelets are observed to be composed of fused nanorod segments aligned in one direction. Ellipsometry confirms that both increasing temperature and number of deposition cycles yield more film deposition in agreement with infrared reflectance-absorbance spectroscopy (IRRAS) that was further used to characterize the chemical binding and orientation in the surface-bound Cu-BDC nanostructures. In addition to observing strong preferred orientation of the nanostructures from the enhancement of the symmetric carboxylate stretch and absence of the antisymmetric stretch, IRRAS results show the emergence of a new binding motif associated with segmented nanoplatelet formation at a higher deposition temperature as well as at high surface coverage after 12 deposition cycles. From the appearance of this higher frequency symmetric carboxylate peak alongside peak splitting in BDC deformation modes, IRRAS data support the presence of a strained configuration that accompanies the appearance of Cu-BDC segmented nanoplatelets observed by AFM.