Real-Space Molecular Resolution Imaging of Graphite-Anchored MOF Surfaces via Atomic Force Microscopy at the Liquid-Solid Interface.

Metal-organic frameworks (MOFs) are crystalline porous materials that have attracted widespread interest owing to their chemical versatility and functional properties. Over the past 30 years, MOF structures have been primarily characterized using diffraction-based methods, which provide ensemble-averaged information about long-range order but are not suited to investigating spatially localized, site-specific structural features. Atomic force microscopy (AFM) at the liquid-solid interface enables real-space, molecular-resolution imaging of MOF crystal surfaces under native conditions, but places stringent demands on sample preparation: crystals must be firmly immobilized and their surfaces preserved in a state compatible with high-resolution imaging. Here, we present a versatile method to anchor MOF crystals on highly oriented pyrolytic graphite (HOPG) via covalent grafting of aryl-carboxylate groups. Crystals of different sizes are grown directly on the functionalized substrate and transferred air-free to the AFM liquid cell, preserving surface integrity by avoiding prior drying or processing. We demonstrate molecular-resolution imaging on three structurally distinct MOFs: MOF-5, HKUST-1, and Zn