In-plane mechanical properties of terephthalate-based two-dimensional metal-organic frameworks

Two-dimensional (2D) metal-organic frameworks (MOFs) are often subjected to mechanical loading in their applications, and the in-plane elastic modulus E_‖ is a critical material property needed to understand and predict the mechanical behaviors of 2D MOFs for improved mechanical reliability and strain engineering of their functional properties. However, the E_‖ values of 2D MOFs are largely unknown, even for those with widely used coordination linkers like 1,4-benzenedicarboxylate (BDC), because of the challenges in in-plane mechanical testing imposed by both the extreme dimensionality and the high sensitivity of 2D MOFs to external factors (e.g., e-beams) due to their hybrid organic-inorganic nature. Here we employed atomic force microscopy (AFM) stretching of suspended thin membranes to measure the E_‖ of three structurally related, BDC-coordinated MOFs. The 2D Zn₃(BDC)₃(H₂O)₂·4(DMF) (DMF = N,N-dimethylformamide) has an E_‖ value of 11.2 ± 2.5 GPa, much lower than that of its 3D analog, (DMA)₂[Zn₃(BDC)₄·1.5H₂O] (DMA = dimethylammonium) (E_‖ = 25.9 ± 6.3 GPa), owing to the absence of interlayer covalent bonding. However, a 2D Mn analog, Mn₃(BDC)₃·4(DMF), exhibits enhanced in-plane stiffness (E_‖ = 25.5 ± 4.9 GPa), likely originating from the strengthened coordination at the nodes. We further compared 2D MOFs to other 2D materials and widely used engineering material systems using a density vs. E_‖ Ashby plot. Our results provide indispensable insights into the structure-mechanical property relationship of 2D MOFs to guide material engineering and selection.