In-plane strains are commonly found in two-dimensional (2D) metal halide organic-inorganic perovskites (HOIPs). The in-plane mechanical properties of 2D HOIPs are vital for mitigating the strain-induced stability issues of 2D HOIPs, yet their structure and mechanical property relationship largely remains unknown. Here, we employed atomic force microscope indentation to systematically investigate the in-plane Young's moduli E of 2D lead halide Ruddlesden-Popper HOIPs with a general formula of (R-NH3)2PbX4, where the spacer molecules R-NH3+ are linear alkylammonium cations (CmH2m+1-NH3+, m = 4, 6, 8, or 12) and X = I, Br, or Cl. Fixing the spacer molecule to butylammonium, we discovered that the E of 2D HOIPs generally follows the trend of Pb-X bond strength, different from the tendency found in the out-of-plane moduli E, showing more prominent effects of the metal halide inorganic framework on E than E. E exhibits nonmonotonic dependence on the chain length of the linear alkyl spacer molecules, which would first decrease and plateau but then increase again. This is likely due to the competition of the bond strength and structural distortion in the inorganic layer, the relative fraction of the soft organic spacers, and the interfacial mechanical coupling associated with the interdigitation of the alkyl chains. The mechanical anisotropy of 2D HOIPs, marked by E/E, shows wide tunability based on structural composition, particularly for iodide-based 2D HOIPs. Our results provide valuable insights into the structure-property relationships regarding the mechanical anisotropy and in-plane mechanical behaviors of 2D HOIPs, which can guide the materials design and device optimization to achieve required mechanical performance in 2D HOIP-based applications.
