Exploring the Factors Affecting the Mechanical Properties of 2D Hybrid Organic-Inorganic Perovskites

Mechanical stability of hybrid organic-inorganic perovskites (HOIPs) is essential to achieve long-term durable HOIP-based devices. While HOIPs in two-dimensional (2D) form offer numerous options in the structure and composition to tune their mechanical properties, little is known about the structure-mechanical-property relationship in this family of materials. Here, we investigated a series of 2D lead halide HOIPs by nanoindentation to explore the impact of critical factors controlling the properties of both the organic and inorganic layers on the materials' out-of-plane mechanical performance. We find that the lead-halide bond in the inorganic framework can significantly influence the mechanical properties of 2D Ruddlesden-Popper (RP) HOIPs with n = 1. Like 3D HOIPs, stronger lead-halide bond strength leads to a higher Young's modulus in these 2D HOIPs, i.e., E_⊥^Cl ≳ E_⊥^Br > E_⊥^I. In contrast, the hardness of 2D RP HOIPs follows a trend of H_Br^2D > H_Cl^2D > H_I^2D, which is different from that found in 3D HOIPs, probably due to the combined effects from the Pb-X bond strength and inorganic framework structural change (e.g., symmetry and distortion). We further show that the interface between the organic layers in 2D HOIPs can be an effective route to engineer the materials' mechanical properties. Replacing the weak CH₃-CH₃ van der Waals forces by covalent bonds or phenyl-phenyl interactions in the interface can lead to a much stiffer and harder 2D HOIPs. Finally, we discover that the mechanical performance of 2D HOIPs with linear aliphatic diammonium spacer molecules is affected by the two basic structural parameters, i.e., the thicknesses of the organic and inorganic layers, in a similar way compared to that of 2D RP HOIPs with linear aliphatic monoammonium spacer molecules. A thinner organic layer and a thicker inorganic layer can result in 2D HOIPs with larger elastic modulus and hardness values. Our results offer intriguing insights into the structure-property relationship of 2D HOIPs from a mechanical perspective, providing guidelines and inspirations to achieve material design with required mechanical properties for applications.