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 HBr2D > HCl2D > HI2D, 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.