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
(C
m
H2m+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.