  2D layered hybrid organic-inorganic perovskites (HOIPs) have demonstrated
improved stability and promising photovoltaic performance. The mechanical
properties of such functional materials are both fundamentally and practically
important to achieve both high performance and mechanical stable (flexible)
devices. Here we report the mechanical properties of a series of 2D layered
lead iodide HOIPs and investigate the role of structural sub-units (e.g.,
variation of the length of the organic spacer molecules -R and the number of
inorganic layer -n) on the mechanical properties. While 2D HOIPs have much
lower nominal elastic moduli and hardness than 3D HOIPs, larger n number and
shorter R lead to stiffer materials. DFT simulations showed a similar trend to
the experimental results. We compared these findings with other 2D layered
crystals and shed light on routes to further tune the out-of-plane mechanical
properties of 2D layered HOIPs.
