  Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are new
members of the 2D materials family with wide tunability, highly dynamic
structural features and excellent physical properties. Ultrathin 2D HOIPs and
their heterostructures with other 2D materials have been exploited for study of
new physical phenomena and novel device applications. The in-plane mechanical
properties of 2D ultrathin HOIPs are critical for understanding the coupling
between mechanical and other physical fields and for integrated devices
applications. Here we report for the first time the in-plane mechanical
properties of ultrathin freestanding 2D lead iodide perovskite membranes and
their dependence on the membrane thickness. The in-plane Young's moduli of 2D
HOIPs are smaller than that of conventional covalently bonded 2D materials.
Both the Young's modulus and breaking strength first decrease and then plateau
as the thickness increases from monolayer to 4 layers due to interlayer
slippage during deformation. Our results show that ultrathin 2D HOIPs exhibit
outstanding breaking strength/Young's Modulus ratio compared to many other
widely used engineering materials and polymeric flexible substrates, which
renders them suitable for application into flexible electronic devices.
