Friction Characteristics of Two-Dimensional Hybrid Organic-Inorganic Perovskites

Background: Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) have been and continue to be of great interest to researchers, given their use in flexible solar cells, wearable sensors, and electronic devices. Frictional properties of 2D HOIPs should be clearly understood to establish the manufacturing processes as well as the durability of the devices. Objective: The aim of this study is to provide a fundamental understanding of the frictional properties of 2D HOIPs at the nanoscale. Methods: The topography of 2D HOIPs with three different chain lengths and five different thicknesses were systematically examined with atomic force microscopy (AFM). The frictional properties of these films were also quantitatively assessed under various normal forces using friction force microscopy (FFM). Results: From the AFM, it was found that the 2D HOIPs exhibited nanoscale surface roughness, presumably due to local disorder of the organic cations or deformation of the inorganic framework. In addition, FFM showed that the intrinsic friction of 2D HOIPs was significantly low. The friction force decreased as molecular chain length increased and thickness-dependent friction characteristics were clearly observed. Conclusions: The trends were due to changes in mechanical properties, surface roughness, and suppression of puckering, but not due to changes in AFM tip adhesion.