Reducing Threshold of Ferroelectric Domain Switching in Ultrathin Two-Dimensional CuInP₂S₆ Ferroelectrics via Electrical–Mechanical Coupling

Abstract

Room-temperature out-of-plane two-dimensional ferroelectrics have promising applications in miniaturized non-volatile memory appliances. The feasible manipulation of polarization switching significantly influences the memory performance of ferroelectrics. However, conventional high-voltage-induced polarization switching inevitably generates charge injection or electric breakdown, and large-mechanical-loading-induced polarization switching may damage the structure of ferroelectrics. Hence, decreasing critical voltage/loading for ferroelectric polarization reversal is highly required. Herein, using atomic force microscopy experiments, the ferroelectric domain switching via both electric field and mechanical loading was demonstrated for an ultrathin (∼4.1 nm) CuInP 2 S 6 nanoflake. The relevant threshold voltage/loading for polarization switching was ∼ −5 V/1095 nN, resulting from the electric field and flexoelectric effect, respectively. Finally, the electrical−mechanical coupling was adopted to reduce the threshold voltage/loading of CuInP 2 S 6 significantly. It can be explained by the Landau−Ginzburg−Devonshire double-well model. This effective way for easily tuning the polarization states of CuInP 2 S 6 opens up new prospects for mechanically written and electrically erased memory devices.

Stress and Curvature Effects in Layered 2D Ferroelectric CuInP2S6

“…Chen et al was able to artificially generate large-scale stripe domains of hundreds of microns by corrugating the CIPS flake, where it is believed that the giant strain gradient is introduced via high curvature . Similar curvature induced domains were observed by several other groups. , Similarly, mechanical switching of polarization in CIPS has been demonstrated. , This effect was attributed to flexoelectricity owing to the formation of a giant strain gradient. Almost at the same time, another work revealed strong second harmonic generation (SHG), ∼160-fold enhancement compared to unstrained region, of wrinkle nanostructures in CIPS, this SHG enhancement can be modulated by the applied strain .…”

“…16 Similar curvature induced domains were observed by several other groups. 14,18 Similarly, mechanical switching of polarization in CIPS has been demonstrated. 14,18 This effect was attributed to flexoelectricity owing to the formation of a giant strain gradient.…”

“…14,18 Similarly, mechanical switching of polarization in CIPS has been demonstrated. 14,18 This effect was attributed to flexoelectricity owing to the formation of a giant strain gradient. Almost at the same time, another work revealed strong second harmonic generation (SHG), ∼160-fold enhancement compared to unstrained region, of wrinkle nanostructures in CIPS, this SHG enhancement can be modulated by the applied strain.…”

“…In addition to the intrinsic excellent physical properties of 2D materials, e.g., high electron mobility and thermal conductivity, new phenomena have been discovered in the twisted bilayers as driven by emergent electronic instabilities. − Over the last several years interest shifted toward the layered materials that support additional functionalities including ferromagnetic and ferroelectric, this opening the route to ferroelectricity at atomic thicknesses. Since 2D materials can withstand significantly large curvature and strain, these materials systems allow to get insight into novel phenomena based on the curvature and strain effects. , With the coupling between polarization and strain effect being significant even for 3D ferroelectrics, this is expected to be even more pronounced in 2D ferroelectric materials that can support larger curvatures. − …”

“…At the same time, stress is one of the well-recognized control parameters in ferroelectric thin films that can couple to polarization and induce transitions between structural variants. For Cu-based layered chalcogenides, stress-induced phase transitions and strain engineering , become especially important in ultrathin films of CuInP 2 (S,Se) 6 , ,, Despite the significant fundamental and practical interest in bulk and nanosized CuInP 2 (S,Se) 6 , the influence of stress and strains on the local switching of its spontaneous polarization is explored only weakly. From a theoretical perspective, there has been very little effort on both mesoscopic and atomistic levels of theory.…”