Mechanically Reconfigurable Electrical Polarization in Two Dimensional α-In₂Se₃

Ferroelectric materials are useful for their application in transistors, nonvolatile memory, solar cells, energy harvesters, and sensors. Under mechanical deformation, the symmetry of a ferroelectric material is broken which offer a facile way to modulate their electronic properties. However, this phenomenon is not fully realized in conventional thin film ferroelectrics made of bulk materials due to their high bending stiffness. In this regard, two-dimensional ferroelectric materials, because of their ultrathin and flexible nature, offer a great opportunity to realize modulation of electronic properties by mechanical means. We focus on α-In₂Se₃, a van der Waals 2D material which has recently been confirmed to show strong out of plane ferroelectricity. The key questions are how does the ferroelectric dipole couple to changes in piezoelectric and flexoelectric moments under large curvatures, and how do energetics of mechanics and bending compare to the switching energy between ferroelectric domains.<br/>In this study, we investigate the modulation of out-of-plane ferroelectric properties in highly localized bends in few layer α-In₂Se₃. We examine two structures: highly localized buckle delamination in multilayer flakes that are spontaneously created because of residual stress in the material during the mechanical exfoliation process, and smooth bends with controlled curvatures made by laminating multilayer α-In₂Se₃ onto graphite steps creating a heterostructure. We use piezoelectric force microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM) to examine tuning of the piezoelectric moment in and near the bends, and scanning transmission electron microscopy (STEM), to examine the atomic structure within the bends.<br/>PFM and KPFM distinguish the local polarization direction and converse piezoelectric coefficient of the α-In₂Se₃. We measure the out of plane converse piezoelectric coefficient of flat α-In₂Se₃to be 4-6 pm/V consistent with the value mentioned in the literature [1]. For the curve-controlled structures, we observe curvature and thickness dependent tuning of the converse piezoelectric coefficient and surface potential at the bends. We observe that the piezoelectric coefficient decreased by 80% and the surface potential increased by 10% for a 25 nm flake on a 30 nm graphite step. For the highly localized buckled structures, STEM reveals a spontaneous flip of polarization across the buckle. The polarization switching resides only in the buckled few layers and does not propagate perpendicular to the basal plane and thus creates two types of domain boundaries, charged lateral domain walls and charge neutral transverse domain walls, which are parallel and perpendicular to the basal plane, respectively. PFM and KPFM reveal that the presence of lateral domain walls within the heterostructure leads to an increase in out of plane converse piezoelectric coefficient of the material by 100% to 800% and surface potential by 40% to 300%. The strong enhancement cannot be explained purely from the net change in polarization. We hypothesize that the difference is explained by the presence of conductive states at the charged domain boundaries.<br/>Our study indicates the potential for dramatically reconfiguring the properties and structure of 2D ferroelectrics using curvature, both through continuous tuning and through domain flipping. The results have applications for 2D ferroelectrics in mechanically reconfigurable devices leading to next generation sensors, memory devices, and energy harvesters by actively utilizing out of plane deformation.<br/> <br/> <br/>References:<br/>[1] F. Xue, et al. <i>ACS Nano </i><b>12_,</b>4976-4983 (2018)