Electric polarization switching in an atomically thin binary rock salt structure

Abstract

Inducing and controlling electric dipoles is hindered in the ultrathin limit by the finite screening length of surface charges at metal-insulator junctions , although this effect can be circumvented by specially designed interfaces . Heterostructures of insulating materials hold great promise, as confirmed by perovskite oxide superlattices with compositional substitution to artificially break the structural inversion symmetry . Bringing this concept to the ultrathin limit would substantially broaden the range of materials and functionalities that could be exploited in novel nanoscale device designs. Here, we report that non-zero electric polarization can be induced and reversed in a hysteretic manner in bilayers made of ultrathin insulators whose electric polarization cannot be switched individually. In particular, we explore the interface between ionic rock salt alkali halides such as NaCl or KBr and polar insulating CuN terminating bulk copper. The strong compositional asymmetry between the polar CuN and the vacuum gap breaks inversion symmetry in the alkali halide layer, inducing out-of-plane dipoles that are stabilized in one orientation (self-poling). The dipole orientation can be reversed by a critical electric field, producing sharp switching of the tunnel current passing through the junction.

Review on Recent Developments in 2D Ferroelectrics: Theories and Applications

“…Another excellent example is inducing the ferroelectric polarization the atomically thin NaCl laid on the Cu substrate by introducing an atomically thin polar insulator Cu 2 N between NaCl and the substrate (Figure 14b,c). [ 84 ] The symmetry breaking and direction‐preferred polarization in the NaCl layer, arising from its asymmetry environment in the out‐of‐plane direction, with Cu 2 N on one side and vacuum on the other are theoretically and experimentally verified. Here, the intercalated atomically thin polar insulator layer not only prevents hybridization between NaCl and the Cu substrate (to guarantee a large tunneling current), but also helps in breaking the inversion symmetry of NaCl.…”

Revealing buckling of an apparently flat monolayer of NaCl on Pt(111)

“…The Cl ions are either above or below the plane defined by the Na ions. Certain atoms are bistable, as has been observed in NaCl layers on a c(2×2) N-Cu(100) surface [38]. Interestingly, the buckling does not have a strong effect on the STM signal due to a trade-off between the Cl ion height over the surface and the magnitude of the LDOS, but can be clearly seen in the AFM signal via the local electrostatic field.…”

Ferroic Phases in Two-Dimensional Materials

“…The development of 2D vdW assembly technology enables the tailoring of physical properties via interface engineering, which may render superior properties compared to pristine 2D materials . For example, the ferroelectric polarization between atomically thin NaCl and a Cu substrate can be induced by introducing an atomically thin polar insulator Cu 2 N between the NaCl and the substrate . Cu 2 N at the interface not only decouples NaCl from the Cu electrode electronically, but also breaks the inversion symmetry of NaCl.…”

“…Cu 2 N at the interface not only decouples NaCl from the Cu electrode electronically, but also breaks the inversion symmetry of NaCl. Theoretical and experimental results confirmed that the polarization in the NaCl layer stems from the vertically asymmetric environment with Cu 2 N on one side and vacuum on the other . In the same year, Xu et al constructed a set of vertically stacked 2D heterostructures consisting of bis­(ethylenedithio)­tetrathiafulvalene/C60 and poly­(3-dodecylthiophene-2,5-diyl) (P3DDT)/C 60 nanosheets prepared by the Langmuir–Blodgett method, which exhibited vertical ferroelectric behavior under an external electric field …”