Spatially Tunable Interfacial Ferroelectricity in Low-Symmetric WTe2

Abstract

Interfacial ferroelectricity, recently discovered in van der Waals (vdW) materials, exhibits switchable dipoles at the interface. Most experiments are realized by stacking high-symmetry two-dimensional (2D) lattices in specific stacking configurations. A prototype based on a synthetic and low-symmetry 2D lattice is robust for switchable dipoles with broken symmetry at the interface. Here, we show that interfacial ferroelectricity can be spatially tunable by controlling the odd-even layer number in the synthetic low-symmetry lattice of 1T'-WTe₂. A high ferroelectric transition temperature (T_C) of >550 K is confirmed. The density functional theory (DFT) calculations indicate that interlayer sliding along the b-axis enables polarization switching of the interfacial dipoles. This study moves a significant step toward spatially tunable interfacial ferroelectricity.

Keywords: WTe2; interfacial ferroelectricity; interlayer sliding; low symmetry; polarization switching.

1

Odd–even layer-dependent symmetry in the synthetic 1T′-WTe₂. (a) Schematic illustration of the 1T′-WTe₂ crystal structures with odd- and even-layer numbers. The violet cross mark indicates an inversion symmetry center. (b) Optical image and surface topography of the thickness-tuned 1T′-WTe₂. The AFM height profile shows the distribution of the bilayer (∼1.5 nm) and trilayer (∼2.1 nm) regions. (c, d) SAED pattern and its simulated diffraction pattern of the (c) odd- and (d) even-layer 1T′-WTe₂. The diffraction spots from {120} planes were highlighted as red circles in (c). Yellow circles in (d) indicate the absence of diffraction spots from {120} planes. The scale bars represent 2 1/nm. (e) Optical image of the single crystalline few-layer 1T′-WTe₂. (f) Polar plot with the copolarized scheme for the A₁ Raman mode at the central region (the yellow spot). The arrow indicates the crystalline orientation of the a-axis. (g) SHG image of the same flake in (e). (h) Polar plot with the copolarized scheme for the SHG at the central region (the red spot) and edge region (the blue spot).

2

Spatially tunable domains of the interfacial ferroelectrics. (a) Surface topography with the height profile of the layer-dependent 1T′-WTe₂. The green arrow indicates the scanning direction. (b) Patterned domains in the bilayer 1T′-WTe₂ illustrate the switchable interfacial dipoles. (c, d) Charge density plot and the interfacial dipoles of the tri- and bilayer 1T′-WTe₂, respectively. The blue arrows indicate the direction of the polarization at the interface.

3

Temperature-dependent symmetry in the even-layer 1T′-WTe₂. (a, b) Optical and SHG images of the few-layer 1T′-WTe₂, respectively. Insets in (a): the AFM height profiles confirm the two different even-layer numbers with the tetra- (green: ∼3.6 nm) and hexalayer (dark green: ∼5.1 nm) regions. (c, d) Normalized SHG intensity with two variable-temperature cycles in the tetra- (c) and hexa- (d) layer 1T′-WTe₂. (e, f) Charge density plot and the interfacial dipoles of the tetra- (e) and hexa- (f) layer 1T′-WTe₂. The blue arrows cancel each other, and the remaining red ones indicate the direction of net polarization.

4

Interlayer sliding of the layer-dependent interfacial ferroelectricity. (a) Energy profile as a function of top layer shift Δy. Inset in (a): FE polarization as a function of top layer shift Δy. (b–d) Charge density plot of the two FE structures (b, d) and the nonpolar structure (c) of bilayer 1T′-WTe₂.