Interfacial ferroelectricity in marginally twisted 2D semiconductors

Abstract

Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of new metamaterials. Here we demonstrate a room temperature ferroelectric semiconductor that is assembled using mono- or few-layer MoS₂. These van der Waals heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of two 2D crystals, enables ferroelectric domains with alternating out-of-plane polarization arranged into a twist-controlled network. The last can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The observed interfacial charge transfer, movement of domain walls and their bending rigidity agree well with theoretical calculations. Furthermore, we demonstrate proof-of-principle field-effect transistors, where the channel resistance exhibits a pronounced hysteresis governed by pinning of ferroelectric domain walls. Our results show a potential avenue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.

Fig. 1. Ferroelectric domains in marginally twisted bilayer MoS₂.

a, Example of BSECCI acquired on unencapsulated twisted bilayer MoS₂ placed onto a graphite substrate. Light and dark domain contrast corresponds to the two dominant stacking orders referred as Mo^tS^b and S^tMo^b. Scale bar, 1 μm. b, Centre: schematic demonstrating the transition from Mo^tS^b to S^tMo^b with perfectly stacked bilayer regions separated by a partial dislocation. Side panels: the cross-sectional alignment of the MoS₂ monolayers, viewed along the armchair direction, assembled within the double-gated device structure. Colour maps overlayed on top of the TMD atomic schematics show calculated charge density transferred between top and bottom layers, with red and blue corresponding to positive and negative charges, respectively. c–g, Domain switching visualized by BSECCI under different values of transverse electric field $D/{\epsilon }_0$ applied in situ. Measurements have been performed on marginally twisted MoS₂ encapsulated in hBN from both sides, placed on a graphite back gate and covered with graphene top gate as shown schematically in b. Large domains mostly retain their shape when the field is removed and practically disappear when the field is inverted; the arrows in e indicate partial dislocations colliding when neighbouring domains of the same orientation try to merge. Micrographs are presented in chronological order. The white oval feature in a and black ring features in c–g are where the intralayer contamination has segregated to form a bubble. Scale bars, 1 μm.

Fig. 2. Domain evolution in double-gated marginally twisted MoS₂ bilayers.

a–c, BSECCI image of a triangular network of small domains undergoes expansion/contraction as a function of applied electric field ($D/{\epsilon }_0=0{\mathrm{Vnm}}^{-1}\left(\mathrm{a}\right),1.4{\mathrm{Vnm}}^{-1}\left(\mathrm{b}\right),-1.4{\mathrm{Vnm}}^{-1}\left(\mathrm{c}\right)$) overlaid with the analytical model equations (2) and (3), yellow lines. Scale bars, 200 nm. d–f, BSECCI image of larger domains, where the partial dislocations that constitute the domain walls merge near the nodes and the energetically disadvantaged domain collapses locally into a PSD at the applied electric field values of 1.4 Vnm⁻¹ (e) and −1.4 V nm⁻¹ (f). Micrographs are presented in their chronological order and the contrast is seen to deteriorate due to beam-induced surface contamination. Scale bars are 200 nm. g–i, Polarization maps for different values of scaling parameter computed using mesoscale relaxation of the bilayer lattice (Supplementary Information) and compared with the analytical model (yellow curves) of the scaled domain evolution given by equations (2) and (3). As the domain walls consist of two partial dislocations with Burgers vectors $\frac{a}{\sqrt{3}}\left(1,0\right)$ and $\frac{a}{\sqrt{3}}\left(\frac{1}{2},\frac{\sqrt{3}}{2}\right)$, line defects observed can be assigned to a PSD with Burgers vector $a\left(\frac{\sqrt{3}}{2},\frac{1}{2}\right)$.

Fig. 3. Electronic properties of ferroelectric domains in MoS₂.

a, Two-pass PM-KPFM map of the surface potential acquired with V_a.c. = 5.75 V, and a time-average probe-sample distance of 37 nm during the second pass. The blue boxed inset magnifies the area used to extract numerical values of 2ΔV. The lower left inset shows the schematic of PM-KPFM measurement set-up. Scale bars 0.5 μm (upper) and 2 μm (lower). b, Typical histogram analysis used to extract surface potential difference. In this case, the data from the blue boxed area in a were used; for details and analysis of other areas see Supplementary Information. a.u., arbitrary units. c, Calculated surface potential distribution for experimentally relevant domain sizes. Inset shows the potential drop across the domain walls calculated considering (narrow line) and ignoring (bold line) piezoelectric charges. Scale bar, 50 nm. d,e, PM-KPFM surface potential map of a sample with no gate voltage (d) and with back-gate voltage applied indicating disappearance of the potential variation when free electrons are introduced (e). Scale bars, 1 μm. f,g, Hysteretic behaviour of electrical conductivity G_sd of our artificially made ferroelectric semiconductors as a function of top-gate electric field (E_t) for different back-gate electric fields (E_b). The shown curves are for 1L MoS₂ on top of 1L MoS₂ at 350 K (f) and 3L/3L MoS₂ at room temperature (g) twisted by 0° to achieve the 3R interface. We used the top gate for recording hysteresis because it covers only the twisted region whereas the bottom gate influences a much larger area, including contact regions. h, Similar measurements on a reference 2L/2L sample twisted by 0°, which produces a 2H interface and displays much smaller hysteresis with the opposite sign (room temperature).