Abstract:2D ferroelectrics with robust polarization down to atomic thicknesses provide building blocks for functional heterostructures. Experimental realization remains challenging because of the requirement of a layered polar crystal. Here, we demonstrate a rational design approach to engineering 2D ferroelectrics from a non-ferroelectric parent compound via employing van der Waals assembly. Parallel-stacked bilayer boron nitride exhibits out-of-plane electric polarization that reverses depending on the stacking order… Show more
“…Recently a new trend in creating truly two-dimensional (2D) ferroelectrics has emerged, which exploits interfacial charge transfer in stacked heterostructures of 2D materials. This possibility has been recently demonstrated in marginally twisted wide band gap insulator, hexagonal boron nitride [9][10][11] and in semi-metallic WTe2 12 with the ability to switch the domain type achieved by sliding atomic planes along the interface. Here, we report an observation of robust room temperature ferroelectricity in marginally twisted semiconducting bilayers of the transition metal dichalcogenide (TMD), MoS2.…”
Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of novel 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 polarisation arranged into a twist-controlled network. The latter can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The interfacial charge transfer for the observed ferroelectric domains is quantified using Kelvin probe force microscopy and agrees well with theoretical calculations. The movement of domain walls and their bending rigidity also agrees well with our modelling results. 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 venue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.
“…Recently a new trend in creating truly two-dimensional (2D) ferroelectrics has emerged, which exploits interfacial charge transfer in stacked heterostructures of 2D materials. This possibility has been recently demonstrated in marginally twisted wide band gap insulator, hexagonal boron nitride [9][10][11] and in semi-metallic WTe2 12 with the ability to switch the domain type achieved by sliding atomic planes along the interface. Here, we report an observation of robust room temperature ferroelectricity in marginally twisted semiconducting bilayers of the transition metal dichalcogenide (TMD), MoS2.…”
Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of novel 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 polarisation arranged into a twist-controlled network. The latter can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The interfacial charge transfer for the observed ferroelectric domains is quantified using Kelvin probe force microscopy and agrees well with theoretical calculations. The movement of domain walls and their bending rigidity also agrees well with our modelling results. 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 venue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.
“…Moiré superlattices induce a plethora of physical effects, such as long-range interlayer hybridization, leading to flat minibands with strongly correlated electronic states [2][3][4][5][6][7][8][9][10] and minibands for excitons in transition metal dichalcogenide (TMD) bilayers [11,12] at twist angles θ 10 • , for which the moiré periodicity exceeds the exciton Bohr radius, thus affecting the system's optoelectronic properties [13][14][15][16][17]. Moreover, piezoelectric effects caused by lattice reconstruction in TMD bilayers [1,18,19] create periodic traps for charge carriers [20,21] and excitons [22], whereas interlayer charge transfer [23,24] induces ferroelectric polarization in these structures [25][26][27].…”
Lattice reconstruction in twisted transition-metal dichalcogenide (TMD) bilayers gives rise to piezo-and ferroelectric moiré potentials for electrons and holes, as well as a modulation of the hybridization across the bilayer. Here, we develop hybrid k • p tight-binding models to describe electrons and holes in the relevant valleys of twisted TMD homobilayers with parallel (P) and antiparallel (AP) orientations of the monolayer unit cells. We apply these models to describe moiré superlattice effects in twisted WSe 2 bilayers, in conjunction with microscopic ab initio calculations, and considering the influence of encapsulation, pressure, and an electric displacement field. Our analysis takes into account mesoscale lattice relaxation, interlayer hybridization, piezopotentials, and a weak ferroelectric charge transfer between the layers, and it describes a multitude of possibilities offered by this system, depending on the choices of P or AP orientation, twist angle magnitude, and electron/hole valley.
“…The h-BN layer exhibits a large electrical bandgap (∼5.97 eV) ( Laturia et al., 2018 ), has an atomically smooth surface with little to no dangling bonds and/or surface trap states (C. R. Dean et al., 2010 ), and also serves as an ideal gate dielectric (6.82–6.93 in-plane) substrate for graphene and other 2D semiconductors ( Laturia et al., 2018 ). Some other notable characteristics of h-BN (as shown in Figure 1 ) are (A) dispersion of solution processed h-BN in water as well as in organic solvents ( Lin et al., 2011 ; Zhu et al., 2015 ), (B) ferroelectricity in AB stacked h-BN ( Yasuda et al., 2021 ), (C) permeability to thermal protons in its monolayer form ( Hu et al., 2014 ; Lozada-Hidalgo et al., 2016 ), (D) interlayer tunneling in a heterojunction solar cell or photodetectors or memory devices ( Vu et al., 2016 ; Wang et al., 2020 ; Won et al., 2021 ), (E) a gate dielectric substrate ( Behura et al., 2017 ), and (F) quantum emission ( Fröch et al., 2020 ; Schell et al., 2018 ; Yim et al., 2020 ). Figure 1 Crystal structure and properties of hexagonal boron nitride (h-BN), a 2D insulator (A–D) The unique properties of h-BN include (A) dispersion inks (Reprinted with permission from ( Zhu et al., 2015 ).…”
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