In-plane Ferroelectricity Research Articles

Purely in-plane ferroelectricity in monolayer SnS at room temperature

2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometer-size monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd–even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics.

Structural and electronic properties of two-dimensional freestanding BaTiO3/SrTiO3 heterostructures

The successful preparation of the freestanding perovskite materials down to the monolayer limit [Ji et al., Nature (London) 570, 87 (2019)] provided the opportunity to make the two-dimensional (2D) oxide and heterostructure, which could be significantly distinctive from the conventional oxide superlattices and other 2D van der Waals heterostructures. By stacking one unit-cell $\mathrm{BaTi}{\mathrm{O}}_{3}$ (BTO) and one unit-cell $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO) on top of each other, we constructed two isolated bilayers of the 2D heterostructure systems. From our density functional theory simulation, their ground states exhibit an in-plane ferroelectricity in both BTO layer and STO layer, while the antiferrodistortive mode of the STO layer is totally suppressed. These two systems show band gaps in the range of 2--2.5 eV (by using HSE06), which are smaller than their monolayer and bulk phases. The layer arrangement strongly influences their electronic properties. We reveal that they adopt the type-II electronic band alignment. The tensile biaxial strain can strongly promote the ferroelectricity and increase the band gaps of these systems. Our results will contribute to the further understanding of layered materials based on the transition metal oxide perovskites and developing relevant experimental devices.

Electronic Band Structure of In-Plane Ferroelectric van der Waals β′-In2Se3

Layered indium selenides ($In_{2}Se_{3}$) have recently been discovered to host robust out-of-plane and in-plane ferroelectricity in the $\alpha$ and $\beta$' phases, respectively. In this work, we utilise angle-resolved photoelectron spectroscopy to directly measure the electronic bandstructure of $\beta '-In_{2}Se_{3}$, and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Due to n-type doping we are able to observe the conduction band minima, and directly measure the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the $\overline{M}$ points, yielding effective masses of 0.21 $m_{0}$ along $\overline{KM}$ and 0.33 $m_{0}$ along $\overline{\Gamma M}$. The measured band structure is well supported by hybrid density functional theory calculations. The highly two-dimensional (2D) bandstructure with moderate bandgap and small effective mass suggest that $\beta'-In_{2}Se_{3}$ is a potentially useful new van der Waals semiconductor. This together with its ferroelectricity makes it a viable material for high-mobility ferroelectric-photovoltaic devices, with applications in non-volatile memory switching and renewable energy technologies.

Emergence of the giant out-of-plane Rashba effect and tunable nanoscale persistent spin helix in ferroelectric SnTe thin films

A non-vanishing electric field inside a non-centrosymmetric crystal transforms into a momentum-dependent magnetic field, namely, a spin–orbit field (SOF). SOFs are of great use in spintronics because they enable spin manipulation via the electric field. At the same time, however, spintronic applications are severely limited by the SOF, as electrons traversing the SOF easily lose their spin information. Here, we propose that in-plane ferroelectricity in (001)-oriented SnTe thin films can support both electrical spin controllability and suppression of spin dephasing. The in-plane ferroelectricity produces a unidirectional out-of-plane Rashba SOF that can host a long-lived helical spin mode known as a persistent spin helix (PSH). Through direct coupling between the inversion asymmetry and the SOF, the ferroelectric switching reverses the out-of-plane Rashba SOF, giving rise to a maximally field-tunable PSH. Furthermore, the giant out-of-plane Rashba SOF seen in the SnTe thin films is linked to the nano-sized PSH, potentially reducing spintronic device sizes to the nanoscale. We combine the two ferroelectric-coupled degrees of freedom, longitudinal charge and transverse PSH, to design intersectional electro-spintronic transistors governed by non-volatile ferroelectric switching within nanoscale lateral and atomic-thick vertical dimensions.

First-principles screening of novel ferroelectric MXene phases with a large piezoelectric response and unusual auxeticity.

Out-of-plane ferroelectricity in the two-dimensional (2D) regime shows great promise for applications in next-generation electronic devices but has been rarely reported. Herein, by using high-throughput search and density functional theory (DFT) calculations, three types of ferroelectric MXene phases (type-I: Nb2CS2 and Ta2CS2; type-II: Sc2CO2 and Y2CO2; and type-III: Sc2CS2 and Y2CS2) are for the first time predicted to be achievable by surface functionalization of 2D MXenes. The identified 2D ferroelectric MXenes not only demonstrate remarkable and reversible spontaneous electric polarization along both the out-of-plane and in-plane orientations but also exhibit giant out-of-plane and in-plane piezoelectric response to the external strain. More excitingly, the type-III ferroelectric MXenes possess rare out-of-plane auxeticity that can be triggered by the tensile strain along both the zig-zag and the arm-chair directions. The intriguing ferroelectricity, large piezoelectric response, and auxeticity render the ferroelectric MXenes versatile candidates for nanoscale electronic and mechanic devices.

Ultrahigh-strain ferroelasticity in two-dimensional honeycomb monolayers: from covalent to metallic bonding

We propose a possible ferroelastic switching pathway of two-dimensional (2D) honeycomb lattice (including graphene, BN, stanene, etc.) that may swap its armchair and zigzag direction, reversing an unprecedented strain of 73.2%. Our ab initio calculations reveal that such pathway cannot work in covalent systems like graphene and BN; for monolayer with metallic bonds like stanene, stanane and InBi that have all been synthesized, however, such pathway can be feasible with a low switching barrier (<0.15 eV) and stress (<graphene upon 1% tensile strain), also with the highest energy/stress point in the elastic region. Their distinct behaviors are attributed to the different feature of covalent bonds and metallic bonds: the former is rigid with directionality, while the latter is malleable with ductility. A general trend of linear decrease in switching barrier with uprising metallicity for the same group compounds is revealed. Similar behaviors can be extended to bulk zinc-blended or wurtzite structure that can be deemed as multilayer stacking of buckled monolayer. Binary compounds like InBi monolayer are even multiferroics with both in-plane and vertical ferroelectricity as well as nontrivial topological properties.

Intrinsic persistent spin helix state in two-dimensional group-IV monochalcogenide MX monolayers ( M=Sn or Ge and X=S , Se, or Te)

Energy-saving spintronics are believed to be implementable on systems hosting the persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically nontrivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide $MX$ monolayer, a new class of the noncentrosymmetric 2D materials having in-plane ferroelectricity. Due to the ${C}_{2v}$ point-group symmetry in the $MX$ monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calculations on various $MX$ ($M=$ Sn, Ge; $X=$ S, Se, Te) monolayers confirmed that such typical spin configuration is observed, in particular, at near the valence-band maximum where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelectric polarization, indicating that an electrically controllable PSH for spintronic applications is plausible.

Gate-Tunable In-Plane Ferroelectricity in Few-Layer SnS.

Ultrathin ferroelectrics hold great promise for modern miniaturized sensors, memories, and optoelectronic devices. However, in most ferroelectric materials, polarization is destabilized in ultrathin films by the intrinsic depolarization field. Here we report robust in-plane ferroelectricity in few-layer tin sulfide (SnS) 2D crystals that is coupled anisotropically to lattice strain. Specifically, the intrinsic polarization of SnS manifests as nanoripples along the armchair direction due to a converse piezoelectric effect. Most interestingly, such nanoripples show an odd-and-even effect in terms of its layer dependence, indicating that it is highly sensitive to changes in inversion symmetry. Ferroelectric switching is demonstrated in field-effect transistor devices fabricated on ultrathin SnS films, in which a stronger ferroelectric response is achieved at negative gate voltages. Our work shows the promise of 2D SnS in ultrathin ferroelectric field-effect transistors as well as nanoscale electromechanical systems.

Room-temperature out-of-plane and in-plane ferroelectricity of two-dimensional β-InSe nanoflakes

Two-dimensional (2D) layered semiconductors have shown great application potential in next generation nanoelectronic devices. The ferroelectric and piezoelectric properties of 2D semiconductors are also highly desirable in many applications, such as ferroelectric nonvolatile memory and switch. In the present work, we experimentally demonstrate the simultaneous out-of-plane and in-plane ferroelectricity of β-indium selenide (β-InSe) nanoflakes at room temperature. The polarization switching in the as-prepared β-InSe with the P63/mmc symmetry is studied by piezoresponse force microscopy. Out-of-plane polarization hysteresis loops are observed in a 7-nm-thick sample, and the in-plane and out-of-plane ferroelectric switching under the forward and reverse direct current bias are obtained in a 10-nm-thick sample at room temperature. These results indicate that β-InSe is a promising intrinsic 2D van der Waals ferroelectric material. Our work has connected the 2D materials with ferroelectric materials and inspired their applications in electronic devices.

Atomic-Scale Observation of Reversible Thermally Driven Phase Transformation in 2D In2Se3

Phase transformation in emerging two-dimensional (2D) materials is crucial for understanding and controlling the interplay between structure and electronic properties. In this work, we investigate 2D In2Se3 synthesized via chemical vapor deposition, a recently discovered 2D ferroelectric material. We observed that In2Se3 layers with thickness ranging from a single layer to ∼20 layers stabilized at the β phase with a superstructure at room temperature. At around 180 K, the β phase converted to a more stable β' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven β-to-β' phase transformation was investigated by temperature-dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, density functional theory calculations reveal in-plane ferroelectricity in the β' phase. Scanning tunneling spectroscopy measurements show that the indirect bandgap of monolayer β' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 with thickness down to the monolayer limit and the associated electronic properties will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications (e.g., nonvolatile memory devices).

Substrate effects on the in-plane ferroelectric polarization of two-dimensional SnTe

Recent experiments confirm that two-dimensional (2D) SnTe films possess strong in-plane ferroelectricity. For controlling or improving this 2D ferroelectricity, the substrate effects should be a prevailing concern. In this work first-principle calculations are performed to investigate the substrate effects on the ferroelectric polarization in this 2D system. Our results indicate that suitable substrates can significantly enhance the ferroelectricity of 2D SnTe. We investigated a series of metals, semiconductors, and insulator materials used as the substrates (Ni, Pd, Pt, Si, Ge, CaO, MgO, etc.), which have different interaction types and bonding strength with the 2D SnTe. For these SnTe/substrate heterostructures, both geometric and electronic structures are explored. It is found that in Pt- and MgO-supported SnTe films, the in-plane polar displacement is significantly enhanced, which can be understood under the theory of lone-pair electrons. Importantly, the substrates show very interesting controlling behaviors for the ferroelectricity of SnTe originating from electronic level, such as enhancing ferroelectric polarization and inducing magnetism. In SnTe/MgO(001) heterostructure, substrate-enhanced ferroelectricity is discussed in detail. In addition, substrate-induced magnetism of SnTe films is observed in Ni-supported SnTe, which indicates a possible scheme to obtain the multiferroics in 2D SnTe. This study suggests that it is a potential method to tune the ferroelectricity of 2D SnTe films by different substrates.

Reversible out-of-plane spin texture in a two-dimensional ferroelectric material for persistent spin helix

Although spin-orbit coupling (SOC) in ferroelectrics contributes to the manipulation of the electron spin by an electric field, it also breaks the spin rotation symmetry resulting in the decay of spin polarization. Here, we showed that the [110] Dresselhaus model in the two-dimensional ferroelectrics with in-plane ferroelectricity can host a persistent spin helix and render the spin lifetime infinite. Taking a ${\mathrm{WO}}_{2}{\mathrm{Cl}}_{2}$ monolayer as an example, we demonstrated from first principles that SOC leads to a sizable [110] Dresselhaus-type band splitting near the conduction-band minimum accompanied by an out-of-plane Dresselhaus-type spin texture which is decisive for a long spin lifetime. More interestingly, such a spin texture can be reversed by switching the ferroelectric polarization, suggesting the fully electrically controllable persistent spin helix. Our findings uncover the possibility of an electrically controllable persistent spin helix in ferroelectrics, offering a promising platform for novel spintronics applications.

Enhanced in-plane ferroelectricity in BaTiO3 thin films fabricated by aqueous chemical solution deposition

Ferroelectric BaTiO3 is widely used in capacitors, but the low Curie temperature limits a further use of BaTiO3. In this work we present an aqueous chemical solution deposition (CSD) route for BaTiO3 thin films, demonstrating that organic solvents are not required for CSD. Textured BaTiO3 thin films were deposited on SrTiO3 substrates. The in-plane dielectric properties were investigated using interdigitated electrodes and ferroelectric switching was observed up to 160±5 °C. The increased Curie temperature is proposed to result from thermal strain due to a mismatch in thermal expansion coefficient between the film and the substrate, and is in good agreement with the theory of strain engineering in BaTiO3. Finally, the decomposition and crystallization of BaTiO3 during thermal treatment were determined by the combination of thermal analysis, IR spectroscopy and X-ray diffraction of powder prepared from the solution.

Room temperature in-plane ferroelectricity in van der Waals In2Se3.

Van der Waals (vdW) assembly of layered materials is a promising paradigm for creating electronic and optoelectronic devices with novel properties. Ferroelectricity in vdW layered materials could enable nonvolatile memory and low-power electronic and optoelectronic switches, but to date, few vdW ferroelectrics have been reported, and few in-plane vdW ferroelectrics are known. We report the discovery of in-plane ferroelectricity in a widely investigated vdW layered material, β'-In2Se3. The in-plane ferroelectricity is strongly tied to the formation of one-dimensional superstructures aligning along one of the threefold rotational symmetric directions of the hexagonal lattice in the c plane. Surprisingly, the superstructures and ferroelectricity are stable to 200°C in both bulk and thin exfoliated layers of In2Se3. Because of the in-plane nature of ferroelectricity, the domains exhibit a strong linear dichroism, enabling novel polarization-dependent optical properties.

Tunable ferroelectricity and anisotropic electric transport in monolayer β -GeSe

Low-dimensional ferroelectricity has attracted tremendous attention due to its huge potential in device applications. Here, based on first-principles calculations, we predict the existence of spontaneous in-plane electrical polarization and ferroelectricity in monolayer $\beta$-GeSe, a polymorph of GeSe with a boat conformation newly synthesized in experiment. The magnitude of the polarization is about $0.16$ nC/m, which is comparable to that of monolayer SnTe studied in recent experiment, and the intrinsic Curie temperature is estimated to be above 200 K. Interestingly, owing to its puckered structure, the physical properties of $\beta$-GeSe can be easily controlled by strain. The Curie temperature can be raised above room temperature by applying a ($\sim 1\%$) tensile strain, and the magnitude of polarization can be largely increased by strains in either armchair or zigzag directions. Furthermore, we find that for the case with electron doping, applying strain can readily tune the anisotropic electric transport with the preferred conducting direction rotated by $90^\circ$, which is connected with a strain-induced Lifshitz transition. The ratio between the effective masses along the two in-plane directions can undergo a dramatic change of two orders of magnitude even by a 2% strain. Our result reveals monolayer $\beta$-GeSe as a promising platform for exploring ferroelectricity in two-dimensions and for nanoscale mechano-electronic device applications.

New Ferroelectric Phase in Atomic-Thick Phosphorene Nanoribbons: Existence of in-Plane Electric Polarization

Ferroelectrics have many significant applications in electric devices, such as capacitor or random-access memory, tuning the efficiency of solar cell. Although atomic-thick ferroelectrics are the necessary components for high-density electric devices or nanoscale devices, the development of such materials still faces a big challenge because of the limitation of intrinsic mechanism. Here, we reported that in-plane atomic-thick ferroelectricity can be induced by vertical electric field in phosphorene nanoribbons (PNRs). Through symmetry arguments, we predicted that ferroelectric direction is perpendicular to the direction of external electric field and lies in the plane. Further confirmed by the comprehensive first-principles calculations, we showed that such ferroelectricity is induced by the electron-polarization, which is different from the structural distortion in traditional ferroelectrics and the recent experimental discovery of in-plane atomic-thick ferroelectrics (Science 2016, 353, 274). Moreover, we found that the value of electronic polarization in bilayer is much larger than that in monolayer. Our results show that electron-polarization ferroelectricity maybe the most promising candidate for atomic-thick ferroelectrics.

Ferroelectricity and Phase Transitions in Monolayer Group-IV Monochalcogenides.

Ferroelectricity usually fades away as materials are thinned down below a critical value. We reveal that the unique ionic-potential anharmonicity can induce spontaneous in-plane electrical polarization and ferroelectricity in monolayer group-IV monochalcogenides MX (M=Ge, Sn; X=S, Se). An effective Hamiltonian has been successfully extracted from the parametrized energy space, making it possible to study the ferroelectric phase transitions in a single-atom layer. The ferroelectricity in these materials is found to be robust and the corresponding Curie temperatures are higher than room temperature, making them promising for realizing ultrathin ferroelectric devices of broad interest. We further provide the phase diagram and predict other potentially two-dimensional ferroelectric materials.

Epitaxial Bi5Ti3FeO15–CoFe2O4 Pillar–Matrix Multiferroic Nanostructures

Epitaxial self-assembled ferro(i)magnetic spinel (CoFe2O4 (CFO)) and ferroelectric bismuth layered perovskite (Bi5Ti3FeO15 (BTFO)) pillar-matrix nanostructures are demonstrated on (001) single-crystalline strontium titanate substrates. The CFO remains embedded in the BTFO matrix as vertical pillars (∼50 nm in diameter) up to a volume fraction of 50%. Piezoresponse force microscopy experiments evidence a weak out-of-plane and a strong in-plane ferroelectricity in the BTFO phase, despite previously reported paraelectricity along the c-axis in a pure BTFO film. Phenomenological Landau-Ginzburg-Devonshire-based thermodynamic computations show that the radial stress induced by the CFO nanopillars can influence these ferroelectric phases, thus signifying the importance of the nanopillars. The CFO pillars demonstrate robust ferromagnetic hysteresis loops with little degradation in the saturation magnetization (ca. 4 μB/f.u.). Thus BTFO-CFO nanocomposites show significant promise as a lead-free magnetoelectric materials system.

Proton transfer in surface-stabilized chiral motifs of croconic acid

The structure and cooperative proton ordering of two-dimensional sheets of croconic acid were studied with scanning tunneling microscopy and first-principles calculations. Unlike in the crystalline form, which exhibits a pleated, densely packed polar sheet structure, the confinement of the molecules to the surface results in hydrogen-bonded chiral clusters and networks. First-principles calculations suggest that the surface stabilizes networks of configurational isomers, which arise from direct hydrogen transfer between their constituent croconic acid monomers. Some of these configurations have a net polarization. It is demonstrated through constrained molecular dynamics simulations that simultaneous proton transfer between any two molecules can occur spontaneously. This finding is a prerequisite for the occurrence of in-plane ferroelectricity based on proton transfer in 2D sheets.

How to Choose In-plane Ferroelectric Polarization States in Rhombohedral BiFeO3

How to Choose In-plane Ferroelectric Polarization States in Rhombohedral BiFeO₃