Room Temperature Ferroelectricity Research Articles

Stabilizing hidden room-temperature ferroelectricity via a metastable atomic distortion pattern

Nonequilibrium atomic structures can host exotic and technologically relevant properties in otherwise conventional materials. Oxygen octahedral rotation forms a fundamental atomic distortion in perovskite oxides, but only a few patterns are predominantly present at equilibrium. This has restricted the range of possible properties and functions of perovskite oxides, necessitating the utilization of nonequilibrium patterns of octahedral rotation. Here, we report that a designed metastable pattern of octahedral rotation leads to robust room-temperature ferroelectricity in CaTiO3, which is otherwise nonpolar down to 0 K. Guided by density-functional theory, we selectively stabilize the metastable pattern, distinct from the equilibrium pattern and cooperative with ferroelectricity, in heteroepitaxial films of CaTiO3. Atomic-scale imaging combined with deep neural network analysis confirms a close correlation between the metastable pattern and ferroelectricity. This work reveals a hidden but functional pattern of oxygen octahedral rotation and opens avenues for designing multifunctional materials.

Simultaneous normal – Anomalous dielectric dispersion and room temperature ferroelectricity in CBD perovskite BaTiO3 thin films

Chemical bath deposition (CBD) method, being an electroless technique, is used to prepare phase pure barium titanate (BaTiO3) thin films using glass as substrate. Molarity of the solution is varied from 0.1 to 1.2 M, whereas, solution temperature is kept constant at 78 °C. XRD analyses show formation of perovskite tetragonal phase of BaTiO3 at all the molarities when films are annealed at 300 °C. Strengthening in perovskite tetragonal phase is observed with increase in solution molarity. Ellipsometric results show high transmission (∼ 83 %) in the visible region for thin films prepared with 1.2 M solution. Variation in direct band gap, 3.31–3.61 eV, is observed for annealed thin films. High dielectric constant (∼ 539.26 at log f = 3) and low tangent loss is obtained for 1.2 M based thin films. Thin films prepared under all the conditions show normal and anomalous / U-shaped frequency dependent dielectric response (i.e. ∼ 2754.24 at log f = 1.5 and ∼ 2004.73 at log f = 7.5). Maximum spontaneous polarization of ∼ 14.78 μC/cm2 and remnant polarization of ∼ 6.94 μC/cm2 are observed for 1.2 M sample. It is important to mention here that U-shaped high dielectric and good ferroelectric properties of BaTiO3 are observed in current study with no change in perovskite structure of tetragonal phase making it a potential candidate for storage devices at low and high frequencies.

Impedance Spectroscopy and Photovoltaic Effect of Oxygen Defect Engineering on KNbO3 Ferroelectric Semiconductors

Perovskite-oxide (1 − x)KNbO3-xBaCo1/2Nb1/2O3−δ (KN-BCN; x = 0.00–0.20) ferroelectric semiconductor ceramics with oxygen defects are successfully prepared via a conventional solid-state sintering method. X-ray diffraction data indicate that the crystal symmetry evolves from orthogonal to tetragonal at increasing x values. Raman spectroscopic analysis confirms the long-range polarization of all compositions. X-ray photoelectron spectroscopy shows that the detailed chemical formula of 0.90KN-0.10BCN ceramics is 0.90KNbO3-0.10BaCo1/2Nb1/2O2.90. Room-temperature ferroelectricity weakens when the x value increases. The optical band gap narrows from 3.25 eV for x = 0.00 to 1.57 eV for x = 0.20, and the minimum value of ∼ 1.28 eV occurs in the 0.90KN-0.10BCN ceramic. Impedance analysis illustrates that the conduction mechanism of grains is mainly internal electron conduction, and that of the grain boundary is intrinsic conduction. The conducting mechanism of the ceramic system follows ohmic behavior by log I–log U curves. The maximum short-circuit photocurrent density and open-circuit photovoltage are 6.68 nA cm−2 and 0.80 V, and stable output is maintained. The KN-BCN ceramic system can be used in photovoltaic materials.

Remarkable enhancement in hybrid improper ferroelectricity of Ca3Ti2O7 ceramics by a simple sol-gel process

Ca3Ti2O7 with Ruddlesden-Popper structure is a typical hybrid improper ferroelectric with large polarization. But the room-temperature ferroelectricity of Ca3Ti2O7 ceramics prepared by a solid-state method is not good enough. In this work, Ca3Ti2O7 ceramics with remarkably enhanced hybrid improper ferroelectricity have been successfully fabricated by a simple tartaric acid sol-gel method. The ceramic sample exhibits the highest remnant polarization (4.32 μC/cm2) to our knowledge and lower coercive electric field (108.7 kV/cm) at room temperature. The enhanced ferroelectricity of Ca3Ti2O7 ceramics results from the larger antiferroelectric distortion displacements and the larger grain with irregular polygon features. These findings are beneficial for developing room-temperature single phase multiferroic materials with strong magnetoelectric coupling and excellent ferroelectricity.

Excellent ferroelectricity of 50 nm-thick doped HfO2 thin films induced by annealing with a rapid-heating-temperature process

The ferroelectric HfO2 thin film has attracted a lot of research interest due to being Pb free and its excellent compatibility with the Si-based semiconductor process. However, methods to obtain thicker HfO2 thin films with strong ferroelectricity have yet to be explored. In this work, a 50 nm-thick La-doped HfO2 thin film was prepared using pulsed laser deposition, and significant room temperature ferroelectricity with a remnant polarization (Pr) of 27 µC/cm2 was achieved through annealing in N2 with a rapid-heating-temperature process. The ferroelectricity is mainly related to the increase in the content of the (002)-oriented orthogonal phase formed by the rapid-heating-temperature treatment. Furthermore, this special annealing process was verified in a 50 nm-thick Tm-doped HfO2 film, and the Pr of 48 µC/cm2 was observed. This value is the highest value reported so far in doped HfO2 films with a thickness of 50 nm or greater. These results provide a new approach to prepare thicker ferroelectric HfO2-based thin films.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity.

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN2O4} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field μ0H from 0 to 2 T decreases linearly from 0.041 to 0.011 μC/cm2 at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Strain-induced room-temperature ferroelectricity in SrTiO3 membranes

Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

Ferroelectric-field accelerated charge transfer in 2D CuInP2S6 heterostructure for enhanced photocatalytic H2 evolution

Abstract The development of ferroelectric photocatalytic materials with polarization electric field is a key approach to realize the spatial separation and fast transfer of charge carriers in visible-light-driven H2 evolution. Generally, replacing traditional 3D perovskite-type ferroelectric materials with 2D ferroelectric materials is disregarded as candidates for photocatalysis. Herein a 2D CuInP2S6 (CIPS) with room-temperature ferroelectricity (a Curie Temperature of around 47 °C) is developed as a new photocatalyst, and 2D/2D heterojunction of CuInP2S6 nanosheet/g-C3N4 ultrathin flake (CIPS/CN) is constructed to further accelerate charge transfer. Benefitting from the synergetic action of the inner polarization electric field of CIPS and 2D/2D heterojunction, CIPS/CN displays a substantially accelerated charge transfer and significantly enhanced photocatalytic H2 evolution rate, which is up to 7.6 times by contrast with that of paraelectric-phase CIPS. This work would provide a new platform for the design of 2D ferroelectric photocatalytic system with highly-efficient charge transfer.

Symmetry degeneration and room temperature ferroelectricity in ion-irradiated SrTiO3

Polar phonon modes associated with room temperature ferroelectricity are observed in SrTiO3 single crystals irradiated with Ti ions. Quantitative strain analysis reveals that irradiation-induced out-of-plane strain drives the centrosymmetric cubic SrTiO3 to a tetragonal-like structure in the maximum damaged region. Energy transfer from ions to electrons during ion irradiation yields defects in SrTiO3 that also plays an important role for the room temperature ferroelectricity. Different from thin film techniques, the ferroelectricity in the ion irradiated SrTiO3 can occur for much larger thicknesses, depending on the energy and type of ion.

Room temperature ferromagnetism and ferroelectricity in double perovskite Y2NiMnO6 thin film

In this work, we synthesized epitaxial Y2NiMnO6 thin film by Pulsed Laser Deposition technique in two different atmospheres (oxygen and argon) and studied their magnetic as well as electric polarization. Room temperature ferromagnetism and ferroelectricity are exhibited by these Y2NiMnO6 thin films, indicating the coexistence of more than one ferroic interaction in this system. Here the surface roughness increases the total surface area and hence the unsaturated surface spins of the films which control both the ferromagnetic as well as ferroelectric order.

Structural, optical, and enhanced multiferroic properties of xCoFe2O4-(1 − x)K0.5Bi0.5TiO3 ferrite–ferroelectric composites

Multiferroic ferrite–ferroelectric composites with general formula xCoFe2O4-(1 − x)K0.5Bi0.5TiO3 (x = 0.0, 0.3, 0.5, 0.7, and 1.0) (xCFO-KBT) were developed by conventional solid-state reaction method. The structural, optical, magnetic and ferroelectric properties for the prepared samples were studied in detail. Two individual structures of spinel-type ferrite and tetragonal-type ferroelectric are characterized through X-ray diffraction while Raman scattering spectra and SEM analyses furtherly confirm the coexistence of the two phases in the composites. The bandgaps calculated for the composites narrow dramatically compared with the pure KBT, which are attributed to Co2+ d–d on-site transitions from the CFO phase. The magnetic property transforms from diamagnetism for pure KBT to ferromagnetism for the composites and pure CFO with saturation magnetization Ms increasing gradually with addition of CFO. Besides, the room temperature ferroelectricity has been detected in the composites, which turns out to be enhanced by space charge contribution of CFO phase. The improvement of optical and multiferroic properties in the composites indicates great prospect on visible-light optical devices and non-volatile data storage.

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.

Room temperature multiferroic properties and polymorphic phase transition-induced noticeable magnetodielectric anomalies in Fe/Co co-doped (K0.52Na0.48)NbO3 ceramics

Abstract Polycrystalline ceramics of (K0.52Na0.48)Nb1-x(FeCo)xO3 (0 ≤ x ≤ 0.02) are synthesized using solid-state reaction method. Appropriate Fe/Co doping (x = 0.010 and 0.015) not only optimizes room temperature ferroelectricity of KNNFC ceramics, but also effectively introduces room temperature ferromagnetism and large magnetodielectric effects. The Fe2+/3+-O-Co2+/3+ ferromagnetic double exchange is considered the main reason for the enhancement in ferromagnetic. Moreover, magnetic anomalies and noticeable magnetodielectric (MD) peaks are found near the ferroelectric rhombohedral-orthorhombic (R–O) polymorphic phase transition, indicating the presence of spin-lattice coupling in these samples. Meanwhile, the Fe2+-Fe3+, Co2+-Co3+, and/or Fe2+/3+-Co2+/3+ local dipoles that begin to appear near the R–O phase transition temperature can also contribute to the MD effect. When x = 0.015, the MD peak is as large as +12% (0.9 T, 106 Hz, 120 K), and the room temperature MD value reaches −7% (0.9 T, 106 Hz).

Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO3 films

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO3. Here, using 3D straining in nanocomposite films of (SmMnO3)0.5((Bi,Sm)2O3)0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with TC,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (−3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)2O3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO3 films. Also, while bulk SmMnO3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.

Room-temperature ferroelectricity in A-site ordered Ruddlesden-Popper Li2CaTa2O7 ceramics

In the present work, the crystal structure, ferroelectric and dielectric properties of the dense single-phase Li2CaTa2O7 ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investigated by the experiments and first-principles calculations. A polar Pna21 phase was determined by the Rietveld refinement against the X-ray diffraction pattern at room temperature, and it was confirmed by its lowest calculated energy and rigid phonon modes. The ferroelectricity was found by observing the ferroelectric hysteresis loop at room temperature, and its remanent polarization was similar to the value calculated from the Berry phase and Born effective charge approaches. The ferroelectricity is a non-conventional proper one since its polarization is induced from the displacement of oxygen anions instead of tantalum cations based on the individual atomic polarization. A first-order transition from an antiferroelectric to paraelectric phase around 900 K was found by the DSC and variable-temperature dielectric measurements. From the present work, the room-temperature ferroelectricity is experimental confirmed in the present ceramics, inspiring the search for new ferroelectricity in the ceramics with A-site ordered Ruddlesden-Popper structures.

(Invited) 2D NC-FET, FE-FET and FeS-FET

The so-called Boltzmann Tyranny defines the fundamental thermionic limit of the subthreshold slope (SS) of a metal-oxide-semiconductor field-effect transistor (MOSFET) at 60 mV/dec at room temperature and, therefore, precludes the lowering of the supply voltage and the overall power consumption. Adding a ferroelectric dielectric as a negative capacitor to the gate stack of a MOSFET may offer a promising solution to bypassing this fundamental barrier. Meanwhile, two-dimensional (2D) semiconductors, such as atomically thin transition metal dichalcogenides (TMDs) due to their low dielectric constant, and ease of integration in a junctionless transistor topology, offer enhanced electrostatic control of the channel. In this talk, we will review the recent progress on negative capacitance field-effect transistors (NC-FET) and ferroelectric field-effect transistors (Fe-FET) using TMDs as the transistor channels. [1,2] More importantly, a new device concept, which we call as ferroelectric semiconductor field-effect transistor (FeS-FET), was proposed and experimentally demonstrated. [3] In this novel FeS-FET, a 2D ferroelectric semiconductor α-In2Se3 is used to replace conventional semiconductor as channel. α-In2Se3 is identified due to its proper bandgap, room temperature ferroelectricity, the ability to maintain ferroelectricity down to a few atomic layers and the feasibility for large-area growth.[1] M. Si et al. Nature Nanotechnology 13, 24-28 (2018).[2] M. Si et al. 2017 IEDM, pp. 573-576 (2017).[3] M. Si et al. Nature Electronics 2, in press (2019).

Morpho-Structural, Electrical and Magnetic Behavior of Chemically Synthesized Multiferroic GdMnO₃ Ceramic.

The observations of room temperature ferroelectricity and ferromagnetism for GdMnO₃ (GMO) ceramics have been reported in this paper. The nanocrystalline GMO powder was synthesized by chemical route and then sintered in the form of bulk pellet at 850 °C for 24 h by following slow step sintering schedule. Bulk GMO sample is observed to be polycrystalline in nature having orthorhombic perovskite structure and Pbnm space group. Well connected grains are observed with higher percentage of porosity in the micrographs obtained from field emission scanning electron microscope. The dielectric anomaly has been observed for the sintered sample at 338 K and a wide ferroelectric hysteresis loop has also been observed at room temperature. Room temperature dielectric constant (ε) value of the sample is high (~2736) at the frequency of 100 Hz. The zero field cooled (ZFC) and field cooled (FC) magnetizations shows the antiferromagnetic behavior around 12 K. Sample shows a weak ferromagnetic behavior at 3 K.

Multiferroic LuFeO3 on GaN by molecular-beam epitaxy

Hexagonal LuFeO3 exhibiting ferroelectricity and weak ferromagnetism is grown on metal-polar GaN by molecular-beam epitaxy. The oxide films exhibit smooth surface morphologies and are found to be single crystalline with an epitaxial relationship related by a 30° in-plane rotation relative to the GaN crystal structure. The LuFeO3 layers grown on GaN exhibit room-temperature ferroelectricity and low-temperature magnetic ordering. This epitaxial integration creates a heterostructure platform to explore and exploit the coupling of the ferroelectricity and magnetism of oxides with the strong spontaneous and piezoelectric polarization and the unique electronic and photonic properties of nitride semiconductors.

Origin and tuning of room-temperature multiferroicity in Fe-doped BaTiO3

Simultaneous co-existence of room-temperature(T) ferromagnetism and ferroelectricity in Fe doped BaTiO$_3$ (BTO) is intriguing, as such Fe doping into tetragonal BTO, a room-T ferroelectric (FE), results in the stabilization of its hexagonal polymorph which is FE only below $\sim$80K. Here, we investigate its origin and show that Fe-doped BTO has a mixed-phase room-temperature multiferroicity, where the ferromagnetism comes from the majority hexagonal phase and a minority tetragonal phase gives rise to the observed weak ferroelectricity. In order to achieve majority tetragonal phase (responsible for room-T ferroelectricity) in Fe-doped BTO, we investigate the role of different parameters which primarily control the PE hexagonal phase stability over the FE tetragonal one and identify three major factors namely, the effect of ionic size, Jahn-Teller (J-T) distortions and oxygen vacancies (OVs), to be primarily responsible. The effect of ionic size which can be qualitatively represented using the Goldschmidt's tolerance (GT) factor seems to be the major dictating factor for the hexagonal phase stability. The understanding of these factors not only enables us to control them but also, achieve suitable co-doped BTO compound with enhanced room-T multiferroic properties.

Low-magnetic-field controlled magnetodielectric coupling at room temperature in a chiral layered diphenylalanine-based 1D-coordination-polymer

Materials with low-magnetic-field control at room temperature is of fundamental importance for the practical application. Herein, a chiral 1D-coordination-polymer ([Ni(DPA)2(H2O)]) possessing coupled ferroelectric and ferromagnetic order is constructed. In addition, [Ni(DPA)2(H2O)] exhibits room-temperature ferroelectricity with giant magnetodielectric coefficency between −18% and −23% in the temperature range from 270 to 310 K, in nearly all measured frequencies, and under applied magnetic field as low as 0.5 T. The asymmetrical distortion of the coordination bond spurs electrical polarization and magnetodielectric effect, and reduces spin frustration in the octahedral geometry due to spin-lattice coupling. In addition, 2D nanosheets with non-centrosymmetric structure can be successfully exfoliated from [Ni(DPA)2(H2O)] microcrystals in retaining the morphological and structural integrity. This observation reveal a new class of magnetodielectric material and represent an important step towards practical device applications using the magnetodielectric coupling by exploiting the asymmetrical distortion of coordination bonds in lead-free organic-inorganic coordination complex.