Flexoelectric Engineering of Bulk Photovoltaic Photodetector

The bulk photovoltaic effect (BPVE) offers an interesting approach to generate a steady photocurrent in a single-phase material under homogeneous illumination, and it has been extensively investigated in ferroelectrics exhibiting spontaneous polarization that breaks inversion symmetry. Flexoelectricity breaks inversion symmetry via a strain gradient in the otherwise nonpolar materials, enabling manipulation of ferroelectric order without an electric field. Combining these two effects, we demonstrate active mechanical control of BPVE in suspended 2-dimensional CuInP2S6 (CIPS) that is ferroelectric yet sensitive to electric field, which enables practical photodetection with an order of magnitude enhancement in performance. The suspended CIPS exhibits a 20-fold increase in photocurrent, which can be continuously modulated by either mechanical force or light polarization. The flexoelectrically engineered photodetection device, activated by air pressure and without any optimization, possesses a responsivity of 2.45 × 10-2 A/W and a detectivity of 1.73 × 1011 jones, which are superior to those of ferroelectric-based photodetection and comparable to those of the commercial Si photodiode.

A Chiral B-N Adduct as a New Frontier in Ferroelectrics and Piezoelectric Energy Harvesting

Within the burgeoning field of electronic materials, B-N Lewis acid-base pairs, distinguished by their partial charge distribution across boron and nitrogen centers, represent an underexplored class with significant potential. These materials exhibit inherent dipoles and are excellent candidates for ferroelectricity. However, the challenge lies in achieving the optimal combination of hard-soft acid-base pairs to yield B-N adducts with stable dipoles. Herein, we present an enantiomeric pair of B-N adducts [R/SC6H5CH(CH3)NH2BF3] (R/SMBA-BF3) crystallizing in the polar monoclinic P21 space group. The ferroelectric measurements on RMBA-BF3 gave a rectangular P-E hysteresis loop with a remnant polarization of 7.65 μC cm-2, a value that aligns with the polarization derived from the extensive density-functional theory computations. The PFM studies on the drop-casted film of RMBA-BF3 further corroborate the existence of ferroelectric domains, displaying characteristic amplitude-bias butterfly and phase-bias hysteresis loops. The piezoelectric nature of the RMBA-BF3 was confirmed by its direct piezoelectric coefficient (d33) value of 3.5 pC N-1 for its pellet. The piezoelectric energy harvesting applications on the sandwich devices fabricated from the as-made crystals of RMBA-BF3 gave an open circuit voltage (VPP) of 6.2 V. This work thus underscores the untapped potential of B-N adducts in the field of piezoelectric energy harvesting.

Ferroelectric polymorphic phenomena in the layered antiferromagnet Cu(OH)2

Ferroic orders and their associated structural phase transitions are paramount in the understanding of a multitude of unconventional condensed matter phenomena. On this note, our investigation focuses on the polymorphic ferroelectric phase transitions of Copper(II) hydroxide, Cu(OH)2, considering an antiferromagnetic ground state. By employing the first-principles studies and group theory analysis, we have provided a systematic theoretical investigation of vibrational properties in the hypothetical Cmcm high-symmetry phase to unveil the symmetry-allowed ferroic phases. We identified a non-polar to polar (Cmc21) phase transition, in which the displacive transformation is primarily responsible for the phase change induced by two B1u(i.e. Γ-2) phonon modes within the centrosymmetric phase. We also observed the existence of two polar structures with the same space group and different degrees of polarization (i.e Ps= 3.06 µC.cm-2and Ps= 42.41 µC.cm-2), emerging from the high symmetry non-polar structure. According to the structural analysis the ferroelectric order, of a geometric nature, is driven by the Γ-2mode in which the O- and H sites displacements lead the polar distortion with a minor contribution from the Cu-sites. Interestingly, the 3d9:Cu2+Jahn-Teller distortion coupled with the orientational shifts of O-H atoms enhances the polarization. .

Ferrite bismuth-based nanomaterials: From ferroelectric and piezoelectric properties to nanomedicine applications

Bismuth ferrite (BiFeO3), a perovskite-type oxide, possesses unique morphology and multiferroicity, rendering it highly versatile for various applications. Recent investigations have demonstrated that BiFeO3 exhibits enhanced Fenton-like and photocatalytic behaviors, coupled with its piezoelectric/ferroelectric properties. BiFeO3 can catalytically generate highly oxidative reactive oxygen species (ROS) when exposed to hydrogen peroxide or light irradiation. Consequently, bismuth ferrite-based nanomaterials have emerged as promising candidates for various biomedical applications. However, the precise fabrication of BiFeO3-based materials with controllable features and applications in diverse biomedical scenarios remains a formidable challenge. In this review, we initially summarize the Fenton reaction property, ferroelectric, and piezoelectric properties of BiFeO3. We further survey the current methodologies for synthesizing BiFeO3 nanomaterials with diverse morphologies. Subsequently, we explore the effects of element doping and heterojunction formation on enhancing the photocatalytic activity of BiFeO3, focusing on microstructural, electronic band structure, and modification approaches. Additionally, we provide an overview of the recent advancements of BiFeO3-based nanomaterials in biomedicine. Finally, we discuss the prevailing obstacles and prospects of BiFeO3 for biomedical applications, offering valuable insights and recommendations for forthcoming research endeavors.

Role of oxygen vacancies in ferroelectric or resistive switching hafnium oxide

HfO2 shows promise for emerging ferroelectric and resistive switching (RS) memory devices owing to its excellent electrical properties and compatibility with complementary metal oxide semiconductor technology based on mature fabrication processes such as atomic layer deposition. Oxygen vacancy (Vo), which is the most frequently observed intrinsic defect in HfO2-based films, determines the physical/electrical properties and device performance. Vo influences the polymorphism and the resulting ferroelectric properties of HfO2. Moreover, the switching speed and endurance of ferroelectric memories are strongly correlated to the Vo concentration and redistribution. They also strongly influence the device-to-device and cycle-to-cycle variability of integrated circuits based on ferroelectric memories. The concentration, migration, and agglomeration of Vo form the main mechanism behind the RS behavior observed in HfO2, suggesting that the device performance and reliability in terms of the operating voltage, switching speed, on/off ratio, analog conductance modulation, endurance, and retention are sensitive to Vo. Therefore, the mechanism of Vo formation and its effects on the chemical, physical, and electrical properties in ferroelectric and RS HfO2 should be understood. This study comprehensively reviews the literature on Vo in HfO2 from the formation and influencing mechanism to material properties and device performance. This review contributes to the synergetic advances of current knowledge and technology in emerging HfO2-based semiconductor devices.

Magnetic and Ferroelectric Manipulation of Valley Physics in Janus Piezoelectric Materials

The realization of multiferroic materials offers the possibility of multifunctional electronic device design. However, the coupling between the multiferroicity and piezoelectricity in Janus materials is rarely reported. In this study, we propose a mechanism for manipulating valley physics by magnetization reversing and ferroelectric switching in multiferroic and piezoelectric material. The ferromagnetic VSiGeP4 monolayer exhibits a large valley polarization up to 100 meV, which can be effectively operated by reversing magnetization. Interestingly, the antiferromagnetic VSiGeP4 bilayers with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, supporting the proposed strategy for manipulating valley physics via ferroelectric switching and interlayer sliding. In addition, the VSiGeP4 monolayer contains remarkable tunable piezoelectricity regulated by electron correlation U. This study proposes a feasible idea for regulating valley polarization and a general design idea for multifunctional devices with multiferroic and piezoelectric properties, facilitating the miniaturization and integration of nanodevices.

A Homochiral Fulgide Organic Ferroelectric Crystal with Photoinduced Molecular Orbital Breaking

Thermally triggered spatial symmetry breaking in traditional ferroelectrics has been extensively studied for manipulation of the ferroelectricity. However, photoinduced molecular orbital breaking, which is promising for optical control of ferroelectric polarization, has been rarely explored. Herein, for the first time, we synthesized a homochiral fulgide organic ferroelectric crystal (E)-(R)-3-methyl-3-cyclohexylidene-4-(diphenylmethylene)dihydro-2,5-furandione (1), which exhibits both ferroelectricity and photoisomerization. Significantly, 1 shows a photoinduced reversible change in its molecular orbitals from the 3 π molecular orbitals in the open-ring isomer to 2 π and 1 σ molecular orbitals in the closed-ring isomer, which enables reversible ferroelectric domain switching by optical manipulation. To our knowledge, this is the first report revealing the manipulation of ferroelectric polarization in homochiral ferroelectric crystal by photoinduced breaking of molecular orbitals. This finding sheds light on the exploration of molecular orbital breaking in ferroelectrics for optical manipulation of ferroelectricity.

Higher harmonics dynamic focalization in single-element ring transducers using biaxial driving

Biaxial driving is a new driving technique that allows the steering of the ultrasound field generated by a single-element piezoceramic transducer. Because of their natural axisymmetric geometry, ultrasound generation with ring transducers can take advantage of the biaxial driving to change the focus of the beam generated by this type of transducer using only two driving signals. In this study, we applied the biaxial driving technique into a single-element PZT ring transducer operating at 500 kHz to produce a change in size and position of the focal spot while using the 1st (482 kHz), 3rd (1.362 MHz) and 5th (2.62 MHz) harmonic excitation. The transducer had a thickness of 2.85 mm, an inner diameter of 9.75 mm and a ring width of 2.0 mm, and two pairs of electrodes as required for biaxial driving. Simulation and experimental results showed that both the focal area and the distance at which the focal area centre was located changed as a function of the phase and power difference between the two driving signals. Experimental results showed that the focal area could be reduced from 31.6 mm² (conventional driving) to 3.4 mm² (89 % reduction) when using the first harmonic excitation. For the third harmonic, the focal area could be reduced from 4.0 mm² (conventional driving) to 3.3 mm² (17.5 % reduction). For the fifth harmonic, the focal area could be reduced from 1.7 mm² (conventional driving) to 1 mm² (41.7 % reduction). Results also demonstrated the centre of the focus could be displaced between 3.0 mm and 9.3 mm from the surface of the transducer when using the first harmonic, between 7.3 mm and 8.4 mm at the third harmonic, and between 4.9 mm and 8.2 mm at the fifth harmonic. The reduction in the focus area, as well as the possibility to displace the focus dynamically will be advantageous for preclinical applications of focused ultrasound, especially on drug delivery and neuromodulation studies in small rodents.

Mixed-Stacking Few-Layer Graphene as an Elemental Weak Ferroelectric Material

Ferroelectricity (Valasek, J. Phys. Rev. 1921, 17, 475), a spontaneous formation of electric polarization, is a solid state phenomenon, usually, associated with ionic compounds or complex materials. Here we show that, atypically for elemental solids, few-layer graphenes can host an equilibrium out-of-plane electric polarization, switchable by sliding the constituent graphene sheets. The systems hosting such effect include mixed-stacking tetralayers and thicker (5-9 layers) rhombohedral graphitic films with a twin boundary in the middle of a flake. The predicted electric polarization would also appear in marginally (small-angle) twisted few-layer flakes, where lattice reconstruction would give rise to networks of mesoscale domains with alternating value and sign of out-of-plane polarization.

Effects of electrode materials on the performance of Zr0.75Hf0.25O2-based ferroelectric thin films via post deposition annealing

In this work, the effects of top electrode (TE) and bottom electrode (BE) on the ferroelectric properties of zirconia-based Zr0.75Hf0.25O2 (ZHO) thin films annealed by post-deposition annealing (PDA) are investigated in detail. Among W/ZHO/BE capacitors (BE = W, Cr or TiN), W/ZHO/W delivered the highest ferroelectric remanent polarization and the best endurance performance, revealing that the BE with a smaller coefficient of thermal expansion (CTE) plays a vital role in enhancing the ferroelectricity of fluorite-structure ZHO. For TE/ZHO/W structures (TE = W, Pt, Ni, TaN or TiN), the stability of TE metals seems to have a larger impact on the performance over their CTE values. This work provides a guideline to modulate and optimize the ferroelectric performance of PDA-treated ZHO-based thin films.

Ferroelectric Control of Magnetic Skyrmions in Two-Dimensional van der Waals Heterostructures

Magnetic skyrmions are chiral nanoscale spin textures which are usually induced by the Dzyaloshinskii-Moriya interaction (DMI). Recently, magnetic skyrmions have been observed in two-dimensional (2D) van der Waals (vdW) ferromagnetic materials, such as Fe₃GeTe₂. The electric control of skyrmions is important for their potential application in low-power memory technologies. Here, we predict that DMI and magnetic skyrmions in a Fe₃GeTe₂ monolayer can be controlled by ferroelectric polarization of an adjacent 2D vdW ferroelectric In₂Se₃. Based on density functional theory and atomistic spin-dynamics modeling, we find that the interfacial symmetry breaking produces a sizable DMI in a Fe₃GeTe₂/In₂Se₃ vdW heterostructure. We show that the magnitude of DMI can be controlled by ferroelectric polarization reversal, leading to creation and annihilation of skyrmions. Furthermore, we find that the sign of DMI in a In₂Se₃/Fe₃GeTe₂/In₂Se₃ heterostructure changes with ferroelectric switching reversing the skyrmion chirality. The predicted electrically controlled skyrmion formation may be interesting for spintronic applications.

Steering single-element lead zirconate titanate ultrasound transducers using biaxial driving

Previous work has shown that biaxial driving using two phase-offset orthogonal electric fields (propagation and lateral) improves the efficiency of ferroelectric materials by reducing coercivity and, hence, energy dissipation. In the current investigation, we demonstrated the capability of the biaxial method to steer ultrasound waves in single-element piezoceramic transducers made of prismatic lead zirconate titanate (PZT). We conducted finite element analysis simulations for 133 kHz (model 1) and 470 kHz biaxial (model 2) transducers models. We performed experimental validation with biaxially driven single-element transducers (n = 3) operating at an average frequency of 131 kHz with the same characteristics as model 1. For both models, we found non-symmetric steering that was a function of both the phase and power of the second electric field. At a constant electrical power (1 W) on the propagation electrodes, simulations for the 133 kHz model predicted maximal steering of 10.3°, 22.6°, and 30.9° for lateral electrode powers of 0.1 W, 0.5 W, and 1.0 W, respectively. Experimentally, for model 1, the maximal steering was 11.7° ± 1.9°, 23.5° ± 3.5°, and 30.2° ± 4.4° for the lateral electrode powers of 0.1 W, 0.5 W, and 1.0 W, respectively. Simulations for the 470 kHz model predicted maximal steering of 8.8°, 16.1°, and 27° for lateral electrode powers of 0.1 W, 0.5 W, and 1.0 W, respectively. Simulations showed that the cause of the steering asymmetry was a non-uniform shear deformation associated with the slightly off-resonance lateral electric field driving frequency. This is the first demonstration of ultrasound steering using a single-element transducer, which can have important applications for ultrasound focusing with phased arrays.

Structure, Performance, and Application of BiFeO3 Nanomaterials

Multiferroic nanomaterials have attracted great interest due to simultaneous two or more properties such as ferroelectricity, ferromagnetism, and ferroelasticity, which can promise a broad application in multifunctional, low-power consumption, environmentally friendly devices. Bismuth ferrite (BiFeO3, BFO) exhibits both (anti)ferromagnetic and ferroelectric properties at room temperature. Thus, it has played an increasingly important role in multiferroic system. In this review, we systematically discussed the developments of BFO nanomaterials including morphology, structures, properties, and potential applications in multiferroic devices with novel functions. Even the opportunities and challenges were all analyzed and summarized. We hope this review can act as an updating and encourage more researchers to push on the development of BFO nanomaterials in the future.

Application of ferroelectric materials for improving output power of energy harvesters

In terms of advances in technology, especially electronic devices for human use, there are needs for miniaturization, low power, and flexibility. However, there are problems that can be caused by these changes in terms of battery life and size. In order to compensate for these problems, research on energy harvesting using environmental energy (mechanical energy, thermal energy, solar energy etc.) has attracted attention. Ferroelectric materials which have switchable dipole moment are promising for energy harvesting fields because of its special properties such as strong dipole moment, piezoelectricity, pyroelectricity. The strong dipole moment in ferroelectric materials can increase internal potential and output power of energy harvesters. In this review, we will provide an overview of the recent research on various energy harvesting fields using ferroelectrics. A brief introduction to energy harvesting and the properties of the ferroelectric material are described, and applications to energy harvesters to improve output power are described as well.

Ferroelectric spontaneous polarization steering charge carriers migration for promoting photocatalysis and molecular oxygen activation

Introducing a polarization electric field in photocatalyst system is regarded asa new concept for photocatalytic activity enhancement. In this work, we first unearth that the spontaneous polarization of ferroelectric BaTiO₃ promotes the photocatalytic and molecular oxygen activation performance of the narrow-band-gap semiconductor BiOI. Ferroelectric tetragonal-phase BaTiO₃ (T-BaTiO₃) were prepared via calcination of nonferroelectric cubic-phase BaTiO₃ (C-BaTiO₃), and their polarization ability was verified via ultrasonication-assisted piezoelectric catalytic degradation. Then, the C-BaTiO₃/BiOI and T-BaTiO₃/BiOI heterostructures are fabricated by a soft-chemical method. To disclose the influence of ferroelectric spontaneous polarization on charge movement behavior, the photocatalytic and molecular oxygen activation properties are monitored by degradation of methyl orange (MO) and superoxide radical (O₂^-) evolution under visible light irradiation (λ>420nm), respectively. The results demonstrated that T-BaTiO₃/BiOI far outperforms C-BaTiO₃/BiOI and pristine BiOI. The ferroelectric spontaneous polarization of T-BaTiO₃ can steer the migration of photogenerated charge carriers and induce efficient separation, accounting for the strengthened photodegradation and reactive oxygen species O₂^- production rate (11.02×10^-7molL^-1h^-1). The study may furnish a new reference for developing efficient tactics to advance the photocatalytic and molecular oxygen activation ability for environmental chemistry and biochemistry applications.

Multiferroic La0.2Pb0.7Fe12O19 ceramics: Ferroelectricity, ferromagnetism and colossal magneto-capacitance effect

The mutual control of the electric and magnetic properties of a multiferroic solid is of fundamental and great technological importance. In this article, the synthesis procedure of La_0.2Pb_0.7Fe₁₂O₁₉ ceramics was briefly described and the data acquired for the materials characterization is presented. This data article is related to the research article-Acta Mater. 2016, 121, 144 (j.actamat.2016.08.083). Electric polarization hysteresis loop and I-V curve, which help to confirm the ferroelectricity of La_0.2Pb_0.7Fe₁₂O₁₉ ceramics, were presented. Strong magnetic polarization data was also presented. The great variation of the dielectric constants along with the magnetic field has been presented which helped to demonstrat the giant magnetocapacitance of La_0.2Pb_0.7Fe₁₂O₁₉. All the datasets were collected at room temperature. Large ferroelectricity, strong magnetism and colossal magneto-capacitance effect have been all realized in one single phase La_0.2Pb_0.7Fe₁₂O₁₉ at room temperature.

PLD prepared bioactive BaTiO3 films on TiNb implants

BaTiO₃ (BTO) layers were deposited by pulsed laser deposition (PLD) on TiNb, Pt/TiNb, Si (100), and fused silica substrates using various deposition conditions. Polycrystalline BTO with sizes of crystallites in the range from 90nm to 160nm was obtained at elevated substrate temperatures of (600°C-700°C). With increasing deposition temperature above 700°C the formation of unwanted rutile phase prevented the growth of perovskite ferroelectric BTO. Concurrently, with decreasing substrate temperature below 500°C, amorphous films were formed. Post-deposition annealing of the amorphous deposits allowed obtaining perovskite BTO. Using a very thin Pt interlayer between the BTO films and TiNb substrate enabled high-temperature growth of preferentially oriented BTO. Raman spectroscopy and electrical characterization indicated polar ferroelectric behaviour of the BTO films.

Inductive crystallization effect of atomic-layer-deposited Hf0.5Zr0.5O2 films for ferroelectric application

Ferroelectric Hf x Zr1-x O2 thin films are considered promising candidates for future lead-free CMOS-compatible ferroelectric memory application. The inductive crystallization behaviors and the ferroelectric performance of Hf0.5Zr0.5O2 thin films prepared by atomic layer deposition were investigated. Inductive crystallization can be induced by the film growth condition and appropriate top electrode selection. In this work, a Ni/Hf0.5Zr0.5O2/Ru/Si stack annealed at 550°C for 30 s in N2 ambient after the Ni top electrode has been deposited was manufactured, and it shows the best ferroelectric hysteresis loop in the dielectric thickness of 25 nm, with a remanent polarization value of 6 μC/cm(2) and a coercive field strength of 2.4 MV/cm measured at 10 kHz. Endurance, retention, and domain switching current characteristics were evaluated well for potential application in the field of ferroelectric field effect transistor (FeFET) and nonvolatile ferroelectric memories (FeRAM).

Synthesis, structure, spectroscopic and ferroelectric properties of an acentric polyoxotungstate containing 1:2-type [Sm(α-PW₁₁O₃₉)₂]¹¹⁻ fragment and D-proline components

An organic-inorganic hybrid mono-Sm(III) substituted phosphotungstate KNa3[HPro]₇[Sm(α-PW₁₁O₃₉)₂]·Pro·₁₈H₂O (1) (Pro=D-proline) has been synthesized in the conventional aqueous solution and structurally characterized by elemental analyses, inductively coupled plasma atomic emission spectrometry (ICP-AES) analyses, IR spectra, UV spectra, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and single-crystal X-ray diffraction. The molecule of 1 consists of a classical 1:2-type [Sm(α-PW₁₁O₃₉)₂](11-) fragment and free D-proline components. It should be pointed out that the synergistic action between the in-situ formed chiral [Sm(α-PW₁₁O₃₉)₂](11-) fragment and chiral D-proline components results in the formation of the chiral 1. The luminescence emission of 1 reveals three characteristic bands that derive from the (4)G₅/₂→(6)H₅/₂, (4)G₅/₂→(6)H₇/₂ and (4)G₅/₂→(6)H₉/₂ transitions of the Sm(III) cation as well as the synergistic contribution of the O→W transitions of [α-PW₁₁O₃₉](7-) moieties and a π(*)-n or π(*)-π transitions of Pro. Its ferroelectric behavior has been measured.

Electric-field-induced phase transitions in co-doped Pb(Zr1-xTix)O3 at the morphotropic phase boundary

The strain- and polarization-electric field behavior was characterized at room temperature for Pb_0.98Ba_0.01(Zr_1-x Ti _x )_0.98Nb_0.02O₃, 0.40 ⩽ x ⩽ 0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.