Ferroelectric Nanocrystals Research Articles

Submillisecond Electric Field Sensing with an Individual Rare-Earth Doped Ferroelectric Nanocrystal.

Understanding the dynamics of electrical signals within neuronal assemblies is crucial to unraveling complex brain functions. Despite recent advances in employing optically active nanostructures in transmembrane potential sensing, there remains room for improvement in terms of response time and sensitivity. Here, we report the development of such a nanosensor capable of detecting electric fields with a submillisecond response time at the single-particle level. We achieve this by using ferroelectric nanocrystals doped with rare-earth ions that produce upconversion (UC). When such a nanocrystal experiences a variation of surrounding electric potential, its surface charge density changes, inducing electric polarization modifications that vary, via a converse piezoelectric effect, the crystal field around the ions. The latter variation is finally converted into UC spectral changes, enabling optical detection of the electric potential. To develop such a sensor, we synthesized erbium and ytterbium-doped barium titanate crystals of ≈160 nm in size. We observed distinct changes in the UC spectrum when individual nanocrystals were subjected to an external field via a conductive atomic force microscope tip, with a response time of 100 μs. Furthermore, our sensor exhibits a remarkable sensitivity of 4.8 kV/cm/, enabling time-resolved detection of a fast-changing electric field of amplitude comparable to that generated during a neuron action potential.

Switchable Bulk Photovoltaic Effect in Intrinsically Ferroelectric 3D All-Inorganic CsPbBr3 Perovskite Nanocrystals.

Ferroelectric all-inorganic halide perovskite nanocrystals with both spontaneous polarization and visible light absorption are promising candidates for designing ferroelectric photovoltaic applications. It remains a challenge to realize ferroelectric photovoltaic devices with all-inorganic halide perovskites that can be operated in the absence of an external electric field. Here we report that a popular all-inorganic halide perovskite nanocrystal, CsPbBr3, exhibits a ferroelectricity-driven photovoltaic effect under visible light in the absence of an external electric field. Pristine CsPbBr3 nanocrystals exhibit intrinsic ferroelectric key properties with a notable saturated polarization of ∼0.15 μC/cm2 and a high Curie temperature of 462 K, driven by the stereochemical activity of the Pb(II) lone pair. Furthermore, application of an external electric field allows the photovoltaic effect to be enhanced and the spontaneous polarization to be switched with the direction of the electric field. CsPbBr3 nanocrystals exhibit a robust fatigue performance and a prolonged photoresponse under continuous illumination in the absence of an external electric field. These findings establish all-inorganic halide perovskite nanocrystals as potential candidates for designing photoferroelectric devices by coupling optical functionalities and ferroelectric responses.

Multi-field Coupled Inverse Hall–Petch Relations for Ferroelectric Nanocrystals

Multi-field Coupled Inverse Hall–Petch Relations for Ferroelectric Nanocrystals

Nonlinear ferroelectric characteristics of barium titanate nanocrystals determined via a polymer nanocomposite approach.

The growing demand for high energy storage materials has garnered substantial attention towards lead-free ferroelectric nanocrystals (NCs), such as BaTiO3 (BTO), for next-generation multilayer ceramic capacitors. Notably, it remains challenging to accurately measure the dielectric constant and polarization-electric field (P-E) hysteresis loop for BTO NCs. Herein, we report on nonlinear ferroelectric characteristics of BTO NCs via a polymer nanocomposite approach. Specifically, poly(vinyl pyrrolidone) (PVP)/BTO nanocomposite films of 3-10 μm thickness, containing 380 nm tetragonal-phased and 60 nm cubic-phased BTO NCs with uniform particle dispersion, were prepared. Theoretical deconvolution of the broad experimental P-E loops of the PVP/BTO NC composite films revealed three contributions, that is, the linear deformational polarization of the nanocomposites, the polarization of BTO NCs (Pp), and the polarization from strong particle-particle interactions. Using different mixing rules and nonlinear dielectric analysis, the overall dielectric constants of BTO NCs were obtained, from which the internal field in the BTO NCs (Ep) was estimated. Consequently, the Pp-Ep hysteresis loops were obtained for the BTO380 and BTO60 NCs. Interestingly, BTO380 exhibited square-shaped ferroelectric loops, whereas BTO60 displayed slim paraelectric loops. This work presents a robust and versatile route to extract the Pp-Ep loops of ferroelectric NCs from polymer/ceramic nanocomposites.

A new perspective for nucleation and nanocrystallization from interfacial energy and nanoscale composition fluctuations in glasses

AbstractNucleation behaviors in glasses/supercooled liquids (SCLs) such as lithium disilicate Li2O–2SiO2, cordierite Mg2Al4Si5O18, fresnoite Ba2TiSi2O8, and K2O–Nb2O5–GeO2/TeO2 glasses were analyzed and discussed from the interfacial energy γ at SCL–nucleus interfaces and nanoscale composition fluctuations. Based on the fragility concept for SCLs and the intrinsic origin of γ, the magnitude order of γ(fragile SCL) < γ(strong SCL) was proposed to be a crucial guide for an understanding of high homogeneous nucleation rates and prominent nanocrystallization. The role of nucleating agent TiO2/ZrO2 in accelerating the nucleation rate in cordierite‐based and β‐spodumene‐type Li2O–Al2O3–SiO2‐based glasses was discussed from the new perspective of γ(homoepitaxial‐like nucleation) < γ(heteroepitaxial‐like nucleation). Ferroelectric nanocrystals such as SrxBa1–xNb2O6 in borate glasses and fluoride nanocrystals such as CaF2 are also well understood from the proposed guidelines. The present article provides a new perspective for nucleation in glasses/SCLs, contributing to the comprehensive composition design of new innovative glass‐ceramics.

Physical properties and energy storage performance of TeO2-based ferroelectric glass-ceramics

xBaO–xTiO2–(100–2x)TeO2 (BTT) glass-ceramics were prepared through a melt-quenching synthesis method. Structural and optical properties, analyzed from Raman spectroscopy and absorption (OA) measurements, revealed that the increase in the TiO2 content promoted a loss in the network connectivity because of the formation of the non-bridging oxygen atoms (NBO’s) as well as the Te–O–Te bonds’ deformation. X-ray diffraction analysis confirmed the formation of ferroelectric nanocrystals (FeNCs) with tetragonal symmetry, related to the BaTiO3 phase, in the studied compositions, whose crystallinity seems to be intensified as the TiO2 concentration increases. The room temperature energy storage performance of these BTT samples, assessed using P-E hysteresis loops, confirmed excellent energy storage response and reveals the potential of the study glass-ceramics for practical applications.

Polar Solomon rings in ferroelectric nanocrystals

Solomon rings, upholding the symbol of wisdom with profound historical roots, were widely used as decorations in ancient architecture and clothing. However, it was only recently discovered that such topological structures can be formed by self-organization in biological/chemical molecules, liquid crystals, etc. Here, we report the observation of polar Solomon rings in a ferroelectric nanocrystal, which consist of two intertwined vortices and are mathematically equivalent to a {4}_{1}^{2} link in topology. By combining piezoresponse force microscopy observations and phase-field simulations, we demonstrate the reversible switching between polar Solomon rings and vertex textures by an electric field. The two types of topological polar textures exhibit distinct absorption of terahertz infrared waves, which can be exploited in infrared displays with a nanoscale resolution. Our study establishes, both experimentally and computationally, the existence and electrical manipulation of polar Solomon rings, a new form of topological polar structures that may provide a simple way for fast, robust, and high-resolution optoelectronic devices.

Solution-processed ferroelectric polymer nanocrystals

Since the first demonstrations of ferro- and piezoelectricity in solution-processed ferroelectric polymer thin films research activities have been stimulated in both academia and industry. Several applications have been reported using ferroelectric polymer thin films as bistable polarization switches in memory devices or as transducers in (micro-)electromechanical devices. In light of the continuing miniaturization of such devices, nanoscaled structures of ferroelectric polymers such as nanomesas are attracting increasing interest. However, nanopatterning ferroelectrics, to develop stable and reliable ferroelectric nanomesas remains challenging. In this work, we demonstrate the fabrication of solution-processed ferroelectric polymer nanocrystals, i.e. ferroelectric nanomesas, and verify their ferroelectric activity. By using the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)), we are able to isolate crystalline domains. Morphology, molecular conformation, (nano-)mechanical and electromechanical properties of the nanocrystals are measured, yielding comparable results to those observed in uniform, continuous thin films. Therefore, synthesizing self-assembled ferroelectric polymer nanocrystals from polymer solution by spin-casting represents a promising approach for the realization of ferroelectric nanomesas.

Topological defects and ferroelastic twins in ferroelectric nanocrystals: What coherent X-rays can reveal about them

Topological defects and ferroelastic twins in ferroelectric nanocrystals: What coherent X-rays can reveal about them

Ferroelectric monodisperse Ln-doped barium titanate cuboidal nanocrystals prepared by a solvothermal route

We demonstrate here that the dielectric, electrical and optical properties of BaTiO3colloidal nanocrystals with a cuboidal shape can be modulated upon aliovalent doping with Ln3+rare earth ions (Ln = La3+, Ce3+, Nd3+, Sm3+, Gd3+, Dy3+, and Tm3+).

Hydroxylation mechanism of phase regulation of nanocrystal BaTiO3 synthesized by a hydrothermal method

Understanding the phase regulation mechanism of BaTiO3 synthesized by a hydrothermal method is a complicated and challenging task. Here, we successfully prepared cubic and tetragonal BaTiO3 single nanocrystals by changing the ratio of water and ethanol in the solvent. We confirm that the BaTiO3 phase is mainly affected by the hydroxylation process due to the reaction with solvent. In particular, ethanol can be catalytically oxidized by titanium atoms, cause BaTiO3 hydroxylation and promote the formation of a cubic phase. In the mixed solution of ethanol and water, the hydroxylation process is suppressed, which facilitates the formation of the tetragonal phase. The relationship framework between the solvent ratio, phase structure and hydroxyl defects of BaTiO3 is established. Tetragonal BaTiO3 with fewer hydroxyl defects can promote charge transfer and surface reaction after polarization, thereby enhancing their photoelectric catalytic performance. This work provides references for the controllable synthesis of ferroelectric nanocrystals by hydrothermal methods and new insight for the utilization of polarization in photoelectrocatalysis applications.

Third- and fifth-order optical nonlinearity in PbO-BaO-Na2O-Nb2O5-SiO2 glass and glass-ceramic nanocomposite dielectrics

We investigate the optical nonlinearity in 6PbO–4BaO–20Na2O–40Nb2O5–30SiO2 glass and glass-ceramic containing ferroelectric Ba2NaNb5O15 nanocrystals and Pb2Nb2O7 nanocrystals using Z-scan method. The values of the third-order (n2) and fifth-order (n4) nonlinear refractive indices are quantitatively determined as 1.76 × 10−17 m2/W and −2.29 × 10−31 m4/W2 in glass phase, respectively, and 2.62 × 10−17 m2/W and −2.89 × 10−31 m4/W2 in glass-ceramic, respectively. The values of three-photon absorption coefficient of the glass and glass-ceramic are obtained as 8.60 × 10−27 m3/W2 and 1.22 × 10−26 m3/W2, respectively. The samples satisfy the criterion for all-optical switching and have potential applications for all-optical photonic devices.

Insights into the tribo-/pyro-catalysis using Sr-doped BaTiO3 ferroelectric nanocrystals for efficient water remediation

Abstract Ferroelectric materials have multiple characteristics in ferroelectric, piezoelectric, pyroelectric properties, which provide an attractive prospect for simultaneously harvesting multiple energy sources for catalytic applications. However, the crucial challenge for wastewater purification lies in the development of ferroelectric materials with improved functions and the design of advanced oxidation processes. Herein, Sr0.3Ba0.7TiO3 (SBT-0.3) nano-catalysts modified by PVP surfactant could significantly promote water recovery efficiency from the rhodamine B (RhB), which is as high as 98% by simultaneously collecting two types of energy sources of mechanical friction and temperature fluctuation from the natural environment for tribo-/pyro-catalysis under dark conditions. It was found that the samples containing PVP surfactant show significantly improved performance in the separation of charge carriers in comparison with those without PVP surfactant. The permanent polarization in SBT-0.3, coupled with the piezo-/pyro-potential, could generate internal field and therefore reduce charge recombination. It also helps to absorb charged species from the dye solution, which favors the reaction with active oxygen to cause the cleavage and breakdown of organic molecules. The mechanism of RhB decomposition mediated by SBT-0.3 ferroelectric is also discussed. This work favors us to in-depth understanding for the tribo-/pyro-catalysis and hence proposes a new strategy to improve the water purification efficiency of ferroelectric nanocrystals.

Needle‐Like Ferroelastic Domains in Individual Ferroelectric Nanoparticles

AbstractSuperior structural, physical, and electronic properties make ferroelectric nanocrystals essential in enabling a range of next‐generation devices. Ferroelectric responses are determined by crystal structure and domain morphology. The ability to reversibly displace, create, and annihilate elastic domains is critical to device applications. Although electric‐field control has been demonstrated for ferroelectric 180° surface domain walls and vortices, similar control of ferroelastic domains and domain boundaries within individual nanocrystals remains challenging. Using controlled external compressive and tensile axial stress, deterministic and reversible control of highly mobile ferroelastic domains and axial polarization in three dimensions is demonstrated. While many studies exist on ferroelastic domains in thin films and bulk, little is known about ferroelastic interactions at the single nanoparticle level, especially involving domain boundaries. Through combining Bragg coherent X‐ray diffractive imaging and Landau theory, strain gradients in individual BaTiO3 nanocrystals are shown to stabilize needle‐like ferroelastic twin domains. These domains are highly labile under applied axial stress, producing a locally enhanced electric polarization mediated by a ferroelectric phase transition. The efficacy of Bragg coherent X‐ray diffractive imaging in studying in operando domains in three dimensions is demonstrated, while synergy with theory provides a paradigm for domain boundary engineering and potential for nanoscale functional devices.

Nanocrystal-Embedded-Insulator (NEI) Ferroelectric Field-Effect Transistor Featuring Low Operating Voltages and Improved Synaptic Behavior

A novel nanocrystal-embedded-insulator (NEI) ferroelectric field-effect transistor (FeFET) is demonstrated to function as a synaptic device for analog neural network (NN) applications. The NEI layer (down to 3.6 nm in thickness) comprises ferroelectric nanocrystals embedded in amorphous Al2O3, resulting in reduced operating voltages and depolarization effects as compared to conventional doped-HfO2 films. With fixed-amplitude 100 ns potentiation/depression pulses, an NEI FeFET synapse achieves weight update with small non-linearity ( $\alpha _{p}= {0.12}$ , $\alpha _{\text {d}}= -{0.09}$ ) and asymmetry factors, advantageous for analog-style NNs with online training. A convolutional NN is designed and emulated for an MNIST dataset, projecting an online training accuracy of 92%.

Facile Fabrication of Size-Tunable Core/Shell Ferroelectric/Polymeric Nanoparticles with Tailorable Dielectric Properties via Organocatalyzed Atom Transfer Radical Polymerization Driven by Visible Light

An unconventional but facile approach to prepare size-tunable core/shell ferroelectric/polymeric nanoparticles with uniform distribution is achieved by metal-free atom transfer radical polymerization (ATRP) driven by visible light under ambient temperature based on novel hyperbranched aromatic polyamides (HBPA) as a functional matrix. Cubic BaTiO3/HBPA nanocomposites can be prepared by in-situ polycondensation process with precursors (barium hydroxide (Ba(OH)2) and titanium(IV) tetraisopropoxide (TTIP)) of ferroelectric BaTiO3 nanocrystals, because precursors can be selectively loaded into the domain containing the benzimidazole rings. At 1200 °C, the aromatic polyamide coating of cubic BaTiO3 nanoparticles are carbonized to form carbon layer in the inert environment, which prevents regular nanoparticles from gathering. In addition, cubic BaTiO3 nanoparticles are simultaneously transformed into tetragonal BaTiO3 nanocrystals after high temperature calcination (1200 °C). The outer carbon shell of tetragonal BaTiO3 nanoparticles is removed via 500 °C calcination in air. Bi-functional ligand can modify the surface of tetragonal BaTiO3 nanoparticles. PMMA polymeric chains are growing from the initiating sites of ferroelectric BaTiO3 nanocrystal surface via the metal-free ATRP technique to obtain core/shell ferroelectric BaTiO3/PMMA hybrid nanoparticles. Changing the molar ratio between benzimidazole ring units and precursors can tune the size of ferroelectric BaTiO3 nanoparticles in the process of polycondensation, and the thickness of polymeric shell can be tailored by changing the white LED irradiation time in the organocatalyzed ATRP process. The dielectric properties of core/shell BaTiO3/PMMA hybrid nanoparticles can be also tuned by adjusting the dimension of BaTiO3 core and the molecular weight of PMMA shell.

Crystalline heterogeneity in single ferroelectric nanocrystals revealed by polarized nonlinear microscopy

Ferroelectric nanocrystals have considerable interest for applications in nanophotonics, optical memories and bio-imaging. Their crystalline nature at the nanoscale remains however poorly known, mostly because structural investigation tools on single nanocrystals are lacking. In this work we apply polarization resolved second harmonic generation (P-SHG) imaging on isolated Barium Titanate (BaTiO3) nanocrystals to unravel their crystalline nature, exploiting the sensitivity of polarized SHG to local non-centrosymmetry and nanocrystals surface responses. We evidence crystalline heterogeneities in BaTiO3 nanocrystals manifested by a centrosymmetric shell around the tetragonal core of the crystals, corroborating hypotheses from previous ensemble structural investigations. This study shows that in contrast to bulk materials, nanocrystals exhibit a complex composition, which is seen to be reproducible among nanocrystals. P-SHG appears furthermore as a powerful methodology that reports structural behaviors in nanoscale dielectrics materials, at the individual nanoparticle scale.

Enhanced ferro-photocatalytic performance for ANbO<sub>3</sub> (A = Na, K) nanoparticles

In this study, NaNbO₃ with average grain size of ~50 nm and KNbO₃ with average grain size of ~300 nm nanocrystals are prepared by the water-based citrate precursor sol-gel process. However, the KNbO₃ sample exhibits better photocatalytic performance than that of the NaNbO₃ sample by Rh B degradation experiment. By Rietveld refinements and piezoelectric displacement measurements, the KNbO₃ with the space group of Bmm2 is ferroelectric while the NaNbO₃ with the space group of Pbma is antiferroelectric. The polarization-modulated built-in electric fields in the ferroelectric KNbO₃ nanoparticles can efficiently enhance the separation of photo-generated charge carries and thus improve the photocatalytic activity. However, there is no internal electric field in the antiferroelectric grain because of the antiparallel spontaneous polarization in the adjacent unit cell. Therefore, KNbO₃ exhibits better oxidizing ability of organic dyes than NaNbO₃. The ferroelectric KNbO₃ nanoparticles exhibit an optimum photocatalytic performance for a complete degradation of Rh B in 100 min under UV-Vis light irradiation with auxiliary ultrasonic excitation. This study demonstrates that the perovskite-type ferroelectric nanocrystals are potentially to design high-performance catalysts for degradation of contaminant.

Enhanced ferro-photocatalytic performance for ANbO3 (A = Na, K) nanoparticles.

In this study, NaNbO3 with average grain size of ~50 nm and KNbO3 with average grain size of ~300 nm nanocrystals are prepared by the water-based citrate precursor sol-gel process. However, the KNbO3 sample exhibits better photocatalytic performance than that of the NaNbO3 sample by Rh B degradation experiment. By Rietveld refinements and piezoelectric displacement measurements, the KNbO3 with the space group of Bmm2 is ferroelectric while the NaNbO3 with the space group of Pbma is antiferroelectric. The polarization-modulated built-in electric fields in the ferroelectric KNbO3 nanoparticles can efficiently enhance the separation of photo-generated charge carries and thus improve the photocatalytic activity. However, there is no internal electric field in the antiferroelectric grain because of the antiparallel spontaneous polarization in the adjacent unit cell. Therefore, KNbO3 exhibits better oxidizing ability of organic dyes than NaNbO3. The ferroelectric KNbO3 nanoparticles exhibit an optimum photocatalytic performance for a complete degradation of Rh B in 100 min under UV-Vis light irradiation with auxiliary ultrasonic excitation. This study demonstrates that the perovskite-type ferroelectric nanocrystals are potentially to design high-performance catalysts for degradation of contaminant.

Dielectric properties of ferroelectric nanocomposites: effects of thermal stresses and filler shape anisotropy

Ferroelectric nanocomposites are intriguing nonhomogeneous materials, which may have unusual phase states and specific physical properties useful for practical applications. Here we theoretically describe dielectric properties of nanocomposites comprising single-domain ferroelectric nanocrystals embedded into a linear dielectric medium. First, small-signal intrinsic permittivities of spheroidal PbTiO3 and BaTiO3 crystallites surrounded by an isotropic matrix with linear elastic properties are calculated with the aid of a nonlinear thermodynamic theory. It is shown that thermal stresses caused by differences in thermal expansion between the inclusions and the matrix may strongly influence the intrinsic permittivities of ferroelectric nanocrystals. Second, macroscopic dielectric responses of ferroelectric–dielectric composites are evaluated in the Maxwell Garnett approximation. For effective permittivities of such composites, generalized relations are derived, which allow for both the shape and dielectric anisotropies of ferroelectric nanocrystals. Numerical calculations of the effective permittivities are performed for composites comprising PbTiO3 and BaTiO3 nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. For nanocomposites involving randomly oriented and similarly aligned spheroidal inclusions, temperature dependences of the effective permittivities are determined with the account of phase transitions occurring in strained ferroelectric nanocrystals and suppression of their dielectric responses due to the depolarizing-field effect created by low-permittivity matrix. Our theoretical calculations show that effective permittivities of composites comprising needle-like or disc-shaped ferroelectric inclusions could strongly exceed the permittivity of the matrix. Most importantly, we predict that BaTiO3–K2O-SiO2 composites with such inclusions should exhibit a broad dielectric peak around the room temperature, which is associated with shifted structural transitions in strained BaTiO3 nanocrystals. The presented theory provides guidelines for the development of ferroelectric nanocomposites with enhanced dielectric responses, which could be suitable for applications in supercapacitors and other advanced electronic devices.