Local Dipole Research Articles

Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids.

We report the growth and photophysical characterization of two polar hybrid lead halide phases, methylenedianiline lead iodide and bromide, (MDA)Pb2I6 and (MDA)Pb2Br6, respectively. The phases crystallize in noncentrosymmetric space group Fdd2, which produces a highly oriented molecular dipole moment that gives rise to second harmonic generation (SHG) upon excitation at 1064 nm. While both compositions are isostructural, the size dependence of the SHG signal suggests that the bromide exhibits a stronger phase-matching response whereas the iodide exhibits a significantly weaker non-phase-matching signal. Similarly, fluorescence from (MDA)Pb2Br6 is observed around 630 nm below 75 K whereas only very weak luminescence from (MDA)Pb2I6 can be seen. We attribute the contrasting optical properties to differences in the character of the halide sublattice and postulate that the increased polarizability of the iodide ions acts to screen the local dipole moment, effectively reducing the local electric field in the crystals.

Ferroelectric photovoltaic response engineered by lattice strain derived from local metal-ion dipoles.

An unfavorable inverse relationship between polarization, bandgap, and leakage always limits the ferroelectric photovoltaic performances. This work proposes a strategy of lattice strain engineering different from traditional lattice distortion by introducing a (Mg2/3Nb1/3)3+ ion group into the B site of BiFeO3 films to construct local metal-ion dipoles. A giant remanent polarization of 98 µC/cm2, narrower bandgap of 2.56 eV, and the decreased leakage current by nearly two orders of magnitude are synchronously obtained in the BiFe0.94(Mg2/3Nb1/3)0.06O3 film by engineering the lattice strain, breaking through the inverse relationship among these three. Thereby, the open-circuit voltage and the short-circuit current of the photovoltaic effect reach as high as 1.05 V and 2.17 µA /cm2, respectively, showing an excellent photovoltaic response. This work provides an alternative strategy to enhance ferroelectric photovoltaic performances by lattice strain derived from local metal-ion dipoles.

Dislocation-tuned ferroelectricity and ferromagnetism of the BiFeO3/SrRuO3 interface

Misfit dislocations at a heteroepitaxial interface produce huge strain and, thus, have a significant impact on the properties of the interface. Here, we use scanning transmission electron microscopy to demonstrate a quantitative unit-cell-by-unit-cell mapping of the lattice parameters and octahedral rotations around misfit dislocations at the BiFeO3/SrRuO3 interface. We find that huge strain field is achieved near dislocations, i.e., above 5% within the first three unit cells of the core, which is typically larger than that achieved from the regular epitaxy thin-film approach, thus significantly altering the magnitude and direction of the local ferroelectric dipole in BiFeO3 and magnetic moments in SrRuO3 near the interface. The strain field and, thus, the structural distortion can be further tuned by the dislocation type. Our atomic-scale study helps us to understand the effects of dislocations in this ferroelectricity/ferromagnetism heterostructure. Such defect engineering allows us to tune the local ferroelectric and ferromagnetic order parameters and the interface electromagnetic coupling, providing new opportunities to design nanosized electronic and spintronic devices.

Magnetic Soft Helical Manipulators with Local Dipole Interactions for Flexibility and Forces.

Magnetic continuum manipulators (MCMs) are a class of continuum robots that can be actuated without direct contact by an external magnetic field. MCMs operating in confined workspaces, such as those targeting medical applications, require flexible magnetic structures that contain combinations of magnetic components and polymers to navigate long and tortuous paths. In cylindrical MCM designs, a significant trade-off exists between magnetic moment and bending flexibility as the ratio between length and diameter decreases. In this study, we propose a new MCM design framework that enables increasing diameter without compromising on flexibility and magnetic moment. Magnetic soft composite helices constitute bending regions of the MCM and are separated by permanent ring magnets. Local dipole interactions between the permanent magnets can reduce bending stiffness, depending on their size and spacing. For the particular segment geometry presented herein, the local dipole interactions result in a 31% increase in angular deflection of composite helices inside an external magnetic field, compared to helices without local interactions. In addition, we demonstrate fabrication, maneuverability, and example applications of a multisegment MCM in a phantom of the abdominal aorta, such as passing contrast dye and guidewires.

Polarization-insensitive electromagnetically induced transparency and its sensing performance based on spoof localized surface plasmons in vanadium dioxide-based terahertz metasurfaces.

The multi-layer terahertz metasurfaces are designed to achieve polarization-insensitive electromagnetically induced transparency (EIT) effect and its sensing performance based on spoof localized surface plasmons (S-LSPs). The unit cell of the proposed metasurfaces is comprised of a metallic spiral (MS) structure, square metal frame (SMF) structure, and vanadium dioxide (VO2) layer. The EIT effect is realized by the bright-bright coupling between spoof electric localized surface plasmons (S-ELSPs) and electric dipole, which can be proved by the multipole scattering theory. The maximum value of transmission amplitude at the transparent window is 0.91, and the modulation depth can reach 51% by adjusting the conductivity of VO2. The theoretical results based on the two-particle model show excellent agreement with the simulated results. Moreover, the change of polarization angle has little effect on the EIT effect and the proposed metasurfaces show polarization-insensitive characteristics. The slow light effect of the proposed metasurfaces can also be dynamically controlled by tuning the conductivity of VO2. Due to the high Q value of the transparent window, the proposed metasurfaces exhibit excellent sensing performance, and the sensitivity is 0.172 THz RIU-1. Our study provides a method for the fabrication of EIT metasurfaces and has a broad application prospect in slow light devices, sensors, and modulators.

AIO3·H6TeO6 (A = NH4+, Rb+): Two telluric acid and iodate Co-crystalline compounds with second harmonic generation response

Two co-crystalline compounds containing telluric acid and iodate, namely AIO3·H6TeO6 (A ​= ​Rb+, NH4+), were investigated as potential nonlinear optical materials. They crystallize into isostructural and consist of isolated [IO3] group and [H6TeO6] molecule linked by hydrogen bonds and charge interactions of Rb+/NH4+. They show phase-matching second harmonic generation responses with moderate intensity of 1.2 ​× ​KDP (Rb+) and 1.1 ​× ​KDP (NH4+). They also possess large optical band gaps over 4.25 ​eV and can be easily grown from aqueous solutions. The local dipole moment calculations based on the structures of AIO3·H6TeO6 (A ​= ​K+, Rb+, NH4+) series of compounds indicate that the change of A+ cation in their structural frame causes a slight change in the arrangement directions of [IO3] and [Te(OH)6] groups, and further induces their SHG responses of different intensities at the macroscopic level.

Real‐Space Observation of Potential Reconstruction at Metallic/Insulating Oxide Interface

AbstractElectric field reconstruction at interfaces plays a crucial role in device performances controlling, for example, Schottky potential barrier and interfacial Rashba effect. Here, scanning transmission electron microscopy (STEM) and ab‐initio calculation are used to estimate the atomic‐scale and large‐scale potential reconstruction at the interface between a metallic oxide SrRuO3 (SRO) thin film and an insulating DyScO3 (DSO) substrate. The intensity and the symmetry of the large‐scale electrostatic reconstruction at the interface is probed by 4D‐STEM discussing the center‐of‐mass shift for different angular ranges detection. Numerical simulations indicate that thermal diffuse scattered (TDS) electrons can be sensitive to large‐scale electric field and experiments based on these diffused electrons near the interface confirm that the electric field extends more in the insulating DyScO3 (DSO) side. The magnitude of the electrostatic drop at the interface estimated by the 4D‐STEM experiment is in accordance with the ab‐initio values for a p‐type reconstruction of the interface plane. Furthermore, an atomically resolved TDS potential asymmetry is observed in real‐space at the SRO/DSO interface by 4D‐STEM. This asymmetry is associated with the formation of a local ferroelectric type dipole at the interfacial unit‐cell revealing unambiguously the balance evolution between antiferrodistortive and ferroelectric instabilities at the interface between a metallic SRO and an insulating DSO.

Formation of Monolayer Charge Density Waves and Anomalous Edge Doping in Na Doped Bulk VSe2

AbstractAlkali atom doping is an efficient way to induce charge transfer and Fermi level tuning in layered materials through intercalation. However, there is a general lack of microscopic understanding of the effect of doping inhomogeneity in geometric and electronic aspects. Here, we report surface doping of a bulk VSe2 crystal by sodium. Na atoms form intercalated subsurface islands that modify the electronic phase of the top layer of VSe2. In addition to n‐doping, the charge density wave of the intercalated VSe2 surface layer changes from the (4 × 4) bulk phase to the () known in monolayer phase of VSe2. Surprisingly, an electronic state at the edges of Na‐intercalated area shift anomalously upward in energy as detected by scanning tunneling spectroscopy. This is explained by a local gating effect resulting from local dipoles at the edges. The study illustrates a clear example of intercalation effect that should be general in alkali‐intercalated bulk layered materials.

High energy density and superior charge/discharge efficiency polymer dielectrics enabled by rationally designed dipolar polarization

Although many dielectric polymers exhibit high energy storage density (Ue) with enhanced dipolar polarization at room temperature, the substantially increased electric conduction loss at high applied electric fields and high temperatures remains a great challenge. Here, we report a strategy that high contents of medium-polar ester group and end-group (St) modification are introduced into a biodegradable polymer polylactic acid (PLA) to synergistically reduce the loss and enhance Ue and charge-discharge efficiency (η). The resultant St-modified PLA polymer (PLA-St) exhibits an Ue of 6.5 J/cm3 with an ultra-high η (95.4%), far outperforming the best reported dielectric polymers. It is worth noting that the modified molecular structures can generate deep trap centers and restrict the local dipole motions in the polymer, which are responsible for the reduction of conduction loss and improvements in high-temperature capacitive performance. In addition, the PLA-St polymer shows intrinsically excellent self-healing ability and cyclic stability surviving over 500 000 charge-discharge cycles. This work offers an efficient route to next-generation eco-friendly dielectric polymers with high energy density, low loss, and long-term stability.

Localized Origin at the Core‐Mantle Boundary of the 1969 Geomagnetic Impulse

Localized Origin at the Core‐Mantle Boundary of the 1969 Geomagnetic Impulse

Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb1.5Nb2O6.5.

Lone pair-driven distortions are a hallmark of many technologically important lead (Pb)-based materials. The role of Pb2+ in polar perovskites is well understood and easily manipulated for applications in piezo- and ferroelectricity, but the control of ordered lone pair behavior in Pb-based pyrochlores is less clear. Crystallographically and geometrically more complex than the perovskite structure, the pyrochlore structure is prone to geometric frustration of local dipoles due to a triangular arrangement of cations on a diamond lattice. The role of vacancies on the O' site of the pyrochlore network has been implicated as an important driver for the expression and correlation of stereochemically active lone pairs in pyrochlores such as Pb2Ru2O6.5 and Pb2Sn2O6. In this work we report on the structural, dielectric, and heat capacity behavior of the cation- and anion-deficient pyrochlore Pb1.5Nb2O6.5 upon cooling. We find that local distortions are present at all temperatures that can be described by cristobalite-type cation ordering, and this ordering persists to longer length scales upon cooling. From a crystallographic perspective, the material remains disordered and does not undergo an observable phase transition. In combination with density function calculations, we propose that the stereochemical activity of the Pb2+ lone pairs is driven by proximity to O' vacancies, and the crystallographic site disorder of the O' vacancies prohibits long range correlation of lone pair-driven distortions. This in turn prevents a low-temperature phase transition and results in an elevated dielectric permittivity across a broad temperature range.

Design Principles to Maximize Non‐Bonding States for Highly Tribopositive Behavior

AbstractThe development of highly tribopositive materials is crucial for realizing high‐performance triboelectric nanogenerators. In this study, a novel protocol to maximize the number of non‐bonding electrons with local dipoles for designing highly tribopositive materials is introduced, and nitrogen‐based dimethylol urea, diazolidinyl urea, and imidazolidinyl urea as promising tribopositive materials are suggested. The mechanism by which nitrogen‐based materials provide highly tribopositive properties is investigated using calculations based on density functional theory. Highly electronegative atoms, such as nitrogen and oxygen, attract electrons from neighboring atoms, resulting in the formation of negative local dipoles in the highest occupied molecular orbital band composed of non‐bonding electrons, thereby creating an electron‐donating environment. The proposed design protocol is confirmed by quantitatively investigating the triboelectric properties of nitrogen‐based materials, and analyzing the charge transfer characteristics of dimethylol urea based on dipole interactions.

High-efficient piezocatalytic hydrogen evolution by centrosymmetric Bi2Fe4O9 nanoplates

Exploring the manifold possibilities of energy conversion mechanisms to realize versatile catalytic reactions, particularly for clean energy, is highly desirable. In this work, Bi2Fe4O9 nanoplates with a centrosymmetric structure are discovered to have unprecedently high piezoelectric catalytic performance in H2 evolution (1058 µmol·g−1·h−1 in pure water and 5723 µmol·g−1·h−1 in 10% Methanol) and degradation of organics. A new perspective is proposed to explain the observed piezoelectric catalysis, that this piezoelectric property could have originated from the existence of local dipoles on exposed surfaces, originating from the non-centrosymmetric ligands in unit cells, as corroborated by synchrotron powder diffraction and Raman spectroscopy. The piezoresponse induced by the surface piezoelectric effect in Bi2Fe4O9 nanoplates has been verified by the 1st and 2nd harmonic piezoresponse measured by piezoelectric force microscopy (PFM) and visualized by fabricating energy conversion device. This work broadens the material candidates for piezoelectric catalysis from traditional piezoelectric materials to nanomaterials with centrosymmetric structures.

Berezinskii-Kosterlitz-Thouless phases in ultrathin PbTiO3/SrTiO3 superlattices

We study the emergence of Berezinskii-Kosterlitz-Thouless (BKT) phases in ${({\mathrm{PbTiO}}_{3})}_{3}/{({\mathrm{SrTiO}}_{3})}_{3}$ superlattices by means of second-principles simulations. Between a tensile epitaxial strain of $\ensuremath{\epsilon}=0.25--1\phantom{\rule{0.16em}{0ex}}%$, the local dipole moments within the superlattices are confined to the film-plane, and thus the polarization can be effectively considered as two-dimensional. The analysis of the decay of the dipole-dipole correlation with the distance, together with the study of the density of defects and its distribution as a function of temperature, supports the existence of a BKT phase in a range of temperature mediating the ordered ferroelectric (stable at low $T$), and the disordered paraelectric phase that appears beyond a critical temperature ${T}_{\mathrm{BKT}}$. This BKT phase is characterized by quasi-long-range order (whose signature is a power-law decay of the correlations with the distance), and the emergence of tightly bound vortex-antivortex pairs whose density is determined by a thermal activation process. The proposed ${\mathrm{PbTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattice model and the imposed mechanical boundary conditions are both experimentally feasible, making it susceptible for an experimental observation of these new topological phases in ferroelectric materials.

Simultaneous Photo-Induced Magnetic and Dielectric Switching in an Iron(II)-Based Spin-Crossover Hofmann-Type Metal-Organic Framework.

Molecular materials possessing photo-tunable polarization switching is promising for optical switches, smart sensors, and data storage devices. However, it is challenging to devise a molecular material featuring simultaneous switchable magnetic and dielectric properties with regard to non-invasive and convenient light stimulus. Herein, we report a new Hofmann-type metal-organic framework (MOF) {Fe(bpt)[Pt(CN)4 ]} ⋅ 0.5anth (1, bpt=2,5-bis(4-pyridyl)thiophen; anth=anthracene), which displays thermo- and photo-switchable magnetic and dielectric properties. Photo-monitored structural analyses revealed that it was the photo-induced deformation of FeII coordination sphere and relative movement of guest anthracene that resulted in the variation of the local electric dipoles. These findings provide a new strategy to realize polarization switching through the light-induced spin crossover, and would be of fundamental significance for future photo-switchable and multifunctional materials.

Simultaneous Photo‐Induced Magnetic and Dielectric Switching in an Iron(II)‐Based Spin‐Crossover Hofmann‐Type Metal‐Organic Framework

AbstractMolecular materials possessing photo‐tunable polarization switching is promising for optical switches, smart sensors, and data storage devices. However, it is challenging to devise a molecular material featuring simultaneous switchable magnetic and dielectric properties with regard to non‐invasive and convenient light stimulus. Herein, we report a new Hofmann‐type metal–organic framework (MOF) {Fe(bpt)[Pt(CN)4]} ⋅ 0.5anth (1, bpt=2,5‐bis(4‐pyridyl)thiophen; anth=anthracene), which displays thermo‐ and photo‐switchable magnetic and dielectric properties. Photo‐monitored structural analyses revealed that it was the photo‐induced deformation of FeII coordination sphere and relative movement of guest anthracene that resulted in the variation of the local electric dipoles. These findings provide a new strategy to realize polarization switching through the light‐induced spin crossover, and would be of fundamental significance for future photo‐switchable and multifunctional materials.

Design of Highly Positive Triboelectric Materials Via Controlling Local Dipole

The design of positive triboelectric materials is highly required to realize high-performance triboelectric generators. Here, we establish a novel design protocol for highly positive triboelectric materials via the maximization of local dipole and suggest nitrogen-based dimethylol urea, diazolidinyl urea, and imidazolidinyl urea as promising positive triboelectric materials. The mechanism that nitrogen-based materials provide highly positive triboelectric properties is investigated using density functional theory calculation. Highly electronegative atoms (e.g. nitrogen and oxygen) attract electrons from neighboring atoms, resulting in the formation of negative local dipoles in the highest occupied molecular orbital consisting of non-bonding electrons, thereby forming an electron-donating environment. We confirmed the proposed design protocol by quantitatively investigating the triboelectric properties of nitrogen-based materials and analyzing the charge transfer characteristics based on local dipole interactions.

Ferroaxial moment induced by vortex spin texture

The nature of an electric ferro-axial moment characterizing a time-reversal-even axial-vector quantity is theoretically investigated under magnetic orderings. We clarify that a vortex spin texture results in the emergence of the ferro-axial moment depending on the helicity. By introducing a multipole description, we show that the ferro-axial nature appears when two types of odd-parity magnetic multipoles become active under the vortex spin texture: One is the magnetic monopole and the other is the magnetic toroidal dipole. We present all the magnetic point groups to possess the ferro-axial moment in the presence of the magnetic orderings with the magnetic monopole and magnetic toroidal dipole. Moreover, we specifically consider a multi-orbital model in a four-sublattice tetragonal system to demonstrate the ferro-axial moment induced by the vortex spin texture. We show that the ferro-axial moment is microscopically characterized by both the cluster-scale and atomic-scale axial-vector quantities: The former is described by the vortex of the local electric dipole and the latter is represented by the outer product of the local orbital and spin angular momenta. Moreover, we find that the atomic-spin orbit coupling and the hybridization between orbitals with different parity are key ingredients to induce the ferro-axial moment. We also apply the result to a magnetic skyrmion with a topologically-nontrivial spin texture and discuss candidate materials.

Engineering Bound States in the Continuum at Telecom Wavelengths with Non‐Bravais Lattices

AbstractVarious optical phenomena can be induced in periodic arrays of nanoparticles by the radiative coupling of the local dipoles in each particle. Probably the most impressive example is bound states in the continuum (BICs), which are electromagnetic modes with a dispersion inside the light cone but infinite lifetime, that is, modes that cannot leak to the continuum. Symmetry‐protected BICs appear at highly symmetric points in the dispersion of periodic systems. Although the addition of nonequivalent lattice points in a unit cell is an easy and straightforward way of tuning the symmetry, BICs in such particle lattice, that is, non‐Bravais lattice, are less explored among periodic systems. Starting from a periodic square lattice of Si nanodisks, three non‐Bravais lattices are prepared by detuning size and position of the second disk in the unit cell. Diffraction‐induced coupling excites magnetic/electric dipoles in each nanodisk, producing two surface lattice resonances at the Γ point with a band gap in between. The high/low energy branch becomes a BIC for the size/position‐detuned array, respectively, while both branches are bright (or leaky) when both size and position are detuned simultaneously. The unexplored role of the interplay between magnetic and electric dipoles in dielectric nanoparticles in connection with the change of BIC to bright character in the detuned arrays is discussed with the aid of a coupled electric and magnetic dipole model. This study gives a simple way of tuning BICs at telecom wavelengths in non‐Bravais lattices, including both plasmonic and dielectric systems, thus scalable to a wide range of frequencies.

Local dipole enhancement of space-charge piezophototronic catalysts of core-shell polytetrafluoroethylene@TiO2 nanospheres

This study demonstrates the hydrothermal treatment of core-shell polytetrafluoroethylene (PTFE) nanoparticles decorated with TiO2 (h-PTFE@TiO2); this material exhibits strain-induced local dipole enhancement of the space-charge piezopotential that improves the photocatalytic effect. Under acoustic cavitation, dielectric barrier discharge and electric dipole formation are initiated in voids within the PTFE nanoparticles and at the TiO2–PTFE interface. The h-PTFE@TiO2 nanoparticles (NPs) have exceptionally high catalytic activity in organic dye degradation because of the local dipole enhancement of photoinduced charge separation with a carrier lifetime of 3.14 ns. The observed rate constant of the h-PTFE@TiO2 NPs in the piezophototronic reaction reaches 0.1388 min−1, which is 17 times that of the photocatalytic reaction (0.0084 min−1) and 66 times that of the piezocatalytic reaction (0.0021 min−1). Computational simulation reveals that large strain-induced space charge piezoelectric polarization induces an internal electric field between the unsaturated PTFE (fluorine vacancies) and TiO2 surface. The piezopotential has a critical role in band bending at PTFE-TiO2 interfaces to enhance the electron-hole separation when PTFE is constrained in TiO2 shells. Results of piezoresponse force microscopy reveal that the piezoelectric coefficient d33 of PTFE is approximately 79.77 pCN−1. The findings provide insights into catalytic activity for environmental remediation.