Dipole Orientation Research Articles

Nanoscale resolved mapping of the dipole emission of hBN color centers with a scattering-type scanning near-field optical microscope

Abstract Color centers in hexagonal boron nitride (hBN) are promising candidates as quantum light sources for future technologies. In this work, we utilize a scattering-type near-field optical microscope (s-SNOM) to study the photoluminescence (PL) emission characteristics of such quantum emitters in metalorganic vapor phase epitaxy grown hBN. On the one hand, we demonstrate direct near-field optical excitation and emission through interaction with the nanofocus of the tip resulting in a subdiffraction limited tip-enhanced PL hotspot. On the other hand, we show that indirect excitation and emission via scattering from the tip significantly increases the recorded PL intensity. This demonstrates that the tip-assisted PL (TAPL) process efficiently guides the generated light to the detector. We apply the TAPL method to map the in-plane dipole orientations of the hBN color centers on the nanoscale. This work promotes the widely available s-SNOM approach to applications in the quantum domain including characterization and optical control.

Cyano‐Modified Multi‐Resonance Thermally Activated Delayed Fluorescent Emitters Towards Pure‐Green OLEDs with a CIEy Value of 0.74

AbstractThe development of pure‐green organic emitters with ideal emission peaks and ultra‐narrow full‐widths at half‐maximum (FWHMs) remains a formidable challenge. Herein, we report two new green emitters, CNBN and MCNBN, which achieve extremely narrow FWHMs by synergistic rigid π‐extension and cyano‐substitution of a sky‐blue multi‐resonance thermally activated delayed fluorescence (MR‐TADF) core. The introduction of cyano groups induces red‐shifts in the emission to the green region and dramatically minimizes the FWHMs. In toluene solution, CNBN and MCNBN exhibit narrowband emission with a maximum at 501 nm and 510 nm with ultra‐narrow FWHMs of 14 nm/0.066 eV and 15 nm/0.071 eV, respectively. Given the near‐unity photoluminescence quantum yields and almost 100 % horizontal dipole orientation, the electroluminescent (EL) devices based on CNBN and MCNBN deliver external quantum efficiencies (EQEs) exceeding 30 % with FWHMs of 16 nm/0.072 eV and 17 nm/0.080 eV, respectively. Notably, the MCNBN‐based device achieves pure‐green emission with a maximum at 517 nm with Commission Internationale de l’Éclairage coordinates of (0.17, 0.74), closely aligning with the BT.2020 green standard.

Enlarging moment and regulating orientation of buried interfacial dipole for efficient inverted perovskite solar cells

Carrier transport and recombination at the buried interface of perovskite have seriously restricted the further development of inverted perovskite solar cells (PSCs). Herein, an interfacial dipolar chemical bridge strategy to address this issue is presented. 2-(Diphenylphosphino) acetic acid (2DPAA) is selected as the linker to reconstruct the interfacial dipole, which effectively enlarges the interfacial dipole moment to 5.10 D and optimizes to a positive dipole orientation, thereby accelerating vertical hole transport, suppressing nonradiative recombination and promoting the perovskite crystallization. The champion inverted device yields a high power conversion efficiency (PCE) of 26.53% (certified 26.02%). Moreover, this strategy is extended to the wide-bandgap perovskite and large-area devices, which delivers high PCEs of 22.02% and 24.11%, respectively. The optimized devices without encapsulation also demonstrate great long-term shelf and operational stability. Our work highlights the importance of interfacial dipole moment and orientation at the buried interface to realize efficient and stable inverted PSCs.

Will Nanodust Reappear in STEREO/WAVES Data?

Abstract Nanodust particles produced near the Sun by collisional breakup of larger grains are accelerated in the magnetised solar wind and reach high speeds outwards of 1 au. Vaporisation and ionization of fast dust grains impacting a spacecraft produce voltage pulses on wave instruments that enable them to act as dust detectors. Wave instruments on STEREO and on Cassini during its cruise phase detected a highly variable flux of fast nanodust. Both detections took place when the orientation of the solar magnetic dipole produced an interplanetary electric field that focused nanoparticles towards the heliospheric current sheet—a geometry that is recurring because of the periodicity of solar activity.

Cyano-Modified Multi-Resonance Thermally Activated Delayed Fluorescent Emitters Towards Pure-Green OLEDs with a CIEy Value of 0.74.

The development of pure-green organic emitters with ideal emission peaks and ultra-narrow full-widths at half-maximum (FWHMs) remains a formidable challenge. Herein, we report two new green emitters, CNBN and MCNBN, which achieve extremely narrow FWHMs by synergistic rigid π-extension and cyano-substitution of a sky-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) core. The introduction of cyano groups induces red-shifts in the emission to the green region and dramatically minimizes the FWHMs. In toluene solution, CNBN and MCNBN exhibit narrowband emission with a maximum at 501 nm and 510 nm with ultra-narrow FWHMs of 14 nm/0.066 eV and 15 nm/0.071 eV, respectively. Given the near-unity photoluminescence quantum yields and almost 100 % horizontal dipole orientation, the electroluminescent (EL) devices based on CNBN and MCNBN deliver external quantum efficiencies (EQEs) exceeding 30 % with FWHMs of 16 nm/0.072 eV and 17 nm/0.080 eV, respectively. Notably, the MCNBN-based device achieves pure-green emission with a maximum at 517 nm with Commission Internationale de l'Éclairage coordinates of (0.17, 0.74), closely aligning with the BT.2020 green standard.

Synthesis of Biomass-Based Linear Aliphatic Polyesters Based on Sebacic Acid and 1,18-Octadecanedioic Acid and Their Thermal Properties and Odd-Even Effect.

A series of biomass-based linear aliphatic polyesters are synthesized by combining sebacic acid (SA) (C10 diacid) and 1,18-octadecanedioic acid (OA) (C18 diacid) with a series of diols with varied alkyl chain lengths (C2 to C10 diols). SA and OA are obtainable from castor oil and palm oil, respectively. The reaction extent (polymerization extent) is high (≥96%) in all cases, and the number-average molecular weight (Mn) is 10000-43000g mol-1 after purification. A possible limitation of currently available biomass-derived polyesters is their relatively low melting temperatures (Tm). The polyesters synthesized using OA with a long alkyl chain (C18 chain) in the present work exhibit relatively high Tm values of 78-93°C, which are rather close to that (105-118°C) of low-density polyethylene (LDPE), and may serve as biomass-based alternatives to LDPE with respect to thermal properties. Scientifically notably, an odd-even effect is observed in the Tm values. Polyesters with an even total-number of carbon atoms in the repeating unit have higher Tm values than their odd total-number counterparts likely due to their different orientations of dipoles of the polar ester groups along the backbone chain.

Pedagogical study of the image of a magnetic dipole in front of a superconducting sphere

Abstract The method of images to solve certain electrostatic boundary-value problems is taught worldwide in undergraduate-level physics courses. Though it is also possible to employ this technique for solving magnetostatic boundary value problems, examples of this usage are rarely found in textbooks or physics pedagogy literature. In particular, the problem of finding the field due to a magnetic dipole kept in front of a superconducting sphere is an interesting one, because (i) it helps the students to compare with the grounded conducting sphere image problem in electrostatics, and (ii) offers a greater degree of difficulty since the source is a dipole (vector), rather than an electric charge (scalar). The present work demonstrates an intuitive way of solving the problem. The case in which the source dipole is oriented radially with respect to the sphere is solved with a single dipole image. In the case of the transverse orientation of the source dipole, we model the dipole as a current loop. Then, we find the image of the radial and transverse current elements that satisfy the boundary conditions. Then, we show that this method can be used to deduce the form of the image dipoles when the dipole is oriented in the transverse direction. This method is very much intuitive and accessible for undergraduate-level students.

Solution-Processed Blue Narrowband OLED Devices with External Quantum Efficiency Beyond 35 % through Horizontal Dipole Orientation Induced by Electrostatic Interaction.

The multiple resonance thermally activated delayed fluorescence (MR-TADF) device has drawn great attention due to their outstanding efficiency and color purity. However, the efficiency of solution-processed MR-TADF devices is still far behind their vacuum-deposited counterparts, due to the uncontrollable horizontal emitting dipole orientation for emitters during solution process, resulting in low light out-coupling efficiency. Here, we proposed a new strategy namely electrostatic interaction between a dendritic host with high positive electrostatic potential (ESP) and dendritic emitter with multiple negative ESP sites, which could induce high horizontal dipole ratio (Θ||) up to 83.0 % in solution-processed films. For this couple, the largest plane of dendritic host tends to anchor on the substrate, and thus the strong positive electrostatic site mainly lies at the exposed tetraphenylsilicon, which could electrostatically attract the multiple negative electrostatic sites of the dendritic emitter, realizing horizontal dipole orientation. Moreover, the highly twisted structure of dendritic host and dendron encapsulation of emitter could effectively suppress aggregation, leading a high photoluminescence quantum yield of 98.6 %. As a result, the solution-processed blue MR-TADF devices exhibit a record-break external quantum efficiency of 35.3 %, as well as narrow bandwidth of 17 nm and pure blue color with CIE coordinates of (0.137, 0.176).

Solution‐Processed Blue Narrowband OLED Devices with External Quantum Efficiency Beyond 35% through Horizontal Dipole Orientation Induced by Electrostatic Interaction

The multiple resonance thermally activated delayed fluorescence (MR‐TADF) device has drawn great attention due to their outstanding efficiency and color purity. However, the efficiency of solution‐processed MR‐TADF devices is still far behind their vacuum‐deposited counterparts, due to the uncontrollable horizontal emitting dipole orientation for emitters during solution process, resulting in low light out‐coupling efficiency. Here, we proposed a new strategy namely electrostatic interaction between a dendritic host with high positive electrostatic potential (ESP) and dendritic emitter with multiple negative ESP sites, which could induce high horizontal dipole ratio (ΘII) up to 83.0% in solution‐processed films. For this couple, the largest plane of dendritic host tends to anchor on the substrate, and thus the strong positive electrostatic site mainly lies at the exposed tetraphenylsilicon, which could electrostatically attract the multiple negative electrostatic sites of the dendritic emitter, realizing horizontal dipole orientation. Moreover, the highly twisted structure of dendritic host and dendron encapsulation of emitter could effectively suppress aggregation, leading a high photoluminescence quantum yield of 98.6%. As a result, the solution‐processed blue MR‐TADF devices exhibit a record‐break external quantum efficiency of 35.3%, as well as narrow bandwidth of 17 nm and pure blue color with CIE coordinates of (0.137, 0.176).

Circular Dichroism and Interlayer Exciton Hall Effect in Transition Metal Dichalcogenides Homobilayers.

In van der Waals (vdW) architectures of transition metal dichalcogenides (TMDCs), the coupling between interlayer exciton and quantum degrees of freedom opens unprecedented opportunities for excitonic physics. Taking the MoSe2 homobilayer as representative, we identify that the interlayer registry defines the nature and dynamics of the lowest-energy interlayer exciton. The large layer polarization (Pn) is proved, which ensures the formation of layer-resolved interlayer excitons. In particular, sliding ferroelectric polarization couples to the dipole orientation of the interlayer exciton, thus achieving the long-sought electric control of excitonic states. In line with the phase winding of the Bloch states under C3 rotational symmetry, we clarify the valley optical circular dichroism, enriching the exciton valleytronics. We also elucidate the Hall effect of the layer- and valley-polarized interlayer excitons, which advances our understanding of the spatial transport properties of the composite particles and provides new insights into the exciton-based applications.

Electrostatically tuning radical addition and atom abstraction reactions with distonic radical ions.

Although electrostatic catalysis can enhance the kinetics and selectivity of reactions to produce greener synthetic processes, the highly directional nature of electrostatic interactions has limited widespread application. In this study, the influence of oriented electric fields (OEF) on radical addition and atom abstraction reactions are systematically explored with ion-trap mass spectrometry using structurally diverse distonic radical ions that maintain spatially separated charge and radical moieties. When installed on rigid molecular scaffolds, charged functional groups lock the magnitude and orientation of the internal electric field with respect to the radical site, creating an OEF which tunes the reactivity across the set of gas-phase carbon-centred radical reactions. In the first case, OEFs predictably accelerate and decelerate the rate of molecular oxygen addition to substituted phenyl, adamantyl, and cubyl radicals, depending on the polarity of the charged functional group and dipole orientation. In the second case, OEFs modulate competition between chlorine and hydrogen atom abstraction from chloroform based on interactions between charge polarity, dipole orientation, and radical polarizability. Importantly, this means the same charge polarity can induce different changes to reaction selectivity. Quantum chemical calculations of these reactions with DSD-PBEP86-D3(BJ)/aug-cc-pVTZ show correlations between the barrier heights and the experimentally determined reaction kinetics. Field effects are consistent between phenyl and cubyl scaffolds, pointing to through-space rather than through-bond field effects, congruent with computations showing that the same effects can be mimicked by point charges. These results experimentally demonstrate how internal OEFs generated by carefully placed charged functional groups can systematically control radical reactions.

Time domain reflectometry studies on benzaldehyde–ethanol binary solutions

Time domain reflectometry (TDR) study has been carried out on benzaldehyde (PhCHO) and ethanol (EtOH) binary solutions in the frequency range from 10 MHz to 25 GHz at different temperatures viz., 288.15, 293.15 and 298.15 K. The complex permittivity spectra have been fitted to the Debye function. Dielectric constant (ε0) vs mole fraction (X2, of EtOH ) graph shows that PhCHO-EtOH H-bond interactions are weaker in the solutions for which 0.0≤X2≤0.50 than in the solutions with 0.50<X2≤1. The relaxation time τ values suggest that lower order EtOH multimer involves in heteromolecular H-bond in PhCHO rich solutions. Antiparallel orientation of dipoles in pure PhCHO continues to exist upto X2 = 0.69. The smaller deviation of gf,εE/fB/ΔFE from the ideal behavior specify that the PhCHO interactions is weaker than EtOH-EtOH interactions. The well-known cyclic and linear dimers of PhCHO and cyclic multimers of EtOH is confirmed by geff and enthalpy values. The average number of hydrogen bonds have been analyzed using Luzar model calculations. EtOH multimers are found to be vulnerable on dilution by PhCHO. It appears that EtOH exhibits the same behavior in the solutions with aromatic carbonyl compounds.

Dipole-induced transitions from Schottky to Ohmic contact at Janus MoSiGeN4/metal interfaces.

Janus MoSiGeN4 monolayers exhibit exceptional mechanical stability and high electron mobility, which make them a promising channel candidate for field-effect transistors (FETs). However, the high Schottky barrier at the contact interface would limit the carrier injection efficiency and degrade device performance. Herein, using density functional theory calculations and machine learning methods, we investigated the interfacial properties of the Janus MoSiGeN4 monolayer and metal electrode contacts. The results demonstrated that the n-type/p-type Schottky and n-type Ohmic contacts can be realized in metal/MoSiGeN4 by changing the built-in electric dipole orientation of MoSiGeN4. Specifically, the contact type of Cu/MoSiGeN4 (Au/MoSiGeN4) transfers from an n-type Schottky (p-type Schottky) contact to an n-type Ohmic (n-type Schottky) contact when the contact side of MoSiGeN4 switches from Si-N to Ge-N. In addition, the Fermi level pinning (FLP) effect of metal/MoSiGeN4 with the Si-N side is weaker than that of metal/MoSiGeN4 with the Ge-N side due to the effect of intrinsic dipole and interface dipole. Notably, a simplified mathematical expression ΔV/WM is developed to describe the Schottky barrier height at metal/MoSiGeN4 interfaces using the machine learning method. These findings offer valuable guidance for the design and development of high-performance Janus MoSiGeN4-based electronic devices.

Fluorescence Interference-Based Polarized Structured Illumination Microscopy for High Axial Accuracy Morphology Imaging of Dipole Orientations

Fluorescence Interference-Based Polarized Structured Illumination Microscopy for High Axial Accuracy Morphology Imaging of Dipole Orientations

Collective Interactions of Quantum-Confined Excitons in Halide Perovskite Nanocrystal Superlattices.

Collective optical properties can emerge from an ordered ensemble of emitters due to interactions between the individual units. Superlattices of halide perovskite nanocrystals exhibit collective light emission, influenced by dipole-dipole interactions between simultaneously excited nanocrystals. This coupling changes both the emission energy and rate compared to the emission of uncoupled nanocrystals. We demonstrate how quantum confinement governs the nature of the coupling between the nanocrystals in the ensemble. The extent of confinement is modified by controlling the nanocrystal size or by compositional control over the Bohr radius. In superlattices made of weakly confined nanocrystals, the collective emission is red-shifted with a faster emission rate, showing the key characteristics of superfluorescence. In contrast, the collective emission of stronger quantum-confined nanocrystals is blue-shifted with a slower emission rate. Both types of collective emission exhibit correlative multiphoton emission bursts, showing distinct photon bunching emission statistics. The quantum confinement changes the preferred alignment of transition dipoles within the nanocrystal and switches the relative dipole orientation between neighbors, resulting in opposite collective optical behaviors. Our results extend these collective effects to relatively high temperatures and provide a better understanding of exciton interactions and collective emission phenomena at the solid state.

Investigation of the Structure-Property Relation of Anthraquinone Dye Molecules with High Dichroism in Guest-Host Liquid Crystal Systems via Computational Methods.

By combining molecular dynamics (MD) simulations and density functional theory (DFT), the influence of dye structure on the optical modulation properties of negative-mode guest-host liquid crystal (GHLC) systems was systematically investigated. Firstly, the reliability of the simulation method was validated by comparing the performance parameters of the GHLC system obtained from simulations with those from experimental results. Subsequently, a series of guest dye molecules, along with their mixtures with negative dielectric anisotropy mesogens, were designed and analyzed. This exploration focused on how variations in dye terminal chain lengths, substitution positions, and substituent group properties affect dye molecular geometry, dye alignment within the host, transition dipole orientation, absorption spectra, and electronic excitation properties. Our findings suggest that dye molecules with a flexible terminal chain substitution of five carbon atoms, positioned at the 2 and 6 locations on the anthraquinone core, exhibit higher order parameters, favorable for enhancing dichroic performance. Moreover, introducing different α-substituents further influences the dye orientation and electronic behavior within the host. These results highlight that structural modifications of anthraquinone-based dyes allow for the design of high-dichroic-ratio materials with customized absorption properties. Overall, our results provide a beneficial understanding of the structure-property relation in GHLC systems, offering valuable guidance for designing high-performance dye molecules and advanced optoelectronic materials in future research.

Effects of droplet deposition on aerosol capture efficiency of bipolarly charged fibers.

Aerosol filters composed of electrostatically charged bipolar fibers are referred to as electret filters. A novel computational model is developed in this work to study the impact of droplet deposition on aerosol capture efficiency of electret fibers. The electret fibers were assumed to have a dipole orientation that was either parallel or perpendicular to the airflow direction. The simulations were conducted using the ANSYS CFD code after it was enhanced with a series of in-house subroutines. Our simulations revealed that droplet deposition on electret fibers decreases their ability to capture airborne particles. More specifically, the simulations were devised to isolate droplet's physical and electrical properties (e.g., surface tension, electrical conductivity…) and quantify their impact on fiber capture efficiency. It was found, in particular, that droplet's electrical conductivity and permittivity have the most adverse impact on the performance of an electret fiber. This is perhaps because higher droplet conductivity results in severe fiber charge neutralization, and higher droplet permittivity leads to a stronger fiber charge shielding. In contrast, fiber wettability was found to have a negligible impact on fiber efficiency. The work presented in this paper offers valuable insights into the complex nature of electret filters used in different industrial and environmental applications.

Ligand discrimination in hOR1A1 based on the capacitive response

Odorant discrimination mechanisms are based on the differential interactions between odorant molecules and olfactory receptors (ORs). Biohybrid sensors based on ORs described to date show selectivity towards specific versus non-specific binding of odorants, being unable to distinguish between specific ligands of different affinity. Here we disclose a method that enables odorant discrimination based on the modulation of the capacitive response of the receptor, which allows the differentiation of three high-affinity hOR1A1 agonists. We performed voltammetry and impedance measurements of the hOR1A1 receptor selectively immobilized on a gold electrode in the absence and presence of the agonists. Binding induces a decrease in the capacitive response of the receptor that is proportional to the ligand potency, reaching up to a 40% decrease for the cognate ligand dihydrojasmone, which is attributed to changes in the magnitude and orientation of the electric dipole in the receptor, thereby regulating its response to the applied electric field.

A Piezocatalysis Strategy to Enable Efficient Redox in Solid‐State Battery

AbstractPiezocatalysis is considered to be a favorable catalytic technology by utilizing external mechanical stimulation to promote the specific redox reactions. The application of piezocatalysis in batteries is promising but faces formidable challenges. Herein, the operation principles of piezocatalysis were successfully demonstrated by constructing solid‐state Li−Se and Li−S battery models with interfacial stress accumulation. Lead zirconate titanate with an ultra‐high piezoelectric coefficient was applied as the piezoelectric catalyst. The uniformity of the dipole orientation in materials was essential for achieving piezocatalysis in batteries. The accumulated high‐stress converts to piezopotential for catalyzing most reaction processes in batteries, while the rapid stress change prevents the internal charge reorganization, ensuring continuous efficient catalysis. The internal stress change alters the internal polarisation strength, driving excess screening charge to participate in the electrochemical reaction. Under piezocatalysis, solid‐state Li−Se batteries exhibited a initial discharge capacity of 670.9 mAh g−1 (99.4 % of the theoretical value) at 0.1 C, and Li−S batteries showed a capacity of 1463 mAh g−1 at 0.2 C, which was still maintained up to 1084 mAh g−1 at an elevated rate of 0.5 C. This piezocatalysis strategy provides a strong theoretical basis and design specification for enhancing Li−Se and Li−S battery reaction kinetics.

A Piezocatalysis Strategy to Enable Efficient Redox in Solid-State Battery.

Piezocatalysis is considered to be a favorable catalytic technology by utilizing external mechanical stimulation to promote the specific redox reactions. The application of piezocatalysis in batteries is promising but faces formidable challenges. Herein, the operation principles of piezocatalysis were successfully demonstrated by constructing solid-state Li-Se and Li-S battery models with interfacial stress accumulation. Lead zirconate titanate with an ultra-high piezoelectric coefficient was applied as the piezoelectric catalyst. The uniformity of the dipole orientation in materials was essential for achieving piezocatalysis in batteries. The accumulated high-stress converts to piezopotential for catalyzing most reaction processes in batteries, while the rapid stress change prevents the internal charge reorganization, ensuring continuous efficient catalysis. The internal stress change alters the internal polarisation strength, driving excess screening charge to participate in the electrochemical reaction. Under piezocatalysis, solid-state Li-Se batteries exhibited a initial discharge capacity of 670.9 mAh g-1 (99.4 % of the theoretical value) at 0.1 C, and Li-S batteries showed a capacity of 1463 mAh g-1 at 0.2 C, which was still maintained up to 1084 mAh g-1 at an elevated rate of 0.5 C. This piezocatalysis strategy provides a strong theoretical basis and design specification for enhancing Li-Se and Li-S battery reaction kinetics.