In-plane Dipole Research Articles

Improved Efficiency and Stability in Pure-Red CdSe Nanoplatelet LEDs Enabled by Gradient Alloyed CdSeS/CdZnS Crown/Shell.

Anisotropic nanoplatelets (NPLs) possess strong in-plane transition dipole moments and out-of-plane emission, which enable a maximum photon out-coupling efficiency of 40% and a high gain coefficient, making them ideal candidates for light-emitting diodes (LEDs) and lasers. However, the unbalanced surface energy between the side and top facets of NPLs results in poor thermal stability and high susceptibility to ripening at elevated temperatures, which complicates the growth of the shell. To address this issue, a gradient crown (CdSeS) around the CdSe NPLs is designed to stabilize the high energy side facets. Consequently, the gradient alloyed shell (CdZnS) is successfully grown, and the CdSe/CdSeS/CdZnS core/crown/shell NPLs exhibited near-unity photoluminescence quantum yield. The CdSeS/CdZnS crown/shell structure suppressed non-radiative Auger recombinations, achieving a record-low amplification spontaneous emission threshold of 2.11 µJcm-2 under femtosecond laser excitation. In addition, by selecting the carrier transport layers with matched energy levels, the NPL-LEDs demonstrate a record-high external quantum efficiency of 30.1% in the pure-red range, driven by the 94% in-plane transition dipole moment distribution of NPL film. The NPL-LEDs also exhibited a long operational lifetime of T95 > 600 h at a luminance of 1000 cdm-2.

Layered Babinet complementary patterns acting as asymmetric negative index metamaterial

Azimuthal orientation and handedness dependence of the optical responses, accompanied by asymmetric transmission and asymmetric dichroism, were demonstrated on multilayers constructed with subwavelength periodic arrays of Babinet complementary miniarrays, illuminated by linearly and circularly polarized light. In case of single-sided illumination asymmetric optical responses were observed at the spectral location of maximal cross-polarization that is accompanied by radiative electric dipoles and weak, slowly-rotating in-plane magnetic dipoles on the nano-objects; where the outgoing waves are elliptically (almost circularly) polarized. The negative index material phenomenon was demonstrated, where the electric and magnetic dipoles overlap both spatially and spectrally. The negative index material (NIM) phenomenon is accompanied by electric multipoles that add up non-radiatively and correlates with the strong, pronouncedly-tilted, rotating magnetic dipoles characteristic on the nano-entities, where the outgoing waves are linearly (slightly elliptically) polarized. By illuminating the multilayer with two counter-propagating circularly polarized beams it was proven that asymmetrical normal component displacement currents at the bounding interfaces, arising along flat and tilted bands, accompany the asymmetric copolarized and cross-polarized transmission. The latter correlates with the asymmetric dichroism in the cross-polarized signal observed in case of single-sided circularly polarized light illumination. The dispersion maps in the single-sided asymmetrical co-polarized reflectance and absorptance indicate flat bands of analogous and complementary extrema, proving the partially dichroic nature of the observed asymmetric phenomena. The Tellegen (chirality) coefficients exhibit a maximum in a spectral region coincident with the asymmetric transmission (maximal polarization rotation in the 90° azimuthal orientation). The multilayer is proposed as an ultrathin NIM and nonreciprocal nanophotonic element.

Mirror-coupled plasmonic nanostructures for enhanced in-plane magnetic dipole emission

Abstract Self-assembling of plasmonic nanoparticles into an oligomer can produce optical nanoantennas with resonant magnetic responses, which constitutes a promising platform to study magnetic light-matter interactions at the nanoscale. However, these self-assembled magnetic nanostructures typically feature a flat-lying geometric configuration, giving rise to out-of-plane magnetic dipole (MD) moments that cannot couple to the far-field light efficiently. Herein, we propose a new geometric configuration of plasmonic nanoantennas to realize in-plane MD responses. This is achieved by elegantly coupling a plasmonic nanoparticle oligomer to one or more adjacent metal mirrors, virtually forming a vertically standing nanoparticle tetramer or trimer, yet with in-plane MD moments. We verified this design strategy by numerically simulating the resonance responses of three typical mirror-coupled nanostructures, all exhibiting pronounced resonant MD characters. Furthermore, optical magnetic emitters can be readily coupled to these plasmonic nanoantennas and gain an emission enhancement of two orders of magnitude while simultaneously featuring a high emission directionality (collection efficiency up to ∼ 70 % evaluated with common microscopy optics). We believe these mirror-enabled magnetic nanoantennas could lead to novel nanophotonic devices fully exploiting the magnetic nature of light.

An Approach to Calculate Source Locations for Thicker Plates when Acoustic Emission Signals are Dominated by Trailing Waves

In many acoustic emission (AE) applications, signal arrival times from an array of sensors are coupled with an appropriate velocity to calculate the source location of AE events. To date, the AE signals present in these applications are Lamb and/or Rayleigh waves. In a previous publication [1], finite element modeled (FEM) AE signals in thicker plates were dominated by trailing wave (TW) packets, when no Rayleigh wave was present. The typical character of the TW dominated signals was described as a spaced-in-time train of short-in-time wave packets. Further, it was pointed out, that when TW dominate the signals, it is not clear how arrival times for source location calculations might be obtained along with an appropriate propagation velocity. The goal of this work is to develop a source location approach for cases of dominant TWs in the AE signals in thicker plates. The FEM-signal database from the previous publication was used for this study. The database consisted of in-plane dipole sources at five different depths and four different steel plate thicknesses (50, 75, 100 and 125 mm). Bottom-surface out-of-plane displacement signals versus time for propagation distances from 250 to 1500 mm were examined. The FEM signals were frequency filtered to examine a broadband case of 80 to 500 kHz and a narrowband case of 100 to 300 kHz. To study the potential source location accuracy, a total of seven four-sensor arrays were employed. The study results describe a process to select appropriate arrival times and obtain a propagation “velocity.” This information was used to calculate source locations for the sensor arrays. The location errors were shown to be less than the plate thickness.

Excitonic signatures of ferroelectric order in parallel-stacked MoS2

Interfacial ferroelectricity, prevalent in various parallel-stacked layered materials, allows switching of out-of-plane ferroelectric order by in-plane sliding of adjacent layers. Its resilience against doping potentially enables next-generation storage and logic devices. However, studies have been limited to indirect sensing or visualization of ferroelectricity. For transition metal dichalcogenides, there is little knowledge about the influence of ferroelectric order on their intrinsic valley and excitonic properties. Here, we report direct probing of ferroelectricity in few-layer 3R-MoS2 using reflectance contrast spectroscopy. Contrary to a simple electrostatic perception, layer-hybridized excitons with out-of-plane electric dipole moment remain decoupled from ferroelectric ordering, while intralayer excitons with in-plane dipole orientation are sensitive to it. Ab initio calculations identify stacking-specific interlayer hybridization leading to this asymmetric response. Exploiting this sensitivity, we demonstrate optical readout and control of multi-state polarization with hysteretic switching in a field-effect device. Time-resolved Kerr ellipticity reveals direct correspondence between spin-valley dynamics and stacking order.

Selective colorimetric sensing of histidine in aqueous media via Ag nanotriangles in the presence of HgCl2

Silver nanotriangles (AgNTs) were successfully synthesized as a colorimetric probe for selective and sensitive histidine detection in aqueous media within a 15–100 µM range and a detection limit of 330 nM using UV–Vis spectroscopy. The interaction of HgCl2 with AgNTs would lead to the formation of disk-shaped Ag/Hg amalgam as observed from the transmission electron images and X-ray diffraction patterns. Histidine prevents these structural and morphological changes and accordingly, the detection approach was developed based on the correlation between the histidine concentration and the in-plane dipole plasmon resonance (DPR) intensity.

Finite-Area Membrane Metasurfaces for Enhancing Light-Matter Coupling in Monolayer Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDCs) are at the forefront of nanophotonics because of their exceptional optical characteristics. The 2D architecture of TMDCs facilitates efficient light absorption and emission, holding tantalizing potential for next-generation nanophotonic and quantum devices. Yet, the atomic thinness limits their interaction volume with light, affecting light-matter interaction and quantum efficiency. The light coupling in the 2D layered TMDCs can be enhanced by integration with photonic structure, and the metasurfaces supporting bound states in the continuum (BICs) offer strong confinement of optical fields, ideal for coupling with 2D TMDCs. Here, we demonstrate enhanced light-matter coupling by integrating TMDC monolayers, including WSe2 and MoS2, with a finite-area membrane metasurface, leading to amplified and high-quality-factor (Q-factor) spontaneous emission from quasi-BIC-coupled TMDC monolayers. The high-Q-factor emission extends over an area with a scale of a few micrometers while maintaining the high-Q factor across the emission area. Notably, the suspended finite-area membrane metasurface, which is freestanding in air rather than positioned atop a substrate, minimizes radiation loss while enhancing light-matter interaction in the TMDC monolayer. Furthermore, the predominantly in-plane dipole orientation of excitons within TMDC monolayers results in distinctive enhancement behaviors for emission, contingent on the excitation power, when coupled with quasi-BIC modes exhibiting TE and TM resonances. This work introduces a nanophotonic platform for robust coupling of membrane metasurfaces with 2D materials, offering possibilities for developing 2D material-based nanophotonic and quantum devices.

(Invited) Mie Resonant Silicon Nanosphere Nanoantenna for Fluorescence Enhancement

A nanoantenna is a device that manipulates light propagation and enhances light-matter interaction at the nanoscale. Integration of an emitter into a nanoantenna capable of increasing local density of photonic states at the emission wavelength results in the enhancement of the spontaneous emission rate (Purcell effect) and modifies the emission spectrum. A nanoantenna can also control the directionality and radiation pattern of an emitter. Traditionally, plasmonic nanoantennas made from gold or silver nanostructures supporting localized surface plasmon resonances had been the main stream of the nanoantenna research. Recently, high refractive index nanoparticles having low-order Mie resonances have been attracting attention as a new type of dielectric nanoantennas. Advantages of a dielectric nanoantenna compared to a plasmonic one are the smaller loss and the possession of magnetic type Mie resonances arising from light-induced displacement current. By properly controlling the overlap of the magnetic and electric multipole resonances, radiation patters of an emitter placed nearby can be tailored with high degree of freedom. Furthermore, magnetic-type Mie resonances can couple with higher-order optical transitions of an emitter such as a magnetic-dipole transition, and thus can enhance and control radiation from otherwise weak forbidden transitions.Recently, we have succeeded in producing perfectly spherical nanoparticles of crystalline silicon (Si) in 100-300 nm in diameters. These Si nanospheres exhibit the electric and magnetic multipole resonances at the optical frequency [1]. In this presentation, we first show scattering and absorption properties of the Mie resonant Si nanospheres. Especially, we show that solutions of size-purified Si nanospheres exhibit vivid structural color and the solutions can be used as structural color inks. We then show that the solutions can preserve helicity of incoming circularly-polarized light and can enhance local chiral fields around nanospheres [2, 3]. This property can be used to improve the sensitivity of enantiomer-selective chiral molecular sensing. We then show some examples of enhanced light-matter interaction by Si nanoparticles. These include enhancement of magnetic dipole transitions of an emitter by magnetic-type Mie resonances of Si nanospheres (magnetic Purcell effect) [4, 5], enhancement of excitation and emission processes of in-plane dipoles of monolayer transition metal dichalcogenides (TMDC) [6], and enhancement of a spin forbidden singlet-to-triplet absorption transition of a molecule by Si nanodisks [7, 8].

Synthesis, Characterization, Acute Dermal Toxicity, Anti-inflammatory, and Wound Healing Potential of Biogenic Silver Nanoparticles in Balb C Mice.

The current study aimed to develop an economic plant-based therapeutic agent to improve the treatment strategies for diseases at the nano-scale. In the current research, silver nanoparticles were synthesized using Trillium govanianum aqueous extract. Characterizations were done using UV-visible spectrophotometer, X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. In vivo biological activities such as acute dermal toxicity, wound healing, and anti-inflammatory were done on Balb C mice. Absorbance at 295 nm corresponds to the out-of-plane quadrupole Plasmonresonance while at 350 nm corresponds to in-plane dipole resonance. SEM images showed the morphology of TGAgNPs is not exactly spherical while XRD analysis shows that highly crystalline TGAgNPs with an average size of 27.94 nm. The FTIR spectrum represents sharp peaks of aldehyde, amide I, aromatic rings, and polysaccharides. The microscopic assessment did not find any epidermal and dermal layer abnormalities in Blab C mice when exposed to TGAgNPs during acute dermal toxicity. Results revealed that 1000 mg/kg is not a lethal dose. In the wound healing activity, no mortality and no abnormal signs were observed when petroleum jelly, nitrofuranose, TGaqu, and TGAgNPs-based ointments were applied. Enhanced epithelization was recorded in TGaqu and TGAgNPs treated mice (p≤0.001). The wound contraction percentage was higher in nitrofuranose-treated mice (74%) followed by TGAgNPs (71%), and TGaqu (69%) compared to vehicle-treated and open-wounded mice. The paw edema model proved the potential use of TGAgNPs and TGaqu as anti-inflammatory agents. Hence, the results proved that both TGaqu and TGAgNPs are not toxic and possessed strong anti-inflammatory and wound-healing effects due to the presence of phytochemical constituents and could be used in various drug production as a therapeutic agent.

Highly tunable ground and excited state excitonic dipoles in multilayer 2H-MoSe2

The fundamental properties of an exciton are determined by the spin, valley, energy, and spatial wavefunctions of the Coulomb-bound electron and hole. In van der Waals materials, these attributes can be widely engineered through layer stacking configuration to create highly tunable interlayer excitons with static out-of-plane electric dipoles, at the expense of the strength of the oscillating in-plane dipole responsible for light-matter coupling. Here we show that interlayer excitons in bi- and tri-layer 2H-MoSe2 crystals exhibit electric-field-driven coupling with the ground (1s) and excited states (2s) of the intralayer A excitons. We demonstrate that the hybrid states of these distinct exciton species provide strong oscillator strength, large permanent dipoles (up to 0.73 ± 0.01 enm), high energy tunability (up to ~200 meV), and full control of the spin and valley characteristics such that the exciton g-factor can be manipulated over a large range (from −4 to +14). Further, we observe the bi- and tri-layer excited state (2s) interlayer excitons and their coupling with the intralayer excitons states (1s and 2s). Our results, in good agreement with a coupled oscillator model with spin (layer)-selectivity and beyond standard density functional theory calculations, promote multilayer 2H-MoSe2 as a highly tunable platform to explore exciton-exciton interactions with strong light-matter interactions.

2D Janus Polarization Functioned by Mechanical Force.

2Dpolarization materials have emerged as promising candidates for meeting the demands of device miniaturization, attributed to their unique electronic configurations and transport characteristics. Although the existing inherent and sliding mechanisms are increasingly investigated in recent years, strategies for inducing 2D polarization with innovative mechanisms remain rare. This study introduces a novel 2D Janus state by modulating the puckered structure. Combining scanning probe microscopy, transmission electron microscopy, and density functional theory calculations, this work realizes force-triggered out-of-plane and in-plane dipoles with distorted smaller warping in GeSe. The Janus state is preserved after removing the external mechanical perturbation, which could be switched by modulating the sliding direction. This work offers a versatile method to break the space inversion symmetry in a 2D system to trigger polarization in the atomic scale, which may open an innovative insight into configuring novel 2D polarization materials.

Probing the in-plane dipole moment vector between ground and excited state of single molecules by the Stark effect.

Single molecules, embedded inside a well-defined insertion site of a single-crystalline host matrix, are sensitive probes of electric field via the induced Stark shift on their lifetime-limited electronic transition. Though the response of molecules to electric field has been shown to be relatively homogeneous, crystal symmetry allows for several, spectroscopically-indistinguishable, orientations of the net permanent dipole moment between the ground and excited state - the dipole vector - and this is problematic for measuring field orientation and magnitude. In this work, we measure for each terrylene molecule, embedded inside a new host matrix, the dipole vector independently by an electric field that we can rotate in the plane of the crystal. This single crystal host matrix, called [1]BenzoThieno[3,2-b]BenzoThiophene, induces a moderate symmetry breaking of the embedded centrosymmetric terrylene molecule, and gives rise to a net dipole moment of 0.28±0.09 Debye. Based on quantum chemistry calculations we propose an insertion site that best matches the experimental findings.

Ultraviolet light scattering by a silicon Bethe hole.

Bethe's theory predicts that scattering by a small hole on a thin perfect electric conductor (PEC) is presented as radiation by an in-plane magnetic dipole of the incident magnetic field direction. Even in the near-infrared range where metals are no more PEC, the magnetic dipole radiation of Bethe holes has been demonstrated. However, such Bethe holes' nature has not been addressed yet in the ultraviolet (UV) range where conductivity of metals becomes severely deteriorated. Meanwhile, UV plasmonics has been elevating its importance in spectroscopy and photochemistry, recognizing silicon (Si) as an alternative plasmonic metal featuring the interband transition in the UV range. In this work, we expanded the Bethe's theory's prediction to the UV range by investigating Si Bethe holes theoretically and experimentally in terms of the scattering pattern and polarization. Simulation results showed that the scattered field distribution resembles that of an in-plane magnetic dipole, and the dipole direction at oblique incidence is roughly given as the incident magnetic field direction with a deviation angle which can be predicted from the Fresnel equations. Simulation with various diameters showed that the magnetic dipole nature maintains with a diameter less than the quarter-wavelength and multipoles becomes noticeable for diameters larger than the half-wavelength. We performed scattering polarization measurement at 69-degree incidence, which confirms the theoretical analysis. The features of Si Bethe holes demonstrated here will be useful for designing UV plasmonic metasurfaces.

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor.

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

Role of peripheral substitution on electronics and self-assembly of discotic semiconductors: The case of diazatruxenone

In this paper we present two sets of diazatruxenones functionalized at the 3,8 or 2,7 positions with two phenyl rings substituted at the para positions with groups of different electronic nature. With the aim to shed light on the role of the positional isomeric effect and of the electronic nature of the attached peripheral substituents on the final properties of these systems, we have performed a joint experimental and theoretical investigation linking thermal analysis, UV–Vis spectroscopy, electrochemistry and SCLC measurements with DFT calculations. The donor–acceptor character of these molecules not only confers them an interesting amphoteric redox behaviour but also creates a significant in-plane dipole moment. We have found that the different functionalization has only a slight impact on the electronic properties of these systems but exerts a significant influence on the strength and directionality of their dipole moment with strong implications in their self-assembly. Particularly, derivatives functionalized with electron acceptors, present the highest dipole moments within the series and form stable columnar mesophases, probably driven by the antiparallel dipole–dipole arrangement of neighbouring molecules. DFT calculations confirms that cofacially stacked dimers with their dipoles opposed allow for efficient electronic couplings helping to rationalize the favourable hole mobilities determined for the 3,8-disubstituted diazatruxenones with electron-acceptor groups by means of the space charge–limited current technique.

Strong Dipolar Repulsion of One-Dimensional Interfacial Excitons in Monolayer Lateral Heterojunctions.

Repulsive and long-range exciton-exciton interactions are crucial for the exploration of one-dimensional (1D) correlated quantum phases in the solid state. However, the experimental realization of nanoscale confinement of a 1D dipolar exciton has thus far been limited. Here, we demonstrate atomically precise lateral heterojunctions based at transitional-metal dichalcogenides (TMDCs) as a platform for 1D dipolar excitons. The dynamics and transport of the interfacial charge transfer excitons in a type II WSe2-WS1.16Se0.84 lateral heterostructure were spatially and temporally imaged using ultrafast transient reflection microscopy. The expansion of the exciton cloud driven by dipolar repulsion was found to be strongly density dependent and highly anisotropic. The interaction strength between the 1D excitons was determined to be ∼3.9 × 10-14 eV cm-2, corresponding to a dipolar length of 310 nm, which is a factor of 2-3 larger than the interlayer excitons at two-dimensional van der Waals vertical interfaces. These results suggest 1D dipolar excitons with large static in-plane dipole moments in lateral TMDC heterojunctions as an exciting system for investigating quantum many-body physics.

Creating a three-dimensional intrinsic electric dipole on rotated CrI3 bilayers

Two-dimensional (2D) materials are being explored as a novel multiferroic platform. One of the most studied magnetoelectric multiferroic 2D materials are antiferromagnetically coupled (AFM) ${\mathrm{CrI}}_{3}$ bilayers. Neglecting magnetism, those bilayers possess a crystalline point of inversion, which is only removed by the antiparallel spin configuration among its two constituent monolayers. The resulting intrinsic electric dipole on those bilayers has a magnitude no larger than 0.04 pC/m, it points out of plane, and it reverts direction when the Ising-like-chromium spins are flipped (toward opposite layers versus away from opposite layers). The combined presence of antiferromagnetism and a weak intrinsic electric dipole makes this material a two-dimensional magnetoelectric multiferroic. Here, we remove the crystalline center of inversion of the bilayer by a relative ${60}^{\ensuremath{\circ}}$ rotation of its constituent monolayers. This process enhances the out-of-plane intrinsic electric dipole tenfold with respect to its magnitude in the nonrotated AFM bilayer and also creates an even stronger and switchable in-plane intrinsic electric dipole. The ability to create a three-dimensional electric dipole is important because it enhances the magnetoelectric coupling on this experimentally accessible 2D material, which is explicitly calculated here as well.

Giant piezoresistivity in a van der Waals material induced by intralayer atomic motions

The presence of the van der Waals gap in layered materials creates a wealth of intriguing phenomena different to their counterparts in conventional materials. For example, pressurization can generate a large anisotropic lattice shrinkage along the stacking orientation and/or a significant interlayer sliding, and many of the exotic pressure-dependent properties derive from these mechanisms. Here we report a giant piezoresistivity in pressurized β′-In2Se3. Upon compression, a six-orders-of-magnitude drop of electrical resistivity is obtained below 1.2 GPa in β′-In2Se3 flakes, yielding a giant piezoresistive gauge πp of −5.33 GPa−1. Simultaneously, the sample undergoes a semiconductor-to-semimetal transition without a structural phase transition. Surprisingly, linear dichroism study and theoretical first principles modelling show that these phenomena arise not due to shrinkage or sliding at the van der Waals gap, but rather are dominated by the layer-dependent atomic motions inside the quintuple layer, mainly from the shifting of middle Se atoms to their high-symmetric location. The atomic motions link to both the band structure modulation and the in-plane ferroelectric dipoles. Our work not only provides a prominent piezoresistive material but also points out the importance of intralayer atomic motions beyond van der Waals gap.

On trailing waves rather than lamb waves being generated by buried in-plane dipoles in thick plates

The potential for trailing waves (TWs) rather than Lamb waves to be observed in the out-of-plane displacement signals from acoustic emission (AE) sources in thick plates was examined. Finite element modeling (FEM) was done for buried in-plane dipole (IPD) sources in steel plates with thicknesses up to 125 mm and propagation distances from 250 mm to 1500 mm. The domain was large enough so that significant reflections were not observed during the primary signal arrivals. The run times extended to 1 ms. “Ring-type” sources were used so that axisymmetric modeling could be done. The rise time of the IPDs was typically 1 μs with a cosine-bell time dependence, and the two 1 N forces were spaced by 200 μm. The IPDs were located at several depths from very near the top surface of the plates to the midplane. The displacements as a function of time were obtained on both the top and bottom surfaces. The analysis primarily considered a bandpass of 80 kHz to 500 kHz. In addition, fast Fourier transform results and time/frequency analysis were used. The typical character of TWs consisted of a train of short wave-packets (each 10 to 25 microseconds). The significant-amplitude TW packets arrived after an arrival time corresponding to the Rayleigh wave velocity. The number of packets in the train increased with increasing propagation distance. The likelihood of the signals exhibiting TWs increased with increases in plate thickness, increases in the distance of the IPD below the surface of interest. The presence of the TWs complicates the determination of fixed velocity arrival times necessary for source location determination. Also, since TWs rather than Rayleigh waves appear for IPD depths at some distance below the surface used to sense AE signals, there are potential issues when the AE approach depends on a Rayleigh wave being present.

Cs-Symmetric, Peripherally Fluorinated Boron Subphthalocyanine–Subnaphthalocyanine Hybrids: Shedding New Light on Their Fundamental Photophysical Properties and Their Functionality as Optoelectronic Materials

Hybridization of boron subphthalocyanine (BsubPc) and boron subnaphthalocyanine (BsubNc) has been modestly explored in the past, and was therefore carried out herein to access a subclass of these chromophores denoted as Ra-FxBsub(Pc3–p-Ncp) hybrids, where Ra is the axial halide substituent (axial chloride, Cl-, or axial fluoride, F-), x = 8, and p = 1. These chromophores were targeted as new candidate materials for organic electronic applications due to their Cs symmetry, in-plane dipole moment, and unique photophysical properties. Cl-/F- F8Bsub(Pc2-Nc1) hybrids were compared to Cl-/F- F8BsubPc hybrids, which are analogous compounds with an identical number of peripheral fluorine atoms but lower degree of π-conjugation. Upon photoexcitation of dilute toluene solutions and doped thin films with polystyrene as a nonemissive host, F8Bsub(Pc2-Nc1) hybrids exhibited distinctively broad absorption spectra and narrow emission profiles with red-shifted peak photoluminescence wavelengths in the 618–623 nm region compared to F8BsubPc hybrids (581–584 nm). Electroluminescence properties were probed in simple solution-cast organic light-emitting diodes (OLEDs) in which all four hybrids were diluted within an F8BT emissive polymer host. Upon electronic excitation, electroluminescence (EL) of Cs-symmetric hybrids followed the same trends as photoluminescence (PL), with OLED devices displaying narrow EL in the orange region (588–592 nm) for F8BsubPc hybrids and in the near-red region for F8Bsub(Pc2-Nc1) hybrids (627–632 nm). The photophysical processes important to OLEDs moderately improved with less π-conjugation in the periphery of the Cs-symmetric hybrids [F8Bsub(Pc2-Nc1) vs F8BsubPc; EQEMax: 0.099% vs 0.129%; luminance: 500 cd/m2 vs 1000 cd/m2 at maximum current density; relative PL quantum yields (QYs) ≈ 17–22% vs ≈ 35–44%]. Electrochemical data showed that Cs symmetry introduces reversibility or quasi-reversibility in the oxidation regime at potentials >1 V, and peripheral fluorination enables reversibility in the reduction process. Overall, F8Bsub(Pc2-Nc1) hybrids embody several unique physical characteristics into one: broad absorption spectra (full width at half-maximum height, FWHMsolabs = 53–55 nm, FWHMfilmabs = 71 nm), narrow near-red electroluminescence (FWHMOLEDEL = 33 nm), and photoluminescence (FWHMsolPL = 25–26 nm, FWHMfilmPL = 32 nm) emission, small energy band gaps (1.95–1.96 eV), high extinction coefficients (ε ≈ (5.65–6.16) × 104 M–1 cm–1), and dual electrochemical versatility. These results provide new physical insights into the material properties of Cs-symmetric macrocycles and advance the consideration of F8Bsub(Pc2-Nc1) and F8BsubPc hybrids for optoelectronic applications.