Browse Articles

The first step of cyanine dye self-assembly: Dimerization

Self-assembling amphiphilic cyanine dyes, such as C8S3, are promising candidates for energy storage and optoelectronic applications due to their efficient energy transport properties. C8S3 is known to self-assemble in water into double-walled J-aggregates. Thus far, the molecular self-assembly steps remain shrouded in mystery. Here, we employ a multiscale approach to unravel the first self-assembly step: dimerization. Our multiscale approach combines molecular dynamics simulations with quantum chemistry calculations to obtain a Frenkel exciton Hamiltonian, which we then use in spectral calculations to determine the absorption and two-dimensional electronic spectra of C8S3 monomer and dimer systems. We model these systems solvated in both water and methanol, validating our model with experiments in methanol solution. Our theoretical results predict a measurable anisotropy decay upon dimerization, which is experimentally confirmed. Our approach provides a tool for the experimental probing of dimerization. Moreover, molecular dynamics simulations reveal that the dimer conformation is characterized by the interaction between the hydrophobic aliphatic tails rather than the π–π stacking previously reported for other cyanine dyes. Our results pave the way for future research into the mechanism of molecular self-assembly in similar light-harvesting complexes, offering valuable insights for understanding and optimizing self-assembly processes for various (nano)technological applications.

Perturbation energy extraction from a fluid via a subsurface acoustic diode with sustained downstream attenuation

Engineered subsurface structures inspired by phononic materials have shown favorable capabilities in flow control. Current efforts rely on tuning a structural resonance to the frequency of an unstable fluid mode, causing the deformations of the interfacing solid to destructively interfere with a wave-like flow instability. Although promising, the technique is most effective at targeting a single frequency mode with high-Q resonance. Additionally, several studies have shown the reduction in the perturbation energy to be spatially localized to the flow region above the subsurface strip, with a severely worsening effect downstream. Motivated by a desire to overcome these limitations, we present a different approach to methodically extract an undesirable pressure disturbance from a fluid column in a manner that capitalizes on the broad frequency range of a phononic bandgap, with the goal of permanently confining the perturbation energy within the subsurface structure. An acoustic diode (AD), i.e., a unidirectional transmitter of mechanical energy, comprised of a bi-layered phononic crystal and an auxiliary medium, interacts with a fluid cavity and provides a terminal energy sink. Two distinct ADs are presented that demonstrate active (time-variant) and passive (strain-dependent) paths to concept realization. The AD’s performance is described in terms of the time-transient energy distribution in the fluid and the interacting structure, as well as spatial wave profiles at critical time instants. The results show the system’s ability to achieve robust extraction of undesirable fluid oscillations with minimal residual energy. The concept is then tested in a fully developed plane channel flow with a superimposed perturbation, demonstrating the sustained nature of the subsurface AD’s energy trapping mechanism in addition to its ability to induce downstream attenuation.

Applicability of the thermodynamic and mechanical route to Young’s equation for rigid and flexible solids: A molecular dynamics simulations study of a Lennard-Jones system model

The wetting properties of a liquid in contact with a solid are commonly described by Young’s equation, which defines the relationship between the angle made by a fluid droplet onto the solid surface and the interfacial properties of the different interfaces involved. When modeling such interfacial systems, several assumptions are usually made to determine this angle of contact, such as a completely rigid solid or the use of the tension at the interface instead of the surface free energy. In this work, we perform molecular dynamics simulations of a Lennard-Jones liquid in contact with a Lennard-Jones crystal and compare the contact angles measured from a droplet simulation with those calculated using Young’s equation based on surface free energy or surface stress. We analyze cases where the solid atoms are kept frozen in their positions and where they are allowed to relax and simulate surfaces with different wettability and degrees of softness. Our results show that using either surface free energy or surface stress in Young’s equation leads to similar contact angles but different interfacial properties. We find that the approximation of keeping the solid atoms frozen must be done carefully, especially if the liquid can efficiently pack at the interface. Finally, we show that to correctly reproduce the measured contact angles when the solid becomes soft, the quantity to be used in Young’s equation is the surface free energy only and that the error committed in using the surface stress becomes larger as the softness of the solid increases.

Misfit accommodation in a single interface atomic layer at a highly lattice-mismatched InN/GaN

Heterostructures of covalent semiconductors provide an invaluable platform for synthesizing the distinct properties of materials, leading to unprecedented functions in electronic and optoelectronic devices. The main challenge has been to forge high-quality interfaces of the heterostructures that guarantee the designed properties. To date, high-quality interfaces have been attained in heterostructures with a lattice mismatch of less than a few percent. However, for highly lattice-mismatched interfaces, such as InN/GaN (0001) (11.1% mismatch), interfacial structures remain unknown. Here, we investigate the atomic structure of the InN/GaN interface using atomic-resolution transmission electron microscopy and large-scale density-functional calculations. Our findings show that an interface structure without any misfit dislocations is formed, where an InN single monolayer at the interface accommodates the entire misfit. We argue that the mechanism underlying the formation of this interface monolayer is the flexibility of the group III–nitrogen bond network.

Reevaluating infrared spectroscopic signatures of polaron trapping in a chemically doped conducting polymer

Charge conductivity in conducting polymers is typically improved by increasing carrier density via chemical oxidation. However, the resulting electrostatic stabilization of the carriers by the dopant ions, combined with their nanostructural environment, are both known to crucially affect charge trapping. Although the effects of charge–ion electrostatic interactions on carrier trapping have been well-characterized using conventional infrared (IR) spectroscopy, the impacts of the polymer chain ordering and energetic environment are difficult to disentangle. In this study, we examine the limitations of conventional IR absorption spectroscopy and introduce a complementary spectroscopic approach capable of discerning polaron trapping more generally. To do so, we investigated films of poly(3-hexylthiophene-2,5-diyl) (P3HT) chemically doped using four different oxidants, of which each preferentially dopes the amorphous and crystalline (lamellar) phases to varying extents. Using this model system, we observed counterintuitive shifts in the polaron IR absorption band, indicating that IR spectroscopy is a clear reporter of trapping only when the carriers exclusively reside in the lamellar phase and in the absence of bipolarons or coupled polarons. Alternatively, we found that polaron excited state dynamics, probed using ultrafast near-infrared transient absorption spectroscopy, more clearly report on charge trapping. This study demonstrates near-infrared transient absorption spectroscopy as a complementary tool for probing charge trapping in conducting polymers when doping induces carriers in different nanostructural environments.

Thermoelectric algebra made simple for thermoelectric generator module performance prediction under constant Seebeck coefficient approximation

Although thermoelectric material performance can be estimated using the dimensionless figure of merit ZT, predicting the performance of thermoelectric generator modules (TGMs) is complex owing to the nonlinearity and nonlocality of the thermoelectric differential equations. Here, we present a simplified thermoelectric algebra framework for predicting TGM performance within the constant Seebeck coefficient approximation (CSA). First, we revisit the constant Seebeck coefficient model (CSM) to transform the differential equations into exact algebraic equations for thermoelectric heat flux and conversion efficiency in terms of the load resistance ratio and relative Fourier heat flux. Next, we introduce the CSA, where the Thomson term is neglected and the device parameters are assumed to be fixed. We define the average thermoelectric properties and device parameters under the zero-current condition using a simple temperature integral. Finally, we derive approximate thermoelectric algebraic equations for voltage, resistance, heat flux, and conversion efficiency as functions of current. We numerically validate that the CSA formalism is superior to other single-parameter theories, such as peak-ZT, integral-ZT, and generic engineering-ZT, in predicting efficiency. The relative standard error of the optimal efficiency is less than 11% for average ZT values not exceeding 2. By combining the CSM and CSA, TGM performance can be easily estimated without requiring calculus or solving differential equations. Therefore, this simplified thermoelectric algebra under the CSA framework has the potential to significantly enhance future TGM analysis and design, facilitating more efficient device-level research and development beyond the traditional focus on material properties.

Time-resolved vacuum-ultraviolet photoelectron spectroscopy of the Ã1Au state of acetylene

Ultrafast time-resolved photoelectron spectra are reported for the vacuum-ultraviolet (VUV) photoionization of acetylene following excitation to the Ã1Au state via UV absorption at 200 nm. The excitation energy lies above the lowest dissociation threshold to C2H X̃2Σ+ + H, as well as above the threshold for adiabatic dissociation of the Ã1Au state to form C2H (Ã2Π) + H. The time-dependent mass spectra and photoelectron spectra provide insight into the intramolecular decay processes of the Ã1Au state. In addition, photoelectron spectra of the Ã1Au state with VUV light access both the X̃2Πu and Ã2Σg+ states of the ion, as well as the predicted, but previously unobserved, 1 2Πg state, which corresponds to a two-hole, one-particle configuration that lies in close proximity to the Ã2Σg+ state. The 1 2Πg state is split into 2A2 + 2B2 and 2Ag + 2Bg states in the cis and trans configurations, respectively. Electronic structure calculations, along with trajectory calculations, reproduce the principal features of the experimental data and confirm the assignment of the 1 2Πg state.

Comprehensive thermal property measurement of semiconductor heterostructures using the square-pulsed source (SPS) method

Accurate thermal property measurements of multilayer heterostructures, especially in wide-bandgap semiconductor devices, are essential for optimizing device performance. While traditional methods, such as Time-Domain Thermoreflectance (TDTR) and Frequency-Domain Thermoreflectance (FDTR), are effective for thin films and interfaces, they encounter challenges with complex multilayer heterostructures. This paper presents an optical Square-Pulsed Source (SPS) method for comprehensive thermal property measurements of heterostructures. By combining the advantages of TDTR and FDTR, the SPS technique enables time-resolved signal observation and offers an adjustable heating frequency range from 1 Hz to 10 MHz. This allows for the simultaneous determination of multiple parameters, including the thermal conductivity and the heat capacity of each layer and the substrate, and interfacial thermal conductance between layers. Applied to epitaxially grown GaN/Si and ɛ-Ga2O3/SiC heterostructures, the SPS method demonstrates its simplicity, robustness, and precision for comprehensive thermal property analysis, essential for effective thermal management in advanced electronic devices.

Simulation of electron and nuclear spin dynamics in many-spin charge-separated states

This study presents a numerical simulation approach to investigate singlet–triplet interconversion effects in organic materials with rigid molecular structures that facilitate the photogeneration of charge-separated (CS) states, such as zwitterions resulting from intramolecular electron transfer. Our approach enables the detailed modeling of electron and nuclear spin-dependent observables, including magnetic field-affected reaction yields (MARY) and chemically induced dynamic nuclear polarization (CIDNP). The equilibrium solution of the stochastic Liouville equation can be obtained with simple algebraic manipulation by noting the relationship between the Laplace transform of the density operator and the time-domain representation of the same operator. Experimental MARY and CIDNP data are modeled as functions of key external and internal system parameters, such as magnetic field strength, hyperfine interactions, and exchange couplings. This allows for exploring processes that are otherwise experimentally inaccessible, providing deeper insights into the spin dynamics of the photoinduced CS state. Understanding these interconversion processes is not only essential for the fundamental photochemistry studies but also for the rational design and development of novel organic materials for photovoltaics and photocatalysis. Our results demonstrate the significant impact of singlet–triplet interconversion on the overall efficiency of charge separation and recombination processes, highlighting the importance of spin dynamics in the design of next-generation organic photovoltaic materials.

Research progress on the principle and application of metalenses based on metasurfaces

The metalens discussed in this article is a planar optical device built on metasurfaces, utilizing precisely engineered subwavelength nanostructured wires to manipulate electromagnetic waves. This enables precise control over the phase, amplitude, and polarization of these waves. Recent advancements in metalens research primarily focus on their design principles, performance optimization, imaging enhancement, manufacturing techniques, and various application domains. This article reviews the progress in metalens development, highlighting their design principles across different frequency bands and their applications in science and technology. Future research directions are expected to emphasize material innovation, simplification of manufacturing processes, and further performance improvements to drive their adoption in cutting-edge fields.

Developing orbital-dependent corrections for the non-additive kinetic energy in subsystem density functional theory

We present a novel route to constructing cost-efficient semi-empirical approximations for the non-additive kinetic energy in subsystem density functional theory. The developed methodology is based on the use of Slater determinants composed of non-orthogonal Kohn–Sham-like orbitals for the evaluation of kinetic energy expectation values and the expansion of the inverse molecular-orbital overlap matrix into a Neumann series. By applying these techniques, we derived and implemented a series of orbital-dependent approximations for the non-additive kinetic energy, which are employed self-consistently. Our proof-of-principle computations demonstrated quantitatively correct results for potential energy curves and electron densities and hinted on the applicability of the introduced empirical parameters to different types of molecular systems and intermolecular interactions. Therefore, we conclude that the presented study is an important step toward constructing accurate and efficient orbital-dependent approximations for the non-additive kinetic energy applicable to large molecular systems.

Erratum: “Electrical and structural characterization of YAlN at high alloy concentrations” [J. Appl. Phys. 136, 185103 (2024)]

Transient 19F photo-CIDNP: A practical tool to distinguish intermediate radical species and determine isotropic hyperfine coupling constants of 19F nuclei

Fluorine-containing flavin derivatives can be used as probes in flavin-binding proteins forming radical pairs to exploit the photo-chemically induced dynamic nuclear polarization (photo-CIDNP) effect. Knowledge of the hyperfine structure is crucial for studying the mechanism of intramolecular radical-pair formation in proteins. Transient 19F photo-CIDNP NMR has so far not been used to determine the isotropic hyperfine coupling constants of 19F nuclei. Here, we show that this method provides reliable results by studying three monofluorinated flavin mononucleotide (FMN) derivatives in conjunction with 6-fluoro-tryptophan. Combining this method with transient 1H photo-CIDNP spectroscopy leads to a more accurate interpretation of the intermediate radical species forming a radical pair. The gathered information can be used to identify the most promising FMN derivative for usage as a probe for formation of radical pairs in proteins.

Nanoscopic structural and optical investigations on blue InGaN single quantum wells serving as layers beneath efficient red active layers

In state-of-the-art red InGaN light-emitting diodes (LEDs), an InGaN-based blue single quantum well (SQW) is used as an underlying layer to improve the red emission efficiency. However, the role of blue SQW is not fully understood. This study investigates the structural and optical properties of blue InGaN SQW by atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy under scanning near-field optical microscopy (SNOM). The AFM images reveal deep and shallow V-pits, corresponding to screw and mixed threading dislocations (TDs). The SNOM-PL intensity image illustrates that all the V-pits correspond to dark spots. We also observe dark spots not associated with the V-pits; given the correspondence between the dark spot density and the edge TD density estimated via x-ray diffraction measurement, these dark spots likely originate from edge TDs. Unlike previous studies, we find that TDs act as nonradiative recombination centers (NRCs) in recent blue SQW, most likely because of the reduction of point defects by InGaN/GaN superlattices. Edge, screw, and mixed TDs have nearly the same impact on the integrated PL intensity of the blue InGaN SQW. Given the correlation of dark emission positions between the blue and red emissions in hybrid red InGaN LEDs in our prior study, all the TDs should function as NRCs in the red emission. The comparable dark spot density between the blue and red SQW suggests that blue SQW suppresses the generation of NRCs in red SQW.

High temperature melting of dense molecular hydrogen from machine-learning interatomic potentials trained on quantum Monte Carlo

We present results and discuss methods for computing the melting temperature of dense molecular hydrogen using a machine learned model trained on quantum Monte Carlo data. In this newly trained model, we emphasize the importance of accurate total energies in the training. We integrate a two phase method for estimating the melting temperature with estimates from the Clausius–Clapeyron relation to provide a more accurate melting curve from the model. We make detailed predictions of the melting temperature, solid and liquid volumes, latent heat, and internal energy from 50 to 180 GPa for both classical hydrogen and quantum hydrogen. At pressures of roughly 173 GPa and 1635 K, we observe molecular dissociation in the liquid phase. We compare with previous simulations and experimental measurements.

Discretizing the bistability of mode-locked electron spin precession: An Overhauser field hysteresis manifestation

Electron-nuclear spin interactions by pulsed optical pumping have been found to polarize the nuclear spin system, leading to the nuclei building up an intrinsic magnetic field known as the Overhauser field. Studies have indicated an Overhauser field hysteresis effect dependent on the sweep direction of an externally applied magnetic field in negatively detuned periodically pumped Si-doped GaAs. Although predictions of bistable mode-locked electron spin precession frequency modes have been made for systems exhibiting this hysteresis, there have been no reports on the experimental observation of said bistable spin precession modes. This report details the evolution of bistable Overhauser field solutions leading to a hysteretic effect in negatively detuned optical excitation of Si-doped GaAs by magneto-optic pump–probe spectroscopy in the Voigt geometry and investigates the resulting consequence of this hysteresis acting on the electron spin system. One manifestation of the Overhauser field hysteresis acting on the electron spin system leads to the discretization of bistable mode-locked electron spin precession modes within a given band of externally applied magnetic fields. A method for preferentially accessing the two different and stable mode-locked spin precession modes within a given band of externally applied magnetic field is outlined, which may be of interest for communities utilizing electron and nuclear spins for information processing protocols.

Combined molecular and spin dynamics simulation of BCC iron with vacancy defects

Utilizing an atomistic computational model, which handles both translational and spin degrees of freedom, combined molecular and spin dynamics simulations have been performed to investigate the effect of vacancy defects on spin wave excitations in ferromagnetic iron. Fourier transforms of space- and time-displaced correlation functions yield the dynamic structure factor, providing characteristic frequencies and lifetimes of the spin wave modes. A comparison of the system with a 5% vacancy concentration with pure lattice data shows a decrease in frequency and a decrease in lifetime for all transverse spin wave excitations observed. In addition, the clearly defined transverse spin wave excitations are distorted with the introduction of vacancy defects, and we observe reduced excitation lifetimes due to increased magnon–magnon scattering. We observe further evidence of increased magnon–magnon scattering, as the peaks in the longitudinal spin wave spectrum become less distinct. Similar impacts are observed in the vibrational subsystem, with a decrease in characteristic phonon frequency and flattening of lattice excitation signals due to vacancy defects.

<i>Ab initio</i> investigation on the effect of solid solution RE on interface and mechanical properties of Al–Mg–Si alloy

Investigating the role of solid-solution rare earth elements (REs) in Al–Mg–Si alloys helps to gain a deeper understanding of the mechanisms of solid-solution elements, enabling the adjustment of alloy composition to improve the microstructure and overall performance of the alloy. This study employs first-principles computational methods to reveal the impact of rare earth element addition on the interface properties of α-Al/β″–Mg5Si6 and α-Al/β–Mg2Si in Al–Mg–Si alloys, as well as the effects on the mechanical properties of β″ and β precipitate phases. The study first constructs two interface models, Al(130)/Mg5Si6(100) and Al(001)/Mg2Si(001), to analyze their interface properties. Based on this, the substitution positions of rare earth elements in the interface models after their addition are further discussed. The results indicate that the atomic substitution positions of rare earth elements are related to crystal structure, interface properties, rare earth atomic radius, and electronegativity. Interface property studies show that the addition of rare earth elements significantly enhances the adhesion of Al(130)/Mg5Si6(100) and Al(001)/Mg2Si(001) interfaces, reduces interface energy, and strengthens interface stability. Additionally, the effects of rare earth element addition on the lattice mismatch of Al(130)/Mg5Si6(100) and Al(001)/Mg2Si(001) interfaces exhibit opposite trends and, to some extent, inhibit the β″ → β phase transformation. Mechanical property studies of the precipitate phases reveal that rare earth atoms in solid solution in β″ and β phases decrease the bulk modulus, shear modulus, and Young modulus, while increasing the Poisson ratio and B/G ratio. This indicates that the introduction of solid-solution rare earth elements reduces the alloy's stiffness and shear resistance while enhancing its plasticity and brittleness.

Analytic calculation of transition dipole moment using four-component relativistic equation-of-motion coupled-cluster expectation value approach

We have developed an efficient scheme for the calculation of transition properties within the four-component relativistic equation-of-motion coupled cluster (EOM-CC) method using the expectation value approach. The calculation of transition properties within the relativistic EOM-CC framework requires the solution of both right and left eigenvectors. The accuracy of the approach has been investigated by calculating low-lying transitions of a Xe atom, a HI molecule, and spin forbidden 1S0 → 3P1 and spin allowed 1S0 → 1P1 transitions in a few closed shell cations. In addition to the valence spectra, the relativistic EOM-CCSD expectation value approach is particularly suitable for simulating the L-edge x-ray absorption spectrum (XAS). The calculated results show good agreement with the earlier reported theoretical studies and experimental values.

Semiconductor-to-metal transition in BaFe12−<i>x</i>(Ru, Re)<i>x</i>O19 (<i>x</i> = 0.1) hexaferrite at high temperatures

Semiconductor-to-metal-like behavior was observed in the BaFe11.9(Ru,Re)0.1O19 ceramic hexaferrite at temperatures above 450 K. X-ray diffraction analysis confirmed its hexagonal structure with minor α-Fe2O3 impurities, while Rietveld refinement revealed significant changes in the lattice parameters, particularly an expansion along the c axis. Atomic positions at the 2b, 4f2, and 12k sites were altered due to the partial substitution of Fe3+ by Ru4+ and Re4+ cations, which have smaller ionic radii than Fe3+. These substitutions modified the bond lengths within the crystal structure, as evidenced by increased Fe–O distances, and led to a partial reduction of Fe3+ to Fe2+, increasing electron density. The AC conductivity, σac(T), showed a transition from semiconducting to metallic-like behavior above ∼450 K. The resistivity, ρac(T), exhibited a plateau near 500 K, indicating a change in the conduction mechanism. The transport mechanism below 450 K was dominated by the non-overlapping small polaron tunneling model, characterized by thermally activated hopping with minimal Coulomb interactions, while above 450 K, the correlated barrier hopping model became dominant. The activation energy for high temperatures, Ea=0.31±0.06 eV, was consistent with the sum of the hopping energy WH=0.20±0.01 eV and half the disorder energy, Ed/2, which increased significantly with temperature. The partial substitution of Ru4+ and Re4+ significantly weakened the magnetic interactions, resulting in reduced saturation magnetization and modifications in coercivity.