Low Energy Region Research Articles

Effect of dipolar state on J-type aggregation of acceptor group modified pyrene-based push-pull systems

The present work demonstrates the ground and excited state aggregation behavior of two pyrene-based push–pull systems (Pyr-MC and Pyr-PyA) both in solid and solution state. The UV–Vis spectra of these molecules show a unique broaden spectral pattern at lower energy region (400–600 nm) in both H2O and solid state. Further, along with the local (intense) emission band, a new (less intense) band is appeared at higher wavelength region (∼550 nm) in water, which is due to the formation of aggregates. Interestingly, it is the lowest energy emission out of all previously reported emission by pyrene derivatives. The experimental results suggest the formation of J-type aggregation in solid and solution. The average lifetime of these molecules is also measured in different solvents. Intriguingly, the ground/excited state J-type aggregation in solid and solution are similar in nature in spite of different mode of intermolecular interactions, and are supported by experimental results. The dipolar state plays an important role on the J-type aggregation process. The QTAIM (quantum theory of atom in molecule) theoretical framework is applied to analyze and understand the properties of molecules in terms of the distribution of electrons and the topological features of the electron density within a molecule. Importantly, bond paths provide information about the nature of chemical bonds, including their strength and directionality.

Thickness-modulated electronic band structures and exciton behavior of non-van-der-waals 2D Bi2O2Se films

Bi2O2Se, as a non-van-der-waals (non-vdW) interlayer coupling two-dimensional material, its electronic band structures and exciton behavior still merit further in-depth studies, especially in the consideration of thickness tuning degrees of freedom. Here, we reveal the broadband excitonic performance and critical point properties of Bi2O2Se films with different thicknesses (3.54–16.33 nm). Five critical points are determined and assigned to specific optical transitions. Moreover, we observe that electronic bandstructures in the Γ valley are stable with variable thicknesses, while the deeper electron levels received more pronounced thickness modulation. And we first report four excitons in wide spectral range (1.24–6.20 eV) through absorption coefficient fitting. Significantly, due to the strong localization of electrons and holes induced by the intralayer built-in electric field, the exciton binding energy (Eb) value of 3.54 nm-Bi2O2Se (234 meV at Γ position) is apparently larger than typical multi-layer TMDCs, which offering unprecedented opportunities in applications in optoelectronic devices. Furthermore, Eb of two excitons at low energy region (Eb1, Eb2) red-shift with increasing thickness, while Eb of higher-energy excitons (Eb3, Eb4) blue-shift, owing to the dispersion characteristic of the effect dielectric screening effects. Our results are vital to wide-ranging applications of Bi2O2Se from fundamental science to optoelectronic devices.

Construction of diabatic potential energy surfaces for the SiH2+ system and dynamics studies of the Si+(2P1/2, 3/2) + H2 reaction.

The construction of diabatic potential energy surfaces (PESs) for the SiH2+ system, related to the ground (12A') and excited states (22A'), has been successfully achieved. This was accomplished by utilizing high-level abinitio energy points, employing a neural network fitting method in conjunction with a specifically designed function. The newly constructed diabatic PESs are carefully examined for dynamics calculations of the Si+(2P1/2, 3/2) + H2 reaction. Through time-dependent quantum wave packet calculations, the reaction probabilities, integral cross sections (ICSs), and differential cross sections (DCSs) of the Si+(2P1/2, 3/2) + H2 reaction were reported. The dynamics results indicate that the total ICS is in excellent agreement with experimental data within the collision energy range studied. The results also indicate that the SiH+ ion is hardly formed via the Si+(2P3/2) + H2 reaction. The results from the DCSs suggest that the "complex-forming" reaction mechanism predominates in the low collision energy region. Conversely, the forward abstraction reaction mechanism is dominant in the high collision energy region.

The Configuration Identification of Guanine Molecules Adsorbed on the Si(100) Surface by Using XPS Shake-up Satellites and NEXAFS.

Bioelectronic devices can be manufactured by organic-inorganic hybrid systems based on biomolecules and silicon semiconductors. The performance of the hybrid systems is largely determined by the adsorption manners of biomolecules on the silicon surface. In this paper, we demonstrated that the X-ray photoelectron spectroscopy (XPS) shake-up satellites and near-edge X-ray absorption fine-structure (NEXAFS) spectra at the carbon K-edges can be used to distinguish the interface of guanine molecules anchored on Si(100) surface. There are only 9 possible stable guanine@Si(100) hybrid systems that have been found based on the density functional theory. According to the characteristic peaks, it is confirmed that NEXAFS spectra are more sensitive to the identification of adsorption configurations. While the first characteristic peak in the low energy region of NEXAFS spectra are capable of distinguishing chemical bonds at the interface of the adsorption configurations. These results may facilitate a better understanding of the interface formations between biomolecules and silicon surfaces, which could be further utilized for the new bioelectronic device design.

Transmissions of an x-ray free electron laser pulse through Al: Influence of nonequilibrium electron kinetics.

A theoretical model for investigating the radiative transfer of an x-ray free electron laser (XFEL) pulse is developed based on a one-dimensional radiative transfer equation. The population dynamics of energy levels is obtained by rate equationapproximation coupling with the Fokker-Planck equation, in which the electron energy distribution function (EEDF) is self-consistently determined. As an illustrative example, XFEL pulse propagation through a solid-density aluminum (Al) is investigated. The characteristics of the temporal evolution of the x-ray pulse shape, level population, and EEDF are demonstrated. The EEDF usually has two parts in XFEL-Al interactions: the near equilibrium part in the lower energy regions and the nonequilibrium part in the higher energy region. The deep gap between the two parts is quickly filled in the solid-density Al plasma. The pulse shape is distorted and the duration shortens as the x-ray pulse propagates through the Al sample. The x-ray transmission spectra were compared with experimental and other theoretical results, and good agreement was found. There are slight discrepancies between the transmission obtained by solving the Fokker-Planck equationand Maxwellian assumptions because nonequilibrium electrons in the higher energy region account for only a small fraction of the total electrons.

Hydrostatic pressure-induced transformations and multifunctional properties of Francium-based halide perovskite FrCaCl3: Insights from first-principles calculations

Under varying hydrostatic pressures ranging from 0 to 150 GPa, first-principles calculations were conducted to investigate the structural, electronic, bonding, optical, elastic, and mechanical characteristics of the Lead (Pb)-free halide perovskite FrCaCl3 using both the GGA and hybrid HSE06 parameterized density functional theory (DFT). Since the FrCaCl3 cubic perovskite has not yet been synthesized experimentally, its structural and thermodynamic stabilities are confirmed by the Goldschmidt tolerance factor, the octahedral factor, and the formation energy. The induction of pressure has caused a simultaneous decrease in both the lattice parameters and the electronic band gap. Applying the hybrid HSE06 potential refines the accuracy of the band gap, with values decreasing from 5.705 to 2.618 eV from 0 to 150 GPa pressure, suggesting improved optoelectronic attributes. Employing pressure facilitates the formation of stronger chemical bonds characterized by reduced bond lengths. The investigation of optical functions demonstrates that with increased pressure ranging to 150 GPa, the optical conductivity along with the absorption coefficient is oriented towards the low-energy region. The FrCaCl3 perovskite has the prospect to be used in X-ray imaging and other fields of nuclear medicine and diagnostics as it contains the radioactive element Francium (Fr). Additionally, it is found via the study of mechanical characteristics that FrCaCl3 is mechanically stable under various applied pressure, and adding pressure makes it more ductile as well as more anisotropic.

Deformation effect on graphene quantum dot/graphane and silicene quantum dot/silicane array

This article presents a design for the two-dimensional heterostructure systems (2DHS) consisting of graphene quantum dot (QD) array in graphane (GQD/Graphane) and silicene QD array in silicane (SiQD/Silicane). First-principles method was used to evaluate the effect of deformation on the magnetism and electronic properties of these 2DHS. For the band engineering, the tuning range of the energy gap of GQD/Graphane and SiQD/Silicane 2DHS can be up to 1.20 and 1.35 eV, respectively, through strains ranging from −7% to 10 %. A strain-sharing effect is found, wherein the hydrogenated region shares a portion of strain within the QD array. This effect is stronger in SiQD/silicane than in GQD/graphane. Strain sharing enhances band coupling between the QDs and their hydrogenated counterpart in the low-energy region. This alters the electronic properties of the 2DHS and the magnetic properties of triangular and parallelogram SiQD/Silicane arrays under large compressive strain. Strain modulates the band gap of 2DHS, triggers a phase transition from semiconductor to metal in SiQD/Silicane systems under homogeneous strain, and removes the magnetism of triangular and parallelogram SiQD arrays under compressive strain. These findings suggest that the 2DHS could be promising for designing nanoelectronic devices and binary logic based on nanoscale magnetism.

The comparative study of structural, optoelectronic and thermoelectric properties of Li-based half-Heusler alloys via GGA and meta-GGA functionals

The present work aims to investigate the effects of meta-GGA functionals on the structural, optoelectronic and thermoelectric properties of LiXY(X=Be, Mg, Zn, Cd and Y=N, P, As, Sb, Bi) half-Heusler alloys using density functional theory (DFT) and classical Boltzmann transport theory. The obtained lattice constants of these alloys are in good agreement with the experimental values, exhibiting a slight decrease slightly with meta-GGA functionals compared with GGA-PBE. Band structure calculations demonstrate that the materials exhibit semiconductor character and provide validation for the use of meta-GGA functionals, particularly the r2SCAN and rSCAN, to predict the band gap values of LiXY more accurately. Optical and thermoelectric properties are calculated and discussed in detail. The results reveal that the meta-GGA functionals predict a significant change compared with GGA-PBE in the low energy and temperature region, while in the high energy and temperature regions, all four functionals present almost the same change trends.

New insights into the bonding, electronic and optical properties of fergusonite and scheelite ReNbO4 (Re = Y, La and Lu) compounds

A systematic study of the structural, bonding, electronic, and optical properties of the ReNbO4 (Re = Y, La, and Lu) compounds with the fergusonite (space group I/2a) and scheelite (space group I41/a) structure was performed using density functional theory calculations. The aim of these calculations was to resolve some doubt about these properties for the fergusonite phase and to fill the existing knowledge gap for the scheelite phase. To carry out geometry optimization, the generalized gradient approximation for solids was used. Based on these optimized crystal structures, the bonding, electronic and optical properties were determined using the modified Becke and Johnson exchange potential. An analysis of the bonds between Nb and O atoms showed that Nb atoms in the fergusonite structure of the rare-earth niobates were octahedrally coordinated to oxygen atoms, unlike in the scheelite phase, which had tetrahedral coordination. The calculated values of the direct energy band gap for the fergusonite YNbO4, LaNbO4 and LuNbO4 compounds were 4.435, 4.427 and 4.431 eV, respectively. The optical spectrum for these systems is characterized by a small exciton binding energy, which affects the experimental prediction of the energy band gap when the Tauc plot method is used. The values for the energy band gap in the scheelite phase were 4.625 eV for YNbO4, 4.844 eV for LaNbO4 and 4.611 eV for LuNbO4 and all were indirect. The real and imaginary parts of the dielectric tensor were computed up to 25 eV for both phases, and were interpreted based on their electronic structure. The lower energy regions of the optical spectra of these systems are mainly due to electronic transitions within the NbO6 polyhedron (for the fergusonite phase) and the NbO4 polyhedron (for the scheelite phase), i.e., electronic transitions from the occupied O-2p states at the top of the valence band to the unoccupied Nb-4d states at the bottom of the conduction band.

Infrared spectroscopic study on Nb-ion-irradiated MgB2 thin films

Magnesium diboride (MgB2) is a two-band superconductor with a high superconducting critical temperature (Tc) of approximately 39 K. Owing to the lack of vortex pinning centers, MgB2 exhibits an abrupt decline in the critical current density (Jc) in an applied magnetic field. Here, we prepared 1 MeV Nb ion-irradiated MgB2 thin-film samples with doses of 3×1013, 7×1013, and 9×1013 ions/cm2. Temperature-dependent magnetization and x-ray diffraction (XRD) measurements were performed to determine the Tc and c-axis lattice constant of each sample. Furthermore, a Fourier transform infrared (FTIR) spectroscopy was performed to obtain the infrared properties of the Nb-ion-irradiated MgB2 thin-film samples. The optical conductivity of each sample in the low-energy region was fitted with two (narrow and broad) Drude modes. We found that the spectral weight redistribution from the low-to high-frequency regions and the broadening of the narrow Drude mode caused by irradiation are closely related to the reduction in Tc.

Variable Peripheral Ligand Donation Tunes Electronic Structure and NIR II Emission in Tetrathiafulvalene Tetrathiolate Diradicaloids.

Near-infrared (NIR) lumiphores are promising candidates for numerous imaging, communication, and sensing applications, but they typically require large, conjugated scaffolds to achieve emission in this low-energy region. Due to the extended conjugation and synthetic complexity required, it is extremely difficult to tune the photophysical properties of these systems for desired applications. Here, we report facile tuning of deep NIR-emitting diradicaloid complexes through simple modification of peripheral ligands. These new lumiphores are rare examples of air-, acid-, and water-stable emissive diradicaloids. We apply a simple Hammett parameter-based strategy to tune the electron donation of the capping ligand across a series of commercially available triarylphosphines. This minor peripheral modification significantly alters the electronic structure, and consequently, the electrochemical, photophysical, and magnetic properties of the tetrathiafulvalene tetrathiolate (TTFtt)-based lumiphores. The resultant ∼100 nm absorption and emission range spans common laser lines and the desirable telecom region (ca. 1260-1550 nm). Furthermore, these lumiphores are sensitive to local dielectrics, distinguishing them as promising candidates for ratiometric imaging and/or barcoding in the deep NIR region.

Low-energy Compton scattering in materials

Low-energy Compton scattering is an important background for sub-GeV dark matter direct-detection and other experiments. Current Compton scattering calculations typically rely on assumptions that are not valid in the low-energy region of interest, beneath ∼50 eV. Here we relate the low-energy Compton scattering differential cross section to the dielectric response of the material. Our new approach can be used for a wide range of materials and includes all-electron, band-structure, and collective effects, which can be particularly relevant at low energies. We demonstrate the strength of our approach in several solid-state systems, in particular, Si, Ge, GaAs, and SiC, which are relevant for current and proposed experiments searching for dark matter, neutrinos, and millicharged particles. Published by the American Physical Society 2024

Effects of external fields and structural parameters on third harmonic generation of GaAs/AlGaAs Manning-like double quantum well structure

In this theoretical work, we investigate how external fields like as electric, magnetic, and intense laser fields, as well as structural factors, affect the third harmonic generation (THG) coefficient of an AlGaAs/GaAs Manning-like double quantum well heterostructure. To achieve our goals, we numerically solve the time-independent Schrödinger equation using the diagonalization approach with the effective mass approximation and then derive the subband energy levels and corresponding wave functions of the structure. After that, we derive the mathematical expression of the THG coefficient by using the compact density matrix method. According to our results, the resonance peaks of the THG coefficient show shifting to the high-energy region with an increase in the field’s magnitude in cases where external fields (electric, magnetic, and intense laser) are applied separately. At the same time, increasing the depth (width) of the quantum well structure causes the THG peaks to shift to the high (low) energy region. We believe that the findings from this search will have a substantial impact on existing experimental device designs and applications.

Theoretical Study of Electron Capture, Excitation, and Ionization Processes in H+−H(2l) Collisions

The processes of single-electron charge exchange, excitation, and ionization during proton impact on H(2l) are investigated. We employ two different theoretical methods that are suitable for different collision energy regions: the full quantum-mechanical molecular orbital close-coupling method for energies from 0.001 to 1 keV u−1 and the two-center atomic orbital close-coupling (TC-AOCC) method for energies between 0.3 and 100 keV u−1. For charge exchange and excitation processes, the total and nl-resolved cross sections to the final reaction channels of H (nl, n = 1–4) have been obtained over a broad energy region. Moreover, the ionization cross sections in the TC-AOCC calculation are also reported for both H(2s) and H(2p) initial target states. The present results are all compared with those from other sources when available. It is found that the magnitude and energy behavior of nl-resolved excitation cross sections for H+–H(2p) collisions are significantly distinct from those of the H(2s) initial state in the entire energy range considered, particularly in the low-energy region. The energy behaviors of the nl-resolved charge exchange cross sections from the H(2p) initial state are similar to those from the H(2s) initial state, but their magnitudes are larger. The present accurate cross-section data are anticipated to provide insight into the research of astrophysics and controlled fusion plasmas.

37Ar source on-demand production and deployment for low-energy nuclear recoil measurement in ReD liquid Argon TPC

The search for light dark matter (< 10 GeV/c 2) has become increasingly important, since no conclusive evidence has been found in the higher dark matter (DM) mass region. In order to explore this light mass range, it is necessary to accurately model the response of the noble liquid time projection chamber (TPC) detectors, used in many experiments aimed at the direct measurement of DM, to low energy (< 1 keV) nuclear recoils (NRs). In this respect, 37Ar provides an ideal calibration source in the low-energy region due to its two low-energy peaks at 0.27 and 2.82 keV following electron capture with a 35-day half-life. We propose a method to produce 37Ar without chemical or heating treatments by using the 40Ca(n,α)37Ar reaction. This can be achieved by irradiating nano-CaO powder with a neutron source (e.g. AmBe) and allowing the produced 37Ar to diffuse into the argon used inside a double-phase TPC. By measuring the NR yields relative to those two low-energy points in the Recoil Directionality (ReD) experiment, other detectors can be cross-calibrated with the same source deployed. In this talk, the 37Ar source production, its deployment, and preliminary results will be presented.

Photoionization process of the hydrogenlike carbon ion embedded in warm and hot dense plasmas.

Complicated many-body interactions between ions and surrounding particles exist in warm and hot dense plasmas. It will significantly alter the atomic structures and dynamic properties of the embedded ions. Recently, the atomic-state-dependent (ASD) screening model has been proposed and shown to be valid for investigating the screening effect in warm and hot dense plasmas over a wide range of electron densities and temperatures. By employing the ASD model, we investigate the photoionization process for the hydrogenlike carbon ion embedded in warm and hot dense plasmas with corresponding Coulomb coupling parameter ranges of 0.05 ≤ Γ ≤ 1.16, where Γ characterizes the ratio of the average potential to thermal energy. It is found that there are stronger plasma screening effects on the ionization energy and photoionization cross section due to the negative-energy electron distributions considered in the ASD model compared to those considering only free electrons. The present results from the ASD model show reasonable agreement with the classical Debye-Hückel (DH) model in weakly coupled plasmas. However, significant deviations of the ionization energy and cross section between these two models are observed in moderately and strongly coupled plasmas, due to the approximate treatment of the plasma-electron density distribution of the DH model. In the region of low photoelectron energies, the positions of the shape resonance peaks of the cross sections obtained from the ASD model differ significantly from those of the DH model due to the different screening effects.

Studying subthreshold resonance using the Trojan horse method

The Trojan horse method was employed to indirectly measure the bare-nucleus reaction cross-section and astrophysical S-factor of the 9Be(p,α)6Li reaction in the low-energy region, utilizing the three-body reaction 2H(9Be,α6Li)n. Comparing the two-body reaction data extracted from the Trojan horse method with that obtained through direct measurements, compatibility is observed in the energy region above approximately 100 keV. Additionally, the THM data successfully reproduces the expected low-energy resonance peak around 270 keV. The THM extraction of the astrophysical factor yields S(0) = 21.0 ± 0.8 MeV b, which surpasses the extrapolation obtained from direct measurements. The 9Be(p,α)6Li reaction channel exhibits a subthreshold resonance with a width of 25 keV, positioned approximately -23 keV below the threshold. However, the strong electron shielding effect near the zero energy position in direct measurements often masks the influence of the subthreshold resonance on the low-energy region. In contrast, the THM method allows us to neglect the electron shielding effect. The THM experimental data were subjected to fitting using the Breit-Wigner function and subsequently compared with directly measured data. Following a comprehensive comparative analysis, it was discerned that the S(0) value obtained through THM exceeded the extrapolated value derived from direct measurements. This disparity was primarily attributed to the influence of the subthreshold resonance.

Theory of electric field effect on the optical properties of elliptical quantum wire

In the approximation of the effective mass, the electric field effect on the optical properties of the elliptical quantum wire (EQW) was investigated. In an elliptic coordinate system, exact solutions of the Schrödinger equation for an electron in an EQW with hard walls are obtained. The electron energy spectrum consists of even and odd energies states, whose wave functions are expressed through even and odd Mathieu functions of the first kind. Using these solutions, an orthonormal basis was constructed. The influence of the electric field perpendicular to the quantum wire and parallel to the ellipse’s major axis on the energies and oscillator strength of quantum electron transitions was calculated using the matrix method. It is shown that the ellipticity of the quantum wire leads to the removal of the degeneracy of the energy spectrum of quasiparticles since the energies of even and odd electronic states depend differently on the ratio of the EQW semi-axes. The electron’s ground state is the even state, which is nondegenerate because there is no corresponding odd state. The electric field has a greater effect on energy states with a lower value of the magnetic quantum number. As the electric field strength increases, the energies of even and odd states shift to the region of lower energies.

Phonon-mediated quantum efficiency measurement in semiconductors

Accurate quantum efficiency measurement not only provides crucial information for the photovoltaic cell industry but also supports experiments aimed at directly detecting dark matter and elastic neutrino interactions. The dark matter direct searches paradigm has recently expanded to include particles with masses below 1,MeV/c2, where the expected signal in an electron–recoil interaction is approximately in the eV range, just above the energy gap for silicon and germanium. A robust calibration method for ionization signals in this lower energy region is essential. This paper presents a method for measuring quantum efficiency and yield (q/E) in semiconductors using phonon-mediated calorimetry. The Neganov–Trofimov–Luke phonon amplification method in low-temperature semiconductor crystals has been employed to indirectly measure ionization down to single-electron accuracy. Specifically, at zero bias, the phonon readout directly quantifies the total energy deposited within the detector, independent of the ionization yield. This eliminates a significant source of systematic uncertainty in quantum efficiency estimates associated with total energy uncertainty. The paper includes results from an updated ionization efficiency measurement in a germanium detector.

The Influence of the Alkylamino Group on the Solvatochromic Behavior of 5-(4-substituted-arylidene)-1,3-dimethylpyrimidine-2,4,6-triones: Synthesis, Spectroscopic and Computational Studies.

Advances in electronics and medical diagnostics have made organic dyes extremely popular as key functional materials. From a practical viewpoint, it is necessary to assess the spectroscopic and physicochemical properties of newly designed dyes. In this context, the condensation of 1,3-dimethylbarbituric acid with electron-rich alkylaminobenzaldehyde derivatives has been described, resulting in a series of merocyanine-type dyes. These dyes exhibit intense blue-light absorption but weak fluorescence. An electron-donating alkylamino group at position C4 is responsible for the solvatochromic behavior of the dyes since the lone electron pair of the nitrogen atom is variably delocalized toward the barbituric ring, which exhibits electron-withdrawing properties. This was elucidated, taking into account the different geometry of the amino group. The intramolecular charge transfer in the molecules is responsible for the relatively high redshift in absorption and fluorescence spectra. Additionally, an increase in solvent polarity moves the absorption and fluorescence to lower energy regions. The observed solvatochromism is discussed in terms of the four-parameter Catalán solvent polarity scale. The differences in the behavior of the dyes were quantified with the aid of time-dependent density functional theory calculations. The obtained results made it possible to find regularities linking the basic spectroscopic properties of the compounds with their chemical structure. This is important in the targeted search for new, practically important dyes.