Low-energy Peak Research Articles

Theoretical investigation of enhanced nonlinear optical properties of silicene and carbon nanotubes: Potential applications in infrared and ultraviolet optoelectronics

This theoretical study investigates the linear and nonlinear optical properties of zigzag carbon nanotubes (CNTs) and silicene nanotubes (SiNTs) with varying radii, focusing on their behavior in the infrared and ultraviolet energy ranges. In the infrared region, absorption spectra exhibit several peaks resulting from allowed transitions between valence and conduction bands. The number of absorption peaks increases with radius for both nanotube types, with SiNTs showing peaks at lower energy ranges and higher intensities compared to CNTs. Conversely, CNTs display markedly higher absorption intensities in the ultraviolet region. The quadratic electronic optic (DC Kerr) effect reveals sharp peaks near the band gap with multiple sign changes, attributed to allowed optical transitions at band edges. The third-order optical susceptibility for both CNT and SiNT structures show several peaks below the band gap energy due to multiphoton resonance absorption. In the infrared region, two highest and lowest subbands near to the Fermi level play a dominant role in the χTHG(3)(3ω) peaks formation. The position and intensity of χTHG(3)(3ω) peaks demonstrate a strong dependence on nanotube radius and type with higher intensity for the SiNTs. The tunable nature of the optical properties of CNTs and SiNTs by their radius and the enhanced nonlinear optical response of SiNTs, characterized by lower energy peaks and higher intensities, show their significant potential for advanced applications in nonlinear optics, optical detection, and high-energy optical systems.

Temperature dependence of Ce luminescence characteristics in LaBr3: Ce crystal

Despite the large interest in the scintillation properties of LaBr3:Ce, a detailed understanding of the underlying mechanism of temperature-dependence properties of Ce luminescence remains elusive. This study introduces a self-designed spectral apparatus to explore these properties in LaBr3:5%Ce. We observed a redshift phenomenon and band changes in the emission peak bands, indicating a reduction of the bond length between Ce and the host with increasing temperature. Moreover, the probability of low-energy peak emission decreases and the probability of high-energy peak emission increases, with increasing temperature was observed, suggesting a correlation with the proximity of Ce's 4f energy level to the valence band. Utilizing intensity parameters from the spectra, we identified the impact of temperature on LaBr3:Ce's self-absorption effect, revealing a significant self-absorption effect at the high-energy peak for the first time. A simple self-absorption model indicated that, despite high quantum efficiency of Ce, the overall self-absorption is minimal, establishing a correlation between the self-absorption coefficient of the high-energy peak and overall absorption. This research offers insights for developing radiation-resistant high-temperature luminescent devices and advances the field of high-temperature luminescent materials.

Electric conductivity of hot and dense nuclear matter

Transport coefficients play an important role in characterising hot and dense nuclear matter, such as that created in ultra-relativistic heavy-ion collisions (URHIC). In the present work we calculate the electric conductivity of hot and dense hadronic matter by extracting it from the electromagnetic spectral function, through its zero energy limit at vanishing 3-momentum. We utilise the vector dominance model (VDM), in which the photon couples to hadronic currents predominantly through the ρ meson. Therefore, we use hadronic many-body theory to calculate the ρ-meson's self-energy in hot and dense hadronic matter, by dressing its pion cloud with π-ρ, π-σ, π-K, N-hole, and Δ-hole loops. We then introduce vertex corrections to maintain gauge invariance. Finally, we analyze the low-energy transport peak as a function of temperature and baryon chemical potential, and extract the conductivity along a proposed phase transition line.

Feasibility of the Compton Suppressed Gamma-ray spectrometry for proton therapy applications

The presence of the Bragg peak and its use in proton therapy cause less damage to healthy tissues. For optimal use of the proton beam in proton therapy, it is necessary to monitor the deviation of its range during treatment. The prompt gamma rays originated from the nuclear reactions of the proton beam with tissue elements can be applied for this purpose. The spatial distribution of prompt gamma rays can be used to estimate the range of the incident charged particles. Detecting a wide energy range of gamma rays, up to 10 MeV, is the most challenging step of the proton dose verification by using prompt gamma spectroscopy. Compton suppression is a system that is used to reduce the contribution of scattered gamma rays. This technique will cause better detection of the peaks of low-energy prompt gamma rays. In this research, the performance of a Compton suppression system has been investigated in two fields for proton therapy and for elemental analysis of materials using isotopes by a wide range of gamma energies. At first, by using the simulated Compton suppression system, the calculations of the prompt gamma ray spectra emitted from the brain phantom during the radiation of 150 MeV protons were presented. The results indicated that the low energy elements peaks such as 40Ca (1.37 MeV) with CSF equal to 1.138 appeared in the suppressed spectrum. Also, it was possible to detect 14N (2.31 MeV) and 16O (6.13 MeV) peaks equivalent to the CSF 2.449 and 2.067 respectively. In the next step, it was possible to detect the low energy peaks of 152Eu and 155Eu isotopes in the suppressed spectrum. For this purpose, the minimum relative intensity required to detect all 152Eu and 155Eu peaks was equivalent to 0.001 and 0.002 respectively. CSF for the lowest energies such as 105 keV and 344 keV was equal to 1.169 and 1.184. The simulation was done using the GEANT4 Monte-Carlo toolkit.

Impact of composition of AlGaInAs confining barriers on emission of InAs quantum dots embedded in AlGaAs/GaAs dot-in-a-well heterostructures

The emission of InAs quantum dots (QDs) grown on Al0.30 Ga0.70As/GaAs heterostructures and integrated into additional capping/buffer quantum wells (QW), known as dot-in-a-well (DWELL) structures, has been investigated. Two different AlGaInAs confining barriers (CBs) and buffer layers (BLs) are compared. The first QD structure includes the Al0.30 Ga0.70As CB (#1) and the In0.15Ga0.85As BL. The second QD structure consists of the Al0.40Ga0.45In0.15As CB (#2) and the In0.25Ga0.75As BL. Comparison of photoluminescence (PL) spectra has revealed that the ground state (GS) emission band in the structure #2 with Al0.40Ga0.45In0.15As CB is characterized by the lower peak energy, smaller PL linewidth (more homogeneous QD sizes) and higher GS emission intensity compared to that parameters in the structure #1 with Al0.30 Ga0.70As CB. The smaller potential barriers at the Al0.40Ga0.45In0.15As CB/QD interfaces in #2 lead to faster thermal decrease in the GS PL intensity at high temperatures compared with that in #1. High-resolution X-ray diffraction (HR-XRD) method was used for the study of the QD structures with the aim of monitoring the sizes and compositions of QDs and QWs. Numerical simulation of HR-XRD scans has shown that the material compositions of QDs and QW in #2 with Al0.40Ga0.45In0.15As CB has not changed in the process of QD structure growth at high temperatures compared to those in #1. The obtained results are interesting for further improvement of InAs/GaAs QD structures for telecommunication technology and optoelectronic applications.

Low-energy peak in the one-particle spectral function of the electron gas at metallic densities

Low-energy peak in the one-particle spectral function of the electron gas at metallic densities

Deconvoluting XPS Spectra of La-Containing Perovskites from First-Principles.

Perovskite-based oxides are used in electrochemical CO2 and H2O reduction in electrochemical cells due to their compositional versatility, redox properties, and stability. However, limited knowledge exists on the mechanisms driving these processes. Toward this understanding, herein we probe the core level binding energy shifts of water-derived adspecies (H, O, OH, H2O) as well as the adsorption of CO2 on LaCoO3 and LaNiO3 and correlate the simulated peaks with experimental temperature-programmed X-ray photoelectron spectroscopy (TPXPS) results. We find that the strong adsorption of such chemical species can affect the antiferromagnetic ordering of LaNiO3. The adsorption of such adspecies is further quantified through Bader and differential charge analyses. We find that the higher O 1s core level binding energy peak for both LaCoO3 and LaNiO3 corresponds to adsorption of water-related species and CO2, while the lower energy peak is due to lattice oxygen. We further correlate these density functional theory-based core level O 1s binding energies with the TPXPS measurements to quantify the decrease of the O 1s contribution due to desorption of adsorbates and the apparent increase of the lattice oxygen (both bulk and surface) with temperature. Finally, we quantify the influence of adsorbates on the La 4d, Co 2p, and the Ni 3p core level binding energy shifts. This work demonstrates how theoretically generated XPS data can be utilized to predict species-specific binding energy shifts to assist in deconvolution of the experimental results.

Understanding the interplay of defects, oxygen, and strain in 2D materials for next-generation optoelectronics

Two-dimensional transition metal dichalcogenides are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include, firstly, the widely debated defect- and strain-induced origins of intense low-energy broad luminescence peaks (L-peaks) observed at low temperatures. Secondly, the role of oxygen in tuning the properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, our physical understanding of the benefits of hexagonal boron nitride (hBN) encapsulation is inadequate. Using a series of samples, we decouple the contributions of oxygen, defects, adsorbates, and strain on the optical properties of monolayer MoS2. The defect origin of the L-peak is confirmed by temperature- and power-dependent photoluminescence (PL) measurements, with a dramatic redshift of ∼130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples compared with exfoliated samples. Anomalously, the O-CVD samples show high A-exciton PL at room temperature (cf exfoliated), but reduced PL at low temperatures, attributed to the strain-induced direct-to-indirect bandgap crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations and are supported by Raman spectroscopy. In the exfoliated samples, the charged O adatoms are identified as thermodynamically favourable defects, and create in-gap states. The beneficial effect of encapsulation originates from the reduction of charged O adatoms and adsorbates. This experimental–theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain, and oxygen on the band structure, and enriches our understanding of the effects of encapsulation. This work proposes O-CVD as a method for creating high-quality materials for optoelectronics.

The origins of dual-peak emission and anomalous exciton decay in 2D Sn-based perovskites.

Two-dimensional (2D) Sn-based perovskites exhibit significant potential in diverse optoelectronic applications, such as on-chip lasers and photodetectors. Yet, the underlying mechanism behind the frequently observed dual-peak emission in 2D Sn-based perovskites remains a subject of intense debate, and there is a lack of research on the carrier dynamics in these materials. In this study, we investigate these issues in a representative 2D Sn-based perovskite, namely, PEA2SnI4, through temperature-, excitation intensity-, angle-, and time-dependent photoluminescence studies. The results indicate that the high- and low-energy peaks originate from in-face and out-of-face dipole transitions, respectively. In addition, we observe an anomalous increase in the non-radiative recombination rate as temperature decreases. After ruling out enhanced electron-phonon coupling and Auger recombination as potential causes of the anomalous carrier dynamics, we propose that the significantly increased exciton binding energy (Eb) plays a decisive role. The increased Eb arises from enhanced electronic localization, a consequence of weakened lattice distortion at low temperatures, as confirmed by first-principles calculations and temperature-dependent x-ray diffraction measurements. These findings offer valuable insights into the electronic processes in the unique 2D Sn-based perovskites.

Vibration-Dependent Dual-Phosphorescent Cu4 Nanocluster with Remarkable Piezochromic Behavior.

The dual emission (DE) characteristics of atomically precise copper nanoclusters (Cu NCs) are of significant theoretical and practical interest. Despite this, the underlying mechanism driving DE in Cu NCs remains elusive, primarily due to the complexities of excited state processes. Herein, a novel [Cu4(PPh3)4(C≡C-p-NH2C6H4)3]PF6 (Cu4) NC, shielded by alkynyl and exhibiting DE, was synthesized. Hydrostatic pressure was applied to Cu4, for the first time, to investigate the mechanism of DE. With increasing pressure, the higher-energy emission peak of Cu4 gradually disappeared, leaving the lower-energy emission peak as the dominant emission. Additionally, the Cu4 crystal exhibited notable piezochromism transitioning from cyan to orange. Angle-dispersive synchrotron X-ray diffraction results revealed that the reduced inter-cluster distances under pressure brought the peripheral ligands closer, leading to the formation of new C-H⋅⋅⋅N and N-H⋅⋅⋅N hydrogen bonds in Cu4. It is proposed that these strengthened hydrogen bond interactions limit the ligands' vibration, resulting in the vanishing of the higher-energy peak. In situ high-pressure Raman and vibrationally resolved emission spectra demonstrated that the benzene ring C=C stretching vibration is the structural source of the DE in Cu4.

Thiocarbonyl-Bridged N-Heterotriangulenes for Energy Efficient Triplet Photosensitization: A Theoretical Perspective.

Structurally-rigid metal-free organic molecules are of high demand for various triplet harvesting applications. However, inefficient intersystem crossing (ISC) due to large singlet-triplet gap ( ) and small spin-orbit coupling (SOC) between lowest excited singlet and triplet often limits their efficiency. Excited electronic states, fluorescence and ISC rates in several thiocarbonyl-bridged N-heterotriangulene ( S-HTG) with systematically increased thione content ( 0-3) are investigated implementing polarization consistent time-dependent optimally-tuned range-separated hybrid. All S-HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low-lying singlets ( ) and triplets ( ), and their excitation nature (i. e., or ) and SOC are determined for these S-HTGs in dichloromethane. Low-energy optical peak displays gradual red-shift with increasing thione content due to relatively smaller electronic gap resulted from greater degree of orbital delocalization. Significantly large SOC due to different orbital-symmetry and heavy-atom effect produces remarkably high ISC rates ( ~1012 s-1) for enthalpically favoured ( ) channel in these S-HTGs, which outcompete radiative fluorescence rates (~108 s-1) even directly from higher lying optically bright singlets. Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non-fluorescent make these thiocarbonylated HTGs ideal candidates for energy efficient triplet harvest including triplet-photosensitization.

Interfacial Charge Transfer in Photoexcited QD-Molecule Composite of Tetrahedral CdSe Quantum Dot Coupled with Carbazole.

Photoexcited charge transfer dynamics in CdSe quantum dots (QDs) coupled with carbazole were explored to model QD-molecule systems for light-harvesting applications. The absorption spectra of QDs with different sizes, i.e., Cd35Se20X30L30 (T1), Cd56Se35X42L42 (T2), and Cd84Se56X56L56 (T3) were simulated with quantum dynamical methods, which qualitatively match the reported experimental spectra. The carbazole is attached with a 3-amino group at the apex position of T1 (namely T1-3A-Cz), establishing proper electronic communication between T1 and carbazole. The spectra of T1-3A-Cz is 0.22 eV red-shifted compared to T1. A time-dependent perturbation was applied in tune with the lowest energy peak (3.63 eV) of T1-3A-Cz to investigate the charge transfer dynamics, which revealed an ultrafast charge separation within the femtosecond time scale. The electronic structure showed a favorable energy alignment between T1 and carbazole in T1-3A-Cz. The LUMO of carbazole was situated below the conduction band of the QD, while the HOMO of carbazole mixed perfectly with the top of the valence band of the QD, developing the interfacial charge transfer states. These states promoted the photoexcited electron transfer directly from the CdSe core to carbazole. A rapid and enhanced charge separation occurred with the laser field strength increasing from 0.001 to 0.005 V/Å. However, T1 connected to the other positions of carbazole did not show charge separation effectively. The photoinduced charge transfer is negligible in the case of T2-carbazole systems due to poor electronic coupling, and it is not observed in T3-carbazole systems. So, the T1-3A-Cz model acts as a perfect donor-acceptor QD-molecule nanocomposite that can harvest photon energy efficiently. Further enhancement of charge transfer can be achieved by coupling more carbazoles to the T1 QD (e.g., T1-3A-Cz2) due to the extension of hole delocalization between T1 and the carbazoles.

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.

Spin-configuration of emission states in zero-dimensional metal halides

ABSTRACT Understanding the spin-configuration of excited states in a luminescent material is essential for tailoring its properties for many applications such as light-emitting diodes and spin-optoelectronic devices. Zero-dimensional organic-inorganic metal halide (0D-OIMH) materials have demonstrated remarkable potential in diverse applications owing to their captivating optoelectronic characteristics. However, the electronic structure and spin-configuration of the frequently observed dual-peak emission in these materials remains a subject of intensive debate. In this study, we employ low-temperature magneto-optical measurements to investigate the excited state structure of a representative 0D-OIMH, namely (Bmpip)2SnBr4. The spin-configurations of the dark and bright states are clearly elucidated by measuring the magneto-polarization of the emissions. Our results reveal that the high-energy peak arises from bright excited states within a higher energy band, whilst the low-energy peak originates from a combination of triplet-bright states and singlet-dark states. These findings provide an unambiguous understanding of the exciton structures of the distinctive 0D-OIMHs.

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.

Investigating the effect of Zn doping and temperature on the photoluminescence behaviour of CuLaSe2 quantum dots.

In this study, CuLaSe2 and ZnCuLaSe2 quantum dots (QDs) with a mean size of ~4nm were synthesized and characterized, and their temperature-dependent photoluminescence (PL) properties were studied in the temperature range from 90 to 300 K for the first time. The results show that the obtained QDs were spherical and revealed excitonic band gaps. The PL intensity for both types of materials decreased when increasing the temperature to 300 K, which was attributed to the nonradiative relaxation and thermal escape mechanisms. As the temperature was increased, the PL linewidths broadened, and PL peak energies were red shifted for both types of QDs due to the exciton-phonon coupling and lattice deformation potential mechanisms. In addition, we found that as the temperature was decreased, the PL spectrum of ZnCuLaSe2 QDs contained two extra components, which could be attributed to the shallow defect sites (low energy peak) and the crystal phase transition process (high energy peak). The spectrum of CuLaSe2 QDs contained one extra component, which could be attributed to the crystal phase transition process.

Changing absorption intensity ratios for methylene violet Bernthsen in normal alcohols and at different temperatures

Absorption spectra of methylene violet Bernthsen (MVB) are reported, obtained in normal alcohols, methanol through 1-octanol, and at different temperatures, 15 °C to 75 °C. Two distinct absorption intensities are apparent. The 565 nm intensity increases relative to the 598 nm intensity as alcohol chain length increases. For each solvent, increasing temperature causes similar intensity ratio effects. Computational spectra of MVB in methanol indicates a decrease in hydrogen-bonding as temperature increases with a corresponding loss of intensity in the low energy peak consistent with the experimental observations. Hydrogen-bonding is indicated as playing a role in the alcoholic solvent effects.

Emergence of a Low-Energy Peak in the Smoothed Absolute Value Circular Dichroism Spectra of Small Helical Gold Nanorods.

We calculate, using time-dependent density functional theory, absorption and circular dichroism (CD) spectra for a series of small helical gold nanorod structures with a width of 0.6 nm and length increasing from 0.7 nm for Au24 to 1.9 nm for Au56. For a low-energy window, ranging from 1.7 to 4.1 eV, broadening the lines in the absorption spectra results in a low energy peak which previous studies have identified as the (localized) plasmon resonance. As expected, the absorption peak position of the plasmon resonance systematically redshifts as the length of the nanorod increases. However, trends in the CD and straightforwardly broadened CD spectra are more difficult to discern. We introduce the idea of an absolute value CD spectrum and show that broadening the lines results in a low energy peak that has not previously been reported. The peak position systematically redshifts as the length of the nanorod increases but over a significantly smaller range than that for the absorption spectrum.

Exploring Epitaxial Structures for Electrically Pumped Perovskite Lasers: A Study of CsPb(Br,I)3 Based on the Ab Initio Bethe-Salpeter Equation.

Halide perovskites are widely used as components of electronic and optoelectronic devices such as solar cells, light-emitting diodes (LEDs), optically pumped lasers, field-effect transistors, photodetectors, and γ-detectors. Despite this wide range of applications, the construction of an electrically pumped perovskite laser remains challenging. In this paper, we numerically justify that mixing two perovskite compounds with different halide elements can lead to optical properties suitable for electrical pumping. As a reference, the chosen model material was CsPbBr3, whose performance as a part of lasers has been widely recognised, with some Br atoms substituted by I at specific sites. In particular, a strong enhancement of the low-energy absorption peaks has been obtained using the ab initio Bethe-Salpeter equation. Based on these results, we propose specific architectures of ordered doping that could be realised by epitaxial growth. Efficient light emission from the bottom of the conduction band is expected.

Broken Symmetry Optical Transitions in (6,5) Single-Walled Carbon Nanotubes Containing sp3 Defects Revealed by First-Principles Theory.

We present a first-principles many-body perturbation theory study of nitrophenyl-doped (6,5) single-walled nanotubes (SWCNTs) to understand how sp3 doping impacts the excitonic properties. sp3-doped SWCNTs are promising as a class of optoelectronic materials with bright tunable photoluminescence, long spin coherence, and single-photon emission (SPE), motivating the study of spin excitations. We predict that the dopant results in a single unpaired spin localized around the defect site, which induces multiple low-energy excitonic peaks. By comparing optical absorption and photoluminescence from experiment and theory, we identify the transitions responsible for the red-shifted, defect-induced E11* peak, which has demonstrated SPE for some dopants; the presence of this state is due to both the symmetry-breaking associated with the defect and the presence of the defect-induced in-gap state. Furthermore, we find an asymmetry between the contribution of the two spin channels, suggesting that this system has potential for spin-selective optical transitions.