Optical Peak Research Articles

Simultaneous optical and X-ray detection of a Thermonuclear Burst in the 2024 outburst of EXO 0748–676

Abstract The neutron star low-mass X-ray binary, EXO 0748–676, recently returned to outburst after a ∼16 year-long quiescence. Since its return, there has been a global effort to capture the previously unseen rise of the source and to understand its somewhat early return to outburst, as it is typical for a source to spend longer in quiescence than in outburst. Here, we report on the simultaneous optical and X-ray detection of a type I X-ray burst, captured by XMM-Newton during a DDT observation on 30th June 2024. The data show 3 X-ray eclipses consistent with the known ephemeris and one type I X-ray burst at 60492.309 MJD. The X-ray burst is reprocessed into the optical band and captured by XMM-Newton’s Optical Monitor during a 4399 s exposure with the B filter in image + fast mode. We determine that the optical peak lags the X-ray peak by 4.46 ± 1.71s. The optical and X-ray rise times are similar, but the optical decay timescale is shorter than the X-ray decay timescale. The reprocessing site is likely within a few light seconds of the X-ray emitting region, so the companion star, accretion disc and ablated material are all plausible.

Structural, Morphological, Optical and Vibrational Properties of a Novel Coral-Like Fibrous CuO Nanostructure Synthesized via Dropwise Precipitation for UV-light Induced Photocatalytic Wastewater Remediation on Aqueous R6G Dye

CuO is a well-known monoclinic structure with unique properties that is extensively investigated for sensor, energy storage, and optoelectronic applications. In this work, the CuO nanostructure was synthesized through facile precipitation by mixing the aqueous copper (II) sulfate pentahydrate with sodium hydroxide. The as-synthesized CuO was characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and UV–visible spectroscopy. A homogeneous growth of a narrow, fine coral-like fibrous morphology was observed on CuO under SEM imaging. The FTIR spectra reveal the existence of some dominant sharp bands in the lower wavenumber region that represent the Cu-O bonds. The high solubility of CuO in water confirms the formation of particles with nanoscale dimensions. CuO exhibits a strong optical absorption peak at 510 nm, and the band gap determined by Tauc's relation is 2.11 eV. CuO is capable of inducing photocatalytic degradation on R6G dye solutions under low-intensity UVC irradiation, which can serve as a future photocatalyst for wastewater treatment.

Green engineering of NiO nanoparticles decorated with Arachis hypogaea shell extract for biomedical applications

Abstract We attempted to synthesize nickel oxide nanoparticles (NiO-NPs) utilizing waste Arachis hypogaea (peanut) shell extract and studied their structural, morphological, and biological performance for biomedical applications. The green engineered NiO-NPs possessed a face-centered cubic structure with an average particle size of 20 nm in highly crystalline form. NiO-NPs were shown to have an optical resonance peak at 327 nm with 3 eV as the optical band gap according to the UV–visible spectra, and the stretching band between Ni–O were evidenced from the FTIR and Raman spectrum. Utilizing green approach the stable nanoparticles were obtained with average particle size of 31 nm from SEM analysis; zeta potential value of −17.6 mV, and PDI as 0.68, revealed the formation of spherical nanoparticles with distinct morphologies without aggregation. XPS analysis confirmed the oxidation states of the elements Ni (2p) and O (1s). This approach may help to increase the surface area, increasing the possibility of nanoparticles interacting with bacterial cells. Furthermore, the presence of nickel and the oxygen oxidation state were confirmed by XPS. Proteus vulgaris, Streptococcus oralis, Bacillus subtilis, and Escherichia coli were found to be susceptible to the antibacterial action of the produced NiO-NPs, with a maximal zone of inhibition of 10.25 mm at 500 μg/ml for P. vulgaris. For P. vulgaris and E. coli, the minimum inhibitory concentrations of NiO were 5.36 and 12.55 %, respectively, at 31.25 μg mL−1. We hereby claim that green engineered NiO NPs decorated with A. hypogaea shell extract have great potential for pharmaceutical and biomedical applications.

The influence of carbon content to the band gap of ABC stacked trilayer hybridized graphene and hexagonal boron nitride

The influence of carbon content on the band gap of ABC stacked trilayer hybridized graphene and hexagonal boron nitride (h-BNC) are calculated by first-principles calculations. The formation energy results indicate that the configurations with cluster of boron(B) and nitrogen(N) atoms are more stable than that with separate. By calculating the band structures of seven doping models, we found that the band gap reduces with the ascendance of carbon atom concentration. With the increase of the carbon atom content, the optical absorption peak shifts to longer wavelength range. Combined with the band structure results, the red-shift is due to the reduction of the bandgap with the increase of the carbon atom content. The density of states, charge densities and Mulliken populations suggest that the collapse of the band gap is attributed to the charge transfer between atoms in the seven doping models of the h-BNC system. This work is valuable for the band structure engineering of h-BNC.

Origin of the High-Frequency Shoulder in the Raman Spectra of CdSe Quantum Dots.

The origin of the high-frequency shoulder (HFS) observed above the longitudinal optical (LO) peak around 230 cm-1 in the Raman spectra of CdSe quantum dots (QDs) has been the subject of intense debate. We use state-of-the-art ab initio density functional theory applied to small CdSe QDs with various realistic surface passivations and find an intense Raman signal around 230 cm-1, which corresponds to a stretching vibration of a defective 2-fold coordinated Se atom. We interpret this signal as being the origin of the HFS. Since the signal disappears in fully passivated and defect-free (magic size cluster) structures, it can be used as a fingerprint to distinguish defective from nondefective structures.

Magnetic field-engineered optical nonlinearity in germanene nanotubes

Abstract The linear and nonlinear optical properties of germanene nanotubes (GeNTs) under the influence of magnetic fields are investigated through theoretical analysis. By employing a tight-binding model beyond the Dirac cone approximation, the electronic band structure, density of states, dipole matrix elements, and third-order nonlinear optical susceptibilities are calculated. The application of a magnetic field induces a Zeeman splitting of the energy levels, breaking the band degeneracy. Consequently, the dipole matrix elements exhibit a magnetic field dependence, altering the intensity and number of allowed transitions. In the infrared and ultraviolet energy regions, the linear optical absorption spectrum displays the splitting of peaks into dual neighboring peaks, with the splitting distance linearly dependent on the magnetic field strength. The third-order nonlinear optical susceptibilities, χTHG(3)3ω and χIDIR(3)ω, reveal the emergence of multiple resonance peaks in different energy regions due to two-photon and three-photon absorption processes. The magnetic field significantly influences the number, position, and height of these optical peaks, enabling effective control over the nonlinear optical response. By investigating the scaled energy (E/E_g) dependence, a universal behavior is observed for the three- and two photon resonance peaks, where their positions remain constant, but their peak intensities are significantly enhanced under the influence of the magnetic field. These findings demonstrate the potential of GeNTs as tunable nonlinear optical materials, with the magnetic field serving as an effective parameter for engineering their optical properties for various applications in optoelectronics, photonics, and related fields.

Clinical correlation of optic nerve head analysis performed by SD-OCT in children with pseudopapilledema and papilledema

PurposeTo investigate some diagnostic indicators in optic nerve head (ONH) analysis of children with ‘Pseudopapilledema’ and ‘Papilledema’ by Spectral Domain Optical Coherence Tomography (SD-OCT). MethodsMedical records of 52 children with optic disc swelling, who were followed up by the departments of pediatric neurology and ophthalmology between May 2018 and May 2019 were reviewed retrospectively. Cases were classified as group 1 (Papilledema secondary to Idiopathic Intracranial Hypertension) including 54 eyes of 27 cases, group 2 (Pseudopapilledema secondary to bilateral optic disc drusen) including 50 eyes of 25 cases and control group. Three-dimensional imaging of ONH, peripapillary retinal nerve fiber layer (RNLF) thickness, Bruch's membrane opening height in the nasal respectively temporal quadrant (BMHN, BMH-T) levels were measured by SD-OCT and B-mode ocular ultrasonography data of all cases were evaluated. ResultsWhen RNFL levels were compared between groups, nasal RNFL levels were found to be significantly higher in group 1 cases compared to group 2 (p < 0.001). In 3D imaging of the ONH among group 1 patients, the mean height of the apex in disc swelling was significantly higher than that of group 2 (p = 0,024). The apex in disc swelling was mostly observed to be localized at the middle and had a diffuse swelling pattern in group 1; whereas, in group 2, the it was localized mostly at the nasal quadrant. This variation was statistically significant (p < 0.001). When, Bruch's membrane opening height in the nasal respectively temporal quadrant (BMHN, BMH-T) levels were compared, the measurements obtained were found to be significantly higher in group 1 (p = 0,050 and p = 0,003 respectively). ConclusionNasal RNFL values of SD-OCT, Bruch's membrane opening height in the nasal respectively temporal quadrant (BMHN, BMH-T) levels, the location of the apex in disc swelling obtained by 3D analysis of the ONH are found to be potential diagnostic parameters when combined with clinical findings. It is important that the nasal quadrant elevation, where the highest peak in 3D imaging was measured, was higher in the papilledema group. The elevation of the optic disc peak in 3D imaging can be used as a parameter to help clinicians distinguish between optic disc drusen (ODD) and papilledema.

The structural, mechanical, electronic, and optical properties of monolayer and bilayer ABC3(A [dbnd] Ga, In; B [dbnd] Si, Ge; C [dbnd] S, Se, Te)

This paper studies the structural, mechanical, optical, and electronic characteristics of monolayer and bilayer ABC3 (AGa, In; BSi, Ge; CS, Se, Te). Their characteristics are obtained using the density functional theory with van der Waals (vdW) correction. The stability is confirmed by examining Born’s criterion, cohesive energy, phonon dispersions, and COHP analysis. In addition, the stability of these monolayers against tensile strain is explored. The electronic properties are extensively studied through band structure, PDOS, and effective masses. The optical properties are investigated for possible applications in optoelectronic devices. In the following, bilayer ABC3 with three different stacking orders is investigated. Most of the monolayers and bilayers ABC3 show an indirect bandgap. The contributions of each element at the valence and conduction bands are explored through the projected density of the state. The effective mass is derived for the valence and conduction band valleys, whereas the Γ-valley shows the lowest effective mass. Additionally, both real and imaginary parts of the dielectric constant, electron energy loss spectra, and optical absorption are studied, which might lead to intriguing usages in photovoltaic and electroluminescent systems. The analysis of the optical properties reveals that the studied monolayers and bilayers have acceptable visible-range absorption characteristics. Furthermore, the monolayer and bilayer ABC3 dielectric function and absorption coefficient spectra display prominent optical peaks in the UV light range. Due to their distinct band gaps and optical characteristics, monolayer and bilayer ABC3 materials are good prospects for upcoming electrical and optoelectronic applications.

Multi-messenger signatures of delayed choked jets in tidal disruption events

Abstract Recent radio observations and coincident neutrino detections suggest that some tidal disruption events (TDEs) exhibit late-time activities, relative to the optical ehmission peak, and these may be due to delayed outflows launched from the central supermassive black hole. We investigate te possibility that jets launched with a time delay of days to months, interact with a debris that may expand outwards. We discuss the effects of the time delay and expansion velocity on the outcomes of jet breakout and collimation. We find that a jet with an isotropic-equivalent luminosity of ≲ 5 × 1045 erg/s is likely to be choked for a delay time of ∼3 months. We also study the observational signatures of such delayed choked jets. The jet-debris interaction preceding the breakout would lead to particle acceleration and the resulting synchrotron emission can be detected by current and near-future radio, optical and X-ray telescopes, and the expanding jet-driven debris could explain late-time radio emission. We discuss high-energy neutrino production in delayed choked jets, and the time delay can significantly alleviate the difficulty of the hidden jet scenario in explaining neutrino coincidences.

Synthesis and Characterization of Carbon Nano Sphere-doped Gd: Alpha Sb2O4 Nanostructure for High-Performance Energy Storage Applications

Background: To enhance the super capacitive properties of nanocomposites, the effective method is to combine carbon nanospheres with mesoporous structures with Gd3+:α-Sb2O4 inorganic nanocomposites (NC) to form hybrid electrodes. An as-prepared hybrid electrode material possesses increased energy density, high rate of reversibility and cyclic stability when incorporated in electrochemical cyclic voltammetric studies. Methods: In the present investigation, various wt % of C-nanospheres (Cx) (5 %, 10% and 20%) were decorated over Gd3+: α-Sb2O4 nanocomposites and were synthesized by coprecipitation method. XRD, SEM, EDX, UV-visible, and XPS are only a few of the analytical techniques used to describe the as-prepared hybrid nanocomposites. Electrochemical cyclic voltammetry was carried out in a 6 M KOH solution, three-electrode system. Results: The crystal structure and morphology of Cx: Gd3+@ α-Sb2O4 NC showed a mixed hexagonal phase and agglomerated tiny irregularly shaped morphology that appeared as the Cx concentration increased. Redshift in optical absorption peak appeared (near UV-edge), and the optical band gap (Eg) value increased from 3.53 eV to 3.65 eV. The electrochemical supercapacitor showed the highest specific capacitance of 989 F/g at the current density of 1 A/g for C20%:Gd3+@α-Sb2O4 NC compared with Cx:Gd3+@α-Sb2O4 (x = 5% and 10%) and undoped Gd3+:α-Sb2O4 NC. The change in phase angle and Rs value of 1.98 was attributed to the ideal supercapacitor properties. The cyclic stability after 5000 cycles with 79.71% capacitive retention was exhibited by C20%:Gd3+@α-Sb2O4 NC. Conclusion: The present research introduces ease of synthesis of hybrid electrode materials possessing high active surface area, increased energy density, high cyclic stability, and reversibility in an aqueous solution.

High-throughput screening on optoelectronic properties of two-dimensional InN/GaN heterostructure from first principles.

A novel 2D InN/GaNlateral heterostructure (LHT) was simulated by stitching monolayer of 2D InN and monolayer of 2D GaN. The structural stability, electronic structure, and optical properties were systematically investigated using first-principle calculations and by considering the effects of strain. The results indicated that the designed heterostructure has a direct bandgap of 2.26eV which is further affected by applied biaxial strain. The bandgap of 2D InN/GaN lateral heterostructure decreases with the increase in biaxial strain, and tensile strain triggers a direct-to-indirect energy gap changeover at + 6%. Additionally, under compressive strain, heterostructure remains a direct bandgap semiconductor. Furthermore, the strain significantly affects the optical characteristics of lateral heterostructure. It has been noticed that the first optical absorption peak moves from 2.51eV (ɛ = - 4%) to 1.40eV (ɛ = 10%). Therefore, 2D InN/GaN lateral heterostructure provides an approachable way for utilizing in optoelectronic devices through the creation of in-plane lateral heterostructures. We performed all the computations using a self-consistent method based upon density functional theory. We used the PBEsol functional in the GGA to account for the exchange-correlation effects. We introduced a 10-Å vacuum region in the z-direction to avoid interaction between periodic images. We considered non-negligible weak dispersion correction in the lateral heterostructure using Grimme's DFT-D3 approach. In this study, we also computed the electrical and optical properties employing the local modified Becke-Johnson (lmBJ) exchange potential under meta-GGA functional to obtain more precise results.

Structure and optical properties of ZnxCd1-xS and Cu:ZnxCd1-xS templated on DNA molecules

One-dimensional ZnxCd1−xS and Cu: Zn x Cd1−x S nanostructures were prepared using DNA as a template to promote growth along the molecular axis. The formation of homogeneously alloyed nanocrystals with cubic zinc blende-type structures was verified using x-ray diffraction and Raman spectroscopy. X-ray photoemission spectra revealed the presence of Cu(I) in the doped Cu: Zn x Cd1−x S nanocrystals. The effectiveness of the DNA template to direct the semiconductor growth in one dimension was demonstrated by AFM and TEM. The nanostructures displayed a granular morphology comprising nanoparticles with an average diameter of 14 nm composed of assemblies of smaller crystallites of 2.0 nm in size. Rope-like assemblies with an average diameter of 48 nm and extending in length to several hundred micrometres were obtained by evaporation-induced self-assembly. UV-Vis absorption and emission spectra indicated that the optical bandgaps (2.89–4.00eV) and photoluminescence peaks (608–819 nm) of the DNA-templated nanocrystals could be precisely controlled by modifying the molar ratios of their Zn/Cd precursors. Doping with Cu(I) gave an increase in photoluminescence intensity and a composition-independent red-shift of 0.23 eV. The preparation of DNA-templated Zn x Cd1−x S and Cu: Zn x Cd1−x S provides a simple, low-temperature route to aqueous dispersions of inorganic materials with controlled optical gap.

Determination of third order nonlinearity (n2 & β) of glycine zinc sulphate pentahydrate semi-organic single crystal using two laser radiations at different intensities

AbstarctThe third order nonlinear optical properties i.e. nonlinear refraction (n2), absorption coefficient (β) and third order susceptibility χ(3) of glycine zinc sulphate pentahydrate (GZS) semi-organic single crystal was investigated under the excitation of Q-switched Nd-YAG laser pulsed radiations (λ = 532 nm & 1064 nm) by familiar Z-scan method. The values of nonlinear refraction (n2), at 532 nm were 1.19 × 10−5 (cm2/GW), 2.09 × 10−5 (cm2/GW), 2.27 × 10−5 (cm2/GW), 2.48 × 10−5 (cm2/GW), and 2.55 × 10−5 (cm2/GW) at incident optical beam intensities of 0.76 (GW/cm2), 1.13 (GW/cm2), 1.17 (GW/cm2), 1.63 (GW/cm2) and 1.88 (GW/cm2) respectively while that of values at 1064 nm were 4.31 × 10−5 (cm2/GW), 4.69 × 10−5 (cm2/GW), 5.41 × 10−5 (cm2/GW) and 6.39 × 10−5 (cm2/GW) at incident optical beam intensities of 1.20 (GW/cm2), 1.25 (GW/cm2), 1.32 (GW/cm2) and 1.38 (GW/cm2) respectively. The values of nonlinear absorption coefficient (β) at 532 nm were −1.45 (cm/GW), −0.79 (cm/GW), −0.5 (cm/GW), 0.12 (cm/GW) and 0.64 (cm/GW) at incident optical beam intensities of 0.76 (GW/cm2), 1.13 (GW/cm2), 1.17 (GW/cm2), 1.63 (GW/cm2) and 1.88 (GW/cm2) respectively while that of values at 1064 nm were 0.62 (cm/GW), 0.74 (cm/GW), 0.77 (cm/GW) and 0.81 (cm/GW) at incident optical beam intensities of 1.20 (GW/cm2), 1.25 (GW/cm2), 1.32 (GW/cm2) and 1.38 (GW/cm2) respectively. The values of third order susceptibilities χ(3) at 532 nm were 0.83 × 10−12 (esu), 1.32 × 10−12 (esu), 1.42 × 10−12 (esu), 1.55 × 10−12 (esu) and 1.56 × 10−12 (esu) at intensities of 0.76 (GW/cm2), 1.13 (GW/cm2), 1.17 (GW/cm2), 1.63 (GW/cm2) and 1.88 (GW/cm2) respectively while that of values at 1064 nm were 2.71 × 10−12 (esu), 2.96 × 10−12 (esu), 3.4 × 10−12 (esu) and 4.02 × 10−12 (esu) at incident optical beam intensities of 1.20 (GW/cm2), 1.25 (GW/cm2), 1.32 (GW/cm2) and 1.38 (GW/cm2) respectively. It was seen that the sample showing saturable absorber to reverse saturable absorber at 532 nm wavelength at incident optical beam peak intensity (I0) of 1.53 GW/cm2. Thus the crystal could be a potential candidate of optical switching application. It was also seen that the material possess positive nonlinearity. It was also found that the values of nonlinear absorption coefficient (β) and nonlinear refraction (n2) increase with increase of excitation intensity irrespective of excitation wavelengths.

Enhanced SERS performance of Ag nanoparticles using hybrid dewetting process for melamine detection

A thermal evaporation system was used to deposit Ag thin films on glass substrates. The films were dewetted using both a regular laser irradiation approach and a hybrid dewetting process that involved laser irradiation followed by Ag thermal deposition. The study would evaluate and compare the optical characteristics and efficiency in melamine detection within these dewetted films. In the traditional dewetting process, as the laser power increased from 15 to 25 W, the nanoparticles (NPs) size decreased from 162 to 125 nm. Furthermore, an evident optical absorption peak range of 540–600 nm was also observed. Through the hybrid dewetting process, a mixture of small NPs (20 nm) and larger NPs (142–167 nm) were formed. It resulted in two distinct Surface Plasmon Resonance (SPR) peaks at 460 nm and 660 nm, respectively. Notably, a strong Raman peak at 701 cm−1 emerged when a melamine aqueous solution was placed on the hybrid dewetted surface. Comparing the two methods, the hybrid dewetted sample exhibited enhanced fingerprint peak intensity and a higher analytical enhancement factor (AEF). When operating at laser power of 25 W and a scanning speed of 100 mm/s, the maximum AEF value reached 1.9 × 105. The limit of detection for melamine solution in the hybrid dewetted sample was established at 10−7 M. Hence, the hybrid dewetting process is a potential method to produce the mixed-sized nanoparticles for diverse applications.

Tuning optical excitations of graphene quantum dots through selective nitrogen doping

Our research has revealed that the nitrogen doping configuration has a significant impact on the absorption properties and band gap of nitrogen doped graphene quantum dots (NGQDs). By analyzing the composition and character of optical transitions, we have observed that nitrogen doping causes a redistribution of oscillator strength between significant peaks and the emergence of new optical features. These changes lead to broken molecular orbital degeneracies, optical peak splitting, and activation of dark states in the visible to near-infrared (NIR) region. These findings shed light on the mechanisms that govern alterations in the spectral properties of NGQDs within the visible and near-infra red (NIR) absorption bands. Furthermore, selective manipulation of optoelectronic properties via distinct N-doping patterns could pave the way for the development of novel optoelectronic nanodevices and functional materials.

Mechanical strain effect on the optoelectronic properties and photocatalysis applications of layered AlN/GaN nanoheterostructure.

The aim of this work is to use first principles calculations to examine the effects of different mechanical strains on the optoelectronic and photocatalytic capabilities of the 2D/2D nanoheterostructure of AlN/GaN. By utilizing the lmBJ (Meta-GGA) and PBEsol (GGA) functional, the bandgap of the nanoheterostructure is calculated and found to be 4.89eV and 3.24eV. Simulated 2D AlN/GaN nanoheterostructure exhibits exceptional optical and electronic characteristics under applied biaxial tensile and compressive strains. The band gap changes from 4.89 to 3.77eV, while the energy gap nature transitions from direct to indirect during tensile strain fluctuations of 0% to 8%. Strain is also found to have a significant effect on the optical absorption peaks. And a 0-8% rise in tensile strain causes the initial absorption peak of the 2D AlN/GaN nanoheterostructure to shift from 4.88 to 4.20eV, which results in a 14% red shift in photon energy for every 2% change in strain. Furthermore, the optimum bandgap and band edge positions of the 2D AlN/GaN nanoheterostructure enable the water redox process to produce hydrogen and oxygen for wide range of pH. Thus, modification via strain may be an effective method for altering the optical as well as electronic characteristics of a 2D AlN/GaN nanoheterostructure, and this study may pave the way for new applications of this material in optoelectronic devices in the future. In the current work, density functional theory is used to explore every attribute of the 2D AlN/GaN nanoheterostructure. To characterize the electronic exchange-correlation, we used the PBEsol functional. In order to prevent any interlayer contact between periodicity of images, a vacuum is produced along the z-direction of approximately 10 Å. To increase the precision of bandgap prediction, the electronic and optical characteristics were computed using the meta-GGA lmBJ functional. To account for interlayer van der Waals interactions, nanoheterostructure computations were performed using the DFT-D3 functional.

Effects of hydrostatic pressure, temperature, and position-dependent mass on the nonlinear optical properties of triple delta-doped GaAs quantum well

In this study, we thoroughly investigate the impacts of hydrostatic pressure, temperature, and position-dependent mass (PDM) on the nonlinear optical properties of asymmetric triple δ-doped GaAs quantum wells. Our analysis covers total optical absorption coefficients, relative refractive index changes, nonlinear optical rectification, second harmonic generation, and third harmonic generation. Initially, we employ PDM to solve the time-independent Schrödinger equation using the diagonalization method under effective mass and parabolic band approaches, considering pressure and temperature dependencies. Utilizing the first four energy eigenvalues and eigenfunctions, we apply the compact density matrix method to compute the system’s nonlinear optical properties numerically. The results indicate a shift in optical property peak positions toward lower (higher) energy spectra with increasing hydrostatic pressure (temperature). Furthermore, the influence of PDM shifts the system’s optical properties toward the higher energy spectrum, resembling the effect of temperature. From an experimental and theoretical perspective, one of the topics that researchers work on most is GaAs-based δ-doped systems (δ-doped heterojunction bipolar transistors, δ-doped field effect transistors, δ-multiple independent gate field effect transistors, etc.). We believe these findings will provide valuable insights for the researchers involved in GaAs-based δ-doped optoelectronic device design.

Strain-bandgap quasi-linear correlation applied to modulation of NiBr2 monolayer optical properties

Recently, two-dimensional transition metal dihalides have shown diverse applications in spintronics, quantum devices, and optoelectronic devices. In this work, density functional theory (DFT) is adopted to investigate the effect of biaxial strain upon the electronic structure and optical properties of NiBr2 monolayer. The results show that NiBr2 is an indirect semiconductor with band gap 2.62 eV (DFT + U) and 3.18 eV (HSE06), which could vary quasi-linearly from 2.73 eV to 1.71 eV modulated by biaxial strain (6 % to −10 %). Combined with the analysis of the density of states (DOS), the primary reason for the alteration in the band gap is that strain significantly affects the behavior of Ni 3d electrons. When the strain changes from positive to negative, the DOS of Ni 3d shifts towards the Fermi level, resulting in a reduction in the band gap. Within this strain range, all of the optical properties exhibit a quasi-linear tendency similar to the band gap variation. The optical peaks of NiBr2 under compressive strain are not only red-shifted into the visible light (VL) region but also significantly enhanced in the VL and ultraviolet (UV) regions. Applying strain can greatly boost the electrical and optical properties of NiBr2 monolayer, making it a promising candidate in the field of ultraviolet detectors.

Influence of the optoelectronic properties of F-MoTe2 systems under the combined action of an external electric field and biaxial tensile-compressive deformation: a first-principles study

The effects of an applied electric field on the electronic structure, charge transfer, and optical characteristics of molybdenum telluride (MoTe2) systems doped with halogen F atoms during biaxial tensile-compressive deformation were explored using first-principles approaches. The results show that the MoTe2 system exhibits an upward shift of the Fermi energy level and a downward shift of the conduction band when crossing the Fermi energy level due to the doping of halogen F atoms. Subsequently, we applied an electric field of −0.4 eV∼−0.4 eV to the F-MoTe2 system. The change of the F-MoTe2 system under the electric field effect is very small. This result indicates that the F-MoTe2 system can be stabilized in this range. Subsequently, we chose to apply a biaxial tensile-compressive deformation of −14%−14% to the F-MoTe2 system under the action of an electric field of +0.4 eV. The F-MoTe2 system underwent a transition from a semiconductor to a metal, with an increase in the carrier concentration and a good shift in electrical conductivity. The compressive strain gives better results than the tensile strain. In terms of optical characteristics, the absorption and emission peaks of the F-MoTe2 system are blueshifted under the combined effect of electric field and stress.

Revealing the Structural, Electronic, Optical, and Thermoelectric Aspects of the Gold‐Based Double Perovskites X2Au+Au3+Br6 (X = Cs, Rb) Using a First‐Principles Approach

Lead‐free double perovskites are now assumed to be suitable candidates for green energy harvesting in particular as active materials for solar cells and thermoelectric generators, which can meet future generation energy needs. Therefore, we explore the Au‐based halide double perovskites X2Au+Au3+Br6 (X = Cs, Rb) from the first principles approach. Density functional theory (DFT) is utilized to explore the electronic structure with DFT code Quantum ESPRESSO. The mechanical, thermodynamic, and structural stability is ensured from Burn‐Haun criterion, formation energies, and Goldschmidt factors, respectively. The examined materials have stable structures with direct band gaps i.e. 1.54 and 1.72 eV. The existence of band gaps in the visible region motivates us to explore the optical properties, which give fascinating outcomes. The absorption coefficients and optical conductivity peaks are found to be significant in the visible region i.e., ≈104 cm−1 and ≈1015 s−1, respectively. Additionally, the thermoelectric properties are also investigated using Boltzmann transport theory. There are several good gestures for the usage in the thermoelectric generators since the values of Seebeck coefficients (446.5, and 225.2 μV K−1), power factors (1.75 × 1011 W mk−2 s, and 1.24 × 1011 W mk−2 s), and figure of merits (0.92 and 0.73) are noteworthy for Cs2AuAuBr6 and Rb2AuAuBr6, respectively, at room temperature T = 300 K.