Optical Absorption Coefficient Research Articles

Al2O3/SnC heterostructure: Physical properties, regulation effect and device design

Two van der Waals (vdW) heterostructures between Al2O3 and SnC monolayers are constructed by the Generalised Lattice Matching (GLM) method. We calculate the elastic constants of monolayers and heterostructure and find that the heterostructures show larger Young’s modulus (220−234) N/m compared with Al2O3 (SnC), the reasonable Poisson’s ratio is about 0.3 and gravity-induced bending is the order of 104, which is comparable to graphene. The electronic properties, optical absorption coefficient and transmission properties of the heterostructure also have been investigated. Some important discoveries can be obtained: The Al2O3/SnC heterostructure shows an indirect bandgap (Eg) semiconductor with 1.70 eV and type II band arrangement when the ferroelectric polarization of Al2O3 points to SnC. While the polarization direction of Al2O3 is opposite, the Eg is zero. The physical properties are manipulated by some conditions (strain and electric field), leading to transitioning between Type II and I semiconductors, semiconductors and metal can be experienced. Moreover, the heterostructures prefer the higher light adsorption coefficient (105 cm−1) compared to monolayers. The transport properties of the Al2O3/SnC linked by two silicene electrodes show an excellent tunnel electroresistance (TER) effect with 108, moreover, the negative differential resistance (NDR) effect and switching effect also can be observed.

Effect of strain on the electronic structure and optical spectra of two-dimensional monolayer GaN

The effect of compressive strain and tensile strain on the band structure and optical spectra of two-dimensional monolayer GaN has been investigated. Computations were performed within density functional-theory. The results show that tensile two-dimensional monolayer-GaN undergoes an indirect-to-direct transition, which makes the material suitable for light-emitting and laser diodes. The material of interest is found to exhibit different optical properties dependent on the strain. Besides, the absorption band becomes wider and the optical absorption coefficient is reduced negligibly by strain, making two-dimensional-GaN a good candidate for application in photovoltaics and flexible optoelectronics.

Hydrostatic pressure and temperature effects on nonlinear optical properties in harmonic-Gaussian asymmetric double quantum wells

The paper examines the linear and non-linear optical characteristics of an electron in harmonic Gaussian asymmetrical double quantum wells, taking into account thermodynamic variables such as temperature and hydrostatic pressure. Numerical calculations by considering the effective mass and parabolic band approximation are performed. The electron contained within an asymmetric double well generated by the sum of a parabolic and Gaussian potential has its eigenvalues and eigenfunctions determined using the diagonalization approach. For nonlinear optical coefficients, the density matrix expansion is used. Wavefunctions and energy levels vary as an effect of the applied fields. In harmonic Gaussian asymmetrical double quantum wells, the total optical absorption coefficient (TOAC), the relative refractive index changes (RRIC), and second harmonic generation (SHG) have all been theoretically investigated. The magnitude and position of the resonant peaks are significantly influenced by the hydrostatic pressure and temperature effects. With controllable coupling and externally applied hydrostatic pressure and temperature, the potential model presented in this study can be used to simulate and manipulate the optical and electronic properties of the asymmetric double-quantum heterostructures, such as double quantum dots and wells.

Central-cell corrections for hydrogenic, silicon (Si), selenium (Se), sulfur (S), and germanium (Ge) donor impurities and pressure–temperature effects on the optical properties of the GaAs/GaAlAs multi-layer quantum disk

GaAs/GaAlAs quantum dots can be doped with various impurities to modify their optoelectronic properties. Common impurities used for doping include: Hydrogenic impurity (HI), Silicon (Si), Selenium (Se), Germanium (Ge), and Sulfur (S) are common n-type dopant in GaAs-based materials. This work has investigated the effects of temperature and pressure on the refractive index changes (RICs) and optical absorption coefficients (OACs) in the multi-layer quantum disk, accounting for central cell correction. We have used the finite element method to calculate the eigenvalues and their corresponding wave-functions. These values are utilized in the computation of the OACs and RICs. Our outcomes can be summarized as follows: (i) The applied pressure induces a red-shift of the OACs peaks, and their magnitude is reduced. Similarly, temperature causes a blue-shift of the OACs peaks, and their magnitude is enhanced. (ii) The presence and type of impurity lead to significant changes in both OACs and RICs.

A Novel Approach to Assessing Light Extinction with Decade-Long Observations of Chemical and Optical Properties in Seoul, South Korea

A Novel Approach to Assessing Light Extinction with Decade-Long Observations of Chemical and Optical Properties in Seoul, South Korea

Te-vacancy defect triggering photocatalytic overall water splitting for MoTe2/GeN Z-scheme heterostructure

Developing proper semiconductor photocatalysts is urgently needed to solve energy and environmental crises. Especially, photocatalysts for water splitting with a Z-scheme mechanism are desirable materials. Herein, a two-dimensional VTe-MoTe2/GeN heterostructure (VTe-HS), including single Te vacancy, is theoretically proved to function as a direct Z-scheme photocatalyst with the wide light-harvesting range, the effective spatial separation for carriers, and the high catalytic activity for water splitting. An enhanced optical absorption coefficient of 105 cm−1 is observed and the absorption edge (∼1.0 eV) is red-shifted into the infrared light region. Separating photogenerated electron–hole pairs enhances the photocatalytic efficiency due to the staggered band alignment. Interlayer charge migration of about 0.154 electrons induces an intrinsic built-in electric field in the heterostructure, allowing forming the Z-scheme mechanism with a reduction potential of –4.01 eV and an oxidation potential of –7.13 eV. Gibbs free energies confirm that water redox reactions can proceed spontaneously under sunshine. Moreover, the Heyrovsky reaction is kinetically more favorable than the Tafel reaction because of a little energy barrier (0.071 eV). These features show that the VTe-HS is a desirable water-splitting photocatalyst, providing a valuable clue to constructing high-efficiency photocatalysts.

Computational predictions of optoelectronic energy materials Cs2SiBr6 Cs2GeBr6 & Cs2SnBr6 for phenomenal photovoltaic applications; a first principles study

Exploring prospective materials for energy storage and production is one of the largest confrontation of the century. Solar optoelectronic energy is one of the utmost exceptional renewable resources now a days, due to their small Environmental impact and wide availability. The perovskite type double halides are promising materials for optoelectronic and solar applications. In this article, the optoelectronic characteristics of newly perovskite double halides Cs2SiBr6 Cs2GeBr6 and Cs2SnBr6 are addressed. The calculated band gaps 2.70 eV (Cs2SiBr6), 1.80 eV (Cs2GeBr6), 2.90 eV (Cs2SnBr6) with raising performance for optoelectronic applications. The optical properties are explored by large dielectric constants, high optical absorption coefficients, small optical loss etc. Moreover, the mechanical properties for the studied materials have been performed to confirm the mechanical stability, comactability and reliability of the materials for photovoltaic applications. The thermodynamic and vibrational stability and compatibility has been verified due to negative values of enthalpy, cohesive energies and no soft phonon mode of vibrations. These optoelectronic energy related calculations confirmed that the perovskites type double halides may Cs2SiBr6, Cs2GeBr6 and Cs2SnBr6 may be novel, stimulating potential materials and can be synthesized experimentally for next generation photovoltaic devices and related applications.

Moving Beyond Simulation: Data-Driven Quantitative Photoacoustic Imaging Using Tissue-Mimicking Phantoms.

Accurate measurement of optical absorption coefficients from photoacoustic imaging (PAI) data would enable direct mapping of molecular concentrations, providing vital clinical insight. The ill-posed nature of the problem of absorption coefficient recovery has prohibited PAI from achieving this goal in living systems due to the domain gap between simulation and experiment. To bridge this gap, we introduce a collection of experimentally well-characterised imaging phantoms and their digital twins. This first-of-a-kind phantom data set enables supervised training of a U-Net on experimental data for pixel-wise estimation of absorption coefficients. We show that training on simulated data results in artefacts and biases in the estimates, reinforcing the existence of a domain gap between simulation and experiment. Training on experimentally acquired data, however, yielded more accurate and robust estimates of optical absorption coefficients. We compare the results to fluence correction with a Monte Carlo model from reference optical properties of the materials, which yields a quantification error of approximately 20%. Application of the trained U-Nets to a blood flow phantom demonstrated spectral biases when training on simulated data, while application to a mouse model highlighted the ability of both learning-based approaches to recover the depth-dependent loss of signal intensity. We demonstrate that training on experimental phantoms can restore the correlation of signal amplitudes measured in depth. While the absolute quantification error remains high and further improvements are needed, our results highlight the promise of deep learning to advance quantitative PAI.

Two-dimensional InBrSe monolayer: A highly efficient photocatalyst for water splitting

Good photocatalysts is one of the best approaches to harvest the solar energy. The performance of two-dimensional photocatalytic materials depends on their distinctive geometric structure and optical absorbed efficiency. However, the tendency to recombine the photogenerated electron-hole pairs in many two-dimensional materials limits their photocatalytic performances. Here, we find a novel FeOCl-type InBrSe monolayer of superior photocatalysis through first-principle calculations. Its electron mobility is 42.53 times greater than that of hole along y-axis, indicating that anisotropic mobilities limit the recombination of photogenerated electron-hole pairs. Its solar to hydrogen efficiency (27 %) exceeds the critical value (10 %) of commercial application of photocatalytic water splitting. Moreover, its suitable band edges are advantageous for spanning water redox potentials to achieve photocatalytic processes. Under biaxial strains, it obtains large optical absorption coefficients within the visible light range. Additionally, it exhibits favorable thermal, mechanical and structural stability, indicating that it is feasible for real-life experimental synthesis. Thus, we predict that the InBrSe monolayer is a promising candidate for photocatalyst.

Constant Photocurrent Method to Probe the Sub‐Bandgap Absorption in Wide Bandgap Semiconductor Films: The Case of α‐Ga2O3

The optical absorption coefficient is one of the fundamental properties of semiconductors and is critical to the development of optical devices. Herein, a revival of the constant photocurrent method is presented to measure sub‐bandgap absorption in wide bandgap semiconductor films. The method involves maintaining a constant photocurrent by continually adjusting the impinging photon flux across the energy spectrum. Under such conditions, the reciprocal of the photon flux for uniformly absorbed light is proportional to the absorption coefficient. This method is applied to α‐Ga2O3 and reveals that it can access the absorption coefficient from 1 × 105 cm−1 at the band edge (5.3 eV) to 0.8 cm−1 close to mid‐bandgap (2.7 eV). Changes in the steepness of the absorption curve in the sub‐bandgap region are in excellent agreement with defect states of α‐Ga2O3 reported by deep level transient spectroscopy, indicating that the technique shows promise as a probe of energetically distributed defect states in thin film wide bandgap semiconductors.

The effect of Coulomb interaction on optical absorption of a quantum well wire

The optical absorption coefficient in GaAs/AlGaAs cylindrical parabolic quantum wire (QW) with an applied static-electric field is studied in detail. The Schrödinger equation is solved within the effective mass approximation and an analytical solution is used for the electronic structure of cylindrical QW. The main aim is to clarify the effect of the Coulomb interaction on the exciton-related optical absorption of a QW with no assumptions about the strength of the confinement. Finally, it is clearly shown that the further confinement due to Coulomb interaction in a quantum wire increases the transition energy, dipole matrix elements, and absorption peaks, and results with the blue shift. One cannot neglect the Coulomb interaction in the absorption calculations.

Influence of temperature on bandgap shifts, optical properties and photovoltaic parameters of GaAs/AlAs and GaAs/AlSb p–n heterojunctions: insights from ab-initio DFT + NEGF studies

The III–V group semiconductors are highly promising absorbers for heterojunctions based solar cell devices due to their high conversion efficiency. In this work, we explore the solar cell properties and the role of electron–phonon coupling (EPC) on the solar cell parameters of GaAs/AlSb and GaAs/AlAs p–n heterojunctions using non-equilibrium Green function method (NEGF) in combination of ab-initio density functional theory (DFT). In addition, the band offsets at the heterointerfaces, optical absorption and bandgap shifts (BGSs) due to temperature are estimated using DFT + NEGF approach. The interface band gaps in heterostructures are found to be lower than bulk band gaps leading to a shift in optical absorption coefficient towards lower energy side that results in stronger photocurrent. The temperature dependent electronic BGS is significantly influenced by the phonon density and phonon energy via EPC. The phonon influenced BGS is found to change the optical absorption, photocurrent density and open-circuit voltage. In case of GaAs/AlSb junction, the interface phonons are found to have significantly higher energies as compared to the bulk phonons and thereby may have important implications for photovoltaic (PV) properties. Overall, the present study reveals the influence of EPC on the optical absorption and PV properties of GaAs/AlSb and GaAs/AlSb p–n heterojunctions. Furthermore, the study shows that the DFT + NEGF method can be successfully used to obtain the reasonable quantitative estimates of temperature dependent BGSs, optical absorption and PV properties of p–n heterojunctions.

Light absorption by free charge carriers in the presence of phonons in an anisotropic quantum wire

The absorption of light by free charge carriers has been theoretically studied considering the processes related to scattering from phonons in an anisotropic parabolic quantum wire. The case in which the electromagnetic wave is polarized along the axis of the wire and the case of non-degenerate electron gas statistics have been considered. In this work, it was determined that the dependence of the location of the peaks of the intraband optical absorption coefficient on the characteristic frequency of the confinement potential in quantum wires with asymmetric parabolic potential was .

Effect of Nd substitution on electronic, thermoelectric, and optical response of WO3

The properties of Nd-doped WO3 thin films are studied to assess their suitability for photovoltaic and optoelectronic applications. The simulations on pristine and Nd-doped WO3 were performed using Tran Blaha modified Becke-Johnson approximation to explore the various properties, whereas the spin coating technique was employed to synthesize thin films. X-ray diffraction analysis revealed the crystalline cubic structure in all synthesized thin films. The morphology contains uniform rod-like features and the width of these were enhanced in doped composition. The projected density of states demonstrated the major prominent contributions of W-d in conduction and O-p in the valence band while Nd-f orbital plays a major contribution in doped compositions. Thermoelectric parameters were found to change significantly with Nd incorporation in WO3. The highest refractive index and real epsilon values were recorded at the higher energy regimes approximately 2.96 and 8.61, respectively, for compositions containing maximum dopant content. The optical conductivity and absorption coefficient showed an enhanced trend in doped composition which is considered favourable for improved optoelectronic and photovoltaic applications. The experimental band gap of WO3 was calculated as 2.03 eV and found to decrease with increment of Nd doping concentration.

Structural and Optical Properties of Interfacial InSe Thin Film.

This study presents a comprehensive investigation of the optical and structural characteristics of the indium selenide (InSe) film prepared on a glass substrate. The structural characteristics of the InSe film were analyzed using characterization techniques including X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy while the UV-vis spectrophotometry method was used in the spectral range between 500 and 1000 nm to examine the optical characteristics. Thus, the UV-vis spectroscopic data were used to extract several optical parameters including extinction coefficient (k), optical band gap (Eg), refractive index (n), absorption coefficient (α), and optical conductivity (σopt). The optical transition of InSe was found as a direct transition. However, the optical analysis of this study has revealed that the InSe film has the potential to be used in various optoelectronic and photovoltaic applications.

Tunable electronic properties and optoelectronic characteristics of MoGe2N4/SiC van der Waals heterostructure

The assembly of van der Waals (vdW) heterostructure with easily regulated electronic properties provides a new way for the expansion of two-dimensional materials and promotes the development of optoelectronics, sensors, switching devices and other fields. In this work, a systematic investigation of the electronic properties of MoGe2N4/SiC heterostructures using density functional theory has been conducted, along with the modulation of electronic properties by vertical strain and the potential application prospects in optoelectronic devices. The results show that MoGe2N4/SiC heterostructure has excellent dynamic and thermal stability and belongs to type-II band alignment semiconductors. This is extremely beneficial for the separation of photo-generating electron-hole pairs, so it has important significance for the development of photovoltaic materials. In addition, under the control of vertical strain, the semiconductor-metal transition occurs in the MoGe2N4/SiC heterostructure when the compressive strain reaches 6%. In the case of compressive strain less than 6% and tensile strain, the MoGe2N4/SiC heterostructure maintains the type-II band alignment semiconductor characteristics. Meanwhile, we find that the MoGe2N4/SiC heterostructure has optical absorption coefficients of up to 105 in the visible and ultraviolet light ranges, which can improve the absorption coefficients of the MoGe2N4 and SiC monolayer in some visible light regions. Finally, the optical conductivity of the MoGe2N4/SiC heterostructure exhibits significant anisotropy, with the armchair direction displaying higher conductivity within the orange light range. In conclusion, the formation of vdW heterostructure by vertically stacking MoGe2N4 and SiC monolayers can effectively improve their electronic and optical properties, which provides a valuable reference for the future development of electronic devices and photovoltaic materials.

Modulation mechanism of electronic and optical properties of Cs2SnX6 (X = Cl, Br and I) under hydrostatic or uniaxial pressure

The air-stable, low-toxicity double perovskite structure [Formula: see text] (X = Cl, Br and I) has attracted great attention due to its excellent potential in optoelectronic and photovoltaic applications. We used density functional theory to theoretically study the pressure response mechanism of double perovskite [Formula: see text]. The results show that pressure can effectively regulate the electronic structure and optical absorption properties of [Formula: see text]. When subjected to hydrostatic pressure ranging from 0 to 50 GPa, the bandgaps of [Formula: see text], [Formula: see text] and [Formula: see text] all exhibit a phenomenon of first decreasing and then increasing during the change. [Formula: see text] exhibits semimetallic properties when the valence band and conduction band begin to overlap under hydrostatic pressure of 2 GPa or higher. In addition, with increasing pressure, the optical absorption peak of [Formula: see text] shows a significant blue shift, and the absorption coefficient in the visible and near-infrared regions gradually increases. When a uniaxial pressure is applied to [Formula: see text] crystal, it significantly reduces in one direction and expands in the other two perpendicular directions. The structure changes from cubic phase to tetragonal phase. The optical absorption coefficient in the direction of pressure first increases and the others decrease.

Optical properties and electric modulus studies of TSP: CH3COONa based biopolymer electrolytes

The research of biopolymers for the creation of solid polymer electrolytes (SPE) for electrochemical devices has resulted from the green revolution. Various strategies have been used to build and characterise biopolymer-based membranes for proton and lithium-ion conduction. However, research on biopolymers based on sodium ions is uncommon in the literature. The solution cast approach is used in this work to create a SPE based on the biopolymer Tamarind Seed Polysaccharide (TSP) incorporated with sodium acetate (CH3COONa). UV–visible optical absorption spectroscopy in the wavelength range of 200–800 nm was used to investigate the optical characteristics. From the studies of optical absorption, optical transmission, optical absorption coefficient, refractive index spectrum, extinction coefficient spectra, direct energy bandgap, indirect energy bandgap, optical absorption edge, Urbach – energy, and optical dielectric loss were calculated. The comparative study of the optical dielectric loss to the optical bandgap showed that the direct allowed transition was the most probable electronic transition for the prepared films. The calculated optical bandgap (Direct allowed) was decreased for 80:20 film from 5.51 eV (for pure TSP) to 4.88 eV. The direct, indirect, and absorption edges for pure TSP were all high, and when the salt concentration of CH3COONa increased, the above characteristics progressively decreased. For 80 % TSP: 20 % CH3COONa concentration, a low direct and indirect energy bandgap value was obtained. Tangent loss and electric modulus experiment results were then presented using AC Impedance data. For the 80:20 film, the tangent energy loss revealed a minimum and the electric modulus spectra exhibited the zeroth tail is at its maximum.

Peculiar band geometry induced giant shift current in ferroelectric SnTe monolayer

The bulk photovoltaic effect (BPVE) occurs when homogeneous noncentrosymmetric materials generate photocurrent or photovoltage under illumination. The intrinsic contribution to this effect is known as the shift current effect. We calculate the shift current conductivities of the ferroelectric SnTe monolayer using first-principles methods. Our results reveal a giant shift-current conductivity near the valley points in the SnTe monolayer. More remarkably, the linear optical absorption coefficient at this energy is very small, resulting in an enormous Glass coefficient that is four orders of magnitude larger than that of BaTiO3. To understand these giant shift-current effects, we employ a three-band model and find that they arise from the nontrivial energy band geometries near the valley points, where the shift-vector diverges. This serves as a prominent example highlighting the crucial role of band geometry in determining the fundamental properties of solids.

Tunable electronic and optical properties of WSe2/Si2H heterojunction via electric field

Van der Waals heterojunctions based on two-dimensional (2D) materials hold great potential applications in photodetectors. Using the density functional theory (DFT) method, the structures, electronic and optical properties of van der Waals WSe2/Si2H heterojunction are investigated. 1.32 eV of indirect bandgap is calculated from the WSe2/Si2H heterojunction, which is 0.3 eV and 0.1 eV smaller than those of its monolayer WSe2 and Si2H. This contributes to the photocarrier generations, and the Type-II heterojunction also benefits to the separation of the photogenerated electron and hole pairs. A significant hole mobility 1.05 × 104 cm2 V−1 s−1 of the heterojunction along the y-direction is obtained. Moreover, a high Ultraviolet light (UV) absorption coefficient is presented in the heterojunction. The heterojunction transforms to Type-I under a vertical electric field, with the bandgap, orientation and amount of transfer electrons modulated sufficiently. As a result, the optical absorption coefficient of the heterojunction is also improved significantly, leading to the red-shift of the absorption spectrum. These excellent properties address the WSe2/Si2H heterojunction one of the good candidates for UV detectors.