Structural Optical Properties Research Articles

Lanthanide ions doped ZnO based photocatalysts

ZnO is a versatile semiconductor that finds prominence in the area of photocatalysis due to their compatible crystal structure and band gap edge potentials that are vital for the pollutant degradation reaction. Thus, fabrication of ZnO based nanomaterials with tailored properties remains as the trending topic in recent times. In this context, doping with lanthanide ion finds interesting because of their unique 4f electronic configuration which offers stupendous opportunity to modulate the optical, luminescent and surface acid-base properties of ZnO. This article provides concise overview about the influence of lanthanide ions dopant on the structure-optical properties of ZnO. The performance of Ln-ZnO towards the degradation of organic pollutants is discussed encompassing both the materials properties and photocatalytic reaction conditions under UV/visible and solar light. The co-doping of Ln-ZnO with non-metal/transition metal ion and modification of Ln-ZnO with carbon materials is also emphasized to underscore the recent advancements in this field. We anticipate that our review article provides adequate information into the rational design and development of Ln-ZnO for various applications.

Rational Molecular Design of Azaacene-Based Narrowband Green-Emitting Fluorophores: Modulation of Spectral Bandwidth and Vibronic Transitions.

A series of green-emitting fluorophores based on a tetra-azaacene core is synthesized by introducing nitrile substituents at different positions. Their molecular structure-optical property relationship [i.e., vibronic transitions in photoluminescence (PL) and electroluminescence (EL) spectra] is investigated to obtain a sharp emission where the vibronic peak ν0-0 should be intensified by suppressing ν0-n (n = 1, 2, 3...) transitions. The intensity ratios (I0-1/I0-0) of the ν0-1 and ν0-0 vibronic transitions in the PL spectra of DBBNP, DBBNP2CN1, and DBBNP2CN2 in hexane are 1.13, 0.80, and 0.67, respectively. Theoretical calculations explain that the CN substitution at positions 8 and 13 in DBBNP2CN2 induces a uniform charge distribution and reduces the Huang-Rhys factors (HRFs) of the vibrational normal modes coupled to the electronic transition. The organic light-emitting diode (OLED) fabricated with DBBNP2CN2 shows a narrower green EL emission at 518 nm with a smaller bandwidth (50 nm) than those of devices adopting DBBNP or DBBNP2CN1. The careful modification of the molecular structures and positions of substituents enables us to reduce the HRFs of vibrations to achieve a narrow emission bandwidth with decreased I0-1/I0-0, which suggests a design strategy to develop narrowband organic fluorophores to improve the color purity for wide-gamut OLED displays.

Pressure-induced bandgap engineering of lead-free halide double perovskite (NH4)2SnBr6.

Lead-free halide double perovskites (HDPs) have recently been proposed as potential stable and environment-friendly alternatives to lead-based halide perovskites. Bandgap engineering plays a vital role in the optoelectronic applications of HDP materials. In this study, methods combining high-pressure techniques with density functional theory calculations were employed to implement the bandgap engineering of a classic HDP-based (NH4)2SnBr6. Under high pressure, (NH4)2SnBr6 exhibits a redshift of the bandgap with increasing pressure up to 6.3 GPa and a sudden blueshift up to 20.2 GPa, followed by a redshift at higher pressures, which is relevant to the cubic-tetragonal phase transition, direct-indirect transition, and amorphization, respectively. Our results enrich the understanding of the structural-optical properties of (NH4)2SnBr6 and reveal the special role of NH4+ cations in pressure-induced bandgap engineering, thus providing important information for application in optoelectronic devices and helping to design ideal materials with higher efficiency.

Pulsed Laser Irradiation of GaAs-Based Light-Emitting Structures

The effects of KrF excimer laser pulses on the crystalline and optical properties of structures with four InxGa1 – xAs/GaAs quantum wells (x ranging from 0.08 to 0.25) are studied. The results obtained by Raman spectroscopy and reflection spectroscopy show that the high crystalline quality of the GaAs cap layer is retained after exposure to laser radiation with an energy density from 200 to 360 mJ/cm2. It is experimentally shown by photoluminescence spectroscopy and laser-annealing simulation, and by solving the problem of heat propagation in a one-dimensional GaAs-based system, that the thermal effects occurring in heterostructures under pulsed laser irradiation below the GaAs melting threshold lead to mechanical-stress relaxation. At the initial stages of this process, point defects appear in InxGa1 – xAs/GaAs quantum wells which lead to a “red” shift of the photoluminescence emission peaks of the quantum wells and serve as nonradiative-recombination centers, which causes photoluminescence quenching.

Improving the surface layer structure in opal-like plasmonic-photonic crystals for efficient excitation of optical Tamm states

We investigate the transmission of light through three-dimensional structures in opal-like plasmonic-photonic crystals with an Au surface layer of different geometries. We study how the inclusion of buffer layers of various forms affects the spectral characteristics of the structures. Three-dimensional photonic crystals have significant advantages over one-dimensional ones since they display a complete photonic bandgap and porosity. Controlling the spectral characteristics of the hybrid plasmonic-photonic mode is an important task that offers the possibility of creating tunable devices based on such structures. In this work we show that the optical properties of structures with different shapes of the surface layer do not depend on light polarization, except for structures with a corrugated Au layer and a corrugated buffer layer. The bandwidth of a structure with a corrugated Au layer is smaller and shows polarization sensitivity in contrast to a structure with a smooth Au layer. The optical characteristics of hybrid structures undergo significant changes when buffer layers of various shapes are introduced between the photonic crystal and the Au layer. The width of the peak in the transmission spectra changes and its intensity increases against the background after the improvement of the structure of the plasmonic-photonic crystal. Thus, opal-like plasmonic-photonic crystals with different structure of the surface layer have spectral features that can be used in integrated circuits, biosensors, and resonators. • The inclusion of buffer layers of various forms affects the spectral characteristics of the plasmonic-photonic crystal. • The optical properties of different structures do not depend on light polarization. • As an exception, the bandwidth of a structure with a corrugated Au&buffer layers shows polarization sensitivity. • The width and intensity of the peak changes after the improvement of the plasmonic-photonic crystal.

Structure-optical property correlation in CdTe/CdS core-shell quantum dots and their effects on Cu2+ sensing mechanisms

A rapid and simple aqueous method has been developed for synthesizing CdTe/CdS core shell quantum dots (QDs) by refluxing glutathione (GSH) with Cd2+ and Te2− precursors at 100 °C without using inert medium. Here GSH has dual role, i.e., as an in-situ source of sulphide for deposition of CdS on CdTe and as a capping agent to stabilize the CdTe/CdS QDs. As the refluxing time was enhanced from 15 min to 105 min, the sizes of the spherical shaped CdTe/CdS QDs increased from 2.8 ± 0.8 nm to 3.9 ± 1.1 nm, the crystalline nature improved, GSH capping content decreased and the CdS shell deposition was increased. The size dependent change in photoluminescence (PL) from green to red was observed. The photoluminescence quantum yield (PLQY) was improved with refluxing time and maximum PLQY was obtained for the batch synthesized by 60 min refluxing. The PL of these QDs were drastically quenched by ppb levels of Cu2+ ions. Instead of the batch with maximum PLQY, the best Cu2+ sensitivity and selectivity was obtained for the batch synthesized by refluxing for 30 min. This is due to optimal GSH capping content required for availability of Cu2+ ions at the surface of the QDs for favorable electron transfer process, confirmed by PL decay kinetic studies.

Tight-Binding Studio: A technical software package to find the parameters of tight-binding Hamiltonian

We present the Tight-Binding Studio (TB Studio) software package that calculates the different parameters of a tight-binding Hamiltonian from a set of Bloch energy bands obtained from first principle theories such as density functional theory, Hartree–Fock calculations or semi-empirical band-structure theory. This will be helpful for scientists who are interested in studying electronic and optical properties of structures using Green’s function theory within the tight-binding approximation. TB Studio is a cross-platform application written in C++ with a graphical user interface design that is user-friendly and easy to work with. This software is powered by Linear Algebra Package C interface library for solving the eigenvalue problems and the standard high performance OpenGL graphic library for real time plotting. TB Studio and its examples together with the tutorials are available for download from tight-binding.com. Program summaryProgram Title: Tight-Binding StudioProgram Files doi:http://dx.doi.org/10.17632/j6x5mwzm2d.1Licensing provisions: LGPLProgramming language: C++External routines: BLAS, LAPACK, LAPACKE, wxWidgets, OpenGL, MathGLNature of problem: Obtaining Tight-Binding Hamiltonian from a set of Bloch energy bands obtained from first-principles calculations.Solution method: Starting from the simplified LCAO method, a tight-binding model in the two-center approximation is constructed. The Slater and Koster (SK) approach is used to calculate the parameters of the TB Hamiltonian. By using non-linear fitting approaches the optimal values of the SK parameters are obtained such that the TB energy eigenvalues are as close as possible to those from first-principles calculations. We obtain the expression for the Hamiltonian and the overlap matrix elements between the different orbitals of the different atoms in an orthogonal or non-orthogonal basis set.

Pressure-Induced Remarkable Enhancement of Self-Trapped Exciton Emission in One-Dimensional CsCu2I3 with Tetrahedral Units.

Self-trapped exciton (STE) emissions derived from inorganic octahedral units make metal halide perovskites promising photoluminescence materials for light-emitting applications. However, there is still little understanding of the intrinsic STE emissions in metal halide perovskites or derivatives with nonoctahedral units. In this work, via high pressure compression, remarkable STE emission enhancement is, for the first time, realized in one-dimensional CsCu2I3 crystals with {CuCl4} tetrahedral units. The intertetrahedral distortionis believed to induce the slight emission enhancement of the ambient phase under initial compression. Notably, the obvious structural distortions of both inter- and intratetrahedra are responsible for the significant emission enhancement of the high pressure phase. This work not only sheds light on the structure-optical property relationships of tetrahedron-based halide complexes, but also may provide guidance for the design and fabrication of highly luminescent metal halides.

Импульсное лазерное облучение светоизлучающих структур на основе GaAs

The effects of KrF excimer laser pulses on the crystalline and optical properties of structures with four InxGa1−xAs/GaAs quantum wells (x ranged from 0.08 to 0.25) were studied. The results obtained by Raman spectroscopy and reflection spectroscopy showed that the high crystalline quality of the GaAs cap layer is retained after exposure to laser radiation with an energy density of 200 to 360 mJ/cm2. It was established experimentally by photoluminescence spectroscopy and by modeling the laser annealing process, which is a solution to the problem of heat propagation in a one-dimensional GaAs-based system, that the thermal effects that occur in heterostructures under pulsed laser irradiation below the GaAs melting threshold lead to relaxation of mechanical stresses. At the initial stages of this process, the point defects appear in InxGa1−xAs/GaAs quantum wells. The latter lead to a “red” shift of the photoluminescence emission peaks of quantum wells and serve as centers of nonradiative recombination, which causes the quenching of the photoluminescence.

A cyanine-derived near-infrared molecular rotor for ratiometric imaging of mitochondrial viscosity in cells

We developed a new cyanine-derived near-infrared (NIR) molecular rotor Mu1, which is superior to the traditional cyanines with high viscosity response, large Stokes shift (˜100 nm), and high photo-stability to detect the microscopic viscosity ratiometrically. The time-dependent density functional theory calculations highlighted the structure-optical properties of Mu1 as molecular rotor. Due to the mitochondria-actived fluorescence characteristics, Mu1 was used to track mitochondrial viscosity changes in live cells with high spatial and temporal resolution. This new type of NIR molecular rotor presented herein may open up new opportunities of NIR sensors for biomedical diagnosis and imaging applications.

Tuning the photo-physical properties of BODIPY dyes: Effects of 1, 3, 5, 7- substitution on their optical and electrochemical behaviours

Five BODIPY derivatives containing different number of styryl units were designed and synthesized based on the Knoevenagel reactions. All compounds were characterized by NMR and HRMS. The single crystal structures of compounds 1 and 3–5 were confirmed by X-ray crystallography. The photophysical and electrochemical properties of these dyes were also investigated by UV/Vis spectroscopy, fluorescence spectroscopy, and cyclic voltammetry. Our study indicates that the optical and electronic properties of the BODIPY derivatives could be easily tuned by the introduction of a various number of styryl units. Their absorption and emission spectra become gradually bathochromically shifted from the red region to the near-infrared region. Their first reduction and first oxidation potential (HOMO-LUMO gap) changes from 2.45 to 1.62 V. The structure-optical property relationships of these compounds are also systematically studied. Furthermore, cellular uptake experiments suggest that compounds 1–3 are membrane permeable and promising fluorophores for bio-labeling/imaging applications.

Investigation of the microstructure, cation distribution and optical properties of nanoscale NixMg1-xAl2O4 spinel pigments

Abstract Nanoscale NixMg1-xAl2O4 spinel pigments were synthesized by a citric acid precursor combined with the gel-casting method. The microstructure, cation distribution and optical properties as a function of calcining temperature and nickel content were investigated by the X-ray diffraction (XRD) Rietveld refinement, transmission/field emission scanning electron microscopy (TEM/FESEM), X-ray photoelectron spectroscopy (XPS), colour measurement and UV–vis–NIR spectrophotometry. Upon increasing the calcining temperature, both Ni2+ and Mg2+ hindered the migration of Al3+ to octahedral sites. When the Ni content increased, the cation site percentage of Ni2+ in the tetrahedral and octahedral sites varied slightly while that of Al3+ and Mg2+ change substantially. The cation exchange resulted in an increase in the inversion parameters and a decrease in the lattice parameters with increasing temperature or Ni content. Furthermore, Rietveld refinement also showed a shrinkage of the tetrahedra and an expansion of the distorted octahedra in the spinel structure. Short-range information based on optical spectra suggests that variation in the splitting energy of tetrahedra and octahedra caused the change in the spectral absorption. This study may deepen the understanding of the structural-optical property relationship of NixMg1-xAl2O4 spinel, which is vital to the further colour modification of ceramics and glazes.

The influence of technological parameters on the optical properties of photosensitive structures based on porous silicon

We study photosensitive structures containing porous silicon layers made on different technological routes. The spectral characteristics of the reflection coefficient of structures with a porous layer and their photosensitivity are investigated. The effect of diffusion on the optical properties of structures. Modeling the reflecting properties of a textured surface allows us to estimate the appreciable contribution of electrochemical treatment to the reduction of the reflection coefficient over the entire spectral range.

Femtosecond two-photon absorption spectroscopy of copper indium sulfide quantum dots: A structure-optical properties relationship

We have interpreted the two-photon absorption spectrum of water-soluble copper indium sulfide (CIS) QDs with stoichiometry 0.18 (Cu), 0.42 (In), and 2 (S) and an average diameter of approximately 2.6 nm. For that, we employed the wavelength-tunable femtosecond Z-scan technique and the parabolic effective-mass approximation model, in which the excitonic transition energies were phenomenologically corrected due to the stoichiometry of the nanocrystal. This model considers a conduction band and three valence sub-bands allowing excitonic transitions via centrosymmetric (Δl = ±1, where l is the angular momentum of the absorbing state) and non-centrosymmetric (Δl = 0) channels. In such case, this became relevant because the CIS QDs with chalcopyrite crystalline structure is a non-centrosymmetric semiconductor. Thus, our experimental results pointed out two 2 PA allowed bands located at 715 nm (2hv = 3.47 eV) and 625 nm (2hv = 3.97 eV) with cross sections of (6.3 ± 1.0) x 102 GM and (4.5 ± 0.7) x 102 GM, respectively. According to the theoretical model, these 2 PA bands can be ascribed to the 1P1/2(h3) → 1S3/2(e) (lower energy band) and 1P1/2(hheavy) → 1S3/2(e) (90%)/(10%)1P1/2(hsplit-off) → 1P3/2(e) (higher energy band) excitonic transitions. A good agreement (magnitude and spectral position) between the experimental and theoretical data were obtained. However, our experimental data suggest that the higher-energy 2 PA band may have other contributions due to the mixing between the heavy- and the light-hole bands, which the effective mass model does not take into consideration.

Experimental characterization and physics-based modeling of the temperature-dependent diffuse reflectance of plasma-sprayed Nd2Zr2O7 in the near to short-wave infrared.

Supercontinuum-laser illumination in conjunction with CO2-laser heating has been implemented to measure the near to short-wave infrared (970-1660nm) diffuse reflectance of plasma-sprayed Nd2Zr2O7 as a function of temperature. Owing to the broadband nature of this experimental technique, the diffuse reflectance of plasma-sprayed Nd2Zr2O7 has been measured at many wavelengths and has been shown to decrease with increasing temperature. A physics-based model for diffuse reflectance predicated on the crystal/electronic band structure of highly scattering semiconductor materials has been constructed to interpret the results of these measurements. Baseline materials characterization has also been performed to assist in the development of crystal/electronic band structure-optical property relationships that could be useful for the design of next-generation environmental barrier coatings. This characterization has included ambient and non-ambient x-ray diffraction as well as room-temperature, integrating-sphere diffuse reflectance spectroscopy.

Influence of the Number of Metallophilic Interactions and Structures on the Optical Properties of Heterometallic Au/Ag Complexes with Mixed-Donor Macrocyclic Ligands.

The reactivity of the polymeric gold(I)/silver(I) compound [Au2Ag2(C6F5)4(OEt2)2] n toward the 12-membered mixed-donor macrocyclic ligands 1,7-diaza-4,10-dithiacyclododecane (L1), 1-aza-4,7,10-trithiacyclododecane (L2), N-quinolinylmethyl-1-aza-4,7,10-trithiacyclododecane (L3), and N, N'-bis(quinolinylmethyl)-1,7-diaza-4,10-dithiacyclododecane (L4) was studied. The reactions were carried out using different molar ratios depending on the coordination properties of the ligands, which were modified by changing the donor atoms present in the macrocyclic framework (sulfur or nitrogen) or by linking one or two methylquinoline pendant-arms at the secondary nitrogen atom(s). X-ray diffraction analysis of the new complexes obtained show a nuclearity that increases on increasing the number of donor atoms in the ligands. The rich structural diversity observed determines different optical responses when the complexes are irradiated with UV-vis light in the solid state and in THF solution. The study of the optical properties reveals that in complexes for which the luminescence is due to metal-metal interactions, higher emission wavelengths are observed as the number of these metallophilic contacts increases, while the luminescence of ionic complexes has its origin in the macrocyclic ligands. TD-DFT calculations were carried out to verify the origin of these interesting structural-optical property relationships.

Application of artificial neural network for accelerated optimization of ultra thin organic solar cells

In this study, we show that design optimization of solar cells can be accelerated using neural networks (NN) effectively. We consider an organic thin film solar cell consisting of a poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric-acid-methyl ester (P3HT:PCBM) absorber, an antireflective indium tin oxide (ITO) layer and an aluminum back reflector layer. Zinc oxide (ZnO) and molybdenum trioxide (MoO3) interlayers are also used as electron and hole transfer layers. Silver nanotextures are embedded within absorber layer to create near field effects thus enhancing optical absorption. Optical properties of structures at sub-wavelength scales are measured by numerically solving first principle electromagnetic equations, e.g., by means of finite difference time domain and finite element methods. These methods are time-consuming, and therefore limit the possibility of exhaustive optimization. Surrogate modeling can be used to overcome this challenge. In the present work, we design a two-layer NN surrogate model to estimate the optical absorptivity of the cell for any given geometry vector as well as any radiation wavelength. After the preliminary optimization which utilizes NN, the result of optimization is obtained within narrowed optimization bounds obtained from the results of surrogate based optimization. A 325% of enhancement in absorption is obtained as a result of optimization.

Investigation of a broadband coherent perfect absorber in a multi-layer structure by using the transfer matrix method

We investigated a multi-layer structure for a broadband coherent perfect absorber (CPA). The transfer matrix method (TMM) is useful for analyzing the optical properties of structures and optimizing multi-layer structures. The broadband CPA strongly depends on the phase of the light traveling in one direction and the light reflected within the structure. The TMM simulation shows that the absorption bandwidth is increased by 95% in a multi-layer CPA compared to that in a single-layer CPA.

Surrogate based modeling and optimization of plasmonic thin film organic solar cells

In this study, we demonstrate that surrogate models can be trained and used to accurately predict the optical properties of thin film solar cells and to optimize their structures. We consider organic an active thin film poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric-acid-methyl ester (P3HT:PCBM) layer coated by indium-free highly conductivepolymer poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) on top with aluminum as cathode, and study optical absorptivity enhancement of such a structure when embedded with periodic spherical silver nano-particles. Metallic nano-structures, including textures, gratings and particles are known to induce plasmonic effects on the surface and inside absorbing semiconductors, thereby increasing light trapping in thin film cells. However, design of such structures requires precise characterization of the dependencies of electromagnetic field distribution to geometry parameters and material choices. Optical properties of structures at sub-wavelength scales are measured by numerically solving first principle electromagnetic equations, e.g., by means of finite difference time domain (FDTD) method. These methods are time-consuming, and therefore limit the possibility of exhaustive optimization. Surrogate modeling can be used to overcome this challenge. In the present work, we design a two-layer neural network (NN) surrogate model to estimate the optical absorptivity of the cell for any given geometry vector as well as any radiation wavelength and incident angle. Training of the network is done using the Levenberg–Marquardt (LM) method with generalization techniques such as early stopping and Bayesian regularization using a pool of training and validation data. The resulting surrogate model is first demonstrated to yield accurate out-of-sample estimation of absorptivity. Then, the model is used to investigate the individual contributions of each input by means of sensitivity analysis. In addition, optimization of the parameters is efficiently done using the surrogate model for different source light irradiance spectra and incident angles. Resulting optimizations are very efficient. At the end, solutions found by surrogate-based optimization demonstrate enhancement factors greater than 270% for optical absorptivity.

A first principles investigation of the effect of aluminum, gallium and indium impurities on optical properties of β-Si3N4 structure

In this study, effects of some impurity atoms included in IIIA group such as Al, Ga, and In on the optical properties of the β-Si3N4 structure have been discussed. The calculations were made using Density Functional Theory (DFT) in 0–15eV range and local density approximation (LDA) as the exchange-correlation. Using the real and the imaginary parts of the complex dielectric function, the basic optical properties of β-Si3N4 such as dielectric coefficient, refractive index, absorption, reflection coefficients have been investigated. As a result of the calculations, it is determined that optical properties of structure have been significantly changed with doping.