Optical Absorption Measurements Research Articles

Photoluminescence of GaAsSb/AlAsSb superlattices for investigating band structure

To study the band structure of GaAsSb/AlAsSb systems grown on InP substrates, optical absorption and photoluminescence (PL) measurements were performed. The measurements utilized GaAsSb/AlAsSb superlattice (SL) samples to investigate the optical properties of the system. By comparing optical absorption and photoluminescence (PL) measurements, it was determined that the non-radiative transition occurred at lower energy levels than the PL transition energy in the sample with the narrowest well width. The excitation power dependence of PL measurements exhibited an almost linear power law of peak intensity, suggesting high-quality luminescence properties of GaAsSb/AlAsSb SLs. In addition, the sample with the narrowest well width exhibited a blueshift of the PL transition energy as a function of the excitation power between square and cubic root laws. To explain these experimental observations, quantized energies in SLs were calculated using the band structures that were suggested and modified using an established band parameter. The most plausible band structure was revealed by comparing experimental observations with the calculations. PL from the SL samples was found to occur within the GaAsSb well (type I) in between the gamma point (Γ) of the conduction band and the heavy hole (HH) energy level in the valence band. To understand the underlying causes of the blueshift phenomenon, a self-consistent calculation that solves the Schrödinger and Poisson equations was performed iteratively as a function of carrier density. As a result, the blueshift of transition energy with an increase in the carrier density was confirmed in the type-I transition between Γ and HH bands if the lowest transition was type II between L for AlAsSb and HH for GaAsSb bands. The Hartree potential due to the separately confined electrons and holes in the SLs slightly narrowed the GaAsSb well width by modifying the conduction band.

The CZTS Thin Films Grown by Sulfurization of Electrodeposited Metallic Precursors: The Effect of Increasing Tin Content of the Metallic Precursors on the Structure, Morphology and Optical Properties of the Thin Films

A study has been carried out to investigate the influence of the amount of Sn in the precursor solution, on some physical properties of CZTS films grown by sulfurization of electrodeposited metallic precursors. The growth of the CZTS samples was achieved by sequential electrodepositon of constituent metallic layers on ITO glass substrates using a 3-electrode electrochemical cell with graphite as a counter electrode and Ag/AgCl as the reference electrode. The Sn-content in the metallic precursor was varied by varying the deposition time of Sn. The stacked elemental layer was then soft annealed in Argon at 350 °C, and subsequently sulfurized at 550° C to grow the CZTS thin films. The structure, morphology and optical properties were investigated. X-ray diffraction studies revealed that, irrespective of the Sn content all the films were polycrystalline and exhibited the Kesterite CZTS structure with preferred orientation along the (112) plane. However, there was an increase in the amount of peaks indexed to the undesirable secondary phases, as the Sn content in metallic precursor was increased. Optical absorption measurements revealed the existence of a direct transition with band gap values decreasing from 1.74 eV to 1.25 eV with increasing amounts of Sn. The lower value for the band gap was attributed to the presence of secondary phases formed in addition to the CZTS film. Morphology of the sulfurized films showed a compact and rocky texture with good coverage across the entire substrate. However, CZTS films with a higher Sn content appeared to have a molten metallic surface with deep cracks which could have adverse effects on the electrical properties of the film. EDAX analysis showed all the films were consistent with the formation of CZTS. It is evident from all the characterization techniques that increasing the Sn content of the stacked metallic precursors beyond stoichiometric amounts had an adverse effect on the structural and optical properties of CZTS films grown by this technique.

Synthesis of Flower-like ZnO and Its Enhanced Sensitivity towards NO2 Gas Detection at Room Temperature

A flower-like ZnO was successfully synthesized via a simple chemical precipitation method at room temperature (RT) in distilled water, without the use of any catalysts or substrates. The sample’s structure was analyzed using various techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (FETEM), and X-ray photoelectron spectroscopy (XPS), which confirmed its hexagonal structure. UV–visible optical absorption measurements also revealed the presence of UV absorption at 365 nm. A reasonable growth mechanism for the formation of flower-like ZnO was proposed based on these analyses. The response of the sample to low concentrations of NO2 (1 ppm) was evaluated at different calcination temperatures, and the results showed that the best response was achieved when the sample was calcined at 600 °C. The flower-like ZnO sample labeled as 6ZnO showed the highest response of 54.18 when exposed to 1 ppm of NO2 gas at RT. Additionally, 6ZnO exhibited good response and recovery properties of 11 s and 93 s, respectively, at low concentrations of NO2 at 1 ppm. The gas sensing mechanism and the mechanism of the enhanced gas response of the flower-like ZnO are discussed.

Charge order, frustration relief, and spin-orbit coupling in U3O8

Research efforts on the description of the low-temperature magnetic order and electronic properties of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ have been inconclusive so far. Reinterpreting neutron scattering results, we use group representation theory to show that the ground state presents collinear out-of-plane magnetic moments, with antiferromagnetic coupling both in-layer and between layers. Charge order relieves the initial geometric frustration, generating a slightly distorted honeycomb sublattice with N\'eel-type order. The precise knowledge of the characteristics of this magnetic ground state is then used to explain the fine features of the band gap. In this system, spin-orbit coupling (SOC) is of critical importance, as it strongly affects the electronic structure, narrowing the gap by $\ensuremath{\sim}38%$, compared to calculations neglecting SOC. The predicted electronic structure actually explains the salient features of recent optical absorption measurements, further demonstrating the excellent agreement between the calculated ground state properties and experiment.

Controllable preparation and photoelectric properties of oriented two-dimensional GeSe2 nanobelt arrays

Germanium diselenide (GeSe2) nanobelts are synthesized by atmospheric-pressure chemical vapor deposition under low temperature by using Se and Ge powders as precursor materials in a quartz tube furnace with double heating zones. The GeSe2 nanobelts thus prepared exhibit growth directionality. Unidirectional nanobelt clusters are tightly spaced and shaped as rectangular nanobelt arrays. Additionally, the thickness of the prepared GeSe2 material is less than 5 nm, and the area of a single array can attain 0.96 mm2. Our experimental results show that hydrogen directly affects the growth of GeSe2. First-principles calculations reveal the electronic properties and in-plane anisotropic optical absorption of the few-layer two-dimensional GeSe2 material. Optical absorbance measurements of GeSe2 nanobelt arrays reveal high ultraviolet absorbance of GeSe2 (200–400 nm). Photodetectors based on GeSe2 nanobelts are p-type, with high responsivity, superior detectivity, and a fast response time. These results show that GeSe2 is an excellent ultraviolet photoelectric material with potential photoelectronic applications.

Raman study of colloidal Cu2ZnSnS4 nanocrystals obtained by “green” synthesis modified by seed nanocrystals or extra cations in the solution

The method of affordable colloidal synthesis of nanocrystalline Cu2ZnSnS4 (CZTS) is developed, which is suitable for obtaining bare CZTS nanocrystals (NCs), cation substituted CZTS NCs, and CZTS-based hetero-NCs. For the hetero-NCs, the synthesized in advance NCs of another material are introduced into the reaction solution so that the formation of CZTS takes place preferably on these “seed” NCs. Raman spectroscopy is used as the primary method of structural characterization of the NCs in this work because it is very sensitive to the CZTS structure and allows to probe NCs both in solutions and films. Raman data are corroborated by optical absorption measurements and transmission electron microscopy on selected samples. The CdTe and Ag NCs are found to be good seed NCs, resulting in a comparable or even better quality of the CZTS compound compared to bare CZTS NCs. For Au NCs, on the contrary, no hetero-NCs could be obtained under the given condition. Partial substitution of Zn for Ba during the synthesis of bare CZTS NCs results in a superior structural quality of NCs, while the introduction of Ag for partial substitution of Cu deteriorates the structural quality of the NCs.

Structural and electronic phase transition in the van der Waals crystal HfS2 under high pressure

The two-dimensional (2D) to three-dimensional (3D) structural evolution of transition metal dichalcogenides (TMDCs) under high pressure is a significant subject since the change of dimensionality could induce a drastic change in various physical properties. A more compact 3D structure of TMDCs could result in metallization and even superconductivity. In this work, we report an unusual 3D state of an $\mathrm{Hf}{\mathrm{S}}_{2}$ crystal that can still maintain its semiconducting nature over a wide pressure range. X-ray diffraction results show that the layered $\mathrm{Hf}{\mathrm{S}}_{2}$ undergoes a reversible transformation to a 3D $Immm$ structure at $\ensuremath{\sim}12$ GPa, accompanied by a semiconductor-to-semiconductor transition. The Raman and optical absorption measurements show that the phase transition may occur as early as $\ensuremath{\sim}9.2$ GPa. The semiconducting state is stable up to $\ensuremath{\sim}65$ GPa, and the metallization occurs at 68.6 GPa, the maximum pressure in this work. This work not only reports the 2D-to-3D structural phase transition in an $\mathrm{Hf}{\mathrm{S}}_{2}$ crystal driven by pressure, but also gives a clear physical picture that shows the intrinsic electrical transport properties and electronic behavior that enriches our understanding of the electronic behavior of TMDCs and sheds light on the future design of novel optoelectronic devices.

Luminescence and energy transfer mechanism in Ce[formula omitted] and Gd[formula omitted] ions in bismuth borate glass

In recent years, researchers have been attracted to the tuning of the solar spectrum through a coverglass as a means to enhance the efficiency of a photovoltaic cell by improving the energy transfer mechanism. In this study, energy transfer mechanism was investigated between Gd3+ and Ce3+ ions in a bismuth borate glass. Cerium ions (0.1–2.5 mol%) were doped into a bismuth borate glass using a versatile melt-quenching technique in an ambient atmosphere under various experimental conditions. The luminescence and optical properties were examined in relation to the concentration of cerium. The structural changes in the glass matrix that occurred as a result of variation in the concentration were investigated using Fourier transform infrared spectroscopy. To ensure the incident light passage through the glass is not compromised due to self-absorption, optical absorption measurements were carried out to investigate the presence of resonant plasmonic absorption of nanoclusters. The energy bandgap of Ce3+-doped glass decreased from 2.772 to 2.671 eV due to dopant concentration-induced structural change in the glass matrix. The photoluminescence properties of the present samples confirmed their suitability for luminescent down converter applications. At UV excitation wavelength of 325 nm, the Ce3+-doped samples down-shifted the solar spectrum and gave the most prominent emission band at 448 nm due to the 5d-4f transition of Ce3+. The optimum concentration was observed as 0.3 mol%, further increase in the dopant concentration led to a decrease in the emission intensity may be due to the conversion of Ce4+ into Ce3+ ions [47,48]. In the energy transfer, Gd3+ transfers energy non-radiatively to Ce3+ due to the equivalent energy level 6P7/2 of Gd3+ and 5d energy level of Ce3+.

A highly sensitive colorimetric approach based on tris (bipyridine) Ruthenium (II/III) mediator for the enzymatic detection of phenylalanine.

The accurate monitoring of phenylalanine concentration plays a prominent role in the treatment of phenylketonuria (PKU). In this study, we present an enzymatic assay based on Phenylalanine Dehydrogenase/NAD+ and tris (bipyridine) Ruthenium (II/III) as a colorimetric mediator for the detection of Phenylalanine concentration. The amount of amino acid was quantitatively recognized by optical absorption measurements at 452nm through the conversion of Ru (byp)3 3+ to Ru (byp)3 2+, which is induced by the neoformed NADH. A detection limit of 0.33µM, a limit of quantification of 1.01µM, and a sensitivity of 36.6a.u nM-1 were obtained. The proposed method was successfully tested using biological specimens from patients affected by hyperphenylalaninemia. The proposed enzymatic assay showed a high selectivity, making it a promising alternative for the development of versatile assays for the detection of phenylalanine in diluted serums.

Low Temperature Optical Properties of Novel Lead‐Free Cs2NaFeCl6 Perovskite Single Crystals

Lead‐free double perovskites have attracted much attention as possible alternatives to lead halide based perovskites in photovoltaic applications. However, to date only few double perovskites have been successfully employed in optoelectronic device prototypes. Therefore, the search for stable and lead‐free materials is ongoing. Here, we present the successful growth of high‐quality Cs2NaFeCl6 single crystals and their temperature‐dependent structural and optical properties. By combining electron paramagnetic resonance (EPR), crystal structure analysis and density functional theory (DFT) we could determine a cubic crystal structure with a spin of 5/2 for this material, showing strongly spin polarized character. Furthermore, combining photoluminescence (PL) and optical absorption measurements we find a bandgap of approximately 2.1 eV at room temperature as well as the presence of excitonic states. Using Elliot's formula, we are able to extract the temperature‐dependent behavior of the bandgap as well as an estimated exciton binding energy of only 20 meV at 80 K.

Reactive sputter deposition of β-Ni(OH)2 thin films in Ar + H2O mixed gas atmosphere at a substrate temperature of −80 °C

Nickel hydroxide [Ni(OH)2] is an electrochemically-active material used for rechargeable batteries, electrochemical capacitors, and electrochromic devices. Although there have been some studies on Ni(OH)2 thin films deposited by sputtering, the Ni(OH)2 formation has not been fully confirmed. In this study, a Ni metal target was reactively sputtered in atmospheres of O2 and Ar + H2O at substrate temperatures of RT (RT, around 20 °C), −80 °C, and −170 °C, and the aging treatment effects in the air at RT were studied. From optical, X-ray diffraction, and IR absorption measurements, β-Ni(OH)2 thin films were found to be formed after aging the films deposited at −80 °C in Ar + H2O, however, NiO thin films were formed at RT. These results corresponded well with a thermodynamic consideration of Ni(OH)2. At −170 °C, mixed metal and oxide films were formed, presumably because of insufficient Ni oxidation.

Stability of the Halide Double Perovskite Cs2AgInBr6.

Cs2AgInBr6 is among the lead-free halide perovskites of interest, predicted by first-principles calculations to be stable with a direct band gap, but there has been only one report of its synthesis. Herein we report the formation of Cs2AgInBr6 thin films through thermal evaporation of CsBr, AgBr, and InBr3 and subsequent annealing between 130 °C and 250 °C. Cs2AgInBr6 appears stable in this temperature range. However, Cs2AgInBr6 thin films are thermodynamically unstable at room temperature, remaining cubic only long enough to be characterized but not long enough to be useful for practical devices. Cs2AgInBr6 decomposed into Cs2AgBr3, Cs3In2Br9, AgBr, and InBr3 upon cooling from 130 °C to 250 °C to room temperature. This conclusion did not depend on illumination, film thickness, annealing environment, or details of the film formation, pointing to an intrinsic thermodynamic instability of the material. Optical absorption measurements may be interpreted as Cs2AgInBr6 having a direct band gap of 1.57 ± 0.1 eV.

Screen-Printed Electrochemical Biosensor for the Detection of Bacteriophage of Lactococcus Lactis for Dairy Production

In the dairy industry, the spreading of phages of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Lactococcus lactis</i> prevent the proper lactic fermentation, causing waste of contaminated products and economic losses. This work presents a cheap biosensing method for rapidly detecting the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L.lactis</i> phages. The detection is based on live <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L. lactis</i> bacteria covering the sensor electrodes, whose electrical response is measured by electrochemical impedance spectroscopy (EIS). Solutions contaminated by phages induce bacteria lysis, clearly reducing the bacteria coverage over the electrodes and leading to evident parametrical shifts in the charge transfer resistance and in the impedance phase at 400 Hz. Experimental tests with laboratory contaminated samples confirm that screen-printed sensors with gold and detection capability compared to interdigitated gold electrodes. Two measurement protocols, called spill-out and drop-in methods, are evaluated to optimize the sensor detection capability and time. The EIS results are compared with optical absorbance measurements at 600 nm, in order to validate the proposed detection method with 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> PFU/mL phages and with a detection time of about 5 hours. Lastly, the proposed method is tested successfully with milk-based solutions. Evident shifts are measured in the charge transfer resistance of more than one order of magnitude and impedance phase differences of 43° in phages contaminated sensors.

Ca2.9La0.1Co4O9/graphene composite electrodes for improved electrochemical performance of cellulose-based supercapacitors

In this study, we report a highly flexible and biodegradable supercapacitor (SC) assembled with electrodes made of cellulose and graphene microplates. The cellulose was extracted from the palocote plant (tithonia tubaeformis), which is considered a weed in Mexico. To enhance the capacitance of the devices, Ca2.9La0.1Co4O9 (CaLaCo) particles with microplate morphology were added to the SC electrodes. The electrochemical characterization indicated that the SCs fabricated without CaLaCo particles exhibited a specific capacitance of 777.3 F/g (at 1 A/g) and an energy density of 108.1 Wh/kg. When the CaLaCo was introduced into the SC electrodes, the above parameters increased to 1320.3 F/g (at 1 A/g) and 182.3 Wh/kg, that is, the capacitive performance was enhanced by ≈ 70%. Also, the device made with the CaLaCo particles exhibited a high capacitance retention of 95.8% after 1500 continuous charging/discharging cycles. Some advantages in the devices made with CaLaCo in comparison with these fabricated without CaLaCo were: 1) longer discharge times and 2) low loss of capacitance (4.1%) at higher current densities (up to 10 A/g). On the other hand, redox peaks were observed in the cyclic voltammetry curves, suggesting the storage of charge by redox reactions. To find the redox centers responsible for the charge storage, optical absorbance, Raman and XPS measurements were carried out and we found that the oxygen vacancy defects and Co2+/Co3+ species were the main redox centers. Furthermore, EIS measurements showed that the series resistance (Rs) and charge transfer resistance (Rct) decreased by 28% and 80%, respectively, after the introduction of the CaLaCo oxide in the SC. The reduction of the Rs and Rct was relevant because it promoted the ion diffusion/storage in the electrodes, which in turn, increased the capacitance of the devices. Thus, the results of this report indicate that our biodegradable and flexible SC could be produced from sustainable materials and can be used as energy source for wearable/portable devices.

Fabrication of Nanoparticle Agglomerate Films by Spark Ablation and Their Application in Surface-Enhanced Raman Spectroscopy

This paper presents a systematic study of the investigation of nanoparticle (NP) agglomerate films fabricated via depositing spark-generated Au, Ag, and Au/Ag NPs onto quartz microscope coverslips in a low-pressure inertial impactor. The primary focus of the study is to characterize these nanostructures and to examine their potential application in surface-enhanced Raman spectroscopy (SERS). The characterization of the produced nanostructures was carried out by performing optical absorbance measurements, morphology, and composition analysis, as well as testing the SERS performance of the NP films at three different excitation laser wavelengths in the visible range. The study aims to investigate the relationship between the optical properties, the morphology, and the enhancement of the produced samples at different excitations, and the results are presented and discussed. The study highlights the potential of using spark ablation and inertial impaction-based deposition as a method for producing nanoparticle films for SERS.

Silver molecular clusters and the properties of radiophotoluminescence of alkali-phosphate glasses at high dose

This paper describes the results of radiophotoluminescence (RPL) measurements at high beta (90Sr/90Y) dose, including measurements of RPL intensity, excitation and emission spectra, and lifetimes, along with optical absorption measurements. Correction of the RPL intensity as the dose increases for absorption of the excitation light during RPL stimulation and the emitted light during RPL emission, reveals that the RPL intensities at emission wavelengths in the blue (470 nm) and orange (630 nm) peak at higher doses than previously reported. The corrected orange emission shows a linear-sublinear growth with dose with a peak intensity occurring at ∼10 kGy. The blue emission shows a linear-supralinear growth with a maximum as a function of dose not observed in the dose range used. This behavior is explained in terms of the growth of Ag molecular clusters, Ag2+ dimers and Ag42+ tetramers, as the dose increases. The growth of these clusters as a function of dose is modeled using rate equations and the simulated behavior is shown to match well the experimentally observed dose-response characteristics of the orange and blue RPL emissions. The growth of the Ag42+ tetramers with dose is also speculated to cause a decrease in the orange RPL sensitivity with increasing Linear Energy Transfer (LET quenching) at high LET values when this material is exposed to charged particles. Consequently it is hypothesized that blue RPL will not exhibit the same degree of LET quenching.

Synthesis, Crystal Structure, Vibrational Study, Optical Properties and Hirshfeld Surface Analysis of a New Centrosymmetric Hybrid Material, Bis(3-Aminoquinolium) Tetrachloridocobaltate (II)

Abstract: The title organic-inorganic hybrid compound (C9H9N2)2[CoCl4] crystallizes at room temperature in the monoclinic C2/c space group. Its atomic arrangement can be described as an alternation of organic-inorganic layers along the c-axis. Crystal cohesion of the compound is achieved through N-H…Cl hydrogen bonds between the anions and cations and through cycle stacking interactions between organic cations. The vibrational properties were studied by Fourier Transform Infrared (FTIR) spectroscopy. The three-dimensional Hirshfeld surface (3D-HS) analysis and the two-dimensional fingerprint plots (2D-FP) were generated to quantify the weak intermolecular interactions present in the crystal. The optical properties were investigated by optical absorption and photoluminescence measurements. The band gap energy indicates that the compound behaves as an insulator material. Photoluminescence results suggest that the title compound can have a potential application as a non-linear optical material.

Exciton dispersion in two-dimensional organic perylene crystal indicates substantial charge-transfer exciton coupling

Two-dimensional, high-quality perylene single crystals were grown with a space-confined strategy method. The grown films crystallize in the $\ensuremath{\alpha}$ form, as is confirmed by a combination of techniques. Polarization-dependent optical absorption measurements show a strong anisotropy in very good agreement with the literature data, and the anisotropic mobility data in field-effect transistors document the very high crystalline order. Momentum-dependent studies using electron energy-loss spectroscopy reveal a negative dispersion of the first exciton along the crystal $b$ direction with an exciton bandwidth of 72 meV. We argue that this behavior is a result of charge-transfer exciton coupling between the perylene dimers in the unit cell.

Optical absorption measurement of spin Berry curvature and spin Chern marker

In two-dimensional time-reversal symmetric topological insulators described by Dirac models, the topological invariant can be described by the spin Chern number. We present a linear response theory for the spin Berry curvature that integrates to the spin Chern number, and introduce its spectral function that can be measured at finite temperature by momentum- and spin-resolved circular dichroism, which may be achieved by pump-probe type of experiments using spin- and time-resolved ARPES. As a result, the sign of the Pfaffian of the invariant can be directly measured. A spin Chern number spectral function is further introduced from the optical spin current response, and is shown to be measurable from the spin-resolved opacity of two-dimensional materials under circularly polarized light, pointing to an optical measurement of the invariant and a frequency sum rule. The spin Chern number expressed in real space is known to yield a spin Chern marker, and we propose that it may be measurable from spin-resolved local heating rate caused by circularly polarized light. A nonlocal spin Chern marker is further proposed to characterize the quantum criticality near topological phase transitions, and is shown to be equivalent to an overlap between spin-selected Wannier states.

Creation of gold nanoparticles in langasite

Langasite (La3Ga5SiO14) is a material with interesting ferroelectric and optical properties for applications in environments with high temperature and/or high ionizing radiation levels. The formation of metallic nanoclusters is, due to their unique nonlinear-optical properties, of high technological interest. The present work investigates the formation and removal of implantation-induced radiation damage as well as the production of Au-nanoparticles. Langasite single crystals were implanted with Au+ ions at an energy of 150 keV and a fluence of 1 × 1016 cm−2. The implanted samples were subjected to annealing in air at temperatures between 600 °C and 1000 °C. Optical absorption measurements indicate that stable nanoparticles are formed in the Au-implanted samples for temperatures of 800 °C and above. RBS/channeling studies show that an almost complete recovery of the crystal structure takes place. On the other hand, samples treated at 600 °C showed neither nanoparticle formation nor extended lattice recovery.