Determination of the thickness and optical constants of amorphous silicon

Nanometric determination of the thickness of aqueous samples for accurate molar absorption coefficients of water-soluble molecules in the mid-infrared region

Absorption spectra of aqueous samples measured by transmission need to be acquired using very thin cells (5-50 μm) when targeting the mid-infrared (mid-IR) region due to the strong background absorbance of liquid water. The thickness of the cell used controls the pathlength of the light through the sample, a value needed to transform absorption spectra into molar absorption coefficient spectra, or to determine solute concentrations from absorption spectra. The most accurate way to determine the thickness of an empty cell (i.e., filled with air) is from the period of an interference pattern, known as interference fringes, that arises when the cell is placed perpendicular to the path of light in the spectrometer. However, this same approach is not directly applicable to determine the thickness of a cell filled with an aqueous solution, due partially to the smaller amplitude of the interference fringes but fundamentally caused by its complex waveform, with a wavenumber-dependent oscillation period. Here, using Fresnel equations, we derived analytical expressions to model interference fringes in absorption spectra obtained by transmission, which are also valid for aqueous samples. We also present a novel Fourier-based analysis of the interference fringes that, in combination with the derived analytical expressions, allowed us to determine the pathlength of aqueous samples with an error below ∼ 50 nm. We implemented this novel approach to analyze interference fringes as a Live Script running in the software Matlab. As an application, we measured the absorption spectra of a 97 mM solution of MES buffer at pH 3.4 and pH 8.4 using cells of various nominal thicknesses (6, 25 and 50 μm), whose actual thicknesses were determined using the present approach. The derived molar absorption coefficient spectrum for both the acidic and basic forms of MES were virtually identical regardless of the cell, indicating that the determined thicknesses were likely very accurate. These results illustrate the utility of the present methodology in obtaining accurate molar absorption coefficient spectra of water-soluble molecules in the mid-IR region.

Influence of film thickness on structural, optical, and electrical properties of sputtered nickel oxide thin films

Utilizing radio frequency magnetron sputtering, we successfully fabricated nickel oxide thin films with different thickness (from 80 to 270 nm), and conducted an in-depth examination of their structural, morphological, optical, and electrical properties. The crystal structure and surface roughness were determined using x-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The XRD analyses showed that the films were composed of cubic nickel oxide, exhibiting a notable orientation along the (200) direction. This crystal texture partially increased when the film thickness reached 270 nm. In addition, a direct correlation between film thickness and crystallite size was observed, with the latter increasing as the former did. AFM analysis provided insights into the surface morphology, revealing metrics like the bearing area, 3D surfaces intersections, and statistical properties of surface height. These insights underscore the relationship between film thickness and surface properties, which in turn influence the overall electrical, and prominently, optical properties of the films. Employing transmittance UV-visible spectroscopy, we characterized the optical behavior of these films, noting a proportional increase in refractive index with film thickness. Additionally, resistivity was observed to increase concomitantly with film thickness. In conclusion, the deposition process's film thickness acts as a pivotal parameter for fine-tuning the structural, morphological, and optical properties of nickel oxide thin films. This knowledge paves the way for optimizing nickel oxide-based devices across various applications. RESEARCH HIGHLIGHTS: We synthesized and characterized of p-type semiconducting NiO thin films sputtered on substrates by using RF magnetron sputtering with different thickness. Advanced crystalline structures and fractal features extracted from XRD and AFM analysis.  The 2D and 3D surface analysis of the samples indicates a complex structure with an imperfect self-similarity that suggests a multifractal structure. We represented graphically the relative representation of higher geometric objects in the AFM image. We attributed the optical and electrical properties of the samples to the crystallite size, and the concurrent reduction in oxygen vacancies and crystalline defects within the films.

Insight into What Is inside Swift Heavy Ion Latent Tracks in PET Film

We present here a novel experimental study of changes after contact electrification in the optical transmission spectra of samples of both pristine and irradiated PET film treated with Kr+15 ions of energy of 1.75 MeV and a fluence of 3 × 1010 cm2. We used a non-standard electrification scheme for injecting electrons into the film by applying negative electrodes to both its surfaces and using the positively charged inner regions of the film itself as the positive electrode. Electrification led to a decrease in the intensity of the internal electric fields for both samples and a hypsochromic (blue) shift in their spectra. For the irradiated PET sample, electrification resulted in a Gaussian modulation of its optical properties in the photon energy range 2.3-3.6 eV. We associate this Gaussian modulation with the partial decay of non-covalent extended conjugated systems that were formed under the influence of the residual radial electric field of the SHI latent tracks. Our studies lead us to suggest the latent track in the PET film can be considered as a variband material in the radial direction. Consideration of our results along with other published experimental results leads us to conclude that these can all be consistently understood by taking into account both the swift and slow electrons produced by SHI irradiation, and that it appears that the core of a latent track is negatively charged, and the periphery is positively charged.

Increasing the Accuracy of the Characterization of a Thin Semiconductor or Dielectric Film on a Substrate from Only One Quasi-Normal Incidence UV/Vis/NIR Reflectance Spectrum of the Sample

OEMT is an existing optimizing envelope method for thin-film characterization that uses only one transmittance spectrum, T(λ), of the film deposited on the substrate. OEMT computes the optimized values of the average thickness, d¯, and the thickness non-uniformity, Δd, employing variables for the external smoothing of T(λ), the slit width correction, and the optimized wavelength intervals for the computation of d¯ and Δd, and taking into account both the finite size and absorption of the substrate. Our group had achieved record low relative errors, <0.1%, in d¯ of thin semiconductor films via OEMT, whereas the high accuracy of d¯ and Δd allow for the accurate computation of the complex refractive index, N˙(λ), of the film. In this paper is a proposed envelope method, named OEMR, for the characterization of thin dielectric or semiconductor films using only one quasi-normal incidence UV/Vis/NIR reflectance spectrum, R(λ), of the film on the substrate. The features of OEMR are similar to the described above features of OEMT. OEMR and several popular dispersion models are employed for the characterization of two a-Si films, only from R(λ), with computed d¯ = 674.3 nm and Δd = 11.5 nm for the thinner film. It is demonstrated that the most accurate characterizations of these films over the measured spectrum are based on OEMR.

Decoding the Atomic Structure of Ga2Te5 Pulsed Laser Deposition Films for Memory Applications Using Diffraction and First-Principles Simulations

Neuromorphic computing, reconfigurable optical metamaterials that are operational over a wide spectral range, holographic and nonvolatile displays of extremely high resolution, integrated smart photonics, and many other applications need next-generation phase-change materials (PCMs) with better energy efficiency and wider temperature and spectral ranges to increase reliability compared to current flagship PCMs, such as Ge₂Sb₂Te₅ or doped Sb₂Te. Gallium tellurides are favorable compounds to achieve the necessary requirements because of their higher melting and crystallization temperatures, combined with low switching power and fast switching rate. Ga₂Te₃ and non-stoichiometric alloys appear to be atypical PCMs; they are characterized by regular tetrahedral structures and the absence of metavalent bonding. The sp³ gallium hybridization in cubic and amorphous Ga₂Te₃ is also different from conventional p-bonding in flagship PCMs, raising questions about its phase-change mechanism. Furthermore, gallium tellurides exhibit a number of unexpected and highly unusual phenomena, such as nanotectonic compression and viscosity anomalies just above their melting points. Using high-energy X-ray diffraction, supported by first-principles simulations, we will elucidate the atomic structure of amorphous Ga₂Te₅ PLD films, compare it with the crystal structure of tetragonal gallium pentatelluride, and investigate the electrical, optical, and thermal properties of these two materials to assess their potential for memory applications, among others.

Characterization of Structural, Optical, Corrosion, and Mechanical Properties of HfO2 Thin Films Deposited Using Pulsed DC Magnetron Sputtering

Various properties of HfO₂, such as hardness, corrosion, or electrical resistance, depend on the method and the conditions of deposition. In this work, a thorough comparison of scarcely investigated mechanical properties of HfO₂ thin films deposited with different conditions of reactive magnetron sputtering process is presented. Four thin films were sputtered in processes that varied in plasma ignition method (continuous or sequential) and target-substrate distance. The structural characteristics of the HfO₂ thin films were examined using Raman spectroscopy and X-ray diffraction measurements. Furthermore, the optoelectronic properties were determined based on transmittance and current-voltage characteristics. The mechanical properties of the HfO₂ thin films were determined using nanoindentation and scratch test. In turn, the corrosion properties were determined by analyzing the voltametric curves. The transparent HfO₂ thin films deposited in the continuous process are characterized by better corrosion resistance than the same layer formed in the sequential process, regardless of the target-substrate distance (8 cm or 12 cm). Furthermore, these samples are also characterized by the highest value of Young's modulus and scratch resistance. The combination of good corrosion and scratch resistance could contribute to the new application of HfO₂ as a corrosion protective material.

Refractive Index-Adaptive Nanoporous Chiral Photonic Crystal Film for Chemical Detection Visualized via Selective Reflection

Photonic crystals (PC) are of great importance in technology, especially in optics and photonics. In general, the structural color of PCs responds to external stimuli primarily by changing their periodicity. Herein, the authors report on refractive index (RI) adaptive PCs. Cross-linked cholesteric films with interconnected nanopores exhibit a very low RI without light scattering. Transparent PC films with maximum reflectance in the ultravoilet (UV) region respond to various chemicals by changing the reflective color of the PC. The authors demonstrate its unique colorimetric chemical detections of hazardous organic liquids. Loading various chemicals into nanopores significantly shifts the structural color into the visible range depending on the chemical's RI. These results are unique in that the structural color of photonic films is mediated by RI changes rather than periodicity changes. In principle, nanoporous photonic crystal films can detect the RI of a chemical substance by its unique color. In contrast to volumetric changes, this sensing mechanism offers several advantages, including durability, excellent sensitivity, fast response time, and wide detection range. These results provide useful insight into stimulus-responsive PCs. The structural color of PC films can be effectively tuned by adjusting average RIs instead of changing periodicity.

Resulting Effect of the p-Type of ZnTe: Cu Thin Films of the Intermediate Layer in Heterojunction Solar Cells: Structural, Optical, and Electrical Characteristics

The microstructural, electrical, and optical properties of Cu-doped and undoped ZnTe thin films grown on glass substrates are covered in this article. To determine the chemical makeup of these materials, both energy-dispersive X-ray (EDAX) spectroscopy and X-ray photoelectron spectroscopy were employed. The cubic zinc-blende crystal structure of ZnTe and Cu-doped ZnTe films was discovered using X-ray diffraction crystallography. According to these microstructural studies, the average crystallite size increased as the amount of Cu doping increased, whereas the microstrain decreased as the crystallinity increased; hence, defects were minimized. The Swanepoel method was used to compute the refractive index, and it was found that the refractive index rises as the Cu doping levels rises. The optical band gap energy was observed to decrease from 2.225 eV to 1.941 eV as the Cu content rose from 0% to 8%, and then slightly increase to 1.965 eV at a Cu concentration of 10%. The Burstein-Moss effect may be connected to this observation. The larger grain size, which lessens the dispersion of the grain boundary, was thought to be the cause of the observed increase in the dc electrical conductivity with an increase in Cu doping. In structured undoped and Cu-doped ZnTe films, there were two carrier transport conduction mechanisms that could be seen. According to the Hall Effect measurements, all the grown films exhibited a p-type conduction behavior. In addition, the findings demonstrated that as the Cu doping level rises, the carrier concentration and the Hall mobility similarly rise, reaching an ideal Cu concentration of 8 at.%, which is due to the fact that the grain size decreases grain boundary scattering. Furthermore, we examined the impact of the ZnTe and ZnTe:Cu (at Cu 8 at.%) layers on the efficiency of the CdS/CdTe solar cells.

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.

Prospects of ZnS and ZnO as smart semiconductor materials in light-activated antimicrobial coatings for mitigation of severe acute respiratory syndrome coronavirus-2 infection

We carried out theoretical and experimental analyses of ZnO and ZnS nanoparticles as smart semiconductor materials in light-activated antimicrobial coating for application in masks. We used low-cost hydrothermally processable precursors to direct the growth of the coatings on cotton fabric. Both ZnO and ZnS coatings had high reactivities as disinfection agents in photocatalysis reactions for the degradation of a methylene blue dye solution. Also, these coatings showed excellent UV protection properties. For understanding at the molecular level, the broad-spectrum biological activities of the ZnO and ZnS coatings against Fusarium Oxysporum fungi, Escherichia coli bacteria, and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) virus and their variants, were investigated computationally. Hexagonal Zn6O6 and Zn6S6 clusters were used as models for the simulations through excited- and ground-state calculations. The theoretical findings show that changes in the local chemical environment in these excited systems have a profound impact on their physical and chemical properties and thus, can provide a better understanding to engineer new functional materials in light-activated antimicrobial coatings for the mitigation of SARS-CoV-2 infection.

A new setup for optical measurements under controlled environment

We present a new analytical instrument for studying the optical properties of materials in different gaseous environments at room and controlled elevated temperatures. The system consists of a vacuum chamber, which is equipped with temperature and pressure controllers, a heating band, and a residual gas analyzer and is connected to a gas feeding line via a leak valve. Two transparent view ports located around a sample holder allow for optical transmission and pump-probe spectroscopy using an external optical setup. The capabilities of the setup are demonstrated by conducting two experiments. In the first experiment, we study the photodarkening and bleaching kinetics of photochromic oxygen-containing yttrium hydride thin films illuminated in ultra high-vacuum and correlate it with changes in partial pressures inside the vacuum chamber. In the second study, we investigate changes in the optical properties of a 50 nm V film upon hydrogen absorption.

Enhancement of Luminescence of PET Films after Swift Heavy Ion Irradiation

The novelty of the study is that the ordering that occurs in a PET film under the action of SHI irradiation manifests itself as an increase in the integral intensity of intrinsic luminescence. The Urbach behaviour of the red shift of the absorption edge is used as a baseline for further analysis of experimental optical transmission spectra of PET films irradiated by swift heavy ions (SHI) previously published by the authors. Negative deviations of the experimental spectra from the Urbach baseline in the visible and UV parts of the spectrum are attributed to enhanced by SHI irradiation intrinsic luminescence. The observed dependence of the integral intensity of luminescence of irradiated PET films on the SHI fluence and ion charge provides further confirmation of the presence of SHI-induced ordering of the molecular structure in SHI latent tracks.

Citric acid tuned negative thermal quenching of all inorganic copper-based perovskites

Copper-based perovskites, with lower electronic dimensions and high photoluminescence quantum yields (PLQY), which are non-toxic and thermally stable, have been reported since 2019 and have immediately attracted great attention. So far, only a few studies have researched the temperature-dependent photoluminescence properties, posing a challenge in ensuring the stability of the material. In this paper, the temperature-dependent photoluminescence properties have been investigated in detail, and a negative thermal quenching of all-inorganic CsCu₂I₃ perovskites has been studied. Moreover, the negative thermal quenching property can be tuned with the assistance of citric acid, which has not been reported before. The Huang-Rhys factors are calculated to be 46.32/38.31, which is higher than for many semiconductors and perovskites.

The effect of ion radius on luminescence for alkali ions doping in Y2O3: Yb3+/Ho3+ thin film

In this paper, alkali ion (Li⁺ Na⁺ K⁺ and Rb⁺)-doped Y₂O₃:Yb³⁺/Ho³⁺ up conversion films were prepared using the sol-gel method. The structures of the films were studied by using X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. A series of high-quality thin films with good crystallization were prepared. For all samples, two emission bands were observed: green emission at 539 (550) nm and red emission at 664 nm, which can be attributed to ⁵F₄ (⁵S₂)→⁵I₇ and ⁵F₅→⁵I₈, respectively. The green emission is dominant, and the red emission is extremely weak. The effect of each alkali-ion dopant on the emission and color adjustment of samples was investigated. The green emission intensity is increased by a factor of 6.33 (Li), 2.03 (Na), 4.82 (K) and 1.92 (Rb) with increasing alkali-ion doping concentration, and red emission is increased by a factor of 7.80 (Li), 1.92 (Na), 4.78 (K) and 1.90 (Rb). The extreme value appears earlier with increasing ion radius. Li⁺ doping boosts luminescence in three ways, and the other alkali ions affect the light emission in two ways. Li⁺ doping and K⁺ doping can be used to adjust the color coordinates towards the 539 nm and 550 nm directions, respectively. Na⁺ and Rb⁺ doping can enhance emission with a stable color. This means that each alkali ion is a suitable choice as a color-regulating ion and can play a role in the regulation of luminescence.

Fabrication of Li-Doped NiO Thin Films by Ultrasonic Spray Pyrolysis and Its Application in Light-Emitting Diodes

The fabrication of NiO films by different routes is important to extend and improve their applications as hole-transporting layers in organic and inorganic optoelectronic devices. Here, an automated ultrasonic pyrolysis spray method was used to fabricate NiO and Li-doped NiO thin films using nickel acetylacetonate and lithium acetate dihydrate as metal precursor and dimethylformamide as solvent. The effect of the amount of lithium in the precursor solution on the structural, morphological, optical, and electrical properties were studied. XRD results reveal that all the samples are polycrystalline with cubic structure and crystallite sizes in the range of 21 to 25 nm, without any clear trend with the Li doping level. AFM analysis shows that the crystallites form round-shaped aggregates and all the films have low roughness. The optical transmittance of the films reaches values of 60% to 77% with tendency upward as Li content is increased. The electrical study shows that the films are p-type, with the carrier concentration, resistivity, and carrier mobility depending on the lithium doping. NiO:Li (10%) films were successfully incorporated into inorganic light emitting diodes together with Mn-doped ZnS and ZnO:Al films, all deposited on ITO by the same ultrasonic spray pyrolysis technique.

Investigation of the Optical Properties for Quaternary Se60-xGe35Ga5Sbx (x = 0, 5, and 10) Chalcogenide Glass

A quenching technique was used to prepare the chalcogenide system of the Se60−xGe35Ga5Sbx (x = 0, 5, and 10 at. %), which was deposited as thin films onto glass substrates using a thermal evaporation technique. X-ray diffraction patterns were used for structure examination of the fabricated compositions, which exposes the amorphous nature of the deposited samples. Meanwhile, the chemical compositions of the prepared samples were evaluated and calculated via the energy-dispersive X-ray spectroscopy (EDX), which was in agreement with the measured compositional element percentages of the prepared samples. Based on the optical reflectance R and transmittance T spectra from the recorded spectrophotometric data ranging from 350 to 2500 nm, the influence of the Sb element on the Se60−xGe35Ga5Sbx thin films’ optical properties was studied. The film thickness and the refractive index were calculated via Swanepoel’s technique from optical transmittance data. It has been observed that the films’ refractive index increases with increasing x value over the spectral range. The refractive index data were used to evaluate the dielectric constants and estimate dispersion parameters Eo and Ed using the Wemple−DiDomenico model. The optical energy gap Egopt was calculated for the tested compositions. The result of the optical absorption analysis shows the presence of allowed direct and indirect transitions.

Exciton Luminescence and Optical Properties of Nanocrystalline Cubic Y2O3 Films Prepared by Reactive Magnetron Sputtering

This paper presents a comprehensive study of the energy structure, optical characteristics, and spectral-kinetic parameters of elementary excitations in a high-purity nanocrystalline cubic Y₂O₃ film synthesized by reactive magnetron sputtering. The optical transparency gaps for direct and indirect interband transitions were determined and discussed. The dispersion of the refractive index was established based on the analysis of interference effects. It was found that the refractive index of the Y₂O₃ film synthesized in this study is higher in order of magnitude than that of the films obtained with the help of other technologies. The intrinsic emission of Y₂O₃ film has been discussed and associated with the triplet-singlet radiative relaxation of self-trapped and bound excitons. We also studied the temperature behavior of the exciton luminescence of Y₂O₃ for the first time and determined thermal activation barriers. The optical energy and kinetic parameters of cubic Y₂O₃ films were analyzed in comparison with those of the monoclinic films of yttrium oxide. The main difference between the optical properties of cubic and monoclinic Y₂O₃ films was established, which allowed for a supposition of their application prospects.

Nonlocal Effective Medium (NLEM) for Quantitative Modelling of Nanoroughness in Spectroscopic Reflectance

Spectroscopic reflectance is a versatile optical methodology for the characterization of transparent and semi-transparent thin films in terms of thickness and refractive index. The Fresnel equations are used to interpret the measurements, but their accuracy is limited when surface roughness is present. Nanoroughness can be modelled through a discretized multi-slice and effective medium approach, but to date, this offered mostly qualitative and not quantitative accuracy. Here we introduce an adaptive and nonlocal effective medium approach, which considers the relative size and environment of each discretized slice. We develop our model using finite-difference time-domain simulation results and demonstrate its ability to predict nanoroughness size and shape with relative errors < 3% in a variety of test systems. The accuracy of the model is directly compared to the prediction capabilities of the Bruggeman and Maxwell–Garnett models, highlighting its superiority. Our model is fully parametrized and ready to use for exploring the effects of roughness on reflectance without the need for costly 3D simulations and to be integrated into the Fresnel simulator of spectroscopic reflectance tools.

Special Issue: Optical Properties of Crystals and Thin Films

Crystalline materials and coatings can be found almost everywhere in the modern world [...]

New method for the determination of photoabsorption from transmittance measurements in the extreme ultraviolet

We have developed a new method for the determination of photoabsorption at extreme ultraviolet wavelengths longer than 20 nm, where reliable refractive index values are sparse or non-existent. Our method overcomes the obstacle of multiple reflections that occur inside thin films in this spectral range, which up until now has prevented the accurate determination of photoabsorption from transmittance measurements. We have derived a mathematical expression that is independent of internal reflection amplitudes, while taking advantage of the transmittance oscillations stemming from such reflections. The method is validated on measurements of aluminum thin films. This advance will enable accurate refractive index values for many important materials for optical instrumentation, thus facilitating high-priority research on topics including coherent light sources, planetary and solar physics, and semiconductor manufacturing.

Temperature dependence of refractive indices of Al0.9Ga0.1As and In0.53Al0.1Ga0.37As in the telecommunication spectral range

In this work, we determine the temperature dependence of refractive indices of In_0.53Al_0.1Ga_0.37As and Al_0.9Ga_0.1As semiconductor alloys at telecommunication wavelengths in the range from room temperature down to 10 K. For that, we measure the temperature-dependent reflectance of two structures: with an Al_0.9Ga_0.1As/GaAs distributed Bragg reflector (DBR) designed for 1.3 µm and with an In_0.53Al_0.1Ga_0.37As/InP DBR designed for 1.55 µm. The obtained experimental results are compared to DBR reflectivity spectra calculated within the transfer matrix method to determine refractive index values. We further show that changes due to the thermal expansion of the DBR layers are negligible for our method.

Laser Structuring and DLC Coating of Elastomers for High Performance Applications

Even though hard, low friction coatings such as diamond like carbon (DLC) would be beneficial for the performance and longevity of rubber seals, a crucial challenge remains. The elastic mismatch of rubber substrate and DLC coating prevents a fracture free coating application. In this work, a nature inspired approach is applied to render the stiff coating flexible and resilient to delamination at the same time by direct patterning. Rubber substrates were laser structured with tile patterns and subsequently DLC coated. Tensile and tribology tests were performed on structured and unstructured samples. Unstructured DLC coatings showed a crack pattern induced by the coating process, which was further fragmented by tensile stress. Coatings with tile patterns did not experience a further fragmentation under load. During continuous tribological loading, less heterogenous damage is produced for tile structured samples. The findings are ascribed to the relief of induced coating stress by the tile structure, meaning a more resilient coating.

The Effect of Annealing on the Optoelectronic Properties and Energy State of Amorphous Pyrochlore Y2Ti2O7 Thin Layers by Sol–Gel Synthesis

Pyrochlore titanate (Y2Ti2O7) is a promising material for a wide range of applications in optoelectronics and photocatalysis due to its advantageous chemical, mechanical, and optical properties. To enhance its potential for such uses, however, a high-quality and scalable synthesis method is required. We here investigate the crystallization of sol–gel produced Y2Ti2O7 layers. We observe a transition of the amorphous pyrochlore phase at annealing temperatures below 700 °C. The transmittances of the Y2Ti2O7 thin layers annealed at 400 to 700 °C are approximately 92.3%. The refractive indices and packing densities of Y2Ti2O7 thin layers annealed at 400–700 °C/1 h vary from 1.931 to 1.954 and 0.835 to 0.846, respectively. The optical bandgap energies of Y2Ti2O7 thin layers annealed at 400–700 °C/1 h reduce from 4.356 to 4.319 eV because of the Moss–Burstein effect. These good electronic and optical properties make Y2Ti2O7 thin layers a promising host material for many potential applications.

Infrared Spectral Intensities of Amine Ices, Precursors to Amino Acids

Here, we address the paucity of infrared (IR) spectral data needed to quantify low-temperature experiments with amine ices, such as the formation of amino acids, by reporting new IR results on solid phases of methylamine (CH₃NH₂) and ethylamine (CH₃CH₂NH₂), precursors to glycine and alanine, respectively. Mid-IR band strengths and absorption coefficients for CH₃NH₂ and CH₃CH₂NH₂, in both amorphous and crystalline forms, are presented, along with measurements of a density and refractive index (670 nm) for each. For these same compounds, we also have calculated IR optical constants, and they are being made available in electronic form. Some applications of our new results are described along with proposals for future investigations. Suggestions are made related to the methods employed in such work, and particularly to the application of Beer's Law to the IR study of compounds of astrobiological interest. Comments are also included on the methods used, and the results presented in a recently published work on amino-acid IR intensities.

Multifractal analysis of Mg-doped ZnO thin films deposited by sol-gel spin coating method

A multifractal analysis has been performed on the 3D (three-dimensional) surface microtexture of magnesium-doped zinc oxide (ZnO:Mg) thin films with doping concentration of 0, 2, 4, and 5%. Thin films were deposited onto the glass substrates via the sol-gel spin coating method. The effect of magnesium doping, on the crystal structure, morphology, and band gap for ZnO:Mg thin films has been analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-Vis spectroscopy. It has been observed that the surface of ZnO thin films is multifractal in nature. However, multifractality and complexity observed to decrease with increasing content of Mg in ZnO thin films due to formation of islands on the surface in accordance with Volmer-Weber growth mechanism. The investigations revealed that crystallinity, microtexture, morphology, and optical properties of the thin films can be tuned by controlling the Mg content within the ZnO lattice. In particular, their optical band gap energies were 3.27, 3.31, 3.34, and 3.33 eV at 0, 2, 4, and 5%, respectively. The prepared thin films of ZnO:Mg with tuned characteristics would have promising applications in optoelectronic devices.

Excited state photochemically driven surface formation of benzene from acetylene ices on Pluto and in the outer solar system

NASA's New Horizons mission unveiled a diverse landscape of Pluto's surface with massive regions being neutral in color, while others like Cthulhu Macula range from golden-yellow to reddish comprising up to half of Pluto's carbon budget. Here, we demonstrate in laboratory experiments merged with electronic structure calculations that the photolysis of solid acetylene - the most abundant precipitate on Pluto's surface - by low energy ultraviolet photons efficiently synthesizes benzene and polycyclic aromatic hydrocarbons via excited state photochemistry thus providing critical molecular building blocks for the colored surface material. Since low energy photons deliver doses to Pluto's surface exceeding those from cosmic rays by six orders of magnitude, these processes may significantly contribute to the coloration of Pluto's surface and of hydrocarbon-covered surfaces of Solar System bodies such as Triton in general. This discovery critically enhances our perception of the distribution of aromatic molecules and carbon throughout our Solar System.

Optimization of coating uniformity in a simple rotation system by using a rotating shadow mask

A theoretically designed rotating shadow mask is proposed to optimize the uniformity of a simple rotation system, which makes full use of the width of the coating chamber. This method can fabricate a large-aperture optical component, the diameter of which is more than half the width of the coating machine. The rotating shadow mask is applied to correct the film thickness uniformity near the center point of simple plane substrate. The factors influencing the effect of the rotating shadow mask are simulated and discussed. Then the shape of the rotating shadow mask is theoretically designed, and the uniformity within a corresponding radius is well corrected. After determining the shape of the rotating shadow mask, an additional fixed shadow mask is calculated and used to improve the uniformity of the entire substrate. Through the application of the two shadow masks together, uniformity about 99.5% is obtained in the diameter of 640 mm on a 1100 mm coating machine.

Synthesis and optical properties of N→Ga coordinated gallium boroxines

Reactions of the N,C,N-chelated organogallium amide LGa(NEt₂)₂ (1), where L is {2,6-(Me₂NCH₂)₂C₆H₃}^-, with organoboronic acids RB(OH)₂ yielded molecular gallium boroxines LGa(O₃B₂R₂) (2: R = OH, 3: R = Ph, 4: R = 4-MeO-C₆H₄, 5: R = 4-CHO-C₆H₄, 6: R = Fc), neutral analogues of gallaborates. The molecular structures revealed the presence of a six-membered central GaB₂O₃ ring. The film forming properties of 5 allowed the deposition of transparent thin films by a spin coating method. The thicknesses, refractive index, energy of the optical gap (Eoptg), activation energy of surface electrical conductivity (Esa) and pre-exponential factor (σ₀) of the thin layers of 5 were measured and they are close to those found for related oxygen glass. Finally, GBO 5 was also used as an additive to printing ink and a thin film of 5 was prepared by the gravure printing technique.

Optical Characterization of H-Free a-Si Layers Grown by rf-Magnetron Sputtering by Inverse Synthesis Using Matlab: Tauc–Lorentz–Urbach Parameterization

Several, nearly-1-µm-thick, pure, unhydrogenated amorphous-silicon (a-Si) thin layers were grown at high rates by non-equilibrium rf-magnetron Ar-plasma sputtering (RFMS) onto room-temperature low-cost glass substrates. A new approach is employed for the optical characterization of the thin-layer samples, which is based on some new formulae for the normal-incidence transmission of such a samples and on the adoption of the inverse-synthesis method, by using a devised Matlab GUI environment. The so-far existing limiting value of the thickness-non-uniformity parameter, Δd, when optically characterizing wedge-shaped layers, has been suppressed with the introduction of the appropriate corrections in the expression of transmittance. The optical responses of the H-free RFMS-a-Si thin films investigated, were successfully parameterized using a single, Kramers–Krönig (KK)-consistent, Tauc–Lorentz oscillator model, with the inclusion in the model of the Urbach tail (TLUC), in the present case of non-hydrogenated a-Si films. We have also employed the Wemple–DiDomenico (WDD) single-oscillator model to calculate the two WDD dispersion parameters, dispersion energy, Ed, and oscillator energy, Eso. The amorphous-to-crystalline mass-density ratio in the expression for Ed suggested by Wemple and DiDomenico is the key factor in understanding the refractive index behavior of the a-Si layers under study. The value of the porosity for the specific rf-magnetron sputtering deposition conditions employed in this work, with an Ar-pressure of ~4.4 Pa, is found to be approximately 21%. Additionally, it must be concluded that the adopted TLUC parameterization is highly accurate for the evaluation of the UV/visible/NIR transmittance measurements, on the H-free a-Si investigated. Finally, the performed experiments are needed to have more confidence of quick and accurate optical-characterizations techniques, in order to find new applications of a-Si layers in optics and optoelectronics.

Influence of the Iron as a Dopant on the Refractive Index of WO3

Results on studies of pure tungsten oxide WO₃ and 2, 3 and 4% Fe-doped WO₃ grown on the sapphire substrates by reactive pulsed laser deposition technique are reported. From X-ray diffraction it results that the crystalline structures changed with the substrate temperature and the peaks diffraction having a small shift by the amount of Fe content in WO₃ lattice was noticed. Scanning electron microscopy presented a random behavior of WO₃ nanocrystallites size with substrate temperatures. In the presence of 2% Fe-doped WO₃, the nanocrystallites size varied gradually from 60 nm to 190 nm as substrate temperature increased. The transmission spectra of the pure and 2, 3 and 4% Fe-doped WO₃ films were obtained within the 300-1200 nm spectral range. The refractive index of WO₃ and Fe-doped WO₃ layers were calculated by the Swanepoel method. The refractive index of pure WO₃ shows a variation from 2.35-1.90 and for 2% Fe-doped WO₃ from 2.30-2.00, as the substrate temperature increased. The contents of 3 and 4% Fe-doped WO₃ presented nearly identical values of the refractive index with pure and 2% Fe-doped WO₃, in error limits, at 600 °C. The optical band gap changes with substrate temperature from 3.2 eV to 2.9 eV for pure WO₃ and has a small variation with the Fe.

Optical properties of anatase and rutile TiO2 films deposited by using a pulsed laser

Transparent titanium dioxide (TiO₂) films were deposited on quartz substrates by using a pulsed-laser deposition technique. The effects of deposition temperatures on the crystalline phases and optical properties of the films were investigated. Phase-pure anatase and rutile TiO₂ films were obtained at temperatures of 600°C and 800°C, respectively. The deposited TiO₂ films were transparent in the visible spectral region, and the linear refractive indices of the films were determined by using the optical transmission spectra. The optical bandgaps were obtained as 3.25 and 3.02 eV for the anatase and rutile TiO₂ films, respectively. The third-order nonlinear optical properties of the films were investigated by using a single-beam z-scan method at a wavelength of 532 nm. The values of the nonlinear absorption coefficient and the nonlinear refractive index of the rutile TiO₂ film were determined to be -7.44×10^-10m/W and -1.36×10^-16m²/W, respectively, which are greater than those of the anatase TiO₂ film. The metal-oxygen bond lengths based on the bond-orbital theory could be used to explain the results.

Spectral fitting method for obtaining the refractive index and thickness of chalcogenide films

Spectral fitting method (SFM) was proposed to obtain the refractive index (RI) and thickness of chalcogenide films based on transmission spectra. It extended the Swanepoel method to the films on the order of hundreds of nanometers in thickness. The RI and thickness of the films can be obtained quickly and accurately by using the SFM based on the transmission spectrum with only one peak and valley. The method's reliability theoretically was evaluated by simulation analysis. The results showed that the accuracy of the RI and thickness was better than 0.2% by using the SFM regardless of thin or thick film. Finally, the RI and thickness of the new ultralow loss reversible phase-change material Sb₂Se₃ films were obtained experimentally by the SFM. This work should provide a useful guideline for obtaining the RI and thickness of the transparent optical films.

Further Increasing the Accuracy of Characterization of a Thin Dielectric or Semiconductor Film on a Substrate from Its Interference Transmittance Spectrum

Three means are investigated for further increasing the accuracy of the characterization of a thin film on a substrate, from the transmittance spectrum T(λ) of the specimen, based on the envelope method. Firstly, it is demonstrated that the accuracy of characterization, of the average film thickness d¯ and the thickness non-uniformity ∆d over the illuminated area, increases, employing a simple dual transformation utilizing the product T(λ)xs(λ), where Tsm(λ) is the smoothed spectrum of T(λ) and xs(λ) is the substrate absorbance. Secondly, an approach is proposed for selecting an interval of wavelengths, so that using envelope points only from this interval provides the most accurate characterization of d¯ and ∆d, as this approach is applicable no matter whether the substrate is transparent or non-transparent. Thirdly, the refractive index n(λ) and the extinction coefficient k(λ) are computed, employing curve fitting by polynomials of the optimized degree of 1/λ, instead of by previously used either polynomial of the optimized degree of λ or a two-term exponential of λ. An algorithm is developed, applying these three means, and implemented, to characterize a-Si and As98Te2 thin films. Record high accuracy within 0.1% is achieved in the computation of d¯ and n(λ) of these films.

Simultaneous Determination of Refractive Index and Thickness of Submicron Optical Polymer Films from Transmission Spectra

High-transparency polymers, called optical polymers (OPs), are used in many thin-film devices, for which the knowledge of film thickness (h) and refractive index (n) is generally required. Spectrophotometry is a cost-effective, simple and fast non-destructive method often used to determine these parameters simultaneously, but its application is limited to films where h > 500 nm. Here, a simple spectrophotometric method is reported to obtain simultaneously the n and h of a sub-micron OP film (down to values of a few tenths of a nm) from its transmission spectrum. The method is valid for any OP where the n dispersion curve follows a two-coefficient Cauchy function and complies with a certain equation involving n at two different wavelengths. Remarkably, such an equation is determined through the analysis of n data for a wide set of commercial OPs, and its general validity is demonstrated. Films of various OPs (pristine or doped with fluorescent compounds), typically used in applications such as thin-film organic lasers, are prepared, and n and h are simultaneously determined with the proposed procedure. The success of the method is confirmed with variable-angle spectroscopic ellipsometry.

Selective C–C Bond Cleavage in Diols and Lignin Models: High-Throughput Screening of Metal Oxide-Anchored Vanadium in Mesoporous Silica

The development of selective and robust heterogeneous oxidation catalysts is an enabling technology for conversion of biomass-derived platform chemicals. Vanadium active sites were incorporated into the structure of mesoporous silica via an ultra-fast, one-pot synthesis method based on microwave-assisted heating. In addition, Al/Ti/Zr/Ce anchoring ions were introduced in order to minimize vanadium leaching and better control its dispersion. The supported V-(Al/Ti/Zr/Ce)-MCM-41 composite materials were assessed as catalysts for aerobic C–C bond cleavage of simple models for lignin (1,2-diphenyl-2-methoxyethanol) and sugar-derived polyalcohols (meso-hydrobenzoin). The TiIV ions proved to be the best anchors to prevent V leaching, while AlIII and ZrIV ions were the best to improve selective conversion of the substrates. The active sites in these catalysts are shown to be 2D VOx layers stabilized on the anchors. In a screen of twelve solvents, weakly polar solvents like toluene were found to be most suitable for this reaction, as was environmentally friendly ethyl acetate. The above properties, together with the high selectivity for C–C bond cleavage, advocate for a heterogeneous catalytic pathway, intrinsically different from that reported previously for molecular oxovanadium (V) catalysts.

Unified Model for Laser Doping of Silicon from Precursors

Laser doping of silicon with the help of precursors is well established in photovoltaics. Upon illumination with the constant or pulsed laser beam, the silicon melts and doping atoms from the doping precursor diffuse into the melted silicon. With the proper laser parameters, after resolidification, the silicon is doped without any lattice defects. Depending on laser energy and on the kind of precursor, the precursor either melts or evaporates during the laser process. For high enough laser energies, even parts of the silicon’s surface evaporate. Here, we present a unified model and simulation program, which considers all these cases. We exemplify our model with experiments and simulations of laser doping from a boron oxide precursor layer. In contrast to previous models, we are able to predict not only the width and depth of the patterns on the deformed silicon surface but also the doping profiles over a wide range of laser energies. In addition, we also show that the diffusion of the boron atoms in the molten Si is boosted by a thermally induced convection in the silicon melt: the Gaussian intensity distribution of the laser beam increases the temperature-gradient-induced surface tension gradient, causing the molten Si to circulate by Marangoni convection. Laser pulse energy densities above H > 2.8 J/cm² lead not only to evaporation of the precursor, but also to a partial evaporation of the molten silicon. Without considering the evaporation of Si, it is not possible to correctly predict the doping profiles for high laser energies. About 50% of the evaporated materials recondense and resolidify on the wafer surface. The recondensed material from each laser pulse forms a dopant source for the subsequent laser pulses.

In situ laser irradiation: the kinetics of the changes in the nonlinear/linear optical parameters of As50Se40Sb10 thin films for photonic applications

The photosensitivity of amorphous chalcogenide thin films brings out light-induced changes in the nonlinear and linear optical parameters upon sub-bandgap and bandgap laser irradiation. The present work reports the in situ laser irradiated changes in the optical properties of As50Se40Sb10 thin films. The thermally evaporated film showed an exponential decrease in optical bandgap and increase in Urbach energy. The decay rate of the bandgap was 6.24 min and growth rate of Urbach energy was 6.67 min respectively. The dynamics of photo-induced changes were observed from the changes in linear refractive index and its dependent parameters such as 3rd order nonlinear susceptibility, nonlinear refractive index, dispersion and dielectric parameters. The conversion of heteropolar to homopolar bonds induced the photodarkening mechanism that changed the dispersion parameters. The decrease in E d and E o reduced the oscillator strength along with the zero-frequency dielectric constant. The optical and electrical conductivity changed significantly with time. The changes were saturated with time which brings stability in the film properties that is useful for various optical applications. However, no structural and compositional changes upon laser irradiation were noticed from the X-ray diffraction and EDX studies respectively. The surface homogeneity was checked from the FESEM picture.

Effects of Different Amounts of Nb Doping on Electrical, Optical and Structural Properties in Sputtered TiO2−x Films

Highly conductive TiO2 films with different Nb doping levels (up to 5 at%) were prepared by reactive DC magnetron sputtering under precise control of the oxygen partial pressure. They were deposited on unheated substrates, covered with a protective Si3N4 layer, and subsequently annealed at 300 °C. The doping efficiency of Nb is greater than 90%. Conductivity is a maximum for a partly oxidized target in the transition range. The best films exhibit a resistivity of 630 µΩ cm and a mobility of 7.6 cm2/Vs combined with a high transparency above 70%. Comparing the behavior of undoped and Nb-containing films, intrinsic limits of the conductivity in the TiO2−x:Nb system could be observed, and a consistent model explaining these findings is presented. The conductivity is limited—by decreasing electron density due to Nb oxidation—by increasing incorporation formation of Nb2O5 clusters as scattering centers with increasing oxygen partial pressure and Nb concentration, by a transition from the crystalline to the amorphous state of the films below a critical oxygen partial pressure.