REFLEX: a program for the analysis of specular X-ray and neutron reflectivity data

Red Emissive 1D Copper(I) Thiolates and Green Emissive 2D Copper(I) Halide Thiolates Displaying Second Harmonic Generation and Two-Photon Absorption Processes

Copper(I), as a d10 metal, is a promising affordable noncritical raw material finding great interest for the development of photoluminescent materials. Halide and thiolate-based copper(I) compounds are known for their efficient emission and good stability. In order to rationalize the effect of these two anions in the structure and photoemission of Cu(I) compounds, two new families of coordination polymers have been synthesized: the copper(I)-thiolates: [Cu(p-SPhX)]n (X = F, Cl, Br), and the copper halide thiolates: [Cu3Cl(p-SPhX)2]n (X = F, Cl, Br). The two families display different structural dimensionalities: 1D [Cu(p-SPhX)]n versus 2D [Cu3Cl(p-SPhX)2]n and they exhibit distinct photophysical properties: [Cu(p-SPhX)]n shows usual solid-state red emission, while [Cu3Cl(p-SPhX)2]n hasintense solid-state green anti-Stokes emission, associated, for the noncentrosymmetric compounds, X = F and Br, to non-linear optical (NLO) response, pointing out the effect of the weak halogen interactions of the thiolate ligands on the symmetry and the properties.

Size-Dependent Electrostatic Adsorption of Polymer-Grafted Gold Nanoparticles on Polyelectrolyte Brushes

Designing a functional surface that selectively adsorbs nanoparticles based on their size and shape is essential for developing an advanced adsorption-based, postsynthesis nanoparticle separation device. We demonstrate selective adsorption of larger nanoparticles from solution onto a polyelectrolyte brush by tuning the salt concentration. Specifically, a positively charged polyelectrolyte brush is created by converting pyridine groups of poly(2-vinylpyridine) to n-methylpyridinium groups using methyl iodide. The adsorption kinetics and thermodynamics of poly(ethylene glycol)-grafted, negatively charged gold nanoparticles (diameters of 12 and 20 nm) were monitored as a function of salt concentration. In a salt-free solution, the polyelectrolyte brush adsorbs gold nanoparticles of both sizes. As the salinity increases, the areal number density of adsorbed nanoparticles monotonically decreases and becomes negligible at high salinity. Interestingly, there is an intermediate range of salt concentrations (i.e., 15-20 mM of NaCl) where the decrease in nanoparticle adsorption is more pronounced for smaller particles, leading to size-selective adsorption of the larger nanoparticles. As a further demonstration of selectivity, the polyelectrolyte brush is immersed in a binary mixture of 12 and 20 nm nanoparticles and found to selectively capture larger particles with ∼90% selectivity. In addition, the size distribution of as-synthesized gold nanoparticles, with an average diameter of 12 nm, was reduced by selectively removing larger particles by exposing the solution to polyelectrolyte brush surfaces. This study demonstrates the potential of a polyelectrolyte brush separation device to remove larger nanoparticles by controlling electrostatic interactions between polymer brushes and particles.

Impact of Irreversible Adsorption on Molecular Ordering and Charge Transport in Poly(3-hexylthiophene) Thin Films on Solid Substrates

We investigate the irreversible adsorption of poly(3-hexylthiophene) (P3HT) polymer thin films on silicon dioxide/silicon (SiO2/Si) substrates during thermal annealing at a temperature below the melting temperature (Tm) but far above the glass transition temperature (Tg), i.e., Tg ≪ T = 170 °C < Tm, and its effect on their crystalline ordering and charge transport properties. It was found that short-time annealing enhances the molecular ordering of P3HT films, while prolonged thermal annealing gradually disrupts the crystalline structures and reduces the overall crystallinity of the film. Concurrently, thermal annealing at this temperature facilitates the slow irreversible adsorption of P3HT chains at the polymer-solid interface, resulting in the formation of a 1.7 Rg-thick (∼18 nm thick) adsorbed layer on SiO2/Si substrates that is fully amorphous and contains a large fraction of loosely adsorbed chains. We postulate that such irreversible adsorption is responsible for the reduced crystalline packing of P3HT at the polymer-solid interface at Tg ≪ T < Tm, which further disrupts the molecular ordering of the entire 46 nm thick P3HT film by a long-range perturbation effect. Electrical measurements using an organic field-effect transistor (OFET) device reveal that the enhanced charge carrier mobility of P3HT films correlates with an optimized annealing time at Tg ≪ T < Tm, which achieves a balance between maximizing molecular ordering and minimizing the impact of irreversible chain adsorption. These findings provide new insights into the underlying mechanism of thermal annealing in tailoring the structure and property of conjugated polymer thin films prepared on solid substrates.

Fast Photoactuation and Environmental Response of Humidity-Sensitive pDAP-Silicon Nanocantilevers

Multi-responsive nanomembranes are a new class of advanced materials that can be harnessed in complex architectures for micro and nano-manipulators, artificial muscles, energy harvesting, soft robotics, and sensors. The design and fabrication of responsive membranes must meet such challenges as trade-offs between responsiveness and mechanical durability, volumetric low-cost production ensuring low environmental impact, and compatibility with standard technologies or biological systems This work demonstrates the fabrication of multi-responsive, mechanically robust poly(1,3-diaminopropane) (pDAP) nanomembranes and their application in fast photoactuators. The pDAP films are developed using a plasma-assisted polymerization technique that offers large-scale production and versatility of potential industrial relevance. The pDAP layers exhibit high elasticity with the Young's modulus of ≈7 GPa and remarkable mechanical durability across 20-80 °C temperatures. Notably, pDAP membranes reveal immediate and reversible contraction triggered by light, rising temperature, or reducing relative humidity underpinned by a reversible water sorption mechanism. These features enable the fabrication of photoactuators composed of pDAP-coated Si nanocantilevers, demonstrating ms timescale response to light, tens of µm deflections, and robust performance up to kHz frequencies. These results advance fundamental research on multi-responsive nanomembranes and hold the potential to boost versatile applications in light-to-motion conversion and sensing toward the industrial level.

Exploring the evolution of mass density and thickness of N-doped Ge-rich GeSbTe during multistep crystallization

Among phase change materials, Ge-rich GeSbTe alloys (GGST) are key alloys for the next generation of embedded phase change memories because of their good thermal stability, allowing their use for the automotive applications. Several studies have investigated GGST crystallization, which takes place in several stages, including phase separation in the amorphous material, the crystallization of the cubic Ge and GST phases before a complete crystallization for higher thermal budget. So far, however, no information is available on the possible changes in density and thickness of such alloys. This paper investigates such variations in density and thickness for a N-doped GGST layer (GGSTN) during isothermal annealing, following the four main stages of its multistep crystallization process. X-ray reflectivity (XRR) and X-ray diffraction were employed for analysis. The study reveals that density and thickness exhibit distinct changes during crystallization, with density increasing by approximately 9% during transition from amorphous to crystalline states. These changes are attributed to alterations in layer morphology, particularly at the Ge crystallization temperature and at the onset of GST crystal formation. Additionally, at high thermal budgets, discrepancies between XRR analysis methods suggest the formation of a thin, lower density layer near the top interface of the GGSTN layer. These results provide insights into the structural evolution of the GGSTN layer, which is crucial for phase change random access memory applications.

X-Ray Calc 3: improved software for simulation and inverse problem solving for X-ray reflectivity

This work introduces X-Ray Calc (XRC), an open-source software package designed to simulate X-ray reflectivity (XRR) and address the inverse problem of reconstructing film structures on the basis of measured XRR curves. XRC features a user-friendly graphical interface that facilitates interactive simulation and reconstruction. The software employs a recursive approach based on the Fresnel equations to calculate XRR and incorporates specialized tools for modeling periodic multilayer structures. This article presents the latest version of the X-Ray Calc software (XRC3), with notable improvements. These enhancements encompass an automatic fitting capability for XRR curves utilizing a modified flight particle swarm optimization algorithm. A novel cost function was also developed specifically for fitting XRR curves of periodic structures. Furthermore, the overall user experience has been enhanced by developing a new single-window interface.

Electronic structure of superconducting VN(111) films

Vanadium nitride (VN) is a transition-metal nitride with remarkable properties that have prompted extensive experimental and theoretical investigations in recent years. However, there is a current paucity of experimental research investigating the temperature-dependent electronic structure of single-crystalline VN. In this study, high-quality VN(111) films were successfully synthesized on α -Al2 O3 (0001) substrates using magnetron sputtering. The crystal and electronic structures of the VN films were characterized by a combination of high-resolution X-ray diffraction, low-energy electron diffraction, resonant soft X-ray absorption spectroscopy, and ultraviolet photoelectron spectroscopy. The electrical transport measurements indicate that the superconducting critical temperature of the VN films is around 8.1 K. Intriguingly, the temperature-dependent photoelectron spectroscopy measurements demonstrate a weak temperature dependence in the electronic structure of the VN films, which is significant for understanding the ground state of VN compounds.

pH-Mediated Size-Selective Adsorption of Gold Nanoparticles on Diblock Copolymer Brushes

Precise control of nanoparticles at interfaces can be achieved by designing stimuli-responsive surfaces that have tunable interactions with nanoparticles. In this study, we demonstrate that a polymer brush can selectively adsorb nanoparticles according to size by tuning the pH of the buffer solution. Specifically, we developed a facile polymer brush preparation method using a symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer deposited on a grafted polystyrene layer. This method is based on the assembly of a PS-b-P2VP thin film oriented with parallel lamellae that remains after exfoliation of the top PS-b-P2VP layer. We characterized the P2VP brush using X-ray reflectivity and atomic force microscopy. The buffer pH is used to tailor interactions between citrate-coated gold nanoparticles (AuNPs) and the top P2VP block that behaves like a polymer brush. At low pH (∼4.0) the P2VP brushes are strongly stretched and display a high density of attractive sites, whereas at neutral pH (∼6.5) the P2VP brushes are only slightly stretched and have fewer attractive sites. A quartz crystal microbalance with dissipation monitored the adsorption thermodynamics as a function of AuNP diameter (11 and 21 nm) and pH of the buffer. Neutral pH provides limited penetration depth for nanoparticles and promotes size selectivity for 11 nm AuNP adsorption. As a proof of concept, the P2VP brushes were exposed to various mixtures of large and small AuNPs to demonstrate selective capture of the smaller AuNPs. This study shows the potential of creating devices for nanoparticle size separations using pH-sensitive polymer brushes.

Process temperature-dependent interface quality and Maxwell-Wagner interfacial polarization in atomic layer deposited Al2O3/TiO2 nanolaminates for energy storage applications

Considering the excellent tunability of electrical and dielectric properties in binary metal oxide based multi-layered nanolaminate structures, a thermal atomic layer deposition system is carefully optimized for the synthesis of device grade Al₂O₃/TiO₂ nanolaminates with well-defined artificial periodicity and distinct interfaces, and the role of process temperature in the structural, interfacial, dielectric and electrical properties is systematically investigated. A marginal increase in interfacial interdiffusion in these nanolaminates, at elevated temperatures, is validated using X-ray reflectivity and secondary ion mass spectrometry studies. With an increase in deposition temperature from 150 to 300 °C, the impedance spectroscopy measurements of these nanolaminates exhibited a monotonic increment in dielectric constant from ∼95 to 186, and a decrement in dielectric loss from ∼0.48 to 0.21, while the current-voltage measurements revealed a subsequent reduction in leakage current density from ∼2.24 × 10^-5 to 3.45 × 10^-7 A cm^-2 at 1 V applied bias and an improvement in nanobattery polarization voltage from 100 mV to 700 mV, respectively. This improvement in dielectric and electrical properties at elevated processing temperature is attributed to the reduction in impurity content along with the significant enhancement in sublayer densities and the conductivity contrast driven Maxwell-Wagner interfacial polarisation. Additionally, the devices fabricated at 300 °C exhibited a higher capacitance density of ∼22.87 fF μm^-2, a low equivalent oxide thickness of ∼1.51 nm, and a low leakage current density of ∼10^-7 A cm^-2 (at 1 V bias), making this nanolaminate a promising material for high-density energy storage applications. These findings highlight the ALD process temperature assisted growth chemistry of Al₂O₃/TiO₂ nanolaminates for superior dielectric performance and multifaceted applications.

Repulsive, but sticky - Insights into the non-ionic foam stabilization mechanism by superchaotropic nano-ions

HYPOTHESIS

The superchaotropic Keggin polyoxometalate α-SiW₁₂O₄₀^4- (SiW) was recently shown to stabilize non-ionic surfactant (C_18:1E₁₀) foams owing to electrostatic repulsion that arises from the adsorption of SiW-ions to the foam interfaces. The precise mechanism of foam stabilization by SiW however remained unsolved.

EXPERIMENTS

Imaging and conductimetry were used on macroscopic foams to monitor the foam collapse under free drainage and small angle neutron scattering (SANS) at a given foam height allowed for the tracking of the evolution of film thickness under quasi-stationary conditions. Thin film pressure balance (TFPB) measurements enabled to quantify the resistance of single foam films to external pressure and to identify intra-film forces.

FINDINGS

At low SiW/surfactant ratios, the adsorption of SiW induces electrostatic repulsion within foam films. Above a concentration threshold corresponding to an adsorption saturation, excess of SiW screens the electrostatic repulsion that leads to thinner foam films. Despite screened electrostatics, the foam and single foam films remain very stable caused by an additional steric stabilizing force consistent with the presence of trapped micelles inside the foam films that bridge between the interfaces. These trapped micelles can serve as a surfactant reservoir, which promotes self-healing of the interface leading to much more resilient foam films in comparison to bare surfactant foams/films.

GenX 3: the latest generation of an established tool

Improvements to the GenX program are discussed, including performance, model building and error analysis.

Since its publication more than 15 years ago the GenX software has been continuously developed and has established itself as a standard package for analyzing X-ray and neutron reflectometry data. The evolution of the software during the last two major revisions is reported here. This includes a simplified model builder for beginners, simple samples, additional sample models, statistical error analysis and the use of just-in-time compilation modules for the reflectometry kernel to achieve higher performance. In addition, the influence of experimental errors on the reflectivity curve is discussed, and new features are described that allow the user to include these in the error statistics to improve the fitting and uncertainty estimation.

Physicochemical Properties of a Bi-aromatic Heterocyclic-Azo/BSA Hybrid System at the Air-Water Interface

The interaction of a heterocyclic azo compound with itself and with bovine serum albumin (BSA) is realized by probing the structural modifications in Langmuir (L) monolayers and Langmuir-Blodgett (LB) films. It was found from the pressure-area/molecule isotherms that the elastic, thermodynamic, and hysteretic properties of the pure azo L monolayer were strongly altered due to the variation of temperature and pH of subphase water. In addition to that, the modification of such properties of the azo L monolayer due to mixing with BSA was also studied. The incorporation of BSA within the azo molecular assembly reduced the elasticity of that assembly. Such reduction of in-plane elasticity of the pure azo monolayer can also be achieved by reducing the temperature and pH of subphase water without adding BSA. A reduction in area per molecule of the azo assembly at the air-water interface associated with the conformational change from horizontal to vertical orientation facilitating π-π interaction was observed with increase in temperature and pH of the subphase. Such parameters also affected the interactions between azo and BSA molecules within the azo/BSA binary system. The structures of pure azo and binary films can be determined after they are transferred to hydrophilic and hydrophobic Si surfaces using the LB technique. Their out-of-plane and in-plane structures, as extracted from two complementary surface sensitive techniques, X-ray reflectivity and atomic force microscopy, were found to be strongly dependent on mixing with BSA, subphase pH, temperature, and substrate nature.

Assessment of nanoparticle immersion depth at liquid interfaces from chemically equivalent macroscopic surfaces

HYPOTHESIS

We test whether the wettability of nanoparticles (NPs) straddling at an air/water surface or oil/water interface can be extrapolated from sessile drop-derived macroscopic contact angles (mCAs) on planar substrates, assuming that both the nanoparticles and the macroscopic substrates are chemically equivalent and feature the same electrokinetic potential.

EXPERIMENTS

Pure silica (SiO₂) and amino-terminated silica (APTES-SiO₂) NPs are compared to macroscopic surfaces with extremely low roughness (root mean square [RMS] roughness ≤ 2 nm) or a roughness determined by a close-packed layer of NPs (RMS roughness ∼ 35 nm). Equivalence of the surface chemistry is assessed by comparing the electrokinetic potentials of the NPs via electrophoretic light scattering and of the macroscopic substrates via streaming current analysis. The wettability of the macroscopic substrates is obtained from advancing (ACAs) and receding contact angles (RCAs) and in situ synchrotron X-ray reflectivity (XRR) provided by the NP wettability at the liquid interfaces.

FINDINGS

Generally, the RCA on smooth surfaces provides a good estimate of NP wetting properties. However, mCAs alone cannot predict adsorption barriers that prevent NP segregation to the interface, as is the case with the pure SiO₂ nanoparticles. This strategy greatly facilitates assessing the wetting properties of NPs for applications such as emulsion formulation, flotation, or water remediation.

Maxwell-Wagner Relaxation-Driven High Dielectric Constant in Al2O3/TiO2 Nanolaminates Grown by Pulsed Laser Deposition

Multilayer nanolaminates (NLs) of alternate ultrathin sublayers of Al₂O₃ and TiO₂ (ATA) with the thickness ranging ∼2 to 0.5 nm were fabricated by optimized pulsed laser deposition (PLD). Maxwell-Wagner (M-W) relaxation-induced interfacial polarization was realized and engineered by precisely controlling the sublayer thicknesses and the number of interfaces. X-ray reflectivity and cross-sectional transmission electron microscopy measurements of ATA NLs revealed an artificial periodicity with well-defined uniformly thick amorphous sublayers with chemically and physically distinct interfaces down to a sublayer thickness of ∼0.8 nm. The dielectric constants and loss of ATA NLs were found to increase from ∼60 to 670 and decrease from ∼0.9 to 0.16, respectively, as sublayer thicknesses reduced from ∼2 to 0.8 nm. However, for a sublayer thickness below 0.8 nm, the trend was reversed. Furthermore, temperature-dependent impedance spectroscopy studies revealed two distinct thermally activated relaxation processes, corresponding to TiO₂ and Al₂O₃ sublayers, corroborating the M-W relaxation. The conductivity contrast between the sublayers of ATA NLs enhanced with reducing sublayer thickness and plateaued at a sublayer thickness of ∼0.8 nm, resulting in dominant M-W interfacial polarization and a high cut-off frequency of ∼50 kHz. These results demonstrate that ATA NLs grown by PLD may find application as potential high-k materials for next-generation nanoelectronic devices.

Determination of optical constants of thin films in the EUV

The determination of fundamental optical parameters is essential for the development of new optical elements such as mirrors, gratings, or photomasks. Especially in the extreme ultraviolet (EUV) and soft x-ray spectral range, the existing databases for the refractive indices of many materials and compositions are insufficient or are a mixture of experimentally measured and calculated values from atomic scattering factors. Since the physical properties of bulk materials and thin films with thicknesses in the nanometer range are not identical, measurements need to be performed on thin layers. In this study we demonstrate how optical constants of various thin film samples on a bulk substrate can be determined from reflection measurements in the EUV photon energy range from 62 eV to 124 eV. Thin films with thickness of 20 nm to 50 nm of pure Mo, Ni, Pt, Ru, Ta, and Te and different compositions of Ni_xAl_x, PtTe, Pt_xMo, Ru_xTa_x, Ru₃Re, Ru₂W, and TaTeN were prepared by DC magnetron sputtering and measured using EUV reflectometry. The determination optical constants of the different materials are discussed and compared to existing tabulated values.

Synergistic and Competitive Adsorption of Hydrophilic Nanoparticles and Oil-Soluble Surfactants at the Oil-Water Interface

Fundamental insights into the interplay and self-assembly of nanoparticles and surface-active agents at the liquid-liquid interface play a pivotal role in understanding the ubiquitous colloidal systems present in our natural surroundings, including foods and aquatic life, and in the industry for emulsion stabilization, drug delivery, or enhanced oil recovery. Moreover, well-controlled model systems for mixed interfacial adsorption of nanoparticles and surfactants allow unprecedented insights into nonideal or contaminated particle-stabilized emulsions. Here, we investigate such a model system composed of hydrophilic, negatively, and positively charged silica nanoparticles and the oil-soluble cationic lipid octadecyl amine with in situ synchrotron-based X-ray reflectometry, which is analyzed and discussed jointly with dynamic interfacial tensiometry. Our results indicate that negatively charged silica nanoparticles only adsorb if the oil-water interface is covered with the positively charged lipid, indicating synergistic adsorption. Conversely, the positively charged nanoparticles readily adsorb on their own, but compete with octadecyl amine and reversibly desorb with increasing concentrations of the lipid. These results further indicate that with competitive adsorption, an electrostatic exclusion zone exists around the adsorbed particles. This prevents the adsorption of lipid molecules in this area, leading to a decreased surface excess concentration of surfactants and unexpectedly high interfacial tension.

Structure and Elastic Properties of an Unsymmetrical Bi-Heterocyclic Azo Compound in the Langmuir Monolayer and Langmuir-Blodgett Film

We study the structure and elastic properties of the bi-heterocyclic azo compound at the air-water interface through surface pressure (π)-area (A) isotherm recording followed by transferring them on hydrophilic and hydrophobic Si surfaces by the Langmuir-Blodgett (LB) deposition method. A substantial change in the area/molecule is observed as a function of subphase pH and temperature. Such parameters strongly influence intramolecular interactions within azo molecules and the interactions between azo molecules and water that manifested in higher surface activity at low temperature and high pH, which in turn modifies the elasticity of azo assembly at the air-water interface. A large pH-dependent hysteresis with negative change in entropy, indicating molecular rearrangements, is observed. Molecular assembly formed at the air-water interface is then transferred onto hydrophilic and hydrophobic Si surfaces at two different surface pressures (5 and 30 mN/m) by the LB technique. The structural analysis performed by X-ray reflectivity and atomic force microscopy techniques suggests that the LB films exhibit an abrupt layered structure on hydrophilic Si, whereas an overall rough film is formed on hydrophobic Si. The coverage and compactness of individual layers are found to increase with the deposition pressure (5 to 30 mN/m).

Silicon oxide-niobium oxide mixture films and nanolaminates grown by atomic layer deposition from niobium pentaethoxide and hexakis(ethylamino) disilane

Amorphous SiO₂-Nb₂O₅ nanolaminates and mixture films were grown by atomic layer deposition. The films were grown at 300 °C from Nb(OC₂H₅)₅, Si₂(NHC₂H₅)₆, and O₃ to thicknesses ranging from 13 to 130 nm. The niobium to silicon atomic ratio was varied in the range of 0.11-7.20. After optimizing the composition, resistive switching properties could be observed in the form of characteristic current-voltage behavior. Switching parameters in the conventional regime were well defined only in a SiO₂:Nb₂O₅ mixture at certain, optimized, composition with Nb:Si atomic ratio of 0.13, whereas low-reading voltage measurements allowed recording memory effects in a wider composition range.

High-temperature X-ray scattering studies of atomic layer deposited IrO2

IrO2 is an important material in numerous applications ranging from catalysis to the microelectronics industry, but despite this its behaviour upon annealing under different conditions has not yet been thoroughly studied. This work provides a detailed investigation of the annealing of IrO2 thin films using in situ high-temperature X-ray diffraction and X-ray reflectivity (HTXRR) measurements from room temperature up to 1000°C in oxygen, nitrogen, forming gas and vacuum. Complementary ex situ scanning electron microscopy and atomic force microscopy measurements were conducted. The combined data show the dependencies of crystalline properties and surface morphology on the annealing temperature and atmosphere. The reduction of IrO2 to Ir takes place at a temperature as low as 150°C in forming gas, but in oxygen IrO2 is stable up to 800°C and evaporates as a volatile oxide at higher temperatures. The IrO2 crystallite size remains constant in oxygen up to 400°C and increases above that, while in the more reducing atmospheres the Ir crystallites grow continuously above the phase-change temperature. The role of HTXRR in the analysis is shown to be important since its high sensitivity allows one to observe changes taking place in the film at temperatures much below the phase change.

Multifitting: software for the reflectometric reconstruction of multilayer nanofilms

Multifitting is a computer program designed specifically for modeling the optical properties (reflection, transmission, absorption) of multilayer films consisting of an arbitrary number of layers in a wide range of wavelengths. Multifitting allows a user to calculate the reflectometric curves for a given structure (direct problem) and to find the parameters of the films from the experimentally obtained curves (inverse problem), either manually or automatically. Key features of Multifitting are the ability to work simultaneously with an arbitrary number of experimental curves and an ergonomic graphical user interface that is designed for intensive daily use in the diagnosis of thin films. Multifitting is positioned by the author as the successor to the IMD program, which has become the standard tool in research and technology groups synthesizing and studying thin-film coatings.