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In this paper, the aqueous copolymer dispersions of methyl (meth)acrylate-grafted poly(vinyl alcohol) are in situ synthesized and studied as a promising platform for the deposition of thin films with advanced applications. A series of dispersions of the varied copolymer structure and composition are obtained at mild reaction conditions by carrying out a redox polymerization at a different monomer-to-PVA ratio and initiator concentration. The obtained colloidal particles are characterized by Fourier transformed infrared spectroscopy, nuclear magnetic resonance, dynamic light scattering, and transmission electron microscopy. The copolymer dispersions are further used for deposition of thin films on silicon substrates. The films are characterized optically through reflectance measurements and non-linear curve fitting. Their suitability for the optical sensing of acetone vapors is confirmed by reflectance measurements before and during their exposure to analyte vapors. The cross sensitivity, sensing repeatability, and recovery of the sensitive films are discussed.

In this study, an emphasis is put on vapor-sensitive Bragg stacks as an important class of optical sensors. All-niobia Bragg stacks were deposited by spin-coating of sol-gel Nb2O5 thin films alternated with mesoporous layers after proper design through optimization of operating wavelength and number of layers in the stack. Mesoporous Nb2O5 films with different morphology and identical structure were obtained using organic templates (Pluronics PE6200 and PE6800) and subsequent annealing. Transmittance measurements were performed as a detection method that offers technological simplicity and accuracy. It was demonstrated that stacks including PE6200 templated films exhibit higher sensitivity than stacks templated with PE6800. It was assumed and verified by computer-aided modelling of experimental data that mesoporous films prepared with addition of PE6200, although less porous, were more stable compared to those templated with PE6800, and did not collapse during the thermal treatment of the stacks. Furthermore, the reproducibility of optical response was studied by sorption and desorption cycles of acetone vapors. The suitability of all-niobia Bragg stacks for optical sensing of VOCs was discussed.

The anodic oxidation method is applied for preparation of TiO2 nanotube (NT) arrays on Ti6Al4V alloy substrates. The NTs were of four different diameters (60, 65, 80 and 120 nm) and wall thicknesses in the range 8–14 nm. The as-prepared samples were characterized and then annealed at 450 °C in air for 2 h. The surface morphology of the samples is characterized by field emission scanning electron microscopy which reveals uniformly distributed nanotube arrays over the substrate. Raman scattering measurements show that the NTs in as-prepared samples are amorphous. In annealed samples rutile phase and a small amount of anatase phase are detected by both Raman scattering and X-ray diffraction methods. It is assumed that the rutile phase is related to a TiO2 film with rutile structure formed during the annealing between NTs and alloy substrate. The retarded formation of anatase phase is connected with inclusion in the TiO2 nanotubes of external atoms coming from the substrate. The amount and crystallinity of anatase phase decreases with decreasing nanotube diameter and/or wall thickness which is assigned to size-induced increase of the NT crystallization temperature. Investigations of the optical response of the samples to ethanol vapors at room temperature reveal relations between the NT size and their ethanol sensitivity. The response of as-prepared nanotube arrays increases with decreasing nanotube diameter while in the annealed samples an opposite trend is observed. The size dependence seen for as-prepared samples is related to an increase of the total adsorbing surface with decreasing NT diameter. The result obtained for the annealed samples is assigned to annealing induced changes in the morphology and composition of the internal surface of NTs.

Aluminum-doped ZnO (AZO) thin films are deposited by economically-versatile fabrication method of electrospraying onto two types of substrates – glass and silicon wafer, at substrate temperature of 300 °C, voltage of 18 kV and emitter to collector distance of 6 cm, Zinc acetate dehydrate dissolved in a mixture of deionized water, ethanol and acetic acid is used for deposition of undoped ZnO film, while for Al-doping one two precursor materials are chosen - aluminum chloride and aluminum nitrate dissolved in ethanol. The doping levels are 5 and 10 wt% and the concentration dependence of films properties is investigated. All films exhibit polycrystalline structure with decreasing crystallites size from 21 nm for undoped films to 19 nm and 17 nm for AZO films doped with 5 and 10 wt% Al, respectively. The shift of the XRD peaks of doped samples towards higher diffraction angles reveals successful incorporation of Al into ZnO lattice. The optical properties are investigated by ellipsometric and photoluminescence measurements. A decrease of refractive index from 1.89 for undoped sample to 1.84 and 1.83 for 5 and 10% AZO films, respectively and an increase of optical band gap from 3.30 eV to 3.40 eV are observed. A substantial decrease of sheet resistance is measured for AZO films: 86 Ω/□ and 61 Ω/□ for 5 wt% and 10% doped samples, respectively. Simultaneously, the transmittance of the films is kept high. The average values in the visible range for films with thickness of 200 nm are 86% for undoped ones and 84% and 80% for 5 and 10% Al-doped films, respectively. Thus, the applicability of the studied films as transparent conductive electrodes is demonstrated.

Thin films, consisting of a sol–gel Nb2O5 matrix doped with zeolite Na-X synthesized from fly ash through ultrasonic-assisted double-stage fusion-hydrothermal alkaline activation, were deposited by the spin-coating method. In order to improve the optical quality and sensing properties of the thin films, zeolites were wet milled for 60, 120 and 540 s prior to incorporation in the film. The liquid adsorption ability of thin films was tested by measuring the reflectance spectra prior to and after exposure to liquid acetone and the change in the reflection coefficient ∆R of the films was calculated. The influence of milling time of zeolites on the sensing and optical properties of the films was studied.

Humidity-sensitive polymer, namely poly(vinyl alcohol-co-vinyl acetal), was deposited on a flexible poly(ethylene terephthalate) (PET) substrate pre-covered with a sputtered Au:Pd thin layer in order to develop an optical flexible humidity sensor. The spin-coating method was applied for the thin polymer film deposition. The optical and sensing properties of the device were studied after repeated bending deformation in the range of 25−1000 times. Transmittance measurements at different levels of relative humidity were conducted in order to examine the sensing properties of the probed flexible sample. The influence of the number of bending deformations on the performance of the sensor is studied, and the possibility for its successful application is demonstrated and discussed.

Thin spin-coated polymer films of amphiphilic copolymer obtained by partial acetalization of poly (vinyl alcohol) are used as humidity-sensitive media. They are deposited on polymer substrate (PET) in order to obtain a flexible humidity sensor. Pre-metallization of substrate is implemented for increasing the optical contrast of the sensor, thus improving the sensitivity. The morphology of the sensors is studied by surface profiling, while the transparency of the sensor is controlled by transmittance measurements. The sensing behavior is evaluated through monitoring of transmittance values at different levels of relative humidity gradually changing in the range 5–95% and the influence of up to 1000 bending deformations is estimated by determining the hysteresis and sensitivity of the flexible sensor after each set of deformations. The successful development of a flexible sensor for optical monitoring of humidity in a wide humidity range is demonstrated and discussed.

Two-dimensional (2D) materials have been intensively studied due to their outstanding chemical, physical, electronic and optical properties potentially offering a breakthrough in the search of a direct integration of these materials with the silicon (Si) technology. Among them, platinum diselenide (PtSe2) is of significant interest for applications in electronics, spintronics, sensors, catalysis, etc. In this paper, we report a large-scale synthesis of PtSe2-layers by thermally-assisted selenization of pre-deposited platinum films in a horizontal quartz-tube chemical vapor deposition (CVD) reactor. The overall structural information and quality of the PtSe2-films is analyzed by Raman spectroscopy and Raman active modes identification. The electrical characterization via V-I characteristics reveals sheet resistance values in the range of ~103 Ω.

Rapid advances in material science and processing methods over the past decade accelerated the development of flexible and stretchable sensors for various innovative applications in healthcare, medicine, biology and environment. Such devices have the potential to be applied to soft surfaces of different shape like textile fabrics or human skin. Flexible substrates allow continuous roll processing, thus providing the potential for efficient and low cost mass production. In this paper a flexible humidity sensor is prepared by spin-coating of moisture-sensitive polymer on three types of substrates: poly(ethylene terephthalate) (PET), polylactide (PLA) and composite polysiloxane. The optical properties, surface morphology and roughness of the substrates are studied by transmittance measurements and surface profiling, respectively. Thin spin-coated polymer films of amphiphilic copolymer obtained by partial acetalization of poly(vinyl alcohol) are used as humidity sensitive media. In order to increase the sensitivity a thin metal layer of Au-Pd is incorporated between the polymer film and substrate. The thickness of metal layer is optimized in order the transmittance level of the sensor to be in the range 40-50%. The sensing properties are probed through transmittance measurements at different levels of relative humidity (RH). The influence of substrate type is studied by comparing hysteresis and sensitivity of flexible sensors with those deposited on glass substrate. The successful application of the designed flexible humidity sensors is demonstrated and discussed.

Present study aims at development of thin films of sol–gel Ta2O5 with tunable refractive index that could be used as an active medium for volatile organic compound (VOCs) optical detection. In order to tune the refractive index, mesoporosity is introduced through soft templating approach where series of co-polymers (Pluronics) are utilized as structure-directing agents. Thin films are prepared by spin-coating method using tantalum sol synthesized from TaCl5 as a precursor and ethanol as a solvent. The temperature annealing at 320 °C finalizes the tantalum oxide lattice formation and decomposes the template being used. The morphology and elemental composition of the films are studied by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy in STEM, respectively, and the amorphous status of the films is confirmed by selected area electron diffraction. Non-linear curve fitting method and non-destructive UV–Vis reflectance measurements are used for determination of optical parameters of the films. The free volume fraction in the films is estimated through modeling of refractive index by Bruggeman effective medium approximation. The films are exposed to acetone vapors as a commonly studied VOC. The reaction of the film due to vapors’ exposure is recorded as a change in the reflection spectra of the film and the adsorbed acetone quantity is estimated. The influence of template composition on optical and sensing properties of Ta2O5 films is revealed and discussed.

In this study, solid waste from coal combustion in thermal power plants (TPPs) was used for the synthesis of zeolite Na-X samples. They were prepared by the long-term alkaline atmospheric conversion of coal ash collected from the electrostatic precipitators in the TPP “AES Galabovo”. When used in the form of thin films/layers, the optical detection of volatile organic compounds (VOCs) is possible due to a change in their reflectance spectra and color. In order to improve the sensing properties of synthesized zeolites, they were wet milled for 60 s and both milled and unmilled zeolites were used as dopants for the niobium oxide matrix in the form of thin films deposited by the spin-coating method on a silicon substrate. The surface morphology and structure of both zeolite powders were studied by scanning electron microscopy, while their size was determined by dynamic light scattering (DLS) spectra. Optical constants (refractive index, n, and extinction coefficient, k) and the thickness of the films were calculated from reflectance measurements. The change in the reflection coefficient ∆R of the films was determined from measured reflectance spectra prior to and after exposure to probe acetone molecules. An increase in the reaction of the films with milled zeolites to acetone, compared to the samples with unmilled zeolites, is demonstrated.

A possible approach for enhancement of Poly(vinyl alcohol) (PVA) humidity-sensing performance using hydrophobically modified PVA copolymers is studied. Series of poly(vinylalcohol-co-vinylacetal)s (PVA&ndash, Ac) of acetal content in the range 18%&ndash, 28% are synthesized by partial acetalization of hydroxyl groups of PVA with acetaldehyde and thin films are deposited by spin-coating using silicon substrates and glass substrates covered with Au&ndash, Pd thin film with thickness of 30 nm. Sensing properties are probed through reflectance measurements at relative humidity (RH) in the range 5%&ndash, 95% RH. The influence of film thickness, post-deposition annealing temperature, and substrate type/configuration on hysteresis, sensitivity, and accuracy/resolution of humidity sensing is studied for partially acetalized PVA copolymer films, and comparison with neat PVA is made. Enhancement of sensing behavior through preparation of polymer&ndash, silica hybrids is demonstrated. The possibility of color sensing is discussed.

Thin, homogeneous ZrO2 films were obtained by spin coating method from ZrOCl2.8H2O solution, modified with polyethylene glycol (PEG, Mw = 400). The films have thickness of 80 nm and refractive index of about 1.45, which varied with the amount of added PEG. The thermal behaviour of the precursor was studied with thermogravimetry and differential thermal analysis (TG-DTA). The X-ray diffraction (XRD) analysis revealed the presence of a mixture of monoclinic and tetragonal ZrO2 polycrystalline phases with nanosized crystallites. The formation of hydrogen bonds among the organic and inorganic components was proved by means of fourier transform infrared spectroscopy (FT-IR) analysis, while the different defect centers were investigated with electron paramagnetic resonance (EPR) spectroscopy. The scanning electron microscopy (SEM) images showed that the samples are dense, crack free with ganglia-like nanostructure. It was established that the addition of polymer resulted in the introduction of free volume in the films, which also varied with the content of PEG in the precursor solution.

These works present the electrochemical deposition of ZnO nanostructured thin films on glass substrates covered by SnO2 thin films. Nanostructured ZnO films are obtained by an electrochemical process using a three-electrode system with a saturated calomel electrode as reference electrode, in aqueous solution containing ZnCl2 and KCl. The influence of ZnCl2 concentration on the structural properties of the obtained ZnO layers is investigated by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and optical profilometry. The SEM micrographs show that the ZnO films consist of nanograins at lower ZnCl2 concentrations that transform in a shape of walls at the highest one. The XRD spectra demonstrate the polycrystalline nature of the films at all studied concentrations with the presence of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) and (1 0 3) characteristic reflexes of the ZnO. The influence of the ZnCl2 concentration on the transmittance and reflectance spectra and haze ratio of ZnO thin films is presented and discussed. The ZnO nanostructured layers exhibit high values of the diffuse transmission and reflection in the 400–900 nm spectral range; they can be applied as rear contacts of thin films solar cells thus increasing the light trapping.

Coupler comprising side-polished optical fiber and tantalum pentoxide (Ta2O5) waveguide coated with 330 nm hydrophobic MFI-type zeolite films was utilized as chemical sensor for acetone vapors. The thickness of the zeolite films was selected through theoretical modeling of the resonant wavelength shift in order to achieve the highest possible sensitivity. The spectral positions of the resonances were determined through solving of the characteristics equations for the coupled waveguides and finding their mode effective refractive indices. The optical quality of the coupler surface, that is responsible for the proper operation of the device, was controlled through 3D optical imaging prior to and after deposition of the zeolite films. The application of side-polished optical fiber–Ta2O5 waveguide coupler covered with hydrophobic zeolite film for acetone vapor sensing was demonstrated and discussed.

Vapor responsive all Nb2O5 Bragg stacks were prepared using spin-coating of alternating dense and porous films with quarter-wavelength thickness. The required porosity in the films was achieved by soft-templating method using micellar solution of Pluronic copolymer as an organic template. The surface morphology and structure of the films were studied by transmission electron microscopy and selected area electron diffraction, respectively, while optical characterization was performed by combination of ellipsometric and reflectance measurements. Vapor sensing response of the films was tested by reflectance measurements of the samples prior to and after exposure to acetone vapors selected as probe molecules. Incorporation of “defect” layer in the multilayered structures was firstly simulated theoretically and then realized experimentally aiming at improvement of the optical response to volatile organic compounds. Furthermore, the potential of all niobia stacks consisting of alternating dense and porous sol–gel Nb2O5 films as chemical sensors with optical read-out is demonstrated and discussed.

Optical sensors fabricated in zeolite nanoparticle composite films rely on changes in their optical properties (refractive index, n, and thickness, d) to produce a measurable response in the presence of a target analyte. Here, ellipsometry is used to characterize the changes in optical properties of Linde Type L (LTL) zeolite thin films in the presence of Cu2+ ions in solution, with a view to improving the design of optical sensors that involve the change of n and/or d due to the adsorption of Cu2+ ions. The suitability of two different ellipsometry techniques (single wavelength and spectroscopic) for the evaluation of changes in n and d of both undoped and zeolite-doped films during exposure to water and Cu2+-containing solutions was investigated. The influence of pre-immersion thermal treatment conditions on sensor response was also studied. Due to the high temporal resolution, single wavelength ellipsometry facilitated the identification of a Cu2+ concentration response immediately after Cu2+ introduction, indicating that the single wavelength technique is suitable for dynamic studies of sensor&ndash, analyte interactions over short time scales. In comparison, spectroscopic ellipsometry produced a robust analysis of absolute changes in film n and d, as well as yielding insight into the net influence of competing and simultaneous changes in n and d inside the zeolite-doped films arising due to water adsorption and the ion exchange of potassium (K+) cations by copper (Cu2+).

Thin films with nanometer thicknesses in the range 100−400 nm are prepared from double hydrophilic copolymers of complex branched structures containing poly(N,N-dimethyl acrylamide) and poly(ethylene oxide) blocks and are used as humidity sensitive media. Instead of using glass or opaque wafer for substrates, polymer thin films are deposited on Bragg stacks and thin (30 nm) sputtered Au−Pd films thus bringing color for the colorless polymer/glass system and enabling transmittance measurements for humidity sensing. All samples are characterized by transmittance measurements at different humidity levels in the range from 5% to 90% relative humidity. Additionally, the humidity induced color change is studied by calculating the color coordinates at different relative humidity using measured spectra of transmittance or reflectance. A special attention is paid to the selection of wavelength(s) of measurements and discriminating between different humidity levels when sensing is performed by measuring transmittance at fixed wavelengths. The influence of initial film thickness, sensor architecture, and measuring configuration on sensitivity is studied. The potential and advantages of using top covered Bragg stacks and polymer/metal thin film structures as humidity sensors with simple optical read-outs are demonstrated and discussed.

This Special Issue is devoted on design and application of thin films and structures with special emphasis on optical applications. It comprises ten papers, five featured and five regular papers, authored by respective scientists all over the world. Diverse materials are studied and their possible applications are demonstrated and discussed: transparent conductive coatings and structures from ZnO doped with Al and Ga and Ti-doped SnO2, polymer and nanosized zeolite thin films for optical sensing, TiO2 with linear and non-linear optical properties, organic diamagnetic materials, broadband optical coatings, CrWN glass molding coatings and silicon on insulator waveguides.

Fly ash (FA) from lignite coal combusted in different Thermal Power Plants (TPPs) was used for the synthesis of zeolites (FAZs) of the Na-X type by alkaline activation via three laboratory procedures. FAZs were characterized with respect to their morphology, phase composition and surface properties, which predetermine their suitability for applications as catalysts and adsorbents. FAZs were subsequently modified with metal oxides (CuO) to improve their catalytic properties. The catalytic activity of non-modified and CuO-modified FAZs in the total oxidation of volatile organic compounds was investigated. FAZs were studied for their potential to retain CO2, as their favorable surface characteristics and the presence of iron oxides make them suitable for carbon capture technologies. Thin films of FAZs were deposited by in situ crystallization, and investigated for their morphology and optical sensitivity when exposed to pollutants in the gas phase, e.g., acetone. This study contributes to the development of novel technological solutions for the smart and valuable utilization of FA in the context of the circular economy and green energy production.

We present a study of the birefringence photoinduced in thin nanocomposite films of an azopolymer (PAZO) doped with different concentrations of nanoparticles (NPs) of ZnO with spherical shape and size in the range 40 – 100 nm. The nanocomposite thin films were prepared by electrospray deposition of ZnO over a thin azopolymer film with a thickness below 200 nm. The different NPs concentrations were obtained through different electrospray deposition times varied from 0 to 10 minutes. Birefringence was induced by a He-Cd laser at a wavelength of 442 nm and measured by a probe diode pumped solid state (DPSS) laser at 635 nm. The birefringence dependence on the ZnO NP deposition time reached its maximum for a deposition time of 90 s.

The optical and humidity-sensing properties are studied of hydrophobically modified PVA, namely poly(vinyl alcohol-co-vinyl acetal)s of varied copolymer composition. Copolymers are synthesized by reacting PVA with acetaldehyde in aqueous solution, thus introducing cyclic acetal functionalities in the polymer chain. Thin polymer films are deposited by spin-coating and their optical properties are studied as a function of the copolymer composition, i.e., degree of acetalization. Reflectance measurements at different relative humidities in the range 5 – 95 % RH are conducted in order to probe the sensing behavior. The comparison with neat PVA thin films confirms the influence of the copolymer’s acetal fraction on the optical and sensing properties and degree of hysteresis. The feasibility of applying poly(vinyl alcohol-co-vinyl acetal) thin films for optical sensing of humidity is demonstrated and discussed.

Bragg reflectors are multi-layered systems of sequentially deposited low and high refractive index materials with quarter-wavelength optical thicknesses exhibiting structural colors due to the presence of high reflection band in the visible spectral range. If Bragg reflector is covered with film sensitive to external stimuli then the change in the environment could be easily detected by monitoring the change of Bragg reflector’s transmittance or reflectance spectra. In the present study as a sensitive material we use linearly branched copolymer containing poly(N,N-dimethyl acrylamide) and Poly(ethylene oxide) blocks deposited in the form of thin films by spin-coating method. The required thickness is pre-optimized through theoretical modelling in order to achieve the highest sensitivity. Two types of Bragg reflectors, having different optical contrast and operating wavelengths are implemented as transducers of humidity changes. The first one consists of sequentially deposited SiO2 and Nb2O5 films prepared by sol-gel and spin-coating methods. The second one comprises alternating dense and porous Nb2O5, the last prepared with addition of organic template. Single films and Bragg reflectors are characterized by transmittance measurements at different humidity levels in the range from 5 to 95 % relative humidity (RH). The influence of number of the layers in the stack, the operating wavelength and optical contrast on the sensitivity is studied. The potential and advantages of using top covered Bragg reflector as humidity sensor with simple optical read-out are demonstrated and discussed. Acknowledgments: The financial support of Bulgarian National Science Fund, grant number DN08-15/14.12.2016 is highly appreciated. R. Georgiev acknowledges World Federation of Scientists for fellowship and project DFNP-17-97/28.07.2017 of the Program for career development of young scientists.

In the current study triblock copolymers (Pluronics) consisting of central poly(propylene oxide) (PPO) block flanked by two poly(ethylene oxide) (PEO) chains are used as organic templates for generating mesoporosity in thin Nb2O5 films. In order to optimize the polymer framework within the Nb2O5 films, two copolymers with different chemical compositions and properties are used, namely Pluronic PE 6200 and PE 9400. The copolymers’ micelle formation is studied by Dynamic Light Scattering (DLS) measurements of the copolymer aqueous solutions at different temperatures and concentrations and by Transmission Electron Microscopy (TEM). Thin films of mesoporous Nb2O5 films are deposited by sol-gel and spin-coating methods and characterized by TEM, UV-VIS reflectance spectroscopy and non-linear fitting for optical properties determination. The porosity of the films is calculated using effective medium approach on the basis of dispersion curves of the calculated refractive indices. The development of porosity in the films is studied as a function of the duration of the annealing. Besides, the sensing properties of mesostructured films are tested with vapors of acetone as a probe molecule of VOC’s (Volatile Organic Compounds) by reflectance measurements prior to and after exposure to vapors. The films optical response is demonstrated and possible application as optical indicator for VOC’s is discussed.

Aluminum-doped zinc oxide (AZO) layers are prepared by using atomic layer deposition technique. The obtained layers possess high optical transmittance at visible and near-infrared spectral range. By varying the Al content, optimal growth compositions of AZO are established where the electrical conductivity of the film is highest. Based on performed optical and electrical characteristics measurements, selected AZO films with optimal conductivity are implemented as transparent electrodes in liquid crystal display devices. The electro-optical modulation characteristics of these devices are found to be comparable to those of devices using commercial ITO electrodes.

The generation of porosity in Nb2O5 sol–gel films through introduction of polymer micelles of commercially available copolymer Pluronic PE6800 [poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)] was studied using direct observation of film morphology by Transmission Electron Microscopy and indirect calculations of free volume fraction by optical means. Mesoporous Nb2O5 films were deposited by the sol–gel and spin-coating methods and templated with aqueous solutions of Pluronic PE6800 with concentrations below and above critical micelle concentration determined by the modified dye solubilization method. The influence of the duration of the postdeposition annealing on total pore volumes was investigated by reflectance measurements and consequent calculation of optical properties and free volume fraction by Bruggeman effective medium theory. The possible application of the films in optical sensing of vapors has been demonstrated through reflectance measurements prior to and after acetone vapors exposure.

The sol-gel and spin-coating methods were used for deposition of thin transparent V2O5 films on optical glass substrates and silicon wafers. Different synthesis and deposition conditions, including synthesis temperatures and post-deposition annealing, were used aiming at obtaining transparent films with high refractive index and good optical quality. The surface morphology and structure of the films were studied by SEM and XRD. The optical properties (refractive index, extinction coefficient and optical band gap) and thickness of the V2O5 films were determined from their transmittance and reflectance spectra. The potential application of the films as building blocks of optical sensors was demonstrated by preparation of multilayered structures comprising both V2O5 and BEA-type zeolite films and testing their response towards acetone vapors.

The possibility for color sensing of humidity at room temperature using thin films from hydrophilic cationic block copolymers of different compositions and thickness is demonstrated. Diblock copolymers of composition poly [(ethylene oxide)-b-(N, N-dimethyl acrylamide)] (PEO-b-PDMAA), the PDMAA block being modified with different amounts of quaternary ammonium groups, are synthesized via redox copolymerization in aqueous media. Silicon wafers and Bragg stacks are used as substrates for spin-coating of thin polymers films with thickness in the range 50–200 nm. The color change is quantified through calculation of CIE tristimulus color coordinates using reflectance spectra of the films measured at relative humidity (RH) of 5% and 97%. The influence offilm thickness, substrate type and chemical modification on the humidity sensitivity and color change of the films is demonstrated and discussed.

High-grade zeolite nanocomposites are synthesized utilizing solid by-products from combustion of coal for energy production in Thermal Power Plants applying alkaline aging, hydrothermal and fusion-hydrothermal activation procedures. The obtained coal ash zeolites were studied with respect to their chemical and phase composition, morphology, surface parameters and thermal properties. It was found that they are distinguished in nanocrystalline morphology and significant content of iron oxide nanoparticles (γ-Fe2O3, α-Fe2O3, γ-Fe3O4) and doping elements (Cu, Co, Mn, V, W, etc.) transferred from the raw coal ash, and therefore they are assumed as nanocomposites. Coal fly ash zeolite nanocomposites are characterized by a mixed micro-mesoporous texture, significant concentration of acidic Brønsted centers due to their high surface insaturation, high chemical and thermal stabilty. This unique combination of compositional and textural properties predetermines the application of these materials as catalysts for thermal oxidation processes, anticorrosion barrier coatings, carbon capture adsorbents, matrices for hosting functional groups, detergents etc. Examples for coal fly ash zeolite applications for substitution of critical raw materials in practice are provided.

The optical properties of thin Nb2O5 films, obtained by spin coating of Nb sol on glass and silicon substrates are investigated. The Nb sol was prepared by a sonocatalytic method using niobium chloride dissolved in ethanol. A calculation procedure for reliable and unambiguous determination of optical constants (refractive index and extinction coefficient) and thickness of films is developed that uses transmittance and reflectance measurements and non-linear curve fitting. By means of the developed procedure, the impacts of the sol ageing and the post deposition annealing on the optical properties and thickness of the films are investigated. It is demonstrated that films with controlled thicknesses and refractive index values in the range from 1.82 to 2.20 (at wavelength of 600 nm) could be obtained using different concentrations of the Nb sol and post deposition annealing in the range 60–650 °C. The possibility of fabrication of single layer anti-reflective coating for silicon is discussed.

The fabrication and characterization of one-dimensional photonic crystals (1D-PC) in the form of quarter-wavelength multilayered stack (Bragg stack) are presented. The 1D-PC comprises alternatively deposited spin-coated polymer PMMA (Poly(methyl methacrylate)) films as low-refractive index material that change thickness upon exposure to chloroform vapors, and both dense and porous V2O5 films providing both high optical contrast. The possibility of using PMMA–V2O5 structure as an omnidirectional reflector in both cases is confirmed experimentally. Besides, the prospect of utilizing the studied structures as optical indicators of chloroform vapors is demonstrated and discussed.

The influence of the macromolecular architecture of block copolymers containing poly(N,N-dimethyl acrylamide) (PDMA) on the optical characteristics and sensing properties of corresponding thin films is discussed. Series of hydrophilic PDMA-based copolymers of different chemical composition and chain architecture such as triblock, star-shaped, and branched were synthesized. The copolymers were characterized using conventional spectroscopic techniques as well as methods for characterization of copolymer macromolecular characteristics in solution, namely size-exclusion chromatography and static light scattering. Thin films of the copolymers of nanometer scale thickness were deposited on silicon substrates by the spin-coating method. The refractive index and extinction coefficient of the copolymer films were calculated from the reflectance spectra by using non-linear curve fitting methods and the composition-structure-optical properties relationships were evaluated. Humidity-sensing properties of the films were studied by measuring reflectance spectra of the films at a relative humidity range from 5 to 95%RH. The implementation of the copolymer films as optical sensors of humidity is justified and discussed.