Structure and behavior of ZrO2-graphene-ZrO2 stacks

Producción CientíficaZrO2-graphene-ZrO2 layered structures were built and their crystallinity was characterized before resistive switching measurements. Thin nanocrystalline ZrO2 dielectric films were grown by atomic layer deposition on chemical vapor deposited graphene. Graphene was transferred, prior to the growth of the ZrO2 overlayer, to the ZrO2 film pre-grown on titanium nitride. Nucleation and growth of the top ZrO2 layer was improved after growing an amorphous Al2O3 interface layer on graphene at lowered temperatures. Studies on resistive switching in such structures revealed that the exploitation of graphene interlayers could modify the operational voltage ranges and somewhat increase the ratio between high and low resistance states.Fondo Europeo de Desarrollo Regional (project TK134)Estonian Research Agency (grants PRG753 and PRG4)Ministerio de Economía, Industria y Competitividad (grant TEC2017-84321-C4-2-R

Mittehomogeensete õhukeste kilede optiline iseloomustamine

Väitekirja elektroonilisest versioonist puuduvad publikatsioonid.Töö tegeleb õhukeste väheneelavate kilede optiliste parameetrite määramisega olukorras, kus kile murdumisnäitajat läbi materjali ei saa lugeda konstantseks. Näidatakse, et mõõdetud läbilaskvusspektri alusel on võimalik määrata murdumisnäitaja struktuuri, kirjeldades kilet Lorentzi dispersioonsõltuvusega, kasutades kahest alakihist koosnevat struktuuri ja mitmeparameetrilist globaalset optimeerimist. Seejuures peab ühe alakihi paksus olema väiksem, kui optiline veerandlaine kasutatavas mõõtepiirkonnas. Demonstreeritakse, et sel viisil arvutuslikult saadud struktuurid on korduvad, kui aurustustingimused pole muutunud. Veel parem lähendus saadakse, kui arvesse on võetud ka sujuv üleminek alakihtide vahel, kuid sel juhul pole kindel, et tulemus kirjeldab kihi struktuuri, aga mitte mõõtmisega kaasnevat müra. Vaakumaurustatud kilede jaoks (MgF2, Y2O3 jt.) annab modelleerimine tulemuseks negatiivse mittehomogeensusega struktuuri, mille korral ülemisel, õhuga kokkupuutes oleval alakihil, on väiksem murdumisnäitaja, kui alumisel, alusega kokkupuutuval alakihil. Kumbal pool paikneb õhuke alakiht, pole võimalik määrata spektrist, kuid neid kaht paigutust ja lineaarse mittehomogeensuse juhtu on võimalik eristada, kui võtta juurde kihi füüsilise paksuse mõõtmise tulemus. Aatomkihtsadestatud kilede jaoks on näidatud, et neil on protsessitingimustest sõltuvalt võimalik nii positiivne kui ka negatiivne mittehomogeensus. TiO2 kilede jaoks on tõestatud, et modelleerimisega on võimalik kihi struktuurist informatsiooni saada ka juhul, kui kile paksusest ei piisa interferentsiekstreemumide tekkeks. Kasvuaegse minimaalse kandegaasi voo korral kujuneb TiO2 kihi korral õhuke, väiksema murdumisnäitajaga, alakiht kontaktis sulakvartsist alusega. Läbilaskvusspektri kasutamine annab võimaluse uurida kile struktuuri lihtsamaid vahendeid kasutades. Reaalsete kilede kirjeldamine murdumisnäitaja mittehomogeensust arvestavas lähenduses lubab ka paremini aru saada mitmekihiliste katete käitumisest ja loodetavasti parandada nende omadusi.A work deals with the optical thin films of low absorption in a case where the film refractive index is a function of film thickness. It is shown that a film on a transparent substrate may be modelled based only on its transmission spectrum if Lorentz dispersion model is applied for its dispersion and a structure consisting of two sublayers is used. It is shown a multiparameter optimization yields reproducible results for the films of different thickness. A smooth transition between the two sublayers still improves a merit function for fitting, but it is not clear whether an obtained improvement demonstrates a real structure or simply better representation of measured spectrum including a noise. For vacuum evaporated films (MgF2, Y2O3 etc.) the fitting yields a negative inhomogeneity of a refractive index (higher refractive index in contact with a substrate and lower in contact with ambient) in which one of the sublayers must have an optical thickness less than quarter of the wavelength for near UV region. While a fit merit function may be the same for the two-layer inhomogeneity and a linear inhomogeneity model, measurement of a physical thickness of a film may help to make a choice. It is shown for electron beam evaporated Y2O3 film a thin sublayer is placed in contact with a fused silica substrate. For atomic layer deposited films the both types of two-layer inhomogeneity (negative and positive) are possible. The analysis of the TiO2 films shows film inhomogeneity type may change depending on process carrier gas flow rate or on temperature. Using a multiparameter fitting makes it possible to obtain the results also for the films too thin for using the usual interference analysis methods. For a lowest carrier gas rate a thin sublayer of less index of refraction is shown to have a place in contact with substrate. The results of this work should help to understand a structure of the thin films based on the transmission measurements, which are less demanding in instrumentation. Taking into account film inhomogeneity should also help in constructing of the thin film optical coatings

Electroreduction of oxygen on Nafion®-coated thin platinum films in acid media

Electrocatalytic oxygen reduction reaction (ORR) kinetics on vacuum-evaporated thin platinum films in sulfuric acid solution is thoroughly investigated employing the rotating disc electrode (RDE) method. The nominal thickness of the Pt films is varied from 0.25 to 20 nm and the effect of applied Nafion® layer on the ORR activity in 0.5 M H2SO4 solution is evaluated. The electroactive surface area of Pt and the corresponding overall ORR activity decreases with the nominal film thickness. An increase in the hydrogen peroxide yield and a decrease of the ORR specific activity of Pt with decreasing the film thickness is also observed. The Nafion® coating slightly increases the hydrogen peroxide production, but does not affect the electrocatalytic activity of Pt. The results obtained with the model system employed herein are important for elucidating the influence of Nafion® layer on the ORR kinetics on Pt-based electrocatalysts

Influence of α-Al2O3 Template and Process Parameters on Atomic Layer Deposition and Properties of Thin Films Containing High-Density TiO2 Phases

High-density phases of TiO2, such as rutile and high-pressure TiO2-II, have attracted interest as materials with high dielectric constant and refractive index values, while combinations of TiO2-II with anatase and rutile have been considered promising materials for catalytic applications. In this work, the atomic layer deposition of TiO2 on α-Al2O3 (0 0 0 1) (c-sapphire) was used to grow thin films containing different combinations of TiO2-II, anatase, and rutile, and to investigate the properties of the films. The results obtained demonstrate that in a temperature range of 300–400 °C, where transition from anatase to TiO2-II and rutile growth occurs in the films deposited on c-sapphire, the phase composition and other properties of a film depend significantly on the film thickness and ALD process time parameters. The changes in the phase composition, related to formation of the TiO2-II phase, caused an increase in the density and refractive index, minor narrowing of the optical bandgap, and an increase in the hardness of the films deposited on c-sapphire at TG ≥ 400 °C. These properties, together with high catalytic efficiency of mixed TiO2-II and anatase phases, as reported earlier, make the films promising for application in various functional coatings

Surface-Active Thermally Responsive Hydrogels by Emulsion Sedimentation for Smart Window Applications

Thermally responsive polymers are a subject of increasing interest in research and development as a basis for a potential smart window technology. Here, we present a concept of preparing thermally responsive hydrogels with a thin active surface layer exhibiting rapid and reversible switching of light scattering in the visible and near-infrared spectral ranges. The process relies on the forced emulsion formation and sedimentation from the aqueous prepolymer solution by using a crosslinker that is engineered to serve as an antisolvent for the prepolymer and at the same time exhibit a suitable solubility profile in the sedimented hydrogel layer with respect to the supernatant aqueous phase. While the method can be employed for different polymer and crosslinker systems, as an example, here, we employ this concept for preparing thermally responsive hydrogels based on ethoxylated trimethylolpropane tri(3-mercaptopropionate) (ETTMP) and glycerol-derived crosslinkers with a dimaleate functionality, enabling crosslinking by the thiol-Michael click reaction. The material exhibits a luminous transmittance of over 95% and solar energy modulation of 59.91%. Moreover, we show that the pH and additives in the aqueous operating solution of the hydrogel enable the choice of the transition temperature in a wide range. The unique thin layer on the surface of the hydrogel, scalability to large surface areas, and robust and fast response at the practically relevant temperature range give this material a strong potential for smart window technology applications