Inorganic Oxide Films Research Articles

Wide-field optical imaging of electrical charge and chemical reactions at the solid–liquid interface

From molecules and particles to macroscopic surfaces immersed in fluids, chemical reactions often endow interfaces with electrical charge which in turn governs surface interactions and interfacial phenomena. The ability to measure the electrical properties of a material immersed in any solvent, as well as to monitor the spatial heterogeneity and temporal variation thereof, has been a long-standing challenge. Here, we describe an optical microscopy-based approach to probe the surface charge distribution of a range of materials, including inorganic oxide, polymer, and polyelectrolyte films, in contact with a fluid. The method relies on optical visualization of the electrical repulsion between diffusing charged probe molecules and the unknown surface to be characterized. Rapid image-based measurements enable us to further determine isoelectric points of the material as well as properties of its ionizable chemical groups. We further demonstrate the ability to optically monitor chemically triggered surface charge changes with millisecond time resolution. Finally, we present a scanning-surface probe technique capable of diffraction-limited imaging of spatial heterogeneities in chemical composition and charge over large areas. This technique will enable facile characterization of the solid-liquid interface with wide-ranging relevance across application areas from biology to engineering.

Freestanding inorganic oxide films for flexible electronics

Recently, flexible electronic devices are of increasing interest due to their wide range of application fields, including information storage, energy conversion, and wearable and implantable electronics. In particular, freestanding inorganic oxide films are proved to be an extraordinary versatile platform for flexible electronics owing to their super elasticity, outstanding functionalities, tunability, and long-term stability. In this Perspective, we review the up-to-date advances of freestanding inorganic oxide films from the perspectives of synthesis methods, physical properties, and device applications. First, preparation strategies based on epitaxial lift-off technologies are classified into physical and chemical aspects that are to be introduced. Second, we discuss the physical properties of freestanding inorganic oxide films, especially in terms of ferroelectricity, magnetism, multiferroics, etc. Third, we highlight several device applications in the fields of data memory, energy storage, and health care. Finally, we conclude with a future perspective into prospects and challenges regarding the syntheses and applications of freestanding inorganic oxide films.

Reversal of triboelectric charges on sol–gel oxide films annealed at different temperatures

Using Kelvin probe force microscopy, we studied the contact-electrification (CE) induced charge density on the surfaces of organic–inorganic composite oxide films, such as Nb2O5, ZrO2, and HfO2 films, prepared by a sol–gel method and annealed at different temperatures. The results show that positive triboelectric charges on the film surface gradually turn into negative ones with the increase in the annealing temperature. This phenomenon is attributed to the broken valence bonds between organics and organic–inorganic composites in the film at high temperature, which causes oxygen vacancy defects and consequently changes the surface states of the film. A surface states model is established to understand the process of CE, which further confirms that electron transfer is the dominant mechanism of CE. Moreover, it indicates that in addition to oxygen vacancies, there are also many other factors that can influence CE, including but not limited to non-stoichiometric ratios, impurities, and other types of defects. These factors will modify the surface states through changing the potential barrier energy of the bound electrons on the surface and thus impact the amount and even the direction of electron transfer.

Flexible and Rewritable Non‐Volatile Photomemory Based on Inorganic Lanthanide‐Doped Photochromic Thin Films

AbstractEmerging wearable electronics are spurring unprecedented enthusiasm in the pursuit of next‐generation memory realizable on flexible substrates. Photochromic materials enable reversible manipulation and possess erasable/rewritable capability, which ensure them to be adequate candidates as optical memories. Nevertheless, it has been challenging to develop a flexible optical memory with organic photochromic elements that simultaneously meets the stringent thermo‐ and photostable requirements. Inorganic lanthanide Er3+‐doped bismuth layer‐structure ferroelectric Na0.5Bi2.5Nb2O9 (NBN:Er) exhibits superior photochromic performance. The coupling between lanthanide upconversion emission and photochromic effect enables rewritable and nondestructive readout characteristics. Low‐dimensional complex oxide materials have superior mechanical properties and can be subject to large strain by integration with flexible substrates. Recent advances in synthesizing high‐quality inorganic oxide films on flexible substrates provide new opportunities to build flexible optical memories with dramatically higher performance. Herein, NBN:Er thin films have been integrated with flexible polyimide substrates. These hybrid heterostructures demonstrate comprehensive robust characteristics that sustain well during the 105 bending cycles, and maintain stability over many write–read–erase cycles. This work addresses the main problems hampering the promising applications of inorganic photochromic materials and paves the way for developing more flexible and compact optical memories.

Study of Polyoxometalates Based Films Formation with High Gas Sensitivity

Introduction Inorganic oxide films activated with different additives are the unique sensitive layers for working at room temperature sensors. The sensor behavior of nanostructured activated silica films produced by sol-gel route was examined for environment control. Polyoxometalates (POMs) of Dawson type were used as additives in the silica matrix for gas sensitivity. POMs have good electronic conductivity and chemical stability. These compounds can act as reversible reagents in the reactions of ligand exchange and redox transformations, accompanied by changes in current carrier concentration or catalyze of such transformations that can be used in chemical sensors. Their redox properties can be varied within broad limits due to changing the heteropolyanion composition [1-4]. Porous silica is a unique matrix for chemical sensors, which has a lot of excellent physical and chemical properties [5]. Method The sensor’s interdigital structure was made by microelectronic planar technology as shown in figure 1. The active area of the sensor was 4 x 4 mm. The thickness of the electrodes was 0,2 mm. The width of finger pairs of electrodes and distance between them were 20 μm. The method of hydrolythic polycondensation from tetraethyl orthosilicate solutions was used for forming complex silica films in this work. Films were deposited by a centrifugation from solutions based on tetraethyl orthosilicate and polyoxometalate. POMs with various compositions have been incorporated into the initial solution. The developed technology of multistep temperature treatment up to 413 K allowed film properties stabilizing.The conditions of film formation varied during the tests: solution composition, temperature, and application modes. Morphology and infrared spectra of the layers were studied using Solver-P47 scanning probe microscope (NT-MDT, Russia) and spectrometer Bruker EQUINOX 55 (Germany). Standard gas mixtures (MGPZ, Russia) and Environics – 4000 gas-mixture generator (Environics, USA) were used to measure the properties of sensors. Electrical parameters of the sensors have been measured in real-time mode. Environment influence on sol preparation and films forming was investigated. Results and Conclusions Films had thickness of 200-300 nm and nanoscale pores. The metrological parameters of sensors depend mainly on different parameters: synthesis conditions and chemical content of sensitive film, temperature of the environment, annealing temperature, storage conditions and duration after heat treatment. Temperature treatment and decreasing of environment humidity lead to acceleration of hydrolythic polycondensation of solutions and stabilization of sensitive films at right conditions as demonstrated in figure 2. Introduction of heteropolyanion into film resulted in film’s mass stabilization. The temperature treatment don’t disturb POM molecule in the film. The polyoxometalates didn’t display their catalytic activity at weight content less than 10 % in the film. The increase in conductivity is observed due to an increase in the concentration of charge carriers on the surface of the film, donors of which are introduced into the film POM molecules. And gas sensitivity maximum decreases at weight content more than 50-60 % in matrix what is explained apparently by the formation of crystalline aggregates POM in the volume of the film. Besides the humidity adsorption on the film’s surface contribute to the sensor response. Adding of Mn3P2Mo18O62 into silica film resulted in high sensitivity to ethanolamine (up to 10 times) more than to ammonia as shown in figure 3. Obviously, this is explained by the reaction of ethanolamine complexation with Mn ions, among the possible valence states of which the strongest complex with ethanolamine molecules is formed by Mn3+. An equilibrium shift towards Mn3+ and a decrease in the redox potential of the Mn3+/Mn2+ pair leads to an intramolecular reduction reaction of the heteropolyanion. This interaction is quick (less than 30 s) and reversible at room temperature.

Controlling Chemical Selectivity in Electrocatalysis with Chiral CuO-Coated Electrodes

This work demonstrates the chiral-induced spin selectivity effect for inorganic copper oxide films and exploits it to enhance the chemical selectivity in electrocatalytic water splitting. Chiral CuO films are electrodeposited on a polycrystalline Au substrate, and their spin filtering effect on electrons is demonstrated using Mott polarimetry analysis of photoelectrons. CuO is known to act as an electrocatalyst for the oxygen evolution reaction; however, it also generates side products such as H2O2. We show that chiral CuO is selective for O2; H2O2 generation is strongly suppressed on chiral CuO but is present with achiral CuO. The selectivity is rationalized in terms of the electron spin-filtering properties of the chiral CuO and the spin constraints for the generation of triplet oxygen. These findings represent an important step toward the development of all-inorganic chiral materials for electron spin filtering and the creation of efficient, spin-selective (photo)electrocatalysts for water splitting.

Antioxidant migration resistance of SiOx layer in SiOx/PLA coated film

ABSTRACTAs novel materials for food contact packaging, inorganic silicon oxide (SiOx) films are high barrier property materials that have been developed rapidly and have attracted the attention of many manufacturers. For the safe use of SiOx films for food packaging it is vital to study the interaction between SiOx layers and food contaminants, as well as the function of a SiOx barrier layer in antioxidant migration resistance. In this study, we deposited a SiOx layer on polylactic acid (PLA)-based films to prepare SiOx/PLA coated films by plasma-enhanced chemical vapour deposition. Additionally, we compared PLA-based films and SiOx/PLA coated films in terms of the migration of different antioxidants (e.g. t-butylhydroquinone [TBHQ], butylated hydroxyanisole [BHA], and butylated hydroxytoluene [BHT]) via specific migration experiments and then investigated the effects of a SiOx layer on antioxidant migration under different conditions. The results indicate that antioxidant migration from SiOx/PLA coated films is similar to that for PLA-based films: with increase of temperature, decrease of food simulant polarity, and increase of single-sided contact time, the antioxidant migration rate and amount in SiOx/PLA coated films increase. The SiOx barrier layer significantly reduced the amount of migration of antioxidants with small and similar molecular weights and similar physical and chemical properties, while the degree of migration blocking was not significantly different among the studied antioxidants. However, the migration was affected by temperature and food simulant. Depending on the food simulants considered, the migration amount in SiOx/PLA coated films was reduced compared with that in PLA-based films by 42–46%, 44–47%, and 44–46% for TBHQ, BHA, and BHT, respectively.

Enhanced Photoelectric Properties in Dye-Sensitized Solar Cells Using TiO2 Pyramid Arrays

In this study, the improved device performance obtained by introducing pyramid-structured photoanodes has been investigated. The periodical pyramid arrays in the TiO2 active layer are fabricated by soft imprint lithography. The pyramid-structured photoanodes are characterized by scanning electron microscope, UV–vis spectroscopy, and electrochemical impedance spectroscopy. The experimental results demonstrate that pyramid-structured photoanodes can reduce the transmission light loss over a broad wavelength region and result in increased light reflection and scattering effect. Hence, dye-sensitized solar cells with periodical pyramid arrays can increase the light absorption in the active layer and enhance the photovoltaic performance. The overall efficiency of such structured devices is increased by 20% in comparison with the device without periodical pyramid arrays and scattering layers. Fabrication of periodical pyramid arrays of mesoporous inorganic oxide films also can be applied for other various devic...

Artificial reticular structure by continuous titanium oxide frameworks

A series of macroporous titania films were fabricated using spherical soft colloids of polystyrene-block-poly(oxyethylene) (PS-b-PEO) and the macrostructure was systematically varied from reticular to spherical and large spherical according to the synthetic conditions. This macroreticular structure, which has never been organized by templating with organic assemblies/aggregates to date, was obtained with the controlled condensation of oxide frameworks in the presence of the soft PS-b-PEO colloids. The formation of reticular type pores was very sensitive to the synthetic conditions, which were optimized one-by-one for improving this unique colloidal PS-b-PEO templating approach for the precise structural design of inorganic oxide film as a more extendable method. Such spaces in the artificial reticular structure by continuous titania frameworks were designed in the range of macroscale to mesoscale. Accordingly, a new family of reticular type porous materials, which are suitable as porous platforms with very high surface area and allow smooth/rapid diffusion of targeted molecules, will be provided by this colloidal templating approach of organic block copolymer combined with the controlled condensation of frameworks for a variety of compositions.

Solar Cells: Hybrid Templated Synthesis of Crack‐Free, Organized Mesoporous TiO2 Electrodes for High Efficiency Solid‐State Dye‐Sensitized Solar Cells (Adv. Funct. Mater. 1/2013)

A solid-state dye-sensitized solar cell with 7.3% efficiency is fabricated using crack-free, organized, mesoporous TiO2 films with high surface areas and good interconnectivity. On page 26, Jong Hak Kim and co-workers demonstrate the role of the hybrid template, i.e., alumina nanoparticles grafted with a hydrophilic polymer. This template can be used for the preparation of various mesoporous inorganic oxide films.

Formation of mesostructured thin films at the air–liquid interface

The growth of free-standing surfactant-templated films of inorganic oxides at the air-solution interface is an attractive route to manufacture unsupported mesostructured membranes for a range of potential applications. So far this synthesis method has been relatively neglected due to the fragility of the initial films. More recent work to understand the mechanism of formation has led to development of thicker, more robust films, as well as providing new information on the general formation mechanisms of mesoporous materials whether in film or particulate form. This review describes the properties of silica and other inorganic oxide films templated by surfactants and grown at the air-solution interface, their formation mechanisms and implications for further development of these materials.

Macroporous Oxide Platforms Templated by Non‐Close‐Packed Spherical Copolymer Aggregates

Exclusive organic templating of macroporous oxide films is reported by using non-close and lose packing of spherical copolymer aggregates, in combination with facile control of condensation degree/density of inorganic oxide frameworks. Unique macroporous oxide films, mainly titania showing highly porous, crystalline, and versatile properties, can be fabricated with continuous design from unusual 3-D net-shape to tunable spherical macrostructures, which expands the preparation of other inorganic oxide films (silica, alumina, and zirconia) and possibly adapts the use of other assembled organic polymers. The macroporous structures are helpful for effective accommodation of bulky biomoleculeshigh and diffusivity of organic molecules (useful for photocatalysts). Unusual structural variation, expansion of spherical voids, is also observed, being useful for fine tuning of optical property.

Surface Dynamics of SiO2‐like Films on Polymers Grown by DBD Assisted CVD at Atmospheric Pressure

AbstractThe development of the morphology and surface roughness of amorphous SiO2‐like films, on various polymeric substrates was analyzed by atomic force microscopy. The inorganic oxide films were deposited from organo‐silicon precursors with different reactive gases using dielectric barrier discharge assisted chemical vapor deposition (DBD‐CVD) at atmospheric pressure in a roll‐to‐roll configuration. Detailed study on the roughness statistical parameters characterizing morphology shows that DBD‐CVD films grow in a conformal and layer‐by‐layer like fashion on polymers very similar to films produced by atomic layer deposition process. Independent of substrate (planarised and non‐planarised polyethylene‐2,6‐napthalate) and methodology used to build the thickness as stacks or single layers, the films were found to be smooth, both locally and globally, and show negligible development of roughness with thickness (≤350 nm). The scaling exponents observed, show that the film growth does not fall into any of the universality classes that have been reported so far.magnified image

Nanopatterning of Mesoporous Inorganic Oxide Films for Efficient Light Harvesting of Dye‐Sensitized Solar Cells

Nanopatterning provides facile process to well-arrayed mesoporous inorganic oxide films at low cost by using readily available pastes and elastomeric nanostamps. The fabricated nanopattern boosted the light-harvesting efficiency of dye-sensitized solar cells (DSSCs) by a light-trapping technique. The iodine-free solid-state DSSCs showed a 40 % increase in the current density and high efficiency (7.03 %).

Nanopatterning of Mesoporous Inorganic Oxide Films for Efficient Light Harvesting of Dye‐Sensitized Solar Cells

Pasten und elastomere Nanostempel werden verwendet, um auf einfache Weise Nanomuster in mesoporösen Filmen aus anorganischen Oxiden zu erzeugen. Die Muster können Licht einfangen und verbessern die Lichtsammeleffizienz in Farbstoffsolarzellen (DSSCs) deutlich. Iodfreie Festkörper-DSSCs zeigten eine um 40 % erhöhte Stromdichte und waren hoch effizient (7.03 %). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

26.3: Development of Nanoporous Anodic Aluminum Oxide (np‐AAO) Thin Template on PET/Ti Flexible Substrate for Flexible Liquid Crystal Display (Flexible‐LCD) Application

AbstractThis study reports implementation and integration of the inorganic nanoporous anodic aluminum oxide (named np‐AAO) film and Liquid crystal (LC) on PET/Ti flexible substrate for flexible liquid crystal display (flexible‐LCD) application. This works is the first time to demonstration of flexible‐LCD with micromachined flexible inorganic nanostructure film. The np‐AAO offers three advantages for flexible‐LCD, (1) the np‐AAO is vertical alignment layer to easily and uniformly align LC molecules, (2) the np‐AAO with vertically nanostructure control in thickness uniformity of LC layer between flexible substrates for flexible‐LCD; and (3) the np‐AAO film is an inorganic, optical, and mechanical materials for flexible‐LCD. In application, the LC with the flexible np‐AAO template for capacity in flexible‐LCD is also demonstrated.

Limitations of dual and complementary inorganic–organic electrochromic device for smart window application and its colorimetric analysis

Inorganic tungsten oxide (WO 3) and organic polyaniline (PANI) films were used as the cathodic and anodic electrodes of an inorganic–organic electrochromic device (IOECD). WO 3 was deposited by sputtering while PANI films were electrodeposited on transparent Indium doped Tin Oxide (ITO) glass substrates, respectively. Optical and electrochemical studies were performed to find the optical attenuation, coloration efficiencies, reversibility and response time separately for the individual films as well as for the assembled IOECD. The change in transmittance of WO 3 film was found to be 63.92%. PANI film showed an optical attenuation of 50.69% while the transmission change through the IOECD was equal to 57%. Colorimetric analysis was done to define the colors in the bleached and colored states in terms of L* ab values and xy chromaticity diagram. Factors limiting the performance of an IOECD were analyzed. It was found that in a charge unbalanced dual complementary IOECD, the coloration efficiency is always less than that of one of the constituent electrodes.

Design and fabrication of compositionally graded inorganic oxide thin films: Mechanical, optical and permeation characteristics

Different types of inorganic oxide films composed of a chemical composition gradient single layer were designed, fabricated and characterized. Compositionally graded thin films were created by power-controlled co-sputtering of alumina (Al2O3) and silica (SiO2) at room temperature, allowing the structural design of the film to be tailored at the nanometer scale. Two distinct graded thin films were fabricated: one with a compositionally asymmetric structure consisting of a SiO2-rich bottom interface and a Al2O3-rich top surface, and the other with a compositionally balanced sandwich structure consisting of both the top surface and bottom interface rich in SiO2 and a core rich in Al2O3 (referred to as SGS for ‘sandwich graded structure’). Smoothly graded thin films without interfacial boundaries were verified by Auger electron spectroscopy profiles. X-ray photoelectron spectroscopy demonstrated that the Al2O3/SiO2 graded structures consisted of Si–O and Al–O bonds, as well as Al–O–Si bonds in the transition layer. Neat Al2O3 or SiO2 and their graded ones were all investigated for their mechanical, optical and permeation properties. A SGS thin film presented the best mechanical stability (i.e., about three times improved film toughness of a neat Al2O3 single layer), demonstrating that balanced internal stresses and alternating bonding structures, achieved via a graded structure without interfaces, are crucial for enhancing mechanical stability. Furthermore, neat and graded thin films exhibited the similar level of optical transmittance and the permeation properties for the graded films were well matched with the behaviors of mechanical stability.

Laser-induced congruent forward transfer of SiO x -layers

Laser-induced forward transfer of inorganic oxide films is demonstrated using the example of SiOx-layers. SiOx with x<2 is strongly UV-absorbing, so that excimer lasers are suited for inducing transfer. When the distance between donor and receiving substrate is reduced to zero, congruent transfer is possible; i.e. the deposited geometry corresponds exactly to the ablated spot. This zero-distance configuration is accomplished by using a polydimethylsiloxane (PDMS) film as the receiving substrate, which is cast directly on the SiOx-coated fused silica donor substrate. After curing of the PDMS, the SiOx-layer is irradiated by a single 25 ns-KrF-excimer laser pulse through the donor substrate. At a fluence of 300 to 600 mJ/cm2 predefined areas of a 830 nm thick SiOx-layer are cleanly transferred to the PDMS-substrate, i.e. the irradiated parts of the layer stick to the PDMS-surface after peeling it off. This way spot arrays or stripes with lateral dimensions of a few μm as well as free-standing grids can be generated. By multipulse exposure, the fabrication of stacks or bridges is possible.

Interface-directed sol-gel: direct fabrication of the covalently attached ultraflat inorganic oxide pattern on functionalized plastics

For plastic electronics and optics, the fabrication of smooth, transparent and stable crack-free inorganic oxide films (and patterning) on flexible polymeric substrates with strong bonding strength and controllable thickness from nanometers to micrometers is a key but still remains a challenge. Among versatile inorganic oxides, silica oxide film as SiO x is especially important because this semiconductor material could provide crucial properties in devices or serve as a base layer for further multilayer construction. In this paper, we describe a new interface-directed sol-gel method to fabricate flexible high quality silicon oxide film onto commodity plastics. The resulting crack-free silica film has strong covalent bonding with polymer substrates, homogeneous morphology with ultralow roughness, highly optical transparency, tunable thickness from nm to μm, and easy patterning ability. Such fabrication strategy relies on a novel photocatalytic oxidation reaction by photosensitive ammonium persulfate (APS), which is able to fabricate highly reactive hydroxyl monolayer surface on inert polymeric substrates. This kind of hydroxylated surface could serve as nucleation and growth sites to initiate surface sol-gel process. As a result, well-defined SiO x film deposition (gelation) occurs, and patterned hydroxylation regions could be easily utilized to induce the formation of patterned oxide film arrays. Our strategy also excludes the requirements of clean room and vacuum devices so as to fulfill low-cost and fast fabrication demands. Two application examples from such high quality SiO x layer onto plastics are given but should not be limited within these. One is that oxygen permeation rate of SiO x deposited polymer film decreases 25 times than pristine polymer substrate, which is good for the potential packaging materials. The other one is that silanization monolayer, for example, 3-aminopropyltriethoxysilane (APTES), could be successfully constructed onto silica layer through classical silanization reaction, which is applicable for many potential purposes, for instance, proteins could be accordingly immobilized onto plastic support with effective signal-to-background ratio. Moreover, we further demonstrate that this interface-directed sol-gel strategy is a general method which could be successfully extended to other high quality oxide film fabrication, e.g., TiO2.