Photochemistry in the Low-Temperature Processing of Metal Oxide Thin Films by Solution Methods.

Photochemistry has emerged in the last few years as a powerful tool for the low-temperature processing of metal oxide thin films prepared by solution methods. Today, its implementation into the fabrication procedure makes possible the integration of amorphous semiconductors or functional crystalline oxides into flexible electronic systems at temperatures below 350 °C. In this review, the effects of UV irradiation at the different stages of the chemical solution deposition of metal oxide thin films are presented. These stages include from the synthesis of the precursor solution to the formation of the amorphous metal-oxygen network in the film and its subsequent crystallization into the oxide phase. Photochemical reactions that can be induced in both the solution deposited layer and the irradiation atmosphere are first described, highlighting the role of the potential reactive chemical species formed in the system under irradiation, such as free radicals or oxidizing compounds. Then, the photochemical effects of continuous UV light on the film are shown, focusing on the decomposition of the metal precursors, the condensation and densification of the metal-oxygen network, and the nucleation and growth of the crystalline oxide. All these processes are demonstrated to advance the formation and crystallization of the metal oxide thin film to an earlier stage, which is ultimately translated into a lower temperature range of fabrication. The reduced energy consumption of the process upon decreasing the processing temperature, and the prospect of using light instead of heat in the synthesis of inorganic materials, make photochemistry as a promising technique for a sustainable future ever more needed in our life.

Low-Temperature Solution Crystallization of Nanostructured Oxides and Thin Films.

As an introduction to this themed issue, a critically selected overview of recent progress on the topic of solution methods for the low-temperature crystallization of nanoscale oxide materials is presented. It is focused on the low-temperature solution processing of oxide nanostructures and thin films. Benefits derived from these methods span from minimizing the environmental impact to reducing the fabrication costs. In addition, this topic is regarded as a key objective in the area because it offers a unique opportunity for the use of these materials in areas like flexible electronics, energy conversion and storage, environmental sciences, catalysis, or biomedicine.

Low-temperature crystallization of solution-derived metal oxide thin films assisted by chemical processes.

Over the last few years the efforts devoted to the research on low-temperature processing of metal oxide thin films have increased notably. This has enabled the direct integration of metal oxide layers (amorphous semiconductors) on low-melting-point polymeric substrates for flexible electronic systems, which adds to the economic and environmental benefits of the use of these processes with reduced energy consumption. More challenging is the preparation of crystalline complex oxide films at temperatures compatible with their direct integration in flexible devices. However, the usually high crystallization temperatures (>600 °C) impede the development of devices that take full advantage of the large variety of oxide functionalities available. This tutorial review analyzes a number of strategies based on wet chemical methods for inducing the crystallization of metal oxide thin films at low temperatures. The key mechanisms are explained in relation to the specific step of the fabrication process reached in an earlier stage: the formation of a defect-free, highly densified amorphous metal-oxygen network or the actual crystallization of the metal oxide. The role of photochemistry, where light can be used as a complementary energy source to induce crystallization, is particularly highlighted. This requires the synthesis of novel photosensitive solutions (modified metal alkoxides, charge-transfer metal complexes or structurally designed molecular compounds) and a precise control over the reactions promoted by UV irradiation (photochemical cleavage, ozonolysis, condensation or photocatalysis). Relevant examples derived from the integration of crystalline metal oxide thin films on flexible substrates (≤350 °C) illustrate the most recent achievements in this field.

Aqueous solutions for low-temperature photoannealing of functional oxide films: reaching the 400 °C Si-technology integration barrier.

Functional oxide films were obtained at low temperature by combination of aqueous precursors and a UV-assisted annealing process (aqueous photochemical solution deposition). For a PbTiO(3) model system, functional ferroelectric perovskite films were prepared at only 400 °C, a temperature compatible with the current Si-technology demands. Intrinsically photosensitive and environmentally friendly aqueous precursors can be prepared for most of the functional multimetal oxides, as additionally demonstrated here for multiferroic BiFeO(3), yielding virtually unlimited possibilities for this low-temperature fabrication technology.

Multiferroic Aurivillius Bi4Ti2-xMnxFe0.5Nb0.5O12 (n = 3) compounds with tailored magnetic interactions.

Aurivillius compounds with the general formula (Bi2O2)(An-1BnO3n+1) are a highly topical family of functional layered oxides currently under investigation for room-temperature multiferroism. A chemical design strategy is the incorporation of magnetically active BiMO3 units (M: Fe3+, Mn3+, Co3+ …) into the pseudo-perovskite layer of known ferroelectrics like Bi4Ti3O12, introducing additional oxygen octahedra. Alternatively, one can try to directly substitute magnetic species for Ti4+ in the perovskite slab. Previous reports explored the introduction of the M3+ species, which required the simultaneous incorporation of a 5+ cation, as for the Bi4Ti3-2xNbxFexO12 system. A larger magnetic fraction might be attained if Ti4+ is substituted with Mn4+, though it has been argued that the small ionic radius prevents its incorporation into the pseudo-perovskite layer. We report here the mechanosynthesis of Aurivillius Bi4Ti2-xMnxNb0.5Fe0.5O12 (n = 3) compounds with increasing Mn4+ content up to x = 0.5, which corresponds to a magnetic fraction of 1/3 at the B-site surpassing the threshold for percolation, and equal amounts of Mn4+ and Fe3+. The appearance of ferromagnetic superexchange interactions and magnetic ordering was anticipated and is shown for phases with x ≥ 0.3. Ceramic processing was accomplished by spark plasma sintering, which enabled electrical measurements that demonstrated ferroelectricity for all Mn4+-containing Aurivillius compounds. This is a new family of layered oxides and a promising alternative single-phase approach for multiferroism.

Low-Temperature Solution Approaches for the Potential Integration of Ferroelectric Oxide Films in Flexible Electronics.

This technical review presents the state of the art in low-temperature chemical solution deposition (CSD) processing of ferroelectric oxide thin films. To achieve the integration of multifunctional crystalline oxides with flexible and semiconductor devices is, today, crucial to meet the demands of coming electronic devices. Hence, amorphous metal-oxide-semiconductors have been recently introduced in thin-film electronics. However, their benefits are limited compared with those of ferroelectric oxides, in which intrinsic multifunctionality would make possible multiple operations in the device. However, ferroelectricity is linked to a noncentrosymmetric crystal structure that is achieved, in general, at high temperatures, over 500 °C. These temperatures exceed the thermal stability of flexible polymer substrates and are not compatible with those permitted in the current fabrication routines of Si-based devices. In addition, the manufacturing of flexible electronic devices not only calls for low-temperature fabrication processes but also for deposition techniques that scale easily to the large areas required in flexible devices. In this regard, CSD processes are the best positioned today to integrate metal oxide thin films with flexible substrates as a large-area, low-cost, high-throughput fabrication technique. Here, we review the progress made in the last years in fabricating at low-temperature crystalline ferroelectric oxide thin films via CSD methods, highlighting the recent work of our group in the preparation of ferroelectric oxide thin films on flexible polyimide substrates.

Relaxor behavior, polarization buildup, and switching in nanostructured 0.92 PbZn1/3Nb2/3O3-0.08 PbTiO3 ceramics.

The relaxor-type behavior, electrical polarization buildup, and switching in 0.92Pb(Zn(1/3)Nb(2/3))O(3)-0.08PbTiO(3) nanostructured ceramics with a grain size of approximately 20 nm is reported for the first time. This composition presents the highest-known piezoelectric coefficients, yet phase stability is an issue. Ceramics can only be obtained by the combination of mechanosynthesis and spark-plasma sintering. The results raise the possibility of using nanoscale, perovskite-relaxor-based morphotropic-phase-boundary materials for sensing and actuation in nanoelectromechanical systems.

Multilayer Ceramic Magnetoelectric Composites with Tailored Interfaces for Enhanced Response.

Composite materials consisting of two dissimilar ferroic phases are an excellent alternative to single-phase multiferroics for a wide range of magnetoelectric technologies. In composites with strain-mediated magnetoelectric coupling the response is strongly dependent on the characteristics of the interface between the two mechanically coupled phases. Among the different material approaches considered, cofired ceramic composites offer improved reliability in applications and are more adequate for free-forming and miniaturization. However, their magnetoelectric response often suffers from poor reproducibility, which has been reiteratively associated with the quality of the interfaces with little experimental support. Here, we report an in-depth study of the local material properties across the interfaces of 0.36BiScO3-0.64PbTiO3/NiFe2O4 multilayer ceramic composites, processed by spark plasma sintering of nanocrystalline powders. Tailored microstructures and low residual stress levels were obtained by adjusting the sintering mismatch between the two ferroic phases, which also resulted in fully functional interfaces and enhanced magnetoelectric responses.

Resonant electron tunnelling assisted by charged domain walls in multiferroic tunnel junctions.

The peculiar features of domain walls observed in ferroelectrics make them promising active elements for next-generation non-volatile memories, logic gates and energy-harvesting devices. Although extensive research activity has been devoted recently to making full use of this technological potential, concrete realizations of working nanodevices exploiting these functional properties are yet to be demonstrated. Here, we fabricate a multiferroic tunnel junction based on ferromagnetic La0.7Sr0.3MnO3 electrodes separated by an ultrathin ferroelectric BaTiO3 tunnel barrier, where a head-to-head domain wall is constrained. An electron gas stabilized by oxygen vacancies is confined within the domain wall, displaying discrete quantum-well energy levels. These states assist resonant electron tunnelling processes across the barrier, leading to strong quantum oscillations of the electrical conductance.

Piezoelectric ultrathin lead titanate films prepared by deposition of aquo-diol solutions.

We report on the preparation of continuous ultrathin ferroelectric films of pure lead titanate by chemical solution deposition (CSD) methods. Aquo-diol solutions highly diluted have been used to obtain films with thickness down to approximately 13 nm, the lowest reported for CSD films. The formation of islands instead of continuous coatings, which has been reported when CSD is used to prepare ultrathin films, is avoided here. The piezoelectric activity of the films has been characterized by piezoresponse. force microscopy, showing that the thinnest film obtained retains a significant piezoelectric activity at the nanoscale, which is promising for their use as transducer elements in nanoelectromechanical systems (NEMS).

Photochemical solution processing of films of metastable phases for flexible devices: the ß-Bi2O3 polymorph.

The potential of UV-light for the photochemical synthesis and stabilization of non-equilibrium crystalline phases in thin films is demonstrated for the ß-Bi2O3 polymorph. The pure ß-Bi2O3 phase is thermodynamically stable at high temperature (450-667 °C), which limits its applications in devices. Here, a tailored UV-absorbing bismuth(III)-N-methyldiethanolamine complex is selected as an ideal precursor for this phase, in order to induce under UV-light the formation of a -Bi-O-Bi- continuous network in the deposited layers and the further conversion into the ß-Bi2O3 polymorph at a temperature as low as 250 °C. The stabilization of the ß-Bi2O3 films is confirmed by their conductivity behavior and a thorough characterization of their crystal structure. This is also supported by their remarkable photocatalytic activity. Besides, this processing method has allowed us for the first time the preparation of ß-Bi2O3 films on flexible plastic substrates, which opens new opportunities for using these materials in potential applications not available until now (e.g., flexible photocatalytic reactors, self-cleaning surfaces or wearable antimicrobial fabrics). Therefore, photochemical solution deposition (PCSD) demonstrates to be not only an efficient approach for the low temperature processing of oxide films, but also an excellent alternative for the stabilization of metastable phases.

Low-temperature liquid precursors of crystalline metal oxides assisted by heterogeneous photocatalysis.

The photocatalytically assisted decomposition of liquid precursors of metal oxides incorporating TiO2 particles enables the preparation of functional layers from the ferroelectric Pb(Zr,Ti)O3 and multiferroic BiFeO3 perovskite systems at temperatures not exceeding 350 ºC. This enables direct deposition on flexible plastic, where the multifunctionality provided by these complex-oxide materials guarantees their potential use in next-generation flexible electronics.

Thin film multiferroic nanocomposites by ion implantation.

Thin film multiferroic nanocomposites might enable a range of potentially disruptive integrated magnetoelectric devices for information storage, spintronics, microwave telecommunications, and magnetic sensing. With this aim, we have investigated ion implantation of magnetic species into ferroelectric single crystal targets as a radically novel approach to prepare film nanoparticulate magnetic-metal ferroelectric-oxide composites. These materials are an alternative to multiferroic oxide epitaxial columnar nanostructures that are under intensive research, but whose magnetoelectric response is far from expectations. Here, we unambiguously demonstrate the preparation of such a thin film multiferroic nanocomposite of Co and BaTiO3 by ion implantation of a high dose of the magnetic species, followed by rapid thermal processing under tailored conditions. Results thus constitute a proof of concept for the feasibility of obtaining the materials by this alternative approach. Ion implantation is a standard technique for the microelectronic industry in combination with well-established patterning procedures.