Transition Metal Oxide Films Research Articles

Thin Layers of Cerium Oxynitride Deposited via RF Sputtering.

Thin films of transition metal oxides and oxynitrides have proven highly effective in protecting stainless steels against corrosion in both chemically aggressive environments and biological fluids. In the present work, cerium zirconium oxynitride thin films were deposited to enhance the corrosion resistance of surgical-grade stainless steel to be used in osteosynthesis processes. Two techniques were employed: co-sputtering and radiofrequency (RF) sputtering, and the morphology and corrosion efficiency of the coatings deposited by each technique were evaluated. X-ray diffraction, X-ray photoelectron spectroscopy and field emission transmission electron microscopy were used to characterize the morphological and chemical structure, respectively. Additionally, the corrosion resistance of the oxynitride-coated surgical grade stainless steel system (ZrCeOxNy-AISI 316L) was assessed using Hank's solution as the corrosive electrolyte, to determine its resistance to corrosion in biological media. The results show that ZrCeOxNy coatings increase the corrosion resistance of surgical grade stainless steel by two orders of magnitude and that the Ce(III)/Ce(IV) equilibrium decreases the corrosion rate, thereby increasing the durability of the steel in a biological environment. The results show that Ce coatings increase the corrosion resistance of surgical grade stainless steel by two orders of magnitude and that the Ce(III)/Ce(IV) equilibrium decreases the corrosion rate, thereby increasing the durability of the steel in a biological environment.

Tailoring of Interface Quality of MoOx/Si Solar Cells

Transition metal oxide films (TMO) as passivating contacts with improved opto-electronic characteristics play an important role in improving the silicon solar cell device efficiency. In this report, the effect of sputtering power on the optical properties of MoOx and the quality of MoOx/n-Si interface for its application in a silicon solar cell as carrier selective contacts has been reported. The optical transmittance of the film greater than 80 % in the visible and near infrared region of the spectrum is observed, which further improved with sputtering power. The creation of oxygen ion vacancies, which acts as positively charged structural defects able to capture one or two electrons led to the decrease of optical band gap from 3.70 eV to 3.23 eV at higher power. The oxygen vacancies occupied by electrons acts as donor centers, which lies close to the valence band, were responsible for modulation in electrical properties. The electrical properties of MoOx/n-Si interface was analyzed using current-voltage (I-V) measurements for its application as selective contact. A significant change in the selectivity parameters, like barrier height, I0 and series resistance of MoOx, has been observed with dc power. These extracted parameters showed that the sputtering power has a great influence on the selectivity of the charge carriers.

(Invited) Potential of Silicon Oxide Films for Low-Cost and High-Performance Resistive Switching Devices

Although resistance switching in transition-metal oxide (TMO) films has been widely studied [1], the physics and chemistry of resistance switching in non-transition-metal oxide (nTMO) films remains under-studied [2]. This must be corrected because inexpensive and chemically stable device configurations are required for the future Internet-of-Things society. It is easily anticipated that the switching electric field of silicon oxide films is apt to be higher than that of TMO films because the dielectric constant of silicon oxide films is smaller than those of the TMO films. However, the use of two-layer stacks like SiOx/hi-k oxide reduces the switching electric field [3]. Therefore, the study of silicon oxide films is still meaningful.Many scientists have recently investigated resistance switching in sputter-deposited silicon oxide films in detail [4,5] because this structure dispenses with the TMO (Fig. 1). Many papers addressed the role of silicon sub-oxide (SiOx) [6] because it is anticipated that the unstable bonds of non-stoichiometric silicon oxide can create degraded, but reversible, conductive paths inside the film. However, it is not yet clear how the SiOx region can trigger resistance switching, how important the SiOx region is, and whether the SiOx region is the only determiner of resistance switching [7] (Figs. 2, 3). The author demonstrated that hot-electron injection from the Si substrate had great potential in triggering resistance switching and lowering the switching voltage of sputter-deposited Si oxide films [5,8]; it was also mentioned that Si precipitates played an important role in realizing repeatable bipolar switching [9] (Fig. 2). Relating to this study, the author also proposed the physical and chemical structure of conductive filaments and their switching behavior based on an analysis of a possibly equivalent circuit model [10] (Fig. 4). However, it was not definitely elucidated why unipolar switching is not easily observed in sputter-deposited silicon oxide films, even though it is not a TMO.Recently, the author performed various Monte Carlo simulations to elucidate the physical and chemical parameters that rule the unipolar switching process in sputter-deposited silicon oxide films. Generations of simple bond breaking, oxygen vacancies, metallic Si sites, and E’’ centers were implemented in the simulation algorithm [8]. All-positive voltage stress mode for both the electroforming process and the reset process will not yield devices with stable, repeatable switching [11]. On the other hand, the all-negative stress mode results in stable, repeatable switching because the recovery of the internal degradation of the Si oxide film is not completed [11] (Fig. 5). This difference stems from the physical asymmetry of the electrode materials (Fig. 1).Though some may consider that silicon oxide films are not preferable to ReRAM devices from the chemical points of view, the theoretical analysis provided by the author in this paper suggests that silicon oxide films can be applied to the ReRAM device.[1] Y. Tokura, Physics Today, vol. 56, pp. 50-55, 2003.[2] T. Yanagida, et al., Sci. Rep., vol. 3, No.1657, pp.1-6, 2013.[3] P. Broqvist and A. Pasquarello, Appl. Phys. Lett., vol. 91, 192905, 2007.[4] J. Yao, et al., Appl. Phys. Lett, vol. 93, pp. 253101-1-253101-3, 2008.[5] R. Yamaguchi, S. Sato, and Y. Omura, Jpn. J. Appl. Phys., vol. 56, pp. 041301-1-041301-6, 2017.[6] A. Mehonic, et al., J. Appl. Phys., vol. 111, pp. 074507-1-074507-9, 2012.[7] Y. Wang, et al., Appl. Phys. Lett., vol.102, pp. 042103-1-042103-5, 2013.[8] Y. Omura, Ind. J. Electrical Eng. & Comput. Sci., vol. 24, pp. 1367-1378, 2021.[9] Y. Omura, R. Yamaguchi, and S. Sato, IEEE Trans. Device Reliab. and Mat. Vol. 17, pp. 561-567 (2017).[10] Y. Omura, ECS J. Solid State Sci. and Technol., vol. 10, pp. 124006-1-124006-10, 2021.[11] Y. Omura, Materials Today Proc., vol. 20, pp. 273-282, 2020; the advanced study will be published in the ECS J. Solid State Sci. and Technol. Figure 1

Deciphering phase evolution in complex metal oxide thin films via high-throughput materials synthesis and characterization

Discovery of structure-property relationships in thin film alloys of complex metal oxides enabled by high-throughput materials synthesis and characterization facilities is demonstrated here with a case-study. Thin films of binary transition metal oxides (Ti–Zn) are prepared by pulsed laser deposition with continuously varying Ti:Zn ratio, creating combinatorial samples for exploration of the properties of this material family. The atomic structure and electronic properties are probed by spatially resolved techniques including x-ray absorption near edge structures (XANES) and x-ray fluorescence (XRF) at the Ti and Zn K-edge, x-ray diffraction, and spectroscopic ellipsometry. The observed properties as a function of Ti:Zn ratio are resolved into mixtures of five distinguishable phases by deploying multivariate curve resolution analysis on the XANES spectral series, under constraints set by results from the other characterization techniques. First-principles computations based on density function theory connect the observed properties of each distinct phase with structural and spectral characteristics of crystalline polymorphs of Ti–Zn oxide. Continuous tuning of the optical absorption edge as a function of Ti:Zn ratio, including the unusual observation of negative optical bowing, exemplifies a functional property of the film correlated to the phase evolution.

Research progress of control of physical properties of topological phase change oxide films by external field

Perovskite transition-metal oxides can undergo significant structural topological phase transition between perovskite structure, brownmillerite structure, and infinite-layer structure under the external field through the gain and loss of the oxygen ions, accompanied with significant changes in physical properties such as transportation, magnetism, and optics. Topotactic phase transformation allows structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for utilizing the redox reaction and oxygen diffusion within transition metal oxides, and establishing great potential applications in solid oxide fuel cells, oxygen sensors, catalysis, intelligent optical windows, and neuromorphic devices. In this work, we review the recent research progress of manipulating the topological phase transition of the perovskite-type oxide films and regulating their physical properties, mainly focusing on tuning the novel physical properties of these typical films through strong interaction between the lattice and electronic degrees of freedom by the action of external fields such as strain, electric field, optical field, and temperature field. For example, a giant photoinduced structure distortion in SrCoO<sub>2.5</sub> thin film excited by photons is observed to be higher than any previously reported results in the other transition metal oxide films. The SrFeO<sub>2</sub> films undergo an insulator-to-metal transition when the strain state changes from compressive state to tensile state. It is directly observed that perovskite SrFeO<sub>3</sub> nanofilament is formed under the action of electric field and extends almost through the brownmillerite SrFeO<sub>2.5</sub> matrix in the ON state and is ruptured in the OFF state, unambiguously revealing a filamentary resistance switching mechanism. Utilizing <i>in situ</i> electrical scanning transmission electron microscopy, the transformation from brownmillerite SrFeO<sub>2.5</sub> to infinite-layer SrFeO<sub>2</sub> under electric field can be directly visualized with atomic resolution. We also clarify the relationship between the microscopic coupling mechanism and the macroscopic quantum properties of charges, lattices, orbits, spin, etc. Relevant research is expected to provide a platform for new materials, new approaches and new ideas for developing high-sensitivity and weak-field response electronic devices based on functional oxides. These findings about the topological phase transition in perovskite oxide films can expand the research scope of material science, and have important significance in exploring new states of matters and studying quantum critical phenomena.

Interdigitated back-contacted crystalline silicon solar cells fully manufactured with atomic layer deposited selective contacts

The interdigitated back-contacted (IBC) solar cell concept has been extensively studied for single-junction cells and more recently as a good choice for three-terminal tandem devices. In this work, carrier-selective contacts based on transition metal oxides deposited by atomic layer deposition (ALD) technique are applied to IBC c-Si(n) devices. In the first part of the study, we develop a hole-selective contact based on thin ALD vanadium oxide (V2O5) layers without using an amorphous silicon interlayer. The ALD process has been optimised, i.e. number of ALD cycles and deposition temperature, as a trade-off between surface passivation and contact resistivity. Noticeable surface passivation with recombination current densities around 100 fA/cm2, as well as reasonable contact resistivity values below 250 mΩcm2 are reached using 200 ALD V2O5 cycles deposited at a deposition temperature of 125 °C (∼10 nm layer thickness). The optimised ALD V2O5-based contact is combined with both an ALD TiO2-based electron-selective contact and an excellent surface passivation in non-contacted regions provided by ALD Al2O3 films, to form a fully ALD IBC c-Si(n) solar cell scheme. Fabricated devices yield photovoltaic efficiencies and pseudo efficiencies, i.e. calculated without series resistance losses, of 18.6% and 21.1% respectively (3 cm × 3 cm device area). These results reveal the potential of the ALD technique to deposit transition metal oxide (TMO) films as selective contacts on high efficiency devices, paving the way of using low thermal-budget, low cost and highly scalable processes for a highly demanding IBC solar cell architecture in the photovoltaic industry.

Superconductivity in Ti2O3 films on MgO substrate

Transition metal oxide films exhibit strong structure-property correlations, which foster innovative physical properties that are not present in their bulk counterparts. This study has successfully prepared different thickness-dependent epitaxial Ti2O3 thin films on MgO (001) single crystal substrate using the pulsed laser deposition method. The results reveal a strong correlation between the crystal structure and superconductivity in Ti2O3 thin films. Synchrotron x-ray diffraction and high resolution transmission electron microscope measurements confirmed that the appearance of superconductivity is closely related to the coexistence of the cubic-Ti2O3 (γ-Ti2O3) and orthorhombic-Ti2O3 (o-Ti2O3) phases. Superconductivity is absent in 80 nm film, which exhibits only a single γ-Ti2O3 phase. The highest superconducting transition temperature (T C) ∼ 7.2 K is achieved in 1200 nm film, which shows about 55% o-Ti2O3 phase and 45% γ-Ti2O3 phase. Room temperature Raman data suggest that E g modes at ∼330 and ∼430 cm−1 may associate with superconductivity, and an E g mode at ∼660 cm−1 is related to the insulating behavior observed in 80 nm film.

Anomalous Hall effect superimposed in polycrystalline SrRuO3 thick film

The electric, magnetic, and thermal properties of transition metal oxide films can be modulated by introducing polycrystalline at the macroscopic grain boundaries. Based on these points, in this work, we studied the two-channel anomalous Hall effect (AHE) in polycrystalline ferromagnetic SrRuO3 (SRO) films. The magnetic regions with different crystal directions have different coercivities, resulting in two opposite AHE channels in the polycrystalline SRO layer. However, single-crystal SRO films prepared under the same conditions are found to exhibit only one AHE. The superposition of the two AHE leads to the hump-like behavior of the Hall resistance loop, which is caused by the change of crystalline. This observation provides a new way to explain the hump-like feature of SRO.

Ionic Liquid Gating Control of Oxygen Vacancies in the La0.8Ba0.2MnO3 Ultrathin Films

In this work, we show ionic liquid field-effect of La0.8Ba0.2MnO3 (LBMO) untrathin films prepared by the sol-gel method. When applying a positive gate voltage (V g) in vacuum, it is found that the film resistivity continuously increases tenfold within hours or minutes depending on the magnitude of V g, accompanied with disappearance of the low-temperature metallic transport behavior. In contrast, the film resistivity changes little in air or under a negative applied V g. Such a difference reveals that the increased resistivity is related to the oxygen depletion in the films under the positive V g, especially at the grain boundaries. After removing the positive V g in vacuum, the room-temperature resistivity begins to fall off and the low-temperature metallic state is partially restored in several tens hours, suggesting oxygen diffusion from the relatively oxygen-rich grains to the seriously oxygen-deficient grain boundaries. Furthermore, the oxygen content in the oxygen-deficient films can be almost fully restored in ten minutes by external annealing (200 °C) or Joule self-heating (21 mA) the films in air. These experimental findings provide an additional method in tuning oxygen vacancies in transition-metal oxide films.

Patterning of WOx, VOx, and MoOx thin-films with picosecond and nanosecond laser sources

Transition metal oxide (TMOs) layers have interesting properties as selective contacts, i.e., hole or electron transport layers for novel semiconductor devices. Especially, oxides of molybdenum (MoO3), vanadium (V2O5), and tungsten (WO3) show good bahaviour acting as front hole-selective contacts for n-type crystalline-silicon heterojunction solar cells. Laser scribing has been widely used for thin-film ablation and seems the appropriate technology for device manufacturing with such non-conventional materials. In this work, we study the laser scribing of non-stoichiometric evaporated WOx, VOx, and MoOx films with three different wavelengths (1064, 532, and 355 nm) with pulse duration in the nanosecond and picosecond regimes. The selection of the proper laser source allows a wide parametric window, with complete removal of the TMO films and no alteration of the silicon substrate. The results on the isolation of diodes and their electrical characteristics show the quality of the laser scribing processes.

Toward Functionalized Ultrathin Oxide Films: The Impact of Surface Apical Oxygen

AbstractThin films of transition metal oxides open up a gateway to nanoscale electronic devices beyond silicon characterized by novel electronic functionalities. While such films are commonly prepared in an oxygen atmosphere, they are typically considered to be ideally terminated with the stoichiometric composition. Using the prototypical correlated metal SrVO3 as an example, it is demonstrated that this idealized description overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not explicitly exposed to air, are shown to severely affect the intrinsic electronic structure of a transition metal oxide film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in the thin films. These findings highlight that it is important to take into account surface apical oxygen or—mutatis mutandis—the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of transition metal oxides near the single unit‐cell limit.

Tuning the Thermoelectric Properties of Transition Metal Oxide Thin Films and Superlattices on the Quantum Scale

Combining advanced growth and characterization techniques with state‐of‐the‐art first‐principles simulations in the frameworks of density functional theory and Boltzmann transport theory, recent advances in the field of transition metal oxide films and superlattices (SLs) as thermoelectric materials are discussed, with particular focus on a selection of quantum‐scale approaches to tune their thermoelectric performance. Specifically, films grown on regular and miscut substrates have enabled experimental confirmation of the large predicted out‐of‐plane Seebeck coefficient of this anisotropic material and also reveal the necessity of a Hubbard‐U parameter on the Co states. Furthermore, oxygen diffusion and incorporation from the substrate lead to a significant enhancement of the high‐temperature Seebeck coefficient in SLs. Next, it is shown how n‐ and p‐type materials can be achieved either by exploiting interface polarity in a SL or using epitaxial strain to shift orbital‐dependent transport resonances across the Fermi level in SLs. Moreover, confinement‐ and strain‐induced metal‐to‐insulator transitions induce high Seebeck coefficients and power factors in short‐period and SLs ( V, Cr, Mn). Finally, a relation between the topologically nontrivial Chern insulating behavior and enhanced thermoelectric response in SLs is established. The article concludes with a discussion of challenges and future topics of research in oxide thermoelectrics.

Solution processable transition metal oxide ultra-thin films as alternative electron transport and hole blocking layers in dye sensitized solar cells

An electron transporting and hole-blocking layer (ETL) between the fluorine-doped tin oxide (FTO) and mesoporous titanium dioxide interface of photoanode is essential in dye-sensitized solar cells (DSCs) for improving their photovoltaic performance. In the present work, we have examined the potential for further improving device performance by using alternative materials for ETLs. Accordingly, the effect of replacing TiO2 based ETLs with spin coated ultra-thin transition metal oxides (TMOs), molybdenum trioxide (MoO3) and tungsten trioxide (WO3) films has been studied. TMO films of 10 nm thickness have been found to be optimal for obtaining good transparency and charge transport properties, without compromising device performance. Further, devices fabricated with surfactant assisted (SA-) MoO3 ETLs have shown efficiency ∼4%, which is comparable with devices based on conventionally prepared TiO2 ETLs. In addition, a significant improvement in efficiency (15 %) has been observed for DSCs prepared using SA-WO3 films. The overall improvement in device efficiency is attributed to the increase in short-circuit current density without loss of open circuit potential and fill factor. Detailed analyses of results suggest that in addition to surface morphology, transparency and charge transport properties; oxygen vacancies in sub-stoichiometric TMOs also plays an important role in determining the device performance. The findings reveal that TMOs have significant potential to replace TiO2 ETL in DSCs and other third generation solar cell, viz. perovskite and polymer solar cells.

Electrodeposition of atmosphere-sensitive ternary sodium transition metal oxide films for sodium-based electrochemical energy storage

We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, NaxMO2 (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures ≥700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion-based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75% dense NaxCoO2 and NaxMnO2 can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na+ ions (∼10-12 cm2⋅s-1), and high reversible areal capacities in the range ∼0.25 to 0.76 mA⋅h⋅cm-2, values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air- and moisture-sensitive high valent multinary structures with extended frameworks.

Development of Transition Metal Oxide Film Coated Platforms for Aptamer Based Electrochemical Detection of Ochratoxin A

A novel aptamer based nanobiosensor for label free detection of Ochratoxin A (OTA) was developed in the present study. Pencil graphite electrode (PGE) surfaces modified with transition metal oxides (MeOx) were used as a platform for immobilizing the 5′aminohexyl linked aptamer sequence representing Ochratoxin A via carbodiimide chemistry. After blocking the residual active surface with bovine serum albumin, aptasensors were incubated with OTA solution to monitor their interaction by Electrochemical Impedance Spectrometry (EIS) in the presence of 5 mM [Fe(CN)6]3−/4− based on the electron transfer resistances (Rct). Operational and solution parameters such as the type of metal oxide (MnOx, MoOx and WOx) and the technique used in their deposition step being cyclic voltammetry (CV) or pulsed deposition (PD) along with the aptamer concentration were optimized by comparing the EIS measurements, and the best performance was obtained with MnOx modified PGEs by PD technique. Selectivity of the designed aptasensor was detected by using Aflatoxin B1 and Zearalenone as nonspecific molecules and from the calibration curves, the limit of detection (LOD) was calculated as 0.03 nM.

Lifetime of electrochromic optical transition cycling of ethyl viologen diperchlorate-based electrochromic devices

Electrochromic materials and devices are enabling a variety of advanced technologies. Gel-based organic electrochromic molecules such as ethyl viologen diperchlorate are attractive options due to their simple device design and low cost processing options relative to the more expensive and complex transition metal oxide films. However, electrochromic devices are subject to extensive cycling in which failure and fatigue can eventually occur. This work presents the lifetime cycling performance of ethyl viologen diperchlorate-based electrochromic devices using two different anodic compounds, hydroquinone and ferrocene, which are cycled at different voltages, 3 V and 1.2 V, respectively. Multiple devices are cycled until failure with periodic device characterization via UV–Vis spectroscopy, electrical resistance and power measurement, and transition duration measurement. Devices with hydroquinone can transition quickly. Cycle times are sim 30 s in these samples, however, these samples also typically fail before 3000 cycles. On the other hand, devices using ferrocene transition more slowly (total cycle time sim 2 min), but have superior cycling performance with all samples surviving beyond 10,000 cycles while complying with ASTM E2141-14 standard.

Structural symmetry evolution in surface and interface of SrRuO3 thin films

Recent advances in thin film technologies enable us to engineer structural details of surface and interface in transition metal oxide films, and provide numerous valuable opportunities to manipulate electric and magnetic properties via strong couplings among charge, spin, and lattice degrees of freedom. In this work, we exploit an optical second harmonic generation technique to determine crystalline symmetries of surface and interface of compressively strained SrRuO3 thin films, which have been widely used as one of representative ferromagnetic metals in oxide devices. We reveal that the structural symmetries of the surface, the inner-bulk part, and the interface of the films are given distinctly owing to the compressive strain and its relaxation. Furthermore, we trace evolutions of the surface/interface structural symmetries with variations of temperature and film thickness, and discuss their similarities and differences compared to the bulk crystal. The structural phase diagram presented here will provide fundamental information of the surface and interface structural symmetry in exploring electronic and magnetic phenomena emerging in relevant oxide heterostructures.

A solution-processed, ultraviolet-irradiation-derived WO3 film as anode interface layer for high-performance non-fullerene organic solar cells

Tungsten(VI) oxide (WO3) with excellent charge transport properties and intrinsic thermal and chemical robustness is a promising anode interface material instead of PEDOT:PSS in organic solar cells (OSCs). Herein, a solution-processed, ultraviolet-irradiation-derived WO3 film as anode interface layer (AIL) is developed based on tungsten(VI) chloride (WCl6) as precursor. The WO3 film is derived from the solution-processed precursor film with simple ultraviolet-irradiation treatment, and the preparation process without high-temperature annealing is very compatible with large-scale production and flexible substrate. The composition of the WO3 film is analyzed by X-ray photoelectron spectroscopy. ITO modified by the WO3 film, namely ITO/WO3, exhibits similarly high optical transmittance as that of bare ITO. The work function of ITO/WO3 measured by ultraviolet photoelectron spectroscopy is obviously lower than that of bare ITO, which can facilitate effective holes extraction. In addition, the WO3 film possesses relatively smooth and hydrophobic surface, beneficial to form excellent interface contact with active layer. Moreover, non-fullerene OSCs based on the WO3 film as AIL are studied with typical and relatively low-cost PBDB-T:ITIC as active layer. The optimal efficiency (8.70%) of the device based on ITO/WO3 is dramatically higher than that (3.32%) of the device based on bare ITO, and comparable to that (8.80%) of the device based on ITO/PEDOT:PSS. Therefore, the solution-processed, ultraviolet-irradiation-derived WO3 film as AIL should have great potential to realize high-performance non-fullerene OSCs, which can also inspire a novel and promising route to prepare other transition metal oxide films in optoelectronic devices.

Atomic Layer Deposition of a Manganese-Iron Mixed Oxide As a Bifunctional Oxygen Catalyst for Zinc-Air Batteries

Carbon-free energy sources will become a necessary step in order to mitigate the effects of climate change. In order to integrate these renewable energy sources, which are usually intermittent and not on-demand, into the power grid, a significant amount of energy storage will be required. The popularity of lithium-ion batteries as an energy storage option has exploded in recent years; however, a safer and less expensive alternative may be zinc-air batteries.Zinc-air batteries combine zinc, an abundant and safe metal, with oxygen from the air. Using oxygen as a battery component, however, presents issues because of the slow kinetics of both oxygen reduction and oxygen evolution, the necessary steps in battery discharge and charge, respectively [1]. To compensate, catalysts are utilized that improve reaction kinetics for both the charge and discharge reactions. The benchmark in oxygen reaction catalysis is rare and expensive noble metals, such as platinum and ruthenium, which hinders practicality. On the other hand, inexpensive and abundant transition metal oxides have also shown promising catalytic activity towards these oxygen reactions [2]. Unfortunately, transition metal oxide catalysts that improve the reaction kinetics of the oxygen reduction reaction are typically not active towards the oxygen evolution reaction, and vice versa. However, a ternary oxide of two transition metals may result in a bifunctional catalyst, which is active towards both charge and discharge reactions.The aim of this work is to produce a bifunctional, mixed oxide catalyst through atomic layer deposition (ALD). ALD employs self-terminating surface reactions of a metal-containing gaseous precursor and an oxygen co-reactant to produce very conformal and thin oxide films. Highly porous carbon paper, which typically serves as the air electrode in metal-air batteries, acts as the substrate for ALD of these transition metal oxide films. ALD can conformally coat the high surface area substrate with minimal loss in porosity. Furthermore, ALD can penetrate deeper into the pores of the electrode as compared with other deposition techniques. Together this allows ALD to yield high surface area nanostructured catalysts that maximizes the three-phase region between gaseous oxygen, liquid electrolyte, and the solid catalyst, thereby improving battery performance [3].Previously studies have shown manganese oxides to be active towards the oxygen reduction reaction, while iron oxide is a component of many oxygen evolution catalysts [3]–[5]. Therefore, a recipe to deposit iron oxide films using ethylferrocene and an oxygen plasma is combined with an already established manganese oxide deposition procedure, with the ultimate goal of creating a mixed transition metal oxide catalyst. Electrochemical characterization techniques are employed to evaluate the bifunctional activity of synthesized transition metal oxides films. The atomic-level control of ALD means that the mixed oxide can be grown in various different proportions. An optimization process to establish the best ALD sequence is showcased, resulting in a bifunctional efficiency of 55% at 20 mA/cm2 for a 10 nm catalytic film.[1] F. Cheng and J. Chen, “Metal-air batteries: From oxygen reduction electrochemistry to cathode catalysts,” Chem. Soc. Rev., vol. 41, no. 6, pp. 2172–2192, 2012.[2] H. Osgood, S. V. Devaguptapu, H. Xu, J. Cho, and G. Wu, “Transition metal (Fe, Co, Ni, and Mn) oxides for oxygen reduction and evolution bifunctional catalysts in alkaline media,” Nano Today, vol. 11, no. 5, pp. 601–625, 2016.[3] M. P. Clark, M. Xiong, K. Cadien, and D. G. Ivey, “High Performance Oxygen Reduction/Evolution Electrodes for Zinc − Air Batteries Prepared by Atomic Layer Deposition of MnOx,” ACS Appl. Energy Mater., vol. 3, no. 1, pp. 603–313, 2020.[4] M. Xiong, M. P. Clark, M. Labbe, and D. G. Ivey, “A horizontal zinc-air battery with physically decoupled oxygen evolution/reduction reaction electrodes,” J. Power Sources, vol. 393, pp. 108–118, 2018.[5] D. Aasen, M. P. Clark, and D. G. Ivey, “(Co,Fe)3O4 Decorated Nitrogen-Doped Carbon Nanotubes in Nano-Composite Gas Diffusion Layers as Highly Stable Bifunctional Catalysts for Rechargeable Zinc-Air Batteries,” Batter. Supercaps, vol. 3, no. 2, pp. 174–184, 2020.

A comparative study on atomic layer deposited oxide film morphology and their electrical breakdown

Transition metal oxide films HfO2, ZrO2, Ta2O5, TiO2, Nb2O5, and Y2O3, together with Al2O3 and SiO2 for reference, were deposited by atomic layer deposition (ALD) and investigated. For all the films ALD process was performed within the same reactor at two temperature ranges 100–125∘C and 250–300∘C. Both water and ozone were used as oxygen sources if processes were available. Most films are amorphous as-deposited, but HfO2, ZrO2, and Y2O3 can be crystalline, depending on deposition temperature and oxygen source.After deposition and initial characterization, the films were annealed to different temperatures up to 1200∘C, depending on the material, to observe the crystallization process.When crystalline phases were observed, grain sizes have been derived from the X-ray diffractograms and compared with feature sizes in SEM images. RMS analysis revealed a general tendency - an increase of roughness with the appearance of crystallization.The temperature-dependent morphological investigations were supported by breakdown field measurements. Those are also strongly affected when a film becomes crystalline. Most values observed for break down fields agree with literature, some values exceed previously reported numbers.