Transition Metal Oxide Thin Films Research Articles

Fabrication of Thermally Evaporated CuIx Thin Films and Their Characteristics for Solar Cell Applications

Carrier-selective contacts (CSCs) for high-efficiency heterojunction solar cells have been widely studied due to their advantages of processing at relatively low temperatures and simple fabrication processes. Transition metal oxide (TMO) (e.g., molybdenum oxide, vanadium oxide, and tungsten oxide) thin films are widely used as hole-selective contacts (HSCs, required work function for Si solar cells > 5.0 eV). However, when TMO thin films are used, difficulties are faced in uniform deposition. In this study, we fabricated a copper (I) iodide (CuI) thin film (work function > 5.0 eV) that remained relatively stable during atmospheric exposure compared with TMO thin films and employed it as an HSC layer in an n-type Si solar cell. To facilitate efficient hole collection, we conducted iodine annealing at temperatures of 100–180 °C to enhance the film’s electrical characteristics (carrier density and carrier mobility). Subsequently, we fabricated CSC Si solar cells using the annealed CuIx layer, which achieved an efficiency of 6.42%.

Enhanced oxygen evolution reaction by modulating the electronic states of a flexible van der Waals membranous catalyst with infinite-layer phase

The precise stoichiometry and lattice ordering of perovskite transition metal oxide (TMO) thin films enable the atomic-level catalytic mechanisms in the process of oxygen evolution reaction (OER), accordingly facilitating the creation and refinement potentially effective catalysts. Nevertheless, the fundamental understanding of the interaction between the electronic and structural properties of infinite-layer structured electrocatalysts, as well as their catalytic efficiency, remains lacking. Herein, the successful acquisition of the membranous LaNiO2 catalysts with infinite-layer phase were achieved via topological reconstruction. Subsequent electrochemical testing revealed a decrease in the OER performance of nickelate after reduction, attributed to the diminished bonding strength between catalyst and oxygen intermediates. Given these unexpected findings, this study aimed to leverage surface strains to manipulate electronic states, thereby modulating the OER performance of LaNiO2. Applying either compressive or tensile strains to LaNiO2 enabled the restoration of its performance to pre-reduction levels induced by hydrogen, owing to the reinforced bond strength between catalyst and oxygen intermediates. This research provides both experimental evidence and theoretical guidance for understanding the importance of defect and stress engineering in OER, offering valuable insights for improving OER performance in flexible nano-membranous electrocatalysts with infinite-layer phase for practical applications.

In-situ hydrogenated V2O5 thin films: Study of the structural dynamics and their behavior as Li-ion batteries cathode

This investigation focuses on a one-step preparation method to produce hydrogenated V2O5 thin films by controlling the atmosphere of the DC reactive magnetron sputtering process. An increasing hydrogen flux during the film deposition promotes the formation of defects such as oxygen vacancies and hydroxyl sites in the oxide matrix. The hydrogenated samples were tested as cathodes in Li-ion batteries and exhibited lower charge capacities but superior stability to cycling compared to pristine V2O5 film. These results pave the way to prepare hydrogenated transition metal oxide thin films by a simple route without further thermal procedures and affordable conditions.

Enhanced nonlinear absorption and photoluminescence properties of Zn, Fe, Cu, V and Ni doped MoO3 transition metal oxide thin films

Abstract In this study, the effect of the dopant element on the linear, nonlinear absorption and optical limiting properties of ultrasonically sprayed MoO3 thin films is presented. The linear optical results showed an increase with the density of the defect states in the bandgap with doping, in conjunction with a decrease in bandgap energy and an increase in Urbach energy. Broad photoluminescence emissions are detected in the range of 350 and 600 nm, decreasing in intensity by doping. To reveal the defect states effects’ on the nonlinear absorption (NA) behavior, OA Z-scan data were analyzed with two theoretical models considering only two photon absorption (2PA) (model 1), and one photon absorption (OPA), 2PA and free carrier absorption (model 2). The NA behavior is observed and found to get enhanced by increasing input intensity and doping atoms due to generation of new oxygen vacancies and formation of further defect states. The NA coefficient values of the thin films in model 2 are 100 times higher than that of 2PA coefficient values in model 1. This result revealed the strong effect of defect states on the NA behavior. Among the investigated dopant atoms, Cu resulted in enhanced NA due to the higher density of defect states. While the genuine 2PA is the dominant NA mechanism for V and Fe doped MoO3 thin films, OPA and 2PA are the dominant NA mechanisms for the Ni, Zn and Cu doped MoO3 thin films due to their higher concentration of defect states. Cu-doped MoO3 thin film has a lower optical limiting threshold of 0.026 mJ/cm2 due to its enhanced NA behavior. Considering the results obtained, this study opens the door to the potential of doped MoO3 thin films to be used as optical limiters in the visible wavelength region.

Magnetic and electrical properties of LaCoO3 - LaNiO3 epitaxial thin films on LaAlO3 substrate

The mixed transition metal oxide thin films having ABO3 structure noticed numerous applications due to their robust electrical and magnetic properties. Among them, Lanthanum cobalt oxide considered as promising material since it exhibits excellent properties as catalyst and mixed ionic-electronic conductors. In this manuscript, we successfully deposited the epitaxial lanthanum cobalt oxide (LaCoO3) and lanthanum nickel oxide (LaNiO3) thin films on single crystal lanthanum aluminate (LAO) substrate. Individual and bilayer thin films are grown by pulsed laser deposition (PLD). X-ray diffraction pattern reveals the epitaxial nature and phase purity of the deposited thin films. Morphological images has been captured using field-emission scanning electron microscopy (FE-SEM). The electrical and magnetic properties of individual and bilayer LaCoO3/LaNiO3 thin films are measured using four-probe and physical property measurement system (PPMS) respectively. There is a clear ferromagnetic transition in LaCoO3 thin film observed at 85 K. Surprisingly, the bilayer of LaNiO3/LaCoO3 thin films exhibited enhanced magnetic properties compared with individual LaCoO3 or LaNiO3 thin films. The bilayer films revealed the high magnetization and less coercive filed (3.76 emu/cm3, 2215H) respectively. These results given an idea that bilayer LaCoO3/LaNiO3 thin films are capable to exhibit enhanced multifunctional properties.

Enhanced oxygen evolution reaction via the tunability of spin polarization and electronic states in a flexible van der Waals membranous catalyst.

The response of the magnetic field and strain engineering in an electrochemical process, such as the oxygen evolution reaction (OER), not only provides a strategy for enhancing catalytic performance through external fields and mechanical stress but also serves as a platform for revealing the functionality of multiple degrees of freedom in catalysts. The perovskite transition metal oxide (TMO) thin film with precise stoichiometry and lattice ordering enables atomic-level catalysis mechanisms in various electrochemical processes, thereby facilitating the design and engineering of promising catalysts. However, the perplexing dominance of spin in an OER process is still a puzzle due to the strong correlation between transition metal d and oxygen p orbitals. In this study, we utilized La0.7Sr0.3MnO3 (LSMO) manganite as a ferromagnetic OER catalyst, which was directly deposited onto a flexible mica substrate. By subjecting LSMO to a tensile stress, we observed an enhanced OER, and the OER performance of LSMO improved by 30% with a +0.2% strain due to the weakened chemisorption of Mn-O. Moreover, it has been observed that the OER performance can be improved by approximately 87%, while the overpotential can be reduced by around 22% through the combination of a 5 kOe magnetic field and +0.2% strain. The OER performance of LSMO changed by ∼153% under 4% strain and 5 kOe magnetic field. Our experiments indicate that the primary source of the observed magnetic response is derived from the triplet state of O2, in which spin-polarized d and oxygen p orbitals decrease the spin potential within OER. This study provides experimental evidence for understanding the spin degree and electronic state regulation in the OER process, thereby facilitating further design and engineering of flexible magnetic electrochemistry catalysts with promising potential.

Mechanisms of metal–insulator transitions in ultrathin SrMoO3 films

As the thickness of a transition metal oxide thin film is reduced to several unit cells, dimensional and interfacial effects modulate its structure and properties, and initiate low-dimension quantum phase transitions different from its bulk counterparts. To check if a metal–insulator transition (MIT) occurs to a low-dimensional 4 electron systems, we investigated SrMoO3 thin films by characterizing and analyzing their lattice structures, electric transport properties and electronic states. Among various dimensional effects and interfacial effects, quantum confinement effect (QCE) was discerned as the dominating mechanism of the thickness-driven MIT. Surface/interface scattering contributes to the residual resistivity while the competition of several interactions modulated by QCE governs the temperature dependence of the resistivity of SrMoO3 ultrathin films.

High-temperature ferromagnetic metallic phase in LaMnO3/Sr3Al2O6 heterostructure

• Oxygen migration occurs at the interface due to the stronger oxygen affinity of Al. • Continuously tunable strain can be generated by adjusting the thickness of SAO. • In-plane tensile strain would stabilizes d x 2 -y 2 occupancy and increases T c . To achieve a flexible single-crystal multifunctional membrane, the freestanding process of a rigid epitaxial transition metal oxide thin film via a buffered water-dissolution sacrificial layer has attracted reasonable attentions. Owing to the difference in chemical potential, specific element affinity, and lattice constant between the target membrane and the sacrificial layer, the freestanding process may cause an indelible change of physics property once the target thin film is sensitive to the above factors. Here, the heterostructures composed of the generally adopted sacrificial layer Sr 3 Al 2 O 6 (SAO) and LaMnO 3 (LMO) have been systematically investigated. The electrical and magnetic properties of LMO show extreme sensitivity to the thickness of SAO ( t SAO ). Then we have also found that LMO/SAO heterostructures can exhibit the coexistence of two ferromagnetic phases, the significantly enhanced Curie temperature ∼ 342 K, and the large magnetoresistance -23.3% at 300 K, which is similar to the optimal-doped manganite such as La 2/3 Sr 1/3 MnO 3 . X-ray diffraction results show that continuously tunable strain from out-of-plane tension to relaxation and then to compression can be generated by adjusting t SAO . This strain can stabilize the migrated oxygen from LMO to SAO, which is induced by the large oxygen affinity difference between B-site Mn and Al. It is believed that these unexpected electrical/magnetic phenomena are originated from the combined effects of interfacial element diffusion and strain. Our study provides a strategy for designing new magnetic phases, and a reference for the fundamental understanding of strongly correlated transition metal oxide systems in the freestanding process.

Enhanced oxygen evolution reaction by stacking single-crystalline freestanding SrRuO3

Single-crystalline transition metal oxide thin film has been employed to perceive the fundamental functions of electronic structure and charge transfer processes in water splitting processes. However, the surface area enlargement and strain tunability in a single-crystalline transition metal oxide are restricted in conventional epitaxy. In this study, we report the oxygen evolution reaction (OER) enhancement by a stack of multilayer SrRuO3 featured single-crystallinity, flexibility, and stackability. The controllable stack of multiple cylindrical SrRuO3 for surface enlargement in a magnitude of order and the emergent electronic structure transition, from t2g(3↑, 1↓) to t2g(3↑)eg(1↑), of Ru efficiently enhances the OER performance, the overpotential can be reduced by ~74 % and ~78 % in KOH and HClO4 respectively at 5 mA/cm2. Our study provides an approach for fine manipulation of single-crystalline freestanding oxide morphologically, and an efficient strategy aiming at the extreme enhancement of the electrochemically active surface area and the electronic structure engineering by strain.

The modulated oxygen evolution reaction performance in La2/3Sr1/3CoO3 by a design of stoichiometry offset

The accurate stoichiometry control of single-crystalline transition metal oxide thin film and the electrochemical performance such as in oxygen evolution reaction (OER) offer a useful pathway towards the mechanism understanding and the strategy of improving electrochemical activity at the atomic level. As a stable, low cost, and high active OER catalyst, La2/3Sr1/3CoO3 (LSCO) has drawn substantial attention recently. Here we report a modulated OER performance of LSCO by different stoichiometry offsets, an overall ∼2074.63% tunability can be seen. Meanwhile, the overpotential of LSCO exhibits a volcano trend as a function of oxygen concentration. These phenomena can be understood on the competition of double- and super-exchanges in Co3+/Co4+ with the help of the overlap of O 2p and Co 3d orbits. Our investigation provides an effective platform for obtaining a narrow window of improved electrochemical activity in correlated transition metal oxides.

Surface Atomic Decoration of a Manganite to a Modulable Oxygen Evolution Reaction.

Aiming at the fundamental understanding of oxygen evolution reaction (OER) in epitaxial perovskite transition metal oxide (TMO) thin films, we evaluate the surface decoration conditions, including lattice orientation and surface morphology, of La2/3Sr1/3MnO3 (LSMO) related to its modulable OER performance. The LSMOs with orientations of (001), (110), and, (111) exhibit different OER activities owing to the discrepant surface density of Mn. Furthermore, tuning of the surface atomic terrace width of LSMO shows a more efficient path to modulate the OER performance by introducing a high-valence Mn state owing to the surface dangling bonds of LSMO. As the electrochemical process is dominated by the interface of the TMO surface and electrolyte, our investigation can approach the fundamental understanding of a perovskite-type TMO surface state and its OER performance while highlighting the role of the nonbulk electron state in a promising TMO electrocatalyst in abundant electrochemical processes.

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.

Optical properties of transition-metal oxide thin films deposited using a pulsed laser

Optical properties of transition-metal oxide thin films deposited using a pulsed laser

Reversible Rapid Hydrogen Doping of WO3 in Non-Acid Solution.

Hydrogenation, an effective way to tune the properties of transition metal oxide (TMO) thin films, has been long awaited to be performed safely and without an external energy input. Recently, metal-acid-TMO has been reported to be an effective approach for hydrogenation, but the requirement of acid limits its application. In this work, the reversible and rapid hydrogen doping of WO3 in NaOH(aq) | Al(s) | WO3(s) is revealed by structural and electrical measurements. Accompanied by the structural phase transition identified by in situ X-ray diffraction, the electric resistance of the WO3 film is found to be able to change by 5 orders of magnitude. A significant electrical response of touching, 8-fold in amplitude and 3 s in a cycle, can be achieved in the low-resistance state. These reactions are reversible at room temperature. This study unambiguously proves that the hydrogenation-driven dynamic phase transition of WO3 in metal-solution-WO3 systems could occur not only in acid solutions but also in some non-acid environments. Unlike the monotonic increase of resistance revealed during HδWO3 to WO3 transition, an intriguing non-monotonic evolution was found for crystal lattice parameter c, indicating that the mechanism of WO3 hydrogenation involves a series of metastable states, more comprehensive and reasonable. This work sheds light on the potential applications of metal-solution-TMO hydrogenation in touching sensors, circuits survey, and information storage.

Pushing the Study of Point Defects in Thin Film Ferrites to Low Temperatures Using In Situ Ellipsometry

AbstractUnveiling point defects concentration in transition metal oxide thin films is essential to understand and eventually control their functional properties, employed in an increasing number of applications and devices. Despite this unquestionable interest, there is a lack of available experimental techniques able to estimate the defect chemistry and equilibrium constants in such oxides at intermediate‐to‐low temperatures. In this study, the defect chemistry of a relevant material such as La1−xSrxFeO3−δ (LSF) with (x = 0.2, 0.4 and 0.5 (LSF20, LSF40 and LSF50 respectively) is obtained by using a novel in situ spectroscopic ellipsometry approach applied to thin films. Through this technique, the concentration of holes in LSF is correlated to measured optical properties and its evolution with temperature and oxygen partial pressure is determined. In this way, a systematic description of defect chemistry in LSF thin films in the temperature range from 350 °C to 500 °C is obtained for the first time, which represents a step forward in the understanding of LSF20, LSF40 and LSF50 for emerging low temperature applications.

Ferroelectricity promoted by cation/anion divacancies in SrMnO3.

We investigate the effect of polar Sr–O vacancy pairs on the electric polarization of SrMnO3 (SMO) thin films using density functional theory (DFT) calculations. This is motivated by indications that ferroelectricity in complex oxides can be engineered by epitaxial strain but also via the defect chemistry. Our results suggest that intrinsic doping by cation and anion divacancies can induce a local polarization in unstrained non-polar SMO thin films and that a ferroelectric state can be stabilized below the critical strain of the stoichiometric material. This polarity is promoted by the electric dipole associated with the defect pair and its coupling to the atomic relaxations upon defect formation that polarize a region around the defect. This suggests that polar defect pairs affect the strain-dependent ferroelectricity in semiconducting antiferromagnetic SMO. For metallic ferromagnetic SMO we find a much weaker coupling between the defect dipole and the polarization due to much stronger electronic screening. Coupling of defect-pair dipoles at high enough concentrations along with their switchable orientation thus makes them a promising route to affect the ferroelectric transition in complex transition metal oxide thin films.

Towards reliable X-ray photoelectron spectroscopy: Sputter-damage effects in transition metal borides, carbides, nitrides, and oxides

Ar+ sputter etching is often used prior to X-ray photoelectron spectroscopy (XPS) analyses with the intention to remove surface oxides and contaminants. Since the XPS probing depth is comparable to the thickness of the ion-beam modified layer the signal from the latter dominates the spectra. We check here the conditions for reliable XPS analysis by studying ion irradiation effects for single-phase Group IVB transition metal (IVB-TM) boride, carbide, nitride, and oxide thin film specimens. The extent of sputter damage, manifested by changes in the surface composition, binding energy shift, peak broadening, and the appearance of new spectral features, varies greatly between material systems: from subtle effects in the case of IVB-TM carbides to a complete change of spectral components for IVB-TM oxides. The determining factors are: (i) the nature of compounds that may form as a result of ion-induced mixing in the affected layer together with (ii) the final elemental composition after sputtering, and (iii) the thickness of the Ar+-affected layer with respect to the XPS probing depth. Our results reveal that the effects of Ar+ ion irradiation on XPS spectra cannot be a priori neglected and a great deal of scrutiny, if not restraint, is necessary during spectra interpretation.

Single-Crystal-like Textured Growth of CoFe2O4 Thin Film on an Amorphous Substrate: A Self-Bilayer Approach

Single crystal or textured growth with preferred orientation of transition-metal oxide thin films is highly desirable for their application in technological devices. Commercially available single-c...

Controlling antiferromagnetic domains in patterned La0.7Sr0.3FeO3 thin films

Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La0.7Sr0.3FeO3 (LSFO) thin films and La0.7Sr0.3MnO3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such that the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction.

Electronic structure of epitaxial perovskite films in the two-dimensional limit: Role of the surface termination

An often-overlooked property of transition metal oxide thin films is their microscopic surface structure and its effect on the electronic properties in the ultrathin limit. Contrary to the expected conservation of the perovskite stacking order in the (001) direction, heteroepitaxially grown SrIrO3 films on TiO2-terminated SrTiO3 are found to exhibit a terminating SrO surface layer. The proposed mechanism for the self-organized conversion involves the adsorption of excess oxygen ions at the apical sites of the IrO2-terminated surface and the subsequent decomposition of the IrO6 octahedra into gaseous molecular IrO3 and the remaining SrO-terminated surface. Whereas the ab initio calculated electronic structure of SrO-terminated SrIrO3 in the monolayer limit exhibits a striking similarity to bulk Sr2IrO4, the broken octahedral symmetry at the IrO2-terminated surface would mix the otherwise crystal field split eg and t2g states, resulting in distinctly different low-energy electronic states.