LaCoO3 Films Research Articles

Micropatterning of LaCoO3 thin films through a novel photosensitive sol-gel lithography technique and their magnetic properties

This paper presents a study on the microstructure and magnetic properties of LaCoO3 (LCO) thin films, and also the micropatterning of LCO thin films using a novel photo-sensitive sol-gel lithography method. It is found that higher heat-treatment temperature can promote the healing of cracks and filling of pores, resulting in refining the grains, and improving the density of LCO film. The film treated at 850 °C has a larger nucleation energy, which makes the distribution of nucleation sites more uniform, promoting the healing of cracks and filling of pores, refining the grains, and improving the density of LCO film. LCO precursor sol, metal chelates with UV photosensitivity were synthesized by adding BzAcH photosensitizer. LCO micro-arrays with diameters of 50 μm, as well as other fine patterns with a minimum line width of about 2 μm were successfully prepared by a UV mask lithography technique. In contrast, our novel lithography method eliminates the need for photoresist, as the fine pattern is formed during the preparation of the LCO gel film, which is the advantages of our lithography method over traditional techniques.

Simultaneous control of ferromagnetism and ferroelasticity by oxygen octahedral backbone stretching

Abstract Coexistence of ferromagnetism and ferroelasticity in a single material is an intriguing phenomenon, but has been rarely found. Here we studied both the ferromagnetism and ferroelasticity in a group of LaCoO3 films with systematically tuned atomic structures. We found that all films exhibit ferroelastic domains with four-fold symmetry and the larger domain size (higher elasticity) is always accompanied by stronger ferromagnetism. We performed synchrotron x-ray diffraction studies to investigate the backbone structure of the CoO6 octahedra, and found that both the ferromagnetism and the elasticity are simultaneously enhanced when the in-plane Co–O–Co bond angles are straightened. Therefore the study demonstrates the inextricable correlation between the ferromagnetism and ferroelasticity mediated through the octahedral backbone structure, which may open up new possibilities to develop multifunctional materials.

Comparing the Impacts of Strain Types on Oxygen-Vacancy Formation in a Perovskite Oxide via Nanometer-Scale Strain Fields.

The utilization of an in-plane lattice misfit in an oxide epitaxially grown on another oxide with a different lattice parameter is a well-known approach to induce strains in oxide materials. However, achieving a sufficiently large misfit strain in this heteroepitaxial configuration is usually challenging, unless the thickness of the grown oxide is kept well below a critical value to prevent the formation of misfit dislocations at the interface for relaxation. Instead of adhering to this conventional approach, here, we employ nanometer-scale large strain fields built around misfit dislocations to examine the effects of two distinct types of strains─tension and compression─on the generation of oxygen vacancies in heteroepitaxial LaCoO3 films. Our atomic-level observations, coupled with local electron-beam irradiation, clarify that the in-plane compression notably suppresses the creation of oxygen vacancies, whereas the formation of vacancies is facilitated under tensile strain. Demonstrating that the defect generation can considerably vary with the type of strain, our study highlights that the experimental approach adopted in this work is applicable to other oxide systems when investigating the strain effects on vacancy formation.

Photoconductive responses to biaxially tensile strain in LaCoO3 epitaxial films

Cobalt-based perovskite-type LaCoO3 epitaxial films, with different thickness (105, 44, 38 and 11 nm), were epitaxially grown on (100) SrTiO3 substrates by polymer assisted deposition. LaCoO3 films have different degree of CoO6 octahedral distortion, i.e., Jahn-Teller-like tetragonal distortion caused by the biaxially tensile strain. The photoconductivity of the LaCoO3 epitaxial films under visible light increases gradually with the decrease of film thickness, accompanied by the increasing of CoO6 octahedral tetragonal distortion. The results reveal that the photoconductive performance of the LaCoO3 epitaxial films is largely related to the biaxially tensile-strained tetragonal distortion of CoO6. Higher degree of tetragonal distortion corresponds to smaller value of crystal field splitting energy as well as the band gap, which is beneficial to photoconductivity.

In-Situ Atomic-Scale Observation of Brownmillerite to Ruddlesden–Popper Phase Transition Tuned by Epitaxial Strain in Cobaltites

Phase transitions involving oxygen ion extraction within the framework of the crystallographic relevance have been widely exploited for sake of superconductivity, ferromagnetism, and ion conductivity in perovskite-related oxides. However, atomic-scale pathways of phase transitions and ion extraction threshold are inadequately understood. Here we investigate the atomic structure evolution of LaCoO3 films upon oxygen extraction and subsequent Co migration, focusing on the key role of epitaxial strain. The brownmillerite to Ruddlesden–Popper phase transitions are discovered to stabilize at distinct crystal orientations in compressive- and tensile-strained cobaltites, which could be attributed to in-plane and out-of-plane Ruddlesden–Popper stacking faults, respectively. A two-stage process from exterior to interior phase transition is evidenced in compressive-strained LaCoO2.5, while a single-step nucleation process leaving bottom layer unchanged in tensile-strained situation. Strain analyses reveal that the former process is initiated by an expansion in Co layer at boundary, whereas the latter one is associated with an edge dislocation combined with antiphase boundary. These findings provide a chemo-mechanical perspective on the structure regulation of perovskite oxides and enrich insights into strain-dependent phase diagram in epitaxial oxides films.

Hydrogen-Driven Low-Temperature Topotactic Transition in Nanocomb Cobaltite for Ultralow Power Ionic-Magnetic Coupled Applications.

We reversibly control ferromagnetic-antiferromagnetic ordering in an insulating ground state by annealing tensile-strained LaCoO3 films in hydrogen. This ionic-magnetic coupling occurs due to the hydrogen-driven topotactic transition between perovskite LaCoO3 and brownmillerite La2Co2O5 at a lower temperature (125-200 °C) and within a shorter time (3-10 min) than the oxygen-driven effect (500 °C, tens of hours). The X-ray and optical spectroscopic analyses reveal that the transition results from hydrogen-driven filling of correlated electrons in the Co 3d-orbitals, which successively releases oxygen by destabilizing the CoO6 octahedra into CoO4 tetrahedra. The transition is accelerated by surface exchange, diffusion of hydrogen in and oxygen out through atomically ordered oxygen vacancy "nanocomb" stripes in the tensile-strained LaCoO3 films. Our ionic-magnetic coupling with fast operation, good reproducibility, and long-term stability is a proof-of-principle demonstration of high-performance ultralow power magnetic switching devices for sensors, energy, and artificial intelligence applications, which are keys for attaining carbon neutrality.

High stability visible-light photoresponse of flexible heterostructures based on LaCoO3 epitaxial films

The research of high-speed photodetector and high-speed optical receiver garners significant attention due to their indispensable properties in many fields including high-temperature event monitoring, security, and ad hoc network wireless communication. Inorganic perovskite oxides LaCoO3 films showcase an extensive prospect in this area owing to their remarkable photoconductive effect. Up to present, however, LaCoO3 films are mostly grown on rigid instead of flexible substrates, greatly limiting their applications to wearable optoelectronic sensors. In this work, we fabricated some flexible LaCoO3 thin films to study their photoresponse. By inserting SrTiO3 and BaTiO3 as buffer layers, we find that LaCoO3 thin films could epitaxially grow on the fluorphlogopite surface and show a stable photoresponse to visible-light. Nevertheless, we also found that the epitaxial strain and the content of surface-adsorbed oxygen could impact the photoconductivity. The mechanical strain generated by bending the fluorphlogopite substrate has been confirmed to has a tunable effect on the photoresponse. Moreover, with the fatigue tests of 105 bending cycles, the flexible LaCoO3 thin films maintain a good photoresponse without any essentially weakening, proving a superior mechanical durability and stability. This work not only indicates the feasibility of the LaCoO3 films applied for flexible photodetectors but also opens a path for other perovskite films applications for flexible substrate.

Atomic Imaging of Lattice and Electron Ordering in Tensile-Strained LaCoO3 Films

Atomic Imaging of Lattice and Electron Ordering in Tensile-Strained LaCoO3 Films

Thickness dependent strain effect on ferromagnetic insulating properties of LaCoO3 thin films

Epitaxial tensile-strain is an important factor to determine the superior ferromagnetic insulating property of LaCoO3 (LCO) epitaxial films. In this paper, the whole development of LCO ferromagnetic insulating properties modulated by thickness-dependent strain effect were studied and we found that the film strain decreased and the ferromagnetic property weakened with increasing film thickness. The measurements of x-ray magnetic circular dichroism confirm that the spin moment of Co3+ ions is significantly larger than its orbital magnetic moment and it also reduces with increasing thickness. The x-ray absorption spectroscopy revealed that the increasing film thickness is unfavorable for the presence of eg electrons, as well as for the intermediate-spin and high-spin states. Overall, the enhancement of film thickness leads to the decrease of the Co-O bond length and the increase of crystal field splitting energy, unfavorable for the electron transfer from t2g to eg orbitals, and in turn for the stability of intermediate-spin and high-spin states. The electronic transport behavior of LCO film was proved to be a good insulating properties and the temperature dependence of the resistivity is consistent with the 3D-Mott’s variable range hopping model. Our results not only demonstrated the feasibility of thickness-induced strain modulating the properties of LCO films, but also strengthen the understanding of the ferromagnetic insulating mechanism in LCO films.

The significant magnetic attenuation with submicrometer scale magnetic phase separation in tensile-strained LaCoO3 films

It is well known that the epitaxial strain plays a vital role in tuning the magnetic states in transition metal oxide LaCoO3 films. Here, we reported a robust long-range ferromagnetic (FM) ground state in a tensile-strained perovskite LaCoO3 film on a SrTiO3 (STO) substrate, which has a very significant attenuation when the thickness ranges from 10 to 50 nm. It is speculated that such attenuation may be caused by the appearance of the cross-hatched grain boundary, which relaxes the tensile strain around the crosshatch, resulting in the local non-FM phases. Magnetic force microscope observation reveals non-FM patterns correlated with the structural crosshatches in the strain-relaxed film even down to a temperature of 2 K and up to a magnetic field of 7 T, suggesting the phase separation origin of magnetization attenuation. Furthermore, the investigations of the temperature-dependent inverse magnetic susceptibility show a deviation from the Curie–Weiss law above the transition temperature in a 50-nm-thick LaCoO3/STO film but not in the LaCoO3/LaAlO3 film, which is ascribed to the Griffiths phase due to the crosshatch-line grain boundaries. These results demonstrated that the local strain effect due to structural defects is important to affect the ferromagnetism in strain-engineered LaCoO3 films, which may have potential implications for future oxide-based spintronics.

Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films.

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it isunambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it isinferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. Thestudy highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO6 octahedral rotations throughout LaCoO3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO3 films while suggesting potential applications toward low-power spintronic devices.

Tuning charge transfer and magnetic properties in LaCoO3/LaMnO3/LaCoO3 trilayers by oxygen content

In this manuscript, perovskite oxide LaCoO3/LaMnO3/LaCoO3 (LLL) trilayers and the single-layer LaMnO3 (LMO) and LaCoO3 (LCO) films are epitaxially grown on (001)-oriented SrTiO3 substrates by polymer-assisted deposition combining with heat treating in Ar, air, and O2 atmospheres, respectively. The crystal structure and magnetic properties are investigated. It is found that the responses of LMO and LCO films to oxygen content are different, and the magnetic property of LMO is more sensitivity to oxygen content than that of LCO. The oxygen content can directly affect the structure and magnetic properties of LMO and LCO layers, which, in turn, modulate the interfacial magnetic properties of the LLL trilayers through interlayer coupling. By comparison with the magnetic properties of the single-layer films, an abnormal ferromagnetic (FM) transition is found in the LLL trilayers at around 216 K, besides two known FM transitions corresponding to those in the LCO and LMO layers, respectively. It is suggested that the abnormal FM transition is attributed to Mn4+–O–Co2+ FM super-exchange interaction, which is induced by the charge transfer from Mn3+ to Co3+ ions and the interlayer coupling between the LMO and LCO layers. The interlayer coupling of the trilayers, including the interfacial coupling and the diffusion-induced inlayer coupling, dominates the interface properties, which are tuned by oxygen content-dependent charge transfer. The cooperative function of the orbital/charge structure and the interlayer coupling dominates the magnetic properties of the LLL trilayer under the tensile strain. These findings provide new avenues to regulate the electronic and magnetic states of perovskite oxide multilayers and another way to the development of multifunctional electronic devices.

Enhanced Ferromagnetism Induced by Chemical Doping and Epitaxial Strain in La0.8Sr0.2CoO3 Films

The perovskite LaCoO3 and La0.8Sr0.2CoO3 thin films were synthesized successfully by a polymer-assisted deposition method. The structural characterization and thermal decomposition measurements indicate that appropriately increasing the annealing temperature is helpful in improving the film crystallinity. Compared with LaCoO3 films, the Curie temperature is enhanced to a higher temperature in La0.8Sr0.2CoO3 films, which is attributed to the changes in both the valence state and spin state induced by the chemical doping. In addition, a broad transition temperature region is observed in La0.8Sr0.2CoO3 films, revealing the existence of inhomogeneous ground states in this system.

Octahedral rotation induced spin state and metal–insulator transition in LaCoO[formula omitted] films

The discussions over the origins of the magnetic and insulating properties in LaCoO3 films has continued unabated even now. We investigate the energy of natural low spin, homogeneous intermediate spin, and mixed spin configurations using first principle calculations, taking into account various distortions in bulk and films. The spin state transition can be controlled by rhombohedral distortion and octahedral rotations in films bulk. The octahedral rotation reserved in the films bulk, and it can also induce the breathing distortion or the Jahn–Teller distortion when increase the mismatch ratio. The breathing distortion or the Jahn–Teller distortion change the electronic structure from a metal to semiconductor. The octahedral rotations can be a efficient regulatory to control electronic structure and spin states. The constrain of mixed spin configuration (IS:LS=1:2) potentially cause a charge disproportionation, resulting in a quasi-breathing distortion similar to magnetoelastic effect.

Photocatalytic reduction of CO2 over LaMO3 (M: Fe, Co, Mn) /CuxO films

LaMO3/CuxO (M: Co, Mn, Fe) thin films were prepared in two steps. First, lanthanum perovskites were deposited by spin coating, and then a CuxO layer by Chemical Vapor Deposition. The X-ray diffraction characterization confirmed the formation of the crystalline perovskite phases. The topography of the films was observed by AFM analysis, where fine needles grains appeared in LaMnO3/CuxO and LaCoO3/CuxO perovskite films, being attributed to the CuxO layer. XPS characterization confirms the presence of a larger concentration of oxygenated species over the surface of the films; besides, it was possible to elucidate a mixed-valence state of copper nanoparticles. DR UV–vis analyses show a redshift displacement for LaCoO3 and LaMnO3 films with the addition of copper nanoparticles, related to improved light absorption. The evaluation of the CO2 photoreduction using the pure perovskite films was performed. The reaction showed that adsorbed oxygenated species (CO32−, OH−, among others) over the surface could hinder active sites causing low performance. Once films were decorated with CuxO nanoparticles, their performance was improved significantly in the formic acid evolution. Copper nanoparticles acted like active sites for CO2 reduction, promoting the separation of the charges.LaMnO3/CuxO films presented the higher production of formic acid associated with low presence of oxygenated species in the surface and the formation of scheme Z between semiconductors, allowing a better charge transfer.

In-situ quantification of the surface roughness for facile fabrications of atomically smooth thin films

This work presents an in-situ technique to quantify the layer-by-layer roughness of thin films and heterostructures by measuring the spectral profile of the reflection high-energy electron diffraction (RHEED). The characteristic features of the diffraction spot, including the vertical to lateral size ratio c/b and the asymmetrical ratio c1/c2 along the vertical direction, are found to be quantitatively dependent on the surface roughness. The quantitative relationships between them are established and discussed for different incident angles of high-energy electrons. As an example, the surface roughnesses of LaCoO3 films grown at different temperatures are obtained using such an in-situ technique, which are confirmed by the ex-situ atomic force microscopy. Moreover, the in-situ measured layer-by-layer roughness oscillations of two LaCoO3 films are demonstrated, revealing drastically different information from the intensity oscillations. The experiments assisted with the in-situ technique demonstrate an outstanding high resolution down to ∼ 0.1 Å. Therefore, the new quantitative RHEED technique with real-time feedbacks significantly escalates the thin film synthesis efficiency, especially for achieving atomically smooth surfaces and interfaces. It opens up new prospects for future generations of thin film growth, such as the artificial intelligence-assisted thin film growth.

Ce-doped LaCoO3 film as a promising gas sensor for ethanol

With increasing exposure to ethanol in various scenarios, including hand sanitizers that combat bacteria and viruses, energy-efficient miniaturized sensors capable of detecting excessive concentrations of ethanol are required in the fields of disinfection and chemical storage. Modified perovskite-type oxides with ABO3 structures are particularly attractive because they can be appropriately modified and have been used in heterogeneous catalysis and gas sensors. In this study, we designed and manufactured a novel thin-film-type LaCeCoO3 gas sensor using lithography technology and precursor-solution doping. The samples exhibited morphologies that contain randomly oriented nanostructures and short rods; the particle size was observed to decrease with Ce-addition. Room-temperature gas-sensing characterization studies revealed high reproducibility for the detection of ethanol. La0.96Ce0.04CoO3 exhibited superior stability and sensitivity, with a high impedance, |Z|, of ∼120 kΩ and a Δ|Z| of up to 77%, with response and recovery times of 16 and 8 s, respectively. This study provides a rational method for the development of LaCeCoO3 as a semiconducting material for ethanol gas-sensing applications.

Tuning of the oxygen vacancies in LaCoO3 films at the atomic scale

Oxygen vacancies (Vo) play significant roles in determining the properties of transition-metal oxides. However, the concentration of Vo cannot be tuned quantitatively by optimizing the preparation conditions, and the precise control of Vo distribution at the atomic scale is even more challenging. Here, by controlling the reversible phase transitions between perovskite LaCoO3 (PV-LCO) and brownmillerite LaCoO2.5, we realize the tuning of Vo in PV-LCO, including the concentration with quantitative precision and the spatial distribution at the atomic scale. With the first principles calculations, we clarify that two thirds of Vo in PV-LCO can be eliminated after a cycle of the reversible phase transitions, and all the residual Vo are confined in specific lattice sites in PV-LCO. Such an ordered distribution of Vo can help to enhance the ferromagnetism of PV-LCO.

Two-dimensional magnetic interplay in the tensile-strained LaCoO3 thin films.

High-quality epitaxial LaCoO3 (LCO) thin films have been deposited on SrTiO3 (STO) substrates with pulsed laser deposition (PLD). We find that the LCO films undergo a typical ferromagnetic-paramagnetic (FM-PM) phase transition at ∼80 K. To understand the nature of magnetic phase transition, various methods, including the modified Arrott plot and critical isotherm analysis, were used to determine the critical exponents, which are β = 0.754(1) with a Curie temperature TC = 79.8(8) K and γ = 1.52(2) with TC = 79.9(2) K. The reliability of these critical exponents was confirmed using the Widom scaling relation and the scaling hypothesis. Further analysis revealed that the spin coupling within the LCO films exhibits two-dimensional (2D) long-range magnetic interaction and the magnetic exchange distance decays as J(r) ∼r-(3.46). Our critical behavior analysis may shed new light on the further understanding of the origin of FM and the relatively fixed TC in LCO thin films.