Epitaxial Stabilization Research Articles

Reviving Sodium Tunnel Oxide Cathodes Based on Structural Modulation and Sodium Compensation Strategy Toward Practical Sodium-Ion Cylindrical Battery.

As a typical tunnel oxide, Na0.44MnO2 features excellent electrochemical performance and outstanding structural stability, making it a promising cathode for sodium-ion batteries (SIBs). However, it suffers from undesirable challenges such as surface residual alkali, multiple voltage plateaus, and low initial charge specific capacity. Herein, an internal and external synergistic modulation strategy is adopted by replacing part of the Mn with Ti to optimize the bulk phase and construct a Ti-containing epitaxial stabilization layer, resulting in reduced surface residual alkali, excellent Na+ transport kinetics and improved water/air stability. Specifically, the Na0.44Mn0.85Ti0.15O2 using water-soluble carboxymethyl cellulose as a binder can realize a capacity retention rate of 94.30% after 1,000 cycles at 2C, and excellent stability is further verified in kilogram large-up applications. In addition, taking advantage of the rich Na content in Prussian blue analog (PBA), PBA-Na0.44Mn1-xTixO2 composites are designed to compensate for the insufficient Na in the tunnel oxide and are matched with hard carbon to achieve the preparation of coin full cell and 18650 cylindrical battery with satisfactory electrochemical performance. This work enables the application of tunnel oxides cathode for SIBs in 18650 cylindrical batteries for the first time and promotes the commercialization of SIBs.

Regulating anionic redox reversibility in Li-rich layered cathodes via diffusion-induced entropy-assisted surface engineering

Cobalt-free lithium- and manganese-rich layered oxides (LMROs) are regarded as effective cathode materials for lithium storage due to their high capacity and cost-effectiveness. However, challenges with structural degradation, resulting in poor cyclability and rate performance, have hindered their widespread use. Essentially, rapid structural degradation arises from irreversible and complex redox reactions, triggering oxygen release, transition metal migration, and electrolyte decomposition. To tackle this issue, we propose an epitaxial entropy-assisted construction approach to develop a sturdy surface with adaptable composition, stabilising the surface crystal structure and preventing undesirable interface reactions. This distinct reconstructed surface comprises diverse heterogeneous elements and composite microstructures. The heteroatom-doped layer, with multi-doping sites like Li, O site, and tetrahedral positions, effectively manages the chemical environment and electronic structure of surface lattice oxygen. This epitaxial entropy stabilisation approach, stemming from multi-element synergy, effectively controls redox progress to limit oxygen release and curb transition metal migration, reducing structural decay. Additionally, the composite coated layer, containing oxygen defects and heterogeneous spinel phases, can hinder electrolyte corrosion and promote Li+ transport. Using these epitaxial entropy surface modifications, the LMRO cathode demonstrates regulated anionic redox reversibility and enhanced cycling stability across diverse operational conditions. This epitaxial entropy-assisted surface engineering offers a promising avenue for stabilising high-energy cathode materials.

Elevating the Superconducting Temperature in Epitaxially Stabilized Rock-Salt NbO

Elevating the Superconducting Temperature in Epitaxially Stabilized Rock-Salt NbO

Peculiar magnetotransport properties in epitaxially stabilized orthorhombic Ru3+ perovskite LaRuO3 and NdRuO3

Complex oxides are interesting materials where multiple physical properties and functionalities can be realized by integrating different elements in a single compound. However, owing to the chemical instability, not all the combinations of elements can be materialized despite the intriguing potential expected from their magnetic and electronic properties. Here we demonstrate an epitaxial stabilization of orthorhombic Ru3+ perovskite oxides: LaRuO3 and NdRuO3, and their magnetotransport properties that reflect the difference between non-magnetic La3+ and magnetic Nd3+. Above all, an unconventional anomalous Hall effect accompanied by an inflection point in magnetoresistance is observed around 1.3 T below 1 K for NdRuO3, which we propose is possibly related to a non-coplanar spin texture on Nd3+ sublattice. These studies not only serve as a new testbed for the interplay between spin-orbit coupling and Coulomb interaction but also open a new avenue to explore topological emergent phenomena in well-studied perovskite oxides.

Atomic Layer Deposition of Epitaxial Ferroelectric Hf0.5Zr0.5O2 Thin Films

AbstractThe groundbreaking discovery of unconventional ferroelectricity in HfO2 opens exciting prospects for next‐generation memory devices. However, the practical implementation, particularly its epitaxial stabilization and a clearer understanding of its intrinsic ferroelectricity has been a significant challenge. The study arouses the potential importance of atomic layer deposition (ALD) for mass production in modern industries, demonstrating its proficiency in achieving epitaxial growth of ferroelectric Hf0.5Zr0.5O2 (HZO) thin films on Yttria‐stabilized zirconia (YSZ) substrates. Moreover, with distinct ferroelectric switching currents, the work reveals the ferroelectric characteristics of epitaxial HZO thin films deposited through ALD on YSZ‐buffered Si substrates, which aligns well with CMOS technology. Overall, the results pave the way for a scalable synthesis system for ferroelectric HfO2‐based materials, hinting at a bright future for low‐temperature epitaxial nanoelectronics.

Epitaxial Stabilization and Persistent Nucleation of the 3C Polymorph of Ba0.6Sr0.4MnO3.

Ba-rich compositions in the BaxSr1-xMnO3 (BSMO) cubic perovskite (3C) system are magnetic ferroelectrics and are of interest for their strong magnetoelectric coupling. Beyond x = 0.5, they only form in hexagonal polymorphs. Here, the 3C phase boundary is pushed to Ba0.6Sr0.4MnO3 for the thin films. Using regular pulsed laser deposition (rPLD), 3C Ba0.6Sr0.4MnO3 could be epitaxially stabilized on DyScO3 (101)o substrates by using a 0.1% O2/99.9% N2 gas mixture. However, the 3C phase was mixed with the 4H polymorph for films 24 nm thick and above, and the films were relatively rough. To improve flatness and phase purity, changes in growth kinetics were investigated and interval PLD (iPLD) was especially effective. In iPLD, deposition is interrupted after completion of approximately one monolayer, and the deposit is annealed for a specific period of time before repeating. Both film flatness and, more importantly, the volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase films. The results imply that iPLD improves the persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.

Epitaxial Stabilization of Perovskite ATeO3 Thin Films

Tellurium oxides of the ATeO3 form typically do not crystallize in perovskite structures. Here, we show that perovskite-like ATeO3 (A = Ca, Sr, Ba) thin films can be grown on perovskite single-crystal substrates via epitaxial stabilization. These films are stable with high optical bandgaps, low dielectric losses, and a high electric breakdown strength. Hysteretic dielectric behavior found in SrTeO3 and BaTeO3 strongly suggests the presence of antiferroelectricity and ferroelectricity, respectively. These properties make perovskite tellurium oxides possibly appealing candidates for thin film coating or insulator materials in advanced microelectronics. Tellurium oxides constitute a largely unexplored class of materials that might show new and interesting functionalities in epitaxial thin-films. Our work encourages new work within this field.

Enhanced Broadband Light Harvesting in Ultrathin Absorbers Enabled by Epitaxial Stabilization of Silver Thin Film Mirrors.

Silver thin film mirrors are attractive candidates for use as specular back reflectors to enhance broadband light absorption via strong optical interference in ultrathin film semiconductor photoabsorbers. However, deposition of metal-oxide absorbers often requires exposure to high temperature in an oxygen atmosphere, conditions that cause thermal etching and degrade the specular reflectance of silver films. Here, we overcome this challenge and demonstrate that epitaxial growth of silver mitigates thermal etching under the high-temperature oxygen-containing environments that cause polycrystalline films to degrade. The degree of thermal etching resistance is related to the epitaxial film structure, where high-quality films completely prevent thermal etching, allowing for direct deposition of metal-oxide thin film photoabsorbers at elevated temperatures without any degradation of the optical properties of the silver layer. As a proof of concept for device applications, a metal-oxide photoanode for photoelectrochemical water splitting is fabricated by directly growing epitaxial SnO2 and Ti-doped α-Fe2O3 (hematite) thin films onto stabilized silver reflectors by pulsed laser deposition. The photoanode displays enhanced broadband light absorption due to strong interference effects enabled by the highly reflective silver film and demonstrates stable operation in a photoelectrochemical cell under conditions of water photo-oxidation in alkaline electrolyte.

Lattice pinning in MoO3 via coherent interface with stabilized Li+ intercalation

Large lattice expansion/contraction with Li+ intercalation/deintercalation of electrode active materials results in severe structural degradation to electrodes and can negatively impact the cycle life of solid-state lithium-based batteries. In case of the layered orthorhombic MoO3 (α-MoO3), its large lattice variation along the b axis during Li+ insertion/extraction induces irreversible phase transition and structural degradation, leading to undesirable cycle life. Herein, we propose a lattice pinning strategy to construct a coherent interface between α-MoO3 and η-Mo4O11 with epitaxial intergrowth structure. Owing to the minimal lattice change of η-Mo4O11 during Li+ insertion/extraction, η-Mo4O11 domains serve as pin centers that can effectively suppress the lattice expansion of α-MoO3, evidenced by the noticeably decreased lattice expansion from about 16% to 2% along the b direction. The designed α-MoO3/η-Mo4O11 intergrown heterostructure enables robust structural stability during cycling (about 81% capacity retention after 3000 cycles at a specific current of 2 A g−1 and 298 ± 2 K) by harnessing the merits of epitaxial stabilization and the pinning effect. Finally, benefiting from the stable positive electrode–solid electrolyte interface, a highly durable and flexible all-solid-state thin-film lithium microbattery is further demonstrated. This work advances the fundamental understanding of the unstable structure evolution for α-MoO3, and may offer a rational strategy to develop highly stable electrode materials for advanced batteries.

Impact of iso-structural template layer on stabilizing pyrochlore Bi2Rh2O7

We present an epitaxial stabilization of pyrochlore Bi2Rh2O7 on the Y-stabilized ZrO2 (YSZ) (111) substrate by inserting a pyrochlore Eu2Ti2O7 template layer, otherwise Bi-based layered structures being formed directly on the YSZ (111) substrate. This result reveals that “iso-structural crystal phase” plays an important role in the interfacial phase control. The Bi2Rh2O7 film exhibits p-type electrical conduction with the lowest longitudinal resistivity (ρxx) among the reported Rh pyrochlore oxides. Such parameters as ρxx, carrier density, and mobility show almost no temperature dependence in the measured range of 2–300 K.

Unusual structural rearrangement and superconductivity in infinite layer cuprate superlattices

Epitaxial stabilization of thermodynamically metastable phases and advances in atomic control of complex oxide thin-film growth can be used effectively for realizing novel phenomena and as an alternative for bulk synthesis under extreme thermodynamic conditions. Here, we investigate infinite layer (IL) based cuprate superlattices, where 7--8 unit cells of ${\mathrm{Sr}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Cu}{\mathrm{O}}_{2}$ (SCCO) are sandwiched between ultrathin spacer layers of $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO), $\mathrm{SrRu}{\mathrm{O}}_{3}$, or $\mathrm{BaCu}{\mathrm{O}}_{2}$ (BCO) and only observe superconductivity in the pure [SCCO/BCO] superlattice (SL) without spacer layers. Apparently, the insertion of an additional STO spacer layer in the latter SL prevents the occurrence of superconductivity. The observed superconductivity in [SCCO/BCO] SL is discussed in terms of a structural model involving the interplay between the $\mathrm{Cu}{\mathrm{O}}_{2}$ plane and the CuO chain similar to the bulk $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ superconductor. The structural origin was found by the identification of a metastable IL-$\mathrm{BaCu}{\mathrm{O}}_{2}$ variant, which deviates highly from its parent bulk crystal structure and exhibits a relatively larger out-of-plane lattice parameter (around $7\phantom{\rule{0.16em}{0ex}}\AA{}$) when sandwiched with SCCO in the form of [SCCO/BCO] SL. However, this variant is absent when STO spacer layers are introduced between SCCO and BCO layers. X-ray absorption spectra of the Cu $L$ edge for BCO exhibits a slightly higher energy satellite peak as compared to the $3{d}^{9}L$ Zhang-Rice character observed in SCCO. This result indicates the existence of contrasting plane and chain-type Cu-O blocks in SCCO and BCO, respectively, which is further corroborated using annular bright field scanning transmission electron microscopy imaging. This work unravels an unexpected structure of BCO which helps in realizing superconductivity in [SCCO/BCO] SL and provides a wider perspective in the growth and design of cuprate-based hybrid structures.

Pressure-Induced Transition from Wurtzite and Epitaxial Stabilization for Thin Films of Rocksalt MgSnN2

The thin-film synthesis of high-pressure phases in inorganic compounds remains a challenge. The synthesis of high-pressure phases in thin-film form opens potential opportunities for creating unique optoelectronic devices because high-pressure phases often exhibit intriguing characteristics that cannot be accessed in ambient phases. We investigated a high-pressure phase of MgSnN2 with the rocksalt structure (rs-MTN) which has only been identified in recent years. rs-MTN is a direct-gap compound, and its (111) plane matches perfectly with GaN(001), which implies that rs-MTN is a promising candidate for optoelectronic materials for light-emitting diodes and tandem solar cells. However, single-phase rs-MTN has never been synthesized in either thin-film or single-crystalline forms. Herein, single-phase rs-MTN thin films were successfully synthesized via two routes. One was the high-pressure heat treatment of wurtzite-type MTN precursor layers, and the other was direct growth onto isostructural MgO(111) substrates using reactive co-sputtering. The former route exploited the pressure-induced wurtzite-to-rocksalt transition and was designed based on first-principles calculations that predicted a transition pressure of ∼8 GPa. The latter route utilized epitaxial stabilization on the (111) plane of MgO. The direct growth of the rs-MTN films with smooth surfaces enabled the investigation into their optoelectronic properties. Consequently, the rs-MTN films were found to be n-type semiconductors with electron densities of an order of 1017 cm–3 and a band gap of 2.3 eV. These findings provide a platform for developing rs-MTN as an optoelectronic semiconductor.

Nano-scale mechanical characteristics of epitaxial stabilization ZrTiN/NbN superlattice coatings

Protective hard thin films are necessary in industrial engineering under extreme condition. In that regard, superlattices with epitaxial stabilization have shown to be a potential approach. The ZrTiN/NbN system was chosen due to the similar lattice parameter and crystal structure. The phase transition from c-NbN to h-NbN was presented while NbN sub-layer was relatively thick in the superlattices, owing to higher VEC (valence electron concentration) of NbN. HRTEM images confirmed that h-NBN led to the loss of epitaxial growth, and thus the coatings demonstrated polycrystalline structure. The superlattices showed no cracks after indentation, possessing stronger crack resistance than ZrTiN (Kc = 3.22 MPa √ m) and NbN (Kc = 2.70 MPa √ m) monolayers. A 3-scan procedure was conducted and multilayers exhibited the staircase-like curve during scratching. The wear volume of multilayers was significantly lower than monolithic films, however, the existence of h-NbN would deteriorate the wear resistance and elastic recovery of the coatings. It can be concluded that epitaxial ZrTiN/c-NbN superlattices exhibited not only favorable mechanical performance but also excellent tribological behavior.

Ferroelectricity and negative piezoelectric coefficient in orthorhombic phase pure ZrO2 thin films

A new approach for epitaxial stabilisation of ferroelectric orthorhombic (o-) ZrO2 films with negative piezoelectric coefficient in ∼ 8nm thick films grown by ion-beam sputtering is demonstrated. Films on (011)-Nb:SrTiO3 gave the oriented o-phase, as confirmed by transmission electron microscopy and electron backscatter diffraction mapping, grazing incidence x-ray diffraction and Raman spectroscopy. Scanning probe microscopy techniques and macroscopic polarization-electric field hysteresis loops show ferroelectric behavior, with saturation polarization of ∼14.3 µC/cm2, remnant polarization of ∼9.3 µC/cm2 and coercive field ∼1.2 MV/cm. In contrast to the o-films grown on (011)-Nb:SrTiO3, films grown on (001)-Nb:SrTiO3 showed mixed monoclinic (m-) and o-phases causing an inferior remnant polarization of ∼4.8 µC/cm2, over 50% lower than the one observed for the film grown on (011)-Nb:SrTiO3. Density functional theory (DFT) calculations of the SrTiO3/ZrO2 interfaces support the experimental findings of a stable polar o-phase for growth on (011) Nb:SrTiO3, and they also explain the negative piezoelectric coefficient.

Technological Peculiarities of Epsilon Ferrite Epitaxial Stabilization by PLD

The present paper describes the technological peculiarities relevant to the nucleation and further epitaxial growth of the metastable epsilon phase of iron oxide by means of pulsed laser deposition (PLD). The orthorhombic epsilon ferrite ε-Fe2O3 is an exotic member of a large family of iron oxide polymorphs, which attracts extensive attention nowadays due to its ultra-high magneto-crystalline anisotropy and room temperature multiferroic properties. Continuing the series of previous publications dedicated to the fabrication of ε-Fe2O3 films on GaN, this present work addresses a number of important requirements for the growing conditions of these films. Among the most sensitive technological parameters, the growth temperature must be high enough to aid the nucleation of the orthorhombic phase and, at the same time, low enough to prevent the thermal degradation of an overheated ε-Fe2O3/GaN interface. Overcoming the contradicting growth temperature requirements, an alternative substrate-independent technique to stabilize the orthorhombic phase by mild aluminum substitution is proposed. The advantages of this technique are demonstrated by the example of ε-Fe2O3 films PLD growth carried out on sapphire—the substrate that possesses a trigonal lattice structure and would normally drive the nucleation of the isostructural and energetically more favorable trigonal α-Fe2O3 phase. The real-time profiling of high-energy electron diffraction patterns has been extensively utilized throughout this work to keep track of the orthorhombic-to-trigonal balance being the most important feed-back parameter at the growth optimization stage.

Epitaxial growth of hexagonal GdFeO3 thin films with magnetic order by pulsed laser deposition

• Hexagonal GdFeO 3 films were prepared using pulsed laser deposition. • The films were obtained on indium tin oxide (ITO(111)) layer. • The hexagonal GdFeO 3 film showed weak ferromagnetism. Hexagonal rare-earth iron oxides, h - R FeO 3 , are promising multiferroic materials. The ferroic properties of h - R FeO 3 can be controlled by varying R . However, h - R FeO 3 exists in a metastable phase, resulting in a limit of available range of R . In this study, we fabricated h -GdFeO 3 phase via epitaxial stabilization. The h -GdFeO 3 epitaxial film with c -axis orientation was deposited on an indium tin oxide (ITO(111)) layer as the bottom electrode via pulsed laser deposition. The metastable hexagonal phase was obtained in a narrow range of deposition parameters. High-crystalline h -GdFeO 3 film was obtained at substrate temperature of 780 °C and oxygen pressure of 13 Pa. The a -axis of the h -GdFeO 3 phase is parallel to the [11–2] directions of ITO(111) layer. The h -GdFeO 3 film exhibited weak ferromagnetism with a Néel temperature of 43 K, which was lower than that of the previously reported h - R FeO 3 films ( R = Dy–Lu, Y and Sc) owing to the longer a -axis.

First-principles indicators of ferroic parameters in epitaxial BiFeO3 and BiCrO3

Density-functional theory is used to validate spin-resolved and orbital-resolved metrics of localized electronic states to anticipate ferroic and dielectric properties of BiFeO3 and BiCrO3 under epitaxial strain. Using previous investigations of epitaxial phase stability in these systems, trends in properties such as spontaneous polarization and bandgap are compared to trends in atomic orbital occupation derived from projected density of states. Based on first principles theories of ferroic and dielectric properties, such as the Modern Theory of Polarization for spontaneous polarization or Goodenough–Kanamori theory for magnetic interactions, this work validates the sufficiency of metrics of localized electronic states to predict trends in multiple ferroic and dielectric properties. Capabilities of these metrics include the anticipation of the transition from G-Type to C-Type antiferromagnetism in BiFeO3 under 4.2% compressive epitaxial strain and the interval of C-Type antiferromagnetism from 3% to 7% tensile epitaxial strain in BiCrO3. The results of this work suggest a capability of localized electronic metrics to predict multiferroic characteristics in the BiXO3 systems under epitaxial strain, with single or mixed B-site occupation.

Synthesis and electronic properties of Ndn+1NinO3n+1 Ruddlesden-Popper nickelate thin films

The rare-earth nickelates possess a diverse set of collective phenomena including metal-to-insulator transitions, magnetic phase transitions, and, upon chemical reduction, superconductivity. Here, we demonstrate epitaxial stabilization of layered nickelates in the Ruddlesden-Popper form, Nd$_{n+1}$Ni$_n$O$_{3n+1}$, using molecular beam epitaxy. By optimizing the stoichiometry of the parent perovskite NdNiO$_3$, we can reproducibly synthesize the $n = 1 - 5$ member compounds. X-ray absorption spectroscopy at the O $K$ and Ni $L$ edges indicate systematic changes in both the nickel-oxygen hybridization level and nominal nickel filling from 3$d^8$ to 3$d^7$ as we move across the series from $n = 1$ to $n = \infty$. The $n = 3 - 5$ compounds exhibit weakly hysteretic metal-to-insulator transitions with transition temperatures that depress with increasing order toward NdNiO$_3$ ($n = \infty)$.

Direct Epitaxial Growth of Polar Hf0.5Zr0.5O2 Films on Corundum.

Single-phase epitaxial Hf0.5Zr0.5O2 films with non-centrosymmetric orthorhombic structure have been grown directly on electrode-free corundum (α-Al2O3) substrates by pulsed laser deposition. A combination of high-resolution X-ray diffraction and X-ray absorption spectroscopy confirms the epitaxial growth of high-quality films belonging to the Pca21 space group, with [111] out-of-plane orientation. The surface of a 7-nm-thick sample exhibits an atomic step-terrace structure with a corrugation of the order of one atomic layer, as proved by atomic force microscopy. Scanning transmission electron microscopy reveals that it consists of grains with around 10 nm lateral size. The polar nature of this film has been corroborated by pyroelectric measurements. These results shed light on the mechanisms of the epitaxial stabilization of the ferroelectric phase of hafnia.

Correlation-induced emergent charge order in metallic vanadium dioxide

Recent progress in growth and characterization of thin-film VO$_2$ has shown its electronic properties can be significantly modulated by epitaxial matching. To throw new light on the concept of `Mott engineering', we develop a symmetry-consistent approach to treat structural distortions and electronic correlations in epitaxial VO$_2$ films under strain, and compare our design with direct experimental probes. We find strong evidence for the emergence of correlation-driven charge order deep in the metallic phase, and our results indicate that exotic phases of VO$_2$ can be controlled with epitaxial stabilization.