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Superconducting films of α-Ta are promising candidates for the fabrication of advanced superconducting qubits. However, α-Ta films suffer from many growth-induced structural inadequacies that negatively affect their performance. We have therefore explored a new synthesis method for α-Ta films, which allows for the growth of these films with an unprecedented quality. Using this method, high quality α-Ta films are deposited at a comparably high substrate temperature of 1150 °C. They are single-phase α-Ta and have a single out-of-plane (110) orientation. They consist of grains ≥2 μm that have one of three possible in-plane orientations. As shown by scanning transmission electron microscopy and electron energy loss studies, the substrate–film interfaces are sharp with no observable intermixing. The obtained insights into the epitaxial growth of body-centered-cubic films on quasi-hexagonal substrates lay the basis for harnessing the high structural coherence of such films in various applications.

Sapphire is a technologically highly relevant material, but it poses many challenges when performing epitaxial thin-film deposition. We have identified and applied the conditions for adsorption-controlled homoepitaxial growth of c-plane sapphire. The films thus grown are atomically smooth, have a controlled termination, and are of outstanding crystallinity. Their chemical purity exceeds that of the substrates. The films exhibit exceptional optical properties, such as a single-crystal-like bandgap and a low density of F+ centers.

Infinite-layer nickelates stand as a promising frontier in the exploration of unconventional superconductivity. Their synthesis through topotactic oxygen reduction from the parent perovskite phase remains a complex and elusive process. This study delves into the nano-scale effects of the topotactic lattice transformation within LaNiO2 crystals. Leveraging high-resolution scanning transmission electron microscopy and spectroscopy, our investigations uncover a panorama of structural alterations, including grain boundaries and coherent twin boundaries, triggered by reduction-induced transformations. In addition, our analyses unveil the formation of an oxygen-rich disordered transition phase encircling impurities and pervading crystalline domains and the internal strain is accommodated by grain boundary formation. By unraveling these nano-scale effects, our findings provide insights into the microscopic intricacies of the topotactic reduction process elucidating the transition from the perovskite to the infinite-layer phase within nickelate bulk crystals.

The growing interest in the growth and study of thin films of low-dimensional metallic delafossites, with the general formula ABO2, is driven by their potential to exhibit electronic and magnetic characteristics that are not accessible in bulk systems. The layered structure of these compounds introduces unique surface states as well as electronic and structural reconstructions, making the investigation of their surface behavior pivotal to understanding their intrinsic electronic structure. In this work, we study the surface phenomena of epitaxially grown PtCoO2, PdCoO2, and PdCrO2 films, utilizing a combination of molecular-beam epitaxy and angle-resolved photoemission spectroscopy. Through precise control of surface termination and treatment, we discover a pronounced 3×3 surface reconstruction in PtCoO2 films and PdCoO2 films, alongside a 2 × 2 surface reconstruction observed in PdCrO2 films. These reconstructions have not been reported in prior studies of delafossites. Furthermore, our computational investigations demonstrate the BO2 surface’s relative stability compared to the A-terminated surface and the significant reduction in surface energy facilitated by the reconstruction of the A-terminated surface. These experimental and theoretical insights illuminate the complex surface dynamics in metallic delafossites, paving the way for future explorations of their distinctive properties in low-dimensional studies.

Abstract Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb1+x Se2. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.

Abstract The polarity of a surface can affect the electronic and structural properties of oxide thin films through electrostatic effects. Understanding the mechanism behind these effects requires knowledge of the atomic structure and electrostatic characteristics at the surface. In this study, we use annular bright-field imaging to investigate the surface structure of a Pr0.8Sr0.2NiO2+x (0 < x

Abstract 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.

Abstract The presence of metal atoms at the edges of graphene nanoribbons (GNRs) opens new possibilities toward tailoring their physical properties. We present here formation and high-resolution characterization of indium (In) chains on the edges of graphene-supported GNRs. The GNRs are formed when adsorbed hydrocarbon contamination crystallizes via laser heating into small ribbon-like patches of a second graphitic layer on a continuous graphene monolayer and onto which In is subsequently physical vapor deposited. Using aberration-corrected scanning transmission electron microscopy (STEM), we find that this leads to the preferential decoration of the edges of the overlying GNRs with multiple In atoms along their graphitic edges. Electron-beam irradiation during STEM induces migration of In atoms along the edges of the GNRs and triggers the formation of longer In atom chains during imaging. Density functional theory (DFT) calculations of GNRs similar to our experimentally observed structures indicate that both bare zigzag (ZZ) GNRs as well as In-terminated ZZ-GNRs have metallic character, whereas in contrast, In termination induces metallicity for otherwise semiconducting armchair (AC) GNRs. Our findings provide insights into the creation and properties of long linear metal atom chains at graphitic edges.

Abstract Inline holography in the transmission electron microscope is a versatile technique which provides real-space phase information that can be used for the correction of imaging aberrations, as well as for measuring electric and magnetic fields and strain distributions. It is able to recover high-spatial-frequency contributions of the phase effectively but suffers from the weak transfer of low-spatial-frequency information, as well as from incoherent scattering. Here, we combine gradient flipping and phase prediction in an iterative flux-preserving focal series reconstruction algorithm with incoherent background subtraction that gives extensive access to the missing low spatial frequencies. A procedure for optimizing the reconstruction parameters is presented, and results from Fe-filled C nanospheres, and MgO cubes are compared with phase images obtained using off-axis holography.

Abstract One of the unsolved problems for using high‐Tc superconducting cuprates for spintronic applications are the short coherence lengths of Cooper pairs in oxides (a few Å), which requires atomically sharp and defect‐free interfaces. This research demonstrates the presence of high‐Tc superconducting La1.84Sr0.16CuO4 in direct proximity to SrLaMnO4 and provides evidence for the sharpness of interfaces between the cuprate and the manganite layers at the atomic scale. These findings shed light on the impact of the chemical potential at the interface of distinct materials on highly sensitive physical properties, such as superconductivity. Additionally, this results show the high stability of ultrathin layers from the same K2NiF4‐type family, specifically one unit cell of Sr2−xLaxMnO4 and three unit cells of La1.84Sr0.16CuO4. This work advances both the fundamental understanding of the proximity region between superconducting cuprates and manganite phases and the potential use of oxide‐based materials in quantum computing.

Negative pressure tailors the physical properties of functional oxide materials. Here, the authors demonstrate an emergent multiferroism with magnetodielectric coupling in EuTiO3 created by a negative pressure control of strong spin-phonon coupling.

Abstract The subtle interplay of strong electronic correlations in a distorted crystal lattice often leads to the evolution of novel emergent functionalities in the strongly correlated materials (SCM). Here, we unravel such unprecedented commensurate (COM) and incommensurate (ICOM) charge ordered (CO) phases at room temperature in a simple transition-metal mono-oxide, namely CoO. The electron diffraction pattern unveils a COM ( $$q_{1}$$ q 1 = $$\frac{1}{2}(1,1,{\bar{1}})$$ 1 2 ( 1 , 1 , 1 ¯ ) and ICOM ( $$q_{2}=0.213(1,1,{\bar{1}})$$ q 2 = 0.213 ( 1 , 1 , 1 ¯ ) ) periodic lattice distortion. Transmission electron microscopy (TEM) captures unidirectional and bidirectional stripe patterns of charge density modulations. The widespread phase singularities in the phase-field of the order parameter (OP) affirms the abundant topological disorder. Using, density functional theory (DFT) calculations, we demystify the underlying electronic mechanism. The DFT study shows that a cation disordering ( $$\mathrm {Co}_{1-\textit{x}}\mathrm {O}, \text {with }{} \textit{x} = 4.17 \%$$ Co 1 - x O , with x = 4.17 % ) stabilizes Jahn-Teller (JT) distortion and localized aliovalent $$\mathrm {Co}^{3+}$$ Co 3 + states in CoO. Therefore, the lattice distortion accompanied with mixed valence states ( $$\mathrm {Co}^{3+}, \mathrm {Co}^{2+}$$ Co 3 + , Co 2 + ) states introduces CO in CoO. Our findings offer an electronic paradigm to engineer CO to exploit the associated electronic functionalities in widely available transition-metal mono-oxides.

For La-doped BaSnO3 thin films grown by pulsed laser deposition, we combine chemical surface characterization and electronic transport studies to probe the evolution of electronic states in the band structure for different La-doping contents. Systematic analyses of spectroscopic data based on fitting the core electron line shapes help to unravel the composition of the surface as well as the dynamics associated with increasing doping. These dynamics are observed with a more pronounced signature in the Sn 3d core level, which exhibits an increasing asymmetry to the high binding energy side of the peak with increasing electron density. The present results expand the current understanding of the interplay between the doping concentration, electronic band structure, and transport properties of epitaxial La:BaSnO3 films.

Hyperbolic materials have unique optical properties such as negative refraction and highly directional polaritons, relevant in super-resolution imaging. Here, the topological insulator Bi2Se3 is shown to host hyperbolic edge-confined exciton polaritons that can be steered via engineered edge defects.

Abstract Pseudo-capacitive mechanisms can provide higher energy densities than electrical double-layer capacitors while being faster than bulk storage mechanisms. Usually, they suffer from low intrinsic electronic and ion conductivities of the active materials. Here, taking advantage of the combination of TiS2 decoration, sulfur doping, and a nanometer-sized structure, as-spun TiO2/C nanofiber composites are developed that enable rapid transport of sodium ions and electrons, and exhibit enhanced pseudo-capacitively dominated capacities. At a scan rate of 0.5 mV s−1, a high pseudo-capacitive contribution (76% of the total storage) is obtained for the S-doped TiS2/TiO2/C electrode (termed as TiS2/S-TiO2/C). Such enhanced pseudo-capacitive activity allows rapid chemical kinetics and significantly improves the high-rate sodium storage performance of TiO2. The TiS2/S-TiO2/C composite electrode delivers a high capacity of 114 mAh g−1 at a current density of 5000 mA g−1. The capacity maintains at high level (161 mAh g−1) even after 1500 cycles and is still characterized by 58 mAh g−1 at the extreme condition of 10,000 mA g−1 after 10,000 cycles.

There is growing interest in designing platforms for coherent electron-driven photon sources for hybrid light and electron spectroscopy. Here the authors demonstrate generation of coherent broadband ultrashort light pulses upon electron irradiation to nanostructured gold plated film.

Zinc–ion hybrid supercapacitors are a promising energy storage device as they simultaneously combine the high capacity of batteries and the high power of supercapacitors. However, the practical application of Zinc–ion hybrid supercapacitors is hindered by insufficient energy density and poor rate performance. In this study, a symmetrical zinc–ion hybrid supercapacitor device was constructed with hollow mesoporous-carbon nanospheres as electrode materials, and aqueous ZnSO4 adopted as an electrolyte. Benefiting from the mesoporous structure and high specific area (800 m2/g) of the hollow carbon nanospheres, fast capacitor-type ion adsorption/de-adsorption on both the cathode and the anode can be achieved, as well as additional battery-type Zn/Zn2+ electroplating/stripping on the anode. This device thus demonstrates outstanding electrochemical performance, with high capacity (212.1 F/g at 0.2 A/g), a high energy density (75.4 Wh/kg at 0.16 kW/kg), a good rate performance (34.2 Wh/kg energy density maintained at a high power density of 16.0 kW/kg) and excellent cycling stability with 99.4% capacitance retention after 2,500 cycles at 2 A/g. The engineering of this new configuration provides an extremely safe, high-rate, and durable energy-storage device.

Zinc-based electrochemistry attracts significant attention for practical energy storage owing to its uniqueness in terms of low cost and high safety. In this work, we propose a 2.0-V high-voltage Zn–MnO2 battery with core@shell Co3O4@MnO2 on carbon cloth as a cathode, an optimized aqueous ZnSO4 electrolyte with Mn2+ additive, and a Zn metal anode. Benefitting from the architecture engineering of growing Co3O4 nanorods on carbon cloth and subsequently deposited MnO2 on Co3O4 with a two-step hydrothermal method, the binder-free zinc-ion battery delivers a high power of 2384.7 W kg−1, a high capacity of 245.6 mAh g−1 at 0.5 A g−1, and a high energy density of 212.8 Wh kg−1. It is found that the Mn2+ cations are in situ converted to Mn3O4 during electrochemical operations followed by a phase transition into electroactive MnO2 in our battery system. The charge-storage mechanism of the MnO2-based cathode is Zn2+/Zn and H+ insertion/extraction. This work shines light on designing multivalent cation-based battery devices with high output voltage, safety, and remarkable electrochemical performances.

A key step in engineering resistive switching is the ability to control the device switching behavior. Here, we investigate the possibility to tune the resistive switching of tantalum oxide (TaOx)-based memristors from a non-switchable state to a switchable state by applying post-fabrication annealing of the devices. The switching of the devices was found to be related to: (1) the oxidation state changes in the TaOx thin film after annealing and (2) the local variations in oxygen stoichiometry in the vicinity of the interface between the TiN electrode and the TaOx active resistive layer. We further discuss the possible mechanism behind the resistive switching after annealing. This experimental approach provides a simple but powerful pathway to trigger the resistive switching in devices that do not show any resistive switching initially.

The NdNiO3 (NNO) system has attracted a considerable amount of attention owing to the discovery of superconductivity in Nd0.8Sr0.2NiO2. In rare-earth nickelates, Ruddlesden–Popper (RP) faults play a significant role in functional properties, motivating our exploration of its microstructural characteristics and the electronic structure. Here, we employed aberration-corrected scanning transmission electron microscopy and spectroscopy to study a NdNiO3 film grown by layer-by-layer molecular beam epitaxy (MBE). We found RP faults with multiple configurations in high-angle annular dark-field images. Elemental intermixing occurs at the SrTiO3–NdNiO3 interface and in the RP fault regions. Quantitative analysis of the variation in lattice constants indicates that large strains exist around the substrate–film interface. We demonstrate that the Ni valence change around RP faults is related to a strain and structure variation. This work provides insights into the microstructure and electronic-structure modifications around RP faults in nickelates.

Abstract Among the range of complex interactions, especially at the interfaces of epitaxial oxide systems, contributing to the occurrence of intriguing effects, a predominant role is played by the local structural parameters. In this study, oxide molecular beam epitaxy grown lanthanum cuprate-based bilayers (consisting of a metallic (M) and an insulating phase (I)), in which high-temperature superconductivity arises as a consequence of interface effects, are considered. With the aim of assessing the role of the dopant size on local crystal structure and chemistry, and on the interface functionalities, different dopants (Ca2+, Sr2+ and, Ba2+) are employed in the M-phase, and the M–I bilayers are investigated by complementary techniques, including spherical-aberration-corrected scanning transmission electron microscopy. A series of exciting outcomes are found: (i) the average out-of-plane lattice parameter of the bilayers is linearly dependent on the dopant ion size, (ii) each dopant redistributes at the interface with a characteristic diffusion length, and (iii) the superconductivity properties are highly dependent on the dopant of choice. Hence, this study highlights the profound impact of the dopant size and related interface chemistry on the functionalities of superconducting oxide systems.

Epitaxial films of high critical temperature ( T c ) cuprate superconductors preserve their transport properties even when their thickness is reduced to a few nanometers. However, when approaching the single crystalline unit cell (u.c.) of thickness, T c decreases and eventually, superconductivity is lost. Strain originating from the mismatch with the substrate, electronic reconstruction at the interface and alteration of the chemical composition and of doping can be the cause of such changes. Here, we use resonant inelastic x-ray scattering at the Cu L 3 edge to study the crystal field and spin excitations of NdBa 2 Cu 3 O 7 − x ultrathin films grown on SrTiO 3 , comparing 1, 2 and 80 u.c.-thick samples. We find that even at extremely low thicknesses, the strength of the in-plane superexchange interaction is mostly preserved, with just a slight decrease in the 1 u.c. with respect to the 80 u.c.-thick sample. We also observe spectroscopic signatures for a decrease of the hole-doping at low thickness, consistent with the expansion of the c-axis lattice parameter and oxygen deficiency in the chains of the first unit cell, determined by high-resolution transmission microscopy and x-ray diffraction.

We studied ZrO2 − La2/3Sr1/3MnO3 pillar–matrix thin films which were found to show anomalous magnetic and electron transport properties. With the application of an aberration-corrected transmission electron microscope, interfacial chemistry, and atomic-arrangement of the system, especially of the pillar–matrix interface were revealed at atomic resolution. Minor amounts of Zr were found to occupy Mn positions within the matrix. The Zr concentration reaches a minimum near the pillar–matrix interface accompanied by oxygen vacancies. La and Mn diffusion into the pillar was revealed at atomic resolution and a concomitant change of the Mn valence state was observed.

Epitaxial CeO2 films with different thickness were grown on Y2O3 stabilised Zirconia substrates. Reduction of cerium ions at the interface between CeO2 films and yttria stabilised zirconia substrates is demonstrated using aberration-corrected scanning transmission electron microscopy combined with electron energy-loss spectroscopy. It is revealed that most of the Ce ions were reduced from Ce4+ to Ce3+ at the interface region with a decay of several nanometers. Several possibilities of charge compensations are discussed. Irrespective of the details, such local non-stoichiometries are crucial not only for understanding charge transport in such hetero-structures but also for understanding ceria catalytic properties.