Controlling Octahedral Rotations in a Perovskite via Strain Doping

Dynamic bandgap modulation in CsPbBr3 perovskite nanocrystals through reversible ammonia intercalation

Modulation of the electronic states of a semiconductor is an intriguing area of research because of its interesting applications. In general, physical methods are used to reversibly manipulate the bandgap of semiconductors. Herein, we have used a simple molecule, ammonia, and allowed it to intercalate inside the crystal lattice of CsPbBr3 perovskites to alter the band positions. The molecular intercalation of ammonia induces strain in the crystal structure of perovskite, which widens the bandgap. Ammonia intercalation results in fall-off of the visible absorption and emission of the CsPbBr3 perovskites and a new absorption emerges in the ultraviolet region. Interestingly, with time, the deintercalation takes place, as a result of the population in the antibonding orbitals formed due to the mixing of s orbital of the Pb and p orbital of N in the intercalated NH3. The deintercalation of gaseous ammonia results in the narrowing of the bandgap which results in the regaining of the visible absorption. Together with the density functional theory calculations, herein, we demonstrate the reversible bandgap modulation in CsPbBr3 perovskite nanocrystals. Aspects discussed here can give directions to develop newer methodologies to tune the band positions of semiconductors by the intercalation of the right molecules inside their crystal lattice.

Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties, such as valence, spin, and orbital degrees of freedom, as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and can be subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultrahigh-conductivity, nonmagnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce the emergence of a net moment on the surrounding transition metal octahedral sites. These highly localized moments communicate through the itinerant metal states, which trigger the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultrahigh-conductivity film and will be critical to applications across spintronics.

Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nanoplatelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr₃ nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable-temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

Strain Modified Oxygen Evolution Reaction Performance in Epitaxial, Freestanding, and Van Der Waals Manganite Thin Films

In perovskite complex oxides, the strain has been established as a promising approach for tuning the oxygen evolution reaction (OER) performance by the manipulated electronic structure and interaction/coupling. In this study, we have employed rigid epitaxial, flexible freestanding, and van der Waals La_2/3Sr_1/3MnO₃ (LSMO) to investigate the strain effects on OER, which are different in stress strength and range via lattice mismatch and curvature change. It was found that the OER performances as a function of strain exhibited volcano and monotonous trends in rigid and flexible LSMO, respectively. The findings suggest that distinguished oxygen activation energy in varied lattice fields also plays a crucial role in the epitaxial LSMO in contrast to the pure strain effect in the flexible LSMO. Our results not only fundamentally clarify the effort of strain but also technologically provide an effective route to engineer the electronic structure for modified OER performance by perovskite complex oxides.

Topological Hall effect in SrRuO3thin films and heterostructures

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO₃thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Dimensional Control of Octahedral Tilt in SrRuO3 via Infinite-Layered Oxides

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO₃ (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO₂ (SCO) layers. The infinite-layered CuO₂ exhibits a structural transformation from "planar-type" to "chain-type" with reduced film thickness. Two orientations dramatically modify the polyhedral connectivity at the interface, thus altering the octahedral distortion of SRO. The local structural variation changes the spin state of Ru and orbital hybridization strength, leading to a significant change in the magnetoresistance and anomalous Hall resistivity. These findings could launch investigations into adaptive control of functionalities in quantum oxide heterostructures using oxygen coordination.

Enhanced Oxygen Evolution Electrocatalysis in Strained A-Site Cation Deficient LaNiO3 Perovskite Thin Films

As the BO₆ octahedral structure in perovskite oxide is strongly linked with electronic behavior, it is actively studied for various fields such as metal-insulator transition, superconductivity, and so on. However, the research about the relationship between water-splitting activity and BO₆ structure is largely lacking. Here, we report the oxygen evolution reaction (OER) of LaNiO₃ (LNO) by changing the NiO₆ structure using compositional change and strain. The 5 atom % La deficiency in LNO resulted in an increase of the Ni-O-Ni bond angle and an expansion of bandwidth, enhancing the charge transfer ability. In-plane compressive strain derives the higher d_z² orbital occupancy, leading to suitable metal-oxygen bond strength for OER. Because of the synergistic effect of A-site deficiency and compressive strain, the overpotential (η) of compressively strained L_0.95NO film is reduced to 130 mV at j = 30 μA/cm² compared with nonstrained LNO (η = 280 mV), indicating a significant enhancement in OER.

Thickness-Dependent Perovskite Octahedral Distortions at Heterointerfaces

In this study, we analyze how the octahedral tilts and rotations of thin films of LaNiO₃ and LaAlO₃ grown on different substrates, determined using synchrotron X-ray diffraction-measured half-integer Bragg peaks, depend upon the total film thickness. We find a striking difference between films grown on SrTiO₃ and LaAlO₃ substrates which appears to stem not only from the difference in epitaxial strain state but also from the level of continuity at the heterointerface. In particular, the chemically and structurally discontinuous LaNiO₃/SrTiO₃ and LaAlO₃/SrTiO₃ interfaces cause a large variation in the octahedral network as a function of film thickness whereas the rather continuous LaNiO₃/LaAlO₃ interface seems to allow from just a few unit cells the formation of a stable octahedral pattern corresponding to that expected only given the applied biaxial strain.

Designing Magnetic Anisotropy through Strain Doping

The coupling between a material's lattice and its underlying spin state links structural deformation to magnetic properties; however, traditional strain engineering does not allow the continuous, post-synthesis control of lattice symmetry needed to fully utilize this fundamental coupling in device design. Uniaxial lattice expansion induced by post-synthesis low energy helium ion implantation is shown to provide a means of bypassing these limitations. Magnetocrystalline energy calculations can be used a priori to estimate the predictive design of a material's preferred magnetic spin orientation. The efficacy of this approach is experimentally confirmed in a spinel CoFe2O4 model system where the epitaxial film's magnetic easy axis is continuously manipulated between the out-of-plane (oop) and in-plane (ip) directions as lattice tetragonality moves from ip to oop with increasing strain doping. Macroscopically gradual and microscopically abrupt changes to preferential spin orientation are demonstrated by combining ion irradiation with simple beam masking and lithographic procedures. The ability to design magnetic spin orientations across multiple length scales in a single crystal wafer using only crystal symmetry considerations provides a clear path toward the rational design of spin transfer, magnetoelectric, and skyrmion-based applications where magnetocrystalline energy must be dictated across multiple length scales.

Electromagnetic Functionalization of Wide-Bandgap Dielectric Oxides by Boron Interstitial Doping

A surge in interest of oxide-based materials is testimony for their potential utility in a wide array of device applications and offers a fascinating landscape for tuning the functional properties through a variety of physical and chemical parameters. In particular, selective electronic/defect doping has been demonstrated to be vital in tailoring novel functionalities, not existing in the bulk host oxides. Here, an extraordinary interstitial doping effect is demonstrated centered around a light element, boron (B). The host matrix is a novel composite system, made from discrete bulk LaAlO₃ :LaBO₃ compounds. The findings show a spontaneous ordering of the interstitial B cations within the host LaAlO₃ lattices, and subsequent spin-polarized charge injection into the neighboring cations. This leads to a series of remarkable cation-dominated electrical switching and high-temperature ferromagnetism. Hence, the induced interstitial doping serves to transform a nonmagnetic insulating bulk oxide into a ferromagnetic ionic-electronic conductor. This unique interstitial B doping effect upon its control is proposed to be as a general route for extracting/modifying multifunctional properties in bulk oxides utilized in energy and spin-based applications.

Defect-Induced Exchange Bias in a Single SrRuO3 Layer

Exchange bias stems from the interaction between different magnetic phases, and therefore, it generally occurs in magnetic multilayers. Here, we present a large exchange bias in a single SrRuO₃ layer induced by helium ion irradiation. When the fluence increases, the induced defects not only suppress the magnetization and the Curie temperature but also drive a metal-insulator transition at a low temperature. In particular, a large exchange bias field up to ∼0.36 T can be created by the irradiation. This large exchange bias is related to the coexistence of different magnetic and structural phases that are introduced by embedded defects. Our work demonstrates that spintronic properties in complex oxides can be created and enhanced by applying ion irradiation.

Local structure and defects in ion irradiated KTaO3

The modification of the local structure in cubic perovskite KTaO₃ irradiated with 3 MeV and 1.1 GeV Au ions is studied by Raman and x-ray absorption spectroscopy, complemented by density functional theory (DFT) calculations. In the case of irradiation with 3 MeV Au ions where displacement cascade processes are dominant, the Ta L₃-edge x-ray absorption measurements suggest that a peak corresponding to the Ta-O bonds in the TaO₆ octahedra splits, which is attributed to the formation of Ta_K antisite defects that are coupled with oxygen vacancies, V _O. This finding is consistent with the DFT calculations. Under irradiation with 1.1 GeV ions, the intense ionization and electronic energy deposition lead to a blue shift and an intensity reduction of active Raman bands. In the case of sequential irradiations, extended x-ray absorption fine structure measurements reveal a decrease in concentration of coupled Ta_K-V _O defects under subsequent irradiation with 1.1 GeV Au ions.

Strain effects on oxygen vacancy energetics in KTaO3

Due to lattice mismatch between epitaxial films and substrates, in-plane strain fields are produced in the thin films, with accompanying structural distortions, and ion implantation can be used to controllably engineer the strain throughout the film. Because of the strain profile, local defect energetics are changed. In this study, the effects of in-plane strain fields on the formation and migration of oxygen vacancies in KTaO₃ are investigated using first-principles calculations. In particular, the doubly positive charged oxygen vacancy (V) is studied, which is considered to be the main charge state of the oxygen vacancy in KTaO₃. We find that the formation energies for oxygen vacancies are sensitive to in-plane strain and oxygen position. The local atomic configuration is identified, and strong relaxation of local defect structure is mainly responsible for the formation characteristics of these oxygen vacancies. Based on the computational results, formation-dependent site preferences for oxygen vacancies are expected to occur under epitaxial strain, which can result in orders of magnitude differences in equilibrium vacancy concentrations on different oxygen sites. In addition, all possible migration pathways, including intra- and inter-plane diffusions, are considered. In contrast to the strain-enhanced intra-plane diffusion, the diffusion in the direction normal to the strained plane is impeded under the epitaxial strain field. These anisotropic diffusion processes can further enhance site preferences.

Isosymmetric structural phase transition of the orthorhombic lanthanum gallate structure as a function of temperature determined by Rietveld analysis

High energy planetary ball milling has been used to synthesize pseudo-cubic highly-pure LaGaO₃ in one hour from its oxide components in an air atmosphere.

Highly efficient white-light emission in a polar two-dimensional hybrid perovskite

A polar two-dimensional hybrid perovskite showing efficient white-light emission and a nonlinear optical effect suggests potential application in solid-state optics.

Designing functionality in perovskite thin films using ion implantation techniques: Assessment and insights from first-principles calculations

Recent experimental findings have demonstrated that low doses of low energy helium ions can be used to tailor the structural and electronic properties of single crystal films. These initial studies have shown that changes to lattice expansion were proposed to be the direct result of chemical pressure originating predominantly from the implanted He applying chemical pressure at interstitial sites. However, the influence of possible secondary knock-on damage arising from the He atoms transferring energy to the lattice through nuclear-nuclear collision with the crystal lattice remains largely unaddressed. Here, we study SrRuO₃ to provide a comprehensive examination of the impact of common defects on structural and electronic properties. We found that, while interstitial He can modify the properties, a dose significantly larger than those reported in experimental studies would be required. Our study suggests that true origin of the observed changes is from combination of secondary defects created during He implantation. Of particular importance, we observe that different defect types can generate greatly varied local electronic structures and that the formation energies and migration energy barriers vary by defect type. Thus, we may have identified a new method of selectively inducing controlled defect complexes into single crystal materials.

Electron-Beam-Induced Antiphase Boundary Reconstructions in a ZrO2-LSMO Pillar-Matrix System

The availability of aberration correctors for the probe-forming lenses makes simultaneous modification and characterization of materials down to atomic scale inside a transmission electron microscopy (TEM) realizable. In this work, we report on the electron-beam-induced reconstructions of three types of antiphase boundaries (APBs) in a probe-aberration-corrected TEM. With the utilization of high-angle annular dark-field scanning transmission electron microscopy (STEM), annular bright-field STEM, and electron energy-loss spectroscopy, the motion of both heavy element Mn and light element O atomic columns under moderate electron beam irradiation are revealed at atomic resolution. Besides, Mn segregated in the APBs was observed to have reduced valence states which can be directly correlated with oxygen loss. Charge states of the APBs are finally discussed on the basis of these experimental results. This study provides support for the design of radiation-engineering solid-oxide fuel cell materials.