Band gap and edge engineering via ferroic distortion and anisotropic strain: the case of SrTiO(3)

Screening of Excitons by Organic Cations in Quasi-Two-Dimensional Organic-Inorganic Lead-Halide Perovskites

Interlayer organic cations in quasi-two-dimensional halide perovskites are a versatile tuning vehicle for the optoelectronic properties of these complex systems, but chemical intuition for this design route is yet to be established. Here, we use density functional theory, the GW approximation, and the Bethe-Salpeter equation approach to understand the contribution of the organic cation to the quasiparticle band gap and exciton binding energy of layered perovskites. We show that organic cations in quasi-two-dimensional perovskites contribute significantly to the dielectric screening in these systems, countering quantum confinement effects on the quasiparticle band gap and the exciton binding energy. Using a simple electrostatics model inspired by parallel-plate capacitors, we decouple the organic cation and inorganic layer contributions to the effective dielectric constants and show that dielectric properties of layered perovskites are broadly tunable via the interlayer cation, providing a direct means of tuning photophysical properties for a variety of applications.

The electronic properties of SrTiO3-δ with oxygen vacancies or substitutions

The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO3 perovskite ferroelectrics - strontium titanate (SrTiO3). The atomic and electronic structure of defects were theoretically investigated using the large-scale first-principles calculations for both bulk crystal and thin films. The results of calculations were experimentally verified by studies of the optical properties at photon energies from 25 meV to 8.8 eV for in-situ prepared thin films. It was demonstrated that substitutions and vacancies prefer locations at surfaces or phase boundaries over those inside crystallites. At the same time, local states in the bandgap can be produced by vacancies located both inside the crystals and at the surface, but by nitrogen substitution only inside crystals. Wide-bandgap insulator phases were evidenced for all defects. Compared to pure SrTiO3 films, bandgap widening due to defects was theoretically predicted and experimentally detected.

In situ anion-doped epitaxial strontium titanate films

Misfit strains arising from a film-substrate mismatch can induce novel phases and properties in the epitaxial films of perovskite oxides. Here we employ yet another effect, namely, strain-assisted formation of oxygen vacancies. We demonstrate the misfit-promoted presence of oxygen vacancies and related substitutional incorporation of anion dopants in the epitaxial films of archetypal perovskite oxide SrTiO₃. Both the oxygen vacancies and hydrogen or nitrogen dopants are introduced in situ during the pulsed-laser deposition of the films using compressive substrates. The films exhibit peculiar chemical expansion and optical properties, which are consistent with substitutional anion doping.

Real-Space Imaging of Orbital Selectivity on SrTiO3(001) Surface

Real-space access of the orbital degree of freedom in complex oxides is still challenging due to intricate electronic hybridization. Here, we report a direct observation of reproducible orbital-selective tunneling on a novel SrTiO₃(001) surface by scanning tunneling microscopy. The electronic structures reversibly switch between two different sets of symmetries depending on the sample bias, which is accompanied by a remarkable change in energy-dependent spectroscopy data. Tunneling spectrum combined with density functional theory calculations elucidates that symmetry-breaking at the surface determines the crystal-splitting field of e_g/t_2g orbitals with a strong in-plane anisotropy so that electrons alternatingly fill e_g and t_2g orbitals during the imaging process with different biases. This surface superstructure provides a new strategy toward understanding orbital textures and orbital selectivity in complex oxides.

Alloy-Free Band Gap Tuning across the Visible Spectrum

We present evidence, from theory and experiment, that ZnSnN_{2} and MgSnN_{2} can be used to match the band gap of InGaN without alloying-by exploiting cation disorder in a controlled fashion. We base this on the determination of S, the long-range order parameter of the cation sublattice, for a series of epitaxial thin films of ZnSnN_{2} and MgSnN_{2} using three different techniques: x-ray diffraction, Raman spectroscopy, and in situ electron diffraction. We observe a linear relationship between S^{2} and the optical band gap of both ZnSnN_{2} (1.12-1.98 eV) and MgSnN_{2} (1.87-3.43 eV). The results clearly demonstrate the correlation between controlled heterovalent cation ordering and the optical band gap, which applies to a broad group of emerging ternary heterovalent compounds and has implications for similar trends in other material properties besides the band gap.

Optical revelation of defects in epitaxial barium titanate films

Allying epitaxial strain and synthesis conditions allows for the introduction of specific point defects in perovskite oxide films. In ferroelectric films, such defects lead to essential polar and electronic properties, which can enable advanced applications. Here, to elucidate the nature of the defects, optical constants are investigated in the spectral range of 0.7-8.8 eV in epitaxial ferroelectric BaTiO₃ films, which are synthesized under different conditions. It is demonstrated that oxygen-vacancy-related defects are responsible for a peculiar transition below the bandgap at ∼2.7-2.9 eV and significant blueshifts of ∼0.3-0.4 eV of the gap and the main interband transitions. These observations suggest that the defects are dipolar complexes comprising titanium cations and oxygen vacancies (Ti³⁺-V_O).

Interface Engineering and Emergent Phenomena in Oxide Heterostructures

Complex oxide interfaces have mesmerized the scientific community in the last decade due to the possibility of creating tunable novel multifunctionalities, which are possible owing to the strong interaction among charge, spin, orbital, and structural degrees of freedom. Artificial interfacial modifications, which include defects, formal polarization, structural symmetry breaking, and interlayer interaction, have led to novel properties in various complex oxide heterostructures. These emergent phenomena not only serve as a platform for investigating strong electronic correlations in low-dimensional systems but also provide potentials for exploring next-generation electronic devices with high functionality. Herein, some recently developed strategies in engineering functional oxide interfaces and their emergent properties are reviewed.

Band Gap Engineering and Room-Temperature Ferromagnetism by Oxygen Vacancies in SrSnO3 Epitaxial Films

Perovskite SrSnO₃ (SSO) thin films were epitaxially grown on LaAlO₃ (001) substrates by pulsed laser deposition at various oxygen pressures. X-ray diffraction was carried out to characterize the microstructure of the films, and the results showed that the unit-cell volume of the films increased gradually with lowering the growth oxygen pressures while remaining the perovskite structure. X-ray photoelectron spectroscopy results indicated that oxygen vacancies (OVs) existed in SSO thin films. Optical property measurements showed that all samples have a transmittance of more than 75% in the visible and near-infrared wavelength region. Furthermore, the band gaps of SSO films were found to increase from 4.56 to 5.21 eV with the decrease of deposition oxygen pressures calculated by linear fitting absorption edges of optical transmittance. In order to further ascertain the effect of OVs on band gaps of SSO films, the as-deposited 10 Pa film was annealed at 10^-4 Pa oxygen pressures and the band gap was found to increase by more than 1 eV. Density functional theory was used to explain the effects of OVs on band gaps and the ferromagnetism of SSO films, and the results suggested that an impurity energy level of OVs appeared near the Fermi level, causing the widening of the band gaps, which is consistent with our experimental results. Meanwhile, the room-temperature ferromagnetism was observed in the SSO films, and saturation magnetization increased gradually from 4.46 to 7.69 emu/cm³ with decreasing the growth oxygen pressures.

Calibrating the Extended Hückel Method to Quantitatively Screen the Electronic Properties of Materials

The extended Hückel (eH) tight-binding method has historically been prized for its computational ease and intuitive chemical clarity. However, its lack of quantitative predictiveness has prevented the eH method from being used as a tool for rapidly screening materials for desired electronic properties. In this work, we demonstrate that when eH input parameters are calibrated using density functional theory (DFT) calculations of carefully chosen sets of simple crystals, the eH parameters retain most of their quantitative accuracy when transferred to more complex, structurally related phases. Using solar-energy-relevant semiconductors and insulators in the Sr–Ti–O family as a case study, we show that calibrated eH parameters can match the features of DFT band structures within about two tenths of an eV, at a tiny fraction of the computational cost of DFT.

Nanoscale Bandgap Tuning across an Inhomogeneous Ferroelectric Interface

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovoltaic effects and the spectral response of the photocurrent were explored to illustrate the reversible bandgap variation (∼0.3 eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the optically related behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters.

Design of new Mott multiferroics via complete charge transfer: promising candidates for bulk photovoltaics

Optimal materials to induce bulk photovoltaic effects should lack inversion symmetry and have an optical gap matching the energies of visible radiation. Ferroelectric perovskite oxides such as BaTiO₃ and PbTiO₃ exhibit substantial polarization and stability, but have the disadvantage of excessively large band gaps. We use both density functional theory and dynamical mean field theory calculations to design a new class of Mott multiferroics–double perovskite oxides A₂VFeO₆ (A = Ba, Pb, etc). While neither perovskite AVO₃ nor AFeO₃ is ferroelectric, in the double perovskite A₂VFeO₆ a ‘complete’ charge transfer from V to Fe leads to a non-bulk-like charge configuration–an empty V-d shell and a half-filled Fe-d shell, giving rise to a polarization comparable to that of ferroelectric ATiO₃. Different from nonmagnetic ATiO₃, the new double perovskite oxides have an antiferromagnetic ground state and around room temperatures, are paramagnetic Mott insulators. Most importantly, the V d⁰ state significantly reduces the band gap of A₂VFeO₆, making it smaller than that of ATiO₃ and BiFeO₃ and rendering the new multiferroics a promising candidate to induce bulk photovoltaic effects.

Temperature-Induced Lattice Relaxation of Perovskite Crystal Enhances Optoelectronic Properties and Solar Cell Performance

Hybrid organic-inorganic perovskite crystals have recently become one of the most important classes of photoactive materials in the solar cell and optoelectronic communities. Albeit improvements have focused on state-of-the-art technology including various fabrication methods, device architectures, and surface passivation, progress is yet to be made in understanding the actual operational temperature on the electronic properties and the device performances. Therefore, the substantial effect of temperature on the optoelectronic properties, charge separation, charge recombination dynamics, and photoconversion efficiency are explored. The results clearly demonstrated a significant enhancement in the carrier mobility, photocurrent, charge carrier lifetime, and solar cell performance in the 60 ± 5 °C temperature range. In this temperature range, perovskite crystal exhibits a highly symmetrical relaxed cubic structure with well-aligned domains that are perpendicular to a principal axis, thereby remarkably improving the device operation. This finding provides a new key variable component and paves the way toward using perovskite crystals in highly efficient photovoltaic cells.

Schottky barrier and band edge engineering via the interfacial structure and strain for the Pt/TiO2 heterostructure

The Schottky barrier of the Pt/TiO₂ interface depends strongly on both the interfacial stain and structure.

Controlling Kondo-like Scattering at the SrTiO3-based Interfaces

The observation of magnetic interaction at the interface between nonmagnetic oxides has attracted much attention in recent years. In this report, we show that the Kondo-like scattering at the SrTiO3-based conducting interface is enhanced by increasing the lattice mismatch and growth oxygen pressure PO2. For the 26-unit-cell LaAlO3/SrTiO3 (LAO/STO) interface with lattice mismatch being 3.0%, the Kondo-like scattering is observed when PO2 is beyond 1 mTorr. By contrast, when the lattice mismatch is reduced to 1.0% at the (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 (LSAT/STO) interface, the metallic state is always preserved up to PO2 of 100 mTorr. The data from Hall measurement and X-ray absorption near edge structure (XANES) spectroscopy reveal that the larger amount of localized Ti(3+) ions are formed at the LAO/STO interface compared to LSAT/STO. Those localized Ti(3+) ions with unpaired electrons can be spin-polarized to scatter mobile electrons, responsible for the Kondo-like scattering observed at the LAO/STO interface.

Optical Properties of Ferroelectric Epitaxial K0.5Na0.5NbO3 Films in Visible to Ultraviolet Range

The complex index of refraction in the spectral range of 0.74 to 4.5 eV is studied by variable-angle spectroscopic ellipsometry in ferroelectric K_0.5Na_0.5NbO₃ films. The 20-nm-thick cube-on-cube-type epitaxial films are grown on SrTiO₃(001) and DyScO₃(011) single-crystal substrates. The films are transparent and exhibit a significant difference between refractive indices Δn = 0.5 at photon energies below 3 eV. The energies of optical transitions are in the range of 3.15–4.30 eV and differ by 0.2–0.3 eV in these films. The observed behavior is discussed in terms of lattice strain and strain-induced ferroelectric polarization in epitaxial perovskite oxide films.

Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO₃ or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology—a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO₃ thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption—reminiscent of piezochromism—which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.

Modern technology such as electronics and photovoltaics requires careful control of optical responses of electronic properties. Here, Sando et al. demonstrate a large variation of optical index and light absorption in multiferroic material BiFeO₃ thin films, tunable by in-film strain or electric field.

Interface Schottky barrier engineering via strain in metal-semiconductor composites

The interfacial carrier transfer property, which is dominated by the interface Schottky barrier height (SBH), plays a crucial role in determining the performance of metal-semiconductor heterostructures in a variety of applications. Therefore, artificially controlling the interface SBH is of great importance for their industrial applications. As a model system, the Au/TiO2 (001) heterostructure is studied using first-principles calculations and the tight-binding method in the present study. Our investigation demonstrates that strain can be an effective way to decrease the interface SBH and that the n-type SBH can be more effectively decreased than the p-type SBH. Astonishingly, strain affects the interface SBH mainly by changing the intrinsic properties of Au and TiO2, whereas the interfacial potential alignment is almost independent of strain due to two opposite effects, which are induced by strain at the interfacial region. These observed trends can be understood on the basis of the general free-electron gas model of typical metals, the tight-binding theory and the crystal-field theory, which suggest that similar trends may be generalized for many other metal-semiconductor heterostructures. Given the commonness and tunability of strain in typical heterostructures, we anticipate that the tunability of the interface SBH with strain described here can provide an alternative effective way for realizing more efficient applications of relevant heterostructures.

High Electron Mobility of Nb-Doped SrTiO₃ Films Stemming from Rod-Type Sr Vacancy Clusters

Achieving high electron mobility in SrTiO3 films is of significant interest, particularly in relation to technological applications such as oxide semiconductors, field-induced superconductors, and thermoelectric generators. One route to achieving high electron mobility is growth of high quality SrTiO3 films with low defect concentrations. Another approach for mobility enhancement is applying a strain to the crystal. However, the maximum mobilities obtainable by these approaches are limited both by external and internal factors (currently available fabrication techniques, and maximum crystal strain, for example). In this paper, we demonstrate a unique crystal engineering approach to alter the strain in Nb-doped SrTiO3 films based on the deliberate introduction of Sr vacancy clusters. Nb-doped SrTiO3 films produced in this manner are found to exhibit remarkably enhanced electron mobilities (exceeding 53,000 cm(2) V(-1) s(-1)). This method of defect engineering is expected to enable tuning and enhancement of electron mobilities not only in SrTiO3 films, but also in thin films and bulk crystals of other perovskite-type materials.

Coupled Nonpolar-Polar Metal-Insulator Transition in 1∶1 SrCrO(3)/SrTiO(3) Superlattices: A First-Principles Study

Using first-principles calculations, we determined the epitaxial-strain dependence of the ground state of the 1∶1 SrCrO(3)/SrTiO(3) superlattice. The superlattice layering leads to significant changes in the electronic states near the Fermi level, derived from Cr t(2g) orbitals. An insulating phase is found when the tensile strain is greater than 2.2% relative to unstrained cubic SrTiO(3). The insulating character is shown to arise from Cr t(2g) orbital ordering, which is produced by an in-plane polar distortion that couples to the superlattice d bands and is stabilized by epitaxial strain. This effect can be used to engineer the band structure near the Fermi level in transition metal oxide superlattices.

Assessment of Strain-Generated Oxygen Vacancies Using SrTiO₃ Bicrystals

Atomic-scale defects strongly influence the electrical and optical properties of materials, and their impact can be more pronounced in localized dimensions. Here, we directly demonstrate that strain triggers the formation of oxygen vacancies in complex oxides by examining the tilt boundary of SrTiO3 bicrystals. Through transmission electron microscopy and electron energy loss spectroscopy, we identify strains along the tilt boundary and oxygen vacancies in the strain-imposed regions between dislocation cores. First-principles calculations support that strains, irrespective of their type or sign, lower the formation energy of oxygen vacancies, thereby enhancing vacancy formation. Finally, current-voltage measurements confirm that such oxygen vacancies at the strained boundary result in a decrease of the nonlinearity of the I-V curve as well as the resistivity. Our results strongly indicate that oxygen vacancies are preferentially formed and are segregated at the regions where strains accumulate, such as heterogeneous interfaces and grain boundaries.

Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3 (KNO), and NaNbO3 (NNO) with a thickness of 10-30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.

First-principles study of electronic structure and photocatalytic properties of MnNiO3 as an alkaline oxygen-evolution photocatalyst

We present a first-principles study of MnNiO₃, a promising oxygen-evolution photocatalyst.

Realizing chemical codoping in TiO2

N and P chemical codoping of TiO₂ has been experimentally demonstrated. The band gap of TiO₂ can be reduced from 3.2 eV to 1.8 eV, without affecting the CBM position.

First-principles characterization of the critical thickness for forming metallic states in strained LaAlO₃/SrTiO₃(001) heterostructure

The emerging two-dimensional electron gas (2DEG) at the interface between polar LaAlO3 (LAO) and nonpolar SrTiO3 (STO) provides potential applications in low-dimensional nanoelectronic devices because of its exceptional electron transport property. To form 2DEG in the LAO/STO heterostructure (HS), a minimum thickness of approximately 4 unit cells of LAO is necessary. Herein, we modeled the n-type (TiO2)(0)/(LaO)(+1) HS by depositing (LAO)n (n = 4, 5, and 6) thin films on the STO substrate and explored strain effects on the critical thickness for forming 2DEG in the LAO/STO HS-based slab systems using first-principles electronic structure calculations. A vacuum layer was added along the [001] direction on the LAO film to resemble the actual epitaxial growth process of the materials. An insulator-to-metal transition is predicted in unstrained (LAO)n/STO systems when n ≥ 5. Our calculations indicate that O 2px/py states give rise to the surface conductivity, while Ti 3dxy states are responsible for the interfacial conductivity. For the tensilely strained HS system, an increased film thickness of LAO (n ≥ 6) is required to form the 2DEG, and a remarkable shift of O 2p orbitals toward higher energy in LAO layers is found, which is caused by the strain-induced change of the electrostatic potential. For the compressively strained HS system, the critical thickness of LAO film for forming 2DEG is between 5 and 6 unit cells of LAO. In addition, our calculations suggest that the interfacial charge carrier density and magnetic moment can be optimized when a moderate tensile strain is applied on the STO substrate in the ab-plane.

BiFeO3 epitaxial thin films and devices: past, present and future

The celebrated renaissance of the multiferroics family over the past ten years has also been that of its most paradigmatic member, bismuth ferrite (BiFeO3). Known since the 1960s to be a high temperature antiferromagnet and since the 1970s to be ferroelectric, BiFeO3 only had its bulk ferroic properties clarified in the mid-2000s. It is however the fabrication of BiFeO3 thin films and their integration into epitaxial oxide heterostructures that have fully revealed its extraordinarily broad palette of functionalities. Here we review the first decade of research on BiFeO3 films, restricting ourselves to epitaxial structures. We discuss how thickness and epitaxial strain influence not only the unit cell parameters, but also the crystal structure, illustrated for instance by the discovery of the so-called T-like phase of BiFeO3. We then present its ferroelectric and piezoelectric properties and their evolution near morphotropic phase boundaries. Magnetic properties and their modification by thickness and strain effects, as well as optical parameters, are covered. Finally, we highlight various types of devices based on BiFeO3 in electronics, spintronics, and optics, and provide perspectives for the development of further multifunctional devices for information technology and energy harvesting.

Oxygen octahedral distortions in LaMO3/SrTiO3 superlattices

In this work we study the interfaces between the Mott insulator LaMnO3 (LMO) and the band insulator SrTiO3 (STO) in epitaxially grown superlattices with different thickness ratios and different transport and magnetic behaviors. Using atomic resolution electron energy-loss spectral imaging, we analyze simultaneously the structural and chemical properties of these interfaces. We find changes in the oxygen octahedral tilts within the LaMnO3 layers when the thickness ratio between the manganite and the titanate layers is varied. Superlattices with thick LMO and ultrathin STO layers present unexpected octahedral tilts in the STO, along with a small amount of oxygen vacancies. On the other hand, thick STO layers exhibit undistorted octahedra while the LMO layers present reduced O octahedral distortions near the interfaces. These findings are discussed in view of the transport and magnetic differences found in previous studies.

Competition and cooperation between antiferrodistortive and ferroelectric instabilities in the model perovskite SrTiO3

We use density functional theory to explore the interplay between octahedral rotations and ferroelectricity in the model compound SrTiO3. We find that over the experimentally relevant range octahedral rotations suppress ferroelectricity as is generally assumed in the literature. Somewhat surprisingly, we observe that at larger angles the previously weakened ferroelectric instability strengthens significantly. By analyzing geometry changes, energetics, force constants and charges, we explain the mechanisms behind this transition from competition to cooperation with increasing octahedral rotation angle.

Interplay of octahedral tilts and polar order in BiFeO3 films

Heterointerface stabilization of a distinct nonpolar BiFeO3 phase occurs simultaneously with changes in octahedral tilts. The resulting phase arises via suppression of polarization by a structural order parameter and can thus be identified as anti-ferroelectric (Fe displacements - bottom panel). The phase is metastable and can be switched into a polar ferroelectric state (top panel) under an applied electric bias.

Emergent superstructural dynamic order due to competing antiferroelectric and antiferrodistortive instabilities in bulk EuTiO3

Microscopic structural instabilities of EuTiO3 single crystals were investigated by synchrotron x-ray diffraction. Antiferrodistortive (AFD) oxygen octahedron rotational order was observed alongside Ti derived antiferroelectric distortions. The competition between the two instabilities is reconciled through a cooperatively modulated structure allowing both to coexist. The combination of electric and magnetic fields increases the population of the modulated AFD order, illustrating how the origin of the large magnetoelectric coupling derives from the dynamic equilibrium between AFD and polar instabilities.