Effects of nonequilibrium growth, nonstoichiometry, and film orientation on the metal-to-insulator transition in NdNiO₃ thin films

Role of electron and hole doping in the NdNi1-xVxO3 nanostructure

Neodymium nickelate, NdNiO3, attracts attention due to the simultaneous occurrence of several phase transitions around the same temperature. The electronic properties of NdNiO3 are extremely complex as structural distortion, electron correlation, charge ordering, and orbital overlapping play significant roles in the transitions. We report the effects of electron and hole injection via doping a single 3d metal, V, in the NdNiO3 nanostructure to understand the variations in the electronic properties without any structural distortion. A reversible resistivity modulation of more than five orders of magnitude via hole doping and complete suppression of the metal to insulator transition via electron doping is observed along with the switching of major charge carriers. The modulation of electronic properties without any structural distortion and external strain opens up new directions to consider the NdNi1-xVxO3 nanostructures applicable as emerging electronic devices.

Limits to the strain engineering of layered square-planar nickelate thin films

The layered square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd₆Ni₅O₁₂ thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd₄Ni₃O₁₀, and subsequent reduction to the square-planar phase, Nd₄Ni₃O₈. We synthesize our highest quality Nd₄Ni₃O₁₀ films under compressive strain on LaAlO₃ (001), while Nd₄Ni₃O₁₀ on NdGaO₃ (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd₄Ni₃O₁₀ on SrTiO₃ (001). Films reduced on LaAlO₃ become insulating and form compressive strain-induced c-axis canting defects, while Nd₄Ni₃O₈ films on NdGaO₃ are metallic. This work provides a pathway to the synthesis of Nd_n+1Ni_nO_2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Stoichiometry, Orbital Configuration, and Metal-to-Insulator Transition in Nd0.8Sr0.2NiO3 Films

The discovery of superconductivity in the infinite-layer nickelate Nd_0.8Sr_0.2NiO₂ has motivated tremendous efforts for its significance toward the understanding of high-temperature superconductivity. However, the synthesis of infinite-layer nickelates is instable and has become a hindrance to experimental progress. Optimizing the growth of precursor Nd_0.8Sr_0.2NiO₃ by pulsed laser deposition is crucial for obtaining infinite-layer nickelates. By systematically investigating the growth of Nd_0.8Sr_0.2NiO₃ with wide range of conditions, we found that the laser fluence plays a critical role in determining the stoichiometry, lattice structure, and electronic properties. A higher Ni deficiency and larger c-axis lattice constant appeared with the lower laser fluence. At 0.6 J/cm², the Ni deficiency is as large as 25%. According to X-ray absorption spectra and X-ray linear dichroism, we further find that (i) there are no obvious changes of the Ni valence and (ii) the energy level of the d_x²-y² orbital gradually increases relative to the d_3z²-r² orbital with increasing Ni deficiency. What is more, the onset temperature and magnitude of the resistivity change at the metal-to-insulator transitions (MITs) also are found to decrease with increasing laser fluence during the growth.

Template Engineering of Metal-to-Insulator Transitions in Epitaxial Bilayer Nickelate Thin Films

Understanding metal-to-insulator phase transitions in solids has been a keystone not only for discovering novel physical phenomena in condensed matter physics but also for achieving scientific breakthroughs in materials science. In this work, we demonstrate that the transport properties (i.e., resistivity and transition temperature) in the metal-to-insulator transitions of perovskite nickelates are tunable via the epitaxial heterojunctions of LaNiO₃ and NdNiO₃ thin films. A mismatch in the oxygen coordination environment and interfacial octahedral coupling at the oxide heterointerface allows us to realize an exotic phase that is unattainable in the parent compound. With oxygen vacancy formation for strain accommodation, the topmost LaNiO₃ layer in LaNiO₃/NdNiO₃ bilayer thin films is structurally engineered and it electrically undergoes a metal-to-insulator transition that does not appear in metallic LaNiO₃. Modification of the NdNiO₃ template layer thickness provides an additional knob for tailoring the tilting angles of corner-connected NiO₆ octahedra and the linked transport characteristics further. Our approaches can be harnessed to tune physical properties in complex oxides and to realize exotic physical phenomena through oxide thin-film heterostructuring.

Anisotropic chemical expansion due to oxygen vacancies in perovskite films

In scientifically intriguing and technologically important multifunctional ABO3 perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.

Thickness-Dependent Photoelectrochemical Water Splitting Properties of Self-Assembled Nanostructured LaFeO3 Perovskite Thin Films

Tuning the intrinsic structural and stoichiometric properties by different means is used for increasing the green energy production efficiency of complex oxide materials. Here, we report on the formation of self-assembled nanodomains and their effects on the photoelectrochemical (PEC) properties of LaFeO₃ (LFO) epitaxial thin films as a function of layer’s thickness. The variation with the film’s thickness of the structural parameters such as in-plane and out-of-plane crystalline coherence length and the coexistence of different epitaxial orientation—<100>SrTiO₃//<001> LFO, <100>SrTiO₃//<110> LFO and [110] LFO//[10] STO, as well as the appearance of self-assembled nanodomains for film’s thicknesses higher than 14 nm, is presented. LFO thin films exhibit different epitaxial orientations depending on their thickness, and the appearance of self-assembled nanopyramids-like domains after a thickness threshold value has proven to have a detrimental effect on the PEC functional properties. Using Nb:SrTiO₃ as conductive substrate and 0.5 M NaOH aqueous solution for PEC measurements, the dependence of the photocurrent density and the onset potential vs. RHE on the structural and stoichiometric features exhibited by the LFO photoelectrodes are unveiled by the X-ray diffraction, high-resolution transmission electron microscopy, ellipsometry, and Rutherford backscattering spectroscopy results. The potentiodynamic PEC analysis has revealed the highest photocurrent density J_photocurrent values (up to 1.2 mA/cm²) with excellent stability over time, for the thinnest LFO/Nb:SrTiO₃ sample, both cathodic and anodic behavior being noticed. Noticeably, the LFO thin film shows unbiased hydrogen evolution from water, as determined by gas chromatography in aqueous 0.5 M NaOH solution under constant illumination.

Spatially Controlled Octahedral Rotations and Metal-Insulator Transitions in Nickelate Superlattices

The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal-insulator transitions (MITs) in tetragonal NdNiO₃/SrTiO₃ superlattices by controlling the NdNiO₃ layer thickness, n in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at n = 8 to a uniform nontilt pattern at n = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing n the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to n = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.

Switching of majority charge carriers by Zn doping in NdNiO3 thin films

We have studied the effects of Zn doping on the structural and electronic properties of epitaxial NdNiO₃ thin films grown on single-crystal LaAlO₃ (001) (LAO) substrates by pulsed laser deposition. The films are deposited in two sets, one with variation in Zn doping, and another with variation in thickness for undoped and 2% Zn doping. The experimental investigations show that Zn occupies Ni-site and that the films are grown with an in-plane compressive strain on LAO. All the films show metal-to-insulator transitions with a thermal hysteresis in the temperature-dependent resistivity curves except 5% Zn-doped film, which remains metallic. The theoretical fits show non-Fermi liquid behaviour, which gets influenced by Zn doping. The Hall resistance measurements clearly show that Zn doping causes injection of holes in the system which affects the electronic properties as follows: i) the metallic conduction increases by two factors just by 0.5% Zn doping whereas, 5% doping completely suppresses the insulating state, ii) a reversal of the sign of Hall coefficient of resistance is observed at low temperature.

Tuning Photovoltaic Performance of Perovskite Nickelates Heterostructures by Changing the A-Site Rare-Earth Element

Perovskite rare-earth nickelates (RNiO₃) have attracted much attention because of their exotic physical properties and rich potential applications. Here, we report systematic tuning of the electronic structures of RNiO₃ (R = Nd, Sm, Gd, and Lu) by isovalent A-site substitution. By integrating RNiO₃ thin films with Nb-doped SrTiO₃ (NSTO), p-n heterojunction photovoltaic cells have been prepared and their performance has been investigated. The open-circuit voltage increases monotonically with decreasing A-site cation radius. This change results in a downward shift of the Fermi level and induces an increase in the built-in potential at the RNiO₃/NSTO heterojunction, with LuNiO₃/NSTO showing the largest open-circuit voltage. At the same time, the short-circuit current initially increases upon changing the A-site element from Nd to Sm. However, the larger bandgaps of GdNiO₃ and LuNiO₃ reduce light absorption which in turn induces a decrease in the short-circuit current. A power conversion efficiency of 1.13% has been achieved by inserting an ultrathin insulating SrTiO₃ layer at the SmNiO₃/NSTO interface. Our study illustrates how changing the A-site cation is an effective strategy for tuning photovoltaic performance and sheds light on which A-site element is the best for photovoltaic applications, which can significantly increase the applicability of nickelates in optoelectric devices.

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.

Influence of tensile-strain-induced oxygen deficiency on metal-insulator transitions in NdNiO3-δ epitaxial thin films

We report direct evidence that oxygen vacancies affect the structural and electrical parameters in tensile-strained NdNiO_3−δ epitaxial thin films by elaborately adjusting the amount of oxygen deficiency (δ) with changing growth temperature T_D. The modulation in tensile strain and T_D tended to increase oxygen deficiency (δ) in NdNiO_3−δ thin films; this process relieves tensile strain of the thin film by oxygen vacancy incorporation. The oxygen deficiency is directly correlated with unit-cell volume and the metal-insulator transition temperature (T_MI), i.e., resulting in the increase of both unit-cell volume and metal-insulator transition temperature as oxygen vacancies are incorporated. Our study suggests that the intrinsic defect sensitively influences both structural and electronic properties, and provides useful knobs for tailoring correlation-induced properties in complex oxides.

Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films

The effect of A- and B-site cation deficiency on the optical absorption spectrum is presented for a series of LaFeO_3−δ epitaxial films providing insights into the relationship between defect chemistry and electronic structure in this semiconducting perovskite oxide.

Enhanced Electrical Resistivity and Properties via Ion Bombardment of Ferroelectric Thin Films

A novel approach to on-demand improvement of electronic properties in complex-oxide ferroelectrics is demonstrated whereby ion bombardment - commonly used in classic semiconductor materials - is applied to the PbTiO₃ system. The result is deterministic reduction in leakage currents by 5 orders of magnitude, improved ferroelectric switching, and unprecedented insights into the nature of defects and intergap state evolution in these materials.

New modalities of strain-control of ferroelectric thin films

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Ultrathin Films of VO2 on r-Cut Sapphire Achieved by Postdeposition Etching

The metal-insulator transition (MIT) properties of correlated oxides thin films, such as VO2, are dramatically affected by strain induced at the interface with the substrate, which usually changes with deposition thickness. For VO2 grown on r-cut sapphire, there is a minimum deposition thickness required for a significant MIT to appear, around 60 nm. We show that in these thicker films an interface layer develops, which accompanies the relaxation of film strain and enhanced electronic transition. If these interface dislocations are stable at room temperature, we conjectured, a new route opens to control thickness of VO2 films by postdeposition thinning of relaxed films, overcoming the need for thickness-dependent strain-engineered substrates. This is possible only if thinning does not alter the films' electronic properties. We find that wet etching in a dilute NaOH solution can effectively thin the VO2 films, which continue to show a significant MIT, even when etched to 10 nm, for which directly deposited films show nearly no transition. The structural and chemical composition were not modified by the etching, but the grain size and film roughness were, which modified the hysteresis width and magnitude of the MIT resistance change.

Oxygen Vacancy Induced Room-Temperature Metal-Insulator Transition in Nickelate Films and Its Potential Application in Photovoltaics

Oxygen vacancy is intrinsically coupled with magnetic, electronic, and transport properties of transition-metal oxide materials and directly determines their multifunctionality. Here, we demonstrate reversible control of oxygen content by postannealing at temperature lower than 300 °C and realize the reversible metal-insulator transition in epitaxial NdNiO₃ films. Importantly, over 6 orders of magnitude in the resistance modulation and a large change in optical bandgap are demonstrated at room temperature without destroying the parent framework and changing the p-type conductive mechanism. Further study revealed that oxygen vacancies stabilized the insulating phase at room temperature is universal for perovskite nickelate films. Acting as electron donors, oxygen vacancies not only stabilize the insulating phase at room temperature, but also induce a large magnetization of ∼50 emu/cm³ due to the formation of strongly correlated Ni²⁺ t(2g)⁶e(g)² states. The bandgap opening is an order of magnitude larger than that of the thermally driven metal-insulator transition and continuously tunable. Potential application of the newly found insulating phase in photovoltaics has been demonstrated in the nickelate-based heterojunctions. Our discovery opens up new possibilities for strongly correlated perovskite nickelates.

Modulation of metal-insulator transitions by field-controlled strain in NdNiO3/SrTiO3/PMN-PT (001) heterostructures

The band width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in RNiO₃ as a prototype correlated materials. In particular, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on transition temperature so far but the results were inconsistent in the previous literatures. Here, we demonstrate dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO₃ (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut - (PMN-PT) single crystal substrates. Despite the difficulty in the NNO growth on rough PMN-PT substrates, the structural quality of NNO thin films has been significantly improved by inserting SrTiO₃ (STO) buffer layers. Interestingly, the MIT temperature in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective T_MI modulation of field-induced strain than substrate-induced strain. This study provides not only scientific insights on band-width control of correlated materials using pure strain but also potentials for energy-efficient electronic devices.