Single Crystal LaAlO3 Research Articles

Influence of Ce doping and thickness on the structure and non-Fermi liquid behavior of LaNiO3 thin films

Thin films of La1-xCexNiO3 (x = 0, 0.1, and 0.3) are grown with two different thickness (8 and 24 nm) on single-crystal LaAlO3 (001) (LAO) substrate using pulsed laser deposition. The films with x = 0 and 0.1 are highly oriented toward the (00l) axis despite different thickness, whereas the higher doped films (x = 0.3) of both thicknesses show two crystallographic orientations. A significant shifting of Raman modes is observed due to increased Ce doping, clearly indicating a large change in Ni–O–Ni bond-angle and NiO6 octahedra distortion. Also, Raman modes show a red-shift in all the thin films with increases in temperature. These films remain metallic until low temperatures. The resistivity increases with increased Ce doping as well as with increased thickness. The resistivity data fit well with the power-law equation, indicating a non-Fermi liquid state. This study helps to distinguish the effects of Ce doping and thickness on the structure and Raman modes.

Structural stability and electronic transitions of NdNi0.98Zn0.02O3−δ thin films

NdNiO3 is a multiband system and shows temperature-driven first-order metal to insulator phase transition. Divalent Zn doping at trivalent Ni-site in NdNiO3 injects holes in the system. To study the effects of oxygen variation in a combination of Zn doping, NdNi1−xZnxO3−δ (x = 0, 0.02) thin films (30 nm) have been deposited on single-crystal LaAlO3 (001) substrate. The broadening of temperature-driven metal–insulator transition reduces drastically just by 2% Zn-doping at Ni-site. Further, the variation in oxygen content in NdNi0.98Zn0.02O3−δ films modifies the structural and electronic properties quite systematically and significantly. The Raman modes related to Ni–O bonds get blue-shifted due to the increase in oxygen content of thin films. A comparative study of the films with and without doping clearly shows that Zn-doping provides stability to the structure for the single-phase formation despite oxygen deficiency. The oxygen-deficient insulating films show activation behaviour at high temperatures, indicating an opening of charge-transfer gap.

Interface reaction‐governed heteroepitaxial growth of YBa2Cu3O7‐δ film on CeO2‐buffered technical substrate

AbstractDue to the high growth rate and environmental‐friendly, fluorine‐free metal‐organic decomposition routes (FF‐MOD) have attracted more attention for growth of high‐quality YBa2Cu3O7‐δ (YBCO) films. Few works have been performed when using technical substrates. In this study, correlation among the sintering process, microstructure, and superconductivity of the YBCO was systematically established on the technical substrates capped with CeO2 layer. We found that the optimal process conditions are mainly related to the enhanced transient liquid phase and BaCeO3. Combined X‐ray diffraction and scanning electron microscopy analyses indicate that high‐quality growth of YBCO film is a trade‐off of two different competition phenomena during sintering: (a) the presence of enhanced transient liquid phase, (b) the formation of BaCeO3 at the interface. The former is beneficial to YBCO epitaxial growth/structure rearrangement, while the latter should be suppressed in view of minimizing YBCO partial decomposition triggered by the interfacial reaction. Moreover, we confirmed that both two aforementioned phenomena are somehow associated with the cross‐linkage between the sintering temperature and pO2 during the YBCO conversion. According to this systemic study, the key parameters are defined to avoid the BaCeO3 formation prior to the YBCO orientation nucleation. Structure and superconductivity of the YBCO film were also investigated. Remarkably, a high Jc value of 3.69 mA/cm2 (77 K, sf) was obtained in the YBCO film grown on the CeO2 technical substrate deposited under optimized deposition conditions, which is rather comparable with that on the LaAlO3 single crystal. TEM cross‐sectional observation reveals that the enhanced Jc (B) properties of the YBCO film are mainly contributed by high density of short stacking faults. This work demonstrates the feasibility of FF‐MOD to fabricate high‐performance YBCO films on the CeO2‐buffered technical substrate.

Luminescence and scintillation properties of rare-earth-doped LaAlO3 single crystals

LaAlO3 perovskite single crystals doped with Ce, Eu or Tb have been grown by micro-pulling-down method. X-ray diffraction analysis confirmed low-temperature rhombohedric phase. Energy-dispersive Laue mapping revealed sub-grains in the grown crystals. For the Ce-doped crystals, very low radioluminescence efficiency was found and the observed emission was ascribed to defects. Photoluminescence spectra were significantly different from the radioluminescence ones. They were significantly quenched at room temperature and the related luminescence showed also unusually high Stokes shift. High radioluminescence efficiency was found for the Tb-doped samples and especially for the Eu-doped ones, which would point to the fact that in LaAlO3 the scintillation process is more favorable for the luminescence ions which tend to capture an electron first.

Structural and superconducting characteristics of YBa2Cu3O7 films grown by fluorine-free metal-organic deposition route

Microstructure and superconducting performance of YBa2Cu3O7 (YBCO) films deposited on LaAlO3 single crystal (LAO) substrates by a fluorine-free metal-organic deposition (FF-MOD) technique, have been studied by means of X-ray reciprocal space mapping (RSM), cross-sectional transmission electron microscopy (TEM) and magneto-optical (MO) imaging. Combining the X-ray diffraction and the TEM cross-sectional analysis, it is revealed that stacking faults, i.e. YBa2Cu4Ox intergrowths, and ab-plane twins are main defects in the FF-MOD YBCO films. Due to the highly epitaxial growth mechanism related to transient liquid phase, the LAO twinned substrate structure is also inherited in the FF-YBCO films. The low-density planar defects containing dislocations parallel to c-axis result in stripy patterns observed in the MO images. For comparison, the low-fluorine (LF) MOD film show a texture mosaic spread in the ab plane and is little influenced by the LAO twinning underneath, implying the severe structural disorder most likely associated with the large amount of small-angle grain boundaries. Moreover, the higher density of stacking faults was also detected by XRD θ−2θ scan in the LF-MOD film. It is suggested that associated partial dislocations formed at the boundary between the stacking faults and YBCO matrix act as strong linear (or dot) pinning centers. These structural characteristics are well in line with the better superconducting performance of the low fluorine-MOD film, in particular under external magnetic field at 77 K. This work offers an in-depth insight into the correlation between the microstructure and superconductivity in the MOD YBCO films.

Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films.

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO3 and SrTiO3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO3. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.

Understanding surface structure and chemistry of single crystal lanthanum aluminate

The surface crystallography and chemistry of a LaAlO3 single crystal, a material mainly used as a substrate to deposit technologically important thin films (e.g. for superconducting and magnetic devices), was analysed using surface X-ray diffraction and low energy ion scattering spectroscopy. The surface was determined to be terminated by Al-O species, and was significantly different from the idealised bulk structure. Termination reversal was not observed at higher temperature (600 °C) and chamber pressure of 10−10 Torr, but rather an increased Al-O occupancy occurred, which was accompanied by a larger outwards relaxation of Al from the bulk positions. Changing the oxygen pressure to 10−6 Torr enriched the Al site occupancy fraction at the outermost surface from 0.245(10) to 0.325(9). In contrast the LaO, which is located at the next sub-surface atomic layer, showed no chemical enrichment and the structural relaxation was lower than for the top AlO2 layer. Knowledge of the surface structure will aid the understanding of how and which type of interface will be formed when LaAlO3 is used as a substrate as a function of temperature and pressure, and so lead to improved design of device structures.

Surface Segregation in Lanthanum Strontium Manganite Thin Films and Its Potential Effect on the Oxygen Reduction Reaction

A limiting factor for solid oxide fuel cell (SOFC) performance is the oxygen reduction reaction (ORR) that occurs at the cathode surface. This study examines changes in the surface composition of (La0.8Sr0.2)0.95MnO3 (LSM-20), a well-known SOFC cathode material. Heteroepitaxial thin films of LSM-20 were grown on lattice matched single crystal LaAlO3 and NdGaO3 substrates. Total Reflection X-ray Fluorescence (TXRF) measurements showed surface manganese enrichment on annealing at 800°C in air. X-ray absorption near edge spectroscopy (XANES) showed that this leads to a redistribution of manganese 3+, 4+, and 2+ valence states at the surface. This was confirmed by defect modeling, which showed that surface Mn segregation leads to an increase in the surface concentration of oxygen vacancies and the Mn+4 species, both of which are expected to be beneficial for ORR.

Influence of the vicinal surface on the anisotropic dielectric properties of highly epitaxial Ba0.7Sr0.3TiO3 thin films.

Epitaxial thin films of Ba0.7Sr0.3TiO3 (BST) were grown on the designed vicinal single-crystal LaAlO3 (001) substrates to systematically investigate the evolution of microstructures and in-plane dielectric properties of the as-grown films under the strains induced by surface step terraces. Anisotropic dielectric properties were observed, which can be attributed to different tetragonalities induced by vicinal LaAlO3 substrates with miscut orientations along the [100] and [110] directions with different miscut angles of 1.0°, 2.75° and 5.0°. A terrace geometric model with both compressive and tensile strained domains in the BST film was established, which is in good agreement with the experimental results. Our experimental studies not only shed new light on the heteroepitaxial growth mechanism, but also provide a promising platform for the design and integration of high performance device applications.

Ni 3d – O 2p hybridization dependent magnetic properties of LaNiO3 thin films

We present here the study on magnetic properties and their relation to the electronic structure for c-axis oriented LaNiO3 films deposited on LaAlO3 single crystals using the rf-magnetron sputtering. Swift heavy ion irradiation induced annealing alters the unusual temperature dependent magnetic behaviour of pristine film to the paramagnetic one. X-ray absorption spectroscopic measurements, performed at O K-edge and Ni L-edge, reveal that NiO hybridization is increased after the initial irradiation. Charge-transfer multiplet calculations of the Ni L-edge indicate that variation of magnetic behaviour can be related to the NiO hybridization strength inducing a further variation in Ni spin state.

Effect of Oxygen Content in Atmospheres of Heat Treatment on the Properties of YBCO Thin films

YBa 2 Cu 3 O 7- x (YBCO) films were prepared on single-crystal LaAlO 3 (100) substrate using a low-fluorine sol-gel method. The effect of the oxygen content in the atmospheres of heat treatment at 725 °C on the critical current density ( J c ) of the obtained YBCO films was investigated. Results show that all the films heat-treated in the oxygen content range of 100∼1700 µL/L exhibit good biaxial-texture characteristics. However, high- J c YBCO films are not obtained at the oxygen contents of 100 and 200 µL/L due to the lower density of the surfaces. With increasing the oxygen content, the film surface becomes dense gradually. When the oxygen content is 300 µL/L, the surface of the film is very dense and J c reaches 4.3 MA/cm 2 . Furthermore, many copper-rich second phases grow up on the surfaces with further increasing oxygen content up to 800 and 1700 µL/L, which is a dominant reason for the decline in J c of the films.

Disappearance of metal-insulator transition in NdNiO3/LaAlO3 films by ion irradiation controlled stress

The effect of strain on the metal-insulator transition (MIT) of the epitaxial NdNiO3 film on the c-axis oriented LaAlO3 single crystal, grown by pulsed laser deposition, has been investigated. Swift heavy ion irradiation was used to vary the strain state of the deposited film. X-ray diffraction confirmed a systematic fluence dependent rise in the in-plane compressive strain, while maintaining the epitaxy of the film. This in-plane compressive strain has been found to reduce the MIT temperature, which finally disappeared for the highest fluence of the irradiation. This is also corroborated with the Raman spectroscopy measurements which suggest that the ion irradiation induced stress is responsible for the suppression of the metal-insulator transition.

Microwave dielectric properties of pure and Mn-doped lead-free Na0.5Bi0.5TiO3epitaxial thin films grown on (001) LaAlO3single crystals using pulsed laser deposition

(001)-epitaxial thin films of pure and Mn-doped lead-free Na0.5Bi0.5TiO3 (NBT) were grown on (00l) LaAlO3 single crystals, by pulsed laser deposition. The determination of the microwave dielectric permittivity, from interdigitated capacitance values treated using the combination of two previously developed analytical models, constitutes the backbone of this work. In the 3–5 GHz frequency range, constant ϵr values of ∼485 and ∼515 for the pure and Mn-doped layers, respectively, were extracted and found in good agreement with literature data for pure NBT bulk ceramics, thus underlining the relevance of the approach. These permittivity values were re-injected in a second step into an electromagnetic simulation of the equivalent electrical circuit. Smith charts revealed the good matching between experimental and simulated data, proving that the employed method appears meaningful. Mn doping seems to lead to a small permittivity increase and clearly to a moderate reduction of the dielectric losses in the 3–5 GHz interval. Finally, the variation of the capacitance versus temperature testifies to the ferroelectric nature of the samples.

Confinement-driven metal-insulator transition and polarity-controlled conductivity of epitaxial LaNiO3/LaAlO3 (111) superlattices

Recently, topological conductivity has been predicted theoretically in LaNiO3(111)-based superlattices. Here we report high-quality epitaxial LaNiO3/LaAlO3 superlattices on (111)-oriented SrTiO3 and LaAlO3 single crystals. For both substrates a metal-insulator transition with decreasing number of LaNiO3 monolayers is found. While the electrical transport is dominated by two-dimensional variable range hopping for superlattices grown on polar mismatched SrTiO3(111), it switches to a thermally activated single gap behavior on polar matched LaAlO3(111). The gap energy of the polar double-layer LaNiO3 superlattices can be tuned via the thickness of the insulating LaAlO3 layers.

Inhomogeneous Spin Diffusion in Thickness-Controlled La 0 . 6 7 Sr 0 . 3 3 MnO 3 /YBa 2 Cu 3 O 7 − δ /La 0 . 6 7 Sr 0 . 3 3 MnO 3 Trilayers

La0.67Sr0.33MnO3/YBa2Cu3O7 − δ /La0.67Sr0.33 MnO3 (LSMO/YBCO/LSMO) trilayers are prepared on single-crystal LaAlO3 (LAO) substrates by pulsed laser deposition (PLD) with the thicknesses of LSMO layers fixed at 50 nm, and the thickness of YBCO sandwich varied from 10 to 40 nm. X-ray diffraction (XRD) and pole figure measurements disclose that all the trilayers are c-oriented and there is little change in the textures of YBCO layer for both in-plane and out-of-plane. Based on inductive, transport, and magnetic measurements, obvious suppression of superconductivity with a decrease in YBCO thickness is observed, particularly at thickness below 20 nm. Importantly, there are great disparities in the superconducting transition temperature (T c) and transition width (ΔT c) values for both inductive and transport measurements. The T c values from an inductive method are much smaller than those from transport measurements, with ΔT c in the opposite direction. The results are explained by the nonuniform diffusion of spin-polarized quasiparticles due to the comparable length scales of the spin diffusion length and the superconducting film thickness.

Substrate effect on the growth of monolayer dendritic MoS2 on LaAlO3 (100) and its electrocatalytic applications

In accommodating the rapid development of two-dimensional (2D) nanomaterials, chemical vapor deposition (CVD) has become a powerful tool for their batch production with desirable characteristics, such as high crystal quality, large domain size, and tunable domain shape. The crystallinity and morphology of the growth substrates usually play a crucial role in the CVD synthesis of high-quality monolayer MoS2, a kind of 2D layered material which has ignited huge interest in nanoelectronics, optoelectronics and energy harvesting, etc. Herein, by utilizing a low-pressure chemical vapor deposition (LPCVD) system, we demonstrate a regioselective synthesis of monolayer MoS2 on the corrugated single-crystal LaAlO3 (100) with twin crystal domains induced by the second-order phase transition. Unique dendritic morphologies with tunable nucleation densities were obtained in different regions of the undulated substrate, presenting a strong substrate modulation effect. Interestingly, the exposure of abundant active edge sites along with the rather high nucleation density makes the monolayer dendritic MoS2 a good electrocatalyst for hydrogen evolution reaction (HER), particularly featured by a rather high exchange current density (70.4 μA cm−2). Furthermore, uniform monolayer MoS2 films can also be obtained and transferred to arbitrary substrates. We believe that this work provides a new growth system for the controllable synthesis of 2D layered materials with unique dendritic morphologies, as well as its great application potential in energy conversion and harvesting.

Enhanced thermoelectric performance in c-axis oriented Ca3Co4O9 films by Ag addition through multiple annealing

The [Formula: see text]-axis oriented Ca3Co4O9 (CCO) films without and with 5 wt.% Ag addition were prepared by chemical solution deposition (CSD) through multiple annealing processing on single crystal LaAlO3 (001) substrates. With Ag addition, the resistivity at 300 K is decreased to 2.25 m[Formula: see text]cm, the Seebeck coefficient at 300 K is enhanced to 106 [Formula: see text]V/K and the power factor at 300 K can reach as high as 0.5 mW[Formula: see text]K[Formula: see text]m[Formula: see text], which is the highest value among CCO films prepared by CSD. The results suggest that Ag addition is a very effective route to improve the thermoelectric properties of CCO films through multiple annealing processing.

(Invited) Tuning the Strain and Physical Properties of Highly Epitaxial Perovskite Thin Films on Vicinal Substrates

Controllable interfacial strain can manipulate the physical properties of epitaxial films and help understand the physical nature of the correlation between the properties and the atomic microstructures. In this presentation, we demonstrated that the interface strain in epitaxial thin films can be well controlled by adjusting the miscutting angle of a vicinal single-crystal substrate via a surface-step-terrace matching growth mode. Epitaxial perovskite or perovskite-like oxide thin films such as CaCu3Ti4O12 (CCTO) and (Ba,Sr)TiO3 were grown on vicinal (001) single-crystal LaAlO3 (LAO) substrates with various miscutting angles of 1.0°, 2.5°, and 5.0° by a polymer assisted deposition (PAD) technique. High-resolution X-ray diffraction were employed to study the epitaxial nature and interface strains induced by the substrate step terrace dimensions. It is found that the strain is determined by the mismatch between the lattice of the thin film and the substrate step terrace dimensions. A model is proposed that both compressive and tensile strained domains would form in the film to fit the size of each step terrace whereas the ratio of the two kinds of domains can be tuned by the miscutting angle. Based on this growth mode, a proper design of the substrate step terrace dimension can tune the properties of the thin films. For example, an optimized design of a CCTO thin film on a 5o-miscutting (001) LaAlO3 substrate can maintain a similar dielectric constant and reduce the dielectric loss by half an order of magnitude compared with the films grown on the regular (001) LaAlO3 substrates, briefly by releasing a large part of the interfacial stress through the complementary strained domains instead of dislocations. This work is supported by the National Basic Research Program of China (973 Program) under Grant No. 2015CB351905, the National Natural Science Foundation of China (Nos. 51172036, 11329402, and 51372034), and the Technology Innovative Research Team of Sichuan Province of China (No.2015TD0005).

Thin (111) oriented CoFe2O4 and Co3O4 films prepared by decomposition of layered cobaltates

The formation and structural characterization of highly (111)-oriented Co3O4 and CoFe2O4 films prepared by a novel procedure from 00l-oriented NaCoO2 and Na(CoFe)O2 is reported. The Na(CoFe)O2 films were deposited on MgO, SrTiO3, LaAlO3, and Zr(Y)O2 single crystals with (100) and (111) orientations by chemical solution deposition method and crystallized at 700°C. Subsequently they were transformed into (111)-oriented spinel phase during post-growth annealing at 800–1000°C. Morphology and structure of the films was investigated by means of scanning electron microscopy and X-ray diffraction. While all spinel films exhibit pronounced out-of-plane orientation irrespective of substrate, the rate of in-plane orientation strongly depend on lattice misfit values. Different epitaxial phenomena ranging from true one-to-one epitaxy to the existence of many-to-one epitaxy involving two or more orientations were determined by full 3D texture analysis.

Fast Atomic-Scale Chemical Imaging of Crystalline Materials and Dynamic Phase Transformations.

Atomic-scale phenomena fundamentally influence materials form and function that makes the ability to locally probe and study these processes critical to advancing our understanding and development of materials. Atomic-scale chemical imaging by scanning transmission electron microscopy (STEM) using energy-dispersive X-ray spectroscopy (EDS) is a powerful approach to investigate solid crystal structures. Inefficient X-ray emission and collection, however, require long acquisition times (typically hundreds of seconds), making the technique incompatible with electron-beam sensitive materials and study of dynamic material phenomena. Here we describe an atomic-scale STEM-EDS chemical imaging technique that decreases the acquisition time to as little as one second, a reduction of more than 100 times. We demonstrate this new approach using LaAlO3 single crystal and study dynamic phase transformation in beam-sensitive Li[Li0.2Ni0.2Mn0.6]O2 (LNMO) lithium ion battery cathode material. By capturing a series of time-lapsed chemical maps, we show for the first time clear atomic-scale evidence of preferred Ni-mobility in LNMO transformation, revealing new kinetic mechanisms. These examples highlight the potential of this approach toward temporal, atomic-scale mapping of crystal structure and chemistry for investigating dynamic material phenomena.