LaAlO3 Single Crystal Substrates Research Articles

Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides

Developing highly efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts is critical for many energy devices. While regulating the proton-coupled electron transfer (PCET) process via introducing additive into the system has been reported effective in promoting OER activity, controlling the PCET process by tuning the intrinsic material properties remains a challenging task. In this work, we take double perovskite oxide PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) as a model system to demonstrate enhancing OER activity through the promotion of PCET by tuning the crystal orientation and correlated proton diffusion. OER kinetics on PBSCF thin films with (100), (110), and (111) orientation, deposited on single crystal LaAlO3 substrates, were investigated using electrochemical measurements, density functional theory (DFT) calculations, and synchrotron-based near ambient X-ray photoelectron spectroscopy. The results clearly show that the OER activity and the ease of deprotonation depend on orientation and follow the order of (100) > (110) > (111). Correlated with OER activity, proton diffusion is found to be the fastest in the (100) film, followed by (110) and (111) films. Our results point out a way of boosting PCET and OER activity, which can also be successfully applied to a wide range of crucial applications in green energy and environment.

Effect of substrate orientation on formation of cluster glass state in epitaxial LaMnO3 thin films

Cluster glass state, which is closely related to some special physical phenomena, has attracted extensive attention. Herein, we report a new phenomenon that is the substrate orientation can influence the formation of the cluster glass state in LaMnO3 (LMO) films. To explore the effect of substrate orientation on the formation of cluster glass state, the LMO thin films are epitaxially grown on (001)-, (110)- and (111)-oriented LaAlO3 (LAO) single crystal substrates by pulsed laser deposition (PLD), and the cluster glass state in which is investigated. The results show that the substrate orientation has a great influence on the formation of cluster glass state in the LMO epitaxial films. The LMO thin film grown on (001)-oriented LAO substrates is more likely to form cluster glass state than those grown on (110)- and (111)-oriented LAO substrates. This can be attributed to the different degrees of the ferromagnetic and antiferromagnetic interactions, which is caused by the different crystalline orientations and strain states of the epitaxial LMO thin films.

Trilayer Films of YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x with Superconducting Properties Prepared via Sol-Gel Method

The YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x (YBCO/LAO/YBCO) heterogeneous trilayer film structures were prepared on the LaAlO3 (001) single-crystal substrate via the sol-gel method. The characteristics of the phase, orientation, and superconducting properties were studied via an X-ray diffractometer and a comprehensive physical property measurement system. The results show that when the LAO layer was thin (<40 nm), the trilayer films were relatively pure and had a high c-axis orientation. When the nominal thickness of the LAO layer increased to 70 nm, non-c-axis oriented grains started to appear in the trilayer film. In this case, the top and bottom YBCO layers exhibited superconducting properties, and the resistance was found to be zero at 80.1 and 72.5 K, respectively. In addition, for the YBCO/LAO (40 nm)/YBCO structure, the intermediate LAO layer was too thin to completely cover the surface of the underlying YBCO. This resulted in point contact and micro-bridge contact in several micro-regions, thus forming a weak connection and exhibiting a current-voltage (I–V) characteristic similar to the direct-current (DC) Josephson effect. On the other hand, for the YBCO/LAO (70 nm)/YBCO structure, the thicker LAO layer could completely cover the underlying YBCO surface, which could isolate the superconducting current between the YBCO layers.

Significant enhancement of magnetization in 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3/La0.70Sr0.30MnO3 heterostructure

In this article, we present a substantial enhancement of magnetization in La0.70Sr0.30MnO3 (LSMO) perovskite thin film (100 nm) with 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) as a ferroelectric capping layer (100 nm) epitaxially grown on (001) oriented LaAlO3 (LAO) single crystal substrate. For PMN-PT/LSMO/LAO and LSMO/LAO, the ferromagnetic ordering temperature (TC) was found to be 316 K and 252 K with corresponding films having saturation magnetization ≃182 kA/m and ≃105 kA/m respectively, at 5 K. The enhancement in magnetization is mainly understood due to the tensile strain between PMN-PT and LSMO layers. Attributable to the self-polarization of the ferroelectric PMN-PT layer due to compressive strain, the accumulation of surplus holes at the interface of the ferromagnetic PMN-PT/LSMO layer might also contribute towards the improvement of magnetization.

Improvement of J c Properties for Hf and La Co-Doped Gd123 Films Fabricated by Fluorine-Free MOD Method

In this study we have successfully fabricated La and Hf co-doped Gd123 thin films on LaAlO3 single crystal substrates by the fluorine-free metal organic deposition method with tetrakishafnium as a new organic metal salt. Hf doping is the effective means to introduce the perovskite nano-structure BaHfO3 as artificial pinning centers (APCs) into REBa2Cu3O y coated conductors. On the other hand, La doping is valid method in promoting high quality epitaxial crystallization of the superconducting phase. By possessing both advantages La 1 mol% and Hf 2 mol% co-doped film achieved high critical current densities of 3.1 MA cm−2 at 0 T, and 0.25 MA cm−2 at 1 T, 77.3 K. The detailed study of the flux pinning revealed that APCs introduced by Hf doping brought about the increase of J c in high fields, and La doping brought about the quality crystallization resulting in the increase of J c in low fields.

Aqueous Chemical Solution Deposition of Functional Double Perovskite Epitaxial Thin Films.

Double perovskite structure (A2 BB'O6 ) oxides exhibit a breadth of multifunctional properties with a huge potential range of applications in fields as diverse as spintronics, magneto-optic devices, or catalysis, and most of these applications require the use of thin films and heterostructures. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films combining high performance with high throughput and low cost. In addition, the physical properties of these materials are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Thus, promoting spontaneous cationic ordering has become a relevant issue. In this work, our recent achievements by using polymer-assisted deposition (PAD) of environmentally friendly, water-based solutions for the growth of epitaxial ferromagnetic insulating double perovskite La2 CoMnO6 and La2 NiMnO6 thin films on SrTiO3 and LaAlO3 single-crystal substrates are presented. It is shown that the particular crystallization and growth process conditions of PAD (very slow rate, close to thermodynamic equilibrium conditions) promote high crystallinity and quality of the films, as well as favors spontaneous B-site cationic ordering.

Elastic strain control of electronic structure, and magnetic properties of [Pr1−xCaxMnO3/SrTiO3]15 superlattices

We report the growth, electronic structure, and in-plane magnetic properties of pulsed laser deposition grown 2D superlattice structures [Pr0.7Ca0.3MnO3/SrTiO3]15 and [Pr0.5Ca0.5MnO3/SrTiO3]15 on (001) oriented SrTiO3 and LaAlO3 single crystal substrates. The x-ray reflectivity measurements reveal well-defined interfaces between the manganite and titanate layers along with the existence of Kiessig fringes, providing the evidence for the smooth periodic superlattice structure. The reciprocal space mapping provides signature of tetragonal distortion in all the superlattices. The electronic structure determined from the x-ray photoelectron spectroscopy reveals divalent Sr and Ca, tetravalent Ti, and mixed valent Mn with a pronounce shift of binding energy peaks toward the higher energy side in the superlattices grown on (001) oriented LaAlO3 as compared to those grown on SrTiO3. These superlattices exhibit highly anisotropic ferromagnetic character. We used the law of approach to saturation to determine the anisotropy field (HK) and cubic anisotropy constant (K1) for all the investigated superlattices. This analysis yields the highest HK∼9 kOe and K1∼8×105 erg/cc for the [Pr0.7Ca0.3MnO3/SrTiO3]15 superlattice system. Furthermore, significant enhancement of the overall magnetic moment and a decrease in TC (<100 K) was observed in the case of LaAlO3 grown superlattice, which indicates a substantial role of residual elastic strain on the magnetic ordering. Our results indicate that the strain induced elongation of MnO6 octahedra leads to finite possibility of non-orthogonal overlapping of orbitals in the presence of large crystal field splitting of eg levels, which, in turn, causes suppression of the ferromagnetic double exchange interaction.

Joule self-heating effects and controlling oxygen vacancy in La0.8Ba0.2MnO3 ultrathin films with nano-sized labyrinth morphology

Electric current induced Joule heating effects have been investigated in La0.8Ba0.2MnO3 ultrathin films deposited on a LaAlO3(001) single crystal substrate with a smaller lattice constant by using the sol–gel method. By applying moderate bias currents (∼10 mA), it is found that Joule self-heating simply gives rise to a temperature deviation between the thermostat and the test sample, but the intrinsic ρ(T) relationship measured at a low current (0.1 mA) changes a little. However, it is noteworthy that the low-temperature transport behavior degrades from the metallic to the insulating state after applying higher bias currents (>31 mA) in vacuum. Furthermore, the metallic transport can be recovered by placing the degraded film in air. The results clearly suggest that the oxygen vacancy in the La0.8Ba0.2MnO3 films is controllable in different atmospheres, particularly with the aid of the Joule self-heating. According to the SEM images, we attribute the controlled oxygen vacancy to the nano-sized labyrinth pattern of the films, where the large surface-to-volume ratio plays a crucial role.

Synthesis and characterization of patterned La2/3Sr1/3MnO3 epitaxial thin films via the ultraviolet-visible spectrum assisted photosensitive sol-gel method

A La2/3Sr1/3MnO3 (LSMO) photosensitive sol was synthesized using La(NO3)3•6H2O, Sr(CH3COO)2•0.5H2O, and Mn(CH3COO)2•4H2O as starting materials and benzoylacetone (BzAc) as a chelating agent, and the corresponding gel films were casted on single crystal LaAlO3 (00l) substrates. The complexation mechanism between metal ions and the chelating agent was investigated via Fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy. The results show that the Mn and La ions were chelated with BzAc, and the as-cast gel film shows photosensitivity to ultraviolet-visible light. Grating-patterned films were prepared by irradiating the gel films for 3 min using a two-beam interference process followed by rinsing in a solution of isopropyl alcohol and n-butyl alcohol (2:1 v/v) for 8 s. It was found that the irradiated portion of the gel film remained intact in the solvent, whereas the unirradiated portion was dissolved, producing the gratings. The shape of the gratings is highly periodic without broken wires or defects. After annealing at 750 °C, the patterned LSMO gel film turned into a c-axis orientated inorganic film. The patterned LSMO film exhibited a saturation magnetization value that is similar to that of the unpatterned LSMO film.

Structural, electronic, transport and magnetic studies of LaCo1 − xNixO3 (x = 0, 0.3) thin films

We present here a study on structural, electronic, transport, and magnetic properties of single-phase (001) oriented compressively strained thin films of LaCoO3 (LCO) and LaCo0.7Ni0.3O3 (LCNO). The films were deposited on the LaAlO3 (001) single crystal substrate using pulsed laser deposition. Electronic structure investigations by x-ray absorption spectroscopy (XAS) measurements at various edges (Co-L3,2, Ni-L3,2, and La-M5,4) confirm +3 charge states for Co, Ni, and La ions in all samples. The XAS study also reveals a transition in the spin state of Co+3 (low spin to intermediate/high spin), which is attributed to induced strain and Ni substitution. Electrical transport measurements suggest the semiconducting behavior for LCO and LCNO thin films in the measured temperature range. We found a drastic change in resistivity (ρ) and magnitude with Ni substitution. An in-depth analysis of the conduction mechanism governing the transport behavior in thin films has been done using various theoretical models. Different conduction mechanisms (Arrhenius law and Effros-Shklovski-type variable range hopping) have been observed to govern conduction in different temperature ranges, suggesting disorder controlled transport in these films. The magnetization measurements show a magnificent increase in saturation magnetization for LCNO and ferromagnetic ordered states, which persists even up to room temperature.

Growth of YBCO film by chemical solution deposition with an optimization fluorine process

Trifluoroacetate metal organic deposition (TFA-MOD) with low fluorine contents would bring benefits for the environment and the epitaxial growth of YBa2Cu3Oy (YBCO) film. In this paper, a simple and effective method which could tune the fluorine content and improve the performance of YBCO film was reported. YBCO Low-Fluorine/YBCO Fluorine-Free composites were prepared on LaAlO3 single crystal substrate. During the pyrolysis process, the fluorine would diffuse across the interface of YBCO Low-Fluorine/YBCO Fluorine-Free composites. Both the fluorine in the YBCO Low-Fluorine and the carbon in the YBCO Fluorine-Free could be reduced. It could significantly improve the epitaxial growth and performance of YBCO film.

Terahertz time domain spectroscopy of GdBCO superconducting thin films

We present terahertz optical properties of GdBa2Cu3O7-x (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a LaAlO3 (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K.

Biaxially textured (Bi, Pb)2Sr2Ca2Cu3Ox thin films on LaAlO3 substrates fabricated via the chemical solution deposition method

In this study, biaxially textured (Bi, Pb)2Sr2Ca2Cu3Ox ((Bi, Pb)-2223) thin films were fabricated epitaxially on LaAlO3 (LAO) single crystal substrates via the chemical solution deposition (CSD) method. In order to prevent the evaporation of Bi and Pb, a sealed face-to-face architecture for sintering was utilized. According to the results of x-ray diffraction measurements including θ/2θ-scan, ω-scan, and ϕ-scan patterns, the optimal sintering temperature was found to be 840 °C. The volume fraction of the (Bi, Pb)-2223 phase of the optimal sample was enhanced to above 80%. Favorable out-of-plane and in-plane textures were also detected, which could be attributed to an epitaxy system of (Bi, Pb)-2223 thin film and an LAO substrate with a 45° rotated alignment. A dense microstructure and thickness of about 100 nm were found by scanning electron microscopy observations of the surface morphology and cross section. The critical temperature of the optimal sample determined with the four-probe method was 103.2 K, and its critical current density values were 0.06 MA cm−2 (77 K, self-field) and 1.91 MA cm−2 (4.2 K, self-field), which were obtained by AC susceptibility measurement and a superconducting quantum interference device, respectively. This study demonstrated the feasibility of fabricating (Bi, Pb)-2223 thin films with superconducting performance via the low-cost CSD method.

Structural, Electronic, and Magnetic Analysis and Device Characterization of Ferroelectric–Ferromagnetic Heterostructure (BZT–BCT/LSMO/LAO) Devices for Multiferroic Applications

Ferroelectricity and ferromagnetism have been investigated in a lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–BCT)/La0.7Sr0.3MnO3 (LSMO) heterostructure for multiferroic (MF) applications. The BZT–BCT thin film has been grown on LSMO/lanthanum aluminate, LaAlO3 (LAO) by pulsed laser deposition (PLD). Prior to that, the LSMO layer was deposited on a single-crystal LAO substrate by PLD. The epitaxial growth of the (001) oriented films was confirmed by X-ray diffraction analysis. The small value of the full-width at half-maximum of the rocking curve peak (0.1°) performed about (002) plane of the BZT–BCT film indicates an out-of-plane orientation of the film. The polarization switching behavior in the heterostructure device was observed with a remnant polarization of $\sim 47~\boldsymbol {\mu \text {C}/}{{\text {cm}}}^{2}$ and a coercive field of ~180kV/cm at an applied voltage of 5 V. The frequency-dependent relative dielectric constant varies in-between 5100 and 4900 in the frequency range from 1 to 50 kHz during the dielectric measurements of the fabricated device. The observed low value of the dielectric loss (0.02) confirms the outstanding quality of the ferroelectric device. A well-saturated room temperature magnetization–applied field curve, with a coercive field of ~1200A/m and a remnant magnetization of ~110kA/m, was observed in the LSMO/LAO system indicating the ferromagnetic behavior of the film. The temperature-dependent magnetization of the LSMO film exhibits a ferromagnetic-to-paramagnetic transition at ~360K. These results on all solid-state ferroelectric–ferromagnetic heterostructure using BZT–BCT and LSMO open viable possibilities for MF applications.

Lead-free epitaxial ferroelectric heterostructures for energy storage applications

With the development of electronic devices towards miniaturization, light weight and integration, dielectric thin film-capacitors for recoverable energy storage applications have attracted ever-growing interests owing to their fast charging and discharging rate. In continuation to our investigations for achieving high energy density, we have grown lead free BaZr0.2Ti0.8O3 (BZT)/Ba0.7Ca0.3TiO3 (BCT) heterostructures and studied the structural, dielectric, ferroelectric and energy density characteristics. The BZT/BCT bilayer epitaxial heterostructures were grown on La0.67Sr0.33MnO3 (LSMO) buffered LaAlO3 (LAO) single crystal substrate by optimized pulsed laser deposition technique. The ferroelectric phase transition has been probed above room temperature with relaxor behavior. These nanostructures show high discharge and charge energy densities of 9.74 and 26.55 J/cm3, respectively. These heterostructures show high dielectric permittivity, large polarization and high energy density characteristics well above room temperature which may be useful for both high power and energy density device applications.

One step preparation of photosensitive Bi2Sr2CaCu2O8+x films and their fine patterns by a photosensitive sol–gel method

An acrylic acid-modified photosensitive sol–gel method is proposed to prepare Bi2Sr2CaCu2O8+x (Bi-2212) gel films and their fine patterns on LaAlO3 (LAO) single crystal substrates. An ultraviolet–visible absorption spectrum showed the UV characteristic absorption peak at around 270 ∼ 280 nm which originates from the π-π* electron transition of acrylic acid in metal complexes, and its intensity decreases as the UV irradiation time increases, indicating that Bi-2212 gel films are photosensitive. Based on the photosensitivity of Bi-2212 gel films, fine-patterned Bi-2212 gel films with the smallest resolution of 3 μm have been prepared. The prepared fine-patterned Bi-2212 superconducting films exhibit c-axis growth orientation and excellent superconducting properties.

Preparation of epitaxial CaMn7O12 film via sol-gel method and its ferromagnetic properties

CaMn7O12 precursor sol was prepared by using Ca(NO3)2·4H2O and Mn(CH3COO)4·4H2O as the raw materials, acetylacetone (AcAcH) as the chelating agent, and methyl alcohol (MeOH) as the solvent. The CaMn7O12 crystalline film was obtained via dip-coating and annealing treatment on the LaAlO3 (001) single-crystal substrate. XRD θ-2θ scan indicated that the as-prepared CaMn7O12 film had strong preferred orientation along the c-axis. In addition, the results of the ω and ϕ scans demonstrated that the film exhibited outstanding out-of-plane and in-plane texture characteristics. The SEM characterization showed that the CaMn7O12 film was dense and free of cracks. The grain size was uniform with an average size of ~180 nm. Vibrating sample magnetometer (VSM) test results indicated the CaMn7O12 film was antiferromagnetic and had a saturation magnetization of 114.2 emu/cm3 at 50 K.

Enhanced flux-pinning performance of YBCO films derived from a fluorine-free sol-gel process with ceria addition

In this work, a facile and effective method which can significantly improve the flux-pinning performance of YBCO films was reported. YBCO/CeO2 composite films were prepared on LaAlO3 (LAO) single crystal substrates through a fluorine-free sol-gel approach with a certain amount of ceria addition. During the nucleation and growth of YBCO films, CeO2 reacted with the barium-contained phase in YBCO precursors to generate the nanosized BaCeO3 phase and thus the redundant Y2O3 and CuO phases remained, leading to the wrong stacking of Cu atoms to form the stacking faults of Cu-O planes, which has been demonstrated by XRD, TEM, SEM and EDS results. The variation of critical current density (Jc) and flux-pinning force (Fp) with the percentage of ceria addition was investigated, and the results show that YBCO film with the amount of 5 mol% of ceria addition exhibits the highest Jc and the strongest Fp at the same temperature and applied magnetic fields. In addition, the stacking faults effectively reduced the degree of dependency of Jc of YBCO film on the applied magnetic field especially at low temperature.

High efficiency preparation of Bi2Sr2CaCu2O8+δ superconducting thin films by an acetate based Sol-Gel process

Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting thin films with c-axis epitaxial orientation were prepared on LaAlO3 (LAO) single crystal substrates via a Sol-Gel technique using metallic acetates as starting materials. The influence of the growth temperature and oxygen partial pressure (pO2) on the phase purity and superconducting properties of Bi-2212 was discussed. The results show that Bi-2212 films prepared at 820 °C with the pO2 of 1.4 KPa exhibit good in-plane and out-of-plane growth textures and excellent superconducting properties with high critical transition temperature (Tc) of 88 K and narrow transition width (ΔTc) of 4.8 K.

The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures

High-TC GdBa2Cu3O7−δ (GdBCO) superconductor has been popular for making superconductive tapes that have much potential for various fields of large-scale applications. We investigated aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO layer on Hastelloy substrate, so called GdBCO coated conductors (CCs), and Ag-coated GdBCO films on LaAlO3 (LAO) single-crystal substrates at cryogenic temperatures and compared them with each other. Unlike the RS of Ag-coated GdBCO films showing significant degradation in 4 weeks, no significant aging effects were found in our Ag-coated GdBCO CCs aged 85 weeks. The reactive co-evaporation deposition and reaction (RCE-DR) method was used for preparing the Ag-coated GdBCO CCs. Such durability of the Ag-coated GdBCO CCs in terms of the RS could be explained by existence of a protective impurity phase, i.e., Gd-free Ba–Cu–O phase as confirmed by transmission electron microscopy study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on transport properties of other kinds of Ag-coated GdBCO CCs could be realized by means of an artificially-grown protective impurity layer.