Single-crystalline BaTiO3-based ferroelectric capacitive memory via membrane transfer

We investigated the catalytic activities of three perovskite oxide (LaMnO3, La0.8Sr0.2MnO3, and La0.8Sr0.2CoO3) thin films grown by pulsed laser deposition when they were used as oxygen electrodes for aqueous rechargeable metal-air batteries. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on perovskite oxide thin films in alkaline solution were observed by linear sweep voltammetry using a rotating disk electrode. Although we confirmed that the resultant perovskite oxide thin films were sufficiently electroconductive to facilitate the ferricyanide reduction reaction on themselves, the ORR activities of perovskite oxide thin films were much lower than those of conventional composite electrodes, which consist of perovskite oxide powder and carbon black. In contrast, the OER activities of thin films and their activity order (La0.8Sr0.2CoO3 > LaMnO3 ≥ La0.8Sr0.2MnO3) were almost the same as those of conventional composite electrodes. We discuss the catalytic roles of perovskite oxides and carbon powder in oxygen electrodes and propose possible reaction paths for the ORR and OER.

Two-dimensional (2D) ferroelectricity have attracted much attention due to their applications in novel miniaturized devices such as nonvolatile memories, field effect transistors, and sensors. Since most of the commercial ferroelectric (FE) devices are based on ABO3 perovskite oxides, it is important to investigate the properties of 2D ferroelectricity in perovskite oxide thin films. Here, based on density functional theory (DFT) calculations, we find that there exist three kinds of in-plane FE states that originate from different microscopic mechanisms: (i) a proper FE state with the polarization along [110] due to the second-order Jahn-Teller effect related to the B ion with empty d-orbitals; (ii) a robust FE state with the polarization along [100] induced by the surface effect; (iii) a hybrid improper FE state with the polarization along [110] that is induced by the trilinear coupling between two rotational modes and the A-site displacement. Interestingly, the ferroelectricity in the latter two cases becomes stronger along with decreasing the thin film thickness, in contrast to the usual behavior. Moreover, the latter two FE states are compatible with magnetism since their stability does not depend on the occupation of the d-orbitals of the B-ion. These two novel 2D FE mechanisms provide new avenues to design 2D multiferroics, as we demonstrated in SrVO and CaFeO thin film cases. Our work not only reveals new physical mechanisms of 2D ferroelectricity in perovskite oxide thin films but also provides a new route to design the high-performance 2D FE and multiferroics.

Strong Coulomb repulsion and spin-orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Here, we report magnetic characteristics of a ${\mathrm{Sr}\mathrm{Mn}}_{0.3}{\mathrm{Ir}}_{0.7}{\mathrm{O}}_{3}$ perovskite thin film, having both 3d and 5d elements on the $B$ sites, with a strong perpendicular magnetic anisotropy (PMA), which is rare in perovskite oxide thin films. The ${\mathrm{Sr}\mathrm{Mn}}_{0.3}{\mathrm{Ir}}_{0.7}{\mathrm{O}}_{3}$ films have been epitaxially deposited on (001)-oriented ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ single-crystal substrates by pulsed-laser deposition, exhibiting a para-to-ferromagnetic transition at about 110 K. The average valence of $\mathrm{Mn}$ and $\mathrm{Ir}$ cations are estimated as +3.61 and +4.17, respectively, by x-ray photoelectron spectroscopy. X-ray absorption spectroscopy reveals that $\mathrm{Mn}$ ${e}_{g}$ electrons occupy preferentially the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital, which produces the observed PMA in the framework of spin-orbital coupling. The ${\mathrm{Sr}\mathrm{Mn}}_{0.3}{\mathrm{Ir}}_{0.7}{\mathrm{O}}_{3}$ thin film shows a large effective anisotropy constant of about 1.2 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ erg ${\mathrm{cm}}^{\ensuremath{-}3}$ at 10 K, which is found to decrease with decreasing tetragonality c/a of the film. First-principles calculations reveal that $\mathrm{Mn}$ and $\mathrm{Ir}$ cations provide opposite magnetizations in ${\mathrm{Sr}\mathrm{Mn}}_{0.3}{\mathrm{Ir}}_{0.7}{\mathrm{O}}_{3}$ and the $\mathrm{Mn}$ ${e}_{g}$ orbital polarization comes from both lattice distortion and oxygen octahedron rotation. The ${\mathrm{Sr}\mathrm{Mn}}_{0.3}{\mathrm{Ir}}_{0.7}{\mathrm{O}}_{3}$ thin film with strong PMA may have applications in future low-power-consumption oxide spintronic devices.

The authors report the direct observation of the electric pulse induced resistance-change effect at the nanoscale on La1−xSrxMnO3 thin films by the current measurement of the atomic force microscopy (AFM) technique. After a switching voltage of one polarity is applied across the sample by the AFM tip, the conductivity in a local nanometer region around the AFM tip is increased, and after a switching voltage of the opposite polarity is applied, the local conductivity is reduced. This reversible resistance switching effect is observed under both continuous and short-pulse-voltage switching conditions. It is important for future nanoscale nonvolatile memory device applications.

Dimensionality and strain-dependent properties of Orthorhombic (100) NaTaO3 thin films: A comprehensive DFT investigation

In recent years, the efficiency of perovskite solar cells (PSCs) has rapidly increased, making PSCs commercially viable and able to compete with silicon solar cells. However, the technology for large-scale production of perovskite thin films still poses a significant challenge to the commercialization of PSCs. This is mainly because the thickness of the perovskite light-absorbing film is less than 1 μm and easily produces pinholes during large-scale production, causing serious charge recombination and reducing the efficiency of the device. As a result, the efficiencies of most large-scale PSC modules are significantly lower than those of small-size devices. Vapor-reaction technology is considered a crucial method to enhance the uniformity of perovskite thin films. However, current techniques are not yet advanced enough to deposit pinhole-free perovskite thin films on a large scale. One major challenge is that lead iodide (PbI2) precursor thin films have a tendency to develop a sheet-like structure with a loose morphology, which contributes to the formation of pinholes in the perovskite film. Additionally, uncoordinated Pb2+ could be easily generated in the PbI2 film during thermal evaporation, causing an increase in trap-states in the perovskite crystal. To address this, we developed a valine treatment process in this study to change the preferred orientation of PbI2 from a sheet-like to a particle-like aggregate and obtained compact PbI2 thin films. Meanwhile, this valine vapor treatment can significantly reduce the uncoordinated Pb2+ in PbI2 films. Based on this, we successfully prepared pinhole-free perovskite thin films using a vapor–solid reaction. Perovskite films based on these valine-treated PbI2 exhibited a reduced trap density and significantly improved PSCs’ performance, with a maximum efficiency exceeding 21%. Large-area PSC mini-modules (5 cm × 5 cm) produced from the perovskite film (30 cm × 30 cm) showed uniformly distributed power conversion efficiency values exceeding 17%, with the highest value reaching 18.78%. We believe that this facile valine vapor treatment holds great potential for the large-scale production of stable and high-efficiency PSC modules.

Perovskite thin films are at the forefront of highly promising photovoltaic technologies due to their remarkable optoelectronic properties. Herein, we explore a low-cost, reproducible, and industry-scalable methodology to synthesize an all-inorganic CsPbI1.5Br1.5 perovskite thin film with additional incorporation of copper and chloride ions into the lattice structure. The synthesis process involves chemical bath deposition of PbS, followed by a gas-solid iodination reaction to yield PbI2. Subsequently, dip-coating incorporates Cs+, Cu2+, Br-, and Cl- ions into PbI2, and annealing at 270 °C produces perovskite thin films. The results show a large coverage area and a uniform thickness of each perovskite thin film. Comprehensive characterization, including X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and photoluminescence, provides the structural, chemical, and optical properties of the synthesized thin films. To evaluate the practical implications of our methodology, we fabricated photodetectors employing CsPbI1.5Br1.5 and (Cs0.95:Cu0.01)PbI1.5Br1.3Cl0.1 perovskite films. A comparative analysis unequivocally demonstrates a significant increase in photodetector performance when utilizing (Cs0.95:Cu0.01)PbI1.5Br1.3Cl0.1 perovskite films. While our findings quantitatively assess the tangible enhancement in photocurrent, we acknowledge the potential for improvement in device fabrication to enhance the overall performance. This study not only affirms the successful low-cost synthesis of perovskite thin films but also emphasizes the pivotal role of Cu2+ and Cl- ions in enhancing the performance of perovskite-based optoelectronic devices.

We have measured first-order reversal curves (FORCs) for Fe-1wt%Cu alloy thermally aged at 753 K up to 20000 min. While hardness exhibits a maximum at around 1000 min, reflecting the formation and growth of Cu precipitates, major-loop coercivity monotonically decreases and becomes almost constant above 100 min.; an increase of coercivity associated with Cu precipitation is masked by a large decrease due to recovery. On the other hand, FORC diagrams exhibit two distribution peaks at low and high switching fields after aging. While the former shifts towards a lower switching field after aging, reflecting recovery, the latter shows up after aging up to ∼1000 min, possibly due to the formation of Cu precipitates. These observations demonstrate that FORCs are useful to separately evaluate competing microstructural changes in thermally aged Fe-Cu alloy where recovery and Cu precipitation take place simultaneously.

Freestanding perovskite thin films display many unprecedented properties and exhibit the potential to be easily integrated on other non-oxide substrates or layers. In this work, we demonstrated a pathway to synthesis freestanding perovskite oxide thin films by using brownmillerite SrCoO2.5 as a sacrificial layer. Four representative freestanding perovskite oxide films, e.g., ferromagnetic SrRuO3, La0.7Sr0.3MnO3, dielectric SrTiO3, and ferroelectric Pb(Zr0.2Ti0.8)O3, were produced by etching SrCoO2.5 in Fe(NO3)3 weak acidic solution at room temperature. A 80 nm SrRuO3, which served as an H+ conduction channel, was deposited as a bottom layer of SrCoO2.5 to trigger a quick dissolution for the exfoliation of SrTiO3 and Pb(Zr0.2Ti0.8)O3 poor H+ conductor. Their crystal structure and physical properties were well retained in transferred films. Our work demonstrated the wide applicability of SrCoO2.5 as a sacrificial layer on the synthesis of freestanding perovskite oxide thin films.

Modeling and simulation of low power ferroelectric non-volatile memory tunnel field effect transistors using silicon-doped hafnium oxide as gate dielectric

Organic-inorganic hybrid perovskite solar cells are fabricated using polycrystalline perovskite thin films, which possess high densities of point and surface defects. The surface defects of perovskite thin films are the key factors that affect the device performance. Therefore, the reduction of harmful defects is the primary task for improving device performance. Therefore, in this study, high-quality perovskite thin films are prepared using an ionic liquid, dithiocarbamate diethylamine (DADA), to passivate the interface. The electron-rich sulfur atom in the DADA molecule chelates with the uncoordinated lead ion in the perovskite films, and the diethylammonium cation forms a hydrogen bond with the free iodine ion, which further improves the passivation. The synergistic passivation and improved morphology of the perovskite thin films substantially reduce the number of charged defects on the film surface and prolong the carrier lifetime. In addition, the DADA surface treatment increases the work function of the perovskite film, which is beneficial for carrier transport. Under standard solar-lighting conditions, the power conversion efficiency (PCE) of the device increases from 19.13 to 21.36%, and the fill factor is as high as 83.17%. Owing to both the hydrophobicity of DADA molecules and the passivation of ion defects, the PCE of the device remains above 80%, even for the device stored for 500 h in air at a relative humidity of 65%, and the device stability is substantially improved.

We examine the effects of substrates on the low temperature photoluminescence (PL) spectra and phase transition in methylammonium lead iodide hybrid perovskite (CH3NH3PbI3) thin films. Structural characterization at room temperature with X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy indicated that while the chemical structure of films deposited on glass and quartz was similar, the glass substrate induced strain in the perovskite films and suppressed the grain growth. The luminescence response and phase transition of the perovskite thin films were studied by PL spectroscopy. The induced strain was found to affect both the room temperature and low temperature PL spectra of the hybrid perovskite films. In addition, it was found that the effects of the glass substrate inhibited a tetragonal to orthorhombic phase transition such that it occurred at lower temperatures.

Due to their high efficiency and low cost, organic-inorganic hybrid perovskite solar cells are attracting growing interest recently. For the most commonly studied perovskite CH3NH3PbI3, optimization of the morphology and crystallinity of CH3NH3PbI3 thin films can greatly improve the efficiency of perovskite solar cells. A homogenous and uniform perovskite film can prevent direct contact between the hole transport layer and the electron transport layer, and thus can significantly reduce charge recombination. And the high crystallinity perovskite film facilitates fast charge transportation and injection. Various studies have proved that solvent has a critical influence on both the morphology and the crystallinity of perovskite thin films. In this work, we thoroughly studied the influence of the normally used N, N-Dimethylformamide (DMF) and r-butyrolactone (GBL) solvents on perovskite morphology, crystallinity, as well as the solar cells efficiency. When using DMF as the solvent, the efficiency is only 2.8%, while the efficiency of the cell obtained based on GBL can reach 10.1%. SEM and HRTEM are employed to study the morphology and crystallinity of these two kinds of perovskite films. The perovskite film prepared using solvent DMF shows a rough capping layer consisting of strip-like perovskite crystals, and the filling of meso-TiO2 is poor. Compared with DMF, the GBL perovskite film shows a better capping layer structure consisting of large perovskite domains, and the filling of meso-TiO2 is improved as well. This great difference in capping layer morphology and meso-TiO2 filling is one reason for the different performance. Besides morphology, different defect concentrations in these two kinds of perovskite films are another crucial issue. By Combined XRD and UV techniques, the mechanisms how perovskite precipitats from DMF and GBL solutions can be disclosed. In DMF, because of its low spoiling point of 153 ℃, most of DMF solvent volatilize by spin-coating, and an intermediate MOF structure of PbI2: MAI: xDMF is formed. During thermal annealing, the unstable MOF structure breaks down and a large amount of dislocations form in perovskite films, which highly restrict the charge transport. However, the spoil point of GBL (206 ℃) is higher than that of DMF, which makes it hard to be fully volatilized by spin-coating. During the following thermal treatment, the solubility of perovskite is lowered with increasing temperature. So perovskite crystallites precipitate from the GBL first and then gradually grow up with the volatilization of the excess solvent. We finally find that coordination between the solvent and the PbI2 plays a big role on the morphology and the crystallinity of the solution-processed perovskite film, and this is responsible for the difference of the device performance.

Micromagnetic simulations are performed to investigate the reversal process and the thermal stability of a grain of a typical perpendicular recording media (Co-Cr). The integration of the LLG equation yields that the reversal process changes slowly and steadily from coherent rotation to nucleation with increasing column length. The region between homogeneous rotation and nucleation becomes smaller and is shifted to smaller column lengths if the damping constant is reduced from /spl alpha/=1 to /spl alpha/=0.02. In the weakly damped case very fast switching modes exist if the switching field is only slightly larger than the coercive field. In this small regime the switching time increases with higher switching fields. Using solutions of LLG simulations, energy barriers between the two stable states at zero field are estimated. For column lengths larger than 30 nm the energy barrier for inhomogeneous reversal processes are smaller than for coherent rotation.

Developing vapor-solid reaction methods to prepare organic-inorganic hybrid perovskite thin films is highly compatible with processes in crystalline silicon solar cells and the thin-film photovoltaic industries, facilitating rapid industrialization. In the vapor-solid reaction, the crystallization quality of perovskite thin films is widely influenced by the crystallinity and microstructure of lead iodide (PbI2) precursor films. During the thermal evaporation process of preparing the PbI2 precursor films, we observed that PbI2 tends to develop a disordered surface morphology and exhibits high crystallinity, which significantly hinders the uniform diffusion of the organic amine salt vapor during the subsequent vapor-solid reaction. This results in a high defect concentration within the perovskite crystals. In this study, we propose a strategy to optimize the PbI2 precursor to address this issue. By employing mercaptoethylammonium Iodide (ESAI) vapor treatment on the PbI2 precursor films, we found that the thiol groups (-SH) in ESAI can bond with uncoordinated Pb2+ ions, effectively regulating the crystallinity of the PbI2 films and forming an ordered surface morphology. The resulting perovskite films exhibited significantly larger grain sizes. Additionally, we discovered that ESAI interacts with formamidinium iodide (FAI) to form hydrogen bonds, stabilizing FA+ ions and effectively passivating defects in the perovskite. By increasing the grain size and reducing the defect density in perovskite thin films, we achieved an inverted device with a maximum power conversion efficiency (PCE) of 20.79%. The unencapsulated device maintained 92% of their initial efficiency after being exposed to air for 1562 h, demonstrating excellent stability.