Hafnia-based ferroelectrics hold promise for nonvolatile ferroelectric memory devices. However, the high coercive field required for polarization switching remains a prime obstacle to their practical applications. A notable reduction in coercive field has been achieved in ferroelectric Hf(Zr)_{1+x}O_{2} films with interstitial Hf(Zr) dopants [Science 381, 558 (2023)SCIEAS0036-807510.1126/science.adf6137], suggesting a less-explored strategy for coercive field optimization. Supported by density functional theory calculations, we demonstrate the Pca2_{1} phase, with a moderate concentration of interstitial Hf dopants, serves as a minimal model to explain the experimental observations, rather than the originally assumed rhombohedral phase. Large-scale deep potential molecular dynamics simulations suggest that interstitial defects promote the polarization reversal by facilitating Pbcn-like mobile 180° domain walls. A simple prepoling treatment could reduce the switching field to less than 1 MV/cm and enable switching on a subnanosecond timescale. High-throughput calculations reveal a negative correlation between the switching barrier and dopant size and identify a few promising interstitial dopants for coercive field reduction.
Abstract Since the first report of ferroelectricity in nanoscale HfO 2 -based thin films in 2011, this silicon-compatible binary oxide has quickly garnered intense interest in academia and industry, and continues to do so. Despite its deceivingly simple chemical composition, the ferroelectric physics supported by HfO 2 is remarkably complex, arguably rivaling that of perovskite ferroelectrics. Computational investigations, especially those utilizing first-principles density functional theory (DFT), have significantly advanced our understanding of the nature of ferroelectricity in these thin films. In this review, we provide an in-depth discussion of the computational efforts to understand ferroelectric hafnia, comparing various metastable polar phases and examining the critical factors necessary for their stabilization. The intricate nature of HfO 2 is intimately related to the complex interplay among diverse structural polymorphs, dopants and their charge-compensating oxygen vacancies, and unconventional switching mechanisms of domains and domain walls, which can sometimes yield conflicting theoretical predictions and theoretical-experimental discrepancies. We also discuss opportunities enabled by machine-learning-assisted molecular dynamics and phase-field simulations to go beyond DFT modeling, probing the dynamical properties of ferroelectric HfO 2 and tackling pressing issues such as high coercive fields.
We compute the thermodynamic and the kinetic properties for the reaction: HCOCN→HCH+CO using the statistical theory and the transition-state theory.The equi- librium constants and the rate coefficients of this reaction are also reported here,and the half lives of formyl cyanide at different temperatures are first estimated in this work.
“Multiscale structural optimization” is assumed a next step to further optimize the unconventional ferroelectricity in HfO 2 for high-performance HfO 2 -based ferroelectrics and devices.
Water, because of its fundamental role in biology, geology, and many industrial applications and its anomalous behavior compared to that of simple fluids, continues to fascinate and attract extensive scientific interest. Building on previous studies of water in contact with different surfaces, in this study, we report results obtained from molecular dynamics simulations of water near hydrophilic and hydrophobic interfaces in the presence of nonionic and ionic amphiphilic molecules, hexaethylene glycol monododecyl ether (C12E6) and sodium dodecyl sulfate (SDS). We elucidate how these surfactants affect the packing (i.e., density profiles) and orientation of interfacial water. The results highlight the interplay of both surfactant charges and the substrate charge distribution predominantly with respect to the orientation of water molecules, up to distances longer than those expected based on simulation results on flat solid surfaces. We also quantify the dynamics of interfacial water molecules by computing the residence probability for water in contact with various substrates. We compare our results to those previously obtained for interfacial water on silica and graphite and also with experimental sum-frequency vibrational spectroscopy results at the air–water interface in the presence of surfactants. Our analysis could be useful for a better understanding of interfacial water not only near solid substrates but also near self-assembled/aggregated molecules at a variety of interfaces.
The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization–electric field (P–E) and strain–electric field (ε–E) hysteresis loops for BaTiO3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole–enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (≈0.1 at. %) of defect dipoles dramatically improves electromechanical responses. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers.
Volatile organic compounds (VOCs) exert a serious impact on the environment and human health. The development of new technologies for the elimination of VOCs, especially those from non-industrial emission sources, such as indoor air pollution and other low-concentration VOCs exhaust gases, is essential for improving environmental quality and human health. In this study, a monolithic photothermocatalyst was prepared by stabilizing manganese oxide on multi-porous carbon spheres to facilitate the elimination of formaldehyde (HCHO). This catalyst exhibited excellent photothermal synergistic performance. Therefore, by harvesting only visible light, the catalyst could spontaneously heat up its surface to achieve a thermal catalytic oxidation state suitable for eliminating HCHO. We found that the surface temperature of the catalyst could reach to up 93.8 °C under visible light, achieving an 87.5% HCHO removal efficiency when the initial concentration of HCHO was 160 ppm. The microporous structure on the surface of the carbon spheres not only increased the specific surface area and loading capacity of manganese oxide but also increased their photothermal efficiency, allowing them to reach a temperature high enough for MnOx to overcome the activation energy required for HCHO oxidation. The relevant catalyst characteristics were analyzed using XRD, measurement of BET surface area, scanning electron microscopy, HR-TEM, XPS, and DRS. Results obtained from a cyclic performance test indicated high stability and potential application of the MnOx-modified multi-porous carbon sphere.
In2O3, a convertible reactant used as an anode, exhibits exceptional capacity in lithium-ion batteries (LIBs). However, its tendency for substantial volume expansion leads to internal fracturing and rearrangement. Two-dimensional double transition metal carbides and nitrides (MXenes) present unique out-of-plane ordering of metal atoms, show promising electrical properties due to their chemical adaptability and complex structure. Yet, MXenes tend to aggregate or stack in lamellar structures, hindering their practical application in energy storage and utilization. To address these challenges, a hierarchical porous microrods In2O3@C@Ti3C2TX (HPMR-In2O3@C@Ti3C2TX) composite anode material was synthesized. This involved electrostatic self-assembly of MIL-68 (In) and Ti3C2TX, followed by carbonization treatment. The resulting continuous one-dimensional microrods structure and hierarchical porous channels in HPMR-In2O3@C provide a significant specific surface area, abundant Li+ storage sites, and expedited charge transfer rates. Additionally, the interlayer space within Ti3C2TX functions as an electrolyte reservoir, facilitating comprehensive electrochemical reactions and accommodating volume changes during charge-discharge cycles. As expected, the HPMR-In2O3@C@Ti3C2TX anode demonstrated impressive attributes, boasting a high initial discharge specific capacity of 1406 mAh g-1 at 0.1 C, outstanding cycling performance, and rate performance. This work provides a promising avenue for the development of high-performance anode materials tailored for lithium-ion batteries.
Long-wave infrared InAs/InAsSb type-II superlattice nBn photodetectors are demonstrated on GaSb substrates. The typical device consists of a 2.2 μm thick absorber layer and has a 50% cutoff wavelength of 13.2 μm, a measured dark current density of 5 × 10−4 A/cm2 at 77 K under a bias of −0.3 V, a peak responsivity of 0.24 A/W at 12 μm, and a maximum resistance-area product of 300 Ω cm2 at 77 K. The calculated generation-recombination noise limited specific detectivity (D*) and experimentally measured D* at 12 μm and 77 K are 1 × 1010 cm Hz1/2/W and 1 × 108 cm Hz1/2/W, respectively.
Correction for 'On-demand quantum spin Hall insulators controlled by two-dimensional ferroelectricity' by Jiawei Huang et al., Mater. Horiz., 2022, DOI: https://doi.org/10.1039/d2mh00334a.
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