Kinetically Controlled Synthesis of Quasi-Square CsPbI3 Nanoplatelets with Excellent Stability.

Nanoplatelets (NPLs) share excellent luminescent properties with their symmetric quantum dots counterparts and entail special characters benefiting from the shape, like the thickness-dependent bandgap and anisotropic luminescence. However, perovskite NPLs, especially those based on iodide, suffer from poor spectral and phase stability. Here, stable CsPbI3 NPLs obtained by accelerating the crystallization process in ambient-condition synthesis are reported. By this kinetic control, the rectangular NPLs into quasi-square NPLs are tuned, where enlarged width endows the NPLs with a lower surface-area-to-volume ratio (S/V ratio), leading to lower surficial energy and thus improved endurance against NPL fusion (cause for spectral shift or phase transformation). The accelerated crystallization, denoting the fast nucleation and short period of growth in this report, is enabled by preparing a precursor with complete transformation of PbI2 into intermediates (PbI3 -), through an additional iodide supplier (e.g., zinc iodide). The excellent color stability of the materials remains in the light-emitting diodes under various bias stresses.

Studying the Thermodynamic Phase Stability of Organic-Inorganic Hybrid Perovskites Using Machine Learning.

As an important photovoltaic material, organic-inorganic hybrid perovskites have attracted much attention in the field of solar cells, but their instability is one of the main challenges limiting their commercial application. However, the search for stable perovskites among the thousands of perovskite materials still faces great challenges. In this work, the energy above the convex hull values of organic-inorganic hybrid perovskites was predicted based on four different machine learning algorithms, namely random forest regression (RFR), support vector machine regression (SVR), XGBoost regression, and LightGBM regression, to study the thermodynamic phase stability of organic-inorganic hybrid perovskites. The results show that the LightGBM algorithm has a low prediction error and can effectively capture the key features related to the thermodynamic phase stability of organic-inorganic hybrid perovskites. Meanwhile, the Shapley Additive Explanation (SHAP) method was used to analyze the prediction results based on the LightGBM algorithm. The third ionization energy of the B element is the most critical feature related to the thermodynamic phase stability, and the second key feature is the electron affinity of ions at the X site, which are significantly negatively correlated with the predicted values of energy above the convex hull (Ehull). In the screening of organic-inorganic perovskites with high stability, the third ionization energy of the B element and the electron affinity of ions at the X site is a worthy priority. The results of this study can help us to understand the correlation between the thermodynamic phase stability of organic-inorganic hybrid perovskites and the key features, which can assist with the rapid discovery of highly stable perovskite materials.

Tailoring Copper Single-Atoms-Stabilized Metastable Transition-Metal-Dichalcogenides for Sustainable Hydrogen Production.

Unconventional 1T' phase transition metal dichalcogenides (TMDs) show great potential for hydrogen evolution reaction (HER). However, they are susceptible to transitioning into the stable 2H phase, which reduces their catalytic activity and stability. Herein, we present a scalable approach for designing thermally stable 1T'-TMDs hollow structures (HSs) by etching Cu1.94S templates from pre-synthesized Cu1.94S@TMDs heterostructures, including 1T'-MoS2, MoSe2, WS2, and WSe2 HSs. Furthermore, taking 1T'-MoS2 HSs as an example, the etched Cu ions can be firmly adsorbed on their surface in the form of single atoms (SAs) through Cu-S bonds, thereby elevating the phase transition temperature from 149 ºC to 373 ºC. Due to the advantages conferred by the 1T' phase, hollow structure, and synergistic effect between Cu SAs and 1T'-MoS2 supports, the fabricated 1T'-MoS2 HSs demonstrate superior HER performance. Notably, their high-phase stability enables continuous operation of designed 1T'-MoS2 HSs for up to 200 hours at an ampere-level current density without significant activity decay. This work provides a universal method for synthesizing highly stable 1T'-TMDs electrocatalysts, with a particular focus on the relationship between their phase and catalytic stability.

Surface-Stabilized CsPbI3 Nanocrystals with Tailored Organic Polymer Ligand Binding.

Perovskite nanocrystals (NCs) exhibit attractive photophysical properties by combining the excellent optoelectronic properties of bulk perovskites with the strong quantum confinement effect at the nanoscale. However, CsPbI3 NCs easily transform into a non-perovskite phase because of the ionic lattice and dynamic ligand binding. Herein, stable black-phase CsPbI3 NCs capped with a new organic ligand, HO-PS-N3 (HOPS), which consists of a polystyrene segment with hydroxyl and azide end groups, are reported. This organic polymer ligand passivated the surface defects and enhanced the stability of CsPbI3 NCs by exposing the linking hydrophobic polystyrene segment. Consequently, the optimized CsPbI3 NCs exhibit significantly improved resistance to moisture or light and maintained 70 % of the original luminous intensity after immersion in water for two months. The theoretical results revealed that the binding energy of the HOPS ligand on the surface of the CsPbI3 NCs is higher than that of the commonly used oleic acid, alleviating the defects-induced degradation during purification. Thus, surface-stabilized CsPbI3 NCs are beneficial for a broad range of optoelectronic applications.

The Effect of Arginine on the Phase Stability of Aqueous Hen Egg-White Lysozyme Solutions.

The effect of arginine on the phase stability of the hen egg-white lysozyme (HEWL) has been studied via molecular dynamics computer simulations, as well as experimentally via cloud-point temperature determination. The experiments show that the addition of arginine increases the stability of the HEWL solutions. The computer simulation results indicate that arginine molecules tend to self-associate. If arginine residues are located on the protein surface, the free arginine molecules stay in their vicinity and prevent the way protein molecules "connect" through them to form clusters. The results are not sensitive to a particular force field and suggest a possible microscopic mechanism of the stabilizing role of arginine as an excipient.

Phase Instability in van der Waals In2 Se3 Determined by Surface Coordination.

van der Waals In2 Se3 has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In2 Se3 have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and ß'-In2 Se3 originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In2 Se3 in air to form amorphous In2 Se3-3x O3x layers and Se hemisphere particles. Both O2 and H2 O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In2 Se3-3x O3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In2 Se3 performance for device applications.

Cation/Anion Dual-Vacancy Pair Modulated Atomically-Thin Sex -Co3 S4 Nanosheets with Extremely High Water Oxidation Performance in Ultralow-Concentration Alkaline Solutions.

The density functional theory calculation results reveal that the adjacent defect concentration and electronic spin state can effectively activate the CoIII sites in the atomically thin nanosheets, facilitating the thermodynamic transformation of *O to *OOH, thus offering ultrahigh charge transfer properties and efficiently stabilizing the phase. This undoubtedly evidences that, for metal sulfides, the atom-scale cation/anion vacancy pair and surface electronic spin state can play a great role in enhancing the oxygen evolution reaction. Inspired by the theoretical prediction, interconnected selenium (Se) wired ultrathin Co3 S4 (Sex -Co3 S4 ) nanosheets with Co/S (Se) dual-vacancies (Se1.0 -Co3 S4 -VS/Se -VCo ) pairs are constructed by a simple approach. As an efficient sulfur host material, in an ultralow-concentration KOH solution (0.1 m), Se1.0 -Co3 S4 -VS/Se -VCo presents outstanding durability up to 165 h and a low overpotential of 289.5 mV at 10 mA cm-2 , which outperform the commercial Co3 S4 nanosheets (NSs) and RuO2 . Moreover, the turnover frequency of Se1.0 -Co3 S4 -VS/Se -VCo is 0.00965 s-1 at an overpotential of 0.39 V, which is 5.7 times that of Co3 S4 NSs, and 5.8 times that of commercial RuO2 . The finding offers a rational design strategy to create the multi-defect structure in catalysts toward high-efficiency water electrolysis.

Intrinsic Phase Stability and Inherent Bandgap of Formamidinium Lead Triiodide Perovskite Single Crystals.

Understanding the intrinsic phase stability and inherent band gap of formamidinium lead triiodide (FAPbI3 ) perovskites is crucial to further improve the performance of perovskite solar cells (PSCs). Herein, we explored the α- to δ-phase transition and band gap of FAPbI3 single crystals grown by an inverse temperature solubility method. We found that the residual γ-butyrolactone solvents in the inner empty space of the FAPbI3 single crystal accelerate the phase transition at kinetics. By adopting 2-methoxyethanol as the solvent, over 2000 h of stable α-FAPbI3 crystals could be acquired. This proves that although FAPbI3 is regarded as unstable at thermodynamics, it could own excellent kinetic stability without any doping or additives because of the slow solid to solid phase transition instead of the fast phase transition assisted by the solvents. Furthermore, we revealed that the bulk FAPbI3 single crystal with a size above 100 µm can have an inherent band gap of 1.41 eV. Thus, our work provides key scientific guidance for high-performance FAPbI3 -based PSCs.

Remarkable Black-Phase Robustness of CsPbI3 Nanocrystals Sealed in Solid SiO2 /AlOx Sub-Micron Particles.

This work combines the high-temperature sintering method and atomic layer deposition (ALD) technique, and yields SiO2 /AlOx -sealed γ-CsPbI3 nanocrystals (NCs). The black-phase CsPbI3 NCs, scattered and encapsulated firmly in solid SiO2 sub-micron particles, maintain in black phases against water soaking, ultraviolet irradiation, and heating, exhibiting remarkable phase stability. A new phase-transition route, from γ via ß to α phase without transferring into δ phase, has been discovered upon temperature increasing. The phase stability is ascribed to the high pressure exerted by the rigid SiO2 encapsulations, and its condensed amorphous structures that prevent the permeation of H2 O molecules. Nanoscale coating of Al2 O3 thin films, which are deposited on the surface of the CsPbI3 -SiO2 by ALD, enhances the protection against O2 infiltration, greatly elevating the high-temperature stability of CsPbI3 NCs sealed inside, as the samples remain bright after 1-h annealing in air at 400 °C. These fabrication and encapsulation techniques effectively prevent the formation of δ-CsPbI3 under harsh environment, bringing the high-pressure preservation of black-phase CsPbI3 from laboratory to industry toward potential applications in both photovoltaic and fluorescent areas.

Design of Low Bandgap CsPb1- x Snx I2 Br Perovskite Solar Cells with Excellent Phase Stability.

Novel all-inorganic Sn-Pb alloyed perovskites are developed aiming for low toxicity, low bandgap, and long-term stability. Among them, CsPb1- x Snx I2 Br is predicted as an ideal perovskite with favorable band gap, but previously is demonstrated unable to convert to perovskite phase by thermal annealing. In this report, a series of CsPb1- x Snx I2 Br perovskites with tunable bandgaps from 1.92 to 1.38 eV are successfully prepared for the first time via low annealing temperature (60 °C). Compared to the pure CsPbI2 Br, these Sn-Pb alloyed perovskites show superior stability. Furthermore, a novel α-phase-stabilization mechanism of the inorganic Sn-Pb alloyed perovskite by reconfiguring the perovskite crystallization process with chloride doping is provided. Simultaneously, a dense protection layer is formed by the coordination reaction between the surface lead dangling bonds and sulfate ion to retard the permeation of external oxygen and moisture, leading to less oxidation of Sn2+ in perovskite film. As a result, the fabricated all-inorganic Sn-Pb perovskite solar cells (PSCs) show a champion power conversion efficiency of 10.39% with improved phase stability and long-term ambient stability against light, heat, and humidity. This work provides a viable strategy in fabricating high-performance narrow-bandgap all-inorganic PSCs.

Phase Stability under Thermal Drifts in Photodiode-Conditioning Transimpedance Amplifiers for Distance Metrology.

Transimpedance amplifiers (TIA) are widely used for front-end signal conditioning in many optical distance measuring applications in which high accuracy is often required. Small effects due to the real characteristics of the components and the parasitic elements in the circuit board may cause the error to rise to unacceptable levels. In this work we study these effects on the TIA delay time error and deduce analytic expressions, taking into account the trade-off between the uncertainties caused by the delay time instability and by the signal-to-noise ratio. A specific continuous-wave phase-shift case study is shown to illustrate the analysis, and further compared with real measurements. General strategies and conclusions, useful for designers of this kind of system, are extracted too. The study and results show that the delay time thermal stability is a key determinant factor in the measured distance accuracy and, without an adequate design, moderate temperature variations of the TIA can cause extremely high measurement errors.

Unravelling thermal history during additive manufacturing of martensitic stainless steel.

In-situ thermal cycling neutron diffraction experiments were employed to unravel the effect of thermal history on the evolution of phase stability and internal stresses during the additive manufacturing (AM) process. While the fully-reversible martensite-austenite phase transformation was observed in the earlier thermal cycles where heating temperatures were higher than Af, the subsequent damped thermal cycles exhibited irreversible phase transformation forming reverted austenite. With increasing number of thermal cycles, the thermal stability of the retained austenite increased, which decreased the coefficient of thermal expansion. However, martensite revealed higher compressive residual stresses and lower dislocation density, indicating inhomogeneous distributions of the residual stresses and microstructures on the inside and on the surface of the AM component. The compressive residual stresses that acted on the martensite resulted preferentially from transformation strain and additionally from thermal misfit strain, and the decrease in the dislocation density might have been due to the strong recovery effect near the Ac1 temperature.

Promoting Thermodynamic and Kinetic Stabilities of FA-based Perovskite by an in Situ Bilayer Structure.

The commonly employed formamidinium (FA)-containing perovskite solar cells (PSCs) exhibit a severe phase instability problem, thereby limiting their commercial applications. Here, both phase stability and energy efficiency of FA-based PSCs were improved by treating the perovskite surface with pyrrolidinium hydroiodide (PyI) salts, resulting in a 1D perovskite structure (PyPbI3), stacked on the original 3D perovskite. By employing in situ XRD measurements, we revealed that the temperature-dependent phase transition activation barrier was enhanced after forming the 1D/3D structure, resulting in a prolonged transition time by 30-40-fold. From the first-principle calculations, we found the thermodynamic energy difference between two phases reduced from -0.16 to -0.04 eV after the stacking of 1D PyPbI3, offering additional lifetime improvement. Moreover, the champion 1D/3D bilayer PSC exhibits a boosted power conversion efficiency of 19.62%, versus 18.21% of the control. Such 1D/3D bilayer structure may be employed in PSCs to enhance their phase stability and photovoltaic performance.

Brownmillerites CaFeO2.5 and SrFeO2.5 as Catalyst Support for CO Oxidation.

The support material can play an important role in oxidation catalysis, notably for CO oxidation. Here, we study two materials of the Brownmillerite family, CaFeO2.5 and SrFeO2.5, as one example of a stoichiometric phase (CaFeO2.5, CFO) and one existing in different modifications (SrFeO2.75, SrFeO2.875 and SrFeO3, SFO). The two materials are synthesized using two synthesis methods, one bottom-up approach via a complexation route and one top-down method (electric arc fusion), allowing to study the impact of the specific surface area on the oxygen mobility and catalytic performance. CO oxidation on 18O-exchanged materials shows that oxygen from SFO participates in the reaction as soon as the reaction starts, while for CFO, this onset takes place 185 °C after reaction onset. This indicates that the structure of the support material has an impact on the catalytic performance. We report here on significant differences in the catalytic activity linked to long-term stability of CFO and SFO, which is an important parameter not only for possible applications, but equally to better understand the mechanism of the catalytic activity itself.

Mechanistic differences in the effects of sucrose and sucralose on the phase stability of lysozyme solutions.

The effect of two disaccharide analogues, sucrose and sucralose, on the phase stability of aqueous lysozyme solutions has been addressed from a mechanistic viewpoint by a combination of experiment and molecular dynamics (MD) simulations. The influence of the added low molecular weight salts (NaBr, NaI and NaNO3) was considered as well. The cloud-point temperature measurements revealed a larger stabilizing effect of sucralose. Upon increasing sugar concentration, the protein solutions became more stable and differences in the effect of sucralose and sucrose amplified. It was confirmed that the addition of either of the two sugars imposed no secondary structure changes of the lysozyme. Enthalpies of lysozyme-sugar mixing were exothermic and a larger effect was recorded for sucralose. MD simulations indicated that acidic, basic and polar amino acid residues play predominant roles in the sugar-protein interactions, mainly through hydrogen bonding. Such sugar mediated protein-protein interactions are thought to be responsible for the biopreserative nature of sugars. Our observations hint at mechanistic differences in sugar-lysozyme interactions: while sucrose does not interact directly with the protein's surface for the most part (in line with the preferential hydration hypothesis), sucralose forms hydrogen bonds with acidic, basic and polar amino acid residues at the lysozyme's surface (in line with the water replacement hypothesis).

The Relative Thermodynamic Stability of Diamond and Graphite.

Recent density-functional theory (DFT) calculations raised the possibility that diamond could be degenerate with graphite at very low temperatures. Through high-accuracy calorimetric experiments closing gaps in available data, we reinvestigate the relative thermodynamic stability of diamond and graphite. For T<400 K, graphite is always more stable than diamond at ambient pressure. At low temperatures, the stability is enthalpically driven, and entropy terms add to the stability at higher temperatures. We also carried out DFT calculations: B86bPBE-25X-XDM//B86bPBE-XDM and PBE0-XDM//PBE-XDM results overlap with the experimental -TΔS results and bracket the experimental values of ΔH and ΔG, displaced by only about 2× the experimental uncertainty. Revised values of the standard thermodynamic functions for diamond are Δf Ho =-2150±150 J mol-1 , Δf So =3.44±0.03 J K-1 mol-1 and Δf Go =-3170±150 J mol-1 .

Recent Advances and Optoelectronic Applications of Lead-Free Halide Double Perovskites.

Organic-inorganic metal halide perovskites (most notably CH3 NH3 PbI3 ) have demonstrated remarkable physical attributes for photovoltaic and diverse optoelectronic applications, whereas concerns about toxicity owing to the use of lead in the chemical composition still motivate further exploration of new, nontoxic candidates. Lead-free halide double perovskites (HDPs), designed by the rational chemical substitution of Pb2+ with other nontoxic candidate elements, have recently attracted interest as a fascinating alternative to their Pb-based counterparts. Herein, recent advances in crystal structures, physical properties, and versatile optoelectronic applications of lead-free HDPs, such as solar cells, photodetectors, X-ray detectors, and light-emitting diodes, are reviewed. Perspectives to improve the physical and photoelectric properties of existing HDP materials are also discussed and will favor future development of new, lead-free HDP candidates.

Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress, Phase Equilibria and Critical Temperatures.

Metal-hydrogen (M-H) systems offer grand opportunities for studies on fundamental aspects of thermodynamics and kinetics. When the system size is reduced to the nanoscale, microstructural defects as well as mechanical stress affect the systems' properties. This is contemplated for the model system of epitaxial niobium-hydrogen (Nb-H) thin films. Hydrogen absorption in metals commonly leads to lattice expansion which is hindered when the metal adheres to a flat rigid substrate. Consequently, high mechanical stress of about -10 GPa for 1 H/Nb are predicted, in theory. However, metals cannot yield such high stresses and respond with plastic deformation, commonly limiting measured stresses to -2 to -3 GPa for 100 nm Nb-H films. It will be shown that the coherency state changes with film thickness reduction, shifting the onset of plastic deformation to larger hydrogen concentrations. Below critical film thicknesses, plastic deformation is fully absent. The system then behaves purely elastic and ultra-high stress of about -10 (±2) GPa can be obtained. Arising stress controls the phase stability of M-H systems, and the coherency state strongly affects the nucleation and growth dynamics of the phase transition. In case of Nb-H thin films of less than 8 nm thickness the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed at 300 K. Related effects can be utilised to optimise metal-hydrides used in applications.

Potassium Incorporation for Enhanced Performance and Stability of Fully Inorganic Cesium Lead Halide Perovskite Solar Cells.

Thermally unstable nature of hybrid organic-inorganic perovskites has been a major obstacle to fabricating the long-term operational device. A cesium lead halide perovskite has been suggested as an alternative light absorber, due to its superb thermal stability. However, the phase instability and poor performance are hindering the further progress. Here, cesium lead halide perovskite solar cells with enhanced performance and stability are demonstrated via incorporating potassium cations. Based on Cs0.925K0.075PbI2Br, the planar-architecture device achieves a power conversion efficiency of 10.0%, which is a remarkable record in the field of inorganic perovskite solar cells. In addition, the device shows an extended operational lifetime against air. Our research will stimulate the development of cesium lead halide perovskite materials for next-generation photovoltaics.

Brute Force Composition Scanning with a CALPHAD Database to Find Low Temperature Body Centered Cubic High Entropy Alloys.

We used the Thermo-Calc High Entropy Alloy CALPHAD database to determine the stable phases of AlCrMnNbTiV, AlCrMoNbTiV, AlCrFeTiV and AlCrMnMoTi alloys from 800 to 2800 K. The concentrations of elements were varied from 1-49 atom%. A five- or six-dimensional grid is constructed, with stable phases calculated at each grid point. Thermo-Calc was used as a massive parallel tool and three million compositions were calculated, resulting in tens of thousands of compositions for which the alloys formed a single disordered body centered cubic (bcc) phase at 800 K. By filtering out alloy compositions for which a disordered single phase persists down to 800 K, composition 'islands' of high entropy alloys are determined in composition space. The sizes and shapes of such islands provide information about which element combinations have good high entropy alloy forming qualities as well as about the role of individual elements within an alloy. In most cases disordered single phases are formed most readily at low temperature when several elements are almost entirely excluded, resulting in essentially ternary alloys. We determined which compositions lie near the centers of the high entropy alloy islands and therefore remain high entropy islands under small composition changes. These island center compositions are predicted to be high entropy alloys with the greatest certainty and make good candidates for experimental verification. The search for high entropy islands can be conducted subject to constraints, e.g., requiring a minimum amount of Al and/or Cr to promote oxidation resistance. Imposing such constraints rapidly diminishes the number of high entropy alloy compositions, in some cases to zero. We find that AlCrMnNbTiV and AlCrMoNbTiV are relatively good high entropy alloy formers, AlCrFeTiV is a poor high entropy alloy former, while AlCrMnMoTi is a poor high entropy alloy former at 800 K but quickly becomes a better high entropy alloy former with increasing temperature.