Rashba and Dresselhaus effects in doped methylammonium lead halide perovskite MAPbI3.

Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI3) perovskite, have been regarded as promising materials for optoelectronics and spintronics. However, the practical applications of these perovskites are limited by lead toxicity and instability under air and pressure. This study investigates the substitution of Pb with Sn and Ge in cubic MAPbI3 perovskite. The properties of the resulting hybrid perovskites are compared using state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with generalized gradient approximation (PBE) and hybrid functional (HSE06), in conjunction with spin-orbit coupling (SOC). Here, we mainly study the Rashba-Dresselhaus (RD) effect, which arises due to two major mechanisms: (i) the breaking of inversion symmetry (static and dynamic) and (ii) SOC, originating from the presence of heavy elements. We find significant spin-splitting effects in the conduction band minimum and valence band maximum for hybrid perovskites. To gain a deeper understanding of the observed spin-splitting, the spin textures are analyzed, and Rashba coefficients are calculated. We find that the Dresselhaus effect comes into play in substituted hybrid structures in addition to the usual Rashba effect observed in the pristine compound. Additionally, we observe that the strength of Rashba spin-splitting is substantially tuned by the application of uniaxial strain (±5%). Moreover, certain hybrid perovskites exhibit mechanical stability and ductility, making them potential candidates in perovskite-based optoelectronics and spintronics applications.

O-Vacancy Mediated Partially Inverted Ferrospinels for Enhanced Activity in the Sulfuric Acid Decomposition for Hydrogen Production.

Understanding the characterization of a tailored Co3O4 spinel with Fe3+ doping poses a challenge due to the surface state complexity in bifunctional catalysts with higher cation diversity. Doping with secondary metal results in a double spinel structure (a hybrid of normal and inverted spinels). This enhances the catalytic properties by generating more active oxygen vacancies. The cobalt-rich (FeCo2O4) hybrid spinel and iron-rich (CoFe2O4) inverted spinel are synthesized using a wet impregnation method, supported over oxidized SiC (SiC-Pretrt) for an improved metal-support interaction. FeCo2O4 on pretreated SiC exhibits the highest catalytic activity (90% conversion at 1173 K) and stability (over 100 h) in sulfuric acid decomposition of the iodine-sulfur process for hydrogen production. This improved performance is attributed to the high electronegativity of Co3+, oxygen vacancies, and strong metal-support interaction. The high electronegativity of Co3+ weakens the S-O bond in M-S-O, enhancing the catalytic activity of the spinels. These results are further corroborated by detailed characterization and density functional theory calculations.

Facile Hydrogenolysis of Sugars to 1,2-Glycols by Ru@PPh3/OPPh3 Confined Large-Pore Mesoporous Silica.

Tandem hydrogenation vis-à-vis hydrogenolysis of xylose to 1,2-glycols remains a major challenge. Although one-pot conversion of xylose to 1,2-glycols requires stringent conditions, a sustainable approach would be quite noteworthy. We have developed a microwave route for the one-pot conversion of pentose (C5) and hexose (C6) sugars into glycol and hexitol, without pressurized hydrogen reactors. A pronounced hydrogenolysis of sugars to glycols is observed by Ru single atom (SA) on triphenylphosphine/phosphine oxide-modified silica (Ru@SiP), in contrast to Ru SA on pristine (Ru@SiC) and 3-aminopropyl-modified silica (Ru@SiN). A promising "ligand effect" was observed through phosphine modification of silica that presents a 70% overall yield of all reduced sugars (xylitol + glycols) from a 99% conversion of xylose with Ru@SiP. A theoretical study by DFT depicts an electronic effect on Ru-SA by triphenylphosphine that promotes the catalytic hydrogenolysis of sugars under mild conditions. Hence, this research represents an important step for glycols from biomass-derived sources.

Boosting Photocatalytic Nitrogen Fixation via Nanoarchitectonics Using Oxygen Vacancy Regulation in W-Doped Bi2MoO6 Nanosheets.

Ammonia and nitrates are key raw materials for various chemical and pharmaceutical industries. The conventional methods like Haber-Bosch and Ostwald methods used in the synthesis of ammonia and nitrates, respectively, result in harmful emission of gases. In recent years, the photocatalytic fixation of N2 into NH3 and nitrates has become a hot topic since it is a green and cost-effective approach. However, the simultaneous production of ammonia and nitrates has not been studied much. In this regard, we have synthesized W-doped Bi2MoO6 nanosheets in various molar ratios and demonstrated their potential as efficient photocatalysts for the simultaneous production of NH3 and NO3- ions under visible light irradiation. It was found that one of the catalysts (BMWO0.4) having an optimal molar ratio of doped tungsten showed the best photocatalytic NH3 production (56 µmol h-1) without using any sacrificial agents along with the simultaneous production of NO3- ions at a rate of 7 µmol h-1. The enhanced photocatalytic activity of the synthesized photocatalysts could be ascribed to oxygen vacancy defects caused by Mo substitution by a more electronegative W atom. Furthermore, density functional theory calculations verified the alteration in the band gap after doping of W atoms and also showed a strong chemisorption of N2 over the photocatalyst surface leading to its activation and thereby enhancing the photocatalytic activity. Thus, the present work provides insights into the effect of structural distortions on tailoring the efficiency of materials used in photocatalytic N2 fixation.

Exploring Chalcohalide Perovskite-Inspired Materials (Sn2SbX2I3; X = S or Se) for Optoelectronic and Spintronic Applications.

Chalcohalide perovskite-inspired materials have attracted attention as promising optoelectronic materials due to their small band gaps, high defect tolerance, nontoxicity, and stability. However, a detailed analysis of their electronic structure and excited-state properties is lacking. Here, using state-of-the-art density functional theory, an effective k·p model analysis, and many-body perturbation theory (within the framework of GW and BSE), we explore the band splitting and excitonic properties of Sn2SbX2I3 (X = S or Se). Our findings reveal that the Cmc21 phase exhibits Rashba and Dresselhaus effects, causing significant band splitting, especially near the conduction and valence band extremes, respectively. Moreover, we find that the exciton binding energy is larger than those of lead halide perovskites but smaller than those of chalcogenide perovskites. We also investigate polaron-facilitated charge carrier mobility, which is found to be similar to that of lead halide perovskites and greater than that of chalcogenide perovskites. These characteristics make these materials promising for applications in spintronics and optoelectronics.

Thermodynamic Window for Size-Controlled Pore Formation in Graphene for Large-Scale Molecular Sieves.

Nanopores in graphene monolayers are a promising option for molecular separation applications, such as desalination and carbon capture. Graphene's atomic thickness allows for an optimal balance between molecular selectivity and permeability, while its chemical stability and robust mechanical properties make it appealing for a wide range of commercial applications. However, scaling to large areas with controlled pore size distribution is an open challenge in ultrathin membranes. Here, using first-principles calculations, we identify a suitable thermodynamic window in a chemical vapor deposition system for directly growing graphene monolayers with a controlled pore size distribution. As an example, our calculations show that a postgrowth annealing step with a supersaturation range of 19.7-25 kJ/mol at 1000 K results in the creation of a controllable pore density at graphene grain boundaries, with pore sizes falling within the range of 5-8 Å. Such pores isolate hydrated Cl ions from water molecules, effectively desalinating seawater. Thus, it allows the design of targeted synthesis of large-scale 2D layers for membrane applications.

Electrocatalytic study of the hydrogen evolution reaction on MoS2/BP and MoSSe/BP in acidic media.

Molecular hydrogen (H2) production by the electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious metal-based electrocatalysts that are earth-abundant and low cost like MoS2. Although it is acid-stable, its applicability is limited by catalytically inactive basal planes, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer boron phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for the HER in an electrochemical double layer model with the BP monolayer, MoS2/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations reveal that the Tafel reaction shows no reaction barrier. Thereafter, for the Heyrovsky reaction, we obtained a low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS2/BP and MoSSe/BP vdW HTSs in the case of 2 × 2 supercell configuration. However, in the case of 3 × 3 and 4 × 4 configurations, MoSSe/BP shows a feasible Heyrovsky reaction with no reaction barrier. The coverages with 1/4H+ concentration (conc.) deduced high coverage with low conc. and low coverage with high conc. to be apt for the HER via the Heyrovsky reaction path. Finally, on observing the activation barrier of the Heyrovsky pathway along with that of second H adsorption at the surface, the Heyrovsky path is expected to be favoured.

Tailoring defects in SrTiO3 by one step nanoarchitectonics for realizing photocatalytic nitrogen fixation in pure water.

Surface contamination of materials by nitrogenous impurities is a major problem that can bias the quantification of ammonia in photocatalytic N2 fixation reactions. In this work, SrTiO3 nanocubes were prepared by using a nitrogenous precursor and engineered with Ti3+ sites and oxygen vacancy defects in a one-step solvothermal approach. It was observed that the synthesized materials were containing surface nitrogenous impurities and therefore a rigorous cleaning procedure was adopted to eliminate them to the best extent. The contribution of unavoidable surface impurities was deduced in the form of adventitious NH3 by employing control experiments and a realistic photocatalytic NH3 generation was achieved. It was found that pristine SrTiO3 showed no photocatalytic activity, whereas one of the defected SrTiO3 materials showed the highest NH3 formation under natural sunlight in pure water, which was ascribed to the tuned defect sites, enhanced surface area and efficient separation of photogenerated charges. Based on the experimental results, a stringent protocol has been suggested for materials synthesis while working with nitrogenous precursors and for subsequent photocatalytic N2 fixation experiments. Thus, the present study provides a simple and affordable procedure for catalyst synthesis for the studied application and expands the scope of perovskite oxide materials to fabricate efficient photocatalysts for sustainable NH3 production.

Coupled Spin-Valley, Rashba Effect, and Hidden Spin Polarization in WSi2N4 Family.

Using first-principles calculations, we report the electronic properties with a special focus on the band splitting in the WSi2N4 class of materials. Due to the broken inversion symmetry and strong spin-orbit coupling, we detect coupled spin-valley effects at the corners of the first Brillouin zone (BZ). Additionally, we observe cubically and linearly split bands around the Γ and M points, respectively. The in-plane mirror symmetry (σh) and reduced symmetry of the arbitrary k-point, enforce the persistent spin textures (PST) to occur in full BZ. We induce the Rashba splitting by breaking the σh through an out-of-plane external electric field (EEF). The inversion asymmetric site point group of the W atom introduces the hidden spin polarization in centrosymmetric layered bulk counterparts. Low energy k.p models demonstrate that the PST along the M-K line is robust to EEF and layer thickness, making them suitable for applications in spintronics and valleytronics.

Vacancy-Ordered Double Perovskites Cs2BI6 (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials.

Vacancy-ordered double perovskites (A2BX6), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much, despite their potential applications. Here, we explore Cs2BI6 (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using state-of-the-art first-principles-based methodologies, viz., density functional theory combined with many-body perturbation theory (G0W0) and spin-orbit coupling. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom, leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well-captured by the Fröhlich mesoscopic model. The lattice anharmonicity is further studied using ab initio molecular dynamics and the electron localization function. The maximum anharmonicity is observed in Cs2PtI6, followed by Cs2PdI6, Cs2TeI6, and Cs2SnI6. Also, the computed average thermoelectric figure of merit (zT) for Cs2PtI6, Cs2PdI6, Cs2TeI6, and Cs2SnI6 is 0.88, 0.85, 0.95, and 0.78, respectively, which reveals their promising renewable energy applications.

Exploring strong and weak topological states on isostructural substitutions in TlBiSe[Formula: see text].

Topological Insulators (TIs) are unique materials where insulating bulk hosts linearly dispersing surface states protected by the Time-Reversal Symmetry. These states lead to dissipationless current flow, which makes this class of materials highly promising for spintronic applications. Here, we predict TIs by employing state-of-the-art first-principles based methodologies, viz., density functional theory and many-body perturbation theory (G[Formula: see text]W[Formula: see text]) combined with spin-orbit coupling effects. For this, we take a well-known 3D TI, TlBiSe[Formula: see text] and perform complete substitution with suitable materials at different sites to check if the obtained isostructural materials exhibit topological properties. Subsequently, we scan these materials based on SOC-induced parity inversion at Time-Reversal Invariant Momenta. Later, to confirm the topological nature of selected materials, we plot their surface states along with calculation of Z[Formula: see text] invariants. Our results show that GaBiSe[Formula: see text] is a strong Topological Insulator, besides, we report six weak Topological Insulators, viz., PbBiSe[Formula: see text], SnBiSe[Formula: see text], SbBiSe[Formula: see text], Bi[Formula: see text]Se[Formula: see text], TlSnSe[Formula: see text] and PbSbSe[Formula: see text]. We have further verified that all the reported TIs are dynamically stable, showing all real phonon modes of vibration.

Controlled synthesis of Ru-single-atoms on ordered mesoporous phosphine polymers for microwave-assisted conversion of biomass-derived sugars to artificial sweeteners.

Atomically dispersed metal-single-atoms have become a frontier in solid catalysis due to their characteristic electronic properties. However, for biomass conversion, employing metal-single-atoms as catalysts is rather challenging since they suffer from poor selectivity and yield due to inadequate metal-support interactions. We show here that Ru/triphenylphosphine (PPh)-based ordered mesoporous polymers afford high yields of reduced sugars, xylitol (yield ∼95%) and sorbitol (yield ∼65%) in a microwave reactor with formic acid as the only hydrogen donor. We have established a unique relationship within Ru/triphenylphosphine that shows an important ligand effect, in contrast to, Ru/triphenylamine and Ru/catechol. The tailored electronic properties in Ru/phosphine were thoroughly examined by using state-of-the-art experimental techniques viz. EXAFS, XANES, XPS, DRIFTS and HAADF-STEM. The resulting phosphine-modified catalysts show a promotion in activity and selectivity towards less vulnerable aldehydes for hydrogenation, further confirmed by DFT calculations. This finding reveals a new protocol to tailor the activity of metal-single-atoms utilizing functional porous polymers as nanoreactors.

Synergistic Effect of Work Function and Acoustic Impedance Mismatch for Improved Thermoelectric Performance in GeTe-WC Composite.

The preparation of composite materials is a promising methodology for concurrent optimization of electrical and thermal transport properties for improved thermoelectric (TE) performance. This study demonstrates how the acoustic impedance mismatch (AIM) and the work function of components decouple the TE parameters to achieve enhanced TE performance of the (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite. The simultaneous increase in the electrical conductivity (σ) and Seebeck coefficient (α) with WC (tungsten carbide) volume fraction (z) results in an enhanced power factor (α2σ) in the composite. The rise in σ is attributed to the creation of favorable current paths through the WC phase located between grains of Ge0.87Mn0.05Sb0.08Te, which leads to increased carrier mobility in the composite. Detailed analysis of the obtained electrical properties was performed via Kelvin probe force microscopy (work function measurement) and atomic force microscopy techniques (spatial current distribution map and current-voltage (I-V) characteristics), which are further supported by density functional theory (DFT) calculations. Furthermore, the difference in elastic properties (i.e., sound velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM, and hence, a large interface thermal resistance (Rint) between the phases is achieved. The correlation between Rint and the Kapitza radius depicts a reduced phonon thermal conductivity (κph) of the composite, which is explained using the Bruggeman asymmetrical model. Moreover, the decrease in κph is further validated by phonon dispersion calculations that indicate the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced α2σ and reduced κph results in a maximum figure of merit (zT) of 1.93 at 773 K for (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite for z = 0.010. It results in an average thermoelectric figure of merit (zTav) of 1.02 for a temperature difference (ΔT) of 473 K. This study shows promise to achieve higher zTav across a wide range of composite materials.

Chalcogenide Perovskites (ABS3; A = Ba, Ca, Sr; B = Hf, Sn): An Emerging Class of Semiconductors for Optoelectronics.

Chalcogenide perovskites have received considerable interest in the photovoltaic research community because of their stability, nontoxicity, and lead-free composition. However, because of the huge computational cost, theoretical study focusing on excitonic and polaronic properties is not explored rigorously. Herein, we capture the excitonic and polaronic effects in a series of chalcogenide perovskites ABS3, where A = Ba, Ca, Sr and B = Hf, Sn, by employing state-of-the-art hybrid density functional theory and many-body perturbative approaches, viz., GW and BSE. We find that they possess an exciton binding energy larger than that of 3D inorganic-organic hybrid perovskites. We examine the interplay of electronic and ionic contributions to the dielectric screening and conclude that the electronic contribution is dominant over the ionic contribution. Using the Feynman polaron model, polaron parameters are computed, and charge-separated polaronic states are less stable than bound excitons. Finally, the theoretically calculated spectroscopic limited maximum efficiency suggests that among all chalcogenide perovskites, CaSnS3 could serve as the best choice for photovoltaic applications.

Ferrocene-derived Fe-metalated porous organic polymer for the core planarity-triggered detoxification of chemical warfare agents.

Herein, we demonstrate the successful construction of two Fe-metalated porous organic polymers having planar (Fe-Tt-POP) and non-planar (Fe-Rb-POP) geometry via the ternary copolymerization strategy for the catalytic oxidative decontamination of different sulfur-based mustard gas simulants (HD). Fe-Tt-POP exhibits superior catalytic performance for the oxidation of thioanisole (TA) in comparison with Fe-Rb-POP. Interestingly, this activity difference can be further explored by in situ operando DRIFTS and DFT computational studies.

SO3 decomposition over silica-modified ß-SiC supported CuFe2O4 catalyst: characterization, performance, and atomistic insights.

The sulfur-iodine (S-I) thermochemical water-splitting cycle is one of the potential ways to produce hydrogen on a large scale. CuFe2O4 was dispersed over modified silica or treated ß-SiC and untreated ß-SiC using the wet impregnation method for SO3 decomposition, which is the most endothermic reaction of the S-I cycle. Various state-of-the-art techniques such as XRD, FT-IR, BET, XPS, TEM, HR-TEM, FESEM-EDS and elemental mapping were employed to characterize both the synthesized catalysts. CuFe2O4 catalyst supported on silica-modified ß-SiC resulted in enhanced catalytic activity and stability due to better metal-support interaction. In order to get a better insight into the reaction mechanism over this bimetallic catalyst, the first principles based simulation under the framework of density functional theory was performed. We have found that the presence of Cu gives rise to an improved charge localization at the O-vacancy site alongside favourable reaction kinetics, which results in an enhanced catalytic activity for the CuFe2O4 nano-cluster compared to that of a single metallic catalyst containing Fe2O3 nano-cluster.

Organogel-assisted porous organic polymer embedding Cu NPs for selectivity control in the semi hydrogenation of alkynes.

Heteroatom-rich porous-organic-polymers (POPs) comprising highly cross-linked robust skeletons with high physical and thermal stability, high surface area, and tunable pore size distribution have garnered significant research interest owing to their versatile functionalities in a wide range of applications. Here, we report a newly developed organogel-assisted porous-organic-polymer (POP) supported Cu catalyst (Cu@TpRb-POP). The organogel was synthesized via a temperature induced gelation strategy, employing Schiff-base coupling between 2,4,6-triformylphloroglucinol aldehyde (Tp) and pararosaniline base (Rb). The gel is subsequently transformed to hierarchical porous organic structures without the use of any additive, thereby offering advantageous features including extremely low density, high surface area, a highly cross-linked framework, and a heteroatom-enriched backbone of the polymer. During the semi-hydrogenation of terminal and internal alkynes, the Cu@TpRb-POP-B catalyst with Cu embedded in the TpRb-POP structure consistently demonstrated improved selectivity towards alkenes compared to Cu@TpRb-POP-A, which contains Cu NPs exposed at the exterior surfaces of the POP support. Additionally, Cu@TpRb-POP-B showed higher stability and reusability than Cu@TpRb-POP-A. The superior performance of the Cu@TpRb-POP-B catalyst is attributed to the steric hindrance effect, which controls the product selectivity, as well as the synergistic interaction between the heteroatom-rich POP framework and the embedded Cu NPs. Both the effects are corroborated by experimental characterization of the catalysts and density functional theory (DFT) calculations.

Origin of Rashba Spin Splitting and Strain Tunability in Ferroelectric Bulk CsPbF3.

Spin-orbit coupling (SOC) in conjunction with broken inversion symmetry acts as a key ingredient for several intriguing quantum phenomena, viz., Rashba-Dresselhaus (RD) effect. The coexistence of spontaneous polarization and the RD effect in ferroelectric (FE) materials enables the electrical control of spin degrees of freedom. Here, we explore the FE lead halide perovskite CsPbF3 as a potential candidate in the field of spintronics by employing state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with semilocal and hybrid functional (HSE06) combined with SOC and many-body perturbation theory (G0W0). For a deeper understanding of the observed spin splitting, the spin textures are analyzed using the k.p model Hamiltonian. We find there is no out-of-plane spin component indicating that the Rashba splitting dominates over Dresselhaus splitting. We also observe that the strength of Rashba spin splitting can be substantially tuned on application of uniaxial strain (±5%). More interestingly, we notice reversible spin textures by switching the FE polarization in CsPbF3 perovskite, making it potent for perovskite-based spintronic applications.

Exploring Exciton and Polaron Dominated Photophysical Phenomena in Ruddlesden-Popper Phases of Ban+1ZrnS3n+1 (n = 1-3) from Many Body Perturbation Theory.

Ruddlesden-Popper (RP) phases of Ban+1ZrnS3n+1 are an evolving class of chalcogenide perovskites in the field of optoelectronics, especially in solar cells. However, detailed studies regarding its optical, excitonic, polaronic, and transport properties are hitherto unknown. Here, we have explored the excitonic and polaronic effect using several first-principles based methodologies under the framework of Many Body Perturbation Theory. Unlike its bulk counterpart, the optical and excitonic anisotropy are observed in Ban+1ZrnS3n+1 (n = 1-3) RP phases. As per the Wannier-Mott approach, the ionic contribution to the dielectric constant is important, but it gets decreased on increasing n in Ban+1ZrnS3n+1. The exciton binding energy is found to be dependent on the presence of large electron-phonon coupling. We further observed maximum charge carrier mobility in the Ba2ZrS4 phase. As per our analysis, the optical phonon modes are observed to dominate the acoustic phonon modes, leading to a decrease in polaron mobility on increasing n in Ban+1ZrnS3n+1 (n = 1-3).

Optoelectronic Properties of Chalcogenide Perovskites by Many-Body Perturbation Theory.

Chalcogenide perovskites have emerged as lead-free, stable photovoltaic materials, having promising optoelectronic properties. However, a detailed theoretical study on excitonic properties is rather demanding task due to the huge computational cost and, therefore, is hitherto unknown. Here, we report the excitonic properties of chalcogenide perovskites AZrS3 (A = Ca, Sr, Ba) using state-of-the-art hybrid density functional theory and many-body perturbation theory (within the framework of GW and BSE). We find the exciton binding energy (EB) is larger than that of conventional halide perovskites. We also observe, by computing the electron-phonon coupling parameters, a more stable charge-separated polaronic state as compared to that of the bound exciton. The ionic contribution to dielectric screening is found to be negligible in this class of materials. On the basis of the direct band gap and the absorption coefficient, the estimated spectroscopic limited maximum efficiency is quite good when these materials are considered as promising environmentally friendly perovskites suitable for photovoltaics.