Pressure-Driven Band Gap Narrowing in Rb₂AgPdCl₅: Toward the Shockley–Queisser Limit of Lead-free Double Perovskites

Hydrostatic pressure is an effective tool that can give rise to novel crystal structures and physical properties. This study presents the structural, electronic, and optical properties of electronically one-dimensional (1D) double perovskite Rb2AgPdCl5 (A2BB′X5) under hydrostatic pressure. At ambient pressure, Rb2AgPdCl5 shows a band gap of 2.20 eV (0.65 eV) at the HSE06 + SOC (PBE) level of theory, and effective carrier masses are 0.44 and 0.64 mo (where mo is the rest mass of an electron) for electrons and holes, respectively. Upon applying the hydrostatic pressure, we observe band gap narrowing, accompanied by piezochromism, and a reduction in effective carrier masses. At a relatively low pressure of 9 GPa, Rb2AgPdCl5 achieves the optimum band gap of 1.36 eV, which is close to the optimal value of the Shockley–Queisser limit. The band gap reduction is attributed to the contraction of the metal-halide bond length and the increase in the overlap of atomic orbitals. The decrease in effective carrier masses is attributed to the increase in the width of conduction and valence bands, indicating improved transport of carriers with external pressure. This work elucidates the effects of hydrostatic pressure on the sensitive tuning of the electronic and optical properties of this perovskite family for vivid optoelectronic applications

Cationic Effect on Pressure Driven Spin-State Transition and Cooperativity in Hybrid Perovskites

Hybrid or metal–organic framework (MOF) perovskites of general composition, ABX₃, are known to show interesting properties that can lead to a variety of technological applications. Our first-principles study shows they are also potential candidates for exhibiting cooperative spin-state transitions upon application of external stimuli. We demonstrate this by considering two specific Fe-based MOF perovskites, namely dimethylammonium iron formate, [CH₃NH₂CH₃]­[Fe­(HCOO)₃], and hydroxylammonium iron formate, [NH₃OH]­[Fe­(HCOO)₃]. Both the compounds are found to undergo high-spin (<i>S</i> = 2) to low-spin (<i>S</i> = 0) transition at Fe­(II) site upon application of moderate strength of hydrostatic pressure, along with large hysteresis. This spin-state transition is signaled by the changes in electronic, magnetic, and optical properties. We find both the transition pressure and the width of the hysteresis to be strongly dependent on the choice of A-site cation, dimethylammonium or hydroxylammonium, implying that tuning of spin-switching properties is achievable by chemical variation of the amine cation in the structure. Our findings open up novel functionalities in this family of materials of recent interest, which can have important usage in sensors and memory devices

Cationic Effect on Pressure Driven Spin-State Transition and Cooperativity in Hybrid Perovskites

Hybrid or metal–organic framework (MOF) perovskites of general composition, ABX₃, are known to show interesting properties that can lead to a variety of technological applications. Our first-principles study shows they are also potential candidates for exhibiting cooperative spin-state transitions upon application of external stimuli. We demonstrate this by considering two specific Fe-based MOF perovskites, namely dimethylammonium iron formate, [CH₃NH₂CH₃]­[Fe­(HCOO)₃], and hydroxylammonium iron formate, [NH₃OH]­[Fe­(HCOO)₃]. Both the compounds are found to undergo high-spin (<i>S</i> = 2) to low-spin (<i>S</i> = 0) transition at Fe­(II) site upon application of moderate strength of hydrostatic pressure, along with large hysteresis. This spin-state transition is signaled by the changes in electronic, magnetic, and optical properties. We find both the transition pressure and the width of the hysteresis to be strongly dependent on the choice of A-site cation, dimethylammonium or hydroxylammonium, implying that tuning of spin-switching properties is achievable by chemical variation of the amine cation in the structure. Our findings open up novel functionalities in this family of materials of recent interest, which can have important usage in sensors and memory devices

Scrupulous Probing of Bifunctional Catalytic Activity of Borophene Monolayer: Mapping Reaction Coordinate with Charge Transfer

We have envisaged the hydrogen evolution and oxygen evolution reactions (HER and OER) on two-dimensional (2D) noble metal free borophene monolayer based on first-principles electronic structure calculations. We have investigated the effect of Ti functionalization on borophene monolayer from the perspective of HER and OER activities enhancement. We have probed the activities based on the reaction coordinate, which is conceptually related to the adsorption free energies of the intermediates of HER and OER, as well as from the vibrational frequency analysis with the corresponding charge transfer mechanism between the surface and the adsorbate. Ti-functionalized borophene has emerged as a promising material for HER and OER mechanisms. We believe that our probing method, based on reaction coordinate coupled with vibrational analysis that has been validated by the charge transfer mechanism, would certainly become as a robust prediction route for HER and OER mechanisms in coming days

Maneuvering the Physical Properties and Spin States To Enhance the Activity of La–Sr–Co–Fe–O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation

Perovskite oxides have attracted considerable attention as durable electrocatalysts for metal–air batteries and fuel cells due to their precedence in oxygen electrocatalysis in spite of the complexities involved with their crystal structure, spin states, and physical properties. Here we report optimization of the activity of a model perovskite system La_1–<i>x</i>Sr_<i>x</i>Co_1–<i>y</i>Fe_<i>y</i>O_3−δ (LSCF; <i>x</i> = 0.301, <i>y</i> = 0.298, and δ = 0.05–0.11) toward electrochemical water oxidation (OER) by altering the calcination temperature of the nonaqueous sol–gel synthesized nanoparticles (NPs). Our results show that improved OER activity is the result of a synergism between its morphology, surface area, electrical conductivity, and spin state of the active transition metal site. With an e_g orbital occupancy of 1.26, the interconnected ∼90 nm LSCF NPs prepared at 975 °C (LSCF-975) outperforms the other distinguishable LSCF morphologies, requiring 440 mV overpotential to achieve 10 mA/cm², a performance comparable to the best-performing perovskite oxide electrocatalysts. While the interconnected NP morphology increases the propensity of electronic conduction across crystalline grain boundaries, the morphology-tuned high spin Co³⁺ ions increases the probability of binding reaction intermediates at the available surface sites. Density functional theory based work function modeling further demonstrates that LSCF-975 is the most favorable OER catalyst among others in terms of a moderate work function and Fermi energy level facilitating the adsorption and desorption of reaction intermediates

Mapping Structural Changes in Electrode Materials: Application of the Hybrid Eigenvector-Following Density Functional Theory (DFT) Method to Layered Li_0.5MnO₂

The migration mechanism associated with the initial layered-to-spinel transformation of partially delithiated layered LiMnO₂ was studied using hybrid eigenvector-following coupled with density functional theory. The initial part of the transformation mechanism of Li_0.5MnO₂ involves the migration of Li into both octahedral and tetrahedral local minima within the layered structure. The next stage of the transformation process involves the migration of Mn and was found to occur through several local minima, including an intermediate square pyramidal MnO₅ configuration and an independent Mn³⁺ to Mn²⁺ charge-transfer process. The migration pathways were found to be significantly affected by the size of the supercell used and the inclusion of a Hubbard U parameter in the DFT functional. The transition state searching methodology described should be useful for studying the structural rearrangements that can occur in electrode materials during battery cycling, and more generally, ionic and electronic transport phenomena in a wide range of energy materials

Highly Sensitive and Selective Gas Detection Based on Silicene

Recent advances in the fabrication of silicene devices have raised exciting prospects for practical applications such as gas sensing. We investigated the gas detection performance of silicene nanosensors for four different gases (NO, NO₂, NH₃, and CO) in terms of sensitivity and selectivity, employing density functional theory and nonequilibrium Green’s function method. The structural configurations, adsorption sites, binding energies and charge transfer of all studied gas molecules on silicene nanosensors are systematically discussed in this work. Our results indicate that pristine silicene exhibits strong sensitivity for NO and NO₂, while it appears incapable of sensing CO and NH₃. In an attempt to overcome sensitivity limitations due to weak van der Waals interaction of those latter gas molecules on the device, we doped pristine silicene with either B or N atoms, leading to enhanced binding energy as well as charge transfer, and subsequently a significant improvement of sensitivity. A distinction between the four studied gases based on the silicene devices appears possible, and thus these promise to be next-generation nanosensors for highly sensitive and selective gas detection

Poor Photovoltaic Performance of Cs₃Bi₂I₉: An Insight through First-Principles Calculations

Bismuth-based halide perovskite derivatives have now attracted huge attention for photovoltaic (PV) applications after the unparalleled success of lead-based halide perovskites. However, the performances of PV devices based on these compounds are poor, despite theoretical predictions. In this Article, we have investigated the electronic structure and defect formation energies of Cs₃Bi₂I₉ using density functional theory (DFT) calculations. The calculated electronic bandstructure indicates an indirect bandgap and high carrier effective masses. Our calculations reveal a large stability region for this compound; however, deep level defects are quite prominent. Even the varying chemical potentials from the stoichiometric region do not eliminate the presence of deep defects, ultimately limiting photovoltaic efficiencies

Differential dynamics of the serotonin_1A receptor in membrane bilayers of varying cholesterol content revealed by all atom molecular dynamics simulation

<p>The serotonin_1A receptor belongs to the superfamily of G protein-coupled receptors (GPCRs) and is a potential drug target in neuropsychiatric disorders. The receptor has been shown to require membrane cholesterol for its organization, dynamics and function. Although recent work suggests a close interaction of cholesterol with the receptor, the structural integrity of the serotonin_1A receptor in the presence of cholesterol has not been explored. In this work, we have carried out all atom molecular dynamics simulations, totaling to 3 μs, to analyze the effect of cholesterol on the structure and dynamics of the serotonin_1A receptor. Our results show that the presence of physiologically relevant concentration of membrane cholesterol alters conformational dynamics of the serotonin_1A receptor and, on an average lowers conformational fluctuations. Our results show that, in general, transmembrane helix VII is most affected by the absence of membrane cholesterol. These results are in overall agreement with experimental data showing enhancement of GPCR stability in the presence of membrane cholesterol. Our results constitute a molecular level understanding of GPCR-cholesterol interaction, and represent an important step in our overall understanding of GPCR function in health and disease.</p

Microwave-Assisted Modified Polyimide Synthesis: A Facile Route to the Enhancement of Visible-Light-Induced Photocatalytic Performance for Dye Degradation

Visible-light-active π-conjugated polymer photocatalysts can effectively harness solar energy, thereby offering pragmatic solutions to eclectic environmental issues. In the present study, a series of ingenious visible-light-responsive, stable, and recyclable modified polyimide (SWO₃/PI) photocatalysts was synthesized via a facile microwave-assisted rapid thermal polymerization strategy. The precursors employed were pyromellitic dianhydride, melamine, thiourea, and tungsten trioxide co-catalyst. The template-free inclusion of sulfur and tungsten oxide species into the PI conformation increased visible-light absorption and enhanced the separation efficiency of the photogenerated electron–hole pairs. The visible-light-induced reactive red 120 (RR 120) photodegradation efficiency exhibited by the SWO₃/PI photocatalyst was over 98% and was approximately 2.3 times higher than that exhibited by pristine PI. Herein h⁺ and OH^• were the principal active species involved in dye degradation. Interestingly, the sizable valence band edge downshift from 2.02 to 3.36 eV induced a remarkable enhancement in the photooxidation ability of the photoinduced holes, despite the fact that the relatively inappropriate position conduction band edge position (1.77 eV) did not favor the participation of photoinduced electrons in the reduction process. The liquid chromatography–mass spectrometry results revealed that photocatalytic degradation of RR 120 had been effectively accomplished