Orthorhombic lead-free hybrid perovskite CH3NH3SnI3 under strain: an ab initio study.

We report a computational study where we explore the possibility of tuning the electronic properties of orthorhombic methylammonium tin iodide CH3NH3SnI3 using strains. According to our findings, a moderate [001] strain, smaller than 2%, would open the band gap up to 1.25 eV and enhance the exciton binding energy, opening up new possibilities for the use of CH3NH3SnI3 in technological applications. To better understand the impact of strain, we also examined its influence on bonding properties. The results reveal that the directional pnictogen and the hydrogen bonding are not altered by strains and that the tuning of the electronic properties is the result of changes induced in the orbital contributions to states near the Fermi level and the tilting of the SnI6 octahedral units.

Tuning the electronic properties of asymmetric YZrCOF MXene for water splitting applications: an ab initio study.

Identifying and evaluating novel and extremely stable materials for catalysis is one of the major challenges that mankind faces today to rapidly reduce the dependence on fossil fuels. To contribute to achieving this goal, we have evaluated within the density-functional framework the properties of a new two-dimensional MXene structure, the asymmetric MXene YZrCOF monolayer. Phonon dispersion calculations at 0 K and 300 K indicate that the studied material is dynamically stable. The calculations also indicate that the material has a rigid crystal structure with a wide band gap, a strong potential difference, and a band-gap alignment that favors the production of both H2 and O2 molecules from water splitting. We also report the outcome of the strain effect on the electrical and photocatalytic characteristics of the studied material. We will demonstrate that even under a large strain, the YZrCOF monolayer is stable and useful for photocatalytic applications.

Theoretical study of electrocatalytic properties of low-dimensional freestanding PbTiO3 for hydrogen evolution reactions.

The discovery of novel materials for catalytic purposes that are highly stable is one of the main challenges nowadays for reducing our dependence on fossil fuels. Here, low-dimensional PbTiO3 is introduced as an electrocatalyst using first-principles calculations. Density-functional theory calculations indicate that 2D-PbTiO3 is dynamically and thermodynamically stable. Our results show that a single oxygen defect vacancy in 2D-PbTiO3 can play a key role in enhancing the hydrogen evolution reaction (HER), together with the Ti atoms. Our study concludes that the Volmer-Heyrovsky mechanism is a more favorable route to achieve HER than the Volmer-Tafel mechanism, including solvation and vacuum conditions.

Unveiling the structural, dynamical, elastic, and electronic properties of cuboid silver tetrathiotungstate by means ofab initiocalculations.

We present for the first time a theoretical study of the structural stability and physical properties of the newly synthesized Ag2WS4. The study contributes to a better understanding of its electronic and vibrational properties, which is fundamental for the optimization of the technological applications of Ag2WS4. Calculations have been carried out by means of density-functional theory. The obtained results support that Ag2WS4is thermodynamically, mechanically, and dynamically stable in a tetragonal layered structure, in good agreement with experiments. Calculations have also been used to obtain phonon frequencies, their assignments, and the Raman scattering spectrum. Furthermore, we show that Ag2WS4has a brittle structure, that is governed by van der Waals interactions, which favors its exfoliation as a low-dimensional structure. Additionally, the results show that Ag2WS4has a band gap of 2.02 eV with a favorable band-edge diagram for water splitting as well as for optoelectronic applications.

Understanding the thermodynamic, dynamic, bonding, and electrocatalytic properties of low-dimensional MgPSe3.

The study of novel two-dimensional structures for potential applications in photocatalysis or in optoelectronics is a challenging task. In this work, first-principles calculations have been carried out to explore the properties of the two-dimensional perovskite-based MgPSe3. Dynamic and mechanical analyses confirm the stability of this low-dimensional material. Our calculated Raman frequencies are in good agreement with previous studies. Furthermore, a topological bonding analysis, based on the electron localization function, indicates a covalent and ionic character for the P-Se and Mg-Se bonds, respectively. From a reactivity point of view, water interacts poorly with MgPSe3 and its associative interaction is physisorbed and governed by weak interactions. Consequently, the low dissociative energy of H2O molecules affects the reaction taking place on the surface of the material, making it unfavorable for both hydrogen and oxygen evolution reactions. However, the computed electronic properties show that MgPSe3 is a promising material for optoelectronic applications.

Effect of the sulfur termination on the properties of Hf2CO2 MXene.

A computational study was carried out to investigate the effect of surface termination on Janus Hf2COS MXene by substituting partly the O-terminated layer with S atoms. Our predictions confirm that this chemical strategy allows one to tailor the band gap of MXenes. Indeed, the semiconducting character of Hf2CO2 MXene decreases by the exchange of O by S atoms. From a structural point of view, dynamical, mechanical, and thermal analysis confirm the thermodynamic stability of the Janus Hf2COS MXene, which shows metallic character. Furthermore, topological chemical analysis indicates an ionic nature of Hf2CO2 MXene that tends to be reduced by increasing the concentration of S atoms, promoting a covalent character. Shortly, the present study illustrates how the properties of MXenes can be tailored by functionalizing them with different chemical terminations.

Theoretical calculations of the effect of nitrogen substitution on the structural, vibrational, and electronic properties of wolframite-type ScTaO4 at ambient conditions.

In this study, the effect of nitrogen substitution in wolframite-type ScTaO4 was investigated using density-functional theory calculations. First, structural and mechanical properties, as well as the dynamical stability of ScTaO4 were examined deeply for the ambient-pressure structure. Subsequently, we studied how lattice vibrations are affected by hydrostatic pressure and determined the elastic moduli of ScTaO4. The results of our study show that the monoclinic structure of ScTaO4 is rigid and non-compressible. In addition, band-structure calculations show that ScTaO4 has a wide direct band-gap of 4.04 eV, which in turn leads to a possible tuning of electronic properties. We have found that this task can be conducted by partially substituting oxygen atoms in the unit cell with nitrogen atoms. Both band-structure calculations and charge-density analyses revealed a narrowing in the band gap caused by the presence of nitrogen atoms, which act as a shallow acceptor state, resulting in weak repulsive interactions and structural distortions in both Sc and Ta coordination polyhedra; reducing the crystal symmetry from monoclinic to triclinic.

Low-dimensional HfS2 as SO2 adsorbent and gas sensor: effect of water and sulfur vacancies.

First-principles based on density functional theory (DFT) calculations were performed to investigate the interaction of two-dimensional (2D) HfS2 with SO2, a harmful gas with implications for climate change. In particular, we describe the effect of water and sulfur vacancies on such interaction. The former promotes the physisorption of SO2, whereas the latter promotes its chemisorption with structural changes on the absorbing surface. The results show that both structures are exothermic to adsorb the SO2 molecules, but the adsorption type is different. The reaction of the stable structure in the presence of water with the sulfur oxides is a physisorption interaction that enhances the band gap value of the isolated monolayer. However, for the defective structure, we have a chemisorption interaction type, where the adsorption of SO2 molecules widens the band gap values. To understand this behavior, we used Bader charge calculations and the noncovalent interactions index. While the water enhances the charge transfer between the monolayer and the adsorbed gas, the results show, however, that the defective structure is a more favorable gas sensor due to the metallic edge of the active site.

Enhancement of optoelectronic properties of layered MgIn 2 Se 4 compound under uniaxial strain, an ab initio study.

We argue that tuning the structure of a semiconductor offers abundant scope for use in a number of applications. In this work, by means of comprehensive density functional theory computations, we demonstrated that layered MgIn 2 Se 4 could be a promising candidate for future electronic and optoelectronic technologies. To do this task, we have applied a uniaxial strain in the z-direction. The results show that MgIn 2 Se 4 can support only a - 2.5 % of deformation without losing its dynamical stability. However, we showed that the effect of strain strongly affects the bonding pattern, which tends to increase the bandgap value. Both the charge density and noncovalent interactions were analyzed to understand this behavior. In addition, we saw that the application of non-hydrostatic pressure also enhanced the photocatalytic/optoelectronic performance of the investigated material, offering useful insights into layered MgIn 2 Se 4 for future development in this area.

Disclosing the behavior under hydrostatic pressure of rhombohedral MgIn2Se4 by means of first-principles calculations.

AM2X4 crystalline materials display important technological electronic, optical and magnetic properties that are sensitive to general stress effects. In this paper, the behavior under hydrostatic pressure of the ambient condition rhombohedral phase of MgIn2Se4 is investigated in detail for the first time. We carried out first-principles calculations within the density functional theory framework aimed at determining the pressure-induced polymorphic sequence of this selenide. To accurately evaluate transition pressures at room temperature, thermal corrections have been included after the computation of phonon dispersion curves in potential candidate phases, namely the initial rhombohedral R3[combining macron]m one, inverse and direct spinels, LiTiO2-type and defective I4[combining macron] structures. Only the transition from the R3[combining macron]m to the inverse spinel phase was found to fulfill the thermodynamic and mechanical stability criteria. The direct spinel could appear as metastable if kinetic effects hinder the above transition. Additionally, electronic band structures and chemical bonding properties were analyzed from the outcome of our quantum-mechanical solutions reporting band gap values and ionicity and noncovalent interaction indexes. It is shown that the investigated compound keeps behaving as a semiconductor, loses its van der Waals interactions, and becomes more covalent as hydrostatic pressure is applied.

Structural, Elastic, Electronic and Optical Properties of SrTMO3 (TM = Rh, Zr) Compounds: Insights from FP-LAPW Study.

The structural, mechanical, electronic and optical properties of SrTMO3 (TM = Rh, Zr) compounds are investigated by using first principle calculations based on density functional theory (DFT). The exchange-correlation potential was treated with the generalized gradient approximation (GGA) for the structural properties. Moreover, the modified Becke-Johnson (mBJ) approximation was also employed for the electronic properties. The calculated lattice constants are in good agreement with the available experimental and theoretical results. The elastic constants and their derived moduli reveal that SrRhO3 is ductile and SrZrO3 is brittle in nature. The band structure and the density of states calculations with mBJ-GGA predict a metallic nature for SrRhO3 and an insulating behavior for SrZrO3. The optical properties reveal that both SrRhO3 and SrZrO3 are suitable as wave reflectance compounds in the whole spectrum for SrRhO3 and in the far ultraviolet region (FUV) for SrZrO3.