Band structure optimization of superconducting photonic crystals based on transmission spectrum calculation.

The design of photonic crystals using novel materials is of great significance for the construction of high-performance, next-generation photonic crystal devices. We propose a universal Band structure-Transmission optimization-Band structure method based on moving asymptotic (MMA) method, which can be widely applied to photonic crystal structures. In this paper, we use the method to optimize the band structure of high temperature superconducting photonic crystal, and obtain a wider photonic bandgap and better band flatness in a specific frequency band. This method avoids the disadvantages of traditional scanning methods such as low efficiency and high resource consumption, allows multi-parameter optimization, and improves the accuracy and effectiveness of band modulation based on the iterative process of numerical calculation. The study provides some insights for the design of novel wide-bandgap optical devices.

A promising type-II ß-AsP/g-C6N6 van der Waals heterostructure photocatalyst for water splitting: a first-principles study.

Photocatalytic water splitting, which can use abundant and clean solar energy, is friendly to the environment and consumes less energy. This work predicts the type-II ß-AsP/g-C6N6 van der Waals (vdW) heterostructure as a promising photocatalyst for water splitting using first-principles calculations. Compared with the g-C6N6 monolayer, the ß-AsP/g-C6N6 heterostructure not only can effectively separate photogenerated electron and hole pairs but also shows considerable light absorption and better photocatalyst performances. The optical absorption coefficient of the ß-AsP/g-C6N6 heterostructure has also extremely increased, more than ten times that of the g-C6N6 monolayer. The ß-AsP/g-C6N6 heterostructure with little compression (-2%) could be conducive to the hydrogen evolution reaction with a Gibbs free energy closer to zero (-0.46 eV) compared to monolayers. The oxygen evolution reaction has a low theoretical overpotential of 0.63 V. These results indicate that it exhibits favorable performances as a photocatalyst. This work predicts the possible application of the ß-AsP/g-C6N6 heterostructure as a photocatalyst and expands the scope of renewable energy devices.

Quasiparticle energies and significant exciton effects of monolayered blue arsenic phosphorus conformers.

Low-dimensional systems have strong multi-body interactions and fewer geometric constraints due to the screening effect of the Coulomb interaction. We use the single-shot GW-Bethe Salpeter equation (G0W0-BSE) to calculate the electronic and optical properties of six-blue arsenic phosphorus (ß-AsP) conformers. The results show significant anisotropic exciton effects of covering visible regions, which apparently changed the light absorption. The maximum exciton binding energy is up to 0.99 eV, which is more extensive than the black phosphorus monolayer (0.9 eV). We predict that the different orbital contributions to valence bands may cause the anisotropic exciton effect difference. Our results indicate that ß-AsP monolayers with the large binding energies of exciton hold a great promise for applications in optoelectronic devices.

Photocatalysis screening for group III-V vdW heterostructures constructed by MX (M = Ga, In and X = P, As) monolayers.

To harvest solar energy for water splitting and produce pollution-free hydrogen and oxygen, high-performance photocatalysts are essential. Here, by combining different two-dimensional (2D) group III-V MX (M = Ga, In and X = P, As) monolayers, we designed 144 van der Waals (vdW) heterostructures to identify efficient photoelectrochemical materials. Using first-principles calculations, we investigated the stabilities, electronic properties, and optical properties of these heterostructures. After a careful screening process, we elected GaP/InP in a BB-II stacking configuration as the most promising candidate. This specific GaP/InP configuration has a type-II band alignment with a gap value of 1.83 eV. The conduction band minimum (CBM) is located at -4.276 eV, and the valence band maximum (VBM) is located at -6.217 eV, fully satisfying the requirements of the catalytic reaction under pH = 0. Additionally, light absorption has been improved through the construction of the vdW heterostructure. These results could help in understanding the properties of the III-V heterostructures and guide the experimental synthesis of these materials for photocatalysis applications.

First-principles calculation of the optical properties of the YBa2Cu3O7-δ oxygen vacancies model.

We used first-principles methods to investigate how oxygen vacancy defects affect the optical properties of YBa2Cu3O7-δ (0 < δ < 1), a high-temperature superconductor with potential applications in optical detectors. We calculated the electronic structure of YBa2Cu3O7-δ with different amounts of oxygen vacancies at three different sites: Cu-O chains, CuO2 planes, and apical oxygens. The formation energy calculations support the formation of oxygen vacancies in the Cu-O chain at higher concentrations of vacancy defects, with a preference for alignment in the same chain. The presence of oxygen vacancies affects the optical absorption peak of YBa2Cu3O7-δ in different ways depending on their location and concentration. The optical absorption peaks in the visible range (1.6-3.2 eV) decrease in intensity and shift towards the infrared spectrum as oxygen vacancies increase. We demonstrate that oxygen vacancies can be used as a powerful tool to manipulate the optical response of YBa2Cu3O7-δ to different wavelengths in optical detector devices.

The Tunable Electronic and Optical Properties of Two-Dimensional Bismuth Oxyhalides.

Two-dimensional (2D) bismuth oxyhalides (BiOX) have attracted much attention as potential optoelectronic materials. To explore their application diversity, we herewith systematically investigate the tunable properties of 2D BiOX using first-principles calculations. Their electronic and optical properties can be modulated by changing the number of monolayers, applying strain, and/or varying the halogen composition. The band gap shrinks monotonically and approaches the bulk value, the optical absorption coefficient increases, and the absorption spectrum redshifts as the layer number of 2D BiOX increases. The carrier transport property can be improved by applying tensile strain, and the ability of photocatalytic hydrogen evolution can be obtained by applying compressive strain. General strain engineering will be effective in linearly tuning the band gap of BiOX in a wide strain range. Strain, together with halogen composition variation, can tune the optical absorption spectrum to be on demand in the range from visible to ultraviolet. This suggests that 2D BiOX materials can potentially serve as tunable novel photodetectors, can be used to improve clean energy techniques, and have potential in the field of flexible optoelectronics.

Computational Investigation of Two-Dimensional Vanadium Boride Compounds for Na-Ion Batteries.

Sodium (Na)-ion batteries have received widespread attention due to their low cost and good safety. The possibility of two-dimensional vanadium boride (V2B2, MBene) as the anode material for Na-ion batteries is explored by first principles. It is found that V2B2 has good dynamic stability, thermodynamic stability, and conductivity. V2B2 has a good performance as anode material: it can adsorb nearly 3 layers of Na ions, and the maximum capacity reaches 814 mAhg-1. It is found that V2B2 has a very low Na ion diffusion barrier, about 0.011 eV, which represents the ultrahigh ion diffusion rate of Na ions on the surface of V2B2. The average open circuit voltage of V2B2 is 0.65 V, and good metallicity is maintained during the entire Na ion adsorption process. The results indicate that two-dimensional V2B2 has a low diffusion barrier, low open circuit voltage, and high theoretical capacity and is a potential anode material for Na-ion batteries.

Defect induced electrocatalytic hydrogen properties of pentagonal PdX2 (X = S, Se).

Searching for catalysts of hydrogen evolution reaction (HER) that can replace Pt is critical. Here, we investigated the HER electrocatalytic activity of pentagonal PdS2 (penta-PdS2) and PdSe2 (penta-PdSe2) by first-principles calculations. Three types of vacancies (VS/Se, VPd, DVS/Se) were constructed to activate the inert basal planes of PdS2 and PdSe2. The results show that S/Se and Pd vacancies significantly improve HER performance, and the Gibbs free energy (ΔG H) of systems can be further regulated by vacancy concentration. Particularly, PdS2 with 2.78% VS, 50% VPd and PdSe2 with 12.5% VSe display the optimal ΔG H value and the highest exchange current density. Further analysis of charge transfer and band structures were described that the introduce of vacancies efficiently regulates the electronic properties, resulting in the diminution of bandgap, and accelerates the charge transfer, thereby contributing to an enhanced electron environment for HER process. Our results provide a theoretical guidance for the applications of pentagonal transition-metal dichalcogenides as catalysts of hydrogen evolution reaction.

First-principles study of dehydration process of potassium dihydrogen phosphate crystal.

KDP crystal is showing a good property in high-power laser systems. However, working in a high-power environment is easy to have damaged-defect. Dehydration of KDP crystal is one of the damage phenomena. We explore the total energy and physical properties of the KDP crystal progressive dehydration by using First-principles calculations. It is found that the band gap of the KDP crystal gradually decreases with the deepening of dehydration, and there are many obvious defect states between 4 eV and 8 eV (the corresponding wavelength region is from 310 nm to 155 nm). It indicates that dehydration causes a reduction in the damage threshold of the KDP crystal. Our results indicate that these defect states are due to the change of hybridization type of P atoms, which is gradually transformed from original sp 3 hybridization to sp 2 hybridization in the dehydration process. An obvious redshift can be observed in the absorption spectrum, producing many distinct absorption peaks. All of the results can provide the good basis for deeply understanding the electronic and optical properties of the KDP crystal.