Electronic structure of random Al0.5Ga0.5As alloys: Test of the "special-quasirandom-structures" description

Unusual Band Gap Bowing in CsCdxPb(1-x)Br3 Alloys Due to a Chemical Mismatch

In this work, the band structure of CsCdxPb1-xBr3 alloys is investigated through first-principles calculations. An unusual upward band gap bowing is revealed, which is consistent with the experimental observations of the blue-shifted gap in Cd doped CsPbBr3. The gap bowing is found to be mainly contributed to by the conduction band minimum. Based on symmetry analysis, it is demonstrated that, at the Pb-rich or Cd-rich regime, the hybridization between the Pb(6p)-driven and Cd(5s)-driven conduction bands is strongly suppressed due to their different symmetries. Such a chemical mismatch leads to an almost independent evolution of the Pb(6p) and Cd(5s) bands. Then, a model of band shrinking and broadening is proposed to explain upward gap bowing. The results highlight the critical role of symmetry in determining the electronic properties of alloys consisting of materials with distinct band edge characters.

Rational design of bimetallic MXene solid solution with High-Performance electrocatalytic N2 reduction

Electrocatalytic N₂ reduction reaction (eNRR) was an effective alternative method for green synthesis of NH₃. By combining the first-principal Density functional theory (DFT) calculations and Monte Carlo (MC) simulation, we systematacially investigated 24 types equal-ratio bimetallic MXene solid solution, involving 88 different catalysts. Our focus was on the catalytic performance of these materials in eNRR. The computational result indicate that MoW(3Mo) has high stability, selectivity (93.8 % against the hydrogen evolution reaction (HER)) and activity (U_L = -0.26 V), which is significantly better than that of monometal Mo₂CO₂ and W₂CO₂. This improvement in catalytic properties is attributed to the unique electronic structure (e.g. d-band center, charge) of bimetallic MXene solid solution. In explicit solvent conditions, the microenvironment of hydrogen bond in aqueous liquid thermodynamically promotes the catalytic property for eNRR and reduce the catalytic property of HER side reaction, but the kinetic barrier is also increased due to the effect of the hydrogen-bond microenvironment on proton migration. Overall, the obtained bimetallic MXene solid solution MoW(3Mo) exhibits excellent catalytic performance in eNRR.

Multi-component (Ag-Au-Cu-Pd-Pt) alloy nanoparticle-decorated p-type 2D-molybdenum disulfide (MoS2) for enhanced hydrogen sensing

Molybdenum disulfide (MoS2) has emerged as a promising material for the development of efficient sensors. Here, we have exfoliated and decorated MoS2 flakes with the novel, single-phase multi-component silver-gold-copper-palladium-platinum (Ag-Au-Cu-Pd-Pt) alloy nanoparticles, popularly named High Entropy Alloy (HEA) nanoparticles, using facile and scalable low-temperature grinding, followed by the sonochemical method. It was found that the decoration of HEA nanoparticles imparts the surface-enhanced Raman scattering effect and reduction in the work function of the material from 4.9 to 4.75 eV as measured by UV photoelectron spectroscopy. This change in the work function resulted in a Schottky barrier between the gold contact and HEA decorated MoS2 flakes as a result of drastic changes in the surface chemical non-stoichiometry. The response to hydrogen gas was studied at temperatures in the range of 30 to 100 °C, and it showed an unusual p-type nature due to surface-adsorbed oxygen species. The nanoscale junction formed between HEA and MoS2 showed a ten-time increase in the response towards hydrogen gas at 80 °C. The experimental observations have been explained with DFT simulation showing more favourable hydrogen adsorption on HEA-decorated MoS2 resulting in an enhanced response.

Optimizing special quasirandom structure (SQS) models for accurate functional property prediction in disordered 2D alloys

2D materials such as MXenes have garnered attention in a wide field of applications ranging from energy to environment to medical. Properties of 2D materials can be tailored via alloying and in some cases, solid-solutions (disordered alloys) are formed. To predict the disordered alloy properties via first-principles, the model structure needs to imitate the random arrangements of alloyants and yet remains computationally tractable. Using density functional theory and the cluster expansion method, we investigate the accuracy of using of special quasirandom structures (SQSs) for predicting disordered 2D alloy properties, evaluating the effect of SQS supercell size on the prediction quality of formation energies, elastic properties, and structural parameters. We illustrate the findings with 5 different disordered binary [Formula: see text] MXene alloy systems (where M  =  Ti and M'  =  Zr, Hf, V, Nb, or Ta), demonstrating that SQSs around 6-8 times the primitive cell (N  =  6-8) are sufficient to attain convergence in the property predictions versus supercell size. For formation energies, SQSs with N  >  4 are found to reproduce the formation energies of the fully disordered phase within ~2.5 meV. For the simulation of the experimentally-synthesized TiNbCO₂, we find convergence in structural parameters and elastic tensors at N ~ 6. We traced the convergence of the predictions to the convergence in the band structure-related properties via analysis of the electronic densities-of-states and the projected crystal overlap Hamilton population. Our findings suggest that modest sized SQSs would reproduce the properties of disordered MXene alloys. The results should help guide the investigations of structure-property relationships in other disordered 2D materials as well.

Special quasirandom structures for perovskite solid solutions

Special quasirandom structures (SQS) are presently generated for disordered (A'1-x [Formula: see text] x )BX3 and A(B'1-x [Formula: see text] x )X3 perovskite solid solutions, with x  =  1/2 as well as 1/3 and 2/3. These SQS configurations are obtained by imposing that the so-called Cowley parameters are as close to zero as possible for the three nearest neighboring shells. Moreover, these SQS configurations are slightly larger in size than those available in the literature for x  =  1/2, mostly because of the current capabilities of atomistic techniques. They are used here within effective Hamiltonian schemes to predict various properties, which are then compared to those associated with large random supercells, in a variety of compounds, namely (Ba1-x Sr x )TiO3, Pb(Zr1-x Ti x )O3, Pb(Sc0.5Nb0.5)O3, Ba(Zr1-x Ti x )O3, Pb(Mg1/3Nb2/3)O3 and (Bi1-x Nd x )FeO3. It is found that these SQS configurations can reproduce many properties of large random supercells of most of these disordered perovskite alloys, below some finite material-dependent temperature. Examples of these properties are electrical polarization, anti-phase and in-phase octahedral tiltings, antipolar motions, antiferromagnetism, strain, piezoelectric coefficients, dielectric response, specific heat and even the formation of polar nanoregions (PNRs) in some relaxors. Some limitations of these SQS configurations are also pointed out and explained.

First-Principles Investigation of Laves Phases in Mg-Al-Ca System

Recent experiments and first-principles calculations in the literature revealed the existence of a C36 laves phase in the Al2Ca-Mg2Ca pseudo-binary system in addition to the C14-Mg2Ca and C15-Al2Ca laves phases. In the present work, special quasirandom structures (SQS) for all three laves phases were constructed. The structures possess local pair and multisite correlation functions that mimic those of the corresponding random structures. First-principles calculations were carried out based on the SQS developed to predict the enthalpy of formation in the Al2Ca-Mg2Ca pseudo-binary system. It was observed that the enthalpy of formation of C36 is very close to that of C14 at the Mg2Ca end and decreases with the addition of small amount of Al, while the enthalpy of formation of C14 increases with the addition of Al. It is thus energetically plausible that C36 is stable in the Al2C- Mg2Ca pseudo-binary system.