We present an equation of state (EOS) that describes how the hydrostatic change in surface area is related to two-dimensional in-plane pressure (F) and yields the measure of a material's resilience to isotropic stretching (the layer modulus $\gamma${}) as one of its fit parameters. We give results for the monolayer systems of graphene and boronitrene, and we also include results for Si, Ge, GeC, and SiC in the isostructural honeycomb structure for comparison. Our results show that, of the honeycomb structures, graphene is the most resilient to stretching with a value of $\gamma${}C~=~206.6 N~m$-${}1, second is boronitrene with $\gamma${}BN~=~177.0 N~m$-${}1, followed by $\gamma${}SiC~=~116.5 N~m$-${}1, $\gamma${}GeC~=~101.0 N~m$-${}1, $\gamma${}Si~=~44.5 N~m$-${}1, and $\gamma${}Ge~=~29.6 N~m$-${}1. We calculate the Young's and shear moduli from the elastic constants and find that, in general, they rank according to the layer modulus. We also find that the calculated layer modulus matches the one obtained from the EOS. We use the EOS to predict the isotropic intrinsic strength of the various systems and find that, in general, the intrinsic stresses also rank according to the layer modulus. Graphene and boronitrene have comparable strengths with intrinsic stresses of 29.4 and 26.0 N~m$-${}1, respectively. We considered four graphene allotropes including pentaheptite and graphdiyne and find that pentaheptite has a value for $\gamma${} comparable to graphene. We find a phase transition from graphene to graphdiyne at F~=~$-${}7.0 N~m$-${}1. We also consider bilayer, trilayer, and four-layered graphene and find that the addition of extra layers results in a linear dependence of $\gamma${} with F.

Metal halide perovskite materials have shown significant advancements in their application as light absorbers in perovskite solar cells, with power conversion efficiencies reaching 27%. However, lead-based perovskites pose a concern due to their toxicity and stability issues in moisture, UV radiation, and heat. This has led to a pressing need to explore substitute materials that do not contain lead but maintain the remarkable characteristics of lead-based perovskites. This review article focuses on halide double perovskites characterised by the A₂B’B”X₆ composition, highlighting their structural, optical, thermoelectric, and mechanical capabilities. Additionally, the review evaluates several materials databases to investigate materials suitable for high-throughput first-principles calculations integrated inside density functional theory. The review aims to identify novel perovskite materials, offer a thorough evaluation of the potential benefits and drawbacks associated with this class of materials, and, from the pedagogical standpoint, discover effective instructional frameworks.

We present a comparative density functional study of the adsorption of hydrogen on bilayer graphene. Six different exchange-correlation functionals are employed to explore the possible configurations of hydrogen adsorption at 50% coverage. Using the four variants of the nonlocal van der Waals density functional, we identify three distinct competing configurations that retain the coupled bilayer structure at 0 K. One of the configurations undergoes a spontaneous transformation from hexagonal to tetrahedral structure, under hydrogenation, with heat of formation ranging between $-${}0.03 eV (vdW-DF) and $-${}0.37 eV (vdW-DFC09x). This configuration has a finite band gap of around 3 eV, whereas all other competing configurations are either semimetallic or metallic. We also find two unique low-energy competing configurations of decoupled bilayer graphene, and therefore suggest the possibility of graphene exfoliation by hydrogen intercalation.

First principles calculations have been performed to investigate the ground state properties of freestanding monolayer hexagonal boronitrene (h-BN). We have considered monolayers that contain native point defects and their complexes, which form when the point defects bind with the boron vacancy on the nearest-neighbor position. The changes in the electronic structure are analyzed to show the extent of localization of the defect-induced midgap states. The variations in formation energies suggest that defective h-BN monolayers that contain carbon substitutional impurities are the most stable structures, irrespective of the changes in growth conditions. The high energies of formation of the boron vacancy complexes suggest that they are less stable, and their creation by ion bombardment would require high-energy ions compared to point defects. Using the relative positions of the derived midgap levels for the double vacancy complex, it is shown that the quasi-donor\char21{}acceptor pair interpretation of optical transitions is consistent with stimulated transitions between electron and hole states in boronitrene.

This paper presents a comparative first principles study of van der Waals heterobilayers derived from the coupling of graphene to silicon carbide and hexagonal boron nitride monolayer. Using the local, semi-local, and van der Waals interaction-corrected density functional theory, it found that the adhesion energy of graphene on SiC and h-BN monolayer is invariant under dispersion corrections. On the other hand, considerably more accurate interlayer distances are obtained using the semi-empirical DFT-D3 correction, whereas non-local corrections consistently yield higher adhesion energies of 3.70 and 2.84 mRyd per carbon for graphene on SiC and h-BN monolayer substrates. It is also observed that the anisotropy induced band gap depends on the evolution of the stacking sequence of heterobilayers under persistent strain. It is suggested that the delicate minimisation of the overlap interactions between interlayer π-bonds induces localised charge puddles in the interfacial electronic structure and opens up a band gap. Using the dependence of the induced band gap on network anisotropy, it is shown that the size of the band gap is tunable by controlling the stacking sequence and the strength of the dipole-induced electric field within the heterobilayer interface