First principle study of ATiO[Formula: see text] (A=Ti,Sr) materials for photovoltaic applications

CONTEXT

The study investigates the impact of Hubbard U correction and spin-orbit coupling (SOC) on the structural, mechanical, electronic, and optical properties of Ti[Formula: see text]O[Formula: see text] and SrTiO[Formula: see text] compounds. The research is motivated by the potential applications of these materials in photovoltaics, with a focus on understanding their properties for such use. The ductility, ionicity, and mechanical stability of both compounds at zero pressure are assessed, indicating their potential as resilient materials. Also, the compounds display high refractive indices and absorption coefficients, indicating their suitability for solar harvesting applications. The predicted bandgaps align primarily with the UV-Vis areas of the electromagnetic spectrum, highlighting their potential in this domain.

METHODS

Computational techniques employed in this study are density functional theory (DFT) with and without spin-orbit coupling, as well as DFT+U methods, implemented using the Quantum ESPRESSO (QE) package. The study adopts the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, while employing a plane-wave basis set with an energy cutoff of 50 Ry for wavefunctions and 500 Ry for charge density.

Band gap engineering of a MoS2 monolayer through oxygen alloying: an ab initio study

Oxygen (O) alloying in a MoS₂ monolayer appearing in different shapes: line-ordered, cluster and random have been theoretically designed, for band gap engineering in order to extend its nanotechnological applications. The thermodynamic stability, structural and electronic properties of these alloy configurations at each concentration have been comparatively studied using the density functional theory methods. Based on the formation energy analysis, the O line-ordered alloys are most stable compared to the well known random and cluster alloys at high concentration, while at low concentration they compete. The lattice constants of all the alloyed systems decrease linearly with the increase in O concentration, consistent with Vegard's law. The Mo-O bond lengths are shorter than Mo-S leading to a reduction in the band gap, based on density of state analysis. The partial charge density reconciling with the partial density of states analysis reveals that the band gap reduction is mainly contributed by the Mo 4d and O 2p orbitals as shown at the band edges of the density of states plots. Creation of stacking of MoS₂ with MoO₂ gives metallic character, with Mo 4d orbital crossing the Fermi level. The O alloys in a MoS₂ monolayer should be considered to be an effective way to engineer the band gap for designing new nanoelectronic devices with novel performance.

Electrically active induced energy levels and metastability of B and N vacancy-complexes in 4H-SiC

Electrically active induced energy levels in semiconductor devices could be beneficial to the discovery of an enhanced p or n-type semiconductor. Nitrogen (N) implanted into 4H-SiC is a high energy process that produced high defect concentrations which could be removed during dopant activation annealing. On the other hand, boron (B) substituted for silicon in SiC causes a reduction in the number of defects. This scenario leads to a decrease in the dielectric properties and induced deep donor and shallow acceptor levels. Complexes formed by the N, such as the nitrogen-vacancy centre, have been reported to play a significant role in the application of quantum bits. In this paper, results of charge states thermodynamic transition level of the N and B vacancy-complexes in 4H-SiC are presented. We explore complexes where substitutional N[Formula: see text]/N[Formula: see text] or B[Formula: see text]/B[Formula: see text] sits near a Si (V[Formula: see text]) or C (V[Formula: see text]) vacancy to form vacancy-complexes (N[Formula: see text]V[Formula: see text], N[Formula: see text]V[Formula: see text], N[Formula: see text]V[Formula: see text], N[Formula: see text]V[Formula: see text], B[Formula: see text]V[Formula: see text], B[Formula: see text]V[Formula: see text], B[Formula: see text]V[Formula: see text] and B[Formula: see text]V[Formula: see text]). The energies of formation of the N related vacancy-complexes showed the N[Formula: see text]V[Formula: see text] to be energetically stable close to the valence band maximum in its double positive charge state. The N[Formula: see text]V[Formula: see text] is more energetically stable in the double negative charge state close to the conduction band minimum. The N[Formula: see text]V[Formula: see text] on the other hand, induced double donor level and the N[Formula: see text]V[Formula: see text] induced a double acceptor level. For B related complexes, the B[Formula: see text]V[Formula: see text] and B[Formula: see text]V[Formula: see text] were energetically stable in their single positive charge state close to the valence band maximum. As the Fermi energy is varied across the band gap, the neutral and single negative charge states of the B[Formula: see text]V[Formula: see text] become more stable at different energy levels. B and N related complexes exhibited charge state controlled metastability behaviour.

Li states on a C–H vacancy in graphane: a first-principles study

The Li ion enhances the V_CH induced magnetism. The −1 charge doping shifts the Fermi level to the CBM further increasing magnetism. The +1 charge doping shifts the Fermi level to the VBM reducing magnetism.

Defect charge states in Si doped hexagonal boron-nitride monolayer

We perform ab initio density functional theory calculations to investigate the energetics, electronic and magnetic properties of isolated stoichiometric and non-stoichiometric substitutional Si complexes in a hexagonal boron-nitride monolayer. The Si impurity atoms substituting the boron atom sites SiB giving non-stoichiometric complexes are found to be the most energetically favourable, and are half-metallic and order ferromagnetically in the neutral charge state. We find that the magnetic moments and magnetization energies increase monotonically when Si defects form a cluster. Partial density of states and standard Mulliken population analysis indicate that the half-metallic character and magnetic moments mainly arise from the Si 3p impurity states. The stoichiometric Si complexes are energetically unfavorable and non-magnetic. When charging the energetically favourable non-stoichiometric Si complexes, we find that the formation energies strongly depend on the impurity charge states and Fermi level position. We also find that the magnetic moments and orderings are tunable by charge state modulation q  =  -2, -1, 0, +1, +2. The induced half-metallic character is lost (retained) when charging isolated (clustered) Si defect(s). This underlines the potential of a Si doped hexagonal boron-nitride monolayer for novel spin-based applications.

Exploring the stability and electronic structure of beryllium and sulphur co-doped graphene: a first principles study

First principles density functional theory calculations have been performed to explore the stability, structural and electronic properties of Be and S co-doped graphene sheets.

Mechanical properties of hydrogenated bilayer graphene

Using first principle methods, we study the mechanical properties of monolayer and bilayer graphene with 50% and 100% coverage of hydrogen. We employ the vdW-DF, vdW-DF-C09x, and vdW-DF2-C09x van der Waals functionals for the exchange correlation interactions that give significantly improved interlayer spacings and energies. We also use the PBE form for the generalized gradient corrected exchange correlation functional for comparison. We present a consistent theoretical framework for the in-plane layer modulus and the out-of-plane interlayer modulus and we calculate, for the first time, these properties for these systems. This gives a measure of the change of the strength properties when monolayer and bilayer graphene are hydrogenated. Moreover, comparing the relative performance of these functionals in describing hydrogenated bilayered graphenes, we also benchmark these functionals in how they calculate the properties of graphite.