Strain Engineering of Antimonene by a First-principles Study: Mechanical and Electronic Properties

In this work, we investigate the mechanical and electronic properties of monolayer antimonene in its most stable beta-phase using first-principles calculations. The upper region of its valence band is found to solely consist of lone pair p-orbital states, which are by nature more delocalized than the d-orbital states in transition metal dichalcogenides, implying superior transport performance of antimonene. The Young's and shear moduli of beta-antimonene are observed to be ~25% higher than those of bulk antimony, while the hexagonal lattice constant of the monolayer reduces significantly (~5%) from that in bulk, indicative of strong inter-layer coupling. The ideal tensile test of beta-antimonene under applied uniaxial strain highlights ideal strengths of 6 GPa and 8 GPa, corresponding to critical strains of 15% and 17% in the zigzag and armchair directions, respectively. During the deformation process, the structural integrity of the material is shown to be better preserved, albeit moderately, in the armchair direction. Interestingly, the application of uniaxial strain in the zigzag and armchair directions unveil direction-dependent trends in the electronic band structure. We find that the nature of the band gap remains insensitive to strain in the zigzag direction, while strain in the armchair direction activates an indirect-direct band gap transition at a critical strain of 4%, owing to a band switching mechanism. The curvature of the conduction band minimum increases during the transition, which suggests a lighter effective mass of electrons in the direct-gap configuration than in the free-standing state of equilibrium. The work function of free-standing beta-antimonene is 4.59 eV and it attains a maximum value of 5.07 eV under an applied biaxial strain of 4%

Effects of Graphene/BN Encapsulation, Surface Functionalization and Molecular Adsorption on the Electronic Properties of Layered InSe: A First-Principles Study

By using first-principles calculations, we investigated the effects of graphene/boron nitride (BN) encapsulation, surface functionalization by metallic elements (K, Al, Mg and typical transition metals) and molecules (tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE)) on the electronic properties of layered indium selenide (InSe). It was found that an opposite trend of charge transfer is possible for graphene (donor) and BN (acceptor), which is dramatically different from phosphorene where both graphene and BN play the same role (donor). For InSe/BN heterostructure, a change of the interlayer distance due to an out-of-plane compression can effectively modulate the band gap. Strong acceptor abilities to InSe were found for the TCNE and TCNQ molecules. For K, Al and Mg-doped monolayer InSe, the charge transfer from K and Al atoms to the InSe surface was observed, causing an n-type conduction of InSe, while p-type conduction of InSe observed in case of the Mg-doping. The atomically thin structure of InSe enables the possible observation and utilization of the dopant-induced vertical electric field across the interface. A proper adoption of the n- or p-type dopants allows for the modulation of the work function, the Fermi level pinning, the band bending, and the photo-adsorbing efficiency near the InSe surface/interface. Investigation on the adsorption of transition metal atoms on InSe showed that Ti-, V-, Cr-, Mn-, Co-adsorbed InSe are spin-polarized, while Ni-, Cu-, Pd-, Ag- and Au-adsorbed InSe are non-spin-polarized. Our results shed lights on the possible ways to protect InSe structure and modulate its electronic properties for nanoelectronics and electrochemical device applications

Exploring the Charge Localization and Band Gap Opening of Borophene: A First-Principles Study

Recently synthesized two-dimensional (2D) boron, borophene, exhibits a novel metallic behavior rooted in the s-p orbital hybridization, distinctively different from other 2D materials such as sulfides/selenides and semi-metallic graphene. This unique feature of borophene implies new routes for charge delocalization and band gap opening. Herein, using first-principles calculations, we explore the routes to localize the carriers and open the band gap of borophene via chemical functionalization, ribbon construction, and defect engineering. The metallicity of borophene is found to be remarkably robust against H- and F-functionalization and the presence of vacancies. Interestingly, a strong odd-even oscillation of the electronic structure with width is revealed for H-functionalized borophene nanoribbons, while an ultra-high work function (~ 7.83 eV) is found for the F-functionalized borophene due to its strong charge transfer to the atomic adsorbates

Large Electronic Anisotropy and Enhanced Chemical Activity of Highly Rippled Phosphorene

We investigate the electronic structure and chemical activity of rippled phosphorene induced by large compressive strains via first-principles calculation. It is found that phosphorene is extraordinarily bendable, enabling the accommodation of ripples with large curvatures. Such highly rippled phosphorene shows a strong anisotropy in electronic properties. For ripples along the armchair direction, the band gap changes from 0.84 to 0.51 eV for the compressive strain up to -20% and further compression shows no significant effect, for ripples along the zigzag direction, semiconductor to metal transition occurs. Within the rippled phosphorene, the local electronic properties, such as the modulated band gap and the alignments of frontier orbitals, are found to be highly spatially dependent, which may be used for modulating the injection and confinement of carriers for optical and photovoltaic applications. The examination of the interaction of a physisorbed NO molecule with the rippled phosphorene under different compressive strains shows that the chemical activities of the phosphorene are significantly enhanced at the top and bottom peaks of the ripples, indicated by the enhanced adsorption and charge transfer between them. All these features can be ascribed to the effect of curvatures, which modifies the orbital coupling between atoms at the ripple peaks

Atomic-scale mechanisms of defect- and light-induced oxidation and degradation of InSe

Layered indium selenide (InSe), a new two-dimensional (2D) material with a hexagonal structure and semiconducting characteristic, is gaining increasing attention owing to its intriguing electronic properties. Here, by using first-principles calculations, we reveal that perfect InSe possesses a high chemical stability against oxidation, superior to MoS2. However, the presence of intrinsic Se vacancy (VSe) and light illumination can markedly affect the surface activity. In particular, the excess electrons associated with the exposed In atoms at the VSe site under illumination are able to remarkably reduce the dissociation barrier of O2 to ~0.2 eV. Moreover, at ambient conditions, the splitting of O2 enables the formation of substitutional (apical) oxygen atomic species, which further cause the trapping and subsequent rapid splitting of H2O molecules and ultimately the formation of hydroxyl groups. Our findings uncover the causes and underlying mechanisms of InSe surface degradation via the defect-photo promoted oxidations. Such results will be beneficial in developing strategies for the storage of InSe material and its applications for surface passivation with boron nitride, graphene or In-based oxide layers

Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-based Cements

Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in carbon capture processes. However, knowledge on the driving forces that control the degree of carbonation and hydration and rate of reactions in this system remains limited. In this work, density functional theory-based simulations are used to investigate the physical nature of the reactions taking place during the fabrication of RMCs under ambient conditions. Parametric indicators such as adsorption energies, charge transfer, electron localization function, adsorption/dissociation energy barriers and the mechanisms of interaction of H2O and CO2 molecules with MgO and brucite (Mg(OH)2) clusters are considered. The following hydration and carbonation interactions relevant to RMCs are evaluated i) carbonation of MgO, ii) hydration of MgO, carbonation of hydrated MgO, iii) carbonation of Mg(OH)2, iv) hydration of Mg(OH)2 and v) hydration of carbonated Mg(OH)2. A comparison of the energy barriers and reaction pathways of these mechanisms shows that the carbonation of MgO is hindered by presence of H2O molecules, while the carbonation of Mg(OH)2 is hindered by the formation of initial carbonate and hydrate layers as well as presence of excessed H2O molecules. To compare these finding to bulk mineral surfaces, the interactions of the CO2 and H2O molecules with the MgO(001) and Mg(OH)2 (001) surfaces are studied. Therefore, this work presents deep insights into the physical nature of the reactions and the mechanisms involved in hydrated magnesium carbonates production that can be beneficial for its development

The first-principles study of the adsorption of NH₃, NO, and NO₂ gas molecules on InSe-like phosphorus carbide

Abstract The adsorption of environmental gas molecules, i.e., NH₃, NO, and NO₂ on the γ-PC surface has been studied using first-principles calculations. The lowest-energy configurations of these molecules on the γ-PC surface were found and the adsorption energies were calculated. The NH₃, NO, and NO₂ molecules were found to be physisorbed on the γ-PC surface. The analysis of the charge transfer between the molecules and the surface predicted NH₃ and NO as donors to γ-PC, while NO₂ acted as an acceptor to γ-PC. Remarkable changes in the band structure of γ-PC were found upon the adsorption of NO₂ on its surface. In addition, significant modulations in the work function of γ-PC were observed after the adsorption of NH₃ and NO