Theoretical prediction of two-dimensional BC2X (X = N, P, As) monolayers: ab initio investigations.

In this work, novel two-dimensional BC[Formula: see text]X (X = N, P, As) monolayers with X atoms out of the B-C plane, are predicted by means of the density functional theory. The structural, electronic, optical, photocatalytic and thermoelectric properties of the BC[Formula: see text]X monolayers have been investigated. Stability evaluation of the BC[Formula: see text]X single-layers is carried out by phonon dispersion, ab-initio molecular dynamics (AIMD) simulation, elastic stability, and cohesive energies study. The mechanical properties reveal all monolayers considered are stable and have brittle nature. The band structure calculations using the HSE06 functional reveal that the BC[Formula: see text]N, BC[Formula: see text]P and BC[Formula: see text]As are semiconducting monolayers with indirect bandgaps of 2.68 eV, 1.77 eV and 1.21 eV, respectively. The absorption spectra demonstrate large absorption coefficients of the BC[Formula: see text]X monolayers in the ultraviolet range of electromagnetic spectrum. Furthermore, we disclose the BC[Formula: see text]N and BC[Formula: see text]P monolayers are potentially good candidates for photocatalytic water splitting. The electrical conductivity of BC[Formula: see text]X is very small and slightly increases by raising the temperature. Electron doping may yield greater electric productivity of the studied monolayers than hole doping, as indicated by the larger power factor in the n-doped region compared to the p-type region. These results suggest that BC[Formula: see text]X (X = N, P, As) monolayers represent a new promising class of 2DMs for electronic, optical and energy conversion systems.

Investigation of vacancy defects and substitutional doping in AlSb monolayer with double layer honeycomb structure: a first-principles calculation.

The experimental knowledge of the AlSb monolayer with double layer honeycomb structure is largely based on the recent publication (Le Qinet al2021ACS Nano158184), where this monolayer was recently synthesized. Therefore, the aim of our research is to consequently explore the effects of substitutional doping and vacancy point defects on the electronic and magnetic properties of the novel hexagonal AlSb monolayer. Besides experimental reports, the phonon band structure and cohesive energy calculations confirm the stability of the AlSb monolayer. Its direct bandgap has been estimated to be 0.9 eV via the hybrid functional method, which is smaller than the value of 1.6 eV of bulk material. The majority of vacancy defects and substitutional dopants change the electronic properties of the AlSb monolayer from semiconducting to metallic. Moreover, the MgSbimpurity has demonstrated the addition of ferromagnetic behavior to the material. It is revealed through the calculation of formation energy that in Al-rich conditions, the vacant site of VSbis the most stable, while in Sb-rich circumstances the point defect of VAlgets the title. The formation energy has also been calculated for the substitutional dopants, showing relative stability of the defected structures. We undertook this theoretical study to inspire many experimentalists to focus their efforts on AlSb monolayer growth incorporating different impurities. It has been shown here that defect engineering is a powerful tool to tune the properties of novel AlSb two-dimensional monolayer for advanced nanoelectronic applications.

Two-dimensional FeTe2 and predicted Janus FeXS (X: Te and Se) monolayers with intrinsic half-metallic character: tunable electronic and magnetic properties via strain and electric field.

Driven by the fabrication of bulk and monolayer FeTe2 (ACS Nano, 2020, 14, 11473-11481), we explore the lattice, dynamic stability, electronic and magnetic properties of FeTeS and FeSeS Janus monolayers using density functional theory calculations. The obtained results validate the dynamic and thermal stability of the FeTeS and FeSeS Janus monolayers examined. The electronic structure shows that the FeTe2 bulk yields a total magnetization higher than the FeTe2 monolayer. FeTeS and FeSeS are categorized as ferromagnetic metals due to their bands crossing the Fermi level. So, they can be a good candidate material for spin filter applications. The biaxial compressive strain on the FeTe2 monolayer tunes the bandgap of the spin-down channel in the half-metal phase. By contrast, for FeTeS, the biaxial strain transforms the ferromagnetic metal into a half-metal. The electric field applied to the FeSeS monolayer in a parallel direction transforms the half-metal to a ferromagnetic metal by closing the gap in the spin-down channel.

Point defects in two-dimensional BeO monolayer: a first-principles study on electronic and magnetic properties.

Very recently, the 2D form of BeO monolayer has been successfully fabricated [Hui Zhang et al., ACS Nano, 2021, 15, 2497]. Motivated by these exciting experimental results on 2D layered BeO structures, the effect of atom adsorption, substitutional doping and vacancy defects on the electronic and magnetic properties of a hexagonal BeO monolayer have been systematically investigated employing density functional theory-based first-principles calculations. We found out that BeO monolayer is a semiconductor with an indirect band gap of 5.9 eV. Next, a plethora of atoms (27 in total) were adsorbed on the surface of BeO monolayer to tailor its electronic properties. The bond length, work function, difference in charge and magnetic moment were also calculated for all modifications covering the vacancy defects and substitutional doping. The band gap is also supplied for these changes, showing how these adjustments can provide amazing opportunities in granting a variety of options in band gap engineering and in transforming the BeO monolayer from a semiconductor to a dilute magnetic semiconductor or half-metal in view of different applications. The formation energy of the defects was also computed as an important indicator for the stability of the defected structures, when created in a real experiment. We have theoretically demonstrated several possible approaches to modify the properties of BeO monolayer in a powerful and controllable manner. Thus, we expect to inspire many experimental studies focused on two dimensional BeO growth and property tuning, and exploration for applications in advanced nanoelectronics.

Van der Waals heterostructure of graphene and germanane: tuning the ohmic contact by electrostatic gating and mechanical strain.

Correction for 'Van der Waals heterostructure of graphene and germanane: tuning the ohmic contact by electrostatic gating and mechanical strain' by A. Bafekry et al., Phys. Chem. Chem. Phys., 2021, DOI: 10.1039/D1CP03632G.

Biphenylene monolayer as a two-dimensional nonbenzenoid carbon allotrope: a first-principles study.

In a very recent accomplishment, the two-dimensional form of biphenylene network (BPN) has been fabricated. Motivated by this exciting experimental result on 2D layered BPN structure, herein we perform detailed density-functional theory-based first-principles calculations, in order to gain insight into the structural, mechanical, electronic and optical properties of this promising nanomaterial. Our theoretical results reveal the BPN structure is constructed from three rings of tetragon, hexagon and octagon, meanwhile the electron localization function shows very strong bonds between the C atoms in the structure. The dynamical stability of BPN is verified via the phonon band dispersion calculations. The mechanical properties reveal the brittle behavior of BPN monolayer. The Young's modulus has been computed as 0.1 TPa, which is smaller than the corresponding value of graphene, while the Poisson's ratio determined to be 0.26 is larger than that of graphene. The band structure is evaluated to show the electronic features of the material; determining the BPN monolayer as metallic with a band gap of zero. The optical properties (real and imaginary parts of the dielectric function, and the absorption spectrum) uncover BPN as an insulator along thezzdirection, while owning metallic properties inxxandyydirections. We anticipate that our discoveries will pave the way to the successful implementation of this 2D allotrope of carbon in advanced nanoelectronics.

Van der Waals heterostructure of graphene and germanane: tuning the ohmic contact by electrostatic gating and mechanical strain.

Recent exciting developments in synthesis and properties study of the Germanane (GeH) monolayer have inspired us to investigate the structural and electronic properties of the van der Waals GeH/Graphene (Gr) heterostructure by the first-principle approach. The stability of the GeH/Gr heterostructure is verified by calculating the phonon dispersion curves as well as by thermodynamic binding energy calculations. According to the band structure calculation, the GeH/Gr interface is n-type Ohmic. The effects of different interlayer distances and strains between the layers and the applied electric field on the interface have been investigated to gain insight into the van der Waals heterostructure modifications. An interlayer distance of 2.11 Å and compressive strain of 6% alter the contact from Ohmic to Schottky status, while the electric field can tune the GeH/Gr contact as p- or n-type, Ohmic, or Schottky. The average electrostatic potential of GeH/Gr and the Bader charge analysis have been used to explain the results obtained. Our theoretical study could provide a promising approach for improving the electronic performance of GeH/Gr-based nano-rectifiers.

Surface modification of titanium carbide MXene monolayers (Ti2C and Ti3C2) via chalcogenide and halogenide atoms.

Inspired by the recent successful growth of Ti2C and Ti3C2 monolayers, here, we investigate the structural, electronic, and mechanical properties of functionalized Ti2C and Ti3C2 monolayers by means of density functional theory calculations. The results reveal that monolayers of Ti2C and Ti3C2 are dynamically stable metals. Phonon band dispersion calculations demonstrate that two-surface functionalization of Ti2C and Ti3C2via chalcogenides (S, Se, and Te), halides (F, Cl, Br, and I), and oxygen atoms results in dynamically stable novel functionalized monolayer materials. Electronic band dispersions and density of states calculations reveal that all functionalized monolayer structures preserve the metallic nature of both Ti2C and Ti3C2 except Ti2C-O2, which possesses the behavior of an indirect semiconductor via full-surface oxygen passivation. In addition, it is shown that although halide passivated Ti3C2 structures are still metallic, there exist multiple Dirac-like cones around the Fermi energy level, which indicates that semi-metallic behavior can be obtained upon external effects by tuning the energy of the Dirac cones. In addition, the computed linear-elastic parameters prove that functionalization is a powerful tool in tuning the mechanical properties of stiff monolayers of bare Ti2C and Ti3C2. Our study discloses that the electronic and structural properties of Ti2C and Ti3C2 MXene monolayers are suitable for surface modification, which is highly desirable for material property engineering and device integration.

Two-dimensional buckled tetragonal cadmium chalcogenides including CdS, CdSe, and CdTe monolayers as photo-catalysts for water splitting.

Pure hydrogen production via water splitting is an ideal strategy for producing clean and sustainable energy. Two-dimensional (2D) cadmium chalcogenide single-layers with a tetragonal crystal structure, namely Tetra-CdX (X = S, Se, and Te) monolayers, are theoretically predicted by means of density functional theory (DFT). Their structural stability and electronic and optical properties are investigated. We find that Tetra-CdX single-layers are thermodynamically stable. Their stability decreases as we go down the 6A group in the periodic table, i.e., from X = S to Se, and Te which also means that the electronegativity decreases. All considered novel monolayers are indirect band gap semiconductors. Using the HSE06 functional the electronic band gaps of CdS, CdSe, and CdTe monolayers are predicted to be 3.10 eV, 2.97 eV, and 2.90 eV, respectively. The impact of mechanical strain on the physical properties was studied, which indicates that compressive strain increases the band gap and tensile strain decreases the band gap. The optical properties of the Tetra-CdX monolayers show the ability of these monolayers to absorb visible light. Due to the suitable band gaps and band edge positions of Tetra-CdX, these newly discovered 2D materials are promising for photocatalytic water splitting.

Two-dimensional carbon nitride C6N nanosheet with egg-comb-like structure and electronic properties of a semimetal.

In this study, the structural, electronic and optical properties of theoretically predicted C6N monolayer structure are investigated by means of Density Functional Theory-based First-Principles Calculations. Phonon band dispersion calculations and molecular dynamics simulations reveal the dynamical and thermal stability of the C6N single-layer structure. We found out that the C6N monolayer has large negative in-plane Poisson's ratios along bothXandYdirection and the both values are almost four times that of the famous-pentagraphene. The electronic structure shows that C6N monolayer is a semi-metal and has a Dirac-point in the BZ. The optical analysis using the random phase approximation method constructed over HSE06 illustrates that the first peak of absorption coefficient of the C6N monolayer along all polarizations is located in theIRrange of spectrum, while the second absorption peak occurs in the visible range, which suggests its potential applications in optical and electronic devices. Interestingly, optically anisotropic character of this system is highly desirable for the design of polarization-sensitive photodetectors. Thermoelectric properties such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity and power factor are investigated as a function of carrier doping at temperatures 300, 400, and 500 K. In general, we predict that the C6N monolayer could be a new platform for study of novel physical properties in two-dimensional semi-metal materials, which may provide new opportunities to realize high-speed low-dissipation devices.

First-principles investigation of nonmetal doped single-layer BiOBr as a potential photocatalyst with a low recombination rate.

Nonmetal doping is an effective approach to modify the electronic band structure and enhance the photocatalytic performance of bismuth oxyhalides. Using density functional theory, we systematically examine the fundamental properties of single-layer BiOBr doped with boron (B) and phosphorus (P) atoms. The stability of the doped models is investigated based on the formation energies, where the substitutional doping is found to be energetically more stable under O-rich conditions than under Bi-rich ones. The results showed that substitutional doping of P atoms reduced the bandgap of pristine BiOBr to a greater extent than that of boron substitution. The calculation of the effective masses reveals that B doping can render the electrons and holes of pristine BiOBr lighter and heavier, respectively, resulting in a slower recombination rate of photoexcited electron-hole pairs. Based on the results of HOMO-LUMO calculations, the introduction of B atoms tends to increase the number of photocatalytically active sites. The top of the valence band and the conduction band bottom of the B doped BiOBr monolayer match well with the water redox potentials in an acidic environment. The absorption spectra propose that B(P) doping causes a red-shift. Overall, the results predict that nonmetal-doped BiOBr monolayers have a reduced bandgap, a slow recombination rate, more catalytically active sites, enhanced optical absorption edges, and reduced work functions, which will contribute to superior photocatalytic performance.

Aligned AlN nanowires by self-organized vapor-solid growth.

Highly oriented AlN single crystal nanowires with aspect ratio up to 600, diameter in the range of 40-500 nm, and 100 microm lengths, have been synthesized via a vapor-solid growth mechanism. The results were obtained at 1750 degrees C and 850 mbar nitrogen pressure on vicinal SiC substrates pretreated by SiC sublimation epitaxy in order to attain distinguishable terraces. It was found that the nanowires change in thickness after they have reached a critical length, and this fact contributes to an understanding of the growth mechanism of AlN nanowires. The nanowires are hexagonally shaped and perfectly aligned along the [0001] direction with a small tilt given by the substrate vicinality. Under nitrogen excess a preferential growth along the c-axis of the wurtzite structure takes place while below some critical value of nitrogen pressure the growth mode switches to lateral. The AlN nanowires are shown to have a dislocation free wurtzite crystal structure. Some possible applications are discussed.