Metal to semiconductor switching in the AgTe monolayer via decoration with alkali metal and alkaline earth metal atoms: a first-principles perspective.

In this work, employing first-principles calculations, we systematically investigate the atomic structure and electronic and optical properties of the AgTe monolayer, as well as the impact of alkali metal (Li, Na, K) and alkaline earth metal (Be, Mg, Ca) atoms decoration. The AgTe monolayer exhibits metallic characteristics. When Li, Na, K, and Mg atoms are decorated on the AgTe monolayer, the decorated AgTe monolayers are dynamically stable. In contrast, with Be and Ca atoms, the decorated structures are found to be dynamically unstable. Interestingly, the decoration of Li, Na, and K atoms into the AgTe monolayer can open the band gaps in the decorated Li-, Na- and K-AgTe monolayers around the Fermi level, leading to the actualization of metal-to-semiconductor transitions. In contrast, the decorated Mg-AgTe monolayer maintains its metallic characteristic. The highest electron and hole mobilities are achieved in the Na-AgTe monolayer among the decorated structures, suggesting the applicability of this structure in photovoltaic applications. The optical study shows that Li-, Na- and K-decorated AgTe monolayers have improved light absorption in the visible light region. Consequently, our findings shed light on the decoration of these 2D material monolayers and can potentially enhance and motivate studies in producing these monolayers for current nanodevices and future applications.

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.

Thermoelectric properties of doped graphene nanoribbons: density functional theory calculations and electrical transport.

We present a detailed study on band structure-dependent properties such as electrical conductivity, the charge of carriers and Seebeck coefficients of graphene nano-ribbons (GNRs) doped with the magnetic impurities Fe and Co since the spin thermopower could be considerably enhanced by impurities. Thermoelectric properties of two-dimensional systems are currently of great interest due to the possibility of heat to electrical energy conversion at the nanoscale. The thermoelectric properties are investigated using the semi-classical Boltzmann method. The electronic band structure of doped nano-ribbons is evaluated by means of density-functional theory in which the Hubbard interaction is considered. Different types of nano-ribbons (armchair-edge and zigzag-edge) and their thermoelectric features such as conductivity and Seebeck coefficient in the presence and absence of magnetic impurities have been studied.

Two-dimensional penta-like PdPSe with a puckered pentagonal structure: a first-principles study.

Low-symmetry penta-PdPSe (Pd4P4Se4) with intrinsic in-plane anisotropy was synthesized successfully [P. Li et al., Adv. Mater., 2021, 2102541]. Motivated by this experimental discovery, we investigate the structural, mechanical, electronic, optical and thermoelectric properties of PdPSe nanosheets via density functional theory calculations. The phonon dispersion, molecular dynamics simulation, and cohesive energy mechanical properties of the penta-PdPSe are verified to confirm its stability. The phonon spectrum represents a striking gap between the high-frequency and the low-frequency optical branches and an out-of-plane flexure mode with a quadratic dispersion in the long-wavelength limit. The Poisson's ratio indicates that penta-PdPSe is a brittle nanosheet. The penta-PdPSe is a semiconductor with an indirect bandgap of 1.40 (2.07) eV using the PBE functional (HSE06 hybrid functional). Optical properties simulation suggests that PdPSe is capable of absorbing a substantial range of visible to ultraviolet light. Band alignment analysis also reveals the compatibility of PdPSe for water splitting photocatalysis application. By combining the electrical and thermal transport properties of PdPSe, we show that a high power factor is achievable at room temperature, thus making PdPSe a candidate material for thermoelectric applications. Our findings reveal the strong potential of penta-PdPSe nanosheets for a wide array of applications, including optoelectronic, water splitting and thermoelectric device applications.

Electronic, optical and thermoelectric properties of a novel two-dimensional SbXY (X = Se, Te; Y = Br, I) family: ab initio perspective.

Recent developments in the synthesis of highly crystalline ultrathin BiTeX (X = Br, Cl) structures [Debarati Hajra et al., ACS Nano 14, 15626 (2020)] have led to the exploration of the atomic structure, dynamical stability, and electronic, optical, and thermoelectric properties of SbXY (X = Se, Te; Y = Br, I) monolayers via density functional calculations. The calculated phonon spectrum, elastic stability conditions, and cohesive energy verified the stability of the studied SbXY monolayers. The mechanical properties reveal that all studied monolayers are stable and brittle. Based on PBE (PBE + SOC) functional calculations, the SbXY monolayers are semiconductors with indirect bandgaps. The calculated bandgaps using HSE (HSE + SOC) for SbSeBr, SbSeI, SbTeBr, and SbTeI monolayers are between 1.45 and 1.91 eV, which are appealing for applications in nanoelectronic devices. The signature of the Rashba effect appears in the SbXY monolayer. The SbXY monolayers are visible-light active. Hole doping can be an efficient way to increase the electricity production of SbXY monolayers from waste heat energy. This study suggests that SbXY (X = Se, Te; Y = Br, I) monolayers represent promising new 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.

Effect of adsorption and substitutional B doping at different concentrations on the electronic and magnetic properties of a BeO monolayer: a first-principles study.

The 2D form of the BeO sheet has been successfully prepared (Hui Zhang et al., ACS Nano, 2021, 15, 2497). Motivated by these exciting experimental results on the 2D layered BeO structure, we studied the effect of the adsorption of B atoms on BeO (B@BeO) and substitutional B atoms (B-BeO) at the Be site at different B concentrations. We investigated the structural stability and the mechanical, electronic, magnetic, and optical properties of the mentioned structures using first-principles calculations. We found out that hexagonal BeO monolayers with adsorbed and dopant B atoms have different mechanical stabilities at different concentrations. B@BeO and B-BeO monolayers are brittle structures, and B@BeO structures are more rigid than B-BeO monolayers (at the same B concentration). The adsorption and the formation energy per B atom decrease as the B concentration increases. In comparison, the work function increases when increasing the B concentration. The work function of B@BeO is higher than the corresponding value of B-BeO (at the same B concentration). The magnetic moment linearly increases as the B concentration increases. BeO is a semiconductor with an indirect bandgap of 5.3 eV. The B@BeO and B-BeO structures are semiconductors, except for 3B-BeO (14.2% doped concentration), which is a metal. The bandgap is 1.25 eV for most of the adsorbed atom concentrations. For B-BeO, the bandgap decreases to zero at a concentration of 14.2%. The bandgap of the B-BeO monolayer at different B concentrations is smaller than the corresponding values of the B@BeO monolayer, which indicates that B substitutional doping has a greater effect on the electronic structure of the BeO monolayer than B adsorption doping. We investigated the optical properties, including the dielectric function and absorption coefficient. The results indicate good optical absorption in the range of infrared and ultraviolet energies for the B adsorbed and doped BeO monolayer.

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.

Novel two-dimensional AlSb and InSb monolayers with a double-layer honeycomb structure: a first-principles study.

In this work, motivated by the fabrication of an AlSb monolayer, we have focused on the electronic, mechanical and optical properties of AlSb and InSb monolayers with double-layer honeycomb structures, employing the density functional theory approach. The phonon band structure and cohesive energy confirm the stability of the XSb (X = Al and In) monolayers. The mechanical properties reveal that the XSb monolayers have a brittle nature. Using the GGA + SOC (HSE + SOC) functionals, the bandgap of the AlSb monolayer is predicted to be direct, while InSb has a metallic character using both functionals. We find that XSb (X = Al, In) two-dimensional bodies can absorb ultraviolet light. The present findings suggest several applications of AlSb and InSb monolayers in novel optical and electronic usages.

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.

Two-dimensional Dirac half-metal in porous carbon nitride C6N7monolayer via atomic doping.

Motivated by the recent experimental discovery of C6N7monolayer (Zhaoet al2021Science Bulletin66, 1764), we show that C6N7monolayer co-doped with C atom is a Dirac half-metal by employing first-principle density functional theory calculations. The structural, mechanical, electronic and magnetic properties of the co-doped C6N7are investigated by both the PBE and HSE06 functionals. Pristine C6N7monolayer is a semiconductor with almost isotropic electronic dispersion around the Γ point. As the doping of the C6N7takes place, the substitution of an N atom with a C atom transforms the monolayer into a dilute magnetic semiconductor, with the spin-up channel showing a band gap of 2.3 eV, while the spin-down channel exhibits a semimetallic phase with multiple Dirac points. The thermodynamic stability of the system is also checked out via AIMD simulations, showing the monolayer to be free of distortion at 500 K. The emergence of Dirac half-metal in carbon nitride monolayer via atomic doping reveals an exciting material platform for designing novel nanoelectronics and spintronics devices.

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.

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.

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.

Two-dimensional Janus semiconductor BiTeCl and BiTeBr monolayers: a first-principles study on their tunable electronic properties via an electric field and mechanical strain.

Motivated by the recent successful synthesis of highly crystalline ultrathin BiTeCl and BiTeBr layered sheets [Debarati Hajra et al., ACS Nano, 2020, 14, 15626], herein for the first time, we carry out a comprehensive study on the structural and electronic properties of BiTeCl and BiTeBr Janus monolayers using density functional theory (DFT) calculations. Different structural and electronic parameters including the lattice constant, bond lengths, layer thickness in the z-direction, different interatomic angles, work function, charge density difference, cohesive energy and Rashba coefficients are determined to acquire a deep understanding of these monolayers. The calculations show good stability of the studied single layers. BiTeCl and BiTeBr monolayers are semiconductors with electronic bandgaps of 0.83 and 0.80 eV, respectively. The results also show that the semiconductor-metal transformation can be induced by increasing the number of layers. In addition, the engineering of the electronic structure is also studied by applying an electric field, and mechanical uniaxial and biaxial strain. The results show a significant change of the bandgaps and that an indirect-direct band-gap transition can be induced. This study highlights the positive prospect for the application of BiTeCl and BiTeBr layered sheets in novel electronic and energy conversion systems.

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.

Ab initio prediction of semiconductivity in a novel two-dimensional Sb2X3 (X= S, Se, Te) monolayers with orthorhombic structure.

[Formula: see text] and [Formula: see text] are well-known layered bulk structures with weak van der Waals interactions. In this work we explore the atomic lattice, dynamical stability, electronic and optical properties of [Formula: see text], [Formula: see text] and [Formula: see text] monolayers using the density functional theory simulations. Molecular dynamics and phonon dispersion results show the desirable thermal and dynamical stability of studied nanosheets. On the basis of HSE06 and PBE/GGA functionals, we show that all the considered novel monolayers are semiconductors. Using the HSE06 functional the electronic bandgap of [Formula: see text], [Formula: see text] and [Formula: see text] monolayers are predicted to be 2.15, 1.35 and 1.37 eV, respectively. Optical simulations show that the first absorption coefficient peak for [Formula: see text], [Formula: see text] and [Formula: see text] monolayers along in-plane polarization is suitable for the absorption of the visible and IR range of light. Interestingly, optically anisotropic character along planar directions can be desirable for polarization-sensitive photodetectors. Furthermore, we systematically investigate the electrical transport properties with combined first-principles and Boltzmann transport theory calculations. At optimal doping concentration, we found the considerable larger power factor values of 2.69, 4.91, and 5.45 for hole-doped [Formula: see text], [Formula: see text], and [Formula: see text], respectively. This study highlights the bright prospect for the application of [Formula: see text], [Formula: see text] and [Formula: see text] nanosheets in novel electronic, optical and energy conversion systems.

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.

Strain, electric-field and functionalization induced widely tunable electronic properties in MoS2/BC 3, /C 3 N and /[Formula: see text] van der Waals heterostructures.

In this paper, the effect of BC 3, C 3 N and [Formula: see text] substrates on the atomic and electronic properties of MoS2 were systematically investigated using first-principles calculations. Our results show that the MoS2/BC 3 and MoS2/C 3 N4 heterostructures are direct semiconductors with band gaps of 0.4 and 1.74 eV, respectively, while MoS2/C 3 N is a metal. Furthermore, the influence of strain and electric field on the electronic structure of these van der Waals heterostructures is investigated. The MoS2/BC3 heterostructure, for strains larger than -4%, transforms it into a metal where the metallic character is maintained for strains larger than -6%. The band gap decreases with increasing strain to 0.35 eV (at +2%), while for strain (>+6%) a direct-indirect band gap transition is predicted to occur. For the MoS2/C3N heterostructure, the metallic character persists for all strains considered. On applying an electric field, the electronic properties of MoS2/C3N4 are modified and its band gap decreases as the electric field increases. Interestingly, the band gap reaches 30 meV at +0.8 V/Å, and with increase above +0.8 V/Å, a semiconductor-to-metal transition occurs. Furthermore, we investigated effects of semi- and full-hydrogenation of MoS2/C3N and we found that it leads to a metallic and semiconducting character, respectively.