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

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.

Biphenylene network as sodium ion battery anode material

Sodium ion batteries possess several advantages for large-scale energy storage, such as low cost and enhanced safety. However, graphite or other anode materials are not satisfactory because the large radius of Na⁺ hinders their embedding and removal in the charge and discharge processes. Recently, a biphenylene network (BPN), a two-dimensional (2D) carbon allotrope, has been synthesized. In this paper, we reveal the potential possibility of BPN as a Na storage material. The theoretical results indicate the advantages of BPN as a sodium battery anode. The maximum specific capacity (413 mA h g^-1) is larger than that of the graphite-Li system (372 mA h g^-1). With low Na⁺ diffusion barrier (<0.6 eV) and small volume expansion in the charging process (∼26%), BPN presents superiority to the graphite-Na system. Our findings show new insights into Na storage in BPN and provide guidance for the use of a BPN anode in sodium ion batteries.

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.

2D multifunctional SiAs2/GeAs2van der Waals heterostructure

The structural and electronic properties of two-dimensional (2D) SiAs₂/GeAs₂van der Waals heterostructure (vdWH) and its applications are investigated by combing first-principles calculations and Silvaco Atlas simulations. The stable SiAs₂/GeAs₂vdWH exhibits an indirect bandgap of 0.99 eV in type II band alignment for light detection and energy harvesting. The vdWH can exhibit a direct bandgap up to 0.66 eV by applying an appropriate electric field (E_ext). Due to theE_extinduced charge redistribution, its band alignment can be transformed from type II to type I for light-emitting. Further simulation shows that the band alignment of SiAs₂/GeAs₂vdWH can be tuned back and forth between type II and type I by gate voltage in a single field-effect transistor for multiple functional applications. These results may be useful for applications of the SiAs₂/GeAs₂heterostructure in future electronic and optoelectronic devices.

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.

Comment on 'Two-dimensional carbon nitride C6N nanosheet with egg-comb-like structure and electronic properties of a semimetal [2021,Nanotechnology, 215702]'

Recently, Bafekryet al(2021Nanotechnology32215702) predicted a novel two-dimensional carbon nitride with an egg-comb-like structure and a C₆N stoichiometry. Their density functional theory results reveal the thermal stability of this structure at 300 K. They also examined the thermoelectric properties of this monolayer up to 500 K. By adopting the same methodology employed in the original work by Bafekryet al(2021Nanotechnology32215702), we show that this monolayer is thermally unstable and cannot be employed for practical applications. Aforementioned theoretical work also includes inaccuracies in the evaluation of thermoelectric properties of the C₆N monolayer, as they did not consider the contribution of the lattice thermal conductivity.

Two-Dimensional Dirac Nodal Line Carbon Nitride to Anchor Single-Atom Catalyst for Oxygen Reduction Reaction

Two-dimensional carbon nitride (2DCN) materials have emerged as an important class of 2D materials beyond graphene. However, 2DCN materials with nodal-line semimetal characteristic are rarely reported. In this work, a new nodal-line semimetal 2DCN with the stoichiometry C₄ N₄ is designed by using density functional theory (DFT) calculations and its application to anchor single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is investigated. C₄ N₄ is a planar covalent network (sp² hybridization) with regular holes formed by the four N atoms, which is dynamically, thermodynamically, and mechanically stable. The nodal line is contributed by the p_z orbitals of C and p_x/y orbitals of N atoms. C₄ N₄ shows an anisotropic Fermi velocity and high electron mobility. Because of its porous structure, C₄ N₄ can anchor heteroatoms as SACs for electrocatalysis. C₄ N₄ anchored with Fe or Co is shown to be highly active for the ORR with a rather high half-wave potential of around 0.90 V, which is higher than those of SACs on other carbon nitrides. These findings may provide a new strategy to design novel substrates for SACs.

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.

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 Mg_Sbimpurity 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 V_Sbis the most stable, while in Sb-rich circumstances the point defect of V_Algets 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.

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.

Ab initiostudies for characterization and identification of nanocrystalline copper pyrophosphate confined in mesoporous silica

Here we employ a novel method for preparing the homogeneous copper pyrophosphate nanocrystals inside silica mesopores. In order to characterize and identify synthesized nanocrystals we performed theab initiostudies of theαphase of Cu₂P₂O₇. The electronic and crystal structure were optimized within the density functional theory with the strong electron interactions in the3dstates on copper atoms and van der Waals corrections included in calculations. The relaxed lattice parameters and atomic positions agree very well with the results of the diffraction measurements for nanocrystalline copper pyrophosphates embedded inside SBA-15 silica pores. The obtained Mott insulating state with the energy gap of 3.17 eV exhibits the antiferromagnetic order with magnetic moments on copper atoms (0.8μB) that is compatible with the experimental studies. The phonon dispersion relations were obtained to study the dynamical properties of the Cu₂P₂O₇lattice and the element-specific atomic vibrations were analyzed using the partial phonon density of states. The calculated Raman spectrum revealed the consistency of typical bands of Cu₂P₂O₇with the experimental data. The investigation that combines a new synthesis of nanomaterials with the first-principles calculations is important for better characterization and understanding of the physical properties relevant for nanotechnological applications.

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.