Investigation of strain and doping on the electronic properties of single layers of C6N6 and C6N8: a first principles study

Sensing potential of C6N8 for ammonia (NH3) and nitrogen triflouride (NF3): A DFT study

The detection of toxic gases (NH3 and NF3) in regulating and monitoring air quality in the atmosphere has drawn a lot of attention. Herein, we explored a novel material (C6N8) for the detection of the important but toxic gases (NH3 and NF3). We investigated the interactions of the NH3 and NF3 with C6N8 through DFT at B3LYP, ωB97XD, and non-DFT M06-2X. Counterpoise interaction energy values (Eint. cp.) of NH3@C6N8 and NF3@C6N8 are -0.45 eV and -3.51 eV (for B3LYP), -0.42 eV and 2.11 eV (for ωB97XD) and -0.44 eV and -3.41eV (for M06-2X), respectively. Complexes having the most stable configurations were then subjected to further analyses including frontier molecular orbitals, H-L gap, and conductivity of complexes. An increase in the H-L gap in complexes (NH3@C6N8 and NF3@C6N8) is observed. The conductivity of NH3@C6N8 and NF3@C6N8 decreases as compared to C6N8. A considerable change in dipole moment was seen in C6N8 before and after complex formation. This is because of the shifting of charge between C6N8 and gases (NH3 and NF3). CHELPG and NBO charge analysis were used to evaluate the amount of charge transfer between C6N8 and gases. These analyses demonstrate that NH3 and NF3 withdraw electron density from C6N8. It was found that NH3 tends to be physically adsorbed on C6N8 while NF3 adsorbs chemically on C6N8. NCI and QTAIM analyses were performed to investigate the kind of interactions between the surface (C6N8) and gases (NH3 and NF3). Furthermore, the recovery time of NH3@C6N8 and NF3@C6N8 shows that C6N8 can be a better choice for sensing NH3 and NF3 gases.

Effect of vacancy defect and strain on the structural, electronic and magnetic properties of carbon nitride 2D monolayers by DFTB method

We investigate the electronic and magnetic properties of CnNm(C₆N₆, C₂N, C₃N and C₃N₄) using density functional tight-binding (DFTB) method. We find that these compounds are dynamically stable and their electronic band gaps are in the range of 0.59-3.28 eV. We show that the electronic structure is modulated by strain and the semiconducting behavior is well preserved except for C₃N at +5% biaxial strain, where a transition from semiconductor to metal was observed. Under +3% biaxial strain, C₃N₄undergoes a transition from an indirect (K-Γ) to a direct (Γ-Γ) band gap. Moreover, band gap of C₂N transforms from direct (Γ-Γ) to indirect (M-Γ) under +4% biaxial strain. However, no change in the nature of the band gap of C₆N₆. Further, when the studied materials under uniaxial tensile strain, their bandgaps reduce. Our theoretical study showed that an indirect-to-direct nature transition may occur for C₆N₆and for C₃N₄, which broadens their applications. On the other hand, magnetism is observed in all N-vacancy defected CnNm, which encourages its application in spintronic. Moreover, calculations of formation energies indicate that N-vacancy is energetically more favorable than C-vacancy in both C₂N and C₃N₄. Opposite behavior found for C₆N₆and C₃N.

Two-dimensional AlN/g-CNs van der Waals type-II heterojunction for water splitting

A type-II van der Waals heterojunction photocatalyst is not only an ideal material for hydrogen production by water splitting, but also an important way to improve efficiency and produce low-cost clean energy. In this work, we unexpectedly found that monolayers of AlN and C₂N, g-C₃N₄, and C₆N₈ all formed type-II heterojunctions according to density functional theory, and we report a comparison of their photocatalytic performance. Among them, the AlN/C₂N heterojunction has an appropriate band gap value of 1.61 eV for visible light water splitting. It has higher carrier mobility than the AlN/g-C₃N₄ heterojunction (electron 253.1 cm² V^-1 s^-1 > 31.6 cm² V^-1 s^-1 and hole 11043.4 cm² V^-1 s^-1 > 524.7 cm² V^-1 s^-1), and an absorption peak similar those of monolayer C₂N in visible light (8 × 10⁴ cm^-1) and monolayer AlN in ultraviolet light (11 × 10⁴ cm^-1). The Bader charge shows that the charge transfer number of the AlN/g-C₃N₄ heterojunction is higher than that of the AlN/C₂N heterojunction, and its Gibbs free energy (-0.22 eV) is smaller than that of single-layer g-C₃N₄ (-0.30 eV). The AlN/C₆N₈ heterojunction also has a perfect band gap of 2.16 eV and an absorption peak of over 10 × 10⁴ cm^-1 in the UV region. Since a type-II heterojunction can effectively promote the separation of photogenerated electron-hole pairs and prevent their rapid recombination, the above heterojunctions are promising candidates for new photocatalysts.

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.

Origin of phonon-limited mobility in two-dimensional metal dichalcogenides

Metal dichalcogenides are novel two-dimensional (2D) semiconductors after the discovery of graphene. In this article, phonon-limited mobility for six kinds of 2D semiconductors with the composition of MX₂is reviewed, in which M (Cr, Mo and W) is the transition metal, and X (S and Se) is the chalcogen element. The review is divided into three parts. In the first part, we briefly introduce the calculation method of mobility, including the empirical model and Boltzmann transport theory (BTE). The application scope, merits and limitations of these methods are summarized. In the second part, we explore empirical models to calculate the mobility of MX₂, including longitudinal acoustic phonon, optical phonon (OP) and polar optical phonon (POP) models. The contribution of multi-valley to mobility is reviewed in the calculation. The differences between static and high-frequency dielectric constants (Δϵ) are only 0.13 and 0.03 for MoS₂and WS₂. Such a low value indicates that the polarization hardly changes in the external field. So, their mobility is not determined by POP, but by deformation potential models. Different from GaAs, POP scattering plays a decisive role in its mobility. Our investigations also reveal that the scattering from POP cannot be ignored in CrSe₂, MoSe₂and WSe₂. In the third parts, we investigate the mobility of MX₂using electron-phonon coupling matrix element, which is based on BTE from the framework of a many-body quantum-field theory. Valence band splitting of MoS₂and WS₂is induced by spin-orbit coupling effect, which leads to the increase of hole mobility. In particular, we review in detail the theoretical and experimental results of MoS₂mobility in recent ten years, and its mobility is also compared with other materials to deepen the understanding.

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.

Investigation of strain behavior and carrier mobility of organic-inorganic hybrid perovskites: (C4H9NH3)2GeI4 and (C4H9NH3)2SnI4

Two dimensional (2D) organic-inorganic hybrid perovskites have attracted great interest due to their tunable band gap and structural stability. In this study, biaxial strain behavior and carrier mobility of monolayers (C₄H₉NH₃)₂GeI₄ and (C₄H₉NH₃)₂SnI₄ are investigated by first principles calculations. (C₄H₉NH₃)₂GeI₄ and (C₄H₉NH₃)₂SnI₄ still retain their structural stability at ε = 13% and ε = 15%, respectively. Ab initio molecular dynamics (AIMD) simulation has confirmed that the system at 300 K is still thermodynamically stable at a biaxial strain of ε = 8%. The band gaps of (C₄H₉NH₃)₂GeI₄ and (C₄H₉NH₃)₂SnI₄ calculated from the HSE06 functional are increased from 2.427 and 1.953 eV at zero strain to 3.002 and 2.626 eV at ε = 8%. Deformation potential (DP) models based on longitudinal acoustic phonon (LAP) and optical phonon (OP) scattering are used to investigate mobility. The mobility of (C₄H₉NH₃)₂GeI₄ is lower than that of (C₄H₉NH₃)₂SnI₄. It is mainly determined by the scattering from OP with lower energy and decreases sharply with an increase in biaxial strain. Compared with Pb based perovskites, (C₄H₉NH₃)₂SnI₄ exhibits high carrier mobility and thermodynamic stability.

Tunable electronic properties of the dynamically stable layered mineral Pt2HgSe3 (Jacutingaite)

Density functional theory calculations are performed in order to study the structural and electronic properties of monolayer Pt₂HgSe₃. Effects of uniaxial and biaxial strain, layer thickness, electric field and out-of-plane pressure on the electronic properties are systematically investigated.

The mechanical, electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si, Ge, Sn) monolayers: a first-principles calculations

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure.

Modulating the electro-optical properties of doped C3N monolayers and graphene bilayers via mechanical strain and pressure

The effect of atomic doping on the electronic properties of C₃N monolayer and graphene bilayer is investigated. We found that doped C₃N monolayer and doped graphene bilayer are a direct semiconductor. Our result show that the electronic properties of the studied structures can be modulated by electric field and mechanical strain.