Multifunctional Binary Monolayers Ge xP y: Tunable Band Gap, Ferromagnetism, and Photocatalyst for Water Splitting

Group III (In/Ga)-V (P/As)-VI (S/Se) Monolayers: A New Class of Auxetic Semiconductors with Highly Anisotropic Electronic/Optical/Mechanical/Thermal Properties

We present a theoretical design of a class of 2D semiconducting materials, namely, group III (In/Ga)-V (P/As)-VI (S/Se) monolayers, whose global-minimum structures are predicted based on the particle swarm optimization method. Electronic structure calculations suggest that all group III-V-VI monolayers exhibit quasi-direct semiconducting characteristics with desirable band gaps ranging from 1.76 to 2.86 eV (HSE06 functional). Moreover, most group III-V-VI monolayers possess highly anisotropic carrier mobilities with large anisotropic ratios (3.4-6 for electrons, 2.2-25 for holes). G0W0+BSE calculations suggest that these monolayers show high optical anisotropy and relatively large exciton binding energies (0.33-0.75 eV), comparable to that (0.5 eV) of MoS2 monolayer. In particular, the GaPS monolayer manifests strikingly anisotropic I-V curves with a large ON/OFF ratio of ∼105 (106 for the GaPS bilayer) and anisotropic lattice thermal conductivity. Furthermore, the GaPS monolayer is predicted to exhibit both in-plane and out-of-plane negative Poisson ratios (NPRs) and prominent anisotropic Young moduli.

Monolayer Ge2Te2P4 as a promising photocatalyst for solar driven water-splitting: a DFT study

The buckling hexagonal structure of Ge2Te2P4 was studied by first-principles calculations. The newly proposed structure was proven to be stable by analyzing its cohesive energy, phonon dispersion, elastic constants and AIMD results. Poisson's ratio of the Ge2Te2P4 monolayer is in the range 0.16-0.18, and Young's modulus is in the range 40.16-43.74 N m-1. The substituted Te atoms enhance the sp2 orbitals which strengthen the σ-bonds and therefore the thickness of the Ge2Te2P4 monolayer is smaller than that of monolayer GeP3. The Ge2Te2P4 monolayer has an indirect band gap of 1.85 eV, which can be narrowed by strains. The compressive band gaps from -2% to -4% change the electronic structure from the indirect band gap into the direct band gap. Strains can also increase the light absorption rate α(ω) in the visible region, which is 2-3 × 105 cm-1 at equilibrium. The Ge2Te2P4 monolayer has a suitable band gap and an appropriate VBM and CBM position for hydrogen generation. Under strain rate of 4% and higher, the VBM and CBM remain at suitable positions for hydrogen production. Another advantage of the Ge2Te2P4 monolayer is that its charge carrier mobilities are really high. The highest electron mobility is 1301.47 cm2 V-1 s-1, and the highest hole mobility is 28627.24 cm2 V-1 s-1, which are much higher than the mobility in monolayer GeP3. The Ge2Te2P4 monolayer has advantages for photocatalytic applications and it is necessary to perform further study on the material.

Penta-MP5 (M = B, Al, Ga, In) monolayers as high-performance photocatalysts for overall water splitting

Two-dimensional (2D) phosphorus-rich phosphides generally preserve the excellent electronic properties of phosphorene, making them promising photocatalysts for water splitting. Despite tremendous efforts in the search for potential photocatalysts in 2D phosphides, few known 2D phosphides fully meet the requirements for photocatalytic water splitting. Herein, we systemically investigate a set of penta-MP5 (M = B, Al, Ga, and In) monolayers by first-principles calculations and identify them as potential photocatalysts for water splitting. These penta-MP5 monolayers are found to feature favorable bandgaps of about 2.70 eV with appropriate band edge positions, a high carrier mobility of 1 × 104 cm-2 V-1 s-1, an excellent optical absorption coefficient (OAC) of 1 × 105 cm-1, and a good solar-to-hydrogen (STH) efficiency of 8%. Meanwhile, free energy calculations indicate that these penta-MP5 monolayers present both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) photocatalytic activities under light conditions. All these excellent properties demonstrate that penta-MP5 monolayers are suitable candidates as photocatalysts for promising applications in overall water splitting.

Anisotropic Mechanical Properties of Orthorhombic SiP2 Monolayer: A First-Principles Study

In recent years, the two-dimensional (2D) orthorhombic SiP2 flake has been peeled off successfully by micromechanical exfoliation and it exhibits an excellent performance in photodetection. In this paper, we investigated the mechanical properties and the origin of its anisotropy in an orthorhombic SiP2 monolayer through first-principles calculations, which can provide a theoretical basis for utilizing and tailoring the physical properties of a 2D orthorhombic SiP2 in the future. We found that the Young's modulus is up to 113.36 N/m along the a direction, while the smallest value is only 17.46 N/m in the b direction. The in-plane anisotropic ratio is calculated as 6.49, while a similar anisotropic ratio (~6.55) can also be observed in Poisson's ratio. Meanwhile, the in-plane anisotropic ratio for the fracture stress of the orthorhombic SiP2 monolayer is up to 9.2. These in-plane anisotropic ratios are much larger than in black phosphorus, ReS2, and biphenylene. To explain the origin of strong in-plane anisotropy, the interatomic force constants were obtained using the finite-displacement method. It was found that the maximum of interatomic force constant along the a direction is 5.79 times of that in the b direction, which should be considered as the main origin of the in-plane anisotropy in the orthorhombic SiP2 monolayer. In addition, we also found some negative Poisson's ratios in certain specific orientations, allowing the orthorhombic SiP2 monolayer to be applied in next-generation nanomechanics and nanoelectronics.

Pentagonal CmXnY6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P) Monolayers: Janus Ternaries Combine Omnidirectional Negative Poisson Ratios with Giant Piezoelectric Effects

Two-dimensional (2D) materials composed of pentagon and Janus motifs usually exhibit unique mechanical and electronic properties. In this work, a class of ternary carbon-based 2D materials, C_mX_nY_6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P), are systematically studied by first-principles calculations. Six of 21 Janus penta-C_mX_nY_6-m-n monolayers are dynamically and thermally stable. The Janus penta-C₂B₂Al₂ and Janus penta-Si₂C₂N₂ exhibit auxeticity. More strikingly, Janus penta-Si₂C₂N₂ exhibits an omnidirectional negative Poisson ratio (NPR) with values ranging from -0.13 to -0.15; in other words, it is auxetic under stretch in any direction. The calculations of piezoelectricity reveal that the out-of-plane piezoelectric strain coefficient (d₃₂) of Janus panta-C₂B₂Al₂ is up to 0.63 pm/V and increases to 1 pm/V after a strain engineering. These omnidirectional NPR, giant piezoelectric coefficients endow the Janus pentagonal ternary carbon-based monolayers as potential candidates in the future nanoelectronics, especially in the electromechanical devices.

MgXN2 (X = Hf/Zr) Monolayers: Auxetic Semiconductor with Highly Anisotropic Optical/Mechanical Properties and Carrier Mobility

Two-dimensional (2D) semiconducting materials with distinct anisotropic physical properties have attracted intense interests. Herein, we show theoretical predictions that MgXN₂ (X = Hf/Zr) monolayers are auxetic semiconductors with highly anisotropic electronic, optical, and mechanical properties. The density functional theory calculations coupled with a PSO algorithm (global-minimum search) suggest that both MgHfN₂ (MgZrN₂) monolayers exhibit orthorhombic symmetry (Pmma) and are direct-gap (indirect-gap) semiconductors with a bandgap of 2.43 eV (2.13 eV). Specifically, the MgHfN₂ monolayer exhibits highly anisotropic hole mobility as well as very high electron mobility (∼10⁴ cm² V^-1 s^-1). G₀W₀+BSE calculations indicate that both monolayers bear notable optical anisotropy and relatively large exitonic binding energy (∼0.6 eV). In addition, both monolayers acquire remarkable mechanical anisotropy with a negative in-plane Poisson's ratio (∼-0.2) and high Young's modulus (∼260 N/m). The combination of highly anisotropic electronic, optical, and mechanical properties endows MgXN₂ monolayers as potentially useful parts in multifunctional nanoelectronic devices.

Prediction of Two-Dimensional Group IV Nitrides AxNy (A = Sn, Ge, or Si): Diverse Stoichiometric Ratios, Ferromagnetism, and Auxetic Mechanical Property

In this work, we unveiled a new class of two-dimensional (2D) group IV nitride A_xN_y (A = Sn, Ge, or Si) prototypes, C2/m A₄N, P3̅m1 A₃N, P3m1 A₂N, P3̅m1 A₃N₂, P6̅m2 AN, P3̅m1 AN, P6̅2m A₃N₄, P3m1 A₂N₃, P4̅2₁m AN₂, and P3̅m1 AN₃, by using evolutionary algorithms combined with first-principles calculations. Using HSE06 functional calculations, a wide range of band gaps from metal to semiconductor (0.405-5.050 eV) and ultrahigh carrier mobilities (1-24 × 10³ cm² V^-1 s^-1) were evidenced in these 2D structures. We found that 2D P3m1 Sn₂N₃, Ge₂N₃, and Si₂N₃ are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N₂ charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D P4̅2₁m SiN₂. Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.

Binary pentagonal auxetic materials for photocatalysis and energy storage with outstanding performances

Since the discovery of penta-graphene, two-dimensional (2-D) pentagonal-structured materials have been highly expected to have desirable performance because of their unique structures and accompanied physical properties. Hence, based on the first-principles calculations, we performed a systematical study on the structure, stability, mechanical and electronic properties, and potential applications on carbon-based pentagonal materials with binary compositions, namely, Penta-C_nX_6-n (n = 1, 2, 4, 5; X = B, N, Al, Si, P, Ga, Ge, As). We found that eleven out of thirty-two Penta-C_nX_6-n have good stability and can be further studied. Among them, two materials, namely, Penta-C₄P₂ and Penta-C₅P are metallic, and others are indirect band gap semiconductors, whose band gaps calculated by the HSE06 functional are in the range of 1.37-6.43 eV, covering the infrared-visible-ultraviolet regions. Furthermore, we found that metallic Penta-C_nX_6-n can become promising anode materials for Na-ion batteries (NIBs) with high storage capacity, while some semiconducting Penta-C_nX_6-n can become excellent water splitting photocatalysts. In addition, Penta-C₄P₂ and Penta-C₂Al₄ were found to have obvious in-plane negative Poisson's ratio (NPR) of -0.083 and -0.077, respectively. More interestingly, we found that Penta-C₂Al₄ exhibits a peculiar in-plane half negative Poisson's ratio (H-NPR) with the fundamental mechanism clarified. These outstanding performances endow binary pentagonal materials with excellent application prospects.

MoS2 and Janus (MoSSe) based 2D van der Waals heterostructures: emerging direct Z-scheme photocatalysts

Two-dimensional (2D) materials, viz. transition metal dichalcogenides (TMD) and transition metal oxides (TMO), offer a platform that allows the creation of heterostructures with a variety of properties. The optoelectronic industry has observed an upheaval in the research arena of MoS₂ based van der Waals (vdW) heterostructures (HTSs) and Janus structures. Therefore, interest towards these structures is backed by the ability to select their electronic and optical properties. The present study investigates the photocatalytic abilites of bilayer, MoS₂ and Janus (MoSSe) based vdW HTSs, viz. MoS₂/TMO, MoS₂/TMD, MoSSe/TMO and MoSSe/TMD, by a first-principles based approach under the framework of (hybrid) density functional theory (DFT) and many body perturbation theory (GW approximation). We have considered HfS₂, ZrS₂, TiS₂ and WS₂, and HfO₂, T-SnO₂ and T-PtO₂ from the families of TMDs and TMOs, respectively. The photocatalytic properties of these vdW HTSs are thoroughly investigated and compared with their respective individual monolayers by visualizing their band edge alignments, electron-hole recombination and optical properties. Strikingly, we observe that, despite most of the individual monolayers not performing optimally as photocatalysts, type II band edge alignment is noticed in vdW HTSs and they appear to be efficient photocatalysts via the Z-scheme. Moreover, these vdW HTSs have also shown promising optical responses in the visible region. Finally, electron-hole recombination, H₂O adsorption and hydrogen evolution reaction (HER) results establish that MoSSe/HfS₂, MoSSe/TiS₂, MoS₂/T-SnO₂ and MoSSe/ZrS₂ are probable highly efficient Z-scheme photocatalysts.

Two-Dimensional IV-V Monolayers with Highly Anisotropic Carrier Mobility and Electric Transport Properties

Two-dimensional (2D) semiconductors with anisotropic properties (e.g., mechanical, optical, and electric transport anisotropy) have long been sought in materials research, especially 2D semiconducting sheets with strong anisotropy in carrier mobility, e.g., n-type in one direction and p-type in another direction. Here, we report a comprehensive study of the carrier mobility and electric transport anisotropy of a class of 2D IV-V monolayers, XAs (X = Si or Ge), by using density functional theory methods coupled with deformation potential theory and non-equilibrium Green's function method. We find that the polarity of room-temperature carrier mobility μ of the 2D XAs monolayer is highly dependent on the lattice direction. In particular, for the SiAs monolayer, the μ values of the electron (e) and hole (h) are 1.25 × 10³ and 0.39 × 10³ cm² V^-1 s^-1, respectively, in the a direction and 0.31 × 10³ and 1.12 × 10³ cm² V^-1 s^-1, respectively, for the b direction. The computed electric transport properties also show that the SiAs monolayer exhibits strong anisotropy in the biased voltage in the range of -1 to 1 V. In particular, the current reflects the ON state in the a direction but the OFF state in the b direction. In addition, we find that the uniaxial strain can significantly improve the electric transport performance and even lead to the negative differential conductance at 10% strain. The unique transport properties of the 2D XAs monolayers can be exploited for potential applications in nanoelectronics.

2D AlP3 with high carrier mobility and tunable band structure

The exploration of new monolayer materials always attracts much attention due to the extraordinary properties and promising applications. Here we predict two monolayered aluminum triphosphides (AlP₃) with C2/m and P3m1 space groups with a tunable bandgap under strain as the new members of the 2D XP₃ family by using the first principles calculations. The stabilities of the predicted structures are confirmed with the phonon dispersion curves and molecular dynamics. Unlike the narrow bandgaps of the reported XP₃ monolayers, the larger bandgaps of 1.78 (HSE06) or 1.91 eV (G₀W₀) for C2/m and 1.42 (HSE06) or 2.14 eV (G₀W₀) for P3m1 AlP₃ monolayers are observed. The high mobility of 1.01  ×  10⁵ and 1.62  ×  10⁴ cm² V^-1 s^-1 are observed for the electron of P3m1 and the hole of C2/m. The optical absorptions of the AlP₃ monolayers, in particular, the one with C2/m, are obviously strong in the visible light range. These results imply that the monolayers are promising in the optoelectronic application. Unfortunately, the undesirable band edges make them not suitable for water splitting in spite of the strong optical absorption coefficient in the visible light range. However, an obvious effect of strain engineering is demonstrated for the monolayers. Under  -2% and  -3% biaxial strain, the band edges of P3m1 AlP₃ can straddle the redox potential of water and meet the requirement of photocatalytic water splitting. Therefore, the P3m1 AlP₃ monolayer can also be a promising candidate for the photocatalytic water splitting to produce hydrogen driven by the visible light.

One-step co-precipitation synthesis of novel BiOCl/CeO2composites with enhanced photodegradation of rhodamine B

BiOCl/CeO₂composites were synthesized by a facile one-step co-precipitation method and showed good photodegradation activity of rhodamine B (RhB).

Even–odd oscillation of bandgaps in GeP3 nanoribbons and a tunable 1D lateral homogenous heterojunction

Band gap of armchair GeP₃ nanoribbons shows strong even-odd oscillation as a function of ribbon width. Based on this unique feature, a one dimensional lateral homogenous heterojunction is designed to investigate the potential application.

P3Cl2: A Unique Post-Phosphorene 2D Material with Superior Properties against Oxidation

Herein, a unique class of post-phosphorene materials, namely, phosphorene halogenides (e.g., α-P₃Cl₂) with superior oxidation resistance and desirable bandgap characteristics, are proposed. Our first-principles computations show that monolayer α-P₃Cl₂ is a direct semiconductor with a wide bandgap of 2.41 eV (HSE06) or 4.02 eV (G₀W₀), while the bandgap exhibits only slight reduction with increasing number of layers. The monolayer α-P₃Cl₂ also possesses highly anisotropic carrier mobility, with both ultrahigh electron mobility (56 890 cm² V^-1 s^-1) and hole mobility (26 450 cm² V^-1 s^-1). Meanwhile, the outstanding optical properties and favorable band alignment of 2D P₃Cl₂ suggest its potential as a photocatalyst for visible-light water splitting. 2D α-P₃X₂ (X = F, Br, I) also exhibit good oxidation resistance and possess wide direct bandgaps ranging from 2.16 to 2.43 eV (HSE06). These unique electronic and optical properties render 2D phosphorene halogenide as promising functional materials for broad applications in electronic and optoelectronic devices.

Two-Dimensional Janus Transition Metal Oxides and Chalcogenides: Multifunctional Properties for Photocatalysts, Electronics, and Energy Conversion

The fast development of high-performance devices for diverse applications requires nanoscale materials with multifunctional properties, motivating theoretical exploration into novel two-dimensional (2D) materials. In this work, we propose a new family of 2D nanomaterials, Janus transition metal oxides and chalcogenides MXY (M = Ti, Zr, or Hf; X = S or Se; Y = O or S; X ≠ Y) monolayers, for their versatile applications. We find that the Janus MXY monolayers are semiconductors with a wide range of band gaps ranging from 0.739 to 2.884 eV. We show that TiSO, ZrSO, and HfSO monolayers are promising candidates for photocatalysis because of their suitable band gaps and optimal redox potentials for water splitting, and ZrSeS and HfSeS monolayers are suitable candidates for nanoscale electronics because of their high carrier mobility. We further show that TiSO, ZrSO, and ZrSeO monolayers possess large piezoelectric properties because of the broken inversion symmetry stemmed from the different atomic sizes and electronegativities of the X and Y elements, which are better or comparable to other 2D and bulk piezoelectric materials. Our study demonstrates that the 2D Janus MXYs may find versatile applications into photocatalysts, electronics, sensors, and energy harvesting/conversion.