Novel two-dimensional HfSi2N4 monolayer with excellent bandgap modulation and electronic properties modulation

The bandgap modulation and electronic properties modulation of two-dimensional HfSi2N4 monolayer induced by strain, electric field and atomic adsorption are studied by first principles. The HfSi2N4 monolayer was found to be dynamically, thermally, and mechanically stable at equilibrium, and it is a direct semiconductor with a bandgap of 1.87 eV. The bandgap of the HfSi2N4 monolayer can be precisely modulated by strain. Under the action of strain, HfSi2N4 monolayer not only transforms from direct semiconductor to indirect semiconductor, but also improves the absorption of visible light. An external electric field in the 0-0.5 eV/Å range can also modulate the bandgap of HfSi2N4 monolayer from 1.87 eV to 0 eV, and most importantly, at an external electric field of 0.5 eV/Å, HfSi2N4 monolayer shows the characteristics of spin gapless semiconductor. The calculated adsorption energy shows that the structures of H, O and F atoms adsorbed by HfSi2N4 monolayer can all exist stably. The bandgap of the configuration after adsorption of O and F atoms is significantly reduced compared with that of HfSi2N4 monolayer. Furthermore, the HfSi2N4 monolayer after adsorption of H and F atoms is transformed into a magnetic semiconductor. METHOD: All calculations were performed using Vienna ab initial simulation package, The electronic structure, mechanical properties, electronic properties and other properties were carried out using generalized gradient approximation (GGA-PBE), supplemented by HSE06 and GGA + U. The total-energy and force convergence are less than 10-6 eV and 0.001 eV/Å, respectively. The vacuum on the z-axis is selected 20 Å. The vdW interactions were corrected using the Grimme scheme (DFT-D3).

External fields effectively switch the spin channels of transition metal-doped β-phase tellurene from first principles

Non-volatile magnetic random-access memories have proposed the need for spin channel switching. However, this presents a challenge as each spin channel reacts differently to the external field. Tellurene is a semiconductor with a tunable bandgap, excellent stability, and high carrier concentration, but its lack of magnetic properties has hindered its application in spintronics. In this work, the influence of an external field on transition metal (TM)-doped β-tellurene is systematically analysed from first principles. First, the active-learning moment-tensor-potential (MTP) is used to verify the thermal stability of the V-doped system with the MTP proving to be 900 times faster than the traditional method. Subsequently, under biaxial strain ranging from -2% to 10%, the V-doped system undergoes a gradual transition from a magnetic semiconductor to a spin-gapless semiconductor, and further to a half-metal and magnetic metal. The band structure can be maintained under an electric field. By examining the magnetic anisotropy energy, the lattice changes profoundly impact the electromagnetic properties, particularly with the TMs being sensitive to strain. Moreover, the band structure is reflected in the spin resolution current of the magnetic tunnel junction. This work investigates the response of doped β-Te to external fields, revealing its potential applications in spintronics.

Transition metal doped WSi2N4monolayer for water splitting electrocatalysts: a first-principles study

High-performance water splitting electrocatalysts are urgently needed in the face of the environmental degradation and energy crisis. The first principles method was used in this study to systematically examine the electronic characteristics of transition metal (Sc, Ti, V, Cr, Mn, Fe, and Ru) doped WSi2N4(TM@WSi2N4) and its potential as oxygen evolution reaction (OER) catalysts. Our study shows that the doping of TM atoms significantly improves the catalytic performance of TM@WSi2N4, especially Fe@WSi2N4shows a low overpotential (ηOER= 470 mV). Interestingly, we found that integrated-crystal orbital Hamilton population and d-band center can be used as descriptors to explain the high catalytic activity of Fe@WSi2N4. Subsequently, Fe@WSi2N4exhibits the best hydrogen evolution reaction (HER) activity with a universal overpotential of 47 mV on N1sites. According to our research, Fe@WSi2N4offers a promising substitute for precious metals as a catalyst for overall water splitting with low OER and HER overpotentials.

Electronic and Excitonic Properties of MSi2 Z4 Monolayers

MA₂ Z₄ monolayers form a new class of hexagonal non-centrosymmetric materials hosting extraordinary spin-valley physics. While only two compounds (MoSi₂ N₄ and WSi₂ N₄ ) are recently synthesized, theory predicts interesting (opto)electronic properties of a whole new family of such two-dimensional (2D) materials. Here, the chemical trends of band gaps and spin-orbit splittings of bands in selected MSi₂ Z₄ (M = Mo, W; Z = N, P, As, Sb) compounds are studied from first-principles. Effective Bethe-Salpeter-equation-based calculations reveal high exciton binding energies. Evolution of excitonic energies under external magnetic field is predicted by providing their effective g-factors and diamagnetic coefficients, which can be directly compared to experimental values. In particular, large positive g-factors are predicted for excitons involving higher conduction bands. In view of these predictions, MSi₂ Z₄ monolayers yield a new platform to study excitons and are attractive for optoelectronic devices, also in the form of heterostructures. In addition, a spin-orbit induced bands inversion is observed in the heaviest studied compound, WSi₂ Sb₄ , a hallmark of its topological nature.

Effect of vacancy defects on the electronic and mechanical properties of two-dimensional MoSi2N4

MoSi₂N₄ is a recently fabricated 2-dimensional indirect bandgap semiconductor material that has attracted interest in various fields due to its promising properties. A defect-based thorough and reliable investigation of its physical properties is indispensable in this regard to explore its industrial applications in the future. In this work, a comprehensive vacancy defect-based analysis of the electronic and mechanical characteristics of this material is conducted with varying defect percentages. We have analyzed the gradual change in electronic properties of MoSi₂N₄ by performing first-principles density functional theory-based investigation and presented a detailed analysis for point vacancies ranging from 0.297% to 14.29%, revealing the transition of this monolayer from the semiconductor to metal phase. The gradual change in mechanical properties due to the defect introduction has also been reported and analyzed, where the Young's modulus, Poisson ratio, elastic constant, etc. are calculated by the stress-strain method using Matrix Sets (OHESS). Further, we extend the investigation to the exploration of thermal and topological characteristics and report the triviality of the MoSi₂N₄ material as well as the effect on specific heat, entropy, and free energy with respect to temperature. We believe that the results presented in this study could assist the process of incorporating MoSi₂N₄ in future 2D electronics.

Piezoelectric ferromagnetism in Janus monolayer YBrI: a first-principles prediction

Coexistence of intrinsic ferromagnetism and piezoelectricity, namely piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI is a PFM, which is dynamically, mechanically and thermally stable. The electronic correlation effects on the physical properties of YBrI are investigated by using generalized gradient approximation plus U (GGA+U) approach. For out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and its valley splitting is larger than 82 meV within the considered U range. The anomalous valley Hall effect (AVHE) can be achieved under an in-plane electric field. However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM) semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of YBrI is always in-plane, and its magnetic anisotropy energy (MAE) varies from 0.309 meV to 0.237 meV (U = 0.0 eV to 3.0 eV). However, the magnetization can be adjusted from the in-plane to out-of-plane direction by an external magnetic field, and then lead to the occurrence of valley polarization. Moreover, the missing centrosymmetry along with broken mirror symmetry results in both in-plane and out-of-plane piezoelectricity in the YBrI monolayer. At a typical U = 2.0 eV, the piezoelectric strain coefficient d₁₁ is predicted to be -5.61 pm V^-1, which is higher than or comparable with the ones of other known two-dimensional (2D) materials. The electronic and piezoelectric properties of YBrI can be effectively tuned by applying biaxial strain. For example, tensile strain can enhance valley splitting and d₁₁ (absolute value). The predicted magnetic transition temperature of YBrI is higher than those of experimentally synthesized 2D FM materials CrI₃ and Cr₂Ge₂Te₆. Our findings of these distinctive properties could pave the way for designing multifunctional spintronic devices, and bring forward a new perspective for constructing 2D materials.

Mini-review of interesting properties in Mn2CoAl bulk and films

Heusler compounds exhibit many interesting properties, such as high thermopower, magnetocaloric properties, and even topological insulator states. Heusler Mn₂CoAl alloy has been experimentally and theoretically proposed as a promising spin-gapless semiconductor with novel electronic, magnetic, spintronic, transport, and topological properties. Furthermore, the spin-gapless semiconducting-like behaviors are also predicted in Mn₂CoAl films by measuring the transport and magnetic properties. This mini-review systematically summarizes the interesting properties of Mn₂CoAl bulk and Mn₂CoAl-based films. This mini-review is hoped to guide further experimental investigations and applications in the particular scientific community.

Nitric oxide electrochemical reduction reaction on transition metal-doped MoSi2N4 monolayers

Nitric oxide electrochemical reduction (NOER) reactions are usually catalyzed by noble metals. However, the commercial applications are limited by the low atomic utilization and high price, which prompt researchers to turn their attentions to single-atom catalysts (SACs). Recently, a novel two-dimensional semiconducting material MoSi₂N₄ (MSN) has been synthesized and is suitable for the substrate of SACs due to its high stability, carrier mobility and mechanical strength. Herein, we employed first principles calculations to investigate the catalytic properties of transition metal doped MoSi₂N₄ monolayers (labelled as TM-MSN, where TM is a transition metal atom from 3d to 5d except Y, Tc, Cd, La-Lu and Hg) in NO reduction. The calculated results demonstrate that the introduction of Zr, Pd, Pt, Mn, Au, or Mo atoms can greatly improve the catalytic NOER performance of a pristine MSN monolayer. Zr-MSN and Pt-MSN monolayers at low coverage exhibit the most superior catalytic activity and selectivity for NH₃ production with a limiting potential of 0 and -0.10 V. This work may help guide the application of MSN monolayer in the area of energy conversion.

Monolayer MSi2P4 (M = V, Nb, and Ta) as Highly Efficient Sulfur Host Materials for Lithium-Sulfur Batteries

Despite the high capacity and low cost of lithium-sulfur (Li-S) batteries, their commercialization is greatly blocked by multiple bottlenecks including the shuttle effect of lithium polysulfides (LiPSs), poor conductivity of sulfur, and sluggish reaction kinetics. Herein, we propose novel two-dimensional MSi₂P₄ (M = V, Nb, and Ta) monolayers as promising sulfur hosts to improve the Li-S battery performance. Our calculations show that MSi₂P₄ monolayers offer moderate binding strengths to the polysulfides, which are expected to effectively inhibit the LiPS shuttling and dissolution. Moreover, the conductive properties of the MSi₂P₄ systems are well maintained after LiPS adsorption, eliminating the insulating nature of sulfur species. Remarkably, MSi₂P₄ monolayers exhibit superior electrocatalytic activity for the sulfur reduction reaction and the Li₂S decomposition reaction, which considerably lowers the energy barriers of LiPS conversions during discharge and charge, thus ensuring the fast redox kinetics and high sulfur utilization of Li-S batteries. This study pioneers the application of MSi₂P₄ monolayers as highly efficient sulfur host materials for Li-S batteries and affords insights for further development of advanced Li-S batteries.

Substitutional transition metal doping in MoSi2N4 monolayer: structural, electronic and magnetic properties

Monolayer MoSi₂N₄ (MoSiN) was successfully synthesized last year [Hong et al., Science369, 670 (2020)]. The MoSiN monolayer exhibited semiconducting characteristics and exceptional ambient stability, calling for more studies of its properties. Here, we conduct first-principle calculations to examine the structural, magnetic, and electronic properties of substitutional doping of MoSiN monolayers with transition metals (TM) at the Mo site (TM-MoSiN). We find that the Sc-, Y-, Ti-, and Zr-MoSiN are metallic systems, while Mn-, Tc-, and Ru-MoSiN are n-type conducting. The Fe-MoSiN is a dilute magnetic semiconductor, and the Ni-MoSiN is a metal (or half-metal). The inclusion of spin-orbit coupling turns them into a half-metal and a semimetal, respectively. We also find that the work function of TM-MoSiN and the bond lengths between the TM and neighbor atoms increase as the atomic radius and electronegativity of the TM atom increase, respectively. The Fe-, Co-, and Ni-MoSiN may be used in spintronic devices, while Mn-, Rh- and Pd-MoSiN could be utilized for spin filter applications.

Coexistence of intrinsic room-temperature ferromagnetism and piezoelectricity in monolayer BiCrX3 (X = S, Se, and Te)

Two-dimensional (2D) materials with intrinsic ferromagnetism and piezoelectricity have received growing attention due to their potential applications in nanoscale spintronic devices. However, their applications are highly limited by the low Curie temperatures (T_C) and small piezoelectric coefficients. Here, using first-principles calculations, we have successfully predicted that BiCrX₃ (X = S, Se, and Te) monolayers simultaneously possess ferromagnetism and piezoelectricity by replacing one layer of Bi atoms with Cr atoms in Bi₂X₃ monolayers. Our results demonstrate that BiCrX₃ monolayers are not only intrinsic ferromagnetic semiconductors with indirect band gaps, adequate T_C values of higher than 300 K, and significant out-of-plane magnetic anisotropic energies, but also exhibit appreciable in-plane and out-of-plane piezoelectricity. In particular, the in-plane piezoelectric coefficients of BiCrX₃ monolayers with ABCAB configuration are up to 15.16 pm V^-1, which is higher than those of traditional three-dimensional piezoelectric materials such as α-quartz. The coexistence of ferromagnetism and piezoelectricity in BiCrX₃ monolayers gives them promising applications in spintronics and nano-sized sensors.

Two-dimensional type-II XSi2P4/MoTe2 (X = Mo, W) van der Waals heterostructures with tunable electronic and optical properties

The type-II MoSi2P4/MoTe2 (WSi2P4/MoTe2) possesses a direct bandgap of 0.258 eV (0.363 eV) at the PBE level and shows promise for application in the nanoelectronic and optoelectronic fields.

Layered post-transition-metal dichalcogenide SnGe2N4 as a promising photoelectric material: a DFT study

First-principles calculations were performed to study a novel layered SnGe₂N₄ compound, which was found to be dynamically and thermally stable in the 2H phase, with the space group P6̄m2 and lattice constant a = 3.143 Å. Due to its hexagonal structure, SnGe₂N₄ exhibits isotropic mechanical properties on the x–y plane, where the Young’s modulus is 335.49 N m⁻¹ and the Poisson’s ratio is 0.862. The layered 2H SnGe₂N₄ is a semiconductor with a direct band gap of 1.832 eV, allowing the absorption of infrared and visible light at a rate of about 10⁴ cm⁻¹. The DOS is characterized by multiple high peaks in the valence and conduction bands, making it possible for this semiconductor to absorb light in the ultraviolet region with an even higher rate of 10⁵ cm⁻¹. The band structure, with a strongly concave downward conduction band and rather flat valence band, leads to a high electron mobility of 1061.66 cm² V⁻¹ s⁻¹, which is substantially greater than the hole mobility of 28.35 cm² V⁻¹ s⁻¹. This difference in mobility is favorable for electron–hole separation. These advantages make layered 2H SnGe₂N₄ a very promising photoelectric material. Furthermore, the electronic structure of 2H SnGe₂N₄ responds well to strain and an external electric field due to the specificity of the p–d hybridization, which predominantly constructs the valence bands. As a result, strain and external electric fields can efficiently tune the band gap value of 2H SnGe₂N₄, where compressive strain widens the band gap, meanwhile tensile strain and external electric fields cause band gap reduction. In particular, the band gap is decreased by about 0.25 eV when the electric field strength increases by 0.1 V Å⁻¹, making a semiconductor–metal transition possible for the layered SnGe₂N₄.

The promising photoelectric semiconductor 2H SnGe₂N₄ has a tunable electronic structure which is favorable for the absorption of light in the infrared and visible regions.

VSi2P4/FeCl2 van der Waals heterostructure: A two-dimensional reconfigurable magnetic diode

The reconfigurable magnetic tunnel diode has recently been proposed as a promising approach to decrease the base collector leakage currents. However, conventional bulk interfaces usually suffer from strong Fermi level...

Structural Symmetry, Spin-Orbit Coupling, and Valley-Related Properties of Monolayer WSi2N4 Family

Recently prepared layered MoSi₂N₄ exhibits excellent stability and semiconductor properties, adding building blocks for two-dimensional families. In this research, we present the spin-orbit coupling and valley-related properties of monolayer WSi₂N₄ family. Better than transition metal dichalcogenides, the structural symmetry of WSi₂N₄ monolayer can be different by changing the stacking of three parts in the monolayers, resulting in a Rashba spin-orbit field. The vertical and horizontal polarization will lift the degeneration of the in-plane and out-of-plane polarized spin, respectively. The gradient of potential energy and the proportion of d orbitals play dominant roles. The in-plane orbitals contribute to the out-of-plane spin polarization, while the out-of-plane orbitals contribute to the in-plane spin polarization. The characteristics of a Rashba semiconductor can be utilized in spin/valley Hall effects, as well as the regulation of the spin direction of the valley electrons, promoting the manipulation of multiple degrees of freedom of electrons in monolayer materials.

Highly adjustable piezoelectric properties in two-dimensional LiAlTe2by strain and stacking

Two-dimensional (2D) piezoelectric materials have attracted wide attention because they are of great significance to the composition of piezoelectric nanogenerators. In this work, we have systematically studied the piezoelectric properties of 2D LiAlTe₂by using the first-principles calculation and found the 2D LiAlTe₂monolayer exhibits both large in-plane piezoelectric coefficientd₁₁(3.73 pm V^-1) and out-of-plane piezoelectric coefficientd₃₁(0.97 pm V^-1). Moreover, the piezoelectric coefficients of 2D LiAlTe₂are highly tunable by strain and stacking. When different uniaxial strains are applied,d₁₁changes dramatically, butd₃₁changes little. When 2% stretching is applied to 2D LiAlTe₂monolayer along thex-axis,d₁₁reaches 7.80 pm V^-1, which is twice as large as the previously reported 2D piezoelectric material MoS₂. Both AA stacking and AB stacking can enhance the piezoelectric properties of 2D LiAlTe₂, but they have different effects on in-plane and out-of-plane piezoelectric coefficients. AA stacking can greatly increased₃₁but has little impact ond₁₁. In the case of four-layer AA stacking, thed₃₁reaches 3.32 pm V^-1. AB stacking can both increased₁₁andd₃₁, butd₁₁grows faster thand₃₁as the number of layers increases. In the case of four-layer AB stacking,d₁₁reaches 18.05 pm V^-1. The excellent and highly tunable piezoelectric performance provides 2D LiAlTe₂greater potential for the application of piezoelectric nano-generators and other micro-nano piezoelectric devices.

Novel Two-Dimensional MA2N4 Materials for Photovoltaic and Spintronic Applications

We have systematically investigated a family of newly proposed two-dimensional MA₂N₄ materials (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; A = Si, Ge) using first-principles calculation. We categorize the potential of these materials into three different applications based on accurate simulation of band gap (using Hybrid HSE06 functional) and the associated descriptors. Three candidate materials (MoGe₂N₄, HfSi₂N₄, and NbSi₂N₄) turn out to be extremely promising for three different applications. MoGe₂N₄ and HfSi₂N₄ monolayers show strong optical absorption in the visible range, including high transition probability from the valence to conduction band. The GW+BSE calculations confirm a strong excitonic effect in both the systems. With a band gap of 1.42 eV, multilayer MoGe₂N₄ shows reasonably large simulated efficiency (∼15.40%) and hence can be explored for possible photovoltaic applications. High optical absorption, suitable band gap/edge positions, and the CO₂ activation make HfSi₂N₄ monolayer a promising candidate for photocatalytic CO₂ reduction. NbSi₂N₄, on the other hand, belongs to a new class of spintronic material called a bipolar magnetic semiconductor, recommended for spin-transport-based applications.

Predicted intrinsic piezoelectric ferromagnetism in Janus monolayer MnSbBiTe4: a first principles study

Two-dimensional (2D) piezoelectric ferromagnetism (PFM) is essential for the development of the next-generation multifunctional spintronic technologies. Recently, the layered van der Waals (vdW) compound MnBi₂Te₄ as a platform to realize the quantum anomalous Hall effect (QAHE) has attracted great interest. In this work, the Janus monolayer MnSbBiTe₄ with dynamic, mechanical and thermal stabilities is constructed from a synthesized non-piezoelectric MnBi₂Te₄ monolayer by replacing the top Bi atomic layer with Sb atoms. The calculated results show that monolayer MnSbBiTe₄ is an intrinsic ferromagnetic (FM) semiconductor with a gap value of 0.25 eV, whose easy magnetization axis is out-of-plane direction with magnetic anisotropy energy (MAE) of 158 μeV per Mn. The predicted Curie temperature T_C is about 20.3 K, which is close to that of monolayer MnBi₂Te₄. The calculated results show that the in-plane d₁₁ is about 5.56 pm V^-1, which is higher than or comparable to those of other 2D known materials. Moreover, it is found that strain engineering can effectively tune the piezoelectric properties of Janus monolayer MnSbBiTe₄. The calculated results show that tensile strain can improve the d₁₁, which is improved to be 21.16 pm V^-1 at only 1.04 strain. It is proved that the ferromagnetic order, semiconducting properties, out-of-plane easy axis and a large d₁₁ are robust against electronic correlations. Our work provides a possible way to achieve PFM with a large d₁₁ in well-explored vdW compound MnBi₂Te₄, which makes it possible to use the piezoelectric effect to tune the quantum transport process.

Intrinsic room-temperature piezoelectric quantum anomalous hall insulator in Janus monolayer Fe2IX (X = Cl and Br)

A two-dimensional (2D) material with piezoelectricity, topological and ferromagnetic (FM) properties, namely a 2D piezoelectric quantum anomalous hall insulator (PQAHI), may open new opportunities to realize novel physics and applications. Here, by first-principles calculations, a family of 2D Janus monolayer Fe₂IX (X = Cl and Br) with dynamic, mechanical, and thermal stabilities is predicted to be a room-temperature PQAHI. In the absence of spin-orbit coupling (SOC), the monolayer Fe₂IX (X = Cl and Br) is in a half Dirac semimetal state. When the SOC is included, these monolayers become quantum anomalous Hall (QAH) states with sizable gaps (more than 200 meV) and two chiral edge modes (Chern number C = 2). It is also found that the monolayer Fe₂IX (X = Cl and Br) possesses robust QAH states against the biaxial strain. By symmetry analysis, it is found that only an out-of-plane piezoelectric response can be induced by a uniaxial strain in the basal plane. The calculated out-of-plane d₃₁ of Fe₂ICl (Fe₂IBr) is 0.467 pm V^-1 (0.384 pm V^-1), which is higher than or comparable with those of other 2D known materials. Meanwhile, using Monte Carlo (MC) simulations, the Curie temperature T_C is estimated to be 429/403 K for the monolayer Fe₂ICl/Fe₂IBr at the FM ground state, which is above room temperature. Finally, the interplay of electronic correlations with nontrivial band topology is studied to confirm the robustness of the QAH state. The combination of piezoelectricity, topological and FM orders makes the monolayer Fe₂IX (X = Cl and Br) become a potential platform for multi-functional spintronic applications with a large gap and high T_C. Our work provides the possibility to use the piezotronic effect to control QAH effects, and can stimulate further experimental works.

The Versatile Electronic, Magnetic and Photo-Electro Catalytic Activity of a New 2D MA2 Z4 Family*

The recent successful growth of MoSi₂ N₄ and WSi₂ N₄ monolayers led to the discovery of a new class of the two-dimensional (2D) MA₂ Z₄ materials with no known 3D layered allotropes, which renders great possibilities to integrate diverse properties by proper design of sandwiched "MZ₂ " building blocks and "A-Z" passivation layers. In this work, the dynamic stability, electronic properties, and surface reactivity of the new MA₂ Z₄ family, in which M is Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, A refers to Si or Ge, and Z is N, P or As, is theoretically probed. Among the proposed 54 possible combinations, about 42 compositions are dynamically stable, which vary from non-magnetic, anti-ferromagnetic, to ferromagnetic semiconductors, metals and half-metals. In particular, the VB (V, Nb, Ta) MA₂ Z₄ possesses robust intrinsic ferromagnetism that is essential for spintronics applications. In regard to surface activity, most MA₂ Z₄ , particularly N- or P-terminated IVB and VB MA₂ Z₄ , have high catalytic potential for hydrogen evolution, and the ▵G_H of non-magnetic MA₂ Z₄ is highly correlated to the highest occupied p electronic states of the surface Z atoms. The photocatalytic activity is also evaluated. MoSi₂ N₄ and WSi₂ N₄ within 4 % tensile strain are capable of photocatalytic overall water splitting. The findings indicate the new 2D MA₂ Z₄ family has fascinating properties and possesses strong potential for applications but not limited to electronics, spintronics and catalysts, which will stimulate the interests of experimental synthesis.

Design of 2D materials - MSi2CxN4-x (M = Cr, Mo, and W; x = 1 and 2) - with tunable electronic and magnetic properties

Two-dimensional (2D) materials have attracted increasing interest in the past decades due to their unique physical and chemical properties for diverse applications. In this work, we present a first-principles design on a novel 2D family, MSi₂C_xN_4-x (M = Cr, Mo, and W; x = 1 and 2), based on density-functional theory (DFT). We find that all MSi₂C_xN_4-x monolayers are stable by investigating their mechanic, dynamic, and thermodynamic properties. Interestingly, we see that the alignment of magnetic moments can be tuned to achieve non-magnetism (NM), ferromagnetism (FM), anti-ferromagnetism (AFM) or paramagnetism (PM) by arranging the positions of carbon atoms in the 2D systems. Accordingly, their electronic properties can be controlled to obtain semiconductor, half-metal, or metal. The FM states in half-metallic 2D systems are contributed to the hole-mediated double exchange, while the AFM states are induced by super-exchange. Our findings show that the physical properties of 2D systems can be tuned by compositional and structural engineering, especially the layer of C atoms, which may provide guidance on the design and fabrication of novel 2D materials with projected properties for multi-functional applications.

Accurate electronic properties and non-linear optical response of two-dimensional MA2Z4

Two-dimensional MA2Z4 (M = Mo, W, V, Nb, Ta, Ti, Zr, Hf, or Cr; A = Si or Ge; Z = N, P, or As) is a new lead in the 2D family, because it exhibits versatile properties by tuning the components M, A and Z. However, theoretical studies on MA2Z4 are quite limited, and electronic properties are mainly studied by standard DFT levels, which seriously underestimates the band gap. Here, we systematically investigated the electronic properties and nonlinear optical response of MA2Z4 using a hybrid HSE06 functional. It was found that replacing component Z changes the lattice constant most, while the lattice influence by component M substitution is only slight. We showed that the gap difference between PBE and HSE06 is generally about 30% but can be up to 101%. (MIV = Hf, Ti, or Zr)Si2N4 possesses multi-valley characteristics. Furthermore, the second-harmonic generation (SHG) responses of various MA2Z4 composites were also calculated. Three non-zero elements of second order non-linear susceptibilities are revealed for MA2Z4 with the relationship: d16 = d21 = d22, indicating that MA2Z4 belongs to the D3H1 space group. HfSi2N4 possesses a multi-valley characteristic, and exhibits the largest susceptibility under broad wavelengths and the value of d21 reaches 3697.04 pm V-1 at band gap resonance energy. Intriguingly, the non-linear coefficients of MoSi2P4 and MoSi2As4 in the IR region are two orders of magnitude larger than those of other well-known non-linear crystals, such as LiGaS2 and BaAl4S7. We further explored the anisotropic SHG response by the polar plot of intensity under different incident light into MA2Z4. Our work provides theoretical guidelines for further experimental explorations of MA2Z4 and paves the way for its utilization in non-linear optic devices.

Novel Two-Dimensional Layered MoSi2Z4 (Z = P, As): New Promising Optoelectronic Materials

Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSi2N4 and WSi2N4 were successfully synthesized in experiments, and a large family of these two 2D materials, namely MA2Z4, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSi2P4 and MoSi2As4 by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSi2Z4 (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSi2Z4-based electronic and optoelectronic devices. We also build a monolayer MoSi2Z4-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSi2Z4 and expands their potential application in nanoscale electronic and optoelectronic devices.