Janus monolayer materials characterize by two asymmetric surfaces exhibit unique properties, holding significant applications in the fields of energy and information storage. Based on the extensive study of twodimensional (2D) transition-metal dichalcogenides, we design a novel class of stable 2D monolayer Janus MoXNH (X = S, Se, Te) materials, and systematically investigate their valleytronic, piezoelectric, and optical absorption properties using first-principles calculations. The computational results indicate that the three Janus monolayers are semiconductors with moderate band gaps (1.822-2.220 eV) and exhibit pronounced absorption in both visible and ultraviolet regions, making them promising catalysts for photocatalytic water splitting to produce hydrogen. Moreover, owing to the broken inversion symmetry and the strong spin-orbit coupling effect, the Janus monolayer MoXNH materials exhibit valleytronic properties protected by time-reversal symmetry, and the spin-valley coupling strength increases with the atomic radius of the X element, reaching as high as 248.4 meV. More importantly, beyond the in-plane piezoelectric response, the vertical asymmetry of the constituent elements in the MoXNH system induces remarkably strong out-of-plane piezoelectricity, with the coefficients e31 and d31 increasing with the atomic number of the X element and reaching the values of -1.571 & times; 10-10 Cm-1 and -0.937 pmV-1, respectively. Our findings propose an innovative approach to constructing Janus monolayers, which not only expands the Janus material family but also unlocks significant opportunities for future applications in information storage and energy conversion.

Abstract The assembly of two-dimensional (2D) materials into van der Waals heterostructures (vdWHs) provides an effective platform for tailoring the physical properties of individual 2D materials. In this work, based on three centrosymmetric monolayers, namely, antimonene, tellurene, and PtTe2, we construct three stable bilayer vdWHs: antimonene (Sb)/tellurene (Te), antimonene (Sb)/PtTe2, and tellurene (Te)/PtTe2. Using first-principles calculations, we systematically investigate their electronic, spintronic, optical, piezoelectric, and transport properties. Our results show that heterostructure formation breaks spatial inversion symmetry and induces strong interlayer hybridization, which together with the strong spin–orbit coupling significantly reduce the bandgaps and trigger pronounced spin splitting in their band structures. Among the three systems, the Sb/PtTe2 bilayer exhibits a type-III broken-gap band alignment, with obvious tunneling effect, suggesting its potential for tunneling field-effect transistor applications. In contrast, the Sb/Te and Te/PtTe2 systems are small-bandgap type-I vdWHs, in which both the valence band maxima and conduction band minima are mainly contributed by tellurene. More importantly, the conduction band minima of the Sb/Te and Te/PtTe2 vdWHs and the valence band top of the Sb/PtTe2 vdWH exhibit extraordinary Rashba effects, with Rashba coefficients reaching 1.08, 2.89, and 2.85 eV Å, respectively. The Rashba effects can be further tuned by an external strain and electric field, and the coefficient of the Te/PtTe2 vdWH can be enhanced to 3.45 eV Å under 6% tensile strain. In addition, compared with their constituent monolayers, the three vdWHs exhibit greatly enhanced optical absorption in the visible-light region and notable in-plane and out-of-plane piezoelectric responses. Meanwhile, the carrier mobilities of the Sb/Te and Te/PtTe2 vdWHs are also significantly improved, up to 104 cm2 V–1 s–1. The designed vdWHs, with their diverse and highly tunable properties, hold great promise for applications in nanoelectronic, optoelectronic, piezoelectric, and spintronic devices.

Both theoretical and experimental studies have confirmed that combining two or more two-dimensional (2D) monolayer materials into heterostructures is an efficient way to tailor the physical properties of single 2D materials. In this paper, we design a 2D bilayer α-tellurene/GaTe (Te/GaTe) van der Waals heterostructure (vdWH) with an extremely strong interlayer hybridization, a distinctive electronic, spin and optical properties using first-principles calculations. Our calculated results show that the Te/GaTe vdWH is an intrinsic type-I vdWH with an indirect band gap of 0.65 eV, presenting a unique optical property and a high carrier mobility up to 103 cm2/V/s. Intriguingly, in the Te/GaTe vdWH, the strong interlayer hybridization together with the broken symmetry drives the significant spin effect, especially the Rashba effect with a Rashba constant of 0.75 eV Å. Meanwhile, the strong hybridization also makes the electronic property and heterostructure type susceptible to strain and an electric field. Our findings provide a novel 2D Te/GaTe semiconductor vdWH that can be used in electronics and spintronics.

Two-dimensional (2D) van der Waals heterostructures (vdWHs) have garnered significant attention due to their potential to extend the applications of single 2D materials. In this study, we use first-principles calculations to design a novel ZnI2/In2Se3 vdWH that combines semiconducting and ferroelectric properties via precise interfacial control. Our results reveal that the reversal of the polarization direction in In2Se3 acts as a non-volatile switch, enabling a reversible transition between type-I and type-II band alignments in the heterostructure with the bandgaps of 1.56 and 1.43 eV, respectively. The ZnI2/In2Se3 vdWH shows enhanced optical absorption with the visible and ultraviolet light absorption coefficients reaching 6% and 27%, respectively, substantially higher than those of the individual monolayers. Furthermore, the type-II ZnI2/In2Se3 vdWH exhibits efficient interlayer charge transfer following a Z-scheme mechanism, promising for photocatalytic hydrogen production through water splitting. Additionally, the electronic structure of ZnI2/In2Se3 vdWH is highly tunable under external electric field and strain, facilitating the reversible switching between type-I and type-II band alignments as well as between direct and indirect bandgap semiconductors. This tunability is crucial for the development of multifunctional optoelectronic devices. Our designed ZnI2/In2Se3 vdWH presents significant opportunities for nano-optoelectronic device integration.

Two-dimensional (2D) van der Waals heterostructures (vdWHs) incorporating ferroelectric properties and Rashba effects hold attractive applications in spintronics. In this work, the 2D alpha-tellurene and typical piezoelectric alpha-In2Se3 are selected to form the bilayer alpha-tellurene/alpha-In2Se3 (Te/In2Se3) vdWHs and the novel electronic, optical and spin characteristics are explored utilizing the first-principles calculations. We show that, firstly, the Te/In2Se3 vdWH exhibits semiconductor nature as well as the super strong optical absorption characteristics in the visible region. Then, the pronounced spin-orbit coupling effect is revealed, which results from the heterostructure disrupting the symmetries of the individual monolayers. Furthermore, reversing the direction of ferroelectric polarization for the In2Se3 monolayer can transform the Te/In2Se3 vdWH between type-I and type-II heterostructures, also tuning the Rashba effect. And such electronic properties and Rashba effect can be tailored by strain and electric field. Our proposed Te/In2Se3 vdWH will be potential candidate for applications in nanodevices, especially spin-field effect transistors.

In this paper, for the newly proposed two-dimensional (2D) Janus MoAZ(3)H (A = Si, or Ge; Z = N, P, or As) monolayer (ML) materials, we theoretically explore the valleytronic and piezoelectric properties using first-principles calculations. The calculated results show that Janus MoAZ(3)H MLs have obvious valleytronic properties due to the broken inversion symmetry and strong spin-orbit coupling (SOC) effects, and the spin-valley coupling of MoAZ(3)H MLs can be enhanced from 173 to 283 meV by changing the A or Z element, comparable to the corresponding MoA(2)Z(4) MLs. The non-zero Berry curvatures with opposite signs at the K and K ' points can induce the valley Hall effect. Furthermore, Janus MoAZ(3)H MLs exhibit obvious in-plane and out-of-plane piezoelectric responses, and their independent d(11) and d(31) piezoelectric coefficients range from 1.62 to 8.60 pm V-1 and from 0.058 to 0.325 pm V-1, respectively. This piezoelectricity is much stronger than those of the corresponding MoA(2)Z(4) and MoSiGeN4 MLs, attributed to the unique Janus structure of the MoAZ(3)H MLs. More importantly, strain can tailor both the spin-valley coupling and piezoelectricity of the MoAZ(3)H MLs. Our findings not only enrich the excellent properties of the MoAZ(3)H MLs, but also indicate a direction for their application in valleytronics and energy conversion devices.

Extensive research has been conducted on silver niobite (AgNbO3)-based antiferroelectric ceramics for their promising applications in energy storage applications, with various compositional modifications explored to improve their energy storage capabilities. In this theoretical study, we have systematically investigated the electronic, structural, and chemical bonding properties of AgNb1-xTaxO3 (x = 0.00, 0.125, 0.25, 0.375, 0.50, abbreviated as ANT100x) solid solutions based on first-principles calculation. Our results reveal that the bandgap increases from 1.82 eV to 1.89 eV, due to the higher energy level of Ta 5d orbitals compared to Nb 4d orbitals. The enlarged bandgap, accompanied with oxygen vacancy formation energy (Delta Ef,vac), contributes to the enhancement of Eb. The Ta substitution of Nb site suppresses the cation displacement, oxygen octahedral distortion, and bond length and angles, indicating an improved stability of antiferroelectric phase. In addition, the electron localization function (ELF) and Bader charge values show weakened covalent bonding of Ta-O bonds compared to Nb-O bonds. These theoretical findings have the potential to aid in the advancement and creation of novel energy storage applications using lead-free AFE perovskites, as well as facilitate the manipulation of their breakdown electric field through bandgap engineering.

Two-dimensional (2D) ferroelectric materials have intrinsic polarized electric field, when they stack with other 2D materials to construct ferroelectric van der Waals heterostructures, they will produce a lot of novel properties, which has aroused extensive research interests. Therefore, based on first-principle calculations, we have comprehensively studied the electronic properties of LaBr2/& alpha;-In2Se3 ferroelectric heterostructures to investigate the reversibility and nonvolatility with information. Our calculations show that, the system is a ferromagnetic semiconductor with intrinsic valley polarization when & alpha;-In2Se3 is in the ferroelectric polarized up state. By switching the ferroelectric polarization of & alpha;-In2Se3, the heterostructure undergoes a transition from semi-conductor to half metal, which can be attributed to the competition between the built-in electric field (Eint) and & RARR; the polarized electric field Epi ,generated by the charge transfer at interface of heterostructure and the bound charge of & alpha;-In2Se3, respectively. Besides, the band arrangement types of LaBr2/& alpha;-In2Se3 heterostructure can be well modulated under extra electric filed and biaxial strain. Furthermore, we have observed valley-submerging under electric filed and strain, which indicates the valleytronic nature can also be on-off in LaBr2/& alpha;-In2Se3 heterostructures. More importantly, the realization of reversible and non-volatile properties depends on the intrinsic characteristics of heterostructure and does not require external mechanisms. Our research not only provides the possibility for the application of the LaBr2/& alpha;-In2Se3 heterostructures in nanodevices, but also give the theoretical support to the study of spintronics and valleytronics.

Two-dimensional ferroelectric monolayer materials with reversible spontaneous polarization provide more regulatory dimensions for their relevant van der Waals heterostructures. Using first-principles calculations, we construct the C2N/In2Se3 bilayer heterostructure and study its physical properties as well as the effects of E-field and strain. The results indicate that the intrinsic polarization of the component In2Se3 monoalyer can significantly adjust the electronic properties of the C2N/In2Se3 heterobilayer. When the polarization of the In2Se3 monolayer points to the interface (up-In2Se3), the C2N/In2Se3 bilayer behaves as the type-I indirect band gap heterostructure, while it transforms to the type-II direct band gap heterostructure after reversing the polarization of the In2Se3 monolayer (dp-In2Se3). Furthermore, the two C2N/In2Se3 heterostructures both have enhanced optical absorption in the visible region than the isolated In2Se3 and C2N monolayers. More importantly, the external electric field and strain can easily regulate the electronic properties of the C2N/In2Se3 heterostructures. The power conversion efficiency (PCE) of the type-II C2N/dp-In2Se3 heterostructure is 8.16%, and the electric field of 0.1 V/Å and the strain of -2% can transform the C2N/up-In2Se3 heterostructure into type-II one, conducive to the high PCE up to 24.03 and 24%, respectively. Our proposed C2N/In2Se3 heterostructure is promising in future luminescent and photovoltaic fields, and our findings also provide a strategy for functionalizing 2D monolayer materials by the intrinsic polarization property of ferroelectric materials.

In this study, DFT calculations are used to analyze the adsorption of industrial waste gases (NO2, SO2, H2S, and NH3) on WSe2 monolayers. The adsorption energy, energy band, density of states, charge transfer, and recovery time of the adsorption structures between the target gas molecules and the Os-doped WSe2 are studied. Compared with pure WSe2 monolayer, Os surface bonding doping WSe2 (Os-modified WSe2) and Os doping with Se vacancy of WSe2 (Os-embedded WSe2) exhibit improved gas molecule adsorption ability. Among them, the adsorption energy of the Os-modified WSe2 monolayer on NO2, SO2, H2S, and NH3 is greater than that of the WSe2 monolayer. At the same time, it is proved that the Os-embedded WSe2 can be used as a gas sensor for H2S and NH3 gas molecules at a high temperature.

In this paper, the photocatalytic ability of the g-C3N4/LNS-TiO2(Cr + C) heterostructure in water splitting process is studied theoretically. Calculations indicate that this hetero-junction is a stable structure. The PDOS results show that the CBM and VBM for this heterojunction come from different components. Charge transfer analysis indicates that when the two materials are joined together to form heterojunction, electrons in g-C3N4 will flow into LNS-TiO2(Cr + C), thus creating a built-in electric field from g-C3N4 to LNS-TiO2(Cr + C). Research on band edge positions and photocatalytic mechanism indicates that this heterostructure is Z-scheme photocatalyst, in which the powerful photogenerated electrons (0.86 eV higher than the H2 producing level) and holes (2.46 eV lower than the O2 producing level) are preserved in different constitutes to participate in photocatalytic reactions. In general, this study enriches the photocatalyst types for water splitting process.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Limited by the cost and complexity, superresolution lithography is hard to achieve through the traditional interference lithography. We here developed the plasmonic interference lithography technique by using a hyperbolic metamaterials (HMMs) / photoresist / metal plasmonic waveguide to push the feature sizes theoretically down to 16 nm and even to 11 nm at the wavelength of 365 nm with TM polarization. The waveguide based on the proposed HMMs can support high-k mode for superresolution lithography. Furthermore, plasmonic mode supported in the proposed lithography structure can be tailored by dimension of HMM and permittivity of the materials, which makes it possible to get higher resolution pattern under conventional UV light. Our findings open up an avenue to pushing the nanolithography node towards 10 nm for low-cost and large area fabrication under conventional UV light source.

Due to the asymmetrical structure in the vertical direction, Janus two-dimensional (2D) monolayer (ML) materials possess some unique physical properties, holding great promise for nanoscale devices. In this paper, based on the newly discovered MoA2Z4 (A = Si, Ge; Z = N, P, As) ML, we propose a class of 2D Janus MoAZ3H ML materials with good stability and excellent mechanical properties using first-principles calculations. We demonstrate that the novel Janus MoAZ3H ML materials are all semiconductors with bandgaps ranging from 0.69 to 2.44 eV, giving rise to good absorption in the visible light region. Especially, both MoSiN3H and MoGeN3H MLs can be used as catalysts for producing hydrogen through water splitting. This catalytic property is much more efficient than that of the MoA2Z4 ML, attributed to the intrinsic electric field induced by the vertical asymmetry effectively separating electrons and holes. More importantly, the carrier mobility of the MoAZ3H ML is up to 103-104 cm2 V-1 s-1 due to the large elastic modulus or small effective mass. Additionally, the electronic properties of the MoAZ3H ML can be easily tuned by strain. Our results suggest a new strategy for designing novel 2D Janus materials, which not only expands the members in the 2D MA2Z4-based Janus family, but also provide candidates with excellent performances in photovoltaic and catalytic fields.

This paper theoretically studies the photocatalytic performance of g-C3N4 self codoped with C–N pair using first-principles. Calculations show that the system with a N2–C1 pair been replaced is the most stable structure. The difference in PDOS after codoping results from the neutral 2p orbital energy difference between the substitution and original atoms, as well as the change in interaction among all atoms after codoping. The band gap can be reduced by 0.41 eV through self C–N codoping. The visible light absorption ability is obviously enhanced through codoping. Both the CBM and VBM of g-C3N4 after codoping are still enough to meet the redox potentials for splitting water. In general, the g-C3N4 self codoped with C and N should have good performance as photocatalyst in water splitting process.

Building two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) is one of the effective methods to regulate the properties of single 2D materials. In this paper, we stack the hexagonal boron nitride (h-BN) monolayer (ML) on the SnSe2 ML to construct the stable h-BN/SnSe2 vdWH, of which the crystal and electronic structures, together with the optical properties, are also analyzed by the first-principles calculations. The results show that the h-BN/SnSe2 vdWH belongs to a type-I heterostructure with an indirect bandgap of 1.33 eV, in which the valence band maximum and conduction band minimum are both determined by the component SnSe2 ML. Interestingly, the h-BN/SnSe2 vdWH under the tensile strain or electric field undergoes the transitions both from type-I to type-II heterostructure and from the indirect to direct bandgap semiconductor. In addition, the carrier mobility of the h-BN/SnSe2 heterostructure has a significant enhancement relative to that of the SnSe2 ML, up to 104 cm2 V-1 s-1. Meanwhile, the h-BN/SnSe2 heterostructure presents the superb optical absorption and unique type-II hyperbolic property. Our findings will broaden the potential applications of SnSe2 ML and provide theoretical guidance for the related experimental studies.

Spin polarized van der Waals (vdW) heterostructures have attracted considerable interest owing to the spin splitting manipulation. In this paper, first-principles calculations are employed to explore the two-dimensional arsenene/CrI3 vdW heterostructure as a promising spin polarized material, as well as the spin polarization under strain and electronic field are investigated. The most stable stacking configuration and ferromagnetic (FM) property of the arsenene/CrI3 vdW heterostructure have been confirmed. The detailed calculations show that the Curie temperature (Tc) of the FM coupling CrI3 layer in the heterostructure can be enhanced up to 61 K, which is attributed to both superexchange interaction and proximity exchange effect. The electronic structures suggest that this heterostructure possess an intrinsic type-II band alignment and diluted magnetic semiconductor property. Interestingly, we found a transition from diluted magnetic semiconductor to half-metal and gradually to metal induced by the biaxial strains, and the extra electronic field can modulate the "maxican hat" of the valance band maximum (VBM) of arsenene. Our work provides not only application prospects of the arsenene/CrI3 vdW heterostructure nanodevices but also theoretical effective support for the research and development of the spin electronics and flexible electronics. (c) 2022 Elsevier B.V. All rights reserved.

Constructing novel van der Waals heterostructures (vdWHs) is one of the effective methods for expanding the properties and applications of single materials. In this contribution, a blue phosphorene (Blue P)/MoSi2N4 vertical bilayer vdWH was constructed, and its crystal and electronic structures as well as optical properties were systematically studied via first principles calculation. It was found that the Blue P/MoSi2N4 vdWH with good thermal and dynamic stabilities belongs to the type-II indirect bandgap semiconductor with the bandgap of 1.92 eV, which can efficiently separate electrons and holes. Additionally, the two band edges straddle the redox potential of water, and the charge transfer follows the Z-scheme mode, making the Blue P/MoSi2N4 vdWH a promising catalyst of hydrogen production through splitting water. Meanwhile, the Blue P/MoSi2N4 vdWH has higher optical absorption than its two component monolayers. Both the external electric field and vertical strain can easily tailor the bandgap of Blue P/MoSi2N4 vdWH while still preserving its type-II heterostructure characteristics. Our proposed Blue P/MoSi2N4 vdWH is a promising photovoltaic two-dimensional material, and our findings provided theoretical support for the related experimental exploration.

The physical properties and potential applications of two-dimensional (2D) materials can be effectively modulated and enriched by constructing van der Waals heterostructures (VDWHs) with two or more 2D monolayer materials. In this work, by using first-principles calculations based on density functional theory (DFT), we have systematically investigated the structural, electronic and optical properties of four α-Se-based VDWHs, that is, α-Se/Ca(OH)2, α-Se/GaSe, α-Se/h-BN and α-Se/MoS2 VDWHs. The results show that both the band alignment and band gap of these four VDWHs can be effectively modulated by interlayer coupling, biaxial strain and an external electric field. Compared with interlayer coupling and biaxial strain, the external electric field can modulate the electronic properties of these VDWHs more significantly, which makes them exhibit more rich electronic properties. Interestingly, the optical property calculations revealed that both α-Se and the four α-Se-based VDWHs have intrinsic hyperbolic properties. In addition, compared with the individual components, the optical absorption of these four VDWHs in the visible and ultraviolet light regions is significantly enhanced. These results enrich the properties of selenene and selenene-based VDWHs and extend their potential applications in electronic and optoelectronic devices.

The tunable angle-selective transparency of hyperbolic metamaterials consisting of various multilayers of Dirac semimetal and dielectric materials are theoretically and numerically studied in the terahertz range. Three stack configurations are considered: alternating, sandwiched, and disordered. It is found that the proposed structures exhibit strong optical angular selectivity induced by photonic topological transition for transverse magnetic waves. Interestingly, the topological transition frequency can be flexibly modulated by changing the Fermi energy, temperature, and the releasing time of the Dirac semimetal, as well as the thickness ratio of the dielectric and semimetal layers. It is also noticed that the angular optical transparency properties are independent of the order of the proposed structure even in alternating/disordered/random configurations if the total thickness ratio of the semimetal to dielectric are the same, which makes the properties particularly easy to realize experimentally. The proposed hyperbolic metamaterial structures present a promising opportunity for wavefront engineering, offering crucial properties for applications in private screens, optical detectors, and light manipulation.

Designing van der Waals (vdW) heterostructures has been considered to be a promising strategy for fabricating high-performance nanosized optoelectronic devices. Based on the first-principles calculations within density functional theory (DFT), we have demonstrated that a BP/MoSi2P4 vdW heterostructure possesses a direct band gap with a typical type-II band alignment, which can facilitate the effective separation of photogenerated electron-hole pairs. Compared with a MoSi2P4 monolayer, not only the hole carrier mobility but also the optical absorption intensity of the BP/MoSi2P4 heterostructure can be enhanced significantly. The predicted photoelectric conversion efficiency (PCE) for the BP/MoSi2P4 heterostructure can reach up to 22.2%, which is competitive with other existing two-dimensional (2D) heterostructures studied previously. Our findings indicate that the 2D BP/MoSi2P4 heterostructure can be a good candidate for new optoelectronic nanodevices.