Emergence of superconductivity by intercalation of alkali metals and alkaline earth metals in Janus transition-metal dichalcogenide heterostructures

Superconductivity in two-dimensional materials has attracted considerable attention. A new material belonging to the family of Janus transition metal dichalcogenides with out-of-plane structural asymmetry has been recently found to show interesting physical and chemical properties. Using density functional theory and density functional perturbation theory, within the generalized gradient approximation with van der Waals correction, we performed a detailed investigation of the electronic structure, phonon dispersion, Eliashberg spectral function, and electron-phonon coupling of Janus MSSe bilayers (M = Mo or W) and the Janus MoSSe/WSSe heterostructure intercalated with alkali metals (Li, Na, and K) or alkaline earth metals (Mg, Ca, and Sr). We found that the Janus MoSSe bilayer, Janus WSSe bilayer, and Janus MoSSe/WSSe heterostructure transform from a direct band gap semiconductor to a metal or semimetal due to charge transfer from intercalated atoms to the Se and S planes. We showed that all compounds are dynamically stable conventional superconductors. In addition, the Janus heterostructure MoSSe/WSSe intercalated with K exhibits the highest electron-phonon coupling of about 2.12 and the highest superconducting transition temperature of about 14.77 K. Our results indicate the potential application of the Janus materials we investigated in superconductivity.

ScSeI Monolayer for Photocatalytic Water Splitting

We theoretically identify the ScSeI monolayer as a promising new 2D material for photocatalysis through first-principles calculations. The most notable feature is the significant difference in carrier mobility, with electron mobility in the horizontal direction being 20.66 times higher than hole mobility, minimizing electron-hole recombination. The ScSeI monolayer exhibits a bandgap of 2.51 eV, with the valence band maximum at -6.37 eV and the conduction band minimum at -3.86 eV, meeting the requirements for water splitting. Phosphorus doping lowers the Gibbs free energy by 1.63 eV, enhancing the catalytic activity. The ScSeI monolayer achieves a hydrogen production efficiency of 17%, surpassing the commercial threshold of 10% and shows excellent mechanical, thermal, and dynamic stability, indicating feasibility for experimental synthesis and practical application. Additionally, the monolayer maintains its photocatalytic properties under tensile strain (-6% to 6%) and in aqueous environments, reinforcing its potential as an effective photocatalyst. Based on these findings, we believe the ScSeI monolayer is a highly promising photocatalyst.

BlueP encapsulated Janus MoSSe as a promising heterostructure anode material for LIBs

In this work, the significance of BlueP-Janus MoSSe heterostructures in LIBs is explored in detail by using density functional theory calculations. The Janus MoSSe possesses two different atomic layers, and hence two different heterostructures, BlueP-SMoSe and BlueP-SeMoS, are taken into account. The heterostructure formation energies are computed to check their stability. Besides, ab initio molecular dynamics simulations and phonon studies are done to check their thermal and dynamical stabilities, respectively. The adsorption and diffusion of Li at different surfaces of both the heterostructures are calculated. Our study reveals that the heterostructures show strong Li intercalation capability with ultrafast Li diffusion barrier energies. The electronic properties of the lithiated heterointerfaces are also explored. Both the heterostructures can hold a maximum of two layers of Li ions on each side of both BlueP and MoSSe to give a large storage capacity, signifying their extraordinary potential to be appropriate as an anode material for Li-ion batteries. Additionally, due to their strong mechanical strength, the 2D BlueP-Janus MoSSe heterostructures can withstand massive volume expansion during the lithiation-delithiation reaction, which is remarkably beneficial for manufacturing flexible anodes. Based on the above findings, the newly designed heterostructures are expected to open a new avenue for the next generation of electronic devices.

Strain-induced excellent photocatalytic performance in Z-scheme BlueP/γ-SnS heterostructures for water splitting

Constructing Z-scheme heterojunction photocatalysts with high solar-to-hydrogen (STH) efficiency is a practical alternative to produce clean and recyclable hydrogen energy on a large scale. This paper presents the design of stable Z-scheme blue phosphorene (BlueP)/γ-SnS heterostructures with excellent photocatalytic activities by applying strains. The first-principles calculations show that the BlueP/γ-SnS heterobilayer is a type-I heterojunction with an indirect bandgap of 1.41 eV and strong visible-light absorption up to 105 cm-1. Interestingly, biaxial strains (ε) can effectively regulate its bandgap width (semiconductor-metal) and induce the band alignment transition (type-I-type-II). Compressive and tensile strains can significantly enhance the interfacial interaction and visible-light absorption, respectively. More intriguingly, compressive strains can not only modulate the heterojunction types but also make the band edges meet the requirements for overall water splitting. In particular, the Z-scheme (type-I) BlueP/γ-SnS bilayer at -8% (-2%) strain exhibits a relatively high STH efficiency of 18% (17%), and the strained Z-scheme system (-8% ≤ ε ≤ -6%) also exhibits high and anisotropic carrier mobilities (158-2327 cm2 V-1 s-1). These strain-induced outstanding properties make BlueP/γ-SnS heterostructures promising candidates for constructing economically feasible photocatalysts and flexible nanodevices.

Janus Zn-IV-VI: Robust Photocatalysts with Enhanced Built-In Electric Fields and Strain-Regulation Capability for Water Splitting

The use of 2D materials to produce hydrogen (H2 ) fuel via photocatalytic water splitting has been intensively studied. However, the simultaneous fulfillment of the three essential requirements-high photon utilization, rapid carrier transfer, and low-barrier redox reactions-for wide-pH-range production of H2 still poses a significant challenge with no additional modulation. By employing the first-principles calculations, it has been observed that the Janus ZnXY2 structures (X = Si/Ge/Sn, Y = S/Se/Te) exhibit significantly enhanced built-in electric fields (0.20-0.36 eV Å-1 ), which address the limitations intrinsically. Compared to conventional Janus membranes, the ductile ZnSnSe2 and ZnSnTe2 monolayers have stronger regulation of electric fields, resulting in improved electron mobility and excitonic nature (Ebinding = 0.50/0.35 eV). Both monolayers exhibit lower energy barriers of hydrogen evolution reaction (HER, 0.98/0.86 eV, pH = 7) and resistance to photocorrosion across pH 0-7. Furthermore, the 1% tensile strain can further boost visible light utilization and intermediate absorption. The optimal AC-type bilayer stacking configuration is conducive to enhancing electric fields for photocatalysis. Overall, Janus ZnXY2 membranes overcome the major challenges faced by conventional 2D photocatalysts via intrinsic polarization and external amelioration, enabling efficient and controllable photocatalysis without the need for doping or heterojunctions.

The thermoelectric properties of CdBr, CdI, and Janus Cd2BrI monolayers with low lattice thermal conductivity

The investigation and development of high thermoelectric value materials has become a research hotspot in recent years. In this work, based on the density functional theory on the Perdew-Burke-Ernzerhof (GGA-PBE) level, the thermoelectric properties of transition metal halides CdBr, Janus Cd2BrI, and CdI monolayers have been systematically investigated using Boltzmann transport theory. The calculation of the electronic band structure shows that these three materials have indirect band gap semiconductor properties. For carrier transport, the electron mobilities for CdBr, Janus Cd2BrI, and CdI monolayers are found to be 74, 16, 21 cm2 s-1 V-1 for p-type doping and 116, 102, 78 cm2 s-1 V-1 for n-type doping. Regarding their phonon transport, the CdBr, Cd2BrI, and CdI monolayers all have very low lattice thermal conductivity (4.78, 2.46, and 1.65 W m-1 K-1, respectively) that decreases with increasing temperature, which is favorable for obtaining large zT values. The electrical transport results show that the performance of p-type doping is better than that of n-type doping. At 300 K, the Seebeck coefficients of p-type doping for the CdBr, Cd2BrI, and CdI monolayers are 217.72, 246.43, and 226.24 μV K-1, respectively. In addition, we predict that the zT values of the CdBr, Cd2BrI, and CdI monolayers are 0.62, 1.64, and 0.87 for p-type doping at 300 K respectively. The zT values increase with the increase of temperature. In particular, the Janus Cd2BrI monolayer has a zT value of 3.03 at 600 K. These results suggest that all these materials can be good candidates for thermoelectric materials.

Two-dimensional Janus Si2OX (X = S, Se, Te) monolayers as auxetic semiconductors: theoretical prediction

The auxetic materials have exotic mechanical properties compared to conventional materials, such as higher indentation resistance, more superior sound absorption performance. Although the auxetic behavior has also been observed in two-dimensional (2D) nanomaterials, to date there has not been much research on auxetic materials in the vertical asymmetric Janus 2D layered structures. In this paper, we explore the mechanical, electronic, and transport characteristics of Janus Si2OX (X = S, Se, Te) monolayers by first-principle calculations. Except for the Si2OTe monolayer, both Si2OS and Si2OSe are found to be stable. Most importantly, both Si2OS and Si2OSe monolayers are predicted to be auxetic semiconductors with a large negative Poisson's ratio. The auxetic behavior is clearly observed in the Janus Si2OS monolayer with an extremely large negative Poisson's ratio of -0.234 in the x axis. At the equilibrium state, both Si2OS and Si2OSe materials exhibit indirect semiconducting characteristics and their band gaps can be easily altered by the mechanical strain. More interestingly, the indirect-direct bandgap phase transitions are observed in both Si2OS and Si2OSe monolayers when the biaxial strains are introduced. Further, the studied Janus structures also exhibit remarkably high electron mobility, particularly along the x direction. Our findings demonstrate that Si2OS and Si2OSe monolayers are new auxetic materials with asymmetric structures and show their great promise in electronic and nanomechanical applications.

First-principles prediction of ferroelectric Janus Si2XY (X/Y = S/Se/Te, X ≠ Y) monolayers with negative Poisson's ratios

Nowadays, two-dimensional (2D) materials with Janus structures evoke much attention due to their unique mechanical and electronic properties. In this work, Janus Pma2-Si2XY (X/Y = S/Se/Te, X ≠ Y) ferroelectric monolayers are firstly proposed and systematically investigated by first-principles calculations. These monolayers exhibit remarkable mechanical properties, including small Young's modulus values, negative Poisson's ratios (NPRs) and large critical strains, reflecting their exceptional flexibility and stretchability. More strikingly, the novel structures of Si2STe and Si2SeTe also endow them with in-plane spontaneous polarization (Ps) and low energy barrier for phase transition, with Ps and energy barrier values being 1.632 × 10-10 C m-1 and 159 meV for Si2STe and 1.149 × 10-10 C m-1 and 196.6 meV for Si2SeTe. The ab initio molecular dynamics (AIMD) simulations reveal high Curie temperatures (Tc) for Si2STe and Si2SeTe, ranging between 1300 K and 1400 K. Additionally, Si2XY monolayers exhibit high anisotropic carrier mobility (∼103 cm2 V-1 s-1) and an extraordinary light absorption coefficient (∼105 cm-1). Our research not only broadens the family of 2D Janus ferroelectric materials, but also demonstrates their potential applications in nanomechanical, nanoelectronic and optoelectronic devices.

The intrinsically low lattice thermal conductivity of monolayer T-Au6X2 (X = S, Se and Te)

Thermal conductivity (κ, which consists of electronic thermal conductivity κe and lattice thermal conductivity κl), as an essential parameter in thermal management applications, is a critical physical quantity to measure the heat transfer performance of materials. To seek low-κ materials for heat-related applications, such as thermoelectric materials and thermal barrier coatings. In this study, based on a complex cluster design, we report a new class of two-dimensional (2D) transition metal dichalcogenides (TMDs): T-Au6X2 (X = S, Se, and Te) with record ultralow κl values. At room temperature, the κl values of T-Au6S2, T-Au6Se2, and T-Au6Te2 are 0.25 (0.23), 0.30 (0.21), and 0.12 (0.10) W m-1 K-1 along the x-axis (y-axis) direction, respectively, exhibiting good thermal insulation. The ultralow κl originates from strong phonon softening and suppression, especially for the phonon with frequency 0-1 THz. In addition, T-Au6Te2 holds the lowest group velocity and phonon relaxation time among the three T-Au6X2 monolayers. Our study provides an alternative approach for achieving ultralow κl through complex cluster replacement. Meanwhile, this new class of TMDs is expected to shine in thermal insulation and thermoelectricity due to their ultralow κl values.

Tuning the Electronic Properties of Two-Dimensional Lepidocrocite Titanium Dioxide-Based Heterojunctions

Two-dimensional (2D) heterostructures reveal novel physicochemical phenomena at different length scales that are highly desirable for technological applications. We present a comprehensive density functional theory study of van der Waals (vdW) heterostructures constructed by stacking 2D TiO2 and 2D MoSSe monolayers to form the TiO2-MoSSe heterojunction. The heterostructure formation is found to be exothermic, indicating stability. We find that by varying the atomic species at the interfaces, the electronic structure can be considerably altered due to the differences in charge transfer arising from the inherent electronegativity of the atoms. We demonstrate that the heterostructures possess a type II or type III band alignment, depending on the atomic termination of MoSSe at the interface. The observed charge transfer occurs from MoSSe to TiO2. Our results suggest that the Janus interface enables the tuning of electronic properties, providing an understanding of the possible applications of the TiO2-MoSSe heterostructure.

Strain engineering on electronic structure, effective mass and charge carrier mobility in monolayer YBr3

In recent years, two-dimensional materials have significant prospects for applications in nanoelectronic devices due to their unique physical properties. In this paper, the strain effect on the electronic structure, effective mass, and charge carrier mobility of monolayer yttrium bromide (YBr3) is systematically investigated using first-principles calculation based on density functional theory. It is found that the monolayer YBr3undergoes energy band gap reduction under the increasing compressive strain. The effective mass and charge carrier mobility can be effectively tuned by the applied compressive strain. Under the uniaxial compressive strain along the zigzag direction, the hole effective mass in the zigzag direction (mao1_h) can decrease from 1.64m0to 0.45m0. In addition, when the uniaxial compressive strain is applied, the electron and hole mobility can up to ∼103cm2V-1s-1. The present investigations emphasize that monolayer YBr3is expected to be a candidate material for the preparation of new high-performance nanoelectronic devices by strain engineering.

Surface asymmetry induced turn-overed lifetime of acoustic phonons in monolayer MoSSe

Recent successful growth of asymmetric transition metal dichalcogenides via accurate manipulation of different chalcogen atoms in top and bottom surfaces demonstrates exotic electronic and chemical properties in such Janus systems. Within the framework of density functional perturbation theory, anharmonic phonon properties of monolayer Janus MoSSe sheet are explored. By considering three-phonons scattering, out-of-plane flexural acoustic (ZA) mode tends to undergo a stronger phonon scattering than transverse acoustic (TA) mode and the longitudinal acoustic (LA) mode with phonon lifetime of ZA (1.0 ps) < LA (23.8 ps) < TA (25.8 ps). This is sharply different from the symmetric MoS₂ where flexural ZA mode has the weakest anharmonicity and is least scattered. Moreover, utilizing non-equilibrium Green function method, ballistic thermal conductance at room temperature is found to be around 0.11 nWK^-1nm^-2, lower than that of MoS₂. Our work highlights intriguing phononic properties of such MoSSe Janus layers associated with asymmetric surfaces.

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.

Janus zirconium halide ZrXY (X, Y = Br, Cl and F) monolayers with high lattice thermal conductivity and strong visible-light absorption

In this work, the structural, mechanical, and electronic properties of Janus zirconium halide monolayers have been systematically investigated using the first-principles calculations. After verifying the mechanical and dynamical stability of these monolayers, their electronic band structures have been predicted. These Janus monolayers have band gaps of 1.51-1.96 eV, which indicates their suitability for visible light absorption. The relaxation time and mobility of charge carriers are estimated using deformation potential theory, and the mobility of these monolayers has been predicted to be of the order ∼10² cm² V^-1 s^-1. The lattice thermal conductivity has been calculated by solving the phonon Boltzmann transport equation using ShengBTE software. At 300 K, the in-plane lattice thermal conductivity has values of 76.94, 54.18, and 95.87 W m^-1 K^-1 for ZrBrCl, ZrBrF, and ZrClF monolayers, respectively. The higher group velocity and small anharmonic three-phonon scattering rate are the main reasons for the high lattice thermal conductivity of the ZrClF monolayer. The real and imaginary parts of the dielectric function are calculated to find the absorption coefficients and these monolayers have a high absorption coefficient of the order ∼10⁶ cm^-1 in the visible light range. Our results show that Janus zirconium halide monolayers are potential candidates for optoelectronic and photocatalytic applications.

Phonon transport in Janus monolayer siblings: a comparison of 1T and 2H-ISbTe

In the last decade, two-dimension materials with reduced symmetry have attracted a lot of attention due to the emerging quantum features induced by their structural asymmetry. Two-dimensional Janus materials, named after the Roman deity of beginnings and endings who has two faces, have a structure with broken mirror symmetry because the two sides of the material have distinct chemical compositions. Extensive study has been undertaken on phonon transport for Janus monolayers for their strong applicability in thermoelectrics compared to their parent material, while Janus materials with the same space group but a distinct crystal protype have received very little attention. Using first-principles calculations and the Boltzmann transport equation accelerated by a machine learning interatomic potential, we explore the phonon transport of 1T and 2H-ISbTe. ISbTe possesses significant intrinsic phonon-phonon interactions, resulting in a low lattice thermal conductivity, as a result of its covalent bonding and low elastic constants. A thorough examination of phonon group velocity, phonon lifetime, and heat carrier identification reveals that 2H has a low lattice thermal conductivity of 1.5 W mK-1, which is 2.3 times lower than its 1T sibling. This study demonstrates Janus ISbTe monolayers have extensive physical phenomena in their thermal transport characteristics, which might provide a new degree of control over their thermal conductivity for applications such as thermal management and thermoelectric devices.

Tunable electronic properties and related functional devices for ferroelectric In2Se3/MoSSe van der Waals heterostructures

In recent years, two-dimensional (2D) materials have attracted increasing attraction in a number of scientific research fields. In particular, ferroelectric materials with reversible spontaneous electric polarization and Janus transition metal dichalcogenides (TMDs) with intrinsic dipoles exhibit novel properties for many practical applications. Here, the electronic properties of van der Waals (vdW) heterostructures consisting of In₂Se₃ and MoSSe were investigated based on a first-principles approach. It was demonstrated that four studied In₂Se₃/MoSSe heterostructures exhibited obvious band gap (E _g) differences, ranging 0.13 to 0.90 eV for PBE (0.47 to 1.50 eV for HSE06) owing to the reversible spontaneous electric polarization of In₂Se₃ and different intrinsic dipole of MoSSe, and different band alignments of type-I or type-II could also be obtained. The energy bands of the four vdW heterostructures could be obviously regulated by varying degrees of vertical (horizontal) strain and vertical interface electric field, and the E _g varied from zero to 1.27 eV. Then, M4-based mechanical switching devices and ferroelectric diodes were designed based on the significant strain and electric field function. These results provide one possible mechanism for how the polarization direction regulates the physical properties of the system due to the different charges on the two surfaces of the out-of-plane polarized ferroelectric material, which may lead to different proximity effects on the face of the material.

A first-principles study on the electronic, piezoelectric, and optical properties and strain-dependent carrier mobility of Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers

Janus transition metal dichalcogenide monolayers (TMDs) have attracted wide attention due to their unique physical and chemical properties since the successful synthesis of the MoSSe monolayer. However, the related studies of Janus monolayers of transition metal halides (TMHs) with similar structures have rarely been reported. In this article, we systematically investigate the electronic properties, piezoelectric properties, optical properties, and carrier mobility of new Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers using first principles calculations for the first time. These Janus TiXY monolayers are thermally, dynamically, and mechanically stable, and their energy bands near the Fermi level (E_F) are almost entirely contributed by the central Ti atom. Besides, the Janus TiXY monolayers exhibit excellent in-plane and out-of-plane piezoelectric effects, especially with an in-plane piezoelectric coefficient of ∼4.58 pm V^-1 for the TiBrI monolayer and an out-of-plane piezoelectric coefficient of ∼1.63 pm V^-1 for the TiClI monolayer, suggesting their promising applications in piezoelectric sensors and energy storage applications. The absorption spectra of Janus TiXY monolayers are mainly distributed in the visible and infrared regions, implying that they are fantastic candidates for photoelectric and photovoltaic applications. The obtained carrier mobilities revealed that TiXY monolayers are hole-type semiconductors. Under uniaxial compressive strain, the hole mobilities of these monolayers are gradually improved, indicating that TiXY monolayers have potential applications in the field of flexible electronic devices.

Lattice thermal conductivity of Janus MoSSe and WSSe monolayers

In this work, the heat transport properties of Janus MoSSe and WSSe monolayers are systematically investigated using non-equilibrium molecular dynamics simulations. Strong size dependence of the thermal conductivity is found in the Janus MoSSe and WSSe monolayers. In the two-dimensional limit, the Mo-based Janus MoSSe monolayer shows a higher thermal conductivity but similar phonon mean free path as MoS₂, while the W-based Janus WSSe monolayer shows a similar thermal conductivity but longer phonon mean free path than WSe₂. These two Janus monolayers also present quite different temperature dependencies. With temperature increasing from 100 K to 350 K, the reduction in thermal conductivity of MoSSe is up to 28.4%, but only 12.75% in WSSe, because of the weak temperature dependence in the phonon density of states. With 2% vacancy density, the thermal conductivity of defective MoSSe is only 16.03% that of pristine MoSSe, while for defective WSSe, the thermal conductivity is 14.04% that of pristine WSSe. The strong dependence on vacancy is explained by atomic heat flux vector analysis. The present study demonstrates rich physical phenomena in the thermal transport properties of Janus transition metal dichalcogenide monolayers, which may offer a new degree of freedom for manipulating their thermal conductivity for applications including thermal management and thermoelectric devices.

Simulation of a New CZTS Solar Cell Model with ZnO/CdS Core-Shell Nanowires for High Efficiency

The numerical modeling of Cu2ZnSnS4 solar cells with ZnO/CdS core-shell nanowires of optimal dimensions with and without graphene is described in detail in this study. The COMSOL Simulation was used to determine the optimal values of core diameter and shell thickness by comparing their optical performance and to evaluate the optical and electrical properties of the different models. The deposition of a nanolayer of graphene on the layer of MoS2 made it possible to obtain a maximum absorption of 97.8% against 96.5% without the deposition of graphene.The difference between generation rates and between recombination rates of electron–hole pairs of models with and without graphene is explored.The electrical parameters obtained, such as the filling factor (FF), the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the efficiency (EFF) are, respectively, 81.7%, 6.2 mA/cm2, 0.63 V, and 16.6% in the presence of graphene against 79.2%, 6.1 mA/cm2, 0.6 V, and 15.07% in the absence of graphene. The suggested results will be useful for future research work in the field of CZTS-based solar cells with ZnO/CdS core-shell nanowires with broadband light absorption rates.

First principles study of SnX2(X = S, Se) and Janus SnSSe monolayer for thermoelectric applications

Tin-based chalcogenides are of increasing interest for thermoelectric applications owing to their low-cost, earth-abundant, and environmentally friendly nature. This is especially true for 2D materials, in which breaking of the structural symmetry plays a crucial role in tuning the electronic properties. 2D materials present a unique opportunity to manipulate the electronic and thermal properties by transforming a monolayer into a Janus monolayer. In the present work, we have investigated the thermoelectric properties of hexagonal SnS₂, SnSe₂monolayer, and Janus SnSSe monolayer. Density functional theoretical calculations points out the hexagonal Janus SnSSe monolayer as a potential high-performing thermoelectric material. Results for the Janus SnSSe monolayer show an ultra-low thermal conductivity originating from the low group velocity of the low-lying optical modes, leading to superiorzTvalues of 0.5 and 3 at 300 K and 700 K for thep-type doping, respectively.

Novel Janus GaInX3 (X = S, Se, Te) single-layers: first-principles prediction on structural, electronic, and transport properties

In this paper, the structural, electronic, and transport properties of Janus GaInX3 (X = S, Se, Te) single-layers are investigated by a first-principles calculations. All three structures of GaInX3 are examined to be stable based on the analysis of their phonon dispersions, cohesive energy, and Born's criteria for mechanical stability. At the ground state, The Janus GaInX3 is a semiconductor in which its bandgap decreases as the chalcogen element X moves from S to Te. Due to the vertical asymmetric structure, a difference in the vacuum level between the two surfaces of GaInX3 is found, leading to work functions on the two sides being different. The Janus GaInX3 exhibit high directional isotropic transport characteristics. Particularly, GaInX3 single-layers have high electron mobility, which could make them potential materials for applications in electronic nanodevices.

Valley polarization transition driven by biaxial strain in Janus GdClF monolayer

The valley degree of freedom of carriers in crystals is useful to process information and perform logic operations, and it is a key factor for valley application to realize valley polarization. Here, we propose a model that the valley polarization transition at different valley points (-K and K points) is produced by biaxial strain. Using first-principles calculations, we illustrate our idea with a concrete example of a Janus GdClF monolayer. The predicted GdClF monolayer is dynamically, mechanically and thermally stable, and is a ferromagnetic (FM) semiconductor with perpendicular magnetic anisotropy (PMA), valence band maximum (VBM) at valley points and a high Curie temperature (T_C). Due to its intrinsic ferromagnetism and spin-orbit coupling (SOC), a spontaneous valley polarization will be induced, but the valley splitting is only -3.1 meV, which provides an opportunity to achieve valley polarization transition at different valley points by strain. In the considered strain range (a/a₀: 0.94-1.06), the strained GdClF monolayer always has an energy bandgap, strong FM coupling and PMA. The compressive strain is in favour of -K valley polarization, while the tensile strain is favorable for K valley polarization. The corresponding valley splittings at 0.96 and 1.04 strains are -44.5 meV and 29.4 meV, respectively, which are higher than the thermal energy at room temperature (25 meV). Due to its special Janus structure, both in-plane and out-of-plane piezoelectric polarizations can be observed. It is found that the direction of in-plane piezoelectric polarization can be overturned by strain, and the d₁₁ values at 0.96 and 1.04 strains are -1.37 pm V^-1 and 2.05 pm V^-1, respectively. Our work paves the way to design ferrovalley materials for application in multifunctional valleytronic and piezoelectric devices by strain.

Ultrahigh thermoelectric performance of Janus α-STe2 and α-SeTe2 monolayers

Janus α-STe2 and α-SeTe2 monolayers are investigated systematically using first-principles calculations combined with semiclassical Boltzmann transport theory.

Armchair Janus MoSSe Nanoribbon with Spontaneous Curling: A First-Principles Study

Based on density functional theory, we theoretically investigate the electronic structures of free-standing armchair Janus MoSSe nanoribbons (A-MoSSeNR) with width up to 25.5 nm. The equilibrium structures of nanoribbons with spontaneous curling are obtained by energy minimization in molecular dynamics (MD). The curvature is 0.178 nm^-1 regardless of nanoribbon width. Both finite element method and analytical solution based on continuum theory provide qualitatively consistent results for the curling behavior, reflecting that relaxation of intrinsic strain induced by the atomic asymmetry acts as the driving force. The non-edge bandgap of curled A-MoSSeNR reduces faster with the increase of width compared with planar nanoribbons. It can be observed that the real-space wave function at the non-edge VBM is localized in the central region of the curled nanoribbon. When the curvature is larger than 1.0 nm^-1, both edge bandgap and non-edge bandgap shrink with the further increase of curvature. Moreover, we explore the spontaneous curling and consequent sewing process of nanoribbon to form nanotube (Z-MoSSeNT) by MD simulations. The spontaneously formed Z-MoSSeNT with 5.6 nm radius possesses the lowest energy. When radius is smaller than 0.9 nm, the bandgap of Z-MoSSeNT drops rapidly as the radius decreases. We expect the theoretical results can help build the foundation for novel nanoscale devices based on Janus TMD nanoribbons.

The coexistence of superior intrinsic piezoelectricity and thermoelectricity in two-dimensional Janus α-TeSSe

Piezoelectric and thermoelectric materials that can directly convert mechanical and thermal energies into electricity have attracted great interest because of their practical applications in overcoming the challenges of the energy crisis. In this research, a new family of two-dimensional (2D) group-VI Janus ternary compounds with α and γ phases are predicted. After the stability testing, only the α-TeSSe monolayer has dynamic and thermal stability. The band structure and the optic, piezoelectric, and thermoelectric performances of the Janus α-TeSSe monolayer are calculated via first-principles calculations. Janus α-TeSSe is a narrow indirect bandgap semiconductor with a value of 0.953 eV at the HSE06 functional considering the spin-orbit coupling (SOC), which is beneficial to its thermoelectric performance, and its excellent absorption coefficients indicate that it may be a promising optoelectronic material. The piezoelectric calculations show that Janus α-TeSSe exhibits not only appreciable in-plane piezoelectricity (d₁₁ = 17.17 pm V^-1) but also superior vertical piezoelectricity (d₃₁ = 0.22 pm V^-1). Furthermore, a new TransOpt code is used to calculate the electrical transport coefficients with a constant electron-phonon coupling approximation, which is more accurate than the constant relaxation time approximation. The origin of ultralow lattice thermal conductivity is also discussed in detail. Finally, ultrahigh ZT values of 0.77 and 1.95 occur in n-type and p-type doping at 600 K, respectively, indicating that it is a promising thermoelectric material. Our work demonstrates that Janus α-TeSSe monolayers have potential applications in optoelectronic, piezoelectric, and thermoelectric devices, which will greatly stimulate research-related experiments.

Stretchable AgX (X = Se, Te) for Efficient Thermoelectrics and Photovoltaics

Transition metal chalcogenides (TMCs) have gained worldwide interest owing to their outstanding renewable energy conversion capability. However, the poor mechanical flexibility of most existing TMCs limits their practical commercial applications. Herein, triggered by the recent and imperative synthesis of highly ductile α-Ag₂S, an effective approach based on evolutionary algorithm and ab initio total-energy calculations for determining stable, ductile phases of bulk and two-dimensional Ag_xSe_1-x and Ag_xTe_1-x compounds was implemented. The calculations correctly reproduced the global minimum bulk stoichiometric P2₁2₁2₁-Ag₈Se₄ and P2₁/c-Ag₈Te₄ structures. Recently reported metastable AgTe₃ was also revealed but it lacks dynamical stability. Further single-layered screening unveiled two new monolayer P4/nmm-Ag₄Se₂ and C2-Ag₈Te₄ phases. Orthorhombic Ag₈Se₄ crystalline has a narrow, direct band gap of 0.26 eV that increases to 2.68 eV when transforms to tetragonal Ag₄Se₂ monolayer. Interestingly, metallic P2₁/c-Ag₈Te₄ changes to semiconductor when thinned down to monolayer, exhibiting a band gap of 1.60 eV. Present findings confirm their strong stability from mechanical and thermodynamic aspects, with reasonable Vickers hardness, bone-like Young's modulus (E) and high machinability observed in bulk phases. Detailed analysis of the dielectric functions ε(ω), absorption coefficient α(ω), power conversion efficiency (PCE) and refractive index n(ω) of monolayers are reported for the first time. Fine theoretical PCE (SLME method ∼11-28%), relatively high n(0) (1.59-1.93), and sizable α(ω) (10⁴-10⁵ cm^-1) that spans the infrared to visible regions indicate their prospects in optoelectronics and photoluminescence applications. Effective strategies to improve the temperature dependent power factor (PF) and figure of merit (ZT) are illustrated, including optimizing the carrier concentration. With decreasing thickness, ZT of p-doped Ag-Se was found to rise from approximately 0.15-0.90 at 300 K, leading to a record high theoretical conversion efficiency of ∼12.0%. The results presented foreshadow their potential application in a hybrid device that combines the photovoltaic and thermoelectric technologies.

Origins of Minimized Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in WS2/WSe2 Lateral Superlattice

We report a configuration strategy for improving the thermoelectric (TE) performance of two-dimensional transition metal dichalcogenide WS₂ based on the experimentally prepared WS₂/WSe₂ lateral superlattice (LS) crystal. On the basis of density function theory combined with a Boltzmann transport equation, we show that the TE figure of merit zT of monolayer WS₂ is remarkably enhanced when forming into a WS₂/WSe₂ LS crystal. This is primarily ascribed to the almost halved lattice thermal conductivity due to the enhanced anharmonic processes. Electronic transport properties parallel (xx) and perpendicular (yy) to the superlattice period are highly symmetric for both p- and n-doped LS owing to the nearly isotropic lifetime of charger carriers. The spin-orbital effect causes a significant split of conduction band and leads to three-fold degenerate sub-bands and high density of states (DOS), which offers opportunity to obtain a high n-type Seebeck coefficient (S). Interestingly, the separated degenerate sub-bands and upper conduction band in monolayer WS₂ form a remarkable stair-like DOS, yielding a higher S. The hole carriers with much higher mobility than electrons reveal the high p-type power factor, and the potential to be good p-type TE materials with optimal zT exceeds 1 at 400 K in WS₂/WSe₂ LS.

Group-IV(A) Janus dichalcogenide monolayers and their interfaces straddle gigantic shear and in-plane piezoelectricity

Inversion symmetry in the 1T-phase of pristine dichalcogenide monolayer MX2 (M = Ge, Sn; X = S, Se) is broken in their Janus structures, MXY (M = Ge, Sn; X ≠ Y = S, Se), which induces an in-plane piezoelectric coefficient, d22 = 4.09 (2.15) pm V-1 and a shear piezoelectric coefficient, d15 = 7.90 (13.68) pm V-1 in the GeSSe (SnSSe) monolayer. High flexibility arising from the small Young's modulus (60-70 N m-1) found in these Group-IV(A) Janus monolayers makes them suitable for large-scale strain engineering. Application of 7% uniaxial tensile strain increases d22 and d15 colossally to 267.07 pm V-1 and 702.34 pm V-1, respectively, thereby reaching the level of bulk piezoelectric perovskite materials. When the Janus GeSSe monolayers are stacked to form a van der Waals (vdW) homo-bilayer, d22 lies between 19.87 and 73.26 pm V-1, while d15 falls into the range between 83.01 and 604.34 pm V-1, depending on the stacking order. The chalcogen exchange energies and overall stabilities of the monolayers and bilayers confirm the feasibility of their experimental synthesis. Moreover, hole mobility in the GeSSe monolayer is greater than the electron mobility along its zigzag directions (μe = 883 cm2 V-1 s-1 and μh = 1134 cm2 V-1 s-1). Therefore, the semiconducting, flexible, and piezoelectric Janus GeSSe monolayer and bilayers are immensely promising for futuristic applications in energy harvesting, nanopiezotronic field-effect transistors, atomically thin sensors, shear/torsion actuators, transducers, self-powered circuits in nanorobotics, and electromechanical memory devices, and biomedical and other nanoelectronic applications.

The first-principles and BTE investigation of phonon transport in 1T-TiSe2

Through the first-principles density functional theory and the phonon Boltzmann transport equation, we investigated the phonon transport characteristics inside 1T-TiSe₂.

Highly Sensitive Gas Sensing Material for Environmentally Toxic Gases Based on Janus NbSeTe Monolayer

Recently, a new family of the Janus NbSeTe monolayer has exciting development prospects for two-dimensional (2D) asymmetric layered materials that demonstrate outstanding properties for high-performance nanoelectronics and optoelectronics applications. Motivated by the fascinating properties of the Janus monolayer, we have studied the gas sensing properties of the Janus NbSeTe monolayer for CO, CO2, NO, NO2, H2S, and SO2 gas molecules using first-principles calculations that will have eminent application in the field of personal security, protection of the environment, and various other industries. We have calculated the adsorption energies and sensing height from the Janus NbSeTe monolayer surface to the gas molecules to detect the binding strength for these considered toxic gases. In addition, considerable charge transfer between Janus monolayer and gas molecules were calculated to confirm the detection of toxic gases. Due to the presence of asymmetric structures of the Janus NbSeTe monolayer, the projected density of states, charge transfer, binding strength, and transport properties displayed distinct behavior when these toxic gases absorbed at Se- and Te-sites of the Janus monolayer. Based on the ultra-low recovery time in the order of μs for NO and NO2 and ps for CO, CO2, H2S, and SO2 gas molecules in the visible region at room temperature suggest that the Janus monolayer as a better candidate for reusable sensors for gas sensing materials. From the transport properties, it can be observed that there is a significant variation of I-V characteristics and sensitivity of the Janus NbSeTe monolayer before and after adsorbing gas molecules demonstrates the feasibility of NbSeTe material that makes it an ideal material for a high-sensitivity gas sensor.

The Effect of Janus Asymmetry on Thermal Transport in SnSSe

Several ternary "Janus" metal dichalcogenides such as {Mo,Zr,Pt}-SSe have emerged as candidates with significant potential for optoelectronic, piezoelectric, and thermoelectric applications. SnSSe, a natural option to explore as a thermoelectric given that its "parent" structures are SnS₂ and SnSe₂ has, however, only recently been shown to be mechanically stable. Here, we calculate the lattice thermal conductivities of the Janus SnSSe monolayer along with those of its parent dicalchogenides. The phonon frequencies of SnSSe are intermediate between those of SnSe₂ and SnS₂; however, its thermal conductivity is the lowest of the three and even lower than that of a random Sn[S_0.5Se_0.5]₂ alloy. This can be attributed to the breakdown of inversion symmetry and manifests as a subtle effect beyond the reach of the relaxation-time approximation. Together with its low favorable power factor, its thermal conductivity confirms SnSSe as a good candidate for thermoelectric applications.

Comparative investigation of the thermal transport properties of Janus SnSSe and SnS2 monolayers

Recently, Janus two-dimensional (2D) materials as a new member of 2D derivatives have been receiving much attention due to their novel properties. In this work, the lattice thermal conductivity κ_L of the Janus SnSSe monolayer is investigated based on first-principles calculations, while that of the SnS₂ monolayer is studied for comparison. It is found the the κ_L values of SnSSe and SnS₂ are 13.3 and 11.0 W m^-1 K^-1 at room temperature, and acoustic branches dominate their thermal transport. Weaker phonon anharmonicity in SnSSe leads to a slightly higher κ_L, though it has weaker phonon harmonicity. The smaller Grüneisen parameters of TA and LA phonons lower than 1 THz in SnSSe indicate weaker phonon anharmonicity, resulting in a higher κ_L. Finally, the size effect and boundary effect are also investigated, exhibiting that the κ_L can further decrease at the nanoscale. Our work suggests that Janus SnSSe and SnS₂ have a much lower κ_L compared with conventional transition metal dichalcogenides (TMDs) and are potential competitors in the thermoelectric field.

Thermoelectric properties of 1 T monolayer pristine and Janus Pd dichalcogenides

In this paper, we investigate the stability and thermoelectric properties of 1 T PdSSe, PdSTe and PdSeTe Janus structures using density functional theory (DFT). All three systems are narrow gap semiconductors with indirect bandgaps of 0.94 eV, 0.33 eV and 0.34 eV respectively. Compared to transition metal dichalcogenide (TMD) monolayers, PdS₂ and PdSe₂ are semiconductors with wider indirect bandgaps of 1.29 eV and 0.69 eV respectively. Phonon dispersion calculations demonstrate that all pristine and Janus structures are mechanically stable despite the presence of negligible negative frequencies around the [Formula: see text] point in PdSTe and PdSeTe. Inspection of the lattice thermal conductivity ([Formula: see text]) shows that these structures are slightly anisotropic in the x and y  directions except for PdSe₂ which shows a higher degree of anisotropy. Influenced by the values of [Formula: see text], the thermal electronic conductivity ([Formula: see text]), the electronic conductivity ([Formula: see text]) and the Seebeck effect (S), the figure of merit along the x (ZT ^xx )and y  (ZT ^yy ) directions register the largest values in the case of electron doping for the PdSe₂ and PdSeTe 2D crystals. Interestingly, the figures of merit of the Janus structures are larger than their corresponding pristine PdX₂ (X  =  S, Se) structures. Once synthesized, such information is crucial for the implementation of the PdXY (Y  =  Se, Te) structures in industrial applications.

Phonon and electron transport in Janus monolayers based on InSe

We systematically investigated the phonon and electron transport properties of monolayer InSe and its Janus derivatives including monolayer In₂SSe and In₂SeTe by first-principles calculations. The breaking of mirror symmetry produces a distinguishable A ₁ peak in the Raman spectra of monolayer In₂SSe and In₂SeTe. The long-range harmonic and anharmonic interactions play an important role in the heat transport of the group-III chalcogenides. The room-temperature thermal conductivity ([Formula: see text]) of monolayer InSe, In₂SSe and In₂SeTe are 44.6, 46.9, and 29.9 W (mK)^-1, respectively. There is a competition effect between atomic mass, phonon group velocity and phonon lifetime. The [Formula: see text] can be further effectively modulated by sample size for the purpose of thermoelectric applications. Meanwhile, monolayer In₂SeTe exhibits a direct band gap of 1.8 eV and a higher electron mobility than that of monolayer InSe, due to the smaller electron effective mass caused by tensile strain on the Se side and smaller deformation potential. These results indicate that 2D Janus group-III chalcogenides can provide a platform to design the new electronic, optoelectronic and thermoelectric devices.

Transport and topological properties of ThOCh(Ch: S, Se and Te) in bulk and monolayer: a first principles study

The present study unveils the topological insulating nature of Th-based oxy-chalcogenides and their transport properties which are less explored. A systematic analysis of electronic, topological, mechanical, dynamical and thermoelectric properties of ThOCh (Ch: S, Se and Te) in bulk and monolayer is presented. The effect of spin-orbit coupling is found to be appreciable in ThOTe compared to ThOS and ThOSe, causing a strong topological nature in bulk ThOTe. The detailed analysis of electronic structure, Z₂ topological invariant and conducting surface states support the strong topological nature in bulk ThOTe. From thermoelectric studies, ThOS and ThOSe are found to be good thermoelectric candidates with heavy carrier doping (around 10²⁰ cm^-3). To explore further, we have applied hydrostatic strain on bulk ThOCh and found that all the compounds are dynamically stable and show topological metallic behavior. The appearance of highly linearized Dirac points in the same energy range at different high symmetry points in the BZ indicate the presence of nodal line in ThOS and ThOSe without spin-orbit coupling and with the inclusion of spin-orbit coupling, the nodal line is found to be disappear. This variation in bands with and without spin-orbit coupling might indicate the topological nature in monolayer ThOCh. The thermoelectric calculations for monolayer shows an enhancement in electrical conductivity scaled by relaxation time by an order of ten compared to bulk and the carrier independent thermoelectric properties in monolayer might fetch good thermoelectric device applications. Overall, the present study explores yet another series of potential candidates for topological and thermoelectric properties in both bulk and layer forms.

Second harmonic generation in Janus MoSSe a monolayer and stacked bulk with vertical asymmetry

A recently synchronized Janus TMD material with broken out-of-plane symmetry offers a vertical dipole to enhance nonlinear optical behavior. Here, by comparing the second harmonic generation properties of MoS₂ and MoSSe monolayers, we investigated the nonzero out-of-plane SHG susceptibilities of a Janus MoSSe 2D material. A three-fold enhancement of out-of-plane SHG susceptibilities exists in three stacked bulks of Janus MoSSe compared to that in the monolayer. A sensitivity to their stack pattern is also found. The broken out-of-plane symmetry, vertical dipole, and intrinsic tunable electronic properties of Janus two-dimensional materials make MoSSe a promising nanomaterial for nonlinear optical devices.

First-principles investigations of the stability and electronic properties of fluorinated Janus MoSSe monolayer

The stability and electronic structures of fluorinated Janus MoSSe (MoSSe-Fx, x = 0–16) were investigated by the first-principles method. Energetically, the Setop site of Janus MoSSe is the most favorable site for F-adsorption. Based on the adsorption, binding and formation energy analysis, it seems the fluorinated Janus MoSSe is stable. The analysis of the electronic density distribution and orbital hybrid shows that the fluorinated Janus MoSSe forms stable structure as well. Then, the electronic structure and work function change with the concentration of F atoms analyzed. With the increase of adsorbed F atoms, the bandgap of Janus MoSSe-Fx decreases from 1.456 (pristine case, [Formula: see text]) to 1.073[Formula: see text]eV (semi-fluorinated case, [Formula: see text]), and changes from direct to indirect bandgap semiconductor. It is noteworthy that there are some additional doping levels near the valence band after F adsorbed, which originated from the F 2[Formula: see text] doping states. The charge population analysis shows that electrons transfer was from Se to F atoms. It is worth noting that the charge on F atom (MoSSe-F16) is two times more than Se atoms in pristine Janus MoSSe (MoSSe-F0). As a result, the built-in electric field pointed from Mo to F atoms is enhanced, resulting in the tremendous increase of the work function for fluorinated Janus MoSSe. The work function changes from 5.22[Formula: see text]eV ([Formula: see text]) in pristine case to 8.30[Formula: see text]eV after semi-fluorinated ([Formula: see text]). Therefore, due to the adjustable work function and built-in electric field, the fluorinated Janus MoSSe monolayer shows better properties for applications in the piezoelectric device, optoelectronic device or photocatalyst.

Born effective charge removed anomalous temperature dependence of lattice thermal conductivity in monolayer GeC

Due to potential applications in nano- and opto-electronics, two-dimensional (2D) materials have attracted tremendous interest. Their thermal transport properties are closely related to the performance of 2D materials-based devices. Here, the phonon transports of monolayer GeC with a perfect planar hexagonal honeycomb structure are investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Without inclusion of Born effective charges (Z ^*) and dielectric constants ([Formula: see text]), the lattice thermal conductivity ([Formula: see text]) decreases almost linearly above 350 K, deviating from the usual [Formula: see text] law. The underlying mechanism is because the contribution to [Formula: see text] from high-frequency optical phonon modes increases with increasing temperature, and the contribution exceeds one from acoustic branches at high temperature. These can be understood by huge phonon band gap caused by large difference in atom mass between Ge and C atoms, which produces important effects on scattering process involving high-frequency optical phonon. When considering Z ^* and [Formula: see text], the phonon group velocities and phonon lifetimes of high-frequency optical phonon modes are obviously reduced with respect to ones without Z ^* and [Formula: see text]. The reduced group velocities and phonon lifetimes give rise to small contribution to [Formula: see text] from high-frequency optical phonon modes, which produces the the traditional [Formula: see text] relation in monolayer GeC. Our works highlight the importance of Z ^* and [Formula: see text] to investigate phonon transports of monolayer GeC, and motivate further theoretical or experimental efforts to investigate thermal transports of other 2D materials.

Predicted Janus SnSSe monolayer: a comprehensive first-principles study

The dynamically and mechanically stable Janus SnSSe monolayer has distinctive electronic, optical, piezoelectric and transport properties.

Enhanced thermoelectric performance of monolayer MoSSe, bilayer MoSSe and graphene/MoSSe heterogeneous nanoribbons

Monolayer MoSSe, bilayer MoSSe and graphene/MoSSe heterostructure nanoribbons have been simulated to exhibit a high thermoelectric figure of merit.

First-principles calculations of thermal transport properties in MoS2/MoSe2 bilayer heterostructure

The van der Waals interaction in a MoS₂/MoSe₂ bilayer heterostructure has a significant effect on its lattice thermal conductivity.

Nonmonotonic strain dependence of lattice thermal conductivity in monolayer SiC: a first-principles study

The lattice thermal conductivities (200, 250, 300 and 400 K) of a SiC monolayer versus strain, showing nonmonotonic strain dependence.

Bandgap tuning in MoSSe bilayers: synergistic effects of dipole moment and interlayer distance

The gap of bilayer MoSSe is affected by the synergistic effects of dipole moment and interlayer distance.