The Janus structure, by combining properties of different transition metal dichalcogenide (TMD) monolayers in a single polar material, has attracted increasing research interest because of their particular structure and potential application in electronics, optoelectronics and piezoelectronics. In this work, Janus SnSSe monolayer is predicted by means of first-principles calculations, which exhibits dynamic and mechanical stability. By using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC), the Janus SnSSe monolayer is found to be an indirect band-gap semiconductor, whose gap can easily be tuned by strain. High carrier mobilities are obtained for SnSSe monolayer, and the hole mobility is higher than the electron mobility. For SnSSe monolayer, a uniaxial strain in the basal plane can induce both strong in-plane and much weaker out-of-plane piezoelectric polarizations, which reveals the potential as a piezoelectric two-dimensional (2D) material. The high absorption coefficients in the visible light region are observed, suggesting a potential photocatalytic application. Calculated results show that SnSSe monolayer has very high power factor, making it a promising candidate for thermoelectric applications. Our works reveal that the Janus SnSSe structure can be fabricated with unique electronic, optical, piezoelectric and transport properties, and can motivate related experimental works.
The discrepancy between the bright theoretical projections for two-dimensional (2D) Janus structures and the lack of experimental realisation of these structures motivated us to study the effect of structural disorder on the stability of MoSSe, SnSSe, PtSSe, In2SSe and GaInSe2. The calculation results demonstrate that the difference between metal-sulfur and metal-selenium bonds makes Janus structures frustrated and less energetically favourable than less ordered allotropes of the same compounds. This result explains the difficulties encountered in experimental fabrication of these materials. I…
A contrastive investigation of the symmetry aspects of phonons in monolayer chromium trihalides and their Janus structures Y$_3$-Cr$_2$-X$_3$ (X, Y = F, Cl, Br, I) by group theory is presented. We first classify all phonons at the Brillouin-zone center ($Γ$) into the irreducible representation. Then the infrared and Raman activity of optic phonons, Raman tensors, and the possible polarization assignments of R active phonons are predicted. Base on these results, we clarify the the discrepancy about the Raman activity o optic modes in monolayer CrI$_3$. Besides, we find that the Raman and infrar…
Janus transition-metal dichalcogenide monolayers are fully artificial materials, where one plane of chalcogen atoms is replaced by chalcogen atoms of a different type. Theory predicts an in-built out-of-plane electric field, giving rise to long-lived, dipolar excitons, while preserving direct-bandgap optical transitions in a uniform potential landscape. Previous Janus studies had broad photoluminescence (>15 meV) spectra obfuscating their excitonic origin. Here, we identify the neutral, and negatively charged inter- and intravalley exciton transitions in Janus WSeS monolayer with $\sim 6$ meV …
We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit-torque memories. Using first principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling …
Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolv…
In an outstanding experimental advance in the field of two-dimensional nanomaterials, cuprous iodide (CuI) and silver iodide (AgI) monolayers have been grown via a novel graphene encapsulation synthesis approach [Adv.Mater.2022, 34, 2106922]. Inspired by this accomplishment, we conduct first-principles calculations to investigate the elastic, phononic thermal transport, electronic, and optical properties of the native CuI and AgI and Janus Cu2BrI and Ag2BrI monolayers. Electronic and excitonic optical properties are elaborately studied using the many-body perturbation theory on the basis of GW…
Due to their great potential in electronics, optoelectronics and piezoelectronics, Janus transition metal dichalcogenide (TMD) monolayers have attracted increasing research interest, the MoSSe of which with sandwiched S-Mo-Se structure has been synthesized experimentally. In this work, the biaxial strain dependence of electronic structures and transport properties of Janus PtSSe monolayer is systematically investigated by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). Calculated results show that SOC has a detrimental effect on power factor of PtSSe monolayer, w…
We present first-principles results on the structural, electronic, and magnetic properties of a new family of two-dimensional antiferromagnetic (AFM) manganese chalcogenides, namely monolayer MnX and Janus XMnY (X, Y= S, Se, Te), among which monolayer MnSe was recently synthesized in experiments [\href{https://pubs.acs.org/doi/abs/10.1021/acsnano.1c05532}{ACS Nano 15 (8),13794 (2021)}]. By carrying out calculations of the phonon dispersion and \textit{ab-initio} molecular dynamics simulations, we first confirmed that these systems, characterized by an unconventional strongly coupled bilayer at…
Achieving combination of spin and valley polarized states with topological insulating phase is pregnant to promote the fantastic integration of topological physics, spintronics and valleytronics. In this work, a spin-valley-coupled quantum spin Hall insulator (svc-QSHI) is predicted in Janus monolayer $\mathrm{CSb_{1.5}Bi_{1.5}}$ with dynamic, mechanical and thermal stabilities. The inequivalent valleys have opposite Berry curvature and spin moment, which can produce a spin-valley Hall effect. In the center of Brillouin zone, a Rashba-type spin splitting can be observed due to missing horizont…
A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculation…
Based on first-principles calculations, we studied the geometric configuration, stability and electronic structure of the two-dimensional Janus MoTeB2. The MoTeB2 monolayer is semimetal, and its attractive electronic structure reveals the perfect electron-hole compensation. Moreover, the electron-type and hole-type bands of the MoTeB2 monolayer are easily adjustable by external stain and charge doping, such as the switch of carrier polarity by charge doping, and the metal-semiconductor transition under tensile stain. These properties allow the MoTeB2 monolayer to be a controllable two-dimensio…
Combined with first-principles calculations and semiclassical Boltzmann transport theory, Janus α-STe2 and α-SeTe2 monolayers are investigated systematically. Janus α-STe2 and α-SeTe2 monolayers are indirect semiconductors with band gaps of 1.20 and 0.96 eV. It is found they possess ultrahigh figure of merit (ZT) values of 3.9 and 4.4 at 500 K, much higher than that of the pristine α-Te monolayer (2.8). The higher ZT originates from Janus structures reduce lattice thermal conductivities remarkably compared with pristine α-Te monolayer. The much higher phonon anharmonicity in Janus monolayers l…
Two-dimensional transition metal di-chalcogenides (TMDCs) have shown great potential as good quality thermoelectric materials at high temperature since past few years due to their suitable band gap tunabilty, low dimensionality and fantastic combination of electrical conductivity and lattice thermal conductivity. Here, a first principles calculations of electronic and thermoelectric performance of two dimensional monolayer HfS 2 , HfSe 2 and their Janus monolayer HfSSe has been performed with the help of density functional theory and Boltzmann transport equation. Thermodynamical stability of a…
Janus monolayer transition metal dichalcogenides, where one of the two chalcogen layers is substituted with a different kind of chalcogen atoms, are pushing the properties of two dimensional materials into new territories. Yet only little is known about this new kind of material class, mainly due to the challenging synthesis. In this work we propose a method to prepare high quality Janus MoSSe monolayers from as-exfoliated MoSe2 by thermal sulfurization. With this we aim to pave a way for more exotic Janus monolayers, which have been out of the experimental reach thus far. The synthesized MoSS…
Two-dimensional (2D) ferromagnetic materials present promising candidates for spintronic devices, and the half-metallic materials with 100% spin polarization at Fermi energy level are highly desired for many spin-based devices. 2D Janus materials have attracted great attention in recent years due to their excellent properties induced by breaking the symmetry. Here, using the density functional theory, we report that the Janus V2AsP monolayer demonstrates a charming ferromagnetic half-metallic feature. It is dynamically stable in view of the absence of imaginary frequency phonon. The half-metal…
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 produce a distinguishable A1 peak in the Raman spectra of monolayer In2SSe and In2SeTe. The room-temperature thermal conductivity (\k{appa}) of monolayer InSe, In2SSe and In2SeTe is 44.6, 46.9, and 29.9 W/(m K), respectively. There is a competition effect between atomic mass, phonon group velocity and phonon lifetime. The \k{appa} can be further effectively modulated by sa…
The new class of Janus two-dimensional (2D) transition-metal dichalcogenides with two different interfaces are currently gaining increasing attention due to their distinct properties different from the typical 2D materials. Here, we show that in-plane anisotropy of a 2D atomic crystal, like ReS$_{2}$ or ReSe$_{2}$, allows formation of a large number of inequivalent Janus monolayers. We use first-principles calculations to investigate the structural stability of 29 distinct ReX$_{2-x}$Y$_{x}$ ($\mathrm{X,Y \in \{S,Se\}}$) structures, which can be obtained by selective exchange of exposed chalco…
Antiferromagnetic skyrmion crystal (AF-SkX), a regular array of antiferromagnetic skyrmions, is a fundamental phenomenon in the field of condensed-matter physics. So far, only very few proposals have been made to realize the AF-SkX, and most based on three-dimensional (3D) materials. Herein, using first-principles calculations and Monte-Carlo simulations, we report the identification of AF-SkX in two-dimensional lattice of Janus monolayer CrSi2N2As2. Arising from the broken inversion symmetry and strong spin-orbit coupling, large Dzyaloshinskii-Moriya interaction is obtained in Janus monolayer…
We present a comprehensive theory of the magnetic phases in monolayer nickel-based Janus dihalides (NiIBr, NiICl, NiBrCl) through a combination of first-principles calculations and atomistic simulations. Phonon band structure calculations and finite temperature molecular dynamics simulations show that Ni-dihalide Janus monolayers are stable. The parameters of the interacting spin Hamiltonian extracted from ab initio calculations show that nickel-dihalide Janus monolayers exhibit varying degrees of magnetic frustration together with a large Dzyaloshinskii-Moriya interaction due to their inheren…
We investigate the magnetic phase diagram of 1T-vanadium dichalcogenide monolayers in Janus configuration (VSeTe, VSSe, and VSTe) from first principles. The magnetic exchange, magnetocrystalline anisotropy and Dzyaloshinskii-Moriya interaction (DMI) are computed using density functional theory calculations, while the temperature- and field-dependent magnetic phase diagram is simulated using large-scale atomistic spin modeling in the presence of thermal fluctuations. The boundaries between magnetic ordered phases and paramagnetic phases are determined by cross-analyzing the average topological …
A two-dimensional (2D) material with piezoelectricity, topological and ferromagnetic (FM) orders, namely 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 $\mathrm{Fe_2IX}$ (X=Cl and Br) with dynamic, mechanical and thermal stabilities is predict to be room-temperature PQAHI. At the absence of spin-orbit coupling (SOC), monolayer $\mathrm{Fe_2IX}$ (X=Cl and Br) is a half Dirac semimetal state. When the SOC is included, these monolayers become qua…
The structural, electronic, and magnetic properties of VSSe, VSeTe, VSTe monolayers in both 2H and 1T phases are investigated via first-principles calculations. The 2H phase is energetically favorable in VSSe and VSeTe, whereas the 1T phase is lower in energy in VSTe. For V-based Janus monolayers in the 2H phase, calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. As such, they should not exhibit a ferromagnetic phase transition, but instead, a Berezinskii-Kosterlitz-Thouless (BKT) transition. A classical XY model with nearest-neighbor coupling estimates critica…
Inspired by the successful synthesis of two-dimensional (2D) V-based Janus dichloride monolayers with intrinsic ferromagnetism and high Curie temperature (T$_{c}$), the electronic structure, spin-valley splitting and magnetic anisotropy of Janus 2H-VSeS monolayers are investigated in detailed using first-principles calculations. The results show that the Janus 2H-VSeS monolayer exhibits a large valley splitting of 105meV, high T$_{c}$ of 278K and good magnetocrystalline anisotropy (0.31meV) contributed by the in-plane d$_{x^{2}-y^{2}}$/d$_{xy}$ orbitals of V atoms. The biaxial strain ($-$8%<$\…
Two-dimensional (2D) intrinsic ferromagnetic semiconductors are expected to stand out in the spintronic field. Recently, the monolayer VI$_{3}$ has been experimentally synthesized but the weak ferromagnetism and low Curie temperature ($T_C$) limit its potential application. Here we report that the Janus structure can elevate the $T_C$ to 240 K. And it is discussed that the reason for high $T_C$ in Janus structure originates from the lower virtual exchange gap between $t_{2g}$ and $e_{g}$ states of nearest-neighbor V atoms. Besides, $T_C$ can be further substantially enhanced by tensile strain …
The valley degrees 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 the 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. By the first-principle calculations, we illustrate our idea with a concrete example of Janus $\mathrm{GdClF}$ monolayer. The predicted $\mathrm{GdClF}$ monolayer is dynamically, mechanically and thermally stable, and is a ferromagnetic (FM) semiconductor with …
We report one-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these two-dimensional semiconductor monolayers takes place upon the thermodynamic equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy and the growth mechanisms a…
Intrinsic characteristic radius drives phonon anomalies in Janus transition metal dichalcogenide nanotubes
School of Mechatronic Engineering and Automation, Shanghai University（上海大学机电工程与自动化学院）；Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai Institute of Applied Mathematics and Mechanics, Shanghai Frontier Science Center of Mechanoinformatics, School of Mechanics and Engineering Science, Shanghai University（上海大学力学与工程科学学院）
Magnetic skyrmions are nano-scale spin structures that are promising for ultra-dense memory and logic devices. Recent progresses in two-dimensional magnets encourage the idea to realize skyrmionic states in freestanding monolayers. However, monolayers such as CrI3 lack Dzyaloshinskii-Moriya interactions (DMI) and thus do not naturally exhibit skyrmions but rather a ferromagnetic state. Here we propose the fabrication of Cr(I,X)3 Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and top-layer Br or Cl atoms. By performing first-principles calculations and Mo…
The electronic correlation may have essential influence on electronic structures in some materials with special structure and localized orbital distribution. In this work, taking Janus monolayer FeClF as a concrete example, the correlation effects on its electronic structures are investigated by using generalized gradient approximation plus $U$ (GGA+$U$) approach. For perpendicular magnetic anisotropy (PMA), the increasing electron correlation effect can induce the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous Hall (QAH) to HVM to FV transitions. For QAH state, there are a u…
In analogy to transition-metal dichalcogenide (TMD) monolayers, which have wide applications in photoelectricity, piezoelectricity and thermoelectricity, Janus MoSSe monolayer has been successfully synthesized by substituting the top Se atomic layer in $\mathrm{MoSe_2}$ by S atoms. In this work, Janus monolayer ZrSSe is proposed by ab initio calculations. For the electron part, the generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is used as exchange-correlation potential, while GGA for lattice part. Calculated results show that the ZrSSe monolayer is dynamically and mech…
First-principles calculations on the mechanical, electronic, magnetic and optical properties of two-dimensional Janus Cr$_2$TeX (X= P, As, Sb) monolayers
The seminal experimental discovery of the remarkably stable MoSi$_2$N$_4$ monolayer has led to a handful of predicted magnetic two-dimensional (2D) materials in the MA$_2$Z$_4$ family (M = transition metals, A = Si, Ge, and Z = N, P, As). These magnetic monolayers were predicted to be dynamically stable, but none of them has been synthesized to date. In this Research Letter, from first-principles thermodynamic stability analysis, we demonstrate that only the nitrides are thermodynamically stable and this occurs under N-rich conditions. Based on this finding, we propose two ferromagnetic, semic…
Multiple magnetic states, valley electronics, and topological phase transitions in two-dimensional Janus XYZH (X = Sc, Y, La, Y = Cl, Br, I, and Z = S, Se, Te): From monolayers to bilayers
Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are a new class of materials with unique physical properties. However, the carrier mobility of most Janus TMDs calculated by deformation potential theory (DPT) is not reliable due to the unconsidered part of lattice scattering. In this work, we propose a new method of Born effective charge (BEC) to calculate the carrier mobility of Janus TMDs by including the important factors that neglected in the DPT. The BEC could be used in the calculation of both pure and defective Janus TMDs by employing density functional perturbation th…
College of Physical Science and Technology, Guangxi Normal University（广西师范大学物理科学与技术学院）；College of Physical Science and Technology, Yangzhou University（扬州大学物理科学与技术学院）
Two-dimensional (2D) XY magnets with easy magnetization planes support the nontrivial topological spin textures whose dissipationless transport is highly desirable for 2D spintronic devices. Here, we predicted that Janus monolayer V$_{2}$XN (X = P, As) with a square lattice are 2D-XY ferromagnets by first-principles calculations. Both the magnetocrystalline anisotropy and magnetic shape anisotropy favor an in-plane magnetization, leading to an easy magnetization $xy$-plane in Janus monolayer V$_{2}$XN. Resting on the Monte Carlo simulations, we observed the Berezinskii-Kosterlitz-Thouless (BKT…
Transition Metal Dichalcogenide (TMD) monolayers have most widely studied due to their unique physical properties. Recently, Janus TMD Monolayer MoSSe with sandwiched S-Mo-Se structure has been synthesized by replacing the top S atomic layer in $\mathrm{MoS_2}$ with Se atoms. In this work, we systematically investigate the phonon transport and lattice thermal conductivity ($κ_L$) of MoSSe monolayer by first-principles calculations and linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Calculated results show that the $κ_L$ of MoSSe monolayer is ver…
The absence of both the inversion symmetry and out-of-plane mirror symmetry together with spin-orbit coupling (SOC) can induce novel electronic and piezoelectric properties. In this work, the piezoelectric properties along with carrier mobilities of Janus monolayer XTeI (X=Sb and Bi) are studied by density functional theory (DFT). By using generalized gradient approximation (GGA) plus SOC, they are found to be indirect gap semiconductors with the Rashba spin splitting. The piezoelectric tensors of Janus monolayer XTeI (X=Sb and Bi) are reported by using density functional perturbation theory (…
Two-dimensional rare-earth Janus 2$\textit{H}$-Gd$\textit{XY}$ ($\textit{X}$,$\textit{Y}$=Cl, Br, I, $\textit{X}$$\neq$$\textit{Y}$) monolayers: Bipolar ferro-magnetic semiconductors with high Curie temperature and large valley polarization
Gas sensing mechanism of H2S, NH3, NO2 and NO toxic gases on transition metal dichalcogenides based Janus MoSSe monolayers are investigated using the density functional theory. The pristine and defect included MoSSe layers are considered as a host material for adsorption study. Three types of defects (i) molybdenum vacancy, (ii) selenium vacancy, and (iii) sulfur/selenium vacancy are studied to understand their impact on electronic properties and sensing of these gas molecules. The formation energy is computed to predict the stability of these defects and noticed that selenium vacancy is the m…
Janus single-layer transition metal dichalcogenides, in which the two chalcogen layers have a different chemical nature, push chemical composition control beyond what is usually achievable with van der Waals heterostructures. Here we report such a novel Janus compound, SPtSe, which is predicted to exhibit strong Rashba spin-orbit coupling. We synthetized it by conversion of a single-layer of PtSe$_2$ on Pt(111) via sulphurization under H$_2$S atmosphere. Our in situ and operando structural analysis with grazing incidence synchrotron X-ray diffraction reveals the process by which the Janus allo…
Tuning physical properties of transition metal dichalcogenide (TMD) monolayers by strain engineering have most widely studied, and recently Janus TMD monolayer MoSSe has been synthesized. In this work, we systematically study biaxial strain dependence of electronic structures and transport properties of Janus TMD MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). It is found that SOC has a noteworthy detrimental influence on power factor in p-type MoSSe, WSSe, n-type WSTe, p-type MoSeTe and WSeTe, and has a negligib…
Recently, the quantum anomalous Hall (QAH) insulators are predicted in Lithium-decorated iron-based superconductor monolayer materials (LiFeX (X=S, Se and Te)) with very high Curie temperature (\textcolor[rgb]{0.00,0.00,1.00}{PRL 125, 086401 (2020)}), which combines the topological and ferromagnetic (FM) orders. It is interesting and useful to achieve coexistence of intrinsic piezoelectricity, ferromagnetism and nontrivial band topology in single two-dimensional (2D) material, namely 2D piezoelectric quantum anomalous hall insulator (PQAHI). In this work, 2D Janus monolayer $\mathrm{Li_2Fe_2SS…
The electron-hole separation efficiency is a key factor that determines the performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and devices. Therefore, searching for novel 2D TMD materials with long timescale of carrier lifetime becomes one of the most important topics. Here, based on the timedomain density functional theory (TD-DFT), we propose a brand new TMD material, namely janus-MoSTe, which exhibits a strong build-in electric field. Our results show that in janus-MoSTe monolayer, the exciton consisting of electron and hole has a relatively wide spatial extension a…
