The Effect of Janus Asymmetry on Thermal Transport in SnSSe

Janus ScYCBr2 MXene as a Promising Thermoelectric Material

Finding green energy resources that contribute to the battle against global warming and the pollution of our planet is an urgent challenge. Thermoelectric electricity production is a clean and efficient method of producing energy; consequently, scientists are currently researching and creating thermoelectric materials to increase the efficiency of thermoelectric electricity production and expand the potential of the thermoelectric effect for clean energy production. This work focuses on a comprehensive study of the thermoelectric properties of two-dimensional ScYCBr2. We report here a computational analysis of this Janus-like MXene, which is predicted to exhibit outstanding thermoelectric properties. The study uses density-functional theory to provide evidence of the important role played by symmetry breaking to promote low-thermal transport by favoring certain phonon scattering channels. Compared to its symmetric parent compounds, the asymmetric Janus-type ScYCBr2 displays additional phonon scattering channels reducing the thermal conductivity. An exhaustive investigation of the dynamical stability for both zero-temperature and high-temperature conditions was also performed to support the stability of ScYCBr2. Our analysis shows that thanks to its asymmetric structure, the ScYCBr2 MXene has thermoelectric properties that largely surpass those of its parent symmetric counterpart Sc2CBr2, being a material with a remarkable thermoelectric high figure of merit. Another advantage of ScYCBr2 is its high carrier mobility. This work not only demonstrates that this material is a promising thermoelectric material but also shows that ScYCBr2 can operate efficiently at high temperatures up to 1200 K.

Thermal Transport Properties of Two-Dimensional Janus MoXSiN2 (X = S, Se, and Te)

The design of Janus materials offers an effective means of regulating both their physical and chemical properties, leading to their application in various fields. However, the underlying mechanism governing the modulation of the thermal transport characteristics through the construction of Janus materials remains unclear. In this work, we introduce VI-group elements into the MoSi2N4 structure, yielding two-dimensional Janus MoXSiN2 (X = S, Se, and Te) and systematically investigate their thermal transport properties based on first-principles calculation methods. Our findings reveal that the lattice thermal conductivities (κl) of MoSSiN2, MoSeSiN2, and MoTeSiN2 are 47.2, 24.3, and 40.6 W/mK at 300 K, respectively, significantly lower than that of MoSi2N4 (224 W/mK). Such low κl values mainly come from the introduction of X atoms, which enhances phonon scattering and reduces phonon vibration frequencies. In addition, MoTeSiN2 exhibits a higher κl compared to MoSeSiN2, contrary to the trend observed in most materials containing VI-group elements, where κl decreases gradually from S to Te. This anomalous behavior can be attributed to the competitive result between its lower phonon vibrational frequency and weaker phonon anharmonicity of MoTeSiN2. This work elucidates the inherent mechanism governing the modulation of thermal transport properties in Janus materials, thereby enhancing the potential application of Janus MoXSiN2 in engineering thermal management.

Plasma-Driven Selenization for Electrical Property Enhancement in Janus 2D Materials

The recent emergence of Janus 2D materials like SnSSe, derived from SnS2, reveals unique electrical and optical features, such as asymmetrical electronic structure, enhanced carrier mobility, and tunable bandgap. Previous theoretical studies have discuss the electronic properties of Janus SnSSe, but experimental evidence is limited. This study presents a two-step method for synthesizing Janus SnSSe, involving hydrogen plasma treatment and in situ selenization. Optimized conditions (38 W, 1.5 min, 250 °C) are determined using Raman spectroscopy and AFM analysis. XPS confirmed SnSSe's elemental composition, while KPFM reveals a significant reduction in the work function (from 5.26 down to 5.14 eV) for the first time, indicating asymmetrically induced n-type doping. Finally, field-effect transistors (FETs) derived from SnSSe exhibited significantly enhanced mobility and on-current, as well as n-type doping, compared to SnS2-based FETs. These findings lay a crucial foundation for developing high-performance 2D electronic and optoelectronic devices.

Ultralow thermal conductivity of W-Janus bilayers (WXY: X, Y = S, Se, and Te) for thermoelectric devices

Lattice thermal conductivity (κ) in tungsten dichalcogenide Janus (WXY, where X, Y = S, Se, and Te) monolayers and heterostructures (HSs) have been investigated using ab initio DFT simulations. Tungsten-based Janus monolayers show semiconducting behavior with the bandgap in the semiconducting range for WSSe (1.70 eV), WSTe (1.26 eV), and WSeTe (1.34 eV). When Janus monolayers are stacked to form HSs with weak van der Waals (vdW) interactions, the bandgap reduces to 0.19 eV, 0.40 eV, and 0.24 eV, respectively, for WSeTe/WSTe, WSSe/WSTe, and WSSe/WSeTe HSs. Thermal vibrational characteristics of Janus monolayers are modified when these are stacked in 2D HSs with the introduction of interlayer hybrid phonon modes. Large longitudinal-transverse optical (LO-TO) splitting is noticed at the Brillouin zone-center (Γ-point): 135 cm-1, 140 cm-1, and 150 cm-1 for WSeTe/WSTe, WSSe/WSeTe and WSSe/WSTe HSs, respectively. Thermal conductivity calculations show ultra-low κ values for WSeTe/WSTe (0.01 W m-1 K-1), WSSe/WSTe (0.02 W m-1 K-1) and WSSe/WSeTe HS (0.004 W m-1 K-1) at 300 K. The results can be attributed to the hybrid phonon modes with frequencies very close to acoustic modes at the gamma point, low Debye temperature (θD) and specific heat capacity. Our results highlight the possible applications of these HSs in designing thermoelectric interfaces at the nanoscale.

First-principles study of magnetic properties and electronic structure of 3d transition-metal atom-adsorbed SnSSe monolayers

We investigated the electronic structure and magnetic characteristics of 3d transition metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed onto monolayer SnSSe by employing first-principles calculations. After the calculation, we found that Sc, Ti, V, Cu, and Zn atoms adsorbed onto monolayer SnSSe do not have magnetic moments, while the rest of the atoms adsorbed onto SnSSe are able to produce magnetic moments, and their magnetic moments in the adsorption systems are in the range of 1.0-3.0 μB, in which the magnetic distance of Mn is the largest. The results of MAE calculations indicate that there is a big difference in the MAE of the systems with TM atoms adsorbed to the S-side and the Se-side; for V adsorbed to the S-side on the Sn atoms, the MAE is the largest, which reaches 8.277 meV f.u.-1, showing an in-plane magnetic anisotropy, and for Co adsorbed to the Se-side on the Sn atoms, the MAE is the smallest, which is -0.673 meV f.u.-1, showing a perpendicular magnetic anisotropy. Calculations of binding energies show that all atoms are able to adsorb stably. Our results indicate the potential application of TM-adsorbed SnSSe monolayers in spintronics and magnetic memory devices.

Theoretical study of Cr2X3S3(X = Br, I) monolayers for thermoelectric and spin caloritronics properties

High curie temperature 2D materials are important for the progress of the field of spin caloritronics. The spin Seebeck effect and conventional thermoelectric figure of merit (ZT) can give a great insight into how these 2D magnetic materials will perform in spin caloritronics applications. Here in this paper, we have systematically studied 2D Janus monolayers based on CrX₃monolayers. We obtain a ZT of 0.31 and 0.21 for the Cr₂Br₃S₃and Cr₂I₃S₃Janus monolayers. The spin Seebeck coefficient obtained at room temperature is also very high (∼1570μVK^-1in the hole-doped region and ∼1590μVK^-1in the electron-doped region). The thermal conductivity of these monolayers (∼22 Wm^-1K^-1for Cr₂Br₃S₃and ∼16 Wm^-1K^-1for Cr₂I₃S₃) are also very similar to other 2D semiconductor transition metals chalcogenides. These findings suggest a high potential for these monolayers in the spin caloritronics field.

How Hydrodynamic Phonon Transport Determines the Convergence of Thermal Conductivity in Two-Dimensional Materials

The phonon Boltzmann transport equation combined with first-principles calculation has achieved great success in exploring the lattice thermal conductivity (κ) of various materials. However, the convergence of the predicted κ is a critical issue, leading to quite scattered results recorded in the literature, even for the same material. In this paper, we explore the origin for the convergence of thermal conductivity in two-dimensional (2D) materials. Two kinds of typical 2D materials, graphene and silicene, are studied, and the bulk silicon is also compared as a control system for a three-dimensional material. The effect of the cutoff radius (rc) in the third-order interatomic force constants on κ is studied for these three materials. It is found that that κ of these three materials exhibits diverse convergence behaviors with respect to rc, which coincides very well with the strength of hydrodynamic phonon transport. By further analyzing the phonon lifetime and scattering rates, we reveal that the dominance of the normal scattering process gives rise to the hydrodynamic phonon transport in both graphene and silicene, which results in long-range interaction and a large lifetime of low-frequency flexural acoustic phonons, while the same phenomenon is absent in bulk silicon. Our study highlights the importance of long-range interaction associated with hydrodynamic phonon transport in determining the thermal conductivity of 2D materials.

A bifunctional GeC/SnSSe heterostructure for highly efficient photocatalysts and photovoltaic devices

Alongside highly efficient photocatalysts, high photovoltaic performance is also a key element for efficiently harvesting solar energy. Developing bifunctional materials which satisfy concurrently these two demands is an appealing strategy for solving the current serious energy and environmental issues. Based on first-principles and quantum transport calculations, we designed this kind of novel bifunctional material: Janus GeC/SnSSe van der Waals heterostructure (vdWH). We demonstrate that it is a highly efficient direct Z-scheme photocatalyst. However, unlike traditional direct Z-scheme photocatalysts, the GeC/SnSSe vdWH possesses a small energy separation between the low conduction band located in SnSSe and the high valence band residing in the GeC layer, which significantly fosters the interlayer charge transfer. Hence, its solar-to-hydrogen conversion efficiency reaches as high as 68.37%. Moreover, we also find that tensile strain promotes an astonishing increase in photovoltaic performance, e.g., 4% tensile strain leads to an increase of the photocurrent by 40%.

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.

Strain-tunable lattice thermal conductivity of the Janus PtSTe monolayer

Using first-principles calculations and Boltzmann transport theory, we study the effect of biaxial tensile strain on phonon transport in a Janus PtSTe monolayer. The band gap between the optical and acoustic phonon branches shrinks with increasing strain, resulting in a highly nonlinear monotonic decrease in the lattice thermal conductivity. That reduction reaches close to an order of magnitude when the gap disappears completely under high strains (>8%). This behavior is attributed to a strong enhancement of the anharmonic scattering of acoustic phonons due to the band overlap. Our findings underscore the potential of strain engineering as a class of methods to tune the thermal transport properties of two-dimensional (2D) Janus nanomaterials.

Design of a noble-metal-free direct Z-scheme photocatalyst for overall water splitting based on a SnC/SnSSe van der Waals heterostructure

Semiconductor photocatalysts, using sunlight to stimulate various photocatalytic reactions, are promising materials for solving the energy crisis and environmental problems. However, the low photocatalytic efficiency and high cost pose major challenges for their widespread application. Mimicking the natural photosynthesis system, we propose a direct Z-scheme photocatalyst based on a Janus van der Waals heterostructure (vdWH) comprising SnC and Janus SeSnS monolayers. From first-principles calculations, the intrinsic built-in electric field of Janus SeSnS and the charge transfer from the SnC to the SeSnS layer give rise to a type-II band alignment. Such a band alignment benefits the formation of spatially separated reductive and oxidative active sites and the reduction of the global bandgap of the Janus vdWH. The proposed material increases the solar-to-hydrogen conversion efficiency to 60.8%. Besides, we also find that the light absorption coefficient is stacking configuration controllable and strain-tunable, e.g., the tensile strain promotes photocatalytic efficiency. Moreover, because Sn, S, and Se are environmentally benign and inexpensive elements, SnC/SeSnS vdWH is a promising noble-metal-free direct Z-scheme photocatalyst.

Hierarchical thermal transport in nanoconfined water

The structure of nanoconfined fluids is particularly non-uniform owing to the wall interaction, resulting in the distinctive characteristic of thermal transport compared to bulk fluids. We present the molecular simulations on the thermal transport of water confined in nanochannels with a major investigation of its spatial distribution under the effects of wall interaction. The results show that the thermal conductivity of nanoconfined water is inhomogeneous and its layered distribution is very similar to the density profile. The layered thermal conductivity is the coupling result of inhomogeneous density and energy distributions that are generally diametrical, and their contributions to the thermal conductivity compensate with each other. However, the accumulative effect of water molecules is really dominating, resulting in a high thermal conductivity in the high-density layers with the low-energy molecules, and vice versa. Moreover, it is found that the adsorptive and repulsive interactions from solid walls have different roles in the hierarchical thermal transport in nanoconfined water. The adsorptive interaction is only responsible for the layered distribution of thermal conductivity, while the repulsive interaction is responsible for the overall thermal conductivity; accordingly, the thermal conductivity is independent of the strength of water-solid interactions. The identified hierarchical thermal transport in nanoconfined water and its underlying mechanisms have a great significance for the understanding of nanoscale thermal transport and even the mass and energy transport of nanoconfined fluids.