High Thermoelectric Performance in Two-Dimensional Janus Monolayer Material WS-X (X = Se and Te)

Flat-Band Electronic Bipolarity in a Janus and Kagome van der Waals Semiconductor Nb3TeI7

Janus materials, a novel class of materials with two faces of different chemical compositions and electronic polarities, offer significant potential for various applications with catalytic reactions, chemical sensing, and optical or electronic responses. A key aspect for such functionalities is face-dependent electronic bipolarity, which is usually limited by the chemical distinction of terminated surfaces and has not been exploited in the semiconducting regime. Here, it is showed that a Janus and Kagome van der Waals (vdW) material Nb3TeI7 has ferroelectric-like coherent stacking of the Janus layers and hosts strong electronic bipolar states in the semiconducting regime. A large potential difference of ∼ 0.7 eV between the I4 and TeI3 terminated surfaces is observed, despite only one fourth of the I atoms being replaced by Te atoms on one side of the layers. Additionally, robust semiconducting properties with the face-dependent n-type and p-type field-effect transistor behaviors are demonstrated. These unique properties are attributed to Nb 4d orbital flat bands of the breathing-Kagome lattice, of which significantly large electron mass makes the semiconducting properties immune to impurity doping, and inherent strong electron correlation enhances asymmetric electron distribution, thereby amplifying a built-in electric field. These findings highlight that naturally-grown Janus and Kagome vdW semiconductors provide a promising material platform for utilizing strong electronic bipolarity in 2D-material-based applications.

Doping Janus MoSSe monolayer with Al/Ga and P/As atoms, and their clusters: effective methods for the band structure and magnetism engineering

In this work, new doping approaches are proposed towards effective band structure and magnetism engineering of Janus MoSSe monolayer. In its pristine form, MoSSe monolayer is a direct gap semiconductor. Magnetic semiconductor nature is obtained by doping with Al/Ga atoms at Mo sublattice and P atom at S sublattice. Herein, overall magnetic moments of 3.00/2.96 and 1.00 μ B are obtained, respectively. Moreover, the spin-polarized states are produced primarily by the first neighboring Mo atoms from doping site and P impurity, respectively. Meanwhile, As doping metallizes the monolayer, maintaining its nonmagnetic nature. Similarly, no magnetism is induced by doping with AlP3, AlAs3, GaP3, and GaAs3 binary clusters. However, the substitution of these clusters causes large band gap reduction up to 78.85%, which can be attributed to new mid-gap subands formed mainly by Mo atoms. Further, doping with AlPAs/GaPAs and AlP3As3/GaP3As3 ternary clusters is also considered. In these cases, magnetic semiconductor and half-metallic natures are obtained, respectively, which are regulated primarily by Mo atoms. Further, Bader charge analysis is carried out to investigate the interactions between impurities/clusters with the host monolayer. Results demonstrate the charge gainer role of Al and Ga atoms, meanwhile P and As impurities act as charge gainer. Our findings may suggest the prospect of the proposed doping approaches to functionalize Janus MoSSe monolayer towards spintronic and optoelectronic applications.

Entropy-Boosting Intrinsic Elemental Disorder for Advanced Thermoelectric Performance in MoSSe

Entropy design, facilitated by disorder, emerges as a crucial strategy for the performance enhancement of thermoelectric materials. The characteristic average multielement composition of Janus MoSSe offers an opportunity to introduce intrinsic elemental disorder by altering the positions of different atoms, thereby boosting entropy. Here, we explored the thermoelectric performance of the initial MoSSe and various constructed disordered structures through first-principles calculations. The results demonstrated that the intrinsic elemental disorder enables simultaneous optimization of power factor and reduction of thermal conductivity, with the optimal disordered structure achieving a ZT of 2.89 at 700 K, approximately 10 times greater than the initial structure. Atomic disorder directly induces charge disorder, decreasing the polarity and enhancing the charge density of the valence band maximum (VBM) to optimize conductivity. This leads to local atomic energy level resonances, which significantly increase the electronic density of states, enhancing the Seebeck coefficient. Moreover, the disorder considerably intensifies phonon scattering through the compression of phonon bandgap and the increased number of peaks for phonon density of states. The lowest thermal conductivity of the disordered structures reaches 1.02 W·m-1·K-1 at 700 K. Comprehensive analyses were carried out through the examination of phonon lifetime, phonon group velocity, and specific heat. Finally, we quantified structural disorder utilizing entropy and derived thermoelectric performance optimization based on the intrinsic elemental disorder of the Janus materials. Our results demonstrate how control over intrinsic elemental disorder offers an effective avenue for tuning the performance of thermoelectric materials.

Thermoelectric properties of XX- and XY-stacked GeS/GeSe van der Waals heterostructures from DFT and BTP calculations

This study utilizes density functional theory (DFT) and the Boltzmann transport equation (BTE) to investigate the structural, electronic, and thermoelectric properties of germanium sulfide (GeS) and germanium selenide (GeSe) monolayers, along with their van der Waals (vdW) heterostructures. We analyzed XX-stacked and XY-stacked configurations, where the XX configuration features direct atomic stacking, while the XY configuration exhibits staggered stacking. Our first-principles calculations indicate that the formation of GeS/GeSe heterostructures results in a reduction of bandgaps compared to their bulk and monolayer counterparts, yielding bandgap values of 0.91 eV for the XX configuration and 0.84 eV for the XY configuration. Stability assessments reveal that the XY configuration is more stable, demonstrating a lattice thermal conductivity of 15.21 W/mK compared to 17.95 W/mK for the XX configuration T 300 K. The thermoelectric properties were systematically evaluated across a temperature range of 300-800 K, revealing high Seebeck coefficients of 1.51 mV/K for the XX heterostructure and 1.39 mV/K for the XY heterostructure. reflecting their excellent charge transport capabilities. Notably, the figure of merit (ZT) at 800 K was calculated to be 0.90 for the XX configuration and 1.01 for the XY configuration, underscoring the superior thermoelectric performance of the XY heterostructure. These findings contribute to a comprehensive understanding of 2D GeS/GeSe heterostructures for thermoelectric applications and provide a solid foundation for future research and technological advancements in this domain.

Two-dimensional BiSbTeX2 (X = S, Se, Te) and their Janus monolayers as efficient thermoelectric materials

Today, there is a huge need for highly efficient and sustainable energy resources to tackle environmental degradation and energy crisis. We have analyzed the electronic, mechanical and thermoelectric (TE) characteristics of two-dimensional (2D) BiSbTeX2 (X = S, Se and Te) and Janus BiSbTeXY (X/Y = S, Se and Te) monolayers by implementing first principles simulations. These monolayers' dynamic stability and thermal stability have been demonstrated through phonon dispersion spectra and ab initio molecular dynamics (AIMD) simulations, respectively. The band structure of these monolayers can be tuned by applying uniaxial and biaxial strains. The investigated lattice thermal conductivity (κl) for these monolayers lies between 0.23 and 0.37 W m-1 K-1 at 300 K. For a more precise calculation of the scattering rate, we implemented electron-phonon coupling (EPC) and spin-orbit coupling effects to calculate the transport properties. For p(n)-type carriers, the power factor of these monolayers is predicted to be as high as 2.08 × 10-3 W m-1 K-2 and (0.47 × 10-3 W m-1 K-2) at 300 K. The higher thermoelectric figure of merit (ZT) of p-type carriers at 300 K is obtained because of their very low value of κl and high power factor. Our theoretical investigation predicts that these monolayers can be potential candidates for fabricating highly efficient thermoelectric power generators.

Enhanced Model for the Analysis of Thermoelectric Effects at Nanoscale: Onsager's Method and Liu's Technique in Comparison

The aim of this paper is twofold. From the practical point of view, an enhanced model for the description of thermoelectric effects at nanoscale is proposed. From the theoretical point of view, instead, in the particular case of the proposed model, the equivalence between two classical techniques for the exploitation of the second law of thermodynamics is shown, i.e., Onsager's method and Liu's technique. An analysis of the heat-wave propagation is performed as well.

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.

Piezoelectric, Thermoelectric, and Photocatalytic Water Splitting Properties of Janus Arsenic Chalcohalide Monolayers

In this study, we systematically investigate the piezoelectric, thermoelectric, and photocatalytic properties of novel two-dimensional Janus arsenic chalcohalide monolayers, AsXX' (X = S and Se and X' = Cl, Br, and I) using density functional theory. The positive phonon spectra and ab initio molecular dynamics simulation plots indicate these monolayers to be dynamically and thermally stable. The mechanical stability of these monolayers is confirmed by a nonzero elastic constant (C ij ), Young's modulus (Y 2D), and Poisson ratio (ν). These monolayers exhibit strong out-of-plane piezoelectric coefficients, making them candidate materials for piezoelectric devices. Our calculated results indicate that these monolayers have a low lattice thermal conductivity (κl) and high thermoelectric figure of merit (zT) up to 1.51 at 800 K. These monolayers have an indirect bandgap, high carrier mobility, and strong visible light absorption spectra. Furthermore, the AsSCl, AsSBr, and AsSeI monolayers exhibit appropriate band alignment for water splitting. The calculated value of the corrected solar-to-hydrogen conversion efficiency can reach up to 19%. The nonadiabatic molecular dynamics simulations reveal the prolonged electron-hole recombination rates of 1.52 0.98, and 0.67 ns for AsSCl, AsSBr, and AsSeI monolayers, respectively. Our findings demonstrate these monolayers to be potential candidates in energy-harvesting fields.

Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al2Te3 and Al2Te5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al2Te5, a square planar chain (SPC) known as polytelluride [Te3]2- is neutralized by the covalently bonded [Al2Te2]2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al2Te5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al2Te5 monolayer. The calculated κL values of 1.8 and 0.5 W m-1 K-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al2Te3, and even lower than monolayers that contain heavy cations like Bi2Te3. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al2Te5, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al2Te5. The pioneering zT of Al2Te5 compared to that of Al2Te3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.

Machine Learning-Assisted Exploration of Intrinsically Spin-Ordered Two-Dimensional (2D) Nanomagnets

The existence of spontaneous spin-ordering in two-dimensional (2D) nanomagnets holds significant importance due to their several unique and promising properties that distinguish them from conventional 2D materials. In recent times, machine learning (ML) has emerged as a powerful tool for effectively exploring and identifying the optimal 2D materials for specific applications or properties within a limited span of time. Here, we have introduced ML-accelerated approaches to specifically estimate the properties, such as the HSE bandgap and magnetoanisotropic energy (MAE) of 2D magnetic materials. Supervised ML algorithms were employed to derive the descriptors that are capable of predicting the properties of intrinsic 2D magnetic materials. Furthermore, the feature selection score is also calculated to reduce the feature space complexity and improve the model accuracy. The input features were obtained from the C2DB database, and models were constructed using linear regression, Lasso, decision tree, random forest, XG Boost, and support vector machine algorithms. The random forest model predicted the HSE band gaps with an unprecedented low root-mean-square error (RMSE) of 0.22 eV, while the linear regression gives the best fit with RMSEs of 0.25 and 0.22 meV for the MAE(x) and MAE(y), respectively. Therefore, the integration of interpretable analytical models with density functional theory offers a swift and reliable approach for uncovering the properties of intrinsic 2D magnetic materials. This collaborative methodology not only ensures speed in analysis but also enriches the material space.

Symmetry lowering through surface engineering and improved thermoelectric properties in Janus MXenes

Despite ample evidence of their influence on the transport properties of two-dimensional solids, the interrelations of reduced symmetry, electronic and thermal transport have rarely been discussed in the context of thermoelectric materials. With the motivation to design new thermoelectric materials with improved properties, we have addressed these by performing first-principles density functional theory based calculations in conjunction with semi-classical Boltzmann transport theory on a number of compounds in the MXene family. The symmetry lowering in parent M2CO2 (M = Ti, Zr, Hf, Mo) MXenes is achieved by replacing the transition metal M on one surface, resulting in Janus compounds MM'CO2 (M = Ti, Zr, Hf and M' = Mo, Zr, Hf; M ≠ M'). Our calculations show that the thermoelectric figure-of-merit can be improved significantly by such surface engineering. We discuss in detail, both qualitatively and quantitatively, the origin behind high thermoelectric parameters for these compounds. Our in-depth analysis shows that the modifications in the electronic band structures and degree of anharmonicity driven by the dispersions in the bond strengths due to the lowering of symmetry, an artefact of surface engineering, are the factors behind the trends in the thermoelectric parameters of the MXenes considered. The results also substantiate that the compositional flexibility offered by the MXene family of compounds can generate a complex interplay of symmetry, electronic structure, bond strength and anharmonicity which can be exploited to engineer thermoelectric materials with improved properties.

Ab-initiotransport model to study the thermoelectric performance of MoS2, MoSe2, and WS2monolayers by using Boltzmann transport equation

The potential for thermoelectric applications of two-dimensional materials is quite promising. Usingab-initiocalculations, we have investigated the electronic band structure, phonon band structure, electronic density of states, and phonon density of states of monolayers MoS2, MoSe2, and WS2. In order to compute the thermoelectric properties of monolayers MoS2, MoSe2, and WS2, we used theab-initiomodel suggested by Faghaniniaet al(2015Phys. Rev.B91235123). Within this model, by using inputs from density functional theory and considering all relevant elastic and inelastic scattering mechanisms, we have calculated the thermoelectric properties of monolayers MoS2, MoSe2, and WS2over various ranges of temperature (T) and carrier concentration (n). The obtained results of Seebeck coefficients (S) and figure of merit (ZT) atT= 300 K for bothn/p-types of monolayers MoS2, MoSe2, and WS2are in good agreement with the findings obtained by other models using the Boltzmann transport equation within a constant relaxation time framework.

Revealing large room-temperature Nernst coefficients in 2D materials by first-principles modeling

Two-dimensional (2D) materials have attracted significant attention owing to their distinctive electronic, thermal, and mechanical characteristics. Recent advancements in both theoretical understanding and experimental methods have greatly contributed to the understanding of thermoelectric properties in 2D materials. However, thermomagnetic properties of 2D materials have not yet received the same amount of attention. In this work, we select promising 2D materials guided by the physics of the Nernst effect and present a thorough first-principles study of their electronic structures, carrier mobilities, and Nernst coefficients as a function of carrier concentration. Specifically, we reveal that trilayer graphene with an ABA stacking exhibits an exceptionally large Nernst coefficient of 112 μV (KT)-1 at room temperature. We further demonstrate that monolayer graphene, ABC-stacked trilayer graphene, and trilayer phosphorene (AAA stacking) have large Nernst coefficients at room temperature. This study establishes an ab initio framework for the quantitative study of the thermomagnetic effects in 2D materials and demonstrates high fidelity with previous experimental data.

Unveiling the temperature-dependent thermoelectric properties of the undoped and Na-doped monolayer SnSe allotropes: a comparative study

Motivated by the excellent thermoelectric (TE) performance of bulk SnSe, extensive attention has been drawn to the TE properties of the monolayer SnSe. To uncover the fundamental mechanism of manipulating the TE performance of the SnSe monolayer, we perform a systematic study on the TE properties of five monolayer SnSe allotropes such asα-,β-,γ-,δ-, andε-SnSe based on the density functional theory and the non-equilibrium Green's functions. By comparing the TE properties of the Na-doped SnSe allotropes with the undoped ones, the influences of the Na doping and the temperature on the TE properties are deeply investigated. It is shown that the figure of meritZTwill increase as the temperature increases, which is the same for almost all the Na-doped and undoped cases. The Na doping can enhance or suppress theZTin different SnSe allotropes at different temperatures, implying the presence of the anomalous suppression of theZT. The Na doping inducedZTsuppression may be caused basically by the sharp decrease of the power factor and the weak decrease of the electronic thermal conductance, rather than by the decrease of the phononic thermal conductance. We hope this work will be able to enrich the understanding of the manipulation of TE properties by means of dimensions, structurization, doping, and temperature.

Hidden Valley Polarization, Piezoelectricity, and Dzyaloshinskii-Moriya Interactions of Janus Vanadium Dichalcogenides

Due to the lack of inversion symmetry and the discovery of room-temperature ferromagnetism, two-dimensional semiconducting vanadium-based van der Waals transition-metal dichalcogenides (V-TMDs) are drawing attention for their possible application in spintronics and valleytronics. Here, we show the functional properties enriched by the broken inversion, out-of-plane mirror, and time-reversal symmetries of Janus H-VXY TMDs (X, Y = S, Se, Te). By first-principles calculations, we reveal the intrinsic xy easy-plane magnetism of the Janus vanadium-based TMD monolayers and systematically study their hidden valley polarization and giant magneto band structure. Their strong nearest-neighbor exchange strengths lead to near-room-temperature magnetic phase transitions. The Janus H-VXY system also exhibits piezoelectricity with nonzero e31 and e21. Interestingly, it is found that the right-handed Dzyaloshinskii-Moriya interaction has nonzero in-plane components in our Janus system, with fluctuating magnitudes determined by competence between relaxed bond-angle and atomic index of ligands.

Janus 2H-MXTe (M = Zr, Hf; X = S, Se) monolayers with outstanding thermoelectric properties and low lattice thermal conductivities

Two-dimensional (2D) materials have been one of the most popular objects in the research field of thermoelectric (TE) materials and have attracted substantial attention in recent years. Inspired by the synthesized 2H-MoSSe and numerous theoretical studies, we systematically investigated the electronic, thermal, and TE properties of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers by using first-principles calculations. The phonon dispersion curves and AIMD simulations confirm the thermodynamic stabilities. Moreover, Janus 2H-MXTe were evaluated as indirect band-gap semiconductors with band gaps ranging from 0.56 to 0.90 eV using the HSE06 + SOC method. To evaluate the TE performance, firstly, we calculated the temperature-dependent carrier relaxation time with acoustic phonon scattering τac, impurity scattering τimp, and polarized scattering τpol. Secondly, the calculation of lattice thermal conductivity (κl) shows that these monolayers possess relatively poor κl with values of 3.4-5.4 W mK-1 at 300 K, which is caused by the low phonon lifetime and group velocity. After computing the electronic transport properties, we found that the n-type doped Janus 2H-MXTe monolayers exhibit a high Seebeck coefficient exceeding 200 μV K-1 at 300 K, resulting in a high TE power factor. Eventually, combining the electrical and thermal conductivities, the optimal dimensionless figure of merit (zT) at 300 K (900 K) can be obtained, which is 0.94 (3.63), 0.51 (2.57), 0.64 (2.72), and 0.50 (1.98) for n-type doping of ZrSeTe, HfSeTe, ZeSTe, and HfSTe monolayers. Particularly, the ZrSeTe monolayer shows the best TE performance with the maximal zT value. These results indicate the excellent application potential of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers in TE materials.

Key phonon modes to determine the phase transition of two dimensional Janus transition metal dichalcogenides: a DFT and tight-binding study

Phase stability and the phase transition of Janus transition metal chalcogenides (TMDs) have become interesting issues that have not been fully resolved since their successful synthesis. By fitting the results from first principles calculations, a tight-binding dynamics matrix of the 1T' phase is constructed and the eigenvectors are also obtained. We propose a method to project the atomic motion causing the phase transition from 2H to 1T' onto these eigenvectors, and identify four key phonon modes which are the major factors to trigger phase transition. Temperature excitation is used to excite the key modes and the free energy criterion is used to determine the phase stability. The relatively large enthalpy difference between the 2H and 1T' phases favours the 2H one as the stable phase at low temperature. While the 1T' phase has a quick increase in vibrational free energy with rising temperature, especially for 1T' Janus TMDs which have a quicker increase in the total free energy than that of 1T' non-Janus TMDs, making them show a lower phase transition temperature. Our work will deepen our understanding of the phase transition behavior of 2D Janus TMDs, and the tight-binding dynamics matrix and the method to obtain the key modes will be a useful tool for further study of the phase transitions of 2D Janus TMDs and other related materials.

The effect of four-phonon interaction on phonon thermal conductivity of hexagonal VTe2 and puckered pentagonal VTe2

The traditional view is that complex structures have lower lattice thermal conductivity. However, it is observed that complex structures have higher lattice thermal conductivity than simple atomic structures in VTe2 systems after considering the four-phonon scattering effect. In this work, we calculate the lattice thermal conductivity of an H-VTe2 monolayer with a simple atomic structure and that of a PP-VTe2 monolayer with a complex atomic arrangement using first-principles calculations combined with the Boltzmann transport theory under the conditions of with and without the four-phonon scattering process. Our findings reveal that the lattice thermal conductivity of the PP-VTe2 monolayer along the x or y direction is 3-4 times lower than that of the H-VTe2 monolayer when only considering the three-phonon scattering process. After taking into account the four-phonon scattering process, the lattice thermal conductivity of both monolayers decreases. For the H-VTe2 monolayer, the lattice thermal conductivity decreases by 88.7% (from 1.33 to 0.15 W m-1 K-1) compared to only considering the three-phonon scattering process, mainly due to strong four-phonon scattering. In addition, the PP-VTe2 monolayer experiences a lower decrease in lattice thermal conductivity, with reductions of 12.5% (from 0.4 to 0.35 W m-1 K-1) and 11.7% (from 0.34 to 0.3 W m-1 K-1) in the x and y directions, respectively, because of the weak four-phonon scattering. Notably, the lattice thermal conductivity with the four-phonon scattering process of the H-VTe2 monolayer is twice as low as that of the PP-VTe2 monolayer. Hence, our findings suggest that even simple atomic structures can exhibit lower lattice thermal conductivity than complex structures when considering four-phonon interaction.

Tunable electronic structures, Rashba splitting, and optical and photocatalytic responses of MSSe-PtO2 (M = Mo, W) van der Waals heterostructures

Binding energies, AIMD simulation and phonon spectra confirm both the thermal and dynamical stabilities of model-I and model-II of MSSe-PtO2 (M = Mo, W) vdWHs. An indirect type-II band alignment in both the models of MSSe-PtO2 vdWHs and a larger Rashba spin splitting in model-II than in model-I provide a platform for experimental design of MSSe-PtO2 vdWHs for optoelectronics and spintronic device applications. Transfer of electrons from the MSSe layer to the PtO2 layer at the interface of MSSe-PtO2 vdWHs makes MSSe (PtO2) p(n)-type. Large absorption in the visible region of MoSSe-PtO2 vdWHs, while blue shifts in WSSe-PtO2 vdWHs are observed. In the case of model-II of MSSe-PtO2 vdWHs, a further blue shift is observed. Furthermore, the photocatalytic response shows that MSSe-PtO2 vdWHs cross the standard water redox potentials confirming their capability to split water into H+/H2 and O2/H2O.

Two-dimensional Janus pentagonal MSeTe (M = Ni, Pd, Pt): promising water-splitting photocatalysts and optoelectronic materials

Inspired by the interesting and novel properties exhibited by Janus transition metal dichalcogenides (TMDs) and two-dimensional pentagonal structures, we here investigated the structural stability, mechanical, electronic, photocatalytic, and optical properties for a class of two-dimensional (2D) pentagonal Janus TMDs, namely penta-MSeTe (M = Ni, Pd, Pt) monolayers, by using density functional theory (DFT) combined with Hubbard's correction (U). Our results showed that these monolayers exhibit good structural stability, appropriate band structures for photocatalysts, high visible light absorption, and good photocatalytic applicability. The calculated electronic properties reveal that the penta-MSeTe are semiconductors with a bandgap range of 2.06-2.39 eV, and their band edge positions meet the requirements for water-splitting photocatalysts in various environments (pH = 0-13). We used stress engineering to seek higher solar-to-hydrogen (STH) efficiency in acidic (pH = 0), neutral (pH = 7) and alkaline (pH = 13) environments for penta-MSeTe from 0% to +8% biaxial and uniaxial strains. Our results showed that penta-PdSeTe stretched 8% along the y direction and demonstrates an STH efficiency of up to 29.71% when pH = 0, which breaks the theoretical limit of the conventional photocatalytic model. We also calculated the optical properties and found that they exhibit high absorption (13.11%) in the visible light range and possess a diverse range of hyperbolic regions. Hence, it is anticipated that penta-MSeTe materials hold great promise for applications in photocatalytic water splitting and optoelectronic devices.

Dimensionality reduction induced synergetic optimization of the thermoelectric properties in Bi2Si2X6 (X = Se, Te) monolayers

Different from three-dimensional bulk compounds, two-dimensional monolayer compounds exhibit much better thermoelectric performance on account of the quantum confinement and interface effect. Here, we present a systematic study on the electronic and thermal transport properties of bulk and monolayer Bi2Si2X6 (X = Se, Te) through theoretical calculations using density functional theory based on first-principles and Boltzmann transport theory. Monolayer Bi2Si2X6 are chemically, mechanically and thermodynamically stable semiconductors with suitable band gaps, and they have lower lattice thermal conductivity (κL) in the a/b direction than their bulk counterparts. The calculated κL of monolayer Bi2Si2Se6 (Bi2Si2Te6) is as low as 0.72 (0.95) W m-1 K-1 at 700 K. Moreover, monolayer Bi2Si2X6 exhibit a higher Seebeck coefficient compared with bulk Bi2Si2X6 owing to the sharper peaks in the electronic density of states (DOS). This results in a significant increase in power factor by dimensionality reduction. Combined with the synergetically suppressed thermal conductivity, the maximum ZT values for monolayer Bi2Si2Se6 and Bi2Si2Te6 are significantly enhanced up to 5.03 and 2.87 with p-type doping at 700 K, which are more than 2 times that of the corresponding bulk compounds. These results demonstrate the superb thermoelectric performance of monolayer Bi2Si2X6 for promising thermoelectric conversion applications.

A computational study of 2D group-III ternary chalcogenide monolayer compounds MNTe2(M, N = In, Ga, Al)

First principle calculations of novel two-dimensional (2D) group-III ternary chalcogenide monolayer (G3TCM) compounds have been carried out using density functional theory. The 2D hexagonal structure has a honeycomb-like appearance from both the top and bottom views. Both pristine and G3TCM compounds are energetically favourable and have been found to be dynamically stable via phonon calculations. Theab-initiomolecular dynamics calculations show the thermodynamical stability of the G3TCM compounds. The G3TCM compounds exhibit semiconductor behaviour with a decreased indirect bandgap compared to the pristine monolayers. Chalcogen atoms contribute mainly to the valence bands, while group-III atoms have a major contribution to the conduction band. A red shift has been observed in the absorption of light, mainly in the visible and ultraviolet regions, and the refractive index is increased compared to the pristine material. Both pristine and G3TCM compounds have been found to be more active in the ultraviolet region, and low reflection has been observed. In the 6-8 eV range of the ultraviolet region, zero reflection and the highest absorption are observed. The monolayer has shown potential applications in optoelectronics devices as an ultraviolet and visible light detector, absorber, coating material, and more. The band alignment of the 2D G3TCM monolayer is calculated to observe its photo-catalyst behaviour.

X-ray and Neutron Diffraction Studies of SrTe2FeO6Cl, an Oxide Chloride with Rare Anion Ordering

The oxychloride SrTe2FeO6Cl is obtained by high-temperature solid-state synthesis under inert conditions in closed reaction vessels. The compound crystallizes in a novel monoclinic crystal structure that is described in the space group P121/n1 (No. 14). The unit cell parameters, a = 10.2604(1) Å, b = 5.34556(5) Å, c = 26.6851(3) Å, and β = 93.6853(4)°, and atomic parameters were determined from synchrotron diffraction data, starting from a model that was obtained from single-crystal X-ray diffraction data. The anion lattice exhibits a rare ordering of oxide and chloride ions: one-dimensional zig-zag ladders of chlorine (squarelike motif) are surrounded by an oxygen matrix. Two different iron sites coordinated solely to oxygen are present in the structure, one octahedral and one square pyramidal, both distorted. Similarly, two different strontium coordinations are present; the first homoleptic coordinated to eight oxygen atoms and the second heteroleptic coordinated to four oxygen and four chlorine atoms in a fac-like manner. The lone pair of Te(IV) is directed toward the larger chlorine atoms. Magnetic susceptibility measurements confirm that Fe is +3 (d5) in the high-spin electronic configuration, exhibiting an almost ideal spin-only moment, μeff = 5.65 μB Fe-1. The slightly negative Weiss constant (θCW = -39 K) suggests dominating antiparallel spin-to-spin coupling in the paramagnetic temperature range, agreeing with an observed long-range antiferromagnetic spin ordering below Néel temperature, TN ∼ 13 K, and a broad second order-like anomaly in the specific heat measurement data. Low-temperature neutron diffraction data reveal that the antiferromagnetic ordered phase is C-type, with a k-vector (1/2, 1/2, 0) and ordered moment of 4.14(7) μB. The spin structure can be described as antiferromagnetic ordered layers stacked along the a-axis, forming layers of squares that alternate along the c-axis.

High Thermoelectric Performance of a Novel γ-PbSnX2 (X = S, Se, Te) Monolayer: Predicted Using First Principles

Two-dimensional (2D) group IV metal chalcogenides are potential candidates for thermoelectric (TE) applications due to their unique structural properties. In this paper, we predicted a 2D monolayer group IV metal chalcogenide semiconductor γ-PbSn₂ (X = S, Se, Te), and first-principles calculations and Boltzmann transport theory were used to study the thermoelectric performance. We found that γ-PbSnX₂ had an ultra-high carrier mobility of up to 4.04 × 10³ cm² V^-1 s^-1, which produced metal-like electrical conductivity. Moreover, γ-PbSn₂ not only has a very high Seebeck coefficient, which leads to a high power factor, but also shows an intrinsically low lattice thermal conductivity of 6-8 W/mK at room temperature. The lower lattice thermal conductivity and high power factors resulted in excellent thermoelectric performance. The ZT values of γ-PbSnS₂ and γ-PbSnSe₂ were as high as 2.65 and 2.96 at 900 K, respectively. The result suggests that the γ-PbSnX₂ monolayer is a better candidates for excellent thermoelectric performance.

High thermoelectric performance of two-dimensional SiPGaS/As heterostructures

Thermoelectric technology holds great promise as a green and sustainable energy solution, generating electric power directly from waste heat. Herein, we investigate the thermoelectric properties of SiPGaS/As van der Waals heterostructures by using computations based on density functional theory and semiclassical Boltzmann transport theory. Our results show that both models of SiPGaS/As van der Waals heterostructures have low lattice thermal conductivity at room temperature (300 K). Applying 4% tensile strain to the models leads to a significant enhancement in the figure of merit (ZT), with model-I and model-II exhibiting ZT improvements of up to 24.5% and 14.8%, respectively. Notably, model-II outperforms all previously reported heterostructures in terms of ZT value. Additionally, we find that the maximum thermoelectric conversion efficiency (η) for model-II at 4% tensile strain reaches 23.98% at 700 K. Our predicted ZT_avg > 1 suggests that these materials have practical potential for thermoelectric applications over a wide temperature range. Overall, our findings offer valuable insights for designing better thermoelectric materials.

Janus β-PdXY (X/Y = S, Se, Te) materials with high anisotropic thermoelectric performance

Two-dimensional (2D) materials have garnered considerable attention as emerging thermoelectric (TE) materials owing to their unique density of states (DOS) near the Fermi level. We investigate the TE performance of Janus β-PdXY (X/Y = S, Se, Te) monolayer materials as a function of carrier concentration and temperature in the mid-range from 300 to 800 K by combining density functional theory (DFT) and semi-classical Boltzmann transport theory. The phonon dispersion spectra and AIMD simulations confirm their thermal and dynamic stability. The transport calculation results reveal the highly anisotropic TE performance of both n and p-type Janus β-PdXY monolayers. Meanwhile, the coexistence of low phonon group velocity and a converged scattering rate leads to a lower lattice thermal conductivity (K_l) of 0.80 W mK^-1, 0.94 W mK^-1, and 0.77 W mK^-1 along the y-direction for these Janus materials, while the high TE power factor is attributed to the high Seebeck coefficient (S) and electrical conductivity, which are due to the degenerate top valence bands of these Janus monolayers. The combination of lower K_l and a high-power factor at 300 K (800 K) leads to an optimal figure of merit (ZT) of 0.68 (2.21), 0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe monolayers, respectively. To evaluate rational electron transport properties, the effects of acoustic phonon scattering (τ_ac), impurity scattering (τ_imp), and polarized phonon scattering (τ_polar) are included in the temperature-dependent electron relaxation time. These findings indicated that the Janus β-PdXY monolayers are promising candidates for TE conversion devices.

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.

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.

High thermoelectric performance of a Sc2Si2Te6 monolayer at medium temperatures: an ab initio study

Thermoelectric (TE) materials have attracted great attention in solving the problems in the waste heat field, while low figure of merit and poor material stability drastically limit their practical applications. In this work, a two-dimensional (2D) Sc₂Si₂Te₆ monolayer was systematically explored as a promising TE material via ab initio methods. The results reveal that the Sc₂Si₂Te₆ monolayer possesses an indirect band gap with a rhombohedral crystal phase and exhibits excellent dynamic stability. The lower electronic/lattice thermal conductivity and higher electron carrier mobility result in good n-type power factor parameters between 6.24 × 10¹⁰ and 1.5 × 10¹¹ W m^-1 s^-1 K^-2 from 300 to 700 K. Such combined merits of low thermal conductivity and high power factor parameters endow the Sc₂Si₂Te₆ monolayer with superior thermoelectric properties with figure of merit (ZT) values of 1.41 and 3.81 at 300 K and 700 K, respectively. This study presented here can shed light on the future design of various 2D materials for thermoelectric applications.

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.

Theoretical analysis of the thermoelectric properties of penta-PdX2 (X = Se, Te) monolayer

Based on the successful fabrication of PdSe2 monolayers, the electronic and thermoelectric properties of pentagonal PdX2 (X = Se, Te) monolayers were investigated via first-principles calculations and the Boltzmann transport theory. The results showed that the PdX2 monolayer exhibits an indirect bandgap at the Perdew–Burke–Ernzerhof level, as well as electronic and thermoelectric anisotropy in the transmission directions. In the PdTe2 monolayer, P-doping owing to weak electron–phonon coupling is the main reason for the excellent electronic properties of the material. The low phonon velocity and short phonon lifetime decreased the thermal conductivity (κl) of penta-PdTe2. In particular, the thermal conductivity of PdTe2 along the x and y transmission directions was 0.41 and 0.83 Wm−1K−1, respectively. Owing to the anisotropy of κl and electronic structures along the transmission direction of PdX2, an anisotropic thermoelectric quality factor ZT appeared in PdX2. The excellent electronic properties and low lattice thermal conductivity (κl) achieved a high ZT of the penta-PdTe2 monolayer, whereas the maximum ZT of the p- and n-type PdTe2 reached 6.6 and 4.4, respectively. Thus, the results indicate PdTe2 as a promising thermoelectric candidate.

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.

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.

Tribo-Piezoelectricity in Group III Nitride Bilayers: A Density Functional Theory Investigation

The notable out-of-plane piezoelectric effect caused by the large electronegativity of the constituent elements makes two-dimensional (2D) group III nitrides appealing for nanoscale energy-harvesting applications. Here, we demonstrate by extensive density functional theory investigations that the vertical piezoelectricity is enhanced significantly in 2D XN (X = B, Al, Ga) bilayers due to in-plane interlayer sliding. The sliding operation generates tribological energy from the vertical resistance force between the monolayers. A maximum shear strength between the monolayers of 1-25 GPa is recorded during vertical sliding. We elucidate the tribo-piezoelectricity generation mechanism of XN bilayers using the tribological energy conversion to overcome the interfacial sliding barrier. The strongest out-of-plane piezoelectricity is found when the bilayers are in the A-A stacking arrangement. Any reduction in the interlayer distance between group III nitride bilayers enhances out-of-plane polarization due to the increase in sliding energy resistances, leading to an increased inductive voltage output. An induced voltage of ∼3.5 V is achieved during vertical compressive sliding of the upper layer. Using these phenomena, we present a compression-slide XN bilayer nanogenerator strategy capable of tuning the produced tribo-piezoelectric energy through sliding and compression.

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.

Low-cost pentagonal NiX2 (X=S, Se, and Te) monolayers with strong anisotropy as potential thermoelectric materials

Pentagonal compounds, as a new family of 2D materials, have recently been extensively studied in the fields of electrocatalysis, photovoltaics, and thermoelectrics. Encouraged by the successful synthesis of pentagonal PdSe2,...

Impressive Thermoelectric Figure of Merit in Two-Dimensional Tetragonal Pnictogens: a Combined First-Principles and Machine-Learning Approach

Over the past decade, two-dimensional materials have gained a lot of interest due to their fascinating applications in the field of thermoelectricity. In this study, tetragonal monolayers of group-V elements (T-P, T-As, T-Sb, and T-Bi) are systematically analyzed in the framework of density functional theory in combination with the machine-learning approach. The phonon spectra, as well as the strain profile, dictate that these tetragonal structures are geometrically stable as well as they are potential candidates for experimental synthesis. Electronic analysis suggests that tetragonal pnictogens offer a band gap in the semiconducting regime. Thermal transport characteristics are investigated by solving the semiclassical Boltzmann transport equation. Exceptionally low lattice thermal conductivity has been observed as the atomic number increases in the group. The high Seebeck coefficient and electrical conductivity as well as the low thermal conductivity of T-As, T-Sb, and T-Bi lead to the generation of a very high thermoelectric figure of merit as compared to standard thermoelectric materials. Furthermore, the thermoelectric conversion efficiency of these materials has been observed to be much higher, which ensures their implications in thermoelectric device engineering.

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.

First principles study of Janus WSeTe monolayer and its application in photocatalytic water splitting

By employing the state-of-the-art density functional theory method, we demonstrate that Janus WSeTe monolayer exhibits promising photocatalytic properties for solar water splitting. The results show that the monolayer possesses thermodynamic stability, suitable bandgap (∼1.89 eV), low excitons binding energy (∼0.19 eV) together with high hole mobility (∼10³cm²V^-1s^-1). Notably, the results suggest that the oxygen evolution reaction can undergo spontaneously without any sacrificial reagents. In contrast, the overpotential of hydrogen evolution reaction can partially be overcome by the external potential under solar light irradiation. Furthermore, the intrinsic electric field induced by the symmetry breaking along the perpendicular direction of Janus WSeTe monolayer not only suppresses the electron-hole recombination but also contributes to the solar-to-hydrogen efficiency, which is calculated to be ∼19%. These characteristics make the Janus WSeTe monolayer to be a promising candidate for solar water splitting.

Excellent Room-Temperature Thermoelectricity of 2D GeP3: Mexican-Hat-Shaped Band Dispersion and Ultralow Lattice Thermal Conductivity

Although some atomically thin 2D semiconductors have been found to possess good thermoelectric performance due to the quantum confinement effect, most of their behaviors occur at a higher temperature. Searching for promising thermoelectric materials at room temperature is meaningful and challenging. Inspired by the finding of moderate band gap and high carrier mobility in monolayer GeP_3, we investigated the thermoelectric properties by using semi-classical Boltzmann transport theory and first-principles calculations. The results show that the room-temperature lattice thermal conductivity of monolayer GeP₃ is only 0.43 Wm⁻¹K⁻¹ because of the low group velocity and the strong anharmonic phonon scattering resulting from the disordered phonon vibrations with out-of-plane and in-plane directions. Simultaneously, the Mexican-hat-shaped dispersion and the orbital degeneracy of the valence bands result in a large p-type power factor. Combining this superior power factor with the ultralow lattice thermal conductivity, a high p-type thermoelectric figure of merit of 3.33 is achieved with a moderate carrier concentration at 300 K. The present work highlights the potential applications of 2D GeP₃ as an excellent room-temperature thermoelectric material.

Theoretical investigations of novel Janus Pb2SSe monolayer as a potential multifunctional material for piezoelectric, photovoltaic, and thermoelectric applications

Two-dimensional Janus nanomaterials, due to their unique electronic, optical, and piezoelectric characteristics resulting from the antisymmetric structures, exhibit great prospects in multifunctional energy application to alleviate the energy crisis. Monolayer Janus Pb₂SSe, with a black phosphorus-like structure and an indirect band gap of 1.59 eV as well as high carrier mobility (526-2105 cm² V^-1 s^-1), displays outstanding potentials in the energy conversion between nanomechanical energy, solar energy, waste heat, and electricity, which has been comprehensively studied utilizing DFT-based simulations. The research results reveal that monolayer Pb₂SSe not only possesses giant in-plane piezoelectricity of d₁₁ = 75.1 pm V^-1 but also superhigh out-of-plane piezoelectric coefficients (d₃₁ = 0.5 pm V^-1 and d₃₃ = 15.7 pm V^-1). Meanwhile, by constructing Pb₂SSe bilayers, the out-of-plane piezoelectric coefficients can be significantly enhanced (d₃₁ = 19.2 pm V^-1 and d₃₃ = 194.7 pm V^-1). In addition, owing to the small conduction band offset, suitable donor band gap and excellent light absorption capability in the Pb₂SSe/SnSe (Pb₂SSe/GeSe) heterostructure, the power conversion efficiencies were calculated to be up to 20.02% (Pb₂SSe/SnSe) and 19.28% (Pb₂SSe/GeSe), making it a promising candidate for solar energy collection. Furthermore, from the thermoelectric electron and phonon transport calculations, it can be found that the Pb₂SSe monolayer is an n-type thermoelectric material with ultrahigh ZT = 2.19 (1.52) at room temperature, which can be traced back to its ultralow κ_L = 0.78 (0.99) W m^-1 K^-1, and superhigh PF = 10.18 (8.25) mW m^-1 K^-2 along the x(y) direction at the optimal doping concentration at 300 K. The abovementioned versatile characteristics in the Janus Pb₂SSe monolayer, along with its comprehensive stabilities (energy, dynamic, thermal, and mechanical stabilities), highlight its potential in clean energy harvesting.

Strong tribo-piezoelectric effect in bilayer indium nitride (InN)

The high electronegativity between the atoms of two-dimensional (2D) group-III nitrides makes them attractive to demonstrating a strong out-of-plane piezo-electricity effect. Energy harvesting devices can be predicted by cultivating such salient piezoelectric features. This work explores the tribo-piezoelectric properties of 2D-indium nitride (InN) as a promising candidate in nanogenerator applications by means of first-principles calculations. In-plane interlayer sliding between two InN monolayers leads to a noticeable rise of vertical piezoelectricity. The vertical resistance between the InN bilayer renders tribological energy by the sliding effect. During the vertical sliding, a shear strength of 6.6-9.7 GPa is observed between the monolayers. The structure can be used as a tribo-piezoelectric transducer to extract force and stress from the generated out-of-plane tribo-piezoelectric energy. The A-A stacking of the bilayer InN elucidates the highest out-of-plane piezoelectricity. Any decrease in the interlayer distance between the monolayers improves the out-of-plane polarization and thus, increases the inductive voltage generation. Vertical compression of bilayer InN produces an inductive voltage in the range of 0.146-0.196 V. Utilizing such a phenomenon, an InN-based bilayer compression-sliding nanogenerator is proposed, which can tune the generated tribo-piezoelectric energy by compressing the interlayer distance between the InN monolayers. The considered model can render a maximum output power density of ~ 73 mWcm-2 upon vertical sliding.

Janus 2D titanium nitride halide TiNX0.5Y0.5 (X, Y = F, Cl, or Br, and X ≠ Y) monolayers with giant out-of-plane piezoelectricity and high carrier mobility

We have proposed a series of Janus 2D titanium nitride halide TiNX_0.5Y_0.5 (X, Y = F, Cl, or Br, and X ≠ Y) monolayers, which have considerable out-of-plane piezoelectricity and high carrier mobility.