Boltzmann Transport Theory Research Articles

Evaluation of battery positive-electrode performance with simultaneous ab-initio calculations of both electronic and ionic conductivities

Battery positive-electrode material is usually a mixed conductor that has certain electronic and ionic conductivities, both of which crucially control battery performance such as the rate capability, whereas the microscopic understanding of the conductivity relationship has not been established yet. Herein, we used Boltzmann transport theory and molecular dynamics at the ab initio level to investigate the electronic and ionic conductivities on the same footing, with the representative layered oxides Lix(Co, Ni)O2. The present calculations successfully demonstrated the electronic conductivities quantitatively and indicated a microscopic origin of the electronic difference between LixCoO2 and LixNiO2 (0.6 < x≤ 1). The calculated ionic conductivities were also consistent with the experimental values. Especially, we observed Ni ion migration to the Li layer at lower x, leading to suppression of Li ion diffusion. Based on these results, we found that the ratio and product of the electronic and ionic conductivities as ζ and κ, respectively, are good descriptors to evaluate the battery positive-electrode performance because these descriptors clearly can distinguish between LixCoO2 and LixNiO2. Referring to the values of the excellent positive electrode LixCoO2, we suggest ζ≈ 106 and κ≈ 10−2 as target measures for the positive-electrode material design.

High thermoelectric performance of TlInSe3 with ultra-low lattice thermal conductivity

Thermoelectric (TE) materials with an excellent thermoelectric figure of merit (ZT) provide an effective way to alleviate energy pressure and protect the environment. By applying the first-principles method, this paper makes a systematic study of the electronic and phonon transport properties of two-dimensional (2D) novel TlInSe3 utilizing the Boltzmann transport theory (BTE). The calculation results reveal that 2D TlInSe3 has an excellent power factor (0.81 × 10−2 W/mK2) and ultra-low lattice thermal conductivity (0.46 W/mK) at 300 K. We find that the low phonon group velocity and strong anharmonicity are the main factors leading to the ultra-low lattice thermal conductivity of TlInSe3. Meanwhile, by discussing the acoustic-optical scattering, we attribute low phonon group velocity and strong anharmonicity to the increase of scattering rates between acoustic mode and optical mode, which further suppresses the lattice thermal conductivity. In the analysis of electron and phonon transport properties, 2D TlInSe3, as a novel TE material, exhibits a ZT value as high as 4.15 at 500 K. Our research results show that TlInSe3 is a potential TE material, and the relevant analysis is significant in exploring new TE materials.

Transport and thermoelectric performance of Fluorine functionalized Ge-carbide sheets

Based on density functional theory simulations, GW approximation + BSE and the semi-classical Boltzmann transport theory, electronic ,optical and thermoelectric response of bidimensional fluorinated Ge-carbides are investigated. Three configurations are considered, namely F-CGe, CGe-F, F-CGe-F to explore the effect of both fluorination distribution and coverage. It is shown that the conduction (valence) band is shifted upwards (downwards) when F decorates C-atoms (Ge-atoms) which tunes the gap energy. Furthermore, full fluorination makes CGe sheet better dielectric structure. The maximum reflectivity and absorption reveal that F-CGe-F structure exhibits better optical response, unlike its two counterparts constrained to absorb incident photon energy to the UV region only. The maximum polarization values are found in the visible region for the half fluorinated structure F-CGe and in the UV spectrum for F-CGe-F and CGe-F. The Fluorination also improves the transport properties of CGe which significantly increases its merit factor (ZT) at room temperature. Th three compounds are found to exhibit an ultralow lattice conductivity which leads and a record high ZT up to 0.65, 0.77, and 0.81 at 900 K in CGe-F, F-CGe and F-CGe-F respectively. Our results evaluate the potential of fluorinated CGe sheets as suitable thermoelectric materials at the ambient temperature.

Tuned electronic band structure and intensified phonon scattering of Ge2Sb2Te5 by strain engineering for thermoelectric performance

The strain engineering effect on the thermoelectric properties of Ge2Sb2Te5 has been investigated using first-principles calculations combined with Boltzmann transport theory. The results show that 3% strain tunes the band gap to a desirable value and leads to a band convergence of conduction bands. The multiband transport and more localized density of state in the conduction bands significantly increase the power factor of n-type Ge2Sb2Te5. The strain-induced phonon vibration spectrum convergence enhances the phonon scattering rate. The low-frequency acoustic-optical branch coupling and the unique high-frequency optical branch scattering at 3% strain together contribute to the reduction of the lattice thermal conductivity. At 700 K, 3% strain results in the maximum enhancement of the thermoelectric properties of Ge2Sb2Te5, enhanced by 30% and 424% for p- and n-type Ge2Sb2Te5 compared with no strain one, respectively.

Structural, electronic, elastic and thermal properties of Cr-doped U3Si2: A DFT study

Alloying has been considered as an effective strategy to make up for the poor oxidation corrosion resistance of U3Si2. However, the theoretical studies on alloyed U3Si2 are rarely reported. In this study, the structural, electronic, elastic and thermal properties of 10at% Cr-doped U3Si2 were studied using first principles calculations and semiclassical Boltzmann transport theory. Our results show that the U3Si1.5Cr0.5 phase is determined to be thermodynamically, dynamically and elastically stable, and U2.5Si2Cr0.5 phase is dynamically and elastically stable. U3Si1.5Cr0.5 is more stable than U2.5Si2Cr0.5 from the thermodynamic calculation results. From the calculated electronic structure results, a mixture of ionic and metallic bonds dominates the bonding of Cr-doped U3Si2. They are ductile materials compared to the brittleness of U3Si2 according to the results of B/G ratio, Cauchy pressure and Poisson's ratio. Moreover, U3Si1.5Cr0.5 is a nearly elastic isotropic material, while U2.5Si2Cr0.5 exhibits strong anisotropy. The thermal conductivity of Cr-alloyed U3Si2 is dominated by the electronic contribution at high temperature, which is lower than U3Si2 but higher than UO2 at service temperature and increases with temperature. Therefore, in addition to significantly improving the resistance to oxidation and corrosion of U3Si2, Cr doping can also convert the brittleness of U3Si2 into toughness and improve its machinability, albeit at the reasonable expense of some mechanical properties and thermal conductivities, while maintaining the high uranium density of U3Si2 fuel form. The results of this work may provide useful clues for the application and improvement of uranium silicide fuels.

Significantly reinforced thermoelectric performance in the novel 1T-Au6Se2 monolayer

Ultra-low lattice thermal conductivity has long been a requirement for the high thermoelectric properties of materials. In this work, the novel 1T-Au6Se2 monolayer was obtained by introducing Au6 clusters into the selenide monolayer, and its electrical and thermal transport characteristics are investigated using first-principles computations supplemented with semi-classical Boltzmann transport theory. The calculation shows that the 1T-Au6Se2 monolayer exhibits ultra-low lattice thermal conductivity and excellent thermoelectric properties owing to its low phonon frequency, group velocity, and extremely strong anharmonicity. Based on strain engineering from 0% to 2%, the lattice thermal conductivity further reduces by restricting the thermal transport on the premise of maintaining outstanding electrical transport properties in the p-type doped system. Thence, the value of ZT for the p-type system increases nearly by 70% compared with the non-stressed state at 700 K. Our investigation indicates the ultra-low thermal conductivity and high ZT of the 1T-Au6Se2 monolayer that might be prepared in the lab, providing new insights into enhancing the thermoelectric performance of the material in the future.

Anharmonic lattice dynamics and the origin of intrinsic ultralow thermal conductivity in AgI materials

Ionic conductors such as AgI with ultralow thermal conductivities $({\ensuremath{\kappa}}_{l})$ are of increasing interest because of their excellent thermoelectric properties. However, the origin of their intrinsic low ${\ensuremath{\kappa}}_{l}$ values remain elusive. In this study, comprehensive theoretical calculations of the lattice dynamics and the thermal transport properties of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$ (zinc-blende structure) and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{AgI}$ (wurtzite structure) as functions of temperature were carried out based on many-body perturbation theory and phonon Boltzmann transport theory. First, the mean-squared displacements (MSDs) of ${\mathrm{Ag}}^{+}$ were significantly larger than those of ${\mathrm{I}}^{\ensuremath{-}}$ in both $\ensuremath{\gamma}\text{\ensuremath{-}}$ and \ensuremath{\beta}-phases below the order-disorder phase transition temperature $({T}_{\mathrm{c}})$, which led to a characteristic ``rattling'' feature and low-frequency, nearly flat local phonon vibrations. According to our previous work [Xie et al., Phys. Rev. Lett. 125, 245901 (2020)], such nondispersive flat phonon band structures are expected to give rise to four-phonon resonance and result in a dramatic increase in the four-phonon scattering over the conventional three-phonon scattering. For $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$, similar four-phonon resonance behavior was also discovered for the low-lying transverse acoustic phonon branches, and it was found that their four-phonon scattering rates were an order of magnitude larger than the corresponding three-phonon scattering rates. Considering the four-phonon scattering, the theoretical ${\ensuremath{\kappa}}_{l}$ of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$ was predicted to be $\ensuremath{\sim}0.32$ W/m K at 300 K, which was in good agreement with the value deduced from our experiments ($\ensuremath{\sim}0.36$ W/m K at 300 K). Compared to $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{AgI}$, the acoustic phonons in $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{AgI}$ were more dispersive, and they intertwined with low-energy optical phonons at the zone boundaries. It was found that three-phonon resonance became as important as four-phonon resonance for the nearly flat longitudinal phonon band. The theoretical ${\ensuremath{\kappa}}_{l}$ for $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{AgI}$ was determined to be around $\ensuremath{\sim}0.32$ W/m K at room temperature, closely reproducing our measurement value $\ensuremath{\sim}0.29$ W/m K. Our results for AgI demonstrate the strong quartic anharmonicity in materials characterized by the rattling of weak bonding atoms as well as dispersionless phonon band structures. It is believed that this intimate relationship between the low-${\ensuremath{\kappa}}_{l}$ and flat phonon dispersion can be employed as a good indicator when searching for material systems with ultralow ${\ensuremath{\kappa}}_{l}$ values, e.g., cagelike rattling structures, quasi-two-dimensional structures, and chainlike structures.

Quantum oscillations as a robust fingerprint of chiral anomaly in nonlinear response in Weyl semimetals

In view of searching the signature of the celebrated chiral anomaly (CA) in Weyl semimetals (WSMs) in ongoing experiments, quantum oscillation in linear response regime has been considered as an important signature in the magneto transports in WSMs, due to its unique relation to CA. Investigating the nonlinear planar effects (NPEs) starting from the semiclassical regime to the ultra-quantum limit within the framework of Boltzmann transport theory incorporating Landau quantization, we here propose the quantum oscillations in NPEs can serve as a robust signature of CA in WSMs. By obtaining analytical expressions, we show that the quantum oscillations of the nonlinear effects exhibit two different period scales in 1/B (B is the magnetic field) compared to the linear responses where only one period scale exists. We find that these quantum oscillations in NPEs are attributed to the deviation of chiral chemical potential (CCP), which is proportional to the finite band tilt as well as transverse electric field and therefore, directly linked to CA in WSMs. In addition, we also show that the CA-induced nonlinear magneto conductivity is linear and independent in the magnetic field in the semiclassical and ultraquantum regimes, respectively. We conclude that the proposed behaviors of NPEs in different regimes uniquely signify the existence of CA and therefore, can serve as a probe of identifying CA in WSMs in experiments.

First-principles study on the thermoelectric properties of Sr2Si and Sr2Ge

The development of environmentally friendly thermoelectric materials composed of earth-abundant, non-toxic elements is highly desirable in thermoelectric technology. In this study, the thermoelectric properties of n-type doped Sr2Si and Sr2Ge were systematically investigated using first-principles density functional theory calculations combined with semi-classical Boltzmann transport theory. The multi-band feature in the conduction band of Sr2Ge leads to a higher Seebeck coefficient than Sr2Si, resulting in a higher power factor. The phonon transport calculations using third-order perturbation theory predict ultra-low lattice thermal conductivity of 0.42 Wm−1K−1 for Sr2Si and 0.33 Wm−1K−1 for Sr2Ge at 900 K. The maximum figure of merit ZT is 1.44 for Sr2Ge, which is approximately 1.25 times higher than that of 1.15 for Sr2Si at 900 K. Our results indicate that the Sr2Ge and Sr2Si are promising candidates for high-performance thermoelectric materials.

Pressure-induced enhancement of thermoelectric performance of CoP3 by the structural phase transition

Using particle swarm optimization and first-principles calculations, we discovered a novel high-pressure orthorhombic structure of CoP3 with the space group of Pnma, which is stable above 24.6 GPa. According to our calculations based on Boltzmann transport theory, the Seebeck coefficient of the Pnma phase is 6.5 times as great as that of the ambient Im-3 phase at a hole concentration of 1 × 1020 cm−3. The calculation further indicates the Pnma-CoP3 exhibits a maximal figure of merit ZT of 0.56 and 0.74 for p- and n-type at 800 K, respectively. Especially for p-type, the Pnma-CoP3 is quintuple better than the ambient phase which has a ZT of 0.1. The higher ZT value can be attributed to the deeper projected density of states generated by hybridization between P p states and Co d states near the Fermi level. Therefore, the high pressure mainly improves the ZT value by modulating electron properties. Current results demonstrate that the Pnma-CoP3 material is a competitive candidate for thermoelectric applications, especially under high temperature condition (800 K). This work provides a convenient alternative route to increase the thermoelectric performance of skutterudites.

Strain Tunable Thermoelectric Material: Janus ZrSSe Monolayer

Thermoelectric (TE) performance of the Janus ZrSSe monolayer under biaxial strain is systematically explored by the first-principles approach and Boltzmann transport theory. Our results show that the Janus ZrSSe monolayer has excellent chemical, dynamical, thermal, and mechanical stabilities, which provide a reliable platform for strain tuning. The electronic structure and TE transport parameters of the Janus ZrSSe monolayer can be obviously tuned by biaxial strain. Under 2% tensile strain, the optimal power factor PF of the n-type-doped Janus ZrSSe monolayer reaches 46.36 m W m-1 K-2 at 300 K. This value is higher than that of the most classical TE materials. Under 6% tensile strain, the maximum ZT values for the p-type- and n-type-doped Janus ZrSSe monolayers are 4.41 and 4.88, respectively, which are about 3.83 and 1.49 times the results of no strain, respectively. Such high TE performance can be attributed to high band degeneracy and short phonon relaxation time under strain, causing simultaneous increase of the Seebeck coefficient and suppression of the phonon thermal transport. Present work demonstrates that the Janus ZrSSe monolayer is a promising candidate as a strain-tunable TE material and stimulates further experimental synthesis.

Effect of Hydrostatic Pressure on Lone Pair Activity and Phonon Transport in Bi2O2S

Dibismuth dioxychalcogenides, Bi2O2Ch (Ch = S, Se, Te), are a promising class of materials for next-generation electronics and thermoelectrics due to their ultrahigh carrier mobility and excellent air stability. An interesting member of this family is Bi2O2S, which has a stereochemically active 6s2 lone pair of Bi3+ cations, heterogeneous bonding, and a high mass contrast between its constituent elements. In the present study, we have used first-principles calculations in combination with Boltzmann transport theory to systematically investigate the effect of hydrostatic pressure on lattice dynamics and phonon transport properties of Bi2O2S. We found that the ambient Pnmn phase has a low average lattice thermal conductivity (κl) of 1.71 W/(m K) at 300 K. We also predicted that Bi2O2S undergoes a structural phase transition from a low-symmetry (Pnmn) to a high-symmetry (I4/mmm) structure at around 4 GPa due to centering of Bi3+ cations with pressure. Upon compression, the lone pair activity of Bi3+ cations is suppressed, which increases κl by almost 3 times to 4.92 W/(m K) at 5 GPa for the I4/mmm phase. The computed phonon lifetimes and Grüneisen parameters show that anharmonicity decreases with increasing pressure due to further suppression of the lone pair activity and strengthening of intra- and intermolecular interactions, leading to an average room-temperature κl of 12.82 W/(m K) at 20 GPa. Overall, this study provides a comprehensive understanding of the effect of hydrostatic pressure on the stereochemical activity of the lone pair of Bi3+ cations and its implications on the phonon transport properties of Bi2O2S.

Phase transition enhanced thermoelectric performance for perovskites: The case of AgTaO3

In general, perovskite materials, such as AgTaO3, can exhibit complex structural phase transitions when the temperature changes. Different phase structures may show distinct thermoelectric properties. However, the effects of phase transition on the thermoelectric performance of AgTaO3 have yet to be systematically investigated. Thus, in this work, the electronic structures and thermoelectric properties of the p-type AgTaO3 in various phase structures are investigated by utilizing first-principles calculations and semi-classical Boltzmann transport theory. The results show that the p-type AgTaO3 in the cubic phase shows a superior power factor because the cubic phase possesses a large Seebeck coefficient and intermediate electrical conductivity. The calculated thermoelectric performance is consistent with the predictions and explanations from the analyses of the electronic structures. When the p-type AgTaO3 is transformed from the tetragonal phase to the cubic phase, its power factor is tripled, suggesting that a suitable phase transition can improve the thermoelectric properties of perovskites.

First principles study on structural, mechanical, and thermoelectric properties of half-Heusler alloy (KLiTe)

In this study, the structural, optoelectronics, elastic, and transport properties of KLiTe were estimated from first principles computations using the FP-LAPW technique and semi-classical Boltzmann transport theory The exchange–correlation is addressed employing the generalized gradient approximation with Perdew-Burke-Ernzerhof scheme (GGA-PBE). In order to distinguish between the brittle and ductile properties, the Poisson and Pugh's ratio was calculated. The semiconductor KLiTe is demonstrated to have a direct band gap. The compound under investigation exhibits significant Seebeck coefficient and power factor values. The high-power factor of 8.3 × 1011 W/K2ms. that KLiTe exhibits at 1200 K makes it a potentially viable material for thermoelectric applications. Due to their high figure of merit values, the results obtained indicate that KLiTe could be a strong contender for thermoelectric generator applications at low and moderate temperature.

A theoretical prediction of novel Janus NiSX (X = O, Se, Te) Monolayers: Electronic, optical, and thermoelectric properties

Using density functional theory and semi-classical Boltzmann transport theory were employed to investigate the electronic structure, phononic and optical properties, dynamic stability, and thermoelectric characteristics of Janus NiSX (X = O, Se, Te). The electronic band structure of NiSTe indicates metallic behavior while NiSO and NiSSe show semiconducting features with indirect bandgap. Phononic properties and dynamic stability analyses reveal the stability of Janus Ni compounds. These compounds can be utilized as promising wave reflectors in the infrared (IR) range. Moreover, NiSO and NiSSe are good optical absorbers in the range of far ultraviolet (FUV). NiSTe exhibits isotropic absorption in the visible as well as FUV ranges. Thermoelectric calculations at room temperature show high Seebeck coefficients of 650 and 550 µV/K for NiSO and NiSSe, respectively. The large power factor of 20.83 × 1010 and 25.16 × 1010 W/msK2 are respectively found for n-type NiSO and NiSSe. The results suggest NiSO and NiSSe as promising thermoelectric materials with figure of merit of one (ZT ∼ 1). Consequently, NiSO and NiSSe can serve as efficient alternatives to conventional thermoelectric devices. The results of the current research can be used in the design of novel nanoelectronic devices as well as providing an excellent database for future studies.

Is Cu3-x P a Semiconductor, a Metal, or a Semimetal?

Despite the recent surge in interest in Cu3-x P for catalysis, batteries, and plasmonics, the electronic nature of Cu3-x P remains unclear. Some studies have shown evidence of semiconducting behavior, whereas others have argued that Cu3-x P is a metallic compound. Here, we attempt to resolve this dilemma on the basis of combinatorial thin-film experiments, electronic structure calculations, and semiclassical Boltzmann transport theory. We find strong evidence that stoichiometric, defect-free Cu3P is an intrinsic semimetal, i.e., a material with a small overlap between the valence and the conduction band. On the other hand, experimentally realizable Cu3-x P films are always p-type semimetals natively doped by copper vacancies regardless of x. It is not implausible that Cu3-x P samples with very small characteristic sizes (such as small nanoparticles) are semiconductors due to quantum confinement effects that result in the opening of a band gap. We observe high hole mobilities (276 cm2/(V s)) in Cu3-x P films at low temperatures, pointing to low ionized impurity scattering rates in spite of a high doping density. We report an optical effect equivalent to the Burstein-Moss shift, and we assign an infrared absorption peak to bulk interband transitions rather than to a surface plasmon resonance. From a materials processing perspective, this study demonstrates the suitability of reactive sputter deposition for detailed high-throughput studies of emerging metal phosphides.

Thermodynamic and electron transport properties of Ca3Ru2O7 from first-principles phonon calculations and Boltzmann transport theory

This work demonstrates a first-principles-based approach to obtaining finite temperature thermal and electronic transport properties which can be employed to model and understand mesoscale structural evolution during electronic, magnetic, and structural phase transitions. A computationally tractable model was introduced to estimate electron relaxation time and its temperature dependence. The model is applied to ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ with a focus on understanding its electrical resistivity across the electronic phase transition at 48 K. A quasiharmonic phonon approach to the lattice vibrations was employed to account for thermal expansion while the Boltzmann transport theory including spin-orbit coupling was used to calculate the electron-transport properties, including the temperature dependence of electrical conductivity.

Improved Thermoelectric Performance of NbCoSb with Intrinsic Nb Vacancies and Ni-Doping-Induced Band Degeneracy

Lately, the nominal 19-electron half-Heusler compound NbCoSb, historically viewed as a metal, has attracted reacquaintance and widespread attention because of its unexpected high thermoelectric (TE) performance. Here, the electronic structures of NbxCo1–yNiySb (x = 0.8, 1; y = 0, 0.1) have been systematically investigated by using the first-principles method and semiclassical Boltzmann transport theory. We demonstrate that Ni doping at Co sites in NbCoSb with 20% intrinsic Nb vacancies (Nb0.8CoSb) leads to a large band degeneracy, and the conduction band edge are mainly provided by the d-orbitals of the Ni, Co, and Nb atoms. The relative localization of the d orbitals not only remarkably increased the density-of-states effective mass near the Fermi energy but also retained a high mobility, which resulted in an optimal electrical conductivity without significant reduction of the Seebeck coefficient (∼−411.08 μV K –1 at 800 K) and thereby a large power factor. Meanwhile, Ni doping still preserved a low lattice thermal conductivity. As a result, a peak zT of 1.18 was achieved at 1100 K in the compound Nb0.8Co0.9Ni0.1Sb with a carrier concentration of 4.35 × 1020 cm–3. The present work identifies that Ni doping is an effective method to improve the TE properties of nominally 19-electron NbCoSb compounds with Nb vacancies and demonstrates the great potential for searching of nonstoichiometric 19-electron half-Heusler compounds with vacancies.

Janus γ-GeSSe Monolayer as a High-Performance Material for Photocatalysis and Thermoelectricity

Two-dimensional Janus materials have attracted increasing attention in recent years because of their potential as materials for energy applications, such as photocatalysis and thermoelectricity (TE). Here, for the first time, we propose a monolayer Janus structure with the Mexican-hat band, γ-GeSSe, by replacing the S atoms on one side of the synthesized γ-GeS with Se atoms. Using first-principles calculations based on density functional theory (DFT), we show that γ-GeSSe is mechanically, dynamically, and thermally stable. The mechanical, electronic, optical, and transport properties of monolayer Janus γ-GeSSe are also investigated in the DFT calculations. We find that the ideal strengths of the Janus γ-GeSSe are 6.08, 8.6, and 10.08 GPa for the armchair, zigzag, and biaxial directions, respectively. The Janus γ-GeSSe is an indirect-gap semiconductor with a band gap of 1.131 eV. Our results show the Janus γ-GeSSe as a photocatalysis material for water splitting with a high solar-to-hydrogen efficiency of up to 28.78%. This is because the combination of features includes an intrinsic electric field, high optical absorption coefficient, and carrier mobility. In addition, with the γ-structure, the Janus γ-GeSSe shows an intrinsic low lattice thermal conductivity of 3.33 W/mK at room temperature, which will contribute to obtaining high TE efficiency. By using the Boltzmann transport theory, we find that the maximum TE power factor and figure of merit of the Janus γ-GeSSe reach 55–85 mW/mK2 and 0.7–0.9 in the temperature range from 300 to 900 K, respectively. Thus, our findings not only contribute to proposing a Janus material but also suggest that it can be used as an energy material with high-performance photocatalysis and TE.

Thermoelectric properties of two-dimensional double transition metal MXenes: ScYC[formula omitted] ([formula omitted]F, OH)

Using first-principles calculations, we research the stability and electronic properties of two-dimensional double transition metal MXenes ScYCT2 (T=F, OH). On this basis, the semiconductor properties of ScYCF2 and ScYC(OH)2 are determined, and their thermoelectric properties are investigated by using Boltzmann transport theory and Slack model. The results show that the investigated MXenes structures have high thermoelectric properties. In particular, the n-type ScYC(OH)2 has the largest power factor of 0.072 W/mK2 at 900 K, and its maximum ZT value exceeds 3. According to the calculation result, we predict the ZT values for both the ScYCF2 and ScYC(OH)2 are greater than 1 at 900 K. The results of this study show that ScYCF2 and ScYC(OH)2 have potential as thermoelectric materials in the range of 300–900 K. And this work provides theoretical support for the experimental study of ScYCF2 and ScYC(OH)2.