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

Fundamental Factors Governing Stabilization of Janus 2D-Bulk Heterostructures with Machine Learning

The more than 6000 2D materials predicted thus far provide a huge combinatorial space for forming functional heterostructures with bulk materials, with potential applications in nanoelectronics, sensing, and energy conversion. In this work, we investigate nearly 1000 heterostructures, the largest number of heterostructures thus far, of 2D Janus and bulk materials' surfaces using ab initio simulations and machine learning (ML) to deduce the structure-property relationships of the complex interfaces in such heterostructures. We first perform van der Waals-corrected density functional theory simulations using a high-throughput computational framework on 51 Janus 2D materials and 19 metallic, cubic phase, elemental bulk materials that exhibit low lattice mismatches and low coincident site lattices. The formation energies of the resultant 1147 Janus 2D-bulk heterostructures were analyzed, and 828 were found to be thermodynamically stable. ML models were trained on the computed data, and we found that they could predict the binding energy and z-separation of 2D-bulk heterostructures with root mean squared errors (RMSEs) of 0.05 eV/atom and 0.14 Å, respectively. The feature importance of the models reveals that the properties of the bulk materials dominate the heterostructures' energies and interfacial structures heavily. These findings are in line with experimentally observed behavior of several well-known 2D materials-bulk systems. The data used within this paper are freely available in the Ab Initio 2D-Bulk Heterostructure Database (aiHD). The fundamental insights into 2D-bulk heterostructures and the predictive ML models developed in this work could accelerate the application of thousands of 2D-bulk heterostructures, thus stimulating research within a wide range of electronic, quantum computing, sensing, and energy applications.

Two-dimensional Janus X2SSe (X = Al, Ga or In) monolayers: potential photocatalysts with low effective mass

Janus 2D materials with high-efficiency solar energy conversion possess fine photocatalytic properties, which are current research hotspots in the field of photocatalysis. Herein, Janus X2SSe (X = Al, Ga or In) monolayers were proposed to explore their photocatalytic activities through first-principles calculation. The results demonstrated that the structures of Janus X2SSe (X = Al, Ga or In) monolayers were both dynamically and thermodynamically stable and exhibited excellent semiconductor properties. Additionally, a low effective mass for the photogenerated electrons and holes was observed. However, the Janus Al2SSe monolayer had an inappropriate band-edge potential, and the light-absorption edge corresponding to the maximum wavelength of the Janus Ga2SSe monolayer was not wide enough, which did not meet the conditions for an effective photocatalyst. Amazingly, the Janus In2SSe monolayer not only completely met the redox conditions for photocatalytic water decomposition but also presented visible-optical absorption with an absorption-band edge at 662 nm because of its indirect band gap of 1.52 eV, revealing its great application potential in photocatalysis. The above findings disclose the potential of the Janus In2SSe monolayer as a high-performance photocatalyst.

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.

Two-dimensional Be2P4 as a promising thermoelectric material and anode for Na/K-ion batteries

Incredibly effective and flexible energy conversion and storage systems hold great promise for portable self-powered electronic devices. Owing to their large surface area, exceptional atomic structures, superior electrical conductivity and good mechanical flexibility, two-dimensional (2D) materials are recognized as an attractive option for energy conversion and storage application. In this work, we examined the stability, electronic, thermoelectric and electrochemical aspects of a novel 2D Be2P4 monolayer by adopting density functional theory (DFT). The Be2P4 monolayer exhibits a direct semiconductor gap of 0.9 eV (HSE06), large Young's modulus (∼198 GPa), high carrier mobility (∼104 cm2 V-1 s-1) and a low excitonic binding energy of 0.11 eV. Our calculated findings suggest that Be2P4 shows a lattice thermal conductivity of 1.02 W m K-1 at 700 K, resulting in moderate thermoelectric performance (ZT ∼ 0.7), encouraging its use in thermoelectric materials. In addition, a higher adsorption energy of -2.28 eV (-2.52 eV) and less diffusion barrier of 0.22 eV (0.17 eV) for Na(K)-ion batteries promote fast ion transport in the Be2P4 monolayer. This material also shows a high specific capacity and superior energy density of 8460 W h kg-1 (8883 W h kg-1) for Na(K)-ion batteries. Thus, our results offer insightful information for investigating potential thermoelectric and flexible anode materials based on the Be2P4 monolayer.

Metavalently bonded tellurides: the essence of improved thermoelectric performance in elemental Te

Elemental Te is important for semiconductor applications including thermoelectric energy conversion. Introducing dopants such as As, Sb, and Bi has been proven critical for improving its thermoelectric performance. However, the remarkably low solubility of these elements in Te raises questions about the mechanism with which these dopants can improve the thermoelectric properties. Indeed, these dopants overwhelmingly form precipitates rather than dissolve in the Te lattice. To distinguish the role of doping and precipitation on the properties, we have developed a correlative method to locally determine the structure-property relationship for an individual matrix or precipitate. We reveal that the conspicuous enhancement of electrical conductivity and power factor of bulk Te stems from the dopant-induced metavalently bonded telluride precipitates. These precipitates form electrically beneficial interfaces with the Te matrix. A quantum-mechanical-derived map uncovers more candidates for advancing Te thermoelectrics. This unconventional doping scenario adds another recipe to the design options for thermoelectrics and opens interesting pathways for microstructure design.

A van der Waals p-n heterostructure of GaSe/SnS2: a high thermoelectric figure of merit and strong anisotropy

In recent years, van der Waals heterostructures (vdWHs) with controllable and peculiar properties have attracted extensive attention in the fields of electronics, optoelectronics, spintronics and electrochemistry. However, vdWHs with good thermoelectric performance are few due to the complex coupling of thermoelectric coefficients. Here, we employ density functional theory and Boltzmann's transport equation to explore the thermoelectric properties of the p-n vdWH of GaSe/SnS2, which has been experimentally observed to exhibit high performance as an optoelectronic device. We reveal that GaSe/SnS2 possesses strong anisotropy in terms of electronic transport resulting from the anisotropic carrier relaxation time. The longer carrier relaxation time in the y-direction for n-type induces a high power factor of 0.084 W m-1 K-2 at 300 K, while it is only 0.0087 W m-1 K-2) in the x-direction. The strong coupling of low-mid frequency phonon branches and the relatively weak Sn-S bond-induced anharmonicity hinder the phonon transport, which results in the lattice thermal conductivity of GaSe/SnS2 (14.61 and 15.43 W m-1 K-1 along the x- and y-directions at 300 K) being much smaller than the average value of GaSe and SnS2 (43.44 W m-1 K-1 at 300 K). The optimal thermoelectric figure of merit at 700 K for GaSe/SnS2 reaches 2.99, which is significantly higher than those of the constituents of GaSe (0.58) and SnS2 (1.04). The present work highlights the potential thermoelectric applications and the understanding of the thermoelectric transport mechanism for the recently synthesized p-n vdWH of GaSe/SnS2 with a high thermoelectric figure of merit and strong anisotropy.

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.