Excellent thermoelectric properties of monolayer RbAgM (M = Se and Te): first-principles calculations

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.

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.

High thermoelectric performance of two-dimensional layered AB2Te4 (A = Sn, Pb; B = Sb, Bi) ternary compounds

The thermoelectric properties of two-dimensional layered ternary compounds AB₂Te₄, in which A (Sn, Pb) and B(Sb, Bi) are group-IV and group-V cations, respectively, were investigated by using first-principles based transport theory. These septuple-atom-layer monolayers have wider band gaps with respect to their bulks, which extend their operating temperature and inhibit the bipolar carrier conduction and thermal conductivity, and more importantly, their energy bands exhibit multiple valence band convergence to a narrow energy range near the Brillouin zone center, which induces an optimal p-type power factor up to 10.94-32.11 W m^-1 K^-2 at room temperature. Moreover, these monolayers contain heavy atomic masses and high polarizability of some chemical bonds, leading to small group velocities of phonons and anharmonic phonon behavior that produce an intrinsic lattice thermal conductivity as low as 0.79-3.13 W m^-1 K^-1 at room temperature. Thus, these monolayers act as p-type thermoelectric materials with thermoelectric figure of merit of up to 2.6-5.5 for SnSb₂Te₄, 0.7-2.2 for PbSb₂Te₄, and 1.6-4.2 for PbBi₂Te₄ in the temperature range of 300 to 750 K, and 4.5-5.9 for SnBi₂Te₄ in the temperature range of 300 to 450 K.

Ultralow lattice thermal conductivity at room temperature in 2D KCuSe from first-principles calculations

Ultralow lattice thermal conductivity is crucial for achieving a high thermoelectric figure of merit for thermoelectric applications. In this work, using first-principles calculations and the phonon Boltzmann transport theory, we investigate the phonon thermal transport properties of 2D KCuSe. Our calculations indicate that the strong acoustic-optical coupling, the low-lying acoustic phonon modes and the strong lattice anharmonic effect with a large Grüneisen parameter and phase space volume result in an ultralow lattice thermal conductivity of 0.021 W m^-1 K^-1 at 300 K for monolayer KCuSe, which is lower than those of recently reported KAgSe (0.26 W m^-1 K^-1 at 300 K) and TlCuSe (0.44 W m^-1 K^-1 at 300 K). Importantly, although the Coulomb interactions and the tensile biaxial strain lead to the increase of lattice thermal conductivity due to the increasing relaxation time (0.056 and 0.28 W m^-1 K^-1 at 300 K without and with 6% tensile strain, respectively), it is still lower than those of most 2D thermoelectric materials. The advantages of being cheap, environmentally friendly and having low lattice thermal conductivity compared to the KAgSe and TlCuSe derivatives make KCuSe a promising candidate for thermoelectric applications, which will stimulate more efforts toward theoretical and experimental studies on this class of 2D ternary semiconductors.

Prediction of stable silver selenide-based energy materials sustained by rubidium selenide alloying

Silver selenide (Ag2Se) is a ductile material with a low lattice thermal conductivity that can be a valuable substitute for both PbSe and Bi2Se3 for Pb toxicity free and Bi scarcity.