Thermoelectric properties of SbXY (X = Se, Te; Y = Br, I) Janus layers

Abstract

We report a comprehensive investigation of the thermoelectric properties of SbXY (X = Se, Te; Y = Br, I) Janus layers (JL) using spin-polarized first-principles calculations. Ab initio molecular dynamics confirm that the 1T phase ($Pm31$) remains stable up to 1000 K, excluding any phase transitions. The calculated mean-square displacement further evidences the structural robustness. The thermal conductivity is strongly suppressed in Br-containing layers due to enhanced Froehlich interactions between optical and acoustic phonons. Electronic structure calculations reveal indirect band gaps of 1.1 to 1.3 eV, with valence and conduction bands dominated by the $p$-orbitals of halogen/chalcogen and of Sb, respectively. The carrier effective mass highlights anisotropic transport with lighter electrons being more mobile, while holes dominate the power factor, which attains values on the order of mW/mK$^2$. Direction-dependent transport indicates superior thermoelectric performance along the $xx$ direction, with negligible contribution along $yy$. The Figure of Merit reaches 0.6 at 1000 K in hole-doped SbSeBr, demonstrating strong potential for high-temperature applications. Our results reveal that the SbXY JLs, particularly SbSeBr, emerge as promising candidates for next-generation thermoelectric devices at elevated temperatures.

Figures (11)

• Figure 1: The (a) cross-sectional and (b) perpendicular views of the sandwich atomic arrangement in SbXY JLs. The X and Y represent the chalcogen (Se or Te) and halide (Br or I) atoms, respectively. The vertical distance between X and Y is taken as the thickness $t$ of JLs, and the calculated $t$ is given in (c) along with the unitcell lattice parameter $a_0$. The energy levels of JLs in (d-g) are derived from the wavefunction coefficients calculated using the BoltzTrap code btp2. The Fermi level is set at $0eV$, represented by the horizontal dashed gray line. The shaded region indicates all possible band gaps between valence and conduction bands. The shift in the valence band maximum (occurs along the $K-\Gamma$ direction) and conduction band minimum (positions at $\Gamma$ point) turns all SbXY JLs into indirect bandgap layers, denoted by green arrows with calculated bandgap (in eV). The Cartesian X and $y$ directions are mentioned by arrows in (b).
• Figure 2: Computed partial density of states in arbitrary units. The valence bands are contributed by $p$ orbitals of both the chalcogen X and halide Y in all layers, whereas Sb-$p$ orbitals dominate the conduction band. The equal up and down spin states depict that the 1T-phases of SbXY JL are essentially non-magnetic. Note that the Sb-$p$ states are magnified by a factor of three for better visibility.
• Figure 3: Summarized are the calculated (a) mobility $\mu$ and (b) relaxation time $\tau_c$ of electrons in SbXY JLs. The $x$- and $y$-components of the presented quantities are denoted by continuous and dashed lines, respectively. The mobility of electrons is higher than that of holes by a factor of $\approx 10$, cf. Fig. S1 in SM. Consequently, holes have a shorter mean free time $\tau_c$ and relax faster. The anisotropies in $\mu$ and $\tau_c$ denote that the SbXY JLs may have different TE efficiency in different directions.
• Figure 4: Computed differential charge density $\Delta \rho$ in SbXY JLs. The yellow (cyan) isosurfaces, visualized at 4e-2 $e$/bohr$^3$ level, denote charge accumulation (depletion from) on atoms. The charge depletion from Sb indicates the cationic character, and accumulation on X and Y denotes their anionic character. The clear separation between charge depletion and accumulation results in bonding that is mostly ionic in nature. The qualitative assessment reveals that the charge distribution in SbTeBr JL extends over a larger area compared to other JLs, indicating improved charge dynamics.
• Figure 5: Temperature-dependent phonon dispersions of SbXY JLs from 0 K – 1000 K. Acoustic branches remain largely rigid with only minor variation at high-symmetry points, whereas the optical modes display temperature-driven hardening. In SbSeI, the presence of I$^-$ suppresses anharmonic effects, yielding a comparatively rigid vibrational spectrum.