Thermoelectric properties of the Janus PtSTe monolayer compared with its parent structures

We use ab initio calculations to study phonon and charge-carrier transport in the Janus $\mathrm{PtSTe}$ monolayer, a lower-symmetry derivative of quasi-two-dimensional transition metal chalcogenides, with a view to characterizing its thermoelectric performance. For the sake of comparison, we also perform the same study on its parent structures, ${\mathrm{PtTe}}_2$ and ${\mathrm{PtS}}_2$. We find a significantly increased thermoelectric figure of merit, driven by modest increases in the Seebeck coefficient but, most importantly, by a drastic reduction in the thermal conductivity. We show that this decrease cannot be explained by a simple inspection of the phonon band structure; instead, it has its roots in a relaxation of the selection rules for three-phonon scattering as a result of the broken symmetry of the Janus structure. This suggests that Janus monolayers have a significant intrinsic advantage and untapped potential for thermoelectric applications among quasi-two-dimensional systems.

Figure 1

Two views of the optimized unit cells of (a) ${\mathrm{PtS}}_2$, (b) $\mathrm{PtSTe}$, and (c) ${\mathrm{PtTe}}_2$.

Figure 2

Calculated electronic band structures, DOS, and $e$-ph scattering rates for (a) ${\mathrm{PtS}}_2$ (purple), (b) $\mathrm{PtSTe}$ (blue), and (c) ${\mathrm{PtTe}}_2$ (green). The Fermi level is taken as zero.

Figure 3

(a) Seebeck coefficient $\mathit{S}$, (b) electrical conductivity $\sigma$, (c) TE power factor, and (d) total thermal conductivity $\kappa$ vs $N$ at $300\mathrm{K}$ (dashed lines) and $800\mathrm{K}$ (solid lines).

Figure 4

TE figure of merit $zT$ vs carrier concentration $\mathit{N}$ for doped (a) ${\mathrm{PtS}}_2$, (b) $\mathrm{PtSTe}$, and (c) ${\mathrm{PtTe}}_2$ at different temperatures.

Figure 5

Calculated ${\kappa }_{\ell }$ vs temperature for the ${\mathrm{PtS}}_2$, $\mathrm{PtSTe}$ and ${\mathrm{PtTe}}_2$ monolayers.

Figure 6

Phonon dispersion curves of (a) ${\mathrm{PtS}}_2$, (b) $\mathrm{PtSTe}$, and (c) ${\mathrm{PtTe}}_2$ along a high-symmetry path in the Brillouin zone. The color of the curves corresponds to the value of the Grüneisen parameter $\gamma$, as indicated by the color bar.

Figure 7

Frequency-dependent anharmonic scattering rates for (a) ${\mathrm{PtS}}_2$, (b) $\mathrm{PtSTe}$, and (c) ${\mathrm{PtTe}}_2$. The arrow in (b) is a visual aid serving to highlight the peculiar low-frequency behavior.

Figure 8

Squared modulus of the three-phonon matrix elements for a hypothetical emission process involving three low-frequency phonons from several acoustic branches (see text for details) in (a) and (d) ${\mathrm{PtS}}_2$, (b) $\mathrm{PtSTe}$, and (c) ${\mathrm{PtTe}}_2$. $b_2$ and $b_3$ denote the branches of the second and third phonon modes: 0 stands for ZA, 1 stands for TA, and 2 stands for LA. The dashed horizontal line is drawn at zero. Lines are colored according to $b_3$, whereas $b_2$ determines the line style.