Historically, finding two-dimensional (2D) magnets is well known to be a difficult task due to the instability against thermal spin fluctuations. Metals are also normally considered poor thermoelectric (TE) materials. Combining intrinsic magnetism in two dimensions with conducting properties, one may expect to get the worst for thermoelectrics. However, we will show this is not always the case. Here, we investigate the spin-dependent TE properties of monolayer chromium pnictides ($\mathrm{Cr}X$, where $X=\text{P}$, As, Sb, and Bi) using first-principles calculations of electron- and phonon-energy dispersion, along with the Boltzmann transport formalism under an energy-dependent relaxation time approximation. All the $\mathrm{Cr}X$ monolayers are dynamically stable and they also exhibit half metallicity with ferromagnetic ordering. Using the spin-valve setup with an antiparallel spin configuration, the half metallicity and ferromagnetism in monolayer $\mathrm{Cr}X$ enable the manipulation of spin degrees of freedom to tune the TE figure of merit $\left(ZT\right)$. At an optimized chemical potential and operating temperature of 500 K, the maximum $ZT$ values ($\approx 0.22$, 0.12, and 0.09) with the antiparallel spin-valve setup in CrAs, CrSb, and CrBi improve up to almost twice the original values ($ZT\approx 0.12$, 0.08, and 0.05) without the spin-valve configuration. Only in CrP, which is the lightest species and less spin polarized among $\mathrm{Cr}X$, the maximum $ZT$ $(\approx 0.34)$ without the spin-valve configuration is larger than that $(\approx 0.19)$ with the spin-valve one. We also find that, at 500 K, all the $\mathrm{Cr}X$ monolayers possess exceptional TE power factors of about 0.02–0.08 W/m ${\mathrm{K}}^2$, which could be one of the best values among 2D conductors.

Materials with Mexican-hat electronic energy dispersions emerging from heterostructures, substrate effects, or spin–orbit couplings are believed to exhibit excellent thermoelectric properties due to its van Hove singularity of density of states in two-dimension. However, within a constant relaxation time approximation, we disprove this belief and we find that the singularity effect is cancelled down by the group velocity contribution in the thermoelectric transport distribution. Nevertheless, the band parameters can still be optimised to reach thermoelectric figure of merit larger than 2 in a wide bandwidth, thus keeping the potential of materials with Mexican-hat bands for thermoelectric applications.

The properties of two-dimensional (2D) materials have been extensively studied and applied in various applications. Our interest is to theoretically investigate the thermal transport and thermoelectric properties of the 2D transition metal dichalcogenides MoX ₂ (X=S, Se, Te). We employ density functional theory and Boltzmann transport theory with relaxation-time approximation to calculate the electronic and transport properties. We also implemented the kinetic-collective model to improve the calculation of lattice thermal conductivity. Our calculations indicate that MoTe₂ has the highest ZT of 2.77 among the other MoX ₂ at 550 K due to its low thermal conductivity and high electrical conductivity. Consequently, we suggest that MoX ₂ monolayers hold promise as materials for energy conversion devices due to their relatively high ZT. Moreover, these results could be beneficial to design 2D material-based high-performance thermoelectric devices.