Thermoelectric Properties of Janus ${\mathrm{\gamma}}$-$\mathrm{Ge_{2}}$$XY(X,Y=~$$\mathrm{S,Se,Te})$ Monolayers with Intrinsically Low Lattice Thermal Conductivity

Abstract Materials with intrinsically low lattice thermal conductivity ($\kappa_l$) are highly desirable for high-performance thermoelectric applications. Using first-principles calculations combined with Boltzmann transport theory, we systematically investigate the thermoelectric properties of a family of two-dimensional Janus semiconductors, ${\mathrm{\gamma}}$-$\mathrm{Ge_{2}}XY$ ($X,Y=$ S, Se, Te). All three compounds exhibit ultralow $\kappa_l$ at room temperature, reaching 6.16, 1.16, and 4.91 W/mK for $\mathrm{\gamma}$-$\mathrm{Ge_{2}SSe}$, $\mathrm{\gamma}$-$\mathrm{Ge_{2}STe}$, and $\mathrm{\gamma}$-$\mathrm{Ge_{2}SeTe}$, respectively. Combined with favorable electronic structures featuring enhanced density of states near the band edges, these materials achieve high thermoelectric performance, with maximum $ZT$ values of 2.48–2.59 at 900 K. Notably, biaxial tensile strain further boosts the thermoelectric efficiency of $\mathrm{\gamma}$-$\mathrm{Ge_{2}SeTe}$, yielding an exceptional n-type $ZT$ of 4.78 at 900 K under 2% strain. This remarkable enhancement originates from strain-induced strengthening of lattice anharmonicity and an increased Seebeck coefficient associated with conduction-band modulation. Our results identify Janus $\gamma$-$\mathrm{Ge_{2}}XY$ monolayers as promising platforms for next-generation thermoelectric energy conversion and highlight strain engineering as an effective route for targeted performance optimization.