Geometry and Greatly Enhanced Thermoelectric Performance of Monolayer MXY Transition‐Metal Dichalcogenide: MoSTe as an Example

2D thermoelectric materials with a high power factor and low lattice thermal conductivity have recently been the focus of cutting‐edge research. Herein, combining first‐principles calculations and semiclassical Boltzmann transport theory, a structural search on the monolayer transition‐metal dichalcogenide MoSTe is conducted and its electronic structure, lattice dynamics, and thermoelectric properties are carefully studied. A new tetragonal blend structure is found that is both energetically and dynamically more stable than the experimentally synthesized hexagonal Janus structure, and this blend structure possesses strong anisotropic in‐plane electron and phonon transport properties. Both structures have insulating ground states, which allow a reasonable Seebeck coefficient. Phonon dispersion reveals that several optical phonon‐branches downshift and overlap with the acoustic branches, leading to an enhanced scattering rate, greatly reduced lattice thermal conductivity, and eventually excellent thermoelectric performance with = 0.34 at 300 K and 1.0 at 600 K for the blend‐MoSTe. The results demonstrate the great potential of the monolayer MXY transition‐metal dichalcogenide in thermoelectric applications.