Two-dimensional monolayer CuXSe2 (X  =  Cl, Br) as a potential medium-temperature thermoelectric material

Numerous two-dimensional materials show remarkable thermoelectric (TE) performances due to quantum confinement effects. However, identifying ideal TE materials with low thermal conductivity and high Seebeck coefficient is challenging because of their strong coupling and conflict. In this work, a novel monolayer semiconductor CuXSe2 (X = Cl, Br) with balanced TE performance is theoretically predicted using first-principles calculations and the Boltzmann transport equation. Phonon spectrum analysis, ab initio molecular dynamics simulations, and mechanical property assessments confirm CuXSe2's thermodynamic, dynamic, and mechanical stability. Studies show the ultralow lattice thermal conductivity of CuXSe2 monolayer at 300 K is 0.13 and 0.03 W/m K along the x axis, and 0.24 and 0.06 W/m K along the y axis, attributed to stronger phonon anharmonic property from weaker binding force. Low phonon group velocity and strong phonon scattering lead to its low room-temperature lattice thermal conductivity. Moreover, 2D CuBrSe2 has a high ZT of 2.36 at 500 K due to low lattice thermal conductivity and excellent electronic transport performance. This study identifies CuXSe2 as a new TE material for promising medium-temperature applications.