  Mechanical stability of the ZnIn2X4 monolayers. The ZnIn2S4 and ZnIn2Se4 are
semiconductors with direct band gaps of 3.94 and 2.77 eV, respectively whereas
the ZnIn2Te4 shows an indirect band gap of 1.84 eV at the G0W0 level. The
optical properties achieved from the solution of the Bethe-Salpeter equation
predict the exciton binding energy of the ZnIn2S4, ZnIn2Se4, and ZnIn2Te4
monolayers to be 0.51, 0.41, and 0.34 eV, respectively, suggesting the high
stability of the excitonic states against thermal dissociation. Using the
iterative solutions of the Boltzmann transport equation accelerated by machine
learning interatomic potentials, the room-temperature lattice thermal
conductivity of the ZnIn2S4, ZnIn2Se4, and ZnIn2Te4 monolayers is predicted to
be remarkably low as 5.8, 2.0, and 0.4 W/mK, respectively. Due to the low
lattice thermal conductivity, high thermopower, and large figure of merit, we
propose the ZnIn2Se4 and ZnIn2Te4 monolayers as promising candidates for
thermoelectric energy conversion systems. This study provides an extensive
vision concerning the intrinsic physical properties of the ZnIn2X4 nanosheets
and highlights their characteristics for energy conversion and optoelectronics
applications.
