Ultralow Thermal Conductivity in Two-Dimensional MoO₃

Monolayer molybdenum trioxide (MoO3) is an emerging two-dimensional (2D) material with high electrical conductivity but unexplored thermal conductivity. Using first-principles calculations and a Boltzmann transport theoretical framework, we predict a record low room-temperature phonon thermal conductivity (κp) of 1.57 and 1.26 W/mK along the principal in-plane directions of the MoO3 monolayer. The behavior is attributed to the combination of soft flexural and in-plane acoustic modes, which are coupled through the finite layer thickness, and to the strong bonding anharmonicity, which gives rise to significant 3- and 4-phonon scattering. These insights suggest new indicators for guiding the search of 2D materials with low κp and motivates κp measurements in MoO3 and its applications as a thermoelectric and thermally protective material.