Thermal conductivity across the van der Waals layers of α-MoO₃ thin films composed of mosaic domains with in-plane 90° rotations

α-Molybdenum trioxide (α-MoO₃) is a typical two-dimensional metal oxide material, in which the covalently bonded layers are bonded by van der Waals forces. Herein, the thermal conductivities across the van der Waals layers of α-MoO₃ thin films with nominal thicknesses of 50, 100, and 125 nm were investigated. The α-MoO₃ thin films were fabricated on the (100) plane of a single-crystalline SrTiO₃ substrate heated up to 400 °C by DC reactive magnetron sputtering using a Mo metal target. The b-axis-oriented α-MoO₃ thin films epitaxially grown on the SrTiO₃ (100) plane were confirmed by x-ray diffraction and x-ray pole figure analyses. Electron diffraction patterns and plane-view transmission electron micrographs revealed that the α-MoO₃ thin films were composed of mosaic domains with a diameter of ∼2 nm, with each domain rotated in-plane by 90° with respect to the neighboring one. The mean thermal conductivity across the van der Waals layers of the three α-MoO₃ thin films was evaluated to be 1.2 ± 0.3 W m⁻¹ K⁻¹, which is comparable to the reported thermal conductivities of layered cobalt oxide thin films. The reduced thermal conductivity is mainly due to phonon scattering at domain boundaries lying in the in-plane direction.