Phonon Anisotropy and Anharmonicity in Epitaxial Al2Mo3O12 Nanoflakes

Abstract

Aluminum molybdate (Al₂Mo₃O₁₂) exhibits superior properties for wide bandgap, chemical flexibility, negative thermal expansion, and good thermal stability. However, Al₂Mo₃O₁₂ prepared by traditional processes still suffered from inefficiency and poor quality so far. Here, we report on epitaxial growth of α-Al₂Mo₃O₁₂ nanoflake arrays with unidirectional domain orientations on c-sapphire via chemical vapor deposition. Optical microscopy, atomic force microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and Raman spectroscopy measurements reveal the high-quality of as-grown samples and the specific epitaxial relationship between α-Al₂Mo₃O₁₂ and c-sapphire: α-Al₂Mo₃O₁₂[021] || sapphire[112̅0] and α-Al₂Mo₃O₁₂[02̅1] || sapphire[11̅00]. Phonon polarizations of α-Al₂Mo₃O₁₂ exhibit a strong anisotropy ratio up to 3.13 for A_g modes, which can be used to identify the crystal orientation. The abnormal temperature dependence of MoO₄ symmetric stretching vibration phonon modes (SSVPMs) reveal giant anharmonic phonon-phonon interaction in α-Al₂Mo₃O₁₂. The ultralow thermal conductivity of α-Al₂Mo₃O₁₂ is predicted by the ultrashort phonon lifetime of ∼0.12 ps. Our findings provide insight into the thermal properties of α-Al₂Mo₃O₁₂ and are helpful for the application in polarization-sensitive optoelectronic detector and thermoelectric material.