Millimeter-Scale Single-Crystal α-MoO ₃ Nanosheets Grown by Alkali Salt-Assisted Chemical Vapor Deposition

High Resolution Image Download MS PowerPoint Slide The orthorhombic van der Waals (vdW) layered crystal α-MoO 3 is a promising material for infrared nanophotonics; in particular, it may host highly confined hyperbolic phonon polaritons (HPhPs) with wavelength-dependent in-plane anisotropy. However, large-area and uniform single crystals are challenging to grow on substrates, as current α-MoO 3 growth methods struggle to manage adverse tendencies in size, texturing, and roughness. In this work, we establish an alkali salt-assisted chemical vapor deposition (SA-CVD) growth technique to produce smooth, high-quality, and millimeter-scale single-crystal α-MoO 3 nanosheets directly on A-plane sapphire substrates. By cosublimating a NaCl source along with α-MoO 3 during growth, we overcome the size and morphology challenges typical of alkali-free deposition, achieving ultrasmooth crystals with lateral dimensions reaching 6 mm and thicknesses ranging from <6 to 480 nm. We attribute the improved morphology to a molten Na 2 O–MoO 3 intermediate, which forms on the substrate surface and induces a self-expanding vapor–liquid–solid (VLS) growth mode. The as-grown single-crystal nanosheets exhibit high crystal and optical quality without evident degradation by residual Na, enabling characteristically high HPhP quality (Q) factors (12–30) and long lifetimes (2.7–7.7 ps) as measured by scattering-type scanning near-field optical microscopy (s-SNOM). We relocate the large-area crystals onto arbitrary substrates using a water-assisted layer transfer technique, which effectively removes Na-containing residue and relieves residual strain. This work unlocks millimeter-scale, high-quality, uniform α-MoO 3 single-crystal growth directly on substrates for large-area implementation in fields including mid-infrared nanophotonics and layered vdW heterostructures.