Building nanoplatelet α-MoO3 films: A high quality crystal anisotropic 2D material for integration

The successful development of nanoplatelet α-MoO3 −films with wavelength-dependent in-plane and out-of-plane birefringence both in the visible and in the medium infrared is demonstrated. The films are prepared by a two-step process starting from structurally amorphous and continuous substoichiometric MoO3-X followed by isothermal annealing at low temperature (250 °C) to yield the formation of a dense network of large α-MoO3 2D nanoplatelets lying in-plane with no overlapping. These α-MoO3 crystals form a film with excellent thickness uniformity (20 nm) and are oriented with the [0 1 0] crystallographic direction parallel to the substrate normal. Finally, we report the film anisotropic optical complex refractive index, both parallel and perpendicular to the plane the full spectral range from 0.7 to 5 eV as determined by spectroscopic ellipsometry, and their characteristic in-plane phonon mode in the IR from FTIR measurements. These results show a promising pathway to the creation of highly functional anisotropic α-MoO3 2D films suitable for the development of integrated nanostructured photonic components.