Spatially Controlled Growth of Ultrathin MoO₂ Polymorphs by Physical Vapor Deposition

Here we study the controlled growth of ultrathin molybdenum dioxide (MoO₂) flakes, a metallic analogue of the widely studied transition metal dichalcogenide MoS₂. This study demonstrates the growth of three distinct MoO₂ polymorphs (monoclinic, tetragonal, and a newly identified hexagonal phase) using physical vapor deposition. Comprehensive characterization through atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirms their unique structures and validates the newly observed hexagonal polymorph, which is also supported through simulations. Computational modeling suggests that the nucleation and coalescence of gas-phase clusters drive the polymorph formation. Optical measurements reveal that these polymorphs exhibit distinct photonic resonances, influenced by their geometry and thickness. This work opens new possibilities for integrating MoO₂ in hybrid structures and photonic devices, leveraging its polymorphic diversity and close relation to MoS₂, for advanced material design.