Se Nanowire Crystal Formation via Oxidation of 2D HfSe₂: A Solid-State, In Situ Reaction Coupling for Heterogeneous Integration Technologies

Effective heterogeneous integration of low-dimensional nanomaterials in applications ranging from quantum electronics to biomedical devices requires a detailed understanding of different formation and interfacing reactions and the ability to synergize these processes. We report the formation of 1D Se nanowires via low-temperature (30-150 °C) atmospheric oxidation of 2D HfSe₂ crystals. The localized, surface-bound process starting from exfoliated HfSe₂ flakes on a SiO₂/Si wafer support does not involve wet chemistry and allows us to implement optical operando reaction screening and explore the relevant parameter space and underpinning mechanisms. Hf oxidation frees Se at the buried hafnia-HfSe₂ interface, which segregates as amorphous Se, forming aggregates, blisters, and interfacial films. We show that upon diffusion to the stack surface, this Se can crystallize into trigonal Se nanowires with diameters ranging from ∼45 nm to 1.9 μm and lengths up to 43 μm depending on temperature and process time. We discuss the coupled reaction kinetics and pathways for application-relevant integrated process designs and connect diverse literature on the oxidation of transition metal dichalcogenides, Se polymerization and crystallization studies, and prior synthetic strategies for producing Se nanowires.