Self-Organized Growth of Catalyst-Free Single Crystal W_nO_3n-2 (n = 25) Nanowire Bundles on Si (111) via Selective He⁺ Ion Irradiation

Tungsten oxides (WO_x) possess unique properties due to a synergy of multiple effects arising from their wide range of stoichiometric and sub-stoichiometric compositions, defect chemistry, and polymorphism. Synthesis and incorporation of 1D WO_x nano-assemblies is an attractive pathway to enable highly efficient next-generation photocatalysts, sensors, and optoelectronic devices offering tunability over electro-optical response in a wide range of the spectrum, from UV–vis to NIR. However, synthesis of the metal oxide nano-patterns represents a technological challenge, often requiring the presence of a catalyst. Herein, a simple and economical method of synthesizing a catalyst-free self-organized sub-stoichiometric W_nO_3n-2 (n = 25) single crystal nanowire bundles by selectively irradiating a Mo-Ni doped WO_x surface with low-energy He⁺ ions (27 eV) at 700 °C is reported. The synergetic effect of multiple factors including temperature, effective local electric field along the exposed area of the sample, and the micro-gap between the mask and the WO_x (Mo – Ni) film, suitable oxygen content, doping, as well as shielding the nanowire growth area from the direct He⁺ ion irradiation is suggested to drive the single-crystal wire growth. Adjustment is also observed in the effective refractive index and extinction coefficient values in the synthesized W_nO_3n-2 nanorods across the solar spectrum.