Self‐Organized Growth of Catalyst‐Free Single Crystal W n O 3n‐2 ( n = 25) Nanowire Bundles on Si (111) via Selective He + Ion Irradiation

Abstract 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 n O 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 n O 3n‐2 nanorods across the solar spectrum.