Lithography-Free random bismuth nanostructures for full solar spectrum harvesting and mid-infrared sensing

A lithography‐free, double‐functional single bismuth (Bi) metal nanostructure is designed, fabricated, and characterized for ultrabroadband absorption in the visible (vis) and near‐infrared (NIR) ranges, and for a narrowband response with ultrahigh refractive index sensitivity in the mid‐infrared (MIR) range. To achieve a large‐scale fabrication of the design in a lithography‐free route, the oblique‐angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It is shown that this fabrication technique can provide a bottom‐up approach to controlling the length and spacing of the design. The characterization findings reveal a broadband absorbance above 0.8 in vis and NIR, and a narrowband absorbance centered around 6.54 µm. Dense architecture and extraordinary permittivity of Bi provide strong field confinement in ultrasmall gaps between nanostructures, and this can be utilized for a sensing application. An ultrahigh sensitivity of 2151 nm refractive‐index unit (RIU–1) is acquired, which is, as far as it is known, the experimentally highest sensitivity attained so far. The simple and large‐scale compatible fabrication route of the design together with the extraordinary optical response of Bi coating makes this design promising for many optoelectronic and sensing applications.