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 mu m. 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.