We present a study of the surface effects and optical properties of the self-assembled nanostructures comprised of vertically aligned 5 nm-diameter Al nanowires embedded in an amorphous Si matrix (a-Si:Al). The controlled (partial) removal of Al nanowires in a selective etching process yielded nanoporous a-Si media with a variable effective surface area. Different spectroscopy techniques, such as X-ray photoelectron spectroscopy (XPS), UV-Vis spectrophotometry and photoluminescence (PL), have been combined to investigate the impact of such nanostructuring on optical absorption and emission properties. We also examine long-term exposure to air ambient and show that increasing level of surface oxidation determines the oxide defect-related nature of the dominant PL emission from the nanoporous structures. The role of bulk, nanosize and surface effects in optical properties has been separated and quantified, providing a better understanding of the potential of such nanoporous a-Si:Al structures for future device developments.

Nanoporous and nanowire structures based on silicon (Si) have a well recognized potential in a number of applications such as photovoltaics, energy storage and thermoelectricity. The immiscibility of Si and aluminum (Al) may be utilized to produce a thin film of vertically aligned Al nanowires of 5 nm diameter within an amorphous silicon matrix (a-Si), providing a cheap and scalable fabrication method for sub 5 nm size Si nanostructures. In this work we study functionalization of these structures by removal of the Al nanowires. The nanowires have been etched by an aqueous solution of HCl, which results in a structure of vertically aligned nanochannels in a-Si with admixture of SiO x . The removal of Al nanowires has been monitored by several electron microscopy techniques, x-ray diffraction, Rutherford backscattering spectroscopy, and optical reflectance. We have established that optical reflectance measurements can reliably identify the complete removal of Al, confirmed by other techniques. This provides a robust and relatively simple method for controlling the nano-fabrication process on a macroscopic scale.