Investigating the Plasmon Resonances of Silicon Nanowires without Oxide Shell

Silicon nanowires have attracted the scientific community's attention due to their exceptional optical properties and potential for device integration. In our previous research, we investigated plasmonic resonance behavior in silicon nanowires to elucidate the physical origin of plasmonic excitations in semiconductor nanostructures and to explore plasmonic responses for potential applications in silicon-based nanophotonic and optoelectronic devices. We directly observed the longitudinal plasmonic resonance and the transverse plasmonic resonance in cylindrical and conical silicon nanowires with diameters of 30-100 nm. In our most recent work, we extended the study to conical-shaped silicon nanowires with quantistic NW tip sizes to further understand the morphological influence on the plasmonic resonances. All our previous findings refer to silicon nanowires enveloped in a silicon oxide shell. It is known that the dielectric shell modulates the local electromagnetic field and resonance conditions. In this paper, we present our investigation of several groups of conical and cylindrical silicon nanowires with varying lengths, without a dielectric shell, suspended in vacuum or deposited on a carbon film. We employed a scanning transmission electron microscope equipped with electron energy-loss spectroscopy. The results indicate that all the investigated SiNW groups exhibit signals with a periodic nature, with a clear, strong dependence of the plasmonic resonance energy on NW length rather than on environmental conditions, and confirm a strong transversal plasmonic resonance signal in all cases. This opens a practical pathway to plasmon engineering in silicon nanowires solely through geometry, simplifying their integration into nanophotonic platforms.