Exploring plasmons in silicon nanowires with different sizes and morphologies

Silicon nanowires (SiNWs) are extensively studied in the scientific community due to their remarkable electrical and optical properties. Recent studies have demonstrated the appearance of Plasmonic Resonance (PR) in SiNWs. It has been recently demonstrated in literature that SiNWs with a cylindrical shape sustain longitudinal plasmonic resonance (LPR) and transversal plasmonic resonances (TPR). In this thesis work, I present the results on SiNWs with tapered morphology. These SiNWs present a length of about 300 nm and a diameter, at the base, of around 15 nm, while at the center, of approximately 12 nm. I have also investigated conical SiNWs with different lengths and demonstrated that the NW size plays a role on the spectral response. I selected two groups of SiNWs, one with a length of 300 nm and the other with 750 nm, with quantistic sizes at the NW tip. All my previous findings refer to SiNWs enveloped in a silicon oxide shell. I extended the study to the PR behavior and investigated several groups of conical SiNWs have lengths ranging from 180 nm to 340 nm, and cylindrical ones range between roughly 230 nm and 330 nm, without a dielectric shell, suspended in vacuum or deposited on a carbon film, which is known to modulate the local electromagnetic field and resonance conditions. I investigated the optical properties of the SiNWs at a high energy and spatial resolution by using scanning transmission electron microscopy (STEM) and in situ electron energy loss spectroscopy (EELS). In the case of tapered SiNWs, despite the different geometry, the results confirm the presence of TPR, as in the cylindrical SiNW. However, the presence of a continuous signal extending all along the Si-NWs length, and for several energies, is observed here for the first time. In the UV region of the spectrum investigated here for the 300 nm and 750 nm groups of SiNW, the experimental evidence suggests the presence of LPR and a clear presence of TPR. We found that, as the NW length increases, the LPR fundamental mode shifts towards higher energies, while the diameter seems to affect the TPR, shifting it to lower energy levels when the diameter increases. The results indicate that all the investigated shell-free 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 through geometry alone, simplifying their integration into nanophotonic platforms.