# Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium
## 4 Comparison of the Plasmonic Response of Simple Geometries: Oxidation Effects
These two materials have been chosen in order to show how the near-field distribution is affected by the presence of an oxide shell. A deep analysis of this topic on Al + Al\({}_{2}\)O\({}_{3}\), Mg + MgO and Ga + Ga\({}_{2}\)O\({}_{3}\) spherical and hemispherical geometries was made by [PERSON] et al. [23]. The plasmonic response of Al and Mg is completely deteriorated by the formation of the oxide layer, specially in the case of Mg, for which the metal content drops to 0% after oxidation. This aggressive oxidation process limits the performance of Mg NPs with sizes smaller than 10 nm. It is surprising how the absorption cross section of the Al sphere (with a metal content of just 1.8%) is comparable to that of Ga and Rh spheres at energies around 6 eV. However, when looking at the value of \(\langle|E|^{2}\rangle\), for Al spherical NPs, it is approximately half the value for Rh and Ga spherical ones. By comparing Ga and Rh (low oxidizing tendency), in general, the former presents LSPRs at lower energies than the latter (coherent with their Frohlich frequencies, 6.5 and 8.1 eV, respectively). Moreover, for energies above 3 eV both \(C_{abs}\) and \(\langle|E|^{2}\rangle\) are higher for Rh NPs than for Ga NPs. The only exception occurs for the sphere geometry, for which Ga presents higher absorption and electric field enhancement between 5 and 6 eV approximately. From this comparison, it can be concluded that oxidation prevents Al and Mg to be optimum candidates as plasmonic materials. On the contrary, Ga and Rh show a good plasmonic response, specially Rh, which presents almost highest values of \(C_{abs}\) and \(\langle|E|^{2}\rangle\) all over the studied spectral range. Electric field hot-spots can be observed in specific locations for each geometry (see Figure 4). Also, the effect that oxide shells have on them is shown in the bottom row for Ga + Ga\({}_{2}\)O\({}_{3}\) NPs. In absence of oxide, hot-spots are produced on the surface of the particle, where they can be in direct contact with the analytes in surface enhanced applications. When an oxide shell is present, for very thin shells and specific geometries, hot-spots produced on the surface of the oxide are more intense than those at the interface metal-oxide (see the Ga + Ga\({}_{2}\)O\({}_{3}\) sphere). However, for other geometries like Ga + Ga\({}_{2}\)O\({}_{3}\) tripod stars and nanocubes, hot-spots produced in the metal-oxide boundary are more intense. So, as for the case of Ga, the oxide shell can have a critical effect on the UV plasmonic performance of metallic nanostructures and its relevance to real applications. Finally, it is important to point out that, although Rh in principle presents not as good plasmonic properties as Al or Mg (lower \(Fa\), see Figure 1), because its does not oxidize, its plasmonic response is more adequate for practical purposes. Figure 4: \(|E|^{2}\) maps (logarithmic scale) at E = 3.82 eV (\(\lambda\) = 325 nm, HeCd laser) for Rh (top row) and Ga + Ga\({}_{2}\)O\({}_{3}\) (bottom row) tripod stars (\(l=10\) nm; \(w=h=5\) nm), cubes (\(L=8.1139\) nm) and spheres (\(R=5.0335\) nm).