# Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium
## 2 High Oxidation Tendency: Aluminum and Magnesium  
### Aluminum  
Aluminum is the third most abundant element in the Earth's crust, only behind oxygen and silicon. This makes it two to three orders of magnitude cheaper than other conventional plasmonic metals like Au or Ag. It presents a strong plasmonic response in the UV range, and unlike precious metals, its interband transitions lie in the near infra-red (NIR around 800 nm) [26]. For instance, Al is compatible with optoelectronic devices and complementary metal-oxide-semiconductor (CMOS) technology. Most importantly, there is a high number of processing methods for manufacturing Al NPs of different sizes and shapes [27]. Among these methods are lithography, laser ablation and chemical synthesis. This includes nanorods [28; 29; 30], nanodisks [22] or flat triangles [31; 32]. The main disadvantage is its inherent high tendency to oxidation. When exposed to air, a thin layer of oxide wraps the NP [33]. Although the layer thickness is small (a few nanometers), it is highly sensitive to ambient conditions: temperature and humidity. As a consequence, plasmonic resonances of Al NPs are spectrally modified. It has been experimentally and theoretically shown that the growth of this oxide layer leads to both a red-shift and a weakening of the resonance peaks in Al NPs with different shapes [22; 23; 31; 34]. On the other hand, the self terminating oxide layer can prevent further oxidation, producing a natural encapsulation. This is why this oxide layer is sometimes described as a \"protective\" layer. Recent experiments on Al nanorods, have shown that high-order resonant modes are more confined inside the oxide-shell [30]. Recently, a lot of attention has been paid to Al in photocatalysis applications. Traditionally, transition metals like Rh, Ru or Pd have been used in heterogeneous catalysis to lower the activation energy of chemical reactions. However, these metals present lower absorption and weaker LSPRs in the UV spectral region. This is a disadvantage for photocatalytic applications due to the poor overlap with either conventional lasers sources or the solar spectrum. For these reasons, it has been proposed to couple a plasmonic antenna directly to the catalytic nanoparticle. This antenna-reactor complex allows absorption enhancements in poorly light-absorbing catalytic metals [35]. The feasibility of these heterostructures has been demonstrated using Al as \"antenna\" and Pd as \"reactor\" heterodimers [35] and decorated spherical nanoparticles [36]. Aluminum-cuprous oxide antenna-reactor nanoparticles have also been proven to be an efficient photocatalytic heterostructure [37]. More recently, it has been reported the possibility of producing Al nanocrystals decorated with transition metal nanoislands [38]. These metals include among others Rh, Ru or Pd, which have already been used in heterogeneous catalysis. Although Al by itself is a poor catalytic metal, it has been demonstrated that Al nanocrystals can also act as photocatalysts for hydrogen dissociation [39]. Densely distributed Al NPs have been also used to assist photocatalytic processes [40; 41]. Aluminum nanostructured films have been widely used as substrate for enhanced fluorescence spectroscopy in the UV for detection of biomolecules [42; 43]. Al nanoapertures have been studied as a tool for the modification of fluorescence decay rate of p-terphenyl dye molecules [44; 45]. [PERSON] et al. [45] reported a lifetime reduction of \(\approx\)7.2\(\times\), exceeding the previously reported value \(\approx\)3.5\(\times\) by [PERSON] et al. [44]. The first fabrication of Al film-over substrate for UV surface-enhanced resonance Raman scattering at the deepest UV wavelength used to date (\(\lambda\) = 229 nm) [46] has been reported. [PERSON] et al. [47] have proposed to use aluminum nanoantennas to enhance the response of a GaN UV detector and, in this way, overcome some of the problems of current UV photodetectors [48].