## 3 Results
### Ga Nanoparticles on Graphene/Cu  
In contrast to the glass insulating substrate, here we study the effect of having a conducting substrate such as Cu. Specifically, the system analyzed consists of Ga NPs on monolayer graphene/Cu. In this system, the Ga NPs are deposited directly on the CVD-grown graphene/Cu. In this way, we avoid the graphene transfer that could contaminate or damage it. Figure 2a shows the real-time evolution of the pseudo-extinction coefficient \(\langle k\rangle\) during Ga NP deposition on the G/Cu support. For comparison, the spectrum \(\langle k\rangle\) of equivalent Ga NPs simultaneously deposited on G/glass is also plotted, showing that the same NPs should give an LSPR peak at approximately 3.3 eV. The first spectrum (blue in Figure 2a) represents the G/Cu system, and its overall shape is significantly different from that of G/glass in Figure 1a because of the dielectric function of copper, i.e., the spectral region below 2 eV is representative of the Drude component of Cu free carriers, while above 2 eV, the spectra show the Cu interband transitions [13] smoothed and decreased by the absorption of graphene. Although Ga NPs are formed on the G/Cu, as shown by the SEM tilted picture shown in Figure 2a, it is clear that when the Ga NPs are on the G/Cu support, there is no excitation of the LO mode. Figure 2b shows the calculated absorption cross-section spectra of Ga NPs on G/glass and G/Cu substrates. It can be seen that when going from the G/glass to the G/Cu substrate, the LSPR is completely quenched. This is consistent with the fact that both G and Cu are conducting, and thus no confinement is produced at the Ga NP-graphene/Cu interface. These results are corroborated by electromagnetic simulations on a Ga hemisphere on the G/Cu substrate illuminated under normal incidence. As shown in Figure 2c, the near-field maps are completely different for the two cases. Specifically, while Ga NPs on G/glass show a high near-field enhancement at the interface between NP and substrate, for the G/Cu the near-field distribution is uniform around the particle, being minimal at the top of NPs. Therefore, in this case, hotspots at the NP-substrate interface that can enhance the Raman signal of the graphene layer are not produced. Consistently, as shown in Figure 2d, there is no enhancement of the Raman signal of the graphene layer before and after deposition of the Ga NPs on the G/Cu substrate. A deeper analysis of the position of the 2D peak measured for different deposited Ga amounts, as reported in Figure 2e, shows a shift of the 2D peak to a lower wavenumber with the increase of Ga. This phenomenon can be explained considering electron transfer from the Ga NPs into the graphene layer and, consequently, an n-type doping of the graphene layer [20]. This electron transfer is also supported by the XPS analysis. Figure 2f shows the spectra of the photoelectron core level Ga3\(d\) on G/Cu (red curve) compared to that of Ga NPs on an insulating glass substrate (blue curve). It can be seen that in both cases, the Ga3\(d\) photoelectron level shows two main components, namely, the component at binding energy BE = 18 eV due to metallic gallium, Ga\({}^{0}\), and the one at a the higher binding energy BE= 20.8 eV due to oxidized gallium, Ga\({}^{3+}\)[21]. For Ga NPs on insulating glass, the Ga is mainly in the metallic state, which is consistent with the evidence of the localized plasmon resonance peak; conversely, for Ga NPs on G/Cu, the Ga\({}^{3+}\) is the main component, which is consistent with the loss of electrons in the Ga NPs deriving into cationic character. This electron transfer from Ga NPs to graphene and then into the Cu substrate, due to the conductivity of substrates, is consistent with the values of the work function \(\phi_{F}\) of each of the involved materials (\(\phi_{F}^{Ga}\) = 4.2 eV, \(\phi_{F}^{G}\) = 4.5 eV and \(\phi_{F}^{Cu}\) = 4.9 eV), as depicted in the schema in Figure 3a. Therefore, the transferred electrons are moved into lower energy states.