# Epitaxial Graphene on 4H-SiC (0001) as a Versatile Platform for Materials Growth: Mini-Review
## 3 Deposition of Noble Metals on Epitaxial Graphene/4H-SiC by dc Magnetron Sputtering
Figure 11 demonstrates a summary of cyclic voltammetry measurements performed to study the reduction-oxidation behavior of mercury, copper, and lead. The knowledge of redox reactions on pristine epitaxial graphene enables the identification of the deposition potential window and can thus be considered as a starting point for further investigation of metal deposition kinetics. The redox behaviour of mercury is quite complicated and slow, since both mercury reduction (Hg\({}^{2+}\) + 1\(e^{-}\) = Hg\({}^{1+}\) + 1\(e^{-}\) = Hg\({}^{0}\)) and oxidation (Hg\({}^{0}\) - 1\(e^{-}\) = Hg\({}^{1+}\) - 1\(e^{-}\) = Hg\({}^{2+}\)) reactions occur in two steps, which is evidenced by the presence of two reduction and two oxidation peaks (Figure 11a-c) [51]. It was found that the copper electrodeposition on pristine monolayer epitaxial graphene on SiC proceeds via one-step bi-electronic reduction avoiding Cu\({}^{1+}\) intermediate involvement during the first cycle (Figure 11d) [52]. Concomitantly, during the second cycle, we observed additional oxidation and reduction peaks related to Cu\({}^{1+}\), which can be attributed to electrochemical processes on Cu\({}^{0}\)-modified epitaxial graphene. In contrast to mercury and copper electrodeposition, Pb-related redox reactions are one-step and involve the transfer of two electrons [50]. Metallic lead deposition starts at \(-\)0.35 V (Figure 11e). To understand the nature of early-stage kinetics of metals on epitaxial graphene in more depth, we then performed chronoamperometry measurements and recorded current transients during metal electrodeposition [50, 51, 52]. There are two mechanisms of early-stage nucleation: instantaneous and progressive. The former envisages the fast nucleation on a small number of reactive sites, followed by the subsequent blocking of the nucleation process as soon as the available nucleation sites are depleted. The other mechanism is associated with slow nucleation on a large number of reactive sites since new nuclei keep popping up during the growth process. The comparison of the experimental current-time curves with theoretical ones predicted by the Hills-Scharifker theory (Figure 12. Note this figure demonstrates only current-time curves collected during Cu and Hg electrodeposition. See the [50], to know more details on Pb case) suggests that the initial kinetics of all considered metals is regulated by the three-dimensional diffusion-controlled instantaneous nucleation mechanism. During the early stages of the electrodeposition process, the epitaxial graphene surface is covered with separated nuclei with discrete diffusion zones, which tend to merge up to the formation continuous metal layer. Once deposited onto the electrode surface, metal atoms interact with graphene, thereby affecting its physical properties. For example, as was demonstrated in our recent work, electrodeposited lithium species can penetrate beneath both the topmost graphene layer and buffer layer, thereby leading to the formation of bilayer epitaxial graphene with different doping levels per layer [54]. This is confirmed by the apparent \(G\) peak splitting into two components. Both lead and lithium electrodeposition caused the generation of defects in graphene, which is manifested by an activation of the initially forbidden Raman modes (\(D,D^{\prime}\) and \(D+G\)) [50, 54]. In line with this, an attempt was made to link the metal-graphene interaction strength to the degree of influence of each metal over the phonon properties of epitaxial graphene. It was found that the largest adsorption energy of Li among the other metals is well correlated with the largest red-shift of 2D peak and increased \(D/G\) ratio, which was observed for lithiated epitaxial graphene. Even a somewhat small  
Figure 11: Cyclic voltammograms registered at the epitaxial graphene electrode in the presence of mercury [51] (**a–c**) (Copyright 2019 by the authors), copper [52] (**d**) (Copyright 2020 by the authors), and lead [50] (**e**) (Copyright the Owner Societies 2018).