# 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  
The behavior of noble metals on epitaxial graphene is a matter of considerable interest, as its deep understanding may boost the development of new sensor designs based on nano-plasmonics. Our recent findings shed light on the interplay between two selected magnetron-sputtered noble metals (silver and gold as representatives of metals with best plasmonic activity) and epitaxial graphene on 4H-SiC [43; 47; 48; 49]. From the theoretical point of view, both considered metals interact with epitaxial graphene through weak van der Waals forces, which is confirmed by a low adsorption energy that is below the lower limit of chemisorption and small charge transfer from metals to graphene [49]. Concomitantly, silver atoms interact stronger with graphene surfaces compared to gold species. A direct comparison of 2 nm-Ag and 2 nm-Au MLG samples showed that the difference in the interaction strengths translates into a difference in morphologies (Figure 6) and Raman responses of graphene. More specifically, gold on epitaxial graphene forms fractal-like nanostructures (coverage area of 36.6%) with a fractal dimension of 1.84 [42], which was due to limited atomic diffusion at island edges. At the same time, 2 nm-silver film consists of small, isolated nano-islands (average island diameter of ~15 nm; coverage area of 32.2%). Such a growth distinction is mainly driven by the difference in the interrelationships between the adsorption energy and the cohesive energy of gold and silver supported by epitaxial graphene. According to our DFT calculations (Figure 7), both metals tend to occupy the hollow site positions of the topmost graphene layer at different heights (2.07 A and 2.23 A for Ag and Au, respectively), demonstrating a clear difference in the adsorption energies (0.355 eV for Ag vs. 0.255 eV for Au). The fact that the adsorption energy of gold is 100 meV smaller than that of silver presupposes a much smaller probability for Au adatoms to be trapped by an epitaxial graphene surface to form early-stage nuclei in comparison to Ag adatoms. Furthermore, the predicted cohesive energy of bulk gold has a more negative value (~3.62 eV) as compared to the cohesive energy of bulk silver (~2.95 eV), suggesting that the Au-Au interaction is more stable and stronger than the Ag-Ag interaction. This difference provides an additional driving force for the MLG-assisted self-assembly of just adsorbed gold into interconnected nanostructures. Back to the silver case, the higher adsorption energy and the less negative cohesive energy seem to be sufficient factors to ensure a high density of small, separated Ag islands. It is interesting to note that the difference in the interaction strength characteristically manifests itself in Raman spectra of epitaxial graphene (Figure 8) [49]. Particularly, the Raman spectrum of pristine epitaxial graphene remains typically intact after gold deposition (except for small displacements of the main characteristic \(G\) and \(2D\) peaks), while silver deposition causes substantial changes in Raman activity reflected in an appearance of defect-related Raman peaks (\(D\), \(D^{\prime}\), \(D+D^{\prime}\), \(D+G\), and \(D\)**) and huge red-shift of the \(2D\) peak. The latter can be attributed to the \(n\)-doping of graphene [48]. Figure 6: SEM images of (**a**) 2 nm-Au and (**b**) 2 nm-Ag films deposited on epitaxial graphene. Copyright 2021 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. The effect of magnetron-sputtered Ag film thickness on the morphological evolution during film growth and phonon properties of epitaxial graphene has been thoroughly investigated in [47; 48]. As can be seen from Figure 9, only 2 nm-Ag film exhibits island-like morphology with isolated circular islands. Figure 8: Typical Raman spectra of pristine epitaxial graphene, 2 nm-Au-covered epitaxial graphene, and 2 nm-Ag-covered epitaxial graphene, respectively. This figure has been adapted with permission from [49].