# Epitaxial Graphene on 4H-SiC (0001) as a Versatile Platform for Materials Growth: Mini-Review
## 2 Key Properties of Epitaxial Graphene as a Substrate for Materials Deposition
(**b**) Raman spectrum of typical monolayer epitaxial graphene sample. \(BuL\) represents the spectral features related to the buffer layer. (**c**) Dependence of \(2D\) peak position on \(G\) peak position. The above figures have been adapted with permission from [42]. Copyright 2021 The Authors. Published by American Chemical Society, Washington, United States. Further permissions related to the material excerpted should be directed to the ACS, Washington, United States. Figure 4: Charge redistribution maps (constructed based on Hirshfeld charges) of (**a**) free-standing graphene and (**b**) monolayer epitaxial graphene on 4H-SiC structure. The charge population analysis was performed within this work using \(8\times 8\) slabs (free-standing graphene and epitaxial graphene on SiC, respectively) at vdW/BH/DZP level of DFT by SIESTA code. Figure 3: Fat band structure (calculated in the frames of the current work) for monolayer epitaxial graphene on 4H-SiC, including contributions of key orbitals to the bands. The size of each circle is proportional to the corresponding orbital contribution to each band. The Fermi level was set to 0 eV. The band-structure calculations were performed using \(32\times 32\times 1\) k-point Monkhorst-Pack mesh at vdW/BH/DZP level of DFT by SIESTA code. The analysis of Figure 2c also indicates that as-grown epitaxial graphene samples were under compressive strain. This is contrary to the results of theoretical calculations, according to which graphene on SiC would undergo a lattice expansion rather than compression. This is because the lattice constant of graphene is smaller than that of SiC. In line with this, our estimations showed that the topmost graphene layer experiences 9% tensile strain. This discrepancy can be explained by the fact that graphene on SiC is forced to compensate for the mismatch in their coefficients of thermal expansions through building up compressive stress after cooling down to room temperature [45]. While the adsorption capability of expanded graphene originates from the formation of localized dangling-bond states that can promote strong binding with environmental adsorbates [46], the mechanism behind the interaction between compressively strained graphene and environmental species is not so obvious. On the one hand, a shrinkage of graphene (C-C bond length shortening) may induce delocalization of electron wavefunction over the planar graphene structure and decrease the density of dangling bond states, which are available for surface reactions [46]. On the other hand, the compressive strain can cause the formation of nano-ripples, which are able to adsorb atomic species at ridge sites [38]. Further, we highlight four most probable highly reactive regions on epitaxial graphene/SiC, which may serve as native seeds for the growth of external materials:  
1. Step edges. Graphene on SiC has terrace-stepped morphology. Adsorption of atomic species on the steps edges is expected to be much stronger than on terraces. 2. Charge-unbalanced electron-hole puddles. The supporting SiC substrate is responsible not only for the doping of graphene but also for the appearance of charge density fluctuations. Randomly distributed negatively and positively charged regions may favor increased adsorption and nucleation rate. 3. Strained regions. Compressive or tensile strain leading to alteration of orbital hybridization and substrate-induced strain fluctuations are considered as another important factor determining an increased reactivity of epitaxial graphene compared to exfoliated graphene. 4. Edges of the bilayer graphene inclusions. Since as-grown monolayer epitaxial graphene usually includes nanoscale overgrown areas related to bilayer graphene, it is reasonable to assume that unsaturated edges of these inclusions may create extra nucleation sites. Figure 5: DFT-predicted adsorption energy of a \(\mathrm{H_{2}O}\) molecule on monolayer graphene versus (\(\mathrm{E_{F}}-\mathrm{E_{D}}\)) values [44].