Despite the large literature focused on the growth of graphene (Gr) on 6H-SiC(0001) by chemical vapour deposition (CVD), some important issues have not been solved and full wafer scale epitaxy of Gr remains challenging, hampering applications in microelectronics. With this study we shed light on the generic mechanism which produces the coexistence of two different types of Gr domains, whose proportion can be carefully controlled by tuning the H2 flow rate. For the first time, we show that the growth of Gr using CVD under H2/Ar flow rate proceeds in two stages. Firstly, the nucleation of free-standing epitaxial graphene on hydrogen (H-Gr) occurs, then H-atoms eventually desorb from either step edges or defects. This gives rise, for H2 flow rate below a critical value, to the formation of (6x6)Gr domains on 6H-SiC(0001). The front of H-desorption progresses proportionally to the reduction of H2. Using a robust and generic X-ray photoelectron spectroscopy (XPS) analysis, we realistically quantify the proportions of H-Gr and (6x6)Gr domains of a Gr film synthetized in any experimental conditions. Scanning tunnelling microscopy supports the XPS measurements. From these results we can deduce that the H-assisted CVD growth of Gr developed here is a unique method to grow fully free-standing H-Gr on the contrary to the method consisting of H-intercalation below epitaxial Gr on buffer layer. These results are of crucial importance for future applications of Gr/SiC(0001) in nanoelectronics, providing the groundwork for the use of Gr as an optimal template layer for Van der Waals homo-and hetero-epitaxy.

Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling bond-free surface is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, e.g. turning the layer-by-layer growth into the Volmer Weber growth promoted by defect-assisted nucleation. In this work, the growth of silicon on chemical vapor deposited epitaxial Gr (1 ML Gr/1ML Gr buffer) on 6H-SiC(0001) substrate is investigated by a combination of atomic force microscopy (AFM), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Raman spectroscopy measurements. It is shown that the perfect control of full-scale almost defect-free 1 ML Gr with a single surface structure and the ultra-clean conditions for molecular beam epitaxy (MBE) deposition of silicon represent key prerequisites for ensuring the growth of extended silicene sheets on epitaxial graphene. At low coverages, the deposition of Si produces large silicene sheets (some hundreds of nanometers large) attested by both AFM and SEM observations and the onset of a Raman peak at 560 cm -1 very close to the theoretical value of 570 cm -1 calculated for freestanding silicene. This vibrational mode at 560 cm -1 represents the highest ever experimentally measured value and is representative of quasi-free standing silicene with almost no interaction with inert non-metal substrates.From a coverage rate of 1ML, the silicene sheets disappear at the expense of 3D Si dendritic islands whose density, size, and thickness increase with the deposited thickness. From this coverage, the Raman mode assigned to quasi-free standing silicene totally vanishes, and the 2D flakes of silicene are no longer observed by AFM. The experimental results are in very good agreement with the results of kinetic Monte-Carlo simulations that rationalize the initial flake growth in solid-state dewetting conditions, followed by the growth of ridges surrounding and covering the 2D flakes. A full description of the growth mechanism is given. This study, which covers a wide range of growth parameters, challenges recent results stating the impossibility to grow silicene on a carbon inert surface and is very promising for large scale silicene growth. It definitely shows that silicene growth can be achieved using perfectly controlled and ultra-clean deposition conditions and an almost defect-free Gr substrate.