Twisted Graphene Growth on Graphene/SiC Template Using Sequential Thermal Processes in Vacuo Without Air Exposure

Twisted few-layer graphene (FLG) has recently attracted great attention due to the emergence of exotic electrical properties [1]. In a previous study, we reported that the FLG was synthesized by overlayer growth of graphene on a monolayer graphene template (MLGT) using a chemical vapor deposition (CVD) method. We found that moiré pattern appears in the lattice structure of the grown graphene two-dimensional (2D) island and the grown 2D graphene islands have random twisted angles. The coalescence process of graphene islands with various twist angles was indicated by temperature and pressure dependence of graphene island size [2]. In this study, based on understanding the growth mechanism of twisted FLG, we further developed the growth conditions of SiC to optimize the MLGT. By reducing the nucleation sites on the MLGT, large-area lateral growth of twisted graphene can be realized.<br/>The MLGT used in previous studies was possibly contaminated with adsorbates due to air exposure during the step-by-step processes of MLGT formation and overlayer graphene growth in different vacuum chambers. In order to obtain a cleaner MLGT surface with fewer nucleation sites, the epitaxial growth process on SiC and the CVD process on MLGT/SiC were continuously performed in an argon atmosphere in an infrared heating furnace. Raman spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM) were used to evaluate the surface features and morphology of the samples.<br/>We obtained high-quality MLGT on 6H-SiC (0001) surfaces at 1700 °C in an argon environment at ~1 bar. We verified the uniform single crystal surface of MLGT by STM and Raman spectroscopy under the epitaxial growth process. Graphene islands with twisted structures were sequentially synthesized in vacuo without air exposure with optimized CVD conditions. Using AFM to measure several representative locations on the surface, we observed twisted FLGs on the MLGT grown under the CVD process. By comparing with previous results [2], the twisted FLG under sequential thermal processes shows a significant reduction in nucleation sites and step-edge nucleation.<br/>The coalescence growth process shows that twisted graphene islands nucleate very closely in the initial nucleation stage. Then, graphene islands gradually approach each other in lateral directions as they grow [2]. This mechanism is exactly the opposite of the thermodynamics-based energetics principle that the chemical potential of carbon atoms and their diffusion length on the graphene surface should be reduced. This suggests that the roughness of the surface directly affects the nucleation efficiency in our growth model. Therefore, in the study of sequential thermal processes in vacuo without air exposure, the clean MLGT surface with reduced initial nucleation sites can effectively provide enough space for lateral growth. It is expected to make further progress in the large-area lateral growth of twisted graphene.<br/><br/>[1] R. Negishi et al., Phys. Status Solidi B <b>257</b>, 1900437 (2020).<br/>[2] Y. Yao et al., Nanotechnol. <b>33</b>, 155603 (2022).