The Morphology and Interface Structure of Titanium on Graphene

Titanium (Ti) is an adhesion and contact metal commonly used in nanoelectronics and two-dimensional (2D) materials research. However, we find that dramatically different film morphologies can result when Ti is deposited on graphene (Gr), depending on the experimental conditions. Through a combination of transmission electron microscopy, atomic force microscopy, and Raman spectroscopy, we show that the most critical parameters are the number of Gr layers, nature of the Gr support, and deposition temperature. Ti on monolayer Gr is particularly distinctive in both its island morphology and the high defect density in Gr, compared to Ti on bilayer or thicker Gr. We attribute these results to the structural and mechanical differences between monolayer and thicker Gr flakes, where monolayer Gr is more flexible, exhibits a larger surface roughness and therefore a lower Ti diffusivity, and is more easily damaged. This is supported by ab initio density functional theory calculations, which suggest that differences in the Ti interaction between monolayer and thicker Gr are due to extrinsic factors such as surface roughness. Our results highlight the extreme sensitivity of Ti morphology on Gr to processing and substrate conditions, allowing us to propose design rules for controlling the Ti-Gr interface properties and morphology and to discuss the implications for other technologically relevant metal deposition processes.