Spatially Structuring the Surface Energy of Monolayer Graphene Through Selective Heterointerface Engineering*

Selective bottom-up chemical synthesis of low dimensional materials with high spatial resolution has long been a goal of crystal growers. The challenge, however, lies in the spatial modification of the surface energy landscape of a substrate, a crucial factor that promotes the diffusion and accumulation of adatoms and/or molecules along the surface energy gradient, consequently facilitating nucleation in regions of reduced surface energy. Herein, we demonstrate the achievement of a highly controllable surface energy landscape of monolayer graphene on diamond like carbon (DLC) substrate through a heterointerface containing trapped gallium in a uniquely designed spatial structure. The process involves three steps: (i) spatial-selectively Ga⁺ ion implantation into DLC substrate to create “hill” features with a step height of 4 nm; (ii) polymer-free transfer of monolayer graphene on top of Ga⁺-implanted DLC; (iii) in-situ high-vacuum annealing process above 300°C for gallium precipitation at the graphene-DLC heterointerface with low energy electron microscope (LEEM). During the annealing process, both gallium precipitation and gallium-catalyzed reconstruction of the DLC structure contribute to a shift in the local surface work function of graphene, resulting in a decrease in the surface energy of graphene compared to that of pristine graphene. At 300°C, the surface work function difference (ΔWF)  between graphene residing on the “hill” features and unmodulated region is -142 meV, corresponding to a surface energy difference (Δγ)  between the modulated graphene region and unmodulated region of -0.23 mN/mm. However, at 500°C, ΔWF is 240 meV with a Δγ of -14.5 mN/mm. Notably, this difference remains consistent even upon cooling down to 300°C due to the irreversibility of gallium precipitation. In summary, a surface energy landscape of graphene with a high level of complexity can be realized by carefully tuning annealing conditions and the spatial arrangement of “hill” features, facilitating the selective area growth of materials in various nanofabrication processes.