Enhanced crystallinity in Ta2C thin films grown on free-standing graphene

In an effort to understand the mechanisms leading to van der Waals epitaxy, using plan-view transmission electron microscopy and diffraction, we compare and contrast the microstructures of ultra-thin (≈ 9 nm) Ta₂C layers sputter-deposited simultaneously on graphene and 8-nm-thick, amorphous silicon nitride (a-SiN_x) membranes supported by holey transmission electron microscopy grids. On both graphene and a-SiN_x membranes, we obtain polycrystalline trigonal $\alpha$-Ta₂C layers with similar grain sizes. However, the Ta₂C layers deposited on graphene exhibit an in-plane orientation relationship: ${\left(2\overline{1}\overline{1}0\right)}_{{\mathrm{Ta}}_{2}C}$||${\left(10\overline{1}0\right)}_{\text{graphene}}$. Our results reveal that the onset of epitaxy can be observed even on one-atom-thick crystals.