Interfacial Coupling Controls Molecular Epitaxy of HMTP on Graphene/SiC

Epitaxial growth critically influences structural and electronic properties of organic semiconductors. Graphene serves as a prominent van der Waals template for molecular self-assembly; however, graphene on SiC is intrinsically heterogeneous, with decoupled monolayer graphene coexisting with residuals of a covalently bound buffer layer, which may affect molecular ordering. Here, we track the ordering of the molecular donor, 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), from the first layer to thin films, combining low-energy electron microscopy and diffraction with X-ray diffraction. HMTP forms highly ordered epitaxial layers on single-layer graphene, whereas growth on the buffer layer initiates as amorphous and evolves into polycrystalline films with weak orientation with respect to the substrate. Crucially, hydrogen intercalation decouples the buffer layer, converting it into quasi-freestanding monolayer graphene and restoring epitaxial growth. These findings demonstrate that interfacial coupling governs molecular epitaxy on graphene/SiC, and interface engineering via hydrogen intercalation provides a scalable route to control organic thin-film crystallinity on graphene.

💡 Research Summary

The authors investigate how the interfacial chemistry of epitaxial graphene on silicon carbide (SiC) governs the epitaxial growth of the planar organic semiconductor 2,3,6,7,10,11‑hexamethoxy‑triphenylene (HMTP). Graphene grown on SiC is intrinsically heterogeneous: a decoupled single‑layer graphene (SLG) coexists with a carbon “buffer layer” that remains partially covalently bonded to the SiC substrate. This buffer layer retains a graphene‑like lattice but its π‑conjugation is disrupted, leading to distinct electronic and structural properties at the nanoscale.

Using a combination of low‑energy electron microscopy (LEEM) and diffraction (LEED) for real‑time, surface‑sensitive monitoring, together with X‑ray diffraction (XRD) pole‑figure, azimuthal, symmetric ω/2θ, and rocking‑curve measurements for bulk crystallinity, the authors compare HMTP growth on the two surface terminations. On SLG, HMTP rapidly nucleates into compact islands that coalesce into a flat, percolated film. XRD shows sharp six‑fold symmetric {101̅1} reflections rotated by ±19.1° with respect to the SiC lattice, indicating two mirror‑related domains with excellent in‑plane alignment (FWHM ≈ 0.5°). The out‑of‑plane mosaicity is exceptionally low (<0.01°), and Laue oscillations around the 0002 reflection reveal uniform film thickness. AFM confirms a smooth film (RMS ≈ 4 nm) covering the graphene.

In contrast, on the buffer layer HMTP initially forms a disordered, amorphous overlayer. XRD pole figures display a ring‑shaped intensity band at the same polar angle (31.7°) but with weak azimuthal modulation, reflecting many randomly oriented crystallites. Additional peaks appear at ±8.2° and ±19.1° with broad FWHM (2–6°), evidencing strong in‑plane mosaicity. The symmetric 0002 reflection is weaker and lacks Laue oscillations, indicating poorer out‑of‑plane coherence and variable crystallite thickness. AFM shows isolated islands (~0.1 µm) with faceted edges, higher roughness (RMS ≈ 9 nm), and random in‑plane rotation. LEEM/LEED time‑resolved imaging on a sample containing both SLG and buffer regions demonstrates that ordered island nucleation occurs only on SLG, while the buffer region exhibits only a gradual intensity decrease consistent with uniform molecular adsorption without ordered nucleation.

The pivotal experiment is hydrogen intercalation. By annealing the sample in a hydrogen atmosphere, Si–C bonds at the buffer layer are passivated, converting the buffer into quasi‑freestanding SLG. After intercalation, LEEM shows the same rapid nucleation and film coalescence on the formerly buffered area as on pristine SLG. Correspondingly, XRD regains sharp, well‑aligned {101̅1} reflections with the same ±19.1° rotation and strong Laue oscillations, confirming that the epitaxial relationship is restored.

These results lead to three major conclusions: (1) The covalent coupling of the buffer layer to SiC suppresses ordered nucleation of HMTP, producing polycrystalline or amorphous films despite the same out‑of‑plane orientation; (2) Decoupled SLG provides a true van‑der‑Waals template that enables highly ordered, epitaxial growth of HMTP with two mirror domains; (3) Interface engineering via hydrogen intercalation can reliably convert heterogeneous graphene/SiC surfaces into uniform, epitaxy‑friendly templates, offering a scalable route for wafer‑scale organic semiconductor integration on graphene.

The study showcases the power of combining real‑time LEEM/LEED with bulk‑sensitive XRD to dissect interfacial effects across length scales, and it underscores that controlling interfacial coupling is a decisive design parameter for organic/2‑D material heterostructures in future electronic and optoelectronic applications.