Intercalant-induced Kekule ordering and gap opening in quasifreestanding graphene

We present a comprehensive investigation of the structural and electronic properties of Sn-intercalated buffer layers on SiC(0001) using low-temperature scanning tunneling microscopy and spectroscopy (LT-STM and LT-STS), spot-profile analysis low-energy electron diffraction, and density functional theory (DFT) calculations. Sn intercalation effectively decouples the buffer layer, yielding quasifreestanding monolayer graphene while introducing local lattice distortions. Bias-dependent STM imaging revealed the coexistence of conventional and Kekulè-ordered graphene domains, governed by the underlying Sn(1×1) reconstruction at the SiC interface. The measured STS spectra exhibit good agreement with DFT results. However, achieving homogeneous Sn(1×1) domains remains challenging, apparently, due to strain within the Sn monolayer, which drives the emergence of Kekulè distortions and the associated electronic band-gap opening homogeneously in graphene. These findings highlight the crucial role of intercalant homogeneity and strain in tuning graphene's structural and electronic properties.