Intercalant-induced Kekule ordering and gap opening in quasi-free-standing graphene

Abstract: 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/STS), spot-profile analysis low-energy electron diffraction (SPA-LEED), and density functional theory (DFT) calculations. Sn intercalation effectively decouples the buffer layer, yielding quasi-free-standing monolayer graphene (QFMLG) while introducing local lattice distortions. Bias-dependent STM imaging revealed the coexistence of conventional and Kekule-ordered graphene domains, governed by the underlying Sn(1x1) reconstruction at the SiC interface. The measured STS spectra exhibit good agreement with DFT results. However, achieving homogeneous Sn(1x1) domains remains challenging, apparently, due to strain within the Sn monolayer, which drives the emergence of Kekule distortions and the associated electronic band-gap opening omogeneously in graphene. These findings highlight the crucial role of intercalant homogeneity and strain in tuning graphene`s structural and electronic properties.