Atomic Engineering of Monolayer Graphene: Inducing Kekulé Bond Order by Adatom Deposition*

The Kekulé distortion (KD) periodically modifies the carbon-carbon bonds in graphene, resulting in a √3×√3 R30° superstructure and symmetry breaking between three previously equivalent hexagonal plaquettes. Previous scanning tunnelling microscopy (STM) experiments have shown that such a distortion induced by vacancies in the graphene substrate can produce charge density modulations corresponding to the superstructure [1]. Here, we induce a KD phase in monolayer graphene by low-flux deposition of a small number of lithium adatoms. Using angle-resolved photoemission spectroscopy (ARPES), we observe backfolding of the Dirac cones to Γ, as well as a gap opening (2Δ = 0.22 ± 0.02 eV) at the Dirac point. Low-energy electron diffraction (LEED) measurements also show the appearance of peaks corresponding to the new periodicity. Finally, we discuss the real-space behaviour of the adatoms by comparing STM data with predictions by a Monte Carlo toy model.

References

[1] C. Gutiérrez, Nature Physics 12, 950–958 (2016).