Exploring graphene-substrate interactions: Plasmonic excitation in Sn-intercalated epitaxial graphene

Graphene plasmons, including those in intercalated graphene, are an important research focus, with the promise of enabling light manipulation and providing a unique platform for gaining fundamental insights into many-body electronic interactions. In the present work, we discuss the results of low-energy plasmonic excitations in epitaxial quasi-free monolayer graphene formed by intercalation of Sn beneath the buffer layer on 4H-SiC(0001). The quantitative analysis of the sheet plasmon dispersion revealed that the Sn-induced ($1\times1$) interface is metallic and results in formation of charge-neutral graphene. A redshift of the 2D plasmon was found, but only after doping with potassium. The Sn-diluted interface, revealing a ($\sqrt{3}\times\sqrt{3}$) reconstruction and resulting in intrinsically n-type doped graphene, behaves comparably to the buffer layer for epitaxial monolayer graphene. Furthermore, it seems that a dipolar coupling of the longitudinal charge density fluctuations in graphene to the interface layer triggers the formation and the loss energy of a plasmonic multipole component, which therefore makes it suitable for studying proximity effects of excitations in electronically weakly coupled 2D heterosystems.