π Band Folding and Interlayer Band Filling of Graphene upon Interface Potassium Intercalation

Abstract

The structural and electronic properties of epitaxial monolayer graphene on SiC(0001) are examined upon potassium intercalation. Notably, the first real‐space observation via scanning tunneling microscopy of the (2 × 2) superstructure in graphene‐based thin films formed by the potassium atoms below the uppermost graphene layer is presented. Therein, additional signatures stemming from quasiparticle interference effects are found allowing investigations of the electronic bands of K‐intercalated epitaxial monolayer graphene on a local scale. Those data are compared to area‐averaged results obtained from photoelectron spectroscopy. In particular, backfolding of the graphene π bands are found as a consequence of the (2 × 2) superstructure of the K atoms with respect to the graphene lattice. This is accompanied by a strong n‐type doping effect that causes a rigid shift of the Dirac bands to higher binding energies and filling of two parabolic interlayer bands at the Γ point of the surface Brillouin zone of the graphene lattice as well. This electronic configuration is promoted by additional penetration of potassium atoms into the interspace between the SiC substrate and the buffer layer that is located between the substrate and the quasi‐freestanding graphene sheet. Consequently, the epitaxial monolayer graphene sample transforms to n‐doped epitaxial bilayer graphene upon K intercalation.

Superconductivity of K‐Intercalated Epitaxial Bilayer Graphene

“…[38] The energy of this feature agrees well with the out-of-plane acoustic mode of AB-stacked BLG. [39] According to our structural analysis, [28] however, K atoms are located between the topmost graphene and the former buffer layer, so that i) adjacent layers are most likely arranged by AA stacking [7,40] and ii) the interlayer spacing is definitely increased compared to freestanding BLG [40] as it is also apparent in K-intercalated graphite. [41] Therefore, the calculated low-energy mode of AB-stacked BLG is most likely modified and does not explain this low-energy feature here.…”

“…[Correction added on March 29, 2023 after initial publication: The following sentence in section 2.2 "There, the K atoms were deposited at low temperatures, [50] which resulted in a severely lower doping level (n = 0.56 × 10 14 cm −2 [49,50] ) than in the sample studied here (n = 0.56 × 10 14 cm −2 [28] )." was corrected to "There, the K atoms were deposited at low temperatures, [50] which resulted in a severely lower doping level (n = 0.56 × 10 14 cm −2 [49,50] ) than in the sample studied here (n = 4.46 × 10 14 cm −2 [28] )." In addition, the Supporting Information file was replaced, as an earlier outdated version was initially published.…”

“…Epitaxial MLG samples were prepared from 6H-SiC(0001) wafers according to the procedure reported in Ref. [28]. The graphene layer count was double-checked using X-ray photoelectron spectroscopy [59] and ARPES.…”

“…A recently published study reports on a similar structural transition from epitaxial MLG on SiC(0001) to epitaxial BLG upon Ca intercalation, although the Ca atoms form a different ( 3 3) × R30° registry. [29] In particular, Toyama et al show that Ca-intercalated epitaxial BLG is a superconductor with critical temperatures up to T c = 5.7 K. In contrast to K-intercalated epitaxial BLG, [28] filling of only one interlayer band is observed upon Ca intercalation. The detailed characterization of the superconducting state via transport measurements in Ref.…”

“…Both the existence of multiple energy gaps that originate from pairing on different Fermi surfaces [22] and various pairing symmetries [23][24][25][26][27] have been predicted for monolayer and few-layer graphene.Detailed structural and electronic properties of our K-intercalated epitaxial bilayer graphene (BLG) on SiC(0001) are characterized in a separate publication. [28] The K atoms form a highly ordered (2 × 2) registry with respect to the graphene lattice below the topmost graphene layer, see Figure 1. However, potassium also intercalates between the SiC substrate and the buffer layer, which causes electronic decoupling of the latter from the underlying substrate.…”