Pb-intercalated epitaxial graphene on SiC: Full insight into band structure and orbital character of interlayer Pb, and charge transfer into graphene

Intercalation is a robust approach for modulating the properties of epitaxial graphene on SiC and stabilizing two-dimensional intercalant layers at the graphene/SiC interface. In this paper, we present synchrotron-based angle resolved photoelectron spectroscopy (ARPES) measurements focusing on the band structure of intercalated Pb under a single layer of epitaxial graphene. The interlayer Pb exhibits a metallic character, a (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>1</a:mn><a:mo>×</a:mo><a:mn>1</a:mn></a:mrow></a:math>) registry with respect to SiC, and free-electron-like bands to first order. Divergences from the free-electron approximation include various band splittings and gaps throughout the Pb Brillouin zone. Light polarization-dependent ARPES measurements indicate a predominant out-of-plane orbital character for the Pb bands, suggesting potential interactions between the interlayer Pb and graphene's <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>π</b:mi></b:math> orbitals that may induce proximity effects in graphene. Density functional theory calculations for a (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mn>1</c:mn><c:mo>×</c:mo><c:mn>1</c:mn></c:mrow></c:math>) Pb monolayer on SiC show reasonable qualitative agreement with the experimentally observed interlayer bands as well as the polarization-dependent measurements. Finally, temperature-dependent ARPES measurements reveal that the nearly charge-neutral graphene layer involves charge transfer from both the interlayer Pb and the substrate SiC.