  Stepwise deposition of Li atoms onto hexagonal boron nitride (hBN) monolayer
on Ir(111) is investigated by means of angle-resolved photoemission
spectroscopy and low-energy electron diffraction. Sequential Li deposition
progressively shifts the band structure of hBN to higher binding energies due
to the induction of a variable electric potential originating from electronic
charge donation by alkali atoms. At small coverages, Li atoms preferably
intercalate under the hBN layer, where they are highly charged and give rise to
a large initial shift of the band structure. Additionally, intercalated Li
atoms effectively decouple hBN from the substrate and consequently reduce its
moir\'e corrugation. As the deposition progresses further, Li atoms also adsorb
on top of hBN, and the average effective charge of both intercalated and
adsorbed Li atoms progressively diminishes due to the Coulomb repulsion
penalty, which is partially screened by the metal substrate in the intercalated
subsystem. This gives rise to a saturation of the respective electric potential
and the band structure shift, and elaborates on the pathways and limitations of
chemical functionalization of epitaxial hBN systems with charge donating
species.
