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Abstract
Free electron like image potential states are observed in scanning tunneling spectroscopy
on graphene quantum dots on Ir(111) acting as potential wells. The spectrum strongly
depends on the size of the nanostructure as well as on the spatial position on top,
indicating lateral confinement. Analysis of the substructure of the first state by
spatial mapping of constant energy local density of states reveals characteristic
patterns of confined states. The most pronounced state is not the ground state, but
an excited state with a favorable combination of local density of states and parallel
momentum transfer in the tunneling process. Chemical gating tunes the confining potential
by changing the local workfunction. Our experimental determination of this workfunction
allows to deduce the associated shift of the Dirac point.