Mapping Image Potential States on Graphene Quantum Dots

TL;DR

This study investigates image potential states in graphene quantum dots on Ir(111), revealing size-dependent spectral features, lateral confinement effects, and the influence of chemical gating on the confining potential and Dirac point shift.

Contribution

It provides the first detailed spatial and spectral mapping of image potential states on graphene quantum dots, highlighting the effects of size, position, and gating.

Findings

Size-dependent spectral shifts observed

Lateral confinement affects state structure

Chemical gating modifies workfunction and Dirac point

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.