Bound States in Nanoscale Graphene Quantum Dots in a Continuous Graphene Sheet

TL;DR

This paper demonstrates the creation and observation of bound electronic states in nanoscale graphene quantum dots embedded within a continuous graphene sheet, using STM to visualize electron trapping and Coulomb phenomena.

Contribution

It introduces a method to realize bound states in graphene quantum dots within a continuous sheet, overcoming previous limitations to quasi-bound states.

Findings

Observation of Coulomb oscillations indicating single-electron charging.

Visualization of concentric electronic density rings in GQDs.

Control of electron tunneling regimes via STM tip-sample distance.

Abstract

Considerable efforts have been made to trap massless Dirac fermions in graphene monolayer, but only quasi-bound states are realized in continuous graphene sheets up to now. Here, we demonstrate the realization of bound states in nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. The GQDs are electronically isolated from the surrounding continuous graphene sheet by circular boundaries, which are generated by strong coupling between graphene and substrate. By using scanning tunneling microscopy (STM), we observe single-electron charging states of the GQDs, seen as Coulomb oscillations in the tunneling conductance. Evolution of single-electron tunneling of the GQDs between the Coulomb blockade regime and the Coulomb staircase regime is observed by tuning the STM tip-sample distances. Spatial maps of the local electronic densities reveal concentric rings inside the GQDs with each ring corresponding to a single Coulomb oscillation of the tunneling spectra. These results indicate explicitly that the electrons are completely trapped inside the nanoscale GQDs.