  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.
