  Graphene quantum dots (GQDs) not only have potential applications on spin
qubit,but also serve as essential platforms to study the fundamental properties
of Dirac fermions, such as Klein tunneling and Berry phase. By now, the study
of quantum confinement in GQDs still attract much attention in condensed matter
physics. In this article, we review the experimental progresses on quantum
confinement in GQDs mainly by using scanning tunneling microscopy (STM) and
scanning tunneling spectroscopy (STS). Here, the GQDs are divided into Klein
GQDs, bound-state GQDs and edge-terminated GQDs according to their different
confinement strength. Based on the realization of quasi-bound states in Klein
GQDs, external perpendicular magnetic field is utilized as a manipulation
approach to trigger and control the novel properties by tuning Berry phase and
electron-electron (e-e) interaction. The tip induced edge-free GQDs can serve
as an intuitive mean to explore the broken symmetry states at nanoscale and
single-electron accuracy, which are expected to be used in studying physical
properties of different two-dimentional materials. Moreover, high-spin magnetic
ground states are successfully introduced in edge-terminated GQDs by designing
and synthesizing triangulene zigzag nanographenes.
