  While two-dimensional topological insulators (2D TI) initiated the field of
topological materials, only very few materials were discovered to date and the
direct access to their quantum spin Hall edge states has been challenging due
to material issues. Here, we introduce a new 2D TI material, Sb few layer
films. Electronic structures of ultrathin Sb islands grown on Bi2Te2Se are
investigated by scanning tunneling microscopy. The maps of local density of
states clearly identify robust edge electronic states over the thickness of
three bilayers in clear contrast to thinner islands. This indicates the
topological edge states emerged through a 2D topological phase transition
predicted between three and four bilayer films in recent theory. The
non-trivial phase transition and edge states are confirmed for epitaxial films
by extensive density-functional-theory calculations. This work provides an
important material platform to exploit miscroscopic aspects of the quantum spin
Hall phase and its quantum phase transition.
