In the 1960s, it was proposed that in small
indirect band-gap materials, excitons can spontaneously form
because the density of carriers is too low to screen the
attractive Coulomb interaction between electrons and holes.
The result is a novel strongly interacting insulating phase
known as an excitonic insulator. Here we employ scanning
tunnelling microscopy (STM) and spectroscopy (STS) to
show that the enhanced Coulomb interaction in quantumconfined
elemental Sb nanoflakes drives the system to the excitonic insulator state. The unique feature of the excitonic insulator,
a charge density wave (CDW) without periodic lattice distortion, is directly observed. Furthermore, STS shows a gap induced
by the CDW near the Fermi surface. Our observations suggest that the Sb(110) nanoflake is an excitonic insulator.