  New theoretical proposals and experimental findings on transition metal
dichalcogenide 1T-TaS$_2$ have revived interests in its possible Mott
insulating state. We perform a comprehensive scanning tunneling microscopy and
spectroscopy experiment on different single-step areas in pristine 1T-TaS$_2$.
After accurately determining the relative displacement of Star-of-David
super-lattices in two layers, we find different stacking orders can correspond
to the similar large-gap spectrum on the upper terrace. When the measurement is
performed away from the step edge, the large gap spectrum can always be
maintained. The stacking order seems rarely disturb the large-gap spectrum in
the ideal bulk material. We conclude that the large insulating gap is from the
single-layer property, which is a correlation-induced Mott gap based on the
single-band Hubbard model. Specific stacking orders can perturb the state and
induce a small-gap or metallic spectrum for a limited area around the step
edge, which we attribute to a surface and edge phenomenon. Our work provides
more evidence about the surface electronic state and deepens our understanding
of the Mott insulating state in 1T-TaS$_2$.
