  Here we use low-temperature scanning tunneling microscopy and spectroscopy to
reveal the roles of the narrow electronic band in two 1$T$-TaS$_2$ related
materials (bulk 1$T$-TaS$_2$ and 4$H_{\rm b}$-TaS$_2$). 4$H_{\rm b}$-TaS$_2$ is
a superconducting compound with alternating 1$T$-TaS$_2$ and 1$H$-TaS$_2$
layers, where the 1$H$-TaS$_2$ layer has weak charge density wave (CDW) pattern
and reduces the CDW coupling between the adjacent 1$T$-TaS$_2$ layers. In the
1$T$-TaS$_2$ layer of 4$H_{\rm b}$-TaS$_2$, we observe a narrow electronic band
located near Fermi level, and its spatial distribution is consistent with the
tight-binding calculations for two-dimensional 1$T$-TaS$_2$ layers. The weak
electronic hybridization between the 1$T$-TaS$_2$ and 1$H$-TaS$_2$ layers in
4$H_{\rm b}$-TaS$_2$ shifts the narrow electronic band to be slightly above the
Fermi level, which suppresses the electronic correlation induced band
splitting. In contrast, in bulk 1$T$-TaS$_2$, there is an interlayer CDW
coupling induced insulating gap. In comparison with the spatial distributions
of the electronic states in bulk 1$T$-TaS$_2$ and 4$H_{\rm b}$-TaS$_2$, the
insulating gap in bulk 1$T$-TaS$_2$ results from the formation of a bonding
band and an antibonding band due to the overlap of the narrow electronic bands
in the dimerized 1$T$-TaS$_2$ layers.
