  We study the electronic structure and superconductivity in CuO$_2$ monolayer
grown recently on $d$-wave cuprate superconductor
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Density functional theory calculations
indicate significant charge transfer across the interface such that the CuO$_2$
monolayer is heavily overdoped into the hole-rich regime yet inaccessible in
bulk cuprates. We show that both the Cu $d_{x^2-y^2}$ and $d_{3z^2-r^2}$
orbitals become important and the Fermi surface contains one electron and one
hole pocket associated with the two orbitals respectively. Constructing a
minimal correlated two-orbital model for the $e_g$ complex, we show that the
spin-orbital exchange interactions produce a nodeless superconductor with
extended $s$-wave pairing symmetry and a pairing energy gap comparable to the
bulk $d$-wave gap, in agreement with recent experiments. The findings point to
a direction of realizing new high-$T_c$ superconductors in ozone grown
transition-metal-oxide monolayer heterostructures.
