  The pairing mechanism of high-temperature superconductivity in cuprates
remains the biggest unresolved mystery in condensed matter physics. To solve
the problem, one of the most effective approaches is to investigate directly
the superconducting CuO2 layers. Here, by growing CuO2 monolayer films on
Bi2Sr2CaCu2O8+{\delta} substrates, we identify two distinct and spatially
separated energy gaps centered at the Fermi energy, a smaller U-like gap and a
larger V-like gap on the films, and study their interactions with alien atoms
by low-temperature scanning tunneling microscopy. The newly discovered U-like
gap exhibits strong phase coherence and is immune to scattering by K, Cs and Ag
atoms, suggesting its nature as a nodeless superconducting gap in the CuO2
layers, whereas the V-like gap agrees with the well-known pseudogap state in
the underdoped regime. Our results support an s-wave superconductivity in
Bi2Sr2CaCu2O8+{\delta}, which, we propose, originates from the
modulation-doping resultant two-dimensional hole liquid confined in the CuO2
layers.
