  Interlayer coupling between individual unit layers has played a critical role
for layer-dependent properties in two-dimensional (2D) materials. While recent
studies have revealed the significant degrees of interlayer interactions, the
overall electronic structure of the 2D material has been mostly addressed by
the intralayer interactions. Here, we report the direct observation of a highly
dispersive single electronic band along the interlayer direction in puckered 2D
PdSe2 as an experimental hallmark of strong interlayer couplings. Remarkably
large band dispersion along kz-direction near Fermi level, which is even wider
than the in-plane one, is observed by the angle-resolved photoemission
spectroscopy measurement. Employing the X-ray absorption spectroscopy and
density functional theory calculations, we reveal that the strong interlayer
coupling in 2D PdSe2 originates from the unique directional bonding of Pd d
orbitals associated with unexpected Pd 4d9 configuration, which consequently
gives rise to the strong layer-dependency of the band gap.
