Direct Observation of Orbital Driven Strong Interlayer Coupling in Puckered Two-Dimensional PdSe₂

Interlayer coupling between individual unit layers is known to be critical in manipulating the layer-dependent properties of two-dimensional (2D) materials. While recent studies have revealed that several 2D materials with significant degrees of interlayer interaction (such as black phosphorus) show strongly layer-dependent properties, the origin based on the electronic structure is drawing intensive attention along with 2D materials exploration. Here, 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 is reported. Remarkably large band dispersion along the k_z -direction near Fermi level, which is even wider than the in-plane one, is observed by the angle-resolved photoemission spectroscopy measurement. Employing X-ray absorption spectroscopy and density functional theory calculations, it is revealed 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 plays a decisive role for the strong layer-dependency of the band gap.