Ultra‐Thin Metal Oxide Superstructure of Pd(001) as Passivation Interlayer at Organic/Metal Interface

At organic molecule/metal interfaces for electronic applications, it is required of the metal surface to be passivated in view of preserving the molecular properties of the ordered organic layer. This can be achieved by screening the metal with a single atomic layer of O, namely, ultra-thin metal oxide (UTMO) layers. Cobalt tetraphenylporphyrins (CoTPP) on oxygen passivated Fe(001), with 1 ML O coverage, have revealed a molecule/substrate decoupling effect due to the formation of an ultra-thin Fe oxide layer at the interface. However, the threshold concentration of surface O required to observe the decoupling effect has not been assessed yet. In this work, the possibility of stabilizing different ultra-thin Pd oxide superstructures, characterized by a different number of O atoms per unit cell, is exploited to investigate the O decoupling effect on CoTPP films. Two Pd oxide superstructures are considered: Pd(001)-p(2 x 2)O and Pd(001)-p(root 5 x root 5)R27 degrees O, with 0.25 and 0.80 ML O coverages, respectively, which are characterized by low-energy electron diffraction (LEED), X-ray and ultra-violet photoelectron spectroscopies (XPS/UPS) and inverse photoemission spectroscopy (IPES). The results suggest a lower limit of 0.80 ML O coverage as a passivation interlayer to obtain an ordered and decoupled CoTPP monolayer on Pd(001).It is demonstrated, for the first time, that oxygen passivated Pd(001), particularly the superstructure Pd(001)-p(root 5 x root 5)R27 degrees O, acts as a passivation interlayer at the porphyrins/Pd interface, thus facilitating organic/metal decoupling essential for organic electronics, the evidences of which are: a clear molecular reconstruction diffraction pattern, and preservation of molecular electronic features of a porphyrin monolayer, deposited on Pd(001)-p(root 5 x root 5)R27 degrees O. image