Understanding Organic Adlayer Formation on CoOx-Terminated PtCoO2

PtCoO2 is an attractive candidate for next-generation interconnects due to its exceptionally low in-plane resistivity, but its susceptibility to degradation under reducing conditions necessitates effective surface passivation strategies. In this work, the deposition of organic adlayers on PtCoO2 was systematically investigated using octadecylphosphonic acid (ODPA), octylphosphonic acid (OPA), 1-octadecanethiol (ODT), and trifluoroacetic acid (TFA). The influence of immersion time (0.5–48 h) and surface pre-treatment (untreated, ethanol-cleaned, and UV-ozone-treated surfaces) was evaluated using water contact angle (WCA), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Low-energy ion scattering (LEIS) confirmed a CoOx-rich surface termination, which strongly influenced adsorption behaviour. Among the molecules studied, phosphonic acids exhibited the strongest interaction with the surface, with ODPA producing the highest WCAs (97–99°) and clear evidence of phosphonate bonding. Comparison with OPA revealed the importance of alkyl chain length in promoting hydrophobicity and molecular organisation, yielding values of 77–88°. In contrast, ODT exhibited heterogeneous adsorption, sulphur oxidation, and aggregate formation, while TFA showed limited surface retention. Overall, ODPA was identified as the most effective adsorbate, establishing phosphonic acid-based self-assembled monolayers as promising candidates for the passivation of PtCoO2 and related conductive oxide materials.