Site-specific characterization of Ho atoms on MgO

We investigate the site-dependent relative charge of holmium (Ho) atoms adsorbed on MgO/Ag(100) using Kelvin Probe Force Microscopy (KPFM) by means of a nc-AFM/STM qPlus sensor . By manipulating a particular Ho atom between oxygen (Ho-O) and bridge (Ho-B) adsorption sites, we determine their relative charge via local contact potential difference (LCPD) measurements. KPFM parabolas reveal that Ho-O is positively charged, while Ho-B is negatively charged. Measurements at varying tip-sample distances show that atom-specificity is lost at far distances but is recovered at close approach, with significant charge variations between species. We also perform KPFM at constant height over the different absorption sites to spatially resolve the charge distribution along a particular direction. The results are analyzed in the context of electrostatic interactions, work function differences, and ab-initio modeling, providing insight into the absolute charge determination via KPFM height dependence.