Unraveling the Inner Electronic Structure of Chromium-Oxide Films by Probing the Layer-by-Layer Evolution of Their...

Low-temperature tunneling spectroscopy has been employed to probe the workfunction ϕ of atomically flat Cr-oxide single- and double-stack films grown on Pt(111). While the single-stack Cr₃O₆ trilayer has a ϕ value of ∼7.0 eV, it decreases to ∼5.0 eV for a Cr₆O₁₁ double-stack. The charge redistribution underlying this workfunction drop has been analyzed by density functional theory. The remarkably high ϕ value of the Cr₃O₆/Pt trilayer, making it a highly electronegative substrate, originates from a massive electron transfer from the Pt(111) into empty Cr-states that reduces 2/3 of the oxide cations from their formal 4+ to a 3+ charge state. The negative surface dipole diminishes upon growing a Cr-O honeycomb layer on top of the trilayer, forming a Cr₆O₁₁ double-stack. The adlayer acts as electron donor, enabling the charge transfer from the Pt support to decrease substantially. The charge redistribution not only triggers the detected workfunction drop but also stabilizes the double-stack with respect to a single-stack geometry. A comparison of the observed CrO_x/Pt behavior to that of hypothetical double-stack films, made of an interfacial TMO₂ trilayer (TM = Ti, V, Mn, Fe) and a capping Cr-O honeycomb plane, allows us to correlate the charge transfer between the individual oxide layers and the substrate to the overall stability of the metal-oxide system.