Electrolytic scanning tunneling microscopy and point contact studies at electrochemically polished Au(111) substrates with and without Pb adsorbates

Abstract

Electrochemically polished Au(111) substrates have been investigated by scanning tunneling microscopy in 0.5M NaClO<inf>4</inf> in absence and presence of Pb underpotential adsorbates. The nm-scale morphology of the electropolished electrodes features densely terraced dome-shaped domains with heights up to more than 5 nm, whereas only small atomically flat surface regions of ∼5 nm width are observed. During polarization in the ideally polarizable potential range in absence of Pb adsorbates, considerable smoothening and lateral displacement of the dome-shaped domains occurs within a time scale of minutes. Extended polarization in presence of a full Pb adsorbate coverage leads to a marked increase in substrate corrugation, up to the scale of several nm. At Pb-free substrates, controlled variations of the tip-sample separation in the transition regime from tunneling to point contact exhibit a similar distance dependence of the tip current as observed in previous vacuum studies, and allow for the first time an approximate assessment of the tunneling distance range in an electrolytic system. In the case of Au tips, the transition from tunneling to point contact is observed at tunneling resistances of ∼2 × 10⁴ Ω, in good agreement with previous vacuum results. A tunneling regime with R<inf>T</inf> < 10⁷ Ω is assigned approximately to a substrate-tip separation range < 1 nm.