Coulomb barriers and adsorbate effects in scanning tunneling microscopy

Even in ultrahigh vacuum most surfaces studied by scanning tunneling microscopy (STM) and especially the STM tip are coated by adsorbates containing localized states. These localized states adjacent to the tip exhibit very small effective capacitances. For these small capacitances the Coulomb charging energy ${\mathit{e}}^2$/2C of an electron is large compared with the thermal energy kT even at room temperature. As a result, a Coulomb barrier shows up as the reduced tunnel current increases I∝${\mathit{V}}^2$ for eV<${\mathit{e}}^2$/2C. Spectroscopic I(V) data were analyzed for Nb and Au samples at different temperatures and tip-sample distances for wet-etched W tips. The I(V) characteristics revealed a Coulomb barrier in the resonant tunnel channel. Direct tunneling, which decreases more rapidly with tunnel distance, does not show this Coulomb barrier. The resonant tunnel channel shows that the Coulomb barrier seems to be related to the adsorbate (${\mathrm{H}}_2$O?) at the W-${\mathrm{WO}}_3$ tip dominating tunneling under these circumstances. This explains the often reduced barrier height or atomic resolution in STM measurements.