Work Function Changes Induced by Charged Adsorbates: Origin of the Polarity Asymmetry

A theoretical analysis of charged adsorbates on a metal surface reveals a pronounced polarity asymmetry between electropositive and electronegative species, thus reproducing a well known but so far not properly understood experimental fact. For ionic adsorbates on metal surfaces, we analyze the several, often canceling, terms that contribute to the change of the interface dipole and, hence, to work-function changes, $\Delta \varphi$. We demonstrate that for the prototypic case of I on Cu(111) the magnitudes and the signs of these terms can be understood on the basis of their physical and chemical origins. An important consequence of their cancellation is that negatively charged adsorbates can lead to a paradoxical $\Delta \varphi <0$ rather than the expected $\Delta \varphi >0$.

Figure 1

Density difference plots for ${\mathrm{Cu}}_{32}\mathrm{I}$ showing (a) the Cu polarization and (b) the I polarization and donation. Solid (blue) lines are density increases and dashed (red) lines are density decreases. The plotting plane passes through I and the adsorption site Cu atom; the positions of the Cu atoms, projected onto the plotting plane, are shown as shaded circles. (a) $\Delta \rho =\rho [\mathrm{V}(\mathrm{Cu})]-\rho [\mathrm{\text{Pauli}}\mathrm{FO}]$, (b) $\Delta \rho =\rho [\mathrm{V}(\mathrm{I})]-\rho [\mathrm{V}(\mathrm{Cu})]$.