Significant difference in charge transfer at steps on Ag(100) and Cu(100) surfaces revealed by field emission resonance

We employ field emission resonance (FER) to observe the Smoluchowski effect on Ag(100) and Cu(100) surfaces, which is a charge transfer phenomenon, leading to electric dipole formation at surface steps. On Ag(100), pronounced charge transfer results in a discontinuity in FER energies at step sites. In contrast, this discontinuity is absent on Cu(100), indicating that the Smoluchowski effect is negligible. Density functional theory calculations confirms this significant difference in charge transfer at the step. By analyzing FER energies using the triangular potential model, we extract the spatial variation of the work function around the step on both surfaces. Our results for Cu(100) demonstrate that a reduction in the work function can occur even without a step electric dipole, contrary to the widely accepted explanation that the Smoluchowski effect reduces the work function. Furthermore, while it is generally accepted that as charge transfer occurs, local negative (positive) surface charge raises (lowers) the work function, our results for Ag(100) reveal the opposite trend. Additionally, the extracted work function enables spatially resolving the positive and negative charge densities within a step electric dipole, which has not yet been achieved with other local probe techniques.