Tuning the work function of stepped metal surfaces by adsorption of organic molecules

Understanding the binding mechanisms for aromatic molecules on transition-metal surfaces,
especially with defects such as vacancies, steps and kinks, is a major challenge in designing
functional interfaces for organic devices. One important parameter in the performance of
organic/inorganic devices is the barrier of charge carrier injection. In the case of a metallic
electrode, tuning the electronic interface potential or the work function for electronic level
alignment is crucial. Here, we use density-functional theory (DFT) calculations with
van der Waals (vdW) interactions treated with both screened pairwise (vdWsurf) and
many-body dispersion (MBD) methods, to systematically study the interactions of benzene
with a variety of stepped surfaces. Our calculations confirm the physisorptive character
of Ag(2 1 1), Ag(5 3 3), Ag(3 2 2), Ag(7 5 5) and Ag(5 4 4) surfaces upon the adsorption of
benzene. The MBD effects reduce the adsorption energies by about 0.15 eV per molecule
compared to the results from the DFT + vdWsurf method. In addition, we find that the higher
the step density, the larger the reduction of the work function upon the adsorption of benzene.
We also study the effect of vdW interactions on the electronic structure using a fully selfconsistent
implementation of the vdWsurf method in the Kohn–Sham DFT framework. We
find that the self-consistent vdWsurf effects increase the work function due to the lowered
Fermi level and the increased vacuum level. As a result, the benzene/Ag(2 1 1) system has
the lowest work function (3.67 eV) among the five adsorption systems, significantly smaller
than the work function of the clean Ag(1 1 1) surface (4.74 eV). Our results provide important
insights into the stability and electronic properties of molecules adsorbed on stepped metal
surfaces, which could help in designing more appropriate interfaces with low work functions
for electron transfer.