Ordered Chlorine Layer Formation from a Supersaturation of Chloroform

The adsorption of ions on metallic surfaces is a powerful method to alter their electronic structure and thus tune their reactivity. A prominent example is chlorine on Ag(111). We investigate chlorine created by the room-temperature adsorption of chloroform on Ag(111) at supersaturation and the structures it forms from individual monomers to a full layer by using low-temperature scanning tunneling microscopy. The data is supplemented by temperature-programmed desorption and X-ray photoelectron spectroscopy after low-temperature adsorption under ultrahigh-vacuum conditions. Data interpretation is supported by density functional theory (DFT) calculations that account for dispersion forces. At low chlorine coverages, each chlorine locally alters the electronic structure of the surface. The adsorbed chlorine-induced local environment modification thereby creates preferential adsorption sites for other chlorines in their vicinity, stabilizing extended chlorine structures on Ag(111). Oligomer formation leads to distance-dependent cooperative effects of the charge transfer and thus impacts the electronic structure of the surface beyond the change by individual chlorines. At intermediate chlorine coverage, chlorine forms meandering chains with atoms adsorbed in alternating hcp and fcc hollow sites at distinct chlorine-chlorine distances. The one-dimensional structures convert to an open network at intermediate coverages and a two-dimensional hexagonal superstructure at saturation coverage. The DFT calculations suggest that the charge density extracted from the surface into the chlorines and the interaction between chlorine and silver atoms is improved as chlorines are adjoined closer at intermediate and high coverages.