The multifaceted roles of halogen adatoms in surface chemistry

Halogen adatoms are the most common by-products generated during the on-surface reactions aimed at fabricating functional low-dimensional carbon-based nanostructures. Recently, an increasing number of studies have demonstrated the crucial roles of halogen adatoms in various aspects of surface chemistry, revealing them to be far more than mere by-products of on-surface synthesis. Understanding the comprehensive effects of surface halogen adatoms on the chemical and physical properties of their co-adsorbates, along with the underlying molecular-scale mechanisms, is therefore of great significance. This review focuses on scanning tunneling microscopy (STM) studies of this topic. The electronic interactions between halogen adatoms and metal surfaces are first discussed to elucidate the origin of the halogens’ tuning effects. The review then examines the multifaceted roles of halogen adatoms and intercalated adlayers in steering adsorption configurations, self-assemblies, surface reactions, and spin states of the co-adsorbed molecules. Additionally, surface reactions involving halogen adatoms as reactants are briefly summarized. This review aims to provide fundamental insights for the rational design and efficient construction of functional surface systems through the strategic use of halogen adatoms.