N-Heterocyclic Carbenes on a III-V Semiconductor: From Chain Formation to Ordered Monolayers

N-Heterocyclic carbenes (NHCs) are established ligands for various surfaces, known for their strong binding, ability to form self-assembled monolayers, and modular structure. However, semiconductor surface modification with NHCs is still in its infancy despite its technological importance. Although previous studies focused on silicon, III-V compound semiconductors offer direct bandgaps and high electron mobilities, making them ideal for (opto-)electronics. This study examines the adsorption of different NHCs on GaAs, a prototypical III-V material, using scanning tunneling microscopy, density functional theory, X-ray photoelectron spectroscopy, low-energy electron diffraction, and reflectance anisotropy spectroscopy. Covalent binding to the surface and the formation of well-ordered NHC monolayers are observed, along with exceptionally large work function reductions. The unique structural features of NHCs enable precise control over film structure, ordering, and electronic properties.