Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

The production of ordered arrays of organic molecules on metallic surfaces by means of self-assembly is one of the most powerful methods for controlled patterning on the nanometer scale. Although the self-assembly of sulfurbased ligands has been studied for decades, the thermal and oxidative instability of these systems introduces challenges in many potential applications. In recent years, it has been shown that a new ligand class, N-heterocyclic carbenes (NHCs), bind to metal surfaces via a metal–carbon covalent bond, resulting in monolayers with much greater stability. However, fundamental questions surrounding self-assembly in this new ligand class remain unanswered, including the simple questions of what controls NHC orientation on the surface and under what conditions they self-assemble. Herein we describe how NHC structure, surface density, deposition temperature, and annealing temperature control mobility, thermal stability, NHC surface geometry, self-assembly, and the exact chemical nature of the surface structures. These data provide the first general set of guidelines to enable the rational design of highly ordered NHC-based monolayers. Considering that NHCs may supplant thiols as the functionalization agent of choice in a wide range of applications, a detailed understanding of their surface chemistry is crucial for the success of these next-generation monolayers.