As an organic nanostructure, self-assembled monolayers (SAMs) play a central role in many aspects of nanotechnology, including molecular electronics. In this work, we show that SAMs based on N-heterocyclic carbenes on a Au(111) substrate offer a high level of crystallinity and also exhibit the highest possible packing density. As a result of this structural optimization, defect concentrations were reduced by 2–3 orders of magnitude and thermal stability was ∼100 K higher than those of any other SAMs on Au. The conductivity of these SAMs is ∼4 orders of magnitude lower than that of standard alkanethiols of comparable length, which together with very low defect concentration and high thermal stability makes them a highly interesting material for potential application in organic thin film transistors. The self-assembly of such dense, highly crystalline, and notably stable structures is associated with strong C–Au bonding and the rational design of assembled molecules, resulting in the high mobility of both adsorbate and substrate atoms, as confirmed by the size of the molecular domains and the adsorbate-driven modification of the Au(111) substrate, respectively.

Self‐assembled monolayers (SAMs) on metal substrates are an important part of modern interfacial chemistry and nanotechnology. The robustness of SAMs strongly depends on their thermal stability, which, together with electric conductivity, crucial for their applications in molecular/organic electronics. In this context, using a multidisciplinary approach, the structure, stability, and conductivity properties of conjugated aromatic SAMs featuring the naphthalene backbone and S, Se, or COO group, mediating bonding to the Ag substrate are addressed. Whereas thermal stability of these SAMs exhibits a strong dependence on anchoring group, their conductivity is similar, which is rationalized by tentative model considering redistribution of charge density along the molecular framework. The thermal stability of model naphthalenethiol SAM, emphasized by desorption energy of ≈1.69 eV, is better than that of typical N‐heterocyclic carbene (NHC) monolayers considered currently as the most stable SAMs on metal substrates. However, in contrast to NHC SAMs, which are highly insulating, the naphtalene‐based SAM, with S, Se or COO anchoring groups, are highly conductive, even in comparison with analogous oligophenyl SAMs (by a factor of 10). A unique combination of the ultimate thermal stability and superior conductivity for the naphthalenethiol SAM on Ag makes it highly attractive for applications.

Ion-induced desorption was successfully applied for the analysis of the stability of chemical bonds at the molecule–metal interface.

Self-assembled monolayers (SAMs) of N-heterocyclic carbenes (NHCs) on metal substrates are currently one of the most promising systems in context of molecular-scale engineering of surfaces and interfaces, crucial for numerous applications. Interest in NHC SAMs is mainly driven by their assumingly higher thermal stability compared to thiolate SAMs most broadly used at the moment. Most of the NHC SAMs utilize imidazolium as an anchoring group for linking molecules to the metal substrate via carbene C atom. It is well established in the literature that standing up and stable NHC SAMs are built only when using bulky side groups attached to nitrogen heteroatoms in imidazolium moiety, which, however, leads to monolayers exhibiting much lower packing density compared to thiolate SAMs. Here, by combined X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and temperature-programmed secondary ion mass spectrometry analysis, we demonstrate that using NHCs with small methyl side groups in combination with simple solution-based preparation leads to the formation of aromatic monolayers exhibiting at least double surface density, upright molecular orientation, and ultra-high thermal stability compared to the NHC SAMs reported before. These parameters are crucial for most applications, including, in particular, molecular and organic electronics, where aromatic SAMs serve either as a passive element for electrode engineering or as an active part of organic field effect transistors and novel molecular electronics devices.