Sulfur‐to‐Metal Single‐Atom Editing of a Thiourea Self‐Assembled Monolayer into an Adatom‐Bound N‐Heterocyclic Carbene Monolayer on Coinage Metal Surfaces

N-Heterocyclic carbene (NHC) self-assembled monolayers (SAMs) are emerging as robust and versatile surface ligands, yet their direct on-surface construction with atomic precision remains elusive. Here, we report a reagent-free, thermally driven on-surface strategy for single-atom editing, providing an alternative to convert a heteroatom-anchored SAM into a functional NHC layer. N-Heterocyclic thiourea (NHT) with carefully chosen substituents undergoes a S-to-metal adatom exchange, yielding adatom-bound NHC on coinage metal surfaces. Using low-temperature scanning tunneling microscopy (LT-STM), synchrotron-radiation X-ray photoelectron spectroscopy (SR-XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and density functional theory (DFT), we identify two distinct classes of SAMs: (i) NHT-SAM, in which sulfur coordinates to the metal surface and (ii) NHC-SAM, characterized by M─C σ-bonds between the carbene center and a surface adatom. NHT undergoes a clean SAM-to-SAM conversion, achieving covalent single-atom editing within an intact monolayer. DFT calculations show this transformation is energetically favorable and highly tunable by substrates and N-substituents, establishing a general framework for atom-precise molecular editing at surfaces.