On-surface synthesis (OSS) is a successful approach to the creation of carbon-based nanostructures that cannot be obtained via standard solution chemistry. It consists of a bottom-up strategy, based on the use of carefully designed molecular precursors as building blocks. These precursors, are equipped with functional groups that, when activated, become active sites for the intermolecular covalent coupling and allow the creation of one- or two-dimensional (1D, 2D) nanostructures. These reactions can be triggered by means of various stimuli, such as heat, light, and others. The shape and properties of the precursors determine those of the target nanostructures, offering great tuneability. Typically, atomically flat, clean metal surfaces of single crystals in ultrahigh vacuum (UHV) conditions are used as substrates, but the OSS also extends to different environments (ambient, solid-liquid interfaces) and substrates (thin films, insulating/semiconducting layers, bulk insulators). The reaction steps and products are usually monitored and characterized by scanning probe microscopies, such as scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and by photoelectron spectroscopies, such as X-ray/UV photoelectron spectroscopy (XPS/UPS). More advanced approaches, such as temperature programmed XPS (TP-XPS), allow the tracking of relevant signals during the reaction, and the understanding of fundamental mechanistic details.

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