Photothermal bottom-up graphene nanoribbon growth kinetics.

We present the method of laser-induced photothermal synthesis of atomically precise graphene nanoribbons (GNRs). The kinetics of photothermal bottom-up GNR growth are unravelled by \emph{in situ} Raman spectroscopy. We photothermally drive the elementary reactions steps by short periods of intense laser irradiation and subsequently analyze the Raman intensities of the vibrations of the reactants in the irradiated area. Growth kinetics of chevron GNRs (CGNRs) and 7 atoms wide armchair GNRs (7-AGNRs) are investigated. The reaction rate constants for polymerization, cyclodehydrogenation and interribbon fusion are experimentally determined. We find that the limiting rate constants for CGNR growth are several hundred times smaller than for 7-AGNR growth and that interribbon fusion is an important elementary reaction step that occurs during 7-AGNR growth. The narrow width of the Raman modes confirms the high quality of the synthesized product. Our work also highlights that photothermal synthesis and \emph{in situ} Raman is a powerful tandem for the investigation of on-surface reactions.