Molecular-Vapor-Assisted Low-Temperature Growth of Graphene Nanoribbons

Conventional on-surface synthesis under ultrahigh vacuum deposition has been used to synthesize various types of graphene nanoribbons (GNRs). The dehydrogenation reaction from a prepolymer to GNR requires catalysis with a metal substrate at high temperatures, which suffers from undesired side reactions that decompose introduced functional substituents during conversion to GNRs. To overcome these drawbacks, we have developed a new method, molecular-vapor-assisted low-temperature growth (MVLTG), based on a massive excess of gaseous hydrogen-accepting molecules (2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) or oxygen) relative to the prepolymer on the metal surface. MVLTG reduces the GNR growth temperature of prepolymers with the Z-type precursor, 4′,5″-dibromo-1,1′:2′,1′:2″-quaterphenyl from 500 °C, used in conventional thermal annealing, to 350 °C. To investigate the reaction mechanism of MVLTG, a kinetic study using Raman spectroscopy to obtain GNR yields as a function of the hydrogen acceptor concentration and time was in good agreement with a simulation based on the gas-2D solid-state reaction model. These studies suggest a successful “Scholl reaction” in the gas phase, in which gaseous hydrogen acceptors (DDQ and oxygen) efficiently extract hydrogens from prepolymers at lower temperatures, converting them into GNRs. Additionally, MVLTG successfully promoted dehydrogenation reactions of prepolymers made from functionalized precursors of 4′,5″-dibromo-4-butoxy-1,1′:2′,1″:2″,1″′-quaterphenyl or 4′,5″-dibromo-4-butoxy-1,1′:2′,1″:2″,1‴:4‴,1″-quinquephenyl, suppressing decomposition of substituents. MVLTG not only realizes a new class of GNRs but also enables unprecedented on-surface synthesis.