  The growth of controlled 1D carbon-based nanostructures on metal surfaces is
a multistep process whose path, activation energies and intermediate metastable
states strongly depend on the employed substrate. Whereas this process has been
extensively studied on gold, less work has been dedicated to silver surfaces,
which have a rather different catalytic activity. In this work, we present an
experimental and theoretical investigation of the growth of poly-$p$-phenylene
(PPP) chains and subsequent narrow graphene ribbons starting from
4,4''-dibromo-$p$-terphenyl molecular precursors deposited at the silver
surface. By combing scanning tunneling microscopy (STM) imaging and density
functional theory (DFT) simulations, we describe the molecular morphology and
organization at different steps of the growth process and we discuss the
stability and conversion of the encountered species on the basis of calculated
thermodynamic quantities. Unlike the case of gold, at the debromination step we
observe the appearance of organometallic molecules and chains, which can be
explained by their negative formation energy in the presence of a silver adatom
reservoir. At the dehydrogenation temperature the persistence of intercalated
Br atoms hinders the formation of well-structured graphene ribbons, which are
instead observed on gold, leading only to a partial lateral coupling of the PPP
chains. We numerically derive very different activation energies for Br
desorption from the Ag and Au surfaces, thereby confirming the importance of
this process in defining the kinetics of the formation of molecular chains and
graphene ribbons on different metal surfaces.
