On-surface synthesis and evolution of self-assembled poly($p$-phenylene) chains on Ag(111): a joint experimental and theoretical study

TL;DR

This study combines experimental STM imaging and DFT simulations to investigate the on-surface synthesis of poly(p-phenylene) chains on Ag(111), revealing substrate-dependent growth mechanisms, stability, and activation energies distinct from gold surfaces.

Contribution

It provides new insights into the growth process of PPP chains on silver, highlighting the formation of organometallic intermediates and substrate-specific kinetic barriers.

Findings

Organometallic molecules form at debromination on Ag(111).

Intercalated Br atoms hinder graphene ribbon formation on silver.

Activation energies for Br desorption differ significantly between Ag and Au.

Abstract

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.