The growth of poly-p-phenylene (PPP) chains from molecular precursors and their evolution into graphene nanoribbons have been investigated in detail both from an experimental and theoretical point of view over different metal surfaces. While most of studies had previously focused on gold surfaces, within the project we also considered the case of silver, which presents a very different reactivity and growth dynamics. At all steps of the growth process, we have investigated supramolecular structures by scanning tunnelling microscopy (STM), low energy electron diffraction, X-ray photoelectron spectroscopy and near edge X-Ray absorption fine structure. With respect to previous studies, we employed long annealing times to ensure that a steady state was reached at each intermediate state of the growth, allowing us to observe the appearance of metastable organometallic complexes already at room temperature. Experimental observations have been complemented by simulations conducted in the framework of the density functional theory (DFT) that brought an atomic level description of the molecular organization and reshaping at the metal surface. The excellent agreement between the experimental and simulated STM images provided from one side a validation of the computational approach employed and from another side a deeper understanding of the re-conformation of the molecules induced by the substrate. Growth differences between gold and silver substrates have been discussed on the basis of calculated formation energy for various supra-molecular organizations considering the important role played by intermediate organometallic complexes. Finally, a comparative analysis of H and Br desorption paths and activation energies on gold and silver has stressed the importance of these processes in defining the kinetics of formation of molecular chains and graphene ribbons.
Poly-p-phenylene has been proposed as an initial building block for more complex uni-dimensional electronic components. A direct access to the electronic structure of this system was acquired by angular-resolved photoemission spectroscopy (ARPES) over well aligned chains over stepped vicinal surfaces. A rich information brought by this spectroscopy can be extracted by comparing experimental data with first principle simulations. However, in order to minimize spurious stress effects, very large supercells are required, whose electronic structure results in highly folded bands for both the molecule and the substrate, which can be difficult to correlate with the measurements. To overcome this complication, we have employed an unfolding post-processing which provides a primitive cell effective band structure with a great interpretative value since it can be directly related to ARPES experiments. The simulated unfolded band structure provides a very fine description of the role of the metal substrate in defining the ultimate electronic structure of the chains and of the adsorbed bromine atoms in tuning the band alignment between the metal and the molecules.
Within the project the host group has also successfully synthesized graphyne molecular wires on gold surfaces. By first principles simulations, we have studied the organizations and stability of both the molecular precursors and polymeric chains, showing that interactions with the substrate do not induce a substantial re-conformation of the molecules. The combination of first principles and STM image simulations has permitted to correlate high intensity features observed in experiments with the HOMO and LUMO location within the adsorbed molecular precursors and chains. This information has allowed us to provide a precise description of the location of molecules with respect to the underlying metal surface, an information which is not directly accessible with the experiments.
