Spectroscopy at atomic scale group

Undoubtedly, the development of the field of on-surface chemistry has considerably contributed to our understanding of the steps of a few chemical reactions. The visualization of the reactants and reaction products present on the surface template by means of a scanning tunneling microscope has provided snapshots and new detailed insights into the reaction mechanisms. These studies have highlighted the catalytic role of the surface and enabled the synthesis of nanostructures with chosen shape, (opto)-electronic, and magnetic properties required for the further development of quantum technologies. Despite the relevance and success of this technique, most of the chemical reactions characterized so far dealt with the synthesis of polymer nanostructures using molecular precursors functionalized by one single reacting group. To cite one example, graphene nanostructures are, in most cases, formed on metal surfaces by the thermal activation of a molecular de-halogenation process followed by the carbon-carbon coupling between neighboring molecules. The possibility of performing chemical reactions between precursors equipped with different reacting groups, i.e. inducing chemical bonding between different atomic species, as can be a carbon and a chalcogen atom was not explored, so far. Indeed, the detachment of the chalcogen atom from the organic structure is quite frequent upon their interaction with traditional metal catalysts, such as Au, Ni, or Pd, thus, preventing their cycloaddition and the functionalization of polymeric structures.
In this context, this collaborative work between DIPC, CFM, and Ikerbasque, in which successfully C-S bonds are successfully formed on Au(111) surfaces using molecular precursors functionalized simultaneously with a halogen and a sulfhydryl group, brings novelties to the field of surface synthesis. The reaction resulted in the selective thioetherification of the molecules without adding a dreaded degree of complexity due to competing different chemical reactions. Despite the affinity of sulfur with gold, the thermal energy provided to the 4′-Bromo-4-mercaptobiphenyl (Br-MBP) molecules adsorbed on the Au surface, resulted in the formation of extended polymeric chains. A series of four reaction steps involving sulfhydryl or bromine molecular functional groups and leading to the formation of intermolecular C-S bonds have been identified through complementary techniques such as scanning tunneling microscopy, spectroscopy, and first-principles calculations. The study brings evidence that to form the thioether polymer and overcome the competitive common formation of C-C bonds, two reaction steps, namely, the dehalogenation and the dissociation of the S-Au bond, must occur simultaneously. This work provides new insight into the synthesis of such polymeric structures, showing how the electronic properties of the precursors change upon bonding and according to the length of the molecular precursors.

Regardless of the optical or thermal excitation or field-driven injection of electrons, their interaction with organic structures is critically important for the development of many applications. The immediate, albeit transitory, electron occupation of well-defined molecular orbitals, or the related energy exchange process, is beneficial for the activation of chemical reactions. However, for the same reason, the electron interaction itself constitutes one of the main limits to the structural stability of organics in (opto)-electronics.This work addresses the mechanism of molecular dissociation under applied bias by injecting electrons in tunneling conditions. Specifically, we have correlated the energy of debromination of an aryl group with its density of states in a self-assembled dimeric structure of 4’-bromo-4-mercaptobiphenyl adsorbed on an Au(111) surface. We have observed that the electron- energy range where the molecule is chemically stable can be extended, shifting the bias threshold for the rupture of the –C–Br bond continuously from about 2.4 to 4.4 V by changing the electron current. Correspondingly, the power needed for the dissociation changes, however, surprisingly it drops sharply to about 20% of the original value at 3.6 V. The abrupt change identifies two different reaction regimes and the contribution of additional molecular resonance states.

In the last decade, a considerable scientific attention has been devoted to the formation of ordered organic structures ensuring molecular functionalities and good electrical properties. In this context, a valuable approach was offered by Ullmann cross-coupling reactions where halogenated molecules catalyzed by metal surfaces promoted the synthesis of extended structures through the molecular polymerization. This chemical approach results in good mechanical and electrical contacts between components. In this work, we have shown that the electron conductance of benzothiadiazole molecules, extensively used in electronics, can be enhanced even before their polymerization. Indeed, despite the formation of a porous network, two adjacent, periodic and isoenergetic contributions, namely a molecular electronic resonance and the confined surface-state, sum-up forming one-dimensional conductance channels, observable in energy-resolved maps of a 2D-metal-organic network. Though they do not contribute directly to the conductance, the adsorption configurations of Br atoms, inorganic byproduct of the redox-reacted 4,7-dibromobenzo[c]-1,2,5-thiadiazole molecules on the copper surface critically control the channel continuity. These halogen atoms enables the delocalization of the molecular electronic resonance into a continuous channel acting on the confinement of the Cu(110) surface state on the pores. Small displacements of the Br atoms change the local surface potential misaligning the energy levels and creating discontinuity into the channels. This work opens new perspectives on charge-transport mechanisms controlled by an order-disorder transition determined by the movement of single atoms limiting carrier’s mobility in two-dimensional organic networks.

The detailed knowledge of the surface atomic structures able to promote chemical reactivity is of paramount importance for solid-state nanochemistry. In this work, we have shown that the loss of surface planarity observed in two-dimensional (2D) systems increases significantly the adsorption selectivity and additionally controls the surface chemistry. Specifically, we have characterized the reactivity of H2-phthalocyanine (H2-PC) molecules adsorbed on periodic structures characterized by atomic-buckling. The periodic displacement of single atoms orthogonally to the surface plane shown by the GdAu₂/Au(111) surface develops spontaneously due to the not commensurable periodicity of the two interfaced structures. Thus, this system is characterized by variable lattice deformations, which reflect the local atomic interaction with the underlying surface. Specific single-atom sites are exposed, allowing us to explore the relationship between the dehydrogenation reaction of the H2-PC molecules and the atomistic structure of the supporting layer. We have demonstrated that this atomic displacement of the Gd atoms in the GdAu₂/Au(111) system naturally promotes site selectivity and specificity of the chemical reactions. The energetics of a dehydrogenation reaction of the H2-PC molecules is indeed related to the degree of atomic buckling.

By means of scanning tunneling microscopy, X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations, we give evidence that the vertical displacement of the Gd atoms is responsible for the dehydrogenation of the macrocycle of only site-selected H2-PC molecules. Thus, at most, one-third of the monolayer (ML) of adsorbed molecules, corresponding to H2-PC molecules occupying well-defined positions of the Moiré superlattice, undergoes a dehydrogenation reaction. The H2-PC dehydrogenation strengthens the Gd−N interaction inducing structural relaxation effects in the alloy geometry. The deformation of this atomistic order may have possible consequences for the stability of the reported in-plane ferromagnetic character of the alloy layer. The present work provides valuable perspective in the selective activation of chemical reactions on surfaces. As atomistic buckling has been commonly reported on several two-dimensional systems, observed as Moiré patterns, we believe that these findings might apply generally.