Spectroscopy at atomic scale group


February, 2023

Thioetherification of Br-Mercaptobiphenyl molecules on Au(111)

This work addresses the formation of C-S bonds between double functionalized molecules. We describe a serie of 4 chemical steps leading to the thioetherification of the molecules and teh formation of polymeric chains.

Figure: Graphical representation of the thermal induced thioetehrigication of Br-MBP molecules and its electronic fingerprint.

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.

Nano letters 23, 1350-1354 (2023)

August, 2021

Power discontinuity and shift of the energy onset of a molecular de-bromination reaction induced by hot-electron tunneling

Understanding the structural and chemical stability of organics under applied electrical bias is fundamental to further progress in (electro)-catalysis and (opto)-electronics. This work addresses the dissociation mechanism of halogen atoms and reports on its sharp dependence on the applied electrical power and molecular density of states.

Figure: Graphical representation of the electron induced dissociation of Br atoms in the Au(Br-MBP)2 complex. The reaction onset can be controlled and shifted for almost 2eV from 2.4eV. At 3.6eV, however, the reaction power changes sharply dropping to 20% of the pristine value, suggesting that an additional molecular resonance is involved.

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.

Nanoscale 13, 15215 (2021)

July, 2020

Anisotropic Electron Conductance Driven by Reaction Byproducts on a Porous Network of Dibromobenzothiadiazole on Cu(110)

Organics offers a variety of functional properties eligible almost “a-la-carta”. However, the application of organics into devices requires in addition excellent charge transport properties at energies larger than 2eV, where electronics works. Here, we show a mechanism enabling one-dimensional conductance-channels connecting discrete molecular states at 2.1eV through the pores of a metal-organic network on Cu(110)

Figure: Graphical representation of the conductance channels and the topographic structure that enables them. The molecular modeling of the 4,7-dibromobenzo[c]-1,2,5-thiadiazole (2Br-BTD) molecules on the copper surface is superposed: Br atoms are depicted as bright red dots in the sketch.

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.

Angewandte Chemie Int.Ed 59, 15599 (2020)

February, 2019

Can Atomic Buckling Control a Chemical Reaction? The Case of Dehydrogenation of Phthalocyanine Molecules on GdAu2/Au(111)

The efficiency of chemical reactions on surfaces is traditionally related to the atomic structure and catalytic activity of the substrate. Periodic out-of-plane lattice distortions of supported two-dimensional layers is an alternative strategy to promote reactions, strategy that has been scarcely investigated, so far. Here, we show that the variable buckling geometry of a GdAu2 Moiré overlayer supported on the Au(111) surface exposes specific single-atom sites that trigger the selective dehydrogenation process of phthalocyanine (H2-PC) molecules. However, a reaction limit to about 1/3 of the monolayer is observed. This self-limited reaction can be explained considering the lattice mismatch between the substrate and the alloy layer, which leads to the previously reported outward displacement of distinct Gd sites.

Figure: Sketch of the dehydrogenation reaction of the H2-PC macrocycle according to specific corrugation of the GdAu2/Au(111) system. Only when the Gd atoms buckle towards the vacuum region can promote the bonding to the nitrogen atoms of the H2-PC molecule, triggering their dehydrogenation.

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.


November, 2018

Cooperative action for molecular debromination reaction on Cu(110)

The metal-catalyzed coupling of halobenzene derivatives leading to biaryls and larger carbon-based structures is a fundamental reaction in chemical synthesis. Here, the mechanistic steps of this reaction, named by Ullmann, have been investigated by means of scanning probe techniques and first-principle calculations. This work shows that the scission of the strongly bound bromine atoms requires the cooperative action of neighboring molecular precursors, in the studied system.

Figure: Concerted interaction of neighboring intermediate Br-Cu-DBT preceding the molecular debromination. a) Structural model of 2Br-2Cu-DBT dimer. b) The interaction of two self-assembled dimeric pairs promotes the detachment of the Br atoms (red circles between dimers). c) Constant energy maps demonstrating the absence of the electronic state associated to the Br in the inter-dimeric region.

The metal-catalyzed coupling of halobenzene derivatives leading to biaryls and larger carbon-based structures is undergoing a considerable revival in the last decade. The attention to the Ullmann cross-coupling reaction is justified by the reliable synthesis of extended polymers as polyphenyls, graphene structures of well-defined shape, or covalently assembled structures. The unprecedented opportunity to access the molecular functionality with increased mechanical stability and engineered electronic properties offered by this reaction is an essential advance in the realization of organic-based electronics.
The paradigmatic catalyzer of the classical Ullmann reaction is copper. Whereas there is general agreement that at a certain point of the reaction copper-coordinated molecular intermediates and copper halides, as side reaction products, form preceding the cross-coupling reaction, the mechanism responsible for the initial debromination step is still under debate. Indeed, latest investigations on copper surfaces have allowed visualizing the final reaction products as the intermediate Cu-coordinated phase and the aryl-aryl coupling-reaction. Still, the debromination mechanism, which is the rate-limiting step of the reaction, has been scarcely investigated and the question whether the formation of radicals, organocopper complexes or the oxidative addition process precedes the formation of the biaryl remains opens.

We have demonstrated that the interaction of a halogenated molecular precursor with a metal surface held at an opportune temperature may be an insufficient condition for its debromination reaction. The cooperative interaction with another molecule might be necessary for the dehalogenation process, which is preliminary to their cross-coupling reaction.
Specifically, the mechanistic steps of a molecular debromination have been visualized in the electron density of states by characterizing a prototypical molecule, namely 4,7-dibromobenzo[c]1,2,5-thiadiazole (2Br-BTD), deposited on a Cu(110) surface. Based on a step-by-step characterization of the adsorption of isolated single molecules, it is shown that the detachment of the halogen atom is subsequential to the formation of –C-Cu-Br complexes and the concerted action of neighboring complexes. Thus, some light has been shed on the debated role of the Cu atom in the debromination reaction.

J.Am.Chem.Soc 2018, 140, 15631

September, 2017

Self-texturizing electronic properties of a 2-dimensional GdAu2 layer on Au(111): the role of out-of-plane atomic displacement

We demonstrate the spontaneous texturing of the electronic and chemical properties of a 2-dimensional layer GdAu2 on Au(111) as a consequence of layer-buckling and out-of-plane atomic displacement.

Figure: 3D topographic image of the GdAu2/Au(111) forming a Moire' pattern and theoretical modeling of the variable out-of-plane atomic relaxation. As a consequence of this layer buckling, the electronic properties of the layer are patterned from the Fermi level to the Gd4f states.

One of the major issues of nanoscience and technology is the patterning of electronic properties and chemical reactivity of 2-dimensional layers at nanoscale. in this colaborative work, we have demonstrated that the electronic properties of a weakly interacting 2-dimensional layer can spontaneously texturize. The two fundamental parameters that allow achieving the desired periodic modulation are the variable adsorption stacking configurations and structural relaxation processes. Interfaces characterized by mismatched two-dimensional layers in weak interaction are expected to be an excellent playground to template the electronic structure and chemical reactivity. Notwithstanding that such geometrical configurations are commonly observed in Moire' superstructures, little attention has been paid on the effect of layer-buckling on the electronic properties. Here, we have investigated the induced periodic modulation of the electronic properties by scanning probe techniques and theoretical modeling to gain insights into the correlation between its electronic properties and layer planarity. Specifically, we have characterized a monolayer of a bi-metallic alloy, namely GdAu2 , obtained upon evaporation of gadolinium atoms on the annealed Au(111) surface. This results in a weakly interacting Moire' superstructure, characterized by a variable adsorption stacking configuration. This induces a layer-buckling at the GdAu2 /Au(111) interface and modulates the electronic properties and chemical reactivity of the system. The layer/substrate coupling and the induced out-of-plane displacement of the Gd atoms is sufficient to locally open an energy gap of about 0.5 eV at the Fermi level in an otherwise metallic layer. Additionally, this buckling changes the character of the hybridized Gd-pd and Au-sp states and controls the energy of the occupied and unoccupied Gd 4f multiplet proportionally to the lattice distortion. We demonstrate that the resulting template shows different chemical reactivity, which may find important applications.

Nanoscale 2017,9, 17342-17348