Probing dynamic covalent chemistry in a 2D boroxine framework by in situ near-ambient pressure X-ray photoelectron spectroscopy.

Dynamic covalent chemistry is a powerful approach to design covalent organic frameworks, where high crystallinity is achieved through reversible bond formation. Here, we exploit near-ambient pressure X-ray photoelectron spectroscopy to elucidate the reversible formation of a two-dimensional boroxine framework. By in situ mapping the pressure-temperature parameter space, we identify the regions where the rates of the condensation and hydrolysis reactions become dominant, being the key to enable the thermodynamically controlled growth of crystalline frameworks.

In Situ Observation of C-C Coupling and Step Poisoning During the Growth of Hydrocarbon Chains on Ni(111).

The synthesis of high-value fuels and plastics starting from small hydrocarbon molecules plays a central role in the current transition towards renewable energy. However, the detailed mechanisms driving the growth of hydrocarbon chains remain to a large extent unknown. Here we investigated the formation of hydrocarbon chains resulting from acetylene polymerization on a Ni(111) model catalyst surface. Exploiting X-ray photoelectron spectroscopy up to near-ambient pressures, the intermediate species and reaction products have been identified. Complementary in situ scanning tunneling microscopy observations shed light onto the C-C coupling mechanism. While the step edges of the metal catalyst are commonly assumed to be the active sites for the C-C coupling, we showed that the polymerization occurs instead on the flat terraces of the metallic surface.

Exploiting Cooperative Catalysis for the On-Surface Synthesis of Linear Heteroaromatic Polymers via Selective C-H Activation.

Regiospecific C-H activation is a promising approach to achieve extended polymers with tailored structures. While a recent on-surface synthetic approach has enabled regioselective homocoupling of heteroaromatic molecules, only small oligomers have been achieved. Herein, selective C-H activation for dehydrogenative C-C couplings of hexaazatriphenylene by Scholl reaction is reported for the first time. By combining low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we revealed the formation of one-dimensional polymers with a double-chain structure. The details of the growth process are rationalized by density functional theory (DFT) calculations, pointing out a cooperative catalytic action of Na and Ag adatoms in steering the C-H selectivity for the polymerization.

Imaging Charge Localization in a Conjugated Oligophenylene.

Polaron formation in conjugated polymers has a major impact on their optical and electronic properties. In polyphenylene, the molecular conformation is determined by a delicate interplay between electron delocalization and steric effects. Injection of excess charges is expected to increase the degree of conjugation, leading to structural distortions of the chain. Here we investigated at the single-molecule level the role of an excess charge in an individual oligophenylene deposited on sodium chloride films. By combining sub-molecular-resolved atomic force microscopy with redox-state-selective orbital imaging, we characterize both structural and electronical changes occurring upon hole injection. While the neutral molecule exhibits a delocalized frontier orbital, for the cationic radical the excess charge is observed to localize, inducing a partial planarization of the molecule. These results provide direct evidence for self-trapping of the excess charge in oligophenylenes, shedding light on the interplay of charge localization and structural distortion.

Manipulating and Probing the Distribution of Excess Electrons in an Electrically Isolated Self-Assembled Molecular Structure.

Exploiting single electrical charges and their mutual interactions for computation has been proposed as a concept for future nanoelectronics. Controlling and probing charge transfer in electrically isolated atomic-scale structures are fundamental to push its experimental realization. Here, we controllably inject individual excess electrons and study their distribution in a self-assembled structure supported on a nonconductive substrate. The self-assembly ensures structural order down to the atomic scale. Depending on the charge state of the molecular assembly, intermolecular electron hopping and specific electron distributions have been resolved by atomic force microscopy, clarifying the charge-transfer pathways in the tunnel-coupled structure. When mutual charge interactions were exploited, control over specific charge distributions in the self-assembled structure has been achieved with single-molecule precision, paving the way toward the design of data processing platforms based on molecular nanostructures.

Charge-Induced Structural Changes in a Single Molecule Investigated by Atomic Force Microscopy.

Intramolecular structural relaxations occurring upon electron transfer are crucial in determining the rate of redox reactions. Here, we demonstrate that subangstrom structural changes occurring upon single-electron charging can be quantified by means of atomically resolved atomic force microscopy (AFM) for the case of single copper(II)phthalocyanine (CuPc) molecules deposited on an ultrathin NaCl film. Imaging the molecule in distinct charge states (neutral and anionic) reveals characteristic differences in the AFM contrast. In comparison to density functional theory simulations these changes in contrast can be directly related to relaxations of the molecule's geometric structure upon charging. The dominant contribution arises from a nonhomogeneous vertical relaxation of the molecule, caused by a change in the electrostatic interaction with the surface.

Accessing a Charged Intermediate State Involved in the Excitation of Single Molecules.

Intermediate states are elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces.

The new FAST module: A portable and transparent add-on module for time-resolved investigations with commercial scanning probe microscopes.

Time resolution is one of the most severe limitations of scanning probe microscopies (SPMs), since the typical image acquisition times are in the order of several seconds or even few minutes. As a consequence, the characterization of dynamical processes occurring at surfaces (e.g. surface diffusion, film growth, self-assembly and chemical reactions) cannot be thoroughly addressed by conventional SPMs. To overcome this limitation, several years ago we developed a first prototype of the FAST module, an add-on instrument capable of driving a commercial scanning tunneling microscope (STM) at and beyond video rate frequencies. Here we report on a fully redesigned version of the FAST module, featuring improved performance and user experience, which can be used both with STMs and atomic force microscopes (AFMs), and offers additional capabilities such as an atom tracking mode. All the new features of the FAST module, including portability between different commercial instruments, are described in detail and practically demonstrated.

Resolving the Unpaired-Electron Orbital Distribution in a Stable Organic Radical by Kondo Resonance Mapping.

The adsorption geometry and the electronic structure of a Blatter radical derivative on a gold surface were investigated by a combination of high-resolution noncontact atomic force microscopy and scanning tunneling microscopy. While the hybridization with the substrate hinders direct access to the molecular states, we show that the unpaired-electron orbital can be probed with Ångström resolution by mapping the spatial distribution of the Kondo resonance. The Blatter derivative features a peculiar delocalization of the unpaired-electron orbital over some but not all moieties of the molecule, such that the Kondo signature can be related to the spatial fingerprint of the orbital. We observe a direct correspondence between these two quantities, including a pronounced nodal plane structure. Finally, we demonstrate that the spatial signature of the Kondo resonance also persists upon noncovalent dimerization of molecules.

Doping of epitaxial graphene by direct incorporation of nickel adatoms.

Direct incorporation of Ni adatoms during graphene growth on Ni(111) is evidenced by scanning tunneling microscopy. The structure and energetics of the observed defects is thoroughly characterized at the atomic level on the basis of density functional theory calculations. Our results show the feasibility of a simple scalable method, which could be potentially used for the realization of macroscopic practical devices, to dope graphene with a transition metal. The method exploits the kinetics of the growth process for the incorporation of Ni adatoms in the graphene network.

The Environment-Dependent Behavior of the Blatter Radical at the Metal-Molecule Interface.

Stable organic radicals have potential applications for building organic spintronic devices. To fulfill this potential, the interface between organic radicals and metal electrodes must be well characterized. Here, through a combined effort that includes synthesis, scanning tunneling microscopy, X-ray spectroscopy, and single-molecule conductance measurements, we comprehensively probe the electronic interaction between gold metal electrodes and a benchtop stable radical-the Blatter radical. We find that despite its open-shell character and having a half-filled orbital close to the Fermi level, the radical is stable on a gold substrate under ultrahigh vacuum. We observe a Kondo resonance arising from the radical and spectroscopic signatures of its half-filled orbitals. By contrast, in solution-based single-molecule conductance measurements, the radical character is lost through oxidation with charge transfer occurring from the molecule to metal. Our experiments show that the stability of radical states can be very sensitive to the environment around the molecule.

Mapping orbital changes upon electron transfer with tunnelling microscopy on insulators.

Electron transfer plays a crucial part in many chemical reactions1,2, including photosynthesis, combustion and corrosion. But even though redox-state transitions change the electronic structure of the molecules involved, mapping these changes at the single-molecule level is challenging. Scanning tunnelling microscopy provides insights into the orbital structure3 of single molecules and their interactions4,5, but requires the use of a conductive substrate that keeps molecules in a given charge state and thereby suppresses redox-state transitions. Atomic force microscopy can be used on insulating substrates to obtain structural6 and electrostatic7,8 information but does not generally access electronic states. Here we show that when synchronizing voltage pulses that steer electron tunnelling between a conductive atomic force microscope tip and a substrate with the oscillation of the tip, we can perform tunnelling experiments on non-conductive substrates and thereby map the orbital structure of isolated molecules as a function of their redox state. This allows us to resolve previously inaccessible electronic transitions in space and energy and to visualize the effects of electron transfer and polaron formation on individual molecular orbitals. We anticipate that our approach will prove useful for the investigation of complex redox reactions and charging-related phenomena with sub-ångström resolution.

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces.

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

Real-time imaging of adatom-promoted graphene growth on nickel.

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

Imaging on-surface hierarchical assembly of chiral supramolecular networks.

The bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,10'-dibromo-9,9'-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling.

Crystallization of a Two-Dimensional Hydrogen-Bonded Molecular Assembly: Evolution of the Local Structure Resolved by Atomic Force Microscopy.

Structures of the aromatic N-heterocyclic hexaazatriphenylene (HAT) molecular synthon obtained by surface-assisted self-assembly were analyzed with sub-Å resolution by means of noncontact atomic force microscopy (nc-AFM), both in the kinetically trapped amorphous state and in the thermodynamically stable crystalline phase. These results reveal how the crystallization governs the length scale of the network order for non-flexible molecular species without affecting the local bonding schemes. The capability of nc-AFM to accurately resolve structural relaxations will be highly relevant for the characterization of vitreous two-dimensional supramolecular materials.

Activation of Cu(111) surface by decomposition into nanoclusters driven by CO adsorption.

The (111) surface of copper (Cu), its most compact and lowest energy surface, became unstable when exposed to carbon monoxide (CO) gas. Scanning tunneling microscopy revealed that at room temperature in the pressure range 0.1 to 100 Torr, the surface decomposed into clusters decorated by CO molecules attached to edge atoms. Between 0.2 and a few Torr CO, the clusters became mobile in the scale of minutes. Density functional theory showed that the energy gain from CO binding to low-coordinated Cu atoms and the weakening of binding of Cu to neighboring atoms help drive this process. Particularly for softer metals, the optimal balance of these two effects occurs near reaction conditions. Cluster formation activated the surface for water dissociation, an important step in the water-gas shift reaction.

Switchable graphene-substrate coupling through formation/dissolution of an intercalated Ni-carbide layer.

Control over the film-substrate interaction is key to the exploitation of graphene's unique electronic properties. Typically, a buffer layer is irreversibly intercalated "from above" to ensure decoupling. For graphene/Ni(111) we instead tune the film interaction "from below". By temperature controlling the formation/dissolution of a carbide layer under rotated graphene domains, we reversibly switch graphene's electronic structure from semi-metallic to metallic. Our results are relevant for the design of controllable graphene/metal interfaces in functional devices.

Nanobubbles at GPa Pressure under Graphene.

We provide direct evidence that irradiation of a graphene membrane on Ir with low-energy Ar ions induces formation of solid noble-gas nanobubbles. Their size can be controlled by thermal treatment, reaching tens of nanometers laterally and height of 1.5 nm upon annealing at 1080 °C. Ab initio calculations show that Ar nanobubbles are subject to pressures reaching tens of GPa, their formation being driven by minimization of the energy cost of film distortion and loss of adhesion.

Temperature-driven changes of the graphene edge structure on Ni(111): substrate vs hydrogen passivation.

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.