Joule Heating in Single-Molecule Point Contacts Studied by Tip-Enhanced Raman Spectroscopy.

Heating and cooling in current-carrying molecular junctions is a crucial issue in molecular electronics. The microscopic mechanism involves complex factors such as energy inputs, molecular properties, electrode materials, and molecule-electrode coupling. To gain an in-depth understanding, it is a desired experiment to assess vibrational population that represents the energy distribution stored within the molecule. Here, we demonstrate the direct observation of vibrational heating in a single C60 molecule by means of tip-enhanced Raman spectroscopy (TERS). The heating of respective vibrational modes is monitored by anti-Stokes Raman scattering in the TERS spectra. The precise control of the gap distance in the single-molecule junction allows us to reveal a qualitatively different heating mechanism in distinct electron transport regimes, namely, the tunneling and single-molecule point contact (SMPC) regimes. Strong Joule heating via inelastic electron-vibration scattering occurs in the SMPC regime, whereas optical heating is predominant in the tunneling regime. The strong Joule heating at the SMPC also leads to a pronounced red shift of the Raman peak position and line width broadening. Furthermore, by examining the SMPC with several types of contact surfaces, we show that the heating efficiency is related to the current density at the SMPC and the vibrational dissipation channels into the electrode.

Resolving Atomic-Scale Defects in Conjugated Polymers On-Surfaces.

Atomic scale defects significantly affect the mechanical, electronic, and optical properties of π-conjugated polymers. Here, isolated atomic-scale defects are deliberately introduced into a prototypical anthracene-ethynylene π-conjugated polymer, and its local density of states is carefully examined on the atomic scale to show how individual defects modify the inherent electronic and magnetic properties of this one-dimensional systems. Scanning tunneling and atomic force microscopy experiments, supplemented with density functional theory calculations, reveal the existence of a sharp electronic resonance at the Fermi energy around certain defects, which is associated with the formation of a local magnetic moment accompanied by substantial mitigation of the mobility of charge carriers. While defects in traditionally synthesized polymers lead to arbitrary conformations, the presented results clearly reflect the preferential formation of low dimensional defects at specific polymer sites, which may introduce the possibility of engineering macroscopic defects in surface-synthesized conjugated polymers.

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation.

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C60-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Electrically Tunable Reactivity of Substrate-Supported Cobalt Oxide Nanocrystals.

First-row transition metal oxides are promising materials for catalyzing the oxygen evolution reaction. Surface sensitive techniques provide a unique perspective allowing the study of the structure, adsorption sites, and reactivity of catalysts at the atomic scale, which furnishes rationalization and improves the design of highly efficient catalytic materials. Here, a scanning probe microscopy study complemented by density functional theory on the structural and electronic properties of CoO nanoislands grown on Au(111) is reported. Two distinct phases are observed: The most extended displays a Moiré pattern (α-region), while the less abundant is 1Co:1Au coincidental (ß-region). As a result of the surface registry, in the ß-region the oxide adlayer is compressed by 9%, increasing the unoccupied local density of states and enhancing the selective water adsorption at low temperature through a cobalt inversion mechanism. Tip-induced voltage pulses irreversibly transform α- into ß-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.

On-surface synthesis of organocopper metallacycles through activation of inner diacetylene moieties.

The design of organometallic complexes is at the heart of modern organic chemistry and catalysis. Recently, on-surface synthesis has emerged as a disruptive paradigm to design previously precluded compounds and nanomaterials. Despite these advances, the field of organometallic chemistry on surfaces is still at its infancy. Here, we introduce a protocol to activate the inner diacetylene moieties of a molecular precursor by copper surface adatoms affording the formation of unprecedented organocopper metallacycles on Cu(111). The chemical structure of the resulting complexes is characterized by scanning probe microscopy and X-ray photoelectron spectroscopy, being complemented by density functional theory calculations and scanning probe microscopy simulations. Our results pave avenues to the engineering of organometallic compounds and steer the development of polyyne chemistry on surfaces.

Tuning the Magnetic Anisotropy of Lanthanides on a Metal Substrate by Metal-Organic Coordination.

Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal-organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal-organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal-organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.

Dysprosium-directed metallosupramolecular network on graphene/Ir(111).

The interest in exploiting the unique properties of lanthanides has led to the recent design of two-dimensional coordination networks incorporating f-block elements on metallic surfaces. In order to take this field to the next step of progression, it is necessary to electronically decouple these two-dimensional architectures from the metallic surface underneath. As a first step in this direction, we report the formation of dysprosium-directed metal-organic networks employing three-fold ligands as molecular linkers equipped with terminal carbonitrile functional groups on weakly interacting substrates such as Au(111) and graphene/Ir(111). We observe on both substrates identical quasi-hexagonal Dy-carbonitrile coordination networks based on majority five-fold nodes. Our findings provide perspectives for the formation of lanthanide coordination networks on graphene and related sp2 materials grown on metals.

On-surface synthesis of doubly-linked one-dimensional pentacene ladder polymers.

On-surface synthesis has recently become an essential approach toward the formation of carbon-based nanostructures. Special emphasis is set on the synthesis of π-conjugated polymers taking into consideration their relevance and potential in organic electronics, optoelectronics and spintronics. Here, we report the on-surface synthesis of conjugated ladder polymers consisting of pentacene units doubly-linked via ethynylene-like bonds on the Au(111) surface under ultra-high vacuum conditions. To this aim, we have sublimed pentacene-like precursors equipped with four :CBr2 functional groups to steer the desired reaction upon annealing on the surface. The atomically precise structure of the obtained polymers has been unambiguously characterized via low-temperature scanning tunneling microscopy and non-contact atomic force microscopy. In addition, scanning tunneling spectroscopy complemented with density-functional theory calculations reveal the narrow bandgap of the polymer. Our results provide potential for the synthesis of π-conjugated polymers with prospects in functional carbon-based nanomaterials that exploit multiple connections between molecular backbones.

Tailoring π-conjugation and vibrational modes to steer on-surface synthesis of pentalene-bridged ladder polymers.

The development of synthetic strategies to engineer π-conjugated polymers is of paramount importance in modern chemistry and materials science. Here we introduce a synthetic protocol based on the search for specific vibrational modes through an appropriate tailoring of the π-conjugation of the precursors, in order to increase the attempt frequency of a chemical reaction. First, we design a 1D π-conjugated polymer on Au(111), which is based on bisanthene monomers linked by cumulene bridges that tune specific vibrational modes. In a second step, upon further annealing, such vibrational modes steer the twofold cyclization reaction between adjacent bisanthene moieties, which gives rise to a long pentalene-bridged conjugated ladder polymer featuring a low bandgap. In addition, high resolution atomic force microscopy allows us to identify by atomistic insights the resonance form of the polymer, thus confirming the validity of the Glidewell and Lloyd´s rules for aromaticity. This on-surface synthetic strategy may stimulate exploiting previously precluded reactions towards π-conjugated polymers with specific structures and properties.

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation.

Tip-enhanced Raman scattering (TERS) in ångström-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to atomic point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which determines the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not observed for the chemically inert NaCl.

Tailoring topological order and π-conjugation to engineer quasi-metallic polymers.

Topological band theory predicts that a topological electronic phase transition between two insulators must proceed via closure of the electronic gap. Here, we use this transition to circumvent the instability of metallic phases in π-conjugated one-dimensional (1D) polymers. By means of density functional theory, tight-binding and GW calculations, we predict polymers near the topological transition from a trivial to a non-trivial quantum phase. We then use on-surface synthesis with custom-designed precursors to make polymers consisting of 1D linearly bridged acene moieties, which feature narrow bandgaps and in-gap zero-energy edge states when in the topologically non-trivial phase close to the topological transition point. We also reveal the fundamental connection between topological classes and resonant forms of 1D π-conjugated polymers.

On-Surface Synthesis of Ethynylene-Bridged Anthracene Polymers.

Engineering low-band-gap π-conjugated polymers is a growing area in basic and applied research. The main synthetic challenge lies in the solubility of the starting materials, which precludes advancements in the field. Here, we report an on-surface synthesis protocol to overcome such difficulties and produce poly(p-anthracene ethynylene) molecular wires on Au(111). To this aim, a quinoid anthracene precursor with =CBr2 moieties is deposited and annealed to 400 K, resulting in anthracene-based polymers. High-resolution nc-AFM measurements confirm the nature of the ethynylene-bridge bond between the anthracene moieties. Theoretical simulations illustrate the mechanism of the chemical reaction, highlighting three major steps: dehalogenation, diffusion of surface-stabilized carbenes, and homocoupling, which enables the formation of an ethynylene bridge. Our results introduce a novel chemical protocol to design π-conjugated polymers based on oligoacene precursors and pave new avenues for advancing the emerging field of on-surface synthesis.

Long-Range Orientational Self-Assembly, Spatially Controlled Deprotonation, and Off-Centered Metalation of an Expanded Porphyrin.

Expanded porphyrins are large-cavity macrocycles with enormous potential in coordination chemistry, anion sensing, photodynamic therapy, and optoelectronics. In the last two decades, the surface science community has assessed the physicochemical properties of tetrapyrrolic-like macrocycles. However, to date, the sublimation, self-assembly and atomistic insights of expanded porphyrins on surfaces have remained elusive. Here, we show the self-assembly on Au(111) of an expanded aza-porphyrin, namely, an "expanded hemiporphyrazine", through a unique growth mechanism based on long-range orientational self-assembly. Furthermore, a spatially controlled "writing" protocol on such self-assembled architecture is presented based on the STM tip-induced deprotonation of the inner protons of individual macrocycles. Finally, the capability of these surface-confined macrocycles to host lanthanide elements is assessed, introducing a novel off-centered coordination motif. The presented findings represent a milestone in the fields of porphyrinoid chemistry and surface science, revealing a great potential for novel surface patterning, opening new avenues for molecular level information storage, and boosting the emerging field of surface-confined coordination chemistry involving f-block elements.

One-Dimensionally Disordered Chiral Sorting by Racemic Tiling in a Surface-Confined Supramolecular Assembly of Achiral Tectons.

The aggregation of (pro)chiral/achiral molecules into crystalline structures at interfaces forms conglomerates, racemates, and solid solutions, comparable to known bulk phases. Scanning tunneling microscopy and Monte Carlo simulations were employed to uncover a distinct racemic phase, expressing 1D disordered chiral sorting through random tiling in surface-confined supramolecularly assembled achiral 4,4''-diethynyl-1,1':4',1''-terphenyl molecules. The configurational entropy of the 1D disordered racemic tiling phase was verified by analytical modeling, and found to lie between that of a perfectly ordered 2D racemate and a racemic solid solution.

Thermal selectivity of intermolecular versus intramolecular reactions on surfaces.

On-surface synthesis is a promising strategy for engineering heteroatomic covalent nanoarchitectures with prospects in electronics, optoelectronics and photovoltaics. Here we report the thermal tunability of reaction pathways of a molecular precursor in order to select intramolecular versus intermolecular reactions, yielding monomeric or polymeric phthalocyanine derivatives, respectively. Deposition of tetra-aza-porphyrin species bearing ethyl termini on Au(111) held at room temperature results in a close-packed assembly. Upon annealing from room temperature to 275 °C, the molecular precursors undergo a series of covalent reactions via their ethyl termini, giving rise to phthalocyanine tapes. However, deposition of the tetra-aza-porphyrin derivatives on Au(111) held at 300 °C results in the formation and self-assembly of monomeric phthalocyanines. A systematic scanning tunnelling microscopy study of reaction intermediates, combined with density functional calculations, suggests a [2+2] cycloaddition as responsible for the initial linkage between molecular precursors, whereas the monomeric reaction is rationalized as an electrocyclic ring closure.

Surface-Supported Robust 2D Lanthanide-Carboxylate Coordination Networks.

Lanthanide-based metal-organic compounds and architectures are promising systems for sensing, heterogeneous catalysis, photoluminescence, and magnetism. Herein, the fabrication of interfacial 2D lanthanide-carboxylate networks is introduced. This study combines low- and variable-temperature scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) experiments, and density functional theory (DFT) calculations addressing their design and electronic properties. The bonding of ditopic linear linkers to Gd centers on a Cu(111) surface gives rise to extended nanoporous grids, comprising mononuclear nodes featuring eightfold lateral coordination. XPS and DFT elucidate the nature of the bond, indicating ionic characteristics, which is also manifest in appreciable thermal stability. This study introduces a new generation of robust low-dimensional metallosupramolecular systems incorporating the functionalities of the f-block elements.

Morphological self-assembly of enantiopure allenes for upstanding chiral architectures at interfaces.

Chiroptically active allenes are employed for the construction of surface-confined nanostructures. Morphological complementarity between the homochiral units leads to self-assembly of two highly-ordered, upstanding, diastereomeric architectures. The novel, intertwined self-assembled layer structures feature reactive terminal alkynes for further functionalization and carry potential for widespread applications exploiting chiroptical amplification.

Synthesis of extended graphdiyne wires by vicinal surface templating.

Surface-assisted covalent synthesis currently evolves into an important approach for the fabrication of functional nanostructures at interfaces. Here, we employ scanning tunneling microscopy to investigate the homocoupling reaction of linear, terminal alkyne-functionalized polyphenylene building-blocks on noble metal surfaces under ultrahigh vacuum. On the flat Ag(111) surface, thermal activation triggers a variety of side-reactions resulting in irregularly branched polymeric networks. Upon alignment along the step-edges of the Ag(877) vicinal surface drastically improves the chemoselectivity of the linking process permitting the controlled synthesis of extended-graphdiyne wires with lengths reaching 30 nm. The ideal hydrocarbon scaffold is characterized by density functional theory as a 1D, direct band gap semiconductor material with both HOMO and LUMO-derived bands promisingly isolated within the electronic structure. The templating approach should be applicable to related organic precursors and different reaction schemes thus bears general promise for the engineering of novel low-dimensional carbon-based materials.