Porous Nanographenes, Graphene Nanoribbons, and Nanoporous Graphene Selectively Synthesized from the Same Molecular Precursor.

We demonstrate a family of molecular precursors based on 7,10-dibromo-triphenylenes that can selectively produce different varieties of atomically precise porous graphene nanomaterials through the use of different synthetic environments. Upon Yamamoto polymerization of these molecules in solution, the free rotations of the triphenylene units around the C-C bonds result in the formation of cyclotrimers in high yields. In contrast, in on-surface polymerization of the same molecules on Au(111) these rotations are impeded, and the coupling proceeds toward the formation of long polymer chains. These chains can then be converted to porous graphene nanoribbons (pGNRs) by annealing. Correspondingly, the solution-synthesized cyclotrimers can also be deposited onto Au(111) and converted into porous nanographenes (pNGs) via thermal treatment. Thus, both processes start with the same molecular precursor and end with a porous graphene nanomaterial on Au(111), but the type of product, pNG or pGNR, depends on the specific coupling approach. We also produced extended nanoporous graphenes (NPGs) through the lateral fusion of highly aligned pGNRs on Au(111) that were grown at high coverage. The pNGs can also be synthesized directly in solution by Scholl oxidative cyclodehydrogenation of cyclotrimers. We demonstrate the generality of this approach by synthesizing two varieties of 7,10-dibromo-triphenylenes that selectively produced six nanoporous products with different dimensionalities. The basic 7,10-dibromo-triphenylene monomer is amenable to structural modifications, potentially providing access to many new porous graphene nanomaterials. We show that by constructing different porous structures from the same building blocks, it is possible to tune the energy band gap in a wide range.

Tunable Magnetic Coupling in Graphene Nanoribbon Quantum Dots.

Carbon-based quantum dots (QDs) enable flexible manipulation of electronic behavior at the nanoscale, but controlling their magnetic properties requires atomically precise structural control. While magnetism is observed in organic molecules and graphene nanoribbons (GNRs), GNR precursors enabling bottom-up fabrication of QDs with various spin ground states have not yet been reported. Here the development of a new GNR precursor that results in magnetic QD structures embedded in semiconducting GNRs is reported. Inserting one such molecule into the GNR backbone and graphitizing it results in a QD region hosting one unpaired electron. QDs composed of two precursor molecules exhibit nonmagnetic, antiferromagnetic, or antiferromagnetic ground states, depending on the structural details that determine the coupling behavior of the spins originating from each molecule. The synthesis of these QDs and the emergence of localized states are demonstrated through high-resolution atomic force microscopy (HR-AFM), scanning tunneling microscopy (STM) imaging, and spectroscopy, and the relationship between QD atomic structure and magnetic properties is uncovered. GNR QDs provide a useful platform for controlling the spin-degree of freedom in carbon-based nanostructures.

Autonomous Molecular Structure Imaging with High-Resolution Atomic Force Microscopy for Molecular Mixture Discovery.

Due to its single-molecule sensitivity, high-resolution atomic force microscopy (HR-AFM) has proved to be a valuable and uniquely advantageous tool to study complex molecular mixtures, which hold promise for developing clean energy and achieving environmental sustainability. However, significant challenges remain to achieve the full potential of the sophisticated and time-consuming experiments. Automation combined with machine learning (ML) and artificial intelligence (AI) is key to overcoming these challenges. Here we present Auto-HR-AFM, an AI tool to automatically collect HR-AFM images of petroleum-based mixtures. We trained an instance segmentation model to teach Auto-HR-AFM how to recognize features in HR-AFM images. Auto-HR-AFM then uses that information to optimize the imaging by adjusting the probe-molecule distance for each molecule in the run. Auto-HR-AFM is the initial tool that will lead to fully automated scanning probe microscopy (SPM) experiments, from start to finish. This automation will allow SPM to become a mainstream characterization technique for complex mixtures, an otherwise unattainable target.

Formation and Evolution of Metallocene Single-Molecule Circuits with Direct Gold-π Links.

Single-molecule circuits with group 8 metallocenes are formed without additional linker groups in scanning tunneling microscope-based break junction (STMBJ) measurements at cryogenic and room-temperature conditions with gold (Au) electrodes. We investigate the nature of this direct gold-π binding motif and its effect on molecular conductance and persistence characteristics during junction evolution. The measurement technique under cryogenic conditions tracks molecular plateaus through the full cycle of extension and compression. Analysis reveals that junction persistence when the metal electrodes are pushed together correlates with whether electrodes are locally sharp or blunt, suggesting distinct scenarios for metallocene junction formation and evolution. The top and bottom surfaces of the "barrel"-shaped metallocenes present the electron-rich π system of cyclopentadienyl rings, which interacts with the gold electrodes in two distinct ways. An undercoordinated gold atom on a sharp tip forms a donor-acceptor bond to a specific carbon atom in the ring. However, a small, flat patch on a dull tip can bind more strongly to the ring as a whole through van der Waals interactions. Density functional theory (DFT)-based calculations of model electrode structures provide an atomic-scale picture of these scenarios, demonstrating the role of these bonding motifs during junction evolution and showing that the conductance is relatively independent of tip atomic-scale structure. The nonspecific interaction of the cyclopentadienyl rings with the electrodes enables extended conductance plateaus, a mechanism distinct from that identified for the more commonly studied, rod-shaped organic molecular wires.

Cyclopropenylidenes as Strong Carbene Anchoring Groups on Au Surfaces.

The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC-CO2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.

On-Surface Synthesis and Spectroscopic Characterization of Laterally Extended Chevron Graphene Nanoribbons.

We report the on-surface synthesis and spectroscopic study of laterally extended chevron graphene nanoribbons (GNRs) and compare them with the established chevron GNRs, emphasizing the consistency of bandgap reduction of semiconducting GNRs with increased width. The laterally extended chevron GNRs grown on Au(111) exhibit a bandgap of about 2.2 eV, which is considerably smaller than the values reported for chevron GNRs in similar studies.

Self-assembling of formic acid on the partially oxidized p(2 × 1) Cu(110) surface reconstruction at low coverages.

Carbon dioxide (CO2) reduction for synthetic fuel generation could be an integral part of a sustainable energy future. Copper (Cu) is the leading electrocatalyst for CO2 reduction to produce multiple C-containing products such as C1 and C2 hydrocarbons and oxygenates. Understanding the mechanisms leading to their production could help optimize these pathways further. Adsorption studies of the many possible intermediates on well-characterized surfaces are crucial to elucidating these mechanisms. In this work, we explore the adsorption configurations of formic acid (HCOOH) on the surface of the partially oxidized p(2 × 1) reconstruction of the Cu(110) surface, using low-temperature scanning tunneling and atomic force microscopy, in conjunction with density functional theory modeling. We find that HCOOH adsorbs favorably on the CuO chain comprising the reconstruction. The adsorption interactions involve dative bonding of the carbonyl O to the oxidized Cu and hydrogen bonding of the OH group to the surface O or to an adjacently adsorbed HCOOH molecule. Cooperative adsorption of the molecules occurs, forming two- to three-molecule-long oligomer chains, facilitated by intermolecular hydrogen bonding and mutual polarization of the CuO acid-base adsorption sites.

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials.

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

Characterization of one-dimensional molecular chains of 4,4'-biphenyl diisocyanide on Au(111) by scanning tunneling microscopy.

The morphology and electronic structure of vapor deposited 4,4'-biphenyldiisocyanide (BPDI) on a Au(111) surface were investigated using variable-temperature scanning tunneling microscopy. When deposited at room temperature, BPDI molecules form one-dimensional molecular chains similar to that recently observed for the structurally related 1,4-phenyl diisocyanide (PDI). Compared to PDI, the longer periodicity for the BPDI molecular chains is consistent with the addition of a second phenyl ring and supports a structural model in which the BPDI molecules lie parallel to the surface and interconnected by Au-adatoms. The molecular chains are mostly aligned along the 11̄0 direction of the Au(111) substrate, but exhibit frequent changes in angle that are consistent with directions between fcc and hcp three-fold hollow sites. Dispersion-corrected density functional theory calculations for one-dimensional chains of BPDI molecules bound end-to-end via their isocyanide groups to Au-adatoms reproduce the observed periodicity of the chains and show that this morphology is energetically favored over upright binding with one free -NC group. The spatially resolved conductance (dI/dV) map for BPDI on Au(111) exhibits a feature centered at -0.67 eV below the Fermi level which are delocalized along the chain with maxima at the Au-adatom and biphenyl positions. This occupied resonant feature is close to that previously observed for the PDI in both photoemission and conductance measurements and is attributed to an occupied interfacial state resulting from BPDI-Au interactions.

Hydrogen-bonded cyclic water clusters nucleated on an oxide surface.

We report the observation and molecular-scale scanning probe electronic structure (dI/dV) mapping of hydrogen-bonded cyclic water clusters nucleated on an oxide surface. The measurements are made on a new type of cyclic water cluster that is characterized by simultaneous and cooperative bonding interactions among molecules as well as with both metal and oxygen sites of an oxide surface. Density functional theory + U + D calculations confirm the stability of these clusters and are used to discuss other potential water-oxide bonding scenarios. The calculations show that the spatial distributions of electronic states in the system are similar in character to those of the lowest unoccupied molecular orbitals of hydrogen-bonded water molecules. On the partially oxidized Cu(111) investigated here, experiment and theory together suggest that Cu vacancies in the growing islands of cuprous oxide inhibit water adsorption in the centers of the islands (which have reached thermodynamic equilibrium). A stoichiometric, less stable cuprous oxide likely exists at island edges (the growth front) and selectively binds these water clusters.

How Precisely Can Individual Molecules Be Analyzed? A Case Study on Locally Quantifying Forces and Energies Using Scanning Probe Microscopy.

Recent advances in scanning probe microscopy methodology have enabled the measurement of tip-sample interactions with picometer accuracy in all three spatial dimensions, thereby providing a detailed site-specific and distance-dependent picture of the related properties. This paper explores the degree of detail and accuracy that can be achieved in locally quantifying probe-molecule interaction forces and energies for adsorbed molecules. Toward this end, cobalt phthalocyanine (CoPc), a promising CO2 reduction catalyst, was studied on Ag(111) as a model system using low-temperature, ultrahigh vacuum noncontact atomic force microscopy. Data were recorded as a function of distance from the surface, from which detailed three-dimensional maps of the molecule's interaction with the tip for normal and lateral forces as well as the tip-molecule interaction potential were constructed. The data were collected with a CO molecule at the tip apex, which enabled a detailed visualization of the atomic structure. Determination of the tip-substrate interaction as a function of distance allowed isolation of the molecule-tip interactions; when analyzing these in terms of a Lennard-Jones-type potential, the atomically resolved equilibrium interaction energies between the CO tethered to the tip and the CoPc molecule could be recovered. Interaction energies peaked at less than 160 meV, indicating a physisorption interaction. As expected, the interaction was weakest at the aromatic hydrogens around the periphery of the molecule and strongest surrounding the metal center. The interaction, however, did not peak directly above the Co atom but rather in pockets surrounding it.

Direct Observation of Twin van der Waals Molecular Chains.

The van der Waals (vdW) assemblies are the most common structures of materials. However, direct mapping of intermolecular electron clouds of a vdW assembly has never been obtained, even though the intramolecular electron clouds were visualized by atomic-resolution techniques. In this report, we unprecedentedly mapped the intermolecular electron cloud of the assemblies of ethanol molecules via ethyl groups with high-resolution atomic force microscopy and scanning tunneling microscopy at 5 K, leading to the first visualization of vdW molecular chains, in which ethanol molecules assemble into twin vdW molecular chains in a reverse parallel configuration on the Ag(111) plane. Furthermore, spontaneous order-disorder transitions in the chain were dynamically observed, suggesting its unusual properties different from those of 2D vdW materials. These findings provide an "eye" to see the atomic world of vdW materials.

Micrometre-scale single-crystalline borophene on a square-lattice Cu(100) surface.

Borophene, a crystalline monolayer boron sheet, has been predicted to adopt a variety of structures-owing to its high polymorphism-that may possess physical properties that could serve in flexible electronics, energy storage and catalysis. Several borophene polymorphs have been synthesized on noble metal surfaces but for device fabrication larger single-crystal domains are typically needed with, ideally, weak borophene-substrate interactions. Here we report the synthesis of borophene on a square-lattice Cu(100) surface and show that incommensurate coordination reduces the borophene-substrate interactions and also leads to a borophene polymorph different from those previous reported. Micrometre-scale single-crystal domains formed as isolated faceted islands or merged together to achieve full monolayer coverage. The crystal structure of this phase has ten boron atoms and two hexagonal vacancies in its unit cell. First-principles calculations indicate that charge transfer, rather than covalent bonding, binds this two-dimensional boron to the Cu(100) surface. The electronic band structure of this material features multiple anisotropic tilted Dirac cones.

Hydrogen bonded trimesic acid networks on Cu(111) reveal how basic chemical properties are imprinted in HR-AFM images.

High resolution non-contact atomic force microscopy measurements characterize assemblies of trimesic acid molecules on Cu(111) and the link group interactions, providing the first fingerprints utilizing CO-based probes for this widely studied paradigm for hydrogen bond driven molecular self assembly. The enhanced submolecular resolution offered by this technique uniquely reveals key aspects of the competing interactions. Accurate comparison between full-density-based modeled images and experiment allows to identify key structural elements in the assembly in terms of the electron-withdrawing character of the carboxylic groups, interactions of those groups with Cu atoms in the surface, and the valence electron density in the intermolecular region of the hydrogen bonds. This study of trimesic acid assemblies on Cu(111) combining high resolution atomic force microscopy measurements with theory and simulation forges clear connections between fundamental chemical properties of molecules and key features imprinted in force images with submolecular resolution.

Hypothetical Efficiency of Electrical to Mechanical Energy Transfer during Individual Stochastic Molecular Switching Events.

There are now many examples of single molecule rotors, motors, and switches in the literature that, when driven by photons, electrons, or chemical reactions, exhibit well-defined motions. As a step toward using these single molecule devices to perform useful functions, one must understand how they interact with their environment and quantify their ability to perform work on it. Using a single molecule rotary switch, we examine the transfer of electrical energy, delivered via electron tunneling, to mechanical motion and measure the forces the switch experiences with a noncontact q-plus atomic force microscope. Action spectra reveal that the molecular switch has two stable states and can be excited resonantly between them at a bias of 100 mV via a one-electron inelastic tunneling process which corresponds to an energy input of 16 zJ. While the electrically induced switching events are stochastic and no net work is done on the cantilever, by measuring the forces between the molecular switch and the AFM cantilever, we can derive the maximum hypothetical work the switch could perform during a single switching event, which is ∼55 meV, equal to 8.9 zJ, which translates to a hypothetical efficiency of ∼55% per individual inelastic tunneling electron-induced switching event. When considering the total electrical energy input, this drops to 1 × 10-7% due to elastic tunneling events that dominate the tunneling current. However, this approach constitutes a general method for quantifying and comparing the energy input and output of molecular-mechanical devices.

Large-area single-crystal sheets of borophene on Cu(111) surfaces.

Borophene, a theoretically proposed two-dimensional (2D) boron allotrope1-3, has attracted much attention4,5 as a candidate material platform for high-speed, transparent and flexible electronics6-9. It was recently synthesized, on Ag(111) substrates10,11, and studied by tunnelling and electron spectroscopy12. However, the exact crystal structure is still controversial, the nanometre-size single-crystal domains produced so far are too small for device fabrication and the structural tunability via substrate-dependent epitaxy is yet to be proven. We report on the synthesis of borophene monitored in situ by low-energy electron microscopy, diffraction and scanning tunnelling microscopy (STM) and modelled by ab initio theory. We resolved the crystal structure and phase diagram of borophene on Ag(111), but found that the domains remain nanoscale for all growth conditions. However, by growing borophene on Cu(111) surfaces, we obtained large single-crystal domains, up to 100 µm2 in size. The crystal structure is a novel triangular network with a concentration of hexagonal vacancies of η = 1/5. Our experimental data, together with first principles calculations, indicate charge-transfer coupling to the substrate without significant covalent bonding. Our work sets the stage for fabricating borophene-based devices and substantiates the idea of borophene as a model for development of artificial 2D materials.

Phenyl Functionalization of Atomically Precise Graphene Nanoribbons for Engineering Inter-ribbon Interactions and Graphene Nanopores.

Graphene nanoribbons (GNRs) attract much attention from researchers due to their tunable physical properties and potential for becoming nanoscale building blocks of electronic devices. GNRs can be synthesized with atomic precision by on-surface approaches from specially designed molecular precursors. While a considerable number of ribbons with very diverse structures and properties have been demonstrated in recent years, there have been only limited examples of on-surface synthesized GNRs modified with functional groups. In this study, we designed a nanoribbon, in which the chevron GNR backbone is decorated with phenyl functionalities, and demonstrate the on-surface synthesis of these GNRs on Au(111). We show that the phenyl modification affects the assembly of the GNR polymer precursors through π-π interactions. Scanning tunneling spectroscopy of the modified GNRs on Au(111) revealed that they have a band gap of 2.50 ± 0.02 eV, which is comparable to that of the parent chevron GNR. The phenyl functionalization leads to a shift of the band edges to lower energies, suggesting that it could be a useful tool for the GNR band structure engineering. We also investigated lateral fusion of the phenyl-modified GNRs and demonstrate that it could be used to engineer different kinds of atomically precise graphene nanopores. A similar functionalization approach could be potentially applied to other GNRs to affect their on-surface assembly, modify their electronic properties, and realize graphene nanopores with a variety of structures.

Ensemble Control of Kondo Screening in Molecular Adsorbates.

Switching the magnetic properties of organic semiconductors on a metal surface has thus far largely been limited to molecule-by-molecule tip-induced transformations in scanned probe experiments. Here we demonstrate with molecular resolution that collective control of activated Kondo screening can be achieved in thin-films of the organic semiconductor titanyl phthalocyanine on Cu(110) to obtain tunable concentrations of Kondo impurities. Using low-temperature scanning tunneling microscopy and spectroscopy, we show that a thermally activated molecular distortion dramatically shifts surface-molecule coupling and enables ensemble-level control of Kondo screening in the interfacial spin system. This is accompanied by the formation of a temperature-dependent Abrikosov-Suhl-Kondo resonance in the local density of states of the activated molecules. This enables coverage-dependent control over activation to the Kondo screening state. Our study thus advances the versatility of molecular switching for Kondo physics and opens new avenues for scalable bottom-up tailoring of the electronic structure and magnetic texture of organic semiconductor interfaces at the nanoscale.

Dense monolayer films of atomically precise graphene nanoribbons on metallic substrates enabled by direct contact transfer of molecular precursors.

Atomically precise graphene nanoribbons (GNRs) of two types, chevron GNRs and N = 7 straight armchair GNRs (7-AGNRs), have been synthesized through a direct contact transfer (DCT) of molecular precursors on Au(111) and gradual annealing. This method provides an alternative to the conventional approach for the deposition of molecules on surfaces by sublimation and simplifies preparation of dense monolayer films of GNRs. The DCT method allows deposition of molecules on a surface in their original state and then studying their gradual transformation to polymers to GNRs by scanning tunneling microscopy (STM) upon annealing. We performed STM characterization of the precursors of chevron GNRs and 7-AGNRs, and demonstrate that the assemblies of the intermediates of the GNR synthesis are stabilized by π-π interactions. This conclusion was supported by the density functional theory calculations. The resulting monolayer films of GNRs have sufficient coverage and density of nanoribbons for ex situ characterization by spectroscopic methods, such as Raman spectroscopy, and may prove useful for the future GNR device studies.

Sticking with the Pointy End? Molecular Configuration of Chloro Boron-Subphthalocyanine on Cu(111).

In this combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study, we investigate self-assembly of the dipolar nonplanar organic semiconductor chloro boron-subphthalocyanine (ClB-SubPc) on Cu(111). We observe multiple distinct adsorption configurations and demonstrate that these can only be understood by taking surface-catalyzed dechlorination into account. A detailed investigation of possible adsorption configurations and the comparison of experimental and computational STM images demonstrates that the configurations correspond to "Cl-up" molecules with the B-Cl moiety pointing toward the vacuum side of the interface, and dechlorinated molecules. In contrast to the standard interpretation of adsorption of nonplanar molecules in the phthalocyanine family, we find no evidence for "Cl-down" molecules where the B-Cl moiety would be pointing toward the Cu surface. We show computationally that such a configuration is unstable and thus is highly unlikely to occur for ClB-SubPc on Cu(111). Using these assignments, we discuss the different self-assembly motifs in the submonolayer coverage regime. The combination of DFT and STM is essential to gain a full atomistic understanding of the surface-molecule interactions, and our findings imply that phthalocyanines may undergo surface-catalyzed reactions hitherto not considered. Our results also indicate that care has to be taken when analyzing possible adsorption configurations of polar members of the phthalocyanine family, especially when they are adsorbed on comparably reactive surfaces like Cu(111).