Redirecting On-surface Cycloaddition Reactions in a Self-assembled Ordered Molecular Array on Graphite

The synthesis of atomically precise carbon nanostructures in ultra-high vacuum has seen extensive progress on metal surfaces. However, this remains challenging on chemically inert surfaces. It is because the thermally activated C-C coupling encounters a severe "desorption problem" on weakly interacting substrates. In this study, we report an extraordinary [2+2]+[2+2] cycloaddition triggered by mild annealing (~210°C) in a highly ordered π-conjugated molecular array on graphite using scanning tunneling microscopy. In contrast to irregular dendritic fragments typically obtained on metal substrates, large supramolecular islands are observed here with cycloaddition products and other polymers over 30 %, which are embedded as defective individuals or chains (grain boundaries). First-principles calculations reveal that the energy barriers of the multiple dehydrohalogenation and cycloaddition reactions are reduced by catalytic Fe atoms but remain energetically unfavorable. A distinct driving mechanism is proposed for redirecting the reactions on graphite surface, where additional intermolecular coupling, steric hindrance, and interfacial interactions play significant roles. This study introduces a new paradigm for understanding on-surface synthesis on non-metal substrates.

Cationic Magnetically Active Nitrogen-Doped Polycyclic Aromatic Hydrocarbon with Record Low Band Gap

Polycyclic aromatic hydrocarbons (PAHs) have attracted significant interest in material chemistry, particularly if they own extremely low band gaps and magnetic properties. However, challenges remain regarding the synthetic accessibility and energy saturation issues. In this study, we introduce NR-11, which consists of eleven aromatic rings in its main conjugation and is separately doped with two electron-rich nitrogen atoms. This unique structure imparts intriguing oxidation characteristics to NR-11. The cationic radical NR-11+⋅ exhibits enhanced stability and demonstrates strong absorption in the range of 1250 nm to 3000 nm, peaking at 2570 nm. As a result, the optical energy gap of NR-11+⋅ is one of the lowest reported to date. Additionally, X-ray crystal structure analysis reveals that NR-11+⋅ displays unusual symmetry-broken charge separation. For the dication, variable-temperature NMR and variable-temperature EPR studies indicate that NR-112+ exhibits a high diradical character with a ▵ES-T of approximately -1 kcal/mol. Additionally, its spins are polarized at two ends of the PAH. Meanwhile, its strong absorption in the near-infrared II region suggests promise in photoacoustic (PA) conversion applications. This work underscores the significance of cationic species of extended long PAHs, highlighting their exceptional properties and potential applications.

Recent advances in endohedral metallofullerenes

Fullerenes are a collection of closed polycyclic polymers consisting exclusively of carbon atoms. Recent remarkable advancements in the fabrication of metal-fullerene nanocatalysts and polymeric fullerene layers have significantly expanded the potential applications of fullerenes in various domains, including electrocatalysis, transistors, energy storage devices, and superconductors. Notably, the interior of fullerenes provides an optimal environment for stabilizing a diverse range of metal ions or clusters through electron transfer, resulting in the formation of a novel class of hybrid molecules referred to as endohedral metallofullerenes (EMFs). The utilization of advanced synthetic methodologies and the progress achieved in separation techniques have played a pivotal role in expanding the diversity of the encapsulated metal constituents, consequently leading to distinctive structural, electronic, and physicochemical properties of novel EMFs that surpass conventional ones. Intriguing phenomena, including regioselective dimerization between EMFs, direct metal-metal bonding, and non-classical cage preferences, have been unveiled, offering valuable insights into the coordination interactions between metallic species and carbon. Of particular importance, the recent achievements in the comprehensive characterization of EMFs based on transition metals and actinide metals have generated a particular interest in the exploration of new metal clusters possessing long-desired bonding features within the realm of coordination chemistry. These clusters exhibit a remarkable affinity for coordinating with non-metal atoms such as carbon, nitrogen, oxygen, and sulfur, thus making them highly intriguing subjects of systematic investigations focusing on their electronic structures and physicochemical properties, ultimately leading to a deeper comprehension of their unparalleled bonding characteristics. Moreover, the versatility conferred by the encapsulated species endows EMFs with multifunctional properties, thereby unveiling potential applications in various fields including biomedicine, single-molecule magnets, and electronic devices.

Catalytic effects of iron adatoms in poly(para-phenylene) synthesis on rutile TiO2(110)

n-Armchair graphene nanoribbons (nAGNRs) are promising components for next-generation nanoelectronics due to their controllable band gap, which depends on their width and edge structure. Using non-metal surfaces for fabricating nAGNRs gives access to reliable information on their electronic properties. We investigated the influence of light and iron adatoms on the debromination of 4,4''-dibromo-p-terphenyl precursors affording poly(para-phenylene) (PPP as the narrowest GNR) wires through the Ullmann coupling reaction on a rutile TiO2(110) surface, which we studied by scanning tunneling microscopy and X-ray photoemission spectroscopy. The temperature threshold for bromine bond cleavage and desorption is reduced upon exposure to UV light (240-395 nm wavelength), but the reaction yield could not be improved. However, in the presence of codeposited iron adatoms, precursor debromination occurred even at 77 K, allowing for Ullmann coupling and PPP wire formation at 300-400 K, i.e., markedly lower temperatures compared to the conditions without iron adatoms. Furthermore, scanning tunneling spectroscopy data reveal that adsorbed PPP wires feature a band gap of ≈3.1 eV.

Cyclodehydrogenation of molecular nanographene precursors catalyzed by atomic hydrogen

Atomically precise synthesis of graphene nanostructures on semiconductors and insulators has been a formidable challenge. In particular, the metallic substrates needed to catalyze cyclodehydrogenative planarization reactions limit subsequent applications that exploit the electronic and/or magnetic structure of graphene derivatives. Here, we introduce a protocol in which an on-surface reaction is initiated and carried out regardless of the substrate type. We demonstrate that, counterintuitively, atomic hydrogen can play the role of a catalyst in the cyclodehydrogenative planarization reaction. The high efficiency of the method is demonstrated by the nanographene synthesis on metallic Au, semiconducting TiO2, Ge:H, as well as on inert and insulating Si/SiO2 and thin NaCl layers. The hydrogen-catalyzed cyclodehydrogenation reaction reported here leads towards the integration of graphene derivatives in optoelectronic devices as well as developing the field of on-surface synthesis by means of catalytic transformations. It also inspires merging of atomically shaped graphene-based nanostructures with low-dimensional inorganic units into functional devices.

Advancements in π-Magnetism and Precision Engineering of Carbon-Based Nanostructures

The emergence of π-magnetism in low-dimensional carbon-based nanostructures, such as nanographenes (NGs), has captured significant attention due to their unique properties and potential applications in spintronics and quantum technologies. Recent advancements in on-surface synthesis under ultra-high vacuum conditions have enabled the atomically precise engineering of these nanostructures, effectively overcoming the challenges posed by their inherent strong chemical reactivity. This review highlights the essential concepts and synthesis methods used in studying NGs. It also outlines the remarkable progress made in understanding and controlling their magnetic properties. Advanced characterization techniques, such as scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM), have been instrumental in visualizing and manipulating these nanostructures, which highlighting their critical role in the field. The review underscores the versatility of carbon-based π-magnetic materials and their potential for integration into next-generation electronic devices. It also outlines future research directions aimed at optimizing their synthesis and exploring applications in cutting-edge technologies.

Width-dependent continuous growth of atomically thin quantum nanoribbons from nanoalloy seeds in chalcogen vapor

Nanoribbons (NRs) of atomic layer transition metal dichalcogenides (TMDs) can boost the rapidly emerging field of quantum materials owing to their width-dependent phases and electronic properties. However, the controllable downscaling of width by direct growth and the underlying mechanism remain elusive. Here, we demonstrate the vapor-liquid-solid growth of single crystal of single layer NRs of a series of TMDs (MeX2: Me = Mo, W; X = S, Se) under chalcogen vapor atmosphere, seeded by pre-deposited and respective transition metal-alloyed nanoparticles that also control the NR width. We find linear dependence of growth rate on supersaturation, known as a criterion for continues growth mechanism, which decreases with decreasing of NR width driven by the Gibbs-Thomson effect. The NRs show width-dependent photoluminescence and strain-induced quantum emission signatures with up to ≈ 90% purity of single photons. We propose the path and underlying mechanism for width-controllable growth of TMD NRs for applications in quantum optoelectronics.

Pressure enabled organic reactions via confinement between layers of 2D materials

Confinement of reactants within nanoscale spaces of low-dimensional materials has been shown to provide reorientation of strained reactants or stabilization of unstable reactants for synthesis of molecules and tuning of chemical reactivity. While few studies have reported chemistry within zero-dimensional pores and one-dimensional nanotubes, organic reactions in confined spaces between two-dimensional materials have yet to be explored. Here, we demonstrate that reactants confined between atomically thin sheets of graphene or hexagonal boron nitride experience pressures as high as 7 gigapascal, which allows the propagation of solvent-free organic reactions that ordinarily do not occur under standard conditions. Specifically, we show that cyclodehydrogenation of hexaphenylbenzene without catalysts as a proof of concept and oxidative polymerization of dopamine into sheet-like crystalline structure are enabled by the effective high pressure experienced by the reactants between the graphene layers. Our results demonstrate a facile, general approach for performing high-pressure chemistry based on confinement of reactants within two-dimensional materials.

Achieving metal-like catalysis from semiconductor for on-surface synthesis

Free of posttransfer, on-surface synthesis (OSS) of single-atomic-layer nanostructures directly on semiconductors holds considerable potential for next-generation devices. However, due to the high diffusion barrier and abundant defects on semiconductor surfaces, extended and well-defined OSS on semiconductors has major difficulty. Furthermore, given semiconductors' limited thermal catalytic activity, initiating high-barrier reactions remains a significant challenge. Herein, using TiO2(011) as a prototype, we present an effective strategy for steering the molecule adsorption and reaction processes on semiconductors, delivering lengthy graphene nanoribbons with extendable widths. By introducing interstitial titanium (Tiint) and oxygen vacancies (Ov), we convert TiO2(011) from a passive supporting template into a metal-like catalytic platform. This regulation shifts electron density and surface dipoles, resulting in tunable catalytic activity together with varied molecule adsorption and diffusion. Cyclodehydrogenation, which is inefficient on pristine TiO2(011), is markedly improved on Tiint/Ov-doped TiO2. Even interribbon cyclodehydrogenation is achieved. The final product's dimensions, quality, and coverage are all controllable. Tiint doping outperforms Ov in producing regular and prolonged products, whereas excessive Tiint compromises molecule landing and coupling. This work demonstrates the crucial role of semiconductor substrates in OSS and advances OSS on semiconductors from an empirical trial-and-error methodology to a systematic and controllable paradigm.

Deposition temperature-mediated growth of helically shaped polymers and chevron-type graphene nanoribbons from a fluorinated precursor

Graphene nanoribbons (GNRs) of precise size and shape, critical for controlling electronic properties and future device applications, can be realized via precision synthesis on surfaces using rationally designed molecular precursors. Fluorine-bearing precursors have the potential to form GNRs on nonmetallic substrates suitable for device fabrication. Here, we investigate the deposition temperature-mediated growth of a new fluorine-bearing precursor, 6,11-diiodo-1,4-bis(2-fluorophenyl)-2,3-diphenyltriphenylene (C42H24F2I2), into helically shaped polymer intermediates and chevron-type GNRs on Au(111) by combining scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory simulations. The fluorinated precursors do not adsorb on the Au(111) surface at lower temperatures, necessitating an optimum substrate temperature to achieve maximum polymer and GNR lengths. We compare the adsorption behavior with that of pristine chevron precursors and discuss the effects of C-H and C-F bonds. The results elucidate the growth mechanism of GNRs with fluorine-bearing precursors and establish a foundation for future synthesis of GNRs on nonmetallic substrates.

Roto-Cyclization of 4-Bromopicene in On-Surface Synthesis

Progress toward single-molecule electronics relies on a thorough understanding of local physico-chemical processes and development of synthetic routines for controlled hetero-coupling. We demonstrate a structurally unexpected ring closure process for a homo-coupled 4,4'-bipicenyl, realized in on-surface synthesis. An initial covalent C-C coupling of 4-bromopicene locks at lower temperatures the position and geometrically shields part of 4,4'-bipicenyl. Employing this effect of shielding might offer a path toward controlled stepwise hetero-coupling. At higher temperatures, a thermally activated three-dimensional rotation upon hydrogen dissociation, a dehydrogenative roto-cyclization, lifts the surface-dimensionality restriction, and leads to the formation of a perylene. Thereby, the shielded molecular part becomes accessible again.

Topologically Localized Vibronic Excitations in Second-Layer Graphene Nanoribbons

It is of fundamental importance to characterize the intrinsic properties, like the topological end states, in the on-surface synthesized graphene nanoribbons (GNRs), but the strong electronic interaction with the metal substrate usually smears out their characteristic features. Here, we report our approach to investigate the vibronic excitations of the topological end states in self-decoupled second-layer GNRs, which are grown using an on-surface squeezing-induced spillover strategy. The vibronic progressions show highly spatially localized distributions at the second-layer GNR ends, which can be ascribed to the decoupling-extended lifetime of charging through resonant electron tunneling at the topological end states. In combination with theoretical calculations, we assign the vibronic progressions to specific vibrational modes that mediate the vibronic excitations. The spatial distribution of each resolved excitation shows evident characteristics beyond the conventional Franck-Condon picture. Our work by direct growth of second-layer GNRs provides an effective way to explore the interplay between the intrinsic electronic, vibrational, and topological properties.

Unzipping Carbon Nanotubes to Sub-5-nm Graphene Nanoribbons on Cu(111) by Surface Catalysis

Graphene nanoribbons (GNRs) are promising in nanoelectronics for their quasi-1D structures with tunable bandgaps. The methods for controllable fabrication of high-quality GNRs are still limited. Here a way to generate sub-5-nm GNRs by annealing single-walled carbon nanotubes (SWCNTs) on Cu(111) is demonstrated. The structural evolution process is characterized by low-temperature scanning tunneling microscopy. Substrate-dependent measurements on Au(111) and Ru(0001) reveal that the intermediate strong SWCNT-surface interaction plays a pivotal role in the formation of GNRs.

Phase diagram and crossover phases of topologically ordered graphene zigzag nanoribbons: role of localization effects

We computed the phase diagram of zigzag graphene nanoribbons as a function of on-site repulsion, doping, and disorder strength. The topologically ordered phase undergoes topological phase transitions into crossover phases, which are new disordered phases with non-universal topological entanglement entropy that exhibits significant variance. We explored the nature of non-local correlations in both the topologically ordered and crossover phases. In the presence of localization effects, strong on-site repulsion and/or doping weaken non-local correlations between the opposite zigzag edges of the topologically ordered phase. In one of the crossover phases, bothe-/2solitonic fractional charges and spin-charge separation were absent; however, charge-transfer correlations between the zigzag edges were possible. Another crossover phase contains solitonice-/2fractional charges but lacks charge transfer correlations. We also observed properties of non-topological, strongly disordered, and strongly repulsive phases. Each phase on the phase diagram exhibits a different zigzag-edge structure. Additionally, we investigated the tunneling of solitonic fractional charges under an applied voltage between the zigzag edges of undoped topologically ordered zigzag ribbons, and found that it may lead to a zero-bias tunneling anomaly.

Atomically Thin Current Pathways in Graphene through Kekulé-O Engineering

We demonstrate that the current flow in graphene can be guided on atomically thin current pathways by the engineering of Kekulé-O distortions. A grain boundary in these distortions separates the system into topologically distinct regions and induces a ballistic domain-wall state. The state is independent of the orientation of the grain boundary with respect to the graphene sublattice and permits guiding the current on arbitrary paths. As the state is gapped, the current flow can be switched by electrostatic gates. Our findings are explained by a generalization of the Jackiw-Rebbi model, where the electrons behave in one region of the system as Fermions with an effective complex mass, making the device not only promising for technological applications but also a test-ground for concepts from high-energy physics. An atomic model supported by DFT calculations demonstrates that the system can be realized by decorating graphene with Ti atoms.

Barth J, Lobo-Checa J, Corso M, Rogero C. Enhancing Haloarene Coupling Reaction Efficiency on an Oxide Surface by Metal Atom Addition

The bottom-up synthesis of carbon-based nanomaterials directly on semiconductor surfaces allows for the decoupling of their electronic and magnetic properties from the substrates. However, the typically reduced reactivity of such nonmetallic surfaces adversely affects the course of these reactions. Here, we achieve a high polymerization yield of halogenated polyphenyl molecular building blocks on the semiconducting TiO2(110) surface via concomitant surface decoration with cobalt atoms, which catalyze the Ullmann coupling reaction. Specifically, cobalt atoms trigger the debromination of 4,4″-dibromo-p-terphenyl molecules on TiO2(110) and mediate the formation of an intermediate organometallic phase already at room temperature (RT). As the debromination temperature is drastically reduced, homocoupling and polymerization readily proceed, preventing presursor desorption from the substrate and entailing a drastic increase of the poly-para-phenylene polymerization yield. The general efficacy of this mechanism is shown with an iodinated terphenyl derivative, which exhibits similar dehalogenation and reaction yield.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

Long and isolated graphene nanoribbons by on-surface polymerization on Au(111)

Low electronic gap graphene nanoribbons (GNRs) are used for the fabrication of nanomaterial-based devices and, when isolated, for mono-molecular electronics experiences, for which a well-controlled length is crucial. Here, an on-surface chemistry protocol is monitored for producing long and well-isolated GNR molecular wires on an Au(111) surface. The two-step Ullmann coupling reaction is sequenced in temperature from 100 °C to 350 °C by steps of 50 °C, returning at room temperature between each step and remaining in ultrahigh vacuum conditions. After the first annealing step at 100 °C, the monomers self-organize into 2-monolayered nano-islands. Next, the Ullmann coupling reaction takes place in both 1st and 2nd layers of those nano-islands. The nano-island lateral size and shape are controlling the final GNR lengths. Respecting the above on-surface chemistry protocol, an optimal initial monomer coverage of ~1.5 monolayer produces isolated GNRs with a final length distribution reaching up to 50 nm and a low surface coverage of ~0.4 monolayer suitable for single molecule experiments.

Exploration of the origin of the excellent charge-carrier dynamics in Ruddlesden-Popper oxysulfide perovskite Y2Ti2O5S2

Although the efficient separation of electron-hole (e-h) pairs is one of the most sought-after electronic characteristics of materials, due to thermally induced atomic motion and other factors, they do not remain separated during the carrier transport process, potentially leading to rapid carrier recombination. Here, we utilized real-time time-dependent density functional theory in combination with nonadiabatic molecular dynamics (NAMD) to explore the separated dynamic transport path within Ruddlesden-Popper oxysulfide perovskite Y2Ti2O5S2 caused by the dielectric layer and phonon frequency difference. The underlying origin of the efficient overall water splitting in Y2Ti2O5S2 is systematically explored. We report the existence of the bi-directional e-h separate-path transport, in which, the electrons transport in the Ti2O5 layer and the holes diffuse in the rock-salt layer. This is in contrast to the conventional e-h separated distribution with a crowded transport channel, as observed in SrTiO3 and hybrid perovskites. Such a unique feature finally results in a long carrier lifetime of 321 ns, larger than that in the SrTiO3 perovskite (160 ns) with only one carrier transport channel. This work provides insights into the carrier transport in lead-free perovskites and yields a novel design strategy for next-generation functionalized optoelectronic devices.

On-Surface Synthesis of Edge-Extended Zigzag Graphene Nanoribbons

Graphene nanoribbons (GNRs) have gained significant attention in nanoelectronics due to their potential for precise tuning of electronic properties through variations in edge structure and ribbon width. However, the synthesis of GNRs with highly sought-after zigzag edges (ZGNRs), critical for spintronics and quantum information technologies, remains challenging. In this study, a design motif for synthesizing a novel class of GNRs termed edge-extended ZGNRs is presented. This motif enables the controlled incorporation of edge extensions along the zigzag edges at regular intervals. The synthesis of a specific GNR instance-a 3-zigzag-rows-wide ZGNR-with bisanthene units fused to the zigzag edges on alternating sides of the ribbon axis is successfully demonstrated. The resulting edge-extended 3-ZGNR is comprehensively characterized for its chemical structure and electronic properties using scanning probe techniques, complemented by density functional theory calculations. The design motif showcased here opens up new possibilities for synthesizing a diverse range of edge-extended ZGNRs, expanding the structural landscape of GNRs and facilitating the exploration of their structure-dependent electronic properties.

Mutual Information and Correlations across Topological Phase Transitions in Topologically Ordered Graphene Zigzag Nanoribbons

Graphene zigzag nanoribbons, initially in a topologically ordered state, undergo a topological phase transition into crossover phases distinguished by quasi-topological order. We computed mutual information for both the topologically ordered phase and its crossover phases, revealing the following results: (i) In the topologically ordered phase, A-chirality carbon lines strongly entangle with B-chirality carbon lines on the opposite side of the zigzag ribbon. This entanglement persists but weakens in crossover phases. (ii) The upper zigzag edge entangles with non-edge lines of different chirality on the opposite side of the ribbon. (iii) Entanglement increases as more carbon lines are grouped together, regardless of the lines' chirality. No long-range entanglement was found in the symmetry-protected phase in the absence of disorder.

On-Surface Synthesis of Graphene Nanoribbons with Atomically Precise Structural Heterogeneities and On-Site Characterizations

Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and in situ tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations. However, there are still obstacles and challenges lying on the way toward the understanding of the intrinsic structure-property relations, such as the strong screening and Fermi level pinning effect of the normally used transition metal substrates and the lack of collective tip-based techniques that can cover multi-internal degrees of freedom of the GNRs. In this Perspective, we briefly review the recent progress in the on-surface synthesis of GNRs with diverse structural heterogeneities and highlight the structure-property relations as characterized by the noncontact atomic force microscopy and scanning tunneling microscopy/spectroscopy. We furthermore motivate to deliver the need for developing strategies to achieve quasi-freestanding GNRs and for exploiting multifunctional tip-based techniques to collectively probe the intrinsic properties.

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

Magnetic Coupling Control in Triangulene Dimers

Metal-free magnetism remains an enigmatic field, offering prospects for unconventional magnetic and electronic devices. In the pursuit of such magnetism, triangulenes, endowed with inherent spin polarization, are promising candidates to serve as monomers to construct extended structures. However, controlling and enhancing the magnetic interactions between the monomers persist as a significant challenge in molecular spintronics, as so far only weak antiferromagnetic coupling through the linkage has been realized, hindering their room temperature utilization. Herein, we investigate 24 triangulene dimers using first-principles calculations and demonstrate their tunable magnetic coupling (J), achieving unprecedented strong J values of up to -144 meV in a non-Kekulé dimer. We further establish a positive correlation between bandgap, electronic coupling, and antiferromagnetic interaction, thereby providing molecular-level insights into enhancing magnetic interactions. By twisting the molecular fragments, we demonstrate an effective and feasible approach to control both the sign and strength of J by tuning the balance between potential and kinetic exchanges. We discover that J can be substantially boosted at planar configurations up to -198 meV. We realize ferromagnetic coupling in nitrogen-doped triangulene dimers at both planar and largely twisted configurations, representing the first example of ferromagnetic triangulene dimers that cannot be predicted by the Ovchinnikov rule. This work thus provides a practical strategy for augmenting magnetic coupling and open up new avenues for metal-free ferromagnetism.

Rapid production of kilogram-scale graphene nanoribbons with tunable interlayer spacing for an array of renewable energy

Graphene nanoribbons (GNRs) are widely recognized as intriguing building blocks for high-performance electronics and catalysis owing to their unique width-dependent bandgap and ample lone pair electrons on both sides of GNR, respectively, over the graphene nanosheet counterpart. However, it remains challenging to mass-produce kilogram-scale GNRs to render their practical applications. More importantly, the ability to intercalate nanofillers of interest within GNR enables in-situ large-scale dispersion and retains structural stability and properties of nanofillers for enhanced energy conversion and storage. This, however, has yet to be largely explored. Herein, we report a rapid, low-cost freezing-rolling-capillary compression strategy to yield GNRs at a kilogram scale with tunable interlayer spacing for situating a set of functional nanomaterials for electrochemical energy conversion and storage. Specifically, GNRs are created by sequential freezing, rolling, and capillary compression of large-sized graphene oxide nanosheets in liquid nitrogen, followed by pyrolysis. The interlayer spacing of GNRs can be conveniently regulated by tuning the amount of nanofillers of different dimensions added. As such, heteroatoms; metal single atoms; and 0D, 1D, and 2D nanomaterials can be readily in-situ intercalated into the GNR matrix, producing a rich variety of functional nanofiller-dispersed GNR nanocomposites. They manifest promising performance in electrocatalysis, battery, and supercapacitor due to excellent electronic conductivity, catalytic activity, and structural stability of the resulting GNR nanocomposites. The freezing-rolling-capillary compression strategy is facile, robust, and generalizable. It renders the creation of versatile GNR-derived nanocomposites with adjustable interlay spacing of GNR, thereby underpinning future advances in electronics and clean energy applications.

Remote-Triggered Domino-like Cyclodehydrogenation in Second-Layer Topological Graphene Nanoribbons

Cyclodehydrogenation reactions in the on-surface synthesis of graphene nanoribbons (GNRs) usually involve a series of C_sp2-C_sp2 and/or C_sp2-C_sp3 couplings and just happen on uncovered metal or metal oxide surfaces. It is still a big challenge to extend the growth of second-layer GNRs in the absence of necessary catalytic sites. Here, we demonstrate the direct growth of topologically nontrivial GNRs via multistep C_sp2-C_sp2 and C_sp2-C_sp3 couplings in the second layer by annealing designed bowtie-shaped precursor molecules over one monolayer on the Au(111) surface. After annealing at 700 K, most of the polymerized chains that appear in the second layer covalently link to the first-layer GNRs that have partially undergone graphitization. Following annealing at 780 K, the second-layer GNRs are formed and linked to the first-layer GNRs. Benefiting from the minimized local steric hindrance of the precursors, we suggest that the second-layer GNRs undergo domino-like cyclodehydrogenation reactions that are remotely triggered at the link. We confirm the quasi-freestanding behaviors in the second-layer GNRs by measuring the quasiparticle energy gap of topological bands and the tunable Kondo resonance from topological end spins using scanning tunneling microscopy/spectroscopy combined with first-principles calculations. Our findings pave the avenue to diverse multilayer graphene nanostructures with designer quantum spins and topological states for quantum information science.

Proximity-Induced Superconductivity in Atomically Precise Nanographene on Ag/Nb(110)

Obtaining a robust superconducting state in atomically precise nanographene (NG) structures by proximity to a superconductor could foster the discovery of topological superconductivity in graphene. On-surface synthesis of such NGs has been achieved on noble metals and metal oxides; however, it is still absent on superconductors. Here, we present a synthetic method to induce superconductivity of polymeric chains and NGs adsorbed on the superconducting Nb(110) substrate covered by thin Ag films. Using atomic force microscopy at low temperature, we characterize the chemical structure of each subproduct formed on the superconducting Ag layer. Scanning tunneling spectroscopy further allows us to elucidate the electronic properties of these nanostructures, which consistently show a superconducting gap.

Steering On-Surface Reactions by Kinetic and Thermodynamic Strategies

On-surface synthesis has emerged as a powerful tool to fabricate various functional low-dimensional nanostructures with atomic precision, thus becoming a promising platform for the preparation of next-generation semiconductive, magnetic, and topological nanodevices. With the aid of scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy, both the chemical structures and physical properties of the obtained products can be well characterized. A major challenge in this field is how to efficiently steer reaction pathways and improve the yield/quality of products. To address this problem, in recent years various kinetic and thermodynamic strategies have been successfully employed to control on-surface reactions. In this Perspective, we discuss these strategies in view of basic reaction steps on surfaces, including molecular adsorption, diffusion, and reaction. We hope this Perspective will help readers to deepen the understanding of the mechanisms of on-surface reactions and rationally design reaction procedures for the fabrication of high-quality functional nanomaterials on surfaces.

Disorder-tuned conductivity in amorphous monolayer carbon

Correlating atomic configurations-specifically, degree of disorder (DOD)-of an amorphous solid with properties is a long-standing riddle in materials science and condensed matter physics, owing to difficulties in determining precise atomic positions in 3D structures1-5. To this end, 2D systems provide insight to the puzzle by allowing straightforward imaging of all atoms6,7. Direct imaging of amorphous monolayer carbon (AMC) grown by laser-assisted depositions has resolved atomic configurations, supporting the modern crystallite view of vitreous solids over random network theory8. Nevertheless, a causal link between atomic-scale structures and macroscopic properties remains elusive. Here we report facile tuning of DOD and electrical conductivity in AMC films by varying growth temperatures. Specifically, the pyrolysis threshold temperature is the key to growing variable-range-hopping conductive AMC with medium-range order (MRO), whereas increasing the temperature by 25 °C results in AMC losing MRO and becoming electrically insulating, with an increase in sheet resistance of 109 times. Beyond visualizing highly distorted nanocrystallites embedded in a continuous random network, atomic-resolution electron microscopy shows the absence/presence of MRO and temperature-dependent densities of nanocrystallites, two order parameters proposed to fully describe DOD. Numerical calculations establish the conductivity diagram as a function of these two parameters, directly linking microstructures to electrical properties. Our work represents an important step towards understanding the structure-property relationship of amorphous materials at the fundamental level and paves the way to electronic devices using 2D amorphous materials.

Unveiling the formation mechanism of the biphenylene network

We have computationally studied the formation mechanism of the biphenylene network via the intermolecular HF zipping, as well as identified key intermediates experimentally, on the Au(111) surface. We elucidate that the zipping process consists of a series of defluorinations, dehydrogenations, and C-C coupling reactions. The Au substrate not only serves as the active site for defluorination and dehydrogenation, but also forms C-Au bonds that stabilize the defluorinated and dehydrogenated phenylene radicals, leading to "standing" benzyne groups. Despite that the C-C coupling between the "standing" benzyne groups is identified as the rate-limiting step, the limiting barrier can be reduced by the adjacent chemisorbed benzyne groups. The theoretically proposed mechanism is further supported by scanning tunneling microscopy experiments, in which the key intermediate state containing chemisorbed benzyne groups can be observed. This study provides a comprehensive understanding towards the on-surface intermolecular HF zipping, anticipated to be instructive for its future applications.

On-Surface Synthesis of Nanographenes and Graphene Nanoribbons on Titanium Dioxide

The formation of two types of nanographenes from custom designed and synthesized molecular precursors has been achieved through thermally induced intramolecular cyclodehydrogenation reactions on the semiconducting TiO₂(110)-(1×1) surface, confirmed by the combination of high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements, and corroborated by theoretical modeling. The application of this protocol on differently shaped molecular precursors demonstrates the ability to induce a highly efficient planarization reaction both within strained pentahelicenes as well as between vicinal phenyl rings. Additionally, by the combination of successive Ullmann-type polymerization and cyclodehydrogenation reactions, the archetypic 7-armchair graphene nanoribbons (7-AGNRs) have also been fabricated on the titanium dioxide surface from the standard 10,10'-dibromo-9,9'-bianthryl (DBBA) molecular precursors. These examples of the effective cyclodehydrogenative planarization processes provide perspectives for the rational design and synthesis of molecular nanostructures on semiconductors.

Electronic band structure of 1D π-d hybridized narrow-gap metal-organic polymers

One-dimensional (1D) metal-organic (MO) nanowires are captivating from fundamental and technological perspectives due to their distinctive magnetic and electronic properties. The solvent-free synthesis of such nanomaterials on catalytic surfaces provides a unique approach for fabricating low-dimensional single-layer materials with atomic precision and low amount of defects. A detailed understanding of the electronic structure of MO polymers such as band gap and dispersive bands is critical for their prospective implementation into nanodevices such as spin sensors or field-effect transistors. Here, we have performed the on-surface reaction of quinoidal ligands with single cobalt atoms (Co-QDI) on a vicinal Au(788) surface in ultra-high vacuum. This procedure promotes the growth and uniaxial alignment of Co-QDI MO chains along the surface atomic steps, while permitting the mapping of their electronic properties with space-averaging angle-resolved photoemission spectroscopy. In the direction parallel to the principal chain axis, a well-defined 1D band structure with weakly dispersive and dispersive bands is observed, confirming a pronounced electron delocalization. Low-temperature scanning tunneling microscopy/spectroscopy delves into the atomically precise structure of the nanowires and elucidates their narrow bandgap. These findings are supported with GW₀ band structure calculations showing that the observed electronic bands emanate from the efficient hybridization of Co(3d) and molecular orbitals. Our work paves the way towards a systematic search of similar 1D π-d hybridized MO chains with tunable electronic and magnetic properties defined by the transition or rare earth metal atom of choice.

Solution-Synthesized Extended Graphene Nanoribbons Deposited by High-Vacuum Electrospray Deposition

Solution-synthesized graphene nanoribbons (GNRs) facilitate various interesting structures and functionalities, like nonplanarity and thermolabile functional groups, that are not or not easily accessible by on-surface synthesis. Here, we show the successful high-vacuum electrospray deposition (HVESD) of well-elongated solution-synthesized GNRs on surfaces maintained in ultrahigh vacuum. We compare three distinct GNRs, a twisted nonplanar fjord-edged GNR, a methoxy-functionalized "cove"-type (or also called gulf) GNR, and a longer "cove"-type GNR both equipped with alkyl chains on Au(111). Nc-AFM measurements at room temperature with submolecular imaging combined with Raman spectroscopy allow us to characterize individual GNRs and confirm their chemical integrity. The fjord-GNR and methoxy-GNR are additionally deposited on nonmetallic HOPG and SiO2, and fjord-GNR is deposited on a KBr(001) surface, facilitating the study of GNRs on substrates, as of now not accessible by on-surface synthesis.

Dibenzotropylium-Capped Orthogonal Geometry Enabling Isolation and Examination of a Series of Hydrocarbons with Multiple 14π-Aromatic Units

A series of six dications composed of pure hydrocarbons with one to six non-substituted 9,10-anthrylene units end-capped with two dibenzotropyliums were designed and synthesized to elucidate the electronic properties of huge oligo(9,10-anthrylene) backbones. Their structures were successfully determined by X-ray analyses even in the case of eight planar 14π-electron units, revealing that all dications adopt almost orthogonally twisted structures between neighboring units. Spectroscopic and voltammetric analyses show that neither the significant overlap of orbitals nor the delocalization of electrons between 14π-electron units occurs due to the orthogonally twisted geometry even in solution. As a result, sequential oxidation processes were observed with the reversible formation of multivalent cations with the release of the same number of electrons as the number of anthrylene units. Upon two-electron reduction, a closed-shell butterfly-shaped form was obtained from the dication containing one anthrylene unit, whereas open-shell twisted biradicals were isolated as stable entities in the cases of derivatives containing three to six anthrylene units. Notably, from the derivative with two anthrylene units, a metastable open-shell isomer was obtained quantitatively and underwent slow thermal conversion to the most stable closed-shell isomer (E_a = 23.1 kcal mol^-1). There is a drastic change in oxidation potentials between two neutral species (ΔE = 1.32 V in CH₂Cl₂). Since the present dications were regenerated upon oxidation of the isolated reduction products, these systems may contribute to the development of advanced response systems capable of switching color, magnetic properties, and oxidative properties by using a "cation-capped orthogonal geometry".

Structural Design and Fabrication of Multifunctional Nanocarbon Materials for Extreme Environmental Applications

Extreme environments represent numerous harsh environmental conditions, such as temperature, pressure, corrosion, and radiation. The tolerance of applications in extreme environments exemplifies significant challenges to both materials and their structures. Given the superior mechanical strength, electrical conductivity, thermal stability, and chemical stability of nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, they are widely investigated as base materials for extreme environmental applications and have shown numerous breakthroughs in the fields of wide-temperature structural-material construction, low-temperature energy storage, underwater sensing, and electronics operated at high temperatures. Here, the critical aspects of structural design and fabrication of nanocarbon materials for extreme environments are reviewed, including a description of the underlying mechanism supporting the performance of nanocarbon materials against extreme environments, the principles of structural design of nanocarbon materials for the optimization of extreme environmental performances, and the fabrication processes developed for the realization of specific extreme environmental applications. Finally, perspectives on how CNTs and graphene can further contribute to the development of extreme environmental applications are presented.

Synthetic carbon nanomaterials for electrochemical energy conversion

Carbon nanomaterials have attracted widespread attention in electrochemical energy conversion due to their large surface area, excellent electrical/thermal conductivity and good chemical stability. However, the structure-activity relationship of carbon nanomaterials remains unclear. This review is thus on the synthesis methods of carbon nanomaterials including two-dimensional graphene, graphene nanoribbons, nanographene, heteroatom doped porous carbon and graphdiyne as electrocatalysts for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction in fuel cells, electrolyzers and CO₂ reduction. The correlation between the electronic/chemical properties and electrochemical performance of synthetic carbon nanostructures will be profoundly discussed. Additionally, the emerging challenges and some perspectives on the development of synthetic carbon nanomaterials for electrochemical energy conversion are discussed.

Carbon-based nanostructures as a versatile platform for tunableπ-magnetism

Emergence ofπ-magnetism in open-shell nanographenes has been theoretically predicted decades ago but their experimental characterization was elusive due to the strong chemical reactivity that makes their synthesis and stabilization difficult. In recent years, on-surface synthesis under vacuum conditions has provided unprecedented opportunities for atomically precise engineering of nanographenes, which in combination with scanning probe techniques have led to a substantial progress in our capabilities to realize localized electron spin states and to control electron spin interactions at the atomic scale. Here we review the essential concepts and the remarkable advances in the last few years, and outline the versatility of carbon-basedπ-magnetic materials as an interesting platform for applications in spintronics and quantum technologies.

On-Surface Synthesis toward Two-Dimensional Polymers

Two-dimensional (2D) polymers have garnered widespread interest because of their intriguing physicochemical properties. Envisaged applications in fields including nanodevices, solid-state chemistry, physical organic chemistry, and condensed matter physics, however, demand high-quality and large-scale production. In this perspective, we first introduce exotic band structures of organic frameworks holding honeycomb, kagome, and Lieb lattices. We further discuss how mesoscale ordered 2D polymers can be synthesized by means of choosing suitable monomers and optimizing growth conditions. We describe successful polymerization strategies to introducing a non-benzenoid subunit into a π-conjugated carbon lattice via delicately designed monomer precursors. Also, to obviate transfer and restore the intrinsic properties of π-conjugated polymers, new paradigms of aryl-aryl coupling on inert surfaces are discussed. Recent achievements in the photopolymerization demonstrate the need for monomer design. We conclude the potential applications of these organic networks and project the future possibilities in providing new insights into on-surface polymerization.

New disordered anyon phase of doped graphene zigzag nanoribbon

We investigate interacting disordered zigzag nanoribbons at low doping, using the Hubbard model to treat electron interactions within the density matrix renormalization group and Hartree-Fock method. Extra electrons that are inserted into an interacting disordered zigzag nanoribbon divide into anyons. Furthermore, the fractional charges form a new disordered anyon phase with a highly distorted edge spin density wave, containing numerous localized magnetic moments residing on the zigzag edges, thereby displaying spin-charge separation and a strong non-local correlation between the opposite zigzag edges. We make the following new predictions, which can be experimentally tested: (1) In the low doping case and weak disorder regime, the soft gap in the tunneling density of states is replaced by a sharp peak at the midgap energy with two accompanying peaks. The [Formula: see text] fractional charges that reside on the boundary of the zigzag edges are responsible for these peaks. (2) We find that the midgap peak disappears as the doping concentration increases. The presence of [Formula: see text] fractional charges will be strongly supported by the detection of these peaks. Doped zigzag ribbons may also exhibit unusual transport, magnetic, and inter-edge tunneling properties.

On-Surface Chemistry on Low-Reactive Surfaces

Zero-dimensional (0D), mono-dimensional (1D), or two-dimensional (2D) nanostructures with well-defined properties fabricated directly on surfaces are of growing interest. The fabrication of covalently bound nanostructures on non-metallic surfaces is very promising in terms of applications, but the lack of surface assistance during their synthesis is still a challenge to achieving the fabrication of large-scale and defect-free nanostructures. We discuss the state-of-the-art approaches recently developed in order to provide covalently bounded nanoarchitectures on passivated metallic surfaces, semiconductors, and insulators.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies

The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface. From a single molecular precursor correspondingly following a trans- and cis-coupling, the DV8-aGNR and another porous nanographene are respectively formed at step edges and on terraces at 720 and 570 K. Combining scanning tunneling microscopy/spectroscopy, atomic force microscopy, and first-principles calculations, we determine the out-of-plane conformation, wide bandgap (∼3.36 eV), and wiggly shaped frontier orbitals of the DV8-aGNR. Nudged elastic band calculations further quantitatively reveal that the additional steric hindrance effect in the cyclodehydrogenative reactions has a higher barrier of 1.3 eV than that in the planar porous nanographene, which also unveils the important role played by the monatomic Au step and adatoms in reducing the energy barriers and enhancing the thermodynamic preference of the oxidative cyclodehydrogenation. Our results provide the first case of GNRs containing periodic pores as small as divacancies with a nonplanar configuration and demonstrate the strategy by utilizing the chemical heterogeneity of a substrate to promote the formation of novel carbon nanomaterials.

Nanographenes and Graphene Nanoribbons as Multitalents of Present and Future Materials Science

As cut-outs from a graphene sheet, nanographenes (NGs) and graphene nanoribbons (GNRs) are ideal cases with which to connect the world of molecules with that of bulk carbon materials. While various top-down approaches have been developed to produce such nanostructures in high yields, in the present perspective, precision structural control is emphasized for the length, width, and edge structures of NGs and GNRs achieved by modern solution and on-surface syntheses. Their structural possibilities have been further extended from "flatland" to the three-dimensional world, where chirality and handedness are the jewels in the crown. In addition to properties exhibited at the molecular level, self-assembly and thin-film structures cannot be neglected, which emphasizes the importance of processing techniques. With the rich toolkit of chemistry in hand, NGs and GNRs can be endowed with versatile properties and functions ranging from stimulated emission to spintronics and from bioimaging to energy storage, thus demonstrating their multitalents in present and future materials science.

Catalytic Growth of Ultralong Graphene Nanoribbons on Insulating Substrates

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 µm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of ≈1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. This study provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.

Synthesis of a monolayer fullerene network

Two-dimensional (2D) carbon materials, such as graphene, have attracted particular attention owing to the exceptional carrier transport characteristics that arise from the unique π-electron system in their conjugated carbon network structure^1-4. To complement zero-bandgap graphene, material scientists have devoted considerable effort to identifying 2D carbon materials^5-8. However, it is a challenge to prepare large-sized single-crystal 2D carbon materials with moderate bandgaps^5,9. Here we prepare a single-crystal 2D carbon material, namely monolayer quasi-hexagonal-phase fullerene (C₆₀), with a large size via an interlayer bonding cleavage strategy. In this monolayer polymeric C₆₀, cluster cages of C₆₀ are covalently bonded with each other in a plane, forming a regular topology that is distinct from that in conventional 2D materials. Monolayer polymeric C₆₀ exhibits high crystallinity and good thermodynamic stability, and the electronic band structure measurement reveals a transport bandgap of about 1.6 electronvolts. Furthermore, an asymmetric lattice structure endows monolayer polymeric C₆₀ with notable in-plane anisotropic properties, including anisotropic phonon modes and conductivity. This 2D carbon material with a moderate bandgap and unique topological structure offers an interesting platform for potential application in 2D electronic devices.

Bandgaps of atomically precise graphene nanoribbons and Occam's razor

Rationalization of the "bulk" (ΔE_ac) or "zigzag-end" (ΔE_zz) energy gaps of atomically precise armchair graphene nanoribbons (AGNRs), which are directly related to fundamental applications in nanoelectronics, could be challenging and largely controversial with respect to their magnitude, origin, substrate influence (ΔE_sb), and spin-polarization, among others. Hereby a simple self-consistent and "economical" interpretation is presented, in full accordance with Occam's simplicity principle, which is highly successful (within less than 1%) in predicting all energy gaps of the 5-, 7-, and 9-AGNRs, in contrast to other complicated and/or contradicting prevailing views in the literature for ΔE_ac, ΔE_zz, and ΔE_sb. The present approach is based on "appropriate" DFT (TDDFT) calculations, general symmetry principles, and plausibility arguments. The excellent agreement with experiments and the new insight gained is achieved by invoking the approximate equivalence of Coulomb correlation energy with the staggered sublattice potential. Breaking established stereotypes, we suggest that the measured STS gaps are virtually independent of the substrate, essentially equal to their free-standing values, and that the "true" lowest energy state is a closed singlet with no conventional magnetism. The primary source of discrepancies is the finite length of AGNRs together with inversion/reflection symmetry conflict and the resulting topological end/edge states. Such states invariably mix with other "bulk" states making their unambiguous detection/distinction difficult. This can be further tested by eliminating end-states (and ΔE_zz), by eliminating "empty" zigzag rings.

Initial Coupling and Reaction Progression of Directly Deposited Biradical Graphene Nanoribbon Monomers on Iodine-Passivated Versus Pristine Ag(111)

The development of widely applicable methods for the synthesis of C-C-bonded nanostructures on inert and insulating surfaces is a challenging yet rewarding milestone in the field of on-surface synthesis. This would enable studies of nearly unperturbed covalent nanostructures with unique electronic properties as graphene nanoribbons (GNR) and π-conjugated 2D polymers. The prevalent Ullmann-type couplings are almost exclusively carried out on metal surfaces to lower the temperature required for initial dehalogenation well below the desorption threshold. To overcome the necessity for the activation of monomers on the target surface, we employ a recently developed Radical Deposition Source (RaDeS) for the direct deposition of radicals onto inert surfaces for subsequent coupling by addition reactions. The radicals are generated en route by indirect deposition of halogenated precursors through a heated reactive tube, where the dehalogenation reaction proceeds. Here, we use the ditopic 6,11-diiodo-1,2,3,4-tetraphenyltriphenylene (DITTP) precursor that afforded chevron-like GNR on Au(111) via the usual two-staged reaction comprised of monomer-coupling into covalent polymers and subsequent formation of an extended GNR by intramolecular cyclodehydrogenation (CDH). As a model system for inert surfaces, we use Ag(111) passivated with a closed monolayer of chemisorbed iodine that behaves in an inert manner with respect to dehalogenation reactions and facilitates the progressive coupling of radicals into extended covalent structures. We deposit the DITTP-derived biradicals onto both iodine-passivated and pristine Ag(111) surfaces. While on the passivated surface, we directly observe the formation of covalent polymers, on pristine Ag(111) organometallic intermediates emerge instead. This has decisive consequences for the further progression of the reaction: heating the organometallic chain directly on Ag(111) results in complete desorption, whereas the covalent polymer on iodine-passivated Ag(111) can be transformed into the GNR. Yet, the respective CDH proceeds directly on Ag(111) after thermal desorption of the iodine passivation. Accordingly, future work is aimed at the further development of approaches for the complete synthesis of GNR on inert surfaces.

On-surface products from de-fluorination of C60F48 on Ag(111): C60, C60Fx and silver fluoride formation

By employing diverse surface sensitive synchrotron radiation spectroscopies we demonstrate that the fluorine content of initial C₆₀F₄₈ deposited at room temperature on Ag(111) varies with molecular coverage. At the very early stages of deposition, C₆₀F₄₈ fully de-fluorinates and transforms into C₆₀. Strong indications of silver fluoride formation are provided. The chemical footprint of fluorinated fullerenes emerges at relatively low molecular coverage indicating that the degree of fullerene de-fluorination decreases (from total to partial de-fluorination) as molecules are deposited. Full de-fluorination stops well before the substrate surface is completely covered by fullerenes. At the molecular level, the fluorine loss observed by spectroscopic techniques is supported by scanning tunneling microscopy imaging. Both molecules and metal surface are importantly involved in the process.

Magnetic Interactions Between Radical Pairs in Chiral Graphene Nanoribbons

Open-shell graphene nanoribbons have become promising candidates for future applications, including quantum technologies. Here, we characterize magnetic states hosted by chiral graphene nanoribbons (chGNRs). The substitution of a hydrogen atom at the chGNR edge by a ketone effectively adds one p_z electron to the π-electron network, producing an unpaired π-radical. A similar scenario occurs for regular ketone-functionalized chGNRs in which one ketone is missing. Two such radical states can interact via exchange coupling, and we study those interactions as a function of their relative position, which includes a remarkable dependence on the chirality, as well as on the nature of the surrounding ribbon, that is, with or without ketone functionalization. Besides, we determine the parameters whereby this type of system with oxygen heteroatoms can be adequately described within the widely used mean-field Hubbard model. Altogether, we provide insight to both theoretically model and devise GNR-based nanostructures with tunable magnetic properties.

Stepwise on-surface synthesis of thiophene-based polymeric ribbons by coupling reactions and the carbon-fluorine bond cleavage

The rational synthesis of thiophene-based cross-coupled polymers on surfaces has been attracting more attention recently. Here, we report the stepwise activation of 5,5'-(2,3-difluoro-1,4-phenylene)bis(2-bromothiophene) as a precursor to synthesize thiophene-based polymeric ribbons on the Au(111) surface. Scanning tunneling microscopy studies showed that the precursor adopted different conformations in the self-assembled structure, organometallic species, and covalent polymers. On annealing the sample at a relatively low temperature (150 °C), the conversion of the organometallic structure into a covalent product with straight lines was observed, in which the Br adatoms arranged between the neighboring chains. On further annealing the sample at 270 °C, the detached Br adatoms played a key role in promoting the C-H bond activation. The cross-linked polymer was achieved by a combination of Ullmann and dehydrogenative coupling. When the annealing temperature was up to 390 °C, the C-F bond activation was triggered, which led to the formation of polymeric ribbons resulting from the cyclodehydrogenation of the fluorinated polymer. This study further supplements the reaction mechanism of thiophene-based dehalogenative, dehydrogenative and defluorinative coupling, and provides us a rational way for synthesizing cross-linked functional materials.