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Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks. Outstanding examples are excitations with charge e /3 in the fractional quantum Hall effect 1 , 2 , solitons in one-dimensional conducting polymers 3 , 4 and Majorana states in topological superconductors 5 . Fractionalization is also predicted to manifest itself in low-dimensional quantum magnets, such as one-dimensional antiferromagnetic S  = 1 chains. The fundamental features of this system are gapped excitations in the bulk 6 and, remarkably, S  = 1/2 edge states at the chain termini 7 – 9 , leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order 10 , 11 . Here, we use on-surface synthesis 12 to fabricate one-dimensional spin chains that contain the S  = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunnelling microscopy and spectroscopy at 4.5 K, we probe length-dependent magnetic excitations at the atomic scale in both open-ended and cyclic spin chains, and directly observe gapped spin excitations and fractional edge states therein. Exact diagonalization calculations provide conclusive evidence that the spin chains are described by the S  = 1 bilinear-biquadratic Hamiltonian in the Haldane symmetry-protected topological phase. Our results open a bottom-up approach to study strongly correlated phases in purely organic materials, with the potential for the realization of measurement-based quantum computation 13 . Using scanning tunnelling microscopy and spectroscopy, fractional edge excitations are observed in nanographene spin chains, enabling the potential to study strongly correlated phases in purely organic materials.

Boundaries between distinct topological phases of matter support robust, yet exotic quantum states such as spin–momentum locked transport channels or Majorana fermions 1 – 3 . The idea of using such states in spintronic devices or as qubits in quantum information technology is a strong driver of current research in condensed matter physics 4 – 6 . The topological properties of quantum states have helped to explain the conductivity of doped trans -polyacetylene in terms of dispersionless soliton states 7 – 9 . In their seminal paper, Su, Schrieffer and Heeger (SSH) described these exotic quantum states using a one-dimensional tight-binding model 10 , 11 . Because the SSH model describes chiral topological insulators, charge fractionalization and spin–charge separation in one dimension, numerous efforts have been made to realize the SSH Hamiltonian in cold-atom, photonic and acoustic experimental configurations 12 – 14 . It is, however, desirable to rationally engineer topological electronic phases into stable and processable materials to exploit the corresponding quantum states. Here we present a flexible strategy based on atomically precise graphene nanoribbons to design robust nanomaterials exhibiting the valence electronic structures described by the SSH Hamiltonian 15 – 17 . We demonstrate the controlled periodic coupling of topological boundary states 18 at junctions of graphene nanoribbons with armchair edges to create quasi-one-dimensional trivial and non-trivial electronic quantum phases. This strategy has the potential to tune the bandwidth of the topological electronic bands close to the energy scale of proximity-induced spin–orbit coupling 19 or superconductivity 20 , and may allow the realization of Kitaev-like Hamiltonians 3 and Majorana-type end states 21 . Graphene nanoribbons are used to design robust nanomaterials with controlled periodic coupling of topological boundary states to create quasi-one-dimensional trivial and non-trivial electronic quantum phases.

Zigzag edges of graphene nanostructures host localized electronic states that are predicted to be spin-polarized. However, these edge states are highly susceptible to edge roughness and interaction with a supporting substrate, complicating the study of their intrinsic electronic and magnetic structure. Here, we focus on atomically precise graphene nanoribbons whose two short zigzag edges host exactly one localized electron each. Using the tip of a scanning tunnelling microscope, the graphene nanoribbons are transferred from the metallic growth substrate onto insulating islands of NaCl in order to decouple their electronic structure from the metal. The absence of charge transfer and hybridization with the substrate is confirmed by scanning tunnelling spectroscopy, which reveals a pair of occupied/unoccupied edge states. Their large energy splitting of 1.9 eV is in accordance with ab initio many-body perturbation theory calculations and reflects the dominant role of electron–electron interactions in these localized states. The zigzag edges of graphene host edge-localized electronic states with aligned electron spins, but these states strongly interact with metallic substrates. Here, the authors measure the electronic structure of graphene nanoribbons after transferring them to an insulating support.

The chirality-controlled synthesis of single-walled carbon nanotubes (SWCNTs) is a major challenge facing current nanomaterials science. The surface-assisted bottom-up fabrication from unimolecular CNT seeds (precursors), which unambiguously predefine the chirality of the tube during the growth, appears to be the most promising approach. This strategy opens a venue towards controlled synthesis of CNTs of virtually any possible chirality by applying properly designed precursor molecules. However, synthetic access to the required precursor molecules remains practically unexplored because of their complex structure. Here, we report a general strategy for the synthesis of molecular seeds for the controlled growth of SWCNTs possessing virtually any desired chirality by combinatorial multi-segmental assembly. The suggested combinatorial approach allows facile assembly of complex CNT precursors (with up to 100 carbon atoms immobilized at strictly predefined positions) just in one single step from complementary segments. The feasibility of the approach is demonstrated on the synthesis of the precursor molecules for 21 different SWCNT chiralities utilizing just three relatively simple building blocks. Bottom-up synthesis from rationally designed precursor molecules is one of the most promising routes to single-walled carbon nanotubes of any desired chirality. Here, the authors present a combinatorial approach to easily assemble a variety of these complex nanotube precursors from simple complementary segments.

Present preparation methods fail to meet fully the demand for structurally pure single-walled carbon nanotubes; surface-catalysed cyclodehydrogenation reactions are now shown to convert precursor molecules deposited on a platinum(111) surface into ultrashort nanotube seeds that can then be grown further into defect-free and structurally pure single-walled carbon nanotubes of single chirality. Controlled synthesis of single-chirality carbon nanotubes The electronic properties of single-walled carbon nanotubes (SWCNTs) are extraordinarily sensitive to their precise structure. To exploit their technological potential fully, samples containing only one SWCNT type are needed. Juan Ramon Sanchez-Valencia et al . have combined synthetic chemistry with materials engineering to develop a strategy that, with further optimization, could provide a route to nanotube-based materials for use in light detectors, photovoltaics, field-effect transistors and sensors. They use a surface-catalysed cyclodehydrogenation reaction to fold rationally designed precursor molecules deposited on a Pt(111) surface to produce 'end caps' that act as seeds for the growth of defect-free and structurally pure SWCNTs. The technique requires only modest temperatures and is fully compatible with today's complementary metal oxide semiconductor technologies. Cover: Konstantin Amsharov. Over the past two decades, single-walled carbon nanotubes (SWCNTs) have received much attention because their extraordinary properties are promising for numerous applications 1 , 2 . Many of these properties depend sensitively on SWCNT structure, which is characterized by the chiral index ( n , m ) that denotes the length and orientation of the circumferential vector in the hexagonal carbon lattice. Electronic properties are particularly strongly affected, with subtle structural changes switching tubes from metallic to semiconducting with various bandgaps. Monodisperse ‘single-chirality’ (that is, with a single ( n , m ) index) SWCNTs are thus needed to fully exploit their technological potential 1 , 2 . Controlled synthesis through catalyst engineering 3 , 4 , 5 , 6 , end-cap engineering 7 or cloning strategies 8 , 9 , and also tube sorting based on chromatography 10 , 11 , density-gradient centrifugation, electrophoresis and other techniques 12 , have delivered SWCNT samples with narrow distributions of tube diameter and a large fraction of a predetermined tube type. But an effective pathway to truly monodisperse SWCNTs remains elusive. The use of template molecules to unambiguously dictate the diameter and chirality of the resulting nanotube 8 , 13 , 14 , 15 , 16 holds great promise in this regard, but has hitherto had only limited practical success 7 , 17 , 18 . Here we show that this bottom-up strategy can produce targeted nanotubes: we convert molecular precursors into ultrashort singly capped (6,6) ‘armchair’ nanotube seeds using surface-catalysed cyclodehydrogenation on a platinum (111) surface, and then elongate these during a subsequent growth phase to produce single-chirality and essentially defect-free SWCNTs with lengths up to a few hundred nanometres. We expect that our on-surface synthesis approach will provide a route to nanotube-based materials with highly optimized properties for applications such as light detectors, photovoltaics, field-effect transistors and sensors 2 .

Bottom-up synthesized graphene nanoribbons and graphene nanoribbon heterostructures have promising electronic properties for high-performance field-effect transistors and ultra-low power devices such as tunneling field-effect transistors. However, the short length and wide band gap of these graphene nanoribbons have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel ( L ch  ~ 20 nm) devices with a thin, high- κ gate dielectric and a 9-atom wide (0.95 nm) armchair graphene nanoribbon as the channel material, we demonstrate field-effect transistors with high on-current ( I on  > 1 μA at V d  = −1 V) and high I on /I off  ~ 10 5 at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel field-effect transistors with bottom-up synthesized armchair graphene nanoribbons. Graphene nanoribbons show promise for high-performance field-effect transistors, however they often suffer from short lengths and wide band gaps. Here, the authors use a bottom-up synthesis approach to fabricate 9- and 13-atom wide ribbons, enabling short-channel transistors with 10 5 on-off current ratio.

Curved π-conjugated polycyclic aromatic hydrocarbons, buckybowls, constitute an important class of materials with wide applications in materials science. Heteroatom doping of buckybowls is a viable route to tune their intrinsic physicochemical properties. However, synthesis of heteroatom-doped buckybowls is a challenging task. We report on a combined in-solution and on-surface synthetic strategy toward the fabrication of a buckybowl containing two fused nitrogen-doped pentagonal rings. We employ ultra-high-resolution scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations to characterize the final compound. The buckybowl contains a unique combination of non-hexagonal rings at its core, identified as the inverse Stone–Thrower–Wales topology, resulting in a distinctive bowl-opening-down conformation of the buckybowl on the surface. Our controlled design of non-alternant, heteroatom-doped polycyclic aromatic frameworks with established bottom-up fabrication techniques opens new opportunities in the synthesis of carbon nanostructures with the perspective of engineering properties of graphene-based devices. Heteroatom doping of buckybowls is a viable route to tune their intrinsic physico-chemical properties, but their synthesis remains challenging. Here, the authors report on a combined in-solution and on-surface synthetic strategy towards the fabrication of a buckybowl containing two fused nitrogen-doped pentagonal rings.

Ribbon development Graphene nanoribbons, narrow straight-edged strips of the single-atom-thick sheet form of carbon, are predicted to exhibit remarkable properties, making them suitable for future electronic applications. Before this potential can be realized, more chemically precise methods of production will be required. Cai et al . report a step towards that goal with the development of a bottom-up fabrication method that produces atomically precise graphene nanoribbons of different topologies and widths. The process involves the deposition of precursor monomers with structures that 'encode' the topology and width of the desired ribbon end-product onto a metal surface. Surface-assisted coupling of the precursors into linear polyphenylenes is then followed by cyclodehydrogenation. Given the method's versatility and precision, it could even provide a route to more unusual graphene nanoribbon structures with tuned chemical and electronic properties. Graphene nanoribbons (GNRs) have structure-dependent electronic properties that make them attractive for the fabrication of nanoscale electronic devices, but exploiting this potential has been hindered by the lack of precise production methods. Here the authors demonstrate how to reliably produce different GNRs, using precursor monomers that encode the structure of the targeted nanoribbon and are converted into GNRs by means of surface-assisted coupling. Graphene nanoribbons—narrow and straight-edged stripes of graphene, or single-layer graphite—are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices 1 , 2 , 3 . In particular, although the two-dimensional parent material graphene 4 , 5 exhibits semimetallic behaviour, quantum confinement and edge effects 2 , 6 should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical 7 , 8 , 9 , sonochemical 10 and lithographic 11 , 12 methods as well as through the unzipping of carbon nanotubes 13 , 14 , 15 , 16 , the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling 17 , 18 of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation 19 , 20 . The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots 21 , superlattice structures 22 and magnetic devices based on specific graphene nanoribbon edge states 3 .

Acenes are an important class of polycyclic aromatic hydrocarbons which have recently gained exceptional attention due to their potential as functional organic semiconductors. Fundamentally, they are important systems to study the convergence of physico-chemical properties of all-carbon sp 2 -frameworks in the one-dimensional limit; and by virtue of having a zigzag edge topology they also provide a fertile playground to explore magnetism in graphenic nanostructures. The study of larger acenes is thus imperative from both a fundamental and applied perspective, but their synthesis via traditional solution-chemistry route is hindered by their poor solubility and high reactivity. Here, we demonstrate the on-surface formation of heptacene and nonacene, via visible-light-induced photo-dissociation of α-bisdiketone precursors on an Au(111) substrate under ultra-high vacuum conditions. Through combined scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy investigations, together with state-of-the-art first principles calculations, we provide insight into the chemical and electronic structure of these elusive compounds. The synthesis of large acenes via traditional solution-chemistry routes is hindered by their poor solubility and high reactivity under ambient conditions. Here, the authors demonstrate the on-surface formation of large acenes, via visible-light-induced photo-dissociation of α-bisdiketone molecular precursors on an Au(111) substrate.

Polyacetylene (PA) comprises one-dimensional chains of sp 2 -hybridized carbon atoms that may take a cis or trans configuration. Owing to its simple chemical structure and exceptional electronic properties, PA is an ideal system to understand the nature of charge transport in conducting polymers. Here, we report the on-surface synthesis of both cis- and trans -PA chains and their atomic-scale characterization. The structure of individual PA chains was imaged by non-contact atomic force microscopy, which confirmed the formation of PA by resolving single chemical bond units. Angle-resolved photoemission spectroscopy suggests a semiconductor-to-metal transition through doping-induced suppression of the Peierls bond alternation of trans -PA on Cu(110). Electronically decoupled trans -PAs exhibit a band gap of 2.4 eV following copper oxide intercalation. Our study provides a platform for studying individual PA chains in real and reciprocal space, which may be further extended to study the intrinsic properties of non-linear excitons in conducting polymers. Polyacetylene is an ideal system to probe to gain a better understanding of the nature of charge transport in conducting polymers. Now, individual atomically precise polyacetylene chains have been synthesized on a copper surface and characterized using a range of techniques, revealing a doping-induced semiconductor-to-metal transition.