The surface‐assisted polymerization and cyclodehydrogenation of specifically designed organic precursors provides a route toward atomically precise graphene nanoribbons, which promises to combine the ...outstanding electronic properties of graphene with a bandgap that is sufficiently large for room‐temperature digital‐logic applications. Starting from the basic concepts behind the on‐surface synthesis approach, this report covers the progress made in understanding the different reaction steps, in synthesizing atomically precise graphene nanoribbons of various widths and edge structures, and in characterizing their properties, ending with an outlook on the challenges that still lie ahead.
Surface‐assisted polymerization and cyclodehydrogenation of molecular building blocks provide a route toward atomically precise graphene nanoribbons, which combines the outstanding electronic properties of graphene with a bandgap that is sufficiently large for digital‐logic applications. This report covers the progress made in understanding the on‐surface reactions, in synthesizing atomically precise nanoribbons of various shapes, and in measuring their properties.
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.
Triangular zigzag nanographenes, such as triangulene and its π‐extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building ...blocks for high‐spin networks with long‐range magnetic order, which are of immense fundamental and technological relevance. As a first step towards these lines, we present the on‐surface synthesis and a proof‐of‐principle experimental study of magnetism in covalently bonded triangulene dimers. On‐surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4‐phenylene spacer. The chemical structures of the dimers have been characterized by bond‐resolved scanning tunneling microscopy. Scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy measurements reveal collective singlet–triplet spin excitations in the dimers, demonstrating efficient intertriangulene magnetic coupling.
The on‐surface synthesis of covalently bonded triangulene dimers with or without a 1,4‐phenylene spacer was achieved on Au(111). Scanning tunneling spectroscopy measurements revealed collective magnetism in the dimers in the form of singlet–triplet spin excitations, demonstrating efficient and tunable intertriangulene magnetic coupling.
The state of vanishing friction known as superlubricity has important applications for energy saving and increasing the lifetime of devices. Superlubricity, as detected with atomic force microscopy, ...appears when sliding large graphite flakes or gold nanoclusters across surfaces, for example. However, the origin of the behavior is poorly understood because of the lack of a controllable nanocontact. We demonstrated the superlubricity of graphene nanoribbons when sliding on gold with a joint experimental and computational approach. The atomically well-defined contact allows us to trace the origin of superlubricity, unraveling the role played by ribbon size and elasticity, as well as by surface reconstruction. Our results pave the way to the scale-up of superlubricity and thus to the realization of frictionless coatings.
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.
Dehydrogenation reactions are key steps in many metal‐catalyzed chemical processes and in the on‐surface synthesis of atomically precise nanomaterials. The principal role of the metal substrate in ...these reactions is undisputed, but the role of metal adatoms remains, to a large extent, unanswered, particularly on gold substrates. Here, we discuss their importance by studying the surface‐assisted cyclodehydrogenation on Au(111) as an ideal model case. We choose a polymer theoretically predicted to give one of two cyclization products depending on the presence or absence of gold adatoms. Scanning probe microscopy experiments observe only the product associated with adatoms. We challenge the prevalent understanding of surface‐assisted cyclodehydrogenation, unveiling the catalytic role of adatoms and their effect on regioselectivity. The study adds new perspectives to the understanding of metal catalysis and the design of on‐surface synthesis protocols for novel carbon nanomaterials.
Using scanning tunneling microscopy and electronic structure theory, we show that, contrary to previous consensus, surface adatoms can play a central role in the highly chemoselective cyclodehydrogenation of organic polymers.
Over the past two decades, single-walled carbon nanotubes (SWCNTs) have received much attention because their extraordinary properties are promising for numerous applications. 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. Controlled synthesis through catalyst engineering, end-cap engineering or cloning strategies, and also tube sorting based on chromatography, density-gradient centrifugation, electrophoresis and other techniques, 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 holds great promise in this regard, but has hitherto had only limited practical success. 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.
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
~ 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
> 1 μA at V
= -1 V) and high I
/I
~ 10
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
on-off current ratio.
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