Aran Garcia-Lekue

Spin‐split states localized on zigzag edges have been predicted for different free‐standing graphene nanostructures. However, the stability of the edge magnetism can be severely compromised by interfacial interactions when supported on metallic substrates. This imposes a serious challenge for the surface‐assisted synthesis and characterization of magnetic graphene nanostructures, and their application in metal supports. In this work we demonstrate that graphene nanoislands with well‐defined zigzag edges, synthesized by ultra‐high vacuum chemical vapour deposition, can retrieve their spin‐split edge states by minimizing interfacial interactions through metal intercalation. Using scanning tunneling microscopy and spectroscopy we detect edge states with subnanometer spatial localization and 1.2 eV energy splitting, comparable to the spin‐polarized states predicted by ab‐initio calculations. The fabrication of magnetic nanographenes by CVD opens a new avenue for the fabrication of 2D nanostructures that cannot be achieved by on‐surface synthesis, and enables the implementation of demonstrated processes for scaling up synthesis.

The implementation of graphene in semiconductor technology requires bandgap tuning, which can be achieved by chemical doping, geometric confinement in nanoribbons or with the insertion of nanopores and non‐hexagonal rings. The bottom‐up on‐surface synthesis approach in ultra‐high vacuum allows for the synthesis of atomically well‐defined graphene or biphenylene nanoribbons and nanoporous graphene (NPG) structures suitable for device applications. Here, a novel 2D carbon allotrope is synthesized in the form of a functional NPG with periodically spaced biphenylene segments. First, 12‐armchair porous graphene nanoribbons (12‐pGNRs) on Au(111) and Au(788) using 7,10‐dibromo‐1,4‐diphenyl‐triphenylene (DBDT) molecular precursor are grown. Low‐temperature scanning tunneling microscopy/spectroscopy (LT‐STM/STS) and non‐contact atomic force microscopy (nc‐AFM) measurements reveal the presence of high‐quality semiconducting 12‐pGNRs. Thermal annealing of densely packed 12‐pGNRs at 550 °C triggers their lateral fusion into diverse NPG structures featuring either graphene‐type or biphenylene‐type junctions. The structural and electronic properties are again characterized by LT‐STM and nc‐AFM in combination with density functional theory (DFT) calculations. DFT shows that while graphene‐type NPGs are direct bandgap semiconductors, biphenylene‐type NPGs manifest a smaller and indirect bandgap. The NPGs are stable upon oxygen and air exposure, and the nanopores feature a noticeable affinity to carbon monoxide, making them appealing systems for chemical sensors.

The magnetic complex Fe(II) porphyrin was studied on superconducting Pb(100). Low-temperature scanning tunneling microscopy was performed on isolated molecules and molecular networks prepared via Ullmann coupling. Isolated molecules adopt two distinct adsorption configurations with different magnetic excitations, namely, Yu-Shiba-Rusinov states and spin excitations. They may be understood in terms of their different magnetic anisotropies and exchange couplings with the substrate, as illustrated by the density functional theory calculations. In networks, strong intermolecular interactions lead to diverse structures with distinct magnetic and electronic properties. This work demonstrates a degree of control over the magnetic quantum states of a molecular complex on a superconductor.

We report the surface‐assisted synthesis of a non‐planar cyclophenylene derivative containing four meta‐ and two para‐ connected phenylene moieties on Au(111), via hierarchical Ullmann coupling of a 1,10‐dibrominated angular [3] phenylene and subsequent C−C bond cleavage at the four‐membered rings. Scanning tunneling microscopy and spectroscopy (STM/STS) were used for the characterization of its chemical structure and electronic properties. Density functional theory (DFT) calculations support the experimental observations.

Nanoporous graphene (NPG), laterally bonded carbon nanoribbons, is a promising platform for controlling coherent electron propagation in the nanoscale. However, for its successful device integration NPG should ideally be on a substrate that preserves or enhances its anisotropic transport properties. Here, using an atomistic tight-binding model combined with nonequilibrium Green's functions, we study NPG on graphene and show that their electronic coupling is modulated as a function of the interlayer twist angle. At small twist angles (θ ≲ 10°), strong hybridization leads to substantial interlayer transmission and Talbot-like interference in the current flow on both layers. Besides, injected currents exhibit chiral features due to the twist-induced mirror-symmetry-breaking. Upon increasing the twist angle, the coupling is weakened and the monolayer electronic properties are restored. Furthermore, we predict spectroscopic signatures that allow to probe the twist-dependent interlayer coupling via scanning tunneling microscopy.

Advancements in the on-surface synthesis of atomically precise graphene nanostructures are propelled by the introduction of innovative precursor designs and reaction types. Until now, the latter has been confined to cross-coupling and cyclization reactions that involve the cleavage of specific atoms or groups. In this article, we elucidate how the migration of phenyl substituents attached to graphene nanoribbons can be harnessed to generate arrays of [18]-annulene pores at the edges of the nanostructures. This sequential pathway is revealed through a comprehensive study employing bond-resolved scanning tunneling microscopy and ab-initio computational techniques. The yield of pore formation is maximized by anchoring the graphene nanoribbons at steps of vicinal surfaces, underscoring the potential of these substrates to guide reaction paths. Our study introduces a new reaction to the on-surface synthesis toolbox along with a sequential route, altogether enabling the extension of this strategy towards the formation of other porous nanostructures.

Correction for ‘Deciphering the chemical bonding of the trivalent oxygen atom in oxygen doped graphene’ by Andoni Ugartemendia et al., Chem. Sci., 2024, 15, 6151–6159, https://doi.org/10.1039/D4SC00142G.

Recently, planar and neutral tricoordinated oxygen embedded in graphene has been imaged experimentally (Nat. Commun., 2019, 10, 4570–4577). In this work, this unusual chemical species is studied utilizing a variety of state-of-the-art methods and combining periodic calculations with a fragmental approach. Several factors influencing the stability of trivalent oxygen are identified. A σ-donation and a π-backdonation mechanism between graphite and oxygen is established. π-Local aromaticity, with a delocalized 4c–2e bond involving the oxygen atom and the three nearest carbon atoms aids in the stabilization of this system. In addition, the framework in which the oxygen is embedded is crucial too to the stabilization, helping to delocalize the “extra” electron pair in the virtual orbitals. Based on the understanding gathered in this work, a set of organic molecules containing planar and neutral trivalent oxygen is theoretically proposed for the first time.

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.

We investigated the Kondo effect of cobalt(II)-5-15-bis(4'-bromophenyl)-10,20-bis(4'-iodophenyl)porphyrin (CoTPPBr₂I₂) molecules on Au(111) with low-temperature scanning tunneling microscopy under ultrahigh vacuum conditions. The molecules exhibit four adsorption configurations at the top and bridge sites of the surface with different molecular orientations. The Kondo resonance shows extraordinary sensitivity to the adsorption configuration. By switching the molecule between different configurations, the Kondo temperature is varied over a wide range from ≈8 up to ≈250 K. Density functional theory calculations reveal that changes of the adsorption configuration lead to distinct variations of the hybridization between the molecule and the surface. Furthermore, we show that surface reconstruction plays a significant role for the molecular Kondo effect.