Several sp2 carbon-based two-dimensional (2D) materials, including graphynes and conjugated covalent organic frameworks, have been predicted to host graphene-like semimetallic electronic structures. Careful electronic structure calculations increasingly indicate that many such systems favour correlated open-shell antiferromagnetic (AFM) ground states. These results call into question the intrinsic chemical stability of many graphynes and, consequently, their synthetic accessibility. Here, we use first principles hybrid density functional theory to examine how silicon (Si) substitution modifies the electronic ground state of three representative carbon-based 2D materials, namely graphene, alpha-graphyne, and delta-graphyne. These materials all possess networks of sp2-nodes connected in honeycomb-like topologies with two sublattices (i.e. A and B). We systematically assess substitutional Si configurations across different Si:C ratios and Si spatial ordering, with particular focus on how AB sublattice symmetry breaking governs their electronic structure. In graphene, Si substitution patterns that break AB symmetry open a finite band gap, whereas symmetry-preserving arrangements largely maintain the semimetallic parent state. In alpha-graphyne and delta-graphyne, AB symmetry breaking suppresses the tendency for open-shell AFM ground states and stabilises closed-shell semiconducting electronic structures. Conversely, Si substitution patterns that conserve AB sublattice symmetry tend to reinforce the less chemically stable AFM character. Across all systems, the magnitude of the induced band gap correlates with the extent of sublattice symmetry breaking. These results identify AB sublattice symmetry engineering through Si substitution as a general route to simultaneously tune the electronic structure and enhance the chemical stability of graphyne-based materials.

One dimensional conjugated polymers (1DCPs) that host periodic arrays of unpaired electrons are gaining increasing attention as atomically-precise correlated materials for future quantum technologies. 1DCPs based on triarylmethyls (TAM) are particularly interesting, due to the persistent nature of TAM radicals and the possibility to control the delocalization of their unpaired electrons via different means. However, the known strategies to effectively control the quantum ground-state of these organic systems is still limited. Here, by means of first principles density functional theory (DFT) calculations, we propose the use of a rational periodic substitution of radical sp2 carbon (C) sites in TAM 1DCPs by sp2 nitrogen (N) atoms, as a means to tailor the quantum state of the resulting mixed-valence (mv) 1DCP. In particular, we explore a fully alternating N-substitution pattern (NCNC) and a semi-alternating one (NNCC), and show that the former gives rise to a robust multiradical open-shell configuration, whereas the latter leads to a closed-shell quinoidal state. Via ab initio molecular dynamics simulations, we demonstrate that such quantum engineering of mv-1DCPs is robust to thermal fluctuations at room temperature, which highlights the technological viability of our approach for future molecular scale quantum electronics and spintronics.

Bottom-up on-surface synthesis enables the fabrication of carbon nanostructures with atomic precision. Good examples are graphene nanoribbons (GNRs), 1D conjugated polymers, and nanoporous graphenes (NPGs), which are gathering increasing attention for future carbon nanoelectronics. A key step is the ability to manipulate current flow within these nanomaterials. Destructive quantum interference (QI), long studied in the field of single-molecule electronics, has been proposed as the most effective way to achieve such control with molecular-scale precision. However, for practical applications, it is essential that such QI-engineering remains effective near or above room temperature. To assess this important point, here we combine large-scale molecular dynamics simulations and quantum transport calculations and focus our study on NPGs formed as arrays of laterally bonded GNRs. By considering various NPGs with different inter-GNR chemical connections we disentangle the different factors determining electronic transport in these carbon nanomaterials at 300 K. Our findings unequivocally demonstrate that QI survives at room temperature, with thermal vibrations weakly restricting current flow along GNRs while completely blocking transport across GNRs. Our results thus pave the way towards the future realization of QI-engineered carbon nanocircuitry operating at room temperature, which is a fundamental step towards carbon-based nanoelectronics and quantum technologies.

Bottom-up on-surface synthesized nanoporous graphenes (NPGs), realized as 2D arrays of laterally covalently bonded pi-conjugated graphene nanoribbons (GNRs), are a family of carbon nanomaterials which are receiving increasing attention for nanoelectronics and biosensing. Recently, a so-called hybrid-NPG (hNPG) was synthesized, featuring an alternating sequence of doped and non-doped GNRs, resulting in a band staggering effect in its electronic structure. Such a feature is appealing for photo-catalysis, photovoltaics and even carbon nanocircuitry. However, to date, little is known about the transport properties of hNPG and its derivatives, which is key for most applications. Here, via Green's functions simulations, we study the quantum transport properties of hNPGs. We find that injected carriers in hNPG spread laterally through a number of GNRs, though such spreading may take place exclusively through GNRs of one type (doped or non-doped). We propose a simple model to discern the key parameters determining the electronic propagation in hNPGs and explore alternative hNPG designs to control the spreading/confinement and anisotropy of charge transport in these systems. For one such design, we find that it is possible to send directed electric signals with sub-nanometer precision for as long as one micrometer - a result first reported for any NPG.

The development of efficient and economical electrocatalysts for the oxygen evolution reaction (OER) that are stable and free of noble metals remains a significant scientific and technological challenge. The use of redox mediators (RMs) offers a promising approach to enhance the efficiency of electrocatalysts for a range of applications. However, the migration of the RM molecules between the electrodes, also known as shuttle effect, leads to undesirable redox side reactions and a reduction of the OER performance. Here, we introduce a novel approach to overcome this limitation by showing how covalently attaching RMs to the electrode surface through self-assembled monolayers (SAMs) is a promising route to immobilize them and prevent their diffusion into the electrolyte. For this purpose, we prepared different SAMs using two types of RMs based on tetrathiafulvalene (TTF) derivatives and using indium tin oxide (ITO) and fluorine doped tin oxide (FTO) as substrates. All electrodes showed efficient electrocatalytic activity under alkaline conditions. In this small proof-of-concept set of systems, we could achieve an OER performance with an overpotential of 400 mV at 0.25 mA cm-2 and a Tafel slope of 103 mV dec-1. We rationalise these experimental findings with computational chemical modelling, which suggests that further improvements could be achieved through targeted chemical modifications to tune the highest occupied molecular orbital energy in these TTF-based RMs. Thus, this work demonstrates that covalent immobilization of RMs via SAMs offers a robust platform for the rational molecular engineering of electrocatalysts.

Bottom-up on-surface synthesis has demonstrated an impressive capability to realize desired carbon nanomaterials with atomic precision, also referred to as carbon nanoarchitectures. By using chemically tailored organic building blocks, it is possible to obtain virtually any carbon nanoarchitecture, with equally tunable electronic and magnetic properties. Among all known carbon nanoarchitectures, graphene nanoribbons (GNRs) have become the most extensively studied for nanoelectronics, due to their conductive π-conjugated structure and semiconducting nature. In this review, we summarize the progress made on a particular type of nanoporous graphenes (NPGs), conceived as 2D arrays of laterally bonded GNRs. Due to their relative novelty, these GNR-based NPGs have not yet acquired the same global attention as their predecessors (GNRs). However, recent progress suggests that these nanomaterials may play a central role in future carbon nanoelectronics and spintronics. This is due in large part to the ability to fine tune, both by chemical design and by external means, the electronic coupling between neighbouring GNRs within the NPG, thereby enabling precise control over the anisotropic properties of these materials, as demonstrated by various theoretical studies. In this review, we summarize the different approaches that have been proposed to tune such inter-ribbon coupling and, thus, the anisotropy. Overall, these studies underscore the unique platform that GNR-based NPGs provide for tailoring quantum electronic properties and two-dimensional anisotropy. As the field progresses, this capability could be harnessed for targeted applications at the molecular scale or even the atomic scale.

In recent years, it has become possible via on-surface bottom-up synthesis to engineer the topological character of carbon nanostructures. Graphene nanoribbons and 1D conjugated polymers (1DCPs) have thus tailored so as to host either topologically trivial or non-trivial phases. Molecular design is the primary means to set the topological class of these nanomaterials. However, external control over topology is also demonstrated via electric fields or top-down hydrogenation. Inspired by the connection between topology and pi-conjugation, here it is demonstrated via first-principles calculations that aryl ring twist angles also serve as topological knobs. Focusing on rationally designed 1DCPs composed of triarylmethyl (TAM) units, it is shown that rotation of certain aryl rings enables a transition from the trivial to the topologically non-trivial phase. Accordingly, fixing a particular twist angle configuration (e.g., via chemical functionalization) is equivalent to robustly setting a targeted topological phase. It is also found that in considered 1DCPs, the quantum phase transition occurs without the electronic band gap closing, due to a multiradical antiferromagnetic phase emerging at the transition point. All in all, this study highlights the potential of aryl ring twisting for engineering topological properties in carbon nanomaterials and establishes TAM 1DCPs as exotic topological 1D systems. Here it is demonstrated that aryl ring twist angles may be used to tailor the topological nature of 1D conjugated polymers. Based on that, a simple chemical design strategy is established to robustly set the topological class of the system. These findings open the door to externally controlling topology via uniaxial strain or applied electric fields, both known to induce aryl ring twisting. image

During the last 15 years bottom-up on-surface synthesis has been demonstrated as an efficient way to synthesize carbon nanostructures with atomic precision, opening the door to unprecedented electronic control at the nanoscale. Nanoporous graphenes (NPGs) fabricated as two-dimensional arrays of graphene nanoribbons (GNRs) represent one of the key recent breakthroughs in the field. NPGs interestingly display in-plane transport anisotropy of charge carriers, and such anisotropy was shown to be tunable by modulating quantum interference. Herein, using large-scale quantum transport simulations, we show that electrical anisotropy in NPGs is not only resilient to disorder but can further be massively enhanced by its presence. This outcome paves the way to systematic engineering of quantum transport in NPGs as a novel concept for efficient quantum devices and architectures.

In the growing field of nanoelectronics, spin filtering devices are a topic of great interest. In recent years, there have been major advances in the synthesis of slim zigzag graphene nanoribbons (ZGNRs) that show inherent spin polarization along the edges, suggesting their potential as spintronic devices. In the following contribution, we show through theoretical investigations how a nitrogen doping pattern that is used to stabilize longer ZGNRs may turn a 2-ZGNR into an efficient spin filtering device through either simple physical or chemical transformations. Modeling of the electronic structure is performed using density functional theory, and we access global and local transport properties in the quasi-stationary limit from nonequilibrium Green's function simulations. Our work shows that both negative gating and double protonation change the nature of the ground state of these systems, which enables efficient spin-selective transport.

Two-dimensional conjugated polymers (2DCPs)─organic 2D materials composed of arrays of carbon sp2 centers connected by π-conjugated linkers─are attracting increasing attention due to their potential applications in device technologies. This interest stems from the ability of 2DCPs to host a range of correlated electronic and magnetic states (e.g., Mott insulators). Substitution of all carbon sp2 centers in 2DCPs by nitrogen or boron results in diamagnetic insulating states. Partial substitution of C sp2 centers by B or N atoms has not yet been considered for extended 2DCPs but has been extensively studied in the analogous neutral mixed-valence molecular systems. Here, we employ accurate first-principles calculations to predict the electronic and magnetic properties of a new class of hexagonally connected neutral mixed-valence 2DCPs in which every other C sp2 nodal center is substituted by either a N or B atom. We show that these neutral mixed-valence 2DCPs significantly energetically favor a state with emergent superexchange-mediated antiferromagnetic (AFM) interactions between C-based spin-1/2 centers on a triangular sublattice. These AFM interactions are surprisingly strong and comparable to those in the parent compounds of cuprate superconductors. The rigid and covalently linked symmetric triangular AFM lattice in these materials thus provides a highly promising and robust basis for 2D spin frustration. As such, extended mixed-valence 2DCPs are a highly attractive platform for the future bottom-up realization of a new class of all-organic quantum materials, which could host exotic correlated electronic states (e.g., unusual magnetic ordering, quantum spin liquids).

Molecular switches based on functionalized graphene nanoribbons (GNRs) are of great interest in the development of nanoelectronics. In experiment, it was found that a significant difference in the conductance of an anthraquinone derivative can be achieved by altering the pH value of the environment. Building on this, in this work we investigate the underlying mechanism behind this effect and propose a general design principle for a pH based GNR-based switch. The electronic structure of the investigated systems is calculated using density functional theory and the transport properties at the quasi-stationary limit are described using nonequilibrium Green’s function and the Landauer formalism. This approach enables the examination of the local and the global transport through the system. The electrons are shown to flow along the edges of the GNRs. The central carbonyl groups allow for tunable transport through control of the oxidation state via the pH environment. Finally, we also test different types of GNRs (zigzag vs. armchair) to determine which platform provides the best transport switchability.

During the last 15 years bottom-up on-surface synthesis has been demonstrated as an efficient way to synthesize carbon nanostructures with atomic precision, opening the door to unprecedented electronic control at the nanoscale. Nanoporous graphenes (NPGs) fabricated as two-dimensional arrays of graphene nanoribbons (GNRs) represent one of the key recent breakthroughs in the field. NPGs interestingly display in-plane transport anisotropy of charge carriers, and such anisotropy was shown to be tunable by modulating quantum interference. Herein, using large-scale quantum transport simulations, we show that electrical anisotropy in NPGs is not only resilient to disorder but can further be massively enhanced by its presence. This outcome paves the way to systematic engineering of quantum transport in NPGs as a novel concept for efficient quantum devices and architectures.

Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics.

During the past decades π-conjugated bi-radicals have attracted increasing attention, due to the existence of two close-in-energy resonant electronic configurations with very distinct characteristics: the open-shell bi-radical and the closed-shell quinoidal. The chemical design of the bi-radical structure has been shown to be very effective to shift the balance towards one, or the other, electronic distribution. Some reports have experimentally studied the analogous 1D oligomers and polymers, however, only the open-shell multi-radical configuration has been detected, and it is yet not very clear which structural and chemical parameters are relevant in such extended systems. In this work, via first principles quantum chemical simulations, we study a series of π-conjugated 1D polymers based on triarylmethyl radicals with different chemical functionalization. We find that dihedral angles of the aryl rings connecting the radical centres are the key conformational parameter determining the electronic balance. This provides a simple recipe to use chemical functionalization of aryl rings as a tool to shift the system towards either the electron paired or unpaired configurations. Additionally, we find such conformational control is also effective under the effect of thermal fluctuations, which highlights its potential technological applicability.

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials.

The discovery of graphene has catalyzed the search for other 2D carbon allotropes, such as graphynes, graphdiynes, and 2D π-conjugated polymers, which have been theoretically predicted or experimentally synthesized during the past decade. These materials exhibit a conductive nature bound to their π-conjugated sp2 electronic system. Some cases include sp-hybridized moieties in their nanostructure, such as acetylenes in graphynes; however, these act merely as electronic couplers between the conducting π-orbitals of sp2 centers. Herein, via first-principles calculations and quantum transport simulations, we demonstrate the existence of an acetylene-meditated transport mechanism entirely hosted by sp-hybridized orbitals. For that we propose a series of nanostructured 2D materials featuring linear arrangements of closely packed acetylene units which function as sp-nanowires. Because of the very distinct nature of this unique transport mechanism, it appears to be highly complementary with π-conjugation, thus potentially becoming a key tool for future carbon nanoelectronics.

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal–insulator transition on half‐filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex‐CORFs) based on single layers of half‐filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex‐CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out‐of‐plane compressive pressure. This work establishes 2D hex‐CORFs as a class of versatile single‐layer quantum materials to advance the understanding of low dimensional correlated electronic systems.

In this Article, we explore the potential of triarylmethyl (TAM) oligomers as devices for organic molecular spintronic applications. We use the Landauer formalism to compute global spin transport and local current analysis to understand transport at finite bias voltages within the nonequilibrium Green's function framework and using density functional theory calculations. Important variations of the total spin current are observed in chlorinated TAM dimers as a function of the electronic structure in the device. On the other hand, the spin-resolved local transport mechanism and the geometry of the nanodevice are found to depend weakly on the electronic structure, even at relatively high voltages. Such dimers could potentially be used in transistor applications. Further, devices based on chlorinated TAM monomers are shown to possess strong differential spin-transport properties at low bias voltage, a property that could be used for spin filtering.