Unveiling the potential of a BCN-biphenylene monolayer as a high-performance anode material for alkali metal ion batteries: a first-principles study

Inspired by a freshly synthesized two-dimensional biphenylene carbon network, which features a captivating combination of hexagonal, square, and octagonal rings, we explored a similar biphenylene network composed of boron, carbon, and nitrogen (bpn-BCN) using first-principles calculations. There are six possible phases of borocarbonitrides, which are isoelectronic to biphenylene carbon networks with a stoichiometric ratio of 1 : 1 : 1 for boron (B), carbon (C), and nitrogen (N) atoms. All possible isoelectronic structures of the BCN combination of biphenylene networks are found to be stable, according to first-principles calculations. However, the geometry has a relatively large number of robust C-C and B-N bonds and strong partially ionic-covalent B-C and C-N bonds inside these bpn-BCN monolayers are effectively more stable. Furthermore, we employed first-principles calculations to investigate the electrochemical properties of the most stable geometry of BCN biphenylene as a potential anode material for alkali metal (AM) ion batteries. A global search has been made to find the most favourable alkali metal ion adsorption sites. The biphenylene monolayer has octagonal, square, and hexagonal motifs with different adsorption strengths. Furthermore, the partially ionic bond of B-N (due to the electronegativity difference) also supports the alkali metal ions for adsorption. The electronic properties of the stable phase of bpn-BCN reveal its narrow bandgap semiconductor nature. The ion diffusion calculations show a low activation barrier for Li, Na, and K of 0.65 eV, 0.26 eV, and 0.23 eV, respectively, indicating a fast charge/discharge rate. Furthermore, the theoretical capacities of the BCN biphenylene monolayer for Li (1057.33 mA h g-1), Na (647.27 mA h g-1), and K (465.98 mA h g-1) are found to be greater than those of commercial graphite. The average open-circuit voltage for AM decreases with increasing metal ion concentrations. It falls within a reasonable range of 0.34-1.89 V. Our results show that the BCN biphenylene monolayer could be a promising anode material in alkali metal ion rechargeable batteries.

Schwarzites and Triply Periodic Minimal Surfaces: From Pure Topology Mathematics to Macroscale Applications

Schwarzites are porous (spongy-like) carbon allotropes with negative Gaussian curvatures. They are proposed by Mackay and Terrones inspired by the works of the German mathematician Hermann Schwarz on Triply-Periodic Minimal Surfaces (TPMS). This review presents and discusses the history of schwarzites and their place among curved carbon nanomaterials. The main works on schwarzites are summarized and are available in the literature. Their unique structural, electronic, thermal, and mechanical properties are discussed. Although the synthesis of carbon-based schwarzites remains elusive, recent advances in the synthesis of zeolite-templates nanomaterials have brought them closer to reality. Atomic-based models of schwarzites are translated into macroscale ones that are 3D-printed. These 3D-printed models are exploited in many real-world applications, including water remediation and biomedical ones.

Unconventional Band Structure via Combined Molecular Orbital and Lattice Symmetries in a Surface-Confined Metallated Graphdiyne Sheet

Graphyne (GY) and graphdiyne (GDY)-based monolayers represent the next generation 2D carbon-rich materials with tunable structures and properties surpassing those of graphene. However, the detection of band formation in atomically thin GY/GDY analogues has been challenging, as both long-range order and atomic precision have to be fulfilled in the system. The present work reports direct evidence of band formation in on-surface synthesized metallated Ag-GDY sheets with mesoscopic (≈1 µm) regularity. Employing scanning tunneling and angle-resolved photoemission spectroscopies, energy-dependent transitions of real-space electronic states above the Fermi level and formation of the valence band are respectively observed. Furthermore, density functional theory (DFT) calculations corroborate the observations and reveal that doubly degenerate frontier molecular orbitals on a honeycomb lattice give rise to flat, Dirac and Kagome bands close to the Fermi level. DFT modeling also indicates an intrinsic band gap for the pristine sheet material, which is retained for a bilayer with h-BN, whereas adsorption-induced in-gap electronic states evolve at the synthesis platform with Ag-GDY decorating the (111) facet of silver. These results illustrate the tremendous potential for engineering novel band structures via molecular orbital and lattice symmetries in atomically precise 2D carbon materials.

Transition Metal (Co, V, W, Zr) Single-Atom Decorated Biphenylene for Enhancing the Sensing Performance of SF6 Decomposition Molecules

The highly sensitive gas sensors used to monitor the decomposition of toxic gases in the dielectric materials of electrical equipment are vital in preventing safety problems arising from corrosion of the equipment. Recently, biphenylene (BPN) has been prepared through surface interpolymer hydrofluorination (HF zipper) reaction, whereas potential gas-sensitive devices based on the BPN monolayer have lacked in-depth investigation. The stable geometries, adsorption energies, interlayer distances, and charge transfers of small molecules of toxic gases (H2S, SO2, SOF2, SO2F2) produced by SF6 chalcogenide molecules of decomposition adsorbed on the original BPN monolayer are systematically researched by using nonequilibrium Green's function methods and density functional theory. The results indicated that all small molecules adsorbed on the BPN monolayer are physisorbed, while the type of adsorption turned from physisorption to chemisorption when BPN carried out adsorption with adsorbing a transition metal atom (TMA). In addition, the characteristics of current-voltage (I-V) curves of H2S and SO2 based on the TMA-BPN gas sensors revealed that the currents in BPN-based gas sensors displayed an obvious anisotropy, and the currents in the zigzag direction are larger than that in the armchair orientation regardless of the molecular adsorption cases. Moreover, the difference of currents for TMA-decorated BPN sensors changed more remarkably before and after the adsorption of H2S and SO2 in the zigzag direction. This work offers insights into the design of gas-sensitive devices through the adsorption of small molecules on the TMA-decorated BPN monolayer.

On-surface polymerization reactions of dibrominated hexaphenylbenzene influenced by densely packed self-assembly

Controlled bottom-up fabrication of molecular nanostructures through on-surface reactions of tailor-made precursors is of scientific and technological interest. Recently, on-surface polymerization reactions influenced by precursor self-assembly have been reported. Thus, a fundamental understanding of the reaction process is a prerequisite for controlled formation. Herein, we report on the influence of molecular self-assembly of dibrominated hexaphenylbenzene (Br2-HPB) on the on-surface polymerization reactions on a Au(111) substrate. By using low-temperature scanning tunnelling microscopy (STM), we find that the polymerization of Br2-HPB proceeds while maintaining the long-range ordered self-assembly, despite a decrease in HPB-HPB distance due to debromination and successive covalent bonding of Br2-HPB. From the STM investigations of the polymerization process, we conclude that the polymerization of Br2-HPB is accompanied by molecular rotations to maintain the periodic array of the self-assembled structure, contrary to the conventional understanding of the polymerization of the self-assembled precursor layer.

PHOTH-graphene: a new 2D carbon allotrope with low barriers for Li-ion mobility

The continued interest in 2D carbon allotropes stems from their unique structural and electronic characteristics, which are crucial for diverse applications. This work theoretically introduces PHOTH-Graphene (PHOTH-G), a novel 2D planar carbon allotrope formed by 4-5-6-7-8 carbon rings. PHOTH-G emerges as a narrow band gap semiconducting material with low formation energy, demonstrating good stability under thermal and mechanical conditions. This material has slight mechanical anisotropy with Young modulus and Poisson ratios varying between 7.08-167.8 GPa and 0.21-0.96. PHOTH-G presents optical activity restricted to the visible range. Li atoms adsorbed on its surface have a migration barrier averaging 0.38 eV.

Graphsene as a novel porous two-dimensional carbon material for enhanced oxygen reduction electrocatalysis

Graphene allotropes with varied carbon configurations have attracted significant attention for their unique properties and chemical activities. This study introduces a novel two-dimensional carbon-based material, termed Graphsene (GrS), through theoretical study. Comprising tetra-, penta-, and dodeca-carbon rings, GrS's cohesive energy calculations demonstrate its superior structural stability over existing graphene allotropes, including graphyne and graphdiyne families. Phonon dispersion analysis confirms GrS's dynamic stability and its relatively low thermal conductivity. All calculated GrS elastic constants meet the Born criteria, ensuring mechanical stability. Ab-initio molecular dynamic simulations show GrS maintains its structure at 300 K. HSE06 calculations reveal a narrow electronic bandgap of 20 meV, with the electronic band structure featuring a highly anisotropic Dirac-like cone due to its intrinsic structural anisotropy along armchair and zigzag directions. Notably, GrS is predicted to offer exceptional catalytic performance for the oxygen reduction reaction, favoring the four-electron reduction pathway with high selectivity under both acidic and alkaline conditions. This discovery opens promising avenues for developing metal-free catalyst materials in clean energy production.

Directional dependence of the electronic and transport properties of biphenylene under strain conditions

In this study, we investigated the electronic and electronic transport properties of biphenylene (BPN) using first-principles density functional theory (DFT) calculations combined with the non-equilibrium Green's function (NEGF) formalism. We have focused on understanding the electronic properties of BPN, and the anisotropic behavior of electronic transport upon external strain. We found the emergence of electronic stripes (ESs) on the BPN surface and the formation of type-II Dirac cone near the Fermi level. In the sequence, the electronic transport results reveal that such ESs dictate the anisotropic behavior of the transmission function. Finally, we show that the tuning of the (anisotropic) electronic current, mediated by external mechanical strain, is ruled by the energy position of the lowest unoccupied states with wave-vectors perpedicular to the ESs. This control could be advantageous for applications in nanoelectronic devices that require precise control of current direction.

Engineering Two-Dimensional Nodal Semimetals in Functionalized Biphenylene by Fluorine Adatoms

We propose a band engineering scheme on the biphenylene network, a newly synthesized carbon allotrope. We illustrate that the electronic structure of the biphenylene network can be significantly altered by controlling conditions affecting the symmetry and destructive interference of wave functions through periodic fluorination. First, we investigate the mechanism for the appearance of a type-II Dirac fermion in a pristine biphenylene network. We show that the essential ingredients are mirror symmetries and stabilization of the compact localized eigenstates via destructive interference. While the former is used for the band-crossing point along high symmetry lines, the latter induces highly inclined Dirac dispersions. Subsequently, we demonstrate the transformation of the biphenylene network's type-II Dirac semimetal phase into various Dirac phases such as type-I Dirac, gapped type-II Dirac, and nodal line semimetals through the deliberate disruption of mirror symmetry or modulation of destructive interference by varying the concentration of fluorine atoms.

Silver Surface-Assisted Dehydrobrominative Cross-Coupling between Identical Aryl Bromides

Cross-coupling reactions represent an indispensable tool in chemical synthesis. An intriguing challenge in this field is to achieve selective cross-coupling between two precursors with similar reactivity or, to the limit, the identical molecules. Here we report an unexpected dehydrobrominative cross-coupling between 1,3,5-tris(2-bromophenyl)benzene molecules on silver surfaces. Using scanning tunneling microscopy, we examine the reaction process at the single-molecular level, quantify the selectivity of the dehydrobrominative cross-coupling, and reveal the modulation of selectivity by substrate lattice-related catalytic activity or molecular assembly effect. Theoretical calculations indicate that the dehydrobrominative cross-coupling proceeds via regioselective C-H bond activation of debrominated TBPB and subsequent highly selective C-C coupling of the radical-based intermediates. The reaction kinetics plays an important role in the selectivity for the cross-coupling. This work not only expands the toolbox for chemical synthesis but also provides important mechanistic insights into the selectivity of coupling reactions on the surface.

Universal inter-molecular radical transfer reactions on metal surfaces

On-surface synthesis provides tools to prepare low-dimensional supramolecular structures. Traditionally, reactive radicals are a class of single-electron species, serving as exceptional electron-withdrawing groups. On metal surfaces, however, such species are affected by conduction band screening effects that may even quench their unpaired electron characteristics. As a result, radicals are expected to be less active, and reactions catalyzed by surface-stabilized radicals are rarely reported. Herein, we describe a class of inter-molecular radical transfer reactions on metal surfaces. With the assistance of aryl halide precursors, the coupling of terminal alkynes is steered from non-dehydrogenated to dehydrogenated products, resulting in alkynyl-Ag-alkynyl bonds. Dehalogenated molecules are fully passivated by detached hydrogen atoms. The reaction mechanism is unraveled by various surface-sensitive technologies and density functional theory calculations. Moreover, we reveal the universality of this mechanism on metal surfaces. Our studies enrich the on-surface synthesis toolbox and develop a pathway for producing low-dimensional organic materials.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Proposing TODD-graphene as a novel porous 2D carbon allotrope designed for superior lithium-ion battery efficiency

The category of 2D carbon allotropes has gained considerable interest due to its outstanding optoelectronic and mechanical characteristics, which are crucial for various device applications, including energy storage. This study uses density functional theory calculations, ab initio molecular dynamics (AIMD), and classical reactive molecular dynamics (MD) simulations to introduce TODD-Graphene, an innovative 2D planar carbon allotrope with a distinctive porous arrangement comprising 3-8-10-12 carbon rings. TODD-G exhibits intrinsic metallic properties with a low formation energy and stability in thermal and mechanical behavior. Calculations indicate a substantial theoretical capacity for adsorbing Li atoms, revealing a low average diffusion barrier of 0.83 eV. The metallic framework boasts excellent conductivity and positioning TODD-G as an active layer for superior lithium-ion battery efficiency. Charge carrier mobility calculations for electrons and holes in TODD-G surpass those of graphene. Classical reactive MD simulation results affirm its structural integrity, maintaining stability without bond reconstructions at 2200 K.

First-principles study on the electronic properties of biphenylene, net-graphene, graphene+, and T-graphene based nanoribbons

Since the successful separation of graphene, carbon materials with the excellent physical and chemical properties have attracted the interest of a large number of researchers. In this paper, density functional theory combined with non-equilibrium Green's function is used to systematically study the electronic structures of two-dimensional biphenylene, net-graphene, graphene+ and T-graphene, and to reveal the electron transport properties of net-graphene nanodevices under asymmetric regulation. The results show that biphenylene, net-graphene, graphene+, and T-graphene all show metallic properties, in which biphenylene and net-graphene show anisotropy, while graphene+ and T-graphene show isotropy. In addition, for the one-dimensional new carbon based nanoribbons, except for the armchair-edged net-graphene and biphenylene nanoribbons, which exhibit semiconductor properties and a band gap value of 0.08 eV, the rest of the carbon nanoribbons display metal properties. Interestingly, two of them showed a tendency to oscillate and decrease the band gap value with increasing width, while BPN-2 biphenylene nanoribbons directly changed from exhibiting semiconductor to metallic properties with increasing width combination with no oscillation. The electronic transport properties of net-graphene nanoribbons based nanodevice models for electrons transform along zigzag and armchair directions are systematically studied. An obvious negative differential resistance characteristic along the armchair and zigzag directions can be found. Overall, these interesting results show that these new net-graphene nanodevices have good practical application prospects in future electronic nanodevices.

Prediction of highly stable 2D carbon allotropes based on azulenoid kekulene

Despite enormous interest in two-dimensional (2D) carbon allotropes, discovering stable 2D carbon structures with practically useful electronic properties presents a significant challenge. Computational modeling in this work shows that fusing azulene-derived macrocycles - azulenoid kekulenes (AK) - into graphene leads to the most stable 2D carbon allotropes reported to date, excluding graphene. Density functional theory predicts that placing the AK units in appropriate relative positions in the graphene lattice opens the 0.54 eV electronic bandgap and leads to the appearance of the remarkable 0.80 eV secondary gap between conduction bands - a feature that is rare in 2D carbon allotropes but is known to enhance light absorption and emission in 3D semiconductors. Among porous AK structures, one material stands out as a stable narrow-multigap (0.36 and 0.56 eV) semiconductor with light charge carriers (me = 0.17 m0,  mh = 0.19 m0), whereas its boron nitride analog is a wide-multigap (1.51 and 0.82 eV) semiconductor with light carriers (me = 0.39 m0,  mh = 0.32 m0). The multigap engineering strategy proposed here can be applied to other carbon nanostructures creating novel 2D materials for electronic and optoelectronic applications.

Tailoring giant quantum transport anisotropy in nanoporous graphenes under electrostatic disorder

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.

Transition metal small clusters anchored on biphenylene for effective electrocatalytic nitrogen reduction

The synthesis of ammonia via an electrochemical nitrogen reduction reaction (NRR, N2 + 6H+ + 6e- → 2NH3), which can weaken but not directly break an inert NN bond under mild conditions via multiple progressive protonation steps, has been proposed as one of the most attractive alternatives for the production of NH3. However, the development of appropriate catalyst materials is a major challenge in the application of NRRs. Recently, single- or multi-metal atoms anchored on two-dimensional (2D) substrates have been demonstrated as ideal candidates for facilitating NRRs. In this work, by applying spin-polarized density functional theory and ab initio molecular dynamic simulations, we systematically explored the performances of nine types of transition metal multi-atoms anchored on a recently developed 2D biphenylene (BPN) sheet in nitrogen reduction. Structural stability and NRR performance catalyzed by TMn (TM = V, Fe, Ni, Mo, Ru, Rh, W, Re, Ir; n = 1-4) clusters anchored on BPN sheets were systematically explored. After a strict six-step screening strategy, it was found that W2, Ru2 and Mo4 clusters loaded on BPN demonstrate superior potential for nitrogen reduction with extremely low onset potentials of -0.26, -0.36 and -0.17 V, respectively. Electronic structure analysis revealed that the enhanced ability of these multi-atom catalysts to effectively capture and reduce the N2 molecule can be attributed to bidirectional charge transfer between the d orbitals of transition metal atoms and molecular orbitals of the adsorbed N2 through a "donation-back donation" mechanism. Our findings highlight the value of BPN sheets as a substrate for designing multi-atom nitrogen reduction reaction catalysts.

Masked alkynes for synthesis of threaded carbon chains

Polyynes are chains of sp1 carbon atoms with alternating single and triple bonds. As they become longer, they evolve towards carbyne, the 1D allotrope of carbon, and they become increasingly unstable. It has been anticipated that long polyynes could be stabilized by supramolecular encapsulation, by threading them through macrocycles to form polyrotaxanes-but, until now, polyyne polyrotaxanes with many threaded macrocycles have been synthetically inaccessible. Here we show that masked alkynes, in which the C≡C triple bond is temporarily coordinated to cobalt, can be used to synthesize polyrotaxanes, up to the C68 [5]rotaxane with 34 contiguous triple bonds and four threaded macrocycles. This is the length regime at which the electronic properties of polyynes converge to those of carbyne. Cyclocarbons constitute a related family of molecular carbon allotropes, and cobalt-masked alkynes also provide a route to [3]catenanes and [5]catenanes built around cobalt complexes of cyclo[40]carbon and cyclo[80]carbon, respectively.

Insights into the DHQ-BN: mechanical, electronic, and optical properties

Computational materials research is vital in improving our understanding of various class of materials and their properties, contributing valuable information that helps predict innovative structures and complement empirical investigations. In this context, DHQ-graphene recently emerged as a stable two-dimensional carbon allotrope composed of decagonal, hexagonal, and quadrilateral carbon rings. Here, we employ density functional theory calculations to investigate the mechanical, electronic, and optical features of its boron nitride counterpart (DHQ-BN). Our findings reveal an insulating band gap of 5.11 eV at the HSE06 level and good structural stability supported by phonon calculations and ab initio molecular dynamics simulations. Moreover, DHQ-BN exhibits strong ultraviolet (UV) activity, suggesting its potential as a highly efficient UV light absorber. Its mechanical properties, including Young's modulus (230 GPa) and Poisson's ratio (0.7), provide insight into its mechanical resilience and structural stability.

Deciphering the Mechanism of On-Surface Dehydrogenative C-C Coupling Reactions

On-surface synthesis has proven to be a powerful approach for fabricating various low-dimensional covalent nanostructures with atomic precision that could be challenging for conventional solution chemistry. Dehydrogenative Caryl-Caryl coupling is one of the most popular on-surface reactions, of which the mechanisms, however, have not been well understood due to the lack of microscopic insights into the intermediates that are fleetingly existing under harsh reaction conditions. Here, we bypass the most energy-demanding initiation step to generate and capture some of the intermediates at room temperature (RT) via the cyclodehydrobromination of 1-bromo-8-phenylnaphthalene on a Cu(111) surface. Bond-level scanning probe imaging and manipulation in combination with DFT calculations allow for the identification of chemisorbed radicals, cyclized intermediates, and dehydrogenated products. These intermediates correspond to three main reaction steps, namely, debromination, cyclization (radical addition), and H elimination. H elimination is the rate-determining step as evidenced by the predominant cyclized intermediates. Furthermore, we reveal a long-overlooked pathway of dehydrogenation, namely, atomic hydrogen-catalyzed H shift and elimination, based on the observation of intermediates for H shift and superhydrogenation and the proof of a self-amplifying effect of the reaction. This pathway is further corroborated by comprehensive theoretical analysis on the reaction thermodynamics and kinetics.

Cycl[2,2,4]azine-embedded non-alternant nanographenes containing fused antiaromatic azepine ring

The development of non-alternant nanographenes has attracted considerable attention due to their unique photophysical properties. Herein, we reported a novel aza-doped, non-alternant nanographene (NG) 1 by embedding the cycl[2,2,4]azine unit into the benzenoid NG framework. Single-crystal X-ray diffractometry suggests saddle or twisted nonplanar geometry of the entire backbone of 1 and coplanar conformation of the cycl[2,2,4]azine unit. DFT calculation together with solid structure indicates that NG 1 possesses significant local antiaromaticity in the azepine ring. By oxidative process or trifluoroacetic acid treatment, this nanographene can transform into a mono-radical cation, which was confirmed by UV/Vis absorption, 1H NMR, and electron paramagnetic resonance (EPR) spectroscopy. The antiaromaticity/aromaticity switching of the azepine ring on 1˙+ from 1 enables the high stability of this radical cation, which remained intact for over 1 day. Due to the electron-donating nature of the nitrogen and the unique electronic structure, NG 1 exhibits strong electron-donating properties, as proved by the intermolecular charge transfer towards C60 with a high association constant. Furthermore, selective modification of NG 1 was accomplished by Vilsmeier reaction, and the derivatives 7 and 8 with substituted benzophenone were obtained. The photophysical and electronic properties can be tuned by the introduction of different electronic groups in benzophenone.

Theoretical insights into the structural, electronic and thermoelectric properties of the inorganic biphenylene monolayer

Being motivated by a recently synthesized biphenylene carbon monolayer (BPN), using first principles methods, we have studied its inorganic analogue (B-N analogue) named I-BPN. A comparative study of structural, electronic and mechanical properties between BPN and I-BPN was carried out. Like BPN, the stability of I-BPN was verified in terms of formation energy, phonon dispersion calculations, and mechanical parameters (Young's modulus and Poisson's ratio). The chemical inertness of I-BPN was also investigated by adsorbing an oxygen molecule in an oxygen-rich environment. It has been found that the B-B bond favours the oxygen molecule to be adsorbed through chemisorption. The lattice transport properties reveal that the phonon thermal conductivity of I-BPN is ten times lower than that of BPN. The electronic band structure reveals that I-BPN is a semiconductor with an indirect bandgap of 1.88 eV, while BPN shows metallic behaviour. In addition, we investigated various thermoelectric properties of I-BPN for possible thermoelectric applications. The thermoelectric parameters, such as the Seebeck coefficient, show the highest peak value of 0.00289 V K-1 at 300 K. Electronic transport properties reveal that I-BPN is highly anisotropic along the x and y-axes. Furthermore, the thermoelectric power factor as a function of chemical potential shows a peak value of 0.057 W m-1 K-2 along the x-axis in the p-type doping region. The electronic figure of merit shows a peak value of approximately unity. However, considering lattice thermal conductivity, the peak value of the total figure of merit (ZT) reduces to 0.68(0.46) for p-type and 0.56(0.48) for n-type doping regions along the x(y) direction at 900 K. It is worth noting that our calculated ZT value of I-BPN is higher than that of many other reported B-N composite materials.

Defective Biphenylene as High-Efficiency Hydrogen Evolution Catalysts

Electrocatalysts play a pivotal role in advancing the application of water splitting for hydrogen production. This research unveils the potential of defective biphenylenes as high-efficiency catalysts for the hydrogen evolution reaction. Using first-principles simulations, we systematically investigated the structure, stability, and catalytic performance of defective biphenylenes. Our findings unveil that defect engineering significantly enhances the electrocatalytic activity for hydrogen evolution. Specifically, biphenylene with a double-vacancy defect exhibits an outstanding Gibbs free energy of -0.08 eV, surpassing that of Pt, accompanied by a remarkable exchange current density of -3.08 A cm-2, also surpassing that of Pt. Furthermore, we find the preference for the Volmer-Heyrovsky mechanism in the hydrogen evolution reaction, with a low energy barrier of 0.80 eV. This research provides a promising avenue for developing novel metal-free electrocatalysts for water splitting with earth-abundant carbon elements, making a significant step toward sustainable hydrogen production.

Topological edge and corner states in biphenylene photonic crystal

The biphenylene network (BPN) has a unique two-dimensional atomic structure, where hexagonal unit cells are arranged on a square lattice. Inspired by such a BPN structure, we design a counterpart in the fashion of photonic crystals (PhCs), which we refer to as the BPN PhC. We study the photonic band structure using the finite element method and characterize the topological properties of the BPN PhC through the use of the Wilson loop. Our findings reveal the emergence of topological edge states in the BPN PhC, specifically in the zigzag edge and the chiral edge, as a consequence of the nontrivial Zak phase in the corresponding directions. In addition, we find the localization of electromagnetic waves at the corners formed by the chiral edges, which can be considered as second-order topological states, i.e., topological corner states.

Bespoke Tailoring of Graphenoid Sheets: A Rippled Molecular Carbon Comprising Cyclically Fused Nonbenzenoid Rings

The incorporation of nonbenzenoid rings into the hexagonal networks of graphenoid nanostructures is of immense importance for electronic, magnetic, and mechanical properties, but the underlying mechanisms of nonbenzenoid ring fusion are rather unexplored. Here, we report the synthesis and characterization of a rippled C84 molecular carbon, which contains 10 nonbenzenoid rings (five-, seven-, and eight-membered rings) that are contiguously fused to give a cyclic geometry. The fused nonbenzenoid rings impart high solubility, configurational stability, multiple reversible redox behaviors, unique aromaticity, and a narrow band gap to the system. Moreover, this carbon nanostructure allows for further functionalization via electrophilic substitution and metalation reactions, enabling access to finely tuned derivatives. Interestingly, both the bowl-shaped and planar conformations of the core in molecular carbon are observed in the solid state. Additionally, this molecular carbon displays ambipolar transport characteristics.

Biphenylite as Anode Materials for Alkali Metal Ion Batteries with Self-Enhanced Storage Mechanism

In this work, we theoretically investigate the feasibility of biphenylite, the van der Waals layered bulk structure from experimental biphenylene network monolayers, as an anode material for alkali metal ions. The results indicate that the theoretical properties of Li, Na, and K in biphenylite are generally beyond those in graphite. Li-biphenylite exhibits a high specific capacity of 744 mAh·g-1, with a corresponding voltage range of 0.90-0.36 V, low diffusion barrier (<0.30 eV), and small volume change (∼9.9%), far exceeding those of Li-graphite. Moreover, a novel self-enhanced storage mechanism is observed and unveiled, in which the heavy intercalation of Li atoms (i.e., electron doping) induces puckered distortion of the nonhoneycomb carbon frameworks to enhance the intercalation ability and capacity of Li ion via a chemical activation of carbon frameworks. Possessing excellent anode performance beyond graphite, biphenylite is a promising "all-around" anode material candidate for alkali metal ion batteries, especially for lithium ion batteries.

Chiraltube, rolling 2D materials into chiral nanotubes

Carbon nanotubes (NTs) are graphene sheets rolled into a 1D material, with a specific chirality that defines its structure and properties. Graphene has triggered the development of thousands of 2D materials, which in principle could also be rolled into 1D NTs. However, most of these NTs have not been proposed due to difficulties in the generation of atomic coordinates for chiral NTs from 2D materials with a non-hexagonal lattice or multi-layered materials. In this paper we present Chiraltube, an open-source Python code that allows the quick generation of a complete NT with any chirality from the unit cell of its original 2D material. We explain the inner workings of the code as well as the theoretical background on which it is built, generalizing concepts from the construction of chiral and achiral carbon NTs to work on any other 2D material. We show various examples of the resulting chiral NT structures built from phosphorene, MoS2 and Ti3C2, and present some analysis on the interatomic distortion in the outermost layers of these NTs, as well as the results of ab initio electronic structure calculations on a set of phosphorene NTs generated by the program, showing the immediate practicality and usefulness of the program. We also explore some limitations and details of the tool as well as further work to be done.

2D Metal-Free BSi5 with an Intrinsic Metallicity and Remarkable HER Activity

One of the most urgent and attractive topics in electrocatalytic water splitting is the exploration of high-performance and low-cost catalysts. Herein, we have proposed three fresh two-dimensional nanostructures (BSi5, BSi4, and BSi3) with inherent metallicity contributed by delocalized π electrons based on first-principles calculations. Their planar atoms arrangement, akin to graphene, is in favor of the availability of active atoms and H adsorption/deadsorption. Among them, the BSi5 monolayer shows the best HER activity, even superior to a commercial Pt catalyst. Moreover, its extraordinary HER activity can be maintained under high H coverage and large biaxial strain, mainly originating from the fact that B 2pz orbital electrons are responsible for the B-H interaction. Further analysis reveals that there appears to be a linear correlation between the magnitude of B 2pz DOS at the Fermi level and Gibbs free energy in both three proposed nanostructures and five hypothetical B-Si nanostructures. Our work represents a significant step forward toward the design of metal-free HER catalysts.

Anisotropic thermal expansion of silicon monolayer in biphenylene network

Materials with a negative thermal expansion property are of great importance in the emerging family of two-dimensional materials. For example, mixing two materials with negative and positive coefficients of thermal expansion avoids volume changing with temperature. In this work, based on first-principles calculations and Grüneisen's theory, we investigated the thermal expansion properties of a silicon monolayer in biphenylene networks. Our results show that the thermal expansion is greatly negative and anisotropic, as the linear thermal expansion coefficient along the a-direction is significantly smaller than the one along the b-direction, even at high temperatures. At 300 K, the thermal expansion coefficients along the two lattice directions are -17.010 × 10-6 K-1 and -2.907 × 10-6 K-1, respectively. By analyzing the Grüneisen parameters and the elastic compliance, we obtained an understanding of the giant negative thermal expansion of the material. Rigid unit modes are also responsible for the negative thermal expansion behavior. Our work provides fundamental insights into the thermal expansion of silicon monolayer in biphenylene networks and should stimulate the further exploration of the possible thermoelectric and thermal management applications of the material.

Graphene oxide-based large-area dynamic covalent interfaces

Dynamic materials, being capable of reversible structural adaptation in response to the variation of external surroundings, have experienced significant advancements in the past several decades. In particular, dynamic covalent materials (DCMs), where the dynamic covalent bonds (DCBs) can reversibly break and reform under defined conditions, present superior dynamic characteristics, such as self-adaptivity, self-healing and shape memory. However, the dynamic characteristics of DCBs are mainly limited within the length scale of covalent bonds, due to the local position exchange or the inter-distance variation between the chemical compositions involved in the reversible covalent reactions. In this minireview, a discussion regarding the realization of long-range migration of chemical compositions along the interfaces of graphene oxide (GO)-based materials via the spatially connected and consecutive occurrence of DCB-based reversible covalent reactions is presented, and the interfaces are termed "large-area dynamic covalent interfaces (LDCIs)". The effective strategies, including water adsorption, interfacial curvature and metal-substrate support, as well as the potential applications of LDCIs in water dissociation and humidity sensing are summarized. Additionally, we also give an outlook on potential strategies to realize LDCIs on other 2D carbon-based materials, including the interfacial morphology and periodic element doping. This minireview provides insights into the realization of LDCIs on a wider range of 2D materials, and offers a theoretical perspective for advancing materials with long-range dynamic characteristics and improved performance, including controlled drug delivery/release and high-efficiency (bio)sensing.

On-Surface Synthesis and Real-Space Visualization of Aromatic P3 N3

On-surface synthesis is at the verge of emerging as the method of choice for the generation and visualization of unstable or unconventional molecules, which could not be obtained via traditional synthetic methods. A case in point is the on-surface synthesis of the structurally elusive cyclotriphosphazene (P3 N3 ), an inorganic aromatic analogue of benzene. Here, we report the preparation of this fleetingly existing species on Cu(111) and Au(111) surfaces at 5.2 K through molecular manipulation with unprecedented precision, i.e., voltage pulse-induced sextuple dechlorination of an ultra-small (about 6 Å) hexachlorophosphazene P3 N3 Cl6 precursor by the tip of a scanning probe microscope. Real-space atomic-level imaging of cyclotriphosphazene reveals its planar D3h -symmetric ring structure. Furthermore, this demasking strategy has been expanded to generate cyclotriphosphazene from a hexaazide precursor P3 N21 via a different stimulation method (photolysis) for complementary measurements by matrix isolation infrared and ultraviolet spectroscopy.

Emergence of π-Magnetism in Fused Aza-Triangulenes: Symmetry and Charge Transfer Effects

On-surface synthesis has paved the way toward the fabrication and characterization of conjugated carbon-based molecular materials that exhibit π-magnetism such as triangulenes. Aza-triangulene, a nitrogen-substituted derivative, was recently shown to display rich on-surface chemistry, offering an ideal platform to investigate structure-property relations regarding spin-selective charge transfer and magnetic fingerprints. Herein, we study electronic changes upon fusion of single molecules into larger dimeric derivatives. We show that the closed-shell structure of aza-triangulene on Ag(111) leads to closed-shell dimers covalently coupled through sterically accessible carbon atoms. Meanwhile, its open-shell structure on Au(111) leads to coupling via atoms displaying a high spin density, resulting in symmetric or asymmetric products. Interestingly, whereas all dimers on Au(111) exhibit similar charge transfer properties, only asymmetric ones show magnetic fingerprints due to spin-selective charge transfer. These results expose clear relationships among molecular symmetry, charge transfer, and spin states of π-conjugated carbon-based nanostructures.

Studying the Adsorption of Gas Molecules and Defects on Modulating the Electronic Transport Characteristics of Monolayer Penta-BN2-Based Devices

Two-dimensional atomic layer materials, as an important part of the post-Moore era, have recently become an ideal choice for the preparation of high-efficiency, low-power, and miniaturized gas sensors. In this work, our study utilized density functional theory and the nonequilibrium Green's function method to investigate the electronic properties of the pentagonal BN2 (P-BN2) monolayer, as well as its gas-sensing properties for organic and inorganic gases. We also investigated how defects affect the quantum transport properties of the P-BN2-based device. Our findings demonstrate that the CO, H2S, NH3, SO2, C2H5OH, C3H6OH, CH3OH, and CH4 undergo physisorption on the P-BN2 monolayer, while NO, NO2, C2H2, C2H4, and HCHO undergo chemisorption. Then, we analyzed the impact of gas molecules chemisorbed on the P-BN2 monolayer on the electronic transport properties of the P-BN2-based gas sensor. When these five gas molecules are adsorbed, the current of the P-BN2-based gas sensor is greatly reduced. In addition, the effect of defects on the quantum transport properties of the P-BN2-based device is investigated. The results indicate that defects of N, B, and BN atoms lead to a decrease in the current of P-BN2-based nanodevices. Moreover, both the adsorption of gas molecules and the formation of vacancy defects leading to a decrease in device current can be revealed by the local device density of states near the zero-bias Fermi level, elucidating their microscopic mechanisms. Finally, gas molecules can also cause a decrease in the current of defect systems. These theoretical studies are of great significance for exploring two-dimensional atomic layer materials as high-efficiency gas sensors.

Two-Dimensional Biphenylene-Based Carbon Allotrope Family with High Potassium Storage Ability

The development of new carbon materials with novel properties and excellent applications is essential and urgent in many fields, such as potassium-ion batteries (PIBs). In this study, a family of 30 two-dimensional biphenylene carbon allotropes (2D-BCAs) have been systematically extended in theory. The energies of these allotropes are slightly higher than that of graphene, which can be well described by a quantitative energy equation. The 2D-BCAs show high synthesizability consistent with the experimental biphenylene network via "HF-zipping" reactions. The 2D-BCAs are metallic or semimetallic. Six representative 2D-BCAs exhibit good lattice dynamical and thermal stability, excellent anisotropic mechanical properties, and ORR catalytic activity. Moreover, the selected 2D-BCAs demonstrate ultrahigh theoretical potassium-storage capacities of 1116-1489 mAh·g-1, low migration barriers of 0.03-0.22 eV, and low open-circuit voltages of 1.10-0.02 V. The remarkable properties render 2D-BCAs as promising anode materials in PIBs, electrocatalysts, and conductors in electronics and iontronics.

On-surface synthesis of a doubly anti-aromatic carbon allotrope

Synthetic carbon allotropes such as graphene1, carbon nanotubes2 and fullerenes3 have revolutionized materials science and led to new technologies. Many hypothetical carbon allotropes have been discussed4, but few have been studied experimentally. Recently, unconventional synthetic strategies such as dynamic covalent chemistry5 and on-surface synthesis6 have been used to create new forms of carbon, including γ-graphyne7, fullerene polymers8, biphenylene networks9 and cyclocarbons10,11. Cyclo[N]carbons are molecular rings consisting of N carbon atoms12,13; the three that have been reported to date (N = 10, 14 and 18)10,11 are doubly aromatic, which prompts the question: is it possible to prepare doubly anti-aromatic versions? Here we report the synthesis and characterization of an anti-aromatic carbon allotrope, cyclo[16]carbon, by using tip-induced on-surface chemistry6. In addition to structural information from atomic force microscopy, we probed its electronic structure by recording orbital density maps14 with scanning tunnelling microscopy. The observation of bond-length alternation in cyclo[16]carbon confirms its double anti-aromaticity, in concordance with theory. The simple structure of C16 renders it an interesting model system for studying the limits of aromaticity, and its high reactivity makes it a promising precursor to novel carbon allotropes15.

Transition in electronic and magnetic properties of transition metal embedded semimetallic B-graphyne

Spintronics is extremely important in the future development of information technology. Notably, two-dimensional carbon materials with atomically thick and p-electron systems have great potential for application in ultrathin spintronic devices. B-graphyne (B-GY) is a recently proposed two-dimensional carbon allotrope with double Dirac cones. It is a promising nanomaterial for high-speed spintronic devices due to its ultra-high Fermi velocity and thermodynamic stability. We tune the electronic and magnetic properties of B-GY by doping 3d transition metals (TM) (Cr, Mn, Fe, Co, Ni) based on first-principles calculations. After doping, TM forms strong covalent bonds (Fe, Co, Ni) and ionic bonds (Cr, Mn) with adjacent C atoms. The system of TM-doped B-GY (TM@B-GY) is transformed from a semimetal for B-GY to a metal (Cr, Mn, Fe, Co), but Ni@B-GY is still semimetal. Among them, Co@B-GY is approximately a half-metal. Moreover, TM (except Ni) can induce the magnetism of B-GY to undergo spin splitting. The TM d-orbitals are strongly coupled to the C p-orbitals, which play an important role in inducing magnetism. The results show that the tunable electronic and magnetic properties of TM@B-GY are promising as a high-speed spintronic device. Our research helps advance the study of semimetallic carbon allotropes in the field of spintronics.

Synthesis of a Porous [14]Annulene Graphene Nanoribbon and a Porous [30]Annulene Graphene Nanosheet on Metal Surfaces

The electrical and mechanical properties of graphene-based materials can be tuned by the introduction of nanopores, which are sensitively related to the size, morphology, density, and location of nanopores. The synthesis of low-dimensional graphene nanostructures containing well-defined nonplanar nanopores has been challenging due to the intrinsic steric hindrance. Herein, we report the selective synthesis of one-dimensional (1D) graphene nanoribbons (GNRs) containing periodic nonplanar [14]annulene pores on Ag(111) and two-dimensional (2D) porous graphene nanosheet containing periodic nonplanar [30]annulene pores on Au(111), starting from a same precursor. The formation of distinct products on the two substrates originates from the different thermodynamics and kinetics of coupling reactions. The reaction mechanisms were confirmed by a series of control experiments, and the appropriate thermodynamic and kinetic parameters for optimizing the reaction pathways were proposed. In addition, the combined scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations revealed the electronic structures of porous graphene structures, demonstrating the impact of nonplanar pores on the π-conjugation of molecules.

Electronic properties and carrier mobilities of nanocarbons formed by non-benzoidal building blocks

Exotic 1D and 2D carbon nanostructures have been grown in the laboratory in the last few years by means of surface-assisted chemical routes. In these processes, the strategical choice of a molecular precursor plays a dominant role in the determination of the synthesized nanocarbon. Further variations of these techniques are able to produce non-benzoidal carbon quantum-dots (QDs). Considering this experimental scenario as motivation, we propose a series of nanoribbon systems based on concatenating recently synthesized carbon QDs containing pentagonal, hexagonal, and heptagonal rings. We use density functional theory (DFT) simulations to reveal their properties can range from metallic to semiconducting depending on the concatenation hierarchy used to form the nanoribbons. This DFT implementation is based on a LCAO approach to describe valence wavefunctions and most of the simulations employ the PBE-GGA functional. Since this functional is known to underestimate band gaps, we also use the B3LYP functional in a plane-wave DFT approach for a selected case for comparison purposes. These systems show a different gap versus width relationship compared to conventional graphene nanoribbons setups and a particular set of carrier mobility values. We further discuss the interplay between the QD's frontier states and the electronic properties of the nanoribbons in light of their structural details.

Electronic, Thermal and Mechanical Properties of Carbon and Boron Nitride Holey Graphyne Monolayers

In a recent experimental accomplishment, a two-dimensional holey graphyne semiconducting nanosheet with unusual annulative π-extension has been fabricated. Motivated by the aforementioned advance, herein we theoretically explore the electronic, dynamical stability, thermal and mechanical properties of carbon (C) and boron nitride (BN) holey graphyne (HGY) monolayers. Density functional theory (DFT) results reveal that while the C-HGY monolayer shows an appealing direct gap of 1.00 (0.50) eV according to the HSE06(PBE) functional, the BNHGY monolayer is an indirect insulator with large band gaps of 5.58 (4.20) eV. Furthermore, the elastic modulus (ultimate tensile strength) values of the single-layer C- and BN-HGY are predicted to be 127(41) and 105(29) GPa, respectively. The phononic and thermal properties are further investigated using machine learning interatomic potentials (MLIPs). The predicted phonon spectra confirm the dynamical stability of these novel nanoporous lattices. The room temperature lattice thermal conductivity of the considered monolayers is estimated to be very close, around 14.0 ± 1.5 W/mK. At room temperature, the C-HGY and BN-HGY monolayers are predicted to yield an ultrahigh negative thermal expansion coefficient, by more than one order of magnitude larger than that of the graphene. The presented results reveal decent stability, anomalously low elastic modulus to tensile strength ratio, ultrahigh negative thermal expansion coefficients and moderate lattice thermal conductivity of the semiconducting C-HGY and insulating BN-HGY monolayers.

Generalization of the Tensor Product Selected CI Method for Molecular Excited States

In a recent paper [JCTC, 2020, 16, 6098], we introduced a new approach for accurately approximating full CI ground states in large electronic active-spaces called Tensor Product Selected CI (TPSCI). In TPSCI, a large orbital active space is first partitioned into disjoint sets (clusters) for which the exact, local many-body eigenstates are obtained. Tensor products of these locally correlated many-body states are taken as the basis for the full, global Hilbert space. By folding correlation into the basis states themselves, the low-energy eigenstates become increasingly sparse, creating a more compact selected CI expansion. While we demonstrated that this approach can improve accuracy for a variety of systems, there is even greater potential for applications to excited states, particularly those which have some excited-state character. In this paper, we report on the accuracy of TPSCI for excited states, including a far more efficient implementation in the Julia programming language. In traditional SCI methods that use a Slater determinant basis, accurate excitation energies are obtained only after a linear extrapolation and at a large computational cost. We find that TPSCI with perturbative corrections provides accurate excitation energies for several excited states of various polycyclic aromatic hydrocarbons with respect to the extrapolated result (i.e., near exact result). Further, we use TPSCI to report highly accurate estimates of the lowest 31 eigenstates for a tetracene tetramer system with an active space of 40 electrons in 40 orbitals, giving direct access to the initial bright states and the resulting 18 doubly excited (biexcitonic) states.

On the CO[Formula: see text] adsorption in a boron nitride analog for the recently synthesized biphenylene network: a DFT study

CONTEXT

Recent advances in nanomaterial synthesis and characterization have led to exploring novel 2D materials. The biphenylene network (BPN) is a notable achievement in current fabrication efforts. Numerical studies have indicated the stability of its boron nitride counterpart, known as BN-BPN. In this study, we employ computational simulations to investigate the electronic and structural properties of pristine and doped BN-BPN monolayers upon CO[Formula: see text] adsorption. Our findings demonstrate that pristine BN-BPN layers exhibit moderate adsorption energies for CO[Formula: see text] molecules, approximately [Formula: see text]0.16 eV, indicating physisorption. However, introducing one-atom doping with silver, germanium, nickel, palladium, platinum, or silicon significantly enhances CO[Formula: see text] adsorption, leading to adsorption energies ranging from [Formula: see text]0.13 to [Formula: see text]0.65 eV. This enhancement indicates the presence of both physisorption and chemisorption mechanisms. BN-BPN does not show precise CO[Formula: see text] sensing and selectivity. Furthermore, our investigation of the recovery time for adsorbed CO[Formula: see text] molecules suggests that the interaction between BN-BPN and CO[Formula: see text] cannot modify the electronic properties of BN-BPN before the CO[Formula: see text] molecules escape.

METHODS

We performed density functional theory (DFT) simulations using the DMol3 code in the Biovia Materials Studio software. We incorporated Van der Waals corrections (DFT-D) within the Grimme scheme for an accurate representation. The exchange and correlation functions were treated using the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA). We used a double-zeta plus polarization (DZP) basis set to describe the electronic structure. Additionally, we accounted for the basis set superposition error (BSSE) through the counterpoise method. We included semicore DFT pseudopotentials to accurately model the interactions between the nuclei and valence electrons.

Examining O[Formula: see text] adsorption on pristine and defective popgraphene sheets: A DFT study

CONTEXT

Popgraphene (PopG) is a two-dimensional carbon-based material with fused pentagonal and octagonal rings. Like graphene, it exhibits a metallic band gap and exceptional thermal, dynamic, and mechanical stability. Here, we theoretically study the electronic and structural properties of PopG monolayers, including their doped and vacancy-endowed versions, as O[Formula: see text] adsorbers. Our findings show that pristine and vacancy-endowed PopG sheets have a comparable ability to adsorb O[Formula: see text] molecules, with adsorption energies ranging from [Formula: see text]0.57 to [Formula: see text]0.59 eV (physisorption). In these cases, octagonal rings play a dominant role in the adsorption mechanism. Platinum and Silicon doping enhance the O[Formula: see text] adsorption in areas close to the octagonal rings, resulting in adsorption energies ranging from [Formula: see text]1.13 to [Formula: see text]2.56 eV (chemisorption). Furthermore, we computed the recovery time for the adsorbed O[Formula: see text] molecules. The results suggest that PopG/O[Formula: see text] interaction in pristine and vacancy-endowed cases can change the PopG electronic properties before O[Formula: see text] diffusion.

METHODS

Density Functional Theory (DFT) simulations, with Van der Waals corrections (DFT-D, within the Grimme scheme), were performed to study the structural and electronic properties of PopG/O[Formula: see text] systems using the DMol3 code within the Biovia Materials Studio software. The exchange and correlation functions are treated within the generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) functional. We used the double-zeta plus polarization (DZP) for the basis set in these cases. We also considered the BSSE correction through the counterpoise method and the nuclei-valence electron interactions by including semi-core DFT pseudopotentials.

Theoretical prediction of 2D biphenylene as a potential anchoring material for lithium-sulfur batteries

Searching for good anchoring materials that can suppress the shuttle effect is critical to large-scale commercialization of lithium-sulfur (Li-S) batteries. In this work, the adsorption behavior of lithium polysulfides (LiPSs, such as S8 and Li2Sn, n = 1, 2, 4, 6, and 8), the sulfur reduction reaction (SRR), the decomposition processes of Li2S and the diffusion behavior of Li atoms on intrinsic and doped 2D biphenylene (BIP) are systematically investigated by employing the first-principles calculation method. Calculations show that the adsorption energies of LiPSs on the electrolyte (DOL and DME) are smaller than those on the intrinsic/B doped BIP. The moderate anchoring strength (0.8-2.0 eV) between LiPSs and the BIP can effectively suppress the shuttle effect. Moreover, the Gibbs free energy barrier for SRR is 0.72/0.64 eV on intrinsic/B doped BIP. The dissociation energy barrier of Li2S on intrinsic/B doped BIP is 1.35 eV, while the diffusion energy barrier of Li atoms on intrinsic/B doped BIP is 0.18 eV/0.30 eV. Lower energy barriers are conducive to enhancing the discharging and charging efficiency. Therefore, intrinsic and B doped BIP are predicted as good anchoring materials for Li-S batteries.

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.

A Comparative Study of the Electronic Transport and Gas-Sensitive Properties of Graphene+, T-graphene, Net-graphene, and Biphenylene-Based Two-Dimensional Devices

The electronic transport properties of the four carbon isomers: graphene+, T-graphene, net-graphene, and biphenylene, as well as the gas-sensing properties to the nitrogen-based gas molecules including NO2, NO, and NH3 molecules, are systematically studied and comparatively analyzed by combining the density functional theory with the nonequilibrium Green's function. The four carbon isomers are metallic, especially with graphene+ being a Dirac metal due to the two Dirac cones present at the Fermi energy level. The two-dimensional devices based on these four carbon isomers exhibit good conduction properties in the order of biphenylene > T-graphene > graphene+ > net-graphene. More interestingly, net-graphene-based and biphenylene-based devices demonstrate significant anisotropic transport properties. The gas sensors based on the above four structures all have good selectivity and sensitivity to the NO2 molecule, among which T-graphene-based gas sensors are the most prominent with a maximum ΔI value of 39.98 μA, being only three-fifths of the original. In addition, graphene+-based and biphenylene-based gas sensors are also sensitive to the NO molecule with maximum ΔI values of 29.42 and 25.63 μA, respectively. However, the four gas sensors are all physically adsorbed for the NH3 molecule. By the adsorption energy, charge transfer, electron localization functions, and molecular projection of self-consistent Hamiltonian states, the mechanisms behind all properties can be clearly explained. This work shows the potential of graphene+, T-graphene, net-graphene, and biphenylene for the detection of toxic molecules of NO and NO2.

Structural, mechanical, electronic and optical properties of biphenylene hydrogenation: a first-principles study

Biphenylene networks typically exhibit a metallic electronic nature, while hydrogenation can open the band gap changing it to a semiconductor. This property makes hydrogenated biphenylene a promising candidate for use in semiconductor optoelectronic materials and devices. In this work, three representative configurations of hydrogenated biphenylene, denoted by α, β and γ, were investigated. The structural, mechanical, electronic, and optical properties of these hydrogenated biphenylene configurations were calculated by first-principles calculations. Band gaps with HSE correction were 4.69, 4.42 and 4.39 eV for α, β, and γ configurations, respectively. Among these three configurations, β presents the best electronic performance and special elastic properties (negative Poisson's ratio), while γ exhibits the best elastic properties. In addition, we comprehensively analyze the mechanical properties of these configurations and provide evidence that hydrogenated biphenylene possibly exhibits a negative-Poisson's-ratio along the zigzag and armchair directions when hydrogen atoms are added to biphenylene in certain ways. Furthermore, although the electronic properties of γ are weaker than those of β, they are also excellent. In addition, the binding energies of β and γ are relatively lower, which indicates that β and γ are more stable. Our findings demonstrate that the hydrogenated biphenylene is a promising material with significant application potential in optoelectronic devices.

Size-tunable energy gaps of hydrogen-terminated biphenylene segments

The electronic properties of hydrogen-terminated biphenylene (BP) segments of different sizes on the sub-nanoscale are explored using density functional theory, and the size dependence of the energy gap is evaluated using a structural parameter as a function of the bond lengths and the electronic density contributions. More importantly, the energy gap is observed to decrease linearly with the reduced hydrogen-to-carbon ratio of the corresponding structures, while the decrease-rate undergoes a diminution of four times at a gap of 0.5 eV due to the transformed distribution of the lowest unoccupied molecular orbital. The results give a deep insight into the size-tunable energy gaps of BPs and provide a possibility for the preparation of hydrogen-terminated carbon materials with a desirable energy gap.

Computational Studies on Diverse Characterizations of Molecular Descriptors for Graphyne Nanoribbon Structures

Materials made of graphyne, graphyne oxide, and graphyne quantum dots have drawn a lot of interest due to their potential uses in medicinal nanotechnology. Their remarkable physical, chemical, and mechanical qualities, which make them very desirable for a variety of prospective purposes in this area, are mostly to blame for this. In the subject of mathematical chemistry, molecular topology deals with the algebraic characterization of molecules. Molecular descriptors can examine a compound's properties and describe its molecular topology. By evaluating these indices, researchers can predict a molecule's behavior including its reactivity, solubility, and toxicity. Amidst the captivating realm of carbon allotropes, γ-graphyne has emerged as a mesmerizing tool, with exquisite attention due to its extraordinary electronic, optical, and mechanical attributes. Research into its possible applications across numerous scientific and technological fields has increased due to this motivated attention. The exploration of molecular descriptors for characterizing γ-graphyne is very attractive. As a result, it is crucial to investigate and predict γ-graphyne's molecular topology in order to comprehend its physicochemical characteristics fully. In this regard, various characterizations of γ-graphyne and zigzag γ-graphyne nanoribbons, by computing and comparing distance-degree-based topological indices, leap Zagreb indices, hyper leap Zagreb indices, leap gourava indices, and hyper leap gourava indices, are investigated.

A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites

Carbon nitrides that expose atomically dispersed single-atom metals in the form of M-N-C (M = metal) sites are attractive earth-abundant catalyst materials that have been demonstrated in electrocatalytic conversion reactions. The catalytic performance is determined by the abundance of N-doped sites and the type of metal coordination to N, but challenges remain to synthesize pristine carbon nitrides with a high concentration of the most active sites and prepare homogeneously doped materials that allow for in-depth characterization of the M-N-C sites and quantitative evaluation of their catalytic performance. Herein, we have synthesized and characterized a well-defined monolayer carbon nitride phase on a Au(111) surface that exposes an exceedingly high concentration of Co-N4 sites. The crystalline monolayer carbon nitride, whose formation is controlled by an on-surface reaction between Co atoms and melamine on Au(111), is characterized by a dense array of 4- and 6-fold N-terminated pockets, whereof only the 4-fold pocket is found to be holding Co atoms. Through detailed characterization using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory modeling, we determine the atomic structure and chemical state of the carbon nitride network. Furthermore, we show that the monolayer carbon nitride structure is stable and reactive toward the electrocatalytic oxygen reduction reaction in alkaline electrolyte, with a quantitative performance metric that significantly exceeds comparable M-N-C-based catalyst types. The work demonstrates that high-density active catalytic sites can be created using common precursor materials, and the formed networks themselves offer an excellent platform for onward studies addressing the characteristics of M-N-C sites.

On-Surface Two-Dimensional Polymerization: Advances, Challenges, and Prospects

Two-dimensional polymers (2DPs) are molecularly thin networks consisting of monomers covalently linked in at least two directions in the molecular plane. Because of the unique structural features and emergent physicochemical properties, 2DPs promise application potentials in catalysis, chemical sensing, and organic electronic devices. On-surface synthesis is of great interest to fabricate 2DPs with atomic precision, and the properties of the 2DPs can be characterized in situ through scanning probe techniques. In this Perspective, we first introduce the recent developments of on-surface 2D polymerization, including the design principle, the synthetic reactions, and the factors affecting the synthesis of 2DPs on surface. Then, we summarize some major challenges in this field, including the fabrication of high-quality 2DPs and the study of the intrinsic electronic properties of 2DPs, and we discuss some of the available solutions to address these issues.