Zigzag Edges Research Articles

Spin and charge interactions between nanographene host and ferrocene.

Ferrocene (FeCp2) was introduced as a non-magnetic guest molecule to activated carbon fibers (ACFs) as a nanographene-based host having localized spins originating from zigzag edges of graphene. The introduction of the guest molecule was confirmed by FTIR for ACFs-FeCp2 introduced at 55 (150) °C (FeCp2-ACFs-55(150)). The appearance of satellite Fe2p peaks and the increase in shake-up peak intensity of the C1s in the XPS spectrum proved the emergence of charge-transfer host-guest interaction in FeCp2-ACFs-150, supported by the red-shift of the G-band in the Raman spectrum. The six-times enhancement in the spin concentration in FeCp2-ACFs-150 compared with ACFs indicates the spin magnetism of the non-magnetic guest FeCp2+ molecule induced by a charge-transfer host-guest interaction in the nanographene host. The larger ESR linewidth than that expected from the dipolar interaction estimated by the localized spin concentration suggests the exchange interaction between the nanographene and FeCp2 spins. The narrowing of the ESR linewidth of FeCp2-ACFs-55 upon higher excitation microwave power suggests the inhomogeneity of the environment for FeCp2+ molecules in the nanographene host. The observed induction of spin magnetism by the interfacial interactions between the nanographene host and the guest molecules will be a promising strategy for developing a new class of molecular magnets.

Exploring edge states in square–octagon moiré lattice

Moiré lattices, achieved by the superposition of two or more twisted identical periodic lattices, are of interest to various fields because they provide additional degrees of freedom. Here, we theoretically and experimentally study the edge states in a square–octagon moiré lattice. This moiré lattice is created by superimposing two identical square sublattices with an antiphase and a special twist angle. Five different edges, named type-I zigzag edge, type-II zigzag edge, type-I bearded edge, type-II bearded edge, and armchair edge, are explored. Through band structure analysis and numerical simulation of edge excitation, we find that all five edges support edge states. The topological property of the type-I edge states is verified by calculating the 2D polarization of the lattice. Furthermore, the edge mode distribution manifests that multiple bands support identical edge states at the armchair and type-II bearded edges. In the experiment, the moiré lattice is generated by the CW-laser-writing technique, thereby observing all the edge states with corresponding edge excitations. This study broadens the understanding of edge states in the coupled moiré photonic lattices and provides a new platform for exploring topological physics.

Magneto-plasmon of AA-stacked bilayer graphene nanoribbons at finite temperature

Magneto-band structures of AA-stacked bilayer graphene nanoribbons with armchair and zigzag (BLAGNR and BLZGNR) edges are calculated by the pz-orbital tight-binding model. At zero field, BLAGNR is a semiconductor or metal determined by its width. Magnetic field can reduce band-gap and exhibit crossing bands. For metallic BLZGNRs, crossing bands shift and parabolic bands are separated more widely with magnetic field; however, partial flat bands keep unchanged. Temperature significantly increases plasmon frequency and strength for semiconducting BLAGNRs, but slightly affects those of metallic BLAGNRs and BLZGNRs. Magnetic field, for gapped BLAGNRs, can effectively reduce the threshold temperature for inducing plasmon. Whether magneto-plasmon frequency and critical momentum of dispersion relation increase or decrease is strongly sensitive to nanoribbon’s geometry. Field-modulated plasmons with a wide range of frequency could provide potential applications in waveguides and optical sensors.

Enhancing the 3D printing fidelity of vat photopolymerization with machine learning-driven boundary prediction

Like many pixel-based additive manufacturing (AM) techniques, digital light processing (DLP) based vat photopolymerization faces the challenge that the square pixel based processing strategy can lead to zigzag edges especially when feature sizes come close to single-pixel levels. Introducing greyscale pixels has been a strategy to smoothen such edges, but it is a challenging task to understand which of the many permutations of projected pixels would give the optimal 3D printing performance. To address this challenge, a novel data acquisition strategy based on machine learning (ML) principles is proposed, and a training routine is implemented to reproduce the smallest shape of an intended 3D printed object. Through this approach, a chessboard patterning strategy is developed along with an automated data refining and augmentation workflow, demonstrating its efficiency and effectiveness by reducing the deviation by around 30%.

Insights into the electronic, optical, and catalytic properties of finite biphenylene nanoribbons: First-principles study

The electronic, optical, and catalytic properties of finite two-dimensional biphenylene nanoribbons are investigated using density functional theory calculations. The nanoribbons are energetically and dynamically stable as confirmed by the negative formation energy ∼ -7 eV and the real vibrational frequencies, respectively. Increasing the length of nanoribbons in the zigzag or armchair directions reduces the energy gap, with modest oscillations in the former case. In contrast to zigzag graphene nanoribbons, biphenylene ones have nonmagnetic spin ordering due to the passivation of the unpaired electrons at the zigzag edges by the 4C- rings connecting the benzene rings. The UV–vis spectra exhibit redshifts, with size increases in both the armchair and zigzag directions, which validate the electronic computations' results. Simulating the catalytic performance of oxygen evolution reaction shows remarkable performance with a low overpotential of 0.12 V. Thus, biphenylene nanoribbons with desirable electronic and catalytic properties are attractive anode materials for water-splitting cells.

Atomically Precise Bottom‐Up Fabrication of Ultra‐Narrow Semiconducting Zigzag BiP Nanoribbons

Abstract1D semiconductors with atomically precise edge and well‐controlled width hold significant promise as channel materials for next‐generation electronics. Here a method to fabricate the narrowest zigzag‐edged bismuth phosphide (BiP) nanoribbons (NRs) is presented, achieving widths of three atoms (≈0.7 nm), through molecular beam epitaxy on bismuthene in a wide P coverage range. Using scanning tunneling microscopy and first‐principles calculations, it is revealed that these BiP NRs exhibit a blue‐phosphorene‐like structure, with a theoretical bandgap of 0.38 eV. Notably, first‐principles calculations reveal spin‐polarized states located on the zigzag edges, presenting an option for spintronics applications. Formation of these uniform BiP NRs is attributed to tensile strain from lattice‐registry confinement. During epitaxial growth, P clusters act dually as feedstock and catalysts, suggesting a self‐catalyzed growth mechanism. The bottom‐up strategy offers an effective approach for the atomically precise fabrication of 1D BiP NRs, paving the way for the creation of diverse low‐dimensional binary materials with tailored chemical and electronic properties, facilitated by selecting suitable elemental 2D materials as substrates.

Phase diagram and crossover phases of topologically ordered graphene zigzag nanoribbons: role of localization effects

We computed the phase diagram of zigzag graphene nanoribbons as a function of on-site repulsion, doping, and disorder strength. The topologically ordered phase undergoes topological phase transitions into crossover phases, which are new disordered phases with non-universal topological entanglement entropy that exhibits significant variance. We explored the nature of non-local correlations in both the topologically ordered and crossover phases. In the presence of localization effects, strong on-site repulsion and/or doping weaken non-local correlations between the opposite zigzag edges of the topologically ordered phase. In one of the crossover phases, both e−/2 solitonic fractional charges and spin-charge separation were absent; however, charge-transfer correlations between the zigzag edges were possible. Another crossover phase contains solitonic e−/2 fractional charges but lacks charge transfer correlations. We also observed properties of non-topological, strongly disordered, and strongly repulsive phases. Each phase on the phase diagram exhibits a different zigzag-edge structure. Additionally, we investigated the tunneling of solitonic fractional charges under an applied voltage between the zigzag edges of undoped topologically ordered zigzag ribbons, and found that it may lead to a zero-bias tunneling anomaly.

Mechanistic study of direct coupling of CO2 and C2H4 over atomically dispersed metal at graphene edges

Direct coupling of CO2 and ethylene (hereinafter DCCE) to acrylic acid is valuable for valorizing CO2 to manufacture acrylate-derived products. However, previous studies in DCCE have been limited on molecular catalysts with challenges in improving catalytic performance. In this work, we employed density functional theory calculations and ab initio molecular dynamics simulations to investigate the heterogeneous catalysis of DCCE over atomically dispersed metal centers at nitrogen-doped zigzag edge of graphene. Based on competitive adsorption and structural stability, Mo, Cr, V, Ru, and Ni active sites are chosen to explore the reaction kinetics. We find that the activation barriers are determined by the charge redistribution at transition states, which explains the trend of activity for the C-C coupling and the hydrogen transfer, two key steps in DCCE. Furthermore, we show that the intramolecular hydrogen transfer (rate-limiting step) is hindered due to the lack of local coordinate at the active sites. We thus propose to use co-adsorbed water as a “proton-exchanger” following a water-assisted route, and show that the activation barriers are reduced over all metal centers. Particularly, water promotes the hydrogen transfer over metals with strong CO2-ethylene co-activation and facile C-C coupling kinetics, which could be considered promising for DCCE. In both mechanisms, the stability of metallactone intermediate can be used to predict the catalytic activity. It is anticipated that the insights from this work can provide guidelines for mimicking well-defined multifunctional active sites in molecular catalysts to design heterogeneous catalysts for such C-C coupling, which advances catalytic utilization of CO2.

Nox impact on mercury removal based on TAC: A comprehensive DFT and XAFS analysis

The simplification of surface chemical reactions is crucial for comprehending the role of nitrogen oxides (NOx) in the adsorption behavior of elemental mercury (Hg0) onto carbon-based sorbents, thereby contributing to technological advancements in reducing mercury emissions. In this study, we developed two fundamental models for activated carbon and investigated the interactions among the carbon surface, NO/NO2, and Hg0 using density functional theory (DFT) calculations. Additionally, thermally-treated activated carbon (TAC) was utilized for Hg0 adsorption, and X-ray absorption fine structure (XAFS) analysis was conducted to examine the morphology and bonding environment of captured Hg. The results suggest that the carbon surface exhibits weak physisorption towards Hg0, but effectively adsorbs NO/NO2. Adsorption of NO/NO2 on the carbon surface with a zigzag edge leads to a decrease in the ADCH charge and surrounding electrostatic potential (ESP) of neighboring carbon vertices, thereby enhancing Hg0 adsorption in those regions through the Eley-Rideal reaction mechanism. The interaction between Hg0 and the surface with ZNO1, ZNO2, or ZNOO2 configurations involves electron transfer and subsequent formation of Hg-C ionic bonds, resulting in adsorption energies ranging from −91.33 to −87.56 kJ/mol. Notably, the adsorption of Hg0 onto the surface with ZNOO1 configuration results in a more extensive electron transfer, facilitating the formation of a robust Hg-O ionic bond and leading to a further reduction in Hg0 adsorption energy to −430.02 kJ/mol. XAFS analysis confirms that predominant formation of Hg-O species occurs following Hg0 adsorption on TAC under syngas containing NO2.

Graphene nanoribbons grown in hBN stacks for high-performance electronics.

Van der Waals encapsulation of two-dimensional materials in hexagonal boron nitride (hBN) stacks is a promising way to create ultrahigh-performance electronic devices1-4. However, contemporary approaches for achieving van der Waals encapsulation, which involve artificial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination and unscalable. Here we report the transfer-free direct growth of high-quality graphene nanoribbons (GNRs) in hBN stacks. The as-grown embedded GNRs exhibit highly desirable features being ultralong (up to 0.25 mm), ultranarrow (<5 nm) and homochiral with zigzag edges. Our atomistic simulations show that the mechanism underlying the embedded growth involves ultralow GNR friction when sliding between AA'-stacked hBN layers. Using the grown structures, we demonstrate the transfer-free fabrication of embedded GNR field-effect devices that exhibit excellent performance at room temperature with mobilities of up to 4,600 cm2 V-1 s-1 and on-off ratios of up to 106. This paves the way for the bottom-up fabrication of high-performance electronic devices based on embedded layered materials.

Evidence for a different dispersion of the topological edge state of germanene at armchair and zigzag edges

Evidence for a different dispersion of the topological edge state of germanene at armchair and zigzag edges

Tritium storage and release mechanism in nuclear graphite using first-principles combined thermal desorption theory

Nuclear graphite (NG) is widely applied in nuclear facilities, and its strong interaction with tritium is an essential factor for tritium transport behavior in various reactors and the treatment of radioactive NG waste. Given that NG is a polycrystalline and porous material, interactions between tritium and NG exist in various microstructures, including graphite edges, graphene basal planes, and amorphous structures. In this study, density functional theory which is integrated in Vienna ab initio simulation package (VASP) was combined with thermal desorption theory to elucidate the underlying mechanism of hydrogen isotope storage corresponding to the experimentally observed desorption peaks in NG. The four desorption temperature peaks, ranging from low to high, were mainly attributed to hydrogen isotope desorption from the graphite basal plane surface, amorphous carbon surface, unsaturated armchair and zigzag edges, respectively. The interaction between hydrogen isotopes and carbon nanostructures provides deeper insights into the understanding of tritium enrichment on the NG surface, which is primarily attributed to the adsorption of tritium on graphite basal plane-like surfaces at relatively low temperatures. This study provides a practical method to bridge the gap between the atomic binding structure and desorption temperature peaks from a kinetic perspective, providing insights into temperature-dependent hydrogen desorption in typical carbon structures and deepening our understanding of tritium transport behaviors in advanced nuclear energy systems.

Chiral symmetry breaking and topological charge of zigzag graphene nanoribbons

Interacting quasi-one-dimensional zigzag graphene nanoribbons display gapped edge excitations. Although the self-consistent Hartree–Fock fields break chiral symmetry, our work demonstrates that zigzag graphene nanoribbons maintain their status as short-range entangled symmetry-protected topological insulators. The relevant symmetry involves combined mirror and time-reversal operations. In undoped ribbons displaying edge ferromagnetism, the band gap edge states with a topological charge form on the zigzag edges. An analysis of the anomalous continuity equation elucidates that this topological charge is induced by the gap term. In low-doped zigzag ribbons, where the ground state exhibits edge spin density waves, this topological charge appears as a nearly zero-energy edge mode. Our system is outside the conventional classification for topological insulators.

Top-down nanostructured multilayer MoS2 with atomically sharp edges for electrochemical hydrogen evolution reaction

Cost-efficient and readily scalable platinum-free electrocatalysts are crucial for a smooth transition to future renewable energy systems. Top-down activation of MoS2 promises the production of sustainable hydrogen evolution electrocatalysts from the Earth-abundant molybdenite ore. Here, the deterministic nanopatterning of multilayer MoS2 with numerous zigzag edges is explored as a pathway to enhance hydrogen evolution reaction (HER). Nanopatterned single-nanosheet MoS2 electrodes are assessed by two highly localized electrochemical techniques: selected area voltammetry (with lithography-defined regions of electrode-electrolyte contact) and Scanning ElectroChemical Microscopy (SECM). The nanopatterning effect is the most pronounced after prolonged electrochemical cycling in an acidic electrolyte. The electrocatalytic hydrogen evolution activity of edge-enriched electrodes is dramatically enhanced: the maximum electrochemical current density (jmax) achieved at -510 mV vs. reversible hydrogen electrode (mVRHE) is increased by two orders of magnitude, reaching >300 mA⋅cm−2. Both the η10 and η100 overpotentials are significantly reduced as well. Meanwhile, pristine MoS2 shows just ≈6 times jmax increase (≈30 mA⋅cm−2) after the very same cycling. The increased electrocatalytic activity comes with electrode morphology degradation, evidenced by ex-situ scanning electron microscopy. SECM directly visualizes stronger HER activity in the regions with densely located zigzag edges. Intense white light illumination significantly boosts HER on MoS2 electrodes due to the photo-enhanced MoS2 conductivity. These results improve the understanding and reveal the limitations of MoS2-based electrocatalytic water splitting.

Strain-Modulated Electronic Transport Properties in Two-Dimensional Green Phosphorene with Different Edge Morphologies

Based on two-dimensional green phosphorene, we designed two molecular electronic devices with zigzag (Type 1) and whisker-like (Type 2) configurations. By combining density functional theory (DFT) and non-equilibrium Green’s function (NEGF), we investigated the electronic properties of Types 1 and 2. Type 1 exhibits an interesting negative differential resistance (NDR), while the current characteristics of Type 2 show linear growth in the current–voltage curve. We studied the electronic transport properties of Type 1 under uniaxial strain modulation and find that strained devices also exhibit a NDR effect, and the peak-to-valley ratio of device could be controlled by varying the strain intensity. These results show that the transport properties of green phosphorene with different edge configuration are different, and the zigzag edge have adjustable negative differential resistance properties.

Seeded growth of single-crystal black phosphorus nanoribbons.

Two-dimensional materials have emerged as an important research frontier for overcoming the challenges in nanoelectronics and for exploring new physics. Among them, black phosphorus, with a combination of a tunable bandgap and high mobility, is one of the most promising systems. In particular, black phosphorus nanoribbons show excellent electrostatic gate control, which can mitigate short-channel effects in nanoscale transistors. Controlled synthesis of black phosphorus nanoribbons, however, has remained an outstanding problem. Here we report large-area growth of black phosphorus nanoribbons directly on insulating substrates. We seed the chemical vapour transport growth with black phosphorus nanoparticles and obtain uniform, single-crystal nanoribbons oriented exclusively along the [100] crystal direction. With comprehensive structural calculations, we discover that self-passivation at the zigzag edges holds the key to the preferential one-dimensional growth. Field-effect transistors based on individual nanoribbons exhibit on/off ratios up to ~104, confirming the good semiconducting behaviour of the nanoribbons. These results demonstrate the potential of black phosphorus nanoribbons for nanoelectronic devices and also provide a platform for investigating the exotic physics in black phosphorus.

Investigation of the mechanical characteristics and thermal behavior of a polymer matrix by addition of different graphene oxide nanosheets: A molecular dynamics simulation

Molecular dynamics simulation (MDS) was applied to examine the effect of graphene oxide nanosheets (GONs) with different edges (armchair and zig-zag edges) on the mechanical behavior (MB) and thermal behavior (TB) of the base polymer matrix (PM). The physical quantities, such as the maximum stress on the samples, stress–strain diagram, ultimate strength (US), Young's modulus (YM), the order parameter (OP), and length changes (LC) were measured for 10 ns. Lennard-Jones (LJ), Tersoff, and Coulomb potentials were used. The PM's maximum stress, US, and YM values were 0.26, 0.22, and 2.23 GPa, respectively. After 10 ns, the OP of PM decreased from 0.79 to 0.52. In studying the PM, the LC was 17 Å when increasing structure temperature to 350 K. The TB and MB of zig-zag nanosheet were higher than for the armchair type. By adding GONs with an atomic ratio of 1% to PM, maximum stress, US, and YM values in structures with armchair nanosheets were 0.34, 0.28, and 2.78 GPa, respectively, and for samples with zig-zag nanosheets were 0.35, 0.30, and 2.96 GPa. After 10 ns, LC increased from 10 to 11 Å for the sample with a zig-zag edge, and from 10 to 12 Å for an armchair edge. This atomic behavior indicated that the atomic structure of Detda and Dgeba matrixes with GONs with a zig-zag edge had higher thermal stability.

Computational investigation of the oxygen reduction reaction on the edges of differently-sized, shaped and terminated graphene nanoclusters

Graphene nanoclusters (GNCs), part of the graphene family of nanomaterials, are attracting interest as promising Pt free catalysts for the Oxygen Reduction Reaction (ORR) at the cathode of the fuel cells. This investigation focuses on the theoretical prediction of their possible edge activity towards ORR determined by their shape, size and termination type. In this framework, using density functional theory (DFT) we examined the zigzag (zz) and armchair terminated (ac) triangular (T), rhombohedral (R) and hexagonal (H) GNCs of various sizes, ranging from C13 to C114. The theoretical onset overpotential (ηth,onset) indicate that several GNC edges could be active for ORR. The model indicates that the zigzag edges of all triangular GNCs (C13–C46) as well as of the largest investigated rhombohedral shapes (C48, C70) are active for ORR. A correlation between the band gaps, spin densities and p-band centers of the edge carbon atoms and the adsorption energies of the ORR intermediate moieties (HOO*/O*/HO*) are discussed and which are directly related to their activity. One trend is related to the decrease of the adsorption energies with the decrease of the band gap and accordingly an increase of their ORR activities. The case studies, show that the explicit water model developed by using machine learning potentials and combined with DFT, stabilizes the ORR intermediates stronger than the implicit one, thus increasing slightly the onset overpotential. Overall, this computational study enhances our fundamental understanding of the edges of GNC for their potential application in electrocatalysis without doping.

Two-Dimensional Nonbenzenoid Heteroacene Crystals Synthesized via In-Situ Embedding of Ladder Bipyrazinylenes on Au(111).

Precisely introducing topological defects is an important strategy in nanographene crystal engineering because defects can tune π-electronic structures and control molecular assemblies. The synergistic control of the synthesis and assembly of nanographenes by embedding the topological defects to afford two-dimensional (2D) crystals on surfaces is still a great challenge. By in-situ embedding ladder bipyrazinylene (LBPy) into acene, the narrowest nanographene with zigzag edges, we have achieved the precise preparation of 2D nonbenzenoid heteroacene crystals on Au(111). Through intramolecular electrocyclization of o-diisocyanides and Au adatom-directed [2+2] cycloaddition, the nonbenzenoid heteroacene products are produced with high chemoselectivity, and lead to the molecular 2D assembly via LBPy-derived interlocking hydrogen bonds. Using bond-resolved scanning tunneling microscopy, we determined the atomic structures of the nonbenzenoid heteroacene product and diverse organometallic intermediates. The tunneling spectroscopy measurements revealed the electronic structure of the nonbenzenoid heteroacene, which is supported by density functional theory (DFT) calculations. The observed distinct organometallic intermediates during progression annealing combined with DFT calculations demonstrated that LBPy formation proceeds via electrocyclization of o-diisocyanides, trapping of heteroarynes by Au adatoms, and stepwise elimination of Au adatoms.

Exploring Spin Distribution and Electronic Properties in FeN4-Graphene Catalysts with Edge Terminations.

Understanding the spin distribution in FeN4-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN4 defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN4 defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN4 defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.