Armchair Edges Research Articles

Dibenzo-peri-Heptacene: A Stable Open-Shell Graphene Fragment With a Balanced Combination of Armchair and Zigzag Edge Structures.

Atomically precise open-shell graphene fragments, such as extended peri-acenes, hold significant interest for electronics and spintronics. However, their inherent high reactivity poses challenges for synthesis and application. In this study, a novel approach is introduced: the fusion of a zigzag-edged peri-tetracene with an all-armchair-edged hexa-peri-hexabenzocoronene (HBC) via two shared benzene rings to produce a stable open-shell hydrocarbon, named dibenzo-peri-heptacene (DBPH). The DBPH derivative 1 exhibits considerably enhanced stability, with a half-life (t1/2) of 46 days in toluene solution under ambient conditions. This improved stability is attributed to peri-benzannulation, enhanced aromatic stabilization, and kinetic protection of the reactive sites along the zigzag edges. The structure of 1 is unequivocally verified through single-crystal X-ray diffraction analysis. With a balanced combination of armchair and zigzag edge structures, derivative 1 displays a diradical character of 39.2% and a singlet-triplet gap of ≈-3.16 kcalmol-1. It features a narrow electrochemical energy gap (0.87 eV) and exhibits amphoteric redox behavior. Notably, its dication and dianion states manifest a closed-shell singlet ground state, representing doubly charged structures where a HBC unit is fused with a benzo[f]tetraphene moiety. This research paves the way for synthesizing novel open-shell graphene fragments with adjustable electronic properties and exceptional stability.

Edge-state-mediated RKKY coupling in graphene nanoflakes

We investigate the long-range behavior and size dependence of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in hexagonal and triangular graphene nanoflakes with zigzag and armchair edges. We employ the tight-binding model with exact diagonalization to calculate the RKKY interaction as a function of the distance between magnetic impurities, nanoflake size, and edge geometry. Our findings demonstrate a strong dependency of the RKKY interaction on edge geometry and flake size, with notable changes in the RKKY interaction strength. We further analyze the influence of structural defects on the interaction strength of exchange interactions.

Assessment of Edge Modification of Nanographene.

Carboxy groups on the edges of nanographene (NG) enable functionalization for realizing NG-organic hybrid materials. Therefore, assessment of the edge-functionalization of the electronic structures of NGs is valuable for the rational design of functional carbon materials. In this study, the structures of model NGs comprising 174 carbon atoms with armchair edges and various functional groups at the edges were computed. To achieve the greatest possible similarity between the computed structure and the real one, the carbon framework was designed based on experimental observations. The functional groups can be accessed via suitable chemical reactions. The computations predicted that although the conversion of carboxyl groups with electron-withdrawing/donating groups influences the orbital energies, the HOMO-LUMO (H-L) gap is not significantly affected, except in a few cases. Among the evaluated examples, π-extension had the greatest influence on the H-L gap. Interestingly, for the Pd2+-coordinated NG, the participation of the low-lying LUMO localized on Pd2+ in the surface-to-metal transitions seemingly narrowed the H-L gap, and a surface-to-ligand transition was observed.

Unlocking the potential of borophene nanoribbons for efficient hydrogen storage

This research explores the structural, electronic, optical, and hydrogen storage properties of borophene nanoribbons (BNRs) with armchair (ANR-B-H) and zigzag (ZNR B-H) edges. Computational simulations optimized these structures, revealing that 7ZNR-B-H has a superior binding energy. Chemical modifications, such as fluorine passivation and functionalization, influenced bond parameters and quantum properties. Bilayer BNRs showed increased stability and enhanced electrical conductivity. Our study demonstrated promising hydrogen storage capabilities, with passivated and functionalized BNRs achieving suitable adsorption energies and a significant gravimetric storage capacity of 20.32 wt%, exceeding DOE standards. NH2 functionalization notably improved adsorption energy, enhancing potential for efficient hydrogen storage. Changes in absorption spectra post-H2 adsorption further highlight BNRs’ potential for hydrogen storage applications. These findings provide valuable insights into BNRs, paving the way for their use in electronic devices and hydrogen storage systems.

Combined effect of strain and intrinsic spin–orbit coupling on band gap engineering of GNRs: a first-principles study

By employing first-principles calculations, we theoretically investigate the impact of uniaxial strain and intrinsic spin-orbit coupling (SOC) on the electronic properties of zigzag and armchair edge hydrogen (H)-passivated graphene nanoribbons (GNRs). We find that band structure and density of states of 4-zigzag graphene nanoribbon (ZGNR) and 15-armchair graphene nanoribbon (AGNR) are highly sensitive to the combined effect of strain and intrinsic SOC. In the case of H-passivated 4-ZGNR, SOC with a strain>10% increases the energy band by increasing spin-polarized states at the opposite edges. In contrast to 4-ZGNR, the oscillatory behavior of band gap of H-passivated 15-AGNR is preserved in the presence of strain and SOC. Moreover, for both types of GNRs (zigzag and armchair), the presence of strain and intrinsic SOC preserve spin symmetry.

Peculiarities of the Landau level collapse in graphene ribbons in crossed magnetic and in-plane electric fields

Employing the low-energy effective theory alongside a combination of analytical and numerical techniques, we explore the Landau level collapse phenomenon, uncovering previously undisclosed features. We consider both finite-width graphene ribbons and semi-infinite geometries subjected to a perpendicular magnetic field and an in-plane electric field, applied perpendicular to both zigzag and armchair edges. In the semi-infinite geometry the hole (electron)-like Landau levels collapse as the ratio of electric and magnetic fields reaches the critical value +(−)1. On the other hand, the energies of the electron (hole)-like levels remain distinct near the edge and deeply within the bulk, approaching each other asymptotically for the same critical value. In the finite geometry, we show that the electron (hole)-like levels become denser and merge, forming a band. Published by the American Physical Society 2024

Electronic and Thermoelectric Properties in SnS-Nanoribbon-Based Heterojunctions

Abstract As an earth-abundant and environmentally friendly material, tin sulfide (SnS) is not only a high-performance photovoltaic material, but also a new promising thermoelectric material. Despite extensive research on the thermoelectric properties of this material in recent years, the room-temperature thermoelectric figure of merit (ZT) of SnS has not been broke through 2 [2022 Sci. China Mater. 65 1143]. In this work, based on a combination of density functional theory and non-equilibrium Green’s function method, the electronic and thermoelectric properties in SnS-nanoribbon-based heterojunctions are studied. The results show that although SnS nanoribbons (SNSNRs) with zigzag edges (ZSNSNRs) and armchair edges (ASNSNRs) both have semiconductor properties, the bandgaps of ASNSNRs are much wider than those of ZSNSNRs, which induces much wider conductance gaps of N-ASNSNR (N is the number of tin-sulfide lines across the ribbon width)). In the positive energy region, the ZT peaks of L-SNS-Au are much larger than those of L-SNS-GNR (L represents the number of longitudinal repeating units of SNSNR in the scattering region). While in the positive energy region, the ZT peaks of L-SNS-GNR are larger than those of L-SNS-Au. Further calculations reveal that the figure of merit will be over 3.7 in L-SNS-Au and 2.2 in L-SNS-GNR at room temperature, and over 4 in L-SNS-Au and 2.6 in L-SNS-GNR at 500 K.

Investigating Transmission Coefficients of AB-Stacked Bilayer Graphene Nanoribbons with Varied Edge Configurations

In this study, the transmission coefficients of AB-stacked bilayer graphene nanoribbons (AB-BGNRs) with different edge configurations, specifically zigzag and armchair edges, are comprehensively investigated. These coefficients are modeled and simulated using the tight-binding (TB) model and non-equilibrium Green’s function (NEGF) formalism. The impact of edge structures on the electronic properties of AB-BGNRs is highlighted, providing insights into their potential applications in nanoelectronics devices. Significant variations between zigzag and armchair-edged bilayer graphene nanoribbons (BGNRs) are demonstrated, emphasizing the importance of edge configuration in device performance. This study contributes to the fundamental understanding of edge effects in bilayer graphene and paves the way for the design of more efficient graphene-based electronic devices.

Linear Non-benzenoid Isomer of Acene Fusing Chrysene with Azulene Units.

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received considerable attention owing to their distinctive optical and electrical properties. Nevertheless, the synthesis and optoelectronic application of non-benzenoid PAHs remain challenging. Herein, we present a facile synthesis of linear non-benzenoid PAH with an armchair edge, diACh, by fusing chrysene with two azulene units. We systematically investigated the optical and electrical properties, which were also compared to its isomers, including benzenoid and non-benzenoid zigzag edge isomers. diACh exhibits global aromaticity, good planarity, and suitable highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels. The protonation of diACh in solution successively forms a stable tropylium cation and dication. Moreover, the neutral, cationic, and dicationic states of diACh can be transformed with remarkable reversibility during the protonation-deprotonation process, as confirmed by ultraviolet-visible absorptions, fluorescence spectra, 1H nuclear magnetic resonance, and theoretical calculations. Additionally, we fabricate p-type organic field-effect transistor (OFET) devices based on diACh with hole mobility up to 0.026 cm2 V-1 s-1, and we further develop OFET-based acid vapor sensors with good sensitivity, recyclability, and selectivity. These findings underscore the unique properties of linear non-benzenoid PAHs with an armchair edge engendered by the fusion of azulene with the acene backbone, showcasing prospective applications in organic optoelectronics.

Schottky barrier diode and photoelectric device based on two dimensional noble transition-metal sulfides vertical heterojunction

In this work, we use first-principles method to investigate the properties of two-dimensional noble transition-metal sulfides MS2 (M = Ni, Pd, Pt) and the electronic and optoelectronic applications of their vertical heterojunctions. The vertically bilayer or trilayer stacking can modulate the band structures and optical absorption spectra of MS2 (M = Ni, Pd, Pt) obviously. Therefore, we create the atomically thin Schottky barrier diodes (SBD) by vertically stacking the MS2 (M = Ni, Pd, Pt) on monolayer MS2 (M = Ni, Pd, Pt). All SBDs reveal more excellent current rectification behavior along the armchair edge (AE) transport direction than that along the zigzag edge (ZE) transport direction. SBD based on PdS2 bilayer and monolayer heterojunction has the high current value and the significant rectification ratio (exceeding 105) along AE transport direction. The total photocurrent densities of all SBDs along the ZE transport direction are bigger than that along the AE transport direction. SBD based on PdS2 bilayer and monolayer heterojunction has the largest total photocurrent density (1208.07 μA/mm2) along the ZE transport direction. Our work provides foundations for the application of two-dimensional noble transition-metal sulfides in rectification and optoelectronics.

Thermodynamics and electronic structure of edges in monolayer MoSi2N4

MoSi2N4 is a two-dimensional ternary nitride semiconductor that has attracted attention for its excellent mechanical and thermal properties. Theoretical studies predict that zigzag edges of this material can host magnetic edge states and Dirac fermions, but the stability of such edges has not been examined. Here, we present a density functional theory study of the electronic and thermodynamic properties of MoSi2N4 edges. We develop a (partial) ternary phase diagram that identifies a region of chemical potentials within which MoSi2N4 is stable over competing elemental or binary phases. Based on this phase diagram, we determine the thermodynamic stability of several armchair and zigzag edges and elucidate their electronic structures. Bare zigzag edges, predicted to host exotic electronic states, are found to be substantially higher in energy than armchair edges and, thus, unlikely to occur in practice. However, with hydrogen passivation, these zigzag edges can be stabilized relative to their armchair counterparts while retaining metallicity and magnetic order. Our analysis provides a solid thermodynamic basis for further exploration of MoSi2N4 in nanoscale electronics and spintronics.

Edge-dependent Majorana corner modes in an s-wave superconductor

In contrast to Majorana edge modes in first-order topological superconductors, Majorana corner modes are usually related to spatial symmetries and further rely on specific corner of the sample. In this paper, we study Majorana corner modes in an extended Kane-Mele model with arbitrary phase. Majorana corner mode exists at both corner between zigzag edge and armchair edge and corner between zigzag edge and zigzag edge for the phase at π/2. However, while the Majorana corner mode at the corner between zigzag edge and armchair edge remains as the phase deviates from π/2, Majorana mode at the corner between zigzag edge and armchair edge disappears. These Majorana corner modes are edge dependent and originate from the sign change of Dirac mass or Dirac velocity for edge states at adjacent edges tuned by the phase.

Laser printing of certain pixels of graphene nanoribbons

A possibility of laser printing of graphene nanoribbon pixels while preserving the integrity of the structure and shape on the silicon substrate in accordance with the irradiated laser spot is demonstrated. To provide the transfer, a target consisting of a transparent sapphire plate and an absorbing thin titanium film (500 nm thick) coated with a film consisting of graphene nanoribbons is irradiated with a KrF excimer laser (λ=248 nm, τ=20 ns). Optimal conditions for laser irradiation are determined and a technique is developed for transferring a carbon nanomaterial from a growth nickel surface to a titanium film aimed at creating stronger bonds between the nanoribbons. Raman spectroscopy confirms the preservation of structural features of the synthesized atomically precise 7-atoms-wide graphene nanoribbons with an armchair edge during laser transfer.

Theoretical insights into the activity of the N doped graphene quantum dots for oxygen reduction reaction as a function of the shape and edge terminations

We have investigated the possible activity of the N-doped graphene quantum dots (N-GQD) toward the oxygen reduction reaction (ORR) using density functional theory (DFT) method. The effect of the shape (triangular (N-GQDT), rhombohedral (N-GQDR) and hexagonal (N-GQDH)), connected both with the zigzag (zz) and armchair (ac) edge terminations of the GQD and with the location of the N atom across the sheets are investigated. The activity of part of the studied N-GQDTac, GQDRac, N-GQDHac and N-GQDHzz structures improves significantly compared to the similar pristine structures. On the other hand, no matter on the location of the N in the N-GQDTzz and in the N-GQDRzz, their activity gets worsen, when compared with the activity of the pristine structures or with the activity of the other four investigated structures. N-GQDTac and N-GQDRac are the structures with the most possible locations of N, that generates active sites. CN bond breaking takes place in the triangular shapes (N-GQDTzz and N-GQDTac) when oxygen adsorption takes place. These bonds are less likely to regenerate and new reaction sites are created. The new reaction site created on the N-GQDTzz represents a possible active site compared to the site generated on the N-GQDTac, that is highly inactive.

Homoepitaxy of Boron Nitride on Exfoliated Hexagonal Boron Nitride Flakes.

Although large efforts have been made to improve the growth of hexagonal boron nitride (hBN) by heteroepitaxy, the non-native substrates remain a fundamental factor that limits the quality. This problem can be solved by homoepitaxy, which is the growth of hBN on hBN substrates. In this report, we demonstrate the homoepitaxial growth of triangular BN grains on exfoliated hBN flakes by Metal-Organic Vapor Phase Epitaxy and show by atomic force microscopy and photoluminescence that the stacking of these triangular islands can deviate from the AA' stacking of hBN. We show that the stacking order is enforced by the crystallographic direction of the edge of the exfoliated hBN flakes, with armchair edges allowing for centrosymmetric stacking, whereas zigzag edges lead to the growth of noncentrosymmetric BN polytypes. Our results indicate pathways to grow homoepitaxial BN with tunable layer stacking, which is required to induce piezoelectricity or ferroelectricity.

Theoretical insight into NO formation and reduction at biochar N-sites: Influence of different oxygen-containing functional groups

Biochar is a cost-efficient and promising porous carbonaceous material for the efficient removal of flue gas NOx. The N-site located at the edge of biochar can be a crucial gateway for NO reduction to N2 or for NO formation by oxidation, which can be significantly affected by the oxygen-containing functional groups (OFGs) on the biochar edge. However, the roles of different OFGs in N-site-involved reactions remain elusive. Based on the biochar characterization results, reasonable theoretical models of biochar with different N-sites and OFGs were constructed. Subsequently, the effects of distinct OFGs on the interaction between NO and N-sites as well as the NO liberation from N-sites were elucidated, by employing density functional theory (DFT) and electronic structure analysis. The results reveal that all OFGs, encompassing -CHO, -COOH, and -OH, manifest their roles through the direct inherent electrostatic properties of O/H in OFGs (O*/H*) or altering the reactivity of edge atoms indirectly. For pyridinic nitrogen (N-6) at the zigzag edge, unsaturated (-CHO, -COOH)/saturated (-OH) OFGs inhibit/enhance NO reduction with N-6 by electrostatic properties of O*/H*, and all OFGs inhibit NO generation from N-6. Besides, all OFGs enhance the NO reduction with pyrrolic nitrogen (N-5) at the armchair edge while inhibiting the release of NO, by influencing the edge activity. The present study gives a mechanism insight into the roles of distinct OFGs in reactions involving biochar N-sites.

Chlorinated graphene and graphene nanoribbons: A density functional theory study

Functional groups based on halides, such as fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), are crucial for understanding the chemical reactivity of graphitic nanomaterials. Except for I, halogens exhibit electronegativity greater than carbon (C); therefore, charge transfer from carbon to halogen is expected. First-principles density functional theory calculations were performed to determine the role of different Cl-functional groups (methyl-trichloride, ethyl-trichloride, chloride, acyl-chloride, vinyl-chloride, acetyl hypochlorite, chloramines, sulfonyl chloride, and more) on the electronic properties of graphene and graphene nanoribbons (GNRs). GNRs with zigzag edges (ZGNRs) and armchair edges (AGNRs) were studied. We analyzed the optimized structures, band structure, density of states, cohesive energy, and band gap. Our results revealed that the based-Cl functional groups can provide an alternative route to activate the borders and surfaces of sp2 carbon materials. Methyl-trichloride and acyl-chloride can induce magnetism and metallicity. Chloride and acyl-chloride are the most energetically stable functional groups attached to the edges. Surprisingly, methyl-trichloride or acyl-chloride functionalizing the surface of the AGNRs showed a direct (indirect) band gap for states with spin-up (spin-down). The results of aromatic (chlorobenzene- and dichlorobenzene-like structures) functionalization considering F, Cl, Br, and I are also shown. Finally, –F2 and –ClF functionalization cases are discussed.

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.

First-principles study on shape of intrinsic hBN island nucleated during CVD initial growth on Cu(111)

Using the first-principles calculation, the shape of the hexagonal boron nitride (hBN) islands was investigated, with a focus on the edge of the small monolayer hBN islands on Cu(111) and intrinsic nucleation during the CVD initial growth. Several key observations were made. First, desorption does not play a decisive role in shaping the islands. Second, for small islands, there is no chemical potential range where the armchair edge is stable. Third, the bonds between Cu atoms on the surface and N atoms at the edge are strong, whereas the bonds between Cu atoms on the surface and B atoms at the edge are comparatively weaker. Finally, triangular islands with N edge tend to grow more spontaneously than those with B edge because the critical size is smaller for those with N edge across a wide chemical potential range.