Ribbon Edges Research Articles

Probing divacancy defects in a zigzag graphene nanoribbon through an RKKY exchange interaction

We investigate the effects of vacancy defects on the electronic and magnetic properties of zigzag graphene nanoribbons (zGNRs) by making use of the Green’s function formalism in combination with a tight-binding Hamiltonian. We explain the evolution of indirect exchange coupling, known as the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, including single, double, and multiple 5-8-5 divacancy (DV) defects. Our numerical calculations show that changes in the electronic structure and exchange coupling of zGNRs depend to a significant degree on the location of DV defects with respect to the ribbon edges and the number of DV defects. In the case where both impurities are located on the edge, the magnitude of the exchange coupling is several orders of magnitude higher than when they are placed on the interior of the nanoribbon. Furthermore, a periodic DV causes a dramatic change in the magnetic ground state of the ribbon. In the limit of a high vacancy potential, the strength of the RKKY interaction is approximately independent of the Fermi energy. We demonstrate that, on the one hand, the defect engineering of atomic vacancies is a promising way to modify the magnetic properties of graphene nanoribbons, and, on the other hand, unusual RKKY oscillations around the DVs are a new technique for directly probing the local vacancies in a zGNR through the RKKY exchange interaction.

Quantum transport simulation of graphene-nanoribbon field-effect transistors with defects

We present a theoretical study of the effect of defects on the charge-transport properties of gate-all-around graphene nanoribbons field-effect transistors. Electronic transport is treated atomistically using an efficient method we have recently proposed that makes use of a Bloch-wave basis obtained from empirical pseudopotentials and solves the Schrodinger equation with open boundary conditions using the quantum transmitting boundary method. The defects considered here consist in single vacancies at different locations in the ribbon (center and edge of the ribbon; in the source or drain regions or along the channel). We have found that vacancies located at different locations along the ribbon width alter differently the Kekule patterns: Defects at the edge reduce the $$I_{\mathrm{on}}/I_{\mathrm{off}}$$ more than defects located near the center of the ribbon, and the effect is stronger in narrow ribbons. These results show that any proposed technology based on graphene nanoribbons must be able to control the quality of the material down to a single atom.

Photodetection Using Atomically Precise Graphene Nanoribbons

In the search for high-sensitivity, low-noise, and high-bandwidth photodetectors, materials are a key ingredient. One- and two-dimensional materials are of particular interest in this area due to their extraordinary properties such as ballistic transport. Here, we demonstrate nanoscale photoconductive photodetectors using aligned atomically precise seven-atom wide armchair-edge graphene nanoribbons. The detector responsivity is 0.035 mAW–1 at a bias voltage of 2 V. The dark current is below 30 pA for a bias voltage of 1.5 V, which is orders of magnitude lower than that of typical graphene photodetectors. The possibility to align the nanoribbons and to tune their optical and electronic properties by choice of ribbon width and edge structure enables nanoscale polarization-resolving photodetectors optimized for specific spectral ranges. Graphene nanoribbons with identical electronic and optical properties can be prepared on a large scale using bottom-up synthesis, making them a highly interesting material for electronics and optoelectronics.

Reversible edge spin currents in antiferromagnetically proximitized dichalcogenides

We explore proximity effects on transition metal dichalcogenide ribbons deposited on antiferromagnetic (AFM) insulating substrates. We model these hybrid heterostructures using a tight-binding model that incorporates exchange and Rashba fields induced by proximity to the AFM material. The robust edge states that disperse in the midgap of the dichalcogenide are strongly affected by induced exchange fields that reflect different AFM ordering in the substrate. This results in enhanced spin-orbit coupling effects and complex spin projection content for states on zigzag ribbon edges. Gated systems that shift the Fermi level in the midgap range are also shown to exhibit spin-polarized currents on these edges. Antiparallel exchange fields along the edge results in spin currents that can reverse polarization with the applied field. The added functionality of these hybrid structures can provide spintronic devices and versatile platforms to further exploit proximity effects in diverse material systems.

Thermal quantum correlations in zigzag graphene nanoribbons

Based on the effective spin-ladder model, we compute the negativity and geometric quantum discord (GQD) between two edge spins in zigzag graphene nanoribbons (ZGNRs) thermalized with a reservoir at temperature T (canonical ensemble). The results show that the entanglement only exists in antiferromagnetically coupled spin pairs, and completely vanishes between any two spins at the same ribbon edge. Intriguingly, the dominant entanglement always occurs in the nearest inter-edge coupled spin pairs, which is very robust even at room temperature. By contrast, almost all edge spin pairs can hold non-zero GQD, which strongly depends on the ribbon width. To get further insight into the properties of entanglement, the multiparticle entanglement (ME) shared between three spins is studied. The results show that ME always exists among spins located at different edges, and it is more robust than the corresponding two-spin entanglement in against the temperature. These remarkable properties make ZGNRs very promising for possible applications in quantum information processing.

Quantum confinement and edge effects on electronic properties of zigzag green phosphorene nanoribbons

First-principles density-functional theory calculations were performed to investigate quantum confinement and edge effects on electronic properties of zigzag green phosphorene nanoribbons (ZGPNRs) with edge chemical species including H, OH, F, Cl, O, and S for the ribbons width in the range of 0.5–3.7 nm. The ZGPNRs were obtained from relaxed two-dimensional green phosphorene monolayer with different cutting strategies and the most energetically favorable ribbon configuration was selected for further exploration of size and edge effects. It was found that the electronic properties of the ZGPNRs are strongly associated with the ribbon width and edge chemical species. They show either semiconducting or metallic features depending on the edge functionalization species. The ZGPNRs show semiconducting behavior with the edge species of H, OH, F, or Cl (Group I), while they exhibit metallic characteristics with pristine or O, S edges (Group II). The conduction band minimum and valence band maximum of the ZGPNRs with the Group I edge are primarily located at the inner P atoms and the edge P and functionalization atoms have little contribution. However, for the Group II edge, the electronic bands crossing the Fermi level are dominantly contributed by the edge atoms. It was also found that the band gap and work function of the ZGPNRs are sensitively tunable by varying ribbon width and edge functionalization species.

Fabrication of morphologically modified strong supramolecular nanocomposite antibacterial hydrogels based on sodium deoxycholate with inverted optical activity and sustained release

Low Molecular Weight (LMWG) gelators are small molecules that form supramolecular self-assembly involving physical forces and are highly biocompatible. However, fragility of these physical gels restricts their applicability where gels of higher mechanical strength are required. Herein, we have developed two different types of 2-D carbon nanomaterials viz. graphene oxide (GO) and carbon nanosheet (CNS) embedded sodium deoxycholate (NaDC) hydrogels. XRD, scanning electron microscopy (SEM), rheology and CD studies suggest significant modification of morphological, mechanical, viscoelastic and optical properties of the nanocomposite gels which is ascribed to the presence of the 2D nanotemplates and participation of different surface functionalities of GO and CNS in the gelation process. The overall shear resistance of both the nanocomposite hydrogels upto a shear rate of 300 shears/s−1 and above reveals tremendously improved mechanical stability with respect to the pure gels. The increased shear strength of the GO/NaDC and CNS/NaDC hydrogels is attributed to their 3–4 times broader and longer ribbon like structures in comparison to the fibrous structure of pure gels. The intact ribbon like morphology and greater entanglement impart 10 folds greater viscosity to GO-NaDC hydrogels as compared to better elasticity of CNS-NaDC hydrogels possessing broken ribbon edges. Most interestingly both GO and CNS influence the optical activity of the gels and presence of GO results in inversion of optical activity. The GO-NaDC gels are also found to demonstrate antibacterial activity against E. coli, and S. aureus. Thus, these extraordinarily modified mechanically strong gels have enhanced potential for use in tissue engineering, enantioselective and sustained drug delivery, topical antibiotics and other biomedical applications.

Edge Doping in Graphene Devices on SiO2 Substrates

The conductivity of single layer and bilayer graphene ribbons 0.5–4 μm in width fabricated by oxygen plasma etching is studied experimentally. The dependence of the electron concentration in graphene on the ribbon width is revealed. This effect is explained by the edge doping of graphene due to defects arranged in SiO2 near the ribbon edges. The method of the formation of abrupt p–n junctions in graphene and structures with a constant electron concentration gradient in the graphene plane with the help of edge doping is proposed.

Correlated Topological States in Graphene Nanoribbon Heterostructures.

Finite graphene nanoribbon (GNR) heterostructures host intriguing topological in-gap states (Rizzo, D. J.; et al. Nature2018, 560, 204). These states may be localized either at the bulk edges or at the ends of the structure. Here we show that correlation effects (not included in previous density functional simulations) play a key role in these systems: they result in increased magnetic moments at the ribbon edges accompanied by a significant energy renormalization of the topological end states, even in the presence of a metallic substrate. Our computed results are in excellent agreement with the experiments. Furthermore, we discover a striking, novel mechanism that causes an energy splitting of the nonzero-energy topological end states for a weakly screened system. We predict that similar effects should be observable in other GNR heterostructures as well.

Optimized Substrates and Measurement Approaches for Raman Spectroscopy of Graphene Nanoribbons

The on‐surface synthesis of graphene nanoribbons (GNRs) allows for the fabrication of atomically precise narrow GNRs. Despite their exceptional properties which can be tuned by ribbon width and edge structure, significant challenges remain for GNR processing and characterization. Herein, Raman spectroscopy is used to characterize different types of GNRs on their growth substrate and track their quality upon substrate transfer. A Raman‐optimized (RO) device substrate and an optimized mapping approach are presented that allow for the acquisition of high‐resolution Raman spectra, achieving enhancement factors as high as 120 with respect to signals measured on standard SiO2/Si substrates. This approach is well suited to routinely monitor the geometry‐dependent low‐frequency modes of GNRs. In particular, the radial breathing‐like mode (RBLM) and the shear‐like mode (SLM) for 5‐, 7‐, and 9‐atom‐wide armchair GNRs (AGNRs) are tracked and their frequencies are compared with first‐principles calculations.

One-dimensional topological superconductivity at the edges of twisted bilayer graphene nanoribbons

Twisted bilayer graphene is one of the simplest van der Waals structures, and its inhomogeneous interlayer coupling can induce rich electronic properties. In twisted bilayer graphene nanoribbons (tBLGNRs), the interlayer coupling strengths are different for two ribbon edges due to the inhomogeneous bonding, which splits the edge states into two individuals in energy. The lower-energy state, localizing at the ribbon edge with the stronger interlayer coupling, is a good candidate to generate one-dimensional (1D) topological superconductivity in the presence of Rashba spin-orbit coupling, Zeeman field, and $s$-wave superconductivity. Majorana zero modes (MZMs) are found to be localized at both ends of this edge. The topological invariants of the system are explored by evaluating the Berry phase for infinite-length ribbons and Majorana polarization for quasi-1D ribbons, giving the same topological phase diagram. More importantly, by adjusting interlayer dislocation and uniaxial strain of tBLGNRs across the critical values, the lower-energy edge changes and 1D topological superconductivity can ``jump'' from one ribbon edge to the other one. Finally, by applying a gate voltage bias between bilayers or changing the interlayer distance, a MZM can transfer along the ribbon edge. The tBLGNRs provide an alternative platform to study 1D topological superconductivity and MZMs.

Knots connected by wide ribbons

A ribbon is a smooth mapping (possibly self-intersecting) of an annulus [Formula: see text] in 3-space having constant width [Formula: see text]. Given a regular parametrization [Formula: see text], and a smooth unit vector field [Formula: see text] based along [Formula: see text], for a knot [Formula: see text], we may define a ribbon of width [Formula: see text] associated to [Formula: see text] and [Formula: see text] as the set of all points [Formula: see text], [Formula: see text]. For large [Formula: see text], ribbons, and their outer edge curves, may have self-intersections. In this paper, we analyze how the knot type of the outer ribbon edge [Formula: see text] relates to that of the original knot [Formula: see text]. Generically, as [Formula: see text], there is an eventual constant knot type. This eventual knot type is one of only finitely many possibilities which depend just on the vector field [Formula: see text]. The particular knot type within the finite set depends on the parametrized curves [Formula: see text], [Formula: see text], and their interactions. We demonstrate a way to control the curves and their parametrizations so that given two knot types [Formula: see text] and [Formula: see text], we can find a smooth ribbon of constant width connecting curves of these two knot types.

Spin Filtering and Rectification in Lateral Heterostructures of Zigzag-Edge BC3 and Graphene Nanoribbons: Implications for Switching and Memory Devices

Heterostructures of different atomically thin nanomaterials provide an ideal platform of admixing their unique properties to achieve new functionalities. Here, we study the lateral heterostructures of graphene and BC3 nanoribbons with zigzag edge terminations within first-principles calculations. These heterostructures show localization of one kind of spin along the open graphene zigzag edge, whereas the other edge does not show any spin polarization. Therefore, the systems stabilize in the ferromagnetic ground state, inducing net magnetization and broken time reversal symmetry in each valley. Owing to this asymmetric spin distribution, the system shows spin-polarized current that is confirmed by further first-principles-based quantum transport calculations. The conduction behavior can be tuned further by changing the polarity of the side-gates in either sides of the ribbon edges. We observe higher spin polarization of current under forward gate bias but lower spin polarization of current under reverse gate bias. This functionality provides a unique way of spin filtering and rectification by changing the electrode polarity, which can find major applications in switching and memory storage devices.

Geometry-dependent conductance and noise behavior of a graphene ribbon with a series of randomly spaced potential barriers

We perform an envelope-function based numerical analysis of the effect of a sequence of randomly spaced potential barriers on the conductance and shot noise of an armchair graphene ribbon. The behavior is dominated by Klein tunneling and by resonant tunneling and strongly depends on the geometrical details of the device. Klein tunneling effectively filters the modes that can propagate through the device. For a large number of cascaded barriers, this gives rise to different transport regimes for metallic and semiconducting ribbons, with diverging shot noise behaviors. Resonant tunneling is instead energy selective and has quite a different effect depending on whether the barriers are identical or not. We also explore the effect of tilting the barriers with respect to the ribbon edges, observing a transition toward a diffusive transport regime and a one-third shot noise suppression. We investigate this effect, and we find that it takes place also in more traditional semiconducting materials. The results of our analysis could be instrumental for the fabrication of mode-filtering and energy-filtering graphene-based nanodevices. Moreover, our study highlights the importance of the measurement of shot noise as a probe for the nature of the transport regime.

First-principle study of NO2 adsorption and detection on the edges of zigzag nitride nanoribbons

Since from the inception, 2-D materials have gained considerable attention for sensing applications due to larger surface to volume ratio. Here, we compared the sensing capability of nitride based zigzag edged nanoribbons (ZXNNR, X = B/Al/Ga) for harmful environmental gas. NO2 was considered as the target gas and its adsorption on ZXNNR has been modelled in various possible configurations. It is revealed that the adsorption of NO2 is mediated via formation of chemical bond with the ribbon edges and the process is exothermic in nature for all considered ribbons (BN, AlN and GaN). Moreover, the presence of NO2 molecules caused a significant alteration in the electronic properties of ZXNNR. The reduction in the band gap as compared to pristine configurations is noticed for selective edge adsorption. On the other hand, a pure metallic character is witnessed for NO2 adsorption only at either edge of the ribbon. Interestingly, a perfect negative differential resistance (NDR) is also obtained in some of the structures with a high peak to valley ratio (1.92–20.28). Present findings hold potential of ZXNNR towards NO2 detection. The observed NDR behavior suggests that selective NO2 adsorbed ZXNNR may also find applications in fast switching devices, oscillators and other inorganic nano-devices.

Magnetic structure and magnetic transport properties of armchair arsenene nanoribbons

Inducing magnetism for arsenene is important for developing high-performance spintronic devices. Here, we investigate the electronic and magnetic properties of armchair arsenene nanoribbons doped with transition metal Mn atom by the ab initio electronic structure calculations and quantum transport simulations. It is found that such structures are stable by calculated formation energy and molecular dynamics simulation. Magnetic states can be obtained by doping Mn atom in such ribbons at different doping-positions with the distance from the Mn atom to the ribbon edges. AAsNR-Mn experiences a transition from a half-semiconductor to a half-metal by applying an external electric field. Moreover, it possesses an outstanding magnetic device nature, such as perfect spin filtering effect (100%), and a spin-valve feature with giant magnetoresistance (GMR) approaching 106%. Our calculation indicates AAsNRs-Mn has a promising application for multifunctional spintronic devices.

Bio-inspired carbon doped graphitic carbon nitride with booming photocatalytic hydrogen evolution

In this work, kapok fiber (KF), known as a versatile biomass, has been mixed with melamine to produce carbon modified graphitic carbon nitride (CCN) by one-step pyrolysis. The bio-char ribbon edges formed after the decomposition of KF act as the substrate for the epitaxial growth of CCN. The photocatalytic activity in hydrogen (H2) generation from water splitting has been investigated. Nash Equilibrium from Game Theory has been firstly applied in the analysis of the H2 generation rates among catalysts. The bio-char ribbon edges at the thin CN layers after carbon doping from KF decomposition improve the charge separation and transfer for the surface H2 generation reaction. The CCN exhibits superior visible-light-driven photocatalytic activity and the H2 evolution rate (18.89 μmol/h) is 67.5 times higher than that of the pristine CN (0.28 μmol/h). The apparent quantum yields are calculated to be 4.1%, 1.4%, 0.66% for monochromatic light λ = 420, λ = 470 and λ = 550 nm, respectively.

Modulating electronic and magnetic properties of zigzag MoSe2 nanoribbons with different edge structures

In this paper, we performed the first-principles calculations to investigate the effect of different edge structures on stability, electronic and magnetic properties of zigzag MoSe2 nanoribbons and their corresponding variation by saturating the ribbon edges with H. We have taken twelve patterns of edge structure into consideration due to the chemical nonequivalence of Mo and Se atoms. We found that H-NR-u and H-NR-s have the highest stability by analysing binding energies. Specially, all of the zigzag MoSe2 nanoribbons exhibit obvious metallicity, whether they are hydrogenated or not. What's more, the pristine and hydrogenated zigzag nanoribbons have stable ferrimagnetism with a wide range of total magnetic moment 0.27–3.59 μB. The Mo-terminated nanoribbon has the largest magnetic moment and hydrogenation can significantly reduce the magnetic moment of nanoribbons. We also reveal zigzag MoSe2 nanoribbons with same terminal have very similar stability, electronic and magnetic properties, which have nothing to do with the symmetry of the system. Above all, these unique properties of zigzag MoSe2 nanoribbons have the great potential for electronic, spintronic and magnetoresistive nano-devices in the future.

Краевое легирование в графеновых приборах на подложках SiO-=SUB=-2-=/SUB=-

AbstractThe conductivity of single layer and bilayer graphene ribbons 0.5–4 μm in width fabricated by oxygen plasma etching is studied experimentally. The dependence of the electron concentration in graphene on the ribbon width is revealed. This effect is explained by the edge doping of graphene due to defects arranged in SiO_2 near the ribbon edges. The method of the formation of abrupt p – n junctions in graphene and structures with a constant electron concentration gradient in the graphene plane with the help of edge doping is proposed.

Modulation of the electronic properties and spin polarization of 2H VS2 nanoribbons by tuning ribbon widths and edge decoration.

The band structures and spin-polarization characteristics of armchair and zigzag VS2 nanoribbons with different terminated edges are investigated based on density functional theory (DFT) calculations with a spin polarized meta-GGA. The results reveal that zigzag 2H VS2 nanoribbons exhibit metal, half-metal, or semiconductor electrical characteristics with different edge decorations or ribbon widths. And the spin polarized ratio can achieve 100% self-polarization for the zigzag VS2 nanoribbons with V atom edges. The Curie temperatures (TC) estimated by mean field approximation simulations for the zigzag 2H VS2 nanoribbons with terminated edges of V systems are 276 K. These preliminary findings offer an effective treatment option for controllable and adjustable spintronic devices.