Line Defects In Graphene Research Articles

Boron-doped graphene topological defects: unveiling high sensitivity to NO molecule for gas sensing applications.

Global air quality has deteriorated significantly in recent years due to large emissions from the transformation industry and combustion vehicles. This issue requires the development of portable, highly sensitive, and selective gas sensors. Nanostructured materials, including defective graphene, have emerged as promising candidates for such applications. In this work, we investigated the B-doped topological line defect in graphene as a sensing material for various gas molecules (CO, CO2, NO, and NH3) based on a combination of density functional theory and the non-equilibrium Green's function method. The electronic transport calculations reveal that the electric current can be confined to the line defect region by gate voltage control, revealing highly reactive sites. The B-doped topological line defect is metallic, favoring the adsorption of NO and NH3 over CO and CO2 molecules. We notice changes in the conductance after gas molecule adsorption, producing a sensitivity of 50% (16%) for NO (NH3). In addition, the recovery time for nitride gases was calculated for different temperatures and radiation frequencies. At 300 K the ultraviolet (UV) has a fast recovery time compared to the visible (VIS) one by about two orders of magnitude. This study gives an understanding of how engineering transport properties at the microscopic level (by topological line defect and chemical B-doping) leads to promising nanosensors for detecting nitride gas.

A generalised FK model for the 558 line defect in graphene

ABSTRACT Defects in two-dimensional nanomaterials do not always affect their properties negatively, but often have useful functionality. The 558 line defect in graphene distinctively acts as a quasi-one-dimensional metallic wire and a valley filter, which is already experimentally realised. To have a thorough understanding of the 558 line defect in graphene, we have extended the conventional FK model and have applied the model to theoretically investigate the structure of the 558 line defect, which agrees well with the first-principle simulation result. In the generalised FK model, it is vital to take into account bond energy correction to the interatomic force of atoms on the defect line. The generalised FK model can be used to predict structures of one-dimensional topological defects in other two-dimensional materials.

Influence of local deformation on valley transport properties in the line defect of graphene

The line defect of graphene has significant applications in valleytronics, which has received extensive attention in recent years. It is found experimentally that there exists local deformation around the line defect. Current studies generally believe that the influence of local deformation on the valley transport properties can be negligible, because the modifications to the nearest neighbour hopping energy is less than 5% under the small deformation. Based on the first-principles calculations and the non-equilibrium Green’s function method, we investigated the effect of local deformation on the valley transport properties of two different kinds of line defects, the 58 ring line defect and the 57 ring line defect. It is found that for the 58 ring line defect, the effect of local deformation on the valley transmission coefficient is not evident at lower energies. However, at higher energies, the impact of local deformation is obvious, and the maximum valley transmission coefficient does not decrease with increasing energy, but can be maintained 1 within a large energy range. In contrast, the influence of local deformation on the valley transmission coefficient of the 57 ring line defect indeed can be negligible, regardless of the level of energy. Further investigation indicates that the change of the C—C bond length connected to the two defect atoms in the 58 ring plays a key role in the transmission of the valley states across the line defect. If this part of the influence is not taken into account, the valley transmission coefficient is nearly unaffected by the local deformation. The valley state enters the right side of the line defect directly through the bond connected to the line defect, so the change in bond length connected to the line defect has a significant impact on the valley transmission. This special structure does not exist in the 57 ring, where the valley states will have to pass through a narrow region containing 57 ring to enter the right side of the line defect, resulting in different valley scattering phenomena. By constructing two parallel line defects, the 100% valley polarization can be achieved in a large angular range with the 58 ring line defect. The finding has important implications for the design of graphene line defect based valley filters.

Ab initio study of lithium decoration of popgraphene and hydrogen storage capacity of the hybrid nanostructure

The successful realization of extended ‘5-5-8’ line defects in graphene in a controlled way has suggested the possible formation of a new 2D carbon allotrope consisting in pentagonal-octagonal-pentagonal carbon rings. The mechanical and thermal stability of this nanostructure, called popgraphene, has recently been confirmed on the basis of first-principles calculations. Moreover, it has been proposed as a promising anode material for use in Li-ion batteries with fast charge/discharge rates. In the present paper we perform density functional theory calculations to investigate the hydrogen storage ability of popgraphene. Our calculations show that popgraphene can be decorated in a very stable way by placing Li atoms on the pentagonal carbon rings of both sides of the sheet, leaving free the octagonal carbon rings. In that condition, the Li-decorated popgraphene sheet can bind up to four H2 molecules per unit cell, with an average adsorption energy in a range between physisorption and atomic chemisorption, which would allow for reversible hydrogen storage at moderate temperatures and pressures. Moreover, the gravimetric density of the Li-decorated popgraphene is 4.24 wt%, which is almost equal to the threshold specified by the U.S. Department of Energy for novel hydrogen-storage materials. All these results show that Li-decorated popgraphene nanostructures could be good materials for hydrogen storage.

Structural Defects Modulate Electronic and Nanomechanical Properties of 2D Materials.

Two-dimensional materials such as graphene and molybdenum disulfide are often subject to out-of-plane deformation, but its influence on electronic and nanomechanical properties remains poorly understood. These physical distortions modulate important properties which can be studied by atomic force microscopy and Raman spectroscopic mapping. Herein, we have identified and investigated different geometries of line defects in graphene and molybdenum disulfide such as standing collapsed wrinkles, folded wrinkles, and grain boundaries that exhibit distinct strain and doping. In addition, we apply nanomechanical atomic force microscopy to determine the influence of these defects on local stiffness. For wrinkles of similar height, the stiffness of graphene was found to be higher than that of molybdenum disulfide by 10-15% due to stronger in-plane covalent bonding. Interestingly, deflated graphene nanobubbles exhibited entirely different characteristics from wrinkles and exhibit the lowest stiffness of all graphene defects. Density functional theory reveals alteration of the bandstructures of graphene and MoS2 due to the wrinkled structure; such modulation is higher in MoS2 compared to graphene. Using this approach, we can ascertain that wrinkles are subject to significant strain but minimal doping, while edges show significant doping and minimal strain. Furthermore, defects in graphene predominantly show compressive strain and increased carrier density. Defects in molybdenum disulfide predominantly show tensile strain and reduced carrier density, with increasing tensile strain minimizing doping across all defects in both materials. The present work provides critical fundamental insights into the electronic and nanomechanical influence of intrinsic structural defects at the nanoscale, which will be valuable in straintronic device engineering.

Identifying DNA Nucleotides via Transverse Electronic Transport in Atomically Thin Topologically Defected Graphene Electrodes.

Extended line defects in graphene (ELDG) sheets have been found to be promising for biomolecule sensing applications. By means of the consistent-exchange van der Waals density-functional (vdW-DF-cx) method, the electronic, structural, and quantum transport properties of the ELDG nanogap setup has been studied when a DNA nucleotide molecule is positioned inside the nanogap electrodes. The interaction energy (Ei) values indicate charge transfer interaction between the nucleotide molecule and electrode edges. The charge density difference plots reveal that charge fluctuates around the ELDG nanogap edges adjacent to the nucleotides. This charge redistribution grounds the modulation of electronic charge transport in the ELDG nanogap device. Further, we study the electronic transverse-conductance and tunnelling current-voltage (I-V) characteristics across two closely spaced ELDG nanogap electrodes using the density functional theory and the nonequilibrium Green's function methods when a DNA nucleotide is translocated through the nanogap. Our outcomes indicate that the ELDG nano gap device could allow sequencing of DNA nucleotides with a robust and consistent yield, giving the tunneling electric current signals that vary by more than 1 order of magnitude electric current (I) for the different DNA nucleotides. So, we predict that the ELDG nanogap-based tunneling device can be suitable for sequencing DNA nucleobases.

Rippling of two-dimensional materials by line defects

Two-dimensional materials and their mechanical properties are known to be profoundly affected by rippling deformations. However, although ripples are fairly well understood, less is known about their origin and controlled modification. Here, motivated by recent reports of laser-controlled creation of line defects in graphene, we investigate how line defects could be used to control rippling in graphene and other two-dimensional materials. By sequential multiscale coupling of density-functional tight-binding and continuum elasticity simulations, we quantify the amount of rippling when the number and the cumulative length of the line defects increase. Simulations show that elastic sheets with networks of line defects create rippling that induces considerable out-of-plane rigidification and in-plane softening with nonlinear elastic behavior. We hope that these insights help to guide experimental attempts to modify the mechanical properties of graphene and other two-dimensional materials.

Structure and electronic properties of closed-ring defects in epitaxial graphene

A number of past studies have focused on point and line defects in graphene epitaxially grown on SiC substrates. However, few studies have investigated closed-ring defects formed within grain boundary loops. The present study addresses this issue by applying low-temperature scanning tunneling microscopy/spectroscopy to investigate the atomic structures of closed-ring defects in graphene epitaxially grown on 4H-SiC, and to evaluate their effects on the electron state density. The results indicate that the orientations of the graphene lattice inside and outside of grain boundary loop structures are rotated uniformly by an angle of 30° relative to each other, suggesting that closed-ring defects are highly ordered and are mainly composed of clusters of pentagon-heptagon carbon rings and highly ordered pentagon-heptagon chains. In addition, the spectroscopy results reveal for the first time that the density of electron states inside a closed-ring defect is strongly localized and position-dependent. Moreover, these closed-ring defects can eliminate intervalley scattering while maintaining intravalley scattering. These findings are not only helpful for contributing to a deeper understanding of the effects of closed-ring defects in graphene, but also present a potentially useful valley-filtering mechanism for charge carries that can be applied to the practical development of all-electric valley-based devices.

Extended topological line defects in graphene for individual identification of DNA nucleobases

The TOC features a scheme of solid-state nanochannel-based DNA sequencing techniques. DNA nucleobases can be analyzed at the single-molecule level by adsorption on topologically extended line defects in the graphene-based electrode setup.

Topological Line Defects Around Graphene Nanopores for DNA Sequencing

Topological line defects in graphene represent an ideal way to produce highly controlled structures with reduced dimensionality that can be used in electronic devices. In this work, we propose using extended line defects in graphene to improve nucleobase selectivity in nanopore-based DNA sequencing devices. We use a combination of quantum mechanics/molecular mechanics and nonequilibrium Green’s function methods to investigate the conductance modulation, fully accounting for solvent effects. By sampling over a large number of different orientations generated from molecular dynamics simulations, we theoretically demonstrate that distinguishing between the four nucleobases using line defects in a graphene-based electronic device appears possible. The changes in conductance are associated with transport across specific molecular states near the Fermi level and their coupling to the pore. Through the application of a specifically tuned gate voltage, such a device would be able to discriminate the four types of...

Topological line defects in graphene for applications in gas sensing

Topological line defects in graphene synthesized in a highly controlled manner open up new research directions for nanodevice applications. Here, we investigate two types of extended line defects in graphene, namely octagonal/pentagonal and heptagonal/pentagonal reconstructions. A combination of density functional theory and non-equilibrium Green's function methods was utilized in order to explore the application potential of this system as an electronic gas sensor. Our findings show that the electric current is confined to the line defect through gate voltage control, which combined with the enhanced chemical reactivity at the grain boundary, makes this system a highly promising candidate for gas sensor applications. As a proof of principle, we evaluated the sensitivity of a prototypical device toward NO2 molecule, demonstrating that it is indeed possible to reliably detect the target molecule.

The valley filter efficiency of monolayer graphene and bilayer graphene line defect model

In addition to electron charge and spin, novel materials host another degree of freedom, the valley. For a junction composed of valley filter sandwiched by two normal terminals, we focus on the valley efficiency under disorder with two valley filter models based on monolayer and bilayer graphene. Applying the transfer matrix method, valley resolved transmission coefficients are obtained. We find that: (i) under weak disorder, when the line defect length is over about , it functions as a perfect channel (quantized conductance) and valley filter (totally polarized); (ii) in the diffusive regime, combination effects of backscattering and bulk states assisted intervalley transmission enhance the conductance and suppress the valley polarization; (iii) for very long line defect, though the conductance is small, polarization is indifferent to length. Under perpendicular magnetics field, the characters of charge and valley transport are only slightly affected. Finally we discuss the efficiency of transport valley polarized current in a hybrid system.

Line defects in graphene: How doping affects the electronic and mechanical properties

Graphene and carbon nanotubes have extraordinary mechanical and electronic properties. Intrinsic line defects such as local non-hexagonal reconstructions or grain boundaries, however, significantly reduce the tensile strength, but feature exciting electronic properties. Here, we address the properties of line defects in graphene from first-principles on the level of full-potential density-functional theory, and assess doping as one strategy to strengthen such materials. We carefully disentangle the global and local effect of doping by comparing results from the virtual crystal approximation with those from local substitution of chemical species, in order to gain a detailed understanding of the breaking and stabilization mechanisms. We find that n-type doping or local substitution with nitrogen increases the ultimate tensile strength significantly. In particular, it can stabilize the defects beyond the ultimate tensile strength of the pristine material. We therefore propose this as a key strategy to strengthen graphenic materials. Furthermore, we find that doping and/or applying external stress lead to tunable and technologically interesting metal/semi-conductor transitions.

Line defects and quantum Hall plateaus in graphene

Line defects in graphene can be either tailored-growth or arise naturally and are at the center of many discussions. Here we study the multiterminal conductance of graphene with an extended line defect in the quantum Hall regime analyzing the effects of the geometry of the setup, disorder and strain on the quantum Hall plateaus. We show that the defect turns out to affect the local and non-local conductance in very different ways depending on the geometrical configuration. When the defect is parallel to the sample edges one gets an equivalent circuit formed by parallel resistors. In contrast, when the defect bridges opposite edges, the Hall conductance may remain unaltered depending on the geometry of the voltage/current probes. The role of disorder, strain and the microscopic details of the defect in our results is also discussed. We show that the defect provides a realization of the electrical analog of an optical beam splitter. Its peculiar energy dependent inter-edge transmission allows it to be turned on or off at will and it may be used for routing the chiral edge states.

Self-doping and magnetic ordering induced by extended line defects in graphene

Based on first-principles calculations, we reveal that the interactions between extended line defects (ELDs) of type ``585'' (formed by five and eight membered rings) ELDs embedded in graphene can induce ordered magnetism and self-doping of graphene. By reducing the distance between 585 ELDs, a distinct charge transfer is predicted from the center of 585 ELDs to their edges, which induces a Dirac point shift below the Fermi level, resulting in distance- or density-dependent $n$-type doping in the graphene. Relevant to the above finding, we found a distance-dependent spin polarization at the edges of 585 ELDs, attributable to the rigidity of the \ensuremath{\pi} electronic structure. Our finding suggests a promising approach for achieving $n$-type graphene for spintronic devices by creating 585 ELDs.

Magnetoresistance of Mn-decorated topological line defects in graphene

We study the spin polarized transport through Mn-decorated 8-5-5-8 topological line defects in graphene using the nonequilibrium Green's function formalism. Strong preferential bonding overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced $\ensuremath{\pi}$ magnetism, in particular for full coverage of all octagonal hollow sites by Mn atoms. In contrast to the magnetoresistance of graphene nanoribbon edges, the proposed system is well controlled and therefore suitable for applications.

Tunable valley polarization by a gate voltage when an electron tunnels through multiple line defects in graphene

By means of an appropriate wave function connection condition, we study the electronic structure of a line defect superlattice of graphene with the Dirac equation method. We obtain the analytical dispersion relation, which can simulate well the tight-binding numerical result about the band structure of the superlattice. Then, we generalize this theoretical method to study the electronic transmission through a potential barrier where multiple line defects are periodically patterned. We find that there exists a critical incident angle which restricts the electronic transmission through multiple line defects within a specific incident angle range. The critical angle depends sensitively on the potential barrier height, which can be modulated by a gate voltage. As a result, non-trivial transmissions of K and K′ valley electrons are restricted, respectively, in two distinct ranges of the incident angle. Our theoretical result demonstrates that a gate voltage can act as a feasible measure to tune the valley polarization when electrons tunnel through multiple line defects.

Controlled growth of a line defect in graphene and implications for gate-tunable valley filtering

Atomically precise tailoring of graphene can enable unusual transport pathways and new nanometer-scale functional devices. Here we describe a recipe for the controlled production of highly regular ``5-5-8'' line defects in graphene by means of simultaneous electron irradiation and Joule heating by applied electric current. High-resolution transmission electron microscopy reveals individual steps of the growth process. Extending earlier theoretical work suggesting valley-discriminating capabilities of a graphene 5-5-8 line defect, we perform first-principles calculations of transport and find a strong energy dependence of valley polarization of the charge carriers across the defect. These findings inspire us to propose a compact electrostatically gated ``valley valve'' device, a critical component for valleytronics.

Half-metal behaviour mediated by self-doping of topological line defect combining with adsorption of 3d transition-metal atomic chains in graphene

We present a systematic study of 3d transition-metal (TM) atomic chains adsorbed on a line defect consisting of octagonal and pentagonal carbon rings embedded in a perfect graphene sheet. The results suggest that Sc, Ti, V, Cr and Mn atoms can be spontaneously adsorbed at octagonal sites and form an atomic chain along the line defect in graphene, while Fe, Co and Ni atomic chains are adsorbed at pentagonal sites. Moreover, the Sc, Ti, V, Cr, Mn and Fe chains embedded in line defect of graphene structures show half-metallic characters. These results propose a new route to explore the potential applications of the system in spintronic devices.

Quasi-one-dimensional electronic states induced by an extended line defect in graphene: an analytic solution

Analytic solutions of the quasi-one-dimensional (q1D) electron states around an extended line defect in a graphene lattice are derived within the tight-binding model. Then, the electronic properties of this kind of boundary state in graphene are studied in detail. It is found that one subband composed of the even-parity boundary states emerges in the vicinity of the Dirac point. In particular, when the bulk band is gapped, such a one-dimensional subband remains in the bandgap, spanning two inequivalent valleys. In addition, this boundary state subband exhibits nontrivial dispersion, which can carry the valley polarized charge current flowing along the extended line defect. As a result, the line defect behaves like a one-dimensional channel for electronic transport. Moreover, its appearance in graphene or a hexagonal boron nitride sheet provides a promising way to print electric circuits in these two-dimensional materials.