Stone-Wales Research Articles

The influence of Stone–Thrower–Wales defect on vibrational characteristics of single-walled carbon nanotubes incorporating Timoshenko beam element

The influence of Stone–Thrower–Wales (STW) defect is scrutinized on the fundamental frequency of single walled carbon nanotube invoking molecular mechanics approach. The modal finite element analysis is carried out by employing Timoshenko׳s beam element to construct the Carbon–Carbon bond of CNT lattice structure. Hence, three configurations of defective CNTs are taken into account by applying two kinds of boundary conditions. The results demonstrate that the frequencies are dependent on boundary conditions, CNT length, chirality and defect position as the critical position of STW defect is near the fixed end for both cantilever and bridge boundary conditions. Likewise, the results reveal that the natural frequencies extremely depend on the orientation of C–C bond concentrated at the STW defect. Meanwhile, by increasing the number of defects, the models indicate different behaviors ascribing to bond rearrangement imposed by STW defect. The results are in good agreement with those from other literatures.

A real-space study of random extended defects in solids: Application to disordered Stone–Wales defects in graphene

We propose here a first-principles, parameter free, real space method for the study of disordered extended defects in solids. We shall illustrate the power of the technique with an application to graphene sheets with randomly placed Stone–Wales defects and shall examine the signature of such random defects on the density of states as a function of their concentration. The technique is general enough to be applied to a whole class of systems with lattice translational symmetry broken not only locally but by extended defects and defect clusters. The real space approach will allow us to distinguish signatures of specific defects and defect clusters.

Modulating magnetic ordering of the zigzag-edge trigonal graphene by functionalizations

A pristine zigzag-edge trigonal graphene (ZTG) is a magnetic semiconductor, thus its spin polarization is extremely low. Here, we report the calculated results on enhancing the spin magnetism of a zigzag-edge trigonal graphene (ZTG) by functionalizations, including the heteroatom doping, edge modifications, and introducing topologic defects. It is found that the ZTG features a good tuning ability for functionalizations to improve its spin polarization. When one boron (B) atom is doped to replace one carbon atom in the B sublattice of graphene, a higher spin polarization can be achieved, and the edge modification by Cu, Co, O or B atom can modulate the magnetic ordering significantly due to the spin-polarized charge transfer between the ZTG and terminations, especially for O and Co terminations. And also, the introduced defect (a vacancy and a Stone–Wales defect) can obviously tune local magnetic structures owing to geometrically structural deformations (variations of bond length and bond angle). For these behaviors, in-depth analyses are performed. Our findings suggest that the desirable functionalized ZTG structures might promise importantly potential applications for developing nano-scale spintronics devices.

Enhanced Gas Adsorption on Graphitic Substrates via Defects and Local Curvature: A Density Functional Theory Study

Using van-der-Waals-corrected density functional theory calculations, we explore the possibility of engineering the local structure and morphology of high-surface-area graphene-derived materials to improve the uptake of methane and carbon dioxide for gas storage and sensing. We test the sensitivity of the gas adsorption energy to the introduction of native point defects, curvature, and the application of strain. The binding energy at topological point defect sites is inversely correlated with the number of missing carbon atoms, causing Stone–Wales defects to show the largest enhancement with respect to pristine graphene (∼20%). Improvements of similar magnitude are observed at concavely curved surfaces in buckled graphene sheets under compressive strain, whereas tensile strain tends to weaken gas binding. Trends for CO2 and CH4 are similar, although CO2 binding is generally stronger by ∼4 to 5 kJ mol–1. However, the differential between the adsorption of CO2 and CH4 is much higher on folded graphene sheet...

A Large-Scale Molecular Dynamics Study of the Divacancy Defect in Graphene

We report on the dynamical behavior of single divacancy defects in large graphene sheets as studied by extensive classical molecular dynamics (MD) simulations at high temperatures and static calculations. In the first part of the paper, the ability of the used interatomic potential to properly render the stability and dynamics (energy barriers) of such defects is validated against electronic structure calculations from the literature. Then, results from MD simulations are presented. In agreement with recent TEM studies, some mobility is observed through a series of Stone–Wales-like bond rotations involving the 5–8–5, 555–777, and 5555–6–7777 reconstructions. Although these three structures are by far the most probable structures of the DV defect, not less than 18 other full reconstructions, including the experimentally observed 55–66–77 defect, were occasionally observed in the ≈1.5 μs of MD trajectories analyzed in this work. Most of these additional reconstructions have moderate formation energies and can be formed by a bond rotation mechanism from one of the aforementioned structures, with a lower activation energy than the one required to form a Stone–Wales defect in graphene. Therefore their future experimental observation is highly probable. The results presented here also suggest that the barrier to a conventional Stone–Wales transformation (the formation of two pentagon/heptagon pairs from four hexagons) can be significantly reduced in the vicinity of an existing defect, strengthening a recently proposed melting mechanism for graphene based on the aggregation of Stone–Wales defects. From a structural point of view, in addition to pentagons, heptagons, and octagons, these new DV reconstructions can also contain four- and nine-member rings and show a particularly large spatial extent of up to 13 rings (42 atoms) against three (14 atoms) for the original 5–8–5 defect.

The effect of Stone–Thrower–Wales defects on mechanical properties of graphene sheets – A molecular dynamics study

The mechanical properties of graphene may be strongly affected by defects. In our work, the effects of the orientation and tilting angles of Stone–Thrower–Wales (STW, 5-7-7-5) defects on the mechanical properties of graphene have been investigated based on molecular dynamics simulations. When there is one centred STW defect including STW-1 defect and STW-2 defect, our study reveals that the orientation with respect to the chirality governs the mechanical properties of graphene. For STW-1 defective graphene, the strength of the armchair direction is much lower than that of zigzag direction. While the circumstance for STW-2 defective graphene is opposite, the strength of the armchair direction is much higher than the strength of the zigzag direction. Furthermore, when there is more than one STW defect, the mechanical properties of graphene depends on the tilting angle of STW defects. The breaking strength of graphene decreases with the increasing tilting angle. Our present work could provide significant insights into the effect of STW defects on the mechanical properties of graphene.

Visualizing the influence of point defects on the electronic band structure of graphene

The supercell approach enables us to treat the electronic structure of defective crystals, but the calculated energy bands are too complicated to understand or compare with angle-resolved photoemission spectra because of inevitable zone folding. We discuss how to visualize supercell band structures more effectively by incorporating unfolded spectral weights and orbital decompositions into them. We then apply these ideas to gain a better understanding of the band structure of graphene containing various types of point defects, including nitrogen impurity, hydrogen adsorbate, vacancy defects and the Stone–Wales defect.

N2O reduction over hexagonal BN nanosheet: effects of Stone–Wales defect and carbon pair doping

Nitrous oxide (N2O) adsorption on the pristine and Stone–Wales (SW)-defected hexagonal BN nanosheets were investigated using density functional calculations including dispersion correction. It was found that N2O is weakly adsorbed on the pristine sheet (h-BN) through van der Waals interaction with adsorption energy of −1.2 kcal/mol. SW-defected sheet was found to be more reactive toward N2O molecule having no significant change in electronic properties. However, the formation of B–B and N–N bond pairs in SW-defected sheet can be avoided, if there is a C–C pair doped in sheet (C2-SW-h-BN). In this case, a strong adsorption is found due to large adsorption energy (−23.7 kcal/mol) and short bond length compared to the SW-h-BN complex. Interestingly, it was indicated that the N2O molecule could be reduced into the N2 on the C2-SW-h-BN.

Gas adsorption on silicene: A theoretical study

The adsorption of several common gas molecules on silicene has been studied using density functional theory (DFT). The most stable configurations, the corresponding adsorption energies, charge transfer, and electronic properties of several common gas molecules on silicene are thoroughly discussed. We find that silicene exhibits significantly high reactivity towards NO2, O2, and SO2 with the adsorption energies being larger than 1.00eV, suggesting its potential applications for the development of metal-free catalysts. Moreover, NO and NH3 can be adsorbed on silicene with a moderate adsorption energy (0.35 and 0.60eV), indicating that silicene could be a good NO or NH3 sensor. Moreover, the band gap of silicene is opened upon adsorption of NO, O2, NH3, and SO2 in various ways, while NO2 adsorption makes silicene half-metallic nature. In addition, we find that the Stone–Wales defect and Ag(111) substrate can enhance the chemical reactivity of silicene. Our results may be useful not only to deeply understand the properties of silicene, but also to initiate one to further explore its potential applications in catalysis, gas detecting as well as electronics.

An embedded simulation approach for modeling the thermal conductivity of 2D nanoscale material

The thermal property of single layer graphene sheet is investigated in this work by using an embedded approach of molecular dynamics (MD) and soft computing method. The effect of temperature and Stone–Thrower–Wales (STW) defects on the thermal conductivity of graphene sheet is first analyzed using MD simulation. The data obtained using the MD simulation is then fed into the paradigm of soft computing approach, multi-gene genetic programming (MGGP), which was specifically designed to model the response of thermal conductivity of graphene sheet with changes in system temperature and STW defect concentration. We find that our proposed MGGP model is able to model the thermal conductivity of graphene sheet very well which can be used to complement the analytical solution developed by MD simulation. Additionally, we also conducted sensitivity and parametric analysis to find out specific influence and variation of each of the input system parameters on the thermal conductivity of graphene sheet. It was found that the STW defects has the most dominating influence on the thermal conductivity of graphene sheet.

Vibrational characterization of two-dimensional networks: Effects of patterns and defects

We investigated normal-mode vibrations of two-dimensional network structures by using an eigenmode analysis. In small-amplitude limits, we derived eigenvalue equations represented by a set of linear Lagrange equations and determined the eigenvalue distributions of the normal modes. The eigenvalue distributions were analyzed for various pristine networks in the form of kagome, trigonal and hexagonal lattices. Patterns of the normal modes were realized by using the determined eigenvectors and were verified in terms of effective spring constants by using a geometrical analysis. Various defect models, such as inhomogeneities, vacancies, and Stone-Wales defects, were introduced for more realistic characterizations on the networks. Defects, depending on their topological characteristics, were shown to have different effects on the eigenvalue distributions. The symmetry of the eigenvalue distributions was found to be directly related to the bipartite characteristics of the network’s geometry.

The Stone-Wales rotation in fullerenes and their derivatives

The Stone-Wales rotation in fullerenes and their derivatives

A comparative study on hydrogen interaction with defective graphene structures doped by transition metals

In the present work, the interaction of hydrogen molecules with defective graphene structures doped by transition metal (TM) atoms is investigated by using first principles density functional theory (DFT). Defective graphene structures include Stone–Wales (SW), 585 and 555-777 and transition metals include early TMs, i.e. scandium (Sc), titanium (Ti) and vanadium (V). It is found that in comparison with the pristine graphene, presence of defects significantly enhances the metal binding. Among three defects, 585 divacancy leads to the strongest binding between graphene and metal. Hydrogen adsorption is then evaluated by sequential addition of hydrogen molecules to the system. The results reveal that in comparison with other structures, 555-777 defective structure doped by Sc has the maximum capacity for hydrogen molecules. Also it is indicated that none of hydrogen molecules were dissociated during relaxation, indicating that all hydrogen molecules are accessible for reversible storage. Moreover, it is found that binding energies for adsorption of hydrogen molecules over 555-777/ Sc system are in the favorable range of 0.2–0.4eV/H2.

Dynamics of Topological Defects in Single-Walled Carbon Nanotubes during Catalytic Growth

Nucleation and healing of structural defects in single-walled carbon nanotubes (SWCNTs) studied through reactive molecular dynamics simulations (RMD) and RMD trajectories reveal formation and healing mechanisms of various topological defects on the catalyst surface. A quality percentage of nanotubes is measured by calculating the amount of hexagons per carbon atom relative to the same quantity for a perfect nanotube of the same length. Following this approach, the concentration of defects is estimated for nanotubes grown on catalysts with different sizes and morphologies and for various temperatures and gas-phase densities. From this analysis, we identify specific catalyst morphologies that favor the growth of SWCNTs with low defect concentration. Vacancies, 5–7, and Stone–Wales defects are observed to nucleate distinctly in the tubes depending on the catalyst morphology. We find that a strong interaction between the catalyst surface and the graphitic lattice of the nanotube is absolutely necessary for he...

Effects of various defects on the electronic properties of single-walled carbon nanotubes: A first principle study

The geometries, formation energies and electronic band structures of (8, 0) and (14, 0) singlewalled carbon nanotubes (SWCNTs) with various defects, including vacancy, Stone-Wales defect, and octagon-pentagon pair defect, have been investigated within the framework of the density-functional theory (DFT), and the influence of the concentration within the same style of defect on the physical and chemical properties of SWCNTs is also studied. The results suggest that the existence of vacancy and octagon-pentagon pair defect both reduce the band gap, whereas the SW-defect induces a band gap opening in CNTs. More interestingly, the band gaps of (8, 0) and (14, 0) SWCNTs configurations with two octagon-pentagon pair defect presents 0.517 eV and 0.163 eV, which are a little smaller than the perfect CNTs. Furthermore, with the concentration of defects increasing, there is a decreasing of band gap making the two types of SWCNTs change from a semiconductor to a metallic conductor.

Defect engineering of graphene for effective hydrogen storage

Although several modifications to graphene have been proposed to improve its hydrogen binding to practical levels, current theoretical studies largely neglect the role of topological defects. In this paper we analyze the effect of these defects and their possible use in a hydrogen storage system. Hydrogen physisorption on five types of point defects (Stone-Wales, single vacancy and three types of double vacancy) was investigated using density functional theory with the PBE-GGA functional. Point defects were also repeated with the vdW-DF2 functional to better represent long range van der Waals interactions. Although none of the defects were found to be detrimental to hydrogen anchoring, only the single vacancy showed promising hydrogen binding in the ideal range. Model systems combining different defects were also explored, including a defect-anchored metal system, a bilayer graphene system and a grain-boundary system. Finally, two high defect density structures constructed using vacancies and combined Stone-Wales defects and vacancies yielded gravimetric densities of 5.81% and 7.02%, respectively, with the vdW functional. This study suggests that graphene can be defect-engineered to develop effective hydrogen storage media.

Catalytic Mechanisms of Sulfur-Doped Graphene as Efficient Oxygen Reduction Reaction Catalysts for Fuel Cells

Density functional theory (DFT) was applied to study sulfur-doped graphene clusters as oxygen reduction reaction (ORR) cathode catalysts for fuel cells. Several sulfur-doped graphene clusters with/without Stone–Wales defects were investigated and their electronic structures, reaction free energy, transition states, and energy barriers were calculated to predict their catalytic properties. The results show that sulfur atoms could be adsorbed on the graphene surface, substitute carbon atoms at the graphene edges in the form of sulfur/sulfur oxide, or connect two graphene sheets by forming a sulfur cluster ring. These sulfur-doped graphene clusters with sulfur or sulfur oxide locating at graphene edges show electrocatalytic activity for ORR. Catalytic active sites distribute at the zigzag edge or the neighboring carbon atoms of doped sulfur oxide atoms, which possess large spin or charge density. For those being the active catalytic sites, sulfur atoms with the highest charge density take a two-electron tran...

Effect of Stone–Wales and vacancy defects on elastic moduli of carbon nanotubes and their composites using molecular dynamics simulation

Carbon nanotubes (CNTs) containing no defect exhibit excellent mechanical properties. However, CNTs suffer from defects such as vacancies, hybridization, doping and Stone–Wales (SW) defects which can appear during purification, production or can be deliberately introduced using chemical treatment. The study conducted here is based on comparative investigation of effect produced by SW and vacancy defects on the elastic moduli of SWCNTs. A (4,4) armchair SWCNT has been used in this study and the number of defects is varied from 1 to 4. Molecular dynamics (MD) simulation has been used to study the effects of these defects on mechanical properties of single-walled carbon nanotubes (SWCNTs). MD simulation has also been used to study the effect of variation of radius of defective CNTs on their elastic moduli. Results have also been obtained for defective SWCNT/polypropylene composites. Results show that a gradual degradation of elastic moduli is observed with increasing diameter and the number of defects of armchair CNTs. As the number of defects increases, the moduli of CNTs with SW defects decreases more rapidly in comparison to the decrease in moduli for CNTs with vacancy defects. Percentage increase in different moduli with increase in CNT volume fraction is greater for CNTs with vacancy defects in comparison to CNTs with SW defects.

Core level binding energies of functionalized and defective graphene.

X-ray photoelectron spectroscopy (XPS) is a widely used tool for studying the chemical composition of materials and it is a standard technique in surface science and technology. XPS is particularly useful for characterizing nanostructures such as carbon nanomaterials due to their reduced dimensionality. In order to assign the measured binding energies to specific bonding environments, reference energy values need to be known. Experimental measurements of the core level signals of the elements present in novel materials such as graphene have often been compared to values measured for molecules, or calculated for finite clusters. Here we have calculated core level binding energies for variously functionalized or defected graphene by delta Kohn–Sham total energy differences in the real-space grid-based projector-augmented wave density functional theory code (GPAW). To accurately model extended systems, we applied periodic boundary conditions in large unit cells to avoid computational artifacts. In select cases, we compared the results to all-electron calculations using an ab initio molecular simulations (FHI-aims) code. We calculated the carbon and oxygen 1s core level binding energies for oxygen and hydrogen functionalities such as graphane-like hydrogenation, and epoxide, hydroxide and carboxylic functional groups. In all cases, we considered binding energy contributions arising from carbon atoms up to the third nearest neighbor from the functional group, and plotted C 1s line shapes by using experimentally realistic broadenings. Furthermore, we simulated the simplest atomic defects, namely single and double vacancies and the Stone–Thrower–Wales defect. Finally, we studied modifications of a reactive single vacancy with O and H functionalities, and compared the calculated values to data found in the literature.

Atomistic Studies on Tensile Mechanics of BN Nanotubes in the Presence of Defects

Boron nitride nanotubes (BNNTs) are of immense importance due to their many interesting functional features, notably biocompatibility and piezoelectricity and dominant mechanical strength as compared to carbon nanotubes (CNTs). The reliable implementation of these structures in an application is inherently related to its mechanical characteristics under external loads. The presence of defects in these structures severely affects the tensile properties. The effect of presence of point, line and Stone–Wales (SW) defects on the tensile behavior of BNNTs is systematically investigated by applying reactive force fields in molecular dynamics (MD) framework. Reactive force fields effectively describe the bond breaking and bond forming mechanism for BNNTs that are important for a practical situation. The Young's modulus of single-walled (10,0) BNNTs of length 100 nm has been found to be nearly 1.098 TPa, in good agreement with the available reports. The presence of defects has been shown to significantly reduce the tensile strength of the tube, while the number and separation of the defects effectively contribute to the percentage reduction. In addition, the effect of tube diameter and also the initial temperature are observed to strongly influence the tensile characteristics of BNNTs, indicating increased auxetic behavior than CNTs.