Stone-Wales Research Articles

The role of structure defects in the deformation of anthracite and their influence on the macromolecular structure

The impacts of stress on the physical and optical properties of coals are well recognized, while the influence on the chemical structure is seldom considered. In the light of mechanochemistry research that mechanical force can act on the molecule directly to initiate or accelerate reactions by deforming the chemical bonds, it is meaningful to consider how stress works on the macromolecule of coals. In this work, some insights are given based on anthracites with different tectonic deformation from Qinshui Basin, Shanxi Province, China. The deformation degree was measured by bireflectance (Ro,max−Ro,min), and the macromolecular structure was characterized by Raman spectroscopy. For samples from the same colliery, there is a positive relationship between Raman area ratio AD/AG and bireflectance, suggesting that the deformation of anthracite is related with the generation of structure defects at the atomic scale. Further quantum chemistry calculations demonstrate that accompanying the generation of one Stone-Wales (SW) defect (induced via in-plane rotation of C-C bond by 90°), the molecular geometries of anthracite, such as chemical bonds and angles, change. The deviation of atoms from their equilibrium geometries reflects the local force distribution and transfers 303.48kJ/mol mechanical energy into chemical energy. These changes allow chemical bonds to adjust to the applied stress without breakage, so that anthracite will accommodate plastic deformation. Additionally, the existence of SW defect slightly reduces the energy needed to produce carbon monoxide from carbonyl in anthracite. The current study helps to understand the potential influence of stress on the chemical structure evolution of coals.

Empirical potential influence and effect of temperature on the mechanical properties of pristine and defective hexagonal boron nitride

The major objective of this work is to present results of a classical molecular dynamics study to investigate the effect of changing the cut-off distance in the empirical potential on the stress–strain relation and also the temperature dependent Young’s modulus of pristine and defective hexagonal boron nitride. As the temperature increases, the computed Young’s modulus shows a significant decrease along both the armchair and zigzag directions. The computed Young’s modulus shows a trend in keeping with the structural anisotropy of h-BN. The variation of Young’s modulus with system size is elucidated. The observed mechanical strength of h-BN is significantly affected by the vacancy and Stone–Wales type defects. The computed room temperature Young’s modulus of pristine h-BN is 755 GPa and 769 GPa respectively along the armchair and zigzag directions. The decrease of Young’s modulus with increase in temperature has been analyzed and the results show that the system with zigzag edge shows a higher value of Young’s modulus in comparison to that with armchair edge. As the temperature increases, the computed stiffness decreases and the system with zigzag edge possesses a higher value of stiffness as compared to the armchair counterpart and this behaviour is consistent with the variation of Young’s modulus. The defect analysis shows that presence of vacancy type defects leads to a higher Young’s modulus, in the studied range with different percentage of defect concentration, in comparison with Stone–Wales defect. The variations in the peak position of the computed radial distribution function reveals the changes in the structural features of systems with zigzag and armchair edges in the presence of applied stress.

Hydrogen storage in Titanium-doped single-walled carbon nanotubes with Stone-Wales defects

The hydrogen storage capacity of Titanium-doped single-walled carbon nanotubes (SWCNT) containing the Stone-Wales 5,8 defects was studied using molecular dynamics simulations. The equilibrium doping sites and their stability were estimated using density functional theory. Although introduction of structural defects and dopant atom decreases the formation and cohesive energies, the drop in these energies is not large enough to hinder the thermodynamic feasibility of formation of these structures. Moreover, we observed that the stability of SWCNTs, where Ti is doped by replacing two carbon atoms is similar to that of the defect-free nanotube. This particular novel configuration (D5) was also obtained by rearranging the bonds in the 5 and 8-member rings of the Stone-Wales defect. Doping Ti on the defective rings has a more significant effect on the adsorption of hydrogen than doping on the regular 6-member rings. The D5 SWCNT showed the highest gravimetric and volumetric storage capacities at a temperature of 298K and a moderate pressure of 140atm. We also compared the performance of the D5 SWCNT with a recently reported Ti-doped porphyrin SWCNT and observed that the storage capacity of the D5 SWCNT was significantly higher at similar conditions. Our results suggest that Ti-doped SWCNTs with the Stone-Wales (5,8) defects show a promising potential to meet the ultimate goal set by the US Department of Energy for hydrogen storage.

Formation of hierarchical porous graphene films with defects using a nanosecond laser on polyimide sheet

The cost of effective preparation of graphene-based nanomaterials is a challenge in high-performance flexible electrodes. We demonstrated the formation of hierarchical porous graphene (HPG) films with defects from polyimide (PI) sheets using a high repetition rate nanosecond fiber laser. The honeycomb structure with mesopores and macropores can be rapidly induced on the polyimide by the localized focused laser beam in air atmosphere. Employing laser direct writing method, the one-step synthesis and patterning of conductive HPG films were achieved directly on the surface of polyimide sheets. The results show that the unique honeycomb porous structure on HPG film is composed of few-layer graphene or graphene stacks. The lattice structure of graphene nanoplatelets contains the Stone-Wales defects. Furthermore, there are a lot of small-size graphene nanoplatelets on the surface of HPG films with high content of edge defects. These two defects can not only enhance the adsorption without compromising on high diffusivity of ions, but also contribute to the infiltration and flow of electrolyte on the surface of electrode. The proposed one-step laser direct writing technique with highly valuable suitable for developing large-scale fabrication of conductive HPG based flexible electrodes at low-cost.

Coherent resonance of quantum plasmons in Stone–Wales defected graphene–silver nanowire hybrid system

Defected graphene has a more important practical significance than graphene. Silver nanoparticles can modify the optical properties of defected graphene. We present herein a detailed theoretical analysis about the coherent resonance of quantum plasmons in the Stone–Wales (SW) defected graphene–silver nanowire hybrid system by using time-dependent density functional theory. The plasmon coherent effect is mainly attributed to the electromagnetic field coupling between the Stone–Wales defected graphene and silver nanowires. As a result, the optical response of the hybrid system exhibits a remarkable enhancement. Plasmon resonance, which depends on polarization and selectable tuning, is enhanced in wide frequency regions. Moreover, it reveals that the resonance frequency of an optical absorption spectrum depends on the space configuration of the SW defected graphene in the hybrid system. This investigation provides a better understanding of the plasmon enhancement effect used in a graphene-based photoelectric device. The study also offers an effective means of detecting the defects existing in graphene.

Electronic structure of bilayer graphene with defects under an external electric field

The electronic structure of bilayer graphene, where one of its layers possesses divacancies (DVs) or Stone-Wales (SW) defects, under an external electric field is studied within the framework of the density functional theory. Our calculations show that the Fermi level of bilayer graphene with DVs strongly depends on gate voltage, while that of bilayer graphene with SW defects is insensitive to gate voltage, indicating that the Fermi level tuning of graphene thin films with defects depends on the defect species and relative arrangement to the gate electrode. The electronic energy band of bilayer graphene under an external electric field is asymmetrically modulated with respect to the gate arrangements.

(Invited) Molecular-Scale Evidence Provides New Insights into Fullerene Formation Mechanism

It has been proposed that the fullerene formation mechanism involves either a top-down or bottom-up pathway. Despite of different starting points, either mechanism approved that particular fullerenes or metallofullerenes are formed through a consecutive stepwise process involving Stone-Wales transformations (SWTs) and C2 insertion or extrusion. However, the formation pathway has been seldom defined at the molecular level due to the missing link fullerenes. Herein, we present a series of previously missing species, including Sc2O@C2v(24107)-C78 and Sc2O@D3h(24109)-C78, Sc2O@C2(7892)-C70 and Sc2C2@D3h(14246)-C74. Each of them can be converted to other isomers or larger or smaller species via a simple conversion route involving Stone-Wales (SW) transformation and C2 insertion or extrusion. Thus, this study might provide molecular structural evidence for the less confirmed fullerene formation mechanism.

Electronic properties of graphene with single vacancy and Stone-Wales defects

The first principles calculations have been performed based on self-consistent charge density functional tight-binding in order to examine the electronic properties of graphene with single vacancy (SV) and Stone-Wales (SW) defects. We have optimized structures of pristine graphene and graphene with SV and SW defects. The bond lengths, current-voltage curve and transmission probability have been calculated. We found that the bond length for relaxed graphene is 1.43Å while for graphene with SV and SW defects the bond lengths are 1.41Å and 1.33Å, respectively. For the SV defect, the arrangement of atoms with three nearest neighbors indicates sp2 bonding. While for SW defect, the arrangement of atoms suggests nearly sp bonding. From the current-voltage curve for graphene with defects we have determined that the behavior of the I–V curves is nonlinear. It is also found that the SV and SW defects cause to decrease the current compared to the pristine graphene case. Furthermore, the single vacancy defect reduces the current more than the Stone-Wales defect. Moreover, we observed that by increasing the voltage from zero to 1V new peaks near Fermi level in the transmission probability curves have been created.

Atomistic simulations on interwall sliding behaviour of double-walled carbon nanotube: effects of structural defects

In this study, we investigated the interwall sliding behaviours of double-wall carbon nanotubes (DWCNTs) using molecular dynamics (MD) simulations, focusing on the effects of different structural defects including the vacancy, adsorbed atom (Adatom) and Stone-Wales (SW) defects. The simulation results showed that structural defects, especially the Adatom ones, caused large fluctuations and decreased the overall pull-out force. Stick-slip motions were observed in the interwall sliding processes of DWCNTs containing multiple structural defects. Among three types of structural defects, the Adatom defects most significantly weaken the interwall load transferring capability and degrade the interface shear strength (IFSS). This work provides useful information for promoting DWCNTs’ applications in Micro/Nano Electro-Mechanical Systems (M/NEMS).

Effects of defects on thermoelectric properties of carbon nanotubes

Carbon nanotubes (CNTs) have recently attracted attention as materials for flexible thermoelectric devices. To provide a theoretical guideline of how defects influence the thermoelectric performance of CNTs, we theoretically studied the effects of defects (vacancies and Stone-Wales defects) on their thermoelectric properties; thermal conductance, electrical conductance, and Seebeck coefficient. The results revealed that the defects most strongly suppress the electron conductance, and deteriorate the thermoelectric performance of a CNT. By plugging in the results and the intertube-junction properties into the network model, we further show that the defects with realistic concentrations can significantly degrade the thermoelectric performance of CNT-based networks. Our findings indicate the importance of the improvement of crystallinity of CNTs for improving CNT-based thermoelectrics.

Cyclosarin nerve agent interaction with the pristine, Stone Wales defected, and Si-doped BN nanosheets: Theoretical study

Never agent identification and disposal is vital for both civilian and military defense resources. Herein, using density functional theory calculations, the reactivity and electronic sensitivity of pristine, Stone Wales (SW) defected, and Si-doped BN (Si-BN) nanosheets toward cyclosarin nerve agent were investigated. It was found that the interaction of cyclosarin with the pristine BN sheet is very weak and also that is not energetically favorable with SW defected one. Unlike the SW defect, replacing a B atom by Si atom significantly makes the cyclosarin adsorption energetically favorable. Calculations show that the carbonyl and etheric oxygen atoms of cyclosarin attack the Si atom of Si-BN with the adsorption energies of −73.5 and −136.9kJ/mol, respectively. The cyclosarin nerve agent can be decomposed by the Si-BN sheet which is thermodynamically highly favorable. Upon this process, the HOMO and LUMO levels are significantly unstabilized and the HOMO-LUMO gap significantly changed by about 24.2%. The cyclosarin presence and its decomposition by Si-BN sheet can be recognized because of the electrical conductivity change of the sheet.

Local equilibrium configurations and minimum energy path of carbon nanotubes with Stone-Wales defects and their related pentagon-heptagon lattice defects

The energy landscape over configurational change of defects in carbon nanotube (CNT) is clarified for a variety of distributions of Stone–Wales defects and pentagon–heptagon (5–7) defects using the Peierls–Nabarro dislocation model. Based on the theoretical model, the configurational energy is estimated by relative displacement without fatal loss of accuracy, but the calculation of energy during the configurational change of defect is simplified with reduced degrees of freedom in the displacement field. The equilibrium configurations are obtained using the conjugate-gradient method, and the saddle point on the minimum-energy path between two equilibrium configurations for evolution of defects, such as the nucleation/annihilation and movement of defects, is estimated using the nudged elastic-band method. Many equilibrium configurations stably exist because of the lattice trapping of the 5–7 defects due to the relatively large Peierls–Nabarro barrier. The total energy does not only depend on the mean value of the relative displacement, but also on the complexity of the configurations of the defects, and we draw a directed graph which shows the possible paths of the configuration change of the defects. The results provide fundamental information related to the stability of the thermal activation of defects in CNT.

Tuning Cd adsorption behaviours on graphene by introducing defects: a first-principles study

Adsorption of Cadmium (Cd) on pristine and defective graphenes, including Stone–Wales (SW) defect and single vacancy (SV) defect, was studied by first-principles calculations. It is found that Cd adatom is weakly adsorbed on the pristine and SW defective graphene, while SV defect shows a substantial increase in the bonding with Cd adatom. Analysis of electronic density-of-state indicates that the effect of SV defect on the adsorption process can be mainly ascribed to the hybridisation between C-2p and Cd-5s orbitals. The results of charge density changes due to the Cd adsorption show the substantial amount of electrons are transferred from Cd to graphene, and the magnetic moment of substrate is increased after the adsorption of Cd. These experimental results suggest that SV graphene could be a good sensor for the detection of heavy metal ions.

An investigation on tensile properties of coiled carbon nanotubes using molecular dynamics simulation

A coiled carbon nanotube (CCNT) can be formed from the distortion of parent toroidal carbon nanotube with a uniform pitch length and a uniform spring rise angle. In this research molecular dynamics simulation was carried out to assess the tensile properties of three CCNT having indexes of (3,3), (4,4), and (5,5). The results indicated that Stone-Wales defects are necessary for thermodynamic stability of the CCNTs. The stress-strain curves showed that the yield stress, yield strain, and failure strain are decreased with increase in temperature. The force-displacement curves revealed that the spring constant of these materials is highly depended on the tube diameter and rising angle, while it is not noticeably depended on temperature. However, the toughness of the (4,4) CCNT at different temperatures and rising angles indicated that the CCNT has the highest toughness at lower temperatures and rising angles. Also, it was shown that a (4,4) CCNT presents its largest toughness at temperature of 100K and the rising angle of 12.5°.

Interfacial strengthening between graphene and polymer through Stone-Thrower-Wales defects: Ab initio and molecular dynamics simulations

In this study, we revealed the interfacial strengthening mechanism between a Stone- Thrower-Wales (STW) defective single layer graphene and polypropylene (PP), through a density functional theory (DFT) simulation and atomistic molecular dynamics simulations. In quantum mechanical simulation, the adhesion energy of propylene monomer on STW defective graphene is calculated with van der Waals interaction. An improved adsorption characteristic of propylene to the STW defective graphene is clearly observed, compared with a pristine counterpart. For deeper understanding of the adsorption, the electronic structure calculation and geometrical analysis of the adsorbed structures are also performed. In molecular dynamics simulation, three transversely isotropic nanocomposite unit cell structures consisting of PP and single layer graphene having a different number of STW defects are constructed. The stress-strain curves of nanocomposites according to the density of STW defects are obtained from uniaxial tension and shear tests. Since the properties of graphene itself are degraded by the STW defects, the overall stress-strain characteristics of nanocomposites involving the deformation of graphene are degraded by the addressed STW defects. However, in longitudinal shearing, where interfacial shearing between graphene and PP is involved, the STW defect can critically improve the shear load bearing capability. The increased interfacial shear load transfer is mostly attributed to the rippling of graphene at the STW defective sites, and the resultant surface roughness of graphene.

Structural characterization of carbon nanotubes via the vibrational density of states

The electrical and chemical properties of carbon nanotubes vary significantly with different chirality and diameter, making the experimental determination of these structural properties important. Here, we show that the vibrational density of states (VDOS) contains information on the structure of carbon nanotubes, particularly at low frequencies. We show that the diameter and chirality of the nanotubes can be determined from the characteristic low frequency L and L′ modes in the VDOS. For zigzag nanotubes, the L peak splits into two peaks giving rise to another low energy L″ peak. The significant changes in the frequencies and relative intensities of these peaks open up a route to distinguish among structurally different nanotubes. A close study of different orientations of Stone–Wales defects with varying defect density reveals that different structural defects also leave distinct fingerprints in the VDOS, particularly in the L and L′ modes. With our results, more structural information can be obtained from experiments which can directly measure the VDOS, such as inelastic electron and inelastic neutron spectroscopy.

Enhancement of fracture toughness of graphene via crack bridging with stone-thrower-wales defects

Ever increasing applications of graphene motivates researchers to synthesize large scale graphene sheets. In general, large scale graphene sheets are synthesized with some inherent topological defects such as vacancies, Stone Thrower Wales (STW) and grain boundaries. In this article, effect of STW defects on the fracture toughness of graphene was studied with the help of molecular dynamics based simulations. In this numerical study, different atomistic configurations of graphene containing a centrally embedded crack with or without STW defects were modelled. It can be predicted from the simulations that STW defects could be used for tailoring the fracture toughness as well as fracture behaviour of graphene. Significant improvement in the fracture toughness of graphene has been observed while applying load in zig-zag direction with STW defects were positioned next to the crack faces. Interaction of stress fields generated from crack tip as well as from STW defects helps in tailoring the fracture behaviour of graphene.

Estimation of graphene surface stability against the adsorption of environmental and technological chemical agents

The ability of graphene to resist the chemical transformations caused by chemisorption of several environmental and technological chemical agents (H2, O2, H2O, H2O2) was studied using DFT ab initio calculations of potential energy surfaces of adsorption on perfect and defect (single vacancy and Stone–Wales defect) graphene with zero and finite curvature. Beside the evident result that structural imperfections such as curvature and atomic defects make graphene chemically vulnerable, it was found that the two kinds of impact of vacancies and Stone–Wales defects on the chemical stability of a sp2‐hybridized carbon lattice are essentially different. In the latter case, chemisorption barriers for any kind of chemical agents decrease with increasing curvature but, in the case of single vacancies the finite curvature lowers the adsorption probability for oxygen‐based molecules. In general, it was shown that single‐layered graphene could be used as an effective protecting coating against environmental and technological chemical agents.

Numerical Investigation of the Fracture Mechanism of Defective Graphene Sheets.

Despite the unique occurrences of structural defects in graphene synthesis, the fracture mechanism of a defective graphene sheet has not been fully understood due to the complexities of the defects. In this study, the fracture mechanism of the monolayer graphene with four common types of defects (single vacancy defect, divacancy defect, Stone–Wales defect and line vacancy defect) were investigated systematically for mechanical loading along armchair and zigzag directions, by using the finite element method. The results demonstrated that all four types of defects could cause significant fracture strength loss in graphene sheet compared with the pristine one. In addition, the results revealed that the stress concentration occurred at the carbon–carbon bonds along the same direction as the displacement loading due to the deficiency or twist of carbon–carbon bonds, resulting in the breaking of the initial crack point in the graphene sheet. The fracture of the graphene sheet was developed following the direction of the breaking of carbon–carbon bonds, which was opposite to that of the displacement loading.

Preferable binding site of gas molecules on graphene nanoribbon with Stone–Wales defect

The issue of sensitivity of sensing device has focused on the development of sensing devices by using new materials, such as graphene. The gas molecules in different positions such as on, near and far from the defect are placed in the same binding site in two graphene configurations for fair comparison. The interaction between two different graphene configurations such as (pristine armchair graphene nanoribbon (P-AGNR) and Stone–Wales defect on graphene surface (SW-AGNR)) with gas molecules (e.g. O2, N2 and NH3) have been investigated to observe the preferential position site of adsorbate gas molecules. The preferable position sites are investigated by using Extended-Huckel Theory. It is found that, the electronic properties of each configuration are strongly depends on the position of gas molecules and graphene system. Meanwhile, the binding site of the gas molecules on the defective site of graphene surface is a significant factor in determining the sensing behavior of graphene based gas defection device.