Double Vacancy Defects Research Articles

The effect of vacancy defects on the conductive properties of SiGe

Electronic band structures of defective silicon–germanium (SiGe) were described using ab initio calculations, and the influences of vacancy defects on the conductive properties of SiGe were studied. The analysis of formation energies of the defects indicates that a Si atom is easier to lose than a Ge one to form a single vacancy (SV) defect, and double vacancy (DV) defects might be more frequently found than two adjacent SV defects in SiGe. A vacancy could be formed more easily in Ge-rich than Si-rich SiGe. The band structures of SiGe compounds are affected conspicuously by defects, making most of the SixGe8−x compounds with SVs show a trend to become to p-type, and almost all of the SixGe8−x compounds with DVs turn to n-type. The p-type property might come from the vacancies with +2 charged state, and n-type property from the vacancies with −2 charged state.

Effects of sulfur line vacancy defects on the electronic and optical properties of armchair MoS2 nanoribbon

Abstract Based on the density functional theory (DFT) calculations, we study the electronic and optical properties of armchair MoS2 nanoribbons (AMoS2NR) with single and double sulfur line vacancy defects in two different directions, i.e. parallel to the length (longitudinal) and width (transversal) of the structure. The formation energy calculations show that the structures with longitudinal vacancies are easier to create than the transversal ones. Investigating the band structures, it is observed that all the line defects proposed in this study decrease the band gap of the AMoS2NR. In fact, the band gap of 0.68 eV in perfect nanoribbon reduces to the lowest value of 0.11 eV in transversal double vacancy structure (T-DV). Analysis of the total and projected density of states (TDOS and PDOS) of the perfect and proposed defective structures indicates that in the defective structures the contribution of central atoms becomes more dominant especially in the case of the T-DV structure which has the most number of non-terminated bonds. In order to investigate the optical properties of the perfect and defective AMoS2NR, we present the real and imaginary parts of dielectric function, absorption coefficient and reflectance spectra for two different polarizations of light, along and perpendicular to the nanoribbon. The results show that all the structures in this study are anisotropic with respect to the type of polarization in the energies below 12 eV and are isotropic above it. It is also depicted that the light polarized along the nanoribbon has higher absorption in the energies below 3 eV. In addition, we show that the proposed line defects cause redshifts in the first peaks of the optical spectra which provides a tunable optical characteristic that can extend the application of AMoS2NR in a wide frequency range especially IR and visible.

Study of graphene device electronic properties with horizontal and vertical double vacancy defects

Graphene device electronic properties with double vacancy (DV) defects for two cases, along the direction and perpendicular to the current pathways graphene device, were investigated by using the first principles calculations in combination with density functional theory. The bond lengths, density of states, transmission probability, and current-voltage curves are computed. For relaxed pristine graphene the bond length is around 1.43 Å. However, the bond lengths near the defects for relaxed graphene for DV case are modified to 1.40-1.49 Å. It is also observed that I − V graph is nonlinear based on the current-voltage curve of graphene device which contain DV defects. Furthermore, it has been shown that having the DV defects lead to reduce the current relative to the case of perfect graphene device. Moreover, we noted that when the voltage is increased from zero to one volt new peaks are created near Fermi level in the transmission spectrum graphs. In addition, we noted that the current for the vertical DV defect is smaller than the pristine and horizontal DV device because the number of blocked electrons current pathways in vertical DV defect is larger than the two other cases, namely the pristine and horizontal DV defect cases The obtained results can be useful for the construction of new nanoelectronic devices and may have practical applications.

Research Progress on Thermal Conductivity of Graphdiyne Nanoribbons and its Defects: A Review.

Graphdiyne has a unique pi-conjugated structure, perfect pore distribution and adjustable electronic properties of sp2, sp hybrid planar framework. Due to the presence of acetylenic bonds, it has more excellent properties compared to grapheme, such as a unique structure-dependent Dirac cone, abundant carbon bonds and a large bandgap. As one of the important raw materials for nanodevices, it is extremely important to study the thermal properties of graphdiyne nanoribbon. This paper mainly introduces and discusses recent academic research and patents on the preparation methods and thermal conductivity of graphdiyne nanoribbons. Besides, the applications in engineering and vacancy defects in the preparation process of graphdiyne are described. Firstly, taking thermal conductivity as an index, the thermal conductivity of graphdiyne with various vacancy defects is discussed from the aspects of length, defect location and defect type. In addition, the graphdiyne nanoribbons were laterally compared with the thermal conductivity of the graphene nanoribbons. The thermal conductivity of graphdiyne with defects increases with the length and width, which is lower than the intrinsic graphdiyne. The thermal conductivity of the acetylene chain lacking one carbon atom is higher than the one lacking the benzene ring. Typically, the thermal conductivity is larger in armchair than that of zigzag in the same size. Moreover the thermal conductivity of nanoribbons with double vacancy defects is lower than those nanoribbons with single vacancy defects, which can also decrease with the increase of temperature and the number of acetylene chains. The thermal conductivity is not sensitive to shear strain. Due to the unique structure and electronic characteristics, graphdiyne has provoked an extensive research interest in the field of nanoscience. Graphdiyne is considered as one of the most promising materials of next-generation electronic devices.

Potential Difference Driving Electron Transfer via Defective Carbon Nanotubes toward Selective Oxidation of Organic Micropollutants.

Nanocarbon-based persulfate oxidation emerges as a promising technology for the elimination of organic micropollutants (OMPs). However, the nature of the active site and its working mechanism remain elusive, impeding developments of high-performance oxidative technology for water treatment practice. Here, we report that defect-rich carbon nanotubes (CNTs) exhibit a superior activity in the activation of peroxymonosulfate (PMS) for OMP oxidation. Quantitative structure-activity relationship studies combined with theoretical calculations unveil that the double-vacancy defect on CNTs may be the intrinsic active site, which works as a conductive bridge to facilitate the potential difference-dominated electron transfer from the highest occupied molecular orbital of OMPs to the lowest unoccupied molecular orbital of PMS. Based on this unique mechanism, the established CNTs@PMS oxidative system achieves outstanding selectivity and realizes the target-oriented elimination of specific OMPs in a complicated aquatic environment. This work sheds new light on the mechanism of carbocatalysis for selective oxidation and develops an innovative technology toward remediation of practical wastewater.

Effect of vacancy defects on electronic properties and wettability of coal surface

The water wettability of coal is a major factor affecting its flotation performance, and thus it is imperative to explore the wetting mechanism. Previous studies on wettability mainly focused on the contact angle and interfacial tension, and there are only a few reports on the electronic properties that affect the adsorption behavior of H2O. In this study, the electronic properties of perfect and defective surfaces of coal, the adsorption behavior of H2O on the surface, and wettability were tested using molecular simulations. X–ray diffraction (XRD) analysis of anthracite was performed, and two–layered graphite was used as an anthracite model. The results show that the aromatic structure of anthracite is very similar to the crystal structure of graphite; the difference lies only in the crystallite size: anthracite crystallites are smaller than graphite crystallites. Therefore, the modified graphite structure can be used to simulate the local structure of coal. The bandgap of defective surfaces is lower, their electronic density of states near Fermi energy is greater, and their electronic activities are better than those of perfect surfaces. Defects decrease the energy gap of frontier orbitals (ΔELUMO–HOMO) between H2O and the surface, and strengthen the interaction between H2O and the surface·H2O adsorbs physically on the coal surface with two hydrogen atoms facing the surface. Defects increase the wettability of the coal surface, and the order of wettability improvement by defects is: double vacancy defect > single vacancy defect > Stone–Wales defect.

Influence of vacancy defects on vibration analysis of graphene sheets applying isogeometric method: Molecular and continuum approaches

The main objective of this research paper is to consider vibration analysis of vacancy defected graphene sheet as a nonisotropic structure via molecular dynamic and continuum approaches. The influence of structural defects on the vibration of graphene sheets is considered by applying the mechanical properties of defected graphene sheets. Molecular dynamic simulations have been performed to estimate the mechanical properties of graphene as a nonisotropic structure with single- and double- vacancy defects using open source well-known software i.e., large-scale atomic/molecular massively parallel simulator (LAMMPS). The interactions between the carbon atoms are modelled using Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. An isogeometric analysis (IGA) based upon non-uniform rational B-spline (NURBS) is employed for approximation of single-layered graphene sheets deflection field and the governing equations are derived using nonlocal elasticity theory. The dependence of small-scale effects, chirality and different defect types on vibrational characteristic of graphene sheets is investigated in this comprehensive research work. In addition, numerical results are validated and compared with those achieved using other analysis, where an excellent agreement is found. The interesting results indicate that increasing the number of missing atoms can lead to decrease the natural frequencies of graphene sheets. It is seen that the degree of the detrimental effects differ with defect type. The Young\'s and shear modulus of the graphene with SV defects are much smaller than graphene with DV defects. It is also observed that Single Vacancy (SV) clusters cause more reduction in the natural frequencies of SLGS than Double Vacancy (DV) clusters. The effectiveness and the accuracy of the present IGA approach have been demonstrated and it is shown that the IGA is efficient, robust and accurate in terms of nanoplate problems.

First-principles study of defects in blue phosphorene

Using first-principles density functional theory calculations, we investigate the energetics and electronic properties of Stone Wales and vacancy defects in blue phosphorene. Among these defects, the Stone Wales defect has the lowest formation energy of 1.49 eV. Single and double vacancy defects have much larger formation energies (2.35 ∼ 2.90 eV). All the defects induce mid-gap bands that are relatively flat, which indicates the localized nature of the defect states. Our spin-polarized calculations further show that a single vacancy defect exhibits local magnetic moments (1.0 μB), which can be attributed to the existence of an unpaired electron. Besides, we propose new types of defects with irregular buckling configurations. These new defects have even lower formation energies (1.01 and 1.30 eV). The present work may serve as an important guidance for designing and defect engineering of blue phosphorene-based devices.

Tunable electronic and magnetic properties of graphene/carbon-nitride van der Waals heterostructures

In this paper, we explore the electronic properties of C3N, C3N4 and C4N3 and graphene (Gr) van der Waals heterostructures by conducting extensive first-principles calculations. The acquired results show that these heterostructures can show diverse electronic properties, such as the metal (Gr on C3N), semiconductor with narrow band gap (Gr on C3N4) and ferromagnetic-metal (Gr on C4N3). We furthermore explored the effect of vacancies, atom substitution, topological, antisite and Stone-Wales defects on the structural and electronic properties of considered heterostructures. Our results show that the vacancy defects introduce localized states near the Fermi level and create a local magnetic moment. The Gr/C3N heterostructures with the single and double vacancy defects exhibit a ferromagnetic-metal, while Stone-Wales defects show an indirect semiconductor with the band gap of 0.2 eV. The effects of adsorption and insertion of O, C, Be, Cr, Fe and Co atoms on the electronic properties of Gr/C3N have been also elaborately studied. Our results highlight that the electronic and magnetic properties of garphene/carbon-nitride lateral heterostructures can be effectively modified by point defects and impurities.

Double-Vacancy Controlled Friction on Graphene: The Enhancement of Atomic Pinning.

The vacancy-enhanced contact friction of graphene is mainly attributed to the vacancy-enhanced out-of-plane deformation flexibility of the graphene and the climbing of the tip out of the vacancy trap (which actually acts as a step edge). However, this mechanism does not apply for explaining the enhanced friction caused by small-sized vacancies that are unable to accommodate the tip, such as single vacancy and double vacancies, which also commonly exist in the graphene. In the present study, by performing a set of classic molecular dynamics simulations, we demonstrated that the double-vacancy defect in graphene substantially enhanced the contact friction when the tip slides over it and the pinning effect of the reconstructed lattice of the double-vacancy defect with atoms at the bottom of the tip dominated such an influence. The underlying mechanism of such an atomic pinning effect and the influence of the normal load, sliding direction, and the sliding velocity were unveiled by analyzing the obtained friction evolution and the atomic configuration and interaction between the tip and the graphene. We believe that the findings presented in this study complete the state-of-art understanding of the nanoscale friction behaviors of vacancy-defected graphene, which is essential for the implementation of their potential control.

A computational study on nuclear magnetic resonance parameters of defects of graphene

Abstract The defects in graphene have significant influence on its properties. However, there are few literatures about how these defects influence nuclear magnetic resonance parameters of 13C nuclei. In this work, we provide first-principles quantum calculations of NMR parameters of three types of defects: Stone-Wales, double vacancy, and single vacancy in modeled graphene nanoflakes. The calculated 13C chemical shifts and nuclear spin-spin coupling constants are significantly influenced by different defects. For single vacancy model, the averages and standard deviations of chemical shifts of carbon nuclei of defected circle always are much higher than those in the Stone-Wales and double vacancy defects. For one-bond spin-spin coupling constants of 13C–13C in the defected circles of different models, the averages and the standard deviations also have similar trends, but the differences among these models are not large. Our results indicate that every kind of defect shows a different pattern of the NMR parameters from the others, which may be used to identify the type of defect in graphene.

Defected Graphene as Ammonia Sensor: Theoretical Insight

Defects are often symbolized as deformity in material that deteriorate its performance. However, in nanoscale regime, defects lead to generate a useful and novel material for device applications. In the present report, the vacancy defects, i.e., single vacancy and double vacancy defects (with different symmetry) on graphene sheet have been analyzed to understand the electronic as well as transport properties using density functional theory and NEGF approach. Conductance, current–voltage, and sensitivity analysis of these defected graphene sheets have been examined for its suitability for gas sensing application particularly for ammonia gas. The study observes that a single vacancy defected graphene is a good candidate for ammonia molecule sensing in comparison to double vacancy.

The characterization of electronic defect states of single and double carbon vacancies in graphene sheets using molecular density functional theory

ABSTRACTA detailed picture of the electronic states manifolds of single- and double-vacancy defects in molecular models of graphene based on polycyclic aromatic hydrocarbons (PAHs) is presented. DFT calculations using various density functionals including long-range corrected ones have been performed for pyrene, circumpyrene and 7a,7z-periacene. It has been found for pyrene defect models that DFT results reproduced well the set of closely-spaced singlet and triplet states predicted by the CCSD(T) and previous MRCI + Q calculations, indicating the applicability of DFT for accessing the excited states manifolds also for larger graphene models. For the single-carbon vacancy defect, all structures have a triplet ground state. As expected, in the largest system, 7a,7z-periacene-1C, the lowest lying states are much closer in energy. For all double-vacancy defect structures, a significant rearrangement of the electronic states with increasing size of the sheet is observed. The closed-shell 1Ag state in the smallest systems is destabilised in the extended 7a,7z-periacene system, which has the 3B2u state as the ground state. As observed for the single-vacancy defect, the lowest lying states are closer in energy for the larger systems, since there are more π orbitals close in energy available. For all states, the formation of the bridging bonds for the double vacancy leads to distances shorter than for the single vacancy defect indicating a larger rigidity of the former structure which does not allow stronger distortions.

The Inhibition Mechanism of Lithium Dendrite on Nitrogen-Doped Defective Graphite: The First Principles Studies

The initial Li-ions nucleation on the basal plane of graphite anode with N-doped defects during the charge progress of lithium-ion battery was investigated theoretically by first-principle calculations using double-layered graphene (DLG) model. Based on the geometric structure optimization, the Li-ions are found to be accumulated on the top surface of N-doped DLG with single vacancy defect (SV-3N) in the near 2D structure. Also for the N-doped DLG with double vacancy defect (DV-4N), the Li-ions are deposit on its top surface, as well as could enter the interlayer space. The electronic analysis proves the ion character of Li layer. Therefore, the initial Li-ions nucleation on the basal plane of graphite with both N-doped SV and DV defects can be suppressed with different detailed mechanisms. The calculation of diffusion energy and lithiation potential shows that the Li-ions nucleation is inhibited via an adsorption-like and intercalation-like process on the DLG with SV-3N and DV-4N, respectively. Additionally, the calculated adsorption energy suggests that more Li atoms are adsorbed due to the presence of N, and thus the capacity of N doped graphite anode can be enhanced accordingly.

Simulation of static and dynamic mechanical characteristics of carbon nanotubes and carbon nano-peapods with defects

The static and dynamic mechanical characteristics of carbon nanotubes with double and multiple vacancy defects are simulated by the molecular dynamics method. Firstly, the effects of diatomic and polyatomic vacancy defects on the quasi-static mechanical properties of carbon nanotubes are discussed. Then, the effects of defects and axial pre-stress on the dynamics of C60 molecular oscillation in carbon nano-peapods are discussed. The results show that the ultimate stress, ultimate strain and elastic modulus of carbon nanotube containing different types of diatomic vacancies are significantly reduced as compared with those of non-defective carbon nanotubes. When the carbon nanotubes have many defective atoms and the defects are connected together to form a crack, the axial compressive properties of the carbon nanotubes are greatly reduced. Compared with the circumferential development of cracks, the cracks along the axis greatly reduce the compressive capacity of carbon nanotubes, which is similar to that of shell models with cracks. The oscillation frequency of C60 molecular in defective carbon nano-peapods is affected by the number of missing atoms. The single vacancy defect increases the oscillation frequency of C60 molecule, while with the further increase of vacancy number, the oscillation frequency of C60 molecule decreases gradually. When the defective carbon nano-peapod has axial tensile or compressive pre-stress, the oscillation of the C60 molecule is affected not only by the defects, but also by the axial pre-stress, which makes the oscillation of C60 molecule more complicated.

First‐Principles Studies of Li Nucleation on Double‐Layered Defective Graphene

AbstractLithium‐dendrite growth is one of the most challenging problems affecting the safety performance of Li‐ion batteries. Understanding the evolution process of Li‐dendrite growth at the atomic scale is critical for solving this problem. In this paper, the adsorption processes, geometrical configurations, and electronic structures of Li clusters on double‐layered graphene with two types of defects were investigated by first‐principles calculations. It was found that single vacancy (SV) defects tend to promote the nucleation of Li dendrites, and the adsorption energy of Li clusters near the SV defect decreases with increasing number of Li atoms. Meanwhile, the Li atoms accumulated on the surface of SV‐defect graphene with distances between the Li atoms similar to those in bulk metallic Li. However, in the case of double vacancy (DV) defects, the Li atoms could diffuse freely in the direction perpendicular to the graphene sheets through DV defects at the top‐layer graphene, thus hindering nucleation and dendrite growth. Density of states analyses suggested that the Li atoms on SV‐defect graphene transform from the fully ionized state to the metallic state with continuously increasing number, while the ionic properties of the Li atoms remain and even increase on DV‐defect graphene. Our conclusions can help to understand the Li‐graphite interaction from an atomistic point of view and provide theoretical hints for the development of graphite anodes with high charge‐rate properties.

Comparison of fracture behavior of defective armchair and zigzag graphene nanoribbons

Molecular dynamics simulations of armchair graphene nanoribbons and zigzag graphene nanoribbons with different sizes were performed at room temperature. Double vacancy defects were introduced in each graphene nanoribbon at its center or at its edge. The effect of defect on the mechanical behavior was studied by comparing the stress–strain response and the fracture toughness of each pair of pristine and defective graphene nanoribbon. Results show that the effect of vacancies in zigzag graphene nanoribbon is more profound than in armchair graphene nanoribbon. Also, the effect of double vacancy defect on the ultimate failure stress is greater in zigzag graphene nanoribbons than in armchair graphene nanoribbon due to bond orientation with respect to loading direction. Strength reduction can be as high as 17.5% in armchair graphene nanoribbon with no significant difference between single and double vacancies, while for zigzag graphene nanoribbon, the strength reduction is up to 30% for single vacancy and 43% for double vacancy defects. It is observed that for zigzag graphene nanoribbon with double vacancy at the edge, the direction of the failure plane is oriented at ±30° with respect to the loading direction while it is always perpendicular to the direction of loading in armchair graphene nanoribbon. Results have been verified through studying the fracture toughness parameters in each case as well.

Characterization of point defects in monolayer arsenene

Topological defects that are inevitably found in 2D materials can dramatically affect their properties. Using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) method, the structural, thermodynamic, electronic and magnetic properties of six types of typical point defects in arsenene, i.e. the Stone-Wales defect, single and double vacancies and adatoms, were systemically studied. It was found that these defects were all more easily generated in arsenene with lower formation energies than those with graphene and silicene. Stone-Wales defects can be transformed from pristine arsenene by overcoming a barrier of 2.19 eV and single vacancy defects tend to coalesce into double vacancy defects by diffusion. However, a type of adatom defect does not exhibit kinetic stability at room temperature. In addition, SV defects and another type of adatom defect can remarkably affect the electronic and magnetic properties of arsenene, e.g. they can introduce localized states near the Fermi level, as well as a strongly local magnetic moment due to dangling bond and unpaired electron. Furthermore, the simulated scanning tunneling microscopy (STM) and Raman spectroscopy were computed and the types of point defects can be fully characterized by correlating the STM images and Raman spectra to the defective atomistic structures. The results provide significant insights to the effect of defects in arsenene for potential applications, as well as identifications of two helpful tools (STM and Raman spectroscopy) to distinguish the type of defects in arsenene for future experiments.

Interface adhesion property between graphene film and surface of nanometric microstructure

The performance of graphene can be influenced by its surface mophology, while the surface morphology of graphene is closely related to the substrate. The adsorption and peeling process of graphene on a corrugated surface can provide a theoretical basis for the functional preparation and transfer of graphene. In this work, the adhesion properties and peeling process of graphene on nanostructured substrate are investigated by using molecular dynamics (MD) simulation. As an effective tool of atomic collision theory, MD simulation can provide detailed information about the adsorption configuration and peeling properties of graphene on the nanostructure surface, making up for the deficiency of experiment. The results indicate that graphene can conformably coat on the surface, partially adhere to or remain flat on the top of the stepped substrate. We find that the continuous transition occurs in the adsorption configuration of graphene on the stepped substrate, but the repeated process appears in the transition from partial adherence to conformable coating. When graphene coats on the nanostructured substrate conformably, the adsorption energy can reach its peak value. The adsorption configuration of graphene can change from suspension to partial adhesion after the adsorption energy has exceeded 360 eV -2. It is also shown that the average peeling force fluctuates periodically when the absorption configuration of graphene is conformably coated or suspended on the stepped substrate. Two kinds of behaviors can be noticed in the peeling process. The graphene can directly slide over the bottom while it is fully coated on the surface. The graphene is separated directly from the corrugated surface while it suspends or partially adheres to the surface. If the absorption configuration of graphene is in the suspension state, the average peeling force appears to change drastically within a section of peeling distance and then decreases immediately below zero. Although the flexural stiffness of graphene can be overcome, the interfacial friction between graphene and the substrate is also an essential factor affecting the final adsorption configuration. In this paper, we propose a theoretical formula for the average peeling force according to the changes of size parameters on the nanostructured substrate. The theoretical formula is validated by the simulation results. In addition, with the increase of peeling angle, the average peeling force first increases and then becomes smaller. As a result, a larger average peeling force can be found when the graphene with Stone-Wales defect structure is peeled from the flat substrate. With the increase of double vacancy defect, the maximum peeling force decreases in a certain range, whereas it increases beyond this range. This work can provide a theoretical reference for exploring the peeling property and the adhesion mechanism of graphene on nanostructure surface.

Vibration analysis of defective graphene sheets using nonlocal elasticity theory

Many papers have studied the free vibration of graphene sheets. However, all this papers assumed their atomic structure free of any defects. Nonetheless, they actually contain some defects including single vacancy, double vacancy and Stone-Wales defects. This paper, therefore, investigates the free vibration of defective graphene sheets, rather than pristine graphene sheets, via nonlocal elasticity theory. Governing equations are derived using nonlocal elasticity and the first-order shear deformation theory (FSDT). The influence of structural defects on the vibration of graphene sheets is considered by applying the mechanical properties of defective graphene sheets. Afterwards, these equations solved using generalized differential quadrature method (GDQ). The small-scale effect is applied in the governing equations of motion by nonlocal parameter. The effects of different defect types are inspected for graphene sheets with clamped or simply-supported boundary conditions on all sides. It is shown that the natural frequencies of graphene sheets decrease by introducing defects to the atomic structure. Furthermore, it is found that the number of missing atoms, shapes and distributions of structural defects play a significant role in the vibrational behavior of graphene. The effect of vacancy defect reconstruction is also discussed in this paper.