Defective Graphene Sheets Research Articles

Atomic insights into the fluorinated pristine and defected graphene sheets for inducing the electronic and magnetic properties with the aid of scandium metal decoration- Acumen from density functional theory

To scrutinize the electronic and magnetic properties of fluorinated pristine, stone wales, di-vacancy, and two stone wales defected graphene sheets by scandium (Sc) metal decoration, the density functional theory is used. The obtained maximum binding and interaction energy values confirm that the two Sc metals decorated on the direct top and bottom of pentagonal rings of the two stone wales graphene sheet have stronger interactions than others. To validate the interactions, natural population analysis and electron density difference map are performed, revealing that Sc metal is the electron donor in all complexes. After the formation of defects in the pristine sheet, energy gap values are reduced in the defected sheets. The density of states infers the shift in the fermi level of all complexes after defect formation and metal decoration. The ultraviolet–visible spectral analysis exposes the highest absorption peaks of all bare sheets at the UV region and after Sc metal decoration, the peaks get red shifted. The results show that Sc metal decorated on two stone wales graphene sheet interacts better with Sc metals than other complexes. The decoration of single Sc metal in all four bare sheets transforms the sheet from a non-magnetic to a magnetic state but when two Sc metals are decorated it became non-magnetic. The highest degree of spin polarization, P = 4.15 is obtained while decorating a single Sc metal on the hexagonal ring of the pristine sheet. The outcome of this work suggests that Sc metal decoration on pristine and defected graphene sheets has great potential to improve the electronic and magnetic properties of graphene for designing spintronic and data storage devices.

Influence of flexoelectric effect on the bending rigidity of a Timoshenko graphene-reinforced nanorod

Abstract The focus of this work is to study the influence of flexoelectric phenomenon on the electromechanical response of graphene-reinforced nanocomposite (GNC) nanorods. An analytical model has been derived by utilizing the Timoshenko beam theory and the principle of variational work by incorporating flexoelectric effects. The GNC nanorod is subjected to a concentrated load acting downward for clamped-free and simply supported support types. The GNC is reinforced with a defective graphene sheet as it is known to show enhanced polarization. The elastic properties of defective graphene sheets have been evaluated using molecular dynamic simulations. The outcome of our model shows that the flexoelectric effect must be considered for accurate modeling of nanostructures. Irrespective of the support type, flexoelectric effect improves the stiffness of the nanorod. We also observed that the stiffness of the nanorod is significantly influenced by the support type. This work presents an opportunity for the development of high-performance graphene-based nanoactuators/sensors.

Crumpling Defective Graphene Sheets.

Upon crumpling, graphene sheets yield intriguing hierarchical structures with high resistance to compression and aggregation, garnering a great deal of attention in recent years for their remarkable potential in a variety of applications. Here, we aim to understand the effect of Stone-Wales (SW) defects, i.e., a typical topological defect of graphene, on the crumpling behavior of graphene sheets at a fundamental level. By employing atomistically informed coarse-grained molecular dynamics (CG-MD) simulations, we find that SW defects strongly influence the sheet conformation as manifested by the change in size scaling laws and weaken the self-adhesion of the sheet during the crumpling process. Remarkably, the analyses of the internal structures (i.e., local curvatures, stresses, and cross-section patterns) of crumpled graphene emphasize the enhanced mechanical heterogeneity and "glass-like" amorphous state elicited by SW defects. Our findings pave the way for understanding and exploring the tailored design of crumpled structure via defect engineering.

Modification of micro-crystalline graphite and carbon black by acetone, toluene, and phenol.

The chemical interactions of two types of graphite and two types of carbon black (CB) with acetone, toluene, and phenol were studied in order to evaluate the influence of chemical treatment on the structure and morphology of the carbon phases. The experimental treatment of carbon phases was carried out at room temperature for 1 hour. The chemical and phase composition were studied by x-ray photoelectron (XP) and Raman spectroscopies, while the morphology and structure were determined by powder x-ray diffraction, as well as transmission electron microscopy techniques. To shed light on the most probable explanation of the observed results, we performed simulations and calculations of the binding energies of acetone, toluene, and phenol with model carbon phases: a perfect graphene sheet and a defective graphene sheet containing various structural defects (vacancies as well as zigzag and armchair edges). Simulations show that all non-covalent and most covalent coupling reactions are exothermic, with acetone coupling having the higher calorimetric effect. Based on the results of the simulations and the XP spectroscopy measurements, the probable reactions taking place during the respective treatments are outlined. The conducted studies (both theoretical and experimental) show that the treatment of graphite powders and CB with acetone, toluene, or phenol can be used as a preliminary stage of their modification and/or functionalization, including their conversion into graphene-like (defective graphene, reduced graphene oxide, and/or graphene oxide) phases. For example, the treatment of SPHERON 5000 with acetone significantly facilitates their subsequent modification with laser radiation to graphene-like phases.

Defect induced tunable nitrogen configuration in holey graphene for enhanced supercapacitive performance

Nitrogen (N) doped graphene has attracted various attention in many fields due to its adjustable physicochemical properties. However, directional control of graphitic N content in graphene sheets under mild conditions without decreasing the total N content remains a challenge. Here, by combining the defect engineering and lattice restoring agent, tuning the graphitic N contents in a wide range can be achieved at low temperature. X-ray photoelectron spectroscopy (XPS) was used for identifying the N configuration and N content. The content of total N changes from 5.62% to 6.61%, and the content of graphitic N changes from 10.73% to 82.34% with the increase of defects in graphene sheets. Results from electrochemical impedance spectroscopy (EIS) suggest that the ion diffusion capacity increases with the increasing defect level. Results from scanning electron microscopy (SEM) suggest that higher defect level leads to smaller wrinkle structure. Nitrogen adsorption-desorption and electrochemical tests suggest that the specific pore volume, specific surface area and specific capacitances of the optimum sample are 4.35, 2.06 and 1.89 times higher than those samples without defect-introducing- treatment. The supercapacitor assembled with thus modified graphene has a high energy density of 13.8 Wh·Kg−1 and a high power density of 13 000 W·Kg−1. Moreover, the assembled supercapacitor holds a high capacitance retention of 86.8% after 10,000 charge-discharge cycles at 20 A·g−1. This simple and effective method of tuning surface chemistry of graphene provides a reference for the functionalization of graphene.

Investigation of unified impact of Ti adatom and N doping on hydrogen gas adsorption capabilities of defected graphene sheets

In this work, we studied the hydrogen adsorption capabilities of functionalized graphene sheets containing a variety of defects (D-G) via molecular dynamics (MD) simulations that govern the mechanisms involved in hydrogen adsorption. Specifically, the graphene sheets containing monovacancy (MV), Stone-Wales (SW), and multiple double vacancy (DV) defects were functionalized with Ti and N atoms to enhance their hydrogen adsorption capacity. We measured the adsorption capacities of the N-/D-G sheets with varying concentrations of Ti adatoms at 300 K and 77 K temperatures and various pressures. Our study revealed that the increasing concentration of Ti adatoms on the D-G sheets led to a significant improvement in the hydrogen adsorption capacity of the graphene sheets. The DV(III)-G sheets showed the maximum adsorption capacity at 300 K because the DV(III)-G sheets had a small number of large-sized pores that bind hydrogen with high binding energy. Thus, hydrogen remained adsorbed even at higher temperatures (300 K). The N doping on the D-G sheets initially reduced their hydrogen adsorption capabilities; however, the N-D-G sheets enhanced their hydrogen adsorption capacity with the increasing concentrations of Ti adatoms. Compared to all other defect types, the Ti–N-DV(III)-G sheet with a Ti concentration of 10.5% showed a hydrogen uptake of 5.5 wt% at 300 K and 100 bar pressure. Thus, the N doping and Ti implantations improved the hydrogen storage capabilities of the graphene sheets, and these findings helped design solid-state hydrogen storage systems operating at ambient conditions and moderate pressure ranges.

Periodic Structural Defects in Graphene Sheets Engineered via Electron Irradiation.

Artificially-induced defects in the lattice of graphene are a powerful tool for engineering the properties of the crystal, especially if organized in highly-ordered structures such as periodic arrays. A method to deterministically induce defects in graphene is to irradiate the crystal with low-energy (<20 keV) electrons delivered by a scanning electron microscope. However, the nanometric precision granted by the focused beam can be hindered by the pattern irradiation itself due to the small lateral separation among the elements, which can prevent the generation of sharp features. An accurate analysis of the achievable resolution is thus essential for practical applications. To this end, we investigated patterns generated by low-energy electron irradiation combining atomic force microscopy and micro-Raman spectroscopy measurements. We proved that it is possible to create well-defined periodic patterns with precision of a few tens of nanometers. We found that the defected lines are influenced by electrons back-scattered by the substrate, which limit the achievable resolution. We provided a model that takes into account such substrate effects. The findings of our study allow the design and easily accessible fabrication of graphene devices featuring complex defect engineering, with a remarkable impact on technologies exploiting the increased surface reactivity.

Molecular Dynamic Simulation of Defective Graphene Nanoribbons for Tension and Vibration.

As deformation and defects are inevitable during the manufacture and service of graphene resonators, comprehensive molecular dynamic (MD) simulations are performed to investigate the vibrational properties of the defective single-layer graphene sheets (SLGSs) during tension. Perfect SLGSs, SLGSs with single vacancy, SLGSs with low-concentration vacancies, and SLGSs with high-concentration vacancies are considered, respectively. The frequencies of the perfect and defective SLGSs at different stretching stages are investigated in detail. The effects of different external forces are also taken into account to study the vibration properties of the defective SLGSs. Results show that the perfect and defective SLGSs both successively perform four stages, i.e., the elastic stage, the yield stage, the hardening stage, and the fracture stage during stretching, and the elastic properties of the SLGSs are insensitive to the vacancy defects, while the ultimate strain is noticeably reduced by the vacancies. The single vacancy has no effect on the vibration properties of SLGS, while the frequency decreases with the increasing vacancy concentration for SLGS at the elastic stage. The frequency of yielded SLGS with a certain vacancy concentration is almost constant even with a varying external force.

Rational design for the microplasma synthesis from vitamin B9 to N-doped carbon quantum dots towards selected applications

N-doped carbon quantum dots (NCQD) are rationally designed and synthesised, for the first time, from folic acid (Vitamin B9) by a non-thermal microplasma jet. A new conceptual design was developed to synthesise the desirable NCQD for three main applications (nanopesticides, water purification, and theranostic treatment). The structural and analytical characterisation confirmed an average size of 3.1 nm for the synthesised NCQD with the multi-functional groups (-OH, –COOH, –NH2) on their surface. The TEM results indicated that the core of NCQD was a multilayered structure, including single defected graphene sheets of graphitic-nitrogen and pyrrolic-nitrogen. In addition, fluorescence performance and stability of the as-prepared NCQD were determined. The quantum yield of NCQD was 35%, which is relatively high, with a strong blue fluorescence. A basis for predicting colloidal behaviours based on balancing molecular attractive and repulsive forces was elucidated by applying the Derjaguin, Landau, Vervey, and Overbeek (DLVO) theory. Finally, compared with other similar microplasma-assisted synthesis processes, this developed method has proven the ability to provide a tailored and scalable synthesis process of high-quantum-yield NCQD at gram-scale production.

Sulfur and nitrogen codoped cyanoethyl cellulose‐derived carbon with superior gravimetric and volumetric capacity for potassium ion storage

AbstractWe fabricated sulfur and nitrogen codoped cyanoethyl cellulose‐derived carbons (SNCCs) with state‐of‐the‐art electrochemical performance for potassium ion battery (PIB) and potassium ion capacitor (PIC) anodes. At 0.2, 0.5, 1, 2, 5, and 10 A g−1, the SNCC shows reversible capacities of 369, 328, 249, 208, 150, and 121 mA h g−1, respectively. Due to a high packing density of 1.01 g cm−3, the volumetric capacities are also uniquely favorable, being 373, 331, 251, 210, 151, and 122 mA h cm−3 at these currents, respectively. SNCC also shows promising initial Coulombic efficiency of 69.0% and extended cycling stability with 99.8% capacity retention after 1000 cycles. As proof of principle, an SNCC‐based PIC is fabricated and tested, achieving 94.3 Wh kg−1 at 237.5 W kg−1 and sustaining over 6000 cycles at 30 A g−1 with 84.5% retention. The internal structure of S and N codoped SNCC is based on highly dilated and defective graphene sheets arranged into nanometer‐scale walls. Using a baseline S‐free carbon for comparison (termed NCC), the role of S doping and the resultant dilated structure was elucidated. According to galvanostatic intermittent titration technique and electrochemical impedance spectroscopy analyses, as well as COMSOL simulations, this structure promotes rapid solid‐state diffusion of potassium ions and a solid electrolyte interphase that is stable during cycling. X‐ray diffraction was used to probe the ion storage mechanisms in SNCC, establishing the role of reversible potassium intercalation and the presence of KC36, KC24, and KC8 phases at low voltages.

Changes in electronic structure of graphene by adsorption of low melamine coverages

Using first-principles simulations at Density Functional Theory (DFT) level, we concluded a comprehensive study of the molecular doping of graphene by melamine adsorption. We characterized the different possible positions achieved by the molecule on a pristine graphene or a Stone-Wales defect (5-7-7-5) were characterized. We also analyzed adsorption energies, deformations experienced by melamine and charge separation involved in the process. The defective graphene sheet turned out to be more flexible than the pristine one, exhibiting greater deformations. Melamine adsorption on Stone-Wales defect is preferred and it can be used as a useful strategy to carry out the molecular doping of graphene. Due to adsorption, a charge separation is generated between melamine and graphene, giving rise to an organic/organic interface with a net interfacial dipole. This change in the electronic structure of graphene favors the dynamics of the excited electronic state, tuning it an interesting material used in optoelectronic devices.

Effect of defects and boundary conditions on the vibrational behavior of carbon nanotube and graphene: A molecular dynamics perspective

The presence of defects has a significant effect on the mechanical and vibrational properties of carbon nanotubes (CNTs) and graphene. The vibrational properties of single-layered graphene sheets (SLGS) and single-wall carbon nanotubes (SWCNT) have been investigated in this research. Molecular dynamics simulations have been conducted on pristine and defective zigzag and armchair CNTs, with clamped-free (C-F) boundary conditions as well as pristine and defective armchair graphene sheets with different boundary conditions to investigate the effect of various parameters such as chirality, aspect ratio, boundary conditions, and types and number of defects on the longitudinal and transverse vibrational behavior of CNTs and graphene sheets. It was observed that for the cantilever boundary condition, shorter tubes give better vibrational sensitivity as compared to the larger ones for a constant diameter of the tube. The chirality has not a considerable effect on the natural frequency of CNTs. Regarding graphene sheets, it was observed that increasing the size of the graphene sheet decreased the fundamental natural frequency of SLGS. As compared to Stone-Wales defects, the vacancy defects were found to have more impact on the degradation of the natural frequency of CNTs and GS.

Nitrogen-doped carbon anchored ruthenium nanoparticles for biofuel upgrade

The development of efficient catalysts is of great significance for the hydrodeoxygenation (HDO) of renewable biomass into value-added biofuel and chemicals to mitigate the environmental and energy menace. Herein, we report a sustainable strategy to fabricate ruthenium nanoparticles (NPs) anchored on defective nitrogen-doped carbon (Ru@NC) via a facile pyrolysis of a mixture of ruthenium trichloride and urea with carbon support. The highly distributed Ru NPs, constituted by N-enriched graphene shells, have been established as an excellent catalyst for the selective HDO of lignin- and furan- derivatives toward biofuel upgrade. Impressively, Ru@NC exhibits enhanced catalytic performance to commercial Ru/C, which could be attributed to the presence of the ensemble effects resulting from defective graphene sheet coverage on the Ru surface. Notably, the prepared catalyst was not affected after five successive reaction cycles, demonstrating superior stability and reproducibility. We envision that this work may pave the way for the development of highly efficient novel metal catalysts for heterogeneous catalytic HDO reactions in sustainable energy conversion.

Effect of reconstructed vacancy defects on the crumpling behavior of graphene sheets

Understanding the influence of reconstructed single vacancy (SV) defects on the crumpling behavior of graphene sheets is crucial for successful implementation of graphene in advanced technologies. Here, we report the results of a systematic coarse-grained molecular dynamics (CG-MD) simulation study of the crumpling process of graphene sheets at varying degree of reconstructed SV defects. We find that two-dimensional (2D) defective graphene sheets with larger degree of SV defects reconstruction exhibit more wrinkling in equilibrium than pristine graphene and less reconstructed defective sheets. By evaluating the evolution of potential energy of the sheet during the crumpling process, our results show that the reconstructed SV defects reduce the adhesion properties while greatly increase the angle potential energy. Moreover, the shape descriptor analysis shows less self-folding and self-adhering upon crumpling for sheets with larger degree of defects reconstruction. The analyses of the sheet pressure and bulk modulus indicate that the larger the degree of defects reconstruction, the more difficult it is to compact the sheet into a crumple, corresponding to a greater bulk modulus. The evaluation of stress distribution of the sheet further reveals the stress heterogeneity arising from defects and crumpling. Our findings highlight the critical role of reconstructed SV defects in the crumpling process of graphene sheets, which has significant implications for the tailored design of crumpled defective materials.

Work Function Modulation of Few-Layer Graphene by Swift Heavy Ion Irradiation.

Here, we report the structural and electronic modification induced in chemical vapor deposited graphene by using swift heavy ions (70 MeV Ni6+).Raman spectroscopy was used to quantify the irradiation-induced modification in vibrational properties. The increase in defect density with fluence causes an increase in the intensity ratio of its characteristic Raman D and G band. The increase in defect density also results in a decrease in crystallite size. The changes in the crystal structure are observed from X-rays diffraction measurement. Swift heavy ion irradiation induced defect, modified the surface roughness and surface potential of graphene thin film as measured from atomic force microscopy and scanning Kelvin probe microscopy respectively. The increase in the work function, surface roughness as well as defect concentration with fluence, indicate the possibility of linear correlation between them. Presence of defects in graphene sheets strongly affects surface electronic and optical properties of the material that can be used to tailor the optoelectronics device performance.

Elastic modulus of defected graphene sheets

In this paper, the elastic modulus of single-layered graphene sheets (SLGSs) with and without defects is investigated using the finite element method. The SLGSs with two chiralities (armchair and zigzag) are modeled by beam elements. At first, the SLGSs without defects are investigated then the carbon atoms and corresponding beam elements are removed and the elastic modulus of SLGSs is determined. The increasing number of defects apparently decreased the elastic modulus of graphene sheets.

Mechanical properties of graphene nanoplatelets-reinforced concrete prepared with different dispersion techniques

In this paper, an experimental investigation is carried out to study the effects of graphene nanoplatelets (GNPs) on the mechanical properties of cementitious composites with coarse aggregates. The concrete mixtures with GNPs concentrations ranging from 0.025% to 0.10% by weight of the cement are prepared, where a wet dispersion technique that employs high shear mixing and polycarboxylate-based superplasticizer to disperse GNPs in water. The effects of various dispersion parameters including different high shear mixing durations as well as the use of ultrasonication together with high shear mixing on the dispersion of GNPs are studied. The dispersion quality of GNPs is assessed through optical microscopy, Raman spectroscopy, and Scanning Electron Microscopy tests. Compressive strength and flexural strength tests are conducted to assess the effects of GNPs on mechanical properties of the fabricated GNP-reinforced concrete specimens. Results show that all the dispersion procedures considered in this study can disperse GNPs in water without causing any basal or vacancy defects in graphene sheets and reduce the size of graphene flakes. The use of ultrasonication together with high shear mixing leads to the smallest size graphene sheets. When the GNPs are added to the concrete mixture at a dosage of 0.025%, a maximum increase of 17% in compressive strength is observed, while no significant effect of GNPs on the flexural strength is noticed.

Computational assessment of Stone-Wales defects on the elastic modulus and vibration response of graphene sheets

In this work we assess the influence of Stone-Wales defects on the elastic modulus and vibration responses of single-layered graphene sheets. To this end, the atomic position of the C-atoms within the defective layers are inserted as initial conditions into finite element calculations used to estimate the mechanical properties and natural frequencies of carbon monolayers. Moreover, we consider two sets of initial atomic positions: the harmonic solution predicted by discrete dislocation theory and its fully-relaxed configuration, which is a novelty in this type of study. Thus, the mechanical properties of the beam elements are depicted based on the equivalence between the interatomic potential energy of the atomic model and the strain energy of the equivalent continuum model. Furthermore, based on the results, we propose an extension of the classical beam model that accurately defines the mechanical properties of the elements in terms of the actual length of each atomic bond. The finite element model is implemented in the commercial software Abaqus/Standard. The results obtained are in reasonable agreement with previous data presented by other authors. In this work we have specifically ascertained the importance of using a non-linear model to predict the Young’s modulus and vibration responses of defective graphene sheets.

Effect of Defects on the Interfacial Thermal Conductance between n-Heneicosane in Solid and Liquid Phases and a Graphene Monolayer

The molecular level mechanism of heat transport across the interface between solid and liquid n-heneicosane and monolayer graphene with three types of defects (single-vacancy, multivacancy (MV), and Stone–Wales (SW) (SW1 and SW2 cases are considered based on the orientation of the defects) has been studied using nonequilibrium molecular dynamics simulations. The influence of the alignment of an ideal crystal structure (heneicosane molecules positioned perpendicular and parallel to the graphene basal plane) and two heating modes (in the “heat-matrix” mode, heat enters the defective graphene sheet from one side of its basal plane and leaves from the other side, and in the “heat-graphene” mode, the heat is entering from the heated graphene layer to the cooled heneicosane from both sides of the basal plane) on the thermal conductance has been examined. With an increase in the defect percentage (up to 9.0%), the thermal conductance is found to be increasing for all types of defects under both heating modes. It is observed that both MV and SW2 defects in graphene result in the largest enhancement in the conductance under the heat-matrix mode, whereas the SW1 defect yields maximum improvement under the heat-graphene mode. Spectral analysis indicates that the vibrational modes of all frequencies are important for the interfacial heat transfer.

Effects of Orientation and Temperature on the Tensile Strength of Pristine and Defective Bi-Layer Graphene Sheet – A Molecular Dynamics Study

Molecular dynamics simulations with adaptive intermolecular reactive empirical bond order (AIREBO) potential were carried out to study the effect of temperature and orientation on the tensile strength of pristine and defective bilayer graphene (BLG) sheet. Results obtained reveal that the fall in tensile strength of pristine AA stacked BLG due to the presence of vacancy is significant at room temperature (300 K) but decreases at higher temperature (1073 K). Interestingly, this phenomenon reverses in case of AB stacked BLG, wherein the percentage fall in strength at higher temperature due to defect is more than that at room temperature. In order to understand these discrepancies in the results obtained, three case studies were conducted, and the results obtained suggested that when defects are present in armchair direction, this phenomenon occurs. The study also reveals that in case of AB stacked BLG, zigzag direction is more defect tolerant at room and high temperatures. Interestingly, variation of tensile strength due to the orientation is in good agreement with projections from potential energy concepts and theoretical calculations. We envisage that the study will provide useful information to the device engineers for the optimisation of the mechanical properties and convenient structural adaptation of bilayer graphene while working at wide range of temperatures.