Imperfect Graphene Research Articles

Nonlinear aero-thermo-elastic flutter analysis of stiffened graphene platelets reinforced metal foams plates with initial geometric imperfection

Due to its lightweight design and high load-bearing capacity, the stiffened plate structure is extensively applied in the passive control of aircraft flutter. However, the evolution mechanism of the nonlinear response of stiffened plates within the unstable region remains unclear. In particular, there is a lack of research on the nonlinear aero-thermo-elastic properties of stiffened plates with initial geometric imperfection. Inspired by this, this article investigates the stability and post-instability behavior of stiffened geometrical imperfect graphene platelets reinforced metal foams (GPLRMF) plates under combined thermal load and aerodynamic pressure. Graphene platelets (GPLs) and foam are distributed evenly or unevenly in the plates, while the material properties are calculated by modified Halpin-Tsai model and rule of mixture. Considering initial geometric imperfection, the models of plates and stiffeners are established by first-order shear deformation (FOSD) plate and Timoshenko beam theories, respectively. Using the linear piston theory to simulate supersonic aerodynamic pressure. The stability boundary of stiffened plates is determined by eigenvalue analysis. The Newton-Raphson approach is used to simulate the aero-thermo-elastic static response of stiffened plates, and the dynamic post-flutter behavior of the stiffened plate is determined through the Runge-Kutta method. Finally, the effects of initial geometric imperfection, material parameters (foam distribution type, porosity coefficient, GPLs weight fraction and GPLs distribution type), and stiffeners (size, position, and stiffening scheme) on the thermal aeroelastic behavior of stiffened GPLRMF plates are revealed. It can be found that the presence of initial geometric imperfection will significantly change the nonlinear flutter behavior of the stiffened plates.

The Role of Structural Defects in the Growth of Two-Dimensional Diamond from Graphene.

The presented work is devoted to the study of the formation of the thinnest diamond film (diamane). We investigate the initial stages of diamond nucleation in imperfect bilayer graphene exposed by the deposition of H atoms (chemically induced phase transition). We show that defects serve as nucleation centers, their hydrogenation is energy favorable and depends on the defect type. Hydrogenation of vacancies facilitates the binding of graphene layers, but the impact wanes already at the second coordination sphere. Defects influence of 5|7 is lower but promotes diamondization. The grain boundary role is similar but can lead to the final formation of a diamond film consisting of chemically connected grains with different surfaces. Interestingly, even hexagonal and cubic two-dimensional diamonds can coexist together in the same film, which suggests the possibility of obtaining a new two-dimensional polycrystal unexplored before.

Natural frequency analysis of imperfect GNPRN conical shell, cylindrical shell, and annular plate structures resting on Winkler-Pasternak Foundations under arbitrary boundary conditions

The goal of the present research is to examine the Natural Frequencies (NFs) of perfect and imperfect Graphene Nanoplatelet Reinforced Nanocomposite (GNPRN) structures of revolution (conical and cylindrical shells and annular plate structures) resting on elastic foundations under general boundary conditions (BCs). The graphene nanoplatelet material is implemented to compose the nanocomposite enhanced by a polymeric matrix including porosities. The springs technique is applied to define the general BCs of GNPRN structures of revolution. Elastic foundations are described by two parameters, i.e., Winkler-Pasternak Foundations (WPFs). Additionally, Donnell's hypothesis and first-order shear deformation theory (FSDT) are employed to figure out the primary formulations associated with the GNPRN structures of revolution. In contrast, the equations of motion are found using Hamilton's principle. Then the equations of motion are then discretized using the well-known Generalized Differential Quadrature Method (GDQM). Next, the standard eigenvalue solution is assigned to obtain the NFs of the perfect/imperfect GNPRN structures of revolution. Finally, various benchmarks are addressed to verify the approach suggested for evaluating the NFs of the perfect/imperfect GNPRN structures of revolution. Furthermore, several novel examples highlight the effects of the change in geometrical and material properties, arbitrary boundary conditions, and WPFs on the NFs of the perfect/imperfect GNPRN structures of revolution.

The Magic Kingdom of imperfect graphene

The Magic Kingdom of imperfect graphene

A dual-role theory of the aspect ratio of the nanofillers for the thermal conductivity of graphene-polymer nanocomposites

The extremely low thickness-to-length ratio (aspect ratio) of graphene nanofillers has long been considered beneficial to the overall thermal conductivity of graphene-polymer nanocomposites as it creates large surface area and extra channels for phonon transport. From time to time, however, a lower aspect ratio of the nanofillers did not yield a higher thermal conductivity as expected. To explain such phenomena, we explored the dual role played by the aspect ratio of graphene nanofillers in the effective thermal conductivity via an effective-medium approximation based on Maxwell's far-field matching with consideration of imperfect graphene interfaces. On one hand, we found that the initiation of the graphene thermal networks relies solely on the aspect ratio. A lower aspect ratio tends to boost the establishment of the thermal channels. On the other, we discovered a persistent exponential law that relates the graphene interfacial thermal coefficient to the aspect ratio regardless of the matrix material. A lower aspect ratio causes a higher interfacial thermal resistance, thus a possible lower effective thermal conductivity. These two effects are competing against each other in the framework of this theory. The origin of the second effect was discussed and the effect cannot be neglected with an aspect ratio below 0.01. This dual-role theory was confirmed by a large number of experimental observations.

Non-uniform self-folding of impure graphene

Planar graphene with extreme flexibility folds into three-dimension structures, holding promises in quantum- and nano-electronics with interesting geometries and properties. However, it is challenging to synthesize high-quality and uniform graphene on a large scale. The inevitable inhomogeneity, such as geometry variation and material impurity, yields imperfect graphene and tunable folded structures. This enables the diverse design of nanostructures with targeted configurations and engineered performances. The mechanics of non-uniform self-folding of impure graphene is explored through a finite-deformation varying-section beam model, the critical folding conditions, folded profiles, and maximum strains dependent on the non-uniform half-length are theoretically determined. Molecular dynamics (MD) simulations are performed to verify theoretical solutions. Our study is helpful for the design of programmable quantum- and nano-electronics to achieve desired properties.

Pattern Directive Sensing Selectivity of Graphene for Wearable Multifunctional Sensors via Femtosecond Laser Fabrication

AbstractGraphene has great potential in wearable sensors. However, the sensing selectivity of graphene‐based sensors is still a big challenge, limiting their application in the multifunctional sensors. Herein, different types and distributions of defects are introduced into graphene by femtosecond laser patterning to realize sensing selectivity for wearable multifunctional sensors. The imperfect graphene with four pattern arrays, circle, square, triangle, and hexagon with none, right, acute, and obtuse angle, is fabricated by laser to control the types and distributions of defects. The graphene with different patterns shows remarkable sensing selectivity, owing to the dangling bonds and vacancies on the edge of patterns. The graphene‐based sensor with the circle pattern array is used to detect the strain variation; the triangle one is for temperature detection; and the hexagon one can collect the information of gas. The gauge factor is demonstrated to be as high as ≈104. The as‐produced sensor with the above‐mentioned four patterns can simultaneously detect body pulse, temperature, and harmful gas by attaching to human body or clothes, offering real‐time health monitoring and protection. The patterned graphene‐based sensors with high sensing selectivity are expected to develop multifunctional sensor platform and versatile artificial electronic skin.

Retracted Article: Pre-thermal buckling and frequency characteristics of imperfect graphene nanoplatelets reinforced composite curved open cylindrical micropanel under nonlinear thermal loading

We, the editors and publisher of the journal Mechanics Based Design of Structures and Machines have retracted the following article: Hu Jun, Liu Jian, Xu Junxin, Yi Hong & Aghil Shavalipour (2022) Pre-thermal buckling and frequency characteristics of imperfect graphene nanoplatelets reinforced composite curved open cylindrical micropanel under nonlinear thermal loading, Mechanics Based Design of Structures and Machines, 50:5, 1723–1746, DOI: 10.1080/15397734.2020.1761384 It has come to our attention that the authorship list for this manuscript was significantly changed after the submission of the article. We have contacted the authors for a full explanation, but we have not received a satisfactory explanation. As determining authorship is core to the integrity of published work, we are therefore retracting the article. The corresponding author listed in this publication has been informed. We have been informed in our decision-making by our editorial policies and the COPE guidelines. The retracted article will remain online to maintain the scholarly record, but it will be digitally watermarked on each page as ‘Retracted’.

Dynamic characteristics of an electrostatically actuated imperfect graphene drum resonator

Recent experiments have shown the applicability of graphene in Nanoelectromechanical Systems (NEMS) devices. Because it has very high Young’s modulus, high strength, excellent conductivity and small thickness. In our paper, we present a continuum model of graphene drum resonator for analysing dynamic characteristics. The investigated graphene drum resonator is electrostatically actuated and it has nearly circular shape. This devices have applications as NEMS sensors and actuators. We have modelled a graphene sheet as circular plate in finite element analysis (FEA) software for investigation. In this FE model, geometric and electrostatic force nonlinearities inherently present. The static and dynamic characteristics of graphene drum resonator have been studied in detail. The relationship between the natural frequency and electrostatic actuation force has been explored for both circular and imperfect circular plate. We have observed that the higher natural frequencies of circular graphene sheet splits into two distinct natural frequencies with introduction of imperfection in circular geometry. We have introduced imperfection in circular geometry by modelling the graphene sheet as an elliptical sheet. We have systematically investigated the split natural frequency behaviour of the electrostatically actuated imperfect circular graphene sheet.

In-situ electrochemically deposited Fe3O4 nanoparticles onto graphene nanosheets as amperometric amplifier for electrochemical biosensing applications

A novel in-situ approach based on electrochemical techniques has been developed for incorporating iron oxide nanoparticles onto CVD-grown few-layered graphene nanosheets. The embedment of Fe3O4 nanoparticles within graphitic planes was realized by sweeping conductive Gr electrode in a diluted solution of ultrafast redox probe K3[Fe(CN)]6. The morphology and chemical composition of the modified graphene sheets were probed by Scanning Electron Microscopy and Energy Dispersive X-ray analysis, respectively. X-ray photoelectron spectroscopy technique was also employed to provide deeper insights on material structure. Electrochemical behavior of the material was then examined by Cyclic Voltammetry and Electrochemical Impedance Spectroscopy. The results clearly revealed that graphene sheets were evenly decorated by Fe3O4 particles with relatively uniform size of less than 50 nm. The presence of such ‘ceramic’ particles has provided active sites for promoting electron transfer from redox-active species in the solution. Meanwhile, the electrochemical reduction of imperfect graphene sheets has greatly restricted the oxygen moieties, thus improving the conductivity of the material. As a consequence, an increase in amperometric current response of at least 20-fold was recorded after the graphene sheets were electrochemically tailored with iron-based nanoparticles. The graphene-Fe3O4 nanocomposites were then applied for sensing H2O2 (non-enzymatic), glucose and acetylthiocholine (enzymatic). The developed sensors had excellent performances with detection limits of 4.4, 8.2 and 8.35 μM for H2O2, glucose and acetylthiocholine, respectively.

Quenching of the Resonant States of Single Carbon Vacancies in Graphene/Pt(111)

For more than a decade, investigations of single carbon vacancies in graphene have sought to increase the fundamental understanding of the local electronic, magnetic, and mechanical properties of such vacancies. The single C vacancy in graphene has been known to generate a resonant state through the integration of π orbitals near the missing C atom. Here, we examine single C vacancies in graphene/Pt(111) to explore the effects of graphene–substrate interactions on the local electronic properties of imperfect graphene. Our scanning tunneling microscopy, scanning tunneling spectroscopy, and related density functional theory calculations show the resulting modifications, including the complete disappearance of the resonant state attributable to strong graphene–substrate coupling near the vacancy. The different relative positions of single C vacancies corresponding to the Pt atoms lead both to varying C–Pt bonding structures and strengths and to corresponding changes in the local density of states.

Axisymmetric deformation of geometrically imperfect circular graphene sheets

In this study, we investigate the axisymmetric deformation of a geometrically imperfect circular graphene sheet subjected to a uniform radial load using nonlocal elasticity theory. Due to the imperfection of the graphene sheet, an inhomogeneous version of Bessel’s equation is derived as a nonlocal governing equation of the system. Closed-form expressions are obtained to predict the deformations of the graphene sheet as functions of the radius, small-scale coefficient, initial imperfection, and bending rigidity of the graphene sheet. Furthermore, relations are proposed to determine critical radial loads. The present model indicates that it is necessary to include the effect of an initial imperfection as well as the small-scale effect.

Simulations of Graphene Sheets Based on the Finite Element Method and Density Functional Theory: Comparison of the Geometry Modeling under the Influence of Defects

In the present research, imperfect graphene sheets were generated and their vibrational property was studied via finite element analysis. The effect of vacant sites in the arrangement of these nano-structures was examined. The fundamental frequency of the defect free and imperfect nano-sheets was acquired based on two different approaches. The first approach was a pure finite element simulation. The second approach for comparison purpose was a recently reported refined finite element simulation at which the vicinity of a defect was first evaluated according to the density functional theory (DFT) and then the refined geometry was implemented into a finite element model. The findings of this research show that the fundamental frequency of graphene sheets decreases by presenting microscopic imperfection to the formation of these nano-materials. In addition, it was pointed out that the geometry based on the more precise DFT simulations gives a higher decrease in the fundamental frequency of the sheets for all considered cases.

First-principles investigation on thermal properties and infrared spectra of imperfect graphene

In this study we used first-principles density functional theory to investigate the thermal and optical properties of graphene. Graphene phonon properties were first calculated by the density-functional perturbation theory and then used to acquire thermal properties such as the specific heat, free and total energy, and entropy, as well as the infrared and Raman spectra. Results show that the peaks of phonon density of states at about 40 and 45.5THz in the perfect graphene (G) were shifted to 40.5 and 46THz in the imperfect graphene (G-D), respectively. There are peaks at 16.5, 19, 25, and 43.5THz in the G-D curve, while there is no obvious peak at same frequencies in that of the G. The specific heat and entropy are lower for the G-D than for the G at temperature >280K, but the tendency is slightly reversed at temperature <200K. The total energy, change in vibrational internal energy, and change in the vibrational Helmholtz free energy are all lower for the G-D than for the G. The Helmholtz free energy for the G-D is higher at temperature >1300K, but lower at temperature below 1250K than for the G. In the infrared (IR) spectrum, no absorption peak exists for perfect graphene, but strong absorption is found at about 233, 830 and 1392cm−1 for the G-D. The character peak of sp2 carbon atom in-plane vibration in the Raman spectrum is shifted from 1589cm−1 for the G to 1530cm−1 for the G-D. The defects in the G-D caused the peaks and splits in the IR and Raman spectra.

Modeling of Forced Desorption Processes in a Regenerable Graphene Sorbent for Elemental Mercury Capture

Most sorbents display poor capacity for elemental mercury at elevated temperatures. Graphene is the potential candidate among different high-temperature sorbents. We have studied the physical properties of mercury films on partially hydrogenated imperfect graphene, as well as their heating and bombardment with xenon clusters, by means of molecular dynamics. Hydrogenated edges of a graphene sheet containing Stone–Wales defects withstand heating to 1100 K. Formation of the droplet leads to a decrease in the blunt contact angle. The bombardment of a target with a Xe13 cluster beam at energies of 5–30 eV and incidence angles of 0–60° aiming to remove a mercury film from imperfect graphene has been performed. The graphene is completely cleaned of mercury at a cluster energy of EXe ≥ 15 eV. Mercury is removed from the graphene film via sputtering of single atoms and droplet detachment. A stress in graphene resulting from forces normal to the sheet plane is noticeably higher than that due to forces acting in its...

Computer simulation of the xenon-cluster bombardment of mercury on graphene

The bombardment of a target with Xe13 clusters with kinetic energies ranging from 5 to 30 eV and an incident angle of θ = 0° is studied using the molecular dynamics method. The target consists of mercury on partially hydrogenated imperfect graphene. The complete cleaning of a graphene sheet from mercury is not reached after 125 cluster impacts. The radial distribution functions and vertical density profiles indicate the formation of Hg monomer vapor around the bombarded target. The Hg film has a tendency to curl into a droplet. For all cluster energies, the Hg atom mobility and stresses in the Hg film in the horizontal directions are higher than in the vertical direction. The roughness of the graphene sheet increases continuously under bombardment.

Mercury removal from graphene by bombardment with xenon clusters: Computer simulation

Molecular dynamics simulation of the bombardment of a target with a Xe13cluster beam at energies of 5–30 eV and incidence angles of 0°–60° aiming to remove a mercury film from partially hydrogenated imperfect graphene has been performed. The graphene is completely cleaned of mercury at a cluster energy of EXe ≥ 15 eV. It has been revealed that the mercury film tends to form a droplet. Mercury is removed from the graphene film via sputtering of single atoms and droplet detachment. The energy of interaction of mercury with graphene is very low and weakly depends on the incident beam energy. The horizontal mobility of atoms in the liquid metal film has a significantly higher value than their vertical mobility. A stress in graphene resulting from forces normal to the sheet plane is noticeably higher than that due to forces acting in its plane. Bombardment at an angle of incidence of 45° is more efficient than that at incidence angles of 0° and 60° and leads to lower graphene roughness.

Computer modeling of the structure and properties of mercury films on graphene

The physical properties of mercury films on partially hydrogenated imperfect graphene are studied by means of molecular dynamics at 300 K. Films prepared on the basis of three various types of the atomic interaction potential for mercury and other constant interaction potentials are considered. It is shown that the one most promising is the Schwerdtfeger potential function, at which mercury atoms do not fall into the divacancies present on graphene and atom packing with the lowest energy are realized in a liquid film and the film gradually folds into a drop. The horizontal and vertical mobilities of mercury atoms, the radial distribution function of Hg film, and the roughness of graphene are calculated.

Effects of graphene membrane parameters on diaphragm-type optical fibre pressure sensing characteristics

Considering the potential application of graphene in optical fibre Fabry–Perot pressure-sensitive membrane, a constitutive model for strain-induced deformation characteristics of graphene membrane was established based on the large deflection elastic theory of circular membrane. The effects of membrane parameters, including pre-stressing force, Young's modulus and membrane layer number on deflection mechanical behaviours were studied by using finite element method. The simulation results demonstrated that the pre-stressing force had a significant impact on the deflection deformation and natural frequency with small loads. And the values of Young's modulus should also be considered with large applied loads, because of the imperfect high-quality and uniform graphene preparation methods. Then we carried out a computational study of the effects of membrane thickness on its reflectivity with Fresnel's equations. It was shown that a single-layered graphene membrane with 25 μm diameter can achieve an approximate sensitivity of 80 nm/kPa for a 40 μm Fabry–Perot cavity.

Computer simulation of cluster bombardment of a lead film on graphene

The problem of removal of a lead film from partially hydrogenated imperfect graphene by bombarding the target with a beam of Xe13 clusters having energies of 5–30 eV and an incident angle of 0° has been solved using the molecular dynamics method. Graphene is completely cleaned of lead at cluster energies of 10 and 15 eV. At higher beam energies, the lead can percolate through divacancies to the back side of graphene or, by virtue of plastic deformation, adhere to the front side. The separation of lead from the graphene film follows the cluster mechanism. The metal film torn off the graphene and acquired the cluster form has the lowest value of the horizontal mobility of the atoms among all of the systems under consideration and quite low values of vertical mobility. The separation of the lead film from the graphene can also result in a significant decrease in the value of certain components of the metal-film stress tensor and an increase in graphene roughness.