Stress Of Graphene Research Articles

Quantitative estimation of atom-scaled ripple structure using transmission electron microscopy images

Atom-scaled ripple structure can be intrinsically formed because of thermal instability or induced stress in graphene or two-dimensional (2D) materials. However, it is difficult to estimate the period, amplitude, and shape of such a ripple structure. In this study, by applying the geometrical phase analysis method to atomically resolved transmission electron microscopy images, we demonstrate that the atom-scaled ripple structure of MoS2 nanosheet can be quantitatively analyzed at the subnanometer scale. Furthermore, by analyzing the observed ripple structure of the MoS2 nanosheet, we established that it is inclined by approximately 7.1° from the plane perpendicular to the incident electron beam; it had 5.5 and 0.3 nm in period and amplitude, respectively. For quantitative estimation of ripple structure, our results provide an effective method that contributes to a better understanding of 2D materials in the sub-nanometre scale.

Effect of patterned silicon nitride substrate on Raman scattering and stress of graphene

Graphene is widely used for nano-devices due to its distinctive band structure and fascinating properties. The substrates could significantly affect the properties of graphene and related devices. In this work, we investigate the effect of surface morphology and roughness of patterned silicon nitride substrates on Raman scattering and stress of graphene. We find that the Raman scattering of graphene depend strongly on surface morphology and roughness of patterned substrates. It is concluded that the peak red-shifts in 2D and G bands was due to the strain induced by patterns with different surface morphology (holes and trenches) and roughness. Furthermore, the effects of morphology of patterned area are much greater than the role of surface roughness in the induced strain of graphene. Due to the larger surface area (about 1.65 times), the strain in the grooves is greater than in holes, in spite of greater surface roughness in holes. Our results also reveal that the effect of pattern depth should be taken into account to understand the Raman peak shift and the strain change of graphene. Our work is fundamentally important for understanding the graphene properties on dependence of surface morphology of substrates and enhancing the interfacial strength of graphene-based devices.

Mechanical properties of defective cyanoethynyl (2D polyaniline – C3N): A comparative molecular dynamics study versus graphene and hexagonal boron nitride

Abstract Remarkable properties that free-standing 2D nanomaterials offer have allured academic and industrial interest in them beyond graphene. One of these materials is cyanoethynyl, which is a 2D monocrystalline carbon nitride material with a C3N stoichiometry. Investigations have confirmed its high thermal stability at elevated temperatures and ferromagnetism at low temperatures, yet very little is known about its mechanical properties. This paper reports a molecular dynamics (MD) insight into this gap. The simulations are validated against density functional theory calculations. The tensile properties of defect-free C3N sheets at/under different temperatures and strain rates are investigated. The influences of single vacancy (SV), Stone-Waals (SW), and clustered vacancy (CV) defects on these properties are also studied. The findings are compared against those reported for graphene and hexagonal boron nitride (hBN). The results show that the strength of C3N decreases with increasing temperature and/or decreasing the strain rate. They also indicate that CV defects deteriorate the mechanical properties of C3N to a greater degree than SV and SW defects. At the same defect concentration, C3N shows a lower sensitivity in its stress and strain at failure to the defects compared with graphene and hBN. For example, 3% SV can reduce the failure stress of graphene and hBN by 75% and 73%, respectively, while this reduction is 54% for C3N. For the failure strain, these reductions are 72%, 63%, and 48% for graphene, hBN, and C3N, respectively.

Graphene Stress Transducer-Based Thermo-Mechanically Fractured Micro Valve

This paper presents the design, analysis, and testing of a single-shot thermo-mechanically triggered micromachined micro-valve with graphene on a silicon nitride (Si x N y ) membrane. The graphene serves as a 2-D resistive heater for producing thermo-mechanical stress, to fracture the bilayer formed between nitride and the nickel used to make electrodes. It is demonstrated that the ability of graphene to carry much higher current density per unit than metal heaters enables high thermo-mechanical stress to be generated. The valve is demonstrated as an enabling component of polymer substrate-based vaporizable electronics. It is used for sealing rubidium (Rb) in polymer chambers, and then triggering and exposing it to ambient air to produce heat by exothermic oxidation. This heat energy is used to vaporize the polymers. The nominal valve tested in this work is a $750\,\,\mu \text{m}\,\,\times 750\,\,\mu \text{m}$ membrane of Si x N y . An analytical and finite-element model are presented for the valve, which agree with measured membrane peak-to-peak deflection of $4.1~\mu \text{m}$ just before fracturing, and a peak fracture stress of 395 MPa. The pulsed-power for triggering the valve is nominally 142.1 ± 13.5 mW with a trigger time of 15.4 ± 3.9 ms, resulting in a very low input trigger energy of 2.2 mJ. This trigger energy is $100\times $ lower compared with that of previously reported single-use valves in the literature. [2017–0317]

Raman scattering studies of graphene under high pressure

The effect of pressure – by definition a 3‐dimensional concept – on 2‐dimensional systems brings a number of fundamental questions, which have been partly answered through newly engineered samples and experimental setups. In this review of the high‐pressure Raman studies of graphene, we will in particular underline the importance of the presence of a supporting substrate and its role for the production of biaxial strain conditions in high pressure experiments. Raman shifts observed during these experiments may be related to the strain induced by the substrate rather than by the applied pressure, graphene stress, and hydrostatic pressure not being straightforwardly equivalent in this case of a composite 2D‐on‐3D material. We will also consider the effects of the nature of the substrate surface, the nature of the pressure transmitting media, as well as the effect of the number of layers on the high‐pressure response of the 2D–system, by using the phonon behavior as strain probe as pressure is applied. The effects of adhesion and of strain transfer from the substrate to the 2D–system will be at the heart of our discussion. Copyright © 2017 John Wiley & Sons, Ltd.

Elasticity of damaged graphene: A damage mechanics approach

In this work, the authors discuss a systematic method of characterizing the damage process in graphene. The mathematical formulation is limited to the elastic range. In the elastic constitutive equation for graphene, both the second-order and third-order elastic stiffnesses are considered. The formulation is performed within the framework of Continuum Damage Mechanics. Thus, both the hypotheses of elastic strain equivalence and elastic energy equivalence are utilized. In addition to the classical damage variable that is defined in terms of the cross-sectional area, a new damage variable is introduced and defined in terms of the surface area. This particular damage variable is suited for general nanomaterials, especially graphene. Both the scalar and tensorial formulations are presented. The special case of plane stress in graphene is illustrated as an example using the derived general equations and some interesting results are obtained.

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...

Nonlinear anisotropic deformation behavior of a graphene monolayer under uniaxial tension.

Based on first principle calculations, a graphene monolayer shows a complicated deformation behavior under uniaxial tension. The maximum stress of graphene is reached when the bond stretching ratio is far less than its breaking value, which means that graphene shows the typical "ductile-like" behavior but not the conventionally considered "brittle-like" behavior. Although the graphene monolayer shows isotropic behavior in strength, it is strongly anisotropic in deformation (i.e., the ultimate strain is highly different along the different directions). Under uniaxial tension along the zigzag/armchair direction, the overall deformation is only supported by the C-C bonds in one orientation, whereas the C-C bonds in the other orientation and the C-C-C bond angle have almost no contribution, which cannot be correctly predicted by the empirical potential simulations. The complicated bond deformation means that the conventional constitutive model (σ = Eε + Dε(2)) cannot accurately describe the tensile behavior of the graphene monolayer. According to the bond deformation under uniaxial tension, graphene can be simplified as a spring-network including both nonlinear springs (resisting both the tensile and compressive load) and a very strong compressive angle-spring (resisting the decrease of the C-C-C bond angle).

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.

Correlation between electron-irradiation defects and applied stress in graphene: A molecular dynamics study

Molecular dynamics (MD) simulations are performed to study the correlation between electron irradiation defects and applied stress in graphene. The electron irradiation effect is introduced by the binary collision model in the MD simulation. By applying a tensile stress to graphene, the number of adatom-vacancy (AV) and Stone–Wales (SW) defects increase under electron irradiation, while the number of single-vacancy defects is not noticeably affected by the applied stress. Both the activation and formation energies of an AV defect and the activation energy of an SW defect decrease when a tensile stress is applied to graphene. Applying tensile stress also relaxes the compression stress associated with SW defect formation. These effects induced by the applied stress cause the increase in AV and SW defect formation under electron irradiation.

Corrugated epitaxial graphene/SiC interfaces: photon excitation and probing.

Localized energy exchange and mechanical coupling across a few nm gap at a corrugated graphene-substrate interface remain great challenges to study. In this work, an infrared laser is used to excite an unconstrained epitaxial graphene/SiC interface to induce a local thermal non-equilibrium. The interface behavior is uncovered using a second laser beam for Raman excitation. Using Raman peaks for dual thermal probing, the temperature difference across a gap of just a few nm is determined precisely. The interfacial thermal conductance is found to be extremely low: 410 ± 7 W m(-2) K(-1), indicating poor phonon transport across the interface. By decoupling of the graphene's mechanical and thermal behavior from the Raman wavenumber, the stress in graphene is found to be extremely low, uncovering its flexible mechanical behavior. Based on interface-enhanced Raman, it is found that the increment of interface separation between graphene and SiC can be as large as 2.9 nm when the local thermal equilibrium is destroyed.

The effect of a thin gold layer on graphene: a Raman spectroscopy study

We examined graphene on sapphire with various thicknesses of a gold layer. Raman maps were used to study distribution of the doping levels and stress in graphene on sapphire substrate.

Effects of Heat Treatment on Raman Spectra of Two‐Layer 12C/13C Graphene

The Raman spectra of two-layered graphene on a silicon substrate were studied in the temperature range from 298 to 1073 K in an inert atmosphere. Isotopic engineering was used to fabricate two-layer graphene specimens containing (13)C atoms in the top layer and (12)C atoms in the bottom layer, which allowed the behavior of each particular layer to be distinguished as a function of temperature. It is demonstrated that the top layer exhibits much lower Raman temperature coefficients than the bottom one for both the G and the G' modes. We suggest that the changes in the Raman spectra of graphene observed during thermal cycling are predominantly caused by a superposition of two effects, namely, the mechanical stress in graphene exerted by the substrate and the intrinsic changes in the graphene lattice caused by the temperature itself. The top graphene layer is proposed to be more relaxed than the bottom graphene layer and thus reflects almost exclusively the temperature variations as a freestanding graphene layer would.

Density-functional study of edge stress in graphene

The definition of edge stress is introduced in order to quantify the energy cost to deform a pre-existing edge in a two-dimensional crystal. Using density-functional ab initio calculations, edge stresses, as well as edge energies, are determined for the armchair and zigzag edges in graphene. It is found that both edges are under compression along the edge and the magnitude of compressive edge stress of armchair edge is larger than that of zigzag edge. In addition, hydrogen termination results in stress-free state for both edges. This excess edge quantity is expected to contribute to understanding various edge-related phenomena in graphene such as edge reconstructions and edge functionalizations.

Tunable Stress and Controlled Thickness Modification in Graphene by Annealing

Graphene has many unique properties which make it an attractive material for fundamental study as well as for potential applications. In this paper, we report the first experimental study of process-induced defects and stress in graphene using Raman spectroscopy and imaging. While defects lead to the observation of defect-related Raman bands, stress causes shift in phonon frequency. A compressive stress (as high as 2.1 GPa) was induced in graphene by depositing a 5 nm SiO(2) followed by annealing, whereas a tensile stress ( approximately 0.7 GPa) was obtained by depositing a thin silicon capping layer. In the former case, both the magnitude of the compressive stress and number of graphene layers can be controlled or modified by the annealing temperature. As both the stress and thickness affect the physical properties of graphene, this study may open up the possibility of utilizing thickness and stress engineering to improve the performance of graphene-based devices. Local heating techniques may be used to either induce the stress or reduce the thickness selectively.