Number Of Graphene Layers Research Articles

Determining the number of graphene layers based on Raman response of the SiC substrate

In this report we demonstrate a method for direct determination of the number of layers of hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semiinsulating vanadium-compensated on-axis 6H-SiC(0001). The method anticipates that the intensity of the substrate’s Raman-active longitudinal optical A1 mode at 964 cm−1 is attenuated by 2.3% each time the light passes through a single graphene layer. Normalized to its value in a graphene-free region, the A1 mode relative intensity provides a greatly enhanced topographic image of graphene and points out to the number of its layers within the terraces and step edges, making the technique a reliable diagnostic tool for applied research.

Manipulating the self-assembly behavior of graphene nanosheets via adenine-functionalized biodegradable polymers

Electrically conductive supramolecular polymer composites, based on a combination of natural graphite and the adenine-functionalized biodegradable oligomer 3A-PCL, are able to directly exfoliate crystalline graphite into highly disordered graphene nanosheets with well-tailored structural and physical properties. Herein, we discover 3A-PCL can self-assemble into well-ordered lamellar nanostructures on the surface of graphite due to the strong affinity between 3A-PCL and graphite, and thus lead to direct exfoliation of graphite into well-suspended graphene nanosheets in an organic solvent. Furthermore, after drying, the resulting graphite/3A-PCL composites exhibit stable thermoreversible phase transition behavior between viscous and elastic states, and the number of graphene layers exfoliated can be controlled by tuning the content of 3A-PCL within the composites. Importantly, these newly developed graphene composites possess a low electrical resistivity of 1.5 ± 0.7 mΩ·cm at a graphite loading of 23 wt%, which is more than an order of magnitude lower than that of pristine graphite. Given the simplicity of the production process, well-tailored physical properties and excellent conductive performance, this development offers a highly efficient process for the fabrication of multifunctional supramolecular graphene nanosheets that holds great potential for conductive device applications.

Hydrodynamic cavitation for scalable exfoliation of few-layered graphene nanosheets

A scalable manufacturing method for the production of biocompatible fewlayered graphene nanosheets is developed using hydrodynamic cavitation. Scalable exfoliation is induced by employing hydrodynamic cavitation and a serum albumin protein. Unlike acoustic cavitation, the primary means of bubble collapse in hydrodynamic cavitation is caused laterally, thereby separating two adjacent flakes through a shear effect. In this process, bovine serum albumin, a known protein, was employed to act as an effective exfoliation agent and provide desired stability by preventing restacking of the graphene layers. This method was used to study the effect of time of graphene exfoliation in a novel hydrodynamic cavitation system. The fabricated products were characterized using Raman spectroscopy, Transmission electron microscopy, Fourier transform infrared spectroscopy and differential scanning calorimetry. The results showed that with increasing the time of exfoliation, the number of graphene layers decreased based on the I 2D/I G ratio but disorder increased based on the I D/I G ratio. At 3 h, the I 2D/I G ratio was at 0.39 and the I D/I G ratio was 0.25, while at 6 h the I 2D/I G ratio was 0.35 and I D/I G ratio was 0.29. The results of the theoretical and computational analysis this research outlines are needed to obtain an effective cavitation model that can be used to potentially improve graphene synthesis and quality. The captured images of bubble propagation in the solution imply that this fluidic phenomenon could assist the graphene exfoliation. To prove this, a simple cavitation model using a needle valve was designed. The needle valve cavitation setup was able to identify that cavitation assists in graphene exfoliation and this was proved using the graphene characterization data. Based on these findings, the simulation models were designed in ANSYS and COMSOL. Specifically, through the ANSYS simulation, we were able to calculate cavitation numbers for specific flow rates and fluid temperatures.

Emergence of intrinsically isolated flat bands and their topology in fully relaxed twisted multilayer graphene

We study the electronic structure and band topology of fully relaxed twisted multi-layer graphene (TMLG). Isolated flat bands emerge in TMLG with the number of layers [$M+N$ with $M$ the layer number of the bottom few-layer graphene (FLG)] up to 10 and with various stacking orders, and most of them are on the hole side. The touched bands of FLGs around the Fermi level are split by the moir\'{e} coupling through the electron-hole asymmetry in low-energy bands of FLGs and by the vertical hopping between next-nearest layers. The full structural relaxation leads to global gaps that completely isolate a flat band. For TMLG with given $M$ and $N$, the highest magnitude of Chern numbers ($|C|$) of the separable flat bands reaches $M+N-1$ and can be hosted by certain isolated bands. The $|C|=9$ occurs in the isolated flat valence band of several configurations with 10 layers. Such high $|C|$ originates from the lifting of the band-state degeneracy in the weak regime of moir\'{e} coupling or from the topological phase transitions induced by the strong moir\'{e} coupling. Moreover, large orbital magnetic moments arise in isolated flat bands with high $|C|$ and depend on the structural configurations of TMLG.

Delamination of multilayer graphene stacks from its substrate through wrinkle formation under high pressures

There is a need to find new paths for van der Waals 2D-systems detachment and transfer or to control their adhesion state in different environments. We have observed that supported multilayer graphene immersed in a fluid can be detached from a substrate through pressure application. The process is based on the development of wrinkles originated by the difference of in-plane-compressibility between the graphene stacks and the substrate. Graphene stacks comprised between 9 and 110 layers and immersed in various fluids allowed to investigate the growth and evolution of wrinkles with increasing pressure. The detachment from the substrate stops at the pressure-induced fluid solidification. Methanol, ethanol or their mixtures favor the pressure-induced wrinkle formation in SiO2/Si substrates. In these cases, the pressure evolution of the delamination process follows a universal behavior independently of the number of graphene layers with a complete delamination at ∼4GPa. The quantitative analysis of the wrinkle geometry evolution can be consistently interpreted as due to a pressure-induced increase of the bending stiffness of the graphene stacks, or a reduction of the adhesion forces between the sample and the substrate, or both. These results should also be of practical use in high-pressure experiments of van der Waals systems.

Mechanical Properties of Solution-Blended Graphene Nanoplatelets/Polyether-Ether-Ketone Nanocomposites.

A lightweight composite with outstanding damping effects and impact resistance is significant for the design of functional structures in advanced equipment, such as aircraft and space vehicles. In this paper, the mechanical properties of solution-blended graphene nanoplatelets (GNPs)/polyether-ether-ketone (PEEK) nanocomposites were characterized by both experimental and numerical methods. It can be found that the layer number and packing configuration of graphene layers are critical to the efficiency of energy dissipation in the composite, while a pack of six- layer graphene and the perpendicular arrangement to the shockwave direction provide the most outstanding energy dissipation ability. The reflection of shockwave caused by graphene reinforcements in the nanocomposite was found to be the dominating reason for the enhanced energy dissipation effect. Physical mechanisms of energy dissipation are investigated by a molecular modeling method to provide insights into the cross-scale design of graphene-reinforced nanocomposites as structural materials.

Strain Hardening in Graphene Foams under Shear.

Strain hardening is an important issue for the design and application of materials. The strain hardening of graphene foams has been widely observed but poorly understood. Here, by adopting the coarse-grained molecular dynamics method, we systematically investigated the microscopic mechanism and influencing factors of strain hardening and related mechanical properties of graphene foams under shear loading. We found that the strain hardening is induced by cumulative nonlocalized bond-breakings and rearrangements of microstructures. Furthermore, it can be effectively tuned by the number of graphene layers and cross-link densities, i.e., the strain hardening would emerge at a smaller shear strain for the graphene foams with thicker sheets and/or more cross-links. In addition, the shear stiffness G of graphene foams increases linearly with the cross-link density and exponentially with the number of graphene layers n by G ∼ n1.95. These findings not only improve our understanding of the promising bulk materials but also pave the way for optimizing structural design in wide applications based on their mechanical properties.

Large cross-polarization rotation of light on graphene

A large cross-polarization (CP) rotation of light is found on the graphene surface experimentally and applied to measuring the layer number of graphene. First, a general propagation model for the polarization of light reflected on the graphene is established. By using this model, it is found that the CP effect rotates remarkably with the incident polarization near the pseudo-Brewster angle. More importantly, such a rotation differs noticeably for graphene surfaces with distinct layers, thereby providing a simple and efficient approach to detect the graphene layer number. Finally, the CP rotation is shown to be accompanied by the rotation of the spin Hall effect of light. Further experiments are performed to validate the theoretical results.

Estimation of Number of Graphene Layers Using Different Methods: A Focused Review.

Graphene, a two-dimensional nanosheet, is composed of carbon species (sp2 hybridized carbon atoms) and is the center of attention for researchers due to its extraordinary physicochemical (e.g., optical transparency, electrical, thermal conductivity, and mechanical) properties. Graphene can be synthesized using top-down or bottom-up approaches and is used in the electronics and medical (e.g., drug delivery, tissue engineering, biosensors) fields as well as in photovoltaic systems. However, the mass production of graphene and the means of transferring monolayer graphene for commercial purposes are still under investigation. When graphene layers are stacked as flakes, they have substantial impacts on the properties of graphene-based materials, and the layering of graphene obtained using different approaches varies. The determination of number of graphene layers is very important since the properties exhibited by monolayer graphene decrease as the number of graphene layer per flake increases to 5 as few-layer graphene, 10 as multilayer graphene, and more than 10 layers, when it behaves like bulk graphite. Thus, this review summarizes graphene developments and production. In addition, the efficacies of determining the number of graphene layers using various characterization methods (e.g., transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectra and mapping, and spin hall effect-based methods) are compared. Among these methods, TEM and Raman spectra were found to be most promising to determine number of graphene layers and their stacking order.

Influence of the Underlying Substrate on the Physical Vapor Deposition of Zn-Phthalocyanine on Graphene.

Graphene shows great promise not only as a highly conductive flexible and transparent electrode for fabricating novel device architectures but also as an ideal synthesis platform for studying fundamental growth mechanisms of various materials. In particular, directly depositing metal phthalocyanines (MPc’s) on graphene is viewed as a compelling approach to improve the performance of organic photovoltaics and light-emitting diodes. In this work, we systematically investigate the ZnPc physical vapor deposition (PVD) on graphene either as-grown on Cu or as-transferred on various substrates including Si(100), C-plane sapphire, SiO2/Si, and h-BN. To better understand the effect of the substrate on the ZnPc structure and morphology, we also compare the ZnPc growth on highly crystalline single- and multilayer graphene. The experiments show that, for identical deposition conditions, ZnPc exhibits various morphologies such as high-aspect-ratio nanowires or a continuous film when changing the substrate supporting graphene. ZnPc morphology is also found to transition from a thin film to a nanowire structure when increasing the number of graphene layers. Our observations suggest that substrate-induced changes in graphene affect the adsorption, surface diffusion, and arrangement of ZnPc molecules. This study provides clear guidelines to control MPc crystallinity, morphology, and molecular orientations which drastically influence the (opto)electronic properties.

Mechanical properties of graphene, defective graphene, multilayer graphene and SiC-graphene composites: A molecular dynamics study

In this article, using molecular dynamics (MD) simulation, several mechanical properties of graphene were calculated for pristine single layer graphene sheet (SLGS), defective SLGS, multilayer graphene sheets (MLGS) without defects and for composites of SiC and graphene. Firstly, the room temperature elastic moduli (Young's, shear and bulk), constants and Poison's ratios were simulated for the above structures and then their properties such as stress vs strain under tensile and compressive loading were simulated within the temperature range of 300K–2000K. The simulations were performed in both zigzag and armchair directions of graphene surface and differences were observed in the mechanical properties. Simulations revealed that the location of defects as well as their type influence the elastic moduli and constants at room temperature and that their tensile and compressive properties vary with temperature. For both MLGS, and SiC-graphene composites, the properties improved with increase in number of continuous graphene layers.

Atomistic simulation on frictional response of nanoscratched Al–graphene system

Frictional behaviors of graphene, which have been generally investigated on elastic or rigid substrates, are carefully discussed here in metal–graphene systems through molecular dynamics simulations. The nanoscratched process and topography analysis are conducted to study the friction on a composite surface, where the layer number, embedding depth, and interval of graphene are considered as major factors. The friction coefficient of monolayer graphene on an Al surface is obviously higher than the reported results on an elastic or rigid substrate, while the variety of substrates seems to make no much difference to frictional results on multilayer graphene. Graphene is actually helpful for reducing friction on composite surfaces, but the friction coefficient on composite surfaces will rocket from 0.045 to 0.835 with the rise in the embedding depth of graphene, and this range of data is consistent with experimental values measured on Al–graphene composite coatings. Moreover, a larger distance between graphene may lead to an increase in the overall friction coefficient, which can also be proved by electroplating experiments under a low concentration of graphene.

High-Quality Transferred GaN-Based Light-Emitting Diodes through Oxygen-Assisted Plasma Patterning of Graphene.

Two-dimensional (2D) release layers are commonly used to realize flexible nitride films. Here, high-quality, large-area, and transferable nitride films can be precisely controlled grown on O2-plasma-assisted patterned graphene. The first-principles calculation indicates that the patterned graphene introduced by O2 plasma changes the original wettability of sapphire and the growth behavior of Al atoms is related with layer number of graphene, which is consistent with experimental results. The as-fabricated violet GaN-based light-emitting diodes (LEDs) show high stability and high light output power (LOP). This work provides a general rule for the growth of high-quality and transferable III-nitride films on graphene from the atomic scale and provide actual demonstration in LED. The advantages of the proposed new growth method can supply new ways for electronic and optoelectronic flexible devices of group III nitride semiconductors.

Plasmon Resonances of Graphene-Dielectric-Metal Structures Calculated by the Method of Recurrence Relations

Graphene supports surface plasmons in the terahertz range, and compared with noble-metal plasmons, they show an extreme level of field confinement and relatively long propagation distances, with the advantage of being highly tunable via electrostatic field. Nevertheless, its interaction with light is normally rather weak. To obtain a more powerful capability of excite plasmons, a combination of graphene and artificial structures (metamaterials) present a powerful tunability for enhancing light-matter interaction. These features make graphene metamaterials a promising candidate for plasmonics and surface plasmon resonance for biological sensors. In this work, we study the plasmon spectra in a finite number of graphene layers on a metallic-dielectric substrate surrounded by materials with different dielectric constants. It is shown that using standard electromagnetic boundary conditions and solving the recurrence relation (a suitable alternative to transfer matrix method) for the coefficients of the electric potential between graphene layers, an explicit effective dielectric function of the metamaterial can be obtained giving the plasmon dispersion relations. It is found that the metal-dielectric-layered graphene structure supports both high-energy optical plasmon oscillations and out-of-phase low-energy acoustic charge density excitations. Experimentally, the Kretschmann configuration can be used to excite the surface plasmon resonances. It is based on the observation of a sharp minimum in the reflection coefficient versus angle (or wavelength) curve.

Tunable Goos-Hänchen Shift Surface Plasmon Resonance Sensor Based on Graphene-hBN Heterostructure.

In this paper, a bimetallic sensor based on graphene-hexagonal boron nitride (hBN) heterostructure is theoretically studied. The sensitivity of the sensor can be improved by enhancing the Goos–Hänchen (GH) shift in the infrared band. The theoretical results show that adjusting the Fermi level, the number of graphene layers and the thickness of hBN, a GH shift of 182.09 λ can be obtained. Moreover, sensitivity of 2.02 × 105 λ/RIU can be achieved with monolayer graphene, the thickness of gold layer is 20 nm, silver layer is 15 nm, and the hBN thickness of 492 nm. This heterogeneous infrared sensor has the advantages of high sensitivity and strong stability. The research results will provide a theoretical basis for the design of a new high-sensitivity infrared band sensor.

Impedance Characteristics of Biofilms Formed on Graphene Films and Their Substrates

Biofilms are inhomogeneous film-like matters formed on materials by bacterial activities. They bring about many detrimental problems in various fields such as cooling pipes, kitchens, toilets, baths, marine structures, food industries, medical applications, hospitals etc., as well as beneficial purposes. Therefore, they should be controlled properly and the sensing is very important therefore. We have confirmed so far that graphene as one of representative 2D materials has high sensitivity to biofilm formation due to their pi electron clouds. The characteristics would lead to the application of biofilm sensors. In this experiment, we formed one layer, two layers and multi-layers graphenes were formed by a thermal CVD process on copper substrates and in some cases, they were moved to the surfaces of polyethylene and biofilms were formed on graphene films with those substrates. Then the impedance characteristics were measured and the relation between the impedance characteristics and biofilms formation/growth behaviors were investigated and discussed. As a result, we obtained the following results.1. In the case of graphene films on all substrates, the characteristic peaks of impedance were observed at some specific frequencies.2. The impedance peaks were different between copper substrates and PE ones.3. The impedance peaks were also different according to the numbers of graphene layers.4. The impedance peaks became more remarkable for non-freeze hydrated specimens.5. In the case of polymeric substrates, the impedance characteristics of biofilms showed different patters.We presume that the impedance peaks were obtained, since external polymeric substances (EPS) in biofilms would have absorbed electromagnetic waves. Therefore, the polymeric substrates could have affected the absorbance behaviors of electromagnetic waves differently. The application possibilities of the experimental results to biofilm sensors were finally discussed.

Solution Electrochemical Approach to Synthesis of Low Stage Graphite Bisulphate

Since their discovery in 1841, Graphite Intercalation Compounds (GICs) have found tremendous applications in various industrial and research fields such as supercapacitors, superconductors, heterogeneous catalysts, anode materials, hydrogen and lithium-ion storage and notably in graphene synthesis. GICs are a type of graphitic compounds formed by the insertion of a different atomic, ionic or molecular species, known as intercalants between the layers of graphite host. Intercalation occurs in stages, and the phenomenon of staging is characterised by stage index n, which is the number of carbon layers present between two successive intercalate layers. The concentration of intercalant in graphite increases as the stage index decreases.Oxyacids readily intercalate in graphite. Researchers have studied intercalation of graphite in a wide variety of such acids such as H2SO4, HNO3, H2SeO4, HClO4 and H3PO4, and concluded that sulphuric acid is the best intercalant for graphite owing to higher efficiency and lesser reaction times. Intercalation in sulphuric acid gives Graphite Bisulphate (GB). Conventional GB chemical synthesis methods, pose an environmental and safety hazard due to the usage of hazardous and explosive oxidants. Moreover, chemical methods such as Hummers’ are primarily oxidation routes for the synthesis of Graphene Oxide (GO) wherein GB is believed to be the process intermediate, which is difficult to isolate.Electrochemical synthesis of GB is inexpensive, scalable and provides an easy defect-free transfer to the application substrate due to its ability to deliver the product in a suspension. Furthermore, the electrochemical approach provides an environment- friendly solution to the synthesis of GB because it replaces harsh chemical oxidants with an external polarisation potential. The quantity of electricity passed can also be controlled by changing the applied potential to obtain desired stages of GB.However, the synthesis of low stage GB by the electrochemical process in aqueous sulphuric acid solutions is an arduous task as a result of the excess water present in the system. Water molecules react with surface C atoms to form bulky surface groups which hinder the process of intercalation. Furthermore, undissociated water molecules may also co-intercalate with the bisulphate ions from aqueous sulphuric acid. Electrochemical oxidation of this intercalated water causes reduction of bisulphate ions and ultimately results in the vigorous evolution of oxygen and sulphur dioxide, which exfoliates graphite. Thus, GB formed before exfoliation is disintegrated and lost.On the other hand, water also facilitates intercalation by opening up graphitic grain boundaries for intercalation by carrying out a nucleophilic attack on the edge sites. Thus an optimal concentration of water in aqueous sulphuric acid along with an optimal positive polarisation potential is required for the synthesis and stability of GB. The present study aims at the electrochemical synthesis and characterisation of low stage GB and determination of optimal reaction conditions for the process. A pure stage GB synthesised and isolated this way can be particularly useful for the synthesis of layer-specific graphene flakes. Intercalation reduces the force of attraction between layers of graphite and hence makes selective cleaving possible by sonication. The stage index is strongly correlated with the number of graphene layers produced upon electrochemical exfoliation and is thus a key parameter to tune the electrochemical exfoliation of graphite.

Experimental Study of Few‐Layer Graphene: Optical Anisotropy and Pseudo‐Brewster Angle Shift in Vacuum Ultraviolet Spectral Range

The optical properties of mono‐ and trilayer graphene on SiO2/Si substrate are studied at hydrogen Lyman‐alpha (121.6 nm) spectral line for the first time. The optical anisotropy of graphene at this wavelength is experimentally demonstrated by retrieving the anisotropic “effective” optical constants. The results confirm that the axis of symmetry is nearly perpendicular to the surface and coherently related to the π‐orbitals’ structural orientation. Furthermore, it is observed that graphene strongly affects the performances of the substrate by inducing a pseudo‐Brewster angle downshift, which depends on the number of graphene layers. This finding is in contrast with what occurs in the visible spectral range, where the upshift of the pseudo‐Brewster angle is experienced in similar samples.

Electron transmission through suspended graphene membranes measured with a low-voltage gated Si field emitter array

We experimentally demonstrate the transmission of electrons through different number (1, 2, and 5) of suspended graphene layers at electron energies between 20 and 250 eV. Electrons with initial energies lower than 40 eV are generated using silicon field emitter arrays with 1 μm pitch, and accelerated towards the graphene layers supported by a silicon nitride grid biased at voltages from −20 to 200 V. We measured significant increase in current collected at the anode with the presence of graphene, which is attributed to the possible generation of secondary electrons by primary electrons impinging on the graphene membrane. Highest output current was recorded with monolayer graphene at approximately 90 eV, with up to 1.7 times the incident current. The transparency of graphene to low-energy electrons and its impermeability to gas molecules could enable low-voltage field emission electron sources, which often require ultra-high vacuum, to operate in a relatively poor vacuum environment.

Wettability of graphene and interfacial water structure

<h2>Summary</h2> Understanding the wettability of graphene is important for various applications, such as water desalination, energy storage, and catalysis. However, the detailed water hydrogen-bonding structure at the water-graphene interface is not yet fully understood. Using vibrational sum-frequency-generation (VSFG) spectroscopy, we elucidate the water structure at a water-graphene interface. As the number of graphene layers increases, water molecules with dangling OH groups become more populated, indicating the transition from transparency to translucency. We compare the water contact angles on the graphene surfaces with VSFG results, showing an excellent correlation between the water adhesion energy of graphene and the fraction of dangling OH groups estimated from the VSFG spectrum. This observation suggests that the VSFG could be an incisive technique for measuring water's adhesion energy on any spatially confined interface where the water contact angle cannot be measured. We anticipate that the VSFG results will help to elucidate the wettability of low-dimensional materials. <h3>Video Abstract</h3>