Triple-layer Graphene Research Articles

A novel top-down approach for high yield production of graphene from natural graphite and its supercapacitor applications

The cost-effective and sustainable mass production of graphene is of utmost importance for its comprehensive application across various industries. Despite ball milling being the most efficacious method for obtaining high-quality graphene, industrial-scale production poses a significant challenge due to a lowered yield, heterogeneous media and reduced energy transfer efficiency. Furthermore, the consistent properties of graphene pose another major challenge when it comes to scaled-up processes. To address these challenges, we conducted a study aiming to produce high-quality graphene from natural graphite sourced from Indian ore through the utilisation of a horizontal planetary ball mill. Our analysis revealed that the graphene sheets produced via this method exhibit excellent structural integrity, rendering them highly beneficial for a wide range of applications. The diffraction pattern of the graphene sheets is dominated by the (002) plane, as shown by XRD data. The Raman spectrum indicates a compatible I2D/IG ratio, suggesting double- to triple-layer graphene sheets. The graphene sheets were seen to be layered and irregularly shaped in the SEM and TEM pictures, which is consistent with the sample's polycrystalline makeup. The findings demonstrate the large surface area and strong electrical conductivity of the synthesized graphene. Furthermore, the high specific capacitance value of 30F/g is shown by in-house graphene-based supercapacitors. Overall, the findings of this work show that manufacturing high-quality graphene from synthetic and natural graphite is feasible and effective when carried out on a horizontal planetary ball mill, and each batch of 200 g graphene requires $25 in production costs. This strategy promises to supply the rising demand for graphene by enabling more widespread, cost-effective manufacturing.

Evaluation of transmission characteristics of CVD-grown graphene and effect of tuning electrical properties of graphene up to 50 GHz

Graphene has been investigated as a transparent conductive film for use in a variety of devices, and in recent years it has shown promise for use in millimeter-wave devices as 5G technology. In this study, we applied single-layer (SL), triple-layer (3L), and P-type doped 3L graphene to coplanar waveguide (CPW) transmission lines and obtained transmission characteristics (S21) from 1 to 50 GHz, which covered the 5G band. Furthermore, an equivalent circuit model of the CPW used in the measurements was constructed and simulations were performed, which showed good agreement with the measured results. The results validated the transmission properties of the graphene and the contact impedance at the interface between electrodes and the graphene in CPW circuits, which are necessary parameters for designing antennas using graphene. In addition, by comparing the transmission loss of three types of graphene, the parameters for improving the transmission characteristics were clarified.

Magic angles and flat Chern bands in alternating-twist multilayer graphene system

Based on the effective continuum model, we study alternating-twist multilayer graphene system and emergence of magic angles and flat band topology. All the alternating-twist multilayer graphene system (from triple layers to few layers) are found to have flat bands at magic angles where the area of AA stacking equals n-fold (n is an integer) electron cyclotron area. From the pseudo-Landau-level representation, there is always an isolated Dirac band in the alternating-twist graphene system constructed by odd number of layers. Since each pair of flat bands can be perceived as the zeroth pseudo-Landau-levels in two dimensional Dirac fermions, electron in the flat band pair can feel a pseudo-magnetic field with the same magnitude but the opposite sign. Calculated Chern number for each flat band is +1 (or -1) which can be tuned by twisting in the vicinity of magic angles or by gating. The concurrent appearance of strong correlation and band topology of flat bands in the alternating-twist multilayer graphene may pave an avenue for the new understanding of superconductivity observed in triple-layered graphene, and supply a new playground for realizing (quantum) anomalous Hall effect.

Hydrophobic-to-hydrophilic affinity change of sub-monolayer water molecules at water–graphene interfaces

Investigating the interfacial properties between water and graphene is important, as the water–carbon interface influences various applications. Graphene is the fundamental unit of various carbon allotropes and composed of a single atomic species, although slight oxygen groups are normally attached on it. Therefore, adsorption properties on graphene is inevitable to advance fundamental understating on surface sciences. However, the detection of water adsorption on graphene is considerably hard, because amount of water is tiny and mass production of graphene to a detectable level using a standard adsorption apparatus is extremely difficult. We succeeded to detect water vapor adsorption on several layer graphene using lab-made adsorption apparatus with a quartz crystal microbalance technique. This study derives the properties of water on single-, triple-, and six-layer graphene from the contact angles of water droplets and the water vapor adsorption isotherms. The surface hydrophobicity evaluated from the contact angles and water vapor adsorption isotherms decreased in the following order: single-layer graphene > triple-layer graphene > six-layer graphene. At a relative pressure of 0.4 and water surface densities of 0.5–1.5 mg m −2 (corresponding to 2–5 water layers), the water stability was 1.0 kJ mol −1 smaller on single-layer graphene than on triple- and six-layer graphene. The weak interaction between water molecules and single-layer graphene induced slight water adsorption on graphene. As the water amount increased, the water–graphene interaction energy increased with steps of 0.17–0.18 mg m −2 (corresponding to nearly 50% of a water monolayer), and the water structure correspondingly transformed from cluster to layer. The structure transformation of adsorbed water induced the affinity change from hydrophobic to hydrophilic on graphene interfaces. The mechanism of water–graphene interfaces can be exploited in applications of graphene-based carbon nanomaterials. • Water contact angles of graphene were 57–80°, indicating hydrophobic interfaces. • Water vapor on single-layer graphene was rarely adsorbed below P/P 0 = 0.4. • Water-adsorbed structure was changed from cluster to layer form at 0.5 water layer. • Cluster-to-layer transformation triggers affinity change of water on graphene.

Low temperature chemical treatment of graphene films made by double self-assembly process to improve sheet resistance

In this study, a low temperature hydro iodic acid (HI) vapor treatment of the self-assembled graphene films has been developed, and the electrical, optical, structural and morphological properties were investigated by four-point probe, UV–Visible spectroscopy, Raman spectroscopy and scanning electron microscopy (SEM). Mono-, double- and triple-layer of graphene flakes were deposited on glass substrates by using the Double Self-Assembly (DSA) process. The self-assembled graphene films were treated with HI vapors at 40 °C for different time intervals between 1 and 24 h. In addition, graphene deposition and HI-vapor treatment (at 40 °C for 1 h) was enforced three times to the same substrate. The optical transparency values of the self-assembled mono- (MGFs), double- (DGFs) and triple-layer graphene flakes (TGFs) were measured as 91, 85 and 80%, respectively (values at 550 nm). Due to the HI-vapor treatment, the sheet resistance of MGFs significantly reduced from 1.1 × 107 Ω □−1 to 2.9 × 104 Ω □−1, the transparency of the graphene films slightly reduced by 2–5%, the ID/IG ratio of the DGFs decreased from 1.01 to 0.81, while the I2D/IG ratio increased from 0.43 to 0.48 in the Raman spectrum. Thanks to its impressive reducing effect on sheet resistance, HI-vapor treatment can be a suitable method to improve the conductivity of low-cost large area graphene films.

Suspended triple-layer graphene modulator with two modulation depths and ultra-high modulation speed

The suspended triple-layer graphene modulator is firstly investigated theoretically. We find there appear two modulation depths for electro-absorption modulation. The light-graphene interaction is enhanced to its maximum by our designed waveguide structure. The highest modulation depth for electro-absorption modulation can be 0.834 dB/µm, causing a 3-dB footprint of only 0.94 µm2. For electro-refractive modulation, there appear several 100% modulations with a much smaller π-phase shift length of only 11.3 µm. This modulator also shows great potential for high-speed modulation with a prediction value of 759.85 GHz, while the switch energy can be as low as 0.61 fJ/bit with low applied voltage. Moreover, the verification simulation by COMSOL is also presented, which shows very good agreement with our calculation results, and the figure of merit (defined as the ratio of modulation depth to insertion loss) of this modulator can be 2105. We believe these results can pave the way to design practical high-speed, compact-footprint, and high-efficiency devices.

Layer-by-Layer Graphene Growth on β-SiC/Si(001).

The mechanism of few-layer graphene growth on the technologically relevant cubic-SiC/Si(001) substrate is uncovered using high-resolution core-level and angle-resolved photoelectron spectroscopy, low-energy electron microscopy, and microspot low-energy electron diffraction. The thickness of the graphitic overlayer supported on the silicon carbide substrate and related changes in the surface structure are precisely controlled by monitoring the progress of the surface graphitization in situ during high-temperature graphene synthesis, using a combination of microspectroscopic techniques. The experimental data reveal gradual changes in the preferential graphene lattice orientations at the initial stages of the few-layer graphene growth on SiC(001) and can act as reference data for controllable growth of single-, double-, and triple-layer graphene on silicon carbide substrates.

Scale Effects on the Ballistic Penetration of Graphene Sheets

Carbon nanostructures are promising ballistic protection materials, due to their low density and excellent mechanical properties. Recent experimental and computational investigations on the behavior of graphene under impact conditions revealed exceptional energy absorption properties as well. However, the reported numerical and experimental values differ by an order of magnitude. In this work, we combined numerical and analytical modeling to address this issue. In the numerical part, we employed reactive molecular dynamics to carry out ballistic tests on single, double, and triple-layered graphene sheets. We used velocity values within the range tested in experiments. Our numerical and the experimental results were used to determine parameters for a scaling law. We find that the specific penetration energy decreases as the number of layers (N) increases, from ∼15 MJ/kg for N = 1 to ∼0.9 MJ/kg for N = 350, for an impact velocity of 900 m/s. These values are in good agreement with simulations and experiments, within the entire range of N values for which data is presently available. Scale effects explain the apparent discrepancy between simulations and experiments.

Nonlinear dynamic responses of triple-layered graphene sheets under moving particles and an external magnetic field

This paper reports the result of an investigation into the nonlinear dynamic responses of triple-layered graphene sheets subjected to moving particles (gas molecules, lithium ion, catalyst molecules, electron, etc.) and an external magnetic field. Based on Von Kármán nonlinear geometric relations and nonlocal elasticity theory, the nonlinear dynamic nonlocal governing equations are found by utilizing the Hamilton's principles and Galerkin method, where Van der Waals forces between two layers of triple-layered graphene sheets are considered. A reformulated differential quadrature method (DQM) is demonstrated to solve the nonlinear dynamic nonlocal governing equations. Effects of some key factors on the nonlinear dynamic characteristics of triple-layered graphene sheets are calculated and described in examples. Results show that the deflection response magnitude for the nonlinear solution is smaller than that for linear, the small scale effect could induce the decline of natural frequencies, the dynamic response is significantly affected by particles moving speed, to increase magnetic field strength could induce the decrease of the dynamic deflection and the increase of resonant frequency at the same time. The presented meaningful results could serve as useful references in the application and design of lithium batteries, nano sensors, and other micro or nano-devices based on laminated graphene sheets.

Interfacial Li-Ion Storage Between Graphene Layers

The ever increased development of portable devices and electrical vehicles demands the electrochemical energy storage devices with high energy and power densities. Graphite has been the most widely used anode materials for Li-ion batteries. However, because of its low theoretical capacity (372 mAh/g), there has been significant efforts in developing electrode materials with much higher energy densities. Graphene, a one-atom-thick carbon materials showed numerous advantages including superior electrical conductivity, thermal conductivity and high surface area; and is expected to be a good candidate as a high-power and high-energy electrode material for Li-ion batteries. Although, previous studies used graphene as anode materials, it showed significantly enhanced capacity >1000 mAh/g; the energy storage mechanism in graphene is still unclear. This is due to the high density of defects in chemically synthesized graphene. Few studies have been using chemical vapor deposition (CVD)-grown defect-free single layer graphene (SLG) on copper foil as anodes in Li-ion batteries.1 Despite this, the exact mechanism of Li ions storage in the SLG is still a subject of controversy. Theoretical study has shown that the absorption of Li-ions on defect-free SLG is energetically unfavorable.2 This is in controversy with the experiments which claims that CVD-grown SLG is promising electrode materials for Li anodes. In this study, we investigated the Li-ion storage in CVD-grown SLG graphene in comparison with Cu substrate, our results confirmed that the capacity obtained for SLG on Cu is mainly originated from the contribution of Cu substrate. However, by using bilayer or triple layer graphene which is prepared by a layer-by-layer transfer process, we found that the Li ion storage capacity will be significantly enhanced when additional layer graphene was placed on the top of initial SLG. The capacitance increased from 0.05 F/cm2 for Cu substrate to 0.08 F/cm2 for bilayer graphene on Cu and further increased to 0.129 F/cm2for triple layer graphene on Cu substrate. The areal capacity of the bilayer graphene is ~ 2 times of the capacity available for Cu substrate after 50 cycles. Our study indicates that the interfacial space between graphene layers plays a critical role for high performance Li-ion storage.

Buckling analysis of multi-layered graphene sheets based on a continuum mechanics model

In this paper, buckling analysis of biaxially compressed multi-layered graphene sheets with a continuum plate model is reported. The equations of motion are analytically solved to obtain closed-form solution for buckling loads of all edges simply supported multi-layered graphene sheets. The interaction of van der Waals (vdWs) pressure between the layers is incorporated in the formulation to determine the buckling behavior of simply supported MLGSs. Explicit formulae are derived for predicting the critical buckling loads of double- and triple-layered graphene sheets, and they clearly indicate the effect of vdW interaction on the critical buckling loads. The critical buckling loads are calculated for various numbers of layered graphene sheets, and the obtained results show that the vdW force has no effect on the first critical buckling load of an MLGS, but plays a significant role in all higher first critical buckling loads for all combinations of m and n.

Sensitivity investigation for the dependence of monolayer and stacking graphene NH3 gas sensor

The influence of various stacking chemical vapor deposition (CVD) graphene on ammonia sensing is investigated. Three interdigitated electrodes (IDEs) gas sensor with non-stacking monolayer (NSM), less stacking double layer (LSDL) and more stacking few layer (MSFL) CVD graphene were fabricated for ammonia gas detection. NSM sensor is more sensitive than LSDL and MSFL ones towards NH3. Especially for 12,500ppm NH3, the sensitivity of NSM device is more than twice the LSDL device and almost 7 times of MSFL device. Moreover, the repeatability and response time of NSM device are all much better than the other two. The simulation results show that the change ratios of integral electrons in total DOS changed 26.65% after NH3 adsorption for monolayer graphene, which is three times more than triple layer graphene of 8.29%. This is the reason why NSM has the highest sensitivity towards NH3.

Hybrid Tunnel Junction-Graphene Transparent Conductive Electrodes for Nitride Lateral Light Emitting Diodes.

Graphene transparent conductive electrode (TCE) applications in nitride light emitting diodes (LEDs) are still limited by the large contact resistance and interface barrier between graphene and p-GaN. We propose a hybrid tunnel junction (TJ)-graphene TCE approach for nitride lateral LEDs theoretically and experimentally. Through simulation using commercial advanced physical models of semiconductor devices (APSYS), we found that low tunnel resistance can be achieved in the n(+)-GaN/u-InGaN/p(+)-GaN TJ, which has a lower tunneling barrier and an enhanced electric field due to the polarization effect. Graphene TCEs and hybrid graphene-TJ TCEs are then modeled. The designed hybrid TJ-graphene TCEs show sufficient current diffusion length (Ls), low introduced series resistance, and high transmittance. The assembled TJ LED with the triple-layer graphene (TLG) TCEs show comparable optoelectrical performance (3.99 V@20 mA, LOP = 10.8 mW) with the reference LED with ITO TCEs (3.36 V@20 mA, LOP = 12.6 mW). The experimental results further prove that the TJ-graphene structure can be successfully incorporated as TCEs for lateral nitride LEDs.

Enhancement of electrochemical properties by polysulfide trapping in a graphene-coated sulfur cathode on patterned current collector.

A sulfur cathode on a honeycomb-shape-patterned Al current collector was prepared and successfully sealed using triple-layered graphene. Graphene layers on the sulfur cathode well confined the dissolved polysulfide in the electrode, leading to significantly enhanced electrochemical properties including cycle retention and Coulombic efficiency.

Structure and field emission of graphene layers on top of silicon nanowire arrays

Monolayer graphene was grown on copper foils and then transferred on planar silicon substrates and on top of silicon nanowire (SiNW) arrays to form single- to quadruple-layer graphene films. The morphology, structure, and electron field emission (FE) of these graphene films were investigated. The graphene films on the planar silicon substrates were continuous. The single- to triple-layer graphene films on the SiNW arrays were discontinuous and while the quadruple-layer graphene film featured a mostly continuous area. The Raman spectra of the graphene films on the SiNW arrays showed G and Gʹ bands with a singular-Lorentzian shape together with a weak D band. The D band intensity decreased as the number of graphene layers increased. The FE efficiency of the graphene films on the planar silicon substrates and the SiNW arrays varied with the number of graphene layers. The turn-on field for the single- to quadruple-layer graphene films on planar silicon substrates were 4.3, 3.7, 3.5 and 3.4V/μm, respectively. The turn-on field for the single- to quadruple-layer graphene films on SiNW arrays decreased to 3.9, 3.3, 3.0 and 3.3V/μm, respectively. Correlation of the FE with structure and morphology of the graphene films is discussed.

Low-Energy Electron Potentiometry: Contactless Imaging of Charge Transport on the Nanoscale.

Charge transport measurements form an essential tool in condensed matter physics. The usual approach is to contact a sample by two or four probes, measure the resistance and derive the resistivity, assuming homogeneity within the sample. A more thorough understanding, however, requires knowledge of local resistivity variations. Spatially resolved information is particularly important when studying novel materials like topological insulators, where the current is localized at the edges, or quasi-two-dimensional (2D) systems, where small-scale variations can determine global properties. Here, we demonstrate a new method to determine spatially-resolved voltage maps of current-carrying samples. This technique is based on low-energy electron microscopy (LEEM) and is therefore quick and non-invasive. It makes use of resonance-induced contrast, which strongly depends on the local potential. We demonstrate our method using single to triple layer graphene. However, it is straightforwardly extendable to other quasi-2D systems, most prominently to the upcoming class of layered van der Waals materials.

Nanoscale Modeling and Elastic Properties of Portlandite and Graphene Based on Atomic Finite Element Method

The development of multi-scale modeling methods reveals to be of undeniable practical importance, especially to describe and predict the mechanical properties of structural materials. The present work aims to relate the atomic scale with the macro-scale performances. To this purpose a model of a crystalline structure based on the Atomic Finite Element Method (AFEM) is developed. The interatomic bonding forces of Van der Waals, the Coulomb electrostatic force and the covalent chemical bond are taken into account. It is then applied to Portlandite (CH) as well as to graphene (triple-layer graphene sheet, TLGSs). Elastic modulus of these structures based on AFEM is determined. Then, modeling of a single crystal can be traced back to the homogenized elastic properties of polycrystals. Elastic constants and elastic modulus by AFEM algorithm are in quite good agreement with literature experiment. These modeling method and algorithm provide some basic reference to other hexagonal structures.

Atomic resolution electrostatic potential mapping of graphene sheets by off-axis electron holography

Off-axis electron holography has been performed at atomic resolution with the microscope operated at 80 kV to provide electrostatic potential maps from single, double, and triple layer graphene. These electron holograms have been reconstructed in order to obtain information about atomically resolved and mean inner potentials. We propose that off-axis electron holography can now be used to measure the electrical properties in a range of two-dimensional semiconductor materials and three dimensional devices comprising stacked layers of films to provide important information about their electrical properties.

Nanoscale vibration characteristics of multi-layered graphene sheets

A continuum-based plate model is proposed to study the nanoscale vibration characteristics of multi-layered graphene sheets (MLGSs) that are increasingly being proposed for important engineering applications such as THz resonators. Generalized Differential Quadrature (GDQ) method is used to predict the natural frequencies and their associated vibration modes of single-layered and triple-layered graphene sheets, as well as general MLGSs. Numerical simulations are carried out to examine the effects of van der Waals (vdW) interactions, which are present as bonding forces between the layers, on nanoscale vibration natural frequencies and their mode shapes. The results show that for a general MLGSs, vibration modes can be classified into 3 families—lower classical synchronized modes which are independent of van der Waals forces, middle van der Waals enhanced modes which are largely determined by the presence of van der Waals interactions and higher mixed modes which are combinations of the classical synchronized modes and van der Waals enhanced modes. Detailed characterizations of these modes from their derived mode shapes have been achieved for the typical case of triple-layered GSs, as well as general MLGSs. Effects of different boundary conditions, aspect ratios and the number of layers on nanoscale vibration properties have been examined in detail. The results presented in this paper, for the first time, provide accurate and wholesome studies and characterizations on the interesting nanoscale vibration properties of multi-layered graphene sheets and the results obtained will certainly be useful to those who are concerned with the dynamics of graphene sheets which are increasingly being deployed for various innovative engineering applications.

Enhanced Current Drivability of CVD Graphene Interconnect in Oxygen-Deficient Environment

Graphene has been considered as a candidate for interconnect metal due to its high carrier mobility and current drivability. In this letter, the breakdown mechanism of single-layer chemical-vapor-deposited (CVD) graphene and triple-layer CVD graphene has been investigated at three different conditions (air exposed, vacuum, and dielectric capped) to identify a failure mechanism. In vacuum, both single- and triple-layer graphenes demonstrated a breakdown current density as high as ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is similar to that of exfoliated graphene. On the other hand, the breakdown current of graphene exposed to air was degraded by one order of magnitude from that of graphene tested in vacuum. Thus, oxidation initiated at the defect sites of CVD graphene was suggested as a major failure mechanism in air, while Joule heating was more dominant with dielectric capping and in vacuum.