Multilayer Graphene Membranes Research Articles

The study of multilayer graphene membrane performance in O2 purification process: Molecular dynamics simulation

We use molecular dynamics (MD) method to describe the atomic behavior of Graphene nanostructure for Oxygen molecules (O2) separation from Carbon dioxide (CO2) molecules. Technically, for the simulation of graphene-based membrane and O2-CO2 gas mixture, we used Tersoff and DREIDING force fields, respectively. The result of equilibrium process of these structures indicated the good stability of them. Physically, this behavior arises from the appropriate MD simulation settings. Furthermore, to describe the purification performance of graphene-based membrane, we report some physical parameters such as purification value, impurity rate, and permeability of membrane after atomic filtering process. Numerically, by defined membranes optimization, the purification value of them reach to 97.31%. Also, by using these atomic structures the CO2 impurity which passed from graphene-based membrane reach to zero value.

Effect of air-loading on the performance limits of graphene microphones

As a consequence of their high strength, small thickness, and high flexibility, ultrathin graphene membranes show great potential for pressure and sound sensing applications. This study investigates the performance of multi-layer graphene membranes for microphone applications in the presence of air-loading. Since microphones need a flatband response over the full audible bandwidth, they require a sufficiently high mechanical resonance frequency. Reducing membrane thickness facilitates meeting this bandwidth requirement, and therefore, also allows increasing compliance and sensitivity of the membranes. However, at atmospheric pressure, air-loading effects can increase the effective mass, and thus, reduce the bandwidth of graphene and other 2D material-based microphones. To assess the severity of this performance-limiting effect, we characterize the acoustic response of multi-layer graphene membranes with a thickness of 8 nm in the pressure range from 30 to 1000 mbar, in air and helium environments. A bandwidth reduction by a factor ∼2.8× for membranes with a diameter of 500 μm is observed. These measurements show that air-loading effects, which are usually negligible in conventional microphones, can lead to a substantial bandwidth reduction in ultrathin graphene microphones. With analytical and finite element models, we further analyze the performance limits of graphene microphones in the presence of air-loading effects.

Beyond the single-layer limit for desalination based on conical channels of multilayer nanoporous graphene membranes

Nanoporous graphene monolayer has been proved to be a candidate for desalination with high permeability due to its ultrathin nature, however, it is still challenging to produce at large scales, which hampers its practical applications. Here, we propose a multilayer membrane strategy for high-efficiency desalination beyond the single-layer limit using conical channels formed by graphene multilayer of nanopores. By varying the number of layers and the cone angle, molecular dynamics simulations demonstrate that the water flux for the channels made of four-layer graphene at the cone angles of 8.6°, 19.1° and 35° is much larger than that the monolayer graphene with the pore size identical to that of the smallest pore of multilayer. Based on the channel with cone angle of 19.1°, we further find that N-functionalized conical channels exhibit promising desalination performance in terms of both water permeability and salt rejection. This work provides a useful guideline for the application of multilayer graphene membranes in the field of desalination.

High-Sensitivity Graphene MOEMS Resonant Pressure Sensor.

Nanomechanical resonators made from suspended graphene exhibit high sensitivity toward pressure variations. Nevertheless, these devices exhibit significant energy loss in nonvacuum environments due to air damping, as well as inevitably weak gas leakage within the reference cavity because of the slight permeation of graphene. We present a new type of graphene resonant pressure sensor utilizing micro-opto-electro-mechanical systems technology, which features a multilayer graphene membrane that is sealed in vacuum and adhered to pressure-sensitive silicon film with grooves. This approach innovatively employs an indirectly sensitive method, exhibiting 60 times smaller energy loss in atmosphere, and solving the long-standing issue of gas permeation between the substrate and graphene. Notably, the proposed sensor exhibits a high pressure sensitivity of 1.7 Hz/Pa, which is 5 times higher than the sensitivity of the silicon counterparts. Also, the all-optical encapsulating cavity structure contributes a high signal-to-noise ratio of 6.9 × 10-5 Pa-1 and a low temperature drift (0.014%/◦C). The proposed method offers a promising solution for long-term stability and energy loss suppression of pressure sensors using two-dimensional materials as the sensitive membrane.

Nanoporous multilayer graphene oxide membrane for forward osmosis metal ion recovery from spent Li-ion batteries

The recovery of Li and rare metals from the spent electrodes of Li-ion batteries (LIBs) is becoming increasingly important. Herein, nanoporous multilayer graphene oxide (NMG) membranes were fabricated via the confined thermal annealing method to achieve Li-ion selectivity. A graphene oxide (GO) membrane was prepared by coating the membrane with an aqueous layer of GO liquid crystal via the scalable bar coating method, followed by hot-pressing. The influence of the GO interlayer spacing and nanopore presence on the ion separation performance was investigated. The NMG membrane shows different ion permeance phenomena depending on ion concentration. The permeation rate of divalent ions (Co, Ni, Mn) through the NMG membrane was faster than that of Li ions in solution with low ionic strength, whereas the membrane was Li-ion selective in mixed ionic solutions or at high ionic strength. This result indicates that electrostatic interaction by oxygen groups is important at low ionic strength, but size exclusion by interlayer spacing and adsorption by nanopore is critical at high ionic strength. The ion permeation mechanisms were further examined based on molecular calculations, which revealed that the binding energies between the divalent ions and nanopores or basal plane of graphene were high, resulting in slow ion permeation. When the NMG membrane was combined in a forward osmosis system, Li-ion can be separated continuously from the mixture solution simulated from the spent Li-battery electrode. This approach shows the high potential of nanoporous multilayer graphene membrane for Li extraction in particular with dense pore structure and around 7 Å of d-spacing.

MoS2 Passivated Multilayer Graphene Membranes for Li-Ion Extraction From Seawater.

Abundant Li resources in the ocean are promising alternatives to refining ore, whose supplies are limited by the total amount and geopolitical imbalance of reserves in Earth's crust. Despite advances in Li+ extraction using porous membranes, they require screening other cations on a large scale due to the lack in precise control of pore size and inborn defects. Herein, MoS2 nanoflakes on a multilayer graphene membrane (MFs-on-MGM) that possess ion channels comprising i) van der Waals interlayer gaps for optimal Li+ extraction and ii) negatively charged vertical inlets for cation attraction, are reported. Ion transport measurements across the membrane reveal ≈6- and 13-fold higher selectivity for Li+ compared to Na+ and Mg2+ , respectively. Furthermore, continuous, stable Li+ extraction from seawater is demonstrated by integrating the membrane into a H2 and Cl2 evolution system, enabling more than 104 -fold decrease in the Na+ concentration and near-complete elimination of other cations.

Design of Multi-Layer Graphene Membrane with Descending Pore Size for 100% Water Desalination by Simulation Using ReaxFF.

The performance of a desalination membrane depends on a specific pore size suitable for both water permeability and salt rejection. To increase membrane permeability, the applied pressure should be increased, which creates the need to improve membrane stability. In this research article, a molecular dynamics (MD) simulation was performed using ReaxFF module from Amsterdam Modeling suite (AMS) software to simulate water desalination efficiency using a single and multi-layer graphene membrane. The graphene membrane with different pore sizes and a multi-layer graphene membrane with descending pore size in each layer were designed and studied under different pressures. The stability of the membrane was checked using Material Studio 2019 by studying the dynamics summary. The single-layer graphene membrane was evaluated under pressures ranging from 100 to 500 MPa, with the salt rejection ranging from 95% to 82% with a water permeability of 0.347 × 10-9 to 2.94 × 10-9 (mm.g.cm-2s-1.bar-1), respectively. Almost 100% salt rejection was achieved for the multi-layer graphene membrane. This study successfully demonstrated the design and optimization of graphene membrane performance without functionalization.

Sensitive Transfer-Free Wafer-Scale Graphene Microphones.

During the past decades micro-electromechanical microphones have largely taken over the market for portable devices, being produced in volumes of billions yearly. Because performance of current devices is near the physical limits, further miniaturization and improvement of microphones for mobile devices poses a major challenge that requires breakthrough device concepts, geometries, and materials. Graphene is an attractive material for enabling these breakthroughs due to its flexibility, strength, nanometer thinness, and high electrical conductivity. Here, we demonstrate that transfer-free 7 nm thick multilayer graphene (MLGr) membranes with diameters ranging from 85–155 to 300 μm can be used to detect sound and show a mechanical compliance up to 92 nm Pa–1, thus outperforming commercially available MEMS microphones of 950 μm with compliances around 3 nm Pa–1. The feasibility of realizing larger membranes with diameters of 300 μm and even higher compliances is shown, although these have lower yields. We present a process for locally growing graphene on a silicon wafer and realizing suspended membranes of patterned graphene across through-silicon holes by bulk micromachining and sacrificial layer etching, such that no transfer is required. This transfer-free method results in a 100% yield for membranes with diameters up to 155 μm on 132 fabricated drums. The device-to-device variations in the mechanical compliance in the audible range (20–20000 Hz) are significantly smaller than those in transferred membranes. With this work, we demonstrate a transfer-free method for realizing wafer-scale multilayer graphene membranes that is compatible with high-volume manufacturing. Thus, limitations of transfer-based methods for graphene microphone fabrication such as polymer contamination, crack formation, wrinkling, folding, delamination, and low-tension reproducibility are largely circumvented, setting a significant step on the route toward high-volume production of graphene microphones.

Water desalination by charged multilayer graphene membrane: A molecular dynamics simulation

Graphene, due to its unique excellent properties, is proposed as a developing method with high efficiency compared to classical water desalination methods. In this regard, charging the membrane is considered a promising and effective approach to enhance the performance of the graphene membrane. In this research, by using molecular dynamics simulations, the water desalination by charged multilayer graphene is evaluated and the influence of electric charge amount and geometric parameters, including the pore diameter and the interlayer distance, are investigated. According to the results, the multilayer nanoporous graphene with 16.35 Å pore diameter, in which the electric charge is distributed with the proper amount and particular arrangement on the carbon atoms on the periphery of the pores, shows a significant enhancement of 59.4% in the water flow rate and improves the salt rejection efficiency from 67.27% to the desired value of 94.54%, compared to the uncharged multilayer graphene with 12.6 Å pore diameter. Moreover, it is demonstrated that increasing the interlayer distance of the membrane diminishes the effect of electric charge on improving the water flux.

Preparation and Evaluation of the Polyethylene Film Deposited With a Multilayer Graphene Membrane for Tensile Properties

This study aims to improve the tensile properties of the polyethylene film deposited with a multilayer graphene membrane, in order to establish the understanding of the influence of the methane to hydrogen ratio on the tensile properties of the multilayer graphene membrane. Multilayer graphene membranes were prepared using the chemical vapor deposition method. Four types of multilayer graphene membranes were prepared with different ratios of methane to hydrogen before depositing a membrane on the polyethylene film. Experiments showed that the tensile strength of the polyethylene films with multilayer graphene deposition increased 7 times and the Young’s modulus 5 times more than that of pure polyethylene films, when the ratio of methane to hydrogen was set to 35/100 sccm. A compromise between hydrogen and methane mixture is required to achieve uniform growth of graphene. Insufficient hydrogen cannot activate the surface bound carbon that is necessary for continuous growth. Continuous and well-defined multilayer graphene was synthesized when the ratio of methane to hydrogen reached up a proper value.

Enhancing the Ultraviolet Photocurrent and Response Speed of Zinc Oxide Nanoflowers using Surface Plasmons of Gold Nanoparticles and a Graphene Membrane

The enhancement effect of the plasmonic resonance of AuNPs and a graphene membrane on the optoelectronic properties of ZnO nanoflowers is investigated. A layered structure is fabricated, consisting of ZnO nanoflowers grown on a conductive fluorine‐doped tin oxide (FTO) glass substrate, decorated with gold nanoparticles, and covered by a multilayer graphene membrane. An excellent photocurrent response as high as 1650 A W−1 and a millisecond level response time are achieved. Compared with the pristine ZnO device, which has a responsivity of 2.98 A W−1 and a response time of tens of seconds, the photocurrent response is dramatically increased by 500‐fold and the response time is decreased by over 1000‐fold. The significant enhancement of the photocurrent can be attributed to the coupling of plasmonic resonance modes of AuNPs and the graphene membrane, according to finite‐difference time‐domain simulation results; whereas the tremendous improvement on response time can be attributed to the high electron mobility of graphene. The work is essential for further study of the plasmonic enhancement of ZnO optoelectronic performance and provides a new path for the fabrication of ZnO‐based high‐responsivity, high‐speed, and low‐cost ultraviolet emission and detection devices.

On the desalination performance of multi-layer graphene membranes; A molecular dynamics study

In this study, the efficiency of a multi-layer graphene membrane with fully aligned pores was discussed in the desalination process. By changing the configurations and parameters such as pore size, pressure, thickness, ion concentration, and pore geometry, the water transfer rate has been observed from the scale 100 to 1000 L/m2 h bar, which is very close to the transfer rate of water through single-layer graphene sheet and three times larger than conventional reverse osmosis membranes. Small pores (with 0.6 nm in diameter) in the multi-layered graphene membrane pass a small water flow rate even when the high pressures. More water flow rate could be achieved in larger pores, but in the pores with diameters larger than 1.2 nm, the salt rejection is weakened. Increasing the number of layers from five to ten can completely cease the water flow in the same pore diameter. With a 40% reduction in the concentration of ions in the feed area, the water transfer rate is increased by about half. The results of this study can be useful for making a superior membrane for future water crises.

Large area continuous multilayer graphene membrane for water desalination

This manuscript reports the preparation of a large area (84 cm2) desalination membrane based on multilayers (11 nm thickness) of B,N-codoped defective graphene. The process consists of coating a porous ceramic α-Al2O3 support (100 nm pore size) with a continuous nanometric (50 nm) chitosan film containing adsorbed (NH4)3BO3. Subsequent pyrolysis in the presence of hydrogen converts chitosan into multilayer defective B,N-codoped graphene. The partial removal of B and N dopant atoms by H2 during the pyrolysis causes the generation of subnanometric pores due to atom vacancy, as determined by control experiments in the absence of this gas. A NaCl and KCl removal efficiency from brackish water higher than 95% for a permeate flux of 24.3 L m−2 h−1 at 10 bars were achieved.

Mapping nanoscale dynamic properties of suspended and supported multi-layer graphene membranes via contact resonance and ultrasonic scanning probe microscopies

Graphene’s (GR) remarkable mechanical and electrical properties—such as its Young’s modulus, low mass per unit area, natural atomic flatness and electrical conductance—would make it an ideal material for micro and nanoelectromechanical systems (MEMS and NEMS). However, the difficulty of attaching GR to supports, coupled with naturally occurring internal defects in a few layer GR can significantly adversely affect the performance of such devices. Here, we have used a combined contact resonance atomic force microscopy (CR-AFM) and ultrasonic force microscopy (UFM) approach to characterise and map with nanoscale spatial resolution GR membrane properties inaccessible to most conventional scanning probe characterisation techniques. Using a multi-layer GR plate (membrane) suspended over a round hole, we show that this combined approach allows access to the mechanical properties, internal structure and attachment geometry of the membrane providing information about both the supported and suspended regions of the system. We show that UFM allows the precise geometrical position of the supported membrane-substrate contact to be located and provides an indication of the local variation of its quality in the contact areas. At the same time, we show that by mapping the position sensitive frequency and phase response of CR-AFM response, one can reliably quantify the membrane stiffness, and image the defects in the suspended area of the membrane. The phase and amplitude of experimental CR-AFM measurements show excellent agreement with an analytical model accounting for the resonance of the combined CR-AFM probe-membrane system. The combination of UFM and CR-AFM provide a beneficial combination for the investigation of few-layer NEMS systems based on two dimensional materials.

The voltage-dependent manipulation of few-layer graphene with a scanning tunneling microscopy tip

Strain and deformation alter the electronic properties of graphene, offering the possibility to control its transport behavior. The tip of a scanning tunneling microscope is an ideal tool to mechanically perturb the system locally while simultaneously measuring the electronic response. Here we stretch few- and multi-layer graphene membranes supported on SiO2 substrates and suspended over voids. An automated approach-retraction method stably traces the graphene deflection hysteresis curve hundreds of times across four samples, measuring the voltage-dependent stretching, from which we extract the hysteresis width. Using a force-balance model, we are able to reproduce the voltage-dependent hysteretic graphene extension behavior. We directly observe a voltage-dependent interplay where electrostatic forces dominate at high voltage and van der Waals forces at low voltage. The relative contribution of each force is dependent on the graphene and tunneling resistance, giving rise to different observed voltage-dependent behavior between samples. Understanding the voltage dependence of these forces impacts scanning probe measurement of 2D materials and informs oscillating graphene device design where similar forces act from the side walls of cavities, leading towards strain engineering of layered 2D systems.

Atomistic insights into the separation mechanism of multilayer graphene membranes for water desalination.

Graphene-based membranes have been extensively explored owing to their excellent separation properties. In this paper, multiple factors regarding desalination performance were investigated by molecular dynamics (MD) simulations. These factors include the interlayer spacing distance (H), the gap width (dG), offset (O), and the number of gaps and layers in a multilayer graphene membrane (MGM). It is found that salt rejection is influenced significantly by the interlayer spacing distance owing to the largest free energy between ions and graphene sheets as well as the relatively larger size of the hydration layer around the ions. The optimal desalting parameter (dG = 1 nm, H = 0.8 nm) was selected; MGM systems based on the optimized parameter exhibited excellent salt rejection for NaCl, MgCl2 and CaCl2 solutions. These results can provide some ideas for the future design of graphene-based membranes.

Graphene membrane-based NEMS for study of interface interaction

Abstract The scheme and operational principles of graphene-based nanoelectromechanical system (NEMS) for study of interaction between graphene and surface of a sample is proposed. In such a NEMS multilayer graphene membrane bends due to van der Waals attraction between surface of graphene membrane and surface of a sample attached to a manipulator. An analysis of the NEMS total energy balance shows that the NEMS is bistable and abrupt transition between the stable states occurs if the sample is moved toward and backward the membrane. The detection of the interface distances corresponding to these transitions can be used to fit parameters of interatomic potentials for interaction between atoms of the surfaces of the graphene membrane and the sample. The analytical expression for dependences of this transition distances on NEMS sizes and parameters of the potential are derived on example of Lennard-Jones potential. For graphene-graphene interaction the transition distance is estimated to be from several nanometers to several tens nanometers for possible sizes of the proposed NEMS and thus can be measurable for example by transmission electron microscopy . Possibility of this NEMS implementation and application to study graphene-metal interaction are discussed.

Breakdown of continuum model for water transport and desalination through ultrathin graphene nanopores: insights from molecular dynamics simulations.

In the quest for identifying a graphene membrane for efficient water desalination, molecular dynamics simulations were performed for the pressure-driven flow of salty water across a multilayer graphene membrane. Water transport through the graphene membranes was tuned as a function of pore size, external pressure, and salt concentration. The results predicted that water permeability through the graphene channel (width of h = 7 Å) is two orders of magnitude higher than that through the conventional thin film membranes. The breaking of continuum assumption in graphene nanopores was captured by the appearance of a layered water structure and plug-like velocity profiles. Furthermore, the fluidity under nano confinement of graphene was examined in terms of shear viscosity, friction coefficient, and slip length, which were found to depend on the separation of the confining graphene walls and the external pressure. Furthermore, the MD results revealed that the macroscopic water flux through the graphene nanopores can be linked to the microscopic diffusion of water. The calculated viscosity and diffusion coefficient under the graphene pores did not follow the Stokes-Einstein relation, indicating the failure of the hydrodynamic theory. The confined state of water in the graphene pores was also explored via the translational density of states (TDOS) and entropy, which displayed a significant change in the translational entropy with change in the pore size and applied pressure and thus revealed the interconnectivity of the structure, dynamics, thermodynamics, and hydrodynamics of water in the graphene nanopores. Such a linking of the microscopic parameters with the macroscopic profiles provides direct evidence to the experiments of pressure-driven flow through the graphene membranes and might be helpful in examining the performance of graphene membranes for the factors that have large implications on their application in the reverse osmosis (RO) process and other biological channels.

Molecular Dynamics Study on the Reverse Osmosis Using Multilayer Porous Graphene Membranes.

In this study, the reverse osmosis (RO) of a salt solution was investigated using a molecular dynamics method to explore the performance of a multilayer porous graphene membrane. The effects of the salt solution concentration, pressure, layer separation and pore offset on the RO performance of the membrane were investigated and the influences of the number of layers and the gradient structure were determined. The results show that as the salt solution concentration increases, the energy barrier of the water molecules passing through the bilayer porous graphene membranes changes slightly, indicating that the effect of the water flux on the membrane can be ignored. The salt rejection performance of the membrane improves with an increase in the concentration of the salt solution. When the pressure is increased, the energy barrier decreases, the water flux increases and the salt rejection decreases. When the layer separation of the bilayer porous graphene membrane is the same as the equilibrium spacing of the graphene membrane, the energy barrier is the lowest and the membrane water flux is the largest. The energy barrier of the bilayer porous graphene membrane increases with increasing layer separation, resulting in a decrease in the water flux of the membrane. The salt rejection increases with increasing layer separation. The water flux of the membrane decreases as the energy barrier increases with increasing pore offset and the salt rejection increases. The energy barrier effect is more pronounced for a larger number of graphene layers and the water flux of the membrane decreases because it is more difficult for the water molecules to pass through the porous graphene membrane. However, the salt rejection performance improves with the increase in the number of layers. The gradient pore structure enhances the energy barrier effect of the water molecules permeating through the membrane and the water flux of the membrane decreases. The salt rejection performance is improved by the gradient pore structure. The research results provide theoretical guidance for research on the RO performance of porous graphene membranes and the design of porous graphene membranes.

Influence of electric fields on the efficiency of multilayer graphene membrane.

Multilayer graphene membranes could be considered as an efficient membrane in water desalination processes based on the reverse osmosis (RO) method. In this study, we designed multilayer graphene channels using the molecular dynamics (MD) simulation approach. The effects of different parameters, such as channel width and length, and the pressure on the operation of the designed channels were examined, in the absence and presence of electric fields with various amplitudes and directions. The results indicated that the ion separation and water flow through the channels were modified under the application of the electric fields. Additionally, it has been shown that salt rejection and water flow could be controlled by the channel's structural parameters mentioned above. The obtained results of this study at the molecular level can improve the knowledge of designing membranes for water purification processes. Graphical abstract Using MD method a multilayer graphene membrane was designed to separate Na+ and Cl- ions from a NaCl solution by the aid of external electric field, which can significantly effect the membrane operation.