Freestanding Graphene Membranes Research Articles

Growth of Highly Oriented (VNbMoTaW)S2 Layers.

Compositional tunability, an indispensable parameter for modifying the properties of materials, can open up new applications for van der Waals (vdW) layered materials such as transition-metal dichalcogenides (TMDCs). To date, multielement alloy TMDC layers are obtained via exfoliation from bulk polycrystalline powders. Here, we demonstrate direct deposition of high-entropy alloy disulfide, (VNbMoTaW)S2, layers with controllable thicknesses on free-standing graphene membranes and on bare and hBN-covered Al2O3(0001) substrates via ultra-high-vacuum reactive dc magnetron sputtering of the VNbMoTaW target in Kr and H2S gas mixtures. Using a combination of density functional theory calculations, Raman spectroscopy, X-ray diffraction, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, we determine that the as-deposited layers are single-phase, 2H-structured, and 0001-oriented (V0.10Nb0.16Mo0.19Ta0.28W0.27)S2.44. Our synthesis route is general and applicable for heteroepitaxial growth of a wide variety of TMDC alloys and potentially other multielement alloy vdW compounds with the desired compositions.

(Invited) Harvesting Energy from Freestanding Graphene Thermal Fluctuations

Highly flexible, electrically conductive freestanding graphene membranes hold great promise for vibration energy harvesting applications. In this study, we present numerical results for a graphene ripple treated as a Brownian particle coupled to an energy harvesting circuit. When circuit and particle are at the same temperature, the second law forbids harvesting energy from the thermal motion of the Brownian particle, even if the circuit contains a rectifying diode. However, when the circuit contains a junction followed by two diodes wired in opposition, a special ultraslow convergence to equilibrium may be imposed. Detailed balance is temporarily broken as current flows between the two diodes and charges two storage capacitors. The harvested energy is taken from the surrounding thermal environment, and we show that the system obeys the first and second laws of thermodynamics. Experimental results will also be presented for a custom designed integrated circuit built by TSMC combined with post-processing lithography on bare die followed by graphene suspension. Figure 1

Mirror buckling analysis of freestanding graphene membranes by coarse-grained molecular dynamics method

Up to now, the analysis has rarely been conducted of thermal-mechanical mirror buckling behavior of freestanding graphene membranes discovered in scan tunneling microscope experiments. One of the potential applications of the out-of-plane deformational behavior of graphene membranes is energy harvesting system. Whether in the experiments or for energy harvesting systems, the size of graphene membrane needs to be down to micron scale. According to previous researches, traditional molecular dynamics method is a suitable method to characterize nano-scale mirror buckling. However, owing to the limit of algorithm, when dealing with micro size model by molecular dynamics method, two problems arise: low computational efficiency and too long calculation time. Therefore, for analyzing the mirror buckling of micro size graphene membranes, the coarse-grained molecular dynamics method is utilized in this work. Graphene membranes with a fan-shaped cross section and various depth-span ratios are under mechanical or thermal loads. Effects of each factor on the mirror buckling are investigated. The calculations indicate that for graphene membranes with various depth-span ratios under mechanical load mirror buckling can be observed. And the critical loading increases with the depth-span ratio increasing. Under thermal load graphene membranes only with low depth-span ratios can undergo complete flipping phenomenon. For high depth-span ratio graphene, the center height decreases with temperature rising. However, it is hard to flip over completely. The understanding of the effects of various factors on the mirror buckling phenomenon of graphene membranes can provide theoretical guidance for designing the energy harvesting systems.

Macro and micro methodologies for thermomechanical mirror buckling of freestanding graphene membranes

The thermal mirror buckling behavior of freestanding graphene, which is a particular phenomenon observed in scanning tunneling microscope experiments, has not been analyzed systematically in the existing literature. However, because loads imposed by the scanning tunneling microscope tip are both mechanical and thermal, it is appropriate to regard this behavior as thermomechanical mirror buckling. The mirror buckling behavior is related to geometrical characteristics of a model, such as structure curvature. Therefore, appropriate analytical methods should be determined before examining the deep-seated origin of mirror buckling of graphene membranes. In this study, three methods are used separately in both the macroscale and microscale analyses, namely, continuum mechanics, molecular structural mechanics, and molecular dynamics, to analyze the mirror buckling of graphene. A comparison of the theoretical analysis and experimental results shows that, of the three methods, the molecular dynamics method is the most effective and suitable method for examining the thermomechanical mirror buckling behavior of freestanding graphene. The effects of the load magnitudes and graphene membrane radius on the deflection are also analyzed.

Array of Graphene Variable Capacitors on 100 mm Silicon Wafers for Vibration-Based Applications.

Highly flexible, electrically conductive freestanding graphene membranes hold great promise for vibration-based applications. This study focuses on their integration into mainstream semiconductor manufacturing methods. We designed a two-mask lithography process that creates an array of freestanding graphene-based variable capacitors on 100 mm silicon wafers. The first mask forms long trenches terminated by square wells featuring cone-shaped tips at their centers. The second mask fabricates metal traces from each tip to its contact pad along the trench and a second contact pad opposite the square well. A graphene membrane is then suspended over the square well to form a variable capacitor. The same capacitor structures were also built on 5 mm by 5 mm bare dies containing an integrated circuit underneath. We used atomic force microscopy, optical microscopy, and capacitance measurements in time to characterize the samples.

Evidence of synergistic electrocatalysis at a cobalt oxide–graphene interface through nanochemical mapping of scanning transmission X-ray microscopy

Free-standing graphene membranes are a promising candidate for use as in-situ environmental windows in X-ray (electron) microscopy. In this study, graphene membranes were used as the working electrode, and cobalt nanoparticles (Co-NPs) were grown directly on top of the graphene through electrochemical deposition for interfacial variation. The electronic structure and the chemical bonding states of the Co-NPs and the graphene materials were examined by using the high spatial resolution and element-specific properties of scanning transmission X-ray microscopy. X-ray absorption spectra of C, O, and Co revealed that the Co-NP size increased in accordance with the oxidation state (Co0/2+/3+), depending on the configuration of carbon bonding (C–C/C–OH/HO–C=O/O–C(O)–O/C=O–like state). We conducted a spectral comparison of the dipped graphene and the electrodeposited Co–graphene sample, which revealed an increase in C–OH formation before Co-NPs growth. In addition to electron transfer and electrochemical reduction, the oxidation evolution from C–OH to HO–C=O (or a defect) and the O–C(O)–O (or C=O) state paralleled the increase in Co-NPs size. We curve-fitted the results to demonstrate the reduction in chemical structure from mixing Co2+/3+ to Co3+/2+/0, and to explain the interfacial modulation and the unique metal Co0 layer on the surface of the Co-NPs. Our results provide information for the design of a reliable graphene window and offer an example for the interpretation of experimental X-ray (electron) microscopy.

Sensitive capacitive pressure sensors based on graphene membrane arrays

The high flexibility, impermeability and strength of graphene membranes are key properties that can enable the next generation of nanomechanical sensors. However, for capacitive pressure sensors, the sensitivity offered by a single suspended graphene membrane is too small to compete with commercial sensors. Here, we realize highly sensitive capacitive pressure sensors consisting of arrays of nearly ten thousand small, freestanding double-layer graphene membranes. We fabricate large arrays of small-diameter membranes using a procedure that maintains the superior material and mechanical properties of graphene, even after high-temperature annealing. These sensors are readout using a low-cost battery-powered circuit board, with a responsivity of up to 47.8 aF Pa−1 mm−2, thereby outperforming the commercial sensors.

Origin of Non‐Gaussian Velocity Distribution Found in Freestanding Graphene Membranes

In this study, an analytic derivation is made for the truncated Cauchy‐Lorentz velocity distribution experimentally observed in freestanding graphene membranes. Three methods are used and discussed, including the Fokker‐Planck‐Kolmogorov equation, the maximum nonsymmetric entropy principle, and the Bayesian inference. From these results, a physical mechanism is provided for the non‐Gaussian velocity distribution in terms of carbon atom arrangement in freestanding graphene. Moreover, a new theoretical foundation is proposed for future studies of the anomalous dynamics of carbon atoms in graphene membranes.

Electrically Conductive Thin Films Derived from Bulk Graphite and Liquid–Liquid Interface Assembly

AbstractConfinement of particles to fluid–fluid interfaces provides a unique interaction environment, allowing the directed assembly of particles using lateral capillary forces. The particle laden interfacial layers can be deposited onto a variety of substrates for the fabrication of thin film coatings, designed to have structural or functional properties resulting from the interface‐specific structures. For the fabrication of electrically conducting films and specifically graphene‐based coatings, interfacial deposition techniques could offer a low cost and environmentally favorable alternative to conventional gas phase production methods, with possibly a broader choice of substrates. In this work liquid‐phase electrochemical exfoliation is used to produce platelets of few‐layer graphene from bulk graphite which are directly introduced to the water–air interface. These interfaces are characterized through compression experiments and interfacial shear rheology to probe the mechanical properties of the resulting monolayer films and identify mechanical percolation. Further, in situ measurements of electrical conductivity are integrated as a direct indication of electrical percolation. This directly verifies sufficient film quality as an important characteristic of the deposition process. Once deposited on a solid substrate, these films retain their electrical conductivity. But the mechanical properties of the films allow for facile production of freestanding microporous graphene membranes.

Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS2 Films.

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS2) films. To this end, we deposit a model heterostructure of thin amorphous MoS2 films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS2 into crystalline MoS2 domains, thereby emulating widely used elevated temperature MoS2 synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS2 films in situ and at the atomic scale. Our observations suggest that during MoS2 processing, various MoS2 polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS2 layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS2 crystallization can be influenced by a weak MoS2/graphene support epitaxy. Our atomic-scale in situ approach thereby visualizes multiple fundamental processes that underlie the varied MoS2 morphologies observed in previous ex situ growth and processing work. Our work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.

Functional reduced graphene oxide-based membranes with selective ion transport channels for zwitterionic ions separation based on the pH gradient.

In this work, we report a method for the fabrication of a functional free-standing graphene membrane (FFGM) with high mechanical strength, enlarged interlayer spacing and ion channels for zwitterionic ions separation. To obtain the FFGM, the anionic dye Eosin Y (EY) was introduced into a graphene oxide (GO) and hydroquinone (HQ) mixture to prepare functional graphene-based membranes on Cu foil using simply a drop-casting method. In comparison with a GO membrane, the molar flux and the mechanical strength of the FFGM were dramatically increased. The FFGM was then equipped on custom-built glass reservoirs for zwitterionic amino acids (AAs) separation based on the inner pH gradient, which was formed by controlling H+ and OH- (in the feed and receiver solution) migration in rGO/GO sheets via an external electric field. With the help of the inner pH gradient and external electric field, AAs could change their charge behaviors. The ionized AAs transport through the FFGM and finally separation was realized.

A convenient method for large-scale STM mapping of freestanding atomically thin conductive membranes.

Two-dimensional atomically flat sheets with a high flexibility are very attractive as ultrathin membranes but are also inherently challenging for microscopic investigations. We report on a method using Scanning Tunneling Microscopy (STM) under ultra-high vacuum conditions for large-scale mapping of several-micrometer-sized freestanding single and multilayer graphene membranes. This is achieved by operating the STM at unusual parameters. We found that large-scale scanning on atomically thin membranes delivers valuable results using very high tip-scan speeds combined with high feedback-loop gain and low tunneling currents. The method ultimately relies on the particular behavior of the freestanding membrane in the STM which is much different from that of a solid substrate.

Selective protein transport through ultra-thin suspended reduced graphene oxide nanopores.

The nanoporous free-standing graphene membrane is of great interest in high performance separation technology. In particular, the separation of biological molecules with similar sizes is one of the key challenges in the purification of biomaterials. Here, we report a reliable, cost-effective, and facile method for the fabrication of a graphene-based nanosieve and its application in the separation of similar-size proteins. A suspended reduced graphene oxide (rGO) nanosieve with ultra-thin, large-area, well-ordered, and dense 15 nm-sized pores was fabricated using block copolymer (BCP) lithography. The fabricated 5 nm-ultrathin nanosieve with an area of 200 μm × 200 μm (an ultra-high aspect ratio of ∼40 000) endured pressure up to 1 atm, and effectively separated hemoglobin (Hb) from a mixture of hemoglobin and immunoglobulin G (IgG), the common proteins in human blood, in a highly selective and rapid manner. The use of the suspended rGO nanosieve is expected to provide a simple and manufacturable platform for practical biomolecule separation offering high selectivity and a large throughput.

Monolayer graphene as dissipative membrane in an optical resonator

We experimentally demonstrate coupling of an atomically thin, free-standing graphene membrane to an optical cavity. By changing the position of the membrane along the standing-wave field of the cavity, we tailor the dissipative coupling between the membrane and the cavity, and we show that the dissipative coupling can outweigh the dispersive coupling. Such a system, for which controlled dissipation prevails dispersion, will prove useful for novel laser-cooling schemes in optomechanics. In addition, we have determined the continuous-wave optical damage threshold of free-standing monolayer graphene of $$1.8(4)~\hbox {MW}/\hbox {cm}^2$$ at 780 nm.

Free-standing graphene slit membrane for enhanced desalination

This study considers two novel ideas to further explore and enhance the graphene membrane for desalination. Firstly, while earlier molecular dynamics (MD) simulations studies have used frozen membranes, free-standing membrane is considered here. Since 2D membranes are usually embedded on porous support in the experimental reverse osmosis (RO) process, the free-standing membrane can more accurately model the behavior expected during operation. This study showed, using MD simulations, that a free-standing nanoporous graphene membrane can provide a higher salt rejection, but lower water permeability as compared to frozen membrane. Secondly, the performance of a slit membrane as compared to a membrane with circular pore is studied. At a pressure of 268 MPa, the critical diameter of circular pore that can maintain perfect salt rejection is found to be 5 Å and the critical size for a slit is determined to be 2.28 Å. It is shown that a slit membrane at its critical size can achieve water flux 3.5 times higher than a membrane with circular pore at its critical radius. This finding highlights the importance of slits over circular pores which can potentially widen the options for the design and fabrication of 2D graphene membranes for experimental verification of RO.

Anomalous Dynamical Behavior of Freestanding Graphene Membranes.

We report subnanometer, high-bandwidth measurements of the out-of-plane (vertical) motion of atoms in freestanding graphene using scanning tunneling microscopy. By tracking the vertical position over a long time period, a 1000-fold increase in the ability to measure space-time dynamics of atomically thin membranes is achieved over the current state-of-the-art imaging technologies. We observe that the vertical motion of a graphene membrane exhibits rare long-scale excursions characterized by both anomalous mean-squared displacements and Cauchy-Lorentz power law jump distributions.

Ultra-large suspended graphene as a highly elastic membrane for capacitive pressure sensors.

In this work, we fabricate ultra-large suspended graphene membranes, where stacks of a few layers of graphene could be suspended over a circular hole with a diameter of up to 1.5 mm, with a diameter to thickness aspect ratio of 3 × 10(5), which is the record for free-standing graphene membranes. The process is based on large crystalline graphene (∼55 μm) obtained using a chemical vapor deposition (CVD) method, followed by a gradual solvent replacement technique. Combining a hydrogen bubbling transfer approach with thermal annealing to reduce polymer residue results in an extremely clean surface, where the ultra-large suspended graphene retains the intrinsic features of graphene, including phonon response and an enhanced carrier mobility (200% higher than that of graphene on a substrate). The highly elastic mechanical properties of the graphene membrane are demonstrated, and the Q-factor under 2 MHz stimulation is measured to be 200-300. A graphene-based capacitive pressure sensor is fabricated, where it shows a linear response and a high sensitivity of 15.15 aF Pa(-1), which is 770% higher than that of frequently used silicon-based membranes. The reported approach is universal, which could be employed to fabricate other suspended 2D materials with macro-scale sizes on versatile support substrates, such as arrays of Si nano-pillars and deep trenches.

Measuring the proton selectivity of graphene membranes

By systematically studying the proton selectivity of free-standing graphene membranes in aqueous solutions, we demonstrate that protons are transported by passing through defects. We study the current-voltage characteristics of single-layer graphene grown by chemical vapour deposition (CVD) when a concentration gradient of HCl exists across it. Our measurements can unambiguously determine that H+ ions are responsible for the selective part of the ionic current. By comparing the observed reversal potentials with positive and negative controls, we demonstrate that the as-grown graphene is only weakly selective for protons. We use atomic layer deposition to block most of the defects in our CVD graphene. Our results show that a reduction in defect size decreases the ionic current but increases proton selectivity.

Probing the Young’s modulus and Poisson’s ratio in graphene/metal interfaces and graphite: a comparative study

By analyzing phonon dispersion, we have evaluated the average Young’s modulus and Poisson’s ratio in graphite and in graphene grown on Ru(0001), Pt(111), Ir(111), Ni(111), and BC3/NbB2(0001). In both flat and corrugated graphene sheets and in graphite, we find a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. The unique exception is graphene/Ni(111), for which we find different values because of the stretching of C-C bonds occurring in the commensurate overstructure (0.36 and 310 N/m for the Poisson’s ratio and Young’s modulus, respectively). Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. The high crystalline quality of graphene grown on metal substrates leads to macroscopic samples with high tensile strength and bending flexibility for use in technological applications such as electromechanical devices and carbon-fiber reinforcements.

Free-Standing and Transparent Graphene Membrane of Polyhedron Box-Shaped Basic Building Units Directly Grown Using a NaCl Template for Flexible Transparent and Stretchable Solid-State Supercapacitors.

Transparency has never been integrated into freestanding flexible graphene paper (FF-GP), although FF-GP has been discussed extensively, because a thin transparent graphene sheet will fracture easily when the template or substrate is removed using traditional methods. Here, transparent FF-GP (FFT-GP) was developed using NaCl as the template and was applied in transparent and stretchable supercapacitors. The capacitance was improved by nearly 1000-fold compared with that of the laminated or wrinkled chemical vapor deposition graphene-film-based supercapacitors.