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

Piezoresistive Platinum Diselenide Pressure Sensors with Reliable High Sensitivity and Their Integration into Complementary Metal-Oxide-Semiconductor Circuits

Membrane-based sensors are an important market for microelectromechanical systems (MEMS). Two-dimensional (2D) materials, with their low mass, are excellent candidates for suspended membranes to provide high sensitivity, small footprint sensors. The present work demonstrates pressure sensors employing large-scale-synthesized 2D platinum diselenide (PtSe2) films as piezoresistive membranes supported only by a thin polymer layer. We investigate three different synthesis methods with contrasting growth parameters and establish a reliable high yield fabrication process for suspended PtSe2/PMMA membranes across sealed cavities. The pressure sensors reproducibly display sensitivities above 6 × 10-4 kPa-1. We show that the sensitivity clearly depends on the membrane diameter and the piezoresistive gauge factor of the PtSe2 film. Reducing the total device size by decreasing the number of membranes within a device leads to a significant increase in the area-normalized sensitivity. This allows the manufacturing of pressure sensors with high sensitivity but a much smaller device footprint than the current state-of-the-art MEMS technology. We further integrate PtSe2 pressure sensors with CMOS technology, improving the technological readiness of PtSe2-based MEMS and NEMS devices.

Modeling and Simulation of 2D Transducers Based on Suspended Graphene-Based Heterostructures in Nanoelectromechanical Pressure Sensors

Graphene-based two-dimensional (2D) heterostructures exhibit excellent mechanical and electrical properties, which are expected to exhibit better performances than graphene for nanoelectromechanical pressure sensors. Here, we built pressure sensor models based on suspended heterostructures of graphene/h-BN, graphene/MoS2, and graphene/MoSe2 by using COMSOL Multiphysics finite element software. We found that suspended circular 2D membranes show the best sensitivity to pressures compared to rectangular and square ones. We simulated the deflections, strains, resonant frequencies, and Young's moduli of suspended graphene-based heterostructures under the conditions of different applied pressures and geometrical sizes, built-in tensions, and the number of atomic layers of 2D membranes. The Young's moduli of 2D heterostructures of graphene, graphene/h-BN, graphene/MoS2, and graphene/MoSe2 were estimated to be 1.001 TPa, 921.08, 551.11, and 475.68 GPa, respectively. We also discuss the effect of highly asymmetric cavities on device performance. These results would contribute to the understanding of the mechanical properties of graphene-based heterostructures and would be helpful for the design and manufacture of high-performance NEMS pressure sensors.

Graphene MEMS and NEMS

Graphene is being increasingly used as an interesting transducer membrane in micro- and nanoelectromechanical systems (MEMS and NEMS, respectively) due to its atomical thickness, extremely high carrier mobility, high mechanical strength, and piezoresistive electromechanical transductions. NEMS devices based on graphene feature increased sensitivity, reduced size, and new functionalities. In this review, we discuss the merits of graphene as a functional material for MEMS and NEMS, the related properties of graphene, the transduction mechanisms of graphene MEMS and NEMS, typical transfer methods for integrating graphene with MEMS substrates, methods for fabricating suspended graphene, and graphene patterning and electrical contact. Consequently, we provide an overview of devices based on suspended and nonsuspended graphene structures. Finally, we discuss the potential and challenges of applications of graphene in MEMS and NEMS. Owing to its unique features, graphene is a promising material for emerging MEMS, NEMS, and sensor applications.

Cyclododecane-based high-intactness and clean transfer method for fabricating suspended two-dimensional materials

The high-intactness and ultraclean fabrication of suspended 2D materials has always been a challenge due to their atomically thin nature. Here, we present a universal polymer-free transfer approach for fabricating suspended 2D materials by using volatile micro-molecule cyclododecane as the transfer medium, thus ensuring the ultraclean and intact surface of suspended 2D materials. For the fabricated monolayer suspended graphene, the intactness reaches 99% for size below 10 µm and suspended size reaches 36 µm. Owing to the advantages of ultra-cleanness and large size, the thermal conductivity reaches 4914 W m - 1 K - 1 at 338 K. Moreover, this strategy can also realize efficient batch transfer of suspended graphene and is applicable for fabricating other 2D suspended materials such as MoS2. Our research not only establishes foundation for potential applications and investigations of intrinsic properties of large-area suspended 2D materials, but also accelerates the wide applications of suspended graphene grid in ultrahigh-resolution TEM characterization.

Ultralarge suspended and perforated graphene membranes for cell culture applications

With its high mechanical strength and its remarkable thermal and electrical properties, suspended graphene has long been expected to find revolutionary applications in optoelectronics or as a membrane in nano-devices. However, the lack of efficient transfer and patterning processes still limits its potential. In this work, we report an optimized anthracene-based transfer process to suspend few layers of graphene (1-, 2- and 4-layers) in the millimeter range (up to 3 mm) with high reproducibility. We have explored the advantages and limitations for patterning of these membranes with micrometer-resolution by focused ion beam (FIB) and picosecond pulsed laser ablation techniques. The FIB approach offers higher patterning resolution but suffers from the low throughput. We demonstrate that cold laser ablation is a fast and flexible method for micro-structuring of suspended graphene. One promising field of application of ultimately thin, microporous graphene membranes is their use as next-generation cell culture supports as alternative to track-etched polymer membranes, which often exhibit poor permeability and limited cell-to-cell communication across the membranes. To this end, we confirmed good adhesion and high viability of placental trophoblast cells cultivated on suspended porous graphene membranes without rupturing of the membranes. Overall, there is high potential for the further development of ultrathin suspended graphene membranes for many future applications, including their use for biobarrier cell culture models to enable predictive transport and toxicity assessment of drugs, environmental pollutants, and nanoparticles.

Carbon Nanotube Network Topology-Enhanced Iontronic Capacitive Pressure Sensor with High Linearity and Ultrahigh Sensitivity

Flexible capacitive pressure sensors with high sensitivity over a wide pressure range are highly anticipated in the fields of tactile perception and physiological signal monitoring. However, despite the introduction of microstructures on the electrolyte layer, the deformability is still limited due to the size limitation of the microstructures, making it difficult to significantly improve the sensitivity of iontronic capacitive pressure sensors (ICPSs). Here, we propose an innovative strategy of combining carbon nanotubes (CNTs) topological networks and ionic hydrogel micropyramid array microstructures that can significantly enhance the sensitivity of flexible ICPSs for ultrasensitive pressure detection. Compared with other previously reported ICPSs, the sensor developed in this work exhibits an unprecedented sensitivity (Smin > 1050 kPa-1) and a high linear response (R2 > 0.99) in a wide pressure range (0.03-28 kPa) enabled by CNT percolation networks inside the microstructred electrolyte layer. This ultrasensitive and flexible ICPS also can effectively detect pressure from a variety of sources, including sound waves, lightweight objects, and tiny physiological signals, such as pulse rate and heartbeat. This work provides a general strategy to achieve an ICPS with both broader pressure-response range and higher sensitivity, which provides a stable and efficient way for a low-cost, scalable sensor for sensitive tactile sensing in human-computer interaction applications.

Significant Enhanced Mechanical Properties of Suspended Graphene Film by Stacking Multilayer CVD Graphene Films

Suspended graphene film is of great significance for building high-performance electrical devices. However, fabricating large-area suspended graphene film with good mechanical properties is still a challenge, especially for the chemical vapor deposition (CVD)-grown graphene films. In this work, the mechanical properties of suspended CVD-grown graphene film are investigated systematically for the first time. It is found that monolayer graphene film is hard to maintain on circular holes with a diameter of tens of micrometers, which can be improved greatly by increasing the layer of graphene films. The mechanical properties of CVD-grown multilayer graphene films suspended on a circular hole with a diameter of 70 µm can be increased by 20%, and multilayer graphene films prepared by layer-layer stacking process can be increased by up to 400% for the same size. The corresponding mechanism was also discussed in detail, which might pave the way for building high-performance electrical devices based on high-strength suspended graphene film.

Mechanically Stable Ultrathin Layered Graphene Nanocomposites Alleviate Residual Interfacial Stresses: Implications for Nanoelectromechanical Systems

Advanced nanoelectromechanical systems made from polymer dielectrics deposited onto 2D-nanomaterials such as graphene are increasingly popular as pressure and touch sensors, resonant sensors, and capacitive micromachined ultrasound transducers (CMUTs). However, durability and accuracy of layered nanocomposites depend on the mechanical stability of the interface between polymer and graphene layers. Here we used molecular dynamics computer simulations to investigate the interface between a sheet of graphene and a layer of parylene-C thermoplastic polymer during large numbers of high-frequency (MHz) cycles of bending relevant to the operating regime. We find that important interfacial sliding occurs almost immediately in usage conditions, resulting in more than 2% expansion of the membrane, a detrimental mechanism which requires repeated calibration to maintain CMUTs accuracy. This irreversible mechanism is caused by relaxation of residual internal stresses in the nanocomposite bilayer, leading to the emergence of self-equilibrated tension in the polymer and compression in the graphene. It arises as a result of deposition-polymerization processing conditions. Our findings demonstrate the need for particular care to be exercised in overcoming initial expansion. The selection of appropriate materials chemistry including low electrostatic interactions will also be key to their successful application as durable and reliable devices.

Fabrication of a 100 × 100 mm2 nanometer-thick graphite pellicle for extreme ultraviolet lithography by a peel-off and camphor-supported transfer approach

An extreme ultraviolet (EUV) lithography pellicle is used to physically protect a mask from contaminants during the EUV exposure process and needs to have a high EUV transmittance. The EUV pellicle should be fabricated using a freestanding thin film with several tens of nanometer thickness in an area of 110 × 142 mm², which is a challenging task. Here, we propose a peel-off approach to directly detach the nanometer-thick graphite film (NGF)/Ni film from SiO₂/Si wafer and significantly shorten the etching time of the Ni film. Combined with the residue-damage-free transfer method that used camphor as a supporting layer, we successfully fabricated a large-area (100 × 100 mm²) NGF pellicle with a thickness of ∼20 nm, and an EUV transmittance of ∼87.2%.

Nano-optomechanical Resonators for Sensitive Pressure Sensing

Nanomechanical sensors made from suspended graphene are sensitive to pressure changes. However, these devices typically function by obtaining an electrical signal based on the static displacement of a suspended graphene membrane and so, in practice, have limited sensitivity and operational range. The present work demonstrates an optomechanical Au/graphene membrane-based gas pressure sensor with ultrahigh sensitivity. This sensor comprises a suspended Au/graphene membrane appended to a section of hollow-core fiber to form a sealed Fabry-Pérot cavity. In contrast to conventional nanomechanical pressure sensors, pressure changes are monitored via resonant sensing with an optical readout. A miniature pressure sensor based on this principle was able to detect an ultrasmall pressure difference of 1 × 10-7 mbar in the ultrahigh-vacuum region with a pressure range of 4.1 × 10-5 to 8.3 × 10-6 mbar. Furthermore, this pressure sensor can work over an extended pressure range of 7 × 10-6 mbar to 1000 mbar at room temperature, outperforming commercial pressure sensors. Similar results were obtained using both the fundamental and higher-order resonant frequencies but with the latter providing improved sensitivity. This sensor has a wide range of potential applications, including indoor navigation, altitude monitoring, and motion detection.

Green syntheses of graphene and its applications in internet of things (IoT)-a status review

Internet of Things (IoT) is a trending technological field that converts any physical object into a communicable smarter one by converging the physical world with the digital world. This innovative technology connects the device to the internet and provides a platform to collect real-time data, cloud storage, and analyze the collected data to trigger smart actions from a remote location via remote notifications, etc. Because of its wide-ranging applications, this technology can be integrated into almost all the industries. Another trending field with tremendous opportunities is Nanotechnology, which provides many benefits in several areas of life, and helps to improve many technological and industrial sectors. So, integration of IoT and Nanotechnology can bring about the very important field of Internet of Nanothings (IoNT), which can re-shape the communication industry. For that, data (collected from trillions of nanosensors, connected to billions of devices) would be the 'ultimate truth', which could be generated from highly efficient nanosensors, fabricated from various novel nanomaterials, one of which is graphene, the so-called 'wonder material' of the 21st century. Therefore, graphene-assisted IoT/IoNT platforms may revolutionize the communication technologies around the globe. In this article, a status review of the smart applications of graphene in the IoT sector is presented. Firstly, various green synthesis of graphene for sustainable development is elucidated, followed by its applications in various nanosensors, detectors, actuators, memory, and nano-communication devices. Also, the future market prospects are discussed to converge various emerging concepts like machine learning, fog/edge computing, artificial intelligence, big data, and blockchain, with the graphene-assisted IoT field to bring about the concept of 'all-round connectivity in every sphere possible'.

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.

Strain-Based Chemiresistive Polymer-Coated Graphene Vapor Sensors

Suspended chemiresistive graphene sensors have been fabricated using well-established nanofabrication techniques to generate sensors that are highly sensitive to pyridine and with excellent discrimination between polar and nonpolar analytes. When coated with a polymer surface layer and suspended on 3-D patterned glass electrodes, a hybrid combination of polymer and graphene yields chemiresistive vapor sensors. Expansion and contraction of the polymer layer produces strain on the suspended graphene (Gr). Hence, when organic vapors permeate into the polymer layer, the high gauge factor of the graphene induces substantial electrical resistive changes as folds and creases are induced in the graphene. The hybrid suspended polymer/Gr sensor exhibits substantial responses to polar organic vapors, especially pyridine, while also exhibiting reversibility and increased discrimination between polar and nonpolar analytes compared to previous approaches. This sensor design also allows for potential tunability in the types of polymers used for the reactive surface layer, allowing for use in a variety of potential applications.

Robustness of large-area suspended graphene under interaction with intense laser

Graphene is known as an atomically thin, transparent, highly electrically and thermally conductive, light-weight, and the strongest 2D material. We investigate disruptive application of graphene as a target of laser-driven ion acceleration. We develop large-area suspended graphene (LSG) and by transferring graphene layer by layer we control the thickness with precision down to a single atomic layer. Direct irradiations of the LSG targets generate MeV protons and carbons from sub-relativistic to relativistic laser intensities from low contrast to high contrast conditions without plasma mirror, evidently showing the durability of graphene.

Graphene-enabled wearable sensors for healthcare monitoring

Wearable sensors in healthcare monitoring have recently found widespread applications in biomedical fields for their non- or minimal-invasive, user-friendly and easy-accessible features. Sensing materials is one of the major challenges to achieve these superiorities of wearable sensors for healthcare monitoring, while graphene-based materials with many favorable properties have shown great efficiency in sensing various biochemical and biophysical signals. In this paper, we review state-of-the-art advances in the development and modification of graphene-based materials (i.e., graphene, graphene oxide and reduced graphene oxide) for fabricating advanced wearable sensors with 1D (fibers), 2D (films) and 3D (foams/aerogels/hydrogels) macroscopic structures. We summarize the structural design guidelines, sensing mechanisms, applications and evolution of the graphene-based materials as wearable sensors for healthcare monitoring of biophysical signals (e.g., mechanical, thermal and electrophysiological signals) and biochemical signals from various body fluids and exhaled gases. Finally, existing challenges and future prospects are presented in this area.

A Review on the Applications of Graphene in Mechanical Transduction

A pressing need to develop low-cost, environmentally friendly, and sensitive sensors has arisen with the advent of the always-connected paradigm of the internet-of-things (IoT). In particular, mechanical sensors have been widely studied in recent years for applications ranging from health monitoring, through mechanical biosignals, to structure integrity analysis. On the other hand, innovative ways to implement mechanical actuation have also been the focus of intense research in an attempt to close the circle of human-machine interaction, and move toward applications in flexible electronics. Due to its potential scalability, disposability, and outstanding properties, graphene has been thoroughly studied in the field of mechanical transduction. The applications of graphene in mechanical transduction are reviewed here. An overview of sensor and actuator applications is provided, covering different transduction mechanisms such as piezoresistivity, capacitive sensing, optically interrogated displacement, piezoelectricity, triboelectricity, electrostatic actuation, chemomechanical and thermomechanical actuation, as well as thermoacoustic emission. A critical review of the main approaches is presented within the scope of a wider discussion on the future of this so-called wonder material in the field of mechanical transduction.

Defects Produced during Wet Transfer Affect the Electrical Properties of Graphene

Graphene has been widely used due to its excellent electrical, mechanical and chemical properties. Defects produced during its transfer process will seriously affect the performance of graphene devices. In this paper, single-layer graphene was transferred onto glass and silicon dioxide (SiO2) substrates by wet transfer technology, and the square resistances thereof were tested. Due to the different binding forces of the transferred graphene surfaces, there may have been pollutants present. PMMA residues, graphene laminations and other defects that occurred in the wet transfer process were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. These defects influenced the square resistance of the produced graphene films, and of these defects, PMMA residue was the most influential; square resistance increased with increasing PMMA residue.

Mechanical sensors based on two-dimensional materials: Sensing mechanisms, structural designs and wearable applications

Compared with bulk materials, atomically thin two-dimensional (2D) crystals possess a range of unique mechanical properties, including relatively high in-plane stiffness and large bending flexibility. The atomic 2D building blocks can be reassembled into precisely designed heterogeneous composite structures of various geometries with customized mechanical sensing behaviors. Due to their small specific density, high flexibility, and environmental adaptability, mechanical sensors based on 2D materials can conform to soft and curved surfaces, thus providing suitable solutions for functional applications in future wearable devices. In this review, we summarize the latest developments in mechanical sensors based on 2D materials from the perspective of function-oriented applications. First, typical mechanical sensing mechanisms are introduced. Second, we attempt to establish a correspondence between typical structure designs and the performance/multi-functions of the devices. Afterward, several particularly promising areas for potential applications are discussed, following which we present perspectives on current challenges and future opportunities

Exploring the mechanical properties of nanometer-thick elastic films through micro-drop impinging on large-area suspended graphene

In this work, the dependence of effective Young's modulus on the thickness of suspended graphene was confirmed through a drop impingement method. Large area suspended graphene (LSG) layers with a diameter of up to 400 μm and a nanometer thickness were prepared through transferring chemical vapor deposition grown graphene from copper substrates. 4, 8, and 12-layer LSG samples were found to be crumpled yet defect-free. The mechanical properties of LSG were first studied by observing its interaction with impinging droplets from an ink-jet nozzle. First, the effective Young's modulus was calculated by fitting the instant deformation captured by high speed photography within microseconds. Next, droplets deposited on LSG caused deformation and generated wrinkles and the effective Young's modulus was calculated from the number of wrinkles. The above methods yielded effective Young's modulus values ranging from 0.3 to 3.4 TPa. The results from these methods all indicated that the effective Young's modulus increases with the decreasing thickness or size of suspended graphene layers. Moreover, the crumpled LSG yields higher effective Young's modulus than ideal flat graphene. These comprehensive results from complementary methodologies with precise LSG thickness control down to the nanometer scale provide good evidence to resolve the debate on the thickness dependence of mechanical strength for LSG.

Single pixel wide gamut dynamic color modulation based on a graphene micromechanical system

Dynamic color modulation in the composite structure of a graphene microelectromechanical system (MEMS)-photonic crystal microcavity is investigated in this work. The designed photonic crystal microcavity has three resonant standing wave modes corresponding to the three primary colors of red (R), green (G) and blue (B), forming strong localization of light in three modes at different positions of the microcavity. Once graphene is added, it can govern the transmittance of three modes. When graphene is located in the antinode of the standing wave, it has strong light absorption and therefore the structure's transmittance is lower, and when graphene is located in the node of the standing wave, it has weak light absorption and therefore the structure's transmittance is higher. Therefore, the graphene absorption of different colors of light can be regulated dynamically by applying voltages to tune the equilibrium position of the graphene MEMS in the microcavity, consequently realizing the output of vivid monochromatic light or multiple mixed colors of light within a single pixel, thus greatly improving the resolution. Our work provides a route to dynamic color modulation with graphene and provides guidance for the design and manufacture of high resolution, fast modulation and wide color gamut interferometric modulator displays.

Transfer Printing of Freestanding Nanoassemblies: A Route to Membrane Resonators with Adjustable Prestress

Freestanding nanoassemblies represent a new class of functional materials with highly responsive optical, electrical, and mechanical properties. Hence, they are well-suited for applications in advanced sensor devices. Here, it is shown that transfer printing enables the well-controlled fabrication of freestanding membranes from layered nanoassemblies: Using a polydimethylsiloxane (PDMS) stamp, thin films (thickness: ∼45 to ∼51 nm) of 1,6-hexanedithiol cross-linked gold nanoparticles (diameter: ∼3.9 ± 0.8 nm) were transferred onto surface-oxidized silicon substrates featuring square microcavities with edge lengths of ∼78 μm. After adjusting the contact pressure to 1.8 bar, intact membranes were printed in yields of ∼70%. The prestress of printed membranes was determined by measuring their resonance frequencies under electrostatic actuation. In general, the prestress values were in the ∼10 MPa range with standard deviations below 10% for parallel printed resonators. The deviations in average prestress between resonators printed onto different substrates were 21% or less. By increasing the temperature during the final transfer step from 5 to 48 °C, it was possible to tune the average prestress from ∼14 to ∼28 MPa. This effect was attributed to the pronounced thermal expansion of the PDMS stamp. Finally, by transfer printing layered films of graphene oxide/silk fibroin (GO/SF), it is shown that the approach can be adapted for the fabrication of freestanding membranes from very different nanomaterials.

Fabrication of extreme ultraviolet lithography pellicle with nanometer-thick graphite film by sublimation of camphor supporting layer

An extreme ultraviolet (EUV) pellicle consists of freestanding thin films on a frame; these films are tens of nanometers in thickness and can include Si, SiN_X, or graphite. Nanometer-thick graphite films (NGFs), synthesized via chemical vapor deposition on a metal catalyst, are used as a pellicle material. The most common method to transfer NGFs onto a substrate or a frame is to use polymethyl methacrylate (PMMA) as a supporting layer. However, this PMMA-mediated technique involves several disadvantages in term of manufacturing NGF EUV pellicles. When removing the PMMA using acetone or O₂plasma, defects or deflections can occur in the NGFs. Furthermore, PMMA residues are generally present on large-area NGFs. In this study, a transfer method using camphor instead of PMMA as the supporting layer was developed to overcome these problems. After the camphor/NGF was formed on the frame, camphor was removed via sublimation in an atmosphere of ethanol vapor. This study investigated the deposition and sublimation of camphor, and confirmed that no residue was present and no deflection or defects were observed in the NGFs. Thus, a large-area NGF pellicle was successfully fabricated using the camphor transfer process.

Ultra-Narrow Metallic Nano-Trenches Realized by Wet Etching and Critical Point Drying

A metallic nano-trench is a unique optical structure capable of ultrasensitive detection of molecules, active modulation as well as potential electrochemical applications. Recently, wet-etching the dielectrics of metal-insulator-metal structures has emerged as a reliable method of creating optically active metallic nano-trenches with a gap width of 10 nm or less, opening a new venue for studying the dynamics of nanoconfined molecules. Yet, the high surface tension of water in the process of drying leaves the nano-trenches vulnerable to collapsing, limiting the achievable width to no less than 5 nm. In this work, we overcome the technical limit and realize metallic nano-trenches with widths as small as 1.5 nm. The critical point drying technique significantly alleviates the stress applied to the gap in the drying process, keeping the ultra-narrow gap from collapsing. Terahertz spectroscopy of the trenches clearly reveals the signature of successful wet etching of the dielectrics without apparent damage to the gap. We expect that our work will enable various optical and electrochemical studies at a few-molecules-thick level.

Mechanics of free-standing inorganic and molecular 2D materials

The discovery of graphene has triggered a great interest in inorganic as well as molecular two-dimensional (2D) materials. In this review, we summarize recent progress in the mechanical characterization of free-standing 2D materials, such as graphene, hexagonal boron nitride (hBN), transition metal-dichalcogenides, MXenes, black phosphor, carbon nanomembranes (CNMs), 2D polymers, 2D metal organic frameworks (MOFs) and covalent organic frameworks (COFs). Elastic, fracture, bending and interfacial properties of these materials have been determined using a variety of experimental techniques including atomic force microscopy based nanoindentation, in situ tensile/fracture testing, bulge testing, Raman spectroscopy, Brillouin light scattering and buckling-based metrology. Additionally, we address recent advances of 2D materials in a variety of mechanical applications, including resonators, microphones and nanoelectromechanical sensors. With the emphasis on progress and challenges in the mechanical characterization of inorganic and molecular 2D materials, we expect a continuous growth of interest and more systematic experimental work on the mechanics of such ultrathin nanomaterials.

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.

Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications

In recent years, innovative research has been widely conducted on flexible devices for wearable electronics applications. Many examples of wearable electronics, such as smartwatches and glasses, are already available to consumers. However, strictly speaking, the sensors used in these devices are not flexible. Many studies are underway to address a wider range of wearable electronics and the development of related fields is progressing very rapidly. In particular, there is intense interest in the research field of flexible pressure sensors because they can collect and use information regarding a wide variety of sources. Through the combination of novel materials and fabrication methods, human-machine interfaces, biomedical sensors, and motion detection techniques, it is now possible to produce sensors with a superior level of performance to meet the demands of wearable electronics. In addition, more compact and human-friendly sensors have been invented in recent years, as biodegradable and self-powered sensor systems have been studied. In this review, a comprehensive description of flexible pressure sensors will be covered, and design strategies that meet the needs for applications in wearable electronics will be presented. Moreover, we will cover several fabrication methods to implement these technologies and the corresponding real-world applications.

Nanoelectromechanical Sensors Based on Suspended 2D Materials

The unique properties and atomic thickness of two-dimensional (2D) materials enable smaller and better nanoelectromechanical sensors with novel functionalities. During the last decade, many studies have successfully shown the feasibility of using suspended membranes of 2D materials in pressure sensors, microphones, accelerometers, and mass and gas sensors. In this review, we explain the different sensing concepts and give an overview of the relevant material properties, fabrication routes, and device operation principles. Finally, we discuss sensor readout and integration methods and provide comparisons against the state of the art to show both the challenges and promises of 2D material-based nanoelectromechanical sensing.

Electron-Induced Perpendicular Graphene Sheets Embedded Porous Carbon Film for Flexible Touch Sensors

Graphene-based materials on wearable electronics and bendable displays have received considerable attention for the mechanical flexibility, superior electrical conductivity, and high surface area, which are proved to be one of the most promising candidates of stretching and wearable sensors. However, polarized electric charges need to overcome the barrier of graphene sheets to cross over flakes to penetrate into the electrode, as the graphene planes are usually parallel to the electrode surface. By introducing electron-induced perpendicular graphene (EIPG) electrodes incorporated with a stretchable dielectric layer, a flexible and stretchable touch sensor with "in-sheet-charges-transportation" is developed to lower the resistance of carrier movement. The electrode was fabricated with porous nanostructured architecture design to enable wider variety of dielectric constants of only 50-μm-thick Ecoflex layer, leading to fast response time of only 66 ms, as well as high sensitivities of 0.13 kPa-1 below 0.1 kPa and 4.41 MPa-1 above 10 kPa, respectively. Moreover, the capacitance-decrease phenomenon of capacitive sensor is explored to exhibit an object recognition function in one pixel without any other integrated sensor. This not only suggests promising applications of the EIPG electrode in flexible touch sensors but also provides a strategy for internet of things security functions.

Towards large-scale graphene transfer

The transfer process is crucial for obtaining high-quality graphene for its large-scale industrial application. In this review, graphene transfer methods are systematically classified along with an analysis of the contamination or impurity of graphene that is introduced during the transfer process. Two key processes are emphasized, the substrate removal process and the direct/indirect transfer of graphene. Based on the efficiency and cost factors of industrial scale production, various transfer methods are summarized and evaluated. Potential transfer technologies and future research directions for industrial application are prospected.

Large Suspended Monolayer and Bilayer Graphene Membranes with Diameter up to 750 µm

In this paper ultra clean monolayer and bilayer Chemical Vapor Deposited (CVD) graphene membranes with diameters up to 500 µm and 750 µm, respectively have been fabricated using Inverted Floating Method (IFM) followed by thermal annealing in vacuum. The yield decreases with size but we show the importance of choosing a good graphene raw material. Dynamic mechanical properties of the membranes at room temperature in different diameters are measured before and after annealing. The quality factor ranges from 200 to 2000 and shows no clear dependence on the size. The resonance frequency is inversely proportional to the diameter of the membranes. We observe a reduction of the effective intrinsic stress in the graphene, as well as of the relative error in the determination of said stress after thermal annealing. These measurements show that it is possible to produce graphene membranes with reproducible and excellent mechanical properties.

Manufacture and characterization of graphene membranes with suspended silicon proof masses for MEMS and NEMS applications

Graphene's unparalleled strength, chemical stability, ultimate surface-to-volume ratio and excellent electronic properties make it an ideal candidate as a material for membranes in micro- and nanoelectromechanical systems (MEMS and NEMS). However, the integration of graphene into MEMS or NEMS devices and suspended structures such as proof masses on graphene membranes raises several technological challenges, including collapse and rupture of the graphene. We have developed a robust route for realizing membranes made of double-layer CVD graphene and suspending large silicon proof masses on membranes with high yields. We have demonstrated the manufacture of square graphene membranes with side lengths from 7 µm to 110 µm, and suspended proof masses consisting of solid silicon cubes that are from 5 µm × 5 µm × 16.4 µm to 100 µm × 100 µm × 16.4 µm in size. Our approach is compatible with wafer-scale MEMS and semiconductor manufacturing technologies, and the manufacturing yields of the graphene membranes with suspended proof masses were >90%, with >70% of the graphene membranes having >90% graphene area without visible defects. The measured resonance frequencies of the realized structures ranged from tens to hundreds of kHz, with quality factors ranging from 63 to 148. The graphene membranes with suspended proof masses were extremely robust, and were able to withstand indentation forces from an atomic force microscope (AFM) tip of up to ~7000 nN. The proposed approach for the reliable and large-scale manufacture of graphene membranes with suspended proof masses will enable the development and study of innovative NEMS devices with new functionalities and improved performances.

Spectroscopic and Electrical Characterizations of Low-Damage Phosphorous-Doped Graphene via Ion Implantation

Development of n-/p-type semiconducting graphenes is a critical route to implement in graphene-based nanoelectronics and optronics. Compared to the p-type graphene, the n-type graphene is more difficult to be prepared. Recently, phosphorous doping was reported to achieve air-stable and high mobility of n-typed graphene. The phosphorous-doped graphene (P-Gra) by ion implantation is considered as an ideal method for tailoring graphene due to its IC compatible process; however, for a conventional ion implanter, the acceleration energy is in the order of kiloelectron volts (keV), thus severely destroys the sp² bonding of graphene owing to its high energy of accelerated ions. The introduced defects, therefore, degrade the electrical performance of graphene. Here, for the first time, we report a low-damage n-typed chemical vapor deposition (CVD) graphene by an industrial-compatible ion implanter with an energy of 20 keV where the designed protection layer (thin Au film) covered on as-grown CVD graphene is employed to efficiently reduce defect formation. The additional post-annealing is found to heal the crystal defects of graphene. Moreover, this method allows transferring ultraclean and residue-free P-Gra onto versatile target substrates directly. The doping configuration, crystallinity, and electrical properties on P-Gra were comprehensively studied. The results indicate that the low-damaged P-Gra with a controllable doping concentration of up to 4.22 at % was achieved, which is the highest concentration ever recorded. The doped graphenes with tunable work functions (4.85-4.15 eV) and stable n-type doping while keeping high-carrier mobility are realized. This work contributes to the proof-of-concept for tailoring graphene or 2D materials through doping with an exceptional low defect density by the low energy ion implantation, suggesting a great potential for unconventional doping technologies for next-generation 2D-based nanoelectronics.

High-range noise immune supersensitive graphene-electrolyte capacitive strain sensor for biomedical applications

This paper presents development and performance assessment of an innovative and a highly potent graphene-electrolyte capacitive sensor (GECS) based on the supercapacitor model. Although graphene has been widely researched and adapted in supercapacitors as electrode material, this combination has not been applied in sensor technology. A low base capacitance, generally the impeding factor in capacitive sensors, is addressed by incorporating electric double layer capacitance in GECS, and a million-fold increase in base capacitance is achieved. The high base capacitance (∼22.0 μF) promises to solve many inherent issues pertaining to capacitive sensors. GECS is fabricated by using thermally reduced microwave exfoliated graphene oxide material to form interdigitated electrodes coated with solid-state electrolyte which forms the double layer capacitance. The capacitance response of GECS on subjecting to strain is examined and an enormous operating range (∼300 nF) is seen, which is the salient feature of this sensor. The GECS showed an impressive device sensitivity of 11.24 nF kPa-1 and good immunity towards noise i.e. lead capacitance and stray capacitance. Two regimes of operation are identified based on the procedure of device fabrication. The device can be applied to varied applications and one such biomedical application of breath pattern monitoring is demonstrated.

Surface Micro-/Nanotextured Hybrid PEDOT:PSS-Silicon Photovoltaic Cells Employing Kirigami Graphene

Kirigami graphene allows a two-dimensional material to transform into a three-dimensional structure, which constitutes an effective transparent electrode candidate for photovoltaic (PV) cells having a surface texture. The surface texture of an inverted pyramid was fabricated on a Si substrate using photolithography and wet etching, followed by metal-assisted chemical etching to obtain silicon nanowires on the surface of the inverted pyramid. Kirigami graphene with a cross-pattern array was prepared using photolithography and plasma etching on a copper foil. Then, kirigami graphene was transferred onto hybrid heterojunction PV cells with a poly(ethylene terephthalate)/silicone film. These cells consisted of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the p-type semiconductor, Si(100) as the inorganic n-type semiconductor, and a silver comb electrode on top of PEDOT:PSS. The conductivity of PEDOT:PSS was greatly improved. This improvement was significantly higher than that achieved by the continuous graphene sheet without a pattern. Transmission electron microscopy and Raman spectroscopy results revealed that the greater improvement with kirigami graphene was due to the larger contact area between PEDOT:PSS and graphene. By using two-layer graphene having a kirigami pattern, the power conversion efficiency, under simulated AM1.5G illumination conditions, was significantly augmented by up to 9.8% (from 10.03 to 11.01%).

Structure-Property Relationships in Graphene-Based Strain and Pressure Sensors for Potential Artificial Intelligence Applications

Wearable electronic sensing devices are deemed to be a crucial technology of smart personal electronics. Strain and pressure sensors, one of the most popular research directions in recent years, are the key components of smart and flexible electronics. Graphene, as an advanced nanomaterial, exerts pre-eminent characteristics including high electrical conductivity, excellent mechanical properties, and flexibility. The above advantages of graphene provide great potential for applications in mechatronics, robotics, automation, human-machine interaction, etc.: graphene with diverse structures and leverages, strain and pressure sensors with new functionalities. Herein, the recent progress in graphene-based strain and pressure sensors is presented. The sensing materials are classified into four structures including 0D fullerene, 1D fiber, 2D film, and 3D porous structures. Different structures of graphene-based strain and pressure sensors provide various properties and multifunctions in crucial parameters such as sensitivity, linearity, and hysteresis. The recent and potential applications for graphene-based sensors are also discussed, especially in the field of human motion detection. Finally, the perspectives of graphene-based strain and pressure sensors used in human motion detection combined with artificial intelligence are surveyed. Challenges such as the biocompatibility, integration, and additivity of the sensors are discussed as well.

Integration of Nanomaterials into Three-Dimensional Vertical Architectures

A novel layer-by-layer three-dimensional (3D) architecture allowing one to expand device fabrication in the vertical direction and integrating functional nanomaterials is presented by emulating civil engineering. The architecture uses SU-8 pillars as structural columns, which support multiple horizontal suspended thin films. The films then serve as platforms for the integration of nanomaterials and nanodevices. Multiple graphene layers suspended across SU-8 pillars with precise control on their vertical spacing are demonstrated. In addition to graphene, silicon nitride films that offer high strength yield and thickness control are also presented. Metallic microstructures, plasmonic nanostructures, semiconducting quantum dots, and monolayer graphene on the suspended films are achieved to prove the capability of integrating functional nanomaterials. This work provides the potential to integrate highly compact micro/nanoscale devices at different vertical levels with high surface density, which allows for more capabilities and functionalities in a single device.

Strategies for Preparing Graphene Liquid Cells for Transmission Electron Microscopy

A graphene liquid cell for transmission electron microscopy (TEM) uses one or two graphene sheets to separate the liquid from the vacuum in the microscope. In principle, graphene is an excellent material for such an application because it allows the highest possible spatial resolution, provides a flexible covering foil, and effectively protects the liquid from evaporating. Examples in open literature have demonstrated atomic-resolution TEM using small liquid pockets and the coverage of whole biological cells with graphene sheets. A total of three different basic types of liquid cells are discerned: (i) one graphene sheet is used to cover a liquid sample supported by a thin membrane of another material (for example, silicon nitride, SiN), (ii) two graphene sheets pressed together leaving liquid pockets with graphene at both sides, and (iii) a spacer material with liquid pockets covered at both sides by graphene. A total of four different process flows are available for liquid cell assembly, but there is not yet a consensus on the best routes, and a number of variations exist. The key step is the transfer of graphene to a liquid sample, which is complicated by practical issues that arise from imperfections in the graphene sheets, such as cracks. This review provides an overview of these different approaches to assembling graphene liquid cells and discusses the main obstacles and ideas to overcome them with the prospect of developing the nanoscale technology needed for graphene liquid cells so that they become available on a routine basis for electron microscopy in liquid. It also provides guidance in selecting the appropriate type of graphene liquid cell and the best assembly method for a specific experiment.

Water graphene contact surface investigated by pairwise potentials from force-matching PAW-PBE with dispersion correction

A pairwise additive atomistic potential was developed for modeling liquid water on graphene. The graphene-water interaction terms were fit to map the PAW-PBE-D3 potential energy surface using the adaptive force matching method. Through condensed phase force matching, the potential developed implicitly considers the many-body effects of water. With this potential, the graphene-water contact angle was determined to be 86° in good agreement with a recent experimental measurement of 85° ± 5° on fully suspended graphene. Furthermore, the PAW-PBE-D3 based model was used to study contact line hysteresis. It was found that the advancing and receding contact angles of water do agree on pristine graphene, however a long simulation time was required to reach the equilibrium contact angle. For water on suspended graphene, sharp peaks in the water density profile disappear when the flexibility of graphene was explicitly considered. The water droplet induces graphene to wrap around it leading to a slightly concave contact interface.

Nanopatterned High-Frequency Supporting Structures Stably Eliminate Substrate Effects Imposed on Two-Dimensional Semiconductors

Despite the outstanding physical and chemical properties of two-dimensional (2D) materials, due to their extremely thin nature, eliminating detrimental substrate effects such as serious degradation of charge-carrier mobility or light-emission yield remains a major challenge. However, previous approaches have suffered from limitations such as structural instability or the need of costly and high-temperature deposition processes. Herein, we propose a new strategy based on the insertion of high-density topographic nanopatterns as a nanogap-containing supporter between 2D materials and substrate to minimize their contact and to block the substrate-induced undesirable effects. We show that well-controlled high-frequency SiO _x nanopillar structures derived from the self-assembly of Si-containing block copolymer securely prevent the collapse or deformation of transferred MoS₂ and guarantee excellent mechanical stability. The nanogap supporters formed below monolayer MoS₂ lead to dramatic enhancement of the photoluminescence emission intensity (8.7-fold), field-effect mobility (2.0-fold, with a maximum of 4.3-fold), and photoresponsivity (12.1-fold) compared to the sample on flat SiO₂. Similar favorable effects observed for graphene strongly suggest that this simple but powerful nanogap-supporting method can be extensively applicable to a variety of low-dimensional materials and contribute to improved device performance.

Capacitive Pressure Sensor with High Sensitivity and Fast Response to Dynamic Interaction Based on Graphene and Porous Nylon Networks

Flexible pressure sensors are of great importance to be applied in artificial intelligence and wearable electronics. However, assembling a simple structure, high-performance capacitive pressure sensor, especially for monitoring the flow of liquids, is still a big challenge. Here, on the basis of a sandwich-like structure, we propose a facile capacitive pressure sensor optimized by a flexible, low-cost nylon netting, showing many merits including a high response sensitivity (0.33 kPa^-1) in a low-pressure regime (<1 kPa), an ultralow detection limit as 3.3 Pa, excellent working stability after more than 1000 cycles, and synchronous monitoring for human pulses and clicks. More important, this sensor exhibits an ultrafast response speed (<20 ms), which enables its detection for the fast variations of a small applied pressure from the morphological changing processes of a droplet falling onto the sensor. Furthermore, a capacitive pressure sensor array is fabricated for demonstrating the ability to spatial pressure distribution. Our developed pressure sensors show great prospects in practical applications such as health monitoring, flexible tactile devices, and motion detection.

Static Capacitive Pressure Sensing Using a Single Graphene Drum

To realize nanomechanical graphene-based pressure sensors, it is beneficial to have a method to electrically readout the static displacement of a suspended graphene membrane. Capacitive readout, typical in micro-electromechanical systems, gets increasingly challenging as one starts shrinking the dimensions of these devices because the expected responsivity of such devices is below 0.1 aF/Pa. To overcome the challenges of detecting small capacitance changes, we design an electrical readout device fabricated on top of an insulating quartz substrate, maximizing the contribution of the suspended membrane to the total capacitance of the device. The capacitance of the drum is further increased by reducing the gap size to 110 nm. Using an external pressure load, we demonstrate the successful detection of capacitance changes of a single graphene drum down to 50 aF, and pressure differences down to 25 mbar.

Recent Progress of Self-Powered Sensing Systems for Wearable Electronics

Wearable/flexible electronic sensing systems are considered to be one of the key technologies in the next generation of smart personal electronics. To realize personal portable devices with mobile electronics application, i.e., wearable electronic sensors that can work sustainably and continuously without an external power supply are highly desired. The recent progress and advantages of wearable self-powered electronic sensing systems for mobile or personal attachable health monitoring applications are presented. An overview of various types of wearable electronic sensors, including flexible tactile sensors, wearable image sensor array, biological and chemical sensor, temperature sensors, and multifunctional integrated sensing systems is provided. Self-powered sensing systems with integrated energy units are then discussed, separated as energy harvesting self-powered sensing systems, energy storage integrated sensing systems, and all-in-on integrated sensing systems. Finally, the future perspectives of self-powered sensing systems for wearable electronics are discussed.

Capacitive pressure sensing with suspended graphene-polymer heterostructure membranes

We describe the fabrication and characterisation of a capacitive pressure sensor formed by an ultra-thin graphene-polymer heterostructure membrane spanning a large array of micro-cavities each up to 30 μm in diameter with 100% yield. Sensors covering an area of just 1 mm² show reproducible pressure transduction under static and dynamic loading up to pressures of 250 kPa. The measured capacitance change in response to pressure is in good agreement with calculations. Further, we demonstrate high-sensitivity pressure sensors by applying a novel strained membrane transfer and optimising the sensor architecture. This method enables suspended structures with less than 50 nm of air dielectric gap, giving a pressure sensitivity of 123 aF Pa^-1 mm^-2 over a pressure range of 0 to 100 kPa.

Structural Engineering for High Sensitivity, Ultrathin Pressure Sensors Based on Wrinkled Graphene and Anodic Aluminum Oxide Membrane

Nature-motivated pressure sensors have been greatly important components integrated into flexible electronics and applied in artificial intelligence. Here, we report a high sensitivity, ultrathin, and transparent pressure sensor based on wrinkled graphene prepared by a facile liquid-phase shrink method. Two pieces of wrinkled graphene are face to face assembled into a pressure sensor, in which a porous anodic aluminum oxide (AAO) membrane with the thickness of only 200 nm was used to insulate the two layers of graphene. The pressure sensor exhibits ultrahigh operating sensitivity (6.92 kPa^-1), resulting from the insulation in its inactive state and conduction under compression. Formation of current pathways is attributed to the contact of graphene wrinkles through the pores of AAO membrane. In addition, the pressure sensor is also an on/off and energy saving device, due to the complete isolation between the two graphene layers when the sensor is not subjected to any pressure. We believe that our high-performance pressure sensor is an ideal candidate for integration in flexible electronics, but also paves the way for other 2D materials to be involved in the fabrication of pressure sensors.

Large-Scale Suspended Graphene Used as a Transparent Substrate for Infrared Spectroscopy

Due to weak interactions between micrometer-wavelength infrared (IR) light and nanosized samples, a high signal to noise ratio is a prerequisite in order to precisely characterize nanosized samples using IR spectroscopy. Traditional micrometer-thick window substrates, however, have considerable IR absorption which may introduce unavoidable deformations and interruptions to IR spectra of nanoscale samples. A promising alternative is the use of a suspended graphene substrate which has ultrahigh IR transmittance (>97.5%) as well as unique mechanical properties. Here, an effective method is presented for fabrication of suspended graphene over circular holes up to 150 µm in diameter to be utilized as a transparent substrate for IR spectroscopy. It is demonstrated that the suspended graphene has little impact on the measured IR spectra, an advantage which has led to the discovery of several missing vibrational modes of a 20 nm thick PEO film measured on a traditional CaF₂ substrate. This can provide a better understanding of molecules' fine structures and status of hanging bands. The unique optical properties of suspended graphene are determined to be superior to those of conventional IR window materials, giving this new substrate great potential as part of a new generation of IR transparent substrates, especially for use in examining nanoscale samples.

Micropatterned Pyramidal Ionic Gels for Sensing Broad-Range Pressures with High Sensitivity

The development of pressure sensors that are effective over a broad range of pressures is crucial for the future development of electronic skin applicable to the detection of a wide pressure range from acoustic wave to dynamic human motion. Here, we present flexible capacitive pressure sensors that incorporate micropatterned pyramidal ionic gels to enable ultrasensitive pressure detection. Our devices show superior pressure-sensing performance, with a broad sensing range from a few pascals up to 50 kPa, with fast response times of <20 ms and a low operating voltage of 0.25 V. Since high-dielectric-constant ionic gels were employed as constituent sensing materials, an unprecedented sensitivity of 41 kPa^-1 in the low-pressure regime of <400 Pa could be realized in the context of a metal-insulator-metal platform. This broad-range capacitive pressure sensor allows for the efficient detection of pressure from a variety of sources, including sound waves, a lightweight object, jugular venous pulses, radial artery pulses, and human finger touch. This platform offers a simple, robust approach to low-cost, scalable device design, enabling practical applications of electronic skin.

Recent Advances in Graphene-Based Pressure Sensors

With the promising applications in artificial intelligence systems and wearable health care devices, great efforts have been devoted to develop advanced pressure sensors. Graphene-based materials are promising pressure sensor materials due to the excellent electrical conductivity, outstanding mechanical properties and large surface area. This review summarizes the recent advances and progress in graphene and graphene-based pressure sensors. Perspectives and challenges in this exciting field are also highlighted and discussed.