Graphene MEMS: AFM probe performance improvement

Graphene Failure under MPa: Nanowear of Step Edges Initiated by Interfacial Mechanochemical Reactions

The low wear resistance of macroscale graphene coatings does not match the ultrahigh mechanical strength and chemical inertness of the graphene layer itself; however, the wear mechanism responsible for this issue at low mechanical stress is still unclear. Here, we demonstrate that the susceptibility of the graphene monolayer to wear at its atomic step edges is governed by the mechanochemistry of frictional interfaces. The mechanochemical reactions activated by chemically active SiO2 microspheres result in atomic attrition rather than mechanical damage such as surface fracture and folding by chemically inert diamond tools. Correspondingly, the threshold contact stress for graphene edge wear decreases more than 30 times to the MPa level, and mechanochemical wear can be described well with the mechanically assisted Arrhenius-type kinetic model, i.e., exponential dependence of the removal rate on the contact stress. These findings provide a strategy for improving the antiwear of graphene-based materials by reducing the mechanochemical interactions at tribological interfaces.

Dissipation Mechanisms and Superlubricity in Solid Lubrication by Wet-Transferred Solution-Processed Graphene Flakes: Implications for Micro Electromechanical Devices

Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS₂ and WS₂. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.

Discovery of Graphene-Water Membrane Structure: Toward High-Quality Graphene Process

It is widely accepted that solid-state membranes are indispensable media for the graphene process, particularly transfer procedures. But these membranes inevitably bring contaminations and residues to the transferred graphene and consequently compromise the material quality. This study reports a newly observed free-standing graphene-water membrane structure, which replaces the conventional solid-state supporting media with liquid film to sustain the graphene integrity and continuity. Experimental observation, theoretical model, and molecular dynamics simulations consistently indicate that the high surface tension of pure water and its large contact angle with graphene are essential factors for forming such a membrane structure. More interestingly, water surface tension ensures the flatness of graphene layers and renders high transfer quality on many types of target substrates. This report enriches the understanding of the interactions on reduced dimensional material while rendering an alternative approach for scalable layered material processing with ensured quality for advanced manufacturing.

Ultrafast Growth of Highly Conductive Graphene Films by a Single Subsecond Pulse of Microwave

Currently, graphene films are expected to achieve real applications in various fields. However, the conventional synthesis methods still have intrinsic limitations, especially not being applicable on a surface with high curvature. Herein, an ultrafast synthesis method was developed for graphene and turbostratic graphite growth by a single subsecond pulse of microwaves generated by a household magnetron. We succeeded in growing high-quality around 10-layered turbostratic graphite in 0.16 s directly on the surface of an atomic force microscope probe and maintaining a tip curvature radius of less than 30 nm. The thus-produced probes showed high conductivity and tip durability. Moreover, turbostratic graphite film was also demonstrated to grow on the surface of dielectric Si flat substrates in a full coverage. Graphene can also grow on metallic Ni tips by this method. Our microwave ultrafast method can be used to grow high-quality graphene in a facile, efficient, and economical way.

High-reliability graphene-wrapped nanoprobes for scanning probe microscopy

The nanoprobe is a powerful tool in scanning probe microscopy (SPM) that is used to explore various fields of nanoscience. However, the tips can wear out very fast due to the low stability of conventional probes, especially after the measurement of high currents or lateral friction, which results in image distortion and test imprecision. Herein, a novel functional nanoprobe is presented using graphene sheets in a high-quality graphene solution wrapped round a plasma-treated conventional Pt-Ir coated nanoprobe, which shows highly stability and resistance to degradation, leading to a significantly increased lifetime. Furthermore, we show that the graphene-wrapped nanoprobes have the advantages of enhanced electrical conductivity and reduced tip-sample friction, compared with Pt-Ir coated nanoprobes. The simplicity and low cost of this method make it valuable to various functional graphene-wrapped nanoprobes and applications.

Ultra-Fast, Chemical-Free, Mass Production of High Quality Exfoliated Graphene

As graphene penetrates into industries, it is essential to mass produce high quality graphene sheets. New discoveries face formidable challenges in the marketplace due to the lack of proficient protocols to produce graphene on a commercial scale while maintaining its quality. Here, we present a conspicuous protocol for ultrafast exfoliation of graphite into high quality graphene on the sub-kilogram scale without the use of any intercalants, chemicals, or solvent. We show that graphite can be exfoliated using a plasma spray technique with high single-layer selectivity (∼85%) at a very high production rate (48 g/h). This is possible because of the inherent characteristics of the protocol which provides sudden thermal shock followed by two-stage shear. The exfoliated graphene shows almost no basal defect (I_d/I_g: 0) and possesses high quality (C/O ratio: 21.2, sp² %: ∼95%), an indication of negligible structural deterioration. The results were reproducible indicating the adeptness of the protocol. We provided several proofs-of-concept of plasma spray exfoliated graphene to demonstrate its utility in applications such as mechanical reinforcements; frictionless, transparent conductive coatings; and energy storage devices.

Recent Advancements of N-Doped Graphene for Rechargeable Batteries: A Review

Graphene, a 2D carbon structure, due to its unique materials characteristics for energy storage applications has grasped the considerable attention of scientists. The highlighted properties of this material with a mechanically robust and highly conductive nature have opened new opportunities for different energy storage systems such as Li-S (lithium-sulfur), Li-ion batteries, and metal-air batteries. It is necessary to understand the intrinsic properties of graphene materials to widen its large-scale applications in energy storage systems. In this review, different routes of graphene synthesis were investigated using chemical, thermal, plasma, and other methods along with their advantages and disadvantages. Apart from this, the applications of N-doped graphene in energy storage devices were discussed.

Review: Cantilever-Based Sensors for High Speed Atomic Force Microscopy

This review critically summarizes the recent advances of the microcantilever-based force sensors for atomic force microscope (AFM) applications. They are one the most common mechanical spring-mass systems and are extremely sensitive to changes in the resonant frequency, thus finding numerous applications especially for molecular sensing. Specifically, we comment on the latest progress in research on the deflection detection systems, fabrication, coating and functionalization of the microcantilevers and their application as bio- and chemical sensors. A trend on the recent breakthroughs on the study of biological samples using high-speed atomic force microscope is also reported in this review.

Recent Applications of Advanced Atomic Force Microscopy in Polymer Science: A Review

Atomic force microscopy (AFM) has been extensively used for the nanoscale characterization of polymeric materials. The coupling of AFM with infrared spectroscope (AFM-IR) provides another advantage to the chemical analyses and thus helps to shed light upon the study of polymers. This paper reviews some recent progress in the application of AFM and AFM-IR in polymer science. We describe the principle of AFM-IR and the recent improvements to enhance its resolution. We also discuss the latest progress in the use of AFM-IR as a super-resolution correlated scanned-probe infrared spectroscopy for the chemical characterization of polymer materials dealing with polymer composites, polymer blends, multilayers, and biopolymers. To highlight the advantages of AFM-IR, we report several results in studying the crystallization of both miscible and immiscible blends as well as polymer aging. Finally, we demonstrate how this novel technique can be used to determine phase separation, spherulitic structure, and crystallization mechanisms at nanoscales, which has never been achieved before. The review also discusses future trends in the use of AFM-IR in polymer materials, especially in polymer thin film investigation.

Application of Electrochemical Atomic Force Microscopy (EC-AFM) in the Corrosion Study of Metallic Materials

Electrochemical atomic force microscopy (EC-AFM), a branch of a scanning probe microscopy (SPM), can image substrate topography with high resolution. Since its inception, it was extended to a wide range of research areas through continuous improvement. The presence of an electrolytic cell and a potentiostat makes it possible to observe the topographical changes of the sample surface in real time. EC-AFM is used in in situ corrosion research because the samples are not required to be electrically conductive. It is widely used in passive film properties, surface dissolution, early-stage corrosion initiation, inhibitor efficiency, and many other branches of corrosion science. This review provides the research progress of EC-AFM and summarizes the extensive applications and investigations using EC-AFM in corrosion science.

Integration of sharp silicon nitride tips into high-speed SU8 cantilevers in a batch fabrication process

Employing polymer cantilevers has shown to outperform using their silicon or silicon nitride analogues concerning the imaging speed of atomic force microscopy (AFM) in tapping mode (intermittent contact mode with amplitude modulation) by up to one order of magnitude. However, tips of the cantilever made out of a polymer material do not meet the requirements for tip sharpness and durability. Combining the high imaging bandwidth of polymer cantilevers with making sharp and wear-resistant tips is essential for a future adoption of polymer cantilevers in routine AFM use. In this work, we have developed a batch fabrication process to integrate silicon nitride tips with an average tip radius of 9 ± 2 nm into high-speed SU8 cantilevers. Key aspects of the process are the mechanical anchoring of a moulded silicon nitride tip and a two-step release process. The fabrication recipe can be adjusted to any photo-processable polymer cantilever.

Recovery from mechanical degradation of graphene by defect enlargement

The extraordinary properties of graphene have made it an elite candidate for a broad range of emerging applications since its discovery. However, the introduction of structural defects during graphene production often compromises the theoretically predicted performance of graphene-based technologies to a great extent. In this study, a counterintuitive defect enlargement strategy to recover from defect-induced mechanical degradation is explored, of which the realization may lead to an enhanced operating efficiency and manufacturing feasibility. Our molecular-dynamics simulation results show that the enlargement of a preexisting defect to an elliptical shape can potentially recover from the mechanical degradation that the very defect has caused. For a defective graphene sheet having a failure strain of 48% of the pristine graphene sheet, enlarging the defect can enhance the failure strain up to 80% of the pristine graphene sheet. The mechanism of degradation recovery lies in a reduced change in curvature during deformation, which is further solidified by theoretical quantification and stress-field analysis. This theory can also predict and pinpoint the location of the initiation of the fracture-where the curvature changes most significantly during the deformation. In addition, the influence of an elliptical defect on the mechanical properties of a graphene sheet is systematically studied, which is not well understood today. Finally, the degradation recovery potential of defect of various sizes is examined, showing that the initial defect that can create the highest degree of geometric asymmetry has the best potential for degradation recovery. This study investigates the recovery from defect-induced mechanical degradation and the influence of elliptical defects on the mechanical properties of a graphene sheet, which widens our understanding of the possibility of fine-tuning mechanical properties via defect engineering and has the potential to improve materials for emerging technologies such as supercapacitor devices.

Double-Vacancy Controlled Friction on Graphene: The Enhancement of Atomic Pinning

The vacancy-enhanced contact friction of graphene is mainly attributed to the vacancy-enhanced out-of-plane deformation flexibility of the graphene and the climbing of the tip out of the vacancy trap (which actually acts as a step edge). However, this mechanism does not apply for explaining the enhanced friction caused by small-sized vacancies that are unable to accommodate the tip, such as single vacancy and double vacancies, which also commonly exist in the graphene. In the present study, by performing a set of classic molecular dynamics simulations, we demonstrated that the double-vacancy defect in graphene substantially enhanced the contact friction when the tip slides over it and the pinning effect of the reconstructed lattice of the double-vacancy defect with atoms at the bottom of the tip dominated such an influence. The underlying mechanism of such an atomic pinning effect and the influence of the normal load, sliding direction, and the sliding velocity were unveiled by analyzing the obtained friction evolution and the atomic configuration and interaction between the tip and the graphene. We believe that the findings presented in this study complete the state-of-art understanding of the nanoscale friction behaviors of vacancy-defected graphene, which is essential for the implementation of their potential control.

Selective Carbon Material Engineering for Improved MEMS and NEMS

The development of micro and nano electromechanical systems and achievement of higher performances with increased quality and life time is confronted to searching and mastering of material with superior properties and quality. Those can affect many aspects of the MEMS, NEMS and MOMS design including geometric tolerances and reproducibility of many specific solid-state structures and properties. Among those: Mechanical, adhesion, thermal and chemical stability, electrical and heat conductance, optical, optoelectronic and semiconducting properties, porosity, bulk and surface properties. They can be affected by different kinds of phase transformations and degrading, which greatly depends on the conditions of use and the way the materials have been selected, elaborated, modified and assembled. Distribution of these properties cover several orders of magnitude and depend on the design, actually achieved structure, type and number of defects. It is then essential to be well aware about all these, and to distinguish and characterize all features that are able to affect the results. For this achievement, we point out and discuss the necessity to take into account several recently revisited fundamentals on carbon atomic rearrangement and revised carbon Raman spectroscopy characterizing in addition to several other aspects we will briefly describe. Correctly selected and implemented, these carbon materials can then open new routes for many new and more performing microsystems including improved energy generation, storage and conversion, 2D superconductivity, light switches, light pipes and quantum devices and with new improved sensor and mechanical functions and biomedical applications.

Deposition of Multiscale Thickness Graphene Coating by Harnessing Extreme Heat and Rapid Quenching: Toward Commercialization

Deposition of graphene as a coating material over large-scale areas is an intense topic of research because of complexities involved in the existing deposition techniques. Higher defects and compromised properties restricted in realizing the full potential of graphene coating. This work aims to deposit graphene coatings by adopting a traditional technique, that is, plasma spraying, which has inherent merits of extremely high cooling rate (∼10⁶ K/s) and low plasma exposure time (∼0.1-10 μs). Graphene nanoplatelets (GNPs) were spray-dried into spherical agglomerates (∼60 μm dia.) and coatings were deposited over a wide range of surfaces. Continuous monitoring of temperature and velocity of in-flight GNPs was done using a diagnostic sensor. Deposition of GNP coatings was the result of striking of quasi-2D melted GNPs with higher velocity (∼197 m/s) toward the substrate. Postcharacterizations confirmed that GNPs did not collapse even after being exposed to harsh environments in plasma. Instead, high temperatures proved to be beneficial in purifying the commercial GNPs. The coatings were transparent even in the short-wavelength infrared region and remained electrically conductive. A proof-of-concept was established by carrying out preliminary corrosion and antifriction tests. Outstanding reduction of ∼3.5 times in corrosion rate and 3 times in coefficient of friction was observed in GNP-deposited coating. It is envisaged that graphene coating by plasma spraying can bring a revolution in commercial sectors.

Direct synthesis of graphene on silicon oxide by low temperature plasma enhanced chemical vapor deposition

Direct graphene growth on silicon with a native oxide using plasma enhanced chemical vapour deposition at low temperatures [550 °C-650 °C] is demonstrated for the first time. It is shown that the fine-tuning of a two-step synthesis with gas mixtures C2H2/H2 yields monolayer and few layer graphene films with a controllable domain size from 50 nm to more than 300 nm and the sheet resistance ranging from 8 kΩ sq-1 to less than 1.8 kΩ sq-1. Differences are understood in terms of the interaction of the plasma species - chiefly atomic H - with the deposited graphene and the native oxide layer. The proposed low temperature direct synthesis on an insulating substrate does not require any transfer processes and improves the compatibility with the current industrial processes.

One Second Formation of Large Area Graphene on a Conical Tip Surface via Direct Transformation of Surface Carbide

Graphene functionalized nanotips are expected to possess promising potential for various applications based on the outstanding electrical and mechanical properties of graphene. However, current methods, usually requiring a high growth temperature and identical crystal surface to match graphene lattice, are suitable for graphene formation on a flat surface. It remains a big challenge to grow graphene on a nanosized convex surface and fabricate functionalized nanotips with high quality graphene at the apex. In this work, a novel ultrafast annealing method is developed for growing large area graphene on Ni nanotips within 1-2 s. Few layered or multiple layered graphene is presented on the apex or sidewall of the conical tip surface. Direct experimental evidences support that thus-produced graphene is formed via the direct conversion of nickel carbide at the outer surface under the instantaneous high temperature, which is different from the conventional segregation mechanism. This newly developed ultrafast method provides a new route to produce graphene efficiently and economically, promising for both convex surfaces and flat substrates. Moreover, the graphene functionalized nanotips exhibit a great potential for nanoelectrical measurements and conductive scanning probe microscopy (SPM) applications.

>1000-Fold Lifetime Extension of a Nickel Electromechanical Contact Device via Graphene

Micro-/nano-electromechanical (M/NEM) switches have received significant attention as promising switching devices for a wide range of applications such as computing, radio frequency communication, and power gating devices. However, M/NEM switches still suffer from unacceptably low reliability because of irreversible degradation at the contacting interfaces, hindering adoption in practical applications and further development. Here, we evaluate and verify graphene as a contact material for reliability-enhanced M/NEM switching devices. Atomic force microscopy experiments and quantum mechanics calculations reveal that energy-efficient mechanical contact-separation characteristics are achieved when a few layers of graphene are used as a contact material on a nickel surface, reducing the energy dissipation by 96.6% relative to that of a bare nickel surface. Importantly, graphene displays almost elastic contact-separation, indicating that little atomic-scale wear, including plastic deformation, fracture, and atomic attrition, is generated. We also develop a feasible fabrication method to demonstrate a MEM switch, which has high-quality graphene as the contact material, and verify that the devices with graphene show mechanically stable and elastic-like contact properties, consistent with our nanoscale contact experiment. The graphene coating extends the switch lifetime >10³ times under hot switching conditions.

Scalable High-Performance Ultraminiature Graphene Micro-Supercapacitors by a Hybrid Technique Combining Direct Writing and Controllable Microdroplet Transfer

Miniaturization of energy storage devices can significantly decrease the overall size of electronic systems. However, this miniaturization is limited by the reduction of electrode dimensions and the reproducible transfer of small electrolyte drops. This paper reports first a simple scalable direct writing method for the production of ultraminiature microsupercapacitor (MSC) electrodes, based on femtosecond laser reduced graphene oxide (fsrGO) interlaced pads. These pads, separated by 2 μm spacing, are 100 μm long and 8 μm wide. A second stage involves the accurate transfer of an electrolyte microdroplet on top of each individual electrode, which can avoid any interference of the electrolyte with other electronic components. Abundant in-plane mesopores in fsrGO induced by a fs laser together with ultrashort interelectrode spacing enables MSCs to exhibit a high specific capacitance (6.3 mF cm^-2 and 105 F cm^-3) and ∼100% retention after 1000 cycles. An all graphene resistor-capacitor (RC) filter is also constructed by combining the MSC and a fsrGO resistor, which is confirmed to exhibit highly enhanced performance characteristics. This new hybrid technique combining fs laser direct writing and precise microdroplet transfer easily enables scalable production of ultraminiature MSCs, which is believed to be significant for practical application of micro-supercapacitor microelectronic systems.

Suppressing Nanoscale Wear by Graphene/Graphene Interfacial Contact Architecture: A Molecular Dynamics Study

Nanoscale wear is one of the key factors hindering the performance and lifetime of micro- and nanosystems, such as the scanning tip wear in atomic force microscopy (AFM), the head-disk interface in magnetic storage system, and the moving components in micro- or nanoelectromechanical systems (MEMS/NEMS). Here, we propose to construct the graphene/graphene interfacial architecture to suppress the nanoscale wear. Molecular dynamics simulations show that the atomic roughness of the sliding surfaces with either stepped or amorphous structure can lead to strong inhomogeneity of the local contact pressure distribution. By coating graphene on both sides of the frictional surfaces, the local contact pressure fluctuations due to the atomic roughness are suppressed. Moreover, this trend is more evident with the increasing layer number of the graphene coating. Furthermore, the nanoscratching simulation suggests that the rupture of graphene is driven by the inhomogeneous pressure distribution-induced lateral atomic interlocking between the rough tip and substrate and the consequent in-plane lattice deformation and C-C bond breaking during sliding. By coating graphene on the rough amorphous carbon tip, the critical normal load for wear failure of graphene is significantly increased, due to the weakening effect of the atomic interlocking by improving the contact conditions with atomically smooth graphene/graphene sliding interface. This investigation reveals a strategy for reducing nanowear by suppressing the local contact pressure fluctuations via graphene/graphene sliding interface architecture, which provides a theoretical guidance for designing wear-resistant coatings for the longevity of AFM probes and MEMS/NEMS systems.

Phase imaging and nanoscale energy dissipation of supported graphene using amplitude modulation atomic force microscopy

We investigate the phase imaging of supported graphene using amplitude modulation atomic force microscopy (AFM), the so-called tapping mode. The phase contrast between graphene and the neighboring substrate grows in hard tapping conditions and the contrast is enhanced compared to the topographic one. Therefore, phase measurements could enable the high-contrast imaging of graphene and related two-dimensional materials and heterostructures, which is not achievable with conventional AFM based topographic measurements. Obtained phase maps are then transformed into energy dissipation maps, which are important for graphene applications in various nano-mechanical systems. From a fundamental point of view, energy dissipation gives further insight into mechanical properties. Reliable measurements, obtained in the repulsive regime, show that the energy dissipation on a graphene-covered substrate is lower than that on a bare one, so graphene provides certain shielding in tip-substrate interaction. Based on the obtained phase curves and their derivatives, as well as on correlation measurements based on AFM nanoindentation and force modulation microscopy, we conclude that the main dissipation channels in graphene-substrate systems are short-range hysteresis and long-range interfacial forces.

Graphene Coated Nanoprobes: A Review

Nanoprobes are one of the most important components in several fields of nanoscience to study materials, molecules and particles. In scanning probe microscopes, the nanoprobes consist on silicon tips coated with thin metallic films to provide additional properties, such as conductivity. However, if the experiments involve high currents or lateral frictions, the initial properties of the tips can wear out very fast. One possible solution is the use of hard coatings, such as diamond, or making the entire tip out of a precious material (platinum or diamond). However, this strategy is more expensive and the diamond coatings can damage the samples. In this context, the use of graphene as a protective coating for nanoprobes has attracted considerable interest. Here we review the main literature in this field, and discuss the fabrication, performance and scalability of nanoprobes.

Ultralow friction of ink-jet printed graphene flakes

We report the frictional response of few-layer graphene (FLG) flakes obtained by the liquid phase exfoliation (LPE) of pristine graphite. To this end, we inkjet print FLG on bare and hexamethyldisilazane-terminated SiO₂ substrates, producing micrometric patterns with nanoscopic roughness that are investigated by atomic force microscopy. Normal force spectroscopy and atomically-resolved morphologies indicate reduced surface contamination by solvents after a vacuum annealing process. Notably, the printed FLG flakes show ultralow friction comparable to that of micromechanically exfoliated graphene flakes. Lubricity is retained on flakes with a lateral size of a few tens of nanometres, and with a thickness as small as ∼2 nm, confirming the high crystalline quality and low defects density in the FLG basal plane. Surface exposed step edges exhibit the highest friction values, representing the preferential sites for the origin of the secondary dissipative processes related to edge straining, wear or lateral displacement of the flakes. Our work demonstrates that LPE enables fundamental studies on graphene friction to the single-flake level. The capability to deliver ultralow-friction-graphene over technologically relevant substrates, using a scalable production route and a high-throughput, large-area printing technique, may also open up new opportunities in the lubrication of micro- and nano-electromechanical systems.

Robust microscale superlubricity under high contact pressure enabled by graphene-coated microsphere

Superlubricity of graphite and graphene has aroused increasing interest in recent years. Yet how to obtain a long-lasting superlubricity between graphene layers, under high applied normal load in ambient atmosphere still remains a challenge but is highly desirable. Here, we report a direct measurement of sliding friction between graphene and graphene, and graphene and hexagonal boron nitride (h-BN) under high contact pressures by employing graphene-coated microsphere (GMS) probe prepared by metal-catalyst-free chemical vapour deposition. The exceptionally low and robust friction coefficient of 0.003 is accomplished under local asperity contact pressure up to 1 GPa, at arbitrary relative surface rotation angles, which is insensitive to relative humidity up to 51% RH. This ultralow friction is attributed to the sustainable overall incommensurability due to the multi-asperity contact covered with randomly oriented graphene nanograins. This realization of microscale superlubricity can be extended to the sliding between a variety of two-dimensional (2D) layers.

Enhanced plasmonic nanofocusing of terahertz waves in tapered graphene multilayers

We investigate the plasmonic nanofocusing of terahertz waves in tapered graphene multilayers separated by dielectrics. The nanofocusing effect is significantly enhanced in the graphene multilayer taper compared with that in a single layer graphene taper due to interlayer coupling between surface plasmon polaritons. The results are optimized by choosing an appropriate layer number of graphene and the field amplitude has been enhanced by 620 folds at λ = 50 μm. Additionally, the structure can slow light to a group velocity ~1/2815 of the light speed in vacuum. Our study provides a unique approach to compress terahertz waves into deep subwavelength scale and may find great applications in terahertz nanodevices for imaging, detecting and spectroscopy.

Moving graphene devices from lab to market: advanced graphene-coated nanoprobes

After more than a decade working with graphene there is still a preoccupying lack of commercial devices based on this wonder material. Here we report the use of high-quality solution-processed graphene sheets to fabricate ultra-sharp probes with superior performance. Nanoprobes are versatile tools used in many fields of science, but they can wear fast after some experiments, reducing the quality and increasing the cost of the research. As the market of nanoprobes is huge, providing a solution for this problem should be a priority for the nanotechnology industry. Our graphene-coated nanoprobes not only show enhanced lifetime, but also additional unique properties of graphene, such as hydrophobicity. Moreover, we have functionalized the surface of graphene to provide piezoelectric capability, and have fabricated a nano relay. The simplicity and low cost of this method, which can be used to coat any kind of sharp tip, make it suitable for the industry, allowing production on demand.

Direct manufacturing of ultrathin graphite on three-dimensional nanoscale features

There have been many successful attempts to grow high-quality large-area graphene on flat substrates. Doing so at the nanoscale has thus far been plagued by significant scalability problems, particularly because of the need for delicate transfer processes onto predefined features, which are necessarily low-yield processes and which can introduce undesirable residues. Herein we describe a highly scalable, clean and effective, in-situ method that uses thin film deposition techniques to directly grow on a continuous basis ultrathin graphite (uG) on uneven nanoscale surfaces. We then demonstrate that this is possible on a model system of atomic force probe tips of various radii. Further, we characterize the growth characteristics of this technique as well as the film’s superior conduction and lower adhesion at these scales. This sets the stage for such a process to allow the use of highly functional graphite in high-aspect-ratio nanoscale components.

Three-Dimensional Integration of Graphene via Swelling, Shrinking, and Adaptation

The transfer of graphene from its growth substrate to a target substrate has been widely investigated for its decisive role in subsequent device integration and performance. Thus far, various reported methods of graphene transfer have been mostly limited to planar or curvilinear surfaces due to the challenges associated with fractures from local stress during transfer onto three-dimensional (3D) microstructured surfaces. Here, we report a robust approach to integrate graphene onto 3D microstructured surfaces while maintaining the structural integrity of graphene, where the out-of-plane dimensions of the 3D features vary from 3.5 to 50 μm. We utilized three sequential steps: (1) substrate swelling, (2) shrinking, and (3) adaptation, in order to achieve damage-free, large area integration of graphene on 3D microstructures. Detailed scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and electrical resistance measurement studies show that the amount of substrate swelling as well as the flexural rigidities of the transfer film affect the integration yield and quality of the integrated graphene. We also demonstrate the versatility of our approach by extension to a variety of 3D microstructured geometries. Lastly, we show the integration of hybrid structures of graphene decorated with gold nanoparticles onto 3D microstructure substrates, demonstrating the compatibility of our integration method with other hybrid nanomaterials. We believe that the versatile, damage-free integration method based on swelling, shrinking, and adaptation will pave the way for 3D integration of two-dimensional (2D) materials and expand potential applications of graphene and 2D materials in the future.

Moisture-responsive graphene paper prepared by self-controlled photoreduction

A facile and cost-effective preparation of moisture-responsive graphene bilayer paper by focused sunlight irradiation is reported. The smart graphene paper shows moisture-responsive properties due to selective adsorption of water molecules, leading to controllable actuation under humid conditions. In this way, graphene-based moisture-responsive actuators including a smart claw, an orientable transporter, and a crawler paper robot are successfully developed.

Striped nanoscale friction and edge rigidity of MoS2layers

Lateral force microscopy (LFM) is used to probe the nanoscale elastic and frictional characteristics of molybdenum disulfide (MoS₂).

Graphene-based patterning and differentiation of C2C12 myoblasts

This study aims at generating highly aligned functional myotubes using graphene as the underlying scaffold. Graphene not only supports the growth of C2C12 muscle cells but also enhances its differentiation and leads to spontaneous patterning of myotubes.

A Review on Resistive Switching in High-k Dielectrics: A Nanoscale Point of View Using Conductive Atomic Force Microscope

Metal-Insulator-Metal (MIM) structures have raised as the most promising configuration for next generation information storage, leading to great performance and fabrication-friendly Resistive Random Access Memories (RRAM). In these cells, the memory concept is no more based on the charge storage, but on tuning the electrical resistance of the insulating layer by applying electrical stresses to reach a high resistive state (HRS or “0”) and a low resistive state (LRS or “1”), which makes the memory point. Some high-k dielectrics show this unusual property and in the last years high-k based RRAM have been extensively analyzed, especially at the device level. However, as resistance switching (in the most promising cells) is a local phenomenon that takes place in areas of ~100 nm², the use of characterization tools with high lateral spatial resolution is necessary. In this paper the status of resistive switching in high-k materials is reviewed from a nanoscale point of view by means of conductive atomic force microscope analyses.

Nanogap based graphene coated AFM tips with high spatial resolution, conductivity and durability

After one decade of analyzing the intrinsic properties of graphene, interest into the development of graphene-based devices and micro electromechanical systems is increasing. Here, we fabricate graphene-coated atomic force microscope tips by growing the graphene on copper foil and transferring it onto the apex of a commercially available AFM tip. The resulting tip exhibits surprising enhanced resolution in nanoscale electrical measurements. By means of topographic AFM maps and statistical analyses we determine that this superior performance may be related to the presence of a nanogap between the graphene and the tip apex, which reduces the tip radius and tip-sample contact area. In addition, the graphene-coated tips show a low tip-sample interaction, high conductivity and long life times. The novel fabrication-friendly tip could improve the quality and reliability of AFM experiments, while reducing the cost of AFM-based research.