Surface Properties of CVD-Grown Graphene Transferred by Wet and Dry Transfer Processes.

Graphene, an atomically thin material, has unique electrical, mechanical, and optical properties that can enhance the performance of thin film-based flexible and transparent devices, including gas sensors. Graphene synthesized on a metallic catalyst must first be transferred onto a target substrate using wet or dry transfer processes; however, the graphene surface is susceptible to chemical modification and mechanical damage during the transfer. Defects on the graphene surface deteriorate its excellent intrinsic properties, thus reducing device performance. In this study, the surface properties of transferred graphene were investigated according to the transfer method (wet vs. dry) and characterized using atomic force microscopy, Raman spectroscopy, and contact angle measurements. After the wet transfer process, the surface properties of graphene exhibited tendencies similar to the poly(methyl methacrylate) residue remaining after solvent etching. The dry-transferred graphene revealed a surface closer to that of pristine graphene, regardless of substrates. These results provide insight into the utilization of wet and dry transfer processes for various graphene applications.

Embedded Reverse-Offset Printing of Silver Nanowires and Its Application to Double-Stacked Transparent Electrodes with Microscale Patterns.

We propose an embedded reverse-offset printing (EROP) method, which generates silver nanowire (AgNW) transparent electrodes for display applications. The proposed EROP method can solve the two critical issues of microscale pattern formation and surface planarization. The AgNW electrode had a transmittance of 82% at 550 nm, a sheet resistance of 12.2 Ω/sq, and a 3.27 nm smooth surface. We realized the roll-based pattern formation of AgNW on a plastic substrate as small as 10 µm with negligible step differences to facilitate the proposed method. The proposed EROP method also produced a double-stacked AgNW electrode, enabling the simultaneous operation of separately micropatterned devices. To verify the usefulness of EROP, we fabricated an organic light-emitting diode (OLED) device to demonstrate leakage current reduction and efficiency improvement compared with a conventional indium tin oxide (ITO)-based OLED device. The EROP-based OLED showed 38 and 25% higher current efficiencies than an insulator-patterned AgNW OLED and a conventional ITO-based OLED, respectively.

A New Active Locomotion Capsule Endoscopy under Magnetic Control and Automated Reading Program.

Capsule endoscopy (CE) is the first-line diagnostic modality for detecting small bowel lesions. CE is non-invasive and does not require sedation, but its movements cannot be controlled, it requires a long time for interpretation, and it has lower image quality compared to wired endoscopy. With the rapid advancement of technology, several methods to solve these problems have been developed. This article describes the ongoing developments regarding external CE locomotion using magnetic force, artificial intelligence-based interpretation, and image-enhancing technologies with the CE system.

Comparison of Frictional Properties of CVD-Grown MoS2 and Graphene Films under Dry Sliding Conditions.

In the present study, dry friction and wear properties of atomically thin CVD-grown graphene and MoS2 films on SiO2/Si substrates were compared at low (72 MPa) and high (378 MPa) contact pressures. Analysis of atomic force microscopy images of these films verified that the MoS2 films, which were directly grown on the SiO2/Si substrates, had clean surfaces and made conformal contacts with the substrates. In contrast, the graphene film showed many contaminants on its surface and was loosely bonded with its SiO2/Si substrate due to its wet transfer from a Cu foil to the substrate. The MoS2 film exhibited friction and wear properties superior to those of the graphene film both at low and high contact pressures. We found that the clean sliding surface and strong bonding with SiO2/Si were the main causes of the superiority of the MoS2 film compared to the graphene film. Mild wear occurred in a layer-by-layer fashion at low contact pressure for the MoS2 film. At high contact pressure, severe wear occurred due to failure at the boundary between the MoS2 films and the underlying substrates. At both contact pressures, friction did not increase immediately after the removal of the MoS2 film from the SiO2/Si substrate because the film transferred onto the counter sliding surface and served as a lubricant.

Graphene-based stretchable and transparent moisture barrier.

We propose an alumina-deposited double-layer graphene (2LG) as a transparent, scalable, and stretchable barrier against moisture; this barrier is indispensable for foldable or stretchable organic displays and electronics. Both the barrier property and stretchability were significantly enhanced through the introduction of 2LG between alumina and a polymeric substrate. 2LG with negligible polymeric residues was coated on the polymeric substrate via a scalable dry transfer method in a roll-to-roll manner; an alumina layer was deposited on the graphene via atomic layer deposition. The effect of the graphene layer on crack generation in the alumina layer was systematically studied under external strain using an in situ micro-tensile tester, and correlations between the deformation-induced defects and water vapor transmission rate were quantitatively analyzed. The enhanced stretchability of alumina-deposited 2LG originated from the interlayer sliding between the graphene layers, which resulted in the crack density of the alumina layer being reduced under external strain.

Covalent bonding-assisted nanotransfer lithography for the fabrication of plasmonic nano-optical elements.

Many high-resolution patterning techniques have been developed to realize nano- and microscale applications of electric devices, sensors, and transistors. However, conventional patterning methods based on photo or e-beam lithography are not employed to fabricate optical elements of high aspect ratio and a sub-100 nm scale due to the limit of resolution, high costs and low throughput. In this study, covalent bonding-assisted nanotransfer lithography (CBNL) was proposed to fabricate various structures of high resolution and high aspect ratio at low cost by a robust and fast chemical reaction. The proposed process is based on the formation of covalent bonds between silicon of adhesive layers on a substrate and oxygen of the deposited material on the polymer stamp. The covalent bond is strong enough to detach multiple layers from the stamp for a large area without defects. The obtained nanostructures can be used for direct application or as a hard mask for etching. Two nano-optical applications were demonstrated in this study, i.e., a meta-surface and a wire-grid polarizer. A perfect absorption meta-surface was generated by transferring subwavelength hole arrays onto a substrate without any post-processing procedures. In addition, a wire-grid polarizer with high aspect ratio (1 : 3) and 50 nm line width was prepared by the nano-transfer of materials, which were used as a hard mask for etching. Therefore, CBNL provides a means of achieving large-area nano-optical elements with a simple roll-to-plate process at low cost.

Ultimate Control of Rate-Dependent Adhesion for Reversible Transfer Process via a Thin Elastomeric Layer.

Adhesion between a stamp with an elastomeric layer and various devices or substrates is crucial to successfully fabricate flexible electronics using a transfer process. Although various transfer processes using stamps with different adhesion strengths have been suggested, the controllable range of adhesion is still limited to a narrow range. To precisely transfer devices onto selected substrates, however, the difference in adhesion between the picking and placing processes should be large enough to achieve a high yield. Herein, we report a simple way to extend the controllable adhesion range of stamps, which can be achieved by adjusting the thickness of the elastomeric layer and the separation velocity. The adhesion strength increased with decreasing layer thickness on the stamp due to a magnification of the confinement and rate-dependent effects on the adhesion. This enabled the controllable range of the adhesion strength for a 15 µm-thick elastomeric layer to be extended up to 12 times that of the bulk under the same separation conditions. The strategy of designing stamps using simple adhesion tests is also introduced, and the reversible transfer of thin Si chips was successfully demonstrated. Tuning and optimizing the adhesion strength of a stamp according to the design process suggested here can be applied to various materials for the selective transfer and replacement of individual devices.

A graphene meta-interface for enhancing the stretchability of brittle oxide layers.

Oxide materials have recently attracted much research attention for applications in flexible and stretchable electronics due to their excellent electrical properties and their compatibility with established silicon semiconductor processes. Their widespread uptake has been hindered, however, by the intrinsic brittleness and low stretchability. Here we investigate the use of a graphene meta-interface to enhance the electromechanical stretchability of fragile oxide layers. Electromechanical tensile tests of indium tin oxide (ITO) layers on polymer substrates were carried out with in situ observations using an optical microscope. It was found that the graphene meta-interface reduced the strain transfer between the ITO layer and the substrate, and this behavior was well described using a shear lag model. The graphene meta-interface provides a novel pathway for realizing flexible and stretchable electronic applications based on oxide layers.

Uniform growth of high-quality oxide thin films on graphene using a CdSe quantum dot array seeding layer.

Graphene displays outstanding properties as an electrode and a semiconducting channel material for transistors; however, the weak interfacial bond between graphene and an inorganic oxide material-based insulator presents a major constraint on these applications. Here, we report a new approach to improving the interface between the two materials using a CdSe quantum dot (QD)-based seeding layer in an inorganic material-graphene junction. CdSe QDs were electrochemically grown on graphene without degrading the properties of the graphene layer. The graphene structure was then used as the electrode in an oxide semiconductor by depositing a zinc oxide thin film onto the graphene coated with a QD seed layer (QD/G). The zinc oxide film adhered strongly to the graphene layer and provided a low contact resistance. A high-k dielectric layer in the form of an HfO2 film, which is an essential element in the fabrication of high-performance graphene-based field effect transistors, was also uniformly formed on the QD/G sheet using atomic layer deposition. The resulting transistors provided a relatively good performance, yielding hole and electron mobilities of 2600 and 2000 cm(2)/V·s.

Double-layer CVD graphene as stretchable transparent electrodes.

The stretchability of CVD graphene with a large area is much lower than that of mechanically exfoliated pristine graphene owing to the intrinsic and extrinsic defects induced during its synthesis, etch-out of the catalytic metal, and the transfer processes. This low stretchability is the main obstacle for commercial application of CVD graphene in the field of flexible and stretchable electronics. In this study, artificially layered CVD graphene is suggested as a promising candidate for a stretchable transparent electrode. In contrast to single-layer graphene (SLG), multi-layer graphene has excellent electromechanical stretchability owing to the strain relaxation facilitated by sliding among the graphene layers. Macroscopic and microscopic electromechanical tensile tests were performed to understand the key mechanism for the improved stretchability, and crack generation and evolution were systematically investigated for their dependence on the number of CVD graphene layers during tensile deformation using lateral force microscopy. The stretchability of double-layer graphene (DLG) is much larger than that of SLG and is similar to that of triple-layer graphene (TLG). Considering the transmittance and the cost of transfer, DLG can be regarded as a suitable candidate for stretchable transparent electrodes.

Fracture characteristics of monolayer CVD-graphene.

We have observed and analyzed the fracture characteristics of the monolayer CVD-graphene using pressure bulge testing setup. The monolayer CVD-graphene has appeared to undergo environmentally assisted subcritical crack growth in room condition, i.e. stress corrosion cracking arising from the adsorption of water vapor on the graphene and the subsequent chemical reactions. The crack propagation in graphene has appeared to be able to be reasonably tamed by adjusting applied humidity and stress. The fracture toughness, describing the ability of a material containing inherent flaws to resist catastrophic failure, of the CVD-graphene has turned out to be exceptionally high, as compared to other carbon based 3D materials. These results imply that the CVD-graphene could be an ideal candidate as a structural material notwithstanding environmental susceptibility. In addition, the measurements reported here suggest that specific non-continuum fracture behaviors occurring in 2D monoatomic structures can be macroscopically well visualized and characterized.

Synthesis of CVD-graphene on rapidly heated copper foils.

Most chemical vapor deposition (CVD) systems used for graphene growth mainly employ convection and radiation heat transfer between the heating source and the metal catalyst in order to reach the activation temperature of the reaction, which in general leads to a long synthesis time and poor energy efficiency. Here, we report a highly time- and energy-efficient CVD setup, in which the metal catalyst (Cu) is designed to be physically contacted with a heating source to give quick heat transfer by conduction. The induced conduction heating enabled the usual effects of the pretreatment and annealing of Cu (i.e., annihilation of surface defects, impurities and contaminants) to be achieved in a significantly shorter time compared to conventional CVD. Notably, the rapid heating was observed to lead to larger grains of Cu with high uniformity as compared to the Cu annealed by conventional CVD, which are believed to be beneficial for the growth of high quality graphene. Through this CVD setup, bundles of high quality (∼252 Ω per square) and large area (over 16 inch) graphenes were able to be readily synthesized in 40 min in a significantly efficient way. When considering ease of scalability, high energy effectiveness and considerable productivity, our method is expected to be welcomed by industrialists.

Monatomic chemical-vapor-deposited graphene membranes bridge a half-millimeter-scale gap.

One of the main concerns in nanotechnology is the utilization of nanomaterials in macroscopic applications without losing their extreme properties. In an effort to bridge the gap between the nano- and macroscales, we propose a clever fabrication method, the inverted floating method (IFM), for preparing freestanding chemical-vapor-deposited (CVD) graphene membranes. These freestanding membranes were then successfully suspended over a gap a half-millimeter in diameter. To understand the working principle of IFM, high-speed photography and white light interferometry were used to characterize and analyze the deformation behaviors of the freestanding graphene membranes in contact with a liquid during fabrication. Some nanoscale configurations in the macroscopic graphene membranes were able to be characterized by simple optical microscopy. The proposed IFM is a powerful approach to investigating the macroscopic structures of CVD graphene and enables the exploitation of freestanding CVD graphene for device applications.

Anisotropic adhesion of micropillars with spatula pads.

Natural gecko adhesive structures consisting of angled setae, branched into thin spatulas, have remarkable properties including easily attachable and releasable anisotropic adhesion. The geometrically asymmetric structures lead to anisotropic adhesive properties. Inspired by the gecko, we fabricated an array of micropillars with asymmetric spatula pads from elastomeric materials. This paper describes the anisotropic properties of the micropillars with spatula pads as established by experimental measurements and observation together with finite element analysis. The results indicate that the structural difference of the spatula pad at one edge of the micropillar provides the anisotropic adhesive properties.

Dimensional dependence of phonon transport in freestanding atomic layer systems.

Due to the fast development of nanotechnology, we have the capability of manipulating atomic layer systems such as graphene, hexagonal boron nitride and dichalcogenides. The major concern in the 2-dimensional nanostructures is how to preserve their exceptional single-layer properties in 3-dimensional bulk structures. In this study, we report that the extreme phonon transport in graphene is highly affected by the graphitic layer stacking based on experimental investigation of the thermal conduction in few-layer graphene, 1-7 layers thick, suspended over holes of various diameters. We fabricate freestanding axisymmetric graphene structures without any perturbing substrate, and measure the in-plane transport property in terms of thermal conduction by using Raman spectroscopy. From the difference in susceptibility to substrate effect, size effect on hot-spot temperature variation and layer number dependence of thermal conductivity, we show that the graphitic membranes with 2 or more layers have characteristics similar to 3-dimensional graphite, which are very different from those of 2-dimensional graphene membranes. This implies that the scattering of out-of-plane phonons by interlayer atomic coupling could be a key mechanism governing the intrinsic thermal property.

Mechanical and environmental stability of polymer thin-film-coated graphene.

A uniform polymer thin layer of controllable thickness was bar-coated onto a chemical vapor deposition (CVD) grown monolayer graphene surface. The effects of this coating layer on the optical, electric, and tribological properties were then investigated. The thin polymer coating layer did not reduce the optical transmittance of the graphene films. The variation in the sheet resistance of the graphene films after the coating depended on the interaction between polymer and graphene. The top coating layer can maintain the high conductivity of chemical doped graphene films under long-term ambient conditions compared with uncovered doped samples. Friction tests demonstrated that the polymer coating layer can enhance both the friction force and the coefficient of friction of the graphene films and protect the graphene against damage in the repeated sliding processes.

Chemical vapor deposition-grown graphene: the thinnest solid lubricant.

As an atomically thin material with low surface energy, graphene is an excellent candidate for reducing adhesion and friction when coated on various surfaces. Here, we demonstrate the superior adhesion and frictional characteristics of graphene films which were grown on Cu and Ni metal catalysts by chemical vapor deposition and transferred onto the SiO(2)/Si substrate. The graphene films effectively reduced the adhesion and friction forces, and multilayer graphene films that were a few nanometers thick had low coefficients of friction comparable to that of bulk graphite.

The effect of temperature on the nanoscale adhesion and friction behaviors of thermoplastic polymer films.

Adhesion and friction tests were carried out in order to investigate the effect of temperature on the tribological characteristics of poly(methylmethacrylate) (PMMA) film using AFM. The pull-off and friction forces on the PMMA film were measured under a high vacuum condition (below 1 × 10(-4) Pa) as the temperature of the PMMA film was increased from 300 to 420 K (heating) and decreased to 300 K (cooling). Friction tests were also conducted in both high vacuum and air conditions at room temperature. When the temperature was 420 K, which is 25 K higher than the glass-transition temperature (T(g)) of PMMA, the PMMA film surface became deformable. Subsequently, the pull-off force increased in proportion to the maximum applied load during the pull-off force measurement. In contrast, when the temperature was under 395 K, the pull-off force did not depend on the maximum applied load. The friction force began to increase when the temperature rose above 370 K, which is 25 K lower than the T(g) of PMMA, and rapidly increased at 420 K. Decrease of the PMMA film stiffness and plastic deformation of the PMMA film were observed at 420 K in force-displacement curves. After the heating to 420 K, the coefficient of friction was measured under air condition at room temperature and was found to be lower than that measured before the heating. Additionally, the RMS roughness increased as a result of heating.