A Stretchable Graphene Thin-Film Sensor for Detecting All of Lateral and Vertical Strains.

In this work, we propose a stretchable graphene film sensor that can detect all of lateral and vertical strain with unique architecture in single sensor element since most approaches so far are only available for detecting either lateral or vertical strain, but not both. The sensor is fabricated with percolative networks of graphene nanoplatelet using spray-coating method for constructing strain sensing channel and electrode simultaneously. The sensor exhibits a high stretchability of 150% with a gauge factor of 8.56 (0-100%) and 19.8 (100-150%) in the two regimes, for lateral strain. The sensor also presents a high sensitivity ((ΔR/R0)/ΔP of -0.026 kPa-1) for vertically applied pressure in the range of 100-20,000 Pa, belonging to general human pressure perception range. Based on the sensing properties demonstrated, the proposed graphene sensor is a promising candidate for sensor that can detect both lateral and vertical strains in single sensor element.

A Flexible Graphene-Polydimethylsiloxane Nanocomposite Force Sensor with Linear Response Across a Wide Pressure Detection Range.

As a flexible force sensor operating in the pressure range covering the entire general human pressure detection range, we developed a piezoresistive nanocomposite using graphene flakes as the conducting filler with polydimethylsiloxane (PDMS) as the polymer matrix. The homogeneous dispersion of graphene flakes allows their uniform distribution in the PDMS matrix with a low percolation threshold owing to their geometrically high aspect ratio, thus resulting in a linear piezoresistive response across a wide pressure detection range (100 Pa-1,020 kPa), when static forces are externally applied. Furthermore, the sensor shows sensitive piezoresistive responses to dynamically applied forces. Based on the characteristics demonstrated and described in this study, graphene- PDMS nanocomposites can be considered promising materials for flexible force sensors capable of describing human pressure perception ability.

A Highly Efficient Absorbent for Various Organic Solvents Using Hydrophobic Graphene-Sponge Composite.

We propose a hydrophobic graphene-sponge composite produced by embedding graphene flakes within polyurethane sponge as a selective absorbent for hydrophobic liquids from a contaminated water mixture. The self-aggregation nature of graphene flakes effectively allows higher graphene content to be added inside the polyurethane sponge with repeated dip-coating. High hydrophobicity due to the intrinsic nature graphene surface allows the selective absorption of liquid contaminants from a water mixture. Given the porous structure of the composite, absorption capability of 134-233 times its own weight towards various hydrophobic liquids is possible.

Ecoflex-Passivated Graphene-Yarn Composite for a Highly Conductive and Stretchable Strain Sensor.

We present a flexible strain sensor based on a graphene-yarn composite obtained by spray coating of graphene nanoplates. To improve the stretchability, graphene nanoplates were spray-coated instead of dip-coated on pre-stretched yarn. The spray-coating method yielded not only 3.68 times higher conductivity but also 2.1 times higher stretchability compared to the dip-coating method. The sensor spray-coated 400 times showed a high stretchability of 310%. Here, the relative resistance change (ΔR/R0) was 2.27 when a tensile strain of 50% was applied to the strain sensor. In addition, the fabricated sensor was coated with a protective layer of Ecoflex to minimize environmental effects. The passivated graphene-yarn composite sensor had a higher resistance than the unpassivated sensor because the Ecoflex film penetrated the conductive graphene nanoplates; however, the response to strains of up to 200% did not degrade after passivation. Furthermore, we demonstrated that our sensor can be used in wearable applications for monitoring individual finger movements and the wrist pulse.

Reduction of hole doping of chemical vapor deposition grown graphene by photoresist selection and thermal treatment.

The doping effect on graphene by photoresists were studied in this article. Polymethyl methacrylate (PMMA) is the usual choice for graphene transfer, but it is known to leave a significant amount of residue. PMMA results in strong hole doping and reduction of mobility of the graphene devices. Not only PMMA, but photoresists also leave residues during the lithographic steps and dope the graphene in strong hole-doping states along with water and oxygen molecules. In this article, we tested three types of photoresists for their effects on graphene's electrical properties. It was found that a specific photoresist can significantly reduce the amount of hole-doping of the graphene transistor more than other photoresists. The use of hydrophobic substrates and additional thermal treatment can help reducing the hole-doping further.

Optical and Electrical Characteristics of Graphene Double Layer Formed by a Double Transfer of Graphene Single Layers.

We demonstrate formation of double layer graphene by means of a double transfer using two single graphene layers grown by a chemical vapor deposition method. It is observed that shiftiness and broadness in the double-resonance of Raman scattering are much weaker than those of bilayer graphene formed naturally. Transport characteristics examined from transmission line measurements and field effect transistors show the similar behavior with those of single layer graphene. It indicates that interlayer separation, in electrical view, is large enough to avoid correlation between layers for the double layer structure. It is also observed from a transistor with the double layer graphene that molecules adsorpted on two inner graphene surfaces in the double layered structure are isolated and conserved from ambient environment.

Electric Characteristics of the Carbon Nanotube Network Transistor with Directly Grown ZnO Nanoparticles.

We report on the electrical characteristics of field effect transistors fabricated with random networks of single-walled carbon nanotubes with surfaces modified by ZnO nanoparticles. ZnO nanoparticles are directly grown on single-walled carbon nanotubes by atomic layer deposition using diethylzinc (DEZ) and water. Electrical observations show that ZnO nanoparticles act as charge transfer sources that provide electrons to the nanotube channel. The valley position in ambipolar transport of nanotube transistors is negatively shifted for 3V due to the electronic n-typed property of ZnO nanoparticles. However, the Raman resonance remains invariant despite the charge transfer effect produced by ZnO nanoparticles.

The roles of wetting liquid in the transfer process of single layer graphene onto arbitrary substrates.

Wet transfer is crucial for most device structures of the proposed applications employing single layer graphene in order to take advantage of the unique physical, chemical, bio-chemical and electrical properties of the graphene. However, transfer methodologies that can be used to obtain continuous film without voids, wrinkles and cracks are limited although film perfectness critically depends on the relative surface tension of wetting liquids on the substrate. We report the importance of wetting liquid in the transfer process with a systematic study on the parameters governing film integrity in single layer graphene grown via chemical vapor deposition. Two different suspension liquids (in terms of polar character) are tested for adequacy of transfer onto SiO2 and hexamethyldisiloxane (HMDS). We found that the relative surface tension of the wetting liquid on the surfaces of the substrate is related to transfer quality. In addition, dimethyl sulfoxide (DMSO) is introduced as a good suspension liquid to HMDS, a mechanically flexible substrate.

The geometrical effect of single walled carbon nanotube network transistors on low frequency noise characteristics.

The noise characteristics of randomly networked single walled carbon nanotubes grown directly by plasma enhanced chemical vapor deposition with field effect transistor. Geometrical complexity due to the large number of tube-tube junctions in the nanotube network is expected to be one of the key factors for the noise power of 1/f dependence. We investigated low frequency noise as a function of channel length (2-10 microm) and found that increased with longer channel length. Percolational behaviors of nanotube network that differs from ordinary semiconducting and metallic materials can be characterized by a geometrical picture with electrical homo- and hetero-junctions. Fixed nanotube density provides a test conditions to evaluate the contributions of junctions as a noise center. Hooge's empirical law is applied to investigate the low frequency noise characteristics of single walled carbon nanotube random network transistors. The noise power shows the dependence of the transistor channel length. It is understood that nanotube/nanotube junctions act as a noise center. However, the differences induced by channel length in the noise power are observed as not so significant. We conclude that tolerance of low frequency noise is important property for SWNT networks as an electronic device application.

Post annealing effect on the electrical properties of top-gate SWNT network transistors.

High performance top-gate single walled carbon nanotube network transistors are fabricated with aluminum oxide (Al2O3) layer as a gate dielectric by atomic layer deposition. It exhibits large on/off ratio (>10(4)) due to selective growth of semiconducting tubes by the plasma enhanced chemical vapor deposition. I-V characteristics show p-type or n-type depending on the deposition temperature. We investigate the type dependent characteristics for the carrier polarities with the post annealing effect on the top-gate SWNT network transistors. The dramatic change in the polarity of the top-gate SWNT network transistors, from n-type to p-type due to conversion of I-V characteristics is observed by post-annealing at 350 degrees C for 30 minutes under vacuum. Our observation suggests that competition between electron transfer from the Al2O3 layers to the SWNT surface and electron capture by oxygen molecules adsorbed on the tube walls seems to be the key point for the V(th) change as a function of Al2O3 deposition temperature.

Thin film transistors using preferentially grown semiconducting single-walled carbon nanotube networks by water-assisted plasma-enhanced chemical vapor deposition.

Nearly perfect semiconducting single-walled carbon nanotube random network thin film transistors were fabricated and their reproducible transport properties were investigated. The networked single-walled carbon nanotubes were directly grown by water-assisted plasma-enhanced chemical vapor deposition. Optical analysis confirmed that the nanotubes were mostly semiconductors without clear metallic resonances in both the Raman and the UV-vis-IR spectroscopy. The transistors made by the nanotube networks whose density was much larger than the percolation threshold also showed no metallic paths. Estimation based on the conductance change of semiconducting nanotubes in the SWNT network due to applied gate voltage difference (conductance difference for on and off state) indicated a preferential growth of semiconducting nanotubes with an advantage of water-assisted PECVD. The nanotube transistors showed 10(-5) of on/off ratio and approximately 8 cm2 V(-1) s(-1) of field effect mobility.

Recognition, classification, and prediction of the tactile sense.

The emulation of the tactile sense is presented with the encoding of a complex surface texture through an electrical sensor device. To achieve a functional capability comparable to a human mechanoreceptor, a tactile sensor is designed by employing a naturally formed porous structure of a graphene film. The inherent tactile patterns are achievable by means of proper analysis of the electrical signals that the sensor provides during the event of touching the interacting objects. It is confirmed that the pattern-recognition method using machine learning is suitable for quantifying human tactile sensations. The classification accuracy of the tactile sensor system is better than that of human touch for the tested fabric samples, which have a delicate surface texture.

Thin film transistors of single-walled carbon nanotubes grown directly on glass substrates.

We report a transistor of randomly networked single-walled carbon nanotubes on a glass substrate. The carbon nanotube networks acting as the active components of the thin film transistor were selectively formed on the transistor channel areas that were previously patterned with catalysts to avoid the etching process for isolating nanotubes. The nanotube density was more than 50 microm(-2), which is much larger than the percolation threshold. Transistors were successfully fabricated with a conducting and transparent ZnO for the back-side gate and the top-side gate. This allows the transparent electronics or suggests thin film applications of nanotubes for future opto-electronics.

A tactile sensor using single layer graphene for surface texture recognition.

Tactile sensors capable of texture recognition are essential for artificial skin functions. In this work, we describe a tactile sensor with a single sensor architecture made of single layer graphene that can recognize surface texture based on the roughness of the interacting surface. Resistance changes due to the local deformation of a local area of the single layer graphene are reflected in the resistance of the entire sensor. By introducing microstructures inspired by human finger prints, surface texture was successfully defined through fast Fourier transform analysis, and spatial resolution was easily achievable. This work provides a simple method utilizing a single sensor for surface texture recognition at the level of human sensation without using a matrix architecture which requires high density integration technology with force and vibration sensor elements.

A tactile sensor using a conductive graphene-sponge composite.

For sensors that emulate human tactile perception, we suggest a simple method for fabricating a highly sensitive force sensor using a conductive polyurethane sponge where graphene flakes are self-assembled into the porous structure of the sponge. The complete sensor device shows a sensitive and reliable detection response for a broad range of pressure and dynamic pressure that correspond to human tactile perception. Sensitivity of the sensor to detect vibration is also confirmed with vertical actuations due to slipping over micro-scale ridge structures attached on the sensors. Based on the sensor's ability to detect both pressure and vibration, the sensor can be utilized as a flexible tactile sensor.

Touch stimulated pulse generation in biomimetic single-layer graphene.

Detecting variation in contact pressure is a separate sensing mode in the human somatosensory system that differs from the detection of pressure magnitude. If pressure magnitude and variation sensing can be achieved simultaneously, an advanced biomimetic tactile system that better emulates human senses may be developed. We report on a novel single-layer graphene based artificial mechanoreceptor that generates a resistance pulse as the contact stimulus passes a specific threshold pressure, mimicking the generation of action potentials in a biological fast-adapting mechanoreceptor. The electric field from a flexible membrane gate electrode placed above a graphene channel raises the Fermi level from the valence band as pressure deflects the membrane. The threshold pressure is reached when the Fermi level crosses the Dirac point in the graphene energy band, which generates a sharp peak in the measured resistance. We found that by changing the gate potential it was possible to modulate the threshold pressure and using a series of graphene channels, a train of pulses were generated during a transient pressurizing stimulus demonstrating biomimetic behaviour.

Vertically aligned carbon-nanotube arrays showing Schottky behavior at room temperature.

Vertically aligned carbon-nanotube (CNT) arrays were fabricated in the thin-film anodic aluminum oxide (AAO) templates on silicon wafers utilizing a niobium (Nb) thin film as the source electrode. The average diameter of the CNTs was 25 nm, and the number density was 3 x 10(10) cm(-2). The CNT arrays synthesized at 700 degrees C and above exhibited Schottky behavior even at 300 K, with energy gaps between 0.2 eV and 0.3 eV. However, individual CNTs obtained by removal of the template behaved as resistors at 300 K. The CNT/Nb oxide/Nb junction is thought to be responsible for the Schottky behavior. This structure can be a useful cornerstone in the fabrication of nanotransistors operating at room temperature.

A highly sensitive pressure sensor using a double-layered graphene structure for tactile sensing.

In this paper, we propose a graphene sensor using two separated single-layered graphenes on a flexible substrate for use as a pressure sensor, such as for soft electronics. The working pressure corresponds to the range in which human perception recognizes surface morphologies. A specific design of the sensor structure drives the piezoresistive character due to the contact resistance between two graphene layers and the electromechanical properties of graphene itself. Accordingly, sensitivity in resistance change is given by two modes for low pressure (-0.24 kPa(-1)) and high pressure (0.039 kPa(-1)) with a crossover pressure (700 Pa). This sensor can detect infinitesimal pressure as low as 0.3 Pa with uniformly applied vertical force. With the attachment of the artificial fingerprint structure (AFPS) on the sensor, the detection ability for both the locally generated shear force and actual human touch confirms recognition of the surface morphology constructed by periodic structures.

Fabrication of a Flexible Graphene Pressure Sensor.

The electromechanical properties of single-layer graphene have inspired specific application to force sensors, sinceit is capable of sensing within the range of human pressure perception. In this study, we present a pressure sensor for vertical force that is flexible and transparent by introducing a single graphene layer on a polyethylene naphthalate substrate. This substrate is commonly used as a force absorber in sensors. By employing it with a pressure amplifying structure, the performance of the sensor shows a reliable resistance change of 0.15% per 1 kPa of applied vertical pressure. Detection for the motion of the finger joint and touching are demonstrated with the sensor equipped on the human body.

Interfacial magnetic anisotropy of Co90Zr10 on Pt layer.

Spin Transfer Torque (STT) is of great interest in data writing scheme for the Magneto-resistive Random Access Memory (MRAM) using Magnetic Tunnel Junction (MTJ). Scalability for high density memory requires ferromagnetic electrodes having the perpendicular magnetic easy axis. We investigated CoZr as the ferromagnetic electrode. It is observed that interfacial magnetic anisotropy is preferred perpendicular to the plane with thickness dependence on the interfaces with Pt layer. The anisotropy energy (K(u)) with thickness dependence shows a change of magnetic-easy-axis direction from perpendicular to in-plane around 1.2 nm of CoZr. The interfacial anisotropy (K(i)) as the directly related parameters to switching and thermal stability, are estimated as 1.64 erg/cm2 from CoZr/Pt multilayered system.