Jong-Hyun Ahn

Synthesis of wafer-scale uniform molybdenum disulfide films with control over the layer number using a gas phase sulfur precursor

We describe a method for synthesizing large-area and uniform molybdenum disulfide films, with control over the layer number, on insulating substrates using a gas phase sulfuric precursor (H2S) and a molybdenum metal source. The metal layer thickness was varied to effectively control the number of layers (2 to 12) present in the synthesized film. The films were grown on wafer-scale Si/SiO2 or quartz substrates and displayed excellent uniformity and a high crystallinity over the entire area. Thin film transistors were prepared using these materials, and the performances of the devices were tested. The devices displayed an on/off current ratio of 10(5), a mobility of 0.12 cm(2) V(-1) s(-1) (mean mobility value of 0.07 cm(2) V(-1) s(-1)), and reliable operation.

Graphene for displays that bend

Jong-Hyun Ahn and Byung Hee Hong discuss how graphene can be used in the development of flexible electronics.

A graphene-based transparent electrode for use in flexible optoelectronic devices

Graphene, a monolayer of carbon atoms arranged in a honeycomb structure, is a unique material with outstanding properties that may be useful in applications ranging from electronic devices to energy storage devices. The versatile properties of graphene make it suitable for use in flexible and transparent optoelectronics, biological sensors, energy storage and conversion devices, electromechanical devices, and heat spreaders. This review focuses on recent progress in methods for graphene growth, modification, and transfer, and the uses of graphene as a transparent conducting electrode in flexible organic optoelectronic devices. Although prototypical laboratory-scale graphene-based devices have been prepared to demonstrate the advantages of graphene, many challenges must be addressed before such devices can be realized commercially.

Effect of PEDOT Nanofibril Networks on the Conductivity, Flexibility, and Coatability of PEDOT:PSS Films.

The use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in electrodes and electrical circuits presents a number of challenges that are yet to be overcome, foremost amongst which are its relatively low conductivity, low coatability on hydrophobic substrates, and decreased conductivity at large strains. With this in mind, this study suggests a simple way to simultaneously address all of these issues through the addition of a small amount of a nonionic surfactant (Triton X-100) to commercial PEDOT:PSS solutions. This surfactant is shown to considerably reduce the surface tension of the PEDOT:PSS solution, thus permitting conformal coatings of PEDOT:PSS thin film on a diverse range of hydrophobic substrates. Furthermore, this surfactant induces the formation of PEDOT nanofibrils during coating, which led to the high conductivity values and mechanical stability at large strains (ε=10.3%). Taking advantage of the superior characteristics of these PEDOT:PSS thin films, a highly flexible polymer solar cell was fabricated. The power conversion efficiency of this solar cell (3.14% at zero strain) was preserved at large strains (ε=7.0%).

Graphene-based conformal devices

Despite recent progress in bendable and stretchable thin-film transistors using novel designs and materials, the development of conformal devices remains limited by the insufficient flexibility of devices. Here, we demonstrate the fabrication of graphene-based conformal and stretchable devices such as transistor and tactile sensor on a substrate with a convoluted surface by scaling down the device thickness. The 70 nm thick graphene-based conformal devices displayed a much lower bending stiffness than reported previously. The demonstrated devices provided excellent conformal coverage over an uneven animal hide surface without the need for an adhesive. In addition, the ultrathin graphene devices formed on the three-dimensionally curved animal hide exhibited stable electrical characteristics, even under repetitive bending and twisting. The advanced performance and flexibility demonstrated here show promise for the development and adoption of wearable electronics in a wide range of future applications.

Ultrathin Organic Solar Cells with Graphene Doped by Ferroelectric Polarization

Graphene has been employed as transparent electrodes in organic solar cells (OSCs) because of its good physical and optical properties. However, the electrical conductivity of graphene films synthesized by chemical vapor deposition (CVD) is still inferior to that of conventional indium tin oxide (ITO) electrodes of comparable transparency, resulting in a lower performance of OSCs. Here, we report an effective method to improve the performance and long-term stability of graphene-based OSCs using electrostatically doped graphene films via a ferroelectric polymer. The sheet resistance of electrostatically doped few layer graphene films was reduced to ∼70 Ω/sq at 87% optical transmittance. Such graphene-based OSCs exhibit an efficiency of 2.07% with a superior stability when compared to chemically doped graphene-based OSCs. Furthermore, OSCs constructed on ultrathin ferroelectric film as a substrate of only a few micrometers show extremely good mechanical flexibility and durability and can be rolled up into a cylinder with 7 mm diameter.

Detection of graphene domains and defects using liquid crystals

The direct observation of the domain size and defect distribution in a graphene film is important for the development of electronic applications involving graphene. Here we report a promising method for observing graphene domains grown by chemical vapour deposition. The unavoidable development of crack or pinhole defects during the growth and transfer processes is visualized using a liquid crystal layer. Liquid crystal molecules align anisotropically with respect to the graphene domains and exhibit distinct birefringence properties that can be used to image the graphene domains. This approach is useful for visualizing the crack distributions and their generation process in graphene films under external strain. This type of simple observation method provides an effective route to evaluating the quality and reliability of graphene sheets for use in various electronic devices.

Tuning Optical Conductivity of Large-Scale CVD Graphene by Strain Engineering

A controllable optical anisotropy in CVD graphene is shown. The transparency in the visible range of pre-strained CVD graphene exhibits a periodic modulation as a function of polarization direction. The strain sensitivity of the optical response of graphene demonstrated here can be effectively utilized towards novel ultra-thin optical devices and strain sensing applications.

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.

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.