Showing papers by "Jong Hyun Ahn published in 2014"

Giant spin Hall effect in graphene grown by chemical vapour deposition.

Abstract: Manipulating spin currents in graphene by the spin–orbit interaction is important for many technological developments. Here, the authors show that the presence of residual metallic adatoms in chemical vapour deposition graphene enhances its spin–orbit coupling by three orders of magnitude.

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

Abstract: 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 105, a mobility of 0.12 cm2 V−1 s−1 (mean mobility value of 0.07 cm2 V−1 s−1), and reliable operation.

Graphene for displays that bend

Abstract: 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

Abstract: 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.

Self‐Healing Reduced Graphene Oxide Films by Supersonic Kinetic Spraying

Abstract: The industrial scale application of graphene and other functional materials in the field of electronics has been limited by inherent defects, and the lack of simple deposition methods. A simple spray deposition method is developed that uses a supersonic air jet for a commercially available reduced graphene oxide (r-GO) suspension. The r-GO flakes are used as received, which are pre-annealed and pre-hydrazine-treated, and do not undergo any post-treatment. A part of the considerable kinetic energy of the r-GO flakes entrained by the supersonic jet is used in stretching the flakes upon impact with the substrate. The resulting “frozen elastic strains” heal the defects (topological defects, namely Stone-Wales defect and C2 vacancies) in the r-GO flakes, which is reflected in the reduced ratio of the intensities of the D and G bands in the deposited film. The defects can also be regenerated by annealing.

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

Abstract: 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 (e=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 (e=7.0%).

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

Abstract: 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.

Organic solar cells using CVD-grown graphene electrodes.

Abstract: We report on the development of flexible organic solar cells (OSCs) incorporating graphene sheets synthesized by chemical vapor deposition (CVD) as transparent conducting electrodes on polyethylene terephthalate (PET) substrates. A key barrier that must be overcome for the successful fabrication of OSCs with graphene electrodes is the poor-film properties of water-based poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) when coated onto hydrophobic graphene surfaces. To form a uniform PEDOT:PSS film on a graphene surface, we added perfluorinated ionomers (PFI) to pristine PEDOT:PSS to create 'GraHEL', which we then successfully spin coated onto the graphene surface. We systematically investigated the effect of number of layers in layer-by-layer stacked graphene anode of an OSC on the performance parameters including the open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). As the number of graphene layers increased, the FF tended to increase owing to lower sheet resistance, while Jsc tended to decrease owing to the lower light absorption. In light of this trade-off between sheet resistance and transmittance, we determined that three-layer graphene (3LG) represents the best configuration for obtaining the optimal power conversion efficiency (PCE) in OSC anodes, even at suboptimal sheet resistances. We finally developed efficient, flexible OSCs with a PCE of 4.33%, which is the highest efficiency attained so far by an OSC with CVD-grown graphene electrodes to the best of our knowledge.

Graphene-based conformal devices.

Abstract: 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.

Abstract: 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.

Double-layer CVD graphene as stretchable transparent electrodes

Abstract: 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.

Detection of graphene domains and defects using liquid crystals.

Abstract: Determining graphene domain size and distribution is important for realizing functional electronic devices. Here, the authors use liquid crystals to study graphene surfaces, via the liquid crystal molecules aligning with the domains, and use nematic to smectic transitions to study defects.

Photo-patternable ion gel-gated graphene transistors and inverters on plastic.

Abstract: We demonstrate photo-patternable ion gel-gated graphene transistors and inverters on plastic substrates. The photo-patternable ion gel can be used as a negative photoresist for the patterning of underlying graphene as well as gate dielectrics. As a result, an extra graphene-patterning step is not required, which simplifies the device fabrication and avoids a side effect arising from the photoresist residue. The high capacitance of ion gel gate dielectrics yielded a low voltage operation (~2 V) of the graphene transistor and inverter. The graphene transistors on plastic showed an on/off-current ratio of ~11.5, along with hole and electron mobilities of 852 ± 124 and 452 ± 98 cm(2) V(-1) s(-1), respectively. In addition, the flexible graphene inverter was successfully fabricated on plastic through the potential superposition effect from the drain bias. These devices show excellent mechanical flexibility and fatigue stability.

Highly Conductive Freestanding Graphene Films as Anode Current Collectors for Flexible Lithium-Ion Batteries

Abstract: The electrodes in lithium-ion batteries (LIBs) are typically films that are arranged on metal foil current collectors with a thickness of several tens of μm. Here, we report on the preparation of a thick free-standing graphene film synthesized by CVD as an alternative to Cu foil as an anode current collector. As a model system, MoS2 anodes with a flower-like morphology were anchored onto the surface of the thick graphene film. A hybrid and binder free anode without a conventional metal current collector exhibited an excellent capacity value of around 580 mAh/g (@50 mA/g) and reasonable charge/discharge cyclability. The work presented here may stimulate the use of graphene films as replacements for conventional current collectors and additive free electrode in LIBs.

Graphene protective film for preventing gas and water, method of forming the same and uses of the same

Abstract: PURPOSE: A graphene protective film for blocking gas and moisture, a formation method thereof and a use thereof are provided to maintain electrical features of the device for a long time by improving gas and moisture blocking features. CONSTITUTION: The formation method of the graphene protective film for blocking gas and moisture is as follows. The graphene film(20) reacts a material by adding reaction gas containing a carbon source and heat to the material(10). Coating transfers the graphene film manufactured by a dry process, a wet process or roll-to-roll process on a flexible plate. The flexible plate(32) includes a plastic plate, an OLED plate or a sheet for food and beverage containers.

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

Abstract: 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 appli...

Graphene-Based Heat Spreader for Flexible Electronic Devices

Abstract: Graphene known for its superb physical properties, such as high transparency and thermal conductivity, is proposed as a solution to the problem of thermal management of the electronic devices, requiring transparency and cooling. It is shown that graphene heat spreader layer drives the heat out of the device more efficiently as compared with the commercially used metal thin films for integrated circuit cooling. An application of graphene heat spreader is proposed and tested in chip-on-film packaging. Graphene performance is compared with a gold layer with a similar transparency experimentally and theoretically as a proof of the efficient thermal management capability of graphene.

Improvement of work function and hole injection efficiency of graphene anode using CHF3 plasma treatment

Abstract: We report improvement of hole injection efficiency of a graphene anode by tuning its work function (WF) via surface fluorination. We used chemical vapor deposition to synthesize high-quality graphene sheets and then treated them with CHF3 plasma to induce fluorination. We used x-ray photoelectron spectroscopy to examine the fluorine coverage and the kind of chemical bonds in fluorinated graphene (FG). Also, we used ultraviolet photoelectron spectroscopy to systematically study the changes in the WF and sheet resistance of the FG sheets with varying plasma exposure time (0, 10, 30, 60, 90 s) to find an optimum fluorination condition for hole injection. The WF of graphene sheets was increased by up to 0.74 eV, as a result of the formation of carbon-fluorine bonds that function as negative surface dipoles. We fabricated hole-only devices and conducted dark injection space-charge-limited-current transient measurement; the fluorination greatly increased the hole injection efficiency of graphene anodes (from 0.237 to 0.652). The enhanced hole injection efficiency of FG anodes in our study provides wide opportunities for applications in graphene-based flexible/stretchable organic optoelectronics.

Observation of the inverse giant piezoresistance effect in silicon nanomembranes probed by ultrafast terahertz spectroscopy.

Abstract: The anomalous piezoresistance (a-PZR) effects, including giant PZR (GPZR) with large magnitude and inverse PZR of opposite, have exciting technological potentials for their integration into novel nanoelectromechanical systems. However, the nature of a-PZR effect and the associated kinetics have not been clearly determined yet. Even further, there are intense research debates whether the a-PZR effect actually exists or not; although numerous investigations have been conducted, the origin of the effect has not been clearly understood. This paper shows the existence of a-PZR and provides direct experimental evidence through the performance of well-established electrical measurements and terahertz spectroscopy on silicon nanomembranes (Si NMs). The clear inverse PZR behavior was observed in the Si NMs when the thickness was less than 40 nm and the magnitude of the PZR response linearly increased with the decreasing thickness. Observations combined with electrical and optical measurements strongly corroborate ...

Graphene modelocked VECSELs

Abstract: In the past decade, passively modelocked optically pumped vertical external cavity surface emitting lasers (OPVECSELs), sometimes referred to as semiconductor disk lasers (OP-SDLs), impressively demonstrated the potential for generating femtosecond pulses at multi-Watt average output powers with gigahertz repetition rates. Passive modelocking with a semiconductor saturable absorber mirror (SESAM) is well established and offers many advantages such as a flexible design of the parameters and low non-saturable losses. Recently, graphene has emerged as an attractive wavelength-independent alternative saturable absorber for passive modelocking in various lasers such as fiber or solid-state bulk lasers because of its unique optical properties. Here, we present and discuss the modelocked VECSELs using graphene saturable absorbers. The broadband absorption due to the linear dispersion of the Dirac electrons in graphene makes this absorber interesting for wavelength tunable ultrafast VECSELs. Such widely tunable modelocked sources are in particularly interesting for bio-medical imaging applications. We present a straightforward approach to design the optical properties of single layer graphene saturable absorber mirrors (GSAMs) suitable for passive modelocking of VECSELs. We demonstrate sub-500 fs pulses from a GSAM modelocked VECSEL. The potential for broadband wavelength tuning is confirmed by covering 46 nm in modelocked operation using three different VECSEL chips and up to 21 nm tuning in pulsed operation is achieved with one single gain chip. A linear and nonlinear optical characterization of different GSAMs with different absorption properties is discussed and can be compared to SESAMs.

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

Abstract: 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 s...

Graphene saturable absorbers for VECSELs

Abstract: We report mode-locking of an optically pumped VECSEL using a graphene-based saturable absorber mirror (GSAM). Self-starting and stable modelocked operation is demonstrated with 473 fs pulses at 1.5 GHz repetition rate and 949 nm center wavelength. Wavelength tuning is achieved over a 46 nm bandwidth. We discuss the mirror design, the fabrication of the GSAMs, and give an outlook on further optimization of the design, including dielectric top coatings to protect the graphene and to increase the flexibility in the design.