Showing papers by "Jong Hyun Ahn published in 2012"

Extremely efficient flexible organic light-emitting diodes with modified graphene anode

Abstract: Although graphene films have a strong potential to replace indium tin oxide anodes in organic light-emitting diodes (OLEDs), to date, the luminous efficiency of OLEDs with graphene anodes has been limited by a lack of efficient methods to improve the low work function and reduce the sheet resistance of graphene films to the levels required for electrodes1,2,3,4. Here, we fabricate flexible OLEDs by modifying the graphene anode to have a high work function and low sheet resistance, and thus achieve extremely high luminous efficiencies (37.2 lm W–1 in fluorescent OLEDs, 102.7 lm W–1 in phosphorescent OLEDs), which are significantly higher than those of optimized devices with an indium tin oxide anode (24.1 lm W–1 in fluorescent OLEDs, 85.6 lm W–1 in phosphorescent OLEDs). We also fabricate flexible white OLED lighting devices using the graphene anode. These results demonstrate the great potential of graphene anodes for use in a wide variety of high-performance flexible organic optoelectronics. By replacing conventional indium tin oxide (ITO) anodes with high-work-function, low-sheet-resistance graphene anodes, researchers demonstrate flexible fluorescent organic LEDs with extremely high luminous efficiencies of 37.2 lm W–1 for fluorescent devices and 102.7 lm W–1 for phosphorescent devices. These values are significantly higher than those of optimized organic LEDs based on ITO anodes.

All Graphene-Based Thin Film Transistors on Flexible Plastic Substrates

Abstract: High-performance, flexible all graphene-based thin film transistor (TFT) was fabricated on plastic substrates using a graphene active layer, graphene oxide (GO) dielectrics, and graphene electrodes. The GO dielectrics exhibit a dielectric constant (3.1 at 77 K), low leakage current (17 mA/cm2), breakdown bias (1.5 × 106 V/cm), and good mechanical flexibility. Graphene-based TFTs showed a hole and electron mobility of 300 and 250 cm2/(V·s), respectively, at a drain bias of −0.1 V. Moreover, graphene TFTs on the plastic substrates exhibited remarkably good mechanical flexibility and optical transmittance. This method explores a significant step for the application of graphene toward flexible and stretchable electronics.

Stretchable electronics: materials, architectures and integrations

Abstract: Stretchable electronics, ie elastic electronics that can be bent and stretched, is a new, emerging class of electronics, based on building electronic circuits or devices on stretchable substrates The potential applications range from fully conformable, stretchable, skin sensors for robotic devices, wearable electronic devices, to flesh-like biodevices One of the challenges in the development of stretchable electronics is to retain full functionality under high external strains in stretching In this paper, we review a few approaches recently developed for stretchable electronics and highlight recent research efforts on multi-directional writing for stretchable, three-dimensional structures (Some figures may appear in colour only in the online journal)

A high performance PZT ribbon-based nanogenerator using graphene transparent electrodes

Abstract: We report a simple and effective approach for high performance PbZr0.52Ti0.48O3 (PZT) based flexible and semi-transparent NGs that exploit the electrical, mechanical and transparent properties of graphene. PZT NGs are successfully demonstrated for continuous driving of a liquid crystal display screen and a light emitting diode. A good quality PZT film was deposited on Pt/Ti/SiO2/Si wafer by the sol–gel method, exhibiting a typical hysteresis loop in the low voltage region. A graphene film was used in the interdigitated electrode form to improve the PZT/graphene interface under mechanical stress. Further improvement in NGs performance was realized by p-type doping in graphene, resulting in an increase in current density. NGs showed a high output voltage ∼2 V, current density ∼2.2 μA cm−2 and power density ∼88 mW cm−3 at an applying force of 0.9 kgf. This can efficiently run commercially available electronic components in a self-powered mode, without any external electrical supply.

Graphene-based flexible and stretchable thin film transistors.

Abstract: Graphene has been attracting wide attention owing to its superb electronic, thermal and mechanical properties. These properties allow great applications in the next generation of optoelectronics, where flexibility and stretchability are essential. In this context, the recent development of graphene growth/transfer and its applications in field-effect transistors are involved. In particular, we provide a detailed review on the state-of-the-art of graphene-based flexible and stretchable thin film transistors. We address the principles of fabricating high-speed graphene analog transistors and the key issues of producing an array of graphene-based transistors on flexible and stretchable substrates. It provides a platform for future work to focus on understanding and realizing high-performance graphene-based transistors.

Towards industrial applications of graphene electrodes

Abstract: Since the first isolation of graphene in 2004 by mechanical exfoliation from graphite, many people have tried to synthesize large-scale graphene using various chemical methods. In particular, there has been a great number of advances in the synthesis of graphene using chemical vapor deposition (CVD) on metal substrates such as Ni and Cu. Recently, a method to synthesize ultra-large-scale ( 30 inch) graphene films using roll-to-roll transfer and chemical doping processes was developed that shows excellent electrical and physical properties suitable for practical applications on a large scale. Considering the outstanding scalability/processibility of roll-to-roll and CVD methods as well as the extraordinary flexibility/conductivity of graphene films, we expect that transparent graphene electrodes can replace indium tin oxide in the near future.

Quasi-periodic nanoripples in graphene grown by chemical vapor deposition and its impact on charge transport.

Abstract: The technical breakthrough in synthesizing graphene by chemical vapor deposition methods (CVD) has opened up enormous opportunities for large-scale device applications. To improve the electrical properties of CVD graphene grown on copper (Cu-CVD graphene), recent efforts have focused on increasing the grain size of such polycrystalline graphene films to 100 μm and larger. While an increase in grain size and, hence, a decrease of grain boundary density is expected to greatly enhance the device performance, here we show that the charge mobility and sheet resistance of Cu-CVD graphene is already limited within a single grain. We find that the current high-temperature growth and wet transfer methods of CVD graphene result in quasi-periodic nanoripple arrays (NRAs). Electron-flexural phonon scattering in such partially suspended graphene devices introduces anisotropic charge transport and sets limits to both the highest possible charge mobility and lowest possible sheet resistance values. Our findings provide ...

Flexible and Platinum‐Free Dye‐Sensitized Solar Cells with Conducting‐Polymer‐Coated Graphene Counter Electrodes

Abstract: Typically, DSSCs are composed of a mesoporous titaniananocrystal electrode on a transparent conductive oxide (TCO)substrate with ruthenium-based sensitizers on the titania nano-crystals, platinum on the TCO substrate as a counter electrode,and iodine/iodide electrolyte between the two TCO sub-strates.

2\mu m Solid-State Laser Mode-locked By Single-Layer Graphene

Abstract: We report a 2\mu m ultrafast solid-state Tm:Lu2O3 laser, mode-locked by single-layer graphene, generating transform-limited~410fs pulses, with a spectral width~11.1nm at 2067nm. The maximum average output power is 270mW, at a pulse repetition frequency of 110MHz. This is a convenient high-power transform-limited laser at 2\mu m for various applications, such as laser surgery and material processing.

Coplanar-Gate Transparent Graphene Transistors and Inverters on Plastic

Abstract: Transparent flexible graphene transistors and inverters in a coplanar-gate configuration were presented for the first time using only two materials: graphene and an ion gel gate dielectric. The novel device configuration simplifies device fabrication such that only two printing steps were required to fabricate transistors and inverters. The devices exhibited excellent device performances including low-voltage operation with a high transistor-on-current and mobility, excellent mechanical flexibility, environmental stability, and a reasonable inverting behavior upon connecting the two transistors.

Graphene induced tunability of the surface plasmon resonance

Abstract: Tunability of the surface plasmon resonance wavelength is demonstrated by varying the thickness of Al2O3 spacer layer inserted between the graphene and nanoparticles. By varying the spacer layer thickness from 0.3 to 1.8 nm, the resonance wavelength is shifted from 583 to 566 nm. The shift is due to a change in the electromagnetic field coupling strength between the localized surface plasmons excited in the gold nanoparticles and a single layer graphene film. In contrast, when the graphene film is absent from the system, no noticeable shift in the resonance wavelength is observed upon varying the spacer thickness.

Graphene induced tunability of the surface plasmon resonance

Abstract: Tunability of the surface plasmon resonance wavelength is demonstrated by varying the thickness of Al2O3 spacer layer inserted between the graphene and nanoparticles. By varying the spacer layer thickness from 0.3 to 1.8 nm, the resonance wavelength is shifted from 583 to 566 nm. The shift is due to a change in the electromagnetic field coupling strength between the localized surface plasmons excited in the gold nanoparticles and a single layer graphene film. In contrast, when the graphene film is absent from the system, no noticeable shift in the resonance wavelength is observed upon varying the spacer thickness.

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

Abstract: 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.

Three-Dimensional Writing of Highly Stretchable Organic Nanowires

Abstract: Three-dimensional (3D) writing is a promising approach to realize stretchable electronics, but is so far limited to microscale features. We developed accurate 3D writing for highly stretchable organic nanowire arrays using a nanoscale polymer meniscus. Specifically, 3D nanoarches of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) with unprecedented stretchability, over 270%, and no compromise on the electrical characteristics were fabricated. Then, we integrated nanoarches into photoswitches, electrochemical transistors, and electrical interconnects. The impact of these successful tests goes well beyond these specific devices and opens the way to new classes of stretchable nanodevices based on organic materials.

Dynamic spin injection into chemical vapor deposited graphene

Abstract: We demonstrate dynamic spin injection into chemical vapor deposition (CVD) grown graphene by spin pumping from permalloy (Py) layers. Ferromagnetic resonance measurements at room temperature reveal a strong enhancement of the Gilbert damping at the Py/graphene interface, indeed exceeding that observed in Py/platinum interfaces. Similar results are also shown on Co/graphene layers. This enhancement in the Gilbert damping is understood as the consequence of spin pumping at the interface driven by magnetization dynamics. Our observations suggest a strong enhancement of spin-orbit coupling in CVD graphene, in agreement with earlier spin valve measurements.

Shifting of surface plasmon resonance due to electromagnetic coupling between graphene and Au nanoparticles.

Abstract: Shifting of the surface plasmon resonance wavelength induced by the variation of the thickness of insulating spacer between single layer graphene and Au nanoparticles is studied. The system demonstrates a blue-shift of 29 nm as the thickness of the spacer layer increases from 0 to 15 nm. This is due to the electromagnetic coupling between the localized surface plasmons excited in the nanoparticles and the graphene film. The strength of the coupling decays exponentially with a decay length of d/R = 0.36, where d is the spacer layer thickness and R is the diameter of the Au nanoparticles. The result agrees qualitatively well with the plasmon ruler equation. Interestingly, a further increment of the spacer layer thickness induces a red-shift of 17 nm in the resonance wavelength and the shift saturates when the thickness of the spacer layer increases above 20 nm.

Shifting of surface plasmon resonance due to electromagnetic coupling between graphene and Au nanoparticles

Abstract: Shifting of the surface plasmon resonance wavelength induced by the variation of the thickness of insulating spacer between single layer graphene and Au nanoparticles is studied. The system demonstrates a blue shift of 29 nm as the thickness of the spacer layer increases from 0 to 15 nm. This is due to the electromagnetic coupling between the localized surface plasmons excited in the nanoparticles and the graphene film. The strength of the coupling decays exponentially with a decay length of d/R=0.36, where d is the spacer layer thickness and R is the diameter of the Au nanoparticles. The result agrees qualitatively well with the plasmon ruler equation. Interestingly, a further increment of the spacer layer thickness induces a red shift of 17 nm in the resonance wavelength and the shift saturates when the thickness of the spacer layer increases above 20 nm.

Synthesis and applications of graphene electrodes

Abstract: The near explosion of attention given to graphene has attracted many to its research field. As new studies and findings about graphene synthesis, properties, electronic quality control, and pos sible applications simultaneous burgeon in the scientific community, it is quite hard to grasp the breadth of graphene history. At this stage, graphene’s many fascinating qualities have been amply reported and its potential for various electronic applications are increasing, pulling in ever more newcomers to the field of graphene. Thus it has become important as a community to have an equal understanding of how this material was discovered, why it is stirring up the scientific com munity and what sort of progress has been made and for what purposes. Since the first discovery, the hype has expediently led to near accomplishment of industrial-sized production of graphene. This review covers the progress and development of synthesis and transfer techniques with an emphasis on the most recent technique of chemical vapor deposition, and explores the potential applications of graphene that are made possible with the improved synthesis and transfer.

Stable graphene film and preparing method of the same

Abstract: The present disclosure relates to a stable graphene film, a preparing method of the stable graphene film, a graphene transparent electrode including the stable graphene film, and a touch screen including the stable graphene film.

Low-temperature growth and direct transfer of graphene–graphitic carbon films on flexible plastic substrates

Abstract: We demonstrate low-temperature growth and direct transfer of graphene-graphitic carbon films (G-GC) onto plastic substrates without the use of supporting materials. In this approach, G-GC films were synthesized on copper layers by using inductively coupled plasma enhanced chemical vapor deposition, enabling the growth of few-layer graphene (G) on top of Cu and the additional growth of graphitic carbon (GC) films above the graphene layer at temperatures as low as 300 °C. The patterned G-GC films are not easily damaged or detached from the polymer substrates during the wet etching and transfer process because of the van der Waals forces and π-π interactions between the films and the substrates. Raman spectroscopy reveals the two-dimensional hexagonal lattice of carbon atoms and the crystallinity of the G-GC films. The optical transparency and sheet resistance of the G-GC films are controlled by modulating the film thickness. Strain sensors are successfully fabricated on plastic substrates, and their resistance modulation at different strains is investigated.

Mechanical flexibility of zinc oxide thin-film transistors prepared by transfer printing method

Abstract: In the present study, we demonstrate the performance of Zinc oxide thin film transistors (ZnO TFTs) array subjected to the strain under high bending test and the reliability of TFTs was confirmed for the bending fatigue test of 2000 cycles. Initially, ZnO TFTs were fabricated on Si substrate and subsequently transferred on flexible PET substrate using transfer printing process. It was observed that when the bending radius reached ≥ 11 mm then cracks start to initiate first at SiO2 bridges, acting as interconnecting layers among individual TFT. Whatever the strain is applied to the devices, it is almost equivalently adopted by the SiO2 bridges, as they are relatively weak compared to rest of the part. The initial cracking of destructed SiO2 bridge leads to the secondary cracks to the ITO electrodes upon further increment of bending radius. Numerical simulation suggested that the strain of SiO2 layer reached to fracture level of 0.55% which was concentrated at the edge of SiO2 bridge layer. It also suggests that the round shape of SiO2 bridge can be more fruitful to compensate the stress concentration and to prevent failure of device.

Touch sensor sensing position and pressure using graphene

Abstract: PURPOSE: A simultaneous pressure and position detecting touch sensor is provided to implement a multi-touch function by applying a griffin electrode to a touch sensor. CONSTITUTION: A first griffin electrode pattern is formed in a first material and includes griffins(450). An FSR(Force Sensing Resistor) layer is formed on the first griffin electrode pattern. A second griffin electrode pattern is formed in a second material and includes griffin electrodes. A controller applies an electrode to the first and second griffin electrode patterns and senses resistance variation of the FSR.

Electrode and electronic device comprising the same

Abstract: A graphene electrode having a surface modified to have a high work function, and an electronic device including the same.

Preparing method of graphene, graphene shheet and device using the same

Abstract: PURPOSE: A method for manufacturing graphene, a graphene sheet, an element using the same are provided to simplify manufacturing processes by forming graphene on a substrate without a catalyst and omitting a process for eliminating a catalyst layer CONSTITUTION: A method for manufacturing graphene forms graphene(20) on a substrate(12) by supplying heat and gas containing carbon source on the substrate to be reacted The reaction is implemented at a temperature lower than or equal to 1000 degrees Celsius The graphene is formed based on inductively coupled plasma-chemical vapor deposition, low pressure chemical vapor deposition, or atmospheric pressure chemical vapor deposition The graphene is further cooled and patterned The substrate is transparent and is patterned

Electrode and electronic element including the same

Abstract: PROBLEM TO BE SOLVED: To provide an electrode having excellent conductivity and a high work function, and an electronic element including the electrode.SOLUTION: An electrode includes: a graphene-containing layer; and a layer having work function gradient formed on the graphene-containing layer. The layer having work function gradient is a single layer comprising a first surface that is in contact with the graphene-containing layer and a second surface that is opposite to the first surface, and a work function of the layer having work function gradient gradually increases along in a direction from the first surface to the second surface.

Numerical Design of SiO$_{2}$ Bridges in Stretchable Thin Film Transistors

Abstract: An amorphous indium–gallium–zinc oxide based inverter (a-IGZO inverter) was fabricated. Subsequently, its mechanical characteristics were investigated. A numerical method was adopted to optimize the a-IGZO inverter design. This optimization secured mechanical stability. The curvature at the edge of the SiO2 pad and the thickness of the indium–tin-oxide (ITO) electrode were accounted for in the models. The new model improved the mechanical stability when it was stretched by a total of 5% of its length along the x-axis and did not exhibit fractures or cracks. In contrast, the referenced model fractured under the same condition. It has been verified by both experiments and simulations that newly designed models obtain mechanical stability.

Elektrode und diese enthaltende elektronische Vorrichtung

Abstract: Graphen-Elektrode mit einer Oberflache, die derart modifiziert ist, dass sie eine hohe Austrittsarbeit aufweist, und eine elektronische Vorrichtung, welche die Graphen-Elektrode umfasst.