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Seo S.-W.,Sungkyunkwan University | Chae H.,Sungkyunkwan University | Seo S.J.,SKKU Advanced Institute of Nanotechnology SAINT | Chung H.K.,SKKU Advanced Institute of Nanotechnology SAINT | Cho S.M.,Sungkyunkwan University
Applied Physics Letters | Year: 2013

We report on an extremely bendable moisture barrier for the thin-film encapsulation of organic light-emitting diodes (OLEDs). Hybrid barriers with various dyads of alternating aluminum oxide (Al2O3) and plasma-polymerized layers, which are utilizable for the thin-film encapsulation of flexible OLEDs, were prepared by atomic layer deposition and plasma chemical vapor deposition, respectively. When the total thickness of Al2O 3 was fixed at 20 nm, an ultimate 200-dyad multilayer barrier showed change of less than 20 in water vapor transmission rate from its initial value of the order of 10-4 g/m2/day, even after 10 000 times of bending with a bending radius of 5 mm. © 2013 AIP Publishing LLC.

Seo S.-W.,Sungkyunkwan University | Jung E.,Sungkyunkwan University | Joon Seo S.,SKKU Advanced Institute of Nanotechnology SAINT | Chae H.,Sungkyunkwan University | And 2 more authors.
Journal of Applied Physics | Year: 2013

Flexible organic light-emitting diodes require flexible thin-film encapsulation to ensure both a long lifetime and flexibility of the device. Although an aluminum oxide layer grown by atomic layer deposition can protect the device from oxidative species, such as moisture, it does not provide sufficient flexibility for a flexible device. Thus, organic-inorganic multilayer structures were prepared and tested for both the moisture-barrier property and flexibility to achieve fully flexible thin-film encapsulation for organic light-emitting diodes. The flexible thin-film encapsulation developed in this study resulted in an initial water vapor transmission rate of 3 × 10 -4 g/m2/day that decreased by only 10% even after a 10 000 severe bending cycles at a bending radius of 0.3 cm. This result was possible by reducing the thickness of the aluminum-oxide sub-layer as low as possible down to only 1 cycle of atomic layer deposition and placing it in a neutral stress plane. Theoretical estimations of tensile strain supported the experimental results. © 2013 AIP Publishing LLC.

Trung T.Q.,Sungkyunkwan University | Ramasundaram S.,Korea Institute of Science and Technology | Hong S.W.,Korea Institute of Science and Technology | Lee N.-E.,Sungkyunkwan University | And 2 more authors.
Advanced Functional Materials | Year: 2014

A new class of temperature-sensing materials is demonstrated along with their integration into transparent and flexible field-effect transistor (FET) temperature sensors with high thermal responsivity, stability, and reproducibility. The novelty of this particular type of temperature sensor is the incorporation of an R-GO/P(VDF-TrFE) nanocomposite channel as a sensing layer that is highly responsive to temperature, and is optically transparent and mechanically flexible. Furthermore, the nanocomposite sensing layer is easily coated onto flexible substrates for the fabrication of transparent and flexible FETs using a simple spin-coating method. The transparent and flexible nanocomposite FETs are capable of detecting an extremely small temperature change as small as 0.1 C and are highly responsive to human body temperature. Temperature responsivity and optical transmittance of transparent nanocomposite FETs were adjustable and tuneable by changing the thickness and R-GO concentration of the nanocomposite. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lee C.,SKKU Advanced Institute of Nanotechnology SAINT | Baik S.,SKKU Advanced Institute of Nanotechnology SAINT
Carbon | Year: 2010

A membrane filter possessing both superhydrophobicity and superoleophilicity is of great interest for the possible separation of oil and water. Such a filter was realized in this study by synthesizing vertically-aligned multi-walled carbon nano-tubes on a stainless steel mesh. The dual-scale structure, nano-scale needle-like tubes on the mesh with micro-scale pores, combined with the low surface energy of carbon amplified both hydrophobicity and oleophilicity. For the tests, diesel was selected as a representative of low viscosity oils. The contact angles for diesel and water were 0° and 163 ± 4°. The nano-tube filter could separate diesel and water layers, and even surfactant-stabilized emulsions. The successful phase separation of the high viscosity lubricating oil and water emulsions was also carried out. The separation mechanism can be readily expanded to a variety of different hydrophobic and oleophilic liquids. The simple nano-tube filter might be practically employed in environmental and chemical separation processes including oil spill cleanup. © 2010 Elsevier Ltd. All rights reserved.

Park J.,Center for Superfunctional Materials | Lee W.H.,Pohang University of Science and Technology | Huh S.,Center for Superfunctional Materials | Sim S.H.,SKKU Advanced Institute of Nanotechnology SAINT | And 6 more authors.
Journal of Physical Chemistry Letters | Year: 2011

We have devised a method to optimize the performance of organic field-effect transistors (OFETs) by controlling the work functions of graphene electrodes by functionalizing the surface of SiO2 substrates with self-assembled monolayers (SAMs). The electron-donating NH2- terminated SAMs induce strong n-doping in graphene, whereas the CH 3-terminated SAMs neutralize the p-doping induced by SiO2 substrates, resulting in considerable changes in the work functions of graphene electrodes. This approach was successfully utilized to optimize electrical properties of graphene field-effect transistors and organic electronic devices using graphene electrodes. Considering the patternability and robustness of SAMs, this method would find numerous applications in graphene-based organic electronics and optoelectronic devices such as organic light-emitting diodes and organic photovoltaic devices. © 2011 American Chemical Society.

Ni G.-X.,National University of Singapore | Zheng Y.,National University of Singapore | Bae S.,SKKU Advanced Institute of Nanotechnology SAINT | Kim H.R.,SKKU Advanced Institute of Nanotechnology SAINT | And 8 more authors.
ACS Nano | Year: 2012

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 Î/m 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 guidance for further improving the CVD graphene growth and transfer process. © 2012 American Chemical Society.

Lai S.,SKKU Advanced Institute of Nanotechnology SAINT | Lai S.,Center for Human Interface Nanotechnology | Kyu Jang S.,SKKU Advanced Institute of Nanotechnology SAINT | Jae Song Y.,SKKU Advanced Institute of Nanotechnology SAINT | And 4 more authors.
Applied Physics Letters | Year: 2014

We report a simple and accurate method for detecting graphene defects that utilizes the mild, dry annealing of graphene/Cu films in air. In contrast to previously reported techniques, our simple approach with optical microscopy can determine the density and degree of dislocation of defects in a graphene film without inducing water-related damage or functionalization. Scanning electron microscopy, confocal Raman and atomic force microscopy, and X-ray photoelectron spectroscopy analysis were performed to demonstrate that our nondestructive approach to characterizing graphene defects with optimized thermal annealing provides rapid and comprehensive determinations of graphene quality. © 2014 AIP Publishing LLC.

Kang J.,SKKU Advanced Institute of Nanotechnology SAINT | Kim H.,Korea Electronics Technology Institute | Kim K.S.,Sejong University | Lee S.-K.,Sungkyunkwan University | And 9 more authors.
Nano Letters | Year: 2011

We demonstrate high-performance, flexible, transparent heaters based on large-scale graphene films synthesized by chemical vapor deposition on Cu foils. After multiple transfers and chemical doping processes, the graphene films show sheet resistance as low as ∼43 Ohm/sq with ∼89% optical transmittance, which are ideal as low-voltage transparent heaters. Time-dependent temperature profiles and heat distribution analyses show that the performance of graphene-based heaters is superior to that of conventional transparent heaters based on indium tin oxide. In addition, we confirmed that mechanical strain as high as ∼4% did not substantially affect heater performance. Therefore, graphene-based, flexible, transparent heaters are expected to find uses in a broad range of applications, including automobile defogging/deicing systems and heatable smart windows. © 2011 American Chemical Society.

Kim B.J.,Soongsil University | Jang H.,SKKU Advanced Institute of Nanotechnology SAINT | Jang H.,Sungkyunkwan University | Lee S.-K.,SKKU Advanced Institute of Nanotechnology SAINT | And 6 more authors.
Nano Letters | Year: 2010

A high-performance low-voltage graphene field-effect transistor (FET) array was fabricated on a flexible polymer substrate using solution-processable, high-capacitance ion gel gate dielectrics. The high capacitance of the ion gel, which originated from the formation of an electric double layer under the application of a gate voltage, yielded a high on-current and low voltage operation below 3 V. The graphene FETs fabricated on the plastic substrates showed a hole and electron mobility of 203 ± 57 and 91 ± 50 cm2/(V•s), respectively, at a drain bias of -1 V. Moreover, ion gel gated graphene FETs on the plastic substrates exhibited remarkably good mechanical flexibility. This method represents a significant step in the application of graphene to flexible and stretchable electronics. © 2010 American Chemical Society.

Xu J.,SKKU Advanced Institute of Nanotechnology SAINT | Xu J.,Center for Human Interface Nanotechnology | Jang S.K.,SKKU Advanced Institute of Nanotechnology SAINT | Jang S.K.,Center for Human Interface Nanotechnology | And 6 more authors.
Journal of Physical Chemistry C | Year: 2014

We report a new method for the codoping of boron and nitrogen in a monolayer graphene film. After the CVD synthesis of monolayer graphene, BN-doped graphene is prepared by performing power-controlled plasma treatment and thermal annealing with borazine. BN-doped graphene films with various doping levels, which were controlled by altering the plasma treatment power, were found with Raman and electrical measurements to investigate exhibit p-doping behavior. Transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy were used to demonstrate that the synthesized BN-doped graphene films have a sp2 hybridized hexagonal structure. This approach to tuning the distribution and doping levels of boron and nitrogen in monolayer sp2 hybridized BN-doped graphene is expected to be very useful for applications requiring large-area graphene with an opened band gap. © 2014 American Chemical Society.

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