Showing papers by "Jong Hyun Ahn published in 2016"
TL;DR: The production and fabrication methods used for target device applications, including logic devices, energy-harvesting devices, sensors, and bioinspired devices, and the various types of flexible and stretchable electronic devices enabled by graphene are discussed.
Abstract: Graphene provides outstanding properties that can be integrated into various flexible and stretchable electronic devices in a conventional, scalable fashion. The mechanical, electrical, and optical properties of graphene make it an attractive candidate for applications in electronics, energy-harvesting devices, sensors, and other systems. Recent research progress on graphene-based flexible and stretchable electronics is reviewed here. The production and fabrication methods used for target device applications are first briefly discussed. Then, the various types of flexible and stretchable electronic devices that are enabled by graphene are discussed, including logic devices, energy-harvesting devices, sensors, and bioinspired devices. The results represent important steps in the development of graphene-based electronics that could find applications in the area of flexible and stretchable electronics.
515 citations
TL;DR: A conformal tactile sensor based on MoS2 and graphene is demonstrated that exhibits excellent sensitivity, high uniformity, and good repeatability in terms of various strains.
Abstract: A conformal tactile sensor based on MoS2 and graphene is demonstrated. The MoS2 tactile sensor exhibits excellent sensitivity, high uniformity, and good repeatability in terms of various strains. In addition, the outstanding flexibility enables the MoS2 strain tactile sensor to be realized conformally on a finger tip. The MoS2 -based tactile sensor can be utilized for wearable electronics, such as electronic skin.
337 citations
TL;DR: Self-junctioned copper nanofiber transparent flexible films are produced using electrospinning and electroplating processes that provide high performances of T = 97% and Rs = 0.42 Ω sq(-1) by eliminating junction resistance at wire intersections.
Abstract: Self-junctioned copper nanofiber transparent flexible films are produced using electrospinning and electroplating processes that provide high performances of T = 97% and Rs = 0.42 Ω sq(-1) by eliminating junction resistance at wire intersections. The film remains conductive after being stretched by up to 770% (films with T = 76%) and after 1000 cycles of bending to a 5 mm radius.
139 citations
TL;DR: In this article, a triboelectric nanogenerators (TENGs) that can form directly on human skin and operate wearable devices without recharging process are presented. But their long-term operation is a key requirement for the widespread use of wearable devices.
138 citations
TL;DR: Ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer provide excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices.
Abstract: Enhancing the device performance of organic memory devices while providing high optical transparency and mechanical flexibility requires an optimized combination of functional materials and smart device architecture design. However, it remains a great challenge to realize fully functional transparent and mechanically durable nonvolatile memory because of the limitations of conventional rigid, opaque metal electrodes. Here, we demonstrate ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer. The strong crystallographic interaction between PVDF-TrFE and graphene results in the orientation of the crystals with distinct symmetry, which is favorable for polarization switching upon the electric field. The epitaxial growth of PVDF-TrFE on a graphene layer thus provides excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices. Fur...
111 citations
TL;DR: In this paper, the surface-enhanced Raman scattering of an organic fluoropore (Rhodamine 6G, R6G) monolayer adsorbed onto graphene and two-dimensional (2D) molybedenium disulfides (MoS2) phototransistors was studied.
Abstract: We studied the surface-enhanced Raman scattering of an organic fluoropore (Rhodamine 6G, R6G) monolayer adsorbed onto graphene and two-dimensional (2D) molybedenium disulfides (MoS2) phototransistors and compared the results with the Raman scattering of R6G on 2D tungsten diselenides system (WSe2). The Raman enhancement factor of the R6G film adsorbed onto WSe2 was comparable to the corresponding value on graphene at 1365 cm–1 and was approximately twice this value at 615 cm–1. The amplitude of the charge transfer was estimated in situ by measuring the photocurrent produced in a hybrid system consisting of physisorbed R6G layer and the 2D materials. We found that the enhanced Raman scattering of R6G adsorbed onto the 2D materials was closely correlated with the charge transfer between the adsorbed molecules and the 2D materials. We also revealed that the intensity of Raman scattering generally decreased as the layer number of the 2D materials increased. For the R6G on the MoS2 nanosheet, a single layer sy...
103 citations
TL;DR: Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108.
Abstract: This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.
74 citations
TL;DR: This study reveals a novel method to fabricate and use patterned MoS2 and graphene-MoS2 heterojunctions, which could be generalized to the rational design of other 2D materials, heterojunction and devices in the future.
Abstract: Application-oriented patterned growth of transition metal dichalcogenides (TMDCs) and their heterojunctions is of critical importance for sophisticated, customized two-dimensional (2D) electronic and optoelectronic devices; however, it is still difficult to fabricate these patterns in a simple, clean, and high controllability manner without using optical lithography. Here, we report the direct synthesis of patterned MoS2 and graphene–MoS2 heterojunctions via selective plasma treatment of a SiO2/Si substrate and chemical vapor deposition of MoS2. This method has multiple merits, such as simple steps, a short operating time, easily isolated MoS2 layers with clean surfaces and controllable locations, shapes, sizes and thicknesses, which enable their integration into the device structure without using a photoresist. In addition, we demonstrate the direct growth of patterned graphene–MoS2 heterojunctions for the fabrication of transistor. This study reveals a novel method to fabricate and use patterned MoS2 and graphene–MoS2 heterojunctions, which could be generalized to the rational design of other 2D materials, heterojunctions and devices in the future.
68 citations
TL;DR: Flexible MgO barrier magnetic tunnel junction (MTJ) devices are fabricated using a transfer printing process in this article, which yield significantly enhanced tunneling magnetoresistance of ≈300% and improved abruptness of switching.
Abstract: Flexible MgO barrier magnetic tunnel junction (MTJ) devices are fabricated using a transfer printing process. The flexible MTJ devices yield significantly enhanced tunneling magnetoresistance of ≈300% and improved abruptness of switching, as residual strain in the MTJ structure is released during the transfer process. This approach could be useful for flexible electronic systems that require high-performance memory components.
59 citations
TL;DR: Lee et al. as mentioned in this paper used an easily controlled and low-temperature processable charge generation layer with lithium nitride (Li3N), which simultaneously provided efficient stacking of EL units and enhanced compatibility of the flexible device on a thin plastic substrate.
Abstract: Ultimate flexible organic light-emitting diodes (OLEDs) should have an ultra-high device efficiency, a low-efficiency roll-off at a high luminance and excellent flexibility. Here, we realized flexible tandem OLEDs using a graphene anode with a very high electroluminescent efficiency of ~205.9 cd A−1, 45.2% (~396.4 cd A−1, 87.3% with a hemispherical lens) and a very low efficiency roll-off at a high luminance of ~6.6% at 10 000 cd m−2 (~3.8% with a hemispherical lens) by stacking two organic electroluminescence (EL) units. For the first time, we used an easily controlled and low-temperature processable charge generation layer with lithium nitride (Li3N). This simultaneously provided efficient stacking of EL units and enhanced compatibility of the flexible device on a thin plastic substrate. The flexible tandem OLEDs with a graphene anode also showed great flexibility against bending up to a bending strain of 6.7%. These results represent a significant advancement towards the production of next-generation flexible displays and solid-state lighting that use a graphene anode. Bendable displays with nearly unmatched strain resistance and device efficiency can be realized using graphene-based transparent electrodes. Organic light-emitting diodes (OLEDs) are integral to today's touch-screen technology, but future applications are limited by a reliance on brittle, indium tin oxide electrodes. Researchers led by Tae-Woo Lee from Pohang University of Science and Technology in South Korea have made an alternative OLED prototype that assembles multi-electroluminescent units of organic materials onto high-quality, graphene electrodes transferred to thin plastic substrates. The team improved on previous graphene OLED designs by inserting a charge-generation layer - an organic material mixed with lithium ions that can be evaporated at low temperatures - between separate electroluminescent stacks. This setup provides ample charge carriers and mechanical strength to keep the OLED efficiency high, even after 1,000 bending cycles. We realized highly flexible tandem OLEDs using a graphene anode with very high electroluminescent efficiency ~205.9 cd A−1, 45.2% (~396.4 cd A−1, 87.3% with a hemispherical lens) and very low efficiency roll-off at high luminance ~6.6% at 10 000 cd m−2 (~3.8% with a hemispherical lens) by stacking two organic electroluminescence units using a easily controllable and low-temperature processable charge generation layer on thin flexible plastic substrates. The flexible OLEDs showed great flexibility against bending stress up to bending strain of 6.7%.
48 citations
TL;DR: Lee et al. as mentioned in this paper proposed a method to solve the problem of particle filtering in the context of nanotechnologies, which is similar to the one we are investigating in this paper.
Abstract: H. Kim, J. Byun, S.-J. Kwon, Y. Lee, Dr. S.-Y. Min, C. Wolf, H.-K. Seo, Prof. T.-W. Lee Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH) Pohang , Gyungbuk 790-784, South Korea E-mail: twlee@postech.ac.kr , taewlees@gmail.com S.-H. Bae, Prof. J.-H. Ahn School of Electrical and Electronic Engineering Yonsei University Seoul 120–749, South Korea Dr. T. Ahmed Theoretical Division Los Alamos National Laboratory Los Alamos , NM 87545, USA Dr. J.-X. Zhu Center for Integrated Nanotechnologies Los Alamos National Laboratory Los Alamos , NM 87545, USA
TL;DR: In this paper, a review mainly focuses on basic flexibility and stretchability concepts that can be integrated and applied to all inorganic-material-based electronic devices, and possible fabrication methods are also discussed.
Abstract: Electronic applications that offer flexibility and stretchability have attracted increasing interest over the past few years because of their potential applications in, for example, electronic skin and bio-inspired devices, where electronics based on current wafer-based technology are not suitable. Metaloxide-based binary and ternary systems offer the opportunity for large-area uniform synthesis of materials with excellent electrical performance. This review mainly focuses on basic flexibility and stretchability concepts that can be integrated and applied to all inorganic-material-based electronic devices. Possible fabrication methods are also discussed, and oxide-based electronic applications summarized in this context. Finally, the review is concluded with a discussion of future prospects of oxides in flexible and stretchable devices.
TL;DR: The fabricated hetero-CMOS inverters which are composed of two isolated transistors exhibit a novel high performance air-stable voltage transfer characteristic with different supply voltages, with a maximum voltage gain of ≈16, and sub-nano watt power consumption.
Abstract: 2D semiconductor materials are being considered for next generation electronic device application such as thin-film transistors and complementary metal–oxide–semiconductor (CMOS) circuit due to their unique structural and superior electronics properties. Various approaches have already been taken to fabricate 2D complementary logics circuits. However, those CMOS devices mostly demonstrated based on exfoliated 2D materials show the performance of a single device. In this work, the design and fabrication of a complementary inverter is experimentally reported, based on a chemical vapor deposition MoS2 n-type transistor and a Si nanomembrane p-type transistor on the same substrate. The advantages offered by such CMOS configuration allow to fabricate large area wafer scale integration of high performance Si technology with transition-metal dichalcogenide materials. The fabricated hetero-CMOS inverters which are composed of two isolated transistors exhibit a novel high performance air-stable voltage transfer characteristic with different supply voltages, with a maximum voltage gain of ≈16, and sub-nano watt power consumption. Moreover, the logic gates have been integrated on a plastic substrate and displayed reliable electrical properties paving a realistic path for the fabrication of flexible/transparent CMOS circuits in 2D electronics.
TL;DR: Li4Ti5O12 (LTO) nanowire arrays were used in this paper to construct a high-power, lightweight, and flexible lithium-ion batteries (FLIBs) to fulfill the energy requirements of future flexible electronic devices.
Abstract: Interest in high-power, lightweight, and flexible lithium-ion batteries (FLIBs) to fulfill the energy requirements of future flexible electronic devices is increasing. To fabricate a thin FLIB, which consists of flexible components, we herein describe, as a model study, a hybrid electrode structure that includes no binders, conductive agents or metal current collectors. A freestanding ultrathin graphite (FSG) film was used as both the current collector and the anode, over which vertically aligned Li4Ti5O12 (LTO) nanowire (NW) arrays were grown. For comparison, two different additive-free electrodes consisting of an LTO NW array on titanium (LTO–Ti) and an LTO NW array on FSG (LTO–FSG) were prepared; they exhibited discharge capacities of 158 and 154 mA h g−1, respectively. The capacities remained approximately stable even after 500 cycles at a 20C charge/discharge rate, revealing outstanding cycling stability of the electrodes. The high-rate performance of the additive-free LTO–FSG electrode revealed excellent rate capability. Finally, the flexible LTO–FSG electrode was used to assemble an FLIB. This novel approach of preparing an additive-free and flexible hybrid electrode enabled the fabrication of a high-power, thin, and lightweight FLIB that exhibited no physical deterioration or performance loss.
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25 Apr 2016TL;DR: Choi et al. as discussed by the authors reported a simple and effective approach for enhancing the electrical properties of graphene by selective graphene-defect decoration with Pd nanoparticles (Pd NPs) using a wet-chemistry-based galvanic displacement reaction.
Abstract: Inherent defects, such as grain boundaries (GBs), wrinkles and structural cracks, present on chemical vapor deposition (CVD)-grown graphene are inevitable because of the mechanism used for its synthesis. Because graphene defects are detrimental to electrical transport properties and degrade the performance of graphene-based devices, a defect-healing process is required. We report a simple and effective approach for enhancing the electrical properties of graphene by selective graphene-defect decoration with Pd nanoparticles (Pd NPs) using a wet-chemistry-based galvanic displacement reaction. According to the selective nucleation and growth behaviors of Pd NPs on graphene, several types of defects, such as GBs, wrinkles, graphene regions on Cu fatigue cracks and external edges of multiple graphene layers, were precisely confirmed via spherical aberration correction scanning transmission electron microscopy, field-emission scanning electron microscopy and atomic force microscopy imaging. The resultant Pd-NP-decorated graphene films showed improved sheet resistance. A transparent heater was fabricated using Pd-decorated graphene films and exhibited better heating performance than a heater fabricated using pristine graphene. This simple and novel approach promises the selective decoration of defects in CVD-grown graphene and further exploits the visualization of diverse defects on a graphene surface, which can be a versatile method for improving the properties of graphene. Decorating defects with palladium nanoparticles enhances the electrical properties of chemical vapour deposited graphene. Unlike other methods of synthesizing graphene, chemical vapour deposition can produce large areas of the two-dimensional material, but it also tends to create defects. Han-Bo-Ram Lee from Incheon National University and Taeyoon Lee from Yonsei University in South Korea, together with colleagues across the country, have shown that palladium nanoparticles automatically attach themselves to imperfections in chemical vapour deposited graphene, thereby improving its electrical properties. They chemical vapour deposited graphene on copper and dipped it in a palladium chloride solution. The greater chemical reactivity of defect sites in graphene causes palladium atoms to selectively attach to imperfections such as wrinkles, cracks and grain boundaries, creating nanoparticles at these sites. The researchers anticipate that the method could be used in the manufacture of smart mirrors and displays. Selective decoration of palladium nanoparticles on diverse graphene-defect sites. Graphene grown by chemical vapor deposition method has structural imperfections, such as grain boundaries, wrinkles and topological cracks, which cause reduced graphene-based device performance. Herein, we selectively decorated graphene defects with metal nanoparticles (Pd and Ag) by using a wet-chemistry-based galvanic displacement reaction within a few minutes without generating damage on graphene. The defect-decorated graphene showed a noticeable improvement in electrical characteristics, and defect-decorated graphene-based transparent heater showed better heating performance than a heater fabricated using pristine graphene.
TL;DR: Ahn et al. as mentioned in this paper proposed a transfer-process-based approach to reduce the number of defects in silicon and enabled devices to be transferred to plastic substrates to realize flexible devices.
Abstract: Strained thin-film transistors have been made by releasing the silicon layer of a silicon-on-insulator (SOI) wafer to realize better devices. Stretching a silicon thin film by a transfer process makes it easier for electrons to flow through the film and hence improves the performance of electronic devices. Jong-Hyun Ahn from Yonsei University and colleagues have realized a simple and cost-effective way for achieving this. After fabricating devices on a SOI wafer, they etched the handle silicon layer of a SOI wafer to leave a suspended silicon–silicon dioxide membrane. Residual oxidation-induced compressive strain in the buried oxide layer of the wafer created the desired strain and introduced it to the top silicon layer. This transfer-process-based approach reduced the number of defects in silicon and enabled devices to be transferred to plastic substrates to realize flexible devices.
Posted Content•
TL;DR: Flexible MgO barrier magnetic tunnel junction devices are fabricated using a transfer printing process that yields significantly enhanced tunneling magnetoresistance and improved abruptness of switching, as residual strain in the MTJ structure is released during the transfer process.
Abstract: Flexible electronic devices require the integration of multiple crucial components on soft substrates to achieve their functions. In particular, memory devices are the fundamental component for data storage and processing in flexible electronics. Here, we present flexible MgO barrier magnetic tunnel junction (MTJ) devices fabricated using a transfer printing process, which exhibit reliable and stable operation under substantial deformation of the device substrates. In addition, the flexible MTJ devices yield significantly enhanced tunneling magnetoresistance (TMR) of ~300 % and improved abruptness of switching, as residual strain in the MTJ structure induced by the fabrication process is released during the transfer process. This approach could be useful for a wide range of flexible electronic systems that require high performance memory components.
TL;DR: In this paper, the instability of an amorphous indium-gallium-zinc oxide (IGZO) field effect transistor is investigated upon water treatment, and it is observed that after a few days of water exposure, an IGZO field effects transistor (FET) shows relatively more stable behavior as compared to before exposure.
Abstract: The instability of an amorphous indium-gallium-zinc oxide (IGZO) field effect transistor is investigated upon water treatment. Electrical characteristics are measured before, immediately after and a few days after water treatment in ambient as well as in vacuum conditions. It is observed that after a few days of water exposure an IGZO field effect transistor (FET) shows relatively more stable behaviour as compared to before exposure. Transfer characteristics are found to shift negatively after immediate water exposure and in vacuum. More interestingly, after water exposure the off current is found to decrease by 1-2 orders of magnitude and remains stable even after 15 d of water exposure in ambient as well as in vacuum, whereas the on current more or less remains the same. An x-ray photoelectron spectroscopic study is carried out to investigate the qualitative and quantitative analysis of IGZO upon water exposure. The changes in the FET parameters are evaluated and attributed to the formation of excess oxygen vacancies and changes in the electronic structure of the IGZO bulk channel and at the IGZO/SiO2 interface, which can further lead to the formation of subgap states. An attempt is made to distinguish which parameters of the FET are affected by the changes in the electronic structure of the IGZO bulk channel and at the IGZO/SiO2 interface separately.
TL;DR: A tactile sensor based on a MoS2 strain gauge and a graphene electrode is integrated on a finger tip by M. Kim, J. Ahn, and co-workers, expected to provide great opportunities for electronic-skin and wearable-electronics applications.
Abstract: A tactile sensor based on a MoS2 strain gauge and a graphene electrode is integrated on a finger tip by M.-S. Kim, J.-H. Ahn, and co-workers, as described on page 2556. The MoS2 and graphene can be conformally attached onto a thumbprint thanks to their outstanding mechanical flexibility. The MoS2 -based tactile sensor, showing excellent sensing properties, is expected to provide great opportunities for electronic-skin and wearable-electronics applications.
01 Jul 2016
TL;DR: A novel method to synthesise of ultrathin graphite film (UTGF) directly on Cu foil using a well-known chemical vapour deposition process to fabricate additive-free and metallic current collector-free electrodes.
Abstract: Due to the unique properties of carbon nano materials, they exhibit great potential in energy storage and conversion devices such as batteries and supercapacitors. The present work reports a novel method to synthesise of ultrathin graphite film (UTGF) directly on Cu foil using a well-known chemical vapour deposition process. Such electrode materials are attractive for use in supercapacitors as electrodes, due to their high stability, surface area and low fabrication. Existing approaches for fabricating additive-free (binder- and carbon black-free) and metallic current collector-free electrodes are complex. In order to overcome this complexities, we have directly grown UTGF over Cu foil without using any additive.
05 Jun 2016
TL;DR: In this paper, the authors demonstrate infrared photodetectors based on graphene-Bi 2 Se 3 heterostructures with high responsivity (≥ 1.9 A/W) at room temperature.
Abstract: We demonstrate infrared photodetectors based on graphene-Bi 2 Se 3 heterostructures with high responsivity (≥1.9 A/W) at room temperature. Strong photogating effect across the tunneling barrier and built-in potential enables the internal quantum efficiency larger than 100 %.
Patent•
04 May 2016TL;DR: In this article, an apparatus for controlling a droplet according to an embodiment includes a flexible substrate (120) having a hydrophobic or oleophobic surface, a dimple formation unit (140), which locally deforms a bottom surface of the flexible substrate to form a dimone, and a driving unit (160) which moves the dimple to a lower side of the substrate.
Abstract: Provided is an apparatus and method for controlling a droplet. The apparatus for controlling the droplet does not contaminate/damage a sample, has a high degree of freedom in droplet control, and is capable of being repeatedly used for a long time. An apparatus (100) for controlling a droplet according to an embodiment includes a flexible substrate (120) having a hydrophobic or oleophobic surface, a dimple formation unit (140), which locally deforms a bottom surface of the flexible substrate (120) to form a dimple, and a driving unit (160), which moves the dimple formation unit (140) at a lower side of the flexible substrate.
08 Apr 2016