Showing papers by "Jong Hyun Ahn published in 2013"

Graphene-P(VDF-TrFE) multilayer film for flexible applications.

Abstract: A flexible, transparent acoustic actuator and nanogenerator based on graphene/P(VDF-TrFE)/graphene multilayer film is demonstrated. P(VDF-TrFE) is used as an effective doping layer for graphene and contributes significantly to decreasing the sheet resistance of graphene to 188 ohm/sq. The potentiality of graphene/P(VDF-TrFE)/graphene multilayer film is realized in fabricating transparent, flexible acoustic devices and nanogenerators to represent its functionality. The acoustic actuator shows good performance and sensitivity over a broad range of frequency. The output voltage and the current density of the nanogenerator are estimated to be ∼3 V and ∼0.37 μAcm–2, respectively, upon the application of pressure. These values are comparable to those reported earlier for ZnO- and PZT-based nanogenerators. Finally, the possibility of rollable devices based on graphene/P(VDF-TrFE)/graphene structure is also demonstrated under a dynamic mechanical loading condition.

2 μm solid-state laser mode-locked by single-layer graphene

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

Ultrafast and widely tuneable vertical-external-cavity surface-emitting laser, mode-locked by a graphene-integrated distributed Bragg reflector

Abstract: We report a versatile and cost-effective way of controlling the unsaturated loss, modulation depth and saturation fluence of graphene-based saturable absorbers (GSAs), by changing the thickness of a spacer between SLG and a high-reflection mirror. This allows us to modulate the electric field intensity enhancement at the GSA from 0 up to 400%, due to the interference of incident and reflected light at the mirror. The unsaturated loss of the SLG-mirror-assembly can be reduced to$\sim$0. We use this to mode-lock a VECSEL from 935 to 981nm. This approach can be applied to integrate SLG into various optical components, such as output coupler mirrors, dispersive mirrors, dielectric coatings on gain materials. Conversely, it can also be used to increase absorption (up to 10%) in various graphene based photonics and optoelectronics devices, such as photodetectors.

Ultrafast and widely tuneable vertical-external-cavity surface-emitting laser, mode-locked by a graphene-integrated distributed Bragg reflector

Abstract: We report a versatile way of controlling the unsaturated loss, modulation depth and saturation fluence of graphene-based saturable absorbers (GSAs), by changing the thickness of a spacer between a single layer graphene (SLG) and a high-reflection mirror. This allows us to modulate the electric field intensity enhancement at the GSA from 0 up to 400%, due to the interference of incident and reflected light at the mirror. The unsaturated loss of the SLG-mirror-assembly can be reduced to ∼0. We use this to mode-lock a vertical-external-cavity surface-emitting laser (VECSEL) from 935 to 981 nm. This approach can be applied to integrate SLG into various optical components, such as output coupler mirrors, dispersive mirrors or dielectric coatings on gain materials. Conversely, it can also be used to increase the absorption (up to 10%) in various graphene based photonics and optoelectronics devices, such as photodetectors.

Graphene based field effect transistors: Efforts made towards flexible electronics

Abstract: The integration of flexibility in existing electronics has been realized as a key point for practical application of unusual format electronics that can extend the application limit of biomedical equipments and of course daily routine kind of electronic devices. Graphene showed the great potentiality for flexible format owing to its excellent electronic, mechanical and optical properties. Field effect transistor (FET) is a basic unit for digital and analog electronics thus enormous efforts have been attempted to fabricate the flexible FETs in order to get the high performance. This article reviews the recent development of graphene based FETs including the fabrication and active layers material compatibility in flexible format.

Fabrication of metallic nanomesh: Pt nano-mesh as a proof of concept for stretchable and transparent electrodes

Abstract: Two-dimensional ordered arrays of honeycomb morphology of platinum are fabricated by using anodized aluminum oxide template and metal sputtering methods. The resulting metal films are highly conductible (71 Ω/sq), stretchable (16.8%), and transparent (75.2% at 550 nm). The presented synthetic strategy is scalable to large area without noticeable defects by incorporating the deposition of a thin layer of silver. In addition, both the pore size and wall thickness of platinum nanomesh films are straightforwardly controlled with sputtering time. As a proof of concept, the metal nanomesh films using AAO template suggest a new concept of synthesizing transparent and stretchable metal electrodes for future electronic devices.

Graphene/liquid crystal based terahertz phase shifters

Abstract: Due to its high electrical conductivity and excellent transmittance at terahertz frequencies, graphene is a promising candidate as transparent electrodes for terahertz devices. We demonstrate a liquid crystal based terahertz phase shifter with the graphene films as transparent electrodes. The maximum phase shift is 10.8 degree and the saturation voltage is 5 V with a 50 µm liquid crystal cell. The transmittance at terahertz frequencies and electrical conductivity depending on the number of graphene layer are also investigated. The proposed phase shifter provides a continuous tunability, fully electrical controllability, and low DC voltage operation.

Load-Controlled Roll Transfer of Oxide Transistors for Stretchable Electronics

Abstract: A stretchable and transparent In-Ga-Zn-O (IGZO) thin film transistors with high electrical performance and scalability is demonstrated. A load-controlled roll transfer method is realized for fully automated and scalable transfer of the IGZO TFTs from a rigid substrate to a nonconventional elastomeric substrate. The IGZO TFTs exhibit high electrical performance under stretching and cyclic tests, demonstrating the potentiality of the load-controlled roll transfer in stretchable electronics. The mechanics of the load-controlled roll transfer is investigated and simulated, and it is shown that the strain level experienced by the active layers of the device can be controlled to well below their maximum fracture level during transfer.

Graphene/liquid crystal based terahertz phase shifters

Abstract: Due to its high electrical conductivity and excellent transmittance at terahertz frequencies, graphene is a promising candidate as transparent electrodes for terahertz devices. We demonstrate a liquid crystal based terahertz phase shifter with the graphene films as transparent electrodes. The maximum phase shift is 10.8 degree and the saturation voltage is 5 V with a 50 um liquid crystal cell. The transmittance at terahertz frequencies and electrical conductivity depending on the number of graphene layer are also investigated. The proposed phase shifter provides a continuous tunability, fully electrical controllability, and low DC voltage operation.

Graphene Films for Flexible Organic and Energy Storage Devices.

Abstract: Graphene and its derivatives have been the subject of extensive research in fundamental science and have viable applications in current and future technology. The exceptionally high electronic and thermal conductivity, optical transparency, and high specific surface area, combined with excellent mechanical flexibility and environmental stability leave graphene poised to be a material of the future. This perspective introduces the importance of graphene electrodes, discusses the synthesis of graphene and transfer onto desired substrates and the role of graphene in electrodes for a broad range of flexible devices such as photovoltaic, electronic, and electrochemical energy storage.

Flexible graphene-PZT ferroelectric nonvolatile memory.

Abstract: We report the fabrication of a flexible graphene-based nonvolatile memory device using Pb(Zr0.35,Ti0.65)O3 (PZT) as the ferroelectric material. The graphene and PZT ferroelectric layers were deposited using chemical vapor deposition and sol–gel methods, respectively. Such PZT films show a high remnant polarization (Pr) of 30 μC cm−2 and a coercive voltage (Vc) of 3.5 V under a voltage loop over ±11 V. The graphene–PZT ferroelectric nonvolatile memory on a plastic substrate displayed an on/off current ratio of 6.7, a memory window of 6 V and reliable operation. In addition, the device showed one order of magnitude lower operation voltage range than organic-based ferroelectric nonvolatile memory after removing the anti-ferroelectric behavior incorporating an electrolyte solution. The devices showed robust operation in bent states of bending radii up to 9 mm and in cycling tests of 200 times. The devices exhibited remarkable mechanical properties and were readily integrated with plastic substrates for the production of flexible circuits.

Graphene-based transparent conductive films

Abstract: Graphene is a promising alternative to indium tin oxide for use in transparent conducting electrodes. We review recent progress in production methods of graphene and its applications in optoelectronic devices such as touch panel screens, organic photovoltaic cells, organic light emitting diodes and thin film transistors. In addition, we discuss important criteria such as optical transmittance, electrical conductivity and work function, which are critical considerations in the integration of graphene conductive films with optoelectronic devices.

High-quality Si3N4 circuits as a platform for graphene-based nanophotonic devices.

Abstract: Hybrid circuits combining traditional nanophotonic components with carbon-based materials are emerging as a promising platform for optoelectronic devices. We demonstrate such circuits by integrating single-layer graphene films with silicon nitride waveguides as a new architecture for broadband optical operation. Using high-quality microring resonators and Mach-Zehnder interferometers with extinction ratios beyond 40 dB we realize flexible circuits for phase-sensitive detection on chip. Hybrid graphene-photonic devices are fabricated via mechanical transfer and lithographic structuring, allowing for prolonged light-matter interactions. Our approach holds promise for studying optical processes in low-dimensional physical systems and for realizing electrically tunable photonic circuits.

Quantum Confinement Effects in Transferrable Silicon Nanomembranes and Their Applications on Unusual Substrates

Abstract: Two dimensional (2D) semiconductors have attracted attention for a range of electronic applications, such as transparent, flexible field effect transistors and sensors owing to their good optical transparency and mechanical flexibility. Efforts to exploit 2D semiconductors in electronics are hampered, however, by the lack of efficient methods for their synthesis at levels of quality, uniformity, and reliability needed for practical applications. Here, as an alternative 2D semiconductor, we study single crystal Si nanomembranes (NMs), formed in large area sheets with precisely defined thicknesses ranging from 1.4 to 10 nm. These Si NMs exhibit electronic properties of two-dimensional quantum wells and offer exceptionally high optical transparency and low flexural rigidity. Deterministic assembly techniques allow integration of these materials into unusual device architectures, including field effect transistors with total thicknesses of less than 12 nm, for potential use in transparent, flexible, and stret...

Thermal stability of metal Ohmic contacts in indium gallium zinc oxide transistors using a graphene barrier layer

Abstract: The excellent impermeability of graphene was exploited to produce stable ohmic contact at the interface between Al metal and a semiconducting indium gallium zinc oxide (IGZO) layer after high-temperature annealing. Thin film transistors (TFTs) were fabricated with and without a graphene interlayer between the Al metal and the IGZO channel region. Metal contact at the interface prepared without a graphene interlayer showed serious instabilities in the IGZO TFT under thermal annealing; however, the insertion of a graphene interlayer between the IGZO channel and the Al metal offered good stability under repeated high-temperature annealing cycles and maintained ohmic contact.

Graphene Based Nanogenerator for Energy Harvesting

Abstract: Development of energy harvesting system becomes one of the most important necessities of today. In this context, nanogenerators (NGs) have attracted considerable attentions in recent years due to their potential applications such as self-powered portable devices. This review article addresses the significant development of NGs systems based on semiconducting and insulating piezoelectric materials. Further, the need of mechanical flexibility and optical transparency on the demand of various electronic applications has been highlighted. In addition, we discussed some recent studies on graphene-based NGs which have been explored for stable performance of NGs.

Conductance modulation in topological insulator Bi2Se3 thin films with ionic liquid gating

Abstract: A Bi2Se3 topological insulator field effect transistor is investigated by using ionic liquid as an electric double layer gating material, leading to a conductance modulation of 365% at room temperature. We discuss the role of charged impurities on the transport properties. The conductance modulation with gate bias is due to a change in the carrier concentration, whereas the temperature dependent conductance change is originated from a change in mobility. Large conductance modulation at room temperature along with the transparent optical properties makes topological insulators as an interesting (opto)electronic material.

Conductance modulation in topological insulator Bi2Se3 thin films with ionic liquid gating

Abstract: A Bi2Se3 topological insulator field effect transistor is investigated by using ionic liquid as an electric double layer gating material, leading to a conductance modulation of 365% at room temperature We discuss the role of charged impurities on the transport properties The conductance modulation with gate bias is due to a change in the carrier concentration, whereas the temperature dependent conductance change is originated from a change in mobility Large conductance modulation at room temperature along with the transparent optical properties makes topological insulators as an interesting (opto)electronic material

Flexible transparent heating element using graphene and method for manufacturing the same

Abstract: The present invention relates to a flexible transparent heating element using graphene and a method for manufacturing the same. The heating element comprises a flexible transparent substrate; a graphene layer formed to at least one side of the flexible transparent substrate; and an electrode connected with the graphene layer.

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. In order to improve the electrical properties of CVD graphene grown on copper (Cu-CVD graphene), recent efforts have focussed on increasing the grain size of such polycrystalline graphene films to 100 micrometers 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.

Technology of Flexible Semiconductor/Memory Device

Abstract: Recently flexible electronic devices have attracted a great deal of attention because of new application possibilities including flexible display, flexible memory, flexible solar cell and flexible sensor. In particular, development of flexible memory is essential to complete the flexible integrated systems such as flexible smart phone and wearable computer. Research of flexible memory has primarily focused on organic-based materials. However, organic flexible memory has still several disadvantages, including lower electrical performance and long-term reliability. Therefore, emerging research in flexible electronics seeks to develop flexible and stretchable technologies that offer the high performance of conventional wafer-based devices as well as superior flexibility. Development of flexible memory with inorganic silicon materials is based on the design principle that any material, in sufficiently thin form, is flexible and bendable since the bending strain is directly proportional to thickness. This article reviews progress in recent technologies for flexible memory and flexible electronics with inorganic silicon materials, including transfer printing technology, wavy or serpentine interconnection structure for reducing strain, and wafer thinning technology.

Critical role of presenilin-dependent γ -secretase activity in DNA damage-induced promyelocytic leukemia protein expression and apoptosis

Abstract: Promyelocytic leukemia (PML) is a major component of macromolecular multiprotein complexes called PML nuclear-bodies (PML-NBs). These PML-NBs recruit numerous proteins including CBP, p53 and HIPK2 in response to DNA damage, senescence and apoptosis. In this study, we investigated the effect of presenilin (PS), the main component of the γ-secretase complex, in PML/p53 expression and downstream consequences during DNA damage-induced cell death using camptothecin (CPT). We found that the loss of PS in PS knockout (KO) MEFs (mouse embryonic fibroblasts) results in severely blunted PML expression and attenuated cell death upon CPT exposure, a phenotype that is fully reversed by re-expression of PS1 in PS KO cells and recapitulated by γ-secretase inhibitors in hPS1 MEFs. Interestingly, the γ-secretase cleavage product, APP intracellular domain (AICD), together with Fe65-induced PML expression at the protein and transcriptional levels in PS KO cells. PML and p53 reciprocally positively regulated each other during CPT-induced DNA damage, both of which were dependent on PS. Finally, elevated levels of PML-NB, PML protein and PML mRNA were detected in the brain tissues from Alzheimer's disease (AD) patients, where γ-secretase activity is essential for pathogenesis. Our data provide for the first time, a critical role of the PS/AICD-PML/p53 pathway in DNA damage-induced apoptosis, and implicate this pathway in AD pathogenesis.

Wavelength tunable graphene modelocked VECSEL

Abstract: We passively modelock an optically pumped VECSEL by using a single-layer graphene saturable absorber mirror, resulting in pulses as short as 473 fs. A broad wavelength tuning range of 46 nm is achieved with three different VECSEL chips, with a single chip 21 nm are covered.

Multiple Virtual Tunneling of Dirac Fermions in Granular Graphene

Abstract: Graphene charge carriers behave as massless Dirac fermions, opening the exciting possibility to observe long-range virtual tunneling of electrons in a solid. In granular metals, electron hops arising from series of virtual transitions are predicted to yield observable currents at low-enough temperatures, but to date experimental evidence is lacking. We report on electron transport in granular graphene films self-assembled by hydrogenation of suspended graphene. While the log-conductance shows a characteristic T−1/2 temperature dependence, cooling the samples below 10 K drives a triple crossover: a slope break in log-conductance, simultaneous to a substantial increase in magneto-conductance and onset of large mesoscopic conductance fluctuations. These phenomena are signatures of virtual transitions of electrons between distant localized states, and conductance statistics reveal that the high crossover-temperature is due to the Dirac nature of granular graphene charge carriers.

Touch panel and producing method of the same

Abstract: The present invention relates to a touch panel and a method of producing the same. The touch panel and the method of producing the same according to the embodiment of the present invention include a substrate; a graphene electrode which is formed on the substrate; a protection layer which includes insulating polymer which is formed on the graphene electrode; an insulating layer which is formed on the protection layer; and an electrode material which is formed on the insulating layer. According to one embodiment of the present invention, the thickness of the protection layer is 1 nm to 1000 nm.

Graphene roll and element

Abstract: PROBLEM TO BE SOLVED: To provide a method and an apparatus for roll-to-roll transfer of graphene which enable transferring a large-area graphene layer, easily at low costs, to a variety of substrates having flexibility and allowing drawing, graphene roll produced by a roll-to-roll transfer step and its applications.SOLUTION: A roll-to-roll graphene transfer method comprises forming a laminate 50 including a substrate 10, a graphene layer 20 and a first flexible substrate 31 from the graphene layer 20 formed on the substrate 10 and the first flexible substrate 31 brought into contact with the graphene layer by using a first roller portion 110, transferring the graphene layer 20 to the first flexible substrate 31, while removing the substrate 10 from the laminate 50 by immersing the laminate 50 in an etching solution 60 and passing through the etching solution 60, by means of a second roller portion 120, transferring the graphene layer 20 on the first flexible substrate 31 to a second flexible substrate 32 by using a third roller portion 130 and taking up in a rolled form.

Passively modelocked VECSEL using a single-layer graphene saturable absorber mirror

Abstract: Optically pumped ultrafast vertical external cavity surface emitting lasers (VECSELs), also referred to as semiconductor disk lasers (SDLs), are very attractive sources for ps- and fs-pulses in the near infrared [1]. So far VECSELs have been passively modelocked with semiconductor saturable absorber mirrors (SESAMs, [2]). Graphene has emerged as a promising saturable absorber (SA) for a variety of applications [3-5], since it offers an almost unlimited bandwidth and a fast recovery time [3-5]. A number of different laser types and gain materials have been modelocked with graphene SAs [3-4], including fiber [5] and solid-state bulk lasers [6-7]. Ultrafast VECSELs are based on a high-Q cavity, which requires very low-loss SAs compared to other lasers (e.g., fiber lasers). Here we develop a single-layer graphene saturable absorber mirror (GSAM) and use it to passively modelock a VECSEL.