Dong Ick Son

Flexible Organic Bistable Devices Based on Graphene Embedded in an Insulating Poly(methyl methacrylate) Polymer Layer

The electrical properties of flexible nonvolatile organic bistable devices (OBDs) fabricated with graphene sandwiched between two insulating poly(methyl methacrylate) (PMMA) polymer layers were investigated. Current-voltage (I-V) measurements on the Al/PMMA/graphene/PMMA/indium-tin-oxide/poly(ethylene terephthalate) devices at 300 K showed a current bistability due to the existence of the graphene, indicative of charge storage in the graphene. The maximum ON/OFF ratio of the current bistability for the fabricated OBDs was as large as 1 x 10(7), and the endurance number of ON/OFF switchings was 1.5 x 10(5) cycles, and an ON/OFF ratio of 4.4 x 10(6) was maintained for retention times larger than 1 x 10(5) s. No interference effect was observed for the scaled-down OBDs containing a graphene layer. The memory characteristics of the OBDs maintained similar device efficiencies after bending and were stable during repetitive bendings of the OBDs. The mechanisms for these characteristics of the fabricated OBDs are described on the basis of the I-V results.

Photoresponse mechanisms of ultraviolet photodetectors based on colloidal ZnO quantum dot-graphene nanocomposites

Ultraviolet (UV) photodetectors were fabricated using the wet spin-coating for ZnO quantum dots (QDs) and the transfer method for the graphene sheet. High-resolution transmission electron microscopy images showed that the ZnO QDs were uniformly distributed between the voids of the surface circumferences on the graphene layers. Current-voltage measurements on the UV photodetector at 300 K showed that the ratio of the photocurrent to the dark current was about 1.1 × 104. The rise and the decay times of the UV photodetector were approximately 2 and 1 s, respectively. The photoresponse mechanisms are described on the basis of the experimental results.

Inverted Quantum Dot Light Emitting Diodes using Polyethylenimine ethoxylated modified ZnO

Colloidal quantum dots (QDs) are an emerging class of new materials due to their unique physical properties. In particular, colloidal QD based light emitting diodes (QDLEDs) have been extensively studied and developed for the next generation displays and solid-state lighting. Among a number of approaches to improve performance of the QDLEDs, the most practical one is optimization of charge transport and charge balance in the recombination region. Here, we suggest a polyethylenimine ethoxylated (PEIE) modified ZnO nanoparticles (NPs) as electron injection and transport layer for inverted structure red CdSe-ZnS based QDLED. The PEIE surface modifier, incorporated on the top of the ZnO NPs film, facilitates the enhancement of both electron injection into the CdSe-ZnS QD emissive layer by lowering the workfunction of ZnO from 3.58 eV to 2.87 eV and charge balance on the QD emitter. As a result, this device exhibits a low turn-on voltage of 2.0–2.5 V and has maximum luminance and current efficiency values of 8600 cd/m2 and current efficiency of 1.53 cd/A, respectively. The same scheme with ZnO NPs/PEIE layer has also been used to successfully fabricate green, blue, and white QDLEDs.

Inverted CdSe–ZnS quantum dots light-emitting diode using low-work function organic material polyethylenimine ethoxylated

Inverted quantum dot based light-emitting diodes (QDLED) were simply fabricated by an all solution processing. Polyethylenimine ethoxylated (PEIE) was used as a surface modifier in the device, to reduce the indium tin oxide (ITO) electrode work function below 3.08 eV. Based on transmission electron microscopy (TEM) results, CdSe–ZnS QDs with an 8 nm size were uniformly distributed to form a monolayer on a PEIE/ITO glass substrate. In this inverted QDLED, hybrid polymers [poly(N-vinylcarbazole) + poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine were adopted as a hole transporting layer (HTL) to enhance the hole transport property. At a low-operating voltage of 3 V, the device was turned on and emitted a spectrally red color light with a maximum luminance of 2900 cd m−2 and a current efficacy of 0.35 cd A−1.

Polymer?ultrathin graphite sheet?polymer composite structured flexible nonvolatile bistable organic memory devices

We present data, which were obtained before bending and after bending, for the electrical bistabilities, memory stabilities, and memory mechanisms of three-layer structured flexible bistable organic memory (BOM) devices, which were fabricated utilizing the ultrathin graphite sheets (UGS) sandwiched between insulating poly(methylmethacrylate) (PMMA) polymer layers. The UGS were formed by transferring UGS (about 30 layers) and using a simple spin-coating technique. Transmission electron microscopy (TEM) measurements were performed to investigate the microstructural properties of the PMMA/UGS/PMMA films. Current-voltage (I-V) measurements were carried out to investigate the electrical properties of the BOM devices containing the UGS embedded in the PMMA polymer. Current-time (I-t) and current-cycle measurements under flat and bent conditions were performed to investigate the memory stabilities of the BOM devices. The memory characteristics of the BOM maintained similar device efficiencies after bending and were stable during repeated bendings of the BOM devices. The mechanisms for these characteristics of the fabricated BOM are described on the basis of the I-V results.

Charge separation and ultraviolet photovoltaic conversion of ZnO quantum dots conjugated with graphene nanoshells

ZnO-graphene quasi core-shell quantum dot (QD) structures in which the inner ZnO QDs are covered with graphene nanoshells have been synthesized via a simple solution process method. The outer graphene nanoshells were identified as a single graphene layer using high resolution transmission electron microscopy (HR-TEM). Zn-O-C (graphene) chemical bonds between the inner ZnO QDs and the oxygen-containing functional groups introduced into the graphene layer are believed to be important in the formation of the consolidated quasi core-shell QD structure. A multilayer structure organic ultraviolet (UV) photovoltaic (PV) device was fabricated using ZnO-graphene core-shell QDs as the absorption layer. A quenching behavior as large as 71% near the UV emission peak for the ZnO-graphene core-shell QDs was observed in the photoluminescence. Density of state (DOS) calculations for the graphene using density functional theory (DFT) revealed that the static quenching can be attributed to a faster charge separation via the direct electron transfer from the conduction band (CB) of the ZnO QDs to the induced lowest unoccupied molecular orbitals (LUMO) of the graphene nanoshell resulting from the Zn-O-C bonding. This charge separation mechanism was confirmed experimentally using time-correlated single photon counting (TCSPC) measurements. The calculated average lifetime of 0.13 ns and 0.165 ns of the 375 and 383 nm UV emissions, respectively, for the ZnO-graphene core-shell QDs were approximately 10 times faster than those of 1.86 ns and 1.83 nm for the reference ZnO QDs; this is indicative of the existence of an additional high efficiency relaxation channel. The observed saturation current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) were 196.4 μA/cm2, 0.99 V, 0.24, and 2.33%, respectively. In this study, it was found that the UV power conversion efficiency of ZnO QDs could be significantly improved by invoking a fast photoinduced charge separation and the subsequent transport of carriers to the collecting electrodes through conjugation with highly conductive graphene nanoshell acceptors to the ZnO QDs donor.Graphical abstract

High efficiency ultraviolet photovoltaic cells based on ZnO–C60 core–shell QDs with organic–inorganic multilayer structure

We report on the successful conjugation of C60 molecules on the surface of ZnO quantum dots (QDs) and their application in multi-layer structured ultraviolet (UV) photovoltaic (PV) devices. In situgrowth of C60 on the surface of ZnO QDs with a core–shell structure was realized via a mild solution-process method, which resulted in an improvement in photo-induced charge separation and transport of carriers to the collecting electrodes in a fabricated device. The conjugation of the C60 with ZnO QDs leads to a PL quenching of about 99.8%, which can be attributed to an efficient transfer of photo-induced electrons from the ZnO QDs to the C60 through a Zn–O–C chemical bonding. UV PV cells with ZnO–C60 core–shell QDs as an active layer and fabricated through an all layer simple chemical method exhibit high power conversion efficiency as much as about 3.02%.

STRUCTURAL AND OPTICAL PROPERTIES OF ZnO THIN FILMS GROWN ON FLEXIBLE POLYIMIDE SUBSTRATES

Nominally undoped ZnO thin films were grown on polyimide (PI) substrates at various temperatures by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the root mean squares of the average surface roughnesses for the ZnO thin films grown on the PI substrates at 27°C, 100°C, 200°C, and 300°C were 4.08, 4.50, 4.18, and 3.89 nm, respectively. X-ray diffraction patterns showed that the crystallinity of the ZnO films had a preferential (0001) direction and that the full width at half-maxima for the (0002) ZnO diffraction peak for the ZnO thin films grown on the PI substrates at 27°C, 100°C, 200°C, and 300°C were 0.22, 0.22, 0.22, and 0.23, respectively. The average optical transmittances in the visible ranges between 550 and 750 nm for the ZnO/PI heterostructures grown at 27°C, 100°C, 200°C, and 300°C were 87%, 83%, 87%, and 78%, respectively.

Characterization and Photocatalytic Performance of Potassium-Doped Titanium Oxide Nanostructures Prepared via Wet Corrosion of Titanium Microspheres

Potassium doped titanium oxide (KTiOx) nanowires were prepared by the wet corrosion process (WCP) and their photocatalytic effects were systematically characterized. For the synthesis of KTiOx, the potassium hydroxide concentration of the WCP was varied in order to obtain nanostructures with different surface area and surface charge. Structural and crystalline properties of KTiOx were studied by means of X-ray diffraction, scanning and transmission electron microscopy. Chemical composition was determined by X-ray fluorescence and energy-dispersive X-ray analysis. Photocatalytic performance was investigated as a function of the surface area, pH, and crystalline structures by studying the degradation of methylene blue, cardiogreen, and azorubine red dyes upon UV irradiation. The negatively charged crystalline KTiOx nanostructures with high surface area showed significantly higher photocatalytic degradation compared to their TiOx counterpart. They also showed high efficiency for recovery and re-use. Annealing KTiOx nanostructures improved structural properties leading to well-ordered layered structures and improved photocatalysis. However, annealing at temperatures higher than 600 °C yielded formation of rutile grains at the surface of nanowires, significantly affecting the photocatalytic performance. We believe that KTiOx nanostructures produced by WCP are very promising for photocatalysis, especially due to their high photocatalytic efficiency as well as their potential for re-use and durability.

Environment-friendly, durable, electro-conductive, and highly transparent heaters based on silver nanowire functionalized keratin nanofiber textiles

We demonstrate the fabrication of highly transparent, electro-conductive, durable, and eco-friendly nanofiber (NF) textiles based on keratin using highly conductive silver nanowire (Ag NW) networks on transparent nano-fibrous substrates for multifunctional, high-performance flexible heaters. These NF-based textiles, including keratin, have a high transparency of more than 89% in the visible area, making them very useful for manufacturing transparent wearable electronics. The Ag NW networks on the nanofibers provide a low sheet resistance of 25 Ω sq−1, with a maximum optical transparency of 85% and superior flexibility with mechanical durability up to 1000 cycles of bending and release. Flexible and transparent wearable electronic devices, such as heaters, with an exceptionally high mechanical stability and foldability with negligible changes in electrical conductivity have been successfully fabricated by using this simple method. Also, transparent conductive NFs based on keratin are a promising wearable electronic material for a wide range of high performance, low cost, biocompatible and wearable electronics applications. This study successfully demonstrates a flexible heater based on transparent conductive NFs with a wide operating temperature up to 65.75 °C, a good recovery time of about 10 s and low power consumption (0.38 W) for wearable electronics applications.