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Efficiency roll-off is a significant issue in blue light-emitting diodes (LEDs), but its origin still remains controversial. We report CsPbBr3−xClx/PbBry (or PbCly) (core/shell) quantum dot LEDs exhibiting the emission peak spanning from λpeak = 492 to λpeak = 435 nm by varying the Cl composition (0 ≤ x ≤ 2.21). PLQY and EQE significantly decreased with the increase of the Cl composition. Careful analyses of the composition and the energy structure using EL, PL, TRPL, UPS, XPS and STEM-EDX measurements reveal that the luminance efficiency roll-off in CsPbBr3−xClx LEDs is attributed to the following three combined effects: (1) luminescence quenching by the non-radiative trap-assisted recombination, (2) the hole transfer process at the CsPbBr3−xClx/PbBry core/shell, and (3) imbalanced charge injection caused by a high energy barrier for the hole conduction at the PVK/QD heterointerface (Δh′).

Organic electronic devices such as organic light-emitting diodes (OLEDs), quantum dot LEDs, and organic photovoltaics are promising technologies for future electronics. However, achieving long-term stability of organic-based optoelectronic devices has been regarded as a crucial problem to be solved. In this work, a simple and reproducible fabrication method for ultralow water permeation barrier films having a triple-layered (triad) hydrogenated silicon nitride (a-SiNx:H)/nanosilicon oxynitride (n-SiOxNy)/hybrid silicon oxide (h-SiOx) multistructure is presented. Two triad (a-SiNx:H/n-SiOxNy/h-SiOx)n=2 multistructure barrier films are deposited on both sides of a poly(ethylene terephthalate) substrate using a combination of low-pressure plasma-enhanced chemical vapor deposition and dip coating. The deposited films show a high average transmittance (400-700 nm) of 84% and an ultralow water vapor transmission rate of 2 × 10-6 g/m2/day. In the electroluminescence characteristics of OLEDs encapsulated with two triad barrier films, the operational lifetime (T50) of OLEDs is 1584 h, which is almost similar to that (1416 h) of OLEDs encapsulated with a glass lid.

Owing to the growth of demand for highly integrated electronic devices, high heat dissipation of thermal management materials is essential. Epoxy composites have been prepared with vertically aligned (VA) three-dimensional (3D)-structured SiC sheet scaffolds. The required VA-SiC sheet scaffolds were prepared by a novel approach starting with a graphene oxide (GO) scaffold. The VA-GO scaffolds were reduced to VA-graphene scaffolds in an argon environment, and the latter were subsequently transformed into VA-SiC sheet scaffolds by a template-assisted chemical vapor deposition method. Epoxy resin was filled in the empty spaces of the 3D scaffold of SiC sheets to prepare the composite mass. The material so prepared shows anisotropic thermal property with ultrahigh through-plane conductivity of 14.32 W·m-1·K-1 at a SiC sheet content of 3.71 vol %. A thermal percolation is observed at 1.78 vol % SiC filler. The SiC sheet scaffold of covalently interconnected SiC nanoparticles plays a vital role in the formation of the thermal conductive network to significantly enhance the thermal conductivity of epoxy composites. The application of the VA-SiC/epoxy composite as an efficient thermal dissipating material has also been presented. The VA-SiC/epoxy composites have a strong potential for preparing heat-dissipating components in integrated microelectronics.

Ecofriendly cation metal oxide quantum dots (QDs) are one of the promising candidates to replace for QDs containing expensive indium (In) or hazardous Cd- and Pb-elements. Super-Eg excitation wavel...

With the development of portable and flexible electronic devices, demends for high-performance thermal management materials with high thermal conductivity and electrical insulation are growing. Recently, graphene fluoride has received numerous attention from science community due to its high thermal conductivity and electrical insulation. Here, we report an advanced exfoliated graphene fluoride (EGF) film as a high performance heat spreader and thermal interface material for flexible electronic devices, which simultaneously exhibits ultrahigh thermal conductivity, excellent electrical insulation, and satisfactory flexibility. Commercially available graphite fluoride was directly exfoliated into graphene fluoride solution using ball milling method, followed by preparing ultrathin graphene fluoride films to take full advantage of graphene fluoride properties for heat dissipating applications. The resultant films show a tunable thermal conductivity by altering the thickness of films and show a superior in-plane thermal conductivity of 242 W m−1 K−1 and exceptional through-plane thermal conductivity of 21.8 W m−1 K−1 at the thickness of 10 μm. More interestingly, the graphene fluoride films possess excellent flexibility. The newly developed EGF film provides a creative opportunity for fabrication of multifunctional highly flexible materials, which may promote the development of heat dissipation materials in next generation flexible electronics.

Environmentally friendly metal oxide quantum dot (QD) nanostructures have become attractive alternative materials for replacing QDs containing toxic elements of Cd and Pb. Although many attempts have been proposed for optoelectronic applications of hybrid metal oxide QDs with tailored opto-electronic properties, the fabrication of heterogeneous QD–fullerene hybrids as an emissive material has been studied little for optoelectronic devices. Herein, we report the preparation of ZnO–fullerene C70 (ZnO–C70) QDs by a facile chemical reaction with a zero length linker and demonstrate whitish light-emitting diodes by fullerene induced multicolor emission from the ZnO–C70 QD heterostructure. The electroluminescence (EL) is attributed to three new emission features in the visible region that originate from the electron transitions from three split lowest unoccupied molecular orbital levels of C70 molecules to the valence band of ZnO QDs. Compared with photoluminescence, the EL emission was red-shifted by 50–130 meV due to the interaction between ZnO–C70 QDs in the solid state. The energy levels of C70 were also suitably aligned to those of ZnO QDs with a lower electron transfer barrier of 0.22 eV, which offers an advantageous route for electron transport. Consequently, the environmentally friendly ZnO–C70 QDs with extraordinary properties promise great potential for use as novel encapsulating materials in the field of opto-electronic applications adopting QDs.

A structure of flexible glass/flexible transparent conducting electrode (TCE) comparable with the conventional glass/indium tin oxide (ITO) substrate is proposed by combining an Ag NW + PEDOT:PSS hybrid TCE (h‐TCE) with triple‐layer a‐SiNx:H/n‐SiOxNy/h‐SiOx ultrahigh barrier (UHB) films. The UHB films have dual functions of both moisture barriers with a water vapor transmission rate (WVTR) of 1.6 × 10−5 × g m−2 day−1 and antireflection (AR) with a gradient refractive index. Flexible PTB7:PC70BM‐based organic photovoltaics (OPVs) with the AR‐UHB film achieve a maximum power conversion efficiency (PCE) of ηmax = 8.33%, saturation current density (Jsc) of 17.67 mA cm−2, and external quantum efficiency (EQE) of 63.3%, which are higher than ηmax = 7.63%, Jsc = 15.25 mA cm−2, and EQE = 59.26% of the OPVs on glass/ITO. These result from the increase in the absorption of the donor PTB7 due to the AR effect of the UHB films. A long‐term stability of the AR‐UHB encapsulation layer in damp heat (40 °C/85% relative humidity [RH]) is confirmed by the result that the measured T80 and T50 values for the OPV device on glass/ITO encapsulated with the AR‐UHB film are about 1008 h (42 days) and 2205 h (92 days), respectively.

Quantum dot light-emitting diodes (QLEDs) are expected to be the basis of next-generation displays and have consequently been extensively investigated with the aim of commercialization. Herein, QLE...

This study demonstrates quantum-dot light-emitting diodes (QD-LEDs) with a function of resistive switching memory, capable of on/off operation at the same driving current depending on reset/set state. The QD-LEDs were fabricated by spin-coating process and experienced two different annealing conditions, which yielded defective or less-defective V2O5–x layer. One of the annealing conditions produced QD-LEDs with the unusual electrical behaviors of negative differential resistance (NDR), capacitance oscillation, and voltage–current hysteresis curves, signifying so-called resistive switching characteristics. X-ray and ultraviolet photoelectron spectroscopies were used to examine the chemical state of the differently annealed V2O5–x layers. The less stoichiometric V2O5–x layer was found to be responsible for the resistive switching behaviors of the NDR and the low and high resistance states (LRS and HRS, respectively). We discuss the LRS/HRS of V2O5–x for resistive switching in terms of a conductive filament effect, induced by microstructural changes caused by oxygen drift and vacancy annihilation processes in the high defect density V2O5–x layer.

In our study, to optimize the electron–hole balance through controlling the electron transport layer (ETL) in the QD-LEDs, four materials (ZnO, ZnGaO, ZnMgO, and ZnGaMgO NPs) were synthesized and applied to the QD-LEDs as ETLs. By doping ZnO NPs with Ga, the electrons easily inject due to the increased Fermi level of ZnO NPs, and as Mg is further doped, the valence band maximum (VBM) of ZnO NPs deepens and blocks the holes more efficiently. Also, at the interface of QD/ETLs, Mg reduces non-radiative recombination by reducing oxygen vacancy defects on the surface of ZnO NPs. As a result, the maximum luminance (Lmax) and maximum luminance efficiency (LEmax) of QD-LEDs based on ZnGaMgO NPs reached 43 440 cd m−2 and 15.4 cd A−1. These results increased by 34%, 10% and 27% for the Lmax and 450%, 88%, and 208% for the LEmax when compared with ZnO, ZnGaO, and ZnMgO NPs as ETLs.

Because of its zero bandgap, graphene has not been considered to be useful for optoelectronic devices that utilize absorption or photoemission phenomena. Even though the finite size 0D graphene quantum dots can open the bandgap due to quantum confinement, graphene composites through hybridization have been investigated to control optical properties. Photoluminescence (PL) quenching or enhancement was achieved by resonance energy transfer to graphene or by the localized surface plasmon of graphene. Reduced graphene oxide (rGO) and graphene oxide (GO) composites with metal oxide (MO) also reduced or increased the PL intensity of the band-edge emission, or visible light emission related to the defect in MO was ascribed to charge transfer from MO to rGO or GO and depended on the amounts of sp2 C–C bonding in rGO and GO. These graphene-based functional composites have been also extensively developed as an active light-emitting material and a very promising candidate for flexible lighting and light-emitting diode.

In this study, Iridium (Ir) metallic layer as an ultra-thin surface modifier (USM) was deposited on ITO coated glass substrate using radio frequency magnetron sputtering for improving the photo-conversion efficiency of organic photovoltaic cells. Ultra-thin Ir acts as a surface modifier replacing the conventional hole transport layer (HTL) PEDOT:PSS in organic photovoltaic (OPV) cells with two different active layers P3HT:PC60BM and PTB7:PC70BM. The Ir USM (1.0 nm) coated on ITO glass substrate showed transmittance of 84.1% and work function of >5.0 eV, which is higher than that of ITO (4.5–4.7 eV). The OPV cells with Ir USM (1.0 nm) exhibits increased power conversion efficiency of 3.70% (for P3HT:PC60BM active layer) and 7.28% (for PTB7:PC70BM active layer) under 100 mW/cm2 illumination (AM 1.5G) which are higher than those of 3.26% and 6.95% for the same OPV cells but with PEDOT:PSS as HTL instead of Ir USM. The results reveal that the chemically stable Ir USM layer could be used as an alternative material for PEDOT:PSS in organic photovoltaic cells.

Surface engineering of the electron collecting layers (ECLs) is a straightforward and practical strategy to develop high performance inverted structure organic solar cells (OSCs). Here, we systematically investigate four different types of surface modified ECLs to implement high performance low-energy band gap poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-bA]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b ]-thiophenediyl] (PTB7) and [6,6]-phenyl-C70-butyric acid methyl ester (PC 70 BM) based inverted OSCs: (1) single PEIE, (2) ZnO:PEIE mixture (1:1 vol ratio), (3) PEIE/ZnO bilayer, and (4) ZnO/PEIE bilayer. OSCs with the ZnO/PEIE ECL show the highest power conversion efficiency (PCE) value of 8.61% while all the other devices exhibit PCE values of less than 7.80%. The excellent surface coverage as well as proper band alignment of the ZnO/PEIE bilayer leads to the highest device performance.

Fabrication of highly flexible, inverted, indium tin oxide (ITO)-free top-emitting quantum-dot light-emitting diodes (QDLEDs) on elastic polyurethane (PU) substrates is reported here. To cope with the irregular surfaces of the PU substrate, polyvinyl alcohol (PVA) is used as a buffer layer. By adopting the PVA buffer layer, we demonstrate red-color and green-color top-emitting QDLEDs that show turn-on voltages from 4.0 V to 4.5 V, and maximum luminance values of 1200 cd m−2 (red) and 4800 cd m−2 (green). In addition, when the QDLEDs are stuck on the curved surfaces (vials and a tumbler), the devices still exhibit a clear light emission. These highly flexible QDLEDs can pave the way for further development of next-generation wearable or attachable displays, or solid-state lighting.

Although various colloidal quantum dot (QD) coating and patterning techniques have been developed to meet the demands in optoelectronic applications over the past years, each of the previously demonstrated methods has one or more limitations and trade-offs in forming multicolor, high-resolution, or large-area patterns of QDs. In this study, we present an alternative QD patterning technique using conventional photolithography combined with charge-assisted layer-by-layer (LbL) assembly to solve the trade-offs of the traditional patterning processes. From our demonstrations, we show repeatable QD patterning process that allows multicolor QD patterns in both large-area and microscale. Also, we show that the QD patterns are robust against additional photolithography processes and that the thickness of the QD patterns can be controlled at each position. To validate that this process can be applied to actual device applications as an active material, we have fabricated inverted, differently colored, active QD light-emitting device (QD-LED) on a pixelated substrate, which achieved maximum electroluminescence intensity of 23 770 cd/m

Two-dimensional (2D) van der Waals (vdW) atomic crystals have been extensively studied and significant progress has been made. The newest 2D vdW material, called black phosphorus (BP), has attracted considerable attention due to its unique physical properties, such as its being a singlecomponent material like graphene, and its having a high mobility and direct band gap. Here, we report on a high-performance BP nanosheet based ferroelectric field effect transistor (FeFET) with a poly(vinylidenefluoride-trifluoroethylene) top-gate insulator for a nonvolatile memory application. The BP FeFETs show the highest linear hole mobility of 563 cm2/Vs and a clear memory window of more than 15 V. For more advanced nonvolatile memory circuit applications, two different types of resistive-load and complementary ferroelectric memory inverters were implemented, which showed distinct memory on/off switching characteristics.

2D van der Waals atomic crystal materials have great potential for use in future nanoscale electronic and optoelectronic applications owing to their unique properties such as a tunable energy band gap according to their thickness or number of layers. Recently, black phosphorous (BP) has attracted significant interest because it is a single-component material like graphene and has high mobility, a direct band gap, and exhibits ambipolar transition behavior. This study reports on a charge injection memory field-effect transistor on a glass substrate, where few-layer BPs act as the active channel and charge trapping layers, and Al2O3 films grown by atomic layer deposition act as the tunneling and blocking layers. Because of the ambipolar properties of BP nanosheets, both electrons and holes are involved in the charge trapping process, resulting in bilateral threshold voltage shifts with a large memory window of 22 V. Finally, a memory circuit of a resistive-load inverter is implemented that converts analog signals (current) to digital signals (voltage). Such a memory inverter also shows a clear memory window and distinct memory on/off switching characteristics.

We demonstrate the polyol synthesis of ultrathin Ag nanowires with diameters of 20 nm and an aspect ratio as high as ∼1000 under high-pressure conditions. The increase in pressure increases density and enhances solubility, making the reactive solution rapidly reach super-saturation and increase the nucleation rate to effect a near-instantaneous formation of Ag nuclei. Clearly, the particle size was small at high pressure and an increase in pressure promoted the formation of small-size Ag seed-particles and small diameter wires. As a result, the Ag nanostructures in this study were initially formed as Ag seed particles with a diameter of 20 nm and subsequently grew into well-defined Ag nanowires with a uniform and narrow diameter distribution in the range of 16–22 nm, with a long dimension of up to 20 μm. Fabrication of random networks of the 20 nm diameter Ag nanowires synthesized lead to the fabrication of flexible transparent electrodes with excellent optoelectronic performance, with a sheet resistance of 30 Ω sq−1 and 94% transmittance with a very low haze value of ≤1.0%, making them suitable for electronic display applications.

ZnO-Graphene conjugated thin films were prepared using spin coating technique for different spin rates. Prior to the deposition, ZnO-Graphene nanoparticles were synthesized and their particle size and conjugation was studied through Transmission electron microscope (TEM). The deposited films were characterized using grazing incidence x-ray diffractometer (GIXRD), atomic force microscope (AFM) and UV-Visible spectrometer for their crystallinity, surface topographic features and optical properties. GIXRD patterns confirms the presence of both ZnO and Graphene related crystalline peaks supports the TEM results, which shows the quasi core-shell type conjugation of ZnO-Graphene particles. The crystallinity as well as thickness of the films found to decrease with increase of spin rate. AFM results reveal the uniform, smooth and homogeneity of films and also good adhesivity of ZnO-Graphene with glass substrates. No significant change in the transmittance and absorption with spin rate is observed, while the band gap energy found to decrease due to the reduction in the thickness of the films and conjugation of ZnO-Graphene. All films exhibit∼90 % transmittance in the visible wavelength region, could be potential candidates for optoelectronics and transparent conducting oxide (TCO) applications.

Black phosphorus, or BP, has found a lot of applications in recent years including photonics. The most recent studies have shown that the material has an excellent optical nonlinearity useful in many areas, one of which is in saturable absorption for passive mode-locking. A direct interaction scheme for mode-locking, however, has a potential to optically cause permanent damage to the already delicate material. Evanescent field interaction scheme has already been proven to be a useful method to prevent such danger for other 2-dimensional nanomaterials. In this report, we have utilized the evanescent field interaction to demonstrate that the optical nonlinear characteristics of BP is sufficiently strong to use in such an indirect interaction method. The successful demonstration of the passive mode-locking operation has generated pulses with the pulse duration, repetition rate, and time bandwidth product of 2.18 ps, 15.59 MHz, and 0.336, respectively.

Two-dimensional van der Waals (2D vdWs) materials are a class of new materials that can provide important resources for future electronics and materials sciences due to their unique physical properties. Among 2D vdWs materials, black phosphorus (BP) has exhibited significant potential for use in electronic and optoelectronic applications because of its allotropic properties, high mobility, and direct and narrow band gap. Here, we demonstrate a few-layered BP-based nonvolatile memory transistor with a poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric top gate insulator. Experiments showed that our BP-based ferroelectric transistors operate satisfactorily at room temperature in ambient air and exhibit a clear memory window. Unlike conventional ambipolar BP transistors, our ferroelectric transistors showed only p-type characteristics due to the carbon-fluorine (C-F) dipole effect of the P(VDF-TrFE) layer, as well as the highest linear mobility value of 1159 cm(2) V(-1) s(-1) with a 10(3) on/off current ratio. For more advanced memory applications beyond unit memory devices, we implemented two memory inverter circuits, a resistive-load inverter circuit and a complementary inverter circuit, combined with an n-type molybdenum disulfide (MoS2) nanosheet. Our memory inverter circuits displayed a clear memory window of 15 V and memory output voltage efficiency of 95%.

Indium free consisting of three alternating layers GTO/Ag/GTO has been fabricated by radio-frequency (RF) sputtering for the appli-cations as transparent conducting electrodes and the structural, electrical and optical properties of the gallium tin oxide (GTO) films werecarefully studied. The gallium tin oxide thin films deposited at room temperature are found to have an amorphous structure. Hall Effectmeasurements show a strong influence on the conductivity type where it changed from n-type to p-type at 700 o C. GTO/Ag/GTO mul-tilayer structured electrode with a few nm of Ag layer embedded is fabricated and show the optical transmittance of 86.48% in the vis-ible range (λ = 380~770 nm) and quite low electrical resistivity of ~10 -5 Ω cm. The resultant power conversion efficiency of 2.60%of the multilayer based OPV (GAG) is lower than that of the reference commercial ITO. GTO/Ag/GTO multilayer is a promising trans-parent conducting electrode material due to its low resistivity, high transmittance, low temperature deposition and low cost components.Keywords: Transparent conductive oxide, Gallium Tin Oxide (GTO), RF Sputtering, Structural and optical Properties, Bulkhetero-junction Organic Photovoltaic’s Cells (BHJ-OPVs), and power conversion efficiency

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.

Mn-doped SnO2 thin films were fabricated by a continuous composition spread (CCS) method on a glass substrate at room temperature to find optimized compositions. The fabricated materials were found to have a lower resistivity than pure SnO2 thin films because of oxygen vacancies generated by Mn doping. As Mn content was increased, resistivity was found to decrease for limited doping concentrations. The minimum thin film resistivity was 0.29 Ω-cm for a composition of 2.59 wt % Mn-doped SnO2. The Sn–O vibrational stretching frequency in FT-IR showed a blue shift, consistent with oxygen deficiency. Mn-doped SnO2/Ag/Mn-doped SnO2 multilayer structures were fabricated using this optimized composition deposited by an on-axis radio frequency (RF) sputter. The multilayer transparent conducting oxide film had a resistivity of 7.35 × 10–5 Ω-cm and an average transmittance above 86% in the 550 nm wavelength region.

Transparent and conductive multilayer thin films consisting of three alternating layers FZTO/Ag/WO3 have been fabricated by radiofrequency (RF) sputtering for the applications as transparent conducting oxides and the structural and optical properties of the resulting films were carefully studied. The single layer fluorine doped zinc tin oxide (FZTO) and tungsten oxide (WO3) films grown at room temperature are found to have an amorphous structure. Multilayer structured electrode with a few nm Ag layer embedded in FZTO/Ag/ WO3 (FAW) was fabricated and showed the optical transmittance of 87.60 % in the visible range (λ = 380~770 nm), quite low electrical resistivity of ~ 10-5 Ω cm and the corresponding figure of merit (T10/Rs) is equivalent to 3.0×10-2Ω-1. The resultant power conversion efficiency of 2.50% of the multilayer based OPV is lower than that of the reference commercial ITO. Asymmetric D/M/D multilayer is a promising transparent conducting electrode material due to its low resistivity, high transmittance, low temperature deposition and low cost components.

Aim To evaluate the safety and efficacy of repeated pelvic arterial embolisation (PAE) for uncontrolled postpartum haemorrhage (PPH) after a single session of PAE and to compare angiographic findings between the two sessions of PAE. Materials and methods A total of 23 consecutive patients (age range, 23–44 years) who underwent repeated PAE for uncontrolled PPH between March 2001 and January 2016 in Severance Hospital were reviewed. The interval times between the two sessions of PAE, the angiographic findings, embolic materials, arteries embolised during PAE, and the clinical outcomes were reviewed retrospectively. Results Overall clinical success was achieved after repeated PAE in 21 of 23 patients (91.3%). There were no procedure-related, major complications. On angiography, active bleeding from the uterine collateral arteries was more frequently observed in the second session of PAE (p>0.05), and embolisation of the anterior division of the internal iliac artery was significantly higher during the second session of PAE. Use of permanent embolic materials was significantly higher during the second session of PAE. Recanalisation of a previously embolised artery was identified in 14 patients (60.9%) during the second session. Conclusion Repeated PAE is safe and effective for managing recurrent bleeding after a single session of PAE. Repeated PAE is related to a higher chance of embolisation of the anterior division of the internal iliac artery, with the use of permanent embolic materials. Recanalisation of a previously embolised artery seems to be a principal source of rebleeding during a repeated session of PAE.

Two-dimensional van der Waals (2D vdWs) materials are a class of new materials that can provide important resources for future electronics and materials sciences due to their unique physical properties. Molybdenum disulfide (MoS2) is one of the most promising n-type TMD semiconductors. Several research groups reported on MoS2 nanosheet based transistors that exhibit satisfactory carrier mobility values with high on/off current ratios. On the other hand, a newly discovered 2D vdWs material, called black phosphorous (BP), has generated considerable scientific and technological interest in the research community. 2D BP also has considerable potential for electronic and optoelectronic applications. This is evidenced by recent research on FETs, diodes, and photodetectors involving few-layered BP flakes. Here, we report on a high performance MoS2 and BP nanosheet based nonvolatile memory transistors with a poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric top gate insulator. The MoS2 ferroelectric field-effect transistor (FeFET) shows a highest linear electron mobility value of 175 cm2/Vs with a high on/off current ratio more than 107, and a very clear memory window over 15 V. The program and erase dynamics and static retention properties are also well demonstrated. Our BP ferroelectric FETs (FeFETs) also exhibit a clear memory window of 15 V and a highest linear mobility value of 1159 cm2V-1s-1 with a 103 on/off current ratio at room temperature in ambient air. In order to explore advanced memory applications beyond unit memory devices, we implement two kinds of memory inverter circuits: a resistive-load inverter circuit and a CMOS inverter circuit combined with n-type MoS2.

One of a wide-bandgap semiconductor, Zinc oxide (ZnO) has a near ultraviolet bandgap (3.37 eV) and an exciton binding energy of 60 meV at room temperature (RT), and has several favorable properties, such as high electron mobility, high oscillator strength, and good transparency. In the photoluminescence (PL) spectra of ZnO nanoparticles, the near band edge ultraviolet (UV) emission at 378 nm relevant to direct bandgap of ZnO, and blue light emissions centered at 410, 435, and 465 nm corresponding to Zn interstitial (Zni) to valence band maximum (VBM), and to Zn vacancies (VZn) and green light emission at 540 nm corresponding to conduction band maximum (CBM) to oxygen vacancy (Vo). Ultra-small size quasi consolidated ZnO-graphene nanoparticles was synthesized in which graphene outer layer was chemically attached with ZnO inner core. After attaching graphene to ZnO, green emission completely disappeared whereas the intensity of blue emission was greatly increased. Enhanced blue emission could be well described by both fast electron transfer from CBM of ZnO to graphene having similar molecular energy level with Zni and transition to VBM and Vzn. Glass/ITO/PEDOT:PSS/poly-TPD/ZnO-graphene/Cs2CO3/Al were fabricated and showed the blue emission centered at 435 nm with FWHM of about 90 nm.

Electroluminescence efficiency is crucial for the application of quantum-dot light-emitting diodes (QD-LEDs) in practical devices. We demonstrate that nitrogen-doped carbon nanodot (N-CD) interlayer improves electrical and luminescent properties of QD-LEDs. The N-CDs were prepared by solution-based bottom up synthesis and were inserted as a hole transport layer (HTL) between other multilayer HTL heterojunction and the red-QD layer. The QD-LEDs with N-CD interlayer represented superior electrical rectification and electroluminescent efficiency than those without the N-CD interlayer. The insertion of N-CD layer was found to provoke the Förster resonance energy transfer (FRET) from N-CD to QD layer, as confirmed by time-integrated and -resolved photoluminescence spectroscopy. Moreover, hole-only devices (HODs) with N-CD interlayer presented high hole transport capability, and ultraviolet photoelectron spectroscopy also revealed that the N-CD interlayer reduced the highest hole barrier height. Thus, more balanced carrier injection with sufficient hole carrier transport feasibly lead to the superior electrical and electroluminescent properties of the QD-LEDs with N-CD interlayer. We further studied effect of N-CD interlayer thickness on electrical and luminescent performances for high-brightness QD-LEDs. The ability of the N-CD interlayer to improve both the electrical and luminescent characteristics of the QD-LEDs would be readily exploited as an emerging photoactive material for high-efficiency optoelectronic devices.

ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO-graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystalline-electrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy.

A hybrid polymer-nanorod (NR) light-emitting diode (LED), consisting of a hole-conducting polymer poly (9-vinyl carbazole) (PVK) and ZnO nanorod (NR) composite, with the device structure of glass/indium-tin-oxide (ITO)/PEDOT:PSS/(PVK + ZnO nanorods)/Al is fabricated through a simple spin coating technique. TEM images shows inhomogeneous deposition and the agglomeration of ZnO NRs, which is explained through their low probability of adsorption on PVK due to two-dimensional structural property. In the current-voltage characteristics, negative differential resistance (NDR) phenomenon is observed corresponding to device structure without ZnO NRs. The carrier transport behavior in the LED device is well described by both ohmic and space-chargelimited-current (SCLC) mechanisms. Broad blue electroluminescence (EL) consisting of two sub peaks, are centered at 441 nm and the other at 495 nm, is observed, which indicates that the ZnO nanorod play a role as a recombination center for excitons. The red shift in the position of the EL compared to that photoluminescence is well explained through band offsets at the heterojunction between the PVK and ZnO NRs.

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of KoreaReceived 30 November 2013, revised 14 May 2014, accepted 21 May 2014Published online 30 June 2014Keywords electrical properties, optical absorption, organic solar cells, silver, transparent electrodes, zinc tin oxide

Transparent and conductive thin films of fluorine doped zinc tin oxide (FZTO) were deposited on glass substrates by radio-frequency (RF) magnetron sputtering using a 30 wt% ZnO with 70 wt% SnO2 ceramic targets. The F-doping was carried out by introducing a mixed gas of pure Ar, CF4, and O2 forming gas into the sputtering chamber while sputtering ZTO target. The effect of annealing temperature on the structural, electrical and optical performances of FZTO thin films has been studied. FZTO thin film annealed at 600 °C shows the decrease in resistivity 5.47 × 10−3 Ω cm, carrier concentration ∼1019 cm−3, mobility ∼20 cm2 V−1 s−1 and an increase in optical band gap from 3.41 to 3.60 eV with increasing the annealing temperatures which is well explained by Burstein–Moss effect. The optical transmittance of FZTO films was higher than 80% in all specimens. Work function (ϕ) of the FZTO films increase from 3.80 eV to 4.10 eV through annealing and are largely dependent on the amounts of incorporated F. FZTO is a possible potential transparent conducting oxide (TCO) alternative for application in optoelectronics.

An inverted-type quantum-dot light-emitting-diode (QD LED), employing low-work function organic material polyethylenimine ethoxylated (PEIE) as electron injection layer, was fabricated by all solution processing method, excluding anode electrode. From transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies, it was confirmed that CdSe@ZnS QDs with 7 nm size were uniformly distributed as a monolayer on PEIE layer. In this inverted QD LED, two kinds of hybrid organic materials, [poly (9,9-di-n-octyl-fluorene-alt-benzothiadiazolo)(F8BT) + poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (poly-TPD)] and [4,4′-N,N′-dicarbazole-biphenyl (CBP) + poly-TPD], were adopted as hole transport layer having high highest occupied molecular orbital (HOMO) level for improving hole transport ability. At a low-operating voltage of 8 V, the device emits orange and red spectral radiation with high brightness up to 2450 and 1420 cd/m2, and luminance efficacy of 1.4 cd/A and 0.89 cd/A, respectively, at 7 V applied bias. Also, the carrier transport mechanisms for the QD LEDs are described by using several models to fit the experimental I–V data.

Intensely pulsed white light (IPWL) treatment was tested as an ultrafast, large-area processable optical technique for the control of the nanostructure of a polymeric bulk-heterojunction photoactive layer to improve the efficiencies of polymer solar cells. Only 2 s of IPWL irradiation of a polymer:fullerene photoactive layer under ambient conditions was found to enhance significantly the power conversion efficiencies of the tested polymer solar cells to values approaching that of typical devices treated with thermal annealing. Consecutive white-light pulses from the xenon lamp induce the self-organization of the polymeric donor into an ordered structure and result in the optimized phase segregation of the polymeric donor and the fullerene acceptor in the photoactive layer, which enhances the light absorption and hole mobility and results in efficient photocurrent generation. The effects of varying the pulse conditions on device performance, including the irradiation fluence, pulse duration time, and number of pulses, were systematically investigated. Finally, it was successfully demonstrated that the IPWL treatment produces flexible polymer solar cells. The proposed IPWL process is suitable for the efficient industrial roll-to-roll production of polymer solar cells.

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.

We fabricated an a-IGZO thin film transistor (TFT) with AZO/Ag/AZO transparent multilayer source/drain contacts by rf magnetron sputtering. a-IGZO TFT with AZO/Ag/AZO multilayer S/D electrodes (W/L = 400/50 µm) showed a subs-threshold swing of 3.78 V/dec, a minimum off-current of 10-12 A, a threshold voltage of 0.41 V, a field effect mobility of 10.86 cm2/Vs, and an on/off ratio of 9ƒ 109. From the ultraviolet photoemission spectroscopy, it was revealed that the enhanced electrical performance resulted from the lowering of the Schottky barrier between a-IGZO and Ag due to the insertion of an AZO layer and thus the AZO/Ag/AZO multilayer would be very appropriate for a promising S/D contact material for the fabrication of high performance TFTs.

Colloidal quantum dots (QDs) have been extensively studied for optoelectronic and biological applications due to their unique physical and optical properties. In particular, among the optoelectronics applications, the white light emitting diode (WLED) has great potential in flat panel displays and solid-state lighting. Herein, we demonstrate a novel, facile, and efficient technique for the synthesis of CdTe/ZnO/GO quasi-core–shell–shell hybrid quantum dots containing the CdTe core with multi shells of ZnO and graphene oxide (GO) and fabrication of WQDLEDs. The CdTe/ZnO/GO quasi-core–shell–shell QDs have a unique strong photoluminescence (PL) peak at 624 nm related to the CdTe core and new weak peaks at 382, 404, 422, and 440 nm due to conjugation with ZnO and GO. Also, in the electroluminescence (EL), multiple emission peaks are observed, which can be correlated to the recombination process inside the CdTe core and also recombination of electrons in the lowest unoccupied molecular orbital (LUMO) and LUMO+2 of GO and holes in the valence band (VB) of ZnO. The QDLEDs show clear white color emission with a maximum luminance value of about 480 cd m−2 with Commission Internationale de l'Eclairage (CIE) color coordinates of (0.35, 0.28).

In ZnO–graphene (–C60) based UV photovoltaic devices, the maximum PCE of 3.02 % for ZnO–C60 is greater than that of 2.33 % for ZnO–graphene QDs, which can be ascribed to difference in the energy level alignment of C60 and graphene with poly-TPD (hole transport layer) and Cs2CO3/Al(electron transport layer). The ZnO–graphene quantum-dot layer LED pixels shows uniform luminescence with the maximum blue luminance of 798 cd/m2 at 15 V. In the photoelectrochemical devices, the rate constant for ZnO was only 0.0088 and slightly increased to 0.053 for ZnO–graphene. ZnO–C60 shows a k value more than 10-times higher than that of bare ZnO because of the higher charge transportation property of ZnO–C60. The normalized photocurrent density to time curves demonstrate that the ZnO–graphene core–shell structures have more improved long-term stability performance than those of ZnO–C60 because 2-Dim graphene can conformally cover the surface of the ZnO QD core better than 0-Dim C60 can.

Al-doped ZnO (AZO) thin films were deposited at room temperature on glass substrates by rf magnetron sputtering with simultaneous dc bias through an external inductor coil. The deposition rates of AZO films deposited using simultaneous rf and dc power along with an inductor coil were 20% higher than those deposited using only rf power. The effects of simultaneous rf and dc bias voltage during the deposition of AZO films were investigated in terms of their resistivity and compressive stress. It was observed that the AZO films deposited at 120 W rf power with 600 μH inductor coil exhibit the lowest resistivity of 6.71 × 10 −4 Ω⋅cm.

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 (J sc), open circuit voltage (V oc), 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.

Al doped ZnO(AZO) thin films were deposited at different substrate temperatures by a continuous composition spread(CCS) method. Various compositions of Al doped ZnO thin films deposited at substrate temperatures between 0 and 250 °C were explored to find excellent electrical and optical properties. The AZO thin film deposited at 100 °C had the lowest resistivity, 9×10-4 Ω·cm and its average transmittance at the 400 to 700 nm wavelength region was 92 %. Optimized composition of the AZO thin film which had the lowest resistivity and high transmittance was 3.13 wt% Al doped ZnO.

Transparent conducting multilayer structured electrode of a few nm Ag layer embedded in tin oxide thin film SnO x /Ag/SnO x was fabricated on a glass by RF magnetron sputtering at room temperature. The multilayer of the SnO x (40 nm)/Ag(11 nm)/SnO x (40 nm) electrode shows the maximum optical transmittance of 87.3% at 550 nm and a quite low electrical resistivity of 6.5 × 10 − 5 Ω cm, and the corresponding figure of merit (T 10 /R S ) is equivalent to 3.6 × 10 − 2 Ω − 1 . A normal organic photovoltaic (OPV) structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/polythiophene:phenyl-C60-butyric acid methyl ester/Al was fabricated on glass/SnO x /Ag/SnO x to examine the compatibility of OPV as a transparent conducting electrode. Measured characteristic values of open circuit voltage of 0.62 V, saturation current of 8.11 mA/cm 2 and fill factor of 0.54 are analogous to 0.63 V, 8.37 mA/cm 2 and 0.58 of OPV on commercial glass/indium tin oxide (ITO) respectively. A resultant power conversion efficiency of 2.7% is also very comparable with the 3.09% of the same OPV structure on the commercial ITO glass as a reference, and which reveals that SnO x /Ag/SnO x can be appropriate to OPV solar cells as a sound transparent conducting electrode.

Hybrid nanostructures combining inorganic materials and graphene are being developed for applications such as fuel cells, batteries, photovoltaics and sensors. However, the absence of a bandgap in graphene has restricted the electrical and optical characteristics of these hybrids, particularly their emissive properties. Here, we use a simple solution method to prepare emissive hybrid quantum dots consisting of a ZnO core wrapped in a shell of single-layer graphene. We then use these quantum dots to make a white-light-emitting diode with a brightness of 798 cd m(-2). The strain introduced by curvature opens an electronic bandgap of 250 meV in the graphene, and two additional blue emission peaks are observed in the luminescent spectrum of the quantum dot. Density functional theory calculations reveal that these additional peaks result from a splitting of the lowest unoccupied orbitals of the graphene into three orbitals with distinct energy levels. White emission is achieved by combining the quantum dots with other emissive materials in a multilayer light-emitting diode.

The electrical and optical properties of Ga-doped ZnO (GZO) thin films deposited at different substrate temperatures have been studied by a continuous composition spread (CCS) method. The full range of GZO compositions deposited at different substrate temperatures was explored to find excellent electrical and optical properties. Optimized GZO thin films with a low resistivity of 9.6 × 10 −4 Ω cm and an average transmittance above 89% in the 400–700 nm wavelength regions were able to be formed at a substrate temperature of 100 °C. Optimized composition of the GZO thin film which had the lowest resistivity and highest transmittance was found at 0.8 wt% Ga 2 O 3 doped ZnO.

SnO2-based ceramics substituted with manganese as a new p-type oxide semiconductor were prepared by conventional solid state reaction. The Mn was ranged from 5 to 20 mol%, and the microstructure as well as the physical and chemical properties was characterized. Single-phase rutile of Mn:SnO2 solid solution was obtained in all compositions. Lattice parameter was decreased with the increase of amount of Mn. The compositional change and electrical properties of the Mn:SnO2 ceramics were confirmed by X-ray photoelectron spectroscopy and Hall effect measurement. The 5–10 mol% Mn:SnO2 solid solutions exhibited electrically p-type behavior. The simultaneous presence of Mn2+, Mn3+ and Mn4+ states was approved and Sn4+ in Mn:SnO2 ceramics was partially substituted with Mn3+ which contributes p-type behavior. SnO2 substituted with higher contents of Mn3+ of 50% exhibits p-type semiconductor.

ZnO nanoparticles (NPs) of 4-5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to N D = 6.9 × 1021 cm-3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16-0.33 eV by degeneracy is explained well by the Burstein-Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60 °C, 90 °C, 140 °C, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 °C to 180 °C, the defect density related to oxygen vacancy (V O) in ZnO NPs is reduced from 34.4% to 17.8%, whereas the defect density of interstitial Zn (Zni) is increased. Increased Zni reduces the width (W) of the depletion region from 0.21 to 0.12 nm and lowers the Schottky barrier (ФB) between ZnO NPs and the Al electrode from 1.19 to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zni above the conduction band minimum, and effectively described by space charge limited current and trap charge limited current models.