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A series of LEDs that have a green CdSe@ZnS quantum dot (QD) emitting layer (EML) in response to treatment with formic acid (FA), 1,3-benzenedithiol (BDT), and 1,2-ethanedithiol (EDT) were fabricated and investigated. The initial long-chained ligands of oleic acid-based CdSe@ZnS QD layers were partially replaced by short-chained and more conductive ligands of FA, BDT, and EDT that can offer to improve the performance of QD-LEDs (QLEDs). Comparison of the operation characteristics of QLEDs without and with various solution treatments revealed that the control of charge injection and recombination conditions in QD EMLs is the key step for the improvement of the efficiency of our devices. The best device performance was observed for EDT treatment with the maximum luminance and luminous efficiency of 3776 cd m−2 and 1.47 cd A−1, respectively, which are increased by 487% and 270% compared with those of the untreated QLED.

In this work, we demonstrated a phonon-polariton in the terahertz (THz) frequency range, generated in a crystallized lead halide perovskite film coated on metamaterials. When the metamaterial resonance was in tune with the phonon resonance of the perovskite film, Rabi splitting occurred due to the strong coupling between the resonances. The Rabi splitting energy was about 1.1 meV, which is larger than the metamaterial and phonon resonance line widths; the interaction potential estimation confirmed that the strong coupling regime was reached successfully. We were able to tune the polaritonic branches by varying the metamaterial resonance, thereby obtaining the dispersion curve with a clear anticrossing behavior. Additionally, we performed

Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS2 on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS2 is monolayer with high quality and uniformity. The MoS2 field effect transistors based on the as-grown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104 while showing large photoresponse. Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications.

Metal-insulator-semiconductor (MIS) structures are widely used in Si-based solar water-splitting photoelectrodes to protect the Si layer from corrosion. Typically, there is a tradeoff between efficiency and stability when optimizing insulator thickness. Moreover, lithographic patterning is often required for fabricating MIS photoelectrodes. In this study, we demonstrate improved Si-based MIS photoanodes with thick insulating layers fabricated using thin-film reactions to create localized conduction paths through the insulator and electrodeposition to form metal catalyst islands. These fabrication approaches are low-cost and highly scalable, and yield MIS photoanodes with low onset potential, high saturation current density, and excellent stability. By combining this approach with a p+n-Si buried junction, further improved oxygen evolution reaction (OER) performance is achieved with an onset potential of 0.7 V versus reversible hydrogen electrode (RHE) and saturation current density of 32 mA/cm2 under simulated AM1.5G illumination. Moreover, in stability testing in 1 M KOH aqueous solution, a constant photocurrent density of ~22 mA/cm2 is maintained at 1.3 V versus RHE for 7 days., Authors demonstrate Si-based MIS photoanodes using Al thin-film reactions to create localized conduction paths through the insulator and Ni electrodeposition to form metal catalyst islands. These approaches yielded low onset potential, high saturation current density, and excellent stability.

We report on the facile synthesis of alloyed CdS x Se1−x quantum dots (QDs) via a one-pot method using the simultaneous injection of Se and S source into a solution of the Cd precursor dissolved in a coordinating mixture of hexadecylamine and trioctylphosphine, during which the formation of CdS x Se1−x nanocrystals was controlled by growth time at a temperature of 260 °C. In particular, the emission peak and full width at half maximum of the photoluminescence (PL) of alloyed CdS x Se1−x QDs were tunable in the range of 588–604 nm and 36–38 nm, respectively, with a PL quantum yield of up to 55% by a reaction time of 60 min. Importantly, the structural advantage of alloyed CdS x Se1−x QDs-based light emitting devices have been fabricated and their electroluminescence properties characterized. A good performance device with a maximum luminance and luminous efficiency of 761 cd m−2 and 0.82 cd A−1, respectively, was obtained.

Bi-Te based materials have been used for near-room-temperature thermoelectric applications. However, their properties dramatically decrease at high temperatures (over 100 °C), limiting their use in power generation. In this study, we investigated the enhanced thermoelectric properties of Bi-Te based materials by Cu doping and employing the melt-spinning (MS) process that can be utilized especially at elevated temperatures. By changing the doping amount, we could modulate the temperature dependence of thermoelectric properties, where the maximum ZT temperature could be shifted from room temperature to 450 K. The highest ZT value, 1.34, was achieved at 400 K for 2% Cu-doped Bi0.5Sb1.5Te3, which is due to the enhancement in power factor and reduction in lattice thermal conductivity. The average ZT value between room temperature and 530 K was 1.17 for 2% Cu-doped Bi0.5Sb1.5Te3, which is 46% higher than that of pristine Bi0.5Sb1.5Te3. Consequently, the synergetic effect of MS process and Cu incorporation can be a promising method to widen the application of Bi-Te based thermoelectric materials for mid-temperature power generation.

We demonstrate subcycle electron pulse generation in a nanogap of graphene when irradiated by a femtosecond laser pulse in the near-infrared region (800 nm). A strong photoinduced emission was prod...

We demonstrate highly efficient multi-colored semitransparent perovskite solar cells that can create high angular tolerant controllable transmissive colors up to 60°, based on phase-compensated microcavities. The efficiency of the semitransparent colors was improved by impedance matching, which was enabled by placing a dielectric functional layer on top of traditional optical microcavities, with negligible influence on color pureness. The vast majority of the visible part of solar radiation is efficiently utilized for solar energy harvesting, achieving 10.47%, 10.66%, and 11.18% of efficiency for red, green, and blue (RGB) colored solar cells, respectively, while a very small proportion of the visible solar spectrum is used for structural coloration that can be readily tuned by altering the cavity medium thickness. The approach described herein can be suitable for a variety of applications such as display systems with ultra-low power consumption, highly efficient colorful solar panels, low-power wearable electronics, and energy-efficient optoelectronics.

Electrical characteristics of ion gels prepared by loading different amounts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are investigated and compared with those of ion liquid, [EMIM][TFSI] for possible application as a gate stack for flexible electronic devices. Capacitance and impedance as a function of frequency are measured, which can be well accounted for by a simple circuit model identifying the local device components. The operation of a flexible field effect transistor based on graphene and the ion gel as a top gate stack is also demonstrated.

We demonstrated imaging of the depletion layer in a MoS2/graphene heterojunction fabricated by chemical vapor deposition and obtained their transport parameters such as diffusion length, lifetime, and mobility by using scanning photocurrent microscopy (SPCM). The device exhibited a n-type operation, which was determined by the MoS2 layer with a lower mobility. The SPCM revealed the presence of the depletion layer at the heterojunction, whereas graphene provided an excellent electrical contact for the MoS2 layer without resulting in a rectifying behavior, even if they were anchored within a very short range. The polarity of the photocurrent signal switched when we applied a drain–source bias voltage, from which we extracted the potential barrier at the junction. More importantly, a bias-dependent SPCM allowed us to simultaneously record the diffusion lengths of both majority and minority carriers for the respective MoS2 and graphene layers. By combining the diffusion lengths with the lifetimes measured by ...

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-rich nanoprecipitates and sub-micron FeTe2 inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe2 incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively hig...

•High capacitance on In0.53Ga0.47As epitaxial layers grown on Si substrates•Achievement of capacitance equivalent thickness lower than 1.5nm with low leakage current•Enhancement of electrical performances by a surface treatment using trimethylaluminum.

We report an in situ characterization of transition-metal dichalcogenide (TMD) monolayers and twisted bilayers using a high-speed second-harmonic generation (SHG) imaging technique. High-frequency laser modulation and galvano scanning in the SHG imaging enabled a rapid identification of the crystallinity in the TMD, including the orientation and homogeneity with a speed of 1 frame/s. For a twisted bilayer MoS2, we studied the SHG peak intensity and angles as a function of the twist angle under a strong interlayer coupling. In addition, rapid SHG imaging can be used to visualize laser-induced ablation of monolayer and bilayer MoS2 in situ under illumination by a strong femtosecond laser. Importantly, we observed a characteristic threshold behavior, the ablation process occurred for a very short time duration once the preheating condition was reached. We investigated the laser thinning of the bilayer MoS2 with different twist angles. When the twist angle was 0°, the SHG decreased by approximately one-fourth of the initial intensity when one layer was removed. Conversely, when the twist angle was approximately 60° (the SHG intensity was suppressed), the SHG increased abruptly close to that of the nearby monolayer when one layer was removed. Precise layer-by-layer control was possible because of the unique threshold behavior of the laser-induced ablation.

This paper demonstrates a time-of-flight mass spectrometer (TOFMS) fabricated using simple micro electromechanical system technologies. It consists of two components: a triode-type field emitter for electron-impact ionization and ion separator with a repeller, an acceleration electrode, and a flight tube. As a cold-cathode, the field emitter with carbon nanotube is a promising electron source and offers several advantages over conventional thermionic electron sources, such as compact size, low power consumption because of the absence of heating elements, excellent durability, and high electron emission density. Micromachined TOFMS successfully detects signals of Ar and CH3I. This is practically proved with preliminary experiments on ionization and mass spectrum tests with Ar and CH3I, ions, including finite element modeling and theoretical analysis.

We report a 47.1% efficiency improvement resulting from the combination of device-architecture modification and inclusion of poly(3-hexylthiophene-2,5-diyl) nanowires (P3HT NWs) in bulk heterojunction (BHJ) active layers (ALs). Modelling of ellipsometry spectra shows substantial changes in inhomogeneity and optical constants of BHJ ALs with P3HT-NW inclusion. Furthermore, finite-difference time-domain simulation results based on actual device structures with inhomogeneous AL models indicate that enhanced light harvesting is a main contributing factor to efficiency improvement. On the contrary, P3HT-NW inclusion has no significant effect on charge carrier collection, other than suppressing occurrence of cul-de-sac in hole-transport pathways and unfavourable indene-C60 bisadduct domains near top anodes.

Biological polymers offer new opportunities for flexible electronics, which are necessary for applications in soft and biological systems, because of their bio- and eco-friendly material traits. However, integrating an arbitrarily patterned electrode, the backbone of electronic devices, is still challenging when transparency of the electrode is required. Here, we report the fabrication of patterned silver nanowire (AgNW) networks buried at the surface of a silk fibroin film, which are suitable for bioelectronic applications that require biocompatibility, flexibility, high conductivity, and optical transparency. The AgNW-buried silk film exhibits excellent low sheet resistance of ∼15 Ω sq.−1 and high optical transparency of over 80%, along with smoothness of the surface. A light-emitting diode (LED) chip is successfully integrated on the patterned electrodes and can be stably turned on and off. Furthermore, we can produce a transparent resistor and a radio-frequency (RF) antenna on the silk film, and use them together as a food sensor that responds to the decreased conductivity caused by rancidity of spoiled food.

We report systematic efficiency variations of green-emitting CdSe@ZnS quantum-dot (QD) LEDs (QLEDs) in response to in situ treatments with 1,2-ethanedithiol (EDT) solutions at various concentrations. The main effect of in situ EDT treatment on a QD layer spin-coated onto a ZnO layer was vacuum-level shift due to dipole moments on the surface of the QD layer and at the interface between QD and ZnO layers. Competing contributions of these dipole moments were responsible for changes in energy level configurations and, accordingly, electron and hole barriers that resulted in discrepancies in electron- and hole-current variations. QLED efficiency was best when treated with an EDT solution of 4 mM, attributable to the largest increase in the hole- to electron current ratio. The maximum luminous yield of the 4 mM EDT-treated QLED was 5.43 cd A−1, which is 10 times higher than that of an untreated device. Furthermore, the luminous yield of this treated device remained as high as 2.56 cd A−1 at a luminance of 500 cd m−2.

We report on a substantial increase in luminance and luminous efficiency of green-light emitting devices (LEDs) that use colloidal CdSe@ZnS quantum dots (QDs) as a light-emitting material in response to treatment with 1,2-ethanedithiol (EDT). The maximum luminance increased from 1146 to 8075 cd m-2, and luminous yield from 0.15 to 1.41 cd A-1 as a result of treating an incomplete device with drops of EDT right after spin-coating QDs onto a ZnO-nanoparticle layer. Based on systematic studies on substrate-dependent change in photoluminescence, and current-voltage and luminance-voltage characteristics, we propose that passivation of intra-gap defect states and relative shifts of energy levels relevant to the operation of QD LEDs are two main results of EDT treatment. In particular, we argue that energy-level shift without emission-color change can be attributed to surface-dipole effects.

The ultrafast bouncing motion of a liquid droplet has been investigated for droplet manipulation with a single droplet actuator using an electrostatic force for the first time. Under an electrostatic field, various kinds of liquid droplets (e.g. deionized water and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) solution) were rapidly bounced between two superhydrophobic electrodes, one from a capacitive electrode and another from a stage electrode. They were formed by sequential processes of a galvanic displacement method and the coating of a self-assembled monolayer on circular-patterned Cu films on a printed circuit board substrate. The frequency of the bouncing motion of the droplets reached up to 0.57 kHz and 0.71 kHz for water and PEDOT:PSS solution, respectively. Furthermore, the effects of the droplet volume, different types of liquid material such as water and conducting polymer solution, and applied voltages on the droplet bouncing motion were fully analyzed. A charge transfer between the droplet surface and the super-hydrophobic electrodes leads to droplet bouncing and its repetitive charging/discharging process across two electrodes results in a continuous bouncing operation. We successfully measured the transferred current of nA level from several single droplets under rapidly-bouncing motion via the stage electrode. This rapidly-bouncing droplet actuator using electrostatic force can be effectively used for various chemical and biological applications due to its high-speed, contamination-free and facile method.

We fabricated and studied the operation characteristics of light emitting devices (LEDs) based on green CdSe/ZnS quantum dots (QDs). A series of devices with different concentrations of QDs in toluene showed typical electroluminescence (EL) peaks that are consistent with green photoluminescence of colloidal QDs emitting layer (EML), but their bandwidth were varied due to the parasitic emission originating from the adjacent organic layers. Upon the increase of the QD concentration, the contribution of the color contamination to emission light from QD-LED (QLED) devices was decreased resulting in the narrowing of EL spectra. On the other hand, the maximum efficiency of QLEDs increased in the lower QD concentration range, while decreased in the higher range. The optimum luminous efficiency of 1.22 cd/A and the external quantum efficiency of 0.43% at standard video brightness (200 cd/m2) was observed from the device with a QD concentration of 6 mg/ml which can be attributed to the enhanced charge recombination in QD EML.

We fabricated high efficiency yellow-color organic light-emitting diodes (OLEDs) by co-doping 2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb) and tris(8-hydroxyquinolinato)aluminum (Alq 3 ) guests in a N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) host. Co-doping of electron-transporting Alq 3 with yellow dopant TBRb in hole-dominant NPB layers resulted in substantial luminance-yield improvement compared to TBRb-only counterparts without color contamination from Alq 3 green emission. The luminance yield of 11.4 cd/A, corresponding to the co-doping of 8% TBRb and 2% Alq 3 , is larger than previously reported luminance-yield values of conventional TBRb-based yellow OLEDs. Another advantage of the TBRb-Alq 3 co-doped OLEDs is an insignificant roll-off of efficiency at high current-density and/or brightness levels.

With poly(3-hexylthiophene) (P3HT) nanowire (NW) inclusion in active layers (ALs), organic solar cells (OSCs) based on P3HT donor and indene-C60 bisadduct (ICBA) acceptor showed power conversion efficiency (PCE) improvements for both bulk heterojunction (BHJ)- and bilayer (BL)-structure AL devices. The PCE increase was approximately 14 % for both types of P3HT:ICBA OSCs. However, improvements in short-circuit current density (Jsc ) were about 4.4 and 6.4 % for BHJ- and BL-type AL devices, respectively. A systematic study showed that the addition of P3HT NWs did not result in enhanced internal quantum efficiencies for either type of device. However, the difference in light-harvesting efficiency was important in accounting for Jsc variations. Interestingly, there was no correlation between Jsc and PCE variations, whereas the open-circuit voltage (Voc ) and fill factor (FF) showed correlations with the PCE. The variation in FF is discussed in terms of Voc and equivalent-circuit parameters based on a nonideal diode model.

Gradient-layered $i$ a-Si:H passivation is shown to enhance minority carrier lifetimes in silicon heterojunction solar cells compared to single or dual-layer passivation structures. The gradient layered passivation contains optimized interfacial and capping layers for passivating the $i$ a-Si:H/c-Si interface and a gradient layer for suppressing charge traps in bulk $i$ a-Si:H. Gradient layers were obtained by changing the hydrogen dilution ratio continuously during deposition of $i$ a-Si:H. The minority carrier lifetime of the c-Si substrate using gradient-layered passivation was as high as $2451\mu \mathbf{s}$ , compared to 1831 μs for dual-layered passivation at $2\times 10^{15}$ cm−1injection level. JV characteristics of Silicon heterojunction (SHJ) solar cell employing these gradient passivation schemes will also be discussed. When the gradient-layered passivation was implied to interdigitated back contact (IBC) SHJ solar cells, there was an improvement in efficiency, compared to other passivation schemes such as single and dual passivation.

The band alignment at an Al2O3/SrTiO3 heterointerface forming a two-dimensional electron gas (2DEG) was investigated using scanning photocurrent microscopy (SPCM) in an electrolyte-gated environment. We used a focused UV laser source for above-the-bandgap illumination on the SrTiO3 layer, creating electron-hole pairs that contributed to the photocurrent through migration towards the metal electrodes. The polarity of the SPCM signals of a bare SrTiO3 device shows typical p-type behavior at zero gate bias, in which the photogenerated electrons are collected by the electrodes. In contrast, the SPCM polarity of 2DEG device indicates that the hole carriers were collected by the metal electrodes. Careful transport measurements revealed that the gate-dependent conductance of the 2DEG devices exhibits n-type switching behavior. More importantly, the SPCM signals in 2DEG devices demonstrated very unique gate-responses that cannot be found in conventional semiconducting devices, based on which we were able to perform detailed investigation into the electronic band alignment of the 2DEG devices and obtain the valence band offset at the heterointerface.

We fabricated plasmonic devices operating in the terahertz (THz) frequency range using silver nanowire (AgNW) network films. AgNW films exhibit high conductivity and good transparency in the visible range, with a figure of merit comparable to that of conventional transparent conducting oxide films. The THz conductivity of AgNW films can be improved by post-treatment procedures such as welding using graphene oxide flakes. Using photolithography, we fabricated the slot antenna arrays whose resonance behaviors are determined by geometric parameters such as the length of individual elements. The plasmonic resonance varied with the sheet resistances of the film, enabling us to manipulate the quality factors and the peak position of the resonance, in particular, by controlling the films thickness and by the post-procedures such as the chemical vapor treatment. (C) 2017 Optical Society of America

In the present study, we visualize ultrafast carrier dynamics in one-dimensional nanoscale devices, such as Si nanowire and carbon nanotube transistors using femtosecond photocurrent microscopy. We investigate transit times of ultrashort carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Conversely, pure diffusion motion is observed when the pump pulse is located in the middle of the nanowires. Carrier dynamics have been addressed for various working conditions, in which we found that the carrier velocity and pulse width can be manipulated by the external electrodes. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias because of the increased field strength at the Schottky barrier.

We fabricated hybrid light emitting devices based on colloidal CdSe/ZnS core/shell quantum dots and a solution-processed NiO layer. The use of a sol-gel NiO layer as a hole injection layer (HIL) resulted in overall improvement in device operation compared to a control device with a more conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) HIL. In particular, luminous efficiency increased substantially because of the suppression of excessive currents and became as large as 2.45 cd/A. To manifest the origin of current reduction, temperature- and electric field-dependent variations of currents with respect to bias voltages were investigated. In a low bias voltage range below the threshold for luminance turn-on, the Poole-Frenkel (PF) emission mechanism was responsible for the current-density variation. However, the space-charge-limited current modified with PF-type mobility ruled the current-density variation in high bias voltage range above the threshold.

We report the fabrication of efficient indium-tin-oxide-free organic solar cells based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). All layers of the devices from the lowermost silver nanowire cathode to the uppermost conducting polymer anode are deposited from solution and processed at plastic-compatible temperatures

We report the characterization of individual carbon nanotube and Si nanowire field-effect transistors through high-speed scanning photocurrent microscopy with a scanning speed of 1 frame/s and a photocurrent sensitivity of less than 1 pA. This enables us to record photocurrent images that are free from hysteresis effects that modify the field configurations applied by the gate bias voltage. We can clearly resolve the photocurrent signals with polarity inversion near the metallic contacts under gate bias conditions which cause severe hysteresis effects in the nanowire devices. We also studied the dynamics of the hysteresis effects for different gate bias configurations. This high-speed photocurrent imaging technique is particularly useful for obtaining two-dimensional, localized optoelectronic characteristics and their correlation with overall device performance without encountering undesired dynamic responses.

In CH3NH3PbI3-based high efficiency perovskite solar cells (PSCs), tiny amount of PbI2 impurity was often found with the perovskite crystal. However, for two-step solution process-based perovskite films, most of findings have been based on the films having different morphologies between with and without PbI2. This was mainly due to the inferior morphology of pure perovskite film without PbI2, inevitably produced when the remaining PbI2 forced to be converted to perovskite, so advantages of pure perovskite photoactive layer without PbI2 impurity have been overlooked. In this work, we designed a printing-based two-step process, which could not only generate pure perovskite crystal without PbI2, but also provide uniform and full surface coverage perovskite film, of which nanoscale morphology was comparable to that prepared by conventional two-step solution process having residual PbI2. Our results showed that, in two-step solution process-based PSC, pure perovskite had better photon absorption and longer carrier lifetime, leading to superior photocurrent generation with higher power conversion efficiency. Furthermore, this process was further applicable to prepare mixed phase pure perovskite crystal without PbI2 impurity, and we showed that the additional merits such as extended absorption to longer wavelength, increased carrier lifetime and reduced carrier recombination could be secured.

We determined the carrier diffusion lengths in nanoporous layers of dye-sensitized solar cells by using scanning photocurrent microscopy. The diffusion lengths were found to be 60-100 μm for the conventional cells. In addition, we found a correlation between the carrier diffusion lengths and the cell efficiency, which proved that improvement in the diffusion length is one of the crucial factors for optimizing device performance. The diffusion length was measured for various operating conditions by varying parameters such as solar light intensity and applied electrical voltage. In particular, we observed electric-field-driven, carrier transport phenomena (i.e., drift current) in modified cells. Fitting with the drift-diffusion model enabled us to extract the electric field strengths present in the TiO2 nanoporous layer.

Universal use of single-walled carbon-nanotube based saturable absorber devices for mode-locking of bulk solid-state lasers between 0.8 and 2 μm is discussed. The advantages in comparison to semiconductor saturable absorbers are emphasised. We briefly describe the manufacturing process and the essential optical properties, and review experimental results obtained with various types of femtosecond and picosecond solid-state lasers in the steady-state regime. We also demonstrate that a single hybrid saturable absorber used in transmission can be used to mode-lock four different types of lasers operating between 1 and 2 μm.

A series of inorganic–organic hybrid light-emitting devices (HLEDs) based on orange CdSe/ZnS quantum dots were fabricated and their operation characteristics were investigated. Other than replacing an organic light-emitting layer (EML) with a CdSe/ZnS-QD EML spin-coated from a colloidal QD solution, the structure of HLEDs was kept identical to that of conventional organic light-emitting devices. Comparison of the operation characteristics of two types of CdSe/ZnS-QD HLEDs with a either Bebq2 or Alq3 electron transport layer (ETL) showed that the control of the injection and transport of electrons is the key step for the improvement of the luminance yield, power efficiency, and color purity of our HLEDs. The most common ETL material Alq3 turned out to be sufficient to make HLEDs with promising device performance.

Inorganic–organic hybrid light emitting devices (HLEDs) with CdSe/ZnS quantum dot (QD) emitting layers are fabricated, and the variation of their operation characteristics with respect to a cathode material selection among Au, Al, and LiF/Al is investigated. All three types of QD-based HLEDs show orange-color emission, centered at 580 nm with a full-width at half-maximum of 50 nm, regardless of cathode material. However, quantitatively, the QD-HLED with a LiF/Al-cathode shows much larger current density and luminance compared to the other QD-HLEDs, owing to better electron injection. Moreover, the respective maximum values of power efficiency and luminous yield of the LiF/Al-cathode HLED are as large as 1.27 lm/W and 2.42 cd/A.

Single-walled carbon-nanotube absorbers are experimentally demonstrated for laser mode-locking. A saturable absorber device is used to mode-lock three different bulk solid-state lasers in a 500 nm-wide wavelength interval. The devices exhibit a low saturation fluence of

We developed a room-temperature encapsulation process based on multi-stack of ultra thin Al 2 O 3 and polyurea layers for top-emission organic light-emitting devices (TEOLEDs). Device structure, including a capping layer for refractive-index matching and a thick polyurea buffer layer, was optimized to enhance light extraction without distorting electroluminescence spectrum. The efficiency of a TEOLED encapsulated with 5 pairs of Al 2 O 3 (50 nm)/polyurea(20 nm) layers was better than that of a glass-encapsulated TEOLED, whereas their color coordinates were almost identical. Moreover, the half-decay lifetime of a TEOLED encapsulated with 5 pairs of Al 2 O 3 /polyurea layers was 86% of that of a glass-encapsulated TEOLED. Water vapor transition rate of 5 pairs of Al 2 O 3 (50 nm)/polyurea(20 nm) layers on PET film was measured as low as 5 × 10 −4 g/m 2 day.

Vertically aligned ZnO nanorods have been grown on silicon substrates pre-coated with thin, less than 10 nm, textured ZnO seeding layers via a vapor-solid mechanism. The ZnO seeding layers, which were essential for vertical alignment of ZnO nanorods without using any metal catalyst, were prepared by decomposing zinc acetate. The structure and the luminescence properties of the ZnO nanorods synthesized onto ZnO seeding layers were investigated and their morphologies were compared with those of single-crystalline GaN substrates and silicon substrates covered with sputtered ZnO flms. Patterning of ZnO seed layers using photolithography allowed the fabrication of patterned ZnO-nanorod arrays.

Cathodoluminescence (CL) emissions from individual ZnO nanostructures with diameters of 30−100 nm are investigated to correlate their optical and electrical properties. Two types of ZnO nanostructu...

In this paper, we demonstrate the fabrication of liquid flow sensors employing partially suspended single-walled carbon nanotubes (SWNTs). We have found that the sign of the conductance change in SWNT flow sensors is not influenced by the direction of water flow for both supported and suspended devices. Therefore, the streaming potential is not the principal mechanism of the SWNT sensor response. Instead, the conductance change is more likely due to a reduction in the cation density in the electrical double layer, whose equilibrium conditions are determined by the liquid flow rate. More importantly, we have found that the sensitivity of suspended SWNT devices is more than 10 times greater than that of supported SWNT devices. A reduced screening effect and an increase in effective sensing volume are responsible for the enhanced sensitivity, which is consistent with the ion depletion model. We also have measured conductance as a function of gate bias at different flow rates and have determined the flow-rate dependent effective charge density, which influences the electrostatic configuration around SWNT devices.

The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility–lifetime (μ–τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.

We designed and fabricated three different types of triode field emitters with planar carbon-nanoparticle (CNP) cathodes such as a normal-gate structure, a double-gate structure, and a well-structure. A normal-gate structure CNP triode emitter showed good field emission properties. The field emission started at the gate-voltage of 45 V, and the anode current reached the level of ∼120 nA at the gate-voltage of 60 V. However, in general, normal-gate structure suffered from large gate current. Using the double-gate structure, we successfully reduced the gate current to the level less than 4% of anode currents up to the anode current of ∼250 nA. To simplify fabrication process while maintaining the gate current reduction effect of the double-gate structure, a triode emitter with a well-structure cathode was fabricated via reactive-ion etching of a heavily doped n-type silicon wafer. The triode emitter with a well-structure cathode and a recessed gate structure showed negligible gate current.

We designed a series of metallo-dielectric multilayers transparent in visible-wavelength range using dielectric materials with high, medium, and low refractive indexes, for a systematic investigation on the coupling between resonators. However, the transparent metal systems fabricated using sputtering method failed to reach the designed maximum transmission levels. Non-destructive depth profiling of metallo-dielectric multilayers revealed that structural imperfection, such as surface roughness and interface layers, were responsible for the observed discrepancy in the maximum transmission level. Respective substitutions of ZnS–SiO 2 and APC (an alloy of Ag, Palladium, and Copper), which are the materials that tend to grow with less roughness, for ZnS and Ag resulted in a significant improvement in the maximum transmission level.

Ultra-thin and ultra-smooth gold (Au) films are appealing for photonic applications including surface plasmon resonances and transparent contacts. However, poor adhesion at the Au-dielectric interface prohibits the formation of a mechanically stable, ultra-thin, and ultra-smooth Au film. A conventional solution is to use a metallic adhesion layer, such as titanium and chromium, however such layers cause the optical properties of pure Au to deteriorate. Here we report the use of silk protein to enhance the adhesion at the Au-dielectric interface, thus obtaining ultra-smooth sub-10 nm Au films. The Au films that were deposited onto the silk layer exhibited superior surface roughness to those deposited on SiO2, Si, and poly(methyl methacrylate), along with improved adhesion, electrical conductivity, and optical transparency. Additionally, we confirm that a metal-insulator-metal optical resonator can be successfully generated using a silk insulating layer without the use of a metallic adhesion layer.

Composite films of Au and dielectric material (TiO2 and/or mixed ZnS–SiO2) with a wide range of Au fractions were fabricated through a co-sputtering method utilizing a multi-target sputtering system. The optical absorption spectra of these composite films demonstrated an absorption peak due to surface plasmon resonance of embedded Au nanoclusters. It was found that, depending on the dielectric matrix, Au concentration, and post-deposition annealing, the wavelength of the surface plasmon resonance of nanoclusters shifted by 130 nm: from 550 to 680 nm. The third-order non-linear susceptibility χ(3) of these composite films, which also originates from field enhancement due to the surface plasmon resonance, demonstrated a corresponding variation.

International audience; We have designed and fabricated a new type of field electron source for a novel onboard mass spectrometer. The new electron source, which is a field effect emitter in a triode configuration, consists of a CNT-column array cathode and an extraction gate with holes that are aligned concentrically with respect to the cylindrical CNT columns. In triode mode operation, cathode currents as large as ~420 μA have been emitted with an anode-to-gate current ratio of ~1.5. To account for the observed emission characteristics of the new electron source, we have carried out multi-scale simulations that combine a three-dimensional (3D) microscopic model in the vicinity of an actual emission site with a two-dimensional (2D) macroscopic model that covers the whole device structure. Because the mesh size in the microscopic 3D model is as small as 100 nm, the contributions of the extruding CNT bundle at the top edge of an electron column can be examined in detail. Unlike the macroscopic 2D simulation that shows only small field enhancement at CNT column's top edge, the multi-scale simulation successfully reproduced the local electric field strongly enough to emit the measured cathode currents and the electric field distribution which is consistent with the measured anode-to-gate current ratio.

We performed time-domain terahertz (THz) spectroscopy on reduced graphene oxide (rGO) network films coated on quartz substrates from dispersion solutions by spraying method. The rGO network films demonstrate high conductivity of about 900 S/cm in the THz frequency range after a high temperature reduction process. The frequency-dependent conductivities and the refractive indexes of the rGO films have been obtained and analyzed with respect to the Drude free-electron model, which is characterized by large scattering rate. Finally, we demonstrate that the THz conductivities can be manipulated by controlling the reduction process, which correlates well with the DC conductivity above the percolation limit. (C) 2013 Optical Society of America

Three planar CH3NH3PbI3 (MAPbI3) solar cells having the same structure except a hole-extraction layer (HEL) showed distinctive difference in operation characteristics. Analysis of frequency-dependent capacitance and dielectric-loss spectra of the three MAPbI3 devices showed two types of recombination-loss channels with different time constants that we attributed respectively to interface and bulk defects. Discrepancy in defect formation among the three devices with a HEL of PEDOT:PSS, NiOx, or Cu-doped NiOx was not surprising because grain-size distribution and crystalline quality of MAPbI3 can be affected by surface energy and morphology of underlying HELs. We were able to quantify interface and bulk defects in these MAPbI3solar cells based on systematic and simultaneous simulations of capacitance and dielectric-loss spectra, and current–voltage characteristics by using the device simulator SCAPS. Defect density reduction is essential for efficient planar solar cells. Defect density in MAPbI3 layers, with respect to which an open-cell voltage and a fill factor vary, can be investigated by using capacitance–frequency spectra.