Enhancing the efficiency of solution-processed inverted quantum dot light-emitting diodes via ligand modification with 6-mercaptohexanol.

In this Letter, the surface hydrophilicity of the quantum dot (QD) emitting layer (EML) was modified via a ligand exchange to prevent QD EML damage upon hole transport layer (HTL) deposition for all-solution-processed inverted QD-light-emitting diodes (QLEDs). The conventional hydrophobic oleic acid ligand (OA-QDs) was partially replaced with a hydrophilic 6-mercaptohexanol (OH-QDs) through a one-pot ligand exchange. Owing to this replacement, the contact angle of a water droplet on the OH-QD films was reduced to 71.7° from 89.5° on the OA-QD films, indicating the conversion to hydrophilic hydroxyl ligands. The OH-QD EML maintained its integrity without any noticeable damage, even after HTL deposition, enabling all-solution processing for inverted QLEDs with well-organized multilayers. Inverted QLEDs with the OH-QD EMLs were compared with those with OA-QD EMLs; the maximum current efficiency of the device with the OH-QD EML significantly improved to 39.0 cd A-1 from 5.3 cd A-1, and the peak external quantum efficiency improved to 9.3% from 1.2%, which is a seven-fold increase over the OA-QD device. This approach is believed to be effective for forming solid QD films with resistance to chlorobenzene, a representative HTL solvent, and consequently contributes to high-efficiency all-solution-processed inverted QLEDs.

Polyethylenimine-ethoxylated dual interfacial layers for highly efficient and all-solution-processed inverted quantum dot light-emitting diodes.

Inverted quantum dot light-emitting diodes (QLEDs) were fabricated through all-solution processing by sandwiching quantum dot (QD) emitting layers (EMLs) between dual polyethylenimine-ethoxylated (PEIE) layers. First, a PEIE layer as EML protecting layer (EPL) was formed on a QD EML to protect the EML from the hole transport layer (HTL) solvents and to facilitate the formation of a well-organized structure in the all-solution-processed inverted QLEDs. Second, another PEIE layer was introduced as an electron-blocking layer (EBL) on the zinc oxide (ZnO) electron transport layer (ETL) and effectively suppressed the excessive electron injection to the QD EML, thereby enhancing device efficiency.

Phenethylamine ligand engineering of red InP quantum dots for improving the efficiency of quantum dot light-emitting diodes.

In this Letter, red-emitting multi-shelled indium phosphide (InP) quantum dots (QDs) were synthesized using the safe phosphorus precursor tris(dimethylamino)phosphine ((DMA)3P). The long-chain ligands of oleylamine (OAm) in the (DMA)3P phosphide source-based InP QDs were partially exchanged with short-chain ligands of phenethylamine (PEA) in the core formation process, and the resulting InP QDs were applied to quantum dot light-emitting diodes (QLEDs). The short-chain ligands of PEA with the π-conjugated benzene ring improved the charge transport and electrical conduction of the QLEDs with (DMA)3P phosphide source-based InP QDs. The PEA-engineering of InP QDs improved their maximum quantum yield from 71% to 85.5% with the full-width at half-maximum of 62 nm. Furthermore, the maximum external quantum efficiency of QLEDs with the PEA-engineered InP QDs improved from 1.9% to 3.5%, and their maximum power efficiency increased from 2.8 to 6.0 lm/W. This Letter demonstrates that engineering the core formation process with the short-chain ligands of PEA provides an efficient and facile way to improve the charge transport and electrical conduction in (DMA)3P phosphide source-based InP QLEDs for electroluminescent devices.

Stability and dispersion improvement of quantum-dot films by hydrosilylation between quantum-dot ligands and a siloxane matrix.

Quantum-dot (QD) ligands were modified and hydrosilylated with a siloxane matrix to improve the quantum efficiency and stability of the QDs. Conventional oleic acid (OA) ligands were exchanged with vinyl ligands without any reduction in the quantum yield. After ligand modification, hydrosilylation was induced between the vinyl ligands on the QDs (vinyl QDs) and a siloxane matrix, resulting in a uniform QD dispersion in the matrix. The hydrosilylated QDs in siloxane showed 23% higher photoluminescence intensity than OA QDs blended in siloxane after storage for 30 days at 85 °C under 85% relative humidity. The QDs also showed 22.3% higher UV/thermal stability than OA QDs in siloxane after 29 h under a high LED photon flux. This study demonstrates that the chemical reaction of QD ligands with polymer matrices can improve the QDs' dispersion and stability.

Interlayer doping with p-type dopant for charge balance in indium phosphide (InP)-based quantum dot light-emitting diodes.

A 2,3,4,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) doping interlayer was developed to improve charge imbalance and the efficiency in indium phosphide (InP)-based quantum dot light-emitting diodes (QLEDs). The doping layer was coated between a hole injecting layer (HIL) and a hole transport layer (HTL) and successfully diffused with thermal annealing. This doping reduces the hole injection barrier and improves the charge balance of InP-based QLEDs, resulting in enhancement of an external quantum efficiency (EQE) of 3.78% (up from 1.6%) and a power efficiency of 6.41 lm/W (up from 2.77 lm/W). This work shows that F4-TCNQ interlayer doping into both HIL and HTL facilitates hole injection and can provide an efficient solution of improving charge balance in QLED for the device efficiency.

Surface Planarization of Low-Temperature Flowable Silicon Oxide for Atomic Layer Deposition Al2O3 Thin Film Encapsulation.

In this research, a flowable chemical vapor deposition (FCVD) process was developed to planarize particle-scattered surfaces for thin film encapsulation by atomic layer deposition (ALD). Nanometer-thick ALD layers are known to have good barrier properties owing to the conformal deposition of the films and their high density, but those barrier properties are vulnerable to degradation because of surface particles on the substrates. In this study, FCVD silicon oxide layer was applied to particlescattered surfaces as a planarization interlayer. Flowable silicon oxide thin films were deposited with tetrabutoxysiline and O2 in an inductively coupled plasmas reactor. The chemical bonding structure of the flowable silicon oxide was verified with Fourier transform infrared spectroscopy. To confirm the planarization effect, particles 2 µm in diameter were intentionally spread on the substrates by electrospray processing and nanometer-thick Al2O3 layers were deposited on top of the planarization interlayers. With the flowable silicon oxide interlayer and the same particle density on flexible substrates, the water vapor transmission rate was reduced to 1.2×10-3 g/(m² day) from 2.0×10-3 g/(m² day). The flowable silicon oxide layers are thus demonstrated to be effective interlayers to reduce the influence of particle contamination for ALD barrier films.

Light extraction from surface plasmon polaritons and substrate/waveguide modes in organic light-emitting devices with silver-nanomesh electrodes.

Light out-coupling of organic light-emitting diodes (OLEDs) with silver (Ag)-nanomesh electrodes was examined. Experimental results for the OLEDs with nine different nanomesh dimensions were compared with simulation results to elucidate the dimensional effect of the nanomesh on the light out-coupling behavior of the devices. The Ag-nanomesh electrodes did not only increase the transparency of the Ag electrode due to periodic nanoholes but also enhanced light extraction from surface plasmon polaritons and substrate/waveguide modes in the devices. The simulation results show similar trends with the experimental results for the optical transmittance, emission spectrum, and efficiency enhancement. With a nanomesh dimension of 480 nm period and 50% fill factor, the OLEDs with the Ag-nanomesh electrode showed 1.66 times higher efficiency than those with a planar Ag electrode did. Using our validated simulation, we construct an external quantum efficiency map in full ranges of the period and fill factor of the Ag-nanomesh electrode to find out the optimum nanomesh dimension.

Fabrication of aligned nanofibers by electric-field-controlled electrospinning: insulating-block method.

Aligned nanofiber arrays and mats were fabricated with an electrospinning process by manipulating the electric field. The electric field was modified by insulating blocks (IBs) that were installed between the nozzle and the substrate as guiding elements to control the trajectory of the electrospinning jet flow. Simulation results showed that the electric field was deformed near the IBs, resulting in confinement of the electrospinning jet between the blocks. The balance of the electric field in the vertical direction and the repulsive force by space charges in the confined electrified jet stream was attributed to the aligned motion of the jet. Aligned arrays of 200 nm thick polyethylene oxide nanofibers were obtained, exhibiting wave-shaped and cross patterns as well as rectilinear patterns. In addition, 40 µm thick quasi-aligned carbon-nanofiber mats with anisotropic electrical property were also attained by this method.

Improved electroluminescence of quantum dot light-emitting diodes enabled by a partial ligand exchange with benzenethiol.

In this study, benzenethiol ligands were applied to the surface of CdSe@ZnS core@shell quantum dots (QDs) and their effect on the performance of quantum dot light-emitting diodes (QD-LEDs) was investigated. Conventional long-chained oleic acid (OA) and trioctylphosphine (TOP) capping ligands were partially replaced by short-chained benzenethiol ligands in order to increase the stability of QDs during purification and also improve the electroluminescence performance of QD-LEDs. The quantum yield of the QD solution was increased from 41% to 84% by the benzenethiol ligand exchange. The mobility of the QD films with benzenethiol ligands approximately doubled to 2.42 × 10(-5) cm(2) V(-1) s(-1) from 1.19 × 10(-5) cm(2) V(-1) s(-1) compared to the device consisting of OA/TOP-capped QDs, and an approximately 1.8-fold improvement was achieved over QD-LEDs fabricated with bezenethiol ligand-exchanged QDs with respect to the maximum luminance and current efficiency. The turn-on voltage decreased by about -0.6 V through shifting the energy level of the QDs with benzenethiol ligands compared to conventional OA and TOP ligands.

Enhancement of electrical conductivity of silver nanowires-networked films via the addition of Cs-added TiO2.

This study proposes a novel method of improving the electrical conductivity of silver nanowires (NWs)-networked films for the application of transparent conductive electrodes. We applied Cs-added TiO2 (TiO2:Cs) nanoparticles onto Ag NWs, which caused the NWs to be neatly welded together through local melting at the junctions, according to our transmission and scanning electron microscopy analyses. Systematic comparison of the sheet resistance of the samples reveals that these welded NWs yielded a significant improvement in conductivity. OLED devices, fabricated by using the NW film planarized via embedding the wires into PMMA, demonstrated device performance was comparable with the reference sample with indium tin oxide electrode.

CdSe/ZnS quantum dot thin film formation by an electrospray deposition process for light-emitting devices.

About 30 nm quantum-dot thin films are formed by electrospray deposition (ESD) process and quantum-dot-light-emitting-diodes (QD-LEDs) are demonstrated. Maximum brightness of 23 000 cd m(-2) and current efficiency of 5.9 cd A(-1) are achieved with the ESD process. The ESD process can be a potential solution for large area quantum dot layers with simple and flexible control.

Characterization of low temperature graphene synthesis in inductively coupled plasma chemical vapor deposition process with optical emission spectroscopy.

Low-temperature graphene was synthesized at 400 degrees C with inductively coupled plasma chemical vapor deposition (PECVD) process. The effects of plasma power and flow rate of various carbon containing precursors and hydrogen on graphene properties were investigated with optical emission spectroscopy (OES). Various radicals monitored by OES were correlated with graphene film properties such as sheet resistance, I(D)/I(G) ratio of Raman spectra and transparency. C2H2 was used as a main precursor and the increase of plasma power enhanced intensity of carbon (C2) radical OES intensity in plasma, reduced sheet resistance and increased transparency of graphene films. The reduced flow rate of C2H2 decreased sheet resistance and increased transparency of graphene films in the range of this study. H2 addition was found to increase sheet resistance, transparency and attributed to reduction of graphene grain and etching graphene layers. OES analysis showed that C2 radicals contribute to graphite networking and sheet resistance reduction. TEM and AFM were applied to provide credible information that graphene had been successfully grown at low temperature.

Electrospray deposition of carbon nanotube thin films for flexible transparent electrodes.

Flexible transparent carbon nanotube (CNT) electrodes were fabricated by electrospray deposition, a large-area scalable and cost-effective process. The carbon nanotubes were dispersed in N,N-dimethylformamide (DMF) and deposited on polyethylene terephthalate (PET) substrates by electrospray deposition process at room temperature and atmospheric pressure. Major process variables were characterized and optimized for the electrospray process development such as electric field between nozzle and substrates, CNT solution flowrate, gap between nozzle and substrates, solution concentration, solvent properties and surface temperature. The sheet resistance of the electrospray deposited CNT films were reduced by HNO3 doping process. 169 Omega/sq sheet resistance and 86% optical transmittance was achieved with low surface roughness of 1.2 nm. The films showed high flexibility and transparency, making them potential replacements of ITO or ZnO in such as solid state lighting, touch panels, and solar cells. Electrospray process is a scalable process and we believe that this process can be applied for large area carbon nanotube film formation.

Control of the Reaction Kinetics of Monodispersed InP/ZnSeS/ZnS-Based Quantum Dots Using Organophosphorus Compounds for Electroluminescent Devices.

Green emissive InP-based quantum dots (QDs) remain less developed than red QDs because of the difficulty of controlling the reactivity of small InP cores. Herein, we report the synthesis of monodispersed green InP-based QDs using tris(dimethylamino)phosphine, a considerably inexpensive and safer phosphorus source compared to conventional tris(trimethylsilyl)phosphine. An organophosphorus compound, trioctylphosphine, was used to control the reaction kinetics by slowing the progression of the nucleation process, which weakened the aggregation behavior of the clusters and improved the size distribution. The synthesized green emissive InP/ZnSeS/ZnS QDs exhibited a photoluminescence (PL) peak at 515 nm with an enhancement of the full width at half-maximum from 66 to 46 nm and the PL quantum yield from 61% to 70%. An electroluminescent device was fabricated, and the electron transport layer was optimized by changing the layer thickness. The optimized device structure improved the charge balance and increased the external quantum efficiency from 2.1% to 3.5%.

Electrospray deposition of silver nanowire films for transparent electrodes.

Silver nanowire (AgNWs) films were fabricated as transparent electrodes by electrostatic spray deposition (ESD) at atmospheric pressure and room temperature. The effects of solution concentration, spray flow rate, applied high voltage, and annealing temperature were characterized to obtain uniform films. AgNWs thin film was produced with ca. 20 Ω/[square] sheet resistance and 83% transparency in the visible range. Morphologies, optical and electrical properties, and stabilities of the films were investigated in this work. A maximum ratio of DC to optical conductivity of 288 was achieved in a 120 nm thick AgNW thin film. Chemical stability was evaluated in various solvents and we found that solvents had little effect on conductivity.

Characterization of low-k dielectric SiCOH films deposited with decamethylcyclopentasiloxane and cyclohexane.

Ultra low-k dielectric SiCOH films were deposited with decamethylcyclopentasiloxane (DMCPSO, C10H30O5Si5) and cyclohexane (C6H12) precursors by plasma-enhanced chemical vapor deposition at the deposition temperature between 25 and 200 degrees C and their chemical composition and deposition kinetics were investigated in this work. Low dielectric constants of 1.9-2.4 were obtained due to intrinsic nanoscale pores originating from the relatively large ring structure of DMCPSO and to the relatively large fraction of carbon contents in cyclohexane. Three different deposition regions were identified in the temperature range. Deposition rates increased with temperature below 40 degrees C and decreased as temperature increased to 75 degrees C with apparent activation energies of 56 kJ/mol x K at < 40 degrees C, -26 kJ/mol x K at 40-100 degrees C, respectively. In the temperature region of 40-100 degrees C hydrocarbon deposition and decomposition process compete each other and decomposition becomes dominant, which results in apparent negative activation energy. Deposition rates remain relatively unaffected with further increases of temperature above 100 degrees C. FTIR analysis and deposition kinetic analysis showed that hydrocarbon deposition is the major factor determining chemical composition and deposition rate. The hydrocarbon deposition dominates especially at lower temperatures below 40 degrees C and Si-O fraction increases above 40 degrees C. We believe that dielectric constants of low-k films can be controlled by manipulating the fraction of deposited hydrocarbon through temperature control.

Self-Assembled 4-Aminopyridine Monolayer as a Nucleation-Inducing Layer for Transparent Silver Electrodes.

The role of a self-assembled monolayer obtained by vacuum deposition of 4-aminopyridine (4-AP), a small organic molecule having amine and pyridine groups, as a metal nucleation inducer and adhesion promoter was verified, and the applicability was evaluated. 4-AP deposited to an extremely thin thickness effectively changed the substrate surface properties, increasing the nucleation density of silver (Ag) more than 3 times and eventually forming a more transparent, low-resistance Ag thin film. The optical transmittance of the Ag thin film, which was less than 60% when 4-AP was not applied, could be increased to about 77% by simply applying 4-AP, and the electrical resistance could be lowered from 37 to 14 Ω/square at the same time. Transmittance could be further improved to higher than 90% by depositing an antireflection layer for use as a transparent Ag electrode. It was also verified that 4-AP not only serves as a nucleation inducer but also contributes to improving interfacial adhesion. The Ag transparent electrode using 4-AP provided the improved performance of the organic light-emitting device due to higher transmittance, lower resistance, and surface roughness. Small organic molecules including functional groups that can be vacuum deposited, such as 4-AP, are expected to be used as surface pretreatment materials for various depositions because they can be easily patterned and can efficiently modify the surface even with extremely thin thickness.

Precise and Prompt Analyte Detection via Ordered Orientation of Receptor in WSe2-Based Field Effect Transistor.

Field-effect transistors (FET) composed of transition metal dichalcogenide (TMDC) materials have gained huge importance as biosensors due to their added advantage of high sensitivity and moderate bandgap. However, the true potential of these biosensors highly depends upon the quality of TMDC material, as well as the orientation of receptors on their surfaces. The uncontrolled orientation of receptors and screening issues due to crossing the Debye screening length while functionalizing TMDC materials is a big challenge in this field. To address these issues, we introduce a combination of high-quality monolayer WSe2 with our designed Pyrene-based receptor moiety for its ordered orientation onto the WSe2 FET biosensor. A monolayer WSe2 sheet is utilized to fabricate an ideal FET for biosensing applications, which is characterized via Raman spectroscopy, atomic force microscopy, and electrical prob station. Our construct can sensitively detect our target protein (streptavidin) with 1 pM limit of detection within a short span of 2 min, through a one-step functionalizing process. In addition to having this ultra-fast response and high sensitivity, our biosensor can be a reliable platform for point-of-care-based diagnosis.

Supramolecular assembly of protein building blocks: from folding to function.

Several phenomena occurring throughout the life of living things start and end with proteins. Various proteins form one complex structure to control detailed reactions. In contrast, one protein forms various structures and implements other biological phenomena depending on the situation. The basic principle that forms these hierarchical structures is protein self-assembly. A single building block is sufficient to create homogeneous structures with complex shapes, such as rings, filaments, or containers. These assemblies are widely used in biology as they enable multivalent binding, ultra-sensitive regulation, and compartmentalization. Moreover, with advances in the computational design of protein folding and protein-protein interfaces, considerable progress has recently been made in the de novo design of protein assemblies. Our review presents a description of the components of supramolecular protein assembly and their application in understanding biological phenomena to therapeutics.

Effect of Secondary Structures on the Adsorption of Peptides onto Hydrophobic Solid Surfaces Revealed by SALDI-TOF and MD Simulations.

The adsorption of peptides and proteins on hydrophobic solid surfaces has received considerable research attention owing to their wide applications to biocompatible nanomaterials and nanodevices, such as biosensors and cell adhesion materials with reduced nanomaterial toxicity. However, fundamental understandings about physicochemical hydrophobic interactions between peptides and hydrophobic solid surfaces are still unknown. In this study, we investigate the effect of secondary structures on adsorption energies between peptides and hydrophobic solid surfaces via experimental and theoretical analyses using surface-assisted laser desorption/ionization-time-of-flight (SALDI-TOF) and molecular dynamics (MD) simulations. The hydrophobic interactions between peptides and hydrophobic solid surfaces measured via SALDI-TOF and MD simulations indicate that the hydrophobic interaction of peptides with random coil structures increased more than that of peptides with an α-helix structure when polar amino acids are replaced with hydrophobic amino acids. Additionally, our study sheds new light on the fundamental understanding of the hydrophobic interaction between hydrophobic solid surfaces and peptides that have diverse secondary structures.