Overcoming the Trade-Off between Optical Transmittance and Areal Capacitance of Transparent Supercapacitors for Practical Application.

It is substantially challenging for transition metal oxide nanoparticle (NP)-based electrodes for supercapacitors to achieve high transparency and large capacity simultaneously due to the inherent trade-off between optical transmittance (T) and areal capacitance (CA ). This study demonstrates how this trade-off limitation can be overcome by replacing some electrode NPs with transparent tin oxide (SnO2 ) NPs. Although SnO2 NPs are non-capacitive, they provide effective paths for charge transport, which simultaneously increase the CA and T550nm of the manganese oxide (Mn3 O4 ) NP electrode from 11.7 to 13.4 mF cm-2 and 82.1% to 87.4%, respectively, when 25 wt% of Mn3 O4 are replaced by SnO2 . The obtained CA values at a given T are higher than those of the transparent electrodes previously reported. An energy storage window fabricated using the mixed-NP electrodes exhibits the highest energy density among transparent supercapacitors previously reported. The improved energy density enables the window to operate various electronic devices for a considerable amount of time, demonstrating its applicability in constructing a reliable and space-efficient building-integrated power supply system.

Improvement of Dynamic Performance and Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors by Incorporating Conjugated Polymers.

Colloidal quantum dots (CQDs) have a unique advantage in realizing near-infrared (NIR) photodetection since their optical properties are readily tuned by the particle size, but CQD-based photodetectors (QPDs) presently show a high dark current density (Jd) and insufficient dynamic characteristics. To overcome these two problems, we synthesized and introduced two types of conjugated polymers (CPs) by replacing the p-type CQD layer in the QPDs. The low dielectric constant and insulating properties of CPs under dark conditions effectively suppressed the Jd in the QPDs. In addition, the energy-level alignment and high-hole mobility of the CPs facilitated hole transport. Therefore, both the responsivity and specific detectivity were highly enhanced in the CP-based QPDs. Notably, the dynamic characteristics of the QPDs, such as the -3 dB cut-off frequency and rising/falling response times, were significantly improved in the CP-based QPDs owing to the sizable molecular ordering and fast hole transport of the CP in the film state as well as the low trap density, well-aligned energy levels, and good interfacial contact in the CP-based devices.

RuO2 Thin Films Electrodeposited on Polystyrene Nanosphere Arrays: Growth Mechanism and Application to Supercapacitor Electrodes.

Two-dimensionally (2D) arrayed polystyrene (PS)/ruthenium oxide (RuO2) core/shell nanospheres are successfully prepared by the electrodeposition of RuO2 nanoparticles on a hexagonal close-packed PS monolayer. This nanosphere structure is entirely different from the structure previously reported for other transition metal oxides electrodeposited on the PS nanosphere arrays. The different growth behavior is analyzed, and a possible deposition mechanism is proposed based on the morphological evolution and photoelectron spectroscopy measurements. As an electrode for supercapacitors, this 2D arrayed nanosphere structure exhibits superior capacitive properties such as significantly large areal capacitance, tight binding with current collectors, and retarded saturation of the capacitance, compared to a planar RuO2 film electrode.

Coaxial RuO2-ITO nanopillars for transparent supercapacitor application.

Supercapacitive properties of ruthenium oxide (RuO2) nanoparticles electrodeposited onto the indium tin oxide (ITO) nanopillars were investigated. Compared to conventional planar current collectors, this coaxially nanostructured current collector-electrode system can provide increased contact for efficient charge transport, and the internanopillar spacing allows easy access of electrolyte ions. The morphological and electrochemical properties depended on the thickness of the RuO2 layers, i.e., the number of electrodeposition cycles. A maximum specific capacitance, Csp, of 1235 F/g at a scan rate of 50 mV/s was achieved for the 30-cycle deposited RuO2-ITO nanopillars. The other capacitive properties such as electrochemical reversibility and Csp retention at high scan rates also improved greatly.

Preparation of superhydrophobic, long-neck vase-like polymer surfaces.

Polymer surfaces comprising nanopillars with various geometries were prepared by nanoimprinting the surface using anodic aluminium oxide templates. In particular, a simple fabrication method for long-neck vase-like stepped nanopillars was established, and the surface showed considerable enhancement in the water contact angle, for example from 95.7° to 150.6° for the polystyrene surface. This enhanced hydrophobicity could be explained by the desirable reduction in the area of the solid-liquid interface and reduced sticking between the nanopillars.

Effects of surface characteristics of dielectric layers on polymer thin-film transistors obtained by spray methods.

The effect of surface characteristics of dielectric layers on the molecular orientation and device performance of sprayed organic field-effect transistors (OFETs) obtained by a novel solvent-assisted post-treatment, called the solvent-sprayed overlayer (SSO) method, were investigated. The OFETs were fabricated by the spray method using regioregular poly(3-hexylthiophene) (RR-P3HT) as an active material. The SSO treatment was applied on the as-sprayed active layers to arrange the molecular ordering. Bare thin SiO(2) layers and octadecyltrichlorosilane (OTS)-treated SiO(2) (OTS-SiO(2)) were employed as the dielectric materials. The resulting chain orientation, crystallinity, and device performance were correlated as a function of SSO treatment and dielectric layers. The intrinsic limitation of spray methods for polymer film formation was overcome regardless of the type of dielectric layer using the SSO treatment. The orientation direction of RR-P3HT was controlled by SSO treatment to an edge-on dominant orientation that is preferential for charge transport, regardless of the type of dielectric layer. The crystal growth was further enhanced on the OTS-SiO(2) layers because of the reduced nucleation sites. These effects were successfully reflected in the device performance, including an orders-of-magnitude increase in charge mobility. The SSO method is a powerful external treatment method for reorienting the molecular ordering of solidified active films of OFETs to the preferential edge-on packing. The growth of crystals was further optimized by controlling the surface characteristics of the dielectric layers. The purpose of this study was to find the full capabilities of the SSO treatment method that will facilitate the development of high-throughput, large-area organic electronic device manufacturing.

Effect of incorporated PVP/Ag nanoparticles on ZnPc/C60 organic solar cells.

Various sizes of PVP-capped Ag nanoparticles were incorporated in the PEDOT:PSS layer of ZnPc/C60-based small-molecule organic solar cells. The incorporated nanoparticles partially block the incident light, but this was offset by the scattering effect and consequent increase in path lengths through the active organic layers. As a result, the overall power conversion efficiency of the cell increased by approximately 15% when nanoparticles with an average diameter of 24 nm were used.

Two-dimensional photonic crystal arrays for polymer:fullerene solar cells.

We report the application of two-dimensional (2D) photonic crystal (PC) array substrates for polymer:fullerene solar cells of which the active layer is made with blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The 2D PC array substrates were fabricated by employing a nanosphere lithography technique. Two different hole depths (200 and 300 nm) were introduced for the 2D PC arrays to examine the hole depth effect on the light harvesting (trapping). The optical effect by the 2D PC arrays was investigated by the measurement of optical transmittance either in the direction normal to the substrate (direct transmittance) or in all directions (integrated transmittance). The results showed that the integrated transmittance was higher for the 2D PC array substrates than the conventional planar substrate at the wavelengths of ca. 400 nm, even though the direct transmittance of 2D PC array substrates was much lower over the entire visible light range. The short circuit current density (J(SC)) was higher for the device with the 2D PC array (200 nm hole depth) than the reference device. However, the device with the 2D PC array (300 nm hole depth) showed a slightly lower J(SC) value at a high light intensity in spite of its light harvesting effect proven at a lower light intensity.

Effective Control of Chlorine Contents in MAPbI3- xCl x Perovskite Solar Cells Using a Single-Source Vapor Deposition and Anion-Exchange Technique.

In this study, a new method is developed to control the Cl-to-I ratio in MAPbI3- xCl x perovskite solar cells (PSCs) more easily and precisely using single-source vapor deposition of MAPbCl3 thin films and a subsequent anion exchange by repeated spin-coatings of methylammonium iodide (MAI) solution. This method can overcome the problems of previous vapor-deposition techniques for PSCs such as the occurrence of morphological defects in the films and difficulty in controlling the stoichiometry of the elements. The repetitive MAI treatments gradually fill the interstitial voids in the perovskite film and increase the average grain size up to 1.2 µm, which improves the charge-transfer property of the cells. The atomic Cl content, i.e., the x value, of the MAPbI3- xCl x film can also be simply controlled by changing the number of MAI treatments. The energy levels and resistive elements of the cells are strongly dependent on the x value of the MAPbI3- xCl x film. A maximum power conversion efficiency of 19.1% is achieved at x = 0.005.

Enhanced capacitive properties of all-metal-oxide-nanoparticle-based asymmetric supercapacitors.

The major problem of transition metal oxide (TMO)-based supercapacitors is their low specific energy (E sp) due to the poor electrical conductivity of the TMO electrodes and narrow operating voltage window. To solve these limitations simultaneously, we propose asymmetric supercapacitors (ASCs) consisting of two composite TMO electrodes working in different potential ranges. Titanium dioxide (TiO2) nanoparticle (NP)-incorporated iron oxide (Fe2O3) and manganese oxide (MnO2) NPs were used as electrode materials covering the negative and positive potential window, respectively. The specific capacitance (C sp) of this asymmetric TiO2-Fe2O3‖TiO2-MnO2 supercapacitor is comparable to that of the symmetric TiO2-MnO2‖TiO2-MnO2 supercapacitor. However, the ASC can operate over a doubly extended voltage range, which resulted in a significant enhancement in the specific energy of the device. The E sp value of the ASC at a specific power of 1000 W kg-1 is 48.6 W h kg-1, which is 34.1 and 8.1 times, respectively, larger than that of the two symmetric devices.

Enhanced cycle stability of a NiCo2S4 nanostructured electrode for supercapacitors fabricated by the alternate-dip-coating method.

Nanostructured nickel cobalt sulfide (NiCo2S4) electrodes are successfully fabricated using a simple alternate-dip-coating method. The process involves dipping a TiO2 nanoparticles-covered substrate in a nickel/cobalt precursor solution and sulfur precursor solution alternately at room temperature. The fabricated bimetallic sulfide electrode exhibits a synergetic improvement compensating for the disadvantages of the two single metal sulfide electrodes, i.e. the poor cycle stability of the nickel sulfide electrode and the low specific capacitance (Csp) of the cobalt sulfide electrode. The two capacitive properties are optimized by adjusting the ratio of nickel and cobalt concentrations in the metal precursor solution, reaching a Csp of 516 F g-1 at a current density of 1 mA cm-2, with its retention being 99.9% even after 2000 galvanostatic charge-discharge cycles.

Retarded saturation of the areal capacitance using 3D-aligned MnO2 thin film nanostructures as a supercapacitor electrode.

The supercapacitive properties of manganese oxide (MnO2) thin films electrodeposited on three-dimensionally (3D) aligned inverse-opal nickel nanostructures are investigated. Compared to conventional planar or two-dimensionally (2D) aligned nanostructures, 3D-aligned nanostructures can provide considerably increased and controllable contacts between the electrode and electrolyte. As a result, saturation of the areal capacitance with the electrode thickness and associated decrease of the specific capacitance, C sp , become much slower than those of the planar and 2D-aligned electrode systems. While, for planar MnO2 electrodes, the C sp of a 60-cycle electrodeposited electrode is only the half of the 10-cycle electrodeposited one, the value of the 3D-nanostructured electrode remains unchanged under the same condition. The maximum C sp value of 864 F g-1, and C sp retention of 87.7% after 5000 cycles of galvanostatic charge-discharge are obtained. The voltammetric response is also improved significantly and the C sp measured at 200 mV s-1 retains 71.7% of the value measured at 10 mV s-1. More quantitative analysis on the effect of this 3D-aligned nanostructuring is also performed using a deconvolution of the capacitive elements in the total capacitance of the electrodes.

Supercapacitive Properties of 3D-Arrayed Polyaniline Hollow Nanospheres Encaging RuO2 Nanoparticles.

A major limitation of polyaniline (PANi) electrodes for supercapacitors is the slow rate of ion transport during redox reactions and the resultant easy saturation of areal capacitance with film thickness. In this study, three-dimensionally (3D)-arrayed PANi nanospheres with highly roughened surface nanomorphology were fabricated to overcome this limitation. A hierarchical nanostructure was obtained by polymerizing aniline monomers on a template of 3D-arrayed polystyrene (PS) nanospheres and appropriate oxidative acid doping. The structure provided dramatically increased surface area and porosity that led to the efficient diffusion of ions. Thus, the specific capacitance (Csp) reached 1570 F g-1, thereby approaching a theoretical capacitance of PANi. In addition, the retention at a high scan rate of 100 mV s-1 was 77.6% of the Csp at a scan rate of 10 mV s-1. Furthermore, 3D-arrayed hollow PANi (H-PANi) nanospheres could be obtained by dissolving the inner PS part of the PS/PANi core/shell nanospheres with tetrahydrofuran. The ruthenium oxide (RuO2) nanoparticles (NPs) were also encaged in the H-PANi nanospheres by embedding RuO2 NPs on the PS nanospheres prior to polymerization of PANi. The combination of the two active electrode materials indicated synergetic effects. The areal capacitance of the RuO2-encaged PANi electrode was significantly larger than that of the RuO2-free PANi electrode and could be controlled by varying the amount of encaged RuO2 nanoparticles. The encagement could also solve the problem of detachment of RuO2 electrodes from the current collector. The effects of the nanostructuring and RuO2 encagement were also quantitatively analyzed by deconvoluting the total capacitance into the surface capacitive and insertion elements.

An Aptamer-Based Electrochemical Sensor That Can Distinguish Influenza Virus Subtype H1 from H5.

The surface protein hemagglutinin (HA) mediates the attachment of influenza virus to host cells containing sialic acid and thus facilitates viral infection. Therefore, HA is considered as a good target for the development of diagnostic tools for influenza virus. Previously, we reported the isolation of single-stranded aptamers that can distinguish influenza subtype H1 from H5. In this study, we describe a method for the selective electrical detection of H1 using the isolated aptamer as a molecular probe. After immobilization of the aptamer on Si wafer, enzyme-linked immunosorbent assay (ELISA) and field emission scanning electron microscopy (FE-SEM) showed that the immobilized aptamer bound specifically to the H1 subtype but not to the H5 subtype. Assessment by cyclic voltammetry (CV) also demonstrated that the immobilized aptamer on the indium thin oxide-coated surface was specifically bound to the H1 subtype only, which was consistent with the ELISA and FE-SEM results. Further measurement of CV using various amounts of H1 subtype provided the detection limit of the immobilized aptamer, which showed that a nanomolar scale of target protein was sufficient to produce the signal. These results indicated that the selected aptamer can be an effective probe for distinguishing the subtypes of influenza viruses by monitoring current changes.

Molecular-orientation-induced rapid roughening and morphology transition in organic semiconductor thin-film growth.

We study the roughening process and morphology transition of organic semiconductor thin film induced by molecular orientation in the model of molecular semiconductor copper hexadecafluorophthalocyanine (F16CuPc) using both experiment and simulation. The growth behaviour of F16CuPc thin film with the thickness, D, on SiO2 substrate takes on two processes divided by a critical thickness: (1) D ≤ 40 nm, F16CuPc thin films are composed of uniform caterpillar-like crystals. The kinetic roughening is confirmed during this growth, which is successfully analyzed by Kardar-Parisi-Zhang (KPZ) model with scaling exponents α = 0.71 ± 0.12, ß = 0.36 ± 0.03, and 1/z = 0.39 ± 0.12; (2) D > 40 nm, nanobelt crystals are formed gradually on the caterpillar-like crystal surface and the film growth shows anomalous growth behaviour. These new growth behaviours with two processes result from the gradual change of molecular orientation and the formation of grain boundaries, which conversely induce new molecular orientation, rapid roughening process, and the formation of nanobelt crystals.

Isolation of single-stranded DNA aptamers that distinguish influenza virus hemagglutinin subtype H1 from H5.

Surface protein hemagglutinin (HA) mediates the binding of influenza virus to host cell receptors containing sialic acid, facilitating the entry of the virus into host cells. Therefore, the HA protein is regarded as a suitable target for the development of influenza virus detection devices. In this study, we isolated single-stranded DNA (ssDNA) aptamers binding to the HA1 subunit of subtype H1 (H1-HA1), but not to the HA1 subunit of subtype H5 (H5-HA1), using a counter-systematic evolution of ligands by exponential enrichment (counter-SELEX) procedure. Enzyme-linked immunosorbent assay and surface plasmon resonance studies showed that the selected aptamers bind tightly to H1-HA1 with dissociation constants in the nanomolar range. Western blot analysis demonstrated that the aptamers were binding to H1-HA1 in a concentration-dependent manner, yet were not binding to H5-HA1. Interestingly, the selected aptamers contained G-rich sequences in the central random nucleotides region. Further biophysical analysis showed that the G-rich sequences formed a G-quadruplex structure, which is a distinctive structure compared to the starting ssDNA library. Using flow cytometry analysis, we found that the aptamers did not bind to the receptor-binding site of H1-HA1. These results indicate that the selected aptamers that distinguish H1-HA1 from H5-HA1 can be developed as unique probes for the detection of the H1 subtype of influenza virus.

Effective surface oxidation of polymer replica molds for nanoimprint lithography.

In nanoimprint lithography, a surface oxidation process is needed to produce an effective poly(dimethylsiloxane) coating that can be used as an anti-adhesive surface of template molds. However, the conventional photooxidation technique or acidic oxidative treatment cannot be easily applied to polymer molds with nanostructures since surface etching by UV radiation or strong acids significantly damages the surface nanostructures in a short space of time. In this study, we developed a basic oxidative treatment method and consequently, an effective generation of hydroxyl groups on a nanostructured surface of polymer replica molds. The surface morphologies and water contact angles of the polymer molds indicate that this new method is relatively nondestructive and more efficient than conventional oxidation treatments.