Spectrally narrow band-edge photoluminescence from AgInS2-based core/shell quantum dots for electroluminescence applications.

This study presents a facile synthesis of cadmium-free ternary and quaternary quantum dots (QDs) and their application to light-emitting diode (LED) devices. AgInS2 ternary QDs, developed as a substitute for cadmium chalcogenide QDs, exhibited spectrally broad photoluminescence due to intrinsic defect levels. Our group has successfully achieved narrow band-edge PL by a coating with gallium sulfide shell. Subsequently, an intrinsic difficulty in the synthesis of multinary compound QDs, which often results in unnecessary byproducts, was surmounted by a new approach involving the nucleation of silver sulfide followed by material conversion to the intended composition (silver indium gallium sulfide). By fine-tuning this reaction and bringing the starting material closer to stoichiometric compositional ratios, atom economy was further improved. These QDs have been tested in LED applications, but the standard device encountered a significant defective emission that would have been eliminated by the gallium sulfide shells. This problem is addressed by introducing gallium oxide as a new electron transport layer.

Electrocatalyst Fabrication Using Metal Nanoparticles Prepared in Ionic Liquids.

Metal nanoparticle-based electrocatalysts are widely used in electronic devices, which serve for electrochemical reactions like oxygen reduction reaction, alcohol oxidation and CO2 reduction reaction. These catalyst-dependent reactions are the key of the emerging clean energy systems. Catalyst design and synthesis therefore have received keen attention in past decades. We are motivated to study synthesis approaches of metal nanoparticle-based electrocatalysts using ionic liquids (ILs), which are promising solvents for the nanoparticle preparation because of their unique physicochemical properties. In this personal account, we review our previous and present works on nanoparticle preparation in IL and utilization of the obtained nanoparticles as electrocatalysts.

Development of Cu-In-Ga-S quantum dots with a narrow emission peak for red electroluminescence.

Narrowing the emission peak width and adjusting the peak position play a key role in the chromaticity and color accuracy of display devices with the use of quantum dot light-emitting diodes (QD-LEDs). In this study, we developed multinary Cu-In-Ga-S (CIGS) QDs showing a narrow photoluminescence (PL) peak by controlling the Cu fraction, i.e., Cu/(In+Ga), and the ratio of In to Ga composing the QDs. The energy gap of CIGS QDs was enlarged from 1.74 to 2.77 eV with a decrease in the In/(In+Ga) ratio from 1.0 to 0. The PL intensity was remarkably dependent on the Cu fraction, and the PL peak width was dependent on the In/(In+Ga) ratio. The sharpest PL peak at 668 nm with a full width at half maximum (fwhm) of 0.23 eV was obtained for CIGS QDs prepared with ratios of Cu/(In+Ga) = 0.3 and In/(In+Ga) = 0.7, being much narrower than those previously reported with CIGS QDs, fwhm of >0.4 eV. The PL quantum yield of CIGS QDs, 8.3%, was increased to 27% and 46% without a PL peak broadening by surface coating with GaSx and Ga-Zn-S shells, respectively. Considering a large Stokes shift of >0.5 eV and the predominant PL decay component of ∼200-400 ns, the narrow PL peak was assignable to the emission from intragap states. QD-LEDs fabricated with CIGS QDs surface-coated with GaSx shells showed a red color with a narrow emission peak at 688 nm with a fwhm of 0.24 eV.

On-Surface Metathesis of an Ionic Liquid on Ag(111).

We investigated the adsorption, surface enrichment, ion exchange, and on-surface metathesis of ultrathin mixed IL films on Ag(111). We stepwise deposited 0.5 ML of the protic IL diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) and 1.0 ML of the aprotic IL 1-methyl-3-octylimidazolium hexafluorophosphate ([C8 C1 Im][PF6 ]) at around 90 K. Thereafter, the resulting layered frozen film was heated to 550 K, and the thermally induced phenomena were monitored in situ by angle-resolved X-ray photoelectron spectroscopy. Between 135 and 200 K, [TfO]- anions at the Ag(111) surface are exchanged by [PF6 ]- anions and enriched together with [C8 C1 Im]+ cations at the IL/vacuum interface. Upon further heating, [dema][PF6 ] and [OMIm][PF6 ] desorb selectively at ∼235 and ∼380 K, respectively. Hereby, a wetting layer of pure [C8 C1 Im][TfO] is formed by on-surface metathesis at the IL/metal interface, which completely desorbs at ∼480 K. For comparison, ion enrichment at the vacuum/IL interface was also studied in macroscopic IL mixtures, where no influence of the solid support is expected.

Composition control of alloy nanoparticles consisting of bulk-immiscible Au and Rh metals via an ionic liquid/metal sputtering technique for improving their electrocatalytic activity.

AuRh bimetallic alloy nanoparticles (NPs) were successfully prepared by simultaneous sputtering of Au and Rh in a room-temperature ionic liquid (RTIL) of N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF4). Bimetallic AuRh alloy NPs of 1-2 nm in size were formed in the RTIL. The alloy composition was controllable by changing the surface areas of Au and Rh plates used as sputtering targets. Loading thus-obtained AuRh NPs on carbon black (CB) powders increased the size of AuRh NPs to ca. 2-8 nm, depending on the Au/Rh ratio. The electrocatalytic activity for oxygen reduction reaction (ORR) of AuRh NP-loaded CB catalysts showed a volcano-type dependence on their composition, in which AuRh NPs with Au surface coverage of 62% exhibited the optimal ORR activity, the specific activity being ca. 5 times higher than that of pure Rh NPs.

Adsorption, Wetting, Growth, and Thermal Stability of the Protic Ionic Liquid Diethylmethylammonium Trifluoromethanesulfonate on Ag(111) and Au(111).

We have studied the adsorption, wetting, growth, and thermal evolution of the protic IL diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) on Au(111) and Ag(111). Ultrathin films were deposited at room temperature (RT) and at 90 K, and were characterized in situ by angle-resolved X-ray photoelectron spectroscopy. For both surfaces, we observe that independent of temperature, initially, a closed 2D wetting layer forms. While the film thickness does not increase past this wetting layer at RT, at 200 K and below, "moderate" 3D island growth occurs on top of the wetting layer. Upon heating, on Au(111), the [dema][TfO] multilayers desorb at 292 K, leaving an intact [dema][TfO] wetting layer, which desorbs intact at 348 K. The behavior on Ag(111) is much more complex. Upon heating [dema][TfO] deposited at 90 K, the [dema]+ cations deprotonate in two steps at 185 and 305 K, yielding H[TfO] and volatile [dema]0. At 355 K, the formed H[TfO] wetting layer partly desorbs (∼50%) and partly decomposes to form an F-containing surface species, which is stable up to 570 K.

Luminescent Quaternary Ag(InxGa1-x)S2/GaSy Core/Shell Quantum Dots Prepared Using Dithiocarbamate Compounds and Photoluminescence Recovery via Post Treatment.

Cadmium-free quantum dots (QDs) consisting of silver-indium-gallium-sulfide (AIGS) quaternary semiconductors were successfully synthesized using a metal-dithiocarbamate complex with sufficiently high reactivity to produce metal sulfides. The introduction of a gallium diethyldithiocarbamate precursor decreased the reaction temperature to produce active intermediates, which were subsequently converted into AIGS QDs at 150 °C with silver and indium acetates. Because of the low reaction temperature, AIGS QDs with a tetragonal crystal phase were produced selectively, which favorably generated band-edge emission whose full width at half-maximum is smaller than 40 nm after they were coated with gallium sulfide (GaSy) shells. The compositional indium/gallium ratio was varied by changing the mixing ratio of the precursors used for the synthesis of the AIGS core, and the band-edge photoluminescence (PL) generated from the AIGS/GaSy core/shell QDs was blue-shifted with an increase in the gallium content in the core. Consequently, a pure green emission centered at 518 nm was obtained with a PL quantum yield as high as 68%.

Epoxy-Containing Ionic Liquids with Tunable Functionality.

New types of ionic liquids (ILs) with an epoxy group on a piperidinium-type cation were successfully synthesized by the simple anion exchange reaction of a solid 1-allyl-1-(oxiran-2-ylmethyl)piperidinium bromide, which was designed in this study. Unfortunately, the physicochemical properties, e.g., viscosity and ionic conductivity, of the ILs were inferior to those of common ILs such as 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][Tf2N]). However, the resulting ILs are of great interest as reaction intermediates: For example, the epoxy group on the cation could react with various reagents, including CO2. Consequently, the modification of the cation structure in the ILs was possible. This is particularly interesting because it is very difficult to modify commonly used ILs. The approach established in this article will provide a favorable synthetic route for creating novel functional ILs in the future.

SEM as a Facile Tool for Real-Time Monitoring of Microcrystal Growth during Electrodeposition: The Merit of Ionic Liquids.

Due to the extremely low vapor pressure of ionic liquids (ILs), electrochemical deposition/dissolution of hexagonal hollow CuSn alloy tubes in IL can be real-time observed with a scanning electron microscope (SEM). Two specially made electrochemical cells are used for the top-view and side-view observations. A series of clear SEM images reveals the hexagonal cones are formed prior to further developing hexagonal tubes. At the beginning, many small nuclei are formed on the electrode surface. Due to the electrodeposition rate being slow enough for the nuclei to crystallize into hexagonal structures, the electrodeposition rate is faster on the periphery than the inside of the structures. As the electrodeposition progresses, the tube wall will grow thicker, and a secondary tube emerges inside the primary tube. Finally, shell-by-shell hexagonal tubes are formed. Furthermore, the in situ SEM observation also reveals that the dissolution of tube starts from its inner wall followed by a sudden collapse of its wall. In this paper, we establish a simple and convenient method, which can be broadly applied to the study of metal, alloy, and semiconductor growth in real time.

Photocatalytic Properties of TiO2 Composites Immobilized with Gold Nanoparticle Assemblies Using the Streptavidin-Biotin Interaction.

A method using biomolecules to precisely fabricate the morphology of metal nanoparticles immobilized on the surface of a semiconductor using biomolecules is described. A biotin moiety (Biot) is introduced onto the surface of a gold nanoparticle (AuNP) by covalent coupling with α-lipoic acid to assemble AuNPs in the presence of streptavidin (STV). The assembly of Biot-AuNP/STV is immobilized on the surface of TiO2 chemically modified with 1-(3-aminopropyl)silatrane (APS) to provide a positively charged surface. The Au content immobilized on the surface of TiO2 is clearly increased to 9.5 wt % (Au) as a result of the STV-biotin interaction and the electrostatic interaction between negatively charged Biot-AuNPs and the positively charged surface of APS/TiO2. Transmission electron microscopy (TEM) analysis reveals that the composite has an ordered surface geometry in which Biot-AuNPs are spread over the composite surface in two dimensions. The photocatalytic activity toward decomposition of methyl orange dye promoted by this composite is 55%, which is higher than that of the other composites. The Biot-AuNP/STV@APS/TiO2 composite efficiently reduces O2 molecules at Eonset = -0.23 V vs Ag|AgCl, which is more positive than that of other composites (Eonset = -0.40 to -0.32 V). The result suggests that an increased number of AuNPs immobilized in close contact with the TiO2 surface facilitates photoinduced charge transfer. This strategy, which takes advantage of the specific interactions provided by biomolecules and the chemical modification on the surface, has remarkable potential for efficient fabrication of metal nanoparticles on the surface of the semiconductor, which accelerates the reduction of oxygen molecules.

Basolateral Mg2+ extrusion via CNNM4 mediates transcellular Mg2+ transport across epithelia: a mouse model.

Transcellular Mg(2+) transport across epithelia, involving both apical entry and basolateral extrusion, is essential for magnesium homeostasis, but molecules involved in basolateral extrusion have not yet been identified. Here, we show that CNNM4 is the basolaterally located Mg(2+) extrusion molecule. CNNM4 is strongly expressed in intestinal epithelia and localizes to their basolateral membrane. CNNM4-knockout mice showed hypomagnesemia due to the intestinal malabsorption of magnesium, suggesting its role in Mg(2+) extrusion to the inner parts of body. Imaging analyses revealed that CNNM4 can extrude Mg(2+) by exchanging intracellular Mg(2+) with extracellular Na(+). Furthermore, CNNM4 mutations cause Jalili syndrome, characterized by recessive amelogenesis imperfecta with cone-rod dystrophy. CNNM4-knockout mice showed defective amelogenesis, and CNNM4 again localizes to the basolateral membrane of ameloblasts, the enamel-forming epithelial cells. Missense point mutations associated with the disease abolish the Mg(2+) extrusion activity. These results demonstrate the crucial importance of Mg(2+) extrusion by CNNM4 in organismal and topical regulation of magnesium.

Direct Observation of Short-Range Structural Coherence During a Charge Transfer Induced Spin Transition in a CoFe Prussian Blue Analogue by Transmission Electron Microscopy.

The local structure within the Co-Fe atomic array of the photoswitchable coordination polymer magnet, K0.3Co[Fe(CN)6]0.77·nH2O, is directly observed during charge transfer induced spin transition (CTIST), a solid-solid phase change, using high-resolution transmission electron microscopy (HRTEM). Along with the low-spin (LS) or thermally quenched high-spin (HS) states normally observed in CTIST solids at low temperature, slow cooling of K0.3Co[Fe(CN)6]0.77·nH2O results in an intermediate phase containing both HS and LS domains with short coherence length. By mapping individual metal-metal distances, the nanometer-scale HS domains are directly visualized within the LS array. Temperature-dependent analyses allow monitoring of HS domain coarsening along the warming branch of the CTIST, providing direct visualization of the elastic process and insight into the mechanism of phase propagation. Normally sensitive to electron beam damage, the low-temperature TEM measurements of the porous coordination polymer are enabled by using appropriate ionic liquids instead of usual conductive thin-film coatings, an approach that should find general utility in related classes of materials.

Single-step preparation of two-dimensionally organized gold particles via ionic liquid/metal sputter deposition.

Sputtering of noble metals, such as Au, Ag, Pd, and Pt, onto room-temperature ionic liquids (RTILs) enabled the formation of monoparticle films composed of spherical noble metal nanoparticles on the liquid surface only when the RTILs used contained hydroxyl-functionalized cations as a component. Sputter deposition of these metals under the same conditions simply produced well-dispersed metal particles without the formation of any films on the solution surface when pure RTILs with non-functionalized cations were employed. Anionic species, even those containing a hydroxyl group, did not significantly affect the formation of the particle film on the RTIL surface or dispersion of particles in the solution. The size of Au particles could be controlled by varying the sputtering condition regardless of the two-dimensional particle density, which was determined by the composition of RTILs used. An Au monoparticle film on the RTIL surface was easily transferred onto various solid substrates via the horizontal liftoff method without large aggregation even when the substrate surface was highly curved.

Atomic resolution imaging of gold nanoparticle generation and growth in ionic liquids.

Recent advances in in situ transmission electron microscopy (TEM) techniques have provided unprecedented knowledge of chemical reactions from a microscopic viewpoint. To introduce volatile liquids, in which chemical reactions take place, use of sophisticated tailor-made fluid cells is a usual method. Herein, a very simple method is presented, which takes advantage of nonvolatile ionic liquids without any fluid cell. This method is successfully employed to investigate the essential steps in the generation of gold nanoparticles as well as the growth kinetics of individual particles. The ionic liquids that we select do not exhibit any anomalous effects on the reaction process as compared with recent in situ TEM studies using conventional solvents. Thus, obtained TEM movies largely support not only classical theory of nanoparticle generation but also some nonconventional phenomena that have been expected recently by some researchers. More noteworthy is the clear observation of lattice fringes by high-resolution TEM even in the ionic liquid media, providing intriguing information correlating coalescence with crystal states. The relaxation of nanoparticle shape and crystal structure after the coalescence is investigated in detail. The effect of crystal orientation upon coalescence is also analyzed and discussed.

Cadmium-free sugar-chain-immobilized fluorescent nanoparticles containing low-toxicity ZnS-AgInS2 cores for probing lectin and cells.

Sugar chains play a significant role in various biological processes through sugar chain-protein and sugar chain-sugar chain interactions. To date, various tools for analyzing sugar chains biofunctions have been developed. Fluorescent nanoparticles (FNPs) functionalized with carbohydrate, such as quantum dots (QDs), are an attractive imaging tool for analyzing carbohydrate biofunctions in vitro and in vivo. Most FNPs, however, consist of highly toxic elements such as cadmium, tellurium, selenium, and so on, causing problems in long-term bioimaging because of their cytotoxicity. In this study, we developed cadmium-free sugar-chain-immobilized fluorescent nanoparticles (SFNPs) using ZnS-AgInS2 (ZAIS) solid solution nanoparticles (NPs) of low or negligible toxicity as core components, and investigated their bioavailability and cytotoxicity. SFNPs were prepared by mixing our originally developed sugar-chain-ligand conjugates with ZAIS/ZnS core/shell NPs. In binding experiments with lectin, the obtained ZAIS/ZnS SFNPs interacted with an appropriate lectin to give specific aggregates, and their binding interaction was visually and/or spectroscopically detected. In addition, these SFNPs were successfully utilized for cytometry analysis and cellular imaging in which the cell was found to possess different sugar-binding properties. The results of the cytotoxicity assay indicated that SFNPs containing ZAIS/ZnS have much lower toxicity than those containing cadmium. These data strongly suggest that our designed SFNPs can be widely utilized in various biosensing applications involved in carbohydrates.

Controllable electronic energy structure of size-controlled Cu2ZnSnS4 nanoparticles prepared by a solution-based approach.

Cu2ZnSnS4 nanoparticles with sizes of 2-5 nm, synthesized in hot organic solutions, exhibited size-dependent photoelectrochemical properties due to the quantum size effect. The potentials of the valence band edge and conduction band edge of the nanoparticles, experimentally determined by photoelectrochemical measurements, were shifted more positively and more negatively, respectively, with a decrease in particle size.

In situ SEM study of a lithium deposition and dissolution mechanism in a bulk-type solid-state cell with a Li2S-P2S5 solid electrolyte.

In situ SEM observation of a lithium deposition and dissolution process in an all-solid-state lithium metal battery using a sulfide-based solid electrolyte (SE) was carried out. We revealed visually that the morphology of lithium deposition varies with the operating current densities. At current densities higher than 1 mA cm(-2), local lithium deposition triggers large cracks, leading to a decrease in the reversibility of lithium deposition and dissolution. On the other hand, at a low current density of 0.01 mA cm(-2), its homogeneous deposition, which enables the reversible deposition and dissolution, hardly brings about the occurrence of unfavorable cracks. These results suggest that homogeneous lithium deposition on the SE and the suppression of the growth of lithium metal along the grain boundaries inside the SE are keys to achieve the repetitive lithium deposition and dissolution reaction without deterioration of the SE.

Composition-dependent electrocatalytic activity of AuPd alloy nanoparticles prepared via simultaneous sputter deposition into an ionic liquid.

Homogeneously alloyed bimetallic particles of AuPd with an average size of ca. 2 nm were successfully prepared by simultaneous sputter deposition of Au and Pd in an ionic liquid in the absence of any additional stabilizing agents. The chemical composition of the AuPd alloy was tunable depending on the area fraction of Au plates in the Au-Pd binary targets for sputtering. The particles were immobilized on an HOPG surface by heat treatment along with the increase in the average size of particles from ca. 2 nm to ca. 7 nm. Ionic liquid species adsorbed on the as-prepared AuPd nanoparticle films on HOPG caused the prevention of electrocatalytic reactions, but repetition of potential sweep cycling in a basic aqueous solution removed the adsorbed ionic species, resulting in electrocatalytic oxidation of ethanol at the AuPd alloy nanoparticle-immobilized HOPG electrode. The electrocatalytic activity of AuPd nanoalloy particles varied upon changing the fraction of Au and Pd in the particles, and alloy particles having an Au fraction of ca. 0.61 exhibited the maximum activity against ethanol oxidation, being higher than the activity of the pure Pt surface.

Quantum-Dot Light-Emitting Diodes Exhibiting Narrow-Spectrum Green Electroluminescence by Using Ag-In-Ga-S/GaSx Quantum Dots.

Quantum dots (QDs), which have high color purity, are expected to be applied as emitting materials to wide-color-gamut displays. To enable their use as an alternative to Cd-based QDs, it is necessary to improve the properties of QDs composed of low-toxicity materials. Although multielement QDs such as Ag-In-Ga-S are prone to spectrally broad emission from defect sites, a core/shell structure covered with a GaSx shell is expected to enable sharp emission from band-edge transitions. Here, QD light-emitting diodes (QD-LEDs) embedded with Ag-In-Ga-S/GaSx core/shell QDs (AIGS QDs) were fabricated, and their electroluminescence (EL) was observed. The EL spectra from the AIGS QD-LEDs were found to contain a large defect-related emission component not observed in the photoluminescence (PL) spectra of the AIGS QD films. This defect-related emission was caused by electrons injected into defect sites in the QDs. Therefore, the AIGS QDs and the electron injection layer (EIL) of ZnMgO were treated with Ga compounds such as gallium chloride (GaCl3) and gallium tris(N,N'-diethyldithiocarbamate) (Ga(DDTC)3) to improve the luminescence properties of the QD-LEDs. The added Ga compounds effectively compensated for defect sites on the surface of the QDs and suppressed direct electron injection from the EIL into defect sites. As a result, the defect-related emission components in the EL were successfully suppressed, and the EL exhibited a color purity comparable to the PL of the AIGS QD films. The QD-LEDs exhibited EL spectra with a full width at half-maximum of 33 nm, which is extremely sharp for a low-toxicity QD, and the chromaticity coordinates (0.260, 0.695) for green EL were achieved.

One-pot synthesis of Ag-In-Ga-S nanocrystals embedded in a Ga2O3 matrix and enhancement of band-edge emission by Na+ doping.

I-III-VI-based semiconductor quantum dots (QDs) have been intensively explored because of their unique controllable optoelectronic properties. Here we report one-pot synthesis of Na-doped Ag-In-Ga-S (AIGS) QDs incorporated in a Ga2O3 matrix. The obtained QDs showed a sharp band-edge photoluminescence peak at 557 nm without a broad-defect site emission. The PL quantum yield (QY) of such QDs was 58%, being much higher than that of AIGS QDs without Na+ doping, 29%. The obtained Na-doped AIGS/Ga2O3 composite particles were used as an emitting layer of green QD light-emitted diodes. A sharp electroluminescence (EL) peak was observed at 563 nm, being similar to that in the PL spectrum of the QDs used. The external quantum efficiency of the device was 0.6%.