Length-Controllable Gold-Coated Silver Nanowire Probes for High AFM-TERS Scattering Activity.

Tip-enhanced Raman scattering (TERS) microscopy is an advanced technique for investigation at the nanoscale that provides topographic and chemical information simultaneously. The TERS probe plays a crucial role in the microscopic performance. In the recent past, the development of silver nanowire (AgNW) based TERS probes solved the main tip fabrication issues, such as low mechanical strength and reproducibility. However, this fabrication method still suffers from low control of the protruded length of the AgNW. In this work, a simple water-air interface electrocutting method is proposed to achieve wide controllability of the length. This water cutting method was combined with a succedent Au coating on the AgNW surface, and the probe achieved an up to 100× higher enhancement factor (EF) and a 2× smaller spatial resolution compared to pristine AgNW. Thanks to this excellent EF, the water-cut Au-coated AgNW probes were found to possess high TERS activity even in the nongap mode, enabling broad applications.

Proton conductivity dependence on the surface polymer thickness of core-shell type nanoparticles in a proton exchange membrane.

The proton exchange membrane (PEM) is the main component that determines the performance of polymer electrolyte fuel cells. The construction of proton-conduction channels capable of fast proton conduction is an important topic in PEM research. In this study, we have developed poly(vinylphosphonic acid)-block-polystyrene (PVPA-b-PS)-coated core-shell type silica nanoparticles prepared by in situ polymerization and a core-shell type nanoparticle-filled PEM. In this system, two-dimensional (2D) proton-conduction channels have been constructed between PVPA and the surface of silica nanoparticles, and three-dimensional proton-conduction channels were constructed by connecting these 2D channels by filling with the core-shell type nanoparticles. The proton conductivities and activation energies of pelletized PVPA-coated core-shell type nanoparticles increased depending on the coated PVPA thickness. Additionally, pelletized PVPA-b-PS-coated silica nanoparticles showed a good proton conductivity of 1.3 × 10-2 S cm-1 at 80 °C and 95% RH. Also, the membrane state achieved 1.8 × 10-4 S cm-1 in a similar temperature and humidity environment. Although these proton conductivities were lower than those of PVPA, they have advantages such as low activation energy for proton conduction, suppression of swelling due to water absorption, and the ability to handle samples in powder form. Moreover, by using PS simultaneously, we succeeded in improving the stability of proton conductivity against changes in the temperature and humidity environment. Therefore, we have demonstrated a highly durable, tough but still enough high proton conductive material by polymer coating onto the surface of nanoparticles and also succeeded in constructing proton-conduction channels through the easy integration of core-shell type nanoparticles.

Highly luminescent MAPbI3 perovskite quantum dots with a simple purification process via ultrasound-assisted bead milling.

Organic-inorganic hybrid lead halide perovskite quantum dots (QDs) have various excellent optical properties, and they have drastically enhanced the field of light-emitting diode (LED) research. However, red-emissive CH3NH3 (MA) PbI3 QDs have worse optical properties compared with those of green-emissive MAPbBr3 QDs due to their instability under high-moisture and high-temperature conditions. Therefore, it is quite difficult to prepare MAPbI3 QDs with good optical properties via bottom-up methods using conditions involving high temperature and high-solubility solvents. On the other hand, top-down methods for preparing MAPbI3 QDs under an air atmosphere have attracted attention; however, there are issues, such as PL emission with a wide FWHM being obtained due to the wide particle-size distribution. In this research, red-emissive MAPbI3 QDs were prepared via an ultrasound-assisted bead milling (UBM) method, and the MAPbI3 QDs were purified using various carboxylate esters. As a result, we solved the issue of the wide particle-size distribution unique to top-down methods via purifying the MAPbI3 QDs, and they achieved the following excellent optical properties: a FWHM of 44 to 48 nm and a PLQY of over 60%. Notably, a fabricated LED device with MAPbI3 QDs purified using methyl acetate showed a PL peak at 738 nm and a FWHM of 49 nm, resulting in an excellent EQE value of 3.2%.

Gel permeation chromatography process for highly oriented Cs3Cu2I5 nanocrystal film.

The emergence of green materials has attracted considerable attention in the field of optoelectronics. Copper-based lead-free metal halide (with a near-unity quantum yield) obtained from Cs3Cu2I5 nanocrystals (NCs) can exhibit blue emission with a wavelength of 440 nm and provide outstanding stability for various applications. However, in practical applications, colloidal dispersion purity and film quality are inadequate toward a high-performance device. In this study, antisolvent-free gel permeation chromatography is used to purify Cs3Cu2I5 NCs. The purified Cs3Cu2I5 NCs exhibit a high photoluminescent quantum yield and provide a highly oriented single-crystal film. Density functional theory calculation results indicate that the iodide-rich surface in the NCs makes them highly stable. In addition, it has been demonstrated for the first time that the mixture of polymethyl methacrylate (PMMA) and Cs3Cu2I5 NCs has waterproofing capabilities. The composite film consisting of Cs3Cu2I5 NCs and PMMA can survive in water for several days. This result opens up more possibilities for the application of these green material.

Novel Filler-Filled-Type Polymer Electrolyte Membrane for PEFC Employing Poly(vinylphosphonic acid)-b-polystyrene-Coated Cellulose Nanocrystals as a Filler.

Low-acidity polymer electrolyte membranes are essential to polymer electrolyte fuel cells (PEFCs) and water electrolysis systems, both of which are expected to be next-generation energy and hydrogen sources. We developed a new type of high-performance polymer electrolyte membrane (PEM) in which the core particles are precisely electrolyte polymer coated and filled into binder resin. Cellulose nanocrystals (CNCs), which have attracted attention as light, rigid, and sustainable materials, were selected as the core material for the filler. The CNC surface was coated with a new block copolymer containing a proton conductive polymer of poly(vinylphosphonic acid) (PVPA) and a hydrophobic polymer of polystyrene (PS) using RAFT polymerization with particles (PwP) we developed. The pelletized fillers and the filler-filled polycarbonate membranes achieved proton conductivities of over 10-2 S/cm with lower activation energies and much weaker acidity than the Nafion membrane.

Single-Component Organic Solar Cells Based on Intramolecular Charge Transfer Photoabsorption.

Conjugated donor-acceptor molecules with intramolecular charge transfer absorption are employed for single-component organic solar cells. Among the five types of donor-acceptor molecules, the strong push-pull structure of DTDCPB resulted in solar cells with high JSC, an internal quantum efficiency exceeding 20%, and high VOC exceeding 1 V with little photon energy loss around 0.7 eV. The exciton binding energy (EBE), which is a key factor in enhancing the photocurrent in the single-component device, was determined by quantum chemical calculation. The relationship between the photoexcited state and the device performance suggests that the strong internal charge transfer is effective for reducing the EBE. Furthermore, molecular packing in the film is shown to influence photogeneration in the film bulk.

Li@C60 thin films: characterization and nonlinear optical properties.

Organic materials have attracted considerable attention in nonlinear optical (NLO) applications as they have several advantages over inorganic materials, including high NLO response, and fast response time as well as low-cost and easy fabrication. Lithium-containing C60 (Li@C60) is promising for NLO over other organic materials because of its strong NLO response proven by theoretical and experimental studies. However, the low purity of Li@C60 has been a bottleneck for applications in the fields of solar cells, electronics and optics. In 2010, highly purified Li@C60 was finally obtained, encouraging further studies. In this study, we demonstrate a facile method to fabricate thin films of Li@C60 and their strong NLO potential for high harmonic generation by showing its comparatively strong emission of degenerate-six-wave mixing, a fifth-order NLO effect.

Water-mediated polyol synthesis of pencil-like sharp silver nanowires suitable for nonlinear plasmonics.

We report a simple method to control the end shape of silver nanowires by adding pure water in the conventional polyol synthesis. The use of 0.2-0.4% (v/v) water in ethylene glycol as a solvent provides pencil-like silver nanowires with sharp ends in a high yield. We have demonstrated remote excitation of SHG on the sharp nanowires, promising a point light source for super resolution microscopy.

Preparation of Hierarchic Porous Films of α-MnO2 Nanoparticles by Using the Breath Figure Technique and Application for Hybrid Capacitor Electrodes.

The honeycomb-structured film has advantages such as high wettability and high surface area. This structure and properties are suitable for the capacitor electrode. In this study, the electrode structure is controlled by the synthesis of MnO2 nanoparticles using the breath figure method. The electrode performance was calculated by electrochemical measurements. As a result, the capacitance value was 100.5 F/g at 1 mV s-1, which was improved 2.7 times as compared with that without structure control.

Photoconductive Properties of Dibenzotetrathiafulvalene-Tetracyanoquinodimethane (DBTTF-TCNQ) Nanorods Prepared by the Reprecipitation Method.

Charge-transfer complex crystals have been extensively studied because of their metallic conductivity, photoconductivity, ambipolar charge transport, and high career mobility. Numerous studies of their applications for organic electric devices such as organic field effect transistors and solar cells have reported. However, bulky single crystals of charge-transfer complexes are difficult to handle, specifically to be made into a form of a thin film. Recently, nano/micro crystallization of charge-transfer crystal is attracted to realize thin film applications. In this paper, charge transfer complex nanorods composed of dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) were prepared by the reprecipitation method. The as-formed nanorods possess a kinetically metastable crystal structure different from the thermodynamically stable bulk crystal prepared by slow evaporation of the solvent. From photoconductive measurement, nanorod stacks show a significant photosensitivity (354.57 µA/W) on par with bulk crystal (417.14 µA/W). These results suggest dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) nanorods have a favorable crystal structure for carrier transport due to the difference of molecular stacking assembly.

Polymeric Engineering of Nanoparticles for Highly Efficient Multifunctional Drug Delivery Systems.

Most targeting strategies of anticancer drug delivery systems (DDSs) rely on the surface functionalization of nanocarriers with specific ligands, which trigger the internalization in cancer cells via receptor-mediated endocytosis. The endocytosis implies the entrapment of DDSs in acidic vesicles (endosomes and lysosomes) and their eventual ejection by exocytosis. This process, intrinsic to eukaryotic cells, is one of the main drawbacks of DDSs because it reduces the drug bioavailability in the intracellular environment. The escape of DDSs from the acidic vesicles is, therefore, crucial to enhance the therapeutic performance at low drug dose. To this end, we developed a multifunctionalized DDS that combines high specificity towards cancer cells with endosomal escape capabilities. Doxorubicin-loaded mesoporous silica nanoparticles were functionalized with polyethylenimine, a polymer commonly used to induce endosomal rupture, and hyaluronic acid, which binds to CD44 receptors, overexpressed in cancer cells. We show irrefutable proof that the developed DDS can escape the endosomal pathway upon polymeric functionalization. Interestingly, the combination of the two polymers resulted in higher endosomal escape efficiency than the polyethylenimine coating alone. Hyaluronic acid additionally provides the system with cancer targeting capability and enzymatically controlled drug release. Thanks to this multifunctionality, the engineered DDS had cytotoxicity comparable to the pure drug whilst displaying high specificity towards cancer cells. The polymeric engineering here developed enhances the performance of DDS at low drug dose, holding great potential for anticancer therapeutic applications.

Facet-Dependent Diol-Induced Density of States of Anatase TiO2 Crystal Surface.

Owing to their fundamental importance and practical applications, anatase TiO2 crystals with well-defined {001} and {101} facets attracted intensive research interests. In this study, we systematically investigated solvent dependence of the photoreaction of the different coexposed crystal facets during noble metal photodeposition. By examining the deposition position in each solvent, we revealed that solvents play a pivotal role on the facet selectivity. On the basis of density functional theory calculations, the solvent molecules were found to modify both the crystal facet electronic structure and the {001}-{001} heterojunction. These modifications are not only the origin of diverse charge-carrier pathways but are also responsible for carrier accumulation at specific facets that increase their reductive power. These findings are vital for a better understanding of photocatalytic materials and an improved design for the next-generation materials.

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices.

Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.

Single Benzene Green Fluorophore: Solid-State Emissive, Water-Soluble, and Solvent- and pH-Independent Fluorescence with Large Stokes Shifts.

Benzene is the simplest aromatic hydrocarbon with a six-membered ring. It is one of the most basic structural units for the construction of π conjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal. Presented herein is 2,5-bis(methylsulfonyl)-1,4-diaminobenzene as a novel architecture for green fluorophores, established based on an effective push-pull system supported by intramolecular hydrogen bonding. This compound demonstrates high fluorescence emission and photostability and is solid-state emissive, water-soluble, and solvent- and pH-independent with quantum yields of Φ=0.67 and Stokes shift of 140 nm (in water). This architecture is a significant departure from conventional extended π-conjugated systems based on a flat and rigid molecular design and provides a minimum requirement for green fluorophores comprising a single benzene ring.

Solution-processed inorganic-organic hybrid electron injection layer for polymer light-emitting devices.

A lithium quinolate complex (Liq) has high solubility in polar solvents such as alcohols and can be spin-coated onto emitting polymers, resulting in a smooth surface morphology. A polymer light-emitting device fabricated with spin-coated Liq as an electron injection layer (EIL) exhibited a lower turn-on voltage and a higher efficiency than a device with spin-coated Cs2CO3 and a device with thermally evaporated Ca. The mixture of ZnO nanoparticles and Liq served as an efficient EIL, resulting in a lower driving voltage even in thick films (∼10 nm), and it did not require a high-temperature annealing process.

Direct deposition of two nanomaterials with the same surface charge using a liquid-liquid interface.

Two negatively charged nanoparticles (SDS-coated SWCNT and polydiacetylene nanocrystals) were sequentially adsorbed onto the same water-hexane interface. The absorbed film can be transferred onto a solid substrate. Repeating the adsorption and transfer process enables assembly of the two nanoparticles in a layer-by-layer growth fashion up to three bi-layers.

Radical-initiator-Induced solid-state polymerization of butadiyne nanocrystals in water and their dispersion stabilization.

Butadiyne nanocrystals in water are usually polymerized by UV or gamma-ray irradiation to give polydiacetylene (PDA) nanocrystals. In this study, we confirmed that solid-state polymerization of 1,6-di(N-carbazolyl)-2,4-hexadiyne (DCHD) and 5,7-dodecadiyn-1,12-diyl bis[N-(butoxycarbonyl-methyl)carbamate] (4BCMU) could be stimulated by water-soluble radical initiators. The radical initiators used were potassium peroxodisulfate, three kinds of azo-type compounds and a redox initiator. In all cases, the solid-state polymerization was confirmed by color change into blue indicating that PDA modified by the radical residues at the end was formed. However, nanocrystal cohesion occurred especially when the concentration of the initiators was high or the dispersion was kept for a long time. In order to improve the dispersion stability, two kinds of surfactants, i.e., sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium chloride (DTMAC), were added to the DCHD nanocrystal aqueous dispersion. As a result, when anionic SDS was added, the solid-state polymerization of nanocrystals proceeded without coagulation and quantitative conversion was confirmed for all initiators. Cationic DTMAC has no effect on dispersion stabilization. PDA nanocrystal surfaces in water are negatively charged in nature and electric interaction of nanocrystals with the cations results in decrease of surface charge and aggregation of nanocrystals.

In situ and ex situ observations of the growth dynamics of single perylene nanocrystals in water.

The growth dynamics of fluorescent perylene nanocrystals, which are fabricated by the reprecipitation method, was investigated using in situ and ex situ single-particle fluorescence measurements. A red shift in the emission maxima as the aging time increased was observed by single-particle fluorescence spectral measurements. The number and size of the nanocrystals increased with the increasing aging time in water. It was concluded that the metastable intermediates, such as clusters and initial nanoparticles, are relevant for the early stages of nucleation and growth of the perylene nanocrystals.

Synthesis and characterization of monodispersed polymer/polydiacetylene nanocrystal composite particles.

Monodispersed polymer/polydiacetylenecomposite particles were synthesized by soap-free seeded emulsion polymerization of styrene andmethyl methacrylate; the products were characterized by XRD, SEM, TEM, UV-visible spectroscopy, and single particle scattering spectroscopy. In the synthesis process, polydiacetylene nanocrystals were found to act as inhibitor, and consequently a relatively low concentration was necessary. Different monomers lead to the differences in reaction condition and particle morphology; the PMMA composite particles were simpler in preparation than polystyrene particles, but the latter havebetter spherical morphology. The composite particles were composed of polymer shells and polydiacetylene cores, which kept their crystal structure and optical properties. A high percentage of cored particles could be achieved with optimized reaction conditions where the amount of seed was sufficient and the oily oligomer by-product was suppressed.