Self-Organizing, Environmentally Stable, and Low-Cost Copper-Nickel Complex Inks for Printed Flexible Electronics.

Cost-effective copper conductive inks are considered as the most promising alternative to expensive silver conductive inks for use in printed electronics. However, the low stability and high sintering temperature of copper inks hinder their practical application. Herein, we develop rapidly customizable and stable copper-nickel complex inks that can be transformed in situ into uniform copper@nickel core-shell nanostructures by a self-organized process during low-temperature annealing and immediately sintered under photon irradiation to form copper-nickel alloy patterns on flexible substrates. The complex inks are synthesized within 15 min via a simple mixing process and are particle-free, air-stable, and compatible with large-area screen printing. The manufactured patterns exhibit a high conductivity of 19-67 µΩ·cm, with the value depending on the nickel content, and can maintain high oxidation resistance at 180 °C even when the nickel content is as low as 6 wt %. In addition, the printed copper-nickel alloy patterns exhibit high flexibility as a consequence of the local softening and mechanical anchoring effect between the metal pattern and the flexible substrate, showing strong potential in the additive manufacturing of highly reliable flexible electronics, such as flexible radio-frequency identification (RFID) tags and various wearable sensors.

Layer-By-Layer Printing Strategy for High-Performance Flexible Electronic Devices with Low-Temperature Catalyzed Solution-Processed SiO2.

Additive printing techniques have been widely investigated for fabricating multilayered electronic devices. In this work, a layer-by-layer printing strategy is developed to fabricate multilayered electronics including 3D conductive circuits and thin-film transistors (TFTs) with low-temperature catalyzed, solution-processed SiO2 (LCSS) as the dielectric. Ultrafine, ultrasmooth LCSS films can be facilely formed at 90 °C on a wide variety of organic and inorganic substrates, offering a versatile platform to construct complex heterojunction structures with layer-by-layer fashion at microscale. The high-resolution 3D conductive circuits formed with gold nanoparticles inside the LCSS dielectric demonstrate a high-speed response to the transient voltage in less than 1 µs. The TFTs with semiconducting single-wall carbon nanotubes can be operated with the accumulation mode at a low voltage of 1 V and exhibit average field-effect mobility of 70 cm2  V-1  s-1 , on/off ratio of 107 , small average hysteresis of 0.1 V, and high yield up to 100% as well as long-term stability, high negative-gate bias stability, and good mechanical stability. Therefore, the layer-by-layer printing strategy with the LCSS film is promising to assemble large-scale, high-resolution, and high-performance flexible electronics and to provide a fundamental understanding for correlating dielectric properties with device performance.

Dual Surface Architectonics for Directed Self-Assembly of Ultrahigh-Resolution Electronics.

The directed self-assembly of electronic circuits using functional metallic inks has attracted intensive attention because of its high compatibility with extensive applications ranging from soft printed circuits to wearable devices. However, the typical resolution of conventional self-assembly technologies is not sufficient for practical applications in the rapidly evolving additively manufactured electronics (AMEs) market. Herein, an ultrahigh-resolution self-assembly strategy is reported based on a dual-surface-architectonics (DSA) process. Inspired by the Tokay gecko, the approach is to endow submicrometer-scale surface regions with strong adhesion force toward metallic inks via a series of photoirradiation and chemical polarization treatments. The prepared DSA surface enables the directed self-assembly of electronic circuits with unprecedented 600 nm resolution, suppresses the coffee-ring effect, and results in a reliable conductivity of 14.1 ± 0.6 µΩ cm. Furthermore, the DSA process enables the layer-by-layer fabrication of fully printed organic thin-film transistors with a short channel length of 1 µm, which results in a large on-off ratio of 106 and a high field-effect mobility of 0.5 cm2  V-1  s-1 .

Fabrication of Two-Dimensional Crystalline Organic Films by Tilted Spin Coating for High-Performance Organic Field-Effect Transistors.

We developed a facile method for fabricating large-area, two-dimensional (2D), organic, highly crystalline films and extended it to organic thin-film transistor arrays. Tilted spinning provided oriented flow at the three-phase contact line, and a 2D crystalline film that consisted of layer-by-layer stacked 2,7-diocty[1]benzothieno[3,2- b]benzothiophene (C8-BTBT) was obtained facilely for organic thin-film transistors (OTFTs). The extracted field-effect mobility is 4.6 cm2 V-1 s-1, but with nonideal features. By applying this method to microdroplet arrays, an oriented crystal was fabricated, and the channel region for OTFTs was covered by adjusting the spinning speed. By tuning the tilt angle (θ) of the revolving substrate, we fabricated high-performance OTFT arrays with average and maximum mobilities of 7.5 and 10.1 cm2 V-1 s-1, respectively, which exhibited high reliability factors of over 90% and were close to that of ideal transistors. These results suggest that high-quality crystalline films can be obtained via a facile tilted-spinning method.

High-Resolution Electronics: Spontaneous Patterning of High-Resolution Electronics via Parallel Vacuum Ultraviolet (Adv. Mater. 31/2016).

On page 6568, T. Minari and co-workers describe spontaneous patterning based on the parallel vacuum ultraviolet (PVUV) technique, enabling the homogeneous integration of complex, high-resolution electronic circuits, even on large-scale, flexible, transparent substrates. Irradiation of PVUV to the hydrophobic polymer surface precisely renders the selected surface into highly wettable regions with sharply defined boundaries, which spontaneously guides a metal nanoparticle ink into a series of circuit lines and gaps with the widths down to a resolution of 1 µm.

Spontaneous Patterning of High-Resolution Electronics via Parallel Vacuum Ultraviolet.

A spontaneous patterning technique via parallel vacuum ultraviolet is developed for fabricating large-scale, complex electronic circuits with 1 µm resolution. The prepared organic thin-film transistors exhibit a low contact resistance of 1.5 kΩ cm, and high mobilities of 0.3 and 1.5 cm(2) V(-1) s(-1) in the devices with channel lengths of 1 and 5 µm, respectively.

Origin of surface trap states in CdS quantum dots: relationship between size dependent photoluminescence and sulfur vacancy trap states.

Monodisperse cadmium sulphide (CdS) quantum dots (QDs) with a tunable size from 1.4 to 4.3 nm were synthesized by a non-injection method, and their surface states were characterized by photoluminescence spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The steady state photoluminescence study identified that the proportion of the trap state emission increased with the QD size decrease, while from the photoluminescence decay study, it appeared that the trap state emission results from the emission via a surface deep trap state. The XPS measurements revealed the existence of surface Cd with sulfur vacancy sites which act as electron trap sites, and the population of these sites increases with the QD size decrease. These results are consistent to conclude that the trap state emission mainly originates from the surface deep trapped electrons at the surface Cd with sulfur vacancy sites.

Covalent assembly of silver nanoparticles on hydrogen-terminated silicon surface.

Synthesis of ω-alkenyl-terminated silver nanoparticles (AgNPs) and then their immobilization on a hydrogen-terminated silicon surface in two-dimensional arrangement through covalent interaction are demonstrated. The thermal-induced hydrosilylation at mild conditions facilitate nanoparticles assembly through interaction between terminal alkenyl (CH(2)=CH-) groups of AgNPs and hydrogen-terminated silicon surface. The assembly of AgNPs on a silicon surface is characterized by FESEM and XPS. Adequate coating of 10-undecene-1-thiol (UDT) on AgNPs and mild temperature hydrosilylation impede the fusion or aggregation of nanoparticles, while they immobilized on a silicon surface, which is very crucial to preserve the discrete entities of nanoparticles. This elegant and facile approach provides stable monolayer of AgNPs with very good coverage area and promises potential to fabricate electronic devices and solar cells, where nanoparticles needs to be directly attached to the silicon surface without an interfacial oxide thin film.

Large-scale synthesis of high-quality metal sulfide semiconductor quantum dots with tunable surface-plasmon resonance frequencies.

High-quality CdS and Cu(7)S(4) quantum dots (QDs) were synthesized with N,N-dibutylthiourea (DBTU) as an organic sulfur source. In this method, nucleation and growth reactions were controlled simply by the heating rate of the reaction. The mild oxidation conditions gave monodisperse CdS QDs exhibiting pure band-edge emission with relatively high photoluminescence quantum yield. During the synthesis of Cu(7)S(4) QDs, the addition of dodecanethiol to the reaction system controlled the reaction rate to give monodisperse spherical or disk-shaped QDs. A hundred-gram scale of copper precursor could be used to generate the high-quality Cu(7)S(4) QDs, indicating that an industrial-scale reaction is achievable with our method. As observed in anisotropic noble-metal nanocrystals, larger disk-shaped Cu(7)S(4) QDs showed lower localized-surface-plasmon resonance energy in the near-infrared region. The disk-shaped Cu(7)S(4) QDs could be used effectively as templates to form cation-exchanged monodisperse disk-shaped CdS QDs.

Electric-field enhancement inducing near-infrared two-photon absorption in an indium-tin oxide nanoparticle film.

An enhancing factor: the enhancement of the electric properties of a dye molecule (IR26) by indium-tin oxide nanoparticles (ITO NPs, see picture) has been shown by measuring the near-infrared two-photon-excited transient absorption spectra. The dye molecule was excited much more efficiently in the presence of an ITO NP layer.

Preparation of core-shell-structured nanoparticles (with a noble-metal or metal oxide core and a chromia shell) and their application in water splitting by means of visible light.

Core-shell-structured nanoparticles, consisting of a noble metal or metal oxide core and a chromia (Cr(2)O(3)) shell, were studied as promoters for photocatalytic water splitting under visible light. Core nanoparticles were loaded by impregnation, adsorption or photodeposition onto a solid solution of gallium nitride and zinc oxide (abbreviated GaN:ZnO), which is a particulate semiconductor photocatalyst with a band gap of approximately 2.7 eV, and a Cr(2)O(3) shell was formed by photodeposition using a K(2)CrO(4) precursor. Photodeposition of Cr(2)O(3) on GaN:ZnO modified with a noble metal (Rh, Pd and Pt) or metal oxide (NiO(x), RuO(2) and Rh(2)O(3)) co-catalyst resulted in enhanced photocatalytic activity for overall water splitting under visible light (lambda>400 nm). This enhancement in activity was primarily due to the suppression of undesirable reverse reactions (H(2)-O(2) recombination and/or O(2) photoreduction) and/or protection of the core component from chemical corrosion, depending on the core type. Among the core materials examined, Rh species exhibited relatively high performance for this application. The activity for visible-light water splitting on GaN:ZnO modified with an Rh/Cr(2)O(3) core-shell configuration was dependent on both the dispersion of Rh nanoparticles and the valence state. In addition, the morphology of the Cr(2)O(3) photodeposits was significantly affected by the valence state of Rh and the pH at which the photoreduction of K(2)CrO(4) was conducted. When a sufficient amount of K(2)CrO(4) was used as the precursor and the solution pH ranged from 3 to 7.5, Cr(2)O(3) was successfully formed with a constant shell thickness (approximately 2 nm) on metallic Rh nanoparticles, which resulted in an effective promoter for overall water splitting.

Drastic structural transformation of cadmium chalcogenide nanoparticles using chloride ions and surfactants.

In the present work, we studied a unique and facile method for the drastic structural transformation of hydrophobic small CdE (E = S, Se, Te) nanoparticles into large, high-quality pencil-shaped nanoparticles through an Ostwald ripening process induced by Cl(-) and surfactants (oleic acid and oleylamine). This study revealed that Cl(-) is the effective anion for the controlled structural transformation of CdE nanoparticles. This transformation reaction can be readily extended to the formation of various functional materials.

Plasmon hybridization in individual gold nanocrystal dimers: direct observation of bright and dark modes.

We apply a nanomanipulation technique to assemble pairs of monodispersed octahedral gold nanocrystals (side length, 150 nm) along their major axes with a varying tip-to-tip separation (25-125 nm). These pairs are immobilized onto indium tin oxide coated silica substrates and studied as plasmonic dimers by polarization-selective total internal reflection (TIR) microscopy and spectroscopy. We confirm that the plasmon coupling modes with the scattering polarization along the incident light direction result from the transverse-magnetic-polarized incident light, which induces two near-field-coupled dipole moments oriented normal to the air-substrate interface. In such cases, both in-phase (antibonding) and antiphase (bonding) plasmon coupling modes can be directly observed with the incident light wave vector perpendicular and parallel to the dimer axis, respectively. The observation of antiphase plasmon coupling modes ("dark" plasmons) is made possible by the unique polarization nature of the TIR-generated evanescent field. Furthermore, with decreasing nanocrystal separation, the plasmon coupling modes shift to shorter wavelengths for the incident light perpendicular to the dimer axis, whereas relatively large red shifts of the plasmonic coupling modes are found for the parallel incident light.

Indium tin oxide nanoparticles with compositionally tunable surface plasmon resonance frequencies in the near-IR region.

Here we report the synthesis of conducting indium tin oxide (ITO) nanoparticles (NPs) and their surface plasmon resonance (SPR) properties. The SPR peaks of the ITO NPs can be easily tuned by changing the concentration of Sn doping from 3 to 30 mol %. The shortest SPR wavelength of 1618 nm in 10% Sn-doped ITO NPs may reflect the highest electron carrier density in the ITO NPs. The controllable SPR frequencies of metal oxides may offer a novel approach for noble-metal-free SPR applications. Unlike noble-metal nanostructures, ITO has no inter- and intraband transitions in the vis-near-IR region and represents a free-electron conduction, allowing us to systematically study the origin of optical effects arising from the SPRs of conduction electrons.

CdPd sulfide heterostructured nanoparticles with metal sulfide seed-dependent morphologies.

Seed-mediated growth synthesis has provided us with anisotropically phase-segregated CdPd sulfide heterostructured nanoparticles with seed-dependent morphologies and crystal structures.

Thermally stable silica-coated hydrophobic gold nanoparticles.

We have successfully developed a method for silica coating on hydrophobic dodecanethiol-protected Au nanoparticles with coating thickness ranging from 10 to 40 nm. The formation of silica-coated Au nanoparticles could be accomplished via the preparation of hydrophilic Au nanoparticle micelles by cationic surfactant encapsulation in aqueous phase, followed by hydrolysis of tetraethylorthosilicate on the hydrophilic surface of gold nanoparticle micelles. Silica-coated Au nanoparticles exhibited quite high thermal stability, that is, no agglomeration of the Au cores could be observed after annealing at 600 degrees C for 30 min. Silica-coated Au nanoparticles could serve as a template to derive hollow nanoparticles. An addition of NaCN solution to silica-coated Au nanoparticles led the formation of hollow silica nanoparticles, which were redispersible in deionized water. The formation of the hollow silica nanoparticles results from the mesoporous structures of the silica shell and such a mesoporous structure is applicable to both catalyst support and drug delivery.

Room-temperature size evolution of thiol-protected gold nanoparticles assisted by proton acids and halogen anions.

In the present work, a mild size-control method for thiol-protected Au NPs was developed. When thiol-protected Au NPs were reacted with tetra-n-octylammonium bromide and trifluoroacetic acid, the size of Au NPs was tuned in the range from 2 to 6 nm at room temperature. Larger Au NPs could be obtained by mild heating at 55 degrees C. This size evolution method assisted by proton acid and bromide anion is a first example of the size tuning of thiol-protected Au NPs at room temperature. Because of the mild reactivity of acid and bromide ions to organic molecules, various thiol derivatives sensitive to heat, base, and reducing agent can be utilized for this method.