Yasuyuki Kusaka

Metal-enhanced fluorescence platforms based on plasmonic ordered copper arrays: wavelength dependence of quenching and enhancement effects.

Ordered arrays of copper nanostructures were fabricated and modified with porphyrin molecules in order to evaluate fluorescence enhancement due to the localized surface plasmon resonance. The nanostructures were prepared by thermally depositing copper on the upper hemispheres of two-dimensional silica colloidal crystals. The wavelength at which the surface plasmon resonance of the nanostructures was generated was tuned to a longer wavelength than the interband transition region of copper (>590 nm) by controlling the diameter of the underlying silica particles. Immobilization of porphyrin monolayers onto the nanostructures was achieved via self-assembly of 16-mercaptohexadecanoic acid, which also suppressed the oxidation of the copper surface. The maximum fluorescence enhancement of porphyrin by a factor of 89.2 was achieved as compared with that on a planar Cu plate (CuP) due to the generation of the surface plasmon resonance. Furthermore, it was found that while the fluorescence from the porphyrin was quenched within the interband transition region, it was efficiently enhanced at longer wavelengths. It was demonstrated that the enhancement induced by the proximity of the fluorophore to the nanostructures was enough to overcome the highly efficient quenching effects of the metal. From these results, it is speculated that the surface plasmon resonance of copper has tremendous potential for practical use as high functional plasmonic sensor and devices.

Overlay of semi-dried functional layers in offset printing for rapid and high-precision fabrication of flexible TFTs

We achieved a reduction in the misregistration of overlying patterns printed on a flexible plastic film and a drastically shorter processing time with fully printed thin-film transistor (TFT) fabrication. This was achieved using a newly developed wet-on-wet (WoW) printing process wherein a subsequent layer can be printed on a previous semi-dried (not-sintered) layer. In the WoW process, as examined by rheological measurements, a semi-dried (highly solidified) state of ink was attained before transferring by utilizing the solvent uptake of a PDMS blanket in offset printing to ensure the structural integrity of the ink layer, and to reduce the inter-contamination of adjoining layers. Loss-on-drying tests and resistivity measurements indicated that molecular penetration at the boundary of adjoining layers with a length of c.a. 70 nm occurred in the WoW process; however, with thicker electrodes, we successfully fabricated a WoW-processed TFT whose performance was comparable with a TFT formed by a conventional printing process.

Hydrophobic attraction between silanated silica surfaces in the absence of bridging bubbles.

The interaction forces between silanated silica surfaces on which there were neither nanobubbles nor a gas phase were measured using colloidal probe atomic force microscopy (AFM). To obtain hydrophobic surfaces without attached nanobubbles, an aqueous solution was introduced between the surfaces after an exchange process involving several solvents. In the approaching force curves obtained, an attractive force was observed from a distance of 10-25 nm, indicating the existence of an additional attractive force stronger than the van der Waals attraction. In the retracting force curves, a strong adhesion force was observed, and the value of this force was comparable to that of the capillary bridging force. The data clearly showed that although the bridging of nanobubbles is responsible for long-range hydrophobic attraction, there also exists an additional attractive force larger than the van der Waals attraction between hydrophobic surfaces without nanobubbles. Both the ionic strength and the temperature of the solution had little influence on the force. The possible origin of the force is discussed on the basis of the obtained results.

Fabrication of embedded electrodes by reverse offset printing

We developed a novel offset-printing process that permits the fabrication of silver-nanoparticle electrodes embedded in a dielectric layer. We succeeded in embedding approximately 1 µm thick silver electrodes to a dielectric layer with thickness ratio of 1:1.4. The surface-height difference between the embedded electrode and the surrounded dielectric layer was less than 80 nm. By virtue of the surface uniformity of this embedding process, the interconnect breakage of orthogonal wires printed on top of the underlying wire was drastically reduced to 4%, compared to 41% for non-embedded wires. The electric conductivity of embedded electrodes with thickness ≈110 nm was about 10 µΩ cm−1, which was comparable to that of a silver pattern formed on a glass substrate alone. We also examined the characteristics of fully printed thin-film transistors composed of the embedded electrodes, and measured an average mobility of 0.07 cm2 V−1s−1. These results demonstrate the applicability of the proposed technique to the fabrication of printed circuits and devices including active elements.

Continuous fine pattern formation by screen-offset printing using a silicone blanket

Screen-offset printing combines screen-printing on a silicone blanket with transference of the print from the blanket to a substrate. The blanket absorbs organic solvents in the ink, and therefore, the ink does not disperse through the material. This prevents blurring and allows fine patterns with widths of a few tens of micrometres to be produced. However, continuous printing deteriorates the patterns shape, which may be a result of decay in the absorption abilities of the blanket. Thus, we have developed a new technique for refreshing the blanket by substituting high-boiling-point solvents present on the blanket surface with low-boiling-point solvents. We analyse the efficacy of this technique, and demonstrate continuous fine pattern formation for 100 screen-offset printing processes.

Extending microcontact printing for patterning of thick polymer layers: semi-drying of inks and contact mechanisms

We investigate the applicability of the microcontact printing technique for the patterning of polymeric etch-resistant layers with thicknesses in the order of micrometers. In contrast to small molecular materials such as thiols and silane coupling agents typically used in microcontact printing, the patterning of thick layers requires tuning of the rheological properties of an ink film to prevent pattern deformation and attain high-quality transfer. By evaluating the swelling rate of a microcontact stamp material (i.e. poly(dimethylsiloxane) (PDMS)) and the evaporation rate of solvents, we find an optimal ink formulation to attain the desired semi-dried state for the printing of polymer layers. In polymer films with solid content below the optimal limit, split- or wrinkle-type deformations were found depending on the adhesion force and deformability of ink films, while overly-dried polymer films failed to be transferred. These phenomena are in qualitative agreement with deformation curves obtained from colloidal probe microscopy measurements that successfully revealed the deformability and adhesion of semi-dried polymer films. Further investigation of the effects of stamp stiffness on pattern formation reveals that a pattern region in which the thickness profile has a small curvature radius failed to be transferred when a stiffer PDMS stamp was used. This type of defect is thought to be caused by incomplete contact between the film and substrate due to a semi-circular cap structure of the polymer film and insufficient deformation of the stamp. Herein, a detailed contacting mechanism for high-quality patterning is discussed on the basis of the Hertz contact model. Using the developed etch-resistant ink and optimized printing process conditions, a finely defined etched structure for a silicon substrate is obtained.

Microcontact patterning of conductive silver lines by contact inking and its layer-transfer mechanisms

We developed a contact inking technique for microcontact printing aiming at the fabrication of conductive silver-nanoparticle (Ag NP) lines with rectangular cross section and constant layer thickness, irrespective of pattern size and shape. In the proposed process, Ag NP ink was first coated on a blanket and then inking was carried out by a contact with a microcontact stamp. The ink transferred onto the top of the stamp was finally settled on a workpiece by pressing to complete the printing process. To achieve robust inking to the stamp, the peel strengths between the Ag NP layer and the blankets and between the Ag NP layer and the stamp were investigated using poly (dimethylsiloxane) (PDMS) materials with different surface energies. Interestingly, it was revealed that the transferability of Ag NP from the blanket toward the stamp was not solely determined by the surface energy difference but also by the extent of solvent uptake by the PDMS blanket during inking. The solvent-containing PDMS significantly lowered its adhesion strength against adjacent ink layers and, as a consequence, the ink transfer was successfully achieved even if the ink passed from a higher to a lower energy surface. Furthermore, by the solvent-vapour annealing of contact-inked semi-dried patterns, arbitrarily iterated transfers between PDMS surfaces became possible. With the contact-inking process developed here, we demonstrate a finely defined printed structure of Ag NP conductive lines with widths of up to 1 μm.

High-resolution patterning of silver conductive lines by adhesion contrast planography

We developed printed electronics compatible planographic printing methods that enable single-micrometer-order patterning with high rectangularity and thickness uniformity. Instead of conventional planographic printing methods where selective wetting is used for pattern generation, an adhesive latent image produced on a silicone surface is exploited for patterning in the proposed printing methodologies. We further investigated the fundamental mechanisms of the proposed methods by focusing on adhesion contrasts between the blanket, printing area, and non-printing area of a printing plate (PP) and determined that printing is feasible when a simple magnitude relation of adhesions is satisfied for thin layers of size ranging from approximately 50 nm to 100 nm. Latent image formation can be carried out via a simple ultraviolet exposure of the silicone surface, thereby enabling the rapid prototyping of printed device fabrications. The easily preparable, single material-based flat PPs developed in this study have the advantages of flexibility in pattern designs, washing process, fabrication cost, and pattern-rewriting capability compared with the conventional printing methods in which raised surfaces such as stamps or cliches are required for patterning.

Photocurrent enhancement of porphyrin molecules over a wide-wavelength region based on combined use of silver nanoprisms with different aspect ratios

We succeeded in controlling the wavelength range in which the photocurrent of porphyrin is enhanced by tuning as well as expanding the wavelength ranges in which the localized surface plasmon resonance (LSPR) occurs. We fabricated photoelectric conversion systems consisting of 5,10,15,20-tetrakis(p-carboxyphenyl)porphyrin (TCPP) and silver nanoprisms with small (SAgPRs) and large (LAgPRs) aspect ratios as plasmonic nano-antennae. Their photocurrents were much larger than those from TCPP-modified Ag planar electrodes at the specific wavelengths corresponding to their LSPR bands (SAgPRs: 460–610 nm; LAgPRs: 610–690 nm). The maximum enhancement factors (EFs) for the SAgPRs and the LAgPRs were 37 and 35, respectively. In order to enhance the photocurrents, we expanded the LSPR bands by the combined use of SAgPRs and LAgPRs. The system consisting of the mixture (MAgPRs) showed enhancement of the photocurrent over the entire Q-band region (480–690 nm). Finally, the total EFs of the photocurrents were evaluated by irradiation with AM1.5G sunlight through a long-pass filter of 480 nm and the results revealed that the EFs were in the order of MAgPRs > SAgPRs > LAgPRs. Furthermore, the system showed stability without loss of the enhancement property for at least 10 min under the solar irradiation.

Development of Plasmonic Cu2O/Cu Composite Arrays as Visible- and Near-Infrared-Light-Driven Plasmonic Photocatalysts

We describe efficient visible- and near-infrared (vis/NIR) light-driven photocatalytic properties of hybrids of Cu2O and plasmonic Cu arrays. The Cu2O/Cu arrays were prepared simply by allowing a Cu half-shell array to stand in an oxygen atmosphere for 3 h, which was prepared by depositing Cu on two-dimensional colloidal crystals with a diameter of 543 or 224 nm. The localized surface plasmon resonances (LSPRs) of the arrays were strongly excited at 866 and 626 nm, respectively, at which the imaginary part of the dielectric function of Cu is small. The rate of photodegradation of methyl orange was 27 and 84 times faster, respectively, than that with a Cu2O/nonplasmonic Cu plate. The photocatalytic activity was demonstrated to be dominated by Cu LSPR excitation. These results showed that the inexpensive Cu2O/Cu arrays can be excellent vis/NIR-light-driven photocatalysts based on the efficient excitation of Cu LSPR.