Mitsuhiro Honda

Nanoscale heating of laser irradiated single gold nanoparticles in liquid.

Biological applications where nanoparticles are used in a cell environment with laser irradiation are rapidly emerging. Investigation of the localized heating effect due to the laser irradiation on the particle is required to preclude unintended thermal effects. While bulk temperature rise can be determined using macroscale measurement methods, observation of the actual temperature within the nanoscale domain around the particle is difficult and here we propose a method to measure the local temperature around a single gold nanoparticle in liquid, using white light scattering spectroscopy. Using 40-nm-diameter gold nanoparticles coated with thermo-responsive polymer, we monitored the localized heating effect through the plasmon peak shift. The shift occurs due to the temperature-dependent refractive index change in surrounding polymer medium. The results indicate that the particle experiences a temperature rise of around 10 degrees Celsius when irradiated with tightly focused irradiation of ~1 mW at 532 nm.

Plasmon-enhanced UV photocatalysis

We report plasmonic nanoparticle enhanced photocatalysis on titanium dioxide (TiO2) in the deep-UV range. Aluminum (Al) nanoparticles fabricated on TiO2 film increases the reaction rate of photocatalysis by factors as high as 14 under UV irradiation in the range of 260–340 nm. The reaction efficiency has been determined by measuring the decolorization rate of methylene blue applied on the TiO2 substrate. The enhancement of photocatalysis shows particle size and excitation wavelength dependence, which can be explained by the surface plasmon resonance of Al nanoparticles.

Efficient UV photocatalysis assisted by densely distributed aluminum nanoparticles

Aluminum nanoparticles fabricated by oblique angle deposition (OAD) successfully increased the yield and reaction rate of UV photocatalysis due to the localized surface plasmon resonance (LSPR) effect. Nanoparticles 20–60 nm in size were formed in an area larger than ~1 cm2 when the film was highly tilted during the thermal deposition process. The size and density of these nanoparticles were readily controlled by the deposition thickness and speed. The yield of photocatalytic reactions increased by a factor of ~2, while the reaction rate increased by up to ~10 times. The aluminum nanostructures presented here are of tremendous advantage for future applications in photocatalysis through efficient coupling with UV light.

Room temperature ethanol sensor based on ZnO prepared via laser ablation in water

The present work reports on room-temperature ethanol sensing performance of ZnO nanospheres and nanorods prepared using pulsed laser ablation in water. Nanosecond and millisecond lasers were used to prepare ZnO nanomaterials with hexagonal wurtzite crystal structure. The two contrasting nanostructures were tested as gas sensors towards volatile compounds such as ethanol, ammonia, and acetone. At room temperature, devices based on both ZnO nanomaterials demonstrated selectivity for ethanol vapor. The sensitivity of nanospheres was somewhat higher compared to that of nanorods, with response values of ~19 and ~14, respectively, towards 250 ppm. Concentrations as low as 50 ppm could be easily detected.

Defects in ZnO nanoparticles laser-ablated in water?ethanol mixtures at different pressures

The effect of liquid medium and its pressure on the photoluminescence of ZnO nanoparticles prepared via laser ablation of Zn targets in various water-ethanol mixtures is studied. As the ethanol content increases, the photoluminescence of the product changes, while metallic zinc is observed to emerge in nanomaterials prepared in ethanol-rich environments. The applied pressure had a less profound effect, mainly affecting materials produced in water or water-ethanol, and much less those generated in pressurized ethanol. Tuning the reactivity of the liquid and pressurizing it during laser ablation is demonstrated to be promising for tailoring the emission properties of the product.

Nanostructures prepared via laser ablation of tin in water

In this study, nanomaterials prepared via laser ablation of tin in water are systematically studied and compared. Tin targets were ablated by both millisecond and nanosecond pulsed lasers, resulting in core@shell product nanostructures with different chemistries and morphologies. Depending on laser fluence, the obtained core@shell nanoparticles had either Sn or SnO cores and SnOx shells with varied surface hydration degree. Optical emission spectra of laser-generated plasmas were taken, giving additional support to nanoparticle formation mechanisms. Finally, gas sensing at room temperature is demonstrated as one of the potential applications for such nanostructures.

UV plasmonic device for sensing ethanol and acetone

In the present study, we demonstrate efficient detection of volatile organic vapors with improved sensitivity, exploiting the localized surface plasmon resonance of indium nanograins in the UV range (UV-LSPR). The sensitivity of deep-UV-LSPR measurements toward ethanol was observed to be 0.004 nm/ppm, which is 10 times higher than that of a previously reported visible-LSPR device based on Ag nanoprisms [Sensors 11, 8643 (2011)]. Although practical issues such as improving detection limits are still remaining, the results of the present study suggest that the new approach based on UV-LSPR may open new avenues to the detection of organic molecules in solid, liquid, and gas phases using plasmonic sensors.

Direct growth of densely aligned ZnO nanorods on graphene

Densely aligned ZnO nanorods were directly grown on graphene sheets. On graphene prepared via a chemical vapor deposition technique, ZnO nanorods were synthesized by a hydrothermal method. The rod density was ~1.4 × 109/cm2 and the nanorods were observed to be well aligned on graphene by scanning electron microscopy. The formation of such ZnO structures is considered to be induced by carbon vacancies in graphene in accordance with Raman spectroscopic results.

Individual TiO2 nanocrystals probed by resonant Rayleigh scattering spectroscopy

Individual titanium dioxide (TiO2) nanocrystals with bandgaps in the deep ultraviolet (DUV) wavelength range were investigated using resonant Rayleigh scattering spectroscopy. A microscopy system that was switchable from dark-field imaging to scattering spectroscopy was specifically constructed for a broadband UV light source. Weak Rayleigh scattering from a single nanocrystal a few nanometers in size was obtained through the UV excitation resonance and high positional reproducibility of the switching optics. Individual nanocrystals exhibited specific intrinsic bandgaps depending on their size, shape, and crystallinity, greatly affecting their photocatalytic efficiency.

The effect of annealing graphene in O2 and N2 flow

Thermal annealing is potentially capable of removing surface contaminants of polymers on graphene while annealing parameters are crucial to prevent graphene from unintended damages. We studied the influence of O2 flow and N2 flow during an anneal treatment on few-layer graphene through transmittance and Raman spectroscopy. Annealing in O2 atmosphere at 400 °C was observed to provide graphene with being etched and damaged. On the other hand, by annealing in N2, a noticeable change in the defect state was hardly observed while amorphous carbon was found on the surface, which is expected to be due to the decomposition of polymer residues at the graphene surface.Thermal annealing is potentially capable of removing surface contaminants of polymers on graphene while annealing parameters are crucial to prevent graphene from unintended damages. We studied the influence of O2 flow and N2 flow during an anneal treatment on few-layer graphene through transmittance and Raman spectroscopy. Annealing in O2 atmosphere at 400 °C was observed to provide graphene with being etched and damaged. On the other hand, by annealing in N2, a noticeable change in the defect state was hardly observed while amorphous carbon was found on the surface, which is expected to be due to the decomposition of polymer residues at the graphene surface.