Kosei Ueno

Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source

We demonstrate the possibility to achieve optical triggering of photochemical reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.

Near-infrared plasmon-assisted water oxidation

We report the stoichiometric evolution of oxygen via water oxidation by irradiating a plasmon-enhanced photocurrent generation system with near-infrared light (λ: 1000 nm), in which gold nanostructures were arrayed on the surface of TiO2 electrode. It is considered that multiple electron holes generated by plasmon-induced charge excitation led to the effective recovery of water oxidation after the electron transfer from gold to TiO2. The proposed system containing a gold nanostructured TiO2 electrode may be a promising artificial photosynthetic system using near-infrared light.

Plasmonic antenna effects on photochemical reactions.

Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale implementation hinges on the availability of high-efficiency modules. For maximum efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively separate photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as amorphous silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelectric conversion characteristics of solar cells generally decrease in the infrared wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addition, the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-infrared wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochemical reactions between photons and molecules close to an optical antenna through the local field enhancement. This technology has the potential to induce highly efficient photoexcitation between photons and molecules over a wide spectrum of sunlight, from visible to near-infrared wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochemical reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithography and a lift-off technique to afford high resolution and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymerization on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochemical reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO(2) and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-infrared (1300 nm) wavelengths.

Optical properties of nanoengineered gold blocks.

We report on control over the extinction spectrum of tetragonal gold nanoblocks by changing their lateral aspect ratio. Nanoblocks were patterned on a glass substrate by electron-beam lithography and were 40-400 nm in lateral dimensions and spanned from 1 to 9 in aspect ratio. This allowed us to tune the localized surface plasmon bands from 700 nm to 1.5 microm (longitudinal mode) and from 700 to 550 nm (transverse mode). Unprecedented polarization selectivity of the transverse and longitudinal plasmon bands was achieved via alignment and 3D control of the dimensions of the nanoblocks.

Plasmon‐Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation

We have successfully developed a plasmon-induced technique for ammonia synthesis that responds to visible light through a strontium titanate (SrTiO3) photoelectrode loaded with gold (Au) nanoparticles. The photoelectrochemical reaction cell was divided into two chambers to separate the oxidized (anodic side) and reduced (cathodic side) products. To promote NH3 formation, a chemical bias was applied by regulating the pH value of these compartments, and ethanol was added to the anodic chamber as a sacrificial donor. The quantity of NH3 formed at the ruthenium surface, which was used as a co-catalyst for SrTiO3, increases linearly as a function of time under irradiation with visible light at wavelengths longer than 550 nm. The NH3 formation action spectrum approximately corresponds to the plasmon resonance spectrum. We deduced that plasmon-induced charge separation at the Au/SrTiO3 interface promotes oxidation at the anodic chamber and subsequent nitrogen reduction on the cathodic side.

Inverse silica opal photonic crystals for optical sensing applications

This work reports fabrication of inverse silica opal photonic crystal structures from direct polystyrene micro sphere opals using low-temperature sol-gel infiltration of silica, and examines performance of these photonic crystals as environmental refractive index sensors. Sensitivity of the spectral position and optical attenuation of photonic stop gaps is found to allow detection of the index changes by the amount of ~10(-3). The high value of sensitivity, which is comparable with those of other optical sensing techniques, along with simplicity of the optical detection setup required for sensing, and the low-temperature, energy-efficient fabrication process make inverse silica opals attractive systems for optical sensing applications.

Highly controlled surface-enhanced Raman scattering chips using nanoengineered gold blocks.

Well defined gold nanostructures of various sizes are fabricated on glass substrates using high-resolution electron-beam lithography/lift-off techniques and detailed surface-enhanced Raman scattering (SERS) properties of crystal violet molecules are studied in order to elucidate electromagnetic (EM) field enhancement effects on the fabricated structures. SERS measurements are performed with high reproducibility using in situ Raman microspectroscopy in aqueous solution. An analysis based on EM theory is performed using field-enhancement factors obtained from finite-difference time-domain (FDTD) simulations and the analysis reproduces experimental results very well. It is noteworthy, furthermore, that the proposed analytic method of EM effects on SERS allows the estimate of the ideal local temperature of gold nanostructures by canceling out the difference in EM field factors at Stokes and anti-Stokes Raman scattering wavelengths. Thus, these experimental results demonstrate that quantitative analysis based on EM theory can be obtained using highly controlled gold nanostructures for SERS measurements with high reproducibility, a result that is promising for the construction of a SERS analysis chip. Although no SERS chip reported so far has been usable for quantitative analysis, this study opens the door for construction of a quantitative SERS chip.

Homogeneous nano-patterning using plasmon-assisted photolithography

We report an innovative lithography system appropriate for fabricating sharp-edged nanodot patterns with nanoscale accuracy using plasmon-assisted photolithography. The key technology is two-photon photochemical reactions of a photoresist induced by plasmonic near-field light and the scattering component of the light in a photoresist film. The scattering component of the light is a radiation mode from higher order localized surface plasmon resonances scattered by metallic nanostructures.

Direct imaging of nanogap-mode plasmon-resonant fields

We perform direct local-field imaging of a plasmon-resonant gold nanoparticle pair separated by a gap of several nanometers using a scattering-type near-field optical microscope with a sharp silicon tip of atomic force microscope. The sharp tip allows the access for the nanogap and the high spatial resolution. Our results provide experimental evidence that the nanogap structure produces an optical spot with the size of a single nanometer (<10 nm). This is not only of fundamental importance in the field of nanophotonics, but also provide significant information for the development of plasmonic devices with the nanogap structures.

Plasmon-Induced Water Splitting Using Metallic-Nanoparticle-Loaded Photocatalysts and Photoelectrodes.

Visible- and near-infrared-light-driven water splitting, which splits water molecules to generate hydrogen and oxygen gases, is a significant subject in artificial photosynthesis with the goal of achieving a low-carbon society. In recent years, considerable attention has been paid to studies on the development of a plasmon-induced water-splitting system responding to visible light. In this review, we categorized water-splitting systems as gold-nanoparticle-loaded semiconductor photocatalytic particles system and metallic-nanoparticles-loaded semiconductor photoelectrode systems, and introduce the latest studies according to these categories. Especially, we describe the studies that optimize a material or a structural design of metallic-nanoparticle-loaded semiconductor photoelectrodes and consider a whole water-splitting system, including a cathode design. Furthermore, we discuss important points when studying plasmon-induced water splitting, and we describe a methodology that enhances plasmon-induced water-splitting efficiency.