Kyoko Namura

Thermochromic VO2 nanorods made by sputter deposition : Growth conditions and optical modeling

Reactive dc magnetron sputtering onto glass-based substrates yielded deposits of thermochromic VO2 with well-developed nanorods and nanowires. Their formation was promoted by high substrate temperature (above ∼500 °C), sufficient film thickness, proper inlet of the reactive gas, dispersed gold “seeds,” and pronounced substrate roughness. Rutherford back scattering ascertained mass thicknesses, scanning electron microscopy depicted the nanostructures, and glancing incidence X-ray diffraction proved that single-phase VO2 was normally formed. Spectrophotometric measurements of total and diffuse transmittance and reflectance on VO2 thin films, at room temperature and ∼100 °C, allowed us to determine complex dielectric functions below and above the “critical” temperature for thermochromic switching (∼68 °C). These data were then used in computations based on the Bruggeman effective medium theory applied to randomly oriented prolate spheroidal structural units to derive the optical properties of the deposits. Experimental and computed data on spectral absorptance were found to be in good qualitative agreement.

Photothermally controlled Marangoni flow around a micro bubble

We have experimentally investigated the control of Marangoni flow around a micro bubble using photothermal conversion. Using a focused laser spot acting as a highly localized heat source on Au nanoparticles/dielectric/Ag mirror thin film enables us to create a micro bubble and to control the temperature gradient around the bubble at a micrometer scale. When we irradiate the laser next to the bubble, a strong main flow towards the bubble and two symmetric rotation flows on either side of it develop. The shape of this rotation flow shows a significant transformation depending on the relative position of the bubble and the laser spot. Using this controllable rotation flow, we have demonstrated sorting of the polystyrene spheres with diameters of 2 μm and 0.75 μm according to their size.

Heat-generating property of a local plasmon resonator under illumination

We have investigated the heat generation from gold nanoparticles resulting from their local plasma resonance. We have demonstrated the self-assembly of Au nanoparticle arrays/dielectric layer/Ag mirror sandwiches, i.e., a local plasmon resonator, using a dynamic oblique deposition technique. The thicknesses of the Au and dielectric layers were changed combinatorially on a single substrate. As a result, local plasmon resonator chips were successfully fabricated. Because of strong interference, their optical absorption can be controlled between 0.0% and 97% in the near-IR region, depending on the thickness of the dielectric layer. We evaluated the heat generation from Au nanoparticles by measuring the temperature of water with which a cell prepared on a chip is filled under laser illumination. The change in the water temperature is proportional to the optical absorption of the local plasmon resonator chips. This suggests that the photothermal conversion efficiency can be controlled by interference. These features make the application of the local plasmon resonator to nanoheaters, which can spatiotemporally control heat generation, suitable.

Sheathless particle focusing in a microfluidic chamber by using the thermoplasmonic Marangoni effect

We experimentally investigated the modes of the Marangoni flow around a microbubble in a 50-μm-thick water chamber and found a transition flow mode that enables sheathless particle focusing. A temperature gradient was thermoplasmonically induced around the laser spot on a gold nanoisland film, and Marangoni flows were generated around the microbubble to drive submicron particles dispersed in the water. When the laser spot was slightly displaced from the bubble center, the particles were continuously collected by the bubble underneath and leaked in one direction to form a focused particle stream. The generation of the particle-focusing Marangoni flow was attributed to the appropriate balance of the temperature gradient in the perpendicular and horizontal directions of the chamber, which was controlled by the laser spot position against the bubble center. Temporally controlling this flow mode with laser power caused the periodic emission of clustered particles from the bubble underneath. This particle handling method with a thermoplasmonic Marangoni flow can be useful for improving the efficiency of reaction or sensing processes that take place in a microfluidic chamber.

Photoacoustic emission from Au nanoparticles arrayed on thermal insulation layer

Efficient photoacoustic emission from Au nanoparticles on a porous SiO(2) layer was investigated experimentally and theoretically. The Au nanoparticle arrays/porous SiO(2)/SiO(2)/Ag mirror sandwiches, namely, local plasmon resonators, were prepared by dynamic oblique deposition (DOD). Photoacoustic measurements were performed on the local plasmon resonators, whose optical absorption was varied from 0.03 (3%) to 0.95 by varying the thickness of the dielectric SiO(2) layer. The sample with high absorption (0.95) emitted a sound that was eight times stronger than that emitted by graphite (0.94) and three times stronger than that emitted by the sample without the porous SiO(2) layer (0.93). The contribution of the porous SiO(2) layer to the efficient photoacoustic emission was analyzed by means of a numerical method based on a one-dimensional heat transfer model. The result suggested that the low thermal conductivity of the underlying porous layer reduces the amount of heat escaping from the substrate and contributes to the efficient photoacoustic emission from Au nanoparticle arrays. Because both the thermal conductivity and the spatial distribution of the heat generation can be controlled by DOD, the local plasmon resonators produced by DOD are suitable for the spatio-temporal modulation of the local temperature.

Highly localized photothermal conversion in two-dimensional Au nanoparticle arrays

We have investigated, both theoretically and experimentally, highly localized photothermal conversions in Au nanoparticle array/dielectric layer/Ag mirror sandwiches, namely local plasmon resonators. The depth profile of the optical absorption in the local plasmon resonators was calculated using a simple model comprising homogeneous multilayers. The calculation results show highly localized light absorption in the ∼ 10-nm-thick Au nanoparticles layer (more than 99% of total optical absorption). The photoacoustic measurements, which are sensitive to the surface temperature of the sample, were performed on the fabricated local plasmon resonators. The photoacoustic amplitude of the local plasmon resonator possessing a high optical absorption (A = 0.97) was 15 times larger than the absorbance of the bulk Si wafer (A = 0.67) and 8 times larger than the absorbance of graphite (A = 0.85). These results suggest that the photothermal conversion is localized in the thin Au nanoparticles layer, which enables rapid m...

Nanostructured copper/porous silicon hybrid systems as efficient sound-emitting devices.

In the present work, the photo-acoustic emission from nanostructured copper/porous silicon hybrid systems was studied. Copper nanoparticles were grown by photo-assisted electroless deposition on crystalline silicon and nanostructured porous silicon (nanoPS). Both the optical and photo-acoustic responses from these systems were determined. The experimental results show a remarkable increase in the photo-acoustic intensity when copper nanoparticles are incorporated to the porous structure. The results thus suggest that the Cu/nanoPS hybrid systems are suitable candidates for several applications in the field of thermoplasmonics, including the development of sound-emitting devices of great efficiency.

Quasi-stokeslet induced by thermoplasmonic Marangoni effect around a water vapor microbubble.

Rapid Marangoni flows around a water vapor microbubble (WVMB) is investigated using the thermoplasmonic effect of a gold nanoisland film (GNF). By focusing a laser onto the GNF, a stable WVMB with a diameter of ~10 μm is generated in degassed water, while an air bubble generated in non-degassed water is larger than 40 μm. Under continuous heating, the WVMB involves significantly rapid Marangoni flow. This flow is well-described by a stokeslet sat ~10 μm above the surface of GNF, from which the maximum flow speed around the WVMB is estimated to exceed 1 m/s. This rapid flow generation is attributed to the small bubble size, over which the temperature is graded, and the superheat at the bubble surface in contact with the GNF. It is expected to be useful not only for microfluidic mixing but also for fundamental research on viscous flow induced by a single stokeslet.

Microfluidic control on nanoplasmonic thin films using Marangoni effect

Abstract. The rapid switching of flow direction in a thin microfluidic chamber filled with water is demonstrated using the thermoplasmonic Marangoni effect. A gold island film is used as a thermoplasmonic heater on which a continuous-wave laser is focused to generate a microbubble and develop Marangoni flows around it. The direction of the observed flow significantly changes depending on the laser power. When the laser power is square-wave modulated, the flow direction instantaneously switches in response to the power, creating a discrete pattern of polystyrene microspheres. Flow direction-switching is observed for laser power modulation frequencies of up to 40 Hz, which indicates that the time constant of the flow direction switching is on the order of at least several milliseconds. This rapid flow direction switching is attributed to the fast response of both the thermoplasmonic effect of the gold nanoparticles and the Marangoni effect on the bubble surface.

Surface-Controlled Metal Oxide Resistive Memory

To explore the surface effect on resistive random-access memory (ReRAM), the impact of surface roughness on the characteristics of ZnO ReRAM was studied. The thickness-independent resistance and the higher switching probability of ZnO ReRAM with rough surfaces indicate the importance of surface oxygen chemisorption on the switching process. Furthermore, the improvements in switching probability, switching voltage, and resistance distribution observed for ReRAM with rough surfaces can be attributed to the stable oxygen adatoms under various ambience conditions. The findings validate the surface-controlled stability and the uniformity of ReRAM and can serve as the guideline for developing practical device applications.