Kazunari Ozasa

Stimulus-Responsive Azobenzene Supramolecules: Fibers, Gels, and Hollow Spheres

Novel, stimulus-responsive supramolecular structures in the form of fibers, gels, and spheres, derived from an azobenzene-containing benzenetricarboxamide derivative, are described. Self-assembly of tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxamide (Azo-1) in aqueous organic solvent systems results in solvent dependent generation of microfibers (aq DMSO), gels (aq DMF), and hollow spheres (aq THF). The results of a single crystal X-ray diffraction analysis of Azo-1 (crystallized from a mixture of DMSO and H2O) reveal that it possesses supramolecular columnar packing along the b axis. Data obtained from FTIR analysis and density functional theory (DFT) calculation suggest that multiple hydrogen bonding modes exist in the Azo-1 fibers. UV irradiation of the microfibers, formed in aq DMSO, causes complete melting while regeneration of new fibers occurs upon visible light irradiation. In addition to this photoinduced and reversible phase transition, the Azo-1 supramolecules display a reversible, fiber-to-sphere morphological transition upon exposure to pure DMSO or aq THF. The role played by amide hydrogen bonds in the morphological changes occurring in Azo-1 is demonstrated by the behavior of the analogous, ester-containing tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxylate (Azo-2) and by the hydrogen abstraction in the presence of fluoride anions.

Reversible transition between InGaAs dot structure and InGaAsP flat surface

We have studied the in situ modification of coherently grown InGaAs dots by interaction with phosphorus. By monitoring the intensity of reflection high-energy electron diffraction transmission spot, the in situ phosphorus (precracked PH3) supply on the InGaAs dots was examined at 480 °C. It was found that the phosphorus exposure induces a surface structure change from a dot structure to a flat surface. The change is caused by the replacement of arsenic in the dots by phosphorus, which reduces the strain between the InGaAs(P) dots and the GaAs substrate. By switching AsH3/PH3 beams in situ, a reversible transition of the surface structure between the InGaAs dot structure and the InGaAsP flat surface was observed. A transitional state between the dot structure and the flat surface was metastabilized by tuning the AsH3/PH3 beam ratio. The metastabilized surface was observed ex situ using a high-resolution scanning electron microscope.

Gas/liquid sensing via chemotaxis of Euglena cells confined in an isolated micro-aquarium

We demonstrate on-chip gas/liquid sensing by using the chemotaxis of live bacteria (Euglena gracilis) confined in an isolated micro-aquarium, and gas/liquid permeation through porous polydimethylsiloxane (PDMS). The sensing chip consisted of one closed micro-aquarium and two separated bypass microchannels along the perimeter of the micro-aquarium. Test gas/liquid and reference samples were introduced into the two individual microchannels separately, and the gas/liquid permeated through the PDMS walls and dissolved in the micro-aquarium water, resulting in a chemical concentration gradient in the micro-aquarium. By employing the closed micro-aquarium isolated from sample flows, we succeeded in measuring the chemotaxis of Euglena for a gas substance quantitatively, which cannot be achieved with the conventional flow-type or hydro-gel-type microfluidic devices. We found positive (negative) chemotaxis for CO2 concentrations below (above) 15%, with 64 ppm as the minimum concentration affecting the cells. We also observed chemotaxis for ethanol and H2O2. By supplying culture medium via the microchannels, the Euglena culture remained alive for more than 2 months. The sensing chip is thus useful for culturing cells and using them for environmental toxicity/nutrition studies by monitoring their motion.

Two-dimensional optical feedback control of Euglena confined in closed-type microfluidic channels

We examined two-dimensional (2D) optical feedback control of phototaxis flagellate Euglena cells confined in closed-type microfluidic channels (microaquariums), and demonstrated that the 2D optical feedback enables the control of the density and position of Euglena cells in microaquariums externally, flexibly, and dynamically. Using three types of feedback algorithms, the density of Euglena cells in a specified area can be controlled arbitrarily and dynamically, and more than 70% of the cells can be concentrated into a specified area. Separation of photo-sensitive/insensitive Euglena cells was also demonstrated. Moreover, Euglena-based neuro-computing has been achieved, where 16 imaginary neurons were defined as Euglena-activity levels in 16 individual areas in microaquariums. The study proves that 2D optical feedback control of photoreactive flagellate microbes is promising for microbial biology studies as well as applications such as microbe-based particle transportation in microfluidic channels or separation of photo-sensitive/insensitive microbes.

Facets, indium distribution, and lattice distortion of InGaAs/GaAs quantum dots observed by three-dimensional scanning transmission electron microscope

Multiazimuth 360° observation of InGaAs/GaAs quantum dots (QDs) was performed with a 300 kV scanning transmission electron microscope, where both cross-sectional and plan-view images of the same specific QDs can be taken for a single specimen. The facet structure of truncated pyramids was reconstructed from facet-enhanced bright-field images newly observed with the incident axes slightly off from 〈−552〉 or 〈−332〉, resulting in high contrast for the lattice distortion on (110) facets of InGaAs QDs. Dark-field images for a large QD clearly indicate indium distribution inside the large QD, originating from the coalescence of two small QDs during growth. Localized relaxation of the lattices was observed, for the same large QD with indium content fluctuation, as disturbed/disappeared moire-fringes in the images taken with the incidence around 〈−552〉.

Euglena-based neurocomputing with two-dimensional optical feedback on swimming cells in micro-aquariums

We report on neurocomputing performed with real Euglena cells confined in micro-aquariums, on which two-dimensional optical feedback is applied using the Hopfield-Tank algorithm. Trace momentum, an index of swimming activity of Euglena cells, is used as the input/output signal for neurons in the neurocomputation. Feedback as blue-light illumination results in temporal changes in trace momentum according to the photophobic reactions of Euglena. Combinatorial optimization for a four-city traveling salesman problem is achieved with a high occupation ratio of the best solutions. Two characteristics of Euglena-based neurocomputing desirable for combinatorial optimization are elucidated: (1) attaining one of the best solutions to the problem, and (2) searching for a number of solutions via dynamic transition between the best solutions. Mechanisms responsible for the two characteristics are analyzed in terms of network energy, photoreaction ratio, and dynamics/statistics of Euglena movements. The spontaneous fluctuation in input/output signals and reduction in photoreaction ratio were found to be key factors in producing characteristic (1), while the photo-insensitive Euglena cells or the accidental evacuation of cells from non-illuminated areas causes characteristic (2). Furthermore, we show that the photophobic reactions of Euglena involves various survival strategies such as adaptation to blue-light or awakening from dormancy, which can extend the performance of Euglena-based neurocomputing toward deadlock avoidance or program-less adaptation. Finally, two approaches for achieving a high-speed Euglena-inspired Si-based computation are described.

In situ composition control of self-organized InGaAs dots

Abstract In situ composition change of self-organized InGaAs dots has been investigated by means of the control of arsenic/phosphorus supply in chemical beam epitaxy. Due to the replacement of arsenic/phosphorus atoms in the dots by ambient phosphorus/arsenic, surface structure changes from the InGaAs dots to an InGaAsP flat surface and vice versa according to the arsenic/phosphorus exposure. Transitional surfaces between the dot structure and a flat surface were metastabilized and observed ex situ to figure out the dot formation process. It was found that the density of well-developed dots (not the size of dense dots) increases during the dot formation. Two new approaches for modifying the dot structure in situ by using the arsenic/phosphorus replacement technique were demonstrated. For InAs dot reformation, smaller InAs dots disappeared and larger InAs dots were formed instead. In the examination of additional supply of indium or gallium during step-by-step dot reformation with pulse AsH 3 exposure, the coalescence of dots occurred due to excess strain only in the case of indium addition.

Deposition of gallium oxide and indium oxide on GaAs for in situ process use by alternating supply of TEGa, TMIn, and H2O2 as surge pulses

Deposition of gallium oxide and indium oxide on GaAs (001) was investigated by the alternating supply of triethylgallium (TEGa) or trimethylindium (TMIn), and hydrogen peroxide (H2O2). A pressure‐control method was newly developed to produce precisely controlled surge pulses of source gases. Strong temperature dependence of the growth rate per source cycle obtained for the oxide deposition was caused by the decomposition of metalorganics, and by the thermal desorption of the oxide during the deposition. A critical thickness of 20 nm for the thermal desorption of gallium oxide was observed, which gives the upper limit of the promotive effect of underlying GaAs on thermal desorption of gallium oxide. With implications for in situ process use, deposition of GaAs on gallium oxides and on indium oxides was performed by chemical beam epitaxy, and the differences between the two oxides were discussed from the viewpoint of gallium‐atom‐induced desorption and selectivity for GaAs deposition.

Excitation-wavelength-dependent photoluminescence evolution of CdSe∕ZnS nanoparticles

The wavelength-dependent photoluminescence (PL) evolution of CdSe∕ZnS (core/shell) nanoparticles (NPs) was investigated from the viewpoint of excitation wavelength dependence. In addition, the mechanisms involved in the photoinduced changes of the NP PL spectrum are discussed. The NPs of CdSe∕ZnS placed on a thin film of an insulator (GaAsOx∕GaAs or SiO2∕Si) exhibited PL decay and blueshifts on a time scale of minutes depending on the excitation wavelength, i.e., higher photon energy induces a faster PL decay with a larger and faster blueshift. The PL decay with 435nm excitation was a single exponential in temporal evolution, while the delayed acceleration of decay was observed with 365nm excitation. The excitation-wavelength dependence of the blueshift was much larger than that of the PL decay, indicating that they originate from independent photoinduced reactions. This was confirmed by an ultrahigh vacuum measurement where only PL decay was observed without any blueshift. This result clearly reveals tha...

Simulation of neurocomputing based on the photophobic reactions of Euglena with optical feedback stimulation

To explore possible forms of unconventional computers that have high capacities for adaptation and exploration, we propose a new approach to developing a biocomputer based on the photophobic reactions of microbes (Euglena gracilis), and perform the Monte-Carlo simulation of Euglena-based neural network computing, involving virtual optical feedback to the Euglena cells. The photophobic reactions of Euglena are obtained experimentally, and incorporated in the simulation, together with a feedback algorithm with a modified Hopfield-Tank model for solving a 4-city traveling salesman problem. The simulation shows high performances in terms of (1) reaching one of the best solutions of the problem, and (2) searching for a number of solutions via dynamic transition among the solutions. This dynamic transition is attributed to the fluctuation of state variables, global oscillation through feedback instability, and the one-by-one change of state variables.