In mesoscopic materials fields, mesostructure designing has been performed taking advantage of chemical-physical interactions such as "Self-Assembly" and "Oriented Attachment". The structure has been controlled in solution process, such as "sol-gel method" and "hydrothermal synthesis". The unique properties of meso/macroscopic objects have been developed by arranging the shape and the orientation of nanoparticles and nanostructures.
As an example, fluorescence organic-inorganic nanocomposite materials have been developed using the specific interaction at the interface between luminescenct nanocarbon(C-dots) and inorganic matrix, for the solid-state lasing applications.
Carbon dots (C-dots) are luminescent nanomaterials composed of carbon-based materials with below 10 nm in size (Fig.(a)). C-dots have several advantages, such as abundant resources, simple synthesis, low cost and excellent dispersibility in water. Therefore it is expected to be used as a novel luminescent material in many fields, such as biomedical, electronic, optical applications (Fig.(b)).
Now we work on nanocomposites of C-dots with metal oxides matrices (ZnO, TiO2, SiO2) to induce some interactions at the interface of the materials. The targets of the research are the improvement of luminescent efficiency and the tuning of the emission range as a solid-state emitting device. As an example, when C-dots are dispersed in the semiconductive matrix of zinc oxide (ZnO), energy transfer is induced from ZnO to C-dots and the luminescent of C-dots is sensitized. The ZnO－C-dots nanocomposite films are prepared by sol-gel process and we achieved to obtain the various luminescent color depending on the energy transfer frequency (Fig.(c)).
Nanocrystals of metal and metal oxide, show unique properties depending on the shape, such as cube, octahedron, sheet, sphere, wire, and so on. In order to apply the unique properties in practical devices, we work on the development of scale-up technology with keeping the anisotropy of nanocrystals.
As an example, bismuth ferrite (BiFeO3) nanocrystals which are the perovskite multiferroic materials, are prepared by hydrothermal synthesis. By changing the stability in solution (supersaturation) by KOH concentration, aggregation and crystal growth processes are controlled, and as a result, shape-controlled BiFeO3 aggregates of tens micron in size, which reflects the shape of primary nanocrystals, are obtained (Figure).
Multi-layered films, which have different mechanical property in each layer, form periodic surface wrinkle microstructures by stress mismatching at the interlayer. Organic-inorganic hybrid films with wrinkle microstructures are expected to be applied in flexible electronic device, microlens, microflow channel, and so on.
Organic-inorganic hybrid films composed of photo-monomer and silicon alkoxide are prepared by solution process. The films became three-layered structures, which are polymer surface layer, intermediate layer and silica bottom layer, by UV irradiation, resulting in nested periodic wrinkle structures (Fig. upper-right). The nested wrinkle structures show a stimuli-response against humidity. Using the unique response, micro-particles are caught and released size-selectively depending on the period of wrinkle structure in polydispersed particles' solution (Fig. lower-right).