Nanocube and Assembly Leading Innovation in Ceramics
Kazumi Kato
National Institute of Advanced Industrial Science and Technology

Nanocube which is single crystal in nanometer-size has attracted much attention as a building block to evolve hierarchical structures where the nanocubes are assembled with the 4-fold symmetry along both vertical and horizontal directions. Single crystalline {100} dominant BaTiO3 nanocubes possesing edge length of almost 15 nm were synthesized by the hydrothermal method. Small capacitors consisting of the BaTiO3 nanocube ordering assemblies after heating at 1123 K exhibited higher dielectric constant and relatively low loss of 4000 and 0.07, respectively. By in-situ TEM analysis of the nanocube at 773 K, the hopping movement of atoms at the surface and the surface reconstruction were confirmed although the inner atomic arrangement has been fixed as a perfect perovskite structure. On the other hand, after heating at a certain temperature range of 1023 K to 1173 K, the adjacent nanocubes with small tilting angles were found to bond each other to generate misfit strain in the interface region. The consequent geometric appearances were consistent with the enhanced dielectric properties. Recently, monolayer assemblies of BaTiO3 nanocubes having relatively larger area without cracks and containing nanocube ordering with the 4-fold symmetry in 2 dimensions were developed. It is not until the monolayer that we open our knowledge of nano-sized ferroelectrics. Tip-enhanced Raman spectroscopy (TERS) of individual nanocubes in the monolayer indicated the sole nanocube without post heat treatment was in tetragonal phase and exhibited phase transition. The construction of boundary between the nanocubes with heat treatment was confirmed. These newly given basic results were identified with our previous results of the 3D assemblies. In addition, electrospray deposition method was conducted using BaTiO3 nanocubes. Elementary insights of the relation of the individual nanocube polar and the externally applied voltage might be available in the construction of future devices.
This work was supported by the Japan Science and Technology Agency (JST) under the Advanced Low Carbon Technology Research and Development Program (ALCA) and the Collaborative Research Based on Adaptable and Seamless Technology Transfer Program (A-Step), and the New Energy and Industrial Technology Development Organization (NEDO) under the Advanced Research Program for Energy and Environmental Technologies.
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