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Selective graphene growth on copper twin crystals by chemical vapor deposition has been achieved. Graphene ribbons can be formed only on narrow twin crystal regions with a (001) or high-index surface sandwiched between Cu crystals having (111) surfaces by tuning the growth conditions, especially by controlling the partial pressure of CH(4) in Ar/H(2) carrier gas. At a relatively low CH(4) pressure, graphene nucleation at steps on Cu (111) surfaces is suppressed, and graphene is preferentially nucleated and formed on twin crystal regions. Graphene ribbons as narrow as ~100 nm have been obtained in experiments. The preferential graphene nucleation and formation seem to be caused primarily by a difference in surface-dependent adsorption energies of reactants, which has been estimated by first principles calculations. Concentrations of reactants on a Cu surface have also been analyzed by solving a diffusion equation that qualitatively explains our experimental observations of the preferential graphene nucleation. Our findings may lead to self-organizing formation of graphene nanoribbons without reliance on top-down approaches in the future.

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Mixing heterogeneous Li-ion conductive materials is one potential way to enhance Li-ion conductivity more than that of the parent materials. However, the huge number of possible compositions of parent materials impedes the development of an optimal mixture by using conventional methods. In this study, we employed machine learning to optimize the composition ratio of ternary Li3PO4–Li3BO3–Li2SO4 for Li-ion conductivity. We found the optimum composition of the ternary mixture system to be 25:14:61 (Li3PO4:Li3BO3:Li2SO4 in mol %), whose Li-ion conductivity is measured as 4.9 × 10–4 S/cm at 300 °C. Our X-ray structure analysis suggested that Li-ion conductivity of the mixed systems tends to be enhanced by the coexistence of two or more phases. Although the mechanism enhancing Li-ion conductivity is not simple, our results demonstrate the effectiveness of machine learning for the development of materials.

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