No abstract

The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our "embedded-ZnO nanowire structure" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

Layered silicene with deformed buckled structure attracts great interest as a next generation 2D Dirac thermoelectric material beyond conventional layered materials. However, the difficulty of modulating atomic positions in silicene prevents its realization. This study proposes a method to deform buckled structure in layered silicene by controlling the intercalated atoms, which can dramatically enhance its thermoelectric properties. Silicene buckled structure is deformed in epitaxial CaSi2 thin films, Ca‐intercalated layered silicenes, on Si(111) substrates, which is related to the composition of intercalated Ca. Therein, buckling height of silicene is changed. This CaSi2 film with deformed silicene exhibits not only metal‐like electrical conductivity but also three times larger Seebeck coefficient than the theoretically predicted value, resulting in ≈3000 times larger power factor (≈40 μW cm−1 K−2) than that of the reported CaSi2 film at room temperature. This result experimentally demonstrates that power factor can be greatly enhanced by deforming the silicene buckled structure.

We investigated thermal conductivity of epitaxial germanane films: stacked structure of hydrogenated germanenes. It was confirmed that single crystalline germanane films were epitaxially grown on Ge(111). The films exhibited low out-of-plane thermal conductivity of 1.1 ± 0.3 W m−1 K−1 which is lower than other layered materials composed of heavy atoms. This came from weak van der Waals interlayer interaction related to weak polarization in germanane composed of smaller atoms. This demonstrates that choice of small constituent atoms for weakening van der Waals interlayer interaction is a promising thermal conductivity reduction outline for developing ecofriendly high performance thermoelectric layered materials.

For realization of new informative systems, the memristor working like synapse has drawn much attention. We developed isolated high-density Fe 3 O 4 nanocrystals on Ge nuclei/Si with uniform and high resistive switching performance using low-temperature growth. The Fe 3 O 4 nanocrystals on Ge nuclei had a well-controlled interface (Fe 3 O 4 /GeO x /Ge) composed of highcrystallinity Fe 3 O 4 and high-quality GeO x layers. The nanocrystals showed uniform resistive switching characteristics (high switching probability of~90%) and relatively high Off/On resistance ratio (~58). The high-quality interface enables electric field application to Fe 3 O 4 and GeO x near the interface, which leads to effective positively charged oxygen vacancy movement, resulting in high-performance resistive switching. Furthermore, we successfully observed memory effect in nanocrystals with well-controlled interface. The experimental confirmation of the memory effect existence even in ultrasmall nanocrystals is significant for realizing non-volatile nanocrystal memory leading to neuromorphic devices.