Michiko Sasaki

Phonons with long mean free paths in a-Si and a-Ge

We investigated phonons with long mean free paths (MFPs) in amorphous Si (a-Si) and amorphous Ge (a-Ge). The thermal conductivity of a-Si and a-Ge thin films prepared by magnetron sputtering was found to depend on film thickness and deposition temperature. From the film thickness dependence, we conclude that phonons with MFPs longer than 100 nm contribute to heat transport in a-Si and a-Ge. Also, as deposition temperature was increased, phonons with MFPs ranging from 100 to 250 nm in a-Si and from 15 to 250 nm in a-Ge increased.

Thermal conductivity of sputtered amorphous Ge films

We measured the thermal conductivity of amorphous Ge films prepared by magnetron sputtering. The thermal conductivity was significantly higher than the value predicted by the minimum thermal conductivity model and increased with deposition temperature. We found that variations in sound velocity and Ge film density were not the main factors in the high thermal conductivity. Fast Fourier transform patterns of transmission electron micrographs revealed that short-range order in the Ge films was responsible for their high thermal conductivity. The results provide experimental evidences to understand the underlying nature of the variation of phonon mean free path in amorphous solids.

Thermal boundary resistance at Au/Ge/Ge and Au/Si/Ge interfaces

Amorphous Ge (a-Ge), crystalline Ge (c-Ge), and amorphous Si (a-Si) thin films were deposited on a Ge substrate at different temperatures by magnetron sputtering. We measured thermal boundary resistance (TBR) in Au/Ge/Ge and Au/Si/Ge three-layer samples. The measured TBR in Au/a-Ge/Ge and Au/a-Si/Ge decreased slightly with increasing deposition temperature. The measured TBR values were larger than the values predicted by the diffuse mismatch model. Furthermore, it is interesting to note that the measured TBR in Au/c-Ge/Ge was twofold larger than that in Au/a-Ge/Ge. Cross-sectional transmission electron microscopy was conducted to investigate interfacial morphology of the samples. The results indicate that the crystalline state of the deposited thin films play an important role in TBR by modifying phonon density of states and interfacial properties. Our findings are of great importance for applications involving thermal management of micro- and optoelectronic devices, and for the development of thermal barrier coatings and thermoelectric materials with high figures-of-merit.

Reduction of Friction Force by Light

The reduction of the friction force between a silicon nitride cantilever coated with pyrene molecules and a sapphire substrate in water was observed upon light irradiation. The friction force was analyzed by lateral force microscopy in water. The wavelength of the irradiated light was changed using an optical monochromator combined with a Xe lamp. The pyrene molecules on the surface of the cantilever absorbed the light and were photoexcited. As a result of the photoexcitation, the interaction between the pyrene molecules and sapphire in water was changed, thereby changing the friction force. This observation may open a new area of research in the field of tribology.

Combinatorial investigation of spin-orbit materials using spin Peltier effect

Conversion between spin and charge currents is essential in spintronics, since it enables spin-orbit-torque magnetization switching, spin-current-driven thermoelectric generation, and nano-scale thermal energy control. To realize efficient spin-charge conversion, a variety of mechanisms, including spin Hall effects, Rashba-Edelstein effects, and spin-momentum locking in topological insulators, have been investigated and more comprehensive material exploration is necessary. Here we demonstrate high-throughput screening of spin-charge conversion materials by means of the spin Peltier effect (SPE). This is enabled by combining recently-developed SPE-imaging techniques with combinatorial materials science; using a composition-spread alloy film formed on a magnetic insulator, we observe the SPE-induced temperature change due to the spin Hall effect and obtain a continuous mapping of its composition dependence from the single sample. The distribution of the SPE signals reflects local spin-charge conversion capability in the alloy owing to unique heat-generation nature of the SPE. This combinatorial approach will accelerate materials research towards high-performance spintronic devices.

Ultra-low thermal conductivity of high-interface density Si/Ge amorphous multilayers

Owing to their phonon scattering and interfacial thermal resistance (ITR) characteristics, inorganic multilayers (MLs) have attracted considerable attention for thermal barrier applications. In this study, a-Si/a-Ge MLs with layer thicknesses ranging from 0.3 to 5 nm and different interfacial elemental mixture states were fabricated using a combinatorial sputter-coating system, and their thermal conductivities were measured via a frequency-domain thermo-reflectance method. An ultra-low thermal conductivity of κ = 0.29 ± 0.01 W K−1 m−1 was achieved for a layer thickness of 0.8 nm. The ITR was found to decrease from 8.5 × 10−9 to 3.6 × 10−9 m2 K W−1 when the interfacial density increases from 0.15 to 0.77 nm−1.

Control of friction force by light observed by friction force microscopy in a vacuum

The friction force (FF) between Coumarin 6 molecules on a silicon cantilever and a sapphire single-crystal substrate was controlled by laser irradiation in a vacuum. The molecules on the tip surface of the cantilever absorbed the laser light and were photoexcited. It was found that the interaction between the Coumarin 6 molecules and the sapphire in the vacuum was changed rapidly by the irradiation, thus increasing the FF. After turning off the laser, the FF returned to its original value. This phenomenon is expected to be applied to control the performance of micro-electro-mechanical systems by light.

Frictional Property Depended on Crystal Preferred Orientation Analyzed by a Combinatorial Technique

An advanced technique for investigating the frictional property of materials depended on crystal preferred orientation is newly proposed. The crystal preferred orientation of ZnO coating films on stainless-steel substrates was markedly changed with the magnitude of an applied load, which was observed by X-ray diffraction analysis with micropoint focus X-ray optics. Also the dependence of the friction coefficient on the crystal preferred orientation was measured. Using the new method, it is possible to find the appropriate crystal preferred orientation for coatings with the desired value of the friction coefficient. Therefore, the technique is expected to be an effective method for tribology research.

Properties of Molecular Nanojets in Different Solutions

Molecular nanojets generated by sub-nanosecond pulsed laser irradiation have been observed under different conditions such as in glucose–water solutions with different viscosities and in salt-water solutions with different concentrations. Time-resolved shadowgraphy imaging of the molecular nanojets under the different conditions was performed. The molecular nanojets were ejected through the center of a cavitation bubble, which was generated by pulsed laser irradiation of the liquid. The speed and shape of the molecular nanojets and the size of the cavitation bubble changed markedly with the conditions. The speed of a molecular nanojet can be controlled by changing of the viscosity of the glucose–water solution and the concentration of the salt-water solution.

Synthesis of Polystyrene Nanowires Doped with Iron Oxide Nanoparticles Using a Pulsed Laser

We have developed a technique to synthesize polystyrene (PS) nanowires doped with iron oxide nanoparticles (nanoFeO) by sub-nanosecond pulsed laser irradiation. Scanning electron microscopy (SEM) images of the polymer nanowires with uniform diameters were obtained. The structure of the nanoFeO-PS nanowires was analyzed by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray analysis (EDX). The polymer nanowires grew perpendicular to the surface of the source film, which contained iron oxide nanoparticles and dye molecules as a photoabsorber.