Hai Jun Cho

Thermal transistors have potential as thermal management devices because they can electrically control the thermal conductivity (κ) of the active layer. Recently, we realized solid-state electrochemical thermal transistors by utilizing the electrochemical redox reaction of SrCoO_y (2 ≤ y ≤ 3). However, the guiding principle to improve the on/off κ ratio has yet to be clarified because the κ modulation mechanism is unclear. This study systematically modulates κ of SrCo_1-xFe_xO_y (0 ≤ x ≤ 1, 2 ≤ y ≤ 3) solid solutions used as the active layers in solid-state electrochemical thermal transistors. When y = 3, the lattice κ of SrCo_1-xFe_xO_y is ∼2.8 W m^-1 K^-1 and insensitive to x. When x = 0 and y = 3, κ increases to ∼3.8 W m^-1 K^-1 due to the contribution of the electron κ. When y = 2, κ slightly depends on the ordered atomic arrangement. Materials that are high electrical conductors with highly ordered lattices when the transistor is on but are electrical insulators with disordered lattices when the transistor is off should be well-suited for the active layers of solid-state electrochemical thermal transistors.

Thermal transistors that electrically control heat flow have attracted growing attention as thermal management devices and phonon logic circuits. Although several thermal transistors are demonstrated, the use of liquid electrolytes may limit the application from the viewpoint of reliability or liquid leakage. Herein, a solid‐state thermal transistor that can electrochemically control the heat flow with an on‐to‐off ratio of the thermal conductivity (κ) of ≈4 without using any liquid is demonstrated. The thermal transistor is a multilayer film composed of an upper electrode, strontium cobaltite (SrCoO_x), solid electrolyte, and bottom electrode. An electrochemical redox treatment at 280 °C in air repeatedly modulates the crystal structure and κ of the SrCoO_x layer. The fully oxidized perovskite‐structured SrCoO₃ layer shows a high κ ≈3 .8 W m⁻¹ K⁻¹, whereas the fully reduced defect perovskite‐structured SrCoO₂ layer shows a low κ ≈ 0.95 W m⁻¹ K⁻¹. The present solid‐state electrochemical thermal transistor may become next‐generation devices toward future thermal management technology.

Thermoelectric energy conversion technology has attracted attention as an energy harvesting technology that converts waste heat into electricity by means of the Seebeck effect. Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Here, we show a reliable high-ZT thermoelectric oxide, Ba_1/3CoO₂. We fabricated Ba_1/3CoO₂ epitaxial films by the reactive solid-phase epitaxy method (Na_3/4CoO₂) followed by ion exchange (Na⁺ → Ba²⁺) treatment and performed thermal annealing of the film at high temperatures and structural and electrical measurements. The crystal structure and electrical resistivity of the Ba_1/3CoO₂ epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ∼1.2 mW m^-1 K^-2 and the thermal conductivity gradually decreased to ∼1.9 W m^-1 K^-1 with increasing temperature up to 600 °C. Consequently, the ZT reached ∼0.55 at 600 °C in air.

The importance of transparent oxide semiconductors is growing immensely due to their unprecedented dual properties: high electrical conductivity and optical transparency. They have been widely used in many transparent electronics devices due to their excellent electronic properties. In this Review, we discuss our recent research progresses on transparent ASnO3 (A = Ba, Sr, and Ca)-based thin films and thin film transistors (TFT). Here, we have explored the underlying materials physics through the investigation of fundamental properties such as effective mass, effective channel thickness, carrier mobility, electrical characteristics, and optical properties. High Hall mobility and wide bandgap are the key deciding parameters to consider when choosing ASnO3 for transparent electronic applications. Thus, carrier mobility improvisation was also carried out via modifying thin film preparation conditions such as using the highly oxidative atmosphere, vacuum annealing, and increasing the film thickness. Furthermore, we clarified the operating mechanism of BaSnO3-SrSnO3 solid solution-based TFTs and succeeded in fabricating the deep-UV La-doped SrSnO3 TFTs, which has great potential in biological applications. We have also demonstrated that the optoelectronic properties ASnO3 may be controlled by changing the A-site, which is consistent with expectations from the lattice parameter changes. This Review provides better options for designing ASnO3-based transparent devices in future.

Since nanomaterials have high surface/volume ratio, vibrational wave transmission at solid interfaces is of great importance in nanotechnologies and have been extensively examined for the past 30 y...

Since the optoelectronic properties of VO₂ are modulated around the critical temperature (T_c) of insulator‐to‐metal transition, VO₂ is a promising candidate material for smart window. However, the T_c of bulk VO₂ is rather high 341 K (68 °C) and therefore, needs to be decreased down to near temperature for practical applications. Although T_c can be reduced to room temperature if rutile TiO₂ crystal is used as the substrate, it is not technologically viable for large‐scale applications because of the size limitation of available TiO₂ crystal. Here, this work shows that the T_c can be reduced to near room temperature using M‐plane sapphire as the substrate. VO₂/TiO₂ bilayer films are fabricated with varied thicknesses on M‐plane sapphire [(100) α‐Al₂O₃] substrates. The 5.5‐nm‐thick VO₂ film on 200‐nm‐thick TiO₂ buffered sapphire substrate shows clear insulator‐to‐metal transition at ≈305 K (32 °C). It is also found that the insulator‐to‐metal transition is sensitive to the in‐plane lattice distortion, which induces carrier generation. The systematic study on the effect of in‐plane lattice distortion on the insulator‐to‐metal transition of VO₂ will be useful for the practical application of VO₂ as an active material of smart window.

We have measured and analyzed the in-plane thermal diffusivity of the Mott insulator Ca2RuO4 around room temperature under external currents with an infrared camera as a contactless thermometer. We...