Kazutaka Nishikawa

Experimental investigation of radiative thermal rectifier using vanadium dioxide

Vanadium dioxide (VO2) exhibits a phase-change behavior from the insulating state to the metallic state around 340 K. By using this effect, we experimentally demonstrate a radiative thermal rectifier in the far-field regime with a thin film VO2 deposited on the silicon wafer. A rectification contrast ratio as large as two is accurately obtained by utilizing a one-dimensional steady-state heat flux measurement system. We develop a theoretical model of the thermal rectifier with optical responses of the materials retrieved from the measured mid-infrared reflection spectra, which is cross-checked with experimentally measured heat flux. Furthermore, we tune the operating temperatures by doping the VO2 film with tungsten (W). These results open up prospects in the fields of thermal management and thermal information processing.

A hot-carrier solar cell with optical energy selective contacts

The hot-carrier solar cell (HC-SC) is an ambitious approach to solar energy conversion which in principle can achieve high efficiency (84%) from a single bandgap semiconductor. Here we propose a method of utilising hot-carriers within a photovoltaic device in which energy is extracted optically from a hot-carrier distribution rather than through the usual approach of electrical conduction. Depending on the optical extraction rate, the concept proposed here may attain an upper efficiency approaching that of the conventional HC-SC.

Extremely long carrier lifetime over 200 ns in GaAs wall-inserted type II InAs quantum dots

To realize highly efficient intermediate-band (IB) solar cells, long lifetime of photo-generated carriers in the IB is essential. For this purpose, we propose a concept for IB absorbers using GaAs wall-inserted type II InAs quantum-dots (QDs), in which electrons at the IB of the InAs QDs and holes in the valence band of the GaAsSb layers are farther separated compared to those in conventional type II QDs. We fabricated InAs/GaAs/GaAs0.82Sb0.18 type II QDs and performed time-resolved photoluminescence spectroscopy. The obtained lifetime was as long as 220 ns for electrons at the IB.

Multilevel radiative thermal memory realized by the hysteretic metal-insulator transition of vanadium dioxide

Thermal information processing is attracting much interest as an analog of electronic computing. We experimentally demonstrated a radiative thermal memory utilizing a phase change material. The hysteretic metal-insulator transition of vanadium dioxide (VO2) allows us to obtain a multilevel memory. We developed a Preisach model to explain the hysteretic radiative heat transfer between a VO2 film and a fused quartz substrate. The transient response of our memory predicted by the Preisach model agrees well with the measured response. Our multilevel thermal memory paves the way for thermal information processing as well as contactless thermal management.

Over 100 ns intrinsic radiative recombination lifetime in type II InAs/GaAs1−xSbx quantum dots

We report very long intrinsic radiative recombination lifetime τrad in type II InAs quantum dots embedded in GaAs1−xSbx. The dependence of photoluminescence (PL) decay time τPL on both the Sb composition (x = 0–0.18) and excitation intensity (38–460 mW/cm2) was systematically investigated by time-resolved PL measurements with a time-correlated single-photon counting method. All PL decay curves exhibited non-exponential profiles, and τPL was strongly dependent on the excitation intensity. These properties were well explained by solving rate equations of carrier density with neglecting nonradiative process, in which τrad is inversely proportional to carrier density. The 18% Sb sample exhibited a τPL of over 100 ns under weak excitation, which is longer than twice the previously reported values. We evaluated the value of τrad in InAs/GaAs1−xSbx QDs relative to that in type I InAs/GaAs QDs based on an effective mass approximation and found that the observed extremely long τPL corresponds to τrad.

Dynamic Modulation of Radiative Heat Transfer beyond the Blackbody Limit

Dynamic control of electromagnetic heat transfer without changing mechanical configuration opens possibilities in intelligent thermal management in nanoscale systems. We confirmed by experiment that the radiative heat transfer is dynamically modulated beyond the blackbody limit. The near-field electromagnetic heat exchange mediated by phonon-polariton is controlled by the metal-insulator transition of tungsten-doped vanadium dioxide. The functionalized heat flux is transferred over an area of 1.6 cm2 across a 370 nm gap, which is maintained by the microfabricated spacers and applied pressure. The uniformity of the gap is validated by optical interferometry, and the measured heat transfer is well modeled as the sum of the radiative and the parasitic conductive components. The presented methodology to form a nanometric gap with functional heat flux paves the way to the smart thermal management in various scenes ranging from highly integrated systems to macroscopic apparatus.

Synthesis of a calcium-bridged siloxene by a solid state reaction for optical and electrochemical properties

A Ca-bridged siloxene (Ca-siloxene) resulting from the deintercalation of Ca from CaSi2 was synthesized via a solid state reaction with ethanol wash using TaCl5. The presence of fragmented, two-dimensional siloxene planes with Ca bridging was confirmed. The Ca-siloxene exhibited tunable optical properties, as well as stable Li storage performance by Ca bridging.

Thermoelectric properties of molten Bi2Te3, CuI, and AgI

To achieve large figure of merit (ZT) and superior thermal durability at high temperatures, we have investigated thermoelectric properties of molten Bi2Te3, CuI, and AgI up to 1173 K. Molten Bi2Te3 was found to have large electrical conductivity between 1800 and 2000 (Ω ⋅ cm)−1. Molten CuI and AgI, however, exhibited small electrical conductivity less than 1 (Ω ⋅ cm)−1, nevertheless they show very large Seebeck coefficients over 800 μV/K. We estimated thermal conductivity using Wiedemann Franz law and the model established by Regel et al. [Phys. Status Solidi A 5, 13 (1971)]. The evaluated ZT for CuI is over 0.1.

Extremely long carrier lifetime at intermediate states in wall-inserted type II quantum dot absorbers

To realize highly efficient intermediate-band solar cells (IB-SCs), a long lifetime of photo-generated carriers in the IB is essential. We propose a new concept for this purpose based on IB absorbers using quantum-dots (QDs). By inserting potential walls between QDs and barriers that form a type II band alignment, electrons in the IB and holes in the valence band are farther separated compared to those in a conventional type II QD material, leading to significant reduction of radiative recombination. We designed a concrete structure using InAs QDs, GaAs1−xSbx barriers, and GaAs walls to find the suitable GaAs wall thickness and Sb content being 2 nm and x = 0.18, respectively, and demonstrated a lifetime of electrons excited to the IB as long as 220 ns.

Near-infrared localized surface plasmon resonance of self-growing W-doped VO2 nanoparticles at room temperature

Nanoparticles (NPs) of vanadium dioxide (VO2) in the metal state exhibit localized surface plasmon resonance (LSPR) at 1200–1600 nm, which fills the gap between the absorption ranges of silicon and the LSPR of conventional transparent conductor NPs (ZnO:Al, In2O3:Sn, etc.). However, two issues of the lithographic process for NP formation and the metal-insulator transition temperature (69 °C) higher than room temperature have made it difficult to use VO2 NPs for applications such as energy conversion devices, near infrared (NIR) light detectors, and bio-therapy. In this study, we developed a self-growing process for tungsten (W)-doped VO2 NPs that are in the metal state at room temperature, using sputter deposition and post-lamp annealing. The changes in the LSPR peak wavelengths with the NP size were well controlled by changing the deposited film thickness and oxygen pressure during the post-annealing treatment. The presented results resolve the difficulties of using the metal-insulator transition materia...