Jin-Sang Kim

High-performance shape-engineerable thermoelectric painting.

Output power of thermoelectric generators depends on device engineering minimizing heat loss as well as inherent material properties. However, the device engineering has been largely neglected due to the limited flat or angular shape of devices. Considering that the surface of most heat sources where these planar devices are attached is curved, a considerable amount of heat loss is inevitable. To address this issue, here, we present the shape-engineerable thermoelectric painting, geometrically compatible to surfaces of any shape. We prepared Bi2Te3-based inorganic paints using the molecular Sb2Te3 chalcogenidometalate as a sintering aid for thermoelectric particles, with ZT values of 0.67 for n-type and 1.21 for p-type painted materials that compete the bulk values. Devices directly brush-painted onto curved surfaces produced the high output power of 4.0u2009mWu2009cm−2. This approach paves the way to designing materials and devices that can be easily transferred to other applications.

Vertically ordered SnO2 nanobamboos for substantially improved detection of volatile reducing gases

Vertically ordered SnO2 nanorods with Au nanoparticles deposited in multiple steps, namely, SnO2 nanobamboos are synthesized by a glancing-angle deposition technique. The highly ordered porous structures enable us to detect sub-ppb levels of volatile reducing gases with a fast response speed by maximizing the sensitization effects of the Au nanoparticles.

Tunable conductivity at LaAlO3/SrxCa1−xTiO3 (0 ≤ x ≤ 1) heterointerfaces

The two-dimensional electron gas formed at LaAlO3/SrTiO3 heterointerfaces exhibits a variety of interesting physical properties. Herein, we report on tunable conductivity at LaAlO3/SrxCa1−xTiO3 heterostructures. By changing the Sr content in the SrxCa1−xTiO3 (0u2009≤u2009xu2009≤u20091) layers, the orthorhombicity of the films, which inevitably accompanies TiO6 octahedral distortions in the unit cells, could be varied. As a result, the interfacial conductivity can be tuned over 6 orders of magnitude. We suggest that the use of pseudosubstrates with chemical substitution or alloying is a promising route to finely tune conductivity at oxide heterointerfaces.

Impact of parasitic thermal effects on thermoelectric property measurements by Harman method

Harman method is a rapid and simple technique to measure thermoelectric properties. However, its validity has been often questioned due to the over-simplified assumptions that this method relies on. Here, we quantitatively investigate the influence of the previously ignored parasitic thermal effects on the Harman method and develop a method to determine an intrinsic ZT. We expand the original Harman relation with three extra terms: heat losses via both the lead wires and radiation, and Joule heating within the sample. Based on the expanded Harman relation, we use differential measurement of the sample geometry to measure the intrinsic ZT. To separately evaluate the parasitic terms, the measured ZTs with systematically varied sample geometries and the lead wire types are fitted to the expanded relation. A huge discrepancy (∼28%) of the measured ZTs depending on the measurement configuration is observed. We are able to separately evaluate those parasitic terms. This work will help to evaluate the intrinsic thermoelectric property with Harman method by eliminating ambiguities coming from extrinsic effects.

Electric-field-induced shift in the threshold voltage in LaAlO3/SrTiO3 heterostructures.

The two-dimensional electron gas (2DEG) at the interface between insulating LaAlO3 and SrTiO3 is intriguing both as a fundamental science topic and for possible applications in electronics or sensors. For example, because the electrical conductance of the 2DEG at the LaAlO3/SrTiO3 interface can be tuned by applying an electric field, new electronic devices utilizing the 2DEG at the LaAlO3/SrTiO3 interface could be possible. For the implementation of field-effect devices utilizing the 2DEG, determining the on/off switching voltage for the devices and ensuring their stability are essential. However, the factors influencing the threshold voltage have not been extensively investigated. Here, we report the voltage-induced shift of the threshold voltage of Pt/LaAlO3/SrTiO3 heterostructures. A large negative voltage induces an irreversible positive shift in the threshold voltage. In fact, after the application of such a large negative voltage, the original threshold voltage cannot be recovered even by application of a large positive electric field. This irreversibility is attributed to the generation of deep traps near the LaAlO3/SrTiO3 interface under the negative voltage. This finding could contribute to the implementation of nanoelectronic devices using the 2DEG at the LaAlO3/SrTiO3 interface.

Thermoelectric Properties of n-Type Bi2Te3/PbSe0.5Te0.5 Segmented Thermoelectric Material

To investigate the effects of segmentation of thermoelectric materials on performance levels, n-type segmented Bi2Te3/PbSe0.5Te0.5 thermoelectric material was fabricated, and its output power was measured and compared with those of Bi2Te3 and PbSe0.5Te0.5. The two materials were bonded by diffusion bonding with a diffusion layer that was ∼18xa0μm thick. The electrical conductivity, Seebeck coefficient, and power factor of the segmented Bi2Te3/PbSe0.5Te0.5 sample were close to the average of the values for Bi2Te3 and PbSe0.5Te0.5. The output power of Bi2Te3 was higher than those of PbSe0.5Te0.5 and the segmented sample for small ΔT (300xa0K to 400xa0K and 300xa0K to 500xa0K), but that of the segmented sample was higher than those of Bi2Te3 and PbSe0.5Te0.5 when ΔT exceeded 300xa0K (300xa0K to 600xa0K and 300xa0K to 700xa0K). The output power of the segmented sample was about 15% and 73% higher than those of the Bi2Te3 and PbSe0.5Te0.5 samples, respectively, when ΔT was 400xa0K (300xa0K to 700xa0K). The efficiency of thermoelectric materials for large temperature differences can be enhanced by segmenting materials with high performance in different temperature ranges.

The Effect of Grain Size and Density on the Thermoelectric Properties of Bi2Te3-PbTe Compounds

The effects of microstructure on thermoelectric properties were investigated in Bi2Te3-PbTe compounds of different grain size and density. Powders of two different sizes [0.1xa0μm to 1xa0μm (micropowder) and <50xa0nm (nanopowder)] were prepared from Bi2Te3-PbTe ingots by ball milling and high-energy ball milling. Three different samples were spark plasma sintered from each powder and the mixture of the two powders. The grain size and relative density of the sintered samples varied from 100xa0nm to a few micrometers and 89.7% to 97.3%, respectively. The dimensionless figure of merit zT of the sample sintered from nanopowder was about 0.50 at 500xa0K, being about 3.3 times larger than that of the sample sintered from micropowder (∼0.15 at 500xa0K), when the relative density of the former and the latter were 89.7% and 97.3%, respectively. The improved thermoelectric performance of the samples may originate from the decrease of the thermal conductivity, which was caused by the decrease of the grain size and the increase of the amount of pores.

Structural properties of ZnSe layers grown on (001) GaAs substrates tilted toward [110] and [010]

We have investigated the structural properties of ZnSe epilayers that were molecular beam epitaxially grown on (001) GaAs substrates with different tilt angles and tilt directions. We measured the properties of the epilayers by x-ray diffraction, transmission electron microscopy, and etch pit density analysis. Tilting the (001) GaAs substrate toward [010] was very effective in reducing the surface defect density of the ZnSe layers, while tilting toward the [110] direction was of no use. We could observe the increasingly two-dimensional nature of the initial growth mode in the (001) GaAs substrate tilted toward [010]. Growth of a 1.8-μm-thick ZnSe layer on (001) GaAs tilted 4° toward [010] resulted in a very low surface defect density of 1×104u2009cm−2. Such a low defect density has seldom been obtained in ZnSe, without growing a GaAs buffer layer below the ZnSe layer.

Thermoelectric Properties of Indium-Selenium Nanocomposites Prepared by Mechanical Alloying and Spark Plasma Sintering

Indium-selenium-based compounds have received much attention as thermoelectric materials since a high thermoelectric figure of merit of 1.48 at 705xa0K was observed in In4Se2.35. In this study, four different compositions of indium-selenium compounds, In2Se3, InSe, In4Se3, and In4Se2.35, were prepared by mechanical alloying followed by spark plasma sintering. Their thermoelectric properties such as electrical resistivity, Seebeck coefficient, and thermal conductivity were measured in the temperature range of 300xa0K to 673xa0K. All the In-Se compounds comprised nanoscaled structures and exhibited n-type conductivity with Seebeck coefficients ranging from −159xa0μVxa0K−1 to −568xa0μVxa0K−1 at room temperature.

Fabrication of high-performance p-type thin film transistors using atomic-layer-deposited SnO films

Here, we demonstrate high-performance p-type thin film transistors (TFTs) with a SnO channel layer grown by atomic layer deposition (ALD). The performance of the SnO TFTs relies on hole carriers and defects in SnO and near the back-channel surface of SnO as well as the quality of the gate dielectric/SnO interface. The growth of SnO films at a high temperature of 210 °C effectively suppresses the hole carrier concentration, leading to a high on-current/off-current (Ion/Ioff) ratio. In addition, the SnO films grown at 210 °C achieve high field effect mobility (μFE) compared with the SnO films grown at lower temperatures because of their large grain size and lower impurity contents. However, the SnO films grown at 210 °C still contain defects and hole carriers, especially near the back-channel surface. The post-deposition process – back-channel surface passivation with ALD-grown Al2O3 followed by post-deposition annealing at 250 °C – considerably alleviates the defects and hole carriers, resulting in superior TFT performance (Ion/Ioff: 2 × 106, subthreshold swing: 1.8 V dec−1, μFE: ∼1 cm2 V−1 s−1). We expect that the SnO ALD and subsequent process will provide a new opportunity for producing high-performance p-type oxide TFTs.