Kyu Hyoung Lee

Bio: Kyu Hyoung Lee is an academic researcher from Yonsei University. The author has contributed to research in topics: Thermoelectric effect & Thermoelectric materials. The author has an hindex of 32, co-authored 205 publications receiving 5093 citations. Previous affiliations of Kyu Hyoung Lee include Samsung & Kyung Hee University.

Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

Abstract: The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit ( zT ). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi 0.5 Sb 1.5 Te 3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

Peierls distortion as a route to high thermoelectric performance in In4Se3―δ crystals

Abstract: Thermoelectric materials, which convert heat into electricity, are much studied for their potential in energy-saving applications — for example as a way of recovering waste heat in cars. At present, though, these materials are inefficient, with very few of them achieving a thermoelectric figure of merit (ZT) above one in the mid-temperature range (500–900 K). Now a figure of merit of 1.48, notably high for a bulk material, is reported for indium selenide crystals (In4Se3–δ) at 705 K. The high thermoelectric performance of this material is related to a Peierls distortion of the crystal lattice at 710 K. This work suggests a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves. Thermoelectric materials, which can convert heat into electricity, are of great interest for energy sustainability. The problem is the low efficiency of these materials, quantified by a coefficient, ZT, which for mid-temperature materials is usually around 1. The realization of a material, In4Se3–δ, which achieves the ZT value of 1.48 at 705 K, could open up a new avenue in the research to generate high ZT materials. Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit1. Here we report a binary crystalline n-type material, In4Se3-δ, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

Thermoelectrical properties of A-site substituted Ca1- xRexMnO3 system

Abstract: CaMnO3 is an electron-doped compound which belongs to the perovskite family. Despite its high Seebeck coefficient S value, the figure of merit at high temperature remains low due to its large resistivity ρ(ρ300K=2Ωcm). To optimize the performance of this material in terms of thermoelectric properties, several substitutions have been attempted on the Ca site to decrease the ρ. Structure and thermoelectric properties of polycrystalline samples Ca1−xAxMnO3 (A=Yb, Tb, Nd, and Ho) have been investigated. Although ρ strongly depends on the ionic radius ⟨rA⟩ and carrier concentration, we have shown that the thermal conductivity κ is mainly driven by the atomic weight of the A site and decreases with it. Therefore, it seems that the S, ρ, and κ could be controlled separately. For instance, the highest dimensionless ZT (=0.16) has been obtained at 1000K in the air for Ca0.9Yb0.1MnO3.

Surfactant‐Free Scalable Synthesis of Bi2Te3 and Bi2Se3 Nanoflakes and Enhanced Thermoelectric Properties of Their Nanocomposites

Abstract: Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

Abstract: 2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post-silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high-performance devices while adapting for large-area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS2 devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD-grown MoS2 film and a Ag electrode as an interfacial layer. The MoS2 field-effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field-effect mobility of 35 cm2 V-1 s-1 , an on/off current ratio of 4 × 108 , and a photoresponsivity of 2160 A W-1 , compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n-doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes. This demonstration of contact interface engineering with CVD-grown MoS2 and graphene is a key step toward the practical application of atomically thin TMDC-based devices with low-resistance contacts for high-performance large-area electronics and optoelectronics.

Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals

Abstract: The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion Enhancements above the generally high threshold value of 25 have important implications for commercial deployment, especially for compounds free of Pb and Te Here we report an unprecedented ZT of 26 ± 03 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell This material also shows a high ZT of 23 ± 03 along the c axis but a significantly reduced ZT of 08 ± 02 along the a axis We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Gruneisen parameters, which reflect the anharmonic and anisotropic bonding We attribute the exceptionally low lattice thermal conductivity (023 ± 003 W m(-1) K(-1) at 973 K) in SnSe to the anharmonicity These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance

Van der Waals heterostructures and devices

Abstract: Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics. With a dangling-bond-free surface, two dimensional layered materials (2DLMs) can enable the creation of diverse van der Waals heterostructures (vdWHs) without the conventional constraint of lattice matching or process compatibility. This Review discusses the recent advances in exploring 2DLM vdWHs for future electronics and optoelectronics.

Copper ion liquid-like thermoelectrics

Abstract: Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling. Worldwide efforts to find materials with thermoelectric figure of merit, zT values significantly above unity, are frequently focused on crystalline semiconductors with low thermal conductivity. Here we report on Cu_(2−x)Se, which reaches a zT of 1.5 at 1,000 K, among the highest values for any bulk materials. Whereas the Se atoms in Cu_(2−x)Se form a rigid face-centred cubic lattice, providing a crystalline pathway for semiconducting electrons (or more precisely holes), the copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary ‘liquid-like’ behaviour of copper ions around a crystalline sublattice of Se in Cu_(2−x)Se results in an intrinsically very low lattice thermal conductivity which enables high zT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.