Thermoelectric materials and devices are energy-harvesting devices that can effectively recycle waste heat into electricity. Thermoelectric power generation is widely used in factories, engines, and even in human bodies as they continuously generate heat. However, thermoelectric elements exhibit poor performance and low energy efficiency; research is being conducted to find new materials or improve the thermoelectric performance of existing materials, that is, by ensuring a high figure-of-merit (zT) value. For increasing zT, higher σ (electrical conductivity) and S (Seebeck coefficient) and a lower к (thermal conductivity) are required. Here, interface engineering by atomic layer deposition (ALD) is used to increase zT of n-type BiTeSe (BTS) thermoelectric powders. ALD of the BTS powders is performed in a rotary-type ALD reactor, and 40 to 100 ALD cycles of ZnO thin films are conducted at 100°C. The physical and chemical properties and thermoelectric performance of the ALD-coated BTS powders and pellets are characterized. It is revealed that electrical conductivity and thermal conductivity are decoupled, and thus, zT of ALD-coated BTS pellets is increased by more than 60% compared to that of the uncoated BTS pellets. This result can be utilized in a novel method for improving the thermoelectric efficiency in materials processing.

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• Enhancing Electrical Properties of N-type Bismuth Telluride Alloys through Graphene Oxide Incorporation in Extrusion 3D Printing
  Jinhee Bae, Seungki Jo, Kyung Tae Kim
  journal of Korean Powder Metallurgy Institute.2023; 30(4): 318.     CrossRef

The semiconductor industry faces physical limitations due to its top-down manufacturing processes. High cost of EUV equipment, time loss during tens or hundreds of photolithography steps, overlay, etch process errors, and contamination issues owing to photolithography still exist and may become more serious with the miniaturization of semiconductor devices. Therefore, a bottom-up approach is required to overcome these issues. The key technology that enables bottom-up semiconductor manufacturing is area-selective atomic layer deposition (ASALD). Here, various ASALD processes for elemental metals, such as Co, Cu, Ir, Ni, Pt, and Ru, are reviewed. Surface treatments using chemical species, such as self-assembled monolayers and small-molecule inhibitors, to control the hydrophilicity of the surface have been introduced. Finally, we discuss the future applications of metal ASALD processes.

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• Selective Atomic Layer Deposition of Co Thin Films Using Co(EtCp)2 Precursor
  Sujeong Kim, Yong Tae Kim, Jaeyeong Heo
  Korean Journal of Materials Research.2024; 34(3): 163.     CrossRef

Thermoelectric materials can reversely convert heat and electricity into each other; therefore, they can be very useful for energy harvesting from heat waste. Among many thermoelectrical materials, SnSe exhibits outstanding thermoelectric performance along the particular direction of a single crystal. However, single-crystal SnSe has poor mechanical properties and thus it is difficult to apply for mass production. Therefore, polycrystalline SnSe materials may be used to replace single-crystal SnSe by overcoming its inferior thermoelectric performance owing to surface oxidation. Considerable efforts are currently focused on enhancing the thermoelectric performance of polycrystalline SnSe. In this study, we briefly review various enhancement methods for SnSe thermoelectric materials, including doping, texturing, and nano-structuring. Finally, we discuss the future prospects of SnSe thermoelectric powder materials.

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• The Mechanism Behind the High zT of SnSe₂ Added SnSe at High Temperatures
  JunSu Kim, Seong-Mee Hwang, Hyunjin Park, Yinglu Tang, Won-Seon Seo, Chae Woo Ryu, Heesun Yang, Weon Ho Shin, Hyun-Sik Kim
  Korean Journal of Metals and Materials.2023; 61(11): 857.     CrossRef

The SnSe single crystal shows an outstanding figure of merit (ZT) of 2.6 at 973 K; thus, it is considered to be a promising thermoelectric material. However, the mass production of SnSe single crystals is difficult, and their mechanical properties are poor. Alternatively, we can use polycrystalline SnSe powder, which has better mechanical properties. In this study, surface modification by atomic layer deposition (ALD) is chosen to increase the ZT value of SnSe polycrystalline powder. SnSe powder is ground by a ball mill. An ALD coating process using a rotary-type reactor is adopted. ZnO thin films are grown by 100 ALD cycles using diethylzinc and H₂O as precursors at 100°C. ALD is performed at rotation speeds of 30, 40, 50, and 60 rpm to examine the effects of rotation speed on the thin film characteristics. The physical and chemical properties of ALD-coated SnSe powders are characterized by scanning and tunneling electron microscopy combined with energy-dispersive spectroscopy. The results reveal that a smooth oxygenrich ZnO layer is grown on SnSe at a rotation speed of 30 rpm. This result can be applied for the uniform coating of a ZnO layer on various powder materials.

Atomic layer deposition (ALD) is widely used as a tool for the formation of near-atomically flat and uniform thin films in the semiconductor and display industries because of its excellent uniformity. Nowadays, ALD is being extensively used in diverse fields, such as energy and biology. By controlling the reactivity of the surface, either homogeneous or inhomogeneous coating on the shell of nanostructured powder can be accomplished by the ALD process. However, the ALD process on the powder largely depends on the displacement of powder in the reactor. Therefore, the technology for the fluidization of the powder is very important to redistribute its position during the ALD process. Herein, an overview of the three types of ALD reactors to agitate or fluidize the powder to improve the conformality of coating is presented. The principle of fluidization its advantages, examples, and limitations are addressed.

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• High-performance of ZnO/TiO2 heterostructured thin-film photocatalyst fabricated via atomic layer deposition
  Ji Young Park, Jeong Hwan Han, Byung Joon Choi
  Journal of Vacuum Science & Technology A.2024;[Epub]     CrossRef
• Atomic layer deposition of ZnO layers on Bi2Te3 powders: Comparison of gas fluidization and rotary reactors
  Myeong Jun Jung, Myeongjun Ji, Jeong Hwan Han, Young-In Lee, Sung-Tag Oh, Min Hwan Lee, Byung Joon Choi
  Ceramics International.2022; 48(24): 36773.     CrossRef

Carbon nanofibers (CNF) are widely used as active agents for electrodes in Li-ion secondary battery cells, supercapacitors, and fuel cells. Nanoscale coatings on CNF electrodes can increase the output and lifespan of battery devices. Atomic layer deposition (ALD) can control the coating thickness at the nanoscale regardless of the shape, suitable for coating CNFs. However, because the CNF surface comprises stable C–C bonds, initiating homogeneous nuclear formation is difficult because of the lack of initial nucleation sites. This study introduces uniform nucleation site formation on CNF surfaces to promote a uniform SnO₂ layer. We pretreat the CNF surface by introducing H₂O or Al₂O₃ (trimethylaluminum + H₂O) before the SnO₂ ALD process to form active sites on the CNF surface. Transmission electron microscopy and energy-dispersive spectroscopy both identify the SnO₂ layer morphology on the CNF. The Al₂O₃-pretreated sample shows a uniform SnO₂ layer, while island-type SnOx layers grow sparsely on the H₂Opretreated or untreated CNF.

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• Atomic layer deposition of ZnO layers on Bi2Te3 powders: Comparison of gas fluidization and rotary reactors
  Myeong Jun Jung, Myeongjun Ji, Jeong Hwan Han, Young-In Lee, Sung-Tag Oh, Min Hwan Lee, Byung Joon Choi
  Ceramics International.2022; 48(24): 36773.     CrossRef
• Effects of SnO2 layer coated on carbon nanofiber for the methanol oxidation reaction
  Dong Ha Kim, Dong-Yo Shin, Young-Geun Lee, Geon-Hyoung An, Jeong Hwan Han, Hyo-Jin Ahn, Byung Joon Choi
  Ceramics International.2018; 44(16): 19554.     CrossRef

Lithium-ion batteries (LIBs) are rapidly improving in capacity and life cycle characteristics to meet the requirements of a wide range of applications, such as portable electronics, electric vehicles, and micro- or nanoelectromechanical systems. Recently, atomic layer deposition (ALD), one of the vapor deposition methods, has been explored to expand the capability of LIBs by producing near-atomically flat and uniform coatings on the shell of nanostructured electrodes and membranes for conventional LIBs. In this paper, we introduce various ALD coatings on the anode, cathode, and separator materials to protect them and improve their electrochemical and thermomechanical stability. In addition, we discuss the effects of ALD coatings on the three-dimensional structuring and conduction layer through activation of electrochemical reactions and facilitation of fluent charge collection.

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• Atomic Layer Deposition for Powder Coating
  Seok Choi, Jeong Hwan Han, Byung Joon Choi
  Journal of Korean Powder Metallurgy Institute.2019; 26(3): 243.     CrossRef
• Formation of Uniform SnO2 Coating Layer on Carbon Nanofiber by Pretreatment in Atomic Layer Deposition
  Dong Ha Kim, Doh-Hyung Riu, Byung Joon Choi
  journal of Korean Powder Metallurgy Institute.2018; 25(1): 43.     CrossRef

The triboelectric property of a material is important to improve an efficiency of triboelectric generator (TEG) in energy harvesting from an ambient energy. In this study, we have studied the TEG property of a semiconducting SnO₂ which has yet to be explored so far. As a counter triboelectric material, PET and glass are used. Vertical contact mode is utilized to evaluate the TEG efficiency. SnO₂ thin film is deposited by atomic layer deposition on bare Si wafer for various thicknesses from 5.2 nm to 34.6 nm, where the TEG output is increased from 13.9V to 73.5V. Triboelectric series are determined by comparing the polarity of output voltage of 2 samples among SnO₂, PET, and glass. In conclusion, SnO₂, as an intrinsic n-type material, has the most strong tendency to be positive side to lose the electron and PET has the most strong tendency to be negative side to get the electron, and glass to be between them. Therefore, the SnO₂-PET combination shows the highest TEG efficiency.

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• Triboelectric charge generation by semiconducting SnO2 film grown by atomic layer deposition
  No Ho Lee, Seong Yu Yoon, Dong Ha Kim, Seong Keun Kim, Byung Joon Choi
  Electronic Materials Letters.2017; 13(4): 318.     CrossRef

Pt nanopowder-dispersed SiO₂ (SOP) films were prepared by RF co-sputtering method using Pt and SiO₂ targets in Ar atmosphere. The growth rate and Pt content in the film were controlled by means of manipulating the RF power of Pt target while that of SiO₂ was fixed. The roughness of the film was increased with increasing the power of Pt target, which was mainly due to the increment of the size and planar density of Pt nanopowder. It was revealed that SOP film formed at 10, 15, 20 W of Pt power contained 2.3, 2.7, and 3.0 nm of spherical Pt nanopowder, respectively. Electrical conductivity of SOP films was exponentially increased with increasing Pt power as one can expect. Interestingly, conductivity of SOP films from Hall effect measurement was greater than that from DC I-V measurement, which was explained by the significant increase of electron density.