Cho, Kyeongjae
Permanent URI for this collectionhttps://hdl.handle.net/10735.1/3651
Kyeongjae Cho is a Professor of Materials Science. His research interests include:
- Computational modeling study of nanomaterials with applications to nanoelectronic devices
- Renewable energy technology
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Browsing Cho, Kyeongjae by Author "369148996084659752200 (Cho, K)"
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Item Enhanced P-Type Behavior in 2D WSe2 via Chemical Defect Engineering(Institute of Electrical and Electronics Engineers Inc.) Rai, A.; Park, J. H.; Zhang, Chenxi; Kwak, I.; Wolf, S.; Vishwanath, S.; Lin, X.; Furdyna, J.; Xing, H. G.; Cho, Kyeongjae; Kummel, A. C.; Banerjee, S. K.; 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Zhang, Chenxi; Cho, KyeongjaeDefect engineering of 2D semiconducting transition metal dichalcogenides (TMDCs) has been demonstrated to be a promising way to tune both their bandgaps and carrier concentrations. Moreover, controlled introduction of defects in the source/drain access regions of a TMDC FET can boost its performance by decreasing the contact resistance at the metallTMDC interface [1]. While chemical functionalization offers a facile route towards defect engineering in 2D TMDCs, several chemically-treated TMDCs have not been fully understood at the molecular level. In this study, chemical sulfur treatment (ST) utilizing ammonium sulfide [(NH4)2S] solution is shown to enhance the p-type behavior in 2D WSe2 via introduction of acceptor defect states near its valence band edge (VBE), with the results verified using detailed scanning tunneling microscopy (STM)/spectroscopy (STS) studies, field-effect transistor (FET) measurements and theoretical density-of-states (DOS) calculations.Item First-Principles Study of Metal-Graphene Edge Contact for Ballistic Josephson Junction(American Physical Society, 2019-06-05) Lee, Yeonghun; Hwang, Jeongwoon; Zhang, Fan; Cho, Kyeongjae; 0000-0003-4623-4200 (Zhang, F); 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Lee, Yeonghun; Hwang, Jeongwoon; Zhang, Fan; Cho, KyeongjaeEdge-contacted superconductor-graphene-superconductor Josephson junctions have been utilized to realize topological superconductivity, and have shown superconducting signatures in the quantum Hall regime. We perform first-principles calculations to interpret electronic couplings at the superconductor-graphene edge contacts by investigating various aspects in hybridization of molybdenum d orbitals and graphene π orbitals. We also reveal that interfacial oxygen defects play an important role in determining the doping type of graphene near the interface. © 2019 American Physical Society.Item Investigation of the Hydrothermal Aging of an Mn-Based Mullite SmMn₂O₅ Catalyst of NO Oxidation(Royal Society of Chemistry, 2017-10-20) Xue, L.; Xiong, K.; Chen, H.; Cho, Kyeongjae; Wang, Weichao; 0000-0003-2698-7774 (Cho, K); 0000-0001-5931-212X (Wang, W); 369148996084659752200 (Cho, K); Cho, Kyeongjae; Wang, WeichaoHydrothermal aging tests are important to carry out when evaluating the hydrothermal durability of heterogeneous catalysts in vehicle exhaust emission. Here, we explored the effect of aging on an efficient Mn-based mullite catalyst (SmMn₂O₅) of NO oxidation. The mullite catalyst was prepared via the hydrothermal method and was subsequently aged in air with a 10% H2O stream at 750 °C for 16 hours. The fresh and aged catalysts were structurally characterized using Powder X-ray diffraction(XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), high resolution-transmission electron microscope (HR-TEM), Brunauer-Emmett-Teller (BET) and temperature programmed desorption (TPD). For the performance evaluations, the samples were placed into a U-shape reactor furnace, and NO and NO2 concentrations were in situ recorded with an NOx analyzer. In contrast to fresh mullite, the aged sample showed a 25 °C higher light-off temperature and 11% conversion loss at its maximum conversion temperature of 300 °C. O2-TPD of the aged sample displayed a large decrease of the desorption area, consistent with an ∼3-fold loss of the BET specific surface area. Moreover, HRTEM, XPS and Raman spectroscopy results together indicated that a small portion of the mullite decomposed into perovskite SmMnO3 and Mn2O3, which further reduced the total quantity of Mn active sites. The reduction of the BET surface area and mullite decomposition together caused the decrease of the catalytic performance. We therefore expect maintaining the specific surface area to be important for preventing the loss of catalytic performance during the hydrothermal aging process. © 2017 The Royal Society of Chemistry.Item Quantum Transport and Band Structure Evolution under High Magnetic Field in Few-Layer Tellurene(American Chemical Society) Qiu, G.; Wang, Y.; Nie, Yifan; Zheng, Yongping; Cho, Kyeongjae; Wu, W.; Ye, P. D.; 0000-0003-4771-3633 (Nie, Y); 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Nie, Yifan; Zheng, Yongping; Cho, KyeongjaeQuantum Hall effect (QHE) is a macroscopic manifestation of quantized states that only occurs in confined two-dimensional electron gas (2DEG) systems. Experimentally, QHE is hosted in high-mobility 2DEG with large external magnetic field at low temperature. Two-dimensional van der Waals materials, such as graphene and black phosphorus, are considered interesting material systems to study quantum transport because they could unveil unique host material properties due to the easy accessibility of monolayer or few-layer thin films at the 2D quantum limit. For the first time, we report direct observation of QHE in a novel low-dimensional material system, tellurene. High-quality 2D tellurene thin films were acquired from recently reported hydrothermal method with high hole mobility of nearly 3000 cm2/(V s) at low temperatures, which allows the observation of well-developed Shubnikov-de Haas (SdH) oscillations and QHE. A four-fold degeneracy of Landau levels in SdH oscillations and QHE was revealed. Quantum oscillations were investigated under different gate biases, tilted magnetic fields, and various temperatures, and the results manifest the inherent information on the electronic structure of Te. Anomalies in both temperature-dependent oscillation amplitudes and transport characteristics were observed that are ascribed to the interplay between the Zeeman effect and spin-orbit coupling, as depicted by the density functional theory calculations. ©2018 American Chemical Society.Item Superior Low-Temperature NO Catalytic Performance of PrMn₂O₅ over SmMn₂O₅ Mullite-Type Catalysts(Royal Society of Chemistry, 2019) Thampy, Sampreetha; Ashburn, Nickolas; Liu, C.; Xiong, K.; Dillon, Sean; Zheng, Yongping; Chabal, Yves J.; Cho, Kyeongjae; Hsu, Julia W. P.; 0000-0002-7821-3001 (Hsu, JWP); 0000-0002-6435-0347 (Chabal, YJ); 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Thampy, Sampreetha; Ashburn, Nickolas; Dillon, Sean; Zheng, Yongping; Chabal, Yves J.; Cho, Kyeongjae; Hsu, Julia W. P.By studying their surface chemistry, metal-oxygen bond strength, and critical energy barrier heights, we elucidate the differences in the NO oxidation catalytic performance of PrMn₂O₅ and SmMn₂O₅ mullite-type oxides. The 50% conversion temperature is lower (230 °C vs. 275 °C) and the maximum conversion efficiency is higher (81% at 282 °C vs. 68% at 314 °C) for PrMn₂O₅ compared to SmMn₂O₅, despite having a ∼15% lower specific surface area. Furthermore, PrMn₂O₅ exhibits higher maximum efficiency compared to Pt/Al₂O₃. Combined experimental and theoretical findings indicate that the superior catalytic performance of PrMn₂O₅ at low temperatures arises from the presence of more labile and reactive surface lattice oxygen due to weaker Mn-O bond strength and lower thermal stability of surface NOₓ ad-species. ©2019 The Royal Society of Chemistry.Item The Band Structure Change of Hf₀.₅Zr₀.₅O₂/Ge System upon Post Deposition Annealing(Elsevier B.V., 2019-05-25) Feng, Z.; Peng, Y.; Liu, H.; Sun, Y.; Wang, Y.; Meng, M.; Liu, H.; Wang, J.; Wu, R.; Wang, Xinglu; Cho, Kyeongjae; Han, G.; Dong, H.; 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Wang, Xinglu; Cho, KyeongjaeHafnium zirconium oxide films have been utilized in negative capacitance (NC) field-effect transistors (FETs). The band alignment of semiconductor and HfZrOₓ film is critical to obtain high device performance. The band alignment of Hf₀.₅Zr₀.₅O₂/SiOₓ/Ge system before and after post deposition annealing at 500 °C is studied via angle resolved X-ray photoelectron spectroscopy, synchrotron radiation photoemission spectroscopy and UV–Visible spectroscopy. The band gap of Hf₀.₅Zr₀.₅O₂ is seen narrowed 0.27 ± 0.05 eV, and the valence band offset between Hf₀.₅Zr₀.₅O₂ and Ge decreases 0.25 eV ± 0.05 eV after PDA at 500 °C. Therefore, the conduction band offset is nearly unchanged. This work gives insights into the interface physics about Hf₀.₅Zr₀.₅O₂/SiOₓ and is valuable for Ge-based NC pFETs. ©2019 Elsevier B.V.Item Tunable H₂ Binding on Alkaline and Alkaline Earth Metals Decorated Graphene Substrates from First-Principles Calculations(Pergamon-Elsevier Science Ltd, 2017-04-13) Wen, Yanwei; Xie, Fan; Liu, Xiaolin; Liu, Xiao; Chen, Rong; Cho, Kyeongjae; Shan, Bin; 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Cho, Kyeongjae; Shan, BinBased on first-principles calculations, the H-2 adsorptions onto six types of modified graphene substrates decorated with light metals (Li, Na, K, Be, Mg, Ca) are investigated to shed light on the factors affecting the H-2 binding energies. It is demonstrated that the introduction of defects and dopants into graphene substrates is essential to prevent the metal clustering and achieve dispersed metal atoms desirable for H-2 adsorption. The interaction between H-2 and alkali/alkali-earth metal decorated graphene systems is attributed to the electrostatic effect induced by polarized dipole-dipole interaction. Via introducing defects and hetero-atoms to modify the electronegativity of the local structure, the H-2 adsorption energy can be tuned by choosing the combination of suitable metals and substrates. The calculated H-2 binding strength is positively correlated to the charge transfer from the metal to the substrates and the dipole momentum of metal decorated substrates. Compared the cases with different metals decoration, Mg and Ca are expected to the most promising candidates for multiple H-2 adsorptions.Item ZnO Composite Nanolayer with Mobility Edge Quantization for Multi-Value Logic Transistors(Nature Publishing Group, 2019-04-30) Lee, L.; Hwang, Jeongwoon; Jung, J. W.; Kim, J.; Lee, H. -I; Heo, S.; Yoon, M.; Choi, S.; Van Long, N.; Park, J.; Jeong, J. W.; Kim, Jiyoung; Kim, K. R.; Kim, D. H.; Im, S.; Lee, B. H.; Cho, Kyeongjae; Sung, M. M.; 0000-0003-2781-5149 (Kim, J); 0000-0003-2698-7774 (Cho, K); 70133685 (Kim, J); 369148996084659752200 (Cho, K); Hwang, Jeongwoon; Kim, Jiyoung; Cho, KyeongjaeA quantum confined transport based on a zinc oxide composite nanolayer that has conducting states with mobility edge quantization is proposed and was applied to develop multi-value logic transistors with stable intermediate states. A composite nanolayer with zinc oxide quantum dots embedded in amorphous zinc oxide domains generated quantized conducting states at the mobility edge, which we refer to as “mobility edge quantization”. The unique quantized conducting state effectively restricted the occupied number of carriers due to its low density of states, which enable current saturation. Multi-value logic transistors were realized by applying a hybrid superlattice consisting of zinc oxide composite nanolayers and organic barriers as channels in the transistor. The superlattice channels produced multiple states due to current saturation of the quantized conducting state in the composite nanolayers. Our multi-value transistors exhibited excellent performance characteristics, stable and reliable operation with no current fluctuation, and adjustable multi-level states. ©2019, The Author(s).