Cho, Kyeongjae

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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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Recent Submissions

Now showing 1 - 20 of 29
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    Kinetic Stability of Bulk LiNiO₂ and Surface Degradation by Oxygen Evolution in LiNiO₂-Based Cathode Materials
    (Wiley-VCH Verlag Gmbh, 2018-11-02) Kong, Fantai; Liang, Chaoping; Wang, Luhua; Zheng, Yongping; Perananthan, Sahila; Longo, Roberto C.; Ferraris, John P.; Kim, Moon J.; Cho, Kyeongjae; Kong, Fantai; Liang, Chaoping; Wang, Luhua; Zheng, Yongping; Perananthan, Sahila; Longo, Roberto C.; Ferraris, John P.; Kim, Moon J.; Cho, Kyeongjae
    Capacity degradation by phase changes and oxygen evolution has been the largest obstacle for the ultimate commercialization of high-capacity LiNiO₂-based cathode materials. The ultimate thermodynamic and kinetic reasons of these limitations are not yet systematically studied, and the fundamental mechanisms are still poorly understood. In this work, both phenomena are studied by density functional theory simulations and validation experiments. It is found that during delithiation of LiNiO₂, decreased oxygen reduction induces a strong thermodynamic driving force for oxygen evolution in bulk. However, oxygen evolution is kinetically prohibited in the bulk phase due to a large oxygen migration kinetic barrier (2.4 eV). In contrast, surface regions provide a larger space for oxygen migration leading to facile oxygen evolution. These theoretical results are validated by experimental studies, and the kinetic stability of bulk LiNiO₂ is clearly confirmed. Based on these findings, a rational design strategy for protective surface coating is proposed.
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    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.
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    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, Kyeongjae
    Edge-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.
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    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, Kyeongjae
    Hafnium 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.
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    WSe₂ Homojunctions and Quantum Dots Created by Patterned Hydrogenation of Epitaxial Graphene Substrates
    (IOP Publishing Ltd, 2019-01-17) Pan, Y.; Fölsch, S.; Lin, Y. -C; Jariwala, B.; Robinson, J. A.; Nie, Yifan; Cho, Kyeongjiae; Feenstra, R. M.; 0000-0003-2698-7774 (Cho, K); 0000-0003-4771-3633 (Nie, Y); Nie, Yifan; Cho, Kyeongjiae
    Scanning tunneling microscopy (STM) at 5 K is used to study WSe₂ layers grown on epitaxial graphene which is formed on Si-terminated SiC(0 0 0 1). Specifically, a partial hydrogenation process is applied to intercalate hydrogen at the SiC-graphene interface, yielding areas of quasi-free-standing bilayer graphene coexisting with bare monolayer graphene. We find that an abrupt and structurally perfect homojunction (band-edge offset ∼0.25 eV) is formed when WSe₂ overgrows a lateral junction between adjacent monolayer and quasi-free-standing bilayer areas in the graphene. The band structure modulation in the WSe₂ overlayer arises from the varying work function (electrostatic potential) of the graphene beneath. Scanning tunneling spectroscopy measurements reveal that this effect can be also utilized to create WSe₂ quantum dots that confine either valence or conduction band states, in agreement with first-principles band structure calculations. ©2019 IOP Publishing Ltd.
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    Tuning the Structure of Bifunctional Pt/SmMn₂O₅ Interfaces for Promoted Low-Temperature CO Oxidation Activity
    (Royal Society of Chemistry, 2019-01-30) Liu, X.; Yang, J.; Shen, G.; Shen, M.; Zhao, Y.; Cho, Kyeongjae; Shan, Bin; Chen, R.; 0000-0003-2698-7774 (Cho, K); 0000-0001-7800-0762 (Shan, B); Cho, Kyeongjae; Shan, Bin
    The interfacial structure of metal-oxide composite catalysts plays a vital role in heterogeneous catalysis, which is crucial to the adsorption and activation of reactants. Herein, the interfacial effects of bare and Fe/Co/Ni doped Pt/SmMn₂O₅ mullite oxide supported Pt clusters on CO oxidation have been investigated by first-principles based microkinetics analysis. A robust formation of Pt/Mn₂ trimer structures is demonstrated at the bifunctional interfaces irrespective of the Pt_{n} cluster's size, which can provide spatially separated sites for CO adsorption and O₂ dissociation. The binding strength of CO at the interfacial Pt sites is in the optimal range due to the charge transfer from Pt clusters to oxide, while the strong polarization of Mn₂ dimers induced by Pt clusters with stable three-dimensional morphologies can lower the energy barrier of O₂ dissociation. Based on the microkinetics analysis, the O₂ dissociation is the rate-determining step in the full CO oxidation cycle, and the introduction of Mn-Fe hetero-dimers at the interface is predicted to further enhance the low temperature CO oxidation activity of Pt/SmMn₂O₅ catalysts. © The Royal Society of Chemistry 2019.
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    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, Kyeongjae
    A 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).
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    Threshold Voltage Modulation of a Graphene–ZnO Barristor Using a Polymer Doping Process
    (Blackwell Publishing Ltd, 2019-05-06) Kim, S. -Y; Hwang, Jeongwoon; Kim, Y. J.; Hwang, H. J.; Son, M.; Revannath, N.; Ham, M. -H; Cho, Kyeongjie; Lee, B. H.; 0000-0003-2698-7774 (Cho, K); Hwang, Jeongwoon; Cho, Kyeongjie
    A method to modulate the threshold voltage of a graphene–ZnO barristor is investigated. Two types of polymers, polyethyleneimine (as an n-type dopant) and poly(acrylic acid) (as a p-type dopant), are used to pre-set the initial Fermi level of the graphene. The threshold voltage of the graphene barristor can be modulated between −2.0 V (n-type graphene) and 1.2 V (p-type graphene) while modulating the Fermi level of the graphene by 120 meV. This process provides a scalable and facile method to adjust the threshold voltage of graphene–semiconductor junction-based devices, which is a crucial function required to implement graphene-based electronic devices in integrated circuits. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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    A Fermi-Level-Pinning-Free 1D Electrical Contact at the Intrinsic 2D MoS₂–Metal Junction
    (Wiley-VCH Verlag, 2019-05-08) Yang, Z.; Kim, C.; Lee, K. Y.; Lee, M.; Appalakondaiah, S.; Ra, C. -H; Watanabe, K.; Taniguchi, T.; Cho, Kyeongiae; Hwang, E.; Hone, J.; Yoo, W. J.; 0000-0003-2698-7774 (Cho, K); Cho, Kyeongiae
    Currently 2D crystals are being studied intensively for use in future nanoelectronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier-free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky–Mott rule and resulting in significant suppression of carrier transport in the device. Here, MoS₂ polarity control is realized without extrinsic doping by employing a 1D elemental metal contact scheme. The use of high-work-function palladium (Pd) or gold (Au) enables a high-quality p-type dominant contact to intrinsic MoS₂, realizing Fermi level depinning. Field-effect transistors (FETs) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 and 432 cm² V⁻¹ s⁻¹ at 300 K, respectively. The ideal Fermi level alignment allows creation of p- and n-type FETs on the same intrinsic MoS₂ flake using Pd and low-work-function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at V_{D} = 5 V. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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    Ab-Initio Design of Novel Cathode Material LiFeP₁₋ₓSiₓO₄ for Rechargeable Li-Ion Batteries
    (Elsevier Ltd, 2019-04-27) Yi, S.; Moon, J.; Cho, M.; Cho, Kyeongiae; 0000-0003-2698-7774 (Cho, K); Cho, Kyeongiae
    In this study, newly designed cathode material LiFeP₁₋ₓSiₓO₄, with silicon mixed in LiFePO₄ is investigated using the density functional theory. Its most optimized structure is the olivine structure of the Pnma space group. Bonding length show the anti-site defect which hinders Li diffusivity is prevented in the LiFeP₁₋ₓSiₓO₄. Lithium migration energy barriers in the (010) path of LiFeP₁₋ₓSiₓO₄ (x = 0, 0.5, and 1) are calculated by using nudged elastic band calculations, and the average values are determined as 0.180, 0.245, and 0.280 eV for LiFePO₄, LiFeP₀․₅Si₀․₅O₄, and LiFeSiO₄, respectively. This signifies that the Li ionic diffusivity is degraded thermodynamically, which is contrary to that indicates by the calculated bonding length, however, the difference is negligibly small. Furthermore, the intercalation voltage increases up to 4.97 V, depending on the Si ratio to P, and is much higher than that of the pristine cathode materials LiFePO₄ (~3.47 V) enabling voltage optimization by Si substitution. The energy density is proportional to the intercalation voltage, hence the energy density is increased, respectively. Finally, the Total density of states show that the electronic conductivity of LiFeP₁₋ₓSiₓO₄ (x = 0–1) is better than that of LiFePO₄.
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    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, Kyeongjae
    Quantum 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.
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    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, Kyeongjae
    Defect 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.
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    Nucleation and Growth of WSe₂: Enabling Large Grain Transition Metal Dichalcogenides
    (IOP Publishing Ltd, 2017-09-22) Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Cha, Pil-Ryung; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.; 0000-0002-2910-2938 (Liang, C); Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.
    The limited grain size (< 200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe₂ is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe₂ thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).
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    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, Weichao
    Hydrothermal 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.
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    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, Bin
    Based 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.
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    A Kinetic Monte Carlo Simulation Method of Van Der Waals Epitaxy for Atomistic Nucleation-Growth Processes of Transition Metal Dichalcogenides
    (Nature Publishing Group, 2018-08-31) Nie, Yifan; Liang, Chaoping; Cha, Pil-Ryung; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; 0000-0003-4771-3633 (Nie, Y); Nie, Yifan; Liang, Chaoping; Cha, Pil-Ryung; Wallace, Robert M.; Cho, Kyeongjae
    Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.
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    Underlying Mechanisms of the Synergistic Role of Li₂MnO₃ and LiNi_{1/3}Co_{1/3}Mn_{1/3}O₂ in High-Mn, Li-Rich Oxides
    (Royal Society of Chemistry) Lim, J. -M; Kim, D.; Park, M. -S; Cho, M.; Cho, Kyeongjae; 0000-0003-2698-7774 (Cho, K)
    For large-scale energy storage applications requiring high energy density, the development of Li-rich oxides with enhanced cyclic stabilities during high-voltage operations and large specific capacities is required. In this regard, high-Mn, Li-rich oxides (HMLOs; xLi₂MnO₃3 (1 - x)LiNi_{1/3}Co_{1/3}Mn_{1/3}O₂ at x > 0.5) warrant an in-depth study because of their good cyclic performance at high operating voltages and potentially large specific capacities. Here, to understand the synergistic effects and enhanced cyclic stability of HMLOs, mechanically blended HMLO (m-HMLO) and chemically bonded HMLO (c-HMLO) were prepared and investigated. c-HMLO exhibits relatively high reaction voltages, large specific capacities, and enhanced cyclic stabilities (∼99%) at a high operating voltage (∼4.8 V vs. Li/Li⁺) compared with m-HMLO. First-principles calculations with electronic structure analysis were performed using an atomic model developed by Rietveld refinement using as-synthesised c-HMLO. The redox mechanisms of Ni, Co, and Mn ions were determined via the partial density of states of the ground states predicted using the cluster expansion method, which elucidates that LiNi_{1/3}Co_{1/3}Mn_{1/3}O₂ stabilises the transition metal (TM) layer of Li₁MnO₃ and separates Li delithiation potentials in Li₁MnO₃ in the HMLO. Kinetic analyses including electronic structures revealed that the interlayer migration of TMs from the TM layer to the Li layer depends on the crystal field stabilisation. Thus, TMs with reduced character in the tetrahedral sites than the octahedral sites owing to the effects of crystal field stabilisation, such as Ni ions, in HMLOs would face a higher interlayer migration barrier, impeding phase transformation into spinel phases. Furthermore, Cu ions could constitute a doping source for HMLOs to improve the material's cyclic stability through this mechanism. These characteristics may be widely applied to explain experimental phenomena and improve the properties of cathode materials for Li-ion batteries.
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    Tuning Electronic Transport in Epitaxial Graphene-Based Van Der Waals Heterostructures
    (RSC Pub) Lin, Yu-Chuan; Li, Jun; de la Barrera, Sergio,C.; Eichfeld, Sarah M.; Nie, Yifan; Addou, Rafik; Mende, Patrick C.; Wallace, Robert M.; Cho, Kyeongjae; Feenstra, Randall M.; Robinson, Joshua A.; 0000-0001-5566-4806 (Wallace, RM); 0000-0003-2698-7774 (Cho, K); Nie, Yifan; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae
    Two-dimensional tungsten diselenide (WSe₂) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe₂-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe₂-graphene interface. Electrical transport measurements reveal a lower resistance between WSe₂ and fully hydrogenated epitaxial graphene (EGFH) compared to WSe₂ grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe₂ and graphene and employ a charge transfer model to determine the WSe₂ carrier density in both cases. The results indicate that WSe₂-EGFH displays ohmic behavior at small biases due to a large hole density in the WSe₂, whereas WSe₂-EGPH forms a Schottky barrier junction.;
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    Organic-Inorganic Hybrid Semiconductor Thin Films Deposited Using Molecular-Atomic Layer Deposition (MALD)
    (Royal Society of Chemistry) Huang, Jie; Zhang, Hengji; Lucero, Antonio; Cheng, Lanxia; KC, Santosh; Wang, Jian; Hsu, Julia W. P.; Cho, Kyeongjae; Kim, Jiyoung; 0000 0003 8600 0978 (Hsu, JWP); 0000-0003-2698-7774 (Cho, K); 0000-0003-2781-5149 (Kim, J); Huang, Jie; Zhang, Hengji; Lucero, Antonio; Cheng, Lanxia; KC, Santosh; Wang, Jian; Hsu, Julia W. P.; Cho, Kyeongjae; Kim, Jiyoung
    Molecular-atomic layer deposition (MALD) is employed to fabricate hydroquinone (HQ)/diethyl zinc (DEZ) organic-inorganic hybrid semiconductor thin films with accurate thickness control, sharp interfaces, and low deposition temperature. Self-limiting growth is observed for both HQ and DEZ precursors. The growth rate remains constant at approximately 2.8 Å per cycle at 150°C. The hybrid materials exhibit n-type semiconducting behavior with a field effect mobility of approximately 5.7 cm² V⁻¹ s⁻¹ and an on/off ratio of over 103 following post annealing at 200°C in nitrogen. The resulting films are characterized using ellipsometry, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), UV-Vis spectroscopy, transistor behavior, and Hall-effect measurements. Density functional theory (DFT) and many-body perturbation theory within the GW approximation are also performed to assist the explanation and understanding of the experimental results. This research offers n-channel materials as valuable candidates for efficient organic CMOS devices. © 2016.
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    Electrode-Electrolyte Interface for Solid State Li-Ion Batteries: Point Defects and Mechanical Strain
    Santosh, KC; Longo, Roberto C.; Xiong, Ka; Cho, Kyeongjae; 0000-0003-2698-7774 (Cho, K)
    In this work, we present an ab-initio investigation of point defects in solid electrolyte γ-Li₃PO₄ and in negative electrode-electrolyte interface (Li/γ-Li₃PO₄). Our results on Li defects on γ-Li₃PO₄ exhibit that Li interstitial defects dominate over vacancy defects, and that Li vacancy-interstitial pair defect formation energy in-the-interface is comparable to the sum of-Li vacancy defect in the electrode and Li ion interstitial defects in the electrolyte region. Our study reveals that the high Li ion defect formation energy is the determining factor for the low ionic conductivity across Li metal/electrolyte interface. Moreover, in a realistic interface, the mechanical strain at the interface increases with the concentration of the impurities produced as a result of the cycling of the battery or due to surface impurities, also affecting the electrostatic potential and charge distribution. Thus, the study of the Li metal/electrolyte interface provides information on the defect formation and mechanical stability and, hence, it helps to understand the realistic modeling of the interface-as a way to-improve the ionic conductivity and stability of future solid state Li-ion batteries.

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