Browsing by Author "Cho, Kyeongjae"
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Item A First Principles Approach to Closing the “10-100 eV GAP” for Electron Thermalization in Wurtzite GaN(May 2023) Nielsen, Dallin O 1993-; Fischetti, Massimo V.; Akin, Bilal; Vandenberghe, William; Cho, Kyeongjae; Gelb, Lev D.Since the 1960s, when radiation-induced disruption of electronic devices in space was first observed, the study of the effects of ionizing radiation on electronics has grown into an extensive field of its own. The present work is concerned with accurately modelling the energy-loss processes that control the thermalization of hot carriers (electrons and/or electron-hole pairs) that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-device community has done extensive work in the lower-energy range (below ~10 eV). However, the processes that control the energy losses and thermalization of electrons and holes in the intermediate energy range of about 10-100 eV are poorly known (the “10-100 eV gap”). The aim of this research is to close this gap. To this end, Density Functional Theory (DFT) is utilized to obtain the band structure of GaN for bands reaching energies above 100 eV. Furthermore, charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission) are calculated, using the DFT results and first-order perturbation theory (Fermi’s Golden Rule). With this information, the thermalization of electrons starting at 100 eV is simulated in a Monte Carlo code, allowing the electrons to interact stochastically according to the calculated interaction rates and generate electron-hole pairs as they go, which are also tracked in the simulation. Full thermalization of electrons is complete within 1 ps, and that of holes is complete in approximately half the time. Electrons lose 90% of their energy (90 eV) during the first few ~0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remainder is lost more slowly as phonon emission dominates at lower energies (below ~10 eV). During the thermalization, hot electrons generate electron-hole pairs with an average energy of ~8.9 eV/pair (11-12 pairs per hot electron). Additionally, upon full thermalization, the average electron displacement from its original position is found to be on the order of 100 nm.Item Electrode-Electrolyte Interface for Solid State Li-Ion Batteries: Point Defects and Mechanical StrainSantosh, 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.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 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 Studies on Oxide Semiconductors for Back-end-ofline Transistor Applications(May 2023) Hu, Yaoqiao; Cho, Kyeongjae; Chen, Feng; Toher, Cormac; Young, Chadwin D.; Quevedo-Lopez, ManuelThe development of high-performance p-type and n-type oxides with good carrier mobilities and wide band gaps is critical for the applications of metal oxide (MO) semiconductors in back-endof-line (BEOL) CMOS devices. [S. Salahuddin et al. Nat Electron. 1, 442 (2018)] Currently available oxide semiconductors are limited to n-type conduction, and p-type oxides have inferior performance due to carrier mobilities and dopability which are significantly lower than that of their n-type counterparts. This thesis devotes to studying and identifying novel high mobility p-type oxide candidates with wide band gaps and robust phase stabilities. Using first principles studies, we have identified several promising candidates including Rb2Sn2O3, TiSnO3, Ta2SnO6, and Sn5(PO5)2 that would be of interest as high-mobility p-type oxides. An engineering method is also developed to enhance the bandgap and hole mobility in widely investigated p-type oxide SnO. Amorphous phase engineering is revealed effectively improving the hole dopability and hole transport. Electron transport study on n-type oxides sheds light on the defect controlling and film density engineering in improving the n-type BEOL device performances. Our results provide fundamental materials insights into rational design of high mobility oxides semiconductors and serve as a guide for experimental realization of oxide semiconductors based BEOL transistors.Item First-Principle Prediction on STM Tip Manipulation of Ti Adatom on Two-Dimensional Monolayer YBr₃(Wiley-Hindawi, 2019-02-04) Liu, Pan; Wu, Maokun; Liu, Hui; Lu, Feng; Wang, Wei-Hua; Cho, Kyeongjae; 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Cho, KyeongjaeScanning tunneling microscopy (STM) is an important tool in surface science on atomic scale characterization and manipulation. In this work, Ti adatom manipulation is theoretically simulated by using a tungsten tip (W-tip) in STM based on first-principle calculations. The results demonstrate the possibility of inserting Ti adatoms into the atomic pores of monolayer YBr₃, which is thermodynamically stable at room temperature. In this process, the energy barriers of vertical and lateral movements of Ti are 0.38eV and 0.64eV, respectively, and the Ti atoms are stably placed within YBr₃ by >1.2eV binding energy. These theoretical predictions provide an insight that it is experimentally promising to manipulate Ti adatom and form artificially designed 2D magnetic materials.Item First-Principle Study on Electronic Structures of Two-Dimensional Transition Metal Dichalcogenides(2018-05) Zhang, Chenxi; 0000-0002-9114-1014 (Zhang, C); Cho, KyeongjaeTwo-dimensional (2D) transition metal dichalcogenides (TMDs), because of their sizable direct band gap, ultrathin body, good electron mobility, and versatile electronic structures, have attracted a lot of attention. They have shown their potential in future electronic, optoelectronic, photovoltaic, phase change, and photocatalytic devices. To examine their applicability to these devices, understanding their fundamental electronic structure and structural stability is a prerequisite. A systematic study of the electronic structure of monolayer transition metal dichalcogenides (TMDs) is performed by the density functional theory (DFT) method. Their band alignments are summarized and several representative bilayer heterostructures of two types of monolayer TMDs are investigated to understand the band realignment in the process of stacking. A formula is developed to predict the band realignment in the TMD bilayer heterostructure. Different to graphene, there exist several polymorphs of the same TMDs (e.g., 2H-MoTe2 and 1T-MoTe2). The total energies of various phases of TMDs are compared to determine the most energetically favorable phase. The electronic structure of TMDs, depending on the phase, can vary from metallic to semiconducting. This characteristic correlation between atomic and electronic structures opens up the possibility of controlling the electrical property by phase engineering, external field or charging. The charge-driven phase transition in monolayer WxMo1-xTe2 alloy has been investigated. Critical charge concentration and composition to induce phase transition is investigated. Doping, as an important technique of modulating the electronic properties of semiconductors, has been also investigated by DFT in monolayer MoS2. Substitutional doping of halogen group elements and nitrogen group elements is found to introduce n-type and p-type doping, respectively. The reduced dielectric screening will affect the exciton binding energy in 2D TMDs which will further change the impurity levels. NO2 adsorption-doping at the surface of monolayer WSe2 is also investigated showing p-type doping characteristic on WSe2. These findings have enabled the utilization of two-dimensional TMDs in the realization of future electronic devices.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 IoT Data Discovery and Learning(2022-08-01T05:00:00.000Z) Tran, Trung Hieu; Yen, I-Ling; Bastani, Farokh; Cho, Kyeongjae; Khan, Latifur; Wu, WeiliThe massive number of Internet-of-Things (IoT) creates a torrent of data. These data may be stored and hosted by nodes dispersed over the edge of the Internet, forming peer-to-peer (p2p) IoT database networks (IoT-DBNs) that can be dynamically discovered and used to enhance daily operations and solve real-world problems. The issues toward making use of the massive amount of IoT data include how to discover the IoT data streams from the IoT-DBN and how to learn and extract useful knowledge from the discovered data to help cope with dynamically arising tasks. In this dissertation, we consider these two problems and develop solutions for them. First, we consider the IoT data discovery problem in growing IoT-DBNs. We show the benefits of p2p unstructured routing for IoT data discovery and point out the space efficiency issue that has been overlooked in keyword-based routing algorithms. As the first in the field, this work investigates routing table designs and various compression techniques to support effective and space efficient IoT data discovery routing. Novel summarization algorithms are proposed, including alphabetical-based, hash-based, and meaning-based summarization and their corresponding coding schemes. We also consider routing table design to support summarization without degrading lookup efficiency for discovery query routing. To evaluate our approach, we collected 100K IoT data streams from various IoT resources and distributed them over a simulated Internet. Then, our data discovery routing with the summarization techniques is applied for handling discovery queries. The results show that our summarization solutions can reduce the routing table size by 20 to 30 folds with a 2-5% increase in latency compared with other peer-to-peer discovery routing algorithms. Our approach outperforms DHT-based approaches by 2 to 6 folds in latency and communication cost. After IoT data discovery and retrieval, a prominent problem is how to learn from the data to address real-world tasks. Since different applications require different learning schemes, we choose to focus on one example application, the estimated time of arrival (ETA) problem, which is very important in intelligent transportation systems and has received a lot of attention recently. Though many tools exist for ETA, ETA for special vehicles, such as ambulances, fire engines, etc., is still challenging due to the scarcity or non-existence of data. To tackle it, we propose a deep transfer learning framework TLETA for the ETA of special vehicles, namely TLETA. TLETA constructs cellular level spatial-temporal knowledge for fine-grained extraction of driving patterns. The learning network contains transferable layers to support knowledge transfer between different categories of vehicles. Importantly, our transfer models only train the last layers to map the transferred knowledge, significantly reducing the training time to achieve real-time learning. We also introduce the inter-region transfer method to build a mapping function between vehicle domains within a region. The mapping functions of top-k region spatial-temporal similarity are then used to construct the predictor in regions whose target data is unavailable. The experimental studies show that our model outperforms many state-of-the-art approaches in accuracy and training time.Item 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, KyeongjaeControlled 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.Item 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, KyeongjaeCapacity 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.Item Modulation of Contact Resistance between Metal and Graphene by Controlling the Graphene Edge, Contact Area, and Point Defects: An Ab Initio Study(Amer Inst Physics) Ma, Bo; Gong, Cheng; Wen, Yanwei; Chen, Rong; Cho, Kyeongjae; Shan, BinA systematic first-principles non-equilibrium Green's function study is conducted on the contact resistance between a series of metals (Au, Ag, Pt, Cu, Ni, and Pd) and graphene in the side contact geometry. Different factors such as the termination of the graphene edge, contact area, and point defect in contacted graphene are investigated. Notable differences are observed in structural configurations and electronic transport characteristics of these metal-graphene contacts, depending on the metal species and aforementioned influencing factors. It is found that the enhanced chemical reactivity of the graphene due to dangling bonds from either the unsaturated graphene edge or point defects strengthens the metal-graphene bonding, leading to a considerable contact resistance reduction for weakly interacting metals Au and Ag. For stronger interacting metals Pt and Cu, a slightly reduced contact resistance is found due to such influencing factors. However, the wetting metals Ni and Pd most strongly hybridize with graphene, exhibiting negligible dependence on the above influencing factors. This study provides guidance for the optimization of metal-graphene contacts at an atomic scale.Item MoS₂ Functionalization for Ultra-Thin Atomic Layer Deposited DielectricsAzcatl, Angelica; McDonnell, Stephen; KC, Santosh; Peng, Xin; Dong, Hong; Qin, Xiaoye; Addou, Rafik; Mordi, Greg I.; Lu, Ning; Kim, Jiyoung; Kim, Moon J.; Cho, Kyeongjae; Wallace, Robert M.; 70133685 (Kim, J)The effect of room temperature ultraviolet-ozone (UV-O₃) exposure of MoS₂ on the uniformity of subsequent atomic layer deposition of Al₂O₃ is investigated. It is found that a UV-O₃ pre-treatment removes adsorbed carbon contamination from the MoS₂ surface and also functionalizes the MoS₂ surface through the formation of a weak sulfur-oxygen bond without any evidence of molybdenum-sulfur bond disruption. This is supported by first principles density functional theory calculations which show that oxygen bonded to a surface sulfur atom while the sulfur is simultaneously back-bonded to three molybdenum atoms is a thermodynamically favorable configuration. The adsorbed oxygen increases the reactivity of MoS₂ surface and provides nucleation sites for atomic layer deposition of Al₂O₃. The enhanced nucleation is found to be dependent on the thin film deposition temperature.Item Multi-Scale Simulation and Experimental Study of High Voltage High Capacity Cathode Materials for Lithium Ion Battery(2017-05) Kong, Fantai; Cho, KyeongjaePresented in this dissertation is combined research of multi-scale simulation and experimental study of high voltage high capacity cathode materials for Lithium ion batteries. The materials under study are Li-Mn-rich layered oxides and Ni-rich layered oxides, which are widely regarded as the next generation cathode materials. However, they both face different challenges towards final commercialization. Many of these challenges have not been well understood, resulting in the difficulties of rationally optimizing battery performances. Therefore, we applied the ab initio method (density functional theory) to understand the underlying mechanisms that determine various important properties of these oxides, including redox potential, structural stability, ionic conductivity, electronic conductivity, cation mixing, etc. Based on these understandings, we proposed some rationalized optimizing strategies. Some of the strategies have also been experimentally validated by chemical synthesis and electrochemical performance testing via assembling coin cell type devices. Furthermore, as a way of extending the simulation limit (time scale and space scale) of the ab initio method, we have developed a new interatomic potential method by introducing dynamic charge transfer potential into the modified embedding atomic method (CT-MEAM). The potential method has been successfully applied to Li-Mn-O, Mn-O and Li-Ni-O systems with validated high accuracy in reproducing and predicting redox potentials, Li dynamics, surface effects, phase stabilities, structural parameters, phase diagrams, etc. These works could not only stimulate the large scale simulation of cathode materials for Li ion batteries, but also other materials involving strong charge transfer effects and electrochemical reactions.Item Multicomponent Silicate Cathode Materials for Rechargeable Li-ion Batteries: An Ab Initio StudyLongo, Roberto C.; Xiong, Ka; Cho, KyeongjaeA first principles investigation is performed to study the structural and electrochemical properties of new multicomponent silicate materials that can be suitable for the cathode of rechargeable Li-ion batteries. The distribution of different transition metals in the silicate structure alters the structural and electronic properties of the crystal, affecting its kinetics, redox potentials and both ionic and electronic conductivities. We also explain the effect of the multiple interactions between Li ions and the transition metals. These multicomponent structures represent a very powerful strategy to control the electrochemical performance of the silicates. In this work, we finally address the implications of such strategy on the design of Li-ion batteries.Item Multiscale Simulation Method Development and Application in Two-Dimensional Functional Materials(2018-12) Nie, Yifan; 0000-0003-4771-3633 (Nie, Y); Cho, KyeongjaeThe unique properties of two-dimensional (2D) materials make them the potential solutions to a wide variety of engineering challenges, especially in electronic device engineering. In order to realize their application potentials, three tasks must be fulfilled, namely the thorough understanding of the material properties, the suitable application design, and synthesis route development for the mass production of the materials. Currently, the first two tasks are in relatively mature stages; however, the bottom-up synthesis method development of 2D materials, especially 2D compound materials, is still at an early stage, due to a limited theoretical understanding of the mechanisms of the submonolayer growth of the 2D crystalline compounds. In this dissertation, the author presents the development and application of multiple simulation tools, including density functional theory, kinetic Monte Carlo method, and phase field method to study the growth mechanism of 2D compounds. Transition metal dichalcogenides (TMDs) are taken as a representative example. The multiscale simulation methods can be used to analyze the nucleation and growth processes during the deposition of TMDs. The formation and influence of material imperfections, such as point defects, metal clustering, screw dislocations, etc., are also studied with these simulation tools. With the help of the established simulation toolbox, the author contributes to the understanding of the physics behind this complex catalog of atomic processes involved in the synthesis of TMDs. The simulation tools and the results thereof provide guidance to the experimental efforts, and have supported the joint efforts towards the realization of large-scale production of electronic-grade 2D semiconductors. In addition, the author also presents his theoretic works on the investigation of electric properties of 2D functional materials and their contact/combination systems, the novel device design based on these understandings, and the method pathfinding in neural-networks based interatomic potentials. The studies presented in this dissertation, from one facet, exhibit the wide application potential of multiscale simulation tools on materials science research and engineering.Item 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).Item Organic Photovoltaics for Outdoor and Indoor Applications(August 2022) Zhang, Boya; Hassanipour, Fatemeh; Hsu, Julia W.P.; Stefan, Mihaela C.; Vandenberghe, William G.; Cho, KyeongjaeOrganic Photovoltaics (OPVs) have been actively researched during the past three decades due to their low cost, flexibility, solution processability, and robustness. With the development of novel non-fullerene acceptor (NFA) materials, the power conversion efficiency (PCE) exceeds 18% under the one-sun condition for single-junction devices. However, developing absorber material alone is not enough to advance OPV performance because the opposite type of charge carriers is likely to recombine at the electrodes when the active layer contacts intimately with the anode and cathode. To mitigate the charge carrier recombination, it is critical to developing appropriate interfacial materials to selectively pass the desired charges, i.e., positive ones, while blocking the undesired negative ones. In Chapter 2 of this dissertation, solution-processed Mg doped CuCrO2 nanoparticles have been demonstrated as efficient hole transport layers. Mg doping effects from structural, chemical, morphological, optical, and electronic properties of CuCrO2 are investigated. Under the indoor condition, several high-performance indoor OPVs (IOPVs) have been reported with the highest PCE surpassing 30%. Many groups have studied the light intensity dependence of IOPVs. However, the IOPV performance as a function of color temperature has not been carefully examined before. In Chapter 4 of this dissertation, the PCE as a function of correlated color temperature (CCT) in several organic donor-acceptor systems is studied. Due to the different absorption spectra of organic materials, two groups of behaviors, CCT-independent and CCTdependent, are seen, which provides a guidance on selecting suitable IOPVs for different applications. In Chapter 5 of this dissertation, we focus on understanding the photocurrent generation in OPV devices using two popular commercial NFAs. We show charge transport determines the significant photocurrent difference. Understanding the mechanism behind higher photocurrent is pivotal for developing NFA-based OPVs.Item 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, JiyoungMolecular-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.