Browsing by Author "Kim, Moon J."
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Item 2D Materials: Theoretical Study of Magnetic and Contact Properties(December 2023) Reyntjens, Peter Dirk Jan 1994-; Liu, Jin; Vandenberghe, William; Kim, Moon J.; Fischetti, Massimo V.; Sorée, BartThe integrated circuit is without a doubt one of the most influential inventions in all of human history. While every technological revolution has had massive impacts across human societies, modern electronic circuits have increased the rate of change by orders of magnitude and this process shows no signs of stopping. As society has become accustomed to the rapid pace of technological development, the expectations for further improvements are more and more demanding. The silicon transistor was the ideal vehicle for such a rapid development, as transistors typically become more powerful and less costly to make when their size is decreased. With the added bonus of being able to cram more transistors into the same chip, the electronics revolution started, and a snowball effect of increasingly complexity and performance was unleashed onto the market, leading to the highly interconnected society we live in today. However, the benefits of decreasing transistor dimensions cannot last forever. There are certain extremely fundamental limits, at the nanometer scale, to how far one can go in making smaller and smaller devices. At some point, transistors begin to suffer from all sorts of performance-degrading issues, such as short-channel effects, increased leakage, fabrication difficulties, etc. Even more fundamental issues arise once the device dimensions go down to only a few nanometers, where quantum effects can seriously degrade traditional silicon-based transistors. It is with these scaling limitations in mind that researchers started looking very seriously at a relatively new class of materials: two-dimensional (2D) materials. 2D materials are atomically thin materials, consisting of a single layer not bound covalently in the out-of-plane direction. The 2D nature of these materials is of course in stark contrast with more “normal” materials, such as silicon or iron, which have covalent bonds in three dimensions. It turns out that due to the special structure of 2D materials, the physical properties are also extremely interesting, and worth investigating seriously. At present, various classes of 2D materials have been found, and many 2D materials have corresponding stacked layered versions with their own special properties. Add in, for example, the fact that one can dope these materials of make heterostructures out of several different kinds, then one can start to appreciate the vast parameter space that can be explored in the search for interesting applications. In this work, I focus on the applications of 2D materials in logic and memory devices. More specifically, I discuss the studies done by myself and my collaborators on the magnetic properties of layered WSe2 and PtSe2, and the calculation of the contact resistance between a metal and a 2D semiconductor. In the first part of the thesis, I share our investigation on the nature and stability of magnetic phases of doped intercalated WSe2 and PtSe2. We showed that, depending on the dopant, the stable magnetic phase at low temperature can be drastically different in both stability and type (ferro- or antiferromagnetic). We further showed that the presence of W or Pt vacancies in the lattice can be used to control the thermodynamic stability of the intercalated structures. Finally, we investigated the effect of the Pt vacancies on the magnetism in intercalated PtSe2. We showed that even though the spin polarization around the Pt atoms is very small, the Pt electronic cloud mediates longer magnetic interactions. Therefore, the presence or absence of Pt vacancies has a strong impact on the magnetic phases in the intercalated PtSe2. In the second part of this thesis, transport properties at a metal-2D semiconductor contact are the main topic. More specifically, I, along with my collaborators, have created a flexible model that can be used to efficiently simulate metal-2D semiconductor contacts and extract key parameters, such as the contact resistance. We studied the effects of device parameters, such as backgate bias, but also simulation parameters, such as the size of the simulation domain used to solve the Poisson equation. Crucially, we found that the contact resistance can be underestimated by over an order of magnitude when the Poisson domain is too small. In the final chapter, I provide an overview of the main achievements of the thesis and discuss potential avenues for future research.Item Al₂O₃ on WSe₂ by Ozone Based Atomic Layer Deposition: Nucleation and Interface StudyAzcatl, Angelica; Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Azcatl, Angelica; Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.In this work, the atomic layer deposition process using ozone and trimethylaluminum (TMA) for the deposition of Al₂O₃ films on WSe₂ was investigated. It was found that the ozone-based atomic layer deposition enhanced the nucleation of Al₂O₃ in comparison to the water/TMA process. In addition, the chemistry at the Al₂O₃ / WSe₂ interface and the surface morphology of the Al₂O₃ films exhibited a dependence on the deposition temperature. A non-covalent functionalizing effect of ozone on WSe₂ at low deposition temperatures 30 ⁰C was identified which prevented the formation of pinholes in the Al₂O₃ films. These findings aim to provide an approach to obtain high-quality gate dielectrics on WSe₂ for two-dimensional transistor applications.Item Atomic Scale Study of Polar Lomer-Cottrell and Hirth Lock Dislocation Cores in CdTe(International Union of Crystallography) Paulauskas, T.; Buurma, C.; Colegrove, E.; Stafford, B.; Guo, Z.; Chan, M. K. Y.; Sun, Kuei; Kim, Moon J.; Sivananthan, Sivalingam; Klie, R. F.Dislocation cores have long dominated the electronic and optical behaviors of semiconductor devices and detailed atomic characterization is required to further explore their effects. Miniaturization of semiconductor devices to nanometre scale also puts emphasis on a material's mechanical properties to withstand failure due to processing or operational stresses. Sessile junctions of dislocations provide barriers to propagation of mobile dislocations and may lead to work-hardening. The sessile Lomer-Cottrell and Hirth lock dislocations, two stable lowest elastic energy stair-rods, are studied in this paper. More specifically, using atomic resolution high-angle annular dark-field imaging and atomic-column-resolved X-ray spectrum imaging in an aberration-corrected scanning transmission electron microscope, dislocation core structures are examined in zinc-blende CdTe. A procedure is outlined for atomic scale analysis of dislocation junctions which allows determination of their identity with specially tailored Burgers circuits and also formation mechanisms of the polar core structures based on Thompson's tetrahedron adapted to reactions of polar dislocations as they appear in CdTe and other zinc-blende solids. Strain fields associated with the dislocations calculated via geometric phase analysis are found to be diffuse and free of 'hot spots' that reflect compact structures and low elastic energy of the pure-edge stair-rods.Item Atomically Thin Resonant Tunnel Diodes Built from Synthetic van der Waals Heterostructures(Nature Pub. Group) Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M.; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J.; Li, Lain-Jong; Wallace, Robert M.; Datta, Suman; Robinson, Joshua A.; A-5283-2008 (Wallace, RM); A-2297-2010 (Kim, MJ)Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂). The realization of MoS₂-WSe₂-graphene and WSe₂-MoS₂-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.;Item Creating a Single Twin Boundary between Two CdTe (111) Wafers with Controlled Rotation Angle by Wafer BondingSun, Ce; Lu, Ning; Wang, Jinguo; Lee, Jihyung; Peng, Xin; Klie, R. F.; Kim, Moon J.The single twin boundary with crystallographic orientation relationship (1‾ 1‾ 1 ‾)//(111) [0 1‾1]// [011‾] was created by wafer bonding. Electron diffraction patterns and high-resolution transmission electron microscopy images demonstrated the well control of the rotation angle between the bonded pair. At the twin boundary, one unit of wurtzite structure was found between two zinc-blende matrices. High-angle annular dark-field scanning transmission electron microscopy images showed Cd- and Te-terminated for the two bonded portions, respectively. The I-V curve across the twin boundary showed increasingly nonlinear behavior, indicating a potential barrier at the bonded twin boundary.Item Cubic Crystalline Erbium Oxide Growth on GaN(0001) by Atomic Layer Deposition(Amer Inst Physics, 2018-10-22) Chen, Pei-Yu; Posadas, Agham B.; Kwon, Sunah; Wang, Qingxiao; Kim, Moon J.; Demkov, Alexander A.; Ekerdt, John G.; Kwon, Sunah; Wang, Qingxiao; Kim, Moon J.Growth of crystalline Er₂O₃, a rare earth sesquioxide, on GaN(0001) is described. Ex situ HCl and NH₄OH solutions and an in situ N₂ plasma are used to remove impurities on the GaN surface and result in a Ga/N stoichiometry of 1.02. Using atomic layer deposition with erbium tris(isopropylcyclopentadienyl) [Er((^{i}PrCp)₃] and water, crystalline cubic Er₂O₃ (C-Er₂O₃) is grown on GaN at 250 ⁰C. The orientation relationships between the C-Er₂O₃ film and the GaN substrate are C-Er₂O₃(222)Item Engineering The Palladium-WSe₂ Interface Chemistry for Field Effect Transistors with High-Performance Hole Contacts(Amer Chemical Soc, 2018-12-07) Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); 0000-0003-0690-7423 (Young, CD); 0000-0003-2781-5149 (Kim, J); 0000-0002-6688-8626 (Walsh, LA); 0000-0002-5485-6600 (Hinkle, CD); 0000-0002-5454-0315 (Addou, R); 70133685 (Kim, J); Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.Palladium has been widely employed as a hole contact to WSe₂ and has enabled, at times, the highest WSe₂ transistor performance. However, there are orders of magnitude variation across the literature in Pd-WSe₂ contact resistance and I-ON/I-OFF ratios with no true understanding of how to consistently achieve high-performance contacts. In this work, WSe₂ transistors with impressive I-ON/I-OFF ratios of 10(6) and Pd-WSe₂ Schottky diodes with near-zero variability are demonstrated utilizing Ohmic-like Pd contacts through deliberate control of the interface chemistry. The increased concentration of a PdSeₓ intermetallic is correlated with an Ohmic band alignment and concomitant defect passivation, which further reduces the contact resistance, variability, and barrier height inhomogeneity. The lowest contact resistance occurs when a 60 min post-metallization anneal at 400 degrees C in forming gas (FG) is performed. X-ray photoelectron spectroscopy indicates this FG anneal produces 3x the concentration of PdSeₓ and an Ohmic band alignment, in contrast to that detected after annealing in ultrahigh vacuum, during which a 0.2 eV hole Schottky barrier forms. Raman spectroscopy and scanning transmission electron microscopy highlight the necessity of the fabrication step to achieve high-performance contacts as no PdSeₓ forms, and WSe₂ is unperturbed by room temperature Pd deposition. However, at least one WSe₂ layer is consumed by the necessary interface reactions that form PdSeₓ requiring strategic exploitation of a sacrificial WSe₂ layer during device fabrication. The interface chemistry and structural properties are correlated with Pd-WSe₂ diode and transistor performance, and the recommended processing steps are provided to enable reliable high-performance contact formation.Item Enhanced Shape Stability of Pd-Rh Core-Frame Nanocubes at Elevated Temperature: In Situ Heating Transmission Electron MicroscopyLu, Ning; Wang, Jinguo; Xie, S.; Xia, Y.; Kim, Moon J.Shape stability of Pd-Rh core-frame nanocubes was studied by in situ heating transmission electron microscopy. Pd-Rh nanocubes could maintain cubic shape at elevated temperature compared with pure Pd. The surface diffusion process of Rh onto {100} side surfaces is believed to postpone the degradation to higher temperature.Item Enhanced Thermal Conductivity in Cu/Diamond Composites by Tailoring the Thickness of Interfacial TiC Layer(Elsevier Ltd) Wang, Luhua; Li, J.; Catalano, Massimo; Bai, G.; Li, N.; Dai, J.; Wang, X.; Zhang, H.; Wang, Jinguo; Kim, Moon J.; Wang, Luhua; Catalano, Massimo; Wang, Jinguo; Kim, Moon J.Diamond particles reinforced Cu matrix (Cu/diamond) composites were fabricated by gas pressure infiltration using Ti-coated diamond particles with Ti coating from 65 nm to 850 nm. The scanning transmission electron microscopy (STEM) characterizes that the Ti coating transforms from elemental Ti to TiC after infiltration, and the crystallographic orientation relationship between diamond and TiC is [1 1 0]_{diamond}//[1 1 0]_{TiC} and (1 1 1)_{diamond}//(1 1 1)_{TiC}. The thermal conductivity of the Cu/Ti-diamond composites firstly increases and then decreases with increasing Ti coating thickness, giving a maximal value of 811 W m⁻¹ K⁻¹ at 220 nm Ti-coating layer. The results clearly manifest the effect of interfacial layer thickness on the thermal conductivity of Cu/diamond composites.Item Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature(Wiley-VCH Verlag, 2019-06-11) Lo, C. -L; Catalano, Massimo; Khosravi, Ava; Ge, W.; Ji, Y.; Zemlyanov, D. Y.; Wang, Luhua; Addou, Rafik; Liu, Y.; Wallace, Robert M.; Kim, Moon J.; Chen, Z.; 0000-0001-5566-4806 (Wallace, RM); Catalano, Massimo; Khosravi, Ava; Wang, Luhua; Addou, Rafik; Wallace, Robert M.; Kim, Moon J.The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaSₓ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaSx ultrathin film is industry-friendly, BEOL-compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaSₓ compared to the TaN/Ta bilayer. This single-stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimItem Facile Synthesis of Pd-Ir Bimetallic Octapods and Nanocages Through Galvanic Replacement and Co-Reduction, and their use for Hydrazine DecompositionLiu, M.; Zheng, Y.; Xie, S.; Li, N.; Lu, Ning; Wang, Jinquo; Kim, Moon J.; Guo, L.; Xia, Y.This article describes a facile synthesis of Pd-Ir bimetallic nanostructures in the forms of core-shell octapods and alloyed nanocages. The success of this synthesis relies on the use of Pd nanocubes as the sacrificial templates and interplay of two different processes: the galvanic replacement between an Ir precursor and the Pd nanocubes and the co-reduction of Pd²⁺ and Ir³⁺ by ethylene glycol. The galvanic replacement played a dominant role in the initial stage, through which Pd atoms were dissolved from the side faces whereas Ir atoms were deposited at the corner sites to generate Pd-Ir core-shell octapods. As the concentration of Pd²⁺ in the reaction mixture was increased, co-reduction of Pd²⁺ and Ir³⁺ occurred in the late stage of synthesis. The resultant Pd and Ir atoms were deposited onto the octapods while the Pd atoms in the interiors continued to be etched away due to the galvanic replacement, finally leading to the formation of Pd-Ir alloyed nanocages. The octapods and nanocages were then evaluated as catalysts for the selective generation of hydrogen from the decomposition of hydrous hydrazine. The nanocages exhibited better selectivity for hydrogen generation than octapods (66% versus 29%), which can be attributed to the presence of an alloyed, porous structure on the surface.Item In Situ Characterization of Phase Transition and Defect Dynamics in Molybdenum Ditelluride(2020-04-17) Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.Transition metal dichalcogenides (TMDs) are regarded as promising materials for emerging applications, including electronic devices, photonic devices, biosensors, and energy storage, etc. Owing to their novel structures and extraordinary properties, they have provided the researchers with an excellent platform to explore low-dimensional physics. However, some challenges need to be resolved before their practical application. The phases and defects in TMDs can significantly affect their properties. Therefore, understanding the phase transition and defects in TMDs would be of great importance to advance their further application. This dissertation focuses on the identification and characterization of a novel phase transition from two dimensional MoTe2 phase to one dimensional Mo6Te6 nanowire phase during the vacuum annealing. Furthermore, the thermal stability of MoTe2 is extensively investigated. In particular, the inversion domain boundaries formed during the vacuum annealing are identified to be a defective interface in MoTe2. The role of Te vacancy to the evolution of inversion domain boundaries is extensively studied. Also, a possible strategy to improve its thermal stability is demonstrated.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 Low Temperature Synthesis of Graphite on Ni Films Using Inductively Coupled Plasma Enhanced CVD(Royal Soc Chemistry) Cheng, Lanxia; Yun, Kayoung; Lucero, Antonio; Huang, Jie; Meng, Xin; Lian, Guoda; Nam, Ho-Seok; Wallace, Robert M.; Kim, Moon J.; Venugopal, Archana; Colombo, Luigi; Kim, Jiyoung; A-5283-2008 (Wallace, RM); A-2297-2010 (Kim, MJ); 70133685 (Kim, J)Controlled synthesis of graphite at low temperatures is a desirable process for a number of applications. Here, we present a study on the growth of thin graphite films on polycrystalline Ni films at low temperatures, about 380 ⁰C, using inductively coupled plasma enhanced chemical vapor deposition. Raman analysis shows that the grown graphite films are of good quality as determined by a low I-D/I-G ratio, ~0.43, for thicknesses ranging from a few layers of graphene to several nanometer thick graphitic films. The growth of graphite films was also studied as a function of time, precursor gas pressure, hydrogen concentration, substrate temperature and plasma power. We found that graphitic films can be synthesized on polycrystalline thin Ni films on SiO₂/Si substrates after only 10 seconds at a substrate temperature as low as 200 ⁰C. The amount of hydrogen radicals, adjusted by changing the hydrogen to methane gas ratio and pressure, was found to dramatically affect the quality of graphite films due to their dual role as a catalyst and an etchant. We also find that a plasma power of about 50 W is preferred in order to minimize plasma induced graphite degradation.Item Luminescent LaF₃:Ce-Doped Organically Modified Nanoporous Silica XerogelsYao, Mingzhen; Hall, Ryan; Chen, Wei; Mohite, Dhairyashil P.; Leventis, Nicholas; Lu, Ning; Wang, Jinguo; Kim, Moon J.; Luo, Huiyang; Lu, HongbingOrganically modified silica compounds (ORMOSILs) were synthesized by a sol-gel method from amine-functionalized 3-aminopropyl triethoxylsilane and tetramethylorthosilicate and were doped in situ with LaF3:Ce nanoparticles, which in turn were prepared either in water or in ethanol. Doped ORMOSILs display strong photoluminescence either by UV or X-ray excitation and maintain good transparency up to a loading level of 15.66% w/w. The TEM observations demonstrate that ORMOSILs remain nanoporous with pore diameters in the 5-10 nm range. LaF3:Ce nanoparticles doped into the ORMOSILs are rod-like, 5 nm in diameter and 10-15 nm in length. Compression testing indicates that the nanocomposites have very good strength, without significant lateral dilatation and buckling under quasi-static compression. LaF3:Ce nanoparticle-doped ORMOSILs have potential for applications in radiation detection and solid state lighting.Item Metal-Organic Chemical Vapor Deposition of High Quality, High Indium Composition N-Polar InGaN Layers for Tunnel Devices(American Institute of Physics Inc, 2018-08-24) Lund, C.; Romanczyk, B.; Catalano, Massimo; Wang, Qingxiao; Li, W.; DiGiovanni, D.; Kim, Moon J.; Fay, P.; Nakamura, S.; DenBaars, S. P.; Mishra, U. K.; Keller, S.; Catalano, Massimo; Wang, Qingxiao; Kim, Moon J.In this study, the growth of high quality N-polar InGaN films by metalorganic chemical vapor deposition is presented with a focus on growth process optimization for high indium compositions and the structural and tunneling properties of such films. Uniform InGaN/GaN multiple quantum well stacks with indium compositions up to 0.46 were grown with local compositional analysis performed by energy-dispersive X-ray spectroscopy within a scanning transmission electron microscope. Bright room-temperature photoluminescence up to 600 nm was observed for films with indium compositions up to 0.35. To study the tunneling behavior of the InGaN layers, N-polar GaN/In0.35Ga0.65N/GaN tunnel diodes were fabricated which reached a maximum current density of 1.7 kA/cm2 at 5 V reverse bias. Temperature-dependent measurements are presented and confirm tunneling behavior under reverse bias. © 2017 Author(s).Item Modelling topological and magnetic materials for charge and spin-based devices(2022-05-01T05:00:00.000Z) Tiwari, Sabyasachi; Vandenberghe, William; Wong, W. Eric; Fischetti, Massimo V.; Kim, Moon J.; Soree, BartThe imminent halt of Moore’s law and discontinuation of scaling of transistors based on three-dimensional materials, e.g., silicon, has prompted researchers to look for different ma- terials and device systems apart from the conventional ones to form the backbone of the electronics industry of the future. Topological insulators (TIs) open a vast avenue to realize devices with high ON current and low power consumption. TIs are a class of materials with topologically protected edge states which are spin-polarized and robust against impurity scattering. The possibility of spin-polarization in TIs and efficient transfer of spin-current in soft-layered magnets opens another avenue of research for realizing fast memory devices. In this dissertation, first, we model carrier transport through imperfect two-dimensional (2D) TI ribbons. In particular, we investigate the impact of vacancy defects on the carrier trans- port of 2D TIs. We show that carrier transport through the topologically protected edge states is robust against a high percentage of defects (up to 2%), whereas the carrier trans- port through the bulk state is strongly suppressed due to backscattering. We show that the suppression of bulk transport in 2D TIs can be used to design devices using 2D TI ribbons. Next, we develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature from the first principles, taking into account both the magnetic anisotropy as well as the out-of-plane interactions. We ap- ply our method on Cr-compounds: CrI3, CrBr3, and CrGeTe3, and FeCl2, and show that our predictions match well with experimental values. Using the same model we next inves- tigate the magnetic order in two-dimensional (2D) transition-metal-dichalcogenide (TMD) monolayers: MoS2, MoSe2, MoTe2, WSe2 , and WS2 substitutionally doped with period-four transition-metals (Ti, V, Cr, Mn, Fe, Co, Ni). We show that five distinct magnetically or- dered states can exist among the 35 distinct TMD-dopant combinations including the non- magnetic (NM), the ferromagnetic (FM) with out-of-plane spin polarization (Z FM), the out-of-plane polarized clustered FMs (clustered Z FM), the in-plane polarized FMs (X–Y FM), and the anti-ferromagnetic (AFM) state. Most remarkably, we find from our study that V-doped MoSe2 and WSe2, and Mn-doped MoS2, are the most suitable candidates for realizing a room-temperature FM at a 16–18% atomic substitution. We then compare three first-principles methods (the MC, the Green’s function, and the RNSW) of calculating the Curie temperature in 2D FMs in the presence of exchange anisotropy, modeled using the Heisenberg model. We find that the Curie temperature obtained from the Green’s function in high-anisotropy regimes is higher compared to MC, whereas the Curie temperature cal- culated using the renormalized spin-waves (RNSW) is lower compared to the MC and the Green’s function for all anisotropies. Finally, we present a theoretical model to simulate spin- dynamics and spin-induced switching in a semiconductor-ferromagnet heterostructure. Our theoretical model combines the non-equilibrium Green’s function method for spin-dependent electron transport and time-quantified Monte-Carlo for simulating magnetization dynamics. We use the adiabatic approximation for combining the electron dynamics and the magne- tization dynamics. We study spin-induced switching in a 2D TI-FM interface. Finally, we show that for a certain range of magnetic exchange parameters (or certain materials), it is possible to change magnetic domains in a 2D FM using spin-torque from TIs, which can be used for designing high-speed memories.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 New Intrinsic Mechanism on Gum-Like Superelasticity of Multifunctional AlloysLiu, J. -P; Wang, Y. -D; Hao, Y. -L; Wang, Y.; Nie, Z. -H; Wang, D.; Ren, Y.; Lu, Z. -P; Wang, Jinguo; Wang, H.; Hui, X.; Lu, Ning; Kim, Moon J.; Yang, R.Ti-Nb-based Gum Metals exhibit extraordinary superelasticity with ultralow elastic modulus, superior strength and ductility, and a peculiar dislocation-free deformation behavior, most of which challenge existing theories of crystal strength. Additionally, this kind of alloys actually displays even more anomalous mechanical properties, such as the non-linear superelastic behavior, accompanied by a pronounced tension-to-compression asymmetry, and large ductility with a low Poisson's ratio. Two main contradictory arguments exist concerning the deformation mechanisms of those alloys, i.e., formation of reversible nanodisturbance and reversible martensitic transformation. Herein we used the in-situ synchrotron high-energy X-ray scattering technique to reveal the novel intrinsic physical origin of all anomalous mechanical properties of the Ti-24Nb-4Zr-8Sn-0.10O alloy, a typical gum-like metal. Our experiments provide direct evidence on two different kinds of interesting, stress-induced, reversible nanoscale martensitic transitions, i.e., the austenitic regions with B2 structure transform to α" martensite and those with BCC structure transform to ō martensite.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).