Browsing by Author "Nie, Yifan"
Now showing 1 - 6 of 6
- Results Per Page
- Sort Options
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 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 Quantum Transport and Band Structure Evolution under High Magnetic Field in Few-Layer Tellurene(American Chemical Society) Qiu, G.; Wang, Y.; Nie, Yifan; Zheng, Yongping; Cho, Kyeongjae; Wu, W.; Ye, P. D.; 0000-0003-4771-3633 (Nie, Y); 0000-0003-2698-7774 (Cho, K); 369148996084659752200 (Cho, K); Nie, Yifan; Zheng, Yongping; Cho, KyeongjaeQuantum Hall effect (QHE) is a macroscopic manifestation of quantized states that only occurs in confined two-dimensional electron gas (2DEG) systems. Experimentally, QHE is hosted in high-mobility 2DEG with large external magnetic field at low temperature. Two-dimensional van der Waals materials, such as graphene and black phosphorus, are considered interesting material systems to study quantum transport because they could unveil unique host material properties due to the easy accessibility of monolayer or few-layer thin films at the 2D quantum limit. For the first time, we report direct observation of QHE in a novel low-dimensional material system, tellurene. High-quality 2D tellurene thin films were acquired from recently reported hydrothermal method with high hole mobility of nearly 3000 cm2/(V s) at low temperatures, which allows the observation of well-developed Shubnikov-de Haas (SdH) oscillations and QHE. A four-fold degeneracy of Landau levels in SdH oscillations and QHE was revealed. Quantum oscillations were investigated under different gate biases, tilted magnetic fields, and various temperatures, and the results manifest the inherent information on the electronic structure of Te. Anomalies in both temperature-dependent oscillation amplitudes and transport characteristics were observed that are ascribed to the interplay between the Zeeman effect and spin-orbit coupling, as depicted by the density functional theory calculations. ©2018 American Chemical Society.Item 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, KyeongjaeTwo-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.;Item 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, KyeongjiaeScanning 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.