Browsing by Author "Zhu, Hui"
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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 EMIC Wave Parameterization in the Long-Term Verb Code Simulation(Amer Geophysical Union) Drozdov, A. Y.; Shprits, Y. Y.; Usanova, M. E.; Aseev, N. A.; Kellerman, A. C.; Zhu, Hui; 0000-0003-3556-8096 (Zhu, H); Zhu, HuiElectromagnetic ion cyclotron (EMIC) waves play an important role in the dynamics of ultrarelativistic electron population in the radiation belts. However, as EMIC waves are very sporadic, developing a parameterization of such wave properties is a challenging task. Currently, there are no dynamic, activity-dependent models of EMIC waves that can be used in the long-term (several months) simulations, which makes the quantitative modeling of the radiation belt dynamics incomplete. In this study, we investigate Kp, Dst, and AE indices, solar wind speed, and dynamic pressure as possible parameters of EMIC wave presence. The EMIC waves are included in the long-term simulations (1 year, including different geomagnetic activity) performed with the Versatile Electron Radiation Belt code, and we compare results of the simulation with the Van Allen Probes observations. The comparison shows that modeling with EMIC waves, parameterized by solar wind dynamic pressure, provides a better agreement with the observations among considered parameterizations. The simulation with EMIC waves improves the dynamics of ultrarelativistic fluxes and reproduces the formation of the local minimum in the phase space density profiles.Item Modeling Energetic Electron Nonlinear Wave-Particle Interactions with Electromagnetic Ion Cyclotron Waves(Blackwell Publishing Ltd, 2019-04-15) Zheng, Liheng; Chen, Lunjin; Zhu, Hui; 0000-0003-2489-3571 (Chen, L); 0000-0001-9068-4431 (Zheng, L); 0000-0003-3556-8096 (Zhu, H); Zheng, Liheng; Chen, Lunjin; Zhu, HuiElectromagnetic ion cyclotron (EMIC) waves in duskside plasmasphere and plasmaspheric plume scatter megaelectron volt electrons into the loss cone and are considered a major loss mechanism for the outer radiation belt. Wave-particle interaction between energetic electrons and EMIC waves has been studied extensively by the quasi-linear diffusion theory. However, EMIC waves are typically strong enough to trigger nonlinear wave-particle interaction effects and transport electrons in very different ways from quasi-linear diffusion. New mathematical method is therefore in demand to study the evolution of energetic electron distribution in response to nonlinear wave-particle interaction. In this work, we present a Markov chain description of the wave-particle interaction process, in which the electron distribution is represented by a state vector and is evolved by the Markov matrix. The Markov matrix is a matrix form of the electron response Green's function and could be determined from test particle simulations. Our modeling results suggest that electron loss rate is not significantly affected by phase bunching and phase trapping, but for strong EMIC waves, electron distribution is more saturated near loss cone than quasi-linear theory prediction, and negative electron phase space density slope develops inside loss cone. ©2019. American Geophysical Union. All Rights Reserved.Item Modulation of Locally Generated Equatorial Noise by ULF Wave(Blackwell Publishing Ltd, 2019-04-23) Zhu, Hui; Chen, Lunjin; Liu, Xu; Shprits, Y. Y.; 0000-0003-3556-8096 (Zhu, H); 0000-0003-2489-3571 (Chen, L); 0000-0002-7211-0546 (Liu, X); Zhu, Hui; Chen, Lunjin; Liu, XuIn this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon. ©2019. American Geophysical Union. All Rights Reserved.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 Surface and Interface Characterization of 2D Materials: Transition Metal Dichalcogenide and Black Phosphorous(2017-12) Zhu, Hui; 0000-0003-3556-8096 (Zhu, H); Wallace, Robert M.Transition metal dichalcogenides (TMDs) and black phosphorous (black-P) are representative two dimensional (2D) materials with versatile electronic, optical, physical, and chemical properties that can be manipulated for novel electronic and optoelectronic device applications in nanoscale science and technology. However, many challenges remain associated with the nature of defects, crystal synthesis, thickness control, chemical stabilities, doping strategies, and Schottky contacts. This dissertation focuses on the surface and interface understanding of 2D materials to propose desired attributes and surface engineering to overcome those challenges. In particular, the interfacial qualities between the atomic layer deposited Al₂O₃ and black-P, MoTe₂, or WTe₂ are investigated by in situ X-ray photoelectron spectroscopy to lower the interfacial damage possibility. Then the chemical and structural properties of MoS₂ under remote O₂ plasma and thermal treatments are studied to propose a two-step atomic layer etching method. Also, the thermal and structural properties of MoTe₂ are investigated to avoid thermal damage as well as explore possible phase engineering applications.