Browsing by Subject "2D materials"

Now showing 1 - 7 of 7
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    Atomic scale electro-mechanics of two dimensional materials using modeling and simulations
    (2018-06) Verma, Deepti
    Objective molecular dynamics (OMD) is a generalization of the universally adopted periodic boundary conditions (PBCs) used in atomic simulations. By taking advantage of the translational symmetry, molecular dynamics under PBCs reduces the overall number of atoms that need to be simulated in crystalline bulk materials. Unfortunately, PBCs are not generally applicable to structures having helical and rotational symmetries. OMD replaces the use of translational symmetry with helical and rotational symmetries, which are widely present in a number of nanostructures and nano materials. Such structures include carbon nanotubes, mechanically deformed nanomaterials such as 2D films subjected to bending and torsion. The recent capability resulted from the coupling of self-consistent charge density functional tight binding (SCC-DFTB) with OMD enables unprecedented calculations on helical and mechanically deformed two-dimensional materials with quantum mechanical accuracy. This coupling is especially important for simulating materials with complex bonding such as zinc oxide thin films, which contains charged species. It also captures the charge redistribution under mechanical deformation in layer bending. In this thesis, we use this capability to simulate bending of 2D materials and build continuum elastic models based on the atomistic data. These structures of interest include phosphorene, boron nitride and zinc oxide. Mechanical properties of 2D materials are studied mainly via bending deformations. Bending deformations lead to charge distribution across the thickness in two-dimensional materials. This effect can produce structural effects in nano-films. Zinc oxide layers are important because of their properties of piezo-electric effects with potential applications as important energy materials. In addition, we also study bending and in-plain deformation of boron nitride. This understanding of bending deformations will help advance the development of strain engineering technology for these 2D materials.
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    Coupling of far-infrared light to the surface phonon-polaritons in Transition metal dichalcogenides
    (2021-05) Saha, Subhodip
    Enhanced light-matter interactions through a plethora of dipole-typepolaritonic excitations started to emerge in two-dimensional (2D) layered materials in recent years. 2D Van der Waals (vdWs) polar crystals sustaining phonon-polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications within the mid-infrared (mid-IR) to terahertz ranges. One of the fundamental hurdles in polaritonics is the trade-off between electromagnetic field confinement and the coupling efficiency with free-space light, a consequence of the large momentum mismatch between the excitation source and polaritonic modes. Our group recently demonstrated the fundamental problem of momentum mismatch can be overcome with a graphene acoustic plasmon resonator with nearly perfect absorption (94%) of incident mid-infrared light. This high efficiency is achieved by utilizing a two-stage coupling scheme: free-space light coupled to conventional graphene plasmons, which then couple to ultraconfined acoustic plasmons. To date, experimental demonstration of excitation of the surface phonon-polaritons (SPhPs) in transition metal dichalcogenides (TMDs) remains unexplored, so here we demonstrate novel strategies for dynamically controlling of far-infrared light using unique optical properties of TMDs in particular ZrS2. In comparison to other vdW materials, the phonon modes of these TMDs are much softer and exhibits phonon peaks within the far-infrared (far-IR) regime. Here we experimentally demonstrate the excitation of SPhPs in ZrS2 acoustic resonator by the far-IR absorption spectroscopy. The surface optical (SO) phonon modes of these TMDs were tuned and tailored to lie anywhere within the reststrahlen band, by controlling the ribbon width, which brings extreme tunability. In addition, the absorption can be pushed beyond 90% with the integration of gold reflectors and can become promising material for far-IR biosensing. The results demonstrate TMDs as a new platform for studying phonon-polaritons exhibiting good quality factors and excellent tunability which enable far-IR nanophotonics devices. Recently our group demonstrated ultra strong coupling (USC) between polar phonons and mid-IR light in coaxial nanocavities. Here we push the boundary further up to the far-IR range and we propose to demonstrate USC coupling between polar phonons and far-IR light using the coaxial nanocavity platform. Our numerical simulation predicts a level splitting of strongly coupled polaritons of 95% of the resonant frequency, enabled by epsilon-near-zero (ENZ) responses in the far-IR of the ZrS2 filled coaxial nanocavities. The ability to reach the USC regime in mass-produced nanocavity systems can open up new avenues to explore non-perturbatively coupled light-matter systems, multiphoton effects, as well as higher-order nonlinear effects, which may lead to novel applications in sensing, spectroscopy, and nanocavity optomechanics.
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    Data supporting Holey Substrate-Directed Strain Pattering in Bilayer MoS2
    (2021-11-10) Zhang, Yichao; Choi, Moon-Ki; Haugstad, Greg; Tadmor, Ellad B; Flannigan, David J; flan0076@umn.edu; Flannigan, David J
    This data set contains transmission electron microscopy (TEM), atomic force microscopy (AFM), and atomistic simulation data supporting "Holey Substrate-Directed Strain Pattering in Bilayer MoS2" manuscript cited in referenced by.
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    Engineering Novel Transistors Based On Black Phosphorus
    (2019-02) Robbins, Matthew
    Black phosphorus (BP), a layered 2D semiconductor that can be isolated to one monolayer thicknesses re-emerged in 2014 because of its promise for use in applications such as high performance MOSFETs, optoelectronic devices, novel devices like tunneling-field-effect-transistors (TFETs), and flexible electronics. The promise of BP comes from its unique material properties such as a high mobility, crystal anisotropy, a tunable direct band gap, an anisotropic effective mass, and the ability to scale to sub-1 nm thicknesses while retaining good electronic properties. These properties make BP particularly interesting as a possible post-silicon channel material in advanced logic transistors which could enable the continuation of transistor scaling beyond the foreseeable future. However, most experimental demonstrations of BP transistors have displayed poor OFF-state performance caused by gate-induced-drain-leakage (GIDL) which limits the device's overall usefulness. In this dissertation, novel BP transistors that utilize the unique properties of BP to improve OFF-state performance are demonstrated. These novel devices include a heterostructure BP MOSFET which utilizes the thickness-tunable band gap of BP to supress GIDL current, an electrostatically doped BP MOSFET which takes advantage the thin body of BP with a novel device structure used to effectively dope the source and drain regions of the BP, to again suppress GIDL current, and BP TFETs which utilize the anisotropic effective mass in order to open a path for realizing transistors with a subthreshold slope (SS) of less than 60 mV/dec in BP.
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    Modeling of Transport Phenomena in Two-Dimensional Semiconductors
    (2016-12) Liu, Yue
    Recently, transition metal dichalcogenides and black phosphorus (BP) emerged as new 2D semiconductors due to the advantages of moderate energy band gap, high carrier mobility, ultra thin film and high anisotropy. Together with graphene, 2D materials have been utilized in the development of biomedical devices, touch screen and display technologies, and flexible applications such as wearable electronics and IoT devices. They also open up new opportunities in research fields including spintronics, optoelectronics and next generation post-silicon transistor. In this dissertation, we present theoretical modeling for several topics related to 2D materials. Starting with the fundamental tight-binding theory of graphene, we review electronic properties for graphene including massless 2x2 Dirac Hamiltonian and pseudo-spin wave function. Followed by discussion of ballistic transport, a detailed analysis on graphene diffusive transport is provided. Ionized impurity scattering and carrier screening effect is considered in the model. The momentum relaxation time and mobility for graphene is modeled. A non-linear Thomas Fermi screening is introduced to improve the simulation accuracy. Taking the real spin into account, the new Hamiltonian is a 4x4 matrix. An external field perpendicular to the graphene breaks the reflection symmetry and introduces a Rashba spin-orbit interaction, which couples pseudo-spin and real spin. The relevant charge carrier states are no longer spin eigenstates. Rashba interaction is found to be quite small compared to Coulomb impurity scattering. To characterize the spin-polarized electrons tunneling from electrodes and transport in graphene, a spin valve device modeling and magnetoresistance calculation is developed. Black phosphorus possesses excellent properties like other 2D materials for high performance nanoelectronic applications. Moreover, there is a uniquely high in-plane anisotropy in BP due to its puckered crystal structure. To model the anisotropic transport, a model based on the BTE is developed, considering the full anisotropic electronic structure. For zero temperature calculation with ionized impurity limited scattering, anisotropy ratio 3-4 can be obtained from the model. Due to the dominating effect of screening, mobility is found to decrease weakly with increasing temperature. For , a smaller anisotropy ratio of 1.8-3.5 matching experimental measurements indicates that impurity scattering is an important mechanism for black phosphorus.
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    Supporting data for Atomistically-informed continuum modeling and isogeometric analysis of 2D materials over holey substrates
    (2022-12-20) Choi, Moon-ki; Pasetto, Marco; Shen, Zhaoxiang; Tadmor, Ellad; Kamensky, David; choi0652@umn.edu; Choi, Moon-ki; University of Minnesota Tadmor group; University of California San Diego Kamensky group
    Data includes LAMMPS input script for MoS2 test problems and MATLAB data for generating figures in the paper.
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    Two-Dimensional Black Phosphorus for High Performance Field Effect Transistors
    (2017-06) Haratipour, Nazila
    Two-dimensional (2D) materials are a potential platform for scaled logic devices, sensor applications, flexible electronics and other innovative device concepts. Black phosphorus (BP) has recently emerged as a new promising layered semiconductor due to its unique material properties. BP has high electron and hole mobility, tunable band-gap ranging from 0.3 eV (bulk) to 1-2 eV (monolayer) and highly asymmetric effective mass. BP metal-oxide-semiconductor field-effect transistors (MOSFETs) have the potential to outperform other 2D semiconductors mainly due to the lighter effective mass of BP, which leads to higher mobility, and narrower band gap, which can reduce contact resistance due to the Schottky barrier height lowering. In this dissertation, BP n- and p-type MOSFETs with record performance are demonstrated. A comprehensive experimental and theoretical evaluation of the design and operating parameters that limit the off-state performance and subthreshold slope in BP MOSFETs is performed. Next, for the first time, the effect of asymmetric crystal orientation on BP MOSFET performance is quantified and the anisotropic mobility in a realistic MOSFET geometry is analyzed. Finally, contact engineering is utilized to achieve record-low contact resistance in BP p-MOSFETs.