Browsing by Subject "MoS2"
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Item Applications of Transmission Electron Microscopy on Free-standing and Embedded Two-dimensional Materials(2019-12) Wu, RyanIn the last decade, 2D nanosheets, more commonly referred to as 2D, layered, or van der Waals materials, have garnered significant scientific interest because of their novel material properties at the nanoscale regime compared to their bulk. Their rise in popularity is commonly attributed to the isolation and study of graphene by Geim and Novoselov in 2004 for which they were awarded the Nobel prize in physics in 2010. Since then, more than 1000 unique 2D chemical compounds have been at least theorized if not experimentally isolated. Many of these materials exhibit favorable mechanical, optical, or electronic properties that may also be tunable by controlling their number of layers. With novel materials being continuously synthesized and applied at such a feverish pace, there exists a critical need to characterize and understand the structures and properties of these novel materials that may have been nothing but theoretical predictions a mere decade ago. Herein, analytical scanning transmission electron microscopy (STEM) supported by computational methods is used to study the atomic and electronic structure of numerous free standing 2D materials as well as 2D materials embedded in devices with a spatial resolution of < 1 Å and an energy resolution of < 0.5 eV. Two computational applications are first presented to introduce and highlight the complexities of electron-sample interactions which can be used to extract additional information from experimental results. The first uses experimentally observed Moire patterns to correlate and understanding rotational misalignments of Bi2Se3; the second exploits the channeling of the electron beam in addition to sample tilt to determine the thickness of atomically thin MoS2. The thickness determination method is then experimentally proven using annular dark field-STEM imaging (ADF-STEM) and applied to MoS2 layers of various thicknesses to test the limits of measuring layer-dependent properties in the TEM using electron energy loss spectroscopy (EELS). Subsequently, the atomic and electronic structure of black phosphorus is thoroughly examined using STEM. Its crystal structure including its lattice parameters and stacking order is unambiguously determined by ADF-STEM. Its electronic structure including its conduction band density of states and plasmon excitations are measured using EELS and compared to density functional theory (DFT) calculations. Additionally, the effect of oxidation, a well-known phenomenon when using black phosphorus, on its properties is measured using a similar approach. The results, as measured using the aforementioned techniques in addition to energy dispersive x-ray spectroscopy (EDX), show that oxidation amorphizes black phosphorus transforming the semiconductor into an insulating oxide. Finally, STEM-EELS is applied to study 2D material embedded field effect transistors (FET) in cross-section. Using a layer-by-layer approach, the interactions between MoS2 and metal device contacts are measured to show the non-idealities of the contact/channel interface. These results, supplemented by DFT calculations, are used to understand the phenomenon of Fermi level pinning and the interaction of the metal contact with the MoS2 layers when deposited onto its surface. The results suggest that the chemistry of the metal-MoS2 bond is important in determining the efficacy of the FET and point toward the ultimate limits of which metals and alloys can and cannot be used when ultra-thin mono- and bi- layer MoS2 channels are desired.Item Modeling and Fabrication of Low Power Devices and Circuits Using Low-Dimensional Materials(2016-07) Kshirsagar, ChaitanyaAs silicon approaches its ultimate scaling limit as a channel material for conventional semiconductor devices, alternate mechanisms and materials are emerging rapidly to replace or complement conventional silicon based devices. Attractive semiconducting properties such as high mobility, excellent interface quality, and better scalability are the properties desired for materials to be explored for electronic and photonic device applications. Hybrid III-V semiconductor based tunneling field effect transistors (TFETs) can provide a strong alternative due to their attractive properties such as subthreshold slopes less than 60 mV/decade, which can lead to aggressive power supply scaling. Here, InAs-SiGe-Si based TFETs are studied in detail. Simulations predict that subthreshold slopes as low as 18 mV/decade and on currents as high as 50 µA/µm can be achieved using such a device. However, the simulations also show that the device performance is limited by (1) the low density of states in the source which induces a trade-off between the source doping and the subthreshold slope, limiting power supply scaling, and (2) direct source-to-drain tunneling which limits gate length scaling. Another approach to explore low power alternatives to conventional semiconductor device can be to use emerging two-dimensional (2D) materials. In particular, the transition metal dichalcogenides (TMDs) are promising material group that, like graphene, these material exhibit 2D nature, but unlike graphene, have a finite band gap. In this work, the off-state characteristics are modelled for MoS2 MOSFETs (metal–oxide–semiconductor field-effect transistors), and their circuit performance is predicted. MoS2 Due to its higher effective masses and large band gap compared to silicon it is shown that MoS2 MOSFETs are well suited for dynamic memory applications. Two of such circuits, one transistor one capacitor (1TIC) and two transistor (2T) dynamic memory cells have been fabricated for the first time. Retention times as high as 0.25 second and 1.3 second for the 1T1C and 2T cell, respectively, are demonstrated. Moreover, ultra-low leakage currents less than femto-ampere per micron are extracted based on the retention time measurements. These results establish the potential of 2D MoS2 as an attractive material for low power device and circuit applications.Item Nanoscale Coherent-Acoustic-Phonon Dynamics in Molybdenum Disulfide Using Ultrafast Electron Microscopy(2021-06) Zhang, YichaoIn this dissertation, photoexcited, defect-mediated anisotropic acoustic-phonon dynamics in molybdenum disulfide (MoS2) have been directly imaged on the nanometer-picosecond spatiotemporal scales in real space. MoS2 is a prototypical material system of transition metal dichalcogenides extensively studied due to the exceptional tunability of many properties (e.g., electronic band structure) via layer number and strain, and thus attracts interests in a broad range of device applications. Defects have been demonstrated to impact local properties and dynamics in a spatial range from a single atom to hundreds of nanometers. The combined nanometer spatial resolution and femtosecond temporal resolution of ultrafast electron microscopy (UEM) enables direct visualization of photoinduced anisotropic acoustic-phonon dynamics localized at nanoscale defects in freestanding, multilayer MoS2.Excitation of the phonon dynamics is achieved via uniformly illuminating the specimens with 515-nm, 300-fs laser pulses. Propagation of the waves locally deforms the crystal lattice, leading to a modulation of the local Bragg conditions. Visualization of the waves is thus achieved by monitoring modulation of diffraction contrast features (e.g., bend contours). When viewed along the [0001] direction, photoexcited individual phonon wave trains were observed to be emitted from crystal-crystal interfaces (e.g., step edge and terrace) and propagate along a single wave vector perpendicular to the interfaces at frequencies in the tens of gigahertz (GHz) range and at approximately the in-plane speed of sound (7 nm/ps). When viewed along a high-index zone axis (i.e., large angle between specimen normal and the incident electron wave vector), the observed diffraction-contrast dynamics exhibits no propagations but only oscillations. Such specimen configuration allows for projection of c-axis wave dynamics onto the image plane. The c-axis phonon velocity was extracted from the oscillation frequency and specimen thickness, consistent with c-axis speed of sound (2.9 nm/ps). The onset of the c-axis dynamics occurs a few picoseconds earlier than that of the in-plane dynamics, suggesting photoinduced modulation of interlayer spacing leading to launching of in-plane compression waves. In addition to serving as launching sites of phonon wave trains, step edges can mediate localized dynamics that are distinct from that observed in pristine regions. The c-axis phonon modes exhibit dephased oscillations at an individual step edge owing to different specimen thicknesses, and even one-unit-cell of thickness difference manifests a few GHz of frequency shift. Step edges have also been demonstrated to induce new relaxation states extending to several hundred nanometers. Subsequently, a frequency reduction in the c-axis phonon oscillation (i.e., phonon softening) at an individual, nanoscale step edge was observed, indicative of associated softening of the elastic constant. This is consistent with results obtained with finite element modeling. In the process of preparing ultrathin, pristine specimens for studying photoinduced structural dynamics in mono- and bilayer MoS2, substrate-directed spontaneous strain patterning was observed in the term of twelve-fold zone-axis patterns and six-fold centroidal Voronoi tessellation patterns. Vertical deformation of up to 35 nm for several bilayer MoS2 crystals were measured with atomic force microscopy. The formation mechanism of such pattern was elucidated with atomistic simulations.Item Strongly-Bound Excitons In Transition Metal Dichalcogenides And Organic Semiconductors(2020-05) Schulzetenberg, AaronAtomically-thin, semiconducting transition metal dichalcogenides (TMDs) and organic semiconductors such as rubrene hold exceptional promise for unique and niche electronic applications which cannot be solved with conventional semiconducting crystalline materials. In particular, the process by which excitons relax in thin TMDs controls device engineering considerations including charge carrier mobility and exciton diffusion length. The decay mechanism and time scales can critically depend on interfaces, method of sample preparation and temperature. Here, I present ultrafast transient reflectivity studies of several chemical vapor deposition (CVD) grown TMD structures, including few-layer 2H MoTe2 on SiO2, MoTe2 1T’-2H homojunctions and monolayer MoS2-WS2 lateral heterojunctions on sapphire. The transient reflectivity of CVD-grown, few-layer (5-10 layers) 2H MoTe2 carried out a both room temperature and cryogenic temperatures demonstrates a temperature and fluence dependence consistent with defect-mediated exciton decay. The optical properties of MoTe2 were additionally found to be stable over the course of 8 months air exposure. The biexponential decay dynamics of monolayer MoS2 and WS2 were shown to be consistent with previous investigations. Both studies of interfaces, including the 2H-1T’ MoTe2 homojunctions and the MoS2-WS2 heterojunction were unable to observe signatures of interfacial charge transfer due to lack of sufficient spatial resolution near the interface crossover. In addition to studies on TMDs, the low-wavenumber Raman modes of both isotopically substituted 13C Rubrene and those of a structural analog to rubrene, fm-rubrene, were measured and compared to native rubrene. The 13C rubrene demonstrated a uniform shift to lower energy intermolecular mode vibrations. The modes of fm-rubrene were characterized for the first time and compared to a predicted computational Raman spectrum showing large (~4%) deviations with theory at low vibrational energies (<200cm-1), suggesting intermolecular coupling becomes influential at this threshold.