Browsing by Subject "2D Materials"

Now showing 1 - 5 of 5
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    Applications of Transmission Electron Microscopy on Free-standing and Embedded Two-dimensional Materials
    (2019-12) Wu, Ryan
    In 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.
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    Mesoscopic Transport in Novel Graphene Quantum Devices
    (2023-12) Zhang, Xi
    Two-dimensional (2D) materials are a class of extraordinarily thin materials that are normally referred to being one or two atoms thick. Compared to their bulk counterparts, they would exhibit distinct unique physical properties due to the constrains of the size in one dimension. The remarkable advantages could be offered by 2D materials, such as the electrical, mechanical, and thermal properties, intriguing intense scientific research and great interests from various fields. Being the most well-studied 2D material of them all, graphene becomes a shining star in the family, providing a promising platform of quantum phenomena studies in a 2D regime. The later exploration of transition metal dichalcogenides (TMDs) also provides scientists new insight of the 2D devices applications in terms of the electronics and optoelectronics. Undoubtedly, the research of the novel 2D materials uncovers unprecedented advantages and provides one of the essential building blocks for next generation devices. This thesis centers itself among various exotic physics quantum phenomena and demonstrates several different approaches in terms of the quantum phenomena investigation in 2D material devices. One of the approaches is to change the materials band structure through twist and stacking. The moiré potential and the proximity effect between the layers in van der Waals structures could change the fillings of the states, leading to the emergence of strong correlation, generate charge transfer, therefore resulting into the change of the properties and observations of novel quantum phenomena. The second approach is to utilize the gate-defined structures to locally tune material band structure by changing the electrical potential or applying the displacement field. Due to the fine structure of the designed gate configurations, electronic states can be controlled within a nano-scale regime. The provided flexibility of the gate structures gives the possibility of creating, controlling, or modifying quantum phenomena, as well as achievement of the relevant applications.
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    Modeling and Fabrication of Low Power Devices and Circuits Using Low-Dimensional Materials
    (2016-07) Kshirsagar, Chaitanya
    As 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.
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    Tailoring the Microstructure of 2D Molecular Sieve Materials for Thin Film Applications
    (2018-05) Shete, Meera
    Zeolites and metal organic frameworks (MOFs) are microporous materials, with pores of molecular dimensions, that are of interest in a variety of applications including catalysis, adsorption, ion-exchange, separation membranes etc. With a global need of developing clean energy resources and reducing the carbon footprint of existing processes, they are being increasingly sought after as catalysts for the conversion of biomass to chemicals and fuels, in separation membranes to replace the existing energy intensive industrial separations with clean energy-efficient processes and for capture and storage of carbon dioxide. Their performance in these applications depends mainly on their pore size but also on our ability to tune their microstructure (crystal morphology and size, orientation, phase purity, defect densities etc.) as desired for an optimum performance. Recent advances in synthesis of molecular sieve materials have resulted in the development of advanced morphologies such as hierarchical materials, core-shell catalysts, two-dimensional nanosheets and thin films. However, a lot of the reports in the literature focus on conventional crystals and studies focusing on nanoscale crystal growth control are still in their infancy. This dissertation focuses on developing synthetic methods that will enable us to tailor the microstructure of 2D molecular sieve materials at a nanoscale approaching single-unit-cell dimensions with a goal of optimizing their performance in thin film applications. A novel coating technique was applied to isolate 2D MFI zeolite nanosheets and form monolayer coatings on versatile supports such as Si wafers. Using this as a prototype, growth conditions were developed that lead to unprecedented control of zeolite MFI growth at a scale approaching single-unit-cell dimensions. It was demonstrated that these growth conditions are robust enough and can be used to grow zeolite MFI crystals of varied sizes and morphology. It also enabled us to precisely control the microstructure of MFI thin films leading to the development of a material that had one of the lowest reported dielectric constant. Furthermore, the nanoscale growth control also allowed us to tailor the design of hierarchical catalysts by controllably thickening the zeolite domains in them and open opportunities to design multifunctional catalysts. A scalable and direct synthesis of Cu(BDC) MOF nanosheets was developed. Hybrid nanocomposites incorporating the MOF nanosheets in polymer matrices were fabricated which demonstrated significantly improved performance for CO2/CH4 separation.
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    Theoretical Insights into the Molecular Transformations Governing Redox Chemistry
    (2020-09) Udyavara, Sagar
    Redox reactions are one of the most prevalent chemistries and a ubiquitous component in naturally occurring processes as well as in the production of fuels and various chemicals involving pharmaceuticals, agrochemicals, and other industrially relevant chemicals. Research efforts in the past have focused on the development of safe and environmentally benign redox processes using alternative resources to deal with issues pertaining to global warming and depletion of the existing crude oil reserves. In addition, in the area of organic synthesis, new synthesis strategies are being developed to access important redox derived chemical intermediates via use of less toxic and easily available redox reagents. However, as is the case with most newly developed catalytic processes, the fundamental challenges associated with poor reaction selectivity and activity hinder the effectiveness of these new reactive transformations and technologies, making them less desirable compared to the currently used approaches. Understanding the molecular transformations occurring during these processes thus remains the key ingredient in the further development of these new processes. Gaining mechanistic and kinetic insights into the reaction would allow us to identify factors that control the selectivity and activity for the given reaction, which would then guide the development of more active and selective reaction systems. Along these lines, electronic structure calculations based mainly on density functional theory (DFT) has emerged as a powerful tool to model the reaction kinetics and also to gain important perspectives about the chemical reactions that are not observable experimentally. My dissertation thus focuses on using density functional theory (DFT) to gain important molecular level insights into the transformations that control the oxidation and reduction pathways occurring via thermal or electrocatalytic routes for five independent redox chemistries. Further, we also look into the salient features of redox chemistry that come into play during fabrication of electronic devices, particularly for MoS2 based transistor devices, which ultimately affect its ensuing performance. The first part of the thesis focuses on ab-initio study of three particular systems of interest in oxidation chemistry. Each of these studies separately report on three distinct catalytic features including the nature of the sites, molecularity of the catalyst, and the presence of surface coverages that affect the selectivity towards the desired product. The initial focus is on processes involving partial oxidation of feedstock chemicals which suffer from issues of low selectivity since the processes of over-oxidation is thermodynamically more favorable. One such example of a system that suffers from over-oxidation is sulfur oxidative coupling of methane (SOCM), which involves selective production of ethylene from methane using sulfur as an oxidant. Herein, we report a detailed kinetic and mechanistic study for SOCM reaction done over a sulfided Fe3O4 catalyst (FeS2). Experimental mechanistic analysis involving Delplot and contact time studies reveal a reaction network for SOCM that is different compared to the traditional OCM. Further computational analysis done for this reaction network suggests a site-specific formation of the selective C2 products and unselective carbon-di-sulfide, allowing for potential tuning of the reaction selectivity. The second system of interest that displays challenges with over-oxidation is the dehydrogenation of cyclohexanone. In this study, we examine the role of the homogeneity of the catalyst (Pd(DMSO)2(TFA)2) in selective control of the dehydrogenation reaction of cyclohexanone. Our DFT calculations indicate that the dehydrogenation proceeds via a rate determining intermolecular deprotonation step, activating the C-H bond at the alpha position. Using distortion-interaction analysis, the further activation of the cyclohexenone intermediate formed is shown to be inhibited due to its higher C-H bond strengths as well as due to its weaker interactions with the hydrogen abstracting entity, TFA- compared to cyclohexanone. Besides issues pertaining to over-oxidation affecting the selectivity, for reacting species with multiple reaction centres or functionalities, multiple parallel reaction pathways can emerge which could also lead to lowering of selectivity towards the desired intermediate. Primary electro-oxidation of glycerol over Pt catalyst is one such example wherein the product distribution is dependent on the carbon center that is being activated (primary versus secondary). Using DFT calculations, we hereby report the influence of coverages on directing the oxidation of the C-H bond at the primary versus the secondary position. We show that under high coverage conditions, oxidation at the primary C-H bond to form glyceraldehyde is preferred despite the stronger C-H bond strengths whereas under low coverage conditions, the weaker secondary C-H bond is preferentially activated to form dihydroxyacetone. We further examine the influence of the reaction conditions such as the pH, metal surface, and the operating potentials on the resultant product distribution for electrocatalytic glycerol oxidation over Pt. The second part of the thesis involves a mechanistic investigation of some important reduction chemistries, in particular - the hydrogen evolution reaction (HER) and the electrochemical Birch reduction. In the HER, we hereby examine MoS2 as a non-precious catalyst for use in fuel cells for production of hydrogen from water, and report enhancements in its electrocatalytic activity via application of an external electric field normal to the MoS2 electrode surface (back-gating). DFT studies aimed to understand the mechanistic nuances of the observed increased activity show that the excess electron densities induced via back-gating increase the binding energies of the hydrogen on the MoS2 surface, which in turn lead to improvements in the electrocatalytic activity of MoS2 for HER. In the next topic, we look at the Birch reduction process wherein we report a practical, safe, and scalable electrosynthetic strategy for the reduction of the arenes to dienes and for other similar reductive transformations and subsequently determine the mechanistic nuances of the reaction in the presence of various reagents and additives. Through electroanalytical and computational investigations, we have shown the reaction pathway to proceed via the reduction of the substrate near the electrode surface with the protonation step as the rate limiting step of the reaction. Further, we have deciphered the unique role of each of the reagents used in the study – electrolyte, lithium bromide (LiBr); solvent, tetrahydrofuran (THF); proton source, dimethyl urea (DMU); and the additive, tris(pyrrolidino)phosphoramide (TPPA), to understand how these components co-operatively aid in promoting the desired transformations towards higher reaction yields. In the final part of the thesis, diverging from the theme of redox reactions in chemical production, we investigate the influence of the redox reactions in device fabrication processes with the aim to mitigate some of the issues concerning the contact problem typically observed at the interface. We study here a system of MoS2-metal contacts, which have applications in development of sub-nanometric transistors. Via our “single-atom addition” approach used here, we show that the nature of the metal deposited strongly influences the resulting structural and electronic properties of the MoS2-metal interface. In conjunction with analytical scanning tunnelling electron microscopy (STEM) studies, we screen and characterize the interfacial properties for different metal contacts including Sc, Ti, Cu, In, and Au, with the aim to design systems that minimize the contact resistance at these interfaces.