1,721,017 research outputs found
Nanobeam 4D-STEM raw data of monolayer WS2-WSe2 lateral heterojunctions
No full text<p>The sample under investigation comprises a monolayer WS2-WSe2 lateral heterojunction featuring in-plane epitaxial interfaces. The datasets were acquired using an electron microscope pixel array detector (EMPAD) at Cornell University in 2016, with the following specifications: Magnification: 27.5 kx, Convergence angle: 1.2 mrad, C2 aperture size: 70, Spot size: 9. This dataset is from the same batch of datasets referenced in the paper by Han et al., Nano Letters 18, 3746-3751 (2018). Additionally, data from the same batch are also cited in the publication by Shi et al., npj Computational Materials 8, 114 (2022). Further details regarding the materials synthesis can be found in the paper by Xie et al., Science 359, 1131-1136 (2018). </p>
Investigation of the structure and elemental composition of 2D Interfaces in Transition Metal Dichalcogenides
No full textThe electronics boom happened with the invention of semiconductor materials with Si becoming the number one in the race. But now, since Si electronics seems to be reaching its limit, the researchers are switching their attention to 2D materials because of their unique electrical, mechanical, or optical properties. Transition metal dichalcogenides (TMDs) are semiconductor materials from this family that show promise to be the next generation of nanoelectronics as well as give possibility to improve energy or informational storage. However, for maintaining high efficiency, the structure of TMDs should have minimum number of defects, misfits, or inclusions of other elements. This indicates a need to understand the structural and elemental composition of the 2D materials to atomic scales, and this is where techniques like (scanning) transmission electron microscopy (S)TEM and energy-dispersive x-ray spectroscopy (EDX) come to aid. In this study, we will discuss sample preparation and analysis techniques for of SnSe, MoS2, WS2, WSe2, and graphene/Al heterostructures on silicon wafer and sapphire substrates, the methods of uncovering the elemental and layer composition of these materials and provide image and signal processing approaches to increase the signal-to-noise ratio of the currently employed techniques. All of these will help us reach a conclusion about atomic structure and elemental composition of the materials and show the possible traps and pitfalls that a microscopist might encounter during similar analysis for either TMDs or other types of 2D materials
Advancing Four-Dimensional Scanning Transmission Electron Microscopy for the Strain Analysis of Deformed Thin Films
No full textThis study presents significant advancements in Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM) for analyzing strain and crystal orientation in thin films by introducing three novel methods. First, we developed an area-selective filtering technique that leverages unsupervised learning to reduce noise in 4D-STEM datasets. This approach achieved up to a 70% noise reduction for WS2-WSe2 superlattice data. Second, we introduce a strain correction method tailored for buckled two-dimensional materials. Guided by kinematical diffraction simulations, this method produces surface morphology maps that enable surface tilt and strain decoupling. Its application to MoSe2-MoS2 heterojunction data successfully reduced compressive strain measurements from an overestimated 6.5% to a more accurate ~1.5%. Lastly, we present a technique for precisely mapping crystal orientation in thin films. This technique was effectively applied to a gold nanoplate using a combination of 4D-STEM data, abTEM multislice simulations, and electron tomography validation. These advancements significantly improve the accuracy of strain measurements and crystallographic analysis, thereby enhancing our understanding of deformed nanofilms and expanding the capabilities of 4D-STEM for future materials science research
Advancing Nanobeam Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM) for Strain Analysis
No full textThis dissertation delves into the realm of nanobeam four-dimensional scanning transmission electron microscopy (4D-STEM), and its application for characterizing the intricate microstructures of two-dimensional (2D) materials and nano-catalysts. It leverages recent technological breakthroughs in pixelated, fast direct electron detectors that enable the comprehensive collection of momentum-space data at every scan in STEM. This integration of spatial and momentum dimensions enables the generation of rich 4D datasets. The wealth of information captured by these advanced detectors, though vast, poses interpretative challenges due to its complexity. Consequently, this thesis is dedicated to the development and application of novel analytical methodologies for the extraction of crystallographic information from such voluminous 4D-STEM datasets, addressing key problems in materials science.
In Chapter 2, the study applies 4D-STEM to investigate the broad structural characteristics of van der Waals 2D ferroelectric SnSe. The research uncovers significant in-plane lattice distortions and out-of-plane stacking variations. This has led to the discovery of considerable lattice strains and distinctive ferroelectric-to-antiferroelectric domain walls which hold implications for the material's physical properties and potential device applications.
Chapter 3 shifts the focus to the surface strain of core-shell nano-catalysts' structure. Through meticulous analysis via 4D-STEM, it is demonstrated that cube-shaped Au@Pd particles with sharp-tipped cores exhibit a coherent, dislocation-free heteroepitaxial interface even when the shell thickness is considerably greater than that of comparable nanocatalysts with rounded cores. This finding suggests a route to enhancing the strain stability of such structures, which is paramount in their application as catalysts.
Chapter 4 ventures into the machine leaning methods to process extensive 4D-STEM datasets autonomously. This innovative, data-driven approach effectively discerns various material deformations, including strain, lattice distortions, and bending contours. Such detailed comprehension of lattice alterations is crucial for the advancement of material characterization techniques and the ensuing implications for materials science.
Overall, the thesis advances the understanding of complex material systems through innovative 4D-STEM analysis and machine learning, potentially impacting the design and application of nanoscale materials and advancing technological frontiers across various disciplines
Mechanical Properties of Two-dimensional Nano-composites and Oxides
No full textTwo-dimensional (2D) materials have attracted enormous interests owing to
their extraordinary properties due to their atomic-level thickness and robust in-
plane atomic bonding, positioning them as promising materials for advanced
technological applications across electronics, photonics, sensing, energy storage,
and structural composites. However, their practical applications have been hindered
by intrinsic brittleness, susceptibility to defects, and relatively low fracture
toughness. This thesis systematically investigates intrinsic and extrinsic toughening
mechanisms, along with anisotropic fracture properties, in selected novel 2D
materials and composites to enhance their mechanical robustness and reliability.
The intrinsic toughening mechanisms are explored through detailed studies
on monolayer amorphous carbon (MAC) nanocomposites, investigating how
structural heterogeneities (crystalline and amorphous domain) influence fracture
resistance. In-situ scanning electron microscopy (SEM) tensile testing with
molecular dynamics (MD) simulations provides comprehensive insights into
fracture processes and toughening behaviors.
Extrinsic toughening strategies are investigated through two-dimensional
covalent organic framework sandwich structures, demonstrating significant
improvements in fracture toughness. Additionally, anisotropic fracture behavior is
studied in monolayer titania nanosheets, emphasizing the role of crystallographic
orientation and defect distributions in determining mechanical performance.
Overall, this thesis provides fundamental insights into fracture mechanics
and toughening mechanisms in 2D materials, offering practical strategies for
designing mechanically robust and reliable nanocomposite systems. The findings
not only advance the fundamental understanding of 2D material behavior but also
expand their potential applications in next-generation engineering technologies
Non-Equilibrium, Ultra-Fast Heating Techniques for Material Synthesis, PFAS Mineralization and Upcycling.
No full textThe increasing demand for sustainable technologies and materials has led to a critical need for efficient, scalable, and environmentally friendly solutions for material synthesis, environmental remediation, and resource recovery. Among the innovative technologies addressing these challenges, Flash Joule Heating (FJH) has emerged as a versatile and transformative technique. This thesis explores the application of FJH in four significant areas: heteroatom-substituted graphene synthesis, the destruction of per- and polyfluoroalkyl substances (PFAS), the recovery of critical metals from lithium-ion batteries (LIBs) aided by waste PFAS and the synthesis of silicon carbide nanowires using waste glass.
Graphene, a two-dimensional carbon-based material, is renowned for its extraordinary properties, including high electrical and thermal conductivity, mechanical strength, and chemical versatility. These properties make graphene a highly sought-after material for applications in energy storage, electronics, and catalysis. The functionality of graphene can be further enhanced by heteroatom substitution, which involves incorporating non-carbon atoms, such as nitrogen, boron, sulfur, and fluorine, into its lattice structure. These heteroatoms modify the electronic and chemical properties of graphene, expanding its range of potential applications. Traditional methods for heteroatom substitution, such as chemical vapor deposition and solvothermal processes, are often time-consuming, resource-intensive, and difficult to scale. In contrast, FJH offers a rapid, energy-efficient, and scalable alternative for producing high-quality, heteroatom-substituted graphene. By subjecting pre-formed graphene or carbon precursors to rapid high-temperature heating in the presence of heteroatom-containing precursors, FJH enables precise control over doping levels and ensures structural integrity, making it a promising method for scalable graphene functionalization. This is covered in chapter one of this thesis.
In addition to its role in material synthesis, FJH provides a novel solution to a pressing environmental challenge: the destruction of PFAS, often referred to as "forever chemicals." PFAS are a class of synthetic organofluorine compounds widely used in industrial and consumer applications, including firefighting foams, non-stick coatings, and water-resistant materials. The strong carbon-fluorine bonds in PFAS make them highly resistant to degradation, leading to their accumulation in the environment and posing significant risks to human health and ecosystems. Current remediation techniques, such as adsorption onto granular activated carbon (GAC), capture PFAS but do not degrade them, leaving behind secondary waste. FJH addresses this limitation by degrading PFAS adsorbed onto GAC through high-temperature treatment, breaking the carbon-fluorine bonds and converting PFAS into benign byproducts. This process not only eliminates PFAS but also enables the upcycling of PFAS-contaminated GAC into valuable materials, such as graphene, demonstrating a sustainable approach to waste management. This is covered in chapter two of this thesis.
We then demonstrate that FJH offers a sustainable and efficient approach for resource recovery from spent lithium-ion batteries (LIBs), which play a crucial role in modern energy storage systems. LIBs contain valuable metals, such as lithium and cobalt, whose extraction and processing are energy-intensive and environmentally damaging. The growing demand for these metals, driven by the proliferation of electric vehicles and renewable energy technologies, has raised concerns about resource scarcity and the environmental impact of conventional recycling methods. FJH offers a rapid and solvent-free approach to metal recovery, facilitating the fluorination of lithium into lithium fluoride (LiF) and the reduction of cobalt into metallic form. These transformations occur within a few seconds, minimizing energy consumption and environmental impact while enabling the efficient separation of metals for reuse. FJH addresses the challenges associated with LIB recycling as well as waste PFAS degradation. This is covered in chapter three.
Finally, we demonstrate a flash process for upcycling waste glass into SiC nanowires within seconds. By introducing fluorine, iron oxide present in the waste glass is activated, catalyzing the formation of one-dimensional (1D) SiC nanowires. The resulting SiC nanowires exhibit superior performance in composite reinforcement compared to conventional SiC powders. Additionally, a life cycle assessment (LCA) and techno-economic analysis (TEA) reveal that our process significantly reduces environmental impact and production costs compared to conventional synthesis methods. This work highlights fluorine as a versatile and cost-effective agent for modulating nanomaterial growth kinetics and tailoring morphology, providing a sustainable and scalable approach for advanced material synthesis and is discussed in chapter 4 of this thesis.
This thesis underscores the versatility and scalability of FJH as a platform for material innovation, environmental remediation, and resource recovery. Through its application in heteroatom-doped graphene synthesis, PFAS destruction, metal recovery from LIBs, and synthesis of one-dimension materials, FJH demonstrates its potential to bridge the gap between fundamental research and practical solutions, advancing both sustainability and technological progres
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Beyond Flash Joule Heating: Advanced Non-equilibrium Synthesis and Electron Microscopy Characterization
No full textFlash Joule heating (FJH) has emerged as a highly efficient method for synthesizing a diverse array of advanced materials. This ultrafast, non-equilibrium technique has demonstrated significant potential in transforming various carbon sources into turbostratic flash graphene, synthesizing refractory materials such as metal carbides, converting 2H-phase MoS2 into 1T-phase MoS2, and extracting valuable hydrogen gas from plastic waste. Despite these impressive achievements, FJH still faces critical limitations, including its requirement for material conductivity, restricted generality, and an inability to facilitate non-solid-state reactions.
To overcome these limitations, several innovative strategies have been developed. One notable approach involves introducing immiscible conductive additives, specifically copper (Cu), into highly resistive amorphous boron (B), effectively enabling flash Joule heating. Surprisingly, during this modified process—termed plasma flash Joule heating (PFJH)—we observed spontaneous plasma generation, surpassing conventional temperature limits (~3000 K) and facilitating rapid crystallization of amorphous boron into crystalline form. Subsequent analysis revealed an unexpected and significant discovery: even after thorough removal of excess copper, copper atoms remained homogeneously embedded within the crystalline boron matrix, resulting in the unprecedented synthesis of copper-doped crystalline boron (Cu-B).
Further investigation using 3D micro electron diffraction demonstrated that the rapid heating and cooling intrinsic to PFJH prevented phase segregation, effectively trapping immiscible copper atoms within the boron lattice. This Cu-B composite exhibited notably altered mechanical properties, including a reduced modulus (~267 GPa) and Vickers hardness (~20 GPa), alongside a remarkable optical transition from indirect to direct bandgap. This unexpected change in bandgap structure induced pronounced photoluminescence (PL), revealing a novel material functionality unattainable through conventional methods. These findings highlight PFJH's potential as an ultrafast, non-equilibrium approach for synthesizing novel materials exhibiting extraordinary and unforeseen physical properties.
To expand the versatility of Joule heating further, we introduced an indirect heating method called Flash-within-Flash (FWF) Joule heating. This innovative approach greatly broadens the applicability of FJH, enabling the synthesis of 22 different compounds with properties comparable or superior to commercially available materials. Emphasizing sustainability, FWF addresses critical issues such as energy efficiency, minimal water consumption, scalability, and diverse material synthesis. FWF rapidly produces 10 transition metal dichalcogenides (TMDs), 3 Group-XIV dichalcogenides, and 9 non-TMD materials, with each synthesis completed within five seconds under ambient conditions. Moreover, FWF uniquely allows phase-selective synthesis and generates single-crystalline bulk powders. The enhanced tribological performance of FWF-produced MoSe2 compared to commercial materials further demonstrates the technique's effectiveness. Additionally, the versatility of FWF in facilitating atom substitution and doping establishes it as a robust protocol for general inorganic material synthesis.
Acknowledging the limitations associated with non-solid-state reactions, we developed Flash Vapor Deposition (FVD), a novel reactor design combining chemical vapor deposition (CVD) with the rapid heating principles of FWF Joule heating. Traditional CVD methods suffer from slow temperature ramping, restricting their effectiveness for rapid monolayer synthesis and complex coatings. Our redesigned FVD apparatus incorporates an outer tube containing carbon felt within an inert atmosphere, enabling rapid, uniform radiative heating. The inner tube, isolated from direct contact with heating components, contains reagents and substrates, with precise mass transport achieved through controlled gas flow. This innovative design facilitates the rapid synthesis of high-quality monolayer TMDs directly on substrates such as Si/SiO2.
Collectively, these advancements significantly enhance the versatility and efficacy of flash Joule heating, transforming it into a comprehensive, ultrafast, non-equilibrium synthesis platform. By overcoming previous limitations, these developed techniques pave the way for broad and impactful applications across various domains in materials science, positioning flash Joule heating as a critical enabler of future material innovations. The logical sequence of the PhD work and findings are summarized in Figure 1 for visualization
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
