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Connecting Microstructural Defects in Molybdenum Disulfide and Amorphous Silicon Systems to Macroscale Performance using Mixed Computational Methods
Get PDFMicrostructure defects are the source of many interesting physical phenomena in materials. The influence of microscopic imperfections are visible among a background of a crystalline material and can noticeably change macroscopic materialproperties. Ab initio computations are a wide range of computational methods used to study materials at such microscopic levels. Classical and quantum-level computations are often viewed as competing methods, but we instead take a complementary approach. We combine strengths of both methods to study two classes of materials—silicon and the two-dimensional (2D) transition metal dichalcogenide (TMD) molybdenum disulfide (MoS2). In MoS2, we interrogate the effects of the presence of dopants. Conversely in Si, we take amorphization as a disruption to the base crystalline structure. In both, we are interested in how these changes to microstructure affect energetic efficiencies. In a-Si, the photovoltaic efficiency is hampered by the presence of dangling bonds, while in MoS2, mechanical efficiencies of lubrication are enhanced when doped.Both systems pose their unique challenges and deviations from standard workflows. We will review the overarching strategies used to study them in Ch. 2. Throughout our studies, we have developed several techniques to overcome specific challenges which are detailed in their respective sections. With a-Si, a classical potential Monte Carlo code is used to generate realistic, non-biased, fully amorphous coordinates faster than what could be achieved by currently available quantum-mechanical methods. Then, density functional theory is used to relax the structures. With MoS2, higher-accuracy DFT energy computations are used to parametrize a classical force-field for the computation of larger systems which would otherwise be difficult with DFT alone.We cover a wide range of analyses between both materials. We have developed methodologies for analyzing the amorphous and 2D material systems using classical methods and density functional theory. We have computed a range of material properties for MoS2 and amorphous Si and hydrogenated a-Si:H. With Si, we modified the Wooten-Winer-Weaire algorithm to produce amorphous networks with included voids by application of initial strain. The size of the voids is somewhat controlled by the strain value. These voids emerge naturally as a part of the amorphization process.We find that the amorphous networks generated by simple Keating springs when applied in the WWW method is retained when relaxed by DFT. Structure-scale approximations of the Keating potential yields the result that ∆θ explains a large portion of the structure’s energy, and this holds even in density functional theory.With 2D materials, we outline a specific multi-step method to quantify the sliding of defected materials in Ch. 6. Successive increases in the degrees of freedom while sliding allow us to access different components of sliding—namely the potential barrier differences, low-energy sliding pathways, and slip planes. The sliding potential in MoS2, even while intercalated, is composed of pairwise interactions of the MoS2 interfaces. This means that computing arbitrarily sized systems can be theoretically computed by only considering interactions of their interfaces. We find tetrahedrally intercalated Ni-doped MoS2 to be stable and thus more important than is considered in the literature. When intercalated, we find Ni can bind layers together, explaining the material’s increased resistance to wear, or material loss during sliding.Re-doped MoS2 has shown an increase in friction with an increasing layer count as measured with atomic force microscopy. This is counter to typical 2D materials. We find that intercalated Re can explain this relationship as an alteration to outof-plane stiffness. For this material, in computing the vibrational spectroscopy we overcame a difficulty in computing its Raman spectra. It has a metallic character, thereby limiting our Raman intensity computation due to an infinite dielectric constant. We developed a method to circumvent this computation and approximate the spectra by substitution of the atomic Raman tensor
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Investigation of local strain, symmetry, and elastic properties of methylammonium lead iodide (MAPI) hybrid perovskite using density functional theory
Get PDFHybrid organic metal-halide perovskites are promising materials for next generation solar cell application. Methylammonium lead iodide (CHNHPbI), sometimes called MAPI, is one of the favorable perovskites for making solar cells. Most of the research in the last decade about this material is aimed towards improving its photoconversion efficiency (PCE) and stability. In this work, I have done a detailed study on the three phases (orthorhombic, tetragonal, and cubic) of MAPI to understand how these different phases behave under stress. The total work is divided into 5 chapters. In chapter 1, I give an overview on perovskites for solar cell applications and discuss briefly about the theories that are involved in my calculations. In chapter 2, I investigate the effect of uniaxial strain on the pseudo-cubic structure and identify the most favorable vibrational modes to measure local strain using IR and Raman spectroscopy. In chapter 3, I investigate the same for low-temperature orthorhombic and room-temperature tetragonal phases. In addition to this, I explained about an improvement I made to the Quantum ESPRESSO code to enable these calculations. In chapter 4, I discussed how an analytical method we developed can help to understand hidden symmetries in the tetragonal perovskite and can be useful for any approximately symmetric structure to use symmetry for spectroscopic studies. In chapter 5, the last chapter, I studied the elastic properties of all three phases in detail and tried to determine the root cause behind the discrepancies in earlier published results. We also provide accurate reference values and an appropriate general methodology for elastic properties of metal halide perovskites. This work opens a way for a standard non-destructive bench-top characterization method to be usable for analyzing the critical role of local strain in hybrid perovskite photovoltaics. It provides an analytical method to calculate irreducible representations of vibrational modes for any approximately symmetric crystal structure which can be helpful for spectroscopic studies. Calculated detailed elastic properties of metal-halide perovskite will be useful for future reference for commercial application of perovskites for solar cell and flexible electronics
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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