Synthesis of a silyl complex and studies of mechanistic pathways in chemical vapor deposition of metal-silicon-nitrogen ternary and zirconium carbide thin films

This dissertation describes research on the synthesis of a tantalum silyl complex, chemical vapor deposition (CVD) of Ti-Si-N ternary thin films, and mechanistic pathways in the CVD processes to form the Ti-Si-N ternary thin films and zirconium carbide (ZrC). An overview of the Ph.D. research appears in Chapter 1. Chapter 2 describes the preparation of a thermally unstable bis(alkylidene) complex (RCH2)4Ta2(=CHR)2(Cl)2 (R = SiMe3, 1) and kinetic studies of its conversion to an unstable alkylidene-alkylidyne complex (RCH2)3Ta2(=CHR)(≡CR)(Cl)2 (3). The synthesis and characterization of the first silyl bis(alkylidyne) complex (RCH2)(R3Si)Ta(µ-CR)2Ta(CH2R)2 (4) are also reported. Chapter 3 presents CVD of Ti-Si-N ternary thin films from the reaction of Ti(NMe2)4 with SiH4. Analyses of the thin films by XPS (X-ray photoelectron spectroscopy), RBS (Rutherford backscattering spectrometry), and SEM (scanning electron microscopy) showed that the films likely contained TiN and Si3N4. The reaction between Ti(NMe2)4 and SiH4 at room temperature yielded H2, HSi(NMe2)3 and a black solid. The reaction mixture, after heating, gave powders which were found to be likely ternary materials containing TiN and Si3N4. The formation of these two solid compounds in both the CVD processes and room temperature reaction (followed by heating), rather than a single phase Ti-Si-N solid, TiSin-Si3N4 or TiSin-TiN, is discussed with the use of a Ti-Si-N ternary phase diagram. The reaction between Ti(NMe2)4 and SiH4, and the mechanistic pathways to TiN and Si3N4 were further investigated. These studies are described in Chapter 4. The reactions of M(NMe2)4 (M = Ti, Zr, Hf) with H2SiR\u272 (R\u272 = HPh, Ph2, MePh) were found to give aminosilanes and metal amide hydride species. Hydride amide complexes [(Me2N)3M(µ-H)(µ-NMe2)2]2M [M = Zr (7); Hf (8)] were isolated from the reactions of M(NMe2)4 (M = Zr, Hf) with H2SiPh2 and characterized by NMR and single-crystal X-ray crystallography. Ad initio quantum chemical calculations on the mechanism of the reactions of Ti(NR2)4 (R = Me, H) with SiH4 supported the experimental observations that these reactions preferentially gave aminosilanes and metal hydride amide species. An equilibrium, (Me2N)6ZrSi(SiMe3)3 (5b) + H2SiPh2 = (MeSi3)3SiZr-(H)(NMe2)2 (6) + HSi(NMe2)Ph2, was observed. This equilibrium suggested that the reactions between MNR2 and H-Si bonds to give M-H and Si-NR2 bonds were reversible. The results here indicated that the role of SiH4 in its reaction with Ti(NMe2)4 was to remove amide ligands as amidosilanes. The removal of amide ligands might be incomplete, and thus the reaction gave Ti amide hydride species as black solids. Subsequent heating of the black solids and aminosilanes perhaps yielded TIN and Si3N4, respectively, as the Ti-Si-N materials. The mechanistic studies of CVD of ZrC from Zr(CH2CMe3)4 (11b) are described in Chapter 5. Ab initio MO calculations predicted that, unlike its titanium analogue, the first step in the thermolysis of Zr(CH2CMe3)4 (11b) was a γ-hydrogen abstraction reaction. Experimental studies were designed to confirm the theoretical prediction. Analyses of the volatile products in CVD of ZrC from Zr(CH2CMe3)4 (11b) and Zr(CD2CMe3)4 (11b-d8) showed that the major products were neopentane and isobutene in a ratio of 2.3 : 1. In addition, the ratio of neopentane-d2/neopentane-d6 produced in the CVD involving Zr(CD2CMe3)4 (11b-d8) was ca. 4.9. These results are discussed in terms of y-hydrogen abstraction as the preferred first step in the thermolysis of Zr(CH2CMe3)4 (11b)

Urban land markets and city development: Sub-Saharan Africa

Rapidly growing cities in sub-Saharan Africa face immense population pressures. Weak institutions and outdated regulations inherited from the colonial era threaten to stifle their progress. This paper examines the institutions underlying the operation of urban land markets in Sub-Saharan Africa, focusing on property rights, the evolution of cities and their spatial layout, planning, and property taxes. Countries typically have a dual system of property rights, with in theory formal rights in cities, communal tenure in rural areas, and a transition between the two systems at growing city boundaries. However, large portions of cities operate outside these systems under informal rights. Using case studies and within city variation, we review the historical evolution of these systems in a number of African cities. We argue that cities lacking formal property rights tend to build lower and less intensively, often with slums persisting near the city center, where there is much higher value alternative use. We further explore the relationship between lack of owner occupancy and wealth inequality, as partially affected by the transition to private property rights. Next, we discuss the critical role of planning. Francophone countries for instance historically imposed comprehensive planning on urban land markets compared to Anglophone counterparts. This resulted in greater contiguity and density of land use, gridded urban layouts, and less leapfrogging in new developments. Where planning is weak, special initiatives such as sites and services may impose planning on certain greenfield neighbourhoods, with benefits accruing in the future. The paper then examines problems in property tax enforcement and collection, discussing reforms to improve collections. The paper concludes with a discussion of policy considerations and a research agenda

Design Guidelines for Prompt Engineering Text-to-Image Generative Models

Text-to-image generative models are a new and powerful way to generate visual artwork. However, the open-ended nature of text as interaction is double-edged; while users can input anything and have access to an infinite range of generations, they also must engage in brute-force trial and error with the text prompt when the result quality is poor. We conduct a study exploring what prompt keywords and model hyperparameters can help produce coherent outputs. In particular, we study prompts structured to include subject and style keywords and investigate success and failure modes of these prompts. Our evaluation of 5493 generations over the course of five experiments spans 51 abstract and concrete subjects as well as 51 abstract and figurative styles. From this evaluation, we present design guidelines that can help people produce better outcomes from text-to-image generative models

Timing Matters: How Delaying College Enrollment Affects Earnings Trajectories

High school graduates often delay college enrollment, but few studies have looked at the effects of this choice on their educational attainment and success in the labor market. Using data from the National Longitudinal Survey of Youth 1997, this paper compares the academic and labor market outcomes of high school graduates who delay college enrollment (“delayers”) and those who enroll in college immediately (“on-time enrollees”) up to 13 years after high school completion. The results show that delaying college enrollment decreases individuals’ likelihood of enrolling in college and increases their tendency to enroll in two-year colleges if they do return to school. Delayers experience early earnings benefits, but these fade out after their mid-20s and turn to significant losses over time. Differences in student characteristics only explain one third of the pay gap between delayers and on-time enrollees; 60 percent of the pay gap is explained by delayers’ reduced likelihood of attending and obtaining a degree at a four-year college

Happy Together? The Peer Effects of Dual Enrollment Students on Community College Student Outcomes

Nationally, 15% of first-time community college students were high school students taking college coursework through dual enrollment in the fall of 2010, and the percentage has risen since then. The growing numbers of dual enrollment students at community colleges raises concerns about how high school peers might influence traditionally aged college enrollees. Using administrative data from a large state community college system, this paper examines whether being exposed to a higher percentage of dual enrollment peers influences non-dual enrollment enrollees’ performance in college courses. Focusing on entry-level (or gateway) math and English courses and employing a two-way fixed effects model, the authors find that non-dual enrollment college enrollees exposed to a higher proportion of dual enrollment peers had lower pass rates and grades in gateway courses, higher course repetition, and lower subject persistence

396 Characterizing inpatient hospitalizations for hidradenitis suppurativa and assessing the impact of outpatient dermatology care on hospitalizations

Introduction: Hidradenitis suppurativa (HS) is associated with a significant disease burden. The use of high-cost settings care are common among HS patients. Objective: To explore factors that may influence hospital admissions and readmissions among HS patients. Methods: Using ICD-9/10 codes (705.83 and L73.2), we extracted the medical records of adult HS patients who visited the Henry Ford Health System (HFHS) ED between 2010 and 2020. Results: Of the 100 HS patients, 52 (52%) were admitted to an inpatient service. Hypertension (OR:2.55,95% CI:1.11-5.83, p value=0.027), diabetes mellitus (OR:2.42, 95%CI:1.05-5.61, p value =0.039), cellulitis (OR: 19.28, 95%CI:4.23-87.96 p\u3c0.001), sepsis (OR:10.25, 95%CI:1.34-89.24, p value=0.025), and depression (OR:3.32, 95%CI:1.10-10.04, p value =0.002) were significant predictors of admission. Chronic kidney disease (OR:3.05, 95% CI:1.00-9.23,p value=0.049), congestive heart failure (OR:4.06, 95%CI:1.19-13.80, p value =0.025), coronary artery disease (OR:15.20, 95%CI:2.80-82.65, p value=0.002), chronic obstructive pulmonary disease (OR:8.94, 95%: 1.51-52.86, p value =0.003), cellulitis (OR:4.62, 95%CI:1.66-12.88, p=0.003), sepsis (OR:3.75, 95%CI:1.02-13.82,p value =0.047), and depression (OR:4.50, 95%CI:1.54-13.18, p value=0.006) were positively associated with readmission. Those who received outpatient dermatology care had a lower risk of being admitted (n=87, 28.7% vs n=13,100%, p \u3c0.001) and readmitted (n=10, 11.5% vs n=5, 38.5%, p value =0.0108). Discussion: In this study, we demonstrate that certain comorbidities, that are common among HS patients, are significant determinants of admission to an inpatient service. Furthermore, the increase access to outpatient dermatology care significantly reduces the likelihood of HS patients being admitted and readmitted. Conclusion: The findings of this study illuminate the pivotal role of dermatologists in improving patients’ health outcomes while minimizing the avoidable use of high-cost settings care

Generative Disco: Text-to-Video Generation for Music Visualization

Visuals are a core part of our experience of music, owing to the way they can amplify the emotions and messages conveyed through the music. However, creating music visualization is a complex, time-consuming, and resource-intensive process. We introduce Generative Disco, a generative AI system that helps generate music visualizations with large language models and text-to-image models. Users select intervals of music to visualize and then parameterize that visualization by defining start and end prompts. These prompts are warped between and generated according to the beat of the music for audioreactive video. We introduce design patterns for improving generated videos: "transitions", which express shifts in color, time, subject, or style, and "holds", which encourage visual emphasis and consistency. A study with professionals showed that the system was enjoyable, easy to explore, and highly expressive. We conclude on use cases of Generative Disco for professionals and how AI-generated content is changing the landscape of creative work