Growth of graphene/hexagonal boron nitride heterostructures using molecular beam epitaxy

Zweidimensionale (2D) Materialien bieten eine Vielzahl von neuartigen Eigenschaften und sind aussichtsreich Kandidaten für ein breites Spektrum an Anwendungen. Da hexagonales Bornitrid (h-BN) für eine Integration in Heterostrukturen mit anderen 2D Materialien geeignet ist, erweckte dieses in letzter Zeit großes Interesse. Insbesondere van-der-Waals-Heterostrukturen, welche h-BN und Graphen verbinden, weisen viele potenzielle Vorteile auf, verbleiben in ihrer großflächigen Herstellung von kontinuierlichen Filmen allerdings problematisch. Diese Dissertation stellt eine Untersuchung betreffend des Wachstums von h-BN und vertikalen Heterostrukturen von Graphen und h-BN auf Ni-Substraten durch Molekularstrahlepitaxie (MBE) vor. Zuerst wurde das Wachstum von h-BN mittels elementarer B- und N-Quellen auf Ni als Wachstumssubstrat untersucht. Kristalline h-BN-Schichten konnten durch Raman-spektroskopie nachgewiesen werden. Wachstumsparameter für kontinuierliche und atomar dünne Schichten wurden erlangt. Das Keimbildungs- und Wachstumsverhalten so wie die strukturelle Güte von h-BN wurden mittels einer systemischen Veränderung der Wachstumstemperatur und -dauer untersucht. Die entsprechenden Beobachtungen wie der Änderungen der bevorzugten Keimbildungszentren, der Kristallgröße und der Bedeckung des h-BN wurden diskutiert. Ein Wachstum von großflächigen vertikalen h-BN/Graphen Heterostrukturen (h-BN auf Graphen) konnte mittels einem neuartigen, MBE-basierenden Verfahren demonstriert werden, welche es h-BN und Graphen jeweils erlaubt sich in der vorteilhaften Wachstumsumgebung, welche von Ni bereitgestellt wird, zu formen. In diesem Verfahren formt sich Graphen an der Schnittstelle von h-BN und Ni durch Präzipitation von zuvor in der Ni-Schicht eingebrachten C-Atomen. Schließlich konnte noch ein großflächiges Wachstum von Graphen/h-BN-Heterostrukturen (Graphen auf h-BN) durch das direkte abscheiden von C auf MBE-gewachsenen h-BN gezeigt werden.Two-dimensional (2D) materials offer a variety of novel properties and have shown great promise to be used in a wide range of applications. Recently, hexagonal boron nitride (h-BN) has attracted significant attention due to its suitability for integration into heterostructures with other 2D materials. In particular, van der Waals heterostructures combining h-BN and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. This thesis presents an investigation regarding the growth of h-BN and vertical heterostructures of graphene and h-BN on Ni substrates using molecular beam epitaxy (MBE). The growth of h-BN from elemental sources of B and N was investigated initially by using Ni as the growth substrate. The presence of crystalline h-BN was confirmed using Raman spectroscopy. Growth parameters resulting in continuous and atomically thin h-BN films were obtained. By systematically varying the growth temperature and time the structural quality as well as the nucleation and growth behavior of h-BN was studied. Corresponding observations such as changes in preferred nucleation site, crystallite size, and coverage of h-BN were discussed. Growth of h-BN/graphene vertical heterostructures (h-BN on graphene) over large areas was demonstrated by employing a novel MBE-based technique, which allows both h-BN and graphene to form in the favorable growth environment provided by Ni. In this technique, graphene forms at the interface of h-BN/Ni via the precipitation of C atoms previously dissolved in the thin Ni film. No evidence for the formation of BCN alloy could be found. Additionally, the suitability of ultraviolet Raman spectroscopy for characterization of h-BN/graphene heterostructures was demonstrated. Finally, growth of large-area graphene/h-BN heterostructures (graphene on h-BN) was demonstrated via the direct deposition of C on top of MBE-grown h-BN

Autonomous robots path planning: An adaptive roadmap approach

Developing algorithms that allow robots to independently navigate unknown environments is a widely researched area of robotics. The potential for autonomous mobile robots use, in industrial and military applications, is boundless. Path planning entails computing a collision free path from a robots current position to a desired target. The problem of path planning for these robots remains underdeveloped. Computational complexity, path optimization and robustness are some of the issues that arise. Current algorithms do not generate general solutions for different situations and require user experience and optimization. Classical algorithms are computationally extensive. This reduces the possibility of their use in real time applications. Additionally, classical algorithms do not allow for any control over attributes of the generated path. A new roadmap path planning algorithm is proposed in this paper. This method generates waypoints, through which the robot can avoid obstacles and reach its goal. At the heart of this algorithm is a method to control the distance of the waypoints from obstacles, without increasing its computational complexity. Several simulations were run to illustrate the robustness and adaptability of this approach, compared to the most commonly used path planning methods

Avian Species-Area Relationships and Environmental Covariates in Natural and Constructed Wetlands of Northeastern Alberta

I examined the patterns of species distribution and richness as they relate to area in boreal wetlands of northeastern Alberta. I conducted point counts of bird species in natural and constructed wetlands of various sizes. Plant species richness, habitat attributes, and habitat heterogeneity were also estimated to determine whether these factors influenced the strength of the avian species-area relationship. The species-area relationship was statistically significant in natural but not in constructed wetlands. Plant richness varied independently of area for both wetland classes. Area and anthropogenic disturbance were significant predictors of avian species richness in natural wetlands, but richness was uncorrelated with all variables in constructed wetlands except for habitat heterogeneity. Although mean avian species richness was similar between natural and constructed wetlands overall, community composition differed markedly and was most likely related to natural wetlands\u27 greater age, larger size and distance from busy roads relative to constructed wetlands

Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature

Resonances: An Examination of Republication Through Four Case Studies

Republication, with or without textual changes, keeps a work in circulation. This protects the work from destruction but also affects how we receive it, because publication is always a socializing act. Despite its consequences for works and their reception, republication has not yet been theorized in textual studies. My dissertation addresses this research gap by employing the term resonance to discuss the relationships—between versions, contexts, and ideas—that develop out of republication. I explore republication at its extremes with four case studies of works that underwent major changes in republication. The first chapter examines Oscar Wilde’s The Picture of Dorian Gray with a mixed-methods analysis of the reviews of its 1890 and 1891 publications in relationship to the textual revisions. The second chapter argues that in Evelyn Waugh’s Brideshead Revisited, the much later moment of republication is a moment of nostalgic return. My chapter on Audre Lorde’s Sister Outsider shows how periodical/conference publications place Lorde’s essays into larger conversations that the monograph republication replaces with a unified presentation of Lorde as a theorist. Finally, my fourth chapter brings in adaptation theory to discuss the complexities of republication for digital-born works with Andrew Hussie’s Homestuck. This dissertation enriches our understanding of republication’s consequences for four specific works. It also theorizes republication as a vital object of study that affects not just the versions of works we read but the histories that develop alongside them, and it creates a foundation for further studies of the phenomenon

Fault detection and isolation in spacecraft attitude control system using parity space method

In this thesis, the problem of fault detection and diagnosis in the spacecraft attitude control system is considered. For this purpose, a fault or a failure in a spacecraft is first defined. This is followed up by description and details on the spacecraft architecture, and specifically the spacecraft attitude control system (ACS). The method developed for fault detection and diagnosis is the parity space or the parity equation approach. This method is applied to one type of actuators, namely reaction wheels that are commonly used in the spacecraft attitude control system. The capabilities of this method for fault detectability are studied through extensive numerical simulations. The proposed method is validated by comparing the obtained results with those of a fault detection method based on linear observer approach. Among the various variables in a reaction wheel of the attitude control system, bus voltage and actuator current components are of particular interest in this thesis, due to their influence in performance of the ACS of the spacecraft. Different types of fault signatures are used for these two components, and the simulated results are compared through two different approaches for fault detection. The advantages of the proposed and designed parity space-based method over a linear observer-based approach for fault detection and diagnosis is shown to be significant especially in detecting faults in the pitch axis of the spacecraft

The Prevalence of Shiga toxin-producing Escherichia coli in Patients with Gastroenteritis and Sources of Infections in Iran: A Systematic Review Study Protocol

Hemolytic Uremic Syndrome (HUS) is a leading cause of acute kidney injury in infants and children. The majority of HUS in children is secondary to Shiga toxin induced Escherichia Coli (STEC). Many countries have a strategy to identify the source of infection and annually estimate the rate of this infection but there is no prevention strategy to identify this infection in Iran.The aim of this study is to find the percentage of STEC identified in various samples, in addition to find the outcome of STEC infected patients in Iran.We will search PubMed, EMBASE, Google Scholar, Google, OVID, SCOPUS, Web of Sciences, IranMedex, MagIran, and SID, dociran, PDFiran, ganj.irandoc, and all online university database for thesis, and abstract books of regional or international congresses  between January 1985 and January 2016.One author will do the search screen for potentially eligible studies and two authors independently assess whether the studies meet the inclusion criteria. Discrepancies between the two authors will be resolved through discussion and arbitration by a third author. Data from the selected studies shall be extracted using a data collection form. The methodological quality of studies will be assessed using the STORBE checklist and risk of bias assessment tool developed by Hoy et al for assessing risk of bias for epidemiological studies. We will use random effects models to assess inter-study heterogeneity. Dichotomous data will be analyzed using relative risk and continuous will be assessed with data mean differences, both with 95% CIs.Keywords: Gastroenteritis; Humans; Iran; Prevalence; shiga Yoxin 2; shiga-Toxigenic Escherichia coli

Spectrally narrow exciton luminescence from monolayer MoS2 exfoliated onto epitaxially grown hexagonal BN

The strong light-matter interaction in transition Metal dichalcogenides (TMDs) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation, which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS$_2$ and MoSe$_2$ MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then we cover them with exfoliated hBN to finally obtain an encapsulated sample : exfoliated hBN/TMD ML/MBE hBN. We observe an improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO$_2$ substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge free substrate for fabricating TMD-based heterostructures on a larger scale.Comment: 5 pages, 3 figure