Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Fluorescent Carbon Nanotubes as Molecular Sensors and Color-Center Hosts(2022) Qu, Haoran; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates the application of single-walled carbon nanotubes (SWCNTs) as single-digit nanopores for molecular sieving and addresses a fundamental challenge pertaining to controlled synthesis of organic color-centers (OCCs) on the sp2 carbon lattice of SWCNTs. First, I describe a hyperspectral single-defect photoluminescence imager system that provides both hyperspectral imaging and super-resolution capabilities in the shortwave infrared. Second, I aim to understand the relationship between nanotube photoluminescence and encapsulated molecules. Using carbon nanotubes with sub-1 nm pores, I demonstrate molecular sieving of n-hexane from cyclohexane, which are nearly identical in size. Furthermore, I discovered a light irradiation method to drive structural transformation of OCCs which allow us to narrow the spectral distribution of defect emissions by 26%. Finally, I show that [2+2] cycloaddition can efficiently create OCCs. Remarkably, this novel defect chemistry reduces the number of OCC bonding configurations from six, which are commonly observed with monovalent defect chemistries, to just three. This work may have broad implications to the potential applications of SWCNTs and OCCs in chemical sensing, bioimaging, and quantum information science.Item Controlling Nanostructures for in-situ TEM Characterization(2015) Nilsson, Hanna Magdalena; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Low dimensional nanostructures, such as nanotubes and 2D sheets, have unique and promising material properties both from a fundamental science and an application standpoint. Theoretical modelling and calculations predict previously unobserved phenomena that experimental scientists often struggle to reproduce because of the difficulty in controlling and characterizing the small structures under real-world constraints. The goal of this dissertation is to controlling these structures so that nanostructures can be characterized in-situ in transmission electron microscopes (TEM) allowing for direct observation of the actual physical responses of the materials to different stimuli. Of most interest to this work are the thermal and electrical properties of carbon nanotubes, boron nitride nanotubes, and graphene. The first topic of the dissertation is using surfactants for aqueous processing to fabricate, store, and deposit the nanostructures. More specifically, thorough characterization of a new surfactant, ammonium laurate (AL), is provided and shows that this new surfactant outperforms the standard surfactant for these materials, sodium dodecyl sulfate (SDS), in almost all tested metrics. New experimental set-ups have been developed by combining specialized in-situ TEM holders with innovative device fabrication. For example, electrical characterization of graphene was performed by using an STM-TEM holder and depositing graphene from aqueous solutions onto lithographically patterned, electron transparent silicon nitride membranes. These experiments produce exciting information about the interaction between graphene and metal probes and the substrate that it rests on. Then, by adding indium to the backside of the membrane and employing the electron thermal microscopy (EThM) technique, the same type of graphene samples could be characterized for thermal transport with high spatial resolution. It is found that reduced graphene oxide sheets deposited onto a silicon nitride membrane and displaying high levels of wrinkling have higher than expected electrical and thermal conduction properties. We are clearly able to visualize the ability of graphene to spread heat away from an electronic hot spot and into the substrate.Item Heat dissipation in current carrying multiwalled carbon nanotubes(2014) Voskanian, Norvik; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding thermal transport is of great interest in combatting the excess heat generated in current electronic circuits. In this dissertation we provide insight and progress in thermal transport in current carrying MWCNT. Chapter 1 gives an overview of the work presented in this dissertation, quickly discusses the motivation for studying heat dissipation in current carrying carbon nanotubes, and outlines the key findings. The chapter outlines the unique remote heating phenomena observed in Joule heated MWCNTs, as well as, the process in which the research led to the discovery of a detection method for near-field heat transfer. The physical properties of carbon nanotubes are discussed in Chapter 2 and the relevant heat transfer mechanisms are introduced. Chapter 3 outlines some of the previous experimental work in studying thermal properties of nanotubes. The results presented in this dissertation rely on previously measured thermal conductivity and thermal contact resistance for nanotubes and thus a discussion of these results is critical. The fabrication process for the measured devices is presenter in Chapter 4. In addition, chapter 4 provides a detailed discussion of the measurement technique employed to probe the thermal properties of the devices presented in Chapter 5 and 6. Chapter 5 discusses the findings in regard to heat dissipation for a current carrying MWCNT supported on a SiN substrate. The results provide definitive proof of substrate heating via hot electrons; a process which can not be explained using traditional Joule heating model and requires the presence of an additional remote heating mechanism. Analysis of the results indicate a reduction in remote Joule heating which led to a series of controlled experiments presented in Chapter 6 in an effort to study substrate thermal conductivity, kSiN, variations as a function of voltage. In this chapter we outline the experimental and simulated results which indicate the remarkable ability of our technique to detect near-field thermal radiation. The enhanced thermal transport via near-field radiation is of great interest for scientific and engineering purposes but its detection has proven difficult. This thesis provides evidence of the sensitivity of the electron thermal microscopy technique to measure near-field radiation.