Doctor of Philosophy / Department of Chemistry / Emily McLaurin / Quantum dots are nanocrystalline semiconductors in which the size is so small that optoelectronic properties are size dependent. QDs have a lot of applications in displays, solar cells, lasers, light emitting diodes, etc. The optoelectronic properties of QDs depend on their size, composition, the shape of the particles and also the surface chemistry of the QDs. Phosphine based precursors have been mostly used in the synthesis of QDs. Due to the lack of tunable reactivity, this class of precursors, QDs with different shape are obtained by under different reaction conditions. With that, branched QDs are less likely to be obtained in one step reaction using phosphine based precursors. To synthesis QDs with a branched structure, in a single step synthesis, mixtures of precursors with different reactivity were used. Using dichalcogenides mixture, CdSe₁-xSx hyperbranched supra-quantum dots (HSQDs) where synthesized in a one-step microwave-assisted synthesis and shape evolution mechanism of formation of NCs studied. It is shown that the NCs formed in three steps of nucleation, aggregation, and growth. By controlling the reaction conditions, simple branched tetrapod NCs are prepared, but the obtained NCs have no emission due to unpassivated surface and defects which work as trap. To obtain luminescent NCs obtained through doping. Hyperbranched Mn²+:ZnSe₁-xSx NCs also prepared using a mixture of Ph₂Se₂ and Me₂S₂. The shape evolution mechanism of the formation of NCs was studied and it is shown that the NCs are formed via oriented attachment of initially formed nanoparticles. The NCs used for thiol sensing, and it observed that they have a better sensitivity and detection limit than spherical QDs. Although hyperbranched NCs have higher sensitivities over nonbranched NCs but, the spherical NCs have better detection limit and can dispersed in aqueous medium by ZnS shell growth followed by silica shell formation. To study the effect of ZnS shell thickness on sensing property of NCs, a set of spherical Mn:ZnSe@ZnS with different ZnS shell thickness were prepared and used for thiol sensing. It observed that in organic medium, thinner ZnS layer gives the highest sensitivity and QDs with thick ZnS shell layer have less sensitivity. For measurement in aqueous medium, QDs transferred to PBS buffer after formation of silica shell over QDs. It observed that NCs with a thin ZnS shell layer lose their emission and sensing completely. Thick ZnS shell protects NCs in the silica shell formation step but they show very low sensitivity to thiol compounds as well. ZnS shell with medium thickness gives the best sensitivity in an aqueous medium. The emission of Mn:ZnSe@ZnS QDs originated from d-d electron transition of Mn(II) ions and is independent to the size of QDs. To extend our study to QDs with band edge emission, preparation of luminescent InP QDs by post-synthetic modification is studied. InP NCs were synthesized using heat up method and successive injection of precursors. Narrow size distribution NCs obtained after size selection precipitation. Emissive NCs obtained after etching using InCl3 and fluoride containing salts. The study showed that more InCl3 case more etching and presence of fluoride-containing salt is necessary for band edge emission of the NCs.

abstract: Ultrasonication-mediated liquid-phase exfoliation has emerged as an efficient method for producing large quantities of two-dimensional materials such as graphene, boron nitride, and transition metal dichalcogenides. This thesis explores the use of this process to produce a new class of boron-rich, two-dimensional materials, namely metal diborides, and investigate their properties using bulk and nanoscale characterization methods. Metal diborides are a class of structurally related materials that contain hexagonal sheets of boron separated by metal atoms with applications in superconductivity, composites, ultra-high temperature ceramics and catalysis. To demonstrate the utility of these materials, chromium diboride was incorporated in polyvinyl alcohol as a structural reinforcing agent. These composites not only showed mechanical strength greater than the polymer itself, but also demonstrated superior reinforcing capability to previously well-known two-dimensional materials. Understanding their dispersion behavior and identifying a range of efficient dispersing solvents is an important step in identifying the most effective processing methods for the metal diborides. This was accomplished by subjecting metal diborides to ultrasonication in more than thirty different organic solvents and calculating their surface energy and Hansen solubility parameters. This thesis also explores the production and covalent modification of pristine, unlithiated molybdenum disulfide using ultrasonication-mediated exfoliation and subsequent diazonium functionalization. This approach allows a variety of functional groups to be tethered on the surface of molybdenum disulfide while preserving its semiconducting properties. The diazonium chemistry is further exploited to attach fluorescent proteins on its surface making it amenable to future biological applications. Furthermore, a general approach for delivery of anticancer drugs using pristine two-dimensional materials is also detailed here. This can be achieved by using two-dimensional materials dispersed in a non-ionic and biocompatible polymer, as nanocarriers for delivering the anticancer drug doxorubicin. The potency of this supramolecular assembly for certain types of cancer cell lines can be improved by using folic-acid-conjugated polymer as a dispersing agent due to strong binding between folic acid present on the nanocarriers and folate receptors expressed on the cells. These results show that ultrasonication-mediated liquid-phase exfoliation is an effective method for facilitating the production and diverse application of pristine two-dimensional metal diborides and transition metal dichalcogenides. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2018

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22988号 / 理博第4665号 / 新制||理||1669(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 田中 耕一郎, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

abstract: Two-dimensional quantum materials have garnered increasing interest in a wide variety of applications due to their promising optical and electronic properties. These quantum materials are highly anticipated to make transformative quantum sensors and biosensors. Biosensors are currently considered among one of the most promising solutions to a wide variety of biomedical and environmental problems including highly sensitive and selective detection of difficult pathogens, toxins, and biomolecules. However, scientists face enormous challenges in achieving these goals with current technologies. Quantum biosensors can have detection with extraordinary sensitivity and selectivity through manipulation of their quantum states, offering extraordinary properties that cannot be attained with traditional materials. These quantum materials are anticipated to make significant impact in the detection, diagnosis, and treatment of many diseases. Despite the exciting promise of these cutting-edge technologies, it is largely unknown what the inherent toxicity and biocompatibility of two-dimensional (2D) materials are. Studies are greatly needed to lay the foundation for understanding the interactions between quantum materials and biosystems. This work introduces a new method to continuously monitor the cell proliferation and toxicity behavior of 2D materials. The cell viability and toxicity measurements coupled with Live/Dead fluorescence imaging suggest the biocompatibility of crystalline MoS2 and MoSSe monolayers and the significantly-reduced cellular growth of defected MoTe2 thin films and exfoliated MoS2 nanosheets. Results show the exciting potential of incorporating kinetic cell viability data of 2D materials with other assay tools to further fundamental understanding of 2D material biocompatibility. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019

Tin disulfide (SnS2) is a two-dimensional (2D) material with excellent properties and high prospects for low-cost solutions to catalytic and optoelectronic applications. In this work, vertical nanoflakes of SnS2 have been synthesized using custom-designed close space sublimation (CSS) system and investigated for applications in photoelectrochemical (PEC) water oxidation and metal-semiconductor-metal (MSM) photodetector. For the PEC application, vertical SnS2 nanoflakes grown directly on transparent conductive substrates have been used as photoanodes, which produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte. This remarkable performance has been attributed to three main reasons: (1) high intrinsic carrier mobility of 330 cm2 V−1 s−1 and long photoexcited carrier lifetime of 1.3 ns in the nanoflakes, (2) the nanoflake height that balances the competing requirements of light absorption and charge transport, and (3) the unique stepped morphology of these nanoflakes that improves photocurrent by exposing multiple edge sites in every nanoflake. In another application, these SnS2 nanoflakes have been used to enhance the performance of lead sulfide quantum dot (PbS QDs) photodetectors by providing a high-mobility channel for photoexcited charges from PbS QDs, which results in 2 orders of magnitude enhancement in responsivity. The physical models and experimental findings presented in this dissertation can help engineer more cost-effective solutions for PEC water splitting and optoelectronics based on 2D metal dichalcogenides.

Thesis advisor: Michael J. Naughton / In this dissertation I present studies of plasmonic interactions in different coupling regimes, from zero to strong coupling and approaching ultrastrong coupling. Different physics are manifest in each regime, with different possible applications. The first project uses finite element electromagnetic simulations to model plasmonic waveguides that couple near field light into the far-field for sub-diffraction limited microscopy. Wavelength/32 resolution is shown by minimizing coupling between adjacent waveguiding nanowires, with minimal attenuation over a few microns. The next two projects, by contrast, seek to maximize coupling between plasmons and excitons into the strong coupling regime where the optoelectronic properties are modified and quantum coherent phenomena may be observed. Strong exciton–plasmon coupling in MoS2 is shown experimentally at room temperature and found to be a general phenomenon in other semiconducting transition metal dichalcogenides using transfer matrix modeling. A semiclassical oscillator model is fit to the experimental data to discover coherent hybridization between the ground and first excited states of MoS2. Enhanced coupling is found at the third excitonic transition, approaching the ultrastrong coupling regime where exotic properties are predicted to emerge, such as ground state virtual photons. Our strong coupling studies motivate further studies of the TMDCs as a platform for coherent quantum physics with possible applications in quantum computing and cryptography. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21315号 / エネ博第371号 / 新制||エネ||73(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 松田 一成, 教授 佐川 尚, 教授 大垣 英明 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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This past decade, 2D materials beyond graphene, and most specifically transition metal dichalcogenides (TMDCs) have gained remarkable attention due to their novel applications in electronics and optoelectronics applications. This work reports large-area growth and structural, optical, and electronic transport properties of few-layer MoS2 thin films fabricated using a hybrid approach based on the magnetron sputtering method. In the first part of this dissertation, properties of optimally annealed MoS2 on different substrates such as amorphous BN, SiO2, Si, Al2O3 are discussed using diffraction, spectroscopic, and transport techniques. Later, we show that the physical properties of large-area sputtered MoS2 thin films can be dramatically improved by an ex-situ high-temperature sulfurization process as it leads to the formation of defect-free MoS2 by removing sulfur vacancies. Sharp film-substrate interface along with high bulk structural order is demonstrated as inferred from diffraction and spectroscopic methods. We show that sulfur vacancies can obscure the MoS2 A-B exciton peaks along with a sharp increase in dc conductivity of MoS2. In the last part of my dissertation, we outline the growth of a novel thermoelectric material (SnSe) and new magnetic inverse-Heuslers (of nominal composition MnxFeSi) using the co-sputtering method. These are some of the first attempts, to our knowledge, to grow such materials in thin-film form. Detailed structure-property relations are thoroughly discussed.