Browsing by Subject "Materials Science"

Now showing 1 - 5 of 5
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    Chemical Vapor Deposition Growth of Two-Dimensional Transition Metal Dichalcogenides and Related Heterostructures
    (2018-09) DeGregorio, Zachary
    Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are atomically thin, layered materials with unique physical and electronic properties relative to their bulk forms. Due to these properties, 2D TMDCs show promise for many applications, including catalysis, nanoelectronics, optoelectronics, and spin- and valleytronics. To utilize TMDCs for these applications, they must first be reproducibly isolated. Much previous work in this area has resulted in material batches with low yield, small crystal sizes, and little control over the crystal morphology and orientation. Here, I present the reproducible chemical vapor deposition (CVD) growth of a wide array of 2D TMDCs, including MoS2, WS2, MoTe2, NbS2, and WSe2. Control of the growth of these materials is achieved through the optimization of many parameters, including substrate surface chemistry and synthetic growth parameters. Through the optimization of these parameters, I demonstrate control over the resulting material thickness, phase, and morphology. These high-quality TMDCs are subsequently used to grow many relevant heterostructures, including MoS2/WS2 lateral and vertical heterostructures, MoO2/MoS2 core/shell plates, 2H-1T´ MoTe2 few-layer homojunctions, and WS2/NbS2 lateral heterostructures, and the utility of these heterostructures is assessed. MoS2/WS2 heterostructures show promise as a semiconductor-semiconductor heterostructure in which the nature of the alignment is controlled by the initial MoS2 seed crystal. MoO2/MoS2 core/shell plates are freestanding and show epitaxial alignment with the underlying crystal substrate, with potential applications in catalysis. 2H-1T´ MoTe2 few-layer homojunctions are grown using a patternable phase engineering procedure, and devices fabricated from these homojunctions show reduced contact resistance relative to 2H MoTe2 devices with noble metal contacts. Finally, WS2/NbS2 lateral heterostructures show promise as an alternative metal-semiconductor heterostructure system for creating 2D TMDC devices with low contact resistance. The controlled CVD growth of these materials and heterostructures bolsters their future use for relevant applications.
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    Energy conversion using phase transformation in multiferroic materials
    (2013-08) Song, Yintao
    The history of using first order phase transformations to convert heat into other forms of energy stretches back as far as the 1600's, when the first steam engine was invented. This method can be further applied to any first order phase transformation beyond the liquid-vapor systems. Multiferroic materials undergoing phase transformations, during which a ferroic property changes drastically, are promising candidates, especially in the small temperature difference regime. In this thesis I investigate the conversion from heat into electricity by this new method. A family of alloys undergoing martensitic phase transformation with a big change of magnetization is demonstrated to be capable of energy conversion. It is shown by construction of a demonstration that the proposed concept is feasible. Also the idea of using a temperature gradient for this new energy conversion method is examined. The analysis shows that it is possible to convert a temperature gradient to a temperature oscillation by automatically moving the specimen in two conservative force fields: gravity and magnetic field. Quasi-static and finite-time thermodynamic models are developed. Based on the models, the efficiency and power output of this new method is evaluated theoretically, and the directions of design improvement are proposed. The Clausius-Clapeyron relation (the effect of magnetic field on the transformation temperature) is found to be a key thermodynamic relation in this method. The utilization of other types of muliferroic phase-transforming materials is surveyed.
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    First-principles Study of Lattice Dynamics in Crystals
    (2022-06) Li, Shutong
    Lattice dynamics is a key component in solid state physics. It helps the understanding of many physical properties like structural phase transitions and ferroelectricity. Density functional theory, as a first-principles method, is used to investigate the lattice dynamics in this thesis. Followed by an introduction of density functional theory and lattice dynamics, I first study the strain-suppresed polarization switching barriers in layered perovskites. It is shown that the epitaxial strain is strongly coupled with the free energy of different crystal structures, which enables us to tune the energy difference between stable and transition states. The concept of distortion symmetry group is also utilized here to model the switching process accurately. Second, the idea of free-carriers-induced ferroelectricity is introduced. Free charge carriers is typically detrimental to proper ferroelectricity, but it is not the case for hybrid improper ferroelectrics. This unexpected phenomenon will be explained by the electron-enhancement of oxygen octahedral rotation. Group theory analysis and Landau free energy are also carefully looked into in this system. Third, the nature of chemical bonding in transition metal dichalcogenides (TMD) is investigated using Wannier functions. My DFPT results indicate anomalous ionic charges of HfS2 in the in-plane direction, which is also confirmed by infrared and Raman spectrum from our collaborators. The study of Wannier functions attributes this robust ionicity to the hybridization of Hf and S orbitals. Finally, this dissertation is concluded by a brief comment of future opportunities and challenges in this research field.
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    Impact of Air Gap Length and Waveguide Refractive Index on Luminescent Solar Concentrator Optical Behavior
    (2022) Hoernemann, Diana;
    The optical properties of luminescent solar concentrators (LSCs) containing CdSe/CdS core/shell nanocrystal luminophores in a matrix with a high index of refraction were characterized using UV-Vis spectroscopy, photoluminescence (PL), and PL lifetime measurements. The high refractive index matrix was realized by spin-coating solutions containing polyvinylpyrrolidone (PVP), titania, butanol, and luminophores onto glass substrates. The proportion of titania in the solutions was varied in order to change the refractive indices of the resulting samples. The transmission and reflection spectra of the samples showed evidence of thin film oscillations, known as Fabry-Perot modes, that increased in magnitude with the amount of titania in the sample, suggesting that increasing the amount of titania in the matrix of a sample likely increased its refractive index. The PL peaks of the luminophore blue-shifted from 630 to 627 nm as the concentration of titania increased. The PL lifetime of these samples was found to be 1.8-2.7 ns, with an outlier of 0.5 ns for one sample. Complementary Monte Carlo ray tracing simulations were used to explore the impact of a variable thickness air gap on the optical efficiency of an LSC system. It was found that an air gap of 0.05 mm reduced the peak optical efficiency by 14.6% as compared to a system with no air gap. This decrease is attributed to escape cone loss pathway as photons couple out the air gap as opposed to another loss pathway. Therefore, it is suggested the thickness of an adhesive layer between the waveguide and solar cells should be no greater than 0.05 mm to preserve the optical performance of the device.
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    Structure and mechanical properties of elastomeric block copolymers.
    (2010-12) Alfonzo, Carlos Guillermo
    This research presents the synthesis (by anionic polymerization and catalytic hydrogenation) and characterization of two types of block copolymers: CMC and XPX. In CMC, C is glassy poly(cyclohexylethylene) and M, the matrix, can be semicrystalline poly(ethylene) E, rubbery poly(ethylene-alt-propylene) P, or rubbery poly(ethylethylene) EE, or a combination to yield: CPC, CEEC, CEC, CPEEC and CEPC, with fC ≈ 0.18 – 0.30. XPX materials have X = CEC, fC ≈ fE, and fP ≈ 0.40 – 0.60. Block copolymer phase behavior and morphology were examined through a combination of DSC, rheology, SAXS, WAXS and TEM. CMC materials are meltordered due to block thermodynamic incompatibility with TODT > Tg (C) ≈ 147 °C and show lamellar or C cylinder morphologies. The design of XPX yields melt disordered materials up to high Mn with microphase segregation induced by E crystallization. Two high Mn XPX polymers are melt ordered above Tm(E) and show two correlation lengths in SAXS assigned to the C – E and X – P length scales. TEM images indicate that all XPX materials, irrespective of melt segregation, are characterized by composite glassy and crystalline hard domains dispersed in rubbery P at room temperature. Tensile and recovery testing at room temperature show that CMC and XPX materials, with the exception of plastic CEC, behave as thermoplastic elastomers with tunable properties. Interestingly, melt disordered XPX materials have competitive mechanical properties comparable to the strongest CMC polymers, but with advantageous processing. For melt ordered CMC, Tprocess > TODT, which is dependent on Mn, while for melt disordered XPX, Tprocess > Tm(E) ≈ 100 °C independent of Mn. The deformation of melt disordered XPX materials, probed by recovery studies and WAXS, suggests that deformation is first taken by P, then E and finally C, which causes ultimate failure, as agreed in the literature for conventional SBS and SIS thermoplastic elastomers. This implies that strain recovery in XPX materials can be comparable to that of CPC if materials contain low hard block content or are stretched to strains below the onset of E deformation. Finally, a collection of data of mechanical properties, namely modulus E, strain at break εb, tensile strength σTS and tension set εs, obtained from CMC, XPX and previously reported materials were examined. Most notably, E and εs were found to be strongly correlated with the volume fractions of C and E, as properties increase with (fC + fE)δ, where δ = 1 – 2.4. Ultimate properties such as σTS and εs are unaffected by changes in composition as failure is dictated by that of the hard domains and values are similar above a minimum amount of hard block. In addition, E, σTS, and εb are inversely correlated to rubber entanglement molecular weight Me, which implies that modulus and ultimate properties are affected by the ability of the rubber network to redistribute stress by entanglement slippage. However, εs is unresponsive to Me variations, which indicates that irrecoverable deformation in these materials results from deformation of the hard domains.