Materials Science & Engineering
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Item Electron Beam Induced Current in Wide Bandgap Semiconductors using Scanning Transmission Electron Microscopy(2020) Warecki, Zoey; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wide bandgap semiconductors are those with a larger bandgap than silicon; this property allows them to operate at higher voltages, higher driving frequencies, and higher operating temperatures. Gallium nitride (GaN) in particular is attractive for its high critical electric field and thus high breakdown strength allowing for the design of a thinner drift region for a given blocking voltage. It is for these same reasons that GaN is also more radiation resistant than Si, and thus is attractive for satellite or space applications. With the recent commercial availability of free standing GaN substrates, there are many fundamental properties of GaN-on-GaN devices that are still not understood. One of the main characterization techniques used to classify GaN device quality is the measurement of the minority carrier diffusion length via electron beam induced current (EBIC). One of the main limitations of the traditional scanning electron microscopy (SEM) EBIC technique is due to the size of the electron beam/specimen interaction volume at > 5 kV, as well as large collection losses due to carrier recombination at the surface at < 5 kV. This dissertation addresses the previous issues of SEM EBIC with a non-traditional bulk scanning transmission electron microscopy (STEM) EBIC technique which allows for high resolution measurements of the hole diffusion length in n-GaN/Ni Schottky diodes. A reproducible, non-invasive bulk STEM sample preparation technique for n-GaN/Ni Schottky diodes is developed for the use of collecting bulk STEM EBIC micrographs. Despite the large interaction volume in this system at 100-200 kV, quantitative bulk STEM EBIC imaging is possible due to the small STEM probe beam diameter and sustained collimation of the incident electron beam in the sample. Using a combination of experimental bulk STEM EBIC measurements, Monte Carlo simulations, and numerical simulations, a hole diffusion length of 250 ± 15 nm was determined for homoepitaxial n-GaN samples with a threading dislocation of approximately 10^6 cm^-2. In-situ reverse biasing measurements allowed for the measurement of depletion region growth with increasing bias. Furthermore, accumulated electron irradiation damage was studied at 200 kV. An accumulated dose of 24 x 10^16 electrons cm^-2 caused a 35% reduction in the minority carrier diffusion length which is attributed to knock-on damage of the N sublattice. Additionally, the design and development of a custom STEM holder for in-situ liquid cell electrochemical microscopy is discussed.Item ATOMIC LAYER DEPOSITION OF LEAD ZIRCONATE-TITANATE AND OTHER LEAD-BASED PEROVSKITES(2019) Strnad, Nicholas Anthony; Phaneuf, Raymond J; Polcawich, Ronald G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lead-based perovskites, especially lead zirconate-titanate (PbZrxTi1-xO3, or PZT), have been of great technological interest since they were discovered in the early 1950s to exhibit large electronic polarization. Atomic layer deposition (ALD) is a thin-film growth technique capable of uniformly coating high aspect-ratio structures due to the self-limited nature of the precursor chemisorption steps in the deposition sequence. In this thesis, a suite of related processes to grow lead-based perovskites by ALD are presented. First, a new process to grow ferroelectric lead titanate (PbTiO3, or PTO) by ALD using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)2] and tetrakis dimethylamino titanium [TDMAT] as the lead and titanium cation precursors, respectively, is discussed. A 360-nm thick PTO film grown by ALD displayed a maximum polarization of 48 µC/cm2 and remanent polarization of ±30 µC/cm2. Second, a new process (similar to the ALD PTO process) to grow PZT by ALD is demonstrated by partial substitution of TDMAT with either tetrakis dimethylamino zirconium or zirconium tert-butoxide. The 200 nm-thick ALD PZT films exhibited a maximum polarization of 50 µC/cm2 and zero-field dielectric constant of 545 with leakage current density < 0.7 µA/cm2. Third, a new ALD process for antiferroelectric lead hafnate (PbHfO3, or PHO) is presented along with electrical characterization showing a field-induced antiferroelectric to ferroelectric phase transition with applications for capacitive energy storage. Finally, ALD lead hafnate-titanate (PbHfxTi1-xO3, or PHT), considered to be an isomorph of PZT, is demonstrated by combining the process for PTO and PHO. The thin-film PHT grown by ALD is shown to have electronic properties that rival PZT grown at compositions near the morphotropic phase boundary (MPB). The processes for both ALD PZT and PHT are shown to yield films with promising properties for microelectromechanical systems (MEMS) actuators and may help to dramatically increase the areal work density of such devices.Item MULTI-FUNCTIONAL NANOSTRUCTURED FILMS FROM CELLULOSE NANOFIBERS(2017) Jang, Soo-Hwan; Hu, Liangbing; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cellulose nanofibrils (CNF) are one of most popular materials in nanotechnologies due to its favorable properties, such as biodegradability and high mechanical performance. However, their nano-/microscopric structure is not fully understood. In this thesis, we studied the structural features of CNF by using atomic force microscopy (AFM) and scanning electron microscopy (SEM), and then assessed the dimensions of single fibers from different wood species. We studied the dependence of cellulose nanopaper strength and toughness on the size of cellulose fibers using dynamic mechanical analysis (DMA). Interestingly, we found that both the strength and toughness increased as the fiber aspect ratio increased. Additionally, stability tests of carbon nanotubes and cellulose nanofibrils (CNT-CNF) solution were conducted by rheological measurement. The solution showed high stability and no visible precipitation. Based on these properties, we fabricated functionalized nanostructured films from CNF and observed promising results from the novel materials.Item ALD-ENABLED CATHODE-CATALYST ARCHITECTURES FOR LI-O2 BATTERIES(2015) Schroeder, Marshall Adam; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Li-O2 electrochemical redox couple is one of the prime candidates for next generation energy storage. Known for its impressive theoretical metric for specific energy, even current practically obtainable values are competitive with state of the art Li-ion intercalation chemistries and the achievable performance of batteries featuring this nascent technology will continue to improve as fundamental scientific challenges in each component of the device are addressed. The positive electrode is particularly complicated by its role as a scaffold for oxygen reduction and evolution, exhibiting sluggish kinetics, poor chemical stability, and limited cyclability due to parasitic side reactions. Fortunately, recent Li-O2 research has shown some success in improving the performance and cyclability of these O2 cathodes by shifting toward nanostructured architectures with catalytic functionalizations. Atomic layer deposition (ALD) is one of the most promising enabling technologies for fabricating these complex heterostructures. Offering precise control of film thickness, morphology, and mass loading with excellent conformality, this vapor-phase deposition technique is applied in this work to deposit thin film and particle morphologies of different catalyst chemistries on mesostructured carbon scaffolds. This thesis dissertation discusses: (1) development of a lab-scale infrastructure for assembly, electrochemical testing, and characterization of Li-O2 battery cathodes including a custom test cell and a state of the art integrated system for fabrication and characterization, (2) design, fabrication, testing, and post-mortem characterization of a unique 3D cathode architecture consisting of vertically aligned carbon nanotubes on an integrated nickel foam current collector, (3) atomic layer deposition of heterogeneous ruthenium-based catalysts on a multi-walled carbon nanotube sponge to produce a freestanding, binder-free, mesoporous Li-O2 cathode with high capacity and long-term cyclability, (4) evaluation of dimethyl sulfoxide as an electrolyte solvent for non-aqueous Li-O2 batteries, and (5) investigation of the relative importance of passivating intrinsic defects in carbon redox scaffolds vs. introduction of heterogeneous OER/ORR catalysts for improving the long-term stability and cyclability of these Li-O2 electrodes.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.Item REAL-TIME INVESTIGATION OF INDIVIDUAL SILICON NANOSTRUCTURED ELECTRODES FOR LITHIUM-ION BATTERIES(2013) Karki, Khim Bahadur; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silicon-based anode materials are an attractive candidate to replace today's widely-utilized graphitic electrodes for lithium-ion batteries because of their high gravimetric energy density (3572 mAh/g vs. 372 mAh/g for carbon) and relatively low working potential (~ 0.5V vs. Li/Li+). However, their commercial realization is still far away because of the structural instabilities associated with huge volume changes of ~300% during charge-discharge cycles. Recently, it has been proposed that silicon nanowires and other related one-dimensional nanostructures could be used as lithium storage materials with greatly enhanced storage capacities over that for graphite in the next generation of lithium-ion batteries. However, the studies to date have shown that the nanomaterials, while better, are still not good enough to withstand a large number of lithiation cycles, and moreover, there is little fundamental insight into the science of the improvements or the steps remaining before widespread adoption. This dissertation seeks to understand the basic structural properties and reaction kinetics of one dimensional silicon nanomaterials, including Si-C heterostructures during electrochemical lithiation/delithiation using in-situ transmission electron microscopy (TEM). I present my work in three parts. In part I, I lay out the importance of lithium-ion batteries and silicon-based anodes, followed by experimental techniques using in-situ TEM. In part II, I present results studied on three different nanostructures: Si nanowires (SiNWs), Si-C heterostructures and Si nanotubes (SiNTs). In SiNWs, we report an unexpected two-phase transformation and anisotropic volume expansion during lithiation. We also report an electrochemically-induced weld of ~200 MPa at the Si-Si interface. Next, studies on CNT@α-Si heterostructures with uniform and beaded-string structures with chemically tailored carbon-silicon interfaces are presented. In-situ TEM studies reveal that beaded-string CNT@ α-Si structures can accommodate massive volume changes during lithiation and delithiation without appreciable mechanical failure. Finally, results on lithiation-induced volume clamping effect of SiNTs with and without functional Ni coatings are discussed. In Part III, a conclusion and a brief outlook of the future work are outlined. The findings presented in this dissertation can thus provide important new insights in the design of high performance Si electrodes, laying a foundation for next-generation lithium ion batteries.Item Nanomechanical Properties and Buckling Instability of Plasma Induced Damaged Layer on Polystyrene(2012) Lin, Tsung-Cheng; Phaneuf, Raymond J; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we report on an investigation of an elastic buckling instability as a driving force for the roughening of polystyrene, a model resist, during Ar+ plasma etching. Polystyrene films etched by pure Ar+ plasma with different ion energies were characterized using both atomic force microscopy topography and force curve measurements. By using height-height correlation function in analyzing the AFM measured topography images, we find that surface corrugation of etched polystyrene film surfaces all display a dominant wrinkle wavelength (ë), which is a function of ion energy. Next, we characterized the mechanical properties of these samples using AFM force curve measurements in an controlled ambient environment. We analyzed the measured force curves using a systematic algorithm based on statistical fitting procedures, and taking into account the adhesive interaction, in order to determine the effective elastic modulus of the films. We find that the effective elastic modulus (EBL) of the etched samples increases monotonically with increasing ion energy, but the changes are rather subtle as compared to the elastic modulus (EPS) of the unetched one. In order to test the validity of a buckling instability as the mechanism for surface roughening in our polystyrene-Ar plasma system, the elastic modulus of individual layer (i.e. ion-damaged layer plus unmodified foundation) needs to be determined. We present a determination of the damaged layer elastic modulus (EDL) from the effective elastic modulus of the damaged layer/polystyrene bilayer structure (EBL), based upon a finite element method simulation taking into account the thickness and elastic modulus of the damaged layers. We extract the damaged layer elastic modulus versus etching ion energy initially within the approximation of a spherical tip in contact with a flat sample surface. We next extend our model, by considering a periodic corrugated film surface, with its amplitude and wavelength determined by AFM, to take into account the effect of roughness induced by plasma exposure. The damaged layer elastic modulus extracted from these two approximations gives of quantitative agreement, and thus evidence for the correlation between buckling instability and plasma-induced roughening.Item Synthesis and Characterization of Functional One Dimensional Nanostructures(2011) Lee, Kwan; Ouyang, Min; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One- dimensional (1D) nanostructures have received growing interest due to their unique physical and chemical properties and promising nanodevice applications, as compared with their bulk counterparts. Complex 1D nanostructures with tunable properties and functionalities have been successfully fabricated and characterized in this thesis. I will show our recent efforts on precise controlled 1D nanostructures by template- assisted electrochemical synthesis as well as fundamental understanding of their physical behavior and growth mechanism of as-synthesized nanostructures. Particularly, three topics are presented: Firstly, a constant current (CC) based anodization technique is newly demonstrated to fabricate and control the structure of an anodic aluminum oxide (AAO) template. This technique has enabled the formation of long- range self- ordered hexagonal nanopore patterns with broad range of tunability of interpore distance (Dint). In addition, the combination of CC based anodization and conventional CV anodization can offer a fast, simple, and flexible methodology to achieve new degrees of freedom for engineering planar nanopore structures. This work also facilitates our understanding of the self- ordering mechanism of alumina membranes and complex nanoporous structure. Secondly, functional 1D nanostructures including pure metallic, magnetic and semiconducting nanowires and their heterostructure are demonstrated by versatile template- based electrochemical deposition under feasible control. This study has enabled the creation of high quality and well- controlled 1D nanostructures that can be applied as a model system for understanding unique 1D physics. Some preliminary investigations including exciton confinement, anisotropic magnetism and surface plasmon resonance are also presented. Lastly, a novel and universal non-epitaxial growth of metal-semiconductor core-shell lattice-mismatched hybrid heterostructures is presented. Importantly, a new mechanical stress driven crystalline growth mechanism is developed to account for non-epitaxial shape and monocrystalline evolution kinetics.Item SPECTROSCOPIC ENHANCEMENT FROM NOBLE METALLIC NANOPARTICLES(2011) Tsai, Shu-Ju; Phaneuf, Raymond J.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Resonant coupling of localized surface plasmon resonances (LSPRs) in noble metallic nanostructures to incident radiation and the related subject of localized behavior of electromagnetic waves are currently of great interest due to their potential application to sensors, biochemical assays, optical transmission, and photovoltaic devices. My thesis research is made up of two related parts. In part one I examined enhanced fluorescence in dye molecules in proximity to Ag nanostructures. In part two I studied the effect of Au nanostructure arrays on the performance of poly(3-hexylthiophene-2,5-diyl) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) organic solar cells (OSCs). Nanostructures were fabricated by two different methods: e-beam lithography (top down) and spray pyrolysis (bottom up). Using e-beam lithography, we produced arrays of nanostructures with well defined shapes, sizes, and spacings. By systematically varying these topographical parameters, we measured their effect on nanometer-sized metallic structure-enhanced fluorescence (nMEF) and on absorption and external quantum efficiency (EQE) in OSC devices as a function of optical wavelength. In analyzing experimental results, we carried out numerical simulations of the local electric field under incident light, across plasmonic resonances. The comparison between the calculated local field squared and measured fluorescence/EQE provides physical insight on the configuration- dependence of these two processes. Our results indicate that local field enhancement near nanostructures is dominant in nMEF, and that the local field is strongly affected by the substrate and device architectures. For the OSCs, both measurements and calculations show that absorbance within the active layer is enhanced only in a narrow band of wavelengths (~640-720 nm) where the active layer is not very absorbing for our prototype nanopillar-patterned devices. The peak enhancement for 180 nm wide Au nanopillars was approximately 60% at 675nm. The corresponding resonance involves both localized surface plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. Finally, we explore the role of the size of the nanostructures in such a device on the optical absorption in the OSC active layer. We find that small Au nanopillars produce strong internal absorption resulting in Joule heating, and suppressing the desired enhancement in EQE in OSC devices.Item INFLUENCE OF POLYMER STRUCTURE ON PLASMA-POLYMER INTERACTIONS IN RESIST MATERIALS(2010) Bruce, Robert Lawson; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The controlled patterning of polymer resists by plasma plays an essential role in the fabrication of integrated circuits and nanostructures. As the dimensions of patterned structures continue to decrease, we require an atomistic understanding underlying the morphological changes that occur during plasma-polymer interactions. In this work, we investigated how plasma surface modifications and the initial polymer structure influenced plasma etch behavior and morphological changes in polymer resists. Using a prototypical argon discharge, we observed polymer modification by ions and vacuum ultraviolet (VUV) radiation from the plasma. A thin, highly dense modified layer was formed at the polymer surface due to ion bombardment. The thickness and physical properties of this ion-damaged layer was independent of polymer structure for the systems examined here. A relationship was observed that strongly suggests that buckling caused by ion-damaged layer formation on a polymer is the origin of roughness that develops during plasma etching. Our results indicate that with knowledge of the mechanical properties of the ion-damaged layer and the polymer being processed, plasma-induced surface roughness can be predicted and the surface morphology calculated. Examining a wide variety of polymer structures, the polymer poly(4-vinylpyridine) (P4VP) was observed to produce extremely smooth surfaces during high-ion energy plasma etching. Our data suggest that VUV crosslinking of P4VP below the ion-damaged layer may prevent wrinkling. We also studied another form of resists, silicon-containing polymers that form a SiO2 etch barrier layer during O2 plasma processing. In this study, we examined whether assisting SiO2 layer formation by adding Si-O bonds to the polymer structure would improve O2 etch behavior and reduce polymer surface roughness. Our results showed that while adding Si-O bonds decreased etch rates and silicon volatilization during O2 plasma exposure, the surface roughness became worse. Enhanced roughening was linked to the decrease in glass transition temperature and elastic modulus as Si-O bonds were added to the polymer structure. For polymers used as resists it is required that the mechanical properties of the ion-damaged layer and the polymer be taken into account to understand their roughening behavior.