A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Silicon modulates multi-layered defense against powdery mildew in Arabidopsis(Springer Nature, 2020-03-27) Wang, Lili; Dong, Min; Zhang, Qiong; Wu, Ying; Hu, Liang; Parson, James F.; Eisenstein, Edward; Du, Xiangge; Xiao, ShunyuanSilicon (Si) has been widely employed in agriculture to enhance resistance against pathogens in many crop plants. However, the underlying molecular mechanisms of Si-mediated resistance remain elusive. In this study, the Arabidopsis-powdery mildew pathosystem was employed to investigate possible defense mechanisms of Si-mediated resistance. Because Arabidopsis lacks efficient Si transporters and thus is a low Si-accumulator, two heterologous Si influx transporters (from barley and muskmelon) were individually expressed in wild-type Arabidopsis Col-0 and a panel of mutants defective in different immune signaling pathways. Results from infection tests showed that while very low leaf Si content slightly induced salicylic acid (SA)-dependent resistance, high Si promoted PAD4-dependent but largely EDS1- and SA-independent resistance against the adapted powdery mildew isolate Golovinomyces cichoracearum UCSC1. Intriguingly, our results also showed that high Si could largely reboot non-host resistance in an immune-compromised eds1/pad4/sid2 triple mutant background against a non-adapted powdery mildew isolate G. cichoracearum UMSG1. Taken together, our results suggest that assimilated Si modulates distinct, multi-layered defense mechanisms to enhance plant resistance against adapted and no-adapted powdery mildew pathogens, possibly via synergistic interaction with defense-induced callose.Item MONOCRYSTALLINE SUPERSATURATED ALUMINUM LAYERS BURIED IN EPITAXIAL SILICON(2019) Kim, Hyun soo; Iliadis, Agis; Pomeroy, Joshua M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum information device performance in semiconductors and superconductors is limited by the quality of materials and interfaces, particularly the interface to the oxide layer. By merging semiconductors and superconductors in a single crystal material, the oxide layer can be eliminated, and the advantage of both systems can be realized. To explore interface free circuits in quantum computing, I have synthesized and studied a new two-dimensional hole gas in silicon using aluminum layers sandwiched between single crystal Si layers. At high enough Al density, this system is expected to behave as a superconductor in single crystal Si. The samples were fabricated by low temperature molecular beam epitaxy (MBE) with modulation doping of elemental Al. Scanning tunneling microscope (STM) and scanning transmission electron microscope (STEM) images show epitaxial Si layers with low surface defects and no crystalline defects in the Al enriched region. Electrical measurement shows that holes are the dominant carrier in this system with charge carrier densities of $\approx$ 1.39 x 10$^{14}$ cm$^{-2}$, and Hall mobilities of $\approx$ 20 cm$^{2}$/(Vs). The charge carrier density corresponds to $\approx$ (0.93 $\pm$ 0.1) hole per Al dopant atom. Unfortunately, no superconductivity was observed down to 300 mK. The likely reason for this is found to be re-distribution of Al dopants over $\approx$ (17 to 25) nm due to thermal annealing up to 550 $^{\circ}$C, which decreases the peak Al concentration in Si below the critical density for superconductivity. Al has not been well studied as a dopant in Si due to its low solid solubility, low vapor pressure, and tendency to segregate. To better understand Al as a dopant, the structures and electrical properties of incorporated Al in Si(100) are studied using STM. The scanning tunneling spectroscopy (STS) spectra show shifts of band edges on incorporated Al compared to (2x1) Si(100) dimers. To test the compatibility of elemental Al for STM lithography with a hydrogen resist layer, a standard experimental protocol is tested. Elemental Al is evaluated using 3 different metrics: 1) sticking coefficient contrast, 2) effective enthalpy of sublimation contrast, and 3) surface diffusivity by deposition rates. Elemental Al is shown incompatible with STM lithography and hydrogen masking. Our study suggests that other dopants may overcome this difficulty. Finally, a new method using ion implanted wires is a promising technique for making electrical contacts to devices in Si fabricated with STM lithography. Here, I report a new in situ method for detecting ion implanted wires using STM and STS with a novel lock-in technique. Using the ion implanted wires, a-first-of-its-kind STM-patterned nano-wire made of P dopants is demonstrated.Item A particle erosion model of monocrystalline silicon for high heat flux microchannel heat exchangers(2017) Squiller, David; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As package-level heat generation pushes past 1 kW/cm3 in various military, aerospace, and commercial applications, new thermal management technologies are needed to maximize efficiency and permit advanced power electronic devices to operate closer to their inherent electrical limit. In an effort to align with the size, weight and performance optimization of high temperature electronics, cooling channels embedded directly into the backside of the chip or substrate significantly reduce thermal resistances by minimizing the number of thermal interfaces and distance the heat must travel. One implementation of embedded cooling considers microfluidic jets that directly cool the backside of the substrate. However, as fluid velocities exceed 20 m/s the potential for particle erosion becomes a significant reliability threat. While numerous particle erosion models exist, seldom are the velocities, particle sizes, materials and testing times in alignment with those present in embedded cooling systems. This research fills the above-stated gaps and culminates in a calibrated particle-based erosion model for single crystal silicon. In this type of model the mass of material removed due to a single impacting particle of known velocity and impact angle is calculated. Including this model in commercial computational fluid dynamics (CFD) codes, such as ANSYS FLUENT, can enable erosion predictions in a variety of different microfluidic geometries. First, a CFD model was constructed of a quarter-symmetry impinging jet. Lagrangian particle tracking was used to identify localized particle impact characteristics such as impact velocity, impact angle and the percentage of entrained particle that reach the surface. Next, a slurry erosion jet-impingement test apparatus was constructed to gain insight into the primary material removal mechanisms of silicon under slurry flow conditions. A series of 14 different experiments were performed to identify the effect of jet velocity, particle size, particulate concentration, fluid viscosity and time on maximum erosion depth and volume of material removed. Combining the experimental erosion efforts with the localized particle impact characteristics from the CFD model enabled the previously developed Huang et al. cutting erosion model to be extended to new parameter and application ranges. The model was validated by performing CFD erosion simulations that matched with the experimental test cases in order to compare one-dimensional erosion rates. An impact dampening coefficient was additionally proposed to account for slight deviations between the CFD erosion predictions and experimental erosion rates. The product of this research will ultimately enable high fidelity erosion predictions specifically in mission-critical military, commercial and aerospace applications.Item Optical and Thermal Properties of Nanoporous Material and Devices(2015) Kim, Kyowon; Murphy, Thomas E; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we investigate the optical and thermal properties of porous silicon and its applications. In first part, porous silicon's optical properties and application as a highly sensitive refractive index sensor is studied. An integrated Mach-Zehnder interferometer waveguide fabricated from nanoporous silicon is shown to exhibit high sensitivity and measurement stability that exceeds previously demonstrated porous sensors. In second part, we discuss experimental methods to characterize the thermal conductivity of nanoporous silicon films. We use the 3-ω method to characterize the exceptionally low thermal conductivity of porous silicon. Finally, we employ an improved heat conduction analysis method for the 3-ω method to measure the anisotropy in thermal conductivity. Our measurement show that porous silicon has very low in-plane thermal conductivity compared to cross-plane conductivity. We confirmed this anisotropy using direct numerical simulation of the anisotropic heat equation.Item Silicon-based terahertz waveguides(2015) Li, Shanshan; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present the design, fabrication and application of two types of silicon-based terahertz waveguides. The first is anisotropically etched highly doped silicon surface for supporting terahertz plasmonic guided wave. We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with subwavelength structures. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of structures. The second is silicon dielectric ridge waveguide used to confine terahertz pulses and study silicon's terahertz intensity-dependent absorption. We observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanisms underlying this nonlinear effect. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation.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 A Platform Towards In Situ Stress/Strain Measurement in Lithium Ion Battery Electrodes(2012) Baron, Sergio Daniel; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates the design, fabrication and testing of a platform for in situ stress/strain measurement in lithium ion battery electrodes. The platform - consisting of a Microelectromechanical System (MEMS) chip containing an electrochemical cavity and an optical sensing element, a custom electrochemical package and an experimental setup - was successfully developed. Silicon was used as an active electrode material, and a thin-film electrochemical stack was conceived and tested. Finally, multiple experiments showed correlation between the active material volume change inside the battery and a signal change in the optical sensing element. The experimental results, combined with the MEMS implementation of the sensing element provide a promising way to evaluate electrochemical reaction-induced stress monitoring in a simple and compact fashion, while experiments are carried out in situ.