Theses and Dissertations from UMD
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Item Phonon Modeling in Nano- and Micro- scale Crystalline Systems(2018) VanGessel, Francis; Chung, Peter; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Submicrometer phonon systems are becoming increasingly relevant in modern day technology. Phonon mechanisms are notably relevant in a number of solid-state devices including lasers, LEDs, transistors, and thermoelectrics. Proliferation of these devices has been driven by advancements in silicon-on-insulator technology. These advancements have allowed for the manufacture of devices with complex nanostructures and dimensions deep in the sub-microscale regime. However, accompanying improvements in the manufacture and design of novel crystalline systems is the requirement for accurate computational approaches for phonon modeling in nanostructured, anisotropic, and complex materials. The phonon Boltzmann transport equation is uniquely well suited to modeling energy transfer at the nano- and micro- meter length scales and is therefore an excellent candidate for this simulation task. However, current Boltzmann modeling approaches utilize a range of assumptions and simplifications that restrict their validity to isotropic, nominally one or two dimensional, or compositionally simple systems. In this dissertation we present an original finite volume-based methodology for the solution of the three dimensional full Brillouin zone phonon Boltzmann transport equation. This methodology allows for separate real and reciprocal space discretization. By taking a sampling of vibrational modes throughout the first Brillouin zone our methodology captures three unique sources of phonon anisotropy. We investigate the effect of phonon anisotropy in a fin field effect transistor, calculating the effect that incorporating various sources of anisotropy has on the resultant temperature fields. In a second study, we consider phonon flow through silicon nanowires with a modified boundary geometry. The three-dimensional flow fields are calculated and thermal transport below the Casimir limit is observed. Reduction in thermal conductivity is a result of maximizing the phonon backscatter that occurs in our phononic system. The backscatter serves to create regions of highly misaligned phonon flux. In addition, our silicon nanowire geometry has properties analogous with a high-pass phonon filter. In the final study we apply our Boltzmann transport methodology to the simulation of phonon transport in the energetic material, RDX. We study phonon transport in the vicinity of a material hotspot, the location at which chemistry initiates in the material. By applying Boltzmann modeling, applied for the first time to this material, we gain valuable insights into the interplay between thermal transport and phonon modes linked with initiation.Item Formation and Characterization of Transversely Modulated Nanostructures in Metallic Thin Films using Epitaxial Control(2013) Boyerinas, Brad Michael; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes a fundamental investigation into the formation, characterization, and modeling of epitaxially-controlled self-assembly at the nanoscale. The presence of coherent nanophases and the clamping effect from an epitaxial substrate enables the formation of transversely modulated nanostructures (TMNS) resulting in improved functionality, which was previously observed through increased piezoelectric response in BiFeO3. The ability to fabricate high quality epitaxial films presents opportunity to investigate coherent phase decomposition in other material systems with multifunctional response. The research herein aims to extend the concept of nanoscale self assembly in metallic systems, including Ag-Si and Pd-PdH. First, the effect of annealing a Ag-Si couple was examined, and ordered, nanoscale Ag crystallites were observed along the interface with the epitaxial Si wafer. It is demonstrated that Ag foil can be used in place of doped Ag paste (commonly used in solar cell metallization) to achieve TMNS at the interface. It was proved that annealing the Ag-Si couple in air is necessary for the self-assembly reaction to take place, as doing so prevents bulk diffusion and eutectic melting. Electron backscatter diffraction was used to verify the epitaxial relation between the Ag nanostructures and Si crystal. A method to fabricate ordered, nanoscale PdH precipitates in epitaxial Pd thin films via high temperate gas phase hydrogenation was established. Epitaxial Pd films were deposited via e-beam deposition and a V buffer layer was necessary to induce epitaxy. This novel self-assembled nanostructure may enable hysteresis-less absorption and desorption, thus improving functionality with regard to hydrogen sensing and storage. The epitaxial Pd film was characterized before and after hydrogenation with x-ray diffraction and atomic force microscopy to determine composition and nanostructure of the film. A thermodynamic model was developed to demonstrate the possibility to control or eliminate thermodynamic hysteresis via balance of elastic interaction between the coherent interfaces of metal and metal-hydride phases and the film-substrate interface. This model can be extended to other metal-hydride systems which demonstrate coherent phase decomposition.Item TOBACCO MOSAIC VIRUS BASED THREE DIMENSIONAL ANODES FOR LITHIUM ION BATTERIES(2011) Chen, Xilin; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silicon and tin are promising anodic materials with both the high gravimetric and volumetric capacities for the next generation lithium-ion batteries. To prevent silicon or tin electrodes from a structure failure due to the volume change during lithiation and delithiation, a genetically modified Tobacco mosaic virus (TMV1cys) template is used to fabricate a 3D current collector for the silicon or tin electrode. The 3D current collector can effectively enhance the stabilities of the silicon or tin anodes. The TMV1cys particle can vertically self assemble onto the metal (i.e. Au, Ni, Fe) surfaces in a buffer solution ( PH=7 ). The abundant cysteine-derived thiol groups on the outer surface of the TMV1cys particle can react with metals to form near-covalent bonds. Thus it is very simple to form a 3D current collector by reducing metal such as nickel onto the TMV1cys surface by an electroless metal deposition. The 3D structure increases the electrode surface area by 10-fold. In order to investigate the effect of the 3D structure on the silicon anode, a physical vapor deposition methodology is used to deposit silicon onto the 3D current collector to form a nickel-silicon core-shell nano-rod anode. The abundant free spaces in the electrode accommodate the volume change during cycling and thus the cycleability of the silicon anode is greatly enhanced. The retention capacity at 1C is more than 1100 mAh/g after 340 cycles. Furthermore, a simple electrodeposition method is used to replace the complex physical vapor deposition methodology to make a uniform silicon deposition on the 3D current collector. The electrodeposition methodology is also used to prepare a tin anode. The electrodeposited silicon anode has comparable performance to those silicon anodes prepared by the physical vapor deposition technique. In order to enhance the electrochemical kinetics in silicon anode, the phosphorus doped n-type silicon is used to replace the pure silicon for preparing a high-rate-performance 3D silicon anode. Since the electrochemical reactions take place on the interface between the silicon and the electrolyte, the n-type silicon provides a quicker diffusion path for the involved electrons. The rate capability of the silicon anode has been increased and the capacity difference enlarges with the increasing current density.Item MOLECULAR DYNAMICS SIMULATION OF DICARBOXYLIC ACID COATED AQUEOUS AEROSOL: STRUCTURE AND PROCESSING OF WATER VAPOR(2010) Ma, Xiaofei; Zachariah, Michael R; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Low molecular weight dicarboxylic acids constitute a significant fraction of water-soluble organic aerosols in the atmosphere. They have a potential contribution to the formation of cloud condensation nuclei (CCN) and are involved in a series of chemical reactions occurring in atmosphere. In this work, molecular dynamics simulation method was used to probe the structure and the interfacial properties of the dicarboxylic acid coated aqueous aerosol. Low molecular weight dicarboxylic acids of various chain lengths and water solubility were chosen to coat a water droplet consisting of 2440 water molecules. For malonic acid coated aerosol, the surface acid molecules dissolved into the water core and form an ordered structure due to the hydrophobic interactions. For other nanoaerosols coated with low solubility acids, phase separation between water and acid molecules was observed. To study the water processing of the coated aerosols, the water vapor accommodation factors were calculated.Item SYNTHESIS, CHARACTERIZATION, AND KINETIC STUDIES OF IONIZING RADIATION-INDUCED INTRA- AND INTER-CROSSLINKED POLY(VINYL PYRROLIDONE) NANOHYDROGELS(2007-11-26) An, Jung-Chul; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A polymer nanohydrogel can be defined as a three-dimensional polymer network composed of hydrophilic crosslinked macromolecular chains filled with liquid and possessing a diameter of 1-102 nanometers. Nanohydrogels have drawn huge interest due to their potential applications, such as target-specific drug delivery carriers, absorbents, chemical/biological sensors, and bio-mimetic materials. However, the conventional methods of nanohydrogel synthesis require toxic chemicals (e.g., initiators, crosslinking agents) to form the gel structure. The additional steps required to remove unreacted or residual (undesired) substances cause nanohydrogel fabrication to be complicated, environmentally unfriendly, and unsuitable for biomedical use. This study aims to develop simple and efficient methods of producing nanohydrogels from polymeric, aqueous solutions using ionizing radiation. Poly(vinyl pyrrolidone) (PVP) nanohydrogels of various sizes and molecular weights were prepared by pulsed electron beam and steady-state gamma irradiation at different doses (5 and 10 kGy; 1Gy = 1 J kg-1) and temperatures (20 to 77 °C). The pervaded volume of the PVP chains becomes smaller at high temperatures (above 50 °C) due to the disruption of hydrogen bonds between water and PVP molecules which reduces the size and the molecular weight of the synthesized PVP nanohydrogels. The synthesis parameters (e.g., irradiation temperature, pulse repetition rate, dose rate, and solution concentration) were varied in order to control the size and the average molecular weight of the irradiated sample. In the absence of oxygen, the radiolytically produced free radicals of the thermally collapsed PVP molecules primarily underwent intra-crosslinking reactions, along with a minor contribution from inter-crosslinking reactions. The predominance of the intra-crosslinking mechanism was exhibited at high irradiation temperature (77 °C) in dilute solutions (c = 0.9 x 10-2 mol L-1). The formation of carbon-centered free radicals along the backbone of the PVP chain at higher pulse repetition rate (300 pulses per second) was found to enhance the intra-crosslinking reaction, thereby leading to the formation of smaller nanohydrogel molecules containing an average hydrodynamic radius (Rh) of 9.9 ± 0.1 nm.Item ENGINEERING OF SELF-ASSEMBLED MULTIFERROIC NANOSTRUCTURES IN PbTiO3-CoFe2O4 THIN FILMS(2006-01-31) Li, jianhua; Roytburd, Alexander L; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiferroic materials which display a coexistence of ferroelectric and ferromagnetic properties attract considerable interest for their potential for novel device applications as well as for the interesting physics and materials science underlying their functional responses. In multiferroic composite, electromagnetic coupling facilitate elastic interaction between ferroelectric and ferromagnetic components via piezoeffect and magnetostriction. The major goal of our research is designing the transverse epitaxial multiferroic nanostructures with controlled morphologies. The PbTiO3-CoFe2O4 system was selected for this study because of the (i) large spontaneous strain associated with the ferroelectric phase transition in PbTiO3 (6.5%), which should create strong elastic interactions between the two phases accompanying the piezoelectric effect, and (ii) large magnetostriction of ferrimagnetic CoFe2O4. We successfully fabricate self-assembling multiferroic nanostucture films of CoFe2O4-PbTiO3 by PLD deposition on SrTiO3 substrates of different orientations. X-ray and TEM characterization show that all films have columnar architecture and 3D epitaxial relationships between phases and each phase and substrates. The morphology of nanostructures has been controlled by changing orientation of a substrate. It has been shown that it is possible to obtain the ferromagnetic (CoFe2O4) rods with a diameter about 10-20 nm in the ferroelectric PbTiO3 matrix in (001) films of composition 0.67PbTiO3-0.33CoFe2O4, and vise versa: ferroelectric rods in ferrimagnetic matrix in (111) films of composition 0.33PbTiO3-0.67CoFe2O4. The lamellate morphology with a specific crystallographic orientation of lamellae corresponding to {111} planes has been obtained in (110) films. The measurements of lattice parameters of the constitutive phases at different temperature allows us to determine the level of internal stresses due to misfit between phases. The measurements of piezo- and magnetic responses of the films prove that the films are ferroelectric and ferromagnetic simultaneously. The piezo- and magnetic responses are considerable suppressed due to mutual constraints between phases. This suppression indicates the strong elastic interactions between the phases which allows us to suggest the strong electro-magnetic coupling in the films. Combining theoretical and experimental studies of self-assembled multiferroic nanostructures in epitaxial films has revealed that the elastic interactions caused by epitaxial stresses play the dominate role in defining the morphology of the nanostructures and their magnetic and electric responses.Item MECHANISTIC STUDIES OF PLASMA-SURFACE INTERACTIONS DURING NANOSCALE PATTERNING OF ADVANCED ELECTRONIC MATERIALS USING PLASMA(2005-12-21) Hua, Xuefeng; Oehrlein, Gottlieb S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photolithographic patterning of photoresist materials and transfer of these images into electronic materials using directional plasma etching techniques plays a critical role in the fabrication of integrated circuits. As critical device dimensions are reduced below 100 nm, precise control of the interactions of process plasmas with materials is required for successful integration. This requires a scientific understanding of plasma-surface interaction mechanisms that control the properties of the ultimate devices and ICs produced. Fluorocarbon discharges are commonly used for dielectric etching, e.g. SiO2. In this work we have studied surface-chemical aspects of the interaction of C4F8/Ar discharges with SiO2 and Si. Free fluorine atoms that are liberated from fluorocarbon species during ion bombardment are driven to the surface and react with the substrate, a process called defluorination. The defluorination is dependent on the plasma properties and the penetration of reactive species is limited within 10nm below the surface. Future device requires novel materials, i.e. nanoporous silica, to replace conventional SiO2.When some O atoms in Si-O matrix are replaced with nano cavities (pores), the plasma-induced modifications are extended to the deep subsurface region and the modification scale can be a few hundred nanometers. This modification is correlated with overall porosity and also strongly depends on plasma properties. O2 N2 and H2 discharges likely induce carbon depletion and material densification on nanoporous silica. Novel approach, i.e. shutter approach, is employed to study the issues of plasma processing of advanced photoresist materials at nanometer dimension. Hydrogen depletion, material densification and graphitization of these polymers are important processes during short exposure time with the plasma. High roughening rates are also observed within this time range. Subsequently, dedensification, i.e. surface roughening, dominates in the plasma-photoresist interactions. Depending on the molecular structures, the roughness scale can be well beyond the molecular size and RMS roughness does not saturate even after a long exposure time. For the etching of features, rough edges induced by initial plasma exposure on the top of the lines in the features form local masks and striations are formed on the sidewalls during long exposure times, which could lead failures of the devices.