Matching Items (67)
Description
In this dissertation, the surface interactions of fluorine were studied during atomic layer deposition (ALD) and atomic layer etching (ALE) of wide band gap materials. To enable this research two high vacuum reactors were designed and constructed for thermal and plasma enhanced ALD and ALE, and they were equipped for in-situ process monitoring. Fluorine surface interactions were first studied in a comparison of thermal and plasma enhanced ALD (TALD and PEALD) of AlF3 thin films prepared using hydrogen fluoride (HF), trimethylaluminum (TMA), and H2-plasma. The ALD AlF3 films were compared ¬in-situ using ellipsometry and X-ray photoelectron spectroscopy (XPS). Ellipsometry showed a growth rate of 1.1 Å/ cycle and 0.7 Å/ cycle, at 100°C, for the TALD and PEALD AlF3 processes, respectively. XPS indicated the presence of Al-rich clusters within the PEALD film. The formation of the Al-rich clusters is thought to originate during the H2-plasma step of the PEALD process. The Al-rich clusters were not detected in the TALD AlF3 films. This study provided valuable insight on the role of fluorine in an ALD process.
Reactive ion etching is a common dry chemical etch process for fabricating GaN devices. However, the use of ions can induce various defects, which can degrade device performance. The development of low-damage post etch processes are essential for mitigating plasma induced damage. As such, two multistep ALE methods were implemented for GaN based on oxidation, fluorination, and ligand exchange. First, GaN surfaces were oxidized using either water vapor or O2-plasma exposures to produce a thin oxide layer. The oxide layer was addressed using alternating exposures of HF and TMG, which etch Ga2O3 films. Each ALE process was characterized using in-situ using ellipsometry and XPS and ex-situ transmission electron microscopy (TEM). XPS indicated F and O impurities remained on the etched surfaces. Ellipsometry and TEM showed a slight reduction in thickness. The very low ALE rate was interpreted as the inability of the Ga2O3 ALE process to fluorinate the ordered surface oxide on GaN (0001).
Overall, these results indicate HF is effective for the ALD of metal fluorides and the ALE of metal oxides.
ContributorsMessina, Daniel C (Author) / Nemanich, Robert J (Thesis advisor) / Goodnick, Stephen (Committee member) / Ponce, Fernando A (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2021
Description
The increasing demand for structural materials with superior mechanical properties has provided a strong impetus to the discovery of novel materials, and innovations in processing techniques to improve the properties of existing materials. Methods like severe plastic deformation (SPD) and surface mechanical attrition treatment (SMAT) have led to significant enhancement in the strength of traditional structural materials like Al and Fe based alloys via microstructural refinement. However, the nanocrystalline materials produced using these techniques exhibit poor ductility due to the lack of effective strain hardening mechanisms, and as a result the well-known strength-ductility trade-off persists. To overcome this trade-off, researchers have proposed the concept of heterostructured materials, which are composed of domains ranging in size from a few nanometers to several micrometers. Over the last two decades, there has been intense research on the development of new methods to synthesize heterostructured materials. However, none of these methods is capable of providing precise control over key microstructural parameters such as average grain size, grain morphology, and volume fraction and connectivity of coarse and fine grains. Due to the lack of microstructural control, the relationship between these parameters and the deformation behavior of heterostructured materials cannot be investigated systematically, and hence designing heterostructured materials with optimized properties is currently infeasible. This work aims to address this scientific and technological challenge and is composed of two distinct but interrelated parts. The first part concerns the development of a broadly applicable synthesis method to produce heterostructured metallic films with precisely defined architectures. This method exploits two forms of film growth (epitaxial and Volmer-Weber) to generate heterostructured metallic films. The second part investigates the effect of different microstructural parameters on the deformation behavior of heterostructured metallic films with the aim of elucidating their structure-property relationships. Towards this end, freestanding heterostructured Fe films with different architectures were fabricated and uniaxially deformed using MEMS stages. The results from these experiments are presented and their implications for the mechanical properties of heterostructured materials is discussed.
ContributorsBerlia, Rohit (Author) / Rajagopalan, Jagannathan (Thesis advisor) / Sieradzki, Karl (Committee member) / Peralta, Pedro (Committee member) / Crozier, Peter (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2021
Description
Polarization detection and control techniques play essential roles in various applications, including optical communication, polarization imaging, chemical analysis, target detection, and biomedical diagnosis. Conventional methods for polarization detection and polarization control require bulky optical systems. Flat optics opens a new way for ultra-compact, lower-cost devices and systems for polarization detection and control. However, polarization measurement and manipulating devices with high efficiency and accuracy in the mid-infrared (MIR) range remain elusive. This dissertation presented design concepts and experimental demonstrations of full-Stokes parameters detection and polarization generation devices based on chip-integrated plasmonic metasurfaces with high performance and record efficiency. One of the significant challenges for full-Stokes polarization detection is to achieve high-performance circular polarization (CP) filters. The first design presented in this dissertation is based on the direct integration of plasmonic quarter-wave plate (QWP) onto gold nanowire gratings. It is featured with the subwavelength thickness (~500nm) and extinction ratio around 16. The second design is based on the anisotropic thin-film interference between two vertically integrated anisotropic plasmonic metasurfaces. It provides record high efficiency (around 90%) and extinction ratio (>180). These plasmonic CP filters can be used for circular, elliptical, and linear polarization generation at different wavelengths. The maximum degree of circular polarization (DOCP) measured from the sample achieves 0.99998.
The proposed CP filters were integrated with nanograting-based linear polarization (LP) filters on the same chip for single-shot polarization detection. Full-Stokes measurements were experimentally demonstrated with high accuracy at the single wavelength using the direct subtraction method and over a broad wavelength range from 3.5 to 4.5mm using the Mueller matrix method. This design concept was later expanded to a pixelized array of polarization filters. A full-Stokes imaging system was experimentally demonstrated based on integrating a metasurface with pixelized polarization filters arrays and an MIR camera.
ContributorsBai, Jing (Author) / Yao, Yu (Thesis advisor) / Balanis, Constantine A. (Committee member) / Wang, Liping (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2021
Description
In this dissertation, atomic layer processing and surface characterization techniques were used to investigate surface conditions of wide band gap materials, gallium nitride (GaN) and gallium oxide (Ga2O3). These studies largely focused on mitigation and removal of defect formation induced by ions used in conventional plasma-based dry etching techniques. Band bending measured by x-ray photoelectron spectroscopy (XPS) was used to characterize charge compensation at the surface of GaN (0001) and determine densities of charged surface states produced by dry etching. Mitigation and removal of these dry-etch induced defects was investigated by varying inductively coupled plasma (ICP) etching conditions, performing thermal and plasma-based treatments, and development of a novel low-damage, self-limiting atomic layer etching (ALE) process to remove damaged material. Atomic layer deposition (ALD) and ALE techniques were developed for Ga2O3 using trimethylgallium (TMG). Ga2O3 was deposited by ALD on Si using TMG and O2 plasma with a growth rate of 1.0 ± 0.1 Å/cycle. Ga2O3 films were then etched using HF and TMG using a fully thermal ALE process with an etch rate of 0.9 ± Å/cycle. O2 plasma oxidation of GaN for surface conversion to Ga2O3 was investigated as a pathway for ALE of GaN using HF and TMG. This process was characterized using XPS, in situ multi-wavelength ellipsometry, and transmission electron microscopy. This study indicated that the etch rate was lower than anticipated, which was attributed to crystallinity of the converted surface oxide on GaN (0001).
ContributorsHatch, Kevin Andrew (Author) / Nemanich, Robert J (Thesis advisor) / Ponce, Fernando A (Committee member) / Smith, David J (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2021
Description
The present study aimed to compare brain activity changes related to proactive and reactive control strategies in patients with Parkinson’s disease during “On” levodopa and “Off” levodopa conditions. The study consisted of two participants who had received a prior diagnosis of Parkinson’s Disease. The participants completed AX-CPT task as a measure of attention control in two sessions: a) “On Levodopa” and b) “Off Levodopa” while they were in the fMRI scanner. Prior to the analysis, the T1- weighted anatomical scan images and the BOLD multiband functional images of both the participants were BIDS (Brain Imaging Data Structure) validated and preprocessed using the standard FMRIPrep pipeline. The imaging data was then analyzed using SPM12 (Statistical parametric mapping) software. Individual-level analysis of the imaging data was conducted by creating General Linear models for both the participants on “ON” and “OFF” levodopa conditions. The BOLD responses were compared using AY>BY and BX > BY contrasts. Where BX >, BY contrast, measured BOLD activity related to reactive control strategy and AY> BY contrast measured BOLD activity related to the proactive control strategy. It was observed that participants tended towards reactive control strategy in both “On” and “Off” levodopa conditions.
ContributorsDatta, Kalyani (Author) / Brewer, Gene (Thesis advisor) / Braden, B. Blair (Committee member) / Peterson, Daniel (Committee member) / Arizona State University (Publisher)
Created2022
Description
The Bayesian paradigm provides a flexible and versatile framework for modeling complex biological systems without assuming a fixed functional form or other constraints on the underlying data. This dissertation explores the use of Bayesian nonparametric methods for analyzing fluorescence microscopy data in biophysics, with a focus on enumerating diffraction-limited particles, reconstructing potentials from trajectories corrupted by measurement noise, and inferring potential energy landscapes from fluorescence intensity experiments. This research demonstrates the power and potential of Bayesian methods for solving a variety of problems in fluorescence microscopy and biophysics more broadly.
ContributorsBryan IV, J Shepard (Author) / Presse, Steve (Thesis advisor) / Ozkan, Banu (Committee member) / Wadhwa, Navish (Committee member) / Shepherd, Doug (Committee member) / Arizona State University (Publisher)
Created2023
Description
As a rapidly evolving field, nucleic acid nanotechnology focuses on creating functional nanostructures or dynamic devices through harnessing the programmbility of nucleic acids including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), enabled by the predictable Watson-Crick base pairing. The precise control over the sequence and structure, along with the development of simulation softwares for the prediction of the experimental implementation provides the base of designing structures or devices with arbitrary topology and operational logic at nanoscale. Over the past 40 years, the thriving field has pushed the boundaries of nucleic acids, from originally biological macromolecules to functional building blocks with applications in biomedicine, molecular diagnostics and imaging, material science, electronics, crystallography, and more have emerged through programming the sequences and generating the various structures or devices. The underlying logic of nucleic acid programming is the base pairing rule, straightforward and robust. While for the complicated design of sequences and quantitative understanding of the programmed results, computational tools will markedly reduced the level of difficulty and even meet the challenge not available with manual effort. With this thesis three individual projects are presented, with all of them interweaving theory/computation and experiments. In a higher level abstraction, this dissertation covers the topic of biophysical understanding of the dynamic reactions, designing and realizing complex self-assembly systems and finally super-resolutional imaging. More specifically, Chapter 2 describes the study of RNA strand displacement kinetics with dedicated model extracting the reaction rates, providing guidelines for the rational design and regulation of the strand displacement reactions and eventually biochemical processes. In chapter 3 the platform for the design of complex symmetry of the self-assembly target and first experimental implementation of the assembly of pyrochlore lattices with DNA origamis are presented, which potentially can be applied to manipulate lights as optical materials. Chapter 4 focuses on the in solution characterization of the periodicity of DNA origami lattices with super-resolutional microscopy, with algorithms in development for three dimensional structural reconstruction.
ContributorsLiu, Hao (Author) / Yan, Hao (Thesis advisor) / Sulc, Petr (Thesis advisor) / Guo, Jia (Committee member) / Heyden, Matthias (Committee member) / Arizona State University (Publisher)
Created2023
Description
In recent years, the scientific community around the synthesis and processing of nanoporous metals is striving to integrate them into powder metallurgy processes such as additive manufacturing since it has a potential to fabricate 3D hierarchical high surface area electrodes for energy applications. Recent research in dealloying – a versatile method for synthesizing nanoporous metals – emphasized the need in understanding its process-structure relationships to independently control the relative density, ligament and pore sizes with good process reproducibly. In this dissertation, a new understanding of the dealloying process is presented for synthesizing (i) nanoporous gold thin-films and (ii) nanoporous Cu spherical powders with an emphasis on understanding variability in their process-structure relationships and process scalability. First, this work sheds the light on the nature of the dealloying front and its percolation along the grain boundaries in nanocrystalline gold-silver thin films by studying the early stages of ligament nucleation. Additionally, this work analyses its variability by investigating new process variables such as (i) equilibration time and (ii) precursor aging and their impacts in achieving process reproducibility. The correlation of relative density with ligament size is contextualized with state-of-the-art data mining research. Second, this work provides a new methodology for large scale production of nanoporous Cu powder and demonstrates its integration with powder casting to fabricate porous conductive electrode. By understanding the influence of etching solution concentration and titration methodology on the structure and composition of nanoporous Cu, it was possible to fabricate precipitate-free powders at high throughputs. Further, the nature of oxygen incorporation into porous Cu powder was studied as a function of surface-to-volume ratio of powder in atmospheric conditions. To consolidate powders into parts via open-die casting, this work harvests Ostwald Ripening phenomena associated with thermal coarsening in nanoporous metals to weld them at low temperatures (approximately one-third of its melting temperature). This work represents a major step towards the integration of nanoporous Cu feedstocks into additive manufacturing.
ContributorsNiauzorau, Stanislau (Author) / Azeredo, Bruno (Thesis advisor) / Sieradzki, Karl (Committee member) / Song, Kenan (Committee member) / Chawla, Nikhilesh (Committee member) / Arizona State University (Publisher)
Created2022
Description
The objective of this dissertation is to study the optical and radiative properties of inhomogeneous metallic structures. In the ongoing search for new materials with tunable optical characteristics, porous metals and nanowires provides an extensive design space to engineer its optical response based on the morphology-dependent phenomena.This dissertation firstly discusses the use of aluminum nanopillar array on a quartz substrate as spectrally selective optical filter with narrowband transmission for thermophotovoltaic systems. The narrow-band transmission enhancement is attributed to the magnetic polariton resonance between neighboring aluminum nanopillars. Tuning of the resonance wavelengths for selective filters was achieved by changing the nanopillar geometry. It concludes by showing improved efficiency of Gallium-Antimonide thermophotovoltaic system by coupling the designed filter with the cell.
Next, isotropic nanoporous gold films are investigated for applications in energy conversion and three-dimensional laser printing. The fabricated nanoporous gold samples are characterized by scanning electron microscopy, and the spectral hemispherical reflectance is measured with an integrating sphere. The effective isotropic optical constants of nanoporous gold with varying pore volume fraction are modeled using the Bruggeman effective medium theory.
Nanoporous gold are metastable and to understand its temperature dependent optical properties, a lab-scale fiber-based optical spectrometer setup is developed to characterize the in-situ specular reflectance of nanoporous gold thin films at temperatures ranging from 25 to 500 oC. The in-situ and the ex-situ measurements suggest that the
ii
specular, diffuse, and hemispherical reflectance varies as a function of temperature due to the morphology (ligament diameter) change observed.
The dissertation continues with modeling and measurements of the radiative properties of porous powders. The study shows the enhanced absorption by mixing porous copper to copper powder. This is important from the viewpoint of scalability to get end products such as sheets and tubes with the requirement of high absorptance that can be produced through three-dimensional printing.
Finally, the dissertation concludes with recommendations on the methods to fabricate the suggested optical filters to improve thermophotovoltaic system efficiencies. The results presented in this dissertation will facilitate not only the manufacturing of materials but also the promising applications in solar thermal energy and optical systems.
ContributorsRamesh, Rajagopalan (Author) / Wang, Liping (Thesis advisor) / Azeredo, Bruno (Thesis advisor) / Phelan, Patrick (Committee member) / Yu, Hongbin (Committee member) / Rykaczewski, Konrad (Committee member) / Arizona State University (Publisher)
Created2022
Description
The colloidal solutions of nanoparticles have been seen as promising solutions forheat transfer enhancement. Additionally, there has been an accelerated study on the effects
of ultrasound on heat transfer enhancement in recent years. A few authors have studied the
combined impact of Al2O3 nanofluids and ultrasound on mini channels. This study focused
on the combined effects of Al2O3 nanofluids and ultrasound on heat transfer enhancement
in a circular mini channel heat sink. Two concentrations of Al2O3-water nanofluids, i.e.,
0.5% and 1%, were used for the experiments in addition to two heat input conditions,
namely 40 W and 50 W providing a constant heat flux of 25000 W m-2 and 31250 W m-2
respectively. The effect on the nanofluids using 5 W ultrasound was analyzed.
Experimental observations show that the usage of ultrasound increased the heat transfer
coefficient. The heat transfer coefficient also increased with increasing nanoparticle
concentration and high heat flux. The average heat transfer coefficient enhancement for
0.5% and 1% nanofluid due to increased heat flux in the absence of ultrasound was 12.4%
and 9% respectively. At a constant heat input of 40 W, the induction of ultrasound
enhanced the heat transfer coefficient by 22.8% and 23.9% for 0.5% and 1% nanofluid
respectively. Similarly, for a constant heat input of 50 W, the usage of ultrasound enhanced
the heat transfer coefficient by 19.8% and 22.9% for 0.5% and 1% nanofluid respectively
Also, interesting findings are reported with low heat input with ultrasound vs. high heat
input without ultrasound (i.e., 40 W with US vs. 50 W without US). The heat transfer
coefficient and Nusselt number for 0.5% and 1% concentrations was enhanced by 9.2%
and 13.6%, respectively. Furthermore, for fixed heat input powers of 40 W and 50 W, increasing the concentration from 0.5% to 1% along with ultrasound yielded an average
enhancement in Nu of 38.3% and 32.4% respectively
ContributorsMastoi, Faisal Ali (Author) / Phelan, Patrick E (Thesis advisor) / Milcarek, Ryan (Committee member) / Kwon, Beomjin (Committee member) / Arizona State University (Publisher)
Created2022