Mountain Scholar :: Browsing by Author "Wu, Mingzhong, committee member"

Browsing by Author "Wu, Mingzhong, committee member"

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Open Access
Block-based detection methods for underwater target detection and classification from electro-optical imagery
(Colorado State University. Libraries, 2010) Kabatek, Michael Jonathan, author; Azimi-Sadjadi, Mahmood R., advisor; Pezeshki, Ali, committee member; Wu, Mingzhong, committee member
Detection and classification of underwater mine-like objects is a complicated problem due to various factors such as variations in the operating and environmental conditions, presence of spatially varying clutter, target obstruction and occlusion variations in target shapes, compositions, and orientation. Also contributing to the difficulty of the problem is the lack of a priori knowledge about the shape and geometry of new non-mine-like objects that may be encountered, as well as changes in the environmental or operating conditions encountered during data collection. Two different block-based methods are proposed for detecting frames and localization of mine-like objects from a new CCD-based Electro-optical (EO) imaging system. The block-based methods proposed in this study serve as an excellent tool for detection in low contrast frame sequences, as well as providing means for classifying detected objects as target or non-target objects. The detection methods employed provide frame location, automatic object segmentation, and accurate spatial locations of detected objects. The problem studied in this work is the detection of mine-like objects from a new CCD imagery data set which consists of runs containing tens to hundreds of frames (taken by the CCD camera). The goal is to detect frames containing mine-like objects, as well as locating detected objects and segmenting them from the frame to be subsequently classified as mine-like objects or background clutter. While object segmentation and classification of detected objects are also required as with the previous EO systems, the main challenge is successful frame detection with low false alarm rate. This has prompted research on new detection methods which utilize block- based snapshot information in order to identify potential frames containing targets, and spatially localize detected objects within those detected frames. More specifically, we have addressed CCD object detection problem by developing block-based Gauss-Gauss and matched subspace formulations. The block-based detection framework is applied to raw CCD data directly from the sensor without the need for computationally expensive filtering or pre-processing as with the previous methods. The detector operates by measuring the log-likelihood ratio in each block of a given frame and provides a spatial 'likelihood map'. This detection process pro- vides log-likelihood measurements of blocks in a given EO image which can then be thresholded to generate regions of interest within frame to be subsequently classified. This two-step process in both the Gauss-Gauss and matched subspace detectors consists of first measuring the log-likelihood, and determining frame of interest and then the regions of interest (ROI), and finally classifying the detected object ROIs, based upon shape-dependent features. Complex Zernike moments are extracted from each region of interest which are subsequently used to classify detected objects. The shape-based Zernike moments provide rotational invariance, and robustness to noise which are desirable characteristics for classification. This block-based framework provides flexibility in the detection methods used prior to object classification, and solves the problem of having to invoke a classification system on every CCD frame by determining frames containing only potential targets. A comprehensive study of the block-based detection and classification methods is carried out on a CCD imagery data set. A comparison is made on the detection and false alarm rate performance for the Gauss-Gauss and matched subspace detectors on the CCD data sets acquired from the Applied Signal Technologies in Sunnyvale, CA. In addition a neural-network based classification system is employed to perform object classification based upon the extracted Zernike moments. The tested data set from AST consist of ten runs over the mine field each run containing up to several hundred frames. The total number of frames tested totals 1317, with 16 frames containing a single or partial targets in five of the data runs. Results illustrating the effectiveness of the proposed detection methods are presented in terms of correct detection and false alarm rates. It is observed that the low-rank Gauss-Gauss detector provides an overall frame detection rate of 100% at the cost of a false alarm rate of 36.9%. The matched subspace detector outperforms the Gauss-Gauss method and reduces the false frame detection rate by 16.9%. Using the Zernike features extracted from the matched subspace detector's output and an artificial neural network classifier yields a true frame detection rate of Pd = 100% at the cost of Pfd = 16:8% reducing the detected false frames detected by 3.3%. The reduced-rank Gauss-Gauss detector has a detection rate of Pd = 100% at the cost of probability of false detection Pfd = 36:9%, using features extracted from the reduced-rank Gauss-Gauss detector's output passed to the neural network classifier yields a true detection rate of Pd = 100% at the cost of Pfd = 21:7% which significantly reduces the detected false frames by 15.1%.
Open Access
Design strategies for high-efficiency CdTe solar cells
(Colorado State University. Libraries, 2017) Song, Tao, author; Sites, James R., advisor; Kanevce, Ana, committee member; Gelfand, Martin, committee member; Wu, Mingzhong, committee member; Sampath, W. S., committee member
With continuous technology advances over the past years, CdTe solar cells have surged to be a leading contributor in thin-film photovoltaic (PV) field. While empirical material and device optimization has led to considerable progress, further device optimization requires accurate device models that are able to provide an in-depth understanding of CdTe device physics. Consequently, this thesis is intended to develop a comprehensive model system for high-efficiency CdTe devices through applying basic design principles of solar cells with numerical modeling and comparing results with experimental CdTe devices. Four key topics about high-efficiency CdTe cells are covered in this dissertation: (a) material optimization of CdTe absorber, (b) roles of emitter/absorber interface on carrier transport, (c) substrate choices for monocrystalline CdTe cells, and (d) back contact configurations for thin-film polycrystalline CdTe cells. Finally, comparisons between simulation and experiment are carried out to identify both beneficial and detrimental mechanisms for CdTe cell performance and to guide future cell optimization. The CdTe absorber is central to cell performance. Numerical simulation has shown the feasibility of high energy-conversion efficiency (open-circuit voltage VOC > 1000 mV, efficiency η > 25%), which requires both high carrier density (p >1016 cm-3) and long minority carrier lifetime (τn > 100 ns). As the minority carrier lifetime increases (τn > 10 ns), the carrier recombination at the back surface becomes a limitation for cell performance with absorber thickness < 3 µm. Hence, either a thicker absorber or an appropriate back-surface-field layer is a requisite for reducing the back-surface recombination. When integrating layers into devices, more careful design of interfaces is needed. One consideration is the emitter/absorber interface. It is shown that a positive conduction-band offset ΔEC ("spike") at the interface is beneficial to cell performance, since it can induce a large valence-band bending which suppresses the hole injection near the interface for the electron-hole recombination, but too large a spike is detrimental to photocurrent transport. In a heterojunction device with many defects at the emitter/absorber interface (high SIF), a thin and highly-doped emitter can induce strong absorber inversion and hence help maintain good cell performance. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. In terms of specific emitter materials, the calculations suggest that the (Mg,Zn)O alloy with 20% Mg, or a similar type-I heterojunction partner with moderate ΔEC (e.g., Cd(S,O) or (Cd,Mg)Te with appropriate oxygen or magnesium ratios) should yield higher voltages and would therefore be better candidates for the CdTe-cell emitter. The CdTe/substrate interface is also of great importance, particularly in the growth of epitaxial monocrystalline CdTe cells. Several substrate materials (CdTe, Si, GaAs, and InSb) have been discussed and all have challenges. These have generally been addressed through the addition of intermediate layers between the substrate and CdTe absorber. InSb is an attractive substrate choice for CdTe devices, because it has a close lattice match with CdTe, it has low resistivity, and it is easy to contact. However, the valence-band alignment between InSb and p-type CdTe, which can both impede hole current and enhance forward electron current, is not favorable. Three strategies to address the band-offset problem are investigated by numerical simulation: (a) heavy doping of the back part of the CdTe layer, (b) incorporation of an intermediate CdMgTe or CdZnTe layer, and (c) formation of an InSb tunnel junction. Each of these strategies is predicted to be helpful for higher cell performance, but a combination of them should be most effective. In addition, the CdTe/back contact interface plays a significant role in carrier transport for conventional polycrystalline thin-film CdTe devices. A significant back-contact barrier φb caused by metallic contact with low work function can block hole transport and enhance the forward current and thus result in a reduced VOC, particularly with fully-depleted CdTe devices. A buffer contact layer between CdTe absorber and metallic contact is strongly needed to mitigate this detrimental impact. The simulation has shown that a thin tellurium (Te) buffer as well as a highly doped p-type CdTe layer can assume such a role by reducing the downward valence-band bending caused by large φb and hence enhancing the extraction of the charge carriers. Finally, experimental CdTe cells are discussed in parallel with the simulation results to identify limiting mechanisms and give guidance for future efficiency improvement. For the monocrystalline CdTe cells made at NREL, it is found that the sputter damage causing large numbers of defect states near the Cd(S,O)/CdTe interface plays an important role in limiting cell performance, particularly for cells with low oxygen Cd(S,O) (with a "cliff" band offset). Other effects, such as the large series resistance and reflection, also reduce the cell performance. A lattice-matched material with less deposition damage and with a type-I interface is suggested to introduce less interfacial recombination in future emitter growth on epitaxial CdTe absorbers. For polycrystalline CdTe solar cells made at CSU, it is demonstrated that an MZO emitter forms a spike at the MZO/CdTe interface and a Te buffer layer mitigates large back-contact barrier φb. Both play very important roles in achieving good cell performance (VOC ~ 860 mV, η ~ 18.3%). The simulation has also shown that the electron reflector would be an effective approach to further increase VOC even with a relative low CdTe carrier concentration (~1014 cm-3).
Open Access
Distortions to current-voltage curves of CIGS cells with sputtered Zn(O,S) buffer layers
(Colorado State University. Libraries, 2013) Song, Tao, author; Sites, James R., advisor; Wu, Mingzhong, committee member; Sampath, Walajabad, committee member
Sputtered-deposited Zn(O,S) is an attractive alternative to CdS for Cu(In,Ga)Se2 (CIGS) thin-film solar cells' buffer layer. It has a higher band gap and thus allows greater blue photon collection to achieve higher photon current. The primary goal of the thesis is to investigate the effects of the secondary barrier at the buffer-absorber interface on the distortions to current-voltage (J-V) curves of sputtered-Zn(O,S)/CIGS solar cells. A straightforward photodiode model is employed in the numerical simulation to explain the physical mechanisms of the experimental J-V distortions including J-V crossover and red kink. It is shown that the secondary barrier is influenced by both the internal material properties, such as the conduction-band offset (CBO) and the doping density of Zn(O,S), and the external conditions, such as the light intensity and operating temperature. A key parameter for the sputter deposition of Zn(O,S) has been the oxygen fraction in the argon beam. It is found that the CBO varies with the oxygen fraction in the argon beam at a fixed temperature. With a greater CBO (∆EC > 0.3 eV), the resulting energy barrier limits the electron current flowing across the interface and thus leads to the J-V distortion. Two different ZnS targets, non-indium and indium-doped one, were used to deposit the Zn(O,S) buffer layer. At the same oxygen fraction in argon beam, a non-In-doped Zn(O,S) buffer with a smaller amount of doping forms a greater secondary barrier to limit the electron current due to the compensation of the Zn(O,S) buffer layer. In addition, the temperature-dependent J-V crossover can be explained by the temperature-dependent impact of the secondary barrier - at lower temperature in the dark, the maximum distortion-free barrier is reduced and results in a more serious current limitation, indicating a greater J-V crossover. It is also found that, under low-intensity illumination, there is a lower doping density of Zn(O,S) due to a smaller amount of photons with hν > Eg(Zn(O,S)) which can excite the buffer layer to release the trapped electrons from the deep-level defect state. The result is a greater secondary barrier to limit the electron current through the interface and shift the light J-V curve right towards the dark J-V curve at high bias (V > VOC) which reduces the J-V crossover. Finally, the quantitative comparison of J-V distortion between simulation and experiment is employed to examine the credibility of the secondary barrier theory.
Open Access
Effect of destabilized reactions using lithium amide (LiNH2) and doping using titanium based catalyst on the desorption characteristics of lithium aluminium hydride (LiAlH4)
(Colorado State University. Libraries, 2012) Paravasthu, Siddharth, author; James, Susan P., advisor; Sampath, Walajabad S., committee member; Wu, Mingzhong, committee member
In the past few decades there has been a tremendous increase in hydrogen storage research. Numerous materials and material systems have been studied as potential candidates for hydrogen storage, but unfortunately none of those materials demonstrate enough hydrogen releasing capacity under suitable temperature range to be used for hydrogen storage. Research promises to unlock the potential of these materials and ultimately lead to the commercialization of this technology. LiAlH4 is one of those materials that have been exclusively studied as a candidate for hydrogen storage due to its high theoretical hydrogen storage capacity, and its ability to release hydrogen in more than one step at different temperature ranges. Jun Lu and Zhigang Zak Fang studied the effects of titanium based catalyst (TiCl3.1/3AlCl3) and destabilization reactions using LiNH2 on LiAlH4, but did not demonstrate the effects of ball milling on the system. In the present work we have investigated the effects of ball milling, and the effects of destabilization reaction using LiNH2 on the hydrogen release characteristics of LiAlH4 doped with TiCl3. The current market scenario for fuel cell technology and the possibility and consequences of introducing the current system in the market has been briefly discussed. X-ray powder diffraction, thermo-gravimetric analysis and scanning electron microscopy were employed for the characterization of the samples. Both the compounds LiNH2, and TiCl3 worked in effecting the dehydrogenation kinetics of LiAlH4. The duration of ball milling required to affect the dehydrogenation kinetics of LiAlH4 using TiCl3 was optimized. A hydrogen release of 7.3 wt% was observed from the final system i.e. (LiAlH4/LiNH2 doped with 2% TiCl3) at temperatures below 4000C.
Open Access
FLASH holographic microscopy using a compact extreme ultraviolet table top laser
(Colorado State University. Libraries, 2015) Monserud, Nils C., author; Marconi, Mario, advisor; Menoni, Carmen, committee member; Wu, Mingzhong, committee member
Microscopes allow our eyes to visualize objects at micro- and nanoscales. But there application are not limited to static images. The visualization of dynamic processes is necessary to understand complex systems on the micro- and nanoscales, Thus the need for microscopes capable of visualizing nanoscale processes, to further extend the development on micro- and nano-electromechanical devices (MEMS and NEMS). Conventional microscopy will not be sufficient for this purpose for two reasons the first is the spatial resolution is not sufficient to capture nanoscale objects and secondly if the object is moving out of plane the image taken needs to be adjusted using methods of post processing. To this end Fourier transform holography using and EUV light source was utilized to provide us with a method recording sub-micron oscillators. We recorded the oscillation of sub-micron pillars using time resolved extreme ultraviolet (EUV) Fourier transform Holography. The source utilized was a 46.9 nm tabletop capillary discharge with an EUV wavelength of 46.9nm, which provided large flux of coherent illumination. The bright illumination allowed for a modified Fresnel Zone plate to be used as a beam splitter. The modified Fresnel zone plate was able to produce a reference and object beam. This reference and object beam interfered creating a hologram. The reference wave is created by the first order focus while a central opening in the zone plate illuminates the object. Single-shot holograms allowed for the composition of a movie featuring the fast oscillation. Three-dimensional displacements of the object were determined as well by numerical back-propagation, or "refocusing" of the electromagnetic fields during the reconstruction of a single holography.
Open Access
Improved methods for calculating the multifractal spectrum for small data sets
(Colorado State University. Libraries, 2014) Anderson, Leif, author; Eykholt, Richard, advisor; Robinson, Raymond Steve, committee member; Gelfand, Martin, committee member; Wu, Mingzhong, committee member; Shipman, Patrick, committee member
There are multiple definitions for fractal dimension, and those definitions disagree. The multifractal spectrum provides a unifying framework for a broad family of definitions of dimension, but it requires large amounts of data to compute. We provide a description of the multifractal spectrum, one existing improvement that improves convergence for small data sets: the Extended Box Algorithm (EBA), and develop several further improvements: Local and Global Norm modifications to the EBA, the a-Norm, the Variable Box size Algorithm (VBA), and the Patchwork method.
Open Access
Investigation of the dynamics of magnetic vortices and antivortices using micromagnetic simulations
(Colorado State University. Libraries, 2017) Asmat-Uceda, Martin Antonio, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Wu, Mingzhong, committee member; Shores, Matthew, committee member
This thesis is focused on investigating the dynamic properties of spin textures in patterned magnetic structures by using micromagnetic simulations. These textures become particularly relevant at sub-micron length scales where the interplay between magnetostatic and exchange energy leads to unique properties that are of great interest both from a fundamental perspective and for the development of new technologies. Two different systems, a magnetic antivortex (AV) stabilized in the intersection of perpendicular microwires, and three interacting vortices in an equilateral arrangement, were considered for this study. For the first system, the AV, the formation process and the excitation spectra were investigated. Since the AV is a metastable state, the design of a host structure capable of stabilizing it requires careful consideration and it is desirable to have general guidelines that could help to optimize the AV formation rate. The role of the shape anisotropy and the field dependence of the AV formation process is discussed in detail. Micromagnetic simulations along with magneto-optical Kerr effect and magnetic force microscopy measurements demonstrated that the asymmetry in the structure can be used to promote the formation of such AV's and that regions with lower shape anisotropy lead the reversal process, while simulations of the dynamic response show that when the system is excited with in-plane and out-of-plane external magnetic fields, normal modes with azimuthal and radial characteristics are found, respectively, in addition to the low frequency gyrotropic mode. The modes are influenced by the spin texture in the intersection, which offers additional possibilities for manipulating spin waves (SW). For the second system, three interacting vortices are simulated and compared with a simple analytical model that considers only dipolar interactions. It was found that when a fitting parameter is introduced to the model, the main features of the simulations are captured better than more complex models, which suggest that this simple framework can be used to accurately model more complex vortex networks.
Open Access
Investigations into photocatalysis and electronic structure for transition metal and actinide complexes
(Colorado State University. Libraries, 2018) Higgins, Robert F., author; Shores, Matthew P., advisor; Rappé, Anthony K., committee member; Neilson, James R., committee member; McNally, Andrew, committee member; Wu, Mingzhong, committee member
Presented herein are investigations into the electronic structure of various metal complexes and how they effect reactivity. The first chapters are centered on how [Cr(Ph2phen)3]3+ reacts as a photooxidant. The latter part of this work concerns magnetic properties of various first row transition metal and actinide complexes. In Chapter 1, I provide a background on how understanding electronic structure of transition metal complexes has motivated later work in reactivity. This Chapter also includes a detailed background in photoredox catalysis and different electronic structures of Ru-, Ir- and Cr-containing photosensitizers. It ends with a lead-in to our initial hypotheses and motivations for using Cr as a paramagnetic, Earth-abundant congener to Ru photosensitizers in photoredox manifolds. Chapters 2-4 illustrate our mechanistic studies into transformations using Cr as a photooxidant to perform [4+2] cycloaddition reactions between (trans and cis)-anethole and dienes. Chapter 2 focuses on the interactions of oxygen (O2) in the reaction of trans-anethole and isoprene mediated by [Cr(Ph2phen)3]3+. We determined three separate, yet invaluable roles that oxygen performs in this reaction, which include: (1) protection of the catalyst through excited-state energy-transfer giving 1O2, (2) 1O2 oxidation of the reduced form of the catalyst, regenerating the ground state species and giving 2O2•- as well as (3) 2O2•- reduction of the radical cation of the [4+2] product, completing the catalytic cycle. In Chapter 3, I discuss the association that trans-anethole and similar dienophiles show with [Cr(Ph2phen)3]3+ and how this affects the overall reactivity. Interestingly, diamagnetic analogues do not show the same association. Finally, in Chapter 4, trans-anethole is replaced with cis-anethole to determine how the overall reactivity changes. These data are supported by reactivity, kinetic and quenching studies to probe the reactivity. Chapters 5-7 concern similar mechanistic details involving [Cr(Ph2phen)3]3+ in photocatlytic cycloaddition reactions, except that trans-anethole, which is electron-rich, is replaced by 4-methoxychlacone, which is electron-poor. Chapter 5 discusses the synthetic utility of this reaction manifold and initial mechanistic details of the transformation, which reveal an orthogonal mechanism which proceeds through energy transfer when compared to the reactivity of trans-anethole with [Cr(Ph2phen)3]3+. In Chapter 6, the observation of enhanced regioselectivity that is observed when [Cr(Ph2phen)3]3+ is used is investigated, specifically in comparison to all other Cr- and Ru-photooxidants attempted. This regioselectivity is manifested in the stabilization of a one-bond intermediate, as well as an association between 4-methoxychalcone and [Cr(Ph2phen)3]3+. To conclude this section, Chapter 7 focuses on the interesting solution-phase equilibria of 4-methoxychalcone and how the association of 4-methoxychlacone with itself and [Cr(Ph2phen)3]3+ impacts the overall reaction mechanism. Chapter 8 provides an interesting method of using ferrocenium as an inexpensive and abundant electron-transfer reagent in reactions similar to common photoredox reactions. This uncommon reaction pathway provides an interesting reactivity compared to traditional pericyclic reactions. The remaining Chapters (9-13) explore the magnetic properties and electronic structures of a variety of first-row and actinide complexes and clusters. Chapter 9 focuses on spin-state switching through oxidation chemistry of both iron and nitrogen atoms in organometallic complexes. The ground states of these complexes can be controllably tuned through sequential oxidation reactions. In Chapter 10, I present the synthesis and magnetic properties of mono- and bis-terpyridine Co(II) complexes. These Co complexes display a variety of coordination geometries which affect their dynamic magnetic properties. Chapter 11 focuses on the reactivity and magnetic properties of a family of U-acetylide species, where interesting redox chemistry is noted upon addition of redox-inactive crown ether molecules. In Chapter 12, I discuss the magnetic properties of 3 different families of uranium complexes measured in collaboration with Prof. Suzanne Bart's group at Purdue University. Finally, in Chapter 13, I give some broad conclusions about what was learned in the mechanistic studies of Cr-photocatalysis and possible interesting avenues for future work.
Open Access
Modeling and analysis of nanoscale surface patterns produced by broad beam ion bombardment
(Colorado State University. Libraries, 2020) Loew, Kevin M., author; Bradley, R. Mark, advisor; Gelfand, Martin, committee member; Shipman, Patrick, committee member; Wu, Mingzhong, committee member
When a solid surface is exposed to broad beam ion bombardment, nanoscale patterns may spontaneously form. This physical phenomenon is of interest to both the academic and nanofabrication communities. Ion bombardment has the potential to provide a cost-efficient method of producing nanoscale patterns over a large area. As such, it has gathered substantial interest and has been the focus of numerous studies, both experimental and theoretical. However, despite more than half a century of study, there are still many unknowns which limit the application of this method to fabrication. In this dissertation, I present contributions to the field of ion bombarded surfaces (IBS). The first is the development of a Python module which facilitates the rapid production and analysis of simulations. This module provides a well-documented tool to allow collaborators to numerically integrate a user-defined partial differential equation, specifically with IBS in mind. Second is a study of dispersive effects on IBS. Dispersion can lead to the formation of raised and depressed triangular regions traversed by parallel-mode ripples, highly ordered parallel-mode ripples, protrusions and depressions that are elongated along the projected beam direction even when there is no transverse instability, and needle-like protrusions that are visually similar to structures observed in experimental studies. Finally, we applied deep learning techniques to estimate the parameters in the underlying equation of motion from an image of a surface exposed to broad beam ion bombardment at a particular fluence. Our trained neural network will allow experimentalists to quickly ascertain the parameters for a given sputtering experiment.
Open Access
NMR thermometry of vanadium based ligand-to-metal-charge transfer complexes
(Colorado State University. Libraries, 2023) Grundy, Josef, author; Zadrozny, Joe, advisor; Crans, Debbie, advisor; Wu, Mingzhong, committee member
Magnetic resonance imaging (MRI) is a noninvasive imaging technique that utilizes safe, nonionizing radiation for the diagnosis of early-stage diseases, including cancer. Although this technique provides a wealth of anatomical information, there are limitations in the information that MRI provides, such as the accurate local temperature in specific parts of the body. 51V is a promising candidate for a high-resolution NMR thermometer due to its intrinsically low quadrupolar moment and subsequent narrow linewidth. Unlike other nuclei, such as 59Co, design strategies for increasing the temperature sensitivity of 51V complexes are currently unexplored. We present a route of amplifying temperature sensitivity of the 51V chemical shift via ligand-to-metal charge transfer electronic structure design criteria. We demonstrate that this design strategy can boost the temperature dependence of the 51V chemical shift by an order of magnitude.
Open Access
Periodic metallic nanostructures fabricated by coherent Talbot lithography in a table top system
(Colorado State University. Libraries, 2013) Li, Wei, author; Marconi, Mario C., advisor; Menoni, Carmen S., committee member; Wu, Mingzhong, committee member
This thesis describes a novel technique of extreme ultraviolet (EUV) lithography. A compact nanofabrication system that combines Talbot lithography and a table top extreme ultraviolet laser illumination is presented. The lithographic method based on the Talbot effect provides a robust and simple experimental setup that is capable to print arbitrary periodic structures over millimeter square areas free of defects. Test structures were printed and metalized by ion beam etching system which was rebuilt and calibrated as part of this work. The results demonstrate that a complete coherent extreme ultraviolet lithographic process based on a table top system has the capability to fabricate functional periodic metallic nanostructures. Preliminary results and prospects for future work are also presented at the end of this thesis.
Open Access
Progress in coherent lithography using table-top extreme ultraviolet lasers
(Colorado State University. Libraries, 2016) Li, Wei, author; Marconi, Mario C., advisor; Menoni, Carmen S., advisor; Wu, Mingzhong, committee member; Krapf, Diego, committee member
Nanotechnology has drawn a wide variety of attention as interesting phenomena occurs when the dimension of the structures is in the nanometer scale. The particular characteristics of nanoscale structures had enabled new applications in different fields in science and technology. Our capability to fabricate these nanostructures routinely for sure will impact the advancement of nanoscience. Apart from the high volume manufacturing in semiconductor industry, a small-scale but reliable nanofabrication tool can dramatically help the research in the field of nanotechnology. This dissertation describes alternative extreme ultraviolet (EUV) lithography techniques which combine table-top EUV laser and various cost-effective imaging strategies. For each technique, numerical simulations, system design, experiment result and its analysis will be presented. In chapter II, a brief review of the main characteristics of table-top EUV lasers will be addressed concentrating on its high power and large coherence radius that enable the lithography application described herein. The development of a Talbot EUV lithography system which is capable of printing 50nm half pitch nanopatterns will be illustrated in chapter III. A detailed discussion of its resolution limit will be presented followed by the development of X-Y-Z positioning stage, the fabrication protocol for diffractive EUV mask, and the pattern transfer using self- developed ion beam etching, and the dose control unit. In addition, this dissertation demonstrated the capability to fabricate functional periodic nanostructures using Talbot EUV lithography. After that, resolution enhancement techniques like multiple exposure, displacement Talbot EUV lithography, fractional Talbot EUV lithography, and Talbot lithography using 18.9nm amplified spontaneous emission laser will be demonstrated. Chapter IV will describe a hybrid EUV lithography which combines the Talbot imaging and interference lithography rendering a high resolution interference pattern whose lattice is modified by a custom designed Talbot mask. In other words, this method enables filling the arbitrary Talbot cell with ultra-fine interference nanofeatures. Detailed optics modeling, system design and experiment results using He-Ne laser and table top EUV laser are included. The last part of chapter IV will analyze its exclusive advantages over traditional Talbot or interference lithography.
Open Access
Spectroscopic ellipsometry as a process control tool for manufacturing cadmium telluride thin film photovoltaic devices
(Colorado State University. Libraries, 2013) Smith, Westcott P., author; Kirkpatrick, Allan T., advisor; James, Susan, advisor; Puttlitz, Christian, committee member; Sampath, W. S., committee member; Wu, Mingzhong, committee member
In recent decades, there has been concern regarding the sustainability of fossil fuels. One of the more promising alternatives is Cadmium Telluride (CdTe) thin–film photovoltaic (PV) devices. Improved quality measurement techniques may aid in improving this existing technology. Spectroscopic ellipsometry (SE) is a common, non-destructive technique for measuring thin films in the silicon wafer industry. SE results have also been tied to properties believed to play a role in CdTe PV device efficiency. A study assessing the potential of SE for use as a quality measurement tool had not been previously reported. Samples of CdTe devices produced by both laboratory and industrial scale processes were measured by SE and Scanning Electron Microscopy (SEM). Mathematical models of the optical characteristics of the devices were developed and fit to SE data from multiple angles and locations on each sample. Basic statistical analysis was performed on results from the automated fits to provide an initial evaluation of SE as a quantitative quality measurement process. In all cases studied, automated SE models produced average stack thickness values within 10% of the values produced by SEM, and standard deviations for the top bulk layer thickness were less than 1% of the average values.
Open Access
Studies of U6+ with the RESIS method: difficulties and future directions
(Colorado State University. Libraries, 2015) Smith, Christopher Scott, author; Lundeen, Stephen R., advisor; Bartels, Randy A., committee member; Roberts, Jacob L., committee member; Wu, Mingzhong, committee member
This dissertation analyzes the details of previous resonant excitation Stark ionization spectroscopy (RESIS) measurements carried out on U5+ Rydberg states, aiming to determine properties of the ground state of U6+. These measurements were unsuccessful for apparently two reasons, a large background, and unexpectedly small signal sizes. It has been concluded that the background is a result of a prodigious amount of metastable states within the initial U6+ ion beam. Detailed simulations of the metastable population within the beamline showed the metastable hypothesis is plausible. Some reduction of the background within the RESIS technique was achieved with a redesign of the detection region in the apparatus. Detailed simulations of the RESIS signal size showed that the experimental resolved signals of the U5+ Rydberg states were no more than 1/4 the size expected from just ground state signal ions. A satisfactory explanation was proposed that metastable Rydberg ions bound to the lowest metastable level (J = 1) are forbidden to autoionize and can contribute to the measured signal. These metastable states compose 75% of the measured signal, diluting the ground state signal and preventing identification of the resolved signals. A possible future approach to the U5+ Rydberg state experiment is proposed that probes the Rydberg electron energies with a 1.5 μm laser, and looks like it could successfully measure the properties of the U6+ ion.
Open Access
Synthesis and characterization of complex metal hydrides for hydrogen technology
(Colorado State University. Libraries, 2010) Nitsche, David Martin, author; Manivannan, Venkatesan, advisor; Wu, Mingzhong, committee member
The limitation as well as the environmental impact of fossil fuels has encouraged the development of alternative energies. As an alternative, hydrogen and its applications have been established due to its favorable physical and chemical properties. Production as well as storage of hydrogen is seen to be key challenges to for a transition to a hydrogen based infrastructure. A possible storage method is seen to be chemical hydrides, where hydrogen is chemically bonded to matrix materials and can be controllable released. The chemical synthesis of these hydrides can become complex with increasing hydride structures. Therefore, mechanochemical synthesis methods have been developed, which simplify the synthesis process. The material characteristics are being determined with an XRD apparatus as well as TGA and DSC analysis. In the present study, hydrides with the general composition MH, MH2, MAlH4, and MAlH6 have been synthesized and tested for desorption characteristics, where M stands for Li, Na, K, Ca, Mg, and Ti. Doping of the materials is being applied to improve desorption characteristics of the material, where catalyst materials are implemented into the hydride structure and cause a change in the structural composition linked with a decrease of the hydrogen desorption temperature.
Open Access
Synthesis and characterization of lithium-ion cathode materials in the system (1-x-y) LiNi0.8Co0.15Al0.05O2.xLi2MnO3.yLiCoO2
(Colorado State University. Libraries, 2013) Yerramilli, Anish, author; James, Susan P., advisor; Prieto, Amy, committee member; Wu, Mingzhong, committee member
Energy storage technology has been dominated by lithium ion batteries, which are considered the most promising with higher energy density compared to any other battery technologies. The market for lithium ion batteries has increased rapidly from 2007. Goals set by the U.S Department of Energy for hybrid electric vehicles have not been met by any of the existing cathode materials. The objective of this thesis was to find a material composition that has better cyclability and lower cost than the standard battery materials. A ternary composition with low cost materials like Al, Mn and Ni were used instead of high amounts of Co to reduce the cost of the battery. It was hypothesized that there are cathode compositions in the system (1-x-y) LiNi0.8Co0.15Al0.05O2.xLi2MnO3.yLiCoO2 that when tested for discharge capacities and cyclability will show better properties than the current generation lithium ion cathode materials. The system (1-x-y) LiNi0.8Co0.15Al0.05O2.xLi2MnO3.yLiCoO2 is synthesized using a simple sol-gel synthesis. The materials LiNi0.8Co0.15Al0.05O2, Li2MnO3 and LiCoO2 were used as end points in a ternary composition diagram. Twenty eight cathode compositions spanning the entire ternary composition diagram were synthesized under the same conditions and characterized using X-ray diffraction (XRD) and an Arbin BT2000 battery testing system. XRD results showed α-NaFeO2 structure with a space group of R3m. The results from electrochemical testing revealed a wide range of electrochemical capacities and cyclabilities. The regions close to Li2MnO3 showed high capacities and cyclability. The material with composition Li1.5 Ni0.133Co0.358Al0.008Mn0.5 had an initial discharge capacity of 216.3 mAh/g and retained this capacity even after multiple cycles in the voltage range of 4.6-2 V at a rate of C/15. Statistical analysis was done using SAS/STAT 9.2 with the ADX procedure to fit a general linear model with three linear terms and three two way interactions to map capacities and cyclabilities. This analysis was used to choose the compositions with best capacities and cyclability. Inductively couple plasma (ICP) analysis was carried out on the chosen samples to find the error between calculated composition and the theoretical composition. XPS (X-ray photoelectron spectroscopy) was conducted for the chosen samples and the oxidation states of the elements were determined. The material with composition Li1.5 Ni0.133Co0.358Al0.008Mn0.5 was found to be the promising material for commercialization. Before going into the market additional changes like synthesis conditions and surface treatments should be conducted on the material.
Open Access
Synthesis and characterization of lithium-ion cathode materials in the system (1-x-y)LiNi1/3Mn1/3Co1/3O2 ∙ xLi2MnO3 ∙ yLiCoO2
(Colorado State University. Libraries, 2012) Paravasthu, Rushendra, author; James, Susan P., advisor; Prieto, Amy, committee member; Wu, Mingzhong, committee member
Considering various technologies for storing energy the usage of lithium (Li) - ion batteries still stands as one of the most promising options, especially for the on-going huge demand for electric and plug-in hybrid vehicles. The main limiting factor in the performance of a Li-ion battery is the cathode material. The current cathode material that is being used in the present market is LiCoO2 cathode which is effective but is expensive and toxic. The objective of this thesis is to find a cathode material which is advantageous and a probable replacement for LiCoO2. Based on the previously studied work on ternary solid solutions and its advantages, this system (1-x-y)LiNi1/3Mn1/3Co1/3O2•xLi2MnO3•yLiCoO2 was chosen. This was made using a ternary composition diagram which is a combination of LiNi1/3Mn1/3Co1/3O2, Li2MnO3 and LiCoO2 materials. Points inside the ternary diagram were chosen in an arrangement conducive to mathematical modeling and compositions of the cathodes were processed accordingly. All the 28 samples in the system were synthesized using the sol-gel method, each sample was characterized using X-ray diffraction (XRD) scans and electrochemical testing was performed by using an Arbin BT2000 battery testing system with MITS Pro Arbin software. The XRD results showed that all the samples established a α-NaFeO2 structure and a space group of R3m with varying degrees of purity in the crystal structure. The compounds in this system showed fairly consistent charge/ discharge curves during the electrochemical testing with initial discharge capacities varying from 122mAh/g to 240mAh/g and high capacity compositions were found in the region of high Li2MnO3 content. The highest capacity found was Li1.222Mn0.5Ni.056Co0.222O2 composition with a discharge capacity of 242mAh/gin the voltage range 4.6 - 2V at a C/15 rate. A statistical based analysis was carried out using JMP version 7.0.1 with the design of experiments (DOE) procedure for a mixture design to find variations in capacity and cyclability trends over the composition triangle. Inductively couple plasma (ICP) analysis was carried out on 4 samples which were found to be most promising basing on the statistical analysis. ICP results confirmed the formation of optimum compositions for the specified synthesis conditions. All these 4 samples were synthesized and tested again to confirm consistent performance across repeat samples. XPS (X-ray photoelectron spectroscopy) was conducted for these samples to determine the chemical environment of transition metals. The composition with the highest capacity was found to be Li1.222Mn0.5Ni0.056Co0.222O2 electrode. Its high capacity is attributed to the Ni+2/Ni+4 and Co+3/Co+4 red-ox couple and the Li-ion extraction form Li2MnO3 at a voltage >4.5V which results in an extended discharge profile. This cathode composition is very promising because it shows high capacity and good cyclability, and would be much cheaper than conventional LiCoO2 cathodes since it has more manganese which is less expensive than cobalt.
Open Access
Synthesis and characterization of sterically and electronically tuned ligands toward magnetic control of iron and cobalt complexes
(Colorado State University. Libraries, 2015) Klug, Christina M., author; Shores, Matthew P., advisor; Rappé, Anthony K., committee member; Ackerson, Christopher J., committee member; Levinger, Nancy E., committee member; Wu, Mingzhong, committee member
Presented within this dissertation are the syntheses and characterizations of iron and cobalt complexes featuring ligands designed to tune the magnetic properties. Two key magnetic phenomena are of interest: spin crossover and single-molecule magnetism. Both of these topics are known to be significantly influenced by subtle changes in coordination and inter- and intramolecular interactions. The overarching goal is to understand how the magnetic properties of the metal center can be controlled via electronic and steric modifications. In Chapter 1, I offer a brief introduction into the background and motivation of the works presented in this dissertation in the realm of spin crossover and single-molecule magnetism. The first section of this chapter is focused on spin crossover and how host:guest interactions can be exploited to alter the magnetic behavior of first-row transition metals. Examples of Fe(II) complexes that display anion-dependent spin state behaviors in both the solid-state and in solution are discussed. Functionalized tripodal Schiff-base ligands are placed into context as an extension of previous research into tripodal ligands for use as metal-based anion-receptors and tripodal spin crossover complexes. The second section of Chapter 1 gives a brief introduction into single-molecule magnetism. An examination of mononuclear Co(II) complexes displaying slow magnetic relaxation and application of acetylide-bridged metal centers to enhance magnetic communication are also given. In Chapter 2, I discuss the preparation and characterizations of a Fe(II) complex coordinated by the alcohol functionalized hexadentate tripodal iminopyridine L6-OH with varying anions. Solid-state magnetic susceptibility measurements of [FeL6-OH]X2 (X = OTf-, Br-, I-, or BPh4-) reveal an anion-dependence on the magnetic behavior. Magnetostructural correlations indicate that stronger hydrogen-bonding interactions are achieved with larger anions, which are better able to undergo bifurcated interactions with the hydroxyl groups from two of the arms. Removal of the tether between the ligand arms leads to the formation of [Fe(L2)2](OTf)2, a bis(tridentate) complex that remains high spin at all temperatures. Variable temperature magnetic measurements in d3-methanol reveal that the high spin state of [FeL6-OH]2+ persists regardless of the anion down to 183 K. In Chapter 3, attempts towards synthesizing the heteroarmed tris(imine) [FeL556]2+ and analogous bis(imine)-mono(amine) [FeL556-NH]2+ complexes are discussed. Several routes are attempted to synthesize the tris-iminopyridine species including selective deprotonation of tris(2-aminoethyl)amine*3 HCl, in situ complex formation via metal-templated self-assembly, and use of presynthesized ligands. Analyses of the reaction mixtures by mass spectrometry suggest that mixtures of products are formed regardless of the method. An anion and solvent dependence leads to preferential formation of the low-spin species [FeL5-ONHtBu]2+, while using solvents such as acetonitrile and ethanol lead to increased production of the desired [FeL556]2+. To test if anion-dependent magnetic behavior can be observed with this ligand type, the comparable complex [FeL556-NH]2+ was synthesized and characterized. Variable temperature solution measurements in d3-acetonitirile suggest that host:guest interactions in solution induce a stabilization of the low-spin state for [FeL556-NH]2+ as indicated by a decrease in susceptibility at lower temperatures for the Cl- salt. In Chapter 4, the preparation, structural, and magnetic characterizations for a family of Fe(II) complexes of tripodal ligands based on L5-ONHtBu are presented. The series of ligands aim to tune the ligand field by selectively reducing imines to amines, producing the ligands L5-(NH)x (x = 1 - 3, number of amines). In the solid state, the three Fe(II) complexes formed are high spin, but significant differences in the structural distortion of both the coordination environment of the Fe(II) center as well as the anion-binding pocket of the amides are noted. In solution, the complexes [FeL5-(NH)3]2+ and [FeL5-NH]2+ are high spin between 183 and 308 K in d6-acetone but interestingly, [FeL5-(NH)2]2+ undergoes a spin-state change with decreasing temperature. Variable temperature studies in d6-acetone and anion titrations in d3-acetonitrile at room temperature monitored by Evans' method of [FeL5-(NH)2]2+ show host:guest interactions stabilize the high spin state. These studies suggest a viable method of ligand tuning for spin-state control by host:guest interactions. In Chapter 5, I discuss the structural and magnetic properties of [Co5-ONHtBu]X2 (X = Cl-, Br-, I-, and ClO4-). These hexadentate Co(II) complexes vary only in the charge-balancing anion, but marked differences in their magnetic properties are observed. Investigation of the magnetic anisotropy of the various salts reveal that the chloride salt possesses the most axial anisotropy, which manifests as an exhibition of slow magnetic relaxation under application of an external field. To my knowledge this is the first example of anion-binding influencing the magnetic anisotropy and 'turning on' single-molecule magnet-like behavior. Lastly, Chapter 6 describes the syntheses and magnetic properties of a series of mono-and dinuclear Fe(III) complexes bridged by ethynylmesitylene ligands. Inclusion of steric bulk onto the bridging-aryl ligand is predicted to increase orbital overlap between the singly-occupied molecular orbital of the metal center and the π-system of the aryl linker. The addition of methyl groups to the aryl ring cements the desired equatorial ligand orientation with respect to the π-system. This leads to an increase in ferromagnetic coupling between the metal centers.
Open Access
Synthesis and characterization of uranium(IV) compounds: from mononuclear complexes to multinuclear assemblies
(Colorado State University. Libraries, 2011) Newell, Brian S., author; Shores, Matthew P., advisor; Anderson, Oren P., advisor; Chen, Eugene Y., committee member; Levinger, Nancy E., committee member; Wu, Mingzhong, committee member
This dissertation describes the synthesis of multinuclear compounds that possess magnetically-coupled actinide, namely uranium-238, clusters. These assemblies are supported by both acetylide-type ligands as well as triamidoamine or softer phosphine ligands. Synthetic inorganic chemists have been able to synthesize molecules and clusters with increased spin, S, or axial anisotropy, D, in an effort to augment the spin-reversal barriers and create better single-molecule magnets (SMMs). However, efforts to simultaneously increase these parameters are complicated. One potential route utilizes heavy atoms as a result of their larger single-ion anisotropy and believed ability to modulate the magnetism of other systems. My research is placed in this context in Chapter 1, where recent efforts to incorporate heavy atoms into expanded clusters are discussed. In Chapter 2, the preparation and magnetic property investigations of a structurally related family of mono-, di- and trinuclear U(IV) aryl acetylide complexes are presented. The reaction between [(NN′3)UCl] and lithiated aryl acetylides leads to the formation of hexacoordinate compounds. In contrast, combining the uranacycle [(bit-NN′3)U] (bit-NN′3 = [N(CH2CH2NSitBuMe2)2(CH2CH2SitBuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords pentacoordinate arylacetylide complexes, where NN′3 = [N(CH2CH2NSitBuMe2)3]. The measured magnetic susceptibilities for these compounds trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the DC magnetic susceptibility data, using the spin Hamiltonian. Geometry-optimized Stuttgart/6-31g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes and the electrochemistry of the monomeric phenylacetylide complex exhibits a reversible redox couple at -1.02 V versus [Cp2Fe]+/0, assignable to an oxidation of U(IV) to U(V). Efforts to study the magnetic correlations as a result of cubic ligands fields are presented in Chapter 3, whereby a neutral bidentate phosphine ligand was utilized. In the course of structurally characterizing previously reported complexes based on the 1,2-bis(dimethylphosphino)ethane)) (dmpe) ligand ([(dmpe)2UCl4] (3.1) and [(dmpe)2UMe4] (3.2)), we found that adjusting the U:dmpe ratio leads to an unprecedented species. Whereas the use of two or three equivalents of dmpe relative to UCl4 produces 3.1 as a blue-green solid, use of a 1:1 dmpe:UCl4 stoichiometry yields [(dmpe)4U4Cl16]•2CH2Cl2•(3.3•2CH2Cl2) as a green solid. In turn, 3.3 is used to prepare a mixed-chelating ligand complex featuring the bidentate ligand 4,4′-dimethyl-2,2′-bipyridine (dmbpy), [(dmpe)(dmbpy)UCl4] (3.4). The measured magnetic susceptibilities for 3.1-3.4 trend toward non-magnetic ground states at low temperatures. In Chapter 4, we hypothesized that preparing complexes that contain U(IV) in a cubic ligand field environment, using acetylide ligands, might allow for the isolation of compounds exhibiting enhanced magnetic coupling. In that vein, we report the synthesis and characterization of [(dmpe)2U(CCPh)4] (4.1) (CCPh = phenylacetylide) and [(dmpe)2U(CCPh)5(Li∙Et2O)] (4.2). No reproducible magnetic susceptibility data were obtained and a discussion about these difficulties is presented. In the course of studying the crystal structure of the mixed-chelating ligand complex [(dmpe)(dmbpy)UCl4] (3.4) an interesting effect on the U-Cl⋯H was observed. Several computation methods were utilized to determine that the M-Cl⋯HC distance based on approach angles is suggestive that Cl is acting more like chlorine and less like chloride. This provides a route to study U-L bonding and is presented in Chapter 5. Finally, in Chapter 6, efforts to synthesize a mixed-metal complex are discussed and preliminary characterization of a dinuclear ethynylbenzene 5f-3d complex (6.3) is presented. While an unambiguously paramagnetic metal-complex was not isolated, initial electrochemical studies indicate a redox process takes place. A short discussion about the temperature dependence of the magnetic susceptibility is given.
Embargo
Synthetic control of magnetic resonance properties towards metal-based electron paramagnetic resonance imaging
(Colorado State University. Libraries, 2023) Campanella, Anthony John, author; Zadrozny, Joseph, advisor; Shores, Matthew, committee member; Bandar, Jeff, committee member; Wu, Mingzhong, committee member
Electron paramagnetic resonance imaging (EPRI) is the electron-spin analogue to conventional biological (nuclear) magnetic resonance imaging (MRI) whereby unpaired electron spins are probed in order to generate an image. The greater sensitivity of electron spins to their environment can thus be leveraged to capture detailed chemical information from the surroundings, producing an image of the local physiology that adds an extra dimension to the already powerful anatomical information gained from MRI. To move EPRI a step closer to common utilization, paramagnetic probes must be developed to sense the local environment using safe low-frequency microwaves at high (ca. 1.5 T) magnetic fields. Paramagnetic metal complexes are ideal candidates due to their electronic structures but have not been investigated for such purposes. The goal of this dissertation is to develop fundamental design principles to improve the utility of metal complexes as EPRI probes. Presented herein is the first comprehensive collection of experimental investigations to this end. Firstly, a method for improving signal sharpness is investigated, where exhaustive spectroscopic and computational studies suggest differences in relaxation dynamics as being a key factor in spectral linewidth (Chapters 2 and 3). A highly tunable clathrochelate structure is developed, inducing an unusual coordination geometry around the Ni(II) ion affording an 11 cm−1control of zero-field splitting (Chapter 4). The temperature dependence of zero-field splitting is examined in a series of Mn(II) complexes where an unusually high temperature sensitivity is found in the solid state (Chapter 5). Finally, the utility of metal complexes as environmental sensors is demonstrated with a pair of Mn(II) complexes showing that increasing magnetic anisotropy is a design strategy for enhancing microviscosity sensitivity (Chapter 6). The learned design principles will serve as a foundation for the design of metal-based EPRI agents towards improving the non-invasive diagnostic capabilities.