Browsing by Author "Gelfand, Martin P., committee member"

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Open Access
A spectral analysis of the Crab nebula and other sources with HAWC
(Colorado State University. Libraries, 2016) Gussert, Michael, author; Harton, John, advisor; Mostafa, Miguel, advisor; Toki, Walter, committee member; Anderson, Chuck, committee member; Gelfand, Martin P., committee member
The High Altitude Water Cherenkov observatory (HAWC) is an extensive air shower particle detection array designed to study cosmic gamma (γ) rays in the Very High Energy (VHE) regime (100 GeV to 100 TeV). One of the most thoroughly studied sources in this energy range is the Crab nebula, a pulsar wind nebula created by the aftermath of supernova 1054. The core of this analysis revolves around the determination of the differential flux spectrum of the Crab nebula using a process known as forward folding. Forward folding allows energy spectra to be fit without requiring a direct measurement of the primary energy of individual extensive air showers. The energy resolution of HAWC is very poor (on the order of 50% or more), and so this method is ideal for any spectral analysis carried out with HAWC data. The differential spectra are modeled as a power law with a normalization (Φ0), spectral index (γ), and a cutoff energy (Ec): dN/dE = Φ0(E/E0)γe−E/Ec . The normalization of the Crab nebula was found to be 1.03±0.091 0.083 stat ±0.19 sys)×10−12(TeV−1 cm−2 s −1 ) with an index of −2.54 ± 0.095 stat ± 0.27 sys and a cutoff of 91.0 ±174 59 stat with E0 =4.0 TeV. This method was also applied to 11 other sources, and the minimum detection significance required to constrain a spectrum was found to be between 10 and 14 σ.
Open Access
Brillouin light scattering study of linear and nonlinear spin waves in continuous and patterned magnetic thin films
(Colorado State University. Libraries, 2014) Liu, Hau-Jian Jason, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Neilson, James R., committee member
This thesis focuses on the use of the Brillouin light scattering (BLS) technique to measure spin waves or magnons in thin films. BLS is an experimental technique that measures the inelastically scattered light from photon-magnon interactions. Broadly, three different experiments are presented in this thesis: the measurements of spin wave properties in iron cobalt (FeCo), yttrium iron garnet (YIG), and microstructures involving Permalloy (Ni80Fe20) and cobalt nickel (CoNi). First, conventional backward scattering BLS was used to measure the spin waves in a set of Fe65Co35 films that were provided by Seagate Technologies. By fitting the spin wave frequencies that were measured as a function of the external magnetic field and film thickness, the quantum mechanical parameter responsible for short range order, known as the exchange parameter, was determined. Second, nonlinear spin waves were measured in YIG using conventional forward scattering BLS with time resolution. Two nonlinear three wave processes were observed, namely, the three magnon splitting and confluence. The nonlinear power threshold, the saturation magnetization, and the film thickness were determined independently using network analyzer measurements. The spin wave group velocities were determined from the space- and time-resolved BLS data and compared to calculations from the dispersion relations. Back calculations showed the location where the three magnon splitting process took place. Lastly, spin waves in Permalloy and CoNi microstrips were measured using a recently developed micro-BLS. The micro-BLS, with a spatial resolution of 250 nm, allows for measuring the effects on the lateral confinement of spin waves in microstrips. The confinement of spin waves led to modifications to the dispersion relations, which were compared against the spin wave frequencies obtained from the micro-BLS. The Permalloy experiments shows non-reciprocity in surface spin wave modes with opposite wavevectors and provides a quantitative measure of the difference in excitation efficiency between the surface spin wave and the backward volume spin wave modes. Measurements were also conducted in the Permalloy microstrips at zero external magnetic field, showing evidence that propagating spin waves can be observed by exploiting the effects of shape anisotropy. Finally, preliminary measurements were done on CoNi microstrips with perpendicular anisotropy. A magnetic signal was detected, however further investigation will be needed to determine the exact origin of the observed signal and to definitively answer the question as to whether or not BLS can be used to measure spin waves in perpendicularly magnetized films. Overall, the experiments and results presented in this thesis show that BLS is a useful tool for measuring spin wave properties in magnetic thin films.
Open Access
Damping mechanisms in magnetic recording materials & microwave-assisted magnetization reversal
(Colorado State University. Libraries, 2014) Lu, Lei, author; Wu, Mingzhong, advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Marconi, Mario C., committee member; Patton, Carl E., committee member
Understanding the damping of magnetization precession in magnetic recording materials is of both fundamental and practical significance. From the practical perspective, the relaxation processes not only set a natural limit to the time of magnetization switching which determines recording data rates, but also play critical roles in advanced magnetic recording techniques such as microwave-assisted magnetic recording and two-dimensional magnetic recording. Experimental and theoretical studies of magnon-electron scattering and two-magnon scattering (TMS) contributions to magnetization relaxations in magnetic recording head and media materials were conducted for the first time in this dissertation. The accuracy of ferromagnetic resonance (FMR) measurements was increased by the use of vector network analyzer (VNA) FMR techniques. Working equations of the grain-to-grain TMS and grain boundary TMS processes were developed based on the TMS models of Krivosik and Mo, and were applied to understand the relaxation mechanisms in various recording-related thin film materials. The dependences of the FMR behavior and relaxation rates on the external field orientation, the microwave frequency, and the temperature were investigated experimentally in the following three domains: the exchange-coupled composite media, the free layers of tunnel magneto-resistance readers, and FeCo alloy films for future writers. The theoretical models were used to analyze the experimental data and to understand the relaxation mechanisms. Microwave-assisted magnetization reversal (MAMR) is considered as a promising mechanism for further increasing the recording area density and pushing it beyond the super-paramagnetic limit. The MAMR operation was demonstrated with a 700-Gbit/in2 perpendicular media sample in this thesis study. For microwaves with frequencies close to the FMR frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.
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
Ion properties from high-L Rydberg atom spectroscopy: applications to nickel
(Colorado State University. Libraries, 2012) Woods, Shannon L., author; Lundeen, Stephen R., advisor; Krueger, David A., committee member; Gelfand, Martin P., committee member; Krummel, Amber T., committee member
An effective potential model describing high-L Rydberg states was systematically derived. The model assumes that the response of the core ion to the electric field of the Rydberg electron is at least approximately adiabatic; in other words, the excitation energies of the core ion are large compared to the typical energies of the Rydberg levels. The resulting model should describe a wide variety of high-L Rydberg systems. It can be used, in combination with experimental measurements of fine structure patterns, to extract measurements of core ion properties that control long-range interactions between the core and the Rydberg electron. These include permanent electric and magnetic moments, and electric polarizabilities. As an example application of the model, the fine structure pattern in n = 9 Rydberg levels of nickel was measured using the Microwave Resonant Excitation Stark Ionization Spectroscopy (RESIS) method. Properties of the 2D5/2 ground state of Ni+ extracted from these measurements include quadrupole and hexadecapole moments Q = -0.4705(2) a.u. and ∏ = 0.27(9) a.u., scalar and tensor dipole polarizabilities αD,0 = 7.925(10) a.u. and αD,2 = 1.043(33) a.u., and scalar quadrupole polarizability αQ,0 = 71(9) a.u. In addition, evidence for a permanent magnetic octupole moment of Ni+ was seen, parameterized by the coefficient CM3 = -0.346(57) a.u.
Open Access
Luminescence measurements inform a strategy for unlocking the full potential of CdTe-based photovoltaics
(Colorado State University. Libraries, 2023) Jundt, Pascal M., author; Sites, James R., advisor; Sampath, Walajabad S., committee member; Yost, Dylan C., committee member; Gelfand, Martin P., committee member; Kuciauskas, Darius, committee member
Cadmium telluride (CdTe) photovoltaics are characterized by simplicity and speed of fabrication with low usage of materials, all of which translate into low cost. These significant advantages have earned CdTe the second-highest adoption rate of all photovoltaic technologies. However, conversion efficiencies, while functional, are significantly lower than the theoretical limit for this material. This discrepancy is almost entirely a discrepancy in voltage, and the so-called "voltage deficit" of CdTe has stubbornly persisted for decades. While many strategies are being pursued to attempt to reduce the voltage deficit, this issue is fundamentally one of excessive nonradiative recombination due to defects within the absorber material, as will be demonstrated in this dissertation. Recombination is evaluated primarily by luminescence measurements, and as such this class of measurements is particularly relevant to the challenges faced by CdTe research today. The rate of recombination is parameterized by the carrier lifetime, and time-resolved photoluminescence (TRPL) is the most common method of determining this parameter in CdTe. Historically, accurate determination of bulk lifetime was as simple as extracting the time constant of the slowest component of a TRPL decay. However, significant gains in material passivation and doping over the last few years have both decreased the relative influence of trap-assisted recombination and increased the influence of p-n junction fields on TRPL measurements. Consequently, when measurements are performed on complete cells, extracting the tail time constant from a TRPL decay no longer necessarily gives an accurate representation of the bulk material lifetime, and the result is distorted by field effect contributions. This fact is not necessarily well-known by the CdTe community, and extraction of the tail time constant is still the most common way to report lifetimes, even in measurements on complete state-of-the-art cells. This dissertation demonstrates the skewing effects of junction fields, and identifies under which conditions they manifest and how. To probe field effects, external electrical bias was incorporated during TRPL measurements, which allows fairly precise manipulation of fields. Biased TRPL measurements were performed on a variety of samples, and a model was developed to substantiate and better explain the results. It was found that the same characteristics which enable good performance (high lifetime, doping, and mobility) are the same which add complexity to TRPL interpretation. It was also found that field effects can be effectively suppressed by significant forward bias, leading to far more accurate determination of bulk lifetime. TRPL and external radiative efficiency (ERE) luminescence measurement results have indicated very low rates of nonradiative recombination and associated very high lifetime for some CdTe-based materials deposited at Colorado State University, particularly the cadmium selenium telluride (CdSeTe) alloy. While these attributes should allow voltages approaching 1 V and efficiencies on the order of 25%, when incorporated into "traditional" cell architectures these materials typically achieve middling performance at best, and often no performance at all. To unlock the great potential indicated by luminescence measurements, a different cell architecture is proposed which aims to accommodate these materials and take advantage of their characteristics. In an n-i-p configuration, an intrinsic absorber material is sandwiched between two carrier-selective contacts, at least one of which must be transparent. This design eliminates the requirement that the absorber be doped, which penalizes lifetime. Based on the findings of modeling reported here, undoped CdSeTe appears to be an ideal intrinsic layer material. The currently-utilized SnO2:F/MgZnO front contact appears to be excellent as the n-type electron-selective layer. The one missing component is the p-type hole-selective layer; modeling in this dissertation describes in detail what attributes are required of this material. Most important is band alignment with CdSeTe, which should produce a valence band offset as close to zero as possible, and a conduction band offset which forms a sufficiently high electron barrier. Sufficient p-type doping is also quite important. Based on these criteria, ZnTe was identified as a suitable candidate material, and several cells were fabricated with this architecture. While preliminary cells achieved relatively poor performance compared with traditional designs, J-V curves were surprisingly well-behaved, and the almost immediate achievement of functioning cells using an entirely new approach is promising. Luminescence characterization of these structures identified several areas for improvement, namely the use of a p-type dopant other than copper and the replacement of ZnTe with another material with similar band structure but more compatible lattice constant.
Open Access
New approaches to fluoromodifications of electron acceptor molecules for organic photovoltaics
(Colorado State University. Libraries, 2021) Brook, Colin P., author; Strauss, Steven H., advisor; Prieto, Amy L., committee member; Kennan, Alan J., committee member; Gelfand, Martin P., committee member
The overall goal of this work is to advance fundamental science and applications of organic electron acceptors based on fluorinated fullerenes and polycyclic aromatic hydrocarbons. The synthetic part of the dissertation focused on the development of new synthetic approaches towards the fluoromodification of large conjugated organic molecules and on the improvements of existing methods for the preparation of high-performing fullerene-based n-type semiconductors. Chapter 1 describes development and application of a new configuration of the gradient-temperature gas-solid reactor for the efficient and high-yielding trifluoromethylation of fullerenes. Significant improvements were achieved in the yields and selectivity of bis-trifluoromethylated C60 and C70 fullerenes: 2-fold and 10-fold yield increase compared to prior state of the art, respectively. An approach to maintain a constant reactive gas pressure in the reactor has been introduced by creating a liquid-gas reservoir of CF3I by submerging the reservoir in one of several low-temperature slush baths available that resulted in improvements in both yields and selectivity for trifluoromethylfullerenes, while also solving a problem of previously unproductive use of the gaseous reagent. Chapter 2 presents the author's work in partnership with the National Renewable Energy Lab (NREL) aimed at investigation of the prominent stabilizing effect of perfluoroalkylated fullerenes on the rate of photobleaching of common high-performance donor-polymers used in OPV devices, compared to the pure polymer films and blends with prototypical non-fluorinated fullerene, PC60BM. It is demonstrated that rationalizing complex photobleaching behaviour ultimately required consideration of the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene blend. The ability of the bis-trifluoromethylfullerene and Fauxhawk fullerene to stabilize certain donor materials against photodegradation, to blend well with fluorinated (and even certain non-fluorinated) polymers, and to quench excited states efficiently was thoroughly studied and correlated with structure-property relationships amongst several polymer donor and fullerene acceptor combinations. Chapter 3 describes a new approach to the direct fluoromodification of polycyclic aromatic hydrocarbons based on replacing commonly used perfluoroalkyl groups (CnF2n+1, or RF) with perfluorobenzyl groups (C6F5CF2, or BnF). For the first time, solution-phase direct perfluorobenzylation of an electron-rich perylene (PERY) and electron-poor perylene diimide (PDI), has been achieved. Five new bay- and peri-substituted compounds of perylene, PERY-(BnF)n, where n = 1, 2 and 3; and three new bay-substituted compounds of perylene diimide, PDI-(BnF)n, where n = 1, 2; were synthesized and fully characterized, revealing that electron withdrawing BnF group is comparable to RF in increasing acceptor strength of new compounds. Chapter 4 deals with a new type of annulative pi-extension (APEX) reaction discovered in this work that occurs via reductive dehydrofluorination/aromatization reactions involving perfluorobenzylated PDI compounds, that afforded fluorinated benzoperylene and coronene-based derivatives with prodigious electron acceptor properties. Another type of annulation leading to the transition-metal free formation of new compounds with all-carbon seven-membered rings across the bay regions of PDIs, consequently forming as rare examples of a newly-recognized fundamental type of conformational isomers, named akamptisomers, is also reported here for the first time. Studies of the likely reaction pathways in both types of reactions and effects of varying reaction parameters on the preferred product formation are presented. Single crystal crystallographic studies of many of the new compounds prepared in this work provide rich and unique data for in-depth analysis of the solid-state packing motifs and influences of the type and position of fluorinated functional groups on the intermolecular interactions, and ultimately, charge transport in these new organic n-type semiconductor materials.
Open Access
Synthesis of fluoromodified carbon rich electron acceptors and exploration of their structural, electronic, and device properties
(Colorado State University. Libraries, 2020) DeWeerd, Nicholas J., author; Strauss, Steven H., advisor; Shores, Matthew P., committee member; Ackerson, Chris J., committee member; McCullagh, Martin J., committee member; Gelfand, Martin P., committee member
The electronic and structural characterization of fluoro-modified carbon-rich compounds is critical to the successful implementation of these materials by physicists, biochemists, materials scientists, medicinal chemists, and most significantly for this work, organic electronics chemists. By adding powerful electron-withdrawing groups, the electron acceptor and solid-state structural properties of carbon rich substrates such as polyaromatic hydrocarbons (PAHs) and fullerenes can be improved, making these derivatives attractive semiconductor materials for organic electronics applications. This work will discuss research which has focused on expanding the library of electron acceptor compounds, elucidating the electronic and structural properties of those compounds, and exploring their physicochemical properties, focusing on properties that are important for the performance of organic electronic devices. This was accomplished by exploring reaction conditions which had not been previously reported at pressures and temperatures exceeding the operational limits of conventional reactors, developing purification methods that allow for chromatographic separation of constitutional isomers, and structural characterization of those purified materials by mass spectrometry, NMR, and most importantly X-ray crystallography. As a complement to this research, the stability of organic electronic active layers was studied to better understand how organic semiconductor active layer's degradation affects device performance over time and to better inform which active layer material properties should be pursued. Based on those findings and literature precedent, one family of compounds, C60 and C70 fauxhawk fullerenes, found to have favorable characteristics were then utilized in OFET devices as n-type semiconductors resulting in record-setting charge carrier mobilities.