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Department Library

2019

Berg Daniel Dodson (Masters Thesis, December 2019, Advisor: Richard Vanfleet )

Abstract

Carbon nanotube templated microfabrication (CNT-M) is a method that allows high-aspect ratio structures to be made for microelectromechanical systems (MEMS) devices. One concern when making monolithic electrical devices using CNT-M is that the aluminum oxide diffusion barrier will create too large of a resistance in the device. However, in developing CNT based MEMS devices, it has been observed that an electrical DC current is capable of transport from a conductive substrate, across the aluminum oxide, and through to the CNT structure grown on top of it. This thesis attempts to determine the mechanisms responsible for current being able to cross the aluminum oxide diffusion barrier easily through sample characterizations. Principally, current-voltage measurements, electron microscopy, XEDS, and SIMS analysis are used to characterize the various samples and determine the process responsible for the observed phenomenon. Through these techniques, it is determined exposure to ethylene gas during the CNT growth recipe used in our lab, regardless of whether CNTs grow on the sample or not, is necessary to cause a drop in resistance across the aluminum oxide, but the that the overall content of iron and carbon in the aluminum oxide do not correlate with this drop in resistance.

2018

David Alan Kane (Masters Thesis, December 2018, Advisor: Richard Vanfleet )

Abstract

Atomic Layer Deposition (ALD) of Al2O3 on tall multiwalled carbon nanotube forests shows concentration variation with the depth in the form of discrete steps. While ALD is capable of extremely conformal deposition in high aspect ratio structures, decreasing penetration depth has been observed over multiple thermal ALD cycles on 1.3 mm tall multiwalled carbon nanotube forests. SEM imaging with Energy Dispersive X-ray Spectroscopy elemental analysis shows steps of decreasing intensity corresponding to decreasing concentrations of Al2O3. A study of these steps suggests that they are produced by a combination of diffusion limited delivery of precursors with increasing precursor adsorption site density as discrete nuclei grow during the ALD process. This conceptual model has been applied to modify literature models for ALD penetration on high aspect ratio structures, allowing several parameters to be extracted from the experimental data. The Knudsen diffusion constant for trimethylaluminum (TMA) in these carbon nanotube forests has been found to be 0.3 cm2s-1. From the profile of the Al2O3 concentration at the steps, the sticking coefficient of TMA on Al2O3 was found to be 0.003.

2017

Berg Dodson (Senior Thesis, May 2017, Advisor: Richard Vanfleet )

Abstract

Metal electroplating of carbon nanotube (CNT) forests is viable method for fabricating metal microelectromechanical systems (MEMS). Recent work has demonstrated that electroplating an interconnected carbon nanotube structure is possible through direct electrical contact to the CNT structure. However, this is not ideal since features in MEMS fabrication often need to be both electrically and mechanically isolated from each other. Current efforts have been directed toward electroplating CNT patterns through electrical contacts on the substrate. In this case contacting to CNTs through metallic leads that sit between the substrate and the grown CNT forests. Since the CNT forest catalyst layer involves an insulating, alumina diffusion barrier between the iron catalyst layer and the metallic substrate, this is not as simple as it sounds. Results on the measured resistance of this structure are presented in this thesis.

2016

Kendall Berry (Senior Thesis, April 2016, Advisor: Richard Vanfleet )

Abstract

X-ray spectroscopy effectively gives insight into material development and other fields by detecting characteristic x-ray emissions from a sample. Measurement of lower Z-number elements is limited by poor transmission through the detector-sample chamber interface, or x-ray window. A thinner, more transmissive window would allow detection of these elements, but it would also need additional support to withstand the required differential pressures. A process for fabricating submicron-thick nanoporous polymer support membranes was developed using electron beam lithography, thermal nickel evaporation, and reactive ion etching. Bulge testing the resulting films gave insight into material strength and properties. Preliminary results are inconclusive but seem to indicate that membranes similar to the ones fabricated here may be able to serve as support structures for more transmissive x-ray windows.

Andrew Davis (Senior Thesis, April 2016, Advisor: Richard Vanfleet )

Abstract

Here we demonstrate the fabrication of two types of fluid filters developed using carbon-nanotube-templated microfabrication (CNT-M). The first of these are high strength microsieves for both liquid and gas filtration. The sieves were fabricated by growing patterned forests of vertically aligned carbon nanotubes followed by an infiltration of the forest with nanocrystalline carbon. The process is compatible with the fabrication of sieves with pore sizes from below one micron up to tens of microns. Straight, vertical channels result in low flow resistance and the high strength carbon material is compatible with high pressure filtering. Initial filtration testing on sample sieves with 5 micron pores showed greater than 99.5% removal efficiencies of 6 micron particles. The second filter type also uses an adaptation of CNT-M to make high throughput filters by hierachically patterning the structure to achieve ultrahigh surface areas. These filters have pores on the order of 50-100 nm and therefore are compatible with bacterial filtration. Preliminary flow testing has shown flow rates of over 180 times greater than standard Millipore mixed cellulose ester sterilizing disk filters.

2015

Dallin Barton (Senior Thesis, August 2015, Advisor: Richard Vanfleet )

Abstract

This thesis presents processes and techniques of electroplating two different metals, nickel and copper, onto patterned ozone-treated carbon-coated carbon nanotube (CNT) forests. Through nickel-electroplating CNTs, we have created a new composite material. The Young’s modulus of the nickel-electroplated CNT forest is 42 GPa with a tensile strength reaching 400 MPa and a density of 85% of bulk nickel. Several different variables were tested to find the fullest and most uniform fill. The parameters we have used to find the most uniform fill include: a nickel chloride boric acid solution, temperature kept at 50C, pH between 3 and 4, current density of 8 mA/mm^3, pulsing with 3ms on 27ms off, agitation, electrical contact, anode of sulphur depolarized nickel rounds, ozone surface treatment, pre-soak time of 30 minutes, and run time of nine hours. The variables that yield the most uniform copper-electroplating results are similar to the variables that yield the most uniform nickel-electroplating results. A base copper solution without extra additives and with an electroplating process similar to nickel electroplating yields the most uniform plating so far.

Gregory J Sutherland (Masters Thesis, June 2015, Advisor: Richard Vanfleet )

Abstract

A method was found for creating ordered nanoparticles whose size and theoretical order-disorder temperature are ideal for study in the TEM. Specifically FePt, NiPt, FeNiPt and AuCu nanoparticles were studied. We were able to show how a nanoparticle's size affects its order-disorder temperature (Tod). When the particles were around 6 nm in diameter there was a shift downward of the Tod of 10-15 percent compared to the bulk. While particles around 10 nm in diameter experienced a downward shift of 0-6 percent compared to the bulk. One can approximate that particles less than 10-15 nm in diameter would show significant decreases in order-disorder temperature. We confirmed that alumina prevents copper losses, compositions were well within percent error. In addition we showed that when the alumina used is thin enough the images are not adversely affected and charging is not an issue.

2014

Jordan Batschi (Senior Thesis, August 2014, Advisor: Richard Vanfleet )

Abstract

Understanding of the nanoparticle order-disorder temperature is important and vital to ongoing research, yet experimental analysis is lacking and the scientific base of knowledge relies on computational models. Tripod polishing techniques were used to prepare Au-Cu TEM samples for experimentally determining the order-disorder temperature of Au-Cu nanoparticles. The tripod polishing process with diamond lapping films proved to affect the samples, with TEM EDS showing significant copper deficiency. Alternative methods of sample preparation are shown and compositionally analyzed. Through coating the nanoparticles with alumina, the sample was shown to be compositionally stable with the desired 50-50 ratio. Further research is required using the alumina capped Au-Cu samples to find the order-disorder temperature.

Darren Wood (Senior Thesis, December 2014, Advisor: Richard Vanfleet )

Abstract

Analysis of the L10 order-disorder transition is an area of ongoing research. Current methods of sample preparation, high-resolution imaging, diffraction analysis and energy dispersive X-ray spectroscopy (EDS) analysis were used to analyze samples of 2-12 nm CoPt nanoparticles with the aim of discovering how the order-disorder temperature changes in the shift from the bulk to nano regime. Previous experimentation failed to yield ordered nanoparticles. In response, particles were annealed twice; once to increase size, then to produce ordering. A size increase of 346% failed to yield ordering.

2012

Amy Jackson (Honors Thesis, July 2012, Advisor: Richard Vanfleet )

Abstract

Analysis of the order-disorder transition in metallic alloy nanoparticles is a vital yet challenging area of ongoing research. Current methods of sample preparation and diffraction analysis were used to analyze samples of 5-10 nm FePt nanoparticles with the aim of evaluating and calculating new standards for energy dispersive X-ray spectroscopy (EDS) analysis. Through comparison with Rutherford backscattering spectrometry (RBS) data, a Pt L edge k-factor of 4.291 ± 0.272 was obtained for use in further research into FePt and NiFePt nanoparticle ordering in BYU’s electron microscopy facility.

Thomas O McConkie (Masters Thesis, August 2012, Advisor: Richard Vanfleet )

Abstract

Initial investigation of Moxtek wire grid polarizers composed of Al and coated with SiO2 - SiX - SiO2 (where SiX is used to indicate a Si rich layer whose complete composition is not to be disclosed for proprietary reasons) showed a growth of 3x in the inner (closest to Al) SiO2 layer after baking. Upon removing the X and varying rib composition and layering composition and geometries in 12 sets of before and after samples, no obvious growth was observed. Even baking the original unbaked sample yielded no growth. Our data suggest that the initial conclusion of buried oxide growth was flawed and that the observed changes in optical properties upon baking are either very sensitive to layer thicknesses (smaller than we can confidently observe) or due to some other mechanism. Here we present our sample preparation and analysis using the Focused Ion Beam (FIB), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive Xray Spectroscopy (EDXS).

Felipe Rivera (PhD Dissertation, March 2012, Advisor: Richard Vanfleet )

Abstract

Vanadium dioxide (VO2) is a material of particular interest due to its exhibited metal to insulator phase transition at 68◦C that is accompanied by an abrupt and significant change in its electronic and optical properties. Since this material can exhibit a reversible drop in resistivity of up to five orders of magnitude and a reversible drop in infrared optical transmission of up to 80%, this material holds promise in several technological applications. Solid phase crystallization of VO2 thin films was obtained by a post-deposition annealing process of a VOx,x≈2 amorphous film sputtered on an amorphous silicon dioxide (SiO2) layer. Scanning electron microscopy (SEM) and electron-backscattered diffraction (EBSD) were utilized to study the morphology of the solid phase crystallization that resulted from this post-deposition annealing process. The annealing parameters ranged in temperature from 300◦C up to 1000◦C and in time from 5 minutes up to 12 hours. Depending on the annealing parameters, EBSD showed that this process yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. In addition to these films on SiO2, other VO2 thin films were deposited onto a-, c-, and r-cuts of sapphire and on TiO2(001) heated single-crystal substrates by pulsed-laser deposition (PLD). The temperature of the substrates was kept at ∼500◦C during deposition. EBSD maps and orientation imaging microscopy were used to study the epitaxy and orientation of the VO2 grains deposited on the single crystal substrates, as well as on the amorphous SiO2 layer. The EBSD/OIM results showed that: 1) For all the sapphire substrates analyzed, there is a predominant family of crystallographic relationships wherein the rutile VO2{001} planes tend to lie parallel to the sapphire’s {1010} and the rutile VO2{100} planes lie parallel to the sapphire’s {1210} and {0001}. Furthermore, while this family of relationships accounts for the majority of the VO2 grains observed, due to the sapphire substrate’s geometry there were variations within these rules that changed the orientation of VO2 grains with respect to the substrate’s normal direction. 2) For the TiO2, a substrate with a lower lattice mismatch, we observe the expected relationship where the rutile VO2 [100], [110], and [001] crystal directions lie parallel to the TiO2 substrate’s [100], [110], and [001] crystal directions respectively. 3) For the amorphous SiO2 layer, all VO2 crystals that were measurable (those that grew to the thickness of the deposited film) had a preferred orientation with the the rutile VO2 [001] crystal direction tending to lie parallel to the plane of the specimen. The use of transmission electron microscopy (TEM) is presented as a tool for further characterization studies of this material and its applications. In this work TEM diffraction patterns taken from cross-sections of particles of the a- and r-cut sapphire substrates not only solidified the predominant family mentioned, but also helped lift the ambiguity present in the rutile VO2{100} axes. Finally, a focused-ion beam technique for preparation of cross-sectional TEM samples of metallic thin films deposited on polymer substrates is demonstrated.

Richard Wyman (Senior Thesis, April 2012, Advisor: Richard Vanfleet )

Abstract

We have developed a bulge test model valid in the large deflection limit. We provide a derivation of this model and compare it with other theoretical equations. This model is useful because current models are not valid in the large deflection limit. Bulge testing characterizes Young’s modulus and intrinsic stress of thin films. A back pressure is applied to a thin film causing the film to bulge upwards. We discuss the theoretical and experimental principles of bulge testing in depth. A computational tool has been developed to aid in interpreting bulge test data. We tested Duracoat (proprietary coating) and amorphous carbon films for potential use in x-ray windows. We found Duracoat/polymer thin films have a substantially higher Young’s modulus than amorphous carbon/polymer films or polymer films. Cracking in Duracoat films may be attributable to their higher Young’s modulus.

2010

James Brady (Capstone, January 2010, Advisor: Richard Vanfleet )

Abstract

We conducted SEM and TEM imaging of some specific Ge2Sb2Te5 samples in order to investigate the previously discovered areas of low resistance. The theory was that the areas may have been the result of crystalline seeds. We found no evidence of crystalline seeds.

Nathaniel Ralston (Senior Thesis, November 2010, Advisor: Richard Vanfleet )

Abstract

Many intermetallic systems are characterized by chemical ordering, and when in the ordered phase display many scientifically interesting and technology useful properties. The materials system investigated in this document is CuAu, which can chemically order in the L10 phase, which is a layer by layer stacking sequence of Cu and Au atoms on an FCC lattice. Multislice simulations are used to study the electron scattering intensity of chemically ordered CuAu. A chemical order parameter (S) is used to quantify the degree of ordering in the material. This chemical order parameter is determined by the ratio of the intensity of the (110) peak to that of the (220) peak. The relation between peak intensities and the chemical order parameter is found for fifteen individual thicknesses ranging from ~6 A to ~100 A. The (110) peak to (220) peak intensity ratio increases for increasing thickness and increasing order parameter. The calculated intensity ratios can now be compared to experimental electron scattering results to extract the sample order parameter.

Ryan Sayer (Senior Thesis, October 2010, Advisor: Richard Vanfleet )

Abstract

Solar cells operate under opposing constraints. Cells that are thinner absorb less sunlight but are better at generating current. Cells that are thicker absorb more sunlight but lose charge before it reaches the electrodes. By rotating cells and orienting them vertically toward the sun they can absorb more sunlight and still maintain a short spacing between conducting electrodes at the edges. The increase in power output of vertically-oriented cells is predicted by comparing their improved light absorption to the value of traditional horizontal cells. I determined that vertically-oriented amorphous silicon cells can generate about 18.9% more power than their planar counterparts.

2009

Scott Black (Capstone, July 2009, Advisor: Robert Davis, Richard Vanfleet )

Abstract

A new growth recipe for height maximization of carbon nanotubes (CNTs) by thermal chemical vapor deposition is found using an ethylene purity of 99.5%. This new recipe yields fast growthrates with average growth heights of 1146 μm in 10 minutes of growth. The CNT forest growth is very uniform but also has tears in structures, bowed vertical growth, and rough sidewalls indicating a poor quality CNT forest. These poor features seem to be a result of the thickness of the iron catalyst layer. With a thicker catalyst layer, high quality CNT forests may potentially be grown using 99.5% pure ethylene with faster growth rates than the current standard growth recipe for CNTs using 99.95% pure ethylene.

Emmalee Jones (Capstone, April 2009, Advisor: Robert Davis, Richard Vanfleet )

Abstract

The purpose of the capstone project was to develop a less costly process of patterning vertical structures in amorphous silicon for use in photovoltaics. Two processes were followed in trying to develop the mold. The first process centered on using a 3 micron layer of photoresist SU-8 10, a viscous polymer creating a pattern using lithography and spun on hydrogen silsesquioxane (HSQ) to create a reusable reverse glass mold. This process resulted in an unusable pattern with unclean lines and definition. A second process was planned after the failure of the first process. The second process centered on polyimide, a different polymer, using lithography to create a reusable reverse mold. The initial results with polyimide are promising and will continue to be investigated.

2008

Arthur Brown (Senior Thesis, August 2008, Advisor: Richard Vanfleet )

Abstract

Resistance of crystalline thin lms of vanadium dioxide with varying grain sizes were measured using a tungsten carbide tipped four-point probe and computer data collection. Resistivity was obtained through a conversion factor dependent on sample geometry. Temperature was measured concurrently, and was slowly varied within a range from 295 K to 360 K. As temperature is increased, a metal-to-insulator transition (MIT) was observed near 340 K. Hysteresis is observed upon cooling where the reverse transition occurs at a lower temperature. The upper (heating) transition temperature decreases as the grain size gets smaller and smaller grains were seen to have a wider hysteresis gap.

Christopher Johnson (Capstone, April 2008, Advisor: Richard Vanfleet )

Abstract

TEM analysis techniques: high resolution, bright-field and dark-field imaging; electron diffraction, and STEM showed crystallite area density in a sputtered GST (GexSbyTez) thin film to be less than one crystallite per 0.0527 μm2, which restated is less than 19 crystalites/μm2 . Beyond the quantified data, a qualitative search of the sample failed to find any crystallites.

Gordon Mancuso (Senior Thesis, August 2008, Advisor: Richard Vanfleet )

Abstract

Gamma phase lithium aluminate ( -LiAlO2) has been proposed as a possi- ble substrate for gallium nitride growth. A corrugation or stripe pattern of beta phase LiAlO2 forms on the surface of -LiAlO2 wafers. We attempt to determine the cause of the phase transformation. The beta phase re-forms immediately after removal. This supports the conclusion that the beta phase is more stable at room temperature than the gamma phase, and suggests that the growth of the beta phase is strain limited.

2007

Adam Callens (Capstone, May 2007, Advisor: Richard Vanfleet )

Abstract

The purpose of this capstone project was to create a sample holder to serve several purposes. A sample holder was created that can be put into the specified vacuum chamber. Additionally, the sample holder is capable of holding samples less than three millimeters in diameter and larger than 2 millimeters in diameter. The sample holder also proved capable of heating the samples to a sustained temperature of 120◦C or more. This project has been a success and has met its goals.

Brady Cox (Senior Thesis, April 2007, Advisor: Richard Vanfleet )

Abstract

Vanadium dioxide undergoes a phase transition from semiconductor to metal near 68C, and consequently has many potential optical and electronic applications. The phase change is a structural change between two crystalline phases. Initially amorphous films of vanadium dioxide (VO2) were annealed to achieve crystallization. These thermally annealed thin films of vanadium dioxide were then characterized through use of Transmission Electron Microscopy (TEM). TEM imaging was used to determine if crystallinity had been achieved at various temperatures. Samples annealed at temperatures as low as 400C were shown to exhibit crystal formation. The crystal grain size and surface morphology were seen to vary significantly with changing annealing temperature and time. Different crystal sizes are shown to result in different electrical properties [4]. The transition between the two crystal phases has also been observed in electron diffraction patterns of the annealed samples. By heating the samples in the TEM and analyzing changes in electron diffraction patterns, we have begun to characterize the transition temperature for different particle sizes.

Mark Esty (Senior Thesis, April 2007, Advisor: Richard Vanfleet )

Abstract

The usefulness of ferritin as a catalyst in the chemical vapor deposition process for carbon nanotube growth is analyzed. It is found that restricting the ferritin deposition time and rinsing the sample reduced the density to a single, well-spaced layer which significantly decreases the amalgamation of the iron cores. The density of the deposited ferritin is found to be significantly affected by the choice of substrate. Annealing studies were done to simulate the nanotube growth conditions between 600C and 800C. The mobility of the iron cores during the annealing process is decreased by removing the protein shells of the ferritin molecules through exposure to an oxygen plasma prior to the anneal. The suggested process can minimize the spread in the catalyst diameters near 600C; however, it is unlikely that this process can prevent the catalyst particles from amalgamating at temperatures near 800C, where most nanotube growths are performed.

Felipe Rivera (Masters Thesis, December 2007, Advisor: Richard Vanfleet )

Abstract

Crystalline films of vanadium dioxide were obtained through thermal annealing of amorphous vanadium dioxide thin films sputtered on silicon dioxide. An annealing process was found that yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. Orientation Imaging Microscopy (OIM) was used to characterize and study the phase and the orientation of the vanadium dioxide crystals obtained, as well as to differentiate them from other vanadium oxide stoichiometries that may have formed during the annealing process. There was no evidence of any other vanadium oxides present in the prepared samples. Indexing of the crystals for the orientation study was performed with the Kikuchi patterns for the tetragonal phase of vanadium dioxide, since it was observed that the Kikuchi patterns for the monoclinic and tetragonal phases of vanadium dioxide are indistinguishable by OIM. It was found that a particle size of 100 nm was in the lower limit of particles that could be reliably characterized with this technique. It was also found that all VO2 crystals large enough to be indexed by OIM had a preferred orientation with the C axis of the tetragonal phase parallel to the plane of the specimen.

Michael Tanner (Honors Thesis, December 2007, Advisor: Richard Vanfleet )

Abstract

The crystal structure of Al2O3 is found naturally in at least seven different atomic arrangements or phases. By heating gibbsite, an aluminum hydroxide, to different temperatures, the , , and  phases were obtained. The phase of each was confirmed by diffraction analysis. The samples were then investigated using electron energy-loss spectroscopy (EELS) in order to obtain an EELS spectra fingerprint for each phase. An EELS fingerprint was obtained for each phase, allowing phase identification in nanometer scale regions that cannot be measured by other means.

2006

Clark Blockburger (Senior Thesis, December 2006, Advisor: Richard Vanfleet )

Abstract

Using Transmission Electron Microscopy we have investigated the structure of two Fischer-Tropsch catalysts, looking specifically for the formation of discrete metal nanoparticles approximately 10 nm in diameter to form on the gamma aluminum oxide support. Using Scanning Transmission Electron Microscopy and Energy Dispersive Xray analysis we have studied the two samples. The data shows that in the case of the cobalt sample that the cobalt has been secured to the alumina support, but exists as cobalt oxide, not elemental cobalt. Large cobalt oxide crystals were also found unattached to the alumina support. The iron catalyst showed similar oxidized iron particles on the alumina support, but without any unattached iron oxide particles. The iron sample also showed the presence some elemental iron particles on the alumina support. A brief description of Transmission Electron Microscopy theory and techniques, as well as a short explanation of Fischer-Tropsch catalysis is included.

Jason Neff (Senior Thesis, April 2006, Advisor: Richard Vanfleet )

Abstract

n/a

2005

David Cullen (Capstone, April 2005, Advisor: Richard Vanfleet )

Abstract

The alpha and gamma phases of aluminum oxide were fingerprinted using electron energy-loss spectroscopy (EELS) with an energy resolution of 0.4 eV. Electron diffraction and powder x-ray diffraction were used to determine the phase of micron and nanopowder alumina samples. The results from indexing peak and ring diffraction patterns proved to be ambiguous and were resolved by comparing EELS spectra with published fingerprints. EELS spectra were generated from the powders with an energy resolution of 0.4 eV. This energy resolution was achieved using a Tecnai F20 Analytical Scanning Transmission Electron Microscope (STEM) equipped with a monochromator, Gatan energy filter (GIF) and high resolution spectrometer in STEM mode. These fingerprints are compared with published data and discrepancies between the spectra are discussed. The significance of using a monochromator with STEM mode for nanoparticle phase identification is addressed, followed by a brief roadmap delineating the goals of future EELS phase identification research.