Browsing by Subject "Thin film"

Now showing 1 - 13 of 13
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    Complexity at cobaltite interfaces: the interplay between strain, stoichiometry, magnetism and transport
    (2014-12) Bose, Shameek
    Thin films and heterostructures of the perovskite cobaltites are of great interest, not only from the point of view of fundamental physics and materials science, but also for technological applications such as solid oxide fuel cells and gas membranes. Their properties are, however, severely deteriorated from the bulk, being dominated by the presence of interfacial "dead layers". Working with the prototypical SrTiO3 (001)/La1-xSrxCoO3 (LSCO) system, our group recently discovered that this degradation in the magnetism and electronic transport at the interface is caused by nanoscopic magneto-electronic phase separation. This was shown to occur primarily due to accumulation of oxygen vacancies near the interface, driven by the interplay between the strain state and the ordering of oxygen vacancies. In the present work we show how this understanding allows for engineering of the interfacial magnetic and electronic transport properties via manipulation of this oxygen vacancy superstructure. We first demonstrate a synthesis technique that utilizes a unique high pressure oxygen plasma to sputter LSCO thin films over a wide doping range 0.05  x  0.80. Then, using reciprocal space mapping and transmission electron microscopy, we demonstrate the ability to control, via the vacancy ordering, the critical strain relaxation thickness by changing the sign of the strain (from tensile on SrTiO3 to compressive on LaAlO3) and crystallographic orientation ((001) vs. (110)). We then provide cross sectional electron energy loss spectroscopy data to show that this strain and orientation control preserves both oxygen and hole carrier concentration at the LaAlO3(001)/LSCO and SrTiO3(110)/LSCO interfaces, strikingly different to the severely depleted SrTiO3(001)/LSCO interface. SQUID magnetometry, polarized neutron reflectometry (PNR) and magneto-transport confirm the concomitant mitigation of the interfacial degradation for LSCO films grown on LaAlO3(001) and SrTiO3(110), as compared to films grown on SrTiO3 (001). Finally, we use scanning tunneling microscopy to provide direct real space images of the magneto-electronic phase separation in ultrathin LSCO on SrTiO3(001). Our work thus demonstrates the ability to utilize oxygen vacancy ordering as a tunable control parameter to tailor interfacial electronic and magnetic properties, with profound implications for the myriad other systems that exhibit unique properties due to such ordering.
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    Computed tear film and osmolarity dynamics on an eye-shaped domain
    (University of Minnesota. Institute for Mathematics and Its Applications, 2014-07) Li, Longfei; Braun, Richard J.; Driscoll, Tobin A.; Henshaw, William D.; Banks, Jeffrey W.; King-Smith, P. Ewen
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    Electromechanical Switches Fabricated by Electrophoretic Deposition of Single Wall Carbon Nanotube Films
    (2015-08) Lim, Jun Young
    Power dissipation is a critical problem of CMOS devices especially for mobile applications. Many efforts have been made to solve the problem, but there are still major issues associated with scaling the device size. Micro electromechanical (MEMS) and nano electromechanical (NEMS) devices are one candidate to solve the problems because of their excellent standby leakage. However, the switches have a tradeoff between low operating power and high device speed. Suspended beams with low mass density and good mechanical properties provide a way to optimize the device. Carbon nanotubes (CNTs) have the low mass density and excellent mechanical properties to enable high performance MEMS/NEMS devices. However, the high temperature required for the direct synthesis for CNTs makes it difficult for them to be compatible with a substrate containing transistors. Therefore, continuous film deposition techniques are investigated with low temperature (< 300 C). Electrophoretic deposition (EPD) is a simple and versatile processing method to deposit carbon nanotubes on the substrate at room temperature. The movement of the charged CNTs in suspension occurs by an applied electric field. The deposited CNT film thickness can be controlled through the applied voltage and process time. We demonstrate the use of an EPD process to deposit various thicknesses of CNT films. Film thicknesses are studied as a function of, deposition time, electric field strength, and suspension concentration. The deposition mechanism of the EPD process for carbon nanotube layers was explained with experimental data. We determined the film mass density and electrical/optical properties of SWCNT films. Rutherford backscattering spectroscopy was used to determine the film mass density. Films created in this manner had a mass density that varies with thickness from 0.12 to 0.54 g/cm3 and a resistivity of 2.1410-3 Ω∙cm. For the mechanical property measurements, we describe a technique to fabricate free-standing thin films using modified Langmuir-Blodgett method. Then we extracted the Young’s modulus of the film from the load-displacement data from nanoindentation using the appropriate modeling. The Young’s modulus had a range of 4.72 to 5.67 GPa, independent of deposited thickness. We fabricated two-terminal fixed beam switches with SWCNT thin films using the EPD process. Device pull-in voltages under 1V were achieved by decreasing the air-gap. The pull-in voltages were compared with the calculated results using the device geometry and extracted Young’s modulus from nanoindentation. Generally good agreement was observed. Also, we found a range of 2.4 to 3.5 MHz resonant frequency. However, we encountered several problems with the device including a gradual turn-on, hysteresis between pull-in and pull-out voltage, changes in the pull-in voltages with repeated on-off cycling, and early failure due to moisture absorption during testing in the air. Mechanisms for these observations are postulated. Further work is needed to improve device performance and reliability.
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    Epitaxial Growth of thin film strontium cobaltite: a feasibility study.
    (2012-06) Gulden, Tobias
    In this work we present a feasibility study of epitaxial growth of thin films of strontium cobaltite, SrCoO3-\delta. The properties of strontium doped lanthanum cobaltite, La1-xSrxCoO3, have been widely studied for dopant concentration x<0.5, but little work has been performed on the x=1 member of the series. The main issue is that this is not a thermodynamically preferable state and close to stochiometric SrCoO3 in polycrystalline samples can only be obtained under high pressures of oxygen (>10kbar) or by electrochemical oxidation. However, theoretical calculations predict a phase change with respect to strain in epitaxially grown samples, from ferromagnetic-metallic behaviour in the bulk state to insulating-ferroelectric-antiferromagnetic behaviour for strongly strained films. This provides strong motivation for epitaxial growth of SrCoO3-d films. In this work we will present a feasibility study by using the methods of high-pressure oxygen sputtering (typically 1.0-4.0mbar) on SrTiO3(001) and LaAlO3(001) substrates. As anticipated, the presence of oxygen vacancies is a severe problem, but also epitaxial stabilization of non-cubic phases, an unexpected issue, arises. These are found to grow in multiple orientations. Overall, the samples exhibit only weak or no ferromagnetism, even though bulk SrCoO3 is known to be a strong ferromagnet. Based on the results, we present an outline for suggested further research on this topic.
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    FePt based advanced magnetic recording media
    (2013-02) Wang, Hao
    In future extremely high density magnetic recording, FePt is considered as a promising candidate for future recording media materials. In this thesis work, FePt media with composite structure have been systematically studied in the forms of both granular media and bit patterned media (BPM). Continuous FePt films with surface roughness of less than 0.3 nm are achieved in FePt hard magnetic films, exchanged coupled composite (ECC) films and graded films. Nanoimpriting and block-copolymer lithography are employed to fabricate BPM. The switching field distribution (SFD) broadening and degradation of FePt BPM are studied. The reduction of SFD has been achieved using a post-annealing process. Both ECC and graded FePt BPM with sub-30 nm dot size have been experimentally demonstrated on large substrates for the first time. It is confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample does. Ultra-thin FePt granular media with graded composition was directly fabricated using a spontaneous layer diffusion process between the FePt and Pt layers during film deposition. A large gain factor of 3.74 was found in this spontaneously formed FePt graded granular media. A nanopatterning process, named as the Embedded Mask Patterning (EMP), is proposed and experimentally demonstrated based on the FePt magnetic recording media. In this process the granular structure is defined by a sputtering-deposited mask layer, while the magnetic properties are determined by the FePt continuous film. Grain size can be decreased by optimizing the mask layer only. A non-ideal surface anisotropy effect has been observed on the magnetization reversal process of both L10 phase FePt nanoparticles, and (001) textured L10 FePt thin film with island structure. The broken symmetry of the surface creates surface anisotropy and also weakens the exchange coupling. The elimination of the surface effect has been experimentally demonstrated by epitaxially capping a Pt layer on FePt. After being embedded in a Pt matrix, the exchange coupling between the surface portion and internal portion of FePt islands was enhanced.
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    Fluid Interfacial Dynamics: From Droplets to Thin Films
    (2024-05) Chen, Zih-Yin
    Many natural processes and industrial applications involve the understanding and modeling of interfacial fluid dynamics. Examples include the coating of a substrate with a liquid, the removal of liquid films from substrates, transport processes, droplets impact on substrates, etc. Given the ubiquity and significance of interfacial fluid flows, the overall goal of this thesis is to advance our fundamental understanding of interfacial fluid dynamics through theoretical modeling. To achieve this goal, we conduct three studies related to interfacial dynamics involving droplets and liquid films: (i) droplet impact dynamics with granular materials, (ii) droplet dynamics with an air jet, and (iii) dynamics of a liquid film with an undulating surface. Motivated by spray coating processes, we investigate how the presence of particles on a substrate changes the droplet impact dynamics. We analyze the spreading dynamics and splashing criterion of an impacting droplet on a layer of particles. A theoretical model is developed to consider the momentum of the spreading liquid and the time-dependent distribution of particles. Additionally, we establish a droplet splashing criterion based on the interaction between the impacting droplet and the particles. Our results provide insight into how the presence of particles lowers the critical impact velocity at which a droplet exhibits splashing, as the particle area fraction is systematically increased. We then focus on the dynamics of a partially-wetting droplet under an impinging air jet. We built a two-dimensional lubrication model of the droplet that incorporates the external pressure of the impinging turbulent jet, in addition to the capillary and hydrostatic pressures of the droplet. Also, the simulations of the contact-line motion by using precursor film and disjoining pressure allows us to capture the physics of different droplet behaviors, which had previously been observed experimentally. Our simulations exhibit a comparable time-scale of droplet deformations and similar outcomes as the experimental observations. We also obtain the analytical steady-state solutions of the droplet shapes and construct the minimum criteria for droplet splitting and depinning. Lastly, we investigate the interfacial dynamics of a thin liquid film over an undulating solid substrate. To explore the physical mechanisms of free surface flows driven by periodic undulations, we developed a two-dimensional thin-film mathematical model. The model combines the effects of inertia, viscosity, gravity, and surface tension in a tractable way. Our model reveals that in the regime where gravity dominates over surface tension (i.e., large Bond number, Bo), the effects of surface tension drop out of the analysis, allowing the flow rate to only depend on Re and Ca/Bo, where Ca is the capillary number. In this same regime, we learn that inertia (Re) tends to enhance the flow rate, while increasing Ca/Bo reduces the flow rate.
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    Growth and Characterization of Wide Bandgap CIAGS Solar Cell Material and Devices
    (2018-12) Hwang, Sehyun
    In this study, we present the development of copper-indium-aluminum-gallium-selenium (Cu(In1-x-yAlyGax)Se2, or CIAGS) as a wide bandgap top cell absorber for tandem photovoltaic (PV) applications. Realizing a tandem PV structure could lead to a breakthrough for high efficiency solar cells. CIAGS absorbers were grown in a single-step process using a custom-designed co-evaporation system under an ultra-high vacuum. The material properties of CIAGS thin films were analyzed in terms of grain morphology, elemental composition, and energy bandgap. The bandgap of CIAGS is tuned by controlling the elemental composition of group III elements. The relation between energy bandgap and elemental composition was empirically established for CIAGS absorbers with varying bandgaps. The CIAGS grown here targeted a bandgap of ~1.65 eV which is optimal for a tandem top cell. CIAGS solar cell devices were fabricated and characterized electrically by J-V measurements. The highest efficiency obtained was 12.8%, although the efficiency tends to decrease as the bandgap increases. Poor film adhesion or delamination is a major problem in wide bandgap CIAGS solar cells. Delamination occurs at the interface between the CIAGS absorber and the Mo back contact layer. We suggest two possible delamination mechanisms caused by interfacial molybdenum diselenide (MoSe2) in the wide bandgap CIAGS. The CIAGS/Mo interface was characterized mechanically (adhesion) and electrically (contact resistance). A TiN diffusion barrier to selenization improves the CIAGS/Mo interfacial adhesion and provides a potential solution to the delamination problem in the wide bandgap absorbers such as CIAGS.
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    Nanoporous materials from ABAC tetrablock terpolymers
    (2013-07) Jackson, Elizabeth
    This dissertation describes efforts towards the preparation of tough nanoporous membranes from ABAC tetrablock terpolymers. This architecture was strategically chosen to combine an etchable C block, PLA, with a mechanically tough ABA triblock into one ABAC terpolymer. Multiple series of poly(styrene-b-isoprene-bstyrene-lactide) (PS-PI-PS-PLA) tetrablock terpolymers were synthesized. Morphological behavior was characterized for terpolymers containing both a 50:50 and 30:70 PS:PI ratio with between 0 and ~20% PLA by volume. Observed bulk morphologies include hexagonally packed cylinders (HEX), core(PLA)-shell(PS) cylinders (CSC), and a PLA sphere in cylinder morphology. Mechanical properties of PS-PEEP-PS-PLA tetrablocks were also investigated. All materials exhibited mechanical properties characteristic of tough thermoplastic elastomers. Composite membranes were prepared from a thin film of PS-PI-PS-PLA terpolymer and a macroporous polyethersulfone support. Described within are the efforts related to the fabrication and filtration performance of these nanoporous PSPI-PS composite membranes. As part of this process, solvent casting and annealing conditions were varied to investigate effects on tetrablock thin film morphology. Optimum conditions were determined to achieve PS-PI-PS-PLA films with perpendicular PLA cylinder orientation. These conditions included use of a mixed solvent system and the addition of a small amount of homopolymer PLA. Highly ordered films with vertically oriented nanopores were obtained.
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    Nanostructures, Nanoparticles, and 2D Materials from Nonthermal Plasmas
    (2021-02) Beaudette, Chad
    The bottom-up synthesis of thin films, nanostructures, and nanoparticles from nonthermal plasmas has been limited largely to both gas-phase and highly-volatile carbon precursors. This has stymied the application of nonthermal plasmas to several new types of materials as there are often no gas-phase or highly volatile precursors that exist for their synthesis. The sublimation of solid and low volatility liquid precursors are used here to expand the realm of new materials towards sulfide Van der Waals 2D materials, high surface area nitride nanostructured plasmonic materials, nitrogen-doped oxide nanoparticles, and crystalline metal aluminum nanoparticles. Plasmonic photodetectors and photocatalytic nanoparticles are demonstrated herein to show the utility of some of the as produced materials. Moreover, traditional nanoparticle reactor limitations such as metallic film deposition between the exciting electrode and the plasma are discussed and new reactors are developed to overcome such limitations. In addition, parameters such as the location of the powered electrode and the location of the gas inlets relative to one another are critical to the production of better materials and examples will be demonstrated herein.
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    Self-assembly of block copolymers in thin films
    (2013-08) Kim, Sangwon
    The self-assembly of block copolymers in thin films has been a subject of recent studies from both academic and industrial perspectives. One of its potential applications is nanolithography; block copolymers can function as novel mask materials intended for fabrication of small features, not easily realizable by current optical lithography. This dissertation addresses several fundamental issues associated with thin-film block copolymers. The bulk and interfacial wetting properties of partially epoxidized poly(styrene-b-isoprene) diblock copolymers, denoted as PS-PEI, were studied while varying the degree of the chemical modification. The incorporation of the random copolymer architecture induced decoupling between the bulk and the thin-film thermodynamics. The tunable surface wetting, a consequence of the partial modification, permitted control over the orientation of the domains in thin films. The morphologies of thin-film block copolymers were investigated using two different boundary conditions that involve one neutral interface and one preferential interface. The neutralities at the free surface and the underlying substrate were attained independently by the partial epoxidation in PS-PEI and the composition adjustment of random copolymer mats, respectively. For both boundary conditions, thin-film block copolymers formed an island/hole motif, characterized by 0.5 L0 step heights (L0: bulk lamellar periodicity). The thin-film behavior of PS-PEI block copolymers with random copolymer architecture was examined as the segregation strength (χN) was adjusted systematically across the order-disorder transition. Unlike in the bulk, the random copolymer architecture did not generate abnormal behavior in thin-film thermodynamics compared to plain linear architecture. With decreasing segregation strength, the thin-film system exhibited fluctuation-pervaded morphologies prior to reaching a disordered state. An agreement was found between the order-disorder transition temperatures in three dimensions (bulk) and in two dimensions (thin film). Lastly, the bulk properties and the thin-film structures of lamellae-forming poly(styrene-b-isoprene-b-methyl methacrylate) (SIM) triblock copolymers were studied. The thin-film morphology exhibited the dependence on the size of the poly(isoprene) (PI) middle block. While perpendicular lamellae were observed for the thin-film SIM block copolymer with a small PI volume fraction, complex behavior was observed for the sample with a large PI volume fraction.
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    Solving the two-interface problem in vibrational sum frequency generation spectroscopy applied to multilayer thin film systems
    (2014-03) O'Brien, Daniel Bruce
    This dissertation describes advances made in applying sum frequency generation spectroscopy (SFG, in particular vibrational SFG or VSFG) to multilayer thin film systems. Application of VSFG to thin film systems is motivated by the challenge of characterizing molecular structure at the active boundary in organic field-effect transistors, these are inherently buried interfaces. VSFG is a surface-selective probe of molecular structure; however, when VSFG is applied to an organic thin film, the detected signal has contributions from two potential sources - the two interfaces of the organic - which must be separated. The problem is further confounded by optical interferences inherent in multilayer thin film systems. An intuitive mathematical model is developed; postulating a solution to the two-interface problem of SFG applied to planar and stratified multilayer structures. The model system for this dissertation is thin films of the small molecule N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) vapor deposited on silica thin film substrates, consistent with an oFET thin film geometry. The interference model is used for an extensive simulation analysis that reveals intricacies contained in the intensity data of VSFG applied to that system. VSFG experiments performed on samples with PTCDI-C8 deposited as gradient thicknesses provide compelling evidence that the model gives an accurate description of optical interference effects and that it can be used to separate contributions to the total VSFG signal intensity. The supplementary materials contain a collection of Mathematica notebooks that can be used to investigate optical interference effects on SFG data collected from systems composed of an arbitrary number of thin film layers.
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    Supporting data for Synthesis, microstructure, and properties of high molar mass polyglycolide copolymers with isolated methyl defects
    (2021-06-15) Altay, Esra; Jang, Yoon-Jung; Kua, Xiang Qi; Hillmyer, Marc; hillmyer@umn.edu; Hillmyer, Marc; University of Minnesota, Hillmyer Lab, Department of Chemistry
    An efficient, fast and reliable method for the synthesis of high molar mass polyglycolide (PGA) in bulk using bismuth (III) subsalicylate through ring-opening transesterification polymerization is described.The difference between the crystallization (Tc ≈ 180 °C) / degradation (Td ≈ 245 °C) temperatures and the melting temperature (Tm ≈ 222 °C) significantly impacts the ability to melt process PGA homopolymer. To expand these windows, the effect of copolymer microstructure differences through incorporation of methyl groups in pairs using lactide or isolated using methyl glycolide (10% methyl) as comonomers on the thermal, mechanical and barrier properties were studied. Structures of copolymers were characterized by Nuclear Magnetic Resonance (1H and 13C NMR) spectroscopies. Films of copolymers were obtained, and the microstructural and physical properties were analyzed. PGA homopolymers exhibited approximately 30 °C difference between Tm and Tc, which increased to 50 °C by incorporating up to 10% methyl groups in the chain while maintaining overall thermal stability. Oxygen and water vapor permeation values of solvent cast non-oriented films of PGA homopolymers were found to be 4.6 (cc.mil.m-2.d-1.atm-1) and 2.6 (g.mil.m-2.d-1.atm-1), respectively. Different methyl distributions in the copolymer sequence, provided through either lactide or methyl glycolide impacted the resulting barrier properties. At 10% methyl insertion using lactide as a comonomer significantly increased both O2 (32 cc.mil.m-2.d-1.atm-1) and water vapor (12 g.mil.m-2.d-1.atm-1) permeation. However, when methyl glycolide was utilized for methyl insertion at 10% Me content, excellent barrier properties for both O2 (2.9 cc.mil.m-2.d-1.atm-1) and water vapor (1.0 g.mil.m-2.d-1.atm-1) were achieved.
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    Thin zinc oxide and cuprous oxide films for photovoltaic applications.
    (2010-08) Jeong, SeongHo
    Metal oxide semiconductors and heterojunctions made from thin films of metal oxide semiconductors have broad range of functional properties and high potential in optical, electrical and magnetic devices such as light emitting diodes, spintronic devices and solar cells. Among the oxide semiconductors, zinc oxide (ZnO) and cuprous oxide (Cu2O) are attractive because they are inexpensive, abundant and nontoxic. As synthesized ZnO is usually an intrinsic n - type semiconductor with wide band gap (3.3 eV) and can be used as the transparent conducting window layer in solar cells. As synthesized Cu2O is usually a p - type semiconductor with a band gap of 2.17 eV and has been considered as a potential material for the light absorbing layer in solar cells. I used various techniques including metal organic chemical vapor deposition, magnetron sputtering and atomic layer deposition to grow thin films of ZnO and Cu2O and fabricated Cu2O/ZnO heterojunctions. I specifically investigated the optical and electrical properties of Cu2O thin films deposited on ZnO by MOCVD and showed that Cu2O thin films grow as single phase with [110] axis aligned perpendicular to the ZnO surface which is (0001) plane and with in-plane rotational alignment due to (220)Cu2O || (0002)ZnO; [001]Cu2O || [1210]ZnO epitaxy. Moreover, I fabricated solar cells based on these Cu2O/ZnO heterojunctions and characterized them. Electrical characterization of these solar cells as a function of temperature between 100 K and 300 K under illumination revealed that interface recombination and tunneling at the interface are the factors that limit the solar cell performance. To date solar cells based on Cu2O/ZnO heterojunctions had low open circuit voltages (~ 0.3V) even though the expected value is around 1V. I achieved open circuit voltages approaching 1V at low temperature (~ 100 K) and showed that if interfacial recombination is reduced these cells can achieve their predicted potential.