Browsing by Subject "Nanostructured materials"

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Aspects of metal and Si-based nanomaterials: synthesis, stability and properties
(2006) Elechiguerra Joven, José Luis; Yacaman, Miguel Jose
Metal and Si-based nanostructures have drawn increasing interest due to their potential uses in catalysis, biological sensors, and nanoelectronics among others. Therefore, in the present work, several nanostructures were produced, characterized and tested. In particular, the conventional synthesis of noble-metal nanostructures through the polyol method was modified by replacing poly-vinyl pyrrolidone PVP with poly-diallyl dimethyl ammonium chloride PDDA. As PDDA is a cationic polyelectrolyte, the initial strong electrostatic interaction between PDDA and the anionic metal precursors produce the formation of stable ion pairs, so the reactivity of the different species can be tailored and particles with different internal structure, i.e. crystallinity, can be produced.
Biological approaches to synthesis and assembly of semiconductor and metallic nanomaterials
(2005) Sweeney, Rozamond Yvonne; Iverson, Brent L.
The goal of this work is to use proteins, viruses, and whole organisms to direct the growth and assembly of semiconductor and metallic materials. The motivation for this work was to find a new way to build inorganic materials and devices with greater ease, more precise control, and smaller features than is possible with current synthetic methods. A biological method to efficiently synthesize large quantities of cadmium sulfide nanocrystals in the bacteria E. coli was discovered. The physical properties of the nanocrystals were characterized by electron microscopy and photoluminescence spectroscopy. Next, the genetic and physiological parameters that play a role in the synthesis of cellular nanocrystals were explored. In particular, a strain and growth phase dependence for E. coli nanocrystal formation was determined, indicating that the capacity for nanocrystal synthesis in E. coli is intrinsic and can be genetically controlled. This result is a first step towards understanding this mechanism of biologically-encoded nanomaterial synthesis, and it suggests the possibility of genetically engineering E. coli to produce nanocrystals with precise control over composition, size, and crystal type. Recently, it was discovered that filamentous viruses can be genetically engineered to direct the formation of semiconductor and magnetic nanowires. To follow-up on this project, a method for precisely directing the assembly of the viruses was developed. In order to begin ordering the viruses, the viral coat proteins were engineered to display a type of protein domain, called a leucine zipper, which can form non-covalent dimeric, trimeric, or tetrameric interactions with other leucine zippers. The leucine zipper, attached at the ends of the virus, caused individual viruses to adhere to each other end-to- end, producing one-and two-dimensional arrays. This method was also shown to be an effective way to alternate assembly of different types of viruses. By controlling the placement of the virus-templated nanowires from the bottom-up, the nanowires might become technologically useful for applications that require precise ordering, such as electronic and photonic circuits, sensors, or liquid crystal displays.
Characterization of nanomaterials by transmission electron microscopy and related techniques
(2005) Gao, Xiaoxia; Yacaman, Miguel Jose
In the development of nanomaterials, their characterization is very important. Transmission Electron Microscopy (TEM) is one of the most useful techniques in the characterization of nanomaterials since TEM probe size is ideal for nanocale study and provides structural & chemical information to the angstrom level. Image techniques, electron diffraction, chemical analysis can be obtained in a single instrument. The analytical capability and information limit of TEM are further increased with the use of highly coherent field emission electron sources with reduced thermal energy spread. In this work I will enhance the application of this advanced technique in the characterization of nanomaterials. The first example of application was the study of carbon nanocages. Carbon is the element with richest chemistry and can form many fascinating structures. Here we report that carbon coated Ag nanoparticles form necklace-structures and the entrapped Ag nanoparticles can jump outside of carbon nanocages by electron beam irradiation. By combining TEM, HAADF-STEM and EELS techniques, we were able to explain the formation of fullerene-open structure. The second example of application was to study nanoparticles (Au nanoparticles and Co nanoparticles in Y-Zeolite). When nanostructures are observed by TEM, forbidden spots are presented in their electron diffraction patterns because their nanometric size. In order to properly interpret the images, the study and understanding of these spots are necessary. In this work we study them and it is shown that they can be well used to obtain 3D structural information in conventional TEM dark field images when weak-beam TEM images of the nanoparticles are generated. In case of Co nanoparticles in zeolites, HAADF-STEM was applied to study the nanopaticles in the channel of zeolites, and we were able to observe 1 or less than 1 nm Co nanoparticles. The third example of application was the study of soft materials in a drug delivery agent. HAADF-STEM was applied for the first time to drug delivery study in particular to the agent with polyvinylpyrrolidone (PVP). The successful application in danazol with PVP proved this technique would be a valuable asset in other research area like polymer composites characterization without using stain.
The dynamic mechanical response of polymer-based nanocomposites and network glasses
(2004) Putz, Karl William; Green, Peter F. (Peter Fitzroy)
The structural entities, atomic or molecular, that constitute a material and their spatial organization within the material largely determine the properties of the material. Different classes of materials respond to external forces often in ways that are fundamentally connected to their structure. Polymer melts exhibit time-dependent viscoelastic behavior in response to external stresses. If the deformation is rapid then the response of the polymer is largely elastic, whereas for sufficiently slow deformations the response is time-dependent and dissipative. In the solid state, an amorphous polymer responds to sufficiently small forces through local relaxations of segments of the molecules. Inorganic network glass melts typically exhibit viscoelastic behavior in the vicinity of the glass transition temperature, Tg, whereas below Tg ionic entities that compose the structure undergo hopping processes in response the external perturbations. This dissertation is largely devoted to understanding the response, or relaxations, of two different classes of materials: polymer based nanocomposites and inorganic network glasses, to external periodic mechanical perturbations. Polymer nanocomposites, materials in which a polymer serves as the host for nanomaterials, may possess properties that differ substantially from those of the pure polymer. Interactions between polymer segments and the foreign particles and the geometry, size, and dispersion of the foreign particles profoundly influence the properties of the composite. Small concentrations (~ few percent) of particles of nano-scale dimensions have been shown to be particularly effective at modifying the properties of the polymer because of the large interfacial area. In this dissertation the influence of C60 fullerenes and of single walled carbon nanotubes on the dynamical mechanical and rheological properties of PS and PMMA is examined. The second problem examined was the mechanical response of inorganic network glasses based on tellurium oxide to mechanical perturbations. Often alkali oxides are included in the glass composition because they improve processability of the glass melt by decreasing Tg. Through a series of dynamic mechanical and rheological experiments, it is shown that the relaxations which characterize the response of the glass increase by up to an order of magnitude when two distinct types of alkali ions are present compared to one.
Electrothermal and optothermal characterization of thermal transport in two-dimensional materials
(2018-08) Coloyan, Gabriella Marie Gregson; Shi, Li, Ph. D.; Ho, Paul; Wang, Yaguo; Dodabalapur, Ananth; Akinwande, Deji
One significant challenge in the development of micro- and nano-electronic devices is thermal management. Improper thermal management can affect a device’s performance and lifetime. Nanostructured materials are one possible solution. A material’s thermal conductivity is an indicator of its viability for thermal management. However, characterizing thermal conductivity of nanostructured materials is challenging due to the inherent difficulties associated with characterizing something so small. Furthermore, thermal conductivity does not describe the entirety of thermal transport in nanostructured materials. This dissertation presents methods of thermal transport characterization in three such materials: germanane, silicon germanium nanowires, and molybdenum disulfide. The thermal conductivities of sub-micron thick polycrystalline and amorphous germanane flakess were measured and compared to simulated values, demonstrating that thermal conductivity can be used as an indicator of a material’s thermal conductivity. The thermal conductivity of silicon germanium nanowires was measured for different suspended segments along the same wire as a means of determining the mean free path and dominant scattering mechanisms in the nanowires. Finally, non-equilibrium phonon transport in molybdenum disulfide is investigated by probing the laser-induced temperature rises of different phonon modes and exploring the mechanisms behind this non-equilibrium response.
Evolving biomolecular control and peptide specificity for the synthesis and assembly of II-VI semiconductor nanomaterials
(2003-05) Flynn, Christine Elizabeth; Belcher, Angela M.
Peptides were selected using an evolutionary screening process utilizing engineered virus libraries to isolated peptides that recognized, nucleated and controlled II-VI semiconductor materials. Specifically, materials screened for this project included polycrystalline and single crystal surfaces of ZnS, CdS, PbS, and ZnSe. Once a positive peptide recognition sequence was isolated using phage display screening, the population of peptides found for ZnS or CdS were successfully tested to decipher relative binding affinities, results that further verified the consensus motifs identified. The peptides selected were then used to nucleate nanocrystals, specifically controlling nanoparticle sizes and directing crystal phases of ZnS and CdS. The ZnS-specific A7 and Z8 peptides were isolated from virus screenings of ZnS and tested for ZnS nucleation ability. Upon HRTEM analysis of the resultant nanocrystals, two different phases of ZnS were grown in the presence of A7 and Z8. A7 directed the wurtzite structure crystal phase of ZnS while Z8 directed the sphalerite crystal phase of ZnS. Two CdS-specific peptides, J140 and J182, were isolated from virus screenings of CdS and were further tested, not only for their CdS recognition ability, but for their CdS nucleating ability. Upon HRTEM analysis of the resultant nanocrystals, two different phases of CdS were grown in the presence of J140 and J182, a trend parallel to that seen with ZnS specific phage grown nanocrystals. J140 directed the wurtzite structure crystal phase while J182 directed the zinc blende crystal phase. Further, orientation of the materials using display of the specific peptides as fusions on the viral protein coat indicated that relative order of nanocrystals over several hundred nanometers could be achieved, while maintaining the crystal phase and size selectivity that was seen on the smaller scale.
Fabrication and characterization of a double torsional mechanical oscillator and its applications in gold micromass measurements
(2008-12) Lu, Wei, 1975-; Markert, John T.
We report the design and fabrication of a micro-mechanical oscillator for use in extremely small force detection experiments such as transverse force measurements of a moving vortex and Nuclear Magnetic Resonance Force Microscopy (NMRFM). We study the basic physics of the double torsional mechanical oscillator, and pursue double torsional oscillators with small spring constants, high resonance frequencies, and high quality factors. Using a series of semiconductor manufacturing techniques, especially using the electron-beam lithography technique, we successfully micro-fabricate double torsional mechanical oscillators from silicon-on-insulator wafers. We conduct characterization experiments to extract important parameters of a mechanical oscillator, including the resonance frequencies, spring constants, and quality factors. We focus on the four typical resonance modes of these oscillators, and then compare the force detection sensitivity of each mode. Eventually we apply these force sensitive oscillators to gold micro-mass measurements, and achieve very small mass detection. In the future we are going to continue to micro-fabricate thinner oscillators to reduce the spring constants, and improve the quality factors by designing more suitable geometric shapes and by pursuing annealing studies. Thus, we might be able to achieve single nuclear spin measurements using NMRFM.
Fabrication and characterization of thin films and optical nanocomposites
(2005-05) Baek, Jonghoon, 1970-; Becker, Michael F.
Aluminum nitride thin films were fabricated and characterized using Pulsed Laser Deposition with varying processing conditions in order to exploit the feasibility as an encapsulating matrix for nonlinear nanocomposites. We have studied the dependence of optical properties, structural properties and their correlations for these AlN films. Low optical absorption, textured polycrystalline AlN films can be produced by PLD on sapphire substrates using a background nitrogen pressure of 4.5×10-4 Torr at 99.9% purity. In order to accurately extract optical properties of non-uniform films, we have developed and successfully applied the Optimum Parameter Extraction (OPE) numerical method. The OPE method can accommodate films with two-dimensional thickness variation. Previous methods rendered significant errors in values of refractive index and film thickness when applied to absorbing and wedged films. GaN nanocomposite coated with AlN film and silver nanocomposite coated with NdAlO3 were successfully synthesized and their nonlinear optical properties were characterized by ultrafast laser. The enhancement of THG in GaN nanocomposite did not occur because the LAM process would change crystalline GaN feedstock into semi-amorphous GaN nanoparticles. The THG signal was enhanced in bare silver nanopaprticles due to the plasmon resonance. On the contrary, in the case of Ag NPs coated with NdAlO3 the THG intensity was decreased by 40% at intermediate fields because the plasmon resonance wavelength was redshifted and moved off the twophoton resonance wavelength of the laser.
Mechanical properties of an irradiated nanocluster strengthened high-chromium ferritic alloy
(2008-08) McClintock, David Allen, 1978-; Landsberger, Sheldon
Advanced nano-structured ferritic alloys (NFAs) containing a high density of ultra-fine (2-5 nm) nanoclusters (NCs) enriched in Y, Ti, and O are considered promising candidates for structural components in future nuclear systems. The superior tensile strengths of NFAs relative to conventional oxide dispersion strengthened (ODS) ferritic alloys are attributed to the high number density of NCs, which may provide effective trapping centers for point defects and transmutation products generated during neutron irradiation. This study consists of production, irradiation, and characterization of an advanced NFA, designated 14YWT, currently being developed at Oak Ridge National Laboratory (ORNL), in Oak Ridge, Tennessee. The purpose of this study was to characterize the tensile and fracture toughness properties of 14YWT produced during this project at ORNL before and after irradiation to evaluate it's resistance to radiation-induced changes in mechanical properties. Another alloy, designated 14WT, was produced during this project using identical production parameters used for 14YWT but without the Y2O3 addition during ball milling required for NC formation. Tensile and fracture toughness specimens were produced from both alloys and irradiated in small "rabbit" capsules in the High Flux Isotope Reactor (HFIR) at ORNL. Five other structural alloys that are currently being evaluated for applications in nuclear environments were irradiated and tested during this project to serve as comparison materials. Microstructural characterization was performed using optical microscopy, scanning electron microscopy, transmission electron microscopy, and atom probe tomography. Tensile strengths for 14YWT were found to be far superior to the other alloys for both irradiated and unirradiated conditions, with yield strength for 14YWT decreasing from ~1,450 MPa at 26°C to ~700 MPa at 600°C. Moderate radiationinduced hardening (50-200 MPa) and reduction in ductility was observed for 14YWT for all irradiation conditions and test temperatures. Fracture toughness results showed 14YWT in the unirradiated condition had a fracture toughness transition temperature (FTTT) around -150°C and upper-shelf K[subscript JIc] values around 175 MPa m. Results from irradiated 14YWT fracture toughness tests were found to closely mirror the unirradiated data and no shift in FTTT or decrease in K[subscript JIc] values were observed following neutron irradiation to 1.5 dpa at 300°C. Master curve analysis of the fracture toughness data show 14YWT to have a T[subscript o] reference temperature of -188 and -176°C in the unirradiated and irradiated condition, respectively, which is unprecedented for a high-strength dispersion strengthened ferritic alloy. The results from this study show 14YWT to be resistant to radiation-induced changes in mechanical properties and a promising candidate for structural applications in future nuclear systems.
Micro- and nano-periodic-structure-based devices for laser beam control
(2007) Gu, Lanlan, 1975-; Chen, Ray T.
With the progress of microfabrication and nanofabrication technologies, there has been a reawakened interest in the possibility of controlling the propagation of light in various materials periodically structured at a scale comparable to, or slightly smaller than the wavelength. We can now engineer materials with periodic structures to implement a great variety of optical phenomena. These include well known effects, such as dispersing a variety of wavelength to form a spectrum and diffracting light and controlling its propagation directions, to new ones such as prohibiting the propagation of light in certain directions at certain wavelengths and localizing light with defects in some artificially synthesized dielectric materials. Advances in this field have had tremendous impact on modern optical and photonic technologies. This doctoral research was aimed at investigating some of the physics and applications of periodic structures for building blocks of the optical communication and interconnection system. Particular research emphasis was placed on the exploitation of innovative periodic structure-based optical and photonic devices featuring better functionality, higher performance, more compact size, and easier fabrication. Research topics extended from one-dimensional periodic-structure-based wavelength-division-multiplexing (WDM) optical interconnects (beam wavelength selection devices), and liquid crystal beam steerers (beam steering devices), to two-dimensional periodic-structure-based silicon photonic-crystal thermo-optic and electro-optic modulators (beam switching devices). This research was specifically targeted to seek novel and effective solutions to some long-standing technical problems, such as the limited wavelength coverage of coarse WDM devices, small bandwidth of highly dispersed dense WDM devices, low deflection efficiency of high-resolution liquid crystal beam steerers, slow switching speed, large device size, and high power consumption of silicon optical modulators, among others. For each subtopic, research challenges were presented and followed by the proposed solutions with extensive theoretical analysis. The proposals were then verified by experimental implementations. Experimental results were carefully interpreted and the future improvements were also discussed.
Micro/nano fabrication of polymeric materials by DMD-based micro-stereolithography and photothermal imprinting
(2006) Lu, Yi; Chen, Shaochen
The revolutionary advancement in semiconductor device manufacturing promoted micro/nano fabrication technologies viable for research and applications in broader fields such as biology and optics. This dissertation is aimed at developing parallel fabrication technologies for polymeric micro/nano structures that can potentially be used in biomedical or optical devices. The objective of the dissertation is three told: a) develop and characterize a digital micro-mirror device (DMD)-based micro-stereolithographic system and explore the fabrication of hydrogel tissue engineering scaffolds, b) use the micro-stereolithographic system to fabricate microlens arrays, c) develop a photothermal imprinting technique to pattern nanostructures on the surface of polymer composites. In the first part of the dissertation, we demonstrated a simple and fast, layer-by-layer micro-stereolithographic system based on DMD dynamic photomask that allows fabrication of complex internal features along the precise spatial distribution of biological factors inside a single scaffold. Photo-crosslinkable poly(ethylene glycol) diacrylate and diamethacrylate were used as the scaffold material. In situ encapsulation of fluorescently-labeled micro-particles and cells was demonstrated. We investigated the photopolymerization process and its effects on the properties of the scaffolds. This technique could provide a powerful tool in studying progenitor cell behavior and differentiation under biomimetic, three-dimensional (3D) culture conditions. In the second part, we developed a novel fabrication technique for microlens arrays using a modified DMD-based micro-stereolithographic system. The DMD can generate high resolution images with quasi-continuous intensity gradient, thanks to its high density mirror elements with a bandwidth of 10 KHz. The projected UV patterns were simply drawn in a computer software. Topographic patterns were created in photocurable resin by spatially controlling the curing depth. Spherical microlens arrays were fabricated and their optical performance was characterized. This technique is capable of fabricating optical elements with any surface topography. In the third part, we discussed the photo-induced radical polymerization. A numerical model was established to correlate the geometry of the resulting gels and system parameters. In the fourth part, we reported a laser-assisted photothermal imprinting method for directly patterning carbon nanofiber reinforced polyethylene nanocomposite. A single laser pulse was used to melt/soften a thin skin layer of polymer nanocomposite. Meanwhile, high resolution patterns were transferred from a quartz mold to the surface of the composite.
Nanocomposites of poly(acrylonitrile-butadiene-styrene) and montmorillonite clay: dispersion and mechanical properties
(2005) Stretz, Holly Ann; Paul, Donald R.
Polymer/montmorillonite clay (MMT) nanocomposites have produced significant commercial interest due to the excellent balance of properties, but the issues controlling proper clay dispersion are poorly understood. Current studies examine the effects of polymer and organoclay structure on properties of melt-processed poly(styrene-coacrylonitrile) (SAN)/MMT, where SAN models the more complex ABS/MMT composites used in computer housings. Initially we examined the effects of organoclay surfactant structure on filler dispersion and composite mechanical properties. The composite which exhibited the highest modulus and greatest particle viii aspect ratio (~50) was produced from an organoclay with the lowest molecular weight surfactant. Swelling of the MMT particles, measured by x-ray diffraction, was more strongly related to reduced surfactant molecular weight than surfactant functionality. The composite moduli were compared to Halpin-Tsai theoretical predictions from TEM-based aspect ratios. Given a range of surfactant structures, we then explored the appropriateness of the SAN matrix as a model for ABS. Electron microscopy showed that clay particles in ABS/MMT composites reside in the SAN matrix phase, accumulating at rubber particle surface. Modulus enhancement patterns were the same for a given organoclay, but reinforcement in ABS was lower due to poor orientation of particles at the rubber surface. Interactions between the polymer and silicate surface were probed by varying the SAN copolymer composition, accounting for variations in matrix modulus and melt viscosity. TEM-based image analysis coupled with Mori-Tanaka composite theory gave predictions which fit experimental moduli better than Halpin-Tsai. Higher acrylonitrile content lead to increased reinforcement in the 0-58 weight % acrylonitrile range. TEM-based specific particle densities reached ~8 particles/μm2 compared to well-exfoliated nylon 6 composites at 100 particles/μm2. Improvements in exfoliation were also noted for higher screw rpm. ix Based on enhancement in exfoliation for polyolefin-g-maleic anydride composites, the effect of maleic anhydride in SMA-based nanocomposites was studied. These materials produced the same properties on a weight percent basis as SAN-based nanocomposites, but particle densities remained lower than for polyolefin-g-MA mixtures. This behavior is explained by repulsive interactions between styrene and the alkyl tail of the surfactant, suggesting that polar surfactant tails could lead to improved exfoliation in styrene copolymer-based/montmorillonite nanocomposites.
Nanomaterials characterization and bio-chemical sensing using microfabricated devices
(2004) Yu, Choongho, 1971-; Shi, Li, Ph. D.
A variety of nanostructured materials have been synthesized in recent years. These nanomaterials have potential applications in areas spanning computing, energy conversion, sensing, and biomedicine. Because of size confinement effects, furthermore, these nanomaterials are expected to show very different physical properties from those of their bulk counterparts. The measurement of their properties, however, has been very challenging due to their small dimensions. Similarly, it remains a challenge to detect chemical and biomolecular species due to their small dimensions. This dissertation presents the development of microelectromechanical systems (MEMS) devices for the characterization of thermophysical properties of nanomaterials and for the detection of chemical species and biological cells. The thermophysical property of one-dimensional (1D) nanomaterials was measured using a batch-fabricated microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams. Three methods were developed to assemble nanomaterials with the measurement devices. Those three methods include a wet deposition process, an in-situ chemical vapor deposition technique, and an electric-field-assisted assembly method. During the measurement, one membrane is Joule-heated to cause heat conduction through the nanomaterials to the other membrane, allowing for the measurement of thermal conductance and Seebeck coefficient. The electrical conductance can also be measured using the microdevice. The temperaturedependent properties of an individual single-wall carbon nanotubes (SWCNs) and SWCN bundles were measured. Measurement sensitivity, errors, and uncertainty were examined. The obtained thermal conductivity of an individual SWCN is found to be much higher than bundles of SWCNs in the range of 2000-11000 W/m-K at room temperature, in agreement with theoretical predictions. Furthermore, the thermal conductivity of bundles of SWCNs are found to be suppressed by contact resistance between interconnected SWCNs in the bundle. The microdevice has also been integrated with metal oxide nanobelts for chemical sensing. The sensing mechanism is based on surface oxidation-reduction (redox) processes that change the electrical conductance of the nanobelt. The sensor was found to be highly sensitive to inflammable and toxic gas species including nitrogen dioxide (NO2), ethanol, and dimethyl methylphosphonate (DMMP). Furthermore, it eliminated the sensor poisoning effects that have limited the wide use of polycrystalline metal-oxide based sensors. The experiment is a step towards the large scale integration of nanomaterials with microsystems, and such integration via an electric-field-directed assembly approach can potentially enable the fabrication of low-power, ultra-sensitive, and selective integrated nanosensor systems. The electric field manipulation technique has not only been used to assemble nanomaterials with MEMS, but also been used to focus biological cells in a microfluidic channel for cytometry applications. Flow cytometry is a powerful and versatile method of rapidly analyzing large populations of cells and other particulate or molecular analytes that have been captured on the surface of carrier particles. However, the key components of the system, hydrodynamic focusing and optical systems, make conventional cytometers complex, large, and expensive. To eliminate these drawbacks, a dielectrophoretic particle focusing technique combined with MEMS is explored to replace the hydrodynamic focusing mechanism. To focus particles, microelectrodes are patterned on the circumference of the channel to generate AC fringing fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the channel. An ellipticlike microfluidic channel has been fabricated by isotropic etching of soda lime glass wafers and a subsequent wafer-bonding process. Experiments with microbeads and human leukemia HL60 cells and an analysis using a thin shell model indicate that biological cells can be focused using an AC voltage of an amplitude up to 15 Vp-p and a frequency below 100 kHz, respectively. This design eliminates the sheath flow and the fluid control system that makes conventional cytometers bulky, complicated, and difficult to operate, and offers the advantages of a portable standalone instrument as well as a module that could potentially be integrated with on-chip impedance or optical sensors into a micro total analysis system.
Nanoscale orthogonal biofunctionalization imprint lithography and its applications for studying nanoscale cell surfaces interactions
(2007-12) Gaubert, Harold Edward, 1979-; Frey, Wolfgang, doctor of physics
Surfaces with nanopatterned biological functionality are important prerequisites for many applications including developing biosensors, tissue engineering scaffolds and Bio-MEMS devices. This work presents a versatile technique, termed nanoscale orthogonal biofunctionalization imprint lithography, which allows "top-down" highprecision nanopatterning of proteins that can meet the demands of various applications. To show applicability of this technique, it was used to create disposable large scale arrays of nanopatterned cell adhesion proteins for cell culture for the purpose of investigating the influence of nanoscale geometrical parameters on cell-surface interactions. These cell culture arrays were used to systematically vary the size, spacing and density of fibronectin adhesion clusters, which are expected to modulate the signaling induced by the cell adhesion, the clustering of adhesion molecules and the force generated in the cytoskeleton. As a result, it was first determined that the nanopatterned adhesion sites provided an upper limit to the size of a corresponding cell focal adhesion. Cell morphology, actin stress fibers, vinculin distribution, proliferation and motility were all influenced by nanoscale fibronectin island size, and in some cases, the distance between patterns. Several parameters depended biphasically on the pattern size, indicating a very fine regulation of the associated cell signaling. Adhesion area and local stress on the adhesion are modulated by the adhesion size, and the cell response on the nanopattern shows strong parallels to the response on elastic adhesion substrates. In addition, chemical signaling may be influenced directly by changing the activity of associated enzymes. The results of this work build a basis for an understanding of adhesion on the nanoscale level and offer design criteria for the engineering of biomaterials and tissue scaffolds.
Nanostructured anode materials for Li-ion and Na-ion batteries
(2013-08) Lin, Yong-Mao; Mullins, C. B.; Heller, Adam (Professor of chemical engineering)
The demand for electrical energy storage has increased tremendously in recent years, especially in the applications of portable electronic devices, transportation and renewable energy. The performances of lithium-ion and sodium-ion batteries depend on their electrode materials. In commercial Li-ion batteries with graphite anodes the intercalation potential of lithium in graphite is close to the reversible Li/Li⁺ half-cell potential. The proximity of the potentials can result in unintended electroplating of metallic instead of intercalation of lithium in the graphite anode and frequently leads to internal shorting and overheating, which constitute unacceptable hazards, especially when the batteries are large, as they are in cars and airplanes. Moreover, graphite cannot be readily used as the anode material of Na-ion batteries, because electroplating of metallic sodium on graphite is kinetically favored over sodium intercalation in graphite. This dissertation examines safer Li-ion and Na-ion battery anode materials.
Nanostructured molybdenum chalcogenides: synthesis, structure and catalytic properties
(2005) Camacho Bragado, Gloria Alejandra; Yacaman, Miguel Jose
The catalytic properties of nanostructured molybdenum oxides and sulfides were investigated. Several synthesis methods were tested in order to determine a reliable and reproducible way to produce structurally and chemically homogeneous products. The structure of synthesized molybdenum oxides and sulfides was thoroughly studied by electron microscopy techniques, X-ray diffraction and X-ray photoelectron spectroscopy among others. It was found that the synthesis method of choice produced high yield, single phase, single crystalline molybdenum oxide nanoparticles. These oxide crystals are susceptible to reduction and sulfidation, which enable their use as precursor in the synthesis of molybdenum sulfides. The sulfidation of molybdenum oxide nanocrystals produced highly textured molybdenum sulfide nanostructures. Pseudo one-dimensional structures were identified in these samples. The structural model proposed for such structures implies the presence of sulfur atoms decorating the edges. This is particularly important since these extra-sulfur atoms may lead to an electronic structure different from the bulk molybdenum sulfide. The catalytic properties of these compounds were studied with the model reaction of hydrodesulfurization (HDS) of dibenzothiophene (DBT). DBT is considered an appropriate compound for the investigation of the activity and reaction mechanisms of HDS catalysts, and it is widely used in the literature, which facilitates the comparison with reported data. The nanostructured molybdenum compounds showed typical to high activity values, however they presented enhanced selectivity. The oxides were more efficient in the desulfurization of DBT, while the sulfides for hydrogenation.
One-dimensional electron systems on graphene edges
(2007-12) Hill, Jason Edward, 1978-; MacDonald, Allan H.
In this dissertation several aspects on one-dimensional edge states in grapheme are studied. First, a background in the history and development of graphitic forms is presented. Then some novel features found in two-dimensional bulk graphene are presented. Here, some focus is given to the chiral nature of the Dirac equation and the symmetries found in the grahene. Magnetism and interactions in graphene is also briefly discussed. Finally, the graphene nanoribbon with its two typical edges: armchair and zigzag is introduced. Gaps due to finite-size effects are studied. Next, the problem of determining the zigzag ground state is presented. Later, we develop this in an attempt to add the Coulomb interaction to the zigzag flat-band states. These nanoribbons can be stimulated with a tight-binding code on a lattice model in which many different effects can be added, including an A/B sublattice asymmetry, spin-orbit coupling and external fields. The lowest Landau level solutions in the different ribbon orientations is of particular current interest. This is done in the context of understanding new physics and developing novel applications of graphene nanoribbon devices. Adding spin-orbit to a graphene ribbon Hamiltonian leads to current carrying electronic states localized on the sample edges. These states can appear on both zigzag and armchair edges in the semi-finite limit and differ qualitatively in dispersion and spin-polarization from the well known zigzag edge states that occur in models that do not include spin-orbit coupling. We investigate the properties of these states both analytically and numerically using lattice and continuum models with intrinsic and Rashba spin-orbit coupling and spin-independent gap producing terms. A brief discussion of the Berry curvature and topological numbers of graphene with spin-orbit coupling also follows.
Polyamide-layered silicate nanocomposites by melt processing
(2003) Fornes, Timothy Dean; Paul, Donald R.
Polyamide-layered silicate nanocomposites based on nylon-6, 11, and 12 and organically modified montmorillonites (organoclay) were prepared by twin screw extrusion. Carefully designed component structure-nanocomposite morphology and property investigations on these materials were executed to understand why nylon-6 readily exfoliates organoclay. The polyamide structure strongly influences the extent of clay platelet delamination and level of property enhancement, as determined by X-ray, transmission electron microscopy and stress-strain analyses. High molecular weight nylon-6 materials lead to better organoclay exfoliation and greater nanocomposite moduli and yield strengths than lower molecular weight materials; this is attributed to higher levels of shear stress imparted on the clay by the higher viscosity polymer. The ratio of amide to methylene units in the repeat structure of nylon-6 appears to affect the polymer- organoclay affinity since a large increase in aliphatic content, i.e., nylon-6 versus nylon-12, results in less organoclay dispersion and lower reinforcing efficiency. The structure of the organoclay is also critical for producing wellexfoliated nylon-6 nanocomposites. Alkyl ammonium surfactants that cover less montmorillonite surface in the organoclay are more effective at exfoliating clay and generating improved nanocomposite stiffness and strength; such surfactants facilitate more desirable polyamide-silicate interactions, yet maintain sufficient organoclay gallery spacings needed both to overcome the cohesive forces between neighboring platelets and to facilitate polymer intercalation. The source of sodium montmorillonite used to form the organoclay is also important. The superior properties observed in nylon-6 nanocomposites may be explained by conventional ideas of reinforcement as predicted by composite theories like those of Halpin-Tsai or Mori-Tanaka. Based on good agreement between experimental nanocomposite moduli and model predictions it is clear that superior reinforcement stems from the high modulus and aspect ratio of montmorillonite; however, montmorillonite particles clearly affect the proprieties of the polymer phase which may have additional effects on the composite. Differential scanning calorimetry and X-ray analyses show that the clay can alter the nucleation, growth, and type of nylon-6 crystals formed under certain crystallization conditions. Furthermore, exposure of organoclay surfaces during processing can cause considerable polymer degradation and color formation depending upon the type of nylon-6 used and the surfactant structure in the organoclay.
Polymer-layered silicate nanocomposites by melt processing
(2006) Shah, Rhutesh Kishorkant; Paul, Donald R.
Polymer-layered silicate nanocomposites formed from the organically modified clay mineral montmorillonite and related materials have attracted a great deal of technological and scientific interest in the past decade. These composites offer the promise of greatly improved mechanical, thermal, and barrier properties over those of the matrix polymer owing to the nanoscale reinforcement and constraints of the polymer caused by dispersing the one nanometer thick, high aspect ratio aluminosilicate (clay) layers. The central scientific issue is how to achieve a high level of dispersion, and ultimately full exfoliation of the clay platelets within the polymer matrix since this is necessary to realize the large filler aspect ratios. Although several factors play a role in organoclay exfoliation, it seems to be largely dependent upon a complex array of interactions between the polymer matrix and the organoclay. Recently, there has been a strong commercial drive for producing such nanocomposites from low cost polymers like polyolefins. Unfortunately, polyolefins are highly inefficient at exfoliating the organoclays by themselves, since there is no favorable interaction with the polar aluminosilicate surface of the clay. Hence, the principal goal of this research work was to explore the various routes to improve polyolefin-organoclay interactions, and thus, organoclay exfoliation in these systems. Three mutually exclusive strategies were employed to achieve this objective. First, the polyolefin matrix was made more polar by several techniques viz., surface treating the polyolefin particles, grafting of maleic anhydride on the polyolefin backbone, copolymerizing with polar monomers like methacrylic acid, and incorporating ionic groups (ionomers). These modifications resulted in significant improvements in organoclay exfoliation. Second, the organoclay structure was engineered to improve polyolefin-organoclay compatibility. It was determined that surfactants whose structure lead to more shielding of the silicate surface or increased alkyl material within the organoclay galleries result in improved levels of exfoliation. Finally, the melt processing conditions were fine tuned to generate optimum amounts of shear, and reduce thermal degradation of the surfactant during the preparation of nanocomposites. Once sufficient levels of organoclay exfoliation were attained, these materials were tested for barrier film applications.
Polymer-organoclay nanocomposites by melt processing
(2009-05) Cui, Lili, 1977-; Paul, Donald R.
Polymer-layered silicate nanocomposites based on a variety of polymer matrices and several organoclays were prepared by melt processing. A detailed characterization of the thermal degradation of several commercial and experimental organoclays often used to form polymer nanocomposites was reported. The surfactant type, loading, and purification level of organoclay significantly affect their thermal stability; however, broadly speaking, the results suggest that these differences in thermal stability do not appear to have much effect on the morphology and properties of the nanocomposites formed from them. It seems that the thermal stability of organoclays is not the key factor in organoclay exfoliation in melt processed polymer nanocomposites, since the exfoliation/dispersion process may have been completed on a time scale before the degradation of surfactant progresses to a detrimental level. Polymer nanocomposites have been made from a variety of polymers; however, few matrices have demonstrated the ability to readily exfoliate the organoclay as well as nylon 6, especially for highly hydrophobic materials like polyolefins. Hence, a significant part of this research work was devoted to explore various routes to improve polyolefinorganoclay interactions, and thus, organoclay exfoliation in these systems. Amine grafted polypropylenes and a conventionally used maleic anhydride grafted polypropylene were used as compatibilizers for polypropylene based nanocomposites to improve the organoclay exfoliation. A series of ethylene vinyl acetate copolymers, the polarity of which can be adjusted by varying their vinyl acetate contents, based nanocomposites were prepared as the model system to address the relationship between the polarity of the polymers and their preferences over various organoclay structures. Attempts were made to explore the effect of degree of neutralization of acid groups in ionomers on the morphology and properties of nanocomposites, and it seems that the ionic units on the polymer chain provide a more favorable interaction between the polymer matrix and the organoclay compared to acid units and, thus, lead to better dispersion of the clay particles. It was determined that surfactants whose structure lead to more shielding of the silicate surface result in improved levels of exfoliation in all the above mentioned unmodified and modified polyolefin based nanocomposites.