Browsing by Author "Rau, Carl"
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Item Carbon Nanotube Doping Procedures for Three-Dimensional Macro-Structures and Gallium-Nitride Functionalization(2014-05-05) Hashim, Daniel Paul; Ajayan, Pulickel M.; Lou, Jun; Rau, CarlCarbon nanotubes (CNTs) in all of their forms are considered “gamechanger” materials that will revolutionize the modern world through many diverse applications. Over 20 years of research has gone into CNT materials, yet we still see their limited use in feasible real-world applications. Part of the reason is because it still remains a challenge for materials scientists to engineer these extraordinary nano-scale building blocks into covalently interconnected three-dimensional (3-D) structures, and to realize macro-scaled sizes via a bulk synthesis process. Another challenge is being able to create CNT-semiconductor hybrid materials by covalently joining other useful semiconductor compounds with CNTs in order to harness their value for electronics applications. The experimental research compiled in the first part of this thesis pioneers an innovative approach to synthesize 3-D macro-structured forms of CNTs by utilizing a heteroatom doping strategy via chemical vapor deposition (CVD). The importance of substitutional doping effects of boron on CNT structural morphology is characterized experimentally and theoretically for the first time so as to create a robust, solid, 3-D networked, CNT “sponge” form. The CNT “sponge” was characterized to exhibit an exotic combination of multifunctional properties including high porosity, high surface area, low density, superhydorphicity, oleophilicity, ferromagnetism, and good elastic mechanical performance. It was also demonstrated that 3-D porous CNT “sponges” could be used for environmental needs as reusable oil spill sorbent materials in seawater. In an effort to combine group III–V semiconductors with CNTs, the second part of this thesis involve a simple solution-based technique for gallium functionalization of nitrogen-doped multi-wall carbon nanotubes. With an aqueous solution of a gallium salt (GaI3), it was possible to form covalent bonds between the Ga3C ion and the nitrogen atoms of the doped carbon nanotubes to form a gallium nitride–carbon nanotube hybrid at room temperature. This functionalization was evaluated by x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy.Item Electrical characterization of thin-film lithium niobate(1985) Fabiny, Larry; Rabson, Thomas A.; Wilson, William L.; Rau, CarlThe electrooptic properties of a thin film of lithium niobate were studied. The film was deposited on an n-type silicon substrate by RF sputtering. The short circuit photocurrent was measured as a function of time, and the voltage dependence of current and capacitance under light and dark conditions was measured. Photocurrents up to 16.1 pA were obtained with a light intensity of .5 mW/cm . With one exception, all photocurrent flowed from the A1 contact to the Si substrate. The current was linear for | Vapp | < .5 volts. At higher voltages, a thermal current was generated. It was found that the current required several minutes to reach its steady state value when the applied voltage was changed. The film was shown to possess some of the ferroelectric properties present in bulk crystals. Although no crystallographic studies have been done, the electrical measurements indicate that the LN film is polycrystalline.Item Ferromagnetism of novel ultrathin films and multilayers: A study of Ru/C(0001) and Fe/Mn/Fe/Pd(100) using magneto-optic Kerr effect and spin-sensitive electron spectroscopies(1997) Steierl, Gerold Josef; Rau, CarlThe possibility of the existence of ferromagnetic order in Ru monolayers on the C(0001) graphite surface is studied using electron induced spin-polarized secondary electron emission (SPSEE). Below a surface Curie temperature of approximately 250K we find ferromagnetic order of the Ru monolayer. The growth mode of Ru on C(0001) is investigated by Auger electron spectroscopy (AES). We find, that the initial growth occurs laterally until a homogenous monolayer is formed. Our study provides the first observation of spontaneous, two-dimensional ferromagnetic order of a 4d transition element. To investigate the spin-polarization and domain pattern in ultrathin films and multilayers, we develop a novel technique: scanning ion microscopy with polarization analysis (SIMPA). A highly focused Ga$\sp+$-ion beam from a liquid metal ion source is moved over a sample to locally eject secondary electrons, whose spin-polarization is determined by using a medium energy Mott polarimeter. We show that the sputtering by the ion beam can be well controlled and utilized for nano-lithography and spin-resolved sputter depth profiling. We use SIMPA and the surface magneto-optic Kerr effect to study exchange coupling in Fe/Mn/Fe/Pd(100) multilayers. The intervening Mn layer is grown in a wedge shape ranging in thickness from 0 to 9 monolayers. Our results show that both Fe layers are coupled ferromagnetically for the entire Mn thickness range. Growth mode and structure are analyzed by AES and medium electron energy diffraction.Item Functionalization of Nanocarbons for Composite, Biomedical and Sensor Applications(2013-07-24) Kuznetsov, Oleksandr; Billups, W. Edward; Hauge, Robert H.; Rau, CarlNew derivatives of carbon nanostructures: nanotubes, nano-onions and nanocrystalline diamonds were obtained through fluorination and subsequent functionalization with sucrose. Chemically modified nanocarbons show high solubility in water, ethanol, DMF and can be used as biomaterials for medical applications. It was demonstrated that sucrose functionalized nanostructures can find applications in nanocomposites due to improved dispersion enabled by polyol functional groups. Additionally, pristine and chemically derivatized carbon nanotubes were studied as nanofillers in epoxy composites. Carbon nanotubes tailored with amino functionalities demonstrated better dispersion and crosslinking with epoxy polymer yielding improved tensile strength and elastic properties of nanocomposites. Reductive functionalization of nanocarbons, also known as Billups reaction, is a powerful method to yield nanomaterials with high degree of surface functionalization. In this method, nanocarbon salts prepared by treatment with lithium or sodium in liquid ammonia react readily with alkyl and aryl halides as well as bromo carboxylic acids. Functionalized materials are soluble in various organic or aqueous solvents. Water soluble nanodiamond derivatives were also synthesized by reductive functionalization of annealed nanodiamonds. Nanodiamond heat pretreatment was necessary to yield surface graphene layers and facilitate electron transfer from reducing agent to the surface of nanoparticles. Other carbon materials such as activated carbon and anthracite coal were also derivatized using reductive functionalization to yield water soluble activated carbon and partially soluble in organic solvents anthracite. It was shown that activated carbon can be effectively functionalized by Billups method. New derivatives of activated carbon can improve water treatment targeting specific impurities and bio active contaminants. It was demonstrated that functionalized carbon nanotubes are suitable for real time radiation measurements. Radiation sensor incorporating derivatized carbon nanotubes is lightweight and reusable. In summary, functionalization of carbon nanomaterials opens new avenues for processing and applications ranging from biomedicine to radiation sensing in space.Item Magnetic and critical behavior of thin terbium(0001)/tungsten(110) films studied by electron capture spectroscopy (ECS)(1988) Jin, Changming; Rau, CarlEpitaxial films of hcp Tb(0001) on bcc W(110) substrates are prepared and their geometric, electronic and magnetic properties are studied. The atomic flatness and cleanness and the single-crystalline state of the W(110) substrates and the Tb films are checked using scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low-energy and high-energy electron diffraction (LEED and RHEED). Using electron capture spectroscopy (ECS), the surface electron spin polarization of the Tb(0001) films is studied as a function of temperature. The topmost layer is found to order ferromagnetically below a surface Curie temperature T$\sb{\rm C\sb s}$ = 249.96 K, which lies above both the bulk Curie and Neel temperatures T$\sb{\rm C\sb b}$ = 220 K and T$\sb{\rm N\sb b}$ = 228 K, respectively, and exhibits strongly non-monotonic behavior close to 245 K. Novel critical behavior is observed near T$\sb{\rm C\sb s}$, suggesting strong surface anisotropies, in accord with a recently predicted and previously unobserved surface phase transition.Item Novel Material Behavior in Carbon Nanotube/Elastomer Composites(2012-09-05) Carey, Brent; Ajayan, Pulickel M.; Barrera, Enrique V.; Rau, CarlComposites are multiphasic materials with individual constituent parts that work cooperatively to produce some desired result. For the common case of structural composites, the use of nanoscale additives does not always yield a predictable outcome due to the complex interactions that occur in the interfacial region where a reinforcing filler meets the supporting matrix. It stands to reason, however, that the thoughtful and deliberate exploitation of unusual effects in this region could lead to the development of nanocomposite materials with extraordinary properties. In this thesis work, I will introduce two such responses in a compliant nanocomposite consisting of highly-aligned carbon nanotubes (CNTs) encased within a poly(dimethylsiloxane) (PDMS) matrix. It is first demonstrated that the material exhibits extremely anisotropic dynamic mechanical behavior. The composite will behave in a way that is evocative of the neat polymer when deformed orthogonal to the CNT alignment direction, yet will exhibit strain softening when cyclically compressed along their axis due to the collective buckling of the nanotube struts. Next, it is shown that this nanocomposite material has the ability to respond and adapt to applied loads. Independent, yet complimentary tests reveal that the structure of the polymer in the presence of nanoscale interstitials will evolve during dynamic stressing, an effect that was predicted nearly 50 years ago. With support from both recent and established literature, an updated mechanism is proposed. Collectively, these results provide insight into the complicated mechanics between polymer matrices and embedded nanoparticles, and assist in the design of advanced synthetic materials with unique physical properties.Item Spin-polarized electron emission spectroscopy (SPEES): A new and novel technique in surface science and ferromagnetism(1989) Waters, Kelley Lyle; Rau, CarlA new technique, Spin-Polarized Electron Emission Spectroscopy (SPEES), capable of investigating surface magnetism, has been successfully developed. SPEES allows us to energy- and spin-analyze electrons emitted during the interaction of a grazing-incidence ion beam with a solid surface. The energy and spin information obtained from the emitted electrons helps us to unravel the processes involved in ion-surface interactions at grazing angles. SPEES data obtained at Ni(110) picture-frame single crystals exhibit new characteristic features in the electron energy distribution that are strikingly different from that of electron-induced spectra. For the first time, two electron capture measurements, which are sensitive to "local" magnetic order existing on an atomic scale, have been performed at low energies (15-30 keV) at surfaces of the amorphous ferromagnet Fe$\sb{80}$B$\sb{20}$; the non-zero electron spin polarization amounts to 55%. These two new techniques open the way to study surface electronic and magnetic properties with unprecedented sensitivity.Item Surface magnetic order of ultra thin epitaxial vanadium films on silver(1988) Xing, Guoqiang; Rau, CarlVanadium ultra-thin (1-7 monolayers) films are deposited epitaxially on well-defined single crystalline Ag(100) substrate. The topmost layers of the films are studied by electron capture spectroscopy(ECS). ECS is a surface-sensitive technique for the investigation of magnetic order existing at surfaces. It is found that the topmost atomic layer of V(100) films are ferromagnetic for all film thicknesses, in contrast to bulk vanadium which is paramagnetic at all temperatures. The films of thickness of 5 monolayers possess a surface Curie temperature T$\sb{\rm cs}$ = 475.1 K, and the critical behavior of the magnetization of the films is identical to that predicted by the well-known two-dimensional Ising Model of ferromagnet.Item Surface magnetism study of nickel(110) and platinum-manganese-antimony using spin polarized electron emission spectroscopy (SPEES)(1991) Waters, Kelley Lyle; Rau, CarlA spin polarized electron emission spectroscopy (SPEES) study of several ferromagnetic samples has been performed. SPEES studies involve the energy- and spin-analysis of electrons emitted normal to the surface of a sample during grazing angle ion-surface reflection. The information obtained from the emitted electrons allows us to characterize the surface electronic, magnetic, and chemical structure, as well as processes occurring during ion-surface interactions. Studies were made on Ni(110) surfaces bombarded with H$\sp+$ and He$\sp+$ ions at an energy of 25 keV. Electron intensity distributions exhibit distinct features that are strikingly different from large-angle ion- and electron-induced electron emission spectra. Polarization data obtained allow us to unravel various physical processes occurring at the surface. Studies made on PtMnSb samples again show distinct and pronounced peaks and features which are more apparent in SPEES spectra than in any other spectroscopy. The studies made on these samples, using SPEES, show the power of this new spectroscopy to characterize and understand particle-surface interaction processes previously not well understood.Item Ultra high vacuum scanning tunneling microscopy to study thin film growth(1994) Nunes, Nicole Lynne; Rau, CarlAn ultra high vacuum chamber was constructed to study thin film growth using scanning tunneling microscopy (STM). The sample stage is specially designed so that samples can be transferred in situ among various film preparation techniques. For first studies, Ru thin films were deposited at sub-monolayer coverages on C(0001) substrates. From our STM studies, it is found that Ru diffuses easily along the graphite surface, that Ru atoms tend to nucleate at defect sites, and that the first monolayer of Ru grows homogeneously on graphite. These results are in agreement with our recent Auger electron spectroscopy data.Item Water Drop Tribology of Graphene and Polymer Nanocomposites(2013-09-16) Cox, Paris; Ajayan, Pulickel M.; Barrera, Enrique V.; Rau, CarlBasic physics teaches us that the frictional force (lateral force) needed to move objects on surfaces are proportional to load (normal force) – Amonton’s Laws. In tribology, this force is proportional to contact area, whereas Amonton is just a special case for contact area scaling with load. Such established laws do not seem to apply to small drops on flat, smooth surfaces in which frictional forces have an inverse relation to contact area and have time component prior to movement. Such phenomena can be explained by Shanahan-deGennes were intermolecular forces are considered for a deformed surface. Graphene is a special case where no time component is observed and frictional forces are attributed to its chemical homogeneity and stability. In the second part of this thesis, graphene is considered as nanofiller to build up polymer nanocomposites via Layer by Layer (LbL). Graphene Nanoribbons derived from multi-walled carbon nanotubes (MWCNT) offers a special case for thermoplastic polyurethane nanocomposites in that of thermally activated twisting morphology influences nanocomposite properties. Finally an electric field driven transdermal hydrogel drug delivery device has been demonstrated by just using CNTs, polyvinyl-borax gel and a CNT membrane