Epitaxial growth, or the oriented growth of a crystalline monolayer on an ordered substrate, appears in a wide range of systems and applications, from novel device fabrication to freshwater remediation. Despite this, methodical studies of the phenomenon are rare, and the mechanisms governing epitaxial growth are poorly understood. This investigation employs AFM techniques to monitor the epitaxial growth of ion crystal systems at the initial stages of growth. By using systems with well-known physical properties, we are able to relate growth modes to two key parameters, crystal lattice mismatch, Δr/r₀, and affinity between the overgrowth and the substrate ions, ξ. We found wetting growth occurs for systems in which Δr/r₀ is expansive (overgrowth lattice must expand to accommodate substrate) or mildly compressive (overgrowth compresses to accommodate substrate). Additionally, a strong affinity between the substrate and overgrowth ions, in combination with an expansive system, allows for epitaxial growth from undersaturated solutions. We also have observed several instances where the lateral force contrast on the growing film exhibits a strong dependence on the time of exposure to the growth solution and on the driving force for growth (solute concentration). We present results for three epitaxial growth systems in aqueous solutions: CaSO₃ on CaCO₃, PbSO₄ on BaSO₄, and BaSO₃ on BaSO₄. Chemically and topographically identical regions grown at higher concentrations exhibit higher friction than regions grown at lower concentrations. These observations suggest that epitaxial growth occurs by a fast condensation step incorporating a high defect density.

This thesis reflects two advances in atomic force microscopy. The first half is a new lateral force calibration procedure, which, in contrast to existing procedures, is independent of sample and cantilever shape, simple, direct, and quick. The second half is a high-throughput method for processing, fitting, and analyzing force curves taken on Pseudomonas aeruginosa bacteria in an effort to inspire better care for statistics and increase measurement precision.

Friction between nanoscale objects has been a subject of great interest and intense research effort for the last two decades. However, the vast majority of the work done in this area has focused upon the sliding friction between two rigid, atomically smooth surfaces. Thus the parameter most explored has been the corrugation in the atomic potentials and how this affects the force required to slide one object across another. In truth, many nanoscale objects whose translation force is of practical interest are more spherical in nature. We hypothesize that the factors that determine the translation force will be related, not only to the interfacial adhesion, but also to the mechanical properties of the translating object and its underlying surface. The dependence on these quantities of the friction is not known. In this dissertation we have utilized Atomic Force Microscopy and Force Spectroscopy to study the tribological properties of submicron scale polymeric particles to explore how the friction between these submicron spherical objects translating over planar substrates is related to interfacial energy and the mechanical properties for these particles. A technique for modifying the mechanical properties was developed and used to provide a set of samples over which we had control of the elastic modulus without corresponding changes in the chemical bonds. The modified mechanical properties were tested against the Flory-Rehner theory. Lateral force microscopy was used to measure the force required to translate asymmetric, nanoscale particles of controlled size, surface chemistry and moduli. Silicon wafers were used as the substrate. The effects of work of adhesion, elastic modulus of polystyrene microspheres, and contact radius between particle and substrate have been studied for the different modes of particle translation under an external force.

In the first part of this study, frictional and normal forces in aqueous solutions were measured between a glass particle and oligopeptide films grafted from a glass plate. Homopeptide molecules consisting of 11 monomers of different amino acids were each "grafted from" an oxidized silicon wafer using microwave-assisted solid phase peptide synthesis. Oligopeptides increased the magnitude of friction compared to a bare hydrophilic silicon wafer. Friction was a strong function of the nature of the monomer unit and was lower for hydrophilic films. There was a strong adhesion and therefore friction between surfaces of opposite charges. Changes in adhesion and friction depended on the hydrophobicity and electrostatic forces: hydrophobic films and oppositely charged films produced high friction, whereas hydrophilic and like-charges produced low friction. Friction was lower in phosphate buffered saline than in pure water due to the screening of the double layer attraction for oppositely charged surfaces and additional lubrication by hydrated salt ions. We also investigated antimicrobial action of poly (allyl amine) (PA) when covalently bonded to glass. Glass surfaces were prepared by a two-step procedure where the glass was first functionalized with epoxide groups using 3-glycidoxypropyltrimethoxy silane (GOPTS) and then exposed to PA to bind via reaction of a fraction of its amine groups. Antibacterial properties of these coatings were evaluated by spraying aqueous suspensions of bacteria on the functionalized glass slides, incubating them under agar, and counting the number of surviving cell colonies. The PA film displayed strong anti-microbial activity against both Gram-positive and Gram-negative bacteria. Films that were prepared by allowing the PA to self assemble onto the solid via electrostatic interactions were ineffective antimicrobials. Such films had an insufficient positive charge and did not extend far from the solid. Thus we found that antimicrobial activity was correlated with a combination of the ability of the polymer chain to extend into solution and a positive surface potential. / Ph. D.

An alkyl-containing self-assembled monolayer is grafted on the silicon surface by a nature process in solutions. The alkyl thin film was used as the lubricant for the silica interface, usually applied to the MEMS or NENS domains. The ability of reducing friction for silica device at room temperature was improved, but little was known as the thin films existed at higher temperature during device was working or operating. In this study, we used Hexyltrichlorosilane (C6), Dodecyltrichlorosilane (C12), and Octadecyltrichlorosilane (C18) molecules to form self-assembled monolayers (SAMs) on silicon, and these monolayers exhibited different molecular packing properties due to different interactions between the molecules. Fourier transform infrared spectroscopy (FTIR) revealed that the short chain-length (C6) molecules exhibited poor packing on the surface at room temperature, and that the molecular packing of C6 was thermally stable up to 500 K. But the C12 and C18 monolayers exhibited abrupt blue shifts in FTIR at temperatures between 300 and 575 K, with stable packing observed over several temperature ranges. Furthermore, water contact angle measurements showed the C6, C12, and C18 molecular films changed from hydrophobic to hydrophilic as the sample temperature was increased. Atomic force microscopy (AFM) images revealed that pits had formed in the C18 monolayer after the temperature was increased to 460 K, which were caused by the molecular reorganization of C18 on the surface. This resulted in an abrupt change in the friction coefficient for the C18 monolayer at 460K as compared to the short C6 and C12 monolayers. However, the friction coefficients for all the SAM films still increased with temperature. Understanding the temperature-dependent behavior of SAM film molecules will assist in the design of better anti-wear monolayers to improve performance and increase lifetimes in modern MEMS and NEMS devices.

Orientador: Fernando Galembeck / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-24T05:29:50Z (GMT). No. of bitstreams: 1 Burgo_ThiagoAugustodeLima_D.pdf: 54614783 bytes, checksum: 04e6b01b521de52d69d91a83d891d034 (MD5) Previous issue date: 2013 / Resumo: A triboeletrização de polímeros gera domínios macroscópicos de carga positiva e negativa, verificando a geometria fractal de mosaicos de carga previamente detectados por microscopia de sonda elétrica. Utilizando métodos analíticos adequados (espectroscopia de perda de energia de elétrons, microespectroscopia infravermelho/Raman e carbonização/colorimetria) e cálculos teóricos, espécies positivas foram identificadas como hidrocarbocátions e espécies negativas como fluorocarbânions. A triboeletrização de PTFE com PE é entendida como segue: a ruptura homolítica mecanoquímica da cadeia é seguida por transferência eletrônica dos radicais livres de hidrocarbonetos para os radicais mais eletronegativos do fluorocarbono. Os íons gerados pelos fragmentos de cadeia sofrem auto-ordenamento de acordo com a teoria de Flory- Huggins, formando os padrões macroscópicos observados experimentalmente. Estes resultados mostram que a triboeletrização só pode ser compreendida considerando os eventos químicos complexos provocados por ação mecânica, acoplados a conceitos bem estabelecidos pela físico-química. Além disso, o atrito entre superfícies dielétricas produz padrões de carga fixos e estáveis que contribuem para interações eletrostáticas entre superfícies. Medidas de coeficiente de resistência de rolamento de esferas de vidro sobre PTFE triboeletrizado, bem como de atrito estático de pellets de polietileno e imagens de microscopia de força lateral, mostraram que as forças de atrito aumentam muitas vezes em superfícies triboeletrizadas, nas quais as contribuições das interações eletrostáticas podem suplantar todas outras contribuições. Estes resultados são explicados considerando um mecanismo segundo o qual a indução de carga por uma superfície triboeletrizada no contato com outra superfície dissipa energia durante o atrito, sugerindo novas abordagens para controlar atrito em interfaces pelo controle da formação de tribocargas / Abstract: Tribocharged polymers display macroscopically patterned positive and negative domains, verifying the fractal geometry of electrostatic mosaics previously detected by electric probe microscopy. Using suitable analytical techniques (electron energy-loss spectral imaging, infrared/Raman microspectrophotometry and carbonization/colorimetry) and theoretical calculations, the positive species were identified as hydrocarbocations and the negative species as fluorocarbanions. A comprehensive model is presented for PTFE tribocharging with PE: mechanochemical chain homolytic rupture is followed by electron transfer from hydrocarbon free radicals to the more electronegative fluorocarbon radicals. Polymer ions self-assemble according to Flory-Huggins theory, thus forming the experimentally observed macroscopic patterns. These results show that tribocharging can only be understood by considering the complex chemical events triggered by mechanical action, coupled to well-established physicochemical concepts. Also, friction between dielectric surfaces produces patterns of fixed, stable electric charges that in turn contribute electrostatic components to surface interactions between the contacting solids. The coefficient of rolling resistance of glass beads on tribocharged PTFE was measured, as well as the static friction coefficient of polyethylene pellets on PTFE and friction force on the scanning probe during lateral-force microscopy imaging. Results show that friction coefficients may increase many-fold on tribocharged surfaces and the contribution of electrostatic interactions may supersede all other contributions to friction. These results are explained considering a mechanism according to which charge induction by an electrified surface on a contacting body accounts for energy dissipation during friction suggesting new approaches to control friction coefficients, by controlling tribocharge formation / Doutorado / Físico-Química / Doutor em Ciências

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[pt] Folhelhos desempenham tanto a função de rocha capeadora, camada de rocha de baixa permeabilidade que contém os hidrocarbonetos no reservatório , quanto de rocha geradora de petróleo em sistemas petrolíferos convencionais. Mais recentemente passaram a ser exploradas como reservatórios não convencionais. As suas propriedades mecânicas são importantes na determinação da estabilidade estrutural de poços e na avaliação da fraturabilidade de reservatórios não convencionais. Além disso, permitem a conexão entre os dados de prospecção de petróleo e as características geoquímicas da rocha geradora. Nesta tese, as propriedades nanomecânicas de amostras de folhelho Irati, provenientes da Bacia do Paraná, foram caracterizadas por microscopia de força atômica e nanoindentação em conjunto com microscopia eletrônica de varredura. Procedimentos de processamento de imagens foram desenvolvidos para construir imagens de propriedades tribomecânicas dos minerais e da matéria orgânica presentes na superfície. A identificação mineralógica realizada no microscópio eletrônico permitiu associar o contraste verificado nas imagens obtidas no microscópio de força atômica às propriedades tribomecânicas dos minerais e matéria orgânica. Esses constituintes do folhelho também foram caracterizados por nanoindentação para permitir a medida de duas propriedades mecânicas por um método independente. Por fim, um conjunto de nanoindentações aleatoriamente distribuídas na superfície foi executada em uma amostra da mesma região, sendo o resultado estatisticamente analisado para permitir a comparação com as propriedades macroscópicas. / [en] Shales perform both the cap rock, low permeability rock layer that restrain hydrocarbons in the reservoir, and petroleum source rock in conventional petroleum systems. More recently they have been exploited as unconventional reservoirs. Their mechanical properties are important in determining the structural stability of wells and in evaluating the fracability of unconventional reservoirs. In addition, they allow the connection between oil prospecting data and the geochemical characteristics of the source rock. In this thesis, the nanomechanical properties of Irati shale samples from the Paraná Basin were characterized by atomic force microscopy and nanoindentation together with scanning electron microscopy. Image processing procedures were developed to construct images of tribomechanical properties of minerals and organic matter present on the surface. The mineralogical identification performed by electron microscopes allowed to associate the contrast found in the images obtained with the atomic force microscope to the tribomechanical properties of minerals and organic matter. These shale constituents were also characterized by nanoindentation to allow the measurement of two mechanical properties by an independent method. Finally, a set of randomly distributed nanoindentations on the surface was performed on a sample from the same region, and the result was statistically analyzed to allow comparison with macroscopic properties.

Organic semiconductors show promise to yield a novel class of bendable electronic devices, and much research efforts have focused on the optimization of these films for device performance. It is well known that the structure of organic films has a large influence over the electronic properties. In particular, the carrier mobility is often highly anisotropic, and domain boundaries have a detrimental effect on charge transport. Therefore the domain structure and lattice orientation are of particular interest. However, little is known about the domain structure of organic films, and techniques to study these properties have only begun to emerge in recent years. In this thesis, we apply two experimental techniques, Grazing-Incidence X-ray Diffraction (GIXD) and Lateral Force Microscopy (LFM), toward studying the lattice and domain structure of tetracene films grown on the silicon(001)-monohydride surface. We describe the necessary steps toward optimizing the sensitivity of these techniques to the domain structure. Results show that the crystalline tetracene films form a layered morphology in which the a-b plane lies parallel to the substrate surface. The film lattice structure is similar to bulk tetracene, and the lattice is confined to two orthogonal orientations, forming a partially-commensurate relationship with the substrate surface lattice along the film 'a' axis. LFM images reveal two types of polycrystalline domains. The first type ("major domains") are tens of microns in size, and are classified by their lattice orientation. They are subdivided into the second type ("sub-domains"), which range from 0.1 to 5um in size, and are argued to represent regions of uniform molecular tilt direction. The GIXD data show that the single-crystal domains which comprise these two larger domain types are anisotropic in size, being up to two times longer along the film 'b' axis than along 'a'. The single-crystal domains range from 0.05 to 0.2um in size, depending on lattice orientation and film thickness. The mathematical basis for these single-crystal domain size calculations is presented. The single-crystal domain sizes are thickness-dependent, and are two orders of magnitude smaller than a typical surface island observed in atomic-force microscopy (AFM) topographs. Substrate steps can also significantly influence the film structure by inducing boundaries in the single-crystal domains and sub-domains, but not in the major domains. This detailed knowledge of the domain structure of organic thin-films may assist in our understanding of the factors which affect charge transport in thin films, and may help to direct research efforts in optimizing the film structure for device performance. / Natural Sciences and Engineering Research Council (NSERC), Canadian Foundation for Innovation (CFI), Ontario Innovation Trust (OIT).