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Filmes de Langmuir de fosfolipídios e ácidos graxos foram caracterizados através de medidas de pressão e potencial de superfície e condutância lateral. O objetivo principal foi o de se fazer um estudo crítico de artigos apresentados na literatura, os quais trazem interpretações errôneas para resultados do potencial de superfície, sugerem a heterogeneidade dos filmes, ou a inexistência do aumento da condutância devido a compressão de um filme de Langmuir. Comprovamos, inicialmente, a homogeneidade macroscópica dos filmes de ácidos graxos e fosfolipídios, medindo-se o potencial com a prova de Kelvin em três posições diferentes. Nenhuma alteração significativa foi registrada. O potencial e a pressão de superfície são nulos para grandes áreas por molécula, sendo que o potencial só se torna não nulo após um aumento abrupto, que ocorre a uma certa área critica. Estes resultados, juntamente com a ausência de histerese nas medidas de pressão e potencial de superfície nos ciclos de compressão-expansão, denotam a inexistência de agregados de dimensões macroscópicas. Eles contradizem, entretanto, a asserção de alguns pesquisadores na ultima conferencia de filmes Langmuir-Blodgett, de que a presença de domínios microscópicos nos filmes de Langmuir deveria afetar os resultados do potencial de superfície. Podemos concluir, portanto, que quaisquer irreprodutibilidades do potencial de superfície para grandes áreas por molécula devem ser atribuídas a impurezas do sistema de medidas, principalmente da subfase. As medidas de condutância foram realizadas diretamente, medindo-se a corrente entre dois eletrodos de platina parcialmente imersos na subfase. Para grandes áreas por molécula, a condutância medida é devido a condutividade de volume da água ultrapura, mas quando o filme e comprimido para áreas menores do que uma certa área critica (a mesma na qual e observado o aumento abrupto do potencial), a condutância sobe ate a pressão de superfície começar a subir, atingindo valores da ordem de 10-8s acima da condutância da subfase. Foram realizadas medidas para um fosfolipídio e um fosfato. Com isso, corroboramos os resultados de condutância, da mesma ordem de grandeza, obtidos em Bangor, no Reino Unido, ao mesmo tempo que são refutadas as observações feitas por grupos de pesquisa dos Estados Unidos e da Rússia. As possíveis causas para a não observação da condutância por esses últimos grupos podem ser o efeito do menisco - queda da área imersa dos eletrodos quando do aumento da pressão de superfície - que mascara o aumento da condutância, ou artefatos experimentais, tais como impurezas na subfase / Langmuir films from phospholipids and fatty acids were characterized using surface pressure, surface potential and lateral conductance measurements. The main aim of this project was to undertake a critical study of articles published in the literature which present erroneous interpretations for surface potential results, suggest non-homogeneity of Langmuir films, or rule out the possible increase in conductance upon the compression of a monolayer. First, we demonstrate that phospholipids and fatty acid mono layers are homogeneous at the macroscopic level, by measuring their surface potential with the Kelvin probe on three different positions. No significant difference was observed. The surface potential and surface pressure is zero for large areas per molecule, and the surface potential only becomes non-zero when an abrupt increase takes place at a given critical area. Together with the absence of hysteresis in the surface pressure and surface potential measurements in the compression-expansion cycles, these results indicate the absence of aggregates (or islands) of macroscopic dimensions. They contradict, however, the opinion of some authors at the last Conference on Langmuir-Blodgett Films, who took the view that the presence of micro domains on Langmuir monolayers should affect the surface potential measurements. We can conclude, therefore, that any non-reproducibility of surface potentials at large areas per molecule must be attributed to impurities in the system, especially in the sub-phase. The lateral conductance measurements were performed by measuring the current between two platinum electrodes, which were partially immersed into the sub-phase. For large areas per molecule, the conductance is due to the bulk conductivity of ultra pure water, but when the monolayer was compressed below a given critical area (the same in which a sharp increase in surface potential was observed), the conductance increases until the surface pressure starts to rise, reaching values of the order of 10-8s above the water conductance. Measurements were taken for one phospholipids and one phosphate. This corroborates the results published by the Bangor Group (UK), at the same time that observations made by an American Group and a Russian Group is contradicted. The possible causes for these groups being unable to observe the increase in conductance can be the meniscus effect caused by the decrease of the electrodes immersed area when the pressure rises - which overrun the conductance increase, and/or experimental artifacts, possibly caused by impurities in the sub-phase

Filmes de Langmuir de fosfolipídios e ácidos graxos foram caracterizados através de medidas de pressão e potencial de superfície e condutância lateral. O objetivo principal foi o de se fazer um estudo crítico de artigos apresentados na literatura, os quais trazem interpretações errôneas para resultados do potencial de superfície, sugerem a heterogeneidade dos filmes, ou a inexistência do aumento da condutância devido a compressão de um filme de Langmuir. Comprovamos, inicialmente, a homogeneidade macroscópica dos filmes de ácidos graxos e fosfolipídios, medindo-se o potencial com a prova de Kelvin em três posições diferentes. Nenhuma alteração significativa foi registrada. O potencial e a pressão de superfície são nulos para grandes áreas por molécula, sendo que o potencial só se torna não nulo após um aumento abrupto, que ocorre a uma certa área critica. Estes resultados, juntamente com a ausência de histerese nas medidas de pressão e potencial de superfície nos ciclos de compressão-expansão, denotam a inexistência de agregados de dimensões macroscópicas. Eles contradizem, entretanto, a asserção de alguns pesquisadores na ultima conferencia de filmes Langmuir-Blodgett, de que a presença de domínios microscópicos nos filmes de Langmuir deveria afetar os resultados do potencial de superfície. Podemos concluir, portanto, que quaisquer irreprodutibilidades do potencial de superfície para grandes áreas por molécula devem ser atribuídas a impurezas do sistema de medidas, principalmente da subfase. As medidas de condutância foram realizadas diretamente, medindo-se a corrente entre dois eletrodos de platina parcialmente imersos na subfase. Para grandes áreas por molécula, a condutância medida é devido a condutividade de volume da água ultrapura, mas quando o filme e comprimido para áreas menores do que uma certa área critica (a mesma na qual e observado o aumento abrupto do potencial), a condutância sobe ate a pressão de superfície começar a subir, atingindo valores da ordem de 10-8s acima da condutância da subfase. Foram realizadas medidas para um fosfolipídio e um fosfato. Com isso, corroboramos os resultados de condutância, da mesma ordem de grandeza, obtidos em Bangor, no Reino Unido, ao mesmo tempo que são refutadas as observações feitas por grupos de pesquisa dos Estados Unidos e da Rússia. As possíveis causas para a não observação da condutância por esses últimos grupos podem ser o efeito do menisco - queda da área imersa dos eletrodos quando do aumento da pressão de superfície - que mascara o aumento da condutância, ou artefatos experimentais, tais como impurezas na subfase / Langmuir films from phospholipids and fatty acids were characterized using surface pressure, surface potential and lateral conductance measurements. The main aim of this project was to undertake a critical study of articles published in the literature which present erroneous interpretations for surface potential results, suggest non-homogeneity of Langmuir films, or rule out the possible increase in conductance upon the compression of a monolayer. First, we demonstrate that phospholipids and fatty acid mono layers are homogeneous at the macroscopic level, by measuring their surface potential with the Kelvin probe on three different positions. No significant difference was observed. The surface potential and surface pressure is zero for large areas per molecule, and the surface potential only becomes non-zero when an abrupt increase takes place at a given critical area. Together with the absence of hysteresis in the surface pressure and surface potential measurements in the compression-expansion cycles, these results indicate the absence of aggregates (or islands) of macroscopic dimensions. They contradict, however, the opinion of some authors at the last Conference on Langmuir-Blodgett Films, who took the view that the presence of micro domains on Langmuir monolayers should affect the surface potential measurements. We can conclude, therefore, that any non-reproducibility of surface potentials at large areas per molecule must be attributed to impurities in the system, especially in the sub-phase. The lateral conductance measurements were performed by measuring the current between two platinum electrodes, which were partially immersed into the sub-phase. For large areas per molecule, the conductance is due to the bulk conductivity of ultra pure water, but when the monolayer was compressed below a given critical area (the same in which a sharp increase in surface potential was observed), the conductance increases until the surface pressure starts to rise, reaching values of the order of 10-8s above the water conductance. Measurements were taken for one phospholipids and one phosphate. This corroborates the results published by the Bangor Group (UK), at the same time that observations made by an American Group and a Russian Group is contradicted. The possible causes for these groups being unable to observe the increase in conductance can be the meniscus effect caused by the decrease of the electrodes immersed area when the pressure rises - which overrun the conductance increase, and/or experimental artifacts, possibly caused by impurities in the sub-phase

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This dissertation describes new measurements of the van der Waals (vdW) potential energy for atoms near a surface. The measurements presented here were accomplished by studying diffraction a beam of atoms transmitted through a nanograting. I will describe how we improved precision by a factor of 10 over previous diffraction measurements by studying how different types of atoms interact with the same surface. As a result of this new precision, we were able to show for the first time the contribution of atomic core electrons to the atom-surface potential, and experimentally test different atomic structure calculation methods.In addition, this dissertation will describe how changing the width of the grating bars to achieve a particular "magic" grating bar width or rotating a grating to a particular "magic" angle allows us to determine both the atom-surface potential strength and the geometry of the grating. This represents an improvement over several recent studies where uncertainties in the nanograting geometry limited precision in the measurements of the vdW potential.For a complementary measurement, also discussed in this dissertation, we collaborated with the Vigue group in Toulouse, France. In this collaboration we used an atom interferometer to measure the phase shift due to transmission through a nanograting. By combining diffraction data from Tucson with interferometry data from Toulouse we improved the precision of interferometry measurements of the atom-surface potential of a single atomic species by almost a factor of 10 over previous interferometric measurements of the vdW potential. These interferometry measurements also serve to measure the shape of the vdW potential and set a limit on non-Newtonian gravitational interactions at 1-2 nm length scales.Finally, this dissertation will discuss how nanogratings with optimized geometry can improve atom interferometers, for example, with blazed gratings. We discuss next generation atom-surface potential measurements and examine new ways of analyzing diffraction data.

Because the surface plays an important role in the electrical and optical properties of GaN devices, an improved understanding of surface effects should help optimize device performance. In this work, atomic force microscopy (AFM) and related techniques have been used to characterize three unique sets of n-type GaN samples. The sample sets comprised freestanding bulk GaN with Ga polar and N polar surfaces, epitaxial GaN films with laterally patterned Ga- and N-polar regions on a common surface, and truncated, hexagonal GaN microstructures containing Ga-polar mesas and semipolar facets. Morphology studies revealed that bulk Ga-polar surfaces treated with a chemical-mechanical polish (CMP) were the flattest of the entire set, with rms values of only 0.4 nm. Conducting AFM (CAFM) indicated unexpected insulating behavior for N-polar GaN bulk samples, but showed expected forward and reverse-bias conduction for periodically patterned GaN samples. Using scanning Kelvin probe microscopy, these same patterned samples demonstrated surface potential differences between the two polarities of up to 0.5 eV, where N-polar showed the expected higher surface potential. An HCl cleaning procedure used to remove the surface oxide decreased this difference between the two regions by 0.2 eV. It is possible to locally inject surface charge and measure the resulting change in surface potential using CAFM in conjunction with SKPM. After injecting electrons using a 10 V applied voltage between sample and tip, the patterned polarity samples reveal that the N-polar regions become significantly more negatively charged as compared to Ga-polar regions, with up to a 2 eV difference between charged and uncharged N polar regions. This result suggests that the N-polar regions have a thicker surface oxide that effectively stores charge. Removal of this oxide layer using HCl results in significantly decreased surface charging behavior. A phenomenological model was then developed to fit the discharging behavior of N-polar GaN with good agreement to experimental data. Surface photovoltage (SPV) measurements obtained using SKPM further support the presence of a thicker surface oxide for N polar GaN based on steady state and restoration SPV behaviors. Scanning probe microscopy techniques have therefore been used to effectively discriminate between the surface morphological and electrical behaviors of Ga- vs. N-polar GaN.