<p> Viable methods for bacterial biofilm remediation require a fundamental understanding of biofilm mechanical properties and their dependence on dynamic environmental conditions. Mechanical test data, quantifying elasticity or adhesion, may be used to perform physical modeling of biofilm behavior, thus enabling the development of novel remediation strategies. To achieve real-time, dynamic measurements of these properties, a novel analysis platform consisting of a microfluidic flowcell device has been designed and fabricated for <i> in situ</i> analysis using atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). The flowcell consists of microfluidic channels for biofilm establishment that are then converted into an open architecture, laminar flow channel for AFM measurement in a liquid environment. Computational fluid dynamics (CFD) was used to profile fluid conditions within the device during biofilm establishment. The validity of the AFM nanoindentation measurement mechanism was confirmed in the context of the system through the elastic characterization of several non-living reference materials. Force-mode AFM was used to measure the elastic properties of mature <i>Pseudomonas aeruginosa</i> PAO1 biofilms and observe a dynamic response to a chemical antagonist. Elastic moduli ranging from 0.58 to 2.61 kPa were determined for the mature biofilm, which fall within the range of moduli previously reported by optical, rheometric, and microindentation techniques. A modified version of the flowcell was employed to perform similar elastic characterization of mouse submandibular glands (SMGs), demonstrating the adaptability of the system to perform <i>ex situ</i> analyses of a broader set of biological materials. These results demonstrate the validity of the microfluidic flowcell system as an effective platform for future investigations of the mechanical and morphological response of biofilms and other soft biomaterials to dynamic environmental conditions. </p>

<p> In the past decades, nanostructured materials have opened new and fascinating avenues for research. Nanopolycrystalline solids, which consist of nano-sized crystalline grains and significant volume fractions of amorphous grain boundaries, are believed to have substantially different response to the thermal-mechanical-electric-magnetic loads, as compared to the response of single-crystalline materials. Nanopolycrystalline materials are expected to play a key role in the next generation of smart materials. </p><p> This research presents a framework (1) to generate full atomistic models, (2) to perform non-equilibrium molecular dynamics simulations, and (3) to study multi-physics phenomena of nanopolycrystalline solids. This work starts the physical model and mathematical representation with the framework of molecular dynamics. In addition to the latest theories and techniques of molecular dynamics simulations, this work implemented principle of objectivity and incorporates multi-physics features. Further, a database of empirical interatomic potentials is established and the combination scheme for potentials is revisited, which enables investigation of a broad spectrum of chemical elements (as in periodic table) and compounds (such as rocksalt, perovskite, wurtzite, diamond, etc.). The configurational model of nanopolycrystalline solids consists of two spatial components: (1) crystalline grains, which can be obtained through crystal structure optimization, and (2) amorphous grain boundaries, which can be obtained through amorphization process. Therefore, multi-grain multi-phase nanopolycrystalline material system can be constructed by partitioning the space for grains, followed by filling the inter-grain space with amorphous grain boundaries. </p><p> Computational simulations are performed on several representative crystalline materials and their mixture, such as rocksalt, perovskite and diamond. Problems of relaxation, mechanical loading, thermal stability, heat conduction, electrical field response, magnetic field response are studied. The simulation results of the mechanical, thermal, electrical and magnetic properties are expected to facilitate the rational design and application of nanostructured materials. </p>

<p> This dissertation is a compilation of research results of extensive Fast Scanning Calorimetry studies of two non-crystalline materials: Toluene and Water. </p><p> Motivation for fundamental studies of non-crystalline phases, a brief overview of glassy materials and concepts and definitions related to them is provided in Chapter 1. Chapter 2 provides fundamentals and details of experimental apparata, experimental protocol and calibration procedure. </p><p> Chapter 3 &amp; 4 provides extensive studies of stable non-crystalline toluene films of micrometer and nanometer thicknesses grown by vapor deposition at distinct deposition rates and temperatures and probed by Fast Scanning Calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor -deposited samples of toluene during heating with rates in excess 100,000 K/s follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics correlates with the surface roughness scale of the substrate, which is interpreted as evidence for kinetic anisotropy of the samples. We also show that out-of-equilibrium relaxation kinetics and possibly the enthalpy of vapor-deposited (VD) films of toluene are distinct from those of ordinary supercooled (OS) phase even when the deposition takes place at temperatures above the glass softening (Tg). The implications of these findings for the formation mechanism and structure of vapor deposited stable glasses are discussed. </p><p> Chapter 5 and 6 provide detailed Fast Scanning Calorimetry studies of amorphous solid water in bulk and confining geometry (ultrathin films and nano-aggregates). Bulk-like water samples were prepared by vapor-deposition on the surface of a tungsten filament near 140 K where vapor-deposition results in low enthalpy glassy water films. The vapor deposition approach was also used to grow nano-aggregates (2- 20 nm thick) and multiple ultrathin (approximately 50 nm thick) water films alternated with benzene and methanoic films of similar dimensions. When heated from cryogenic temperatures, the ultrathin water films underwent a well manifested glass softening transition at temperatures 20 degrees below the onset of crystallization. The thermograms of nano-aggregates of ASW films show two endotherms at 40 and 10 K below the onset temperatures of crystallization. However, no such transition was observed in bulk-like water samples prior to their crystallization. These results indicate that water in confined geometry demonstrates glass softening dynamics which are dramatically distinct from those of the bulk phase. We attribute these differences to water's interfacial glass transition which occurs at temperatures tens of degrees lower than that in the bulk. Implications of these finding for past studies of glass softening dynamics in various glassy water samples are discussed in chapter 5 and 6.</p>

<p> Collective magnetic behavior in nanostructures is a phenomenon commonly observed in various magnetic systems. It arises due to competing inter/intra&ndashparticle; interactions and size distribution and can manifest in phenomena like magnetic freezing, magnetic aging, and exchange bias (EB) effect. In order to probe these rather complex phenomena, conventional DC and AC magnetic measurements have been performed along with radio&ndashfrequency; transverse susceptibility (TS) measurements. We also demonstrate the magnetic entropy change as a parameter sensitive to subtle changes in the magnetization dynamics of nanostructures. The focus of this dissertation is to study the collective magnetic behavior in core-shell nanostructures of Fe/&gamma;&ndashFe;₂O₃ and Co/CoO, La_0.5Sr_0.5MnO₃ nanowires, and LaMnO₃ nanoparticles.</p><p> In the case of core/shell Fe/&gamma;&ndashFe;₂O₃, we found the particles to critically slow down below the glass transition temperature, below which they exhibit aging effects associated with a superspin glass (SSG) state. We demonstrate that it is possible to identify individual magnetic responses of the Fe core and the &gamma;&ndashFe;₂O₃ shell. Consistently, a systematic study of the magnetocaloric effect (MCE) in the Fe/&gamma;&ndashFe;₂O₃ system reveals the development of inverse MCE with peaks associated with the individual magnetic freezing of the core and the shell. From these obtained results, we establish a general criterion for EB to develop in core/shell nanostructures, that is when the core is in the frozen state and the magnetic moments in the shell begin to block. This criterion is shown to be valid for both ferromagnetic/ferrimagnetic (FM/FIM) Fe/&gamma;&ndashFe;₂O₃ and ferromagnetic/antiferromagnetic (FM/AFM) Co/CoO core&ndashshell; nanostructures. We also elucidate the physical origin of the occurrence of asymmetry in field-cooled hysteresis loops and its dependence on magnetic anisotropy in the Co/CoO system by performing a detailed TS study.</p><p> We have performed a detailed magnetic study on hydrothermally synthesized single crystalline La_0.5Sr_0.5MnO₃ nanowires. The temperature and field dependent evolution of the different magnetic phases leading to development of the inverse MCE and EB in the nanowires is discussed. Finally, we have studied the collective magnetic behavior of LaMnO₃ nanoparticles synthesized by the sol&ndashgel; technique. The nanoparticle ensemble shows the unusual co&ndashexistence; of super-ferromagnetism (SFM), as well as the SSG state, which we term the &lsquoferromagnetic; superglass&rsquo; (FSG) state. The existence of FSG and the characteristics of its magnetic ground state are discussed.</p>

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2008. / Committee Chair: Landman, Uzi; Committee Member: Chou, Mei-Yin; Committee Member: Gao, Jianping; Committee Member: Glezer, Ari; Committee Member: Luedtke, W. D.

Thesis (Ph. D.)--Duke University, 2007. / Includes bibliographical references.

Thesis (Ph.D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Zhong lin Wang; Committee Member: Christopher J. Summers; Committee Member: Kenneth A. Gall; Committee Member: Robert L. Snyder; Committee Member: Russell D. Dupuis.

Thesis (Ph. D.)--Drexel University, 2007. / Includes abstract and vita. Includes bibliographical references (leaves 101-108).

Le sujet de cette thèse est l'observation de parois de domaines ferromagnétiques dans des nanofils cylindriques, ainsi que leur dynamique sous champs magnétiques appliqués. Ces nanostructures ont été électrodéposées par mes collègues dans des membranes d'alumine nanoporeuse servant de gabarits à géométrie adaptable. Les matériaux sont des alliages magnétiques doux de FeNi ou CoNi ; les diamètres vont de 150 nm à 250-300 nm, avec une longueur typique de 30 µm.Mon travail a d'abord englobé des développements instrumentaux de porte-échantillons et d'électronique haute fréquence visant au déplacement de parois sous champ. J'ai étudié cette dernière grâce au dichroïsme magnétique circulaire des rayons X couplé à la microscopie électronique de photoémission (XMCD-PEEM), en géométrie dite de transmission ou d'ombre. Cette technique synchrotron permet le suivi de la configuration interne de paroi avant et après déplacement ; en raison de la forte reproductibilité requise par le XMCD-PEEM résolu en temps, la dynamique en temps réel est pour le moment inaccessible.La réponse des parois de domaines ferromagnétiques à un champ magnétique est notoirement caractérisée par leur mobilité, c'est-à-dire le rapport de la vitesse atteinte sur le champ. Dans les nanofils cylindriques, un ingrédient nouveau apparaît dans le cas d'un type de paroi absent dans les bandes plates : la paroi à point de Bloch (Bloch point wall, BPW). Non seulement cette paroi comporte une singularité micromagnétique, c'est-à-dire un point où l'aimantation disparaît (le point de Bloch), mais elle possède également un degré de liberté discret représentant le sens d'enroulement d'aimantation autour de l'axe du fil. Il a été prédit que le déplacement de BPW sous champ suffisamment intense résulte en la sélection de l'un des deux seuls sens possibles. En d'autres termes, un des deux enroulements devient instable. Dans cette thèse, je rapporte l'observation expérimentale de cette sélection dans une majorité de déplacements de BPW.Il n'a pas été possible de mener des mesures de mobilité, néanmoins, mes expériences ont mis en évidence des transformations jusqu'ici non prévues en simulation entre types de parois. La BPW contenant un défaut topologique (le point de Bloch lui-même), ce comportement inattendu remet en question la protection topologique parfois attribuée aux textures topologiquement non-triviales. Bien que rappelant la conversion entre parois transverse et vortex dans les bandes, ces transformations dans les nanofils cylindriques impliquent des configurations micromagnétiques topologiquement non-équivalentes, par contraste avec les parois des bandes sus-mentionnées. De plus, la toute relative stabilité observée des types de parois suggère la prudence dans l'interprétation de future mesures de mobilité dans de tels systèmes dès lors que la configuration interne de paroi n'est pas résolue.En-dehors de tels échantillons électrodéposés, j'ai également étudié un nanofil cœur-coquille crû verticalement par des collègues. Cette nanostructure cylindrique créée par dépôt induit par faisceau d'électron focalisé (FEBID) possèdait un cœur nanocristallin de cobalt et une coquille de platine. Sa configuration magnétique a été également étudiée par XMCD-PEEM en transmission. Contrairement au fils précédemment mentionnés et posés sur leur substrat sur toute leur longueur, cet échantillon cœur-coquille était vertical et sans modulations de diamètre. En revanche, la géométrie coudée du fil a été conçue pour favoriser le piégeage de parois. Dans cette configuration innovante d'imagerie, le défi a été de remonter autant que possible à l'état magnétique du fil ; il m'a été possible de démontrer la présence d'au moins une paroi de domaine. / The thesis is concerned with the observation of ferromagnetic domain walls in cylindrical nanowires, as well as their dynamics under applied magnetic fields. These nanostructures were electrodeposited by colleagues of mine into nanoporous alumina templates with a tailored pore geometry. The materials are soft FeNi or CoNi alloys; the diameters range from 150 nm to 250-300 nm, with a typical length of 30 µm.My work first comprised experimental developments of sample holders and high-frequency electronics towards field-induced domain wall motion. The latter I investigated with X-ray Magnetic Circular Dichroism coupled to transmission PhotoEmission Electron Microscopy (XMCD-PEEM). This synchrotron-based technique allows to monitor the internal domain wall configuration before and after displacement; due to the stringent requirements of time-resolved XMCD-PEEM experiments in terms of reproducibility, the real-time dynamics is out of reach as of yet.The response of ferromagnetic domain walls to applied magnetic fields is notably characterized by their mobility, i.e. the ratio of attained velocity to field strength. In cylindrical nanowires, a novel ingredient emerges in the case of one domain wall type that is absent in flat strips: the Bloch point domain wall. Not only does this domain wall host a micromagnetic singularity, that is to say a point where magnetization vanishes (the Bloch point), but it also possesses a discrete degree of freedom representing the sense of magnetization winding around the nanowire axis. It has been predicted that Bloch point wall motion under sufficiently high fields leads to this degree of freedom selecting one of its only two possible values. In other words, one winding becomes unstable. I report in this thesis experimental evidence of such a selection in a majority of Bloch point wall motion events.Although mobility measurements could not be carried out, my experiments have furthermore evidenced transformations between domain wall types that had not been predicted in simulations. Since the Bloch point wall contains a topological defect (the Bloch point itself), this unexpected behaviour questions the sometimes argued protection attributed to topologically non-trivial textures. While reminiscent of the well-known conversion between transverse and vortex walls in strips, these transformations in cylindrical nanowires involve topologically non-equivalent micromagnetic configurations, in contrast with the aforementioned transverse and vortex walls. Moreover, the observed only relative stability of domain wall types suggests caution in the interpretation of future mobility measurements in such systems, if the internal wall configuration cannot be resolved.Aside from such electrodeposited samples, I have also studied an upright core-shell nanowire grown by colleagues with Focused-Electron-Beam-Induced Deposition. This nanostructure featured a nanocrystalline cobalt core and a platinum shell. Its magnetic configuration was investigated with transmission XMCD-PEEM as well. Contrary to the aforementioned horizontally-lying wires, the core-shell sample was vertical with no diameter modulations. On the other hand, the geometry featured bends engineered to favour domain wall pinning. In this novel imaging configuration, the challenge was to recover as much of the nanowire's magnetic state as possible. I was able to demonstrate the presence of at least one domain wall.

SENA, Silvia Helena Roberto de. Transporte e estados confinados de portadores de carga em nanoestruturas de grafeno. 2010. 63 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-10-15T18:18:41Z No. of bitstreams: 1 2010_dis_shrsena.pdf: 25739738 bytes, checksum: 888d0934048fb00b5f59e71f4745d983 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-10-21T20:30:44Z (GMT) No. of bitstreams: 1 2010_dis_shrsena.pdf: 25739738 bytes, checksum: 888d0934048fb00b5f59e71f4745d983 (MD5) / Made available in DSpace on 2015-10-21T20:30:44Z (GMT). No. of bitstreams: 1 2010_dis_shrsena.pdf: 25739738 bytes, checksum: 888d0934048fb00b5f59e71f4745d983 (MD5) Previous issue date: 2010 / In this work we investigate the behavior of charge carriers in a graphene sheet subjected to a one-dimensional electrostatic potential U(x). At first we consider two coupled quantum wells potential. For this structure we calculate the energy spectrum for the bound states, as well as the probability density behavior for some particular states. Some results for the electronic transmission coefficient through these structures are also presented. Next, we consider the electrostatic potential as a mutibarrier structure and then a correlated disorder was introduced in the barrier width. For this system, the transmission of these carriers through this potential as well as the conductance are investigated for different disorder strengths. Finally a quasiperiodic supperlattice that follows the Fibonacci serie was taken into account and the results for transmission were also presented for this structure. / Nesse trabalho, investigamos o comportamento dos portadores de carga em uma folha de grafeno quando a mesma encontra-se sujeita à influência de um potencial eletrostático unidimensional U(x). Primeiramente, consideramos dois poços de potencial acoplados por uma barreira central. Para essa estrutura, calculamos o espectro de energia dos estados confinados bem como o comportamento da densidade de probabilidade para alguns estados específicos. Apresentamos também, alguns resultados para o coeficiente de transmissão eletrônico através dessas estruturas. Em seguida, consideramos uma super-rede formada por múltiplas barreiras de potencial e introduzimos uma desordem correlacionada na largura das barreiras. Para esse sistema, resultados para a transmissão dos portadores através dessa estrutura bem como a condutância para vários valores de desordem são apresentados. Finalmente uma super-rede quase-periódica que segue a série de Fibonacci foi considerada e resultados para a transmissão através dessa estrutura também foram apresentados.