Understanding thermal transport across length scales lays the foundation to developing high-performance electronic devices. Although many experiments and models of the past few decades have explored the physics of heat transfer at nanoscale, there are still open questions regarding the impact of periodic nanostructuring and coherent phonon effects, as well as the interaction of strain and thermal transport. Thermomechanical effects, as well as strains applied in flexible electronic devices, impact the thermal transport. In the simplest kinetic theory models, thermal conductivity is proportional to the phonon group velocity, heat capacity, and scattering times. Periodic porous nanostructures impact the phonon dispersion relationship (group velocity) and the boundaries of the pores increase the scattering times. Strain, on the other hand, affects the crystal structure of the lattice and slightly increases the thermal conductivity of the material under compression. Intriguingly, applying strain combined with the periodic porous structures is expected to influence both the dispersion relation and scattering rates and yield the ability to tune thermal transport actively. But often these interrelated effects are simplified in models.<br><br>This work evaluates the combination of structure and strain on thermal conductivity by revisiting some of the essential methods used to predict thermal transport for a single layer of graphene with a periodic porous lattice structure with and without applied strain. First, we use the highest fidelity method of Non-Equilibrium Molecular Dynamics (NEMD) simulations to estimate the thermal conductivity which considers the impact of the lattice structure, strain state, and phononic band structure together. Next, the impact of the geometry of the slots within the lattice is interrogated with Boltzmann Transport Equation (BTE) models under a Relaxation Time Approximation. A Monte Carlo based Boltzmann Transport Equation (BTE) solver is also used to estimate the thermal conductivity of phononic crystals with varying pore geometry. Dispersion relations calculated from continuum mechanics are used as input here. This method which utilizes a simplified pore geometry only partially accounts for the effects of scattering on the pore boundaries. Finally, a continuum level model is also used to predict the thermal conductivity and its variations under applied strain. As acoustic phonon branches tend to carry the most heat within the lattice, these continuum models and other simple kinetic theories only consider their group velocities to estimate their impact on phonon thermal conductivity. As such, they do not take into account the details of phonon transport across all wavelengths.<br><br>By comparing the results from these different methods, each of which has different assumptions and simplifications, the current work aims to understand the effects of changes to the dispersion relationship based on strain and the periodic nanostructures on the thermal conductivity. We evaluate the accuracy of the kinetic theory, ray tracing, and BTE models in comparison to the MD results to offer a perspective of the reliability of each method of thermal conductivity estimation. In addition, the effect of strain on each phononic crystal with different pore geometry is also predicted in terms of change to their in-plane thermal anisotropy values. To summarize, this deeper understanding of the nanoscale thermal transport and the interrelated effects of geometry, strain, and phonon band structure on thermal conductivity can aid in developing lattices specifically designed to achieve the required dynamic thermal response for future nano-scale thermoelectric applications.

<p dir="ltr">Metamaterials are artificially engineered nanoscale systems with a three-dimensional repetitive arrangement of certain components, and present exceptional optical properties for applications in nanophotonics, solar cells, plasmonic devices, and more. Self-assembled oxide-metal vertically aligned nanocomposites (VANs), with metallic phase as nanopillars embedded in the matrix oxide, have been recently proposed as a promising candidate for metamaterial applications. However, precise microstructural control and the structure-property relationships in VANs are still in high demand. Thus, by employing multiple approaches for structural design, this dissertation attempts to investigate the mechanisms of nanostructure evolutions and the corresponding optical responses.</p><p dir="ltr">In this dissertation, the precise control over the nanostructures has been demonstrated through morphology tuning, nanopillar orderings, and strain engineering. Firstly, Au, a well-known plasmonic mediator, has been selected as the metallic phase that forms nanopillars. Based on the previously proposed strain compensation model which describes the basic formation mechanism of VAN morphology, two oxides were then considered: La_0.7Sr_0.3MnO₃(LSMO) and CeO₂. In the first two chapters of this dissertation, LSMO was considered due to its similar lattice (a_LSMO= 3.87 Å, a_Au= 4.08 Å) and its enormous potential in nanoelectronics and spintronics. Deposited on SrTiO₃ (001) substrate through pulsed laser deposition (PLD), LSMO-Au nanocomposites exhibit ideal VAN morphology as well as promising hyperbolic dispersions in response to the incident illuminations. By substrate surface treatment of annealing at 1000°C, and variation of STO substate orientations from (001), to (111) and (110), the improved and tunable in-plan orderings of Au nanopillars have been successfully achieved. In the third chapter, a new oxide-metal VAN system of <a href="" target="_blank">CeO₂</a>-Au (a_CeO2= 5.411 Å, and a_CeO2/= 3.83 Å) has been deposited. The intriguing 45° rotated in-plan epitaxy presents an unexpected update to the strain compensation model, and tuning of Au morphology from nanopillars, nanoantennas, to nanoparticles also shows an effective modulation of the LSPR responses. COMSOL simulations have been exploited to reveal the relationships between Au morphologies and optical responses. In the last chapter, the two VAN systems of LSMO-Au and CeO₂-Au have been combined to form a complex layered VAN thin film. Investigations into the strain states, the nature of complex interfaces, and the according hybrid properties, show dramatic possibilities for further strain engineering. In summary, this dissertation has provided multiple routes for highly tailorable oxide-metal nanocomposite designs. And the two proposed material systems present great potential in optical metamaterial applications including biosensors, photovoltaics, super lenses, and more.</p>

High-temperature superconductors belong to the group of strongly correlated materials. In these compounds, complex repulsive electron interactions and a large number of degrees of freedom lead to a rich variety of states of matter. Exotic phases like the pseudogap, charge-, spin- and pair-density waves, but also the remarkable phenomenon of superconductivity emerge, depending on doping level and temperature. However, up to now it is unclear what exactly causes these states, to what extent they are coexisting or competing, and where their borders in the phase diagram lie. A better understanding could help in finding the mechanism behind high-temperature superconductivity, but would also provide a better insight into the puzzling behavior of strongly correlated materials. This thesis tries to resolve some of these questions with focus on the underdoped pseudogap regime. Mesa structures of bismuth-based cuprate superconductors were studied using intrinsic tunneling, which allows spectroscopic characterizations of electronic density of states inside the material. A micro/nano fabrication method was developed to further reduce mesa areas into the sub square-micrometer range, in order to minimize the effect of crystal defects and measurement artifacts caused by heating induced by the measurement current. The comparison of energy scales in Bi-2201 and Bi-2212 cuprates shows that the pseudogap phenomenon is not connected to superconductivity, but possibly represents a competing spin-singlet order that is universal to all cuprates. The analysis of the upper critical field in Bi-2201 reveals a low anisotropy, which gives evidence of paramagnetically limited superconductivity. Furthermore, a new electrical doping method is demonstrated, which enables the reversible tuning the doping level of Bi-2212 and study a broad doping range upon a single sample. Using this method, two distinct critical points were observed under the superconducting dome in the phase diagram: one at the overdoped side, associated with the onset of the pseudogap and a metal to insulator transition, and one at optimal doping, associated with an enhanced "dressed" electron energy. Finally, a novel angular-dependent magnetotunneling technique is introduced, which allows for the separation of the superconducting and non-superconducting contributions to the pseudogap phenomenon. The method reveals that after an abrupt decay of the energy gap for T→Tc, weak superconducting correlations persist up to several tens of degrees above Tc.

During recent years the demand for nanoscale materials with tailor-made functional properties as bulk species, is continuously and progressively rising for such fields as e.g. micro- and nano-electronics, plasmonics, spintronics, bio-technology, bio-sensing and life sciences. Preserving and / or improving properties of functional materials with their simultaneous size reduction and high-resolution site-specific positioning is indeed very challenging, for both conductors and insulators. One of the advanced nanoprototyping methods that can be utilized for this purpose is the Electron-Beam-Induced Deposition, or shortly EBID. This process is based on a local decomposition by a focused electron beam of a precursor gas molecules adsorbed on the sample’s surface. The beauty of this method is that it gives a unique possibility of rapid creation of site-specific nanoscale 3D structures of precise shape in a single operation. It’s an additive process that can be easily combined with other patterns. However, besides all the benefits, EBID has some constraints, in particular low purity of the deposited materials, due to the organometallic nature of the used precursors. Chemical composition of EBID patterns is strongly dependent on the chosen gas chemistry, the substrate, many deposition parameters and post-treatment processes applied to the deposited structures. In our research we focused on deposition of Co, Au, SiO2, C, W and Pt, their purification and shape control. And this thesis presents an overview of our accomplishments in this field. Depending on the gas chemistry of interest, three major purification approaches of EBID-grown materials were tested out: - Post-deposition annealing: in air and in the controlled atmosphere, - Deposition onto a preheated substrate, - Deposition in the presence of reactive gases. As a result, a dramatic purity improvement was observed and a significant advancement was achieved in creation of high-purity gold, cobalt and silicon dioxide nanoscale structures. In particular: 1)   For the Me2Au(acac) precursor, we developed a nanofabrication routine combining application of wetting buffer layers, fine tuning of EBID parameters and subsequent post-annealing step, which led to formation of high-purity planar and high aspect ratio periodic Au nanopatterns. We also describe the adopted and gently adjusted wet etching method of undesirable buffer layer removal, required in some cases for the further device application. 2)   For the Co2(CO)8 precursor, in-situ seeded growth in conjunction with EBID at the elevated substrate temperature resulted in a deposition of pure nanocrystalline Co with magnetic and transport properties close to the bulk material. 3)   For the tetraethyl orthosilicate precursor, or shortly TEOS, assisting of the deposition process with the additional oxygen supply led to the EBID of carbon-free amorphous insulating Si-oxide, with the absorption and refraction properties comparable to those for fused silica. Several applications of EBID nanopatterns are also discussed. / <p>QC 20131028</p>

No description available.

Biomolecules on cell surfaces play critical roles in diverse biological and physiological processes. However, conventional bulk scale techniques are unable to clarify the density and distribution of specific biomolecules in situ on single, living cell surfaces at the micro or nanoscale. In this work, a single cell analysis technique based on Atomic Force Microscopy (AFM) is developed to spatially identify biomolecules and characterize nanomechanical properties on single cell surfaces. The unique advantage of these AFM-based techniques lies in the ability to operate in situ (in a non-destructive fashion) and in real time, under physiological conditions or controlled micro-environments. First, AFM-based force spectroscopy was developed to study the fundamental biophysics of the heparin/thrombin interaction at the molecular level. Based on force spectroscopy, a force recognition mapping strategy was developed and optimized to spatially detect single protein targets on non-biological surfaces. This platform was then translated to the study of complex living cell surfaces. Specific carbohydrate compositions and changes in their distribution, as well as elasticity change were obtained by monitoring Bacillus cells sporulation process. The AFM-based force mapping technique was applied to different cellular systems to develop a cell surface biomolecule library. Nanoscale imaging combined with carbohydrate mapping was used to evaluate inactivation methods and growth temperatures effects on Yersinia pestis surface. A strategy to image cells in real time was coupled with hydrophobicity mapping technique to monitor the effect of antimicrobials (antimicrobial polymer and copper) on Escherichia coli and study their killing mechanisms. The single spore hydrophobicity mapping was used to localize the exosporium structure and potentially reconstruct culture media. The descriptions of cell surface DNA on single human epithelial cells potentially form a novel tool for forensic identification. Overall, these nanoscale tools to detect and assess changes in cell behavior and function over time, either as a result of natural state changes or when perturbed, will further our understanding of fundamental biological processes and lead to novel, robust methods for the analysis of individual cells. Real time analysis of cells can be used for the development of lab-on-chip type assays for drug design and testing or to test the efficacy of antimicrobials.

This thesis deals with optimization and application of assays for determination of ecotoxicity of nanomaterials based on nanoscale zero-valent iron (nZVI), which are used in remedial technologies. After in situ application of nZVI, a significant decrease in toxicity of polluted environment was detected; however, a potential negative effect of nanoparticles has not been sufficiently investigated yet. Standard used tests were found to be incompatible with nZVI for toxicity determination. Specific characteristics of nZVI, such as high reactivity and sorption, complicate determining the toxicity by routinely used ecotoxicity tests. Concentration ranging from 0,1 to 10 g/l that are used in practise for decontamination were tested. These concentrations resulted in formation of turbidity, which prevented the use of standard tests. In this work, a new method has been optimized for in vitro toxicity testing of nZVI and derived nanomaterials using bacteria. The principle of this assay is determination of oxidative stress (OS). The disbalance between formation and degradation of reactive oxygen species (i.e. OS) leads to irreversible changes in biomolecules of organisms and formation of undesirable products. A toxic and mutagenic product - malondialdehyde (MDA) is formed during lipid peroxidation and it is a...

O fenômeno da capilaridade na escala macroscópica é descrito pela teoria capilar (TC) que se utiliza de superfícies contínuas para modelar as interfaces formadas entre dois meios, sendo um líquido e o outro líquido, gasoso, sólido. A TC é empregada em diversas áreas da biologia, ambientes de microgravidade e em aplicações na escala nanométrica, como no microscópio de força atômica. Essa aproximação por superfícies contínuas pode não ser adequada para sistemas na escala nanométrica, em que são reportados comportamentos anômalos como no preenchimento de líquidos em nanocanais e nanotubos de carbono, oscilações nas medidas de força de adesão capilar e grandes valores de pressões de Laplace negativas. Esses fatos motivam o estudo do fenômeno da capilaridade na escala nanométrica por meio de simulações computacionais. Aqui, utilizamos a dinâmica molecular para estudar a interface de gotas e pontes capilares constituídas de água do modelo SPC/E com volumes da ordem de 100 nanômetros cúbicos e aderidas a placas de cristobalita hidrofóbicas/hidrofílicas. Comparamos as propriedades dessas gotas e pontes capilares com as previsões da TC macroscópica, que são baseadas nos ajustes dos perfis e em cálculos analíticos. Especificamente, confrontamos os perfis das interfaces, os ângulos de contato, as forças de adesão capilar, as pressões de Laplace e o valor da tensão superficial da água. Essas análises foram divididas em três etapas. Na primeira etapa, estudamos as gotas e pontes capilares com simetrias axial e translacional, em que a altura da ponte capilar permaneceu constante. Na segunda etapa, focamos nossos estudos nas pontes capilares com simetria axial (ponte SA) e estudamos o processo de ruptura dessa. Finalmente, na terceira etapa, estudamos as flutuações, que não são previstas pela TC, em sistemas mais simples, como no caso de gotas livres, que não estão aderidas a placas, e em gotas com simetria axial. Mostramos que a TC macroscópica é capaz de explicar satisfatoriamente sistemas com volumes da ordem de 100 nanômetros cúbicos, em que submetemos nossos resultados a comparações rigorosas das soluções analíticas da TC, sendo essa capaz de prever a dependência do ângulo de contato nas alturas das rupturas das ponte SA e os volumes das gotas formadas após a ruptura. / The capillarity phenomenon at macroscopic scale are described by the capillarity theory (CT), which uses continuous surfaces to model the interfaces formed between two media, wherever one medium is liquid and the other can be liquid, gas or solid. The CT is employed in several areas ranging from biology, microgravity environments and applications on the nanoscale, such as in the atomic force microscope. However, the continuous approach may not be adequate for systems at nanoscale, where anomalous behaviors have been reported, such as the filling of liquids in nanochannels and carbon nanotubes, oscillations in measurements of capillary adhesion force and large negative values of Laplace pressures. These facts motivate the study of capillarity phenomenon at the nanometric scale by computational simulations. Here, we use the molecular dynamics to study the droplets and capillary bridges interfaces composed of SPC/E water model and volumes in the order of 100 cubic nanometer, and attached to hydrophobic/hydrophilic cristobalite walls. We have compared the droplets and capillary bridges properties with the macroscopic CT predictions, which are based on profile fitting and analytic calculations. Specifically, we have compared the interface profiles, the contact angles, the capillary adhesion forces, the Laplace pressures and the water surface tension. These analyzes were divided into three steps. In the first step, we have studied droplets and capillary bridges with axial and translational symmetries, where the capillary bridge height remained constant. In the second step, we have focused our studies on capillary bridges with axial symmetry (AS bridge), and we have studied the bridges rupture process. Finally, in the third step, we have studied the fluctuations, which are not predicted by the CT, in simpler systems, such as free droplets, which are not attached to walls, and droplets with axial symmetry. We have shown that the macroscopic CT is able to satisfactorily predict systems with volumes in the order of 100 cubic nanometer, in which we have been submitted our results to rigorous comparisons to the analytic CT solutions, which is able to predict the dependence of contact angle on the AS bridge rupture heights, and the volumes of droplets formed after rupture.

In this thesis, I present a microscopic theory of quantum circuits based on interacting electron spins in quantum dot molecules. We use the Linear Combination of Harmonic Orbitals-Configuration Interaction (LCHO-CI) formalism for microscopic calculations. We then derive effective Hubbard, t-J, and Heisenberg models. These models are used to predict the electronic, spin and transport properties of a triple quantum dot molecule (TQDM) as a function of topology, gate configuration, bias and magnetic field. With these theoretical tools and fully characterized TQDMs, we propose the following applications: 1. Voltage tunable qubit encoded in the chiral states of a half-filled TQDM. We show how to perform single qubit operations by pulsing voltages. We propose the "chirality-to-charge" conversion as the measurement scheme and demonstrate the robustness of the chirality-encoded qubit due to charge fluctuations. We derive an effective qubit-qubit Hamiltonian and demonstrate the two-qubit gate. This provides all the necessary operations for a quantum computer built with chirality-encoded qubits. 2. Berry's phase. We explore the prospect of geometric quantum computing with chirality-encoded qubit. We construct a Herzberg circuit in the voltage space and show the accumulation of Berry's phase. 3. Macroscopic quantum states on a semiconductor chip. We consider a linear chain of TQDMs, each with 4 electrons, obtained by nanostructuring a metallic gate in a field effect transistor. We theoretically show that the low energy spectrum of the chain maps onto that of a spin-1 chain. Hence, we show that macroscopic quantum states, protected by a Haldane gap from the continuum, emerge. In order to minimize decoherence of electron spin qubits, we consider using electron spins in the p orbitals of the valence band (valence holes) as qubits. We develop a theory of valence hole qubit within the 4-band k.p model. We show that static magnetic fields can be used to perform single qubit operations. We also show that the qubit-qubit interactions are sensitive to the geometry of a quantum dot network. For vertical qubit arrays, we predict that there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.

In this thesis, I present a microscopic theory of quantum circuits based on interacting electron spins in quantum dot molecules. We use the Linear Combination of Harmonic Orbitals-Configuration Interaction (LCHO-CI) formalism for microscopic calculations. We then derive effective Hubbard, t-J, and Heisenberg models. These models are used to predict the electronic, spin and transport properties of a triple quantum dot molecule (TQDM) as a function of topology, gate configuration, bias and magnetic field. With these theoretical tools and fully characterized TQDMs, we propose the following applications: 1. Voltage tunable qubit encoded in the chiral states of a half-filled TQDM. We show how to perform single qubit operations by pulsing voltages. We propose the "chirality-to-charge" conversion as the measurement scheme and demonstrate the robustness of the chirality-encoded qubit due to charge fluctuations. We derive an effective qubit-qubit Hamiltonian and demonstrate the two-qubit gate. This provides all the necessary operations for a quantum computer built with chirality-encoded qubits. 2. Berry's phase. We explore the prospect of geometric quantum computing with chirality-encoded qubit. We construct a Herzberg circuit in the voltage space and show the accumulation of Berry's phase. 3. Macroscopic quantum states on a semiconductor chip. We consider a linear chain of TQDMs, each with 4 electrons, obtained by nanostructuring a metallic gate in a field effect transistor. We theoretically show that the low energy spectrum of the chain maps onto that of a spin-1 chain. Hence, we show that macroscopic quantum states, protected by a Haldane gap from the continuum, emerge. In order to minimize decoherence of electron spin qubits, we consider using electron spins in the p orbitals of the valence band (valence holes) as qubits. We develop a theory of valence hole qubit within the 4-band k.p model. We show that static magnetic fields can be used to perform single qubit operations. We also show that the qubit-qubit interactions are sensitive to the geometry of a quantum dot network. For vertical qubit arrays, we predict that there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.