This thesis aims to study external field induced alignment of semiconductor quantum dot by utilizing single molecule spectroscopy. Wurtzite structure semiconductors, such as CdSe, exhibit strong electric dipole moment along its c-axis. It is proposed that quantum dot can be aligned along the applied field with sufficient strength. Experiments with two kind of matrix: PMMA mixing with wax, and liquid crystal thin film, were performed for that quantum dots are able to rotate freely in the matrix. Experiments with PMMA matrix were also performed as its rigid matrix for comparison. Interdigitated structure electrodes was deposited on the cover glass for the electric field experiments. The topical transition (absorption and emission) of CdSe quantum dots has a bright plane perpendicular to its c-axis, and a dark axis along the c-axis. It thus used for characterizing the field alignment. For each observing quantum dot, we record the fluorescence intensity, anti-bunching, polarization anisotropy, and fluorescence lifetime information. In addition, we also analyze the fluorescence correlation spectroscopy to probe the small modulation signal from the fluctuating fluorescence intensity. However, the results indicate that we didn¡¦t observe the field induced change with the field up to 1E7(v/m).

No description available.

It is commonly known that water plays a crucial role in many natural and industrial processes. One of these processes is the formation of water trees, and the subsequent breakdown of polyethylene used for high voltage cable insulation purposes. It has been shown that the mechanism for water molecules diffusing through amorphous polyethylene includes the formation of small water clusters. Gibbs Ensemble Monte Carlo molecular simulations has been performed to study the clustering of vapour phase water under vapour - liquid equilibrium conditions at temperatures ranging from 300 K to 600 K. The increase in vapour density with increasing temperature leads to a radical increase in the fraction of molecules belonging to clusters with two or more water molecules. It is also seen that the size of the clusters increases with temperature. The topologies of the smaller clusters, up to pentamers, have also been studied. A structural transition is observed from a large percentage with cyclic topology, which is the minimum energy configuration, at lower temperatures to predominantly linear clusters, favoured by entropic effects, at higher temperatures. Similar water properties have been obseved in simulations where the vapour phase has been replaced with a hydrocarbon rich phase ( n-alkanes and polyethylene ). Application of an external electric field to the polymer system reduces the water solubility and affects the water structure. A dramatic increase in water solubility in the hydrocarbon phase is observed when two oppositely charged ions are introduced in the hydrocarbon. The structure of the water have changed from several small clusters to a single large cluster with a rod-like shape. The cluster is extremely stable during the simulation. Application of an external electric field may enhance or reduce the effect of the ions depending on the direction of the field. Based on these observations is an alternative mechanism for water tree propagation proposed.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O estudo de propriedades caÃticas de alguns sistemas complexos vem se desenvolvendo amplamente a partir do avanÃo tecnolÃgico proporcionado pelos computadores. Dentre esses sistemas, podemos ressaltar os bilhares, que devido à sua simplicidade e enorme aplicabilidade vem se destacando dentre os demais modelos. Neste trabalho, estudamos as propriedades de alguns bilhares sujeitos à aÃÃo de um campo externo. Aqui, o campo externo à representado por meio de um campo de velocidade que à obtido a partir do escoamento de um fluido. Este fluido exerce uma forÃa de arrasto sobre a partı́cula que encontra-se confinada pela fronteira que define o bilhar. Ao contrÃrio dos outros trabalhos realizados com bilhares, propomos uma abordagem totalmente numÃrica levandoem consideraÃÃo vÃrios fatores que poderiam influenciar na dinÃmica da partı́cula, como por exemplo, a forÃa elÃstica que a partı́cula sofre ao colidir com a fronteira, o regime de escoamento do fluido e o efeito do arraste na trajetÃria da partı́cula, causado por uma forÃa que à proporcinal a uma potÃncia da velocidade na forma F ∝ v γ . A partir desta abordagem, foi possı́vel observar um decaimento na energia da partı́cula, que se deu de forma linear (γ = 1.0), polinomial do segundo grau (γ = 1.5) e exponencial (γ = 2.0). AlÃm disso, quando analisamos o espaÃo de fase dos sistemas (bilhares), o efeito do parÃmetro dissipativo contribuiu para o surgimento de um mar de caos em alguns casos, e ainda, para o surgimento de um atrator, cujo efeito à oriundo tambÃm da aÃÃo do campo externo. / The study of the chaotic properties of some complex systems has been largely developed from the technological advancement provided by computers. Among these systems, we can highlight the billiards, which, due to their simplicity and enormous applicability, they have stood out among the other models. In this work, we study the properties of some billiards, when subject to the action of an external field. Here the external field is represented by a velocity field, wich is obtained from the flow of a fluid. Such fluid exerts a drag force on the particle bounded by the boundary defining the billiard. In contrast to the other works done with billiards, we propose a totally numerical approach, taking into account several factors that could exercise influence on the particle dynamics, such as the elastic force that the particle suffers when it collides with the boundary, the outflow regime of the fluid, and the drag effect on the trajectory of the particle, caused by a force that is proportional to a potential of the velocity in the form F ∝ v γ . From this aproach we could observe a decay on the energy of the particle, which occurred in a linear form (γ= 1.0), polynomial of the second degree (γ = 1.5) and exponential (γ = 2.0). Moreover, as we analyzed the phase space of the systems (billiards), the effect of the dissipation parameter contributed to the emerging of a sea of chaos in some cases, and also to the appearance of an attractor, whose effect also originates from the action of the external field.

No description available.

Ultrasound directed self-assembly (DSA) relies on the acoustic radiation force associated with a standing ultrasound wave to organize particles dispersed in a fluid medium into specific patterns. State-of-the-art ultrasound DSA methods can only organize particles into (quasi-)periodic patterns, limited by the use of single-frequency ultrasound wave fields. Acoustic holography and acoustic waveguides provide alternatives to assembling complex patterns of particles, but generally provide low spatial accuracy and are not re-configurable because they require custom hardware for each specific pattern of particles, which is impractical. We introduce multi-frequency ultrasound wave fields to organize particles in non-periodic patterns. We theoretically derive and experimentally validate a solution methodology to determine the operating parameters (frequency, amplitude, phase) of any number and spatial arrangement of ultrasound transducers, required to assemble spherical particles dispersed in an inviscid fluid medium into any specific two-dimensional pattern. The results show that multi-frequency ultrasound DSA enables the assembly of complex, non-periodic patterns of particles with substantially fewer ultrasound transducers than single-frequency ultrasound DSA, and without incurring a penalty in terms of accuracy. The results of this work fundamentally transform the state-of-the-art knowledge of ultrasound DSA. Multi-frequency ultrasound wave fields enable a near-unlimited complexity of patterns of particles that can be assembled, increasing the relevance of the technology to practical implementation in engineering applications such as manufacturing of engineered composite materials that derive their properties from the spatial organization of the filler in the matrix material. Although this work focuses specifically on ultrasound wave fields, the theoretical model is valid for all wave phenomena. / Master of Science / Ultrasound directed self-assembly (DSA) is the process where particles dispersed in a fluid medium assemble into specific patterns due to their interactions with a sound wave and/or other particles. Current ultrasound DSA methods use a single-frequency ultrasound wave to assemble particles into specific patterns, which creates repeating patterns within the fluid medium. Other methods of assembling particles that allow for more complex, non-repeating patterns generally provide low spatial accuracy and do not allow dynamically changing the pattern as they require custom hardware for each specific pattern of particles, rendering these methods impractical. We use many ultrasound waves each with a different frequency to organize particles into complex, non-repeating patterns, which we call multi-frequency ultrasound DSA. We theoretically derive and experimentally validate a method that allows us to assemble any specific two-dimensional pattern of particles using multi-frequency ultrasound DSA. The results show that multi-frequency ultrasound DSA enables the assembly of complex, non-repeating patterns of particles with substantially resources than single-frequency ultrasound DSA, and without incurring a penalty in terms of accuracy. Multi-frequency ultrasound DSA enables a near-unlimited complexity of patterns of particles that can be assembled, increasing the relevance of the technology to practical implementation in engineering applications.

Estudamos o diagrama de fases para uma classe de modelos de Ising ferromagnéticos em $ \\mathbb^2 $, com campo magnético externo periódico. O campo externo assume dois valores: $ h $ e $ -h $, onde $ h> 0 $. Os sítios associados a valores positivos e negativos do campo externo, formam uma configuração em forma de tabuleiro de xadrez (nós chamamos de {\\it cell-board configuration}), com células retangulares de tamanho $ L_1 \\times L_2 $ sítios, de tal forma que o valor total do campo externo é zero. Como principal resultado, mostramos a presença de uma transição de fase de primeira ordem. A transição de fase existe para $ h <\\frac + \\frac $, onde $ J $ é uma constante de interação. A prova é construida usando o método de {\\it reflection positivity (RP)}. Aplicamos uma desigualdade que é normalmente referida como a estimativa de {\\it chessboard}. Além disso, incluímos uma região de unicidade da medida de Gibbs em $h>4J$, isto usando um critério baseado nas ideias de percolação em desacordo. / We study the low-temperature phase diagram for a ferromagnetic Ising model on $\\mathbb^2$, with a periodical external magnetic field. The external field takes two values: $h$ and $-h$, where $h>0$. The sites associated with positive and negative values of external field form a cell-board configuration with rectangular cells of sides $L_1\\times L_2$ sites, such that the total value of the external field is zero. As a main result, we show the presence of a first-order phase transition. The phase transition holds if $h<\\frac+ \\frac$, where $J$ is an interaction constant. We use the reflection positivity (RP) method. We apply a key inequality which is usually referred to as the chessboard estimate. Furthermore, we prove uniqueness for Gibbs measure in $h>4J$, using a uniqueness condition obtained in terms of disagreement percolation.

Estudamos o diagrama de fases para uma classe de modelos de Ising ferromagnéticos em $ \\mathbb^2 $, com campo magnético externo periódico. O campo externo assume dois valores: $ h $ e $ -h $, onde $ h> 0 $. Os sítios associados a valores positivos e negativos do campo externo, formam uma configuração em forma de tabuleiro de xadrez (nós chamamos de {\\it cell-board configuration}), com células retangulares de tamanho $ L_1 \\times L_2 $ sítios, de tal forma que o valor total do campo externo é zero. Como principal resultado, mostramos a presença de uma transição de fase de primeira ordem. A transição de fase existe para $ h <\\frac + \\frac $, onde $ J $ é uma constante de interação. A prova é construida usando o método de {\\it reflection positivity (RP)}. Aplicamos uma desigualdade que é normalmente referida como a estimativa de {\\it chessboard}. Além disso, incluímos uma região de unicidade da medida de Gibbs em $h>4J$, isto usando um critério baseado nas ideias de percolação em desacordo. / We study the low-temperature phase diagram for a ferromagnetic Ising model on $\\mathbb^2$, with a periodical external magnetic field. The external field takes two values: $h$ and $-h$, where $h>0$. The sites associated with positive and negative values of external field form a cell-board configuration with rectangular cells of sides $L_1\\times L_2$ sites, such that the total value of the external field is zero. As a main result, we show the presence of a first-order phase transition. The phase transition holds if $h<\\frac+ \\frac$, where $J$ is an interaction constant. We use the reflection positivity (RP) method. We apply a key inequality which is usually referred to as the chessboard estimate. Furthermore, we prove uniqueness for Gibbs measure in $h>4J$, using a uniqueness condition obtained in terms of disagreement percolation.