<p>Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the easurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical diversity and ambient temperature low cost manufacturing.</p><p> In this thesis we investigate the switching and conducting properties of photochromic dithienylethene derivatives. Such molecules change their conformation in solution when acted upon by light. Photochromic molecules are attractive candidates for use in molecular electronics because of the switching between different states with different conducting properties. The possibility of optically controlling the conductance of the molecule attached to leads may lead to new device implementations.</p><p> The switching reaction is investigated with potential energy calculations for different values of the reaction coordinate between the closed and the open isomer. The electronic and atomic structure calculations are performed with density functional theory (DFT). It is concluded that there is a large potential energy barrier separating the open and closed isomer and that switching between open and closed forms must involve excited states. </p><p>The conducting properties of the molecule inserted between gold leads is calculated within the Non Equilibrium Green Function theory. The transmission function is calculated for the two isomers with different basis sizes for the gold contacts, as well as the electrostatic potential, for finite applied bias voltages. We conclude that a Au 6s basis give qualitatively the same result as a Au spd basis close to the Fermi level. The transmission coefficient at the Fermi energy is around 10 times larger in the closed molecule compared to the open. This will result in a large difference in conductivity. It is also found that the large difference in conductivity will remain for small applied bias voltages. The results are consistent with earlier work.</p>

Transport through nanostructures is studied at the many-body level using exact diagonalization and nonequilibrium Green's functions. Organic molecular junctions are a particular focus because of their technological promise. Work is presented regarding: (1) A π-electron model of organic molecular junctions developed using effective field theory; (2) series transmission and transmission node structure in interacting systems; (3) the effect of interactions on quantum interference and thermoelectricity in polycyclic junctions; and (4) nanoscale transport calculations using self-consistent statistical ensembles.

Magistro baigiamajame darbe išnagrinėta antros eilės paprastoji diferencialinė lygtis naudojant faktorizacijos bei konstantų variavimo metodus. Antros eilės diferencialinė lygtis suvesta į integralinę lygtį. Išnagrinėti du kraštiniai uždaviniai šiai lygčiai ir abiem atvejais sukonstruotos kraštinio uždavinio Gryno funkcijos. / In this work, we study the second – order ordinary differential equation, using the factorization and variation of constant methods. The second – order differential equation boils down to the integral equation. Two boundary value problems have been analyzed and the boundary value problems Greenˊs functions have been constructed in both cases.

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abstract: A proposed visible spectrum nanoscale imaging method requires material with permittivity values much larger than those available in real world materials to shrink the visible wavelength to attain the desired resolution. It has been proposed that the extraordinarily slow propagation experienced by light guided along plasmon resonant structures is a viable approach to obtaining these short wavelengths. To assess the feasibility of such a system, an effective medium model of a chain of Noble metal plasmonic nanospheres is developed, leading to a straightforward calculation of the waveguiding properties. Evaluation of other models for such structures that have appeared in the literature, including an eigenvalue problem nearest neighbor approximation, a multi- neighbor approximation with retardation, and a method-of-moments method for a finite chain, show conflicting expectations of such a structure. In particular, recent publications suggest the possibility of regions of invalidity for eigenvalue problem solutions that are considered far below the onset of guidance, and for solutions that assume the loss is low enough to justify perturbation approximations. Even the published method-of-moments approach suffers from an unjustified assumption in the original interpretation, leading to overly optimistic estimations of the attenuation of the plasmon guided wave. In this work it is shown that the method of moments approach solution was dominated by the radiation from the source dipole, and not the waveguiding behavior claimed. If this dipolar radiation is removed the remaining fields ought to contain the desired guided wave information. Using a Prony's-method-based algorithm the dispersion properties of the chain of spheres are assessed at two frequencies, and shown to be dramatically different from the optimistic expectations in much of the literature. A reliable alternative to these models is to replace the chain of spheres with an effective medium model, thus mapping the chain problem into the well-known problem of the dielectric rod. The solution of the Green function problem for excitation of the symmetric longitudinal mode (TM01) is performed by numerical integration. Using this method the frequency ranges over which the rod guides and the associated attenuation are clearly seen. The effective medium model readily allows for variation of the sphere size and separation, and can be taken to the limit where instead of a chain of spheres we have a solid Noble metal rod. This latter case turns out to be the optimal for minimizing the attenuation of the guided wave. Future work is proposed to simulate the chain of photonic nanospheres and the nanowire using finite-difference time-domain to verify observed guided behavior in the Green's function method devised in this thesis and to simulate the proposed nanosensing devices. / Dissertation/Thesis / M.S. Electrical Engineering 2013

L’antennerie acoustique, aussi appelée formation de voies, est une technique d’identification acoustique basée sur un modèle de propagation analytique entre les sources de bruit et les microphones, la fonction de Green. Dans le cas de l’étude de sources aéroacoustiques en configuration réaliste, le milieu de propagation entre les sources et les microphones est la plupart du temps constitué d’un écoulement inhomogène et/ou d’une géométrie complexe. La fonction de Green n’est alors généralement pas connue et l’utilisation d’une fonction non adaptée conduit à une localisation ainsi qu’une mesure du niveau acoustique des sources erronées. L’objectif de cette thèse est le développement d’une méthode permettant d’estimer, grâce au code de propagation numérique de l’Onera résolvant les équations d’Euler (sAbrinA_v0), les composantes de la fonction de Green entre chaque point de focalisation (point source possible) et chaque microphone pour des cas de propagations complexes. Un seul calcul de propagation est effectué avec un ensemble de points sources positionnés en chacun des points de focalisation. La fonction de Green est ensuite estimée à partir de la résolution de problèmes inverses faisant intervenir les signaux sources et signaux calculés aux positions des microphones. Afin de valider l’approche, la formation de voies est ensuite mise en œuvre avec la fonction estimée, dans le but d’identifier des sources de bruit pour des cas 2D simples, puis des cas 2D avec un écoulement et/où une géométrie complexe. Les signaux microphoniques correspondant au rayonnement des sources à identifier sont obtenus analytiquement ou numériquement selon les cas. / The acoustic Beamforming, also called microphone array processing, is an acoustic identification technique based on an analytical propagation model between noise sources and microphones: the Green function. In the case of the study of aeroacoustic sources in realistic configuration, the propagation medium between sources and microphones is most often made up of an inhomogeneous flow and / or complex geometry. Green's function becomes then analytically difficult to determine and the use of an unsuitable function leads to spurious source localization and level measurement. The aim of this thesis is to overcome these difficulties by employing the Onera’s Euler solver sAbrinA_v0 to determine the Green's function components between each focus point (possible source point) and each microphone for complex cases. One propagation calculation is performed with a set of source points positioned in each of the focal points. The Green's function is then estimated from the resolution of inverse problems involving source signals and signals calculated at microphones position’s. To validate the approach, Beamforming is computed with the estimated function in order to identify noise sources for simple 2D case and 2D cases with a flow or/ with complex geometry. The microphone signals corresponding to the radiation of the sources to identify are obtained analytically or numerically depending on the case.

In this thesis, anisotropic Heisenberg magnets with arbitrary spin are investigated within the second-order Green-function theory. Three models are considered. First, the second-order Green-fuction theory for one-dimensional and two-dimensional Heisenberg ferromagnets with arbitrary spin S in a magnetic field is developed. For the determination of the introduced vertex parameters sum rules, higher-derivative sum rules, and regularity conditions are derived, and the equality of the isothermal and the longitudinal uniform static Kubo susceptibilities is required. Thermodynamic quantities, such as the specific heat, magnetic susceptibility, transverse and longitudinal correlation lengths are calculated. Empirical formulas describing the dependence of the position and height of the susceptibility maximum on the magnetic field are given. An anomal behavior of the longitudinal correlation length is observed. The appearance of two maxima in the temperature dependence of the specific heat is discussed. Further, as an example of a system with an anisotropy in the spin space, the S=1 ferromagnetic chain with easy-axis single-ion anisotropy is studied. Justified by the up-down symmetry of the model with respect to $S_i^z -> -S_i^z$, $\\langle S_i^z \\rangle=0$ is set. Two different ways of the determination of the introduced vertex parameters are presented. The transverse nearest-neighbor correlation function, spin-wave spectrum and longitudinal correlation length are analyzed. The effects of the single-ion anisotropy on the transverse and longitudinal uniform static susceptibilities as well as on the appearance of two maxima in the temperature dependence of the specific heat are examined. Finally, as examples of spatial anisotropic spin systems,layered Heisenberg ferromagnets and antiferromagnets with arbitrary spin are studied within the rotation-invariant Green-function theory. The long-range order is described by the condensation term, which is determined from the requirement that in the ordered state the static susceptibility has to diverge at the ordering wave vector. For determination of the introduced vertex parameters, the sum rule and the isotropy condition are used and also assumptions regarding the temperature dependence of some parameters are made. The main focus is put on the calculation of the specific heat, the Curie temperature, and the Néel temperature in dependence on the interlayer coupling and the spin-quantum number. Empirical formulas describing the dependence of the transition temperatures on the ratio of interlayer and intralayer couplings are given. For all three models, the results of the Green-function theory are compared to available results of exact approaches (Quantum Monte Carlo, exact diagonalization, Bethe-ansatz method) and to available experimental data.

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[EN] This thesis studies the optical properties of random arrays of metal nanoparticles in multilayered substrates such as a solar cell, as well as the electrooptic consequences for those substrates. This study difers from traditional models which assume independent spherical particles in an homogeneous medium. Moreover, the efects beyond the near field range are studied because substrates thicker than 150µm are used. The study in this thesis uses two main approaches: a) A theoretical approach based on simulations and analytical models. Starting with the traditional methods (Mie), alternatives are considered for considering the substrate efect, the shape of the nanoparticles as well as the efect of the surrounding nanoparticles. For this, the use of Green functions and the Sommerfeld identity are presented as interesting strategies against traditional numerical model that are not suitable due to the complexity of the system that leads to huge power, time and memory consumptions. Nevertheless, the analytical approach has its limits and dificulties, that are analysed in this thesis. The results obtained in the thesis are compared with experimental data and a critical analysis is performed to check the real suitability and the scope of this strategy for simulating these kinds of systems. b) An experimental approach, in which special attention has been paid to the self-aggregation method as a quick way of integrating the nanoparticles on the final device. Some issues have been detected and studied related with the degradation of the nanoparticles, and some strategies to minimise this efect are presented. Integrated samples have been prepared using diferent integration approaches. From the measurements and their analysis the infuence of the substrate and other factors on the nanoparticle behaviour is confrmed, and the enhancement potential of the solar cell is studied. This thesis has been carried out at Valencia Nanophotonics Technology Center (NTC, in Spain) partly in the context of the LIMA european project (FP7-ICT-2009.3.8) and has included a short term scientific mission at the Laboratory of Photonics and Nanostructures (CNRS-LPN) at Marcoussis (France). / [ES] En esta tesis se realiza un estudio de las propiedades ópticas de agrupaciones aleatorias de nanopartículas metálicas cuando éstas se depositan en un sustrato multicapa como una célula solar, así como las consecuencias electroópticas sobre dicho sustrato. Este estudio supone una diferencia importante con respecto a las hipótesis de modelos tradicionales en los que se suponen partículas individuales, perfectamente esféricas y en medios homogéneos. Además, estudia los efectos más allá del campo cercano al utilizar sustratos de más de 150µm de grosor. El trabajo de esta tesis gira en torno a dos enfoques principales: a) Un enfoque más teórico basado en simulaciones y modelos analíticos. Partiendo de los métodos tradicionales (Mie), se estudian métodos para incluir el efecto del sustrato, de la forma de las partículas y el efecto de las partículas cercanas. Para este fin, el uso de funciones de Green y de la identidad matemática de Sommerfeld se presentan como alternativas de gran interés frente al uso de modelos numéricos, inviables dada la complejidad del sistema y los recursos de memoria y tiempo necesarios. Aún así, los modelos analíticos presentan sus propias limitaciones y difcultades que son analizadas en esta tesis. Las soluciones obtenidas con estos modelos se han comparado con datos experimentales y un análisis crítico se ha llevado a cabo para determinar el alcance y la fabilidad de estas estrategias de simulación. b) Un enfoque más experimental, en el que se ha hecho especial hincapié en la autoagregación de capas finas como vía rápida para integrar las partículas en el dispositivo fnal. También se han estudiado los problemas asociados a la estabilidad de las nanopartículas con el tiempo y a cómo minimizar la degradación. Por otro lado, se han preparado varios dispositivos integrados siguiendo distintas estrategias y de cuyas medidas y análisis se ha confrmado el efecto del sustrato y otros factores sobre el comportamiento de las nanopartículas, así como estudiado la potencial mejora de la eficiencia en células solares. Esta tesis se ha realizado en su mayoría en el Centro de Tecnología Nanofotónica de Valéncia (NTC, en España) enmarcada parcialmente en el proyecto europeo LIMA (FP7-ICT-2009.3.8) y ha incluido una estancia investigadora en el Laboratorio de Fotónica y Nanoestructuras (CNRS-LPN) en Marcoussis (Francia). / [CA] En aquesta tesi es realitza un estudi de les propietats òptiques d'agrupacions aleatòries de nanopartícules metàl·liques quan aquestes es depositen sobre un substrat multicapa com una cel·lula solar, així com les consequències electroòptiques resultants en el substrat. Aquest estudi presenta una difèrencia important amb les hipotesis de models tradicionals en els quals es suposa una partícula tota sola, perfectament esfèrica i en un medi homogeni. A més a més, s'estudiaran els efectes més enlla del camp proper a l'utilitzar substrats de més de 150µm d'espessor. El treball d'aquesta tesi es fara mitjançant dues estratègies principalment: a) Un enfocament més teòric emprant simulacions i models analítics. Començant amb models tradicionals (Mie), s'estudiaran estratègies per a incloure l'efecte d'un substrat, de la forma de les partícules així com el de la presència de partícules al voltant. Amb aquesta fnalitat, les funcions de Green i la identitat matemàtica de Sommerfeld es presenten com unes eines de gran interés comparat amb l'ús de mètodes numèrics tradicionals, els quals tenen uns requeriments excessius de memòria i temps de càlcul. Amb tot, aquests models analítics també tenen les seues limitacions i dificultats que són estudiades en la tesi. Les solucions obteses amb aquests models s'han comparat amb dades experimentals i s'ha fet un anàlisi crític per determinar l'abast de la validesa i la fiabilitat d'aquestes estrategies de simulació. b) Un enfocament més experimental, en el qual s'ha posat l'accent en l'auto-agregació de pel·lícules fines com a estratègia per a l'integració de les partícules en el dispositiu fnal. També s'han estudiat els problemes associats a l'estabilitat de les partícules amb el temps així com vies per a minimitzar aquesta degradació. D'altra banda, s'han preparat diversos dispositius integrats mitjannant diferents estratègies i a partir de les mesures de les quals s'ha confirmat l'efecte del substrat i d'altres factors en el comportament de les nanopartícules i s'ha estudiat la potencial millora de l'eficiència de la cèl·lula solar. Aquesta tesi s'ha dut a terme majoritàriament en el Centre de Tecnologia Nanofotonica de Valéncia (NTC) parcialment enmarcada en el projecte europeu LIMA (FP7-ICT-2009.3.8), i inclou la realització d'una estància al Laboratori de Fotònica i Nanoestructures (CNRS-LPN) en Marcoussis (França). / Cortés Juan, F. (2015). Theoretical and experimental study of light-nanoparticle interactions in high efficiency solar cells [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/59404

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