Single-walled carbon nanotubes (SWCNTs) are promising for use in solar cells and photodetectors because of their strong optical absorption in most of the solar spectrum. There have been many reports about the photovoltaic effect in nanoelectronic devices based on individual SWCNTs, but they have been limited by complicated fabrication and miniscule absorption. There has been a growing trend for merging SWCNTs into micro-and macroscopic devices to provide more practical applications. Here we report the photoresponse of macroscopic SWCNT films with a p-n junction at room temperature. Photovoltage (PV) and photocurrent (PC) due to the photothermoelectric (PTE) effect were observed at the junction, and they were larger by one order of magnitude as compared with their values at the metal-SWCNT interfaces. Various factors affecting PV amplitude and response time have been studied, including junction length, substrate, and doping level. The maximal responsivity we observed was 1V/W with samples on Teflon tape, while a fast response time 80 S was observed with samples on AlN substrates. Hence an optimal combination of photoresponse time and amplitude can be found by proper choice of substrate. It was found that PV increased nonlinearly with increase in n-doping concentration, indicating the existence of an optimal doping level. This result also suggests the possibility to further improve photoresponse by changing p-doping level. Finally, we checked the photoresponse in wide wavelength range (360-900 nm), and PV was observed throughout, indicating that the device could potentially be used as a broadband photodetector.

This research entails the first comprehensive and systematic study on a heat-driven, self-cooling application based on the thermoelectric generation effect. The system was studied using the first and second laws of thermodynamics to provide a solid and basic understanding of the physical principles governing the system. Multiphysics equations that relate heat transfer, fluid dynamics and thermoelectric generation are derived. The equations are developed with increasing complexity, from the basic Carnot heat engine to externally and internally irreversible engines. A computational algorithm to systematically use the fundamental equations has been presented and computer code is implemented based on the algorithm. Experiments were conducted to analyze the geometric and system parameters affecting the application of thermoelectric based self-cooling in devices. Experimental results show that for the highest heat input studied, the temperature of the device has been reduced by 20-40% as compared to the natural convection case. In addition, it has been found that in the self-cooling cases studied, convection thermal resistance could account for up to 60% of the total thermal resistance. A general numerical methodology was developed to predict steady as well as transient thermal and electrical behavior of a thermoelectric generation-based self-cooling system. The methodology is implemented by using equation modeling capabilities to capture the thermo-electric coupled interaction in TEG elements, enabling the simulation of major heating effects as well as temperature and spatial dependent properties. An alternative methodology was also presented, which integrates specialized ANSI-C code to integrate thermoelectric effects, temperature-dependent properties and transient boundary conditions. It has been shown that the computational model is able to predict the experimental data with good accuracy (within 5% error). A parametric study has been done using the model to study the effect of heat sink geometry on device temperature and power produced by TEG arrays. In addition, a dynamic model suited for integration in control systems is developed. Therefore, the study has shown the potential for a heat driven self-cooling system and provides a comprehensive set of tools for analysis and design of thermoelectric generation.

Anisotropic thermoelectric effects can be measured in certain materials. Anisotropy can also be simulated using a repeated, layered structure of two materials cut at an angle. Various aspect ratios and angles of inclination are investigated in device geometry in order to maximize the thermopower. Eddy currents have been shown to occur in thermoelectric devices, and evidence of these currents are revealed in finite element analysis of the artificially synthesized anisotropic Peltier effect.

Efeitos termoelétricos descrevem o surgimento de campos elétricos em função de gradientes de temperatura e vice-versa. Neste trabalho investigamos as propriedades termoelétricas de materiais de baixa dimensionalidade e nanoestruturas através de cálculos de primeiros princípios das propriedades de transporte destes sistemas, usando o código TRANSAMPA, que é baseado em funções de Green fora do equilíbrio e do código SIESTA, baseado em teoria do funcional da densidade. Inicialmente estudamos nanofitas de grafeno e como estas são alteradas pela presença de impurezas substitucionais de Boro e Nitrogênio. Entre os principais resultados, mostramos que fitas na configuração ferromagnética apresentam efeito Seebeck dependente do spin, que pode ser ajustado por efeito de campo. A seguir, vemos que o coeficiente Seebeck (S) em bicamadas de grafeno pode ser ajustado por potenciais de gate, de forma a escolher os portadores de carga, atingindo S =_250 _V/K. Também estudamos a dependência de S com a temperatura (T) e o tamanho do gate, calculamos a condutividade térmica por dinâmica molecular e a eficiência termoelétrica (ZT). Na seqüencia, mostramos que grafeno dopado com Mn mostra caloritrônica de spin ajustável via gate e como a termocorrente varia com T e _T. Finalmente, calculamos as propriedades termoelétrica de uma junção molecular Au-BDT-Au e como elas variam em função do alongamento da junção. Também propomos um esquema geral para maximizar ZT de junções moleculares em geral. / Thermoelectric effects describe how electric fields arise in response to temperature gradients and vice versa. In this thesis we investigate the thermoelectric properties of low-dimensional materials and nanostructures theoretically. We perform ab initio calculations of the electronic transport properties using the TRANSAMPA code, based in nonequilibrium Greens functions, and the SIESTA code, based in density functional theory. First, we study graphene nanoribbons and how their properties are altered by substitutional impurities. Among our main results for this system, we show that ribbons in the ferromagnetic configuration present spin-dependent Seebeck effect, which can be tuned by a field effect. We show that the Seebeck coefficient (S) of bilayer graphene is highly tunable by a gate potential, with ambipolar behavior, reaching S = _250 _V/K. We also study how S varies with temperature (T) and gate length. We calculate its thermal conductivity by molecular dynamics, and its thermoelectric efficiency (ZT ). Then, we show that Mn doped graphene features a gate-tunable spin-dependent S, which is robust under changes in T and _T, rendering this material suitable for spin caloritronics. Finally, we calculate how the thermoelectric properties of an Au-BDT-Au molecular junction vary with mechanical stretching, and propose a general recipe to improve ZT in molecular junctions in general.

Efeitos termoelétricos descrevem o surgimento de campos elétricos em função de gradientes de temperatura e vice-versa. Neste trabalho investigamos as propriedades termoelétricas de materiais de baixa dimensionalidade e nanoestruturas através de cálculos de primeiros princípios das propriedades de transporte destes sistemas, usando o código TRANSAMPA, que é baseado em funções de Green fora do equilíbrio e do código SIESTA, baseado em teoria do funcional da densidade. Inicialmente estudamos nanofitas de grafeno e como estas são alteradas pela presença de impurezas substitucionais de Boro e Nitrogênio. Entre os principais resultados, mostramos que fitas na configuração ferromagnética apresentam efeito Seebeck dependente do spin, que pode ser ajustado por efeito de campo. A seguir, vemos que o coeficiente Seebeck (S) em bicamadas de grafeno pode ser ajustado por potenciais de gate, de forma a escolher os portadores de carga, atingindo S =_250 _V/K. Também estudamos a dependência de S com a temperatura (T) e o tamanho do gate, calculamos a condutividade térmica por dinâmica molecular e a eficiência termoelétrica (ZT). Na seqüencia, mostramos que grafeno dopado com Mn mostra caloritrônica de spin ajustável via gate e como a termocorrente varia com T e _T. Finalmente, calculamos as propriedades termoelétrica de uma junção molecular Au-BDT-Au e como elas variam em função do alongamento da junção. Também propomos um esquema geral para maximizar ZT de junções moleculares em geral. / Thermoelectric effects describe how electric fields arise in response to temperature gradients and vice versa. In this thesis we investigate the thermoelectric properties of low-dimensional materials and nanostructures theoretically. We perform ab initio calculations of the electronic transport properties using the TRANSAMPA code, based in nonequilibrium Greens functions, and the SIESTA code, based in density functional theory. First, we study graphene nanoribbons and how their properties are altered by substitutional impurities. Among our main results for this system, we show that ribbons in the ferromagnetic configuration present spin-dependent Seebeck effect, which can be tuned by a field effect. We show that the Seebeck coefficient (S) of bilayer graphene is highly tunable by a gate potential, with ambipolar behavior, reaching S = _250 _V/K. We also study how S varies with temperature (T) and gate length. We calculate its thermal conductivity by molecular dynamics, and its thermoelectric efficiency (ZT ). Then, we show that Mn doped graphene features a gate-tunable spin-dependent S, which is robust under changes in T and _T, rendering this material suitable for spin caloritronics. Finally, we calculate how the thermoelectric properties of an Au-BDT-Au molecular junction vary with mechanical stretching, and propose a general recipe to improve ZT in molecular junctions in general.

Este trabalho tem por objetivo apresentar um estudo de viabilidade técnica da utilização de módulos termoelétricos para condicionamento de ar automotivo, com ênfase ao sistema de resfriamento. O sistema termoelétrico de refrigeração baseiase no efeito descoberto por Jean Charles Athanase Peltier em 1834, onde uma corrente elétrica flui na junção de dois materiais semicondutores de propriedades diferentes fazendo com que haja um gradiente de temperatura na junção dos materiais. Os módulos termoelétricos são fabricados por materiais semicondutores selados entre duas placas e, conforme o sentido que a corrente flui, uma superfície da placa torna-se aquecida e a outra resfriada. A refrigeração proveniente dos fenômenos termoelétricos surgiu como uma alternativa aos meios convencionais de refrigeração, pois não gera impactos ambientais, não produz barulho, não possui partes móveis, podendo ser montando em variadas posições de acordo com a necessidade, além de ocupar um volume pequeno em comparação com os outros sistemas. Os efeitos termoelétricos e suas aplicações são apresentados neste estudo, assim como o modelo matemático do sistema de refrigeração, que permite determinar o desempenho do sistema. Dissipadores de calor comercialmente disponíveis são analisados para a determinação da melhor troca térmica para o sistema e protótipos são montados para a obtenção de resultados experimentais de um refrigerador, a fim de verificar sua viabilidade e funcionalidade. / This paper aims to present a technical feasibility study of the use of thermoelectric modules for automotive air conditioning, with emphasis on the cooling system. The thermoelectric cooling system is based on the effect discovered by Jean Charles Athanase Peltier in 1834, where an electric current flows at the junction of two semiconductor materials of different properties so that there is a temperature gradient at the junction of the materials. The thermoelectric modules are manufactured by semiconductor material sealed between two sheets and, as the current flows, a plate surface becomes heated and the other cooled. Thermoelectric cooling from phenomena like Peltier effect has emerged as an alternative to conventional cooling systems, it does not generate environmental impact, do not produce noise, has no moving parts, the system can be mounted in various positions according to the need, and occupies a small volume in comparison with other systems. Thermoelectric effects and their applications are presented in this study, as well as the mathematical modeling of the cooling system, which determines system performance. Heat sinks commercially available are analyzed for determining the best heat transfer to the system and prototypes are assembled to obtain the experimental results for a refrigerator in order to verify its feasibility and functionality.

Cette thèse porte sur le mouvement de particules colloïdales induit par l’effet thermoélectrique (ou effet Seebeck). Dans un électrolyte soumis à un gradient de température, les ions ont tendance à migrer à des vitesse qui différent d'une espèce à l'autre. On observe alors une accumulation de charge aux bords de l’échantillon. Ce déséquilibre induit un champ électrique qui agit sur les colloïdes chargés présents dans la solution. Cette contribution électrophorétique dans le champ de Seebeck s'additionne à la contribution directe de thermodiffusion. Comme résultat principal,nous obtenons la vitesse phorétique en fonction de la fraction volumique des particules et, dans le cas de polyélectrolytes, du poids moléculaire. Dans la seconde partie, nous étudions l’effet thermoélectrique pour une particule chauffée par absorption d’un faisceau laser. Le gradient de température est alors radial et l’effet Seebeck induit une charge nette dans le voisinage de la particule. Enfin, nous discutons les applications possibles de ce phénomène de thermocharge / In this work we study the motion induced in a colloidal dispersion by thethermoelectric or Seebeck effect. As its basic principle, the ions of the electrolytesolution start moving in a temperature gradient. In general, the velocity of one iondiffers from another. As a result, one observes a charge separation and a macroscopicelectric field. This thermoelectric field, in turn, acts upon the charged colloidalparticle present in the solution. Thus thermophoresis of the particle comprises of anelectrophoretic motion in the thermoelectric or Seebeck field. As an important result,we derive how the corresponding velocity of a colloidal particle depends upon thecolloidal volume fraction or on molecular weight for polymers. In a second part, westudy the thermoelectric effect due to a hot colloidal particle where a radialtemperature gradient is produced by the particle itself. In this temperature gradientthe same Seebeck effect takes place in the electrolyte solution. We find that the hotparticle carries a significant amount of charge around it. Whereas the amount ofsurface charges present at the boundaries of the sample container in the onedimensionalcase is rather insignificant. Possible applications of this thermochargingphenomenon are also discussed.

The electrical and thermal conductance and the Seebeck coefficient are calculated for one-dimensional systems, and their behavior as a function of temperature and chemical potential is investigated. It is shown that the conductances are proportional to an average of the transmission probability around the Fermi level with the average taken for the thermal conductance being over a wider range. This has the effect of creating less well-defined plateaus for thermal-conductance quantization experiments. For weak non-linearities, the charge and entropy currents across a quantum point contact are expanded as a series in powers of the applied bias voltage and the temperature difference. After that, the expansions of the Seebeck voltage in temperature difference and the Peltier heat in current are obtained. Also, it is shown that the linear thermal conductance of a quantum point contact displays a half-plateau structure, almost flat regions appearing around half-integer multiples of the conductance quantum. This structure is investigated for the saddle-potential model.

Cette thèse rapporte les mesures de transport réalisées sur des jonctions moléculaires à l'état solide large échelle, mettant en évidence des effets d'interférence quantique. Le premier chapitre pose les bases théoriques de ce phénomène et introduit le formalisme des fonctions de green hors équilibre adapté à la description du couplage molécules/interfaces métalliques. Le second chapitre présente l'état de l'art expérimental dans ce domaine et résume les principales expériences ayant permis de mettre en évidence des effets d'interférences à l'échelle moléculaire. Le troisième chapitre décrit les étapes de fabrication mises en place pour construire les dispositifs mesures pendant ce travail de thèse. Les résultats expérimentaux obtenus sur les mesures de conductance des jonctions moléculaires sont décrits dans le quatrième chapitre et compares a plusieurs modèles théoriques qui confirme la présence d'interférences quantiques. Le dernier chapitre aborde les effets de thermoélectricité qui peuvent avoir lieu dans ces jonctions en présence d'interférence / This thesis reports the transport measurement performed on large scale solid state molecular junctions, highlighting quantum interference effect. First chapter set the theoretical basis of such a phenomenon and introduces the out of equilibrium green's functions formalism which is adapted to the description of coupling molecules/metallic interfaces. Second chapter presents the corresponding experimental state of the art and summarizes the experiments that have contributed to highlight interference effect at the molecular scale. Third chapter describes the fabrication steps optimized to build the devices measured during the thesis work. Experimental results obtained on conductance measurements are described and compared to several theoretical models that confirm the presence of quantum interference. Last chapter deals with thermoelectric effect that can occur in presence of interference

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