Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of great interest to contemporary scientists because of their ability to directly convert temperature differences into electricity, and are regarded as a promising mode of alternative energy. The TE conversion efficiency is determined by the Carnot efficiency, η_C and is relevant to a commonly used figure of merit ZT of a material. Improving the value of ZT is presently a core mission within the TE field. In order to advance our understanding of thermoelectric materials and improve their efficiency, this dissertation investigates the low-temperature behavior of the p-type thermoelectric Cu2Se through chemical doping and nanostructuring. It demonstrates a method to separate the electronic and lattice thermal conductivities in single crystal Bi2Te3, Cu, Al, Zn, and probes the electrical transport of quasi 2D bismuth textured thin films. Cu2Se is a good high temperature TE material due to its phonon-liquid electron-crystal (PLEC) properties. It shows a discontinuity in transport coefficients and ZT around a structural transition. The present work on Cu2Se at low temperatures shows that it is a promising p-type TE material in the low temperature regime and investigates the Peierls transition and charge-density wave (CDW) response to doping [1]. After entering the CDW ground state, an oscillation (wave-like fluctuation) was observed in the dc I-V curve near 50 K; this exhibits a periodic negative differential resistivity in an applied electric field due to the current. An investigation into the doping effect of Zn, Ni, and Te on the CDW ground state shows that Zn and Ni-doped Cu2Se produces an increased semiconducting energy gap and electron-phonon coupling constant, while the Te doping suppresses the Peierls transition. A similar fluctuating wave-like dc I-V curve was observed in Cu1.98Zn0.02Se near 40 K. This oscillatory behavior in the dc I-V curve was found to be insensitive to magnetic field but temperature dependent [2]. Understanding reducing thermal conductivity in TE materials is an important facet of increasing TE efficiency and potential applications. In this dissertation, a magnetothermal (MTR) resistance method is used to measure the lattice thermal conductivity, κ_ph of single crystal Bi2Te3 from 5 to 60 K. A large transverse magnetic field is applied to suppress the electronic thermal conduction while measuring thermal conductivity and electrical resistivity. The lattice thermal conductivity is then calculated by extrapolating the thermal conductivity versus electrical conductivity curve to a zero electrical conductivity value. The results show that the measured phonon thermal conductivity follows the e^(Δ_min⁄T) temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. These low-temperature experimental data and analysis on Bi2Te3 are important compliments to previous measurements and theoretical calculations at higher temperatures, 100 – 300 K. The MTR method on Bi2Te3 provides data necessary for first-principles calculations [4]. A parallel study on single crystal Cu, Al and Zn shows the applicability of the MTR method for separating κ_e and κ_ph in metals and indicates a significant deviation of the Lorenz ratio between 5 K and 60 K [3]. Elemental bismuth is a component of many TE compounds and in this dissertation magnetoresistance measurements are used investigate the effect of texturing in polycrystalline bismuth thin films. Electrical current in bismuth films with texturing such that all grains are oriented with the trigonal axis normal to the film plane is found to flow in an isotropic manner. By contrast, bismuth films with no texture such that not all grains have the same crystallographic orientation exhibit anisotropic current flow, giving rise to preferential current flow pathways in each grain depending on its orientation. Textured and non-textured bismuth thin films are examined by measuring their angle-dependent magnetoresistance at different temperatures (3 – 300 K) and applied magnetic fields (0 – 90 kOe). Experimental evidence shows that the anisotropic conduction is due to the large mass anisotropy of bismuth and is confirmed by a parallel study on an antimony thin film [5]. [1] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril Opeil, J. Alloys Compd. 699, 718 (2017). [2] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril P. Opeil, J. Materiomics 3, 150 (2017). [3] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal metals: Al, Cu and Zn, Mengliang Yao, Mona Zebarjadi, and Cyril P. Opeil, under review. [4] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal bismuth telluride, Mengliang Yao, Stephen Wilson, Mona Zebarjadi, and Cyril Opeil, under review. [5] Albert D. Liao, Mengliang Yao, Ferhat Katmis, Mingda Li, Shuang Tang, Jagadeesh S. Moodera, Cyril Opeil, Mildred S. Dresselhaus, Appl. Phys. Lett. 105, 063114 (2014). / Thesis (PhD) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The standard material by which all materials exhibiting magnetocaloric effect are measured is Gadnolinium. In this work we are attempting to understand how nanostructuring can impact the magnetocaloric effect, to this end we have grown Gd in various thin film structures. The samples made were grown via magnetron sputtering on MgO(100) substrates. Samples of thick Gd (2000 A) were grown and sandwiched between two layers of Cr or W and annealed at increasing temperatures to study how this can perturb the magnetic and structural properties of the Gd. Another set of samples was grown in which Gd (at various thicknesses) is in a multilayer system with W. Here the purpose is to explore how changing the thickness of the Gd can change its magnetic properties. Using the appropriate Maxwell relation, the magnetic entropy change was observed to increase with increasing annealing temperature. In a 0-4T magnetic field change, the peak entropy was found to go from approximately 1.5 J/kg-K for the unannealed sample to 4.4 J/kg-K when annealed to 600°C. The multilayers were found to all have a T C near 280 K, in contrast with what is predicted by finite size scaling. This is likely due to pinholes in the W layers allowing the Gd to act as one magnetic material.

C'est la surface, et non le matériau qui interagit avec l'environnement. Par conséquent, en modifiant la surface d'un matériau de manière contrôlée, nous pouvons moduler ces interactions avec son environnement. Les sels d'aryles diazonium semblent très adaptés pour modifier les propriétés de surface de matériaux de par leurs diversités structurelles et leur capacité à modifier des surfaces conductrices par électrochimie. Ce travail de thèse se concentre sur l'étude du transfert électronique au travers de couches organiques de différentes épaisseurs (monocouches, couches ultraminces et multicouches), générées par électro-réduction de sels d'aryles diazonium. La molécule électroactive étudiée peut être alors soit fixée à la surface du matériau ou en solution. Différentes méthodes électrochimiques ont été utilisées au cours de cette thèse : CV, EIS et SECM. Dans un premier temps, l'étude des propriétés électrochimiques de surfaces carbonées modifiées par des monocouches d'alkyle-ferrocène a été entreprise dans différents solvants ; ainsi que leur évaluation pour des applications en stockage d'énergie. La deuxième étude s'intéresse à l'utilisation d'une approche « bottom-up » pour la fabrication de surfaces organisées. Des substrats de carbone et d'or ont été modifiés par électro-réduction d'un sel d'aryle diazonium pré-organisé en forme de tétraèdre. Ceci aboutit à l'obtention d'un film organique ultra-mince possédant des propriétés de tamisage moléculaire et de rectification de courant électrochimique vis-à-vis de sondes redox en solution. La troisième étude s'est ensuite focalisée sur la réaction de réduction du dioxygène et de ses intermédiaires, qui présentent un intérêt général aussi bien dans des processus naturels qu'en industrie. La détection de ces intermédiaires a été entreprise par SECM, utilisant une stratégie « d'empreinte » utilisant différentes couches organiques sensibles. L'influence du potentiel appliqué et de l'électrolyte a été étudiée. Dans ce travail, nous avons démontré que les propriétés électrochimiques de sondes redox en solution ou greffées à la surface d'un matériau peuvent être modulées par l'utilisation de couches organiques. Ces recherches fondamentales présentent un intérêt dans des domaines tels que le stockage d'énergie et la catalyse. / It is the surface, not the bulk material that interacts with the surrounding environment; hence by altering the surface in a controlled manner we can modulate the properties of the material towards its environment. Aryldiazonium salts are suitable to tailor the surface properties since their structural diversity and their electrochemically-assisted bonding ability to modified conducting surfaces. This thesis focuses on the study of the electron transfer through different aryl layers by aryldiazonium electro-reduction at three different thickness levels, monolayer, near-monolayer, and multilayer, when the electroactive molecule is attached to the surface or in solution. Three different electrochemical methods have been used throughout this thesis, CV, EIS and SECM. The first study of this thesis focused on the investigation of the electrochemical properties of alkyl-ferrocene on-carbon monolayers in different solvents and its evaluation for improving the global charge density of carbon materials for energy storage applications. The second study used a bottom-up approach for the fabrication of well-organized surfaces. Carbon and gold substrates were modified by electro-reduction of a tetrahedral-shape preorganized aryldiazonium salt resulting in an ultrathin organic film that showed molecular sieving and current rectification properties towards redox probes in solution. The third study then focused on the oxygen reduction reaction and its intermediates, which are of general importance in natural and industrial processes. Detection of intermediates was achieved by SECM in a foot-printing strategy based on the use of different sensitive aryl multilayers. The role of the applied potential and electrolytes was investigated. Here we have demonstrated that the electrochemical properties of redox probes attached to a surface or in solution can be modulated by introducing aryl layers allowing fundamental research investigations of interest in fields such as energy storage and catalysis.

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No. of bitstreams: 2 Tese - Flávio Silva de Carvalho - 2015.pdf: 7229360 bytes, checksum: c61e47d7d8698a2f433f115ce5265cb2 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2015-03-10 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / Neoplasmas have been a major cause of death worldwide, with that the search for new candidates for prototypes of more effective anti-tumor drugs, safe and with fewer side effects when compared to those already available already in therapy, it is essential and fundamental importance. With this research goal, we developed the design, synthesis and pharmacological evaluation in order to obtain new candidate of heterocyclic antitumor drugs prototypes (46a-46Q, 48a-48s, 51a-51m, 50m-50a, 53b, 54b, 56b and 58b) and vectorized micronutrient (60) and nanostructures (63) drawn from LQFM 030 (37) prototypes, in that it (37) was obtained by simplifying molecular strategy nutlin from compound (34) . The route chosen to obtain the heterocyclic compounds (46Q-46a, 48a-48s, 51a-51m, 50m, 50a, 53b, 54b, 56b and 58b) resulted in yields ranging from 32-77%, where all synthesized compounds were elucidated through the use of Nuclear Magnetic Resonance dimensional and two-dimensional (NMR) and Infrared (IR). Two methodologies have been developed (A and B) for the synthesis of intermediate pyrazole compounds and formylated acid, wherein the first method was conventional and the second was by heating with microwaves, which have obtained gain in time and / or increasing the yield of these synthetic steps when comparing with the conventional method (A). The synthesized compounds were tested for cytotoxicity assays by MTT method on the cell line K562 leukemic ie, and B16F10 melanoma ie, where the cytotoxic profile was evaluated using the IC50 = 70 uM for K562 cells and IC 50 = 64, 7 uM to line B16F10, both using an exposure time of 48 hours. K562 cells for six compounds LQFM ie 88, 108, 165, 166, 168 and 169 showed higher cytotoxic profile when compared to the prototype LQFM030 compound (37). Regarding the B16F10 line, six other compounds LQFM ie 126, 127, 165, 166 and 169 showed a better cytotoxic profile when compared to the prototype compound LQFM030. Given the results, we can conclude that the planning strategy employed to obtain the study compounds was validated, and against the cytotoxic research profile of 46a-46Q, 48a-48s, 51a- 51m, 50m-50a, 53b, 54b , 56b and 58b, and have a perspective vectorization and nanostructuring of the best compounds evaluated for each type of cell line K562 and B16F10 ie, aiming at optimizing the antitumor activity / As neoplasias têm sido uma das principais causas de morte no mundo, com isso a busca de novos candidatos a protótipos de fármacos antitumorais mais efetivos, seguros e que apresentem menos efeitos colaterais, quando já comparados aos já disponíveis na terapêutica, é imprescindível e de fundamental importância. Com esse objetivo de pesquisa, foi desenvolvido o planejamento, síntese e avaliação farmacológica visando a obtenção de novos candidatos a protótipos de fármacos antitumorais heterocíclicos (46a-46q, 48a-48s, 51a-51m, 50a-50m, 53b, 54b, 56b e 58b), e vetorizados com micronutrientes (60) e nanoestruturados (63), desenhados a partir dos protótipos LQFM 030 (37), em que este (37) foi obtido por meio de estratégia de simplificação molecular a partir do composto Nutlin (34). A rota eleita para a obtenção dos compostos heterocíclicos (46a- 46q, 48a-48s, 51a-51m, 50a-50m, 53b, 54b, 56b e 58b) resultou em rendimentos que variaram entre 32-77 %, onde todos os compostos sintetizados foram elucidados através do emprego de Ressonância Magnética Nuclear Unidimensionais e Bidimensionais (RMN) e Infravermelho (IV). Foram desenvolvidas duas metodologias (A e B), para a síntese dos compostos intermediários pirazola, formilado e ácido, em que a primeira metodologia foi a convencional e a segunda foi por meio de aquecimento com micro-ondas, onde obtivemos ganho em tempo e/ou aumento no rendimento dessas etapas sintéticas ao serem comparadas com a metodologia convencional (A). Os compostos sintetizados foram submetidos aos ensaios de citotoxicidade pelo método MTT, sobre a linhagem celular K562 i.e. leucêmica, e B16F10 i.e. melanoma, onde o perfil citotóxico foi avaliado utilizando-se do IC50= 70 µM para a linhagem K562 e IC50=64,7 µM para a linhagem B16F10, ambos utilizando um tempo de exposição de 48h. Para a linhagem K562, seis compostos i.e. LQFM 88, 108, 165, 166, 168 e 169 apresentaram melhor perfil citotóxico, quando comparado ao composto protótipo LQFM030 (37). Com relação à linhagem B16F10, outros seis compostos i.e. LQFM 126, 127, 165, 166 e 169 demonstraram um melhor perfil citotóxico ao serem comparados ao composto protótipo LQFM030. Diante dos resultados obtidos, podemos concluir que a estratégia de planejamento empregada para a obtenção dos compostos de estudo foi validada, e face ao perfil de investigação citotóxico dos 46a-46q, 48a-48s, 51a-51m, 50a-50m, 53b, 54b, 56b e 58b, e têm-se como perspectiva a vetorização e nanoestruturação dos melhores compostos avaliados para cada tipo de linhagem celular i.e. K562 e B16F10, visando à otimização da atividade antitumoral

The goals of this work are the optimization of the magnetocaloric effect in Gadolinium thin film structures. We approach this issue from two directions, that of process optimization and of interface effects. Past results showed Gd2O3 in our Gadolinium thin films, and the presence of such oxide seemed to grow with the temperature at which the film was grown or annealed. Comparison of samples grown without chamber gettering to those that were gettered show differences in their structural and magnetic properties, and we conclude that gettering is an effective step in enhancing the quality of Gd thin film samples. Early work with Gd/W heterostructures showed a diminished magnetization of the interfacial gadolinium, which reduces the magnetocaloric response as magnetic entropy is proportional to m2/3. It is known that Fe interfaces can boost the Gd moments per atom to above that seen in bulk. As such, we fabricated a series of Fe/Gd heterostructures to study the effects on the structural and magnetic properties of Gd thin films. The use of Fe as a base layer shows increased high frequency oscillations in X-ray reflectivity measurements, indicating sharp interfaces between Gd and Fe. The magnetocaloric measurements produce a magnetic entropy curve with a novel tail extending leftward, making this an improved material over Gd for applications around 240K. All the same, vector magnetometry is needed to ensure that such tail is not due to rotations within the plane and is a direction for further study.

L'intérêt porté au développement de matériaux thermoélectriques est grandissant car ils permettent de créer des sources d'énergie renouvelable, dites « vertes », ce qui s'inscrit pleinement dans la stratégie de lutte contre le réchauffement climatique. A ce jour le rendement de tels systèmes reste faible, le coût de développement élevé, et les plages de températures d'utilisation sont limitées. Dans ces travaux de thèse différentes pistes sont explorées pour développer des matériaux innovants à base de chalcogènes, principalement le tellure. Les principaux résultats portent sur les points suivants. (i) Une étude par spectroscopies couplée à des calculs théoriques a permis de mieux comprendre les phénomènes de conduction dans les verres du système Cu-As-Te. (ii) La recristallisation complète de verres de formulation Ge20Te77Se3 dopés a été réalisée pour pousser à son terme la logique dite du Phonon Glass Electron Crystal (PGEC).(iii) Différents modes de synthèses ont été mis en œuvre pour suivre les propriétés thermoélectriques de matériaux de formulation CuPb18SbTe20 (frittage, SPS, flash-SPS, hybrid flash-SPS). (iv) Accroissement de 170% des performances d'alliage du système Pb-Sb-Te en générant des vacances de sites (composés non-stœchiométriques). (v) Le suivi des conséquences du dopage de GeTe par un seul élément a montré la nécessité d'un co-dopage pour simultanément accroître la conductivité électronique et le Seebeck. (vi) Le co-dopage In-Bi de GeTe a permis de créer des niveaux résonants (In) et d'accroitre la diffusion thermique (Bi). (vii) Enfin, le résultat le plus remarquable porte sur le co-dopage Ga-Sb de GeTe qui permet d'effectuer de l'ingénierie de structure de bandes. Couplé à une synthèse par hybrid flash SPS ces matériaux prometteurs permettent d'obtenir un zT 2 sur une large gamme de température (600–773 K). / With the performance of direct conversion between thermal and electrical energy, thermoelectric materials, which are crucial in the renewable energy conversion roadmap, provide an alternative for power generation and refrigeration to solve the global energy crisis. But the low efficiency of the current materials, their usual costs, availability, and limited working temperatures, drastically constrain their application. Hence, the search for new and more efficient thermoelectric materials is one of the most dynamic objectives of this thesis. The key milestones achieved from this thesis work includes: (i) elucidating the mechanism for hole conductivity in Cu-As-Te glasses by X-ray absorption spectroscopy and quantum simulations; (ii) formulating a novel approach to achieve phonon-glass electron-crystal mechanism by crystallizing the Ge20Te77Se3 glasses by excess doping with metals or semi-metals (glass-ceramics); (iii) demonstrating the effect of processing route on the thermoelectric performance of CuPb18SbTe20 and highlighting the advantage of hybrid-flash spark plasma sintering technique, i.e., better optimization of electrical and thermal transport properties and achieving multi-scale hierarchical architectures; (iv) improving the thermoelectric performance of Pb-Sb-Te alloys (enhancement by 170%) by tuning their cation vacancies (Pb deficiencies); (v) understating the impact of doping just a group-11 coinage metal, or group-13 element on GeTe solid-state solution and recapitulating the need for pair substitution; (vi) substantially enhancing the average zT of In-Bi codoped GeTe; (vii) achieving a remarkably high and stable zT of close to 2 over a wide temperature range (600 – 773 K) by manipulating the electronic bands in Ga-Sb codoped GeTe, which has been processed by hybrid flash-spark plasma sintering, thus making it a serious candidate for energy harvesting systems.

Orientador: Fernando Alvarez / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T13:13:51Z (GMT). No. of bitstreams: 1 OchoaBecerra_ErikaAbigail_D.pdf: 5724102 bytes, checksum: c62c9d58c0f942077a9cfd1ba1d75d94 (MD5) Previous issue date: 2007 / Resumo: A presente tese trata o estudo as propriedades físicas resultantes em sistemas metálicos com superfícies nanoestrutura as pelo bombardeio com gás nobre e posteriormente nitretados com técnicas basea as em plasma. A busca e novas condições e tratamento que aumentem a velocidade a ifusão o nitrogênio é e grande interesse para a modificação e superfícies metálicas. Para isto, o pré-tratamento a superfície o material, através e métodos e refinamento os grãos a superfície, é fundamental na melhora a incorporação e nitrogênio no material. Este trabalho visa o estudo e sistemas basea os em ferro, especialmente aços e interesse tecnológico, com superfícies refinadas até a escala nanométrica e submeti os ao processo e nitretação. Neste caso, a superfície a amostra considera a po e ser nanoestrutura a por bombar eiodiônico com gases nobres ("atomic attrition") e posteriormente nitreta a usan o feixe e íons. O material escolhi o para o presente trabalho é o aço enomina o AISI 4140, um aço e baixa liga. O tratamento superficial ("atomic attrition") prévio ao processo e nitretação possibilita o aumento o conteúdo e nitrogênio em profundidade permitindo ainda que a nitretação possa ser realizada a temperaturas relativamente mais baixas (T ~ 300 °C). A caracterização as amostras pré-trata as e/ou nitretadas é realizada in-situ por espectroscopia e elétrons fotoemitidos garantindo condições únicas para o estudo o fenômeno e nanoestruturação e sua influência na ifusão o nitrogênio. Outras técnicas usuais e caracterização utiliza as foram a nano-in entação, raios-X (em nosso grupo e no LNLS) e microscopia óptica e eletrônica. Os resultados indicam o sucesso na nanoestruturação a superfície as amostras pela formação e caminhos e rápida difusão e nitrogênio e assim aumentando a dureza resultante o material / Abstract: The present thesis treats the stu y of the physical resultant properties in metallic systems with nano-structure surfaces by the bombar ment with noble gas an subsequently nitri e with techniques base on plasma. The search for new con itions of treatment that increase the spee of diffusion of the nitrogen is of great interest for the modification of metal surfaces. For this, the pre-treatment of the material surface, through methods of refinement of the grains at the surface, is fundamental in the improvement of the incorporation of nitrogen in the material. This work aims at the study of systems base on iron, especially steels of technological interest, with surfaces refine up to the nanometric scale an subjecte to the nitriding process. In this case, the surface of the considere sample can be nano - structure by ionic bombar ment with noble gases ("atomic attrition") an subsequently nitride using bundle of ions. The material chosen for the present work is the steel AISI 4140, a steel of low alloy. The superficial treatment ("atomic attrition") prior to the process of nitriding makes possible the increase of the content of nitrogen in epth allowing still that nitriding could be carrie out to relatively lower temperatures (T ~ 300 °C). The characterization of the samples pre-treate an / or nitride is carrie out in-situ by spectroscopy of photo-emitte electrons guaranteeing singular conditions for the study of the phenomenon of nano-structuring an its influence in the nitrogen iffusion. Other usual techniques of characterization use were nano - in dentation, X-rays (in our group an at LNLS), optical an electronic microscopy. The results indicate the success in the nano - structuring of the sample surface for the formation of ways of quick diffusion of nitrogen an so increasing the resultant har ness of the material / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Commercial media and surfaces for cell cultivation do not promote conditions for cell cultivation and proliferation with specific interactions cell-material surface. The aim of this thesis is developing surfaces for cell cultivation which mimic conditions in living tissues. The approach introduced in this thesis is based on applying biomimetic peptide ligands on inert supporting materials with non-fouling properties. Considering that a choice of a ligand sequence and distance between peptides can dramatically influence the outcome, a few model peptides with varying parameters were synthesized and investigated. The cell adhesive peptides were synthesized by solid phase peptide synthesis and scrambled peptides were synthesized and tested as well. Spatial deposition of peptides is another highly important point of study. The 'click'- reaction was used to successfully immobilize the peptides on produced surfaces. The distance between the peptide molecules on the surfaces was controlled by their concentration in a reaction mixture. The reference samples were immobilized with radiolabeled peptides for quantitative estimation of the peptides present. Then, the materials with different types of peptides and range of concentrations were examined via cultivation of cell cultures. The experiments were focused on...

Aujourd’hui, les énergies renouvelables comme l’énergie éolienne, l’énergie solaire, l’énergie hydroélectrique et la thermoélectricité jouent un rôle essentiel dans la couverture de nos besoins en énergie. Parmi ces différentes sources d’énergie, la thermoélectricité, qui permet de convertir la chaleur en électricité ou inversement, attire une grande attention grâce à son large champ d’application. Les actuelles avancées dans la recherche thermoélectrique visent l’amélioration du rendement de conversion des modules thermoélectriques, à travers l’optimisation des propriétés thermoélectriques intrinsèques des matériaux utilisés (coefficient de Seebeck, conductivité électrique et conductivité thermique). Pour cela, différentes approches ont été étudiées (dopage, nouveau alliages, nanostucturation …). Parmi ces approches, la nanostructration des matériaux a été largement étudiée pour mener à bien cet objectif. Dans ce travail de thèse, nous nous sommes intéressés à étudier l’effet de la nanostructuration de surface des matériaux (silicium mesoporeux et oxyde de titane déposé en couches minces) par faisceaux laser ultra-court (picoseconde et femtoseconde) sur l’évolution de leurs propriétés thermoélectriques. Dans un premier temps, nous nous sommes focalisés sur l’étude des différents phénomènes physiques impliqués durant l’interaction laser-matière ainsi que sur la formation des différentes nanostructures résultantes (en forme de ripples, spikes, dots et autres) en fonction de la dose laser appliquée (la fluence et le nombre de pulses). La formation de ces nanostructures a été étudiée suivant deux régimes (stationnaire et dynamique). Après l’optimisation des paramètres conduisant à la formation de ces nanostructures, la caractérisation du coefficient de Seebeck et la conductivité électrique avant et après la nanostructuration de ces matériaux a été réalisée grâce à un nouveau dispositif de mesure (ZT-meter) développé au laboratoire GREMI. Les résultats de mesures montrent une importante amélioration du coefficient de Seebeck et la conductivité électrique après la nanostrucutration. Un facteur d’augmentation de la puissance thermoélectrique a été observé pour les deux matériaux étudiés ; notamment dans le cas de couches minces d’oxyde de titane (jusqu’à 500 fois). / Today, renewable energies such as wind, solar, hydropower and thermoelectricity play an essential role to cover our energy needs. Among these different sources of energy, thermoelectricity, which offers the ability to convert a heat into electricity or vice versa, has attracted a great attention due to its wide field of potential applications. The current advances in thermoelectric research are focusing on the improvement of the conversion efficiency of thermoelectric devices through optimizing and improving the thermoelectric properties of the thermoelectric materials (Seebeck coefficient, electrical conductivity and thermal conductivity). For this, different approaches (doping, new materials, nanostucturing...) have been investigated in the literature. Among these approaches, nanostructuring of materials is the most studied in the literature in order to improve the thermoelectric properties of materials. In this thesis work, we aimed to study the effect of surface nanostructuring of materials (mesoporous silicon and titanium oxide deposited in thin film) by ultra-short laser beams (picosecond and femtosecond) on the evolution of their thermoelectric properties. First, we focused on the study of various physical phenomena involved during the laser-matter interaction that yield to the formation of very different nanostructures in form of ripples, spikes, dots and others as function of the applied laser dose (fluence and number of pulses). The formation of these nanostructures has been studied in two regimes (stationary and dynamic). After optimizing the laser parameters leading to the formation of such nanostructures, a characterization of Seebeck coefficient and the electrical conductivity before and after the nanostructuring of these materials was carried out by using a new experimental setup (ZT-meter) designed and validated in GREMI laboratory. The results of measurements showed an important improvement of Seebeck coefficient and electrical conductivity after nanostructuring. This important improvement observed with the both materials leaded to a strong increase in the thermoelectric power factor (reaching roughly 50000%).

The development of new materials with novel properties plays an important role in improving our lives and welfare. Research in Nanotechnology can provide e.g. cheaper and smarter materials in applications such as energy storage and sensors. In order for this development to proceed, we need to be able to characterize the material properties at the nano-, and even the atomic scale. The ultimate goal is to be able to tailor them according to our needs. One of the great challenges concerning the characterization of nano-sized objects is how to achieve the physical contact to them. This thesis is focused on the contacting of nanoobjects with the aim of electrically characterizing them and subsequently understanding their electrical properties. The analyzed nanoobjects are carbon nanosheets, nanotetrapods, nanoparticles and molecular systems. Two contacting strategies were employed in this thesis. The first strategy involved the development of a focused ion beam (FIB) based nanocontact platform. The platform consists of gold nanoelectrodes, having nanogaps of 10-30 nm, on top of an insulating substrate. Gold nanoparticles, double-stranded DNA and cadmium telluride nanotetrapods have been trapped in the gaps by using dielectrophoresis. In certain studies, the gold electrodes have also been coated with conducting or non-conducting molecules, prior to the trapping of gold nanoparticles, in order to form molecular junctions. These junctions were subsequently electrically characterized to evaluate the conduction properties of these molecular systems. For the purpose of better controlling the attachment of molecules to the nanoelectrodes, a novel route to synthesize alkanedithiol coated gold nanoparticles was developed. The second contacting strategy was based on the versatility of the FIB instrument as a platform for in-situ manipulation and electrical characterization of non-functionalized and functionalized carbon nanosheets, where it was found that the functionalized samples had an increased conductivity by more than one order of magnitude. Both contacting strategies proved to be valuable for building knowledge around contacting and electrical characterization of nanoobjects