An in operando study of chemical expansion and oxygen surface exchange rates in epitaxial GdBaCo2O5.5 electrodes in a solid-state electrochemical cell by time-resolved X-ray diffraction

This report explores the fundamental characteristics of epitaxial thin films of a mixed ionic electronic conducting GdBaCoO (GBCO) material with a layered perovskite structure, relevant for use as an active electrode for the oxygen reduction and evolution reactions in electrochemical devices. Time-resolved X-ray diffraction in combination with voltage step chrono-amperometric measurements in a solid state electrochemical cell provides a deeper insight into the chemical expansion mechanism in the GBCO electrode. The chemical expansion coefficient along the c-axis, α, shows a negative value upon the compound oxidation contrary to standard perovskite materials with disordered oxygen vacancies. Chemical expansion also shows a remarkable asymmetry from α = -0.037 to -0.014 at δ 0, respectively. This change in chemical expansion is an indication of a different mechanism of the structural changes associated with the variable Co cation oxidation state from Co → Co → Co. Since redox reactions are dominated by oxygen surface exchange between the GBCO electrode and gas atmosphere, monitoring the time response of the structural changes allows for direct determination of oxygen reduction and evolution reaction kinetics. The reaction kinetics are progressively slowed down upon reduction in the δ 0 region, in agreement with the structural changes and the electronic carrier delocalization when crossing δ = 0. This work validates the time-resolved XRD technique for fast and reversible measurements of electrode activity in a wide range of oxygen non-stoichiometry in a solid-state electrochemical cell operating under realistic working conditions

On the thermoelectric properties of Nb-doped SrTiO3epitaxial thin films

The exploration for thermoelectric thin films of complex oxides such as SrTiO3-based oxides is driven by the need for miniaturized harvesting devices for powering the Internet of Things (IoT). However, there is still not a clear consensus in the literature for the underlying influence of film thickness on thermoelectric properties. Here, we report the fabrication of epitaxial thin films of 6% Nb-doped SrTiO3 on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) single crystal using pulsed laser deposition (PLD) where the film thickness was varied from 2 nm to 68 nm. The thickness dependence shows a subtle increase of tetragonality of the thin film lattice and a gradual drop of the electrical conductivity, the density of charge carriers, and the thermoelectric Seebeck coefficient as the film thickness decreases. DFT-based calculations show that ∼2.8% increase in tetragonality results in an increased splitting between t2g and eg orbitals to ∼42.3 meV. However, experimentally observed tetragonality for films between 68 to 13 nm is only 0.06%. Hence, the effect of thickness on tetragonality is neglected. We have discussed the decrease of conductivity and the Seebeck coefficient based on the decrease of carriers and change in the scattering mechanism, respectively.The research leading to these results has received funding from the European Union's H2020 Programme under Grant Agreement no 824072 – HARVESTORE

Thermoelectric performance of layered cobaltate epitaxial films deposited by pulsed laser evaporation

El efecto termoeléctrico o efecto Seebeck es un fenómeno característico de un conductor eléctrico y que proporciona información fundamental como la estructura electrónica del material entorno a la energía de Fermi. Desde un punto de vista tecnológico la fabricación de dispositivos termoeléctricos es importantes debido a su capacidad para generar un voltaje eléctrico a partir de un gradiente de temperatura. Sin embargo, la baja eficiencia que generalmente se obtiene a partir de materiales termoeléctricos convencionales, limita su aplicación práctica. Para el diseño de materiales termoeléctricos con una eficiencia adecuada es importante que el material posea simultáneamente varias características: alta conductividad eléctrica, baja conductividad térmica y un elevado poder termoeléctrico S. Por lo tanto, el diseño de materiales termoeléctricos eficientes es a la vez un reto científico así como un desafío de ingeniería. Entre los diferentes materiales termoeléctricos explorados en la literatura los compuestos de estructura laminar presentan unas importantes características, la mayoría de ellas relacionadas con la anisotropía en sus propiedades. De esta manera el crecimiento de películas epiaxiales (o altamente orientadas) permite explorar las propiedades físicas en una determinada dirección cristalográfica. Al mismo tiempo, el uso de películas epitaxiales permite modular las propiedades de transporte electrónico, así como las propiedades de transporte térmico y termoeléctrico, modificando la tensión epitaxial, la presencia de defectos, o las características de la intercara en la heteroestructura. Esta tesis se centra en la investigación de las propiedades termoeléctricas de películas epitaxiales o con un alto grado de orientación preferencial de óxidos de cobalto. La tesis incluye una descripción detallada de los diferentes métodos posibles para el ajuste de la estequiometría de oxígeno de las películas. La tesis se divide en varios capítulos. El capítulo 1 proporciona una breve introducción general del efecto termoeléctrico (TE), para seguidamente ofrecer una descripción de las propiedades TE de capas de óxidos de cobalto. Al mismo tiempo se presenta una breve revisión de la literatura donde se prestó especial atención a los aspectos científicos y resultados experimentales relacionados. En el Capítulo 2 se presentan la metodología experimental utilizada en este estudio. En los capítulos 3, 4 y 5 se describen los resultados de las propiedades termoeléctricas de las películas de cobaltitas, al mismo tiempo que se discuten para extraer conclusiones específicas. En el Capítulo 6 se presenta el experimento realizado para el control de la estequiometría de oxígeno mediante un dispositivo electroquímico de estado sólido. Por último, en el capítulo 7 se presenta un resumen de las conclusiones generales y se discuten las perspectivas de futuro. En resumen, la exploración de las propiedades termoeléctricas en películas delgadas de óxidos de cobalto lleva a la conclusión de que, contrariamente a la creencia actual, la degeneración spin-órbita no contribuye al límite de alta temperatura del coeficiente de Seebeck, y que únicamente la distribución estadística de los electrones en los estados disponibles es relevante para la determinación de dicho límite. En consecuencia, se propone una modificación de la fórmula de Heike, que incluye información del tamaño efectivo de los polarones responsables del transporte de carga.Thermoelectric Seebeck effect is a very important phenomenon of a charge conductor as it provides fundamental information such as the electronic band structure near Fermi energy. Thermoelectric devices are very important from a technological point of view because of its ability to convert electricity from a temperature gradient. However, the low efficiency; of the thermoelectric materials, limits their use for practical applications. Therefore, it is important to design thermoelectric materials that are good electrical conductor, poor thermal conductor and at the same time have large S. Thus, design of efficient thermoelectric materials is a scientific and also an engineering challenge. Among the different thermoelectric materials explored in the literature, layered compounds have shown very important characteristics most of them related to their anisotropic properties. In that sense, the growth of epitaxial (or highly oriented) thin films allows exploring the physical properties in a particular crystallographic direction and, therefore is ideal for exploring the possible anisotropy. At the same time, it allows to engineer the electronic, thermoelectric as well as the thermal transport properties by playing with epitaxial strain, defect chemistry and interface between multilayers. This thesis reports the investigation of thermoelectric properties of epitaxial and highly oriented layered cobaltates films includes a detailed description of the different possible methods to tune the oxygen stoichiometry of thin films. The thesis is divided into several chapters. A brief introduction of the thermoelectric phenomena is provided in chapter 1. Then, the description is focused on the TE properties of layered cobaltates. A short literature review is presented where attention was paid to the status of scientific problems and experimental results. Chapter 2 shows the techniques used to perform the experiments. Thermoelectric properties of layered cobaltates films are presented from chapter 3 to 5 in order to address specific questions in each chapter. Chapter 6 shows the control over oxygen stoichiometry of cobaltate thin film by means of solid state electrochemical approach. Finally, a summary of the conclusions and perspectives are discussed in chapter 7. The exploration of the thermoelectric properties of layered cobaltates thin films led to the conclusion that, contrary to the current belief, spin-orbit degeneracy does not contribute to the high-temperature limit of the Seebeck coefficient, and only the statistical distribution of electrons in the available states is relevant for determining such limit. Consequently, a modification of the Heike’s formula, containing the information of polaron size, is proposed

Thermoelectric performance of layered cobaltate epitaxial films deposited by pulsed laser evaporation

El efecto termoeléctrico o efecto Seebeck es un fenómeno característico de un conductor eléctrico y que proporciona información fundamental como la estructura electrónica del material entorno a la energía de Fermi. Desde un punto de vista tecnológico la fabricación de dispositivos termoeléctricos es importantes debido a su capacidad para generar un voltaje eléctrico a partir de un gradiente de temperatura. Sin embargo, la baja eficiencia que generalmente se obtiene a partir de materiales termoeléctricos convencionales, limita su aplicación práctica. Para el diseño de materiales termoeléctricos con una eficiencia adecuada es importante que el material posea simultáneamente varias características: alta conductividad eléctrica, baja conductividad térmica y un elevado poder termoeléctrico S. Por lo tanto, el diseño de materiales termoeléctricos eficientes es a la vez un reto científico así como un desafío de ingeniería. Entre los diferentes materiales termoeléctricos explorados en la literatura los compuestos de estructura laminar presentan unas importantes características, la mayoría de ellas relacionadas con la anisotropía en sus propiedades. De esta manera el crecimiento de películas epiaxiales (o altamente orientadas) permite explorar las propiedades físicas en una determinada dirección cristalográfica. Al mismo tiempo, el uso de películas epitaxiales permite modular las propiedades de transporte electrónico, así como las propiedades de transporte térmico y termoeléctrico, modificando la tensión epitaxial, la presencia de defectos, o las características de la intercara en la heteroestructura. Esta tesis se centra en la investigación de las propiedades termoeléctricas de películas epitaxiales o con un alto grado de orientación preferencial de óxidos de cobalto. La tesis incluye una descripción detallada de los diferentes métodos posibles para el ajuste de la estequiometría de oxígeno de las películas. La tesis se divide en varios capítulos. El capítulo 1 proporciona una breve introducción general del efecto termoeléctrico (TE), para seguidamente ofrecer una descripción de las propiedades TE de capas de óxidos de cobalto. Al mismo tiempo se presenta una breve revisión de la literatura donde se prestó especial atención a los aspectos científicos y resultados experimentales relacionados. En el Capítulo 2 se presentan la metodología experimental utilizada en este estudio. En los capítulos 3, 4 y 5 se describen los resultados de las propiedades termoeléctricas de las películas de cobaltitas, al mismo tiempo que se discuten para extraer conclusiones específicas. En el Capítulo 6 se presenta el experimento realizado para el control de la estequiometría de oxígeno mediante un dispositivo electroquímico de estado sólido. Por último, en el capítulo 7 se presenta un resumen de las conclusiones generales y se discuten las perspectivas de futuro. En resumen, la exploración de las propiedades termoeléctricas en películas delgadas de óxidos de cobalto lleva a la conclusión de que, contrariamente a la creencia actual, la degeneración spin-órbita no contribuye al límite de alta temperatura del coeficiente de Seebeck, y que únicamente la distribución estadística de los electrones en los estados disponibles es relevante para la determinación de dicho límite. En consecuencia, se propone una modificación de la fórmula de Heike, que incluye información del tamaño efectivo de los polarones responsables del transporte de carga.Thermoelectric Seebeck effect is a very important phenomenon of a charge conductor as it provides fundamental information such as the electronic band structure near Fermi energy. Thermoelectric devices are very important from a technological point of view because of its ability to convert electricity from a temperature gradient. However, the low efficiency; of the thermoelectric materials, limits their use for practical applications. Therefore, it is important to design thermoelectric materials that are good electrical conductor, poor thermal conductor and at the same time have large S. Thus, design of efficient thermoelectric materials is a scientific and also an engineering challenge. Among the different thermoelectric materials explored in the literature, layered compounds have shown very important characteristics most of them related to their anisotropic properties. In that sense, the growth of epitaxial (or highly oriented) thin films allows exploring the physical properties in a particular crystallographic direction and, therefore is ideal for exploring the possible anisotropy. At the same time, it allows to engineer the electronic, thermoelectric as well as the thermal transport properties by playing with epitaxial strain, defect chemistry and interface between multilayers. This thesis reports the investigation of thermoelectric properties of epitaxial and highly oriented layered cobaltates films includes a detailed description of the different possible methods to tune the oxygen stoichiometry of thin films. The thesis is divided into several chapters. A brief introduction of the thermoelectric phenomena is provided in chapter 1. Then, the description is focused on the TE properties of layered cobaltates. A short literature review is presented where attention was paid to the status of scientific problems and experimental results. Chapter 2 shows the techniques used to perform the experiments. Thermoelectric properties of layered cobaltates films are presented from chapter 3 to 5 in order to address specific questions in each chapter. Chapter 6 shows the control over oxygen stoichiometry of cobaltate thin film by means of solid state electrochemical approach. Finally, a summary of the conclusions and perspectives are discussed in chapter 7. The exploration of the thermoelectric properties of layered cobaltates thin films led to the conclusion that, contrary to the current belief, spin-orbit degeneracy does not contribute to the high-temperature limit of the Seebeck coefficient, and only the statistical distribution of electrons in the available states is relevant for determining such limit. Consequently, a modification of the Heike’s formula, containing the information of polaron size, is proposed

Synthesis of Site-specific DNA adducts of Aromatic Amine and Nitro-aromatic Carcinogens and Study of their Replication Properties

A large number of nitro polycyclic aromatic hydrocarbons (nitro-PAHs) and heterocyclic aromatic amines (HAAs) are present in the environment and many compounds of these families are carcinogenic. 2ʹ-Deoxyribonucleic acid (DNA) adduct or lesion formation by these class of compounds play an important role in the process of carcinogenesis. Upon metabolic activation by cytochrome P450 family of enzymes, these nitro-PAHs or HAAs form electrophilic metabolites, which then reacts with the nucleic acid bases of DNA to form DNA-carcinogen adducts. Subsequent to DNA damage, activation of DNA damage checkpoints helps to arrest the growth of the damaged cell and allows the DNA repair mechanism to rectify the error. However, sometimes prior to DNA repair, cells can undergo replications, but replicative DNA polymerases, in most cases, cannot bypass a DNA damage. Depending on the nature of the damage, a special group of DNA polymerases known as translesion synthesis (TLS) DNA polymerases help to bypass this bulky adducts. But this damaged DNA replication by TLS polymerases can lead to mutations, which, in turn, may lead to production of altered proteins. If the mutation occurs in a tumor suppressor gene or oncogene, it can lead to human cancers. 3-Nitrobenzanthrone (3-nitro-7H-benz-[d,e]anthracen-7-one, 3-NBA), a member of the nitro polycyclic aromatic hydrocarbon (PAH), is found in exhausts from diesel and gasoline engines owing to incomplete combustion of the fuel and is considered an environmental pollutant and a suspected human carcinogen. 3-NBA is much more mutagenic than the well-studied PAH benzo[a]pyrene (B[a]P) and the most abundant nitroarene found in diesel exhaust, 1-nitropyrene (1-NP). It causes DNA damage and exhibits mutagenicity in bacterial and mammalian cells. 3-NBA is one of the most potent mutagens in the Ames’ assay. Upon metabolic activation, it forms N-(deoxyguanosin-8-yl)-3-amino-benzanthrone (C8-dG-ABA), the major alkylation adduct at the C8-position of 2ʹ-deoxyguanosine (dG). To investigate its replication properties, I have used a total synthesis approach to synthesize the C8-dG-ABA adduct. The adduct was incorporated into a defined site of a dodecamer oligodeoxynucleotide, 5’-CATG*ATGACGCT-3’, where G* represents the adducted dG and the bypass mechanism was analyzed using crystallographic techniques with the incoming 2ʹ-deoxycytosine triphosphate (dCTP). The environmental pollutant 6-nitrochrysene (6-NC) is a potent mutagen and mammary carcinogen in rats. 6-NC is the most potent carcinogen ever tested in the newborn mouse assay. Upon metabolic activation, 6-NC forms two major adducts with dG, one at the C8-position, N-(dG-8-yl)-6-AC, and the other at the N2-position, 5-(dG-N2-yl)-6-AC. Here, we report the total synthesis of site-specific oligonucleotides containing the 6-NC-derived C8 dG adduct, N-(dG-8-yl)-6-AC by Pd-catalyzed Buchwald-Hartwig cross coupling of 6-aminochrysene with protected C8-bromo-dG derivative. The monomer for solid-phase DNA synthesis was prepared from this adduct by its deprotection followed by conversion to the corresponding phosphoramidite, which was used to synthesize the site-specific oligonucleotides 5’-GTGCG*TGTTTGT-3, which contains the local DNA sequence of the tumor suppressor gene p53 (codon 273 is underlined), where G* represents the C8-dG-6-Ac adduct. The adduct-containing oligonucleotides were purified by reversed phase HPLC followed by denaturing polyacrylamide gel electrophoresis and characterized by mass spectrometry. Oligonucleotide 5’-GTGCG*TGTTTGT-3 was then incorporated into a plasmid and replicated in human embryonic kidney 293T cells, which showed that the DNA adduct is mutagenic inducing largely G→T transversions. We also investigated the roles of several translesion synthesis DNA polymerases in its bypass using siRNA knockdown approach. 2-Amino-9H-pyrido [2, 3-b] indole (AαC) is the most abundant heterocyclic aromatic amine (HAA) and a possible liver and gastrointestinal carcinogen found in the mainstream tobacco smoke. Upon cytochrome P450 mediated N-oxidation of the exocyclic amine of AαC, it reacts to DNA to form N-(deoxyguanosin-8-yl)-AαC adduct (C8-dG-AαC). To study the structural and biological effects of C8-dG-AαC lesion, we conceived a total synthesis approach using 9-benzyl-9H- pyrido [2, 3-b] indol-2-amine and protected C8-bromo-2\u27-deoxyguanosine derivative as precursors to furnish the 2\u27-dG-C8-AαC adduct. The DNA adduct will be incorporated into defined sites of a dodecamer oligodeoxynucleotide, 5’-GTGCG*TGTTTGT-3’, as in the previous case to understand the carcinogenic potency and critical roles of human DNA polymerases in its translesion synthesis

An in operando study of chemical expansion and oxygen surface exchange rates in epitaxial GdBaCo2O5.5 electrodes in a solid-state electrochemical cell by time-resolved X-ray diffraction

This report explores the fundamental characteristics of epitaxial thin films of a mixed ionic electronic conducting GdBaCo2O5.5±δ (GBCO) material with a layered perovskite structure, relevant for use as an active electrode for the oxygen reduction and evolution reactions in electrochemical devices. Time-resolved X-ray diffraction in combination with voltage step chrono-amperometric measurements in a solid state electrochemical cell provides a deeper insight into the chemical expansion mechanism in the GBCO electrode. The chemical expansion coefficient along the c-axis, αc, shows a negative value upon the compound oxidation contrary to standard perovskite materials with disordered oxygen vacancies. Chemical expansion also shows a remarkable asymmetry from αc = −0.037 to −0.014 at δ 0, respectively. This change in chemical expansion is an indication of a different mechanism of the structural changes associated with the variable Co cation oxidation state from Co2+ → Co3+ → Co4+. Since redox reactions are dominated by oxygen surface exchange between the GBCO electrode and gas atmosphere, monitoring the time response of the structural changes allows for direct determination of oxygen reduction and evolution reaction kinetics. The reaction kinetics are progressively slowed down upon reduction in the δ 0 region, in agreement with the structural changes and the electronic carrier delocalization when crossing δ = 0. This work validates the time-resolved XRD technique for fast and reversible measurements of electrode activity in a wide range of oxygen non-stoichiometry in a solid-state electrochemical cell operating under realistic working conditions.The authors acknowledge the support from projects: MINECO ref. MAT2016-77100-C2-1-P; CNRS-CSIC PICS Project ref. 261091, the EU for funding through project H2020-MSCA-RISE-2014 ref. 645658, and the AGAUR agency for 2017SGR. ICN2 is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant no. SEV-2013-0295). A. C. particularly acknowledges the support of Severo Ochoa programme for his PhD grant.Peer reviewe

An in operando study of chemical expansion and oxygen surface exchange rates in epitaxial GdBaCo2O5.5 electrodes in a solid-state electrochemical cell by time-resolved X-ray diffraction

This report explores the fundamental characteristics of epitaxial thin films of a mixed ionic electronic conducting GdBaCoO (GBCO) material with a layered perovskite structure, relevant for use as an active electrode for the oxygen reduction and evolution reactions in electrochemical devices. Time-resolved X-ray diffraction in combination with voltage step chrono-amperometric measurements in a solid state electrochemical cell provides a deeper insight into the chemical expansion mechanism in the GBCO electrode. The chemical expansion coefficient along the c-axis, α, shows a negative value upon the compound oxidation contrary to standard perovskite materials with disordered oxygen vacancies. Chemical expansion also shows a remarkable asymmetry from α = -0.037 to -0.014 at δ 0, respectively. This change in chemical expansion is an indication of a different mechanism of the structural changes associated with the variable Co cation oxidation state from Co → Co → Co. Since redox reactions are dominated by oxygen surface exchange between the GBCO electrode and gas atmosphere, monitoring the time response of the structural changes allows for direct determination of oxygen reduction and evolution reaction kinetics. The reaction kinetics are progressively slowed down upon reduction in the δ 0 region, in agreement with the structural changes and the electronic carrier delocalization when crossing δ = 0. This work validates the time-resolved XRD technique for fast and reversible measurements of electrode activity in a wide range of oxygen non-stoichiometry in a solid-state electrochemical cell operating under realistic working conditions

Thermoelectric performance of layered cobaltate epitaxial films deposited by pulsed laser evaporation /

El efecto termoeléctrico o efecto Seebeck es un fenómeno característico de un conductor eléctrico y que proporciona información fundamental como la estructura electrónica del material entorno a la energía de Fermi. Desde un punto de vista tecnológico la fabricación de dispositivos termoeléctricos es importantes debido a su capacidad para generar un voltaje eléctrico a partir de un gradiente de temperatura. Sin embargo, la baja eficiencia que generalmente se obtiene a partir de materiales termoeléctricos convencionales, limita su aplicación práctica. Para el diseño de materiales termoeléctricos con una eficiencia adecuada es importante que el material posea simultáneamente varias características: alta conductividad eléctrica, baja conductividad térmica y un elevado poder termoeléctrico S. Por lo tanto, el diseño de materiales termoeléctricos eficientes es a la vez un reto científico así como un desafío de ingeniería. Entre los diferentes materiales termoeléctricos explorados en la literatura los compuestos de estructura laminar presentan unas importantes características, la mayoría de ellas relacionadas con la anisotropía en sus propiedades. De esta manera el crecimiento de películas epiaxiales (o altamente orientadas) permite explorar las propiedades físicas en una determinada dirección cristalográfica. Al mismo tiempo, el uso de películas epitaxiales permite modular las propiedades de transporte electrónico, así como las propiedades de transporte térmico y termoeléctrico, modificando la tensión epitaxial, la presencia de defectos, o las características de la intercara en la heteroestructura. Esta tesis se centra en la investigación de las propiedades termoeléctricas de películas epitaxiales o con un alto grado de orientación preferencial de óxidos de cobalto. La tesis incluye una descripción detallada de los diferentes métodos posibles para el ajuste de la estequiometría de oxígeno de las películas. La tesis se divide en varios capítulos. El capítulo 1 proporciona una breve introducción general del efecto termoeléctrico (TE), para seguidamente ofrecer una descripción de las propiedades TE de capas de óxidos de cobalto. Al mismo tiempo se presenta una breve revisión de la literatura donde se prestó especial atención a los aspectos científicos y resultados experimentales relacionados. En el Capítulo 2 se presentan la metodología experimental utilizada en este estudio. En los capítulos 3, 4 y 5 se describen los resultados de las propiedades termoeléctricas de las películas de cobaltitas, al mismo tiempo que se discuten para extraer conclusiones específicas. En el Capítulo 6 se presenta el experimento realizado para el control de la estequiometría de oxígeno mediante un dispositivo electroquímico de estado sólido. Por último, en el capítulo 7 se presenta un resumen de las conclusiones generales y se discuten las perspectivas de futuro. En resumen, la exploración de las propiedades termoeléctricas en películas delgadas de óxidos de cobalto lleva a la conclusión de que, contrariamente a la creencia actual, la degeneración spin-órbita no contribuye al límite de alta temperatura del coeficiente de Seebeck, y que únicamente la distribución estadística de los electrones en los estados disponibles es relevante para la determinación de dicho límite. En consecuencia, se propone una modificación de la fórmula de Heike, que incluye información del tamaño efectivo de los polarones responsables del transporte de carga.Thermoelectric Seebeck effect is a very important phenomenon of a charge conductor as it provides fundamental information such as the electronic band structure near Fermi energy. Thermoelectric devices are very important from a technological point of view because of its ability to convert electricity from a temperature gradient. However, the low efficiency; of the thermoelectric materials, limits their use for practical applications. Therefore, it is important to design thermoelectric materials that are good electrical conductor, poor thermal conductor and at the same time have large S. Thus, design of efficient thermoelectric materials is a scientific and also an engineering challenge. Among the different thermoelectric materials explored in the literature, layered compounds have shown very important characteristics most of them related to their anisotropic properties. In that sense, the growth of epitaxial (or highly oriented) thin films allows exploring the physical properties in a particular crystallographic direction and, therefore is ideal for exploring the possible anisotropy. At the same time, it allows to engineer the electronic, thermoelectric as well as the thermal transport properties by playing with epitaxial strain, defect chemistry and interface between multilayers. This thesis reports the investigation of thermoelectric properties of epitaxial and highly oriented layered cobaltates films includes a detailed description of the different possible methods to tune the oxygen stoichiometry of thin films. The thesis is divided into several chapters. A brief introduction of the thermoelectric phenomena is provided in chapter 1. Then, the description is focused on the TE properties of layered cobaltates. A short literature review is presented where attention was paid to the status of scientific problems and experimental results. Chapter 2 shows the techniques used to perform the experiments. Thermoelectric properties of layered cobaltates films are presented from chapter 3 to 5 in order to address specific questions in each chapter. Chapter 6 shows the control over oxygen stoichiometry of cobaltate thin film by means of solid state electrochemical approach. Finally, a summary of the conclusions and perspectives are discussed in chapter 7. The exploration of the thermoelectric properties of layered cobaltates thin films led to the conclusion that, contrary to the current belief, spin-orbit degeneracy does not contribute to the high-temperature limit of the Seebeck coefficient, and only the statistical distribution of electrons in the available states is relevant for determining such limit. Consequently, a modification of the Heike's formula, containing the information of polaron size, is proposed

Improved High Temperature Thermoelectric Properties in Misfit Ca3Co4O9 by Thermal Annealing

Ca3Co4O9, a p-type thermoelectric material based on transition-metal oxides, has garnered significant interest due to its potential in thermoelectric applications. Its unique misfit-layered crystal structure contributes to low thermal conductivity and a high Seebeck coefficient, leading to a thermoelectric figure of merit (zT) of ≥1 at 1000 K. Conventionally, it has been believed that thermopower reaches its upper limit above 200 K. However, our thermopower measurements on polycrystalline Ca3Co4O9 samples have revealed an unexpected increase in thermopower above 380 K. In this study, we investigate the effects of high oxygen pressure annealing on Ca3Co4O9 and provide an explanation based on the mixed oxide states of cobalt and carrier hopping. Our results demonstrate that annealing induces modifications in the defect chemistry of Ca3Co4O9, leading to a decrease in electron hopping probability and the emergence of a thermal activation-like behavior in thermopower. These findings carry significant implications for the design and optimization of thermoelectric materials based on misfit cobaltates, opening new avenues for enhanced thermoelectric performance