Le graphène a des propriétés électroniques et mécaniques extraordinaires en raison de sa structure de bande linéaire. Toutefois, l'absence d’une bande interdite limite l’utilisation du graphène dans les dispositifs électroniques. Ajuster la bande interdite de graphène permettrait un contrôle précis des porteurs de charge. Une solution prometteuse consiste à modifier le graphène par des briques élémentaires de molécules organiques. Les molécules organiques avec un ion métallique (métal-porphyrine et métal-phtalocyanine) sont des candidats potentiels en raison de leur structure robuste et de leurs propriétés de charge et de spin qui peuvent être modulées. Dans cette thèse, le graphène a été préparé par la sublimation d’atomes de Si sur les faces de Si et de C- du substrat SiC. Trois molécules qui transportent l'information de spin différents ont été étudiés avec le STM. A travers des collaborateurs les calculs DFT, nous apporte des informations complémentaires. La première molécule utilisée dans notre expérience est la phtalocyanine de Ni (NiPc). L'ion Ni²⁺ a une configuration d'électrons 3d⁸ avec au état de spin de 0. La seconde molécule est la tétraphénylporphyrine de Pt (PtTPP (CO₂Me)₄). L'ion Pt²⁺ montre également une configuration d'électrons 3d8 à un état de spin de zéro. Cependant, l'atome Pt est plus lourd que celui du Ni. Promettant des effets spin orbite plus importants. La troisième molécule est tétraphénylporphyrine de fer (III) chlorure (FeTPPCl). Le Fe³⁺ est dans l'état haut spin (S = 5/2). Chacune de ces trois molécules forment un réseau moléculaire carré bien ordonné sur le graphène. Les directions de réseau moléculaires sont dominées par la symétrie du graphène, tandis que les orientations moléculaires dépendent des interactions inter moléculaires. Les couplages électroniques entre chaque molécule et le graphène sont transmis par la force de Van der Waals, qui donne lieu à des interfaces capacitifs entre la couche de graphène et les molécules. Les interactions électroniques entre les molécules FeTPP et graphène sont plus fortes que celles entre NiPc ou PtTPP et le graphène. Les études des molécules organiques avec adsorbées sur le graphène des spins différents a le potentiel d’ouvrir la voie à l'application de l'interface organométallique molécules/ graphène dans les dispositifs de spintronique. / Graphene has extraordinary properties because of its linear band structure and zero band gap. However, the lack of a band gap hinders the implementation of graphene in electronics; tuning the band gap of graphene would enable a precise control of the charge carriers. One of the promising solutions is to modify graphene with organic molecular building blocks. Organic molecules with a metal ion (metal- porphyrin, metal- phthalocyanine) are potential candidates, because of their robust structure and the fact that their charge and spin properties can be tuned. In this thesis, graphene was prepared by sublimating Si atoms from both Si and C- terminated SiC substrates. Three molecules which carry different spin information were studied by STM experiments. Through collaborations, DFT calculations were used to improve our understanding of the molecule- graphene interaction.The first molecule used in our experiment is Ni- phthalocyanine (NiPc). The Ni²⁺ ion has a 3d⁸ electron configuration, giving a spin- state of 0. The second molecule is Pt- tetraphenylporphyrin (PtTPP(CO₂Me)₄). The Pt²⁺ ion also shows a d8 electron configuration with a spin state of zero. However, the Pt atom is heavier than Ni, which should increase the spin- orbit effects. The third molecule is tetraphenylporphyrin iron(III) chloride (Fe(TPP)Cl). The Fe³⁺ in Fe(TPP)Cl is stable in the high spin state (S=5/2). These three molecules each form well- ordered nearly square lattice molecular networks on graphene. The molecular lattice directions are dominated by the graphene symmetry, while the molecular orientations depend on the molecule- molecule interactions. The electronic couplings between each of three molecules and graphene are via the Van der Waals forces, which gives rise to the capacitive molecular- layer/ graphene interfaces. The electronic interactions between FeTPP molecules and graphene are stronger than those between NiPc or PtTPP molecules and graphene. The studies of the organic molecules with different spin information on the graphene has the potential to pave the way for the application of organometallic molecules/graphene interface in spintronic devices.

Polymer-derived ceramics (PDCs) are a new class of high-temperature materials synthesized by thermal decomposition of polymeric precursors. These materials possess many unique features as compared with conventional ceramics synthesized by powder metallurgy based processing. For example, PDCs are neither amorphous nor crystalline. Instead, they possess nano-domain structures. Due to the direct chemical-to-ceramic processing, PDCs can be used for making components and devices with complex shapes. Thus, understanding the properties and structures of these materials are of both fundamental and practical interest. In this work, the structures and electronic behavior of polymer-derived amorphous silicon carbonitrides (SiCNs) were investigated. The materials were synthesized by pyrolysis of a commercially available liquid precursor. Ceramic materials with varied structures/properties were successfully synthesized by modifying the precursor and using different pyrolysis temperatures. The structures of the obtained materials were studied using XRD, solid state NMR, EPR, FTIR and Raman Spectroscope. The electronic behavior of the materials was investigated by measuring I-V curves, Hall effects, temperature dependent conductivity. The experiments were also performed to measure UV-Visible absorption and dielectric properties of the materials. This work leads to the following significant progresses: (i) developed quantitative technique for measuring free carbon concentration; (ii) achieved better understanding of the electronic conduction mechanisms and measured electronic structures of the materials for the first time; and (iii) demonstrated that these materials possess unusual dielectric behavior and provide qualitative explanations.

Understanding structure-property relationship of molecules is imperative for designing efficient materials for organic semiconductors. Organic semiconductors are based on π-conjugated molecules, either small molecules or macromolecules such as dendrimers or polymers. Charge transport through organic materials is one of the most important processes that drive organic electronic devices. We have investigated the charge transport properties in various molecular designs based on dendrons, dendron-rod-coil molecular triads, and conjugated oligomers. The charge transport properties were studied using bottom contact field effect transistors, in which the material was deposited by spin coating. In case of dendrons, their generation and density of charge transporting functionalities were found to play a significant role in influencing the charge transport properties. In case of macromolecules such as dendron-rod-coil molecules, the solid state morphology plays a significant role in influencing the charge transport properties. While these molecules exhibit only electron transporting behavior in field-effect transistor measurements, ambipolar charge transport is observed in the diode configuration. Short conjugated oligomers, based on donor-acceptor-donor design, provide model systems for conjugated polymers. Effect of varying the donor functionality on optoelectronic and charge transport properties was studied in short donor-acceptor-donor molecules. While donor-acceptor-donor molecules are well known in the literature, the effect of molecular composition on the charge transport properties is not well understood. We designed molecules with 2,1,3-benzothiadiazole as the acceptor and thiophene based donor functionalities. These molecules exhibit a reduced bandgap, good solution processability and charge mobility making them interesting systems for application in organic photovoltaics. Cyclopentadithiophene (CPD) based materials have been widely utilized as organic semiconductors due to their planar nature which favors intermolecular charge transport. While most CPD based materials are hole transporting, incorporation of electron withdrawing fluorinated substituents imparts n-type behavior to these molecules. This change in charge transport properties has often been attributed to the lowering of the LUMO energy level due to the increased electron affinity in the molecule. We designed CPD based semiconductors in which the bridgehead position was functionalized with electron withdrawing ketone or dicyanomethylene group and the -positions were substituted with phenyl or pentafluorophenyl groups. Both the phenyl substituted molecules are p-type materials, even though the dicyanomethylene group lowers the LUMO by 500 meV as compared to the carbonyl compound. The pentafluorophenyl substituted molecules are n-type materials even as their LUMO energy levels are about 300 meV higher than the corresponding phenyl substituted molecules. This indicates that charge transport behavior is not an exclusive function of the frontier orbital energy levels.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24446号 / 理博第4945号 / 新制||理||1706(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 北川 宏, 教授 有賀 哲也, 教授 吉村 一良 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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Neste trabalho desenvolvemos um potencial empírico para descrever as ligações químicas entre os átomos de silício, nitrogênio e hidrogênio usando a forma funcional de Tersoff. Exploramos as propriedades estruturais do nitreto de silício amorfo (a-SiNx:Hy) via simulação Monte Carlo e comparamos com dados experimentais. A boa descrição do sistema a-SiNx:Hy, para uma faixa de valores da concentração de nitrogênio (0<x<1,5) e da concentração de hidrogênio (0<y<40%) mostra que o modelo é realístico. Dependendo da concentração de nitrogênio, o hidrogênio possui preferência química diferente para ligar-se ao nitrogênio ou ao silício, o que é corroborado por resultados experimentais. Além do que, a incorporação do hidrogênio reduz consideravelmente a concentração de átomos subcoordenados no material. A estrutura eletrônica do amorfo e defeitos pontuais no nitreto de silício cristalino foram estudadas usando cálculos de primeiros princípios resolvendo as equações de Kohn-Sham. Para tal estudo as configurações inicialmente foram criadas via modelo empírico e serviram como entrada para o cálculo ab initio da energia total e das forças. Nossos resultados mostram que o hidrogênio tem um papel importante nas propriedades estruturais e eletrônicas do nitreto de silício amorfo. Em particular, demonstramos que o hidrogênio remove parcialmente níveis do gap de energia devido a saturação de ligações pendentes. / In this work we developed an empirical potential to describe the chemical bond among silicon, nitrogen and hydrogen atoms using the Tersoff functional form. We explored the structural properties of amorphous silicon nitride (a-SiNx:H,) through the Monte Carlo simulations and compared with experimental data. The good description of the a-SiNx:Hy, systems for a wide range of nitrogen contents (0<x<1.5) and hydrogen contents (0<y<40%) show the reliability of this model. Depending on nitrogen content, hydrogen has a different chemical preference to bond to either nitrogen or silicon, which is corroborated by experimental finding. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material. Electronic structure of amorphous and point defects in crystalline silicon nitride were studied using first-principles calculations solving the Kohn-Sham equations. For such study the configurations were created initially by empirical model as input for the ab initio total energy and forces calculations. Our results show that hydrogen plays an important role in the structural and electronic properties of amorphous silicon nitride. In particularly we demonstrated the role played by hydrogen to remove partially the levels in the energy gap due saturation of the dangling bond.

Die Untersuchungen der nichtskalierbaren Eigenschaften von Clustern in dem Größenregime, in dem jedes Atom zählt, zeigten, daß hier neuartige Phänomene und Funktionalität entstehen können. Dadurch motiviert wurden in dieser Arbeit: i) strukturelle und elektronische Eigenschaften sowie die Reaktivität von Metall Clustern, ii) stationäre optische Eigenschaften und iii) zeitabhängige Eigenschaften und optimale Kontrolle von ultraschnellen Prozessen in Edelmetallcluster und in nonstoichiometrischen Natrium-Fluorid Cluster, untersucht. / The study of the nonscalable properties of clusters in the size regime in which each atom counts have shown that fully new phenomena and striking new unexpected properties of small clusters can emerge. In this work three aspects have been addressed: i) the structural and electronic properties and reactivity of metal clusters, ii) stationary optical propertis and iii) real time investigation and control of ultrafast processes in noble metal and in nonstoichiometric sodium fluoride clusters.

Neste trabalho desenvolvemos um potencial empírico para descrever as ligações químicas entre os átomos de silício, nitrogênio e hidrogênio usando a forma funcional de Tersoff. Exploramos as propriedades estruturais do nitreto de silício amorfo (a-SiNx:Hy) via simulação Monte Carlo e comparamos com dados experimentais. A boa descrição do sistema a-SiNx:Hy, para uma faixa de valores da concentração de nitrogênio (0<x<1,5) e da concentração de hidrogênio (0<y<40%) mostra que o modelo é realístico. Dependendo da concentração de nitrogênio, o hidrogênio possui preferência química diferente para ligar-se ao nitrogênio ou ao silício, o que é corroborado por resultados experimentais. Além do que, a incorporação do hidrogênio reduz consideravelmente a concentração de átomos subcoordenados no material. A estrutura eletrônica do amorfo e defeitos pontuais no nitreto de silício cristalino foram estudadas usando cálculos de primeiros princípios resolvendo as equações de Kohn-Sham. Para tal estudo as configurações inicialmente foram criadas via modelo empírico e serviram como entrada para o cálculo ab initio da energia total e das forças. Nossos resultados mostram que o hidrogênio tem um papel importante nas propriedades estruturais e eletrônicas do nitreto de silício amorfo. Em particular, demonstramos que o hidrogênio remove parcialmente níveis do gap de energia devido a saturação de ligações pendentes. / In this work we developed an empirical potential to describe the chemical bond among silicon, nitrogen and hydrogen atoms using the Tersoff functional form. We explored the structural properties of amorphous silicon nitride (a-SiNx:H,) through the Monte Carlo simulations and compared with experimental data. The good description of the a-SiNx:Hy, systems for a wide range of nitrogen contents (0<x<1.5) and hydrogen contents (0<y<40%) show the reliability of this model. Depending on nitrogen content, hydrogen has a different chemical preference to bond to either nitrogen or silicon, which is corroborated by experimental finding. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material. Electronic structure of amorphous and point defects in crystalline silicon nitride were studied using first-principles calculations solving the Kohn-Sham equations. For such study the configurations were created initially by empirical model as input for the ab initio total energy and forces calculations. Our results show that hydrogen plays an important role in the structural and electronic properties of amorphous silicon nitride. In particularly we demonstrated the role played by hydrogen to remove partially the levels in the energy gap due saturation of the dangling bond.

Les alliages de tellurure de plomb (PbTe) sont reconnus depuis longtemps comme d’excellents matériaux thermoélectriques avec un ZT de l’ordre de 1,0 pour des applications en génération d’électricité à hautes températures où le tellurure de bismuth (Bi2Te3) ne peut plus être utilisé. Cependant, la présence de plomb rend problématique une commercialisation à grande échelle de dispositifs thermoélectriques contenant ce composé. Le tellurure d’étain (SnTe), étudié il y a plus de 40 ans comme un analogue à PbTe, présente des performances thermoélectriques médiocres du fait de la présence d’une concentration élevée en lacunes de Sn. Toutefois, la dernière décennie a vu une recrudescence importante des recherches sur ce composé visant à améliorer ses performances thermoélectriques à hautes températures. Le composé SnTe présente des déviations par rapport à la stœchiométrie idéale (lacunes d’étain) quelles que soient la méthode de synthèse utilisée. Dans ce travail, nous dévoilons l’influence de la déviation par rapport à la stœchiométrie idéale (composition chimique) et des conditions de synthèse (avec trempe, sans trempe, recuit de saturation et melt-spinning) sur les propriétés de transport électrique et thermique de ces matériaux. Pour ce faire, des techniques de synthèse par métallurgie de poudres ont été mises en œuvre. Les matériaux résultants ont été ensuite caractérisés finement aussi bien d’un point de vue structural que physico-chimique. Ainsi, une étude détaillée de leur structure cristalline a été menée en combinant des mesures de diffraction des rayons X sur poudre et des analyses de microscopie électronique à balayage et à transmission à haute résolution. Des mesures de propriétés électriques et thermiques ont été menés à basses températures (5 – 300 K) pour identifier les mécanismes microscopiques qui gouvernent le transport et à hautes températures (300 – 800 K) afin de déterminer le domaine d’application optimal. Ces mesures ont confirmé le potentiel de ces composés pour des applications en génération d’électricité à températures moyennes. De nombreuses possibilités de substitutions sur le site de l’étain ou/et du tellure ont été entreprises afin de tenter d’optimiser davantage les performances thermoélectriques de ces composés. Des éléments en substitution ont été choisi pour augmenter le pouvoir thermoélectrique à travers la diminution de la concentration en trous ou par ingénierie de la structure de bande et/ou diminuer la conductivité thermique de réseau via la formation de solutions solides ou de précipités dans la matrice. Le choix de ces éléments a notamment été guidé par des calculs de structure de bande électronique. Les résultats expérimentaux ont été modélisé par un modèle à deux bandes non dégénérées afin de dévoiler les principaux facteurs qui gouvernent le transport. La conductivité thermique de réseau a été analysée en utilisant le modèle de Callaway afin d’étudier les mécanismes de diffusion des phonons à basses températures et de mieux appréhender l’influence des lacunes sur le transport thermique / Lead telluride (PbTe) alloys are among the most efficient thermoelectric materials with a ZT of 1.0 for electricity generation applications in the mid to high temperature region where bismuth telluride (Bi2Te3) can no longer be used. Despite their excellent environmental stability, the perceived toxicity of lead chalcogenides can frustrate its development and large-scale application. Tin telluride (SnTe), studied more than 40 years ago as a analogue of PbTe shows poor thermoelectric performances because of its lower Seebeck coefficient. The latter is due to heavy intrinsic doping arising from spontaneous Sn vacancies. However, recent studies unambiguously show that SnTe has a strong potential of being a promising thermoelectric at high temperatures. In fact, regardless of the synthesis method used, SnTe compound is in deviation from the ideal stoichiometry (Sn vacancies). In this work, we unveil the influence of this deviation (chemical composition) and of the synthesis conditions (with quenching, without quenching, annealing and melt-spinning) on the electrical and thermal transport properties of these materials. Hence, for the synthesis of these materials different powder metallurgy techniques were implemented. The resulting materials were then finely characterized by structural and physico-chemical point of view. Thus, a detailed study of their crystalline structure was carried on by combining X-ray powder diffraction, scanning and transmission electron microscopy analyzes. The electrical and thermal properties measurements were effectuated at low temperatures (5 - 300 K) to identify the microscopic mechanisms that govern transport and at high temperatures (300 - 800 K) to determine the optimal domain of application. These measurements have confirmed the strong potential of these compounds for electricity generation applications at high temperatures. Numerous substitution possibilities at the tin and / or tellurium site have been undertaken in an attempt to further optimize the thermoelectric performance of these compounds. Substitute elements were chosen to increase the thermoelectric power through the decrease in the hole concentration or by engineering the band structure and / or decrease the lattice thermal conductivity via the formation of solid solutions or precipitates in the matrix. The choice of these elements was guided by electronic band structure calculations (DOS). The experimental results were modeled by a non-degenerate two-band model to reveal the main factors that govern the electronic transport. The lattice thermal conductivity was analyzed using the Callaway model to study the phonon scattering mechanisms at low temperatures and to better understand the influence of Sn vacancies on the thermal transport