One of the challenges in graphene fabrication is the production of large scale, high quality sheets. To study a possible approach to achieve quasi-freestanding graphene on a substrate by the intercalation of alkali metal atoms, Cs intercalation between graphene and Ni(111) is investigated. It is known that direct contact between graphene and Ni(111) perturbs the Dirac states. Cs intercalation restores the linear dispersion characteristic of Dirac fermions, which agrees with experiments, but the Dirac cone is shifted to lower energy, i.e., the graphene sheet is n-doped. Cs decouples the graphene sheet, while the spin polarization of Ni(111) does not extend through the intercalated atoms to the graphene sheet, for which we find virtually spin-degeneracy. In order to employ graphene in electronic applications, one requires a finite band gap. We engineer a band gap in metallic bilayer graphene by substitutional B and/or N doping. Specifically, the introduction of B-N pairs into bilayer graphene can be used to create a band gap that is stable against thermal fluctuations at room temperature. Introduction of B-N pairs into B and/or N doped bilayer graphene likewise hardly modifies the band dispersions, however, the size of the band gap is effectively tuned. We also study the influence of terrace edges on the electronic properties of graphene, considering bare edges and H, F, Cl, NH2 terminations. Periodic structural reconstruction is observed for the Cl and NH2 edge terminations due to interaction between the terminating atoms/groups. We observe that Cl edge termination p-dopes the terraces, while NH2 edge termination results in n-doping.

Les préoccupations environnementales actuelles ont conduit à un regain d’intérêt pour la conversion d'énergie par effets thermoélectriques au cours de ces 20 dernières années. Le challenge lié à cette technologie consiste à découvrir des matériaux qui possèdent à la fois une faible conductivité thermique, une forte conductivité électrique et un fort pouvoir thermoélectrique. Les travaux présentés dans ce mémoire se sont orientés vers l'étude de phases cristallines complexes à base de Mo3Sb7. Contrôler finement les propriétés électriques et thermiques de ces matériaux par le biais de substitutions appropriées et relier les propriétés physiques aux propriétés structurales et électroniques ont été au coeur de ces travaux de recherche. Des résultats significatifs ont ainsi pu être obtenus tant au niveau de la synthèse et de la caractérisation physico-chimique qu'au niveau des propriétés magnétiques et de transport. En particulier, nous avons pu mettre en évidence les propriétés exotiques du composé Mo3Sb7 dont la compréhension s'est révélée indispensable pour l'étude des propriétés de transport des matériaux substitués ternaires et quaternaires. Les différentes possibilités de substitution ont alors permis d'améliorer de façon substantielle les performances thermoélectriques du composé Mo3Sb7 et ont, de ce fait, conduit à la découverte de nouveaux matériaux surpassant les meilleurs matériaux connus à ce jour (Si-Ge) et utilisés sur la gamme 900 - 1200 K dans des applications en génération d'électricité / Due to current environmental concerns, a resurgence of interest in thermoelectricity have been witnessed by the last 20 years. The challenge raised by this technology lies in identifying materials that display low thermal conductivity as well as both high electrical conductivity and thermopower. The work presented in this manuscript deals with a thorough study on molybdenum-antimony based complex crystalline structure. To finely control the thermal and electrical properties of these compounds through judicious substitutions and to link up physical and structural properties were at the heart of this in-depth study. Not only did we obtain outstanding results regarding the synthesis and both the chemical and structural characterizations but we also discovered intriguing magnetic and transport properties. Particularly, we emphasized the exotic properties exhibited by the binary Mo3Sb7 compound whose a deep understanding were essential to study the transport properties of the ternary and quaternary alloys. The different substitutions we have considered were found to substantially improve the thermoelectric properties of the Mo3Sb7 compound and thus, led to the synthesis of new prospective thermoelectric materials that surpass the best compounds discovered up-to-now (Si-Ge) and used in power generation applications in the 900 – 1200 K temperature range

The dilute magnetic semiconductor (Ga,Mn)As , which is the most interesting and promising material for spintronics applications, has been theoretically studied by using Density Functional Theory. First of all, calculations on GaAs were done and it was found that GaAs is a semiconductor with a direct band gap. The calculated value of the band gap is ~ 0.5eV. Secondly, the material iron was considered and it was confirmed that iron is a ferromagnetic metal with 2.2µB net magnetic moment. Then a magnetic impurity of manganese, Mn was introduced in the nonmagnetic GaAs and it became ferromagnetic with a net magnetic moment of 4µB. The origin of the ferromagnetic behaviour is discussed and also the Curie temperature TC of the material. It appeared that (Ga,Mn)As is a suitable material for DMS but TC has to be increased before (Ga,Mn)As could be used for spintronics applications and on that account some methods of increasing TC are considered at the end. / Den magnetiska halvledaren (Ga,Mn)As som är det mest intressanta och lovande materialet för spinelektroniska tillämpningar har teoretiskt undersökts med hjälp av Täthetsfunktionalteorin. Först gjordes beräkningar på GaAs och det visade sig att GaAs är en halvledare med direkt bandgap. Det beräknade värdet på bandgapet är ca 0.5eV. Sedan var det järn som undersöktes och det blev bekräftat att järn är en ferromagnetisk metall med netto magnetisk moment lika med 2.2μB. Då magnetiska störningar i form av mangan atomer, Mn, infördes i det omagnetiska GaAs blev halvledaren ferromagnetisk med netto magnetisk moment lika med 4μB. Orsakerna till den ferromagnetiska ordningen diskuteras och även Curie temperaturen TC för materialet. Det visade sig att (Ga,Mn)As är ett lämpligt material för tillverkning av magnetiska halvledare men TC måste ökas innan (Ga,Mn)As skulle kunna användas i spinntroniska tillämpningar och av det skälet anges i slutet vissa metoder för att öka TC.

Carbon nanotubes have unique electronic, optical, mechanical, and transport properties which make them an important element of nanoscience and nanotechnology. However, successful application and integration of carbon nanotubes into new nanodevices requires fundamental understanding of their property changes under the influence of many external factors. This dissertation presents qualitative and quantitative theoretical understanding of property changes, while carbon nanotubes are exposed to the deformations, defects, external electric fields, and adsorption. Adsorption mechanisms due to Van der Waals dispersion forces are analyzed first for the interactions of graphitic materials and biological molecules with carbon nanotubes. In particular, the calculations are performed for the carbon nanotubes and graphene nanoribbons, DNA bases, and their radicals on the surface of carbon nanotubes in terms of binding energies, structural changes, and electronic properties alterations. The results have shown the importance of many-body effects and discrete nature of system, which are commonly neglected in many calculations for Van der Waals forces in the nanotube interactions with other materials at nanoscale. Then, the effect of the simultaneous application of two external factors, such as radial deformation and different defects (a Stone Wales, nitrogen impurity, and mono-vacancy) on properties of carbon nanotubes is studied. The results reveal significant changes in mechanical, electrical, and magnetic characteristics of nanotubes. The complicated interplay between radial deformation and different kinds of defects leads to the appearance of magnetism in carbon nanotubes which does not exist in perfect ones. Moreover, the combined effect of radial deformation and external electric fields on their electronic properties is shown for the first time. As a result, metal-semiconductor or semiconductor-metal transitions occur and are strongly correlated with the strength and direction of external electric field and the degree of radial deformations.

Results of this research are available online in two published papers. / The main focus of this research was to synthesize and then to characterize the potential high-performance thermoelectric materials. In this regard, we have prepared a series of pure RECuSe2 (with RE = Pr, Sm, Gd, Dy and Er) and RECuTe2 (with RE = Er, Dy and Gd) and analyzed their crystal structure, electronic and physical properties. We used powder and single crystal X-ray diffraction techniques to analyze their crystal structures and employed energy dispersive X-ray spectrometry (EDS) to verify their chemical compositions. The temperature stability of the synthesized samples was examined by differential thermal and gravimetrical analysis. The high-purity consolidated pellets were prepared for physical properties measurements. We analyzed the relationship between their crystal structures and pertinent electronic properties through the LMTO calculations. The RECuSe2 phases adopt two structures, monoclinic and trigonal. The monoclinic structure (P21/c, z = 4) is observed for lighter rare earths (RE = Pr, Sm and Gd) and Cu-disordered trigonal structure for heavier rare earths (P m1, z = 1, RE = Dy and Er). The resistivity and Seebeck coefficient measurements indicate that the studied selenides are p-type semiconductors with relatively small activation energies (0.045-0.12 eV). However, their electrical resistivities are too high (0.49-220 Ohmcm at room temperature) to make them competitive thermoelectric materials. Electronic structure calculations indicate presence of a band gap in the RECuSe2 phases. The synthesized RECuTe2 phases (RE = Er, Dy and Gd) adopt a monoclinic-distorted variant (C2/m, z = 2) of the trigonal structure (P m1, Z= 1) observed for the RECuSe2 (with RE = Dy, Er). While such disorder may be beneficial for lowering their thermal conductivity, large values of electrical resistivity (0.02-0.87 Ohmcm at room temperature) make these phases unsuitable for practical applications. Comparing to the corresponding semiconducting selenides, the tellurides have lower resistivities, and display a metallic type resistivity. Such behavior stems from the closure of band gaps, which is verified by the electronic structures calculations. Structurally the RECuTe2 phases (with RE = Er, Dy and Gd) are similar to RECuSe2 with the P m1 structure. The monoclinic distortion in RECuTe2 is driven by Cu displacement inside the larger tetrahedral voids in the hexagonal close packing of the Te atoms. Most likely, Cu shifts to one side of the Te tetrahedra to optimize the Cu-Te interactions. / Thesis / Candidate in Philosophy

Electronic band structure of various crystal orientations of relaxed and strained bulk, 1D and 2D confined semiconductors are investigated using nonlocal empirical pseudopotential method with spin-orbit interaction. For the bulk semiconductors, local and nonlocal pseudopotential parameters are obtained by fitting transport-relevant quantities, such as band gap, effective masses and deformation potentials, to available experimental data. A cubic-spline interpolation is used to extend local form factors to arbitrary q and the resulting transferable local pseudopotential V(q) with correct work function is used to investigate the 1D and 2D confined systems with supercell method. Quantum confinement, uniaxial and biaxial strain and crystal orientation effects of the band structure are investigated. Regarding the transport relavant quantities, we have found that the largest ballistic electron conductance occurs for compressively-strained large-diameter [001] wires while the smallest transport electron effective mass is found for larger-diameter [110] wires under tensile stress.

No description available.

Au cours de ces travaux de thèse, nous avons étudié par microscopie magnétique à émission d’électrons balistiques (BEMM) les propriétés de magnétotransport d’électrons chauds de la vanne de spin Fe/Au/Fe épitaxiée sur GaAs(001). Dans ces expériences, la pointe d’un microscope à effet tunnel (STM) injecte localement un courant d’électrons chauds à la surface de la vanne de spin. La mesure sous champ magnétique du courant d’électrons balistiques collecté à l’arrière de l’échantillon donne accès aux propriétés locales de magnétoconductance de l’échantillon. Nous avons dans un premier temps étudié les propriétés de magnétotransport de vannes de spin planaires. Les mesures BEMM démontrent un magnétocourant d’électrons chauds pouvant atteindre 500 % à température ambiante. Ces forts effets de magnétoconductance ne sont que très faiblement dépendants des épaisseurs des électrodes de fer et ne peuvent donc être dus à l’asymétrie en spin de la longueur d’atténuation des électrons chauds dans les couches de fer. Dans cette structure épitaxiée, la polarisation en spin du faisceau d’électrons chauds s’acquiert principalement aux interfaces via des effets de structure électronique. L’électron traversant les couches minces métalliques se propage comme un état de Bloch. Sa transmission aux différentes interfaces se fait en conservant d’une part la composante transverse k║ du vecteur d’onde électronique, et d’autre part, la symétrie de la fonction d’onde. Au-dessus de la barrière Schottky, les électrons chauds sont collectés dans la vallée Г du GaAs se projetant à l’interface dans la direction k║=0. Dans cette direction k║=0, la conservation de la symétrie de la fonction d’onde à l’interface Fe/Au conduit au filtrage des états de Bloch de symétrie Δ1 du fer. Ces états de symétrie Δ1, totalement polarisés en spin, sont responsables des forts magnétocourants d’électrons chauds observés. Cette analyse est confirmée expérimentalement par l’observation d’une corrélation entre amplitude du magnétocourant et masse effective du substrat semiconducteur. En augmentant la masse effective du semiconducteur, on ouvre le collimateur filtrant le courant d’électrons chauds autour de la direction k║=0, et le magnétocourant diminue sans modifier la vanne de spin. Dans un second temps, tirant partie de la résolution latérale du microscope et de sa sensibilité au magnétisme, des microstructures de fer préparées sous ultra-vide par évaporation à travers un masque (méthode du nanostencil) ont été étudiées. Dans ces structures, la modulation du courant collecté par la structure locale en domaines magnétiques a permis la réalisation d’images magnétiques avec une haute résolution spatiale. Les contrastes observés sur ces microstructures sont en excellent accord avec les images BEMM calculées à partir de simulations micromagnétiques ouvrant la voie à une microscopie magnétique quantitative à forte sensibilité et résolution latérale nanométrique. / During this thesis work, we studied by ballistic electron magnetic microscopy (BEMM) the hot electron magnetotransport properties of epitaxial Fe/Au/Fe/GaAs(001) heterostructures. In these experiments, hot electrons are injected from an STM tip through the metallic base. The measurement of the ballistic electron current collected at the back of the substrate under magnetic field gives access to the local magnetoconductance properties of the sample. The first part of this work consists in the study of a planar heterostructures. BEMM measurements on epitaxial Fe/Au/Fe/GaAs(001) samples demonstrate hot electron magnetocurrent as high as 500% at room temperature. This high magnetocurrent value is observed to be almost independent of the Fe layers thickness, and thus can not be explained by the spin asymmetry of the electron attenuation length in the iron layers. In this epitaxial heterostructure, the hot electron beam is mainly spin-polarized at the interfaces due to band structure effects. In the metallic thin films, electrons propagate as Bloch states. The electron wave function transmission at the interfaces should satisfy two selection rules: the transverse momentum (k║) of the electron wave vector and the symmetry of the electron wave function should be conserved. Above the Schottky barrier height, hot-electrons are collected in the Г valley of GaAs selecting thus only electrons with a transverse momentum (k║) close to zero. Among these k|| ≈ 0 states, conservation of the electron wave-function symmetry at the Fe/Au epitaxial interfaces additionally selects electrons with the Δ1 symmetry. These Δ1 states are fully spin-polarized and are responsible for the observed high magnetocurrent in these heterostructures. This analysis is experimentally confirmed by the observation of a correlation between the magnetocurrent value and the semiconductor effective mass. By increasing the semiconductor effective mass, we open the collimator which filters the electronic states around k║=0 and the magnetocurrent value decreases. To take advantage of the lateral resolution of the microscope and of its high sensitivity to magnetism, the second part of this work was devoted to the study of sub-micrometric iron structures prepared under UHV by evaporation through a nanostencil. In these structures, the modulation of the collected current by the local magnetic domain structure in the Fe dots allows magnetic imaging with a high spatial resolution. The experimental magnetocontrasts observed on these sub-micrometric Fe dots are in excellent agreement with BEMM current maps calculated from micromagnetic simulation results. This opens the way to a quantitative magnetic microscopy with high contrast and nanometric lateral resolution.

The electronical and structural properties of TlSe-type chain-like crystals are the main topic of this study. A computational method which is Tight Binding method is introduced and used to obtain the electronic band structure of TlSe and TlInSe2 . For both materials the partial and total density of states are calculated. The results are compared with the other theoretical results.

Ce travail de thèse porte sur l’élaboration et la caractérisation des propriétés physiques et chimiques d’une nouvelle famille de composés thermoélectriques, et plus particulièrement le composé BiCuSeO. Les composés de cette famille, dite 1111, présentent une structure en couche de type ZrCuSiAs. L’une des particularités de cette structure est la nature distincte des couches qui la composent, la couche Bi2O2 étant décrite comme isolante tandis que la couche Cu2Se2 est appelée couche conductrice. L’étude approfondie du composé BiCuSeO montre qu’en dépit d’un facteur de puissance (S²σ) relativement modéré, ce composé est un matériau thermoélectrique prometteur, notamment à haute température. En effet, BiCuSeO présente une conductivité thermique remarquablement faible, qui permet d’atteindre des facteurs de mérite relativement élevés. De plus, BiCuSeO présente de nombreuses voies d’améliorations possibles. L’une d’elle concerne l’étude d’un dopage aliovalent sur le site du bismuth. L’analyse des résultats a montré que l’insertion d’un élément divalent permet d’optimiser la concentration des porteurs de charges, entrainant ainsi une forte augmentation du facteur de mérite du composé. Une autre voie possible d’exploration est l’étude de l’influence de l’ion chalcogène, au travers notamment de la substitution du sélénium par le tellure, avec l’obtention d’une solution solide complète BiCuSe(1-x)Te(x)O. L’étude des propriétés électriques des composés de cette série a permis de mettre en évidence la présence d’une transition métal – semi-conducteur – métal pour les fractions de tellure inférieures à 0.5. Ainsi, bien que l’influence du tellure sur le facteur de puissance soit relativement limitée en raison de cette anomalie, des résultats intéressants ont été obtenus pour les fractions de tellure élevées. Par ailleurs, des problématiques autour d’une méthode de synthèse alternative du matériau ainsi que sa stabilité sous air sont également abordées dans ce travail. / This thesis addresses the issues of the elaboration and the characterization of the chemical and physical properties of a new family of thermoelectric materials, the oxychalcogenides with the general formula BiCuSeO. This compound, called 1111, cristallises in the ZrCuSiAs structure-type. One feature of this structure lies in the fact that the layers are considered as electronically distinct: the Bi2O2 layers are described as the insulating layers whereas the chalcogenide layers Cu2Se2 are presented as the conductive ones. The study of BiCuSeO exhibits that in spite of a relatively moderate power factor (S²σ), this compound is very promising as possible thermoelectric material, especially at high temperature. Indeed, BiCuSeO shows a remarkably low thermal conductivity, which can achieve relatively high figures of merit. In addition, BiCuSeO offers many ways for improvement. One of them concerns the study of aliovalent doping on the bismuth site. The results showed that the insertion of a divalent element optimizes the charge carriers concentration, leading to a sharp increase in the figure of merit of the compound. Another possible way of exploration lies the study of the influence of the chalcogen ion, notably through the substitution of selenium and tellurium, with a complete solid-solution BiCuSe(1-x)Te(x)O. The study of the electrical properties of this solid solution has highlighted the presence of a metal - semiconductor - metal transition for tellurium fractions below 0.5. Thus, although the influence of tellurium on the power factor is relatively limited due to this anomaly, interesting results were obtained for the high tellurium fractions. In addition, issues around an alternative method of synthesis of the material and its stability in air are also discussed in this work.