Estudamos o comportamento estático e os mecanismos de inversão da magnetização de arranjos auto-organizados de nanofios de Ni e Co com alta anisotropia de forma. Os arranjos são obtidos a partir da anodização em dois passos de lâminas de Al e subseqüente eletrodeposição do metal magnético. Sua caracterização estrutural é realizada por microscopia eletrônica de varredura, microscopia de força atômica e magnética e difração de raios X. Seu comportamento magnético é estudado a partir da medida de curvas de inversão de primeira ordem (FORCs) a temperatura ambiente, via magnetometria SQUID ou de amostra vibrante. A análise FORC consiste na construção de uma mapa da resposta magnética do material a partir dos dados experimentais e pode ser interpretado a partir da analogia direta com o modelo de histerese de Preisach. Apresentamos resultados da influência dos diâmetros dos nanofios e do ângulo do campo externo aplicado em relação ao eixo de anisotropia sobre as principais características dos diagramas FORC. Estudamos também a dinâmica de spins em superfícies de O-Fe/W(001) através de espectroscopia por perda de energia de elétrons spin-polarizados (SPEELS). As amostras consistem em 30 monocamadas atômicas de Fe crescidas sobre um monocristal de W(001), via epitaxia por feixe molecular a temperatura ambiente. Subseqüentemente, a amostra é exposta a 5 langmuirs de O2 e sofre um suave annealing a 500 K. A estrutura e a pureza da amostra são analisadas por difração de elétrons de baixas energias e espectroscopia de elétrons Auger. A caracterização magnética é realizada por magnetometria por efeito Kerr magneto-óptico, resultando num filme com alta magnetização remanente no plano. Os espectros de SPEELS revelam uma rica profusão de picos inelásticos tipo spin-flip e nãospin-flip, que exibem clara dispersão ao longo de toda a zona de Brillouin de superfície. Os resultados são atribuídos a modos de superfície de fônons e mágnons (ondas de spin), de acordo com resultados da literatura e com modelos teóricos atuais. / We study the static behavior and the mechanisms of magnetization reversal of arrays of self-organized Ni and Co nanowires with high shape anisotropy. The arrays are obtained by two-step anodization of Al foils and subsequent electrodeposition of the magnetic metal.Their structural characterization is obtained by scanning electron microscopy, atomic and magnetic force microscopy and X-ray diffraction. Their magnetic behavior is studied from the measurement of first order reversal curves (FORCs) at room temperature, via SQUID or vibrating sample magnetometry. The FORC analysis method consists in building a map of the magnetic response of the material from the experimental data and it can be interpreted from the direct analogy with the Preisach model of hysteresis. We present results of the influence of the diameter of the nanowires and the angle of the applied external field in relation to the anisotropy easy-axis on the main features of the FORC diagrams. We also study the spin dynamics of O-Fe/W(001) surfaces through spin-polarized electron energy loss spectroscopy (SPEELS). The samples consist of 30 atomic monolayers of Fe grown on a W(001) single crystal via molecular beam epitaxy at room temperature. Subsequently, the samples are exposed to 5 langmuirs of O2 and suffer a mild annealing at 500 K. The structure and purity of the samples are analyzed by low-energy electron diffraction and Auger electron spectroscopy. The magnetic characterization is performed by magneto-optical Kerr effect magnetometry, resulting in films with high remanent in-plane magnetization. SPEEL-spectra reveal a rich profusion of inelastic spin-flip and non-spin-flip peaks, which exhibit clear dispersion for wave vectors throughout the whole surface Brillouin zone. The results are attributed to surface modes of phonons and magnons (spin waves), in accordance with the literature and with current theoretical models.

O entendimento e o controle das propriedades de materiais nanoestruturados em função do seu tamanho, forma e composição, por exemplo, é fundamental para o avanço da chamada nanotecnologia. Nanofios metálicos, em particular, são interessantes pois possibilitam a investigação de propriedades de sistemas com baixa dimensionalidade, além de serem considerados candidatos a elemento de interligação de unidades fundamentais de uma eletrônica no nível molecular. Efeitos de temperatura sobre o rompimento de nanofios monoatômicos de ouro puros e com impurezas de hidrogênio ou carbono foram investigados de modo sistemático, através da utilização do método de Dinâmica Molecular ab initio, na temperatura de 300 K. De acordo com a metodologia utilizada e as impurezas estudadas, os resultados mostraram que os sistemas são estáveis para longo tempo de simulação (20 ps) e que o hidrogênio é o candidato mais apropriado para explicar as distâncias Au-Au da ordem de 3.6 ºA que são observadas experimentalmente. Questões associadas à ruptura, tais como o entendimento do mecanismo físico envolvido no processo, o papel das flutuações térmicas e o efeito da presença de impureza, são discutidas com base em um modelo de triplas de átomos e de dados estatísticos obtidos de simulações de dinâmica molecular. A partir do modelo, a ruptura pôde ser entendida através de instabilidades observadas no perfil da superfície de energia potencial para ligações suficientemente estressadas. As flutuações térmicas seriam então as res- ponsáveis por levar o tamanho das triplas para os valores instáveis. Este modelo foi capaz ainda de explicar fatos como a não observação de eventos de ruptura em ligações do tipo Au-X (X=H,C), e a probabilidade maior de um fio com impureza de H ou C romper na ligação Au-Au mais afastada da impureza. O estudo de efeitos de tempe- ratura foi estendido para 106T6500 K. Nanofios com outros tamanhos de cadeia (3, 4 ou 6 átomos), na temperatura de 300 K, também foram estudados. De forma geral, os resultados mostraram que a temperatura possui essencialmente o efeito de aumentar a amplitude das flutuações, não modificando os valores médios das distâncias interatômicas da cadeia. Um estudo estatístico das simulações permitiu ainda entender o comportamento destas flutuações, que escala com a raiz quadrada da temperatura do sistema. Um aspecto importante das simulações envolvendo átomos de hidrogênio refere-se a efeitos quânticos que estariam sendo negligenciados. De acordo com os resultados obtidos da dinâmica, o movimento vibracional transversal do H conferia ao sistema uma instabilidade que supostamente seria fruto de uma abordagem inapropriada, já que graus de liberdade clássicos estariam sendo excitados indevidamente. Foi proposto então uma metodologia onde o movimento vibracional do H é substituído por um movimento \"adiabático\", de modo que ele se acomoda (quase) instantaneamente ao movimento mais lento do resto do sistema, através de seu posicionamento no mínimo do potencial local. Dentro desta perspectiva, esta metodologia seria mais realista que a dinâmica realizada de forma convencional, fornecendo, portanto, valores com maior nível de confiança. A distância Au-H-Au aumentou com a utilização desta aproximação, concordando com medidas experimentais de distâncias Au-Au em cadeias monoatômicas da ordem de 3.6 ºA. / The understanding and control of the properties of nanostructured materials as a function of their length, shape and composition, for example, is fundamental to improve the so called nanotechnology. Gold nanowires, in particular, are interesting since they not only allow the investigation of the properties of low-dimensional systems, but have also been thought of as candidates for nanometric interconnection elements. Temperature effects in the stability of pure, H or C doped atomically thin gold nanowires were systematically investigated with ab initio Molecular Dynamics simulations at temperature of 300 K. The results showed that the systems are stable for long time simulations (20 ps), and within the present hypothesis, the hydrogen is the best candidate to explain the large Au-Au distances of order of 3.6 ºA that are experimentally observed. Questions about the nanowires rupture, such as the understanding of the physical mechanism involved, the role of the thermal fluctuations and the effect of impurities, are discussed in accordance with a model of triplet of atoms and the statistical results obtained from the molecular dynamics simulations. The triplets model allowed the understanding of the rupture through instabilities observed in the potential energy surface profile when the bonds are su±ciently stressed. Thermal fluctuations would be responsible to lead to these unstable distances. Additionally, this model was able to explain facts such as why the rupture never occurred at Au-X bonds (X=H,C), and the higher probability that a nanowire with H or C impurity has to break on the Au-Au bond more distant from impurity. The study of temperature effects was extended to 106T6500 K. Nanowires with other length chains (3, 4 or 6 atoms) at temperature of 300 K were also studied. In general, the results showed that the effect of temperature is basically to increase the amplitude of the fluctuations, however, it does not modify the average interatomic distances of the chain. A statistical study also allowed to understand the behavior of these fluctuations, which scale with the square root of the temperature. An important aspect of the simulations involving hydrogen atoms is associated with quantum effects that are not taken into account. According to the molecular dynamics results, the transversal vibration of the H atom provided an instability to the system, that supposedly would be produced by an inappropriate treatment, since these degrees of freedom would be inappropriately excited. So, a methodology was proposed where the vibrational motion of the H is replaced by an \\adiabatic\" motion, with the hydrogen following (quasi) instantaneously the slower motion of the remainder system, being positioned at the local minimum of the potential. In this picture, this methodology would be more realistic than the conventional dynamics, allowing to obtain more reliable results. The Au-H-Au distance increased in this approximation, being in good agreement with the Au-Au distances measured experimentally in monoatomic chains, of the order of 3.6 ºA.

Ever rising power densities and smaller transistor dimensions are increasing the challenge of thermal management within integrated-circuit chips and their surrounding packaging. In addition, the need for sustainable energy has placed urgent emphasis on energy conversion. Thermoelectric phenomena, involving the conversion of heat to electrical current, provide a central focus for both needs. Specifically, there is a need to engineer materials or composites with low thermal conductivity and high electrical conductivity for energy conversion and the opposite for heat management. In this presentation, experimental results will be presented of the specific heat and thermal conductivity of cobalt nanowires (CoNW), carbon nanotubes (CNT) and polymer-carbon nanotubes, in various composite arrangements with our high precession Calorimetric technique. Due to the nature of these samples, boundary and defect scattering of phonons in nanomaterials can dominate. This scattering phenomena shows decreasing thermal conductivity in metal nanowires, turns to be good for thermoelectric application. For the CNT, and possibly due to the high volume per atom leading to ballistic phonon propagation, the observed thermal conductivity along the nanotube direction, which leads to manage the heat dissipation problem in integrated circuits (ICs) and microprocessors. The thermal conductivity of a single Single-Wall Carbon Nanotube (SWCNT) was found to be 6600 W/mK, theoretically, twice that of diamond. When such high thermal conductivity materials are dispersed in a low thermal conducting polymer (PMMA), the effective thermal conductivity and thermal stability of the composite can change dramatically. The experimental results show good agreement with theoretical model proposed by Nelsen, Hamilton, Crosse, Geometric, and Xue. The thermal relaxation phenomena such as glass transition temperature (Tg) and dynamics of the molecules in the polymer-nanotubes composites, changes significantly different than the pure polymers during thermal treatment and is one of the focusing point of this presentation. Liquid crystalline materials confined to restrictive nano-channels are of great interest in many potential applications of electro-optics and display technology. This part of the presentation investigates the unexplored phenomenon of the coating and filling of 8CB and 10CB liquid crystals inside ~200nm diameter Multi-Wall Carbon nanopipes. The phase transition characteristics of the confined liquid crystal films were studied using MDSC technique and will be the last part of this presentation.

The crystalline iron-based nanowires encapsulated by multiwalled carbon nanotubes have been the subject of numerous studies owing to the range of potential applications. The presence of a-Fe (bcc)/y -Fe(fcc) junctions o ers the possibility of exploitation of the exchange bias effect, an interfacial magnetic phenomenon that plays a major role in magnetocaloric cooling, spintronic and high-density magnetic storage devices. This work is concerned with the synthesis and microstructural characterization of Fe-based and NiFe nanowires encapsulated by multiwall carbon nanotube radial structures. The known attributes of these structures are well matched to the magnetocaloric application. The primary aim of this work was to determine the unknown microstructural details of the encapsulated nanowire that are of relevance to the magnetocaloric application (junction types, location and orientation relative to the nanotube axis). The secondary aim was to explore the modi cation of the synthesis route to promote desirable attributes. This is the first report of a-Fe/y -Fe sequential junctions and a-Fe/Fe3C concentric junctions in encapsulated Fe-based nanowires. The presence of a-Fe/y -Fe junctions was inferred from the observation of a-Fe nanowires terminated by a ~100 nm length y-Fe crystallites of larger diameter. The a-Fe/Fe3C junctions exhibit the Bagaryatski orientation relationship: [110 ]bcck[100 ]orth. The degree of substrate roughness was found to be a means of tailoring details of the structure and composition of the encapsulated nanowires. NiFe encapsulated nanowires were found to contain crystallites of a-NiFe, y-NiFe and Ni3Fe and the sequential junctions -NiFe/Ni3Fe and a-NiFe/y-NiFe junctions.

Neste trabalho se realiza a síntese e caracterização de nanofios de óxido de zinco. Adicionalmente se apresenta o processo de montagem de um dispositivo para medidas elétricas deste material. Estuda-se complementarmente o efeito do tratamento de plasma sobre as propriedades de fotoluminescência do material. Nanofios foram sintetizados pelo mecanismo vapor-líquido-sólido (VLS), utilizando ouro como catalizador e safira c-plane como substrato. As amostras foram caracterizadas utilizando microscopia eletrônica de varredura, fotoluminescência a temperatura ambiente, difração de raios X, e microscopia eletrônica de transmissão. Os nanofios obtidos têm seção transversal com formato quase hexagonal, e larguras de aproximadamente 46 nm. O comprimento deles varia de 3 a 10 μm. Os resultados de difração de raios x e microscopia eletrônica de transmissão mostram que eles são monocristalinos com rede cristalina tipo wurtzita, e com direção de crescimento no eixo c. Foram estudados os efeitos da potência de plasma de oxigênio (O2) na fotoluminescência dos nanofios a temperatura ambiente. A diferença na fotoluminescência após diferentes tratamentos de plasma de O2 mostra que a razão entre a emissão da região do band gap e da banda do visível pode ser modificada pelo tratamento. Este efeito corrobora com a hipótese de que a banda verde de luminescência está relacionada às vacâncias de zinco. A variação percentual da razão entre as duas regiões apresenta uma dependência linear com a potência do plasma. / In this work, we performed the synthesis and characterization of zinc oxide nanowires. We also report an assembly process to measure the electrical properties of this material. We study the plasma treatment effect on the photoluminescence spectra of the nanowires. Nanowires were synthesized via vapor-liquid-solid mechanism, using gold as catalyst and c-plane sapphire as substrate. The samples were characterized using scanning electron microscopy, room temperature photoluminescence, x-rays diffraction and transmission electron microscopy. Our nanowires show a quasi-hexagonal cross section, with diameters of approximately 46 nm. Their lengths ranged from 3 to 10 μm. Our results show monocrystalline wurtzite crystal nanowires with c growth direction. We also study the plasma power effect of oxygen (O2) plasma treatment on the room temperature photoluminescence spectra of the nanowires. Our results show that the deep level emission to near band emission ratio decreases with the plasma treatment. This effect supports the hypothesis that claims the green band luminescence is related to the oxygen vacancies. Furthermore, the relative ratio change depends linearly on the plasma power.

La miniaturisation des transistors a progressé par noeud technologique avec l’introduction successive de nouveaux matériaux (high k) et de nouvelles architectures (FinFET, NWFET). Pour les noeuds technologiques avancés, une nouvelle rupture en matériau est envisagée pour remplacer le Silicium du canal de conduction par des matériaux à forte mobilité (2D, III-V). Les matériaux III-V sont de bons candidats pour répondre à cette problématique grâce à leur forte mobilité de type n (InGaAs, InAs, InSb) ou de type p (GaSb). Au cours de cette thèse, un intérêt particulier a été porté au couple de matériaux InAs/GaSb, qui offre un avantagesupplémentaire de par son accord de paramètre de maille permettant d’accéder dans une même structure à des couches de mobilités n et p. La croissance de matériau III-V directement sur substrat (001)-Si 300mm est aujourd’hui un challenge d’intérêt majeur pour proposer des procédés compatibles avec les plateformes industriels CMOS. Ces croissances restent complexes à cause de la formation de défauts : parois d’antiphase, dislocations, fissures ; générées respectivement par la différence depolarité, de paramètre de maille et de coefficient d’expansion thermique, entre le Silicium et les matériaux III-V. Dans cette thèse nous présentons une première démonstration de croissance par MOVPE de GaSb directement sur substrat (001)-Si nominal 300mm compatible avec les plateformes industrielles CMOS. Les couches de GaSb présentent une rugosité de surface sub-1nm, et une qualité cristalline au niveau de l’état de l’art en MBE. La croissance d’une couche d’InAs a ensuite permis la réalisation d’un démonstrateur FinFET à canaux multiples d’InAs. Ce derniera été élaboré via une technique lithographique alternative à haute résolution basée sur l’utilisation de copolymère à bloc. Ce procédé simple pour réaliser des canaux de conduction permet d’accéder à une forte densité de fils, de faibles dimensions, et en seulement cinq étapes de fabrication. / The transistors’s miniaturization evolved through technological nodes with the successive introduction of new materials (high k) and new architectures (FinFET, NWFET). For the advanced technological nodes, a new break in material is considered to replace the silicon of the conduction channel with high mobility materials (2D, III-V). III-V materials are good candidates to address a solution to this problem thanks to their n-type (InGaAs, InAs, InSb)or p-type (GaSb) high mobility. During this PhD, a particular interest has been given to the InAs/GaSb pair of materials, which offers an additional advantage by its lattice parameter agreement making it possible to access n-type and p-type high mobility layers in the same structure.Nowadays, the growth of III-V materials directly on (001) -Si 300mm substrates is a challenge of major interest to develop industrial platforms compatible processes. These growths remain complex because of defects formation: antiphase boundaries, dislocations, cracks; generated respectively by the difference in polarity, lattice mismatch and difference in thermal expansion coefficient, between the silicon and III-V materials. In this PhD, we present a first demonstration of GaSb growth by MOVPE directly on nominal (001) -Si 300mm substrate compatible with industrial platforms. The GaSb layers have a sub-1nm surface roughness, and an equal to MBE state of the art crystalline quality. The growth of a InAs layer then allowed the realization of an InAs FinFET multi-channel demonstrator. The latter was developed via a high resolution alternative lithographic technique based on the use of block copolymer. This simple method for producing conduction channels makes it possible to access a high density of wires, of small dimensions, and in only five manufacturingsteps.

Orientador: Guillermo Gerardo Cabrera Oyarzun / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-24T18:53:24Z (GMT). No. of bitstreams: 1 CorreaJunior_Clovis_M.pdf: 2475896 bytes, checksum: c134096367f5dc832f4c32ec8a9e224b (MD5) Previous issue date: 2009 / Resumo: Nesta dissertação de mestrado são descritas as características fundamentais dos condutores mesoscópicos, e as origens e propriedades das várias fontes de ruído em condutores. Primeiramente, descrevemos o ruído com distintos métodos e enfatizamos as propriedades de cada tipo de ruído. Em seguida, apresentamos a abordagem da matriz de espalhamento para condutores com coerência de fase, a qual permite-nos tratar as propriedades de transporte de forma unificada. Estudamos um modelo proposto para descrever as propriedades de transporte de nanofios e nanocontatos dos metais magnéticos de transição. É assumido que os orbitais de condução são do tipo s, o que permite a existência de dois canais de condução devido ao spin. Da mesma forma, consideramos os orbitais d como fontes de momentos de dipolos magnéticos locais. O modelo é aplicado ao caso de nanocontatos constituídos de dois átomos, os quais estão acoplados a dois eletrodos magnéticos. Usando um pequeno campo externo, é possível controlar os estados de polarização dos eletrodos: paralelamente e anti-paralelamente. Nesse nanocontato, são estudados as propriedades do coe½ciente de transmissão, da condutância, do ruído shot quântico, do fator de Fano e da magnetoresistência / Abstract: This dissertation describes the fundamental caracteristics of mesoscopic conductors, and the origins and properties of the sources of noise in conductors. Firstly, we describe the noise through different methods and emphasize the properties of each kind of noise. In the following, we present the scattering approach for coherent phase conductors, which allows us to get the transport properties from a unified picture. It is studied a particular model to describe the transport properties of magnetic transsition metal nanowires and nanocontacts. It is assumed that conduction orbitals are s-like, with the occurrence of only two conductions channel due to spin. In turn, d-like orbitals are sources of local magnetic moments. The model is applied to a simple nanocontact built of two atoms, which are coupled to two magnetic electrodes. Using small external fields, one can handle the polarization state of the electrods: in parallel or antiparallel alignment. From that nanocontact, we investigate the properties of the transmission coe°cient, the conductance, the quantum shot noise, the Fano factor and the magnetoresistance / Mestrado / Física da Matéria Condensada / Mestre em Física

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-01T20:30:15Z No. of bitstreams: 1 WesdneySantosMelo.pdf: 2720501 bytes, checksum: 50d1b40146745c9b22c97ac40da583a3 (MD5) / Made available in DSpace on 2017-06-01T20:30:15Z (GMT). No. of bitstreams: 1 WesdneySantosMelo.pdf: 2720501 bytes, checksum: 50d1b40146745c9b22c97ac40da583a3 (MD5) Previous issue date: 2015-12-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this thesis we studied the structutal, eletronic and magnetic properties of Fe- lled regular and at (8,0), (9,0), (6,6) and (7,7) carbon nanotubes. After the attening of these systems were subjected to hydrogenation and oxygenation. To investigate these properties we use rst principles calculations based on Density Functional Theory, implemented in SIESTA computer code. Our results show that the system Fe- lled at nanotube is stable, and this stability is caused by the interaction between the metal atoms and carbon atoms. We observed that the attening caused a misalignment of wire atoms and dimerization of the nanowire does not cause considerable changes in the distances between an atom and its neighbors. It was observed that when the systems (n, n) were underwent hydrogenation and oxygenation occurred the spontaneous formation of two nanoribbons with zigzag edges. When systems formed by (n, 0) CNT are subjected to hydrogenation and oxygenation, there is no spontaneous production of nanoribbons. Furthermore, it was found that systems formed by a metallic wire between nanoribbons are energetically more stable that systems formed by Fe- lled at (n,0) carbon nanotubes. / Neste trabalho estudamos as propriedades estruturais, eletrônicas e magnéticas de fios lineares de ferro e encapsulados por nanotubos de carbono (8,0), (9,0), (6,6) e (7,7) normais e achatados. Após ́o achatamento estes sistemas foram submetidos à hidrogenação e oxigenação. Para investigar essas propriedades utilizamos cá́lculos de primeiros princípios baseados na teoria do funcional da densidade, implementados no código computacional SIESTA. Nossos resultados mostram que o tubo achatado com o nanofio encapsulado é estável e esta estabilidade deve-se às interações entre os átomos metá́licos e os á́tomos de carbono. Percebeu-se que devido ao achatamento os átomos do fio ficam desalinhados e a dimerização do nanofio nã̃o provoca alterações considerá́veis nas distâncias entre um átomo e seus vizinhos. Observou-se que quando os sistemas (n,n) foram submetidos à hidrogenação e oxigenação houve a formação espontânea de duas nanofitas com bordas zigzag. Os sistemas formados CNTs (n,0) não produziram espontaneamente nanofitas quando submetidos à hidrogenação e à oxigenação. Além disso, verificou-se que energeticamente nanofitas intercaladas com fio metá́lico sã̃o mais está́veis que nanofios encapsulados por nanotubos (n,0) achatados.

>Magister Scientiae - MSc / Solar cells offer SA an additional energy source. While Si cells are abundantly available they are not at an optimal efficiency and the cost is still high. One technology that can enhance their performance is SiNW. However, material properties such as the diameter, porosity and length determine their effectiveness during application to solar cell technology. One method of growing SiNW uses Sn catalysts on a Si substrate. As the properties of the Sn nanoparticle govern the properties of the SiNW, this thesis investigates their formation and properties by depositing a Sn layer on a Si wafer and then subjecting it to different temperatures, during process the layer forms into nanoparticles. At each temperature the morphology, composition and crystallinity will be determined using XPS, SEM, TEM and EDS. Thus, in Chapter 1 there is an overview, Chapter 2 deals with techniques used in this study, Chapter 3 will give the quantitative and qualitative results on the XPS analysis and Chapter 4 will illustrate the structural behaviour of the annealed Sn film samples.

Ce travail est consacré à l'épitaxie en phase gazeuse d'organométallique de nanostructures de nitrures en forme de fil et de pyramide, pour lesquelles nous cherchons à comprendre les mécanismes de croissance mis en jeu. Une étude paramétrique complète est présentée pour optimiser et mieux appréhender la croissance de nanofils GaN auto-assemblés non-catalysés. Nous démontrons notamment que l'injection de silane est un paramètre-clé pour la croissance des nanofils grâce à la formation d'une couche SiNx de passivation sur les facettes latérales qui joue le rôle d'un masque favorisant ainsi la croissance verticale. Un nouveau procédé de croissance de nanofils sans silane est aussi proposé dans ce travail en utilisant de très faibles flux de précurseurs qui favorise la formation de facettes verticales. De tels nanofils présentent d'excellentes propriétés structurales et optiques grâce à l'absence de silicium. Par ailleurs, nous montrons que la polarité joue un rôle crucial sur la croissance des nanostructures de GaN puisque la forme des nanostructures peut être simplement déterminée par l'orientation de la polarité: une polarité N résulte en fils alors qu'une polarité Ga en pyramides. Par conséquent, la forme fil/pyramide des nanostructures peut être directement choisie en contrôlant la polarité sur des substrats de saphir ou de GaN. Nous avons justement exploité cette méthode pour obtenir des réseaux ordonnés de fils et de pyramides de GaN en utilisant la croissance sélective à travers un masque nanostructuré par lithographie. De telles nanostructures ont été utilisées pour la croissance d'hétérostructures InGaN/GaN pour obtenir soit des puits quantiques non-polaires sur les flans des nanofils, soit des boîtes quantiques d'InGaN aux sommets des pyramides. / This work reports the metal-organic vapour phase epitaxy of III-Nitride wire- or pyramid-shaped nanostructures and focuses on the growth mechanisms related to these two types of GaN nanostrcutures. A complete parametric study is presented in order to optimize and to understand the catalyst-free self-assembled GaN nanowire growths. We demonstrate that the silane flux injection is a key-parameter for nanowire growth thanks to the formation of SiNx passivation layer along the sidewall facets that acts as a mask favoring the vertical growth. A novel silane-free nanowire growth is also proposed in this work using ultra-low precursor flux that favors the formation of vertical facets. Such nanowires exhibit excellent structural and optical properties due to the absence of silicon. In addition, the polarity is found to play a key-role for GaN nanostructure growth, since the nanostructure shape can be basically determined by the polarity orientation: N-polar nanostructure results in wire, whereas Ga-polar in pyramid. Consequently, the shape wire/pyramid of nanostructure can be chosen depending on the polarity control on sapphire or GaN substrates. This method is applied to get ordered arrays of GaN wires and pyramids using selective area growth on patterned mask. Such nanostructures can be used as template for InGaN/GaN heterostructure growth to get either non-polar multi-quantum wells along the wire sidewalls or InGaN quantum dots at the pyramid apex.