This thesis is devoted to the investigation of mechanical properties and self-assembly process of materials at the nanoscale. Various nanostructured materials such as nanoparticles, nanotubes, nanowires and thin films are used as constituent elements of nanodevices. Hence, knowledge of the mechanical properties of materials at the nanoscale is extremely important for understanding their functionality in nanodevices. Mechanical properties of nanostructured materials may significantly differ from those of their bulk counterparts due to the high surface to volume ratio in nanostruc-tures. We particularly focus on the role of the surface region on the stiffness of nanomaterials. We have shown that the stiffness of a nanomaterial can be tuned over a wide range by introducing appropriate coating on the nanostructure surface. We have also explored the effects of the surface region on the stability of various phases in a nanostructure. In the second part of this thesis, we have described the self-assembly process of nanostructures mediated by drendrimers. Self-assembly techniques are frequently used to decorate nanostructures into specific networks. The motivation of this study is to investigate the mechanisms which control the effective interaction and the inter-particle distance between nanoparticle-dendrimer compos-ites. Control over the inter-particle separation is very important since it has a strong influence on the electronic and optical properties of the nanostructures. In the following paragraphs, we sum-marize the results of our study. We start with a brief introduction to the mechanical properties and self-assembly process of nanostructures in the first chapter. A brief review of the work done on these topics in the recent past is presented in this chapter. We discuss the results and conclusions of various experimental and numerical studies on these topics. We also mention the motivation for the studies we have carried out. At the end, we briefly describe the numerical methods (molecular dynamics (MD) and density functional theory (DFT)) which have been used in our investigations. In the second chapter, we discuss the effects of the surface region on the mechanical properties of nanostructures. We have investigated the size and growth direction dependence of the mechanical properties of ZnS nanowires and thin films as a case study. We observe that the Young’s modulus of nanowires and thin films strongly depends on their size and growth direction. This size and growth direction dependence of the stiffness of nanostructured materials can be explained in terms of their surface modifications. Since the energy of the surface region is usually higher than that of the core region in a nanostructure, the surface atoms move their positions to minimize the surface energy. As a result, bond lengths at the surface region are usually different from their bulk values. We observe that in ZnS nanowires and thin films, the average bond length at the surface region is lower than that in the core region which remains unchanged from its bulk value. This decrease in the bond length (or equivalently increase in the bond energy) increases the effective stiffness of the entire nanostructure. As the size of the nanowire/thin film increases, the effect of the surface region gradually decreases and hence the Young’s modulus value converges to the bulk value. Since the surface region has a strong influence on the mechanical properties of nanostructures, the stiffness of a nanostructure can be tuned by modifying the surface region with other materials. In chapter three, we have shown that the stiffness of ZnS nanowires can be tuned by introducing a thin CdS shell on top of the ZnS surface. In general, the stiffness of a nanostructure can be increased (decreased) by coating the surface region with a stiffer (less stiff) material. However, the stiffness of the core/shell nanostructures strongly depends on the properties of the interface between the core and the shell. We observe that the binding energy between the core and shell regions is relatively low due to the lattice mismatch at the interface region of core/shell nanostructures. This lower binding energy strongly affects the stiffness of core/shell nanostructures. We have also shown that thermal properties such as thermal conductivity and melting temperature of core/shell structures can be tuned by changing the coating material. In chapter four, we discuss the effects of the surface region on the stability of various phases in a nanostructure. The surface atoms may stabilize a particular phase in a nanostructure which is not a stable phase in the bulk material. In this chapter, we investigate the stability of the h-MgO phase, an intermediate structure found during the wurtzite to rock salt transformation, in CdSe nanostructures. We observe that this five-fold coordinated phase is more stable at lower temperatures and smaller sizes of the nanowires. The appearance of this phase has not been observed till now in experiments. We show that this phase is not stable for larger CdSe nanocrystals on which the experiments have been done. In the rest of the thesis, we have presented the results of our studies of self-assembly of nanostructures mediated by DNAs and dendrimers. First we describe in chapter five the nature of the effective interaction between two PAMAM dendrimers. Dendrimers are frequently used to coat surfaces of nanoparticles to prevent the nanoparticles from aggregation. The interaction between such nanoparticle-dendrimer composites depends strongly on the nature of the effective interac-tion between dendrimers. We have used fully atomistic MD simulations to calculate the potential of mean force (PMF) between two PAMAM dendrimers. We show that the effective interaction strongly depends on the size (generation) and protonation level of the dendrimers. The PMF profiles of nonprotonated dendrimers show a global minimum which represents the attractive nature of the interaction between the dendrimers up to a certain center-to-center distance. On the other hand, the interaction between protonated dendrimers is repulsive throughout their interaction re-gion. The PMF profiles are fitted very well by a sum of an exponential and a Gaussian function. This observation is in contradiction with some of the results of existing coarse-grained simulations which predicted the effective interaction between dendrimers to be Gaussian. Our atomistic simulation which includes all the local fluctuations is expected to give more accurate results. Information about the effective interaction between two dendrimers helps in understanding how dendrimer molecules can be used to control the interaction strength and the preferred inter-particle distance between two nanostructures. In chapter six, we discuss the effective interaction between two dendrimer grafted gold nanoparticles. We observe that dendrimer molecules can get adsorbed spontaneously on the surface of a gold nanoparticle. These grafted dendrimers significantly alter the interaction between the gold nanoparticles. We have explored the effects of proto-nation level and the density of the grafted dendrimers on the effective interaction between two gold nanoparticle-dendrimer composites. We observe that these nanoparticle-dendrimer composites at-tract each other at low grafting density. However, the interaction strength and the inter-particle distance at the minimum of the potential are much lower and higher, respectively than those between two bare gold nanoparticles. Interestingly at higher grafting density, the nature of the interaction between the nanocomposites depends on the protonation level of the grafted dendrimers. Nanoparticles grafted with nonprotonated dendrimers still attract each other but with lower inter-action strength and higher inter-particle distance compared to the values for low grafting density. On the other hand, nanocomposites grafted with protonated dendrimers repel each other at high grafting density. Thus we show that the effective interaction and the optimal inter-particle distance between the nanostructures can be tuned over a wide range by using a suitable grafting density and protonation level of the dendrimers. In the seventh chapter, we describe a strategy to assemble dendrimers with the help of sin-gle stranded DNA (ssDNA). We attach an ssDNA to one dendrimer and a complementary ssDNA to a second dendrimer. These two complementary ssDNAs bind with each other through base pair formation to assemble the dendrimers into a single structure. The complementary ssDNAs form a dsDNA which is rigid enough to maintain the inter-dendrimer distance almost the same as the length of the DNA. The inter-dendrimer distance can be tuned by changing the DNA length. However, this method strongly depends on the protonation level of the dendrimers. It works well only for nonprotonated dendrimers. Since the protonated dendrimers are positively charged, they strongly interact with the negatively charged ssDNAs through electrostatic interaction. As a result, ssDNAs wrap the dendrimer surface and hence the inter-dendrimer distance can not be controlled. We have also verified that this method works for multiple nonprotonated dendrimers as well. In the final chapter of this thesis, we summarize the main results and conclude with a brief discussion of future directions of research on the problems considered in the thesis.

La thèse a pour objet l’analyse de mécanismes d’assemblage permettant la création de structures auto-assemblées à base d’ADN, fonctionnalisées par des protéines. Les PDNA sont des molécules synthétiques associant de manière covalente une séquence d’ADN et un module protéique. Basés sur le domaine hèmique du cytochrome b5, les PDNA présentent des propriétés d’absorption optique dépendant de l’état d’oxydoréduction. Pour comparer leurs propriétés d’assemblage à celles des ADN correspondants, différentes briques ont été assemblées en ensembles finis ou ouverts. Un premier chapitre développe des approches expérimentales et de modélisation pour caractériser les paramètres contrôlant ces assemblages dans le cas des ADN. Un second chapitre transpose ces approches aux assemblages de PDNA afin de mettre en évidence l’influence des domaines protéiques. Dans les deux cas, les travaux sont menés à la fois à un niveau macroscopique et de molécule unique par AFM classique ou à haute vitesse. Une troisième partie vise à comprendre comment ces assemblages peuvent être multiplexés sur des puces au sein de matrices poreuses de dextran. Nous avons modélisé l’influence de la matrice sur l’assemblage et utilisé ces modèles pour définir les relations entre observables et paramètres microscopiques. Une dernière partie vise à lever les limitations (pièges d’assemblages) mises en évidence dans les réactions sous contrôle cinétique. Nous avons développé des assemblages dans des conditions douces et contrôlées par le pH impliquant des structures ADN en motifs-i. Leur mécanisme de formation et de polymérisation en suprastructures a été étudié en combinant biochimie et analyse structurale par AFM. / The work aimed at the analysis of assembly mechanisms allowing the creation of DNA-based self-assembled structures functionalized with proteins. PDNA are synthetic molecules covalently associating a DNA strand to a protein domain. The ones based on the heme binding domain of cytochrome b5 feature optic properties that depend on oxydoreduction state. In order to compare self-assembly properties of PDNA and corresponding DNA, different building blocks were assembled into converging or open ensembles. A first part develops experimental and modeling approaches to characterize parameters controlling such assemblies of DNA blocks. A second part extends these approaches to PDNA assemblies, bringing in evidence the influence of the protein domain on nucleic parts. Studies were both performed at macroscopic and single molecule levels using conventional and high-speed AFM. A third part describes how chip multiplexing of those assemblies onto a porous dextran matrix influences behaviors. Modeling the matrix influence on assembly allowed defining relations between experimental and macroscopic parameters. A last part aims at bypassing bottlenecks related to the formation of trapped structures when reactions are kinetically driven. We developed i-motif based DNA structures that can be assembled into mild, pH controlled conditions. Their mechanism of formation and polymerization into supramolecular structures were studied combining physico-chemical and structural analyses using AFM.

>Magister Scientiae - MSc / In the current study, we give a report when oxalic acid was used as an electrolyte to synthesize an AAO template with hexagonal pore array. Optimum parameters were observed as 0.4 M of oxalic acid, anodizing voltage of 45 V, temperature of approximately 8 °C and the period of 120 minutes. Atomic force microscope (AFM) and High resolution scanning electron microscope (HRSEM) showed that template has an average pore diameter of 103 nm. Co and MnOx (x = 1,2) nanostructures were selectively deposited in the pores of the template using a novel atomic layer deposition (ALD) technique. The diameter sizes and the array of the nanostructures and the template were corresponding. Energy dispersive xrays (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co and MnOx (x =1,2) on the samples while x-ray diffraction (XRD) provided an indication of their orientations. Magnetic force microscopy as main characterization tool showed the existence of multi-domains on both Co and MnOx (x =1,2) nanostructures. / South Africa

No description available.

Graphene is a single layer of carbon atoms arranged in honeycomb lattice. Over a long period of time it was treated as a hypothetical material to understand the properties of other allotropes of carbon, such as graphite, carbon nanotube etc. Half decade back, a single layer of graphene was finally isolated and since then the field has observed a flurry of activities. Low energy excitations in graphene are massless Dirac Fermions and quantum electrodynamic effects can be observed at room temperature in graphene, which makes it very popular among the condensed matter community. In addition graphene also shows many interesting mesoscopic effects, which is the focus of the present work. We study the electronic, magnetic and structural properties of the graphene nanostructures. The entire thesis based on the results and findings obtained from the present investigation is organized as follows. Chapter 1: provides a general introduction to the properties of graphene and graphene based nanostructures. Chapter2:describes the theoretical tools used in this thesis to investigate the properties of graphene nanoribbons. The first two chapters are meant to give the reader an overview about the field of graphene and a few of the computational techniques commonly used to investigate the properties of graphene. The following chapters are the new findings reported in this thesis. Chapter3:shows how zigzag graphene nanoribbons respond in a non-linear fashion when edges are subjected to some external potential such as magnetic field. Such response originates from the edge states present in zigzag ribbons and thus not observed in armchair nanoribbons. In the limit of ribbon width W→∞, an edge magnetic field produces a moment of ~ 1/3 per edge atom even for an infinitesimally small field, which is clearly a signature of non-linear response. Response of a finite width nanoribbon is size dependent and also depends on ln(V), the applied field. This is akin to Weber-Fechner law of audio visual perceptions. It is interesting to note that nature does provide a “quantum realization” of this in the form of biological sensing organs like the ear and eye. The magnetic response is found to scale inversely with the ribbon width. Chapter4:deals with the magnetic properties of the zigzag graphene nanoribbon. This is also a special property of the geometry of the zigzag edges and not observed in armchair nanoribbons. Our investigation reveals that the electron-electron repulsion (Hubbard U) energy creates a delta function like edge magnetic field in zigzag graphene nanoribbons. Starting from this, magnetic properties of zigzag graphene nanoribbons can be qualitatively and quantitatively explained from the non-linear response of zigzag nanoribbons. Zigzag graphene nanoribbons can exist in two possible ‘magnetic states’: antiferro (AF) where the two opposite edges have antiparallel magnetic moment and ferro (FM) where moment is parallel in the two opposite edges. First we describe the properties of undoped zigzag nanoribbons. They have AF ground state. Continuum theory can explain the size dependent bandgap and magnetic moment of the ground state. We present the first explicit derivation of the gap. Then we show that hole doping can change the ground state to FM, which is metallic. Thus the system has the property of magnetoresistance, which can be exploited by doping zigzag graphene nanoribbons externally with some gate voltage or internally by some electron acceptor element, such as boron. The critical doping for transition depends inversely with the ribbon width. We have found that the ‘phase transition’ on hole doping is a common phenomena for zigzag terminated nanostructures, such as hexagonal nanodots. Chapter5:discusses the effects of random edge shapes and random potential (Anderson disorder) on the magnetic properties of zigzag graphene nanostructures. Defects and disorders in the form of edge shape randomness and random potentials arising from substrate are very common in graphene. Our study reveals that edge state magnetism is very robust to shape randomness of the terminating edges of nanostructures; as long as there are three to four repeat units of a zigzag edge, the edge state magnetism is preserved. We also discover some “high energy” edges (ones where the edge atoms have only one nearest neighbor) can have very large moments compared to even the zigzag edges. Edge magnetism is also found to be robust to relatively small Anderson disorders, because a slowly varying small potential does not scatter the edge states. Chapter6:reveals how edge functionalization by O atom and OHgroup changes the properties of the zigzag graphene nanoribbons. Functionalization by various different molecules is a very popular method of tuning the properties of graphene. We have shown that it is possible to tune the properties of zigzag graphene nanoribbons by edge functionalization. Further, we have found that structures with clustered functionalization leads to “spatially” varying electronic structure, which can lead to interesting possibilities for electronic devices. Chapter7:describes structural stability, electronic and magnetic properties of graphene nanoribbons in presence of topological defects such as Stone-Wales defects. Our study reveals that the sign of stress induced by a SW defect in a graphene nanoribbon depends on the orientation of the SW defect with respect to the ribbon edge and the relaxation of the structure to relieve this stress determines its stability. Local warping or wrinkles arise in graphene nanoribbon when the stress is compressive, while the structure remains planar otherwise. The specific consequences to armchair and zigzag graphene nanoribbon can be understood from the anisotropy of the stress induced by a SW defect embedded in bulk graphene. We also have found localized electronic states near the SW defect sites in a nanoribbon. However, warping results in delocalization of electrons in the defect states. We have observed that, in zigzag graphene nanoribbons magnetic ordering weakens due to the presence of SW defects at the edges and the ground state is driven towards that of a nonmagnetic metal.

The hybrid nanostructures exhibit excellent performances in various fields such as catalysis, sensing, and energy conversion as compared to their individual ones. The thesis deals with the new methods for the synthesis of different type of hybrids with doped/pristine carbon nanostructures in the form of graphene, multiwall carbon nanotubes (MWCNTs) as one component and metals nanostructures (Ag, Pd, Pt and Au), carbide (Fe3C), metal chalcogenides (Ni3S2 and Co9S8) and oxide (CoO) as the other components. Various synthesis techniques such as modified galvanic replacement reaction at room temperature, hydrothermal, microwave and pyrolysis have been explored for the synthesis of different hybrid nanostructures. Furthermore, various hybrid nanostructures have been explored for various catalytic activities such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and 4-nitrophenol (4-NP) reduction. It may be noted that the ORR and OER which are undoubtedly vital for their applications in fuel cells, metal-air batteries and water oxidation reaction. Interestingly, the catalytic activities of these hybrid nanostructures are comparable or better as compared to the commercial benchmark precious catalysts.

Les propriétés électroniques et thermoélectriques de matériaux basés sur Mg2Si ont été étudiées par calculs DFT et semi-classiques (théorie de Boltzmann). Les effets d’abaissement de dimensionalité et de contraintes ont été étudiés. Les calculs ont été effectués sur les films monocristallins orientés 001, 110 et 111 et sur les films polycristallins. Seul le film monocristallin orienté 110 a montré des propriétés thermoélectriques intéressantes. Trois types de contraintes ont été investiguées: uniaxiale, biaxiale et isotrope. L’augmentation de la contrainte sur Mg2Si produit un décalage du maximum du facteur de puissance (PF) vers les basses températures. Comparé à Mg2Si non contraint, le coefficient Seebeck (S) augmente uniquement sous contrainte isotrope. On montre l’équivalence des propriétés thermoélectriques entre Mg2Si contraint dans la direction [110] et celles du film orienté 110. Les contraintes de tension isotropes ont été modélisées en insérant des atomes Sb dans Mg2Si massif conduisant aux structures Mg2Si:Sb, Mg2Si:3Sb and Mg2Si:4Sb. Seul Mg2Si:4Sb produit une contrainte isotrope. Les effets de substitutions de Sn pour Si dans Mg2Si massif sont similaires à ceux observés pour Mg2Si sujet à des contraintes en tension uniaxiales et biaxiales. Pour les films Mg2Si1−xSnx orientés 110 le S du matériau dopé p est supérieur à celui des massifs Mg2Si et Mg2Si1−xSnx. Concernant les nanostructures, le super-réseau Mg2Si/Mg2Sn est le plus intéressant lorsque faiblement dopé p et à basse température. Les assemblages de fils sont les meilleurs en tant que matériaux faiblement dopés n et à basse température: le PF est quasiment doublé par rapport à celui de Mg2Si massif. / The electronic and thermoelectric properties of Mg2Si-based materials have been investigated by means of DFT calculations and semi-classical Boltzmann theory. The low-dimensional and strain effects on these properties have been studied. The properties have been investigated on 001-, 110- and 111-oriented Mg2Si monocrystalline films, and on polycrystalline Mg2Si film. Only the 110-oriented monocrystalline film has been found to have interesting thermoelectric properties. Three types of strains have been investigated: uniaxial, biaxial and isotropic. Increasing the intensity of the strain on Mg2Si induces a shift of the power factor (PF) maximum towards low temperature. Compared with unstrained Mg2Si, the Seebeck coefficient (S) increases only under isotropic strain. We evidence an equivalence in the thermoelectric properties between Mg2Si material constrained in the [110] direction and the 110-oriented Mg2Si film. Isotropic tensile strains have been modeled by inserting Sb atoms in bulk Mg2Si leading to the stuctures Mg2Si:Sb, Mg2Si:3Sb and Mg2Si:4Sb. Only Mg2Si:4Sb is found to induces such type of constraints. The effects of the Sn for Si substitutions in bulk Mg2Si are very similar to those observed for Mg2Si subjected to uniaxial and biaxial tensile strains. For (110)-oriented Mg2Si1−xSnx films S of the n−doped material outperforms that of the bulk Mg2Si and bulk Mg2Si1−xSnx. Regarding nanostructures, the Mg2Si/Mg2Sn superlattice is most interesting as a p-doped material at low carrier concentration/low temperature. The stick assemblage is best as a n-doping material at low carrier concentration/low temperature where its PF is almost twice as high as that of bulk Mg2Si.

Le besoin incessant de débits toujours plus élevés dans les systèmes de télécommunications a un impact sur tous les éléments composant la chaine de transmission. Ainsi, pour faire face à l’augmentation croissante du volume de données échangées à travers le monde, le développement de nouvelles sources optiques semi-conductrices est absolument nécessaire. La modulation directe de lasers nanostructurés constitue une alternative bas coût et à faible consommation énergétique qui permettra de remplacer progressivement les diodes lasers à puits quantiques actuelles. De nombreux efforts en recherche ont été consacrés au cours des dernières années en vue d’améliorer les performances dynamiques des lasers nanostructurés notamment en terme de bande passante, de facteur de couplage phase-amplitude (facteur α) et de dérive de fréquence (chirp). Pour les applications aux très grands réseaux et systèmes de communication, la croissance d’îlots ou de fils quantiques déposés sur substrat InP permet de réaliser des dispositifs nanostructurés émettant dans le proche infra-rouge autours de 1550 nm. Dans ce mémoire, la dynamique de modulation des lasers nanostructuré est étudiée en régime de modulation directe. Les caractéristiques analysées comprennent: la modulation en amplitude (AM) et en fréquence (FM), le chirp, et les réponses en régime grandsignal. Grâce à une approche semi-analytique, il est démontré que la bande passante et l’amortissement sont fortement limités par les phénomènes de capture et de relaxation des porteurs de charge dans les nanostructures. Afin d’étudier les propriétés du facteur α et du chirp, un nouveau modèle dynamique a été proposé, prenant en compte la contribution à l’indice optique des porteurs de charge dans des états hors résonance. Il est ainsi montré que, contrairement au cas des lasers à puits quantiques, le facteur α dépend fortement du courant de pompe et de la fréquence de modulation. Le facteur α reste constant à basses fréquences (<0,1 GHz) et supérieur aux valeurs obtenues à hautes fréquences (au-delà de quelques GHz) à partir de la technique FM/AM. Ces caractéristiques sont essentiellement attribuées aux contributions des porteurs dans les états hors résonance. Les simulations montrent que le facteur α peut être réduit en augmentant la séparation énergétique entre l’état fondamental résonant (GS) et les états hors résonance. En particulier, un effet laser sur 1’état excité des nanostructures (ES) constitue une solution prometteuse pour améliorer les performances dynamiques, en accroissant notamment la bande passante de modulation et en réduisant le facteur α d’environ 40%. Les techniques d’injection optique sont également intéressantes pour régénérer les performances dynamiques des lasers. Le couplage phase-amplitude et le gain optique y sont substantiellement modifiés via le contrôle de l’amplitude et du désaccord en fréquence du faisceau injecté. Dans ce cadre, ce travail propose une nouvelle technique dérivée de la méthode Hakki-Paoli, permettant de mesurer, sous injection optique, le facteur α à la fois en dessous et au-dessus du seuil. Les lasers à cascade quantique (QCL) sont basés sur des transitions électroniques inter-sous-bandes dans des hétérostructures à puits quantiques. Ces lasers présentent une bande passante (AM) relativement de quelques dizaines de GHz et sans résonance ce qui est prometteur pour les transmissions en espace libre. De manière surprenante, les calculs montrent que les QCL présentent une largeur de bande FM extrêmement large de l’ordre quelques dizaines de THz, environ trois ordres de grandeur supérieure à la largeur de bande AM. L’injection optique dans ces lasers présente les mêmes avantages que ceux procurés dans les lasers à transitions interbandes. Des désaccords positifs ou négatifs en fréquence augmentent notamment la largeur de la bande passante. / High performance semiconductor lasers are strongly demanded in the rapidly increasing optical communication networks. Low dimensional nanostructure lasers are expected to be substitutes of their quantum well (Qwell) counterparts in the next-generation of energy-saving and high-bandwidth telecommunication optical links. Many efforts have been devoted during the past years to achieve nanostructure lasers with broad modulation bandwidth, low frequency chirp, and reduced linewidth enhancement factor. Particularly, 1.55-μm InP-based quantum dash (Qdash)/dot (Qdot) lasers are preferable for long-haul transmissions in contrast to the 1.3-μm laser sources. In this dissertation, we investigate the dynamic characteristics of InPbased nanostructure semiconductor lasers operating under direct current modulation, including the amplitude (AM) and frequency (FM) modulation responses, the linewidth enhancement factor (also known as α-factor), as well as large-signal modulation responses. Using a semi-analytical analysis of the rate equation model, it is found that the modulation bandwidth of the quantum dot laser is strongly limited by the finite carrier capture and relaxation rates. In order to study the α- factor and chirp properties of the quantum dot laser, we develop an improved rate equation model, which takes into account the contribution of carrier populations in off-resonant states to the refractive index change. It is demonstrated that the α-factor of quantum dot lasers is strongly dependent on the pump current as well as the modulation frequency, in comparison to the case of Qwell lasers. The α-factor remains constant at low modulation frequencies (<0.1 GHz) and higher than the value derived at high modulation frequencies (beyond several GHz) from the FM/AM technique. These unique features are mostly attributed to the carrier populations in off-resonant states. Further simulations show that the α-factor can be reduced by enlarging the energy separation between the resonant ground state (GS) and off-resonant states. Lasing from the excited state (ES) can be a promising alternative to enhance the laser’s dynamic performance. The laser exhibits a broader modulation response and the α-factor can be reduced by as much as 40%. The optical injection technique is attractive to improve the laser’s dynamical performance, including bandwidth enhancement and chirp reduction. These are demonstrated both theoretically and experimentally. The phase-amplitude coupling property is altered as well in comparison with the free-running laser and the optical gain depends on the injection strength and the frequency detuning. This work proposes a new method derived from the Hakki-Paoli method, enabling to measure the α-factor of semiconductor lasers under optical injection both below and above threshold. In addition, it is demonstrated theoretically that the α-factor in nanostructure lasers exhibits a threshold discontinuity, which is mainly attributed to the unclamped carrier populations in the off-resonant states. It is a fundamental limitation, preventing the reduction of the α-factor towards zero. Quantum cascade (QC) lasers rely on intersubband electronic transitions in multi-quantum well heterostructures. QC lasers show flat broadband AM response (tens of GHz) without resonance, which constitutes promising features for free-space communications. Surprisingly, calculations show that the QC laser exhibits an ultrabroad FM bandwidth on the order of tens of THz, about three orders of magnitude larger than the AM bandwidth. Optically injection-locked QC lasers also exhibit specific characteristics by comparison to interband semiconductor lasers. Both positive and negative frequency detunings enhance the modulation bandwidth.

Du fait de leur facilité de fabrication et de leurs propriétés physiques uniques, les nanofils (NFs) de semi-conducteurs présentent des potentialités d’application importantes elles pouvaient être comme briques élémentaires de nombreux dispositifs nano- et opto-électroniques. Différents procédés de fabrication ont été développés pour fabriquer et organiser ces nanofils en épitaxie sur silicium. Cependant, un des principaux problèmes réside dans le manque de reproductibilité des NFs produits naturellement. Pour obtenir un meilleur contrôle de leur périodicité, localisation, forme et taille, différents types de gravure ont été mis au point. Aujourd’hui, des incertitudes importantes persistent quant à leurs propriétés fondamentales, en raison d’un manque de corrélation entre les propriétés électroniques et optiques et les détails microscopiques (composition, structure, chimie ...etc.). L’objectif de ce travail est de développer deux types de procédés de fabrication : le premier "top-down" est basé sur la nanogravure directe par faisceau d’ions focalisés (FIB)de couches bi-dimonsionnelles de SiGe. Ce procédé permet de contrôler la taille des NFs, les déformations, et leur localisation précise. Il permet de fabriquer des réseaux de larges piliers. Les NFs réalisés par cette technique sont peu denses et de diamètre important. Le second procédé est de type "Bottom-Up" ; il s’appuie sur la croissance VLS à partir de catalyseurs métalliques (AuSi). Les NFs réalisés ont étudiés à l’échelle locale afin de mesurer la taille moyenne de contrainte ainsi que leur effet sur le confinement quantique et sur la structure de bande des NFs. / SiGe/Si core/shell nanowires (NWs) and nanodots (NDs) are promising candidates for the future generation of optoelectronic devices. It was demonstrated that the SiGe/Si heterostructure composition, interface geometry, size and aspect ratios can be used to tune the electronic properties of the nanowires. Compared to pure Si or Ge nanowires, the core-shell structures and exhibit extended number of potential configurations to modulate the band gap by the intrinsic strain. Moreover, the epitaxial strain and the band-offsets produce a better conductance and higher mobility of charge carriers. Recent calculations reported that by varying the core-shell aspect ratio could induce an indirect to direct band gap transition. One of the best configurations giving direct allowed transitions consists of a thin Si core embedded within wide Ge shell. The Germanium condensation technique is able to provide high Ge content (> 50%) shell with Si core whom thickness of core and shell can be accurately tuned. The aim of this work is to develop two types of synthesis processes: the first "top-down" will be based on direct nanoetching by focused ion beam (FIB) of 2D SiGe layer. This process allows the control of the size of NWs, and their precise location. The NWs achieved by this technique are not very dense and have a large diameter. The second processes called "bottom-up"; are based on the VLS growth of NWs from metal catalysts (AuSi). Grown NWs have been studied locally in order to measure the mean size and the strain and their effects on the quantum confinement and band structure of NWs.

Recently, efforts have been made for self-powering the batteries and portable electronic devices by piezoelectric nanogenerators. The piezoelectric nanogenerators can work as a power source for nano-systems and also as an active sensor. The piezoelectric nanogenerator is a device that converts random mechanical energy into electrical energy by utilizing the semiconducting and piezoelectric properties. Also, the mechanical energy is always available in and around us for powering these nano devices. The aim of the present thesis work is to explore 1D and 2D ZnO nanostructures (nanorods and nanosheets) on metal alloy substrates for the development of piezoelectric nanogenerators in energy harvesting and sensors applications. Hydrothermal synthesis method was adopted for the growth of ZnO nanostructures. The nanogenerators were fabricated by using the optimized synthesis parameters and subsequently studied their performance for power generation and as an active speed sensor. These 1D and 2D nanostructures based nanogenerators have opened up a new window for the energy harvesting applications and sensors development. The thesis is divided into following six chapters. Chapter 1: This chapter gives a general introduction about energy harvesting devices such as nanogenerators, available energy sources, mechanical energy harvesting, ZnO material and the details on hydrothermal synthesis process. A brief literature survey on different applications of piezoelectric nanogenerators is also included. Chapter 2: A novel flexible metal alloy (Phynox) and its properties along with its applications are discussed in this chapter. Details on the synthesis of 1D ZnO nanorods on Phynox alloy substrate by hydrothermal method are presented. Further, the optimization of parameters such as growth temperature, seed layer annealing and substrate temperature effects on the synthesis of ZnO nanorods are discussed in detail. As-synthesized ZnO nanorods have been characterized using XRD, FE-SEM, TEM and XPS. Chapter 3: It reports on the fabrication of piezoelectric nanogenerator on Phynox alloy substrate as power generating device by harvesting the mechanical energy. Initially, the performance of the nanogenerator for power generation due to finger tip impacts was studied and subsequently its switching polarity test was also carried out. Output voltage measurements were carried out using the in-house developed experimental setup. Stability test was also carried out to see the robustness of the nanogenerator. Finally, the output voltage response of the nanogenerator was studied for its use as an active speed sensor. Chapter 4: Synthesis of Al doped 2D ZnO nanorsheets on Aluminum alloy (AA-6061) substrate by hydrothermal method is reported in this chapter. The optimized parameters such as growth temperature and growth time effects on the synthesis of ZnO nanosheets are discussed. As-synthesized ZnO nanosheets were characterized using XRD, FE-SEM, TEM and XPS. The Al doping in ZnO is confirmed by EDXS and XPS analysis. Chapter 5: Cost effective fabrication of Al doped 2D ZnO nanosheets based nanogenerator for direct current (DC) power generation is reported in this chapter. The performance of the nanogenerator for DC power generation due to finger tip impacts was studied and subsequently its switching polarity test was also carried out. Output voltage measurements were carried out using the in-house developed experimental setup. Stability test was also carried out to see the robustness of the nanogenerator. Finally, the DC output voltage response of the nanogenerator was studied for its use as an active speed sensor. Chapter 6: The first section summarizes the significant features of the work presented in this thesis. In the second section the scope for carrying out the further work is given.