Core-shell nanohybrids have wide applications in pollutant degradation. In this study, core-shell nanohybrid was formed through heteroaggregation between neutral nanoparticles (i.e., hematite nanoparticles or HemNPs) and charged nanoparticles (i.e., carboxylated polystyrene nanoparticles or PSNPs). In the dispersant solution of 1 mM NaCl at pH 6.3, HemNPs were neutral and underwent favorable homoaggregation, whereas PSNPs were negatively charged and underwent no homoaggregation. When the two types of particles were mixed, homoaggregation of HemNPs and heteroaggregation between HemNPs and PSNPs took place simultaneously, forming HemNPs-PSNPs heteroaggregates. The transmission electron microscopy images of heteroaggregates show that HemNPs and PSNPs formed core-shell structure in which HemNPs were the cores and PSNPs were the shells. The size of the core-shell nanohybrids can be controlled by varying the concentration ratio of HemNPs to PSNPs. The increase of the size of charged nanoparticles resulted in larger nanohybrids. This new method has lower energy footprint than existing ones. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

The diverse field of material research has been steadily expanding with a great help from computational physics, especially in the investigation of the fundamental properties of materials. This has driven the computational physics to become one of the main branches of physics, allowing for density functional theory (DFT) to develop as one of the cornerstones of material research. Nowdays, DFT is the method of choice in a great variety of studies, from fundamental properties, to materials modelling and searching for new materials. In this thesis, DFT is employed for the study of a small part of this vast pool of applications. Specifically, the microscopic characteristics of Zn1-xCdxS alloys are studied by looking into the evolution of the local structure. In addition, the way to model the growth of graphene on Fe(110) surface is discussed. The structural stability of silicon nanocrystals with various shapes is analysed in detail, as well. DFT is further used in studying different properties of semiconductor nanocrystals. The size evolution of the character of the band gap in silicon nanocrystals is investigated in terms of changes in the character of the states around the band gap. The influence of various surface impurities on the band gap, as well as on the electronic and optical properties of silicon nanocrystals is further studied. In addition, the future use of silicon nanocrystals in photovoltaic devices is examined by studying the band alignment and the charge densities of silicon nanocrystals embedded in a silicon carbide matrix. Furthermore, the electronic and optical properties of different semiconductor nanocrystals is also investigated. In the case of the CdSe/CdS and CdS/ZnS core-shell nanocrystals the influence of the nanocrystal size and different structural models on their properties is analysed. For silicon nanocrystal capped with organic ligands, the changes in the optical properties and lifetimes is thoroughly examined with changes in the type of organic ligand.

L'objectif de cette thèse est de réaliser l'intégration monolithique de nanofils (NFs) à base de l’alliage Al0.2Ga0.8As sur des substrats de Si par épitaxie par jets moléculaires via la méthode vapeur-liquide-solide (VLS) auto-assistée et de développer une cellule solaire tandem (TSC) à base de ces NFs.Pour atteindre cet objectif, nous avons tout d'abord étudié la croissance de NFs GaAs, étape clé pour le développement des NFs p-GaAs/p.i.n-Al0.2Ga 0.8As coeur/coquille, qui devraient constituer la cellule supérieure de la TSC. Nous avons montré, en particulier, l'influence de l'angle d'incidence du flux de Ga sur la cinétique de croissance des NFs GaAs. Un modèle théorique et des simulations numériques ont été réalisées pour expliquer ces résultats expérimentaux.Nous avons ensuite utilisé le savoir-faire acquis pour faire croître des NFs p-GaAs/p.i.n-Al0,2Ga0,8As coeur/coquille sur des substrats de Si prêts pour l'emploi. Les caractérisations EBIC réalisées sur ces NFs ont montré qu'ils sont des candidats potentiels pour la réalisation d’une cellule photovoltaïque. Nous avons ensuite fait croître ces NFs sur des substrats de Si patternés afin d'obtenir des réseaux réguliers de ces NFs. Nous avons développé un protocole, basé sur un pré-traitement thermique, qui permet d'obtenir des rendements élevés de NFs verticaux (80-90 %) sur une surface patternée de 0,9 x 0,9 mm2.Enfin, nous avons consacré une partie de notre travail à définir le procédé de fabrication optimal pour la TSC, en concentrant notre attention sur le développement de la jonction tunnel de la TSC, l'encapsulation des NFs et le contact électrique supérieur du réseau de NFs. / The objective of this thesis is to achieve monolithical integration of Al0.2Ga0.8As-based nanowires (NWs) on Si substrates by molecular beam epitaxy via the self-assisted vapour-liquid-solid (VLS) method and develop a NWs-based tandem solar cell (TSC).In order to fulfil this purpose, we firstly focused our attention on the growth of GaAs NWs this being a key-step for the development of p-GaAs/p.i.n-Al0.2Ga0.8As core/shell NWs, which are expected to constitute the top cell of the TSC. We have shown, in particular, the influence of the incidence angle of the Ga flux on the GaAs NW growth kinetic. A theoretical model and numerical simulations were performed to explain these experimental results.Subsequently, we employed the skills acquired to grow p-GaAs/p.i.n-Al0.2Ga0.8As core/shell NWs on epi-ready Si substrates. EBIC characterizations performed on these NWs have shown that they are potential building blocks for a photovoltaic cell. We then committed to growing them on patterned Si substrates so as to obtain regular arrays of NWs. We have developed a protocol, based on a thermal pre-treatment, which allows obtaining high vertical yields of such NWs (80-90 %) on patterned Si substrates (on a surface of 0.9 x 0.9 mm2).Finally, we dedicated part of our work to define the optimal fabrication process for the TSC, focusing our attention to the development of the TSC tunnel junction, the NW encapsulation and the top contacting of the NWs.