Submitted by Irene Nascimento (irene.kessia@ufpe.br) on 2016-06-28T16:52:16Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação Denilson Digital.pdf: 4230195 bytes, checksum: 254e94141d3bf5cb07e451a017faacd5 (MD5) / Made available in DSpace on 2016-06-28T16:52:16Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação Denilson Digital.pdf: 4230195 bytes, checksum: 254e94141d3bf5cb07e451a017faacd5 (MD5) Previous issue date: 2015-04-10 / FACEPE / A necessidade de metodologias sintéticas verdes vem aumentando significativamente nos últimos anos. O uso de materiais tóxicos e de metodologias caras vem sendo evitadas, sendo necessário o desenvolvimento de novas metodologias de síntese para Quantum Dots (QDs). Portanto, com o objetivo de baratear e diminuir a toxicidade dos QDs formados, devido à presença de agentes redutores presentes nas metodologias de síntese tradicionais, nesse trabalho foi desenvolvida uma metodologia de síntese eletroquímica para síntese de QDs de CdTe e CdSe estabilizados pelo ácido tioglicólico (TGA). A metodologia de síntese eletroquímica desenvolvida consiste na redução eletroquímica de alguns calcogênios (telúrio e selênio) em solução aquosa de NaOH 0,2 mol L-1 (pH = 13) e atmosfera inerte, para posterior reação com o precursor metálico,CdCl2/TGA. Os QDs obtidos a partir dessa metodologia apresentaram boa estabilidade e alta luminescência, com tamanhos médios de 3,9 nm para o CdTe e de 2,6 nm para o CdSe. Também foi avaliada a fluorescência dos QDs na presença do íon Hg2+, sendo observado que estas nanopartículas podem ser considerados promissores sensores químicos para determinação analítica de íons metálicos. / The demand for green synthetic methodologies has increased significantly in recent years. The use of toxic materials and expensive methodologies has been avoided, requiring the development of new methods of synthesis for Quantum Dots (QDs). Therefore, in order to cheapen and reduce the toxicity of QDs sintetizados, due to the need of reducing agents in the traditional synthetic methods, in this work we developed an electrochemical method for the synthesis of CdSe and CdTe QDs, stabilized by thioglycolic acid (TGA ). The electrochemical methodology developed involves the electrochemical reduction of some chalcogenes (tellurium and selenium) in aqueous solution (pH = 13) and inert atmosphere, for subsequent reaction with the metal precursor, CdCl2/TGA. The QDs obtained from this methodology showed good stability and high luminescence, with average sizes 3.9 nm for CdTe and 2.6 nm for CdSe. The fluorescence of the QDs in the presence of Hg2+ ion was also evaluated. It was observed that the electrochemically synthesized nanoparticles can be considered promising chemical sensors for analytical determination of metal ions.

Research in nanoscience has gained noteworthy interest over the past three decades. As novel chemical and physical properties that are vastly different from extended solids are realized in nanosized materials, nanotechnology has become the center of attention for material in research community. Much to our amazement, investigations in the past two decades revealed that the nanocrystalline semiconductors are “THE PRIME CANDIDATES” to meet the growing energy demand, sensor development, cellular imaging and a number of other optoelectronic applications. Nonetheless, synthesis of nanostructures with control over physical parameters is not sufficient, yet assembling them into functional nanoarchitectures with unique and tunable physical properties is critical for device integration studies. Among bottom-up assembling methods, sol-gel method has received noteworthy interest to produce macroscopic nanostructures of metal and semiconductor NPs with no use of intervening ligands or supports. In 2005, condensation of pre-formed semiconductor NPs (CdSe, CdS, ZnS and PbS) into voluminous gels is reported via controlled destabilization of the surfactant ligands. The resultant chalcogenide aerogels are reported to exhibit extremely low density, high surface area and porosity, and quantum confined optical properties of the NP building blocks. More recently, this method has been extended for the assembly of metal NPs, where transparent and opaque nanostructures (aerogels) of Ag and Au/Ag NPs were produced. The aerogels produced by condensation of NPs are low dimensional (fractal) nanostructures and exhibit a physically connected network of colloidal NPs. Interactions between NPs in a gel structure are intermediate of those of the ligand stabilized NPs and core/shell hetero-nanostructures (e.g. Au@CdSe NPs) with the potential to couple chemically dissimilar systems. In this research study, NP condensation strategy has been utilized to efficiently integrate the plasmonic and excitonic properties of metal and semiconductor nanostructures to produce high-efficiency hybrids that exhibit unique tunable physical and photophysical properties. Two hybrid systems composed of spherical CdSe/Ag hollow NPs and rod shaped CdSe/Ag hollow NPs were investigated for the fabrication of metal-semiconductor hybrid aerogels. The first excitonic energy of spherical CdSe NPs is overlapped with the plasmonic energy of Ag hollow NPs at 515 - 530 nm. The second excitonic energy of rod shaped CdSe is overlapped with the plasmonic energy of Ag hollow NPs at 490 - 505 nm. The photophysical properties of both systems were thoroughly probed through UV-Visible absorption, photoluminescence (PL), and time-resolved (TR) PL spectroscopy. A novel hybrid emission emerged at 640 nm (for spherical CdSe/Ag hollow NPs) and 720 nm (for rod shaped CdSe/Ag hollow NPs) with ~0.2-1% Ag loading. TRPL studies revealed 685 ns and 689 ns PL decay times for hybrid emissions, which are vastly different from the band-edge and trap state emission of phase pure spherical and rod shaped CdSe aerogels respectively, supporting the generation of novel radiative decay pathways. Overall, synthesis of CdSe/Ag hybrid aerogels with novel/tunable photophysical properties will add to the toolbox of semiconductor aerogels with the potential application in future light harvesting technologies.

Quantum confinement structures are attractive for their unconventional size dependence of the optical and electrical properties. There are still challenges to control the size uniformity for the application. The thesis studies the correlation between the size distribution of CdSe/ZnS quantum dots (QD), and the fluorescence properties to understand the shape and size influences of their fluorescence properties. Results from the transmission electron microscopy (TEM) provide the structure and size distribution of the samples. Excitation dependent fluorescence spectra as well as the PL excitation at various emission wavelength confirm that the inhomogeneous distribution of the samples. The results show that the samples are mostly composed of QDs with quasi-spherical structure (aspect ration between 1.1 and 1.5 ;76%) and spherical structure ( aspect ratio < 1.1; 12.8%). In addition, it exhibits a distribution of the long axis of 5.4nm¡Ó1.3nm. By measuring the fluorescence spectra of individual QDs, we construct the distribution. The peaks of the fluorescence spectra show a Gaussian distribution with center at 615.7 nm and width 13.8 nm. In addition, the spectra exhibit a width of 19.7¡Ó8.0nm. This is consistent with the ensemble measurement of the fluorescence from a solution (peak at 616 nm, and width 25 nm). Results of the fluorescence lifetime on the individual QDs indicate the lifetime distribution of 10.3¡Ó5.6ns. Further analyze the size distribution by constructing the size ¡V fluorescence spectrum relationship. By analysis the distribution of the fluorescence spectra, it results the corresponding size distribution of width 0.7 nm. This is much narrower than the size distribution of the long axis measured by TEM, but is more consistent with the corrected size distribution considering the short axis contribution. We conclude that the deviation results from the non-spherical structures in the samples.

L’expression "Nanoparticules Janus" est utilisée pour se référer aux nanoparticules colloïdales faites de deux moitiés qui présent deux propriétés physiques et/ou chimiques différentes. Au cours des dernières années, plusieurs études théoriques ont été publiées sur les possibilités d’auto-assemblage offertes par ces particules (en particulier par Sciortino, F. et al.), mais peu de travail expérimental a été fait sur ce sujet. Les études théoriques suggèrent que beaucoup de comportements intéressants apparaissent quand la taille des particules s’approche de la portée d’interaction des forces en jeu (des dizaines de nanomètres dans le cas des forces de Van der Waals et des forces hydrophobes). Dans ce manuscrit, nous montrons la formation d’agrégats des hétérodimères de Au–SiO₂ d’une taille inférieure à 100nm. L’auto-assemblage a été déclenché par un échange du ligand hydrophile sur la surface de l’or par un ligand hydrophobe induisant une interaction attractive. L’assemblage a été suivi par spectroscopie d’absorption résolue dans le temps et diffusion des rayons X aux petits angles. Nous avons constaté que les thiols les plus courts ont une période d’induction plus longue et forcent les particules à se rapprocher davantage, comparé à des thiols avec des chaînes plus longues. Nous étudions également un second système : des nanoplaquettes de CdSe. Celles-ci sont des objets quasi-2D en matériau semiconducteur avec des propriétés optiques uniques. Ces propriétés résultant de leur taille réduite dans une dimension. Du fait de leur nouveauté et de leur particularité, leur nucléation et le remarquable mécanisme de croissance de ces particules sont toujours étudiés. Ainsi nous avons suivi leur synthèse par SAXS et WAXS in situ, afin d’obtenir des informations en ce qui concerne ces deux étapes, et notamment d’étudier la déformation de certains de ces systèmes sous forme de feuillets enroulés de CdSe. Les nanoplaquettes de CdSe ont été aussi utilisés pour créer des structures hybrides CdSe–Au. La combinaison de ces deux matériaux a déjà montré des effets uniques, comme une meilleure efficacité catalytique et, combiné avec la dimension réduite et le contrôle des plaquettes, pourrait aboutir à des caractéristiques encore plus intéressantes. Nous proposons une méthode de synthèse qui aboutit à la formation de petites sphères d’or sur les coins des plaquettes. Nous montrons que la taille des sphères dépend de la quantité de précurseur utilisée, et des images de microscopie électronique à haute résolution mettent en évidence la structure cristalline des deux matériaux. / "Janus nanoparticles" is the term used to refer to colloidal nanoparticles made of two halves with different physical and chemical properties. Over the last years, several theoretical studies have been published on the self-assembly possibilities offered by these particles (in particular by Sciortino, F. et al.), but little experimental work has been done on them. The theoretical studies suggest that many interesting behaviors appear when the size of the particles approaches the interaction range of the forces at play (tens of nanometers in the case of van der Waals and hydrophobic forces). In this manuscript, we show the formation of clusters of Au–SiO₂ heterodimers with sizes of less than 100nm. The self-assembly was induced by exchanging the hydrophilic ligand on the Au surface with a hydrophobic one, which provided the attractive interaction. The assembly was followed by time-resolved absorption spectroscopy and small-angle X-ray scattering. We found that shorter thiols have a longer induction period, and cause the particles to come closer together, compared to thiols with longer tails. We also study a second system: CdSe nanoplatelets. These are semiconducting quasi-2D structures with unique optical properties. These properties result from their reduced size in one of the dimensions. Because of their novelty and particularity, the nucleation and growth mechanism of these particles is still being studied. We followed the synthesis using in-situ SAXS and WAXS, to obtain information with regards to this mechanism and to study the deformation which occurs in some of these systems which leads to rolled up sheets of CdSe. The CdSe nanoplatelets were also used to create hybrid CdSe–Au structures. The combination of these two materials has already proven to produce unique effects such as enhanced catalysis and, combined with the reduced dimensionality and control of the platelets, could result in even more interesting characteristics. We propose a synthesis method which results in the formation of small gold spheres on the corners of the platelets. We show that the size of the spheres depends on the amount of precursor used, and show high resolution electron microscopy images which highlight the crystalline structure of both materials.

Le présent travail a pour but l'élaboration ainsi que la caractérisation structurale et optique de matériaux composites à base de nanocristaux semi-conducteurs à grand gap dispersés dans des matrices optiquement transparentes dans le domaine UV Visible. La première partie du travail expose la problématique du sujet à étudier et met en évidence l'actualité du travail envisagé. Elle donne un aperçu sur les propriétés optiques des nanocomposites à base de semi conducteurs et fait apparaitre les raisons justifiant le choix des matériaux adoptés pour la fabrication des nanocomposites étudiés : CdSe/KBr, CdSe/KCl, CdSe/ PMMA et ZnSe/PMMA. Dans la deuxième partie, sont développées les méthodes d'élaboration des matériaux nanocomposites élaborés (méthode de Czochralski pour les monocristaux CdSe/KBr, CdSe/KCl et méthode du spin-coating pour les couches minces CdSe/PMMA, ZnSe/PMMA). Egalement sont décrites les différentes techniques utilisées pour leur caractérisation structurale et optique. La troisième partie comporte la discussion des résultats de la caractérisation structurale par la diffraction X, la spectroscopie IR, la spectroscopie Raman, la spectroscopie de masse (AFM) et la microscopie optique (MEB). Toutes ces techniques complémentaires confirment l'incorporation des cristallites de CdSe et de ZnSe dans les matrices hôtes KBr, KCl et PMMA. La quatrième partie présente les résultats de la caractérisation optique par l'absorption optique UV-Visible, la photoluminescence ainsi que la mesure de la réponse non linéaire du second et troisième des nanocomposites élaborés. Les résultats obtenus mettent en évidence l'effet d'un confinement quantique induit par la taille nanométrique des cristallites des semi-conducteurs CdSe et ZnSe et l'effet de surface dont l'action se manifeste par un shift des propriétés optiques linéaires et par une exaltation des propriétés optiques non linéaires.

Quantum dots and nanorods are becoming increasingly important structures due to their potential applications that range from photovoltaic devices to medicine. The majority of the research on carrier dynamics in these structures has been in the optical regime, with little work performed at Terahertz frequencies where excitonic dynamics can be more directly probed. In this work, we examine theoretically the interaction of Terahertz radiation with colloidal CdSe nanorods to determine the dynamics of excitons generated via a short optical pulse. We calculate the energies and wavefunctions for the excitons within the envelope function approximation in the low density limit where there is at most one exciton per nanorod. The linear Terahertz transmittance and absorbance is found for nanorods that are approximately 70 nm in length and 7 nm in diameter and are compared with experimental results that have shown the first observation of intra-excitonic transitions in nanorods. We find absorbance peaks at 8.5 THz and 11 THz that result from polarizations in the longitudinal (rod axis) and transverse directions respectively. Our theoretical results show that the 8.5 THz and 11 Thz peaks are due to 1s-2pz and 1s-2px transitions respectively. The theoretical absorbance spectra is in good agreement with the experimental one and show that only the ground state is significantly populated 1 ps after optical excitation. This provides strong evidence of rapid trapping of excited holes into the ligand used to passivate the nanorods. A full set of dynamical equations were then constructed from Heisenberg's equation of motion, and were used to model the excitonic correlations as a function of time. Transmittance and absorbance were calculated for different nanorod orientations and electric field strengths in both the linear and nonlinear regime. These results were then averaged over nanorod orientation in order to more accurately reflect experimental conditions. Nonlinearity was found to become significant at peak pulse field strengths of 7 kV/cm and greater. Due to two-photon processes, we predict the 2pz-3dz transition that is not observed in the linear regime will be clearly seen in the nonlinear absorbance spectrum. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2014-05-14 23:37:58.604

SOUSA, F. E. B. de. Excitons em ponto quântico tipo II de CdTe/CdSe. 2017. 50 f. Dissertação (Mestrado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017. / Submitted by Pós-Graduação em Física (posgrad@fisica.ufc.br) on 2017-09-14T17:04:47Z No. of bitstreams: 1 2017_dis_febsousa.pdf: 4567545 bytes, checksum: 8eb8315592845d3f57cf4a58edb4574e (MD5) / Approved for entry into archive by Giordana Silva (giordana.nascimento@gmail.com) on 2017-09-15T21:39:59Z (GMT) No. of bitstreams: 1 2017_dis_febsousa.pdf: 4567545 bytes, checksum: 8eb8315592845d3f57cf4a58edb4574e (MD5) / Made available in DSpace on 2017-09-15T21:39:59Z (GMT). No. of bitstreams: 1 2017_dis_febsousa.pdf: 4567545 bytes, checksum: 8eb8315592845d3f57cf4a58edb4574e (MD5) Previous issue date: 2017 / Quantum Dots (QD’s) have been source of extensive scientific research in the last years. Among many applications of this type of structure, one candidate has been highlighted as a strong substitute for the major source in the current global energy matrix, the Quantum Dot’s Solar Cells (QDSC). In this work the exciting properties of type II CdTe/CdSe QD’s with cylindrical symmetry are investigated. It was developed a computational model that is able to diagonalize a two-particle hamiltonian of the type H(~re; ~rh) = He +Hh +U(j~re ~rhj), where He;h representing the hamiltonian of individual particles and U(j~re ~rhj) the Coulomb interaction between electrons and holes. Differently from other methods, that make use of perturbation theory, to determine the binding energy of the fundamental exciton, the present method is able to calculate the excitonic spectrum in a wide range of energies, whose width depends solely on the available computational power. It has been possible to identify a dependence of this proprieties on the confinement dimensions. For the first ten confinement states, lower energy levels are obtained with the increase of the cylindrical well radius and the barrier width along the z-axis. An analysis of the symmetry of the spatial part the wave function was made as well. Our results indicate that we are on the right way to obtain another important data observed on a QDSC, as excitonic lifetime and optic absorption, may reveal to us something about electronic extraction and quantum yield of these materials. / Pontos Quânticos (PQ’s) tem sido fonte de extensivas pesquisas científicas nos últimos anos. Entre várias aplicações deste tipo de estrutura uma tem se destacado como forte candidata a substituição de fontes majoritárias na atual matriz energética mundial, a Célula Solar de Ponto Quântico (CSPQ). Neste trabalho são investigadas as propriedades excitônicas de PQ’s tipo II CdTe/CdSe com simetria cilíndrica utilizando-se um método capaz de diagonalizar um hamiltoniano de duas partículas do tipo H(~re; ~rh) =He + Hh + U(j~re ~rhj), onde He;h representa o hamiltoniano das partículas individuais e U(j~re ~rhj) a interação coulombiana entre elétrons e buracos. Diferentemente de outras metodologias que utilizam métodos perturbativos para calcular a energia de ligação do exciton fundamental, a metodologia aqui utilizada permite calcular o espectro de energia de excitons em uma faixa de energia que depende unicamente do poder computacional disponível. Foi possível identificar a dependência das propriedades excitônicas com as dimensões do confinamento. Os níveis de energia para os dez primeiros estados de confinamento diminuem com o aumento do raio do poço cilíndrico e com o aumento da largura da barreira em z. Características da simetria da parte espacial da função de onda também foram observadas. Nossos resultados indicam que estamos no caminho certo para a obtenção de outros dados importantes observados em uma CSPQ, como o tempo de vida do exciton e a absorção óptica, que pode nos revelar algo sobre a extração eletrônica e o rendimento quântico destes materiais.

In this thesis, cadmium selenide (CdSe) quantum dots (QDs) are synthesized and characterized for patterning applications as well as for photovoltaic devices. The QDs were patterned and embedded into various polymers to form fluorescent composites. Their photophysical properties were investigated in detail. Through template assisted deposition the QDs-polymer composites were patterned into fluorescent nanorods. CdSe QDs were combined with multi-wall carbon nanotubes (CNTs) using a synthesized organic perylene derivative dye (N,N'-di(ethanethiol)-perylene-3,4,9,10-tetracarboxyl diimide) (ETPTCDI) as a link between QDs and CNTs. Upon testing, the QDs-ETPTCDI-CNTs nanocomposite displayed photoactive properties. Photophysical quenching studies of QD-ETPTCDI-CNTs provided better understanding of the electron-hole transfer of each component in the nanocomposite. The nanocomposite material was patterned onto microelectrode devices for photocurrent measurements under an AM1.5 solar simulated light source. These nanocomposites can be used as photovoltaic devices. The preliminary characterization studies of the device show excellent photoresponse under AM1.5 solar simulated light. The band gap alignment of each component of the nanocomposite and the charge transfer kinetics are the key to efficient electron-hole transfer. Optimization of the semiconducting material's interface can potentially make these nanocomposites a system for photovoltaic-based devices.

This dissertation presents new insight into the ability of small molecule passivated NCs to achieve intimate approach distances, despite being well passivated, while developing guiding principles in the area of ligand mediated microstructure control and the resulting macroscopic optical and electronic properties that close packing of high quality NCs enables. NC ligand coverage will be characterized quantitatively through thermogravimetric analysis (TGA), and qualitatively by photoluminescence and electroluminescence, in the case of functional devices; illustrating the importance of practitioner dependent control of ligand coverage through variations in the dispersion precipitation purification procedure. A unique examination of the relative contribution of energy and charge transfer in NC LEDs will demonstrate the ability to achieve charge transfer, at a level competitive with energy transfer, to well passivated NCs at various wt% loading in a polymer matrix. The observation of potential dependent recombination zones within an active layer further suggest novel, NC surface passivation mediated control of blend microstructure during solution processing towards the development of a bi-continuous network. Next, NC self-assembly and resulting microstructure dependent optical and electronic properties will be examined through electroluminescence and high-resolution transmission electron microscopy (TEM) micrographs of functional NC/polymer bulk heterojunction LEDs. The joint characterization of NC optical properties, and self-assembly microstructure provide a deeper understanding of the significant and inseparable effects of minimal changes in NC surface passivation on structure and function, and emphasize the potential to rely on strongly passivating ligands to control physical properties and processing parameters concurrently towards higher efficiency devices via low cost processing. Finally, micro-contact printing of blazed transmission gratings, using stable dispersions of core and core/shell NCs will be shown to produce close packed assemblies of NCs forming near-wavelength luminescent superstructures separated in space. We show the dominant contribution of a two-monolayer thick sharp interface CdS shell to the diffraction efficiency, and necessarily the refractive index, of the NCs, independent of core size. Utilization of these gratings as in-coupling elements at various positions within a device architecture are also examined. These new observations were achieved by unprecedented control of NC architecture during dispersion processing, while maintaining high luminescence, made possible by optimized NC surface passivation. These studies enable the formation of new LED architectures, and new optoelectronic devices based on angle resolved, monochromatic fluorescence from diffraction gratings prepared from simple solution processing approaches. Further, the novel observation of angle amplified interfering fluorescence from these features is argued to be a result of long range radiative coupling and superradiance enabled by the monodispersity and high-quality NC surface passivation described herein.

Oligothiophenes are of increasing interest in organic based electronic devices in part due to their high electron and hole mobilities. In an organic photovoltaic (OPV) device, the electronic properties of oligothiophenes make them advantageous as charge transfer junctions. To serve as charge transfer junctions, oligothiophenes must be functionalized to bind to the donor and acceptor parts of the device. The donor and acceptor parts are different materials and the synthesis of asymmetric oligothiophenes is of great interest. Previous researchers in our lab synthesized four asymmetric oligothiophenes, two with two thiophene subunits and two with four. Each set of oligothiophenes contained a pair of constitutional isomers. Here we report the synthesis of another asymmetric oligothiophene, one with three thiophene subunits. This compound is functionalized with bipyridine to bind Ru(bpy)22+ and with phosphonic acid moieties to bind CdSe nanoparticles. The synthesis was carried out by bonding a phosphonic acid moiety to bithiophene and bipyridine to thiophene and then coupling the phosphate-bithiophene and thiophene-bipyridine. Standard Stille couplings were used for carbon-carbon bond formation. The resulting compounds have complex NMR spectra and overlapping Ru MLCT and π-π* transitions at 450 nm with molar extinction coefficient on the order of 3 x 105 M-1 cm-1. The thiophene fluorescence is quenched by Ru(bpy)22+. These optical properties compare closely with the previous compounds synthesized. Solar cells occupy significant attention in the media, politics and science for their promise of continual pollution-free energy. Quantum dots, metal complexes and organic compounds are all under research as viable replacements for expensive silicon solar cells. To test the efficacy of a light harvesting compound before constructing a solar cell, a model system is constructed to show electron transfer from the light harvester into an electron acceptor. We synthesized oligothiophenes and oligothiophene-ruthenium complexes and tested their ability to act as sensitizers and charge transfer junctions. To do this, they were bonded to CdSe nanoparticles and their optical properties were measured. Steady-state photoluminescence and time correlated single photon counting were used to observe the effects on fluorescence and fluorescence lifetime of the CdSe-oligothiophene and CdSe-oligothiophene-ruthenium complexes before and after binding. It was found that CdSe fluorescence was quenched when bound to an oligothiophene ruthenium complex, and that the fluorescence of the oligothiophene was quenched when bound to CdSe in the absence of ruthenium. The fluorescence lifetimes of the quenched species were shortened.