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.

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.

Semiconductor nanocrystals are interesting candidates as new light-absorbing materials for photovoltaic (PV) devices. They can be dispersed in solvents and cheaply deposited at low-temperature on various substrates. Also, the nanocrystals have unique optical properties depending on their size due to the quantum size effect and moreover it is easy to uniformly control their stoichiometry. CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals were selected to synthesize and investigate in order to utilize the benefits of colloidal nanocrystals described above. Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS). The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model. The synthetic method of monodisperse chalcopyrite (tetragonal) CuInSe₂ nanocrystals was also developed. The nanocrystals have trigonal pyramidal shape with one polar and three non-polar surface facets. When drop-cast onto carbon substrates, the nanocrystals self-assemble into close-packed monolayers with triangular (honeycomb) lattice structure. Moreover, the effect of excess Cu precursor (CuCl) was studied for the formation of monodisperse trigonal pyramidal CuInSe₂ nanocrystals. The formation mechanism of monodisperse trigonal pyramidal CuInSe₂ nanocrystals was suggested with regard to excess amount of CuCl precursor, based on the nucleationgrowth model of colloidal nanocrystal formation. A new wurtzite phase of CuInS₂, CuInSe₂, and Cu(InxGa1-x)Se₂ (CIGS) was observed in nanocrystals synthesized by heating metal precursors and Se-(or S-)urea in alkylamine. X-ray diffraction (XRD) showed the predominant phase to be wurtzite (hexagonal) instead of chalcopyrite (tetragonal). High resolution transmission electron microscopy (TEM), however, revealed polytypism in the nanocrystals, with the wurtzite phase interfaced with significant chalcopyrite domains. / text