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 a semiconductor, negative charge is carried by conduction-band electrons and positive charge is carried by valence-band holes. While charge transport properties can be understood by considering the motion of these carriers individually, the optical properties are largely determined by their mutual interaction. The hydrogen-like bound state of an electron with a hole, or exciton, is the fundamental optical excitation in direct-gap materials such as gallium arsenide and cadmium telluride. In this dissertation, we consider charged excitons, or trions. A bound state of an exciton with a resident electron or hole, trions are a relatively pure manifestation of the three-body problem which can be studied experimentally. This is a subject of practical as well as academic interest: Since the trion is the elementary optical excitation of a resident free carrier, the related optical processes can open pathways for manipulating carrier spin and carrier transport. We present three experimental investigations of trion-based optical processes in semiconductor quantum wells. In the first, we demonstrate electromagnetically induced transparency via the electron spin coherence made possible by the trion transition. We explore the practical limits of this technique in high magnetic fields. In the second, we present a direct measurement of trion and exciton oscillator strength at high magnetic fields. These data reveal insights about the structure of the trion's three-body wavefunction relative to that of its next excited state, the triplet trion. In the last, we investigate the mechanism underlying exciton-correlated tunneling, an optically-controllable transport process in mixed-type quantum wells. Extensive experimental studies indicate that it is due to a local, indirect interaction between an exciton and a hole, forming one more example of a trion-mediated optical process. This dissertation includes previously published co-authored material.

Submitted by Raquel Riciati do Couto Vilela null (r.vilela@ymail.com) on 2017-05-10T19:43:47Z No. of bitstreams: 1 Dissertação final - Raquel Vilela.pdf: 4751160 bytes, checksum: fa0956e5e393b6c5ddea05d45779ce6b (MD5) / Approved for entry into archive by Luiz Galeffi (luizgaleffi@gmail.com) on 2017-05-10T19:46:51Z (GMT) No. of bitstreams: 1 vilela_rrc_me_rcla.pdf: 4751160 bytes, checksum: fa0956e5e393b6c5ddea05d45779ce6b (MD5) / Made available in DSpace on 2017-05-10T19:46:51Z (GMT). No. of bitstreams: 1 vilela_rrc_me_rcla.pdf: 4751160 bytes, checksum: fa0956e5e393b6c5ddea05d45779ce6b (MD5) Previous issue date: 2017-03-15 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A incorporação de nanopartı́culas em matrizes hı́bridas produz materiais com grande potencial para aplicações em diversas áreas de estudo. Na óptica, sua funcionalidade surge quando elétrons confinados em estruturas tridimensionais nanométricas são excitados. Em princı́pio, esses materiais exibem nı́vel de energia discreto, apresentando picos estreitos no espectro de absorção. Tal comportamento torna-os atrativos para óptica não-linear e aplicações eletro-ópticas. Neste trabalho, o processo sol-gel foi utilizado para a sı́ntese de nanocompósitos contendo CuCl ou CuBr. Cu2O, HBr e HCl foram utilizados como precursores para os CuBr e CuCl, e a matriz Orgânica/Sı́lica foi preparada a partir dos alcóxidos 3-glicidoxipropiltrimetoxisilano (GPTS) e Tetraetilortosilicato (TEOS). As amostras foram preparadas na forma de filmes finos depositados sobre lâminas de vidro, utilizando a técnica Dip-Coating. Após a secagem, as amostras foram tratadas num forno convencional e/ou expostas à radiação UV utilizando uma luz negra comercial. A espectroscopia de absorção UV-VIS permitiu identificar estreitas bandas de absorção, bem como sua variação. O espectro de absorção (UV-VIS) registrou à temperatura ambiente picos em 418 e 399 nm para as nanopartı́culas de CuBr e a 376 e 380 nm para as nanopartı́culas CuCl, correspondente aos excitons Z1,2 e Z3 respectivamente. Comportamento semelhante foi relatado na literatura. / The incorporation of nanoparticles in hybrid matrices has produced materials with great potential for applications in many fields of study. In optics, its functionality arises when electrons confined in nanometric three-dimensional structures are excited. At first, these materials exhibit discrete energy level, with sharp peaks in the absorption spectrum. Such behavior makes them attractive to non-linear optical and electro-optical applications. In this work, the sol-gel process was used for the synthesis of nanocomposite containing CuCl or CuBr. These complexes decompose during thermal or ultraviolet treatment, and form copper halide nanoparticles. Cu2O, HCl and HBr were used as precursors for the CuBr and CuCl, and the organic matrix/silica was prepared from alkoxides of 3-glycidoxypropyltrimethoxysilane (GPTS) and tetraethylorthosilicate (TEOS). The samples were made in the form of thin films deposited on glass slides using the dip-coating technique. After drying, they were treated in a conventional oven and/or exposed to UV radiation using a commercial black light. The UV-VIS absorption spectrum allowed to identify narrow absorption bands, such as its variation. The absorption spectrum (UV-Vis) recorded at room temperature showed peaks at 418 and 399 nm CuBr nanoparticles and 376 and 380 to CuCl nanoparticles corresponding to the excitons Z1,2 and Z3 respectively. Similar behavior has been reported in the literature.

Orientador: Jose Antonio Brum / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica "Gleb Wataghin" / Made available in DSpace on 2018-07-20T22:01:39Z (GMT). No. of bitstreams: 1 Resende_XimenesRocha_M.pdf: 2188818 bytes, checksum: 081e5c734241d1fd7bfb5f31ef4960fd (MD5) Previous issue date: 1995 / Resumo: Neste trabalho, estendemos o formalismo de Massa Efetiva e Função Envelope aplicado ao cálculo de heteroestruturas para incluir acoplamento entre vales de pontos distintos da zona de Brillouin. Em particular , estudamos a simetria do acoplamento G -X e aplicamos o formalismo desenvolvido ao problema de Excitons confinados em poços quânticos G -X acoplados. analisamos a transição sistema direto/sistema indireto mediada por um campo elétrico externo e calculamos, em função deste campo, alguns parâmetros que são importantes para a dinâmica excitônica neste materiais: massa efetiva perpendicular à interface, dispersão de estados, força de oscilador / Abstract: In this work we have extended the Effective Mass and Envelope Function approximation applied to heterostructures to address the problem of mixing among different valleys in the Brillouin zone. We have studied specifically the symmetry of G -X mixing and we applied the developed model to treat confined excitons in coupled G - X quantum wells. We analyzed the transition between direct and indirect system assisted by an external electric field and calculated some parameters, as a function of this field, that are important to excitonic dynamics: effective mass perpendicular to interfaces, dispersion of states, oscillator strength / Mestrado / Física / Mestre em Física

Orientador: Eliermes Arraes Meneses / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-07-21T11:07:19Z (GMT). No. of bitstreams: 1 Oliveira_JoseBrasBarretode_D.pdf: 2128248 bytes, checksum: 1d576942bf4ef72cea20fa8f5ee0ccee (MD5) Previous issue date: 1996 / Resumo: Neste trabalho estudamos as propriedades ópticas de excitons e de aceitadores de Be, confinados num sistema de múltiplos poços quânticos de GaAS/Ga0.7AlO.3As, usando as técnicas de fotoluminescência e fotoluminescência de excitação. Observamos a emissão relacionada aos excitons ligados os quais acreditamos estar ligados a defeitos ionizados localizados na interface. A dimensão lateral média das microrugosidades das interfaces foi determinada usando medidas de magneto-fotoluminescência e a aplicabilidade do modelo de Singh-Bajaj foi testada. Analisamos o efeito de ilhas planas e extensas, presentes na interface, sobre as energias de ligação e as energias dos estados excitados dos aceitadores de Be confinados. Calculamos, ainda, a forma de linha da fotoluminescência, associada à transição "free-to-bound", comparando com nossos resultados experimentais / Abstract: In this work we have studied the optical properties of excitons and Be acceptors, confined in a GaAS/Ga0.7AlO.3As multiple quantum well system, using photoluminescence and photoluminescence excitation techniques. We observed na emission related to the bound excitons which we believe are bound at the ionized interface defects. The average lateral dimensions of interface microroughness was determined from magneto-photoluminescence measurements and the applicability of the Singh-Bajaj's model was tested. We have performed an analysis of the effect of flat islands, localized at interfaces, on the binding energies and excited states energies of the confirmed Be acceptors. Also, we have calculated the photoluminescence line shape, associated to the free-to-bound transition, and confronted it with our I experimental results / Doutorado / Física / Doutor em Ciências

Orientador: Jose Antonio Brum / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-03T16:28:39Z (GMT). No. of bitstreams: 1 Caetano_RodrigoAndre_M.pdf: 2078146 bytes, checksum: 61839b17f9e944c2a44b2090bc19616c (MD5) Previous issue date: 2003 / Resumo: Neste trabalho, apresentamos um estudo das propriedades ópticas de Caixas Quânticas (QDs) semicondutoras. Iniciamente, um sistema de GaAs / In0.24Ga0.76As /Al0.3Ga0.7As, intensionalmente dopado com material do tipo-n foi considerado, onde ocorre a transferência de elétrons provenientes da dopagem para o poço quântico de In0.24Ga0.76As. Os QDs encontram-se na interface poço/barreira. Apresentamos uma aproximação simples para saber como que a presença QDs influenciam na transferência de cargas. Nossos resultados mostram que a presença dos QDs na amostra não alteraram significativamente a transferência de cargas e, basicamente, o que ocorre é uma redistribuição dos elétrons que deixam o poço quântico e passam ocupar os estados dos QDs. Quando excitados opticamente, o QD é ocupado por um par elétron buraco. Em materiais do tipo-II, o elétron e o buraco estão em materiais diferentes mas podem formar um par de particular ligadas, via interação Coulombiana. Estudamos as excitações ópticas em QDs de InP/GaAs, que apresenta alinhamento de banda do tipo-II, no qual o elétron está confinado na região do InP enquanto o buraco se encontra livre na região do GaAs. Outro problema abordado foi o problema do éxciton carregado negativamente (X-). O elétron extra, que pode ser proveniente da dopagem ou de uma excitação óptica acima da barreira, fica confinado na região do InP alterando o espectro óptico da amostra. Nossos resultados mostram que o buraco é ligado em ambos os casos. A recombinação entre complexos do tipo X- emite fótons mais energéticos do que o fóton emitido na recombinação do éxciton neutro, contribuindo no deslocamento para o azul do espectro de fotoluminescência quando a potência laser é aumentada / Abstract: In this work, we present a study of the optical properties of semiconductor Quantum Dots (QDs). First, we consider a system of GaAs/In0.24Ga0.76As/Al0.3Ga0.7As intentionally doped with type-n material, where the electrons are transfer from the doping layer to the In0.24Ga0.76As quantum well. The QDs are in the well/barreir interface. We present a simple approuch to know how the presence of the QDs influences the charge transfer. Our results show that the presence of the QDs in the sample does not modify significantly the charge transfer and, basically, what happens is a redistribution of the electrons from the quantumwell to the QDs states. When optically excited, the QD is occupied by a pair electron-hole. In type- II material, electron and hole are in different layer but they can form a pair of bound particles, due to Coumlombian interaction. We studied the optical excitations in InP/GaAs QD that presents type-II band alignment. The electron is confined in InP region while the hole are free in GaAs region. Other problem investigated concerns the negative charged exciton (X-). The extra electron can be from the doping or from an optical excitation above the barrier, being confined in the InP region modifying the optical spectrum of the sample. Our results show that the hole is bound in both cases. The recombination of X- complex emits photons with higher energy than the neutral exciton recombination, contributing to the blue shift of the photoluminescence spectrum with the increasing laser power / Mestrado / Física / Mestre em Física

Please read the abstract in the front of the document. / Dissertation (MSc)--University of Pretoria, 2009. / Physics / unrestricted

Energias de ligação para excitons ligados por impurezas doadoras no ZnSe e CdS são calculadas pelo Método Adiabático Hiperesférico. Os acoplamentos não adiabáticos são incluídos na equação radial levando a valores de energias menores que os valores variacionais encontrados na literatura. Estados ressonantes, similares a estados autoionizantes em átomos de dois elétrons, são obtidos acima do primeiro limiar de ionização elétron-impureza. / Binding energy for excitons trapped by impurities in ZnSe and CdS are calculated withing the hyperspherical adiabatic approach. The non adiabatic couplings are included in the radial equations leading to energies lower than the variational values available in the literature. Resonant states similar to autoionizing lines in atoms are predicted to lie above the first electron-impurity ionization threshold.

Le travail présenté porte sur le contrôle de la couleur d'émission d'une diode électroluminescente organique (OLED), élément de base pour des écrans d'affichage ou des nouveaux écrans de télévision ou encore des futures sources de lumière, par l'étude de la diffusion des excitons. Après optimisation de l'épaisseur de la couche d'émetteurs fluorescents jaunes incorporés dans une matrice émettrice bleue, la variation de leur position depuis la zone de recombinaison a permis d'ajuster la couleur d'émission de l'OLED, variant du bleu au jaune avec un passage par un blanc pur de coordonnées chromatiques (0.32; 0.33). Ce contrôle très fin de la couleur émise est obtenu en tirant parti de la diffusion des excitons dans la structure OLED ; cette diffusion des excitons singulets et triplets est étudiée en détails, et les longueurs de diffusions de ces quasi-particules ont été déterminées à partir des mesures d'électroluminescence dans deux matériaux connus, le NPB et le DPVBi. Dans le cas des triplets, il faut faire appel à une technique utilisant une fine couche dopée d'un émetteur phosphorescent dans une matrice épaisse de CBP. Une attention particulière a été accordée aux phénomènes de microcavité, prépondérants dans ce type de diodes, pour une meilleure compréhension des phénomènes mis en jeux dans ces dispositifs.

The spectroscopy of wide-bandgap fluoride materials doped with divalent ytterbium is presented. The structure of impurity-trapped excitons is explored, vacuum ultraviolet excitation is used to investigate the transfer processes between excitations, and the effect of confinement on self-trapped excitons is studied. The excited-state structure of impurity-trapped excitons is measured in the multisite system NaMgF₃:Yb²⁺. A two-colour ultraviolet-infrared pulsed photoluminescence enhancement technique is employed to probe the interlevel transitions and dynamics of impurity-trapped excitons in doped insulating phosphor materials. NaMgF₃:Yb²⁺ exhibits emission from two charge-compensation centres with peaks at 22 300 cm⁻¹ (448 nm) and 24 000 cm⁻¹ (417 nm). The observed photoluminescence enhancement is caused by a combination of intra-excitonic excitation and electron trap liberation. The electron traps are inferred to have a depth of approximately 800 cm⁻¹. Time-resolved VUV spectroscopic studies of emission and excitation spectra of CaF₂:Yb, NaMgF₃Yb and MgF₂:Yb are presented to investigate excitation and relaxation mechanisms of both impurity-trapped excitons and intrinsic excitons in each fluoride host. Host-to-impurity energy transfer mechanisms leading to formation of impurity-trapped excitons are discussed. The 4f¹⁴ → 4f¹³5d CaF₂:Yb²⁺ absorption bands are successfully modeled with a semi-empirical effective Hamiltonian calculation for NaMgF₃:Yb²⁺ and MgF₂:Yb²⁺. The excitation and emission spectra of all studied materials are compared. Results on VUV spectroscopy of 3 and 5 monolayer CdF₂–CaF₂ superlattices show the change in optical behaviour of the self-trapped exciton in CdF₂ when it is confined and give an indication of the radius of the exciton. The decay of the emission is modeled with three components, corresponding to three self-trapped exciton states. Results on the VUV spectroscopy of CdF₂–CaF₂ superlattices show that the confinement effect seems to equally influence the energy of excitonic and bandgap absorption in 3 and 5 monolayer superlattices. At the same time, as the self-trapped exciton is more confined, the emission is blue-shifted by 1600 cm⁻¹ indicating that the effective excitonic radius is about three monolayers.