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A great deal of interest has been dawn on the colloidal chemistry based semiconductor nanocrysallites, also known as quantum dots (QDs). Because of the strong quantum confinement, quantum dots have unique size-dependent optical properties, which are much more superior to the conventional organic fluorescence materials. In addition, strong chemical resistant makes inorganic semiconductor QDs ideal candidate for next-generation of bio-labels and drug delivery vehicles. This report presents a user friendly approach to synthesize high quality biocompatible CdSe QDs in aqueous solution. Size of the dots can be controlled by adjusting the temperature, pH of the solution, and ratio of the precursors. A thin CdS layer was grown on CdSe QDs, forming a CdSe/CdS core/shell structure, to improve the photoluminescence. In order to use these QDs in-vivo, a more chemically robust coating, silica, was grown on the core/shell structure QD. The optical properties of the QDs were characterized by absorption and photoluminescence spectra. X-ray diffraction and transmission electron microscopy were conducted to verify the QDs composition and structure. / Ph. D.

Foram utilizadas proteínas da famÌlia das GFPs e nanocristais fluorescentes de CdSe/ZnS para caracterização da interação e verificação de transferência de energia por ressonância (FRET) entre estes compostos. Formou-se dois pares doador-receptor onde ora uma proteína figurava como doadora, ora um nanocristal ocupava este papel. Verificou-se que, em ambos os casos, o doador sofre supressão da fluorescência após a formação de complexo com o receptor, complexo este motivado por interação eletrostática e dependente de pH. Foi possível comprovar, através da observação de emissão sensitizada e redução da anisotropia, que entre o par formado por nanocristal com emissão no verde e proteína HcRed1 como receptora, de fato ocorre FRET. As distâncias aparentes entre doador e receptor foram determinadas a partir da eficiência da supressão da fluorescência do doador e da distância de Förster. As distâncias assim obtidas são compatíveis com as dimensões das proteínas e dos nanocristais / Proteins belonging to the GFP family were used to characterize their interaction with fluorescent CdSe/ZnS nanocrystals and to verify the occurrence of resonance energy transfer (FRET) among these elements. Two donor-acceptor pairs were established, one having a protein as donor and the other having a nanocrystal as donor. In both cases the donor suffers quenching of its fluorescence after the formation of a complex with the acceptor. The complex formation is dependent on pH and is due to electrostatic interaction. It was possible to prove the occurrence of FRET between CdSe/ZnS nanocrystals emitting green fluorescence as donors and the protein HcRed1 as acceptor, through the detection of sensitized emission and anisotropy reduction. Apparent donor-acceptor distances were determined from efficiency measurements and Förster distances. The obtained distances agreed with the protein and nanocrystal dimensions

Membranas auto-sustentáveis compostas por filmes finos com múltiplas camadas SiO2/ ( 30 nm)/CdSe( 3,0nm)/SiO2 (18 nm) e SiO2( 30 nm)/PbSe( 3,0nm)/SiO2( 18 nm) foram produzidas por magnetron sputtering e submetidas à irradiação com feixe de elétrons em microscópios de transmissão eletrônica convencionais na faixa de energia de 80 a 300 keV em densidades de corrente de 0,3 a 8,0 A cm-2. Variações de contraste observadas em micrografias adquiridas com diferentes doses de elétrons sinalizam uma considerável redistribuição atômica nos filmes semicondutores, tal redistribuição é restrita às regiões iluminadas e possui maior dependência em relação à dose do que à densidade de corrente do feixe. Medidas de difração com área selecionada (Select Area Diffraction - SAD), observações em condição de alta resolução (High Resolution Transmission Microscopy - HRTEM) e medidas de dispersão em energia de Raio-X característico (Energy Dispersive Spectroscopy - EDS) indicam que ambos semicondutores mantém a estrutura cristalina e a quantidade de átomos de Cd, Se e Pb durante irradiação. As membranas SiO2/CdSe/SiO2 apresentam uma retração contínua e homogênea das interfaces CdSe/SiO2 ao longo da superfície irradiada, a nucleação e crescimento de regiões com apenas SiO2 dá origem a uma rede percolada de CdSe que é desmembradas em nanofios nodulares e nanopartículas isoladas. Os filmes de PbSe não apresentam um processo homogêneo e contínuo durante irradiação. Inicialmente, as interfaces PbSe/SiO2 apresentam perda das arestas de alto ângulo Neste caso, as modificações microestruturais são mais intensas após uma dose limite e em regiões específicas, próximas a buracos de SiO2 previamente existentes na amostra como depositada. Medidas SAD, micrografias em condição de campo escuro e análises de imagem HRTEM mostram que a irradiação no PbSe causa separação de fases, identificada por distribuições de nanopartículas de Pb interfaceadas com uma rede planar percolada de PbSe. A conservação de matéria nos sistemas permitiu determinar os fluxos atômicos durante irradiação, o que foi realizado pelo tratamento numérico das micrografias adquiridas em diferentes doses. A investigação do aquecimento da amostra e do comportamento dos sistemas quando irradiados em diferentes energias e densidades de corrente sugerem que os deslocamentos atômicos podem ser correlacionados com as probabilidades de interação entre elétrons e átomos alvo. Isto permitiu a comparação entre fluxos atômicos experimentais, obtidos pelo tratamento numérico das micrografias TEM, com fluxos atômicos deduzidos em função das seções de choque para deslocamentos atômicos diretos, induzidos por colisões elásticas entre elétrons e átomos alvo, e deslocamentos indiretos, causados por radiólise. Os fluxos teóricos consideram variações nas taxas de deslocamento dos átomos de Cd, Se e Pb ao longo da interface semicondutor/SiO2, tais variações são entendidas como consequência de mudanças na energia de coesão das interfaces, que foram calculadas em função da curvatura e energia de superfície através do modelo de gota líquida (Líquid Drop Model - LDM) Comparações entre os fluxos atômicos inferidos das micrografias com os fluxos teóricos, obtidos das seções de choque para espalhamento elástico e inelástico de elétrons, permitiram estimar as energias de deslocamento dos átomos na interfaces e elaborar possíveis mecanismos para as mudanças microestruturais durante a irradiação. Os valores calculados de energia de deslocamento são inferiores às energias necessárias para deslocar átomos na superfície ou no interior da rede cristalina, mas podem ser aproximados às energias de migração atômica em interfaces. Os resultados mostram que os mecanismos de deslocamento atômico nos filmes finos de CdSe e PbSe não são os mesmos. As mudanças microestruturais observadas nos filmes finos de CdSe não podem ser explicadas apenas em termos de colisões balísticas dos elétrons, mas poderiam ocorrer por deslocamentos radiolíticos, principalmente se houverem estados de interface e meia banda que permitam excitações com energias transferidas menores que a largura de banda do CdSe. Já os resultados das irradiações no PbSe podem ser explicados como decorrentes de deslocamentos diretos, causados pela colisão balística dos elétrons nos átomos de Pb e Se pouco coesos nos planos PbSe{111}. Contudo, este estudo não permite excluir um possível processo de múltiplas ionizações como causa dos fluxos atômicos durante irradiação de ambas membranas. / Self-standing membranes compounded for multilayers SiO2/( 30 nm)/CdSe ( 3.0nm) /SiO2 (18 nm) and SiO2( 30 nm)/PbSe( 3.0nm)/SiO2( 18 nm) were irradiated in conventional Transmission Electron Microscopes (TEM) at energy range of 80 - 300 keV, current densities 0.3 - 8.0 A cm-2. The image contrasts of the micrographs acquired at different electron doses show an intense atomic redistribution in the semiconductor films. The effects of irradiation are restricted on the irradiated regions and show a dose dependence instead electric current dependence. Select Area Diffraction (SAD), Energy Dispersive Spectroscopy (EDS) measurements and High Resolution Electron Transmission Microscopy (HRTEM) micrographs show that the both semiconductors mantained the crystal structure and quantity of Cd, Se and Pb atoms after irradiation. The SiO2/CdSe/SiO2 membranes have a homogeneously and continuous retraction of the CdSe/SiO2 interfaces along the irradiated regions. The SiO2 holes grow to produce a percolated planar network of CdSe. In larger doses this network is disrupted, producing nodular nanowires and isolated nanoparticles. Otherwise, the PbSe thin films did not show a homogeneous and continuous process. In the first minutes of irradiation, the SiO2 holes lost the edges of high angles, the retraction of the interfaces PbSe/SiO2 occurs only at a specific electron dose, after which there are growth and nucleation of new holes around the previous ones SAD measurements, dark field micrographs and HRTEM images attest phase separation during electron beam irradiation of the PbSe, the results show isolated Pb nanoparticles connected to a planar percolated network of PbSe. The matter conservation at the systems allowed the calculation of an atomic flux during the irradiation, what was made by the numerical treatment of the micrographs acquired at different electron doses. The investigation of the sample heating and the behavior of the systems when irradiated at different energies and current densities suggest that the atomic displacements can be correlated with the probabilities of electron-atom interactions. This allowed the comparison between the inferred atomic fluxes with atomic fluxes deduced by the cross sections for the ballistic displacement induced by elastic collision of the electrons and the atomic fluxes deduced by the inelastic cross sections, which show the probability of the indirect displacements induced by radiolysis. These fluxes consider changes in the displacement rates of the Cd, Se, and Pb atoms along the semiconductor/SiO2 due to changes in the cohesion energy at the interfaces, what was calculated in function of the curvature and surface energy using the Líquid Drop Model - LDM The comparison between the atomic fluxes inferred by the TEM micrographs with the theoretical fluxes obtained by the elastic and inelastic scaterring cross sections allowed extimations of the displacement energies of the Cd, Se e Pb atoms at the interfaces, what was used to argue some possibles mechanisms for the microstructural changes during the irradiation. The calculated displacement energies are lower than the bulk or surface displacement energies, but can be approximated with the migration energies for the atomic diffusion at the interfaces. The results suggest that the mechanisms of atomic displacement can not be the same for the thin films of CdSe and PbSe. The microstructural changes observed in the CdSe thin films can not be explained only in terms of ballistic displacements, but can be explained by indirect displacements induced by the radiolysis, especially if there are intermediate and middle band states that allow excitations with energies below the CdSe band-gap. In other way, the results of the PbSe can be explained by direct displacements caused by the ballistic collision of the electrons at the Pb and Se atoms placed in the unstable PbSe{111} planes. However, this study can not rule out the possibility of a multiple ionization process as the cause of the atomic fluxes in both membranes.

Membranas auto-sustentáveis compostas por filmes finos com múltiplas camadas SiO2/ ( 30 nm)/CdSe( 3,0nm)/SiO2 (18 nm) e SiO2( 30 nm)/PbSe( 3,0nm)/SiO2( 18 nm) foram produzidas por magnetron sputtering e submetidas à irradiação com feixe de elétrons em microscópios de transmissão eletrônica convencionais na faixa de energia de 80 a 300 keV em densidades de corrente de 0,3 a 8,0 A cm-2. Variações de contraste observadas em micrografias adquiridas com diferentes doses de elétrons sinalizam uma considerável redistribuição atômica nos filmes semicondutores, tal redistribuição é restrita às regiões iluminadas e possui maior dependência em relação à dose do que à densidade de corrente do feixe. Medidas de difração com área selecionada (Select Area Diffraction - SAD), observações em condição de alta resolução (High Resolution Transmission Microscopy - HRTEM) e medidas de dispersão em energia de Raio-X característico (Energy Dispersive Spectroscopy - EDS) indicam que ambos semicondutores mantém a estrutura cristalina e a quantidade de átomos de Cd, Se e Pb durante irradiação. As membranas SiO2/CdSe/SiO2 apresentam uma retração contínua e homogênea das interfaces CdSe/SiO2 ao longo da superfície irradiada, a nucleação e crescimento de regiões com apenas SiO2 dá origem a uma rede percolada de CdSe que é desmembradas em nanofios nodulares e nanopartículas isoladas. Os filmes de PbSe não apresentam um processo homogêneo e contínuo durante irradiação. Inicialmente, as interfaces PbSe/SiO2 apresentam perda das arestas de alto ângulo Neste caso, as modificações microestruturais são mais intensas após uma dose limite e em regiões específicas, próximas a buracos de SiO2 previamente existentes na amostra como depositada. Medidas SAD, micrografias em condição de campo escuro e análises de imagem HRTEM mostram que a irradiação no PbSe causa separação de fases, identificada por distribuições de nanopartículas de Pb interfaceadas com uma rede planar percolada de PbSe. A conservação de matéria nos sistemas permitiu determinar os fluxos atômicos durante irradiação, o que foi realizado pelo tratamento numérico das micrografias adquiridas em diferentes doses. A investigação do aquecimento da amostra e do comportamento dos sistemas quando irradiados em diferentes energias e densidades de corrente sugerem que os deslocamentos atômicos podem ser correlacionados com as probabilidades de interação entre elétrons e átomos alvo. Isto permitiu a comparação entre fluxos atômicos experimentais, obtidos pelo tratamento numérico das micrografias TEM, com fluxos atômicos deduzidos em função das seções de choque para deslocamentos atômicos diretos, induzidos por colisões elásticas entre elétrons e átomos alvo, e deslocamentos indiretos, causados por radiólise. Os fluxos teóricos consideram variações nas taxas de deslocamento dos átomos de Cd, Se e Pb ao longo da interface semicondutor/SiO2, tais variações são entendidas como consequência de mudanças na energia de coesão das interfaces, que foram calculadas em função da curvatura e energia de superfície através do modelo de gota líquida (Líquid Drop Model - LDM) Comparações entre os fluxos atômicos inferidos das micrografias com os fluxos teóricos, obtidos das seções de choque para espalhamento elástico e inelástico de elétrons, permitiram estimar as energias de deslocamento dos átomos na interfaces e elaborar possíveis mecanismos para as mudanças microestruturais durante a irradiação. Os valores calculados de energia de deslocamento são inferiores às energias necessárias para deslocar átomos na superfície ou no interior da rede cristalina, mas podem ser aproximados às energias de migração atômica em interfaces. Os resultados mostram que os mecanismos de deslocamento atômico nos filmes finos de CdSe e PbSe não são os mesmos. As mudanças microestruturais observadas nos filmes finos de CdSe não podem ser explicadas apenas em termos de colisões balísticas dos elétrons, mas poderiam ocorrer por deslocamentos radiolíticos, principalmente se houverem estados de interface e meia banda que permitam excitações com energias transferidas menores que a largura de banda do CdSe. Já os resultados das irradiações no PbSe podem ser explicados como decorrentes de deslocamentos diretos, causados pela colisão balística dos elétrons nos átomos de Pb e Se pouco coesos nos planos PbSe{111}. Contudo, este estudo não permite excluir um possível processo de múltiplas ionizações como causa dos fluxos atômicos durante irradiação de ambas membranas. / Self-standing membranes compounded for multilayers SiO2/( 30 nm)/CdSe ( 3.0nm) /SiO2 (18 nm) and SiO2( 30 nm)/PbSe( 3.0nm)/SiO2( 18 nm) were irradiated in conventional Transmission Electron Microscopes (TEM) at energy range of 80 - 300 keV, current densities 0.3 - 8.0 A cm-2. The image contrasts of the micrographs acquired at different electron doses show an intense atomic redistribution in the semiconductor films. The effects of irradiation are restricted on the irradiated regions and show a dose dependence instead electric current dependence. Select Area Diffraction (SAD), Energy Dispersive Spectroscopy (EDS) measurements and High Resolution Electron Transmission Microscopy (HRTEM) micrographs show that the both semiconductors mantained the crystal structure and quantity of Cd, Se and Pb atoms after irradiation. The SiO2/CdSe/SiO2 membranes have a homogeneously and continuous retraction of the CdSe/SiO2 interfaces along the irradiated regions. The SiO2 holes grow to produce a percolated planar network of CdSe. In larger doses this network is disrupted, producing nodular nanowires and isolated nanoparticles. Otherwise, the PbSe thin films did not show a homogeneous and continuous process. In the first minutes of irradiation, the SiO2 holes lost the edges of high angles, the retraction of the interfaces PbSe/SiO2 occurs only at a specific electron dose, after which there are growth and nucleation of new holes around the previous ones SAD measurements, dark field micrographs and HRTEM images attest phase separation during electron beam irradiation of the PbSe, the results show isolated Pb nanoparticles connected to a planar percolated network of PbSe. The matter conservation at the systems allowed the calculation of an atomic flux during the irradiation, what was made by the numerical treatment of the micrographs acquired at different electron doses. The investigation of the sample heating and the behavior of the systems when irradiated at different energies and current densities suggest that the atomic displacements can be correlated with the probabilities of electron-atom interactions. This allowed the comparison between the inferred atomic fluxes with atomic fluxes deduced by the cross sections for the ballistic displacement induced by elastic collision of the electrons and the atomic fluxes deduced by the inelastic cross sections, which show the probability of the indirect displacements induced by radiolysis. These fluxes consider changes in the displacement rates of the Cd, Se, and Pb atoms along the semiconductor/SiO2 due to changes in the cohesion energy at the interfaces, what was calculated in function of the curvature and surface energy using the Líquid Drop Model - LDM The comparison between the atomic fluxes inferred by the TEM micrographs with the theoretical fluxes obtained by the elastic and inelastic scaterring cross sections allowed extimations of the displacement energies of the Cd, Se e Pb atoms at the interfaces, what was used to argue some possibles mechanisms for the microstructural changes during the irradiation. The calculated displacement energies are lower than the bulk or surface displacement energies, but can be approximated with the migration energies for the atomic diffusion at the interfaces. The results suggest that the mechanisms of atomic displacement can not be the same for the thin films of CdSe and PbSe. The microstructural changes observed in the CdSe thin films can not be explained only in terms of ballistic displacements, but can be explained by indirect displacements induced by the radiolysis, especially if there are intermediate and middle band states that allow excitations with energies below the CdSe band-gap. In other way, the results of the PbSe can be explained by direct displacements caused by the ballistic collision of the electrons at the Pb and Se atoms placed in the unstable PbSe{111} planes. However, this study can not rule out the possibility of a multiple ionization process as the cause of the atomic fluxes in both membranes.

Foram utilizadas proteínas da famÌlia das GFPs e nanocristais fluorescentes de CdSe/ZnS para caracterização da interação e verificação de transferência de energia por ressonância (FRET) entre estes compostos. Formou-se dois pares doador-receptor onde ora uma proteína figurava como doadora, ora um nanocristal ocupava este papel. Verificou-se que, em ambos os casos, o doador sofre supressão da fluorescência após a formação de complexo com o receptor, complexo este motivado por interação eletrostática e dependente de pH. Foi possível comprovar, através da observação de emissão sensitizada e redução da anisotropia, que entre o par formado por nanocristal com emissão no verde e proteína HcRed1 como receptora, de fato ocorre FRET. As distâncias aparentes entre doador e receptor foram determinadas a partir da eficiência da supressão da fluorescência do doador e da distância de Förster. As distâncias assim obtidas são compatíveis com as dimensões das proteínas e dos nanocristais / Proteins belonging to the GFP family were used to characterize their interaction with fluorescent CdSe/ZnS nanocrystals and to verify the occurrence of resonance energy transfer (FRET) among these elements. Two donor-acceptor pairs were established, one having a protein as donor and the other having a nanocrystal as donor. In both cases the donor suffers quenching of its fluorescence after the formation of a complex with the acceptor. The complex formation is dependent on pH and is due to electrostatic interaction. It was possible to prove the occurrence of FRET between CdSe/ZnS nanocrystals emitting green fluorescence as donors and the protein HcRed1 as acceptor, through the detection of sensitized emission and anisotropy reduction. Apparent donor-acceptor distances were determined from efficiency measurements and Förster distances. The obtained distances agreed with the protein and nanocrystal dimensions

Orientadores: Ana Flávia Nogueira, Teresa Dib Zambon Atvars / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-19T07:00:55Z (GMT). No. of bitstreams: 1 Alves_JoaoPaulodeCarvalho_M.pdf: 2190959 bytes, checksum: a321ebe05ad04c15ea1e77782b12c743 (MD5) Previous issue date: 2011 / Resumo: Células solares orgânicas apresentam-se como uma alternativa promissora para conversão de energia solar. Muitos desses dispositivos são produzidos pela mistura de um polímero condutor e um derivado fulereno. O polímero condutor atua como absorvedor de luz, doador de elétrons e transportador de buracos, enquanto o fulereno atua como transportador e aceitador de elétrons. Esses dispositivos destacam-se pela possibilidade de deposição em substratos flexíveis, transparência, diversidade em cores e uso de materiais recicláveis, além do baixo custo. No entanto, células solares orgânicas apresentam baixa mobilidade de carga e absorção em pequena faixa espectral da energia solar quando comparadas aos dispositivos fotovoltaicos baseados em semicondutores inorgânicos. A adição de CdSe ao sistema P3HT/PCBM promoveu o decréscimo na eficiência e fotocorrente dos dispositivos. Esses resultados são diferentes dos observados para o sistema PFT/PCBM, previamente reportado. A diferença observada para os dispositivos com os polímeros P3HT e PFT foram associados com as diferentes interações polímero-nanopartícula, como evidenciado pelas medidas de absorção e emissão e pelos espectros de RMN. A elevada concentração de tiofeno no P3HT pode contribuir fortemente na formação de um complexo entre polímero e CdSe e na desativação do processo de transferência de elétrons entre polímero e PCBM. Para investigar um possível efeito de morfologia pela introdução de CdSe ao sistema P3HT/PCBM, foram obtidas imagens de microscopia de força atômica e microscopia óptica. Há um aumento da rugosidade e tamanho de grãos do sistema com o acréscimo de nanopartículas de CdSe, que pode inibir o processo de separação de cargas e de formação de redes de percolação / Abstract: Organic solar cells are presented as a promising alternative for solar energy conversion. Many of these devices are produced by mixing a conductive polymer and a fullerene derivative. The conducting polymer acts as a light absorber, electron donor and hole transporter, while the fullerene acts as acceptor and electron shuttle. These devices call attention due to the possibility to produce flexible solar cells with high transparency and different colors, allied to the use of recyclable materials and low cost. However, organic solar cells have lower charge mobility and narrower spectral range absorption of solar energy compared to inorganic-based photovoltaic devices. The addition of CdSe to the P3HT/PCBM system promoted the decrease in efficiency and photocurrent of the devices. These results are different from those observed for the PFT/PCBM system, previously reported. The difference observed for the devices with P3HT and PFT was associated with different polymer-nanoparticle interactions, as evidenced by absorption and emission measurements. The high concentration of thiophene in P3HT contributes in the formation of a complex between polymer-CdSe and the deactivation of the electron transfer process between the polymer and PCBM. To investigate the possible contribution of a morphological effect induced by CdSe in P3HT/PCBM system, Atomic Force Microscopy (AFM) images were obtained. There is an increase of roughness and grain size of the system with the addition of CdSe nanoparticles, which can inhibit the charge separation process and formation of percolation networks / Mestrado / Quimica Inorganica / Mestre em Química

The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale.The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale. To achieve reliable measurements a new strategy was developed so that dense and homogeneous monolayers of semiconductor particles could be deposited onto transparent electrodes. The films were obtained by exchanging the original bulky ligand shell of the nanocrystals with a reactive alkoxysilane species and subsequent immersion of the substrate into a solution of the modified nanocrystals. SEM and electrochemical investigations have shown a much higher coverage efficiency in comparison with other methods presently established in the literature, which are based on the approach of prefunctionalizing of the substrates prior to coating. Fractional coverages of 80 % were obtained within 24 h while avoiding the time consuming and complicated step of functionalizing the substrates before deposition. Films of CdSe and CdS nanoparticles deposited on fluorine doped tin oxide (FTO) electrodes were characterized by means of potential modulated absorption spectro-scopy (EMAS). Employing this special spectroelectrochemical technique, bleach signatures in the absorption spectra of the quantum dots induced by electron injection into their respective conduction band states were investigated. The features observed in the spectra and the evaluation of the potential dependence of the signal intensity revealed that only the lowest conduction band state, namely the 1Se state, is populated. The occupancy follows a quasi Fermi-Dirac distribution whose distributional width, in addition to the temperature, also depends on the size distribution of the particle ensemble investigated. On that basis a model was developed to extract the electrochemical potentials of the respective populated lowest conduction band states. For CdSe quantum dots the four energetically lowest excitonic transitions were found to become bleached as the 1Se state is populated, indicating that these transitions promote electrons from different states in the valence band to the same conduction band state. These findings are in excellent agreement with results obtained from ultra fast optical pump probe experiments, which are methods that usually demand much more experimental efforts than the technique presented in these studies. The determination of the potential of the 1Se state versus a known reference potential allows one to map the top valence band states with respect to an absolute energy scale. This provides the opportunity to compare the energy positions obtained for different samples. Determination of the electrochemical band edge potential clearly features a size dependent shift of the conduction band edge and the valence band edge for both CdSe and CdS quantum dots, which is in excellent agreement with the expected behavior due to the quantum confinement effect. Investigations in different electrolytes have shown that the immediate environment has a major impact on the electrochemical potentials of the energy levels of the nanoparticles. This observation is particularly important from a technological point of view, as in many applications the semiconductor material is in direct contact with an electrolyte as for example in quantum dot sensitized solar cells, electrochemical sensors and catalysis. In contrast to other “purely physical” methods such as photoelectron spectroscopy or scanning tunneling spectroscopy, potential-modulated absorption spectroscopy provides the ability to probe the materials under their most likely “working” conditions where such environmental influences can be directly taken into account. Further, it has been shown that potential modulated absorption spectroscopy can be applied to bulk semiconductor electrodes, as long as they are thin enough to allow adequate amounts of light to pass through. The features observed in the EMAS spectra of these samples clearly differ from those obtained for nanoparticle films, as in such materials a continuum of states is progressively filled rather than a single state. Besides band-filling the potential modulation additionally induces changes in the absorption, which can be attributed to the Franz-Keldysh effect resulting from the modulation of the electric field across the space charge layer. The resolution and sensitivity that one can obtain with this comparatively simple and cost-effective setup is quite remarkable. As has been demonstrated it was possible to achieve clearly resolved bleach spectra of submonolayers of quantum dots attached to FTO with optical densities below 0.001. Recently it has been reported that cyclic voltammetry (CV) can be used to study the size dependent positions of the electronic levels of quantum dots. The intention of the last part of this thesis was to reproduce this work for the nanoparticles investigated within this thesis in order to compare the results with those obtained by EMAS. However, the experiments undertaken here reveal that the anodic and cathodic peaks observed in the cyclic voltammograms cannot automatically be assigned to the absolute band edge positions of the particles as the size dependent peak positions and their potential differences do not show any evidence for a correlation with respect to the quantum size effect. Rather the voltammetric responses reflect the solid state electrochemical characteristics of CdSe. Theoretical considerations concerning the response expected in a CV due to band filling of semiconductor nanoparticles confined to an electrode surface revealed that the expected currents are quite similar to that of a pseudo-capacitance. However, pronounced signals are only obtained if appropriate amounts of deposited nanoparticles are present which are electronically addressable without hampering the charge transfer. Hence a clear assignment of the peaks obtained in a cyclic voltammogram to the electronic band edges without employing a complementary technique to confirm ones findings therefore seems to be at best questionable.

Membranas auto-sustentáveis compostas por filmes finos com múltiplas camadas SiO2/ ( 30 nm)/CdSe( 3,0nm)/SiO2 (18 nm) e SiO2( 30 nm)/PbSe( 3,0nm)/SiO2( 18 nm) foram produzidas por magnetron sputtering e submetidas à irradiação com feixe de elétrons em microscópios de transmissão eletrônica convencionais na faixa de energia de 80 a 300 keV em densidades de corrente de 0,3 a 8,0 A cm-2. Variações de contraste observadas em micrografias adquiridas com diferentes doses de elétrons sinalizam uma considerável redistribuição atômica nos filmes semicondutores, tal redistribuição é restrita às regiões iluminadas e possui maior dependência em relação à dose do que à densidade de corrente do feixe. Medidas de difração com área selecionada (Select Area Diffraction - SAD), observações em condição de alta resolução (High Resolution Transmission Microscopy - HRTEM) e medidas de dispersão em energia de Raio-X característico (Energy Dispersive Spectroscopy - EDS) indicam que ambos semicondutores mantém a estrutura cristalina e a quantidade de átomos de Cd, Se e Pb durante irradiação. As membranas SiO2/CdSe/SiO2 apresentam uma retração contínua e homogênea das interfaces CdSe/SiO2 ao longo da superfície irradiada, a nucleação e crescimento de regiões com apenas SiO2 dá origem a uma rede percolada de CdSe que é desmembradas em nanofios nodulares e nanopartículas isoladas. Os filmes de PbSe não apresentam um processo homogêneo e contínuo durante irradiação. Inicialmente, as interfaces PbSe/SiO2 apresentam perda das arestas de alto ângulo Neste caso, as modificações microestruturais são mais intensas após uma dose limite e em regiões específicas, próximas a buracos de SiO2 previamente existentes na amostra como depositada. Medidas SAD, micrografias em condição de campo escuro e análises de imagem HRTEM mostram que a irradiação no PbSe causa separação de fases, identificada por distribuições de nanopartículas de Pb interfaceadas com uma rede planar percolada de PbSe. A conservação de matéria nos sistemas permitiu determinar os fluxos atômicos durante irradiação, o que foi realizado pelo tratamento numérico das micrografias adquiridas em diferentes doses. A investigação do aquecimento da amostra e do comportamento dos sistemas quando irradiados em diferentes energias e densidades de corrente sugerem que os deslocamentos atômicos podem ser correlacionados com as probabilidades de interação entre elétrons e átomos alvo. Isto permitiu a comparação entre fluxos atômicos experimentais, obtidos pelo tratamento numérico das micrografias TEM, com fluxos atômicos deduzidos em função das seções de choque para deslocamentos atômicos diretos, induzidos por colisões elásticas entre elétrons e átomos alvo, e deslocamentos indiretos, causados por radiólise. Os fluxos teóricos consideram variações nas taxas de deslocamento dos átomos de Cd, Se e Pb ao longo da interface semicondutor/SiO2, tais variações são entendidas como consequência de mudanças na energia de coesão das interfaces, que foram calculadas em função da curvatura e energia de superfície através do modelo de gota líquida (Líquid Drop Model - LDM) Comparações entre os fluxos atômicos inferidos das micrografias com os fluxos teóricos, obtidos das seções de choque para espalhamento elástico e inelástico de elétrons, permitiram estimar as energias de deslocamento dos átomos na interfaces e elaborar possíveis mecanismos para as mudanças microestruturais durante a irradiação. Os valores calculados de energia de deslocamento são inferiores às energias necessárias para deslocar átomos na superfície ou no interior da rede cristalina, mas podem ser aproximados às energias de migração atômica em interfaces. Os resultados mostram que os mecanismos de deslocamento atômico nos filmes finos de CdSe e PbSe não são os mesmos. As mudanças microestruturais observadas nos filmes finos de CdSe não podem ser explicadas apenas em termos de colisões balísticas dos elétrons, mas poderiam ocorrer por deslocamentos radiolíticos, principalmente se houverem estados de interface e meia banda que permitam excitações com energias transferidas menores que a largura de banda do CdSe. Já os resultados das irradiações no PbSe podem ser explicados como decorrentes de deslocamentos diretos, causados pela colisão balística dos elétrons nos átomos de Pb e Se pouco coesos nos planos PbSe{111}. Contudo, este estudo não permite excluir um possível processo de múltiplas ionizações como causa dos fluxos atômicos durante irradiação de ambas membranas. / Self-standing membranes compounded for multilayers SiO2/( 30 nm)/CdSe ( 3.0nm) /SiO2 (18 nm) and SiO2( 30 nm)/PbSe( 3.0nm)/SiO2( 18 nm) were irradiated in conventional Transmission Electron Microscopes (TEM) at energy range of 80 - 300 keV, current densities 0.3 - 8.0 A cm-2. The image contrasts of the micrographs acquired at different electron doses show an intense atomic redistribution in the semiconductor films. The effects of irradiation are restricted on the irradiated regions and show a dose dependence instead electric current dependence. Select Area Diffraction (SAD), Energy Dispersive Spectroscopy (EDS) measurements and High Resolution Electron Transmission Microscopy (HRTEM) micrographs show that the both semiconductors mantained the crystal structure and quantity of Cd, Se and Pb atoms after irradiation. The SiO2/CdSe/SiO2 membranes have a homogeneously and continuous retraction of the CdSe/SiO2 interfaces along the irradiated regions. The SiO2 holes grow to produce a percolated planar network of CdSe. In larger doses this network is disrupted, producing nodular nanowires and isolated nanoparticles. Otherwise, the PbSe thin films did not show a homogeneous and continuous process. In the first minutes of irradiation, the SiO2 holes lost the edges of high angles, the retraction of the interfaces PbSe/SiO2 occurs only at a specific electron dose, after which there are growth and nucleation of new holes around the previous ones SAD measurements, dark field micrographs and HRTEM images attest phase separation during electron beam irradiation of the PbSe, the results show isolated Pb nanoparticles connected to a planar percolated network of PbSe. The matter conservation at the systems allowed the calculation of an atomic flux during the irradiation, what was made by the numerical treatment of the micrographs acquired at different electron doses. The investigation of the sample heating and the behavior of the systems when irradiated at different energies and current densities suggest that the atomic displacements can be correlated with the probabilities of electron-atom interactions. This allowed the comparison between the inferred atomic fluxes with atomic fluxes deduced by the cross sections for the ballistic displacement induced by elastic collision of the electrons and the atomic fluxes deduced by the inelastic cross sections, which show the probability of the indirect displacements induced by radiolysis. These fluxes consider changes in the displacement rates of the Cd, Se, and Pb atoms along the semiconductor/SiO2 due to changes in the cohesion energy at the interfaces, what was calculated in function of the curvature and surface energy using the Líquid Drop Model - LDM The comparison between the atomic fluxes inferred by the TEM micrographs with the theoretical fluxes obtained by the elastic and inelastic scaterring cross sections allowed extimations of the displacement energies of the Cd, Se e Pb atoms at the interfaces, what was used to argue some possibles mechanisms for the microstructural changes during the irradiation. The calculated displacement energies are lower than the bulk or surface displacement energies, but can be approximated with the migration energies for the atomic diffusion at the interfaces. The results suggest that the mechanisms of atomic displacement can not be the same for the thin films of CdSe and PbSe. The microstructural changes observed in the CdSe thin films can not be explained only in terms of ballistic displacements, but can be explained by indirect displacements induced by the radiolysis, especially if there are intermediate and middle band states that allow excitations with energies below the CdSe band-gap. In other way, the results of the PbSe can be explained by direct displacements caused by the ballistic collision of the electrons at the Pb and Se atoms placed in the unstable PbSe{111} planes. However, this study can not rule out the possibility of a multiple ionization process as the cause of the atomic fluxes in both membranes.

The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale.The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale. To achieve reliable measurements a new strategy was developed so that dense and homogeneous monolayers of semiconductor particles could be deposited onto transparent electrodes. The films were obtained by exchanging the original bulky ligand shell of the nanocrystals with a reactive alkoxysilane species and subsequent immersion of the substrate into a solution of the modified nanocrystals. SEM and electrochemical investigations have shown a much higher coverage efficiency in comparison with other methods presently established in the literature, which are based on the approach of prefunctionalizing of the substrates prior to coating. Fractional coverages of 80 % were obtained within 24 h while avoiding the time consuming and complicated step of functionalizing the substrates before deposition. Films of CdSe and CdS nanoparticles deposited on fluorine doped tin oxide (FTO) electrodes were characterized by means of potential modulated absorption spectro-scopy (EMAS). Employing this special spectroelectrochemical technique, bleach signatures in the absorption spectra of the quantum dots induced by electron injection into their respective conduction band states were investigated. The features observed in the spectra and the evaluation of the potential dependence of the signal intensity revealed that only the lowest conduction band state, namely the 1Se state, is populated. The occupancy follows a quasi Fermi-Dirac distribution whose distributional width, in addition to the temperature, also depends on the size distribution of the particle ensemble investigated. On that basis a model was developed to extract the electrochemical potentials of the respective populated lowest conduction band states. For CdSe quantum dots the four energetically lowest excitonic transitions were found to become bleached as the 1Se state is populated, indicating that these transitions promote electrons from different states in the valence band to the same conduction band state. These findings are in excellent agreement with results obtained from ultra fast optical pump probe experiments, which are methods that usually demand much more experimental efforts than the technique presented in these studies. The determination of the potential of the 1Se state versus a known reference potential allows one to map the top valence band states with respect to an absolute energy scale. This provides the opportunity to compare the energy positions obtained for different samples. Determination of the electrochemical band edge potential clearly features a size dependent shift of the conduction band edge and the valence band edge for both CdSe and CdS quantum dots, which is in excellent agreement with the expected behavior due to the quantum confinement effect. Investigations in different electrolytes have shown that the immediate environment has a major impact on the electrochemical potentials of the energy levels of the nanoparticles. This observation is particularly important from a technological point of view, as in many applications the semiconductor material is in direct contact with an electrolyte as for example in quantum dot sensitized solar cells, electrochemical sensors and catalysis. In contrast to other “purely physical” methods such as photoelectron spectroscopy or scanning tunneling spectroscopy, potential-modulated absorption spectroscopy provides the ability to probe the materials under their most likely “working” conditions where such environmental influences can be directly taken into account. Further, it has been shown that potential modulated absorption spectroscopy can be applied to bulk semiconductor electrodes, as long as they are thin enough to allow adequate amounts of light to pass through. The features observed in the EMAS spectra of these samples clearly differ from those obtained for nanoparticle films, as in such materials a continuum of states is progressively filled rather than a single state. Besides band-filling the potential modulation additionally induces changes in the absorption, which can be attributed to the Franz-Keldysh effect resulting from the modulation of the electric field across the space charge layer. The resolution and sensitivity that one can obtain with this comparatively simple and cost-effective setup is quite remarkable. As has been demonstrated it was possible to achieve clearly resolved bleach spectra of submonolayers of quantum dots attached to FTO with optical densities below 0.001. Recently it has been reported that cyclic voltammetry (CV) can be used to study the size dependent positions of the electronic levels of quantum dots. The intention of the last part of this thesis was to reproduce this work for the nanoparticles investigated within this thesis in order to compare the results with those obtained by EMAS. However, the experiments undertaken here reveal that the anodic and cathodic peaks observed in the cyclic voltammograms cannot automatically be assigned to the absolute band edge positions of the particles as the size dependent peak positions and their potential differences do not show any evidence for a correlation with respect to the quantum size effect. Rather the voltammetric responses reflect the solid state electrochemical characteristics of CdSe. Theoretical considerations concerning the response expected in a CV due to band filling of semiconductor nanoparticles confined to an electrode surface revealed that the expected currents are quite similar to that of a pseudo-capacitance. However, pronounced signals are only obtained if appropriate amounts of deposited nanoparticles are present which are electronically addressable without hampering the charge transfer. Hence a clear assignment of the peaks obtained in a cyclic voltammogram to the electronic band edges without employing a complementary technique to confirm ones findings therefore seems to be at best questionable.