[pt] PRODUÇÃO E CARACTERIZAÇÃO DE DISPOSITIVOS ORGÂNICOS ELETROLUMINESCENTES (OLEDS) BASEADOS EM COMPLEXOS (BETA)-DICETONATOS DE TERRAS-RARAS

Este trabalho apresenta os resultados de um estudo que envolve a fabricação e a caracterização de dispositivos orgânicos emissores de luz (OLEDs) baseados em complexos β-dicetonatos de terras-raras. O estudo se coloca como continuação lógica da linha de pesquisa em dispositivos eletroluminescentes baseados em íons terras-raras, começada alguns anos atrás neste grupo de pesquisa. Para a produção dos dispositivos foram empregadas várias técnicas de deposição de filmes finos, tais como deposição térmica resistiva, pulverização catódica assistida por plasma (rf-magnetronsputtering) e spin-coating. A síntese dos compostos orgânicos, bem como alguns estudos adicionais puderam ser realizadas através de colaborações com diversos grupos de pesquisas nacionais, os quais dispõem de recursos e capacitação em áreas complementares. Os complexos orgânicos estudados foram divididos em três conjuntos, que chamamos de sistemas. No sistema 1, estudou-se o complexo Eu(bmdm)3(ttpo)2, onde o ligante orgânico bmdm é um conhecido agente absorvedor de radiação UV bastante usado em protetores solares. Os OLEDs baseados neste complexo apresentaram intensa foto- e eletroluminescência com alta pureza de cor dada apenas pelas finas transições características do íon Eu3+. No sistema 2, estudou-se o complexo chamado de binuclear. Este composto tem dois núcleos terras-raras coordenados numa mesma molécula. O primeiro binuclear estudado, o complexo Eu(btfa)3 phenterpy Tb(acac)3, não apresentou as transições características dos íons Tb3+ e Eu3+ como era esperado inicialmente. Por outro lado, apresentou uma eletroluminescência sintonizável em duas situações distintas, a primeira em função da tensão aplicada e a segunda através de mudanças na arquitetura das camadas constituintes. Por causa desse efeito, mostramos a possibilidade de se construir um dispositivo OLED emissor de luz branca. Ainda nesse sistema, foram estudados OLEDs com complexos modificados quimicamente, chamados de binuclear 2 e trinuclear. O complexo binuclear 2 apresentou as linhas de emissão dos íons Tb3+ e Eu3+. Apesar de menos eficiente que o primeiro complexo binuclear, este estudo mostrou que através de manipulações moleculares (nanotecnologia) é possível sintetizar compostos capazes de emitir as linhas características de emissão dos íons terras-raras, ou seja, com um único complexo é possível obter duas emissões distintas. Por último, ainda como sistema 2, o complexo trinuclear, é uma mistura de compostos orgânicos contendo Tm, Tb e Eu e não formam uma única molécula, como no caso dos compostos binucleares. Este estudo foi iniciado recentemente e ainda não foi completamente explorado. Os primeiros testes mostraram que é possível usar este complexo também para fabricar OLEDs com emissão de cor branca, variando-se as quantidades relativas de Tm, Tb e Eu da mistura. Sabendo-se que os ligantes β-dicetonas são os responsáveis pela transferência de energia para os íons TR3+, através do efeito antena, o sistema 3, despontou como grande novidade, mostrando a construção de dispositivos eletroluminescentes baseados em complexos tetrakis(β-dicetonatos) de TR, ou seja, compostos que possuem quatro ligantes β-dicetonas coordenandos a um único íon TR. Com esse sistema conseguimos pela primeira vez uma emissão eficiente e pura das principais transições do íon Tb à temperatura ambiente. O trabalho apontou, também, que tanto a irradiação com luz UV, quanto a exposição aos agentes atmosféricos (oxigênio, água, umidade, etc.) contribuem para uma rápida degradação dos complexos orgânicos com conseqüente decaimento do desempenho dos dispositivos fabricados. Para tanto, iniciamos um estudo para investigar as causas da degradação de alguns dos compostos orgânicos utilizados na fabricação de OLEDs. Os estudos de fotoabsorção e fotoemissão realizados no Laboratório Nacional de Luz Síncrotron foram fundamentais para uma maior compreeIn this work we present the results of a study that involves the manufacture and the characterization of organic eletroluminescent devices (OLEDs) based on (beta)-diketonates Rare-Earth complexes. The investigation reported is a continuation of the research in electroluminescent devices based on rare-earth ions, started some years ago in our Group. For the production of the devices were applied several thin films deposition techniques: thermal resistive, rf-magnetronsputtering and spin-coating. The synthesis of organic compounds, as well some additional studies, were carried on through the collaboration with different brazilian research groups, which have resources and qualification in complementary areas. The organic compounds studied in this thesis have been divided in three groups, named systems. In system 1, the studied complex was Eu(bmdm)3(ttpo)2, where the organic bmdm ligand is a known UV sensitive material, frequently used in sunblockers. The OLEDs based on this complex presented intense photo- and electroluminescence with high pure color emission due to the almost atomic transitions characteristic of the Eu3+ ion. In system 2, a binuclear complex, represented by the molecular formula Eu(btfa)3 phenterpy Tb(acac)3 was studied. This complex has two rare-earth nuclei coordinated in the same molecule. The OLEDs based on this complex did not present the Tb3+ and Eu3+ characteristic transitions as expected. On the other hand, the complex gave us the possibility to develop an OLED with white color emission. Probably the major novelty of this thesis is represented by system 3. Indeed, knowing that the (beta)-diketone ligands are the main responsible for the RE3+ ions energy transference through the antenna effect, in system 3, we investigated the possibility to fabricate electroluminescent devices based on RE (beta)-diketonate tetrakis complexes, which have four coordinated (beta)- diketones ligand to an RE ion in order to enchance the energy transfer and the emission efficiency. With this system we obtained, for the first time, an efficient and pure Tb emission at room temperature

Transparent thermally stable poly (etherimide) film as flexible substrate for OLEDs

5 p. : il.In this work, ITO thin films were deposited onto poly(etherimide) (PEI) substrates at room temperature using r.f. magnetron sputtering and successively they were annealed in the 423–523 K (150–250 °C) temperature range. PEI/ITO substrates were structurally, optically and electrically characterized in order to verify the quality of the deposited ITO films and the PEI thermal stability during the ITO annealing process. A transmittance of about 80% was measured in the visible range. The best electrical properties achieved were: 3.04×10−4 Ω cm, 12.07×1021cm2/V.s, 16.8 × 1021 cm−3, for resistivity, carrier concentration and mobility, respectively. Small molecule Flexible Organic Light Emitting Diodes (FOLED) were then fabricated and characterized onto ITO functionalized PEI substrates. These preliminary results show clearly that PEI can be successfully used as substrate in flexible optoelectronic devices either operating in high temperature or when the process needs high temperatures

Study of Punctual Defects in Monolayer WS₂: Evidence of Correlations Between Raman and Photoluminescence Spectroscopy

The investigation of defects in transition-metal dichalcogenides (TMDs) are of capital interest for future applications because they strongly change the optical, electronic, or vibrational properties of such materials. In this sense, spectroscopic techniques, such as Raman and photoluminescence, are powerful tools to investigate the optoelectronic properties of crystal defects in two-dimensional TMDs. In this work, we observed that defect-activated Raman modes and bound exciton emissions can be strongly correlated. Specifically, we investigated the impact that sulfur vacancies, produced by focused helium ion beam, have on the electronic and phononic properties of WS2 grown by chemical vapor deposition. The photoluminescence spectra show two new emission peaks related to defects in the crystal structure. The defective nature of these bands were corroborated by density functional theory calculations, which showed new electronic states in the band gap associated with sulfur vacancies. Furthermore, by monitoring the evolution of the Raman spectra as a function of the defect concentration, we observed two new defect activated modes. These bands are explained by a second-order double resonant Raman process, similar to the D band of graphene. Finally, we found out that the defect-related Raman peaks become fully resonant for high defect concentrations. It turns out that degenerate electronic states split in two separated levels for high defect concentrations that are involved in the resonance of the Raman processes

Origin of optical bandgap fluctuations in graphene oxide

In this work, we explore the electrical, optical and spectroscopic properties of different Graphene Oxide (GO) samples focusing on new oxidative strategies to tune their physicochemical properties. Three types of GO samples were prepared by changing the oxidative conditions resulting in carbonyl-, epoxy- or hydroxyl-rich GO. These materials were characterized by UV-VIS absorption, Raman spectroscopy and X-ray diffraction. The experimental results indicate that all samples exhibit oxidation and exfoliation degrees typical of graphene oxides obtained by using the modified Hummers’ method. The optical bandgap values were measured using the Tauc’s plot from UV-VIS data and showed that the stoichiometry of GO impacts the width of the bandgap. The carbonyl-rich sample presented the lowest gap around 3.20 ± 0.02 eV, while epoxy- and hydroxyl-rich GOs showed out gaps of about 3.48 ± 0.07 and 3.72 ± 0.05 eV, respectively. These experimental results are consistent with theoretical calculations of bandgaps obtained with coronene and circumcoronene GO models. The calculations were obtained using different theoretical approaches, such as: Huckel, PM3, AM1 and DFT. The present work suggests that a precise tuning of the optical bandgap of GOs can be achieved by only changing their stoichiometry thus allowing their use in a large range of electronic applications

Biodiesel compatibility with carbon steel and hdpe parts

8 p.: il., tab.Compatibility of the new environmentally-friendly alternative of diesel engine fuels, biodiesel, with storage and engine part materials, is still an open issue. In this work, the interaction between three fuels (petroleum diesel and two types of biodiesel — soybean and sunflower) and two materials (carbon steel and high density polyethylene) used in storage and automotive tanks, is analyzed in detail. A wide set of characterization techniques was used to evaluate the changes in both solid and fluid materials, as weight change measurement, optical, scanning electron and atomic force (AFM) microscopies, Raman and FTIR spectroscopies, and differential scanning calorimetry. The AFM technique allowed detecting surface roughness and morphology changes in the metallic material following the trends in the weight losses. In the case of polymeric material, weight gain by fluid absorption occurred, being detected by the spectroscopic techniques. The biodiesel fuels underwent some ageing however this phenomenon did not affect the interaction between the biodiesel fuels and the substrates. The petrodiesel, which did not age, caused more significant degradation of the substrates