Fenômenos de interfaces e aplicações em sistemas orgânicos

Abstract

In this thesis we investigate basic physical properties as well as applications of organic materials. The understanding of interface related phenomena is particularly emphasized. Our choice of experimental techniques is able to reveal structural aspects of our systems, allowing to correlate their physical chemistry and electronic characteristics topotential uses on organic devices. Self-assembled molecules of phosphonic acids (with dielectric behavior), organic semiconductor molecules and light-emitting polymers were investigated in this work using X-ray reflectivity, X-ray diffraction (specular and diffuse),resonant X-ray reflectivity at the carbon K edge and atomic force microscopy. Our results show that the order-disorder transition in phosphonic acid multilayer systems is related to the spatial configuration of the molecules and takes place initially at the borders of crystallinedomains. Besides this result, differences in the chemical reactivity of these molecules where found when they are located at the surface or at the edges of domains. Such difference results on selective chemisorption characteristics, allowing the deposition of inorganic oxides on specific areas. Organic transistors were also fabricated and their structure and electric behavior studied. In order to increase the number of layers and maximize interfacial effects we propose the fabrication of organic transistors with hybrid (organic-inorganic) multilayer dielectrics. Devices with reduced interfacial roughness exhibit considerably higher electronic mobility and operational stability, if compared to conventional organic devices using the same compounds. Light-emitting organic devices were also investigated, with particular emphasis on the understanding of electronic interactions that take place at the interface between donor and acceptor layers. Our results show that the excitonic energy transfer phenomenon depends on the chemical affinity of the interacting polymers at the interface, regardless of the combination of solvents used for the layer deposition. Finally, the distance for molecular interactions was obtained experimentally, pointing out to a spatial threshold considerably smaller than the Förster radius. The interface phenomena discussed in this thesis consist on a good example of the possibilities opened by the combination of structuraland optical/electric characterization techniques.