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Identifier 000290283
Title Ανάπτυξη και εφαρμογή απεικονιστικής τεχνικής για τον χαρακτηρισμό οργανικών υμενίων και υποστρωμάτων
Alternative Title Development and application of an imaging technique for the study of organic films and substrates
Author Φραγκούλη, Δέσποινα Χαραλάμπους
Thesis advisor Κιτσόπουλος, Θεοφάνης
Abstract The use of Self Assembled Monolayers for the construction of gas sensors is very important. The interaction of the molecules of the monolayers with gases induces the change of the dipole moment and consequently it modifies the electronic transmission and the intrinsic properties of the substrates. As a result a very small amount of molecules of the gases is detected selectively. This kind of sensors is applicable in physical chemistry, in medicine or in biology. An interesting approach is their use in the study of the structural molecules of artworks which are in a controlled environment, since it is possible the detection of a small amount of gases which are related to the oxidation processes or to the chemical reactions that occur on the surface. For this reason the study of the electronic properties of these monolayers and the interaction with the substrates is very important. Knowing these mechanisms, the improvement of the selectivity and the sensitivity of the sensors is possible. The aim of this PhD thesis is the development of a new experimental imaging technique for the study of the electronic properties of Self Assembled Monolayers (SAMs) adsorbed on a metallic substrate, the Low Energy Photoelectron Imaging Spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through the monolayers. The electronic properties are studied by recording the work function of the samples and the photoelectron transmission probability through the SAMs while various structural parameters are modified. Information about the way in which the intrinsic properties of the metals such as the work function and the conductivity are affected by the chemical modification of the substrates is deduced. The experimental results confirm the theoretical study using ab initio DFT and semi-empirical methods. Using this method we are able to follow the kinetics of self assembling by measuring the relative work function of alkanethiols on gold substrates in various surface coverages. It was found that the work function kinetics diverges from the Langmuir model due to the interactions between the adsorbates. Moreover the samples work function is in the expected range of values and depends on the properties of the interface and on the organization of the monolayers but not on the chain length. Furthermore it was proved that the main procedure taking place during the transmission of low energy photoelectrons through the SAMs is the tunnelling effect. Using this method the kinetics of the formation of the potential barrier of the interface is followed and it was proved that the kinetics is representative to the degree of adsorption and to degree the self assembling of the adsorbates on the surface. In addition it is recorded the dependence of the photoelectron attenuation coefficient to the photoelectron’s kinetic energy in the very low range of 0-1.2eV. Finally the change of the electronic properties caused by the gradual adsorption of porphyrine/fullerene molecular systems is studied. This study demonstrates the utility of the new experimental technique and the theoretical simulations for the study of the properties of the organic monolayers. The advantage of this method is the simultaneous record of the transmission and the angular distribution of photoelectrons with very low kinetic energies. Consequently information about the structure and the electronic properties of the monolayers are deduced in a fast and simple way. Moreover this study provides the basis for the conception of the corrosion of the artworks and of the initial phases of formation of the organic layers on their surface. This information is expected to help the improvement of the methodologies related to the artwork restoration and conservation.
Language Greek
Issue date 2007-01-18
Collection   Faculty/Department--Faculty of Sciences and Engineering--Department of Chemistry--Doctoral theses
  Type of Work--Doctoral theses
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