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Self-assembling and charge transfer properties of thin organic films
Lanzilotto, Valeria
2012-03-29
Abstract
In the present thesis I dealt the issue of molecular ordering and charge transfer at two types of organic-inorganic interfaces that are representative of the basic constituents of an organic electron device. I investigated i.) the influence of a selected dielectric surface on the ordering of an overlayer of several organic molecules and ii.) the electronic transport properties of a single molecular junction with a metal electrode.
Both systems have been characterized by a structural and electronic point of view. Among the techniques available for structural investigation, I made extensive use of Helium Atom Scattering (HAS) and Scanning Tunneling Microscopy (STM). The electronic properties, with particular emphasis to the charge transfer, have been addressed by two methods chosen according to the dimensionality of the system under consideration. For the charge transfer at laterally extended interfaces I used synchrotron based techniques, like Resonant Photoemssion Spectroscopy (RPES), while for the charge transport through a single molecule I used and developed the STM-based break junction technique (STM-BJ).
For the first type of interface, I focused on the coupling between the TiO2(110)-1x1 surface and different organic semiconductor molecules: C60, pentacene, perylene-tetracarboxilic-acid-diimide (PTCDI) and perylene. The strong anisotropy of the substrate has been found to drive the adsorption geometry of the molecules leading to the formation of ordered phases (at least for the first layer). In particular pentacene, PTCDI and perylene (polycyclic aromatic hydrocarbons, PAHs) display a common self-assembly mechanism, where the molecules lay on the surface with their long axis oriented parallel to the [001] substrate direction. In the transverse direction [1-10] these molecules are observed to match the substrate periodicity by tilting the molecular plane around the long axis by an angle that depends on the molecular width. Nevertheless the molecule-to-substrate interaction is very weak as indicated by the molecular electronic structure, which is observed by X-ray spectroscopy to remain mostly unperturbed in the first molecular layer. Only PTCDI bears a major interaction with the TiO2(110)-1x1 surface, but confined to the molecular orbitals closest to the gap. The main experimental evidence of this interaction is the appearance of a new molecular filled state in the valence band region close to the Fermi level. By a combined RPES and NEXAFS study we have found that this new electronic state is due to the charge transfer occurring from the substrate Ti defect state (i.e. the excess of electrons associated with oxygen vacancies) to the
lowest unoccupied molecular orbital (LUMO).
For the second type of hybrid interface, instead, I exploited the nitrogen-link chemistry in order to bridge a phthalocyanine to two gold electrodes and to measure its conductance. In particular, by using the Tetraaza-Cu-Phthalocyanine
I investigated the pyridine-gold bond that is relatively weak and insensitive to the local structure, a fundamental requirement for the establishment of well defined and stable transport properties. The weak interaction between the molecule and a representative metal electrode, namely the Au(100) surface, has been confirmed by spectroscopic and STM experiments. At RT the molecules have been found to diffuse on the surface and only at LT (55 K) they can be observed to self-organize into large molecular domains. On these domains, reliable and reproducible single molecule conductance measurements have been performed by using the STM-based break junction method. The conductance value obtained for the Tetraaza-Cu-Phthalocyanine (7x10-4 G0) has been rationalized in terms of the molecular length and degree of conjugation, as well as by correlation to the energy level alignment at the junction.
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Publisher
Università degli studi di Trieste
Languages
en
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