Wachstum, thermische Entwicklung und Phasentransformationen von NTCDA und Tetracen auf der Ag(111)-Oberfläche

In this thesis growth and thermal evolution of the organic molecules 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) and Tetracene (Tc) on the Ag(111) surface were investigated in the coverage range of ultra thin layers. The layer preparation was performed via molecular beam epitaxy under ultrahigh vacuum conditions. For investigation infrared reflection absorption spectroscopy (IRAS), spot profile analysis low-energy electron diffraction (SPA-LEED) and thermally programmed desorption (TPD) were used. For NTCDA in the coverage range of one monolayer the transition from the dense packed compressed monolayer, which corresponds to 1,0 monolayer, to the less dense packed relaxed monolayer, which corresponds to 0,9 monolayers, was investigated. The transition takes place in the temperature range between 350 K and 400 K. In both monolayer phases the molecules are oriented parallel with respect to the surface. Via postdeposition of additional material onto the relaxed monolayer a third type of monolayer could be prepared, with molecules standing upright on the surface. In the submonolayer regime a new ordered phase was found which can be obtained from the relaxed monolayer phase by a shift of entire molecular rows. In the coverage range between one and two monolayers a metastable bilayer was found which is also oriented parallel to the surface. The bilayer is largely decoupled from the metal surface. Starting with a coverage of more than two monolayers a multilayer phase is formed without any sign of the bilayer. In the coverage range between two and 20 monolayers two different multilayer phases were found depending on the amount of NTCDA and sample temperature during deposition. The arrangement of the molecules among each other is similar for both multilayer phases and corresponds to the arrangement found for the bulk phase. The multilayer phases differ in the orientation relative to the surface. For Tc in the coverage range of slightly more than two monolayers the beta-phase and its transition into the alpha-phase was presented. Experiments in the coverage range of one monolayer and beneath were done at sample temperatures of 77 K and 28 K using liquid nitrogen respectively liquid helium as cooling liquid. For measurements at a sample temperature of 77 K the results known from literature concerning the repulsive interaction of the Tc molecules have been confirmed. Specifically, the lateral repulsion leads to a disordered phase for coverages less than a monolayer which can be converted into the alpha-phase by adding material. By experiments at a sample temperature of 28 K two further low temperature phases could be detected. They are formed upon cooling slightly below 77 K and therefore could not be seen during the previous measurements at liquid nitrogen temperature. One of the phases could be identified as the gamma-phase which was described in the literature using STM data. The temperature dependence of the individual phases was investigated by performing fast sequences of linescans through selected diffraction spots of the corresponding phases while cooling down the sample. From the obtained data a well resolved phase diagram of Tc/Ag(111) was established.