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O: Fachverband Oberflächenphysik

O 12: Organic Molecules at Surfaces 2: Characterization of Organic Monolayers

O 12.3: Talk

Monday, September 5, 2022, 15:30–15:45, S051

Molecular orientation and phase transitions of DHTAP — •Claudia López-Posadas¹, Michael Györök¹, Antony Thomas², Thomas Leoni², Olivier Siri², Conrad Becker², and Peter Zeppenfeld¹ — ¹Institute of Experimental Physics, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria — ²Aix-Marseille University, CNRS, CINaM, UMR 7325, F-13288 Marseille, France

The structure and orientation of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) layers deposited on Cu(110), Cu(110)-(2x1)O and the Cu(110)/Cu(110)-(2x1)O stripe phase was studied using reflectance difference spectroscopy (RDS), Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED). The evolution of the RDS signal allows to identify the sequential formation of up to three monolayers as well as a phase transition upon completion of the first one. On Cu(110), DHTAP molecules in the first monolayer are always lying flat with their long molecular axis aligned parallel to the [-110]-direction of the Cu(110) surface. However, for subsequent layers the orientation critically depends on the deposition temperature T. At T=240K the DHTAP molecules are mostly aligned parallel to the ones in the first layer, whereas at room temperature and above their preferential orientation is orthogonal to the molecules in the first layer, the coexistence of the two orientations can be observed at a critical temperature of T=270K. Finally, the main optical transitions and the orientation of the transition dipole moments of the DHTAP layers were extracted from the RDS spectra.