Fluorescence spectroscopy of PTCDA molecules on the KCl(100) surface in the limit of low coverages: site selection and diffusion

Abstract

We performed fluorescence (FL) and fluorescence excitation (FLE) spectroscopy on the model molecule perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) for very low coverages (below 1% of a monolayer) on thin (100) oriented KCl films. Two different states of PTCDA molecules can be distinguished in the spectra: an initial state, which is observed directly after deposition of the molecules onto the cold sample at 20 K, and a final state, which is found after intensive optical excitation or thermal annealing of the sample. The spectrum of the final state is blue-shifted with respect to that of the initial state by 130 ± 15 cm(-1) and exhibits lines with significantly reduced widths. This can be explained by diffusion of molecules from initially populated terrace sites to energetically favoured step edge sites. Polarization dependent spectroscopy reveals the same azimuthal orientation of the molecules on both adsorption sites and leads to a model of the adsorption geometry of PTCDA at the KCl step sites. Our experiment demonstrates how optical spectroscopy can be used to investigate kinetic processes of fluorescent molecules on surfaces.

Adsorption‐induced changes of intramolecular optical transitions:PTCDA/NaCl and PTCDA/KCl

“…On KCl the measured S 1 excitation energy is shifted by −992 cm −1 (−0.123 eV) to 19,996 cm −1 (2.479 eV). On NaCl the shift is even more pronounced, −1308 cm −1 (−0.162 eV) to 19,680 cm −1 (2.440 eV) …”

“…Experimental and theoretical studies suggest that the PTCDA molecule adsorbs with its long axis parallel to the [110] or [1 0] direction, the carbonyl oxygen being located above surface cations and the center of the molecule residing above a surface anion . Hence, we used this orientation as a starting point for the geometry optimization.…”

“…Experimental studies have been performed by Müller et al for the archetypal organic semiconductor perylene‐3,4,9,10‐tetracarboxylic acid dianhydride (PTCDA) adsorbed on sodium chloride NaCl(100) and potassium chloride KCl(100) surfaces. To eliminate the intermolecular interaction, they adsorbed very low concentrations of the molecule (below 1% of a monolayer) . There is a considerable number of experimental and theoretical studies on the adsorption geometry, the epitaxial growth, the structure, and optical properties of thin PTCDA films …”

“…Thereby, we create the basis for further investigations on other problems like the different local molecule–substrate interactions at different adsorption sides. These different local molecule–substrate interactions lead to inhomogeneous line broadening in the experiment . According to this, information about the local molecule–substrate interaction can be of relevance for technological applications including thin films of fluorescent molecules which are located close to interfaces, like organic photovoltaic cells …”

“…To eliminate the intermolecular interaction, they adsorbed very low concentrations of the molecule (below 1% of a monolayer). [16,17] There is a considerable number of experimental and theoretical studies on the adsorption geometry, the epitaxial growth, the structure, and optical properties of thin PTCDA films. [18][19][20][21][22][23][24][25] In this work, we describe a methodology, based on timedependent density functional theory (TD-DFT) methods, [26] to examine the optical properties of organic p-conjugated molecules adsorbed on ionic surfaces.…”