Abstract
Perfluoropentacene (PFP) is an organic material that has been widely studied over the last years and has already found applications in organic electronics. However, fundamental physical questions, such as the structural formation and the preferential orientation of the molecules during deposition on metal surfaces, are still not fully understood. In this work, we report on a unique in-plane molecular reorientation during the completion of the first monolayer of PFP on the Ag(110) surface. To characterize the molecular alignment, we have monitored the deposition process in real time using polarization-dependent differential reflectance spectroscopy and reflectance anisotropy spectroscopy. Abrupt changes in the optical signals reveal an intricate sequence of reorientation transitions of the PFP molecules upon monolayer completion and during the formation of the second monolayer, eventually leading to a full alignment of the long molecular axis along the [001] direction of the substrate and an enhanced structural ordering. Scanning tunneling microscopy and low-energy electron diffraction confirm the observed molecular reorientation upon monolayer compression and provide further details on the structural and orientational ordering of the PFP monolayer before and after compression.
Highlights
One of the many advantages of organic thin layers is that their optical and electronic properties can be systematically modified by molecular design
The PFP-related peak at 1.7 eV in the reflectance anisotropy spectroscopy (RAS) and p-DR spectra only appears after the completion of the third monolayer
We have explored the adsorption and growth of PFP on Ag(110) by real-time monitoring using two complementary optical techniques, namely pol-differential reflectance spectroscopy (DRS) and RAS
Summary
One of the many advantages of organic thin layers is that their optical and electronic properties can be systematically modified by molecular design. The measured Δr/rspectrum is directly related to the optical anisotropy (Δε = ε[1̅10] − ε[001]) of the sample with respect to these two orthogonal crystallographic axes of the substrate.[11,33] The measurements were carried out with a home-built RAS instrument In this setup, the light beam from a xenon lamp is first linearly polarized such that the polarization axis is oriented at an angle of 45° with respect to the main crystallographic axes of the substrate surface, as marked by an orange arrow in the inset of Figure 1a. Besides the in situ optical characterization during growth, the structure and morphology of the deposited PFP thin films were investigated at a sample temperature of 110 K using a variable temperature scanning tunneling microscope and at room temperature using low-energy electron diffraction (LEED)
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