Molecular Reorientation during the Initial Growth of Perfluoropentacene on Ag(110)

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.

Superradiance from Two Dimensional Brick-Wall Aggregates of Dye Molecules: The Role of Size and Shape for the Temperature Dependence

Aggregates of interacting molecules can exhibit electronically excited states that are coherently delocalized over many molecules. This can lead to a strong enhancement of the fluorescence decay rate which is referred to as superradiance (SR). To date, the temperature dependence of SR is described by a 1/T law. Using an epitaxial dye layer and a Frenkel-exciton based model we provide both experimental and theoretical evidence that significant deviations from the 1/T behavior can occur for brick-wall-type aggregates of finite size leading even to a maximum of the SR at finite temperature. This is due to the presence of low energy excitations of weak or zero transition strength. These findings are relevant for designing light-emitting molecular materials.

A surface science compatible epifluorescence microscope for inspection of samples under ultra high vacuum and cryogenic conditions

We modified an epi-illumination light microscope and mounted it on an ultra high vacuum chamber for investigating samples used in a surface science experiment. For easy access and bake out, all optical components are placed outside the vacuum and the sample is imaged through a glass window. The microscope can be operated in reflection brightfield or epifluorescence mode to image the sample surface or fluorescent dye molecules adsorbed on it. The homemade sample mounting was made compatible for the use under the microscope; sample temperatures as low as 6 K can be achieved. The performance of the microscope is demonstrated on two model samples: Brightfield-images of a well-prepared Ag(100) surface show a macroscopic corrugation of the surface, although low energy electron diffraction data indicate a highly ordered crystalline surface. The surface shows macroscopic protrusions with flat regions, about 20-200 μm in diameter, in between. Fluorescence images of diluted 3,4,9,10-perylene tetracarboxylicacid dianhydride (PTCDA) molecules adsorbed on an ultrathin epitaxial KCl film on the Ag(100) surface show a shading effect at surface protrusions due to an inclined angle of incidence of the PTCDA beam during deposition. For some preparations, the distribution of the fluorescence intensity is inhomogeneous and shows a dense network of bright patches about 5 μm in diameter related to the macroscopic corrugation of the surface. We propose that such a light microscope can aid many surface science experiments, especially those dealing with epitaxial growth or fluorescent materials.

Surface induced vibrational modes in the fluorescence spectra of PTCDA adsorbed on the KCl(100) and NaCl(100) surfaces

We report a combined experiment-theory study on low energy vibrational modes in fluorescence spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules.

Polarization-dependent differential reflectance spectroscopy for real-time monitoring of organic thin film growth

By monitoring the reflectance of a sample surface during deposition of a thin organic film, one can obtain information with submonolayer resolution in real-time. A special kind of optical spectroscopy is Differential Reflectance Spectroscopy (DRS), which compares the reflectance before and during deposition of a thin film or any other change of the surface optical properties. In this work, we present an extended DRS setup that allows monitoring simultaneously both linear polarization states (s and p) of the reflected light. We implement polarization-dependent DRS to monitor the growth of perflouropentacene thin films on a Ag(110) single crystal. The setup allows us to deduce the optical anisotropy of the sample and, in particular, the preferred orientation of the molecules on the surface.

van der Waals-Induced Chromatic Shifts in Hydrogen-Bonded Two-Dimensional Porphyrin Arrays on Boron Nitride

The fluorescence of a two-dimensional supramolecular network of 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) adsorbed on hexagonal boron nitride (hBN) is red-shifted due to, primarily, adsorbate-substrate van der Waals interactions. TCPP is deposited from solution on hBN and forms faceted islands with typical dimensions of 100 nm and either square or hexagonal symmetry. The molecular arrangement is stabilized by in-plane hydrogen bonding as determined by a combination of molecular-resolution atomic force microscopy performed under ambient conditions and density functional theory; a similar structure is observed on MoS2 and graphite. The fluorescence spectra of submonolayers of TCPP on hBN are red-shifted by ∼30 nm due to the distortion of the molecule arising from van der Waals interactions, in agreement with time-dependent density functional theory calculations. Fluorescence intensity variations are observed due to coherent partial reflections at the hBN interface, implying that such hybrid structures have potential in photonic applications.

Diffusion coefficients from signal fluctuations: influence of molecular shape and rotational diffusion

Analysis of signal fluctuations of a locally fixed probe, caused by molecules diffusing under the probe, can be used to determine diffusion coefficients. Theoretical treatments so far have been limited to point-like particles or to molecules with circle-like shapes. Here we extend these treatments to molecules with rectangle-like shapes, for which also rotational diffusion needs to be taken into account. Focusing on the distribution of peak widths in the signal, we show how translational as well as rotational diffusion coefficients can be determined. We address also the question, how the distribution of interpeak time intervals and autocorrelation function can be employed for determining diffusion coefficients. Our approach is validated against kinetic Monte Carlo simulations.