Directed growth of quinacridone chains on the vicinal Ag(35 1 1) surface.

The formation of self-assembled domains and chains of monomolecular width of quinacridone (QA) on the vicinal Ag(35 1 1) surface was investigated by scanning tunneling microscopy and low-energy electron diffraction. The focus was on the influence of the steps on the QA structures and their preferential azimuthal orientations with the aim of achieving a selective orientation. After deposition at a sample temperature of 300 K, QA forms the same kind of molecular chains as on the nominally flat Ag(100) surface because of strong intermolecular hydrogen bonds, which we reported in a previous publication [Humberg, N.; Bretel, R.; Eslam, A.; Le Moal, E.; Sokolowski, M. J. Phys. Chem. C 2020, 124, 24861-24873]. The vicinal surface leads to one additional chain orientation, which is parallel to the Ag step edges. However, most chains nucleate on the Ag terraces between steps with four distinct azimuthal orientations that are identical to those on Ag(100), and which are determined by the interactions with the (100) surface. At 300 K, the chains grow across the Ag steps, which do not break the azimuthal chain orientations. In contrast, during the deposition at sample temperatures of 400 and 500 K, the nucleation of the chains takes place at the Ag step edges. Hence, these have a strong influence on the azimuthal orientation of the molecules, resulting in a preferential growth of the chains in two of the four azimuthal orientations. We explain this by the adaptation of favorable adsorption sites, which involve the replacement of Ag atoms by QA molecules with specific azimuthal orientations at the step edges.

Increasing the scan speed in high resolution, low energy electron diffraction measurements by presetting the gate time.

We report on a speed-up data acquisition routine for recording intensities in reciprocal space (k-space) with increased scan speed by a single point detector. It is designed for recording low energy electron diffraction (LEED) data with high resolution by a spot profile analysis LEED instrument. It counteracts the problem of long acquisition times that are encountered when larger areas in the reciprocal space are scanned. It exploits the fact that in typical LEED images of ordered surfaces, more than 90% of the data points in k-space belong to the low-intense background, which is often not of interest. Only about 10% of the data points are related to the relevant diffraction features, namely, the LEED spots. Often it is not necessary to measure the background with the same statistical significance as measuring those points that contain information. The data points belonging to the LEED spots can be discriminated from those of the background by their higher intensities. An acquisition routine that sets an increased gate time for the counting in response to higher intensities thus safeguards good statistics for data points of the LEED spots and saves measurement time when recording data points of the background with small gate times. For typical LEED images, a reduction of the total acquisition time by a factor of about 10 is obtained. We give examples of one- and two-dimensional scans from current experiments, recorded with and without the speed-up routine. We further discuss how the routine supports the measurement of energy dependent reciprocal space maps.

Chiral self-organized single 2D-layers of tetramers from a functional donor-acceptor molecule by the surface template effect.

The ability to control the structural properties of molecular layers is a key for the design and preparation of organic electronic devices. While microscopic growth studies of planar, rigid and symmetric π-conjugated molecules have been performed to a larger extent, this is less the case for elongated donor-acceptor molecules with flexible functional groups, which are particularly interesting due to their high dipole moments. Prototypical molecules of this type are merocyanines (MCs), which have been widely studied for the use as efficient absorbers in organic photodetectors. For maximized light absorption and optimized electronic properties the molecular arrangement which is affected by the initial assembly of the films at the supporting substrate interface is decisive. The situation deserves special attention, when the surface nucleation leads to so far not known and bulk-unlike aggregates. Here, we report on the growth of a typical MC (HB238) on the Ag(100) surface, serving as the substrate. In the energetically preferred phase, the molecules adsorb in a face-on geometry and organize in tetramers with a circular dipole arrangement. The tetramers further self-order in large, enantiopure domains with a periodicity that is commensurate to the Ag(100) surface, likely due to a specific bonding of the thiophene and thiazol rings to the Ag surface. Using scanning tunneling microscopy (STM) in combination with low energy electron diffraction we derive the detailed structure of the tetramers. The center of the tetramer, which is most prominent in STM images, consists of four upward pointing tert-butyl groups from four molecules. It is encircled by a ring of four hydrogen bonds between terminal CN-groups and thiophene rings on neighboring molecules. In parallel, the surface interaction modifies the intramolecular dipole, which is revealed from photoemission spectroscopy. Hence, this example shows how the surface template effect leads to an unforeseen molecular organization which is considerably more complex compared to that in the bulk phases of HB238, which feature paired dipoles.

The influence of an interfacial hBN layer on the fluorescence of an organic molecule.

We investigated the ability of a single layer of hexagonal boron nitride (hBN) to decouple the excited state of the organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) from the supporting Cu(111) surface by Raman and fluorescence (FL) spectroscopy. The Raman fingerprint-type spectrum of PTCDA served as a monitor for the presence of molecules on the surface. Several broad and weak FL lines between 18,150 and 18,450 cm-1 can be detected, already from the first monolayer onward. In contrast, FL from PTCDA on a bare Cu(111) surface is present only from the second PTCDA layer onward. Hence, a single layer of hBN decouples PTCDA from the metal substrate to an extent that a weak radiative FL decay of the optical excitation can occur. The different FL lines can be ascribed to different environments of the adsorption sites, namely molecules adsorbed at surface defects, in large ordered domains, and located in the second layer.

Pure and mixed ordered monolayers of tetracyano-2,6-naphthoquinodimethane and hexathiapentacene on the Ag(100) surface.

We report on mixed ordered monolayers of the electron acceptor-type molecule tetracyano-2,6-naphthoquinodimethane (TNAP) and the electron donor-type molecule hexathiapentacene (HTPEN). This investigation was motivated by the general question which type of mixed stoichiometric structures are formed on a surface by molecules that are otherwise typically used for the synthesis of bulk charge-transfer materials. The layers were obtained by vacuum deposition on the Ag(100) surface and analyzed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The formation of the mixed structure occurs spontaneously. An important motif for the structure formation is given by hydrogen bonds between the TNAP molecules. Both molecules, TNAP and HTPEN also form well-ordered monolayers on the Ag(100) surface on their own. In all structures, the molecules are adsorbed in a planar orientation on the surface. We discuss the influence of intermolecular charge transfer on the ordering in the mixed structure.

On the decoupling of molecules at metal surfaces.

We report a method to achieve physical and electronic decoupling of organic molecules from a metal surface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu-O covalent bonds. This results in electronic surface hardening and prevents charge transfer from the metal into perylene-tetracarboxylic dianhydride molecules subsequently deposited on this surface.

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.

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.

Lateral ordering of PTCDA on the clean and the oxygen pre-covered Cu(100) surface investigated by scanning tunneling microscopy and low energy electron diffraction.

We have investigated the adsorption of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on the clean and on the oxygen pre-covered Cu(100) surface [referred to as (√2 × 2√2)R45° - 2O/Cu(100)] by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Our results confirm the (4√2 × 5√2)R45° superstructure of PTCDA/Cu(100) reported by A. Schmidt et al. [J. Phys. Chem. 1995, 99,11770-11779]. However, contrary to Schmidt et al., we have no indication for a dissociation of the PTCDA upon adsorption, and we propose a detailed structure model with two intact PTCDA molecules within the unit cell. Domains of high lateral order are obtained, if the deposition is performed at 400 K. For deposition at room temperature, a significant density of nucleation defects is found pointing to a strong interaction of PTCDA with Cu(100). Quite differently, after preadsorption of oxygen and formation of the (√2 × 2√2)R45° - 2O/Cu(100) superstructure on Cu(100), PTCDA forms an incommensurate monolayer with a structure that corresponds well to that of PTCDA bulk lattice planes.

Finite size line broadening and superradiance of optical transitions in two dimensional long-range ordered molecular aggregates.

The width and asymmetry of the line shape of the optical transition of a sample of two dimensional (2D) molecular J-aggregates was found to be related to a finite-size effect. The 2D aggregates were domains of the ordered monolayer of the fluorescent dye molecule 3,4,9,10-perylenetetracarboxylic acid dianhydride on a KCl(100) surface. Fluorescence and fluorescence excitation (FLE) spectra were measured as a function of temperature. The system shows a pronounced superradiant emission which yields additional information on the number of coherently coupled molecules participating in the emission. From calculations of the spectra within the tight binding model we find that the finite size of the 2D ordered domains of about N = 7 × 7 molecules, in combination with a Poissonian domain-size distribution, explains the line profile. Line broadening mechanisms due to site disorder or thermal effects--although not excludable straightaway--are not needed to explain the observed FLE line profile. This yields insight into the important, but so far not well understood, relation between the line profile and the size of ordered molecular aggregates.

Spectroscopy of isolated PTCDA molecules on the KCl(100) surface: vibrational spectra and azimuthal orientation.

Small amounts of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) were vacuum deposited on epitaxial KCl films on Ag(100). The use of a low substrate temperature (20 K) during deposition hampered molecular diffusion resulting in isolated monomers on the surface. Fluorescence and fluorescence excitation spectroscopy performed on these monomers yielded highly resolved spectra with narrow lines corresponding to individual vibronic modes. This high resolution in our spectra is caused by a very small inhomogeneous broadening due to well-defined adsorption sites of the molecule on the substrate. Indeed, by polarization dependent fluorescence spectroscopy we show that the flat-lying molecules exhibit a preferred azimuthal orientation on the surface, the long molecular axis being oriented along the [011] or the equivalent [011] direction of the substrate. Furthermore, the high resolution in the spectra allowed a detailed analysis of the vibronic modes. The vibrational modes of the adsorbed molecule are very similar to those of the free PTCDA molecule, but due to the presence of the substrate additional low energy modes which are relevant for the full understanding of the spectra couple to the transition.

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

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.

Conformational flexibility of tetralactam macrocycles and their intermolecular hydrogen-bonding patterns in the solid state.

Despite their rigid scaffold, tetralactam macrocycles (TLMs) display a remarkable degree of conformational flexibility, as revealed by analysis of the corresponding X-ray crystal structures. This flexibility is not limited to the rotatability of the TLM amide groups but also applies to the m-xylene rings, and it thus has a great impact on the overall shape of the macrocycle cavity. The conformational properties of the TLMs give rise to a broad variety of intermolecular hydrogen-bonding patterns, including infinite ladders, an interesting catemer motif, and short C-HO=C hydrogen bonds. These results are in accord with previous theoretical calculations, support a structural model proposed earlier for an interpretation of scanning tunneling microscopy images, and substantially contribute to the understanding of the adaptability of macrocyclic scaffolds, which is crucial for guest binding or templated syntheses with TLMs.