The hexagonal boron nitride (hBN) monolayer on the Cu(111) surface has recently been considered an example of an extremely weak hBN/metal interaction, as indicated, e.g., from the presence of an only electronic Moiré-like superstructure that was observed in scanning tunneling microscopy images. From these results, a large bonding distance of the hBN sheet to the topmost Cu layer can be envisaged but has not been proven so far. We report a structural analysis of the hBN/Cu(111) interface based on high resolution low energy electron diffraction and normal incidence X-ray standing wave experiments. We find that both the boron and nitrogen atoms are located at very large vertical distances of dB = 3.25 ± 0.02 Å and dN = 3.22 ± 0.03 Å with respect to the nominal position of the topmost Cu(111) layer. Significant vertical buckling and lateral distortions of the hBN layer can be excluded. These results demonstrate that the hBN monolayer on the Cu(111) surface is indeed well described by a rigid and geometrically well separated sheet.

The formation of intermolecular complexes of two large molecules—a macrocycle and a semiaxle, which have been used in templated syntheses of amide rotaxanes—was studied by scanning tunneling microscopy (STM) and density functional theory (DFT). These experiments mimic the so-called “threading process”, which is based on intermolecular recognition and which is essential for the rotaxane synthesis in solution. First, ordered monolayers of a tetralactam macrocycle (TLM), i.e. the rotaxane wheel, are prepared on a Au(111) surface. Then, semiaxles (SA) are deposited on top of these ordered TLM layers at ca. 140 K. In solution, the SA molecule threads into the TLM cavity by formation of three hydrogen bonds between the amide groups of both molecules. On the Au(111) surface, the scenario is similar, although different in detail due to geometric restrictions given by the underlying Au(111) surface and conformational energy barriers due to the confinement of the TLM geometry in the ordered monolayer structure. Three distinct and defined adsorption sites of the SA molecules with respect to the TLM molecules exist. Notably, the population of these sites is assisted by interaction with the STM tip. Two sites are compatible with a structural model, in which the tail of the SA molecule binds into the TLM cavity, in one case with three H bonds, one to the terminal NH2 group of the SA and two to the central amide group. This SA—TLM adsorption complex formed at low temperatures is metastable and dissociates at higher temperatures. These results demonstrate the possibility to study intermolecular complex formation by STM.

We report a detailed analysis of the line shapes of the 0–0 transitions in the fluorescence (FL) and fluorescence excitation (FLE) spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules adsorbed at terrace and step edge sites of (100)-oriented alkali halide films (KCl and NaCl). At low temperatures (6–20 K), we find very narrow FLE lines with a fwhm of 4.5 cm–1 (3.0 cm–1) on KCl (NaCl). These line shapes are dominated by inhomogeneous broadening related to the structural variation of the environment of the PTCDA molecules. We explain this site broadening on the basis of structural models for the adsorption sites at the step edges from earlier scanning microscopy data and density functional theory calculations. With increasing temperatures, the 0–0 lines in the FL and FLE spectra broaden; e.g., in the FL, the fwhm increases to 26 cm–1 (18 cm–1) at 100 K on the KCl (NaCl) surface. This temperature induced broadening is of Lorentzian shape and can be described by the theory of Hsu and Skinner, based on dephasing by coupling to acoustic phonons of the substrate. Discrepancies remain for experimentally observed small line shifts. We discuss how surfaces can be used and optimized as sample systems for a highly resolved optical spectroscopy of molecules.

•Heteroepitaxial growth of NaCl on KCl(100) analysed by SPA-LEED at 300 K and 500 K•Multilayer island formation deduced from SPA-LEED profiles•Model for limited lateral island growth suggested from misfit at interfaceWe investigated the growth of NaCl on thin (100)-oriented films of KCl by spot profile analysis of low energy electron diffraction (SPA-LEED). The underlying question of this investigation was how the system accommodates to the misfit of − 10% between the NaCl and KCl lattices. The KCl films (3 atomic layers thick) were epitaxially grown on a Ag(100) single crystal. We studied the heteroepitaxial growth of NaCl on KCl at 300 K and at 500 K, respectively. At 300 K, the first NaCl monolayer (ML) grows pseudomorphically on the KCl film. From the second layer onward, the NaCl lattice relaxes. The NaCl multilayers roughen, and a small rotational disorder (± 4°) of the NaCl domains is observed. The roughening results from the formation of multilayer islands of limited lateral size due to the misfit to the pseudomorphic first NaCl layer. At a growth temperature of 500 K, no pseudomorphic NaCl layer forms, instead relaxed multilayer island growth of NaCl is observed from the first layer onward. Similarly to the growth at 300 K, we find NaCl multilayer islands of limited lateral size. For both temperatures, we explain this growth behavior by the misfit that makes the adsorption sites at the island edges of the first relaxed NaCl layer less favorable for larger islands, promoting nucleation of multilayer islands.

We report on ordered binary monolayer structures consisting of a sulfur-containing π-conjugated molecule, namely, tetrabenzo thianthrene (TBTA) or tetrathiatetracene (TTT) as a donor, and tetracyano naphtho quinodimethane (TNAP) as an acceptor on the Au(111) surface. The investigations were performed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and additional density functional theory (DFT) calculations for TTT/TNAP. Both pairs of molecules (TTT/TNAP and TBTA/TNAP) form long-range ordered, commensurate structures on the Au(111) surface with a 1:1 stoichiometry. The structures consist of alternating rows containing only one type of molecule. The TNAP rows are stabilized by hydrogen bonds. Submolecular resolved STM images indicate a net charge transfer from the donor molecules to the TNAP molecules. The reconstruction of the Au(111) surface is modified upon the formation of the ordered binary structures, pointing to a significant surface–molecule interaction. For TTT/TNAP, the surface interaction leads to bonds of the S atoms to the Au atoms. For TBTA/TNAP, there exists an additional porous, TBTA-rich structure in which TBTA builds the framework.

Scanning tunneling microscopy at low temperature and low molecular coverage (∼1%) was combined with density-functional theory calculations to investigate the adsorption of isolated perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) molecules on KCl(100) surfaces. Experimentally, we used epitaxial thin KCl(100) films on Ag(100) of about 3 layers in thickness. After deposition at 100 K, the PTCDA molecules are statistically distributed on the terraces with an azimuthal orientation of the long axis along the polar ⟨110⟩ orientation. After annealing at about 150 to 200 K the molecules are exclusively found at step-edge sites. Thereby, several configurations are observed, the most typical being a site where the PTCDA molecules protrude into the step edge, forming vacancies at the step edge. The total-energy calculations predict this step-edge adsorption site to be energetically favorable compared to the adsorption on terraces. The corrugation of the calculated potential energy surface is below 1 eV, with diffusion barriers of about 0.6 eV, which explains the mobility of the PTCDA molecules at elevated temperatures.

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.

We report experiments by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) on ordered structures of two sulfur-containing π-conjugated molecules on the Au(111)-surface, namely tetrabenzothianthrene (TBTA) and tetrathiotetracene (TTT). These molecules are candidates for donors in charge transfer salts. On Au(111) both molecules form long-range ordered structures that are commensurate to the top-most surface layer. For TBTA, the reconstruction of the Au(111) surface is maintained, whereas it is lifted by TTT. Both molecules lie flat on the surface. For TBTA, the structure indicates that the molecule is planarized upon adsorption.Highlights► Tetrabenzothianthrene and tetrathiotetracene form long-range ordered commensurate structures on the Au(111) surface ► For TBTA, the reconstruction of the Au(111) surface is maintained. ► The reconstruction is lifted by TTT. ► Both molecules adsorb intact and in a planar configuration. ► For TBTA, the structure indicates that the molecule is planarized upon adsorption.

We investigated the epitaxial growth of thin KCl films on Ag(100) by spot-profile-analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). The structural relation of the (100)-oriented KCl film with respect to the Ag(100) surface is incommensurate, nevertheless the structural quality is very high and terraces with an average diameter of 250 Å are obtained. The unit cells of KCl and Ag(100) are aligned, and there is no rotational mosaicity as present in the case of NaCl on Ag(100). We attribute this to a small interaction between KCl and Ag(100) and growth starting at step edges of the metal substrate. In order to demonstrate the high structural quality of the KCl films, we deposited a monolayer of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on these films. We obtained the identical monolayer structure that was observed earlier on bulk KCl. We thus suggest that KCl/Ag(100) is ideal for surface experiments on thin dielectric films.Research highlights► Thin KCl films of high structural quality on Ag(100). ► Incommensurate growth with aligned unit cells. ► High mobility of KCl molecules leads to step nucleation growth. ► Electron based experiments are possible on these films. ► PTCDA monolayers form a brickwall structure on the films.

Quinacridone (QA), a highly stable red dye, was investigated for the fabrication of organic field effect transistors (OFETs) by vacuum sublimation. Field effect mobilities for holes of 10−6 to 10−3 cm2/Vs were found, depending on the purity of the QA and the substrate temperature during the sublimation. The latter influences the structural phase composition of the films. The temperature dependence of the mobilities exhibits a maximum (μ = (1.5 ± 0.2) × 10−3 cm2/Vs) at ∼270 K. Over a period of 50 days, the mobilities are stable and even slightly increase, although the OFETs were exposed to air between measurements.

The structural order of 1,4,9,10-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) monolayers on Ag(1 1 1) has been investigated by spot profile analysis low energy electron diffraction (SPA-LEED). For increasing coverage, we find a sequence of three highly ordered structures: a commensurate structure (α), a uniaxially incommensurate structure (α2), and an incommensurate structure (β) with coverages of 0.9 ML, 0.95 ML, and 1 (saturated) monolayer (ML), respectively. In the high coverage regime, the structures coexist and a coverage increase causes a change of their relative fractions. The α and β structures were known before [U. Stahl, D. Gador, A. Soukopp, R. Fink, E. Umbach, Surf. Sci. 414 (1998) 423], but the β structure was proposed as commensurate, since its very small misfit with respect to a commensurate structure could not be resolved. This misfit leads to a periodic modulation, causing additional Moiré satellites in the diffraction pattern. This finding demonstrates the importance of high resolution methods for the geometry determination of large organic adsorbates.

The superstructures (H1 and H2) of perylenetetracarboxylicacid-dianhydride (PTCDA) monolayers on the (22×3) reconstructed Au(1 1 1) surface reported by Mannsfeld et al. [S. Mannsfeld, M. Toerker, T. Schmitz-Hübsch, F. Sellam, T. Fritz, K. Leo, Org. Electron. 2 (2001) 121] are reinvestigated by high resolution low energy electron diffraction (SPALEED). The purpose of this investigation is to elucidate in detail the point-on-line (p-o-l) structural relation of the homogeneous adsorbate layer with respect to the three reconstruction domains of the Au(1 1 1) surface. For the H2 (non-equilibrium) structure, diffraction patterns without a spot splitting are found, indicating an almost perfect undisturbed overgrowth of the domain boundaries of the (22×3) reconstruction. As a consequence, the superstructure does not follow strictly the p-o-l relation, but only in an averaged manner. Contrary for the H1 phase (equilibrium structure), a spot splitting is observed, which indicates that superstructure domains on different Au(1 1 1) reconstruction domains exhibit slightly different angular orientations, thus adopting p-o-l relations on all Au(1 1 1) reconstruction domains. The PTCDA layer on Au(1 1 1) may be thus considered as a rigid or distorted carpet, depending on the preparation conditions.

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.

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. Using very low coverages, isolated molecules were adsorbed on terrace sites or at sites located at residual steps on (100) oriented alkali halide films (KCl and NaCl). The low energy modes couple to the optical transition only because the PTCDA molecule is geometrically distorted (C2v) upon adsorption on the surface; they would be absent for the parent planar (D2h) PTCDA molecule. The modes differ in number and energy for molecules adsorbed on regular terrace sites and molecules adsorbed at step edge sites. Modes appearing for step edge sites have the character of frustrated rotations. Their coupling to the optical transition is a consequence of the further reduced symmetry of the step edge sites. We find a larger number of vibrational modes on NaCl than on KCl. We explain this by the stronger electrostatic bonding of the PTCDA on NaCl compared to KCl. It causes the optical transition to induce stronger changes in the molecular coordinates, thus leading to larger Franck–Condon factors and thus stronger coupling. Our results demonstrate how optical spectroscopy can be used to gain information on adsorption sites of molecules at low surface concentrations.