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While the crystal structure of the polymorph phase can be studied in three dimensions conveniently by X‐ray methods like grazing‐incidence X‐ray diffraction (GIXD), the first monolayer is only accessible by surface‐sensitive methods that allow the determination of a two‐dimensional lattice. Here, GIXD measurements with sample rotation are compared with distortion‐corrected low‐energy electron diffraction (LEED) experiments on conjugated molecules: 3,4;9,10‐perylenetetracarboxylic dianhydride (PTCDA), 6,13‐pentacenequinone (P2O), 1,2;8,9‐dibenzopentacene (trans‐DBPen) and dicyanovinyl‐quaterthiophene (DCV4T‐Et2) grown by physical vapor deposition on Ag(111) and Cu(111) single crystals. For these molecular crystals, which exhibit different crystallographic lattices and crystal orientations as well as epitaxial properties, the geometric parameters of the three‐dimensional lattice are compared with the corresponding geometry of the first monolayer. A comparison of the monolayer lattice from LEED investigations with the multilayer lattices determined by rotated GIXD experiments reveals a correlation between the first monolayer and the epitaxial growth of three‐dimensional crystals together with lattice distortions and re‐alignment of molecules. The selected examples show three possible scenarios of crystal growth on top of an ordered monolayer: (i) growth of a single polymorph, (ii) growth of three different polymorphs; in both cases the first monolayer serves as template. In the third case (iii) strong lattice distortion and distinct molecular re‐alignments from the monolayer to epitaxially grown crystals are observed. This is the second part of our work concerning the correlation between two‐ and three‐dimensional crystallographic lattices for epitaxial analysis. In the first part, the theoretical basis has been derived which provides a mathematical relationship between the six lattice parameters of the three‐dimensional case and the three parameters obtained for the two‐dimensional surface unit cell, together with their orientation to the single‐crystalline substrate. In this work, a combined experimental approach of GIXD and LEED is introduced which can be used to investigate the effect of the epitaxial monolayer on the structural properties of molecular crystals grown on top. [ABSTRACT FROM AUTHOR]

The epitaxial growth of molecular crystals at single‐crystalline surfaces is often strongly related to the first monolayer at the substrate surface. The present work presents a theoretical approach to compare three‐dimensional lattices of epitaxially grown crystals with two‐dimensional lattices of the molecules formed within the first monolayer. Real‐space and reciprocal‐space representations are considered. Depending on the crystallographic orientation relative to the substrate surface, proper linear combinations of the lattice vectors of the three‐dimensional unit cell result in a rhomboid in the xy plane, representing a two‐dimensional projection. Mathematical expressions are derived which provide a relationship between the six lattice parameters of the three‐dimensional case and the three parameters obtained for the two‐dimensional surface unit cell. It is found that rotational symmetries of the monolayers are reflected by the epitaxial order. Positive and negative orientations of the crystallographic contact planes are correlated with the mirror symmetry of the surface unit cells, and the corresponding mathematical expressions are derived. The method is exemplarily applied to data obtained in previous grazing‐incidence X‐ray diffraction (GIXD) measurements with sample rotation on thin films of the conjugated molecules 3,4;9,10‐perylenetetracarboxylic dianhydride (PTCDA), 6,13‐pentacenequinone (P2O), 1,2;8,9‐dibenzopentacene (trans‐DBPen) and dicyanovinyl‐quaterthiophene (DCV4T‐Et2) grown by physical vapor deposition on Ag(111) and Cu(111) single crystals. This work introduces the possibility to study three‐dimensional crystal growth nucleated by an ordered monolayer by combining two different experimental techniques, GIXD and low‐energy electron diffraction, which has been implemented in the second part of this work. [ABSTRACT FROM AUTHOR]

The epitaxial growth of the mono-functionalized para-quaterphenylene molecule CNHP4 on muscovite mica is investigated. The vacuum deposited molecules aggregate into nanofibers of varying morphology. Due to muscovite's cm symmetry, almost mutually parallel fibers grow. Polarized light microscopy together with X-ray diffraction resolves the projected orientation of the molecules on the substrate surface and within the fibers. Several different contact planes with the substrate are detected. For all of them, the molecules orient with their long molecule axis approximately perpendicular to the grooved muscovite direction, so that the alignment of the molecules on the substrate is uniform. Kelvin probe force microscopy finds vastly different electrostatic properties of different fiber types and facets. [ABSTRACT FROM AUTHOR]

Crystal structure identification of thin organic films entails a number of technical and methodological challenges. In particular, if molecular crystals are epitaxially grown on single‐crystalline substrates a complex scenario of multiple preferred orientations of the adsorbate, several symmetry‐related in‐plane alignments and the occurrence of unknown polymorphs is frequently observed. In theory, the parameters of the reduced unit cell and its orientation can simply be obtained from the matrix of three linearly independent reciprocal‐space vectors. However, if the sample exhibits unit cells in various orientations and/or with different lattice parameters, it is necessary to assign all experimentally obtained reflections to their associated individual origin. In the present work, an effective algorithm is described to accomplish this task in order to determine the unit‐cell parameters of complex systems comprising different orientations and polymorphs. This method is applied to a polycrystalline thin film of the conjugated organic material 6,13‐pentacenequinone (PQ) epitaxially grown on an Ag(111) surface. All reciprocal vectors can be allocated to unit cells of the same lattice constants but grown in various orientations [sixfold rotational symmetry for the contact planes (102) and (102)]. The as‐determined unit cell is identical to that reported in a previous study determined for a fibre‐textured PQ film. Preliminary results further indicate that the algorithm is especially effective in analysing epitaxially grown crystallites not only for various orientations, but also if different polymorphs are present in the film. [ABSTRACT FROM AUTHOR]

Defined crystal growth is highly demanded for technological applications but also fundamental research. Within this work, the crystal growth of the asymmetric molecule caffeine was studied on single crystalline surfaces of muscovite mica, sodium chloride and potassium chloride. While elongated needle-like crystals grow on muscovite mica and sodium chloride, smaller individual “bird-like” structures were observed on potassium chloride. Depending on the surface type and temperature, the disk-shaped caffeine molecules prefer either an edge-on or flat-on orientation with respect to the surface, but in each case, a defined crystallographic relation between the surface and caffeine crystallites was determined by using the X-ray pole figure technique. On muscovite mica and sodium chloride, needle-like crystallites with edge-on oriented molecules aligned mainly with the unit cell c-axis (which coincides with the long needle axis) along the [1−10]mica, [100]mica, [110]mica and [110]NaCl, [1−10]NaCl directions, respectively. Crystals consisting of flat-on oriented molecules on KCl showed also defined alignments with respect to the substrate, but due to the altered molecule–substrate contact, the b-axis aligned along [110]KCl and [1−10]KCl. Growth at elevated temperatures enabled changes in the crystal growth whereby more defined structures formed on NaCl. On KCl, the bird-like structures remained very similar, while caffeine on the mica surface at elevated temperatures resulted in even additional texture forming with the caffeine molecules now also favoring a flat-on orientation with respect to the surface. The systematic variation of various system parameters demonstrates how sensitive the growth behavior of caffeine on this variety of substrates is. [ABSTRACT FROM AUTHOR]

Abstract: This work focuses on the structural and morphological studies of the evolution of epitaxially grown para-sexiphenyl (p-6P) thin films on freshly cleaved KCl(100) surfaces. The film growth is varied using growth times between 5sec and 60min and substrate temperatures in the range of 80–150°C. The study is based on a combination of X-ray diffraction to derive structural information on the molecular scale, and atomic force as well as electron microscopy to obtain morphological information. The initial growth orientation of p-6P crystallites is identified to be -oriented, which consists of edge-on molecules aligned parallel to 〈011〉KCl forming needles that are also parallel to these directions. Furthermore it is shown that surface step edges do not affect the needle growth, but those needles even overgrow step edges. The subsequent epitaxial growth turned out to be crucially dependent on the substrate temperature with respect to the quantitative ratios of material in different growth orientations, their in-plane alignment, the size of crystallites and the mosaicity of the films. It is shown that some crystallites start their growth using the initially grown needles as seeds, resulting in a different crystal orientation. For such crystallites a realignment of needles during their growth is identified, which is found to be strongly dependent on the growth temperature. Furthermore the heights of needles show distinct temperature dependence. In addition well-aligned terraced islands formed by standing molecules were observed. Finally a growth model for this system can be elucidated. [Copyright &y& Elsevier]

A 30 nm thick quinquephenyl (5P) film was grown by molecular beam deposition on a Cu(110)(2×1)O single crystal surface. The thin film morphology was studied by light microscopy and atomic force microscopy and the crystallographic structure of the thin film was investigated by X-ray diffraction methods. The 5P molecules crystallise epitaxially with (201)5P parallel to the substrate surface (110)Cu and with their long molecular axes parallel to [001]Cu. The observed epitaxial alignment cannot be explained by lattice matching calculations. Although a clear minimum in the lattice misfit exists, it is not adapted by the epitaxial growth of 5P crystals. Instead the formation of epitaxially oriented crystallites is determined by atomic corrugations of the substrate surface, such that the initially adsorbed 5P molecules fill with its rod-like shape the periodic grooves of the substrate. Subsequent crystal growth follows the orientation and alignment of the molecules taken within the initial growth stage. [ABSTRACT FROM AUTHOR]

Here, it is shown that pentacene thin films (30 nm) with distinctively different crystallographic structures and molecular orientations can be grown under essentially identical growth conditions in UHV on clean Cu(110) surfaces. By X-ray diffraction, we show that the epitaxially oriented pentacene films crystallize either in the "thin film" phase with standing molecules or in the "single crystal" structure with molecules lying with their long axes parallel to the substrate. The morphology of the samples observed by atomic force microscopy shows an epitaxial alignment of pentacene crystallites, which corroborates the molecular orientation observed by X-ray diffraction pole figures. Low energy electron diffraction measurements reveal that these dissimilar growth behaviors are induced by subtle differences in the monolayer structures formed by slightly different preparation procedures. This study investigates epitaxially oriented pentacene films grown on Cu(110) surfaces crystallizing either in the "thin film" phase with standing molecules or in the "single crystal" structure with molecules lying with their long axes parallel to the substrate. [ABSTRACT FROM AUTHOR]

During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule-surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model. [ABSTRACT FROM AUTHOR]

Hot-wall epitaxy and molecular-beam epitaxy have been employed for growing quaterthiophene thin films on the (010) cleavage face of potassium hydrogen phthalate, and the results are compared in terms of film properties and growth mode. Even if there is no geometrical match between substrate and overlayer lattices, these films are epitaxially oriented. To investigate the physical rationale for this strong orientation effect, optical microscopy, atomic force microscopy, and X-ray diffraction are employed. A clear correlation between the morphology of the thin films and the crystallographic orientation is found. The results are also validated by surface potential calculations, which demonstrate the primary role played by the corrugation of the substrate surface. [ABSTRACT FROM AUTHOR]