Correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. II. Experimental results.

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.

Nonintuitive Surface Self-Assembly of Functionalized Molecules on Ag(111).

The fabrication of nanomaterials involves self-ordering processes of functional molecules on inorganic surfaces. To obtain specific molecular arrangements, a common strategy is to equip molecules with functional groups. However, focusing on the functional groups alone does not provide a comprehensive picture. Especially at interfaces, processes that govern self-ordering are complex and involve various physical and chemical effects, often leading to unexpected structures, as we showcase here on the example of a homologous series of quinones on Ag(111). Naively, one could expect that such quinones, which all bear the same functionalization, form similar motifs. In salient contrast, our joint theoretical and experimental study shows that profoundly different structures are formed. Using a machine-learning-based structure search algorithm, we find that this is due to a shift of the balance of three antagonizing driving forces: adsorbate-substrate interactions governing adsorption sites, adsorbate-adsorbate interactions favoring close packing, and steric hindrance inhibiting certain otherwise energetically beneficial molecular arrangements. The theoretical structures show excellent agreement with our experimental characterizations of the organic/inorganic interfaces, both for the unit cell sizes and the orientations of the molecules within. The nonintuitive interplay of similarly important interaction mechanisms will continue to be a challenging aspect for the design of functional interfaces. With a detailed examination of all driving forces, we are, however, still able to devise a design principle for self-assembly of functionalized molecules.

Fraternal twins: distinction between PbPc and SnPc by their switching behaviour in a scanning tunnelling microscope.

In this contribution, we compare the optical absorbance behaviour and the structural properties of lead(II)-phthalocyanine (PbPc) and tin(II)-phthalocyanine (SnPc) thin films. To this end, we employ a Ag(1 1 1) substrate terminated with a monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride constituting an internal interface whose main effect is an electronic decoupling of the phthalocyanine adlayer from the metal surface. As deduced from low-energy electron diffraction and scanning tunnelling microscopy (STM) measurements, the epitaxial relations and unit cell compositions of the prevailing PbPc monolayer and multilayer domains are confusingly similar to those of SnPc on PTCDA/Ag(1 1 1). However, SnPc and PbPc can be readily distinguished by their STM-induced switching behaviours: while the former is capable of reversible configurational changes, no effect on the latter could be achieved by us under comparable conditions. This corroborates earlier theoretical predictions and even renders the chemical identification of individual shuttlecock-shaped metal-phthalocyanines feasible.

Influence of Film and Substrate Structure on Photoelectron Momentum Maps of Coronene Thin Films on Ag(111).

Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) was measured for one-monolayer coronene films deposited on Ag(111). The (kx ,ky )-dependent photoelectron momentum maps (PMMs), which were extracted from the ARUPS data by cuts at fixed binding energies, show finely structured patterns for the highest and the second-highest occupied molecular orbitals. While the substructure of the PMM main features is related to the 4 × 4 commensurate film structure, various features with three-fold symmetry imply an additional influence of the substrate. PMM simulations on the basis of both free-standing coronene assemblies and coronene monolayers on the Ag(111) substrate confirm a sizable molecule-molecule interaction because no substructure was observed for PMM simulations using free coronene molecules.

Flexible 2D Crystals of Polycyclic Aromatics Stabilized by Static Distortion Waves.

The epitaxy of many organic films on inorganic substrates can be classified within the framework of rigid lattices which helps to understand the origin of energy gain driving the epitaxy of the films. Yet, there are adsorbate-substrate combinations with distinct mutual orientations for which this classification fails and epitaxy cannot be explained within a rigid lattice concept. It has been proposed that tiny shifts in atomic positions away from ideal lattice points, so-called static distortion waves (SDWs), are responsible for the observed orientational epitaxy in such cases. Using low-energy electron diffraction and scanning tunneling microscopy, we provide direct experimental evidence for SDWs in organic adsorbate films, namely hexa-peri-hexabenzocoronene on graphite. They manifest as wave-like sub-Ångström molecular displacements away from an ideal adsorbate lattice which is incommensurate with graphite. By means of a density-functional-theory based model, we show that, due to the flexibility in the adsorbate layer, molecule-substrate energy is gained by straining the intermolecular bonds and that the resulting total energy is minimal for the observed domain orientation, constituting the orientational epitaxy. While structural relaxation at an interface is a common assumption, the combination of the precise determination of the incommensurate epitaxial relation, the direct observation of SDWs in real space, and their identification as the sole source of epitaxial energy gain constitutes a comprehensive proof of this effect.

Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime.

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the first ML with identical unit cell dimensions and orientation but slightly inclined molecules. This suggests that the first ML acts as a template for the epitaxial growth of further layers. Simultaneously, a significant number of second-layer molecules mainly located at step edges or scattered over narrow terraces do not form highly ordered aggregates.

The complex polymorphism and thermodynamic behavior of a seemingly simple system: naphthalene on Cu(111).

Naphthalene, C10H8, is a polycyclic aromatic hydrocarbon (PAH) consisting of two fused benzene rings. From previous studies, it is known to form three different commensurate structures in thin epitaxial films on Cu(111), depending on the preparation conditions. One of these structures even exhibits a chiral motif of molecular rotations within the unit cell. In an attempt to elucidate this polymorphism, we performed in situ low-energy electron diffraction (LEED) as a function of temperature and surface coverage, revealing an unexpected and extraordinarily complex structural and thermodynamic behavior. We present experimental evidence for a phase transition from a two-dimensional gas to a highly ordered molecular solid via an intermediate metastable phase with moderate order (extending over a few lattice constants only) which undergoes a reversible orientational shift upon temperature variation. At monolayer coverage and above, we find that two different point-on-line (POL) coincident epitaxial relations constitute the dominant structures. This is remarkable because, so far, POL structures of naphthalene on Cu(111) and other substrates have either not been recognized or not obtained under the respective experimental conditions. Our results are corroborated by the analysis of characteristic moiré patterns observed in scanning tunneling microscopy (STM), indicative of a noncommensurate epitaxial registry.

To tilt or not to tilt: correction of the distortion caused by inclined sample surfaces in low-energy electron diffraction.

Low-energy electron diffraction (LEED) is a widely employed technique for the structural characterization of crystalline surfaces and epitaxial adsorbates. For technical reasons the accessible reciprocal space is limited at a given primary electron energy E. This limitation may be overcome by sweeping E to observe higher diffraction orders decisively enhancing the quantitative examination. Yet, in many cases, such as molecular films with rather large unit cells, the adsorbate reflexes become less pronounced at energies high enough to observe substrate reflexes. One possibility to overcome this problem is an intentional inclination of the sample surface during the measurement at the expense of the quantitative interpretability of then severely distorted diffraction patterns. Here, we introduce a correction method for the axially symmetric distortion in LEED images of tilted samples. We provide experimental confirmation for micro-channel plate LEED and spot-profile analysis LEED instruments using the (7×7) reconstructed surface of a Si(111) single crystal as a reference sample. Finally, we demonstrate that the correction of this distortion considerably improves the quantitative analysis of diffraction patterns of adsorbates since substrate and adsorbate reflexes can be evaluated simultaneously. As an illustrative example we have chosen an epitaxial monolayer of 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) that is known to form a commensurate superstructure.

Determination and correction of distortions and systematic errors in low-energy electron diffraction.

We developed and implemented an algorithm to determine and correct systematic distortions in low-energy electron diffraction (LEED) images. The procedure is in principle independent of the design of the apparatus (spherical or planar phosphorescent screen vs. channeltron detector) and is therefore applicable to all device variants, known as conventional LEED, micro-channel plate LEED, and spot profile analysis LEED. The essential prerequisite is a calibration image of a sample with a well-known structure and a suitably high number of diffraction spots, e.g., a Si(111)-7×7 reconstructed surface. The algorithm provides a formalism which can be used to rectify all further measurements generated with the same device. In detail, one needs to distinguish between radial and asymmetric distortion. Additionally, it is necessary to know the primary energy of the electrons precisely to derive accurate lattice constants. Often, there will be a deviation between the true kinetic energy and the value set in the LEED control. Here, we introduce a method to determine this energy error more accurately than in previous studies. Following the correction of the systematic errors, a relative accuracy of better than 1% can be achieved for the determination of the lattice parameters of unknown samples.