In recent years, the utilization of gas sensors has increased tremendously in daily life as well as industrial production. Importantly, appropriate material selection is imperative for gas sensors in order...

We present depth-resolved grazing incidence x-ray diffraction, grazing incidence small angle scattering and x-ray reflectivity studies on the structure of mixed C(60) and diindinoperylene (DIP) films as a function of the mixing ratio. We observe enhanced out-of-plane order and smoothing of the mixed films compared to pure films upon coevaporation of DIP:C(60) thin films (in different mixing ratio) which otherwise phase separate. The mixing ratio of molecules can be tuned to alter the in-plane crystallite size as well as the interisland distances of the mixing molecules. Real-time in situ grazing incidence x-ray diffraction measurements show the kinetics and thickness dependence of phase separation, which appears to proceed only after a certain thickness. The crystallite grain size of the individual phase separated components is significantly larger at the top of the film than at the bottom with implications for the understanding of devices.

We investigate the structure of mixed thin films composed of pentacene and diindenoperylene using x-ray reflectivity and grazing incidence x-ray diffraction. For equimolar mixtures we observe vanishing in-plane order coexisting with an excellent out-of-plane order, a yet unreported disordering behavior in binary mixtures of organic semiconductors, which are crystalline in their pure form. One approach to rationalize our findings is to introduce an anisotropic interaction parameter in the framework of a mean field model. By comparing the structural properties with those of other mixed systems, we discuss the effects of sterical compatibility and chemical composition on the mixing behavior, which adds to the general understanding of interactions in molecular mixtures.

We investigate in situ the structural changes during self-assembly of PbS nanocrystals from colloidal solution into superlattices, solvent evaporation, and ligand exchange at the acetonitrile/air interface by grazing incidence small-angle X-ray scattering (GISAXS). We simulate and fit the diffraction peaks under the distorted wave Born approximation (DWBA) to determine the lattice parameters. We observe a continuous isotropic contraction of the superlattice in two different assembly steps, preserving the body-centered cubic lattice with an overall decrease in the lattice constants of 11%. We argue that the first contraction period is due to a combination of solvent evaporation/annealing and capillary forces acting on the superlattice, whereas the second period is dominated by the effect of replacing oleic acid on the nanocrystal surface with the short and rigid cross-linker tetrathiafulvalene dicarboxylate. This work provides guidelines for optimized ligand exchange conditions and highlights the structural particularities arising from assembling NCs on liquid surfaces.

We analyze the structure and morphology of mesocrystalline, body-centered tetragonal (bct) superlattices of PbS nanocrystals functionalized with oleic acid. On the basis of combined scattering and real space imaging, we derive a three-dimensional (3D) model of the superlattice and show that the bct structure benefits from a balanced combination of {100}PbS-{100}PbS and {111}PbS-{111}PbS interactions between neighboring layers of nanocrystals, which uniquely stabilizes this structure. These interactions are enabled by the coaxial alignment of the atomic lattices of PbS with the superlattice. In addition, we find that this preferential orientation is already weakly present within isolated monolayers. By adding excess oleic acid to the nanocrystal solution, tetragonal distortion is suppressed, and we observe assembly into a bilayered hexagonal lattice reminiscent of a honeycomb with grain sizes of several micrometers.

We report on a real-time in situ study of the growth of α-sexithiophene on silicon oxide substrates. Synchrotron-based X-ray diffraction experiments were performed during and directly after the growth in order to monitor the growth process. We observed a coexistence of two different crystal phases for different substrate temperatures. For films prepared at 233 and 308 K a disordered phase (β-phase) seems to be dominant compared to films prepared at 373 K where the so-called lowtemperature bulk crystal phase (LT-phase) is dominant. From real-time measurements during growth we observed a temperature and film thickness dependent effect on the fraction of both phases in one sample. At 373 K the film growth begins primarily in the β-phase, and above a certain thickness the film growth proceeds mainly in the LT-phase. However, at 308 K the film growth is dominated by the β-phase for the entire thickness. We show that for kinetically limited growth conditions (high deposition rate and/or low substrate temperature) substrate induced growth effects are dominant. ■ INTRODUCTIONOrganic electronics are an attractive alternative to common inorganic devices, inter alia due to potentially low preparation costs, low-temperature processing, and the possibility of using flexible substrates. 1−7 Organic semiconductors (OSCs) are also used as active layers in organic light-emitting diodes, organic photovoltaics (OPV), and organic field effect transistors (OFETs). For small molecule OSCs, the crystallinity of the thin films and the relative orientation of the individual molecules constituting the thin film are crucially important for the efficiencies of such devices. Oligothiophenes are an important class of OSC materials. In particular, α-sexithiophene (6T) (Figure 1a) is considered very promising. 6T has shown a high open circuit voltage in combination with diindenoperylene in OPV cells 8 and high hole mobility of up to 4 × 10 −2 cm 2 V −1 s −1 in OFETs. 9 As in many other small molecule OSCs, the crystal structure and the crystal defect density are crucial for the performance. 10−12 In this study we focus on the growth and structure of 6T on native silicon oxide (nSi).In general, depending on the substrate, the preparation conditions and the sample state (i.e., thin film or single crystal), there are several phases of 6T with different crystal structures reported. 13−16 In thin films on substrates with low interaction energies, mostly the so-called low-temperature phase (LTphase) of single crystals, reported by Horowitz et al., 16 and the thin film β-phase 14,15 are found. Figure 1b depicts the unit cell of the LT crystal phase. For 6T on SiO 2 several studies of growth, structure, and charge transport report that 6T molecules are oriented mostly perpendicularly to the substrate. 14,15,17−21 Different anisotropic growth scenarios on fused silica and stretched polyethylene substrates were measured via absorption spectroscopy by Oelkrug et al. 22 On TiO 2 the formation of domains of the LT-phase 16 was observed. 23,24 Highly ordered...

We report on a combined off-specular and specular x-ray scattering growth study of ultrathin films of the prototypical organic semiconductor diindenoperylene (DIP, C 32 H 16). We investigate the evolution of the in-plane correlation length and the growth kinetics of the films including their dependence on the substrate temperature and the growth rate. We observe a temperature-dependent collective rearrangement of DIP molecules from a transient surface induced to the thin-film phase, which can be rationalized by incorporating a thickness-dependent out-of-plane lattice parameter. We further observe that the nucleation behavior of DIP changes from the first to the second monolayer, which we relate to a difference in the diffusion length of the molecules.

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