Substrate-induced phase of a [1]benzothieno[3,2-b]benzothiophene derivative and phase evolution by aging and solvent vapor annealing

Substrate-induced phases (SIPs) are polymorphic phases that are found in thin films of a material and are different from the single crystal or "bulk" structure of a material. In this work, we investigate the presence of a SIP in the family of [1]benzothieno[3,2-b]benzothiophene (BTBT) organic semiconductors and the effect of aging and solvent vapor annealing on the film structure. Through extensive X-ray structural investigations of spin coated films, we find a SIP with a significantly different structure to that found in single crystals of the same material forms; the SIP has a herringbone motif while single crystals display layered π-π stacking. Over time, the structure of the film is found to slowly convert to the single crystal structure. Solvent vapor annealing initiates the same structural evolution process but at a greatly increased rate, and near complete conversion can be achieved in a short period of time. As properties such as charge transport capability are determined by the molecular structure, this work highlights the importance of understanding and controlling the structure of organic semiconductor films and presents a simple method to control the film structure by solvent vapor annealing

Epitaxial relation of carbamazepine and its precursor template extracted from rotating grazing incidence X-ray diffraction

Directed crystal growth often requires considerable experimental effort to achieve epitaxial control. In this work a simple solution step is shown to yield crystals of two different molecules simultaneously within a thin film and with defined epitaxial relations to each other. This was achieved by dissolving simultaneously carbamazepine (CBZ), a drug molecule, and iminostilbene (ISB), its precursor, in a single solution. When the sample is prepared using slow solvent evaporation rates, both molecules grow into needle-shaped crystals. The ISB crystals contact the substrate surface and extend for hundreds of μm. CBZ tends to minimize the contact with the substrate and assembles on top or is ledge-directed. In both cases a defined inclination of CBZ to the underlying ISB crystals is seen with optical microscopy showing a 36° inclination of the elongated crystals. In a similar way to pole figure measurements, rotating grazing incidence X-ray diffraction allows the determination of crystallographic properties and epitaxial relation, i.e. how the unit cells of both molecules align with respect to each other in terms of contact plane and azimuthal orientation. A point-on-line coincidence was identified, which can be related to the herringbone packing in the ISB crystals. Furthermore, changing the solute content as well as the processing times/kinetics, the appearance and the quality of crystal growth changes, but with the epitaxial relations remaining unaffected; crystals grow more frequent on top when processed slowly and when processed fast, growth takes place more often at facet inclined 90° from the top one.The authors want to thank the NAWI Graz for support. Elettra synchrotron is acknowledged for providing beamtime at the XRD1 beamline. The authors want to thank Luisa Barba and Nicola Demitri for excellent support.Peer reviewe

Alkyl chain assisted thin film growth of 2,7-dioctyloxy-benzothienobenzothiophene

An understanding of the thin film growth modes of substrate-induced polymorphs allows a deeper insight into the origin of this class of materials. Their onset of crystallisation, the subsequent crystal growth, the evolution of the thin film morphology and the transfer to the equilibrium bulk structure are still not fully understood. This work investigates the thin film formation of a conjugated molecule with terminal alkyl chains. Thin films of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene were grown by physical vapor deposition on silicon oxide surfaces with varying the film thicknesses from the sub-monolayer regime up to 33 layer thick films. Additionally, the substrate temperature and deposition rate were varied. The films were investigated by atomic force microscopy, X-ray reflectivity and grazing incidence X-ray diffraction. The first growth stage is a closed monolayer with a thickness of 3 nm formed by upright-standing molecules. It is found that the substrate-induced crystal structure is already formed within the first monolayer and continues its growth up to the largest investigated film thickness. The characteristic morphology is terraced islands over the whole thickness range. On top of the first monolayer a morphology with several terrace levels appears, which is associated with a rapid increase of the surface roughness. At larger film thicknesses (≥13 nm) the number of terrace steps does not increase significantly, so that the surface roughness only increases slowly. This work shows that molecules with terminal alkyl chains can form a substrate-induced phase up to large film thicknesses without the appearance of the equilibrium bulk phase.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Substrate-Induced Phase of a [1]Benzothieno[3,2-b]benzothiophene Derivative and Phase Evolution by Aging and Solvent Vapor Annealing

info:eu-repo/semantics/publishe

Thin film phase and local chirality of surface-bound MOP4 nanofibers

Vacuum deposition of the methoxy-functionalized <i>p</i>-quaterphenylene MOP4 on muscovite mica at elevated temperatures led to the formation of several ten micrometer long nanofibers. As shown by X-ray diffraction and polarized optical microscopy, the fibers are formed by lying molecules which grow epitaxially (point-on-line coincidence) along two directions on a single muscovite domain. The crystal structure of the fibers was induced by the substrate. Molecular packing was calculated by molecular dynamics simulations. A wetting layer from the lying molecules was observed using low-energy electron diffraction (LEED) that had a crystal structure different from that of the fibers. Transfer of the sample from a vacuum to ambient conditions led to dewetting and the formation of clusters from the lying molecules. The samples aged via Ostwald ripening due to the presence of water vapor, which was monitored by atomic force microscopy (AFM). The fibers represent chiral organization into single-handed aggregates of both handednesses, which is not affected by aging. On a global level, the sample remained achiral

Polymorphism of dioctyl-terthiophene within thin films: The role of the first monolayer

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