Interface Induced Crystal Structures of Dioctyl-Terthiophene Thin Films

Abstract

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.

Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

“…SIPs have also been observed for even more flexible molecules, this is perhaps unsurprising as flexible molecules have a strong tendency to form multiple polymorphs . Two such systems where SIPs have been observed are for two dialkylated terthiophenes: α,α′ ‐dihexyl‐terthiophene (DHTT) ( 6a ) and α,α′ ‐dioctyl‐terthiophene (DOTT) ( 6b ) (Figure ). In the case of 6a , an SIP with a unit cell different to the two known bulk forms is observed by specular X‐ray diffraction and GIXD in solution cast (spin coated, drop cast and dip coated) films where crystallization is rapid (i.e., far from thermodynamic equilibrium), with the bulk form dominating when crystallization is allowed to proceed more slowly ( Figure ); the SIP is also observed in films produced by physical vapor deposition, a process which is also far from thermodynamic equilibrium .…”

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

“…6,7 These polymorphic phases, which are typically not found as single crystal structures, were first observed in films of the prototypical organic semiconductor pentacene, [8][9][10][11][12][13] and have since been found in other systems. [14][15][16][17][18][19][20] In films of pentacene, the SIP is found to be less energetically favorable than the bulk phase in isolation, but is stabilized close to the substrate due to improved compatibility of the structure with the flat surface of the substrate such that in the presence of the substrate it is the most stable form. 21,22 The structural difference between the SIPs and bulk phases of π conjugated molecules is most often related to a small change in the tilt angle between the approximately upright standing molecules and the substrate, resulting in more favorable molecule substrate interactions and a decrease in the out of plane lattice spacing such that the two phases are similar but distinct from one another.…”

“…13,23,24 Moreover, SIPs are found to be present only up to a certain film thickness after which the bulk form grows on top so that two phases coexist within a thick film. 15,[23][24][25] It is therefore clear that the structure of the SIP may influence charge mobility and potential device performance when present due to its proximity to the substrate.…”

Surface Mediated Structures: Stabilization of Metastable Polymorphs on the Example of Paracetamol

“…The controlled preparation of polymorphic structures and different crystal morphologies has become a hot topic in several research areas such as pharmaceutical technology, 1,2 crystal engineering, 3,4 organic electronics 5 as well as in material science. 6 Various polymorphs of a single substance differ by means of their physical and chemical properties such as solubility, bioavailability, morphology, crystal structure, or even thermodynamic stability.…”

“…It is known that surfaces are able to induce specific polymorphs upon deposition. 5,17,18 Even the occurrence of surface mediated phases can be observed with distinct properties from the bulk. 5 …”