Interface Induced Crystal Structures of Dioctyl-Terthiophene Thin Films

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Interface Induced Crystal Structures of Dioctyl-Terthiophene Thin Films Oliver Werzer,†,‡ Nicolas Boucher,§ Johann P. de Silva,§ Gabin Gbabode,§ Yves H. Geerts,∥ Oleg Konovalov,⊥ Armin Moser,† Jiri Novak,† Roland Resel,*,† and Michele Sferrazza*,§ †

Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 810 Graz, Austria Institute of Pharmaceutical Sciences, Department of Pharmaceutical Technology, Karl-Franzens Universität Graz, Humboldstrasse 46, 8010 Graz, Austria § Départment de Physique, and ∥Laboratoire de Chimie des Polymères, Faculté des Sciences, Université Libre de Bruxelles (ULB), Boulevard du Triomphe, 1050 Brussels, Belgium ⊥ European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, Grenoble, France ‡

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.

I. INTRODUCTION New functional organic materials are under intense study for their potential application in optoelectronic devices such as organic field-effect transistors (OFETs).1 OFETs are commonly prepared by the deposition of thin layers (less than 100 nm thick) of conjugated organic semiconductors such as polymers or small molecules onto gate dielectric layers made from silicon oxide (SiOx),2 polymeric dielectric,3 self-assembled monolayers,4 or biomaterials.5 Soluble alkyl terminated oligothiophenes attract interest as they combine a highly flexible and cost-effective fabrication process with excellent device performance and stability.6 Their ability to form liquid crystalline mesophases allows, due to defect self-healing and self-assembling into large homogeneous domains,2,7 for homogeneous film formation as required in organic field effect transistors.8 Typically, thin films of alkyl terminated oligothiophenes assemble, like many other rod like conjugated molecules,9 in an upright standing orientation on silica with adjacent molecules arranging in a herringbone structure which allows for the formation of highly favorable charge carrier pathways. © 2012 American Chemical Society

Variations in the crystallographic structure (polymorphs) are commonly observed for small rod like molecules;10 changes in the molecular arrangement results in an alteration of the electronic wave function overlap and therefore altered charge transport properties.11 Furthermore the effective charge transport in thin films was shown to be dominated by the lateral domain structure in the first few interfacial molecular layers.7,12 Increased knowledge of the molecular assembling within thin films and especially the structure at the semiconducting−dielectric interface is therefore highly desirable. α,α′−dioctyl terthiophene (DOTT) is a prototype transistor material.13 Its conjugated core provides excellent charge transport properties and the terminal ends assist for the dissolution in conventional organic solvents. DOTT shows a complex phase behavior13b,c with phase transitions at 337 (crystal−crystal G), 344 (crystal G−smectic F), and 358 K (smectic F−smectic C) and melts at 363 K. In the absence of Received: March 22, 2012 Revised: May 11, 2012 Published: May 14, 2012 8530

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herringbone layers. Although the herringbone arrangement is driven by strong thiophene−thiophene interactions, the alkyl chains confine mainly due to space filling constraints. Individual layers are stacked resulting in the presence of terrace-like pyramidal islands with the thiophene units of individual layers separated by the terminal alkyl chains. Typically, thin DOTT films represent a two-dimensional powder. A preferred orientation (001 texture) along the surface normal results from the up-right standing DOTT molecules in herringbone arrangement. The herringbone layer thickness is therefore obtained from specular X-ray diffraction scans and AFM height images. In the in-plane direction no preferred orientation is present (powder-like character), as the isotropic silica substrate does not imply epitaxial constraints. GIXD is used to identify the in-plane order within thin DOTT films. III.1. Specular XRD of Equilibrium Structures. In Figure 1 the in situ specular X-ray diffraction scans of 100 nm thick

defects and grain boundaries, bulk DOTT shows ambipolar charge carrier mobility which decreases with increasing temperature, i.e., charge transport is favored in the bulk crystal and crystal G phases compared to the smectic F or smectic C phase.13c Within this work the temperature dependent crystallographic states and liquid crystal mesophases of DOTT in device relevant thin films of 20−100 nm are investigated by in situ specular X-ray diffraction (sXRD), in situ grazing incidence Xray diffraction (GIXD), and atomic force microscopy (AFM). The combination of these experimental methods allows for a detailed investigation of the bulk material as well as for the identification of interface structures.

II. EXPERIMENTAL METHODS α,α-Dioctyl terthiophene was synthesized by alkylation of commercially available terthiophene and precharacterized previously.13a,b Thin films of DOTT were obtained via spin-coating toluene solutions of 20, 7, and 5 mg/mL onto polished native oxidized silicon wafers giving layer thicknesses of about 100, 35, and 20 nm respectively. The process temperature was kept at 340 K allowing the production of more uniform thin DOTT films.13d,14 In situ specular X-ray diffraction (sXRD) experiments were performed on a Bruker D8 Advance diffractometer in θ/θ geometry using Cu Kα radiation (wavelength (λ) = 0.154 nm) equipped with a Material Research Instruments heating stage for temperature-dependent measurements from room temperature to 360 K. Specular scans were performed by a simultaneous movement of the source and detector (θ/θ) in the range from 1.6° to 20° with an angular resolution of 0.02° and an integration time of 10 s per step. The angular measurements are represented in the scattering vector (qz) notation (qz = (4π/λ) sin θ) and plotted versus the intensity. Grazing incidence diffraction experiments were performed at the beamline ID10B of the European Synchrotron Radiation Facility (Grenoble)15 using λ = 0.155 nm. The samples were mounted on a DHS 900 heating attachment from Anton Paar GmbH (Austria)16 which provided, in addition to defined temperature control, an inert He atmosphere. The diffracted intensities were collected with a onedimensional position sensitive detector (PSD) mounted perpendicular to the sample horizon. GIXD experiments were performed by collecting reciprocal space maps. At a fixed incidence angle the PSD was moved along the sample surface horizon, i.e., out of the coplanar direction resulting in an increase of the in-plane component (qxy) of the scattering vector. The extension of the PSD perpendicular to the sample horizon allowed to collect an out-of plane range (qz = 0−8.5 nm−1) at a given qxy simultaneously. The measured intensities were corrected versus the monitor and refraction effects prior to data evaluation and peak pattern indexation17 using the self-implemented software PyGID. All GIXD patterns are shown in the reciprocal space representation I(q), with q = (qxy, qz). Integrated in-plane line scans are obtained by summing various PSD channels for better statistics which then are plotted versus qxy. In situ atomic force microscopy measurements were performed with a Digital Instruments Multimode microscope in tapping mode using standard sharp end tips. Film thicknesses were determined by ellipsometry using a spectroscopic Jobin-Yvon MM-16 ellipsometer (λ = 400−800 nm) or single wavelength Nanofilm ellipsometer (λ = 532 nm). The Cauchy model has been used to determine the refractive index of the materials; the refractive index niso was determined from the value of the Brewster angle for the system.18

Figure 1. Semilogarithmic plots of the specular X-ray diffraction patterns of 100 and 35 nm (in red) thick α,α′-dioctyl terthiophene thin films at room temperature (a) and 340 (b), 348 (c), and 360 K (d). Samples were measured on heating. Curves are shifted for clarity.

DOTT thin films in the crystalline phase at room temperature (a), in the crystal G phase at 340 K (b), in the smectic F phase at 348 K (c), and in the smectic C phase at 360 K (d) are shown. Such a sXRD scan allows periodicities along the surface normal to be measured and reveal the variation of herringbone layer thickness on heating. At room temperature a Bragg peak at 1.95 nm−1 and three higher order reflections at 3.93, 5.85, and 7.8 nm−1 are observed which correspond to the repeating distance of the DOTT layers (see Figure 1). However, the Bragg peaks are a consequence of two polymorphic phases: a surface induced crystal phase (s-phase) and the crystalline bulk structure (b-phase) with very similar d-spacings of 3.31 and 3.21 nm, respectively. The first and second order peaks are a superposition of reflection from the s-phase and the b-phase which does not allow us to identify individual contributions of each phase. However the third order reflection at around 5.8 nm−1 reveals a splitting which is sufficient to distinguish the sphase (left peak) and the b-phase (right side). Variation of the peak position between qz = 1.88 and 1.95 nm−1 are noted for various samples (data not shown) due to changes of the relative amount of the s-phase and b-phase within the various samples. Such variations are typically observed for polymorphic small molecule structures and depend on the layer thickness, evaporation times, temperature of deposition, and others.9a,19 However, for thinner films, it seems that the s-phase is favored as it is shown in Figure 1 with the X-ray diffraction pattern of a 35 nm thick film (in red). The first order Bragg peak is observed at 1.90 nm−1 which corresponds to a d-spacing of 3.3

III. RESULTS AND DISCUSSION A combination of specular X-ray diffraction, grazing incidence diffraction, and AFM is used to identify the structure of bulk DOTT and its alterations at the interface. DOTT assembles like many other rod like molecules on silica in an upright standing conformation with parallel molecules assembling in 8531

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Figure 2. Grazing incidence reciprocal space maps of 20 (left column) and 100 nm (right column) thick DOTT films at different temperatures ranging from 298 (bottom) to 360 K (top). All measurements are taken at incidence angle αi = 0.15° and are plotted on common axes. Different color scales are used for clarity. HKLb, HKLs, HKLG, and HKLF denote the crystal reflections of the b-phase, s-phase, crystal G phase, and smectic F phase, respectively. 11* denotes the in-plane rod of the interface structure.

In the smectic F phase (SmF) at 360 K (plot c in Figure 1) the Bragg peak position shifts to q z = 1.981 nm −1 corresponding to an increase in the herringbone layer thickness to 3.17 nm which is similar to the repeating distance in the crystalline states. However, the relative intensity of the higher order reflections at around 3.95 and 5.89 nm−1 are strongly reduced compared to the room temperature polymorphs showing that the long-range positional order within layers are smeared due to the strong thermal vibrations of DOTT molecules. In the smectic C phase (SmC) at 360 K the order is further reduced and a broader Bragg reflection at 1.988 nm−1 is observable with its second order reflection at 3.97 nm−1. As the SmC phase transitions is close to the SmC−isotropic phase transition temperature, it is expected that an onset of the melting takes place which results in a reduced Bragg peak intensity compared to the SmF phase. III.2. Grazing Incidence X-ray Diffraction of Equilibrium Structures. Although specular X-ray diffraction reveals the herringbone layer thickness along the surface normal, grazing incidence X-ray diffraction (GIXD) allows an identification of the in-plane order parallel to the surface. In situ GIXD patterns of 20 and 100 nm thick DOTT thin films at temperatures from 298 to 360 K are depicted in Figure 2

nm, typical of the s-phase. This Bragg reflection is surrounded by Laue oscillations indicating the enhanced stacking order of the molecular layers throughout the film. Higher order reflections are also observed at 3.9 and 5.7 nm−1 (and also 7.7 nm−1). They are comparatively much broader than the reflections observed for the 100 nm thick film due to the reduced film thickness. However, it can be noticed that the position of these reflections is close to the position of the higher order reflections corresponding to the s-phase for the 100 nm thick film (see in particular the third order reflection at 5.7 nm−1). As the temperature is increased to 340 K (plot b in Figure 1) the crystal G phase is present. A Bragg reflection at 2.17 nm−1 and two higher order reflections at 4.32 and 6.44 nm−1 are observed suggesting remaining long-range order of the DOTT layers on heating. The peak positions shift to higher qz values which means that the repeating distance along the surface normal is reduced to 2.91 nm. This indicates that DOTT molecules in the crystal G phase are either more tilted with respect of the surface normal, or may be affected by interdigitating alkyl chains. Even alkyl chain melting could be present. 8532

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presence of a larger amount of DOTT in the s-phase. As solutions of higher concentrations are used for the 100 nm film preparation, faster crystallization takes place during the spincast process. This favors the formation of the surface induced crystallographic phase rather than the equilibrium bulk structures in accordance with previous film formation studies of dihexyl−terthiophene.19 Increasing the temperature of the 20 and 100 nm DOTT films to 340 K results in Bragg spots which belong to the crystal G phase. The indexation suggests that the molecules tilt toward the crystallographic a-axis resulting in a monoclinic unit cell with increased lattice constants of a = 0.91 nm and β = 108° while b remains similar to the b-phase (compare Table 1). It

revealing strong changes of the crystallographic order corresponding to phase transitions of DOTT. The 20 nm thin film at ambient temperature of 298 K reveals scattering rods at around qxy = 14, 16.5, and 20 nm−1. A more detailed analysis reveals that each rod splits into two very similar rods due to the presence of two polymorphic phases within this sample. GIXD measurements at different incidence angles (αi) result in different penetration depths for the X-ray beam and allow identification of which crystallographic phase has a majority at the interface. At αi = 0.07° the beam is able more or less to penetrate only 5 nm into the sample. Integrated line scans reveal diffracted intensity for both phases with peaks corresponding to the bulk phase (b-phase) at qxy = 14.11 and 16.47 nm−1, respectively, for the surface induced phase (sphase) at 14.02 and 16.25 nm−1 (see Figure 3, red). Although

Table 1. Crystallographic Unit Cells of the Various DOTT Phases Describing the Bragg Reflections of the 20 and 100 nm Samples in Figure 2 temp [K]

phase

a [nm]

b [nm]

298 298 340 348 360 298b

b-phase s-phase crystal G SmF SmC frozen phase

0.773 0.763 0.912 0.924

0.555 0.548 0.556 0.556

0.81

0.572

c [nm]

β [deg]

6.42 6.62 6.12 3.56 3.16 3.44

90 90 108 117 113.5

HLRDa [nm]

fwhm [nm‑1]

3.21 3.31 2.91 3.17 3.16 3.16

0.09 0.14 0.18 0.36 0.33

a

Herringbone layer repeating distance (HLRD) as obtained from the sXRD measurements (Figure 1). In-plane full width at half maximum (FWHM) of the 11L rod as obtained from the 20 nm DOTT film at 14 nm−1. bAfter rapid cooling from 360 K.

Figure 3. Integrated intensity of summed qz range 0−0.1 nm−1 versus qxy for two different incidence angles (αi).

must be noted that the β angle does not represent a unique solution for the peak pattern, but as strong intensity is expected to be present for the 110 Bragg spot in the crystal G phase, it gives confidence that it is a reasonable choice.20 Besides, the Bragg spots of the crystal G phase, an additional rod at qxy = 14.09 nm−1 is present (see Figure 2, 11*) which is typical for a few ordered layers close to the silica surface. As the qxy value is close to the s-phase rod position, it can be expected that this rod results from surface interacting DOTT molecules in the sphase. Furthermore, the similarity in the d-spacing suggests that the herringbone like arrangement is preserved. Additional rods are absent but may be a result of the weaker scattering from the 20* and 21* rods or due to long-range order along other crystallographic directions being weakened or lost. The full widths at half maxima (fwhm) in the in-plane direction of the 11* rod are strongly increased compared to those of the 11L rod of the s-phase. According to Williamson-Hall such an increase results from either reduced crystallite size or/and increased strain within a crystallite.21 The GIXD experiments do not allow us to elucidate which contribution is responsible for the increase. However both an increase in strain as well as a decrease in the crystallite size are expected to have a strong impact on the electronic performance of DOTT thin films. In the SmF phase of DOTT which is present at 348 K, the long-range order is further reduced, and only two reflections at (qxy, qz) = (1.320, 0.34) and (1.37, 0.7) nm−1 are visible. The DOTT molecules further tilt toward the surface resulting in an increase of the a-axis and β value (compare Table 1). The 11* rod of the surface phase is still present but with an increased fwhm suggesting that long-range order within the interface phase is further reduced on heating.

the positions remain the same for αi = 0.15° with larger X-ray penetration depth of 24 nm, the relative peak intensities vary significantly with the s-phase being more dominant. As the beam is able to be diffracted within the entire thin film and to reach the DOTT−silica interface higher intensity from surface induced phases is detected. This strongly suggests that the sphase is the majority species at the interface and the b-phase grows on top. This is in agreement with the results from specular X-ray diffraction showing a predominance of the sphase for thinner films. DOTT molecules in both phases are in a herringbone assembling; indexation of the Bragg reflections reveals orthorhombic unit cells of a = 0.773 nm, b = 0.555 nm, and c = 6.42 nm for the b-phase and a = 0.763 nm, b = 0.548 nm, and c = 6.62 nm for the s-phase. The measurement on a 100 nm sample reveals the same polymorphic structures as for the 20 nm sample. However, although the splitted strong rods are still present at qxy = 14, 16.5, and 20 nm−1, differences are noted. First, additional rods with defined Bragg spots occur within the reciprocal space map. A comparison with the unit cell obtained from the 20 nm reveal that, within the limits of the experiment, an increase of 4 times of the (in-plane) a- and b-axes is able to represent these additional rods, suggesting that a superstructure or a modulated phase within the DOTT thin films is present which might be a consequence of the terthiophene being able to adapt a parallel or antiparallel conformation or incommensurability in the system. An onset of the superstructure peaks is also noted for the 20 nm sample, but as the film is thin the intensity is nearly indistinguishable from the background signal. Second, the relative intensity of the rod corresponding to the s-phase is higher than that of the b-phase which corresponds to the 8533

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At 360 K all Bragg spots have vanished and only the rod at 14.09 nm−1 remains with a fwhm similar to the measurements in the SmF. In addition, a broad background is noted which is due to the weak in-plane order of the SmC. Such a broad peak may also be associated with the silica substrate, but a comparison with a 100 nm thick DOTT thin film (see Figure 4) reveals a strong increase of intensity with increasing layer thickness giving confidence that the broad background belongs to the SmC order of DOTT. Figure 5. In situ atomic force microscopy height images of a nominal 3 (left) and 50 nm (right) DOTT film at 340 K. White squares denote example areas used for the evaluation of the terrace heights.

nm for layers elevated from the silica surface. This is in excellent agreement with the GIXD results and suggests that the bulk of the film is completely transferable into other phases, while an interface layer remains for all temperatures. The interfacial layer acts as a template for the formation of the various crystallographic states of DOTT. Figure 4. Integrated intensity of summed qz range of 0−0.1 nm−1 versus qxy for two different DOTT thin films at 360 K.

The obtained unit cells from Table 1 allow us to index each Bragg spot within the GIXD pattern. As the DOTT thin film on silica behaves like a 2-dimensional powder, a verification of the lattice parameters compared to powder diffraction data is improved as each HKL value has a well-defined out-of-plane angle as well as a defined absolute wave vector. This gives confidence that the presented lattice constants in Table 1 are an improvement to previous unit cell refinements of DOTT polymorphs.13b III.3. AFM Imaging of Crystal G Phase. AFM height images for DOTT thin films with nominal thicknesses of 3 and 50 nm in the crystal G phase at 340 K are shown in Figure 4 revealing the typical terrace-like morphology of upright standing molecules. For the 3 nm film, a single submonolyer with dendritic island morphology is observed. Gaussian fits to the height distribution data on one square micrometer area reveals a terrace height of about 3.3 nm corresponding to DOTT in the crystalline phase (compare Table 1). As this layer is at the silica interface, this means that the layer formation is determined by the surface interaction and corresponds most likely to the 11* Bragg rods in the GIXD pattern. For the 50 nm film, the morphology reveals a 3-dimensional growth behavior with multiple terraces distributed over the investigated surface area. The height analysis reveals a reduced step height of about 2.8 nm. A larger step height of 3.3 nm was not observed, suggesting that DOTT molecules pack in the crystal G phase far off the silica surface. Whereas within thicker films only one terrace height is observed, the GIXD measurements reveal contributions from a surface near the 11* rod and the crystal G phase simultaneously suggesting that the interfacial layer is buried under the bulk DOTT and is not accessible within the AFM experiment. However, the strong 3-dimensional growth allows us to reach layers closer to the interface as samples with thicknesses less than nominal 20 nm are investigated. The height investigations on various terraces steps reveal the presence of two step heights simultaneously (see Figure 5): a step height of 3.3 nm is observed close to the interface and a step height of about 2.9

Figure 6. Terrace heights of DOTT layers observed for various layer thicknesses at 340 K. Lines represent the crystallographic layer distance as obtained from the X-ray data.

III.4. GIXD Measurement of Nonequilibrium Structure. The effect of rapid cooling on the room temperature structure within DOTT thin films is investigated on a 20 nm DOTT thin film sample after 10 min heat treatment at 360 K and cooled with a rate of 3 K/min. The pattern shows the presence of an additional crystallographic phase at room temperature (see HKLc in Figure 7a). The strong “comet like” smearing shows large mosaicity for this additional phase. Indexation reveals a monoclinic unit cell with a = 0.810 nm, b = 0.572 nm, c = 3.45 nm, and β = 113.5°; an intermediate crystalline state has formed. As the film cools, DOTT molecules rearrange and, depending on the temperature, follow the SmC−SmF−crystal G−b-phase phase transitions. However, as some transitions may be slow, not all DOTT molecules are able to adopt the respective conformation and “freeze” in their positional and conformational order and a nonequilibrium metastable state follows. Figure 7b shows an integrated in-plane line scan for qz = 0 nm−1 revealing the reformation of the b- and s-phase after the heat treatment with their sharp peaks around qxy = 14 nm−1. Although for the b-phase the 20 Lb and the 21 Lb rods are visible, those are absent for the s-phase. This suggests that a reformation of the interfacial in-plane order is present, but recrystallization into the s-phase is strongly disfavored; an sphase with one or only few monolayers of DOTT forms. It is expected that such a nonequilibrium structure tremendously affects the bulk charge transport properties, but further 8534

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ACKNOWLEDGMENTS

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REFERENCES

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We thank the European Synchrotron Radiation Facility for provision of synchrotron radiation. The authors acknowledge funding from Region Wallonne under Project ETIQUEL (Grant No. 04/1/5706), financial support from the European Community’s Seventh Framework Program under Project ONE-P (Grant No. 212311), and financial support from the ARC program of the Communauté Française de Belgique (Grant No. 20061). We thank also the Austrian Science Fund (FWF): [P21094].

(1) (a) Klauk, H. Organic Electronics: Materials, Manufacturing and Application; Wiley-VCH: Weinheim, Germany, 2007. (b) Horowitz, G. Organic Field-Effect Transistors. Adv. Mater. 1998, 10, 365−377. (2) Fichou, D. Structural order in conjugated oligothiophenes and its implications on opto-electronic devices. J. Mater. Chem. 2000, 10, 571−588. (3) Hernandez-Sosa, G.; Simbrunner, C.; Höfler, T.; Moser, A.; Werzer, O.; Kunert, B.; Trimmel, G.; Kern, W.; Resel, R.; Sitter, H. Modification of para-sexiphenyl layer growth by UV induced polarity changes of polymeric substrates. Org. Electron. 2009, 10, 326−332. (4) Pacher, P.; Lex, A.; Proschek, V.; Werzer, O.; Frank, P.; Temmel, S.; Kern, W.; Resel, R.; Winkler, A.; Slugovc, C.; Schennach, R.; Trimmel, G.; Zojer, E. Characterizing Chemically Reactive Thin Layers: Surface Reaction of [2-[4-(Chlorosulfonyl)phenyl]ethyl]trichlorosilane with Ammonia. J. Phys. Chem. C 2007, 111, 12407− 12413. (5) Irimia-Vladu, M.; Troshin, P. A.; Reisinger, M.; Shmygleva, L.; Kanbur, Y.; Schwabegger, G.; Bodea, M.; Schwödiauer, R.; Mumyatov, A.; Fergus, J. W.; Razumov, V. F.; Sitter, H.; Sariciftci, N. S.; Bauer, S. Biocompatible and Biodegradable Materials for Organic Field-Effect Transistors. Adv. Funct. Mater. 2010, 20, 4069−4076. (6) Serban, D. A.; Kilchytska, V.; Vlad, A.; Martin-Hoyas, A.; Nysten, B.; Jonas, A. M.; Geerts, Y. H.; Lazzaroni, R.; Bayot, V.; Flandre, D.; Melinte, S. Low-power dihexylquaterthiophene-based thin film transistors for analog applications. Appl. Phys. Lett. 2008, 92. (7) de Cupere, V.; Tant, J.; Viville, P.; Lazzaroni, R.; Osikowicz, W.; Salaneck, W. R.; Geerts, Y. H. Effect of interfaces on the alignment of a discotic liquid-crystalline phthalocyanine. Langmuir 2006, 22, 7798− 7806. (8) Lengyel, O.; Hardeman, W. M.; Wondergem, H. J.; de Leeuw, D. M.; van Breemen, A. J. J. M.; Resel, R. Solution-Processed Thin Films of Thiophene Mesogens with Single-Crystalline Alignment. Adv. Mater. 2006, 18, 896−899. (9) (a) Werzer, O.; Stadlober, B.; Haase, A.; Oehzelt, M.; Resel, R. Full X-ray pattern analysis of vacuum deposited pentacene thin films. Eur. Phys. J. B 2008, 66, 455−459. (b) Resel, R. Surface induced crystallographic order in sexiphenyl thin films. J. Phys.: Condens. Matter 2006, 20, 184009(1−10). (10) (a) Cheng, H.; Mai, Y.; Chou, W.; Chang, L.; Liang, X. Thickness-Dependent Structural Evolutions and Growth Models in Relation to Carrier Transport Properties in Polycrystalline Pentacene Thin Films. Adv. Funct. Mater. 2007, 17, 3639−3649. (b) Muguruma, H.; Saito, T. K.; Hotta, S. Conformational polymorphs in vacuum evaporated thin film of 5,5‴-bis[(2,2,5,5-tetramethyl-1-aza-2,5-disila-1cyclopentyl)ethyl]-2,2′:5 ′,2″:5″,2‴-quaterthiophene. Thin Solid Films 2003, 445, 26−31. (c) Moulin, J. F.; Dinelli, F.; Massi, M.; Albanetti, C.; Kshirsagar, R.; Biscarini, F. In situ X-raysynchrotron study of organic semiconductor ultra-thin films growth. Phys. Rev. B 2006, 246, 122−126. (11) Nabok, D.; Puschnig, P.; Ambrosch-Draxl, C.; Werzer, O.; Resel, R.; Smilgies, D. Crystal and electronic structures of pentacene thin films from grazing-incidence x-ray diffraction and first-principles calculations. Phys. Rev. B 2007, 76, 235322(1−6). (12) (a) Tang, X.; Baie, X.; Colinge, J.-P.; Crahay, A.; Katschmarskyj, B.; Scheuren, V.; Spote, D.; Reckinger, N.; Van de Wiele, F.; Bayot, V.

Figure 7. Room temperature grazing incidence in-plane scans of a 20 nm thick DOTT thin film after heat treatment at 360 K (a). Integrated line intensity for summed qz range of 0−0.1 nm−1 versus qxy for the 20 nm DOTT thin film after the annealing process (b).

experiments with various cooling rates or quenching temperatures are required to indentify the crystallographic properties in more detail.

IV. CONCLUSION In conclusion, phase coexistence in spin-coated dioctylterthiophene films has been experimentally identified. The in situ investigations reveal strong rearrangements of DOTT molecules within thin films on temperature change. DOTT has two polymorphic phases at room temperature: a crystal G phase at 340 K and mesomorphic phases (SmF and SmC) at higher temperatures. While the bulk material reveals strong variations in the lattice constants (especially along a, c, and β), the interface DOTT order shows only little changes suggested by the transition from the 11 L rod of the s-phase into the 11* of the monolayer with both having nearly the same reciprocal scattering vectors. Furthermore the interface monolayer formation is independent of the layer thickness which is of high interest as charge transport close to or at the very interface have strong impacts on device performances. An additional crystallographic phase at room temperature is observed as intermediate DOTT conformations freeze-in on temperature quenching from high temperatures. It is expected that various such metastable structures can form, which tremendously affects the bulk charge transport properties of DOTT. As the bulk material may provide additional charge transport channels within a thin film device, this metastable phases also may affect the overall performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.R.); [email protected] (M.S.). Notes

The authors declare no competing financial interest. 8535

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dx.doi.org/10.1021/la301213d | Langmuir 2012, 28, 8530−8536