Oriented growth of zinc(II) phthalocyanines on polycarbonate alignment substrates: Effect of substrate temperature on in-plane orientation

Synthetic Metals 161 (2011) 251–258

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Oriented growth of zinc(II) phthalocyanines on polycarbonate alignment substrates: Effect of substrate temperature on in-plane orientation Christelle Vergnat a,1 , Sureeporn Uttiya b,1 , Sirapat Pratontep c,d , Teerakiat Kerdcharoen b , Jean-Franc¸ois Legrand a , Martin Brinkmann a,∗ a

Institut Charles Sadron, CNRS-Université de Strasbourg, 23 rue du loess, 67034 Strasbourg, France Center of Nanoscience and Nanotechnology, Physics Department, Faculty of Science, Mahidol University, Rama VI Road, Bangkok, Thailand College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand d ThEP Center, CHE, 328 Si Ayutthaya Rd., Bangkok 10400, Thailand b c

a r t i c l e

i n f o

Article history: Received 10 September 2010 Received in revised form 18 November 2010 Accepted 18 November 2010 Available online 24 December 2010 Keywords: Thin films Molecular semi-conductors Transmission electron microscopy Alignment layer

a b s t r a c t Highly oriented films of zinc(II) tetra-tert-butyl-phthalocyanine (ttb-ZnPc) and zinc(II) phthalocyanine (ZnPc) have been deposited on oriented and nanostructured substrates of bisphenol A polycarbonate. The polycarbonate substrates were prepared by a simple method that combines (i) the mechanical rubbing of an amorphous PC film and (ii) the solvent induced crystallization generating oriented crystalline lamellae with a high in-plane orientation. The preparation conditions yielding alignment layers with high orienting capability and improved thermal stability have been optimized. The effects of the substrate temperature (Ts ) on the morphology, the in-plane orientation and the optical properties of the phthalocyanine films have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM) and polarized UV–vis absorption spectroscopy. For both phthalocyanines, the in-plane orientation is observed to increase with increasing Ts in the range 100–170 ◦ C as indicated by an increase of the dichroic ratio of the optical absorption with Ts . However, contrary to ZnPc, the high in-plane orientation of ttb-ZnPc is not related to the growth of elongated nanocrystals but simply to the orientation of columnar stacks parallel to the PC lamellae with a rather short 0.33 nm intermolecular stacking period and an inter-stack period of 1.65 nm. For similar growth conditions, a better orientation of ttb-ZnPc is achieved on PC substrates as compared to substrates of friction transferred polytetrafluoroethylene (PTFE). These results show that PC alignment layers are an interesting alternative to substrates of oriented PTFE for a large range of substrate temperatures up to 170 ◦ C. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Numerous applications in optics and optoelectronics, e.g. polarized color filters and liquid crystal displays (LCDs) make use of polymer alignment layers [1–4]. Because of the anisotropic charge transport in discotic semi-conducting molecules like phthalocyanines or substituted hexapericoronenes, in-plane orientation of these systems on orienting polymer substrates is desirable especially in the perspective of fabricating organic field effect transistors showing high charge carrier mobilities [5,6]. A simple and yet powerful method to prepare large areas of polymer alignment layers is based on the mechanical rubbing of a polymer, e.g. a polyimide with a velvet cloth [7,8]. Although rubbed polyimide substrates

∗ Corresponding author. Tel.: +33 0388414047. E-mail address: [email protected] (M. Brinkmann). 1 These authors contributed equally to this work. 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.11.029

exhibit very high orienting capability, the mechanical rubbing generates microgrooves a few tens of nm deep and thus impedes control of the surface roughness [8]. Nevertheless, surface roughness control is very important, especially in organic field effect transistors (OFETs) for which charge transport occurs in the first monolayers in contact with the dielectric substrate [9]. Beside the rubbing technique, there are several other alternative methods to prepare alignment layers like for instance the friction transfer of poly(tetrafluoroethylene) (PTFE) [10]. Oriented PTFE layers have demonstrated exceptional orienting properties for a large class of polymers and small molecules including discotic liquid crystalline semiconductors [11–19]. However, PTFE films show a discontinuous coverage of the supporting film (see Fig. 1) and therefore they cannot be used as oriented polymer gate dielectrics in OFETs. In addition, the preparation of both polyimide and PTFE alignment layers requires annealing of the supporting films at temperatures >200 ◦ C. Alternatively, polymer alignment layers have also been prepared by orientation under polarized UV irradiation

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Fig. 1. Topographic AFM images for an oriented and nanostructured PC substrate obtained for (a) texp = 2 min and (b) texp = 3 min. The arrow indicates the direction of mechanical rubbing. (c) AFM phase mode image of a friction-transferred PTFE layer on a glass substrate. The arrow indicates the direction of friction transfer (PTFE chain direction).

of liquid-crystalline polymers, which implies, however, the synthesis of specific polyimide derivatives [20]. Several groups have also reported the alignment of discotic liquid crystals, e.g. substituted phthalocyanines by using either chemically or topographically patterned substrates [21,22]. Recently, Brinkmann and coworkers have developed a new method to prepare alignment layers showing a periodic and regular nanostructuration, a high level of order at the length scale of polymer chains and a reduced surface roughness [23]. Oriented and nanostructured substrates of polycarbonate (PC) are readily fabricated by (i) rubbing an amorphous PC film and (ii) initiating the growth of oriented PC lamellae by solvent induced crystallization. These PC substrates present a surface with a periodic alternation of oriented crystalline lamellae and amorphous interlamellar zones with a periodicity of ca. 20 nm and a controlable surface roughness. The alignment ability of these PC layers was demonstrated for various molecular materials including acenes, phthalocyanines, coronene, bisazo dyes as well as ordered arrays of gold nanoparticles [23]. Herein, we have studied the orientation of two phthalocyanines, namely a substituted phthalocyanine, e.g. zinc tetra-tert-butylphthalocyanine (ttb-ZnPc) and zinc phthalocyanine (ZnPc) on oriented and nanostructured PC substrates. We have focussed on the impact of the substrate temperature during deposition on the in-plane orientation of the zinc phthalocyanines. The results for ttb-ZnPc are compared to those obtained for ZnPc grown in similar conditions. In addition, we have compared the orientation achieved on oriented PC substrates with that obtained on friction transferred PTFE substrates. The preparation conditions yielding highly orienting PC substrates with improved thermal stability and controlled surface roughness have been determined. All deposition parameters being equal, higher levels of orientation can be obtained on PC substrates as compared to PTFE substrates.

2. Experimental 2.1. Preparation of the substrates and phthalocyanine thin films The preparation of the PC alignment layers implies three main steps: first, amorphous PC films of thickness in the range 200–500 nm were prepared by spin coating (2000 RPM, 500 RPM/s) a 2 wt% solution of bisphenol A polycarbonate (Acros, Mw = 64,000 g/mol) in tetrachloroethane on clean glass slides (Corning 2947). The glass slides were cleaned by sonication in acetone and ethanol for 15 min each. Then, the slides were gently scrubbed with a soft tooth brush in a solution of Hellemanex® (5% in weight) in deionized water. The slides were further sonicated in a

5 wt% solution of Hellemanex® for 15 min and subsequently rinsed and sonicated in deionized water for 15 min. After three rinsing steps, the slides were finally dried in a flow of nitrogen. Second, the amorphous PC layers were rubbed at 25 ◦ C by using a homemade apparatus. A rotating cylinder (4 cm diameter) covered with a velvet cloth was applied with a pressure of 2 bar on the PC film. A rubbing length of 200 cm was used. Exposure of the thin films to acetone vapors was performed at 25 ◦ C in a closed glass vessel (volume of 50 cm3 ) containing 5 ml of acetone. PTFE films were prepared according to the method described elsewhere [10] by sliding a PTFE rod at a constant pressure (5 bar) against a glass slide (Corning 2947) held at 250–300 ◦ C. The cleaning of the glass substrates was identical to that described for the preparation of the PC substrates. ZnPc and ttb-ZnPc thin films were grown by sublimation in high vacuum using an Edwards Auto306 evaporator system from fused quartz crucibles heated by a tungsten filament. The base pressure of the evaporation chamber was 10−6 mbar. The film thickness and the deposition rate  (2.0 nm/min) were controlled by using a quartz microbalance placed in the vicinity of the substrate holder. ttb-ZnPc starting material (Aldrich, 98%) was used as received, whereas ZnPc was purified by sublimation from a quartz crucible under high vacuum (10−5 mbar). To prevent structural modifications of the film by annealing after film deposition, the temperature of the sample holder was rapidly cooled to room temperature (cooling rate: 0.5–1.0 ◦ C/s) by using a liquid nitrogen cooling system.

2.2. Thin film characterization. The PC substrate orientation was first investigated by polarized optical microscopy using a LEICA DM-RX microscope. The optical birefringence of the polymer films n was investigated using a standard method. The samples were placed between two crossed polarizers and rotated by 45◦ from the extinction position. In this configuration, the intensity of the light of wavelength  detected by the camera is I = I0 sin2 (2ı/), with I0 the incident intensity, and ı the optical path difference through the film of thickness t. For samples with a homogeneous birefringence through the thickness: n = ı/t, but here it is preferred to characterize the anisotropy of the samples by the optical path difference ı. The intensity I of the red component of the light was averaged over a 1.5 mm2 area using a Nikon Coolpix 995 camera and the wavelength was taken as  = 700 nm, according to RGB standards. The surface topography of the polymer alignment layers and the phthalocyanine films was investigated by tapping mode AFM on a Nanoscope III using Si3 N4 cantilevers oscillating at a frequency in the range 250–300 kHz. Visualization of the polymer film topography was performed under

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soft tapping conditions (drive amplitude variation below 15% with respect to the free quantilever). The structure and the morphology of the evaporated phthalocyanine films of thickness 50 nm were investigated by transmission electron microscopy (TEM) in the bright field mode (BF) and by electron diffraction (ED). To this aim, the phthalocyanine/substrate samples were coated with an amorphous carbon film, floated off on a diluted aqueous HF solution (5%) and then transferred onto copper microscope grids. The films were studied in the bright field and the diffraction modes with a 120 kV Philips CM12 electron microscope equiped with a MVIII CCD camera (Soft Imaging Systems). Image treatments, e.g. fast Fourier transforms (FFT) were performed by using AnalySIS (Soft Imaging System) software. The optical anisotropy of the oriented phthalocyanine films was investigated by UV–vis-Near IR absorption (300–900 nm) spectroscopy using a Shimadzu UV-2101PC spectrometer with polarized incident light and a spectral resolution of 1 nm. The film orientation with respect to the incident light was controlled by means of a goniometer: the // orientation was obtained when the incident beam polarization was parallel to the PC rubbing direction (cPC axis).

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Fig. 2. Kinetics of solvent induced plasticization and crystallization of rubbed PC thin films as observed by following the evolution of the root mean square (RMS) roughness of the topographic AFM images (2 ␮m × 2 ␮m) and the optical path difference of the birefringent films. The dash-dotted line highlights the separation between the regime of PC plasticization and the regime of solvent induced crystallization.

3. Results and discussion 3.1. Polycarbonate alignment layer In our previous studies, we have proposed a simple method to prepare highly oriented and nanostructured surfaces of PC substrates [23]. The typical surface morphology of PC alignment layers prepared by this method is illustrated in Fig. 1a and b and compared with that of oriented PTFE substrates (c). The surface topography depends on the duration of the solvent exposure step texp . For texp = 2 min, the oriented PC layers exhibit a regular nanostructuration corresponding to the semi-crystalline structure of PC, i.e. the succession of crystalline lamellae and amorphous interlamellar zones with a typical periodicity of ca. 20 nm. The surface topography of the films becomes rougher as the exposure time texp increases to 3 min. This higher roughness has been previously attributed to the propagation of the crystallization front of PC in the depth of the film upon solvent induced crystallization. We will show hereafter that the higher roughness of the PC surface is not detrimental to the orientation ability of the PC substrates. Rather, the thicker crystalline overlayer of the PC films ensures a higher thermal stability of these oriented substrates (vide infra). Fig. 2 shows the evolution of the optical path difference ı and the RMS roughness r measured by AFM (2 ␮m × 2 ␮m surface) of oriented PC alignment layers as a function of increasing solvent vapor exposure duration (texp ). Both, ı and r show the same overall evolution with texp : (i) a decrease until texp = 2 min, (ii) a rapid increase until texp = 3–4 min and (iii) a saturation. The highest birefringence is observed for the rubbed films and it decreases rapidly upon exposure to acetone vapors at room temperature. The decrease of birefringence and film roughness for texp ≤ 2 min reflects the plasticization of the PC layer during the swelling of the film with acetone. Upon plasticization, the microgrooves of the rubbed PC films are removed, suppressing the corresponding birefringence. Whereas the PC films exhibit a very small birefringence for texp = 2 min, AFM reveals a regular nanostructured surface morphology corresponding to a thin semi-crystalline PC overlayer. For longer exposure times, the films show an increasing birefringence which saturates for texp ≥ 3 min. This birefringence arises from the oriented crystallization of the PC films due to solvent induced crystallization. The increase of birefringence for texp >2 min indicates that the crystallization of the oriented PC domains propagates from the surface down into the bulk of the rubbed PC film. However, the saturation

of ı and r for texp ≥ 3 min suggests that the solvent induced crystallization has reached a limit either because the entire film has been crystallized or because only a crystalline top layer has been generated. Grazing-incidence X-ray diffraction measurements (not shown here) confirm the bilayer structure of the PC substrates with an oriented crystalline top layer and an amorphous underlayer [24]. To summarize, the most regular nanostructured PC substrates with the minimum roughness are obtained for an exposure time of 2 min. For higher exposure times, the RMS roughness and the film birefringence increase first and saturate for texp > 3 min. As seen in the following, the thermal stability of the PC alignment layers in terms of orienting ability is related to the thickness of the oriented crystalline overlayer. 3.2. Morphology and structure of the oriented films of ZnPc and ttb-ZnPc Intrinsically, phthalocyanines tend to form crystalline or liquid crystalline phases with characteristic columnar arrangements [25]. In a preliminary experiment, we have verified the stacking tendency of ttb-ZnPc molecules despite the presence of the bulky terta-tert-butyl side groups which are expected to hamper the efficient packing of the molecule. It has been reported that ttbZnPc thin films evaporated on substrates kept at room temperature are essentially amorphous [26]. Accordingly, crystallization of the evaporated films was induced by solvent vapor annealing [27]. The as-deposited ttb-ZnPc films were exposed to vapors of methanol for 1 h. Fig. 3 depicts the High Resolution TEM (HR-TEM) images of ZnPc and ttb-ZnPc crystalline domains. Both ZnPc and ttb-ZnPc molecules tend to stack and form columns with inter-column periods of 1.23 ± 0.5 nm and 1.65 ± 0.5 nm, respectively. The increased inter-column period for ttb-ZnPc is related to the presence of the tert-butyl side groups. This columnar arrangement is expected to lead to oriented thin films when deposited on polymer alignment layers. In Figs. 4 and 5 we compare the bright field (BF) images and the electron diffraction (ED) patterns of the oriented ttb-ZnPc and ZnPc thin films grown on the oriented PC substrates at two different substrate temperatures. The AFM topographic images of ttb-ZnPc and ZnPc thin films grown on PC substrates at different substrate temperatures are shown in Fig. 6. In the case of ZnPc, the films are made of highly oriented and elongated nanocrystals with a typical

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Fig. 3. High resolution transmission electron microscopy image showing the columnar arrangement of the molecules in ZnPc and ttb-ZnPc thin films. The ZnPc film was obtained at Ts = 100 ◦ C on oriented PC substrate whereas the ttb-ZnPc film was deposited on glass substrate and subjected to solvent vapor annealing using methanol (60 min). The insets show the molecular structures of ZnPc and ttb-ZnPc.

average length in excess of 400 nm and a regular width increasing slightly between 120 and 150 nm. As opposed to this, the films of ttb-ZnPc do not show distinct crystalline features, even at elevated substrate temperatures. The TEM BF indicates that the surface of the ttb-ZnPc films grown at Ts = 125 ◦ C is smooth and featureless whereas the AFM topography reveals a granular topography. For

Ts = 170 ◦ C, the surface topography observed by AFM shows some elongated features in the direction perpendicular to the PC chain direction, but no distinct nanocrystals with sharp grain boundaries are observed. The electron diffraction patterns of the phthalocyanine films shown in Fig. 5 give further information on the Ts -dependence of

Fig. 4. TEM bright field images showing the thin film morphology as a function of substrate temperature for oriented thin films of ttb-ZnPc and ZnPc deposited on oriented and nanostructured PC alignment layers (texp = 2 min). For the ttb-ZnPc films, the TEM BF was obtained for a large defocus in order to enhance the contrast and observe the oriented morphology.

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Fig. 5. Evolution of the electron diffraction patterns with substrate temperature for oriented thin films of ttb-ZnPc and ZnPc deposited on oriented and nanostructured PC alignment layers (texp = 2 min). For all samples, the rubbing direction was oriented horizontally. The ED patterns have been filtered with a high-pass filter to enhance the intensity of the weakest reflections.

in-plane orientation. The ED pattern of the ttb-ZnPc films grown at Ts = 140 ◦ C exhibits a rather broad Scherrer ring at dhkl = 0.33 nm, pointing at a very low level of crystallinity and the absence of in-plane orientation. For Ts = 170 ◦ C, the ED pattern of the ttbZnPc films show two sets of arced reflections at dhkl = 1.63 nm and 0.33 nm. The first reticular distance corresponds to the intercolumn period of ttb-ZnPc and is identical to the period observed by HR-TEM in the thin films crystallized by solvent vapor annealing (see Fig. 3). The orientation of this reflection with respect to the rubbing direction of the PC films indicates that the ttb-ZnPc columns are preferentially oriented parallel to the crystalline PC lamellae, similarly to the situation observed for ZnPc [23]. The second reflection at dhkl = 0.33 nm gives the stacking period within the ttb-ZnPc columns [28]. In the case of ttb-ZnPc, the tilting of the molecular plane in the columns is hindered by the steric crowding caused by the bulky tert-butyl side groups. It has been proposed that the ttb-CuPc molecules can form stacks such that (i) the plane of the macrocycle is perpendicular to the column axis and (ii) successive molecules are in an eclipsed configuration, i.e. rotated by 18◦ one with respect to the other to ensure an efficient packing of tetra-tert-butyl side-groups [28]. In contrast to ttb-ZnPc, ZnPc films show well defined ED patterns with arced reflections already for Ts = 100 ◦ C. For Ts = 130 ◦ C, the ED pattern of ZnPc films is typical of the ␣-II polymorph identified by Kobayashi et al. [29]. For Ts = 170 ◦ C, extra reflections at dhkl = 1.01 nm are observed on the equator of the ED pattern. This reflection can be indexed as the 2 0 −1 reflection of the ␤ form

of ZnPc [30]. Accordingly, the ZnPc films grown at Ts = 170 ◦ C are polymorphic and composed of the ␣-II and ␤ crystalline structures. Contrary to ttb-ZnPc, the plane of the phthalocyanine macrocycle is inclined with respect to the stacking axis in both the ␣-II and ␤ structures of ZnPc, which results in larger stacking periods (0.38 nm for ␣-II and 0.485 nm for ␤) [29,30]. As proposed in our earlier study, the in-plane orientation of the phthalocyanine columns in the direction parallel to the lamellae of the polycarbonate substrate is presumably due to anisotropic diffusion of the phthalocyanine molecules on the PC substrate showing a shallow surface nanocorrugation because of the periodic alternation of crystalline lamellae and amorphous interlamellar zones. A nanocorrugation (with an amplitude below 1 nm) is well observed in the AFM topographic Image 1.a for texp = 2 min. Accordingly, the growth mechanism at play bears some similarity with graphoepitaxy which is usually observed in the case of substrates showing a periodic meso or nano-scale pattern [31,32]. The major difference in the growth of ttb-ZnPc and ZnPc concerns the crystallinity of the films. ZnPc molecules can crystallize in the form of long nanocrystals whereas the substituted ttb-ZnPc tend to order on a smaller length scale, presumably in the form of a few columnar stacks oriented parallel to the lamellae of the PC substrate. The preferential alignment of the columns parallel to the lamellae rather than the polymer chain direction is at variance with respect to other oriented polymer substrates like PTFE and PE. For instance, in the case of perylo(1,12-b,c,d)thiophene deposited on highly oriented polyethylene substrates, the nanocrystals orient

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Fig. 6. Comparison of the film morphology of ttb-ZnPc and ZnPc thin films evaporated on oriented and nanostructured polycarbonate substrates for different substrate temperatures. For all the samples, the rubbing direction is oriented vertically as indicated by the arrow.

parallel to the polymer chains, i.e. perpendicular to the crystalline PE lamellae [33]. This orientation was explained in terms of epitaxy involving some lattice matching between the overlayer and substrate unit cells. In the present case, the fact that (i) two phthalocyanines with very different unit cell parameters orient on the PC substrates and (ii) that the columns orient parallel to the PC lamellae rather than the PC chains suggest that orientation is enforced by a graphoepitaxial-like mechanism rather than true epitaxy based on substrate-overlayer unit cell matching. Nevertheless, graphoepitaxy is often related to the existence of preferential nucleation sites, e.g. macrosteps of the substrate [19,34]. This type of oriented growth also termed ledge-directed nucleation has been investigated by Ward and coworkers [34] whereas preferential nucleation at macrosteps was demonstrated by Chakraverty and Pound [35]. In the present case, no macrosteps are present on the oriented PC substrate. As a consequence, the oriented growth mechanism cannot be ascribed to the nucleation of oriented aggregates as in the case of TiOPc and pentacene on PTFE substrates [19]. As shown in a forthcoming study, the preferential growth of phthalocyanines is best described by a dynamic process involving a progressive selection of the in-plane orientation of the phthalocyanine nanocrystals. 3.3. Effect of substrate temperature on the in-plane orientation of the films The level of in-plane orientation of the ttb-ZnPc and ZnPc films is manifested in the anisotropy of the UV–vis absorption. In the following section, ZnPc and ttb-ZnPc films have been deposited on the smoothest PC alignment layers, i.e. for texp = 2 min. Fig. 7 depicts the UV–vis absorption spectra of oriented ZnPc and ttb-ZnPc films grown on oriented PC substrates at two different substrate temperatures. The UV–vis spectra of ZnPc films grown at Ts = 130 ◦ C

shows the expected structure of the Q band for films in the ␣ structure with two components located at 630 nm and 715 nm. A small shoulder at 750 nm is observed for Ts = 170 ◦ C; it points at the development of the ␤ phase in the films, in agreement with the ED results. The overall spectrum of ttb-ZnPc is not significantly modified with increasing Ts . The UV–vis spectra of the ttb-ZnPc film show the characteristic Soret and Q bands centered around 350 nm and 640 nm, respectively. The Q band shows a typical shoulder around 675 nm. The position of the shoulder coincides with the Q band of ttb-ZnPc in solution in xylene [28]. It is accordingly attributed to a fraction of disordered “amorphous” phase present in the evaporated films. This is further consistent with the fact that the intensity of the 675 nm peak decreases with increasing substrate temperature (see Fig. 7). Whereas the ttb-ZnPc films grown on PC substrate at Ts = 125 ◦ C show only marginal anisotropy, a significant anisotropy is observed for Ts = 170 ◦ C. This result is in line with the ED pattern of Fig. 5 showing almost no orientation for the ttb-ZnPc films grown at Ts below 140 ◦ C and a sizable orientation for Ts = 170 ◦ C. Fig. 8 compares the Ts -dependence of the dichroic ratio A// /A⊥ at the maximum of the Q band for ttb-ZnPc and ZnPc thin films. As shown in Fig. 8, the dichroic ratio of the optical absorbance at 640 nm increases substantially as Ts increases between 125 ◦ C and 170 ◦ C reaching a maximum for Ts = 150 ◦ C. The major difference between oriented ttb-ZnPc and ZnPc films lies in the value of Ts corresponding to the maximum of the dichroic ratio. For ZnPc, the dichroic ratio reaches a maximum for Ts = 100–110 ◦ C, whereas for ttb-ZnPc the best orientation is observed for Ts = 150 ◦ C. The decrease of the dichroic ratio for the ZnPc films for Ts > 110 ◦ C coincides with the presence of the ␤ structure in the films which tends to increase in proportion as Ts increases. However, AFM and electron diffraction indicates that the in-plane orientation in the polymorphic ZnPc films is not decreasing for 110 ◦ C ≤ Ts ≤ 150 ◦ C.

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Fig. 9. Dependence of the dichroic ratio of the optical absorption A// /A⊥ of ttb-ZnPc with increasing substrate temperature Ts for various orienting substrates: open triangles: PC substrates obtained for texp = 2 min. Full circles: PC substrates obtained for texp = 3 min. Full stars: PTFE substrate.

3.4. Orienting ability and thermal stability of PC vs. oriented PTFE substrates

Fig. 7. Effect of substrate temperature on the anisotropy of the optical absorption in ZnPc and ttb-ZnPc thin films grown on oriented substrates of PC (texp = 2 min) at different substrate temperatures. The absorption spectra are obtained for the PC chain direction oriented parallel (//) and perpendicular (⊥) to the incident light polarization.

Accordingly, the observed decrease of the dichroic ratio is not related to a loss of orientating ability of the PC substrate. The reason for the decrease in dichroic ratio is the fact that the intrinsic anisotropy of the Q band absorption band is lower for the ␤ form than for the ␣ form [24]. As a consequence, the higher the content of ␤ phase in the films, the lower the observed dichroic ratio as Ts increases in the range 110–150 ◦ C.

As demonstrated in Section 3.1, the thickness of the oriented and crystallized PC layer tends to increase when the exposure time to the solvent vapor increases, especially for 2 min ≤ texp ≤ 3 min. We have accordingly examined the orienting capability of PC substrates corresponding to texp = 2 min (PC2) and texp = 3 min (PC3) and compared it with that observed for a reference substrate of friction transferred PTFE. Figure S1 of the supporting information shows the typical optical absorption spectra for ttb-ZnPc thin films grown on oriented PTFE substrates for the same deposition conditions as for the films grown on PC substrates. The overall spectra of ttb-ZnPc on PTFE are identical to those observed for PC substrates with the typical Soret and Q bands at 350 nm and 640 nm. The Ts -dependence of the dichroic ratio for ttb-ZnPc films on the different substrates is shown in Fig. 9. The PC2 and PC3 substrates induce a similar orientation of ttb-ZnPc until Ts = 150 ◦ C. However, for Ts ≥ 150 ◦ C, the dichroic ratio of ttb-ZnPc thin films continues to increase with Ts for PC3 substrates whereas for the PC2 substrates, it clearly decreases. Since the glass transition temperature of PC, Tg ∼ 150 ◦ C, this observation suggests that the surface structure of the PC2 films is altered when the substrates are annealed at Ts ≥ Tg . As opposed to PC2 substrates, the PC3 films keep their orienting ability presumably because of the thicker crystalline overlayer. In other words, the orienting ability of the PC3 substrate is not altered when the substrates are annealed in vacuum during the deposition of the phthalocyanines. Finally, the orienting ability of the PC substrates was also compared with the reference samples of PTFE deposited by friction transfer. Fig. 9 indicates that for Ts ≤ 125 ◦ C, a slightly higher dichroic ratio is observed for the films grown on the PTFE substrates, whereas for Ts ≥ 140 ◦ C, the opposite tendency is observed for the PC3 substrates with enhanced thermal stability. For PC2 substrates, the dichroic ratio of the ttb-ZnPc films lies close to or slightly below that of the films grown on PTFE substrates. Accordingly, it can be concluded that the oriented and nanostructured PC3 layers are an interesting alternative to the PTFE substrates and can be used at elevated substrate temperatures as high as 170 ◦ C. 4. Conclusion

Fig. 8. Evolution of the dichroic ratio of oriented ZnPc and ttb-ZnPc thin films grown on oriented and nanostructured PC substrates (2 min acetone vapor exposure) as a function of the substrate temperature during deposition.

This study has demonstrated that oriented and nanostructured PC films constitute an interesting alternative to rubbed polyimide layers and friction transferred PTFE films to orient various phthalo-

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cyanines deposited from the vapor phase. Although the orientation of ttb-ZnPc on PC does not lead to the growth of well defined nanocrystals as observed for ZnPc, both phthalocyanines form columnar arrangements oriented preferentially parallel to the crystalline lamellae of the PC substrate. For both ttb-ZnPc and ZnPc, the in-plane orientation of the molecular columns results in an important dichroism. The orientation achieved in the phthalocyanine films is a function of substrate temperature. For the smoothest PC alignment layers (texp = 2 min), the highest level of in-plane orientation is observed around Ts = 110 ◦ C for ZnPc and Ts = 150 ◦ C for ttb-ZnPc. The orienting capability of the PC substrates for T ≥ 150 ◦ C (Tg of PC) is determined by the thermal stability of the PC substrates. Alignment layers with improved orienting capability beyond the glass transition temperature of PC are obtained for texp = 3 min. The growth mechanism responsible for the orientation of the phthalocyanine columns parallel to the crystalline lamellae of the PC substrate is somehow similar to graphoepitaxy but needs to be further clarified, e.g. by investigating the structure of the oriented and crystalline PC overlayer (fiber symmetry versus preferential contact plane of the crystalline lamellae). Acknowledgments Christophe Contal is acknowledged for his valuable help in AFM. Laurent Herrmann and Gérard Strub are gratefully acknowledged for the design and fabrication of the rubbing machine. SU acknowledges financial support by the Franco-Thai PHC “Nanostructured Interfaces in Electroactive Organic Architectures” (project PHC THAI 16599UG). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.synthmet.2010.11.029. References [1] [2] [3] [4]

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