NEXAFS Spectroscopy Reveals the Molecular Orientation in Blade

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NEXAFS Spectroscopy Reveals the Molecular Orientation in BladeCoated Pyridal[2,1,3]thiadiazole-Containing Conjugated Polymer Thin Films Shrayesh N. Patel,†,◊ Gregory M. Su,‡ Chan Luo,†,§ Ming Wang,†,§ Louis A. Perez,◊,‡ Daniel A. Fischer,& David Prendergast,@ Guillermo C. Bazan,†,◊,‡,§,∥ Alan J. Heeger,†,◊,‡,§,# Michael L. Chabinyc,*,†,◊,‡ and Edward J. Kramer†,◊,‡,⊥

Downloaded by UNIV OF SHEFFIELD on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.macromol.5b01647

†

Mitsubishi Chemical Center for Advanced Materials, ◊Materials Research Laboratory, ‡Materials Department, §Center for Polymers and Organic Solids, ∥Department of Chemistry and Biochemistry, ⊥Department of Chemical Engineering, and #Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, United States & National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States @ The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The characterization of the microstructure and molecular orientation is critical to understanding the performance of conjugated polymer semiconductors. In this work, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to study the molecular orientation of bladecoating thin films of regioregular PCDTPT (poly[4-(4,4dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-alt[1,2,5]thiadiazolo[3,4-c]pyridine]) on substrates with and without uniaxial nanogrooves. The prediction of NEXAFS spectra through density functional theory calculations allowed for the interpretation of the experimental spectral features and provided information about molecular orientation. Using the polarization dependence of the Nitrogen 1s to π*-resonance signals, the molecular orientation was quantified through calculations of the order parameters S (out-of-plane) and η (in-plane) for both the top side and bottom side of the film. All films have out-of-plane orientation where the conjugated backbones have a preferential “edge-on” alignment relative to the substrate surface. On the other hand, with increasing blade-coating rates, the greatest degree of in-plane polymer-chain orientation occurs on the bottom side of a film deposited on a nanogrooved substrate. The results demonstrate the utility of the nanogroove method to induce alignment of solution-processed semiconducting polymers.

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INTRODUCTION

the interconnectivity between crystalline domains in terms of tie chains and low-angle domain boundaries plays a critical role in enhancing long-range charge transport.6 Transport along a conjugated polymer backbone is efficient because of the strong electronic coupling between monomers. However, it is nontrivial to align films where the fast transport direction is oriented along the direction of the flow of electrical current in devices. Therefore, a large number of techniques have been applied to obtain aligned films in order to maximize charge carrier mobility.7−30 Some of these methods include using a rubbed polyimide surface as an alignment layer,9,31 directional crystallization,16,32 high-temperature rubbing,24,25 and zone-casting or blade-coating methods.17,20,23

Significant advances have been made in the molecular design of conjugated polymers.1−5 These designs have yielded better control of electronic proprieties and morphologies of thin films that lead to relatively high charge carrier mobility (>1 cm2/(V s)).3 The design of solution-processable conjugated polymers leads to anisotropies in charge transport due to the presence of solubilizing side chains along the polymer backbone, leading to different intermolecular electronic coupling in crystalline or aggregated domains.6 The charge transport is most efficient along the conjugated polymer chain, the π−π stacking direction is the second most efficient route, and the lamellar alkyl stacking direction is essentially insulating. Consequently, the size and the orientation of these domains, relative to the direction of transport, significantly impact the charge carrier mobility. However, these crystalline or aggregated domains are much smaller than the transport distance (10 to 100s of micrometers) in devices such as thin film transistors. Therefore, © XXXX American Chemical Society

Received: July 24, 2015 Revised: August 22, 2015

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DOI: 10.1021/acs.macromol.5b01647 Macromolecules XXXX, XXX, XXX−XXX

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data, we calculated the order parameters S, describing the outof-plane orientation of the conjugated plane relative to the surface normal (i.e., “edge-on” or “face-on”), and η, describing the extent of in-plane orientation of the conjugated polymerchain axis relative to the alignment direction (Figure 2). We


Uniaxial nanogrooved substrates can be used as a topographic guide for the alignment of polymers. Rubbing a substrate (Si/SiO2) with a diamond lapping paper generates uniaxial nanogrooves, which are approximately ∼50−100 nm in width and a few nanometers in depth (Figure S1).33−35 Recent work has focused on the alignment of regioregular PCDTPT (Figure 1),36 a donor−acceptor copolymer consisting of a

Figure 2. Schematic of the parameters used to qauntify the magnitude of biaxial orienation. (a) S describes the conjugated plane orienation relative to the surface normal (out-of-plane orienation). (b) η describes the conjugated polymer chain orienation in the plane of the film relative to the alignment axis (in-plane orienation). See text for more details on the calculation of order parameters. Figure 1. (a) Chemical structure of regioregular PCDTPT. (b) DFToptimized structure showing PT-CDT-PT-CDT repeat unit (with metyyl side chains). Each unique carbon is labeled for both the CDT unit (D1-D5) and PT unit (A1-A5). PT = [1,2,5]thiadiazolo[3,4c]pyridine acceptor unit, and CDT = cyclopenta[2,1-b:3,4-b′]dithiophene donor unit.

observe that in all films the conjugated plane has a preferential “edge-on” orientation. In addition, we find significant in-plane orientation in these films demonstrating the utility of the nanogroove method to induce alignment of solution processed semiconducting polymers.

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cyclopenta[2,1-b:3,4-b′]dithiophene (CDT) donor unit, and a [1,2,5]thiadiazolo[3,4-c]pyridine (PT) acceptor unit. Using nanogrooved substrates, thin films were cast using a slow casting method through capillary action. This casting method generated highly aligned films with high field-effect charge carrier mobility34,35 (>20 cm2/(V s)). Although this casting method yields macroscopically aligned films, the long casting time (∼6−8 h)35 is not practical for industrial scale-up. In this report, we employed the more rapid blade-coating method (doctor blading) to generate aligned thin films of PCDTPT using nanogrooved substrates. We quantify the resulting alignment at both the buried substrate (bottom side) and air (top side) interfaces of these films using surface sensitive nearedge X-ray absorption fine structure (NEXAFS) spectroscopy. Charge transport in thin film transistors occurs in a thin interfacial region, on the order of one to two molecular layers, at the polymer-gate dielectric interface.37 It is therefore important to understand molecular orientation at such interfaces. Polarization-dependent NEXAFS spectroscopy is a useful tool to probe the molecular orientation at the surface of conjugated polymer thin films.38,39 NEXAFS is a soft X-ray technique where the photon energy can be tuned to probe π* or σ* orbitals of atoms like C, N, and S. The absorption of the polarized soft-X-ray is maximized when the electric field vector (E) of the incident X-ray is in the same direction as the transition dipole moment (TDM) of a given 1s to π* or 1s to σ* transition. The absorption process results in the excitation of a core electron to an antibonding orbital (1s to π* or σ*). When the core hole fills, an X-ray photon (fluorescence) or an Auger electron is emitted. Detection of escaped Auger electrons is from the top 2 nm surface of the film, thus permitting highly surface sensitive characterization.40 In this paper, we present the first reported C and N K-edge NEXAFS spectra for PCDTPT and theoretical calculations of the spectra that help facilitate the assignment of the experimental spectra. Using polarization-dependent NEXAFS

METHODS

Regioregular PCDTPT (Figure 1a) was synthesized using a previously reported method.36 The PCDTPT used in this study has a numberaverage molecular weight of (Mn) of 51 000 g/mol (Đ ∼ 3.18) as determined using gel permeation chromatography (GPC) with chloroform as the eluent solvent (polystyrene calibration). Solutions of PCDTPT in chlorobenzene (5 mg/mL) were blade-coated on Si/ SiO2 (300 nm) substrates with a self-assembled monolayer (SAM) of n-decyltrichlorosilane (DTS). Grazing incidence wide-angle X-ray scattering (GIWAXS) images were obtained using beamline 11-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) located on the SLAC National Accelerator Laboratory campus. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments were performed on the U7A NIST/Dow end station at the National Synchrotron Light Source (NSLS) located on the Brookhaven National Laboratory (BNL) campus. Experimental NEXAFS spectra were obtained in partial electron yield (PEY) mode using a channeltron electron multiplier with an adjustable entrance grid bias voltage set to −150 V. Theoretical NEXAFS spectra were calculated through a computational method developed by Prendergast et al. at The Molecular Foundry on the Lawrence Berkeley National Laboratory (LBNL) campus.41,42 Additional method details are available in the Supporting Information.

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RESULTS AND DISCUSSION Blade-Coating Films on Nanogrooved Substrates. Blade-coating is a scalable unidirectional casting technique that allows for rapid fabrication of polymer films. Thin films of PCDTPT were cast using a home-built blade-coating system housed in a N2 atmosphere glovebox. Nanogrooved substrates were obtained through uniaxial rubbing with a diamond lapping paper (Figure S1). After functionalizing the nanogrooved substrate with a DTS SAM, the nanogrooved substrate was placed on a temperature controlled stage. A glass microscope slide was used as the blade, which was placed at a 60° angle relative to the substrate surface and formed a blade-to-substrate gap height of ∼100 μm (Figure 3). The substrate was oriented such that the blade will nominally translate parallel to the B


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Figure 3. Blade-coating schematic where the blade angle is set to 60° and a gap height ∼100 μm. The blade is nominally translated parallel to nanogrooves.

nanogroove direction, unless otherwise stated. Then, ∼8 μL of polymer solution was placed in the gap between the blade and substrate. The motorized translation stage was activated, and the blade translated across the length of the substrate. The resulting thin films were annealed at 200 °C in a N2 atmosphere glovebox for 8 min. This temperature was chosen as it provided optimal field-effect transistor device mobility values according to Tseng et al.33 However, 200 °C does not correspond to a specific phase transition temperature according to differential scanning calorimetry (DSC) experiments where no discernible phase transitions were observed between the experimental window (−20 to 300 °C).36 To study the effect of blade-coating rate on orientation, solutions of PCDTPT in chlorobenzene were coated at three translation velocities and substrate temperatures: 0.03 mm/s at 28 °C, 0.30 mm/s at 50 °C, and 0.6 mm/s at 80 °C, thus spanning slow and moderate coating rates. At these coating speeds and temperatures, the evaporation zone was located near the edge of the blade as it translated across the substrate. These coating conditions resulted in homogeneous films covering the whole surface of the substrate with a thickness between ∼30−40 nm according to atomic force microscopy (AFM) measurements. The elevation in coating temperature was needed at faster speeds due to the high boiling point of chlorobenzene and the nonpolar substrate surface (DTS SAM layer). Attempts to coat films at 0.3 and 0.6 mm/s at room temperature (28 °C) yielded either no film or very nonuniform films. According to previous studies from the group, regioregular PCDTPT thin films are semicrystalline.34,43 PCDTPT crystallizes to form fibrils where the polymer-chain axis is parallel to the long direction of the fibril.34 Blade-coated films form these fibrils as seen in AFM height image of both the top and bottom surfaces shown in Figure S3. GIWAXS Reveals the Orientation of Crystallites. We performed GIWAXS experiments to probe the microstructure of the blade-coated thin films. Samples were oriented such that the incident X-ray was either parallel or perpendicular to the direction of the nanogrooves. In Figure 4, we show representative GIWAXS images for a film blade-coated at 0.3 mm/s at T = 50 °C on a nanogrooved substrate. The scattering profiles show that the lamellar alkyl stacking reflections (h00) are primarily out-of-plane, and the π−π stacking reflection is primarily in-plane; therefore, the crystallites adopt a preferential “edge-on” orientation (Figure S4a). The (100) reflection at q ∼ 2.51 nm−1 indicates that the characteristic lamellar packing distance between the alkyl side chains, d100, is ∼2.50 nm. We assign the peak at q ∼ 17.8 nm−1 to the interchain π−π stacking

Figure 4. 2D Grazing incidence wide-angle X-ray scattering (GIWAXS) images for the case where the grazing incident X-ray is (a) parallel to the nanogroove direction and (b) perpendicular to the nanogroove direction. These patterns are for a film blade-coated at 0.3 mm/s and T = 50 °C. (c) The in-plane line cuts for both parallel and perpendicular GIWAXS patterns. The higher intensity of the π−π peak in the parallel case provides qualitative confirmation of alignment in the films. The three additional reflections (black arrows) are seen at q ∼ 11.1 nm−1, q ∼ 14.5 nm−1, and q ∼ 16.4 nm−1 are described further in the Supporting Information (Figure S4). C


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Macromolecules distance, dπ−π, of ∼0.353 nm. In addition, the higher intensity of the π−π stacking peak for the parallel configuration relative to the perpendicular configuration indicates preferential inplane alignment of the crystallites relative to the nanogroove direction (Figure 4c). It is important to note the difference of the in-plane scattering intensity of the π−π stacking reflection is used only for qualitative confirmation of alignment in the films because we are only probing crystallites where the scattering vector is appropriately aligned to see diffraction (diagram in Figure S4c). Assignment of NEXAFS Spectra. NEXAFS spectroscopy is a powerful tool for probing the molecular orientation at the surface of thin films. While GIWAXS probes the orientation of crystalline domains, NEXAFS spectroscopy probes the molecular orientation of both crystalline and noncrystalline domains. A clear description of the NEXAFS spectral features arising from the chemical composition and unique bonding environment of the molecule is critical before determining orientation information. Traditionally, peaks are assigned or attributed through the comparison to similar materials, and a calibrated library of C K-edge NEXAFS spectra have been reported for a variety of commonly studied conjugated polymers.44 In Figure 5, we show representative experimental C K-edge and N K-edge PEY NEXAFS spectra of PCDTPT. The C K-edge NEXAXS spectrum in Figure 5b shows several peaks from 284 to 286.7 eV, which correspond to C 1s to π* transitions from the CC and CN bonds. The presence of several distinct C 1s to π* peaks are not typically resolved in some semiconducting polymers such as poly(3-hexylthiophene) (P3HT) or 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT) but have been observed in various phenylenevinylene derivatives44 and donor−acceptor polymers such as poly{N,N′-bis(2-octyldodecyl)-1,4,5,8-napthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P[NDIOD-T2]).23 The absorption peaks at 287.6 eV are typically assigned to be C 1s to σ* transitions from C−S and C−H bonds while the broad peaks centered around 293.2 and 305 eV are assigned to be C 1s to σ* transitions from C−C bonds of the backbone and side chains, respectively. With respect to N K-edge spectra (Figure 5b), the broad absorption structures at ∼410 and ∼420 eV are N 1s to σ* transitions. As reported for BT (benzothiadiazole)-containing polymers, the first peak is assigned to a N 1s to π* transition.45 Thus, we can reasonably assign the first peak at ∼400 eV to a π* transition, which arises from the CN bonds in the PT unit. For the case of the BT-containing polymer, it was suggested that the additional N 1s to π* transitions occur below the ionization potential (∼405 eV).45,46 As a result, the peaks at 401.3, ∼402.7, and ∼403.8 eV could be N 1s to π* transitions as well. Density functional theory (DFT) calculations of NEXAFS spectra can further help elucidate the detailed assignment of the experimentally observed transitions and provide the direction of the transition dipole moments.41,42 The DFT-optimized geometry of an A−D−A−D repeat unit (minus side chains) shown in Figure 1b was used as input atomic coordinates for calculating the NEXAFS spectra of PCDTPT. Figure 5a compares the calculated C K-edge spectrum with the experimental spectrum between the E range of 283 and 287 eV. The calculated and experimental spectra are in reasonable agreement with respect to the observed C 1s to π* transitions. It is important to note that the experimental NEXAFS spectrum was collected in PEY mode where the Auger

Figure 5. (a) C K-edge spectrum for PCDTPT. The inset shows the comparison between the experimental and calculated spectrum in the region of C 1s to π* transitions. The vertical red lines correspond to the π*-resonance peak positions for each unique carbon (Figure S5). (b) N K-edge spectrum for PCDTPT and the corresponding calculated N K-edge spectrum. The vertical red lines correspond to the resonant peak positions for each individual nitrogen atom (limited to values less than 404 eV for clarity) (Figure S6). The calculated N Kedge spectrum was shifted an additional +1.25 eV to match the first experimental peak position (refer to Supporting Information for D


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transitions while any signal past ∼401.8 eV starts to include σ* character. Further analysis of the calculated N K-edge spectra reveals unique spectral features that can be used to probe backbone orientation. The conventional route to assess the molecular orientation of a conjugated polymer is through the polarization dependence of the π*-resonance peaks because the π* TDM is always defined as a vector orthogonal to the conjugated plane. On the other hand, due to the aromatic nature of the polymer backbone, the TDM of σ* transitions are not typically anisotropic and consist of both x and y components and thus are not an ideal spectroscopic tool to probe the orientation of conjugated polymers. The x, y, and z components of the calculated N K-edge spectra (Figure S7) indicate the majority of σ*-resonance peaks have both x and y components. However, the σ*-resonance peak at ∼403.6 eV is primarily from the x component of the TDM, thus parallel to the molecular axis. Furthermore, this transition occurs from the excitation of a core electron from both thiadiazole nitrogen atoms (N2 and N3). The electronic dipole moment of the lone-pair electrons on N2 and N3, which are primarily oriented parallel to the x-axis, influences the transitions observed at ∼403.6 eV. In addition, we also observe a small σ*-resonance peak at ∼402.5 eV for core-excited N1, which is primarily from the y component. For the core-excited N1 atom, the lone-pair electrons are nominally oriented along the y-axis, thus potentially influencing the anisotropy of the TDM. Overall, the ability to discover subtle features like the σ*-resonance peaks at ∼402.5 and ∼403.6 eV through theoretical calculations allows us to develop novel routes for probing molecular orientation and demonstrates the need to use simulations to understand details that cannot be determined from experiments alone. Understanding Molecular Orientation Using Polarized NEXAFS. To assess the effect of blade-coating conditions on the extent of biaxial orientation, polarization-dependent NEXAFS spectra were experimentally obtained as a function of both the angle of the incident soft X-ray (θ) and sample configuration as shown in Figure 6. The electric field vector (E) is primarily orthogonal to the incident soft X-ray (polarization factor, P = 0.85). The samples were measured in the

Figure 5. continued


details on the energy alignment of calculated spectra). (c) Molecular orbitals for the excited state corresponding to the first π-resonance of each nitrogen atom (N1 = pyridal unit, N2 and N3 = thiadiazole unit).

electrons escape from the top few nanometers of the film surface. The NEXAFS calculations do not account for escape depth, which may account for the difference in intensity between the π*-resonance peaks. In addition, the experimental spectrum corresponds to a film with orientation, which is not accounted for in calculated spectrum. This dependence on orientation may also contribute to the difference between the intensities. We can further deconvolute the calculated spectrum into contributions from each unique C atom of the CDT and PT units to determine if the excitation of a specific C atom core electron corresponds to a particular π* resonance signal. In general, attributing peaks between donor−acceptor units is a nontrivial process as shown by Gann et al. for naphthalene diimide−thiophene copolymers.47 As shown in Figure S5, the core−electron excitation of each unique carbon contributes to all the observed π-resonance peaks. However, we can deduce that the π* transition at 284.6 eV is primarily from the coreexcited carbon D5 in the CDT unit. In addition, the highest π* transitions near 286.5 eV are from carbons A2 and A3 of the PT and carbons D3 and D4 in the CDT. The N K-edge NEXAFS spectra is complex despite the presence of only three nitrogen atoms in the repeat unit and provides a unique spectroscopic handle to determine the orientation. Figure 5b compares the calculated N K-edge spectrum with the experimental spectrum between the E range of 397 and 415 eV. The calculated and experimental spectra are in reasonable agreement. Because we are only looking at the excitation of electrons from the three nitrogen atoms, the deconvolution analysis is greatly simplified with respect to the carbon K-edge calculations. The calculated NEXAFS spectra for each N atom are shown in Figure S6 while the breakdown between x, y, and z components of the transition dipole moment (TDM) is shown in Figure S7. The z component of the NEXAFS spectra corresponds to the polarization orthogonal to the conjugated plane; hence, in the direction of the π* TDM for the N 1s to π* transitions. The first peak at ∼400 eV corresponds to a π* resonance. The excitation of electrons from the nitrogen atoms from the pyridal unit (N1) and the thiadiazole unit (N2 and N3) all contribute to the first π*-resonance at ∼400 eV. For the core-excited N1, the LUMO is delocalized between both donor and acceptor units (Figure 5c). In contrast, for core-excited N2 and N3, the LUMO is primarily localized around the acceptor unit (PT). For BTcontaining copolymers, which consist of same N atoms as N2 and N3 of the PT unit, Gliboff et al. observed similar localization of the LUMO around the acceptor unit.45 The LUMO from the excitation of the N1 core electron could be more delocalized because the corresponding CN bond is in conjugation with the CDT unit. A strong second π* transition occurs at ∼400.8 eV. Interestingly, this transition is only from the core-excited N1. Additional higher order π* transitions occur at ∼401.5 and ∼402.5 eV primarily from N2 and N3. However, these signals are mixed in with a strong σ*-resonance peak. Therefore, we can conclude the large peak at 400 eV and the small peak at 401.3 eV in the experimental spectrum are π*

Figure 6. NEXAFS experimental geometry for perpendiuclar and parallel configurations. In the perpendicular configuration, the electric field vector is “perpendicular” to the alignment direction at normal incident angle (θ = 90°) and the sample is rotated about the x-axis (alignment axis) to study the polarization dependence. For the parallel configuration, the sample is first rotated 90° about the z-axis. As a result, at θ = 90°, the electric field vector is “parallel” to the alignment direction. Then, the sample is rotated about the y-axis (orthogonal to the alignment axis) to study the polarization dependence. Red arrows correspond to the 1s to π* transition dipole moments. The alignment direction is parallel to the nanogrooves (i.e., parallel to coating direction). The example shown above is for a film with perfect alignment and primarly edge-on orientation. E


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Macromolecules perpendicular and parallel configurations in order to probe the biaxial orientation. In the perpendicular configuration, the sample is first placed where E is perpendicular to the nanogroove direction at normal incidence (θ = 90°). Then, NEXAFS spectra were obtained at various values of θ to characterize the angular dependence of the absorption spectrum at the perpendicular configuration. Next, the sample is manually rotated 90° about the z-axis to obtain the parallel configuration where E is now parallel to the alignment direction at normal incidence. NEXAFS spectra were obtained at various values of θ to characterize the angular dependence of the absorption spectrum at the parallel configuration as well. The NEXAFS spectra were obtained for both the top side (surface) and the bottom side (film−substrate interface) of the film. To perform NEXAFS experiments on the bottom side of the film, the films needed to be delaminated from the substrate surface. NEXAFS experiments of films delaminated from the surface using dilute hydrofluoric acid were challenging due to some unknown CC-containing contaminate molecule from the HF solution (refer to Supporting Information for more details, Figure S8). Therefore, we used a pressure-sensitive adhesive (Scotch Magic Tape) to delaminate the film from the substrate. As shown in Figure S9, a NEXAFS measurement of the substrate after delamination shows no π* resonance signals in the C K-edge spectrum and no signal in the N K-edge spectrum, which indicates the entire thin film was removed off the substrate. The polarization anisotropy of the π*-resonance peaks in the perpendicular and parallel configurations allows for the qualitative description of the extent of biaxial orientation. Figure 7 shows the N K-edge NEXAFS spectra for the bottom side of a film taken in the perpendicular configuration and parallel configuration. This film was blade-coated along the uniaxial nanogrooves (0.3 mm/s, T = 50 °C). As seen in Figure 7, the difference in the intensity of the π*-resonance peaks (at 400 and 401.3 eV) from θ = 30° to 80° corresponds to a clear indication of polarization anisotropy. The increase in the intensity of the π*-resonance peaks from θ = 30° to 80° qualitatively indicates that the aromatic rings tend to adopt a more edge-on orientation about the polymer-chain axis. The polarization-dependence of π*-resonance peaks in the parallel configuration describes the orientation of the polymer-chain axis relative to the nanogroove direction. As shown in Figure 7b, the π*-resonance peaks in the parallel configuration do not show any significant polarization anisotropy and have a lower πresonance intensity with respect to the perpendicular configuration, which indicates a preferred orientation of the polymer-chain axis along the nanogroove direction. As discussed earlier regarding calculated NEXAFS spectra, the σ*-resonance peak at ∼403.8 eV corresponds to the σ* TDM primarily along the molecular axis, which can be confirmed through the experimental polarization-dependent NEXAFS spectra. The intensity of σ*-resonance peak at θ = 80° (near normal incidence) is higher in the parallel configuration (Figure 7b) relative to the peak intensity in the perpendicular configuration (Figure 7a). The higher intensity is not surprising as E is parallel to the polymer-chain axis when there is preferential alignment along the nanogroove direction. This higher intensity experimentally confirms that the TDM of the σ*-resonance peak at ∼403.8 eV is along the molecular axis, thus providing another spectroscopic handle to experimentally probe molecular orientation.

Figure 7. N K-edge partial electron yield (PEY) data for the bottomside of a film blade-coated on a nanogrooved substrates at 0.3 mm/s and T = 50 °C. Scans were taken at θ = 30°, 51°, and 80° in (a) perpendicular and (b) parallel configurations. (c) The peak intensity of the N 1s to π* resonance at ∼400 eV as a function of sin2 θ. The solid markers are the experimental results, and the solid lines are linear fits.

In addition, we can use the polarization-dependent C K-edge NEXAFS results to describe the orientation in the film (Figure F


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Macromolecules Table 1. TDM Orientation Factors (f x, f y, and fz), Order Parameters S, and η for Blade-Coated Films blade-coating condition

top or bottom

0.03 mm/s T = 28 °C

top bottom

0.3 mm/s T = 50 °C

top bottom

0.6 mm/s T = 80 °C

top


bottom

nanogrooves?

fx

fy

fz

S

η

no yes no yes no yes no yes no yes no yes

0.55 0.31 0.41 0.26 0.47 0.39 0.43 0.17 0.31 0.23 0.31 0.12

0.33 0.56 0.52 0.58 0.44 0.54 0.48 0.67 0.60 0.66 0.61 0.76

0.12 0.15 0.07 0.17 0.09 0.07 0.09 0.16 0.12 0.14 0.09 0.10

−0.32 −0.28 −0.40 −0.25 −0.37 −0.40 −0.37 −0.26 −0.32 −0.29 −0.37 −0.35

−0.33 0.38 0.17 0.48 −0.05 0.23 0.075 0.75 0.44 0.65 0.45 0.96

S10). The polarization dependence of the π*-resonance peaks (at 285.2, 285.7, and 286.7 eV) from θ = 30° to 80° in the perpendicular and parallel configuration follows the same trends seen with the N K-edge spectra. The polarization dependence confirms that the aromatic rings tend to adopt a more edge-on orientation and a preferred orientation of the polymer-chain axis along the nanogroove direction. The qualitative agreement between the C K-edge and N K-edge data is not surprising as the 1s to π* TDMs are orthogonal to the polymer-chain axis for both CC and CN bonds and there is no torsion angle between the CDT and PT units. We will confirm later that the order parameters calculated using the π*-resonance peaks from the C or N K-edge are equivalent. Biaxial Order Parameters Reveal Quality of Alignment. The quantification of the molecular orientation using NEXAFS follows the method develop by Stöhr and coworkers.48 Three TDM orientation factors, f x, f y, and fz, are defined to describe the relative alignment of the TDM of the π*-orbital along three orthogonal axes: x (alignment direction), y (in-plane perpendicular to alignment direction), and z (normal to the surface). In other words, the three orientation factors are projections of the π* TDM on the x-, y-, and z-axes, thus the fraction of the TDM of the molecules along three x-, y-, and z-axes. The three TDM orientation factors can be determined from the fit coefficients of a linear relationship describing the polarization-dependent (i.e., angular dependent) 1s to π* resonance intensity (I) 2

I(θ ) = A + B sin (θ )

equal to zero for our polymeric material.7 Using the normalization condition f x + f y + fz = 1 yields the expression Itot =

S=

(2a)

fy =

A + B⊥ Itot

(2b)

(

A⊥ + B 1 − fz =

Itot

1 P

1 (3f − 1) 2 z

(4)

In addition, the biaxiality parameter (η), which gives the orientation of the π* orbitals in the x−y plane, is defined to be the following: η=

3 (f − fx ) 2 y

(5)

When S = 0 and η = 0 (f x = f y = fz = 1/3), π* TDMs are randomly orientated along all three axes. However, we assume that the polymer chains lie in the plane of the film (x−y plane), which is consistent with GIWAXS results. Therefore, f x = f y = 0.25 and fz = 0.5, which means that S and η are 0.2 and 0, respectively. These order parameter values define the case for no orientation in the film. When S = 1 (fz = 1), the π* TDMs are perfectly orientated along z-axis, which means the conjugated planes of the molecule are perfectly face-on relative to the surface. When S = −0.5 (fz = 0), the π* TDMs are orthogonal to the z-axis, which means the conjugated planes of the molecule are perfectly edge-on and all π* TDMs are in the x−y plane. When S is large and negative, η > 0 corresponds to a preference of the polymer-chain axes to orient along the alignment direction (f y > f x), while η < 0 corresponds to a preference of the polymer-chain axes to orient orthogonal to the alignment direction (f x > f y). For the case of S = −0.5 (fz = 0), η = 1.5 for perfect polymer-chain alignment, η = 0 for no polymer-chain alignment, and η = −1.5 for orthogonal polymer-chain alignment.

where A and B are fit coefficients. We apply eq 1 to the parallel configuration and perpendicular configuration to define I∥(θ), I⊥(θ), A∥, A⊥, B∥, and B⊥. Using these fits coefficients, the three TDM orientation factors can be calculated using the following equations:

A⊥ + B Itot

(3)

Using the TDM orientation factors f x, f y, and fz, we can calculate two order parameters: (1) S, the uniaxial order parameter, which describes the orientation of the conjugated plane of the polymer backbone relative to the z-axis (i.e., orientation of the π* TDM relative to surface normal), and (2) η, the biaxiality parameter, which describes the orientation of the polymer-chain axis relative to the alignment direction. fz = cos2 α, where α equal is the angle between the TDM and z-axis. Using the second Legendre polynomial expression for the structural order parameter, S = 1/2(3⟨cos2 α⟩ − 1), we obtain the following equation for the uniaxial order parameter in terms of fz:

(1)

fx =

3 3P − 1 (A + A⊥) + (B + B ⊥ ) 2 2P

) (2c)

where P is the beamline-specific polarization factor (0.85) and Itot is the total integrated intensity. Stöhr et al. used an additional term for the surface tilt angle, γ, which we assume is G


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Macromolecules Order Parameters from Either C or N K-Edges Yield Similar Values. As discussed earlier, the order parameters can be calculated using either the C 1s to π* transitions or N 1s to π* transitions. When using the C 1s to π* transition at 285.2 eV, S = −0.40 and η = 0.23 (Figure S11) for the top side of a thin film blade-coated at 0.3 mm/s. When using the N 1s to π* transition at 400 eV, S = −0.37 and η = 0.21 (Figure S12). The S and η values are quite similar when using either the C K-edge or N K-edge. One may use either the C K-edge or the N Kedge to determine the molecular orientation of PCDTPT films, but due to some background signal from the adhesive in the C K-edge (refer to Supporting Information sections VI and VII), we proceeded to focus on N K-edge polarization-dependent NEXAFS data to calculate order parameters. Influence of Nanogrooves and Processing Methods on Biaxial Orientation. The calculated biaxial order parameters allows us to the compare the effects of bladecoating rates and nanogrooves on the orientation in the film. In Table 1, the values of f x, f y, fz, S, and η are presented for the top and bottom side of films blade-coated and annealed at 200 °C on substrates with and without nanogrooves. When characterizing the orientation for films on substrates without nanogrooves, we define the blade-coating direction as the alignment direction. The values fz and S do not vary significantly for all blade-coating conditions. The values of fz range from 0.07 to 0.17, which correspond to S values of −0.40 to −0.25. In Figure 8a, the values of S are plotted to show the effects of the nanogrooves and blade-coating rates. For the bottom side, the S values are greater (less negative) with the presence of nanogrooves. This difference is greatest at 0.03 mm/s where S = −0.25 with nanogrooves and S = −0.40 without nanogrooves. This trend indicates that the presence of nanogrooves results in a slightly less edge-on orientation (especially at 0.03 and 0.3 mm/s) on the bottom side. Overall, the values of S indicate all films have some degree of out-ofplane uniaxial orientation where the conjugated plane of the polymer backbone adopts a preferential “edge-on” orientation. The calculation of the average conjugated plane tilt angle (γ) relative to the surface normal (γ = 90 − α) ranges from 17° to 24°. These values are comparable to the γ range of 20° to 30° for poly(3-hexylthiophene) (P3HT), poly(2,5-bis(thiophen-2yl)thieno[3,2-b]thiophene) (PBTTT), and poly(3,3′-dialkylquaterthiophene) (PQT)49,50 and the γ range of 7.5° to 22.4° for shear-coated poly(2,5-bis(thiophene-2-yl)-(3,7-diheptadecanyltetrathienoacene) (P2TDC17FT4).51 We observe significant variation in f x, f y, and η, which indicates chain orientation relative to the alignment direction strongly depends on the blade-coating conditions. As presented in Table 1, f x ranges from 0.12 to 0.55. When the polymerchain axis is perfectly oriented along the alignment direction, f x = 0. In Figure 8b, the values of η are plotted to show the effects of the nanogrooves and blade-coating rates. Overall, the alignment is best for films blade-coated on nanogrooved substrates as indicated by large values of η. For example, for the bottom side, η = 0.75 with nanogrooves while η = 0.075 without nanogrooves at 0.3 mm/s. The best alignment occurs at 0.6 mm/s where η = 0.96. Therefore, the presence of the nanogrooves is critical for alignment of polymer chains for the blade-coating rates used in this study. A film was blade-coated perpendicular to the nanogrooves in order to investigative significance of coating direction relative to the nanogroove direction. For a film blade-coated at 0.3 mm/s perpendicular to the nanogrooves, S = −0.46 and η = −0.044 on the bottom

Figure 8. Order parameters (a) S and (b) η as a function of bladecoating rates. The order parameters are calculated for the top and bottom side of the film with and without the presence of nanogrooves. The dashed lines are to serve as a guide to the eye.

side. Therefore, coating perpendicular to the nanogrooves results in a greater edge-on orientation but does not induce polymer-chain alignment on the bottom-side. In addition, higher blade-coating rates results in better alignment. For the bottom-side with nanogrooves, η increases from 0.48 to 0.96 at 0.03 and 0.6 mm/s, respectively. Blade-coating rates have the greatest effect for the top-side without nanogrooves where η increases from −0.33 to 0.435 at 0.03 and 0.6 mm/s, respectively. In addition, the comparison of η between the top and bottom side indicates the polymer chains are more aligned near the substrate interface with nanogrooves. However, we observe the difference decreases with increasing blade-coating rates without nanogrooves. The extent of alignment at 0.6 mm/s is essentially equal without nanogrooves for the top and bottom side (η ∼ 0.45). Thermal Annealing Enhances Orientation. The order parameters were calculated for an as-cast film to determine the effects of annealing at 200 °C. For a film that is bladecoated parallel to the nanogroove direction at 0.3 mm/s but without annealing, we calculate that S = −0.18 and η = 0.52 on the bottom side. Therefore, the as-cast blade-coated film does result in a certain degree of orientation. However, the annealing step increases the edge-on orientation (to S = −0.26) and improves the orientation of the polymer-chain axis relative to the nanogrooves (to η = 0.75). As discussed earlier, the H


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Macromolecules

for deeper grooves). Assuming the extreme case of a fully extended polymer chain, the polymer chain length would be ∼72 nm (Mn = 50 kg/mol) for the PCDTPT used in this study (ignoring molar weight distribution). This length scale of a polymer chain is a good reference point on how easily the polymer-chain axis orients across the width of the grooves (50− 100 nm) or across the length (essentially infinitely long). Also, from the characteristic π−π stacking spacing of ∼0.35 nm, approximately 142−285 polymer chains can exist across the width of the grooves, most likely divided up between distinct fiber bundles. The contact angle at the leading edge of the solution during blade-coating plays an important role in aligning the polymer chains. The determination of the contact angle is nontrivial as the presence of the nanogrooves and the solution shearing will alter the contact angle (contact angle hysteresis). The polymer film will form at the edge of the sheared solution drop. Also, the polymer aggregates will be most concentrated near the contact line where the contact angle is smallest. At the most rapid coating conditions (0.6 mm/s at T = 80 °C), the contact angle may be smaller and the evaporation rate would be more rapid due to the elevated temperature. Therefore, there would be faster polymer solution flow toward the evaporation front along with a higher concentration of polymer aggregates, thus yielding the best alignment after thermal annealing (η ∼ 1).

annealing temperature does not correspond to a specific phase transition with the experimental window of the measurement.36 The film is most likely well above the glass transitions temperature (Tg) for PCDTPT, thus allowing enough chain mobility for improved orientation. Limits of Molecular Alignment. We can qualitatively compare the orientation in our blade-coated films relative to other alignment techniques. Overall, our highly oriented films on nanogrooved substrates have both a preferential out-ofplane orientation (edge-on) and in-plane chain-axis orientation (biaxial orientation). Pattison et al. casted films of poly(9,9dioctylfluoroene-co-bithiophene) (F8T2) on a rubbed polyimide alignment layer, which yielded significant in-plane orientation throughout the film after annealing above the nematic to isotropic transition temperature.7 However, these F8T2 films showed no preferential edge-on or face-on orientation. Similarly, only uniaxial in-plane orientation was observed for directionally crystallized P3HT.16 Similar to our blade-coated films, biaxial oriented thin films were observed for a variety of systems: (1) flow-coated and subsequently meltannealed PBTTT,17,38 (2) slot-coated23 and dip-coated P(NDIOD-T2),29 (3) dip-coated cyclopentadithiophene− benzothiadiazole (CDT-BTZ) copolymer,30 (4) high-temperature mechanical rubbing,25 and (5) strain-aligned P3HT.27 More recent work by Giri et al. looked at solution shear-coating various semiconducting polymers (PQT, pBTTT). Unlike the other methods, the solution shear coated films did not have significant in-plane chain alignment with the reported conditions (only out-of-plane edge-on orientation). While we show several techniques yield aligned films, the alignment process is nontrivial, and driving forces such as our uniaxial nanogrooves are critical to inducing alignment. Mechanism of Biaxial Orientation on Nanogrooved Substrates. The polymer-chain alignment on nanogrooved substrates involves several interrelated processes such as solution shear stress, solution contact angle, polymer aggregation in solution, polymer diffusion, and solvent evaporation. The mechanism of orientation is complicated further because we are simultaneously varying shear rate and evaporation rate for each different coating condition (i.e., changing both blade-coating speed and temperature). Overall, the NEXAFS study indicates that blade-coating parallel to the nanogrooves, and subsequent thermal annealing is critical for obtaining films with optimal polymer-chain alignment. The polymer-chain shape and aggregation play an important role on the solid-state film formation. PCDTPT tends to aggregate in solution according to previously reported solution UV−vis results.36 Therefore, the nanogrooves serve as highdensity nucleation sites for the crystallization and growth of the fibers from solution. The fibers would nucleate and grow ideally along the nanogrooves. Furthermore, the size of the aggregates plays an important role on how they interact with the grooves. Considering the width of the nanogrooves ranges from 50 to 100 nm, the size of aggregates in solution will dictate if they can enter the nanogrooves and how they orient within the nanogrooves. Further work is needed to characterize the size of the polymer aggregates in solution. With respect to the solid-state film, it is important to note the size of the grooves relative to a polymer molecule. Considering that the grooves have a depth of a few nanometers and the film adapts an edge-on orientation with characteristic lamellar alkyl stacking spacing of ∼2.5 nm, we can assume a single molecular layer of PCDTPT chains are in the grooves (perhaps two layers

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CONCLUSIONS We have characterized the molecular orientation of bladecoated films of regioregular PCDTPT using polarizationdependent NEXAFS at the C K-edge and N K-edge. DFT calculations of NEXAFS spectra facilitated the assignment of experimentally observed spectral features. Interestingly, we discovered the σ* transition at 403.6 eV corresponds to a transition dipole moment (TDM) primarily along the polymerchain axis. This transition provides a unique route in studying polymer-chain orientation with respect to the conventional polarization dependence of the π*-resonance peaks. Our work here shows how one may use theoretical NEXAFS calculations to predict the absorption spectrum and discover unique spectroscopic handles to probe the orientation of molecules before running synchrotron experiments, thus maximizing the use of experimental time. Using the polarization dependence of the N 1s to π* resonance signals, the molecular orientation was quantified through the calculation of the order parameters S (out-of-plane orientation) and η (in-plane orientation). We observe all films have uniaxial orientation out-of-plane where the conjugated planes have a preferential “edge-on” orientation (S → −0.5). The bottom side of films on nanogrooved substrates always had the best alignment as indicated by the large positive η values. The best alignment occurs at a more rapid blade-coating rate of 0.6 mm/s and T = 80 ◦C (η = 0.96 and S = −0.35). The extent of in-plane orientation has direct implications on the electrical properties as charge transport is fastest along the conjugated polymer chain. The higher η values implies better alignment of the fast transport direction with respect to the preferred charge transport pathway. Consequently, our results provide guidelines on the field-effect transistor device fabrication where a bottomgate configuration (with nanogrooves) and rapid blade-coating rates yield aligned films needed, in principle, for enhanced charge transport. I


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ASSOCIATED CONTENT

* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01647. Nanogrooved substrate preparation, GIWAXS methods, NEXAFS calculations methods, NEXAFS experimental methods, AFM images, GIWAXS line cuts and schematics, calculated NEXAFS spectra, film delamination details, and additional experimental NEXAFS spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.L.C.). Present Address Downloaded by UNIV OF SHEFFIELD on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.macromol.5b01647

REFERENCES

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L.A.P.: Apeel Sciences, Inc. 819 Reddick Street, Santa Barbara, CA 93103. Notes

The authors declare no competing financial interest. E.J.K.: Deceased, December 27, 2014.

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ACKNOWLEDGMENTS Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. For molecule geometry optimization, we acknowledge support from the Center for Scientific Computing at the CNSI and MRL: an NSF MRSEC (DMR-1121053) and NSF CNS-0960316. Simulations for NEXAFS calculations used resources of the Molecular Foundry and National Energy Research Scientific Computing Center, which was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. AFM images were taken using the MRL Shared Experimental Facilities: supported by the MRSEC Program of the NSF under Award DMR 1121053; a member of the NSF-funded Materials Research Facilities Network. G.M.S. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under Contract DE-AC0506OR23100. L.A.P. acknowledges support from the ConvEne IGERT Program (NSF-DGE 0801627) and a Graduate Research Fellowship from the National Science Foundation (GRFP). Commercial names mentioned do not constitute an endorsement by the National Institute of Standards and Technology. The authors thank Dr. Cherno Jaye, Brandon Wenning, Hilda Buss, and Dr. David Calabrese for experimental NEXAFS assistance. The authors thank Dr. Christopher J. Takacs, Hung Phan, and Prof. Thuc-Quyen Nguyen for helpful discussions. J


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NEXAFS Spectroscopy Reveals the Molecular Orientation in Blade

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