Hierarchical Orientation of Crystallinity by Block-Copolymer Patterning

Article pubs.acs.org/cm

Hierarchical Orientation of Crystallinity by Block-Copolymer Patterning and Alignment in an Electric Field Pola Goldberg-Oppenheimer,*,†,§ Dinesh Kabra,†,⊥ Silvia Vignolini,† Sven Hüttner,† Michael Sommer,‡,∥ Katharina Neumann,‡ Mukundan Thelakkat,‡ and Ullrich Steiner*,† †

Department of Physics, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K. Applied Functional Polymers, Macromolecular Chemistry I, Universität Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany § Department of Electrical Engineering, CAPE, University of Cambridge, Cambridge CB3 0FA, U.K. ∥ Makromolekulare Chemie, Universität Freiburg, Stefan-Meier-Straße 31, 79100 Freiburg, Germany ‡

S Supporting Information *

ABSTRACT: Electron and hole conducting 10-nm-wide polymer morphologies hold great promise for organic electro-optical devices such as solar cells and light emitting diodes. The selfassembly of block-copolymers (BCPs) is often viewed as an efficient way to generate such materials. Here, a functional block copolymer that contains perylene bismide (PBI) side chains which can crystallize via π−π stacking to form an electron conducting microphase is patterned harnessing hierarchical electrohydrodynamic lithography (HEHL). HEHL film destabilization creates a hierarchical structure with three distinct length scales: (1) micrometer-sized polymer pillars, containing (2) a 10-nm BCP microphase morphology that is aligned perpendicular to the substrate surface and (3) on a molecular length scale (0.35−3 nm) PBI π−π-stacks traverse the HEHL-generated plugs in a continuous fashion. The good control over BCP and PBI alignment inside the generated vertical microstructures gives rise to liquid-crystal-like optical dichroism of the HEHL patterned films, and improves the electron conductivity across the film by 3 orders of magnitude. KEYWORDS: hierarchical electrohydrodynamic patterning, electron-conducting block-co-polymer, orientation of crystallinity, anisotropic optical behavior

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In terms of the first issue, progress was recently made with the synthesis of poly(3-hexylthiophene)-block-(perylene bismide acrylate) (P3HT-b-PPerAcr) BCPs,5 which have a welldefined microphase morphology and exhibit good electron and hole transport.6,7 Several approaches have been demonstrated to tackle the second problem. The control over BCP orientation can be achieved by mechanical shear forces,8 temperature gradients,9 epitaxy,10 electric fields,11,12 and annealing in a solvent vapor.13,14 The feasibility of combining flow- and electrically induced BCP alignment has been tested for amorphous dielectric copolymers by destabilizing BCP films in an electric field.15,16 The resulting shear-flow of polymer into micrometer-wide and high columns, in combination with an electrostatic torque, results in the rapid and effective vertical alignment of the BCP morphology within these structures. The ultimate goal of the research effort involving the synthesis of BCPs comprising donor and acceptor blocks and the control of their self-assembly in a device geometry is the creation of a fully functional device. While this has been

INTRODUCTION Vertically oriented microdomains in binary semiconducting polymer films are highly sought after because of the great promise they hold for applications in organic photovoltaic devices. Conceptually, interpenetrated 10-nm-wide domains of hole-transporting (donor) and electron-transporting (acceptor) materials that span across the entire photoactive layer are thought to be ideal to maximize charge separation and transport. Vertically oriented nanodomain morphologies generated by the self-assembly of semiconducting blockcopolymers (BCPs)1 are often mentioned as the possible approach for combining charge transport with sufficient optical absorption on a length scale that is commensurate with the exciton diffusion length.2−4 Although BCP self-assembly of random-coil polymers is a highly efficient process, its use in organic photovoltaics is limited by several complexities that have to be overcome. First, the synthesis of BCPs with conjugated blocks is complex, and their self-assembly differs from the generic case because of the intramolecular interactions of the blocks. Second, the interactions of the blocks with the surfaces typically results in an orientation of the microdomains parallel to the substrate, preventing their use in vertical diode junctions. © 2013 American Chemical Society

Received: November 25, 2012 Revised: March 7, 2013 Published: March 10, 2013 1063

dx.doi.org/10.1021/cm3038075 | Chem. Mater. 2013, 25, 1063−1070

Chemistry of Materials

Article

Figure 1. (a). Molecular structure of the PS-b-PPerAcr block copolymer consisting of an inert polystyrene block and a block with PBI crystalline side chains. (b) Schematic of the HEHL patterning process. The thin BCP film in the capacitor is liquefied and a capillary surface instability is triggered and amplified by applying the voltage U. With time the instability causes the formation of liquid bridges between the two plates. The insets show BCP microphase morphologies before (lying cylinders) and after HEHL patterning (vertical cylinders). (c) SEM micrograph revealing the morphology of a spin-cast thin BCP film. (d) Cross-sectional TEM image perpendicular and parallel (inset) to the surface of a spin-cast film showing a lying-cylinder morphology. Inset: cross-sectional TEM image of an ultrathin sample microtomed parallel to the film and the epoxy support (black on the right-hand side). The images were stained in a ruthenium tetroxide vapor to improve contrast.

attempted for about a decade,5,17−20 the realization of a BCPbased photovoltaic device with ordered and vertically aligned domains remains a grand challenge. Here, we take a significant step forward in this direction by creating a self-assembled functional BCP microstructure consisting of a vertically oriented cylindrical block copolymer spanning across a patterned polymer film. This 10-nm mesophase aligned morphology further induces an additional internal orientation of the crystalline moieties on the molecular length scale. A poly(perylene bisimide acrylate) (PPerAcr) polymerized to a polystyrene macroinitiator forming polystyrene-block-poly(perylene bismide acrylate) (PS-b-PPerAcr) was chosen as a material containing an electron conducting block (Figure 1a). In PPerAcr, perylene bismide (PBI) units are attached to a polyacrylate backbone, exhibiting π−π stacking of

the dangling PBI moieties, leading to the side-chain crystallinity of PPerAcr. This polymer is therefore an excellent electron acceptor and conductor, with electron mobilities up to 10−3 cm2/(V s).6,21 The BCP can be lithographically guided into design patterns by using a topographically structured topelectrode. While BCPs typically form an in-plane microphase morphology in the initial film, shear-gradients combined with electrostatic stresses during the HEHL patterning15,16 at the polymer−polymer interfaces give rise to a rapid alignment of the BCP microphase morphology perpendicular to the substrate surface. This mechanism was characterized in detail by Zhou et al. for lamellae-forming BCPs.22 The HEHL method makes use of a capacitor device that includes a BCP film-covered electrode opposed by a second electrode with either a planar surface or a topographic design pattern. The 1064



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Figure 2. AFM height images of HEHL generated patterns: (a) Array of pillars with local hexagonal symmetry formed in a parallel-plate capacitor by applying an electric field and thermal annealing. Similar structures were obtained while softening the film in a solvent vapor atmosphere. (b) Array of squares replicating a given design top-electrode liquefied by raising the temperature above the softening point of both BCP blocks and (c) square arrays of pillars generated by an additional design top-electrode in film that was liquefied in a solvent vapor atmosphere.

Figure 3. Perpendicularly oriented PS-b-PPerAcr microdomains that have formed inside HEHL generated structures. (a) Surface SEM image showing the surface of a BCP film that was thermally annealed for 24 h at 215 °C in the presence of an electric field generated by an applied voltage of 70 V. (b) Cross-sectional TEM of (a), revealing the orientation of the cylindrical microdomains normal to the substrate. (c) AFM height image of a PS-b-PPerAcr film swollen in chloroform vapor for 5 h with 80 V applied across the capacitor electrodes. This led to improved packing of 13 ± 3 nm wide cylinders. The insets in (a) and (c) are FFTs of the images. (d) Cross-sectional TEM micrograph of (c) revealing the alignment of the cylinders continuously spanning the two electrodes.

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Figure 4. (a) Optical anisotropy analysis of oriented PS-b-PPerAcr patterns compared with the two reference samples. The samples were measured between crossed (black) and parallel (red) polarizing filters. The full circles correspond to the linear dichroism of light transmitted through an oriented PS-b-PPerAcr microphase morphology. Thermally annealed (in the absence of an electric field) and the as-spun samples correspond to the dotted line and squares, respectively. (b) Schematic of the HEHL-generated structural hierarchy consisting of micrometer-sized columns composed of an aligned hexagonal microphase morphology (tens of nm) with molecular order created by the crystalline stacking of the PBI moieties, similar to an earlier study.7.

application of a voltage U across the two capacitor plates gives rise to an electric field in the capacitor gap. The use of a planar top electrode typically results in the formation of a locally hexagonal array of columns. For a structured electrode, the reduced distance between downward protruding topography and the film locally enhances the electric field, triggering film destabilization and focusing the electrohydrodynamic instability toward these protrusions. This yields a replica of the electrode pattern spanning the capacitor gap at the locations of the smallest interelectrode distances. The final height of the polymer structure is determined by the plate spacing. The resulting morphology represents the optimal orientation of the copolymer nanostructure with respect to the substrate allowing to access the stacked PBI nanomorphology, which should give rise to enhanced charge carrier percolation between the two surfaces of the film. Such a charge transport across the entire device is appealing for a range of technological applications, in particular for organic photovoltaic cells.

thermodynamically preferred because of the differing surface energies of the two blocks at one or both interfaces. To carry out HEHL alignment of the cylinder array, the films were confined by assembling a capacitor, leaving a few-hundred nm thick air gap between the film and the upper electrode. The assembly was then heated to T = 215−220 °C, above the glass transition temperature (Tg) of PS and the melting temperature (Tm) of PPerAcr (100 and 189 °C, respectively). Alternatively, the capacitor was placed in an atmosphere of chloroform vapor, where the film liquefies by solvent sorption,23 which is equivalent to temperature annealing above Tg and Tm. Subsequently, a voltage of V = 55−80 V was applied between the two electrodes. The experimental setup is shown in Figure 1b. The electric field across the BCP layer interacts with the film on two length scales, the film thickness and the BCP morphological spacing. At the surface, it gives rise to a destabilizing electrostatic pressure prevailing over the restoring surface tension, yielding an undulation instability with a welldefined wavelength.24,25 Figure 2a reveals the pillar pattern that results from the application of a homogeneous electric field across the BCP film. A similar pattern is obtained during the HEHL destabilization of the PBI homopolymer (not shown). If a topographically structured top-electrode is used, the surface instability couples to the lateral field variation, and the growth of the instability is focused in the direction of the highest electric field toward the protruding patterns of the top electrode. This gives rise to a pattern spanning the substrate and the top electrode at locations of smallest interelectrode spacing. The HEHL replication of a design electrode topography (array of squares) is shown in Figure 2b. A similar result is achieved when annealing the capacitor-device in a chloroform-vapor atmosphere to liquefy the BCP film, which

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RESULTS AND DISCUSSION The electron transporting PS-b-PPerAcr with a molecular weight of 38 kg/mol containing 70 wt % PPerAcr was used in this study (Figure 1a). The synthesis of this polymer is described in an earlier publication.21 This polymer forms a cylindrical microphase morphology showing a high electron carrier mobility.6,21 50−400 nm-thick films were spin-cast from chloroform solution onto doped silicon wafers. This resulted in a cylindrical microphase morphology, with the cylinders lying in the plane of the film. This morphology is revealed by scanning electron microscopy (SEM) in Figure 1c and cross-sectional transmission electron microscopy (TEM) in Figure 1d. Cylinder orientation parallel to the film surfaces is typically 1066



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Figure 5. J-V curves of ITO/BCP/ITO sandwiches, as-cast (black squares), solvent vapor annealed (open triangles), thermally annealed (open circles), HEHL patterned during thermal annealing (solid triangles), and HEHL patterned in a chloroform-vapor atmosphere (open squares), with a conductivity improvement by 3 orders of magnitude compared to the as-cast film. The schematic shows the device setup of these measurements.

supported by a shear-flow, which arises from polymer displacement during the formation of the micrometer-sized pattern. Preliminary experiments with temperature-annealed PS-b-PPerAcr in a capacitor without an air gap showed no measurable cylinder orientation, indicating that shear-flow alignment provides an important contribution to the obtained results. The final vertical electrode spanning microphase morphology is however almost certainly a consequence of the electrostatic torque acting on the dielectric boundaries between the two blocks, aligning parallel to the electric field lines. The alignment mechanism of PBI-containing polymers might differ from the purely dielectric case because of free charges in this system. Under the present conditions, however, PBI crystallinity is molten during copolymer alignment and the polymer melt is therefore expected to behave very similar to the previously studied purely dielectric polymers. Interestingly, the HEHL-patterned samples show a very distinct anisotropic optical effect. Figure 4 shows the variation of the transmitted light intensity through an array of 1 μm wide, 700 nm high pillars as function of sample rotation in parallel and cross-polarization channels. For this measurement, the sample was mounted on a motorized stage allowing rotation around the focal axis of the objective (with steps of 10 degrees). This configuration enables transmission and reflection measurements in different polarization directions with respect to the sample orientation. The maximum transmitted intensity of polarized light corresponds to the (partial) alignment of dichroic molecules parallel to the polarizing filter (see Experimental Section for further details). The patterned film clearly shows linear dichroism, periodically modulating the transmitted intensity with rotation of polarization angle. This is reminiscent of liquid crystal alignment and therefore a clear indication that the HEHL patterning of the block copolymer also ensues PBI crystal orientation. The two control samples (as-cast, and annealed in the absence of an electric field) lack this optical response (i.e., no crystal orientation is obtained, as shown by dotted and square lines in Figure 4a exhibiting no periodic modulation of the light intensity). While the optical results (Figure 4a) clearly suggest BCPalignment with respect to the substrate normal, a direct

results in a comparably high-fidelity replica of a different master electrode (square arrays of pillars) (Figure 2c). The obtained patterns had typical heights of 200−850 nm, and the diameters of protruding structures were 0.7−5 μm. The formed primary HEHL micrometer patterns contain an internal substructure composed of an ordered array of cylindrical microdomains with an average diameter of 13 ± 3 nm and a periodicity of 25 nm (see Supporting Information, Figure S2), which is revealed in the scanning transmission electron micrographs (in scanning mode) of Figure 3a. Note that these images were taken of the unmodified sample, without partial degradation or sputtered-on metal. The FFT first and weak second order rings of Figure 3a (see also Supporting Information, S3) indicate a very well-defined intercolumn spacing. The smeared-out six reflexes on the first-order ring are indicative of local hexagonal order. The vertically aligned cylindrical microdomains parallel to the axis of the micrometersized cylinders (along the electric field lines) extend from the free surface down to the substrate, as monitored by SEM (Figure 3b). The results for the vapor-annealed films in Figure 3c and d are very similar. The sharp 6-fold symmetry of the FFT pattern in Figure 3c indicates a much better and longerranged hexagonal packing compared to Figure 3a. The absence of a second order FFT ring implies however a weaker fidelity in the intercolumn spacing. Both observations point toward a higher polymer chain mobility in the microphase separated structure in the solventswollen film, allowing the microphase morphology to anneal toward a higher lateral symmetry, while giving rise to fluctuations of the cylinders around their equilibrium positions, which, when frozen in, account for the lower degree of regularity in the intercolumn spacing. HEHL alignment of the BCP in a solvent-vapor atmosphere is overall preferable, giving rise to a more rapid and complete alignment of the microphase morphology while significantly reducing the risk of thermal degradation of the polymers. The alignment of the cylindrical morphology by HEHL was recently studied in detail for the case of lamellae-forming coil− coil BCPs.22 While BCP cylinders can be aligned by an electric field alone,26 morphology alignment is much quicker when it is 1067



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These results indicate that the PBI in the as-cast films is either not well stacked or does not form a percolating network that connects the two surfaces of the film. This takes place upon annealing. HEHL patterning and extended annealing aligns the BCP microphase morphology. The difference in conductivity of the thermally and solvent-vapor annealed HEHL patterned films reflects the higher degree of order in the latter, providing further evidence that solvent-annealed films have evolved closer to thermodynamic equilibrium in the presence of an applied field. The improved alignment of the BCP cylinders implies an orientation of the PBI and increases the number and quality of percolating conducting connections across the film.

structural analysis (e.g., by X-ray scattering) is difficult because of the small sample volume and because of the distributed nature of the material. It is therefore useful to compare the present PS-b-PPerAcr to the related P3HT-b-PPerAcr copolymer, in which the PPerAcr phase is similarly compartmentalized in a copolymer microphase morphology. The detailed X-ray scattering study revealed a staggered π−π-stacking of the PBI moieties.7 Based on the combined optical results shown in Figure 4a and the earlier results, Figure 4b schematically shows the three levels of hierarchy in HEHL-structured PS-b-PPerAcr polymers. The electrohydrodynamically driven flow gives rise to (1) vertical micrometer-wide cylinders, which consist (2) of coaxially aligned 10-nm BCP cylinders. (3) The PBI side chains form a crystalline structure within the BCP self-assembled domains. The alignment of the BCP morphology seems to provide a preferential direction for PBI stacking and PPerAcr crystal orientation. The linear birefringence of Figure 4a is in agreement with the model in Figure 4b, providing clear evidence of long-range order of π−π-stacking within the microphase morphology, forming a crystalline or liquidcrystalline phase. For comparison, a liquid-crystalline lowmolecular weight PBI molecule with a preferential in-plane orientation of π-stacks27 show an identical anisotropic behavior (see Supporting Information Figure S1). The oriented crystalline stacking of PBI moieties within the PS-b-PPerAcr microphase morphology suggests advantages for electron transport within these structures. The electrical properties of the patterned BCP pillar array were therefore examined. The conductivity across the film was measured for four types of sandwich geometries, which varied in terms of the processing conditions of the films, schematically shown in Figure 5. A symmetric J-V response was obtained for all the samples. The current density was lowest for the as-cast flat films. Annealing by heating above the glass transition temperature and by placing the sample in a chloroform vapor substantially increased the current density, by improving PBI π−π stacking, with a higher degree of crystallinity observed for the temperature annealed case (see absorption spectra in the Supporting Information, Figure S4). HEHL patterning of the PS-b-PPerAcr films further increased the current density. The highest increase in current density was found for HEHL pattering in a chloroform vapor atmosphere by more than a factor of 15 compared to the as-cast film. Given the fact that HEHL-generated patterns occupy only 25−50% of the overall surface, the increase in conductivity compared to the as-cast films was about 2 orders of magnitude. This is in agreement with a previous study on PS-b-PPerAcr field-effect transistors which found an increased electron-mobility of annealed samples with a lateral orientation of the block copolymer across the source-drain direction.6,28 Interestingly, the sample that was temperature annealed in the absence of an electric field had a conductivity which was only about a factor of 2 lower compared to the HEHLtemperature annealed case. This is a clear indication that PBIstacking during annealing is the dominant cause of the conductivity increase. Similar to the earlier P3HT-b-PPerAcr work, an in-plane meandering PPerAcr microphase that occasionally makes contact with the two film surfaces is the probable reason for the relatively high increase in conductivity upon annealing.29

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CONCLUSIONS The pattern formation of a BCP with side-chain crystallizable PBI units by an electric field gives rise to a hierarchical pattern on three length scales. The electrostatic stress at the film surface causes an HEHL instability, leading to a pattern on the micrometer length scale. The shear flow during this pattern formation process combined with the electrostatic torque acting on the BCP microphase boundaries aligns the cylindrical 10-nm mesophase along the axes of the HEHL-generated plugs. The resulting system exhibits local hexagonal order on both length scales. The PBI moieties form the matrix of the BCP, and the hexagonal PS mesophase introduces an orientation to the PBI crystallization which forms a third length scale on the molecular level. The resulting control over the alignment of crystalline order reveals two functionalities. BCP alignment endows the film with polarization-sensitive transmission, which is revealed by its linear dichroism. The quarter-waveplate optical response is reminiscent of a liquid crystal and is therefore testimony to the oriented stacking of the PBI side-chains in PS-b-PPerAcr. The PBI crystal orientation substantially improves electron conductivity across the film, by 2 orders of magnitude compared to the as-cast film, and by 1 order of magnitude compared to a film that was annealed in the absence of an electric field. These results are highly promising because it is likely that they can be extended to BCPs with two functional blocks, such as P3HT-b-PPerAcr or the replacement of PS with a softdegradable block (e.g., a lactide) followed by the backfilling of a functional component. Extending the present work to the HEHL alignment of electronically fully functional microphase morphologies is therefore a promising approach to improve the photovoltaic efficiency.

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EXPERIMENTAL SECTION

Polystyrene-block-poly(perylene bismide acrylate) (PS-b-PPerAcr) block copolymer with a 70 wt % fraction of PPerAcr and a molecular weight of Mw = 38 kg/mol, number-average molecular weight of 37 kg/mol for PPerAcr and of 20.5 kg/mol for the PS, peak molecular weight (obtained via SEC) of 38.8 kg/mol, (Mn block copolymer-Mn PS block)/825g/mol and a polydispersity of 1.07 was used. The block copolymer was purified by preparative SEC. The weight of the macroinitiator was 20.5 kg/mol. The synthesis of this polymer is described elsewhere.21 This polymer self-assembles into a hexagonal microphase morphology in the bulk.21 Highly polished p-doped silicon (Si) wafers, with ⟨100⟩ crystal orientation, (Wafernet Gmbh, Eching, Germany) were used as substrates. Patterned silicon wafers were obtained from X-lith eXtreme Lithography, Ulm, Germany. Indium tin oxide (ITO)-coated glass slides with a resistance of 112 Ω/cm2 were used as transparent electrodes. Thin block copolymer films were spincast from chloroform solutions resulting in 50−400 nm thick films. 1068



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Prior to spin-coating, the substrates were cleaned in a “Piranha” solution consisting of 3:1 H2SO4 (98%):H2O2 (30%), followed by thorough rinsing with deionized water and drying under nitrogen. The ITO-coated glass slides were cleaned by scrubbing in soap water at 75 °C, washing in an ultrasonic bath with acetone and isopropanol, followed by irradiation for 20−30 min in an UV-ozone cleaner. Immediately before device assembly, all substrates and electrodes were subjected to snow-jet cleaning. Facing the coated substrate, a wafer with a planar or topographically structured surface was mounted at a distance defined by photolithographically prepared spacers, leaving a thin air gap to the film (Figure 1b). Both capacitor electrodes were electrically contacted using silver paint (Electrodag 1415M). Thermally evaporated 50 nm-thick Au layers on the reverse side of the electrodes ensured good electrical contacts. The spin-cast films were liquefied either by annealing above the softening temperatures of the two blocks (T = 215−220 °C) or by exposing them to a controlled chloroform vapor atmosphere to induce chain mobility and facilitate equilibration. The solvent vapor pressure was adjusted using a homemade apparatus.23 Mass-flow controllers (MKS Instruments Model 1179A with a PR4000F controller) regulated the flux of the carrier gas, N2 through two lines. In one line, the N2 was bubbled through a solvent-filled bottle resulting in a solvent-saturated gas stream. Both streams were mixed and passed through the sample chamber. The flow volumes per time were individually regulated to values between 1 and 20 cm3 min−1. The vapor pressure in the mixing chamber can be estimated by the ratio of the saturated (psat) to dry gas (p) flow as determined by the flowmeter readout. All tubes and connectors were made from solventresistant materials (glass and Teflon). The chamber and a regulated water bath containing the solvent bottle and the mixing chamber were held at the same temperature. Typical values for the vapor pressures were p/psat = 0.5−0.7. The films were allowed to swell in the controlled solvent vapor atmosphere until they reached their equilibrium thickness after around 30 min.23 This condition was determined by experiments using a sample chamber lid with a window, providing optical access to the chamber. Using a light microscope in reflection mode the change in film thickness as function of time was qualitatively monitored. A voltage of 55−80 V was then applied between the two electrodes. Cooling sample to room temperature (RT), or removal of the solvent by passing dry nitrogen through the sample chamber solidified the polymer before the voltage was removed, terminating the patterning process. After each experiment, the top electrode was manually removed and the formed pattern was analyzed by light microscopy, atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM and TEM), and scanning transmission electron microscopy (STEM). To note, the top electrodes were rendered hydrophobic by deposition of a 1H,1H,2H,2H-perfluorodecyltrichlorosilane self-assembled monolayer. This results in a reduction of the surface energy of the mask and significantly reduces the adhesion of the polymer to the imposed electrode preserving the structural integrity of the formed patterns during the removal of the top electrode. NanoScope IV Multimode and Dimension 3100 (Digital Instruments, Santa Barbara, CA) atomic force microscopes were employed to map the sample topography. All the measurements were performed in air under ambient conditions using the tapping mode. Height and phase images were analyzed with the Nanoscope software (Digital Instruments). The AFM measurements yielded the film thickness h, the characteristic wavelength λ, and the width and height of the replicated patterns. To improve contrast, patterns were exposed to UV-light and rinsed in cyclohexane to remove some of the PS phase. A LEO ULTRA 55 SEM microscope including a Schottky emitter (ZrO/ W cathode) was used to image the cylindrical block copolymer morphology. For TEM cross sectional imaging, a thin layer of Pt was sputtered onto the block copolymer films, followed by embedding into a Spurr epoxy resin. The substrate was removed and the films were sectioned using a diamond knife in a Leica Ultracut Microtome, yielding 50 nm thick sections . The cross sections were analyzed using a FEI Technai 12 transmission electron microscope at acceleration voltages of 80 or 120 kV. The microtomed samples were stained in

ruthenium tetroxide vapors to improve the contrast. STEM images were obtained using a Hitachi s5500 scanning transmission electron microscope with a cold field emission source and lens detector with 4 Å resolution, allowing adjustable acceptance angle STEM imaging. An Olympus BX51 optical polarizing microscope was used to investigate the optical texture of the samples. The optical anisotropy of oriented samples was evaluated in terms of variations of the intensity of reflected or transmitted light using an attached spectrometer (QE6500, Ocean Optics). The microscope halogen lamp acted as an illumination source for the spectroscopic measurements. Linear polarization measurements were obtained using achromatic polarizers (Thorlabs, Inc.) in cross and parallel configurations. The sample was mounted on a multirotational stage allowing rotation around the focal axis of the objective. This configuration enabled transmission and reflection measurements in different polarization directions with respect to the sample orientation. The maximum transmitted intensity of polarized light corresponds to dichroic molecules that are (partially) aligned parallel to the polarizing filter. Current−voltage (J-V) measurements were carried out using a Kiethley 2400 source-meter in air. The top electrode consisted of a small strip ITO on a glass slide to selectively cover a well-defined area of interest. Both top and bottom ITO electrodes were photolithographically patterned to reduce possible leakage currents.

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ASSOCIATED CONTENT

S Supporting Information *

Further details are given the Supplementary Figures 1−4. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.G.-O.), [email protected] (U.S.). Present Address ⊥

Department of Physics, Indian Institute of Technology Bombay, Powai - Mumbai 400076, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.G.O. acknowledges a studentship by Kodak European Research, Cambridge. This work was partially funded by the EPSRC and DFG (GRK 1640).

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REFERENCES

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