Manipulating Nanoscale Morphologies in Cylinder-Forming Poly

Mar 1, 2013 - Manipulating Nanoscale Morphologies in Cylinder-Forming Poly(styrene-b-isoprene-b-styrene) Thin Films Using Film Thickness and Substrate...
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Manipulating Nanoscale Morphologies in Cylinder-Forming Poly(styrene‑b‑isoprene‑b‑styrene) Thin Films Using Film Thickness and Substrate Surface Chemistry Gradients Ming Luo,† Jonathan E. Seppala,†,§ Julie N. L. Albert,†,∥ Ronald L. Lewis, III,† Nikhila Mahadevapuram,‡ Gila E. Stein,*,‡ and Thomas H. Epps, III*,† †

Department of Chemical & Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States

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S Supporting Information *

ABSTRACT: Controlling the nanostructure of self-assembled block copolymer thin films is critical for applications in nanotemplate design, nanoporous membranes, and organic optoelectronics. In this study, we employed a gradient approach to examine the effects of substrate surface chemistry and film thickness on the self-assembly of cylinder-forming poly(styrene-bisoprene-b-styrene) (SIS) thin films. Using gradients in film thickness from 85 to 120 nm (3.1d to 4.4d), we found that the thin films contained parallel cylinders on both bare silicon substrates and benzyldimethylchlorosilane (benzyl silane)-modified substrates regardless of film thickness, while thin films contained surface patterns of hexagonally arranged dots on nbutyldimethylchlorosilane (n-butyl silane)-modified substrates. These surface patterns were further investigated using film etching, cross-sectional transmission electron microscopy (TEM), and grazing-incidence small-angle X-ray scattering (GISAXS) techniques. We determined that the nanostructures represented a hexagonally perforated lamellar (HPL) morphology in which the parallel cylinder layering was preserved during the phase transformation to HPL. Additionally, controlled vapor deposition was used to generate a nearly linear substrate surface chemistry gradient from benzyl silane to n-butyl silane. Examination of SIS thin films on this surface gradient revealed a morphological transformation from parallel cylinders to HPL with changing substrate surface composition. Thus, we demonstrated the combined usage of film thickness and monolayer substrate surface chemistry gradients to manipulate the nanostructure of block copolymer films, such as SIS, that possess moderate differences in surface energy between individual blocks. Our gradients represent a high-throughput and versatile screening tool that facilitates the examination of new materials and furthers the understanding of block copolymer thin film self-assembly.

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INTRODUCTION

fairly well understood, particularly with respect to surface interactions and commensurability effects.2,8−10,12−20 The thin film phase behavior of block copolymers that form cylinders in the bulk is more complex and less understood. The interplay between confinement and surface effects (e.g., wetting layer formation by the minor component21,22 and surface reconstruction10,23−25) is complicated by system asymmetry.10,26−28 Experimental investigations have demonstrated changes in morphology through alterations in the annealing conditions (solvent vapor annealing)29−34 or manipulations of the substrate surface chemistry.19,35−38 For example, Paik and

Block copolymer (BCP) thin films have garnered significant interest for a variety of applications including, but not limited to, nanolithographic masks,1,2 templates for synthesis of inorganic or organic structures,3,4 and nanoporous membranes,5,6 due to their ability to form self-assembled structures with length scales on the order of 10−100 nm.7 While the phase behavior of bulk BCPs depends primarily on the block interactions (interaction parameter, χ), degree of polymerization (N), and block volume fractions (f i), thin film selfassembly is strongly influenced by commensurability considerations (i.e., film thickness) and surface interactions.8−11 Numerous researchers have studied AB-type lamellar forming BCP thin films, and the nanoscale behavior of these systems is © XXXX American Chemical Society

Received: November 22, 2012 Revised: February 5, 2013

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intermediate film thicknesses. At lower solvent contents in the film (stronger surface fields) the cylindrical nanostructures reconstructed to perforated lamellae.28 Horvat et al. presented DDFT simulations of thin films of ABA triblock copolymers that supported the aforementioned phase behavior, where an effective surface interaction parameter was used to represent the solvent concentration.46 Similarly, using Monte Carlo simulations of cylinder-forming asymmetric ABA thin films, Szamel et al. predicted a sequence of perpendicular cylinder to parallel cylinder to perforated lamellar morphologies upon increasing the film thickness for cases in which the film surfaces were slightly attractive to the majority block.47 Thus, the nanoscale orientation and assembly of BCP structures depends not only on the energetics between the copolymer and the surfaces but also on the film thickness and chain architecture. To gain further insight into these effects on the phase behavior of BCP thin films, we systematically investigated the nanoscale morphologies of cylinder-forming SIS thin films, in a high-throughput fashion, using substrate surface chemistry and film thickness gradients. Though cylinder-forming SIS triblock copolymer thin film orientations have been manipulated in a controlled fashion using solvent vapor annealing techniques,29,30,48 thin films of these triblock materials, whose blocks possess significantly different surface energies,49 have not been studied using systematic substrate surface modifications. This surface energy difference also differentiates the studies herein from those on copolymers such as poly(styrene-b-methyl methacrylate) whose blocks possess similar surface energies.37,38 Thus, our work, in which we coat SIS thin films onto chlorosilane-modified substrates, highlights an expansion of designer surface modification methods for the manipulation of thin film nanostructures. Herein, the effects of film thickness and substrate surface chemistry were examined by optical microscopy and atomic force microscopy (AFM). Additionally, we employed successive ultraviolet ozone (UVO) etching steps followed by AFM imaging, grazing-incidence small-angle X-ray scattering (GISAXS), and cross-sectional transmission electron microscopy (TEM) to assess the through-film morphologies. Using gradients in substrate surface chemistry and film thickness, we followed the evolution of nanoscale morphologies and identified substrate surface and film thickness conditions that produced parallel cylinders, HPL, and mixed nanostructures in our thin films.

co-workers39 employed a solvent vapor annealing approach to demonstrate that the morphology of a poly(α-methylstyrene-b4-hydroxystyrene) [PαMS-b-PHOST] thin film can be reversibly tuned by switching the annealing solvent between tetrahydrofuran (THF) and acetone. Using solvent vapors that were selective to the majority PHOST block (i.e., acetone), a phase transformation from cylinders to spheres was facilitated due to the shift in effective volume fraction upon swelling. Huang and co-workers40 studied poly(styrene-b-butadiene) (SB) and poly(styrene-b-butadiene-b-styrene) (SBS) thin films exposed to different solvent droplets (toluene, benzene, cyclohexane) with tunable solvent evaporation rates. Inverted spherical and cylindrical morphologies were found due to the partitioning of solvent into the minority PS block, which changed the effective volume fraction of the copolymer blocks. Substrate surface effects also have been investigated in detail. For example, Huinink and co-workers applied dynamic density functional theory (DDFT) to simulate thermally annealed cylinder-forming SB thin films and suggested that the surfaces may initiate transformations from cylinders to hexagonally perforated lamellae (HPL), and to lamellae in the case of sufficiently strong surface interactions.41 To study these trends experimentally, Tsarkova et al. examined cylinder-forming SB thin films on two chemically different substrates (SiOx/Si and carbon/SiOx/Si).42 They noted that HPL and lamellar morphologies developed on piranha-treated SiOx/Si surfaces due to strong interactions between the substrate surface and the PB block, while parallel cylinders formed on the carbon/SiOx/ Si substrates. Additionally, several research groups have employed random copolymer brushes to investigate the effect of substrate surface chemistry/energy on copolymer thin film orientation; however, the focus mostly has been concerned with creating “neutral” substrate surfaces to generate perpendicularly oriented cylinders.19,37,38,43 Monolayer deposition methods are a viable alternative to copolymer brushes for block copolymer morphology manipulation.17,35,36 The commercial availability of chlorosilane agents for deposition onto silicon oxide surfaces, along with the simplicity of the coating process, makes this approach highly attractive for select applications. Furthermore, surface energy and surface chemistry can be tuned easily by selecting chlorosilane functionalities according to the block copolymer of interest. Recently, Albert et al. introduced a facile controlled vapor deposition method to generate well-controlled surface chemistry/energy gradients for studying the thin film phase behavior of a poly(styrene-b-methyl methacrylate) (PS-bPMMA) diblock copolymer.44 We employed a similar approach to investigate the phase behavior of a cylinder-forming poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymer in thin film geometries. Although similarities between AB diblock copolymer and ABA triblock copolymer bulk phase behavior have been established,45 the phase behavior of ABA triblock copolymers in thin films, though investigated in several studies as discussed below, still has many open questions. For example, Knoll et al. explored the phase diagram of SBS thin films annealed in chloroform vapor as a function of film thickness and solvent content, where the chloroform was used to screen interactions between the S and B blocks and between the copolymer and the interfaces.28 At higher solvent contents in the film (weaker surface fields), parallel orientations of cylinders were found at commensurate thicknesses (integer number of domain spacings), while perpendicular orientations were formed at

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EXPERIMENTAL SECTION

Substrate Surface Modification and Characterization. Benzyldimethylchlorosilane (benzyl silane) and n-butyldimethylchlorosilane (n-butyl silane) (Gelest, Inc.) were used as received. Toluene was argon-purged and further purified by passage through a neutral alumina column and a Q5 catalyst column before use. Silicon wafers (N⟨100⟩, Wafer World, Inc.) were rinsed with dry toluene, cleaned in a UVO cleaner (model 342, Jelight Co., Inc.) for 1 h, and then rerinsed with dry toluene prior to use. Pure component monolayers were generated using liquid deposition, where the pure chlorosilane (0.2 mL) was dropped onto a 1 in. × 2.5 in. silicon substrate, forming a thin liquid layer, and allowed to react for 2 h.44,50 After liquid deposition, modified substrates were rinsed with dry toluene multiple times. Diiodomethane (99%, stabilized, Acros Organics) and water (purified with a Milli-Q reagent water purification system) contact angles were measured using a contact angle device (First Ten Ångstroms FTÅ 125). Probe liquid drops (2 μL) were dispensed onto the substrate surface with a Distriman pipet, and static contact angle measurements were taken after the drop shape stabilized (0.3 s for diiodomethane, 0.1 s for water). Surface energies of the modified substrates were calculated using the Owens−Wendt method,51 an extension of the B

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the SIS film. Low beam current (1 nA) was used to etch away two trapezoid trenches (10 μm in depth) next to the rectangle to make a film bridge. The film bridge was cut from the silicon substrate and attached to a carbon grid. Next, a lower beam current (100 pA) was applied to fine the thin sample from 2 μm to ∼50 nm in thickness. Finally, the thin sample was stained with osmium tetroxide (OsO4) vapor for 20 min. TEM studies were carried out on a JEM-2010F TEM at an acceleration voltage of 200 keV.

Good−Girifalco geometric mean approximation method.52 (Note: the dispersive components (γD) and polar components (γP) of the contact angle probe liquids used in these calculations are γDdiiodomethane = 50.8 mJ/m2, γPdiiodomethane = 0 mJ/m2, γDwater = 21.8 mJ/m2, and γPwater = 51.0 mJ/m2.53) Substrate chemistry gradients were created by controlled vapor deposition.35,44 For this work, the silicon substrate was placed between two silane (n-butyl and benzyl) reservoirs inside a Teflon insert. Then, the Teflon insert was loaded into a sealed glass chamber, and dynamic vacuum (75 Torr) was applied to facilitate chlorosilane vaporization and cross-deposition of the silanes. After 4 h of vapor deposition, the gradient substrates were rinsed with dry toluene and dried; following drying, diiodomethane and water contact angles were measured as described above. We note that the liquid deposition and vapor deposition techniques offered consistent contact angle results, indicating that the liquid depositions likely are homogeneous and reproducible. Polymer Film Preparation and Characterization. An SIS triblock copolymer was obtained from DEXCO (V4211) and used as received. The SIS polymer had an overall molecular mass of 118 kg/ mol, block volume fractions of f S = 0.134, f I = 0.732, and f S = 0.134, a polydispersity index of 1.09, and a bulk nearest-neighbor spacing of L0 = 33 nm (dbulk = 2π/q* = 29 nm, q* is the primary peak in small-angle X-ray scattering and d is the distance between (10) planes). SIS films were cast on the modified substrates by flow coating54 from a 2.3 wt % solution of SIS in tetrahydrofuran (THF, Fisher Scientific, ACS Optima grade). A 50 μL volume of polymer solution, a gap height of 200 μm, and a constant velocity of v = 12 mm/s were used to produce films with uniform thickness of 90 ± 1 nm. Gradient thickness SIS films were cast in a similar manner but with an initial velocity of v = 7 mm/s and an acceleration of a = 4 mm/s2, to achieve a film thickness range of 85−120 nm over a 32 mm distance on the substrate. In addition, an initial velocity of v = 9 mm/s and an acceleration of a = 1.8 mm/s2 were used to produce a thickness range from 80 to 100 nm to investigate the film thickness effect close to 90 nm. Film thickness was measured using a reflectance spectrometer (Filmetrics F20-UV). The reflectometer measured the intensity of light reflected from a sample surface over a range of wavelengths (400−1100 nm) when the incident light beam was placed normal to the surface. Thickness profiles are provided in Figure S6 of the Supporting Information. The films were stored under vacuum overnight and subsequently annealed under vacuum (20 mTorr) at 135 °C for 24 h. Optical microscopy images were collected on a Nikon microscope equipped with a 5 MP CCD camera (Nikon Eclipse LV100). The free surface morphologies of polymer films were assessed by atomic force microscopy (Veeco Dimension 3100). Silicon probes (Tap 150G, BudgetSensors) were used in tapping mode. A typical set point ratio was 0.9. Ultraviolet ozone etching was conducted in the UVO cleaner in 15 s increments with the samples placed ≈1 cm from the lamp.29 Samples were soaked in isopropyl alcohol (Fisher Scientific, ACS grade) for ≈10 min between exposures and dried under a stream of dry nitrogen. Film thickness measurements and AFM images were collected after each etching step. Grazing-incidence small-angle X-ray scattering (GISAXS) measurements were performed at beamline 8-ID-E at the Advanced Photon Source of Argonne National Laboratory. Samples were placed in a vacuum chamber and illuminated with 7.35 keV radiation at incident angles in the range of 0.1°−0.24°. The off-specular scattering was recorded with a Pilatus 1MF pixel array detector (pixel size = 172 μm) positioned 2185 mm from the sample. Acquisition times were ≈20 s per frame. Each data set was stored as a 981 × 1043 32-bit tiff image (with 20-bit dynamic range). All data are displayed as intensity maps I(2Θ,α), where 2Θ and α denote in-plane and out-of-plane scattering angles, respectively. Thin film sections for the cross-sectional TEM experiment were prepared using a focused ion beam (Auriga 60 FIB/SEM) at room temperature. A gold layer was sputtered onto the SIS film to provide conductivity. Then, a rectangle shape of platinum protective layer (about 2 μm × 15 μm in size, 1 μm in thickness) was deposited onto

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RESULTS AND DISCUSSION Gradient Thickness Films on Pure Component Monolayers. Modified substrates were fabricated by liquid deposition of pure chlorosilanes onto UVO-cleaned silicon substrates. The chlorosilane functionalities were chosen to mimic the molecular structures of the BCP components, with benzyl silane being chemically similar to the polystyrene (PS) block and n-butyl silane resembling the polyisoprene (PI) block. The diiodomethane and water contact angles of the modified surfaces are given in Table 1. Surface energies, Table 1. Contact Angle (deg) Measurements of Pure Component Monolayersa contact angle liquid

benzyl silane monolayer

n-butyl silane monolayer

diiodomethane water

42.4 ± 0.5 80.4 ± 1.2

61.5 ± 0.6 92.1 ± 1.4

The reported contact angles were averaged over five samples with six spots on each sample; the uncertainty represents one standard deviation of the data obtained from the repeated measurements. a

calculated using the Owens−Wendt method,51 were 42.0 ± 0.4 mJ/m2 for the benzyl silane surface and 29.9 ± 0.6 mJ/m2 for the n-butyl silane surface. The surface energies of these modified surfaces correlated well with those of the polymer components: 40.7 mJ/m2 for PS and 32.0 mJ/m2 for PI.55 The optical microscopy images of gradient thickness SIS films on bare silicon, benzyl silane, and n-butyl silane surfaces after thermal annealing are shown in Figure 1. The film thicknesses spanned 85 to 120 nm, which corresponds to 3.15d to 4.44d (interlayer spacing, i.e., (10) plane, d was 27 nm as discussed later in this section). At t = 87 nm (3.22d), polymer chains could stretch to accommodate small deviations from commensurability;56−58 thus, smooth featureless films were exhibited. As the film thickness incommensurability increased, we note the formation of islands (light blue areas) at a film thickness of 91 nm (3.37d) continued growth to a labyrinthinelike surface morphology (also known as “spinodal patterns” by analogy with phase separation12) at a film thickness of 94 nm (3.48d) and finally the formation of holes (navy blue areas) at a film thickness of 101 nm (3.74d). At t = 108 nm (4.00d), commensurability was achieved, and a uniform film was recovered. At t = 116 nm (4.30d), the film exhibited a similar surface pattern with t = 89 nm (3.30d), and we anticipate cyclical changes in morphology with further increases in film thickness.12 The effects of substrate surface chemistry/energy and film thickness on SIS film morphology are highlighted by the AFM images of gradient thickness films, as shown in Figure 2. Parallel cylinders were found in the SIS films on both bare silicon and benzyl silane surfaces regardless of film thickness, whereas a higher number density of “dots” (indicative of perpendicular cylinders or HPL) were found at the free surface of the films on n-butyl silane-coated substrates. Films exhibiting only surface dot patterns were found at narrow thickness ranges t = 89−91 C

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Figure 1. Optical images of gradient thickness SIS films annealed at 135 °C for 24 h. Close to the commensurability condition t/d = 3 or 4, films appear featureless; with increasing thickness, the morphology progresses from islands to spinodal island/hole structures to holes to featureless at the next commensurate thickness. The scale bar represents 10 μm and applies to all images.

Figure 2. AFM phase images of SIS gradient thickness films on bare silicon, benzyl silane, and n-butyl silane surfaces, where the thickness increases from 3.22d to 4.30d. Parallel cylinders were noted for films on bare silicon and benzyl silane surfaces, while dot patterns were noted for films on nbutyl silane surfaces. The scale bar represents 200 nm and applies to all images.

nanostructure was clearest at a thickness of ≈90 nm (3.33d), where the film on the n-butyl silane surface exhibited surface dot patterns (HPL, as will be detailed later in this discussion) and the films on the benzyl silane and bare silicon surfaces displayed parallel cylinders. To verify this HPL region for films on the n-butyl surface, uniform thickness SIS films (90 ± 1 nm) were cast on pure n-butyl silane substrates, and we confirmed the featureless optical micrograph and dot patterns (by AFM) on a 1 in. × 2.5 in. substrate. Thus, the gradient thickness allowed us to identify the thickness region of interest for further detailed analysis. In the following section, the detailed internal structure of 90 nm thick SIS thin films on n-butyl silane substrates was investigated using UVO etching/AFM, FIB/TEM, and GISAXS. Initially, we employed successive UVO etching steps followed by AFM imaging of the SIS films to closely examine the through-film morphology.66 The results of the successive etching steps, shown in Figure 3, reveal that the dot pattern persisted through the film thickness, indicating that the free surface nanostructure was likely influenced (and templated) by interactions at the substrate. Typically, the substrate surface field propagates ∼6d from the substrate interface;37,38 we believe this scenario applied to our films, which spanned ∼3d− 4d in thickness, and thus displayed a noticeable substrate surface effect. Further, we employed a lift-up technique to prepare a specimen for cross-sectional processing and imaging through a combination of FIB and TEM. Although a hexagonal array

nm (3.30d−3.37d) and t = 116 nm (4.30d) on the n-butyl silane surface, whereas mixed nanostructure regions (with a preference for perpendicular cylinders or HPL) were noted at all other film thicknesses. We note that the thickness difference between regions with solely surface dot patterns was ≈27 nm, which was slightly less than the bulk domain spacing (dbulk) of 29 nm. The small reduction of the repeat thickness of the dot patterns relative to the bulk domain spacing is a known phenomenon in block copolymer thin films.57,59−64 For the thickness gradient films, the benzyl silane substrate surface was preferential for the PS end-blocks, which leads to parallel cylinder structures at all film thicknesses. This result is consistent with the conceptual description of thin film free energy proposed by Han et al., which suggested that favorable interactions between the hard wall and the short end blocks promote parallel domains.19 A similar hypothesis could hold for the bare silicon substrate as well. The PI block had a lower surface energy (32.0 mJ/m2) than the PS block (40.7 mJ/m2) and thus was preferred to segregate to the free air and substrate surfaces, leading to a parallel cylinder domain orientation. It is possible that the surface energy difference was significant enough to overcome the entropic penalties for looping the midblock (PI)18 and loss of free chain ends (PS) at the surfaces.65 We also note that the silicon substrate effect was not strong enough to promote any lamellar-like morphology in our SIS system, in contrast to Tsarkova et al.42 By examining the films across a variety of thicknesses, we saw that the substrate surface chemistry/energy effect on D

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Figure 5. (a) Measured GISAXS pattern on benzyl silane surface. (b) DWBA simulation based on ABA stacking of parallel cylinders. (c) Measured GISAXS pattern on n-butyl silane surface. (d) DWBA simulation based on ABA stacking of hexagonal perforations. Film thickness was 90 nm, and incidence angle was 0.18° for all cases. Peak positions corresponding to √3q* and 2q* are marked with black and orange arrows, respectively.

Figure 3. Morphology evolution of an SIS film on an n-butyl silane substrate. AFM phase images corresponding to the residual film thickness (60, 43, and 30 nm) after each etching step. The scale bar indicates 200 nm and applies to all images.

pattern (not highly ordered, see Supporting Information) was noted on the film surface, the cross-sectional TEM image, shown in Figure 4, clearly displayed layered structures stacked

GISAXS patterns of SIS films annealed at 135 °C for 12 h on benzyl silane and n-butyl silane surfaces, respectively. The film thickness was ≈90 nm. The critical angle for total external reflection was ∼0.16° for this SIS copolymer; thus, higher incident angles (0.18°, 0.20°, and 0.22°) were used to probe the full film thickness (0.18° shown here; data for additional angles are included in the Supporting Information). Furthermore, to facilitate interpretation of experimental data, we performed simulations of the GISAXS intensity based on the distortedwave Born approximation (DWBA). These simulations were implemented using an algorithm described elsewhere,70 and outcomes are reported in Figures 5b and 5d for the parallel cylinder and HPL phases, respectively. The GISAXS data revealed changes in the in-plane symmetry on each surface. Films prepared on benzyl silane surfaces exhibited in-plane peak positions of 1:2:3, while films prepared on n-butyl silane surfaces exhibited in-plane peak positions of 1:√3:2. The √3 peak is denoted by the black arrow in Figure 5c,d, while the “2” peak is marked by the orange dashed arrow. Line cuts at α = 0.75° are reported in Figure 6, and the √3

Figure 4. Cross-sectional TEM images of 90 nm thick SIS film on an n-butyl silane substrate. The film section was stained with OsO4.

parallel to the substrate.67 The sample was stained with OsO4, which was selective for the PI block, thus the dark regions represented the PI domains, and the light regions corresponded to PS domains. The TEM image indicated that the PS layers were perforated by PI domains, forming an HPL structure. Noncylindrical phases like HPL or lamellae have been found in thin film block copolymer samples that show cylindrical nanostructures in the bulk.42,68 In our system, the n-butyl silane substrate was preferential for the PI block, which led to a preferred affinity of the PI block to the substrate. This hypothesis is supported by the cross-sectional TEM image (Figure 4), where a wetting layer of PI block was noted close to the silicon substrate, possibly suggesting that the phase transition of cylinder to HPL was triggered by a volume fraction change due to the substrate surface interactions.41,69 Because the cross-sectional TEM only revealed two-dimensional features, the three-dimensional stacking type of the hexagonal perforations (on the n-butyl silane surface) was further investigated by GISAXS. These data were compared with measurements for parallel cylinders (benzyl silane surfaces), which highlights the change in symmetry versus substrate surface chemistry. Figures 5a and 5c show the

Figure 6. In-plane line cuts for films on benzyl silane and n-butyl silane surfaces (taken at α = 0.75°); qxy denotes the in-plane scattering vector. Arrow denotes the √3 peak for hexagonal symmetry.

peak is visible only on the n-butyl silane surface (black arrow). The appearance of a scattering peak at this position is significant, as it indicates a transition from the in-plane “striped” symmetry of parallel cylinders to in-plane hexagonal symmetry. The peak at the “2” position for n-butyl silane surfaces could be the expected √4 peak from hexagonal E

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Figure 7. (a) Contact angle measurements of diiodomethane (orange circles) and water (blue squares) for gradient monolayer. Contact angle measurements for pure component surfaces are provided as a baseline on the secondary y-axis. (b) Surface energy (blue circles) and surface composition (red squares) across the gradient substrate. The error bars in (a) and (b) represent the standard deviations of the measurements.

Figure 8. AFM phase images of an SIS film coated on top of the gradient monolayer following thermal annealing; the film thickness was 90 ± 1 nm. AFM phase images of SIS films on pure benzyl silane and pure n-butyl silane surfaces also are included for comparison. Parallel cylinders were found near the benzyl silane end with a transition to HPL noted as the n-butyl silane composition on the substrate increased. The molar surface composition of n-butyl silane (xn) and the ratio of HPL to parallel cylinders (given as percentage of HPL) are provided for reference at the bottom of each image. Scale bar is 200 nm and applies to all images.

the DWBA simulation assumes L0 = 33 nm, cylinder radius of 9 nm, and layer thickness of d = 30 nm. For the HPL phase, the DWBA simulation describes the perforations as vertical cylinders with height and radius of 7 and 9 nm, respectively, with L0 = 38 nm and d = 30 nm.74 We found that the experimental and simulated scattering patterns were in reasonable agreement for both parallel cylinders and HPL phases, and the layering thickness was preserved through the transition. On the basis of the simulations, we believe the “2” peak on n-butyl silane surfaces is associated with ABA stacking of cylinders (the second-order peak for this structure). Because the predicted scattering intensity for the √4 HPL peak is weak due to a minimum in the form factor. Considering what is known about the order−order transition from cylinders to HPL, it is not surprising that the scattering from HPL symmetry is weak and reflects a “residual” ABA cylindrical symmetry: The cylinder-to-HPL transformation is driven by fluctuations that merge the (10) cylinders into perforated sheets; thus, the process does not directly generate ordered hexagonal domains.73,75 Therefore, compared to other order− order transitions, the formation of a HPL phase with long-range in-plane hexagonal symmetry and a well-defined out-of-plane stacking sequence is very slow.73 Additionally, we compared the GISAXS data for HPL samples to simulations based on an ABC stacking sequence, and we found no agreement between the experiments and simulations. These data are included in the Supporting Information.

symmetry, or it could be associated with coexisting parallel cylinders. The GISAXS data for both systems exhibited elongated Bragg peaks along the qz axis (or α-axis), typical of scattering from a thin crystal.70 The fact that out-of-plane Bragg peaks are found indicates that the sample is characterized by a layered morphology, such as ABA or ABC stacking of in-plane hexagonal perforations. Debye−Scherrer rings were detected for films on benzyl silane surfaces (white contour in Figure 5a), but they were less pronounced for films on n-butyl silane surfaces, indicating the latter system had better out-of-plane order.71 (See Supporting Information for comparison at other angles of incidence.) Similar behavior has been reported in other studies that compare GISAXS data for parallel cylinders and HPL phases.72 We also note that the d-spacing for the nbutyl silane sample (HPL) was ≈4% larger than that for the benzyl silane sample (parallel cylinder), which is consistent with other literature studies on the transformation from cylinders to HPL.73 The positions of the Bragg peaks along the qz axis (or α-axis) are very sensitive to the incident angle αi and the stacking sequence normal to the substrate.70 Thus, we compared the experimental GISAXS patterns with the DWBA simulations for ABA stacking of parallel cylinders and hexagonal perforations. Figure 5 reports the comparison at an incident angle of 0.18°, and the Supporting Information includes the comparison for incident angles of 0.16°, 0.20°, and 0.22°. For parallel cylinders, F

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Block Copolymer Thin Film Deposited on Chlorosilane Monolayer Surface Gradient. Gradient monolayers were created using a vapor deposition device as described by Albert et al.35,44 The specific setup employed in this work consisted of one benzyl silane reservoir (19.0 mm in diameter) and one n-butyl silane reservoir (3.2 mm in diameter) on either end of a cleaned silicon wafer (60 mm in length). Following deposition for 4 h, the gradient monolayer showed the expected changes in diiodomethane and water contact angles with position (Figure 7a). The combined contact angle measurements indicated that a surface gradient had been successfully generated with primarily benzyl silane on one end and primarily n-butyl silane at the other. (Note: the reported contact angles for the gradient monolayer were averaged over five gradient substrates with 11 spots spaced every 5 mm on each substrate.) The surface energy, calculated using the Owens−Wendt method,51 decreased across the substrate from the benzyl silane end to the n-butyl silane end (40 to 32 mJ/m2) as shown in Figure 7b. Additionally, surface chemistry composition was deduced from a correlation between diiomethane contact angle and n-butyl silane molar composition, utilizing Albert and coworkers’ X-ray photoelectron spectroscopy (XPS) studies on gradient substrates.44 (Note: our data were shifted −2° to account for systematic differences in diiodomethane contact angle measurements on the pure component monolayer surfaces.) The gradient monolayer showed a nearly linear profile in composition, spanning approximately 20−80 mol % n-butyl silane as shown in Figure 7b. A uniform thickness film (90 nm, 3.33d) was flow coated onto the gradient monolayer surface described in Figure 7. The film was thermally annealed and then examined using optical microscopy and AFM. The optical images showed a featureless surface, as the film thickness was only moderately incommensurate with d. The substrate effect on the nanostructures of SIS films is shown in the AFM images in Figure 8. The morphology transitioned from parallel cylinders on the benzyl silane end to HPL on the n-butyl silane end, in agreement with the single-component substrate experiments. We utilized an inhouse Java program to quantify the ratio of HPL to parallel cylinders across the gradient, which showed an increasing degree of HPL structure as one moves from the benzyl silane end to the n-butyl silane end (Figure 8). Using this gradient monolayer, we were able to quickly examine the substrate surface effects on the film morphology. A HPL window was identified from ≈76% n-butyl silane to pure n-butyl silane and a parallel cylinder window was identified from 0% n-butyl silane (pure benzyl silane) to ≈28% n-butyl silane, whereas mixed nanostructures were noted between these substrate surface composition windows. These results highlight the use of monolayer substrate surface chemistry gradients to manipulate and screen nanostructures in the SIS system.

particular, the gradients showed a narrow thickness window (89−91 nm or 3.30d−3.37d) on n-butyl silane substrates that led to solely HPL. For these HPL regions, successive UVO etching/AFM imaging indicated that these structures propagated throughout the thickness of the film. Thus, these results show the ability to control SIS thin film nanostructures using only substrate surface energetics. The versatility of our method also permits the study of other factors that influence copolymer nanoscale structure, such as modifications in block size and segregation strength, and these parameters will be investigated in future work. Finally, we believe this combinatorial approach with film thickness and monolayer surface chemistry gradients is feasible for screening new materials and identifying conditions to obtain the desired morphologies in a myriad of situations, as the substrate surface chemistry and film thickness gradients are easily tunable for a variety of complex nanoscale systems.

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ASSOCIATED CONTENT

S Supporting Information *

1D profiles of azimuthally integrated fast Fourier transforms (FFTs) obtained from the AFM phase images presented in Figure 2 in Figure S1; comparison of GISAXS patterns between benzyl silane sample and n-butyl sample in Figure S2; comparison between experiments and DWBA simulations for parallel cylinders and hexagonal perforations, both with ABA stacking sequence in Figure S3; comparison between experiments and DWBA simulations for ABA and ABC stacking of hexagonal perforations in Figure S4; UVO etching/AFM imaging of an 87 nm thick SIS film on n-butyl silane substrate in Figure S5; thickness profile and spectral reflectance data in Figure S6. 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] (T.H.E.); [email protected] (G.E.S.). Present Addresses §

Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. ∥ Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Science Foundation (NSF) CAREER grant (DMR-0645586), NSF DMR-1207041, and a DuPont Young Professor Award to T.H.E. J.N.L.A. was supported by an NSF Graduate Fellowship. We thank Prof. T. Beebe, Jr., Department of Chemistry, University of Delaware, for use of his contact angle measuring system and the W. M. Keck Electron Microscopy Facility at the University of Delaware for use of their AFM, FIB, and TEM. G.E.S. and N.M. acknowledge financial support from NSF CAREER (DMR-1151468) and the Norman Hackerman Advanced Research Program (003652-0017-2011). Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. The authors thank Joseph Strzalka and Jin Wang for assistance with GISAXS

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CONCLUSIONS We systematically examined the effects of film thickness and substrate surface chemistry/energy on the nanostructures in a thermally annealed SIS thin film that forms cylinders in the bulk. We found that parallel cylinders persisted on bare silicon and benzyl silane modified substrates regardless of film thickness, while varying degrees of HPL existed on n-butyl silane modified substrates. Our high throughput gradient approach facilitated the identification of combined substrate surface chemistry/energy and film thickness windows that led to parallel cylinders, HPL, and mixed morphologies. In G

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measurements. The authors also thank Fei Deng for assistance with FIB and TEM sample preparation.

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