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Jun 27, 2012 - Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W .... of directed phase separation of PS/PMMA binary bl...
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Directed Polystyrene/Poly(methyl methacrylate) Phase Separation and Nanoparticle Ordering on Transparent Chemically Patterned Substrates Saman Harirchian-Saei,† Michael C. P. Wang,‡ Byron D. Gates,‡ and Matthew G. Moffitt*,† †

Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W 3V6, Canada Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6 Canada

‡

ABSTRACT: We investigate the surface-directed phase separation of spin-coated polystyrene/poly(methyl methacrylate) (PS/PMMA) blends on prepatterned octadecyltrichlorosilane (OTS)−glass substrates under various experimental conditions. As a result of tandem processes of spinodal decomposition and selective wetting of polymer components during spin-coating, low-energy OTS stripes and high-energy glass surfaces laterally arrange the phase-separated polymers according to the chemical pattern on the substrate. Optimal pattern replication was achieved when the length scale of phase separation, controlled via the polymer concentration of the spin-coating solution, matched the smallest feature dimension in a striped chemical pattern possessing two alternating distances between stripes. It was also shown that polymer blend patterns were most closely registered with the underlying substrate when the PS/PMMA composition ratio (30/70, w/w) matched the areal fraction of OTS on the glass surface (∼30%). The influence of solvents demonstrated that a solvent with a relatively low volatility, such toluene, was required for patterning so that domain feature sizes were able to coarsen to the size of the patterned features before film vitrification. As well, we showed that the technique and optimized conditions developed in this study could be applied to pattern photoluminescent CdS quantum dots into microscale arrays of parallel lines via spin-coating onto transparent OTS−glass substrates.

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INTRODUCTION The self-assembly of polymers on surfaces has spurred intensive research over the past several decades, in part due to its enabling potential for efficient and controllable organization of organic and inorganic elements within thin-film functional materials and devices.1−14 For example, it has been shown that controlled arrangements of metal and semiconductor nanoparticles (NPs) can be obtained by their selective incorporation into specific domains of phase-separated homopolymer blends 5,11,15 or microphase-separated diblock copolymers.2−4,7−10,12,13 In addition to controlling the spatial organization of NPs, which can also be accomplished using other self-structuring processes,16 self-assembling polymers offer additional benefits such as providing a matrix with tunable mechanical, optical, and electronic properties for the resulting composite structures. Phase separation of immiscible polymer blends by spincoating from a common solvent is a well-established method for obtaining microscale structures of polymer domains on surfaces.17−23 In addition to NP ordering, structured polymer blend films have a variety of applications in electronics and optoelectronic devices;24−28 for example, both polymeric photovoltaics 25−28 and organic light-emitting diodes (OLEDs)24 utilize the structures obtained by blends of conjugated polymers. The morphology of polymer structures, © 2012 American Chemical Society

including the size, shape, and orientation of the domains, critically affects the operation and efficiency of such devices.24−28 Bottom-up self-assembly via polymer/polymer phase separation has also been combined with top-down lithographic techniques, in order to obtain more complex device-oriented structures with improved long-range structural ordering.29−41 Since structure evolution in polymer blends is influenced by both spinodal decomposition and preferential wetting of the blend components at the boundary surfaces, spin-coating onto prepatterned chemically heterogeneous substrates, possessing regions of different surface energies, can deliver specific polymer patterns to the resulting films. Boltau et al. first demonstrated this strategy by directing the assembly of blends of PS and polyvinylpyridine (PVP) on patterned self-assembled monolayers of alkanethiols on gold.29 Since then, patterned films of various polymer/polymer blends30−40,42 and polymer/ polymer/NP blends5,43 have been produced by spin-coating onto chemically patterned substrates. Among immiscible polymer blends, the polystyrene/poly(methyl methacrylate) (PS/PMMA) system has been one of Received: March 28, 2012 Revised: June 27, 2012 Published: June 27, 2012 10838

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the most widely investigated, both in bulk6,44−52 and in thin films.17,18,20−23,53−59 Both polymers are extremely common industrial thermoplastics, with rigid mechanical properties and optical transparency that make them appealing matrix materials in optically functional films containing metal or semiconductor NPs. In PS/PMMA blend films, polymer/polymer phase separation has been applied to direct the organization of NPs localized in one of the polymer phases.5,15 As well, refractive index contrast matching of domains has been shown to give rise to transparent PS/PMMA/NP nanocomposite blend films.11,60 However, despite the fundamental and applied interest in this system, to our knowledge there is only one literature example of directed phase separation of PS/PMMA binary blends on chemically patterned substrates5 and no systematic studies to date. In this paper, we describe for the first time the directed assembly of PS/PMMA via spin-coating onto transparent prepatterned substrates consisting of microcontact printed stripes of octadecyltrichlorosilane (OTS) stripes on glass. Along with the utility of amorphous silica glass as an inexpensive and readily available substrate for microscale pattern generation, its optical transparency makes it an appealing support for photonic and OLED devices. In this context, the PS/PMMA pair is particularly well suited for patterning on transparent OTS-patterned glass; along with the films being relatively transparent, even in their phase-separated state, the contrast between lower surface energy PS and higher surface energy PMMA results in those components being attracted to the different surface energy regions of the patterned substrate. Therefore, replication of the underlying micrometerscale OTS pattern occurs via selective wetting of hydrophobic OTS with PS and selective wetting of hydrophilic glass with PMMA. The patterning of PS and PMMA domains is studied under a range of preparation conditions by varying solution concentration, blend composition, and solvent. Finally, we apply our methodology and optimized conditions to pattern photoluminescent PS-coated cadmium sulfide quantum dots (QDs, PS-CdS) onto transparent OTS−glass substrates via simple spin-coating.

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Microcontact Printing (μCP) Glass Substrates with Octadecyltrichlorosilane (OTS). Glass coverslips (VWR scientific, 18 mm × 18 mm) were cleaned by sonication for 10 min in 95% ethanol, followed by 10 min sonication in DI water. To introduce a layer of hydroxyl groups on the glass surface, the coverslips were submerged in a piranha solution at 70 °C for 30 min. Piranha solution consists of a 3:1 (v/v) mixture of concentrated H2SO4 and 30% H2O2. (Warning: piranha solution is a strong oxidizing agent, so extreme care is necessary when using it.) Following piranha treatment, the coverslips were sonicated in DI water for 10 min and then rinsed with methanol, followed by two further repetitions of the sonication/rinsing process. The resulting hydrophilic glass substrates were dried with a UHP N2(g) stream and used immediately for μCP. The ink for μCP was prepared by dissolving octadecytrichlorosilane (OTS, Aldrich) in anhydrous hexane (≥99%, Aldrich) under UHP N2(g) to obtain a 5 mM solution. For inking the PDMS stamp, a nonpatterned smooth block of PDMS was used as an “ink pad”. One drop of the ink solution was spin-coated on the pad at 3000 rpm for 30 s followed by 20 s drying by UHP N2(g). A PDMS stamp was then brought into contact with the inked pad for 10 s. The inked PDMS stamp was then brought into contact with a hydrophilic glass coverslip for 30 s under a 200 g weight, transferring the OTS stripe pattern to the coverslip. Microcontact-printed substrates were used immediately as substrates for spin-coating PS/PMMA or PS-CdS/PMMA blend films. Preparation of PS/PMMA Blend Solutions. Appropriate quantities of PS and PMMA were each dissolved individually in spectroscopic grade of toluene, THF, or MEK (Aldrich) to a polymer concentration of 2.0 wt %. The solutions were stirred for 4 h and allowed to equilibrate overnight in the dark. 20/80, 30/70, and 50/50 (w/w) PS/PMMA solutions were prepared by mixing appropriate amounts of each solution followed by stirring for 4 h. The resulting blend solutions were then allowed to equilibrate overnight in the dark. Finally, blend solutions of the various PS/PMMA compositions were diluted gravimetrically with the appropriate amount of filtered solvent to prepare blend solutions with total polymer concentrations of 2.0, 1.0, or 0.5 wt %. Preparation of PS-CdS/PMMA Blend Solutions. The polystyrene-stabilized cadmium sulfide QDs (PS-CdS) used in this study were previously synthesized in our group via templated growth of CdS in the cores of reverse micelles consisting of the poly(styrene)-blockpoly(acrylic acid) copolymer PS(226)-b-PAA(22), where numbers in parentheses indicate number-average degrees of polymerization for each block. The characterization of a similar PS-CdS sample has also been described.8 In summary, each PS-CdS unit consists of a core of a ∼5 nm CdS QD (determined from UV−vis and using Henglein’s empirical relationship),61,62 with a poly(cadmium acrylate) (PACd) layer at the CdS surface, covalently attached to external PS brush layer. The emission spectrum of PS-CdS shows two peaks associated with CdS quantum dot photoluminescence: a band-edge emission peak centered at ∼450 nm and a broad trap-state emission peak centered at ∼610 nm. Appropriate quantities of PS-CdS and PMMA were each dissolved individually in spectroscopic grade of toluene (Aldrich) to a polymer concentration of 1.0 wt %. The solutions were stirred for 4 h and allowed to equilibrate overnight in the dark. A blend solution of 30/70 (w/w) PS-CdS/PMMA with a total polymer concentration of 1.0 wt % was prepared by mixing appropriate amounts of each solution, followed by stirring for 4 h and equilibration overnight in the dark. Spin-Coating PS/PMMA and PS-CdS/PMMA Blend Films on Glass and OTS−Glass Substrates. Blend films were prepared by spin-coating one drop of each blend solution onto bare glass or OTS− glass coverslips. OTS−glass substrates were prepared by microcontact printing as described above; bare glass control substrates were prepared by sonication in methanol, chloroform, toluene, and finally acetone for 10 min in each solvent and then vacuum drying overnight. All spin-coating experiments were carried out by spinning at a rate of 9000 rpm for 60 s. The drop (∼50 μL) was applied after the substrate reached a rate of 9000 rpm, and it was deposited onto the center of the spinning substrate using a microsyringe. Spin-coated films were

EXPERIMENTAL SECTION

Materials. The polystyrene (PS) homopolymer (Mw = 131 000 g/ mol, PI = 1.01) was previously synthesized in our group by anionic polymerization. The poly(methyl methacrylate) (PMMA) homopolymer (Mw = 120 000 g/mol) was purchased from Aldrich. Microfabrication of Polydimethylsiloxane (PDMS) Stamps for Microcontact Printing. Masters with topographical periodic stripe patterns on the microscale were obtained from commercial compact discs (CDs). First, the protective layer of a CD was removed by scoring with a scalpel and vigorously rinsing with deionized (DI) water. Then, the metallic layer and thin lacquer coat were removed via a quick rinse with methanol and subsequent sonication in a 1:4 (v/v) methanol/deionized water solution for 2 h, followed by a final methanol rinse, revealing the underlying polycarbonate layer with the desired topographic features. The polycarbonate was cut into square pieces ∼2 cm × 2 cm. Prepolymer and curing agent for polydimethylsiloxane (PDMS) (Dow Corning: Sylgard 184 DC-184 A and DC 184-B) were mixed in a 10:1 (v/v) ratio and then poured over the polycarbonate master, followed by 4 h curing at 80 °C. The PDMS was then carefully peeled from the master in the direction tangential to the CD grooves, at a steady rate of ∼1 cm/s. PDMS stamps consist of periodic raised stripes corresponding to the troughs of the polycarbonate masters. Stamps were sonicated in a 1:2 (v/v) solution of ethanol/deionized water immediately prior to use, followed by drying with a stream of ultrahigh-purity (UHP) N2(g). 10839

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uniformly transparent and were dried overnight under vacuum prior to characterization. Selective Removal of Components of the Blend Films. In order to investigate the lateral distribution of polymer phases within the blend film, selective etching of the components by selective solvents was performed. PMMA removal was carried out by placing the films in a glass Petri dish containing a stir bar and acetic acid (glacial grade from Aldrich) for 20 min with constant stirring. The films were then removed and further rinsed with fresh acetic acid followed by 4 h air drying and then further drying under active vacuum at room temperature overnight. For selective etching of PS, cyclohexane (anhydrous grade from Aldrich) was used as the selective solvent, and the same etching and drying procedure as above was applied to the films. Atomic Force Microscopy (AFM). Contact mode AFM was performed on a MFP-3D-SA system from Asylum Research with nonconductive silicon nitride cantilevers from Veeco (model: NP-10). The cantilevers are 0.4−0.7 μm thick, and their resonance frequency ranges from 12 to 75 kHz with spring constants ranging from 0.06 to 0.58 N/m. Each sample was imaged several times at different locations on the substrate to determine the regularity of the film structure. All images presented in this paper were scanned over a 10 μm × 10 μm area at a 512 × 512 resolution with a scan rate of 0.5 Hz and represent typical images of each film. Statistical analysis of feature dimensions was carried out on the AFM images of spin-coated and selectively etched films. A total of at least 60 features were measured to determine mean dimensions and errors, reported as one standard deviation from the mean. Laser Scanning Confocal Fluorescence Microscopy (LSCFM). LSCFM measurements of 30/70 (w/w) PS-CdS/PMMA blend films on OTS-patterned glass substrates were carried out on a Zeiss LSM 700 with a diode laser. Films were excited at λex = 405 nm, and the emitted light was directed to the photomultiplier tube (PMT) detector by a variable dichroic filter. Emission wavelengths in the range λem = 493−1000 nm were collected using a single long-pass filter. A Zeiss 63× oil-immersion objective lens (NA = 1.3) was used for imaging. The pinhole diameter was set to 2.4 Airy units; multiple sections at different depths of the films were obtained for constructing the image. To prepare samples for LSCFM measurements, a blend filmcontaining glass coverslip was taped to a glass microscope slide, such that the film was sandwiched between the slide and coverslip. Zeiss Immersionsoel 518 C oil was deposited between the objective and the coverslip in order to provide a constant-refractive-index media between the objective and the glass.

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Figure 1. (a) μCP OTS on a hydrophilic (hydroxyl-terminated) glass surface by a PDMS stamp generating an alternating striped pattern on the surface; solution drying accumulates OTS at the edges of transferred solution lines and results in a double-stripe pattern. (b) 10 × 10 μm AFM image of the OTS patterned film and height profile along the white line in the AFM image, with higher magnification detail illustrating λ, ws, and wl.

RESULTS AND DISCUSSION The surface-directed patterning of polymer blends described in this work begins with microcontact printing OTS onto glass, as illustrated in Figure 1.63−68 As shown in the schematic (Figure 1a), a PDMS stamp with a periodic microscale stripe pattern was employed to selectively transfer OTS to piranha-treated glass in areas of conformal contact between the stamp and the substrate; the resulting drying process and bond formation between silane and hydroxyl groups on the piranha-treated glass substrate result in a chemically heterogeneous surface with a double-stripe pattern of OTS. From AFM of the resulting patterns (Figure 1b), the average height of the resulting OTS lines is h = 3 ± 1 nm and the periodicity (λ) is 1.4 ± 0.1 μm. Figure 1b clearly shows the double-stripe pattern arising from solution drying that consists of alternating thin and thick hydrophilic bands in between the OTS lines; within each double-stripe repeat unit, the average short and long distances (ws and wl, respectively) between OTS lines are ws = 0.4 ± 0.1 μm and wl = 1.0 ± 0.1 μm, respectively (Figure 1b). These patterned OTS films were used to direct the phase separation of spin-coated PS/PMMA and PS-CdS/PMMA blends.

Before studying the morphology of phase-separated PS/ PMMA domains on patterned surfaces, the morphologies obtained by spin-coating onto clean, nonpatterned glass were determined for comparison. For this, a 1.0 wt % blend solution of 30/70 PS/PMMA was spin-coated onto clean glass and characterized by contact-mode AFM, which reveals the topology of the PS- and PMMA-rich phases. A difference in the relative heights of PS and PMMA in spin-coated blend films is generally observed as a result of the difference in the relative solubilities of the two polymers in the casting solvent.18,19,23,58 Toluene is a better solvent for PS than for PMMA; therefore, the PMMA phase will contain less solvent than the PS phase during spin-coating such that it solidifies at an earlier stage of the solvent evaporation. Further evaporation eventually leads to 10840

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separation and pattern features is critical, as described below.32,35,39,40,69 Fast Fourier transform (FFT) analysis of PMMA domains after PS etching (Figure 2b, inset) reveals a characteristic domain spacing of dFFT = 0.2 ± 0.1 μm for the 30/70 blend under these spin-coating conditions. During spin-coating, polymer/polymer and polymer/substrate interactions act in tandem to influence the lateral organization of the polymer phases.29−40 For example, if following initial phase separation one of the phases has preferential affinity for a specific region of the substrate, it will accumulate in that region and form a wetting layer. In the control film described in Figure 2, both polymers appear to exhibit similar affinity to the glass, since no wetting layer is formed and both laterally separated PS and PMMA domains extend down to the substrate (confirmed by the selective etching experiments described above, since each was removed independently of the other, Figure 2b,c). As exemplified in Figure 2, domain structures formed by spinodal decomposition on nonpatterned substrates are generally isotropic. However, for spin-coating PS/PMMA on the prepatterned substrates applied here, the more polar polymer (PMMA) is expected to wet the high surface energy glass while the less polar polymer (PS) is expected to wet the low surface energy OTS lines. Along with the various polymer/polymer, polymer/solvent, and polymer/substrate interactions that are in play during spincoating, the replication of the underlying chemical pattern of the substrate in the phase-separated polymer film strongly depends on composition30−36,70 and length scale32,35,39,40,69 matching between the blend and the substrate pattern. For spin-coated blend films, the characteristic dimension of phase separation, d, is determined by the extent of domain coarsening during spinodal decomposition before the onset of polymer vitrification by solvent evaporation which kinetically locks the film structure.18 This length scale will vary depending on the polymer solution concentration (cp) and the spinning rate (ω) according to the following empirical scaling law:30−33

collapse of the solvent-swollen PS below the level of the already solidified PMMA domains.18,19,23,58 Hence, the elevated islands observed in the AFM image of the blend film (Figure 2a) are

d ∼ c p/ω1/2

(1)

Equation 1 expresses a direct proportionality between d and cp and an inverse proportionality between d and ω1/2; therefore, by adjusting cp and ω, d can be tuned to improve the commensurability with the substrate pattern periodicity λ (or other pattern dimension), in order to optimize blend pattern replication. Along with d, the film thickness, h, is also a strong function of the spin-coating conditions and will increase with decreasing ω or increasing cp, as these parameters dictate the balance of counteracting viscous and centrifugal forces, respectively.30,71 To first investigate the effect of cp on pattern replication, blend solutions of 30/70 PS/PMMA at three different polymer concentrations of 0.5, 1.0, and 2.0 wt % in toluene were spincoated onto OTS-patterned glass substrates at a constant spin rate of 9000 rpm; AFM images of the resulting blend films are shown in Figure 3a−c, respectively. For the cp = 0.5 wt % blend film (Figure 3a), the underlying pattern is clearly reflected in the overall film topology, with higher areas corresponding to the hydrophilic regions of the substrate and lower trenches corresponding to the OTS lines. However, the higher areas possess a nanoscale internal structure of small phase-separated domains of different heights (Figure 3a, inset), which are assigned to PS (darker, lower) and PMMA (lighter, higher), based upon relative heights of PS and PMMA on bare glass;

Figure 2. 10 × 10 μm AFM images of 30/70 PS/PMMA films spincoated from blend solutions in toluene with polymer concentration cp = 1.0 wt % onto bare glass substrates at 9000 rpm (a) after spincoating, (b) after selective PS removal, and (c) after selective PMMA removal. FFT analysis of PMMA domain features are shown in (b, inset).

expected to be PMMA domains. However, the contact mode AFM image does not provide us with direct information on the identity of the polymer phases; therefore, selective dissolution was performed to remove either the PS or PMMA phase using cyclohexane or acetic acid, respectively. AFM images of the PSor PMMA-etched films are shown in Figure 2, b and c, and confirm the phase distribution proposed above: PS removal etches away the matrix phase, leaving behind discrete domains of PMMA (Figure 2b), whereas PMMA removal etches away the elevated islands, leaving behind holes in the PS matrix (Figure 2c). For directing phase separation on prepatterned substrates, the matching of characteristic length scales for phase 10841

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width of the thinner hydrophilic regions within the doublestripe pattern (ws = 0.4 ± 0.1 μm), such that individual aligned PMMA domains completely fill these regions with almost complete exclusion of PS. In the alternating thicker hydrophilic regions of the substrate (wl = 1.0 ± 0.1 μm), the lower degree of dimension commensurability leads to imperfect registration between the PMMA domains and the substrate pattern, such that two or three separate PMMA domains interspersed with PS span these regions; the remainder of the PS appears to be well-localized in the trenches above the hydrophobic OTS lines. This patterning feature highlights a challenge of achieving perfect registration of blend components on prepatterned substrates with more than one characteristic feature dimension, since only one feature can be best matched with the length scale of phase separation. However, the resulting defects should be regarded as a function of the particular pattern type explored here and are by no means intrinsic to the method, the polymer pair, or the surface functionalities. Moreover, we show below that these defects can be mostly eliminated with a relatively simple solvent development step following spin-coating. When the polymer solution concentration is increased further to cp = 2.0 wt % (Figure 3c), the resulting characteristic dimension of phase separation, dFFT = 0.6 ± 0.2 μm, is now larger than the smallest dimension of the underlying substrate, so that no registration with the underlying pattern is observed; in this film, the isotropic distribution of circular PMMA domains in a PS matrix closely resembles the morphology of the blend film spin-coated onto the nonpatterned substrate (Figure 2a). In the above discussion of spin-coated blend films on OTS− glass in Figure 3, assignments of PS and PMMA phases were based upon the relative heights of observed domains. However, confirmation of this lateral phase distribution along with additional information on actual domain heights and the possible presence of wetting layers in different regions of the substrate requires selective etching experiments. Therefore, to provide a complete three-dimensional picture of PS and PMMA organization in the two cases of low and high pattern registration, selective etching of PS and PMMA was carried out for the cp = 0.5 wt % (Figure 4) and cp = 1.0 wt % (Figure 5) films; from AFM analysis, comparison of height profiles with and without selective etching (also shown) allowed average relative and absolute domain heights to be determined. For the 0.5 wt % film (low registration case), AFM height analysis before selective etching (Figure 4a) show elevated regions with internal height fluctuations in between deep trenches that correspond to the underlying OTS lines; the average height of these regions relative to the bottom of the trenches is h = 2.3 ± 0.4 nm. After selective removal of the PS phase from the spin-coated film (Figure 4b), the film topology on all length scales is very similar to before etching, although with markedly increased height contrast; this confirms that above the hydrophilic regions of the substrate the film consists of nanoscale PMMA islands surrounded by a matrix of PS. After PS removal, the average height of the elevated regions (or depth of the trenches) increases to h = 6 ± 1 nm; the difference in the height of the elevated features in between the trenches before and after selective etching suggests that an ∼4 nm thick PS film wets the OTS lines during spin-coating. After selective PMMA removal from the spin-coated film (Figure 4c), the elevated regions in between the trenches, including both nanostructured PS and PMMA domains, are completely removed; this indicates that in these regions both PS and

Figure 3. 10 × 10 μm AFM images of 30/70 PS/PMMA films spincoated from blend solutions in toluene with different polymer concentrations cp onto OTS-patterned glass substrates at 9000 rpm. (a) cp = 0.5 wt % (dFFT < ws); (b) cp = 1.0 wt % (dFFT = ws); and (c) cp = 2.0 wt % (dFFT > ws). FFT results for each film correspond to the 1 × 1 μm image in (a, inset) and the 10 × 10 μm images in (b) and (c).

clearly the lateral segregation of PS and PMMA in this film is not strongly registered with the underlying surface energies of the substrate, although based upon the low height of trenches corresponding to the OTS lines it does appear that only PS is present in these regions. The poor registration of PS and PMMA domains is attributed to the fact that the small domain dimensions that develop during spin-coating are not commensurate with the feature sizes of the underlying pattern; FFT analysis of the inset image in Figure 3a gives a characteristic dimension of phase separation of dFFT = 0.1 ± 0.1 μm, which is smaller than any of the pattern dimensions: λ = 1.4 ± 0.1 μm, ws = 0.4 ± 0.1 μm, and wl = 1.0 ± 0.1 μm. In contrast, the cp = 1.0 wt % film (Figure 3b) displays larger PMMA domains that more completely fill the hydrophilic regions of the underlying substrate, excluding more PS from these regions and leading to elongation and alignment of PMMA domains along the pattern stripes. The improved domain registration with the substrate pattern in this case is a direct result of the larger length scale of phase separation from the more concentrated polymer solution; from FFT of the image in Figure 3b, the characteristic dimension of phase separation, dFFT = 0.4 ± 0.1 μm, is commensurate with the 10842

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Figure 4. 10 × 10 μm AFM images, with higher magnification insets and corresponding height profiles, of 30/70 PS/PMMA films spincoated from blend solutions in toluene with cp = 0.5 wt % onto OTSpatterned glass substrates at 9000 rpm: (a) as spin-coated (PS/ PMMA), (b) after selective PS removal (PMMA only), (c) after selective PMMA removal (PS only). Estimated positions of the underlying OTS lines are indicated with dashed lines on the height profiles.

Figure 5. 10 × 10 μm AFM images, with higher magnification insets and corresponding height profiles, of 30/70 PS/PMMA films spincoated from blend solutions in toluene with cp = 1.0 wt % onto OTSpatterned glass substrates at 9000 rpm: (a) as spin-coated (PS/ PMMA), (b) after selective PS removal (PMMA only), (c) after selective PMMA removal (PS only). Estimated positions of the underlying OTS lines are indicated with dashed lines on the height profiles.

PMMA domains sit on a PMMA wetting layer at the substrate. What is left behind after this wetting layer is removed are lines, h = 5 ± l nm, registered with the OTS pattern; their height, which agrees within error to the height difference before and after selective PMMA removal, is greater than the OTS lines before spin-coating (h = 3 ± 1 nm), further evidence for a thin layer of PS above the OTS. The presence of PS above the OTS lines was confirmed by a PS etching step after selective PMMA etching, which reduced the height of the lines to h = 3 ± 1 nm, corresponding to the OTS−glass without polymer. We note that the apparent increase in the distance between PS lines in Figure 4c relative to the trenches in Figure 4b, and thus in the distance between estimated OTS line positions in the two samples, is likely an artifact of selective etching attributed to swelling by acetic acid of PS and/or the siloxane bonding OTS to glass. AFM height analysis for cp = 1.0 wt % (high registration case) and AFM height analysis before selective etching (Figure 5a) show that the elevated domains in between the OTS lines, consisting either of individual elongated islands oriented along the stripe direction (ws hydrophilic regions) or irregular droplets with a general arrangement along the pattern direction (wl hydrophilic regions), possess a height of h = 4 ± 1 nm. Selective PS removal (Figure 5b) gives a nearly identical topological pattern although with increased domain height to h = 10 ± 1 nm, indicating that the elevated domains are PMMA surrounded by a small amount of interspersed PS and that the

trenches corresponding to the OTS lines contain PS with a film height of h ∼ 6 nm. Finally, selective etching of PMMA from the spin-coated blend film (Figure 5c) removes the majority of the polymer in between the OTS lines, revealing PS stripes with excellent registration with the underlying OTS pattern, indicating that, like in the cp = 0.5 wt % case, phase-separated PS/PMMA above the hydrophilic regions of the substrate are supported by a PMMA wetting layer. The height of the PS stripe features, h = 6 ± 1 nm, is consistent with the value obtained by comparing the blend film before and after selective removal of PS. We note that the well-ordered and highly registered features in Figure 5c indicate that developing films in acetic acid provides a method for achieving good pattern fidelity in the hydrophobic polymer phase, despite inherent defects following spin-coating (Figure 5a) related to multiple feature dimensions in the chemical pattern. Based on AFM results before and after selective etching of PS and PMMA components, the proposed phase structure for 30/ 70 PS/PMMA blends spin-coated onto prepatterned OTS− glass from 0.5 wt % solution (low registration case) or 1.0 wt % (high registration case) are shown in Figure 6, a and b, respectively. Both blend films show wetting layers of polymer components that are registered to the appropriate surface energy region of the prepatterned substrate, with surface layers of PS on OTS lines and PMMA on hydrophilic glass. However, in the 0.5 wt % case (Figure 6a), the small length scale of phase 10843

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Figure 6. Schematic showing proposed phase structure of PS/PMMA 30/70 films spin-coated from blend solutions in toluene onto OTSpatterned glass substrates at 9000 rpm with (a) cp = 0.5 wt %, low registration case, or (b) or cp = 1.0 wt %, high registration case.

separation relative to the width of the hydrophilic substrate regions results in a fine structure of nanoscale PS/PMMA domains sitting on top of the PMMA wetting layer; as a result, PS is not strongly localized on the OTS lines. As a result, an interesting structural hierarchy exists in these films due to competing length scales of phase separation and substrate wetting. In contrast, the larger length scale of phase separation in the 1.0 wt % case leads to PMMA domains that are well matched to the width of the thinner hydrophilic regions (ws = 0.4 ± 0.1 μm) in the double-stripe pattern, mostly excluding PS from these regions and leading to a greater relative amount of PS above the OTS lines. The alternating thicker hydrophilic regions (wl = 1.0 ± 0.1 μm) contain significantly more PS in between individual PMMA domains than the thinner hydrophilic regions; however, the overall registration of PS to the OTS lines and PMMA to the hydrophilic glass is greatly improved under these spin-coating conditions. Blend composition is another parameter that can strongly influence the fidelity of pattern replication in spin-coated blend films, due to the requirement of composition commensurability between the prepatterned substrate and the blend.30−36,70 For the OTS-glass double-stripe pattern prepared by μCP for this work, the OTS lines have an areal fraction of 0.28 ± 0.04 (determined from image analysis of several AFM micrographs), which matches well with the 30 wt % hydrophobic PS in the blend films described above. To investigate the effect of blend composition, two blends of 20/80 and 50/50 (w/w) PS/ PMMA (both spin-coated at cp = 1 wt %) were spin-coated onto the prepatterned substrates; AFM characterization of the resulting PMMA or PS domain distributions after selective etching is shown in Figure 7. For both films, similar to the 30/70 blend spin-coated at the same polymer concentration, PS is preferentially localized on OTS lines and PMMA is preferentially localized in the hydrophilic glass regions in between the OTS lines. However, comparison of both PS/PMMA = 20/80 and 50/50 cases with the 30/70 case, which has the closest compositional match with the underlying substrate pattern, shows that composition mismatch strongly influences registration of PS and PMMA to the underlying nonpolar and polar regions, respectively. For example, in the 20/80 blend film (Figure 7, a and c), possessing an excess of PMMA relative to the underlying pattern, the raised PMMA domains are connected by multiple bridges that span the OTS lines of the underlying substrate (Figure 7a), while there is insufficient PS to fully wet the OTS lines forming instead small PS droplets localized along the OTS (Figure 7c).

Figure 7. 10 × 10 μm AFM images of (a, c) 20/80 and (b, d) 50/50 PS/PMMA films spin-coated from blend solutions in toluene with cp = 1.0 wt % onto OTS-patterned glass substrates at 9000 rpm. Both spincoated blends were selectively etched and images show films (a, b) after PS removal (PMMA only) and (c, d) after PMMA removal (PS only).

Conversely, in the 50/50 blend film (Figure 7, b and d), which possesses an excess of PS relative to the underlying pattern, the raised PMMA islands are mostly irregular droplets with very little elongation along the pattern direction (Figure 7b); the islands appear to extend from lower PMMA domains, likely wetting layers, localized in the thinner hydrophilic regions of the substrate. The PS domains localized over the OTS lines in this blend are connected by multiple bridges that span the hydrophilic substrate regions (Figure 7d). Average feature heights observed in the blend films before and after etching are listed in Table 1 for 20/80, 30/70, and 50/50 PS/PMMA blends with cp = 1.0 wt %. Table 1. Average Feature Heights (in nm) of PS/PMMA Blends after Spin-Coating onto Patterned OTS−Glass from cp = 1.0 wt % Solutions in Toluene, before and after Selective Etching spin-coated PS-etched (PMMA) PMMA-etched (PS)

20/80

30/70

50/50

6±2 8±3 2±1

4±1 10 ± 1 6±1

8±1 16 ± 1 10 ± 2

We also investigated the effect of different solvents on surface-directed phase separation, by spin-coating solutions of cp = 1.0 wt % 30/70 PS/PMMA in three different solvents toluene, MEK, or THFonto patterned OTS−glass. Although, as mentioned previously, the topology of spin-coated blends is strongly dependent on the relative solubility of the polymer components in the casting solvent, we find that solvent volatility plays the dominant role in pattern replication for the current system. The vapor pressures for these solvents are 5.4, 10.5, and 17.0 kPa at 20 °C, respectively, indicating an order of 10844

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volatility of THF > MEK > toluene.72 The topology of the resulting blend films is illustrated in Figure 8, showing that the

volatility solvents, such as toluene, are optimal for the patterning of PS/PMMA on OTS−glass surfaces. Finally, we applied the method and optimized conditions for PS/PMMA patterning described above to the patterning of photoluminescent CdS QDs on glass in a simple spin-coating step. Previous work from our group showed that spin-coating a blend of polystyrene-functionalized CdS nanoparticles (PSCdS) and PMMA results in microscale QD organization via spinodal decomposition, although without any long-range order.15 Here, we show that directing the phase separation on chemically patterned surfaces enables controllable patterning of the QD-containing structures. For this experiment, a blend of cp = 1.0 wt % 30/70 PS-CdS/PMMA in toluene was spin-coated onto patterned OTS-glass and characterized both by AFM and LSCFM. Clearly, the AFM image (Figure 9a)

Figure 8. 10 × 10 μm AFM images of 30/70 PS/PMMA films spincoated at 9000 rpm from cp = 1.0 wt % blend solutions in (a) toluene, (b) MEK, and (c) THF onto OTS-patterned glass substrates. Figure 9. 10 × 10 μm (a) AFM and (b) LSCFM images of 30/70 PS/ PMMA films spin-coated at 9000 rpm from cp = 1.0 wt % blend solutions in toluene onto OTS-patterned glass. The arrow on the LSCFM image highlights the direction of the underlying OTS lines.

underlying pattern was replicated by the polymer blend structure only in the toluene case. The MEK and THF blends both show smaller and approximately circular raised domains (PS and PMMA, respectively, based on relative component solubilities), isotropically distributed over the entire film with no observed orientation, elongation, or registration with the underlying pattern. Overall, the domain sizes increase in order of decreasing volatility, with only the PMMA domains from the toluene solution being large enough to show commensurability with the pattern features. This is attributed to the relatively fast evaporation rates of MEK and THF compared to toluene during spin-coating, which leads to a more rapid depletion of the solvent, faster polymer vitrification, and thus less time for domain sizes to develop. Therefore, it appears that low-

displays a similar pattern registration to that of the PS/PMMA blend from cp =1.0 wt % in toluene (Figure 3b), from which we could infer that the PS-CdS, and thus the photoluminescent CdS NPs, are localized within the trenches corresponding to the OTS lines. This was confirmed using LSCFM imaging (Figure 9b), with excitation and emission filters selected to excite and collect photoluminescence from the CdS NPs. This image shows localized CdS emission from patterned lines corresponding to the double-stripe pattern of OTS−glass. The “fuzziness” of the lines in the fluorescent images is in part due 10845

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to limitations in the resolution of LSCFM near the diffraction limit and in part due to actual bridging defects similar to those in 30/70 PS/PMMA self-assembly (Figure 5a); as shown previously in Figure 5c, the aforementioned defects can be largely eliminated with a post-spin-coating acetone developing step to remove the PMMA phase. To our knowledge, this represents the first example of microscale patterning of QDs on a transparent substrate via a fast and efficient spin-coating step. This methodology thus provides a new and relatively simple route to patterning polymer/NP films for photonic and display technology.

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CONCLUSIONS This work describes the surface-directed phase separation of PS/PMMA blends via spin-coating blend solutions onto OTSpatterned glass under various experimental conditions. During spin-coating and phase separation, low-energy OTS stripes and high-energy glass laterally arrange the phase-separated polymers based on preferential wetting. Analysis of films prepared from solutions of different polymer concentrations shows that optimal pattern replication occurs when the length scale of phase separation matches the length scale of the smallest feature dimension in the double-stripe chemical pattern. Studying PS/PMMA blend films of different composition demonstrates that composition mismatch with the areal fractions of the underlying surface energy regions increases the extent of defects such as bridging between the raised domains in the patterned film. The effect of solvent was also investigated, and optimal results were obtained for the least volatile solvent, toluene, which allowed domain feature sizes to develop to match the pattern features before film vitrification. Finally, we employ the optimized conditions established for PS/PMMA to pattern photoluminescent CdS QDs into microscale stripe arrays via spin-coating onto the transparent OTS-patterned glass substrates. The procedures developed here establish for the first time the figures of merit for surfacedirected phase separation of the widely studied PS/PMMA immiscible blend pair on chemically patterned surfaces and open routes to patterned functional nanocomposites for applications including photonics, displays, and light-emitting devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: mmoffi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Natural Science and Engineering Research Council (NSERC), B.D.G for the Government of Canada Research Chairs Program, the Canada Research Chairs Program, the Canadian Foundation for Innovation (CFI), and the British Columbia Knowledge Development Fund (BCKDF) for their generous support of the research. This work also made use of 4D LABORATORIES shared facilities supported by CFI, BCKDF, Western Economic Diversification Canada, and Simon Fraser University. The assistance of Prof. Robert Burke in LSCFM imaging is also gratefully acknowledged. 10846

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