Article pubs.acs.org/Macromolecules
Combinatorial Block Copolymer Ordering on Tunable Rough Substrates Manish M. Kulkarni,† Kevin G. Yager,‡ Ashutosh Sharma,§ and Alamgir Karim*,† †
Department of Polymer Science and Polymer Engineering, Akron Functional Materials Center, The University of Akron, Akron, Ohio 44325, United States ‡ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur, UP 208016 India S Supporting Information *
ABSTRACT: Morphology control of block copolymer (BCP) thin films through substrate interaction via controlled roughness parameters is of significant interest for numerous hightech applications ranging from solar cells to high-density storage media. While effects of substrate surface energy (SE) and roughness (R) on BCP morphology have been individually investigated, their synergistic effects have not been explored in any systematic manner. Interestingly, orientation response of BCP to changes in SE can be similar to what can be accomplished with variations in R. Here we present a novel approach for orienting lamellar BCP films of poly(styrene)-blockpoly(methyl methacrylate) (PS−PMMA) on spin-coated xerogel (a dried gel of silica nanoparticle network) substrate with simultaneously tunable surface energy, γs ∼ 29−53 mJ/m2, by UVO exposure and roughness, Rrms ∼ 0.5−30 nm, by sol−gel processing steps of regulating the catalyst concentration and sol aging time. As in previous BCP orientation studies on 20 nm diameter monodisperse silica nanoparticle coated surface, we find a similar but broadened oscillatory BCP orientation behavior with film thickness due to the random rather than periodic rough surfaces. We also find that higher random roughness amplitude is not the necessary criteria for obtaining a vertical orientation of BCP lamellae. Rather, a high surface fractal dimension (Df > 2.4) of the rough substrate in conjunction with an optimal substrate surface energy γs ∼29 mJ/m2 results in 100% vertically oriented lamellar microdomains. The AFM measured film surface microstructure correlates well with the internal 3D BCP film structure probed by grazing incidence small-angle X-ray scattering (GISAXS) and rotational small-angle neutron scattering (SANS). In contrast to tunable self-assembled monolayer (SAM)-coated substrates, the xerogel films are very durable and retain their chemical properties over period of several months. These results also highlight importantly that BCP orientation control for nanotechnology is possible not only on specially prepared patterned substrates but also on industrially viable sol−gel substrates. energy effects structural changes to self-assembling thin film coatings such as block copolymers (BCPs). Here we first demonstrate a versatile sol−gel method to create aperiodic rough substrates with high reproducibility of roughness and surface energy. We use the functionality provided by these novel substrates to demonstrate control over orientation of lamellar BCP phases in thin films. Self-assembly of BCPs has gained recent importance in nanopatterning where relatively simple large-area patterns are required (e.g., hard disk drive patterning) or low-cost, lowfidelity patterning is needed (e.g., organic photovoltaics).10 It can also serve as an alternative to the high resolution patterning needs of semiconductor industry,11 where lithography puts severe limits on the materials that can be used for creating nanostructures.12 Therefore, there is an immediate need for developing alternate low-cost and generic methods that are also
1. INTRODUCTION Rough substrates are used in various scientific and technological applications such as superhydrophobic and oleophobic surfaces,1−3 controlled dewetting of polymer films4 and liquid crystal ordering,5 controlled adhesive properties of surfaces,6 and for cell growth.7 The nature of roughness can be periodic or nonperiodic, although some of the nonperiodic substrates exhibit a “surface pattern” associated with a dominant wavelength. The response of various material coatings and self-assembling systems on periodic patterns has been the subject of a vast number of studies in the past decade.8,9 However, by comparison, the response of thin films and coatings on nonperiodic rough surfaces has been relatively less studied due to difficulties in reproducible control of surface roughness. In addition to the surface roughness, controlling the surface energy of the substrates for adhesion and stability of the coatings against dewetting or to obtain preferential morphology of the polymer films is imperative. Of significant interest then is how the synergy of interaction provided by well-characterized random or aperiodic surface roughness and concurrent surface © 2012 American Chemical Society
Received: January 23, 2012 Revised: April 6, 2012 Published: May 1, 2012 4303
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Figure 1. (i) AFM 3D topographic images of substrates used to coat the BCP films: (a) Xero-R0.5 nm, Rrms = 0.5 (±0.2) nm; (b) Xero-R5 nm, Rrms = 5.1 (±1.8) nm; (c) Xero-R30 nm, Rrms = 30.4 (±2.4) nm; and (d) silica nanoparticle layer coated on silicon substrate (≈80% coverage) SNP80-R5 nm, Rrms = 5.2 (±1.3) nm. (ii) Typical line cross section of AFM topograph for all the substrates. (iii) The smoothness of substrates a and d compared to b and c can be appreciated easily in normalized z-scale. (iv) 2-D isotropic power spectral density plots obtained from the height image. PSD of samples b and c suggest presence of a dominant periodicity indicated by a peak.
using lamellar BCP since usually one of the blocks of lamellar BCP preferentially wets the substrate, and consequently, the majority of lamellae lie horizontal or parallel to the plane of the substrate.18,19 However, the use of BCP in nanopatterning applications requires that the lamellae orient perpendicularly (vertically) to the substrate so that typically one of the blocks can be selectively etched out20,21 to obtain parallel grating-like lines over a large area. We report on this challenge of controlling orientation of lamellar BCP domains with the help
capable of producing defect-free and patterned nanoscale structures over large areas. Self-assembly of BCPs13 is one of the most promising of the emerging technologies14 for soft nanopatterning. The range of phase structures exhibited by BCPs makes it an attractive technique to create nanoscale features of different shapes like sphere, rod (cylinder), lamellae, and gyroid.15 Although it has already been demonstrated that cylindrical BCP thin films can be used for high-density data storage devices,16,17 there are significant challenges in similarly 4304
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Table 1. Surface Properties of the Rough Substrates
a
substrate
roughness Rrms (nm)a
dominant λ (μm)
qRrmsb
fractal dimension Dfa
Xero-R0.5 nm Xero-R5 nm Xero-R30 nm SNP80-R5 nm
0.5 (±0.2) 5.1 (±1.8) 30.4 (±2.4) 5.2 (±1.3)
0.99 0.5 1.6
0.03 0.38 0.019
2.15 (±0.08) 2.32 (±0.07) 2.4 (±0.1) 2.5 (±0.1)
Estimated errors are standard deviation of three different measurements. bq = 2π/λ. were selected based on the difference in their surface features, as depicted in Table 1, to coat the BCP films. In order to generate a surface energy gradient, each of the selected substrate silica films was placed on an accelerating stage. This stage was accelerated such that different sections of the sample were exposed to UV-ozone (UVO) for different amount of time by passing the sample under UV-wand (185 and 254 nm wavelength). Radiation power varied from 5 to 700 mJ/cm2, depending upon exposure time.32 The surface energy gradient was determined by Owen’s method by measuring sessile drop (3 μL) contact angles (Kruss DSA 100 contact angle goniometer) of toluene, water, and diiodomethane on xerogel substrates.33 Fractal dimension (Df) of the substrates was measured from the AFM topography images using a box-counting method.19 A cube lattice with lattice constant L was superimposed on the 512 pixel by 512 pixel topographic scan of the substrate. The surface area is obtained by multiplying number of cubes that contain at least 1 pixel by L2. This procedure was repeated by changing L from 1 to 512, and the fractal dimension was extracted from the scaling of this box count with L. In the next step, deuterated poly(styrene)-block-poly(methyl methacrylate) (PS−PMMA) (Polymer Source, Inc., polydispersity index = 1.14) with PS Mw = 29.5 kg/mol and PMMA Mw = 33.1 kg/ mol was dissolved in toluene (Sigma-Aldrich). This solution (BCP weight fraction 0.03) was flow-coated on the UVO-exposed substrate fixed on an accelerating stage to obtain films with either constant or gradient thickness.34 Specifically, a 50 μL drop of the polymer solution was placed at a wedge between a glass plate held at fixed angle and the substrate, and the stage was accelerated from 1 to 10 mm/s2 with terminal velocities from 3 to 12 mm/s, depending on desired thickness gradient. Uniformly thick films were obtained at higher acceleration of 160 mm/s2. All the films were then annealed in vacuum oven for 15 h at 165 °C whereupon the BCP microphase separates. BCP Film Characterization. Three different techniques, namely AFM phase imaging, rotational small-angle neutron scattering (RSANS), and grazing incidence small-angle X-ray scattering (GISAXS), were used to characterize the microphase-separated BCP films. Dimension 3100 (Veeco Instruments) was used in “tapping” mode to record the topography and phase images of the films. Film thickness was measured by AFM scratch-height analysis. For the lamellae orientation analysis, the phase and amplitude images were segmented by software methods into regions of parallel and perpendicular lamellae as described in ref 31. R-SANS experiments were performed using the NG7-SANS instrument at the NIST Centre for Neutron Research, using an incident neutron beam of wavelength λ = 0.6 nm (Δλ/λ = 0.11). Uniform thickness BCP samples were spin-coated on xerogel and SiNP80 substrates and annealed in a vacuum oven. Scans were performed as a function of sample rotation angle in the range −84° to +84° in 4° increments. The data were normalized and rotated into a sample-aligned coordinate system as described previously.31 Briefly, the RSANS data were interpolated using Delaunay triangulation. This procedure effectively assumes that the orientation distribution is smooth, without variations smaller than the 4 deg step size selected for the experiment. We note that on a series of BCP test samples with different orientation distributions, we measured using a step size of 0.5 deg and saw no evidence of variation over these small tilts. Grazing small-angle X-ray scattering (GISAXS) measurements were performed at Argonne’s Advanced Photon Source using the 8-ID-E beamline. Two-dimensional scattering images were measured using charge-coupled device (CCD) detector at a distance of 1.945 m and an X-ray wavelength of 0.169 nm (photon energy of 7.35 keV). Samples
of the tunable rough substrates based on the sol−gel method mentioned earlier. In order to obtain vertically oriented lamellar BCP domains, several approaches have been proposed including chemical patterning and/or modification of the substrate,22−24 sandwiching the BCP between two neutral surfaces,25 addition of nanoparticles to the polymer films,26,27 coating the BCP films on rough substrates,28 and addition of surfactant molecules.29 The addition of nanoparticles or surfactants to the BCP film introduces an unwanted third component in the film, and the chemical and topographic substrate modification techniques require prepatterning of the substrate (chemical and/or physical) on a length scale of the order of L0 (the BCP repeat spacing), which is of the order of tens of nanometers.8 It is also noteworthy that the BCP nanopatterns are attractive precisely because of the self-assembly of the structures. While using a periodic nano- or micropatterned substrate may add precision in nanopatterning, it loses much of the advantage of the selfassembly process, and further the BCP pattern is limited by the area of the prepatterned substrate. Our sol−gel substrate bypasses these problems and also allows for a fundamental study of how roughness and surface energy synergistically combine to impact BCP orientation.
2. EXPERIMENTAL SECTION Substrate and Film Preparation. Two methods were used to prepare the substrates before coating the BCP films on them. First, a two-step acid−base catalyzed sol−gel polymerization method reported previously to prepare bulk silica xerogels was modified and used to prepare silica xerogel thin films of different surface features.30 Briefly, ethanol (EtOH) diluted tetraethoxysilane (TEOS) (Fluka, purum 98%) solution was mixed with propyltrimethoxysilane (PrTMS) (Aldrich, 97%) hydrophobic reagent. Then aqueous hydrochloric acid (HCl, volume fraction 2%) was added in this mixture such that the molar ratio of TEOS:EtOH:PrTMS:H2O (added to dilute HCl) was around 1:6.9:0.18:3.2, respectively. The mixture was homogenized by shaking using an autoshaker for 15 min, and this solution was used as stock after aging it for ≈24 h. The final silica sol was prepared by mixing the stock solution with aqueous ammonium hydroxide (NH4OH) solution of different molarities from 0.1 to 0.5 mol/L such that the TEOS:H2O (added to dilute NH4OH) molar ratio was about 1:2.2. Prior to the gelation point, the sol was aged for different time periods from 0 to 30 min (counted from NH4OH addition) before spin-coating it on polished silicon wafers. Spin-coating was performed at 209 rad/s (2000 rpm) for 60 s. The films were then dried under atmospheric pressure at 130 °C for about 12 h to remove any residual solvent. Additionally, silicon substrates were also coated by silica nanoparticle by slightly modifying a process reported previously31 such that the layer of nanoparticles covered most of the surface (≈80% area fraction as determined by AFM). For this dispersion (volume fraction 0.2%) of propyl-terminated silica nanoparticles (≈20 nm diameter) in chloroform was spin-coated on polished and cleaned silicon wafers at 209 rad/s (2000 rpm) for 60 s. These films were dried at room temperature for 1 h so that the volatile chloroform evaporated before using them for further processing. The topography of all the films was studied by AFM by scanning 5 μm × 5 μm areas at three different locations, and four films (see Figure 1) 4305
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were measured under vacuum at incident angles both above (0.2°) and below (0.15°) the polymer−air critical angle, which was determined using an X-ray reflectivity scan.
upon addition of base catalyst (NH4OH) and a 3D network of silica particles starts to develop.35 During the sol aging, the size of the silica particles increases initially before the cross-linking of the particles dominates the process. The cross-linking of particles leads to an increase in viscosity of the sol. Thus, by controlling the catalyst concentration as well as sol aging time, the size of the clusters forming the silica network can be tuned, which effectively determines the final roughness of the xerogel substrate. The particle size and viscosity of the sol increased with increase in the aging time and/or NH4OH concentration, and its effect on the xerogel substrate topography was studied by AFM. As the silica network formation begins after addition of NH4OH, the viscosity of the sol starts to increase with time and at the gel point the sol ceases to flow. In a primary experiment, gelation period was noted for bulk sol in a mold. Thin silica xerogel films were prepared by spin-coating the sol on Si wafers before the gelation time, when sol was still in liquid state. The solvent evaporation during spin-coating process leads to quicker gelation of silica films that gelled almost immediately. The topography of xerogel films (sample Xero-R0.5 nm) synthesized using 0.1 M NH4OH with sol aging time of 15 min (Figure 1(i)a) shows that the film surface is very smooth, mainly composed of very small (
