Dual Imprinted Polymer Thin Films via Pattern Directed Self

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Dual Imprinted Polymer Thin Films via Pattern Directed Self-Organization Danielle Grolman, Diya Bandyopadhyay, Abdullah M. Al-Enizi, Ahmed A. Elzatahry, and Alamgir Karim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Applied Materials & Interfaces

Dual Imprinted Polymer Thin Films via Pattern Directed Self-Organization

Danielle Grolman1, Diya Bandyopadhyay1, Abdullah Al-Enizi2, Ahmed Elzatahry3, Alamgir Karim1*

1

Department of Polymer Engineering, University of Akron, Akron, Ohio, 44325, United States

2

Chemistry Department, Faculty of Science, King Saud University, PO Box 2455, Riyadh

11451, Saudi Arabia 3

Materials Science and Technology Program, College of Arts and Sciences, Qatar University,

PO Box 2713, Doha, Qatar.

KEYWORDS: dewetting, confinement, Moiré patterns, thin films, self-organization

ACS Paragon Plus Environment

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ABSTRACT

Synthetic topographically patterned films and coatings are typically contoured on one side, yet many of nature’s surfaces have distinct textures on different surfaces of the same object. Common examples are the top and bottom sides of the butterfly wing or lotus leaf, onion shells, and the inside versus outside of the stem of a flower. Inspired by nature, we create dual (top and bottom) channel patterned polymer films. To this end, we first develop a novel fabrication method to create ceramic line channel relief structures, by converting the oligomeric residue of stamped poly(dimethylsiloxane) (PDMS) nanopatterns on silicon substrates to glass (SiOx, silica) by ultraviolet-ozone (UVO) exposure. These silica patterned substrates are flow coated with polystyrene (PS) films and confined within an identically patterned top confining soft PDMS elastomer film. Annealing of the sandwich structures drives the PS to rapidly mold fill the top PDMS pattern in conjunction with a dewetting tendency of the PS on the silica pattern. Varying the film thickness h, from less than to greater than the pattern height, and varying the relative angle between the top-down and bottom-up patterned confinement surfaces create interesting uniform and non-uniform digitized defects in PS channel patterns, as also a defect free channel regime. Our dual patterned polymer channels provide a novel fabrication route to topographically imprinted Moiré patterns (whose applications range from security encrypting holograms to sensitive strain gauges), and their basic laser light diffractions properties are illustrated and compared to graphical simulations and 2D-FFT of real-space AFM channel patterns. While traditional ‘Geometrical’ and ‘Fringe’ Moiré patterns function by superposition of two misaligned optical patterned transmittance gratings, our topographic pattern gratings are quite distinct, and may allow for more unique holographic optical characteristics with further development.


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1. Introduction Nature routinely creates structured surfaces with diverse characteristics which can be observed from a leaf structure that has veins or ridges on the back side and superhydrophobic structures on the top side. The multifunctionality of a system can potentially be tailored by dualpatterned structures to develop advanced materials with dynamic performance. For instance, the bottom side may be patterned for higher frictional resistance to slip while the top may be patterned for surface wettability control such as superhydrophobic behavior. Many dual combinations can be considered such as diffraction, high surface area adsorption of oil, directional adhesion, sound baffle, and cushioning effects, to name a few. Fabrication of microand nanopatterned structures, in particular, has recently attracted great interest for potential applications in microelectronics and emerging nanotechnology.1-11 There are currently several lithography techniques to generate ultra-high precision dimensions at the microscale and nanoscale such as nanoimprint lithography (NIL),12 holographic photolithography,13 electron beam lithography,

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and focused ion beam (FIB) lithography.

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However, these novel

approaches such as FIB or electron beam lithography have specific limitations with high frequency gratings that restrict its technological feasibility for efficient large-area manufacturing. While NIL is a promising cost-effective technique that offers high resolution of submicron structures, one of the main challenges is the stress accumulating within the hard mold which can be prone to cracking or deformation due to high pressure and thermal treatments. To meet demanding requirements for next-generation nanotechnology, there is a rising need to develop a universal approach for pattern replication with high pattern fidelity over large areas. In this regard, soft lithography proves to be a viable route for large-area pattern replication over other


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imprinting methods due to the elastomeric nature of PDMS which allows for 1) conformal contact between the film surface and stamp without additional applied pressure, 2) surface modification via UVO or plasma exposure, and 3) tunable elastomer modulus by altering the ratio of elastomer base to crosslinking agent during PDMS preparation.17 The self-organization of polymer thin films appears to be a practical alternative to alter surface morphology by utilizing processes such as phase separation or anisotropic dewetting, for instance. These methods provide an efficient means to generate well-ordered structures wherein its size features can be controlled by manipulating parameters such as the film thickness, annealing temperature, molecular weight, and polymer/substrate interactions. Controlled dewetting mechanism of polymer thin films have been extensively studied both theoretically and experimentally.4,18-25 Subjecting a thin polymer film to physical confinement has been observed to direct and guide the dewetting process. Previous studies by Suh et al.3 demonstrated controlled polymer dewetting by 2D patterned confinement, emphasizing the significance of the characteristic length of pattern size and capillary wavelength on ordering. Bandyopadhyay et al. further illustrated this effect by mapping out the film stability regimes of capillary wave confinement as a function of varying amplitudes in ultrathin polymer films.26 These experimental studies utilized top-down confinement via a soft lithography approach to mold and imprint the polymer film with an elastomeric poly(dimethylsiloxane) (PDMS) stamp.27-29 When heated above the polymer glass transition temperature Tg, capillary forces drive the polymer melt to fill up the void space of the channels formed between the polymer and mold, thereby generating a laterally confined geometry. Further confinement of polymer films between two plates plays a role in the perturbation of polymer film dewetting. Verma and colleagues studied the confinement of a thin liquid film between two plates and its resulting pattern formation utilizing


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nonlinear 3D simulations.30 These studies examined the evolution of thin film microstructure as a function of film thickness, air gap, and surface energy in the presence of an additional plate where the dynamics of thin films subjected to dual confinement was illustrated for smooth confining plates and patterned surfaces. Topographically patterned surfaces via dewetting have been pursued, but not utilized to its full potential due to the challenge of fully controlling the long-range order. While chemically patterned31-35 and modified surfaces36 offer a solution to this problem, this tends to be an expensive and time consuming process with additional fabrication steps required. Unidirectional dewetting by symmetry breaking in more than one direction with facile control over relative axes is a particularly interesting concept and Moiré patterns provide a means to alter multi-level quantized dewetting in a continuous way by altering the angle between the bottom and top line channels. Moiré patterns are essentially interferences generated from the overlay of two repetitive patterns of equal dimensions due to a slight misalignment.37-39 By gaining control of the relative degree of misalignment , the patterned topography and spacing dimensions can be carefully tuned with versatility, while maintaining its regularity and high precision as shown in Figure 1. Superimposing patterns of the same dimensions and altering the relative displacement = 10° between the two patterns induces large-scale changes in the resultant Moiré patterns.


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Figure 1. Simulations of 2D Moiré patterns with respect to degree of relative alignment between two patterns of identical dimensions where a) = 0°, b) = 5°, c) = 10° and d) = 20°. In this study, the pattern misalignment is fixed at 10⁰ and the confined polymer film in-between is subjected to 3 quantized confining heights that include: 1) mesa on mesa, 2) mesa on valley and 3) valley on valley.

In this work, we demonstrate a soft fabrication strategy to create dual-patterned polymer films by combining top-down and bottom-up approaches. A low molecular weight (Mw 3000 g/mol) polystyrene model system was utilized for its low viscosity to facilitate the dewetting process. Thin polystyrene (PS) films were subjected to two types of simultaneous patterned confinement: a glassy bottom-up confinement and a soft elastomeric top-down confinement where the PS film is non-wetting on both surfaces. In a novel approach, the bottom glassy patterned confinement was produced from the diffusion of uncrosslinked PDMS oligomers from a patterned elastomer stamp onto a hard silicon substrate due to incomplete curing. While biological applications demand meticulous PDMS curing conditions to minimize any contamination of uncrosslinked residue in microfluidic channels,40-42 we chose instead to capitalize on this phenomenon. In this regard, we use the diffusion of uncrosslinked PDMS oligomers and tunable surface energy to our advantage as a simple means of constructing


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patterned-incorporated glassy substrates to direct pattern formation. While conventional photolithography methods are commonly used for fabrication of nanostructures, we report in this study a soft fabrication strategy to create dual-imprinted polymer thin films via controlled template-directed dewetting where the mold filling time occurs within seconds. The selforganization of polymer thin films provides an advantageous route for large-area patterning that can be incorporated in a roll-to-roll (R2R) in-line assembly process for nanomanufacturing and continuous materials engineering. Our method serves as a versatile, simple, and cost-effective route for fabricating multi-dimensional patterned polymer thin films over large areas with wellordered arrays of periodic structures and controlled feature sizes at the micro- and nanoscale.

2. Experimental Section 2.1. Materials Polystyrene (PS) with weight average molecular weight Mw = 3000 g/mol was purchased from Polymer Source, Inc. and used as received, without any further purification. PS was dissolved in laboratory grade toluene (Sigma-Aldrich, ACS reagent, ≥ 99.5%) to make a 5 wt% solution and mixed for 6 hours on the Vortex. Solutions were filtered with 0.2µm PTFE filters to remove any contaminants. Polished silicon wafers were purchased from University Wafer with (100) orientation and thickness of 625 µm. Glass micro slides (75 x 25 mm) were purchased from VWR International and thoroughly cleaned before use by rinsing with methanol and deionized water. Silicon wafers were exposed to one hour of ultraviolet ozone (UVO) to remove organic impurities prior to film casting.


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2.2. Preparation of PDMS Substrates and Molds for Pattern Transfer Poly(dimethyl siloxane) (PDMS) was prepared by using Sylgard 182 purchased from Dow Corning. The ratio of elastomer base to crosslinking agent was 10:1 and was thoroughly mixed and placed in a desiccator to remove any trapped air bubbles for 30 minutes. The mixture was then poured directly onto the smooth surface of a glass plate and placed in a vacuum oven to allow for curing at 120°C for 2 h. The crosslinked PDMS was removed from the oven and quenched to room temperature. To create elastomer molds, PDMS was prepared as previously described by thoroughly mixing the elastomer base and crosslinking agent in a 10:1 ratio and 25:1 ratio (Sylgard 182, Dow Corning). A commercially available compact disk (CD) with periodic line channels was used as a lithography master wherein the pattern dimensions have a measured amplitude of 120 nm and pitch of 1.5 µm. The foil of the CD was carefully removed from the polycarbonate and the surface was properly cleaned and rinsed with methanol. The mixture was evenly poured onto the polycarbonate mold surface and placed in the oven at 120°C for 2 hours with a vacuum pressure of 10 Pa to allow for PDMS curing via a crosslinking reaction.

2.3 Generation of Sub-micron Pattern Features Imprinted via Soft Lithography The patterned PDMS stamp (25:1 ratio) was placed in contact with the UVO-treated silicon wafer and placed in a vacuum oven at 140℃ to transfer a significant quantity of uncrosslinked oligomers to the substrate. Upon quenching to room temperature, the PDMS mold was removed and nanopatterned channels from uncrosslinked PDMS were exposed to UVO for pattern densification and utilized as bottom-up topographic nanopatterned confinement. Thin polymer films from solutions of low molecular weight polystyrene (Mw 3000 g/mol) in toluene (5 wt%)


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were cast onto the patterned substrates by a conventional flowcoating technique described in detail elsewhere.43 Various concentrations and speeds were adjusted accordingly to achieve the following film thicknesses where ℎ = 40 , 120 , 160 , 240 . The flow coating process involves a finite volume of dilute polymer solution (5 wt% in toluene) deposited between a blade and underlying substrate (fixed to a translational stage) where the material is in the state of a wet film on the substrate which ultimately dries after solvent evaporation. In this regard, it was crucial to remove any residual solvent from the polystyrene film prior to PDMS contact when subjected to top-down confinement. Complete drying of these films for 12 hours at room temperature under vacuum ensured minimal swelling of the PDMS mold from absorption of any organic solvent. The polymer film thickness was measured using Filmetrics UV-20 interferometer software. Furthermore, to study the effect of dual confined polymer films, a PDMS stamp of identical dimensions was placed directly on the polymer film surface with a misalignment of = 10° (with respect to the underlying bottom-up pattern) and annealed in a vacuum oven at 140℃ for each film thickness to replicate patterns from the mold and imprint the polymer surface. The relative degree of pattern alignment was carefully measured on a customized assembly with a laser set-up in transmission mode.

2.4. Characterization and Morphology of Polymer Thin Films Morphology of polymer thin films subjected to confinement was examined using an Olympus BX41 optical microscope and Atomic Force Microscopy (AFM) using Tapping mode for height and phase images (AFM Dimension ICON, Veeco). Nanoscope Analysis and ImageJ software were used for image analysis to obtain line profiles from height micrographs.

X-Ray

Photoelectron Spectroscopy (XPS, PHI 5000 Versa probe II scanning XPS microprobe,


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ULVAC-PHI, Inc.) was employed for confirmation of elemental composition as a function of UVO exposure.

3. Results and Discussion 3.1 Polymer Film Dewetting Properties For a thin film on a substrate, the dewetting mechanism is reported to proceed by the formation of holes in the film due to capillary instability or nucleation. The holes continue to grow until they coalesce, removing matter towards the edge of the holes which forms a rim. This transforms into ribbons which then spontaneously breaks up into droplets of size proportional to the ribbon. The stability of PS on SiOx can be described by its interface potential ∅ℎ expressed in Equation 1 as:

∅ℎ = ⁄ℎ − ⁄12ℎ

(1)

When the second derivative of the interface potential is negative such that ∅"ℎ < 0, the film is unstable and will proceed through a process known as spinodal dewetting. Taking our experimental conditions and parameters into consideration where ℎ = 40 , = 140℃, ~ 6 ! 10"#$ %$ , &'() ~ 2 ! 10"* %, ultrathin polystyrene films on SiOx will spontaneously rupture and dewet as ∅"ℎ < 0 due to amplification of capillary waves. Furthermore, accounting for the surface tension of PS at + = 35 -/, the dominant capillary instability wavelength is calculated to be /0 ≈ 35 2 which agrees well with literature and experimental results.26,44


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Figure 2. Optical microscope images of ultrathin polystyrene films (40nm) annealed at 140℃ for 24 h on (a) SiOx, (b) UVO-treated PDMS, and (c) PDMS. The scale bar for (a) is 200 µm and the scale bars for (b) and (c) are 20µm.

To demonstrate the non-wettability of an ultrathin polystyrene film on each patterned substrate utilized for confinement, control experiments were first conducted. These measurements were sought to distinguish between the dewetting dynamics behavior when physically confined between the two patterns and to understand if there was any preferential wettability of the film when in contact with PDMS or the SiOx substrate. Due to the non-wetting behavior of PS on PDMS, utltrathin polystyrene films ℎ = 40 were first cast onto silicon substrates and adhered to the surface of a PDMS substrate. The conformal contact between the PS (on SiOx) and PDMS allowed for homogeneous film transfer from the silicon to flat PDMS substrate via a water flotation technique where the film was delaminated from the silicon wafer without any external force. This non-destructive method minimized any defects typically associated with water-transfer methods wherein free-standing films can be susceptible to cracking or film folding (overlapping layers). The PS/PDMS system was placed in a desiccator to completely dry the film and remove any residual water. Two PDMS substrates were used for


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comparison where one was not exposed to UV-ozone and the other had undergone 30 minutes of UV-ozone exposure. Similarly, a PS film of the same thickness was flowcoated onto a SiOx substrate. Optical micrographs were captured after annealing at 140℃ for 24 hours under vacuum to observe the final ‘equilibrium’ state for PS on SiOx, PS on PDMS, and PS on PDMS (with UVO). This is shown in Figure 2 where a narrow distribution of correlated dewet droplet sizes can be observed. The PS/SiOx thin film system undergoes spinodal dewetting due to amplification of capillary wave fluctuations upon heating above Tg. Figure 2a shows the characteristic polygonal pattern of spherical droplets resulting from the minimization of PS unfavorable contact with the SiOx substrate. Similar dewetting mechanism of film rupturing into droplets is observed for PS on PDMS, however with a noticeable difference in the kinetics and droplet size. This phenomenon may be attributed to the difference in surface energy for PS and PDMS with reported values of approximately 40 mJ/m2 and 20 mJ/m2, respectively. Accounting for the difference in surface energies, the spreading parameter is negative such that 3 < 0 indicating the unfavorable interactions for low molecular weight PS to wet the PDMS surface. This is in good agreement with theoretical and experimental studies examining the roles of substrate and coating wettability on polymer thin film dewetting mechanics.38 Fingering instabilities have been demonstrated to become far more pronounced for low wettability surfaces, i.e. an ultrathin PS film on a thin PDMS coating which, in turn, alters the hole growth kinetics and droplet sizes.45 This can be ascribed to the increased velocity during the growth of holes and decay of polygonal rims. Similarly for our studies, PS was floated onto a bulk PDMS substrate that was exposed to 30 minutes of UVO prior to annealing. Due to the surface modification and increase in surface energy upon exposure to UVO, it was found to have intermediate dewetting behavior between that of the PS/SiOx and PS/PDMS systems.


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3.2 Fabricating Novel Glassy Nanopatterned Substrates

We next focus on a new strategy to fabricate glassy topographically patterned substrates utilizing the oligomeric (uncrosslinked) residue that transfers onto substrates during a PDMS contact pattern stamping process. Details regarding the fabrication of the PDMS stamp is described in the experimental section. Transfer of PDMS oligomers to the SiOx substrate is significant, and we convert this patterned residue to glassy ridges with UV-ozone exposure as shown in Figure 3 via Atomic Force Microscopy (AFM). Optimization of the deposited quantity was achieved by altering the ratio of PDMS elastomer base to crosslinking agent and the contact time between the substrate and PDMS stamp where amplitude and pitch are = 120 and / = 1.5 2, respectively. The 3-D AFM micrograph displays the representative sample surface in a 10 µm scan size, confirming clean transfer of the nanopatterned PDMS oligomers to the substrate. Line profiles were taken across each micrograph to determine the average nanopattern height as a function of crosslinking agent and curing time. By controlling the crosslinking agent ratio and PDMS-SiOx substrate contact time, the transferred quantity of diffused PDMS oligomers from the stamp to the substrate is noted to vary from 40 to 100 nm.


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Figure 3.

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Physical characterization via AFM height profiles of patterned PDMS residue

stamped on glass substrate utilized for glassy bottom confinement after UV-ozone (UVO) exposure. (a) AFM micrograph is shown of 40 µm scan size in 2-D and (b) 3-D image of a zoomed-in 10 µm region showing clean transfer of oligomers to the silicon/glass substrate. (c) Densification of pattern height observed as a function of UVO exposure time, tUVO. (d) Corresponding height and roughness profiles along the pattern channels of 30 min UVO-exposed PDMS oligomers on SiOx substrate. Scale bar is 10µm.


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To create robust nanopatterns, we further modify and tailor the transferred PDMS patterned surface properties in a novel approach. The PDMS stamped silicon samples were exposed to short-wavelength ultraviolet light that generates ozone (UVO). At short exposure times, the UV-ozone exposure transforms the methyl-terminated surface to hydroxyl-rich,46-51 rendering the surface from hydrophobic to hydrophilic, as confirmed with contact angle measurements. As the UVO exposure time tUVO is increased from 0 to 240 minutes, the pattern height is observed to significantly decrease, giving rise to PDMS densification and glass (SiOx) formation. This densification trend of stamped PDMS patterns is shown in Figure 3 for 0 < tUVO ≤ 20 min, following first-order kinetics of pattern height change. When tUVO ≥ 60 min, this densification starts to plateau with minor pattern height loss observed between 60 ≤ tUVO ≤ 240 min. This is in good agreement with previous studies which have demonstrated the PDMS surface modification upon UVO exposure through x-ray reflectivity, contact angle, near-edge xray absorption fine structure, and fourier transform infrared spectroscopy techniques.47-49 Ye et al. studied the kinetics of this surface modification with sum frequency vibrational spectroscopy and confirmed first order kinetics with exponential depletion of methyl groups on the surface with time.49 Additionally, an interesting observation is the consequent nano-roughness along the transferred patterns induced from UVO exposure as observed from the line profile extracted in Figure 3d. The roughness along the pattern channel has a measured mean value of approximately 2~6.46 and a variance of + ~3.27 . This nano-roughness is likely to play a role in terms of wettability and dewetting mechanics with the associated polymer film it comes in contact with, and will be studied in further detail. The transfer of PDMS oligomers onto the substrate surface was confirmed by x-ray photoelectron spectroscopy (XPS) for elemental composition where the glassy transformation


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was studied as a function of UVO exposure time, tUVO. By probing approximately 5-10 nm from the surface into the samples, the following elements are detected from the surface: oxygen, carbon, and silicon. These elements, apart from hydrogen (which cannot be detected by XPS), comprise the poly(dimethylsiloxane) PDMS structure (C2H6OSi)n confirming the PDMS oligomers were in fact transferred to the glass substrate.51,52 This data can be summarized in Figure 4 as a function of tUVO. Without any exposure to UVO, the atomic percentage for carbon, oxygen, and silicon was found to be 46.3%, 32.5%, and 21.2%, respectively. Upon increasing the tUVO from 0 to 240 min, the methyl groups detected on the surface are converted to hydroxyl groups, resulting in a decrease in the carbon content from 46.3% to 14.3% and a nearly two-fold increase in the oxygen atomic percentage from 32.5% to 62.9%. This is a relatively fast conversion, as 10-20 minutes of UVO exposure shows a noticeable transformation where the carbon percentage in the sample is decreased by half. Such behavior can be correlated with the first-order kinetics observed in the physical characterization and densification of PDMS in Figure 3.


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Figure 4. Characterization of PDMS bottom confinement as a function of UVO exposure via Xray Photoelectron Spectroscopy (XPS). Elemental composition of oxygen, carbon, and silicon is studied from 0 ≤ tUVO ≤ 240 min. Arrow denotes peaks of increasing UVO exposure time.

3.3 Moiré Pattern Fabrication of Polymer Thin Films Under Dual Confinement Upon characterization of the bottom nanopatterned confinement surface on silicon substrate and confirmation of its transformation to a glassy state, a thin film of polystyrene was flow-coated directly on top after UVO exposure, as illustrated in Figure 5. Low molecular weight polystyrene was utilized in this study for its fast dewetting kinetics of unentangled polystyrene chains.53 To study dewetting phenomena as a function of film thickness under


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simultaneous top-down and bottom-up pattern confinement, a PS film with step gradient thickness ranging from 40 to 240 nm was prepared by flow coating. Furthermore, to study the effect of dual confined polymer films, a soft PDMS patterned stamp of identical dimensions was placed on top of the film with a relative degree of pattern alignment fixed at φ = 10° with respect to the underlying line channels, and annealed above Tg at 140℃ for each film thickness. Upon quenching the samples to room temperature, the top nanopatterned PDMS mold was peeled off to observe the PS dewet film for AFM surface measurements. A novel aspect of the work is the role of finite size effect in z-dimension of such Moiré patterns in polymer films. The nanopatterned confinement on the substrate has an average height of 95 ± 8 nm after UVO exposure, whereas the patterned elastomeric stamp utilized for top confinement has a step height of approximately 120 nm. We anticipate, and confirm experimentally, that there exists a critical PS cast film thickness hc, above which the individual top and bottom pattern “range” from each substrate do not interfere with each other during the simultaneous dual confinement approach, i.e. where Moiré pattern interference effects do not occur. In our study, the top mold is removed and the film remains substrate-supported on the patterned substrate for characterization studies regarding surface topography. However, the film can easily be removed from the underlying substrate as a free-standing dual-patterned film by utilizing a sacrificial thin layer. This was confirmed by casting a thin layer of a water-soluble polymer such as PSS directly on top of the silica channels. The PS film (of varying thicknesses) could be directly cast on the PSS-coated silica channels and subjected to patterned confinement to imprint the polymer surface. To this end, we demonstrate a facile assembly of dual-imprinted polymer thin films which can be freestanding and transferred onto a broad range of different substrates through a water flotation method.


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Figure 5. Schematic of dual confined polystyrene thin films subjected to glassy bottom confinement and top patterned elastomeric confinement utilizing a PDMS stamp with dimensions of approximately 120 nm and 1.5 µm. A) A patterned PDMS stamp is placed on a silicon substrate and heated in the vacuum oven at 140℃. B) Low molecular weight oligomers are transferred to the substrate and upon removing from the oven and cooling to room temperature, the patterned PDMS stamp is peeled off. C) Transferred oligomers are subjected to UVO exposure to allow for densification and transformation into a ‘glassy’ state. D) Thin films of polystyrene are flowcoated on top of the glassy nanopatterned substrate at film thicknesses above and below the critical film thickness hc. E) Nanopatterned PDMS stamp is carefully placed on top of the PS films at a specific degree of misalignment = 10° and heated in the oven at 140℃. Upon cooling to room temperature, the stamp is removed, resulting in a dual-patterned film.


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To conduct a thorough analysis, initial cast PS film thicknesses (as measured on smooth silicon) below and above the hc were studied to map out the conditions for creating Moiré patterned surfaces. Polystyrene thin films subjected to confinement were further characterized by taking line profiles across AFM Height micrographs. The glassy bottom confinement was found to have an average height of 95 nm after UVO exposure whereas the pattern height of the elastomeric top confinement was 120 nm. Therefore, to create stable polymer thin films without pattern interference, the critical film thickness (hc) must be hc ≥ 215 nm. As the confined polystyrene thin film is heated above the polymer’s glass transition temperature (Tg) at 140℃, capillary forces drive the polymer melt into the channels between the mold and the polymer, filling the mold. The mold filling process occurs within seconds as determined from the relationship 7 = 289 ⁄:; cos ? where η is the viscosity, z is the pattern height, R is the hydraulic radius, : is the surface tension, and ? is the contact angle. If the thickness of the polymer film is substantial to fill the mold cavity, the polymer surface will obtain the patterned structure of the mold with a residual polymer layer on the substrate. However, if the polymer film is thin and not sufficient to completely fill the mold, this gives rise to a capillary rise effect. The total free energy is reduced from the wetting of the polymer melt on the wall of the PDMS mold. Upon quenching and removing the mold, a negative replica of the mold can be obtained. This can be summarized in Figure 6 where the top row (a-d) shows representative optical micrographs for 40 nm ≤ h ≤ 240 nm films that have been annealed above Tg and its corresponding 40 µm scan sizes from atomic force microscopy (e-h). The insets of (a) and (d) are the corresponding SEM micrographs after annealing, confirming equilibrium structures for film thicknesses of 40 nm and 240 nm, respectively. The Fast Fourier Transform (FFT) was taken for


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each sample of varying film thickness to measure periodicity of the structures and its orientation (i-l).

Figure 6. Optical micrographs (a-d) and corresponding atomic force microscopy images (e-h) of post-annealed thin PS films on SiOx for 24 h at 140℃ as a function of film thickness (h) at (a,e) 40 nm, (b,f) 120 nm, (c,g) 160 nm, and (d,h) 240 nm. FFTs of AFM micrographs (i-l) for each film thickness are shown and compared to diffraction patterns (m-p) observed with a customized laser assembly. Scale bar represents 15 µm. Insets of a) and d) show high-resolution SEM images with scale bar of 10 µm for a polymer film thickness of 40 nm and 240 nm, respectively.


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A third height pattern dimension was used to study phenomena of polymer film dewetting. In our case, Moiré patterns were created by crossing line channels with patterned heights in the form of mesas and valleys. These have 3 quantized confining heights for any confined film in-between that include, from lowest to highest heights: 1) mesa on mesa, 2) mesa on valley and 3) valley on valley. Figure 6 shows the superposition of two periodic diffraction gratings, with the net transmittance giving rise to dual-patterned surfaces. The optical micrographs and AFM images displayed in Figure 6 show arrays of topographic PS structures obtained both at the polymer/air interface from the PDMS mold and from the polymer/substrate interface from the polymer melt filling the trenches on the substrate. This gives rise to two sets of directionalities where one is generated from the replica of channels from the top PDMS stamp and the other is perpendicular to the former’s mesas and valleys. However, the formation of Moiré patterns is profoundly influenced by the film thickness (height) and therefore only observed for ultrathin PS films at approximately 40 nm via optical micrographs and AFM height images in Figure 6a and 6e, respectively. At 40 nm, the polymer film thickness is not sufficient (ℎ < ℎ0 ) to fill the mold and leads to pattern interference, giving rise to spontaneous dewetting that occurs in a controlled manner along the nanopatterns as the thinner PS region tries to minimize contact with SiOx. This can be confirmed with the corresponding FFT in Figure 6i, which denotes regular periodicity as predicted in the Moiré pattern simulations in Figure 1. Regularly spaced holes (pattern “defects”) are detected at early annealing times and observed to grow across the strip until all the holes propagate to the edge in the lateral direction, which leads to regularly separated block formation (“digitized or quantized dewetting”). An annealing time of 24 h was fixed to allow for well-ordered hole formation ultimately leading to structured dewet surfaces. The dimensions of these rectangular PS blocks have an average length, width, and


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height of 5.07 µm, 1.51 µm, and 103.4 nm, respectively. The distance between the PS structures is approximately 3.21 µm, denoting the regions where the PS film fully dewet. When 120 < ℎ < 160 , the polymer melt is still not enough to completely fill the mold ℎ < ℎ0 ~185 and is only partially filled, thus resulting in random holes throughout the film. The number of holes and its area fraction coverage is observed to significantly decrease with increasing film thickness. Upon heating the polymer melt above its Tg when ℎ A ℎ0 , the amount of polymer melt is sufficient to fill the cavity of the mold, and no dewetting was observed.

Figure 7. Average area fraction of holes via directed dewetting as a function of film thickness 40 B ℎ B 240 for low molecular weight polystyrene thin films subjected to dual


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confinement. Three stability regimes are shown: I) uniform digitization, II) non-uniform digitization, and III) stabilization regime.

As PDMS elastomeric walls were utilized as a top-down confining medium, the dewetting capillary wave can propagate only along the strip direction leading to anisotropic hole formation. The consequent film morphology is highly dependent on film thickness of the confined film which was used as a third height pattern dimension to study dewetting dynamics. Three distinct regimes were observed for varying polystyrene (PS) film thickness h: I) uniform digitization 40 B ℎ B 80 , II) non-uniform digitization 80 B ℎ B 200 , and III) stabilization regime where ℎ A 200 . In Regime I, the ultrathin polystyrene films subjected to both top-down and bottom-up confinement are observed to undergo template-directed dewetting. The resulting polymeric film forms highly precise rectangular topographical structures in attempt to undergo mold filling while minimizing unfavorable interactions to reduce the free energy of the system. Regular arrays of holes are observed when ℎ = 40 , with approximately 57% of the bare substrate left exposed upon annealing due to dewetting. The proposed dewetting mechanism for Regime I can be further explained. An ultrathin PS film initially wets the topographically patterned SiOx substrate which has pattern dimensions of average height and wavelength of approximately 95 nm and 1.5 µm, respectively. It is expected that when subjected to a top-down nanopatterned PDMS mold at a temperature above Tg, capillary forces will attempt to drive the polystyrene melt into the mold. However, due to insufficient polymer melt to completely fill the mold, the PS can only wet the edges of the confined PDMS walls while simultaneously trying to minimize contact with the SiOx/air void thus leading to competitive dewetting behavior.


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In Regime II, the area percentage of holes on the surface is observed to significantly decrease with increasing film thickness. Additionally, the dewet structures are rather holes notably smaller in size than in Regime I. Nearly 22% of the surface was attributed to holes from dewetting that were non-uniform and irregular when ℎ = 120 , and decreased considerably to 6% when the film thickness was increased to 160 nm. In Regime III, the PS film thickness exceeds Moiré patterns finite size interference effects so the patterns are individually imprinted on each surface of the film (polymer/substrate and polymer/air interfaces) and dewetting is completely suppressed. When ℎ A 200 , there is complete polymer surface coverage with the PS film obtaining exact replicas of the mold and pattern. Figure 7 summarizes this relationship between the polymer film thickness (h) and consequent holes (area percentage, AF) due to dewetting, following an exponential decay when fit to a curve as in Equation 2: Cℎ = eE.F"*.*GH"I.$J"FK

L

(2)

Finally, in an effort to demonstrate and test the applicability and preciseness of our patterning technique, a laser with wavelength 645 nm was utilized to test the efficacy of our topographic Moiré patterned substrates as diffraction gratings. Thin films were made on both silicon wafers and glass slides with identical results. As a beam of incident light normal to the grating passes through a set of regularly spaced slits, the diffracted beams produce a resulting pattern (Figure 6 m-p). As such, the track spacings from these patterned surfaces can be confirmed over a large area and compared to the AFM micrographs. For a diffraction grating, the condition for maxima in the interference pattern at a particular angle θ is based on the following relationship sin ? = /, where d is the grating spacing, θ is the angle of diffraction, n is the order number, and λ is the wavelength of light. To establish a baseline, the patterned plastic base


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of a compact disc was used as a transmission grating upon removal of the reflective surface. By accounting for the angle of diffraction and wavelength properties, measurements of the diffraction pattern corresponded to a track pitch of approximately 1.5 µm which is in good agreement with the AFM micrograph. The diffraction pattern showed two sharp orders of diffraction on each side of the direct beam. After cross-calibration with the compact disc base structure, we sought to measure the diffraction pattern of the nano-piano structure created from the 40 nm polystyrene film subjected to dual confinement at = 10°. Directing a laser of a single wavelength 645 nm at the ultrathin film produced multiple orders of diffraction on each side of the direct beam, matching that of the FFT in Figure 6i from the real space AFM micrograph. From Figure 6m, up to 2 orders of diffraction = 2 are observed which is clearly indicative of the fact that the pattern transfer, albeit not perfect, is moderately good. This suggests that the large number of closely spaced slits running parallel within the specimen is regular throughout the entire sample, and well-correlated between optical and topographical effects. It should be noted that much care was taken to improve pattern fidelity by optimization of processing conditions to minimize PDMS shrinkage and minimal deviation of the replicated feature sizes from the original master. However, despite these efforts, we speculate that some degree of distortion is inevitable due to the elastic nature of PDMS, which could alter the fidelity of the patterns, broaden the distribution of feature sizes from localized shape distortion, or potentially be subject to collapsed nanostructures during processing or from gravitational forces over time. While the material properties of an elastomeric stamp during pattern transfer have current limitations associated with distortions for high-resolution structure replication, there have been numerous methods recently proposed to address the PDMS shrinkage, swelling, and deformation through PDMS surface modification, optimization of PDMS curing conditions, and


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tailoring the PDMS stiffness.54-57 Nevertheless, the ability to potentially generate threedimensional complex topologies via a soft lithography approach allows for fabrication of microand nanoscale structures over large areas for a broad range of materials and at a relatively low cost. This cost-effective strategy of producing replicated feature sizes is certainly advantageous for continuous materials engineering and has technological importance towards advances in the field of nanomanufacturing.


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4. Conclusion As inspired by nature, we have achieved the fabrication of well-ordered dual-sided nanopatterned polymer thin films and confirmed their morphology and light diffractive capability by microscopy and optical techniques. We believe this study paves the way for future investigations of dual-interface patterned films. By simply controlling the polymer film thickness and fixing the relative angle and pitch of the confining channel patterns, we could finely tune the morphology of the dual-sided nanopatterned PS channels. The area fraction of surface defects, i.e. holes on the patterned PS surface, is related to dewetting characteristics of the PS on the silica patterned surface. While a patterned wetting substrate may have eliminated these defects, that is, however, potentially the subject of a future study. These hole defects decrease exponentially as a function of thickness of the confined PS film. Three regimes of polymer film structure is observed that include hole channel defects with uniform and non-uniform structure, as well as conformal to dual pattern topography in thick films. An interesting aspect of the holes was their digitized or quantized nature, a consequence of dewetting within a channel confinement. Demonstration for potential application as diffraction gratings was further illustrated through a customized laser set-up in transmission and confirmed with its corresponding FFT of AFM measured real space structure. As an outlook, while line channel patterns of similar dimensions were employed in this study to demonstrate the practicality of these dual-patterned films for use as diffraction gratings, more complicated Moiré patterns can potentially be used to generate an immense array of possibilities with regards to customizable spacing, shape, twists, and size for specific applications. Furthermore, by significantly reducing the wavelength, the spacing of the interference fringes could greatly decrease, offering a cheaper


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alternative to commonly used photolithography techniques and of significant technological interest for potential applications in nanophotonics.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was funded by Department of Energy, Basic Energy Sciences via grant DE-FG0210ER4779.

ACKNOWLEDGMENTS This work was supported by Department of Energy, Basic Energy Sciences under grant DEFG02-10ER4779. The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of the Prolific Research Group (PRG-1436-14). The authors gratefully acknowledge Edward Laughlin with the development and fabrication of the customized assembly, Dr. Zhorro Nikolov for assistance with XPS measurements, and Dr. Saumil Samant and Namrata Salunke for fruitful discussions.


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Dual Imprinted Polymer Thin Films via Pattern Directed Self-Organization Table of Contents Graphic


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Dual Imprinted Polymer Thin Films via Pattern Directed Self

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