Article pubs.acs.org/Langmuir
Fabrication of Nanopatterned Poly(ethylene glycol) Brushes by Molecular Transfer Printing from Poly(styrene-block-methyl methacrylate) Films to Generate Arrays of Au Nanoparticles M. Serdar Onses* Department of Materials Science and Engineering, Nanotechnology Research Center (ERNAM), Erciyes University, Kayseri 38039, Turkey S Supporting Information *
ABSTRACT: This article presents a soft lithographic approach using block copolymer (BCP) films to fabricate functional chemically patterned polymer brushes on the nanoscale. Hydroxyl-terminated poly(ethylene glycol) (PEGOH) was transfer printed from the poly(methyl methacrylate) (PMMA) domains of self-assembled poly(styrene-block-methyl methacrylate) films to a substrate in conformal contact with the film to generate patterned PEG brushes mirroring the pattern of BCP domains. A key point in the study is that the chemistry of the functional transferred brushes is different from the chemistry of either block of the copolymer; PEG-OH is miscible only in the PMMA block and therefore transferred only from PMMA domains. The functionality of the PEG brushes was demonstrated by the selective immobilization of citratestabilized Au NPs (15 nm) and validated the generation of high-quality chemical patterns with sub-30-nm feature sizes.
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INTRODUCTION The generation of chemical patterns on the nanometer length scale plays a key role in a range of scientific and technological fields such as the organization of nanomaterials on surfaces,1−3 directed self-assembly of materials,4−6 and probing cell− substrate interactions.7 The selective immobilization of nanoparticles (NPs) on nanoscale chemical patterns is of significant technological interest. Wet chemical synthesis8 provides access to a rich menu of NPs with precise control of the size, shape, structure, and chemistry of the particles. Controlling the spatial position of these NPs at a resolution on the order of the diameter of individual particles is important in studying their individual and collective properties and in fabricating devices. The optical properties of metallic NPs, for example, strongly depend on the characteristics (e.g., size, shape, and structure) of the individual particles and their nanoscale arrangement (e.g., separation, orientation) because of localized surface plasmons.9 The dramatic alteration of the optical properties of NPs with a few nanometers of change in their separation10 highlights the need for templates with feature sizes and pitches down to several nanometers. The development of soft lithographic approaches is needed to enable high-throughput inexpensive chemical patterning of surfaces with small and dense features. Advanced lithographic techniques (electron beam and extreme ultraviolet lithography) are gold standards and enable the chemical patterning of surfaces with high resolution, density, and fidelity.11 However, the wide adaption of these techniques is hindered by their limitations in cost, throughput, and diversity of the materials © XXXX American Chemical Society
and substrates that can be patterned. Soft lithographic techniques hold great potential to overcome these issues. One approach to meet the requirements of low cost, high throughput, and considerable versatility is transfer printing of functional inks from one substrate to another.12 Microcontact printing of alkanethiols13 is the premier of these approaches and has been applied to a wide range of materials and processes. Transfer printing techniques are now well established on the micrometer scale extending down to ∼100 nm.14 Researchers demonstrated feature sizes approaching 50 nm using transfer printing based on nanocontact,15 subtractive printing,16 and supramolecular interactions.17 However, the fidelity of the patterns (e.g., line edge roughness) degrades at the nanoscale in comparison to traditional lithographic methods, and the density of the features was low. Therefore, it remains a challenge to fabricate chemical patterns by transfer printing at a resolution below 50 nm with a high density of features and a high level of fidelity. One material that presents inherent advantages to generating nanoscale chemical patterns by transfer printing is block copolymers (BCPs). When confined in thin films, BCPs selfassemble into nanopatterns (e.g., hexagonal and linear arrays) with high resolution (feature size down to 5 nm) and density (pitch size as low as 10 nm) as a result of the phase separation.18,19 BCPs are highly integrated with the lithographic Received: November 6, 2014 Revised: December 29, 2014
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DOI: 10.1021/la504359f Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir methods,20,21 and directing the self-assembly of BCPs not only provides long-range order and spatial control but also improves the quality and density of the guiding patterns.22 Previously, BCPs have been used in transfer printing experiments in two different ways. In the indirect approach, nanostructures provided by the self-assembly of BCPs were transformed to an inorganic template (e.g., the metallization of one block and the removal of organics). This template was then used as a master to generate nanoscale chemical patterns on a replica substrate based on supramolecular interactions23 and catalytic reactions.24 A recently developed technique, molecular transfer printing (MTP),25 differs from the past studies by the direct use of BCP domains during transfer printing: the thermodynamic interactions19 that drive the self-assembly process into nanoscale dimensions also ensure high-resolution printing. In MTP, nanoscale domains of the BCP film serve as reservoirs of inks that consist of functional homopolymers. Chemical nanopatterns of covalently bound polymer brushes are formed on the replica substrate through the reaction between the functionality of the homopolymer ink and the surface of the substrate. Here we explore nanopatterning of surfaces using MTP with an ink that is chemically different from the individual blocks of the BCP film. The concept of MTP25 has been demonstrated by printing hydroxyl-terminated polystyrene (PS-OH) and hydroxyl-terminated poly(methyl methacrylate) from selfassembled films of poly(styrene-block-methyl methacrylate) (PS-b-PMMA). One way to diversify chemistries patternable with MTP is to use a print and fill approach26,27 that was demonstrated by printing PS-OH brushes from self-assembled PS-b-PMMA films to a replica substrate and functionalizing the remaining areas with hydroxyl-terminated poly(2-vinylpyridine) brushes. It remains a challenge to directly print inks with a chemistry that is different from that of both blocks of the copolymer in order to create nanopatterns with a wide variety of chemical functionalities. In this article, we investigate the generation of functional chemical patterns by printing hydroxyl-terminated poly(ethylene glycol) (PEG-OH) brushes from self-assembled domains of PS-b-PMMA films to guide the assembly of Au nanoparticles (NPs). A key point of the presented approach is that the miscibility rather than the chemical identity is used for segregation of the homopolymer ink to the corresponding block of the copolymer. PEG, also called poly(ethylene oxide), has been reported to be miscible with PMMA in both bulk28 and thin film studies.29 Therefore, PEG-OH is expected to segregate into PMMA domains of the PS-b-PMMA films and form chemical patterns on the replica substrate mirroring the PMMA domains following transfer printing. The functionality of the nanopatterned PEG brushes is demonstrated by controlling the organization of ex-situ-synthesized Au NPs. The interaction between PEG brushes and citrate-stabilized Au NPs, as recently reported,30 renders selective binding of the particles to the nanopatterns. The arrangement and ordering of Au NPs thus provide a reliable indication of the effectiveness of printing PEG brushes from PMMA domains.
Figure 1. Schematic of the process for printing PEG brushes from selfassembled PS-b-PMMA films. (a) A blend of PEG-OH (5 wt %) and PS-b-PMMA (95 wt %) is self-assembled with solvent annealing to yield honeycomb nanopatterns on the surface of the film. The silicon substrate was functionalized with PS brushes to prevent dewetting of the BCP film during solvent annealing. (b) A clean silicon (replica) substrate is brought into close contact with the surface of the SiO2coated BCP film, and PEG brushes are printed from PMMA domains of the BCP film. (c) Separation of the substrates leads to chemical patterns of PEG brushes mirroring PMMA domains. (d) Selective immobilization of Au NPs onto the nanopatterned PEG brushes.
a PS-brush-grafted substrate. Solvent annealing in an environment saturated with acetone vapor resulted in honeycomb nanopatterns on the surface of the film. The PS brush prevented dewetting of the BCP film during solvent annealing, and the morphology on the surface of the film remained unchanged for methyl methacrylate-containing random copolymer brushes. The choice of solvent annealing over thermal annealing (190−230 °C) is based on the thermal degradation and side reactions related to PEG observed at elevated temperatures.31,32 A thin (35 nm) film of SiO2 was vapor deposited on the surface of the self-assembled film to ensure conformal contact during the transfer printing process. A clean silicon (replica) substrate was then brought into contact with the deposited SiO2 film by applying pressure through a custombuilt clamp and annealed in a nitrogen environment at a temperature of 120 °C for 24 h. Two processes simultaneously took place during annealing: (i) PEG-OH that was previously segregated into the PMMA domains of the BCP film diffused and end-grafted onto the SiO2 layer through the reaction between the hydroxyl terminus of the polymer and silanol groups of the conformal layer.25 (ii) The SiO2 layer bonded to the rigid silicon replica substrate. Following the separation of the substrates by dissolving the BCP film, chemical patterns of the PEG brush in a SiO2 matrix formed with a honeycomb geometry on the replica substrate. The functionality and quality of the nanopatterned PEG brushes were determined by the immobilization of citrate-stabilized Au NPs (15 nm diameter). An aqueous solution of Au NPs was placed on top of the replica substrate for 45 min, and then the substrate was washed with water under sonication. The particles selectively immobilized onto the nanopatterned PEG brushes with almost no binding to the SiO2 regions. The choice of conditions for the MTP of PEG brushes from PMMA domains of PS-b-PMMA films was made as a result of
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RESULTS AND DISCUSSION The fabrication of nanopatterned PEG brushes by MTP from PMMA domains of self-assembled PS-b-PMMA films is described in Figure 1. The process started with the selfassembly of a thin (30 nm) film of a blend containing PS-bPMMA (170.0-168.8 kg/mol) and PEG-OH (10.0 kg/mol) on B
DOI: 10.1021/la504359f Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir experiments on homogeneous substrates. For this purpose, blends of PEG-OH (10.0 kg/mol) and PMMA homopolymer (171.5 kg/mol) with various compositions were spin-coated to yield a film with a thickness of 30 nm on piranha-cleaned silicon substrates and annealed at 120 °C for 24 h in a nitrogen environment. The molecular weight of the PMMA and PEGOH, thickness of the film, and processing conditions were chosen to be almost the same as in the MTP experiments. Following the annealing of the PEG-OH/PMMA blends and washing with repeated sonication in chlorobenzene, the substrates were characterized by the ellipsometric thickness of the films and density of bound Au NPs as shown in Figure 2.
should lead to specific particle binding to regions that mirror the PMMA domains. Selective immobilization of Au NPs on the nanopatterns of PEG brushes printed from PMMA domains of the PS-b-PMMA films validated the printing process and demonstrated the functionality of the resulting chemical patterns. Highly ordered templates were prepared by solvent annealing a 30 nm film of a blend containing 5 wt % PEG-OH and 95 wt % PS-b-PMMA. Depending on the duration of solvent annealing, two types of morphologies were obtained on the surface of the film with a periodicity of ∼125 nm (Figure 3a,b): honeycomb and lamellar.
Figure 2. Formation of PEG brushes from blends of PEG-OH (10.0 kg/mol) and PMMA (171.5 kg/mol) on homogeneous substrates. The ellipsometric thickness of the films shown on the right is based on the average thickness obtained from three different substrates. The density of Au NPs bound to the PEG brushes (on the left) represents average values obtained from three different regions across a single substrate. The error bars represent the standard deviation.
Figure 3. SEM images of the BCP templates (on the left) and corresponding replica substrates (on the right) following the immobilization of Au NPs. The templates consist of a blend of PEG-OH (5%, 10.0 kg/mol) and PS-b-PMMA (95%, 170.0−168.8 kg/ mol) assembled with solvent annealing for (a) 16 h and (b) 48 h.
The former morphology obtained at short solvent annealing times is important for this study because of the asymmetric distribution of the individual domains on the surface of the film. The image contrast in electron microscopy and the selective removal of the PMMA block (Supporting Information Figure S1) verified that PS and PMMA domains are located in central bright and surrounding dark regions, respectively. The immobilization of citrate-stabilized Au NPs on the nanopatterned PEG brushes printed from the PS-b-PMMA films resulted in the organization of the particles into arrays mirroring the morphology of the template. In the case of templates with honeycomb morphology, NPs specifically attached to the surrounding regions (Figure 3c), suggesting that PEG-OH was printed from the PMMA domains of the BCP film. The immobilization of particles along the width of linear chemical patterns (Figure 3d) generated using lamellar templates further confirmed that printing was performed from PMMA domains and not from the interface between the PS and PMMA blocks. The strength of the presented approach is the generation of high chemical contrast patterns of PEG brushes with nanoscale dimensions and dense features. The precise placement of Au NPs (15 nm in diameter) onto the honeycomb patterns of PEG brushes with mostly single particles per line width (Figure 3c) is a measure of the high resolution (
