Highly Selective Immobilization of Au Nanoparticles onto Isolated and

Apr 12, 2012 - Chemical patterns consisting of poly(2-vinyl pyridine) (P2VP) brushes in a background of a cross-linked polystyrene (PS) mat enabled th...
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Highly Selective Immobilization of Au Nanoparticles onto Isolated and Dense Nanopatterns of Poly(2-vinyl pyridine) Brushes down to Single-Particle Resolution M. Serdar Onses, Chi-Chun Liu, Christopher J. Thode, and Paul F. Nealey* Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States ABSTRACT: Chemical patterns consisting of poly(2-vinyl pyridine) (P2VP) brushes in a background of a cross-linked polystyrene (PS) mat enabled the highly selective placement of citrate-stabilized Au nanoparticles (NPs) in arrays on surfaces. The cross-linked PS mat prevented the nonspecific binding of Au NPs, and the regions functionalized with P2VP brushes allowed the immobilization of the particles. Isolated chemical patterns of feature sizes from hundreds to tens of nanometers were prepared by standard lithographic techniques. The number of 13 nm Au NPs bound per feature increased linearly with increasing area of the patterns. This behavior is similar to previous reports using 40 nm particles or larger. Arrays of single NPs were obtained by reducing the dimensions of patterned P2VP brushes to below ∼20 nm. To generate dense (center-to-center distance = 80 nm) linear chemical patterns for the placement of rows of single NPs, a block-copolymer (BCP)assisted lithographic process was used. BCPs healed defects associated with the standard lithographic patterning of small dimensions at high densities and led to highly registered, linear, single NP arrays.

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registration, and resolution.8−10,18 Electron beam lithography, for example, can generate resist features of arbitrary geometry of as small as 10 nm. Chemical contrast relates to the difference in affinity for NPs between patterned, chemically functionalized regions and the background areas. Ideally, NPs should bind to patterned functionality at high densities, not to background regions. Chemical contrast becomes more critical with the number of NPs per patterned feature approaching one, where just a few particles nonspecifically bound to the background regions will ruin the process. The transfer of a pattern of resist features into a pattern of binding sites with high chemical contrast is challenging. Instead, researchers deposited NPs on functionalized regions not protected by the resist and then lifted off the resist to remove the particles that were nonspecifically bound to background regions.9,10,18 The selective immobilization of NPs on high-contrast chemical patterns without the need to lift off the resist is highly desirable in providing a platform for the direct interaction between the particles and chemical patterns and for the applicability of the process to a wide variety of materials (e.g., organic solvents for immobilization). The chemical patterning of surfaces for the site-specific placement of Au NPs becomes challenging as the density of patterned features reaches the limits of state-of-the-art lithographic tools. The chemical patterns employed in previous

INTRODUCTION The immobilization of metallic nanoparticles (NPs) on chemically nanopatterned surfaces with high degrees of specificity and control in the pattern dimensions and densities is of scientific and technological interest. Metallic NPs can now1 be synthesized with extraordinary control over their size, shape, composition, and structure. In turn, the properties (e.g., optical) of the metallic NPs depend on the characteristics of the individual2,3 and nanoscale organization4,5 of the particles. To study6 and fabricate devices7 using the individual and collective properties of the particles, it is often required to control the location and organization of the NPs on surfaces. Researchers have explored a variety of approaches to the site-specific placement of NPs, including but not limited to the generation of chemical,8−10 topographical11,12 and charge patterns,13 DNA-mediated assembly,14,15 and direct printing.16,17 Chemical patterning of surfaces is an effective strategy for directing the location and organization of NPs because it allows for robust and controllable interaction (e.g., covalent, electrostatic) between the substrate and particles of various types (e.g., shape, composition) and size. The generation of binding sites in a nonadsorbing background with high resolution and chemical contrast is critical to the site-specific placement of NPs. The ability to control the size and geometry of the binding sites on the order of the size of individual particles is needed in order to benefit from the individual and collective properties of NPs. The majority of chemical patterns used to create NP arrays have been made by lithography, mostly because of superior pattern fidelity, © 2012 American Chemical Society

Received: February 6, 2012 Revised: April 9, 2012 Published: April 12, 2012 7299

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studies were either isolated or low density.8−10,18 Isolated features are especially important to scientific studies in investigating the properties of individual particles or specific assemblies. Dense NP arrays are required to study the coupling between patterned individual particles and to increase the output or signal in applications in which the signal is proportional to the number of particles per area. The pitch (center-to-center distance) of the lithographically prepared chemical patterns for the site-specific placement of Au NPs in previous studies is typically 200−250 nm and higher.8,9 Chemical patterning of high-density features is challenging for two reasons. First, pushing the limits of lithographic tools leads to defects in the patterning of resist features. Second, difficulties in transferring patterns from resist features to chemical binding sites can cause a further reduction in densities that are reachable with the lithographic process. In the case of lifting off the resist layer following the immobilization of Au NPs, for example, increasing the density of features may cause difficulties in the removal of particles from the background regions or the retention of bound particles on the patterns. Previously,19,20 we explored the direct selective immobilization of Au NPs onto nanoscale chemical patterns generated by the functionalization of patterned SiO2 regions with hydroxylterminated, end-grafted poly(2-vinyl pyridine) (P2VP−OH) brushes in a background consisting of hydroxyl-terminated endgrafted polystyrene (PS) brushes or cross-linked PS mats. The PS brushes or mats as deposited prevent the nonspecific adsorption of Au NPs, and the P2VP brushes present a strong binding affinity toward Au NPs, leading to the selective immobilization of the particles in the patterned regions. In one study,19 we investigated a highly parallel soft lithographic approach based on block-copolymer films (molecular transfer printing, MTP) to generate dense nanoscale patterns of unfunctionalized SiO2 in a background of PS brushes. During the functionalization of the SiO2 regions with end-grafted P2VP brushes, however, processing conditions had to be optimized to limit the interpenetration of P2VP into the PS background, leading to the nonspecific binding of NPs. In a second study,20 we showed that for a sufficiently high cross-linking density, background regions composed of PS mats retained their nonadsorbing properties under a wide range of processing conditions, including the functionalization of patterned SiO2 regions with end-grafted P2VP brushes. A key advantage in fabricating NP arrays by the direct selective immobilization of the particles using PS mats is the ease with which high chemical contrast nanoscale patterns can be generated. Here we explore the highly selective immobilization of 13 nm Au NPs onto isolated and dense chemical nanopatterns of P2VP brushes in a background of a cross-linked PS mat. The dimensions of the patterns can be reduced to immobilize NPs with single-particle resolution. Two closely related processes are used for the preparation of two different types of patterns. Isolated chemical patterns of arbitrary geometries are created by pattering a resist layer with electron beam lithography, followed by oxygen plasma etching to reveal SiO2 regions in a cross-linked PS mat background. The functionalization of etched regions with P2VP brushes leads to the selective immobilization of NPs into arrays with precise control of the number of particles per patterned feature down to one. However, the generation of dense chemical patterns (pitch 50− 80 nm) via a similar process results in issues such as difficulty in controlling the number of NPs per patterned region and poor registration between the particles. A self-assembling material is

inserted into the lithographic patterning process to solve these issues. Block copolymers (BCPs) can form small, dense (pitch ∼10−200 nm) nanopatterns such as closely packed hexagonal arrays and fingerprint structures on surfaces.21,22 More importantly, BCPs are highly integrated with the lithographic methods and have been used to improve the quality (e.g., resolution and line-edge roughness) of the resist patterns.23−26 Poly(styrene-block-methyl methacrylate) (PS-b-PMMA) films are directed to assemble on dense, lithographically prepared chemical patterns. Oxygen plasma etching selectively removes the PMMA block and underlying cross-linked mat to reveal patterns of SiO2 for functionalization with P2VP brushes. The self-assembling nature of BCPs improves the registration and heals the defects that arise in the generation of dense chemical patterns and leads to highly registered linear single NP arrays.

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EXPERIMENTAL SECTION

Materials. PS-b-PMMA (85-b-91 kg/mol, PDI = 1.12) and P2VP− OH (6.2 kg/mol, PDI = 1.05) were purchased from Polymer Source Inc. and used as received. Toluene, chlorobenzene, and 1-methyl-2pyrrolidinone were purchased from Fisher Scientific, and N,Ndimethylformamide was purchased from Sigma-Aldrich and used as received. Water was purified with a Millipore Milli-Q purification system. Hydrogen tetrachloroaurate(III) (HAuCl4) was purchased from Strem Chemicals. Sodium citrate (citric acid trisodium salt dihydrate, 99%) was purchased from Acros. The PMMA resist (950 kg/mol, 4 wt % in chlorobenzene) was purchased from MicroChem Inc. Synthesis of the Cross-Linkable Polystyrene (PS). Crosslinkable PS was synthesized as described previously and contained ∼4% glycidyl methacrylate.20 Synthesis of Au NPs. HAuCl4 (200 mL, 1 mM) was brought to boiling while being stirred in a heating mantle, and then sodium citrate (20 mL, 38.8 mM) was added to the boiling solution. The solution was stirred and boiled for 15 min and then cooled to room temperature and diluted to 200 mL with water. The concentration of Au NPs was determined by absorption at a wavelength of 520 nm, as measured with a UV−vis spectrophotometer (Nanodrop). Deposition of the Cross-Linkable PS Mat. Oxygen-plasmacleaned silicon wafers were used for all experiments. Unless otherwise specified, a 0.25 wt % solution (toluene) of a cross-linkable PS was spin-coated at 4000 rpm. The substrate was then annealed at 190 °C under vacuum for 24 h. Following annealing, ungrafted, uncross-linked PS was removed by three cycles of sonication in warm toluene for 3 min/cycle. Patterning of the Cross-Linked Mat with Electron-Beam Lithography. A 120-nm-thick PMMA resist was spin-coated onto the cross-linked PS mat on the substrate. The substrate was then baked at 160 °C for 1 min before patterning to ensure the removal of solvent. The resist was then patterned with electron beam lithography using a LEO 1550-VP SEM equipped with a J. C. Nabity pattern-generation system. Patterns were written using a 20 keV beam energy with a beam current of ∼30.5 pA. The substrate was then developed using a 1:3 v/v mixture of methyl isobutyl ketone/isopropanol (MIBK/IPA) for 60 s and rinsed with IPA for 60 s. The substrate was then etched with oxygen plasma for 40 s (10 mT, 10 sccm oxygen, and 50 W in a Unaxis 790 RIE tool) to remove the cross-linked mat in the regions patterned by electron beam lithography. The remaining resist was then removed by sonication twice in warm 1-methyl-2-pyrrolidinone (NMP) and once in warm toluene (5 min/cycle). The substrate was thoroughly rinsed with toluene before drying to ensure the removal of NMP. The substrate was then functionalized with P2VP−OH brushes and treated with Au NPs as described in the Functionalizing with P2VP−OH section. Determination of Etching Rates of PMMA and PS on Homogeneous Substrates. An ∼80 nm film of PS and an ∼140 nm film of PMMA were spin-coated on silicon substrates and then were subjected to oxygen plasma etching at plasma powers of 50 and 7300

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Figure 1. Generation of isolated and dense chemical patterns for the direct selective immobilization of Au NPs. (4%) in DMF at 4000 rpm and annealing at 160 °C under vacuum for 24 h. Following annealing, the substrates were washed with five cycles of sonication in DMF for 5 min per cycle. The substrates were later annealed at 160 °C under vacuum for 30 min to remove any residual DMF. The substrates were then immersed in an Au NP solution (∼2 nM) for 30 min and washed in water for 2 min under sonication and dried with N2.

150 W for 30 s (at 10 mT and 10 sccm oxygen in the Unaxis 790 RIE tool). The thicknesses of the films before and after etching were measured with an ellipsiometer (Rudolph). The etching rate of PS was 52.1 nm/min at a plasma power of 50 W and 96.5 nm/min at a plasma power of 150 W. The etching rate of PMMA was 104.3 nm/min at a plasma power of 50 W and 209.2 nm/min at a plasma power of 150 W. Patterning of the Cross-Linked Mat for the Directed Assembly of Block Copolymers. A 70-nm-thick PMMA resist was spin-coated onto the cross-linked PS mat. Substrates were then baked at 160 °C for 1 min before patterning to ensure removal of the solvent. The resist was then patterned with extreme ultraviolet interference lithography (EUV-IL) at the Synchrotron Radiation Center (SRC) at the University of WisconsinMadison. A grating pattern consisting of a series of lines with a period of 80 nm and a width of ∼40 nm was written on the resist by EUV-IL. Following the development of the resist (as described above), the substrate was then etched with oxygen plasma twice for 10 s each (10 mT, 8 sccm oxygen, and 100 W in a PE-200 benchtop plasma system) to transform the resist pattern to the chemical prepattern. The plasma treatment oxidized the cross-linked PS mat in the patterned regions and made it preferential to the PMMA domain of the block copolymer. The remaining resist was then removed as described above. Chemical Patterning of the Surface Using a BlockCopolymer-Assisted Lithographic Approach. A 35 or 55 nm film of PS-b-PMMA was spin-coated from toluene onto chemically patterned substrates and then annealed for 24 h at 230 °C under vacuum. The assembled films were then etched with oxygen plasma for varying amounts of time using two different plasma powers of 50 and 150 W (at 10 mT and 10 sccm oxygen in the Unaxis 790 RIE tool). This etching selectively removed the PMMA block and the underlying mat under the PMMA and revealed the silicon oxide. The substrate was then washed with four cycles of sonication in warm toluene for 4 min per cycle to ensure the removal of the remaining PS block. The substrate was then functionalized with P2VP−OH brushes and treated with Au NPs as described in the Functionalizing with P2VP−OH section. Functionalizing with P2VP−OH. The substrates were functionalized with P2VP−OH (6.2 kg/mol) by spin-coating from a solution

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RESULTS AND DISCUSSION A schematic of the processes used for the generation of isolated and dense chemical patterns for the site-specific placement of Au NPs is shown in Figure 1. A thin film of cross-linkable polystyrene (PS) was first deposited and cross-linked on a Si substrate. In the case of isolated chemical patterns, patterns were then written on a resist layer spin-coated on top of the cross-linked PS mat using electron beam lithography. Following the selective removal of the exposed resist by development, the substrate was treated with oxygen plasma etching so as to remove all of the cross-linked mat in the region not protected by the resist to reveal SiO2. The remaining resist was washed away, and then a film of P2VP−OH was spin-coated onto the substrate and annealed under vacuum to graft the brush on the substrate with the condensation reaction of the hydroxyl terminus of the polymer with the silanol groups of the surface. The high cross-linking density of the mat prevented any composition changes in the background and enabled the specific functionalization of the SiO2 regions with P2VP−OH.20 The substrate was then immersed in an aqueous solution of citrate-stabilized Au NPs and then subsequently washed with water under sonication, leading to the specific binding of the particles to the P2VP-functionalized regions. The high chemical contrast and ability to control the geometry and size of the patterns arbitrarily are two key attributes of the process of patterning isolated features. Scanning electron microscopy (SEM) images in Figure 2 7301

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The strength of the presented approach is the generation of high-contrast chemical patterns for the direct selective immobilization of NPs. In most9,10,18,27 of the previous studies, arrays of Au NPs that are chemically bound to substrates were prepared by the functionalization of the patterns and the immobilization of the particles, followed by the lifting off of the resist. The advantage of the lift-off method is that the background regions are defined by the resist and nonspecifically bound particles can be easily removed. The presence of the resist during the immobilization of NPs has several consequences: (i) The processes of the immobilization of particles and the patterning of resist features are coupled. Therefore, materials used in the two processes should be chosen so as not to interfere with each other. (ii) The chemistry of the background regions cannot be controlled. It may be desirable to vary the interaction of the background regions with the particles (e.g., nonbinding to repulsive, hydrophilic to hydrophobic). (iii) The presence of topography can affect the immobilization process. For example, the attachment of particles to the bottom of the trenches may introduce issues as the pattern dimensions approach the limits of lithographic tools. Alternatively,8 patterns of SiO2 in a background of Au were prepared to provide orthogonal surface chemistries for the functionalization of the patterns and background regions with amine-terminated silanes and carboxyl-terminated thiols, respectively. This process led to high-contrast chemical patterns and was efficient for the direct selective immobilization of Au NPs on the nanoscale patterns of functionalized SiO2 regions. The layer of Au in the background regions, however, may interfere with the properties of metallic particles. In our study, the cross-linked PS mat allowed for the specific functionalization of the SiO2 regions with P2VP−OH, providing high-contrast chemical patterns for the direct selective immobilization of Au NPs. Additionally, the use of polymer brushes for the immobilization of Au NPs onto nanoscale chemical patterns contrasts with past studies8−10,18,27 where the binding of particles is provided by self-assembled monolayers. The interaction between brushes and particles can be tailored on the basis of the length, architecture, and density of polymer chains and thus offers routes to the organization of NPs on surfaces.28−30 The precise control of the number of 13 nm Au NPs per feature down to the single-particle level points to the ability to create high-resolution chemical patterns with the presented process. The surface area of the chemical binding site is directly related to the number of immobilized Au NPs. Recently, Manandhar et al.10 systematically investigated the relationship between the number of Au NPs and the surface area of the chemical binding site for particle sizes of 40, 80, and 150 nm. The number of particles per feature increased linearly with the area of the patterned feature for all three sizes. The slope of the lines indicated the number of particles per unit area and increased with decreasing size of the particles. The similar behavior (Figure 2g) for 13 nm Au NPs suggests that the linear dependence of the number of particles on the surface area of the binding site is valid for particles smaller than 40 nm. On the other hand, the direct selective immobilization of 13 nm Au NPs on the chemical patterns into single-particle arrays shows that the process described in this study enables the generation of high chemical contrast patterns with a resolution of ∼20 nm. Arrays of single Au NPs that are chemically bound to substrates have been previously reported; however, either the size of the particles was about 40 nm9,10 or the effective diameter of the

Figure 2. Direct selective immobilization of Au NPs onto isolated chemical patterns consisting of P2VP brushes in a background of a cross-linked PS mat. SEM images of the substrates following the immobilization of Au NPs are given in a−f with decreasing size of the patterns. Scale bars for SEM images: (a−c) 200 nm and (d−f) 100 nm. (g) Average number of particles per P2VP-functionalized patterns of various sizes. Each point is an average derived from 20 different spots. The red line is a best, linear fit to the data. Error bars represent the standard deviation in the average number of particles per spot.

show that particles can be immobilized at precise positions onto isolated features with low levels of nonspecific adsorption to background regions. The cross-linked PS mat provided high resistance to the interpenetration19,20 of P2VP−OH, leading to almost no particles in the background. On the other hand, fully grafted P2VP brushes in the patterned regions resulted in a high density of bound NPs. The size and geometry of the chemical patterns could be controlled within the limits of electron beam lithography. Decreasing the size of patterns from hundreds to tens of nanometers led to the assembly of NPs in clusters down to individual particles. The number of particles per isolated P2VP-functionalized region showed almost a linear dependence on the area of the patterns (Figure 2g). 7302

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Figure 3. Direct selective immobilization of Au NPs onto lithographically prepared, dense, linear chemical patterns consisting of P2VP brushes in a background of a cross-linked PS mat. Shown are SEM images of the Au NPs immobilized onto chemical patterns of increasing density: the pitch of the pattern is given at the top of each image. The scale bar is 200 nm.

Figure 4. Effect of the thickness of PS-b-PMMA on the generation of chemical patterns using the BCP-assisted lithographic approach for the direct selective immobilization of Au NPs. (a) SEM image of the substrate following the directed assembly of a 55 nm film of PS-b-PMMA (85-b-91) on the chemical prepatterns. SEM images of the substrates following the selective removal of the PMMA block and the underlying cross-linked mat, functionalization with P2VP brushes, and immobilization of Au NPs for BCP film thickness of (b) 35 nm and (c) 55 nm. Please note that the SEM image in part c was intentionally chosen to demonstrate the consequences of the defects in the directed assembly of BCPs on the generation of chemical patterns and NP arrays with the current approach. The defect-free directed assembly of a 55 nm film of PS-b-PMMA (85-b-91) on chemical prepatterns was routinely obtained as shown in part a. The scale bar is 200 nm.

dose determines the line width of the patterned resist features. The exposure dose as a function of the position across a grating pattern resembles a sinusoidal rather than a step function, and the width of the lines cannot be controlled precisely as a result of the increased interaction between neighboring lines. We investigated a BCP-assisted lithographic approach to overcoming the difficulties in controlling the dimensions of patterns with increasing density of the features. As illustrated above on isolated chemical patterns (Figure 2), pattern dimensions determined the number of particles immobilized per feature, and reducing the dimensions to ∼20 nm led to arrays of single 13 nm particles. Here we explore the preparation of dense chemical patterns for the direct selective immobilization of NPs into linear single-particle-wide arrays. The generation of dense single NP (13-nm-diameter) arrays requires the chemical patterning of surfaces at a resolution (∼20 nm) that is as high as that achieved with the isolated features. The pure lithographic approach (Figure 3) did not allow such high resolution because of difficulties in patterning resist features at high densities. A BCP film is integrated into the process in order to decouple the patterning of resist features and the generation of chemical patterns (Figure 1, right column). The resist layer was first patterned on the cross-linked PS mat with EUV-IL and then developed. Slight oxygen plasma etching of the substrates changed the wetting behavior of the cross-linked PS mat under the patterned regions and resulted in chemical prepatterns following the removal of the resist.23 A film of lamellae-forming PS-b-PMMA (85−91 kg/mol, bulk

particles were increased using electrostatic interactions for the 20 nm8 and 32 nm31 particles. Therefore, the size of the binding sites was not as small as the chemical patterns presented in our study. Controlling the number of NPs per patterned region becomes challenging as the density of features increases and approaches the limits of the lithographic processes. To investigate the immobilization of NPs onto dense arrays, periodic linear chemical patterns were prepared as described above except that extreme ultraviolet interference lithography (EUV-IL)32,33 was used instead of electron beam lithography to pattern resist features. A set of patterns with pitches of 80−50 nm with 10 nm intervals was prepared on the same substrate and functionalized with P2VP brushes. The immobilization of Au NPs onto these dense chemical patterns revealed that with the increasing density of patterns, the number of NPs per line width had a broader distribution and the registration between particles become poorer. For example, even at a pitch of 80 nm the number of NPs per line width varied from one to three and a linear array of single Au NPs obtained at a pitch of 50 nm contained discontinuities and misalignments. These defects are likely due to issues in patterning resist features at high densities (∼50 times more NPs per area in Figure 3 than in Figure 2) because the same process led to the defect-free assembly of Au NPs on the isolated features. It is well known that the quality of resist patterns prepared by lithographic processes degrades with an increasing density of features as the diffraction limit is approached.25,26 In the case of EUV-IL system, the exposure 7303

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morphological period, L0 = ∼80 nm) was directed to selfassemble on the chemical prepatterns (period Ls = 80 nm). PS and PMMA blocks wetted the resist-protected and oxygenplasma-treated mat regions, respectively. A second oxygen plasma etching was performed on the assembled BCP film to selectively remove the PMMA block and underlying crosslinked PS mat to reveal SiO2. The remaining PS block of the copolymer was then removed by repeated washing in toluene, leading to linear patterns of SiO2 in a background of a crosslinked PS mat. The patterns were then functionalized with P2VP−OH and treated with Au NPs as described above for the isolated chemical patterns. The contrast of the chemical patterns generated with the BCP-assisted lithographic approach depended on the thickness of the PS-b-PMMA film on top of the cross-linked PS mat. The removal of the cross-linked mat under the patterned regions by oxygen plasma etching to reveal SiO2 constituted some differences in the two types of approaches. In the case of the lithographic approach, ∼100 nm of a resist served as a protecting layer for the etching of 12 nm of a cross-linked mat. On the other hand, ∼50 nm of the PS block provided the protection of the background regions to remove ∼50 nm of the PMMA block and the underlying ∼7 nm cross-linked mat. Because the oxygen plasma etched the PS block at the half rate of the PMMA block, a certain thickness of PS block was needed in order to protect the background regions from any unwanted etching effects. To investigate the effect of the thickness of BCP on the contrast of the chemical patterns, 35- and 55-nm-thick films of PS-b-PMMA were directed to assemble on chemical prepatterns, and then the PMMA block was selectively removed with oxygen plasma, followed by functionalization with P2VP− OH and the immobilization of Au NPs. As seen in Figure 4b, a 35-nm-thick PS-b-PMMA film resulted in the nonspecific binding of Au NPs to the cross-linked mat regions under the PS block. For the samples with a 55 nm film of PS-b-PMMA, Au NPs were bound to the linear patterns with high selectivity (Figure 4c). These results showed that, in the case of a thin film, a protective layer was not fully able to protect the underlying cross-linked mat against unwanted etching effects. However, increasing the thickness of the BCP film can lead to increased defect densities in the directed assembly of BCPs (inset Figure 4c). These defects result in irregularities in the NP arrays as defect sites are also etched into the substrate and functionalized with P2VP−OH. Therefore, the thickness of the BCP film should be chosen by considering both the etching and assembly processes. High-contrast defect-free chemical patterns (Figure 5) can be obtained within the thicknesses readily achievable with the PS-b-PMMA system. The dense chemical patterns generated with the BCPassisted lithographic approach enabled the direct selective immobilization of 13 nm Au NPs into linear arrays of single particles. The selective removal of the PMMA block and the underlying cross-linked mat with oxygen plasma etching was the key step in the process used to control the line width of the chemical patterns. We used varied etching times at two different plasma powers of 50 and 150 W at a pressure of 10 mTorr and an oxygen flow rate of 10 sccm. At a plasma power of 150 W, the etching rates of PMMA and PS were about 2 times larger in comparison to those at 50 W. (See the Experimental Section for the etching rate.) The amount of etching was chosen according to the overetching (OE) percentage defined by eq 1 that is based on the etching rate of PMMA on homogeneous substrates and the thickness of the

Figure 5. Site-specific placement of Au NPs on the dense chemical patterns prepared by the BCP-assisted lithographic approach. SEM images of Au NP immobilized on P2VP-functionalized patterns following the selective removal of the PMMA block and the underlying cross-linked mat with oxygen plasma etching for plasma powers of (a) 50 and (b) 150 W. The thickness of the PS-b-PMMA (85-b-91) film used to generate chemical patterns was 55 nm. The scale bar is 100 nm.

PS-b-PMMA film and cross-linked mat. As an example, 20% OE for a total thickness of the BCP film and mat of 100 nm means that the etching is sufficient to remove 120 nm of PMMA on a homogeneous substrate. The thickness of the cross-linked PS mat was doubled in the equation to consider the 2-fold slower etching of PS with respect to PMMA. At a low plasma power of 50 W, the density of Au NPs immobilized on the chemical patterns was very low (results not shown) for an OE of 10− 30%, suggesting the inability to generate SiO2 regions for functionalization with P2VP brushes. Au NPs were specifically attached to the patterned regions at a high density with an OE of ∼50% as shown in Figure 5a. However, a high plasma power of 150 W led to arrays of Au NPs for an OE range of 10−50% (Figure 5b). Besides the OE%, the key difference between the plasma powers of 50 and 150 W is the line width of the chemical patterns. The low plasma power resulted in singleparticle-wide linear arrays of Au NPs, whereas the lines were two to three particles wide in the case of high plasma power. The arrays of Au NPs were highly registered in both cases. ⎛ etching time (min)×etch rate of PMMA (nm/min) OE% = ⎜ ⎝ total thickness of the BCP film and mat (nm) ⎞ − 1⎟100% ⎠ (1)

Improved control of the number of NPs per line width and registration of particle placement using the BCP-assisted process are suggested to be the results of molecular selfassembly based on free-energy minimization. SEM images given 7304

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introducing programmed defects into the process. Resist patterns were designed to have gaps of ∼50 nm in alternating lines as shown in Figure 6a. Au NPs selectively immobilized

in Figure 5 demonstrated that the incorporation of BCPs into the process led to the immobilization of NPs on the dense chemical patterns with a high level of registration and control of the number of particles per line width. Single-particle linear arrays of Au NPs (Figure 5a), for example, showed much better registration than its pure lithographic counterpart (Figure 3). These results are in good agreement with previous studies23,34 on the self-healing capabilities of BCPs assembled on lithographically defined chemical prepatterns. The directed self-assembly of BCPs was shown to form defect-free assemblies with the same domain width on chemical prepatterns with a line-width variation of up to 15%. In this context, BCPs increase the tolerances and margins of the lithographic processes.35 Moreover, it has recently been demonstrated through experiment24,25 and molecular simulations36 that BCPs directed to self-assemble on chemical prepatterns improved the line-edge roughness of the resist patterns. The control of the line width of chemical patterns with the power of oxygen plasma etching suggests that the removal of the PMMA block and underlying cross-linked PS mat depended on the density and scattering of ions generated during etching. The oxygen plasma etching used in this study can be classified as ion-enhanced energy-driven etching.37 The basis of this type of etching is the reaction of oxygen molecules with the organic polymer layer and the removal of the volatile reaction product. The key point is the bombardment of substrates with the ions in one direction providing anisotropic material removal. The oxygen flow and pressure are kept at low values to enhance the ion effects and thereby the anisotropy in the etching. It is known from previous studies that the density and energy of ions are important to selective and anisotropic etching.38,39 In the reactive ion etching system used for the experiments, the plasma power controls the density and energy of oxygen ions simultaneously. Therefore, both the density and energy of the ions were higher at a plasma power of 150 W compared to those at 50 W. The density and energy of ions relate to the chance of reaction with the surface of the film and the scattering of the ions, respectively.38 The significant difference in the line width of the chemical patterns (Figure 5) between the two plasma powers used in the experiments is likely due to the energy of the ions supported by the dissimilarity in the amount of OE needed (≥50% for 50 W and ≥10% for 150 W) for the generation of chemical patterns. The high energy of the ions was effective in the selective material removal from dense, nanoscale chemical patterns provided by the small amount of scattering, whereas the low energy of the ions resulted in slower etching rates on the nanopatterns in comparison to those on homogeneous substrates. The low plasma power, however, enabled the generation of narrow chemical patterns and linear arrays of single NPs because the ions could reach the bottom substrate only over limited regions of the PMMA block as a result of the scattering. The high registry between the individual NPs further suggests that particles were immobilized into locations corresponding to central regions of the PMMA block. Because the PS block was etched at an even smaller rate than was PMMA, it is likely that SiO2 regions were revealed at the bottom of narrow trenches concentrating at the center of the PMMA block. The ability of BCP self-assembly to improve the direct selective immobilization of Au NPs on the lithographically defined dense chemical patterns is further demonstrated by

Figure 6. Pattern rectification using BCPs. SEM images of (a) resist patterns designed to have a gap of 50 nm in alternating lines (black regions correspond to an electron-beam-exposed and -developed resist) and (b) a self-assembled PS-b-PMMA film on the defectcontaining chemical prepatterns prepared by brief oxygen plasma etching of the patterns shown in part a. SEM images of the immobilized Au NPs on the substrates functionalized with P2VP−OH following (c) etching of the defect-containing resist patterns shown in part a, and (d) etching of the BCP film shown in part b to remove the PMMA block and underlying cross-linked mat. The scale bar is 100 nm.

into arrays of particles in a dashed-line geometry with two to three particles per line width in the case of directly processing the resist patterns by oxygen plasma etching followed by functionalization of the patterns with P2VP−OH (Figure 6c). On the other hand, using the BCP-assisted lithographic approach on the defect-containing resist patterns led to continuous linear arrays of single Au NPs (Figure 6d). BCP healed the gaps and assembled into continuous arrays on the chemical prepatterns prepared by brief oxygen plasma etching of the defect-containing resist patterns. The ability of BCP selfassembly to heal defects of similar types has been shown previously.25 These results show that this ability can be used to generate chemical patterns for the selective immobilization of NPs. The selective immobilization of Au NPs onto isolated and dense chemical nanopatterns with a high level of control over the number of particles and registration may enable scientific studies and applications. The reproducible and simultaneous investigation of individual and collective properties of metallic NPs at the single-particle level is highly desirable because more accurate and direct information can be derived in comparision to methods that measure ensemble-averaged values.40 The precise control of the location and number of particles on isolated chemical patterns can be directly applied to a wide 7305

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variety of ex-situ-synthesized metallic NPs,1 therefore presenting a platform for scientific studies. The dense chemical patterns may be particularly useful in investigating the coupling9 between the individual patterns of the Au NP arrays. Another significant aspect of the selective immobilization of NPs on the high-density chemical patterns is in the use of such arrays in various applications. The density of arrays will directly affect the output/signal of the devices by reducing the total area of fabrication or improving the performance.

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CONCLUSIONS In this study, we demonstrated two approaches to the direct selective immobilization of Au NPs onto nanoscale isolated and dense chemical patterns. The strength of both approaches is the ability to generate high-contrast, high-resolution chemical patterns for the selective immobilization of Au NPs. High chemical contrast was provided by the high resistance of crosslinked PS mats to the interpenetration of P2VP−OH, allowing functionalizing patterned regions of SiO2 with the brush without changing the composition of the mat. The top-down approach using electron beam lithography enabled the generation of isolated chemical patterns with high resolution (∼20 nm) to immobilize Au NPs (13 nm diameter) selectively with precise control of the number of particles down to one. The dense (80 nm pitch) chemical patterns prepared by the BCP-assisted lithographic approach resulted in the immobilization of Au NPs into single-particle-wide linear arrays. The chemical patterning approach described in this study presents a model system for future studies, the interaction between the substrate and NPs as well as the shape, structure and size of the particles can be varied for specific scientific studies and technological applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF through the University of Wisconsin Nanoscale Science and Engineering Center (NSEC) (grant no. DMR-0832760). This work was based in part upon research conducted at the Synchrotron Radiation Center, University of WisconsinMadison, which is supported by the NSF under award DMR-0537588. We acknowledge funding from the Air Force Research Laboratory (AFRL) Materials and Manufacturing Directorate.

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REFERENCES

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