Dewetting of Patterned Silicon Substrates Leading to a Selective

Article pubs.acs.org/JPCC

Dewetting of Patterned Silicon Substrates Leading to a Selective Deposition of Micellar-Based Nanoparticles A. Riskin,*,† C. De Dobbelaere,† L. Shan,‡ H. G. Boyen,‡ J. D’Haen,‡,§ A. Hardy,† and M. K. van Bael*,†,‡ †

Inorganic and Physical Chemistry, Institute for Materials Research, Hasselt University, Agoralaan Building D, 3590 Diepenbeek, Belgium ‡ Imec vzw Division IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium § Electrical and Physical Characterization, Institute for Materials Research, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium S Supporting Information *

ABSTRACT: We have applied soft lithography for the indirect patterning of micellar poly(styrene-b-2-vinyl pyridine) diblock copolymers loaded with gold chloric acid with a pattern width below a micrometer. A combination of physical and chemical heterogeneities on the substrate induced a selective deposition of the micelles in between the relief structures without the need for additional liftoff or annealing steps. Micelle size, dip coating speed, and height of the relief pattern were identified as important parameters to achieve a successful selective deposition. Finally, a single layer of patterned gold nanoparticles was formed inside the micropattern using an oxygen plasma treatment.

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INTRODUCTION The fabrication of metallic nanoparticles is of great interest because many of their properties (such as electrical and thermal conductivity, chemical reactivity, color, and other optical properties) are size dependent and can be tailored toward specific applications like chemical reaction catalysis,1−3 biosensing,4,5 carbon nanotube synthesis,6−10 and silicon nanowire growth.11,12 For advanced applications, like plasmonic devices13−16 or trace chemical detectors based on surfaceenhanced Raman scattering,17,18 the nanoparticles must be organized into specific patterns. Poly(styrene-b-2-vinyl pyridine) (PS-P2VP) diblock copolymers are known to form spherical micelles in an apolar solvent such as toluene. The poly-2-vinyl pyridine (P2VP) block, which is more hydrophilic due to the presence of the pyridine unit, will be directed to the inner core of the spherical micelle. The corona of the sphere is then formed by the more hydrophobic polystyrene (PS) strands protruding from the core into the toluene. Metal salts can be complexated by the pyridine units present in the core. When dip coated or spin coated on an unpatterned substrate, the micelles self-assemble in a continuous array which is readily transformed into an array of metal(oxide) nanoparticles by breaking down the polymer structure of the micelles with an oxygen plasma. Changing the lengths of the PS block and the P2VP block changes the final nanoparticle interdistance and size, respectively. This makes block copolymer micelles a very flexible template for the synthesis and ordered deposition of monodisperse metallic nanoparticles.19−23 © 2012 American Chemical Society

Efforts to organize micellar structures in prepatterned templates have already been undertaken. Cheng et al.24−26 arranged empty polystyrene-block-polyferrocenyldimethylsilane (PS-b-PFS) copolymer micelles between a square wave grating with a width below the micrometer range. A micellar solution was spin coated on top of the pattern, after which conformal coverage was achieved. Annealing at 140 °C for several hours melted the polystyrene and allowed the micelles to flow into the valleys and form vertical strips of hexagonally ordered PFS spheres. Glass et al. have used top-down photo- and e-beamlithography to pattern micellar-based nanoparticles with a resolution of 50 nm.27−29 Although this technique allows for a very high resolution, it is expensive and not applicable as a highthroughput technique to pattern larger surfaces. Soft lithography was developed by Whitesides et al.30 as a convenient, low-cost patterning technique in which an elastomeric stamp is used to generate structures with feature sizes from 100 μm to 30 nm. Cong et al.31 combined PS-P2VP micelles with microcontact printing to produce 2−4 μm wide disk- and dish-like arrays or 500 nm concentrated silver nanoparticle aggregates over a large surface. Although the macroscopic placement of the disks and aggregates was regular, the order and monodisperse size distribution of the particles inside the disks were lost due to aggregation of the micelles during drying. Yun et al.32 successfully spin coated a micellar solution on a patterned polydimethylsiloxane (PDMS) stamp Received: December 7, 2011 Revised: April 4, 2012 Published: April 13, 2012 10743

dx.doi.org/10.1021/jp211775q | J. Phys. Chem. C 2012, 116, 10743−10752

The Journal of Physical Chemistry C

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Scheme 1. Preparation of the Patterned Substrate Using Soft Lithography and Micellar Deposition

flushed glovebox with a relative humidity less than 10% to avoid moisture. After a second week of stirring, the micellar solutions were subsequently filtered through 1.0 μm (Glass Fiber GF100/25) and 0.2 μm (PTFE O-20/25) filters from Macherey Nagel to remove any possible polymer aggregates. Substrate Patterning. First, a patterned PDMS stamp was prepared using the polycarbonate base of a blank recordable compact disk (CD-R) as a master.33 20 g of vinyl-terminated dimethylsiloxane base was mixed with 2 g of curing agent from a Sylgard 184 Silicone Elastomer kit purchased from Dow Corning. The mixture was degassed in a desiccator under vacuum to remove air bubbles that could influence the quality of pattern reproduction. After CD disassembly, the degassed mixture was poured over the polycarbonate disk and cured for 1 h at 90 °C. The hardened PDMS stamp was then peeled off and was ready for use. In the second step, silicon wafer pieces measuring 2 × 2 cm (n-type, native oxide layer of 1.2 nm) were cleaned using a 20 min dip in a sulfuric acid peroxide mixture (H2SO4, pro analysis, 95−97% from Merck mixed with H2O2, pro analysis, 35 wt %, from Acros Organics in a 4:1 volume ratio), followed by a 20 min dip in an ammonia peroxide mixture (milli-Q water, H2O2, and NH3, extra pure, 32% from Merck, mixed in a 5:1:1 ratio). After rinsing with milli-Q water (Millipore, 18.2 MΩ·cm resistivity at 25 °C), the substrates were dried with a nitrogen gun. Mr-I 7010E, a commercial thermoplast from micro resist technology GmbH (Berlin, Germany), was spin coated onto the dried substrates at 3000 rpm for 30 s (acceleration 1000 rpm/s) using a Polos MCD200-NPP spin coater. Subsequently, the coated substrates were treated at 140 °C on a hot plate for 2 min to allow the solvent to evaporate and the thermoplast layer to set. For the actual imprint step, the thermoplast layer was reheated to 140 °C (above the reported glass transition temperature of 60 °C), and the patterned PDMS stamp was pressed into the layer with a pressure of 4.3 N/cm2. After 5 min, the heating was turned off. When the assembly temperature dropped below 60 °C, the pressure was released and the stamp removed. Finally, the scum

for microcontact printing and deposited 50 × 100 μm large rectangles as well as 2 μm wide lines of iron oxide nanoparticles. On the edges and on the ridges of the stamp, however, aggregates formed due to an uneven micellar coating on the stamp edge. Although direct microcontact printing is an easy and accessible method for the large-scale patterning of micelles, edge agglomeration will become a dominant problem when the pattern width continues to decrease. In this paper, we propose a new method for the patterning of metal salt loaded micelles without edge agglomeration. PS-b-P2VP micelles are loaded with a gold metal salt and combined with an easily accessible soft-lithographical pattern with a feature size below 1 μm. First, a commercially available thermoplast is spin coated on a substrate and imprinted using a PDMS stamp with the relief structure of a blank CD-R. Chemical and physical heterogeneities on the substrate control the dewetting process after dip coating of the micelles and lead to the formation of micropatterned, ordered micelles which are transformed into metallic nanoparticles after plasma treatment. Careful control of the dewetting simplifies the deposition process because it eliminates additional liftoff or annealing steps. SEM images confirm the absence of aggregation near the edges when compared to the direct microcontact printing techniques.

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EXPERIMENTAL SECTION Micellar Solution Synthesis. A poly(styrene-b-2-vinyl pyridine) (P3999-S2VP, Polymer Source Inc., polydispersity = 1.09) diblock copolymer was dissolved in toluene (Riedel De Haën, 99.5%) at a concentration of 5 mg/mL. The number average molecular weight of the polymer blocks was 8200 and 8300 g/mol for the PS and P2VP block, respectively. After one week of stirring, the micelles were loaded with gold chloric acid (HAuCl4·xH2O, Sigma Aldrich, 99.999% metals basis, FW 339.79) with a loading factor of 0.5, so that theoretically half of the nitrogen atoms in the pyridine units are protonated and bound to gold chloride. This loading took place in a nitrogen10744



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layer between the patterns was removed by applying 20 min of UV-ozone treatment under ambient air (UV-ozone, PSD pro series, Novascan, Ames, USA). This treatment also lowers the height of the actual pattern. Dip Coating of the Micellar Solutions. Using a DC Mono 160 dip coater from Nima Technologies, the samples were first immersed into the micellar solution at a speed of 10 mm/min and kept in the solution for 30 s to allow the micelles to diffuse and ensure full surface coverage. The sample was then withdrawn at 10 mm/min and allowed to slowly dry while hanging in a saturated toluene atmosphere above the micellar solution for 5 min. Spin Coating of the Micellar Solutions. A few drops of micellar solution were deposited until the patterned samples were completely covered (visually). The samples were then spinned for 30 s at a speed of 3000 rpm and a 1000 rpm/s acceleration on a Polos MCD200-NPP spin coater. Plasma Etching. Oxygen plasma etching (0.106 mbar pressure, 150 W power) of the deposited micelles was performed on a physical vapor deposition system. During the first five minutes of treatment, the temperature was gradually increased from room temperature up to 50 °C. In the next three minutes, the temperature was increased to 300 °C and kept at that level for another 32 min, yielding a total plasma treatment time of 40 min. Plasma and heating were shut down at the same time, and the samples were allowed to cool to room temperature before venting and opening the chamber. The entire procedure from stamp preparation to plasma etch is shown in Scheme 1 for additional clarity. Physical Characterization. All secondary electron microscopy images were taken on a Quanta 200 FEG microscope from FEI operating at 15 kV using an Everhardt-Thornley detector. AFM images were acquired on a Veeco Dimension microscope equipped with a Digital Instruments Nanoscope III controller. Scans were performed in tapping mode at a scan speed of 1 Hz using etched Si cantilevers (5−10 nm radius) from NANOSENSORS with a 320 kHz resonance frequency and a 42 N/m force constant. Contact angle measurements were performed on an OCA15 series from Dataphysics.

Figure 1. (a) AFM height image of the PDMS stamp. 1 and 2 are cross-section profiles shown in b and c, respectively. (b) Profile 1 of the PDMS stamp: half-height width of the relief pattern. (c) Profile 2 of the PDMS stamp: interline distance.

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RESULTS AND DISCUSSION A typical CD-R structure consists of a polycarbonate base on which a recording layer (an organic dye), a reflective layer, a protective layer, and finally a label are deposited. Pregrooves are etched into the polycarbonate surface to guide the laser during recording and reading and have an average depth of 100 nm, a half-height width of 500 nm, and a 1000 nm space in between them.34 The negative replica of these pregrooves form the relief structures on the PDMS stamp after curing. AFM images of the patterned PDMS stamp reveal a 110 nm pattern height, a 550 nm half-height width, and 920 nm between the lines at halfheight (Figures 1a, 1b, and 1c, respectively). Spin coating of Mr-I 7010E on the 2 × 2 cm cleaned silicon substrates results in a 100 nm thick homogeneous thermoplast layer after thermal treatment. SEM images taken after embossing with the patterned PDMS stamp show a pattern consisting of 171 ± 10 nm high mesas with a residual scum layer of 22 ± 3 nm (Figure 2). The PDMS stamp only has a 110 nm deep relief structure; however, the final difference between the relief and the scum layer is 150 nm, which is higher. This difference can be attributed to the elasticity of the PDMS, allowing the expansion of the relief structure and thus additional storage of thermoplast evacuated during imprint-

Figure 2. Cross-section SEM image of the thermoplast layer after imprinting with the patterned PDMS stamp.

ing.30 This in turn leads to a thinner residual scum layer between the features, which is advantageous for the next 10745



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for a good deposition behavior of the micelles. Another effect of the UV-ozone is the decrease of the pattern width and the increase of the connected interpattern distance. After 20 min of UV-ozone treatment, the pattern width is now 805 ± 10 nm, and the interpattern distance has increased to 675 ± 14 nm. Therefore, scum layer removal is a trade-off between substrate cleanliness and accuracy of pattern reproduction as well as residual pattern height. A thinner scum layer after embossing therefore requires a shorter UV-ozone treatment time and will lead to a better end result. When these Mr-I 7010E thermoplast patterned substrates are dip coated at 10 mm/min from the gold-loaded micellar PS-bP2VP solutions with the line pattern parallel to the dip coating direction, SEM and AFM images confirm that the micelles (core size 22 ± 2 nm) are only deposited in the valleys between the relief structures (Figures 4 and 5, respectively). The width

treatment step. The line width of the pattern is measured to be 918 ± 8 nm, and the interline distance (measured from edge to edge of the pattern) is 554 ± 21 nm, which is the negative image of the PDMS stamp described in the first paragraph. Contact angle measurements with toluene showed complete wetting of the bare substrate but a wetting angle of 8−11° on a thermoplast covered substrate. Since this difference in angle is thought to be responsible for the dewetting behavior after dip coating and hence the selective micellar deposition guided by the pattern, removal of the scum layer is essential to allow direct contact between the micellar solution and the silicon substrate during deposition. Oxygen plasma and reactive ion etching (RIE) are common procedures to remove a polymerbased scum layer.35,36 However, because of its low equipment cost and ease of use, we opted for a UV-ozone treatment, which was established as an efficient removal technique for organic contamination.37,38 From time-based etching experiments (included as Supporting Information) and SEM images like Figure 3, it was deduced that a 20 min UV-ozone treatment is

Figure 4. SEM image of gold-loaded micelles (core size 22 ± 2 nm) dip coated onto a soft-lithographical thermoplast patterned substrate. Contrast of the image was increased for additional clarity. Figure 3. Cross-section SEM image of the thermoplast layer after 20 min UV-ozone treatment.

of the micellar deposition is about 850 nm, which is wider than the interline distance of the resist (675 nm), indicating the micelles are also deposited on part of the relief structure slope. Micellar deposition on these slopes is disordered but not aggregated, while the micelles in direct contact with the silicon substrate adopt a close packing, as evidenced by the SEM image

sufficient to remove the scum layer. This conclusion is based on two observations: first, the visual disappearance of the scum layer simultaneously with the appearance of the smooth substrate and second, the difference in profile shape between the relief structure and the scum layer and the relief structure and the substrate, respectively. This difference can be seen clearly when comparing Figures 2 and 3. Before UV-ozone treatment, the thermoplast layer is continuous, and the transition from the actual pattern to the residual scum layer is sigmoidal (Figure 2). After UV-ozone treatment, the pattern has a parabolic shape (Figure 3). During etching, the pattern height decreases from 171 ± 10 nm to 70 ± 4 nm. This indicates a different UV-ozone removal rate between the thermoplast which forms the pattern and the thermoplast in the scum layer (5 nm/min for the pattern, 1 nm/min for the scum layer). A possible hypothesis is that this difference in etching rate is due to the difference in density between both layers. During compression by the PDMS stamp, the thermoplast present under the stamp protrusions is not only forced into the neighboring cavity but also probably compressed, leading to a higher local density and hence a slower removal rate. Results in this paper will show that the final height of the pattern is crucial

Figure 5. AFM height image of gold-loaded micelles (core size 22 ± 2 nm) dip coated onto a soft-lithographical thermoplast patterned substrate. 10746



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in Figure 4. If the Mr-I 7010E pattern was a perfect copy of the CD-R master, the theoretical deposition width would be 550 nm. Because of the necessity of scum layer removal and the partial deposition of micelles on the slopes, final deposition will always be wider. Whitesides et al.30 report a minimal feature size of 30 nm for soft lithography. With the use of a dedicated, custom-made master, the width of the deposition could thus be further reduced. Micellar deposition densities were calculated by counting the number of particles on a known surface area in SEM images and extrapolating this number to the amount of particles per square centimeter. For patterned substrates, this deposition density has increased to 1.53 × 1011 micelles/cm2 compared to 1011 micelles/cm2 for an unpatterned substrate, which might be ascribed to a migration of micelles from the top of the pattern to the valleys after the initial deposition by the dip coating process. On a blank, unpatterned silicon substrate, a deposited toluene layer (with the micelles dispersed in it) only thins due to solvent evaporation after dip coating. Because evaporation is the only active process, this layer thins homogeneously, and when the layer finally becomes so thin that it destabilizes and starts to dewet the surface, the micelles have already settled. In the case of patterned substrates, however, we hypothesize that another dewetting process, i.e., convective dewetting,39 takes place at the same time as the evaporation and that this convection is responsible for the observed selective deposition. During convective dewetting, the substrate dewets not only because of the thinning of the liquid film due to evaporation but also because of liquid movement and film breakup induced by chemical or physical substrate heterogeneities. Chemical substrate heterogeneity introduces a potential gradient which will induce a liquid flow from less wettable areas to more wettable areas.40 In the case of deposition on the patterned substrate, toluene will flow from the top of the relief structure into the valleys, as indicated by the higher measured contact angle between the toluene and the relief structure compared to the value between the toluene and the bare substrate. Furthermore, the physical heterogeneity of the substrate induced by the relief pattern leads to the destabilization of the film at the elevated areas where the local thickness will fall below the critical thickness for a stable film more rapidly.40 Also, a film draining because of convective processes exhibits a circular rim or bead at the drying hole edge that collects all the dewetted liquid41 which enhances the ability to carry the micelles away and prevent their deposition. This process is visualized in Scheme 2. Thiele et al.39 discuss the behavior of a dewetting liquid film containing nanoparticles when dewetting and evaporation take place at the same time. Their analysis shows that the evaporation rate plays a critical role in the final particle deposition result. For low evaporation rates, the film dynamics are dominated by convective dewetting, and the micelles will be collected in the valleys before evaporation removes the solvent. When the evaporation rate is high, the influence of convection on the micellar distribution will be negligible. Therefore, the micelles will be deposited in a homogeneous monolayer, ignoring the pattern completely. This concept is illustrated when the micellar solution is spin coated onto the pattern instead of dip coated. Figure 6 shows a conformal deposition after spin coating with the pattern still visible in the background. Spin coating is considered to consist of four stages:42 deposition, spin-up, spin-off, and evaporation. In the

Scheme 2. Schematic of the Drying Process after Micellar Depositiona

a

(a) Immediately after dip coating, the deposited toluene layer starts to thin on the pattern due to the potential gradient induced by the difference in wetting angle. Micelles in solution are already being entrained at this stage. (b) Because of the selective thinning, the toluene layer becomes unstable and breaks at the highest point of the pattern. (c) Once dewetting has started, the liquid film is drained quickly from the pattern into the valleys. Formation of a circular rim aids in entraining the micelles. (d) Deposition after complete drying of the sample.

Figure 6. SEM image of gold-loaded micelles (core size 22 ± 2 nm) spin coated onto a soft-lithographical patterned substrate. Contrast of the image was increased for additional clarity.

final stage of spin coating, the evaporation rate dominates film thinning and is proportional to the square root of the spin speed.43 Because the evaporation rate is much higher than for dip coating due to the airflow across the substrate surface, the solution will dry, and micelles will be deposited before selective convective dewetting has had a chance to take place. This result confirms that maintaining a low evaporation rate is essential to obtain selective deposition in these systems. In conclusion, the fact that chemical and physical heterogeneities coincide on our substrate in combination with a relatively slow evaporation rate of toluene leads to very efficient film destabilization, liquid drain, micellar entrainment, and finally deposition in the valleys between the relief structures. To investigate the effect of the micelle size, micelles with a larger diameter were deposited. Hereto, a block copolymer with a PS and P2VP part of 185 000 and 73 000 g/mol, respectively, was dissolved into toluene and loaded with HAuCl4. This 10747



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polymer forms micellar cores of 32 ± 4 nm (measured from SEM images). When depositing these micellar solutions on MrI 7010E patterned substrates, it was observed that micelles no longer exclusively deposit in the valleys of the pattern. Instead, a small fraction of the micelles remains on the mesas, most often near the middle of the pattern (Figure 7). Because the

Figure 8. Gold-loaded micellar solutions deposited on patterned substrates after 60 min UV-ozone treatment. Average micellar core size is 22 ± 2 nm. Contrast of the image was increased for additional clarity.

Figure 7. SEM image of gold-loaded micelles (core size 32 ± 4 nm) dip coated onto a soft-lithographical patterned substrate. Contrast of the image was increased for additional clarity.

micelles are bigger and thus heavier, we believe that they are carried away less easily during the convective dewetting process. As the toluene layer thins on top of the relief structure and breaks up, it will first do so in the middle of the pattern (see also Scheme 2).40 Since the layer has become very thin at this point, a larger micelle will have a higher chance of not being entrained and ending up on top of the pattern. By the time the liquid dewets the side of the pattern, the circular rim will have formed, and the micelles will be pushed into the valleys effectively. In conclusion, micellar size has an important influence on the selectivity of the deposition. Smaller micelles will be moved by the dewetting liquid more easily and will therefore achieve the envisaged deposition more effectively. A second important parameter is the final resist pattern thickness after scum layer removal by UV-ozone etching. By extending the UV-ozone treatment time from 20 to 60 min, the final pattern thickness is reduced from 70 ± 4 to 23 ± 3 nm. Also, pattern width and interline distance are now measured to be 506 ± 10 and 985 ± 7 nm, respectively. Due to this decreased thickness, SEM images (Figures 8 and 9) show a nonselective deposition compared to previous results (Figures 4 and 7). Although micelles are present all over the substrate, the result is not identical when compared to unpatterned substrates since the deposition is not uniform. As shown in Table 1, the micellar density is lower on the mesas but higher in the pattern valleys compared to the density on an unpatterned substrate for both micelle sizes. When comparing these numbers to the patterned substrates after 20 min of UVozone treatment, the densities measured in the valleys are now lower. Calculations confirm that the decrease of micelles on the mesas when compared to the density achieved normally agrees with the increase in density measured in the valleys. Therefore, we conclude that chemical heterogeneity has caused the migration of some of the toluene, entraining the micelles, but

Figure 9. Gold-loaded micellar solutions deposited on patterned substrates after 60 min UV-ozone treatment. Average micellar core size is 32 ± 4 nm. Contrast of the image was increased for additional clarity.

that the toluene layer did not break up fast enough to completely prevent deposition of the micelles on the mesas because of the lower physical heterogeneity of the substrate. Another parameter investigated was the speed at which the samples were withdrawn from the micellar solutions. It is known from the literature that increasing the dip coating speed increases the carry-over layer thickness.44,45 Specifically for micelles, increasing the deposition speed shortens the intermicellar distance of the deposited array46 because the thicker layer carries more micelles which are squeezed onto the substrate during drying. For the patterned substrates, dip coating withdrawal velocities of 2, 5, 10 (standard), 15, and 20 mm/min were applied for both micelle sizes. The results for the small micelles (22 ± 2 nm) are shown in Figure 10. Increasing the withdrawal velocity to 15 and 20 mm/min increases the presence of micelles on the mesas, hence no selective deposition is achieved (Figure 10a). Also, the amount of micelles present on the mesas increases with increasing dip 10748



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Table 1. Measured Micellar Densities for Depositions on Mr-I 7010E micellar density (# micelles/cm2) block copolymer size (PS-b-P2VP) 8200-b-8300 185000-b-73000

unpatterned

patterned, 20′ UV-ozone, valley

patterned, 20′ UV-ozone, mesa

patterned, 60′ UV-ozone, valley

patterned, 60′ UV-ozone, mesa

patterned, 20′ UVozone, plasma etched, valley

patterned, 20′ UVozone, plasma etched, mesa

1.03 × 1011 1.00 × 1010

1.53 × 1011 1.68 × 1010

/ 1.41 × 109

1.20 × 1011 1.13 × 1010

5.87 × 1010 8.11 × 109

5.07 × 1010 1.64 × 1010

/ 1.38 × 109

Figure 10. SEM image of gold-loaded micelles (core size 22 ± 2 nm) deposited onto a soft-lithographical thermoplast patterned substrate with different dip coating withdrawal speeds: (a) 20 mm/min and (b) 5 mm/min.

Figure 11. SEM image of gold-loaded micelles (core size 32 ± 4 nm) deposited onto a soft-lithographical thermoplast patterned substrate with different dip coating withdrawal speeds: (a) 20 mm/min and (b) 2 mm/min.

based micelles. However, lowering the velocity to 5 mm/min does not produce a selective deposition, yet 3.5 × 109 micelles/ cm2 are still present on the mesas. Only in the experiment that the dip coating speed is set at 2 mm/min, selective deposition is achieved in the case of 185 000−90 000-based micelles (Figure 11b). These results indicate that the optimal dip coating speed is a function of the micelle size deposited. Also, in combination with the previous paragraph, it is clear that pattern height and dip coating velocity are interchangeable parameters in this system and can be altered to achieve different deposition results as a function of micelle size. The final step of the process consists of an oxygen plasma treatment to destroy the pattern, the block copolymers around

coating velocity. Measured deposition densities on the mesa are 4.84 × 1010 micelles/cm2 for 15 mm/min and 1.50 × 1011 micelles/cm2 for 20 mm/min, respectively. The thicker toluene layer deposited in these instances allows the micelles to set on top of the mesas before the layer starts to break up. When dip coating is slower than 10 mm/min, selective deposition remains (Figure 10b), except in the case of 2 mm/min, where the amount of particles deposited is insufficient to form a continuous layer in the valleys. Different results are obtained in the case of larger micelles (32 ± 4 nm). Increasing the dip coating velocity to 15 and 20 mm/min also increases the deposition density on the mesas (Figure 11a) to 8.0 × 109 and 2.3 × 1010 micelles/cm2, respectively, as with the 8200−8300 10749



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Figure 12. (a, b) SEM images of patterned gold nanoparticles after plasma treatment. Micellar size: 32 ± 4 nm. Contrast of the images was increased for additional clarity.

Figure 13. (a, b) SEM images of patterned gold nanoparticles after plasma treatment. Micellar size: 22 ± 2 nm.

the gold salt, reduce the gold, and hence turn the filled micelles into solid metallic nanoparticles. During plasma etching, care has to be taken to ensure that the micelles do not have the opportunity to agglomerate and that every micelle results in a single nanoparticle. Temperature control is essential in this respect. If the temperature in the beginning of the process is too high, the polystyrene corona around the micellar core will melt, and the micelles will lose their hexagonal patterning and agglomerate. However, once the polystyrene has been removed, the micelles are fixed, and then a high temperature is necessary to ensure that every micelle generates exactly one particle. A too low temperature in this stage leads to multiple nanoparticles per micelle. The heating program described in the Experimental Section was selected with this in mind. Figures 12a and 12b show that in the case of 185 000−73 000-based micelles the carryover from micellar deposition to final nanoparticle placement is successful. The final nanoparticle size is around 8 ± 1 nm, and the density on the substrate remains identical to the deposition density of the micelles. In the case of 8200−8300-based micelles, however, some of the micelles seem to have agglomerated after plasma etching (Figures 13a and 13b). Since the plasma treatment was

identical for all samples, we suspect the higher micellar density in the case of the smallest micelles causes some of them to agglomerate on the substrate during the etching. The selective placement of the particles however remains successful.

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CONCLUSIONS

A new technique for the patterned deposition of micellar-based nanoparticles has been developed which extends the achieved dimensions for soft-lithography-based patterning techniques below a micrometer. By combining chemical and physical substrate heterogeneities, convective dewetting of a toluene dispersion after dip coating dominates evaporation, leading to an effective and selective deposition of gold salt loaded micelles in the valleys of our pattern. This indirect approach shows no aggregation of the micelles near the edges of the pattern compared to direct microcontact printing and also does not require additional liftoff or annealing steps, which could be detrimental to the ordering in the case of metal salt loaded micelles. Large micelle sizes lead to a decrease in selectivity of the deposition because they are more difficult to move by the dewetting liquid. By altering the speed at which the substrates are withdrawn from the micellar solution, selective deposition 10750



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can be achieved for both micelle sizes investigated. Pattern height after scum layer removal is also found to be critical since chemical heterogeneity alone leads to a decrease in the number of micelles present on the pattern but not to their complete removal. Plasma treatment shows the carryover from micelle to nanoparticle is accurate, except in the case of small micelles where increased micelle density leads to partial agglomeration of the particles. We believe the use of low cost and easily accessible equipment makes this procedure a viable candidate for medium resolution patterning of large surfaces with nanoparticles in a short time scale when compared to other methods like e-beam lithography.

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ASSOCIATED CONTENT

S Supporting Information *

Time-based UV-ozone etching experiments for the determination of the ideal etching time. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; marlies.vanbael@ uhasselt.be. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from IWT Flanders (SBOMETACEL project), the Methusalem program “NANO” and the Research Foundation Flanders (FWO) under G034609N. A. Hardy is a postdoctoral research fellow of the FWO. Sebastian Pregl and Dr. Jörg Opitz from the Technical University of Dresden are gratefully acknowledged for their help with the etching of the micelles. Benji Stephani is acknowledged for his assistance in the UV-ozone etching studies of the scum layer.

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Dewetting of Patterned Silicon Substrates Leading to a Selective

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