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Identification of Dewetting Stages and Preparation of Single Chain Gold Nanoparticle Rings by Colloidal Lithography Norbert Nagy, Dániel Zámbó, Szilárd Pothorszky, Eszter Gergely-Fülöp,† and András Deák* Institute of Technical Physics and Materials Science, Centre for Energy Research, 1525 Budapest, P.O. Box 49, Hungary S Supporting Information *

ABSTRACT: Massively parallel nanoparticle assembly was carried out by means of colloidal lithographic experiments over a silicon substrate supported (sub)microparticle Langmuir−Blodgett monolayer, using high purity aqueous solution of PEGylated gold nanoparticles. The size of the polystyrene template particles in the monolayer was varied between 608 nm and 2.48 μm, while gold nanoparticles with diameters between 18 and 65 nm were used. Thanks to the PEGylation of the gold nanoparticles, they could be used as tracer objects to follow the drying process. In this way, different dewetting stages could be identified in the confined space between and underneath the template polystyrene spheres. Depending on the concentration of the nanoparticles, the presented approach allows the preparation of singleparticle width necklace structures composed of gold particles. At the same time, the high purity of the substrate as well as of the evolved particle rings is preserved and unwanted particle deposition on the substrate surface is minimized.

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regions between the template particles.11,8,13 To address this problem, more simple model experiments have been carried out recently. Vakarelski and co-workers have investigated the 3D liquid bridge evolution between macroscopic (50−500 μm) particles and the substrate.14,15 Their results indicate that below the equatorial plane of the particles the sudden rupture of the vertical water film first sets in at the centroid between the neighboring particles and the substrate in the plane normal to the substrate. This rupture results in circular solvent remains at particle−particle and particle-substrate touching points. Dai et al. carried out systematic 2D investigations on controlling the spin-coating assisted assembly of nanoparticles around various top-down prepared PMMA pillar structure.6,16 They could prepare nanoparticle “carpets” around the posts, where the connectivity between the posts depended on the structure dimensions that affected the stability of the pinned droplets. Similar results and more importantly compact 3D assemblies could be obtained by Hamon et al. when drying a drop-casted gold nanorod suspension in a micropillar array.17 In this work, we study and follow the dewetting process during capillary lithographic experiments. We use a sacrificial polystyrene (sub)microsphere monolayer as template, which is removed after the drying process. The template monolayer is impregnated by the solution of nanoparticles using a PTFE-ring confined drop-casting approach. PEGylated gold nanoparticles with diameters of 18−65 nm were used as tracer objects, and

INTRODUCTION Controlled dewetting of structured surfaces is gaining much attention, since it offers a convenient approach to interface nano- and microsystems.1 Controlling particle assembly during drying of nanoparticle solutions is challenging due to the complexity and dynamics of the physicochemical phenomena involved in the process. Nevertheless, it can enable massively parallel positioning of nanoparticles at predefined, specific surface sites, often required to unlock the true potential on nanoparticle based structures in optics,2 energy harvesting,3 sensing,4 or surfaces with special wetting properties.5 Positive or negative surface corrugations at the length scale of the assembled nanoparticles have been shown to effectively direct the particle positioning,6,7 whereas self-assembled microparticle monolayers offer a convenient bottom-up template type for the massively parallel assembly of nanoscale objects. The concept of using larger microspheres was successfully applied earlier to create circular or localized patterns form quantum dots,8 quantum dot bioconjugates,9 or even gold nanoparticles.10,11 These examples together with others, where the circular structures are formed directly from precursors in the liquid that was trapped beneath the microspheres,12 show that dewetting in a confined geometry can enable the preparation of circular structures from different materials. Unfortunately, due to the large number of parameters that determine the structure (e.g., template size, surface energies, speed of evaporation), unambiguous identification of the key parameters is rather difficult. An additional difficulty arises from unwanted or spontaneous deposition of nanoparticles, often leading to excess particle coverage also in © 2016 American Chemical Society

Received: November 5, 2015 Revised: December 18, 2015 Published: January 11, 2016 963

DOI: 10.1021/acs.langmuir.5b04084 Langmuir 2016, 32, 963−971

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template particles and Au particles, as well as on the number density of the template particles, the appropriate sol concentration can be estimated for the dewetting experiments based on geometrical considerations (see the Supporting Information for details). The values were estimated to be ca. 5.5 × 108 nanoparticle/mm3 for GNP1, 1.5 × 108 nanoparticle/mm3 for GNP2, and 7.5 × 107 nanoparticle/ mm3 for GNP3. The concentrations of three different solutions were set to this value for the further experiments. Preparation of the Template Langmuir−Blodgett Monolayers. A given volume of polystyrene microparticle solution (see Table S1) was centrifuged and then redispersed in 500 μL of water. Then, a mixture of 500 μL of EtOH and the required amount of a 0.1% v/v ethanolic octylamine solution was added and sonicated for 5 min. The amount of octylamine was calculated to provide the same surface coverage for each polystyrene particle size. The solution of the polystyrene (PS) beads was spread at the air/water interface in a KSV2000 film balance using a Hamilton syringe. After 30 min, the Langmuir film was compressed with three different compression speeds: 1.25 cm2/s until 0.1 mN/m, 0.625 cm2/s until 0.2 mN/m surface pressure, and finally set to 0.125 cm2/s. After reaching 80% of the collapse pressure (measured earlier in a separate experiment), the barrier movement was stopped and the film was allowed to relax for 1 h. The film was transferred onto the Si substrates by vertical deposition using 7 mm/min substrate withdrawal and 0.0625 cm2/s compression speed. Contact Angle and Surface Tension Measurements. Contact angle measurements were carried out using a Krüss DSA30 apparatus. Advancing (ΘA) and receding (ΘR) contact angles as well as contact angle hysteresis were determined using the sessile drop method and the drop build-up technique. In order to obtain information about the wettability of the different materials that are involved in the dewetting process, contact angle measurements were carried out on macroscopic samples and thin films of the respective materials, that is, on Si substrates, PS layers, and PEG-covered Au thin films. To obtain more relevant data for the silicon substrate, before measuring the contact angle, they were subjected to the complete process of template monolayer formation and removal with all template diameters. For polystyrene, the contribution of octylamine to the wettability was taken into account by heating the different template Langmuir− Blodgett (LB) films of PS beads above their glass transition temperature (ca. 100 °C) and using these thin films for the contact angle measurements. The PEGylated gold surface was prepared by soaking electron beam evaporated Au film in aqueous PEG solution: 0.100 mg mPEG-SH (2000 Da) was dissolved in 21 mL of ultrapure water and the substrate was soaked in this solution for 3 h in an orbital shaker. After the soaking, the substrate was rinsed in ultrapure water several times. Due to the surplus of PEG in the solution, the grafting density and thereby the surface properties are almost identical for the plane gold film and for the 18 nm gold particles. The surface tension of the gold nanoparticle solution was determined by the same instrument using the pendant drop method. Drop-Casting and Drying Procedure, Scanning Electron Microscopy. A PTFE ring with an internal diameter of 7 mm was used for the drop-casting step22 providing a uniform liquid layer inside the PTFE ring (Figure 1). First, 150 μL of gold nanoparticle solution

different stages of the structure formation could be identified. The PEG coating of the Au NPs renders them extremely stable18 and at the same time it can effectively prevent spontaneous adsorption of the particles at solid/liquid interfaces. Hence, unwanted particle deposition can be minimalized and the particles are locked at their final position at the surface only during the very last stages of the drying process. Their location and the typical patterns formed by them were investigated by scanning electron microscopy. The assembly of the nanoparticles in the region underneath the template microspheres is driven by the evaporation of the suspending medium, and the morphology of the dry structure is governed by the interplay of evaporation dynamics, wettability of the surfaces and colloidal forces. While previous literature results refer to the high nanoparticle concentration regime of the approach (resulting in the formation of thick, multiparticle films with several particle layers,8,11 or even continuous particle deposits between neighboring particles13), here we show how this technique can be used for the preparation of nanoparticle rings, composed of a single line of nanoparticles.

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EXPERIMENTAL DETAILS

Materials. Tetrachloroauric(III) acid trihydrate, ACS reagent (HAuCl4·3H2O); sodium citrate tribasic dihydrate, ACS reagent, 99%; L-ascorbic acid, ACS reagent 99%; and octylamine, 99% were purchased from Sigma-Aldrich. Ethanol, AnalaR NORMAPUR ACS, Reag. Ph. Eur. analytical reagent (EtOH) from VWR, α-methoxy-ωmercapto polyethylene glycol (mPEG-SH, MW = 2000 and 5000 Da) from Rapp Polymere GmbH, and polystyrene beads with different sizes (608 nm, 909 nm, 1.27 μm, and 2.48 μm) were purchased from Microparticles GmbH. All chemicals were used as received. For all experiments, ultrapure water with a resistivity of 18.2 MΩ·cm was used. Gold Nanoparticle Synthesis. Three gold nanoparticle samples have been prepared. The smallest gold nanoparticles (18 nm) were synthesized by the Turkevich19 method: 222 mL of Milli-Q water and 6 mL of 0.01 M HAuCl4 solution were mixed in a vial and heated to boil under vigorous stirring. Then 6 mL of 38.76 μM sodium citrate solution was added. The solution was boiled continuously for 15 min. The color of the system changed from light yellowish to ruby red and then the sol was allowed to cool down to room temperature. These particles were used as seed particles in the first growth process to obtain 40 nm gold nanoparticles by a modified seeded growth method.20 Six milliliters of the seed particle solution was diluted to 20 mL. Two solutions (A and B) were prepared: solution A contained 4 mL of a 0.2% (w/v) HAuCl4 solution diluted to 20 mL; solution B was prepared by mixing 0.5 mL of a 1% (w/w) citrate and 1 mL of a 1% (w/v) ascorbic acid solution, and the whole solution was diluted to 20 mL. The seed solution was brought to boil, and solutions A and B were added. The color changed from pink to purple, and then to pink again. The mixture was boiled for an additional 30 min and allowed to cool to room temperature. The biggest nanoparticles (65 nm) were synthesized in a second growth process, where 40 nm gold nanoparticles were used as seeds. The nanoparticle concentrations were determined from UV−vis extinction measurements21 using a Thorlabs CCS200 Compact CCD spectrometer and were found to be 8.1 × 109 nanoparticle/mm3 for GNP1, 7 × 107 nanoparticle/mm3 for GNP2, and 1.4 × 107 nanoparticle/mm3 for GNP3. A volume of 15 mL of the resulting nanoparticles was centrifuged at 8500 rcf for 90 min, and redispersed in water. To this solution, 0.0626 g of mPEG-SH dissolved in 1 mL of H2O was added and gently stirred for 3 h. For GNP1 2000 Da, for GNP2 and GNP3 5000 Da mPEG-SH was used. The PEGylated nanoparticles were washed by repeated centrifugation−redispersion processes with water at least 10 times. The effect of PEGylation was also studied by dynamic light scattering (Malvern ZetaSizer NanoZS). Since the number of Au nanoparticles forming a single chain ring depends on the ratio of the diameter of

Figure 1. Schematic of the confined drop-casting method used during colloidal lithographic experiments. 964


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Langmuir was added, and then 100 μL of was removed by an automatic pipet to ensure the full wetting of the surface. The samples were slowly dried in a Memmert UN55 drying oven at constant temperature of 25 °C and ca. 33% relative humidity. The initial amount of nanoparticle solution inside the ring corresponds to a film thickness of 1.3 mm. Top view FESEM images were taken before and after the mechanical removal of the template monolayer using a LEO 1540 XB field emission scanning electron microscope applying 5.00 keV acceleration voltage.

chemically homogeneous interface despite the special sample preparation procedure for Si and PS. For Si surfaces with native oxide, the measured angles are a bit higher than the typically measured value of ca. 45°.24 This might be attributed to the presence of a small amount of octylamine on the Si surface due to the LB-film preparation process. The contact angle of PS surface is near to the values of 86°−88°, obtained earlier for solvent-casted PS layers.25 The value obtained for the PEG layer is in agreement with static contact angles obtained for surface grafted PEG (2000 Da) layers.26 The surface tension of pure water and the aqueous GNP1 sol was determined by the pendant drop method in saturated vapor at the temperature of 25 °C to be 71.64 ± 0.36 and 71.45 ± 0.64 mN/m, respectively. These values remained constant within error even after 90 min and did not show any tendency. (The time necessary for complete drying is less than 50 min at 25 °C.) This long-term measurement indicates that the spontaneous accumulation of the PEGylated gold nanoparticles at the used particle concentration level, and consequently their influence on the surface tension, can be neglected.

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PROPERTIES OF Au NANOPARTICLES AND SURFACES Dynamic Light Scattering. The prepared nanoparticles have a narrow size distribution and could be effectively surface modified with mPEG-SH molecules as shown by the DLS size distribution of the samples (Figure S1 in the Supporting Information). Besides the size increase, due to the replacement of the original surface ligands, there is a significant change in the zeta potential of the particles as well. For GNP1 the zeta potential increased form −32.0 ± 2.7 to −12.2 ± 1.2 mV, whereas for GNP2 and for GNP3 from −31.2 ± 1.4 to −13.9 ± 0.64 mV and −30.8 ± 0.9 to −14.7 ± 0.9 mV, respectively. The measured zeta potential and diameter values are collected in Table 1.

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THE DEWETTING PROCESS Macroscopic Film Rupture. The initial dewetting of the PS LB monolayer can be followed also by naked eye (Figure 3). At the beginning of the process, the whole circular area, defined by the PTFE ring, is completely covered by the sol of the nanoparticles. The initial amount of nanoparticle solution inside the ring is 50 μL that corresponds to an average film thickness of 1.3 mm. The film is gradually thinning due to

Table 1. Zeta Potential and Diameter Values of the Prepared Au Nanoparticles Obtained from DLS Measurements zeta potential

diameter

sample

mean (mV)

SD (mV)

mean (nm)

SD (nm)

GNP1 GNP1-PEG GNP2 GNP2-PEG GNP3 GNP3-PEG

−32.0 −12.2 −31.2 −13.9 −30.8 −14.7

2.7 1.2 1.4 0.6 1.1 0.9

17.7 27.2 39.8 63.6 66.5 105.8

0.2 0.2 0.2 0.3 0.2 0.6

Wettability and Surface Tension Measurements. Since the surface energies involved in the wetting and drying of such colloidal templates are of great importance,23 contact angle values for the different interfaces were obtained. The measurement results are summarized in Figure 2. The small contact angle hysteresis (i.e., difference between ΘA advancing and ΘR receding contact angles) indicates a smooth and

Figure 3. (a) Template monolayer (prepared from 2.48 μm PS beads) covered by the solution of gold nanoparticles. Diffraction colors originate from the ordered structure of the template particle layer. (b) Black film formation and (c) liquid film rapture; the dewetted area with diameter of ca. 4.5 mm evolves within 40 ms. (d) Second drying front appears. (e,f) Slow drying of the template monolayer. The arrows indicate the edge of the receding “postcursor” film.

Figure 2. Advancing (ΘA) and receding (ΘR) contact angles of 10 μL water droplets on (a) PEGylated Au thin film, (b) Si substrate, and (c) polystyrene surface (molten PS LB-film). Pendant drop of (d) pure water and (e) the GNP1-PEG solution used for the capillary lithographic experiments. The volume of both pendant drops is 10 μL. 965


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Langmuir evaporation, and black film formation can be observed in the middle of the circular area (Figure 3b) right before macroscopic film rupture. Upon rupture, a dewetted region with the diameter of ca. 4.5 mm evolves (Figure 3c) within 40 ms (time resolution of the imaging system). A drying time of 49 min was found to be sufficient in all cases and independent of the template diameter at the applied temperature (25 °C). It is important to note, however, that the PS particle monolayer is still wetted by the solution of the nanoparticles. This also indicated by a second drying front that evolves after the sudden rupture of the liquid, and can be followed under appropriate illumination. It recedes, together with the circular meniscus, toward the edge of the PTFE ring at a speed of ca. 4.9 ± 0.9 μm/s. This second drying front is attributed to the actual drying of the template film, and it is usually referred to as a “postcursor” liquid film. However, the explanation of its existence is unclear, hypothetically it is originating from the high surface coverage by the template particles.27 Initial Drying of the Template Monolayer. As it can be observed in the SEM images of the monolayer after completely drying of the film, some gold nanoparticles are deposited in the top region of the template particles (Figure 4). This can be

Figure S3 in the Supporting Information for details), hence spontaneous adsorption is not significant. Drying around the Equatorial Plane of the Template Particles. According to the advancing contact angle, as the water film thickness level drops below the top level of the template particles, this should result in the breakup of the entire film. As the film thickness decreases, menisci are formed between neighboring template particles, which develop into capillary bridges parallel to the substrate upon further evaporation. This results in separating and trapping of small amount of material at the lateral contact point in a later stage of drying.23 These separated liquid rings will vanish only in the final stage, together with the liquid rings underneath the template particles, due to the phenomenon of capillary condensation. After complete drying of the film, this is clearly indicated by the gold nanoparticle rings between the neighboring PS particles (Figure 5). Such structures have

Figure 5. Single particle rings composed of 18 nm Au nanoparticles between (a) 608 nm and (b) 909 nm neighboring template particles. The triangular openings between PS particles represent the evaporation windows once the film thickness passed the equatorial plane of the template monolayer. Figure 4. Deposition of 18 nm gold nanoparticles on the top of (a) 608 nm and (b) 909 nm PS template LB film.

been observed and sintered earlier as well by Yabu as a result of the evaporation based coassembly of 500 nm PS and 5 nm Au particles.10 As observed in Figure 5, the diameter of the resulting Au particle rings is very sensitive not only to the template diameter, but also to the variations in the lateral distance between two neighboring PS template particles. When the liquid level reaches half of the template diameter, the evaporation area is limited to the small triangular openings at the equatorial plane, around the centroid (“triple point”) of three neighboring template particles. This causes a “chimney” effect, that is, solvent convection underneath the template particles in the direction of these “triple points” at the substrate. Due to this, Au particles might accumulate in this region at the substrate. This phenomenon was utilized by Georgiadis et al. with millimeter length scale in IR radiation-assisted evaporative

attributed to the appearance of contact lines between the PS bead, nanoparticle solution, and air when the liquid/air interface reaches the top of the PS beads. The particle deposition interpreted in terms of the well-known coffee-stain effect28 at the initial stage of the template monolayer drying. It is unlikely that spontaneous adsorption of the Au nanoparticles on the PS surface takes place, due to the steric repulsion originating from the surface grafted PEG chains on the gold particles. This steric stabilization provides a repulsive potential that is in itself large enough to counterbalance the van der Waals attraction already shortly after reaching the effective range of steric repulsion. For the present particle sizes and ligand lengths this attraction corresponds only to ca. 1kT (see 966


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Langmuir lithographic experiments to accumulate latex particles under the openings of a shadow mask by evaporation induced convection of the aqueous particle solution.29 Considering the linear correspondence between the typical film dimension and the critical film thickness according to Vrij’s theory30 and the measured values on similar systems,31 it can be speculated,32 that the smallest thickness of the sol at these “triple points” falls below the diameter even of the smallest, 18 nm Au particles. This small film thickness at the “triple points” is a consequence of the close proximity of the template, furthermore, the curved shape of the meniscus around the centroid (Figure 6a).

Figure 6. Schematics of dewetting stages under the equatorial plane of the template particles. The left panel shows the top view of a unit cell, indicating the plane of cross section (dashed line). Right panel show the cross section views. (a) Au particle trapping at the “triple point”, i.e. at thinnest point of the liquid film. (b) Assumed particle trapping at the thinnest point of the free suspended water film between two neighboring PS particle. (c) Au particle trapping at the midpoint of the liquid bridge between two template particles.

Figure 7. Accumulation of 18 nm Au particles at “triple points” of (a) 608 nm, (b) 909 nm, and (c) 1.27 μm PS template particles are indicated by yellow arrows. Small blue arrows point to deposits at midpoints between two PS spheres.

This situation is analogous to the 2D crystallization experiments carried out by Denkov and co-workers33 using micrometer-size PS spheres. Presumably, the mechanism in our case is almost the same as the two-stage mechanism proposed by them. A “nucleus” of the particle array appears when the upper surface of the liquid film presses a particle against the substrate/water interface, hence it occurs in the thinnest region of the water film. Next, the motion of particles starts toward to the “nucleus” because the water film wets it due to the hydrophilic nature of the particles and the evaporation from this site is compensated by the lateral influx of water from the thicker region. In our case it allows particles to be accumulated−partly due to the above-mentioned “chimney” effect−and locked in position, creating single or multiple particle deposits depending on the local concentration of the Au nanoparticles (Figure 7). Furthermore, already deposited particles can stabilize the film against dewetting for a while at these specific locations. Nevertheless, due to the shape of the film profile, these are the areas where the nucleation of the solid/vapor interface first occurs.

Drying below the Equatorial Plane of the Template Particles. Upon nucleation of the solid/vapor interface, the triple line moves suddenly into its energy minimum determined by the contact angles on the Si and PS surfaces. The receding convex meniscus can randomly leave behind Au particles on the Si substrate surface,34 since the van der Waals attraction is somewhat larger compared to the PS surface (but still well below 5kT, see Supporting Infromation Figure S3). As the dry region is formed, free liquid film evolves between two neighboring PS particles and the Si substrate. This film is thinning during the further evaporation and its critical film thickness can be estimated similarly as above, based on previously measured values.32,35 The smallest film thickness can be less than the Au particle diameter. Hence, a mechanism similar to the array formation experiments in suspended liquid films35 can be assumed and presumably trapping of Au particle(s) occurs at the thinnest point of the free film (Figure 6b). Obviously, this cannot be observed in the SEM images. 967


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during the final stage of the drying.15 The continuously decreasing volume of the solvent forces the Au nanoparticles around the points of osculation of the PS beads and substrate. The circular liquid volume between the neighboring PS particles vanishes together with the pendular rings underneath the PS beads in the final stage of drying. It has to be noted that small amount of water can still exist at the junction points between the PS template particles (and the substrate) even after annealing due to the spontaneous water adsorption from ambient moisture as shown earlier.36 Hence, it can be anticipated, that the final distance between the nanoparticles is evolving only in the high vacuum environment of the SEM chamber. Typical SEM images are collected in Figure 9 showing ordered rings composed of 18 nm Au particles evolved underneath the PS template particles. It can be observed that some Au particles with smaller diameter (due to their size distribution) are located closer to the center. The colloidal lithographic experiments were repeated using 45 and 65 nm Au particles and different template diameters. Figure 10 shows single particle chains of 45 and 65 nm particles formed under the template particles, which can be obtained when the local concentration of the nanoparticles is appropriate. This clearly indicates, that the liquid film underneath the template particles is diminishing in such a way, that redistribution of the particles around the ring perimeter is allowed. Due to the absence of any additives used in the assembly procedure, the prepared structures are very clean. Nevertheless, instead of a perfectly ordered structure with evenly spaced particles, the particle gap sizes fluctuate. This can be attributed to the lateral capillary interaction between neighboring nanoparticles. Due to the relatively small gap-sizes compared to the particle diameter, these single chains could be applied in label-free sensing application like Raman spectroscopy, thanks to the emerging electromagnetic hot-spots in the interparticle regions. The final diameter of the particle rings is determined by the geometry, that is, by the diameter of the template particles and the Au nanoparticles. The theoretical internal radii of the particle chains can be calculated as

After the rupture of the suspended films, the liquid bridges between neighboring template particles at the substrate level still exist as shown earlier by Vakarelski et al. at the 100−500 μm scale15 and can be used for the creation of microwire networks at sufficiently high nanoparticle concentrations.13,14 Obviously, the lifetime and critical thickness of these bridges is inversely proportional to the surface tension of the liquid. The liquid bridges have the form of a saddle surface. It means that the meniscus is convex parallel to the connecting line but concave along it and their thinnest point is located at the midpoint between the two PS particles (Figure 6c). Although random deposition of Au particles is possible along the triple line during the drying, Au particles can be trapped at the midpoint, similarly to the concave meniscus of 2D crystallization experiments mentioned above, as shown in Figure 8. Due to the breakup of the liquid bridges that connect the neighboring particles, the remaining solvent is gathered underneath the template particles in the circular liquid film

ρ = (R + r )2 − (R − r )2 − r , where R and r are the radii of the template PS spheres and the Au nanoparticles, respectively (inset in Figure 11). By performing image analysis on SEM images, the internal radii of the rings can be obtained in a fairly straightforward manner. In Figure 11 the experimentally obtained average internal radii (ρ) are shown as a function of the template particle radius (R). The calculated theoretical values are also plotted as solid lines. There is an excellent agreement between the experimental and theoretical values, when the as-synthesized Au particle radii are used for the calculation. This refers to a complete collapse of the PEG chains, which is also supported by the frequently observed close proximity (nearly touching) of two neighboring Au particles in the same chain (Figure 10). The chain collapse can be associated with the complete water removal in the high vacuum environment of the SEM chamber. Effect of Template Order on the Structure. It is clear, that the samples prepared by this approach show variations in the gold nanoparticle ordering and the number of nanoparticles in one ring. Due to the high density pattern created by the templating microbeads, small variations in the template layer structure have a significant effect on the ordering of the small gold nanoparticles. The LB-film fabrication inherently produces

Figure 8. Deposits of 18 nm Au particles at the midpoint between neighboring 608 nm, 909 nm, and 1.27 μm PS template particles are indicated by blue arrows. Small orange arrows point to particle deposits at “triple points”. 968


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Figure 9. Eighteen nm Au nanoparticle rings formed on Si substrates using (a) 608 nm, (b) 909 nm, (c) 1.27 μm, and (d) 2.48 μm PS template particles.

Figure 11. Calculated and measured internal radii of Au nanoparticle rings as a function of template PS sphere radii. The inset shows the geometry of the hard sphere contact model (not to scale).

Figure 10. Single particle chains of (a,b) 45 nm and (c,d) 65 nm particles formed under (a) 608 nm, (b) 909 nm, (c) 1.27 μm, and (d) 2.48 μm PS particles.

the center of the missing PS particle (Figure 12a). Line defects are typically ordering defects; for example, they result in a square lattice instead of the original triangular one along the dislocation. This produces particle deposits between four particles (Figure 12b) in the same way as shown earlier for the triangular lattice (Figure 7) by the convective local accumulation and deposition of the nanoparticles. The hexagonal order is missing at domain boundaries in more extended regions, thereby the local Au particle concentration can be significantly higher. This can result in enhanced random particle deposition, thicker rings, or connecting lines along the liquid bridges (Figure 12c). It should be noted, that performing the experiments in almost saturated atmosphere leads to qualitatively similar results, but the majority of the nanoparticles gather at the defect sites. This implies that the liquid

a monolayer with domain structure, with several defect sites at the domain boundaries (see Supporting Information Figures S4 and S5 for the long-range organization of the structures). Independently of the defect type, lower local order produces a smaller local surface coverage by the template particles compared to a triangular lattice. This in turn translates into higher local gold nanoparticle/template particle ratio, that is, there is always a relative particle accumulation at the defect sites. Template particle vacancies, that is, point defects cause “chimney” effect due to their large evaporative area compared to the small triangular openings of the ordered structure. It results in nanoparticle accumulation on the substrate around 969


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and defect lines in the template film. It was demonstrated that particle trapping can occur at the thinnest point of the concave menisci, i.e. at the “triple point” and at the midpoint between three and two template spheres, before and after the liquid bridge formation at the substrate level, respectively. The twostage mechanism suggested by Denkov seems to be valid also for the small confined spaces involved in the present study, that is, between and underneath the template spheres with the diameter down to 608 nm. The occasional random particle deposition on the substrate surface can be attributed to the receding convex meniscus as the solid/air interface is growing around the centroid points and the liquid bridge saddles. The use of gold nanoparticles as tracer objects can be generally utilized in experiments where wetting−dewetting properties should be investigated on structures with small dimensions. The results obtained here indicate that, under optimized conditions, nanoparticle rings composed of a single chain of nanoparticles can be achieved. These structures might show advantageous properties compared to single particles or other ringlike structures reported earlier, due to strong coupling between the individual particles, allowing them to be used as a potential SERS substrate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04084. Template monolayer preparation; DLS size distribution of gold nanoparticles; estimation of the appropriate sol concentration; estimation of van der Waals attraction forces; low magnification SEM images (long-range organization of the structure) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

Figure 12. Effect of defects of 909 nm template LB-film on the local ordering of Au nanoparticles. (a) Point defect resulting in particle accumulation around the center of the missing PS sphere. (b) Particle accumulation due to “chimney” effect at a line defect. (c) Domain boundary displaying locally a continuous nanoparticle network.

†

E.G.-F.: Forensic Institute of the National Tax and Customs Administration 1631 Budapest, P.O. Box 35, Hungary. Notes

The authors declare no competing financial interest.

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film breakup around the template particles during the last stages of drying is of crucial importance, since it can effectively prevent the extensive lateral convective transport of the nanoparticles to the defect sites.

ACKNOWLEDGMENTS This work received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant No. 310250. The project was supported by the Hungarian Scientific Research Fund OTKA PD-105173 and K-112114. A.D. acknowledges the support of the János Bolyai Research Fellowship from the Hungarian Academy of Sciences. D.Z. and SZ.P. acknowledge the support of the Pro Progressio Foundation and József Varga Foundation.

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CONCLUSIONS The dewetting process was studied during capillary lithographic experiments carried out over a microparticulate monolayer template using gold nanoparticles. The spontaneous accumulation of the PEGylated gold particles at the water/air interface and consequently their influence on the surface tension can be neglected. Hence, the nanoparticles act as tracer objects during the process, allowing the identification of the different consecutive stages of the drying and structure formation. As the liquid level drops below the equatorial plane of the template particles, the gold nanospheres can accumulate at the centroid of three neighboring template particles due to the “chimney” effect in agreement with similar deposits at the defect points

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REFERENCES

(1) Gentili, D.; Foschi, G.; Valle, F.; Cavallini, M.; Biscarini, F. Applications of Dewetting in Micro and Nanotechnology. Chem. Soc. Rev. 2012, 41, 4430−4443. (2) Cai, Y.; Li, Y.; Nordlander, P.; Cremer, P. S. Fabrication of Elliptical Nanorings with Highly Tunable and Multiple Plasmonic Resonances. Nano Lett. 2012, 12, 4881−4888.

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