Controlled 2D Organization of Gold Nanoparticles in Block Copolymer

Article pubs.acs.org/Langmuir

Controlled 2D Organization of Gold Nanoparticles in Block Copolymer Monolayers Samuel S. Lamarre, Cynthia Lemay, Charles Labrecque, and Anna M. Ritcey* Department of Chemistry and CERMA, Université Laval, Pavillon Alexandre-Vachon, 1045 avenue de la Médecine Québec, G1V 0A6, Canada

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S Supporting Information *

ABSTRACT: The organization of organic-capped gold nanoparticles in PS-bPMMA monolayers is investigated. The preferred location of the particles within the block copolymer template is found to depend on both nanoparticle size and the length of the aliphatic capping agent. In the case of relatively short ligands, the particles behave as hard spheres and their incorporation in the polymer matrix can be qualitatively rationalized by entropic considerations. Three distinct arrangements are observed. Particles that are small, relative to the radius of gyration of the host polymer, evenly disperse within the PS domains, whereas the largest particles are considered form ordered, island-like aggregates. Particles of intermediate size exhibit the most striking arrangement, being relegated to the PS-PMMA interface to form organized ring structures. The tendency of these particles to assemble at the interface is sufficiently strong to force a modification of the polymer morphology to accommodate the particles at higher loadings. As the number of particles is increased, the circular PS-b-PMMA surface micelles elongate to form nanostrands.

B

photonic properties of such assemblies depend both on the distance between neighboring particles and the symmetry of the array. Particularly pertinent to the results presented here is the appearance of new coupled modes, including collective magnetic modes, for particles arranged in rings.9,10 These novel magnetic modes correspond to a resonant circulation of displacement current and open the door to the possible development of negative refractive index materials, or so-called metamaterials, of interest for revolutionary applications such as superlens and cloaking.11 Although mixtures of self-assembling block copolymers and inorganic NPs have been studied in film form,12−18 few experiments of this sort have been conducted at the monolayer level on a Langmuir trough.19,20 The present study differs from previous reports in that the impact of varying NP size and NP loading is considered in a systematic way. Importantly, these parameters are found to influence particle distribution and a number of well-defined particle assemblies are identified. Through this work, we hope to develop a new effective way to pattern NPs in a hierarchical bottom-up manner via Langmuir−Blodgett transfer using the subjacent copolymer nanostructured monolayer as a template. On a more fundamental level, the results presented here are not restricted to Langmuir−Blodgett assemblies but could also be relevant to composite copolymer nanostructures in general.

lock copolymers are known to undergo phase separation at the molecular level to form ordered arrays of selfassembled nanodomains. Block copolymer self-assembly has been extensively studied, and the ability to control the resulting morphology by tailoring the length and chemical nature of the blocks is well-documented. Although the majority of the scientific literature is dedicated to bulk samples, self-assembly at the air−water interface has also been investigated, particularly in the case of amphiphilic block copolymers. Several interesting monolayer morphologies have been reported, including that arising from the formation of surface micelles.1,2 Surface micelles are formed by the aggregation of hydrophobic blocks to form hemispherical domains, around which the surface active blocks spread to form a corona. The resulting well-defined, periodic structures have been investigated for several systems, including polystyrene-b-polymethylmethacrylate (PS-bPMMA)3,4 which forms surface micelles with a PS core and a PMMA corona. In this paper, we report the organization of metallic nanoparticles (NPs) within PS-b-PMMA monolayers. Importantly, we demonstrate that the precise spatial distribution of the particles can be modified through particle size and the choice of capping ligand. The ability to control the location of metallic NPs within polymer matrices is relevant to several applications. For example, the introduction of metallic NPs into organic solar cells has been shown to significantly increase light harvesting efficiency as the result of plasmonic scattering.5−8 The influence of the particles on device performance will clearly depend on the precise location of the particles within these multicomponent systems. A second example of the importance of ordered NP assemblies involves plasmon coupling. The © 2013 American Chemical Society

Received: May 17, 2013 Revised: July 18, 2013 Published: July 30, 2013 10891

dx.doi.org/10.1021/la4018932 | Langmuir 2013, 29, 10891−10898

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Article

EXPERIMENTAL SECTION

Gold Core Synthesis and Surface Passivation. Gold nanoparticles of three different sizes were obtained via different synthetic routes. The smallest NPs of diameter 2.0 ± 1.0 nm were prepared by the Brust-Schiffrin phase transfer method.21 Gold(III) chloride trihydrate, HAuCl4·3H2O, (100 mg) was dissolved in 9 mL of Nanopure water (18.2 mΩ·cm) and added to an organic phase composed of 0.5 g of tetraoctyl ammonium bromide (TOABr) and excess alcanethiol ligand (about 0.1 mL) dissolved in chloroform. Under magnetic stirring, a solution of 100 mg of sodium borohydride (NaBH4) in 4 mL of water was added. Stirring was maintained overnight. The organic phase was isolated and washed twice with 0.1 M sulphuric acid and twice with Nanopure water in a separatory funnel. The NPs were isolated from the organic phase by addition of methanol followed by centrifugation (17 000 rpm). Particles were washed several times by suspension in a minimal quantity of chloroform, followed by the addition of methanol and centrifugation. Particles of intermediate size were obtained via a modified version of the above procedure.22 The same quantities of gold salt, phasetransfer agent (TOABr), and reducing agent (NaBH4) were employed. The alcanethiol ligand, however, was introduced by ligand exchange only after the reduction and washing of the particles. Ligand exchange was carried out by the addition of excess alcanethiol to a chloroform suspension of the particles. NPs were washed as described above. With this synthesis, NP diameter and polydispersity varied slightly from one sample to another and with the different capping agents. For the sake of clarity, we will refer to these particles as having a mean diameter of 5.8 ± 2.3 nm, which is calculated from the ensemble of intermediate size samples. The third NP sample was synthesized using oleylamine (OLA) as both the reducing and capping agent.23,24 Gold(III) chloride trihydrate (20 mg) was dissolved in 50 mL of Nanopure water (18.2 mΩ·cm) with magnetic stirring. The solution temperature was adjusted to 50 °C with an oil bath. Oleylamine (250 μL) was then added and the temperature raised to 80 °C for 2 h. NPs were extracted from the aqueous phase by the addition of 250 mL of chloroform and 8 mL of concentrated NaOH to the reaction mixture. The organic phase was isolated with a separatory funnel. The NPs were isolated by the addition of methanol followed by centrifugation (12 000 rpm) and washed as described above. Ligand exchange was achieved by the addition of excess alcanethiol to a concentrated NP suspension in chloroform. The mixture was stirred overnight. NPs were washed as described above. NPs with a diameter of 10 ± 2 nm were obtained. Ligand Selection. In order to evaluate the effect of ligand size, gold cores were capped with alcanethiols of different chain lengths. Alcanethiols composed of 4, 8, and 18 carbon atoms were used. Since larger particles require longer ligands for the preparation of stable suspensions, 5.8 nm diameter NPs could not be dried and redispersed when capped with butanethiol. Similarly, 10 nm NPs could only be redispersed when coated with octadecanethiol. In all, six different NP populations, illustrated in Figure 1, were available for study. Thermogravimetric Analysis. The proportion of organic ligand relative to the inorganic core was measured by thermogravimetric analysis. A TA Instruments model Q5000 was used for the TGA measurements. Thermograms were recorded under nitrogen flow. Samples exhibited a ligand footprint of 16 ± 3 nm Å2/ligand, which is in agreement with literature values for spherical gold NPs of this size.25 This indicates that the surfaces of the particles are saturated with closepacked aliphatic chains and that no free residual thiol is present in the samples. Polymers. PS-b-PMMA samples were obtained either commercially from Polymer Source Inc. (Montréal, Canada) or by in-house synthesis.26 Characteristics of the three near-symmetrical block copolymers investigated are provided in Table 1. The majority of experiments were carried out with the 104 000 g/mol copolymer and, unless otherwise specified, all references to PS-b-PMMA indicate this sample. The two other copolymers are denoted by mention of their total molecular weight. A PMMA homopolymer from Polymer Source

Figure 1. Schematic representation of the six nanoparticle populations investigated in this work. Ligand contour lengths are drawn to scale relative to particle size.

Table 1. Characteristics of the PS-b-PMMA Samples mean molecular weight (g/mol) total

PS block

28 000 104 000 220 000

13 200 50 000 112 000

PMMA block 14 800 54 000 108 000

poly dispersity index

source

1.1 1.1 1.1

synthesis Polymer Source synthesis

with a mean molecular mass of 45 500 g/mol was used in contact angle experiments. Composite Langmuir−Blodgett Films. Monolayers were spread on KSV 3000 Langmuir trough from chloroform solutions containing both PS-b-PMMA and NPs. Solutions were made using HPLC grade chloroform (Laboratoires MAT Inc., Québec, Canada). A PS-bPMMA concentration of 1 mg/mL was employed for all samples. Except for the samples depicted in Figures 3 and 7, a NP concentration of 0.5 mg/mL was used for the 2.0 nm NPs and of 1 mg/mL for the other particles. Solutions were deposited drop by drop in a grid pattern on Nanopure water surface (18.2 mΩ·cm). Five minutes were allowed before compression for the solvent to evaporate. The surface film was symmetrically compressed by mobile barriers advancing at a speed of 10 mm/min. Monolayer films were transferred at a constant surface pressure of 17 mN/m to carbon-coated TEM grids glued on glass substrate with a lift-off speed of 0.5 mm/min. Transmision Electron Microscopy. TEM images were obtained with a JEOL 1230 microscope (80 kV). Particle counting and size analysis was carried out with ImageJ analysis software. Atomic Force Microscopy. AFM analysis was carried out with a Digital Instruments NanoScope MultiMode in tapping mode. Silicon cantilevers (MikroMasch) with a characteristic resonance frequency of 325 kHz and a force constant of 40 N/m were used. Tips had a radius of curvature of 10 nm. Contact Angle Measurements. Contact angles of a sessile water drop deposited on LB films transferred to glass substrates were measured with a FTA200 model goniometer by First Ten Angstrom. Seven replicates were measured for each sample to obtain a mean contact angle value.

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RESULTS AND DISCUSSION NPs Capped with C8SH. TEM images of PS-b-PMMA surface micelles, containing gold NPs coated with octanethiol are shown in Figure 2. The distribution of the particles within the block copolymer matrix depends on particle size. The smaller (2.0 nm) particles are found to disperse evenly within the PS cores (Figure 2a,b), whereas the larger (5.8 nm) particles tend to segregate to the PS/PMMA/air triple junction interface (Figure 2c,d). 10892


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NPs Capped with C18SH. TEM images of composite films containing NPs coated with a longer chain ligand (octadecanethiol) are presented in Figure 4. In this case, three different particles sizes can be considered since the C18 chain is long enough to stabilize suspensions of the largest (10 nm) particles. When capped with C18SH, the gold nanoparticles are incorporated within the PS domains for all three particle sizes considered here. Differences are, however, observed in the exact way in which the NPs are distributed. In the case of the smaller gold particles (2.0 and 5.8 nm), all of the PS domains are occupied by a similar number of particles and no particle ordering is observed. In the case of the larger 10 nm particles, the distribution of the gold particles among the PS domains is less regular. Some domains contain a relatively large number of particles, whereas many PS domains remain empty. This pattern is similar to what is observed by Li et al. for a similar system.20 Furthermore, within the more highly occupied PS domains, the 10 nm gold nanoparticles are observed to form ordered arrays. NP Size and Aggregation Due to van der Waals Interactions. The formation of 2D close-packed hexagonal aggregates, as illustrated in Figure 4f, can be attributed to interparticle van der Waals attractions. These forces increase with increasing particle size. The ordered arrays observed for the 10 nm NPs indicate that, for particles of this size, van der Waals attractions are present at distances extending beyond the C18SH ligands. In fact, similar aggregates have been reported for similar particles spread alone (in the absence of copolymer) at the air−water interface.28 When cospread with the block copolymer, NP aggregation probably occurs upon deposition of the spreading solution. The copolymer molecules then selfassemble to form empty surface micelles or to surround preexisting NP aggregates. This process most likely begins as early as solvent evaporation and continues through barrier compression as the film become denser. In this case, there is no apparent interaction between the PS-b-PMMA matrix and the particles; NP aggregation is independent of monolayer formation. The conclusion that NP ordering occurs before that of the copolymer is supported by the observation that the occupied island-like PS domains are significantly larger than the unoccupied self-assembled surface micelles. Ligand Length Effect on Interaction with PS from the Matrix. The length of the alkyl chain of the capping ligand also influences the distribution of NPs within the polymer matrix.29 This is illustrated by comparing the results obtained with the 5.8 nm diameter gold cores capped with either C8SH or C18SH (Figures 2c,d and 3c,d). Although, in both cases the NPs are evenly distributed among the PS domains, their location within the domains is very different. C8SH-capped particles are relegated to the periphery of the PS domains whereas C18SH

Figure 2. TEM images of C8SH stabilized gold NPs incorporated in a PS-b-PMMA monolayer. Gold core diameter is 2.0 nm in panels a and b and 5.8 nm in panels c and d.

NP Size and Entropy. At first view, the dependence of particle distribution within the polymer film on particle size illustrated in Figure 2 could be explained by entropic considerations.12,14,27 The incorporation of NPs within the PS domains requires the deformation of the polymer chains as they stretch around the particles. This results in a loss in conformational entropy that is dependent on particle size; the smaller the particles, the smaller the entropic loss. This line of thought could explain why the Brust-Schiffrin NPs can be evenly dispersed within the PS domains whereas the larger, 5.8 nm, NPs are excluded to the PS-PMMA interface as the solvent evaporates from the swollen PS domains. As discussed below, however, particle size is not the only parameter that determines the distribution of the NPs within the copolymer matrix. Effect of NP Loading in the Case of Interfacial Assembly. The regular assembly of the C8SH-capped 5.8 nm gold NPs at the interface between the PS core and the PMMA corona of the surface micelles is arguably the most striking feature of this study. We investigated this system further by modifying particle loading in the composite Langmuir monolayers. The TEM images of Figure 3 indicate that, as loading is increased, a further interesting phenomenon is observed. The initially circular PS domains deform to nanostrands. The NPs effectively play the role of a surfactant in reducing the interfacial tension between the PS and PMMA blocks. The tendency of particles to assemble at the interface is sufficiently strong to force a modification of the polymer morphology to accommodate the particles.

Figure 3. TEM images C8SH-capped 5.8 nm gold NPs incorporated in a PS-b-PMMA monolayer at different NP loadings. Surface concentrations are 320 NPs/μm2 in panel a, 730 NPs/μm2 in panel b, and 1900 NPs/μm2 in panel c. 10893


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Figure 4. TEM images of C18SH stabilized gold NPs incorporated in a PS-b-PMMA monolayer. Gold core diameter is 2.0 nm in panels a and b, 5.8 nm in panels c and d, and 10 nm in panels e and f.

capped particles are dispersed, in a disordered fashion, within them. This difference can be attributed to the increased free volume at the surface of the thicker capping layer. In the case of a spherical particle, the effective grafting density decreases with distance r from the particle surface according to σeff = σo(rc/r)2 where σo is the grafting density at the particle surface and rc is the core radius.30 As the length of the ligand alkyl chain is increased, the decrease in lateral packing density permits increased interaction with the polystyrene chains of the matrix. Whereas in the case of hard spheres, entropy considerations predominate leading to exclusion of the metal nanoparticles, the longer ligand allows for more favorable interaction with the matrix and NPs tend to be more randomly dispersed within PS domains. Densely packed C8SH ligands can thus be considered to form a relatively hard organic shell around the NP, while C18SH ligands constitute a softer coating.31 Therefore, in order to isolate the effect of NP size alone on the nature of particle integration in the polymer matrix, it is important to compare particle populations with dense lateral packing of the ligands. This can be achieved by considering only the shortest possible ligand for each particle population. In the present case, a maximum ratio of gold core radius to ligand contour length of about 3 is found. With a higher ratio (larger particles or shorter ligands), NPs coalesce irreversibly upon drying under vacuum and cannot be redispersed in CHCl3. This limit corresponds to C18SH ligands for the 10.0 nm NPs and to C8SH for the 5.8 nm NPs. NPs Caped with C4SH. To complete the series, composite monolayers were prepared from 2.0 nm NPs capped with C4SH. TEM images of the resulting films are provided in Figure 5. With a diameter of 2.0 nm, these NPs are smaller than the thickness of the PS domains, and it is thus unclear, from TEM images alone, whether they are located at the PS/air interface or dispersed within the PS domains. Discrimination between the two possibilities, illustrated in the schematic representation of Figure 6, is nontrivial because of the very small dimensions involved. AFM measurements indicate that the PS domains have a height of 6 nm and there is thus only 3 nm between the domain center and PS/air interface.

Figure 5. TEM images of C4SH-stabilized gold NPs incorporated in a PS-b-PMMA monolayer. Gold core diameter is 2.0 nm in panels a and b.

Figure 6. Schematic cross-sectional representation of a PS domain illustrating the two possible locations for the 2 nm gold NPs: at the domain surface or dispersed within it.

Two indirect experimental methods were employed to probe NP location. In the first, NP loading was varied to establish its effect of the morphology of the copolymer matrix. If the NPs were preferentially located at the PS/air interface, one would predict an increase in the surface density of NPs on top of the PS domains with no change in the dimensions of the surface micellar morphology. In the case of incorporation of the NPs within the PS, the size of the domains can be expected to increase with increased particle loading. The results are presented in Figure 7 and Table 2. The results indicate that the surface area of the PS domains increases with increasing particle loading, consistent with the model of particle dispersion within domains. Furthermore, by considering the PS domains to be truncated spheres, the corresponding volume increase can be calculated and is found to approximate that of the added NPs. (Calculations are provided as Supporting Information.) 10894


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Figure 7. TEM images of composite films of C4SH-stabilized 2.0 nm gold NPs and PS-b-PMMA at different particle loadings. The monolayer in panel a contains no NPs, whereas the spreading solutions for samples in panels b and c contained NP concentrations of 0.5 and 1.0 mg/mL, respectively.

where b is the Kuhn length of PS and N the number of Kuhn segments in the chain. For the present system, a radius of gyration of 6.1 nm is obtained. The ratio of Rg to NP radius is provided in the upper section of Table 4 for the three populations considered here.

Table 2. Average Diameter of PS Domains at Different NP Concentrations in the Spreading Solutiona

a

NPs (mg/mL)

diameter (nm)

0 0.5 1.0

56 60 65

Table 4. Summary of Particle Characteristics and Assembly Morphology for Hard Sphere Samples of Varying Core Diameters, dNPa

The copolymer concentration was fixed at 1 mg/mL.

A second set of experiments involved contact angle measurements for a sessile water drop on various LB films transferred to a glass microscope slide. The results are presented in Table 3. Control samples include the bare glass substrate and C4SH-capped 2.0 nm gold NPs, PMMA and PSb-PMMA LB films deposited on glass. Of these materials, it is the gold particles that present the highest surface energy, characterized by the lowest contact angle. This result agrees with literature reports and reflects the inability of the short C4SH ligand to mask to high polarizability of the metal.32 As expected, a higher contact angle is found for PS-b-PMMA than for PMMA, consistent with the presence of hydrophobic PS domains on the surface. Importantly, the contact angle is found to increase upon the addition of gold NPs. This result is a clear indication that the NPs are not located at the PS surface, as this would lead to a decrease in contact angle. Instead, swelling of the PS domains upon the incorporation of the NPs increases the fraction of the surface occupied by the hydrophobic copolymer component, thus increasing the contact angle. Comparison with Theoretical Predictions. Consideration of the results obtained for the three different particle sizes, each coated with the shortest possible ligand and incorporated in the 104 000 g/mol PS-b-PMMA matrix, leads to the following scheme of patterning: dispersion in the PS domains for C4SH-capped 2.0 nm NPs, segregation at the PS/PMMA/ air interface for C8SH-capped 5.8 nm NPs and formation of close-packed, island-like aggregates for C18SH-capped 10.0 nm NPs. Since these three populations can be considered as hard spheres, it is relevant to compare particle size to the radius of gyration (Rg) of the PS segments Rg = b

N 6

capping ligand

capping ligand contour length (nm)

Rg/rNP [Au + ligand]

observed NP assembly

2.0

C4SH

0.50

4.1

5.8

C8SH

0.97

1.6

10.0

C18SH

2.2

0.9

28 000

2.0

C4SH

0.50

2.1

220 000

5.8

C8SH

0.97

2.4

uniformly dispersed in PS located at the PMMA-PS interface aggregated in the center of PS domains uniformly dispersed in PS located at the PMMA-PS interface

copolymer molar mass (g/mol)

dNP (nm)

104 000

a

Rg/rNP represents the ratio of the radius of gyration of the PS block to particle radii that include the contour length of the ligand [Au + ligand].

These experimental results can be compared with the mean field theory predictions of Thompson et al.14 for mixtures of soft, flexible chains and hard spheres. These authors modeled the dispersion of NPs of two different sizes in the lamellar phase of a block copolymer and predict different spatial distributions depending on NP size relative to the end-to-end distance, R0, of an unperturbed chain of the matrix polymer. A semiquantitative comparison of their predictions to our observations can be made after conversion of R0 to Rg through the simple relationship Rg = R0/√6. Interfacial segregation is predicted for NPs with a Rg/rNP ratio of 2.0, whereas aggregation at the center of the lamellae is predicted for larger NPs with a Rg/rNP ratio of 1.4. We believe these structures to be analogous to the island (Figure 4f) and ring (Figure 2c,d) morphologies, respectively. Thompson et al.14 did not report

(1)

Table 3. Contact Angle of a Sessile Water Drop on Glass or Glass Coated with Either Coatinga contact angle (°) a

no coating

2 nm C4SH- stabilized NPs

PMMA

PS-b- PMMA

composite 1:2

composite 1:1

53 ± 3

41 ± 9

60 ± 2

65 ± 2

69 ± 2

71 ± 1

Composite ratios indicate NP to copolymer mass. Uncertainty values correspond to the standard deviation from 7 measurements. 10895


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Figure 8. AFM images of LB monolayers of symmetric PS-b-PMMA copolymers with mean molecular masses of (a) 28 000, (b) 104 000, and (c) 220 000 g/mol.

calculations for particles with larger Rg/rNP values, corresponding to the smallest NPs investigated here. The uniform dispersion of these particles within the PS domains is, however, consistent with the argument that entropic loss associated with the disruption of polymer conformation is the primary driving force for the expulsion of hard spheres. Effect of the Mean Molecular Mass of the PS-b-PMMA Matrix. In order to validate the interpretation presented above, analogous experiments were carried out with symmetrical PS-bPMMA copolymers of different molar masses. In general, these samples formed less ordered structures and exhibited less reproducible LB transfer behavior than the 104 000 g/mol PSb-PMMA employed for the main part of this study. Typical differences in the monolayer morphologies obtained for the three molar masses are illustrated in Figure 8. Structures assembled with shorter copolymer chains (Figure 8a) have PS domains that are predictably smaller and closer together. The domains, however, are rather polydisperse and of a more irregular shape than those formed by the 104 000 g/mol polymer (Figure 8b). The longer copolymer chains (Figure 8c) assemble to form well-defined, larger, circular domains with a mean diameter of 67 ± 15 nm. The presence of a number of considerably smaller domains is responsible for the large standard deviation and the seemingly low value of the mean diameter. Since all of the samples have relatively narrow molecular weight distributions, the high polydispersity in PS domain size found for the higher molar mass may be the result of kinetic effects during the assembly process. Finally, it should be noted that the experimental parameters for LB film preparation, including spreading solution concentration, were optimized for the 104 000 PS-b-PMMA system and morphological order could possibly be improved by tailoring these parameters for shorter or longer chains. Composite films prepared from the lowest molecular weight (28 000 g/mol) PS-b-PMMA matrix and the smallest NPs (2.0 nm, C4SH-stabilized) can be used to test the limiting value of Rg/rNP for particle exclusion from the PS domains. Results are shown in Figure 9. Despite the reduction of the Rg/rNP ratio to a value of 2.1, the morphology observed is very similar to that reported in Figures 5 and 7, with NPs presumably located within the PS domains. Similarly, C8SH-stabilized and C18SH-stabilized 5.8 nm NPs incorporated within 220 000 g/mol PS-b-PMMA are distributed in the same way as found for the 104 000 g/mol matrix. As shown in Figure 10, the ring morphology is observed for the shorter ligand, whereas C18SH-stabilized NPs are dispersed within the PS domains. It is interesting to note that in this case of the ring morphology, a small number of NPs are also present

Figure 9. TEM images of composite LB films of C4SH-stabilized 2.0 nm gold NPs and 28 000 g/mol PS-b-PMMA.

Figure 10. TEM images of composite films of 5.8 nm gold NPs either C8SH-stabilized (a) or C18SH-stabilized (b) and 220 000 g/mol PS-bPMMA.

within each of the PS domains (Figure 10a). In contrast, no NPs are observed in the center of the PS domains for the analogous system shown in Figure 2b,d. It can be speculated that the presence of particles within the domains of Figure 10a reflects the increased capacity of the longer PS chains to accommodate NPs. Although convenient to explain the results obtained for the 104 000 g/mol PS-b-PMMA copolymer matrix, the quantitative arguments invoked above do not hold up for the analogous composite system assembled with same NPs and copolymers of different lengths. The best example is the presence of ring structures in Figure 10a for a system with a Rg/rNP ratio of 2.4 and the absence of ring structures in Figure 9 for a composite with a Rg/rNP ratio of 2.1. According to the previous reasoning, segregation at the PS/PMMA/air triple interface would have been expected for 2.0 nm C4SH-stabilized NPs in a 28 000 g/ mol PS-b-PMMA matrix, but instead NPs are evenly incorporated within PS domains. While the quantitative agreement between prediction and observation found for the 104 000 g/mol PS-b-PMMA matrix is thus clearly fortuitous, the results do not necessarily negate the general rational that the Rg/rNP ratio is a relevant parameter for NP dispersion in polymer matrices. 10896


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treated as hard spheres. Free volume in the shell allows for interpenetration with the matrix polymer chains, favoring NP dispersion. In the case of particle aggregation at the PS-PMMA interface, increased NP loading can provoke a morphological transition from surface micelles to nanostrands. Although the detailed organization of these complex multiparameter systems cannot yet be predicted with certainty, well-defined NP assemblies are obtained.

Although NP size clearly influences particle distribution within the copolymer matrix, other parameters, such as the nature of the passivating ligand, must be considered. Both theoretical33,34 and experimental studies17,31 indicate that entropy considerations alone cannot explain NP distribution in block copolymer matrices in all cases. Matsen et al.32 modified the theoretical approach of Thompson et al.14 to include a gradual particle−matrix interface and thus allow interpenetration of the ligands and the polymer matrix. In this case, NP location is dictated by the affinity of surface ligands for one or the other of the polymer blocks. Neutral affinity particles are relegated to the interface between the segregated domains, whereas particles with an affinity for one of the blocks locate within the corresponding domain. Interestingly, neutral affinity particles are predicted to remain at the domain interface even as particle size is reduced. However, the smallest NPs considered in these simulations have a Rg/rNP value of 2.0 and it would be interesting to extend the theoretical predictions beyond this size regime. Such predictions could be compared with the experimental results presented in Figure 2 for NPs of two different sizes, both coated with octanethiol. The smaller (2.0 nm) particles are found to disperse evenly within the polystyrene domains (Figure 2a and b), whereas the larger (5.8 nm) particles tend to segregate to the interface. Since the surface ligand is identical for both populations, it is difficult to attribute the difference in location to differences in particle− matrix affinity. Although it may be argued that the increased radius of curvature of the smaller particles could lead to greater ligand-matrix interdigitation, the 2.0 nm NPs remain dispersed in the PS domains even when the ligand length is reduced to butanethiol, as illustrated in Figures 5 and 7. On a final note, the absence of a quantitative model to predict the details of composite assembly should not detract from the interest of the resulting structures as functional materials. To the best of our knowledge, this study constitutes the first report of LB assembly of metal NPs into ring structures. Furthermore, comparison of the images presented in Figures 2c and 10a indicates that ring diameter can be modulated by modifying PS domain size. Ring diameter is, in turn, a key parameter for the tuning of the coupled plasmonic properties in such structures.35 The ability to organize NP assemblies over large surface areas is essential to the fabrication of plasmonic devices for use in metal-enhanced spectroscopies and the nanoscale manipulation of light.

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ASSOCIATED CONTENT

* Supporting Information S

In order to validate the hypothesis that the metal NPs are located within the PS domains, rather than at their surface, the observed volume change of the domains can be compared with the volume occupied by the added particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 418-656-2368. Fax: 418-656-7916. E-mail: anna. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge NanoQuébec, le Fonds Québécois de la recherche sur la nature et les technologies (FQRNT) and the National Sciences and Engineering Research Council of Canada (NSERC) for their financial support. Rodica Plesu is acknowledged for providing PS-b-PMMA samples.

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REFERENCES

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CONCLUSIONS Langmuir−Blodgett film transfer is a useful tool for the bottomup assembly of various composite systems. In particular, it constitutes a convenient route to model systems for the study of ordered block copolymer/inorganic NP composites. With this technique, we have demonstrated that the surface micelle morphology of PS-b-PMMA can be exploited to pattern the assembly of thiol-capped gold NPs. The precise location of the NPs, as well as the nature of their assembly, depends on both particle size and nature of the capping ligand. By tailoring these parameters, NPs can be dispersed in the PS domains, segregated at the PS-PMMA interface or aggregated into larger island-like arrays. This behavior can be, in part, rationalized with entropic considerations. In the case of hard-sphere particles, observations are in qualitative agreement with predicted trends. Particles with longer ligands, relative to their core radii, tend to assemble in a less defined way. In this case, the ligand shell is less dense and the particles cannot be 10897


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Controlled 2D Organization of Gold Nanoparticles in Block Copolymer

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