Template-assisted self-assembly of diblock copolymer micelles for non-hexagonal arrays of Au nanoparticles

Abstract

We report the construction of non-hexagonal arrays of nanoparticles by the template-assisted self-assembly of polystyrene-block-poly(4-vinylpyridine) copolymer micelles. Diblock copolymers were spin-coated onto nanoscale TiO₂ templates, which successfully guided the placement of the micelles to form unconventional assemblies such as linear, zigzag, and Kagome array structures. These arrangements, different from the usual quasi-hexagonal arrays of copolymer micelles spin-coated onto a flat substrate, greatly depend on the physical dimensions of both the template and the micelles. Subsequent treatment of the copolymer micelle assemblies with oxygen plasma resulted in various non-hexagonal arrays of Au nanoparticles while preserving the arrangement of the original micelles on the template.

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Introduction

Nanoparticles have promising applications in various optical, electronic, and sensor devices because of the extraordinary functionalities that their bulk counterparts do not have.¹ When multiple nanoparticles are organized at the nanometer scale, collective phenomena can be caused by the interaction among them, which strongly relies on the morphology of the assembly, i.e., the configuration of their organization and the interparticle distance. For example, depending on the polarization of light, metallic nanoparticles in a one-dimensional linear array exhibit a remarkable difference in absorption spectra owing to their anisotropic configuration;^2–4 moreover, the size and spacing of the arrayed nanoparticles determine the degree of surface plasmon resonance coupling.^5–8 Therefore, accurate control of the interaction and the coupling phenomena that occur among nanoparticles is strongly required to utilize them as building blocks in state-of-the-art nanodevices. For this purpose, robust and feasible techniques to arrange the nanoparticles with tunable organization and dimensions have been extensively explored. One such technique is direct-write lithography, which includes electron-beam lithography^3,6–8 and dip-pen nanolithography.^9,10 These methods have been used for creating arbitrary arrays of nanoparticles on substrates despite the expensive and serial nature of the process. In contrast, the self-assembly of spherical colloidal nanoparticles can easily form a close-packed hexagonal superlattice in an inexpensive and facile manner,^11–15 although their arrangement and interparticle distance in the assemblies are difficult to control.

Template-assisted self-assembly provides a facile method for adjusting the placement factors of arrayed colloidal particles. When colloidal objects are coated on a topographically patterned substrate, aggregates of the particles are assembled in the templates to form well-defined internal structures that rely on geometrical parameters such as the diameters of colloids and the width and depth of the template. This self-organization process is attributable to the capillary force toward the templates, driven by the evaporation of solvent molecules during the coating process. Xia et al. first reported that submicron-sized colloidal polystyrene (PS) beads are self-organized in template holes and trenches and that the alignment can be accurately determined by the geometric shape of the template as well as the relative size ratio of the colloids to the template.^16,17 The dimension of the self-assembling particles in a template was subsequently reduced to the nanometer scale, which was reported in the previous studies of Alivisatos et al. This indicates that the small size of the nanoparticles ensures a more effective confinement in the topographically patterned template because of the stronger capillary force.^18,19 Thus, nanoscale templates enable the creation of various assemblies of nanometer-sized particles with array structures that are distinct from the conventional hexagonal configuration.

In this study, we demonstrated the fabrication of arrayed metal nanoparticles with non-hexagonal lattice structures by the template-assisted self-assembly of diblock copolymer micelles. Diblock copolymers, consisting of two chemically distinct homopolymers linked by a covalent bond, spontaneously formed nanometer-sized spherical micelles in a selective solvent for one of the blocks. Although the copolymer micelles, similar to other nanocolloids, are normally self-organized into a quasi-hexagonal assembly on a substrate,^20–25 we produced an unconventional array of copolymer micelles with assistance from the nanoscale inorganic template. Two different types of inorganic nanotemplates were fabricated from thin films of self-assembled diblock copolymers via a lithography-free approach. By spin-coating the copolymer micelles onto the inorganic nanotemplates, we were able to produce various unusual arrays of the micelles in large area without additional treatment. With characterization by dynamic light scattering (DLS), atomic force microscope (AFM), and scanning electron microscope (SEM), we found that the placement of the diblock copolymer micelles was determined by the shape of the template as well as by the relative size of the micelles, which were controlled by the molecular weight of copolymers. Then, we synthesized nanoparticles from the micellar core blocks by the removal of the copolymers, which led to the effective construction of unusual non-hexagonal arrays of the nanoparticles. Thus, unlike typical self-assemblies of nanoparticles, we were able to show Au nanoparticles in non-close-packed arrays with well-defined spacing. In addition, we applied this approach to generate binary arrays of two different nanoparticles in specific arrangements.

Experimental section

Chemicals and materials

Diblock copolymers of polystryene-block-poly(methyl methacrylate), PS(80)-PMMA(80), PS(140)-PMMA(60), PS(5)-PMMA(5), and polystyrene-block-poly(4-vinylpyridine), PS(32)-P4VP(13), PS(51)-P4VP(18), PS(109)-P4VP(27) were purchased from Polymer Source, Inc. The number in the parenthesis is the average molecular weight (M_n) in kg mol⁻¹. The polydispersity indices (PDIs) were 1.09, 1.08, 1.09, 1.08, 1.09, and 1.15, respectively. Phenethyltrichlorosilane (PETS), titanium tetraisopropoxide (Ti(OCH(CH₃)₂)₄, TTIP), and precursors of metal nanoparticles(HAuCl₄) were obtained from Sigma-Aldrich and used as received.

Fabrication of TiO₂ nanotemplates

The Si wafer was cleaned using piranha solution (70/30 v/v concentrated H₂SO₄ and 30% H₂O₂), thoroughly rinsed with deionized water, and dried with nitrogen. The surface of the Si wafer was neutralized to provide the same interfacial energy toward both the PS and PMMA blocks of copolymers having self-assembled monolayers.²⁶ For depositing PETS self-assembled monolayers, the substrate was immersed into a 0.1% (v/v) anhydrous toluene solution of PETS for 2 h, and then thoroughly rinsed with ethanol. A lamella-forming PS(80)-PMMA(80) toluene solution was spin-coated onto PETS-treated Si substrates to form a thin film having a thickness of 86 nm. A cylinder-forming PS(140)-PMMA(60) toluene solution was mixed with a PS(5)-PMMA(5) toluene solution (mass ratio = 10 : 4) to promote the self-assembly process.²⁷ Then, the resulting mixture was spin-coated onto PETS-treated Si substrates to form a thin film having a thickness of 51 nm. The PS-PMMA thin films were then thermally annealed at 230 °C for 4 h in vacuum to induce self-assembly into perpendicularly ordered periodic nanostructures by microphase separation. The nanodomains of the PMMA block were selectively removed by UV irradiation (254 nm, 15 W) for 2 h, followed by rinsing with acetic acid and deionized water to produce copolymer nanotemplates. Subsequently, the substrates with the polymeric nanotemplate were treated with oxygen plasma performed at 80 W in 0.038 Torr for 3 s. The plasma-treated substrates were deposited with TTIP sol (TTIP 5.632 g + HCl 1.98 g dissolved in isopropanol 22 mL) by coating using a KSV dip-coater (dipping speed = 85 mm min⁻¹, dipping time = 30 s, and withdrawing speed = 10 mm min⁻¹). The substrates were fully dried by exposure to air. The dip-coating process inevitably formed over-coated TiO₂ layers on the copolymer nanotemplates, which were removed by reactive ion etching using CF₄. The substrates were then treated with oxygen plasma performed at 100 W in 0.038 Torr for 5 min to fully remove the copolymer nanotemplates, thus providing arrayed TiO₂ nanogrooves or nanodiscs on the substrate.

Fabrication of non-hexagonally arrayed Au nanoparticles

The PS-P4VP diblock copolymers were dissolved in 0.5 wt% toluene, a strongly selective solvent for the PS block. The solutions were stirred for 24 h at room temperature and for 3 h at 85 °C, and then cooled to room temperature. HAuCl₄, precursors of Au nanoparticles, respectively, were added to the micellar solution. The molar ratio of HAuCl₄ to the pyridine unit in the P4VP block was fixed at 0.5. The solution was stirred at least for 7 days at room temperature. As-prepared PS-P4VP micellar solutions were then spin-coated (typically at 4000 rpm, 60 s) onto the substrate with TiO₂ nanostructures. Substrates were treated with oxygen plasma at 100 W and 0.038 T for 5 min followed by calcination at 400 °C for 30 min in air to synthesize nanoparticles from the copolymer micelles. The spin-coating and oxygen plasma process could be repeated to create a binary array.

Characterization

Polymeric and TiO₂ nanotemplates, PS-P4VP micelles, and Au nanoparticles were analyzed using a Multimode 8 AFM with a Nanoscope V controller (Bruker) in tapping mode with Al-coated Si cantilevers and FE-SEM performed on a Hitachi S-4300 operating at 15.0 kV. DLS measurements were performed with DLS-7000 (Otsuka Electronics).

Results and discussion

The PS-P4VP spontaneously forms spherical micelles having PS corona and P4VP core blocks in toluene, which is a selective solvent for PS blocks;²⁰ moreover, their size can be controlled by the molecular weights of the copolymers. Fig. 1 shows the distributions of the hydrodynamic diameters of the copolymer micelles with different molecular weights, which were measured using DLS. The observed average hydrodynamic diameters of PS(32)-P4VP(13), PS(51)-P4VP(18), and PS(109)-P4VP(27) were 69.3 ± 22.1 nm, 108.7 ± 22.3 nm, and 178.8 ± 41.9 nm, respectively. These results showed that the PS-P4VP micelles with higher molecular weights had larger diameters.

Fig. 1 The intensity distribution of different PS-P4VP micelles in toluene obtained by DLS.

By tuning the spin-coating velocity and concentration of the micellar solution, the PS-P4VP micelles can be spin-coated onto substrates to form a single-layered micellar film with no aggregation and overlapping, which was confirmed by AFM and TEM (Fig. S1†). Fig. 2a shows the AFM image of the micellar film of PS(51)-P4VP(18) on a flat Si substrate. HAuCl₄, which is the precursor of Au nanoparticles, had already been loaded into the copolymer micellar cores prior to spin-coating. Therefore, we were able to synthesize arrayed Au nanoparticles from arrayed copolymer micelles by treating them with oxygen plasma to remove the copolymers.^21,22 Au nanoparticles were synthesized by the identical procedure in our previous publications,^23,24 in which we confirmed the synthesis of Au nanoparticles by a TEM image with characteristic lattice fringes, an electron-diffraction pattern indicating a face-centered cubic structure, and Au(4f) peaks verifying pure Au without oxide formation.

Fig. 2 (a) AFM image of a single-layered array of PS(51)-P4VP(18) micelles in quasi-hexagonal order. (b) FE-SEM image of the arrayed Au(51-18). Scale bars are 200 nm.

Hereafter, we denote Au nanoparticles synthesized from PS(x)-P4VP(y) micelles as Au(x-y). Fig. 2b shows the FE-SEM image of synthesized arrays of Au(51-18) while preserving the order of original PS(51)-P4VP(18) micelles. As expected, the interparticle distance of Au nanoparticles (∼54 nm) was similar to the spacing of PS-P4VP micelles. Note that the copolymer micelles were arranged in a quasi-hexagonal lateral order by spin-coating despite the absence of additional treatment for the organization. Therefore, single-layered films of PS-P4VP micelles have been widely utilized as effective templates for synthesizing hexagonally-ordered nanoparticles, as shown in Fig. 2b. Hence, to fabricate unusual arrays of the nanoparticles rather than the hexagonal one, we fabricated templates in which the copolymer micelles could be arranged in a different configuration using template-assisted self-assembly. Because the aforementioned copolymer micelles can be considered as nanometer-sized soft colloidal particles, the lateral dimension and depth of the template must be of the nanometer scale to result in effective template-assisted self-assembly.

Thus, thin films of diblock copolymers represent a great suitability as the material to create patterned templates for guiding the placement of nanomaterials because they can assemble into various ordered topographic nanostructures with a certain periodicity.^28–31 Moreover, the size and morphology of nanopatterns can be controlled by the molecular weight, volume ratio, and interaction parameter of copolymers. However, polymeric nanotemplates can be thoroughly eliminated during treatment with oxygen plasma for synthesizing Au nanoparticles from PS-P4VP micelles. Thus, we selected TiO₂ as the material of durable inorganic nanotemplates produced from thin films of self-assembled PS-PMMA (Fig. S2†). Because TiO₂ is a representative semiconducting material that is widely employed in photocatalytic and energy-harvesting devices,^32–34 we expected that the nanoscale TiO₂ templates demonstrated here could be utilized as functional nanostructures to interact with arrayed Au nanoparticles.³⁵ Note that detailed information on the fabrication of TiO₂ nanotemplates is provided in the ESI.†

We first spin-coated PS(51)-P4VP(18) micelles onto the nanogroove TiO₂ template (Fig. S2c†). Fig. 3a shows the AFM height mode image of PS(51)-P4VP(18) micelles, in which we can observe that the linearly-aligned arrays of the copolymer micelles were located between the TiO₂ nanogrooves. The alignment of the spherical micelles was more clearly represented in the AFM phase mode image (inset of Fig. 3a), in which the spherical micelles (bright spheres) were located between the TiO₂ nanogrooves (bright lines). The linear arrangement of the copolymer micelles, which was dramatically different from the hexagonally-ordered placement of the micelles spin-coated on a flat substrate (Fig. S1†), was attributed to the capillary force toward the nanogroove TiO₂ template during the spin-coating process. The rapid evaporation of the solvent by spin-coating generated the capillary force and induced the copolymer micelles to be eventually placed between the TiO₂ nanogrooves.

Fig. 3 (a) AFM height mode images of PS(51)-P4VP(18) micelles spin-coated on the nanogroove TiO₂ template. Inset is the magnified AFM phase mode image corresponding to the square-marked region. (b) FE-SEM image of the linear array of Au(51-18) in the nanogroove TiO₂ template. Inset is the magnified image corresponding to the square-marked region. Schematic illustrations are depicted on the corresponding micrographs. Scale bars are 200 nm.

After treatment with oxygen plasma, the linearly-aligned arrays of the copolymer micelles turned into the linear arrays of Au(51-18) between the TiO₂ nanogrooves (Fig. 3b). This arrangement was readily confirmed by the pseudo-color image of Au(51-18) (pink) to obtain a better visualization of the placement of the nanoparticles (inset of Fig. 3b). The spacing of linear Au(51-18) (∼43 nm) was identical to that of the original PS(51)-P4VP(18) micelles (∼43 nm). The linear alignment of Au(51-18) was similar to that of the original PS(51)-P4VP(18) micelles because the nanoparticles were synthesized at the region of micellar cores with no alteration in the original positions of the copolymer micelles. Thus, we confirmed that the template-assisted self-assembly of the copolymer micelles in the nanogroove TiO₂ nanotemplate resulted in the formation of non-hexagonal, linear arrays of Au nanoparticles. It is observed that the final nanostructures consisting of TiO₂ and Au in this work have a similar configuration in the work of Polleux et al.³⁶ They coated PS-P2VP copolymers containing precursors of TiO₂ onto patterns of Au nanoparticles which were synthesized by PS-P2VP micelles. In contrast, we first fabricated nanogroove and nanodisc templates of TiO₂ by PS-PMMA thin films and then synthesized Au nanoparticles into TiO₂ nanotemplates by PS-P4VP micelles. Thus, the synthetic approach and the final nanopatterns here are distinguishable.

Next, the PS(32)-P4VP(13) micelles were spin-coated onto the nanogroove TiO₂ template. As observed using DLS (Fig. 1) and AFM (Fig. S1†), because the size of the PS(32)-P4VP(13) micelles was smaller than that of PS(51)-P4VP(18), arrays of the PS(32)-P4VP(13) micelles were hardly observed in the AFM height mode image (Fig. 4a). However, the AFM phase mode image of the magnified region in Fig. 4a (see inset) clearly provided a better visualization of the zigzag chains of the copolymer micelles (bright small dots) between the TiO₂ nanogrooves, which were obviously different from the linear chains of PS(51)-P4VP(18) micelles (Fig. 3a). According to Xia et al.,^16,17 colloidal particles of which the diameter is less than the width of the linear trench but larger than the half thereof can form a zigzag chain assembly along the longitudinal direction of the trench. We measured and concluded that the average diameter of PS(32)-P4VP(13) micelles (∼36 nm) was appropriate to assemble into a zigzag chain structure in the trench. The different size of the copolymer micelles led to the change in internal structure among the micelles between the TiO₂ nanogrooves. Because of the smaller size, the PS(32)-P4VP(13) micelles were assembled into a zigzag alignment to maximize their occupancy of space between the TiO₂ nanogrooves. The final treatment with oxygen plasma produced another non-hexagonal, zigzag alignment of Au(32-13) from the PS(32)-P4VP(13) micelles between the TiO₂ nanogrooves (Fig. 4b). This is represented by the pseudo-color image of Au(32-13) (yellow) in the inset of Fig. 4b. The spacing of zigzag Au(32-13) (∼38 nm) was comparable to that of the original PS(32)-P4VP(13) micelles (∼36 nm), which indicates that the zigzag alignment of Au(32-13) was similar to that of the PS(32)-P4VP(13) micelles. In addition, we were able to adjust the lattice structure of the arrayed Au nanoparticles in the nanogroove TiO₂ template by varying the size of the copolymer micelles, which was determined by the molecular weights of the copolymers.

Fig. 4 (a) AFM height mode images of PS(32)-P4VP(13) micelles spin-coated on the nanogroove TiO₂ template. Inset is the magnified AFM phase mode image corresponding to the square-marked region. (b) FE-SEM image of the zigzag array of Au(32-13) in the nanogroove TiO₂ template. Inset is the magnified image corresponding to the square-marked region. Schematic illustrations are depicted on the corresponding micrographs. Scale bars are 200 nm.

Note that the diameters of the copolymer micelles that were used were comparable to or less than the spacing of the TiO₂ nanogrooves. We also note that the center-to-center distances of the micelles in the nanogroove (36 nm for PS(32)-P4VP(13), 43 nm for PS(51)-P4VP(18)) was smaller than those in a close packed array (46 nm for PS(32)-P4VP(13), 56 nm for PS(51)-P4VP(18), Fig. S1†), presumably because of the confinement effect of soft micelles of copolymers in hard nanogrooves of TiO₂. If the size of the copolymer micelles was much larger than the spacing, the special arrangements of micelles between the TiO₂ nanogrooves could not be produced due to the size exclusion effect. The AFM image in Fig. S4a† shows that only a few PS(109)-P4VP(27) micelles were located between the TiO₂ nanogrooves after spin-coating. Based on the broad size distribution of PS(109)-P4VP(27) micelles, most copolymer micelles with larger diameters were presumed to be scarcely placed between the TiO₂ nanogrooves and subsequently evicted from the substrate during the spin-coating process.

To prepare different arrays of the Au nanoparticles, the HAuCl₄-containing PS-P4VP micelles were spin-coated onto the nanodisc TiO₂ template (Fig. S2d and S3b†). Fig. 5a shows the AFM image of PS(51)-P4VP(18) micelles and the TiO₂ nanodiscs, which were indistinguishable from each other by size and height even in the magnified image with a higher z-scale contrast (inset of Fig. 5a). Thus, we were not able to obtain detailed information on the arrangement of the PS(51)-P4VP(18) micelles among the TiO₂ nanodiscs from this image. The substrate was then treated using oxygen plasma to synthesize the Au nanoparticles from the copolymer micelles. The FE-SEM image of Fig. 5b shows that the binary superstructures consist of arrayed TiO₂ nanodiscs and Au(51-18). To analyze the morphology of the arrayed Au nanoparticles, the pseudo-colors, solid circles, and dashed guiding lines were added to the magnified images (insets of Fig. 6b). The pseudo-color image clearly indicated that Au(51-18) was placed at the interstitial spaces of the TiO₂ nanodiscs to form the hexagonal array with an interparticle distance of ∼60 nm. This value, which is identical to the spacing of the TiO₂ nanodiscs (Fig. S3b†), was slightly higher than the spacing among Au(51-18) (∼54 nm) from the single-layered film of the PS(51)-P4VP(18) micelles spin-coated onto a flat Si substrate (Fig. 2a).

Fig. 5 (a) AFM height mode images of PS(51)-P4VP(18) micelles spin-coated on the nanodisc TiO₂ template. Inset is a magnified image (500 nm × 500 nm) with the modification of a height contrast scale. (b) FE-SEM image of the hexagonal array of Au(51-18) in the nanodisc TiO₂ template. Inset is the magnified image corresponding to the square marked region. Schematic illustrations are depicted on the corresponding micrographs. Scale bars are 200 nm.

Fig. 6 (a) AFM height mode images of PS(32)-P4VP(13) micelles spin-coated on the nanodisc TiO₂ template. Inset is a magnified image (500 nm × 500 nm) with the modification of a height contrast scale. (b) FE-SEM image of the hexagonal array of Au(32-13) in the nanodisc TiO₂ template. Inset is the magnified image corresponding to the square-marked region. Schematic illustrations are depicted on the corresponding micrographs. Scale bars are 200 nm.

However, the smaller PS(32)-P4VP(13) micelles was discerned from the TiO₂ nanodiscs by size and height in the AFM image (Fig. 6a). The magnified image (inset of Fig. 6a) shows that the spin-coated micelles were placed in every gap region between the two adjacent TiO₂ nanodiscs. These positions were subsequently occupied by the synthesized Au(32-13) in an unconventional array after treatment with oxygen plasma, as shown in the FE-SEM image in Fig. 6b. Joining the position of Au(32-13) with lines (see inset of Fig. 6b) completed a trihexagonal tiling composed of interlaced triangles whose lattice points had four neighboring points each; this structure is known as the Kagome lattice. In addition, the interparticle distance of Au(32-13) (∼33 nm) was nearly a half of the spacing of TiO₂ nanodiscs (∼67 nm), which can be typically found in the Kagome lattice. Thus, we were able to define Au(32-13) in the nanodisc TiO₂ template as Au(32-13) in the Kagome array.

Both the capillary force and the size exclusion effect should be considered together to understand the selective placement of the copolymer micelles among the TiO₂ nanodiscs. Because the strongest capillary force was oriented toward the midpoints of the two neighboring TiO₂ nanodiscs, the PS-P4VP micelles were generally pulled toward these gap regions. Thus, the smaller PS(32)-P4VP(13) micelles formed the Kagome array in the nanodisc TiO₂ template. However, the large size of PS(51)-P4VP(18) micelles did not allow them to be arranged at the gap between the adjacent TiO₂ nanodiscs; instead, they were arranged at the interstitial sites of the TiO₂ nanodiscs. Moreover, the size exclusion effect prevented the much larger PS(109)-P4VP(27) micelles from being placed among the TiO₂ nanodiscs (Fig. S4b†) as observed in the nanogroove TiO₂ template (Fig. S4a†). Only a few of the PS(109)-P4VP(27) micelles were observed to be placed at the interstitial sites. The number of TiO₂ nanodiscs and Au nanoparticles in the FE-SEM images were counted to obtain a better understanding of the two differently arrayed Au nanoparticles shown in Fig. 5b and 6b. Unlike the copolymer micelles observed using AFM, the arrayed Au nanoparticles in Fig. 5b and 6b were clearly distinguishable from the TiO₂ nanodiscs by the size and image contrast, enabling us to obtain their size distributions as shown in Fig. 7. The diameters of the TiO₂ nanodiscs and Au(51-18) calculated from the FE-SEM image in Fig. 5b were 34 ± 4 nm and 12 ± 3 nm, respectively, with a number ratio of 1 : 1.1. The statistical interference was ignored in this result because of the small overlap between the two size distributions. Provided there was a perfect arrangement of Au(51-18) only at interstitial sites of the TiO₂ nanodiscs, each nanodisc was surrounded by three Au(51-18) while each surrounding Au nanoparticle was shared by three adjacent nanodiscs. The theoretical number ratio of the TiO₂ nanodiscs to the Au nanoparticles would then be 1 : 1 (=3/3). This was similar to the calculated value (1 : 1.1) and indicated that the hexagonal array of Au(51-18) was placed at the interstitial sites of the nanodisc TiO₂ template. Moreover, this consideration could be employed for the Kagome array of Au(32-13) in the nanodisc TiO₂ template (Fig. 6b). The diameters of the TiO₂ nanodiscs and Au(32-13) in the Kagome array were 35 ± 3 nm and 8 ± 2 nm, respectively, with a number ratio of 1 : 2.7. Note that the size distributions (Fig. 7b) were clearly separated from each other. For the ideal case, the number of Au(32-13) organized around each nanodisc should be 6 with each Au(32-13) placed at the gap between the two neighboring nanodiscs. Thus, the number ratio of the nanodiscs to Au(32-13) in the perfect Kagome array should be 1 : 3 (=6/2), which is similar to our result of 1 : 2.7. Therefore, this observation indicated that most of Au(32-13) was regularly placed at the midpoints of two adjacent TiO₂ nanodiscs to form the Kagome array.

Fig. 7 The size distribution of the TiO₂ nanodiscs (gray) and (a) Au(51-18) (pink) and (b) Au(32-13) (yellow) in Fig. 5b and 6b, respectively.

The Au nanoparticles having different sizes were selectively placed at certain positions, which allowed us to prepare binary arrays comprising Au(51-18) in a hexagonal arrangement and Au(32-13) with the Kagome lattice structure in the nanodisc TiO₂ template (Fig. 8). To fabricate this nanostructure, PS(51)-P4VP(18) micelles were spin-coated on the TiO₂ nanodiscs with the Kagome arrays of Au(32-13) (Fig. 6b), followed by treatment with oxygen plasma to generate the hexagonal arrays of Au(51-18). The placement of each nanoparticle is shown in the inset of Fig. 8 with different pseudo-colors. Interestingly, when PS(32)-P4VP(13) micelles were spin-coated on the TiO₂ nanodiscs with the hexagonal arrays of Au(51-18) (Fig. 6a), identical binary arrays were hardly realized (not shown here). This indicated that PS(51)-P4VP(18) micelles were spin-coated onto the TiO₂ nanodiscs as if they did not interact with the small Au(32-13) in the Kagome array; however, the arrangement of PS(32)-P4VP(13) micelles was significantly interrupted by the pre-placed large Au(51-18) in a hexagonal array. In other words, the nanoparticles contributed to the templating effect for the subsequent placement of the copolymer micelles in a manner similar to the TiO₂ nanostructures despite their low height.

Fig. 8 FE-SEM image of Au(51-18) in a hexagonal array and Au(32-13) in the Kagome array in the nanodisc TiO₂ template. Inset is a magnified image of the square-marked region with pseudo-colors for Au nanoparticles: pink for Au(51-18) and yellow for Au(32-13). Scale bar is 200 nm.

Conclusions

Various non-hexagonal arrays of Au nanoparticles were synthesized from self-assembled PS-P4VP micelles in the TiO₂ nanotemplates. The nanotemplates effectively guided the placements of the PS-P4VP micelles to have unconventional organizations in the template, which were different from a quasi-hexagonal order in the thin films of PS-P4VP micelles spin-coated on a flat substrate. The self-organization of the micelles strongly depended on the size of the micelles and the geometrical shape of the templates. These factors were effectively controlled by the molecular weights of the diblock copolymers. We subsequently treated the self-organized PS-P4VP micelles in the templates with oxygen plasma, which led to the synthesis of the unconventional linear, zigzag, and Kagome arrays of the Au nanoparticles in the templates. Moreover, we demonstrated that this methodology could be extended to yield a binary superlattice comprising two different Au nanoparticle arrays by repeating the nanoparticle synthesis process in the TiO₂ nanotemplate.

We believe that the template-assisted self-assembly of the diblock copolymer micelles suggested in this study can be applied to various templates with complex nanopatterns to induce more complex, non-hexagonal arrays of nanoparticles. Because the chemical composition of nanoparticles synthesized from the copolymer micelles is governed by the precursors loaded into the micellar cores, we expect that the non-hexagonal arrays of various nanoparticles other than Au nanoparticles can be easily prepared in the templates. Moreover, template-guided arrays of Au nanoparticles can be used for manipulating the photoactivity of the TiO₂ nanotemplates, which can be applied in optical and energy devices in further studies.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014R1A2A2A01002290).

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Footnote

† Electronic supplementary information (ESI) available: AFM images of PS-P4VP micelles spin-coated onto a flat SiO₂ substrate; detailed description on the fabrication of the PS and TiO₂ nanotemplates with AFM and FE-SEM images; AFM images of PS(109)-P4VP(27) micelles spin-coated onto the nanogroove and nanodisc TiO₂ templates. See DOI: 10.1039/c6ra05530c

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