Article pubs.acs.org/Langmuir
Fabrication and Functionalization of Periodically Aligned Metallic Nanocup Arrays Using Colloidal Lithography with a Sinusoidally Wrinkled Substrate Hiroshi Endo,* Yoshiyuki Mochizuki, Masahiro Tamura, and Takeshi Kawai* Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan S Supporting Information *
ABSTRACT: We propose a general strategy for fabricating ultrasmall attoliter-sized (10−18 L) one-dimensional (1D) aligned nanocup arrays embedded in poly(dimethylsiloxane) (PDMS) films based on a combination of colloidal softlithography and wrinkle processing. The nanocup consists of a metallic shell (silver-single or double-layer silver/gold type) with a thickness of several tens of nanometers and whose diameter was ca. 500 nm and cavity depth was ca. 250 nm. First, monodisperse polystyrene (PS) colloids (d = 500 nm) were arranged onto a sinusoidally wrinkled PDMS substrate. Then, the colloid particle arrays were transferred onto another flat PDMS substrate, and a metal film was vacuum deposited over the array to form a nanostructured surface consisting of half-shell metal-coated colloid particle arrays. After the metal-coated PS array was gently transferred onto another soft PDMS substrate prepared by nonthermal curing, the attached films were thermally cured. After that, both films were carefully separated to selectively transfer the metal-coated PS particle arrays, since the metallic shell on the PS surface can adhere to the soft PDMS. Finally, the PS colloids were removed by plasma etching, leaving behind the 1D hemispherical metallic shells, called here the “metallic nanocup array structure”. This structure was evaluated by performing atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy measurements. We further demonstrate chemical modification of the inner nanocup surface through construction of a self-assembled monolayer, and we also fill them with nanomaterials (silica nanoparticles) to demonstrate their application to size-selecting devices. The obtained metallic nanocup arrays could be components in a new class of chemical and/or biological nanoreactors with small reaction vessels, surface-enhanced Raman scattering (SERS)-based sensors, and size separators for nanoparticles.
1. INTRODUCTION Periodic nanoscale objects with finite sizes and shapes have attracted significant amounts of attention owing to their potential for use in the areas of photonic crystals,1−3 data storage,4 lithium ion microbatteries,5,6 antireflective surfaces,7−9 surface wettability,10,11 plasmonic analytical sensors,12,13 etc. One promising approach for constructing periodically welldefined and ordered nanostructures is the colloidal crystal template approach, which provides low-cost, simple control of the thickness and fabrication scale, high quality, and large-scale production. Utilizing self-assembled spherical colloids (usually polystyrene (PS) or silica particles) as a template, various nanostructures have been fabricated, such as nanorings,14 nanorods,15 nanohoneycombs,16 and “semishell” (or “halfshell”) nanostructures with reduced symmetry including nanostrings,17,18 nanocaps,19,20 nanocup (or nanobowl)21−37 arrays, and other nanostructures.38 Among them, the “semishell” nanocups (or nanobowls) with cavities possess useful attributes such as antisymmetric properties (Janus-type) induced by chemical modification of the inner or outer shell or the cup structure itself, an increase in sharp edges (rims) on the surfaces of the particles, and a high ratio of the surface area to volume. Thus far, various nanocups have been prepared with a number of interesting properties and functionalities. © 2013 American Chemical Society
Previously, we have succeeded in fabricating one-dimensional (1D) strings of gold half-shells by depositing a gold film onto a PS particle monolayer that was tilted at an angle between the normal of the monolayer plane and the direction of metal deposition.17,18 On the other hand, Halas and co-workers have developed nanocaps and nanocups prepared from silica nanoparticles with gold shells with interesting optical properties derived from their specific orientation with respect to the angle of light incidence and polarization.21−23 Wang and co-workers have demonstrated ordered arrays of TiO2 nanobowls fabricated using PS sphere templating and atomic layer deposition, which were found to be useful for the size selection of submicrometer spheres and as reusable masks for the fabrication of nanodot patterns.26 Fujii and co-workers have prepared Janus-type 2D colloidal crystal films and femtoliter cup arrays from polypyrrole (PPy)−PS composites using an air−water interface.30 Whitesides and co-workers prepared superhydrophobic surfaces from aggregated structures composed of gold nanocaps covered with alkanethiol.31 Moreover, Yasuda and co-workers have demonstrated DNA hybridization Received: September 5, 2013 Revised: November 11, 2013 Published: November 12, 2013 15058
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Figure 1. Schematic illustration of the fabrication procedure for the 1D metallic nanocup array. After the wrinkled PDMS surface is prepared (a), PS colloids are assembled into the grooves by spin-coating (b). After subsequent transfer of the colloid particle arrays onto another flat PDMS substrate (c) and vacuum deposition of the metal film (d), the metal-coated colloid particle arrays are retransferred onto another adhesive soft PDMS substrate (e). After removal of PS colloids by plasma etching, 1D metallic nanocup arrays are left behind (e).
film and the substrate. The obtained wrinkle wavelengths are well-defined and can be used not only for organization of colloidal particles40,41,48,49 but also to manipulate the material properties such as the surface wettability,50 liquid-crystal state,51 optical properties,52 etc. Here, we report a straightforward procedure to fabricate ultrasmall attoliter-sized (10−18 L) 1D metallic nanocup arrays embedded in PDMS films by colloidal soft-lithography and wrinkle processing. Through orderly arrangement of PS colloids on a wrinkled substrate, metal deposition, and a transfer process, we can obtain nanocup arrays after removing the PS colloids by plasma etching. Moreover, we demonstrate chemical modification of the inner nanocup surface through the construction of a self-assembled monolayer as well as the filling of nanomaterial (silica nanoparticles) into the nanocup.
in Au/Ni double-layer nanocups as small reaction vessels for biomedical devices.33 In addition, other metallic nanobowl structures such as Co,28 Pd,31 Pt,31,35,36 Pt/Ag,36 and Ni37 nanobowls and also have been fabricated. However, these nanocup structures were made mostly built on rigid/stiff substrates, so many useful applications could not be realized, and thus far they have only been arranged into hexagonal closepacked or randomly aggregated states. In contrast, well-aligned and individual 1D nanocup arrays in flexible poly(dimethylsiloxane) (PDMS) films could be used as ultrasmall vessels to develop a new class of lab-on-a-chip tools, since they would enable the development of linear microfluidics systems, which have been recognized for their exquisite performance in rapid, sensitive, and high-throughput analyses, their low demand for analytes, and their low cost.39 Moreover, nanocups composed of metallic (e.g., gold or silver) shells could be chemically modified in various ways through the construction of self-assembled monolayers, and plasmon coupling could be induced, depending on the nature of the surface. To the best of our knowledge, an efficient methodology for preparing high-quality fine regular 1D metallic nanocup arrays on flexible substrates has not been reported yet. One of the most successful approaches to the fabrication of a 1D nanostructure from a 2D structure such as a colloidal particle monolayer is the use of a wrinkle-patterned surface with sinusoidal grooves on a soft substrate. 40,41 Wrinkling phenomena are based on specific mechanical instabilities, the so-called buckling instabilities of thin films.42,43 Typically, wrinkles can be formed by bilayer systems, where a thin hard film coated on top of an expanded soft substrate (e.g., PDMS) undergoes a compressive force. The buckling instability is a result of the balance between the energy required to bend the hard upper film and the energy required to deform the soft underlying substrate. The hard layer on the soft substrate can be prepared using various methods, such as metal deposition,44 plasma oxidation,45 ultraviolet/ozone treatment,46 polymer coating,47 etc. The wavelength of the wrinkling pattern depends on the thickness of the thin hard film and the mechanical properties (Young’s modulus and Poisson’s ratio) of the thin
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(dimethylsiloxane) (PDMS) prepolymer (Sylgard 184) and a curing agent were purchased from Toray Dow Corning, Japan. 4-Mercaptopyridine (4-MP) was purchased from TCI, Japan. Silica nanoparticles with mean diameters of 100, 200, and 300 nm were purchased from Nissan Chemical, Japan. 2.2. Synthesis of Polystyrene (PS) Particles. Styrene (Kanto Chemical) was purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent-grade potassium persulfate (KPS) was used as an initiator without further purification. Monodisperse PS particles with a mean diameter of 500 nm were synthesized by emulsifier-free emulsion polymerization with KPS as the initiator in water.11,53 The styrene monomer (15 g) and 180 mL of water were added to a 300 mL glass reactor, and the temperature was raised to 75 °C under vigorous stirring. An aqueous solution (20 mL) of KPS (0.35 g) was added to the reactor. 2.3. Preparation of Wrinkles. PDMS was prepared by mixing the monomer with a base in a weight ratio of 10:1. The mixture was poured into a clean planar Petri dish, and the 1 mm thick film was cured for 2 h at 65 °C after degassing overnight under ambient conditions. The cross-linked PDMS was cut in to substrates with dimensions of 1 cm × 2 cm. To induce the surface pattern, the substrate was clamped in a custom-made stretching apparatus (Figure S1) and uniaxially stretched to a length of 15% longer than its original length. The PDMS was subsequently oxidized in this prestrained state 15059
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Figure 2. SEM images after each fabrication stage. (a) 1D PS colloid particle arrays on a wrinkled surface and (b) the arrays after the first transfer onto flat PDMS. (c) 1D PS colloid particle arrays after deposition of an Ag film of 40 nm in thickness. (d) Template silver-coated surface after removal of PS colloid particle array and (e) the array after the second transfer onto soft PDMS. by plasma exposure for 5 min at 3 W (PIB-20 plasma ion bombarder, Vacuum Device, Japan), and periodic wrinkles were formed perpendicular to the stretching direction once the strain was released. 2.4. Preparation of 1D Metallic Nanocup Arrays. PS particles were assembled in the grooves of the wrinkles by spin-coating. To ensure a hydrophilic surface, square wrinkled substrates cut to dimensions of 1 cm × 1 cm were activated by 3 min of plasma treatment. Immediately afterward, a PS aqueous suspension (20 μL, 4.7 wt %) was spin-coated onto the wrinkles (2000 rpm, 30 s). Subsequently, the filled wrinkles were brought into contact with another flat PDMS film covered with an ethanol droplet. No external pressure was applied. The flat PDMS and wrinkles stayed in contact until the ethanol was evaporated. Then, the wrinkled PDMS film was carefully lifted off to transfer the PS colloidal particles. Next, a metal (gold and/or silver) film with a controllable thickness was vacuum deposited over the particles (SVC-700TM SG, Sanyu, Japan) at a rate of 0.1 nm/s. After the partially metal-coated particle film was placed onto another adhesive PDMS substrate prepared by nonthermal curing, the attached films were cured for 1 h at 65 °C. Then, both films were carefully separated to transfer the metal-coated PS particles, since the metallic shell side on the PS surface can adhere to the soft PDMS. Finally, the transferred PS colloids were removed by plasma etching for 20 min at 22 W. 2.5. Characterization. Wrinkled or particle-decorated substrates were characterized by atomic force microscopy (AFM: S-image, SII, Japan) in tapping mode. Scanning electron microscope (SEM) measurements were performed with an S-5000 (Hitachi, Japan), for which the samples were sputtered with Pt/Pd (3−5 nm) by ion sputtering (E1030, Hitachi, Japan). X-ray photoelectron spectra (XPS) were measured with an XP spectrometer (JPS-9010MC, JEOL, Japan). Surface-enhanced Raman scattering (SERS) measurements were performed with an NRS-3200 Raman microscope (Jasco, Japan). The excitation line used was the 785 nm line of a He−Ne laser diode. The laser power at the microscope entrance was 0.4 mW. Raman signals were collected through a confocal pinhole with a diameter of 100 μm using a ×100 objective lens. In all cases, the integration time was 10 s/spectrum, and the data were accumulated over two observations.
return to the original state. Subsequently, the colloid particle arrays were transferred onto another flat PDMS substrate (Figure 1b → 1c), and a metal (gold and/or silver) film with a controllable thickness was vacuum deposited over the array to form a nanostructured surface consisting of half-shell (Janustype) metal-coated colloid particle arrays (Figure 1c → 1d). After the metal-coated PS particle array was gently placed onto another adhesive soft PDMS substrate prepared by nonthermal curing, the attached films were thermally cured. After that, both films were carefully separated to selectively transfer the metalcoated PS particle arrays, since the metallic shell side on PS surface can adhere to the soft PDMS (Figure 1d → 1e). Finally, PS colloids were removed by plasma etching, leaving behind the array of 1D hemispherical metallic shells called here the “metallic nanocup array structure” (Figure 1e → 1f). The SEM image in Figure 2a is a typical result of the spincoating assembly of PS colloids fabricated for 30 s of rotation at 2000 rpm. The PS colloids were well ordered along the wrinkle grooves, yielding densely packed 1D colloid particle arrays on almost the whole surface. The ordering of the colloid particle arrays was maintained even after they were transferred to flat PDMS (Figure 2b) and after deposition of 40 nm thick Ag (Figure 2c). Figure 2d shows the template surface after the second transfer process (Figure 1e), where dark and light regions correspond to the voids formed by the removed PS colloid arrays and the mold of the deposited Ag film, respectively. This result reveals that half-shell Ag-coated colloid arrays only were precisely peeled from the template substrate. In addition, the peeled 1D colloid arrays were confirmed after transfer to a soft PDMS surface (Figure 2e). As can be observed, the surfaces of the particles were smooth, indicating that the Ag-shell side on the PS surface adhered to the soft PDMS. Moreover, slight separation between neighboring particles was observed, even though the 1D colloid particle array maintained a densely packed state on the flat PDMS prepared by thermal curing in the first transfer process. This might be caused by penetration of the soft PDMS polymer between the particles, leading to partial embedding of the PS colloids into the PDMS film. The adhesion strength (stickiness) of the soft PDMS surface and the deposited metal thickness are the main important parameters for achieving selective detachment of metal-coated colloid particle arrays. Before it was attached to the metalcoated colloid particle array, the soft PDMS substrate was cured (cross-linked) under ambient room temperature for 12 h;
3. RESULTS AND DISCUSSION The procedure for fabricating 1D metallic nanocup arrays is illustrated in Figure 1. First, monodisperse PS colloids (d = 500 nm) were arranged onto a wrinkled PDMS substrate (wrinkle wavelength and amplitude are 1.30 and 0.22 μm, respectively; Figure S2) by spin-coating (Figure 1a → 1b). This wrinkle substrate was prepared as follows, in three well-known steps:45 (1) uniaxial stretching of PDMS (stretching rate: 15%), (2) plasma treatment (5 min) to form a silica-like SiOx hard layer,54 and (3) releasing of the applied stress to allow the PDMS to 15060
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(depending on the cross-linking density) are necessary to achieve selective detachment, although it is difficult to clearly quantify the relationship between the microscopic adhesion force (or adhesion work) between the PDMS and the deposited Ag film and the effect of the colloid surface (geometric influence) at the present time. After the second transfer, the AFM height profile showed that the obtained colloid particle arrays were halfway (ca. 250 nm) embedded in the PDMS film (Figure 4a). To remove the
typical thermal curing was only done subsequently. When softer PDMS cured for only 5 h was used, the deposited Ag film and the PS colloids were uniformly peeled because of the strong adhesion and infiltration of the PDMS polymer into the colloid particle arrays (Figure 3a). Moreover, selective transfer
Figure 3. SEM images of various PDMS films after the second transfer process. (a) Colloidal particle array coated with 40 nm thick Ag shells transferred onto soft PDMS cured for 5 h. (b) Template surface after detachment of the colloid particle array coated with 40 nm thick Ag shells using PDMS prepared by thermal curing. (c) Colloid particle array coated with 80 nm thick Ag shells transferred onto soft PDMS cured for 12 h. (d) Template surface after detachment of the colloidal particle array coated with Ag/Au shells using soft PDMS cured for 12 h. Figure 4. AFM and SEM images before and after removal of PS colloids embedded in the PDMS film. AFM height profiles (a) before and (b) after 20 min of plasma etching to remove the PS colloid. (c) SEM images of Ag nanocup array after 5 min of plasma etching. (d) SEM and (e) 3D AFM images (scanning area 5 μm × 5 μm) of the obtained Ag nanocup array after 20 min of plasma etching. (f, g) SEM images of obtained Ag/Au bimetallic nanocup array after 20 min of plasma etching.
did not proceed well using (hard) PDMS cured for 24 h or using PDMS prepared by typical thermal curing (2 h at 65 °C) (Figure 3b). In addition, non-metal-coated colloid particle arrays were also selectively transferred in the same condition. The previously determined adhesion pull-off force or energy55,56 was lower for the soft PDMS materials (less cross-linking), so this transfer behavior can be mainly attributed to differences in the degree of cross-linking (lower cross-linking density induces higher chain lengths between chemical nodes for soft samples), quantity of free chains (unlinked to the network), and number of pendent chains (linked to the network by only one extremity). When hard PDMS with a high cross-linking density is used instead of soft PDMS, the higher surface hydrophobicity due to the CH3 groups might affect the detachment. This surface hydrophobicity will be recovered during the heating step when the PDMS is in contact with metal-coated PS film, which can weaken the interface by reducing of surface energy of the PDMS,57 although we further speculate that the heating step will facilitate the rigid support of the colloid particle arrays. For Ag shells of over 70 nm in thickness, the colloid particle arrays could not be selectively transferred. For instance, the 80 nm thick deposited film was entirely transferred including the PS colloids, as when softer PDMS was used (Figure 3c). This might be due to the bridging of the gap between the bottom side of the PS colloids and the attached PDMS surface by the thick deposited film. In contrast, a colloid particle array coated with a bimetallic, layered Ag (second deposition, 20 nm thickness)/Au (first deposition, 20 nm thickness) shell (Figure 1d) could be selectively detached (Figure 3d). Therefore, an appropriate metal-layer thickness and stickiness of the PDMS
embedded PS colloids and generate metal nanocups, we used three methods: dissolution, calcination, and plasma etching. After immersion of the embedded substrate into chloroform, partial removal of the PS colloids (immersion time: 5 s) or swelling damage of PDMS (immersion time: 30 s) was confirmed (Figure S3). Calcination at 300 °C for 90 min did not completely remove the PS colloids, and the infiltrating PDMS separating the cups was collapsed (Figure S4). Moreover, cracks were generated on the PDMS surface after increased baking times. Therefore, neither of these methods is suitable for removing the PS colloids. On the other hand, plasma etching removes material uniformly from all sides, so that the overall particle size is reduced while each particle keeps its original position.49,58 Therefore, the PS colloids were actually gradually reduced in size as the plasma etching time progressed. After 5 min etching, the diameter of the PS particles was decreased from 500 nm to about 300 nm (Figure 4c). The total difference (500 nm) between the initial height before removal of the PS colloid (Figure 4a) and the hemispherical cavity depth obtained after 20 min of etching (Figure 4b) matched the initial diameter of the PS colloid well. Therefore, the embedded PS colloids were perfectly removed from the PDMS film, yielding 1D Ag nanocup arrays (Figure 15061
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4d,e) while maintaining the order and separation between the nanostructures (1.30 μm in Figure 4b). High-quality bimetallic layered Ag/Au nanocup arrays were also constructed (Figure 4f,g). The projections around cup rim (circle areas in Figure 4d,f) might be caused by upheaval of the soft PDMS when the PS colloid is embedded into the film. In our procedure, two transfer processes were performed to obtain the nanocup arrays. If the metal film is deposited on the 1D colloid particle arrays on the wrinkled substrate (Figure 1b → d) instead of after the transfer to flat PDMS, the deposited film and the wrinkled structure would affect the subsequent transfer process and final morphology (Figure S5). To further confirm the complete removal of PS colloids, we performed X-ray photoelectron spectroscopy (XPS) analysis for the two types of metal nanocup arrays (Ag and double-layer Ag/Au nanocups) before and after removal of the PS colloids. A proper metal surface was clearly observed on the interior of the nanocup after removal of the PS colloids. The XPS peaks appearing from the Ag nanocup are attributed to Ag 3d5/2 (368.2 eV) and Ag 3d3/2 (374.3 eV) (Figure 5a), and the peaks
Figure 6. SERS spectra of immobilized 4-MP from (a) the Ag/Au nanocup inner-surface and (b) another PDMS surface area.
cm−1 is known to be very sensitive to the state of the thiol group and is often used as a key band to check for the formation of an Au−S bond because the band is coupled with the C−S stretching mode.59 The appearance of the band at 1097 cm−1 is a characteristic of 4-MP adsorbed on an Au surface via sulfur atoms, whereas the peak position of the band for bulk 4-MP is 1105 cm−1.59 On the other hand, no such band appeared for other PDMS areas on the same substrate (Figure 6b). Therefore, selective and high-quality chemical modification of the nanocup interior was achieved. This regular chemically modifiable nanocup array and its reproducible fabrication will effectively contribute to high-throughput screening, SERSbased microanalysis, and catalytic reactions (combined with the metal surface) as a chemical and/or biological nanoreactor, especially as compared to randomly oriented and/or aggregated nanocup arrays.31,33 As second application, different-sized silica nanoparticles (d = 100, 200, and 300 nm) were filled into the discrete nanocup cavity. After a nanoparticle suspension (1 wt %, 20 μL) was drop-cast onto the nanocup substrate and the substrate was dried for 12 h, adhesive soft PDMS substrate prepared by nonthermal curing was gently placed onto them. Immediately afterward (few seconds), the PDMS substrate was carefully lifted off several times (stamping process) to remove the randomly aggregated nanoparticles except those inside the nanocups (Figure S6).60 This methodology would not work for a nanocup array protruding from the PDMS surface, unlike our embedded nanocup array structure in the planar PDMS surface, because it would be difficult to completely remove the accumulated nanoparticles between the nanocup arrays. Roughly, the number of 20−25 100 nm nanoparticles fit into each nanocup (Figure 7a) based on the cup volume, whereas two of the larger 200 nm particles filled most nanocups in our experiment (Figure 7b,d). Finally, the largest 300 nm particles could fit in the nanocups only one at a time (Figure 7c). Therefore, the nanocups can be used for the size selection of nanoparticles.
Figure 5. XPS spectra of (a) Ag nanocup and (b) Ag/Au nanocup surfaces before and after removal of the PS colloid.
appearing from the Ag/Au nanocup (exposed surface: Au) are attributed to Au 4f7/2 (83.8 eV) and Au 4f5/2 (88.5 eV) (Figure 5b). Therefore, complete removal of PS colloids and exposure of the metal surface was demonstrated. It was observed that the obtained nanocup arrays were embedded in the PDMS film to a depth equal to the cup height. The volume of a single nanocup was calculated to be about 32.7 aL. To demonstrate the usefulness and potential applications of such hemispherical nanocup arrays, two types of experiments were carried out: (1) chemical functionalization (modification) of the nanocup surface and (2) filling of nanomaterial into the Ag/Au nanocup cavity. In experiment 1, a 4-MP self-assembled monolayer was added when the nanocup-arrayed substrate was dipped into an aqueous solution of 4-MP (1 mM) for 20 h. After the array was rinsed with water and N2 gas flow, the modification of inner nanocup was evaluated in terms of its surface-enhanced Raman scattering (SERS) spectrum. Characteristic bands assigned to 4-MP were obviously detected from the nanocup area (Figure 6a). The bands at 1213 and 1061 cm−1 are assigned to the C−H in-plane bending modes of the pyridine ring. Two strong bands at 1097 and 1004 cm−1 are assigned to the ring-breathing modes. The former band at 1097
4. CONCLUSION In summary, we succeeded in fabricating 1D metallic (Ag or Ag/Au bimetallic) hemispherical nanocup arrays with shell thicknesses of several tens of nanometers embedded orderly in a flexible PDMS film using a bottom-up colloidal lithography process and wrinkled structures. The individual nanocups, whose diameters were ca. 500 nm and cavity depths were ca. 250 nm, were calculated to be attoliter-sized (10−18 L), which is quite small and could be useful for a new class of lab-on-a-chip 15062
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tools. An appropriate thickness of the metal shell and stickiness of the PDMS were important to produce highly ordered and large-area 1D metallic arrays using a process of transferring the half-shell metal-coated PS colloid particle arrays. Moreover, plasma etching effectively removed the PS colloids to generate the final the metal nanocups. To demonstrate usefulness and application of the nanocup arrays, we chemically modified the inner nanocup surface through the construction of a selfassembled monolayer and filled the nanocups with nanomaterial (silica nanoparticle). The obtained metallic nanocup arrays could lead to the development of a new class of chemical and/or biological nanoreactors with small reaction vessels, surface-enhanced Raman scattering (SERS)-based sensors, and size separators for nanoparticles. In the future, it will be possible to produce other interesting cup array structures, such as herringbone arrays, using designable wrinkle patterning.
ASSOCIATED CONTENT
* Supporting Information S
Photograph of stretching apparatus, AFM and SEM images of wrinkled structure and embedded PS colloids surface after dissolution and calcination, and schematic procedure for filling silica nanoparticles into the nanocup array. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 7. SEM images after introduction of silica nanoparticles of (a) 100, (b) 200, and (c) 300 nm in diameter within the Ag/Au nanocup. (d) Number distribution of 200 nm silica nanoparticles within each nanocup.
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AUTHOR INFORMATION
Corresponding Authors
*Tel +81-3-5228-8312; Fax +81-5261-4631; e-mail endo@ci. kagu.tus.ac.jp (H.E.). *Tel +81-3-5228-8312; Fax +81-5261-4631; e-mail kawai@ci. kagu.tus.ac.jp (T.K.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by Adaptable and Seamless Technology Transfer Program through Target-driven R&D (ASTEP) from JST, and JGC-S Scholarship Foundation (Nikki Saneyoshi). 15063
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dx.doi.org/10.1021/la403431n | Langmuir 2013, 29, 15058−15064
