Bottom-Up Approach to Creating Three-Dimensional Nanoring Arrays

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Letter

A Bottom-up Approach for Creating Three-Dimensional Nanoring Arrays Comprised of Au Nanoparticles Hiroshi Yabu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la305028t • Publication Date (Web): 11 Jan 2013 Downloaded from http://pubs.acs.org on January 14, 2013

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A Bottom-up Approach for Creating ThreeDimensional Nanoring Arrays Comprised of Au Nanoparticles Hiroshi Yabu1,2* 1

Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University,

2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan 2

Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and

Technology Agency (JST), 4-3-1, Kawaguchi, Saitama, 332-0014 Japan KEYWORDS Nanorings, Bottom-up, Colloidal Crystals, Capillary forces, Metamaterials

A bottom-up approach to creating three-dimensional assemblies of Au nanorings by drying of aqueous dispersions of PS colloidal particles and Au NPs is shown. The evaporation of water from the dispersion allowed for the formation of hexagonally assembled colloidal crystals and nanorings composed of Au nanoparticles among the PS colloidal particles. The size of the nanorings could be controlled on a scale of tens to hundreds of nanometers. After sintering, the Au NPs formed Au nanorings. This simple approach supplies a potentially useful path to novel plasmonic materials and unique metamaterials for the visible light region.

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1. Introduction Nanoscale metal structures are of great interest due to their potential use in plasmonic devices and metamaterials1. Various sub-wavelength nanoscale features2, including nanoring arrays3, have been constructed by top-down microfabrication techniques with the purpose of creating efficient light-material interactions in the visible light region. Recent investigations of nanoring arrays fabricated on a solid substrate reveal unique absorption of the ring arrays in the visible to near-infrared region, as well as strong electromagnetic field enhancement effects due to the electronic, magnetic, and fano-like resonances among nanorings4,5 and due to the metal nanoring itself6,7. Split-ring resonator (SSR) arrays are promising resonator-type metamaterials that have negative-refractive indices8. To realize electromagnetic responses in higher frequency domains, that is, shorter wavelengths than visible light, metallic resonator patterns including SSRs smaller than 100 nm have been fabricated on solid substrates by using top-down microfabrication techniques9. Conventional photolithography, however, provides only twodimensional surface patterns that are typically restricted by the diffraction limit of light used. Although electron beam lithography allows the creation of features and pseudo-threedimensional structures with resolution of one to tens of nanometers by piling up the patterned films, it requires time-consuming, multi-step processes and is applicable to only limited surface areas10. These restrictions have prompted scientists to investigate alternative methods of fabricating three-dimensional, ordered nanoring arrays on large surface areas. The use of colloidal particles as templates in a bottom-up approach to creating nanoring structures has been reported. Chemical vapor deposition (CVD) and physical vapor deposition (PVD) of metals and metal oxides through colloidal assemblies of particles have been used to form two-dimensional ring arrays11. Xia and co-workers have developed a simple method for


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fabricating two-dimensional metal nanoring arrays by edge-spreading lithography using colloidal particles and an etching process12. Goedel and co-workers have reported sub-micrometer gold rings, which were fabricated by imprinting colloidal particles in polymer films and filling the interior of the imprinted surfaces with the metal ions13. Cremer and co-workers have prepared double-ring structures by templating water stains for nanolithography14. Yang and co-workers have reported on the fabrication of ring arrays by embossing spherical particle assemblies onto polymer films15. Kim and co-workers have reported accumulation of graphene-oxide nanoparticles or polystyrene grafted carbon nanotubes (CNTs) around colloidal-sized water droplets16,17. Chen and co-workers show that Au or CNT nanoring formation in block-copolymer micelles18,19. Additionally, patterned inorganic nanoparticle assemblies have been fabricated on solid substrates through the capillary condensation of nanoparticle suspensions20-22. We have found that various micropatterns, including a dot pattern and a line-and-space pattern, can be simply fabricated by first sandwiching a polymer and gold nanoparticle suspension between two parallel substrates23. The gold nanoparticles concentrate at the three-phase contact line of the suspension supplied at the edge of the upper substrate. Then the gold nanoparticles are deposited at the meniscus of the solution when the concentration of the solution at the edge exceeds their solubility24. After the suspension recedes, the concentration gradually increases again, and deposition of materials intermittently occurs, and finally, periodic micropatterns form. Since the gold nanoparticles precipitate at the three-phase line of suspension supplied from the edge of the substrate, the prepared dots and lines are well aligned along to the meniscus edge. By using this capillary condensation mechanism, Lin and co-workers used spherical templates to fabricate various concentric ring patterns from functional materials on the micrometer scale25.


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Recently, Savinova and co-workers have reported two-dimensional gold nanoring arrays fabricated by a colloidal templating technique that uses the reduction of gold ions patterned beneath polymer particles26. Chen et al. reported on the fabrication of nanoring arrays of CdSe nanoparticles on a solid substrate. They used a co-assembly of polystyrene (PS) colloidal particles and CdSe nanoparticles, and simple evaporation of solvent and removal of PS colloidal particles by peeling with an adhesive tape27. These reports show the successful fabrication of two-dimensional nanoring structures on solid substrates, but there are few reports on threedimensional nanoring assemblies. In this paper we report the application of our capillary condensation mechanism to create three-dimensional metallic nanoring arrays. PS colloidal particles and gold nanoparticles were co-assembled on solid substrates from suspensions to form gold nanoparticle rings between PS colloidal particles. Preparation of these gold nanoring-polymer composite films and control of their dimensions are discussed. 2. Experimental Figure 1 illustrates the formation of the co-assembly of PS colloidal particles and gold nanoparticles. Monodisperse colloidal PS particles (diameter: 100 nm to 2 µm) were purchased from Duke Scientific Ltd. An aqueous dispersion of citrate-stabilized gold nanoparticles (Au NPs; diameter: ~5 nm) was purchased from Sigma-Aldrich (USA) and used after dialysis in water over night to remove excess salts and citrate molecules. Solid substrates (glass or Si) were cut into 5 mm squares and cleaned by UV-O3 treatment followed by an ethanol rinse. An aqueous solution of poly(vinyl alcohol) (PVA; 1 mg/mL, 98% saponification, Wako, Tokyo) was spin– cast at 1,000 r.p.m. onto the solid substrate to form a sacrificial layer, and then 1 mg/mL of a


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chloroform solution of PS (Mw ≈ 280,000, Aldrich, USA) was spin-cast at the same spinning rate. After 10 min of UV-O3 treatment, the PS/PVA coated solid substrate surface was changed from hydrophobic to hydrophilic. As reported previously, co-assemblies of PS particles and Au NPs were fabricated by using a simple apparatus to make colloidal crystals28. First, two solid substrates were placed in parallel with a gap of about 500 µm, and an aqueous dispersion of PS particles and Au NPs was injected into the gap between the two glass substrates. Then the upper glass substrate was slid across the bottom at a velocity of 1 µm/s in order to prepare a co-assembly of PS colloidal particles and Au NPs. After complete evaporation of the water, the surface and cross-sectional structures of co-assemblies were observed by scanning electron microscopy (SEM; S-5200, Hitachi, Japan). After removal of the PS colloidal particles by immersing the co-assemblies in chloroform overnight, the two-dimensional patterns formed on the surface of the substrates were observed by atomic force microscopy (AFM; SPI-400, SII, Tokyo). Metal nanorings and polymer composite films were prepared by embedding the prepared co-assemblies into an epoxy resin. The precursor of the epoxy resin (EPOC, Oken Shoji, Tokyo) was cast onto the prepared co-assemblies and cured over night at 75 °C under vacuum. After curing, the substrate was immersed into water to detach the co-assembly and PS thin film from the solid substrate by dissolving the PVA sacrificial layer. Cross sections of the composite films were observed by SEM after cutting them under liquid nitrogen. The cross-sectional images of the films were obtained by transmission electron microscopy (TEM; H-7650, Hitachi, Japan). For the cross-sectional TEM observations, the prepared composite films were cut into 100-300 nm thin slices with an ultramicrotome (Lica, Germany) and placed on a Cu grid covered with collodion membranes.


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3. Results and Discussion As viewed from above, the FE-SEM image (Figure 2(a)) shows hexagonally arranged PS colloidal particles. The darker contrast regions at the connection sites among the PS colloidal particles correspond to high electron densities and indicate the formation of Au NPs assemblies. Figure 2(b) shows a TEM image of the connection site between two PS colloidal particles. PS colloidal particles congregate forming a neck-like shape. Many Au NPs, with diameters of about 5 nm, accumulated at the center of the neck. The cross-sectional FE-SEM image of the coassembly (Figure 2(c)) shows a nanoring structure formed at the connection site. At the interface between PS colloidal particles and substrate, nanoring structures were also observed (inset in Figure 2(c)). Cross-sectional TEM images of the co-assembly embedded in the epoxy resin revealed that a number of Au nanoparticles accumulated and a ring-like morphology was clearly formed (Figure 2(d)). These results show that the co-assemblies consisted of PS colloidal particles and Au NPs were successfully prepared, as well as that Au NPs were assembled into nanoring structures among the PS colloidal particles on the substrate. When the concentration of Au NPs was increased, the nanoring structure was not observed and Au NPs covered the entire region of PS colloidal particles (see Supporting Information, S1). The number of layers of coassemblies increased with increasing concentrations of both PS colloidal particles and Au NPs (see Supporting Information, S2). Note that Au NPs were aggregated at the bottom of the assembly when larger Au NPs ( > 10 nm) were used due to sedimentation. Figure 3 shows a plot of the diameter of PS colloidal particles, ranging from 100 nm to 2 µm, versus the size of nanoring formed. This plot shows that the size of nanorings increased with increasing size of PS colloidal particles (DPS); however, the width of the nanorings saturated around 80 nm. This relationship originates from the nanoring formation mechanism (Figure 4).


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During the co-assembly process, the PS colloidal particles accumulated in the dispersion meniscus because of convectional flows and were initially assembled by lateral capillary force29. After assembly, the PS colloidal particles stuck together forming connecting sites due to the breakup of surface hydrophilic moieties (generally, protective colloids or surfactants) or due to the surface polymer layers whose glass transition temperature (Tg) was lower than that of the bulk23. The area of contact between the PS colloidal particles increased with increasing size of the particles because the local curvature of larger particles at the connecting site was less than that of the smaller particles. The water dispersion of Au NPs remained among the PS colloidal crystals as capillary bridges, and the concentration of Au NPs gradually increased with the evaporation of the water. Au NPs eventually accumulated, through capillary force, along the edge of the connecting sites and formed nanorings. The size of the ring is proportional the interfacial area of connecting sites, but both the concentration of Au NPs and the interfacial area affect the widths of the rings. It is noteworthy that the smallest diameter and width of a nanoring obtained from the co-assembly of 100 nm PS colloidal particles and Au NPs were 38 nm and 18 nm, respectively. These values are comparable with the resolution limit of the most advanced topdown microfabrication systems such as photolithography. After sintering at 150 °C and immersion in chloroform overnight, the surface structures on the glass or silicon substrates were observed by AFM. Figure 5(a) shows an AFM image of the surface structures after immersion of the PS colloidal particles and Au NPs co-assembly at low concentration in chloroform. Hexagonally arranged nanoring structures are clearly imaged. The chloroform solution, which dissolved the PS colloidal particles, was centrifuged and the supernatant was removed. The residue was re-dispersed in chloroform, cast onto a glass substrate, and observed by SEM. Figure 5(b) shows nanoring structures of about 170 nm in


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diameter and 55 nm in width. These values are comparable with the measured values from the TEM image (Figure 2(d)). Based on these results, three-dimensionally arranged nanoring structures can be obtained by co-assembly of PS colloidal particles and Au nanoparticles. After sintering, Au NPs were melted and connected each other. Due to high temperature over glass transition temperature of PS and high mobility of melted Au NPs, holes of some rings were filed with melted Au. As the result, some Au plates were formed. To avoid this imperfection, it is better to use polymer particles whose glass transition temperatures are higher than melting point of Au NPs. 4. Conclusion In this paper we have presented a bottom-up approach to creating three-dimensional assemblies of Au nanorings. The drying of aqueous dispersions of PS colloidal particles and Au NPs allowed for the formation of hexagonally assembled colloidal crystals and nanorings composed of Au nanoparticles among the PS colloidal particles. The size of the nanorings could be controlled on a scale of tens to hundreds of nanometers. After sintering, the Au NPs formed Au nanorings. This simple approach supplies a potentially useful path to novel plasmonic materials and unique metamaterials for the visible light region.


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eva p

sliding

glass

ora

tion

of w ate

r

PS particles Au nanoparticles glass or Si

PS particle/Au nanoparticle co-assembly Figure 1. Schematic of co-assembly preparation.


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(b)

(a)

glass

100 nm

PS

Au NPs

Au NPs glass

500 nm

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100 nm

(d) PS

Au NPs

PS

50 nm 50 nm

Figure 2. (a) SEM image, (b) cross-sectional SEM image, (c) TEM image, and (d) cross-sectional TEM image of a co-assembly of 500 nm PS spheres and 5 nm Au NPs.


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500 inner 400 width

size of ring (nm)

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300 outer

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0 0

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DPS (nm)

Figure 3. Plot of PS sphere diameter (DPS) versus nanoring size. The solid line, dotted line, and dash-dotted line show the outer diameter, inner diameter, and width of nanoring structures as shown in the schematic illustration and cross-sectional SEM image.


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Figure 4. Schematic of the formation mechanism for the nanoring structures.


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(a)

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Figure 5. (a) AFM image of the surface patterns formed on a substrate after sintering and chloroform treatment and (b) SEM image of the residue re-dispersed in chloroform and cast onto a glass substrate.


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AUTHOR INFORMATION Corresponding Author *Hiroshi Yabu. Give contact information for the author(s) to whom correspondence should be addressed.

Author Contributions This work was solely done by HY.

Funding Sources This work is partially supported by a Grant-in-Aid for Priority Area “Metamaterials” (No. 23109502). ACKNOWLEDGMENT H.Y. acknowledges Emiko Yamazaki and Minori Suzuki for help in preparing the co-assemblies and in SEM and TEM observations. REFERENCES [1] Padilla, W. J.; Basov, D. N.; Smith, D. R. Negative refractive index metamaterials , Material Today 2006, 9(7), 28 [2] Luk'yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. The Fano resonance in plasmonic nanostructures and metamaterials, Nat. Mater. 2010, 9(9), 707


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[19] Chen, L.; Yu, S.; Wang, H.; Xu, J.; Liu, C.; Chong, W. H.; Chen, H. General Methodology of Using Oil-in-Water and Water-in-Oil Emulsions for Coiling Nanofilaments, J. Am. Chem. Soc. 2013, DOI: 10.1021/ja310405d. [20] Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops, Nature 1997, 389, 827 [21] Shimomura, M.; Sawadaishi, T. Bottom-up strategy of materials fabrication: a new trend in nanotechnology of soft materials, Curr. Opn. Colloid and Interf. Sci. 2001, 6 (1), 11 [22] Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the coffee-ring effect by shape-dependent capillary interactions, Nature 2011, 476, 308 [23] Yabu, H.; Shimomura, M. Preparation of Self-Organized Mesoscale Polymer Patterns on Solid Substrate: Continuous Pattern Formation from Receding Meniscus, Adv. Funct. Mater. 2005, 15(4), 575 [24] Nakanishi, T.; Hirai, Y.; Yabu, H.; Shimomura, M. Flexible, Optically Transparent, and Conductive Line-and-Space Patterned Films Prepared by Using a Simple Dewetting Process of Gold Nanoparticle Dispersions, Appl. Phys. Exp. 2011, 4(11), 117301 [25] Hong, S. W.; Xu, J.; Lin, Z. Q. Template-Assisted Formation of Gradient Concentric Gold Rings, Nano. Lett. 2006, 6(12), 2949 [26] Bayati, M.; Patoka, P.; Giersig, M.; Savinova, E. R. An approach to fabrication of metal nanoring arrays, Langmuir 2010, 26(5), 3549


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