Accordion-like plasmonic silver nanorod array exhibiting multiple electromagnetic responses

Abstract

We prepared a high-density array of “accordion-like” plasmonic silver nanorods over a large area (2.5 × 2.5 cm2) that exhibited multiple electromagnetic responses to visible and near-infrared (NIR) wavelengths. This array of “accordion-like” silver nanorods was fabricated by confining the lamellae-forming polystyrene-block-poly (methyl methacrylate) copolymer (PS-b-PMMA) inside the cylindrical pores of an aluminum oxide (AAO) template grafted with thin neutral brush layers. PS and PMMA lamellar nanodomains with sizes of 15 nm were alternatively stacked along the nanorod direction. After the AAO template was removed, a 5-nm-thick layer of silver was thermally deposited on only the PS nanodomains. Owing to the multiple resonances exhibited in the visible and NIR regimes, the array could be used for multi-analyte detection. Furthermore, this concept of fabricating sophisticated nanoscale architectures by utilizing block copolymer self-assembly and incorporating plasmonic metals into one nanodomain could be applied to realize large-scale metamaterials that function under visible and NIR wavelengths. A simple technique for creating large array of nanorods that manipulate both visible and near-infrared light has been devised by researchers in South Korea. Metal nanostructures can dramatically influence light. These so-called plasmonic effects usually only occur at specific wavelengths when the light’s frequency matches the natural collective oscillation of surface electrons. The resonant wavelength depends on the dimensions of the nanostructure. Jin Kon Kim, Junsuk Rho, and co-workers from Pohang University of Science and Technology (POSTECH) fabricated nanorods that combine many metal nanostructures of differing dimensions and support multiple resonances. Their structures comprised a stack of alternating layers of polystyrene and poly(methylmethacylate). The outer edges of the polystyrene layers were coated in silver, making the nanorods look like tiny accordions. Arrays of these structures covering several square centimeters were constructed using block copolymer self-assembly. A high density of “accordion-like” silver nanorod array over a large area (~cm2) was fabricated by confining lamellar-forming polystyrene-block-poly (methyl methacrylate) copolymer (PS-b-PMMA) inside cylindrical pores of aluminum oxide (AAO) template grafted by thin neutral brush layers. After removing the AAO template, a 5 nm thick layer of silver was thermally deposited on only PS nanodomains. Owing to combination of hemispherical head and side silver rings, multiple resonances exhibited in the visible and NIR regimes. This sophisticated fabrication utilizing block copolymer self-assembly could be applied to multi-analyte detection and realization of large-scale metamaterials working at visible and NIR wavelengths.