Surface Patterning with SiO₂@PNiPAm Core–Shell Particles

Abstract

Colloidal lithography is a cost-efficient method to produce large-scale nanostructured arrays on surfaces. Typically, colloidal particles are assembled into hexagonal close-packed monolayers at liquid interfaces and deposited onto a solid substrate. Many applications, however, require non close-packed monolayers, which are more difficult to fabricate. Preassembly at the oil/water interface provides non close-packed colloidal assemblies but these are difficult to transfer to a solid substrate without compromising the ordering due to capillary forces acting upon drying. Alternatively, plasma etching can reduce a close-packed monolayer into a non close-packed arrangement, however, with limited interparticle distance and compromised particle shape. Here, we present a simple alternative approach toward non close-packed colloidal monolayers with tailored interparticle distance, high order, and retained spherical particle shape. We preassemble poly(N-isopropylacrylamide)-silica (SiO2@PNiPAm) core–shell particles at the air/water interface, transfer the interfacial spacer to a solid substrate, and use the polymer shell as a sacrificial layer that can be thermally removed to leave a non close-packed silica monolayer. The shell thickness, cross-linking density, and the phase behavior upon compression of these complex particles at the air/water interface provide parameters to precisely control the lattice spacing in these surface nanostructures. We achieve hexagonal non close-packed arrays of silica spheres with interparticle distances between 400 and 1280 nm, up to 8 times their diameter. The retained spherical shape is advantageous for surface nanostructuring, which we demonstrate by the fabrication of gold nanocrescent arrays via colloidal lithography and silicon nanopillar arrays via metal-assisted chemical etching.

Chiral Surface Lattice Resonances

“…The synthesis is described in more detail in ref. [33]. Purchased chemicals and more details can be found in Supporting Information.…”

“…[ 31,32 ] The particles self‐assemble into hexagonally ordered monolayers at the air/water interface, where the polymer shell provides a flexible spacer to separate the inorganic core particles. [ 33 ] Our method opens the possibility of tuning the lattice constant independent of the core particle diameter. We can therefore produce nanostructure arrays with large interparticle distances required to observe SLRs.…”

“…We use core–shell particles consisting of SiO 2 cores ( d = 198 nm) with soft poly( N ‐isopropylacrylamide) (PNiPAm) shells ( Figure a) and self‐assemble them at the air/water interface of a Langmuir trough. [ 33 ] The soft PNiPAm shell separates the silica cores, which form a non‐close‐packed hexagonal lattice. The interparticle distance of the silica particles in the resulting lattice can be controlled via the shell thickness or the degree of compression at the interface (Figure 1b).…”

“…The interparticle distance of the silica particles in the resulting lattice can be controlled via the shell thickness or the degree of compression at the interface (Figure 1b). [ 33,34 ] After transferring the colloidal monolayer to a substrate via the Langmuir–Blodgett technique, we remove the organic PNiPAm shell with oxygen plasma (Figure 1c). Subsequently, we employ the particle monolayer as a mask to produce arrays of chiral crescents, which exhibit structural chirality (Figure 1; details in Figure S1, Supporting Information).…”

“…[ 21 ] The diameter of our templating silica particles is 198 nm ± 3 nm, leading to a polydispersity of about 1.5% of the produced plasmonic crescents. [ 33 ] SEM reveals the hexagonal arrangement of the prepared crescent array ( Figure a,b), while the optical photograph of a sample with the polymeric shell on top visualizes the polycrystallinity of the sample by its structural color (Figure 2c). The grain size exceeds several square millimeters in some cases, as illustrated by the appearance of Bragg peaks under illumination of a colloidal monolayer with a hand‐held 405 nm laser (Figure 2e).…”