Pitch Control of Hexagonal Non-Close-Packed Nanosphere Arrays Using Isotropic Deformation of an Elastomer

Abstract

Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces an ever-increasing range of two-dimensional (2D) ordered nanostructures, although the pattern periodicity is typically bound to the original interparticle spacing. Deformable soft lithography using controlled deformation of elastomeric substrates and subsequent contact printing transfer offer a versatile method to systematically control the lattice spacing and arrangements of the 2D nanosphere array. However, the anisotropic nature of uniaxial and biaxial stretching as well as the strain limit of solvent swelling makes it difficult to create well-separated, ordered 2D nanosphere arrays with large-area hexagonal arrangements. In this paper, we report a simple, facile approach to fabricate such arrays of polystyrene nanospheres using a custom-made radial stretching apparatus. The maximum stretchability and spatial uniformity of the poly(dimethylsiloxane) (PDMS) elastomeric substrate is systematically investigated. A pitch increase as large as 213% is demonstrated using a single stretching-and-transfer process, which is at least 3 times larger than the maximum pitch increase achievable using a single swelling-and-transfer process. Unlike the colloidal arrays generated by the uniaxial and biaxial stretching, the isotropic expansion of radial stretching allows the hexagonal array to retain its original structure across the entire substrate. Upon radial strain applied to the PDMS sheet, the nanosphere array with modified pitch is transferred to a variety of target substrates, exhibiting different optical behaviors and serving as an etch mask or a template for molding.

Ordered non-close packed colloidal array with morphology control

“…This is not possible by most existing techniques for H-NCP formation, which rely on either UVO/ plasma mediated etching and size reduction of an initially created HCP array [39][40][41] or stretching of the substrate. 42,43 Both the approaches will fail if there is polydispersity in the size of the colloids. Further, though Fig.…”

“…On the other hand, most top-down approaches for NCP fabrication rely on initial creation of an HCP array followed by post processing such as etching to reduce the size of the individual colloids, [39][40][41] or mechanical deformation by stretching of a flexible substrate. 42,43 Other approaches of NCP fabrication rely on chemical routes, examples of which include spin coating of monodisperse silica colloids along with triacrylate monomers followed by subsequent polymerization, 44 or interfacial selfassembly of charged colloidal particles confined at the air-water interface of a Langmuir trough which is subsequently transferred to a solid substrate. 45,46 While the latter method allows precise control of inter-particle separation by laterally compressing the film to desired surface densities, it works only for charged colloids having a high contact angle with water, which is essential for subsequent successful transfer of the array onto a solid substrate by the Langmuir-Blodgett technique.…”

Controlling disorder in self-assembled colloidal monolayers via evaporative processes

“…Post-synthetic techniques using mechanical forces, including among others, strain, shear, and scratching, are promising routes to control disorder. 231 Flexible substrates can potentially produce surfaces over which correlation distances could be dynamically changed.…”

Gas sensors using ordered macroporous oxide nanostructures

“…44 Besides these close-packed CCs, a range of 2D and 3D nonclose-packed CCs have also been developed by means of plasma treatment, heat treatment, and elastomeric substrate stretching on their close-packed parent crystals. [45][46][47] The exibility and diversity of CCs can offer us considerable freedom in the fabrication of ordered macroporous nanostructures and their structural engineering for sensing performance tuning.…”