Large-Area Fabrication of Vertical Silicon Nanotube Arrays via Toroidal Micelle Self-Assembly.

Nadezda Prochukhan,Tandra Ghoshal,Michael A Morris,Clive Downing,Ross Lundy,Andrew Selkirk,Elsa C Giraud

doi:10.1021/acs.langmuir.0c03431

Nadezda Prochukhan, Tandra Ghoshal + Show 5 more

Open Access

https://doi.org/10.1021/acs.langmuir.0c03431

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Abstract

We present a highly scalable, room-temperature strategy for fabricating vertical silicon nanotube arrays derived from a toroidal micelle pattern via a water vapor-induced block copolymer (BCP) self-assembly mechanism. A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) BCP system can be self-assembled into toroidal micelle structures (diameter: 400–600 nm) on a PS-OH-modified substrate in a facile manner contrasting with other complex processes described in the literature. It was found that a minimum PS-b-PEO thickness of ∼86 nm is required for the toroidal self-assembly. Furthermore, a water vapor annealing treatment at room conditions (∼25 °C, 60 min) is shown to vastly enhance the ordering of micellar structures. A liquid-phase infiltration process was used to generate arrays of iron and nickel oxide nanorings. These oxide structures were used as templates for pattern transfer into the underlying silicon substrate via plasma etching, resulting in large-area 3D silicon nanotube arrays. The overall simplicity of this technique, as well as the wide potential versatility of the resulting metal structures, proves that such room-temperature synthesis routes are a viable pathway for complex nanostructure fabrication, with potential applicability in fields such as optics or catalysis.

Highlights

We describe a novel strategy to address this issue by controlling the formation of large toroidal structures via water vapor annealing (WVA) of a polystyrene-bpoly(ethylene oxide) (PS-b-PEO) diblock copolymer system atop a PS-modified substrate
24.5k) system in order to investigate whether surface pretreatment and WVA can influence the resultant film morphology
A WVA step was employed for an hour at 25 °C, which was followed by metal ion liquid phase infiltration and pattern transfer

Summary

■ INTRODUCTION

Block copolymer (BCP) self-assembly provides an avenue for the formation of a myriad of nano-scale morphologies, with applications such as optoelectronics, biosensing, filtration, bioactive surfaces, surface coatings, and magnetic applications, among others.[1−10] More usual microphase separated selfassembled architectures include lamellar, cylindrical, spherical, and gyroidal structures as well as various micellar-based morphologies such as helices, tubes, disks, and toroids.[11−13] Micelle formation is defined as the self-assembly of an amphiphilic BCP in a solvent medium to form a structure typically with a core and corona.[11,14] Of the myriad of available micellar structures, toroidal micelles, in particular, have garnered interest owing to their proven applicability in synthesizing unique nanostructured materials with unique plasmonic and magnetic properties.[15,16] Such structures are typically fabricated via incorporating various metals and metal oxides, which upon removal of the polymer matrix produce metallic toroidal or nanoring structures.[17]. These self-assembled toroidal structures offer an ideal template to facilitate the formation of a Si nanotube array using a well-developed BCP lithographic strategy of liquid-phase selective infiltration and using a resultant metal oxide pattern as a mask in an etch process.[18,53] The PEO domain readily coordinates various metals such as iron or nickel ions, which after processing in turn act as a hard mask for ICP etching.[53,54]. The resultant patterns retain uniformity as the feature sizes of the BCP films and metal oxide masks closely follow normal distributions (Supporting Information, Figure S6). We propose that the vertical silicon nanotube array described can find application in catalysis and sensing.[44,45,58] The nanotube array itself can be modified, for example, as demonstrated by Fan and co-workers whereby an O2 environment at high temperatures is employed to oxidize Si to SiO2.44

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES