Assembly of Plasmonic Nanoparticles on Nanopatterns of Polymer Brushes Fabricated by Electrospin Nanolithography.

This paper presents electrospin nanolithography (ESPNL) for versatile and low-cost fabrication of nanoscale patterns of polymer brushes to serve as templates for assembly of metallic nanoparticles. Here electrospun nanofibers placed on top of a substrate grafted with polymer brushes serve as masks. The oxygen plasma etching of the substrate followed by removal of the fibers leads to linear patterns of polymer brushes. The line-widths as small as ∼50 nm can be achieved by precise tuning of the diameter of fibers, etching condition, and fiber-substrate interaction. Highly aligned and spatially defined patterns can be fabricated by operating in the near-field electrospinning regime. Patterns of polymer brushes with two different chemistries effectively directed the assembly of gold nanoparticles and silver nanocubes. Nanopatterned brushes imparted strong confinement effects on the assembly of plasmonic nanoparticles and resulted in strong localization of electromagnetic fields leading to intense signals in surface-enhanced Raman spectroscopy. The scalability and simplicity of ESPNL hold great promise in patterning of a broad range of polymer thin films for different applications.

Macromolecules that are end-grafted to solid substrates show great promise for a broad range of applications and scientific studies. [1][2][3][4] The strong interest in end-grafted polymers (i.e. polymer brushes) results from the robust interface that can be precisely tuned by the structure and chemistry of the macromolecules which exhibit stimuli responsive behavior. [5][6][7] A set of technologically important applications requires patterning of these materials at the nanometer length scale. [8][9][10] An interesting case involves fabrication of plasmonic nanostructures by assembly of colloidal metallic nanoparticles 11,12 (NPs) on polymer brushes patterned at the length scale of the particles. The ability to assemble such plasmonic structures on patterned brushes allows for investigating the structure property relations at the single particle level and offers unprecedented capabilities in fabrication of devices. [13][14][15] A variety of different approaches including electronbeam lithography, dip-pen nanolithography and self-assembly has been used to fabricate nanoscale patterns of polymer brushes. [16][17][18][19][20][21][22] Advanced lithography techniques are highly developed and allow for fabrication of polymer brushes with high levels of resolution and fidelity; however, these techniques typically require high capital and operation costs, and not suitable for some of the emerging applications where there is a need for patterning unusual materials and substrates which, for example, are nonplanar and flexible. 23,24 The requirement for specialized and expensive facilities also impose significant barriers to the broad research community who has limited access to these techniques. Self-assembled templates 25 could be prepared at low-costs, but it remains a challenge to generate non-periodic patterns with independent control over the dimensions and periodicities of the patterns. All these issues motivate development of innovative approaches for fabrication of nanoscale patterns of polymer brushes using methods that are low-cost, simple and amenable for patterning of large surface areas.
Here we present a simple and versatile approach for fabrication of chemically defined nanoscale patterns consisting of polymer brushes for templated assembly of colloidal NPs using electrospun nanofibers (NFs) as masks for selective material removal. The core idea in this work is to benefit from widely available electrospinners in nanopatterning of polymer brushes.
Electrospinning is a low-cost, simple and widely available technique to generate NFs which are one dimensional structures with sub-micron diameters. 26 The ability to vary the diameter of these fibers at nanoscopic length scales together with the rapid generation of such structures over large areas has great promise for nanofabrication; however, the direct use of NFs in surface patterning is challenging because of several reasons: i) NFs are only physically bound to the underlying substrate posing issues in the stability of fabricated structures, limiting subsequent processes such as chemical modification. ii) It is challenging to optimize the electrospinning process for different polymers, iii) Circular cross-section of the fiber may not be suitable in cases where there is a need for smooth and planar patterns. To fabricate ultra-smooth features with a molecular level thickness control and tunable chemistry, we use NFs as masks to pattern substrates modified with polymer brushes. NFs that are placed on top of the substrate prevent removal of the underlying brush material during oxygen plasma etching which leads to chemically defined linear patterns with widths that are smaller than the diameter of the fiber A particular challenge that relates to the process of electrospinning is random deposition of NFs due the whipping instabilities. To overcome this randomness and fabricate well-aligned nanoscale patterns, we used two approaches: conventional (i.e. far-field) electrospinning with a rotating substrate and near-field electrospinning.
The former is advantageous, since it can be performed with a rotating drum that is present in almost all electrospinning systems. The latter, on the other hand, allows for precise alignment and spatial control of the fibers deposited on the substrate. The versatility of the process is demonstrated through patterns of polymer brushes with two different chemistries. The resulting patterns effectively direct the assembly of gold and silver NPs which exhibit specific and intense signals in surface-enhanced Raman scattering (SERS).    directed-self-assembly, electronics and biotechnology.

Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details, additional SEM, AFM and TEM images.

Notes
The authors declare no competing financial interest.