Fabrication of Large-Area Flexible SERS Substrates by Nanoimprint

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Fabrication of Large Area Flexible SERS Substrates by Nanoimprint Lithography Vignesh Suresh, Lu Ding, Ah Bian Chew, and Fung Ling Yap ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00295 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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ACS Applied Nano Materials

Fabrication of Large Area Flexible SERS Substrates by Nanoimprint Lithography Vignesh Suresh, Lu Ding, Ah Bian Chew and Fung Ling Yap* Agency for Science, Technology and Research (A*STAR), Institute of Materials Research and Engineering (IMRE), #08-03, 2 Fusionopolis Way, Innovis, Singapore 138634

KEYWORDS: Nanoimprinting, SERS substrates, Lithography, Gold nanocones, Plasmonics, Chemical sensing

ABSTRACT We demonstrate the nanofabrication of flexible plasmonic sensors comprising of gold nanocones achieved by nanoimprint lithography (NIL) on polycarbonate (PC) sheets. Thermal imprinting has been performed consistently over a large area (roughly the size of a 6” wafer) with a batch process; this can be extended to a continuous process using UV rollto-roll (R2R) nanoimprinting. This provides a process to scale up the fabrication of continuous imprinted rolls of PC sheets at an optimal rate of 3-5 m/min. The geometry of the peaks and the valleys of the nanocones in the as-imprinted PC is defined by the nickel mold used during imprinting, however, the gaps between the nanocones are tailored by varying the thickness of the gold deposited onto the substrate. Two different thicknesses of gold was deposited to study the effect of geometry on plasmonic sensing. The resulting PC sheet with gold coating enable high sensitive detection of analytes by Surface Enhanced Raman Spectroscopy (SERS) by virtue of plasmonic hotspots generated at the valleys, whose

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presence was confirmed by scattering scanning near-field optical microscopy (s-SNOM). This is promising, particularly when the SERS substrate developed are highly reproducible, cost effective, transparent and flexible, finding application in nanoplasmonic sensing and onfield environmental monitoring where rigid SERS substrates would not be appropriate.

1. INTRODUCTION Plasmonic sensing based on nanostructures for detection of molecules rely largely on the substrate topography, and the geometry of the nanofeatures fabricated out of noble metals such as gold and silver as they influence the electric field critical to SERS performance.1-2 It is also established that the the accessible distance (sphere of influence) up to which SERS is observed is strongly dependent on the size of the nanoparticle.3 These Raman signals are highly sensitive to the arrangement and geometry of the metallic nanostructures on the substrate and their dimensions. Hence, there is an inevitable need to have meticulous control over the nanofeatures dimensions and topographies.4-6 Moreover, the fabrication technique utilized is responsible for the homogeneity of the features and the surface roughness, which again determines the reproducibility in SERS performance. Some of the nanofabrication techniques employed to get precise structural arrangement and geometry for SERS applications include electron beam lithography, laser interference lithography, UV photolithography, electro-oxidative lithography and block copolymer self-assembly.7-16 The key advantages of using these patterning approaches are that they enable defining the position of the nanofeatures with geometric attributes that would overwhelm the surface with large number of plasmonic hotspots due to narrowly separated nanogaps, sharp valleys/vertices and edges.17-19 These technique facilitate in achieving highly homogenous and uniform patterned arrangement that in turn offer enhanced and consistent Raman signals. On the downside, these approaches are either expensive, or they have low batch-to-batch reproducibility and


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realizing large area with uniform patterned features becomes practically difficult. Electron beam lithography allows fabrication of features with well-defined nanogaps that is not easily achieved my any other techniques, however, the operational cost involved in creating large areas of substrates is high while the small areas that are typically patterned using the technique offer less throughput and the yield that is crucial for scaling up is compromised.2021

Nanoimprint lithography (NIL) has been used to fabricate highly periodic nanodisc, nanocone and nanohole metal arrays for plasmon-resonant SERS structures on both hard surfaces and soft surfaces such as plastics.14, 22-24 In all cases, the geometry and structure uniformity have played a key role in their high SERS performance. While SERS substrates on silicon and glass are quite common, in the recent years, SERS substrates based on flexible materials have received much attention.5, 25 We recently reported a block copolymer self-assembly based process for fabricating SERS-active tapes that are transparent and robust by a simple ‘stick and peel’ technique12. These SERS-active tapes facilitate the detection of chemical analytes on irregular and non-planar surfaces. Compared to the traditional SERS substrates such as silicon or glass that are rigid and hard, the flexible substrates such as plastics, nanofibers and papers are highly sought after for on-field Raman measurements and real-time monitoring of chemical and biological analytes.26-31 The flexible SERS substrates are recognized as a sensing platform in military applications as well.32 There are some reports on the fabrication of flexible SERS substrates made from paper, plastic and other polymer scaffolds.33-35 In particular, Zhang et al.,34 reported a SERS substrate fabrication method using a continuous roll-to-roll UV NIL similar to this work. However, the AAO mold used and the geometry it defined was significantly different from this report, wherein the latter employed a Ni mold for large area imprinting. The Ni mold used has precisely defined feature-to-feature spacing,



feature diameter and periodicity as compared to features defined by AAO molds that has larger variation and standard deviation in the geometry. As SERS is highly sensitive to the surface geometry, a well-defined pattern arrangement is the most preferred to localize the hot spots uniformly throughput the substrate in order to mitigate point-to-point signal variation and to assist with more accurate signal qualification. In most other cases, the reproducibility in SERS signals remain a concern as the metal deposition necessary for signal enhancement is either inhomogeneous, de-wets into islets or the fabrication results in scattered distribution of nanoparticles. While some of the concerns may be addressed by varying the deposition conditions or fabrication parameters, many SERS substrates still fail to adapt for those measurements that need them to be optically transparent. Thus, in addition to being flexible, the substrates should be robust, significantly transparent, exhibit definite patterned arrangement of metallic features and defined geometry to ensure structural integrity.4 Although, the flexible SERS substrates made from plastic or paper are susceptible to getting burnt by high energy laser or other harsh chemicals during measurements, the advantages they offer are quite bountiful.

To meet the needs for sensitive, ultra-low cost disposable sensor, in this report, we demonstrate the fabrication of transparent, robust, flexible SERS-active plastic substrates on a large scale and over a large area using NIL. This method offers dual advantage of yielding regions on the substrate with significantly large number of plasmonic hotspots, together with precise control over the feature arrangement that facilitate engineering the geometric attributes. While the NIL batch process enables imprinting a 6” PC sheet, the UV Roll-toRoll (each roll measures 30 cm in width and 100 m in length) process facilitates the continuous production, imprinting at the rate of 3-5 m/min of the PC sheet with high precision using a flexible nickel mold. The process is bestowed with impressive benefits such


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as high batch-to-batch reproducibility, high resolution, and high throughput and is greatly economical. The R2R NIL technique can be used to imprint features such as nanopillars, nanopyramids, nanotriangles/cones which are reported to exhibit large number of plasmonic hotspots for SERS based sensing.36-38 Together with the ease in patterning directly onto plastic films with NIL, the technique represents a significant progress in on-field analytics making SERS based on flexible substrates more accessible beyond research laboratories.

2. EXPERIMENTAL 2.1.MATERIALS

Polycarbonate (PC) sheets of thickness ~250 µm were purchased from Goodfellow Cambridge Ltd., England. The nanoimprinting nickel mold with the nanocone structures used for imprinting on PC sheets was purchased from Temicon, Germany. The UV resin, mrUVCur26SF was purchased from Microresist Technology, Germany. Crystal violet (Sigma) was used as received. Point Probe Plus silicon tips for imaging of the imprinted nano-features in tapping mode with atomic force microscopy (AFM) and PtIr5 coated silicon tips for imaging with Neaspec were purchased from Nanoworld (Neuchatel, Switzerland).

2.2.METHODS The PC sheets were cut into pieces of size 5”×5”. The nanocone structured nickel mold was pressed against the PC sheet and imprinting was carried out at 160oC, at 50 bar pressure for 300 seconds using a 6” nanoimprinter (Obducat, Sweden). Before imprinting, the mold was vapour deposited with perfluorodecyltrichlorosilane that facilitates easily demolding. The mold had features of height 300 nm and pitch of 340 nm. Upon NIL, the resulting pattern characteristics on the imprinted PC, viz., the feature heights, width and periodicity were characterized using tapping mode AFM (Dimension Icon, Bruker Corporation, California,



USA) and Scanning electron microscopy (FESEM 6700F, JEOL, Japan). Metallic gold was deposited on a 5”x5” imprinted substrate using an electron beam evaporator (Denton Vacuum Explorer Coating System, New Jersey, USA). The gold target used had a purity of 99.999% and the deposition was carried out at a rate of 2 Å/s in a chamber base pressure of

Fabrication of Large-Area Flexible SERS Substrates by Nanoimprint

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