Stamping plasmonic nanoarrays on SERS-supporting platforms

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Research ArticleReceived: 31 August 2010 Accepted: 8 February 2011 Published online in Wiley Online Library: 6 April 2011

(wileyonlinelibrary.com) DOI 10.1002/jrs.2940

Stamping plasmonic nanoarrays onSERS-supporting platformsDeepak Bhandari,a Sabrina M. Wells,a Alessia Polemi,b Ivan I. Kravchenko,c

Kevin L. Shufordb and Michael J. Sepaniaka∗

The dielectric property of a nanoparticle-supporting film has recently garnered attention in the fabrication of plasmonicsurfaces. A few studies have shown that the localized surface plasmon resonance (LSPR), and hence surface-enhanced Ramanscattering (SERS), strongly depends on the substrate refractive index. In order to create higher efficiency SERS-active surfaces,it is therefore necessary to consider the substrate property along with nanoparticle morphology. However, due to certainlimitations of conventional lithography, it is often not feasible to create well-defined plasmonic nanoarrays on a substrate ofinterest. Here, an additive nanofabrication technique, i.e., nanotransfer printing (nTP), is implemented to integrate electronbeam lithography (EBL) defined high-aspect-ratio nanofeatures on a variety of SERS-supporting surfaces. With the aid ofsuitable surface chemistries, a wide range of plasmonic particles were successfully integrated on surfaces of three physicallyand chemically distinct dielectric materials, namely, polydimethyl siloxane (PDMS), SU-8 photoresist, and glass surfaces, usingsilicon-based relief pillars. These nTP-created metal nanoparticles strongly amplify the Raman signal and complement theselection of suitable substrates for better SERS enhancement. Our experimental observations are also supported by theoreticalcalculations. The implementation of nTP to stamp out metal nanoparticles on a multitude conventional/unconventionalsubstrates has novel applications in designing in-built plasmonic microanalytical devices for SERS sensing and other relatedphotonic studies. Copyright c© 2011 John Wiley & Sons, Ltd.

Supporting information may be found in the online version of this article.

Keywords: nanotransfer printing; surface-enhanced Raman scattering; nanofabrication; SERS substrate; Maxwell’s equation

Introduction

There is considerable interest in the optical properties of metalnanoparticles, mostly due to their ability to create intense localizedsurface plasmon (LSP) fields. However, due to the nanoscaledimensions, controlled fabrication of rationally conceived specialnanostructures was quite difficult until the recent advances innanoscience and nanotechnology.[1 – 3] Conventional lithographictechniques that emerged from microelectronics are now inwidespread use in the fabrication of nanostructures for researchin both physical and biological sciences. Currently, there issubstantial interest in alternative nonconventional techniques likenanoimprint lithography,[4,5] soft lithography,[6,7] near-field opticallithography,[8,9] and so on. These nonconventional techniqueshave garnered attention in creating fine nanostructures over alarge area at a relatively very low cost.

Surface-enhanced Raman scattering (SERS), first observed byFleischmann et al.,[10] exploits near-field plasmonic effects.[11 – 13]

Molecules located at the vicinity of the roughened surfaces,therefore, experience different spatial field intensity dependingon the morphology and arrangement of nanostructures. So far,there have been several theoretical and experimental studiesperformed on the plasmonics of various shapes, sizes, anddistributions of metal nanoparticles.[14,15] More recently, a gooddeal of understanding has emerged on the substrate plasmonics.A number of studies have shown that the substrate dielectricproperty alone has major contribution on localized surfaceplasmon resonance (LSPR).[16 – 19] This understanding has openedup new challenges in SERS. As most of the uniform and higherefficiency SERS substrates are nanoparticle-embedded planer

surfaces, the proper selection of dielectric materials could notablyalter Raman intensity, regardless of a nanoparticle.

Typically, the metal nanoparticle or plasmonic surface isfabricated by top-down nanofabrication[20 – 22] or bottom-up self-assembly approaches.[23,24] Nevertheless, controlled nanostruc-tured arrays having larger field enhancement and reproducibleSERS signals are usually fabricated via the top-down approach.[3]

Electron beam lithography (EBL) is a burgeoning top-down tech-nique for SERS research. However, the access of EBL to generalusers is limited by the exceptionally high cost of EBL instrumen-tation and its limitation to specialized materials. Therefore, toproduce a SERS-active surface at a relatively low cost on a de-sired substrate, it is necessary to develop a novel nanolithographyapproach that can possibly augment EBL in plasmonic research.Here, we recognized that nanostructures created on a siliconstamp were robust to facilitate the stamp-and-repeat advantageand compatible with surface modification chemistry (see below).Silicon stamps were fabricated by EBL and reactive ion etching(RIE). These silicon-based nanotransfer printing (nTP) stamps in-volved extensive optimization of unique fabrication conditions

∗ Correspondence to: Michael J. Sepaniak, Department of Chemistry, Universityof Tennessee, Knoxville, TN 37996, USA. E-mail: [email protected]

a Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA

b Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA

c Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, OakRidge, TN 37831, USA

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Stamping plasmonic nanoarrays for SERS

such as the EBL exposure dose, development time, lift-off pro-cess, and etching protocol. The stamps contain features with adimension of as small as 150 nm diameter to large 5 × 5 µm2

size aggregates, and have a relief pillar height of 375 nm. TheseSERS-active nanofeatures were stamped out, for the first time, onthree most commonly used research materials, i.e. polydimethylsiloxane (PDMS), SU-8 photoresist, and glass. The materials understudy were selected on the basis of their extensive use in micro andnanotechnology and their distinct refractive indices. The processwas optimized for high fidelity transfer printing. Results showedthat nTP-created nanoparticles retain their inherent SERS activityand also were consistent with the recently cited substrate dielec-tric effect and our theoretical calculations. Additional attributesof this work are highlighted by brief discussions on nanogapmanipulation and SERS-active fiber facet. In general, the presentwork develops nTP as a promising route to integrate EBL-creatednanoarrays on various surfaces for improved SERS enhancement.It also shows potential toward the development of microanalyticalplasmonic devices, such as lab-on-chip, fiber optics, and so on byintegrating nanoparticles on various dielectric films.

Experimental

Materials

Prime grade silicon wafer (orientation 100), glass slides, SU-8 50 resist, and Sylgard 184 silicone elastomer were purchasedfrom Wafer World Inc., Fisher Scientific, Microchem Corporation,and Dow Corning Corporation, respectively. Silver shots (99.99%,2–3 mm diameter) and gold coins were purchased from Alfa Aesar.Rhodamine 6G (R6G) was purchased from Allied chemicals andbenzenethiol (BT) from Acros Chemicals. All test solutions wereprepared in 18-M� deionized water (Barnstead, E-Pure).

Nanofabrication of silicon pillars and metal deposition

Silicon pillars or ‘stamps’ were created using EBL and RIE. Thedetailed procedure is explained elsewhere.[1,3,25] Briefly, 300 nmthick positive resist, ZEP 520 A (Zeon Chemicals, KY, USA), wasspin-coated onto the silicon wafer at 6000 rpm for 45 s, baked at180 ◦C for 2 min, and then placed under vacuum in the EBL system.The resist was patterned by EBL (Jeol JBX-9300, 100 keV) and thendeveloped in xylene for 30 s and rinsed with isopropanol.

For the lift-off procedure, 10 nm of chromium was vapor-deposited onto the nanopatterns using a dual-gun electronbeam evaporation chamber (Thermonics Laboratory, VE-240).Following lift-off, silicon wafers were etched to produce 375-nm silicon posts using an Oxford Reactive Ion Etcher. Afteretching, chromium on the substrate was removed using achrome etchant, CR-14S (Cyantek Corp., CA, USA). The siliconstamps thus created were exposed to the releasing agentheptadecafluoro-(1,1,2,2,tetrahydrodecyl)-trichlorosilane (GelestInc., PA, USA) in a vacuum desiccator for 1 h prior to the physicalvapor deposition of silver (Cooke Vacuum Product).

Substrate preparation and nanoscale transfer printing

PDMS elastomer was prepared by mixing Sylgard 184 curingagent and its elastomer in a 1 : 10 (w/w) ratio. The mixture wasthen degassed using a vacuum dessicator and then spin-coatedon a clean silicon wafer at 200 rpm for 30 s. The solution wasfinally cured in an oven for 1 h at 70 ◦C. Similarly, SU-8 50 resist

was spin-coated on a clean glass slide at 3000 rpm for 1 min,prebaked at 90 ◦C for 20 min, exposed to 365-nm UV radiationfor 4 min, and then post-baked for 10–15 min. The glass slidesin the SERS study were used as obtained from the vendor.Cured PDMS elastomer and SU-8 resist were then exposed toUV ozone for 5 min, while the glass slide was treated with aq30 : 70 (v/v) mixture of hydrogen peroxide and sulfuric acid for30 min at 80 ◦C. Each substrate was then treated with a ‘griping’reagent, 5×10−3 M solution of 3-mercaptopropyltrimethoxysilane(3MPTMS) in acetone for PDMS, or in toluene for glass and SU-8,for 30 min. Silver-coated silicon stamps were then brought intocontact to the final substrate using an in-house built pneumaticdevice. The stamp was finally removed, leaving behind metallicnanofeatures on the final substrate.

Scanning electron and atomic force micrographs

Scanning electron microscopy (SEM) images of all nanofeatureswere obtained using a LEO 1525 scanning electron microscopein a secondary electron detection mode. A low voltage (1.5 kVfield emission gun) was applied to reduce charging. Due to theexcessive charge buildup, atomic force microscopy (AFM) wasused to characterize nanofeatures on SU 8 resist. All AFM imageswere taken in the AC mode in air using an Asylum research AFM.Commercial silicon AC160 cantilever tips (Olympus) with a springconstant of 2.45 N/m were used. The tips were tetrahedral in shapewith the tip radius of curvature less than 10 nm.

SERS measurements

All SERS measurements were acquired using a LabRam Spectro-graph from JY-Horiba using 633 nm excitation. Details about theoptics can be found elsewhere.[26] For SERS detection, the sub-strates were drop-coated with 20 µl solution. However, for BT, anextra rinse-off step was carried out to remove unbound analytefrom the nanoparticles’ surface using deionized water. In eachcase, 25 spectra were acquired over an area of 400 µm2. Baselinecorrections were carried out in order to correct the optical back-ground signal from the substrate. All spectra were recorded usingdimer arrays of 300 × 300 × 30 nm3 (width × height × thickness)triangles or circles (300 nm diameter × 30 nm thickness), referredas ‘bowtie’ or ‘dumbbell’ throughout, 50× microscope objective,1 mW laser power, and 2 s acquisition time unless otherwise men-tioned. Test solutions used in SERS experiments were 5 × 10−6 MR6G and 5 × 10−4 M BT, the latter creating a self-assembledmonolayer on the metal surface.

Theoretical calculations

We have performed simulations using CST Microwave Studio,[27]

which implements a finite integration technique[28] to solveMaxwell’s equations. The calculations were performed in thetime domain, and spectral information was obtained via discreteFourier transformation. The grid mesh for both configurationswas 2 nm, and periodic boundary conditions were employed indirections perpendicular to k. The dispersion of the silver has beenincorporated via experimentally determined values of the opticalconstants.[29]

The model geometries under investigation are shown in Fig. 4afor the bowtie (left) and the dumbbell (right) configurations. Theunit cell has dimensions a = 1100 nm and b = 450 nm for bothcases. For the bowtie, w = 650 nm and v = 300 nm, while the

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Figure 1. Schematic depiction of the procedure for incorporating nanofeatures on various material surfaces: (a) conventional nanofabrication (e-beamlithography/reactive ion etching) in creating relief nanopillars on silicon wafer; (b) nanotransfer printing approach. Shown in the insets are SEM imagesof EBL/RIE-fabricated silicon pillars.

gap is g = 50 nm. For the dumbbell, the diameter is d = 300 nmand the gap is g = 50 nm. The silver nanoparticles are assumed tohave a thickness p = 30 nm. The arrays reside on a substrate withrefractive index nm. In particular, we have considered the followingvalues: PDMS nm = 1.47, glass nm = 1.51, and SU8 nm = 1.59. Allthree materials are assumed lossless. The unit cell is illuminated bya normal plane wave at λ0 = 633 nm, with electric field polarizedperpendicular to the dimer axis.

As figures of merit, we defined the volume-integrated modulusof the electric field as an indicator of the average near-field aRaman-active molecule may feel. The volume is defined as a rightparallelepiped, where two faces are the substrate/particle interfaceand the top of the nanoparticle dimer. The remaining four facesare shifted from the outermost locations on the dimer surface by adistance t = 10 nm. For the bowtie, the base is (2t+w)×(2t+v), andheight is the particle thickness p. Equivalently for the dumbbell,the base is (2t + 2d + g) × (2t + d), and the same height. Thus, thefigure of merit F is defined over the appropriate volumes as

F = 1

V

∫V

|E(x, y, z)|dV (1)

Results and Discussion

nTP process description

The current approach of incorporating plasmonic nanoarrays ondielectric substrates involves (1) defining nanoscale features on asilicon wafer (referred as a ‘stamp’) and chemically modifying thestamp for facile release, (2) physical vapor deposition of a metallicthin film on stamp, (3) bringing the stamp and the chemicallymodified substrate into physical contact, and (4) removing thestamp to leave behind nanostructured metallic features on thesubstrate. The schematics of the process design are shown inFig. 1.

Initially, EBL and RIE were used to define high-aspect-rationanofeatures on the silicon wafer (see inset in Fig. 1). EBL canpattern arbitrary features with a resolution of approximately10 nm.[2,3] Silicon stamps were then treated with fluorinated silaneprior to the metal deposition. The presence of a fluorinated silaneinterface between the relief pillar and the metallic film weakens

the adhesion between the latter two and facilitates the transfer ofmetallic disks.[30] Three chemically and physically distinct dielectricmaterials, i.e. PDMS, glass, and SU-8, were chosen for the study.

The nTP technique depends on the interfacial chemical reactionsthat occur during the physical contact between the stamp andthe final substrate.[30,31] To facilitate surface chemistry, hydroxylgroups were created on the substrate of interest. PDMS elastomerand SU-8 photoresist generate a hydrophilic surface upon plasmatreatment, while the glass substrate was soaked in a 3 : 7 volumetricmixture of hydrogen peroxide and concentrated sulfuric acid.Subsequently, substrates were treated with 3MPTMS to facilitatethe co-condensation reaction (Fig. S1, Supporting information).This results into the formation of a layer of 3MPTMS with thiolgroup terminated outward.[32] As thiol groups bind covalently withcoinage metals when the stamp is brought into physical contactwith the 3MPTMS, covalent interactions can assist the transferprocess.

nTP process optimization

Using a rigid silicon stamp, plasmonic nanoarrays were successfullystamped out on both soft and rigid substrates. For the soft PDMSelastomer, external pressure was not needed to bring the stampand substrate into conformal contact. For rigid substrates likeglass and SU-8 resist, adjusting transfer pressure was important forsuccessful nTP. In case of SU-8 resist, we empirically determinedthat a contact pressure of ∼0.5 MPa was sufficient to facilitatethe transfer process, whereas for the glass substrate ∼1 MPa wasrequired. Contact pressure was monitored by an in-house-builtpneumatic device. The pneumatic device used in our experimentwas composed of a nitrogen tank connected via plastic tubing,with an in-line pressure gauge, to an inverted gas-tight glasssyringe (Fig. S2, Supporting information). The contact pressurereported here is the amplified pressure per unit area exerted onthe stamp and is estimated on the basis of the pressure appliedper unit area to the syringe plunger and the area of the stamp,which was kept as small as possible to minimize the adverse effectof unevenness.

In addition to the external pressure, the contact time betweenthe stamp and the substrate was shown to be important to

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Figure 2. Micrographs (AFM (b), SEM (a), (c), and (d) representing transfer-printing of metallic nanoarrays: (a) Silver modified nanofabricated siliconpillars prior to stamping. Nanotransfer-printed bowtie silver array on(b) PDMS, (c) SU-8, and (d) glass slide.

achieve high transfer success rates. Unlike instant transfer printingof electronic circuits (<15 s),[31] at least 2 h of physical contactwas imperative for the defect-free transfer printing of metallicnanoarrays. This is possibly due to the small contact area andgeometry of isolated nanoparticles. Moreover, strong cohesiveforce between the noble metal particles could be anotherpossibility in making a significant difference on transfer printingrate between integrated circuits and particulate patterns. Wehave shown representative bowtie arrays transfer-printed undernonoptimal conditions on PDMS and SU-8, with several defectsat contact times of less than 30 min in Fig. S3 (Supportinginformation). On the other hand, successfully integrated bowtiearrays on various material surfaces are shown in Fig. 2(b–d)along with a 30-nm silver-modified silicon stamp (Fig. 2(a)). Underoptimal transfer conditions, we observed less than 1% missingnanotriangles or nanocircles in a 50 × 50 µm2 array. However,other minor defects, such as scattered silver dots, were occasionallyobserved in a soft PDMS substrate, while it was not the case inrigid substrates.

SERS detection

The stamping technique employed to integrate metal nanopar-ticles on three distinct dielectric substrates were tested for SERSdetection. Studies were carried out using extended arrays ofbowtie and dumbbell structures. All the adjustable parameters onEBL-created stamps were kept constant, whereas the refractiveindices of SERS substrates were different, i.e. PDMS elastomer(1.47),[33] glass slide (1.51),[34] and SU-8 resist (1.59).[35] Self-assembled monolayers (SAMs) of BT and 5×10−6 M R6G were usedas SERS probes. Raman peak intensities of BT and R6G harvestedfrom the respective SERS substrates are shown in Fig. 3. Each datapoint represents the average SERS intensity collected from 25different spots within 20 × 20 µm2 areas. The average values, in-stead of the best intensities, were considered in order to minimizepossible errors in SERS intensity that could have resulted fromminor variations, including the occurrence of satellite particles[36]

on a soft substrate, in the fidelity of the transfer process. For bothprobe molecules, Raman signals from the high-refractive-index

Figure 3. SERS intensities of benzenethiol peak centered at 1575 cm−1

and rhodamine 6G peak centered at 763 cm−1, with respect to variousunderlying substrates using extended arrays of silver (a) bowtie and(b) dumbbell.

SU-8 substrate were stronger than from PDMS or glass substrates.On the basis of refractive index alone, one might expect that aglass substrate would give a larger response than the PDMS, butthat was not observed. This is possibly due to the different surfaceproperty of the PDMS elastomer. PDMS is a cross-linked polymerhaving a non-cross-linked layer on the surface. The presence ofthis layer could possibly result in the formation of subsurface-embedded nanoparticles.[37] In other words, unlike the glass andSU-8 substrates, nanoparticles on PDMS elastomer may be par-tially submerged into the polymer matrix and are exposed to moreof the substrate. As a result, the nanoparticles have plasmon fre-quencies that are altered due to the change in the local dielectric,and hence act differently in SERS. This sort of understanding basedupon effective refractive index considerations has been previouslyreported by Novo et al.[18]

We have investigated the possibility of the nanoparticlessubmerging into the substrate theoretically using classicalelectrodynamics (simulation details are given in the section onTheoretical Calculations). The electric field modulus was integratedover the volume of a rectangular prism that enclosed the


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Figure 4. Theoretical calculations of the figures of merit: (a) Geometry of both nanoparticles configurations within the unit cell, where a = 1100 nm andb = 450 nm. The nanoparticles are composed of silver and reside on a material substrate with refractive index nm . (Left) bowtie: w = 650 nm, v = 300 nm,g = 50 nm, p = 30 nm. (Right) dumbbell: d = 300 nm, g = 50 nm, p = 30 nm. (b) Figures of merit F for glass, PDMS, and SU8 reported relative to themaximum SU-8 value. Blue corresponds to the bowtie and red corresponds to the dumbbell configuration; (c) Variation of F as a function of the sinkingparameter s in PDMS. Again, the results are reported relative to the SU-8 bowtie value.

nanoparticle dimer and the spatial region in between. Figure 4(b)displays the results for the bowtie and dumbbell structure on glass,PDMS, and SU-8. The trend observed theoretically is consistentwith the experimental results shown in Fig. 3 for both probemolecules. Figure 4(c) shows the effect on the integrated fieldgenerated by sinking the particles into a PDMS substrate. Forboth dimers, there is an initial increase followed by a decline.This is due to an increase in the effective refractive index ofthe nanoparticles, with a corresponding shift of the resonanceand an increase in the plasmon field, coupled with a decreasein integrated volume (only spatial regions above the substratewere included to represent regions accessible to molecules). Amaximum is found at a submersion depth of ∼5 nm for the bowtieand ∼10 nm for the dumbbell. It is these maximum values thatare reported in Fig. 4(b) for the PDMS and suggest that the arraysare submerged in this substrate. Note that, if the particles wererestricted to the surface, then the results for glass and PDMSwould be reversed following the expected trend in the refractiveindex of the substrate. An additional consideration for PDMS-based substrates is that the elastomer is a common solid-phaseextraction material and can enhance nanoparticle accessibility forthe analytes.[37 – 39] Accessibility could include the submerged sideas well, although not considered in the enhancement factor (EF)evaluation performed here (see below).

Generally speaking, regardless of nanoparticle morphology,nTP integrated nanostructures are aware of the underlyingsubstrate properties and so the created SERS substrates canexploit this to optimize performance. In other words, the harmonybetween nTP-created SERS substrates and the recently quotedsubstrate dielectric effect provides new insight into the selectionof an appropriate substrate and/or nanoparticle morphology forimproved sensitivity.

Using an extended array of silver bowtie, the SERS reproducibil-ity of the silicon stamp and the corresponding nTP pattern wereinvestigated. In each case, BT-loaded extended arrays of silver

bowtie were mapped over 20×20 µm2 area in a 5-µm subsequentstep. Based on our observation, the relative standard deviation(RSD) of about 9% and 23% were recorded for the silicon stampand the corresponding pattern on SU-8 substrate, respectively. Thehigher RSD of nTP-created SERS-active surface is presumably dueto the minor defects originated during transfer printing. Shown inFig. 5(a) (left) are the superimposed spectra of BT collected from25 different spots on the silver-modified silicon stamp. Each colorpixel in Fig. 5(a) (right) gives information on BT peak intensity cen-tered at 1575 cm−1. Similarly, Fig. 5(b) shows the result collectedfrom the SU-8 substrate.

It is possible to use the known packing density of BT and acomparison between conventional Raman and SERS to estimatethe SERS EF.[1,25,40] EFs of all nTP-SERS substrates employed in thisstudy were estimated. The details of the procedure we followedto calculate EF are presented in our previous publication.[1] Withreference to the 1575 cm−1 peak, we determined that the averageenhancement of nTP-created SERS substrates varied by morethan one order of magnitude, depending on the nature ofsupporting film (Table 1). Although the highest EF observed inour study is 5.37 × 107, with better rationally designed andnanolithographically fabricated nanoarrays and/or a selectionof other suitable substrate, significant improvement in EMenhancement may be possible.

It is now well established that the SERS signal can be obtainedfrom a variety of substrates, ranging from a single nanoparticleto periodic arrays or nanoaggregates of various morphologies.In order to demonstrate the versatility of nTP, a range ofplanar SERS substrates were created on both soft and rigidsubstrates. Shown in Fig. 6(a) and (b) are transfer-printed single-nanoparticle (diameter = 150 nm) and 4 × 4 arrays of similar sizednanoparticles on a glass surface. Such structures are important,especially in understanding the plasmonic behavior of metalnanoparticles, both experimentally and theoretically. On the otherhand, Fig. 6(c) shows a scanning electron micrograph of a relatively


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Figure 5. SERS spectra of benzenethiol collected from (a) silver modified silicon pillars (RSD ∼9%), and (b) similar nanopattern on SU-8 surface (RSD∼23%). Figures shown on the right-hand side are the spectral mapping of probe molecules over 400 µm2, and those on left-hand side are thesuperimposed spectra.

Table 1. Average SERS enhancement factors of dimer nanoarraysstamped on various supporting films

Nanopatterns Supporting films Enhancement factor

Bowtie antenna SU-8 5.4 × 107

PDMS 4.1 × 106

Glass 2.9 × 106

Dumbbell antenna SU-8 1.2 × 107

PDMS 4.0 × 106

Glass 2.3 × 106

large square pattern integrated on a PDMS substrate. A large120×120 µm2 square pattern was cloned with two different kindsof lithographically designed nanoaggregates (LDNAs). Relativelyless dense LDNAs were cloned at the center of a macropatternforming the shape of a ‘T’ and were surrounded by replicas ofdenser LDNAs (Fig. 6(c) and (d)). Our recent LDNAs-SERS studiesshowed that less dense nanoaggregates produce stronger SERSintensity than the denser one.[3] To demonstrate the legitimacy ofnTP-created relatively large SERS-active surfaces, the entire squarepattern on PDMS was mapped. A significant difference in the SERSintensity was observed between the two LDNAs. Results showedthat the average SERS intensity of the T-cloned nanoaggregatesis roughly 3 times stronger than the surrounding nanoaggregates(Fig. 6(e) and, (f)). The observed spectral trend is consistent with theprevious LDNAs-SERS result[3] and hence demonstrates the abilityof nTP-created patterns to retain their inherent SERS activity overlarge areas.

Nanogap manipulation

More than three decades after the discovery of SERS, fundamentalchallenges still exist. One of the most significant involvesthe so-called nanogap effect.[41 – 46] Several theoretical studies

have predicted that the SERS enhancement field stronglydepends on the spacing between plasmonic nanoparticles,with a dramatic field enhancement when distance betweennanoparticles decreases to near-molecular dimensions.[42,43,46]

However, very recently several groups have published resultson the maximum SERS enhancement with a capability of single-molecule detection from a variety of nanostructures, ranging from‘just touching’ dimer[41] to 30-nm-spaced one-dimensional golddisks.[44] Such diverse SERS phenomena can be perplexing tothe scientific community. This is partly due to the difficulty ofroutinely delivering nanoparticles with controllable sub-10-nminterparticle spacing using existing nanofabrication techniques.Therefore, there is a need for a simple, cost-effective, and handyprotocol that enables the manipulation of the nanogap reversibly.In this work, bowtie arrays stamped onto a stretched PDMS filmfollowed by the physical relaxation of a polymer demonstrate thefeasibility of nanogap reduction. Arrays of 150 × 150 × 30 nm3

(width × height × thickness) gold triangles with 100-nm intra-and 300-nm inter-dimer spacings (prior to gold deposition)were transfer-printed onto a ∼150% stretched PDMS elastomer.Bowtie-array-incorporated stretched PDMS was then physicallyrelaxed to its normal position (∼100%) in an in-house-builtstretching/relaxing device. Figure 7 shows that, on relaxing thestretched PDMS, there is a considerable reduction of nanogapin bowtie arrays. The concept presented here opens up thepossibility to monitor nanogap spacing without any rigorousstep and hence facilitates the better understanding of plasmoniceffects related to the nanogap. Efforts to better control gap spacingand environment are ongoing in our laboratory.

Nanoarrays on a microfiber facet

There is also significant interest toward the development ofintrinsic SERS-active fiber optics, which can be utilized forboth biomedical and remote sensing applications.[47,48] Althoughnanolithographic techniques like EBL and nanosphere lithography


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Figure 6. Demonstrating the ability to integrate a single-particle to LDNAs macropattern on various substrates: (a, b) single-particle and 4 × 4 arrays ofsilver nanoparticles on glass substrate. (c, d) relatively large square patterns, consisting of two different types of LDNAs, on a PDMS substrate. (e, f) typicalSERS spectra of BT harvested from two different clones.

Figure 7. Demonstrating the nanogap reduction by physical manipulation of a elastomer: SEMs of bowtie arrays stamped on (a) a stretched PDMS and(b) the same pattern after relaxing elastomer. Figures shown at the top are schematics of the gap manipulation.

are broadly used in creating SERS substrates, such techniques arenot straightforwardly adaptable to the fiber optic geometry. Inthis work, as a proof of concept, silver arrays defined on a siliconstamp were successfully incorporated on a micro-fiber optic facet(core diameter = 400 µm). It is noteworthy that, in order to allowconformal contact between the stamp and an optical fiber, specialattention is required on the surface polishing protocol. Briefly, asmall piece of Teflon disk (2 cm thick and 4 cm diameter) was drilledthrough using a drill bit of the optical fiber outer diameter. The

optical fiber was then carefully fitted into the through hole of theTeflon. In order to fix the fiber firmly, one side of the Teflon disk wascoated with epoxy glue while the uncoated side was sequentiallypolished using various sizes of polishing grits, i.e. 400, 600, 1200,2400, 4000, followed by a 0.05-µm alumina slurry. The polishedfiber surface was then brought into conformal contact to facilitatetransfer printing (Fig. S4, Supporting information). Figures 8a andb show, respectively, the transfer printed silver arrays and thesilicon stamp prior to silver deposition. Our result emphasizes


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Figure 8. SEMs of dumbbell arrays: (a) integrated on the fiber optic facetand (b) original silicon relief pillars prior to silver deposition.

the significance of the stamping protocol to integrate metalnanoparticles on an optical fiber facet, without any harsh step. Theconcept presented here would support the development of SERS-active fiber optic sensors. Similarly, EBL-created nanoparticlescan be incorporated onto micro/nanofluidics channel and otherrelated photonic devices.

Conclusions

For the first time, we incorporated surface chemistry-mediatedplasmonic arrays on three distinct dielectric surfaces using EBL-crafted silicon relief pillars. To facilitate transfer-printing of anisolated nanoparticle, careful optimization is required to bringthe stamp and the substrate into conformal contact. ThesenTP-created plasmonic nanoarrays on dielectric surfaces retaintheir ability to amplify Raman signals. Both experimental andtheoretical results showed that the SERS enhancement variesfrom substrate to substrate, and nTP-created SERS substratesexploit this property for better field enhancement. This workalso introduced SU-8-based SERS substrates. This SERS substrateprovides higher enhancement magnitudes than the concurrentlystudied conventional glass and PDMS substrates. The techniquecan also be easily extended to other dielectric surfaces for betterfield enhancement. An additional attribute of the current approachis toward the incorporation of metal nanodisks on unconventionalenvironments like fiber optics, lab-on-chip, and so on for in situSERS sensing and other related plasmonic applications.

Acknowledgements

This research was supported by the U.S. Environmental ProtectionAgency STAR program under Grant EPA-83274001 with theUniversity of Tennessee. The nanofabricated substrates werecreated at Oak Ridge National Laboratory’s Center for NanophaseMaterial Sciences (CNMS), sponsored by the Scientific UserFacilities Division, Office of Basic Energy Sciences, U.S. Departmentof Energy. KLS thanks Drexel University for start-up funding. Weare thankful to Dr Jon P. Camden and Christopher Bennett of UTK

for helpful discussions and Dr John Dunlap of UTK for assistancewith AFM.

Supporting information

Supporting information may be found in the online version of thisarticle.

References

[1] J. M. Oran, R. J. Hinde, N. Abu Hatab, S. T. Retterer, M. J. Sepaniak,J. Raman Spectrosc. 2008, 39, 1811.

[2] H. Wang, G. M. Laws, S. Milicic, P. Boland, A. Handugan, M. Pratt,T. Eschrich, S. Myhajlenko, J. A. Allgair, B. Bunday, J. Vac. Sci. Technol.B 2007, 25, 102.

[3] S. M. Wells, S. D. Retterer, J. M. Oran, M. J. Sepaniak, Acs Nano 2009,3, 3845.

[4] S. Y. Chou, P. R. Krauss, P. J. Renstrom, J. Vac. Sci. Technol. B 1996,14, 4129.

[5] S. Y. Chou, P. R. Krauss, W. Zhang, L. J. Guo, L. Zhuang, J. Vac. Sci. &Technol. B 1997, 15, 2897.

[6] Y. N. Xia, G. M. Whitesides, Annu. Rev. Mater. Sci. 1998, 28, 153.[7] Y. N. Xia, G. M. Whitesides, Angew. Chem. Int. Ed. 1998, 37, 551.[8] J. Aizenberg, J. A. Rogers, K. E. Paul, G. M. Whitesides, Appl. Opt.

1998, 37, 2145.[9] H. I. Smith, N. Efremow, P. L. Kelley, J. Electrochem. Soc. 1974, 121,

1503.[10] M. Fleischmann, P. J. Hendra, A. J. McQuilla, Chem. Phys. Lett. 1974,

26, 163.[11] J. A. Creighton, C. G. Blatchford, M. G. Albrecht, J. Chem. Soc.

Faraday Trans. II 1979, 75, 790.[12] M. Moskovits, J. Chem. Phys. 1978, 69, 4159.[13] M. Moskovits, in Surface-Enhanced Raman Scattering: Physics

and Applications, vol. 103 (Eds: Katrin Kneipp, Martin Moskovits,Harald Kneipp), Springer-Verlag Berlin; Germany, 2006, p 1.

[14] A. J. Haes, C. L. Haynes, A. D. McFarland, G. C. Schatz, R. R. VanDuyne, S. L. Zou, MRS Bull. 2005, 30, 368.

[15] G. C. Schatz, M. A. Young, R. P. Van Duyne, in Surface-EnhancedRaman Scattering: Physics and Applications, vol. 103 (Eds:Katrin Kneipp, Martin Moskovits, Harald Kneipp), Springer-VerlagBerlin: Germany, 2006, p 19.

[16] M. K. Kinnan, S. Kachan, C. K. Simmons, G. Chumanov, J. Phys. Chem.C 2009, 113, 7079.

[17] M. D. Malinsky, K. L. Kelly, G. C. Schatz, R. P. Van Duyne, J. Phys.Chem. B 2001, 105, 2343.

[18] C. Novo, A. M. Funston, I. Pastoriza-Santos, L. M. Liz-Marzan,P. Mulvaney, J. Phys. Chem. C 2008, 112, 3.

[19] L. J. Sherry, R. C. Jin, C. A. Mirkin, G. C. Schatz, R. P. Van Duyne, NanoLett. 2006, 6, 2060.

[20] M. A. De Jesus, K. S. Giesfeldt, J. M. Oran, N. A. Abu-Hatab,N. V. Lavrik, M. J. Sepaniak, Appl. Spectrosc. 2005, 59, 1501.

[21] N. Felidj, J. Aubard, G. Levi, J. R. Krenn, M. Salerno, G. Schider,B. Lamprecht, A. Leitner, F. R. Aussenegg, Phys. Rev. B 2002, 65,075419.

[22] L. Gunnarsson, E. J. Bjerneld, H. Xu, S. Petronis, B. Kasemo, M. Kall,Appl. Phys. Lett. 2001, 78, 802.

[23] S. E. Hunyadi, C. J. Murphy, J. Mater. Chem. 2006, 16, 3929.[24] Y. G. Sun, Y. N. Xia, Science 2002, 298, 2176.[25] D. Bhandari, S. M. Wells, S. T. Retterer, M. J. Sepaniak, Anal. Chem.

2009, 81, 8061.[26] D. Bhandari, M. J. Walworth, M. J. Sepaniak, Appl. Spectrosc. 2009,

63, 571.[27] www.cst.com.[28] T. Weiland, Int. J. Electron. Commun. (AEU) 1977, 31, 116.[29] E. Palik (Ed.), in Handbook of Optical Constants of Solids, Academic

Press: Orlando, 1985.[30] Y. L. Loo, R. L. Willett, K. W. Baldwin, J. A. Rogers, J. Am. Chem. Soc.

2002, 124, 7654.[31] Y. L. Loo, R. L. Willett, K. W. Baldwin, J. A. Rogers, Appl. Phys. Lett.

2002, 81, 562.[32] M. H. Hu, S. Noda, T. Okubo, Y. Yamaguchi, H. Komiyama, Appl. Surf.

Sci. 2001, 181, 307.[33] A. D. Wellman, M. J. Sepaniak, Anal. Chem. 2007, 79, 6622.


19

24

D. Bhandari et al.

[34] S. Chah, E. Hutter, D. Roy, J. H. Fendler, J. Yi, Chem. Phys. 2001, 272,127.

[35] B. H. Ong, X. C. Yuan, S. C. Tjin, Appl. Opt. 2006, 45, 8036.[36] A. Gopinath, S. V. Boriskina, W. R. Premasiri, L. Ziegler,

B. M. Reinhard, L. Dal Negro, Nano Lett. 2009, 9, 3922.[37] K. S. Giesfeldt, R. M. Connatser, M. A. De Jesus, N. V. Lavrik, P. Dutta,

M. J. Sepaniak, Appl. Spectrosc. 2003, 57, 1346.[38] R. M. Connatser, L. A. Riddle, M. J. Sepaniak, J. Sep. Sci. 2004, 27,

1545.[39] K. S. Giesfeldt, R. M. Connatser, M. A. De Jesus, P. Dutta,

M. J. Sepaniak, J. Raman Spectrosc. 2005, 36, 1134.[40] C. L. Haynes, R. P. Van Duyne, J. Phys. Chem. B 2003, 107, 7426.[41] J. P. Camden, J. A. Dieringer, Y. M. Wang, D. J. Masiello, L. D. Marks,

G. C. Schatz, R. P. Van Duyne, J. Am. Chem. Soc. 2008, 130, 12616.

[42] E. Hao, G. C. Schatz, J. Chem. Phys. 2004, 120, 357.[43] R. J. Hinde, M. J. Sepaniak, R. N. Compton, J. Nordling, N. Lavrik,

Chem. Phys. Lett. 2001, 339, 167.[44] L. D. Qin, S. L. Zou, C. Xue, A. Atkinson, G. C. Schatz, C. A. Mirkin, Proc.

Natl Acad. Sci. U.S.A. 2006, 103, 13300.[45] D. R. Ward, N. K. Grady, C. S. Levin, N. J. Halas, Y. P. Wu, P. Nordlander,

D. Natelson, Nano Lett. 2007, 7, 1396.[46] H. X. Xu, E. J. Bjerneld, M. Kall, L. Borjesson, Phys. Rev. Lett. 1999, 83,

4357.[47] A. Dhawan, J. F. Muth, D. N. Leonard, M. D. Gerhold, J. Gleeson,

T. Vo-Dinh, P. E. Russell, J. Vac. Sci. Technol. B 2008, 26, 2168.[48] E. J. Smythe, M. D. Dickey, J. M. Bao, G. M. Whitesides, F. Capasso,

Nano Lett. 2009, 9, 1132.


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