Self-Assembled Plasmonic Nanohole Arrays

Self-Assembled Plasmonic Nanohole Arrays

(Parte 1 de 3)

DOI: 10.1021/la9020614 13685Langmuir 2009, 25(23), 13685–13693 Published on Web 10/15/2009 pubs.acs.org/Langmuir ©2009 American Chemical Society

Self-Assembled Plasmonic Nanohole Arrays

Si Hoon Lee,†, Kyle C. Bantz,‡, Nathan C. Lindquist,§ Sang-Hyun Oh,*,†,§ and Christy L. Haynes*,‡

†Department of Biomedical Engineering, 312 Church Street SE and ‡Department of Chemistry, 207 Pleasant

Street SE and §Department of Electrical and Computer Engineering, 200 Union Street SE, University of Minnesota, Twin Cities, Minneapolis, Minnesota 55455. These authors contributed equally to this work.

Received June 9, 2009. Revised Manuscript Received September 4, 2009

We present a simple and massively parallel nanofabrication technique to produce self-assembled periodic nanohole arrays over a millimeter-sized area of metallic film, with a tunable hole shape, diameter, and periodicity. Using this method, 30 30 μm2 defect-free areas of 300 nm diameter or smaller holes were obtained in silver; this area threshold is critical because it is larger than the visible wavelength propagation length of surface plasmon waves (∼27 μm) in the silver film. Measured optical transmission spectra show highly homogeneous characteristics across the millimeter-size patterned area, and they are in good agreement with FDTD simulations. The simulations also reveal intense electric fields concentrated near the air/silver interface, which was used for surface-enhanced Raman spectroscopy (SERS). Enhancement factors (EFs) measured with different hole shape and excitation wavelengths on the self-assembled nanoholearrayswere104-106.WithanadditionalAgelectrolessplatingstep,theEFwasfurtherincreasedupto3 106.The periodic nanohole arrays produced using this tunable self-assembly method show great promise as inexpensive SERS substratesas well as surfaceplasmonresonancebiosensing platforms.

Introduction

Since Ebbesen and co-workers discovered extraordinary optical transmission (EOT) through subwavelength noble metal nanohole arrays1,2 there has been significant effort to fabricate nanohole arrays with well-controlled electromagnetic properties.Surfaceplasmon(SP)waves;collectiveoscillationsof conduction electrons;contribute directly to the EOT effect,3,4 while also facilitating surface-enhanced spectroscopies. For the most part, nanohole arrays have been fabricated using expensive and time-intensive high-resolution serial techniques such as electron beam lithography (EBL)5 and focused ion beam (FIB) milling.1,6Insomecases,advancedsoftlithographymethodssuch as PEEL (a combination of phase-shift lithography, etching, electron-beam deposition, and lift-off) or soft nanoimprint techniques have been successfully employed to fabricate nanohole arrays.7,8 In a recent example, Chen et al. fabricated square lattice gold nanohole arrays with sub-250 nm diameter using UV nanoimprint lithography combined with reactive ion etching and a Cr/Au lift-off process.9 With this method, the authors varied the nanohole diameter and periodicity with well-ordered nanoholearraysupto1cm2andexaminedsystematicshifts inthe transmission spectra of structural variations, baths with varied refractive index, and thiol chemisorption. While these methods have facilitated important fundamental studies, the field would greatly benefit from a simpler massively parallel fabrication method that can pattern nanohole arrays with a deep ultraviolet (DUV) patterning resolution, i.e., around 200 nm, without using an exposure tool, photomask, or imprint mold. The work presented herein capitalizes on the nanosphere lithography (NSL) technique conceived of (as “natural lithography”) by Deckman et al.10 and popularized by Van Duyne and co-workers.1 Instead of employing an as-assembled 2D colloidal array as ashadowmaskfornanostructuredeposition,thisworkemploysa reactive ion etching (RIE) step to shrink the nanospheres before metal deposition, facilitating the formation of nanohole arrays after removal of the nanospheres. By controlling the original nanosphere size, etching time, metal deposition thickness, and metal deposition angle,itis possibletotunethe nanoholespacing, size and aspect ratio, and, accordingly, the plasmonic properties.

While EBL and FIB have been employed in the vast majority of nanohole array studies, there have been a few advances in addition to the aforementioned PEEL and nanoimprint techniques toward implementing high throughput methods. As early as 1995,Masudaetal.usedananodicaluminatemplateandmultiple polymer/metal deposition steps to achieve a nanohole array structure; however, this work did not focus on topographic tunability, optical characterization,oruseofthesubstrate asasurfaceenhanced spectroscopy platform.12 A decade later, Jiang and McFarland employed a specialized spin-coating technique to create 2D non-close-packed nanospheres for use as a deposition mask, again focusing only on structural fabrication and characterization.13 More recently, Peng and Kamiya formed randomly

*To whom correspondence should be addressed. E-mail: sang@umn.edu

(S.-H.O); chaynes@umn.edu (C.L.H.). (1) Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F.; Thio, T.; Wolff, P. A. Nature 1998, 391 (6668), 667. (2) Genet, C.; Ebbesen, T. Nature 2007, 445 (7123),3 9-46. (3) Gao, H.; Henzie, J.; Odom, T. W. Nano Lett. 2006, 6 (9), 2104-2108. (4) Liu, H.; Lalanne, P. Nature 2008, 452 (7188), 728-731. (5) Altewischer, E.; Genet, C.; van Exter, M. P.; Woerdman, J. P.; Alkemade,

P. F. A.; van Zuuk, A.; van der Drift, E. W. J. M. Opt. Lett. 2005, 30 (1),9 0-92. (6) Brian, B.; Sepulveda, B.; Alaverdyan, Y.; Lechuga, L. M.; Kaell, M. Opt.

Express 2009, 17 (3), 2015-2023. (7) Kwak, E.-S.; Henzie, J.; Chang, S.-H.; Gray, S. K.; Schatz, G. C.; Odom,

T. W. Nano Lett. 2005, 5 (10), 1963-1967. (8) Stewart, M. E.; Mack, N. H.; Malyarchuk, V.; Soares, J.; Lee, T. W.; Gray,

S. K.; Nuzzo, R. G.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103 (46), 17143-17148. (9) Chen, J.; Shi, J.; Decanini, D.; Cambril, E.; Chen, Y.; Haghiri-Gosnet, A. Microelectron. Eng. 2009, 86 (4-6), 632-635.

Article Lee et al.

orderednanoholearraysbyspin-coatingasolutionofpolystyrene beads onto the surface of a silicon wafer and controlling relative bead density by adjusting the ionic strength of the nanoparticle solution, followed by Au film deposition.14 Nanoholes were then formed by scanning probe microscopy tip machining, where the tip is used to expose the gold-coated polystyrene bead, and then wet chemistry is used to remove the remaining polystyrene. These nanoholes were characterized using atomic force microscopy and used to create nanoscale transistors. With a similar focus on onlyfabricationmethodology,close-packedNSL-assemblednanospheres were recently used by Wu et al. as nanolenses for forming ananoholearray.15Earlierthisyear,Leeandco-workersreported the combination of NSL and vertical angle ion milling to create nanocrescent holes; however, the holes are disordered, with variable spacing throughout the sample. Surface plasmon resonance (SPR) characterization of the nanocrescent holes was completed, but the nanostructures were not assessed as surfaceenhanced spectroscopy substrates.16

While NSL is most often employed as a direct templating method, where metal is deposited into the void space between asassembled nanospheres, there is precedent for creating more complex nanostructures by combining NSL with plasma or reactive ion etching (RIE) as is done in this work. Van Duyne and co-workers combined NSL and RIE to fabricate embedded triangular nanoparticles that are less prone to solvent remodeling than their nonembedded counterpart.17 After assembling the nanosphere mask, they use CF4 to etch the glass between nanospheresandthendepositAgthroughthenanospheresandintothe wells. Variation of CF4 etch time facilitated plasmon tuning, and SERS enhancement factors as high as 108 were estimated for chemisorbed benzenethiol. Huang et al. used colloidal templating to create a double-layer colloidal crystal mask with two different nanosphere diameters and performed an oxygen etch, a Au deposition, and removal of the top colloidal crystal layer to create gold nanohole substrates.18 While plasmonic characterization was not performed, the specular reflection was shown to be attenuated compared to glass substrates. Live and Masson prepared microhole arrays by drop-coating a polymer microsphere suspension onto glass slides followed by plasma treatment to reduce the size of the spheres and then metal evaporation.19 Controlled evaporation of the microsphere solution yielded large defect-free areas. Both the localized and propagating plasmons were characterized for these substrates with varied hole diameters and refractive index environments, suggesting utility for microhole arrays in plasmonic biosensing; however, nanoscale features were not attemptedin thiswork.Ctistisetal. recentlyreportedthe use of the same method to fabricate nanohole arrays but was focused on the size-dependent magnetic properties of nanoholes in a cobalt thin film.20 Earlier this year, Lou et al. published a short report where NSL and RIE were combined to create nanoholes in Au.21 This work characterized plasmonic transmission with a fixed hole diameter and varied spacing and used the nanohole array as a substrate for cell growth. Unfortunately, the typical defect-free domain sizes were small, only a few micrometers, impeding many potential sensor applications. Clearly, there is a great interest and need for massively parallel nanohole fabrication with well-characterized plasmonic and surface-enhanced spectroscopic properties; this is the goal of the work presented herein.

Significant progress has been made toward understanding and modeling the plasmonic properties of nanohole arrays fabricated by traditionalEBL orFIB methods. A recentsystematicstudy by Lee et al. compared the SPR spectra and refractive index sensitivity of EBL-fabricated nanohole and nanoslit arrays.2 K€all and co-workers demonstrated the influence of the dielectric substrate’s refractive index on the SPR shift per refractive index unit (i.e., the sensitivity) that can be achieved using FIB-fabricated nanohole arrays.6 Parsons et al. examined the effect of nanohole periodicity on the resultant SPR both experimentally using FIB fabrication and in finite-difference time-domain (FDTD) modeling.23 Schatz and co-workers performed FDTD modelingofbothisolatedandsquarearraysofnanoholesinAuto predict the electromagnetic fields due to the interaction of the propagating and localized surface plasmons as well as the optical transmission spectra.24 Their modeling predicted concentrated fields at both the top and bottom surface of the nanoholes and transmissionspectrawithmultiplefeaturesfromnanoholearrays. Optical properties have also been characterized in more exotic versions of the nanohole array. For example, Kim et al. characterized optical transmission through arrays of equilateral triangle nanoholes made by ion milling,25 and Brolo and coworkersdemonstrated enhancementsinspontaneousemissionby coupling quantum dots to plasmonic FIB-fabricated nanohole arrays.26 These fundamental studies of nanohole optical properties, and correlations with FDTD modeling, are critical for the application of these substrates.

Several groups have also used nanohole arrays for SPR biosensing and imaging.8,27-31 In recently published work, Oh and co-workers achieved multiplex SPR biosensing and measurement of binding kinetics using FIB-fabricated nanohole arrays in a microarrayformat.32-34Brolo and co-workers alsointegrateda FIB-manufactured nanohole array into a fluidic device where chemical species could flow through the etched nanoholes, rather than just over the top, in an effort to maximize molecular binding events at the hole edges where the electromagnetic fields should be most intense.35 In an alternate approach to achieve maximal refractive index sensitivity, Brolo and co-workers also demonstrated SPR sensing of binding events after coating the

(14) Peng, X.; Kamiya, I. Nanotechnology 2008, 19 (31). (15) Wu, W.; Dey, D.; Memis, O. G.; Katsnelson, A.; Mohseni, H. Proc. SPIE 2008, 7039, 70390P/1-70390P/8. (16) Wu, L. Y.; Ross, B. M.; Lee, L. P. Nano Lett. 2009, 9 (5), 1956-1961. (17) Hicks, E.; Lyandres, O.; Hall, W.; Zou, S.; Glucksberg, M.; Van Duyne, R.

M. Nano Lett. 2009, 9 (1),1 -6. (21) Lou, Y.; Westcott, N.; Mcglade, J.; Muth, J.; Yousaf, M. Mater. Res. Soc. Symp. Proc. 2009, 13.

Gordon, R.; Riordon, J.; Kavanagh, K. L. J. Phys. Chem. B 2006, 110 (16), 8307-8313. (27) Brolo, A. G.; Arctander, E.; Gordon, R.; Leathem, B.; Kavanagh, K. L.

Nano Lett. 2004, 4 (10), 2015-2018. (28) Stark, P. R. H.; Halleck, A. E.; Larson, D. N. Methods 2005, 37 (1),3 7-47. (29) Tetz, K. A.; Pang, L.; Fainman, Y. Opt. Lett. 2006, 31 (10), 1528-1530. (30) Lesuffleur, A.; Im, H.; Lindquist, N. C.; Oh, S. H. Appl. Phys. Lett. 2007, 90, 243110. (31) Yang, J.-C.; Ji, J.; Hogle, J. M.; Larson, D. N. Nano Lett. 2008, 8 (9), 2718-2724. (32) Lesuffleur, A.; Im, H.; Lindquist, N. C.; Lim, K. S.; Oh, S. H. Opt. Express 2008, 16 (1), 219-224. (3) Im, H.;Lesuffleur, A.; Lindquist, N.C.; Oh, S.-H. Anal.Chem. 2009, 81(8), 2854-2859. (34) Lindquist, N. C.; Lesuffleur, A.; Im, H.; Oh, S. H. Lab Chip 2009, 9 (3), 382-387. (35) Eftekhari, F.; Escobedo, C.; Ferreira, J.; Duan, X.; Girotto, E. M.; Brolo, A. G.; Gordon, R.; Sinton, D. Anal. Chem. 2009, 81 (1), 4308-431.

Lee et al. Article nanohole array with a dielectric layer such that the only revealed Au surface was within the interior area of the nanoholes.36 While each of these examples demonstrates the capability of nanohole arraystoactassensorsignaltransductionelements,effortstodate have largely been limited to model analyte systems (e.g., biotin/ streptavidin) partly due to the low throughput and expensive nature of nanohole array fabrication strategies.

Because nanohole arrays show great promise for SPR sensing, they are also inherently promising as surface-enhanced spectroscopy substrates. Surface-enhanced Raman spectroscopy (SERS) is the most commonly employed of the surface-enhanced phenomena. Since its discovery in 1977 by Van Duyne and coworkers,37 at least 9000 papers have been published on the topic of SERS with a drastic uptick in interest in the past decade as nanofabrication methods and electromagnetic modelingtools are refined and as SERS becomes an accepted analytical technique. SERS occurs when a Raman-active molecule is localized within thelarge electromagnetic fields that are generated uponexcitation oftheSPR andprovidesmanyordersofmagnitudeenhancement over normal Raman scattering. To date, there are only a few examples of SERS measurements using nanohole arrays as the enhancing substrate. Brolo and co-workers measured SERS spectra from both copper and gold nanohole arrays fabricated using FIB but did not quantify the enhancement factor based on complications in their experimental system.27,38 Wallace and co-workers also measured SERS spectra from EBL-fabricated Au nanohole arrays, achieving enhancement factors as high as 4.2 105.39 Rowlen and co-workers used EBL-fabricated nanohole arrays to demonstrate that SERS enhancement decreased as hole lattice spacing increased.40 While these results are promising, further demonstration of SERS on nanohole arrays is warranted, especiallyifthiscan beaccomplishedusing arraysproducedusing a massively parallel fabrication strategy.

(Parte 1 de 3)

Comentários