Silver half-shell arrays with controlled plasmonic response for fluorescenceenhancement optimizationCosmin Farcău and Simion Aştilean Citation: Applied Physics Letters 95, 193110 (2009); doi: 10.1063/1.3263193 View online: http://dx.doi.org/10.1063/1.3263193 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spontaneous emission and collection efficiency enhancement of single emitters in diamond via plasmoniccavities and gratings Appl. Phys. Lett. 103, 161101 (2013); 10.1063/1.4817397 Polymeric photovoltaics with various metallic plasmonic nanostructures J. Appl. Phys. 113, 063109 (2013); 10.1063/1.4790504 Controlled addressing of quantum dots by nanowire plasmons Appl. Phys. Lett. 100, 231102 (2012); 10.1063/1.4725490 Excitation of dielectric-loaded surface plasmon polariton observed by using near-field optical microscopy Appl. Phys. Lett. 93, 073306 (2008); 10.1063/1.2973355 Metal-enhanced fluorescence: Surface plasmons can radiate a fluorophore’s structured emission Appl. Phys. Lett. 90, 053107 (2007); 10.1063/1.2435661

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Silver half-shell arrays with controlled plasmonic response for fluorescenceenhancement optimization

Cosmin Farcăua� and Simion AştileanFaculty of Physics, Babes-Bolyai University, M. Kogalniceanu 1, 400084 Cluj-Napoca, Romania

�Received 13 July 2009; accepted 22 October 2009; published online 13 November 2009�

Regular arrays of interconnected silver half-shells �HSs� deposited on self-organized polystyrenemicrospheres are proposed as plasmonic substrates for metal-enhanced fluorescence. An emissionenhancement of 28 times was demonstrated for Rose Bengal fluorophore placed at about 1 nm aboveHSs. The enhancement correlates with the spectral overlap between the fluorophore emission andthe plasmonic resonance of the HSs, indicating a surface plasmon-coupled emission mechanism forthe amplification. As the overlap can be easily tuned by controlling the diameter of underlyingmicrospheres, such plasmonic structures could be relevant for building fluorescence-based sensingdevices with optimized efficiency for any given fluorophore. © 2009 American Institute of Physics.�doi:10.1063/1.3263193�

Noble metal nanostructures can strongly amplify the in-tensity of light emitted by fluorophores placed in theirvicinity.1,2 The enhancement �or quenching� of emission isdetermined by the balance between the fluorophore’s excita-tion and decay rates �both radiative and nonradiative�, whichare modified by the interaction with a plasmonic structure.3–6

Recent studies indicate an enhancement mechanism based onstrong coupling of the emitter’s excited state to surface plas-mons �SP�. In this picture the metal nanoparticles act as plas-monic antennae by converting a part of the fluorophore’snonradiative near field into emitted far-field.2,5–7 To optimizethe efficiency of this process a good spectral overlap betweenthe emission band and the SP resonance band of the metallicsubstrate is needed.7 It is therefore crucial for the advance-ment of metal-enhanced fluorescence �MEF�-based applica-tions, to design and fabricate MEF substrates which offer thepossibility of plasmon resonance tuning, in order to matchany desired fluorophore.

Colloidal suspensions,8 silver island films,9 and rough-ened silver electrodes10 exhibit irregular morphology andtherefore do not allow for systematic SP resonance tuning.On the other hand, ordered arrays of nanoparticles �i.e., plas-monic crystals� defined by electron-beam lithography11 orrelated techniques are reproducible and tunable but imply ahigh cost and low throughput. A good alternative can benanosphere lithography,12 in which close-packed arrays ofself-organized polystyrene nano�micro�spheres are used aslithographic masks or templates for metal film evaporation.Particularly, an interconnected network of metallic half-shells �HSs� is obtained by metal evaporation on top of thespheres array.13 Although their optical properties remainedpoorly studied, such HS arrays were shown to exhibit impor-tant plasmonic activity, they were efficiently employed assubstrates for surface enhanced Raman scattering14,15 and SPresonance spectroscopy.16 However, a study to explore theeffects of such substrates on the characteristics of fluoro-phore emission has not yet been reported. Here we presentsuch a study and demonstrate that regular arrays of silverhalf-shells templated on close-packed arrays of polystyrene

microspheres can be used as excellent substrates for MEFapplications. An analysis of the optical properties of this pe-riodically structured silver film is provided, showing that itsplasmonic response is tunable across the visible spectrum byadjusting the diameter of the silver HSs. The consequence isthat different-sized half-shells exhibit different efficiencies inenhancing the emission of Rose Bengal �RosB� test fluoro-phores. The observed size-dependent enhancement is ana-lyzed in terms of spectral overlap between the SP band of theMEF substrate and the emission band of the fluorophore.

Highly ordered monolayer colloidal crystals were pre-pared from polystyrene spheres �PSs� of three distinct diam-eters 340, 450, and 500 nm �which will define the innerdiameter of the Ag HSs�. A convective self-assemblytechnique,17 using a home-built apparatus was employed.Silver �Ag� films of 50 nm thickness, were deposited overthe PSs by evaporation in vacuum ��10−6 Torr�. Thesamples were functionalized with 11-mercaptoundecanoicacid �MUA� by immersion in a 10−3 M ethanol solution,followed by ethanol rinsing and drying. RosB fluorophoreswere attached by immersing the samples in an aqueous so-lution of RosB �10−5 M�, left overnight, and then rinsed wellwith ultrapure water to ensure a maximum of one molecularmonolayer. Optical absorption, transmission, and reflectionmeasurements were performed by a Jasco V-530 UV-VISspectrophotometer, while fluorescence emission was mea-sured with a Jasco FP-6500 spectrofluorometer. Excitationwavelength was 540 nm in all presented MEF spectra, whichfalls near the maximum of fluorophore absorption band.

Due to the particular complex morphology of this peri-odically ordered metallic structure �i.e., close-packed inter-connected HSs �Ref. 13��, both transmittance T and reflec-tance R were measured in order to infer the amount of lightwhich can be absorbed and leaked into the structure A=1-T-R. Generally the underlying PS periodic arrays arestrongly diffractive in the visible spectral range. However,when the spatial period of the structure matches the wave-length of a guided mode, strong resonant features would beobserved in the transmission and/or reflection. Therefore, inorder to distinguish such modes in the overall properties ofmetallic HSs over PSs, we first studied the guided-waveresonances of the bare PS arrays. The position of the first-a�Electronic mail: cfarcau@phys.ubbcluj.ro.

APPLIED PHYSICS LETTERS 95, 193110 �2009�

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order waveguide mode can be estimated by �max

=2�nef /Gij, where Gij = �4� /�3D��i2+ j2+ ij is a reciprocalvector of the grating. nef and D are the effective index of thePS array and the diameter of the PS spheres, respectively.Consequently, the resonance bands at 433, 571, and 631 nmin Fig. 1�a� are assigned to light being coupled into suchphotonic eigenmodes, which propagate horizontally and leakaway from the structure.18 After silver film deposition, newpredominant bands are observed at 647, 730, and 803 nm forthe 340, 450, and 500 nm samples, respectively �see Fig.1�b��. Note that each of these extinction maxima in the spec-tra of HSs is located at wavelengths which are larger than thecutoff wavelength of the corresponding sample. We can thusundoubtedly attribute the strong absorption bands to excita-tion of resonant SP modes in the Ag HS array, while theweaker bands on the short-wavelength side are mainly due tofar field diffraction and photonic modes in the dielectric PSarray. It is not clear how much of localized or propagative SPresonances are excited. As it was pointed out in a recentstudy19 it is conceivable that both excitations coexist onclose-packed arrays of metallic HSs. With respect to this wecannot rule out that the photonic propagative modes in thePS array could excite plasmon modes on the inner side of theAg HSs.20 However such modes are expected to be highlylocalized in the PSs. The salient feature of these metallic HSarrays is that their SP resonance can be tuned by changingthe diameter of the underlying spheres.

Figure 2�a� presents the enhanced emission spectra ofRosB on the three Ag HS array samples �curves a–c�, com-pared to a reference with RosB on a glass slide �curve d�.The intensities of the MEF spectra are approximately 3.8,7.2, and 28 times larger for the samples with 500, 450, and340 nm diameter HSs, respectively, relative to the reference.The enhanced fluorescence can be readily imaged by nakedeye, and recorded as photographs taken by a digital camerathrough an edge filter. In the white light image in Fig. 2�b�-top a border between Ag HSs region and mirrorlike flat Agfilm is captured. In the bottom part of Fig. 2�b� only the area

with RosB on HSs is visible, while the area with RosB onflat Ag film is dark, when viewing through the filter, and with532 nm laser illumination.

Geometrical estimations show that the total exposed sur-face area remains constant for close-packed arrays of HSs,irrespective of the HS diameter, meaning that roughly thesame number of molecules is probed in each sample. For thereference, the exposed flat surface area is two times smaller,which makes also the number of molecules giving the fluo-rescence signal two times smaller. Therefore a factor of 2could be attributed to the increased surface area, for each ofthe Ag HSs samples.

Several authors have previously noticed an efficient en-hancement when the emission band of fluorophores over-lapped the SP band of the MEF substrate.2,7,21 In order toascertain the mechanism of fluorescence enhancement we at-tempt to correlate the measured enhancement efficiency ofeach substrate with the degree of overlapping between thesubstrate plasmon response and the RosB emission band.The strength of this spectral overlap is evaluated by integrat-ing the shaded areas in Fig. 3, where the SP band is fitted to

FIG. 1. �Color online� �a� Extinction spectra of bare polystyrene micro-sphere arrays. �b� Extinction spectra of Ag half-shells over polystyrene mi-crospheres; note the correspondence between the marked peaks. Extinctionwas inferred from transmittance T and reflectance R. Diameter of PSs isindicated in the figure. The arrows indicate the positions of RosB absorptionand emission.

FIG. 2. �Color online� �a� MEF spectra of RosB on Ag HS arrays of 340 nm�a�, 450 nm �b� and 500 nm �c� diameter, and emission of RosB on glassreference �d�; inset shows a SEM image of the HS array. �b� Top: white-lightimage of the border between Ag HSs region and mirrorlike flat Ag film;bottom: fluorescence image of the same area viewed through an edge filterunder 532 nm laser illumination, revealing a strong contrast between theMEF active surface and the flat film surface.

FIG. 3. �Color online� Gaussian fits of the plasmon absorption bands fromFig. 1�b� �dashed lines� and Lorentzian fit of the RosB emission band fromFig. 2�a� �full line�. The overlap between each plasmon band and the emis-sion band is highlighted. The inset shows the correlation between the area ofthe overlap and the enhancement factor of each sample.

193110-2 C. Farcău and S. Aştilean Appl. Phys. Lett. 95, 193110 �2009�

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a Gaussian lineshape and the emission band to a Lorentzianlineshape, respectively. The results of the integration are pre-sented in the inset of Fig. 3 together with the experimentalenhancement factors of the three Ag HS arrays. The goodagreement proves a dependence of the enhancement effi-ciency on the degree of spectral overlap between the plas-mon resonance of the nanostructure and the emission band.This is an indication that the enhancement is due to the near-field coupling between fluorophore dipole and SPs electronicoscillations at the emission frequency. Since the fluoro-phore’s absorption is located further toward shorter wave-lengths and does not overlap the main plasmon excitationband we expect a negligible contribution from an increasedexcitation rate. By analyzing Fig. 3 one can notice that theenhancement can be further optimized by fabricating sucharrays to complete the overlap between the SP absorptionband and the dye emission. We prospect that for a full over-lap of two bands an enhancement factor of about 32 isachievable for the present linker-fluorophore configuration.

It is important to note that MEF was obtained here froma molecular monolayer placed very close to the metal sur-face, i.e., at about 1 nm, the length of the 11-MUA linker-spacer molecule. There is not a general consensus in theliterature with respect to the dependence of the enhancementmagnitude on the separation between the fluorophore and themetal surface. Anger et al.22 investigated the fluorescencerate of a single molecule as a function of its separation froma gold nanoparticle, and observed a maximum enhancementat about 5 nm. In a report of Ray et al.9 the maximum en-hancement was obtained with the fluorophore in contact withsilver island films. We understand the rich palette of existingresults as being the outcome of different enhancementmechanisms involved in each particular fluorophore-metallicstructure pair, and of the specific orientation of the transitiondipole moments relative to the local polarization of theplasmon-enhanced fields. Although time-resolved measure-ments would be useful to disentangle the contributions ofdifferent enhancement pathways,7,9 in our case the substrate-dependent enhancement factors and spectral positions of SPresonances suggest the involvement of a plasmon-coupledemission mechanism. Our results indicate that plasmonic en-hancement effects can dominate over quenching even atsmall distances as 1 nm. Note that higher enhancements thanthose reported here could be obtained by optimizing thelength of the spacer molecule. Following this discussion, itappears clear that tunable and reproducible MEF substratesas the ones proposed here will be advantageous also for fun-damental studies aiming the comprehension of fluorescenceenhancement mechanisms.

In conclusion, we have demonstrated a noble metalnanostructured substrate for fluorescence enhancement thatpossesses tunable plasmonic properties. By changing the di-

ameter of the spheres template, the SP resonance of thenoble-metal HS arrays can be controlled, which in turn al-lows the optimization of the enhancement efficiency for thedesired fluorophore. The proposed MEF substrate is highlyversatile, providing the ordering, robustness, reproducibility,and tunability of top-down lithographically fabricated struc-tures, complemented by the ease of fabrication and low-costof bottom-up approaches. Therefore, this kind of orderednoble metal nanostructure should straightforwardly find itsway into practical fluorescence-based sensing or analyticalapplications.

This research was supported by the Consiliul National alCercetarii Stiintifice din Invatamantul Superior �CNCSIS�under the Project IDEI No. 477/2007 and the Project TD No.261/2007.

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