Optimizing Surface-Enhanced Raman Scattering by Template Guided Assembling of Closely Spaced Silver Nanocluster Arrays

Eur. Phys. J. D 52, 111–114 (2009) DOI: 10.1140/epjd/e2009-00057-1

Optimizing surface-enhanced Raman scattering by template guided assembling of closely spaced silver nanocluster arrays

C.H.Xu,B.Xie,Y.J.Liu,L.B.HeandM.Han Eur. Phys. J. D 52, 111–114 (2009) DOI: 10.1140/epjd/e2009-00057-1 THE EUROPEAN PHYSICAL JOURNAL D Regular Article

Optimizing surface-enhanced Raman scattering by template guided assembling of closely spaced silver nanocluster arrays

C.H.Xu,B.Xie,Y.J.Liu,L.B.He,andM.Hana National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, 210093 Nanjing, P.R. China

Received 19 September 2008 Published online 18 February 2009 – c EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2009

Abstract. We present an easy approach to synthesize closely spaced regular arrays of silver nanoclusters, which are self-assembled by depositing gas-phase synthesized metal nanoclusters onto pre-patterned triblock copolymer templates. The array has a high particle density of about 2 × 103 particles per µm2, and an average interparticle space of about 20 nm. The surface plasmon resonance wavelength of the array is tuned due to the interparticle plasmon coupling. High SERS sensitivity for less than one layer trans- 1,2-bi-(4-pyridyl) ethylene (BPE) molecule detection, with an enhancement factor of 2.6 × 106,hasbeen demonstrated for a substrate with this array. The enhanced Raman signal was found to be 5 times higher than that measured from the substrate with randomly distributed silver nanoparticles.

PACS. 81.16.Dn Self-assembly – 78.30.-j Infrared and Raman spectra

1 Introduction regular nanoparticle arrays can give rise to more homogeneous and reproducible SERS results due to their spatial Plasmonic nanostructures have recently attracted much uniformity of high density hot spots. As has been pointed interest because of their unique optical properties and out [6], the average enhancement factors have greater rel- novel applications [1]. Many efforts are aimed at applying evance for rational design of optimized SERS substrates. surface enhanced Raman scattering (SERS) with specially In this paper, we introduce a simple way to prepare prepared noble metal substrates for organic molecules SERS substrates by hierarchical self-assembly of silver sensing [2–5]. Optimization of the Raman amplification nanocluster arrays on triblock copolymer templates. The is of paramount importance for applying the SERS tech- shift of the surface plasmon resonance wavelength of the nique for single molecule detection, as well as for optical array due to interparticle near-field coupling was observed. sensors in nanoscale integrated devices. For such optimiza- The advantages of using these regular closely spaced nano- tion, it is a great challenge to find out the connection ex- particle arrays in SERS experiments compared to ran- isting between the local surface plasmon (LSP) resonance domly distributed nanoparticles films were investigated. and the Raman enhancement factor. The Raman enhancement factor for BPE molecule detec- Closely spaced regular nanoparticle arrays composed tion was found to be 5 times improved. of metal particles with nanoscale sizes and interparticle gaps appear to be more suitable substrates for improv- ing our understanding of the mechanisms involved in the 2 Experimental procedures SERS effect than colloid-based system and randomly roug- hed surface. Thanks to their very narrow particle size and Triblock copolymer poly(styrene-b-butadiene-b-styrene) shape distribution, these arrays exhibit very sharp LSP (SBS) (Aldrich Chemical Inc.), with a molecular weight resonances leading us to demonstrate experimentally the of 140 000 Da, a polydispersity index of 1.2, and a PS required condition for the optimization. They have been weight fraction of 30%, was used for fabricating nanoscale found to be the most efficient SERS substrates owing to patterned templates through microphase separation pro- junction plasmons created in the nanometer interparti- cess. The copolymer was dissolved in toluene to pro- cle gaps (hot spots). Maximum LSP enhancement can duce a 4 wt% solution. A small amount of the solution be achieved under resonant excitation conditions, by tun- (∼20 µL) was spin-coated onto the fused quartz substrates ing the plasmon resonance of the substrate to near the at about 2500 rpm for 30 s to form a film. Then the as- excitation-laser wavelength.Furthermore,closely spaced cast films were placed inside a glass vessel with some drops of volatile toluene added as annealing solvent. The vessel a e-mail: [email protected] was carefully sealed to generate a very slow evaporation 112 The European Physical Journal D rate (∼0.001 mL/h) of toluene. The solvent annealing time was usually 3 days followed by 1 day of slow drying by ex- posing the polymer film in air. The surface morphology of the SBS copolymer films was characterized with an atomic force microscope (AFM) (NTEGRA Probe NanoLabora- tory, NT-MDT Co.) working on semi-contact mode. Silver nanoclusters were then deposited onto the pre- patterned SBS copolymer film surface. The clusters were generated with a magnetron plasma aggregation cluster source. The experimental setup and operation parameters for cluster beam generation have been described in de- tail elsewhere [7,8]. In our experiment, the clusters were perpendicularly deposited onto the SBS copolymer templates in a high vacuum chamber (10−7 torr), with a low < cluster coverage of 100%. The deposition was carried Fig. 1. Typical 2 × 2 µm phase image of the SBS template by on at room temperature, and the deposition rate was tapping mode AFM. controlled to be about 2 A/s.˚ Transmission electron mi- croscopy (TEM) was used to characterize the morphologies of silver nanocluster deposits. Optical extinction spectra of the silver nanocluster a b films were measured with a UV-Visible spectrophotome- ter. The silver cluster covered SBS films were used as plasmonic substrates for surface enhanced Raman scattering measurements of trans-1,2-bi-(4-pyridyl) ethylene (BPE). The BPE used in this research was provided by Aldrich Co. SERS spectra were recorded on a confocal Raman spectroscope (NT-MDT NTEGRA Spectra) using a 473 nm laser excitation, which is close to the sur- 200nm 200nm face plasmon resonant wavelength of silver nanocluster arrays [9]. The laser beam was focused with an objective to Fig. 2. TEM images of silver nanoclusters deposited on give an illuminated area with 350 nm in diameter on the (a) SBS pre-patterned template; (b) amorphous carbon sur- sample surface. To check the sensitivity of SERS on SBS face. The inset shows typical 3 × 3 µm height image of Ag molecular probe and reduce the damage to samples, the nanoclusters on silica glass measured by tapping mode AFM. initial laser power (about 10 mW) was attenuated, by the order of 10−1 or 10−2, with a circular variable neutral density (ND) filters. within the chain is about 20 nm. A high particle num- ber density about 2 × 103 particles per µm2,couldbe calculated for this ordered arrays. For comparison, silver 3 Results and discussion nanoclusters were also deposited onto amorphous carbon films supported with copper grid, or on silica glass. From Block copolymers tend to self-assemble into a variety of the TEM image shown in Figure 2b and the correspond- well-ordered nanostructures from several to hundreds of ing AFM image shown in the inset, these two deposits nanometers because of the chemical immiscibility of the have similar morphologies. Silver nanoclusters are ran- covalently linked segmental chains [10]. In our study, the domly distributed and short-range aggregated due to lim- microphase separation of SBS films were controlled to ited diffusion on the substrate surface, and the particle form parallel cylindrical periodic domains, the space be- sizes are significantly uniform in comparing with the case tween the adjacent in-plane PS domain cylinders is around of patterned SBS copolymer substrate. 38 nm (Fig. 1). The long range ordering stripe-like do- The optical extinction spectra of the SBS copolymer mains can serve as a preformed scaffold for silver nanoclus- substrate templated siliver nanocluster arrays manifest an ters. Regular arrays of silver nanoclusters can be obtained intriguing shift on the surface plasmon resonance (SPR) by decorating silver clusters to the PB domains [11]. wavelength, as illustrated in Figure 3. For silver nanoclus- Gas-phase synthesized silver clusters were deposited ters deposited on silica glass, the maximum extinction is at onto the cylindrical SBS copolymer templates perpendic- 395 nm. However, the SPR peak appears at 424 nm with ularly at room temperature, with a low coverage of about a narrower width when silver nanoclusters are deposited 10%. As shown in Figure 2a, the silver nanoclusters form on the SBS template. The SPR properties of metal nan- chain-like arrays with nearly equal inter-chain spaces, ap- oclusters are mainly determined by the size and shape of proximately 38 nm, corresponding to the period of the particles, the nanostructure of particle assembly and the in-plane PS domain cylinders in the SBS copolymer sub- surrounding medium dielectric function [12]. From Fig- strate. The mean diameter of the silver nanoparticle is ure 2, we can find that there is no significant difference about 10 nm, and the average particle-particle distance on the size and shape of the silver nanoclusters between C.H. Xu et al.: Optimizing surface-enhanced Raman scattering by assembling of closely spaced silver nanoclusters 113

0.12

A 0.10 B C

0.08

0.06

0.04 Extinction (a.u.)

0.02

0.00 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Fig. 3. Optical extinction spectra of silver nanocluster assem- Fig. 4. Raman spectra of trans-1,2-bi-(4-pyridyl) ethylene blies. (A) Randomly distributed nanoclusters deposited on sil- (BPE) molecules. (A) On substrate with silver nanoclusters ica glass substrate; (B) silver nanocluster linear arrays on SBS deposited onto SBS template. (B) On substrate with silver nan- template; (C) randomly deposited silver nanoclusters embed- oclusters deposited onto silica glass. (C) On pure silica glass. ded in SBS film. For spectra (A, B, C), 2 × 10−6 M BPE concentration and 100 µW laser power was used. Spectrum D was measured from of BPE molecules on pure silica glass with a 4 × 10−2 MBPE the two kinds of substrates. Furthermore, the SPR peak concentration and 1 mW laser power. The spectrum D was of silver nanoclusters exhibits little difference when they magnified 150 times for clarity. Both spectrum A and B has were embedded in SBS film (see Fig. 3), which excludes an increased baseline, which come from the luminescence back- the influence from the dielectric functions of different sur- ground that always accompanies SERS spectra. rounding mediums. On the other hand, the SPR property change of SBS copolymer templated siliver nanocluster arrays can be explained if we take into account the near regular silver nanocluster arrays is much stronger than field electromagnetic coupling of adjacent nanoclusters in that measured on the substrate with randomly distributed closely spaced nanoparticle arrays. It has been manifested silver nanoclusters. The larger enhancement of the former that the SPR peak of noble metal nanocluster assem- can arise from two facts: (1) under SBS template guid- bly is red-shifted with the decreasing of interclusters dis- ing, closely spaced regular arrays of silver nanoclusters tance [13]. Therefore, we attribute the 29 nm shift on the are formed, which provides homogeneously distributed hot SPR wavelength observed for the regular arrays of closely spots with high density. Great local field enhancement are spaced nanoclusters on SBS template to the interparticle realized by the SPR coupling at the hot spots; (2) the near-field coupling. surface plasmon resonance band of the closely spaced reg- For SERS measurements, about 10 µLof2× ular arrays of silver nanoclusters is shifted to near the 10−6 M BPE methanol solution was dropped onto the excitation-laser wavelength, great LSP enhancement can substrates covered with silver nanoclusters and dried in be achieved under resonant excitation conditions. In fact, air to hold a BPE molecule surface density as low as the Raman intensity varies significantly with moving the 2 × 10−13 mol/mm2. The laser beam was attenuated to detecting position when a silica glass covered with ran- about 100 µW before focusing onto the surfaces of sam- domly distributed silver nanoclusters is used as substrate, ples. Within the laser illumination spot, about 350 nm in while stable Raman signals can be detected at various po- diameter, there’s estimated to be about 1.2 × 104 BPE sitions on the substrate with closely spaced regular arrays molecules. That means the coverage of BPE molecules of silver nanoclusters. Obviously, the later one is more ef- is much less than one monolayer if we assume the ap- ficient for SERS measurement. proximate area of a single BPE molecule is 30 A˚2.With To estimate the SERS enhancement factor per such low BPE concentration, no Raman signal is de- molecule for the silver nanocluster assembly, a reference tectable under the above laser illumination condition if specimen was prepared on silica glass surface by increase only pure BPE specimen is used. However, strong Raman the BPE concentration to 2 × 104 times higher. Under lines can be observed at about 1010, 1200, 1340, 1610, and 1 mW exciting laser power, which was also 1 order of 1640 cm−1, corresponding to the Raman of BPE molecule, magnitude higher than that used for SERS measurements, when the BPE molecules are attached to the silver nan- the reference Raman signal of pure BPE molecules was oclusters covered substrates, as shown in Figure 4. Obvi- measured and shown in Figure 4. Raman gains were then ously, Raman scattering of BPE molecules is significantly calculated approximately. For the SBS templated closely enhanced by the SPR local field near the silver nanopar- spaced regular silver nanocluster arrays, a 2.6×106 Raman ticle surface generated by 473 nm laser. gain was obtained, which is 5 times of that estimated From Figure 4, it is apparent that the SERS of BPE for the randomly distributed sliver nanocluster assembly molecules measured on the substrate with SBS templated (5.1 × 105). 114 The European Physical Journal D

4 Conclusions References

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