Proceedings of the 2007 International Conference on Information AcquisitionJuly 9-11, 2007, Jeju City, Korea

Aligned Copper Nanorod Arrays for Surface- enhancedRaman Scattering

Aiwu Zhao, Tao Mei, Xinhua Lin, Lin Ni, Anyi WuCenterfor Biomimetic sensing and Control Research,

Institute ofIntelligent Machines,Chinese Academy ofScience

Hefei, Anhui,230031, P. R. China{awzhao & tmei & xhlin & nilin}@iim.ac.cn

Abstract - Highly ordered copper nanorod arrays have been experimental and theoretical researches have confirmed thatsynthesized using potentiostatic electrodeposition within the SERS enhancement has long been associated with rough metalconfined nanochannels of a porous anodic alumina membrane surfaces and has been closely linked to the geometry and the(PAAM). These arrays were evaluated as potential surface- dimensions of its surface protrusions, especially the periodicenhanced Raman scattering (SERS) active substrates using characteristic and the nanostructure.[7-15] Up to now, manyRhodamine 6G as a reported molecule. SERS activity was shown . . .

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to depend upon the length of the nanorods exposed to the surface approaches to prepare highly ordered per1odic arrays of nano-which is controlled by the etching time. The Cu nanorods with particles have been made since 1980s. The first orderedaverage lengths of 200 nm exposed to surface exhibited the periodic arrays of nano-particles for SERS active-substratemaximum SERS enhancement. Theoretical analyses indicate that was reported by Liao et al [16]. They obtained regularlythis large SERS enhancement may be partially explained by the ordered SERS substrates by depositing Ag particles overshape, density, and lateral arrangement of the Cu nanorod periodic arrays of silica posts that were fabricated by the ICarrays. lithography. Several theoretical groups have also investigated

field enhancement for SERS from metal nanoparticle arrays.Index Terms - Nanorod arrays, copper, SERS activity. Specifically, Garcia-Vidal and Pendry proposed that very

localized plasmon modes, created by strong electromagneticI. INTRODUCTION coupling between two adjacent metallic objects, dominate the

Trace-level optical detection has a wide range of SERS response in an array of nanostructures [17-19].applications in security screening, health care, and Recently, metal nanostructured substrates with tunableenvironmental monitoring. The Raman scatting spectrum is localized SPR have been used extensively to improve theconsidered as the fingerprint of chemicals and biomolecules as sensitivity and reliability of SERS active substrate [20, 21].it can represent the vibrational frequencies of functional Here we present the fabrication of ordered Cu nanorod arrayschemical bonds in molecules, and it has the advantages over on a porous anodic alumina membrane template by usinginfrared absorption spectroscopy since it has a capability to potentiostatic electrodeposition. The relation between thedetect both symmetric and non-symmetric chemical bonds in length of the nanorods exposed to the surface and SERSvisible and near infrared light region. But the major problem enhancement were investigated. The results indicated that Cufor Raman scatting Spectrum applications is the relatively nanorod array shows SERS effect, which could be useful inweak optical signal. Surface-enhanced Raman scattering has trace-level detection sensors.tremendous potential for solving to this problem. The II. EXPERIMENTAL SECTIONenhancement of the Raman scattered intensity produced byspecial structured active substrates can increase the sensitivity A. Preparation ofTemplatesby orders of magnitude. [1] In general, there are two separated The porous anodic alumina membrane templates we usedmechanisms to describe the SERS effect, electromagnetic during fabrication of Cu nanorod arrays were prepared in amechanism (EM) and chemical mechanism (CM). EM is a two-step anodization process as described previously.[22 ]long-range effect, which does not require the chemical bond High-purity (99.9990/O) aluminum foil of about 0.5mmof the molecules to the metal and can be used to explain the thickness was used as the starting material. Beforeenhancement of molecules distant from the metal surface. anodization, it was annealed at 500 °C for 4 h under high-When the surface has some roughness, under the surface purity nitrogen atmosphere to remove mechanical stresses.plasmon resonance (SPR), the localized electromagnetic field Then it was electropolished in a 1:9 solution of perchloric acidwill be remarkably enhanced. [2-4] So it is important for and ethanol. Anodization was carried on in a 0.3 MSERS that the surface has a certain extent of "surface phosphoric acid electrolyte at for 5h at about 0°C under aroughness." CM [5, 6] is a short-range effect, which involves constant voltage 150V. The formed alumina was thenthe electronic resonance/charge transfer (CT) between a removed by a solution mixture of phosphoric acid andmolecule and a metal surface resulting in an increase in the chromic acid, and the Al sheet was anodized again under thepolarizability of the molecules. A large number of same conditions as those in the first for 10 h. After the

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anodization, the remaining aluminum was removed selectively PAAM template. Fig. 2(a) shows the top-view SEM image ofby multiple steps in a saturated SnCl4 solution. A subsequent the Cu nanorod arrays by the electrodeposition method in aetching treatment was carried out in a 6wt% phosphoric acid PAAM membrane, which top was etched for 30 min in the 500solution at 30 -C for 60 min to remove the barrier layer on the NaOH solution. It reveals that large quantities of Cu nanorodsbottom side of the PAAM to obtain through-hole template. have been deposited within the nanochannels of the PAAM

template. The length of Cu nanorods exposed out of theB. Fabrication ofcopper nanorod arrays PAAM surface could be controlled by changing the etched

A gold layer (about 200 nm) was sputter deposited on one time in the 500 NaOH solution. Fig. 2(b) shows a cross-side of the through-hole PAAM template, and served as the section image of a Cu nanood arrays grown in a PAAMworking electrode for Cu electrodeposition. An aqueous bath template.containing 0.2M CuS04 *5H20 and 0.1M H3BO3 was used toprepare Cu nanorods using a potentiostatic electrodepositiontechnique. The pH of the solution was controlled in the range4.5 to 5.0 by adding 0.1M H2S04 solution. Theelectrodeposition was carried out at a constant current densityr(2.5mA/CM2), with platinum foil serving as the counterelectrode at room temperature for 8 h. After electrodeposition, T)

the sample was then washed with deionized water and dried inair. In order to obtain the different length Cu nanorodsalexposed up to surface, the sample was embedded in 500NaOH solution at 30 -C for different etching time removepartially the PAAM. Fig. 1. Scanning electron microscopy (SEM) image of a PAAM template a)

C. Measurement and Characterization Top-view and b) cross-sectionSERS activity of the Cu nanorod arrays was evaluated

using rhodamine 6G (R6G) as the probing molecule. A 10-6M aqueous rhodamine 6G solution was prepared by magneticstirring. The samples were submerged in the R6G solution for30 min, and then taken out and rinsed thoroughly. SERSspectra were recorded at room temperature on a Labram-HRConfocal Laser Micro Raman spectrometer, excited with anargon laser operating at 514.5 nm.The Cu nanowire arrays embedded in PAAM werecharacterized by scanning electron microscopy (SEM, JEOLJSM-6700F, operated at 10 kV) and transmission electron Fig. 2 SEM images of Cu nanowire arrays after removing the PAAMtemplate:microscopy (TEM, Hitachi H-800 with an accelerating voltage (a) top view and (b) cross-section viewof 200 kV) , selected area electron diffraction (SAED) wereemployed to characterize the individual Cu nanowires. For We examined the phase of the Cu nanorod by powder X-raySEM observation, a thin gold layer was evaporated to form a diffraction measurements (shown in Fig.3). It can be seen thatconducting film for observation. Specimens for TEM there are three Cu peaks in the XRD spectra of the Cuobservation were prepared by dissolving away the PAAM nanowire arrays, (111), (200) and (220).completely, which was accomplished by placing a small pieceof Cu/PAAM in an aqueous solution of 500 NaOH at 30 -Cfor 60 min. The solution was then slowly removed using asyringe and was carefully replaced with distilled water to rinse 1 1the products. The rinse process was repeated three times. Theremaining black solid was collected and ultrasonicallydispersed in ImL of ethanol. A drop of the suspended solutionwas placed on a carbon grid and allowed to dry prior toelectron microscope analysis.

III. RESULT AND DISCUSSION (200) (220)Fig. l(a) shows a typical top-view the scanning electron

microscopy (SEM) image of PAAM template. The diameter ;20 60 80of the holes and the interpore distance are approximately 110 ;2) dand 120 unm, respectively. The nanopores are hexagonallyarranged. Fig.1 (b) shows a SEM image of the cross-section of Fg3XDpteno unnrdary

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All the diffraction peaks can be indexed as face-centeredcubic crystal structures copper. Further structural analysis ofthe individual Cu nanowires was performed by TEM after thePAAM template had been thoroughly dissolved. Fig. 4a and3b show the TEM image of a typical individual Cu nanowiresand corresponding selected area electron diffraction (SAED) <pattern recorded from it, respectively. It indicates that the Cu _nanowires continuous and uniform in diameter throughout theentire length of the wires. The crystalline structures of theindividual Cu nanowires can be indexed as a single crystal Ewith a face-centered cubic (fcc) structure. M

400 600 800 1000 1200 1400 1600 1800Raman shift (cm-,)

.,...... . Rama shfl(m'

Fig. 4 TEM investigations of the individual Cu nanowires: (a)TEM image of Fig.5. SERS spectrum of 10-6 M R6G solution a)on a Cu/PAAM template;the Cu nanowires; (b) corresponding SAED patterns of the Cu nanowire b) on a typical substrate prepared by depositing Cu film on Si.

shown in (a)

Fig. 5 (a) shows high-intensity Raman peaks are observed IV. CONCLUSIONwhen the fluorescence background is quenched to a steady In summary, we have presented a SERS-active substratestate. A few days after the application of the solution, the made of an array of Cu partially embedded in PAAMsimilar results can be obtained at the same condition. The nanochannels. This substrate exhibits a high Raman signallarge SERS signal indicates that the R6G molecules near the enhancement factor due to the unprecedented narrow gapsCu nano- particles are excited by the laser-induced surface between the Cu nanorods. The uniform and highlyplasmon, while the fluorescence quenching suggests that some reproducible SERS-active properties obtained for theof these molecules spontaneously adsorb onto the surface of Cu/PAAM arays will facilitate the use of SERS forthe nanorods. The enhancement factor for the Cu/PAAM chemical/biological sensing applications with extremely higharrays is at least i05 times larger than that of a SERS substrate sensitivity. The substrate can be further improved byprepared by depositing about 30 nm Cu onto a silicon surface, fabricating long range ordered arrays.which does not provide a detectable SERS signal above thefluorescence background. The spectra are shown for AcKNOWLEDGMENTcomparison in Fig. 5(b). The strong enhancement can be

Ti okwsspotdb ainlBscRsacattributed to the fact that the Cu/PAAM has a very high Thisra workwhias supprated206By3Nationa Basic Researchddensity of both Cu nanorods and 'hot-junctions', which likely b th KnwegInoainProgramof Chna(ratCh6Bi047)adSupredeexist in the gaps between neighboring nanorods. It is to be bycthem nowlScedges I(novatio Program-Wofth Cinsnoted that the enhancing power is uniformly consistent over AcdmofSine(GatKJX-WW 1)the entire 1.5 cm2 sample with less than 200 variation; further,

Cu/PAAM array . pyidn adore at asleeecrdCmPs.let2613194

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