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Plasmonics (2011) 6:715–723DOI 10.1007/s11468-011-9255-yLow-Cost Fabrication of Pt Thin Films with ControlledNanostructures and Their Effects on SERSMin-Hye Kim & Eun-Sun Lee & Seong Kyu Kim &Young-Uk KwonReceived: 27 April 2011 /Accepted: 5 July 2011 /Published online: 13 July 2011# Springer Science+Business Media, LLC 2011Abstract We fabricated nanostructured Pt thin films withtwo different morphologies and studied their effects on theRaman spectrum of rhodamine 6G. The syntheses wereachieved by templating mesoporous silica thin films whichare characterized by 8-nm-sized pore channels and 3–4-nmthickwalls in two different orientations of the channels.Therefore, the resultant nanostructured Pt thin films arecomposed of Pt nanorods of 8 nm in diameter verticallystanding in one morphology and horizontally lying in theother. The latter produced stronger Raman signals than theformer. Simulations based the discrete-dipole approximationon model nanostructures showed that the horizontallylying nanorods produce stronger local electromagnetic fieldthan the vertically standing ones, in agreement with theexperimental observations.Keywords Template synthesis . Platinum . Nanostructures .Thin films . Raman spectroscopyM.-H. Kim : Y.-U. Kwon (*)Research Institute of Advanced Nanomaterials (RIAN),Sungkyunkwan University,Suwon 440-746, South Koreae-mail: ywkwon@skku.eduE.-S. Lee : Y.-U. KwonBK21 School of Chemical Materials Science,Sungkyunkwan University,Suwon 440-746, South KoreaS. K. Kim : Y.-U. KwonDepartment of Chemistry, Sungkyunkwan University,Suwon 440-746, South KoreaY.-U. KwonSungkyunkwan Advanced Institute of Nanotechnology (SAINT),Sungkyunkwan University,Suwon 440-746, South KoreaIntroductionSurface-enhanced Raman scattering (SERS) is developingas an important analytical technique to detect very smallquantities of analytes [1–6]. Two mechanisms have beenwidely accepted to explain the SERS effects, electromagneticenhancement (EM) mechanism and chemical enhancement(CE) mechanism, although it is becoming apparent that botheffects are important [1, 7]. In the EM mechanism, thesubstrate generates strong localized electromagnetic fieldin the vicinity of the substrate surface upon illuminationof an excitation beam. The molecules on the substratesurface experience this field and the excitation of theirvibrational modes is greatly enhanced. In the CEmechanism, a resonant charge transfer state is createdbetween a chemisorbed analyte molecule and metallicsubstrate, in which some vibrational modes are stronglyexcited. Since EM mechanism is known to be strongerthan CE mechanism, the major focus of studies in thisfield has been how to effectively generate the localelectromagnetic field.The coinage metals, copper, silver, and gold, have beenstudied almost exclusively as the substrate materials [2–6,8–10]. This is mainly because they are the only materialsthat have strong surface plasmon resonances (SPRs) in thevisible region enabling absorption of the excitation beam.Although much less attention is paid, other metals also havebeen studied as SERS substrate materials, partly because oftheir potential advantages [11–14]. For example, SERS byplatinum would be of great interest for its significance incatalysis. Platinum or other metals, however, have SPRs atwavelengths away from the visible region and a fewresearches using platinum as a SERS substrate have beenreported [15–19]. Therefore, one has to devise ways toincrease the absorption in the visible region. One way to

716 Plasmonics (2011) 6:715–723achieve this is to construct nanostructured films withcontrolled morphologies. For example, Abdelsalam et al.fabricated Pt films by replicating close-packed polymerspheres to produce regular patterns with repeating distancesof a few hundreds of nanometers, and showed some ofthem have good matching with visible light enabling SERSon them [14]. The other possibility is to form regularnanostructured films in which constituting nanoparticlesand/or nanorods interact to broaden the SPR peak and shiftthe SPR band to longer wavelengths. This method also mayhave the advantage of producing narrow gaps betweennanoparticles or nanorods with strongly localized electromagneticfields, which are believed to serve as hot spots forSERS effects. However, there has been quite a few worksdone in this direction.In fact, the importance of the morphology control ofnanostructured films is not limited to Pt. The morphologyeffects of coinage metals have been widely studied. Typically,electron beam lithography [20–22] andLangmuir–Blodgett(LB) technique of colloidal nanoparticles [23–27] have beenexplored toward this end. These methods have beensuccessful in increasing the SERS effects, but also revealsome limitations. As for the electron beam lithography, thefabrication is time-consuming and expensive, prohibitingsubstrates in large areas [28]. Although the LB techniquecan produce nanostructured films with large areas atmuch lower cost, it is difficult to make films with highdensities and regular inter-particle distances.Template-assisted synthesis can produce thin films withregularly ordered nanostructures over large areas at lowcost [29]. However, such possibilities have been exploredfor SERS only in a few cases. Although there have been anumber of cases in which anodized aluminum oxidetemplates are used to synthesize nanorods, the main focushas been on the control of the shape and size of thenanorods, not on the formation of the array structure ofnanorods [30–33]. In this regard, mesoporous thin filmtemplates appear to be advantageous because nanostructuredfilms with very fine features

Plasmonics (2011) 6:715–723 717Fig. 1 Schematic drawing ofthe procedure to synthesizenanostructured Pt thin films byusing mesoporous silica thinfilms as templates. Thehorizontal channels of ~8 nmin diameter in SKU-5 areinterconnected by smallerchannels of ~1 nm in diameterenabling electrochemicaldepositionResults and DiscussionNanostructured Pt FilmsIn this study, we used MSTFs as templates to fabricate Ptfilms with two different nanostructures and investigatedtheir effects on the SERS of Rh6G. The scheme of ourstudy is shown in Fig. 1. First, a MSTF is formed on asquare-cut Pt-coated Si wafer. The synthesis of MSTFs wasachieved by following the procedure our group hasdeveloped. By using precursor solutions with differentcompositions, the pore structure could be controlled. Thepore morphologies of the templates we used are arhombohedral derivative of the Im-3m cubic structure andthe P6mm 2D hexagonal structure, which are denoted asSKU-1 and -5 respectively, in our previous paper [34].Second, Pt was deposited into the pores of the MSTFtemplates by an electrochemical deposition technique. Theamount of deposited Pt was controlled by monitoring theamount of charge passed. We deposited 10, 20, 30, and40 mC for the two types of MSTF templates (the area ofeach Pt film was 1.0×1.0 cm 2 ). Finally, the MSTFtemplates were removed by etching with a HF solution toreveal the nanostructures of the Pt films.Figure 2 shows representative SEM images of the Ptnanostructured films so-obtained. As can be seen in theseFig. 2 SEM images of v-Pt andh-Pt films with differentamounts of Pt. The amount of Ptwas controlled by measuring theamount of charge passed in theelectrochemical deposition. av-Pt with 10 mC/cm 2 , b v-Ptwith 40 mC/cm 2 , c h-Pt with10 mC/cm 2 , and d h-Pt with40 mC/cm 2 . Insets in b–d areenlarged views

718 Plasmonics (2011) 6:715–723Fig. 3 Variation of the surfacearea of each type of nanostructuredPt films with the variationof the amount of deposited Ptimages, our method is capable of forming nanostructuredfilms over a wide area and with a high degree of uniformity.Depending on the type of MSTF template, the nanorodshave different orientations. In case of using SKU-1 as thetemplate, the Pt nanorods are standing vertically to thesubstrate plane. When the amount of Pt is small, themorphology is best described as a nanodot array (Fig. 2a).Each nanodot has a diameter of 8 nm when viewed fromthe top and the dot-to-dot separation (the gap) is 3~4 nm.As the amount of Pt increases, the nanodots grow intovertically standing nanorods with the diameter and the gapsize practically unchanged (Fig. 2b). However, one can seethat long nanorods are bundled to some degree. In case ofusing SKU-5 as the template, long Pt nanorods are alignedhorizontally with respect to the substrate (Fig. 2c and d).The distance between the nanorods are 3~4 nm, the same asin the case of SKU-1 because the wall thicknesses of bothMSTF are the same. When the amount of Pt is small, manyof the nanorods are segmented probably because theamount of Pt is not enough to fill up the first layer ofpores of the SKU-5 template. As the amount of Ptincreases, the pores of the template are filled up to producehorizontally aligned Pt nanorods. Hereafter, the Pt filmsfrom SKU-1 will be denoted as v-Pt (vertical Pt) and thosefrom SKU-5 as h-Pt (horizontal Pt).We measured the electrochemically active surfaceareas (EASs) of these Pt films by using an electrochemicalmethod. A cyclic voltammogram plot on a Ptelectrode in an acidic aqueous solution shows peaks inthe potential range of −0.2~0.0 V (vs Ag/AgCl)associated with the H + adsorption and reduction. Theintegrated peak area has been used to estimate the EAS ofthe Pt electrode by using the reference value of 210 μC/cm 2 [40]. Figure 3 shows the variation of the EAS withineach type of the nanostructures of Pt films with thevariation of the amount of deposited Pt. The ESAincreases proportionally with the amount of deposited Pt.The EASs are not strongly affected by the morphologywithin the experimental errors.Raman SpectroscopyThe nanostructured Pt films were used to record SERS ofRh6G. We chose to use Rh6G because its SERS is knownFig. 4 The Raman spectra ofRh6G on v-Pt and h-Pt withvarious amounts of Pt. a v-Ptand b h-Pt. Note the scale in b isabout two times larger than thatin a

Plasmonics (2011) 6:715–723 719to have little contribution from CE mechanism [42].Therefore, any changes in SERS can be attributed to thestrength of the electromagnetic field of the substrate. Weused v-Pt and h-Pt films with various amounts of Pt. Theamount of Rh6G was controlled to be the same for eachmeasurement. The results are shown in Fig. 4. The spectraagree well with those in the literature on the SERS ofRh6G [43, 44].One can make two important observations from theSERS spectra: (1) within a series of Pt morphology, theSERS intensity increases as the amount of Pt increases. (2)For a given amount of Pt, h-Pt produces a signal about twotimes stronger than v-Pt.The first observation appears to be related with the surfacearea of the Pt nanostructure. As the amount of Pt increases, thesurface area increases. The monolayer of Rh6G moleculesused in the SERS measurements would occupy about 0.3 cm 2 ,which is 3 to 30 times smaller than the EASs of the Pt films.Therefore, it seems plausible that the larger surface areas cangive better chances for the adsorbed molecules to be moreevenly spread to increase the SERS signals. This observationalso can be explained with the concept of hot spot. Becausethe nanorods are closely spaced with gaps of about 4 nm,one can expect that there are many hot spots in the gaps.Because the morphology does not change with the amount ofPt deposited, the number of hot spots is expected to increasewith the surface area, explaining the increase of the SERSintensity with the amount of Pt deposited.DDA SimulationsThe second observation of the effect of the nanostructure onthe SERS appears to be related with the orientation of thenanorods. In order to better explain this observation, weperformed simulation studies on the extinction spectra andelectromagnetic field distribution of various Pt nanostructuresrelated with the morphologies of v-Pt and h-Pt by using theDDA approximation. The excitation wavelength of 514 nmwas used. For the calculations, the propagation direction ofexcitation beam was set to be perpendicular to the substrates.The complex permittivity of Pt for the simulation wascalculated using the parameters in references [40, 41].Although the bulk values of permittivity may not beappropriate due to increased electron damping at the surfaceof particles with nano-size features [45], the DDA methodhas been reasonably successful in predicting their opticalspectra and local fields [46, 47]We first studied the effect of the array structure bycomparing a single Pt nanorod of 8 nm in diameter and60 nm in length and an array of such nanorods with 3 nmgaps between them. We considered two geometries relatedwith the v-Pt and h-Pt morphologies. In the v-Pt relatedgeometry, the nanorods are vertically standing and theFig. 5 Simulated absorption spectra of a single Pt nanorod and anarray of Pt nanorods. a Vertically standing Pt nanorods, b horizontallylying Pt nanorods with transverse polarization, and c horizontallylying Pt nanorods with longitudinal polarization. The spectra in eachpanel are normalized to compare the spectral behaviors

720 Plasmonics (2011) 6:715–723Fig. 6 DDA results for v-Pt at514 nm (a, b, c) and model usedfor simulation (d). a is forY-polarized and b is Z-polarizedcase. The cut plane is X=0 for aand b, and c is XY plane of aexcitation beam direction is parallel to the long axis of thenanorod. In the other geometry, the nanorods are lyingparallel to the planes of view and the excitation beam isperpendicular to these planes.Fig. 7 DDA results for h-Pt at514 nm (a, b) and model usedfor simulation (c). a is forZ-polarized and b is Y-polarizedcases. The cut plane is located atthe center of cylinder

Plasmonics (2011) 6:715–723 721The SPR band of Pt is reported to be at 215 nm [48, 49].The simulated absorption spectrum of a single nanorod inthe vertical geometry reproduced such a behavior with anabsorption tail extended to the visible region (Fig. 5a). Thearray structure red-shifts the SPR band by about 50 nm.The SPR absorption of the horizontally aligned singlenanorod depends on the direction of polarization (Fig. 5band c). The absorption spectrum with the transversepolarization (where the electric field is perpendicular tothe rod axis) shows practically no difference from that ofthe vertically aligned single nanorod. In the longitudinalpolarization (where the electric field is parallel to the rodaxis) the single nanorod has a peak in the near IR regionat 800 nm. Interestingly, the array structure brings thepeak in the visible region with a peak at 540 nm whichwe believe is a higher order SPR mode. The visibleabsorption of the array structures are experimentallyverified in part by Kuroda’s group who observed thatan array of horizontally lying Pt nanowires has absorption inthe visible region [50].The simulated electromagnetic fields (|E| 2 ) with the514 nm excitation of the models for v-Pt and h-Ptnanostructures are shown in Figs. 6 and 7, respectively.As can be seen in these plots, the arrays of Pt nanorods cangenerate electromagnetic fields on visible excitation. On thecontrary, single Pt nanorods in the corresponding geometriesdo not show any field enhancement (data not shown).The two top view plots in the YZ-plane of v-Ptnanostructure in Figs. 6a and b are the distributions of |E| 2at the midpoint of the height of nanorods upon Y- and Z-polarizations. The maxima of |E| 2 appear at the edges of theplots, which are attributed to the artifacts arising from thesharp edges of the model nanorods. In order to avoid theartifacts, the local maxima near the centers of the plots aretaken as the representative values of maximum |E| 2 valueswhich are 20 and 15 for the Y- and Z-polarizations. Thesevalues indicate that the electromagnetic field enhancementof v-Pt is more or less constant regardless of thepolarization direction. The side view plot in Fig. 6c showsthe distribution of the electromagnetic field along thenanorods upon Z-polarization; the case of Y-polarization isnow shown because it is almost the same. Besides theartifact maximum |E| 2 of 28 at the ends of the nanorods, thedistribution is uniform along the length of nanorods.The distribution of |E| 2 of h-Pt is strongly dependent onthe polarization direction (Fig. 7). When the excitationbeam is polarized orthogonal to the direction of nanorods(Z-polarization), the distribution of |E| 2 is uniform along thelength of nanorods (Fig. 7a). The maximum |E| 2 is 23 and33 at around the center and at the ends of the nanorods,respectively. The latter may be due to the artifact of themodel. When the excitation beam is polarized along thedirection of nanorods (Y-polarization), electromagneticfields are accumulated at the edges and there is practicallyno field enhancement around the midpoints of the nanorods.The |E| 2 at the edges of the outermost nanorods is 72.This, however, may be due to the artifact. The nanorod inthe center shows maximum |E| 2 of 25 at the ends.Comparing the |E| 2 values of the two models, h-Ptproduces stronger electromagnetic fields than v-Pt regardlessof the polarization direction. Therefore, the strongerRaman intensity of Rh6G on h-Pt than v-Pt can beconcluded as the result of stronger electromagnetic fieldsof the former than the latter.ConclusionsIn this work, we synthesized nanostructured Pt thin filmswith two different morphologies and studied their effects onSERS of Rh6G. The synthesis method is cost efficient andthe resultant nanostructures have very fine features of 8 nmin diameter of nanorods and 3–4 nm in the gap sizebetween the nanorods, with which the Pt films can produceenhanced local electromagnetic fields. The SERS intensitywas larger with the horizontally lying Pt nanorods than thevertically standing ones. Simulations based on DDA showthat the array structures of Pt nanorods have red-shiftedSPR bands from those of single Pt nanorods for bothhorizontal and vertical geometries. The simulated electromagneticfield of the horizontally lying Pt nanorods isstronger than that of the vertically standing Pt nanorodsin good agreement with the observations. Because themesoporous thin films can have diverse features including thepore structure and the pore dimension (in the range of 2–10 nm), the template-assisted synthesis method in this studyhold many possibilities to be explored in the field of SERS.Acknowledgments This work was supported by grants NRF-20090081018 (Basic Science Research Program), NRF-2010-0060482 (Mid-career Researcher Program), NRF-2010-0029698(Priority Research Center Program), NRF-2010-0029699 (PriorityResearch Center Program), and NRF-2011-0006268 (Basic ScienceResearch Program). We thank KBSI and CCRF for the SEM andRaman data.References1. Aroca R (2006) Surface enhanced vibrational spectroscopy. Wiley,New York2. Qian X, Peng X-H, Ansari DO, Yin-Goen Q, Chen GZ, Shin DM,Yang L, Young AN, Wang MD, Nie S (2008) In vivo tumortargeting and spectroscopic detection with surface-enhancedRaman nanoparticle tags. Nat Biotechnol 26:83–903. 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