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Novel 3D arrays of gold nanostructures on suspended platinum-coated carbon nanotubes as surface-enhanced Raman scattering substrates Moon-Keun Lee a , Jeongeun Seo a , Seok Jin Cho a , Youngdeok Jo b , Seonae Kim a , Youngjong Kang a , Haiwon Lee a, b, a Department of Chemistry, Hanyang University, Seoul 133791, Republic of Korea b Department of Convergence Nanoscience, Hanyang University, Seoul 133791, Republic of Korea abstract article info Article history: Received 7 February 2012 Accepted 25 April 2012 Available online 3 May 2012 Keywords: Surface-enhanced Raman scattering Three dimensional networks of carbon nanotubes Electrochemical deposition Gold nanoparticles Gold nanowires We report electrochemically controlled fabrications and the corresponding enhancement in Raman scatter- ing of the three-dimensional (3D) gold arrays prepared on the template of platinum-coated carbon nanotubes (CNTs) by electrochemical deposition (ECD). The topography of deposited gold was varied from nanoparticles to nanowires by ECD conditions. The increasing surface area of 3D arrays of gold nanoparticles contributed to increase signicantly the sensitivity of surface enhanced Raman scattering (SERS). For the same 3D arrays consisting of gold nanowires but with different surface roughness, much higher SERS activity was obtained when the surface of nanowires was rough. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The fabrication of ordered gold or silver nanostructures has attracted much interest due to their broad applications in sensors based on their unique optical properties [1,2], so controlling both morphology and geometry of metal ensembles has been an important key for their appli- cations as a substrate for surface-enhanced Raman scattering (SERS) [3,4]. For sensor applications especially in microuidic devices, the spa- tial arrangement of metal nanostructure became one of weighty factors to increase the capture efciency of analytes as well as SERS activity [5,6]. Therefore, the three-dimensional (3D) arrays of gold or silver nanostructures are denitely advantageous SERS substrates for offering large specic surface area, spatial disposition with ease access of the analytes and a large number of hot spots within the laser-illuminated area [7]. Ko et al. reported the fabrication of SERS substrates with ordered 3D gold nanoarray showing a signicant Raman enhancement even with- out nominal hot spots [8]. Also, Tan et al. developed well-ordered 3D silver-coated Si nanostructures which can be utilized as SERS-active 3D substrates [9]. Very recently, the plasmon scattering enhancement by nanoscale structure of noble metals on nested carbon nanotubes (CNTs) has been investigated [10,11]. However, the SERS enhancement of highly ordered 3D arrays of gold nanostructures templated on networks of CNTs has not been investigated yet. We already reported the fabrication of hierarchical gold micro- nanostructure based on 3D networks of CNTs in electrochemical application-wise [12]. In this paper, as an extension of it, we discuss electrochemically controlled fabrications and the corresponding SERS enhancements with 3D arrays of gold nanostructures on the platinum-coated CNTs templates. The correlation between the con- trolled morphology of gold nanostructure and the corresponding variations in Raman spectra is also provided by using rhodamine 6G (R6G) as a probe. 2. Experimental Si pillar structures were rst fabricated on highly N-type doped Si (100) wafers by anisotropic deep etching method. CNTs were directly synthesized onto Si pillars (3 to 8 μm height) by low pressure thermal chemical vapor deposition. Pt was coated onto the networks of CNTs with 20 nm thickness using ion sputter system (E-1045, Hitachi) to provide the electrical path for electrochemical deposition (ECD) and to secure suspended CNTs across Si pillars during massive liquid pro- cesses, and then gold was electrochemically deposited on Pt-coated CNT templates in 0.2 M KAu(CN) 2 (Sigma-Aldrich) solution at pH 4.0 by applying DC pulse with a potentiostat system (WPG100, WonaTech). After ECD, the substrates were dipped into the solution of R6G in various concentrations without stirring for 15 min, and then diluted with de-ionized water, and nally dried with high purity nitrogen Materials Letters 81 (2012) 912 Corresponding author at: Department of Chemistry, Hanyang University, Seoul 133-791, Republic of Korea. Fax: +82 2 2296 0287. E-mail address: [email protected] (H. Lee). 0167-577X/$ see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.124 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet

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Materials Letters 81 (2012) 9–12

Contents lists available at SciVerse ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r .com/ locate /mat le t

Novel 3D arrays of gold nanostructures on suspended platinum-coated carbonnanotubes as surface-enhanced Raman scattering substrates

Moon-Keun Lee a, Jeongeun Seo a, Seok Jin Cho a, Youngdeok Jo b, Seonae Kim a,Youngjong Kang a, Haiwon Lee a,b,⁎a Department of Chemistry, Hanyang University, Seoul 133‐791, Republic of Koreab Department of Convergence Nanoscience, Hanyang University, Seoul 133‐791, Republic of Korea

⁎ Corresponding author at: Department of Chemist133-791, Republic of Korea. Fax: +82 2 2296 0287.

E-mail address: [email protected] (H. Lee).

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.04.124

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 February 2012Accepted 25 April 2012Available online 3 May 2012

Keywords:Surface-enhanced Raman scatteringThree dimensional networks of carbonnanotubesElectrochemical depositionGold nanoparticlesGold nanowires

We report electrochemically controlled fabrications and the corresponding enhancement in Raman scatter-ing of the three-dimensional (3D) gold arrays prepared on the template of platinum-coated carbonnanotubes (CNTs) by electrochemical deposition (ECD). The topography of deposited gold was varied fromnanoparticles to nanowires by ECD conditions. The increasing surface area of 3D arrays of gold nanoparticlescontributed to increase significantly the sensitivity of surface enhanced Raman scattering (SERS). For thesame 3D arrays consisting of gold nanowires but with different surface roughness, much higher SERS activitywas obtained when the surface of nanowires was rough.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The fabrication of ordered gold or silver nanostructures has attractedmuch interest due to their broad applications in sensors based on theirunique optical properties [1,2], so controlling both morphology andgeometry ofmetal ensembles has been an important key for their appli-cations as a substrate for surface-enhanced Raman scattering (SERS)[3,4]. For sensor applications especially in microfluidic devices, the spa-tial arrangement of metal nanostructure became one of weighty factorsto increase the capture efficiency of analytes as well as SERS activity[5,6]. Therefore, the three-dimensional (3D) arrays of gold or silvernanostructures are definitely advantageous SERS substrates for offeringlarge specific surface area, spatial disposition with ease access of theanalytes and a large number of hot spots within the laser-illuminatedarea [7].

Ko et al. reported the fabrication of SERS substrates with ordered 3Dgold nanoarray showing a significant Raman enhancement even with-out nominal hot spots [8]. Also, Tan et al. developed well-ordered 3Dsilver-coated Si nanostructures which can be utilized as SERS-active3D substrates [9]. Very recently, the plasmon scattering enhancementby nanoscale structure of noble metals on nested carbon nanotubes(CNTs) has been investigated [10,11]. However, the SERS enhancement

ry, Hanyang University, Seoul

l rights reserved.

of highly ordered 3D arrays of gold nanostructures templated onnetworks of CNTs has not been investigated yet.

We already reported the fabrication of hierarchical gold micro-nanostructure based on 3D networks of CNTs in electrochemicalapplication-wise [12]. In this paper, as an extension of it, we discusselectrochemically controlled fabrications and the correspondingSERS enhancements with 3D arrays of gold nanostructures on theplatinum-coated CNTs templates. The correlation between the con-trolled morphology of gold nanostructure and the correspondingvariations in Raman spectra is also provided by using rhodamine6G (R6G) as a probe.

2. Experimental

Si pillar structures were first fabricated on highly N-type doped Si(100) wafers by anisotropic deep etching method. CNTs were directlysynthesized onto Si pillars (3 to 8 μm height) by low pressure thermalchemical vapor deposition. Pt was coated onto the networks of CNTswith 20 nm thickness using ion sputter system (E-1045, Hitachi) toprovide the electrical path for electrochemical deposition (ECD) andto secure suspended CNTs across Si pillars during massive liquid pro-cesses, and then gold was electrochemically deposited on Pt-coatedCNT templates in 0.2 M KAu(CN)2 (Sigma-Aldrich) solution at pH4.0 by applying DC pulse with a potentiostat system (WPG100,WonaTech).

After ECD, the substrates were dipped into the solution of R6G invarious concentrations without stirring for 15 min, and then dilutedwith de-ionized water, and finally dried with high purity nitrogen

Fig. 1. Morphology and crystalline structure of 3D arrays of gold nanostructures: (a), (b) cross-sectional view, (c) top view (a zoomed-in image of top area of pillar is inserted) ofgold nanoparticles with 8-μm-height pillar, and (d) XRD data of two 3D and one planar gold nanostructures.

Fig. 2. SEM images of the controlled morphology of gold nanostructures by varying the amount of deposited gold to (a) 0.74 μmol/cm2, (b) 1.03 μmol/cm2, (c) 1.53 μmol/cm2,(d) 3.25 μmol/cm2 and (e) 6.84 μmol/cm2. (f) is the measured thickness of gold nanostructure with the amount of deposited gold.

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Fig. 3. SERS spectra of 3D array of gold nanostructures (a) with different amount ofdeposited gold on 3-μm-pillar-height substrates and (b) with different R6G concen-trations at 3.25 μmol/cm2. Each spectrum is the averaged profile of 5 measurementsat the different locations. Inserted are (a) the peak intensity at 1362 cm−1 in thefunction of deposited gold and (b) the peak intensity at 1362 cm−1 versus R6Gconcentration.

11M-K. Lee et al. / Materials Letters 81 (2012) 9–12

gas. Raman measurements were taken with backscattering geometryon a Horiba JY LabRam HR fitted with a liquid-nitrogen cooled CCDdetector. The spectra were collected under ambient conditions usingthe 633 nm line of a He-Ne laser (0.5 mW) which was confined with-in 1 μm between two adjacent pillars. All gold-deposited substrateswere characterized by X-ray diffractometer (XRD, ATX-G, Rigaku)after morphological analysis by scanning electron microscope (SEM,S-4700, Hitachi).

3. Results and discussion

Fig. 1 shows the typical 3D gold nanoparticle arrays fabricated onPt-coated networks of CNTs by ECD with pulsed bias of −1.0 V for120 s (Si pillar height=8 μm). Gold nanoparticles with diameter of20–30 nm were well deposited on Pt-coated networks of CNTs(Fig. 1a), and formed highly-ordered 3D arrays (Fig. 1b, c), and itsmorphology looks uniform on both CNTs and Si pillars as seen inthe inserted SEM image of Fig. 1(c). From the aspect of diffusion-limited transport of analytes, 3D arrangement of gold nanoparticlesis advantageous for sensor applications.

Pulsed bias condition was optimized for gold nanoparticles at−1.2 V and 120 s and for gold nanowires at −1.0 V and 120 s. TheirXRD spectra show similar crystallinities to planar gold substrates(Fig. 1d). Both 3D gold nanostructures and planar gold substratesare polycrystalline with the preferred orientation of (111) plane andcomparable full width at half maximums.

Morphology of gold nanostructures could be controlled by the biasconditions during ECD, thus the amount of deposited gold on CNTs isestimated by the equation below.

amount of deposited gold ðμmol=cm2Þ¼ biased currentðC=sÞ � biased time ðsÞarea of substrate ðcm2Þ� 0:0964853365ðC=μmolÞ

Fig. 2 represents the morphology control of gold nanostructureson CNTs by varying the amount of gold deposited. The calculatedgold amounts are 0.74, 1.03, 1.53, 3.25 and 6.84 μmol/cm2 for thesamples in Fig. 2a–e, respectively. As the amount is increased, thegold nanoparticles are merged to form smooth gold nanowires. Itwas expected to form tubular structures eventually because goldnuclei were templated on CNTs. The measured thickness of goldon CNTs was increased proportionally to the square root of the de-posited gold amount (Fig. 2f). Considering only gold deposited onsingle suspended CNT, this result consists well with the relation-ship between the tube radius (the thickness of gold) and themass (the amount of deposited gold), supporting that the morpho-logy of gold is successfully controlled by the amount of depositedgold.

SERS spectra obtained from 3D arrays of gold nanostructures arepresented in Fig. 3a. Gold nanoparticles prepared with 0.74 μmol/cm2

do not exhibit strong SERS effect because of the small surface area ofgold for the adsorption of R6G molecules. Intensity increase of the peakat 1362 cm−1 from 0.74 to 3.25 μmol/cm2 is likely attributed to the in-crease in surface area of the deposited gold. Besides the enhancementby larger surface area, there may be another enhancement by hot spotsin the ensemble of gold nanoparticles. However, it was not possible todistinguish the enhancement by hot spots from that by the increasingsurface area.

It is noteworthy that the peak intensity for 6.84 μmol/cm2 is lessthan half of that for 3.25 μmol/cm2 although its surface area is larger.It is inferred that there were hot spots in gold nanowires formed at3.25 μmol/cm2. Judging from the difference of morphology betweengold nanowires of 3.25 and 6.84 μmol/cm2, the reduced surfaceroughness of gold nanowires formed at 6.84 μmol/cm2 likely affectsthe enhancement of Raman scattering by the electromagnetic theoryof SERS [13,14]. Presumably hot spots were at the rough boundaries

of merged gold nanoparticles, but would be disappeared at6.84 μmol/cm2. This result is aligned with the report by Duan et al.[15].

Fig. 3b shows the SERS spectra of 3D gold substrates (3.25 μmol/cm2)with R6G concentrations varied from1 μMto 100 pM. The peaks for R6Gmolecules were decreased with the R6G concentration down to 1 nM,and disappeared completely at 100 pM. Thus, the detection limitachieved from this approach is 1 nM.

The SERS surface enhancement factor (EF) is calculated for R6G onthe Au nanostructured samples (Fig. 2d) according to the equationEF=(Isurf /Nsurf)/(IRaman/NRaman) [16,17]. In this expression, Isurf andIRaman stand for the integrated intensities for the 1362 cm−1 peak ofthe R6G adsorbed on the Au surface and R6G on the bare Si surface,respectively; whereas Nsurf and NRaman represent the correspondingnumber of R6G molecules excited by the laser beam. 20 μl of 1 μMand 1 mM R6G methanol solutions were drop-casted on 3D array ofgold substrate and the bare Si wafer which top-view areas are0.85 cm2, respectively. The effective occupied area of single R6G mol-ecule onto 3D array of gold substrate and the bare Si substrate wereabout 7000 and 7 nm2, respectively. Assuming the projected area bylaser at 3D gold SERS substrate is similar to that of the bare Si sub-strate, the maximum EF of 1.4×109 is achieved.

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4. Conclusions

The 3D gold arrays for SERS-active substrate were electrochemi-cally fabricated using the Pt-coated networks of CNTs as a template.The crystallinity of gold nanostructures was polycrystalline with thepreferred orientation of (111) plane. The morphology of depositedgold was successfully controlled from nanoparticles to nanowires byvarying the amount of gold. The intensity of SERS spectra using R6Gwere varied depending on the surface area and the roughness ofgold. The peak intensity of 1362 cm−1 had a maximum within therange of 0.74 and 6.84 μmol/cm2 by the competing effects on the en-hancement between the increased surface area and the reduced sur-face roughness of gold.

Acknowledgments

This work was supported by the grant (2011K000207) from the‘Center for Nanostructured Materials Technology’ under the ‘21st Cen-tury Frontier R&D Programs’ of the Ministry of Education, Science andTechnology of Korea, the Hanyang University (HYU-2001-T), the Inter-national R&D Program of the National Research Foundation of Koreafunded by the Ministry of Education, Science and Technology of Korea(K21003001810-11E0100-01510), Seoul R&D Program (10919) andthe grant (AOARD-114106) from the Air Force Office of ScientificResearch/the Asian Office of Aerospace Research and Development,USA.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.matlet.2012.04.124.

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