ORIGINAL ARTICLE

Varying SiO2 thickness of core–shell noble-grains on a gradientsensing plate to enhance field Raman scattering

Chun-Che Lee & Tung-Kai Liu & Wen-Chi Hung &

I-Min Jiang & Ming-Shan Tsai & Chun-Te Lee &

Wen-Yao Huang

Received: 3 December 2012 /Accepted: 16 December 2013# Springer-Verlag London 2014

Abstract This study demonstrates a strong surface-enhancedRaman scattering (SERS) of crystal violet (CV) dye by usingSiO2 shell/Ag core nanoparticle in gradient-size surface plate.The excitation of CV dye can be enhanced by the localizedsurface plasmons of Ag core/shell SiO2 grains due to electro-magnetic (EM) enhancement induced. For SERS resonance,the change of dielectric environment of grains results in redshift and magnification of spectra in varying SiO2 thickness.Herein, the enhanced SERS conducted the core/shell grainwith an SiO2 thickness of 8.7 nm to magnify the intensityabout 83 %-fold that is a direct evidence in enhanced chargetransport and mutative dielectric environment.

Keywords Thermal deposition . Core/shell nanoparticle .

SERS . Localized surface plasmon . Electromagneticenhancement . Dielectric . Gradient particle

1 Introduction

Surface-enhanced Raman scattering (SERS) is a remark-able technique for spectral detection and characterizationbecause of offering sensitive down to the single-molecule regime and rich structural information [1–4].Much more attention has been paid to chromophoricmolecules attach to a nanoparticle surface for increasingsensitivity and resolution [5–11]. A nanosphere litho-graphic nanoparticle array describes active mechanisms:the enhancement of local electromagnetic field and a“first layer effect” to be considered as the average over“SERS active sites.” The intensive subjects of highlySERS-active sites are not, however, only due to muchinhomogeneous enhancement of electromagnetic field inthe proximity of the metal substrates but are also causedby the strong local differences in the chemical enhance-ment [12–17].

Hildebrandt and Stockburger [18] showed the dye ondifferent adsorption sites of colloidal silver in variedadsorption energy to yield an enhanced SERS more thantwo orders of magnitude for lower adsorption factor.Kneipp et al. [19, 20] also demonstrated that low sur-face concentration of crystal violet is adsorbed strongeron highly SERS-active sites, especially, to activate theSERS response by probably locating in the slits betweennano-metallic clusters. Effectual experiment of strongeradsorption of dye on the highly SERS-active sites hasbeen found for metallic sols due to the possible rear-rangement of nano-metallic grains in the sol status.

Additionally, the rationale of SERS properties can beascribed to (a) the electromagnetic (EM) field effect,caused by the strong surface plasmon resonance ofcurved metal surfaces coupled with incident light and(b) chemical enhancement can be considered as asurface-induced resonance Raman effect, resulting from

Electronic supplementary material The online version of this article(doi:10.1007/s00170-013-5559-8) contains supplementary material,which is available to authorized users.

C.<. Lee :W.<. HuangDepartment of Photonics, National Sun Yat-Sen University,Kaoshuing 80424, Taiwan

C.<. Lee (*)Department of Chemistry, Simon Fraser University, Burnaby, BC,Canada V5A 1S6e-mail: d933050006@student.nsysu.edu.tw

T.<. Liu :W.<. Hung : I.<. JiangDepartment of Physics, National Sun Yat-Sen University,Kaoshuing 80424, Taiwan

M.<. TsaiDepartment of Applied Physics, National Chia Yi University,ChiaYi 60004, Taiwan

C.<. LeeDepartment of Mathematics, Shantou University, Shantou,Guangdong, People’s Republic of China 515063

Int J Adv Manuf TechnolDOI 10.1007/s00170-013-5559-8

the chemisorption of a molecule on the metal surfacefor mechanisms of interacting molecule–metal complexand arising charge transfer (CT) [21–23]. Sun et al. [24]have visualized the direct evidence of CT process forchemical enhancements that SERS responses are due tochange of charge density. The charge redistribution sur-rounding metallic intra-cluster generates an excitation ofcollective oscillation in Förster transfer to enhanceSERS signals by the local EM field enhancement (alsocalled electromagnetic enhancement). However, the ex-citation of Förster transfer in the local EM field enhancementof small clusters is analogous to the plasmon excitation withsize of nanoparticles Rdiam>10 nm in diameter. For generalSERS phenomenon, the plasmon frequency is extremely sen-sitive to the nanoparticle composition, size, shape, dielectricenvironment, plasmon spectral position, sensing volume, andproximity to other nanoparticles. The preferential influence ofthe SERS in local surface plasmon (LSP) resonance revealsthat EM fields affect the metallic nanoparticle assigned to thedielectric function of nanoparticle in closing to the band andexcitation wavelength of LSP resonance [17–20]. Whitneyet al. [25, 26] indicated that the wavelength shift of an LSPresonance, △λmax, of Ag nanoparticles is fabricated by nano-sphere lithography. It can be conducted as a model of therefractive index of propagating surface plasmon on a metallicsurface: LSP resonance equation

△λmax ¼ m△n 1−exp2d

ld

� �� �ð1Þ

where △λmax is the wavelength shift, m is the refractive indexsensitivity, △n is the difference of refractive index that arechanged by altering surrounding adsorbate, d is the effectiveadsorbate layer thickness, and ld is the characteristic electro-magnetic field decay length. Even this oversimplified modeldoes not quantitatively capture all aspects of the LSP reso-nance response, which does provide some guidance for sensoroptimization. In particular, Eq. (1) highlights the importanceof distance dependence as described by the electromagneticfield decay length, ld and refractive index, △n, which is asso-ciated with dielectric environment.

Structural concept is chiefly concerned with the opticalresponse, morphology, and dielectric environment of a metalnanoparticle. In this study, a simultaneous bearing to variousSiO2 shell thicknesses that are covered with Ag nanoparticlein the same sensing plate has been developed. We indicate theSiO2 shell thicknesses to feature a variety of SERS responsesin the versatile core–shell architecture, which were formedfrom thermal depositing and annealing. Both of the CT pro-cess and local EM field enhancement are further to be testifiedby increasing SERS intensity and red shift via a modifieddielectric environment in closing to the excitation of LSPmodes.

2 Experimental methods

2.1 Materials and fabrication

All reagents were of high purity and grade including thecrystal violet dye (CV dye, Sigma-Aldrich Co., Ltd.). TheCV molecule exhibits strong absorption to the probe light inthe wavelength range from 500 to 630 nm (supplementaryS1). A CV dye solution (0.01 mM) was spin casted on thecore–shell noble-grains. Herein, the surface plasmon reso-nances (SPR) of Ag nanoparticles depend on the surroundingenvironment and further consider the SiO2 buffer layer with agradient thickness that results in a spatially gradient modula-tion on the SPR of Ag core particles to affect the enhancedRaman signal of CV dye by light source of 532 nm.

2.2 Instrumentation

A homebuilt setup for measuring the SERS signal of CV dyeon the gradient-size sensing plate can be seen below. A coolingCCD (1,024×256 pixels, Horiba Jobin Yvon, Inc.) combinedwith a monochromator (with 1,200 lines/mm grating) wasemployed. A spectrometer (IHR-550), the Raman scatteringspectra ranged from 400 to 2,000 cm−1 that were obtained bycontinuously accumulating signals within 90 s per spectrumand an integration time of 120 s. A notch filter was used toeliminate unwanted signal feedback from light source of532 nm laser (CVI MG Inc.). A fiber collimator (600 μm)collected the Raman scattering that irradiated from the interac-tion of CV dye and nanoparticles. The spot size and intensity ofprobe laser beam was 2 mm in diameter and 17 mW, respec-tively. For SERS response occurring from chemisorption ofCV dye on SiO2/Ag nano-grain surface, it induced and en-hanced the excitation of LSP resonance field to influence theinterface between nano-grain and CV dye. UV–vis absorptionspectra were recorded by a Perkin Elmer Lambda 35 spectro-photometer. A cross-sectional image of Ag nano-grain withinSiO2 shells was shot by a Transmission Electron MicroscopeJEM-2100JEM (JEOL Inc.). TEM graphic image shows thatthe Ag core was homogeneously covered with different SiO2

shell thickness in various nano-grain sizes. The field-emissionscanning electron microscope (FE-SEM; JEOL JSM-6700F)also revealed a diagram of nano-grain array.

2.3 Sample preparation

The fabrication of Ag nano-grains of sensing plate that arecovered with a SiO2 shell as well as the probed positions of(P1) ~ (P10) are shown in Fig. 1a. Both of the thickness of SiO2

shells and Ag core are concurrent on the sensing plate thatgradually increases from position (P1) to position (P10), whichare apart of 10 mm. The inserted scheme of 3D setup (L-shaped plate) also indicated the dynamically thermal vapor

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deposition. Two steps were taken out to generate core–shellgrains. In Fig. 1b, a gradient variation of core–shell size on thesensing plate was shown. First, the gradient Ag core was takenthrough a thermal deposition rate of 0.2 nm/s with a tiltedangle for 5 min, afterward an annealing treatment of 250 °Cfor 30 min, and the gradient size of Ag nanoparticles aregrained (e.g., the size of P5 are approximately 60 nm of theAg core) gradually. Finally, the preform Ag core was carriedout with the surrounding of SiO2 vapor, subsequently accu-mulated and formed slowly within various SiO2 shell thick-nesses that are located at the different position (P1) ~ (P10) byannealing of 250 °C for 20 min. An L-shaped plate, as a mask,is attached on the glass substrate, as shown in Fig. 1a. For

thermal deposition, the gradient thicknesses of SiO2 shellswere obtained by rotating from the Z-axis. There was a rotat-ing speed of 0.14°/s from the angle (θ) of 0 to 90°. In order toavoid errors in theconcentration, quenching were preparedwith much diluted dye solutions (CV dye dissolved in poly-vinyl alcohol; ~0.01 mM), then spin casted on the sensingplate (see in supplementary S2). For comparison of the en-hanced EM effect in SERS-active sensing plate, each sample(such as annealing and non-annealing treatment) was takenthrough with the same conductions.

3 Results and discussion

These SERS-active particles on sensing plate were prepared totestify the influence of dielectric and electromagnetic fields

Fig. 1 Fabrication of Ag core/SiO2 shell substrate. aRotarydeposition to generate varioussize grain. bGathering size ofnano-particle on the substrate

Table 1 The preparations of specific substrate and its process

Specimen Experiment conditions (fabrications)

Bare Ag film Thermal deposing of Ag film (99.99 %) around 6~7 nmon ITO glass

• Vacuum of chamber: 6×10−6 Torr

• Deposition rate: 0.2 nm/sec

• Tilted angle θ

Ag grains • Precursor substrate as same as prepartion of bare Ag film

• Ag grains gradually generated form annealingprocess holding 30 min at heating Temp of250–300 °C, and nature cooling overnight in the end

Core–shellnanoparticles

• Above Ag grain substrate as a precursor

• SiO2 shell to cover the Ag grain by a rotatory thermaldepositing

• Rotating Z-axis of 0.14°/s, from angle (θ) of 0 to 90°

• In form of various shell thickness with alteringannealing Temp and holding time (An example of P5kept 30 min at 250–300 °C)

A compared setup was shown in inserted scheme of Fig. 1Fig. 2 Gathering SiO2 thickness on different interval point. TEM imageof P5 grain (interval of 4 mm)

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due to gradient size of core–shell nanoparticles. Herein, thesepreparations of core/shell grains are shown in Fig. 1, includingthe interval of check point. Table 1 summarizes these experi-ment condition and parameters for fabrication of sensing plate.

Figure 2 plots the different thickness of the SiO2 shellslocating at the interval of probed positions. As expected, thethickness of SiO2 shells gradually increased in spacing atintervals of 9 mm apart from P1 to P10, and total variation ofSiO2 thickness was 17 nm. A clear insertion of cross-sectionalTEM images revealed a SiO2 thickness of 8.7 nm that locatesat the position of 4 mm (P5). For improving TEM resolution,an extra carbon layer should be performed on the sample.

To approach the efficient contribution of LSP effects onthese elaborated nanoparticles for the purpose of improvingsurface-enhanced Raman spectroscopy (SERS) sensitivity,Fig. 3 shows that the peak of Ag core/SiO2 shell grainsabsorption spectra range from 450–500 nm in varying SiO2

thickness. Consequence of wavelength enhancement and redshift in responses of LSP resonance represented agreed to amodel of the refractive index of propagating surface plasmonon a metallic surface. In Fig. 3(I) and (II), when increasing theSiO2 thickness, peaks show a red shift and magnified signalsthat implied an LSPs resonance enhancement to affect absorp-tion spectra of Ag nano-grains within possessing a dielectric

Fig. 3 IAbsorption spectra forvarious size grains. IIA red shiftalong with increasing of SiO2

shell thickness. III FE-SEMimages of various sizenanoparticles

Fig. 4 The influence of SiO2 thickness to affect the SERS intensity.Inserted illustration for measuring setup

Fig. 5 The SERS spectra in response to an interaction of CV-dye andselective substrate

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buffer (SiO2-shells). Various sizes of the Ag core/SiO2 shellarray are illustrated in Fig. 3(III) by SEM image. The furtherevidential investigations referred to the change of absorptionspectra in Fig. 3(I). The valid excitation is occurred at thereciprocal resonating between CV-dye and nanoscale grainthat directly affect the SERS signal and shift due to an en-hancement of EM field and charge transfer.

Effectual thicknesses of SiO2 shells can induce intense EMfield effects between Ag core and SiO2 shell to enhanceRaman signals that are elucidated in Fig. 4 as well as optimi-zation of grain sizes. A maximum SERS response of wavenumber 1,170 nm−1 occurs at the SiO2 shell thickness of8.7 nm. Notably, for varying SiO2 thickness, the SERS inten-sity increases from the thickness of 0 to 8.7 nm. Then, adescending of SERS intensity accompanied with a thickerSiO2 shells. It well agrees with previous reports to demon-strate a critical condition of the enhanced EM fields and LSPresonance effect depending on ultrathin surrounding and met-al grain size. The inserted illustration shows the homebuiltsetup for measuring the SERS signal of CV dye.

In Fig. 5, there are SERS spectra of the CV dyes for non-anneal Ag film, Ag nanoparticles layer, and gradient Agcore/SiO2 shell grains layer that were collected at position ofP5 (4 mm), respectively. All spectra contained the predomi-nant Stokes modes and enhancement at wave number of 914,1,170, and 1,620 cm−1, which were ascribed to the activationattachments between the Raman active site and CV dye in theform of covalent binding and accumulative deposition. Forcomparing of bare Ag film and Ag nanoparticles, the CVmolecule attached to Ag nanoparticles substrate revealed astronger Raman signal than bare-Ag film. The reason forenhancing CV dye Raman signal is due to the occurrence ofsurface plasmon resonance of Ag nanoparticles. Additionally,for comparison curves of Ag nanoparticles and gradient Agcore/SiO2 shell grains, the Raman signal of the Ag core/SiO2

shell grains fingered that amplified intensity of 60–85%-foldsat the peak of 1,170 cm−1 was higher than without SiO2 shellnanoparticles. Significantly, the Ag nanoparticles and Agcore/SiO2 shell grain possess hybrid elements of intense elec-tromagnetic (EM) field, charge transfer (CT) process, andlocal surface plasmon influence to amplify and vary theSERS response of CV dye.

By varying thickness of the dielectric SiO2 shells in asimultaneous fabrication, notably, two major factors can affectthe magnitude of the SERS responses: there is a local EM fieldenhancement and penetration depth of the near field. Whenthe SiO2 shell thickness exceeds in 8.7 nm, the influence ofpenetration depth of the near field was observed from thedecay of the SERS intensity. The power of EM field decayis due to the distance from the surface of the Ag core/SiO2

shell grains eventually overcome the increasing local EM fieldenhancement within an excitation of wavelength and LSPresonance effect.

4 Conclusion

To enhance SERS signals utilizing chemisorption of CV dyeon the SiO2/Ag nano-grain surface has been demonstrated byvarying gradient size and shell thickness. The enhanced signaldepends on the probed position of sensing substrate as well asgrain-size effect. The maximum intensity of SERS occurred atthe point of P4~P6, where the thickness of SiO2 shells was~8.7 nm. Using thin-shell cover with SERS-active noblenanostructure has several benefits: it keeps the grains fromagglomerating, protects the SERS-active site from contactwith what is being probed, and allows the “smart dust” toconform to different contours of samples by enhancement ofelectromagnetic field and charge transfer effect. There areespecially in varying nanoparticle composition, size, dielectricenvironment, local surface plasmon resonance, and proximityto other nanoparticles. The SiO2 shells also prevent somedemolition on sensing target as the electron transitionoccurred.

In summary, we describe the ability of an Ag core/SiO2

shell nano-grain of gradient SERS-active sensing plate toenhance the SERS intensity. Experimental results indicate agrain with SiO2 thickness of 8.7 nm to increase the spectraresponse of 83 %-fold and was higher than the bare Ag filmand Ag nanoparticles. The enhanced SERS signals likelyoriginated from the contribution of EM field effect and alsoassociated within the excitation of LSP modes for a red shift.

References

1. Li JF, Huang YF, Ding Y et al (2010) Nature 464:3922. Li JF, Anema JR, Yu YC et al (2011) Chem Commun 47:20233. Xinnan Wang, Yuyang Wang, Ming Cong, Haibo Li, Yuejiao Gu, John

R. Lombardi, Shuping Xu, *Weiqing Xu J, small 2013, 9, No. 11, 18954. Ringe E, McMahon JM et al (2010) J Phys Chem C 114:125115. Otto A, Mrozek I, Grabhorn H, Akemann W (1992) J Phys Condens

Matter 4:11436. Jensen L, Zhao LL, Schatz GC (2007) J Phys Chem C 111:47567. Nie S, Emory SR (1997) Science 275:11028. Weimer WA, Dyer MJ (2001) Appl Phys Lett 79:31649. Jackson JB, Westcott SL, Hirsch LR et al (2003) Appl Phys Lett 82:257

10. Pettinger B, Ren B, Picardi G et al (2004) Phys Rev Lett 92:0961011. Huh YS, Lowe AJ, Strickland AD, Batt CA et al (2009) J Am Chem

Soc 131(6):220812. Siemes C, Bruckbauer A, Goussev A et al (2001) J Raman Spectrosc

32:23113. Wittke W, Hatta A, Otto A (1989) Appl Phys A 48:28914. Zhu Z, Zhu T, Liu ZF (2004) Nanotechnology 15:35715. Yu Q, Guan P, Qin D et al (2008) Nano Lett 8:192316. Oates TWH, Noda S (2009) Appl Phys Lett 94:05310617. Beliatis MJ, Henley SJ, Silva SRP (2011) Opt Lett 36:136218. Hildebrandt P, Stockburger M (1984) J Phys Chem 88:593519. Kneipp K, Wang Y, Kneipp H et al (1997) Phys Rev Lett 78:166720. Kudelski A (2005) Chem Phys Lett 414:271

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21. Yoshida K-I, Itoh T, Tamaru H, Biju V, Ishikawa M, Ozaki Y (2010)Phys Rev B 81:115406

22. Capadona LP, Zheng J, Gonzalez JL et al (2005) Phys Rev Lett 94:058301

23. Otto A, Raman J (2005) Spectroscopy 36:497

24. Sun M, Liu S, Chenb M et al (2009) J Raman Spectrosc 40:13725. Whitney AV, Elam JW, Zou S et al (2005) J Phys Chem B

109:2052226. Siiman O, Jitianu A, Bele M (2007) J Colloid Interface Sci

309:8

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