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Au nanoparticle based localized surface plasmon resonance substrates fabricated by dynamic

shadowing growth

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2010 Nanotechnology 21 175303

(http://iopscience.iop.org/0957-4484/21/17/175303)

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Nanotechnology 21 (2010) 175303 (9pp) doi:10.1088/0957-4484/21/17/175303

Au nanoparticle based localized surfaceplasmon resonance substrates fabricatedby dynamic shadowing growthJunxue Fu1 and Yiping Zhao

Department of Physics and Astronomy, Nanoscale Science and Engineering Center,The University of Georgia, Athens, GA 30602, USA

Received 17 November 2009, in final form 9 February 2010Published 6 April 2010Online at stacks.iop.org/Nano/21/175303

AbstractAu nanoparticle (NP) substrates, Au NP/TiO2/Au NP sandwich structures, and Ti coated Au NPsubstrates are fabricated by glancing angle deposition (GLAD) and oblique angle deposition(OAD) methods. Under the same deposition condition, the Au NP substrates produced byGLAD are more uniform and reproducible compared to those fabricated by OAD. The localizedsurface plasmon resonance (LSPR) wavelength of Au NP substrates can be easily tuned bychanging the film thickness, the deposition angle, and the coating of the dielectric layer (TiO2)and metallic layer (Ti). In addition, the thickness and the deposition angle of the Ti coating onAu NP also affect the LSPR wavelength. Our results demonstrate that GLAD is a very versatilefabrication technique to produce reproducible and fine-tuned LSPR substrates.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

As the size of a metal structure shrinks below 100 nm,the collective movement of electrons inside the internalmetal framework becomes restricted. This confined electrondensity can be polarized and oscillates in resonance with anappropriate frequency of light as the wavefront of the lightpasses; this is called localized surface plasmon resonance(LSPR) [1–3]. LSPR properties of the metal nanostructureshave been intensively studied and broadly applied in variousfields. As the shape or size of the nanoparticle changes,i.e. the surface boundaries, the electrons will modify theircollective oscillation accordingly. Furthermore, changing thedielectric constant of the surrounding material will have aneffect on the oscillation frequency due to the varying ability ofthe surface to accommodate electron charge density from thenanoparticles. Since the surrounding dielectric environmentof the metal structure has a significant influence on theplasmon peak wavelength, the LSPR sensing process can beaccomplished by observing the plasmon peak shift due to thebinding of an analyte to the metal surface [1–3]. Using asimple optical setup, many different sensors based on the LSPRprinciple have been developed and applied in the detection

1 Present address: Department of Electrical and Computer Engineering,Hong Kong University of Science and Technology, Hong Kong.

of heavy metal ions [4], toxins [5], proteins [6], glucose [7],nucleic acids [8], biotin–streptavidin [9] and antigen–antibodyreactions [10, 11]. Besides sensor applications, the LSPRproperties of silver and gold nanoparticles have been usedfor surface enhanced vibrational spectroscopy [12–15], andother important optical applications such as non-linear optics,photoluminescence and photonic devices [16–19].

One of the important factors affecting these applicationsis the metal NP fabrication. For sensor application, theuniform NP substrate is able to produce a narrow width ofthe absorbance spectrum and therefore enhance the sensitivity.Therefore, a great amount of effort has been devoted tothe fabrication of uniform LSPR metallic nanostructuresand tuning their LSPR properties [1–3]. For nanoparticlefabrication, reduction methods are widely accepted as practicaland versatile approaches [16, 20, 21]. However, manyapplications require the nanoparticles to be supported ona substrate. Thus the nanoparticles produced by thistraditional approach need to be immobilized uniformly ontoa substrate. This extra step presents a great challenge forpractical applications. To overcome this difficulty, many otherfabrication methods that directly fabricate nanoparticles ontoa substrate have been proposed. In particular, lithographymethods, such as nanosphere lithography (NSL) [22–25] andelectron beam lithography (EBL) [26], provide an accurate

0957-4484/10/175303+09$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 21 (2010) 175303 J Fu and Y Zhao

way to control the size, shape and spacing of the surface-confined nanoarrays. Nevertheless, NSL is limited in termsof the structures and topographies generated, whereas EBL istime-consuming, expensive, and not applicable to large-scaleproduction. Another economical and direct fabrication methodto produce uniformly coated nanoparticle substrate is physicalvapor deposition, such as thermal evaporation [27] andsputtering [28]. The dependence of the LSPR wavelength onthe deposition parameters (substrate temperature, depositionrate, and film thickness) for silver and gold films was exploredquantitatively by Gupta et al, and an empirical relationshipbetween the LSPR wavelength of the Ag and Ag nanoparticlefilms and the deposition parameters was established [27].However, judging from the AFM images they obtained,the particle size distribution was fairly dispersed, and theLSPR absorbance peaks quickly broadened with the depositionthickness, and thus cannot be used for a good sensor. Recentlywe have shown that Ag nanoparticles fabricated by a so-called oblique angle deposition (OAD) are much more uniformcompared to those fabricated by the conventional thin filmdeposition methods, and the LSPR wavelength can be easilytuned from 401 to 666 nm in the visible range by changing thefilm thickness from 5 to 100 nm and the deposition angle from0◦ to 85◦ [29].

OAD is a physical vapor deposition process where thecollimated deposition flux is incident onto a substrate witha large angle (typically greater than 75◦) with respect tothe surface normal, and the shadowing effect is the mainmechanism that controls the growth [30]. This implies thatanother shadowing effect based growth technique, glancingangle deposition (GLAD) can also be used for fabricationof LSPR substrates [31–35]. The GLAD method combinesthe OAD with substrate rotation. During the GLAD process,the substrate is manipulated by two stepper motors insidethe vacuum chamber using custom computer software andelectronics. One motor controls the incident angle andthe other is responsible for the azimuthal rotation of thesubstrate. Similar to the OAD technique, the size and shapeof nanoparticles can be controlled by adjusting fabricationparameters such as film thickness and deposition angle. Inaddition, substrate rotation can also be utilized as anotheradjustable parameter to produce nanostructures with variousshapes [36]. Therefore, many unique three-dimensionalnanorod structures can be fabricated by GLAD throughprogramming the stepper motors [31–35]. Compared to theOAD method, the GLAD method has more flexibility indesigning nanostructures, and the resulting nanoparticles arealso expected to be more uniform compared to those generatedby the OAD method since the substrate is rotating azimuthally.In fact, Gish et al has used the GLAD method to fabricate shortAg nano-posts for LSPR sensors [37]. In their experiments,the Ag nano-post is about 20–300 nm in diameter and 150 nmin height, and we believe that the LSPR sensitivity comesfrom the transverse mode of the LSPR. So far, there isno study on the fabrication of Ag or Au nanoparticles byGLAD in the thin thickness region. In addition, in orderto achieve the goal of tuning the LSPR, one could also coatnanoparticles with transparent oxide to change the dielectric

Figure 1. (a) OAD and GLAD set up in the electron beamevaporator. (b) OAD set up in the thermal evaporator.

environment [16, 21, 23, 38]. The combination of a betternanoparticle fabrication method with the change of the localdielectric environment could give us more flexibility to tunethe LSPR wavelength for specific applications. In this paper,we have compared the LSPR properties of Au nanoparticlefabricated by OAD and GLAD. We have found that GLADcan produce more reproducible Au nanoparticle substrates.To tune the LSPR properties of the GLAD Au nanoparticles,we deposit different thicknesses of Ti at different incidentangles and find the LSPR peak red-shift with the amountof the Ti deposited. We also fabricate a reproducible Aunanoparticle/TiO2/Au nanoparticle sandwich structure. Allof these results demonstrate that GLAD is a very good andreliable way to fabricate LSPR substrates.

2. Experiments

To compare the qualities of the Au nanoparticle substratesfabricated by OAD and GLAD methods, we used a custom-designed electron beam evaporation system (Torr International,Inc.). For the GLAD deposition, as shown in figure 1(a),the deposition angle θ was set to be 85◦ and the azimuthalrotation speed was set to be 0.5 Hz. For the OAD deposition,the substrate rotation speed was set to be 0 Hz and thedeposition angle was set to be 85◦. The Au film thickness ornominal thickness, which was monitored by a quartz crystalmicrobalance (QCM) directly facing the crucible, was set to bed = 20 and 30 nm.

Based on the Au nanoparticle substrates fabricated by theGLAD method (θ = 86◦ and d = 30 nm), we also fabricateda Au NP/TiO2/Au NP sandwich structure. A TiO2 film with aQCM reading thickness of 5 nm was deposited onto Au NPsubstrates fabricated by GLAD; subsequently, another layerof Au with the same nominal thickness (d = 30 nm) wasdeposited onto TiO2 film by GLAD.

To study how a metallic coating will affect the LSPRproperties of Au NPs, we used a thermal evaporator(Thermionics Laboratory, Inc.) and the OAD method to depositthe Au NP substrates and the Ti cover layer. The Au NPsubstrates were fabricated by using angled wedges with preset

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Nanotechnology 21 (2010) 175303 J Fu and Y Zhao

Figure 2. AFM images of Au NPs (d = 30 nm, θ = 85◦) made by (a) OAD and (b) GLAD. The scan size was 1 μm × 1 μm.

angles of θ = 75◦, 80◦ and 85◦. The glass substrates wereloaded on these wedges, which were mounted onto the flatsubstrate holder as shown in figure 1(b). The QCM readingof Au NP deposition was fixed as d = 10 nm for the entireexperiment. In order to see how the Ti coating changes theLSPR property of Au NPs, Ti was deposited onto the aboveAu NPs using a custom-designed electron beam evaporationsystem (Torr International, Inc.) in three different ways: (1) ata fixed deposition angle θTi = 0◦, with different thicknesses(QCM reading) of Ti dTi = 2, 5, 8, 10, 15 and 20 nm depositedonto Au NPs samples and with different deposition angles ofθ = 75◦, 80◦ and 85◦, respectively; (2) at a fixed QCMthickness dTi = 2 nm, Ti deposited at different depositionangles of θTi = 0◦, 45◦, 65◦ and 85◦ onto Au NPs sampleswith different deposition angles of θ = 75◦, 80◦ and 85◦,respectively; (3) at a fixed deposition angle θTi = 85◦, withdifferent QCM thicknesses of Ti dTi = 2, 10, 30 and 50 nmdeposited onto Au NPs samples with different depositionangles of θ = 75◦, 80◦ and 85◦, respectively.

Au (99.99%) and Ti (99.995%) were purchased fromKurt J Lesker Company (Clairton, PA). For all the depositions,the background pressure in all the chambers was 1×10−6 Torr,and the deposition rate was approximately 0.1–0.3 A s−1. Allthe glass slide substrates (Gold Seal® Catalog No. 3010) werecleaned using piranha solution H2SO4:H2O2 (4:1 vol/vol) andwere cut into 1 cm × 2 cm pieces.

The absorbance spectrum of the substrate was measuredby an UV–vis–NIR double beam spectrophotometer (JASCOV-570). The light source was unpolarized. The morphologiesof the Au NPs substrates were characterized by an AFM(VEECO Dimension 3100) with a tapping mode. The scan ratewas 1–2 Hz and the scan size was 1 μm × 1 μm. The particlesize was analyzed by section function in AFM software and theAFM RTESP7 tip had a radius of 10 nm.

3. Results and discussions

3.1. Gold nanoparticle substrates prepared by OAD andGLAD

AFM images of OAD and GLAD samples with a thicknessof d = 30 nm were obtained and are shown in figures 2(a)

and (b), respectively. By estimation, the averaged particlesize of these samples is around 35 nm. In addition, theparticles appear slightly tilted, but we believe that this is causedby the system error. Figure 3(a) shows UV–vis absorbancespectra of Au NPs made by OAD with d = 20 and 30 nm,respectively. For both thicknesses, figure 3(a) shows twospectra collected from two different substrates. For d = 20 nm,the LSPR wavelength λ0 varies from 523 to 531 nm, whilefor d = 30 nm, λ0 changes from 529 to 533 nm. Thepeak absorbance for each Au thickness also varies slightly.The general trend is that when the Au thickness increases,the λ0 red-shift, and the peak absorbance increases, which isconsistent with our previous observations for Ag NP substratesprepared by OAD. Although we have successfully monitoredthe interaction between Biotin and NeutrAvidin by a LSPRsensor based on the Ag NP substrates made by OAD [29], wehave to measure the same location on the substrate to avoidthis non-uniformity issue. We also estimated the width of thesample with d = 30 nm by fitting the curves with a Gaussianfunction, the result being 137.4 ± 0.5 nm. Figure 3(b) showsthe UV–vis absorbance spectra for 8 Au NP substrates preparedby GLAD with d = 30 nm. Although the absorbance spectrafor all the eight substrates do not overlap with each other, theLSPR wavelengths are almost identical, λ0 = 521 nm. Thevariation in the absorbance peak, from 0.645 to 0.663, is notsignificant (<3%). Regarding the width, Gaussian functionfitting produced a result of 118 ± 2 nm. Compared to Au NPsubstrates fabricated by the OAD method, we have reached thefollowing conclusions: (1) at the same nominal thickness d ,the LSPR wavelength of the OAD samples red-shift comparedto that of the GLAD samples; the absorbance at the LSPRpeak for the OAD samples is also slightly larger than thatof the GLAD samples. Both these results imply that theaverage size of the Au NPs fabricated by GLAD is smaller thanthat of the OAD substrate under similar deposition conditions.However, the AFM measurements could not differentiate thesmall difference in particle size corresponding to the ∼10 nmLSPR peak shift. (2) The variation of LSPR wavelengthin the GLAD samples prepared under the same condition ismuch smaller than that of the OAD samples. (3) The LSPRcurves width of the GLAD samples is also smaller than that

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Nanotechnology 21 (2010) 175303 J Fu and Y Zhao

Figure 3. Absorbance spectra of Au NPs made by (a) OAD and (b) GLAD.

Figure 4. (a) Normalized absorbance spectra of substrates A, A/B and A/B/A. A: Au NPs made by GLAD (d = 30 nm, θ = 85◦). B: TiO2

deposition by GLAD (d = 5 nm, θ = 85◦). AFM images of substrates (b) A/B and (c) A/B/A. The scan size was 1 μm × 1 μm.

of the OAD samples, indicating a more uniform particle sizedistribution. But the smaller standard deviation for OADsamples is unexpected, and we believe that this is caused bylimited sample numbers (only two OAD substrate samples).As we know, one of the factors determining the plasmon peakis the particle size. In the OAD configuration, the distance fromthe material source to different sample pieces is varied and thusaffects the actual thickness of Au NPs. Therefore, the particlesizes of Au NPs varied and resulted in different plasmonpeaks. By incorporating azimuthal substrate rotation duringthe deposition, the spatial difference can be eliminated and

the substrates can produce a uniform and consistent plasmonresponse.

3.2. Au/TiO2/Au sandwich structures

To further explore the LSPR properties of Au NPs, AuNPs/TiO2 film/Au NPs sandwich structures were fabricatedas described in the experimental section. Figure 4(a) showsthe normalized absorbance spectra of Au NPs, Au NPs/TiO2

film, and Au NPs/TiO2 film/Au NPs samples. After a QCMreading dTiO2 = 5 nm TiO2 deposition, the plasmon peak

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Nanotechnology 21 (2010) 175303 J Fu and Y Zhao

Figure 5. (a) AFM image of Au nanoparticles (d = 10 nm, θ = 85◦). The scan size was 1 μm × 1 μm. Relationship between the plasmonpeak shift of Au NPs and (b) thickness of Ti dTi at θTi = 0◦; (c) deposition angles of Ti θTi at dTi = 2 nm; (d) thickness of Ti dTi at θTi = 85◦.

of the nanostructures shifted from 521 nm in Au GLADsubstrate to 534 nm, due to the changing dielectric constantaround the Au NPs. Interestingly, another 30 nm Au GLADdeposition did not make the plasmon peak shift further, whichmeans TiO2 acts as a spacer between two Au depositions. Onthe other hand, according to the AFM images (figures 2(b)and 4(b)), before the TiO2 deposition the average size of theAu particle was 35 nm, and after TiO2 deposition the averageparticle size was still 35 nm, i.e., TiO2 deposition did notincrease the size of the NPs. However, after the second AuGLAD deposition, the size of the NPs seemed to increasesignificantly (figure 4(c)); the average NP size became 44 nm.Combining the morphological results and optical absorbancespectra, we suggest that there are two possible morphologiesfor this multilayer structure: one possible morphology is thatthe second Au deposition produces an Au shell covering theTiO2 layer, and that this shell has similar optical propertiesto the first layer of Au NPs with the TiO2 coating; the othermorphology is that the second Au deposition in fact formssimilar sized NPs on top of TiO2 layer but that the AFM tipcannot resolve the multiple Au NPs grown on the TiO2 layerdue to the complicated geometry. With either morphology,our experimental results demonstrate that (1) the depositionof a dielectric layer on top of GLAD Au NPs can slightlytune the LSPR wavelength; and (2) the Au NP/TiO2/Au NPsandwich structure fabricated with the GLAD technique hassimilar optical properties to the TiO2/Au NP structure. Weexpect that this additional layer of TiO2, like the bow-tie

structure predicted in the literature [39], could generate more‘hot spots’ when the structure is used for surface enhancedRaman scattering, or induce a higher surface area (and thusa higher sensitivity) for LSPR sensor applications.

3.3. The effect of metal layer coating on the LASP property ofAu NP substrates

It is well documented in the literature that the LSPRwavelength of Au or Ag NPs will red-shift when the dielectricconstant of the surrounding dielectric media increases [40].However, it is unknown how the LSPR wavelength changeswhen the surrounding environment becomes a differentmetallic layer. In the literature, such bi-metallic nanoparticleshave been applied for a LSPR sensor, where Au acts as aprotective layer for Ag, but the LSPR wavelength change afterAu coating is not given [41]. Some other works discussthat the LSPR wavelengths of Au and Ag compete as thethickness of Au shell varies [42–44]. With the advantagesof the GLAD and OAD techniques, one could coat the AuNPs with different metal layers with different thickness andcoverage, and perform a systematical study on the effect of thesecond metal layer. Here we concentrate on the Au NPs’ (d =10 nm, θ = 85◦) response to the coating of Ti. As shown infigure 5(a), the averaged diameter of the Au NPs is about D =30 nm. Figure 5(b) shows the LSPR wavelength shift �λp as afunction of the Ti coating thickness dTi at the deposition angleθTi = 0◦ for all the Au NPs substrates fabricated at θ = 75◦,

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Nanotechnology 21 (2010) 175303 J Fu and Y Zhao

Figure 6. (a) Theoretical model of Ti coated Au NP at θTi = 0◦; (b) plasmon peak shift �λp versus the thickness of Ti dTi by Br theory.

80◦, and 85◦, respectively. Regardless of the base Au NPsubstrate, the �λp–dTi follows a similar relationship: the LSPRwavelength shift �λp increases with increasing thickness of TidTi at θTi = 0◦, i.e., the LSPR wavelength red-shifts when moreTi is deposited onto Au NPs. When dTi � 5 nm, �λp is similarfor different Au NP substrates. The discrepancy becomes largewhen dTi > 5 nm, although there is no obvious trend fordifferent Au NPs substrates (e.g. the Au NP deposition angle).In fact, researchers have found that the LSPR peak shift is

proportional to (1 − e−2zld ), where z is the distance from the

metal NP and ld is the characteristic decay length of the localelectric field [45, 46]. Because of the exponential increase ofz, i.e. the thickness of the coating layer, the LSPR shift willsaturate at a critical distance zc, which depends on the size,shape distribution, and composition of the metal NPs [47, 48].Combining the results of the Au spherical NP based LSPRsensors from literature [47, 49, 50] and the measured results,we estimate that the critical distance zc for Au NPs in this studyis 20 nm and the maximum �λp is around 100 nm for the Ticovering layer.

Figure 5(c) shows the LSPR wavelength shift �λp

decreases with increasing θTi at dTi = 2 nm. Due to thegeometric shadowing effect, the area receiving the Ti atomson the Au NPs is different for different deposition angles ofTi θTi: with larger incident angle θTi, the shadowing effectbecomes more significant, and it is expected that each Au NPwill receive less Ti atoms with the same QCM thickness, andwill have less Ti coverage as well. Since the smaller coverageon the Au NPs can cause a smaller change of the surroundingmedium, the LSPR shift will be smaller. This interpretationis consistent with the result shown in figure 5(c). It is alsoexpected that the LSPR wavelength shift �λp increases slowlywith the increasing QCM thickness of dTi at a smaller coverage(or larger deposition angle). Figure 5(d) shows the LSPRwavelength shift �λp as a function of the Ti coating thicknessdTi at the deposition angle θTi = 85◦ for all the Au NPssubstrates fabricated at θ = 75◦, 80◦, and 85◦, respectively.Compared to the same situation shown in figure 5(b), thechange �λp versus dTi becomes significantly slower. Forexample, at dTi = 10 nm, �λp is around 95 nm for θTi = 0◦but only 10 nm for θTi = 85◦. This means that using the GLAD

or OAD multilayer deposition method, one could fine tune theLSPR property of Au or Ag NPs through both the depositionthickness and deposition angle of a second layer.

In order to understand Au NPs’ response to the Tidepositions, we construct a simplified theoretical model basedon a single Au NP, as shown in figure 6(a). For the Au NPdeposited with d = 10 nm and θ = 85◦, the NP diameter is∼30 nm. During the Ti deposition at θTi = 0◦, it is reasonableto assume that half of the Au NP is covered with a uniformTi layer. This layer of Ti on a single Au NP can be treated asan effective medium layer (EML) and the thickness of EMLis experimentally determined as 20 nm (see discussion above).The effective medium theory, Maxwell–Garnett (MG) theory,and Bruggeman (Br) theory [51] can be used to homogenizethe EML containing Ti coating:

ε − εTi

εTi + L(ε − εTi)= (1 − fp)

[εa − εTi

εTi + L(εa − εTi)

],

(MG theory) (1)

ε = 14 (β +

√β2 + 8εTiεa), (Br theory) (2)

β = (3 fTi − 1)εTi + [3(1 − fTi) − 1]εa (3)

where εTi and εa are the dielectric constants of Ti and air,respectively; L is the depolarization factor and is 1/3 for thesphere; fTi is the volume fraction of Ti:

fTi = t2 + 30t

2000, (4)

where t is the thickness of Ti. Using Mie theory, we cancalculate the absolute LSPR wavelength shift caused by thechange of the outer layer dielectric constant [52],

Q(λ) = 3π Nd3ε3/2

λ ln(10)

[εi

(εr + 2ε)2 + ε2i

], (5)

where Q is the extinction, N is the area density of Au NPs,d is the diameter of Au NP, ε is the dielectric constant ofthe surrounding medium, λ is the wavelength of the incidentlight, εi and εr are the imaginary portion and the real portionof the Au NP’s dielectric function εAu, respectively, which iscalculated by following Lucia Scaffardi’s work [53].

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Nanotechnology 21 (2010) 175303 J Fu and Y Zhao

Figure 7. (a) Theoretical model of Ti coated Au NP at dTi = 2 nm;(b) plasmon peak shift �λp versus the deposition angles of Ti θTi byBr theory and MG theory. (c) The enlargement of �λp ∼ fm

relationship by MG theory.

When Ti with different thickness was deposited on Au NPsin the normal direction, as shown in figure 6(a), we assume theshape of Ti is a semi-shell with a thickness of t . Thereforethe plasmon peak shift �λp is a function of t and has beenplotted in figure 6(b). According to Br theory, the plasmonpeak first red-shifts about 10 nm and then blue-shifts about60 nm. However, this is not consistent with the experimentalresults (figure 5(b)). In addition, no plasmon peak is observedwhen using MG theory.

When Ti with a nominal thickness of 2 nm was depositedon Au NPs at different deposition angles, the calculation of theTi volume becomes tricky due to the shadowing affect. If westill use the above simplified model (figure 7(a)), the maximumvolume fraction fTi is unity when θTi = 0◦. Therefore, the

volume fractions corresponding to different deposition anglesθTi are within the range of 0–0.032, and they are inverselyproportional to θTi. Figure 7(b) shows the plasmon peak shift�λp as a function of the volume fraction fTi. From Br theory,the plasmon peak first blue-shifts and then starts to slowlyred-shift, except that there is a big jump when fTi � 0.02.The plasmon peak shift �λp vs volume fraction fm by MGtheory is enlarged in figure 7(c), which shows that �λp almostlinearly blue-shifts with the volume fraction. Unfortunately,these results are not consistent with the experimental results(figure 5(c)). We believe that the detailed transportationbehavior at the interface of Au–Ti has to be taken into account,i.e., the electrons in Au NP could be partially reflected andtransmitted at the boundary of the Au–Ti interface. Theeffective medium theory does not take this into account. Weexpect a good theoretical model could be proposed to explainthis phenomenon.

4. Conclusions

In summary, we have shown that both GLAD and OADcan produce Au NP substrates for LSPR sensor applications.However, under the same deposition conditions, the AuNP substrates produced by GLAD are more uniform andreproducible compared to those fabricated by OAD. Anadditional layer of TiO2 coated onto the Au NP substrateby a physical vapor deposition method can fine tune theLSPR wavelength of the Au NP substrates. The AuNP/TiO2/Au NP sandwich structures fabricated by Au GLADmethod could preserve the LSPR property of TiO2/Au NPsubstrates, which may improve the sensitivity of LSPR basedsensors due to the possible electric field enhancement. Thecoating of a Ti layer on Au NP substrates causes theLSPR wavelength to red-shift with the coating thickness andcoverage. Though this phenomenon cannot be explained byconventional effective medium theory, we believe that themultilayer metal nanoparticles have great potential for LSPRsensor development, due to their sensitive and tunable LSPRproperties. Nevertheless, the coating of a metallic layer onAu NPs can also be used to fine tune the LSPR wavelength,and both GLAD and OAD methods are good candidates forthis purpose. All our results demonstrate that GLAD is avery versatile fabrication technique to produce reproducibleand fine-tuned LSPR substrates.

Acknowledgments

This work was partially supported by the National ScienceFoundation under the contract No. ECS-0824728. The authorsthank Justin Abell for proof-reading the manuscript.

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