Exploring the origin of tip‐enhanced Raman scattering

JOHN WILEY & SONS, LTD., THE ATRIUM, SOUTHERN GATE, CHICHESTER P019 8SQ, UK

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Research ArticleReceived: 19 September 2010 Revised: 16 May 2011 Accepted: 20 May 2011 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jrs.3021

Exploring the origin of tip-enhanced Ramanscattering; preparation of efficient TERS probeswith high yieldMehdi Asghari-Khiavi,a Bayden R. Wood,a Pejman Hojati-Talemi,b

Andrew Downes,c Don McNaughtona∗ and Adam Mechlera,d

Tip-enhanced Raman scattering (TERS) spectroscopy is a promising technique for nanoscale chemical analysis. However, thereare several challenges preventing widespread application of this technology, including reproducible fabrication of efficientTERS probes. These problems reflect a lack of clear understanding of the origins of, and the parameters influencing TERS. Itis believed that the coating characteristics at the apex of the tip have a major effect on the near-field optical enhancementand thus the TERS activity of a metalized probe. Here we show that the aspect ratio of the tip can play a significant role in theefficiency of TERS probes. We argue that the electrostatic field arising from the lightning-rod effect has a substantial role inthe observed TERS effect. This argument is supported by ‘edge-enhanced Raman scattering’ which is shown for a noble metalfilm. Furthermore, it is reported that an associated tip-surface-enhanced Raman scattering effect can be achieved by using aTERS-inactive metalized probe on a surface-enhanced Raman spectroscopy-inactive roughened surface. This observation canbe explained by an interparticle enhancement of the electromagnetic •field. Copyright c© 2011 John Wiley & Sons, Ltd.

AQ1

Supporting information may be found in the online version of this article.

Keywords: tip-enhanced Raman spectroscopy; tip fabrication; mechanism of enhancement; top illumination; hematin

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Introduction

Surface-enhanced Raman scattering (SERS) spectroscopy offers avaluable means of characterizing the chemistry of thin surfacelayers, deposited on plasmon-active metal substrates, down to themolecular or submonolayer level. It would be desirable to performmeasurements with similar spatial resolution as well, which inthe case of SERS, however, is limited to ca λ/2 by the diffractionof the laser light. The diffraction limit can be circumvented byemploying tip-enhanced Raman scattering (TERS) spectroscopyin which a nanotip functions as the Raman enhancing unit,providing a spatial resolution limited only by the size of thefunctional zone of the tip.[1] Using TERS, in principle, it is possibleto obtain direct chemical information from almost any sample,with a spatial resolution down to a few nanometers, without theneed for a special substrate. TERS is essentially a hybrid chemicalmicroscopy technique that combines the nanometer resolutionafforded by scanning probe microscopy (SPM) with the highsensitivity and molecular fingerprinting available from SERS. Inthis technique, an Ag or Au metal or metalized SPM tip within afew nanometers of an analyte surface generates a highly localizedfield enhancement when illuminated with a laser, which in turnresults in the enhancement of Raman scattering.[2] Even thoughthe scattered light is collected with conventional far-field optics,the strongly enhanced TER signal overwhelms the ordinary Ramansignal and allows primarily the near-field signal to be observed.[1]

The enhanced Raman bands, therefore, arise mainly from a verysmall number of molecules close to the tip apex.

Since the first demonstration of the tip-enhancement effectin 2000,[1 – 3] TER scattering has been reported at different laserwavelengths for a wide variety of samples indicating the general

applicability of this technique.[4] TERS is now at the forefront inthe development of methods for nanoscale chemical analysis.[5]

However, TER spectroscopy and microscopy is still in the earlystages of development; and obtaining efficient TERS probesis still a major challenge.[6] The number of research groupsreporting successful enhancements has remained rather low. Forwidespread application of the TERS technique, a TERS systemwith more user-friendly attributes, wide applicability to sampleform and consistency of tip performance is needed. Previousstudies suggested that the tip apex properties and the directionand polarization of excitation laser have major effects on theenhancement of the electromagnetic field at the apex of thetip.[4,7 – 9] Hitherto, the most widely used setup has been epi-illumination (transmission mode) which requires a transparentsample and substrate; and the probes usually have been preparedby thermal evaporation or plasma sputtering of a thin metal film

∗ Correspondence to: Don McNaughton, Centre for Biospectroscopy, School ofChemistry, Monash University, 3800 Victoria, Australia.E-mail: [email protected]

a Centre for Biospectroscopy, School of Chemistry, Monash University, 3800Victoria, Australia

b Department of Materials Engineering, Monash University, Clayton, 3800Victoria, Australia

c Institute of Materials and Processes, University of Edinburgh, Edinburgh EH93JL, United Kingdom

d Department of Chemistry, School of Molecular Sciences, La Trobe University,3086 Victoria, Australia

J. Raman Spectrosc. 2011, 42, 0 Copyright c© 2011 John Wiley & Sons, Ltd.

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M. Asghari-Khiavi et al.

Figure 1. A schematic layout of TERS setup used in this study.

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on a standard atomic force microscopy (AFM) tip.[4] However,this is a stochastic process and yields only a small fraction of themetalized tips with the proper alignment of metal grains on thetip apex to provide a ‘hot spot’ capable of producing the TERSeffect.[4,10] An alternative method of preparation of TERS probesis electroless plating.[11,12] Although the fabrication of scanningtunneling microscopy (STM) probes is easier and more controllable,STM-based TERS technique requires conductive samples, whichseverely limits its application.[4,13]

In this paper, an AFM-based TERS setup with upright illuminationcombined with the application of metalized ‘tilted’ tips with highaspect ratio is applied to yield a very high fraction of probes withthe capability to provide the TERS effect. While the transmissionmode can only be applied to transparent and very thin samples,which significantly restrict its application, the reflection modesetup facilitates the application of TERS for investigating almostany kind of sample.[14,15] Moreover, the top illumination geometrydecreases the far-field spectral background compared with theside illumination scheme.[16]

The mechanism of the TERS effect is usually discussed on thesame basis as the SERS effect, i.e. the electromagnetic and thechemical or charge transfer enhancement.[9] Here, we show thatthe electrostatic field arising from the lightning-rod effect and theinterparticle augmented electromagnetic field can play a majorrole in the tip-enhancement process. Moreover, it is shown thatthe wavenumber shifts in TER spectra are more likely to arise fromtip pressure effects rather than charge transfer complexation.

Experimental

Materials and methods

Hematin porcine, silver nitrate and sodium citrate were purchasedfrom Sigma-Aldrich and used without further purification. Theaqueous stock solution of hematin (1 mM) was prepared bydissolving the solid compound in 1 mM NaOH. The citrate-reducedsilver colloidal solution was prepared according to the proceduredescribed by Lee and Meisel.[17] The λmax of the silver hydrosolwas 412 nm. The SERS sample was prepared by mixing 100 µl ofthe hematin solution (10−1 mM) with 1 ml of silver colloid. After30 min, one small drop of the mixture was deposited on a glassslide (precoated with 80 nm aluminum) and air-dried. To preparethe TERS sample, a drop (∼1 µl) of hematin solution (10−2 mM)

was spin-coated onto an Al-coated glass disk (1 cm diameter) andleft in air until the solvent evaporated. TERS spectra were recordedon single submicrometer crystals of hematin of the size shown inthe AFM image in Fig. S1 (Supporting Information).

Instrumentation

An NT-MDT NTegra AFM coupled to a Renishaw inVia Ramansystem, schematically shown in Fig. 1, was used for TERSexperiments. The instrument is equipped with a 100× objective(NA 0.7) and a 532 nm Nd : YAG laser excitation line. The highnumerical aperture objective lens allows illumination of the samplearea below the apex of a ‘tilted’ tip (the tip looks transparent). Thesame lens collects the scattered light in a backscattering geometry.The Raman microprobe is attached to an optical microscope witha modified stage to accommodate the AFM head. Suitable band-pass filters are employed to avoid the photoperturbation effectof the incident and scattered Raman light on AFM imaging whileminimizing the interference of the AFM laser beam (∼670 nm)in Raman spectroscopy. The Raman microscope and piezo-stageactuator maintain rapid optical alignment between the laser spotand the tip apex. Therefore, the Raman signal comes from exactlythe same location as the AFM image. After AFM observation, torecord the Raman spectra of the surface sites of interest, the samplewas moved relative to the fixed laser spot using a piezoelectricmapping stage. The TER spectrum was then collected while thetip was in intermittent contact with the sample. The laser powerat the sample was ∼60 µW. The spectra are presented withoutsmoothing or further data processing.

The probes used were noncontact mode silicon-based AFMcantilevers of ATEC-NC (Nanosensors) and NSG10 (NTMDT) serieswith a typical tip radius of curvature of 10 nm and nominal springconstants of 45 and 11.8 Nm−1, respectively. The AFM imagingexperiments were performed in ambient conditions using tappingmode with settings of 256 pixels/line and 0.3 Hz scan rate. Thetips were coated with silver (Aldrich, 99.99% pure) using a CE12/14 (Dynavac Engineering Pty Ltd) or a MCS 010 (Bal-TEC)evaporation system working, respectively, at 5 × 10−5 Torr air and7.5 × 10−4 Torr argon. The ATEC-NC tips were located on a 30◦

angled substrate so that the tip apex was pointed up and all threesides of the tip could be coated. The coated tips were stored underargon and used within 2 days.

Scanning electron microscopy (SEM) examinations were run ona JEOL JSM-6300F or a Quanta 200F, FEI.

wileyonlinelibrary.com/journal/jrs Copyright c© 2011 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2011, 42, 0

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Preparation of efficient TERS probes with high yield

Figure 2. Top- and side-view SEM images of a NSG10 tip vapor-coated with a thin layer (20 nm) of silver (coating rate 0.5 Å s−1; source–sample distance13 cm; annealed for 1 min at 300 ◦C under Ar).

Figure 3. Side-view SEM image of a NSG10 tip vapor-coated with a thick layer (60 nm) of silver (coating rate ∼5.0 Å s−1; source–sample distance 5 cm)(A); side-view SEM image of an ATEC-NC tip vapor-coated with 30 nm of silver (coating rate ∼5.0 Ås−1; source–sample distance 5 cm) (B).

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Simulations

The calculations of electric field enhancement were performedusing ANSYS Multiphysics by solving Maxwell’s equations[18]

within a finite geometry containing a tip of apex radius 20 nmsilver. The incident light in all simulations presented is p-polarized(parallel to the tip axis) and the illumination wavelength is 532 nm.

Results and Discussion

Preparation of efficient TERS probes with high yield: the originof the TERS effect

In order to develop a high-throughput method for fabricationof TERS probes and to further elucidate the factors affecting theefficiency of the probes, several protocols were examined bychanging different parameters, some of which are outlined in thefollowing. At first, we examined one of the most widely usedprotocols in the literature.[19] Based on this method, a thin layer(20 nm) of silver was vapor-coated on a cantilever and annealedfor 1 min at 300 ◦C under a flow of argon. The source–sampledistance (the distance between silver target and AFM tip) was∼13 cm, while the evaporation rate was set to 0.5 Ås−1. The resultsshowed that this procedure produces isolated metal islands as isclear from Fig. 2. The SEM images showed that, in the majorityof cases, there was at least one silver grain on the tip apex.Nevertheless, this method resulted in a low yield of coated tips

(typically one in five) with the capability to produce the TERS effect,demonstrating that the existence of a nanoparticle on the tip apexmay not guarantee TERS activity, and there should be a suitable‘hot spot’ on the tip apex which may be produced by a cluster ofnanoparticles. The results showed that by using this protocol, theprobability of obtaining a ‘hot spot’ localized on tip apex was low,in accordance with the literature.[4]

On the other hand, when the cantilever was coated with a thicklayer of silver (without post-annealing), there was a big cluster ofnanoparticles on the tip apex which affected the spatial resolutionof the probe significantly. Figure 3A shows an SEM image of asilicon tip vapor-coated with 60 nm silver with a coating rate of∼5.0 Å s−1 while the source–sample distance was set to 5 cm.Due to their poor spatial resolution, the metalized tips with thisprocedure were not efficient TERS probes (even though they mightbe TERS-active).

However, when a medium-thickness layer (30 nm) of silverwas vapor-coated on sharp tetrahedral tips with high aspectratio (without post-annealing), a great yield of efficient tips wasobtained with a spherical apex of ∼50 nm in diameter (Fig. 3B). Weexamined ten metalized tips with this method, of which just onewas not TERS-active. It should be noted that the coating rate usedin this protocol was ∼5.0 Å s−1 which is much higher than the ratesgenerally reported in the literature (∼0.5 Å s−1).[19 – 21] Moreover,the source–sample distance in our experiments (5 cm) was lowerthan the distance reported in the literature (10–15 cm).[21,22] At this

J. Raman Spectrosc. 2011, 42, 0 Copyright c© 2011 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs

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Figure 4. (A) RR spectrum of a hematin sample dried from a concentrated (1 mM) aqueous solution; (B–D) TER spectra of hematin samples dried from an

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aqueous solution (10−2 mM) spin coated on an Al-coated glass disk: the spectra B–D were recorded using tips presented in Figs 2, 3A, and B, respectively;(E) Raman spectrum of the same point of the sample as Fig. 4D after retracting the tip; The exposure time used to acquire the spectra was 10 s for A and1 s for B, C, D and E. Also shown is the chemical structure of hematin.

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close distance, the source temperature should have a considerableinfluence on the resulting Ag film, inducing morphologicalchanges in metal nanostructures.[22,23] Our results show that,in contrast to what is widely believed, the heating of the cantileverand the fast deposition rate do not bend or deform the tip.

As an example, using the metalized tips with these protocols,we recorded the TER spectra of hematin (ferriprotoporphyrin IXhydroxide), which are presented in Fig. 4. FerriprotoporphyrinIX is believed to be the target of antimalarial drugs in malaria-parasite-infected red blood cells, and the application of TERS forhematin may pave the way for a detailed investigation of theinteraction site between heme and antimalarial agents with ananometer-scale resolution. The assignments of heme vibrationalmodes are presented in Table 1. The Raman bands observed with532 nm excitation are mainly porphyrin skeletal modes includingν10 (1627), ν37 (1591), ν19 (1565), ν2 (1565), ν38 (1526), ν3 (1490), ν28

(1462), ν21 (1307), ν30 (1163), ν6 + ν8 (1128), ν15 (756) along withthe oxidation state marker band ν4 (1372) and the bands relatedto periphery groups, particularly, δ(CaH ) at 1307, twisting ofpropionate CH2 at 1223 and ν (Cc − Cd) at 976 cm−1. Whencompared with the resonance Raman (RR) spectra (Fig. 4A), theTER spectra of hematin (Fig. 4B–D) show some changes in bandpositions and intensity profile, which are discussed in the nextsection.

The results described above show that the efficiency of theTERS probes is correlated with the total metal surface coverage,suggesting that a thick layer of metal grains may increase theprobability of formation of a cluster of nanoparticles and thus aSERS ‘hot spot’ on the tip apex. However, surprisingly, glass slidessilver-coated using the optimized protocol for tips were inefficientSERS substrates, although SEM images showed a roughenedsurface (Fig. 5A). We prepared different Ag-metalized substratesby changing the coating parameters (coating time, rate, angle,source–sample distance, and post-annealing) but could not obtaina SER spectrum of hematin. More importantly, we were unableto obtain the SER spectrum of a hematin sample deposited on a

cantilever or chip section of the metalized AFM probes (Fig. 5B),while we could readily obtain the TER spectrum of hematin usingthese probes. The fact that the cantilever and chip section ofthe metalized probes were not SERS-active substrates while theprobes were strongly TERS-active suggests that there are otherfactors, apart from those involved in SERS, contributing to the TERSeffect. Electromagnetic theory predicts that the incident field isamplified on the tip apex due to the excitation of localized surfaceplasmon polaritons by the laser electric field.[26] However, thelocal field can also be enhanced by a nonresonant effect, namelythe lightning-rod effect.[27] It has been shown that at any sharpgeometrical feature (corner or edge) of a metal or semiconductorobject the electromagnetic field is greatly enhanced due to a quasi-electrostatic ‘crowding’ of dipolar fields,[28] and sharper objectswith a higher aspect ratio generate stronger fields.[29,30]

To investigate the influence of the lighting-rod effect and edge-augmented electric field on Raman enhancement, we deposited adrop of hematin solution (10−1 mM) on the edge of the cantileversection of a metalized probe (coated using the optimized protocol)and recorded the far-field Raman spectra of the dried sample.When the excitation laser was focused on the very edge of thecantilever, a considerable enhancement of the vibrational bandswas observed (Fig. 6B), while focusing the laser on the drop edgeat the inner section of the cantilever resulted in just a weakbackground (Fig. 6C). Similarly, a significant enhancement of thebands was observed when the laser was focused on a tip (apex)pretreated with 10−1 mM hematin solution (data not shown). Theseobservations suggest that the electrostatic field arising from thelightning-rod effect may have an integral role in the observedTERS effect. The results also demonstrate that the very high yieldof efficient tips obtained in this study could be related to the use ofa sharp and long tip with high aspect ratio as well as coating a ratherthick film of Ag nanoparticles on the tip which provides coalescednanostructures rather than isolated metal islands (Figs 2 and 3B),since these factors are required to induce a strong lightning-rodeffect. It should be noted that the tip length and aspect ratio of a


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Table 1. Observed Raman bands (cm−1), assignments, local coordinates, symmetry terms (ST) and enhancement factors for hematin samples indifferent experiments at 532 nm •excitation (NR: normal Raman)

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Assignment ST Local coordinates NRSERS onAg sol

SERS at the edgeof cantilever TERS

AssociatedTERS–SERS

ν10 B1g ν(CαCm)as 1627 (9) 1629 (9) 1625 (8) 1623 (6) 1618 (4)

ν37 Eu ν(CαCm)as 1591 (1) 1588 (8) 1589 (8) 1590∗ (1) 1589 (2)

ν19 A2g ν(CαCm)as 1565 (8) 1565 (1) 1565 (1) 1566 (9) 1564 (6)

ν2 A1g ν(CβCβ ) 1565 (8) 1565 (1) 1565 (1) 1566 (9) 1564 (6)

ν11 B1g ν(CβCβ ) 1549∗ (1)

ν38 Eu ν(CβCβ ) 1526 (2) 1521 (2) 1525 (2)

ν3 A1g ν(CαCm)sym 1490 (1) 1491 (3) 1505 (1)

ν28 B2g ν(CαCm)sym 1462 (2) 1456 (0) 1456 (1)

δ( CbH2)sym (1) 1445 (1) 1443 (1)

ν29 B2g ν(pyr quarter-ring) 1398∗ (1) 1394 (2) 1393 (1) 1405 (1) 1408 (2)

ν20 A2g ν(pyr quarter-ring) 1398∗ (1) 1394 (2) 1393 (1) 1405 (1) 1408 (2)

ν4 A1g ν(pyr half-ring)sym 1372(10) 1372 (7) 1374 (10) 1371 (6) 1381 (6), 1360 (1)

ν41 Eu ν(pyr half-ring)sym 1341∗ (1) 1340∗ (1) 1340 (1) 1339 (3)

δ( CbH2)sym (2) 1341∗ (1) 1340∗ (1) 1340 (1) 1339 (3)

ν21 A2g δ(CmH) 1307 (3) 1307 (9) 1309 (9) 1288 (4) 1281 (5)

δ(CaH ) 1307 (3) 1307 (9) 1309 (9) 1288 (4) 1281 (5)

prop δ(CH2) wag 1296 (1)

ν13 B1g δ(CmH) 1233 (1) 1234 (2) 1234 (1)

prop δ(CH2) twist 1223 (2) 1224 (2)

ν30 B2g ν(pyr half-ring)as 1163 (4) 1163 (7) 1166 (7) 1182 (10) 1185 (10)

ν6 + ν8 A1g ν(Cα − Cβ )sym + ν(Fe–N) 1128 (7) 1127(10) 1123 (7) 1144 (8) 1139 (7)

ν22 A2g ν(pyr half-ring)as 1128 (7) 1127(10) 1123 (7) 1144 (8) 1139 (7)

δ( CbH2)as (1) 1094 (1) 1103 (2) 1111 (1)

δ( CbH2)as (2) 1064 (1) 1073 (1) 1069 (1)

γ (CaH ) 1013 (1) 1035 (1)

ν(Cc–Cd) 976 (1) 979 (2) 967 (7) 963 (4)

γ ( CbH2)sym 925 (1)

Ethyl def? 876 (3) 888 (3)

ν32 B2g δ(pyr def)as 797 (1) 797 (1) 807 (1)

ν15 B1g ν(pyr breathing) 756 (4) 756 (6) 759 (3) 742 (1)

? 747 (2)

ν7 A1g δ(pyr def)sym 685 (1) 650 (1) 675 (3) 678 (3)

δ(pyr def)sym? 661 (2), 652 (2)

ν48 Eu δ(pyr def)sym 617 (1) 624 (2)

For experimental details see Figs. 4 and 5.Normalized enhancement factors (1–10) are shown inside parenthesis. The mode notation is based on that proposed by Abe et al. (Ref. [24]) and Huet al. (Ref. [25]).sym, symmetric; as, asymmetric; def, deformation; pyr, pyrrole; porph, porphyrin; prop, propionate; ∗ , observed after calculating the second derivative.

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standard AFM probe (commonly used for TERS examination) aremuch smaller than those of the probes used in this study. Forexample, a tip of NSG10 series has a typical length and aspect ratioof 11 µm and 1.2, while these values are, respectively, 18 µm and4 (both front and side view) for ATEC-NC series (Fig. 7).

It is noteworthy that the lightning-rod effect has already beenproposed as a contributing factor in SERS and TERS effects.For instance, the higher SERS enhancement factor of nanorodscompared to spherical nanoparticles has been explained by thelightning-rod effect,[9,30,31] and it was shown that the SERS intensitydepends critically on the length of nanowires.[32] Moreover,the increase in enhancement when switching from conical topyramidal probes is attributed to the accumulation of chargeat the sharp edges of the pyramid which is driven to the tipend, resulting in large enhancement.[29] Our results show that thelightning-rod effect can have a major role in the tip enhancement

of Raman scattering. The fact that the cantilever and chip sectionof the Ag-metalized probes were not SERS-active substrates whilethese probes were highly TERS-active along with the observationof enhanced Raman scattering at the edge of the Ag-metalizedcantilever and chip strongly suggests that there is substantialcontribution from the lightning-rod effect to the observed TERSenhancement. Although the edge-enhanced Raman scatteringwas recently reported by Poborchii et al.[33] for silicon edges, thisis the first time that the phenomenon is shown for a noble metalfilm.

We argue that the assumption that the nanotip apex behaveslike a single nanoparticle is not completely accurate and the factorsinvolved in Raman enhancement contribute differently to the SERSand TERS effects. While in SERS the enhanced electromagnetic fieldarising from excitation of localized surface plasmons (particularlyin nanoparticle junctions) has often the major role, in TERS the


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Figure 5. •(A) SEM images of a glass slide and (B) cantilever section of an ATEC-NC tip vapor-coated with a 30 nm of silver (coating rate ∼5.0 Å s−1;AQ3

source–sample distance 5 cm).

Figure 6. (A) SER spectrum of hematin (10−1 mM) dried on Ag sol. (B) Raman spectrum of a hematin sample (10−2 mM) dried on a cantilever section of

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a silver-metalized ATEC-NC tip, when excitation laser was focused on the very edge of the cantilever and (C) when it was focused on the drop edge atthe inner section of the cantilever. (D) TER spectrum of a hematin sample (10−2 mM) dried on a roughened silver substrate (ca 30 nm structures), usingan inactive TERS probe, (E) Raman spectrum of the same point after retracting the tip. The exposure time used to acquire the spectra was 5 s for A, B, C,and 1 s for D and E. For the spectra D and E, the tips used were ATEC-NC Nanosensor tips vapor-coated with a thin layer (20 nm) of silver and aligned ina shape that tip axis was at 30◦ with respect to the substrate (horizontal) plane. The diameter of the coated-tip apex was ∼50 nm while the roughenedsurface contained Ag particles of 30 nm diameter.

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edge-augmented electric field (lightning rod effect) can have theprimary role (followed by excitation of the surface plasmons).Although the term ‘hot spot’ is often used for a gap region of adimer or cluster of strongly coupled nanoparticles, in a sense thelocalized electrostatic field arising from the lightning-rod effectcan be considered as a type of ‘hot spot’, as both conceptsdescribe a type of spatial concentration of electromagnetic fieldinto a narrow zone. Our results show that obtaining a ‘hot spot’on the tip apex via optimization of the parameters affecting thelightning-rod effect results in a much higher throughput comparedto producing a nanoparticle cluster on the tip apex via a stochasticprocess of vapor coating.

Another parameter that may contribute to the TERS enhance-ment factor is the illumination configuration. It should be notedthat both the geometrical features of the tip and the direction ofthe incident field are critical for the operation of the lightning-rod effect.[6,9] Recent theoretical studies suggest that the highestenhancement is achieved when the laser is shone with an angle∼50–70◦ from the tip axis.[7,34] Our setup (which is a rather un-usual configuration) has top illumination with a tip tilt angle of

30◦. However, the fact that we were unable to achieve a betteryield from the tips coated with previous protocols suggests thatthe upright setup is not the determining factor in the improvedperformance of TERS observed in this study.

To further investigate the effect of lightning-rod effect on theenhancement of electric filed, we used a finite element model[18]

to calculate the optical enhancement as a function of the tip coneangle. We considered two tips of cone angles 10.5 and 20◦. As canbe seen from the color codes in Fig. 8, the enhancement of thenear-field is much higher at the smaller cone angle tip, which isin accordance with our experimental results and further supportsour conclusion.

TERS with inactive probes

As outlined above, when the tips were coated with a thin layerof silver, the majority of metalized probes did not show the TERSeffect for an analyte deposited on a smooth substrate despite thefact that SEM images showed that there was at least one silvergrain on the tip apex of the most of the metalized probes. We


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Figure 7. (A) Side-view SEM images of a square pyramid standard AFM tip (NSG10) and (B) a tetrahedral ‘tilted’ tip with high aspect ratio used in thisstudy (ATEC-NC).

Figure 8. Finite element simulations of the enhancement of the electric field for silver tips of 20 nm radius in air: (A) a tip of cone angle 10.5◦ ; (B) a tip of

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cone angle 20◦. The angle of incidence is 45◦ to the vertical axis (coming from the top, left corner of the image).

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examined the efficiency of these probes on a roughened substratewhich did not produce a detectable SERS signal. For this purpose,we deposited a small drop of hematin solution (10−2 mM) on asilver nanostructured film (ca 30 nm structures), and examined itsTER spectra using an inactive TERS probe after first determiningthat the surface was SERS-inactive. The results showed a dramaticenhancement of Raman scattering (Fig. 6D), whereas the recordedspectra of the same point of the sample before approaching andafter retracting the tip showed just a background feature (Fig. 6E),demonstrating that the observed enhancement was due to anassociated effect of tip- and surface-enhanced Raman scattering.This effect may be explained by the interparticle enhancement ofthe electromagnetic field or the collective effect of the particleaggregate. It has been shown theoretically that, when twosilver nanoparticles approach each other, their transition dipoles,composed of oscillating electric field in each particle, couple andthe electromagnetic field dramatically enhances in their junctionunder the resonant condition, which in turn results in hugeenhancement of Raman scattering.[35,36] A similar effect is observedin SERS using aggregated nanoparticles where molecular tunneljunctions in a compact cluster function as electromagnetic ‘hotsites’ providing sensitivity down to a single molecule level.[37 – 39]

In this scenario, the ‘classical’ electromagnetic field enhancement,which is based on the electromagnetic characteristics of isolatedparticles, is not valid and the calculation of the field enhancementmust be based on the exciting optical properties of fractalnanocomposites.[40]

Moreover, the so called gap-mode electromagnetic fieldenhancement may have some contribution in the observedassociated tip-surface effect. Theoretical and experimental studiesshow that, when a TERS probe is placed in the vicinity of a silveror gold substrate, its surface plasmon resonance changes as a

result of the electromagnetic coupling of the tip charges withthe induced charges in the substrate, providing an additionalelectric field (and Raman) enhancement at the tip apex.[18,41] Itis noteworthy that Zenobi et al. recently showed that the surfacenanocorrugations (e.g. a 2 nm high sharp step on the Au surface)can induce a significant (extra) enhancement in gap-mode TERS,which was supported by electromagnetic field simulations.[42]

Intensity and wavenumber changes in the spectra

Heme compounds are susceptible to photoreduction and pho-todecomposition; so, very low laser power and a minimumacquisition time were used to prevent photodegradation. Theband position of the oxidation state marker, ν4 at ∼1375 cm−1,indicates that hematin retains its ferric state in all examinations. Asis evident from Figs 4A and 6B, the SER spectrum of hematin on theedge of a silver nanoparticle film is very similar to its RR spectrum.Likewise, the SER spectrum of hematin on Ag sol resembles itsRR spectrum (Figs 4A and 6A). This is consistent with previousreports[43,44] which showed that the band positions and intensityprofile of the SER spectra of metalloporphyrins are quite similarto those observed in their RR spectra, demonstrating that the ad-sorption of metalloporphyrins on the Ag surface happens throughelectrostatic interaction rather than charge transfer complexation.In this case, the molecular electronic states are not appreciablyaltered by the adsorption on the surface, and the RR selection rulesstill determine the relative strength of the bands. Therefore, thegeneral character of the Raman scattering is established by themolecule itself, with the SERS substrate acting as an amplifier.[43]

On the other hand, the TER and associated tip-surface-enhancedRaman spectra of hematin show considerable changes in theenhancement pattern and wavenumber values when compared


8


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to the RR spectrum (Figs 4A, D, and 6D). The appearance ofnew bands can be related to the activation and enhancementof IR active modes in Raman spectra due to the loss of thesymmetry of the molecule as a result of its interaction withprobe silver atoms and/or tip pressure effect. Comparison of theSERS and TERS data leads us to hypothesize that the observedchanges in the TER spectra are mainly arising from the mechanicalinteraction between tip and sample, resulting in compressionand deformation of the surface-bound molecules.[21,45] In thiscase, the effect must have an orientation dependence, andthus it can be used to identify the orientation of the surface-bound molecules and crystals. However, it should be noted thatthe TER spectra presented are typical, and some of the spectrarecorded showed some variations in band positions and intensityratios. The spectral variations are inherent to the TERS techniquebecause of its extreme sensitivity and spatial resolution, and canbe attributed to the different molecular orientations/structuresunder the probe.[21,46] The difference between spectral signaturesat different positions of the sample shows that the TERS techniquecan be exploited to investigate the vibrational properties ofindividual molecules, a small part of a macromolecule or ananoscale area of a sample instead of the common ensembleaverages. Moreover, nanoscale probing of the sample providesa means to explore how the chemical properties of a moleculeor crystal are affected by neighboring co-adsorbed species orsurface constituents. It is thus, in principle, possible to classifythe observed TER spectra of hematin and attribute each class toa specific molecular structure/orientation; but this is outside thescope of this study.

Conclusion

In this paper, we have shown that efficient TERS can be achievedin a reflection mode with top illumination configuration usingsharp and long ‘tilted’ silicon tips with high aspect ratio precoatedwith 30 nm of silver. With this method, the majority of metalizedtips were effective TERS probes having a tip apex diameter of∼50 nm. The results and observations show that the tip shape(aspect ratio) has considerable influence on the lightning-rodeffect which plays a major role in the enhancement mechanismof the probes. Moreover, it is shown that an associated tip-surface-enhanced Raman scattering can be obtained by usinga TERS-inactive metalized nano-tip in combination with a SERS-inactive roughened substrate. This observation can be explainedby an interparticle enhancement of the electromagnetic field.The findings of this study may have important applicationsin the continued development of the TERS technique, whichhas great potential for solving critical problems in science andnanotechnology.

Acknowledgements

This work was carried out under Australian Research CouncilGrants DP0664012 and DP0878464. We are grateful to Mr PeterEllis for experimental assistance. The Monash Centre for ElectronMicroscopy is acknowledged for microcopy facilities. MAK issupported by Monash University scholarships MGS and MFRS.

Supporting information

Supporting information may be found in the online version of thisarticle.

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1

ContentsPublished online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jrs.3021

Research Article

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• • •• Exploring the origin of tip-enhancedRaman scattering; preparation of ef-ficient TERS probes with high yield

000–000

M. Asghari-Khiavi, B. R. Wood, P. Hojati-Talemi,A. Downes, D. McNaughton∗ and A. Mechler

J. Raman Spectrosc. 2011, 42, 0 Copyright c© 2011 John Wiley & Sons, Ltd.

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Exploring the origin of tip‐enhanced Raman scattering