tip-enhanced raman imaging and nanospectroscopy: sensitivity, … · 2020. 6. 12. · raman...

Nanobiotechnol (2007) 3:172–196DOI 10.1007/s12030-008-9015-z

Tip-Enhanced Raman Imaging and Nanospectroscopy:Sensitivity, Symmetry, and Selection Rules

Catalin C. Neacsu · Samuel Berweger ·Markus B. Raschke

Published online: 25 February 2009© Humana Press Inc. 2009

Abstract The fundamental mechanisms of tip-enhanced Raman spectroscopy (TERS) have beeninvestigated, including the role of the plasmonic ex-citation of the metallic tips, the nature of the opticaltip–sample coupling, and the resulting local-fieldenhancement and confinement responsible for ultra-high resolution imaging down to just several nano-meters. Criteria for the distinction of near-fieldsignature from far-field imaging artifacts are addressed.TERS results of molecules are presented. With en-hancement factors as high as 109, single-moleculespectroscopy is demonstrated. Spatially resolved vi-brational mapping of crystalline nanostructures anddetermination of crystallographic orientation and do-mains is discussed making use of the symmetry prop-erties of the tip scattering response and the intrinsicRaman selection rules.

Keywords tip-enhanced Raman imaging ·nanospectroscopy · tip-enhanced Raman spectroscopy

Introduction

Optical spectroscopy provides nondestructive tech-niques for obtaining both structural and real-timedynamic information of molecules and solids. Vibra-tional spectroscopy in particular, by directly couplingto the nuclear motion, offers insight into chemical

C. C. Neacsu (B) · S. Berweger · M. B. RaschkeDepartment of Chemistry, University of Washington,Seattle, WA 98195-1700, USAe-mail: [email protected]

composition, molecular bonds, intermolecular cou-pling, and zone-center phonons in crystalline solids[1, 2]. IR and Raman spectroscopy provide complemen-tary information. Raman spectroscopy features fewerconstraints in terms of selection rules, readily providesaccess to low frequency vibrations [3–6], and is car-ried out in the visible to near IR spectral range in acomparably simple experimental design. However, withscattering cross-sections of ∼ 10−27 − 10−30 cm2, theRaman response is weak, generally requiring probinga large molecular ensemble or bulk solids [7, 8].

It is therefore desirable to combine the intrinsicchemical specificity of Raman spectroscopy with opticalmicroscopy for the investigation of the spatial hetero-geneity and composition of the analyte. In that regard,the optical far-field Raman microscope has become anestablished tool for material characterization on the mi-crometer scale and in a confocal implementation withspatial resolution down to several hundred nanometers[9]. However, for most applications, the desired spatialresolution often exceeds the resolution imposed by far-field diffraction [10, 11].

Scanning near-field optical microscopy (SNOM) pro-vides access to subwavelength scale spatial resolution[12–19]. Aperture-based SNOM using tapered glassfiber tips has been employed for nano-Raman spec-troscopy [20–23]. However, the low optical throughputof the aperture probes (10−3–10−5) severely limits thespatial resolution and the sensitivity that can be ob-tained, resulting in a long imaging time and parasiticRaman signal from the glass tip that could be an im-pediment [22].

High sensitivity in Raman scattering, in general,can be achieved by surface-enhanced Raman spectros-copy (SERS), providing a strongly enhanced Raman

Tip-Enhanced Raman Imaging and Nanospectroscopy 173

response from molecular adsorbates on rough metallicsurfaces or colloidal aggregates [24–26]. SERS is due tothe near-field enhancement of the electromagnetic fieldat single or coupled metal nanostructures often reso-nantly excited at their surface plasmon polariton (SPP)eigenmodes [27–34]. Together with a corresponding butweaker (∼ 101–102) chemical contribution [35] origi-nating from surface bonding or charge transfer, theelectromagnetic enhancement leads to a total increasein Raman signal by up to 14 orders of magnitude,allowing for detection down to the single moleculelevel [36–41]. Despite its potential for chemically spe-cific detection of minute amounts of analytes, it hasremained challenging to develop SERS into a routineanalytic spectroscopic tool mostly due to difficultiesassociated with the reproducible fabrication of SERS-active substrates [42–45].

Better control over the SERS active sites and theirfield enhancement can be achieved by what may beviewed as resorting to an inverse geometry with respectto SERS: suspension of the metal nanostructure provid-ing the field enhancement at a small distance above theanalyte [46]. This is the basis of tip-enhanced Ramanscattering (TERS) making use of a single plasmon-resonant metallic nanostructure provided in the form ofa scanning probe tip of suitable material and geometry.

Fundamentally, TERS is a variant of scattering-type scanning near-field optical microscopy (s-SNOM)[47–51]. All-optical resolution down to just severalnanometers is provided by s-SNOM, in the visible[52–54] and IR spectral regions [55–58]. TERS is theextension of this technique to inelastic light scatteringwith the metallic tip used as an active probe, whichprovides both the local-field enhancement and servesas an efficient scatterer for the Raman emission.

s-SNOM and special aspects of TERS have been ad-dressed in recent reviews [18, 19, 59–62] (and referencestherein), but no comprehensive discussion of the under-lying physical mechanisms has yet been provided. Here,we review our recent contributions to the understand-ing of near-field Raman enhancement and sensitivity,tip–sample coupling, spatial resolution, the importanceof the plasmonic character of the tip, and tip fabrica-tion. In addition, we show that the symmetry proper-ties of the tip-scattering geometry in combination withthe Raman selection rules allows for the determina-tion of crystallographic information on the nanoscale.This, together with the results of other groups, showsthe potential of TERS as a nanoanalytical tool withdiverse applications in material and surface scienceand analytical chemistry for the study of biomolecularinterfaces, molecular adsorbates, nanostructures, andnanocomposites.

Tip-Enhanced Raman Spectroscopy

TERS combines the advantages of SERS with thoseoffered by s-SNOM: the single nanoscopic tip apex pro-vides the local field enhancement at a desired samplelocation without requiring any special sample prepara-tion [63, 64]. With the spatial resolution mainly limitedby the tip apex size, chemical analysis on the nanometerscale is made possible. By raster scanning the sample,spatially resolved spectral Raman maps with nanome-ter resolution can be obtained simultaneously withthe topography in atomic force microscopy (AFM)or surface electronic properties in scanning tunnelingmicroscopy (STM).

The origin of the field enhancement at the tip apexis attributed to the singular behavior of the electro-magnetic field (akin to the lightning-rod effect). Inaddition, the spatial confinement allows for the possibleexcitation of localized SPPs (tip-plasmons) for certaintip materials [65]. With the first effect being geometricalin origin, its magnitude is mainly dependent on thecurvature of the apex. Taking advantage of the exci-tation of tip-plasmons can further increase the overallenhancement by several orders of magnitude, as will bediscussed further on.

Ultra-high sensitivity and nanometer spatial resolu-tion imaging using TERS were obtained on variousmaterials and molecular systems adsorbed on both flatand corrugated surfaces [66–85]. Having large Ramancross-sections, several dye molecules (e.g., malachitegreen (MG), rhodamine 6G, brilliant cresyl blue) wereused and near-field Raman enhancement factors upto ∼ 109 were achieved [68, 71, 77, 81, 82]. UsingAg-coated AFM tips, spatial resolution below 50 nmwas obtained on surface layers of Rhodamine 6G dyemolecules [68, 71]. Lateral resolution as high as 14 nmand a maximum Raman enhancement factor estimatedat ∼104 were obtained in spatially resolved probingvibrational modes along individual carbon nanotubes[72, 73, 78].

In studies of adenine, as well as C60 molecules,the tip-induced mechanical force was shown to leadto mechanical strain-induced frequency shifts of thenormal Raman modes [74, 80]. Furthermore, it wasobserved that, when interacting with individual metalatoms of the tip apex, adenine molecules form differentisomers, demonstrating the potential for TERS to havefor atomic site selective sensitivity [83].

Extension of TERS implementation for coher-ent spectroscopy was shown for coherent anti-StokesRaman scattering (CARS) of adenine molecules in-cluded in a DNA network [75]. Owing to the third-order nonlinearity of the CARS process, the induced

174 Neacsu et al.

polarization at the tip apex is further confined, andhigher lateral resolution is, in principle, possible [76].Concomitant, theoretical studies on TERS report fieldenhancements up to three orders of magnitude in par-ticular wavelength regions [86–91]. However, the ex-pected resulting TERS enhancement of 12 orders ofmagnitude has not yet been observed experimentally.

In recent work from our group, we have refinedthe metallic tip fabrication and experimentally identi-fied and theoretically discussed the importance of theplasmonic properties of the scanning tip to achievehigh Raman sensitivity [92–95]. This has enabled near-field Raman enhancement factors of up to 109 fromMG molecules adsorbed on smooth Au surfaces to beobtained, allowing for the detection of TERS responsewith single molecule sensitivity [81, 96].

The review is organized as follows: The experimen-tal arrangement is presented in the “Experimental”section. This includes laser excitation, Raman detec-tion, metallic tip fabrication by electrochemicaletching, and molecular systems used. “Optical TipCharacterization” discusses the experimental charac-terization of the optical properties of the tips in-cluding their plasmonic behavior and the local-fieldenhancement factor as determined by second harmonicgeneration (SHG). “Calculation of the Near-FieldDistribution at the Tip-Apex” describes the theoreticalanalysis of the near-field distribution at the tip apextogether with its spectral characteristics. The tip–sample optical coupling is discussed in the “OpticalTip–Sample Coupling” section, where its effect onsensitivity, spatial resolution, and spectral shift ofthe plasmon resonance are derived. The near-fieldcharacter vs far-field imaging artifacts in TERS and itspolarization dependence are addressed in the “Near-Field Character and Far-Field Artifacts in TERS”section. The procedure for estimating the near-fieldenhancement factor is detailed and representativevalues discussed. Tip-enhanced near-field spectra ofmonolayer (ML) and a submonolayer of molecularadsorbates on a smooth Au surface are given in the“TERS of Molecular Adsorbates” section. The highsensitivity obtained and the dependence of the spectralfeatures on the enhancement level is discussed. In the“Molecular Bleaching” section, we also address theimportant question of molecular bleaching and possiblechemical contamination paths and show a number ofcontrol experiments. Near-field tip-enhanced Ramanresults with single molecule sensitivity are shown inthe “TERS with Single Molecule Sensitivity” section.This is concluded from the ultra-low molecular cover-age and the observed intensity and spectral temporalfluctuations. We identify and propose in the “Raman

Imaging of Nanocrystals: Near-Field CrystallographicSymmetry” section a new and promising extensionof TERS for determination of both chemical andstructural properties of nanocrystals. The “Outlook”section gives an outlook on TERS, and novel waysto circumvent current instrumental difficulties arediscussed.

Experimental

Various experimental schemes have been employed forTERS experiments. A tip axial illumination and detec-tion geometry has been used, allowing for high numer-ical aperture (NA), but requiring transparent samplesor substrates [73, 97]. A high-NA parabolic mirror canbe used to probe nontransparent samples [98, 99]. Inboth schemes, the tip is illuminated along the axialdirection with the tip apex positioned in the laser focus.For these geometries, polarization conditions requireeither a Hermite–Gaussian beam [100] or radial inci-dent polarization [98, 101]. While allowing for efficientexcitation and detection with the tip, independent po-larization and k-vector control is limited but desirablefor symmetry selective Raman probing.

In contrast, side-on illumination and detection allowsfor greater flexibility in the selection of polarization andk-vector, as well as the use of nontransparent samples.The scanning and tip–sample distance are controlledusing either an atomic force microscope (AFM) or ascanning tunneling microscope (STM), with STM re-stricted to the use of conducting samples.

Figure 1 shows the experimental layout of our side-illuminated TERS experiment. The incident radiation(νi) is focused onto the tip–sample gap and the tip-scattered Raman light (νs) is detected. For the ex-periments described here, a shear-force AFM is used.Shear-force AFM maintains a constant height of sev-eral nanometers above the sample. Due to the short-range tip–sample distance dependence of the opticalfield enhancement, dynamic noncontact AFM is lesssuitable. The time-averaged signal is greatly reduceddue to the oscillating tip. Contact AFM maintains aconstant and small tip–sample distance but the com-parably large forces make it unfavorable for probingmolecular or soft matter samples. In contrast, with thespatial range of shear-forces confined to within 25 nm[102], the shear-force AFM tip is controlled in closeproximity to the sample without actual physical contact.

The control mechanism in shear-force AFM is basedon the near surface vibrational damping of a probetip oscillating parallel to the surface. The nature ofthe shear-force damping mechanism is not yet fully


Fig. 1 Schematic of the experimental arrangement for TERS.The incident light is focused onto the tip–sample gap. The tip-enhanced and -scattered Raman response is spectrally filteredusing a notch filter and is spectrally resolved by an imagingspectrometer with a liquid nitrogen-cooled charge-coupled de-vice array, or integrally detected by means of an avalanchephotodiode (APD). Polarization directions of both incident andscattered light beams can be controlled independently. The bluelayer indicates a thin water film as may be present on the sampleunder ambient conditions.

understood [19], with a variety of mechanisms beingdiscussed [103–106]. It has been suggested [107, 108]that the tip experiences viscous damping from a thinwater layer adsorbed on the surface of the sampleunder ambient conditions [108–110]. This water layer,present on most hydrophilic samples, may play animportant role in the surface diffusion of the analytemolecules and possibly the transition of molecules toadsorb onto the tip.

For an incident light source, a continuous waveHelium–Neon laser, with λi = 632.8 nm (1.92 eV) iscommonly used [2]. In our experiments, after passingthrough a laser-line filter, the light is focused onto thetip–sample gap by means of a long working distance mi-croscope objective (NA = 0.35). The tip-backscatteredlight is collected with the same objective and spectrallyfiltered using a notch filter. The signal is detected usingeither an avalanche photodiode or spectrally resolvedusing a fiber-coupled imaging spectrograph with a liq-uid nitrogen-cooled charge-coupled device detector.Even for large enhancements, the signal intensities areweak, and detector noise is one limiting factor. Wetherefore limit the spectral resolution to 25 cm−1 forthe tip-enhanced experiments. Far-field spectroscopicstudies of molecular MLs serving as reference toquantify the enhancement are conducted using a micro-Raman confocal setup, based on an inverted micro-scope (Zeiss Axiovert 135).

For our experiments, we chose MG, an organic triph-enylmethane laser dye with an absorption peak aroundλ ≈ 635 nm. The absorption peak of MG is very close

to the laser energy used, leading to a resonant Ramanexcitation via the S0–S1 electronic transition of theconjugated π -electron system, as discussed below [111].To limit the rate of the molecular decomposition, themaximum fluence in the focus of the microscope objec-tive was 5 × 103 − 3 × 104 W/cm2. However, molecularbleaching prevails under ambient conditions under res-onant Raman excitation in TERS [81, 112].

Tip Fabrication

The metallic scanning probe tips hold the central func-tion in TERS studies, providing the enhanced elec-tromagnetic field at their apex. Ideally, as discussedbelow, they present strong plasmon resonances in thespectral region of interest, leading to enhanced pump(νi) and scattered Raman fields (νS) at the apex. Sincethe first experiments, the fabrication of suitable tipshas been a major experimental challenge. A vari-ety of methods for their fabrication have been used:including angle-cutting the metal wire [113], DC orAC voltage electrochemical and milling procedures[114–117], focused ion beam milling [54], metal-coatingcommercially available cantilever AFM tips [118, 119],and attaching spherical or other plasmonic nanopar-ticles to a glass tip [120]. Electrochemical etching[98, 121] is most commonly used due to perceivedadvantages [122] of tips fabricated in this manner.

We employ a DC voltage electrochemical etchingmethod for both Au and W tips. It involves the anodicoxidation of the metal wire [123]. In the case of Au, itinvolves the formation of soluble AuCl−4 , which subse-quently diffuses away from the electrode [121].

The schematic of the electrochemical cell used isdepicted in Fig. 2a. After careful cleaning with acetone,the Au wire (φ = 0.125 mm, purity 99.99%, temperas drawn, Advent Research Materials) is partially im-mersed (∼2–3 mm) into the electrolyte solution. As anelectrolyte, a 1:1 mixture of hydrochloric acid (HCl, aq.37%) and ethanol is used. As cathode, a platinum wire(φ = 0.3 mm) circular ring electrode with a diameterof ∼1 cm is utilized. It is held at the surface of theelectrolyte with the Au wire positioned at the centerof the Pt ring. For the etching, a potential of +2.2 V isapplied to the Au anode with respect to the Pt cathode.This voltage was determined by us, as well as others[124], to produce the best tips. This value is well abovethe Au oxidation potential due in part to the activationenergy along the reaction pathway [114].

When placed in the electrolyte solution, the surfacetension causes a concave meniscus to form around thewire, as shown schematically in Fig. 2b. The overallshape and aspect ratio of the tip after etching are

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Fig. 2 Schematics of theelectrochemical etching cell.a The Au wire (anode) ispartially immersed into theelectrolyte solution. It issurrounded by the Pt-ringcathode. The evolution of theetching process is closelyfollowed using a videomicroscope. b The etchingtakes place at the meniscusformed around the Au-wire.The flow of AuCl−4 is shown.For W-tips, a similarprocedure is used (see text).c Time evolution of theetching current for a potentialof 2.2 V. The currentoscillations are periodic forabout 100 s, after which theybecome less frequent,although they maintain theamplitude. For periodiccurrent oscillations (inset),tips with smooth surface andconsistent taper are obtained.

primarily determined by the shape of the meniscus[121]. During etching, a downward flow of AuCl−4 alongthe wire can be observed. The resulting ion concen-tration gradient partially inhibits etching of the lowerportion of the wire, resulting in a necking of the wirenear the meniscus [125]. This proceeds until the lowersection of the wire falls off. The remaining upper partof the wire is then used as a TERS/AFM tip.

Since the tip remains in the solution under themeniscus after the detachment of the lower part, thecircuit has to switch off as rapidly as possible, as furtheretching would result in blunt tips. A comparator breaksthe etching voltage when the current value becomessmaller than an adjustable reference value determinedfrom the current change associated with the drop of thelower tip.

Monitoring the etching current reveals periodic os-cillations of the current, as is shown in Fig 2c. Forthe potential of 2.2 V, the etching will generally reachcompletion in approximately 150 s with an averagebaseline current of ∼2 mA. After a short time of ini-tial fluctuations, the period equilibrates and remainsconstant for the first ∼100 s of the etching duration.Towards the end of the etching process, the oscillationperiod continuously decreases until completion.

These current oscillations have been attributed tothe depletion of Cl− near the surface of the elec-trode [121]. Initially, the Au will react rapidly toform AuCl−4 , depleting the Cl

− near the electrode–

electrolyte interface and resulting in a period of highcurrent. With decreasing local Cl− concentration, theAu will more readily form an electrode–passivatinglayer of Au(OH)3 [126], leading to extended periodsof decreased current. Upon restoration of the localCl− concentration, the passivating layer dissolves andanother current spike occurs [127]. The details of thisoscillating electrochemical process are sensitively de-pendent on the applied potential. An empirically es-tablished etching voltage leads to the most periodicoscillations and results in the highest quality tips.

It is desirable to have a criterion to select suitableTERS tips other than scanning electron microscopy(SEM) which is known to deposit Raman-visible car-bon contamination onto the tips due to electron-beaminduced decomposition of trace organics in the residualgas [128, 129]. Tips etched under a constant oscillationperiod exhibit a smooth surface and consistent taper.In contrast, tips etched with an irregular periodicityof the etching current frequently present deformitiesand large surface irregularities. We could verify in ourexperiments a link between homogeneous and smoothtaper with TERS performance by comparison of TERSactivity with SEM of tip shape as well as the study ofSPP of the tip apex. Our observations here are in goodagreement with previous work by Wang et al. [124].

A similar etching procedure is used for the tungstentips. A W wire (φ = 200 μm) is partially immersed (∼2–3 mm) in aqueous 2 M KOH. A DC voltage of 3 V


is applied between the wire and a stainless steel ringcathode. Using these procedures, tips with apex radiias small as 10 nm are obtained. After etching, the tipsare cleaned in distilled water and stored in isopropanolprior to usage to avoid possible contamination in anotherwise uncontrolled atmosphere.

Optical Tip Characterization

Efficient local-field enhancement and TERS activity is,in general, associated with the excitation of local modesof SPPs at the metallic tip [63, 73, 130–132]. However,determining the details of this correlation has remainedan open problem. In the following section, we presentthe investigation of the spectral characteristics of theelastic light scattering from individual sharp metal tipsand discuss the results in the context of the local plas-monic resonant behavior.

SPPs of Au Tips

Dark-field scattering spectroscopy with white lightillumination would lead to a largely unspecific responsewith the scattering dominated by the tip shaft [133], andthe SPP characteristics of the apex itself would becomedifficult to distinguish.

Therefore, for the plasmonic light tip-scattering ex-periments, we spatially limit the optical excitation tothe near-apex region by use of evanescent wave excita-tion. For that purpose, the tip frustrates the evanescentfield formed by total internal reflection on a prismbase [134, 135], and the tip-scattered light is detectedand spectrally analyzed. This confines the excitationto just several 100 nm from the tip apex. The com-plete description of the setup and the results are givenelsewhere [93].

Figure 3 shows representative scattering spectra fordifferent Au (a, b) and W (c, d) tips. Both the excitationand detected light fields are unpolarized. All spectraare acquired for the tips within a few nanometersabove the prism surface, as controlled by shear-forceAFM. The intensity scale is the same for all four cases,and the spectra are offset for clarity. Electron micro-graphs for the tip structures investigated are shown asinsets.

The pronounced wavelength dependence of the scat-tering of Au tips is characteristic of a plasmon resonantbehavior. Both scattering intensity and spectral posi-tion of the resonance are found to be sensitive to thestructural details of the tips. In general, for regular tipshapes, the resonance is characterized by one (Fig. 3a)distinct spectral feature. Inhomogeneities in the geo-

Fig. 3 Scattering spectra for different Au and W tips. Aplasmon-resonant behavior is observed for Au tips (a, b). Weakerand, in general, spectrally flat signals are observed in the case ofW tips (c, d). The spectral data are juxtaposed with the electronmicroscope image from the corresponding tip. The scale barcorresponds to 100 nm [93].

metric shape are reflected in spectral broadening and/oroccurrence of multiple spectral features (Fig. 3b). Inaddition, the spectral position and shape of the plasmonresonance depend sensitively on the aspect ratio ofthe tip.

For comparison, spectral light scattering by tungstentips of similar dimensions is shown in Fig. 3c and d.Overall weaker emission intensities are observed com-pared to Au tips. For W, a metal with strong polar-ization damping due to absorptive loss in the visibleand near-IR region, no SPP resonance is expected.Here, a spectrally flat optical response is observed withweak overall scattering intensities (Fig. 3c). The spec-tral behavior is found to show little variation with tipradius and tip cone angle, except for the case of veryslender tips, where a modulation is observed as shownin Fig. 3d.

From polarization-dependent studies of the tip scat-tering process, we have found that more intense scat-tering is observed for emission polarized parallel withrespect to the tip axis (p-polarized emission), corre-sponding to the excitation of longitudinal plasmonicmodes. The intensity ratio of p to s (emission ortho-gonal to the tip axis) is typically found to range be-tween 2 and 5, with a maximum value of ∼ 10. Inthe context of the TERS experiments, it must benoted that the s-polarized field is not expected tobe enhanced near a surface. In this case, the optical

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polarization of the tip and the corresponding imagepolarization induced in the sample are oriented antipar-allel, as will be discussed in the “Optical Tip–SampleCoupling” section.

Local-Field Enhancement from Bare Tips

The highly resonant characteristics observed for Autips suggest strong local-field enhancements in thevicinity of the apex. The quantification of the near-field enhancement factor is a difficult task in general.Without an absolute reference, the enhancement factorcannot readily be quantified from linear optical exper-iments. We therefore make use of the symmetry selec-tivity of the second-order nonlinear optical response inthe form of SHG from the apex region of the tips.

SHG is forbidden in the dipole approximation formedia with inversion symmetry [136]. In scatteringgeometry for sagittal illumination and tip-parallel po-larization, the SHG response from the tip is dominatedby the apex region where the macroscopic transla-tional invariance is broken in the axial direction. Withthe symmetry being radially conserved, little signalis expected from the conical near-apex shaft area.1

For the experiments, linearly polarized incident lightfrom a mode-locked Ti:sapphire oscillator (pulse dura-tion < 15 fs, λ = 805 nm) is directed onto the sharp endof the free standing tip and the scattered SHG signal isspectrally selected and detected.

The contribution of the local field-enhancement ofSHG from the metal tips is derived by comparing thesignal strength obtained to that of a planar surface ofthe same material. With the SH-enhancement expectedto be dominated by the tip apex, a SH-enhancementof ∼ 5 × 103 − 4 × 104 was observed for Au tips withr � 20 nm. For a SH-power ∝ E4 [137], this corre-sponds to an amplification of 8–25 for the averageelectric field near the apex, in agreement with estimatesbased on other SHG experiments [138]. For W tips,significantly lower values for the SH enhancement arefound corresponding to local field factors between 3and 6. These results are also in good agreement withtheoretical models despite microscopic variations in thedetails of the tip geometry.

The excitation of the localized SPP in the axial direc-tion is responsible for the field enhancement observed

1In addition to the unique symmetry properties, the local surfaceand nonlocal longitudinal bulk polarizations contributions to thenonlinear polarization and their directional and polarization se-lection rules are directly distinguishable here due to the geometryof the tip as a partial asymmetric (∞mm) nanostructure with themirror symmetry broken along the axis.

experimentally for Au tips. A systematic investigationof the influence of these geometric parameters in termsof cone angle and tip radius would be highly desirable;however, the limitations due to the electrochemicalpreparation procedure render this difficult.

Calculation of the Near-Field Distributionat the Tip-Apex

The near-field distribution and enhancement have beenderived theoretically for a variety of tip model geome-tries and tip and sample material combinations usingdifferent theoretical methods [81, 92, 95, 130, 132, 139–142] (and references therein). The accurate theoreticaltreatment of the problem involves the solutions ofMaxwell’s equations. This can be performed numeri-cally for a chosen model tip-geometry [94, 131, 143,144]. Although this may closely reproduce the exper-imental observations, the approach is computationallyvery demanding. In addition, it has remained difficult toextract the underlying relevant microscopic parametersresponsible for the optical response observed given thatthe effects of tip geometry, tip material, tip–sampledistance, and optical field are coupled.

Taking advantage of the small dimensions of the tipapex compared to the optical wavelength (kr � 1, withk the wave vector and r the tip-apex radius), the prob-lem can be treated in the quasistatic approximation,which allows solving the Laplace equation analyticallyfor certain geometries [95, 130, 132, 140–142] to derivethe local field distribution [87, 145]. For a size of theapex region of r ∼ 10 nm, this implies that the electricfield has the same amplitude and phase across thestructure at any time; thus, retardation effects can beneglected [133, 146]. Despite constraints in terms ofthe tip geometries that can be treated in this approach(paraboloidal, spheroidal, hyperboloidal), this methodprovides direct insight into how the solutions scale withseveral experimentally relevant structural and materialparameters.

Here, we approximate the tip geometry as a hyper-boloid. The influence of different dielectric and struc-tural parameters on the near-field enhancement anddistribution is systematically derived for both bare tipsand tip–sample systems [95]. The optical wavelengthdependence is explicitly taken into account consideringthe frequency dependence of the dielectric functions oftip and sample media [147].

The field distributions and enhancement and theirspectral dependence calculated within the quasistaticapproximation for a hyperbolical tip are found to agreewith other detailed theoretical observations. Using a


fully 3D finite-difference time-domain method, we havecalculated the field distribution [94] with results simi-lar to those obtained in [140, 144, 148]. The compar-ison with both the exact theoretical treatments andexperimental results validates the approach of treatingthe probe tip in the quasistatic approach to a goodapproximation. Despite the simplicity of the model,the essential optical properties and the physical trendscharacteristic for the optical response of the tip–samplesystem are accurately predicted.

Figure 4 shows characteristic local field distributionsand the corresponding enhancement near the apexregion of free standing tips of gold (a) and tungsten(b). The equipotential surfaces are indicated by solidlines. In both cases, the radius of the apex and thecone semiangle are fixed to r = 10 nm and to θ = 20◦,respectively. This closely resembles the conditions inthe TERS experiments by setting the incident light ata wavelength of λ = 630 nm and the polarization alongthe tip axis.

At optical frequencies, even for metallic tip material,the tip surface does not represent an equipotentialin contrast to the pure electrostatic case. The finiteresponse time of the charge carriers with respect to theoptical frequency results in the decay of the field insidethe tip on the length scale given by the skin depth.For gold as a representative material with high con-ductivity, this results in the strongest field enhancement

Fig. 4 Dependence of field enhancement (E/E0) on tip materialfor free-standing Au and W tips with apex radius r = 10 nm, andwavelength of λ = 630 nm. The solid lines represent contours ofconstant potential [95].

of E/E0 ≈ 50 at the apex.2 In contrast, tungsten is apoor conductor in the optical frequency range leadingto a comparably moderate enhancement of ∼12. Thedegree of field enhancement E/E0 at the tip apexdepends sensitively on apex radius and cone semiangledue to their influence on the plasmon resonance, asdiscussed briefly below and in [95]. Typical values rangebetween 10 and 100 for gold tips with 10–20 nm radiusand realistic semiangles.

Despite the necessary approximations inherent tothe quasistatic approach, the theoretical results pre-sented here prove to be sufficiently accurate for mostpractical purposes. This is drawn from comparison ofthe field enhancement factors with the experimentalresults of the “Optical Tip Characterization” section.From the tip-scattered SHG experiments, the local fieldenhancement of 8–25 for Au and 3–6 for W was esti-mated for r = 20-nm apex radii [92]. Considering thatthe experimental enhancement factors are obtained asa spatial average over the apex region, these valuesfall well within the range of the theoretically predictedenhancements given in Fig. 4.

Optical Tip–Sample Coupling

The local field enhancement, as well as the lateralconfinement, can change significantly for the tip in closeproximity to a surface plane. This behavior is of crucialimportance for the optical contrast in scattering near-field microscopy. The optical tip–sample coupling isthe result of the forcing of the boundary conditionsat the surface plane on the field emerging from theapex. With the incident electric field inducing an opticaldipole excitation in the tip, the presence of the samplecan be accounted for by considering a virtual imagedipole located inside the sample, with the resultingfield distribution being a superposition of the fields ofthe two dipoles [149]. This gives rise to a mutual andconstructive tip–sample optical polarization when theelectric field is oriented parallel with respect to the tipaxis (p-polarized). For an s-polarized incident field, thetip-dipole is induced parallel to the sample surface, andthe correspondent image-dipole aligned antiparallel.This leads to a partial cancellation and reduced fieldintensity and scattering [58].

Figure 5 (top panel) displays the evolution of thefield in the tip–sample gap calculated along the axial

2Note that for the calculations that refer to the field at the tipapex, the field is calculated at 0.125 nm below the apex to avoidnumerical artifacts due to finite grid size.

180 Neacsu et al.

Fig. 5 Top: Variation of field enhancement E/E0 along the axialdirection across the tip–sample gap region for different distancesd for an Au tip (r = 10 nm) and Au sample at an excitationwavelength of λ = 630 nm. The tip is at variable z = d posi-tions and extends to the right. The sample surface is located atz = 0 nm and its bulk occupies the range of negative z-values.Bottom: Spectral dependence of field-enhancement with tip–sample distance d for an Au tip (r = 10 nm) approaching anAu surface. The pronounced red shift of the plasmon responseis associated with the strong near-field tip–sample coupling ford ≤ r. The lines represent contours of equal optical field enhance-ment [95].

direction for different distances d for an Au tip ap-proaching a flat Au sample. The tip–sample approach isaccompanied by a significant increase in field enhance-ment in the tip–sample gap. The tip–sample interactionis correlated with apex radius and becomes significantat distances below about twice the tip radius (here,d = 20 nm). Here, the near-field interaction becomeseffective with a particularly fast rise of the field at the

sample surface. The local field enhancement reachesvalues of up to several hundred for d = 2 nm, showingan increase of more than one order of magnitude whencompared with the free-standing tip. In addition, thefield enhancement with decreasing tip–sample distanceis accompanied by a strong lateral confinement of thefield underneath the apex [95]. The equipotential sur-face is forced to align nearly parallel with respect tothe metallic substrate plane, which gives rise to anenhanced lateral concentration of the field. This is im-portant, as it leads to an increased spatial resolutionin scanning probe near-field microscopy for small tip–sample distances.

The details of the near-field distribution in the tip–sample gap depend on the optical properties of thetip and the sample materials that affect both field en-hancement and lateral confinement. The lateral spatialresolution that can be obtained is determined by thenear-field spatial extent and is given to the first orderby the tip apex radius; however, deviations from thissimple scaling behavior can be expected as the apexdimensions become smaller than the typical metal skindepth of ∼20–25 nm at visible frequencies. In addi-tion, the dielectric properties of both tip and sampledetermine the details of the field distribution in the gapregion. In particular, the lateral confinement decreaseswith a decrease of the material optical polarizability, asdiscussed in [95]. Probing metallic samples with metallictips at small tip–sample gaps results in the highestspatial resolution. In contrast, the lateral resolution willbe lower when imaging dielectric surfaces in otherwiseidentical experimental conditions.

One of the virtues of the quasistatic model is thedirect access to the spectral variation of the field en-hancement and its distribution for different tip–samplegeometries. The spectral tip-scattered response can be-come a complex superposition of the tip and the sampleoptical properties, the understanding of which is im-portant in nanospectroscopy. Figure 5 (bottom panel)shows the calculated spectral dependence of the fieldenhancement near the surface for an Au tip (r = 10 nm)approaching an Au surface. As expected, a structuralplasmon resonant behavior is observed. Associatedwith the increase in field enhancement for shorter dis-tances a spectral shift in the plasmon response to longerwavelengths is observed. This red shift is especially pro-nounced for distances d ≤ r, correlated with the onsetof the sharp rise in field enhancement in the regime ofstrong coupling. It is the result of the superposition ofthe dielectric functions of the tip and the sample mate-rial mediated by the tip–sample optical coupling. Thisis a general phenomenon and it is found in calculationsof spheres and other plasmonic nanostructures in close


proximity to metal surfaces [132, 144, 150, 151], and ithas been observed experimentally in TERS and lightemission in inelastic tunneling [152–154].

Predicted spectral peak widths of the order of 0.2 to0.3 eV correspond to what is expected from the elec-tronic dephasing times for SPP in Au of ∼ 4–7 fs [155].Using tungsten as tip material, no plasmon behavior isobtained except for the case when it is combined witha metallic sample that can itself sustain an SPP at thecorresponding wavelength [95].

The determination of the field enhancement for atip–sample coupled system has been experimentallyachieved by TERS from surface MLs of molecular ad-sorbates [73, 77, 81, 85, 94]. From tip–sample distance-dependent Raman measurements in comparison withcorresponding far-field experiments using MG dyemolecules or single-walled carbon nanotubes, near-field enhancements of 60–150 at the sample surfacewere measured, as detailed below. These experimentalvalues are in good agreement with the theoretical pre-dictions, which range from ∼50 to ∼300, as shown inFig. 5 for small tip–sample distances.

Near-Field Character and Far-Field Artifacts in TERS

TERS manifests itself in an enhancement of the Ramanresponse, with the increase confined to the region un-derneath the tip-apex. However, with the illuminationextended on a larger surface region determined by thefar-field focus, the discrimination of the variation of far-field response due to the presence of the tip inside thefocus is difficult (Hartschuh et al. [73]).

Without any lateral scanning or systematic verticaltip–sample distance variations, this does not allow forthe unambiguous assignment of the observed opticaleffect to a near-field process. The apparent Ramansignal rise may be due to far-field effects associated withthe tip being scanned inside the tight laser focus. Thiscan influence both signal generation and detection. Asthe tip penetrates into the focus region, it would scatteradditional otherwise forward-scattered (nonenhanced)far-field Raman light back into the detector. Further-more, the interference of the tip-scattered and surface-reflected contributions with direct incident pump lightresults in locally enhanced pump intensities. With bothprocesses affecting a surface region not confined by theapex area, near-field effects may be obscured by anincrease in far-field signal.

In Fig. 6, tip-scattered Raman results are shownfor single-wall carbon nanotubes and MLs of MG

Fig. 6 Near-field signature vs far-field artifact: Raman spectraof single-walled carbon nanotubes and MG molecules with tipretracted (SWNT, MG) and tip engaged (SWNT tip, MGtip). Inset:tip–sample distance dependence of the Raman signal obtainedunder similar conditions but displaying very different behaviors:the ∼20-nm-length scale increase is characteristic for the near-field signal origin (circles); the few hundred nanometer decaylength (squares) shows a far-field artifact—leading to similarsignal increase as the near-field response. Dashed lines added asguide for the eye.

molecules with the tip in force feedback at d = 0 nm3vs tip retracted by several 100 nm. When the tip iswithin several nanometers above the sample surface, astrong increase in Raman intensity is observed for bothadsorbates (spectra denoted SWNTtip and MGtip). Notethat the difference in noise level from the SWNT to theMG spectra is due to different spectral resolution set-tings of the spectrometer. Although frequently used toassign the observed TERS signal, a simple comparisonof surface vs tip-scattered Raman intensities rendersnear- and far-field processes a priori indistinguishable.The inset of Fig. 6 shows the Raman peak intensity as afunction of the tip–sample distance obtained in two sim-ilar experiments for MLs of MG molecules adsorbedon a flat Au surface. The overall increase in signalis comparable in both cases, and an estimate of theRaman enhancement factor gives G > 106 (vide infra).However, with the distance variation occurring on alength scale correlated with pump wavelength or focusdimensions, in one case, the enhancement can solelybe attributed to far-field effect (squares). A true near-field effect manifests itself in a correlation of the spatialsignal variation with the tip radius (∼20 nm). Here,with high-quality tip (sharp apex, smooth tip shaft),

3Here, d = 0 nm is defined ascorresponding to a 20–30% de-crease in the shear-force amplitude

182 Neacsu et al.

the near-field contribution can dominate the overallsignal (circles). Therefore, for the tip-scattered Ramansignal, only the demonstration of a clear correlation ofthe lateral or vertical tip-molecule distance dependencewith tip radius allows for an unambiguous near-fieldassignment of the optical response [73]. This is true forall near-field microscopies including s-SNOM and thespecial case of TERS [16, 19, 59, 65].

Experimental Quantification of the Near-FieldRaman Enhancement

In contrast to SERS, where the quantification of theenhancement is generally a difficult task, for the tip-scattering experiment, the Raman enhancement factorcan be derived from comparison of tip-enhanced vsfar-field response of the same surface ML.

Figure 7 shows the spectrally resolved tip-scatteredRaman signal during approach of ∼ 1 ML of MG ongold (2 nm/step, 1 s/spectrum acquisition time). Thepump light is polarized along the tip axis (pin) andthe Raman signal is detected unpolarized. Althougha faint Raman signature of the molecules is observedwith the tip at d > 100 nm, a clear molecular fingerprintis obtained only when the tip is within ∼20 nm fromthe sample. The prominent bands around 1,615 and1,365 cm−1 are assigned to combinations of the C=Cstretching vibrations of the phenyl ring and the modeat 1,170 cm−1 is due to a methyl group rocking mode oran in-plane C-H bending mode of the phenyl ring [156].

The enhancement is confined to a tip–sample spac-ing of just several nanometers and correlated with theapex radius of the tip, as expected for the near-fieldsignature. The increase in Raman response is accom-panied by a weak rise in a spectrally broad fluorescencebackground that has been subtracted. With the molec-ular fluorescence being quenched due to the electronic

Fig. 7 Tip–sample distance dependence of spectrally resolvedRaman signal during approach of ∼1 ML of MG on gold. Eachspectrum is acquired for 1 s and the approach is realized with 2-nm increments. Near-field tip-enhanced signal is observed withthe tip within ∼20 nm above the sample, displaying typicalRaman modes for MG molecules [81].

coupling to the metal substrate, this emission couldlargely be attributed to the enhancement of the intrinsictip luminescence in conjunction with excitation of plas-monic modes in the tip–sample cavity [152, 157], i.e., itsorigin is independent of the molecular adsorbates.

With the near-field character of the Raman responseverified, the experimental field-enhancement factor canbe derived from comparison of the tip-enhanced vs far-field response from the same surface ML. For the ex-periments presented here, the integrated Raman signalover the 1,150–1,650 cm−1 spectral region is used afterbackground subtraction.

As shown in the “Calculation of the Near-FieldDistribution at the Tip-Apex” section, the electromag-netic near-field enhancement originates from a sam-ple surface area approximated by the area of the tipapex. For the evaluation of the enhancement factor,the different areas probed in the near-field (TERS) andfar-field (FF) cases are then taken into account. Forthe TERS setup, the illumination focus has a diameterd � (λ/NA) × 1.5 = 2.7 μm (the empirical factor of 1.5accounts for the deviation of the laser beam from aperfect gaussian profile). Considering the ∼70◦ angleof incidence of the pump light with respect to thesurface-normal in our setup, the actual surface regionilluminated is elongated elliptically and larger, with atotal area of ∼ 17μm2. In the case of the confocal–Raman setup (FF) used to record the far-field spectrumof MG, the same laser illuminates an area of 870 nm2.The molecule is estimated to occupy a surface areaof ∼ 0.87 nm2 (actual 3D space filling: 1.18 × 1.39 ×0.98 nm). One ML surface coverage then correspondsto approximately 106 molecules in the focus of thefar-field setup, and only < 200 are responsible for thetip-enhanced signal for a tip with 10-nm apex radius.

In addition, the detection efficiencies for the two dif-ferent experimental arrangements are considered. Thefar-field response is emitted only in half space (solidangle � = 2π) assuming an isotropic dipolar intensitypattern, and with NA = 0.9, a total of ∼ 27% of theRaman signal is detected, in contrast to the TERSexperiments with ∼2% for NA = 0.35 and assuming theemission pattern following a cos2(θ + π2 , ) dependence(θ ∈ [0, π ]).

Taking all these factors into consideration, theRaman enhancement factor can be estimated, and it isfound to range from 106 to 109, with variation mostlydepending on the tip used. This enhancement is ex-pected to be electromagnetic in origin. The moleculesexperience the tip-enhanced local-field Eloc(νi) = L(νi)E(νi), with L(νi) as the enhancement factor of theincident field E(νi). The concomitant enhancement ofthe polarization P(νs) at the Raman-shifted frequency


Fig. 8 Field enhancement (E/E0) for two different Au tipsapproaching ∼ 1 ML of MG on a gold surface. The Ramanresponse is integrated over the dominant peaks in the 1,150–1,650 cm−1 spectral region after background subtraction. Theenhancement factor is calculated according to ITERS ∝ E4.

νs is L′(νs). Therefore, the total field enhancementis given by L(νi) · L′(νs) [93, 137]. With the Ramanintensity I ∝ |L(νi) · L′(νs) · E(νi)|2, the total Ramanenhancement factor G is given by G = |L(νi) · L′(νs)|2[158]. Although different in general, the field enhance-ment factors L(νi) and L′(νs) for pump and Ramanpolarizations, respectively, can be assumed to be similarin this case [159, 160]. This is motivated by both thespectrally broad plasmonic resonance of the tip and itssmall red-shift upon approaching the sample surface[95, 132, 161].

Figure 8 shows the effective field enhancement fac-tors (L(νi)·L′(νs) = E/E0) for the integrated Raman

signal from ∼ 1 ML of MG on gold as a function oftip–sample distance for two different Au tips (bothwith r < 15 nm). Maximum enhancements of ≥90 and≥60 (black and blue curves, respectively) are obtainedconsidering a variation of the tip-scattered Raman re-sponse with the fourth power of the electrical field, asindicated above. The molecules adsorbed on the planarAu surface already experience a field enhancementgiven by the Fresnel factor due to reflection of theincident light at the surface [162]. At 633 nm and θ =70◦, the Fresnel factor is equal to 0.95, resulting in anelectric field enhancement of 1.95. We will thereforealso derive the total enhancement with respect to thefree molecule response.

The study of the polarization dependence of theRaman response offers additional insight into the elec-tromagnetic enhancement of TERS. Figure 9 showsnear-field Raman spectra from ∼ 1 ML of MG ongold for the different polarization combinations of bothpump and Raman light. Incident laser power and ac-quisition times are identical for all spectra. The po-larization directions are defined as parallel (p) andperpendicular (s) with respect to the plane of incidenceformed by the incoming wave vector k(ωi) and thetip axis. No background has been subtracted and thedata are normalized with respect to the intensity ofthe 1,615 cm−1 mode measured in the pin/pout polar-ization configuration (upper left panel).

With the incident field polarized perpendicular tothe tip axis (sin), almost no Raman signal is observed,irrespective of the polarization of the scattered light.In contrast, with the pump polarized along the tipaxis (pin), clear Raman fingerprints of MG molecules

Fig. 9 Polarizationdependence of the near-fieldtip-enhanced Ramanresponse originating from∼ 1 ML of MG on gold. Allspectra are acquired for 30 sand normalized to themaximum intensity value ofthe 1,615-cm−1 mode inpin/(unpol.)out geometry. Avery weak Raman response isobserved for polarizationcombinations other thanpin/pout, where strongnear-field coupling gives riseto a strong Ramanenhancement. The spectrallybroad background is due tothe intrinsic luminescencefrom the Au tip itself.

184 Neacsu et al.

are observed, with the Raman response being pre-dominantly polarized parallel to the tip (pin/pout) asexpected for near-field TERS from isotropically dis-tributed molecules with diagonal Raman tensor com-ponents as is the case of MG. For both the sin/pout

and the pin/sout configuration, only a weak specificvibrational Raman signal is observed due to the ab-sence of the tip–sample optical coupling. For sin/sout,a larger background is observed, albeit with no Ramanenhancement, as expected.

The highly polarized TERS response observed inour experiments indicates the absence of significantnear-field depolarization for the homogeneous andslender tip geometries used. This is required forsymmetry-selective probing in TERS and other non-linear tip-enhanced processes [163–165] (Neacsu et al.,unpublished manuscript) that rely on polarization se-lective and conserving light scattering. In contrast, fora Ag-particle-topped quartz AFM probe as used forprobing the 520-cm−1 Raman band of Si [166], theobserved Raman depolarization has been attributed tothe wide cone angle of the tip [119].

TERS of Molecular Adsorbates

In Fig. 10, representative tip-enhanced Raman spectraare shown for MG on smooth Au surfaces. They areacquired for the same surface coverage of ∼1 ML, butusing different tips exhibiting enhancements of 3 × 108(a), 7 × 107 (b), 1 × 107 (c), and 1 × 106 (d), respec-tively. The experimental uncertainty is estimated at afactor of 3–5 for each value. The tip-enhanced Ramanspectra are reproducible for a given tip. However, asseen from the data, the spectral details vary from tipto tip with increasing enhancement. With the lateralconfinement of the tip-enhancement within a ∼10-nm-diameter surface region and a molecular density of∼ 1/nm2, we estimate that the signal observed inFig. 10a–d originates from ∼100 molecules.

In the lower panel of the figure, the far-fieldRaman spectrum from the same sample is shown forcomparison (black line). The far-field spectrum closelyresembles that in aqueous solution [156], indicatingthat the molecules are physisorbed in isotropic orien-tation at the surface. The blue bars represent normalRaman modes of the MG anion calculated using densityfunctional theory as implemented in Gaussian03 as dis-cussed in [81]. The assignment and spectral position ofthe calculated modes agree well with literature values[156, 167].

For moderate near-field enhancements ≤107, bothspectral positions and relative intensities of the modes

800 1000 1200 1400 1600 1800

Far-field

Ram

an In

tens

ity (

arb.

u.)

Wavenumber (cm-1)

d)

c)

a)Tip-enhanced

b)

Fig. 10 Tip-enhanced Raman spectra for ∼ 1 ML of MG fordifferent degrees of enhancement (a, b, c, d) in comparisonwith the corresponding far-field Raman spectrum (bottom graph)and DFT calculation for the mode assignment (blue bars). TheRaman enhancement factors derived are 3 × 108 (a), 7 × 107 (b),1 × 107 (c), and 1 × 106 (d), respectively. Data are acquired for1 s (a), 100 s (b, c), and 30 s (d), respectively. Spectral resolu-tion is 25 cm−1 for the near-field and 1 cm−1 for the far-fieldspectra [96].

resemble the far-field signature, as seen in Fig. 10c andd and in accordance with other TERS results for similarenhancements [79]. However, with increasing enhance-ment of 7 × 107 (b), and most pronounced for 3 ×108 (a), the vibrational modes start to look markedlydifferent. While some of the modes are present inboth far-field and near-field spectra (e.g., those at: 920,1,170, and 1,305 cm−1), some others are only presentin the highly enhanced near-field results (e.g., 1,365,


1,544 cm−1). The vertical dashed lines in Fig. 10 areadded as a guide to the eye for easier comparison.

The DFT calculation allows for the identificationof the spectral features in the far-field data of Fig. 10.The majority of the intense Raman modes can be at-tributed to modes either localized at the phenyl ring ordelocalized over the two dimethylamino phenyl groups.In the spectral region of 910 to 980 cm−1, severalvibrational modes, typically characterized by in-planeskeletal bending and/or out-of-plane C-H motions, arefound. The 1,170-cm−1 mode may be assigned to amethyl group rocking mode or an in-plane C-H bendingmode of the phenyl ring. Around 1,300 cm−1, in-planeC-H deformation modes and C-C stretching modes ofthe methane group are located.

Furthermore, the calculations show that the newspectral features seen in the highly enhanced near-fieldspectra in Fig. 10 mostly correspond to vibrational nor-mal modes of MG. Among the characteristic near-fieldenhanced modes, e.g., the peak at 1,365 cm−1, whichis very strong in the far-field spectrum but decreaseswith increasing enhancement, can be assigned to com-bination of the C=C stretching motions of the aromaticring. In contrast, the prominent peak at 1,544 cm−1,which dominates for the highest enhancement is veryweak for small enhancements or in the far-field spectra.Here, the calculation shows a mode characterized bystretching motions combined with in-plane C-H bend-ing motions of the conjugated di-methyl-amino-phenylrings. The two modes at 1,585 and 1,615 cm−1, whichcan be assigned to C=C stretching vibrations of thephenyl ring, decrease with enhancement.

This change in both intensity and spectral signa-ture with increasing near-field enhancement togetherwith the vibrational analysis shows that the Ramanpeaks observed correspond to vibrational modes ofMG, whereby different selection rules seem to applyfor the Raman spectra obtained under condition of highenhancements [158].

In the following, we discuss different physical mech-anisms possibly responsible for this mode selectivity.Due to the high localization of the optical near-field,the molecules in the tip–sample gap experience a largefield gradient and different Raman symmetry selectionrules can come into play [168–170]. The relevant termsof the dipole moment μa of a molecule situated in thepresence of a high field-gradient are:

μa ={ (

dαabdQ

)0

E +(

dμpadQ

)0

E + αab(

dEdQ

)0

+ 13

∂ E∂c

(dAabc

dQ

)0

}Q,

with E as the amplitude of the applied electric field,μ

pa the permanent molecular dipole moment, αab the

polarizability tensor at vibrational frequency, and Aabcthe quadrupole polarizability [171]. Q represents thecoordinate of vibration and {a, b , c} are a permutationof the coordinates {x, y, z}.

The first and second terms of the equation describethe normal Raman emission and the IR absorption,respectively. For an IR mode to be active, a variationin the dipole moment along the vibration coordinateis necessary [172]. The presence of the field gradientfulfills this requirement and thus allows for IR modes tobecome Raman active in that case. However, for MG,it was found that the few IR modes show no significantresemblance with the highly enhanced TER spectra(details in [81]).

The third term denotes the gradient field Raman(GFR) effect [171]. The mechanism by which strongfield gradients can influence the molecular Ramanspectra by altering the selection rules require that thepolarizability tensor (αab ) and (dEb/dQ)0 must simul-taneously be nonzero. The resulting selection rules re-semble the surface selection rules [173] and give rise toGFR lines observed in addition to the normal Ramanmodes.

The optical field gradient can also couple to vibra-tions via the derivative of the quadrupole polarizabilityAijk of a mode, as described by the last term of theabove equation [170]. With αab and Aijk transformingdifferently in terms of symmetry and their ratio beinghighly mode-dependent, this could also account for themode selectivity observed in tip-enhanced Raman scat-tering [174]. This might help to resolve the striking ob-servation that the strength of the calculated four modesat 846, 988, 1,029, and 1,544 cm−1 are overestimated byDFT calculation as compared to the far-field spectra,but coincidentally represent modes that are relativelystrongly enhanced in the TER spectra.

While it is difficult to quantify the contribution ofGFR in general [171], the occurrence of normally for-bidden modes in the TERS results suggest that theprocess contributes in this case. Similar differences be-tween far- and near-field Raman spectra were reportedpreviously in fiber-based SNOM experiments on Rb-doped KTiOPO4 [20, 21].

In addition, the resonant Raman excitation leads toa coupling of electronic with vibrational transitions,resulting in different selection rules. Such vibronicallyallowed transitions may further be influenced by thepresence of the strong field gradient and will affect therelative peak amplitude of the observed Raman modes[172]. This effect would resemble the “chemical effect”observed in SERS [35].

186 Neacsu et al.

The pronounced spectral differences between thetip-enhanced and far-field Raman response have sim-ilarities to observations made in SERS, where vibra-tional modes that are normally not Raman allowedhad been found [175–177]. Aside from orientational ef-fects, these spectral variations are typically interpretedto arise from conformational changes and/or transientcovalent binding of the molecule at “active sites” [173].With that being unlikely in the tip-enhanced Ramangeometry discussed here, this indicates that the kindof spectral selectivity observed in our TERS experi-ments should be attributed primarily to electromag-netic mechanisms.

Molecular Bleaching

An important question regarding the appearance of dif-ferent spectral features in the near-field tip-enhancedRaman spectra is the influence of the molecular bleach-ing or other decomposition products [178, 179].

With the molecules under investigation exposedto the strongly localized and enhanced near-field,this leads to a sometimes rapid photo-decompositionprocess, especially in a resonant Raman excitation.

To probe for the possible appearance of photoreac-tion products and their signature in the Raman spectra,the evolution of the Raman emission is monitored intime-series experiments. Figure 11 (left panel) showsconsecutive near-field Raman spectra acquired for 1 s

each for an enhancement of 1.3 × 107 with an inci-dent laser fluence of 3 × 104 W/cm2. The moleculesbleach on a time scale of ∼ 100 s, depending on theenhancement level, and the decay and subsequent dis-appearance of the spectral response is uniform, i.e.,the relative peak amplitudes are maintained. Duringthe bleaching, no new spectral features appear frompossible photoreaction products and the signal decayswith the relative peak amplitudes remaining constant.After complete bleaching, no discernible Raman re-sponse can be observed. The fluctuation observed in thetime series is expected given the small number of ∼ 100molecules probed in the near-field enhanced regionunder the tip apex. The sum over all spectra (top graph)or the sum of any large enough subset even at latertimes, i.e., after substantial bleaching has already oc-curred, closely resembles the far-field response of MGand, thus, allows us to attribute the Raman response toMG molecules.

The same behavior of a gradual and homogeneousdisappearance of the Raman response without a rel-ative change in peak intensity is also observed forlarger enhancements, i.e., the case where a differentmode structure is observed. However, the larger localfield experienced by the molecules leads, in general, todecreasing decay time constants.

Figure 11 (right panel) shows the decay kineticsof the spectrally integrated Raman intensity for thetime series data shown on the left. This is shown incomparison to the integral intensity of the region from

Fig. 11 Left: Time series of100 consecutive near-fieldRaman spectra (acquired for1 s each) for ∼1 ML MG onAu (Raman enhancement1.3 × 107). The signal decaysto zero due to bleaching andno new spectral featuresappear from thephotoreaction products. Thesum Raman spectrum(top panel) clearly resemblesthe far-field spectrum. Right:Bleaching kinetics derived forthe spectrally integratedRaman time series and theregion from 1,550 to1,650 cm−1 (inset) [96].


1,550 to 1,650 cm−1 encompassing only the two promi-nent modes (inset). Assuming an exponential decaybehavior of I/I0 = exp(−t/τ) for the Raman intensity,a decay time τ = 40 ± 5 s is derived for both cases(solid lines). From the applied laser fluence of 3 ×104 W/cm2 and the enhancement of the pump intensityof ∼ √1.3 × 107, the bleaching is induced by a localpump fluence of 4.7 × 107 W/cm2.

An extreme case of molecular bleaching is shown inFig. 12. A time series of 100 consecutive Raman spectrafrom a sample with submonolayer molecular coverage,with each spectrum acquired for 1 s, is recorded. Theestimated Raman enhancement factor is 9 × 107, andthe spectral features deviate from the far-field Ramanspectrum presented in Fig. 10b, akin to the spectrashown in panels a and b of the same figure for highenhancement level (e.g., the mode at 1,544 cm−1). Afteran illumination time of about 50 s, the overall Ramanintensity drops suddenly over the whole spectral region,and most visible for the peak at 1,306 cm−1, but no newspectral features emerge. Thus, even for extreme cases

Fig. 12 Time series of 100 near-field Raman spectra acquiredfor 1 s each. After ∼50 s, the Raman intensity reduces due tobleaching, but no signature of secondary products is visible.

of bleaching, the molecular decomposition products donot contribute to the observed Raman signal.

It was suggested that the molecular bleaching ratecould be used for the derivation of the enhancementfactor [112]. It should be noted that the bleaching rateis not a characteristic physical quantity universal fora given molecule. For example, MG isothiocyanate—asister dye of MG—was found to bleach with a rate con-stant more than two orders of magnitude higher thanthe MG studied here [112], if renormalized to the sameexperimental conditions. Bleaching mechanisms canbe quite diverse [180]. They may include irreversiblephotoinduced or even multiphoton induced reactionssuch as rearrangements, dissociation and fragmenta-tion, elimination or hydrogen abstraction, or perhapsphotooxidation with ambient oxygen via triplet states.It can depend on, e.g., humidity or cleanliness of the tipor sample, and is, hence, not a useful measure to com-pare experiments performed under different conditionsin different laboratories.

With the experiments carried out under ambientconditions, special care must be taken to use clean sam-ples and tips. It is well known [181] that contaminatingorganic compounds and/or carbon residues can adhereto either tip or sample and reveal their Raman signa-ture in the tip-enhanced spectra, e.g., Raman bands ofappreciable intensity at frequencies above 1,750 cm−1for carbon [182].

A carbon cluster Raman response (see Fig. 6 in [96])manifests itself in characteristic spectral features muchdifferent from the spectral response discussed abovefor ML or submonolayer MG. In accordance withprevious observations [181–183], the carbon Ramanresponse is comparatively large and fluctuates rapidlyin an uncorrelated way. In contrast to the data on MG,a distinct spectral feature emerges around 2,000 cm−1,which has been assigned to, e.g., modes within thesegments of carbon chains [182], and which is absentin the TERS spectra of MG. This absence of the carbonRaman response can be understood since the bleach-ing of ML and submonolayer MG coverage leads tosmaller molecular fragments and, subsequently, to adilute surface carbon distribution and, hence, does notlead to extended carbon chains and aggregates, whichcan readily form by multilayer MG decomposition atambient temperatures.

Besides the degradation of the analyte, anotherpotential source of carbon contamination is the near-field probe itself. At room temperature and in anoncontrolled atmosphere, contaminants from the en-vironment could adsorb onto the tip surface and re-veal highly enhanced Raman signals. TERS controlexperiments with the bare tip and clean Au samples

188 Neacsu et al.

prior to MG deposition were carried out to confirm theabsence of vibrational Raman signature. Only intrinsictip luminescence has been observed in that case [81].

TERS with Single Molecule Sensitivity

With the measured tip-enhanced Raman spectra pre-senting a signal-to-noise ratio of more than 40:1 whenprobing ∼100 molecules for 1 s accumulation time, thisdemonstrates the potential for even single-moleculesensitivity. Figure 13 shows near-field tip-enhancedRaman spectra measured in a time series with 1 sacquisition time for each spectrum for a sample pre-pared with submonolayer surface coverage, adjusted toexpect on average < 1 molecule under the tip-confinedarea of ∼ 100 nm2.

Here, the tip has been held at a constant distanceof d = 0 nm above the sample and the total Ramanenhancement was estimated at 5 × 109.4 The observedRaman signal exhibits temporal variations of relativepeak amplitudes and fluctuations in spectral position.These are characteristic signatures of probing a singleemitter in terms of an individual molecule. Similar ob-servations have been made before in SERS [37–39, 41]with the fluctuations in the spectroscopic signature ofa single emitter typically attributed to changes in itslocal environment, its structure, molecular diffusion[36, 184], and changes in molecular orientation [179].With MG only physisorbed, it has to be considered inparticular that the molecules can diffuse in and out ofthe apex-confined probe region while experiencing dif-ferent degrees of enhancement. The diffusion dynamicscan be facilitated by the thin water layer present on thesample surface under ambient conditions.

In the time series in Fig. 13, the apparent bleachingrate seems reduced compared to what is expected fromthe analysis of the ensemble bleaching discussed above.This is a result of the dilute surface coverage where newmolecules directly neighboring the tip–sample gap dif-fuse into the near-field enhanced region. However, thesignal vanishes rapidly after 100 s due to the depletionof molecules in the immediate near-apex region.

A different mode structure of single-moleculeMG Raman is observed compared to the far-field response—especially pronounced for the 1,500–1,600-cm−1 spectral region, akin to the results for theensembles under strong enhancements shown above.Figure 13 (middle panel) displays the sum spectrum

4Here, d = 0 nm is defined as corresponding to a 20–30% de-crease in the shear-force amplitude.

(olive) for the single-molecule response time series(left) in comparison with ensemble spectra of ∼100molecules probed for 1 s (red) showing different de-grees of enhancement: 3 × 108 in a and 7 × 107 in b.Panel c presents the sum spectrum (blue) of the datashown in Fig. 11a that, as for a moderate enhance-ment, closely resembles the far-field spectrum. Theresemblance of the Raman spectrum of a molecularensemble with the sum spectrum over the whole timeseries offers strong evidence of probing single intactMG molecules [40]. Note that the sum over the timesseries shall therefore resemble an ensemble spectrumfor large enhancement rather than the far-field Ramanspectrum. In assessing the resemblance of the spectralcharacteristics, it has to be considered that, at lowcoverage, the molecules have more degrees of freedomto dynamically change orientation and they can diffuse.Given the rather weak response, the signal detected canonly be expected to emerge from the region of largestenhancement, and with the diffusing molecules probingthe spatial variation of the enhancement under the tip,this corresponds to an extreme case of inhomogeneousbroadening. Therefore, while individual spectral fea-tures at positions in accordance with the strongly tip-enhanced near-field response are observed in the timeseries, the sum spectrum no longer exhibits clearly re-solved lines. This interpretation is further corroboratedconsidering, e.g., the improved resemblance of the peakin the 1,550–1,600-cm−1 region of the single moleculesum spectrum with the sum of the two near-field spectra(in a and b) of different enhancement.

Further insight is obtained by studying the statisti-cal behavior of the single-molecule Raman response[36]. Figure 13 (right panel top graph) displays theintegrated 1,430 to 1,650 cm−1 spectral intensity forthe data in the left panel. The signal intensities clusterwith intervals of 170–230 counts·s−1, as already evidentfrom visually inspecting the time-series of integratedintensities (dashed lines), and this manifests itself in thecorresponding histogram in an asymmetric distributionwith discrete peaks (inset). This behavior is qualita-tively reproducible for experiments with the same sur-face coverage, and it can be interpreted as the Ramanemission from n = 0 (noise peak), 1, 2, and 3 moleculesbeing probed under the tip, as suggested for similarfindings in SERS [34, 36]. This assignment is corrob-orated from experiments with different surface cover-ages: for lower coverages, only the n = 0 and 1 peaksremain, and with increasing coverage, the distributionconverges to a narrow random Gaussian distribution,which is observed from a large molecular ensemble, asseen in the lower panel, where 100 consecutive far-fieldspectra were acquired for 100 s each and the signal is


Fig. 13 Left: Time series of tip-scattered Raman spectra fora submonolayer MG surface coverage with Raman enhance-ment of 5 × 109. The spectral diffusion observed is character-istic for observing single MG molecules. Middle: Comparisonbetween sum spectrum of data shown in left panel (olive) and tip-enhanced Raman spectra for different degrees of enhancement(red, a and b) and sum spectrum of data shown in Fig. 11 (blue).Right: Temporal variation of the Raman intensity of the inte-

grated 1,480–1,630 cm−1 of time series shown in left panel (top).From the corresponding histogram (inset) a discretization ofRaman intensities can be seen with 170–230 counts·molecule−1 ·s−1 (dashed line increment). For comparison, temporal variationof the Raman intensity of the integrated 1,480–1,630 cm−1 of bigmolecular ensemble (far-field) is shown (bottom) together withthe corresponding histogram (inset) [81, 96].

integrated over the same spectral range as in the caseof the single-molecule experiment. The details of thehistogram, however, depend on the binning procedure,especially for a small data set, as has been shown tobe insufficient as the sole argument for single-moleculeobservation [185].

The optical trapping and alignment of MG under thetip must also be considered as a possible source of theobserved surface diffusion and intensity fluctuations[186]. However, for MG together with our particularexperimental conditions, this cannot explain the dis-cretization of Raman peak intensities in the single-molecule response, as detailed elsewhere [81].

Our single-molecule TER results are similar to otherrecent experimental findings [85] where brilliant cresylblue (BCB) molecules adsorbed on planar Au wereprobed. For enhancements of ∼107 similar to our inter-mediate values, temporal fluctuations in both intensityand mode frequency were observed, albeit with nosignificant spectral differences between the far-field andnear-field response, as seen here for high enhance-ments. Recently, spatially resolved TERS imaging andidentification of individual BCB molecules directly con-

firmed our singe-molecule assignment in the Ramanresponse in the results discussed above [187].

Raman Imaging of Nanocrystals: Near-FieldCrystallographic Symmetry

With the TERS capability of detecting molecules downto the single emitter level, it is highly desirable toextend the technique to also probe other classes ofmaterials such as crystalline nanostructures. While far-field Raman microscopy has been successfully used forsuch investigations [188, 189], the applicability of TERShas not yet been explored.

In the following, we propose and discuss the po-tential of TERS for the spatially resolved determina-tion of the crystallographic orientation of crystallinematerials by taking advantage of the fundamental in-teraction of the Raman process with lattice phononstogether with the symmetry properties and selectionrules of the tip-enhanced scattering. While the applica-tion of the symmetry properties of the Raman selectionrules in a tip-enhanced geometry has been emphasized

190 Neacsu et al.

previously [119, 190–192], no discussion of the ap-plicability to crystalline nanostructures has yet beenprovided.

With Raman scattering being less invasive than elec-tron or X-ray techniques and applicable in situ, thisapproach will fill a much needed gap in the characteri-zation of nanostructured materials with increasing com-plexity [193, 194]. Transmission electron microscopy,capable of providing atomic resolution [195], requiressamples thin enough to be transparent for electrons,extensive sample preparation, and vacuum condi-tions, making in situ experiments difficult [196]. Like-wise, X-ray microscopy is capable of characterizingnanostructures with atomic resolution [197], but it re-quires a monochromatic brilliant synchrotron radiationsource and radiation beam damage remains a con-cern [198]. Here, the comparable simplicity of TERSfrom an instrumentation perspective makes it highlyattractive, providing complementary information andeven avoiding some of the disadvantages of the existingtechniques.

In Raman spectroscopy, the specific phonon modesprobed depend on the chosen experimental geometry,in terms of the incident and detected polarization, aswell as the propagation direction of light [5, 199]. Thesephonon modes allow determination of the crystallo-graphic orientation of a sample. This has been shownin far-field Raman in, e.g., the study of 90◦ domainswitching in bulk BaTiO3 [200] or the observationof ferroelastic domains in LaNiO4 [201]. However, inRaman microscopy, in the commonly used confocalepi-illumination and detection geometry, this reducesthe available degrees of freedom, thus resulting in theloss of the general capability to probe the symmetry-specific Raman tensor elements. In extending the use ofthe Raman selection rules to a side-illuminated TERSgeometry, these degrees of freedom can be regainedand even further refined by taking into account the tipgeometry.

The intensity of the Raman scattered light from amedium is given by: Is ∝ | �es · R̄ · �ei|2, where �ei and �esare the polarization of the incident and scattered light,respectively, and the Raman tensor R̄ is the derivativeof the susceptibility tensor [202]. As an example, for aRaman-active phonon mode of tetragonal BaTiO3, R̄ isgiven by:

A1(ξ̃ ) =⎛⎝ a 0 00 a 0

0 0 b

⎞⎠ ,

where �ξ denotes the polarization direction of the mode(for polar modes). The symmetry of a given mode, in

this case A1, is determined from group theory and maycontain multiple component phonon modes of differentfrequency [203]. Thus, when the polarization condi-tions, determined from the susceptibility derivative, aresatisfied for a given symmetry mode, the Raman shiftdue to the phonons belonging to that mode can beobserved.

In addition, one can selectively isolate specificphonon modes within a symmetry mode. For polarmodes, the phonons will separate into transverse op-tical (TO) and longitudinal optical (LO) components[204], which, being distinct in frequency, can be spec-trally resolved. The Raman tensor methods describedabove do not account for the distinct frequencies ofTO and LO modes nor describe how to selectively ex-cite these, requiring further refinement of the selectionrules

For a given geometry, the wavevector �q of the prop-agating phonon can be determined by conservationof momentum from the wavevectors of the incidentand scattered light. Based on the relative orientationbetween �q and �ξ , one can selectively excite the LOmode for �q ‖ �ξ , or the TO mode for �k ⊥ �ξ . Thus, theobservation of either a TO or an LO mode providesthe additional information about the orientation of thecrystallographic axes.

Furthermore, drawing on the nanoscopic apex of aplasmonic tip for preferential enhancement of incidentlight polarized along the tip axis allows us to exploitthe unique symmetry selection rules associated with thetip [92], selecting modes with polarizations parallel tothe tip axis. Modes for which either the incident orscattered polarization coincide with the enhancementaxis may also be observed, albeit with a lower intensity.

By appropriate selection of the polarization andpropagation directions of the incident and scatteredlight, specific Raman-active phonon modes may beisolated and detected. From the Raman tensor, onecan obtain the angular dependence of the Raman scat-tering intensity for each specific combination of inci-dent and scattered polarizations. A comparison of thecorresponding experimental and theoretical emissionintensities will allow one to deduce the orientation ofthe crystallographic axes. Thus, in combination with ana priori knowledge of the space group of the crystaland the Raman modes of the material, one can deter-mine the crystallographic orientation of a nanostruc-ture (Berweger et al., unpublished manuscript).

Although the study of nanocrystalline samples opensup a wide range of potential applications for TERS,some fundamental aspects are not yet fully understood.Recent far-field studies of wurtzite CdS nanorods in-dicate a possible depolarization effect in dielectric


nanostructures, leading to a breaking of the Ramantensor selection rules [189]. Furthermore, it has beenshown that the presence of a sharp edge within thenear-field of a photoemitter can affect the polarizationof the emitted light [205], although the resulting effecton Raman scattering is yet unclear. In addition, thelarge field gradient near the tip can fundamentally al-ter the selection rules, making previously silent modesvisible [171]. Although this may render IR and othermodes Raman-active [206], making mode assignmentmore difficult, it would still shed further insight into thefundamental material properties.

Outlook

TERS may emerge as an important analytical tool forchemical and structural identification on the nanoscale.It offers chemical specificity, nanometer spatial reso-lution, single-molecule sensitivity, and symmetry se-lectivity. However, with both sensitivity and spatialresolution critically dependent on the well definedgeometry and related optical properties of the tip, re-producibility has remained an issue in TERS.

Reproducibility can be enhanced performing TERSunder controlled experimental environmental condi-tions. Performing experiments under, e.g., ultrahighvacuum (UHV) conditions offers variable sample tem-perature and combination with other UHV techniquesfor surface analysis [82, 153, 187]. Furthermore, fordirect far-field illumination conditions, it is difficult toa priori distinguish the near-field response from theunspecific far-field imaging artifacts (vide supra).

Future developments in tip design and fabricationmay prove critical. We have recently demonstrated anovel way to generate a nanoconfined light emitter ona nanoscopic probe tip obtained by grating-coupling ofSPPs on the tip-shaft [207]. The adiabatic field con-centration of the propagating SPP, determined by theboundary conditions imposed by the tapered shape ofthe tip, offers an intrinsic nanofocusing effect and, thus,gives rise to confined light emission only from the apexregion, as theoretically predicted [208].

In this experiment, linear gratings are written ontothe shaft of Au tips by focused ion beam milling,∼ 10 μm away from the apex, as schematically shownin Fig. 14a. Upon grating illumination with a broadbandlight source (150 nm spectral bandwidth of Ti–sapphireoscillator), SPPs are excited and launched towards theapex [155, 209], where they are reradiated, as shown inFig. 14b (details discussed in [207]).

a)

b)

Fig. 14 a Principle of the nonlocal excitation of the tip apex.Far-field radiation excites SPP on the grating, which propagatealong the shaft towards the tip apex, where they are reradiatedinto the far-field. b Microscope image recorded for illuminationof the tip-grid demonstrating the efficient nonlocal excitation ofthe tip apex via illumination of the grating [207].

Spatially separating the excitation from the apexitself, this approach is particularly promising as itavoids the otherwise omnipresent far-field backgroundpresent for direct apex illumination. In addition, itwould provide the spatial resolution needed for near-field optical techniques including s-SNOM and TERS.

Summary

A systematic understanding both experimentally andtheoretically of the fundamental processes responsi-ble for field enhancement, spectral tip plasmonic re-sponse, and tip–sample coupling has allowed for reach-ing Raman enhancement factors as high as 109 leadingto single-molecule sensitivity. With lateral resolutiondetermined by the interplay of the tip apex radius,the tip–sample distance and the dielectric propertiesof tip and sample materials, nanometer spatial reso-lution can be obtained. Criteria have been discussedfor experimental TERS implementation and distinc-tion of the near-field signature from far-field imagingartifacts. The combination of the inherent sensitivityand spatial resolution of TERS with the Raman selec-tion rules and the unique symmetry of the scanningtip makes possible the spatially resolved vibrationalmapping on the nanoscale. Its implementation for

192 Neacsu et al.

determining the orientation and domains in crystal-lographic nanostructures has been proposed. Futuredevelopments in tip design and more efficient illumi-nation and detection geometries and scanning probeimplementation will allow for TERS to become a pow-erful nano-spectroscopic analysis tool.

Acknowledgements The authors would like to thank NicolasBehr, Jens Dreyer, Thomas Elsaesser, Christoph Lienau, andClaus Ropers for valuable discussions and support. Funding bythe Deutsche Forschungsgemeinschaft through SFB 658 (“Ele-mentary Processes in Molecular Switches at Surface”) the Na-tional Science Foundation (NSF CAREER grant CHE 0748226and IGERT Fellowship) is greatly acknowledged.

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4. Hendra PJ, Stratton PM. Laser-Raman spectroscopy. ChemRev. 1968;69:325.

5. Hendra PJ, Vear CJ. Laser Raman spectroscopy: a review.The Analyst. 1970;95:321.

6. Yu PP, Cardona M. Fundamentals of semiconductors. 3rded. New York: Springer; 2005.

7. Aroca R. Surface-enhanced vibrational spectroscopy.New York: Wiley; 2006.

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