Tip-enhanced Raman scattering microscopy: Recent advance in tip production

Tip-enhanced Raman scattering (TERS) microscopy is a new technique which enables topographic and Raman imaging to be taken simultaneously with a spatial resolution far beyond the diffraction limit of light. Since the first experimental demonstration in 2000,^1–4) following a conception made 15 years prior in 1985,⁵⁾ this tip-enhanced technique has been widely used to identify molecules at the nanometer scale in fields including biology,^6–10) catalysis,^11–13) materials,^14–22) and surface science.^23–32) Such a high resolution has been achieved by focusing polarized incident laser light on to the apex of a sharp metal scanning probe microscope (SPM) tip. This leads to a strong electromagnetic field being confined specifically at this point, via the excitation of localized surface plasmons with the help of a geometric light antenna effect (the so-called lightning-rod effect). Under the correct conditions a spatial resolution of 10–20 nm is generally made possible using TERS,^29,33–35) and further to this, molecular resolution has recently been realized by Zhang et al.³⁶⁾ TERS has a number of advantages over other "super-resolution" nanoscopic tools, such as electron microscopy or super-resolution fluorescence microscopy:^37–41) (1) no pre-treatment of the sample is required, such as dye labeling or conductive coating, (2) rich molecular vibrational information can be obtained, (3) TERS can be measured in ambient, in vacuum^34,35,42) or even under electrochemically controlled conditions in liquid.⁴³⁾

The critical component of any TERS experiment is the quality of the SPM tip, which serves as the source of the enhancement and consequent high spatial resolution, and as such recent advances in this area will be the focus of this review. Up to this point, chemical etching^44–51) and metal deposition^1,52,53) have often been used for scanning tunneling microscope (STM)- and atomic force microscope (AFM)-TERS, respectively. Spatial resolutions of several nanometers have been demonstrated using these fabrication procedures,^54,55) illustrating that they have enough potential to conduct super-resolution TERS imaging. However, they typically show extremely low fabrication reproducibility: the Raman enhancement factor or induced plasmonic dipole differs from tip to tip,⁵⁶⁾ which has in turn a consequent effect on performing and interpreting experiments. To avoid such problems, several research groups have proposed methods to better control the tip apex structure, e.g., by using "template-stripping"⁵⁷⁾ or use of a chemically-synthesized noble metal nanostructure (with well-defined crystallinity) as a tip.⁵⁸⁾ This will be reviewed in Sect. 3.

Furthermore, recently a new mode for conducting TERS has been proposed: excitation via propagating surface plasmon (remote excitation-TERS). This has been achieved using a variety of "specialized" tips. In such a case laser light is focused onto a light coupling point — such as a grating structure^59–62) or the junction between a metal nanoparticle/nanowire⁶³⁾ — formed on the body of the tip during the fabrication procedure, instead of directly at the tip apex. This introduces a "nano-point source" that is confined to the diameter of the tip apex. In a conventional TERS measurement, where a diffraction-limited laser spot is focused directly onto the end of the tip, Raman signals emanating from the surface will also be generated in the far-field. This serves as a background to the TERS signal, and thus lowers the observed signal-to-noise (S/N) ratio. Since a remote excitation configuration can minimize this excitation volume, these signals can be reduced. This "nano-focusing" method is therefore a promising way to further increase sensitivity of TERS. This will be introduced in Sect. 4.

In this section, we will focus on the process of designing TERS tips from a theoretical standpoint. In TERS, a metal SPM tip is irradiated with laser light exhibiting a specific polarization (often linear polarization for side-illumination and radial polarization for transmission-type configurations), before being brought into close proximity with a sample surface of interest (1–2 nm) under STM or AFM (tapping, shear-force) feedback. By raster scanning the sample across the tip, topographic and Raman images can be obtained simultaneously. When designing suitable tips for such an experiment, the following points must be considered:

(1) Matching of the local surface plasmon resonance with the incident laser: Since the major source of Raman enhancement is the excitation of localized surface plasmons at its apex, the tip (acting as a "nanoantenna") is required to have a high aspect ratio with a very small apex diameter (several to a few tens of nanometers) made from a plasmonically active material, such as gold, silver, aluminum or platinum. The morphology and size of the tip apex strongly effects the position of the resonance wavelength,^56,64) along with the direction of the induced dipole of the excited localized surface plasmons.⁵⁶⁾ Thus, a procedure that can be replicated to give the same structure — especially at the apex — is important to maintain high experimental reproducibility and control the dipole orientation. In actual TERS measurements, tips made of gold or silver have often been used under excitation wavelengths in the visible range (such as 532 or 633 nm), since these materials have a large polarizability owing to their large negative dielectric constants, thereby serving to more strongly enhance the electromagnetic field [note: in the case of gold, an excitation energy higher than 2.45 eV (= 506 nm) is not suitable because of the emergence of a broad emission from gold itself. This emission arises from the radiative recombination of holes in the 5d-band and electrons photo-excited to the 6sp-band⁶⁵⁾]. Computational calculations, such as the finite-difference time-domain (FDTD) method^66–69) or the finite element method (FEM),^70–72) are powerful tools to investigate the plasmon interaction between tip and free-space light.

In addition to the plasmonic response of the tip, the use of a noble metal substrate can give a further enhancement. Here, the incident radiation can act to excite so called "gap-mode" plasmons, which is a hybridized coupling mode of plasmons between two metals.^73,74) Molecules will therefore be "sandwiched" between the tip and the substrate, leading to an additional strong enhancement of the optical signal. So far enhancement factors of 10⁵–10⁶ have been reported with gap-mode TERS,⁷⁵⁾ and have allowed single molecule sensitivity to be reached.

(2) Surface roughness: It is known that surface roughness (i.e., cluster-like structures) on the metal surface can give inelastic light scattering^76–78) that is broadly over the Raman fingerprint region. In TERS with a typical configuration (conventional-TERS), the Raman enhancement factor (EF) with the presence of a noble metal tip can be calculated using the function below:^79,80)

$$\begin{equation*} \mathrm{EF} \approx \left(\frac{I_{\text{tip-in}}}{I_{\text{tip-out}}} - 1\right)\frac{A_{\text{FF}}}{A_{\text{NF}}} \end{equation*}$$

where I_tip-in and I_tip-out are Raman peak intensities measured with the tip in contact and retracted respectively, A_FF is the area of the far-field laser spot, and A_NF is the effective area of the TERS spot (which can be estimated from the diameter of the tip apex). The term inside the parenthesis is expressed as a "TERS contrast", which directly relates to sensitivity and thereby spatial resolution of TERS mapping. As alluded to above, this equation indicates that any "undesired" signal from sample surfaces, such as Raman scattering/emission from contamination or nanosized-metal grains, will contribute to the TERS signal as a background. This therefore means that not only trying to improve EF by fabricating "sharper" tips but also decreasing the background contribution from the far-field is desirable for conducting high contrast TERS measurements (this will be introduced in Sect. 4).

(3) Structural reproducibility: As we mentioned in (1), the resonance wavelength and direction of the induced dipole of the localized surface plasmons around the tip apex are strongly affected by the morphology of the end-structure of the tip. Control of this "tip-apex geometry" is critical in all TERS measurements, and it in addition points to the possibility of conducting polarization-dependent studies,⁵⁶⁾ where samples such as carbon nanotubes, graphene or strained silicon, are measured. Without precise control of apex geometry, TERS experiments suffer from a "try and error" approach, which has so far proved to be a major hindrance for conducting TERS microscopy.

In this section, we will introduce tips which have been used in conventional TERS measurements, i.e., illuminating laser light directly to the apex of the TERS tip (Fig. 1). In TERS, SPM feedback is necessary, in order to maintain a stable tip–sample distance and to avoid crashing the tip into the surface. Three types of feedback mechanisms are typically used: tunneling current (STM), shear-force, and tapping-mode (AFM). In the case of STM, since a high electrical conductivity is required for the tip and substrate, sharpened gold/silver wires are often used.^44–51) These chemically-etched noble metal wires have also often been used for shear-force AFM, where the tip is fixed on to a tuning fork made of quartz (usually wire thinner than 25 µm is used to maintain a high Q factor). On the other hand, in the case of tapping-mode AFM, which requires cantilever based tips, gold/silver is often deposited onto a standard silicon cantilever via vacuum-deposition.^1,52,53) These tips indeed provide TERS imaging with less than 10 nm resolution,^36,54,55) however, one crucial issue is the low fabrication reproducibility resulting from the poor wetness of metal on silicon (oxide) surface: generally only 1–2 out of 20 tips gives rich TERS contrast. To overcome this issue, recently new fabrication methods have been proposed, such as attaching chemically-synthesized noble metal nanowire/nanoparticle onto the tip apex⁵⁸⁾ or using a template.⁵⁷⁾ Below we will briefly go over the above methods, dividing into three sections.

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3.1. Chemically-etched noble metal tips

The most common tip fabrication procedure for STM and shear-force AFM based-TERS is electrochemical etching of gold/silver wires. The wire is dipped into an etching solution together with a counter electrode, e.g., ring shaped gold or a platinum wire [Fig. 2(a)]. An applied DC (or AC) voltage between these two electrodes induces the dissolution of the metal and leads to a sharpening of the wire. Several types of etching solution have been used for this method, which has been shown to produce tips having an apex diameter of ∼10 nm for both gold and silver.^44–51) Examples are shown in Figs. 2(b) and 2(c) for gold.^44,45) An EF of 10⁵–10⁶ has been achieved in a gap-mode TERS configuration in several research groups⁷⁵⁾ and indeed this type of tip has had a notable contribution to nanoscale studies. A key issue, however, is the low structural reproducibility. Figures 2(c)–2(e) show SEM images of gold tips fabricated with the chemical etching method: Gold wire with a diameter of 0.25 mm is submerged for 1–2 mm into an HCl/Ethanol solution (mix with volume ratio $1:1$), after which a DC 2.4 V is applied against the ring shaped counter gold (or platinum) electrode. Although identical conditions — including temperature and humidity — were used whilst etching, the resultant tips have a huge variation in structure, especially at the apex. From our experience, only a few out of 20 tips gives EF larger than 10³ [gap-mode TERS, benzenethiol-modified Au(111)]. We also found that tips having grain structures [as seen in Fig. 2(f)] tend to show strong scattering in the TERS spectrum range, probably due to an increase of surface roughness on the polycrystalline gold wire.^76–78) Such tips are difficult to use for TERS measurements due to a low TERS contrast and in addition make the effects of the tip on the image — the tip convolution — problematic to estimate.

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3.2. Vacuum-deposited noble metal tips

When TERS is operated under tapping-mode AFM, a commercially-available AFM silicon cantilever is coated with gold/silver using vacuum evaporation. The size and structure of the deposited metal at the very end of the tip (which can be approximated as a metal nanoparticle) is known to be affected by several factors, such as temperature, evaporation rate of metal, etc.⁸¹⁾ Figures 3(a)–3(c) show silver tips made using vacuum-deposition.⁵⁶⁾ As with chemically-etched noble metal wires, the end structure of the tip is known to vary significantly in size and structure, meaning that the structural reproducibility of tips made using this method is also low.^56,57,81) Further, this random geometry of the end structure not only gives variation in the TERS sensitivity (spatial resolution), but can lead to a variability in the direction of the dipole oscillation (polarization), leading to discrepancies in TERS imaging from tip to tip. Mino et al. have recently used this potential disadvantage for polarization-selective TERS.⁵⁶⁾ Figures 3(d)–3(f) represent a series of defocused scattering microscopic image of tips corresponding to Figs. 3(a)–3(c), respectively. Generally the specific pattern obtained from the defocused image represents the angular orientation of a dipole emitter/scatter in a three-dimensional (3D) manner. The different patterns in the defocused images indicate how the polarization properties varied from tip to tip. The dipole orientation can be estimated by comparing the defocused pattern to theoretical simulations [estimated values of horizontal (ϕ) and vertical (θ) angular parameters can be found from each defocused image]. Knowing the polarization property of the tip, polarization-selective TERS imaging can then be conducted.

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3.3. Other type of noble metal tips

As previously stated, a low tip production rate is an issue which needs to be overcome in the field of TERS. Although production of TERS tips having high structural reproducibility remains challenging, recently several groups have proposed techniques to improve it. One example has been demonstrated by Johnson et al., who used a so-called template-stripping fabrication technique^82–84) [Fig. 4(a)].⁵⁷⁾ The procedure was as follows: gold is deposited onto a silicon wafer template etched by a standard photolithography technique with an anisotropic silicon nitride mask. This is then chemically removed, before finally a pyramidal tip, having 10 nm diameter at the apex, is picked up and attached to a tungsten wire using a manipulator. The advantages of this technique are the high TERS reproducibility and mass fabrication: more than 95% of tips showed activity for near-field imaging, and 1.5 million identical probes can be made at once. Another example is presented by Yazdanpanah et al., who grew single freestanding nanoneedles of Ag₂Ga on AFM cantilevers, shown in Fig. 4(b).⁸⁵⁾ The tip is made by dipping a silver-coated AFM cantilever into a liquid Ga droplet at room temperature, followed by vertical retraction of the cantilever from the liquid Ga. They state that needles with diameters as small as 25 nm and lengths up to 33 µm can be produced with this method. This type of nanowire tip has some advantages due to its specific shape, e.g., they show a reduced tip convolution effect in scanned images, as well as showing applicability to biological samples, such as cells.^86–90) Indeed, this type of tip is currently commercially available although without a relevant data sheet.⁹¹⁾

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Another way to produce highly reproducible TERS probes is the attachment of chemically-synthesized noble metal nanoparticle/nanowires onto the tip apex. Such nanostructures are promising due to the following characteristics: (1) high controllability of size and shape,⁹²⁾ (2) massive production in one synthesis, (3) low surface polycrystallinity.⁹³⁾ One example of a spherical gold nanoparticle-terminated tip is shown in Fig. 4(c).⁹⁴⁾ The gold nanoparticle is positioned specifically at the tip apex of a silicon-nitride cantilever by dipping a (3-mercaptopropyl)trimethoxysilane pre-functionalized cantilever into a gold nanoparticle suspension. This chemistry based fabrication method allows very small-sized nanoparticles to be attached, which had been challenging with the so called colloidal probe technique⁹⁵⁾ (attachment of micro/nanosized particles using a manipulator and an optical microscope). These kinds of nanoparticle-terminated tips have been used not only for near-filed imaging,^96–100) but also to investigate particle–surface or particle–particle interaction, such as forces or surface potentials.^101,102)

Recently, our group have also developed chemically-synthesized silver nanowires tips for STM-/shear-force AFM-TERS microscopy [Fig. 4(d)].⁵⁸⁾ The silver nanowires were attached onto a pre-etched tungsten tip with an all wet-chemical process using AC-dielectrophoresis.^103,104) The attachment of several nanowires at the junction of the tungsten tip was key for stable STM operation, reducing vibration and increasing electrical conductivity. Thanks to the highly consistent crystal structure at the end of each nanowire, a very high TERS reproducibility has been reached — approaching 100%. Besides this, high resolution surface topographical imaging was possible, as can be seen in the atomic lattice image of graphite shown in Fig. 4(e).

Usually in TERS microscopy, incident light is illuminated at the apex of noble metal tips, to obtain enhanced Raman signals from molecules underneath the tip, via the local confinement of the electromagnetic field in this region. However, since the diameter of the tip apex (10–20 nm) is much smaller than the diffraction-limited spot size of the incident laser light (300–1000 nm, depending on numerical aperture of the objective used and the wavelength of the laser light), an "unnecessary" region on the sample surface is also illuminated and shows far-field Raman scattering or other emission, leading to a decrease of the observed TERS contrast and giving some restriction in sample preparation. In order to solve these issues, the use of "nano-focusing" tips has been proposed/demonstrated, with the help of the excitation of propagating surface plasmons along the tip. Although this approach is innovative, it requires a special type of tip, with several relying on electron beam techniques,^105–111) such as focused ion beam (FIB) milling.^{59,64,110,112–115)} Further, our group has recently fabricated silver nanowire based tips, which enable us to launch propagating surface plasmons along the nanowire, and allows surface-enhanced Raman scattering (SERS)-endoscopy⁹⁰⁾ and TERS⁶³⁾ using a remote excitation configuration to be conducted. This chapter will focus on these topics.

4.1. Adiabatic nano-focusing tips

Nanostructures which allow free-space light to be converted into surface plasmons, using a tapered gap,¹¹⁶⁾ hole array,^117–119) or grating^120,121) for instance, have attracted significant attention, and have shown applicability for highly integrated photonic circuits,^122,123) optical sensing,^124,125) trapping,^126–129) and manipulation.¹³⁰⁾ Recently, this technique/knowledge has been applied to near-field spectroscopy, such as TERS.^59–62) Nano-focusing has been achieved by directing the laser light to a coupling point placed away from the tip apex, allowing for confinement of surface plasmons at the end of the tip through the propagating photonic or plasmonic wave. Several types of light couplers made using electron beam techniques have been proposed, such as with a nanocavity,^110,111,131) photonic cavity¹⁰⁵⁾ or metal grating,^59,60) and are shown in Figs. 5(a)–5(e). The application of this type of tip to TERS was demonstrated by Berweger et al. in 2010.⁶⁰⁾ Here, chemically-etched gold tips were prepared with a grating coupler on the tip body using FIB milling [Fig. 5(e)] and TERS measurements were conducted by approaching the tip onto a dye film deposited on a flat gold surface. They found that a broad background in the TERS spectra was suppressed with grating illumination compared to direct apex illumination [Fig. 5(f)]. Also recently, this nano-focusing technique has been applied with an ultrafast pulsed-laser,^132–134) which may open the possibility to investigate nanoscale light-matter interactions with high temporal resolution.

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4.2. Remote excitation-SERS on silver nanowires

Besides nanostructures created by the electron beam techniques mentioned above, propagating surface plasmons are known to be able to be launched on noble metal nanowires.^135,136) Chemically-synthesised gold or silver nanowires have attracted particular attention in terms of plasmonics, due to the low plasmon loss¹³⁷⁾ and high electron conductivity^138,139) exhibited by these structures. This results from the good crystalline surface of the wires, which are in addition trivial and cost-effective to synthesize. Propagating surface plasmons can be launched along the nanowire, either by simply focusing light to its end (where it is considered that scattered light at the end couples to propagating surface plasmons), or by irradiating at a junction point, such as between a nanowire and nanoparticle or two crossed nanowires (excited localized surface plasmons in the gap between the two structures are converted into propagating surface plasmons). Our group has demonstrated that, by focusing laser light to the end of a nanowire or nanowire-nanoparticle junction, propagating surface plasmons can be excited along the nanowire and Raman scattering can be induced remotely (we call this "remote excitation") as shown in Figs. 6(a)–6(d).^140–142) We have also found that the broad background [indicated by the red dotted lines in Fig. 6(e)] ordinarily seen using a conventional excitation geometry is suppressed with a remote excitation configuration. This background suppression could be a result of the smaller signal contribution from any contamination or emitters around the detection spot with remote excitation than with direct excitation.

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Furthermore, recently we successfully obtained SERS signals from inside live-cells with the use of remote excitation without inducing any noticeable damage.⁹⁰⁾ For this, silver nanoparticles were attached onto silver nanowires, which were in turn fixed upon a pre-etched tungsten tip [Fig. 6(f)]. The tip is then inserted into a live-cell, and finally a remote-SERS signal from the nanowire apex was measured by focusing laser light to the position of the nanoparticle. As stated, this led to the launching of propagating surface plasmons on the silver nanowire [Figs. 6(g) and 6(h)]. This tip could in principle be a good candidate to conduct remote-excitation TERS, however, modification of the tip has been required due to several issues, such as random attachment of the nanoparticle position/number and low stability during approach.

4.3. Remote excitation-TERS with silver nanowire tips

As we mentioned above, remote excitation is a promising way to further improve the sensitivity of near-field spectroscopy. In the case of TERS, given the issue of tip fabrication reproducibility, the realization of remote excitation-TERS with silver nanowire based-tips would be significant, though well controlled fabrication of these tips is technically challenging. Here, we very recently developed a technique to fabricate silver nanowire tips allowing for remote excitation-TERS [Fig. 7(a)], based on an all wet chemistry process.⁶³⁾ A single silver nanowire was first adhered to a pre-etched tungsten tip with non-conductive glue using a micromanipulator, before gold nanoparticles, acting as a light coupling point, are site-specifically attached on the nanowire using AC-dielectrophoresis¹⁰⁴⁾ [Figs. 7(b) and 7(c)]. Although operation under STM feedback was not possible due to the low electrical conductivity between the nanowire and tungsten, surface imaging was possible under shear force-AFM feedback. The activity of remote excitation-TERS has been shown by approaching the tip onto a benzenethiol-modified flat gold surface under shear-force AFM feedback. Figure 7(d) shows normalized TERS spectra (at C–C stretching peak of benzene ring) with tip retracted (gray), with direct excitation (red), and with remote excitation (blue). As similar to the results with the tip fabricated by FIB milling, background suppression was observed. This indicates that the excitation volume has been confined to the tip apex, resulting in a higher TERS sensitivity.

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In this review, we have introduced the current progress made in the design of TERS tips. One of the biggest issues in TERS has been the low controllability of the morphology of the metal tip apex, especially when typical techniques for the fabrication of tips, such as chemical etching or vacuum deposition, are used. Thanks to extensive effort from researchers, methods that yield a very high reproducibility have now been demonstrated.^57,58) Commercialization of such tips would help TERS to emerge as a "standard" tool for nanoscale analysis. However, sophisticated techniques, such as e-beam lithography, are often required for tip fabrications. Mass production of such tips thus still remains challenging.

Beside the issue of tip production, massive backgrounds resulting from far-field scattering/emission are known to disturb TERS contrast (low signal-to-noise ratio), especially when a gap-mode substrate is unavailable. This has been a big problem for samples such as strained silicon. To solve this, nano-focusing tips, i.e., excitation of surface plasmons at the apex via propagating surface plasmon, have been proposed.^59–63) Although special fabrication methods for tips are required, this is a promising way to obtain higher TERS sensitivity by separating the surface from the focal plane of the incident laser light. This new generation of tip is of interest for further increasing the sensitivity and applicability of TERS and related near-field optical techniques.

This work was supported by the Fonds voor Wetenschappelijk Onderzoek FWO (G.0B55.14, G081916N, G.0B94.13, G.0259.12) and the KU Leuven Research Fund (BOF-OT, BOF-C14). The financial support of the Belgian Federal Science Policy Office (IAP-VI/27), Morino foundation and the Japan Science and Technology Agency (JST) PRESTO program are gratefully acknowledged. The research leading to these results has also received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC Grant Agreement PLASMHACAT (No. 280064) to HU and Grant Agreement NANOGRAPH@LSI (No. 340324) to SDF.