Advances in Tip-Enhanced Near-Field Raman Microscopy Using

Review pubs.acs.org/CR

Advances in Tip-Enhanced Near-Field Raman Microscopy Using Nanoantennas Xian Shi,† Nicolás Coca-López,† Julia Janik, and Achim Hartschuh* Department of Chemistry and Center for NanoScience (CeNS), LMU Munich, 81377 Munich, Germany ABSTRACT: Tip-enhanced near-field Raman microscopy spectroscopy is a scanning probe technique that is capable of providing vibrational spectroscopic information on single nanoobjects and surfaces at (sub-) nanometer spatial resolution and high detection sensitivity. In this review, we first illustrate the physical principle of optical nanoantennas used in tip-enhanced near-field Raman microscopy and tip-enhanced Raman scattering (TERS) to efficiently couple light to Raman excitations on nanometer length scales. Although the antennas’ electric near-field distributions are commonly understood to determine the spatial resolution, recent experiments showing subnanometer-resolved optical images put this understanding into question. This is because such images enter a regime in which classical electrodynamical descriptions might no longer be applicable and quantum plasmonic and atomistic effects could become relevant. After summarizing the current understanding of plasmonic phenomena at extremely short length scales, we discuss the different mechanisms contributing to the signal enhancement. In addition to the known contributions from electric-field and chemical enhancement, several new models have been proposed very recently that could provide important guidelines for the optimization of TERS experiments. We then review recent developments in the areas of antenna design, fabrication, and characterization. Finally, we briefly highlight recent applications to illustrate future directions of tip-enhanced near-field Raman microscopy and TERS.

CONTENTS 1. Introduction 2. Principles of Optical Antennas 2.1. Characteristics of an Optical Antenna 2.2. Near-Field of Optical Antennas 2.3. Gap-Mode Configurations 2.4. Quantum and Atomistic Effects 3. Signal Enhancement 3.1. Signal Enhancement Based on Electromagnetic Field Enhancement 3.2. Other Proposed Enhancement Mechanisms: Beyond f4 3.2.1. Optomechanical Coupling 3.2.2. Electric Field Gradient 3.2.3. Self-Interaction 4. Nanoantenna Design, Fabrication, and Characterization 4.1. Nanoantenna Design and Fabrication 4.2. Nanoantenna Characterization 4.2.1. Chemical Composition 4.2.2. Crystallinity 4.2.3. LSPR Energy 4.2.4. Near-Field Polarization 5. Applications of Nanoantennas in Tip-Enhanced Raman Microscopy 6. Summary Author Information Corresponding Author ORCID © XXXX American Chemical Society

Author Contributions Notes Biographies Acknowledgments References

A B B C D D F

M M M M M

1. INTRODUCTION Since the first reports about 16 years ago, tip-enhanced Raman spectroscopy (TERS) has been continuously developed and is now a powerful technique that is capable of providing vibrational information down to subnanometer spatial resolution and single-molecule detection sensitivity. This development, together with that of tip-enhanced near-field optical microscopy (TENOM) in general, has been described by a number of review articles.1−10 Tip-enhanced near-field Raman microsocopy exploits the local signal enhancement obtained in TERS to form high-resolution images of surfaces by raster scanning the tip. Aside from tip-enhanced near-field Raman microscopy, several other techniques have been shown to provide vibrational information on the nanoscale, including (elastic) scattering scanning near-field optical microscopy (sSNOM),11,12 photoinduced force microscopy (pi-FM),13 and photothermal atomic force microscopy (AFM),14 together with inelastic electron tunneling spectroscopy.15,16

F G G G G H H K K K K K K L M M M

Special Issue: Vibrational Nanoscopy Received: September 15, 2016

A

DOI: 10.1021/acs.chemrev.6b00640 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. Left: Optical antennas convert freely propagating electromagnetic radiation in the far-field to strongly localized near-fields and vice versa. Right: An optical antenna can be described by its functional parameters aperture and directivity. Both parameters typically depend on the frequencies of the incident and scattered light. From a functional perspective, the antenna is formed by both the scanning probe and the sample substrate.

considers the antenna and sample as independent objects, does not seem to capture the whole physics of the signal enhancement. We briefly review several newly developed signal-enhancement concepts, including the one based on optomechanical coupling, that have been put forward to explain the subnanometer spatial resolution obtained in gap-mode TERS and the nonlinear power dependence of the antennaenhanced Raman signal (see Figure 7, below).20,21 We then provide an overview of different nanoantenna/tip structures and their fabrication and characterization in sections 4.1 and 4.2 and finally describe recent applications of tip-enhanced Raman microscopy in section 5 that help to illustrate possible future directions.

Because of its direct connection to surface-enhanced Raman spectroscopy (SERS) and plasmonics in general, TERS has benefited strongly from the enormous developments in these areas. Examples include the development of the concept of optical antennas, the design and application of novel antenna structures, and the theoretical modeling of the resulting plasmonic near-fields. In addition, greatly improved nanofabrication capabilities enable the realization of new nanoantenna designs that can be used in a scanning-probe approach.17−19 These developments have been accompanied by an improved understanding of nano-optical fields at very short length scales and different signal-enhancement mechanisms. In the present review, we first illustrate the working principle of tip-enhanced near-field Raman microscopy, which is based on the antenna function of the scanning tip to convert propagating electromagnetic radiation into local energy of a nearby sample object and vice versa (see Figure 1). After introducing the principles of optical nanoantennas in section 2.1, we describe the electrical near-field distribution of nanoantennas that is the origin of subdiffraction spatial resolution in section 2.2. From the perspective of its function, an optical antenna not only is formed by the tip itself, but also includes the sample substrate and, potentially, the sample itself. Particular emphasis is focused on configurations in which the tip is placed on top of a metallic surface allowing for the formation of highly confined plasmonic gap modes. Although the response of the metal plasmonic nanostructures in most of the theoretical modeling of electromagnetic fields has been described using an effective dielectric constant, in recent years, it became clear that, for very small distances, quantum and atomistic effects become essential. This is particularly evident in the case of the gap mode in which nonlocal screening and electron tunneling strongly affect the plasmonic response (see section 2.3). In section 3 of this review, we discuss the signal enhancement provided by the nanoantenna. Based on its close connection to SERS, for which electromagnetic field enhancement and so-called chemical enhancement have been identified as the two mechanisms contributing to the signal enhancement, the same is expected for TERS. The electromagnetic field enhancement scales approximately with the fourth power of the field-enhancement factor f that relates the local electric field at the nanoantenna to the incident electric field. Recently, it became clear that this “f4 rule”, which

2. PRINCIPLES OF OPTICAL ANTENNAS 2.1. Characteristics of an Optical Antenna

Tip-enhanced Raman spectroscopy exploits the extremely short-range near-field interaction between a pointed probe and the sample. The task of efficiently coupling the tip’s nearfield to the far-field, in which the laser source and the detector are located, is the function of an antenna. More specifically, an optical antenna is defined as an object that converts freely propagating optical radiation into localized energy and vice versa17−19,22 (see Figure 1). For a comprehensive characterization of an optical antenna, classical antenna theory can be used. To simplify the description of the antenna parameters, a dipolelike behavior of receiver and transmitter is typically assumed. Two antenna-enhanced processes, namely, absorption and emission of light, have to be considered to obtain a complete descripton of the functioning of an optical antenna. Enhanced absorption can be quantified in terms of the antenna aperture A, which corresponds to the absorption cross section σ in the presence of the tip. It describes the efficiency with which the incident radiation is captured to excite the receiver with the power Pexc, where I is the intensity of the radiation with the polarization npol incident from the direction (θ,ϕ) correspondP ing to A(θ , ϕ , n pol) = exc = σA(θ , ϕ , n pol). The tip enhances I the field at the absorber, and by defining the field in the absence of the tip as E0 and in the presence of the tip with E, the absorption cross section becomes

σ = σ0 B

|n p·E|2 |n p·E0|2

(1) DOI: 10.1021/acs.chemrev.6b00640 Chem. Rev. XXXX, XXX, XXX−XXX

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geometrical singularity represented by the tip and leads to the spatial confinement of the surface charge density at the apex of a tip or tiplike structure.27 This process is essentially nonresonant and will depend primarily on the electrical conductivity of the tip material at the frequency of light used in the experiment. Second, the excitation of localized surface plasmon resonances (LSPRs) in metallic nanostructures such as nanospheres and nanorods depends on the frequency of incident and Stokes-scattered light. Third, length-related antenna resonances occur if the length of the antenna is a multiple of one-half of the wavelength of light. At optical frequencies, metals are not perfect conductors, and therefore, an effective scaling relation has to be used to relate the effective wavelength λeff to the incident wavelength λ. The effective wavelength shows a linear behavior with the plasma wavelength ⎛λ⎞ of the metal λp of the form λeff = n1 + n2⎜ λ ⎟,28 where n1 and ⎝ p⎠ n2 are constants that depend on geometry and dielectric properties.

Here, np is the orientation of the absorption dipole, and the subscript 0 indicates the absence of the antenna. Equation 1 shows that the absorption process depends on the incident direction and polarization of light. Neglecting the vectorial character or assuming the same direction of enhanced and nonenhanced fields, the absorption enhancement can then be expressed by the local field-enhancement factor f = E/E0 as σ/ σ0 = f 2. For metallic nanoantennas, particularly strong local fields can result in the optical regime from localized surface plasmon resonances, collective electron oscillations that are controlled by their shape and their material composition (see below). A practical way to characterize the enhanced emission is the antenna efficiency, which is defined in eq 2. It describes the ratio between the total power dissipated by the antenna P and the radiative power Prad. The total power P is composed of Prad and the power dissipated through other channels, for example, heat Ploss.19 ϵrad =

Prad Prad = P Prad + Ploss

2.2. Near-Field of Optical Antennas

(2)

The antenna’s near-field distribution determines the spatial resolution, the signal enhancement, and the observed image contrast. In general, the enhanced fields of the optical antenna are highly dependent on the shape, material composition, and structure of the antenna−substrate system. In most cases, the resulting near-field distribution is strongly nonuniform in terms of magnitude and polarization as well as phase. Numerous theoretical studies have been performed to characterize the field distribution and the corresponding enhancement for different conditions.29−33 The parameters mentioned in the preceding section show that the field-enhancement factor depends on the directivity D and the polarization of the incoming light. For semi-infinite conelike tip structures, field enhancement is highest when the polarization is parallel to the long axis of the antenna.29,31,34 However, the tip also modifies the polarization with respect to the incoming field. Figure 2 shows the simulated distributions

The antenna’s ability to emit the radiated power into a certain direction can be measured by the directivity D. It considers the angular power density p(θ,ϕ) with θ and ϕ as the angles of the direction of observation following 4π D(θ , ϕ) = P p(θ , ϕ). By taking the polarization into account, rad

one can define the partial directivities as Dθ (θ , ϕ) =

4π 4π p (θ , ϕ), Dϕ(θ , ϕ) = p (θ , ϕ) Prad θ Prad ϕ

(3)

with pθ and pϕ as normalized angular powers. The antenna gain G combines the efficiency and directivity of an antenna to yield the radiation relative to the total power as G(θ,ϕ) = ϵradD(θ,ϕ). Absorption and emission at the same optical frequency can be related following the reciprocity theorem. This leads to the following relationship between the excitation rate Γexc and the spontaneous emission rate Γrad Γexc, θ(θ , ϕ) 0 Γ exc, θ (θ ,

ϕ)

=

Γrad Dθ (θ , ϕ) Γ 0rad Dθ0(θ , ϕ)

(4)

The index θ refers to one polarization state, but it could also be denoted by ϕ, which corresponds to a rotation of the polarization by 90°. Neglecting the vectorial character, the enhancement of the rate of spontaneous emission can be expressed by the local field-enhancement factor f = E/E0 as f 2 = Γrad/Γ0rad corresponding to the relationship for the absorption 0 rate enhancement f 2 = Γexc/Γexc . Various studies have investigated the field-enhancement factor for different antennas and configurations, and values in the range of f = 2−5 for single spherical particles up to f = 1000 for optimized antennas have been reported.23−26 Importantly, the discussed antenna parameters will show a pronounced frequency depedence in most cases that can vary substantially between the indicent and emitted frequencies. Examples include resonant antenna structures that feature distinct spectral modes such as metallic nanospheres or nanorods. In general, three different contributions to the local field enhancement provided by an optical antenna are currently discussed. First, the lightning-rod effect is due to the

Figure 2. (a) System of reference for the (b) x and (c) z components of the field enhancement ξx,z = |Ex,z|/|E0| on the substrate plane (z = 0) for a silver tip with resonant excitation (λ = 505 nm) and a tip− substrate distance of 2 nm. Calculated by Demming et al.30

of the x and z components of the field enhancement in the x−y plane. Here, the heterogeneity of the near-field becomes visible. Whereas the z component (Figure 2c) is highly localized beneath the tip, the x component (Figure 2b) shows two weaker lobes. In particular, the in-plane modes result in a spatially heterogeneous polarization that can complicate the contrast formation. Recently, several studies were performed on samples such as carbon nanotubes35,36 and transition-metal dichalcogenide monolayers,37,38 taking advantage of the heterogeneous polarization of the tip to probe and distinguish differently polarized C


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transitions. Moreover, the polarization sensitivity of both the antenna aperture and directivity can be exploited for polarization-selective enhancement.39,40 For dielectric sample substrates, the confinement of the local fields reflects the geometrical size of the tip. Because the Raman enhancement scales approximately with the fourth power of the field enhancement (see section 3), the spatial resolution obtained in a TERS experiment on dielectric substrates is typically somewhat smaller than the topographic resolution.6,41 Given that the tip’s near-field decays exponentially with the distance between the tip and the substrate, this separation has to be taken into account and should be as short as possible. An increase in tip−substrate distance will lead to weaker fields and a loss of resolution and enhancement.32,42 For dielectric substrates, Raman signal-enhancement factors in the range of 102−107 have been found.4 Depending on the experimental geometry, nontransparent substrates can also be used.6 In the case of metallic substrates, electromagnetic coupling between the surface charges in the tip and the induced charges in the substrate takes place, and for small distances less than 10 nm, a coupled mode can arise, the so-called gap mode (see section 2.3), that can confine the plasmonic fields substantially below the geometrical size of the tip.

Figure 3. Lateral confinement w of the intensity |E|2 obtained as the full width at half-maximum from cross sections through simulated field patterns 0.2 nm from the apex for different tip−sample spacings d. A square-root model for the gap plasmon extent, w = A Rd , was fitted to the data with tip−sample spacings of d ≤ 2 nm, shown as the solid red line. For R = 10 nm, an optimum value of A = 1.57 was found. The red dashed line represents a constant fit to the data with tip−sample spacings of d ≥ 100 nm. Adapted with permission from ref 49. Copyright 2016 American Chemical Society.

resolution of better than 2.5 nm appears to be feasible, because tapers with R ≤ 5 nm can now be achieved.49 For smaller distances, the onset of quantum tunneling sets a lower limit on w, reaching a minimum at about d ≈ 0.3 nm (see section 2.4).56 Aside from Au and Ag, other substrate materials have been investigated. Using finite-difference time-domain and finiteelement simulations, Stadler et al.48 estimated the electromagnetic field enhancements for a wide range of substrates to assess how well a substrate of choice would perform in a gapmode TERS experiment. Even though the electromagnetic field strength |E|2 for many metals would be reduced in comparison with that of Au (according to calculations,48 Cu, 92%; Ag, 81%; Ni, 53%), this opens the possibility of using a broad range of materials, allowing for a more widespread application of the technique. Importantly, the spectral properties of the gap mode that arises from the coupling between tip and substrate plasmons show a pronounced distance dependence, as can be seen from the spectral shifts of the plasmon-mediated background emission detected in TERS.46,55,57 In experiments, special care has to be taken in choosing the excitation laser wavelength with respect to the actual gap plasmon energy to maximize resonance phenomena.20,58 Moreover, tip−sample-distancedependent measurements of the Raman scattering intensity for a fixed laser excitation energy could be influenced by the simultaneously occurring spectral shift of the gap plasmon. Although the gap-mode configuration is clearly advantageous with respect to optimum spatial resolution and detection sensitivity, its applicability is expected to be limited to extremely thin sample materials with thicknesses of less than 10 nm that allow for substantial coupling between tip and substrate plasmons.

2.3. Gap-Mode Configurations

For dielectric sample substrates, the spatial resolution observed in TERS is mostly determined by the geometrical size of the tip and the tip−sample distance, which defines the lateral extent of the enhanced fields (10−20 nm).41,43−45 From a functional perspective, on the other hand, the concept of an antenna as a (metal) structure that converts freely propagating optical radiation into localized energy and vice versa can be directly extended to include the sample substrate as well. Of particular importance are the so-called gap-mode configurations (see, e.g., refs 20 and 46−49), in which the tip is placed above a metallic sample substrate separated by an extremely small gap distance on the order of 1 nm. Upon illumination of the tip with a focused laser beam, a surface charge density is induced in the tip apex that can be approximated by a point dipole. Because of the proximity of the tip and metal substrate, mirror charges are accumulated at the surface of the metal substrate that can be described by an image dipole.50 Upon the approach of the tip, both the tip-apex plasmon mode and the surface plasmon modes cease to exist independently, and a new hybrid mode forms, the so-called gap plasmon mode49 (see Figure 3). Compared to dielectric substrates that do not support the formation of a gap mode, far stronger confinement of the field is obtained in the case of metallic substrates, leading to substantially improved spatial resolution4,33,49,51 (see Figure 3). At the same time, far higher field-enhancement factors can be achieved, which relates to the well-known observation in SERS that large enhancements are observed for interstitial sites and small nanogaps, for instance, in metal particle dimers.52−54 As a result, gap-mode configurations can be used to confine light to dimensions that are not limited by the diameter of the tip. The lateral confinement of gap plasmons was estimated for tunnel junctions between taper probes and flat surfaces.55 According to this rule, the lateral confinement w = A Rd is proportional to the square root of both the probe’s apex radius of curvature R and the gap separation d (see Figure 3). Therefore, for d = 0.5 nm (at the limits of the classical electrodynamics regime; see section 2.4), a lateral optical

2.4. Quantum and Atomistic Effects

A major question in TERS that is yet to be solved is whether there is a physical limit to the spatial confinement and the enhancement of the electromagnetic field generated by the nanoantenna. From the classical electrodynamic description outlined in sections 2.2 and 2.3, both electric-field enhancement and confinement are predicted to continuously increase with decreasing tip−sample distance. This would suggest the D


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simple rule that, to improve spatial resolution and signal enhancement, the distance needs to be reduced. However, as the distance enters the nanometer and then the subnanometer range, the quantum nature of the electrons will become important, which can significantly alter the plasmonic response of the system.59 Specifically, quantum mechanical treatments, such as the jellium model, show that the probability density of electrons extends outside metal surfaces, decaying exponentially with distance.60,61 This spill-out of electrons allows for electron tunneling accross junctions at separations smaller than ∼1 nm, thereby quenching the local field enhancement provided by the gap plasmon59 (see section 2.3). At the same time, a chargetransfer plasmon can form with different spectral characteristics. Another quantum phenomenon that could influence the plasmon resonance is the nonlocal screening of electric fields. Nonlocal screening62 refers to the fact that, because of electron−electron interactions, the motion of the conduction electrons at a given point in space depends not only on the field applied at that position but also on fields at other locations.59 The nonlocal screening prevents sharp charge localization at interfaces and is therefore important for small particles63 and narrow gaps.64 Time-dependent density functional theory (TDDFT) calculations65 showed that the plasmon-induced surface charge density is smeared out over quite a broad region of about 10 Bohr around the surface. The width of the smeared plasmon-induced electron density is determined by the electron spill-out. An increased spill-out density will introduce a further smearing of the induced electron density and can thus reduce the field enhancements even further, relative to classical results. As an example, the authors calculated that the maximum field enhancement for a silver sphere was reduced by 33% in comparison with the classical electromagnetic theory. For elongated structures such as a silver nanorod with an aspect ratio of 3, the reduction increases to 47%, as can be seen in Figure 4a,b. In the case of gap configurations (see section 2.3), electron spill-out and the associated nonlocal screening will create an “effective” gap distance that also differs from the geometrical value (see ref 59 and references therein). For gap distances of less than 1 nm, electron tunneling across the gap at optical frequencies will further modify the plasmonic response of the system with respect to the classical description. Because of the screening of localized surface charges by quantum tunneling and a consequent reduction in the plasmonic coupling, the resonances shift toward the blue. This phenomenon explains the observed smooth transition of the plasmonic response upon variation of the geometry from a subnanometer gap to touching metal surfaces and has been extensively studied for the case of plasmonic dimers.59 Electron tunneling provides an effective “charge-transfer” channel, neutralizing the bonding-plasmoninduced charges of opposite signs across the gap and thus quenching the field enhancement.65 Moreover, such electron tunneling implies a resistance that broadens the plasmon resonances. As the bonding modes vanish, charge-transfer plasmons are established because the two nanoparticles forming the plasmonic dimer are conductively connected through electron tunneling.56,67 This has important consequences for TERS because electron tunneling at the optical frequency is expected to efficiently quench the plasmonic near-field enhancement for extremely small distances, thus limiting the achievable signal enhancement.68

Figure 4. (a) Comparison of the field distributions calculated using classical electromagnetic theory (left panel) and TDDFT (right panel) for a Ag nanorod with an aspect ratio of 3 and a length of 100 Bohr that contains 510 conduction electrons. (b) Electric-field enhancement as a function of position d around the particle surface for an aspect ratio of 3. The classical calculation is shown as a red line, and the TDDFT result is shown as a black line. The comparison shows strong deviations for distances shorter than 0.5 nm due to electron spill-out. Reproduced with permission from ref 65. Copyright 2010 American Chemical Society. (c,d) Local induced-field enhancement in the y−z plane that contains the axis of a Na metallic dimer for the case of a tipto-facet configuration at the gap with a separation of 0.6 nm. The incident plane wave is polarized along the dimer axis. The field is plotted at the frequency of the most intense resonance in the absorption cross section of the dimer. Reproduced with permission from ref 66. Copyright 2015 American Chemical Society.

In addition to the quantum nature of electrons, the detailed atomistic structures of the tip and the sample substrate will also become relevant on nanometer and subnanometer length scales. Using sodium clusters as an example, Barbry et al. presented TDDFT calculations in which atomic-scale features were taken into account together with the nonlocal screening of conduction electrons.66 The authors found that atomic features such as protruding atoms do localize electromagnetic fields down to atomic-scale dimensions, showing resonant (plasmonic) and nonresonant (lightning-rod effect) field enhancement and beating the typical plasmonic confinement imposed by the nanoparticle size. Figure 4c,d illustrates the electrical near-field at atomic-scale features in a Na particle dimer with a distance of 6 Å, yielding a field confinement below 1 nm. These results show that, for the case of gap plasmons, the exact atomistic structure of the junction determines the appearance of extremely localized “hot spots”. However, in large plasmonic systems, the presence of subnanometric hot spots can affect the effective localization area only weakly, because this area will be dominated by the overall plasmonic field structure in the gap.66 In a very recent theoretical study on plasmon-enhanced microscopy, Trautmann and co-workers suggested that protrusions as small as a single atom found on plasmonic particles might act as probing tips.69 They proposed a classical approach in which an entire nanostructure is divided into a simple base structure that determines the plasmon response, such as a metal sphere, and an atomic-scale substructure that mimics a more realistic surface topography. In recent SERS experiments, Benz et al. showed that the light-activated mobilization of surface atoms in a plasmonic hot spot can E


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trigger the formation of additional “picocavities” bounded by a single gold atom.70 These picocavities, spontaneously formed and destroyed under laser illumination, could result in an extreme optical confinement yielding a factor of 106 enhancement of optomechanical coupling between the picocavity field and vibrations of individual molecular bonds (see also section 3.2). The authors suggested that picocavities are always present in SERS and TERS measurements on nanoscale plasmonic hot spots and could be responsible for the single-molecule and atomic resolution obtained in experiments.70 In summary, both quantum and atomistic effects are expected to determine the plasmonic response, namely, field confinement and enhancement, at extremely small distances. Although the quantum phenomena discussed above are expected to limit the field enhancement and confiment and, thus, the observed Raman enhancement in general, atomic-scale features appear to have the potential to reverse this effect. Atomistic features at both the tip and the substrate surface will have limited stability, particularly at room temperature, such that subtle morphological changes could give rise to substantial variations in the plasmonic properties of the antenna over time. The same could be expected for the short-range quantum phenomena. At present, the details of the interplay between quantum and atomistic effects and their actual contribution to the spatial resolution and signal enhancement observed in experiments are not yet fully clear.

SRaman, θ =

1 4π

2π

∫0 ∫0

π

Γexc, θΓradDθ ηθ sin θ dθ dϕ

(5)

The ϕ integration can be limited to the maximum collection/ detection angle ϕm = a sin(NA/n) determined by the numerical aperture NA of the microscope objective and the refractive index n of the enclosing medium. In an experiment, the excitation profile and the angular detection range of the optical microscope should match the antenna aperture and directivity.6,73 Antenna-enhanced Raman spectra can differ substantially from their far-field counterparts. In both cases, the emission spectrum reflects the spectrally varying spontaneous emission rates Γrad connecting the same virtual state to different final vibrational states. Following the discussion in section 2.1 and noting the frequency dependence of the antenna gain G, the antenna can thus substantially modify the spectral shape of emission. In the case of sharp antenna resonances and emission with large spectral bandwidth, this effect will become particularly relevant. Often, the angular and polarization dependence in eq 5 is neglected, together with the vectorial character of the electric fields (eq 1). As a consequence, the total signal enhancement M is found to scale approximately with the fourth power of the field enhancement for small differences between the excitation and emission wavelengths, assuming that the field enhancement at the tip does not depend sensitively on the wavelength MRaman ≈

3. SIGNAL ENHANCEMENT As discussed in section 2, it is the antenna function of a tip that enhances the Raman signal of the sample by increasing both the excitation rate and the spontaneous emission rate, leading to the well-known scaling of the signal enhancement with the fourth power of the field-enhancement factor f4. In addition to this electromagnetic enhancement, which is reviewed in more detail below, chemical enhancement mechanisms, which were also reported for SERS in very early studies, can occur for direct tip−sample contact or extremely small distances.71 Several studies have also shown the possibility of improving the spatial resolution and signal intensity by exerting a pressure on the sample using the tip.72 More recently, the signal enhancement in TERS and SERS again became a topic of great interest, presumably because of the improved theoretical understanding of the electric fields on very short length scales in the context of quantum plasmonics (see section 2.4) and triggered by the ground-breaking results achieved in ref 20. Here, both the achieved subnanometer spatial resolution and the observed nonlinear power dependence of the Raman signal could not be directly explained by conventional electromagnetic models. After reviewing the principles of electromagnetic field enhancement, we thus provide an overview of more recent developments that go beyond the f4 rule.

Γexc Γrad 0 Γ exc Γ 0rad

≈ f4 (6)

For the general case of surface-enhanced Raman scattering (SERS), Raman enhancement factors reaching up to 12 orders of magnitude have been reported for colloidal particles, rough metal films, and particular multiple-particle configurations involving interstitial sites between particles or outside sharp surface protrusions.52−54 Because the signal scales approximately with the fourth power, even a moderate local field enhancement, predicted for a single spherical particle to be in the range of f ≈ 2−552,74 depending on its size and composition, is sufficient for significant signal enhancement. Overviews of the reported field-enhancement and signalenhancement factors for different experimental configurations are provided in refs 4, 6, and 48. A semiquantitative analysis of the signal levels that can be expected in TERS from different sample materials and in different experimental geometries was provided in ref 73. The authors discussed how high a signal level could actually be expected within the physical limitations set by the field enhancement, damage threshold, tip scattering efficiency, collection and detection efficiencies, and Raman scattering cross section of the sample material for different experimental configurations. So far, the discussion of the electromagnetic field enhancement has focused on the signal generated by a single point dipole or point polarizability and would thus be directly applicable to single molecules. For spatially extended samples, such as graphene or semiconductor nanostructures, the Raman response (i.e., the signal intensities) could, to a first approximation, be integrated over the sample volume probed in the near-field. On the other hand, in the case of spatially coherent inelastic scattering caused by the excitation of a phonon mode in a crystalline material, for example, the summation would need to be performed over the scattered

3.1. Signal Enhancement Based on Electromagnetic Field Enhancement

In the case of spontaneous Raman scattering, the detected signal depends on the product of the transition rates Γexc,θ(θ,ϕ) Γrad, the directivity Dθ(θ,ϕ), and the detection efficiency ηθ(θ,ϕ) of the experimental setup. Furthermore, all of these quantities are wavelength-dependent. The total signal at a given wavelength then results from angular integration F


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fields rather than the intensity. In ref 75, Cançado and coworkers developed a theory for the role of spatial coherence in near-field Raman scattering that they applied to graphene. In addition, in discussing the influence of spatial coherence, the authors also considered the total sample response as the coherent sum of the scattered fields generated through different sequences of interactions involving the tip and the Raman scatterer. More specifically, the signal was described as the sum of the field scattered by the sample only, the field scattered by the sample but emitted by the tip antenna, the sample-scattered field after excitation by the tip, and the corresponding field also emitted by the tip.75 From the discussion, the authors predicted different tip−sample distance dependencies of the Raman signals of graphene, which they confirmed by TERS experiments and extracted the spatial correlation length, which was found to be on the order of 30 nm.75,76 A treatment of SERS that also considered the different sequences of light−plasmon− Raman scatterer interactions in terms of a higher-order Raman process was recently presented by Mueller et al.77 Model calculations of the Raman scattering cross section for a molecule coupled to the dipolar plasmon mode predicted an enhancement of the cross section up to 3 orders of magnitude stronger than what would be expected from a conventional electromagnetic treatment.

will be transferred to the optical cavity, and the mechanical vibrations will be quenched. Alternatively, amplification of the mirror vibrations will occur for a blue-detuned illumination. Considering an inelastic off-resonant Raman process as an optomechanical system in which molecular vibrations modify the energy of a nearby plasmonic cavity, a direct analogy between the optomechanical back-action occurring in optical cavities and the interaction of molecular vibrations and localized plasmons can be found.78 This model suggests that the vibrations of a molecule can be amplified in SERS by illuminating a plasmonic cavity with a blue-detuned laser, just as the heating of mirror vibrations in an optomechanical cavity can be induced by a similarly detuned laser, resulting in an increase of the Raman signal.78,80 The hallmark of optomechanical coupling is a nonlinear increase of the Raman signal with increasing intensity of the laser excitation that leads to stronger signal and higher spatial resolution. This new model points out the need to consider the relative position of a vibrational band with respect to the energy of the plasmonic cavities and the incident laser.78 Complementing this approach, Schmidt et al. employed a fully quantum electrodynamical (QED) description of the coherent interaction of plasmons and molecular vibrations that also reveals the emergence of nonlinearities in the inelastic response of the system.79 They predicted the onset of phononstimulated Raman scattering and a counterintuitive dependence of the anti-Stokes emission on the excitation frequency. 3.2.2. Electric Field Gradient. The influence of an electric field gradient associated with strongly localized fields on the Raman response was addressed in ref 81 both theoretically and experimentally using aperture SNOM. Based on theoretical calculations, Meng et al. suggested that the ultrahigh spatial resolution of TERS can be partially attributed to the electric field gradient effect because of its tighter spatial confinement and sensitivity to the infrared- (IR-) active modes of molecules.82 In ref 83, Duan et al. reported a quantum mechanical description of the interaction between a molecule and a highly confined plasmonic field considering the nonuniformity of the electric field on the length scale of the molecule to higher terms. With this approach, the authors were able to model the molecular TERS images published by Zhang et al.20 3.2.3. Self-Interaction. In refs 84 and 85, Zhang et al. pointed out that, for very small gap sizes comparable to the size of the molecule, inelastic scattering of the molecule becomes so strong that the molecular responses to both excitation and the Stokes-scattered field have to be considered. This so-called selfinteraction process can strongly modulate the Raman response of the system with respect to signal intensity and spatial sensitivity. Based on this discussion, the authors attributed the subnanometer resolution achieved in Raman mapping to the strong optical coupling of the molecule with its plasmonic nanogap environment through multiple elastic scattering. In conclusion, the past two years have seen an enormous effort put toward understanding the dominant contributions to the signal enhancement in TERS. At present, the origin of the subnanometer spatial resolution is still under discussion, and the relevance of both quantum plasmonic and atomistic effects (see section 2.4) and the newly proposed mechanisms for signal enhancement has yet to be clarified.

3.2. Other Proposed Enhancement Mechanisms: Beyond f4

3.2.1. Optomechanical Coupling. In nearly all theoretical discussions of SERS and TERS, the optical antenna and Raman scatterer are treated as independent entities. More recently, both Roelli and co-workers78 and Schmidt and co-workers79 studied the coupling between the two. In a microscopic view, the Raman scatterer represents a polarizability that is periodically modulated at its vibrational frequency. This oscillating polarizability will, in turn, modulate the energy of the plasmon resonance of the metallic nanostructure, which couples back to the vibrational excitation, inducing a nonlinear feedback. For plasmon frequencies substantially larger than the vibrational frequency and in the absence of electronic resonances in the sample material, the shift of the plasmon frequency can be described to first order as ωp(xv) = ωp − Gvxv, where xv describes the displacement of the Raman scatter in mode v and Gv determines the optomechanical coupling rate. This rate scales with the Raman polarizability of the scatterer δα/δxv and the mode volume of the optical cavity Vm according

( )

to Gv = ωp

δα δxv

1 . Vm ϵ0

Although the quality factor of plasmonic

resonances is typically low (Q ≈ 10) because of efficient radiative and nonradiative damping, this can be compensated in part by the extremely small mode volume represented by the plasmonic fields.78 The corresponding quantum mechanical Hamilton operator of the coupled system is equivalent to that originally developed within the framework of cavity optomechanics. In this context, a typical optical cavity is formed by two highly reflective mirrors, with circulating photons inside exerting a radiation pressure on them. Through the connection of one of the mirrors to a mechanical oscillator, such as an AFM cantilever that acts as a spring, the optical force can be translated into a mechanical excitation. The mechanical motion shifts the resonance frequency of the cavity, modifying the intensity of the circulating light and, therefore, the radiation pressure. This feedback mechanism, termed optomechanical back-action, can amplify or quench the mirror vibrations. In a cavity illuminated by a red-detuned laser, the vibrational energy G


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4. NANOANTENNA DESIGN, FABRICATION, AND CHARACTERIZATION 4.1. Nanoantenna Design and Fabrication

Optical nanoantennas for scanning probe microscopy come in a huge variety, involving different designs, materials, and fabrication techniques, and are continuously being developed further. In this section, we first highlight the principles governing antenna designs and then summarize the main methods of antenna fabrication for tip-enhanced microscopy with an emphasis on the developments in the past few years. Generally, the following factors need to be considered in the design of nanoantennas: Antenna Aperture and Directivity. Both of these parameters describe the capability and efficiency of the antenna to couple its optical near-field to the far-field. In general, the two parameters depend on the angle of the incident and scattered light, the polarization, and the frequency. Field Enhancement. For a metallic antenna, there are three sources of field enhancement: the lightning-rod effect and the localized surface plasmon resonance (LSPR) as the two main contributions and, in specific cases, length-related antenna resonances (see section 2.1). The shape-induced lightning-rod effect is caused by surface charge accumulation at geometric singularities.27 The LSPR is the collective oscillation of electrons excited by light resonant with the LSPR frequency, resulting in an enhanced electric field in the vicinity of the antenna apex. Thus, the nanoantenna is required to have a high aspect ratio with a sharp end from a few nanometers to tens of nanometers in diameter. Antenna resonances will come into effect if the size of the nanostructure matches multiples of onehalf of the excitation wavelength. It is noteworthy that the field enhancement also depends on the substrate. Substrates of noble metals such as Au and Ag will support the formation of gap-mode plasmons that lead to stronger field enhancement and confinement (see section 2.3), whereas the enhancement with dielectric substrates is usually lower. In addition, the polarization of the enhanced fields could become relevant in the case of an anisotropic response of the sample (see section 2.2). Spatial Resolution. The spatial resolution is mainly determined by the antenna apex diameter and morphology. Generally a better spatial resolution is achieved with a smaller tip diameter. (The roles of gap modes and additional signalenhancement mechanisms in TERS are discussed in sections 2.3 and 2.4.) Practicality and Reproducibility of Fabrication. With a large number of nanoantennas needed, the production process should be low-cost and easy to facilitate. Moreover, because the frequency and induced dipole of LSPR are heavily affected by the size and morphology of the antenna apex, it is very important that the fabrication procedure can be repeated to produce the same structure. Currently, the most widely used nanoantennas for scanning tunneling microscopy (STM) and shear-force-based atomic force microscopy (AFM) systems are solid gold or silver tips fabricated by electrochemical etching6 (Figure 5a). The metal wire is dipped into an etching solution, and a dc or ac voltage is applied between the wire and a circular counter electrode (Figure 5a). The apex diameter of the tips produced in this way can reach 10−20 nm with the precise control of etching current for Au tips86−88 and an extra polishing step for Ag tips.89−91 Gold and silver tips can significantly enhance the field owing to

Figure 5. Schematic diagrams of different fabrication methods that are discussed further in the text. (a) Full conical metal tip from electrochemical etching. (b) Presynthesized nanoparticles attached to an AFM probe. (c) Nanostructures formed at the end of the tip with a voltage applied between the tip and a sacrificial substrate. (d) Noblemetal layer deposited on an AFM probe. (e) Ring-shaped structure created on a metal antenna by FIB milling.

their strong LSPR effects in the visible range. A fieldenhancement factor of about 5 has been determined from tip-enhanced measurements on carbon nanotubes or nanowires on glass substrate.6,92,93 However, the reproducibility is typically limited, and structural details of the apex including the exact diameter, the geometric shape, and the surface morphology cannot be well controlled and replicated. This can lead to a substantial variation in the plasmonic resonance and, hence, in field-enhancement factors among different tips. Another method is to attach plasmonic nanoparticles (NPs) or nanowires (NWs) to commercially available AFM tips. The advantage of this method is the well-defined and more reproducible geometry and low surface polycrystallinity of the plasmonic particles compared to etched metal wires. The approaches can be divided into two categories, depending on whether the nanostructures are presynthesized before attachment or fabricated directly at the tip apex. For presynthesized NPs, the AFM tip is typically functionalized with surfactants and then dipped into the nanoparticle solution to pick them up through covalent bonding (Figure 5b). One of the earliest studies using this technique was performed by Vakarelski and Higashitani.94 In a first step, the whole tip was coated with a passivation monolayer. Then, the part at the end of the tip was removed by scanning the tip across a silica wafer. Afterward, the end was functionalized with surfactants and dipped into a suspension of gold nanoparticles. This approach allows for the attachment of nanoparticles with diameters of 10−40 nm specifically at the tip. In 2010, You and co-workers introduced alternating-current dielectrophoresis (AC-DEP) to bind single-crystalline Ag NWs to electrochemically etched W tips.95 In their setup, a copper ring held a droplet of Ag NW solution with the W tip dipped inside. An ac voltage was applied between the W tip and the copper ring. After optimizing the Ag concentration, they achieved a success rate of more than 50% in making tips with single NWs attached. Fujita et al. modified the method and attached several Ag NWs at the connecting point to improve the electrical contact and mechanical strength.96 H


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Figure 6. Scanning electron microscopy (SEM) images of different antenna structures. (a) W tip with a gold or silver sphere deposited at the apex. Reproduced with permission from ref 97. Copyright 2015 AIP Publishing. (b) Gold trimer antenna consisting of three Au NPs. Reproduced with permission from ref 103. Copyright 2012 American Physical Society. (c) Disconnected Ag grains coated on a SiO2 probe. Reproduced with permission from ref 98. Copyright 2015 The Royal Society of Chemistry. (d) Template-stripped gold pyramids. Reproduced with permission from ref 99. Copyright 2012 American Chemical Society. (e) Scheme and FIB image of a gold tip with an FIB milled groove. Reproduced with permission from ref 100. Copyright 2015 American Chemical Society. (f) Different views of a C-shaped aperture under the tip apex. Reproduced with permission from ref 101. Copyright 2013 Nature Publishing Group.

make a compromise between the topographic resolution and the signal intensity. To overcome this disadvantage, antennas with two or more coupled NPs with decreasing size can be used. Höppener and co-workers reported a trimer antenna of three individual Au NPs with sizes of 80, 40, and 20 nm103 (Figure 6b). The tip was functionalized with aminosilane and picked up Au NPs from a glass substrate upon mechanical contact. The attachment procedure was then repeated using

It is important to note that, although the light confinement becomes stronger with decreasing particle size, the enhancement factor does not necessarily increase. Instead, the field enhancement is determined by the interplay of size-dependent effects such as surface damping of electrons, radiative scattering, and retardation. For example, for an excitation wavelength of 532 nm, a gold NP with a 50-nm diameter provides the maximum field enhancement.102 This means that one must I


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grains (Figure 6c) on an AFM probe by tuning the evaporation rate and substrate temperature. TERS measurements showed that a group of disconnected grains is the most efficient plasmonic antenna, with an enhancement factor as high as 5500. Through simulations, the grain number and the gap distance were optimized. To our knowledge, this is the first time that the effect of nanoparticle arrangement has been studied both experimentally and theoretically. In the past few years, micro-/nanofabrication technology has been used intensively in nanoantenna fabrication because of its precise control of the size and shape and better reproducibility. One example was presented by Johnson and co-workers, who introduced the fabrication of solid gold pyramidal tips by template stripping99 (Figure 6d). First, the inverted pyramidal molds were generated in a silicon wafer through standard photolithography and subsequent chemical etching in potassium hydroxide (KOH). Then, gold was deposited, and the silicon nitride mask was removed in a lift-off procedure. Finally, a pyramidal tip with a 10-nm diameter was picked up and attached to a tungsten wire. The advantage of this technique is that it can be used for large-scale production and produces uniform tips giving an enhancement factor of 10 in a highly reproducible manner. Moreover, the large apex angle of 70.5° is well suited to scatter the near-field optical signal into the farfield. An approach that provides new flexibility is to utilize focusedion-beam (FIB) milling (Figure 5e). Imad et al. milled a ring shape on a silica AFM probe at a certain distance from the end and then deposited Ag on it.111 The width of the ring was 100 nm, which created a mushroom-shaped apex and avoided plasmonic interactions between the apex and the rest of the tip. In this way, the tip behaves as a finite-size metallic antenna, whose plasmonic resonance frequency can be tuned by changing its length. Antennas of different lengths were used for TERS experiments and gave high enhancements when the excitation wavelength was close to the resonance frequency. Similarly, Thiago et al.100 performed FIB milling to etch a single groove near the apex of electrochemically etched gold tips (Figure 6e) and derived a relationship between the length L and the wavelength of the first LSPR mode. The milled gold tip showed an enhancement factor as high as 210. Hoffman et al. developed a multistep process of FIB milling on sputtered gold layers to fabricate nanocones with variable aspect ratios and, hence, with tailored plasmon resonances.112 The nanocones can be attached to a scanning probe or tapered fiber. In addition to optimizing the enhancement ability, another way to achieve a high-performance antenna is to reduce the confocal background. In the “tip-on-aperture” approach, a metallic tip was grown by electron beam deposition (EBD) at the end of a metal-coated glass fiber that concentrated the light passing through the aperture.113,114 In this way, the direct exposure of the sample to the laser was avoided. In other approaches, instead of directly illuminating the antenna apex, the laser is focused onto a light coupler and confines the surface plasmon at the end of the tip through plasmon propagation. For instance, the Raschke group fabricated a grating structure by FIB on the tip shaft and found that the background in TERS measurements was suppressed.115 Another realization utilized the photonic cavity on the top of the tip to funnel the excitation light into surface plasmon polaritons (SPPs).116 Some groups have demonstrated a different configuration in which a strong enhancement is achieved by including a nanogap near the tip end. A three-

1,6-hexandithiol linker molecules to form multiparticle antennas. Transmission electron microscopy (TEM) images confirmed no electrical contact between the Au NPs. In this geometry, the enhanced field of the largest NP excites the next smaller one and finally results in a field confinement given by the smallest NP, achieving a strong enhancement and a high spatial resolution at the same time. Using a wet chemical route, one can also dip the functionalized tip into the precursor solution and trigger the reaction at the end. Wang et al. developed a method to prepare silver, gold, and copper NPs on AFM probes.104 After coating the tip with a self-assembled monolayer, they transferred methyl to carboxylic groups through electrochemical oxidation at the apex and then used the COOH groups to bind positively charged metal ions, after which they performed chemical reduction to form a nanoparticle-terminated AFM probe. One disadvantage is that this method requires delicate experimental operation and long fabrication times. In 2012, Umakoshi et al. fabricated samples with single Ag nanoparticles of different sizes at the tip apex by photoreduction under laser irradiation.105 In other approaches, a voltage is applied between a functionalized tip and a sacrificial metal substrate to selectively synthesize the nanostructures at the tip end (Figure 5c). Ma et al. deposited Au and Ag nanospheres to the apex of W or C tips through the application of short potential pulses between the tip and a sacrificial electrode97 (Figure 6a). The diameter of the nanospheres could be varied from 60 nm to 1 μm by tuning the pulse length, the distance between the tip and the electrode, and the total deposition time. Cathodoluminescence confirmed the excitation of surface plasmon in the nanospheres. Bakhti et al. presented another technique for growing single Au nanofilaments.106 A platinum-coated commercial AFM tip was operated in tapping mode in a fixed lateral position on a mesoporous silica film loaded with gold salt. A negative bias on the tip, together with a small oscillation amplitude and a high relative humidity, led to the local reduction of gold ions and further to filament formation. The radius of the filaments can be as small as 3 nm. Recently, Kim and co-workers used single DNA strands to tether 5-nm Au NPs onto AFM probes and then transferred the tips into enhancing solutions to make core−shell Au−Ag NPs and Au nanostars in high yields.107 TERS and tip-enhanced fluorescence measurements were successfully conducted with these tips, but the low reproducibility of the geometry, such as the particle shape and orientation of spikes, resulted in a large tip-to-tip variation in enhancement factors. Generally, more quantitative tipenhanced measurements need to be done to better evaluate the validity and field-enhancement ability of these new emerging nanostructure-attached antennas. A large number of groups use TERS probes fabricated by depositing a layer of noble metal on AFM tips, mostly by vacuum evaporation (Figure 5d). In this method, the structure of the metal layer is affected by multiple factors including the evaporation rate, temperature, and vapor pressure. To date, it is still challenging to precisely control all of the parameters and fabricate the films with high reproducibility. Quite different morphologies under identical experimental conditions were indeed reported in articles from various groups.108−110 As a result, there are large tip-to-tip variations in the enhancement factor and induced-dipole direction. In another recent work, Taguchi et al.98 used standard vacuum vapor deposition to successfully fabricate smooth Ag film, connected Ag grains (rough film), and disconnected Ag J


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they used the tips in TERS measurements on CNTs. The G+ Raman band intensity was found to be stronger when the tube axis coincided with the direction of the dipole oscillations.35

dimensional tapered metal−insulator−metal (MIM) structure was fabricated for a so-called campanile probe.117 Here, the sample was only illuminated by the confined field in the gap region so the background was strongly reduced. Likewise, Oh’s group combined the technique of template stripping with FIB milling and integrated a C-shaped nanogap around the tip of gold pyramid mentioned before101 (Figure 6f). The C-shaped aperture offers a large transmission and polarization-dependent control while keeping the nanosized hot spot at the end of the tip.

5. APPLICATIONS OF NANOANTENNAS IN TIP-ENHANCED RAMAN MICROSCOPY The past decade has seen numerous successful applications of tip-enhanced Raman microscopy to biological systems, crystalline and semiconductor materials, solar cells, lowdimensional materials, single-molecule detection, catalysis, and even art conservation. With many reviews in this field,6−10 here, we only briefly highlight some very recent advances that can help to illustrate the future prospects of tipenhanced Raman microscopy. In the past few years, several reports showed that tipenhanced Raman microscopy can provide subnanometer spatial resolution.20,21,124,125 Using an STM-based setup at low temperatures and under ultrahigh-vacuum (UHV) conditions, Zhang and co-workers succeeded in visualizing individual porphyrin-type molecules by their Raman response in a gapmode configuration.20 The authors observed a strong dependence of the resulting signal enhancement on the exact position of the plasmon resonance formed by a silver tip and a silver substrate. Single-molecule sensitivity required the matching of the plasmon resonance with the Raman spectrum. Interestingly, the Raman response showed a nonlinear power dependence, which the authors related to the processes in stimulated Raman scattering and which could be connected to the phenomena going beyond f4 discussed in section 3.2. Subsequently, Jiang and co-workers succeeded in distinguishing adjacent molecules with similar chemical structures, one of the ultimate goals of analytical chemistry in terms of nanocharacterization.125 Recently, STM-based tip-enhanced Raman microscopy has also been applied to the study of individual carbon nanotubes (CNTs), providing simultaneous chemical and structural information. Using gold tips and substrates at ambient conditions, Chen and co-workers achieved a spatial resolution down to 1.7 nm.124 Employing a low-temperature UHV setup with silver tips and substrates, a spatial resolution of 0.7 nm was reached by Liao et al.21 Figure 7a shows an STM topograph together with the simultaneously acquired TERS maps of the G- and D-bands of a CNT. Detailed spectral mapping allowed for an investigation of how strain and the local environment affects the Raman G-band and as well as a comparison of the inner side with the outer side of an individual CNT. The authors investigated the origin of the subnanometer resolution by recording the tip−sample distance dependence of the signal intensity (Figure 7d) and the dependence of the Raman Gband intensity on the incident laser power (Figure 7e). The latter showed a clearly nonlinear dependence, suggesting that the conventional model based on electromagnetic field enhancement is not sufficient to account for the observed spatial resolution and contrast (see section 3.2). Nonlinear techniques have the potential to enhance the detection sensitivity, signal-to-background ratio, and spatial resolution of TERS even further.126 Wickramasinghe et al. demonstrated stimulated Raman scattering (sTERS) of azobenzene thiol grafted on Au(111) (Figure 8).127 In their experiment, the intensity of the stimulating beam was detected using a lock-in amplifier in the presence of a polarizationmodulated pump beam using a conventional TERS setup. In the presence of the tip, the stimulated gain was estimated to be 1.0 × 109, in agreement with theoretical calculations. In

4.2. Nanoantenna Characterization

The experimental characterization of nanoantennas is a crucial step in the sense that it reveals the actual performance of the antennas after fabrication and can be used to test the validity of simulation results. However, although substantial improvements have been made, there is still a need for well-defined reference samples.8,118,119 A first basic characterization in most published works is to check the antenna geometry by scanning electron microscopy (SEM) or TEM. In addition, there are some other techniques for investigating different properties of antennas, including both optical and nonoptical measurements, which we briefly review in the following sections. 4.2.1. Chemical Composition. For antennas with nanowires and nanofilaments synthesized at the AFM probe apex, energy-dispersive X-ray (EDX) spectroscopy has been utilized to confirm the presence of the noble metal.106 EDX analysis has also been conducted to determine whether an antenna was contaminated by ion implantation through FIB milling.120 4.2.2. Crystallinity. It was found that polycrystallinity in metal tips is related to plasmon damping and undesired thermal losses,121,122 which will reduce the enhancement factor and cause large tip-to-tip variations. In earlier works, the electron backscattered diffraction (EBSD) in SEM was used to study the crystallinity of electrochemically etched Au tips.121 Recently, high-resolution TEM with corresponding electron diffraction patterns has been widely used to resolve crystal lattice parameters.100,106 4.2.3. LSPR Energy. Some antenna designs aim at tuning the LSPR energy of the antenna by varying the parameters in the fabrication process. To achieve this goal, it is important to measure the resulting resonance energy precisely. One method is white-light scattering spectroscopy, which detects the strong Rayleigh scattering when LSPRs are excited.111,112,120,123 Another approach involves electron energy loss spectroscopy (EELS).100 In ref 100, EELS spectra were acquired in the vicinity of a gold tip before and after FIB manipulation. The appearance of new absorption peaks in the EELS spectrum indicated the excitation of the first three LSPR modes. Furthermore, STM-induced luminescence could also be used to measure the LSPR energy in the case of gap-mode configurations.20 4.2.4. Near-Field Polarization. It is well-known that metallic tips show different polarization properties and enhancement factors even when fabricated under the same conditions. Mino et al. developed a method to analyze the polarization of near-field light from Ag-coated tips under the dipole approximation. They applied the defocused imaging technique; that is, they measured the Rayleigh scattering image of the tip apex at a plane away from the focus. The tilt and twist angles of the dipole were determined from the dark crescents in the defocused pattern. After assigning the dipole directions, K


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Figure 8. (a,b) Set of (a) STM and (b) sTERS scans of azobenzene thiol grafted on Au(111). (c,d) Set of (c) STM and (d) sTERS scans with the pump or stimulating beam momentarily blocked. The vanishing signal supports its assignment to a nonlinear stimulated Raman process. Reproduced with permission from ref 127. Copyright 2014 American Chemical Society.

6. SUMMARY The present review illustrates the concept of optical nanoantennas and their application to tip-enhanced near-field Raman microscopy. From a functional point of view, it is straightforward to generalize the antenna concept to also include the substrate. This generalization is of particular importance in the case of metallic substrates, which support the formation of gap plasmons, which have been shown to provide the highest field enhancement and confinement. Whereas classical electromagnetic models predict a continuously increasing strength of the plasmonic fields for decreasing distance, recent theoretical and experimental studies have revealed the emergence of quantum phenomena on nanometer and subnanometer scales. At these length scales, nonlocal screening and electron tunneling at optical frequencies limit the maximum field enhancement, giving rise to a finite distance at which the highest enhancement is achieved. On the other hand, detailed theoretical modeling at the shortest length scales indicates the emergence of hot spots caused by atomistic structural features of the tip and substrate, leading to field confinement areas below 1 nm2. At present, the details of the interplay between quantum and atomistic effects and their actual contribution to the spatial resolution and signal enhancement observed in the experiment are not clear. Although the spatial extent of the plasmonic fields at the nanoantenna is thought to determine the spatial resolution, this seems to be in conflict with recent experimental studies that demonstrated 0.7 nm spatial resolution. To explain these findings several different novel microscopic models have been proposed based on different phenomena including optomechanical coupling, self-interaction and electrial field gradients. New experiments are now needed to identify the role of these enhancement mechanisms in the contrast formation in tipenhanced near-field Raman microscopy. These fascinating

Figure 7. (a) Simultaneously acquired STM topograph and TERS mappings for the G-band (1450−1650 cm−1) and D-band (1200- 1450 cm−1) of a CNT. The bright feature inside the rectangle marked in panel a is identified as an amorphous carbon cluster impurity. (b,c) Apparent height profile and TERS intensity profiles along the arrow lines marked in panel a for the (b) G-band and (c) D-band. (d) Dependence of the normalized net G-band intensity ITERS on the gap distance (d). (e) Dependence of the net G-band intensity ITERS on the incident laser power (I0). Reproduced with permission from ref 21. Copyright 2016 American Chemical Society.

addtion, the signal-to-noise ratio improved by 3 orders of magnitude. The capability of TERS to monitor catalytic reactions was shown by van Schrojenstein Lantman et al. in ref 128. In their experiments, a Ag-coated AFM probe acted as both a near-field antenna and a catalyst of the photoreduction of p-nitrothiophenol molecules on gold nanoplates. Whereas green laser light was used to trigger the reaction, TERS spectra were recorded under red laser illumination. Thanks to the huge enhancement from TERS, the catalytic activity was confirmed by the appearance of new Raman peaks. Kurouski et al. investigated the nanoscale redox behavior of Nile Blue (NB) using TERS.129 NB was adsorbed on an indium tin oxide (ITO) film, and TERS spectra were acquired at different potentials during cyclic voltammetry. The disappearance and reappearance of a specific Raman band indicated the reversibility of reduction and oxidation. Steplike behaviors were observed in some TERS voltammograms, and the authors interpreted them as the reaction of single or few NB molecules. L


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REFERENCES

conceptual developments and the emergence of novel phenomena combined with substantial progress in antenna design and fabrication create exciting new perspectives.

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +49 89 2180 77515. Fax: +49 89 2180 77145. ORCID

Achim Hartschuh: 0000-0002-0518-6559 Author Contributions †

X.S. and N.C.-L. contributed equally to the manuscript.

Notes

The authors declare no competing financial interest. Biographies Xian Shi earned his B.S. degree in Materials Science and Engineering from Tsinghua University, Beijing, China, and his M.S. degree from Cornell University, Ithaca, NY. In September 2013, he joined the research group of Achim Hartschuh at LMU Munich. Currently, his studies are focused on probing the optoelectronic properties of nanocarbon materials by tip-enhanced near-field microscopy. Nicolás Coca-López obtained his Licentiate degree in Physics at the Universidad Complutense de Madrid (Madrid, Sapin) in 2012 and his Master’s degree at the Technical University of Munich (Munich, Germany) in 2014. During his Master’s thesis term at the Walter Schottky Institut, he studied the coupling between self-assembled quantum dots and plasmonic waveguides. In 2014, he joined the group of Achim Hartschuh at LMU Munich, where he is currently pursuing a Ph.D. degree. His research interests focus on the antenna-enhanced microscopy and spectroscopy of graphene and related materials. Julia Janik studied chemistry at LMU Munich, Munich, Germany, where she received her B.S. degree in 2009 and her M.S. degree in 2012. For her Master’s degree, she focused on physical chemistry and material science. Since 2012, she has been a member of the group of Achim Hartschuh. Her research focuses on the implementation of a new low-temperature antenna-enhanced microscope. Achim Hartschuh is professor of physical chemistry at LMU Munich. He received his Diploma (1996) and Dr. rer. nat. from the University of Stuttgart (2001), both in Physics. He was a postdoc at the Institute of Optics at the University of Rochester (Rochester, NY) from 2001 to 2003, where he worked on tip-enhanced Raman microscopy in the group of Lukas Novotny. From 2003 to 2005, he was Junior Professor at the University of Siegen (Siegen, Germany) and later at the University of Tübingen (Tübingen, Germany) before he joined the LMU Munich in 2006. His research interests focus on the development and application of optical techniques with high spatial and temporal resolution.

ACKNOWLEDGMENTS We thank our collaborators in this field of research. Projects were financially supported by the ERC through Starting Grant 279494 NEWNANOSPEC; by the Deutsche Forschungsgemeinschaft (DFG) through the Nanosystems Initiative Munich (NIM); and by the European Union’s Seventh Framework Programme FP7/2007-2013: POCAONTAS, REA Grant Agreement 316633. M


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Chemical Reviews

Review

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Advances in Tip-Enhanced Near-Field Raman Microscopy Using

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