Tip-Enhanced Raman Spectroscopy - Analytical Chemistry (ACS

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Tip-Enhanced Raman Spectroscopy Tip-enhanced Raman spectroscopy (TERS), a combination of Raman spectroscopy and apertureless near-field scanning optical microscopy using a metallic tip which resonates with the local mode of the surface plasmon, can provide a high-sensitive and high-spatial-resolution optical analytical approach. The basic principle of TERS, common experimental setups, various SPM technologies, and excitation/collection configurations are introduced as well as recent research progress with respect to TERS. Downloaded via 185.2.32.190 on August 2, 2018 at 11:47:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Zhenglong Zhang,†,§,∥,⊥ Shaoxiang Sheng,‡,⊥ Rongming Wang,† and Mengtao Sun*,†,‡ †

School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China Beijing National Laboratory for Condensed Matter Physics, Beijing Key Laboratory for Nanomaterials and Nanodevices, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China § School of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710062, People’s Republic of China ∥ Leibniz Institute of Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena, Germany SM-SERS experiments were performed statistically; no one can really “see” only one molecule in a measured target, so it is difficult to achieve real single molecule resolution in real space.10,13,14 Moreover, the samples are easily contaminated in air, which may affect the results of the detected target molecules through, for example, photobleaching or signals from other pollutant molecules adsorbed on the samples.15−17 It is sometimes very difficult to distinguish whether a Raman signal originates from the target molecules, contamination, or decomposition of the samples, which leads to unreliable results. In 2010, Li et al. reported a shell-isolated nanoparticleenhanced Raman spectroscopy (SHINERS) technique, using 1 ince being found in 1974 by Fleischmann et al. and later Au-core silica-shell nanoparticles, which can make SERS correctly explained independently by Van Duyne2 and universally applicable to surfaces with any composition and 3 Albrecht in 1977, surface-enhanced Raman scattering (SERS) any morphology,18 while a silica or alumina shell reduces both has been profoundly studied and widely used in many areas. chemical and electric field enhancements compared to normal SERS can be attributed to the electric field enhancement and SERS. The spatial resolution is also limited by the excitation 4−9 chemical enhancement mechanisms. The EM mechanism wavelength due to the diffraction limitation in SERS. results from local surface plasmon resonance (LSPR),5−7 which To overcome the shortcomings of substrates and spatial usually enhances the Raman spectrum over a large frequency resolution in SERS, many researchers have attempted to extend range, and the chemical enhancement is due to changes in the the technology of SERS. In 1985, Wessel first demonstrated the electronic structure of molecules adsorbed on metal surfaces, idea of tip-enhanced Raman scattering (TERS), which whereby some Raman peaks undergo enormous selective combines the advantages of surface-enhanced Raman spectros8,9 enhancement. Chemical enhancements are usually intercopy and scanning probe microscopy (SPM) technologies.19 preted by the charge-transfer (CT) mechanism. The physical The spatial resolution can be greatly increased using a sharp mechanism plays a major role in SERS because electric field metal tip, and the enhancement factor can still be maintained enhancement is approximately 108−1011,5−7 whereas chemical for detecting a single molecule. In 2000, Zenobi, Kawata, 2 3 8,9 enhancement is only 10 −10 . Anderson, and Pettinger first independently reported TERS SERS is a technique that provides an opportunity to results, thereby demonstrating the feasibility of TERS.20−23 overcome the limitation of the low Raman scattering cross Since then, TERS has drawn great interest from scientists section, which has been widely used for detecting the vibration around the world. Single-molecule detection and mapping have and chemical information on molecules. Randomly adsorbed been achieved by several groups24−27 with Raman signals at the molecules make SERS signals the averaged information on all resolution level of a single molecule. However, in the beginning, vibrations. For single molecule-SERS (SM-SERS),10−12 the the best results were only at the whole molecular resolved level, fluctuation is due to different bonds of the molecule going into which is insufficient to distinguish the specific parts of a and out of the hot spot. Even if many groups can chemically molecule. Research has also focused on the mechanism of control the adsorption of a molecule at one specific position of TERS, such as controllable gap distance dependent enhancea nanoparticle, it is impossible to distinguish which Raman signal comes from which part (bond) of the molecule if only the theoretical simulations are compared. Moreover, all of the Published: August 29, 2016 ‡

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© 2016 American Chemical Society

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DOI: 10.1021/acs.analchem.6b02093 Anal. Chem. 2016, 88, 9328−9346

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Analytical Chemistry ment, pure system information, absorbance studies, catalysis, and efforts on TERS, etc. Later, great efforts were made to obtain better resolution,24−28 and TERS was developed into a nanoimaging method for surface science, even at the submolecular level. There are several excellent review paper about that.29−31 In this Feature, we mainly focus on the novel phenomenon and submolecule resolution of TERS. The history of TERS will be briefly introduced, and the main principles of TERS will be presented in detail, including the fundamental theory, setups, tips, and sample preparation. Then, we will focus on current advanced reports on TERS to show its powerful abilities and potential in nanoscience research, including submolecule detection and mapping, biomolecules measurement, plasmondriven reactions, TERS for materials, ultrafast TERS, and deep ultraviolet resonance TERS, etc. Finally, some challenges and outlooks will be put forward.

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NOVEL TECHNIQUE: TERS

Diffraction Limit. The spatial resolution of conventional optical microscopy is limited by the optical diffraction according to the Abbe formula:

Δx = 0.61λ /NA

(1)

The resolution depends on the wavelength of incident light and the numerical aperture (NA) of the objective. The highest resolution is approximately 200 nm in the range of visible light, which remains too low for nanoscience. Therefore, conventional Raman microscopy can only obtain the average chemical information on a sample rather than the nanocomposition and distribution of the specimen. Many researchers have attempted to improve the spatial resolution. In 1928, Synge proposed the primary idea of scanning near-field optical microscopy (SNOM) by using a scanning aperture smaller than the wavelength of the illuminating light to overcome the diffraction limit.32 In 1984, inspired by the invention of scanning tunneling microscopy (STM), Pohl first achieved scanning near-field optical microscopy (SNOM) with a resolution of λ/20 using a very narrow aperture fiber tip.33 The next year, Wessel proposed that by combining Raman spectroscopy and STM, the nanoparticles in SERS could be replaced by a sharp metal tip and the distance between the sample and tip could be precisely controlled by the feedback system.5,34 In 2000, the first results using TERS were reported by four groups,20−23 and the spatial resolution was reduced to 50 nm. As a novel technology, TERS is usually based on the technologies of STM, atomic force microscopy (AFM), shear force microscopy (SFM), and SNOM. For STM-TERS, when the focused laser illuminates the apex of the metal or metalcoated tip, it excites localized surface plasmons around the tip apex, and when the tip approaches the metal substrate, it can create a “hot spot” in the gap between the tip and substrate. Raman signals from this hot spot will be largely enhanced and scattered to the far field, as illustrated in Figure 1a. Note that if the light is confined with enhancement as a hot spot, it is nearfield light localized at the tip but does not propagate out much to the far-field. Dark modes or subradiant modes of metal antenna as near-field light at the apex cannot be excited by farfield light nor scattered out to the far field. As the tip scans the surface of the sample, the signals from the tip apex can be collected at each point, and the spatial resolution can be significantly improved beyond limitations of the light

Figure 1. (a) Schematic illustration of TERS. (b−e) Schematic illustration of STM, AFM, SFM, and SNOM. (f) Three different modes of excitation and collection: (i) side excitation and side collection, (ii) bottom excitation and bottom collection, and (iii) top excitation and top collection.

diffraction, which is largely determined by the shape and size of the tip apex. TERS Mechanisms. Combining SPM and Raman microscopy, the gap distance between the tip and substrate can be well controlled to (sub-) nanometers by the SPM controller. A localized electromagnetic enhancement induced by surface plasmon resonance creates a “hot site” to enhance the Raman scattering for TERS. As shown in Figure 1a, although a larger area (5 μm2, due to the size of the laser spot) was illuminated, only the area around the gap could be enhanced. Thus, the TERS signals mostly come from the range of the gap between the tip and substrate, thus realizing a high spatial resolution of 1−10 nm. Similar to SERS, the enhancement mechanism of TERS includes the electric field enhancement and the chemical enhancement. Noble metals (such as Au/Ag) are usually used as the STM-substrates and the STM tips and can give a good optical response and large electric field enhancement in the visible light region.35 In most cases, the electric field enhancement is dominant and can reach 109−11 in the double tip situation.36−38 The near-field distribution near the metallic tip and substrate can been simulated using various numerical methods, such as the finite-difference time-domain (FDTD) method,37,38 finite element method (FEM),39 and boundary element method (BEM).40 9329


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resolution and high control precision is a powerful method for studying single molecules. To improve the chemical bond analysis ability, many spectroscopy technologies, including scanning tunneling spectroscopy (STS), inelastic electron tunneling spectroscopy (IETS), and TERS, have been developed to achieve energy, structure and spatial resolutions in the frequency domains.45 IETS combined with lock-in amplification technology is used to study single-molecular-bond vibration information at low temperatures, and the tunneling electrons between the tip and substrate can be used as an excitation source for the electroluminescence to observe the interior of a single molecule.46,47 Compared with STS and IETS, TERS can provide better vibration information and better spectroscopic resolution and its ultrahigh spatial resolution can be used to map a single molecule.26 However, only a conductive substrate or ultrathin samples distributed on a conducting substrate are required and are mostly used in reflection mode due to the opacity of the sample or substrate. Normally, high-performance STM is usually performed in an ultrahigh vacuum (UHV) and low-vibration-noise environment. To reduce thermal drifts, a low-temperature (LT) environment is usually required.27 Therefore, the STM-TERS has high spatial resolution and high control precision, but it is difficult to build an excellent TERS setup based on LT-UHV STM and it is not suitable for some special samples, e.g., biospecimen. Both AFM/SFM scan primarily based on the tip interaction with the topography of the sample and subsequent change in amplitude/phase (tapping mode) and deflection (contact mode).31 With a sharp silicon tip at the end of a cantilever, AFM works under the atom force between the tip and substrate. As shown in Figure 1c, when the tip approaches the surface of the sample, the forces (attractive or repulsive forces, van der Waals forces, capillary forces, etc.) between the tip and substrate will lead to a deformation of the cantilever. This deformation can be detected using a laser reflected from the back surface of the cantilever and provides the morphology information on the sample surface.48 Therefore, in AFM, no specific samples or treatments are required, and the technique is not limited to conductors. It can be used on any surface, even with a roughness of several micrometers. Moreover, AFM can also be used in a liquid environment,49 thereby making it advantageous for use in the study of biological and organic molecules. Although AFM-TERS presents many advantages, its shortcomings should also be noted. Usually, the spatial resolution of a common AFM is approximately one or a few nanometers, an Au/Ag coated silicon tip needs to create the hot spot, and the Au/Ag coated tips are soft and easily break during AFM scanning. AFM-TERS can be used for any samples, especially biospecimens, which are widely used in many research fields, such as for biological molecules,50−54 semiconductors, 2D materials, etc.42,55−57 The SFM system is also commonly used in the TERS setup. As shown in Figure 1d, a metal tip can be attached to a highquality tuning fork, which is rigidly mounted onto a piezoelectric and driven at resonance, serving as a dither, to detect surface shear force due to the lateral interaction between the tip and sample.58 The amplitude and phase of the tuning fork can be monitored by using high-speed electronics, using feedback to the loop to control the tip−sample separation distance and for surface topography imaging.59 SFM-TERS can be used for any samples and also in liquid systems such as AFM.60 The metal tips used in SFM-TERS can be more stable than the coated tips in AFM-TERS.61,62 However, SFM, with

In TERS, a very sharp tip apex (approximately 10−50 nm) can lead to a highly localized surface charge density and can be a source of the localized electric field enhancement. The distance between the tip and substrate is usually controlled to approximately one or a few nanometers in SPM by using a feedback system, which can induce enormous electric field coupling between the metal substrate and tip. This will create a hot spot, and the enhancement area is strongly confined underneath the tip apex.41 The spatial resolution of TERS can reach a few nanometers,42,43 even down to the submolecule level.26 One of the important concepts in TERS is the enhancement factor. In the calculation, considering that the enhancement mostly originates from the electric field enhancement in TERS, the electric field enhancement can be written as

g = E Tip/E0

(2)

where Etip and E0 are the enhanced electric field under the tip and the incident electric field intensity, respectively. The light intensity is proportional to the square of the EM field, and the enhancement depends on both the incident laser and Raman scattering light intensity; thus, the TERS enhancement factor is EFTERS_EM = glaser 2 × gRaman 2

(3)

The electric field enhancement is frequency dependent; the wavelengths of the illuminating laser and the Raman scattering light are slightly different. If the Raman shift is not very large, it can be assumed that the electric field enhancement from the laser and Raman scattering is almost the same. Thus, EFTERS can be approximated as EFTERS_EM = gLaser 2 × gRaman 2 ≈ g 4

(4)

The electric field enhancement is sensitive to the tip materials/ shapes/curvature radius and the excitation wavelength, etc., which is a new topic in TERS. TERS Setup. Combining SPM technology and optical geometry, TERS requires a complex and challenging setup, especially in an ultrahigh vacuum and/or low-temperature condition, which brings better performance and gives prominence to the excellent power of TERS. One of the crucial problems in TERS is how to easily focus the incident laser on the apex of the tips and effectively collect the scattering signals while maintaining the high performance of the SPM. In this section, the SPM technologies for TERS and corresponding illumination methods will be discussed, along with their application areas and limitations. SPM Technology. The enhancement at the substrate surface decreases dramatically with increasing of the gap distance between tip and substrate because the near field decays exponentially at the tip apex.32 Therefore, it is critical to precisely control the distance. Among the various SPM technologies, STM, AFM, SFM, and SNOM are used to build TERS setups that have different feedback mechanisms and respective technique features. With the help of quantum tunneling, STM is an instrument for imaging surface at an atomic resolution (0.1 nm). As shown in Figure 1b, when a conducting tip is very close to the surface, the electrons can tunnel the barrier between the surface and tip with a bias voltage applied to them, and the tunneling current is very sensitive to changes of gap distance. This allows for adjusting the distance very accurately, with the exponential dependence of tunneling current on the distance.44 STM with atomic 9330


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Figure 2. Schematic illustration of four typical UHV-TERS setups. (a) Pettinger’s group setup, reproduced with permission from ref 74. Copyright 2007, American Institute of Physics. (b) Van Duyne’s group setup, reproduced with permission from ref 76. Copyright 2011, American Chemical Society. (c) HV-TERS setup, reproduced with permission from ref 25. Copyright 2012, Nature Publishing Group. (d) The setup in Dong’s group, reproduced with permission from ref 26. Copyright 2013, Nature Publishing Group.

TERS, and should be used to generate larger field enhancement.66,67 While, because of the objective used in AFM-TERS, p-polarization also can be obtained, which is useful for creating the hot spot around TERS tip. To overcome the limitation of requiring transparent samples in bottom illumination, side illumination setups are usually adopted both in STM and SFM systems. In this case, a long working distance objective is used to focus the laser onto the apex of the tip. However, the NA of the objective will be unavoidably lowered; thus, the excitation and collection efficiency will be limited by the focusing capability and small collection angle. Although we can increase the laser power to compensate for the decreased efficiency, a higher laser power sometimes will lead to a strong heating effect or photodecomposition, which will damage the target molecules.5 Compared with the bottom illumination geometry, the side illumination setup can excite a stronger electric field at the tip apex using p-polarized light on the tip axis,68,69 thus compensating for the decreased NA of the objective and producing a larger enhancement factor.70 Top illumination can be used for both transparent and opaque samples, which are usually used in AFM- and SNOMTERS. It combines the advantages of the bottom and side illumination setups, and a high-NA objective can be employed so that the laser can be tightly focused.71,72 The tip can be effectively excited using a longitudinal wave, and the electric field can be greatly enhanced and localized at the tip apex. However, it requires a special tip or tip holder to avoid excitation and scattering light being blocked by the tip, a more complex configuration, and may miss the strongest emission angle (imaging a radiation pattern of a dipole along the tip axis) if the NA is insufficient. The top illumination setup is less used in STM-TERS systems, especially under high-vacuum and lowtemperature conditions because of the complexity of operation and limitations of the instruments.

complicated operation, has a somewhat poor lateral resolution that strongly depends on the oscillation amplitudes of the tuning fork.62,63 As another important microscopy, SNOM works by exploiting the evanescent waves on the sample’s surface. As shown in Figure 1e, an aperture optical tip is placed close to the specimen surface. Two types of feedback, including constant force (same as AFM) and shear force (same as SFM), are usually used to achieve high resolution and artifact images. Particularly, the resolution is approximately 20 nm, being limited by the size of the detector aperture and not by the excitation laser. To obtain higher resolution, a sharp tip without an aperture is used in the apertureless mode. Optical Design. The optical design is a crucial step in the TERS setup because the performance of the SPM and optical efficiency should be considered simultaneously. The excitation laser should be focused onto the tip apex easily and effectively collect the Raman scattering signals. As shown in Figure 1f, three different geometry designs, using bottom, side, and top illumination, are usually employed in the experimental setups. The selection of the illumination setups is dependent on the configuration/structure of the SPM systems and also based on the sample of interest. In a bottom illumination geometry, mostly used in AFM-TERS, the substrate should be transparent, and a high NA objective can be used to collect more signals.55 In this case, the light can be tightly focused onto the tip apex through the substrate. However, the drawback of this configuration is also obvious: the light must transmit through the sample so that the sample and support frame are transparent and not too thick.64 Therefore, the electric field enhancement is usually limited due to the polarization always being perpendicular to the tip axis (s-polarization), while an electric field component parallel to the tip apex (p-polarization) contributes to the maximum enhancement.65 Hartschuh et al. demonstrated that a radially polarized annular beam can generate a stronger p-polarization component, which leads to a higher field enhancement and improving the image contrast in 9331


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wafer using thermal evaporation in a high vacuum.26,28,77,79 Compared with a single-crystal substrate, the island distribution on the rough film is similar to that of the arrays of metal tips. The coupling between the probe tip and the roughness of the film as nano island or SERS substrate can generate a greater electric field enhancement, resulting in a higher Raman enhancement factor compared with that on flat single-crystal substrates.17,38 However, for a molecule pattern or submolecule TERS measurement, a single crystal substrate is usually needed. The next issue concerns how the molecules can be adsorbed on the substrate. The sample preparation differs between different experimental setups. In some cases, the substrate can be directly immersed into the molecule solutions to obtain a monolayer of self-assembled molecules via chemisorption.80,81 The dying or spin coating method is sometimes used to disperse the samples and have them physically adsorbed onto the substrates.42 The above methods may be used without serious problems for the spectrum measurement. However, it is usually impossible to obtain the topography of a molecule using SPM. In a UHV environment, the molecular beam epitaxial (MBE) method can be employed to evaporate the molecules onto a clean single-crystal substrate. Because there is no other contaminant involved in this process, it guarantees that only the target molecules are on the substrate.24,25,27,29,76 In this case, the spectrum and topography measurements can be performed simultaneously. Tip Preparation. The SPM tip plays a critical role in TERS measurements. A metal or metal-coated tip is usually employed to create a hot spot in TERS. The material composition, diameter, and shape of the tip determine not only the imaging quality of the SPM but also the enhancement factor and spatial resolution of TERS. One of the great remaining challenges in TERS is producing repeatable high-quality tips that are both suitable for high-quality SPM imaging and in resonance with the excitation light to generate a larger enhancement factor.82−86 In the AFM-TERS system, a metal-coated commercially standard Si or Si3N4 tip is usually used. The high-purity metal (silver or gold) is evaporated onto the probe surface by using the thermal evaporation method in a high vacuum. The thickness of the coated metal film is typically several tens of nanometers, and the diameter of the metal-coated tip apex, as the source of electric-field enhancement, is approximately 20− 50 nm.86,87 A typical silver-coated AFM tip is shown in Figure 3a. In the UV−vis region, silver tips usually exhibit higher enhancement than do gold tips; however, chemical stability also needs to be considered because silver is susceptible to oxidation in air. The oxidation can alter the resonance of the tip and lead to a decrease in the enhancement.88 Therefore, using the silver tips immediately after the deposition is preferable, or several nanometers of gold or silicon dioxide can be deposited onto new tips to prevent oxidation.89 Electrochemically etched Au/Ag tips are usually used in STM- and SFM-TERS systems. Although various methods for producing tips for TERS, such as the focused ion beam (FIB) and mechanical cutting methods, have been developed in the past decade, they are either too expensive or sophisticated or have very poor reproducibility. The electrochemical etching method can produce very nice tips suitable for TERS. Considering the resonance effect with different excitation wavelengths, silver wire or gold wire is usually etched as the tips for TERS measurements in various setups.

In addition to the three types of illumination setups introduced above, the top-illumination with a parabolic mirror TERS setup has also been successfully used in STMTERS.25,28,72−75 In this geometry, the light is focused by a high NA parabolic mirror onto the tip apex and sample surface. Thus, the excitation and collection efficiency can be largely improved. However, it is very difficult to align the light and focus it onto the tip apex in this system. Moreover, the sample should be transparent or very small so that the light can illuminate the mirror without being blocked by the sample. Operating Environment. Previous works of TERS were mainly performed in an atmospheric environment, in which the sample could be very easily contaminated by impurities adsorbed on the substrate or by the surroundings. Moreover, it is almost impossible to see a molecule clearly using SPM in an air atmosphere because a clean single-crystal surface cannot be achieved when exposed to air. Considering the limitation of the air environment, (U)HV-TERS has been developed by several groups, and many interesting and exciting results have been reported recently.25−28,76−79 Placing the TERS setup in a vacuum environment can prevent contamination. The vacuum provides a clean environment in which the TERS measurement can be performed. Moreover, the silver tip or substrate can be protected from oxidation in a vacuum. In 2007, the first UHV-TERS work was reported by Pettinger’s group. A schematic of the setup is presented in Figure 2a; a high-NA parabolic mirror is placed between the scanning probe microscope head, and the sample serves to focus the incident laser and collect the scattered light.74 The next year, they performed single-molecule detection using the UHV-TERS setup, which was confirmed by simultaneously acquired STM images.24 Figure 2b shows a UHV-TERS from Van Duyne’s Group. The sample preparation could be performed in situ, and multiple vibrational modes of CuPc adlayers on Ag(111) were resolved in TER spectra with molecular resolution for the first time.76 Another HV-TERS setup with a long working distance objective (Olympus, 50×), placed in a high vacuum chamber at 60° to the STM tip axis, is demonstrated in Figure 2c.25,27 The scattered light is collected in a backscattering geometry by the same objective and then passes through two notch filters into the Raman spectrograph. By adopting this configuration, the laser can be easily focused onto the tip apex, and the TER spectra and STM topography measurement can be performed simultaneously.25,27 Importantly, a setup of LT-UHV-TERS was built by Dong’s Group.26 As shown in Figure 2d, the setup comprises four systems, including a laser source for light excitation, a dark-box for optical filtering and alignment, a low-temperature UHV-STM chamber for sample preparation and characterization, with a built-in lens for both light excitation and collection, and a spectrometer equipped with a highly sensitive CCD detector for Raman spectral measurements.

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TERS SAMPLES AND TIPS Sample Preparation. Usually, a flat mica, glass slide, or silicon wafer can be used as the substrate in TERS.42,51,61 In gap-mode TERS (in which, both the tip and substrate are metal), a gold or silver film/single crystal is not usually used as the substrate. Because gold and silver single crystals are very expensive and difficult to work with in an air atmosphere, to obtain higher enhancement signals many research groups usually use gold or silver films prepared by depositing a 100− 200 nm gold or silver film on a freshly cleaved mica or silicon 9332


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apex.63,82,93 By modifying the shape and size of the tip apex, the frequency of SPR produced from tip can be well controlled, and the EM enhancement in the nano gap of tip−substrate can be also well manipulated. The other aspect is that the coupling efficiency of the incident light to the tip surface plasmons should be improved, which can usually be achieved via grating coupled excitation on the tip shaft, as illustrated in Figure 3d.94,95 There are also plenty of examples of SPM imaging dropcast or spin coat films and chemisorbed monolayers.31 In addition to the radius and shape of the tip, one issue involves the lifetime of the tip. In an air atmosphere, the tips are very easily contaminated by contaminants from the surroundings or target molecules. Even in vacuum, because the molecules are in constant motion at room or higher temperature as a result of the heating effect at the tip apex, contaminants, such as amorphous carbon, will be produced.88,96−100 Additionally, in STM-TERS, the distance between the tip and substrate is usually less than 1 nm and the molecules can easily jump from the substrate to the tip. Even slight contamination on the tip apex can be easily observed on a retracted tip or a tip in the tunneling state because of the enormous electric field enhancement at the tip apex. Therefore, the tip must be very clean during the spectrum measurement.101 The cleanliness and robustness of TERS tips is an ongoing difficulty of the technique, other than working in HV, which has been developed by Agapov and co-workers to ensure clear TERS tip.102

Figure 3. Scanning electron microscopy (SEM) image of (a) a silvercoated metalized cantilever tip, reproduced with permission from ref 87. Copyright 2001, Elsevier B.V. (b) A typical etched Ag tip, reproduced with permission from ref 92. Copyright 2007, American Chemical Society. (c) A typical etched Au tip, reproduced with permission from ref 91. Copyright 2012, American Institute of Physics. (d) A conical metallic tip with a grating coupler on the shaft, prepared by focused ion beam sputtering, reproduced with permission from ref 94. Copyright 2007, American Chemical Society.

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RECENT ADVANCES REGARDING TERS Since the first TERS experimental results were reported in 2000,21−23 the attention of scientists paid to this field has been greatly increased; the technology has been widely developed and studied in past decades, with many excellent results being achieved. TERS has been used in many research fields and has developed into an imaging method for studying the heterogeneous distributions of substrates at the nanoscale.103,104 Scientists have always focused on achieving a greater enhancement factor and higher spatial resolution. To date, single-molecule detection and mapping have been realized,24,25,28,77,94 and the spatial resolution has been improved to the submolecule level.26 TERS has also been used to investigate biological molecules such as DNA/RNA,

In the etching experiments, the gold tips are usually made by etching gold wire with ethanol using the electrochemical method.90,91 The gold tips are usually cone-shaped, with the radius of curvature ranging from 10 to 30 nm, and can be reproducibly produced.91 The silver tips can be fabricated using a similar process.92 Figure 3b,c is typical SEM images of etched silver and gold tips, respectively. In TERS, to obtain stronger signals and a better signal-tonoise ratio, two key aspects of the tips should be considered. One is that a tip should be in resonance with the frequency of the incident light, which can be accomplished by changing the material of the tip and modifying the shape and size of the tip

Figure 4. STM images of five BCB molecules (a) and a single BCB molecule (d) adsorbed on Au(111), the corresponding height cross sections (b, e) and the TER spectra (c, f). Reproduced with permission from ref 24. Copyright 2008, American Physical Society. 9333


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Figure 5. Schematic illustration of TERS setup for detecting intertwined insulin peptide chains (a). (b) AFM topography of insulin fibrils on a gold nanoplate. (c) Selected TERS spectra of the insulin fibril on black line in part b (25 points separated by 0.5 nm). Reproduced with permission from ref 116. Copyright 2012, WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

proteins, cells, etc.105 TERS also shows great potential in researching carbon materials such as C60, carbon nanotubes (CNTs), and graphene because they are very stable during measurement and exhibit strong Raman scattering intensities. Many excellent and interesting results have been reported by several groups.42,43,53−57,106 In addition to conventional studies, other interesting events, including chemical reactions and nonlinear phenomena, have been characterized by TERS.25−28,77,78,107−115 Various representative studies will be presented in the following subsections. Single-Molecule Detection and Submolecule Resolution. SPM technologies provide a feasible method for studying a single molecule on a clean surface with ultrahigh spatial resolution, but they usually have poor chemical analysis ability. Tip-enhanced Raman spectroscopy is an ideal method

Figure 7. (a, b) Single-molecule TERS spectra for an isolated H2TBPP molecule adsorbed on the terrace or at the step edge of Ag (111). Reproduced with permission from ref 26. Copyright 2013, Nature Publishing Group. (c, d) Calculated Raman and IR spectra of H2TBPP. Reproduced with permission from ref 117. Copyright 2015, Nature Publishing Group. The inset image in part a shows the schematic image of a tilted H2TBPP molecule, with a tilt angle of 30°. The inset image in part b shows the schematic image of a flat-lying H2TBPP molecule.

Figure 6. (a) Schematic illustration of tunneling-controlled TERS in a confocal-type side-illumination configuration. (b) Dependence of TERS spectra (red) on the spectral matching among the laser line (green), nanocavity plasmon resonance (blue), and molecular vibrational transitions (brown). (c) Representative single-molecule TERS spectra on the lobe (red) and center (blue) of a flat-lying molecule on Ag (111). (d) The top panels show experimental TERS mapping of a single molecule for different Raman peaks. The bottom panels show the theoretical simulation of the TERS mapping. (e) Height profile of a line trace in the inset STM topography (1 V, 20 pA). (f) TERS intensity profile of the same line trace for the inset Raman map associated with the 817 cm−1 Raman peak. Reproduced with permission from ref 26. Copyright 2013, Nature Publishing Group. 9334


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Figure 8. Local induced-field enhancement at resonance in the midplane of the gap between two Na380 clusters for our three configurations: facetto-facet gap (left column), tip-to-facet (middle column), and tip-to-tip (right column). The incident plane wave is z-polarized. From top to bottom, each case shows a decreasing separation distance for each configuration, from dsep = 20 Å (largely separated particles, on the top row) to dsep = 1 Å (interpenetrating situation on the bottom row). The influence of the atomic scale features on the nanogaps is directly noticeable. Reproduced with permission from ref 118. Copyright 2015, American Chemical Society.

and the far-field Raman signal of the same area could not be detected with the tip retracted 1 μm from the substrate. The spatial resolution of TERS here can be as good as 15 nm by imaging a single BCB molecule with the Raman mode of 568− 572 cm−1.24 In 2012, Deckert’s group demonstrated the direct molecular distinction of selected amino acids on an insulin fibril surface using TERS.116 As observed in Figure 5, by scanning a metalized AFM tip along an insulin fibril on a gold nanoplate with a step size of 0.5 nm, TERS spectra are acquired along a 12 nm profile on the fibril (as indicated by the black line in Figure 5b). In Figure 5c, 14 consecutive spectra are displayed; the various spectra at different positions are distinct, and an assignment with respect to different amino acids is possible. For histidine, the Raman signals at approximately 1331 (H1), 1183

for studying a single molecule with ultrahigh spatial resolution and fingerprint identification ability. In 2007, Pettinger’s group demonstrated the first UHV-TERS system, which exhibited a Raman enhancement of approximately 106 for the resonance molecule brilliant cresyl blue (BCB) adsorbed on a gold crystal.74 The next year, these researchers performed the first TERS observation of a single BCB molecule absorbed on an Au (111) surface in an UHV-TERS system, which was verified by simultaneously recorded STM images. Figure 4a,d shows the single-crystalline gold surface, covered with either five or a single BCB molecule, imaged by STM. Figure 4b,e shows the STM height profiles of one BCB molecule with a lateral size of 1.4 × 0.7 nm2 and a height of 120−140 pm. Moreover, TER spectra were recorded with the molecules located precisely in the gap between the tip and substrate, as shown in Figure 4c,f, 9335


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Figure 9. Local field profile and Raman signal without and with considering the electromagnetic self-interaction of a molecule with the tip and substrate. (a) Calculated intensity profile of local field confined within the gap, involving the molecule in self-interaction via multiple elastic scattering with the Ag tip−substrate gap. The molecular polarizability is β = 0.45 × 10−37 C m2/V); the lateral and longitudinal displacement of molecule to substrate are x0 = 0 nm and d1 = 1.4 nm, respectively. The fwhm of the “hot spot” is approximately 1.3 nm. (b) Raman signal intensity versus the lateral displacement of molecule x0, as predicted by the unmodified (red) and modified (black) theory. The fwhm of the Raman signal is 7.2 and 2.2 nm for the unmodified and modified theory, respectively. Reproduced with permission from ref 120. Copyright 2015, American Chemical Society.

Figure 10. Plasmon catalytic reactions performed by (a) AFM and (b) HV-STM TERS. (c−g) Laser-intensity-controlled dynamics of plasmon-driven chemical reactions. Reproduced with permission from refs 25 and 109. Copyright 2012, Nature Publishing Group.

(H2), and 1494 cm−1 (H3) serve as distinct markers, and their intensities increase to a maximum between 8−9.5 nm from the starting point and then decay simultaneously. A 2 nm lateral resolution for direct molecular distinction of selected amino acids on insulin fibril surfaces was claimed by the authors. A similar intensity change of the characteristic Raman peaks from different amino acids can also be observed.116 This TERS

Figure 12. (a) Measured IR spectra of pyrazine powder. (b) HV-TER spectrum of pyrazine adsorbed on Ag film. (c) Raman spectrum of pyrazine powder. Reproduced with permission from ref 77. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 11. (a) Stokes and (b) anti-Stokes HV-TER spectra of DMAB. Reproduced with permission from ref 25. Copyright 2012, Nature Publishing Group. (c) The distance-dependent wavelength-scanning plasmon-enhanced spectra of TERS. Reproduced with permission from ref 112. Copyright 2013, Royal Society of Chemistry. 9336


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Figure 13. (a) Electric field and (b) electric field gradient intensity along tip axis (z-axis); (c) ratio of the electric field gradient over electric field intensity at 0.5 nm above the substrate; (d) electric field and (e) electric field gradient intensity along the substrate at 0.5 nm above the substrate in the x−y plane; (f) ratio of electric field gradient over electric field intensity in the x−y plane. The unit of the electric field intensity is V/m, and the unit of the electric field gradient is V/ m/au, where au is atomic unit. Reproduced with permission from ref 77. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

measurement of a single insulin fibril demonstrates the feasibility of mapping a fibril by tracking different amino acids by using their marker bands. In 2013, an inconceivable result was reported by Dong’s group.26 The researchers not only obtained SM-TER spectra but also demonstrated the unprecedented spatial resolution of TERS, i.e., submolecule resolution, by mapping a single H2TBPP molecule on a Ag (111) surface using the selected Raman peaks in the UHV (10−10 Torr) and costumed lowtemperature (LT) conditions, thereby demonstrating the great potential of LT-UHV-TERS in probing a single molecule. As illustrated in Figure 6b, by matching the nanocavity plasmon (NCP) resonance (blue curves) and the downward molecular vibronic transition of Qy(1,1), an efficient doubly resonant TERS process can be realized under the “on-resonance” condition. Figure 6c shows that for a single H2TBPP molecule, the TERS spectrum is stronger at the lobe of the molecule than at the center, and only a broad background is observed on the bare silver, thus indicating that the TERS signals are highly localized. The top rows in Figure 6d show the TERS mapping images of a flat-lying H2TBPP molecule, with five selected Raman peaks processed from all the individual TERS spectra acquired at each pixel. The bottom rows are the corresponding simulated images, which are consistent with the experimental results. Figure 6e,f shows that the spatial resolution of the STM topography and the TERS mapping profile are comparable; subnanometer resolution was claimed by the authors.26 For such a high resolution, the authors also gave some discussions. This is analogous to the stimulated Raman scattering (SRS)

Figure 14. (a−c) Tunneling current intensity dependent TER spectra of 2,2′DA-DMAB produced from 2,4-DNBT and (d) the ratio of electric field gradient over the electric field. Reproduced with permission from ref 143. Copyright 2013, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

process, in which the incident laser acts as a Raman pump source and the NCP field generated in situ by the laser acts as the Stokes field for the possible stimulated Raman emission. The NCP field is naturally highly confined in space, causing researchers to expect greatly improved spatial resolution. Moreover, the SRS is a third-order nonlinear optical process, which further improves the spatial resolution. Thanks to the |E|6 relation in the third-order optical process, the signal can be localized within 1−2 nm for a tip with a 25 nm apex radius.26 Sun et al. interpreted the reason for nanometer spatial resolution by using the effect of the electric field gradient.117 An electric field can significantly enhance Raman modes, while the electric field gradient can significantly enhance the IR-active modes. In Dong’s experimental results,26 both Raman and IRactive modes were simultaneously successfully observed; see Figure 7a,b. Figure 7c,d clearly reveals that there are four IR9337


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Figure 15. (a) Raman spectra for KTP, showing differences between near-field spectra, micro-Raman spectra, and far-field spectra through the probe;137 (b) the points show the integral under the 712 cm−1 peak and that under the 787 cm−1 peak in different spectra, obtained by subtracting spectra at the distances indicated from those far from the surface. The solid line is a model for GFR, as described in the text, and the dotted line is what would be expected for the standard Raman case. Reproduced with permission from ref 141. Copyright 2000, American Physical Society.

active modes at the lower frequency shift (see blue dotted vertical lines) and five Raman-active modes at the higher frequency shift (see red dotted vertical lines), respectively.117 Furthermore, the peak intensity ratio of IR-active mode to Raman active mode for these flat-lying H2TBPP molecules is much stronger than that for the molecules being perpendicularly adsorbed on the substrate. This well agrees with the variation trend in Figure 7a,b, where the tilt angle of molecules varies from 0° to 30° and the relative intensity of IR-active modes greatly decreases, while the intensity of Raman-active modes increases. These experimental and theoretical results reveal that the electric field gradient can play a most important role for the horizontal orientation of molecules, while the electric field is more important regarding the vertical orientation; thus, the electric field gradient may have a dominant contribution to the TERS resolution. Barbry et al. also interpreted this atomic-scale resolution in nanophotonics, using the near-field plasmonics at the atomic level.118 Using sodium clusters, they demonstrated that the atomistic structure of the nanoparticle morphologies determined the presence of near-field hot spots at the subnanometric level, which are further enhanced by the action of underlying plasmonic fields. Figure 8 shows the evolution of plasmonic field confinement in the nanogap as a function of the separation distance, dsep, and then the corresponding change of the localization place, where plasmonic field enhancement is demonstrated in the (x, y) midplane between two particles for each configuration (the tip-to-tip in the right column, the tipto-facet in the middle column, and the facet-to-facet in the left column). When particles are far away from each other (dsep = 20 Å), the relatively spatial profile of the near-field can be obtained (top row). For smaller separation (dsep = 10 Å), we can see that the profile of plasmonic near fields reveals atomistic configuration of the cluster surfaces across the nanogap, which shows a triangular structure for the facet-tofacet configuration, a round spot for the tip-to-tip configuration, and a round spot on the top of the triangular background for the tip-to-facet configuration. If dsep is less than 6 Å (see the

Figure 16. (a) STM topography taken simultaneously with TERS images (24 × 12 nm2 with 40 × 20 pixels). The pink profiles highlight the positions of CNT-2 and CNT-3. Scale bar, 5 nm. (b−d) TERS intensity images for three Raman peaks (ID, I−G + I+G, I−2D + I+2D). The accumulation time at each pixel is 0.15 s. Both the G-band and 2Dband are fitted by double Lorentzian functions. (e) Intensity cross section of the D-band signal along the arrowed line in part b and fitted with a Gaussian profile (dashed orange curve). The 10−90% TERS resolution is 1.7 nm. (f−h) Mapping of spectral peak-energies for three Raman bands. The D-band, ωD (f), is fitted with a single Lorentzian, while both G-band and 2D-band are decomposed with two Lorentzian profiles after subtracting the far-field Raman background. The center frequency of higher-energy-peaks (g and h) is selected for mapping because the deviations are huge due to their weak signals. Reproduced with permission from ref 43. Copyright 2014, Nature Publishing Group.

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Figure 17. (a) TERS spectrum acquired for a BaTiO3 nanorod with phonon mode assignment (fluorescence background subtracted). (b) Corresponding tip−sample distance dependence. (c) Spectrally resolved line scan across a BaTiO3 nanorod. Reproduced with permission from ref 149. Copyright 2009, Nature Publishing Group.

the molecule. This can result in spatially resolved highresolution for the molecular Raman image, and the resonant Raman image reflects the electronic transition density of the molecule. Recently, Zhang et al. also investigated the origin of resolution at the level of nanometer through theoretical calculations.120 In a very strong plasmonics nanogap at resonance, the near field self-interaction of the plasmonic field and molecule must be considered because of the multiple elastic scattering of molecules in the plasmonic near field. The TERS enhancement factor is modified as GS(r0) = =1 −

dm 3

|E N(r0 , ω)|4 |E0(r0 , ω)|4

∑ ((rm 3 eik |r − r |)/(|r0 − ri|3)Bi Mi) 0 0

i

i

(5)

where EN(r0, ω) is the modified local field, when considering the plasmonic near field self-interaction of molecules with the incident laser field E0(r0, ω). Because each molecule has a comparable size to the plasmonic near field nanogap, we can conclude that this type of self-interaction can significantly modulate both Raman excitation and the radiation in both the spatial sensitivity and the Raman intensity. When the second term in eq 5 is close to 1 for TERS, GS(r0) can be highly nonlinear with respect to the molecule position r0. And this leads to ultrahigh sensitivity of Raman signal upon the relative position of molecule to the metal nanogap, namely, the Ag tip. The type of self-interaction effect can well create a “super-hot spot” with the size of a nanometer in the near field nanogap, resulting in the subnanometer lateral resolution of the Raman mapping; see Figure 9. Although several theories have been proposed to interpret the nanometer resolution, more experiments are needed to prove the theories and reveal physical origins of such high spatial resolution. Plasmon-Driven Reactions. In addition to the EM field enhancement, localized surface plasmons excited on silver or gold nanoparticle surfaces can decay nonradiatively into “hot electrons” with a high energy between the Fermi and vacuum energy level.121 If hot electrons scatter into an excited state of the absorbed molecules, triggering a chemical reaction by reducing the activation energy, this is called a “plasmon-driven reaction”.25,122−124 Plasmon-driven reactions can be tracked at the nanoscale by SERS,125,126 including graphene-mediated surface-enhanced Raman scattering (G-SERS),127−129 remotely excited SERS,130−132 and single molecule SERS.124 Compared

Figure 18. (A) Waterfall plot of 60 5 s TER spectra of R6G collected using ps irradiation (λ = 532 nm, Pex = 0.05 W/cm2). The initially retracted Ag tip was brought into tunneling range at t = 60 s. (B) CW TER engaged (blue) and retracted (gray) spectra (λ = 532 nm, Pex = 1.3 W/cm2, tacq = 30 s) plotted above averages of 12 ps TER engaged spectra with SNRs above 12 (red) and 12 retracted spectra (gray) from (A). Black lines represent individual and composite peak fits. The included scale bar applies to all plotted spectra. Reproduced with permission from ref 45. Copyright 2016, American Chemical Society.

lower rows), one can see that the tunneling current expels plasmonic fields from the middle of the nanogap. This type of phenomenon is clearly observed for the charge-transfer plasmon mode in the facet-to-facet configuration. Luo and co-workers also reported theoretical simulations of plasmonenhanced Raman images of a single molecule with subnanometer resolution.119 Their results revealed that the optical transition matrix of the molecule becomes dependent on the position and distribution of the plasmonic field when the spatial distribution of the plasmonic field is comparable to the size of 9339


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Figure 19. (a) Hemisphere radius, (b) thickness of the substrate, (c) incident angle, and (d) distance dependent surface plasmon enhancements of DUV-TERS, where the tip is an Al coated silicon tip. Reproduced with permission from ref 162. Copyright 2011, Royal Society of Chemistry.

Figure 20. (A) Schematic of in situ measurements of TERS and TEF, where the color map shows the schematic of the electric field intensity distribution of the plane between the tip and film. (B) Calculated results: (a) dependence of TERS and TEF enhancement factor on tip-film distance. (b) Dependence of TERS and TEF enhancement factor on wavelength at tip-film distances of d = 2 nm. The shadow in part B,a stands for the suitable region for in situ measurement of TERS and TEF. The red dotted line is 632.8 nm laser. Reproduced with permission from ref 167. Copyright 2016, Nature Publishing Group.

with SERS, TERS with high spatial resolution and controllability is an ideal tool for the study of plasmon-driven reactions. Plasmon-driven reactions revealed by TERS were first reported by Sun and Lantman, respectively, in 2012.25,109 As shown in Figure 10a, a monolayer of 4NBT was assembled on a sample of gold nanoplate on glass and placed in TERS based on the AFM system. Excitation and collection of the Raman signals occurred from below, and the silver-coated AFM tip was used to generate strong plasmon intensity to catalyze 4NBT to DMAB. The incident laser could switch between green (532 nm) and red (632.8 nm) for monitoring the reaction in time. Figure 10b shows the plasmon-driven reduced reaction of 4NBT to DMAB revealed by HV-TERS based on the STM system. It was demonstrated that two 4NBT molecules can be

dimerized into one DMAB molecule by using the HV-TERS setup. The reaction process could be well controlled by the plasmon intensity, which strongly depends on the power of the excitation laser. In the TERS experiment, as illustrated in Figure 10c, when the power was below 3% of the full laser power (60 μw), the main peaks at 1073, 1336, 1587 cm−1 were all attributed to 4NBT, thereby indicating that the reaction could not occur. With increasing laser power, it can be seen that the strongly enhanced 1432 cm−1 peak of the −NN− stretching vibrational mode appears, indicating that certain 4NBT molecules combined into DMAB. When the laser power was increased to 100%, the peak of the −NO2 stretching mode of 4NBT at 1336 cm−1 almost disappeared, which indicated that almost all the 4NBT molecules were dimerized into DMAB, as 9340


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in TERS, the electromagnetic field is highly confined at the apex of the metal tip and will be largely enhanced. In such a high electric field, the polarization intensity of P may no longer exhibit a linear relationship with E. The higher order terms of E should be considered as follows:

shown in Figure 10f. Note that when the laser power was again reduced to 0.5%, the TER spectrum in Figure 10g became the same as in Figure 10f rather than that in Figure 10c. This finding indicates that the reaction is irreversible. Furthermore, in the STM-based HV-TERS, the plasmon intensity in the nanogap could also be controlled by changing the tunneling current and bias voltage to tune the distance between the tip and substrate, which offers a way to control chemical reactions. Plasmon-driven oxidized reaction of PATP to DMAB has also been revealed by TERS133 and HV-TERS73,113,134 and be reviewed.122,135 So, DMAB cannot only be catalyzed from DMAB by reduced reaction but also be catalyzed from PATP by oxidized reaction revealed by TERS. In Situ Local Temperature. Local temperature measurements are very important in observing molecular reactions, superconducting transitions, and heterogeneous catalysis. In general, the temperature is measured by using a thermometer or thermocouple, which mainly provides the average temperature of the environment instead of the local temperature. TERS with nanoscale spatial resolution and ultrasensitive Stokes and anti-Stokes Raman scattering provides a novel method for measuring the in situ local temperature. According to the Boltzmann distribution, the temperature where the molecules are located can be fitted precisely using the equation25 IS/IaS = a e(ℏω / kBT )

μ = μp + αE + βE ·E + γE ·E ·E + ···

where β and γ are the third and fourth polarizability tensor, respectively. In this case, the strongly enhanced field at the apex of a sharp metallic tip can be used as a local second-harmonic generation (SHG) source, and hyper-Raman (HR) scattering will appear. The HR scattering follows different selection rules than normal Raman scattering, and they are mutually exclusive in centrosymmetric systems. This is expected to be a powerful super resolution imaging method138,139 The amplitude of the electric field varies dramatically near the hot spot: the dipole moments in such inhomogeneous field can be written as μ = μp + α E +

(6)

( 2λπ )E;

gradient is on the order of i

(10)

thus, the ratio of the

gradient-field Raman (GFR) term to the Raman term is on the order of 2παM/λ, approximately 10−3 in the visible light region. Therefore, the gradient effect in free space can be ignored. While near the metal surface or in the TERS system, the ratio of the GFR to the Raman term is close to 1, which can no longer be neglected.5,140−142 In 2014, Sun et al. demonstrated the GFR effect both by experiment and through theoretical calculations for the TERS system.77,113 Figure 12 shows that the HV-TER spectrum of pyrazine is significantly different from its normal Raman spectrum. There are many additional peaks in HV-TERS. These additional peaks were analyzed, and the IR spectrum of the pyrazine powder was also measured, as shown in Figure 12a. It can be observed that some of the enhanced vibration peaks originated from the IR modes of pyrazine. Moreover, the nonlinear vibrational peaks originating from the Fermi resonance could also be clearly observed in the HV-TER spectrum. To reveal the strong electric field gradient (EFG) effect in HV-TERS, the electric field and EFG along the tip and along the substrate were calculated using the finite element method. As demonstrated in Figure 13, the electric field (a) and EFG intensity (b) along the tip axis were calculated. The ratio of the EFG over the electric field intensity is also shown in Figure 13c. The ratio was approximately 0.013 along the z-axis in atomic units. Considering the different scattering directions of the molecule, the electric field (d) and EFG (e) in the x−y plane were also calculated. The most effective region for the contribution of the EFG was within 1.5 nm < r < 5 nm, and the ratio of the EFG over the electric field was approximately 0.14 in atomic units. On the basis of the calculated electric dipole−dipole polarizability and dipole−quadrupole polarizability of pyrazine, the ratio of the GFR term over the Raman term was approximately 0.1 in the z direction and approximately 1 in the x−y plane. Thus, the additional peaks

(7)

where σ is the Stokes and anti-Stokes Raman scattering cross section, ω0 is the frequency of the incident laser, and A is the Stokes and anti-Stokes Raman scattering enhancement factor. The enhancement factor is wavelength dependent and can be calculated using the FDTD method, as shown in Figure 11c. The tip-enhanced plasmon resonance exhibits a broad width and symmetrical distribution around the incident laser (632.8 nm) in the nanogap between the tip and substrate. Thus, the enhancement factor A for the Stokes and anti-Stokes shift is almost the same (see Figure 11b).112 By measuring the Stokes and anti-Stokes Raman scattering intensity of the HV-TER spectra of DMAB, the local dimerization reaction temperature can be fitted by using the Boltzmann distribution. The local temperature is approximately 327 (±4) K and the experimental parameter a is 2.06 (±0.19).25 Nonlinear TERS. In general, the laser intensity is not very high and the Raman process is usually spontaneous Raman scattering. The dipole moments of the molecules can be written as

μ = μp + αE

1 A ·∇E + ··· 3

where A is the electric dipole−quadrupole polarizability. The third term is the gradient field term. The ratio of (A/α) is on the order of the ratio of the quadrupole to dipole moment of a molecule, which has units of length and is on the order of the molecular dimension of αM as well. In free space, the field

where IS and IaS are the intensities of the Stokes and anti-Stokes Raman peaks, respectively. These intensities can be simultaneously measured by using a Raman spectrograph configured with a notch filter, as shown in Figure 11a,b. The a is an experimental constant, which can be calculated using the following equation:136,137 ⎛ σ(αω)S ⎞⎛ ω0 − ω ⎞4 N A(ω0 − ω)S2 a=⎜ ⎟⎜ ⎟ ∑ ⎝ σ(αω)aS ⎠⎝ ω0 + ω ⎠ i = 1 A(ω0 − ω)aS2

(9)

(8)

where μp is the permanent electric dipole moment and α is the polarizability tensor. However, when the incident laser intensity is very intensive, such as with a pulsed laser or in the nanogap 9341


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measured by using the tip−sample distance dependence. Thus, the TERS has been applied to the nanoimaging method for materials, combining ultrasensitive chemical analysis abilities and ultrahigh spatial resolution. Raschke and co-workers studied optical crystallography at the nanoscale using SFM-TERS.149 Via selective probing of different transverse optical phonon modes, they identified the intrinsic ferroelectric domains of individual BaTiO3 nanocrystals. The SFM-TERS spectrum in Figure 17a reveals the two dominant peaks, which have been assigned to the A1 transverse optical (TO) mode at 516 cm−1 and the E longitudinal optical (LO) mode at 715 cm−1, respectively. The Raman-active LO modes are characteristic of the tetragonal phase,150 and their observation thus reflects the ferroelectric state. Raman response with the tip−sample distance dependence can be seen in Figure 17b. The rise in the near-field signal above the far-field response can be seen when the nanorod tip correlates with the apex radius within 30 nm. Figure 17c reveals that the Raman signal is strongly enhanced up to 104 ∼ 105 when the lateral line scans across a BaTiO3 nanorod. Ultrafast Tip-Enhanced Raman Spectroscopy. Ultrafast and nonlinear surface-enhanced Raman spectroscopy has been developed.151 Van Duyne reported tip-enhanced Raman spectroscopy of rhodamine 6G on Ag(111) system with picosecond excitation in ultrahigh vacuum, see Figure 18.45 It was found that the signal intensity is observed to fluctuate over time in a manner that is comparable to CW UHV-TERS. The UHV environment suppresses diffusion and/or reactive decay chemistry in picosecond (ps) TERS, which can improve the stability in measurement. So, ps TERS is possible used in studying the spatially resolved dynamics of surface-bound molecules. Deep Ultraviolet (DUV) TERS. Taguchi and co-workers first reported DUV-TERRS, in which the tip and substrate were Al coated onto a silicon tip and an aluminum (Al) film, respectively.152 Al is found to be one of the best candidates when searching for DUV-TERS material because of its low absorption, down to a wavelength of 200 nm, due to plasmon resonance and its free-electron-like character in the range of UV. In the region of DUV, the dielectric function of Al demonstrates a reasonably small imaginary part, while the real part is negative.153 DUV-TERRS has been rapidly evolving because of its advantages and potential application in material sciences and bioscience.154−161 Theoretical analysis of DUVTERS has also been performed by Yang et al.,152 and the calculated results strongly supported the applications of DUVTERS. Theoretical results demonstrated that for the strongest local plasmon, the incident angle of light should be around 20°, the thickness of substrate should be larger than 40 nm, and the thickness range of silicon coated on the Al tip should be from 15 to 30 nm. There is not too much influence of the Al2O3 layer on DUV-TERS to obtain DUV-TERS signals, see Figure 19. TERS and Tip-Enhanced Fluorescence (TEF). Tipenhanced fluorescence (TEF), which is from the coupling between the absorption band of fluorophore and LSPR of the tip−substrate system, has also been widely studied.163−166 The combination of TERS and TEF is desirable for improving the accuracy and sensitivity in detection technology.167 Meng and co-workers theoretically proposed a nanoplasmonic strategy for precision in situ measurements of TERS and TEF spectroscopy.167 Figure 20A is the scheme of setup.167 Figure 20B is the calculated results, which revealed that the optimized distance of

were confirmed, from the IR modes, to be due to the strong electric field gradient effect.77 Experimentally, a controlled EFG effect by using HV-TERS has been reported.143 By turning the tunneling current, the distance of the nanogap can also be changed and the ratio of Raman intensity over the IR-active mode intensity can be manipulated (see Figure 14a−c), which is from the ratio variation of the plasmon gradient intensity over plasmon intensity (see Figure 14d). With increasing tunneling current, the ratio of plasmon gradient intensity over plasmon intensity is increased, and then the ratio of IR-activated mode intensity over Raman intensity is also increased. The experimental results are supported by theoretical calculations. Moreover, coherent anti-Stokes Raman spectroscopy (CARS, which is a third-order nonlinear optical process) has also been successfully performed in TERS.107,108 When TERS is combined with high-order nonlinear optical effects, the spatial resolution can be further improved due to the nonlinear processes being confined to a more local space. The SNOM-TERS is also used to observe the EFG effects.138 One of its advantages is that the tip can be retracted from the surface so that the tip can move the high field gradient region precisely away from the surface, which can effectively “turn off” the EFG effect. This is a very important way to confirm the origins of the observed Raman peaks. Figure 15a demonstrates the comparative spectra of a potassium titanyl phosphate (KTP) sample, measured by using micro-Raman, Raman with an NSOM probe retracted from the surface, and SNOMRaman (in the near field).141 It can be clearly seen that two farfield spectra are the same within noise, but the near-field spectrum differs due to the apparent addition of peaks near 680 and 714 cm−1, which are a Raman and strong IR (not Raman) line, respectively. The probe (metal)−sample distance dependence of the line intensity has also been reported for other SNOM-TERS, where the 712 cm−1 mode was enhanced in the near field.141 The distance dependence (Figure 15b) is very consistent with the field-gradient model. Because the 712 cm−1 vibration is a strong IR absorption, the EFG effect can be responsible for that spectrum.141 TERS for Materials. TERS also shows great potential applications in the investigation of material science, for example, semiconductor nanowires and one-dimensional carbon nanotubes (CNTs)144,145 as well as 2D materials such as semiconductors, solar cell thin films, graphene, etc.100,146−148 CNTs have shown very ideal object investigation for the TERS field, and many important experimental results have been reported. By using AFM-TERS, Yano et al. demonstrated the pressure-assisted TERS of single-walled CNTs. It was shown that by increasing the pressure to 3.3 nN, the G-band of a single-walled carbon nanotube could be shifted by more than 10 cm−1. Furthermore, by using such type of pressure-assisted TERS, a spatial resolution of approximately 4 nm could be well obtained.41 Recently, Chen et al. showed the simultaneous chemical analysis and structure of CNTs by using STM-TERS imaging, where the spatial resolution could be down to 1.7 nm; see Figure 16.42 With a step precision of 0.6 nm, Figure 16a demonstrates the STM-TERS images at very high-resolution. The D-band signal for CNT-3 in Figure 16b demonstrates better confinement than the topographic width. The profile of the D-band intensity shows a 10−90% intensity transition along the arrowed line within 1.7 nm (Figure 16e). Thus, high resolution was further supported by magnificent photon confinement in the z direction (along the tip), which was 9342



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tip-film is around 2 nm, in which the distance is suitable for efficient in situ acquisition of TEF and TERS. Furthermore, the spatial resolution of TEF and TERS can be down to 10 and 6.5 nm, respectively.167

Feature

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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Author Contributions ⊥

SUMMARY AND OUTLOOK TERS combines the large enhancement factor of SERS and the high spatial resolution of SPM technology, which substantially breaks the diffraction limit of light and is applied as a powerful technology in researching single molecules and the nanodistribution of matter. UHV-TERS has great potential in studying materials at the single-molecule level. Single-molecule detection and mapping have been achieved within ultrahigh spatial resolution. TERS enables the study of the dynamic processes of chemical reactions at the single-molecule level and offers a new method for manipulating and designing new molecules. Moreover, because of the highly enhanced and localized electric field at the apex of the metal tip, tip-enhanced nonlinear Raman spectroscopy effects, such as the second harmonics and hyper-Raman, can be observed in TERS. Various silent modes and IR modes also appear in TERS as a result of the substantial electric field gradient effect in the nanogap between the tip and substrate. Meng and co-workers theoretically proposed a nanoplasmonic strategy for precision in situ measurements of TERS and TEF spectroscopy.167 It is expected this proposal will be a new orientation for the spectral analysis at the nanoscale. The studies demonstrated the great potentials of HV-TERS in the study of the dynamic process of chemical catalytic reactions and offer a new way to manipulate and design molecules at the single-molecule level. Similar with GSERS,127−129 the tip and the substrate covered by graphene are expected to be used in surface catalytic reaction codriven by plasmon-exciton coupling of metal−graphene hybrid. Although many important achievements have been made with respect to TERS, there are certain urgent issues that need to be considered. The studied objects are rather limited in TERS; usually, resonance molecules or materials with large Raman scattering cross sections are studied. Studying materials at the single-molecule level remains very difficult. However, the electromagnetic field has been greatly enhanced, and in such a strong field, the heating effect and photobleaching cannot be neglected. Because TERS involves fabricating high-quality and repeatable tips to enable greater enhancements, the lifetime of a tip, especially for silver tips, is very limited; moreover, tips could be damaged by mechanical degradation or laser heating and can be contaminated easily. It is expected that UHV-STM-TERS will become an atomicresolution microscopy technique for low temperature. The ability to map the inner structure of a single molecule with atomic resolution makes it a powerful tool in the investigation of catalytic dynamics at the level of a single molecule. The in situ Raman spectra and the topographical imaging can be simultaneously obtained, providing ideal techniques for revealing the dynamic processes of plasmon-driven catalytic reactions. Femtosecond lasers can be applied to this field; subsequently, the time-resolved UHV-STM-TERS spectra can play the most important role in molecular surface catalysis reaction dynamics. TERS will play a very important role in physics, chemistry, biology, and material and life sciences.

Z.Z. and S.S. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Zhenglong Zhang is a Postdoc and an Av-H Fellow at the Leibniz Institute of Photonic Technology, Germany. He obtained his Ph.D. in 2013 from Shaanxi Normal University and the Institute of Physics, Chinese Academy of Sciences, China, working on STM-based HVTERS. His current research interests focus on TERS and plasmonic catalysis. Shaoxiang Sheng is a Ph.D. candidate at the Institute of Physics, Chinese Academy of Sciences, China. He is mainly working on a lowtemperature STM-based UHV-TERS system. Rongming Wang received his Bachelor and Master’s Degrees in Physics from Peking University and a Ph.D. in Materials Science from Beijing Institute of Aeronautical Materials, China. In 2004−2005, he was a visiting scholar in University of California, Berkeley. Then he joined Beihang University as a Professor of Physics. Currently, he is a professor at the University of Science and Technology Beijing. His research interests include magnetic nanomaterials, transmission electron microscopy, and interface science. Mengtao Sun obtained his Ph.D. in 2003 from the State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS). From 2003 to 2006, he worked as a postdoc at the Department of Chemical Physics, Lund University. Since 2006, as an Associate Professor, he has worked at the Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS. In 2016, he is a Full Professor at the University of Science and Technology Beijing. His current research interests focus on HV-TERS, SERS, and plasmon-driven chemical reactions.

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ACKNOWLEDGMENTS This work was supported by National Nature Science Foundation of China (Grant No. 91436102, 11374353, 11504224 and 11474141), National Basic Research Program of China (Grant number 2016YFA02008000), and the Program of Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials.

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