Nanoscale chemical characterization of materials and interfaces by tip-enhanced Raman spectroscopy

Yi-Fan Bao ^a, Meng-Yuan Zhu ^a, Xiao-Jiao Zhao ^a, Hong-Xuan Chen ^a, Xiang Wang *^ab and Bin Ren *^ab
aCollaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. E-mail: wangxiang@xmu.edu.cn; bren@xmu.edu.cn
^bInnovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen 361005, China

First published on 4th September 2024

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Yi-Fan Bao

Yi-Fan Bao received his BSc Degree in Chemistry from Xiamen University in 2019. Currently, he is a PhD student under the supervision of Professor Xiang Wang and Professor Bin Ren at the Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University. His current research focuses on the design and fabrication of highly active TERS tips and electrochemical-tip enhanced Raman spectroscopy (EC–TERS) study of electrocatalytic processes.

Meng-Yuan Zhu

Meng-Yuan Zhu received her BSc Degree in Chemistry from Lanzhou University in 2020. Currently, she is a PhD student under the supervision of Professor Xiang Wang and Professor Bin Ren at the Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University. Her current research focuses on the EC–TERS study of the electrocatalysis.

Xiao-Jiao Zhao

Xiao-Jiao Zhao received her BSc Degree in Materials Chemistry from Shanxi Normal University in 2020. Currently, she is a PhD student under the supervision of Professor Xiang Wang and Professor Bin Ren at the Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University. Her current research focuses on the TERS study of catalytic processes.

Hong-Xuan Chen

Hong-Xuan Chen received his BSc Degree in Chemistry from Xiamen University in 2023. Currently, he is a PhD student under the supervision of Professor Xiang Wang and Professor Bin Ren at the Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University. His current research focuses on the EC–TERS study of electrocatalytic processes.

Xiang Wang

Professor Xiang Wang received both his BSc Degree (2007) and PhD Degree (2013) from Xiamen University supervised by Professor Bin Ren. He joined Xiamen University as an Associate Professor in 2020. His research interests focus on developing plasmon-enhanced Raman spectroscopy (PERS), including TERS and SERS, to correlate the structure and activity of surfaces and interfaces.

Bin Ren

Professor Bin Ren received his PhD Degree from Xiamen University in 1998 supervised by Professor Zhong-Qun Tian. He spent his sabbatical year at the Fritz-Haber Institute, Germany as an AvH Fellow during 2002 and 2003. He was promoted to a Full Professor at Xiamen University in 2004. He was awarded the Young Chemist Award from Chinese Chemical Society, the Chinese Young Electrochemist Award and the Electrochemistry Award of Analytical Chemistry Division, ACS. He was elected as a Fellow of The International Society of Electrochemistry in 2021. The research interests of Professor Bin Ren include the development of high spatial and temporal resolution Raman methods and instruments as well as deep learning-based methods for studying surfaces and interfaces as well as biological systems. He is a now an Associate Editor of Analytical Chemistry (ACS).

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1. Introduction

The development of a large variety of fields highly relies on high-performance materials, including catalysis, energy storage and conversion. With the fast development of nanotechnology, it is generally recognized that when the size of materials is reduced to the nanometer scale, owing to the dramatic increase in their surface area and the quantum confinement effect, nanomaterials exhibit unique physical and chemical properties, especially with impressive optical behaviors and catalytic activities.^1–3 In particular, the surfaces and the interfaces are at the heart of the materials, which connect the material itself with the outside environment and are the core for their applications. Different from their bulk material, the surfaces of materials possess unsaturated coordination sites with unique structural and electronic properties, leading to strong interactions with surface molecular species, and thus a higher activity. Some special sites (the so-called active sites) at nanoscale or even atomic scale on the surfaces or interfaces of the materials may determine their performance.^4–6 Therefore, to clearly correlate the structure with the activity of both the materials and interfaces requires advanced characterization techniques that are capable of providing nanometer scale resolved structural and chemical information at the molecular level.^7,8

In recent years, various advanced techniques with nanoscale or even atomic spatial resolution have started to flourish, offering great opportunities to gain a deeper understanding of the structure–activity relation of both materials and interfaces. Basically, these techniques can be categorized by the probe utilized, as follows: (1) scanning electron microscopy and transmission electron microscopy (TEM), which utilize an electron beam as the incident source with a wavelength shorter than 1 nm, can obtain fine structural information of the target sample. Recently, the fast development of environmental TEM^9,10 and liquid or even electrochemical TEM^11,12 has made it possible to visualize structural changes under the reaction conditions. Combined with electron energy loss spectroscopy, the morphological and chemical information of the target material with a spatial resolution better than 5 nm can be obtained.^13,14 (2) Scanning probe microscopy (SPM) techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), which employ a physical tip to raster scan the sample, can probe a variety of properties of the sample according to the specific interaction between the tip and surface, such as morphology, electronic and mechanical properties. Since the early stage, in situ SPM techniques have been developed to characterize materials and interfaces even at atomic resolution,^15–18 enabling the visualization of dynamic structural information of materials and the molecular orientation of surface species under different environmental conditions.^19,20 Recently, the development of noise-STM has made it possible to directly identify the active sites on the surface of Pt during the hydrogen evolution reaction (HER) by analyzing the noise of the tunneling current at different sites.²¹ In particular, the invention and rapid development of the qPlus sensor significantly improved the sensitivity of AFM,²² allowing the visualization of the chemical structure inside a molecule, intermolecular interactions and the fine structure of the monolayer water on a metal surface.^23–26 (3) With the development of the synchrotron-based light source, X-ray-based techniques, such as X-ray diffraction, X-ray adsorption (XAS), X-ray scattering and X-ray photoelectron spectroscopy, have become a popular method in probing the crystal structure, valence state and local atomic structure of materials.^12,27–33 In a recent reported work that combined XAS with STM (SX-STM), even the elemental and chemical states of just one atom were obtained based on the X-ray-excited electrons detected by STM, paving the way for using X-ray-based techniques to study materials at the single-atom limit.³⁴ (4) Benefitting from their non-destructive analysis with abundant information, optical techniques are widely used for characterizing materials and their interfaces. However, the spatial resolution is limited by the Abbe diffraction limit to around hundreds of nanometers. Thus, different strategies such as scanning near-field optical microscopy^35–38 and single-molecule fluorescence imaging (SMFI)^39,40 have been developed to overcome this limitation. For example, SMFI enabled by super-localization was able to identify the active sites during chemical reactions and measure the kinetics of chemical reactions at different sites.^41,42

Vibrational spectroscopy, including infrared spectroscopy, Raman spectroscopy, and sum-frequency spectroscopy, are the most commonly used methods to obtain chemical information. Among them, Raman spectroscopy is unique, because it can present vibrational modes across a wide frequency region. As a result, abundant information can be extracted from a Raman spectrum, including weak interactions (such as hydrogen bonds^43–45 and interlayer interactions) in the ultralow-frequency region (below 100 cm⁻¹), the lattice vibration,^46–50 the molecule–metal interaction^51–57 in the low frequency region (hundreds of cm⁻¹) and the molecular vibrational modes in the high frequency region (above 1000 cm⁻¹). It is worth mentioning that the interference from water can be neglected because of its small Raman cross section, making Raman spectroscopy a powerful tool for the in situ characterization of chemical and biological processes in aqueous samples. However, Raman spectroscopy suffers from ultralow sensitivity owing to the very small Raman scattering cross section, where approximately one Raman photon is produced with 10⁶–10¹⁰ incident photons.^58,59

The emergence of surface-enhanced Raman spectroscopy (SERS)^60–62 in the 1970s dramatically increased the sensitivity of Raman spectroscopy up to single-molecules level,^63,64 which significantly broadened its applications. The enhancement of SERS mainly originates from the surface plasmon resonance (SPR) effect. Briefly, SPR effect is the collective oscillation of free electrons in metal nanostructures under the excitation of incident light with a suitable wavelength.⁶⁵ Consequently, the electromagnetic field on the surface of the metal nanostructures will be significantly enhanced, thus amplifying the Raman signal of the species located on or in the vicinity of the surface. After 50 years of development, SERS technique has become a routine analytical method in fields of materials science, biology,^66–68 surface science,^18,69–71 etc.^72–74 In particular, the spatial resolution of SERS limited by diffraction was overcome by the development of the tip-enhanced Raman spectroscopy (TERS) technique in 2000.^75–78 TERS is an apertureless near-field optical technique, combining SPM and SERS. In principle, a sharp tip with plasmonic activity is controlled by the SPM system to approach close to the vicinity of the target sample. Then, with the illumination of an incident laser with a suitable wavelength and polarization, a highly enhanced and localized electromagnetic field will be generated underneath the tip apex owing to SPR and lightning-rod effects. This enhanced and confined field serves as a nanoscale “amplifier” to enhance the Raman scattering of the sample beneath the tip. By raster scanning over the sample surface, TERS can simultaneously obtain morphological and the chemical information at nanometer scale spatial resolution with high sensitivity even down to the single-molecule level, providing a promising tool to comprehensively investigate materials^79–82 and their surfaces.^83–86

Recently, a review highlighted the new era of SERS and TERS, especially the understanding and application of angstrom-resolved TERS.⁷⁴ Additionally, some excellent reviews have discussed the general advances of TERS in recent years.^87–94 There are also reviews focusing on TERS tips,^95–97 the characterization of bio-materials,^98,99 ultrahigh-vacuum (UHV) TERS,¹⁰⁰ application of TERS in catalysis,^101,102 TERS in single-molecule chemistry¹⁰³ and electrochemical (EC)-TERS.^104–106 Alternatively, in this review, we mainly focus on the applications of TERS in the nanoscale chemical characterization of materials and interfaces. The way nano-resolved Raman spectroscopy helps to reveal the unique properties and activities of materials and interfaces is discussed in detail. In Section 2, we discuss the theoretical understanding and technical improvements of TERS in recent years, which are the basis to optimize the performance of the TERS technique, including the general interpretation of the enhancement and the spatial resolution of TERS, the development of TERS instruments, and the fabrication of TERS tips. In particular, we also note the applications of algorithms in TERS, which is rapidly growing. In Section 3, we present several experimental issues that need to be addressed to conduct successful and reliable TERS measurements. This seldomly discussed topic is believed to be helpful for beginners in TERS. In Sections 4 and 5, we present an overview of the progress in TERS for characterizing materials and interfaces. The applications of TERS in studying carbon-based materials, two-dimensional materials and other materials are discussed, while the investigation of solid–gas interfaces under either ambient condition or UHV condition and solid–liquid interfaces (especially the electrochemical interface) are included. In the final section, we propose the future developments of TERS to make it a more powerful technique for broader applications.

2. Theoretical understanding and technical improvements of TERS in recent years

After development for more than two decades, it is time to clarify the understanding of enhancement when TERS measurement is conducted on substrates with different dielectric properties. This forms the basis to design optical path and tips to achieve an optimized TERS performance. In addition to the hardware, recently the utilization of artificial intelligence (AI) has introduced a new opportunity to further improve the sensitivity of TERS.

2.1. General understanding of TERS

The enhancement in TERS originates from the lightning-rod effect and the SPR effect. The lightning-rod effect comes from the accumulation of free electrons at the sharp apex of the tips driven by an external electromagnetic field. Therefore, the lightning-rod effect is a non-resonant effect, which exists in all types of TERS tips. Incident light with polarization parallel to the tip shaft will increase the lightning-rod effect. In contrast, the SPR effect is a resonance effect, where the collective oscillation of conduction electrons leads to much more effective accumulation of free electrons at the tip apex, and thus a higher surface charge density than that of the lightning-rod effect. Therefore, to obtain a strong enhancement, the SPR effect needs to be efficiently excited. The dielectric function of metals will dramatically influence the efficiency of the SPR effect. In the visible range, where TERS generally works, coinage metals including Au, Ag and Cu are the best materials to support the SPR effect. In terms of the TERS configuration, there are different ways to excite the SPR effect, including gap plasmon, localized surface plasmon resonance (LSPR) and surface plasmon polariton (SPP).

When the substrate used in the TESR measurement is a coinage metal (so-called gap mode TERS) (Fig. 1a), a so-called gap plasmon will form from the electromagnetic coupling between the tip and the coinage metallic substrate with the illumination of light polarized along the tip shaft. Gap plasmons are the most efficient way to excite the SPR effect and exhibit a strongly enhanced electric field. Consequently, the Raman spectrum of even a single molecule can be obtained in gap mode TERS. In principle, both the tip and substrate made of coinage metals can support the generation of gap plasmons. Considering the good chemical stability of Au in air and liquid, the most widely used configuration in experiment is a Au tip coupled with a Au substrate. Gap plasmons are quite sensitive to the distance between the tip and the substrate (so-called gap distance), where the enhancement exponentially decreases with an increase in the gap distance.^107,108 As a result, only ultra-thin materials can be studied in gap mode TERS. However, a too small gap distance, for example, the gap distance in the tunneling region may reduce the plasmon enhancement owing to the tunneling and nonlocal screening.¹⁰⁹ In addition, the morphology of the TERS tip, including the radius and the cone angle of the tips as well as the illumination configuration will influence the efficiency of gap plasmons. For example, according to the simulation result of side illumination TERS under 633 nm light (Au tip coupled with Au substrate), the enhancement increases when the tip radius increases from 15 nm to 50 nm and does not grow with a further increase in the radius. An interesting result showed that the TERS enhancement will sharply increase when the tip radius decreases from 15 nm to 5 nm, which may originate from an enhancement in the lightning-rod effect.¹⁰⁷ Besides the ultra-high enhancement, gap mode TERS possesses a spatial resolution that is much smaller than the radius of the TERS tip. It is well accepted that the lateral confinement (w) of the electromagnetic field in the gap is in the form of ,¹¹⁰ where R is the radius of the tip and d is the gap distance. Consequently, TERS images with a spatial resolution better than 5 nm can be obtained under ambient conditions.^83,111 The spatial resolution and the enhancement in gap mode TERS can be further improved by introducing some atomistic protrusions at the tip apex,^112–114 which serve as picocavities.¹¹⁵ These picocavities can be stabilized at cryogenic temperatures to confine the electromagnetic field to the atomic scale,^113,114,116 enabling chemical bond-resolved TERS imaging^117,118 with a spatial resolution of up to 0.15 nm.¹¹⁸

When the substrate used in TERS is not a coinage metal such as dielectric materials and transition metals, efficient coupling between the tip and the substrate will not occur. Thus, only the TERS tip offers the full enhancement. This configuration is called non-gap mode TERS (conventional TERS). Here, it should be noted that for a semi-infinite metallic tip alone, as shown Fig. 1b, an optical near-field measurement near the tip apex within 1 nm based on rescattering of the photoemitted electrons showed that the field enhancement factor for tungsten and gold nanotips is governed by a geometric effect, i.e. the lighting-rod effect, while the SPR effect plays a minor role.^89,97,119 However, the lighting-rod effect alone cannot produce sufficient enhancement to study most of the samples. Therefore, the key for effective non-gap mode TERS is to excite efficient SPR effect on the tip.

Given that LSPR can only be excited on metallic nanostructures with a sub-wavelength size, it is necessary to fabricate TERS tips with finite nanoscale features. As illustrated in Fig. 1c, this finite tip can be either isolated nanoparticles attached on an Si tip, groove-based nanotip or size-adjustable nanopyramid. In the case of the nanoparticle-based finite tip, the size and number of nanoparticles will influence the LSPR efficiency.^120,121 Besides nanoparticles, the introduction of a groove with a suitable distance away from the tip apex can turn the semi-infinite metallic tip into a finite nanotip.^121,122 The nanotip can serve as a dipole to support LSPR. As shown in Fig. 1c, the LSPR wavelength of the nanotip is dominated by the distance (L) between the groove and the tip apex. Besides, the cut length (L_C) and the cut depth (D) will also influence the LSPR efficiency. The simulation suggests that the enhancement will first be improved with an increase in both L_c and D, and then reach a plateau when L_c is around 100 nm, while D is around 20 nm.¹²¹ The best parameter for the groove structure varies in different illumination configurations. Compared with the groove-based structure, the LSPR effect can be achieved in a more efficient way using a monopole antenna, where the flat plateau provides the image of the monopole, and thus the field pattern generated at the apex can be regarded as the overlap of half of a fictitious freestanding dipole with the other half from the mirror plane.¹²³ The monopole antenna consists of a size-adjustable nanopyramid grounded on a flat surface. The so-called plasmon-tunable tip pyramid (PTTP) exhibits a stronger enhancement than the groove-based nanotip.^123,124 By utilizing these specially designed tips to efficiently excite the LSPR effect, non-gap mode TERS can also produce a relatively high enhancement to characterize materials, especially two-dimensional materials with a spatial resolution of around 10 nm.¹²³

It should be noted that because of the large penetration depth of light for dielectric materials, which produce strong far-field Raman signals, it is a great challenge to characterize the bulk materials employing non-gap mode TERS. In this case, an effective strategy is to remotely excite the LSPR of the tip apex with SPP (Fig. 1d). Owing to the large momentum mismatch between the incident light and the plasmon, SPP effect cannot be directly excited. Consequently, the core to excite SPP is the design of a suitable coupler (Fig. 1d), such as a grating,^125,126 which converts the incident light into plasmons. During the propagation of plasmons to the apex of the tip, some loss will occur, which depends on the slope of the tip^127,128 and the surface roughness.¹²⁹ Therefore, the coupling efficiency and the loss during propagation need to be considered when designing SPP-based TERS tips.¹³⁰ One of the advantages of SPP-based TERS is that the incident laser is not directly irradiated on the sample but on the coupler (so-called remote-excitation TERS), while the enhanced Raman signal near the tip apex is collected. This process produces a pure TERS signal without a far-field background.¹²⁵ Technically, the setup for remote-excitation TERS will be more complicated than the conventional TERS setup, given that the excitation and collection paths need be separated, such as two separate objectives for excitation and collection, respectively,¹³¹ or noncolinear optical path to separate the excitation and collection with one objective.^125,132,133

Besides the above-mentioned electromagnetic enhancement, there is also a chemical enhancement in TERS.^134–137 Although the chemical enhancement is less discussed in TERS, it may dominate the TERS signal in some special situations. The Kumagai group observed a dramatic enhancement when a so-called atomic point contact (APC) was formed between the tip and the sample (Fig. 1e).^138–140 This dramatic enhancement appeared over the ZnO layer but disappeared over the inert NaCl layer, indicating the sensitivity to the surface reactivity. According to this result, the enhancement was attributed to the hybridization between the sample and both the tip and substrate in the STM configuration, which made charge transfer enhancement operative through the metal-sample-metal system.¹³⁸ It is surprising that this chemical enhancement could even enable the observation of the phonons on the Si(111)-7×7 surface over a bulk Si sample in the non-gap mode configuration.¹³⁹ Recently, the Dong group investigated in detail the mechanism of the APC-based enhancement for molecules in well-defined single-molecule TERS configurations under UHV and ultralow temperature (LT) conditions. The resonance charge transfer mechanism, which is the resonance between the excitation wavelength and the molecule–metal charge transfer transition was ruled out according to the slightly changed TERS signals in the bias voltage-dependent experiment. Then, a combined physicochemical mechanism was proposed, instead of a pure chemical effect to explain the enhancement during the APC. It was found that the weak interaction between the tip and the molecule will enable ground-state charge transfer (GSCT), which will induce vertical Raman polarizability. Subsequently, the vertical Raman polarizability will be further enhanced by the vertical electromagnetic field in the plasmonic nanogap. This GSCT mechanism was also corroborated in wavelength-dependent TERS experiments under different resonance conditions.¹⁴¹

2.2. TERS instruments

From an instrumental viewpoint, a TERS setup generally consists of a Raman microscopy coupled with an SPM system, while a control module is required for communication between the two parts. STM, AFM and shear force microscopy (SFM) have been successfully used in TERS. Among them, STM-based TERS can provide Raman signals with a high signal-to-noise ratio and high-quality morphological images. However, the conductivity of the sample should be sufficiently high to establish the tunneling current-based feedback. Alternatively, working with the atomic force as feedback, AFM- and SFM-based TERS can be used to study either conductive samples, semi-conductive samples or insulated samples. It is important to note that all the SPM techniques used in TERS are ultra-sensitive to the vibrational noise transmitted from the working environment. A large noise makes it difficult to keep the TERS tip in close vicinity of the sample to produce a stable enhancement. In this regard, one of the biggest challenges in the TERS technique is how to realize a high optical excitation/collection efficiency in the limited space of SPM without introducing too much noise. Accordingly, four optical configurations have been employed in TERS setups, including bottom illumination, side illumination, top illumination and parabolic mirror-based illumination.

In the bottom illumination configuration (Fig. 2a), the optical part is mounted at the opposite side of the TERS tip, which makes it possible to use an objective with a high numerical aperture (NA) and a short working distance, such as an oil-immersion objective (NA=1.4) and water-immersion objective (NA=1.0), to guarantee both high excitation and collection efficiencies. The further introduction of radial polarization can efficiently excite the SPR effect to produce a higher TERS enhancement.¹⁴² However, given that the laser and the back-scattered Raman signals need to pass through the sample, only transparent samples and substrates can be studied in the bottom illumination configuration.

In the side illumination configuration (Fig. 2b), a tilted mounted objective is used for the excitation and collection. Limited by the open space of SPM, the working distance of the objective used in side illumination should be long enough. The recent available objective with both a long working distance (6 mm) and a high NA (0.7) can help to improve both the excitation and collection efficiency. The p-polarized laser is used to make the polarization of the incident light parallel to the tip shaft to produce a higher TERS enhancement. The tilted angle should be carefully designed given that it will not only influence the component of the polarized laser parallel to the tip shaft but also determine the amount of Raman signals to be blocked by the sample, especially when an objective with a high NA is employed. Side illumination TERS can be employed to study both transparent and opaque samples. However, the quality of the optical image is deteriorated, which makes it difficult to coarsely mount the tip in the interested region on the sample. Another objective mounted in the top illumination configuration is now available in some commercial AFM-based TERS instruments and quite helpful for obtaining optical images.

Similar to side illumination, top illumination can also be applied to study both transparent and opaque samples. In addition, the top illumination configuration provides both a high collection efficiency and high optical image quality. As shown in Fig. 2c, the objective is mounted on the top side of the tip to focus the laser on the tip apex and collect the back-scattered Raman signals. As a result, the TERS tips used in this configuration need to be tilted to maintain a high optical efficiency. However, part of the incident laser and the Raman signal are still blocked by the tip, which leads to a loss in both the excitation and collection efficiency. In addition, the image quality of SPM, especially STM may decrease with the use of tilted tips.

A very special configuration based on parabolic mirror-based illumination (Fig. 2d) is also used in TERS, where a hole is drilled at the center of the parabolic mirror. This configuration possesses a high NA of up to 1.0, producing both a high excitation and collection efficiency. It was demonstrated that the introduction of radial polarization in this configuration will also improve the excitation efficiency,^143,144 thus improving the TERS enhancement. It should be noted that very accurate alignment between the tip and the parabolic mirror is required in this configuration.

With the development of the above-mentioned basic configurations, various progress in instrumentation has been realized in recent years, including the improvement of sensitivity and the strict control of the working environment.

2.3. TERS tips

The TERS tip serves as the probe to characterize the morphology of the sample as well as the electromagnetic field amplifier to enhance the Raman signal, which is the key to determine the performance of TERS measurements. In the early stage of TERS, the reproducible fabrication of highly active TERS tips was one of the bottlenecks of this technique. After around 20 years, many methods have been developed to fabricate TERS tips,^95–97,166 and commercial TERS tips have become available recently.¹⁶⁷ The improvement in the fabrication of TERS tips extends the applications of TERS and makes it possible for TERS to become a routine analytical technique. In addition, a variety of special TERS tips was designed and fabricated to modulate the TERS performance for specific samples.^{122,123,131,133,168} In the following parts, we discuss the fabrication of TERS tips for different purposes, including gap mode TERS, non-gap mode TERS and remote-excitation TERS.

2.4. AI-assisted TERS

Given that TERS only enhances the Raman signals of the samples underneath the tip apex, only a small amount of samples has been studied via TERS, which requires a strong enhancement to produce a good signal-to-noise ratio (SNR). However, simply extending the acquisition time cannot improve SNR in TERS because a long acquisition time will enlarge the influence of the thermal drift, especially in TERS imaging.

Thus, to solve this problem and speed up the TERS imaging, some denoising algorithms have been developed and applied in TERS. The Wang group developed both the adaptive low-rank matrix approximation (ALRMA)¹⁹⁵ and collaborative low-rank matrix approximation (CLRMA)¹⁹⁶ methods to improve the SNR in TERS. In ALRMA, the TERS imaging data were decomposed into a variety of submatrices by singular value decomposition (SVD). According to the contribution to the SNR, the submatrices were divided into a positive group and negative group. Then, the denoised data were reconstructed from the positive group after discarding the negative group reflecting the noise. However, it was found that it is difficult for ALRMA to distinguish the signal and noise when the SNR is relatively low. Thus, to improve the performance in extracting weak signals, high-SNR reference data was introduced to perform collaborative matrix factorization in CLRMA. The introduction of high-SNR reference data forces the components of factorization to approximate the true signal components or noise components, and thus a better denoising performance was obtained by CLRMA than ALRMA (Fig. 7a). Unfortunately, it may be difficult to obtain the high-SNR reference data in TERS. In addition, both ALRMA and CLRMA depend on a relatively large data volume, making them unavailable to denoise a single TERS spectrum. In this regard, our group developed deep learning-based algorithms for denoising TERS signals.¹⁹⁷ An AUnet neural network was pre-trained to learn the noise characteristics of instruments, thereby predicting and subtracting the instrument noise from the raw spectrum with a low SNR. As a result, a denoised spectrum with a high-SNR will be obtained (Fig. 7b). This denoising method has the advantage of being independent of the data volume, making it suitable for both a single TERS spectrum and TERS imaging. With the dramatically increased SNR, it is possible to realize fast-speed TERS imaging. Fig. 7c shows the TERS imaging obtained by the raw data (fast-speed TERS imaging, 0.1 s pixel⁻¹), AUnet-treated data (fast-speed TERS imaging, 0.1 s pixel⁻¹) and long-time acquisition data (1 s pixel⁻¹) from the same region in the STM image reconstructed from the peak position of the NC triple bond of phenyl isocyanide (PIC) molecules adsorbed on a Pd/Au surface. The TERS image obtained with the data treated by AUnet fits better with the topography compared with the raw data owing to the improved SNR. Furthermore, the influence of the thermal drift was significantly reduced compared with the long-time acquisition TERS image.

Similar to other hyperspectral techniques, two-dimensional (2D) TERS imaging usually contains a large number of spectra, which makes it nearly impossible to manually analyze each spectrum. In this regard, some algorithms are introduced in TERS to help analyze the hyperspectral TERS data. The Lagugne-Labarthet group applied an artificial neural network (ANN) algorithm to distinguish different types of spectra in TERS images. The trained ANN models could identify the location of carbon nanotubes (CNTs) and distinguish different types of spectra at each pixel, enabling the analysis of the properties of different CNTs.¹⁹⁹ The Nanda group used a multi-resolution manifold (MML) learning algorithm to classify the TERS spectra of amorphous silicon (a-Si).¹⁹⁸ The high-dimensional TERS spectra (around 2500 spectra) obtained from the 2D TERS image were first projected to a low-dimensional (i.e. 2D) manifold space by the MML algorithm (Fig. 7d). Then, straightforward data visualizations and structural categorization were performed to reveal the underlying properties of a-Si. As a result, the 2500 TERS spectra of a-Si were first categorized into seven clusters according to their peak intensity and peak position, which determined the degree of Si–Si bond distortion in various regions of a-Si. Further dividing these seven clusters at a finer resolution grid enabled the discovery of a small region with a characteristic peak at 2435 cm⁻¹ (Fig. 7e), which may come from trace amounts of defects (≤0.3%) in a-Si. It was demonstrated that with the help of the AI algorithms, analysis of the TERS data can be more efficient and precise. Some subtle features that are difficult for human to notice can be revealed using AI algorithms to assist in the analysis of TERS imaging data.

3. Implementation for TERS in studying materials and interfaces

After more than 20 years of development, TERS technique has been successfully applied in various fields, such as the characterization of materials and chemical reactions. Recently, commercial TERS instruments and TERS tips have become available, making it possible for more researchers to use the TERS technique. However, there are still a lot of challenges in conducting successful and reliable TERS measurements. As illustrated in Fig. 8, we discuss some experimental issues for TERS in studying materials and interfaces in the following parts.

3.1. Evaluation before experiment

Careful evaluation should be carried out for targeting samples before performing TERS measurements. Here, we summarize some basic principles that can be applied, as follows: (1) the first thing to consider is which mode of TERS is appropriate for the targeted samples, i.e. gap mode TERS or non-gap mode (conventional) TERS. Gap mode TERS possesses a higher enhancement and a higher spatial resolution. However, the interaction between the sample and the coinage metallic substrate (Au, Ag or Cu) may change the intrinsic property of the sample. In this case, a possible solution is to introduce an ultra-thin isolation layer, such as an ultra-thin h-BN layer, in ambient TERS or NaCl with atomic thickness underneath the sample in UHV–TERS to prevent the influence from the metallic substrate but enable the generation of highly efficient gap plasmons.¹⁴¹ Besides this, the sample studied in gap mode TERS should be thin enough (thinner than 5 nm) to maintain a short distance between the tip and the substrate for achieving efficient gap plasmons. Non-gap mode TERS provides a platform to study the pristine property of the sample (without interference from the metallic substrate) at the nanoscale, but specially designed tips with either high LSPR activity or SPP activity are required. (2) Then, a suitable SPM technique should be selected according to the property of the sample. STM-based TERS can only be used to study conductive samples with a relatively low roughness, while usually a relatively higher spatial resolution can be obtained. SFM- and AFM-based TERS can be employed to study both conductive and insulated samples. The development of modern AFM makes AFM-based TERS more powerful, where abundant information beyond morphology and vibrational spectroscopy, such as electrical and mechanical information, can also be obtained. (3) Finally, the working environment should be selected according to the research interests. UHV, liquid and electrochemical TERS provide a much more controllable environment than the ambient TERS but require more sophisticated instrumentation.

3.2. Preparations for TERS

Considering the above-mentioned evaluations, experimental preparations for different specific conditions are required. The sample needs to be well prepared on suitable substrates before the TERS measurements. In the case of gap mode TERS, the most frequently used substrate in ambient or liquid condition is an Au substrate. The Au substrate should be smooth enough for SPM to work properly and avoid the SERS enhancement from the substrate. Among the Au substrates, Au(111)²⁰⁰ or Au(111) on mica provides a clean and atomically smooth surface, which can be regarded as a model substrate for TERS. Another frequently used Au substrate is atomically smooth Au films fabricated by the template-stripping method,²⁰¹ if there is no need to use a single crystal surface. Besides Au substrates, Ag^114,138,152 and Cu^117,202 substrates are also used in UHV–TERS, where the oxidation of the substrate can be avoided. Under the UHV condition, an in situ cleaning process is required to produce a clean surface. In the case of non-gap mode TERS, either a glass slide or Si with a relatively low roughness can be used as the substrate, depending on the illumination configuration.

Then, suitable methods should be selected to prepare the samples on the substrate for TERS measurements. For example, immersion, drop-casting and spin coating can be used to prepare self-assembled monolayer (SAM) samples on Au substrates under ambient conditions. It is important to optimize the procedures for assembling different molecules, including the concentration and the immersion time, which determines the molecular state on the surface.²⁰³ In the case of 2D materials, mechanical exfoliation, which produces 2D materials with high-quality, followed by dry transfer is still the most frequently used method for preparing samples for TERS. In UHV–TERS, the sample is usually prepared inside a UHV chamber, either via vacuum deposition,^{117,150,204,205} dry contact transfer (DCT)^206,207 or epitaxial growth^79,86 can be used depending on the sample. The coverage of the sample can be well controlled by optimizing the experimental details.

With well prepared samples, the next step will be the fabrication of suitable TERS tips. As discussed before in this review, different methods have been developed to fabricate STM-, AFM- and SFM-based TERS tips for either gap mode or non-gap mode TERS. Special pretreatment, such as insulation and coating with a protection layer, is necessary before conducting liquid TERS, especially EC–TERS, which has been discussed in detail in the literature.¹⁰⁶ However, although the fabrication of TERS tips has become mature, different TERS tips may still show varying TERS performances. In this regard, screening tips in terms of enhancement before measuring the target sample will be helpful for successful TERS measurements. When evaluating the enhancement of TERS tips, a highly uniform standard sample is required to confirm that the differences in TERS intensity obtained with different tips originate from the varying enhancement of the tip instead of the sample. In experiments, SAMs on Au(111) will be a good choice for gap mode TERS, while some flat 2D materials assembled on a glass or Si substrate will be a good choice for non-gap mode TERS. It should be note that some defect sites of 2D materials may present stronger Raman signals than that of the basal plane,^81,146 which should be careful when performing evaluation experiments. In addition, given that a Stokes shift exists between the Raman scattering and incident laser, TERS tips with different SPR resonance wavelengths may produce varying enhancements for peaks with different Raman shifts.^208–210 Therefore, it will be much better to choose a standard sample that has Raman peaks close to that of the target samples. Besides the enhancement, spatial resolution is another important parameter of TERS. Currently, the spatial resolution of TERS can be evaluated by either the full width at half maximum (FWHM) or the 10–90% contrast of the spatial distribution of the intensity of the individual Raman peaks in TERS images.^{83,111,117,118} Considering that the spatial profile of the experimentally measured intensity is a convolution of the TERS spatial resolution and the distribution of the sample, it is straightforward and convincing to utilize the signal of one-dimensional materials such as carbon nanotubes or a sharp edge to evaluate the spatial resolution.²¹¹

3.3. Experimental keys for convincing TERS

4. Nanoscale chemical characterization of materials by TERS

4.1. TERS study of CNTs and carbyne

As one-dimensional carbon-based materials, CNTs have been widely used in electronic devices and industry.^213–216 Given that the electronic and optical properties of CNTs are intimately related to their structure,²¹⁷ characterization of CNTs at the nanoscale is the core to understanding their intrinsic structure–property relationship. Owing to their one-dimensional nature with negligible far-field background, CNTs are ideal samples for TERS. In 2003, the Novotny group first characterized single-wall CNTs (SWCNTs) by TERS,²¹⁸ observing the variation inside a single SWCNT with a spatial resolution better than 30 nm. This variation was attributed to the change in the tube structure, which resulted in the modification of the electronic properties, and thus influenced the resonance enhancement. Thereafter, TERS has shown ability for studying localized vibrational modes, chirality changes, the detection of dopants and local deformation in SWCNTs.^219–222 For example, the Jorio and the Novotny group utilized TERS to spatially resolve the charged defect in a single SWCNT. A new peak whose frequency is near the G′ band was observed at the defect sites, marked as . This new peak originated from the existence of charged defects (dopants), which lead to the renormalization of the electron and phonon energies. Furthermore, exhibited a blue-shift for p-doping, while a red-shift for n-doping relative to the G′ peak, which makes the behavior of the band a sensitive probe to reveal the local effect of a single defect on the electronic properties of SWCNT.²²³

To further improve the spatial resolution for finer characterization of the localized property, the Kawata group performed STM-based gap mode TERS in a sealed chamber under an N₂ atmosphere (Fig. 9a). The nanogap formed by the Au tip and Au substrate provided a strong plasmonic enhancement, enabling a spatial resolution of 1.7 nm under ambient conditions (Fig. 9d).¹¹¹ The simultaneously obtained high-resolution STM image and corresponding TERS imaging reconstructed from different vibrational modes of CNT (Fig. 9c and e) enabled visualization of the lateral confinement of each Raman mode in real spaces, respectively. The underlying physical chemistry, including the diameter of tubes, local defects and bundling effect can be studied using the spatially resolved Raman spectra corroborated with topographies. With the further improvement of the spatial resolution to 0.7 nm in UHV–TERS, the Dong group directly visualized the heterogeneity inside a single SWCNT. A stronger D band signal at the end of the tube and overall uniform distribution of the G band signal was observed, providing more structural information than that of the STM image. The D band signals were also observed far away from the tube end and extended to around 3 nm, indicating the presence of defects in the SWCNT and a relaxation length of the D band near a defect. In addition, the spectral evolution of the G band helps to reveal the strain inside a single SWCNT.²⁰⁷ According to the above-mentioned results, we can conclude that the well-correlated topography and chemical information obtained from TERS are beneficial to reveal the properties of CNTs. Recently, the analysis of the interwall interactions in multiwalled carbon nanotubes (MWNTs) was achieved through D band imaging by the Verma group.²²⁴ Nowadays, SWCNTs have been used as standard materials for the evaluation of the spatial resolution in TERS.^{122,123,130,132,190}

As another one-dimensional carbon material, carbyne was successfully synthesized inside MWCNTs or double-wall carbon nanotubes (DWCNTs) (Fig. 9f).^226,227 This new material is predicted to possess a higher stiffness and elastic modulus than any known materials.²²⁸ In addition, the electronic property of carbyne can be tuned by the length of the carbon line as well as the external perturbations.^229–231 However, exploring the properties of carbyne via experiment is quite challenging given that the nanoscale and even atomic scale carbon line is encapsulated in a DWCNT. In this regard, TERS offers nanoscale spatial resolution to directly measure the Raman spectra of the carbyne inside a DWCNT (Fig. 9g–i),^{225,227,232–234} which is a convincing way to ensure that all the spectral features come from single carbyne. Moreover, TERS was also used to investigate the interaction between the inner carbyne within DWCNTs. For example, it was found that the chirality and the radius of the tube will determine the properties of the encapsulated carbyne, and thus could be reflected by the Raman shift. A linear scaling of the C-mode frequency with the diameter of the CNT exists, indicating that the dominant interaction between the nanotube and the encapsulated chain is van der Waals force.²³²

4.2. TERS study of graphene

After the successful mechanical exfoliation of graphene in 2004,²³⁵ it soon became a rapidly rising star in both materials science and condensed-matter physics²³⁶ owing to its quite unique properties, such as “massive” electrons, ultra-high electron mobility at room temperature, and unexpectedly high opacity.^237–240 Raman spectroscopy is a versatile tool for the study of graphene,⁴⁸ where its number of layers,^48,49,241 defects, doping and strain^242–244 can be reflected by its Raman peaks, respectively. However, limited by the spatial resolution of far-field Raman spectroscopy, it is impossible to deliver the local distributions of properties, such as strain, doping, and interlayer interaction. Moreover, the defects will dramatically influence the property of graphene, but the accurate characterization of the defect sites via far-field Raman spectroscopy is quite challenging given that the signals will be easily obscured by the average signal from the pristine material.

Thus, to overcome this limitation, TERS was utilized to obtain localized Raman spectra from the near-field regime. It is important to note that all the vibrational modes of single-layer graphene (SLG) are in-plane modes, while mainly out-plane modes are effectively enhanced in gap mode TERS according to the selection rule. The Jorio group compared the enhancement of graphene in TERS performed on different substrates and obtained a stronger enhancement on a glass substrate than that on a Au substrate.²⁴⁵ In this regard, most of the TERS studies on graphene were performed in the non-gap mode configuration.^246–252

The Su group studied the edge defect of SLG by TERS in detail,^253,254 where an obvious D band at the edge site was observed (Fig. 10a–c) and the spatial distribution of the D band near the edge was analyzed to be around 4.2 nm.²⁵⁰ Recently, the Cançado group directly measured the localization of the D band experimentally to be around 7.8±3.2 nm near the edge by deconvoluting the excitation field of the tip used for the experiment from the TERS image.²⁵² This directly measured value agrees with the previous indirect far-field Raman measurement, which is 4±1 nm.^255,256 These results showed that beyond direct visualization of the specific sites, TERS can be a powerful tool to obtain the spatial distribution of the unique property induced by these sites, such as the different electronic structures compared with the pristine material. In addition, TERS can even visualize some atomic sites such as point defects. The Pollard and the Roy group successfully probed the point defects in flat graphene (Fig. 10d) by TERS using the TERS image reconstructed from the intensity of the D band (Fig. 10e). It can be concluded that although there are no specific defects in the topography, numerous point defects are observed from the appearance of the D peak on the defect site (Fig. 10f).²⁵⁰

Since the discovery of strong correlations and superconductivity in twisted bilayer graphene (TBG),²³⁷ it has become a great challenge to access the modulations and understand the related effects from the superlattice in TBG, given that the modulations are so small that numerous experimental techniques failed in the accurate characterization. Recently, the Jorio group utilized TERS to measure the localization of the lattice dynamics in TBG (Fig. 11a).⁸² The superlattice image in TBG could be clearly visualized in the TERS image compared with the featureless topography, where the spatially resolved Raman spectra could reflect the distribution of the local vibrational state and the electronic structure (Fig. 11b). The nanoscale spatial resolution and high sensitivity of TERS enabled the observation of the different G peaks at different stacking regions of various positions (Fig. 11e). These vibrational states were demonstrated to be localized in specific regions based on the TERS image (Fig. 11d), which agreed well with the predication of the theory (Fig. 11c). In addition, the electron localization was also measured from the analysis of the G′ peak, whose line shape is sensitive to the electronic structure. This TERS-based nano-Raman characterization successfully pushed the understanding of the phonon-related effects to the nanometer and even atomic scales from experiment. With the successful synthesis of a large variety of graphene-related materials, the TERS technique was also used to characterize the local properties of these materials, such as graphene oxide,⁸⁰ functional graphene^257,258 and graphene nanoribbons.^259,260

4.3. TERS study of transition metal dichalcogenides (TMDCs)

TMDCs are a variety of 2D materials containing a sandwich-like structure. Owing to their original band gap and high carrier mobility at atomic thickness, this type of 2D material is regarded as the candidate for next-generation semiconductor transistors.^261–263 However, structural defects, which are generated during the production process, always exist in most TMDCs. Although these defects are usually in the nanometer and even atomic scale, they can dramatically influence the electronic properties of TMDC materials, and thus affect their application.²⁶⁴ In this case, the nanoscale spatial resolution under ambient condition of TERS, which is similar to the real working environment of TMDC-based devices, offers an opportunity to achieve the accurate characterization of the defect sites and the defect-induced transition region.^{81,146,265–268}

The Jeong group successfully unveiled the defect-related Raman mode by TERS, where the D and D′ modes were observed during TERS imaging of monolayer chemical vapor deposition (CVD)-grown WS₂. These two modes were assigned to the S vacancies according to the DFT calculation, and thus could be regarded as a probe to evaluate the quality of 2D materials.^265,269 However, we note that owing to the limited spatial resolution of TERS under ambient conditions, direct characterization of single-point defects is quite challenging, which can be probably achieved with UHV–TERS. The Verma group probed the nanoscale defects and wrinkles on MoS₂ by TERS, observing plenty of nanoscale inhomogeneities, which may come from the preparation process.²⁶⁶ Recently, our group probed the edge-related properties, including the edge-induced reconstruction of band structure and electron density in bilayer MoS₂ (Fig. 12a) by TERS. Compared with the TERS spectra obtained at the basal plane, a new peak at 396 cm⁻¹ was observed at the edge site (Fig. 12b). This new peak was assigned to the LA+TA mode, which originated from the double resonance Raman (DRR) process activated by the edge-induced band bending, and thus sensitive to the electronic band structure. Then, the intensity profile of the LA+TA mode and the A_1g mode (whose intensity is sensitive to the electron density) as a function of the position over the edge site could be obtained from the TERS line-trace (Fig. 12c), which clearly showed the defect-induced variation in electron density and the electronic band structure. After deconvoluting the spatial resolution of TERS, the edge-induced band bending region and the electron transition region (ETR) could be measured experimentally. This quantitative measurement of the defect-induced property will be helpful for the design of the TMDC-based devices in the future.⁸¹

Besides defects, the strain inside the TMDCs will also influence the electronic property of materials. By loading MoS₂ on Au nanotriangles, the Rahaman and Rodriguez group revealed the localized strain by TERS, where around 1.4% of biaxial strain was introduced in MoS₂ according to the frequency shift of up to around 4.2 cm⁻¹ of the strain-sensitive E_2g mode.²⁷² This strain-related property can also be probed by the TEPL technique.^273–275 Based on nanoscale imaging, the strain distribution inside a single wrinkle (the center and the boundary)²⁷⁴ or individual nanobubble²⁷⁵ could be obtained by TEPL, which shed light on the measurement of the strain in the materials.

In addition to the characterization of the properties of a material, TERS can also be employed to study the interaction between TMDC materials and the substrate or the interactions between different TMDC materials. The Frank group investigated the strong interaction between monolayer MoS₂ and Au by TERS. A dramatic frequency shift in the E_2g peak and splitting of the peak in MoS₂ was observed when the substrate was changed from SiO₂/Si to Au. The frequency shift of the E_2g mode originated from the lattice mismatch between MoS₂ and the Au substrate, while the splitting of the mode originated from the electron doping owing to the strong interaction between MoS₂ and the Au substrate.²⁷⁶ The Jorio group investigated the property of the in-plane homojunctions consisting of graphene on talc and graphene on SiO₂. According to the TERS image over the interface, an abrupt change in the 2D band and the G band in terms of both intensity and frequency was observed, which was then assigned to the local doping effect from the talc substrate rather than strain effect. In addition, an oscillation of the 2D/G intensity ratio was observed across the interface, indicating the formation of a charge depletion region.²⁷⁷

Recently, the Frank and the El-Khoury group investigated the interlayer interactions in WSe₂/WS₂ vertical heterostructures (Fig. 12d) by ultralow-frequency TERS (ULF-TERS), which were very sensitive to the interlayer interaction. The TERS image of a nanobubble reconstructed from the intensity of the 22 cm⁻¹ peak (blue) and the 420 cm⁻¹ peak (red) showed a heterogeneous interlayer interaction over the heterostructure (Fig. 12e). The average spectra from three different regions (flat region, peripheral region of the bubble and center of the bubble) (Fig. 12d) showed that the 22 cm⁻¹ peak could be observed in both the flat region and the peripheral region of the bubble, but this peak disappeared and the peak intensity at 420 cm⁻¹ (assigned to single-layer WS₂) increased in the center region of the bubble. This result was attributed to the decoupling of WS₂ and WSe₂ at the center of the bubble.²⁷⁰ The above-mentioned results clearly show the power of TERS in the investigation of the interaction between the material and the substrate as well as the interlayer interactions, which can help to better understand the role of the substrate and the role of the interlayer interaction on the property of the material, and thus help the future application of the material.

Recently, the successful synthesis of an in-plane (lateral) heterostructure,^278–280 which is an atomically thin p–n junction, showed promising applications as semiconductor transistors. The TERS and TEPL techniques were utilized to characterize the transition region of the in-plane heterostructure under ambient condition, which could influence the optoelectronic properties of the heterostructure.^271,281,282 The Su group utilized TERS to characterize the atomic diffusion in CVD-grown lateral heterostructures (Fig. 12g), observing two types of lateral heterostructures according to the size of the transition region. A clear interface with an abrupt change was observed in one type of heterostructure (Fig. 12h), while a broad interface was obtained in the other type of heterostructure (Fig. 12i), which was assigned to the alloy region originating from the CVD process.²⁷¹ A similar alloy region range was observed in a heterostructure by the Kung group.²⁸² These observations demonstrate that TERS can be a promising tool in ambient condition for the fast evaluation of the quality of an interface, which is usually done by electron microscopy under vacuum conditions.

4.4. TERS study of other 2D materials

Beyond graphene and TMDCs, numerous other 2D materials have also been successfully synthesized, such as silicene^283,284 and borophene,^285,286 offering unique properties and possible applications in many fields.^287,288 Thus, TERS has been utilized to probe these materials to investigate their vibrational properties at nanoscale. Owing to the easy oxidation of these 2D materials, UHV–TERS is usually used to provide a clean and oxygen-free environment.

The Wu group studied the vibrational properties of monolayer silicene on an Ag(111) surface by UHV–TERS.⁷⁹ Compared with far-field Raman spectroscopy, the ultra-high spatial resolution of UHV–TERS enabled the distinction of different phases of silicene on Ag(111). The high-resolution STM images of silicene with different phases on Ag(111) and the corresponding TERS spectra clearly showed the difference in the Raman spectra (Fig. 13a–c). Furthermore, TERS makes it possible to distinguish the local vibrational properties induced by domain boundaries in silicene. A 4×4 – β phase stabilized by the strain effect existed at the boundary of different normal 4×4 – α phase domains (Fig. 13d). The TERS spectra at these two regions and the edge site of the α phase showed a slight red-shift of around 4 cm⁻¹ for the A₁ mode (located at 170 cm⁻¹) and a decrease in the intensity of the ZO modes in the β phase compared with that in the α phase (Fig. 13e). An even lower intensity of the A₁ and A₂ peaks was observed at the edge site of the α phase. Considering the sensitivity of the A₂ and ZO peaks to the interaction between the silicene and the substrate, these changes in the Raman spectra in the β phase and at the edge site were attributed to the decreased interaction between the silicene and the substrate at these special sites.

Besides silicene, TERS was also utilized to study the vibrational properties of 2D borophene to clearly determine its structure with β₁₂ and χ₃ phases. The local vibrational properties from commensuration and strain in the smooth β₁₂ phase and striped β₁₂ phase were also observed by TERS.²⁸⁹ Recently, the Jiang group studied the interaction between tetraphenyldibenzoperiflanthene (DBP) and borophene by TERS. It was found that the adsorption of DBP on borophene introduced subtle tensile strain in borophene, and the strain was found to be highly localized in the range of 1–1.5 nm according to the TERS image.²⁹⁰

4.5. TERS study of other materials

Besides the above-mentioned materials, TERS was also used in the nanoscale characterization of some other important materials, including perovskite BaTiO₃,²⁹¹ strontium titanate (SrTiO₃),²⁹² CdSe nanowire,²⁹³ Si nanoribbon,²⁹⁴ a-Si,^155,198 and 2D polymers (2DPs).^295–297 The Zenobi group characterized 2DPs in detail at the nanoscale by TERS. Recently, the in situ characterization of the plasmon-driven polymerization process on Au(111) was achieved. Combining TERS images with DFT calculations, the whole mechanism of polymerization including a hot electron tunneling process and a self-stimulating growth process was proposed.²⁹⁷ The Wu group and the Jiang group investigated Si nanoribbons on the Ag(110) surface, combining noncontact (nc)-AFM and TERS. Both the Si nanoribbon and Si clusters were demonstrated to be composed of purely pentagon rings, instead of the predicted hexagonal model.²⁹⁴ Recently, the Nanda and the Yang group utilized TERS to characterize the nanoscale heterogeneity of the SEI after different electrochemical cycling over a-Si. The composition of the SEI gradually evolved with different species dominating the chemical composition of the SEI. A multilayer structure with different chemical compositions in each layer of the SEI of a-Si was proposed, while the carboxylates with various molecular conformations dominated the outer layer of the SEI.¹⁵⁵

5. Nanoscale chemical characterization of interfaces by TERS

Surfaces and interfaces are bridges between materials and their applications, where numerous key processes such as charge transfer and energy conversion occur. The performance of a material is sometimes dominated by the processes occurring on its surface and interface. Taking chemical reactions as an example, given that almost all the key steps of a chemical reaction including adsorption, reaction, and desorption occur on the surface and interface, the surface structure, molecule–surface interaction and intermolecular interaction will influence the above-mentioned processes, and then result in either different activity or selectivity. Therefore, a complete understanding of the surface and interfacial processes, including the molecular level and nanometer scale information of the surface and interface, becomes the key to understand the origin of the performance and design of highly efficient processes. Since the first demonstration of TERS, this technique has been utilized to characterize surfaces and interfaces benefitting from its nanoscale spatial resolution and abundant chemical information. In the past 20 years, TERS possesses broad application in the nanoscale chemical characterization of the solid–gas interfaces, solid–liquid interfaces and electrochemical interfaces, where the molecular structures, the detailed reaction process and the nanoscale active sites have been probed at the nanoscale and even atomic scale by TERS.

5.1. TERS for solid–gas interface

5.2. TERS for solid–liquid interfaces

Solid–liquid interfaces are omnipresent in nature and at the heart of numerous fields, including biology, surface science and energy science. Since the development of TERS, there has been growing interest in utilizing TERS to characterize solid–liquid interfaces. In 2009, the Zenobi group performed the first liquid TERS. With the use of a SAM-protected Ag tip, they successfully avoided the contamination of the tip and obtained the TERS signal of thiophenol (PhS) on an Au surface in liquid.³²⁵ The Domke group investigated the influence of the STM parameters on the TERS intensity in detail, revealing that the variations in TERS intensity in water were more moderate than that under argon condition.^160,326 Thereafter, the Weckhuysen group investigated the plasmon-driven coupling reaction on an Ag substrate in liquid, where heterogeneous activities were observed according to the hyperspectral TERS imaging.³²⁷ It is worth mentioning that hyperspectral TERS imaging was also successfully performed in an organic liquid,³²⁸ implying the bright future of using TERS to study various interfaces.

As a crucial type of solid–liquid interface, electrochemical interfaces provide a convenient approach to control the surface state by modulating the applied potential, and thus have received increasing attention due to the growing demand for energy storage and conversion. EC–TERS can shed light on characterizing electrochemical interfaces in situ benefitting from its nanometer-resolved chemical information. The first EC–TERS work reported the potential-dependent behavior of PBT molecules at the electrochemical interface (Fig. 20a). The doublet peaks at around 1592 cm⁻¹ and 1600 cm⁻¹ at 0.3 V weakened and became a shoulder peak when the potential negatively shifted to −0.7 V, originating from the partial protonation of the PBT molecules at a negative potential (Fig. 20b and c),¹⁵⁸ and this phenomenon was not observed by electrochemical-SERS (EC-SERS).

The Domke group investigated the potential-dependent variation of adenine adsorbed on the Au(111) surface (Fig. 20d). The intensity of the 734 cm⁻¹ peak decreased and the 260 cm⁻¹ peak showed a blue-shift (top panel of Fig. 20e, assigned to the Au–N bond) with a positive shift in potential, indicating the reorientation of the molecules from tilted to upright to flat relative to Au(111). In addition, the deprotonation process during the positive shift of the potential was revealed by the blue-shift of the peaks at 736 cm⁻¹ (middle panel of Fig. 20e) and 1464 cm⁻¹ (bottom panel of Fig. 20e). Thus, the whole potential-dependent adsorption and reaction process of adenine on Au(111) was proposed (Fig. 20f), including the reorientation and the deprotonation process at different potentials.³²⁹ These results clearly show the advantages of EC–TERS as a model system with well-defined configuration and rich chemical information in revealing the molecular adsorption and molecular structure at the electrochemical interface.

When the potential reaches the onset potential of the electrochemical reaction, there will be charge transfer between the electrode and the reactive species at the electrochemical interface. Using a single crystal as a modern system, EC–TERS provides a platform to monitor the reaction process and reveal the reaction mechanism. Our group investigated the redox process of a non-resonant molecule, thiolacetyl-terminated-phenylene ethynylene-substituted anthraquinone (2-AQ) adsorbed on the Au(111) surface. A partially irreversible reaction was observed according to the potential-dependent EC–TERS spectra, which originated from the synergistic effect of the potential and the laser illumination.¹⁶³ Subsequently, we successfully observed the intermediate of the electrochemical reduction of a nitrobenzene derivative by EC–TERS. A negatively charged intermediate could be captured in neutral solution, while no intermediate could be observed in acidic solution, indicating different reaction processes in acidic and neutral solution.³³⁰ Recently, the Lucas and the Noël group also elucidated the electrochemical reduction of a nitrobenzene derivative by EC–TERS. The prevailing product was the azoxybenzene dimer according to the EC–TERS image and the dynamic EC–TERS results. The absence of azoxybenzene dimer-related bonds under an open circuit potential (OCP) with a higher laser power indicated that the dimerization was initiated by the electrochemical potential instead of the laser illumination.³³¹

Owing to the nano-heterogeneity on the electrode, it will be very interesting for EC–TERS to distinguish the activity of different sites, which is significant in electrochemistry, especially electrocatalysis. The Van Duyne group studied the redox reaction of Nile blue (NB) molecules on an ITO electrode (Fig. 21a), where the potential-dependent TERS signals of the NB molecules were obtained to form TERS voltammetry (TERS-CV).^165,332 A shift in the redox potential acquired from TERS-CV was observed in comparison with that from the conventional CV (Fig. 21b), which was attributed to the perturbation of the electric double layer by the TERS tip.¹⁶⁵ In addition, some step-like behaviors were observed from TERS-CV at different locations, indicating the reduction and oxidation of few molecules and even a single-molecule underneath the tip.³³² Subsequently, the EC–TERS image of the NB molecules was recorded on an Au plate on an ITO electrode by EC–AFM–TERS at a spatial solution of around 80 nm.³³³ By fitting the TERS spectra recorded at each position under different potentials, the site-dependent formal potential of the redox reaction over Au and ITO electrodes can be extracted (Fig. 21c). A more negative formal potential at Au than ITO and a heterogeneous distribution of formal potential on ITO electrode were observed, indicating the nanoscale heterogeneity of the electrode surface. From a technical viewpoint, this work showed the possibility of EC–TERS in revealing the heterogeneous activities in electrochemistry for the first time.

Thereafter, with the development of instrumentation, the spatial resolution of EC–TERS has been improved to be better than 10 nm,^84,145,334 enabling the characterization of the electrocatalysis process at a spatial resolution level of several nanometers. The Van Duyne group in situ characterized the variation in the iron phthalocyanine (FePc) during the oxygen reduction reaction (ORR) by molecule-resolved EC-STM imaging and EC–TERS spectra. The irreversible demetallation of the FePc molecules was observed during electrocatalytic reactions, which led to the loss of activity in ORR. This correlation also indicated that the active site of FePc for the ORR process was the Fe site.³³⁵ In 2019, the Domke group in situ characterized the site-dependent variation on Au(111) during the oxygen evolution reaction (OER) by EC–TERS.⁸⁴ A broad peak located at around 560 cm⁻¹ to 580 cm⁻¹ assigned to AuO_x (Fig. 22b) appeared over the defect sites when the OER process was initiated on these sites (potential over 1.45 V vs. Pd–H, Fig. 22a). The hyperspectral TERS imaging (Fig. 22d) performed in the region of Fig. 22c showed that AuO_x could only be detected at some of the defect sites on the Au(111) electrode, indicating the varying reactivity of different defect sites. This variation was attributed to the atomic active site heterogeneities, which results in site-specific surface charge and work function. In addition, by analyzing the EC–TERS spectra obtained at different positions, two types of Au oxides (Au₂O₃: >565 cm⁻¹ and Au₂O: <565 cm⁻¹) were observed. The relationship between the height and the identity of the Au oxides was proposed by correlating the height from EC-STM and EC–TERS spectra, where Au₂O₃ was generated on the flatter defect-terrace-like region, while Au₂O formed sharper protrusions over the nanodefects.

Besides identification of the active sites, it is crucial to monitor the evolution of the geometric and electronic structures of the active sites, which indeed determine the catalytic performance and establish accurate structure–activity relationships. Our group utilized EC–AFM–TERS to monitor the evolution of the active sites of MoS₂ during the HER process. It is interesting to mention that the variation in the EC–TERS spectra including the peak shift and relative intensity can only be observed under the reaction conditions (Fig. 23a and b). The potential-dependent TERS line-trace (Fig. 23c) over both the basal plane and edge site of MoS₂ showed an obvious change in peak intensity from the basal plane to the edge site. The site-dependent variation in both electron density and the lattice deformation at different potentials can be visualized from the variation in the intensity of the peaks at 406 cm⁻¹ (A_1g, sensitive to the electron density) and 455 cm⁻¹ (2LA, sensitive to the lattice deformation) (Fig. 23d). It was found that the edge site induces an ETR and lattice reconstruction region (LRR). After comparing these two regions obtained at different potentials, the whole evolution of the active site could be visualized (Fig. 23e). Before HER, the edge-induced an ETR of around 10 nm and LRR of around 2 nm in the pristine MoS₂. During the activation process, the adsorption of H atoms on the edge sites will induce the lattice strain and electron density variation at the edge sites. After stabilization, the ETR extends to around 25 nm and the LRR extends to around 21 nm. This reconstruction induced by the HER process can be regarded as a self-optimization step, which improved the activity of the edge sites. Then, during the HER process after activation, the H atoms continuously adsorbed and desorbed at the edge sites, which led to the angle distortion in MoS₂, and thus further increased the ETR and LRR to around 40 nm. This in situ characterization revealed the progressive generation and the dynamic variation of the active sites during the electrocatalysis, which provided new insights into designing highly active electrocatalysts.³³⁶

It is interesting to note that similar to that under ambient condition, the surface plasmon (SP) in EC–TERS can also induce plasmon-driven photochemical reactions. Furthermore, by modulating the potential of the substrate, the energy of the photo-induced hot carriers can be easily modulated, which makes EC–TERS a flexible platform to study SP-based photoelectrochemical reactions at the electrochemical interface at the nanoscale. Recently, our group used EC–TERS to successfully visualize the spatial distribution of SP-induced hot carriers in real space (Fig. 24a).³³⁴ After the TERS tip approached the 4-mercaptobenzoic acid (4-MBA)-adsorbed Au(111) electrode, the feature peaks (998 cm⁻¹ and 1020 cm⁻¹) of TP were observed (Fig. 24b), indicating the conversion of 4-MBA molecules to TP. This irreversible decarboxylation reaction could be controlled by both the potential and SP (Fig. 24c and d), which makes it possible to turn on (before TERS imaging) or turn off (during TERS imaging) the reaction by modulating the potential, respectively. Subsequently, the SP-induced reaction region was visualized from the TERS line-trace when the reaction was off. After deconvolution of the spatial resolution and the initial distribution of the hot carriers, the transport distance of the reactive plasmonic hot carriers was measured to be around 20 nm in real space (Fig. 24e). In addition, a narrower distribution was observed at the negative potential (Fig. 24f), indicating the shorter transport distance of hot carriers with higher energies. With further improvements in sensitivity, we believe that EC–TERS will be able to address more interesting and challenging issues in electrochemical and photoelectrocatalytic systems.

6. Conclusion and perspective

After more than two decades since the first demonstration of TERS in 2000, great efforts have been devoted to making this powerful but challenging technique mature. The progress in theoretical aspects is beneficial to understand the origin of the strong enhancement and high spatial resolution of TERS, which guides experiments to find the optimized parameters for the best performance. Several methods have been developed to fabricate different TERS tips for characterizing different samples. It has become routine in most TERS labs to fabricate normal TERS tips, while the recently introduced nanofabrication method in tip fabrication makes the accurate fabrication of specially designed TERS tips possible. The TERS instruments have also become mature. Commercial ambient TERS and UHV–TERS instruments are available, while the glovebox-based TERS and EC–TERS instruments also experience fast development. These developments have synergistically boosted the TERS technique to become powerful nanoscopy, and thus broaden the applications of TERS in many research fields. However, there are still challenges to further broaden the application of TERS.

6.1. Design and fabrication of TERS tips with stronger enhancement

The TERS tip is the core of the TERS technique, where the tips need to be sharp to produce a morphology image with a high spatial resolution, while they also need to possess effective plasmonic activity to enhance the Raman signal. Although the development in the fabrication of TERS tips has already made it possible to design and fabricate tips with strong enhancement, the pursuit of higher enhancement never stops. With the reported tips, it is still quite challenging to characterize the surface of a bulk material, where the near-field Raman signal will be easily obscured in the large far-field background signal. In addition, it is also quite difficult to probe the reactive species on non-coinage metal surfaces (such as Pt and Pd) owing to the relatively low TERS enhancement on these surfaces as well as the low coverage of the reactive species. The design of highly active TERS tips from simulation, including the optimized radii of the curvature, cone angle and some special structures, combined with accurate nanofabrication strategy, will be significant to fabricate more powerful TERS tips.

6.2. High-speed TERS imaging

TERS imaging provides 2D Raman imaging of a sample at a nanoscale spatial resolution. Analysis of the image with different Raman modes can visualize the spatial distribution of different properties. However, owing to the small cross section of Raman scattering, a long-time (from several minutes to hours) is usually required to obtain a TERS image. This time-consuming TERS imaging not only enlarges the influence of thermal drift, leading to a mismatch between morphology and the TERS image but also prevents the application of TERS to investigate dynamic processes. In this case, the development of high-speed TERS imaging techniques will further broaden the applications of TERS. High-speed TERS imaging may be achieved by combining the improvement of sensitivity and the help of AI algorithms. A major improvement in sensitivity can be achieved by the design and fabrication of TERS tips with a much higher enhancement. In addition, the sensitivity can also be improved from an optical aspect by introducing radially polarized light,³³⁷ modulating the phase of the laser using a SLM^147,148 or supporting the sample with a Fabry–Perot cavity.³³⁸ Some early attempts in the development of AI algorithm-based methods show the possibility to speed up TERS imaging without the loss of SNR.¹⁹⁷

6.3. TERS indirect detection technique

Although the TERS technique has high sensitivity up to the single-molecule level, it is still quite challenging to comprehensively study reaction processes. This challenge is not only attributed to the small Raman cross section, the low coverage and the short lifetime of the reactive species, but also the fact that some important properties of the interface cannot be directly probed by TERS, such as interfacial pH. In this regard, TERS with indirect detection was developed, wherein molecules sensitive to the reactive species or properties (such as the pH) are adsorbed on the tip. As a result, the distribution and dynamic of the reactive species or properties on material or the interface can be measured by the change in the Raman signal of the probe molecules. Pioneering works have been conducted by the Ozaki group using MBA and pATP as the probe to visualized the nanoscale pH distribution at the liquid–solid interface.³³⁹ The core of TERS indirect detection is the choice of suitable probe molecules that are sensitive enough to the change in the target properties and are highly reversible. For example, using an achiral para-mercaptopyridine (pMPY)-modified tip, the Ozaki group recently distinguished the enantiomers according to the variation of the relative intensities of the pMPY molecules.³⁴⁰ In addition, the way to obtain a high spatial resolution needs to be carefully considered in TERS indirect detection given that all the molecules adsorbed on the tip may contribute to the final TERS signal. Furthermore, given that the migration of the reactive species over the interface always occurs, TERS indirect detection may obtain the distribution of the reactive species including the influence of migration, hindering the investigation of the nanoscale heterogeneity, and thus reducing the spatial resolution.

6.4. Characterization of the materials and interface under the working condition

With the fast development of in situ characterization techniques in recent years, it is generally recognized that the structure and properties of materials and interfaces experience dynamic evolution under the working conditions.¹² Therefore, it will be significant to perform operando TERS characterization to visualize the changes in materials and interfaces. Although glovebox-based TERS or EC–TERS can provide a controlled environment for in situ characterization, we believe some further developments in optical path and the design of cells will achieve operando TERS characterization in the future.

6.5. Correlation of the activity and the property of a single active site

Establishing accurate structure–activity relationships at the single-site level is one of the goals in catalysis, which will guide the design of highly active catalysts. The TERS technique, especially EC–TERS can simultaneously obtain the topography and chemical information with nanoscale spatial resolution under the reaction conditions. However, there is still a lack of information on the electrochemical activity of individual sites in TERS, which makes it difficult to determine the real active site. If TERS can be further combined with some techniques that can obtain nano-resolved activity, such as scanning electrochemical microscopy (SECM),³⁴¹ a systematic understanding of the material or the interface will be obtained to establish an accurate structure–activity relationship. However, a lot of challenges need to be solved to achieve this goal, including the design of functional TERS tips, which can be used for EC-SPM, EC–TERS and SECM, and the design of special working modes to obtain all the information from the same site.

6.6. Characterization with a high spatiotemporal resolution

The TERS technique has enabled the characterization of materials and interfaces with a high spatial resolution up to the chemical bond level. However, the temporal resolution of TERS is still in the time scale of seconds. If the temporal resolution can be improved to the picosecond (ps) or femtosecond (fs) level, it will enable the visualization of the dynamics of some physical or chemical processes with a nanoscale spatial resolution. To achieve this goal, the complicated ultra-fast optical system needs to be well coupled with the TERS system without interfering with the stability of the TERS instrument. Some early attempts have been done in either the ambient TERS system^342,343 or UHV–TERS system,^344,345 demonstrating the successful measurement of TERS with ultrashort laser pulses. We believe that in the future, after solving the challenges in the stability (the tip, the plasmonic gap and the sample) as well as the strong background signal, which comes from the metallic tip and substrate,³⁴⁶ TERS measurement with a high spatiotemporal resolution will be available. There is no doubt that this development will deepen the understanding of physical and chemical processes.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial supports from the National Natural Science Foundation of China (Grant No: 22227802, 22021001, 22393901 and 22372141); the Fundamental Research Funds for the Central Universities (20720220018).

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