Visually constructing the chemical structure of a single molecule by scanning Raman picoscopy

Toward Modeling the Complexity of the Chemical Mechanism in SERS

Surface-enhanced Raman scattering (SERS) provides detailed information about the binding of molecules at interfaces and their interactions with the local environment due to the large enhancement of Raman scattering. This enhancement arises from a combination of the electromagnetic mechanism (EM) and chemical mechanism (CM). While it is commonly accepted that EM gives rise to most of the enhancement, large spectral changes originate from CM. To elucidate the rich information contained in SERS spectra about molecules at interfaces, a comprehensive understanding of the enhancement mechanisms is necessary. In this Perspective, we discuss the current understanding of the enhancement mechanisms and highlight their interplay in complex local environments. We will also discuss emerging areas where the development of computational and theoretical models is needed with specific attention given to how the CM contributes to the spectral changes. Future efforts in modeling should focus on overcoming the challenges presented in this review in order to capture the complexity of CM in SERS.

Time-dependent surface-enhanced Raman scattering: A theoretical approach

A new procedure for computing the time-dependent Raman scattering of molecules in the proximity of plasmonic nanoparticles (NPs) is proposed, drawing inspiration from the pioneering Lee and Heller's theory. This strategy is based on a preliminary simulation of the molecular vibronic wavefunction in the presence of a plasmonic nanostructure and an incident light pulse. Subsequently, the Raman signal is evaluated through an inverse Fourier Transform of the coefficients' dynamics. Employing a multiscale approach, the system is treated by coupling the quantum mechanical description of the molecule with the polarizable continuum model for the NP. This method offers a unique advantage by providing insights into the time evolution of the plasmon-enhanced Raman signal, tracking the dynamics of the incident electric field. It not only provides for the total Raman signal at the process's conclusion but also gives transient information. Importantly, the flexibility of this approach allows for the simulation of various incident electric field profiles, enabling a closer alignment with experimental setups. This adaptability ensures that the method is relevant and applicable to diverse real-world scenarios.

Resonant Tip-Enhanced Raman Spectroscopy of a Single-Molecule Kondo System

Tip-enhanced Raman spectroscopy (TERS) under ultrahigh vacuum and cryogenic conditions enables exploration of the relations between the adsorption geometry, electronic state, and vibrational fingerprints of individual molecules. TERS capability of reflecting spin states in open-shell molecular configurations is yet unexplored. Here, we use the tip of a scanning probe microscope to lift a perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecule from a metal surface to bring it into an open-shell spin one-half anionic state. We reveal a correlation between the appearance of a Kondo resonance in differential conductance spectroscopy and concurrent characteristic changes captured by the TERS measurements. Through a detailed investigation of various adsorbed and tip-contacted PTCDA scenarios, we infer that the Raman scattering on suspended PTCDA is resonant with a higher excited state. Theoretical simulation of the vibrational spectra enables a precise assignment of the individual TERS peaks to high-symmetry Ag modes, including the fingerprints of the observed spin state. These findings highlight the potential of TERS in capturing complex interactions between charge, spin, and photophysical properties in nanoscale molecular systems and suggest a pathway for designing single-molecule spin-optical devices.

Theoretical and computational methods for tip- and surface-enhanced Raman scattering

Raman spectroscopy is a versatile tool for acquiring molecular structure information. The incorporation of plasmonic fields has significantly enhanced the sensitivity and resolution of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman spectroscopy (TERS). The strong spatial confinement effect of plasmonic fields has challenged the conventional Raman theory, in which a plane wave approximation for the light has been adopted. In this review, we comprehensively survey the progress of a generalized theory for SERS and TERS in the framework of effective field Hamiltonian (EFH). With this approach, all characteristics of localized plasmonic fields can be well taken into account. By employing EFH, quantitative simulations at the first-principles level for state-of-the-art experimental observations have been achieved, revealing the underlying intrinsic physics in the measurements. The predictive power of EFH is demonstrated by several new phenomena generated from the intrinsic spatial, momentum, time, and energy structures of the localized plasmonic field. The corresponding experimental verifications are also carried out briefly. A comprehensive computational package for modeling of SERS and TERS at the first-principles level is introduced. Finally, we provide an outlook on the future developments of theory and experiments for SERS and TERS.

Signal2signal: Pushing the Spatiotemporal Resolution to the Limit by Single Chemical Hyperspectral Imaging

There is growing interest in developing a high-performance self-supervised denoising algorithm for real-time chemical hyperspectral imaging. With a good understanding of the working function of the zero-shot Noise2Noise-based denoising algorithm, we developed a self-supervised Signal2Signal (S2S) algorithm for real-time denoising with a single chemical hyperspectral image. Owing to the accurate distinction and capture of the weak signal from the random fluctuating noise, S2S displays excellent denoising performance, even for the hyperspectral image with a spectral signal-to-noise ratio (SNR) as low as 1.12. Under this condition, both the image clarity and the spatial resolution could be significantly improved and present an almost identical pattern with a spectral SNR of 7.87. The feasibility of real-time denoising during imaging was well demonstrated, and S2S was applied to monitor the photoinduced exfoliation of transition metal dichalcogenide, which is hard to accomplish by confocal Raman spectroscopy. In general, the real-time denoising capability of S2S offers an easy way toward in situ/in vivo/operando research with much improved spatial and temporal resolution. S2S is open-source at https://github.com/3331822w/Signal2signal and will be accessible online at https://ramancloud.xmu.edu.cn/tutorial.

Optoelectronic Readout of Single Er Adatom's Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9 K)

Integrating nanoscale optoelectronic functions is vital for applications such as optical emitters, detectors, and quantum information. Lanthanide atoms show great potential in this endeavor due to their intrinsic transitions. Here, we investigate Er adatoms on Si(100)-2×1 at 9 K using a scanning tunneling microscope (STM) coupled to a tunable laser. Er adatoms display two main adsorption configurations that are optically excited between 800 and 1200 nm while the STM reads the resulting photocurrents. Our spectroscopic method reveals that various photocurrent signals stem from the bare silicon surface or Er adatoms. Additional photocurrent peaks appear as the signature of the Er adatom relaxation, triggering efficient dissociation of nearby trapped excitons. Calculations using density functional theory with spin-orbit coupling correction highlight the origin of the observed photocurrent peaks as specific 4f→4f or 4f→5d transitions. This spectroscopic technique can facilitate optoelectronic analysis of atomic and molecular assemblies by offering insight into their intrinsic quantum properties.

Nanoscale Chemical Analysis of Thin Film Solar Cell Interfaces Using Tip-Enhanced Raman Spectroscopy

Interfacial regions play a key role in determining the overall power conversion efficiency of thin film solar cells. However, the nanoscale investigation of thin film interfaces using conventional analytical tools is challenging due to a lack of required sensitivity and spatial resolution. Here, we surmount these obstacles using tip-enhanced Raman spectroscopy (TERS) and apply it to investigate the absorber (Sb2Se3) and buffer (CdS) layers interface in a Sb2Se3-based thin film solar cell. Hyperspectral TERS imaging with 10 nm spatial resolution reveals that the investigated interface between the absorber and buffer layers is far from uniform, as TERS analysis detects an intermixing of chemical compounds instead of a sharp demarcation between the CdS and Sb2Se3 layers. Intriguingly, this interface, comprising both Sb2Se3 and CdS compounds, exhibits an unexpectedly large thickness of 295 ± 70 nm attributable to the roughness of the Sb2Se3 layer. Furthermore, TERS measurements provide compelling evidence of CdS penetration into the Sb2Se3 layer, likely resulting from unwanted reactions on the absorber surface during chemical bath deposition. Notably, the coexistence of ZnO, which serves as the uppermost conducting layer, and CdS within the Sb2Se3-rich region has been experimentally confirmed for the first time. This study underscores TERS as a promising nanoscale technique to investigate thin film inorganic solar cell interfaces, offering novel insights into intricate interface structures and compound intermixing.

Noise learning of instruments for high-contrast, high-resolution and fast hyperspectral microscopy and nanoscopy

The low scattering efficiency of Raman scattering makes it challenging to simultaneously achieve good signal-to-noise ratio (SNR), high imaging speed, and adequate spatial and spectral resolutions. Here, we report a noise learning (NL) approach that estimates the intrinsic noise distribution of each instrument by statistically learning the noise in the pixel-spatial frequency domain. The estimated noise is then removed from the noisy spectra. This enhances the SNR by ca. 10 folds, and suppresses the mean-square error by almost 150 folds. NL allows us to improve the positioning accuracy and spatial resolution and largely eliminates the impact of thermal drift on tip-enhanced Raman spectroscopic nanoimaging. NL is also applicable to enhance SNR in fluorescence and photoluminescence imaging. Our method manages the ground truth spectra and the instrumental noise simultaneously within the training dataset, which bypasses the tedious labelling of huge dataset required in conventional deep learning, potentially shifting deep learning from sample-dependent to instrument-dependent.

Local heating and Raman thermometry in a single molecule

Because of the nonequilibrium nature of thermal effects at the nanoscale, the characterization of local thermal effects within a single molecule is highly challenging. Here, we demonstrate a way to characterize the local thermal properties of a single fullerene (C60) molecule during current-induced heating processes through tip-enhanced anti-Stokes Raman spectroscopy. Although the measured vibron populations are far from equilibrium with the environment, we can still define an "effective temperature (Teff)" statistically via a Bose-Einstein distribution, suggesting a local equilibrium within the molecule. With increased current heating, Teff is found to rise up to about 1150 K until the C60 cage is decomposed. Such a decomposition temperature is similar to that reported for ensemble C60 samples, thus justifying the validity of our methodology. Moreover, the possible reaction pathway and product can be identified because of the chemical sensitivity of Raman spectroscopy. Our findings provide a practical method for noninvasively detecting the local heating effect inside a single molecule under nonequilibrium conditions.

AFM-IR for Nanoscale Chemical Characterization in Life Sciences: Recent Developments and Future Directions

Despite the ubiquitous absorption of mid-infrared (IR) radiation by virtually all molecules that belong to the major biomolecules groups (proteins, lipids, carbohydrates, nucleic acids), the application of conventional IR microscopy to the life sciences remained somewhat limited, due to the restrictions on spatial resolution imposed by the diffraction limit (in the order of several micrometers). This issue is addressed by AFM-IR, a scanning probe-based technique that allows for chemical analysis at the nanoscale with resolutions down to 10 nm and thus has the potential to contribute to the investigation of nano and microscale biological processes. In this perspective, in addition to a concise description of the working principles and operating modes of AFM-IR, we present and evaluate the latest key applications of AFM-IR to the life sciences, summarizing what the technique has to offer to this field. Furthermore, we discuss the most relevant current limitations and point out potential future developments and areas for further application for fruitful interdisciplinary collaboration.

On-Surface Synthesis of Graphene Nanoribbons with Atomically Precise Structural Heterogeneities and On-Site Characterizations

Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and in situ tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations. However, there are still obstacles and challenges lying on the way toward the understanding of the intrinsic structure-property relations, such as the strong screening and Fermi level pinning effect of the normally used transition metal substrates and the lack of collective tip-based techniques that can cover multi-internal degrees of freedom of the GNRs. In this Perspective, we briefly review the recent progress in the on-surface synthesis of GNRs with diverse structural heterogeneities and highlight the structure-property relations as characterized by the noncontact atomic force microscopy and scanning tunneling microscopy/spectroscopy. We furthermore motivate to deliver the need for developing strategies to achieve quasi-freestanding GNRs and for exploiting multifunctional tip-based techniques to collectively probe the intrinsic properties.

Plexcitonics: plasmon-exciton coupling for enhancing spectroscopy, optical chirality, and nonlinearity

Plexcitonics is a rapidly developing interdisciplinary field that holds immense potential for the creation of innovative optical technologies and devices. This field focuses on investigating the interactions between plasmons and excitons in hybrid systems. In this review, we provide an overview of the fundamental principles of plasmonics and plexcitonics and discuss the latest advancements in plexcitonics. Specifically, we highlight the ability to manipulate plasmon-exciton interactions, the emerging field of tip-enhanced spectroscopy, and advancements in optical chirality and nonlinearity. These recent developments have spurred further research in the field of plexcitonics and offer inspiration for the design of advanced materials and devices with enhanced optical properties and functionalities.

Revealing Intramolecular Isotope Effects with Chemical-Bond Precision

Isotope substitution of a molecule not only changes its vibrational frequencies but also changes its vibrational distributions in real-space. Quantitatively measuring the isotope effects inside a polyatomic molecule requires both energy and spatial resolutions at the single-bond level, which has been a long-lasting challenge in macroscopic techniques. By achieving ångström resolution in tip-enhanced Raman spectroscopy (TERS), we record the corresponding local vibrational modes of pentacene and its fully deuterated form, enabling us to identify and measure the isotope effect of each vibrational mode. The measured frequency ratio ν_H/ν_D varies from 1.02 to 1.33 in different vibrational modes, indicating different isotopic contributions of H/D atoms, which can be distinguished from TERS maps in real-space and well described by the potential energy distribution simulations. Our study demonstrates that TERS can serve as a non-destructive and highly sensitive methodology for isotope detection and recognition with chemical-bond precision.

Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.

Polarized tip-enhanced Raman spectroscopy at liquid He temperature in ultrahigh vacuum using an off-axis parabolic mirror

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering well below the diffraction limit down to the nanometer range and scanning probe microscopy and, possibly, spectroscopy. In this way, topographic and spectroscopic as well as single- and two-particle information may simultaneously be collected. While single molecules can now be studied successfully, bulk solids are still not meaningfully accessible. It is the purpose of the work presented here to outline approaches toward this objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum TERS. The setup is based on a scanning tunneling microscope and, as an innovation, an off-axis parabolic mirror having a high numerical aperture of ∼0.85 and a large working distance. The system is equipped with a fast load-lock chamber, a chamber for the in situ preparation of tips, substrates, and samples, and a TERS chamber. Base pressure and temperature in the TERS chamber were ∼3 × 10-11 mbar and 15 K, respectively. Polarization dependent tip-enhanced Raman spectra of the vibration modes of carbon nanotubes were successfully acquired at cryogenic temperature. The new features described here including very low pressure and temperature and the external access to the light polarizations, thus the selection rules, may pave the way toward the investigation of bulk and surface materials.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

Chemical Enhancement and Quenching in Single-Molecule Tip-Enhanced Raman Spectroscopy

Despite intensive research in surface enhanced Raman spectroscopy (SERS), the influence mechanism of chemical effects on Raman signals remains elusive. Here, we investigate such chemical effects through tip-enhanced Raman spectroscopy (TERS) of a single planar ZnPc molecule with varying but controlled contact environments. TERS signals are found dramatically enhanced upon making a tip-molecule point contact. A combined physico-chemical mechanism is proposed to explain such an enhancement via the generation of a ground-state charge-transfer induced vertical Raman polarizability that is further enhanced by the strong vertical plasmonic field in the nanocavity. In contrast, TERS signals from ZnPc chemisorbed flatly on substrates are found strongly quenched, which is rationalized by the Raman polarizability screening effect induced by interfacial dynamic charge transfer. Our results provide deep insights into the understanding of the chemical effects in TERS/SERS enhancement and quenching.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Exploiting the Momentum Distribution in Atomically Confined Plasmonic Fields by Inelastic Scatterings

The utilization of atomically confined plasmonic fields has revolutionized the imaging technique. According to the fundamental position-momentum uncertainty principle, such a narrow spatial distribution certainly leads to a broad momentum distribution in the fields, which has however been overlooked. Here we propose a novel exploitation for the momentum distribution by adaptively satisfying the conservation law of momentum in inelastic Raman scatterings in periodic systems, providing a unique optical means of directly measuring the whole phonon dispersions. The proposed technique is particularly useful for measuring phonon dispersions of low-dimensional hydrogen-rich materials, which are completely inaccessible via other techniques. The numerical results for a single all-trans polyacetylene chain demonstrate that all phonon dispersion branches can be conclusively measured from their Raman images for the first time. Our findings highlight a unique advantage of the emerging momentum-based nanophotonics and open the door for exploiting highly confined plasmonic fields in another dimension.

Precise tracking of tip-induced structural variation at the single-chemical-bond limit

Sub-nanometer-resolved TERS provides a systematic way for investigating tip-molecule interaction and molecular motions, enabling a promising approach to examine on-surface reaction mechanisms and catalysis at the microscopic level.

Nanoscale coherent phonon spectroscopy

Coherent phonon spectroscopy can provide microscopic insight into ultrafast lattice dynamics and its coupling to other degrees of freedom under nonequilibrium conditions. Ultrafast optical spectroscopy is a well-established method to study coherent phonons, but the diffraction limit has hampered observing their local dynamics directly. Here, we demonstrate nanoscale coherent phonon spectroscopy using ultrafast laser-induced scanning tunneling microscopy in a plasmonic junction. Coherent phonons are locally excited in ultrathin zinc oxide films by the tightly confined plasmonic field and are probed via the photoinduced tunneling current through an electronic resonance of the zinc oxide film. Concurrently performed tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In contrast to the Raman spectra, the phonon dynamics observed in coherent phonon spectroscopy exhibit strong nanoscale spatial variations that are correlated with the distribution of the electronic local density of states resolved by scanning tunneling spectroscopy.

Fast fabrication and judgement of tip-enhanced Raman spectroscopy-active tips

The quality of the scanning tip is crucial for tip-enhanced Raman spectroscopy (TERS) experiments towards large signal enhancement and high spatial resolution. In this work, we report a controllable fabrication method to prepare TERS-active tips by modifying the tip apex at the atomic scale, and propose two important criteria to in-situ judge the tip's TERS activity for tip-enhanced Raman measurements. One criterion is based on the downshift of the first image potential state to monitor the coupling between the far-field incident laser and near-field plasmon; the other is based on the appearance of the low-wavenumber Raman peaks associated with an atomistic protrusion at the tip apex to judge the coupling efficiency of emissions from the near field to the far field. This work provides an effective method to quickly fabricate and judge TERS-active tips before real TERS experiments on target molecules and other materials, which is believed to be instrumental for the development of TERS and other tip-enhanced spectroscopic techniques.

Conformational heterogeneity of molecules physisorbed on a gold surface at room temperature

A quantitative single-molecule tip-enhanced Raman spectroscopy (TERS) study at room temperature remained a challenge due to the rapid structural dynamics of molecules exposed to air. Here, we demonstrate the hyperspectral TERS imaging of single or a few brilliant cresyl blue (BCB) molecules at room temperature, along with quantitative spectral analyses. Robust chemical imaging is enabled by the freeze-frame approach using a thin Al₂O₃ capping layer, which suppresses spectral diffusions and inhibits chemical reactions and contamination in air. For the molecules resolved spatially in the TERS image, a clear Raman peak variation up to 7.5 cm⁻¹ is observed, which cannot be found in molecular ensembles. From density functional theory-based quantitative analyses of the varied TERS peaks, we reveal the conformational heterogeneity at the single-molecule level. This work provides a facile way to investigate the single-molecule properties in interacting media, expanding the scope of single-molecule vibrational spectroscopy studies.

Tip-enhanced vibrational spectroscopy at room temperature is complicated by molecular conformational dynamics, photobleaching, contaminations, and chemical reactions in air. This study demonstrates that a sub-nm protective layer of Al₂O₃ provides robust conditions for probing single-molecule conformations.

Ultrashort Pulse Excited Tip-Enhanced Raman Spectroscopy in Molecules

Vibrational fingerprints of molecules and low-dimension materials can be traced with subnanometer resolution by performing Tip-enhanced Raman spectroscopy (TERS) in a scanning tunneling microscope (STM). Strong atomic-scale localization of light in the plasmonic nanocavity of the STM enables high spatial resolution in STM-TERS; however, the temporal resolution is so far limited. Here, we demonstrate stable TERS measurements from subphthalocyanine (SubPc) molecules excited by ∼500 fs long laser pulses in a low-temperature (LT) ultrahigh-vacuum (UHV) STM. The intensity of the TERS signal excited with ultrashort pulses scales linearly with the increasing flux of the laser pulses and exponentially with the decreasing gap-size of the plasmonic nanocavity. Furthermore, we compare the characteristic features of TERS excited with ultrashort pulses and with a continuous-wave (CW) laser. Our work lays the foundation for future experiments of time-resolved femtosecond TERS for the investigation of molecular dynamics with utmost spatial, temporal, and energy resolutions simultaneously.

Enhancing electromagnetic field gradient in tip-enhanced Raman spectroscopy with a perfect radially polarized beam

Tip-enhanced Raman spectroscopy (TERS) is a promising label-free super-resolving imaging technique, and the electric field gradient of nanofocusing plays a role in TERS performance. In this paper, we theoretically investigated the enhancement and manipulation of the electric field gradient in a bottom-illumination TERS configuration through a tightly focused perfect radially polarized beam (PRPB). Improvement and manipulation in electric field enhancement and field gradient of the gap-plasmon mode between a plasmonic tip and a virtual surface plasmons (SPs) probe are achieved by adjusting the ring radius of the incident PRPB. Our results demonstrate that the method of optimizing the ring radius of PRPB is to make the illumination angle of incident light as close to the surface plasmon resonance (SPR) excitation angle as possible. Under the excitation of optimal parameters, more than 10 folds improvement of field enhancement and 3 times of field gradient of the gap-plasmon mode is realized compared with that of the conventional focused RPB. By this feat, our results indicate that such a method can further enhance the gradient Raman mode in TERS. We envision that the proposed method, to achieve the dynamic manipulation and enhancement of the nanofocusing field and field gradient, can be more broadly used to control light-matter interactions and extend the reach of tip-enhanced spectroscopy.

Nanoscale Heating of an Ultrathin Oxide Film Studied by Tip-Enhanced Raman Spectroscopy

We report on the nanoscale heating mechanism of an ultrathin ZnO film using low-temperature tip-enhanced Raman spectroscopy. Under the resonance condition, intense Stokes and anti-Stokes Raman scattering can be observed for the phonon modes of a two-monolayer (ML) ZnO on an Ag(111) surface, enabling us to monitor local heating at the nanoscale. It is revealed that the local heating originates mainly from inelastic electron tunneling through the electronic resonance when the bias voltage exceeds the conduction band edge of the 2-ML ZnO. When the bias voltage is lower than the conduction band edge, the local heating arises from two different contributions, namely direct optical excitation between the interface state and the conduction band of 2-ML ZnO or injection of photoexcited electrons from an Ag tip into the conduction band. These optical heating processes are promoted by localized surface plasmon excitation. Simultaneous mapping of tip-enhanced Raman spectroscopy and scanning tunneling spectroscopy for 2-ML ZnO including an atomic-scale defect demonstrates visualizing a correlation between the heating efficiency and the local density of states, which further allows us to analyze the local electron-phonon coupling strength with ∼2  nm spatial resolution.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

Tip-enhanced Raman scattering (TERS) has recently attracted remarkable attention as a novel nano-spectroscopy technique. TERS, which provides site-specific information, can be performed on any material surface regardless of morphology. Moreover, it can be applied in various environments, such as ambient air, ultrahigh vacuum (UHV), solutions, and electrochemical environments. This review reports on one hand progress of TERS for the last two decades, and on the other hand, its challenges in very recent years. Part of the progress of TERS starts with the prehistory and history of TERS, and then, the characteristics and advantages of TERS are described. Significant emphasis is put on the development of TERS instrumentation and equipment such as ultrahigh vacuum TERS, liquid TERS, electrochemical-TERS, and tip-preparations. Applications of TERS, particularly those with nanocarbons, biological materials, and surface and interface analysis, are mentioned in some detail. In the part on challenges, we focus on the very recent advances in TERS; progress in spatial resolution to the angstrom scale is the hottest topic. Recent TERS studies performed under UHV, for example chemical imaging at the angstrom scale and Raman detection of bond breaking and making of a chemisorbed up-standing single molecules at single-bond level, are reviewed. Of course, there is no clear border between the two parts. In the last part the perspective of TERS is discussed.

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C₆₀-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Quantum tunneling effect on the surface enhanced Raman process in molecular systems

In this paper, we theoretically study the effect of quantum tunneling on the surface enhanced Raman scattering (SERS) of a generic molecule confined in sub-nanometer nanocavities formed by metallic dimers. The tunneling effect was described by the quantum corrected model in combination with finite element simulations. The SERS spectra were calculated by a density matrix method. Simulation results demonstrate that both the field enhancement and the molecular SERS spectra are very sensitive to the size of the cavity. By decreasing the gap size, the local field enhancement first increases then starts to be significantly suppressed as a result of the tunneling effect which neutralizes the positive and negative induced charges in the nanocavity. Consequently, the SERS intensity also experienced dramatic decrease in the short gap distance region. We also show that both the plasmonic enhancement to the local field and the enhanced molecular decay rates have to be taken into account to understand the SERS properties of the molecule in such sub-nanometer nanocavities. These results could be helpful for the understanding of the surface enhanced spectral properties of molecular systems at sub-nanometer nanocavities.

Optical Images of Molecular Vibronic Couplings from Tip-Enhanced Fluorescence Excitation Spectroscopy

Tip-based photoemission spectroscopic techniques have now achieved subnanometer resolution that allows visualization of the chemical structure and even the ground-state vibrational modes of a single molecule. However, the ability to visualize the interplay between electronic and nuclear motions of excited states, i.e., vibronic couplings, is yet to be explored. Herein, we theoretically propose a new technique, namely, tip-enhanced fluorescence excitation (TEFE). TEFE takes advantage of the highly confined plasmonic field and thus can offer a possibility to directly visualize the vibronic effect of a single molecule in real space for arbitrary excited states in a given energy window. Numerical simulations for a single porphine molecule confirm that vibronic couplings originating from Herzberg-Teller (HT) active modes can be visually identified. TEFE further enables high-order vibrational transitions that are normally suppressed in the other plasmon-based processes. Images of the combination vibrational transitions have the same pattern as that of their parental HT active mode's fundamental transition, providing a direct protocol for measurements of the activity of Franck-Condon modes of selected excited states. These findings strongly suggest that TEFE is a powerful strategy to identify the involvement of molecular moieties in the complicated electron-nuclear interactions of the excited states at the single-molecule level.

Anchoring and Reacting On-Surface to Achieve Programmability

On-surface synthesis has developed into a modern method to fabricate low-dimensional molecular nanostructures with atomic precision. It impresses the chemistry community mostly via its simplicity, selectivity, and programmability during the synthesis. However, an insufficient mechanistic understanding of on-surface reactions and the discriminations in methodologies block it out from the conventional cognition of reaction and catalysis, which inhibits the extensive implication of on-surface synthesis. In this Perspective, we summarize the empirical paradigms of conceptually appealing programmability in on-surface synthesis. We endeavor to deliver the message that the impressive programmability is related to chemical heterogeneity which can also be coded at the molecular level and deciphered by the catalytic surfaces in varying chemical environments as specific chemical selectivity. With the assistance of structure-sensitive techniques, it is possible to recognize the chemical heterogeneity on surfaces to provide insight into the programmable on-surface construction of molecular nanoarchitectures and to reshape the correlation between the mechanistic understanding in on-surface synthesis and conventional chemistry.

Direct transformation of n-alkane into all-trans conjugated polyene via cascade dehydrogenation

Selective C(sp³) −H activation is of fundamental importance in processing alkane feedstocks to produce high-value-added chemical products. By virtue of an on-surface synthesis strategy, we report selective cascade dehydrogenation of n-alkane molecules under surface constraints, which yields monodispersed all-trans conjugated polyenes with unprecedented length controllability. We are also able to demonstrate the generality of this concept for alkyl-substituted molecules with programmable lengths and diverse functionalities, and more importantly its promising potential in molecular wiring.

Collaborative Low-Rank Matrix Approximation-Assisted Fast Hyperspectral Raman Imaging and Tip-Enhanced Raman Spectroscopic Imaging

Fast acquisition of Raman images is essential for accurately characterizing the analytes' information. In this paper, we developed a collaborative low-rank matrix approximation method for fast hyperspectral Raman imaging as well as tip-enhanced Raman spectroscopy (TERS) imaging. This method combines high signal-to-noise ratio (SNR) data with the target data to perform collaborative singular value decomposition. The high-quality reference data can impose constraints on factorization, which will force its components to approximate the true signal or noise components. The simulation demonstrated that this method offers state-of-the-art signal extraction performance and, thus, can be used to accelerate data acquisition. Specifically, the results indicate that the CLRMA can largely decrease the root-mean-square error by 20.92-54.12% compared with the baseline method of our previous study. We then applied this method to the fast TERS imaging of a Au/Pd bimetallic surface and significantly decreased the integration time down to 0.1 s/pixel, which is about 10 times faster than that of conventional experiments. High-SNR TERS spectra and clear TERS images that are well consistent with scanning tunneling microscopy (STM) images can be obtained even under such a weak signal condition. We further applied this method to the fast Raman imaging of HeLa cells and obtained clear Raman images at a short integration time of 2 s/line, which is about 5 times faster than that of conventional experiments. This method offers a promising tool for TERS imaging as well as conventional Raman imaging where fast data acquisition is required.

Surface-Enhanced Raman Scattering Stimulated by Strong Metal-Molecule Interactions in a C60 Single-Molecule Junction

Specifying the geometric and electronic structures of a metal-molecule interface at the single-molecule level is crucial for the improvement of organic electronics. A single-molecule junction (SMJ) can be used to investigate interfaces because it can be regarded as an elementary unit of the interface structure. Although considerable efforts have been made to this end, the detection of structural changes in SMJs associated with metal-molecule interactions remains challenging. In this study, we detected the surface-enhanced Raman scattering (SERS) signal originating from the metal-molecule interaction change induced by a local structural change in a C₆₀ SMJ. This junction has attracted wide attention owing to its unique electronic and vibronic properties. We fabricated a C₆₀ SMJ using a lithographically fabricated Au electrode and measured the SERS spectra along with the current-voltage (I-V) response. By continuous measurement of SERS for the C₆₀ SMJ, we obtained SERS spectra dependent on the local structural change. The analysis of the I-V response revealed that the vibration energy shift originates from the change in the local structure for different Au-C₆₀ interactions. Based on the discrimination of the states in accordance with the Au-C₆₀ interaction, we found that the probability of SERS for geometry with a large Au-C₆₀ interaction was enhanced.

Atomic-Scale Structural Fluctuations of a Plasmonic Cavity

Optical spectromicroscopies, which can reach atomic resolution due to plasmonic enhancement, are perturbed by spontaneous intensity modifications. Here, we study such fluctuations in plasmonic electroluminescence at the single-atom limit profiting from the precision of a low-temperature scanning tunneling microscope. First, we investigate the influence of a controlled single-atom transfer from the tip to the sample on the plasmonic properties of the junction. Next, we form a well-defined atomic contact of several quanta of conductance. In contact, we observe changes of the electroluminescence intensity that can be assigned to spontaneous modifications of electronic conductance, plasmonic excitation, and optical antenna properties all originating from minute atomic rearrangements at or near the contact. Our observations are relevant for the understanding of processes leading to spontaneous intensity variations in plasmon-enhanced atomic-scale spectroscopies such as intensity blinking in picocavities.

Recent advances in plasmon-enhanced Raman spectroscopy for catalytic reactions on bifunctional metallic nanostructures

Metallic nanostructures exhibit superior catalytic performance for diverse chemical reactions and the in-depth understanding of reaction mechanisms requires versatile characterization methods. Plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and tip-enhanced Raman spectroscopy (TERS), appears as a powerful technique to characterize the Raman fingerprint information of surface species with high chemical sensitivity and spatial resolution. To expand the range of catalytic reactions studied by PERS, catalytically active metals are integrated with plasmonic metals to produce bifunctional metallic nanostructures. In this minireview, we discuss the recent advances in PERS techniques to probe the chemical reactions catalysed by bifunctional metallic nanostructures. First, we introduce different architectures of these dual-functionality nanostructures. We then highlight the recent works using PERS to investigate important catalytic reactions as well as the electronic and catalytic properties of these nanostructures. Finally, we provide some perspectives for future PERS studies in this field.

Advances of surface-enhanced Raman and IR spectroscopies: from nano/microstructures to macro-optical design

Raman and infrared (IR) spectroscopy are powerful analytical techniques, but have intrinsically low detection sensitivity. There have been three major steps (i) to advance the optical system of the light excitation, collection, and detection since 1920s, (ii) to utilize nanostructure-based surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) since 1990s, and (iii) to rationally couple (i) and (ii) for maximizing the total detection sensitivity since 2010s. After surveying the history of SERS and SEIRA, we outline the principle of plasmonics and the different mechanisms of SERS and SEIRA. We describe various interactions of light with nano/microstructures, localized surface plasmon, surface plasmon polariton, and lightning-rod effect. Their coupling effects can significantly increase the surface sensitivity by designing nanoparticle-nanoparticle and nanoparticle-substrate configuration. As the nano/microstructures have specific optical near-field and far-field behaviors, we focus on how to systematically design the macro-optical systems to maximize the excitation efficiency and detection sensitivity. We enumerate the key optical designs in particular ATR-based operation modes of directional excitation and emission from visible to IR spectral region. We also present some latest advancements on scanning-probe microscopy-based nanoscale spectroscopy. Finally, prospects and further developments of this field are given with emphasis on emerging techniques and methodologies.

Atomic Point Contact Raman Spectroscopy of a Si(111)-7 × 7 Surface

Tip-enhanced Raman scattering (TERS) has recently demonstrated the exceptional sensitivity to observe vibrational structures on the atomic scale. However, it strongly relies on electromagnetic enhancement in plasmonic nanogaps. Here, we demonstrate that atomic point contact (APC) formation between a plasmonic tip and the surface of a bulk Si sample can lead to a dramatic enhancement of Raman scattering and consequently the phonons of the reconstructed Si(111)-7 × 7 surface can be detected. Furthermore, we demonstrate the chemical sensitivity of APC-TERS by probing local vibrations resulting from Si-O bonds on the partially oxidized Si(111)-7 × 7 surface. This approach will expand the applicability of ultrasensitive TERS, exceeding the previous measurement strategies that exploit intense gap-mode plasmons, typically requiring a plasmonic substrate.

Plasmonic structure with nanocavity cavities for SERS detection of pesticide thiram

Excessive thiram residues in food have the potential to negatively impact human health. Hence, the development of a convenient and fast detection method is highly desirable. In this study, an efficient, repeatable, and sensitive surface-enhanced Raman scattering (SERS) active chip was manufactured via a low-cost colloidal lithography technique. The plasmonic structure was composed of a series of silver nanospheres and nanowires. Interestingly, this type structure creates a nanocavity space with a characteristic geometry generating a strong electromagnetic field coupling. The finite-different time-domain software was employed to simulate the electromagnetic field distribute on the nanocavity. Accordingly, SERS active chip that displays ultra-low concentration detection of thiram (10^-11 M) was realized. Moreover, the excellent reproducibility of thiram (10^-6 M) practical detection on an apple pericarp has great potential for application in food safety.

Raman Detection of Bond Breaking and Making of a Chemisorbed Up-Standing Single Molecule at Single-Bond Level

Probing bond breaking and making as well as related structural changes at the single-molecule level is of paramount importance for understanding the mechanism of chemical reactions. In this work, we report in situ tracking of bond breaking and making of an up-standing melamine molecule chemisorbed on Cu(100) by subnanometer resolved tip-enhanced Raman spectroscopy (TERS). We demonstrate a vertical detection depth of about 4 Å with spectral sensitivity at the single chemical-bond level, which allows us not only to justify the up-standing configuration involving a dehydrogenation process at the bottom upon chemisorption, but also to specify the breaking of top N-H bonds and the transformation to its tautomer during photon-induced hydrogen transfer reactions. Our results indicate the chemical and structural sensitivity of TERS for single-molecule recognition beyond flat-lying planar molecules, providing new opportunities for probing the microscopic mechanism of molecular adsorption and surface reactions at the chemical-bond level.

Recent advances in plasmonic nanocavities for single-molecule spectroscopy

Plasmonic nanocavities are able to engineer and confine electromagnetic fields to subwavelength volumes. In the past decade, they have enabled a large set of applications, in particular for sensing, optical trapping, and the investigation of physical and chemical phenomena at a few or single-molecule levels. This extreme sensitivity is possible thanks to the highly confined local field intensity enhancement, which depends on the geometry of plasmonic nanocavities. Indeed, suitably designed structures providing engineered local optical fields lead to enhanced optical sensing based on different phenomena such as surface enhanced Raman scattering, fluorescence, and Förster resonance energy transfer. In this mini-review, we illustrate the most recent results on plasmonic nanocavities, with specific emphasis on the detection of single molecules.

Theoretical method for near-field Raman spectroscopy with multipolar Hamiltonian and real-time-TDDFT: Application to on- and off-resonance tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy in combination with scanning tunneling microscopy could produce ultrahigh-resolution Raman spectra and images for single-molecule vibrations. Furthermore, a recent experimental study successfully decoupled the interaction between the molecule and the substrate/tip to investigate the intrinsic properties of molecules and their near-field interactions by Raman spectroscopy. In such a circumstance, more explicit treatments of the near field and molecular interactions beyond the dipole approximation would be desirable. Here, we propose a theoretical method based on the multipolar Hamiltonian that considers full spatial distribution of the electric field under the framework of real-time time-dependent density functional theory. This approach allows us to treat the on- and off-resonance Raman phenomena on the same footing. For demonstration, a model for the on- and off-resonance tip-enhanced Raman process in benzene was constructed. The obtained Raman spectra are well understood by considering both the spatial structure of the near field and the molecular vibration in the off-resonance condition. For the on-resonance condition, the Raman spectra are governed by the transition moment, in addition to the selection rule of off-resonance Raman. Interestingly, on-resonance Raman can be activated even when the near field forbids the π-π^* transition at equilibrium geometry due to vibronic couplings originating from structural distortions.

Probing the deformation of [12]cycloparaphenylene molecular nanohoops adsorbed on metal surfaces by tip-enhanced Raman spectroscopy

[n]Cycloparaphenylene ([n]CPP) molecules have attracted broad interests due to their unique properties resulting from the distorted and strained aromatic hoop structures. In this work, we apply sub-nanometer resolved tip-enhanced Raman spectroscopy (TERS) to investigate the adsorption configurations and structural deformations of [12]CPP molecules on metal substrates with different crystallographic orientations. The TERS spectra for a [12]CPP molecule adsorbed on the isotropic Cu(100) surface are found to be essentially the same over the whole nanohoop, indicating an alternately twisted structure that is similar to the [12]CPP molecule in free space. However, when the [12]CPP molecules are adsorbed on the anisotropic Ag(110) surface, the molecular shape is found to be severely deformed into two types of adsorption configurations: one showing an interesting "Möbius-like" feature and the other showing a symmetric bending structure. Their TERS spectral features are found to be site-dependent over the hoop and even show peak splitting for the out-of-plane C-H bending vibrations. The deformed structural models gain strong support from the spatial distribution of "symmetric" TERS spectra at different positions on the hoop. Further TERS imaging, with a spatial resolution down to ∼2 Å, provides a panoramic view on the local structural deformations caused by different tilting of the benzene units in real space, which offers insights into the subtle changes in the aromatic properties over the deformed hoop owing to inhomogeneous molecule-substrate interactions. The ability of TERS to probe the molecular structure and local deformation at the sub-molecular level, as demonstrated here, is important for understanding surface science as well as molecular electronics and optoelectronics at the nanoscale.

Challenges in Plasmonic Catalysis

The use of nanoplasmonics to control light and heat close to the thermodynamic limit enables exciting opportunities in the field of plasmonic catalysis. The decay of plasmonic excitations creates highly nonequilibrium distributions of hot carriers that can initiate or catalyze reactions through both thermal and nonthermal pathways. In this Perspective, we present the current understanding in the field of plasmonic catalysis, capturing vibrant debates in the literature, and discuss future avenues of exploration to overcome critical bottlenecks. Our Perspective spans first-principles theory and computation of correlated and far-from-equilibrium light-matter interactions, synthesis of new nanoplasmonic hybrids, and new steady-state and ultrafast spectroscopic probes of interactions in plasmonic catalysis, recognizing the key contributions of each discipline in realizing the promise of plasmonic catalysis. We conclude with our vision for fundamental and technological advances in the field of plasmon-driven chemical reactions in the coming years.

Tip-enhanced Raman spectroscopy for nanoscale probing of dynamic chemical systems

Dynamics are fundamental to all aspects of chemistry and play a central role in the mechanism and product distribution of a chemical reaction. All dynamic processes are influenced by the local environment, so it is of fundamental and practical value to understand the structure of the environment and the dynamics with nanoscale resolution. Most techniques for measuring dynamic processes have microscopic spatial resolution and can only measure the average behavior of a large ensemble of sites within their sampling volumes. Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for overcoming this limitation due to its combination of high chemical specificity and spatial resolution that is on the nanometer scale. Adapting it for the study of dynamic systems remains a work in progress, but the increasing sophistication of TERS is making such studies more routine, and there are now growing efforts to use TERS to examine more complex processes. This Perspective aims to promote development in this area of research by highlighting recent progress in using TERS to understand reacting and dynamic systems, ranging from simple model reactions to complex processes with practical applications. We discuss the unique challenges and opportunities that TERS presents for future studies.