High-resolution tip-enhanced Raman scattering probes sub-molecular density changes

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.

Surface-Enhanced Raman Spectroscopy: Current Understanding, Challenges, and Opportunities

While surface-enhanced Raman spectroscopy (SERS) has experienced substantial advancements since its discovery in the 1970s, it is an opportunity to celebrate achievements, consider ongoing endeavors, and anticipate the future trajectory of SERS. In this perspective, we encapsulate the latest breakthroughs in comprehending the electromagnetic enhancement mechanisms of SERS, and revisit CT mechanisms of semiconductors. We then summarize the strategies to improve sensitivity, selectivity, and reliability. After addressing experimental advancements, we comprehensively survey the progress on spectrum-structure correlation of SERS showcasing their important role in promoting SERS development. Finally, we anticipate forthcoming directions and opportunities, especially in deepening our insights into chemical or biological processes and establishing a clear spectrum-structure correlation.

Nanoscale Chemical Probing of Metal-Supported Ultrathin Ferrous Oxide via Tip-Enhanced Raman Spectroscopy and Scanning Tunneling Microscopy

Metal-supported ultrathin ferrous oxide (FeO) has attracted immense interest in academia and industry due to its widespread applications in heterogeneous catalysis. However, chemical insight into the local structural characteristics of FeO, despite its critical importance in elucidating structure-property relationships, remains elusive. In this work, we report the nanoscale chemical probing of gold (Au)-supported ultrathin FeO via ultrahigh-vacuum tip-enhanced Raman spectroscopy (UHV-TERS) and scanning tunneling microscopy (STM). For comparative analysis, single-crystal Au(111) and Au(100) substrates are used to tune the interfacial properties of FeO. Although STM images show distinctly different moiré superstructures on FeO nanoislands on Au(111) and Au(100), TERS demonstrates the same chemical nature of FeO by comparable vibrational features. In addition, combined TERS and STM measurements identify a unique wrinkled FeO structure on Au(100), which is correlated to the reassembly of the intrinsic Au(100) surface reconstruction due to FeO deposition. Beyond revealing the morphologies of ultrathin FeO on Au substrates, our study provides a thorough understanding of the local interfacial properties and interactions of FeO on Au, which could shed light on the rational design of metal-supported FeO catalysts. Furthermore, this work demonstrates the promising utility of combined TERS and STM in chemically probing the structural properties of metal-supported ultrathin oxides on the nanoscale.

Modeling the near-field effect on molecular excited states using the discrete interaction model/quantum mechanical method

Strong light-matter interactions significantly modify the optical properties of molecules in the vicinity of plasmonic metal nanoparticles. Since the dimension of the plasmonic cavity approaches that of the molecules, it is critical to explicitly describe the nanoparticle junctions. In this work, we use the discrete interaction model/quantum mechanical (DIM/QM) method to model the coupling between the plasmonic near-field and molecular excited states. DIM/QM is a combined electrodynamics/quantum mechanical model that uses an atomistic description of the nanoparticle. We extend the DIM/QM method to include the local field effects in the sum-over-state formalism of time-dependent density functional theory. As a test of the method, we study the interactions between small organic chromophores and metal nanoparticles. In particular, we examine how the inclusion of multiple electronic transitions and intermolecular interactions modify the coupling between molecules and nanoparticles. Using the sum-over-state formalism of DIM/QM, we show that two-state models break down when the plasmon excitation is detuned from the molecular excitations. To gain further insight, we compare the simple coupled-dipole model (CDM) with the DIM/QM model. We find that CDM works well for simple systems but fails when going beyond the single molecule or single nanoparticle cases. We also find that the coupling depends strongly on the site of the nanoparticle in which the chromophore couples to. Our work suggests the importance of explicitly describing the cavity to capture the atomistic level local field environment in which the molecule strongly couples to.

Raman Spectroscopy of Formamidinium-Based Lead Mixed-Halide Perovskite Bulk Crystals

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. Nonetheless, the stability of perovskite films and associated solar cells remains a source of uncertainty and necessitates sophisticated characterization techniques. Here, we report low- to mid-frequency resonant Raman spectra of formamidinium-based lead mixed-halide perovskites. The assignment of the different Raman lines in the measured spectra is assisted by DFT simulations of the Raman spectra of suitable periodic model systems. An important result of this work is that both experiment and theory point to an increase of the stability of the perovskite structure with increasing chloride doping concentration. In the Raman spectra, this is reflected by the appearance of new lines due to the formation of hydrogen bonds. Thus, higher chloride doping results in less torsional motion and lower asymmetric bending contributing to higher stability. This study yields a solid basis for the interpretation of the Raman spectra of formamidinium-based mixed-halide perovskites, furthering the understanding of the properties of these materials, which is essential for their full exploitation in solar cells.

First-Principles Simulations of Tip Enhanced Raman Scattering Reveal Active Role of Substrate on High-Resolution Images

Tip-enhanced Raman scattering (TERS) has emerged as a powerful tool to obtain subnanometer spatial resolution fingerprints of atomic motion. Theoretical calculations that can simulate the Raman scattering process and provide an unambiguous interpretation of TERS images often rely on crude approximations of the local electric field. In this work, we present a novel and first-principles-based method to compute TERS images by combining Time Dependent Density Functional Theory (TD-DFT) and Density Functional Perturbation Theory (DFPT) to calculate Raman cross sections with realistic local fields. We present TERS results on free-standing benzene and C60 molecules, and on the TCNE molecule adsorbed on Ag(100). We demonstrate that chemical effects on chemisorbed molecules, often ignored in TERS simulations of larger systems, dramatically change the TERS images. This observation calls for the inclusion of chemical effects for predictive theory-experiment comparisons and an understanding of molecular motion at the nanoscale.

A Full Quantum Mechanical Approach Assessing the Chemical and Electromagnetic Effect in TERS

Tip-enhanced Raman spectroscopy (TERS) is a valuable method for surface analysis with nanometer to angstrom-scale resolution; however, the accurate simulation of particular TERS signals remains a computational challenge. We approach this challenge by combining the two main contributors to plasmon-enhanced Raman spectroscopy and to the high resolution in TERS, in particular, the electromagnetic and the chemical effect, into one quantum mechanical simulation. The electromagnetic effect describes the sample's interaction with the strong, highly localized, and inhomogeneous electric fields associated with the plasmonic tip and is typically the thematic focus for most mechanistic studies. On the other hand, the chemical effect covers the different responses to the extremely close-range and highly position-sensitive chemical interaction between the apex tip atom(s) and the sample, and, as we could show in previous works, plays an often underestimated role. Starting from a (time-dependent) density functional theory description of the chemical model system, comprised of a tin(II) phthalocyanine sample molecule and a single silver atom as the tip, we introduce the electromagnetic effect through a series of static point charges that recreate the electric field in the vicinity of the plasmonic Ag nanoparticle. By scanning the tip over the molecule along a 3D grid, we can investigate the system's Raman response on each position for nonresonant and resonant illumination. Simulating both effects on their own already hints at the achievable signal enhancement and resolution, but the combination of both creates even stronger evidence that TERS is capable of resolving submolecular features.

Inside Block Copolymer Micelles-Tracing Interfacial Influences on Crosslinking Efficiency in Nanoscale Confined Spaces

Recently, several studies have demonstrated the excellent capabilities of tip-enhanced Raman spectroscopyfor in-depth investigations of structural properties of matter with unprecedented resolution and chemical specificity. These capabilities are utilized here to study the internal structure of core-crosslinked micelles, which are formed by self-assembly of the diblock terpolymer poly(ethylene oxide)-block-poly(furfuryl glycidylether-co-tert-butylglycidyl ether). Supplementing force-volume atomic force microscopy experiments address additionally the nanomechanical properties. Particularly, TERS enables investigating the underlying principles influencing the homogeneity and efficiency of the Diels-Alder core-crosslinking process in the confined hydrophobic core. While the central core region is homogenously crosslinked, a breakdown of the crosslinking reaction is observed in the core-corona interfacial region. The results corroborate that a strong crosslinking efficiency is directly correlated to the formation of a mixed zone of the glycidyl ether and PEO corona blocks reaching ≈5 nm into the core region. Concomitantly a strong exclusion of the encapsulated bismaleimide crosslinker from the interfacial region is observed. It is conceivable that a changed structure, chemical composition and altered nanomechanical properties of this interfacial region may also influence the crosslinking efficiency across the entire core region by a modification of the solubility of the crosslinker in the interfacial core-corona region.

Phonon transition across an isotopic interface

Isotopic mixtures result in distinct properties of materials such as thermal conductivity and nuclear process. However, the knowledge of isotopic interface remains largely unexplored mainly due to the challenges in atomic-scale isotopic identification. Here, using electron energy-loss spectroscopy in a scanning transmission electron microscope, we reveal momentum-transfer-dependent phonon behavior at the h-¹⁰BN/h-¹¹BN isotope heterostructure with sub-unit-cell resolution. We find the phonons' energy changes gradually across the interface, featuring a wide transition regime. Phonons near the Brillouin zone center have a transition regime of ~3.34 nm, whereas phonons at the Brillouin zone boundary have a transition regime of ~1.66 nm. We propose that the isotope-induced charge effect at the interface accounts for the distinct delocalization behavior. Moreover, the variation of phonon energy between atom layers near the interface depends on both of momentum transfer and mass change. This study provides new insights into the isotopic effects in natural materials.

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.

Near-field circular dichroism of single molecules

Near-field images of molecules provide information about their excited orbitals, giving rise to photonic and chemical functions. Such information is crucial to the elucidation of the full potential of molecules as components in functional materials and devices at the nanoscale. However, direct imaging inside single molecules with a complex structure in the near-field is still challenging because it requires in situ observation at a higher resolution than the molecular scale. Here, using a proven theoretical method that has demonstrated sub-nanoscale resolution based on photoinduced force microscopy (PiFM) experiment [Nat. Commun.12, 3865 (2021)10.1038/s41467-021-24136-2], we propose an approach to obtaining the near-field imaging with spatial patterns of electronic transitions of single molecules. We use an extended discrete dipole approximation method that incorporates microscopic nonlocal optical response of molecules and demonstrate that PiFM can visualize circular-dichroism signal patterns at sub-nanometer scale for both optically allowed and forbidden transitions. The result will open the possibility for the direct observation of complex spatial patterns of electronic transitions in a single molecule, providing insight into the optical function of single molecules and helping realize new functional materials and devices.

Effect of the focused gap-plasmon mode on tip-enhanced Raman excitation and scattering

As a powerful molecular detection approach, tip-enhanced Raman scattering (TERS) spectroscopy has the advantages of nanoscale spatial resolution, label-free detection and high enhancement factor, therefore has been widely used in fields of chemistry, materials and life sciences. A TERS system enhanced by the focused gap-plasmon mode composed of Surface Plasmon Polariton (SPP) focus and the metal probe has been reported, however, its underlying enhancement mechanism for Raman excitation and scattering remains to be deeply explored. Here, we focus on the different performances of optical focus and SPP focus in the TERS system, and verify that the cooperation of these two focuses can produce maximum enhancement in a local electromagnetic field. Further, the Purcell effect on sample scattering in such a system is studied for the enhancement of Raman scattering collection in the far field. Finally, the local field enhancement and the sample far-field scattering enhancement are combined to show a full view of the whole process of TERS enhancement. This research can be applied to optimize the excitation and collection of Raman signals in TERS systems, which is of great value for the research and development of TERS technology.

Plasmonic-Thermoelectric Nanotweezers for Immersive SERS Mapping

Surface-enhanced Raman spectroscopy (SERS) technology usually uses metallic nanoparticles to enhance Raman scattering signals, thereby significantly adding to molecule-level recognition and detection. However, realization of nanometer-scaled SERS imaging in liquid environments is extremely difficult due to the requirements of both precise scanning of single metallic nanoparticle and high enhancement field and thus has never been achieved before. To overcome this obstacle, we demonstrate an immersive nanometer-scaled SERS mapping technology, based on dynamic scanning of a single metallic nanoparticle with a plasmonic-thermoelectric nanotweezers system. The technology offers greater stability in the plasmonic trapping of gold nanoparticles at relative low power, as well as generating higher electric fields in the gap region. Through its dynamics, two-dimensional nanometer-scaled SERS imaging is achieved successfully. In regard to in liquid environments, this technology provides a mapping method for label-free imaging of ultrathin materials, structures, and biological samples.

Label-Free Super-Resolution Imaging Techniques

Biological and material samples contain nanoscale heterogeneities that are unresolvable with conventional microscopy techniques. Super-resolution fluorescence methods can break the optical diffraction limit to observe these features, but they require samples to be fluorescently labeled. Over the past decade, progress has been made toward developing super-resolution techniques that do not require the use of labels. These label-free techniques span a variety of different approaches, including structured illumination, transient absorption, infrared absorption, and coherent Raman spectroscopies. Many draw inspiration from widely successful fluorescence-based techniques such as stimulated emission depletion (STED) microscopy, photoactivated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM). In this review, we discuss the progress made in these fields along with the current challenges and prospects in reaching resolutions comparable to those achieved with fluorescence-based methods.

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

Molecular vibrational imaging at nanoscale

The demand to visualize the spatial distribution of chemical species based on vibrational spectra is rapidly increasing. Driven by such a need, various Raman and infrared spectro-microscopies with a nanometric spatial resolution have been developed over the last two decades. Despite rapid progress, a large gap still exists between the general needs and what these techniques can achieve. This Perspective highlights the key challenges and recent breakthroughs of the two vibrational nano-imaging techniques, scattering-type scanning near-field optical microscopy and tip-enhanced Raman scattering.

Strong Out-of-Plane Vibrations and Ultrasensitive Detection of Dopamine-like Neurotransmitters

The detection of monoamine neurotransmitters has become a vital research subject due to their high correlations with nervous system diseases, but insufficient detection precisions have obstructed diagnosis of some related diseases. Here, we focus on four monoamine neurotransmitters, dopamine, norepinephrine, epinephrine, and serotonin, to conduct their rapid and ultrasensitive detection. We find that the low-frequency (<200 cm^-1) Raman vibrations of these molecules show some sharp peaks, and their intensities are significantly stronger than those of the high-frequency side. Theoretical calculations identify these peaks to be from strong out-of-plane vibrations of the C-C single bonds at the joint point of the ring-like molecule and its side chain. Using our surface enhanced low-frequency Raman scattering substrates, we show that the detection limit of dopamine as an example can reach 10 nM in artificial cerebrospinal fluid. This work provides a useful way for ultrasensitive and rapid detection of some neurotransmitters.

Wafer-scale nanocracks enable single-molecule detection and on-site analysis

Large-area surface-enhanced Raman spectroscopy (SERS) sensing platforms displaying ultrahigh sensitivity and signal uniformity have potentially enormous sensing applicability, but they are still challenging to prepare in a scalable manner. In this study, silver nanopaste (AgNPA) was employed to prepare a wafer-scale, ultrasensitive SERS substrate. The self-generated, high-density Ag nanocracks (NCKs) with small gaps could be created on Si wafers via a spin-coating process, and provided extremely abundant hotspots for SERS analyses with ultrahigh sensitivity-down to the level of single molecules (enhancement factor: ca. 10¹⁰; detection limit: ca. 10^-18 M)-and great signal reproducibility (variation: ca. 3.6%). Moreover, the Ag NCK arrays demonstrated broad applicability and practicability for on-site detection when combined with a portable 785 Raman spectrometer. This method allowed the highly sensitive detection of a diverse range of analytes (benzo[a]pyrene, di-2-ethylhexyl phthalate, aflatoxins B1, zearalenone, ractopamine, salbutamol, sildenafil, thiram, dimethoate, and methamidophos). In particular, pesticides are used extensively in agricultural production. Unfortunately, they can affect the environment and human health as a result of acute toxicity. Therefore, the simultaneous label-free detection of three different pesticides was demonstrated. Finally, the SERS substrates are fabricated through a simple, efficient, and scalable process that offers new opportunities for mass production.

A Guide to Elucidate the Hidden Multicomponent Layered Structure of Plant Cuticles by Raman Imaging

The cuticle covers almost all plant organs as the outermost layer and serves as a transpiration barrier, sunscreen, and first line of defense against pathogens. Waxes, fatty acids, and aromatic components build chemically and structurally diverse layers with different functionality. So far, electron microscopy has elucidated structure, while isolation, extraction, and analysis procedures have revealed chemistry. With this method paper, we close the missing link by demonstrating how Raman microscopy gives detailed information about chemistry and structure of the native cuticle on the microscale. We introduce an optimized experimental workflow, covering the whole process of sample preparation, Raman imaging experiment, data analysis, and interpretation and show the versatility of the approach on cuticles of a spruce needle, a tomato peel, and an Arabidopsis stem. We include laser polarization experiments to deduce the orientation of molecules and multivariate data analysis to separate cuticle layers and verify their molecular composition. Based on the three investigated cuticles, we discuss the chemical and structural diversity and validate our findings by comparing models based on our spectroscopic data with the current view of the cuticle. We amend the model by adding the distribution of cinnamic acids and flavonoids within the cuticle layers and their transition to the epidermal layer. Raman imaging proves as a non-destructive and fast approach to assess the chemical and structural variability in space and time. It might become a valuable tool to tackle knowledge gaps in plant cuticle research.

Strong histamine torsion Raman spectrum enables direct, rapid, and ultrasensitive detection of allergic diseases

Allergic diseases are closely related to degranulation and release of histamine and difficult to diagnose because non-allergic diseases also exhibit the same clinical symptoms as allergy. Here, we report direct, rapid, and ultrasensitive detection of histamine using low-frequency molecular torsion Raman spectroscopy. We show that the low-frequency (<200 cm-1) Raman spectral intensities are stronger by one order of magnitude than those of the high-frequency Raman ones. Density functional theory calculation and nuclear magnetic resonance spectroscopy identify the strong spectral feature to be from torsions of carbon-carbon single bonds, which produce large variations of the polarizability densities in the imidazole ring and ethyl amino side chain. Using an omniphobic substrate and surface plasmonic effect of Au@SiO2 nanoparticles, the detection limit (signal-noise ratio >3) of histamine reaches 10-8 g/L in water and 10-6 g/L in serum. This scheme thus opens new lines of inquiry regarding the clinical diagnosis of allergic diseases.

Strong Coupling between Localized Surface Plasmons and Molecules by Coupled Cluster Theory

Plasmonic nanocavities enable the confinement of molecules and electromagnetic fields within nanometric volumes. As a consequence, the molecules experience a remarkably strong interaction with the electromagnetic field to such an extent that the quantum states of the system become hybrids between light and matter: polaritons. Here, we present a nonperturbative method to simulate the emerging properties of such polaritons: it combines a high-level quantum chemical description of the molecule with a quantized description of the localized surface plasmons in the nanocavity. We apply the method to molecules of realistic complexity in a typical plasmonic nanocavity, featuring also a subnanometric asperity (picocavity). Our results disclose the effects of the mutual polarization and correlation of plasmons and molecular excitations, disregarded so far. They also quantify to what extent the molecular charge density can be manipulated by nanocavities and stand as benchmarks to guide the development of methods for molecular polaritonics.

Operando Raman Shift Replaces Current in Electrochemical Analysis of Li-ion Batteries: A Comparative Study

Li-rich and catalytically active γ-LixV2O5 (x = 1.48) was investigated as a cathode for its heterogeneous charge transfer kinetics. Using a specially designed two-electrode system lithium half cell, Butler–Volmer analysis was performed, and Raman spectra were acquired in 18 mV intervals. A direct correlation was observed between the Raman shift of the active modes Ag,Bg, Au, and Bu, and the development of the Faraday current at the working electrode. The Raman intensity and the Raman shift were implemented to replace the current in a Tafel plot used for the analysis of Butler–Volmer kinetics. Striking similarities in the charge transfer proportionality constants α were found for current and Raman-based analysis. The potential of this new method of Raman-aided electrochemical detection at the diffraction limit is discussed.

Influence of an atomistic protrusion at the tip apex on enhancing molecular emission in tunnel junctions: A theoretical study

Light emission from the gap of a scanning tunneling microscope can be used to investigate many optoelectronic processes at the single-molecule level and to gain insight into the fundamental photophysical mechanisms involved. One important issue is how to improve the quantum efficiency of quantum emitters in the nanometer-sized metallic gap so that molecule-specific emission can be clearly observed. Here, using electromagnetic simulations, we systematically investigate the influence of an atomic-scale protrusion at the tip apex on the emission properties of a point dipole in the plasmonic nanocavity. We found that such an atomistic protrusion can induce strong and spatially highly confined electric fields, thus increasing the quantum efficiency of molecular fluorescence over two orders of magnitude even when its dipole is oriented parallel to the metal surface, a situation occurring in most realistic single-molecule electroluminescence experiments. In addition, our theoretical simulations indicate that due to the lightning rod effect induced by the protrusion in a plasmonic nanocavity, the quantum efficiency increases monotonically as the tip approaches the dipole to the point of contact, instead of being quenched, thus explaining previous experimental observations with ever-enhancing fluorescence. Furthermore, we also examine in detail how the protrusion radius, height, and material affect the protrusion-induced emission enhancement. These results are believed to be instructive for further studies on the optoelectronic properties of single molecules in tip-based plasmonic nanocavities.

Plasmonic Core-Shell Nanoparticle Enhanced Spectroscopies for Surface Analysis

Probing the properties and components of reactive surfaces is crucial for illustrating reaction mechanisms. However, common surface analysis techniques are restricted to in situ acquisition of surface information at the molecular scale in the human environment and industrial catalysis processes. Plasmonic spectroscopies are promising tools to solve this problem. This Feature is intended to introduce the plasmonic core-shell nanoparticle enhanced spectroscopies for qualitatively and quantitatively analyzing surface trace species. Four different working modalities are designed for meeting varied needs, involving in situ surface species detection, catalytic process monitoring, labeled sensing, and dual mode analysis. These newly developed plasmonic spectroscopies show great potential not only in fundamental research but also in practical applications.

Imaging Plasmons with Sub-2 nm Spatial Resolution via Tip-Enhanced Four-Wave Mixing

Four-wave mixing at plasmonic tip-sample nanojunctions may be used to visualize plasmonic fields with sub-2 nm spatial resolution under ambient laboratory conditions. We illustrate the latter using a gold-coated atomic force microscopy probe irradiated with a pair of near-infrared femtosecond laser pulses and used to image plasmonic gold nanoplates and silver nanocubes. Through diagnostic polarization-dependent tip-only measurements, we illustrate that the four-wave mixing signal is localized to the tip apex. The apex-bound signal is further enhanced when the tip is located at specific locations near plasmonic nanoparticles. Overall, this work paves the way for visualizing chemical transformations as well as coherent electronic and vibrational dynamics with joint femtosecond temporal and few-nanometer spatial resolution under ambient conditions.

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.

The Expanding Frontiers of Tip-Enhanced Raman Spectroscopy

Fundamental understanding of chemistry and physical properties at the nanoscale enables the rational design of interface-based systems. Surface interactions underlie numerous technologies ranging from catalysis to organic thin films to biological systems. Since surface environments are especially prone to heterogeneity, it becomes crucial to characterize these systems with spatial resolution sufficient to localize individual active sites or defects. Spectroscopy presents as a powerful means to understand these interactions, but typical light-based techniques lack sufficient spatial resolution. This review describes the growing number of applications for the nanoscale spectroscopic technique, tip-enhanced Raman spectroscopy (TERS), with a focus on developments in areas that involve measurements in new environmental conditions, such as liquid, electrochemical, and ultrahigh vacuum. The expansion into unique environments enables the ability to spectroscopically define chemistry at the spatial limit. Through the confinement and enhancement of light at the apex of a plasmonic scanning probe microscopy tip, TERS is able to yield vibrational fingerprint information of molecules and materials with nanoscale resolution, providing insight into highly localized chemical effects.

Tip Recycling for Atomic Force Microscopy-Based Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for the characterization of surfaces and two-dimensional materials, delivering both topographical and chemical information with nanometer-scale spatial resolution. Atomic force microscopy (AFM)-TERS combines AFM with a Raman spectrometer and is a very versatile technique, capable of working in vacuum, air, and liquid, and on a variety of different samples. A metalized AFM tip is necessary in order to take advantage of the plasmonic enhancement. The most commonly used metal is Ag, thanks to its high plasmonic activity in the visible range. Unfortunately, though, the tip metallization process is still challenging and not fully reliable, yielding inconsistent enhancement factors even within the same batch of tips; as a consequence, many tips are usually prepared at once (for a single experiment), to ensure that at least one of them is sufficiently active. As the lifetime of an unprotected, Ag-coated plasmonic probe is only a few hours, the procedure is inefficient and results in a substantial waste of materials and money. In this work, we establish a cleaning routine to effectively re-use Ag-coated AFM-TERS probes, drastically reducing costs without compromising the quality of the experimental results.

Single molecule secondary structure determination of proteins through infrared absorption nanospectroscopy

The chemical and structural properties of biomolecules determine their interactions, and thus their functions, in a wide variety of biochemical processes. Innovative imaging methods have been developed to characterise biomolecular structures down to the angstrom level. However, acquiring vibrational absorption spectra at the single molecule level, a benchmark for bulk sample characterization, has remained elusive. Here, we introduce off-resonance, low power and short pulse infrared nanospectroscopy (ORS-nanoIR) to allow the acquisition of infrared absorption spectra and chemical maps at the single molecule level, at high throughput on a second timescale and with a high signal-to-noise ratio (~10-20). This high sensitivity enables the accurate determination of the secondary structure of single protein molecules with over a million-fold lower mass than conventional bulk vibrational spectroscopy. These results pave the way to probe directly the chemical and structural properties of individual biomolecules, as well as their interactions, in a broad range of chemical and biological systems.

Tip-Enhanced Multipolar Raman Scattering

We record nanoscale chemical images of thiobenzonitrile (TBN)-functionalized plasmonic gold nanocubes via tip-enhanced Raman spectroscopy (TERS). The spatially averaged optical response is dominated by conventional (dipolar) TERS scattering from TBN but also contains weaker spectral signatures in the 1225-1500 cm^-1 region. The weak optical signatures dominate several of the recorded single-pixel TERS spectra. We can uniquely assign these Raman-forbidden transitions to multipolar Raman scattering, which implicates spatially varying enhanced electric field gradients at plasmonic tip-sample nanojunctions. Specifically, we can assign observations of tip-enhanced electric dipole-magnetic dipole as well as electric dipole-electric quadrupole driven transitions. Multipolar Raman scattering and local optical field gradients both need to be understood and accounted for in the interpretation of TERS spectral images, particularly in ongoing quests aimed at chemical reaction mapping via TERS.

Multimodal Characterization of Resin Embedded and Sliced Polymer Nanoparticles by Means of Tip-Enhanced Raman Spectroscopy and Force-Distance Curve Based Atomic Force Microscopy

Understanding the property-function relation of nanoparticles in various application fields involves determining their physicochemical properties, which is still a remaining challenge to date. While a multitude of different characterization tools can be applied, these methods by themselves can only provide an incomplete picture. Therefore, novel analytical techniques are required, which can address both chemical functionality and provide structural information at the same time with high spatial resolution. This is possible by using tip-enhanced Raman spectroscopy (TERS), but due to its limited depth information, TERS is usually restricted to investigations of the nanoparticle surface. Here, TERS experiments are established on polystyrene nanoparticles (PS NPs) after resin embedding and microtome slicing. With that, unique access to their internal morphological features is gained, and thus, enables differentiation between information obtained for core- and shell-regions. Complementary information is obtained by means of transmission electron microscopy (TEM) and from force-distance curve based atomic force microscopy (FD-AFM). This multimodal approach achieves a high degree of discrimination between the resin and the polymers used for nanoparticle formulation. The high potential of TERS combined with advanced AFM spectroscopy tools to probe the mechanical properties is applied for quality control of the resin embedding procedure.

The chemical effect goes resonant - a full quantum mechanical approach on TERS

Lately, experimental evidence of unexpectedly extremely high spatial resolution of tip-enhanced Raman scattering (TERS) has been demonstrated. Theoretically, two different contributions are discussed: an electromagnetic effect, leading to a spatially confined near field due to plasmonic excitations; and the so-called chemical effect originating from the locally modified electronic structure of the molecule due to the close proximity of the plasmonic system. Most of the theoretical efforts have concentrated on the electromagnetic contribution or the chemical effect in case of non-resonant excitation. In this work, we present a fully quantum mechanical description including non-resonant and resonant chemical contributions as well as charge-transfer phenomena of these molecular-plasmonic hybrid systems at the density functional and the time-dependent density functional level of theory. We consider a surface-immobilized tin(ii) phthalocyanine molecule as the molecular system, which is minutely scanned by a plasmonic tip, modeled by a single silver atom. These different relative positions of the Ag atom to the molecule lead to pronounced alterations of the Raman spectra. These Raman spectra vary substantially, both in peak positions and several orders of magnitude in the intensity patterns under non-resonant and resonant conditions, and also, depending on, which electronic states are addressed. Our computational approach reveals that unique - non-resonant and resonant - chemical interactions among the tip and the molecule significantly alter the TERS spectra and are mainly responsible for the high, possibly sub-Angstrom spatial resolution.

Vibrational spectra and chemical imaging of cyclo[18]carbon by tip enhanced Raman spectroscopy

Vibrational modes and tip enhanced Raman spectroscopy (TERS) of a new carbon allotrope, cyclo[18]carbon (C18), were studied by density functional theory. A silver cluster tip was used to probe the interaction with C18, which is dependent on the distance and the atomically resolved positions. The TERS images show the position of the C[triple bond, length as m-dash]C bonds, as observed in a recent experimental report.

Resolving Molecular Structures with High-Resolution Tip-Enhanced Raman Scattering Images

Vibrational modes of a single molecule can be visualized by tip-enhanced Raman spectroscopy with atomic resolution. However, the exact vibrations associated with these Raman scattering images are still in debate due to the lack of theoretical interpretation. In this work, we systematically study the Raman scattering images of a single Co(II)-tetraphenylporphyrin molecule. The stable structure whose Raman scattering images consistently match experimental results is discovered. Furthermore, we elucidate the effects of near-field localizations and field gradient on the resolution in Raman scattering images. The approach of locally integrated Raman polarizability density employed in this work provides an intuitive explanation of the origin of the experimental Raman scattering images.