Single-Molecule Imaging Using Atomistic Near-Field Tip-Enhanced Raman Spectroscopy

Near-field induced local excitation dynamics of Na10 and Na10-N2 from real-time TDDFT

Electron dynamics of the Na10 chain and the Na10-N2 complex locally excited by an atomistic optical near-field are investigated using real-time time-dependent density functional theory calculations on real-space grids. Ultrafast laser pulses were used to simulate the near-field excitation under on- and off-resonance conditions. Off-resonance excitation did not lead to the propagation of the excitation through the Na10 chain. In contrast, under the resonance conditions, the excited state is delocalized over the entire Na chain. Analysis of the local dipole moment of each atom in Na10 indicates that this behavior is consistent with the transition density. Adding an N2 molecule to the opposite end of the local excitation region results in energy transfer via the Na10 chain. The energy transfer efficiency of the N2 molecule is well correlated with the absorption spectrum of Na10. The present study paves the way for realizing remote excitation and photonic devices at the atomic scale.

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.

Quantitative analysis of charge transfer plasmons in silver nanocluster dimers using semiempirical methods

Plasmonic metal nanoclusters are widely used in chemistry, nanotechnology, and biomedicine. In metal nanocluster dimers, coupling of the plasmons leads to the emergence of two distinct types of modes: (1) bonding dipole plasmons (BDP), which occurs when charge oscillates synchronously within each nanocluster, and (2) charge transfer plasmons (CTP), which occurs when charge oscillates between two conductively linked nanoclusters. Although TDDFT-based modeling has uncovered some trends in these modes, it is computationally expensive for large dimers, and quantitative analysis is challenging. Here, we demonstrate that the semiempirical quantum mechanical method INDO/CIS enables us to quantify the CTP character of each excited state efficiently. In end-to-end Ag nanowire dimers, the longitudinal states have CTP character that decreases with increasing gap distance and nanowire length. In side-by-side dimers, the transverse states have CTP character and generally larger than in the end-to-end dimers, particularly for the longer nanowires. In side-by-side dimers where one nanowire is shifted along the length of the other, the CTP character of the longitudinal states peaks when the dimer is shifted by two Ag-Ag bond lengths, while the transverse states show decreasing CTP character as displacement increases. In the larger Ag31+ nanorod dimers, CTP character follow a similar distance dependence to that seen in the small nanowire but have smaller overall CTP character than the nanowires. Our study demonstrates that INDO/CIS is capable of modeling metal nanocluster dimers at a low computational cost, making it possible to study larger dimers that are difficult to analyze using TDDFT.

DOX-DNA Interactions on the Nanoscale: In Situ Studies Using Tip-Enhanced Raman Scattering

Chemotherapeutic anthracyclines, like doxorubicin (DOX), are drugs endowed with cytostatic activity and are widely used in antitumor therapy. Their molecular mechanism of action involves the formation of a stable anthracycline-DNA complex, which prevents cell division and results in cell death. It is known that elevated DOX concentrations induce DNA chain loops and overlaps. Here, for the first time, tip-enhanced Raman scattering was used to identify and localize intercalated DOX in isolated double-stranded calf thymus DNA, and the correlated near-field spectroscopic and morphologic experiments locate the DOX molecules in the DNA and provide further information regarding specific DOX-nucleobase interactions. Thus, the study provides a tool specifically for identifying intercalation markers and generally analyzing drug-DNA interactions. The structure of such complexes down to the molecular level provides mechanistic information about cytotoxicity and the development of potential anticancer drugs.

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.

Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopy to remotely control and probe the interfacial charge transfer dynamics with sub-10 nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

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.

Variations in the Photoexcitation Mechanism of an Adsorbed Molecule on a Gold Nanocluster Governed by Interfacial Contact

We performed first-principles calculations on the optical response of a Au147-azobenzene complex to elucidate the role of the interfacial contact between Au147 and azobenzene. Our calculations of photoexcited electron dynamics for different configurations of azobenzene adsorbed on the Au147 nanocluster revealed that the optical properties of the azobenzene moiety change markedly by the interfacial contact, even if the electronic structure in the ground state is almost unchanged. The optical absorption measured for isolated azobenzene weakens when the Au147-azobenzene interaction increases, while the absorption measured using the light field along the Au147-azobenzene alignment strengthens. The electronic excitation analysis showed that the mechanism of the charge-transfer excitation between Au147 and azobenzene changes remarkably depending on the strength of the interfacial interaction. We revealed that the optical property can be governed by the atomic-scale difference in the adsorption structure of azobenzene on a Au147 nanocluster. This study affords novel insights that could enable the photoexcitation mechanism to be controlled by designing the interface between a metal nanoparticle and a molecule.

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.

Accurate Simulations of Scanning Tunneling Microscope: Both Tip and Substrate States Matter

Scanning tunneling microscope (STM) provides an atomic-scale characterization tool. To this end, high-resolution measurements and accurate simulations must closely cooperate. Emerging experimental techniques, e.g., substrate spacers and tip modifications, suppress metallic couplings and improve the resolution. On the other hand, development of STM simulation methods was inactive in the past decade. Conventional simulations focus on the electronic structure of the substrate, often overlooking detailed descriptions of the tip states. Meanwhile, the overwhelming usage of periodic boundary conditions ensures effective simulations of only neutral systems. In this Perspective, we highlight the recent progress that takes the effects of both tip and substrate into account under either Tersoff-Hamann or Bardeen's approximation, which provides an accurate analysis of measured high-resolution STM results, uncovers underlying concepts, and rationally designs experimental protocols for important chemical systems. We hope this Perspective will stimulate broad interest in advanced STM simulations, highlighting the way forward for STM investigations that involve complex geometrical and electronic structures.

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.

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.

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.

Do We Really Need Quantum Mechanics to Describe Plasmonic Properties of Metal Nanostructures?

Optical properties of metal nanostructures are the basis of several scientific and technological applications. When the nanostructure characteristic size is of the order of few nm or less, it is generally accepted that only a description that explicitly describes electrons by quantum mechanics can reproduce faithfully its optical response. For example, the plasmon resonance shift upon shrinking the nanostructure size (red-shift for simple metals, blue-shift for d-metals such as gold and silver) is universally accepted to originate from the quantum nature of the system. Here we show instead that an atomistic approach based on classical physics, ωFQFμ (frequency dependent fluctuating charges and fluctuating dipoles), is able to reproduce all the typical "quantum" size effects, such as the sign and the magnitude of the plasmon shift, the progressive loss of the plasmon resonance for gold, the atomistically detailed features in the induced electron density, and the non local effects in the nanoparticle response. To support our findings, we compare the ωFQFμ results for Ag and Au with literature time-dependent DFT simulations, showing the capability of fully classical physics to reproduce these TDDFT results. Only electron tunneling between nanostructures emerges as a genuine quantum mechanical effect, that we had to include in the model by an ad hoc term.

Molecular Optomechanics Approach to Surface-Enhanced Raman Scattering

Molecular vibrations constitute one of the smallest mechanical oscillators available for micro-/nanoengineering. The energy and strength of molecular oscillations depend delicately on the attached specific functional groups as well as on the chemical and physical environments. By exploiting the inelastic interaction of molecules with optical photons, Raman scattering can access the information contained in molecular vibrations. However, the low efficiency of the Raman process typically allows only for characterizing large numbers of molecules. To circumvent this limitation, plasmonic resonances supported by metallic nanostructures and nanocavities can be used because they localize and enhance light at optical frequencies, enabling surface-enhanced Raman scattering (SERS), where the Raman signal is increased by many orders of magnitude. This enhancement enables few- or even single-molecule characterization. The coupling between a single molecular vibration and a plasmonic mode constitutes an example of an optomechanical interaction, analogous to that existing between cavity photons and mechanical vibrations. Optomechanical systems have been intensely studied because of their fundamental interest as well as their application in practical implementations of quantum technology and sensing. In this context, SERS brings cavity optomechanics down to the molecular scale and gives access to larger vibrational frequencies associated with molecular motion, offering new possibilities for novel optomechanical nanodevices.

The molecular optomechanics description of SERS is recent, and its implications have only started to be explored. In this Account, we describe the current understanding and progress of this new description of SERS, focusing on our own contributions to the field. We first show that the quantum description of molecular optomechanics is fully consistent with standard classical and semiclassical models often used to describe SERS. Furthermore, we note that the molecular optomechanics framework naturally accounts for a rich variety of nonlinear effects in the SERS signal with increasing laser intensity.

Furthermore, the molecular optomechanics framework provides a tool particularly suited to addressing novel effects of fundamental and practical interest in SERS, such as the emergence of collective phenomena involving many molecules or the modification of the effective losses and energy of the molecular vibrations due to the plasmon–vibration interaction. As compared to standard optomechanics, the plasmonic resonance often differs from a single Lorentzian mode and thus requires a more detailed description of its optical response. This quantum description of SERS also allows us to address the statistics of the Raman photons emitted, enabling the interpretation of two-color correlations of the emerging photons, with potential use in the generation of nonclassical states of light. Current SERS experimental implementations in organic molecules and two-dimensional layers suggest the interest in further exploring intense pulsed illumination, situations of strong coupling, resonant-SERS, and atomic-scale field confinement.

In silico design of graphene plasmonic hot-spots

We propose a route for the rational design of engineered graphene-based nanostructures, which feature enormously enhanced electric fields in their proximity. Geometrical arrangements are inspired by nanopatterns allowing single molecule detection on noble metal substrates, and are conceived to take into account experimental feasibility and ease in fabrication processes. The attention is especially focused on enhancement effects occurring close to edge defects and grain boundaries, which are usually present in graphene samples. There, very localized hot-spots are created, with enhancement factors comparable to noble metal substrates, thus potentially paving the way for single molecule detection from graphene-based substrates.

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.

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.

Two orders of magnitude extra SERS enhancement on silver nanoparticle-based substrate induced by laser irradiation in nitrogen ambient

Photo-reduction of silver oxide and light-induced Ag nanoparticle (NP) generations have been applied for Surface-enhanced Raman spectroscopy (SERS) substrate fabricated for years. In this paper, we demonstrate a general method to enhance the SERS activity of conventional Ag NPs-based SERS substrates by performing Raman scattering measurement in a nitrogen ambient after a period of laser irradiation (photoactivation). The Raman characteristic peak intensity of carbonaceous impurities adsorbed on the surfaces of Ag NPs display an additional enhancement of 93 times after photoactivation in nitrogen ambient. A 3-fold extra Raman gain enhancement is also observed in the nitrogen-protected SERS measurement of R6G molecules. The extra SERS enhancement is attributed to the sub-nanometer scale near-field coupling between the Ag NPs and the photo-generated Ag clusters in the surface oxide layer of Ag NPs. This model is verified through the finite-difference time-domain (FDTD) simulations.

Role of metal-nanostructure features on tip-enhanced photoluminescence of single molecules

Tip-enhanced photoluminescence (TEPL) experiments have recently reached the ability to investigate single molecules exploiting resolution at the submolecular level. Localized surface plasmon resonances of metallic nanostructures have the capability of enhancing an impinging electromagnetic radiation in the proximity of their surface, with evident consequences both on absorption and emission of molecules placed in the same region. We propose a theoretical analysis of these phenomena in order to interpret TEPL experiments on single molecules, including a quantum mechanical description of the target molecule equilibrated with the presence of two nanostructures representative of the nanocavity usually employed in STMs. The approach has been applied to the zinc phthalocyanine molecule, previously considered in recent TEPL experiments [Yang et al., Nat. Photonics 14, 693-699 (2020)]. This work has the aim of providing a comprehensive theoretical understanding of the experimental results, particularly focusing on the investigation of the tip features that majorly influence the excitation and fluorescence processes of the molecule, such as the geometry, the dielectric function, and the tip-molecule distance.

Nonlinearity-modulated single molecule trapping and Raman scattering analysis

Single molecule detection and analysis play important roles in many current biomedical researches. The deep-nanoscale hotspots, being excited and confined in a plasmonic nanocavity, make it possible to simultaneously enhance the nonlinear light-matter interactions and molecular Raman scattering for label-free detections. Here, we theoretically show that a nanocavity formed in a tip-enhanced Raman scattering (TERS) system can also achieve valid optical trapping as well as TERS signal detection for a single molecule. In addition, the nonlinear responses of metallic tip and substrate film can change their intrinsic physical properties, leading to the modulation of the optical trapping force and the TERS signal. The results demonstrate a new degree of freedom brought by the nonlinearity for effectively modulating the optical trapping and Raman detection in single molecule level. This proposed platform also shows a great potential in various fields of research that need high-precision surface imaging.

High-Resolution Measurement of Molecular Internal Polarization Structure by Photoinduced Force Microscopy

Near-field interactions between metallic surfaces and single molecules play an essential role in the application of metamaterials. To reveal the near-field around a photo-irradiated single molecule on the metallic surface, high-resolution photo-assisted scanning microscopy is required. In this study, we theoretically propose photoinduced force microscopy (PiFM) measurements of single molecules at the atomic resolution. For experimental demonstration, we performed a numerical calculation of PiFM images of various transition states, including optical forbidden transitions, and interpreted them in terms of the interaction between the molecular internal polarization structures and localized plasmon. We also clarified the critical role of atomic-scale structures on the tip surface for high-resolution PiFM measurements.

Bright Optical Eigenmode of 1 nm^{3} Mode Volume

We report on the discovery and rationale to devise bright single optical eigenmodes that feature quantum-optical mode volumes of about 1  nm^{3}. Our findings rely on the development and application of a quasinormal mode theory that self-consistently treats fields and electron nonlocality, spill-out, and Landau damping around atomistic protrusions on a metallic nanoantenna. By outpacing Landau damping with radiation via properly designed antenna modes, the extremely localized modes become bright with radiation efficiencies reaching 30% and could provide up to 4×10^{7} times intensity enhancement.

Plexcitons, electric field gradient and electron-phonon coupling in tip-enhanced Raman spectroscopy (TERS)

Tip-Enhanced Raman Spectroscopy (TERS) provides very high spatial resolution and detection sensitivity, so it has important applications in nano-scale molecular analysis. Plexciton is a polarization mode caused by a strongly coupled interaction between plasma excitons and excitons. It is a hot topic in plasma photonics research. We introduce the characteristics, production methods, observation methods and some applications of TERS. The electric field gradient (EFG) is an important factor affecting TERS resolution. The electron-phonon interaction is a fundamental inelastic interaction and plays an important role in current-carrying single-molecular junctions. This article summarizes the characteristics and applications of these three parts for readers to gain a preliminary understanding.

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.

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.

Representing molecular ground and excited vibrational eigenstates with nuclear densities obtained from semiclassical initial value representation molecular dynamics

We present in detail and validate an effective Monte Carlo approach for the calculation of the nuclear vibrational densities via integration of molecular eigenfunctions that we have preliminary employed to calculate the densities of the ground and the excited OH stretch vibrational states in the protonated glycine molecule [Aieta et al., Nat Commun 11, 4348 (2020)]. Here, we first validate and discuss in detail the features of the method on a benchmark water molecule. Then, we apply it to calculate on-the-fly the ab initio anharmonic nuclear densities in the correspondence of the fundamental transitions of NH and CH stretches in protonated glycine. We show how we can gain both qualitative and quantitative physical insight by inspection of different one-nucleus densities and assign a character to spectroscopic absorption peaks using the expansion of vibrational states in terms of harmonic basis functions. The visualization of the nuclear vibrations in a purely quantum picture allows us to observe and quantify the effects of anharmonicity on the molecular structure, also to exploit the effect of IR excitations on specific bonds or functional groups, beyond the harmonic approximation. We also calculate the quantum probability distribution of bond lengths, angles, and dihedrals of the molecule. Notably, we observe how in the case of one type of fundamental NH stretching, the typical harmonic nodal pattern is absent in the anharmonic distribution.

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.

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.

Optical scanning tunneling microscopy based chemical imaging and spectroscopy

Through coupling optical processes with the scanning tunneling microscope (STM), single-molecule chemistry and physics have been investigated at the ultimate spatial and temporal limit. Electrons and photons can be used to drive interactions and reactions in chemical systems and simultaneously probe their characteristics and consequences. In this review we introduce and review methods to couple optical imaging and spectroscopy with scanning tunneling microscopy. The integration of the STM and optical spectroscopy provides new insights into individual molecular adsorbates, surface-supported molecular assemblies, and two-dimensional materials with subnanoscale resolution, enabling the fundamental study of chemistry at the spatial and temporal limit. The inelastic scattering of photons by molecules and materials, that results in unique and sensitive vibrational fingerprints, will be considered with tip-enhanced Raman spectroscopy. STM-induced luminescence examines the intrinsic luminescence of organic adsorbates and their energy transfer and charge transfer processes with their surroundings. We also provide a survey of recent efforts to probe the dynamics of optical excitation at the molecular level with scanning tunneling microscopy in the context of light-induced photophysical and photochemical transformations.

Plasmonic Resonances of Metal Nanoparticles: Atomistic vs. Continuum Approaches

The fully atomistic model, ωFQ, based on textbook concepts (Drude theory, electrostatics, quantum tunneling) and recently developed by some of the present authors in Nanoscale, 11, 6004-6015 is applied to the calculation of the optical properties of complex Na, Ag, and Au nanostructures. In ωFQ, each atom of the nanostructures is endowed with an electric charge that can vary according to the external electric field. The electric conductivity between nearest atoms is modeled by adopting the Drude model, which is reformulated in terms of electric charges. Quantum tunneling effects are considered by letting the dielectric response of the system arise from atom-atom conductivity. ωFQ is challenged to reproduce the optical response of metal nanoparticles of different sizes and shapes, and its performance is compared with continuum Boundary Element Method (BEM) calculations.

Multiplexed Discrimination of Single Amino Acid Residues in Polypeptides in a Single SERS Hot Spot

The SERS-based detection of protein sequences with single-residue sensitivity suffers from signal dominance of aromatic amino acid residues and backbones, impeding detection of non-aromatic amino acid residues. Herein, we trap a gold nanoparticle in a plasmonic nanohole to generate a single SERS hot spot for single-molecule detection of 2 similar polypeptides (vasopressin and oxytocin) and 10 distinct amino acids that constitute the 2 polypeptides. Significantly, both aromatic and non-aromatic amino acids are detected and discriminated at the single-molecule level either at individual amino acid molecules or within the polypeptide chains. Correlated with molecular dynamics simulations, our results suggest that the signal dominance due to large spatial occupancy of aromatic rings of the polypeptide sidechains on gold surfaces can be overcome by the high localization of the single hot spot. The superior spectral and spatial discriminative power of our approach can be applied to single-protein analysis, fingerprinting, and sequencing.

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.

Single-molecule resonance Raman effect in a plasmonic nanocavity

Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine molecule adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying molecules on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the electric field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-molecule study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrology of various molecular systems.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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

The strong spatial confinement of a nanocavity plasmonic field has made it possible to visualize the inner structure of a single molecule and even to distinguish its vibrational modes in real space. With such ever-improved spatial resolution, it is anticipated that full vibrational imaging of a molecule could be achieved to reveal molecular structural details. Here we demonstrate full Raman images of individual vibrational modes at the ångström level for a single Mg-porphine molecule, revealing distinct characteristics of each vibrational mode in real space. Furthermore, by exploiting the underlying interference effect and Raman fingerprint database, we propose a new methodology for structural determination, which we have called ‘scanning Raman picoscopy’, to show how such ultrahigh-resolution spectromicroscopic vibrational images can be used to visually assemble the chemical structure of a single molecule through a simple Lego-like building process.

Machine Learning Protocol for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a powerful technique that can capture the electronic-vibrational "fingerprint" of molecules on surfaces. Ab initio prediction of Raman response is a long-standing challenge because of the diversified interfacial structures. Here we show that a cost-effective machine learning (ML) random forest method can predict SERS signals of a trans-1,2-bis (4-pyridyl) ethylene (BPE) molecule adsorbed on a gold substrate. Using geometric descriptors extracted from quantum chemistry simulations of thousands of ab initio molecular dynamics conformations, the ML protocol predicts vibrational frequencies and Raman intensities. The resulting spectra agree with density functional theory calculations and experiment. Predicted SERS responses of the molecule on different surfaces, or under external fields of electric fields and solvent environment, demonstrate the good transferability of the protocol.

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.

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

Tip-enhanced Raman spectroscopy (TERS) exhibits new selection rule and sub-nanometer spatial resolution, which is attributed to the plasmonic near-field confinement. Despite recent advances in simulations of TERS spectra under highly confined fields, a simply physical mechanism has remained elusive. In this work we show that single-molecule TERS images can be explained by local sub-molecular density changes induced by the confined near-field during the Raman process. The local sub-molecular density changes determine the spatial resolution in TERS and the gradient-based selection rule. Using this approach we find that the four-fold symmetry of meso-tetrakis(3,5-di-tert-butylphenyl)porphyrin (H₂TBPP) TERS images observed in experiments arises from the combination of degenerate normal modes localized in the functional side groups rather than the porphyrin ring as previously considered. As an illustration of the potential of the method, we demonstrate how this new theory can be applied to microscopic structure characterization.

Despite recent advances in simulations of tip-enhanced Raman spectroscopy (TERS) under highly confined fields, a simply physical mechanism has remained elusive. Here, the authors show that single molecule TERS images can be explained by local sub-molecular density changes induced by the confined near-field during the Raman process.

A classical picture of subnanometer junctions: an atomistic Drude approach to nanoplasmonics

The description of optical properties of subnanometer junctions is particularly challenging. Purely classical approaches fail, because the quantum nature of electrons needs to be considered. Here we report on a novel classical fully atomistic approach, ωFQ, based on the Drude model for conduction in metals, classical electrostatics and quantum tunneling. We show that ωFQ is able to reproduce the plasmonic behavior of complex metal subnanometer junctions with quantitative fidelity to full ab initio calculations. Besides the practical potentialities of our approach for large scale nanoplasmonic simulations, we show that a classical approach, in which the atomistic discretization of matter is properly accounted for, can accurately describe the nanoplasmonics phenomena dominated by quantum effects.

Organometallic-Constructed Tip-Based Dual Chemical Sensing by Tip-Enhanced Raman Spectroscopy for Diabetes Detection

Tip-enhanced Raman spectroscopy (TERS) is capable of probing specific molecular information with high sensitivity, but dual chemical sensing remains a challenge. Another major hindrance to TERS chemical detection in biosamples such as blood is the interference from the strong absorptions of biomolecules. Herein, we report the preparation of an organometallic-conjugated TERS tip. We demonstrate that organometallic chemistry can be perfectly coupled with TERS for dual-molecule sensing. The unique Raman signals generated by the organometallic compound circumvent signal interference from the biomolecules in blood, allowing the rapid analysis of two important molecules (glucose and thiol) in ultralow volume (50 nL) samples. This enabled a correlation between the thiol and glucose levels in the blood of nondiabetic and diabetic patients to be drawn.

Cell-Nanoparticle Interactions at (Sub)-Nanometer Resolution Analyzed by Electron Microscopy and Correlative Coherent Anti-Stokes Raman Scattering

A wide variety of nanoparticles are playing an increasingly important role in drug delivery. Label-free imaging techniques are especially desirable to follow the cellular uptake and intracellular fate of nanoparticles. The combined correlative use of different techniques, each with unique advantages, facilitates more detailed investigation about such interactions. The synergistic use of correlative coherent anti-Stokes Raman scattering and electron microscopy (C-CARS-EM) imaging offers label-free, chemically-specific, and (sub)-nanometer spatial resolution for studying nanoparticle uptake into cells as demonstrated in the current study. Coherent anti-Stokes Raman scattering (CARS) microscopy offers chemically-specific (sub)micron spatial resolution imaging without fluorescent labels while transmission electron microscopy (TEM) offers (sub)-nanometer scale spatial resolution and thus visualization of precise nanoparticle localization at the sub-cellular level. This proof-of-concept imaging platform with unlabeled drug nanocrystals and macrophage cells revealed good colocalization between the CARS signal and electron dense nanocrystals in TEM images. The correlative TEM images revealed subcellular localization of nanocrystals inside membrane bound vesicles, showing multivesicular body (MVB)-like morphology typical for late endosomes (LEs), endolysosomes, and phagolysosomes. C-CARS-EM imaging has much potential to study the interactions between a wide range of nanoparticles and cells with high precision and confidence.