Tip-enhanced Raman Spectroscopy Research Articles

Chemically Interrogating N-Heterocyclic Carbenes at the Single-Molecule Level Using Tip-Enhanced Raman Spectroscopy.

N-heterocyclic carbenes (NHCs) have been established as powerful modifiers to functionalize metal surfaces for a wide variety of energy and nanoelectronic applications. To fundamentally understand and harness NHC modification, it is essential to identify suitable methods to interrogate NHC surface chemistry at the spatial limit. Here, we demonstrate tip-enhanced Raman spectroscopy (TERS) as a promising tool for chemically probing the surface properties of NHCs at the single-molecule scale. We show that with subnanometer resolution, TERS measurements are capable of not only unambiguously identifying the chemical structure of individual NHCs by their vibrational fingerprints but also definitively determining the binding mode of NHCs on metal surfaces. In particular, by investigating low-temperature NHC adsorption on Ag(111), our TERS studies provide insights into the temperature dependence of the adsorption properties of NHCs. This work suggests the potential of single-molecule vibrational spectroscopy for investigations of NHC surface modification at the most fundamental level.

High-Resolution Distance Dependence Interrogation of Scanning Ion Conductance Microscopic Tip-Enhanced Raman Spectroscopy Enabled by Two-Dimensional Molybdenum Disulfide Substrates.

Scanning ion conductance microscopy (SICM) is a powerful surface imaging tool used in the electrolytic environment. Tip-enhanced Raman spectroscopy (TERS) can give more information in addition to the morphology provided by the SICM by utilizing label-free Raman spectroscopy aided by the localized plasmonic enhancement from the metal-coated probes. In this study, the integration of SICM with TERS is demonstrated through employing a silver-coated plasmonic nanopipette. Leveraging a two-dimensional (2D) molybdenum disulfide (MoS2) as a model system, the SICM-TERS enhancement factor was estimated to be ∼105, supported by finite-difference time-domain (FDTD) simulation. Moreover, the subnanometer distance dependence SICM-TERS study reveals the tensile stress and structural changes caused by the nanopipette. These findings illustrate the potential of SICM-TERS for providing comprehensive morphological and chemical insights into electrolytic environments, paving the way for future investigations of electrocatalytic and biological systems.

Micro Reference Electrode with an Ultrathin Ionic Path.

Reference electrode (RE) plays the core role in accurate potential control in electrochemistry. However, nanoresolved electrochemical characterization techniques still suffer from unstable potential control of pseudo-REs, because the commercial RE is too large to be used in the tiny electrochemical cell, and thus only pseudo-RE can be used. Therefore, microsized RE with a stable potential is urgently required to push the nanoresolved electrochemical measurements to a new level of accuracy and precision, but it is quite challenging to reproducibly fabricate such a micro RE until now. Here, we revisited the working mechanism of the metal-junction RE and clearly revealed the role of the ionic path between the metal wire and the borosilicate glass capillary to maintain a stable potential of RE. Based on this understanding, we developed a method to fabricate micro ultrastable-RE, where a reproducible ultrathin ionic path can form by dissolving a sandwiched sacrificial layer between the Pt wire and the capillary for the ion transfer. The potential of this new micro RE was almost the same as that of the commercial Ag/AgCl electrode, while the size is much smaller. Different from commercial REs that must be stored in the inner electrolyte, the new RE could be directly stored in air for more than one year without potential drift. Eventually, we successfully applied the micro RE in the electrochemical tip-enhanced Raman spectroscopy (EC-TERS) measurement to precisely control the potential of the working electrode, which makes it possible to compare the results from different laboratories and techniques to better understand the electrochemical interface at the nanoscale.

Nanoscale Imaging of Palladium-Enhanced Photocatalytic Reduction of 4-Nitrothiophenol on Tungsten Disulfide Nanoplates.

Two-dimensional (2D) dichalcogenides are modern nanomaterials with unique physical and chemical properties. These materials possess band gaps in the infrared and visible regions of the electromagnetic spectrum that can be tuned by their molecular composition. Excitons generated as a result of such light-matter interactions are capable of catalyzing chemical reactions in molecular analytes present on the dichalcogenide surfaces. However, the photocatalytic properties of such nanomaterials remain poorly understood. In the current study, we utilize tip-enhanced Raman spectroscopy (TERS) to examine photocatalytic reduction of 4-nitrothiophenol (4-NTP) to p,p'-dimercaptoazobisbenzene (DMAB) on tungsten disulfide (WS2) nanoplates and WS2 coupled with palladium nanoparticles (WS2@PdNPs). Our results indicate that although both WS2 and WS2@Pd were capable of reducing 4-NTP into DMAB, the metallic hybrid demonstrated much greater yield and rates of DMAB formation compared to WS2 nanoplate. These results indicate that coupling of catalytic metals to dichalcogenides could be used to enhance their catalytic properties.

Tailored functional monolayers made from mesoionic carbenes

Significant progress has been made over the last decades in surface functionalization of coinage metals using thiols and more recently N-heterocyclic carbenes. As shown in this work, mesoionic carbenes (MICs) provide straightforward access to a novel class of surface ligands including electroactive ones and thus materials. Importantly, MICs are easily accessed from triazolium salts (TS) onto which functional groups may be attached with little synthetic effort. Here, we present a library of TS that were further converted, in situ, into MICs and grafted onto gold surfaces. The modified surfaces were thoroughly characterized by advanced spectroscopic methods such as XPS, infrared and Tip-Enhanced Raman Spectroscopy. Through cyclic voltammetry at 100 Vs-1, we could evaluate the surfacic concentration of the grafted molecules for electroactive MICs. We also prepared mixed MIC/thiol self-assembled monolayers, which opens the route to multifunctional surfaces.

Probing Deuteration-Induced Phase Separation in Supported Lipid Monolayers using Hyperspectral TERS Imaging.

In this study, we investigate the impact of deuteration on the formation of phase-separated domains in supported lipid monolayers using hyperspectral Tip-Enhanced Raman Spectroscopy (TERS) imaging. The intricate organization of biological membranes plays a crucial role in cellular functions. Various factors that influence domain formation have been identified in previous studies such as lipid tail length and cholesterol concentration. Deuterium labeling of lipids has proven useful for probing cellular structures and dynamics, but its impact on lipid phase separation remains underexplored. By examining 1:1 mixed monolayers of dipalmitoylphosphatidylcholine (DPPC) and deuterated DPPC on Au(111) surfaces, we reveal partial segregation of domains rich in deuterated and nondeuterated lipids. This study addresses a gap in knowledge by examining the impact of deuteration on lipid tail behavior, offering new insights into how even subtle structural modifications can influence phase behavior. Furthermore, it demonstrates that TERS can be a powerful, nondestructive, and label-free nanoanalytical tool for analyzing lipid membranes and advance the field of membrane biophysics.

Raman spectroscopy of large extracellular vesicles derived from human microvascular endothelial cells to detect benzo[a]pyrene exposure.

Extracellular vesicles (EVs) have shown great potential as biomarkers since they reflect the physio-pathological status of the producing cell. In the context of cytotoxicity, it has been found that exposing cells to toxicants leads to changes in protein expression and the cargo of the EVs they produce. Here, we studied large extracellular vesicles (lEVs) derived from human microvascular endothelial cells (HMEC-1) to detect the modifications induced by cell exposure to benzo[a]pyrene (B[a]P). We used a custom CaF2-based biochip which allowed hyphenated techniques of investigation: surface plasmon resonance imaging (SPRi) to monitor the adsorption of objects, atomic force microscopy (AFM) to characterise EVs' size and morphology, and Raman spectroscopy to detect molecular modifications. Results obtained on EVs by Raman microscopy and tip-enhanced Raman spectroscopy (TERS) showed significant differences induced by B[a]P in the high wavenumber region of Raman spectra (2800 to 3000cm-1), corresponding mainly to lipid modifications. Two types of spectra were detected in the control sample. A support vector machine (SVM) model was trained on the pre-processed spectral data to differentiate between EVs from cells exposed or not to B[a]P at the spectrum level; this model could achieve a sensitivity of 88% and a specificity of 99.5%. Thus, this experimental setup facilitated the distinction between EVs originating from two cell culture conditions and enabled the discrimination of EV subsets within one cell culture condition.

Nanoscale Chemical Imaging of Amyloid Fibrils in Water Using Total-Internal-Reflection Tip-Enhanced Raman Spectroscopy.

Total-internal-reflection tip-enhanced Raman spectroscopy (TIR-TERS) imaging of amyloid-β (Aβ1-42-L34T) fibrils is performed with nanoscale spatial resolution in water, using TERS tips fabricated by bipolar electrodeposition. Ideal experimental parameters are corroborated by both theoretical simulations and TIR-TERS measurements. TIR-TERS imaging reveals the predominant parallel β-sheet secondary structure of Aβ1-42-L34T fibrils as well as the nanoscale spatial distribution of tyrosine, histidine, and phenylalanine aromatic amino acids. Their proportion in TERS spectra can be qualitatively explained by the combined effect of their localization in the Aβ1-42-L34T fibril structure and their molecular orientation with respect to the excitation laser light polarization. Conclusions drawn from the TERS experiments in water corroborate and significantly enrich our previous study in ambient air, thus confirming that hydration has only a marginal impact on the structure of such amyloid fibrils. This first TIR-TERS study in liquid opens fascinating perspectives for future applications in biology.

Tip‐Enhanced Raman Spectroscopy Coherence Length of 2D Materials: An Application to Graphene

Herein, a protocol is presented for determining the tip‐enhanced Raman spectroscopy (TERS) coherence length (Lc) of the two main Raman bands of graphene. This method involves performing approach curve experiments and plotting the normalized areas of the G and 2D bands as a function of tip–sample distance. The resultant data are fitted using a specific TERS formula to extract Lc. The results indicate different coherence lengths for the G and 2D bands in graphene as and nm. While this protocol is specifically done for graphene, the underlying theoretical framework, based on mode symmetry, can be extended to other 2D materials, such as transition metal dichalcogenides. This demonstrates the versatility and potential of TERS in exploring the coherence properties of various 2D materials.

Theoretical study of tip-enhanced photoluminescence of single molecule in nanocavity formed by apex protrusion and substrate

Tip-enhanced photoluminescence techniques, characterized by ultra-high detection sensitivity and spatial resolution, have exhibited unprecedented capabilities in exploring optoelectronic phenomena and unraveling intrinsic physical mechanism at the single-molecule level. To suppress the quenching of molecular luminescence and achieve high-resolution optical imaging, a rational design of the tip-substrate configuration is required. In this paper, we systematically investigate the tip-enhanced photoluminescence characteristics of single molecules located within the plasmonic nanocavity formed by the tip protrusion and the substrate from a theoretical perspective. The impacts of electron tunneling and electron-hole pair creation on localized electric field and quantum yield are discussed using the quantum correction model and nonlocal dielectric theory, respectively. The crucial roles of atomic-scale protrusion at the tip apex and the decoupling layer on the substrate in enhancing vertical and horizontal electric fields as well as quantum yields have been emphasized. The results indicate that in smaller sub-nanometer cavities, electron tunneling and nonlocal dielectric effects greatly weaken the electric field intensity and fluorescence quantum yield. Meanwhile, due to the sharp lightning rod effect, the presence of atomic-level protrusion at the tip apex can provide a strong and highly localized plasmonic field. We find that the added NaCl layer not only blocks charge transfer between molecules and the substrate but also effectively prevents fluorescence quenching. The maximum fluorescence and Raman enhancement factors near the tip apex approach up to 5 and 12 orders of magnitude, respectively, achieving sub-nanometer spatial resolution. Finally, compared to vertical electric field and dipole, sharp protrusion can excite a stronger horizontal electric field and couple with horizontal dipoles to yield a higher quantum yield, resulting in a three orders of magnitude higher in the fluorescence enhancement of horizontal dipole compared to the tip without protrusion. Our theoretical research not only confirms the experimental results of spectral enhancement with the proximity of the tip to the molecule in tip-enhanced photoluminescence, but also provides effective guidance for a deeper understanding of the mechanism of tip-enhanced fluorescence and Raman spectroscopy, as well as optimizing the experimental system.

Selective excitation of vibrations in a single molecule

The capability to excite, probe, and manipulate vibrational modes is essential for understanding and controlling chemical reactions at the molecular level. Recent advancements in tip-enhanced Raman spectroscopies have enabled the probing of vibrational fingerprints in a single molecule with Ångström-scale spatial resolution. However, achieving controllable excitation of specific vibrational modes in individual molecules remains challenging. Here, we demonstrate the selective excitation and probing of vibrational modes in single deprotonated phthalocyanine molecules utilizing resonance Raman spectroscopy in a scanning tunneling microscope. Selective excitation is achieved by finely tuning the excitation wavelength of the laser to be resonant with the vibronic transitions between the molecular ground electronic state and the vibrational levels in the excited electronic state, resulting in the state-selective enhancement of the resonance Raman signal. Our approach contributes to setting the stage for steering chemical transformations in molecules on surfaces by selective excitation of molecular vibrations.

Advanced Vibrational Spectroscopy and Bacteriophages Team Up: Dynamic Synergy for Medical and Environmental Applications.

Bacteriophages are emerging as a promising alternative in combating antibiotic-resistant bacteria amidst the escalating global antimicrobial resistance crisis. Recently, there has been a notable resurgence of interest in phages, prompting extensive research into their therapeutic potential. Beyond conventional microbiology and virology techniques, such as genomics and proteomics, novel phenotypic and chemical characterization methods are being explored. Among these, there is a growing interest in vibrational spectroscopy, especially in advanced modalities such as surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), and atomic force microscopy-infrared spectroscopy (AFM-IR), which offer improved sensitivity and spatial resolution. This review explores the spectrum of uses of vibrational spectroscopy for bacteriophages, including its role in diagnostics, biosensing, phage detection, assistance in phage-based therapy, and advancing basic research.

Ultralow-Frequency Tip-Enhanced Raman Scattering Discovers Nanoscale Radial Breathing Mode on Strained 2D Semiconductors.

Collective excitations including plasmons, magnons, and layer-breathing vibration modes emerge at an ultralow frequency (<1 THz) and are crucial for understanding van der Waals materials. Strain at the nanoscale can drastically change the property of van der Waals materials and create localized states like quantum emitters. However, it remains unclear how nanoscale strain changes collective excitations. Herein, ultralow-frequency tip-enhanced Raman spectroscopy (TERS) with sub-10nm resolution under ambient conditions is developed to explore the localized collective excitation on monolayer semiconductors with nanoscale strains. A new vibrational mode is discovered at around 12 cm-1 (0.36 THz) on monolayer MoSe2 nanobubbles and it is identified as the radial breathing mode (RBM) of the curved monolayer. The correlation is determined between the RBM frequency and the strain by simultaneously performing deterministic nanoindentation and TERS measurement on monolayer MoSe2. The generality of the RBM in nanoscale curved monolayer WSe2 and bilayer MoSe2 is demonstrated. Using the RBM frequency, the strain of the monolayer MoSe2 on the nanoscale can be mapped. Such an ultralow-frequency vibration from curved van der Waals materials provides a new approach to study nanoscale strains and points to more localized collective excitations to be discovered at the nanoscale.

A feasibility study on scanning imaging of tip-enhanced Raman spectroscopy using a spiral plasmonic lens

A feasibility study on scanning imaging of tip-enhanced Raman spectroscopy using a spiral plasmonic lens

Insights into Heterogeneous Catalysis by Tip‐Enhanced Raman Spectroscopy

AbstractHeterogeneous catalysis is immensely important in the chemical industry. Understanding the molecular mechanism of heterogeneous catalytic processes is fundamental to the design of high‐performance catalysts and the development of energy conversion technology. Tip‐enhanced Raman spectroscopy (TERS) provides invaluable topographic and chemical information with nanometer‐scale spatial resolution, thus making it a promising tool for the in‐depth characterization of heterogeneous catalytic processes. In this perspective, we first introduce the background of heterogeneous catalysis and TERS, and then we systematically discuss the application of TERS in heterogeneous catalysis. Key insights related to catalyst configurations, catalytically active sites, reacting species, spillover of active species, and structure‐reactivity relationship were summarized. Finally, we discuss the challenges and future development in the field.

Insights into the LiMn2O4 Cathode Stability in Aqueous Electrolytes.

LiMn2O4 (LMO) cathodes present large stability when cycled in aqueous electrolytes, contrasting with their behavior in conventional organic electrolytes in lithium-ion batteries (LIBs). To elucidate the mechanisms underlying this distinctive behavior, we employ unconventional characterization techniques, including variable energy positron annihilation lifetime spectroscopy (VEPALS), tip-enhanced Raman spectroscopy (TERS), and macro-Raman spectroscopy (with tens of μm-size laser spot). These still rather unexplored techniques in the battery field provide complementary information across different length scales, revealing previously hidden features. VEPALS offers atomic-scale insights, uncovering cationic defects and subnanometer pores that tend to collapse with cycling. TERS, operating in the nanometric range at the surface, captured the presence of Mn3O4 and its dissolution with cycling, elucidating dynamic changes during operation. Additionally, TERS highlights the accumulation of SO4 2- at grain boundaries. Macro-Raman spectroscopy focuses on the micrometer scale, depicting small changes in the cathode's long-range order, suggesting a slow but progressive loss of crystalline quality under operation. Integrating these techniques provides a comprehensive assessment of LMO cathode stability in aqueous electrolytes, offering multifaceted insights into phase and defect evolution that can help to rationalize the origin of such stability when compared with conventional organic electrolytes. Our findings advance the understanding of LMO behavior in aqueous environments and provide guidelines for its development for next-generation LIBs.

Imaging spatial plasmon mode of nanocavity formed by Au tip and Au nanorod lattice in tip-enhanced Raman spectroscopy

The integration of Au nanorods in tip-enhanced Raman spectroscopy (TERS) presents a significant increase in the enhancement factor, primarily due to the gap-mode effect. By aligning Au nanorods in parallel, we construct an Au nanorod lattice, referred to as the Au nanolattice, which further amplifies the advantages of TERS imaging due to the induced inter-nanorod surface plasmon resonance. A critical aspect in this research involves investigating the distribution of hotspots within the nanolattice during TERS measurements. Additionally, we demonstrate that the tip–lattice nanocavity is a predominant factor in determining both the intensity and spatial distribution of these hotspots. Employing the experimental and simulation results, we illustrate the enhancement effect of the tip–lattice cavity and elucidate the connection between the hotspot intensity and cavity size. This comprehensive approach contributes to our understanding of the nano-lattice’s role in TERS and offers valuable insights for optimizing nanophotonic applications.

Modification of transition pathways in polarized resonance Raman spectroscopy for carbon nanotubes by highly confined near-field light

We observed a modification of transition pathways in polarized resonance Raman spectroscopy during tip-enhanced Raman spectroscopy (TERS) analysis of metallic carbon nanotubes (CNTs). At a spatial resolution reaching up to the sub-nanometer regime, the signal intensity of the typical D-band is observed to be even higher than the intensity of the G-band all over the probed CNTs in TERS imaging. The measured D-band is attributed to the non-vertical transitions of electrons in k-space that are facilitated by highly confined near-field light at the tip–sample junction of our scanning tunneling microscope based TERS system. The D-band signal was observed even when the CNTs were excited by light polarized perpendicular to the tube axis that corresponds to electronic excitations between different cutting line numbers of a CNT. By combining the electron pathways brought about by both the near-field light and its polarization, we found a unique optical transition of electrons of CNTs in near-field Raman spectroscopy.

Nano-Raman spectroscopy of 2D materials

Abstract The use of nano-Raman spectroscopy to study two-dimensional (2D) systems is presented here. The nano (tip-enhanced) Raman spectroscopy technique is briefly introduced, addressing some new theoretical aspects for Raman spectroscopy in the near-field regime, including field coherence, field distribution and the relevance of atomic description and quenching effects. State-of-the-art results in graphene and transition metal dichalcogenides are presented, exploring the connection between micro- and nano-Raman metrology. Various aspects such as defects, homojunctions, twisted-bilayer structures, localized emissions at bubbles, wrinkles, and borders, as well as substrate and coherence effects are addressed in detail. The paper concludes by outlining the perspectives for nano-Raman spectroscopy in 2D systems, highlighting its potential for advancing our understanding of nanoscale phenomena and facilitating further breakthroughs in materials science and characterization.

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

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