Tip-enhanced Raman Spectroscopy Measurements 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.

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.

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.

Spring constant of an AFM cantilever with a thin-film plasmonic waveguide formed at its end

Abstract Atomic-force-microscope (AFM)-based tip-enhanced Raman spectroscopy (TERS) is a promising analytical technique that can identify the physical and chemical properties of a sample’s surface. In the conventional TERS setup, the tip is directly irradiated by an incident light, which causes degradation of the contrast of the TERS signal due to the Raman scattered light from the surface area around the tip. We recently developed an AFM cantilever for indirect illumination AFM-TERS by milling the tip of the conventional cantilever to form a thin-film waveguide. Since the thin-film waveguide is considered as another cantilever attached at the end of the original cantilever, the waveguide cantilever can be treated as cantilevers connected in series. We then analyzed the static spring constant of the waveguide cantilever by both analytical and numerical methods and found that the static spring constant of the waveguide cantilever is lower than that of the original cantilever, which is advantageous in reducing the contact damage during the TERS measurements. We also proposed procedures to experimentally calibrate the static spring constant of the waveguide cantilever.

Mechanistic Understanding of Oxygen Activation on Bulk Au(111) Surface Using Tip-Enhanced Raman Spectroscopy.

Gaining mechanistic understanding of oxygen activation on metal surfaces is a topical area of research in surface science. However, direct investigation of on-surface oxidation processes at the nanoscale and the empirical validation of oxygen activation pathways remain challenging for the conventional analytical tools. In this study, we applied tip-enhanced Raman spectroscopy (TERS) to gain mechanistic insights into oxygen activation on bulk Au(111) surface. Specifically, oxidation of 4-aminothiophenol (4-ATP) to 4-nitrothiophenol (4-NTP) on Au(111) surface was investigated using hyperspectral TERS imaging. Nanoscale TERS images revealed a markedly higher oxidation efficiency in disordered 4-ATP adlayers compared to the ordered adlayers signifying that the oxidation of 4-ATP molecules proceeds via interaction with the on-surface oxidative species. These results were further validated via direct oxidation of the 4-ATP adlayers with H2O2 solution. Finally, TERS measurements of oxidized 4-ATP adlayers in the presence of H2O18 provided the first empirical evidence for the generation of oxidative species on bulk Au(111) surface via water-mediated activation of molecular oxygen. This study expands our mechanistic understanding of oxidation chemistry on bulk Au surface by elucidating the oxygen activation pathway.

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

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

Mechanistic Understanding of Oxygen Activation on Bulk Au(111) Surface Using Tip‐Enhanced Raman Spectroscopy

AbstractGaining mechanistic understanding of oxygen activation on metal surfaces is a topical area of research in surface science. However, direct investigation of on‐surface oxidation processes at the nanoscale and the empirical validation of oxygen activation pathways remain challenging for the conventional analytical tools. In this study, we applied tip‐enhanced Raman spectroscopy (TERS) to gain mechanistic insights into oxygen activation on bulk Au(111) surface. Specifically, oxidation of 4‐aminothiophenol (4‐ATP) to 4‐nitrothiophenol (4‐NTP) on Au(111) surface was investigated using hyperspectral TERS imaging. Nanoscale TERS images revealed a markedly higher oxidation efficiency in disordered 4‐ATP adlayers compared to the ordered adlayers signifying that the oxidation of 4‐ATP molecules proceeds via interaction with the on‐surface oxidative species. These results were further validated via direct oxidation of the 4‐ATP adlayers with H2O2 solution. Finally, TERS measurements of oxidized 4‐ATP adlayers in the presence of H2O18 provided the first empirical evidence for the generation of oxidative species on bulk Au(111) surface via water‐mediated activation of molecular oxygen. This study expands our mechanistic understanding of oxidation chemistry on bulk Au surface by elucidating the oxygen activation pathway.

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

Materials and their interfaces are the core for the development of a large variety of fields, including catalysis, energy storage and conversion. In this case, tip-enhanced Raman spectroscopy (TERS), which combines scanning probe microscopy with plasmon-enhanced Raman spectroscopy, is a powerful technique that can simultaneously obtain the morphological information and chemical fingerprint of target samples at nanometer spatial resolution. It is an ideal tool for the nanoscale chemical characterization of materials and interfaces, correlating their structures with chemical performances. In this review, we begin with a brief introduction to the nanoscale characterization of materials and interfaces, followed by a detailed discussion on the recent theoretical understanding and technical improvements of TERS, including the origin of enhancement, TERS instruments, TERS tips and the application of algorithms in TERS. Subsequently, we list the key experimental issues that need to be addressed to conduct successful TERS measurements. Next, we focus on the recent progress of TERS in the study of various materials, especially the novel low-dimensional materials, and the progresses of TERS in studying different interfaces, including both solid-gas and solid-liquid interfaces. Finally, we provide an outlook on the future developments of TERS in the study of materials and interfaces.

Spatially Coherent Tip-Enhanced Raman Spectroscopy Measurements of Electron-Phonon Interaction in a Graphene Device.

Coherence length (Lc) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. Lc decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity vg, reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.

Probing the Role of Environmental and Sample Characteristics in Gap Mode Tip-Enhanced Raman Spectroscopy.

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical tool for nondestructive and label-free molecular characterization at the nanoscale. However, the influence of environmental factors and sample characteristics on the occurrence of spurious signals, enhancement of TERS signals, and longevity of TERS probes is not well understood yet. Herein, we present a detailed investigation of the influence of oxygen, humidity, and atmospheric carbon contaminants on scanning tunneling microscopy-TERS (STM-TERS) measurements of self-assembled monolayer systems in ambient and inert environments. Our results reveal a consistent increase of TERS signals, significant reduction of spurious signals, and drastically improved longevity of TERS probes in the inert environment. Additionally, sample characteristics such as molecular packing, chemisorption behavior, and hydrophilicity are found to have a direct impact on signal enhancement in the TERS measurements of molecular self-assembled monolayers (SAMs). The novel insights gained in this study are expected to pave the way for a more robust data analysis and improved experimental design in the future gap mode STM- and atomic force microscopy-TERS (AFM-TERS) studies.

Nanocomposite Au/Si Cantilevers for Tip-Enhanced Raman Scattering (TERS) Sensors

In this study, we proposed and tested different procedures for the preparation of Au/Si cantilevers for Tip-enhanced Raman spectroscopy (TERS). The preparation of Au/Si TERS sensors was based on three methods: chemical (electroless) deposition, thermal evaporation of Au on the tip of commercially available cantilevers in a vacuum, and electrochemical etching of Au microwires. We fabricated and tested four types of TERS probes, and then used these probes for TERS measurements using graphene oxide (GO) as the target analyte. The probe tips were characterized using scanning electron microscopy (SEM). This article presents a comparative analysis of the fabrication methods, quality of the obtained probe tips, and enhancement factors (EFs) for the four types of TERS cantilevers (probes) produced by chemical deposition, sputtering, and electrochemical methods.

Ultrashort Pulse Excited Tip-Enhanced Raman Spectroscopy in Molecules.

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

Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe.

Tip-enhanced Raman spectroscopy (TERS) is a nano-optical approach to extract spatially resolved chemical information with nanometer precision. However, in the case of direct-illumination TERS, which is often employed in commercial TERS instruments, strong fluorescence or far-field Raman signals from the illuminated areas may be excited as a background. They may overwhelm the near-field TERS signal and dramatically decrease the near-field to far-field signal contrast of TERS spectra. It is still challenging for TERS to study the surface of fluorescent materials or a bulk sample that cannot be placed on an Au/Ag substrate. In this study, we developed an indirect-illumination TERS probe that allows a laser to be focused on a flat interface of a thin-film waveguide located far away from the region generating the TERS signal. Surface plasmon polaritons are generated stably on the waveguide and eventually accumulated at the tip apex, thereby producing a spatially and energetically confined hotspot to ensure stable and high-resolution TERS measurements with a low background. With this thin-film waveguide probe, TERS spectra with obvious contrast from a diamond plate can be acquired. Furthermore, the TERS technique based on this probe exhibits excellent TERS signal stability, a long lifetime, and good spatial resolution. This technique is expected to have commercial potential and enable further popularization and development of TERS technology as a powerful analytical method.

Nanomechanics of few-layer materials: do individual layers slide upon folding?

Folds naturally appear on nanometrically thin materials, also called “2D materials”, after exfoliation, eventually creating folded edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges, which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number of layers (n). In the case of talc, we obtain κ ∝ n3 for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a silicon substrate is also in agreement with previous results.

Performance Boosts in n-Type Lateral Double-Diffused MOSFET With Process-Induced Strain Using Contact Etch Stop Layer Stressor

In this brief, the contact etch stop layer (CESL) stressor is introduced to improve the performance of n-type lateral double-diffused MOSFET (nLDMOSFET) with various operating voltages. Three groups of nLDMOSFETs were studied to demonstrate the effects of the CESL stressor. The electrical properties were carefully characterized, and the tip-enhanced Raman spectroscopy measurements were carried out to examine the process-induced strain directly. It is found that nLDMOSFETs with the strained SiN CESL have larger output current, transconductance (g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ), and higher breakdown voltage (BV). The strained SiN CESL introduces tensile strain in the device and results in the performance boosts, which are verified by negative Raman peak shift mappings and TCAD simulations. The strained SiN CESL could effectively decrease the specific ON-resistance (Ron) and improve the BV of the nLDMOSFET, due to the mobility enhancement.

Tip‐modified Raman tensor of a porphine molecule

AbstractIt is well established that bringing a plasmonic tip into the proximity of a single molecule inevitably affects its state and vice versa. Due to the enhanced light–matter interaction, spectroscopic information on the molecule captured with tip‐enhanced Raman spectroscopy (TERS) can noticeably differ on that coming from standard far‐field Raman spectroscopy. In this work, we develop a theoretical approach for calculating Raman tensors of a single oblate molecule that takes near‐field depolarization effects into account. Generally speaking, a tip‐dressed molecule exhibits the modified polarizability and, therefore, the Raman tensors. It is reported that the TERS technique can be used to reconstruct Raman tensors of vibration modes of anisotropic single molecules. The symmetric and antisymmetric modes of the molecules nearby a plasmonic tip are enhanced differently. In order to unravel the Raman tensors, two near‐field depolarization factors are necessarily introduced for the oblate molecule. As an example, we consider a free base porphine molecule as a test molecule to demonstrate the enhancement of intensities of Raman bands assigned to different symmetries. Also, we demonstrate an experimental way how to measure the depolarization factors using an ensemble of randomly oriented molecules. We believe that our finding will be beneficial to the interpretation of TERS measurements of single molecules.

Systematic Assessment of Benzenethiol Self-Assembled Monolayers on Au(111) as a Standard Sample for Electrochemical Tip-Enhanced Raman Spectroscopy

A molecular-scale understanding of electrolyte/electrode interfaces has long been a challenging issue in electrochemistry. Spectroscopic tools with high spatial resolution are required for advancing beyond conventional electrochemical measurements such as cyclic voltammetry (CV). In this study, we developed tip-enhanced Raman spectroscopy (TERS), which is based on an electrochemical scanning tunneling microscope (EC-STM), and demonstrated electrochemical TERS (EC-TERS) measurements of benzenethiol self-assembled monolayers (SAMs) on Au(111). A specially designed cell enables us to carry out reproducible CV, EC-STM, and EC-TERS measurements, which indicates consistent results among these techniques for the oxidative desorption of the SAMs. We also present direct evidence that the measured EC-TERS signals originate from molecules adsorbed on Au(111) and not from those on the STM tip.

Comparison and Evaluation of Silver Probe Preparation Techniques for Tip-Enhanced Raman Spectroscopy

In this work, the different procedures for the fabrication of Ag probes for tip-enhanced Raman spectroscopy (TERS) in a top illumination/detection setup are proposed and tested. We focus on technologically simple methods allowing Si tips coated with plasmonic silver nanostructures and bulk metal Ag tips with good shape reproducibility to be produced for atomic force microscopy (AFM) feedback setup. The preparation of Ag TERS probes was based on chemical deposition and vacuum sputtering of Ag on the tips of commercially available Si cantilevers. A straightforward technique for the fabrication of bulk metal Ag probes by the electrochemical etching of Ag microwires was also proposed. Chemically coated, sputtered, and electrochemically etched TERS tips were characterized by scanning electron microscopy (SEM). The produced tips were tested for TERS measurements using graphene oxide (GO) as the target analyte in a top illumination setup. A comparative analysis of enhancement factors (EF) for the different types of tips (probes) is presented in this work.

Tip-Enhanced Raman Spectroscopy: A Tool for Nanoscale Chemical and Structural Characterization of Biomolecules.

Due to its high molecular sensitivity and spatial optical resolution down to sub-nanometer values, tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful microscopy technique for nanoscale characterization. Progress in TERS instrumentation and in the manufacturing of efficient TERS tips allow for chemical and structural analysis under various experimental conditions (different wavelengths, substrates, and surrounding media). Many biological species have been examined by using this technique. Nucleic acids (individual nucleobases, DNA, and RNA) can show specific TERS features that reveal their composition, conformation, and defects. TERS studies on peptides and proteins (such as amyloid fibrils) provide relevant information on their morphology and structure, leading to valuable insight to their functions and behavior. Finally, lipid layers and membranes, viruses, bacteria, and cells can also be finely characterized. Generalizing TERS measurements in liq- uid medium to study biological systems is the main future challenge.

In-situfabrication of gold nanoparticle functionalized probes for tip-enhanced Raman spectroscopy by dielectrophoresis

We report the use of dielectrophoresis to fabricate in-situ probes for tip-enhanced Raman spectroscopy (TERS) based on Au nanoparticles. A typical conductive atomic force microscope (AFM) was used to functionalize iridium-coated conductive silicon probes with Au nanoparticles of 10-nm diameter. Suitable TERS probes can be rapidly produced (30 to 120 s) by applying a voltage of 10 Vpp at a frequency of 1 MHz. The technique has the advantage that the Au-based probes are ready for immediate use for TERS measurements, minimizing the risks of tip contamination and damage during handling. Scanning electron microscopy and energy dispersive x-ray spectroscopy were used to confirm the quality of the probes, and used samples of p-ATP monolayers on silver substrates were used to demonstrate experimentally TERS measurements.