Tip-enhanced Raman Spectroscopy Research Articles

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.

In situ evaluation of plasmonic enhancement of gold tips for plasmon-enhanced imaging techniques.

Plasmonic nanoantennas are at the core of various optical near-field scanning techniques such as tip-enhanced Raman spectroscopy as they provide the amplification and confinement of the electromagnetic field, which ultimately provides sensitivity and spatial resolution. With a cornucopia of different fabrication methods available, the actual performance of a nanoantenna is often only assessed by whether or not near-field imaging is possible, implying the complete alignment and landing procedure of the scanning probe. We present a semi-quantitative approach to assess the plasmonic enhancement of gold tips via localized surface plasmon resonance (LSPR) enhancement of intrinsic gold photoluminescence without the need for interaction with the sample. As the intensity of the plasmon at the apex decreases, a significant change in the shape of the tip signal spectrum is observed, reflecting itself as a decrease in the R2 value (fit quality) for numerical fitting with a Lorentzian, which also provides an approximation for the LSPR wavelength. Our findings suggest that the potential of a tip to perform well as an optical near field antenna may already be assessed in an early stage of the experiment.

Plasmon-Driven Chemistry on Mono- and Bimetallic Nanostructures.

ConspectusHot carriers are highly energetic species that can perform a large spectrum of chemical reactions. They are generated on the surfaces of nanostructures via direct interband, phonon-assisted intraband, and geometry-assisted decay of localized surface plasmon resonances (LSPRs), which are coherent oscillations of conductive electrons. LSPRs can be induced on the surface of noble metal (Ag or Au) nanostructures by illuminating the surfaces with electromagnetic irradiation. These noble metals can be coupled with catalytic metals, such as Pt, Pd, and Ru, to develop bimetallic nanostructures with unique catalytic activities. The plasmon-driven catalysis on bimetallic nanostructures is light-driven, which essentially enables green chemistry in organic synthesis. During the past decade, surface-enhanced Raman spectroscopy (SERS) has been actively utilized to study the mechanisms of plasmon-driven reactions on mono- and bimetallic nanostructures. SERS has provided a wealth of knowledge about the mechanisms of numerous plasmon-driven redox, coupling, and scissoring reactions. However, the nanoscale catalytic properties of both mono- and bimetallic nanostructures as well as the underlying physical cause of their catalytic reactivity and selectivity remained unclear for decades.In this Account, we focus on the most recent findings reported by our and other research groups that shed light on the nanoscale properties of mono- and bimetallic nanostructures. This information was revealed by tip-enhanced Raman spectroscopy (TERS), a modern analytical technique that has single-molecule sensitivity and subnanometer spatial resolution. TERS findings have shown that plasmonic reactivity and the selectivity of bimetallic nanostructures are governed by the nature of the catalytic metal and the strength of the rectified electric field on their surfaces. TERS has also revealed that the catalytic properties of bimetallic nanostructures directly depend on the interplay between the catalytic and plasmonic metals. We anticipate that these findings will be used to tailor synthetic approaches that are used to fabricate novel nanostructures with desired catalytic properties. The experimental and theoretical results discussed in this Account will facilitate a better understanding of TERS and explain artifacts that could be encountered upon TERS imaging of a large variety of samples. Consequently, plasmon-driven chemistry should be considered as an essential part of near-field microscopy.

Probing trans-polyacetylene segments in a diamond film by tip-enhanced Raman spectroscopy

As an important carbon material, a nano-crystalline diamond (NCD) film is very promising in the field of science and technology. However, there are little knowledge about surface physics and chemistry of nanoscale in NCD films. Here, we perform nanoscale imaging of NCD films grown by hot filament chemical vapor deposition, using tip-enhanced Raman spectroscopy. With ~20 nm spatial resolution, we resolve nanoscale spatial correlations of trans-polyacetylene segments on the surface with characterized Raman modes at 1150 cm−1 and 1480 cm−1 that originate from the motion of conjugate polymer skeletons with a substantial contribution from the in-plane bending of carbon‑hydrogen bonds and stretching vibrations of the double carbon‑carbon bonds. Our work is helpful to understand the role of TPA in NCD synthesis reaction in nanoscale and improve the device application based on this material.

Multimodal Nanoscopic Study of Atomic Diffusion and Related Localized Optoelectronic Response of WS2/MoS2 Lateral Heterojunctions.

The atomic diffusion in transition metal dichalcogenides (TMDs) van der Waals heterojunctions (HJs) strongly modifies their optoelectronic properties in the nanoscale. However, probing such localized properties challenges the spatial resolution and the sensitivity of a variety of analytic tools. Herein, a multimodal nanoscopy (based on tip enhanced Raman spectroscopy (TERS) and photoluminescence (TEPL)) combined with the Kelvin probe force microscopy (KPFM) method was used to probe such nanoscale localized optoelectronic properties induced by atomic diffusion. Chemical vapor deposition (CVD)-grown lateral bilayer (2L) WS2/MoS2 HJs were imaged with a spatial resolution better than 40 nm via TERS and TEPL mapping by using intrinsic Raman and photoluminescence (PL) peaks. The contact potential difference (CPD), capacitance, and PL variation in a nanoscale vicinity of the HJ interface can be correlated to the local stoichiometry variation determined by TERS. The diffusion coefficients of W and Mo were obtained to be ∼0.5 × 10-12 and ∼1 × 10-12 cm2/s, respectively, by using Fick's second law. The obtained results would be useful to further understand the localized optoelectronic response of the TMDs HJs.

Nanoscale InN clusters and compositional inhomogeneities in InGaN epitaxial layers quantified by tip-enhanced Raman scattering

We investigate the compositional homogeneity of InGaN thin films with a high In content grown by migration-enhanced plasma-assisted metal-organic chemical vapor deposition. Micro-Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) are used to analyze the local InGaN composition on the micro- and nanoscale. Based on conventional micro-Raman mapping, the InGaN composition for all samples appears uniform but shows indications for intrinsic phase separations. TERS, a nanoscopic technique with a high spatial resolution far below the diffraction limit, verifies the formation of nanoscale compositional inhomogeneities. The dimensions of these compositional fluctuations observed in TERS are confirmed by scattering-type scanning near-field infrared nanoscopy (s-SNIN). In contrast to s-SNIN, we show that TERS furthermore enables the quantification of the In content in the different compositional regions and even allows the identification of InN nanoclusters near the surface of the epitaxial films.

Virtual probe stimulated tip-enhanced Raman spectroscopy: The extreme field enhancement in virtual-real probe dimer

Tip-enhanced Raman spectroscopy (TERS) can be used for scanning imaging, molecular detection, and chemical analysis. The improvement of detection sensitivity, which is related to the electric field enhancement in the TERS substrate, has attracted much attention from researchers. In this work, we numerically studied the local electric field enhancement in the virtual-real probe dimer structure with a vertical gap. We mainly analyzed the influence of the structure parameters on the field enhancement using the finite-difference time-domain method. The Raman enhancement factor could reach up to 1.6×1015. The local field enhancement benefits from plasmon hybridization between the longitudinal component of the virtual probe and the local surface plasmon of the real probe. We also found that the full width at half maximum of the electric field was as narrow as 7.8 nm, and the volume of the hotspot for single-molecule detection can reach a maximum value of 155 nm3. The virtual-real probe dimer structure has ultrahigh field enhancement and spatial resolution, which is promising for high-sensitivity detection and high-resolution imaging.

Plexciton in tip-enhanced resonance Stokes and anti-Stokes Raman spectroscopy and in propagating surface plasmon polaritons

New methods to improve the properties of surface-enhanced Raman scattering and propagating surface plasmon polaritons (PSPPs) are desirable. In this study, we theoretically employ the plasmon–exciton (plexciton) interaction to enhance the spectra in localized surface plasmon (LSP) and PSPP. The tip-enhanced resonance Raman spectroscopy (TERRS) configuration is used to enhance both Stokes and anti-Stokes Raman spectra of rhodamine 6G simultaneously. We achieve a huge enhancement factor of 1013∼ 1015. Furthermore, we obtain the plexciton by employing the Ag nanowire coated with rhodamine 6G, which improves the propagation properties.

Single-particle Observation of Detonation Nanodiamonds by Tip-enhanced Raman Spectroscopy

This paper demonstrates structural analysis for single detonation nanodiamond (DND) particles, with a size below 6 nm, by tip-enhanced Raman spectroscopy (TERS). Single DND particles show Raman spectra different from that of the aggregates obtained by conventional micro-Raman spectroscopy. Our results suggest that single-particle TERS measurement is a promising technique for revealing structural distribution of individual DND particles and its dependence on synthesis conditions.

Symmetry-Forbidden-Mode Detection in SrTiO3 Nanoislands with Tip-Enhanced Raman Spectroscopy

Among the techniques to reveal the chemistry, structure, and dynamics of surfaces, tip-enhanced Raman spectroscopy (TERS) occupies a unique position for the investigation of nonmetallic nanomateria...

Light-Nucleotide versus Ion-Nucleotide Interactions for Single-Nucleotide Resolution.

Several parallel reads of ionic currents through multiple CsgG nanopores provide information about ion-nucleotide interactions for sequencing single-stranded DNA (ss-DNA) using base-calling algorithms. However, the information in ion-nucleotide interactions seems insufficient for single-read nanopore DNA sequencing. Here we report discriminative light-nucleotide interactions calculated from density functional theory (DFT), which are compared with ionic currents obtained from molecular dynamics (MD) simulations. The MD simulations were performed on a system containing a transverse nanochannel and a longitudinal solid state nanopore. We show that both of the transverse and longitudinal ionic currents during the translocation of A16, G16, T16, and C16 through the nanopore, overlapped widely. On the other hand, the UV-vis and Raman spectra of different types of single nucleotides, nucleosides, and nucleobases show relatively higher resolution than the ionic currents. Light-nucleotide interactions provide better information for characterizing the nucleotides in comparison to ion-nucleotide interactions for nanopore DNA sequencing. This can be realized by using optical techniques including surface-enhanced Raman spectroscopy (SERS) or tip-enhanced Raman spectroscopy (TERS), while plasmon excitation can be used to localize light and control the rate of nucleotide flow.

Electromagnetic Field Gradient-Enhanced Raman Scattering in TERS Configurations

Realizing in situ simultaneous measurement of full vibrational modes in tip-enhanced Raman spectroscopy (TERS) is of immense importance for improving the ability of quantitative analysis and measur...

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

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

Nanoscale structural characterization of plasmon-driven reactions

Abstract Illumination of noble metal nanostructures by electromagnetic radiation induces coherent oscillations of conductive electrons on their surfaces. These coherent oscillations of electrons, also known as localized surface plasmon resonances (LSPR), are the underlying physical cause of the electromagnetic enhancement of Raman scattering from analytes located in a close proximity to the metal surface. This physical phenomenon is broadly known as surface-enhanced Raman scattering (SERS). LSPR can decay via direct interband, phonon-assisted intraband, and geometry-assisted transitions forming hot carriers, highly energetic species that are responsible for a large variety of chemical transformations. This review critically discusses the most recent progress in mechanistic elucidation of hot carrier-driven chemistry and catalytic processes at the nanoscale. The review provides a brief description of tip-enhanced Raman spectroscopy (TERS), modern analytical technique that possesses single-molecule sensitivity and angstrom spatial resolution, showing the advantage of this technique for spatiotemporal characterization of plasmon-driven reactions. The review also discusses experimental and theoretical findings that reported novel plasmon-driven reactivity which can be used to catalyze redox, coupling, elimination and scissoring reactions. Lastly, the review discusses the impact of the most recently reported findings on both plasmonic catalysis and TERS imaging.

Nanoscale investigation of two-photon polymerized microstructures with tip-enhanced Raman spectroscopy

We demonstrate the use of tip-enhanced Raman spectroscopy (TERS) on polymeric microstructures fabricated by two-photon polymerization direct laser writing (TPP-DLW). Compared to the signal intensity obtained in confocal Raman microscopy, a linear enhancement of almost two times is measured when using TERS. Because the probing volume is much smaller in TERS than in confocal Raman microscopy, the effective signal enhancement is estimated to be ca. 104. We obtain chemical maps of TPP microstructures using TERS with relatively short acquisition times and with high spatial resolution as defined by the metallic tip apex radius of curvature. We take advantage of this high resolution to study the homogeneity of the polymer network in TPP microstructures printed in an acrylic-based resin. We find that the polymer degree of conversion varies by about 30% within a distance of only 100 nm. The combination of high resolution topographical and chemical data delivered by TERS provides an effective analytical tool for studying TPP-DLW materials in a non-destructive way.

Understanding the Role of Different Substrate Geometries for Achieving Optimum Tip-Enhanced Raman Scattering Sensitivity.

Tip-enhanced Raman spectroscopy (TERS) has experienced tremendous progress over the last two decades. Despite detecting single molecules and achieving sub-nanometer spatial resolution, attaining high TERS sensitivity is still a challenging task due to low reproducibility of tip fabrication, especially regarding very sharp tip apices. Here, we present an approach for achieving strong TERS sensitivity via a systematic study of the near-field enhancement properties in the so-called gap-mode TERS configurations using the combination of finite element method (FEM) simulations and TERS experiments. In the simulation study, a gold tip apex is fixed at 80 nm of diameter, and the substrate consists of 20 nm high gold nanodiscs with diameter varying from 5 nm to 120 nm placed on a flat extended gold substrate. The local electric field distributions are computed in the spectral range from 500 nm to 800 nm with the tip placed both at the center and the edge of the gold nanostructure. The model is then compared with the typical gap-mode TERS configuration, in which a tip of varying diameter from 2 nm to 160 nm is placed in the proximity of a gold thin film. Our simulations show that the tip-nanodisc combined system provides much improved TERS sensitivity compared to the conventional gap-mode TERS configuration. We find that for the same tip diameter, the spatial resolution achieved in the tip-nanodisc model is much better than that observed in the conventional gap-mode TERS, which requires a very sharp metal tip to achieve the same spatial resolution on an extended metal substrate. Finally, TERS experiments are conducted on gold nanodisc arrays using home-built gold tips to validate our simulation results. Our simulations provide a guide for designing and realization of both high-spatial resolution and strong TERS intensity in future TERS experiments.

Tip-enhanced Raman spectroscopy of Aβ(1-42) fibrils

Using tip-enhanced Raman spectroscopy (TERS) imaging with a lateral optical resolution of ~16 nm, the heterogeneity of Aβ(1-42) fibrils implicated in Alzheimer’s disease is investigated. The amount and spatial distribution of TERS bands assigned to aromatic amino acid residues (histidine, tyrosine and phenylalanine) and to parallel β-sheet and random coil secondary structures suggest that both the surface and core of Aβ(1-42) fibrils are probed, confirm the twisted nature of the fibrils and reveal the presence of several β-sheet and disordered fibril regions with different hydrophilicity. This study completes and bears out literature TERS data on Aβ(1-42) amyloid fibrils.

Distilling nanoscale heterogeneity of amorphous silicon using tip-enhanced Raman spectroscopy (TERS) via multiresolution manifold learning

Accurately identifying the local structural heterogeneity of complex, disordered amorphous materials such as amorphous silicon is crucial for accelerating technology development. However, short-range atomic ordering quantification and nanoscale spatial resolution over a large area on a-Si have remained major challenges and practically unexplored. We resolve phonon vibrational modes of a-Si at a lateral resolution of <60 nm by tip-enhanced Raman spectroscopy. To project the high dimensional TERS imaging to a two-dimensional manifold space and categorize amorphous silicon structure, we developed a multiresolution manifold learning algorithm. It allows for quantifying average Si-Si distortion angle and the strain free energy at nanoscale without a human-specified physical threshold. The multiresolution feature of the multiresolution manifold learning allows for distilling local defects of ultra-low abundance (< 0.3%), presenting a new Raman mode at finer resolution grids. This work promises a general paradigm of resolving nanoscale structural heterogeneity and updating domain knowledge for highly disordered materials.

Probing Bias-Induced Electron Density Shifts in Metal-Molecule Interfaces via Tip-Enhanced Raman Scattering.

Surface charging effects at metal-molecule interfaces, for example, charge transfer, charge transport, charge injection, and so on, have a strong impact on the performance of organic electronics. Only having molecules bound or adsorbed on different metals results in a doping-like behavior at the interface by the different work functions of the metals and creates hybrid surface states, which strongly affect the efficiencies. With the ongoing downsizing and thinning of the organic components, the impact of the interface will even further increase. However, most of the investigations only monitor the interface without the additional charging effects from applying a voltage to the interface. In this work we present a spectroscopic approach based on tip-enhanced Raman spectroscopy (TERS) to study metal-molecule interfaces with an applied voltage simulating the electric field strength in real devices. We monitor how an intrinsic inductive effect of partial functional groups in molecules can shift the molecular electron density (ED) distribution when a bias voltage is applied. Therefore, we choose two molecules as model systems, which are similar in size and binding condition to a smooth gold surface, but with different electronic structure. By placing the tip 1 nm over the molecular surface at a fixed position and changing the applied bias voltage, we record electric-field-dependent tip-enhanced Raman spectra. Specific vibrational bands exhibit voltage-dependent intensity changes related to the shift of the local ED inside the molecules. We believe this experiment is valuable to gain deeper insights into charged metal-molecule interfaces.

Underlying Mechanisms of Hot Carrier-Driven Reactivity on Bimetallic Nanostructures

Bimetallic nanostructures exhibit unique catalytic activity and selectivity that are not evident for their monometallic analogues. Such nanostructures contain plasmonic metals, such as gold or silver, which afford highly efficient harvesting of electromagnetic radiation and its conversion into hot carriers. These highly energetic species are transferred to the catalytic metal subcomponent of the bimetallic nanostructure, where a large spectrum of chemical reactions may be catalyzed. The strength of the electric field and the interplay between catalytic and plasmonic metals at the nanoscale are thus critically important for the catalytic activity of bimetallic nanostructures. In this study, we investigate the relationship between the catalytic activity and local electric fields sustained on the surface of gold–palladium (Au@PdNPs) and gold–platinum (Au@PtNPs) nanoplates using tip-enhanced Raman spectroscopy. We image the spatially varying magnitudes of rectified (DC) local electric fields on the surface of these nanostructures and compare them to the fields sustained on the surface of monometallic nanoplates. We find substantially larger electric field magnitudes on Au@PdNPs and Au@PtNPs as compared to their monometallic analogues. These findings suggest that catalytic efficiency of bimetallic nanostructures may be mediated and potentially tuned through precise control of electric fields sustained on their surfaces.