Tip-enhanced Raman Scattering Signal Research Articles

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.

Optimizing the near‐field and far‐field properties of tips in tip‐enhanced Raman scattering

AbstractThe process of tip‐enhanced Raman scattering (TERS) depends critically on the morphology near the apex of the tip used in the experiment. Many tip designs have focused on optimization of electromagnetic enhancement in the near‐field, which is controlled to a large extent by subtle details at the nanoscale that remain difficult to reproduce in the tip fabrication process. The use of focused ion beams (FIB) permit modification of larger features on the tip in a reproducible manner, yet this approach cannot produce sub‐20‐nm structures important for optimum near‐field enhancement. Nonetheless, FIB milling offers excellent opportunities for improving the far‐field radiation properties of the tip‐antenna, a feature that has received relatively little attention in the TERS research community. In this work, we use finite‐difference time‐domain (FDTD) simulations to study both the near‐field and far‐field radiation efficiency of several tip‐antenna systems that can be constructed with FIB techniques in a feasible manner. Starting from blunt etched tips, we find that excellent overall enhancement of the TERS signal can be obtained with pillar‐type tips. Furthermore, by applying vertical grooves on the tip's shaft, the overall efficiency can be improved even more, producing TERS signals that are up to 10‐fold stronger than signals obtained from an ideal (unmodified) sharp tip of 10‐nm radius. The proposed designs constitute a feasible route toward a tip fabrication process that not only yields more reproducible tips but also promises much stronger TERS signals.

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Interactions Between Epitaxial Graphene Grown on the Si- and C-Faces of 4H-SiC Investigated Using Raman Imaging and Tip-Enhanced Raman Scattering.

Interactions between epitaxial graphene grown on Si- and C-faces were investigated using Raman imaging and tip-enhanced Raman scattering (TERS). In the TERS spectrum, which has a spatial resolution exceeding the diffraction limit, a D band was observed not from graphene surface, but from the edges of the epitaxial graphene ribbons without a buffer layer, which interacts with SiC on the Si-face. In contrast, for a graphene micro-island on the C-face, the D band disappeared even on the edges where the C atoms were arranged in armchair configurations. The disappearance of the edge chirality via combination between the C atoms and SiC on the C-face is responsible for this phenomenon. The TERS signals from the C-face were weaker than those from the Si-face without the buffer layer. On the Si-face with a buffer layer, the graphene TERS signal was hardly observed. TERS enhancement was suppressed by interactions on the edges or by the buffer layer between the SiC and graphene on the C- or Si-face, respectively.

Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution.

Tip-enhanced Raman scattering (TERS) is a promising optical and analytical technique for chemical imaging and sensing at single molecule resolution. In particular, TERS signals generated by a gap-mode configuration where a silver tip is coupled with a gold substrate can resolve a single-stranded DNA (ssDNA) molecule with a spatial resolution below 1 nm. To demonstrate the proof of subnanometer resolution, we show direct nucleic acid sequencing using TERS of a phage ssDNA (M13mp18). M13mp18 provides a known sequence and, through our deposition strategy, can be stretched (uncoiled) and attached to the substrate by its phosphate groups, while exposing its nucleobases to the tip. After deposition, we scan the silver tip along the ssDNA and collect TERS signals with a step of 0.5 nm, comparable to the bond length between two adjacent DNA bases. By demonstrating the real-time profiling of a ssDNA configuration and furthermore, with unique TERS signals of monomeric units of other biopolymers, we anticipate that this technique can be extended to the high-resolution imaging of various nanostructures as well as the direct sequencing of other important biopolymers including RNA, polysaccharides, and polypeptides.

Breakdown of Far-Field Raman Selection Rules by Light–Plasmon Coupling Demonstrated by Tip-Enhanced Raman Scattering

We present an experimental study on the near-field light-matter interaction by tip-enhanced Raman scattering (TERS) with polarized light in three different materials: germanium-doped gallium nitride (GaN), graphene, and carbon nanotubes. We investigate the dependence of the TERS signal on the incoming light polarization and on the sample carrier concentration, as well as the Raman selection rules in the near-field. We explain the experimental data with a tentative quantum mechanical interpretation, which takes into account the role of plasmon polaritons, and the associated evanescent field. The driving force for the breakdown of the classical Raman selection rules in TERS is caused by photon tunneling through the perturbation of the evanescent field, with the consequent polariton annihilation. Predictions based on this quantum mechanical approach are in good agreement with the experimental data, which are shown to be independent of incoming light polarization, leading to new Raman selection rules for TERS.

Visualizing Electric Fields at Au(111) Step Edges via Tip-Enhanced Raman Scattering.

Tip-enhanced Raman scattering (TERS) can be used to image plasmon-enhanced local electric fields on the nanoscale. This is illustrated through ambient TERS measurements recorded using silver atomic force microscope tips coated with 4-mercaptobenzonitrile molecules and used to image step edges on an Au(111) surface. The observed two-dimensional TERS images uniquely map electric fields localized at Au(111) step edges following 671 nm excitation. We establish that our measurements are not only sensitive to spatial variations in the enhanced electric fields but also to their vector components. We also experimentally demonstrate that (i) few nanometer precision is attainable in TERS nanoscopy using corrugated tips with nominal radii on the order of 100-200 nm, and (ii) TERS signals do not necessarily exhibit the expected E4 dependence. Overall, we illustrate the concept of electric field imaging via TERS and establish the connections between our observations and conventional TERS chemical imaging measurements.

Plasmonic tip for nano Raman microcopy: structures, materials, and enhancement

Tip-enhanced Raman scattering (TERS) microscopy is becoming an important tool for analyzing advanced nanomaterials and nanodevices because of its high spatial resolution and high sensitivity. However, despite the decade’s efforts since its invention, strong Raman enhancement is still not always reproducible. Here, Author discusses two aspects in plasmonic metal tips to achieve efficient Raman enhancement. The first is the tip structure whose plasmonic properties directly affect the scattering efficiency and thus the enhancement. The second is the plasmonic tip for deep ultraviolet (DUV), with which TERS signal can be further enhanced by incorporating the resonance Raman effect. The materials for DUV-TERS tips are shown. With the efficient tip structures and materials, nano Raman imaging with TERS microscopy becomes more reliable as it should inherently be, bringing TERS microscopy to higher levels as a nanoanalysis tool useful for everyone.

Distinguishing adjacent molecules on a surface using plasmon-enhanced Raman scattering.

Unambiguous chemical identification of individual molecules closely packed on a surface can offer the possibility to address single chemical species and monitor their behaviour at the individual level. Such a degree of spatial resolution can in principle be achieved by detecting their vibrational fingerprints using tip-enhanced Raman scattering (TERS). The chemical specificity of TERS can be combined with the high spatial resolution of scanning probe microscopy techniques, an approach that has stimulated extensive research in the field. Recently, the development of nonlinear TERS in a scanning tunnelling microscope has pushed the spatial resolution down to ∼0.5 nm, allowing the identification of the vibrational fingerprints of isolated molecules on Raman-silent metal surfaces. Although the nonlinear TERS component is likely to help sharpen the optical contrast of the acquired image, the TERS signal still contains a considerable contribution from the linear term, which is spatially less confined. Therefore, in the presence of different adjacent molecules, a mixing of Raman signals may result. Here, we show that using a nonlinear scanning tunnelling microscope-controlled TERS set-up, two different adjacent molecules that are within van der Waals contact and of very similar chemical structure (a metal-centred porphyrin and a free-base porphyrin) on a silver surface can be distinguished in real space. In addition, with the help of density functional theory simulations, we are also able to determine their adsorption configurations and orientations on step edges and terraces.

Tip-Enhanced Raman Scattering with a Nanoparticle-Functionalized Probe

We carried out the tip-enhanced Raman scattering (TERS) with a tip that is functionalized with a Aunanoparticle (AuNP, with a diameter of 250 nm). The AuNP tip is fabricated by a direct mechanical pickup of a AuNP from a flat substrate, and the TERS signal from the AuNP tip - organic monolayer - Au thin film (thickness of 10 nm) is recorded. We find that such a AuNP-tip interacting with a thin film routinely yields signal enhancement larger than ~10 4 , which is sufficient not only for local (with detection area of ~200 nm 2 ) Raman spectroscopy, but also the nanometric imaging of organic monolayers within a reasonable acquisition time (~20 minutes/image).

Tip‐enhanced Raman scattering along a single wall carbon nanotubes bundle

AbstractRaman scattering is a common technique for studying carbon nanotubes due to the possibility to observe the radial breathing mode (RBM), which is used for identification of single wall carbon nanotubes “SWCNT.” Using tip‐enhanced Raman scattering (TERS) we show along a small SWCNTs bundle that the RBMs are well visible in near‐field, even if they are not observable in the far‐field. Different Raman intensities in TERS for the D, G, and 2D modes at six different positions along a SWCNTs bundle were observed. We discuss this strong dependence of the TERS signal on the position of the tip on the sample and on the distance between the tip and the sample.

Optimization of tip material and shape for near‐UV TERS in Si structures

AbstractHigh spatial resolution stress mapping of Si device structures is in high demand throughout electronic industry. This problem can be solved by application of the tip‐enhanced Raman scattering (TERS) to Si structures. This work involves searching for a suitable TERS material for the near‐UV spectral range that is favorable for Raman mapping of Si structures. We show both theoretically and experimentally that Al is a good Raman enhancer at the 363.8 nm wavelength. Taking into account Raman selection rules of Si and high dielectric contrast between Si and air, we employed an inclined Al‐coated Si atomic force microscope (AFM) tip that is also suitable for the top‐illumination geometry of the experiment. We succeeded in detecting of a weak TERS signal of strained Si. Possible ways of improvement of the Si TERS signal/noise ratio are discussed. An Al‐particle‐topped AFM tip is considered as a promising solution to the problem. A prototype tip was fabricated. Copyright © 2009 John Wiley & Sons, Ltd.

Imaging of single GaN nanowires by tip‐enhanced Raman spectroscopy

AbstractRaman imaging of a single [110] GaN nanowire using a combination of an atomic force microscope (AFM) and an inverted confocal microscope was investigated. The Raman spectra and associated maps of this single object, the diameter of which was of 200 nm, were collected in both confocal mode and with the interaction of an AFM tip to benefit from local tip enhancement effect. Beyond the pure tip‐enhanced Raman scattering (TERS) effect, the setup was developed to integrate a commercial AFM and spectrometer to permit simultaneous topography and vibrational imaging with spatial and spectral resolutions determined by the respective instruments. Data exchange and communication of the Raman spectrometer and the AFM were optimized to display both height and Raman intensity for a set of Z positions at the AFM tip for each (x,y) position on the sample. The experiments were conducted with both a straight tip coated with gold at an excitation of λ = 647 nm and with a tilted tip coated with silver at an excitation wavelength of λ = 514 nm. The TERS signal was observed for a given phonon mode of the [110] nanowire, providing an optical resolution that corresponds to the size of the object. Copyright © 2009 John Wiley & Sons, Ltd.

A tetrahedral tip as a probe for tip‐enhanced Raman scattering and as a near‐field Raman probe

AbstractWe use a tetrahedral glass fragment which is coated with a thin film of gold (T‐tip) to form a local tip source by illuminating the tip from within its transparent glass body. We demonstrate the potential of this apertureless tip source for tip‐enhanced Raman scattering (TERS) as an alternative to the standard scheme with an external illumination. We found that T‐tips with different optical properties result from the process of tip fabrication. For some tips, the Raman signal persists for a small number of molecules picked up by the retraction of the tip from a tunnel contact. In these cases, the tip itself gives rise to the TERS signal without a gold substrate close to the tip. Other tips only exhibit a TERS signal when there is a contact to a gold substrate. Tips covered by Raman active nonresonant molecules serve as a new kind of near‐field Raman probe for local properties of a sample. As the Raman spectrum of the coated tip is sensitive to the local light intensity, it may be used to monitor the field‐enhancement properties of metal films and metal nanostructures. A near‐field Raman probe avoids problems due to bleaching of a fluorescent near‐field probe and, furthermore, such a near‐field Raman probe may be highly selective for specific surface properties. Copyright © 2009 John Wiley & Sons, Ltd.

Depolarization effect in reflection-mode tip-enhanced Raman scattering for Raman active crystals

Reflection-mode tip-enhanced Raman scattering (TERS) has the advantage to characterize any sample, particularly opaque, bulk, and multilayered samples. However, the background signal in reflection-mode TERS is huge due to large focus spots associated with an objective lens that has a long working distance. Moreover, for a multilayered and bulk sample, the Raman signal from the bulk layer interferes with the Raman signal on a thin surface layer. This unwanted bulk background signal reduces the sensitivity of the measurement and makes it difficult to get a high-contrast TERS image in the reflection mode. Here, we demonstrate two techniques to suppress the far-field Raman signals coming from the focus area and bulk silicon germanium substrate. First, we reduce the far-field signal by controlling the polarization state of the incident and scattered Raman as well as manipulating the well-defined polarization of a crystalline sample, which strongly depends on the polarization and propagation of the incident light. We used Raman tensor analysis to quantify the far-field Raman intensity at different polarization states of the polarizer and analyzer at varying sample azimuth. The results of the surface-enhanced Raman spectroscopy experiments were in good agreement with the far-field Raman tensor calculation. The polarizer, analyzer, and sample azimuth combination that gives minimum far-field background signals with high contrast in SERS was utilized in the TERS experiment to obtain a high-contrast near-field Raman signal. Both the field enhancement effect and depolarized detection were considered to obtain a high signal-to-noise TERS signal. We found that, for Raman active and thin crystalline samples assembled in bulk materials, the depolarization effect outweighs the field enhancement effect in getting a high-contrast Raman signal.

Temporal Fluctuation of Tip-Enhanced Raman Spectra of Adenine Molecules

Temporal fluctuations of spectral shapes have been observed in tip-enhanced Raman scattering (TERS) spectra of adenine molecules. The TERS spectra of a nanocrystalline cluster of adenine molecules showed distinct temporal changes including fluctuations of peak frequencies and peak intensities and extraordinary enhancements of several peaks. The fluctuating TERS signals originate primarily from the molecules in the first layer of the 5 nm thick adenine nanocrystalline cluster. The possible origin of the temporal changes is attributed to the changes of molecular orientation under the metal-coated tip, which is responsible for changing the adsorption angle and the angle of local incident polarization for the molecules adsorbed to the tip. This result suggests the importance of sample−tip interaction at molecular level in the interpretation of TERS spectra and indicates the possibility of the single molecule sensitivity of TERS spectroscopy of DNA base molecules.