Note: tip enhanced Raman spectroscopy with objective scanner on opaque samples

Facilitating Hotspot Alignment in Tip-Enhanced Raman Spectroscopy via the Silver Photoluminescence of the Probe

A tip-enhanced Raman spectroscopy (TERS) system based on an atomic force microscope (AFM) and radially polarized laser beam was developed. A TERS probe with plasmon resonance wavelength matching the excitation wavelength was prepared with the help of dark-field micrographs. The intrinsic photoluminescence (PL) from the silver (Ag)-coated TERS probe induced by localized surface plasmon resonance contains information about the near-field enhanced electromagnetic field intensity of the probe. Therefore, we used the intensity change of Ag PL to evaluate the stability of the Ag-coated probe during TERS experiments. Tracking the Ag PL of the TERS probe was helpful to detect probe damage and hotspot alignment. Our setup was successfully used for the TERS imaging of single-walled carbon nanotubes, which demonstrated that the Ag PL of the TERS probe is a good criterion to assist in the hotspot alignment procedure required for TERS experiments. This method lowers the risk of contamination and damage of the precious TERS probe, making it worthwhile for wide adoption in TERS experiments.

Deciphering tip-enhanced Raman imaging of carbon nanotubes with deep learning neural networks

Recent release of open-source machine learning libraries presents opportunities to unify machine learning with nanoscale research, thus improving effectiveness of research methods and characterization protocols. This paper outlines and demonstrates the effectiveness of such a synergy with artificial neural networks to provide for an accelerated and enhanced characterization of individual carbon nanotubes deposited over a surface. Our algorithms provide a rapid diagnosis and analysis of tip-enhanced Raman spectroscopy mappings and the results show an improved spectral assignment of spectral features and spatial contrast of the collected images. Using several examples, we demonstrate the robustness and versatility of our deep learning neural network models. We highlight the use of machine learning and data science in tandem with tip-enhanced Raman spectroscopy technique enables a fast and accurate understanding of experimental data, thus leading to a powerful and comprehensive imaging analysis applied to spectroscopic measurements.

Note: Controlling the length of plasmonic tips obtained by pulsed electrochemical etching

We present a method to control the length of plasmonic gold tips through pulsed electrochemical etching. This method uses a cut-off circuit to interrupt the etching when the desired length is achieved, paving the way to tune the plasmonic properties of these tips through their shape. The control of the tip length by monitoring the cell voltage is the result of a study of the etching dynamics. The resulting tips possess a low apex radius and a small opening angle, allowing for high spatial resolution both in topography and in near-field imaging. The plasmonic behavior was confirmed in tip-enhanced Raman spectroscopy.

Tip-enhanced Raman spectroscopy - from early developments to recent advances

An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

Permittivity imaged at the nanoscale using tip-enhanced Raman spectroscopy

Localized surface plasmon resonances are the dominating contribution to the optical enhancement and the lateral resolution in tip-enhanced Raman spectroscopy. This well studied phenomenon may give access to more information about the sample than the enhanced Raman spectra alone due to its sensitivity to the permittivity of the tip environment. In this work, the effects of the permittivity of the sample on the properties of localized surface plasmon resonance are studied through the amplified signal of the luminescence of gold tips.

Two Mechanisms of Tip Enhancement of Raman Scattering by Protein Aggregates

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for probing the surface of biological species with nanometer spatial resolution. Here, we report the TER spectra of an individual insulin fibril, the protein cast film and a short peptide (LVEALYL) microcrystal mimicking the fibril core. Two different types of TER spectra were acquired depending on the "roughness" of the probed surface at the molecular level. A fully reproducible, low-intensity, normal Raman-type spectrum was characteristic of the top flat surface of the microcrystal while highly variable, higher intensity TER spectra were obtained for the edges of the microcrystal, cast film, and fibril. As a result, two tip enhancement mechanisms of Raman scattering, long- and short-range, were proposed by analogy with the physical and chemical enhancement mechanisms, respectively, known for surface-enhanced Raman spectroscopy.

High resolution scanning near field mapping of enhancement on SERS substrates: comparison with photoemission electron microscopy

The need for a dedicated spectroscopic technique with nanoscale resolution to characterize SERS substrates pushed us to develop a proof of concept of a functionalized tip-surface enhanced Raman scattering (FTERS) technique. We have been able to map hot spots on semi-continuous gold films; in order to validate our approach we compare our results with photoemission electron microscopy (PEEM) data, the complementary electron microscopy tool to map hot spots on random metallic surfaces. Enhanced Raman intensity maps at high spatial resolution reveal the localisation of hotspots at gaps for many neighboring nanostructures. Finally, we compare our findings with theoretical simulations of the enhancement factor distribution, which confirms a dimer effect as the dominant origin of hot spots.

Enhanced Vibrational Spectroscopies as Tools for Small Molecule Biosensing

In this short summary we summarize some of the latest developments in vibrational spectroscopic tools applied for the sensing of (small) molecules and biomolecules in a label-free mode of operation. We first introduce various concepts for the enhancement of InfraRed spectroscopic techniques, including the principles of Attenuated Total Reflection InfraRed (ATR-IR), (phase-modulated) InfraRed Reflection Absorption Spectroscopy (IRRAS/PM-IRRAS), and Surface Enhanced Infrared Reflection Absorption Spectroscopy (SEIRAS). Particular attention is put on the use of novel nanostructured substrates that allow for the excitation of propagating and localized surface plasmon modes aimed at operating additional enhancement mechanisms. This is then be complemented by the description of the latest development in Surface- and Tip-Enhanced Raman Spectroscopies, again with an emphasis on the detection of small molecules or bioanalytes.

High-resolution Raman imaging of bundles of single-walled carbon nanotubes by tip-enhanced Raman spectroscopy

Bundles of single-walled carbon nanotubes (SWCNTs) prepared by plasma torch method and further purified, are deposited over a glass coverslip to estimate the spatial resolution of tip-enhanced Raman spectroscopy measurements. For this purpose, near-field Raman maps and spectra of isolated bundles of carbon nanotubes are collected using optimized experimental conditions such as a tightly focused beam using a 1.4 numerical aperture oil immersion microscope objective and a gold coated atomic force microscope probe illuminated by a radially polarized 632.8 nm wavelength to selectively excite the localized surface plasmon confined at the extremity of the tip. The near-field nature of the collected Raman signals is evaluated through measuring the decay of the Raman signal with respect to the tip-sample separation.

A near field optical image of a gold surface: a luminescence study

We have used the luminescence from a gold tip to study the optical near field properties of a gold surface.

Tip-enhanced Raman spectroscopy of graphene-like and graphitic platelets on ultraflat gold nanoplates

TERS was used to investigate the graphene-like platelets in gap mode geometry using radially and linearly polarized excitation.

Localized enhancement of electric field in tip-enhanced Raman spectroscopy using radially and linearly polarized light

Finite-Difference Time-Domain (FDTD) calculations are used to characterize the electric field in the vicinity of a sharp silver or gold cone with an apex diameter of 10 nm. The simulations are utilized to predict the intensity and the distribution of the locally enhanced electric field in tip-enhanced Raman spectroscopy (TERS). A side-by-side comparison of the enhanced electric field induced by a radially and a linearly polarized light in both gap-mode and conventional TERS setup is performed. For this purpose, a radially polarized source is introduced and integrated into the FDTD modeling. Additionally, the optical effect of a thin protective layer of alumina on the enhancement of the electric field is investigated.