Tip-enhanced Raman Spectroscopy Probes Research Articles

Establishing the excitation field in tip-enhanced Raman spectroscopy to study nanostructures within two-dimensional systems

The optical field generated by a nanoplasmonic probe is revealed in tip-enhanced Raman spectroscopy (TERS) experiments. The TERS intensity profile of nano-objects smaller than the probe’s apex has a donut-like shape which resembles the magnitude of the field generated by a point-dipole source, being well described by the Dyadic Green’s function. Having prior knowledge on the excitation field generated by the TERS probe, we measured the width of shear solitons caused by lattice reconstruction in low-angle twisted bilayer graphene, a prominent platform for twistronics, and the extend of defect-induced light emission from graphene edges.

Fabrication of Au and Ag – Coated AFM Probes for Tip-Enhanced Raman Spectroscopy

In this work, approaches for fabrication metal coated probes for Tip Enhanced Raman Spectroscopy (TERS) are considered. It was proposed to use optical characterization of probes to achieve the effective TERS of semiconductor nanoobjects. The shape and size of the metal cluster at the tip apex determines the position of the localized surface plasmon resonance, the electromagnetic field enhancement and, thus, TERS performance. The possibility of optimizing the characteristics of the probes for TERS studies of nanoobjects has been investigated.

Probing the Local Generation and Diffusion of Active Oxygen Species on a Pd/Au Bimetallic Surface by Tip-Enhanced Raman Spectroscopy.

Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.

Coral-like plasmonic probes for tip-enhanced Raman spectroscopy.

Tip-enhanced Raman spectroscopy is a powerful tool for the analysis of system interfaces, enabling access to chemical information with nanometric spatial resolution and sensitivity up to the single molecule level. Such features are due to the presence of proper metallic nanostructures at the TERS probe apex, which, via the excitation of a plasmonic field, confine light to a nanometric region. The nano-sized characteristic of such metallic structures intrinsically renders the fabrication of high performing and reproducible TERS probes still a challenge. In this paper, we present a facile, rapid and effective approach to prepare Ag-based TERS probes. The fabrication process proposed herein is based on spinodal dewetting of Ag-coated AFM-probes through a RF plasma treatment. The obtained probes appear covered with a coral-like silver nanotexture, endowed with an excellent plasmonic activity. Intriguingly, such a texture can be easily tuned by changing some process parameters, such as Ag film thickness and exposure time to the plasma. The as-prepared TERS probes show a high TERS enhancement, reaching 107, and allow a good spatial resolution, down to 10 nm. Finally, we suggest an easy and effective procedure to restore oxidized TERS tips following exposure to ambient air, which can be applied to all types of Ag-based TERS tips.

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.

Tunable, Liquid Resistant Tip Enhanced Raman Spectroscopy Probes: Toward Label-Free Nano-Resolved Imaging of Biological Systems.

Tip enhanced Raman spectroscopy (TERS) has been established as a powerful, noninvasive technique for chemical identification at the nanoscale. However, difficulties, including the degradation of probes, limit its use in liquid systems. Here TERS probes for studies in aqueous environments have been demonstrated using titanium nitride coatings with an alumina protective layer. The probes show enhancement in signal intensity as high as 380% in liquid measurements, and the probe resonance can be tuned by varying deposition conditions to optimize performance for different laser sources and types of samples. This development of inexpensively produced probes suited for studies in aqueous environments enables its wider use for fields such as biology and biomedicine in which aqueous environments are the norm.

Template stripping and bonding of smooth probes with nanoscale features for tip-enhanced Raman spectroscopy

The batch fabrication of scanning microscopy probes for tip-enhanced Raman spectroscopy (TERS) is an often discussed issue and remains a challenging task. In our work, we started from a known metal moulding process and developed a dry release process based on metal bonding. After establishing the release process, scanning tunnelling microscopy probes could reliably be fabricated and used for tip-enhanced Raman measurements. The Raman signal enhancement with laser excitation from the side was investigated on substrates covered by malachite green dye molecules, where clear signal from few molecules could be detected. TERS sensitivity was investigated on substrates coated with ultra-thin amorphous carbon layers and compared to standard confocal Raman spectroscopy.

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.

Molecular Bending at the Nanoscale Evidenced by Tip-Enhanced Raman Spectroscopy in Tunneling Mode on Thiol Self-Assembled Monolayers

The sharp and intense Raman features associated with hexil azobenzenethiol self-assembled monolayers on gold(111) make these molecules ideal probes for tip-enhanced Raman spectroscopy (TERS) investigations. Spectroscopic and electrical measurements at the molecular junction were performed by combining TERS with scanning tunneling microscopy. The azobenzene TERS signal was monitored while changing the tip–surface distance and/or increasing the bias applied to the tip. The TERS signal decreases with increasing bias faster than what would be expected if only the effect of the tip distance from the monolayer and the related drop of the optically enhancing near-field intensity were considered. We develop a simple geometrical model accounting for the molecular bending induced by the increasingly higher local field close to the tip apex, which brings an additional modification of the TERS intensity and is able to accurately describe the experimental data.

Bare and protected sputtered-noble-metal films for surface-enhanced Raman spectroscopy

Sputtered silver and gold films with different surface morphologies have been prepared and coated with a benzenethiol self-assembled monolayer. Rough noble metal films showed strong Raman features assigned to adsorbed benzenethiol molecules upon irradiation over a wide energy range in the visible spectrum, which disclosed the occurrence of a significant surface-enhanced Raman scattering with maximal enhancement factors as high as 6×106. In addition, the adsorption of ethanethiol onto silver surfaces hinders their corrosion over days while preserving mostly intact enhancement properties of naked silver. This study may be applied to develop stable and efficient metalized probes for tip-enhanced Raman spectroscopy.

The controlled fabrication of “Tip-On-Tip” TERS probes

Tip-enhanced near-field Raman spectroscopy has exhibited a great ability to detect in situ chemical and structural information on a sample surface with the highest lateral resolution combined with a high sensitivity at the same time. A key challenge in the TERS field is the development and improvement of the fabrication of reproducible metal tip geometries with a sharper apex. In this study, one novel apertureless “Tip-On-Tip” TERS probe with a special plasmonic nanostructure is designed, and a simple Ar+-ion sputtering route has been developed to fabricate silver nanoneedle arrays on the scanning probe microscopy (SPM) cantilevers for tip-enhanced Raman spectroscopy (TERS) probes. These silver nanoneedles possess a very sharp apex with an apex diameter of 15 nm and an apex angle of 20°. The novel TERS probes exhibit enhanced near-field Raman signals compared to an Ag-coated SPM probe, which are attributed to the intense electromagnetic field around the apexes of the Ag nanoneedles and the periodic structure of the Ag nanoneedles.

Prolonged Blinking with TERS Probes

Tip-enhanced Raman spectroscopy (TERS), an emerging technique combining scanning probe microscopy (SPM) and Raman spectroscopy, provides the sensitivity and selectivity necessary for chemical imaging with nanoscale resolution. We demonstrate that coating the fragile plasmonic structures used to enhance the electric field with a protective layer improves significantly its mechanical, chemical, and thermal stability without loss of the extreme optical enhancement that appears as a “blinking” phenomenon. This observation provides important information on the mechanism of blinking. The fact that TERS blinking from a polymer layer is preserved when the analyte is separated from the plasmonic nanostructure by a 3 nm thick coating proves that chemical enhancement and charge transfer do not play a significant role in the blinking. Rather, thermally activated diffusion appears to be the dominant mechanism. While extreme enhancement persists only for minutes for an unprotected silver-coated tip, it persists for tens of hours with plasmonic structures coated by an ultrathin layer of alumina (Al2O3), opening broader opportunities for the study of blinking and for chemical imaging and sensing based on plasmonic structures.

The SERS and TERS Effects Obtained by Gold Droplets on Top of Si Nanowires

We show that hemispherical gold droplets on top of silicon nanowires when grown by the vapor-liquid-solid (VLS) mechanism, can produce a significant enhancement of Raman scattered signals. Signal enhancement for a few or even just single gold droplets is demonstrated by analyzing the enhanced Raman signature of malachite green molecules. For this experiment, trenches (approximately 800 nm wide) were etched in a silicon-on-insulator (SOI) wafer along <110> crystallographic directions that constitute sidewalls ({110} surfaces) suitable for the growth of silicon nanowires in <111> directions with the intention that the gold droplets on the silicon nanowires can meet somewhere in the trench when growth time is carefully selected. Another way to realize gold nanostructures in close vicinity is to attach a silicon nanowire with a gold droplet onto an atomic force microscopy (AFM) tip and to bring this tip toward another gold-coated AFM tip where malachite green molecules were deposited prior to the measurements. In both experiments, signal enhancement of characteristic Raman bands of malachite green molecules was observed. This indicates that silicon nanowires with gold droplets atop can act as efficient probes for tip-enhanced Raman spectroscopy (TERS). In our article, we show that a nanowire TERS probe can be fabricated by welding nanowires with gold droplets to AFM tips in a scanning electron microscope (SEM). TERS tips made from nanowires could improve the spatial resolution of Raman spectroscopy so that measurements on the nanometer scale are possible.