Tip-enhanced Raman Mapping Research Articles

Exploring the Spatial Distribution of Local Field Gradients by Tip-Enhanced Raman Mapping of a Single-Molecule Probe

Exploring the Spatial Distribution of Local Field Gradients by Tip-Enhanced Raman Mapping of a Single-Molecule Probe

Giant gap-plasmon tip-enhanced Raman scattering of MoS2 monolayers on Au nanocluster arrays.

In this article, we present the results of a gap-plasmon tip-enhanced Raman scattering study of MoS2 monolayers deposited on a periodic array of Au nanostructures on a silicon substrate forming a two dimensional (2D) crystal/plasmonic heterostructure. We observe a giant Raman enhancement of the phonon modes in the MoS2 monolayer located in the plasmonic gap between the Au tip apex and Au nanoclusters. Tip-enhanced Raman mapping allows us to determine the gap-plasmon field distribution responsible for the formation of hot spots. These hot spots provide an unprecedented giant Raman enhancement of 5.6 × 108 and a spatial resolution as small as 2.3 nm under ambient conditions. Moreover, due to strong hot electron doping in the order of 1.8 × 1013 cm-2, we observe a structural change of MoS2 from the 2H to the 1T phase. Owing to the very good spatial resolution, we are able to spatially resolve those doping sites. To the best of our knowledge, this is the first time reporting of such a phenomenon with nm spatial resolution. Our results will open the perspectives of optical diagnostics with nanometer resolution for many other 2D materials.

Tip-enhanced Raman mapping of single-walled carbon nanotube networks in conductive composite materials

Identifying and characterizing the structural integrity of single-walled carbon nanotubes (SWCNTs) that are fully embedded in a polymer matrix without causing any damage to them is a difficult task to achieve for bulk samples. Using tip-enhanced Raman spectroscopy, the surface of a polymer-embedded conductive network of SWCNTs was mapped underneath a thin layer of pure polymer. The technique was also used to detect tube-breaking within the composite sample caused by mechanical stress, beyond the ‘visual’ capabilities of scanning electron microscopy techniques. Results show that tip-enhanced Raman mapping can be used to successfully identify and characterize SWCNTs even underneath a layer of polymer. Copyright © 2016 John Wiley & Sons, Ltd.

Influence of tip geometry on the spatial resolution of tip enhanced Raman mapping**Project supported by the National Natural Science Foundation of China (Grant No. 11434017) and the National Basic Research Program of China (Grant No. 2013CB632704).

In 2013, a breakthrough experiment pushed the Raman mapping of molecules via the tip-enhanced Raman scattering (TERS) technique to a sub-nanometer spatial resolution, going into the single-molecule level. This surprising result was well explained by accounting for the critical role of elastic molecule Rayleigh scattering within a plasmonic nanogap in enhancing both the localization and the intensity level of the Raman scattering signal. In this paper, we theoretically explore the influence of various geometric factors of the TERS system on the spatial resolution of Raman mapping, such as the tip curvature radius, tip conical angle, tip–substrate distance, and tip–molecule vertical distance. This investigation can help to find out the most critical geometric factor influencing the spatial resolution of TERS and march along in the right direction for further improving the performance of the TERS system.

Optical Origin of Subnanometer Resolution in Tip-Enhanced Raman Mapping

Raman spectroscopy and imaging at the single-molecular level in both signal sensitivity and spatial resolution have been a long dream. A recent experimental study of single-molecule Raman mapping with a subnanometer resolution by using a tip-enhanced Raman scattering (TERS) technique is a great step closer to this great dream. However, how the subnanometer spatial resolution is possible with a Raman excitation light spot (“hot spot”) of diameter over 10 nm formed between the tip–substrate nanogap to excite and probe the molecule still remains mysterious. Here we present an optical theory of Raman scattering that accounts for the strong near-field self-interaction of molecule with the plasmonic nanogap due to multiple elastic scattering of light by the molecule. The result shows that the self-interaction effect strongly modulates the Raman excitation and radiation in both the signal intensity and spatial sensitivity, leading to a “super-hot spot” and subnanometer lateral resolution of Raman mapping. The optical theory can help to uncover the full picture of light-matter interaction of atoms and molecules with plasmonic nanostructures and explore unknown frontiers of physics and chemistry at nanoscale.

Tip-enhanced Raman mapping of local strain in graphene

We demonstrate local strain measurements in graphene by using tip-enhanced Raman spectroscopy (TERS). We find that a single 5 nm particle can induce a radial strain over a lateral distance of ∼170 nm. By treating the particle as a point force on a circular membrane, we find that the strain in the radial direction (r) is , in agreement with force-displacement measurements conducted on suspended graphene flakes. Our results demonstrate that TERS can be used to map out static strain fields at the nanoscale, which are inaccessible using force-displacement techniques.

Exchange of Methyl‐ and Azobenzene‐Terminated Alkanethiols on Polycrystalline Gold Studied by Tip‐Enhanced Raman Mapping

Mixed thiol self-assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single-component n-decanethiol or [4-(phenylazo)phenoxy]hexane-1-thiol SAMs on polycrystalline gold substrates. Static contact-angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact-angle values between those of the more hydrophobic n-decanethiol and the more hydrophilic [4-(phenylazo)phenoxy]hexane-1-thiol single-component SAMs. By means of tip-enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point-by-point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown.

Tip-Enhanced Raman Spectroscopy and Mapping of Graphene Sheets

: Single graphene sheets, a few graphene layers, and bulk graphite, obtained via both micromechanical cleavage of highly oriented pyrolytic graphite and carbon vapor deposition methods, were deposited on a thin glass substrate without the use of any chemical treatment. Micro-Raman spectroscopy, tip-enhanced Raman spectroscopy (TERS), and tip-enhanced Raman spectroscopy mapping (TERM) were used for characterization of the graphene layers. In particular, TERM allows for the investigation of individual graphene sheets with high Raman signal enhancement factors and allows for imaging of local defects with nanometer resolution. Enhancement up to 560% of the graphene Raman band intensity was obtained using TERS. TERM (with resolution better than 100 nm) showed an increase in the number of structural defects (D band) on the edges of both graphene and graphite regions.

Tip-enhanced Raman mapping of a single Ge nanowire

We report the nanometer scale mapping of a single Ge nanowire (NW) using tip-enhanced Raman scattering (TERS). The Raman spectra of a single Ge NW show downshift and asymmetric broadening, depending on the diameter, due to the phonon confinement effect. A Raman peak attributed to amorphous Ge has also been observed. Assuming the crystalline core/amorphous shell structure, we estimate the amorphous shell thickness to be 2–6 monolayers from the integrated Raman intensities of crystalline and amorphous Ge.

High Resolution Tip Enhanced Raman Mapping on Polymer Thin Films

AbstractAdvanced tip enhanced Raman mapping (TERM) was applied to high resolution chemical identification on nanoscale. Thin poly(methyl methacrylate)/poly(styrene acrylonitrile) (SAN28/PMMA) blend films were measured at different stages of phase separation. New insights into the phase evolution behavior of the thin films were obtained, when the TERM images were compared. An unexpected morphology transition was observed after a few minutes annealing at 250 °C. No surface enrichment of PMMA was observed, differing from the previous reports on a similar well‐studied system of SAN33/PMMA. The glass transition temperature, the surface and interfacial tension were found to be the main factors responsible for the phase evolution behavior of SAN28/PMMA films.

High-Resolution Chemical Identification of Polymer Blend Thin Films Using Tip-Enhanced Raman Mapping

Nanoscale chemical identification is required for the analysis of functional materials with fine structures. For the first time, highresolution tip-enhanced Raman mapping (TERM) was applied to a polymer system: poly(methyl methacrylate) (PMMA)/ poly(styrene-co-acrylonitrile) (SAN) thin films. Before TERM measurements were performed, the linear enhancement of tipenhancedRamanspectroscopy(TERS)wasoptimizedintermsofmaximumRamanintensity.Upto15timesoflinearenhancement wasobtained,ascomparedtothe(conventional)confocalRamanintensity.TheenhancementfactorofTERSisgreaterthan1500if taking the 100 times smaller probing area into account. As a result, a short exposure time was sufficient for high-resolution TERM measurements. Using TERM, the phase separation behavior of PMMA/SAN thin films was monitored by chemical recognition of local composition. The interface width (∼200 nm) at the early stage of phase evolution was visualized. The comparison of TERM images at different stages of the phase separation process revealed an unexpected transition of PMMA from the dispersed phase to the continuous phase. This morphology transition of PMMA/SAN is briefly discussed in terms of the glass transition temperature, interface, and surface tension.

Tip-enhanced Raman mapping with top-illumination AFM

Tip-enhanced Raman mapping is a powerful, emerging technique that offers rich chemicalinformation and high spatial resolution. Currently, most of the successes in tip-enhancedRaman scattering (TERS) measurements are based on the inverted configuration wheretips and laser are approaching the sample from opposite sides. This results in the limitationof measurement for transparent samples only. Several approaches have been developed toobtain tip-enhanced Raman mapping in reflection mode, many of which involve certaincustomisations of the system. We have demonstrated in this work that it is also possible toobtain TERS nano-images using an upright microscope (top-illumination) witha gold-coated Si atomic force microscope (AFM) cantilever without significantmodification to the existing integrated AFM/Raman system. A TERS image of asingle-walled carbon nanotube has been achieved with a spatial resolution of ∼ 20–50 nm, demonstrating the potential of this technique for studying non-transparentnanoscale materials.