The information content in single-molecule Raman nanoscopy

Tip-enhanced Raman scattering (TERS)-based nanoimaging is gaining ever-increasing popularity for nanoscale spectroscopic imaging of 2D materials due to its much improved sensitivity and spatial resolution compared to conventional confocal Raman microscopy. In this work we report a successful application of TERS for spectroscopic imaging of the WS2–WSe2 vertical heterostructures transferred to gold. Our approach affords the unambiguous identification of the composition of vertical heterobi- and heterotrilayers. Collecting TERS maps of the same area with various excitation wavelengths showed that 671 and 785 nm excitations yield a strong enhancement of Raman bands for both WS2 and WSe2 sublayers in vertical heterostructures. On the other hand, 638 nm excitation seems insensitive to the WSe2 component in vertical heterobilayers. In this context, the use of a single wavelength for the TERS characterization of heterobilayers leads to wrong conclusions when it comes to the composition of this important class of low-dimensional quantum materials. Additionally, in TERS spectra recorded from monolayer WS2 collected with 671 nm excitation, we observe the appearance of a strong resonant band at 324 cm–1, which was absent in TERS spectra collected with 638 and 785 nm excitations. This observation further bolsters the importance of varied excitation wavelengths to match the electronic/excitonic resonances of 2D materials probed through TERS.

This work presents an overview of the latest results and new data on the optical response from spherical CdSe nanocrystals (NCs) obtained using surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS). SERS is based on the enhancement of the phonon response from nanoobjects such as molecules or inorganic nanostructures placed on metal nanostructured substrates with a localized surface plasmon resonance (LSPR). A drastic SERS enhancement for optical phonons in semiconductor nanostructures can be achieved by a proper choice of the plasmonic substrate, for which the LSPR energy coincides with the laser excitation energy. The resonant enhancement of the optical response makes it possible to detect mono- and submonolayer coatings of CdSe NCs. The combination of Raman scattering with atomic force microscopy (AFM) using a metallized probe represents the basis of TERS from semiconductor nanostructures and makes it possible to investigate their phonon properties with nanoscale spatial resolution. Gap-mode TERS provides further enhancement of Raman scattering by optical phonon modes of CdSe NCs with nanometer spatial resolution due to the highly localized electric field in the gap between the metal AFM tip and a plasmonic substrate and opens new pathways for the optical characterization of single semiconductor nanostructures and for revealing details of their phonon spectrum at the nanometer scale.

Tip-enhanced Raman scattering (TERS) microscopy attracts increasing attention for visualization and characterization of strain distribution on crystalline samples at a nanoscale due to nano-sized fields localized at a metal tip. However, as the metal tip approaches close to a sample surface, a force acts between the tip and sample, which can induce unwanted perturbation on the local strain of the sample. Analysis and evaluation of intrinsic strains on samples with a high reliability demands a technique to correlate TERS spectra with the tip-sample force. Here, we present a TERS microscope based on a frequency-modulation atomic force microscope (FM-AFM) using a quartz tuning fork (QTF) as a force sensor. By continuously monitoring a shift in the resonance frequency of the QTF during TERS, the tip-sample force can be directly measured in both attractive and repulsive force regions, which is not possible by other AFMs. TERS spectra of single-walled carbon nanotubes (CNTs) were measured simultaneously while changing the tip-sample distance and hence the tip-sample force acting on the CNTs. We found that TERS occurs at the tip-sample distance where the repulsive force dominantly works and mostly decays out in the attractive force region.

While surface‐enhanced Raman scattering (SERS) is an established technique to significantly enhance the Raman signal from samples bound to metal surfaces, a quantitative interpretation is very difficult because of the rough and arbitrary nature of the substrate. A distinction between concentration and enhancement effects is virtually impossible. Tip‐enhanced Raman scattering (TERS), a scattering type of near‐field optical microscopy technique, circumvents the problem of random enhancement by concentrating the roughness to only a small region directly at the apex of a scanning probe tip. In this paper, TERS spectra of DNA pyrimidine bases have been collected and compared with standard SERS and Raman measurements of the nucleotides and the pure bases. The results are discussed in terms of electromagnetic and chemical effects of the SERS enhancement of the TERS probes. Copyright © 2006 John Wiley & Sons, Ltd.

Step, ridge, and crack submicro/nanostructures of epitaxial graphene on 4H-SiC (0001̅) were characterized using tip-enhanced Raman scattering (TERS) spectroscopy. The nanostructures were created during graphene synthesis due to a difference in the thermal expansion coefficient of graphene and SiC. These structures are a distinctive property of epitaxial graphene, together with other desirable properties, such as large graphene sheet and minimal defects. The results of this study illustrate that the exceptional spatial resolution of TERS allows spectroscopic measurements of individual nanostructures, a feat which normal Raman spectroscopy is not capable of. By analyzing TERS spectra, the change of local strain on the nanoridge and decreased graphene content in the submicrometer crack were detected. Using G′ band positions in the TERS spectra, the strain difference between the ridge center and flat area was calculated to be 1.6 × 10–3 and 5.8 × 10–4 for uniaxial and biaxial strain, respectively. This confirms the proposed mechanism in previous researches that nanoridges on epitaxial graphene form as a relief against compressive strain. With this study, we demonstrate that TERS is a powerful technique for the characterization of individual local nanostructures on epitaxial graphene.

Calf thymus DNA adsorbed on a rough gold substrate or on an atomically smooth gold (111) surface has been investigated by collecting its unique Raman fingerprints using either surface-enhanced Raman scattering (SERS) or tip-enhanced Raman scattering (TERS). A monolayer coverage of DNA strands adsorbed at both the irregular rough edges of evaporated gold grids and at gold nanoparticles is detected by SERS. Highly improved sensitivity down to single DNA strand spectroscopic determination is accomplished by TERS providing an enhancement factor of at least 1400. Based on our experimental results, we propose that TERS is a promising technique to study the DNA–drug molecule interaction on the level of a single DNA strand.

The manipulation of light and light-matter interactions on the nanoscale is enabled by the unique optical properties of plasmonic metal nanostructures and nanoparticles. Indeed, engineered hybrid molecular-plasmonic nanostructures have enabled single molecule detection and identification via surface-enhanced Raman scattering (SERS). Further, the same effect is at the core of visualizing single molecules with sub-nanometer spatial resolution via tip-enhanced Raman scattering (TERS). More generally, modern applications of plasmon-enhanced spectroscopy and microscopy fueled advances in fields as diverse as quantum and semiconductor materials, heterogeneous catalysis, and the health sciences, to name a few. All these exciting developments and many more are touched in this virtual special issue (VSI) on nanophotonics. From ultra-precise chemical imaging measurements performed in a scanning tunneling microscope chamber to single molecule spectroscopic measurements performed at solid-air and solid-liquid interfaces, this VSI includes contributions from active practitioners that are all striving to better-understand increasingly complex interfaces encountered in the biological, materials, and chemical sciences.

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.

Tip-enhanced Raman scattering (TERS) has emerged as a powerful tool to obtain subnanometer spatial resolution fingerprints of atomic motion. Theoretical calculations that can simulate the Raman scattering process and provide an unambiguous interpretation of TERS images often rely on crude approximations of the local electric field. In this work, we present a novel and first-principles-based method to compute TERS images by combining Time Dependent Density Functional Theory (TD-DFT) and Density Functional Perturbation Theory (DFPT) to calculate Raman cross sections with realistic local fields. We present TERS results on free-standing benzene and C60 molecules, and on the TCNE molecule adsorbed on Ag(100). We demonstrate that chemical effects on chemisorbed molecules, often ignored in TERS simulations of larger systems, dramatically change the TERS images. This observation calls for the inclusion of chemical effects for predictive theory-experiment comparisons and an understanding of molecular motion at the nanoscale.

Modified optical fiber tips with micrometer/sub micrometer dimensions are very promising in the field of Tip enhanced Raman Scattering (TERS), Surface Enhance Raman Scattering (SERS), optical trapping, plasmonic sensor etc. A unique axicon fabricated on the optical fiber tip by simple chemical etching in 48% Hydrofluoric acid (HF) followed by thin film gold (Au) coating by magnetron sputtering is demonstrated for anti-BSA and BSA binding. The dimension of conical axicon is ~2 μm apex at the core of fiber, opening diameter of ~70 μm at the tip and total length of ~300 μm. The inner surface of the hollow negative axicon is coated with thin film of gold. The dielectric tip experiences notable optical field enhancement due to the plasmonic coating. Further, such tip generates non-diffracting Bessel beam which has several advantages including collimation. To monitor the binding process of protein bovine serum albumin (BSA) and anti-BSA, the tip is processed for anti-BSA binding on the surface of the negative axicon. The tip is further tested to bind BSA of different concentrations. For every change in concentration of the BSA, time response curve with respect to variation in wavelength shift was recorded. The tip has shown sensitivity to protein binding process for BSA concentrations of range 10-12 mg/ml to 10-4 mg/ml. The sensor saturates for higher concentration of protein BSA around 10-4 mg/ml. The sensor tends to be very sensitive to proteins of different concentration. This deep seated negative axicon tip based probe can be used for sensing low volumetric bio-samples.

Raman Spectroscopy and Imaging of Biomolecules using Targeted Nanoparticles

The research reported in this thesis focuses on the fundamental science and engineering required for the fabrication and optimization of nanoparticles functionalized paper as Surface Enhanced Raman Scattering (SERS) bio-diagnostics platform. It addresses the current Sensitivity, Selectivity, Simplicity and Strength (4S) limitations of bioactive paper. The research vision is to develop a nanoparticles treated paper as a generic diagnostic platform to identify and quantify low concentrations of a specific (bio)analyte in aqueous solutions. The product concept is a “dipstick sensor” based on functionalized gold nanoparticle treated paper (AuNP paper) for SERS detection. Such AuNP paper test would allow the direct, rapid and easy detection of (bio)analyte molecules at very low concentrations (below the nanomolar range), making it suitable for biomedical analysis, such as in cancer detection. It also eliminates the tedious preparation of multiple reactants and washing steps involved in current technology such as the Enzyme-Linked Immunosorbent Assay (ELISA) based bioactive paper. Three parallel research length scales are investigated: nano, micro and macro. This involves quantifying the distribution and adsorption state of nanoparticles on paper (nanoscale). The effects of gold nanoparticles (AuNPs) concentration and 3-dimensional (3D) distribution profile on their adsorption and aggregation states on paper were explored. The surface coverage of AuNPs on paper scaled linearly with their concentration profile in solutions. The SERS performances of the AuNPs-treated papers were evaluated with a model Raman molecule, 4-aminothiophenol (4-ATP), and their SERS intensities increased linearly with the density of AuNPs on paper. The role of z-distribution of AuNPs within the bulk of paper was highlighted; their three-dimensional (3D) architecture was able to quench the background noise through interlayer plasmon coupling, thus amplifying the SERS signal. The retention and aggregation state of nanoparticles on paper was controlled to drastically optimize the SERS performance (microscale). Paper substrates were pre-treated with a series of cationic polyacrylamide (CPAM) solutions to control the AuNPs adsorption and aggregation state on paper. The CPAM pre-treated paper produced a more uniform distribution of AuNPs compared to untreated paper. CPAM chains which adsorbed on paper in a high loops and tails conformation promotes efficient bridging of AuNPs to achieve higher surface coverage and aggregation of AuNPs on paper. This configuration is favored by CPAM solutions of higher concentration, charge density and molecular weight. A simple approach to form and visualize polyelectrolyte-nanoparticles chaplet like structures on paper was developed by using negatively-charged gold nanoparticles (AuNPs) to decorate polymer chain of CPAM. AuNP chaplets were adsorbed in an opposite direction to the cellulose fibers and along the length of the CPAM molecules which were draped over the fibers. The effects of CPAM polymer concentration, charge density and molecular weight on the dimension of AuNP chaplets were quantified. Assembling nanometer-scale components (AuNPs) into micrometer-scale arrays (chaplets) on a porous paper substrate can be a promising strategy for low-cost nanoelectronics applications The SERS properties of nanoparticles functionalized paper were quantified and optimized (macroscale). The effect of CPAM concentration, charge density and molecular weight on the aggregation and surface coverage of AuNPs on paper and ultimately the SERS signal of 4-ATP was quantified. The optimized AuNPs-CPAM paper showed a higher sensitivity and Raman enhancement factor (EF), which was almost an order of magnitude higher than the untreated AuNPs paper. The effect of CPAM dissolution kinetic on the SERS reproducibility was also quantified. SERS reproducibility of AuNPs on CPAM pre-treated paper both increase with dissolution time of CPAM, as the heterogeneity of AuNP distribution and aggregate size decreases; increasing CPAM charge density accentuated this tendency. After the SERS efficiency towards the detection of model Raman molecule (4-ATP) was proven, AuNPs paper was upgraded by functionalizing the AuNPs with biotin/streptavidin assemblies for the detection of antibody-antigen binding. The modification of antibody local structure due to the interaction with antigen was detected. Evidence of antigen binding was elucidated from the SERS spectra, confirming the presence of antigen. Reproducible spectra features were observed for the functionalized AuNP papers which were exposed to different concentration of antigen; their intensity increased as a function of antigen concentration. By conducting these studies and merging these three length scales of research, a novel strategy to engineer AuNPs functionalized paper as a low-cost and generic SERS platform for bio-diagnostic applications was developed.

It has been shown that surface enhanced Raman spectroscopy (SERS) has many promising applications in ultrasensitive detection of Raman signal of substances. However, optimizing the enhancement in SERS signal for different applications typically requires several levels of fabrication of active plasmonic SERS substrates. In this paper, we report the enhancement of SERS signal of a single layer of graphene located on a plasmonic nano-Lycurgus cup array after placing water droplets on it. The experimental data shows that addition of water droplets can enhance the SERS signal of the single layer of graphene about 10 times without requiring any modifications to the nano-Lycurgus cup array. Using fullwave electromagnetic simulations, we show that addition of water droplets enhances the local electric field at the graphene layer, resulting in stronger light-graphene interaction at the excitation pump laser wavelength. We also show that the addition of water droplets on the graphene layer enables us to modify the band diagram of the structure, in order to enhance the local density of optical states at the Raman emission wavelengths of the graphene layer. Numerical calculations of both the excitation field enhancement at the location of the graphene layer, and the emission enhancement due to enhanced local density of optical states, support the experimental results. Our results demonstrate an approach to boost the SERS signal of a target material by controlling the band diagram of the active nanostructured SERS substrate through the use of fluidic dielectrics. These results could find potential applications in biomedical and environmental technologies.

Surface-enhanced Raman scattering of glyphosate on dispersed silver nanoparticles: A reinterpretation based on model molecules

The ability to confine light in small volumes, associated to low background signals, is an important technological achievement for a number of disciplines such as biology or electronics. In fact, decoupling the source position from the sample area allows an unprecedented sensitivity which can be exploited in different systems. The most direct implications are however related to either Surface Enhanced Raman Scattering (SERS) or Tip Enhanced Raman Scattering (TERS). Furthermore, while the combination with super-hydrophobic patterns can overcome the typical diffusion limit of sensors, focused surface plasmons decaying into hot electrons can be exploited to study the electronic properties of the sample by means of a Schottky junction. Within this paper these techniques will be briefly described and the key role played by both surface and localized plasmons will be highlighted. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.