Raman Scattering Spectra Research Articles

On the Measurements of the Surface-Enhanced Raman Scattering Spectrum: Effective Enhancement Factor, Optical Configuration, Spectral Distortion, and Baseline Variation

In this paper, a comprehensive theoretical framework for understanding surface-enhanced Raman scattering (SERS) measurements in both solution and thin-film setups, focusing on electromagnetic enhancement principles, was presented. Two prevalent types of SERS substrates found in the literature were investigated: plasmonic colloidal particles, including spherical and spheroid nanoparticles, nanoparticle diameters, and thin-film-based SERS substrates, like ultra-thin substrates, bundled nanorods, plasmonic thin films, and porous thin films. The investigation explored the impact of analyte adsorption, orientation, and the polarization of the excitation laser on effective SERS enhancement factors. Notably, it considered the impact of analyte size on the SERS spectrum by examining scenarios where the analyte was significantly smaller or larger than the hot spot dimensions. The analysis also incorporated optical attenuations arising from the optical properties of the analyte and the SERS substrates. The findings provide possible explanations for many observations made in SERS measurements, such as variations in relative peak intensities during SERS assessments, reductions in SERS intensity at high analyte concentrations, and the occurrence of significant baseline fluctuations. This study offers valuable guidance for optimizing SERS substrate design, enhancing SERS measurements, and improving the quantification of SERS detection.

Raman Scattering Spectra and DFT Computational Analyzes of Intermolecular Interactions in Trifluoroacetic and Its Solutions

In this work, the mechanisms of molecular clusters formation in liquid trifluoroacetic acid were studied using Raman scattering spectra in different solutions. The polarized components of Raman scattering spectra corresponding of the C=O, O–H stretching bands of pure trifluoroacetic acid consist of three broad bands at 1734, 1754, and 1800 cm−1 with different depolarization ratios. When the acid is strongly dissolved in acetonitrile, the 1800 cm−1 spectral band belonging to the C=O band remains. The intermolecular interactions in the formation of trifluoroacetic acid monomer, dimer, and trimer, as well as clusters with water [CF3COOH + (H2O)n, n = 1–7] and acetonitrile [CF3COOH + (CH3CN)n, n = 1–2] molecules, were analysed using the density functional theory (DFT) method.

The effect of temperature and excitation energy on Raman scattering in bulk

Raman scattering (RS) in bulk hafnium disulfide () is investigated as a function of temperature (5 K − 350 K) with polarization resolution and excitation of several laser energies. An unexpected temperature dependence of the energies of the main Raman-active (A and Eg) modes with the temperature-induced blueshift in the low-temperature limit is observed. The low-temperature quenching of a mode ω 1 (134 cm−1) and the emergence of a new mode at approx. 184 cm−1, labeled Z, is reported. The optical anisotropy of the RS in is also reported, which is highly susceptible to the excitation energy. The apparent quenching of the A mode at T = 5 K and of the Eg mode at T = 300 K in the RS spectrum excited with 3.06 eV excitation is also observed. We discuss the results in the context of possible resonant character of light-phonon interactions. Analyzed is also a possible effect of the iodine molecules intercalated in the van der Waals gaps between neighboring layers, which inevitably result from the growth procedure.

Nanoscale Surface-Enhanced Raman Spectroscopy Investigation of a Polyphenol-Based Plasmonic Nanovector.

The demand for next-generation multifunctional nanovectors, combining therapeutic effects with specific cellular targeting, has significantly grown during the last few years, pursuing less invasive therapy strategies. Polyphenol-conjugated silver nanoparticles (AgNPs) appear as potential multifunctional nanovectors, integrating the biorecognition capability and the antioxidant power of polyphenols, the antimicrobial activity of silver, and the drug delivery capability of NPs. We present a spectroscopic and microscopic investigation on polyphenol-synthesized AgNPs, selecting caffeic acid (CA) and catechol (CT) as model polyphenols and using them as reducing agents for the AgNP green synthesis, both in the presence and in the absence of a capping agent. We exploit the plasmonic properties of AgNPs to collect Surface-Enhanced Raman Scattering (SERS) spectra from the nanosized region next to the Ag surface and to characterize the molecular environment in the proximity of the NP, assessing the orientation and tunable deprotonation level of CA, depending on the synthesis conditions. Our results suggest that the SERS investigation of such nanovectors can provide crucial information for their perspective biomedical application.

Plasmonic nanorod probes' journey inside plant cells for in vivo SERS sensing and multimodal imaging.

Nanoparticle-based platforms are gaining strong interest in plant biology and bioenergy research to monitor and control biological processes in whole plants. However, in vivo monitoring of biomolecules using nanoparticles inside plant cells remains challenging due to the impenetrability of the plant cell wall to nanoparticles beyond the exclusion limits (5-20 nm). To overcome this physical barrier, we have designed unique bimetallic silver-coated gold nanorods (AuNR@Ag) capable of entering plant cells, while conserving key plasmonic properties in the near-infrared (NIR). To demonstrate cellular internalization and tracking of the nanorods inside plant tissue, we used a comprehensive multimodal imaging approach that included transmission electron microscopy (TEM), confocal fluorescence microscopy, two-photon luminescence (TPL), X-ray fluorescence microscopy (XRF), and photoacoustics imaging (PAI). We successfully acquired SERS signals of nanorods in vivo inside plant cells of tobacco leaves. On the same leaf samples, we applied orthogonal imaging methods, TPL and PAI techniques for in vivo imaging of the nanorods. This study first demonstrates the intracellular internalization of AuNR@Ag inside whole plant systems for in vivo SERS analysis in tobacco cells. This work demonstrates the potential of this nanoplatform as a new nanotool for intracellular in vivo biosensing for plant biology.

Study of Conductive Hydrogels Based on Xanthan and PEDOT PSS Using Raman Spectroscopy

This work is devoted to identification of the structural characteristic of poly(3,4-ethylene dioxy-thiophene) polystyrene sulfonate (PEDOT PSS) in the synthesized hydrogel based on xanthan. It is demonstrated that PEDOT PSS forms a stable structure with polysaccharides to a greater degree than with structure-forming polyvinyl alcohol (PVA). In this article, we also propose the technique of comparing the degree of oxidation of PEDOT PSS by means of Raman-scattering spectra, which makes it possible to identify and qualitatively evaluate the effects on the conductivity of the process of structural changes in the PEDOT chain, which can be useful in future studies of such materials.

Intermolecular Hydrogen Bond in Tertiary Butyl Alcohol and Its Solutions. Raman Scattering Spectra

It is found that the 750 cm–1 band in pure tertiary butyl alcohol shows attributes of a complex structure that can be interpreted as a result of the superposition of several closely located lines belonging to different aggregates of alcoholmolecules. As the alcohol is diluted with neutral solvents, the width of this band passes through a maximum that corresponds to the concentration of the mixture with comparable contents of various most stable aggregates in the solution. The performed quantum-chemical calculations confirm the complexity of the 750-cm–1 band of pure alcohol caused by the aggregation of molecules by means of hydrogen bonds. The calculated energy gain due to the formation of a hydrogen bond (dimer) amounts to 19.7 kJ/mole, and the length of the hydrogen bridge equals 2.068 Å.

Pressure-Driven Phase Transitions in Bulk HfS2

The effect of hydrostatic pressure up to 27 GPa on the Raman scattering (RS) in bulk HfS$_2$ is investigated. There are two transformations of RS spectra, which take place during compression at pressure between 5.7 GPa and 9.8 GPa as well as between 12.8 GPa and 15.2 GPa. Seven vibrational modes can be observed after the transformation, as compared to four modes before the transformation. The observed change suggests structural change in the material of yet unknown nature. The frequencies of the RS modes observed above the transformation change linearly with pressure and corresponding pressure coefficients have been determined. The other transition manifests itself as a change in the RS lineshape. While a series of well-defined RS modes are observed under pressure below the transition, broad spectral bands can be seen at higher pressure. The overall lineshape of the spectra resembles that of disordered materials. The lineshape does not change during decompression, which suggests permanent nature of the high-pressure transition.

Ag-decorated TiO2 Nanofibers for Surface-enhanced Raman Scattering

In this report, SERS substrates based on Ag-decorated TiO2 nanofibers electrospun on a glass substrate (Ag/TiO2/glass) were successfully prepared using electrospinning and photodeposition methods. XRD pattern demonstrated that the crystalline structure of TiO2 nanofibers (TiO2 NFs) consisted of both anatase and rutile phase. SEM images show that the randomly oriented on glass substrate TiO2 NFs’ diameters are in the range of 150 nm-300 nm and their lengths are up to a few micrometers. After 15-minute photodeposition, Ag NPs with average diameter of about a few tens of nanometers were uniformly and densely decorated on TiO2 NFs surface. The Ag/TiO2/glass SERS substrate was then immersed in 0.01 mM, 0.1 mM, and 1 mM 4-mercaptobenzoic acid (4-MBA) solution, and was afterwards investigated by UV-Vis absorption and Surface-enhanced Raman Scattering (SERS) spectra. The preliminary results show that the change in UV-Vis absorption corresponds well to the Raman peak intensity upon the adsorption of probed molecules. This might open up a pathway in the detection and identification of probed molecules based on colorimetric-sensing and SERS-sensing.

Anisotropic Optical and Vibrational Properties of GeS

The optical response of bulk germanium sulfide (GeS) is investigated systematically using different polarization-resolved experimental techniques, such as photoluminescence (PL), reflectance contrast (RC), and Raman scattering (RS). It is shown that while the low-temperature (T = 5 K) optical band-gap absorption is governed by a single resonance related to the neutral exciton, the corresponding emission is dominated by the disorder/impurity- and/or phonon-assisted recombination processes. Both the RC and PL spectra are found to be linearly polarized along the armchair direction. The measured RS spectra over a broad range from 5 to 300 K consist of six Raman peaks identified with the help of Density Functional Theory (DFT) calculations: A, A, A, A, B, and B, which polarization properties are studied under four different excitation energies. We found that the polarization orientations of the A and A modes under specific excitation energy can be useful tools to determine the GeS crystallographic directions: armchair and zigzag.

Mechanisms of the Formation of the ν3 Band Contour in Absorption and Raman Scattering Spectra of Tetrafluoromethane in Condensed Phases

Mechanisms of the Formation of the ν3 Band Contour in Absorption and Raman Scattering Spectra of Tetrafluoromethane in Condensed Phases

A Semi-quantitative method for the detection of fentanyl using surface-enhanced Raman scattering (SERS) with a handheld Raman instrument.

A handheld, spatially offset Raman spectroscopy (SORS) system was successfully used to obtain Surface-enhanced Raman Scattering (SERS) spectra of fentanyl under simulated field conditions. A series of aqueous fentanyl solutions were prepared with commercially available gold nanoparticle solution, at concentrations ranging from 0.003 to 1697μM. These SERS spectra were then used to generate two concentration calibration models (via a plot of peak area (1026cm-1 ) versus concentration, and quantitative spectral decomposition using partial least squares (PLS1)). For both models, the relationship followed Langmuir adsorption and became non-linear at concentrations above ~0.2μM, with a limit of detection (LOD) of approximately 3nM. The same technique was successfully used to measure fentanyl in the presence of two common "cutting agents," heroin and glucose, at 1% and 2% fentanyl proportions (w/w). Fentanyl detection was successfully achieved, but mixture interference from the cutting agents prevented a calibration model being generated. Four fentanyl analogues were also investigated-butyrylfentanyl, furanylfentanyl, acetylfentanyl, and ocfentanyl. A concentration calibration model for each species was successfully generated, but differentiation from fentanyl proved more challenging, although several potential diagnostic peaks were identified. These results identified a pathway forward in using handheld equipment for the reliable detection of ultra-low concentrations of fentanyl and fentanyl analogues via SERS, even when mixed with diluents. However, quantitative detection is negatively impacted in the presence of heroin and glucose. This also provides a starting point for a SERS-based spectral library of fentanyl analogues, in combination with a range of different diluents.

Resonance Raman Scattering Spectra of Co(II)- and Cu-5,10,15,20-Tetrakis[4-(N-methylpyridyl)]porphyrin in the dd Excited State and Mechanisms of Its Deactivation in a Solution and in Complexes with DNA

Resonance Raman scattering (RRS) spectra of the complexes of 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin with Co(II) and Cu(II) (CoIITMpyP4 and CuTMpyP4) in various solvents and in complexes with DNA are studied. Under nanosecond pulsed excitation, additional lines have been found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity and in a complex with DNA. At the same time, such lines are absent in the spectra of CoIITMpyP4 in pure DMF, dimethyl sulfoxide, water, and alcohols under the same excitation conditions. To interpret the experimental data, we have calculated the structure and vibrations of the solvate complexes of CoIITMpyP4 and CuTMpyP4 with water, methanol, and formic acid in the ground and in excited states. Based on the obtained data, additional lines in the Raman spectra are assigned to the dd excited state corresponding to the d(z2) → d(x2 – y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in rate constant kic of the internal conversion due to an increase in energy gap ΔE between the ground and dd states of CoIITMpyP4. A decrease in the kic value in the case of binding of CoIITMpyP4 and CuTMpyP4 to DNA is caused by the additional interaction of the extra-ligand water molecule with one of its bases directly or via several water molecules immobilized on DNA surface. This complicates the conformational rearrangement of the water molecule in the dd excited state, which gives rise to an increase in the ΔE value.

In-depth distribution of elements and chemical bonds in the surface region of calcium-doped diamond-like carbon films

In the present work we determined surface composition and chemical bonds at the top surface and in a shallow sub-surface region of the calcium-doped diamond-like carbon (DLC) layers using angular-resolved photoelectron spectroscopy (ARXPS) aided by the Maximum Entropy Method (MEM). The resulting non-destructive depth profiles showed that the composition and the chemical bonding vary significantly on the way from the top surface to the shallow sub-surface region. We demonstrate, for the first time, that Ca doping induced (i) the accumulation of the calcium carbonate, CaCO3, at the surface, while the calcium oxide, CaO, is locatedin a deeper sub-surface region, (ii) structural conversion from the C sp2-rich top surface observed in the undoped DLC layer to the C sp3-rich top surface of the Ca-doped DLC samples, and (iii) conversion in the opposite direction which occurred in a sub-surface region. Our conclusions are consistent with Raman and Ion Surface Scattering spectra. Obtained results can be useful for deeper understanding the interaction between doped DLC layer surfaces with surroundings and particularly with living tissue.

Cubic shaped hematite (α-Fe2O3) micro-structures composed of stacked nanosheets for rapid ethanol sensor application

Herein, the synthesis, characterization, and ethanol gas sensing properties of hematite iron oxide(α-Fe2O3) microstructures were investigated. Detailed examinations using different techniques confirmed the high-density growth and rhombohedral crystal structure of the synthesized Fe2O3 micro-structures with an average crystal size of 38.29 nm. The appearance of several Raman-active A1g(1), A1g(2), and Eg modes in Raman-scattering spectrum further revealed the formation of the hematite phase. The sensor device was fabricated using Fe2O3 micro-structures and tested for various gases such as ethanol, carbon monoxide (CO), and hydrogen (H2). It was observed that the fabricated sensor demonstrated a high response of 13.1for 100 ppm ethanol gas concentration at a temperature of 400 °C. Including the response and recovery times for the fabricated sensors, the transient response for various gases was also recorded and analyzed. The fabricated sensor exhibited a rapid gas response at 400 °C for ethanol gas (13.1) compared to CO (1.95) and H2 (1.71) gases for 100 ppm gas concentrations. A plausible ethanol sensing mechanism was also proposed and presented.

Analysis of fluctuations in the Raman spectra of suspended and supported graphene films

The repeatability of the Raman scattering (RS) spectra was analyzed in monolayer and bilayer CVD-grown graphene films. Position and width of the two main RS lines (G-line and 2D-line) together with their intensity ratio I2D/IG were measured in samples where graphene film was supported by a SiO2/Si substrate and where the film was suspended over pits in the substrate. The results are presented as histograms averaged by a Gaussian with the width W, considered as a measure of non-repeatability. Relatively large fluctuations in the I2D/IG value and linewidth were observed upon repeated measurements at the same point both in supported and suspended samples. The RS measurements in crystalline graphite and mechanically exfoliated graphene deposited on hexagonal boron nitride showed higher repeatability which means that increased W in CVD-grown films reflects the sensitivity of RS processes in these films to changes in charge and deformation. In bilayer CVD samples, the values of W are higher due to additional fluctuations in the twist angle between two misoriented layers. The average position of the RS lines is shifted in opposite directions and makes it possible to distinguish between bilayer and monolayer films, despite the similarity in the RS spectra.

The optical and high-pressure raman scattering study of La(Fe,Mo)0.5O3 ceramics

This study was focused on the optical-vibrational analysis and high-pressure Raman-scattering spectroscopy of La(Fe,Mo)0.5O3 ceramics at room temperature. This material was fabricated by using the sol-gel method. The element composition was checked by X-ray Fluorescence confirming the atomic ratio of La, Fe and Mo. Fourier Transform Infrared spectroscopy confirmed the (Fe/Mo −O) bonding vibration which is typical perovskite structure. The band gap energy investigated by UV-Vis spectra was found to be 2.03 eV. At room temperature, the Raman scattering spectra under high pressure was characterized by the Raman-scattering spectrum in the range of 0.90 to 16.7 GPa. The high-pressure Raman scattering confirmed the structural transition after the pressure surpasses 7.0 GPa. From this phenomena, it well indicates the lattice structure stability is still preserved under 7.0 GPa.

Phonon softening and higher-order anharmonic effect in the superconducting topological insulator SrxBi2Se3

We report the anharmonic effect of the Raman-scattering spectrum in the low-carrier density superconductor SrxBi2Se3, which is dominated by the quartic term. Compared to the parent Bi2Se3, the superconducting SrxBi2Se3 crystals show obvious phonon softening in the three optical phonon modes , , and . Based on the simulations by the Fano function, we present compelling evidence of the enhanced electron–phonon coupling in SrxBi2Se3 for its smaller asymmetric parameter q. Moreover, an anomalous broadening and intensity antiresonance in the characteristic Raman peaks were observed as the temperature decreased to around 160 K. Thus, the superconducting topological material SrxBi2Se3 represents a counterintuitive example where the electron–phonon interaction is strengthened by the accumulation of electron carriers.

Near-Ir Raman Spectroscopy of Graphitic B–C–N Materials

Raman scattering (RS) by graphitic phases BCxN (g-BCxN), obtained by different methods, was studied using an excitation laser operating in the near-IR (NIR) range. With such excitation the most characteristic peaks for g-BCxN are 1300 cm–1 (D-peak) and 1505 cm–1 (G-peak). It is shown that the D- and G-peaks are associated with disordered graphite: the D-peak is observed because the selection rules change, and its intensity is correlated with the size of the graphite cluster sp2 in a polycrystalline graphite; the G-band is associated with thegraphite mode that is RS-active in the E2g plane. The shift of the D-peak in g-BCxN (1300 cm–1 ) as compared with its position in graphite (1355 cm–1 ) can be explained by the positional dispersion of the D-peak with excitation wavelength. The RS line centered at 1260 cm–1, which could be associated with a RS-active mode, is observed only for samples with high crystallinity and for RS spectra obtained at high pressures and high temperatures.

The effect on the optical modes of quaternary chalcogenides upon metal and chalcogen substitution

Raman-scattering spectra of polycrystalline Cu2ZnSnSe4, Ag2ZnSnSe4, and Cu2CdSnTe4 were investigated in order to investigate the vibrational modes upon substitution on the metal and chalcogen sites. The non-symmetric modes shift dramatically with metal substitution in the kesterite materials, with Ag2ZnSnSe4 modes shifting by 50 cm−1 in comparison with that of Cu2ZnSnSe4. Tellurium substitution results in a lower frequency of the symmetric modes. Structural data are employed to elucidate our results. These results are intended to provide further insight into these materials as they continue to be of interest for potential thermoelectric and photovoltaic applications.