Raman Scattering Cross Sections Research Articles

Photoinduced Electron-Transfer-Mediated Differential Recognition of Proteins on Plasmonic Surfaces.

Photoinduced enhanced Raman spectroscopy (PIERS) has emerged as an efficient technique for enhancing the vibrational modes of analyte molecules adsorbed on a plasmonic nanoparticle-semiconductor hybrid material through chemical enhancement governed by electron transfer from the semiconductor to the plasmonic nanoparticles under an additional ultraviolet (UV) preirradiation step. The increase in chemical enhancement is imperative in analyzing and detecting pharmaceutically important moieties, such as amino acids and proteins, with a low Raman scattering cross section, even in complex biological environments. Herein, we demonstrate that UV preirradiation induced the creation of additional oxygen vacancies by introducing a low concentration (≈1%) of Ni as a dopant in the 2D platelike morphology of the BiOCl semiconductor; i.e., defect states in the semiconductor can induce charge transfer from the semiconductor to the plasmonic nanoparticles. This phenomenon facilitates electron transfer to the adsorbed analyte on the plasmonic surface. Additionally, we have shown the usefulness of this method in protein immobilization on the substrate surface, followed by the identification of a specific protein in the mixture of proteins. Proteins containing cysteine residues capture these electrons to form a surface-bound thiol group via a transient disulfide electron adduct radical. This allows differential binding of the protein molecules to the semiconductor plasmonic hybrid depending on the concentration of surface cysteine residues in proteins. Through PIERS and principal component analysis, we demonstrate the possibility of probing and distinguishing biomolecules based on their surface composition and secondary structure components even in their mixtures, thus paving the way for efficient analysis of complex biological systems.

Beyond the Charge Transfer Mechanism for 2D Materials-Assisted Surface Enhanced Raman Scattering.

Two-dimensional (2D) materials have been extensively implemented as surface-enhanced Raman scattering (SERS) substrates, enabling trace-molecule detection for broad applications. However, the accurate understanding of the mechanism remains elusive because most theoretical explanations are still phenomenological or qualitative based on simplified models and rough assumptions. To advance the development of 2D material-assisted SERS, it is vital to attain a comprehensive understanding of the enhancement mechanism and a quantitative assessment of the enhancement performance. Here, the microscopic chemical mechanism of 2D material-assisted SERS is quantitatively investigated. The frequency-dependent Raman scattering cross sections suggest that the 2D materials' SERS performance is strongly dependent on the excitation wavelengths and the molecule types. By analysis of the microscopic Raman scattering processes, the comprehensive contributions of SERS can be revealed. Beyond the widely postulated charge transfer mechanisms, the quantitative results conclusively demonstrate that the resonant transitions within 2D materials alone are also capable of enhancing the molecular Raman scattering through the diffusive scattering of phonons. Furthermore, all of these scattering routines will interfere with each other and determine the final SERS performance. Our results not only provide a complete picture of the SERS mechanisms but also demonstrate a systematic and quantitative approach to theoretically understand, predict, and promote the 2D materials SERS toward analytical applications.

SERS-Based Hydrogen Bonding Induction Strategy for Gaseous Acetic Acid Capture and Detection.

Surface-enhanced Raman scattering (SERS) can overcome the existing technological limitations, such as complex processes and harsh conditions in gaseous small-molecule detection, and advance the development of real-time gas sensing at room temperature. In this study, a SERS-based hydrogen bonding induction strategy for capturing and sensing gaseous acetic acid is proposed for the detection demands of gaseous acetic acid. This addresses the challenges of low adsorption of gaseous small molecules on SERS substrates and small Raman scattering cross sections and enables the first SERS-based detection of gaseous acetic acid by a portable Raman spectrometer. To provide abundant hydrogen bond donors and acceptors, 4-mercaptobenzoic acid (4-MBA) was used as a ligand molecule modified on the SERS substrate. Furthermore, a sensing chip with a low relative standard deviation (RSD) of 4.15% was constructed, ensuring highly sensitive and reliable detection. The hydrogen bond-induced acetic acid trapping was confirmed by experimental spectroscopy and density functional theory (DFT). In addition, to achieve superior accuracy compared to conventional methods, an innovative analytical method based on direct response hydrogen bond formation (IO-H/Iref) was proposed, enabling the detection of gaseous acetic acid at concentrations as low as 60 ppb. The strategy demonstrated a superior anti-interference capability in simulated breath and wine detection systems. Moreover, the high reusability of the chip highlights the significant potential for real-time sensing of gaseous acetic acid.

Development of a universal method for vibrational analysis of the terminal alkyne C≡C stretch.

The terminal alkyne C≡C stretch has a large Raman scattering cross section in the "silent" region for biomolecules. This has led to many Raman tag and probe studies using this moiety to study biomolecular systems. A computational investigation of these systems is vital to aid in the interpretation of these results. In this work, we develop a method for computing terminal alkyne vibrational frequencies and isotropic transition polarizabilities that can easily and accurately be applied to any terminal alkyne molecule. We apply the discrete variable representation method to a localized version of the C≡C stretch normal mode. The errors of (1) vibrational localization to the terminal alkyne moiety, (2) anharmonic normal mode isolation, and (3) discretization of the Born-Oppenheimer potential energy surface are quantified and found to be generally small and cancel each other. This results in a method with low error compared to other anharmonic vibrational methods like second-order vibrational perturbation theory and to experiments. Several density functionals are tested using the method, and TPSS-D3, an inexpensive nonempirical density functional with dispersion corrections, is found to perform surprisingly well. Diffuse basis functions are found to be important for the accuracy of computed frequencies. Finally, the computation of vibrational properties like isotropic transition polarizabilities and the universality of the localized normal mode for terminal alkynes are demonstrated.

Quantitative studies on the microstructure evolution and its impact on the viscosity of a molten Al2O3-Na3AlF6 system by Raman spectroscopy and theoretical simulations

The microstructure of the molten alumina-cryolite system is an important aspect in the dissolution behavior of alumina (α-Al2O3) in cryolite (Na3AlF6) molten salt and impacts the physical properties of the molten system. In this work, the symmetric stretching vibrational wavenumbers of aluminum nonbridging fluorine (AlFx3-x) and aluminum-oxygen-fluorine (Al2OFy4-y, and Al2O2Fz2-z) bonds in the high wavenumber range, as well as their corresponding Raman scattering cross sections (RSCS), were first analyzed and determined by quantum chemistry (QC) ab initio calculations. The species in binary Al2O3-Na3AlF6 melts with varying Al2O3 contents from 0 to 6.0 wt% were then qualitatively and quantitatively analyzed using in situ high-temperature Raman spectroscopy in conjunction with first principles calculation from room temperature to melt. The species of AlF4-, AlF52- and AlF63- were found to be present and predominant in the melts with Al2O3 contents ranging from 0 to 6.0 wt%. The content of Al2OF62- species tended to increase with Al2O3 content from 0 to 3.0 wt%, while Al2O2F42- species only existed in the melts with an Al2O3 concentration of more than 3.0 wt%. Finally, the abundance of various species in the molten Al2O3-Na3AlF6 system was further correlated to the melt viscosity. The contribution to viscosity was primarily determined by the distribution of AlFx3-x (x = 4, 5, and 6) and Al2OF62- species, and the contribution ability of corresponding species to viscosity was Al2OF62-, AlF63-, AlF52- and AlF4- from largest to smallest.

Ag-Au-Cu Trimetallic Alloy Microflower: A Highly Sensitive SERS Substrate for Detection of Low Raman Scattering Cross-Section Thiols.

Trimetallic Ag-Au-Cu alloy microflowers (MFs) with various surface compositions were synthesized on a glass coverslip and used as efficient surface-enhanced Raman spectroscopy (SERS) substrates for highly sensitive label-free detection of smaller Raman scattering cross-section molecules, namely, L-cysteine and toxic thiophenols. MFs of different compositions were synthesized via appropriate mixing of metal-alkyl ammonium halide precursors followed by a single-step thermolysis at 350 °C. While the Ag percentage was kept constant at 90% for all the substrates, the composition of Au and Cu was varied between 1 and 9% sequentially. The synthesized MFs were thoroughly characterized by using field emission scanning electron microscopy (FE-SEM), wide-angle X-ray scattering, X-ray photoelectron spectroscopy (XPS), and X-ray fluorescence techniques. FE-SEM studies revealed that the MFs were present throughout the substrate, and the average size varied from 20 to 40 μm. XPS studies showed that the top surface of the alloy substrates was rich in either Au or Cu atoms, while Ag remained underneath. The performance of the trimetallic MFs as SERS substrates was evaluated using Rhodamine 6G as a probe molecule, which showed that the MFs with Ag-Au-Cu compositions 90-7-3 and 90-3-7 were found to be the best and of equal SERS efficiency. The SERS enhancement factor (EF) of both these MFs was found to be the same, approximately 9 × 107, when calculated using 1,2,3-benzatriazole as the probe molecule. Between the two, the trimetallic substrate with a higher Au percentage (Ag-Au-Cu as 90-7-3) was used for the sensitive SERS-based detection of thiols to exploit the strong Au-S binding interaction. By virtue of the high EF of the substrate, the inherently low Raman scattering cross-sections of the probe molecules were greatly enhanced in SERS mode. The 'limit of quantification (LOQ)' values were found to be 1 nM for aliphatic L-Cysteine and 1-0.1 pM for aromatic thiols using the trimetallic SERS sensor.

Comparison of Raman spectral characteristics and quantitative methods between 13CH4 and 12CH4 from 25 to 400 °C and 50 to 400 bar

In recent years, increasing attention has been given to quantifying the isotopic compositions of gases by Raman spectroscopy. However, related research on the carbon isotopes of CH4 is still lacking. In this study, the Raman spectral characteristics of 13CH4 and 12CH4 in the pure CH4 system and in the CH4–H2O system are comprehensively studied at temperatures ranging from 25 to 400 °C and pressures ranging from 50 to 400 bar. For the pure CH4 system, the peak positions of the symmetric stretching band (ν1) of 13CH4 and 12CH4 both shift to a higher wavenumber with increasing temperature and decreasing pressure, and their full width at half maximum (FWHM) values can reach 4.7 cm−1 and 5.5 cm−1, respectively, with increasing temperature and density. The peak position of the ν1 band of 13CH4 is always 4.6–5.1 cm−1 lower than that of 12CH4 under the same conditions, indicating that 13CH4 and 12CH4 can be distinguished by peak position and FWHM in the Raman spectra. Although the calculated Raman shifts (ν0) at zero-density for 13CH4 and 12CH4 are different, the unified equation for determining the density of CH4-rich fluid inclusions is still applicable, as long as to bracket the sample measurement with measurements of reference standards. For the CH4–H2O system, the peak position of the ν1 band of dissolved 13CH4 is approximately 2.5 cm−1 lower than that of 12CH4, while their FWHM values can be more than 12.2 cm−1, causing more difficulty in distinguishing them in aqueous solutions. In addition, Raman quantitative calibration models of 12CH4 and 13CH4 in the CH4–H2O system are established. The difference in the Raman scattering cross section (RSCS) of dissolved 12CH4 and 13CH4, rather than changes in the molar density or RSCS of water is the main reason leading to the difference in the Raman quantitative calibration models of CH4 concentrations between the 12CH4–H2O system and the 13CH4–H2O system. Our work provides a good reference for the in situ identification and quantification of the carbon isotopic composition of CH4 in high-temperature and high-pressure environments.

Broadband Two-Photon Absorption Spectroscopy with Stimulated Raman Scattering as an Internal Standard.

Two-photon absorption (2PA) spectroscopy provides valuable information about the nonlinear properties of molecules. In contrast with single-wavelength methods, broadband 2PA spectroscopy using a pump-probe approach gives a continuous 2PA spectrum across a wide range of transition energies without tuning the excitation laser. This contribution shows how stimulated Raman scattering from the solvent can be used as a convenient and robust internal standard for obtaining accurate absolute 2PA cross sections using the broadband approach. Stimulated Raman scattering has the same pump-probe overlap dependence as 2PA, thus eliminating the need to measure the intensity-dependent overlap of the pump and probe directly. Eliminating the overlap represents an important improvement because intensity profiles are typically the largest source of uncertainty in the measurement of absolute 2PA cross sections using any method. Raman scattering cross sections are a fundamental property of the solvent and therefore provide a universal standard that can be applied any time the 2PA and Raman signals are present within the same probe wavelength range. We demonstrate this approach using sample solutions of coumarin 153 in methanol, DMSO, and toluene, as well as fluorescein in water.

Quantitative studies on the microstructure of ternary CaO–Al2O3–SiO2 glasses by Raman spectroscopy, 27Al MAS NMR and quantum chemistry ab initio calculation

To investigate the structural behaviors of Al3+ in aluminosilicate systems, the microstructural characteristics of CaO–SiO2-based glassy samples with various Al2O3 contents were examined quantitatively by Raman spectroscopy and 27Al MAS NMR. A sequence of multiple model clusters of aluminosilicate systems modified with Ca2+ and Na ​+ ​cations was designed. Then quantum chemistry (QC) ab initio calculations were performed for geometric optimization, and Raman spectral simulations were carried out. The functional relationship between the Raman scattering cross section (RSCS) and stress index of silicon-oxygen tetrahedron (SIT) for aluminosilicates was established, which was successfully applied to calibrate experimental Raman spectra. Some fivefold coordinated aluminum (AlⅤ, approximately 5%) and less than 2% sixfold coordinated aluminum (AlⅥ) were detected by 27Al MAS NMR, while most aluminum remained in tetrahedral sites (AlⅣ). The ever-finer quantitative results of Raman spectroscopy and NMR showed a gradual production of AlⅣ with the addition of Al2O3, along with a significant adjustment in Qi species distribution. Specifically, Q1, Q2 decreased, fully polymerized Q4 increased and Q3 showed a nonmonotonic variation and obtained the maximum at Al2O3 ​= ​12 ​mol%. Furthermore, the effects of aluminum on bridging oxygen bond types (T-Ob, T ​= ​Si, Al) and the degree of polymerization were discussed in detail. These structural features related to composition are essential theoretic foundations for understanding the properties of these systems.

Estimation of the proportions of aluminum-fluorine species in NaF-AlF3 mixtures by in situ Raman spectroscopy and theoretical simulations

The quantitative distribution of different types of clusters in a binary NaF-AlF3 molten salt system with a series of cryolite ratios (CR, NaF/AlF3 molar ratio) was explored by in situ high-temperature Raman spectroscopy in conjunction with quantum chemistry (QC) ab initio calculations. Density functional theory (DFT) was used to simulate the Raman spectra of solid crystalline Na5Al3F14 (CR = 1.67) and Na3AlF6 (CR = 3.00). Aluminum-fluorine molecular model clusters containing species with typical characteristic microstructures were built in a molten NaF-AlF3 system. Deconvolution of the major stretching vibrational bands of Al-F-Al and Al-F bonds was carried out by using the Voigt function. The anionic fractions of different species present in the molten NaF-AlF3 system were quantitatively analyzed by a function of the microstructure dependent on Raman scattering cross sections (RSCS). AlF63-, AlF52-, and AlF4- were the dominant species in the aluminum-fluorine molten salt system. As the NaF in the melt increased, the content of AlF52- reached the maximum concentration when CR = 2.00. Apart from the recognized existence of both AlF4- and AlF63-, the aluminum bridging fluorine species of Al2F7- has been proven to exist in molten systems when CR ≤ 1.00. The relationship between the viscosity and various (n-3)Na+·AlFn(n-3)- species distributions has been investigated. The results showed that the contribution of different (n-3)Na+·AlFn(n-3)- to viscosity was essentially the same, and simply depended on the number of different species.

Absolute Stimulated Raman Cross Sections of Molecules

Stimulated Raman scattering (SRS) is a fundamental optical process that was discovered more than 60 years ago. While the early SRS spectroscopy studies have provided valuable insights into materials systems, the advent of SRS microscopy has launched a rapidly growing field in biological imaging. However, a fundamental understanding of the molecular response under SRS is still lacking. Herein we present a new framework to introduce molecule-intrinsic stimulated Raman scattering cross sections, σSRS, in the unit of Göppert-Mayer (GM). The absolute SRS cross sections determined for real molecular systems challenge the conventional wisdom that Raman spectroscopy is always a weak process. The enormous rate acceleration of SRS, captured by an apparent SRS cross section, stems from a synergistic effect between the field and the molecule. Our new framework goes beyond the conventional optics-centric view and presents a molecule-inclusive perspective, thus offering a comprehensive foundation for the future growth of SRS spectroscopy and microscopy.

Quantitative Mineralogy of Planetary Silicate Ternary Mixtures Using Raman Spectroscopy

AbstractA 532‐nm‐excited lunar Raman spectrometer (LRS) has been selected as a scientific payload of the Chang'e‐7 mission, exploring mineralogy assemblages in the lunar south polar region. However, the quantification of dark‐colored silicate minerals via Raman spectroscopy is an urgent requirement for upcoming Raman applications in future lunar and planetary explorations. Therefore, we conducted detailed laboratory studies on the Raman quantification of lunar silicate minerals using ternary mixtures of feldspar, olivine, and augite. Quantitative models were established employing the observed linear relationship between Raman integrated intensities and mineral proportions. The significant correlation coefficients (>0.94) and small RMSE (≤4.20 wt.%) confirmed the performance of these models. A series of methods (multipoint sampling, multispectral averaging, peak area extraction, and spectral parameter ratios) were jointly used to ensure that the models were not significantly affected by crystal orientation, chemical inhomogeneity, and instruments. Factors (σ2/σ1) describing the relative Raman scattering cross sections were introduced to calibrate the Raman counts. Our results indicated that the relative Raman scattering efficiency of feldspar, olivine, and augite is 1.4:2.4:1, which can be used to improve the quantitative accuracy of the point‐counting method if polymineralic mixing spectra are dominant. The models were validated across different samples using laboratory mixtures and lunar soil (CE5C0600). The lithology of the Chang'e‐5 soils is basaltic/gabbroic according to the quantitative mineralogy returned from our models that is consistent with the results from traditional methods. This research will be of particular significance for accurately determining the mineral abundances for Chang'e‐7 and other planetary missions. As such, crucial information can be inferred to understand the geological evolution of the exploration regions.

Monolayer Iron Oxychloride with a Resonant Band Structure for Ultrasensitive Molecular Sensing.

Two-dimensional layered materials (2DLMs) are expected to be next-generation commercial sensors for surface-enhanced Raman scattering (SERS) sensing owing to their unique structural features and physicochemical properties. The low sensitivity and poor universality of 2DLMs are the dominant barriers toward their practical applications. Herein, we report that monolayer iron oxychloride (FeOCl) with a naturally suitable band structure is a promising candidate for ultrasensitive SERS sensing. The generally boosted Raman scattering cross section of different analyte-FeOCl systems benefits from the resonant photoinduced charge transfer processes and strong ground-state interactions. In addition, the strong adsorption ability of monolayer FeOCl is crucial for rapid detection in practical applications, which is proven to be much better than those of conventional SERS sensors. Consequently, monolayer FeOCl enables diverse SERS applications, including multicomponent analysis, chemical reaction monitoring, and indirect ion sensing.

Chemical conjugation to differentiate monosaccharides by Raman and surface enhanced Raman spectroscopy.

Sugars play important roles in numerous biological processes, from providing energy to modifying proteins to alter their function. Glycosylation, the attachment of a sugar residue to a protein, is the most common post translational modification. Identifying the glycans on a protein is a useful tool both for pharmaceutical development as well as probing the proteome and glycome further. Sugars, however, are difficult analytes to probe due to their isomeric nature. In this work, Raman spectroscopy and surface enhanced Raman spectroscopy (SERS) are used to identify different monosaccharide species based on the vibrational modes of these isomeric analytes. The weak scattering of the sugars was overcome through conjugation with phenylboronic acid to provide a larger Raman scattering cross section and induce slight changes in the observed spectra associated with the structure of the monosaccharides. Spontaneous Raman, SERS in flow, and static SERS detection were performed in order to discriminate between arabinose, fructose, galactose, glucose, mannose, and ribose, as well as provide a method for identification and quantification for these sugar conjugates.

Thermodynamics of trans/gauche conformational equilibria and vibrational spectra of 1,2-dihaloethanes in various media.

The recently developed efficient protocols to implicit [Grimme et al., J. Phys. Chem. A 125, 4039-4054 (2021)] and explicit quantum mechanical modeling of non-rigid molecules in solution [Katsyuba et al., J. Phys. Chem. B 124, 6664-6670 (2020)] are used to describe conformational equilibria of 1,2-dichloroethane and 1,2-dibromoethane in various media. Two approaches for evaluation of trans/gauche free energy differences, ΔGt-g, are compared: (a) direct ΔGt-g computation in implicit solution; (b) the use, together with experimental intensities, of infrared absorption coefficients and Raman scattering cross sections computed for each explicitly modeled solution. The same cluster model of a solute surrounded by the first solvation shell of solvent molecules was used to simulate both Raman and IR spectra. The good agreement between the two approaches indicates the reliability of both methods. The importance of using correct absorption coefficients and Raman scattering factors for each medium is discussed. The ΔGt-g estimates from both implicit and explicit solvation simulations were combined with experimentally measured enthalpy differences ΔHt-g available in the literature to obtain condensed-state ΔSt-g estimates.

Efficient SERS Response of Porous-ZnO-Covered Gold Nanoarray Chips to Trace Benzene-Volatile Organic Compounds.

Fast and sensitive detection of gaseous volatile organic compounds (VOCs), based on surface-enhanced Raman spectroscopy (SERS), is still a challenge due to their weak interaction with plasmonic metals and overly small Raman scattering cross sections. Herein, we propose a simple strategy to achieve the SERS-based highly efficient detection of trace benzene-VOCs (B-VOCs) based on a composite chip. The composite chip is designed and fabricated via covering the porous zinc oxide on gold nanoarrays by a one-step solution growth method. Such composite chip shows highly selective capture of gaseous B-VOCs (benzene, toluene, nitrobenzene, xylene, and chlorobenzene, etc.), which leads to the rapid and sensitive SERS responses to them. Typically, this chip can response to gaseous toluene within 30 s, and the lowest detectable concentration is below 10 ppb. Further experiments have revealed that there exists an optimal thickness of the ZnO covering layer for the highly efficient SERS response to the B-VOCs, which is about 150 nm. Also, such a composite chip is recoverable in SERS response and hence reusable. The highly efficient SERS response of the composite chip to the B-VOCs is attributed to the porous structure-enhanced molecular adsorption and the electromagnetic-chemical dual-enhancement mechanism. This work not only presents a practical SERS chip for the efficient detection of the typical B-VOCs but also provides a deep understand the interaction between the B-VOCs and the ZnO as well as the chemical enhancement mechanism.

Thermoelectric Field-Assisted Raman Scattering and Photocatalysis with GaN-Plasmonic Metal Composites

The combination of metallic nanoparticles (NPs) with semiconductors used as surface-enhanced Raman scattering (SERS) substrates have been widely reported. However, the additional enhancements provided by the semiconductors are impressively small and have little effect on the SERS signal compared with that from the metallic NPs alone. Herein, thermoelectric semiconductor material gallium nitride (GaN) and silver nanoparticles (Ag NPs) are combined to create an electric-field-induced SERS (E-SERS) substrate, which further improves the SERS signal intensity by an order of magnitude compared to that without electric field induction. Based on the chemical enhancement induced by the thermoelectric potential, the presented E-SERS substrate realizes the detection over a broad kind of molecules, even with small Raman scattering cross sections. We show that the thermoelectric potential could regulate the charge exchange between GaN and Ag NPs and then shift the Fermi level of the Ag NPs over a wide-ranging distribution, which could increase the resonant electron transition probabilities with the detected molecules. Furthermore, the E-SERS substrate is also realized to monitor and manipulate the plasmon-activated redox reactions. Based on the finite element calculations, a detailed and comprehensive theoretical analysis is conducted to deepen the understanding of the chemical SERS and plasmon-activated photocatalyst mechanism.

Probing emergent QED in quantum spin ice via Raman scattering of phonons: Shallow inelastic scattering and pair production

We present an unconventional mechanism for Raman scattering of phonons, which is based on the linear magnetoelastic coupling present in non-Kramers magnetic ions. This provides a direct coupling of Raman-active phonons to the magnet's quasiparticles. We propose to use this mechanism to probe the emergent magnetic monopoles, electric charges, and photons of the emergent quantum electrodynamics (eQED) of the U(1) quantum spin liquid known as quantum spin ice. Detecting this eQED in candidate rare-earth pyrochlore materials, or indeed signatures of topological magnetic phases more generally, is a challenging task. We show that the Raman scattering cross section of the phonons directly yields relevant information, with the broadening of the phonon linewidth, which we compute, exhibiting a characteristic frequency dependence reflecting the two-particle density of states of the emergent excitations. Remarkably, we find that the Raman linewidth is sensitive to the details of the symmetry fractionalization and hence can reveal information about the projective implementation of symmetry in the quantum spin liquid, thereby providing a diagnostic for a $\ensuremath{\pi}$-flux phase. The Raman scattering of the phonons thus provides a useful experimental tool to probe the fractionalization in quantum spin liquids that turns out to closely mirror pair production in quantum electrodynamics and the deep inelastic scattering of quantum chromodynamics. Indeed, the difference to the latter is conceptual more than technical: the partons (quarks) emerge from the hadrons at high energies due to asymptotic freedom, while those in eQED arise from fractionalization of the spins at low energies.

Carrier Injection Observed by Interface-Enhanced Raman Scattering from Topological Insulators on Gold Substrates.

The electron-phonon interaction at the interface between topological insulator (TI), namely, Bi2Se3 and Bi2Te3 two-dimensional (2D) nanoflakes, to a gold substrate as a function of TI flake thickness is studied by means of Raman scattering. We reveal the presence of interface-enhanced Raman scattering and a strong phonon renormalization induced by carriers injected from the gold substrate to the topological surface in contact. We derive the change of the electron-phonon coupling showing a nearly linear behavior as a function of layer thickness. The strongly nonlinear change of the Raman scattering cross section as a function of flake thickness can be associated with band bending effects at the metal-TI interface. Our results provide spectroscopic evidence for a strongly modified band structure in the first few quintuple layers of Bi2Se3 and Bi2Te3 in contact with gold.

(Invited) Kinetics and Thermodynamics of Swcnts and Bnnts Encapsulation with α-Sexithiophene in Liquid Phase

Single-Walled Carbon Nanotubes (SWNTs) and Boron Nitride Nanotubes (BNNTs) come with hollow 1D centers, which can serve to define narrow spaces for encapsulating small organic dyes. The encapsulation process is of scientific interest because it can be used to tailor the optical properties of the resulting dyes@NTs nanohybrids. Past works have shown that the dyes@SWCNT exhibits a strongly enhanced Raman scattering cross section, while the dyes@BNNTs do emit robust, generally red-shifted, luminescence at wavelengths down to the near-IR. In both cases, the nanotube protects the encapsulated dyes from photobleaching, provides high confinement, and reinforces intermolecular interactions between dyes into specific aggregation states.Here, we compare the liquid phase encapsulation process of α-sexithiophene (6T), which is a conjugated rod-like dye, inside SWCNTs and BNNTs. Raman and luminescence imaging experiments are used to monitor the 6T encapsulation process of a large ensemble of individual nanotubes in liquid-phase. This method probes statistically the encapsulation kinetics using hundreds of individual nanotubes according to various parameters (dye concentration and temperature). The results highlight a kinetic model in which single and double aggregates is formed sequentially. This kinetics and the associated thermodynamic parameters for the 6T encapsulation will be presented and discussed so as to gain a better control over the emission properties of the nanohybrids.