Observed Raman Spectra Research Articles

Impact of the outer‐sphere and inner‐sphere association in the surface enhanced Raman spectra of metal complexes and gold nanoparticles

AbstractTransition metal complexes, such as the low‐spin bis (phenylterpyridine) (A) and bis (pyridylterpyrazine)iron (II) (B) complexes, provide didactic chromophore species for demonstrating the Raman, resonance Raman, and the surface‐enhanced Raman scattering (SERS) behavior in coordination chemistry, as well as for elucidating the nature of inner‐sphere and outer‐sphere association with plasmonic nanoparticles. Their electrostatically stabilized ion pairs with citrate–gold nanoparticles have been studied in an aqueous solution, from the pronounced changes in the plasmonic band at 540 nm. Complex A, lacking any coordinating site, can only generate outer‐sphere complexes with citrate–gold nanoparticles, but they are stable enough to give a strong SERS response, even at 10−8 M. At 10−6 M, agglomeration accompanies the decrease of the electrostatic repulsion, resulting in a sharp decay of the plasmon resonance band at 540 nm. This is followed by the rise of a plasmon coupling band above 700 nm. However, at 10−4 M, the excess of the complex in the adsorption layer produces a reverse effect, decreasing agglomeration. The observed Raman spectra are essentially similar for the several concentrations employed because the outer‐sphere interaction implies a SERS electromagnetic mechanism. In contrast, complex B exhibits several pyridine and pyrazine N‐atoms available to form inner‐sphere‐associated species. A selective enhancement of the SERS signals is observed at 10−8 M, clearly indicating a chemical mechanism, consistent with a bridging mode. At 10−6 M and above, the agglomeration leads to a plasmon coupling band at 800 nm, while the SERS response indicates a change in the binding modes dictated by the excess of the complexing molecules.

Atomic-Scale Insights into the Interlayer Characteristics and Oxygen Reactivity of Bilayer Borophene.

Bilayer (BL) two-dimensional boron (i.e., borophene) has recently been synthesized and computationally predicted to have promising physical properties for a variety of electronic and energy technologies. However, the fundamental chemical properties of BL borophene that form the foundation of practical applications remain unexplored. Here, we present atomic-level chemical characterization of BL borophene using ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS). UHV-TERS identifies the vibrational fingerprint of BL borophene with angstrom-scale spatial resolution. The observed Raman spectra are directly correlated with the vibrations of interlayer boron-boron bonds, validating the three-dimensional lattice geometry of BL borophene. By virtue of the single-bond sensitivity of UHV-TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres in UHV. In addition to providing fundamental chemical insight into BL borophene, this work establishes UHV-TERS as a powerful tool to probe interlayer bonding and surface reactivity of low-dimensional materials at the atomic scale.

Atomic‐Scale Insights into the Interlayer Characteristics and Oxygen Reactivity of Bilayer Borophene

AbstractBilayer (BL) two‐dimensional boron (i.e., borophene) has recently been synthesized and computationally predicted to have promising physical properties for a variety of electronic and energy technologies. However, the fundamental chemical properties of BL borophene that form the foundation of practical applications remain unexplored. Here, we present atomic‐level chemical characterization of BL borophene using ultrahigh vacuum tip‐enhanced Raman spectroscopy (UHV‐TERS). UHV‐TERS identifies the vibrational fingerprint of BL borophene with angstrom‐scale spatial resolution. The observed Raman spectra are directly correlated with the vibrations of interlayer boron–boron bonds, validating the three‐dimensional lattice geometry of BL borophene. By virtue of the single‐bond sensitivity of UHV‐TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres in UHV. In addition to providing fundamental chemical insight into BL borophene, this work establishes UHV‐TERS as a powerful tool to probe interlayer bonding and surface reactivity of low‐dimensional materials at the atomic scale.

Disorder origin of Raman scattering in perovskite single crystals

The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the nonmonotonic evolution with temperature of the scattering intensity and present a model for second-order Raman scattering that accounts for this unique trend. By invoking a low-frequency anharmonic feature, we are able to reproduce the Raman spectral line shapes and integrated intensity temperature dependence. Numerical simulations support our interpretation of this low-frequency mode as a transition between two minima of a double-well potential surface. The model can be applied to other dynamically disordered crystal phases, providing a better understanding of the structural dynamics, leading to favorable electronic, optical, and mechanical properties in functional materials.

First principles molecular dynamics study of proton disorder in [formula omitted] phase of H2 hydrate

The proton-disordered and -ordered crystal structures proposed based on X-ray diffraction experiments for the C1′ phase of H2 hydrate were investigated using first-principles (FP) path-integral ring-polymer molecular dynamics (RPMD) simulations. Our simulated Raman spectra obtained from the FP-RPMD trajectories provide unambiguous evidence for static orientational disorder of H2O molecules forming stacked H-bonded rings. This feature is essential to reproduce the characteristics of the observed Raman spectra. Furthermore, the present study shows that van der Waals interactions need to be treated carefully to achieve satisfactory computational results. The findings of this study can help for better understanding of icy materials.

Probing luminescence of rare earth ions in natural pink fluorites using Raman microscopes

AbstractIt is well‐known that many natural fluorite crystals contain rare earth ions. In this work, a series of pink fluorites is studied to demonstrate that the use of Raman microscopes allows to obtain high resolution emission spectra of trivalent rare earth ions. These emission spectra allow in principle to distinguish differences of the local environment related to the charge compensation necessary when the divalent Ca ion is replaced by a trivalent rare earth ion. The pink fluorites from the Alps have grown under hydrothermal conditions, which results in the incorporation of oxygen into the fluorite. In one sample from Juchlistock (Bern, Switzerland), a trigonal Eu3+‐oxygen center is clearly identified. Interestingly, a pink fluorite from China (Huanggang Mine, Inner Mongolia, China) shows a quite different emission spectrum from those of the Alps. In this sample, the emission bands have much lower relative intensity compared with the intensity of the CaF2 Raman peak, indicating a much lower content of Er3+ and Ho3+ which emit around 520–560 nm. Literature examples show that Raman measurements of differently colored fluorite crystals can also present sharp rare earth spectra. By extension, it is thus possible to observe and identify emission spectra of rare earth ions in other mineral crystals using Raman microspectroscopy and avoid misassignments of the corresponding observed Raman spectra.

Comprehensive Data via Spectroscopy and Molecular Dynamics of Chemically Treated Graphene Nanoplatelets

Graphene nanoplatelets (GnPs) are promising candidates for gas sensing applications because they have a high surface area to volume ratio, high conductivity, and a high temperature stability. The information provided in this data article will cover the surface and structural properties of pure and chemically treated GnPs, specifically with carboxyl, ammonia, nitrogen, oxygen, fluorocarbon, and argon. Molecular dynamics and adsorption calculations are provided alongside characterization data, which was performed with Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) to determine the functional groups present and effects of those groups on the structural and vibrational properties. Certain features in the observed Raman spectra are attributed to the variations in concentration of the chemically treated GnPs. XRD data show smaller crystallite sizes for chemically treated GnPs that agree with images acquired with scanning electron microscopy. A molecular dynamics simulation is also employed to gain a better understanding of the Raman and adsorption properties of pure GnPs.

Raman Spectroscopy of an Atypical C15-E,syn Bilin Chromophore in Cyanobacteriochrome RcaE.

Cyanobacteriochromes (CBCRs) belong to the phytochrome superfamily of photoreceptors, the members of which utilize a linear tetrapyrrole (bilin) as a chromophore. RcaE is a representative member of a green/red-type CBCR subfamily that photoconverts between a green-absorbing dark state and red-absorbing photoproduct (Pr). Our recent crystallographic study showed that the phycocyanobilin (PCB) chromophore of RcaE adopts a unique C15-E,syn configuration in the Pr state, unlike the typical C15-E,anti configuration for the phytochromes and other CBCRs. Here, we measured Raman spectra of the Pr state of RcaE with 1064 nm excitation and explored the structure of PCB and its interacting residues under physiologically relevant aqueous conditions. We also performed measurements of RcaE in D2O as well as the sample reconstituted with the PCB labeled with 15N or with both 13C and 15N. The observed Raman spectra were analyzed by quantum mechanics/molecular mechanics (QM/MM) calculations together with molecular dynamics simulations. The Raman spectra and their isotope effects were well-reproduced by the simulated spectra of fully protonated PCB with the C15-E,syn configuration and allowed us to assign most of the observed bands. The present vibrational analysis of the all syn bilin chromophore using the QM/MM method will advance future studies on CBCRs and the related proteins by vibrational spectroscopy.

Understanding first-order Raman spectra of boron carbides across the homogeneity range

Boron carbide, a lightweight, high temperature material, has various applications as a structural material and as a neutron absorber. The large solubility range of carbon in boron, between $\ensuremath{\approx}9%$ and 20%, has been theoretically explained by some of us by the thermodynamical stability of three icosahedral phases at low temperature, with respective carbon atomic concentrations: 8.7% (${\mathrm{B}}_{10.5}\mathrm{C}$, named ${\mathrm{OPO}}_{1}$), 13.0% (${\mathrm{B}}_{6.7}\mathrm{C}$, named ${\mathrm{OPO}}_{2}$), whose theoretical Raman spectra are still unknown, and 20% (${\mathrm{B}}_{4}\mathrm{C}$), from which the nature of some of the Raman peaks are still debated. We report theoretical and experimental results of the first-order, nonresonant, Raman spectrum of boron carbide. Density functional perturbation theory enables us to obtain the Raman spectra of the ${\mathrm{OPO}}_{1}$ and ${\mathrm{OPO}}_{2}$ phases, which are perfectly ordered structures with however a complex crystalline motif of 414 atoms, due to charge compensation effects. Moreover, for the carbon-rich ${\mathrm{B}}_{4}\mathrm{C}$, with a simpler 15-atom unit cell, we study the influence of the low energy point defects and of their concentrations on the Raman spectrum, in connection with experiments, thus providing insights into the sensitivity of experimental spectra to sample preparation, experimental conditions, and setup. In particular, this enables us to propose a new structure at 19.2% atomic carbon concentration, ${\mathrm{B}}_{4.2}\mathrm{C}$, that, within the local density approximation of density functional theory (DFT-LDA), lies very close to the convex hull of boron carbide, on the carbon-rich side. This new phase, derived from what we name the ``3+1'' defect complex, helps in reconciling the experimentally observed Raman spectrum with the theory around 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$. Finally, we predict the intensity variations induced by the experimental geometry and quantitatively assess the localization of bulk and defect vibrational modes and their character, with an analysis of ``chain'' and ``icosahedral'' modes.

Modelling of the 2ν1-ν1 and ν1 band transitions of 13CH4 using high resolution Raman spectroscopy measurements

The pump-probe technique for investigating vibrationally excited states via high resolution Raman spectroscopy was applied to 13CH4 methane isotopologue. The dipole transitions between A1 totally symmetric vibrational states are not active in IR spectra but these states can be efficiently studied using selective high resolution Raman spectroscopy. 80 vibration-rotation transitions, most of which belong to the 2ν1– ν1 band have been assigned in the observed Raman spectra reported in this work. Including the Raman transitions in the simultaneous data fit improves the accuracy of the effective Hamiltonian and also rovibrational upper state levels of the ν1, 2ν1 and 2ν3 (A1) bands. A more accurate model of the ν1 vibration-rotation transitions improves the interpretation of the temperature dependence of Raman spectra involving the Pentad and Tetradecad polyads.

UV–Vis, Raman spectroscopic and density functional theoretical studies on microsolvation 1, 2, 4-triazole-3-thione clusters

Photophysical and photochemical reactions of microsolvation clusters are attracting an increasing attention due to their wide applications in materials science and biology. In this paper, 1, 2, 4-triazole-3-thione (3TT) is investigated in solid, protic, and aprotic solvents using FT-Raman, resonance Raman and electronic absorption spectroscopic experiments. The structures of microsolvation clusters in solvents were confirmed by 488 nm Raman spectroscopy combining with density functional theory (DFT) calculation. Steady-state absorption and resonance Raman spectra of 3TT in different environments indicate that the intermolecular hydrogen bonding may reveal important insights in the photophysical and photochemical process. With the aid of DFT and time-dependent density functional theory (TDDFT) calculations, we assigned the observed Raman spectra to the microsolvation clusters in acetonitrile, water and methanol, and carried out preliminary investigations on spectrum shifts of UV and Raman spectra due to the hydrogen bonding with the solvent molecules. The intermolecular >NH···O and >=S···H hydrogen bonding interactions, which are the key constituents of stable thione structure of 3TT, revealed the obvious spectrum shifts of 3TT, including Raman and absorption shifts in CH3CN, CH3OH and H2O. The hydrogen bond sites were further confirmed to be located on the functional group SCNH of 3TT with CH3CN, H2O and CH3OH.

Raman Spectra of Persistent Radical Anions from Benzophenone, Fluorenone, 2,2'-Bipyridyl, 4,4'-Di-tert-butyl-2,2'-dipyridyl, and Anthracene: Excellent Agreement between DFT and Experiment for Highly Delocalized Radical Systems.

We report detailed Raman spectra for the neutral and radical anion forms of benzophenone, fluorenone, 2,2'-bipyridyl, 4,4'-di-tert-butyl-2,2'-dipyridyl, and anthracene. Density functional theory (DFT) predictions for the Raman spectra of these molecules give additional insight into the assignment of each vibrational mode. While the use of DFT has been problematic in quantifying the thermochemistry of highly delocalized radicals, we find that DFT-predicted spectra using the popular B3LYP functional are in excellent agreement with the observed Raman spectra. In the case of the two bipyridyl compounds, the Raman spectra allowed us to conclude that the cis form of the radical anion complexed to a sodium cation was the preferred configuration. Benzophenone and fluorenone radical anions gave a significantly weakened C═O bond stretching vibrational frequency as expected from the population of an antibonding π* orbital. For benzophenone, the C═O vibration dropped from 1659 to 1403 cm-1 upon reduction. Similarly, fluorenone showed a C═O vibration observed at 1719 cm-1 for the neutral form that decreased to 1522 cm-1 for the radical anion. The structurally rigid anthracene showed relatively smaller Raman band shifts upon single-electron reduction as the π* orbital is more equally delocalized on the entire structure. In total, we correlated 65 DFT-predicted vibrational modes for the neutral molecules with an overall error of 7.1 cm-1 (root-mean-square errors (RMSEs)) and 67 DFT-predicted vibrational modes for radical anions with an overall error of 9.9 cm-1. These comparisons between theory and experiment are another example to demonstrate the power of DFT in predicting the identity and geometry of molecules using Raman spectroscopy.

Cancer Cell Detection on the Surface of Top-Gated Monolayer Graphene via Raman Spectroscopy.

A label-free biosensor is described based on the Raman spectroscopic signatures of monolayer graphene, which are modified in the compartment of cancer cells because of electron-phonon coupling in monolayer graphene. Specifically, the Raman spectra of electrostatically gated monolayer graphene on SiO2/Si substrates, in the voltage range from 0 to 5 V, were studied in the absence and the presence of cancer cells. Density functional theory simulations afforded a correlation between cancer cells and the observed Raman spectra, through the regulation of the intensities of the G and 2D Raman vibrational modes with applied voltage. The C-H and N-H bonds of phenylalanine enabled the detection of this biosensing activity. Significantly, this detection can be carried out even in the absence of cancer cell-culturing steps.

Computationally Guided Investigation of the Optical Spectra of Pure β-UO3

Single-phase β-UO3 is synthesized by flash heating UO2(NO3)·6H2O in air to 450 °C and annealing for 60 h under the same conditions. For the first time, we report the Raman spectra of pure β-UO3. To facilitate the assignment of Raman and infrared vibrational modes, we use density functional theory with density functional perturbation theory. By employing a novel analysis scheme that includes the mode frequencies as well as a quantitative analysis of the mode eigenvectors, we assign the observed spectral features to individual chemical modes. In particular, the density functional theory optimized structure, observed Raman spectrum, and eigenvector analysis suggest the presence of four crystallographically distinct uranyl ions, one more than has previously been suggested.

Combined effect of 13C isotope and vacancies on the phonon properties in AB stacked bilayer graphene

The combined effects of 13C isotope and vacancies on the phonon properties in AB stacked bilayer graphene (BLG) are explored theoretically. We have calculated the phonon density of states (PDOS) by varying the isotope contents (0–100%) and vacancies (0–30%) in both layers and only in the upper layer of the BLG using forced vibrational method. We found that both isotope and vacancy or merging of these two defects significantly affect the PDOS, especially, E2g mode phonon, which is responsible for the Raman G band, shifted downward with the increase of defect concentrations. Moreover, when 13C isotopes are induced only in the upper layer, E2g peak splits into two peaks which corresponds well with the experimental results of 13C/12C dependence G peak splitting in the Raman spectra of BLG. We also explored the defect induced phonon localization in BLG. Our calculated typical mode patterns show that high frequency optical phonons are strongly localized in the vacancy as well as merging 13C isotope and vacancy defected BLG. The calculated average localization length noticed that strong phonon localization exists at 60% 13C isotope concentration. These findings are important for understanding the experimentally observed Raman spectra as well as thermal transport in BLG.

Strain-Engineered Anisotropic Optical and Electrical Properties in 2D Chiral-Chain Tellurium.

Atomically thin materials, leveraging their low-dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin-film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain-engineered anisotropic optical and electrical properties in solution-grown, sub-millimeter-size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral-chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain-engineered functionalities.

Mechanism Study of Molecular Deformation of 2,2',5',2″-Tetramethylated p-Terphenyl-4,4″-dithiol Trapped in Gold Junctions.

Molecular junctions hold great potential for future microelectronics, while the practical utilization has long been limited by the problem of conformational deformation during charge transport. Here we present a first-principles theoretical study on the surface-enhanced Raman spectroscopy (SERS) characterization of the p-terphenyl-4,4″-dithiol molecule and its 2,2',5',2″-tetramethylated analogue in gold junctions to investigate the molecular deformation mechanism. The effects of charge injection and external electric field were examined, both of which could change π-conjugation by varying the dihedral angle between the central and ending rings (DIPT). The induced significant structural deformations then change SERS responses. Only the SERS responses under an external electric field can account for the experimentally observed Raman spectra, and those of charge injections cannot. Moreover, applying a strong electric field could enlarge the conductivities of the two molecular junctions, agreeing well with experiments. This information not only elaborates that the electric field effect constitutes one important mechanism for molecular deformation but also provides useful insights for the control of charge transport in molecular junctions.

Towards polarization-based excitation tailoring for extended Raman spectroscopy.

Undoubtedly, Raman spectroscopy is one of the most elaborate spectroscopy tools in materials science, chemistry, medicine and optics. However, when it comes to the analysis of nanostructured specimens or individual sub-wavelength-sized systems, the access to Raman spectra resulting from different excitation schemes is usually very limited. For instance, the excitation with an electric field component oriented perpendicularly to the substrate plane is a difficult task. Conventionally, this can only be achieved by mechanically tilting the sample or by sophisticated sample preparation. Here, we propose a novel experimental method based on the utilization of polarization tailored light for Raman spectroscopy of individual nanostructures. As a proof of principle, we create three-dimensional electromagnetic field distributions at the nanoscale using tightly focused cylindrical vector beams impinging normally onto the specimen, hence keeping the traditional beam-path of commercial Raman systems. In order to demonstrate the convenience of this excitation scheme, we use a sub-wavelength diameter gallium-nitride nanostructure as a test platform and show experimentally that its Raman spectra depend sensitively on its location relative to the focal vector field. The observed Raman spectra can be attributed to the interaction with transverse and pure longitudinal electric field components. This novel technique may pave the way towards a characterization of Raman active nanosystems, granting direct access to growth-related parameters such as strain or defects in the material by using the full information of all Raman modes.

Theory of birefringence correction for polarization-controlled CARS.

Polarization-controlled coherent Raman spectroscopy is used as a high-throughput method to characterize the anisotropic nature of a molecular system, such as the molecular orientation distribution. However, optical birefringence originating from the molecular anisotropy can cause the observed Raman spectrum to be significantly distorted, making it extremely challenging to obtain quantitative information from polarization Raman measurements. Here, the birefringence effect on the signal intensity and the spectral shape of a polarization-controlled coherent anti-Stokes Raman scattering (CARS) is theoretically described using a uniaxially symmetrical model system. Due to the complexity, the effect of phase delay in the incident lights is not considered but only that of the generated CARS signal is considered. A new analytical method is presented to eliminate the birefringence contribution from polarization-controlled CARS data by analyzing polarization intensity profiles and retrieving the resonant Raman susceptibility spectra. This method is tested with two sets of polarization-controlled CARS data simulated with various combinations of symmetries of multiple underlying Raman modes. The analysis result clearly demonstrates that the effect of birefringence can be corrected for polarization-controlled CARS data and the symmetry tensor elements of all underlying Raman modes can be quantitatively characterized.

Automatic removal of binary background components expecting Raman big data and its application to human hair imaging

We developed an automated method for removing binary background components from observed Raman spectra by tuning the scaling factors to seek the minimum lengths of the subtracted spectra. This method is effective, especially for large data including imaging data. For application, 400 Raman imaging spectra of a sliced cross section of a strand of gray human hair, fixed by glue on glass, were subjected to the proposed method by removing the glass and glue information. After the binary background removal, principal component analysis successfully detected small but important signals of tryptophan, which is peculiar to the hair cortex.