Raman-active Vibrational Modes Research Articles

Enhancement in photocurrent conversion efficiency via recrystallization of zinc tin hydroxide nanostructures

Herein, the synthesis of cubic phase zinc tin hydroxide is reported by the hydrolysis assisted co-precipitation method followed by calcination in 150–850 °C. The recrystallization of zinc tin hydroxide (ZnSn(OH)6) to zinc tin oxide (Zn2SnO4) phase is observed with the intermediate amorphous phase (ZnSnO3) by increasing the calcination temperature. Increase in calcination temperature also causes the partial alteration in the cubic morphology and diminution in average particle size. The M-O-M and M-O vibration modes have distinct peaks in the FT-IR spectra below 1500 cm−1. Four Raman active vibration modes in ZnSn(OH)6 and two Raman active vibration modes in Zn2SnO4 phase are observed, while no Raman active mode is present in intermediate amorphous ZnSnO3. Chemical states of various elements and compositions are examined by X-ray photoelectron spectroscopy. The ZnSnO3 electrodes has the lowest charge transfer resistance and higher lifetime of the photogenerated charge carriers. Photoelectrochemical measurements rendered enhanced photocurrent density for ZnSnO3 and Zn2SnO4 phases, which is approximate 85 and 50 times higher as compared to ZnSn(OH)6 based photoelectrode, respectively. Due to crystalline nature, the Zn2SnO4 phase has better photo-stability.

Polarized Moiré Phonon and Strain-Coupled Phonon Renormalization in Twisted Bilayer MoS2

The ability to control the atomic-scale behavior in quasi-two-dimensional crystals via twisting the vicinal layers and generating patterned moiré potentials provides an additional degree of freedom to study correlated physics. Under the existence of macroscopic external parameters such as strain, we study the high-frequency Raman active vibrational modes of chemical vapor deposition (CVD)-grown monolayer, stacked (AA′ and AB), and twisted bilayer MoS2. The polarized Raman scattering, being an efficient characterization tool for these moiré structures, is employed to investigate the effects of strain on the different phonon modes. The recently discovered moiré phonons (FA1g), i.e., zone center phonons in a twisted bilayer MoS2, stem from the folding of the off-center phonons of its monolayer constituents and are found to be Raman polarized. Moreover, the Raman shift impressions of these moiré phonons in twist angle in the range between 0 and 30° are reproduced, showing the strain-coupled modifications of the previously reported sinusoidal behavior. Our findings establish an effective method to probe the strain-induced anisotropy in twisted van der Waals systems using polarized Raman spectroscopy showing future directions to moiré-related physics.

Effect of rare earth (Nd3+) metal doping on structural, morphological, optical and magnetic traits of Zn–Mg nano-ferrites

Herein, we report the synthesis of Zn0.7Mg0.3NdxFe2–xO4 (where, x = 0.0, 0.01, 0.02) ferrite nanoparticles by employing the sol–gel auto-combustion technique. The X-ray diffraction (XRD) pattern suggests the formation of a pure cubic structure, without any impurity phase, with an Fd3m space group at room temperature. With increasing doping amount, the crystallite size is reported as 35–41 nm, while the lattice parameters rise from 0.8381 to 0.8395 nm. Field emission scanning electron microscopy (FESEM) images show the formation of spherical grains with agglomerated morphology in all the samples, with grain sizes ranging from 49 to 103 nm. Energy dispersive X-ray spectroscopy (EDX) and elemental mapping investigation confirm the chemical purity of all the samples. Fourier transform infrared (FTIR) analysis shows the presence of two prominent peaks around 440 and 560 cm−1 that correspond to the octahedral and tetrahedral positions. In addition, the existence of five Raman active vibrational modes in all produced specimens again confirms the structural purity of all the samples. The M−H curve shows that saturation magnetization (Ms) first increases from 14.98 to 28.22 emu/g and then decreases to 18.98 emu/g with increasing doping amount. This is due to the A-B type super-exchange interaction for the synthesized samples. The variation in coercivity (Hc) and magnetic anisotropy (K1) suggest the thermal stability of all the samples and can be utilized in transformers and solenoids.

(Digital Presentation) Characterization of Anisotropic Strain in Anelastic Material By Raman Spectroscopy

Raman spectroscopy has recently been applied for non-destructive characterization of strain in crystalline thin films. It makes use of the numerical value of the mode Grüneisen parameter γ , which equals (minus) the ratio of the relative change in the frequency of a Raman-active vibrational mode and the strain-induced relative change in the unit cell volume. In the presence of in-plane, compressive biaxial strain, aliovalent doped CeO2-films exhibit values of γ which are ~30% smaller than the literature values for the bulk ceramics under isostatic stress. This discrepancy is due in large part to the negative contribution from the anelastic (time-dependent) mechanical properties of oxygen-deficient ceria, thereby raising questions concerning the relationship between Raman mode frequency and anelastic strain. Here we propose a way to "separate" anelastic and elastic contributions to the Grüneisen parameter.500-650nm thick films , x=0.1-0.2, were deposited by RF magnetron sputtering on (100) p-Si wafers (1Ω-cm, tSi=280mm). The films (S-films) were in the fluorite phase with pronounced (111) texture and exhibited biaxial , compressive strain. The in- and out-of-plane lattice parameters (aS || , aS ⊥ ) of the S-films were determined by XRD at different sample tilt angles. The in-plane stress (σbi) in the S-films was determined by profilometry from the change in Si-wafer curvature following film deposition. From σbi and the film biaxial modulus, the elastic component of the in-plane compressive strain (ue ) was calculated (~ -0.2%).Strain-free (relaxed) R-films were obtained by partial substrate removal, forming 2 mm diameter self-supported membranes. Pronounced membrane buckling indicated that residual strain was negligible (aR || ≈ aR ^ ). From the increase in area upon substrate removal, measured with optical profilometry, the total S-film in-plane strain (ut ) was calculated from aS ||/aR-1 ≈ -0.5%. The anelastic strain component may be calculated as ua= ut-ue . The small, anisotropic strain-induced blue shift of the frequency of the F2g -phonon mode for S-films suggests that the F2g mode responds primarily to the elastic component of the strain.As a control, 430nm thick films of mechanically elastic Y2O3 were deposited by sputtering on a Si wafer covered with a 1.5 µm thick PECVD deposited SiO2 layer. This layer supported the Y2O3-films upon membrane formation, but prevented complete strain relaxation. The Y2O3-films were in the double fluorite phase with strong (222) texture and, as with the S-films, were under in-plane compressive strain. The Y2O3 S-film lattice parameters were determined by acquiring XRD-patterns at different sample tilt angles, while membrane in- and out-of-plane lattice parameters were determined with XRD in the transmission and reflection modes, respectively. The values of γ calculated from the Raman frequency of the [Fg +Ag ] peak position were close to the literature values for bulk yttria under isostatic stress.This work should serve as a sample protocol for strain characterization of doped ceria thin films. It supports the concept that the discrepancy between the behavior of Raman modes of isostatically and anisotropically strained doped ceria is related to anelasticity. Our results shed light upon strain characterization by Raman spectroscopy and may lay the foundation for future studies to elucidate selective sensitivity to various strain components.

Lattice Dynamics of Bi2Те3 and Vibrational Modes in Raman Scattering of Topological Insulators MnBi2Te4·n(Bi2Te3)

This work is devoted to the experimental study and symmetry analysis of the Raman-active vibration modes in MnBi2Te4·n(Bi2Te3) van der Waals topological insulators, where n is the number of Te–Bi–Te–Bi–Te quintuple layers between two neighboring Te–Bi–Te–Mn–Te–Bi–Te septuple layers. Confocal Raman spectroscopy is applied to study Raman spectra of crystal structures with n = 0,1,2,3,4,5,6, and ∞. The experimental frequencies of vibration modes of the same symmetry in the structures with different n are compared. The lattice dynamics of free-standing one, three, and four quintuple layers, as well as of bulk Bi2Те3(n = infty ) and MnBi2Te4(n = 0), is considered theoretically. Vibrational modes of the last two systems have the same symmetry, but different displacement fields. These fields in the case of a Raman-active mode do not contain displacements of manganese atoms for any finite n. It is shown that two vibrational modes in the low-frequency region of the spectrum (35–70 cm–1) of structures with n = 1,;2,;3,;4,;5, and 6 practically correspond to the lattice dynamics of n free quintuple Bi2Те3 layers. For this reason, the remaining two vibration modes, which are observed in the high-frequency region of the spectrum (100–140 cm–1) and are experimentally indistinguishable in the sense of belonging to quintuple or septuple layer or to both layers simultaneously, should also be assigned to vibrations in quintuple layers under immobile atoms of septuple layers.

Photocatalytic dye degradation efficiency and reusability of Cu-substituted Zn-Mg spinel nanoferrites for wastewater remediation

A series of unmodified and copper-modified zinc‑magnesium nanoferrites [Zn0.7Mg0.3-xCuxFe2O4 (x = 0.0, 0.02, 0.04, 0.06)] were prepared to degrade methylene blue (MB), a widely used textile dye and a threat to humans and the environment. The X-ray diffraction data reveals a spinel cube-like crystal structure with no additional phases. The sharpness and frequency of XRD peaks highlighted the nanocrystalline structure of fabricated photocatalysts. The calculated crystallite size (t) shows inconsistent behavior within 13–25 nm on doping copper ions within the crystal matrix of Zn-Mg nanoferrites. The morphological analysis showed the existence of spherical and aggregated nanoparticles for the synthesized Zn0.7Mg0.3Fe2O4 (x = 0.0), and Zn0.7Mg0.28Cu0.02Fe2O4 (x = 0.02) specimens. In the fabrication of Zn0.7Mg0.3Fe2O4 (ZMF1), Zn0.7Mg0.28Cu0.02Fe2O4 (ZMF2), Zn0.7Mg0.26Cu0.04Fe2O4 (ZMF3) and Zn0.7Mg0.24Cu0.06Fe2O4 (ZMF4) nanoferrites, the predicted band gaps were 2.30 eV, 2.18 eV, 2.07 eV, and 1.94 eV, respectively. The FTIR spectrum shows stretching vibrations in Fe-O complexes at the A-sites and B-sites, whereas the Raman spectrum exhibits five Raman active vibrational modes at 230–800 cm−1. The nanophotocatalysts showed excellent magnetization values around 4.44–12.92 emu/g with an applied field of 6000 Oe. In the photocatalytic degradation of methylene blue, Zn0.7Mg0.24Cu0.06Fe2O4 (x = 0.06) nanophotocatalyst showed a maximum rate constant of 0.0252 min−1 and 90% photocatalytic degradation efficiency. The nanoferrites showed excellent photocatalytic activity for five cycles and were easily separated using an external magnet.

Comparative Raman spectroscopy of magnetic topological material EuCd2X2 (X = P, As)

EuCd2X2 (X = P, As) is a new class of magnetic topological materials discovered recently. The electronic structure and the band topology are intimately coupled with its magnetism, giving rise to interesting properties such as spin fluctuation and colossal magnetoresistance. Phonon excitation can contribute to the quasi-particle response of the topological matters through spin-lattice and electron–phonon coupling. However, the phonon properties of this material family remain unexplored. Here we report a comparative study of Raman-active vibration modes in EuCd2X2 (X = P, As) by means of angle-resolved, temperature-resolved, and magnetic-field-resolved Raman spectroscopy together with the first-principle calculations and Raman tensor analysis. The phonon properties can be tuned by chemical potential and temperature within the material family. All the phonon modes are softened with increased chemical pressure by replacing P with As. Angle-resolved polarized Raman spectroscopy reveals the configuration-sensitive Raman activity and the isotropic intensity response. In addition, the magneto-Raman spectrum indicates the stability of Raman-active vibration modes against the magnetic field at room temperature. Our work sheds light on the phonon dynamics of magnetic topological matters, which are potentially coupled with the topological charge and spin excitation.

Two-Colour Sum-Frequency Generation Spectroscopy Coupled to Plasmonics with the CLIO Free Electron Laser

Nonlinear plasmonics requires the use of high-intensity laser sources in the visible and near/mid-infrared spectral ranges to characterise the potential enhancement of the vibrational fingerprint of chemically functionalised nanostructured interfaces aimed at improving the molecular detection threshold in nanosensors. We used Two-Colour Sum-Frequency Generation (2C-SFG) nonlinear optical spectroscopy coupled to the European CLIO Free Electron Laser in order to highlight an energy transfer in organic and inorganic interfaces built on a silicon substrate. We evidence that a molecular pollutant, such as thiophenol molecules adsorbed on small gold metal nanospheres grafted on silicon, was detected at the monolayer scale in the 10 µm infrared spectral range, with increasing SFG intensity of three specific phenyl ring vibration modes reaching two magnitude orders from blue to green–yellow excitation wavelengths. This observation is related to a strong plasmonic coupling to the thiophenol molecules vibrations. The high level of gold nanospheres aggregation on the substrate allows us to dramatically increase the presence of hotspots, revealing collective plasmon modes based on strong local electric fields between the gold nanoparticles packed in close contact on the substrate. This configuration favors detection of Raman active vibration modes, for which 2C-SFG spectroscopy is particularly efficient in this unusual infrared spectral range.

Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 59Co NMR Molecular Thermometers.

Understanding the mechanisms governing temperature-dependent magnetic resonance properties is essential for enabling thermometry via magnetic resonance imaging. Herein we harness a new molecular design strategy for thermometry─that of effective mass engineering via deuteration in the first coordination shell─to reveal the mechanistic origin of 59Co chemical shift thermometry. Exposure of [Co(en)3]3+ (1; en = ethylenediamine) and [Co(diNOsar)]3+ (2; diNOsar = dinitrosarcophagine) to mixtures of H2O and D2O produces distributions of [Co(en)3]3+-dn (n = 0-12) and [Co(diNOsar)]3+-dn (n = 0-6) isotopomers all resolvable by 59Co NMR. Variable-temperature 59Co NMR analyses reveal a temperature dependence of the 59Co chemical shift, Δδ/ΔT, on deuteration of the N-donor atoms. For 1, deuteration amplifies Δδ/ΔT by 0.07 ppm/°C. Increasing degrees of deuteration yield an opposing influence on 2, diminishing Δδ/ΔT by -0.07 ppm/°C. Solution-phase Raman spectroscopy in the low-frequency 200-600 cm-1 regime reveals a red shift of Raman-active Co-N6 vibrational modes by deuteration. Analysis of the normal vibrational modes shows that Raman modes produce the largest variation in 59Co δ. Finally, partition function analysis of the Raman-active modes shows that increased populations of Raman modes predict greater Δδ/ΔT, representing new experimental insight into the thermometry mechanism.

Unconventional Disorder by Femtosecond Laser Irradiation in Fe2O3.

This paper demonstrates that femtosecond laser-irradiated Fe2O3 materials containing a mixture of α-Fe2O3 and ε-Fe2O3 phases showed significant improvement in their photoelectrochemical performance and magnetic and optical properties. The absence of Raman-active vibrational modes in the irradiated samples and the changes in charge carrier emission observed in the photocurrent density results indicate an increase in the density of defects and distortions in the crystalline lattice when compared to the nonirradiated ones. The magnetization measurements at room temperature for the nonirradiated samples revealed a weak ferromagnetic behavior, whereas the irradiated samples exhibited a strong one. The optical properties showed a reduction in the band gap energy and a higher conductivity for the irradiated materials, causing a higher current density. Due to the high performance observed, it can be applied in dye-sensitized solar cells and water splitting processes. Quantum mechanical calculations based on density functional theory are in accordance with the experimental results, contributing to the elucidation of the changes caused by femtosecond laser irradiation at the molecular level, evaluating structural, energetic, and vibrational frequency parameters. The surface simulations enable the construction of a diagram that elucidates the changes in nanoparticle morphologies.

Obvious phase transition status induced by He+-ions implantation in KTN crystal

We report on the formation of a critical ferroelectric state induced by the He+ ion implantation in potassium tantalate niobate crystal. Obvious phase change has been observed in the ion irradiated region, which is mostly related to the stable polarized nanometric regions formed during the ion implantation process. Under the irradiation of 2 MeV He+ ions, two distinguishable layers corresponding to different energy transfer modes (elastic nuclear collision and inelastic electronic collision, respectively) between the incident He+ ions and the intrinsic lattices have been formed beneath the irradiated surface. Lattice dynamics before and after the ion implantation process are investigated by the confocal µ-Raman system. And the variations of typical Raman-active vibrational modes demonstrate the presence of lattice distortion in the irradiated region. X-ray diffraction experiments further suggest the mostly uniform lattice elongation in this region. Piezo-response force characteristic measurements reveal the existence of stable polarized nanometric regions with more intense polarization and verify that the crystal with such a phase status possesses extraordinary microscopic disorders, which is different from the traditional ferroelectric or paraelectric phase. Optical transmission experiments demonstrate that the irradiated region possesses relatively low propagation loss. The ion implantation method provides a new approach to form a temperature-stable critical ferroelectric state in relaxor ferroelectric materials. Analyses of the modification on the lattice dynamics of the irradiated region can help us build a clear awareness of the physical essence of this critical state and the relaxor ferroelectricity. Also, with good optical transmittance, the irradiated region is capable of promising optical functional devices.

Phase-Sensitive Vibrationally Resonant Sum-Frequency Generation Microscopy in Multiplex Configuration at 80 MHz Repetition Rate.

Vibrationally resonant sum-frequency generation (VR SFG) microscopy is an advanced imaging technique that can map out the intensity contrast of infrared and Raman active vibrational modes with micron to submicron lateral resolution. To broaden its applications and to obtain a molecular level of understanding, further technical advancement is needed to enable high-speed measurements of VR SFG microspectra at every pixel. In this study, we demonstrate a new VR SFG hyperspectral imaging platform combined with an ultrafast laser system operated at a repetition rate of 80 MHz. The multiplex configuration with broadband mid-infrared pulses makes it possible to measure a single microspectrum of CH/CH2 stretching modes in biological samples, such as starch granules and type I collagen tissue, with an exposure time of hundreds of milliseconds. Switching from the homodyne- to heterodyne-detected VR SFG hyperspectral imaging can be achieved by inserting a pair of optics into the beam path for local oscillator generation and delay time adjustment, which enables self-phase-stabilized spectral interferometry. We investigate the relationship between phase images of several different C-H modes and the relative orientation of collagen triple-helix in fibril bundles. The results show that the new multiplex VR SFG microscope operated at a high repetition rate is a powerful approach to probe the structural features and spatial arrangements of biological systems in detail.

A polarizability driven ab initio molecular dynamics approach to stimulating Raman activity: Application to C20

We describe a novel variant of the driven molecular dynamics (DMD) method derived for probing Raman active vibrations. The method is an extension of the conventional μ-DMD formulation for simulating IR activity by means of coupling an oscillating electric field to the molecule’s dipole moment, μ, and inducing absorption of energy via tuning the field to a resonant frequency. Presently, we modify the above prescription to invoke Raman activity by coupling two electric fields, i.e. a ‘Pump’ photon of frequency ωP and a Stokes photon of frequency ωS to the molecule’s polarizability tensor, α, with the difference in the frequencies ω = ωP – ωS corresponding to the Stokes Raman shift. If ω is close to a Raman active vibrational frequency, energy absorption ensues. Varying ω allows identifying and assigning all Raman active vibrational modes, including anharmonic corrections, by means of trajectory analysis. We show that only one element of the full polarizability tensor, and its nuclear derivative, is needed for an α-DMD trajectory, making this method well suited for ab initio dynamics implementation. Numerical results using first-principles calculations are presented and discussed for the vibrational fundamentals, combination bands, overtones of H2O, CH4, and the C20 fullerene.

Role of V-V dimers on structural, electronic, magnetic, and vibrational properties of VO2 by first-principles simulations and Raman spectroscopic analysis

We investigate the vibrational properties of VO2, particularly the low temperature M1 phase by first-principles calculations using the density functional theory as well as Raman spectroscopy. We perform the structural optimization using SCAN meta-GGA functional and obtain the optimized crystal structures for metallic rutile and insulating M1 phases satisfying all expected features of the experimentally derived structures. Based on the harmonic approximation around the optimized structures at zero temperature, we calculate the phonon properties and compare our results with experiments. We show that our calculated phonon density of states is in excellent agreement with the previous neutron scattering experiment. Moreover, we reproduce the phonon softening in the rutile phase as well as the phonon stiffening in the M1 phase. By comparing with the Raman experiments, we find that the Raman-active vibration modes of the M1 phase is strongly correlated with the V-V dimer distance of the crystal structure. Our combined theoretical and experimental framework demonstrates that Raman spectroscopy could serve as a reliable way to detect the subtle change of V-V dimer in the strained VO$_2$.

Operando Measurement of Layer Breathing Modes in Lithiated Graphite

Despite their ubiquitous usage and increasing societal dependence on Li-ion batteries, there remains a lack of detailed empirical evidence of Li intercalation/deintercalation into graphite even though this process dictates the performance, longevity, and safety of the system. Here, we report direct detection and dissociation of specific crystallographic phases in the lithiated graphite, which form through a stepwise staging process. Using operando measurements, LiC18, LiC12, and LiC6 phases are observed via distinct low-frequency Raman features, which are the result of displacement of the graphite lattice by induced local strain. Density functional theory calculations confirm the nature of the Raman-active vibrational modes, to the layer breathing modes (LBMs) of the lithiated graphite. The new findings indicate graphene-like characteristics in the lithiated graphite under the deep charged condition due to the imposed strain by the inserted Li. Moreover, our approach also provides a simple experimental tool to measure induced strain in the graphite structure under full intercalation conditions.

Enhanced electrical and ferrimagnetic properties of bismuth substituted yttrium iron garnets

Y3-xBixFe5O12 (0 ≤ x ≤ 1.0) powders are successfully prepared by sol-gel auto-combustion method. The structural, morphological, electrical and magnetic properties of bismuth substituted YIG are systematically studied and reported. The phase formation and position of vibrational modes are studied using XRD, FTIR and Raman Spectroscopy respectively. The XRD patterns of Bi-YIG powders confirm the formation of polycrystalline, single phasic and cubic garnet structure with average crystallite size around 39 nm. FTIR spectra show well resolved bands corresponding to the tetrahedral and octahedral voids of Fe–O in YIG which is characteristic of garnets. The observed translational Raman active vibrational modes confirm the presence of cubic YIG phase. SEM micrographs reveal the formation of adense homogenous network of spherical shaped Y3-xBixFe5O12 grains. Magnetic properties of Bi-YIG show an enhancement compared to the host YIG, confirming the presence of ferrimagnetic ordering at room temperature. The observation of hysteresis loop along with low coercivities are typical of soft magnetic materials. Impedance measurements of Bi-YIG show an increase in electrical resistance up to two orders compared to YIG. The Bi-YIG sample having x = 0.6 exhibits a low dielectric loss equal to 0.045 and a low conductivity of the order of 10−6 Scm−1 at 100 kHz. The observations from magnetic and electrical studies illustrate the distinct role played by bismuth ions in the YIG lattice making it a potential candidate for microwave and magnetic-optical applications.

Obvious Phase Transition Status Induced by He <sup>+</sup>-Ions Implantation in KTN Crystal

We report on the formation of a critical ferroelectric state induced by the He+ ion implantation in potassium tantalate niobate crystal. An obvious phase change has been observed in the ion irradiated region, which is mostly related to the stable polarized nanometric regions formed during the ion implantation process. Under the irradiation of 2 MeV He+ ions, two distinguishable layers corresponding to different energy transfer modes (elastic nuclear collision and inelastic electronic collision, respectively) between the incident He + ions and the intrinsic lattices have been formed beneath the irradiated surface. Lattice dynamics before and after the ion implantation process are investigated by a confocal μ-Raman system. And the variations of typical Raman-active vibrational modes demonstrate the presence of lattice distortion in the irradiated region. X-ray diffraction experiments further suggest the uniform lattice elongation in this region. Piezo-response force characteristic measurements reveal the existence of stable polarized nanometric regions with more intense polarization and verify that the crystal with such a phase status possesses extraordinary microscopic disorders, which is different from the traditional ferroelectric or paraelectric phase. The ion implantation method provides a new approach to form a temperature-stable critical ferroelectric state in relaxor ferroelectric materials. Also, analyses of the modification on the lattice dynamics of the irradiated region can help us build a clear awareness of the physical essence of this critical state and the relaxor ferroelectricity.

Quantifying dynamic pressure and temperature conditions on fault asperities during earthquake slip

New insights into the pressure and temperature conditions on fault surfaces during seismic slip are provided by Raman-active vibrational modes of SiO2 glass. We performed triaxial stick-slip experiments at room temperature and high normal stresses on pre-ground, high-purity silica glass surfaces. During slip, velocities exceed 0.32 ms−1 over durations of less than one millisecond, generating frictional heat and locally melting the fault surfaces. Temperature increases permit structural rearrangement within the melt; these changes are preserved by rapid quenching. Using Raman spectroscopy, we analyse melt-welded regions and show that these areas exhibit systematic changes in the spectra of silica. Changes result from a decrease in the inter-tetrahedral Si-O-Si bond angle and are correlated to increasing silica glass density in the slip regions. Densification results from both rapid cooling rates and exposure to very high pressures at asperity contacts. We use data from other experiments to calibrate these effects, estimating quench temperatures up to 1800 K and pressures of ∼180 MPa. These results provide the first quantitative evidence for the effects of quench rates and high inter-asperity pressures on the physics of melting and quenching during seismic slip and its impact on fault behaviour.

Enhanced photophysical properties of silica-methylene blue@amorphous carbon as fluorochromes with modulated Raman-active vibration modes

As a kind of near-infrared dye, methylene blue (MB) molecule is proneness to form excimers in solid powder or solution at high concentration, which results in weak photoluminescence intensity during clinic diagnostic and cell labeling applications. Aiming to improve the quantum yield of MB, silica-MB particles are herein prepared and treated hydrothermally (HT) to synthesize silica-MB-HT and silica-MB@amorphous carbon with glucose as carbon source (silica-MB@AmC). Significantly, hydrothermal treatment causes destruction of Si-O-Si network, modulation of aggregation state, variations of energy gap and release behavior of MB. In comparison with that of silica-MB, emission intensities of both silica-MB-HT and silica-MB@AmC increase greatly and their photoluminescence peaks are blue-shifted, which are closely related with Raman-active vibration modes of MB in a locally-excited state. Enhanced emissions of both silica-MB-HT120 and silica-MB@AmC120 are observed with increased intensities of Raman peaks assigned to in-plane bending vibrations of CH bond during skeletal deformation of CC in ring. In contrast, the in-plane bending vibration for CH bonds at end group of MB is considered as a way of radiation decay, and strong vibration of this Raman-active mode causes weak emission intensities of silica-MB-HT180 and silica-MB@AmC160. The stabilities of photophysical properties are compared after 72 h of incubation in buffered mediums or 5 h of illumination under ultraviolet light. The photoluminescence intensity of silica-MB@AmC120 fluorophore increase remarkably after illumination, meeting the requirements of reduced photobleaching, minized solvatochromic shift and increased fluorescent efficiency during versatile applications.

High-pressure and high-temperature vibrational properties and anharmonicity of carbonate minerals up to 6 GPa and 500 °C by Raman spectroscopy

AbstractCarbonate minerals play a dominant role in the deep carbon cycle. Determining the high-pressure and high-temperature vibrational properties of carbonates is essential to understand their anharmonicity and their thermodynamic properties under crustal and upper mantle conditions. Building on our previous study on aragonite, calcite (both CaCO3 polymorphs), dolomite [CaMg(CO3)2], magnesite (MgCO3), rhodochrosite (MnCO3), and siderite (FeCO3) (Farsang et al. 2018), we have measured the pressure- and temperature-induced frequency shifts of Raman-active vibrational modes up to 6 GPa and 500 °C for all naturally occurring aragonite- and calcite-group carbonate minerals, including cerussite (PbCO3), strontianite (SrCO3), witherite (BaCO3), gaspeite (NiCO3), otavite (CdCO3), smithsonite (ZnCO3), and spherocobaltite (CoCO3). Our Raman and XRD measurements show that cerussite decomposes to a mixture of Pb2O3 and tetragonal PbO between 225 and 250 °C, smithsonite breaks down to hexagonal ZnO between 325 and 400 °C, and gaspeite to NiO between 375 and 400 °C. Spherocobaltite breaks down between 425 and 450 °C and otavite between 375 and 400 °C. Due to their thermal stability, carbonates may serve as potential reservoirs for several metals (e.g., Co, Ni, Zn, Cd) in a range of crustal and upper mantle environments (e.g., subduction zones). We have determined the isobaric and isothermal equivalents of the mode Grüneisen parameter and the anharmonic parameter for each Raman mode and compare trends in vibrational properties as a function of pressure, temperature, and chemical composition with concomitant changes in structural properties. Finally, we use the anharmonic parameter to calculate the thermal contribution to the internal energy and entropy, as well as the isochoric and isobaric heat capacity of certain carbonates.