Raman-active Bands Research Articles

Characterization of polycrystalline bulk ferroelectric ZnSnS3 synthesized by hydrothermal method for photovoltaic application

ZnSnS3, a newly theoretically predicted ferroelectric material which shows promising properties for solar cell and photovoltaic applications, was successfully grown in our laboratory by hydrothermal method. The high intensity x-ray diffraction pattern from the (211), (101̅), (200), (210), (310), (320), and (321̅) planes reveal the crystalline character of the trigonal ZnSnS3 phase. The microstructural property and elemental distribution were evaluated using SEM and TEM studies. Diffuse reflectance measurement at room temperature shows a direct bandgap of 2.62 eV along with a high energy direct transition of 3.13 eV. Two broad photoluminescence peaks at various temperatures (4–300 K) were also detected around (2.61–2.73 eV) and (3.04–3.11 eV). The emission characteristics are explained considering the shallow donor level to valence band transitions. Raman study elucidates that the synthesized ZnSnS3 possess a good number of Raman active bands. A phase transition from the ferroelectric to non-ferroelectric phase of ZnSnS3 is observed around 330 °C in DSC and dielectric measurements. The P-E measurement showed that the material is ferroelectric in nature with saturation polarization value of 21.85μC/cm2. The current-voltage characteristics showgood photovoltaic response of the fabricated Ni/ZnSnS3/Ni device in the visible range indicating its application in PV devices and photodetector.

Kinetic study of adsorption and photocatalytic degradation of methylene blue dye using TiO2 nanoparticles with activated carbon

Activated carbon (AC) exhibits limited adsorption capacity for pollutants. Conversely, titanium dioxide (TiO2) demonstrates excellent photocatalytic performance, making it a popular choice for pollutant removal. This study investigates the removal of methylene blue (MB) dye from wastewater using AC, TiO2@AC-2, and TiO2@AC-10 samples via adsorption and photocatalysis. The Energy Dispersive analysis of x-rays (EDAX) has confirmed the presence of Ti, C and O in the prepared samples without any impurities. All the diffraction peaks in x-ray diffractograms indicated the presence of pure anatase TiO2 (tetragonal phase) with no evidence of any other phase. Fourier transform infrared spectroscopy (FTIR) analysis identified a peak around 545 cm−1 in the TiO2@AC-2 sample, indicative of O-Ti-O stretching vibrations. This peak shifted to 602 cm−1 in the TiO2@AC-10 sample. Raman spectroscopy confirmed the presence of carbon (D and G bands) at 1310–1347 cm−1 and 1582–1597 cm−1. Additionally, characteristic Raman active bands for anatase TiO2 were observed at 154 cm−1 (Eg), 204 cm−1 (Eg), 398 cm−1 (B1g), 508 cm−1 (A1g), and 628 cm−1 (Eg). N2 adsorption–desorption isotherms revealed a mesoporous structure for all samples (AC, TiO2@AC-2, and TiO2@AC-10) with hysteresis loops, indicating pores ranging from 2 nm to 50 nm in diameter. Reflectance spectra of TiO2@AC-2 and TiO2@AC-10 displayed absorption edges at 368 nm and 385 nm, respectively, corresponding to a direct band gap of approximately 3.22 eV. Subsequently, these prepared samples were effectively employed for the removal of methylene blue (MB) dye from wastewater utilizing both adsorption and photocatalysis method. Under dark conditions, 20 mg L−1 doses of TiO2@AC-2 and TiO2@AC-10 resulted in 60% and 36% dye adsorption within 60 min respectively. In the presence of UV radiation, the degradation of dye was observed to be 74% and 95% by TiO2@AC-2 and TiO2@AC-10 respectively. This observation indicates that TiO2 nanoparticles along with AC leads to enhanced photocatalytic activity. The Langmuir–Hinshelwood model reveals lower rate constants for AC compared to the composite samples. This is likely because AC lacks inherent catalytic activity, requiring UV light to trigger adsorption. Conversely, TiO2@AC-10 exhibits the highest rate constants (K1 = 24.25 × 10−3 min−1 and K2 = 82.71 × 10−3 min−1), aligning with its higher TiO2 content confirmed by EDAX analysis. This suggests a significantly faster photocatalytic rate and superior degradation efficiency, even at a low sample concentration (20 mg L−1).

Exploring the photocatalytic activity of surfactant-free ZnO micro-flowers synthesized by microwave-assisted method

In present study, surfactant-free synthesis of highly crystalline ZnO micro-flowers was carried out by using the microwave-assisted method with a 4 min reaction time. The effect of different microwave powers on crystal structure, morphology, and photocatalytic activity was studied. The ZnO synthesized at 450 W showed better physico-chemical and photocatalytic performance. XRD pattern showed the formation of the highly crystalline wurtzite/hexagonal crystal structure with an average crystallite size of 31 nm. The presence of Raman active bands validated the formation of wurtzite/hexagonal crystal structure of the ZnO. Moreover, the XPS analysis confirmed the presence of Zn and O elements while the highest specific area of 24.2 m2/g was observed from BET. The morphological studies showed the formation of the ZnO microflowers with an average size of 5 μm. Synthesized ZnO showed a band gap of 3.19 eV. To check the applicability, the photodegradation of methylene blue by ZnO micro-flowers was studied under solar light. For 450 W, due to the highly crystalline and sharp-edged microstructure, high photodegradation efficiency is obtained at about 90 % with recyclability of 72.8 % after 3 cycles. Based on the obtained results, a microwave with low power can be utilized to prepare highly crystalline microstructures of ZnO for better photocatalytic activity with very short reaction time.

Influence of Oxygen on PdCoO2: Structure, Thermogravimetry, Photoemission, Magnetic and μ‐Raman Spectroscopic Studies

AbstractMolten salt synthesized PdCoO2 annealed in argon and air exhibits a reversible weight change of about 2.5 %. Le Bail refinements confirm rhombohedral structures without any appreciable change in the lattice parameters for the given weight change. X‐ray photoemission studies show that oxygen uptake/loss did not change the oxidation states of Pd from its nominal +1 state whereas Co4+ is detected in addition to the Co3+ and the fraction of Co4+ increases marginally with oxygen gain. The impact of weight change on the magnetization of PdCoO2 is very low with a minimum difference. As the low‐spin Co3+ ions are non‐magnetic, the observed magnetization with a ferromagnetic transition at ~25 K is attributed to the low‐spin Co4+ ions in the lattice. Raman active bands observed at 700 cm−1 (A1g) and 508 cm−1 (Eg) correspond to the Pd−O stretching and bending modes respectively with Co3+ environment. Additional peaks at 681 and 473 cm−1 with relatively less intensity arise from the weaker Pd−O bond with oxygen linked to Co4+.

Temperature-dependent Raman spectra and thermal expansion of MgP2O6

The Raman spectra and thermal expansion of MgP2O6 metaphosphate were investigated at various temperatures up to 1073 K at ambient pressure. No temperature-induced phase transition was observed in this study. The effect of temperature on the Raman active vibrations and unit cell parameters was determined. All the observed Raman active bands of MgP2O6 showed linear temperature dependences in the range of −2.61 × 10−2 ∼ −0.49 × 10−2 cm−1 K−1. The thermal expansion coefficient of MgP2O6 was estimated to be 3.21(2) × 10−5 K−1. An axial anisotropic thermal expansion exists and the c-axis shows the smallest thermal expansion. The isobaric mode Grüneisen parameters of MgP2O6 were calculated. The obtained results were compared with other compounds in the MgO-P2O5 system.

Impact of rGO Concentration on the Physical Characteristics of CuO/rGO Nanocomposite for Sensing and Optoelectronic Applications

Facile synthesis demonstrated formation of CuO/rGO composite for enhanced optical and electrical characteristics for sensing and photonic devices. CuO nanoparticles synthesized using sol-gel method and various rGO percentages (10%–30%) were loaded to form composite via ultra-sonic assisted technique. Structural study using XRD and TEM confirms the formation of CuO polyhedral nanoparticles with monoclinic structure showing deviations in the unit cell parameters, crystallite size, axis strain. These deviations cause transformation of polyhedral particles into rod shaped nanocomposites with embedded CuO single crystals with changed rGO. X-ray photoelectron spectroscopy showed varied elemental composition of CuO/rGO nanocomposites having Cu2+ chemical state. Optical measurements exhibit modified direct (1.54 eV–1.51 eV) and indirect bandgap (1.38 eV–1.31 eV) having higher absorption in Visible to NIR region for photovoltaic applications. Raman spectroscopy and FTIR confirms the presence of Raman active bands and functional groups corresponding to Cu-O. Electrical measurements shows decreased resistance with increased incorporation of rGO. The higher presence of oxygen sites and low resistance facilitate easy electron transport alongwith an optimum bandgap (1.51 eV) and higher absorption in Visible to NIR region showed possible utility of the grown nanoparticles and composites in gas/photo sensing and optoelectronic applications.

Study of photodegradation performance of Zn‐doped CeO2 nanoparticles for wastewater contaminants remediation under visible light

The presented paper introduces a green route for the arrangement of pure and 1%, 2%, and 4% zinc doped cerium oxide nanoparticles (Zn‐doped CeO2 NPs) by involving the extract of Salvadora persica. The analytical results of powder X‐ray diffraction (PXRD), ultraviolet–visible (UV–Vis), field emission scanning electron microscopy (FESEM), energy dispersive X‐ray (EDX), and Raman techniques confirmed the successful synthesis of NPs. According to PXRD outcomes, pure CeO2 NPs have a cubic phase. Moreover, as a result of the zinc doping within the crystal framework of this material, a noticeable alteration was observed in the placement of CeO2 (101) peak. The size of pure CeO2 NPs was measured by the FESEM analysis to be around 20–25 nm, which was decreased with the doping of zinc into CeO2 structure. According to the UV–Vis outcomes, appeared peak in 343 cm−1 confirmed the formation of CeO2 NPs; and this peak shifted by the doping of zinc. The observed Raman active band F2g at 459 cm−1 in the graph of CeO2 can be related to the fluorite type of CeO2 construction. Moreover, we evaluated the photocatalytic functionality of the prepared samples towards acid orange 7 (AO7) dye at visible light. The gathered data indicated 2% Zn‐doped CeO2 as the optimum sample due to eliminating 83% of AO7 dye at the volume of 15 mg L−1 in pH = 7. Therefore, performing a doping process on these NPs was a suitable choice for improving the photocatalytic activity of CeO2 on AO7 dye.

Influences of substituting of (Ni1/3Nb2/3)4+for Ti4+on the phase compositions, microstructures, and dielectric properties of Li2Zn[Ti1−x(Ni1/3Nb2/3)x]3O8(0 ≤x≤ 0.3) microwave ceramics

Complex ions substitution is gaining more attention as an appealing method of modifying the structure and performance of microwave ceramics. In this work, Li₂Zn[Ti_1-<em>x</em>(Ni_1/3Nb_2/3)<em>_x</em>]₃O₈ (LZTNN<em>x</em>, 0 ≤ <em>x</em> ≤ 0.3) ceramics were designed based on complex ions substitution strategy, following the substitution rule of radius and valence to investigate the relationships between phase composition (containing oxygen vacancies and Ti³⁺ ions), microstructure, and microwave dielectric characteristics of LZTNN<em>x</em> ceramics. The samples maintained a single Li₂ZnTi₃O₈ solid solution phase as <em>x</em> ≤ 0.2 whereas the <em>x</em> = 0.3 sample produced a secondary phase with LiNbO₃ structure. The appropriate amount of (Ni_1/3Nb_2/3)⁴⁺ substitution could slightly improve the densification of LZTNN<em>x</em> ceramics due to the formation of Li₂ZnTi₃O₈ solid solution accompanied by a decrease in average grain size. The presence of a new <em>A</em>_1g Raman active band at about 848 cm^-1 indicated that the local symmetry changed, affecting the atomic interactions of LZTNN<em>x</em> ceramics. The variation in dielectric constant (ε<em>_r</em>) was closely related to molar volume ionic polarizability, and the temperature coefficient of resonant frequency (τ<em>_f</em>) was related to the bond valence of Ti. The increase in density, the absences of Ti³⁺ ions and oxygen vacancies, and the reduction in damping behavior were responsible for decreased dielectric loss. The LZTNN0.2 ceramic sintered at 1120 ^oC exhibited favorable microwave dielectric properties: ε<em>_r</em> = 22.13, quality factor (<em>Q</em>×<em>f)</em> = 97350 GHz, and τ<em>_f</em> = −18.6 ppm/^oC, which might be a promising candidate for wireless communication applications in highly selective electronics.

New polymorphic phase of arachidic acid crystal: structure, intermolecular interactions, low-temperature stability and Raman spectroscopy combined with DFT calculations.

Saturated monocarboxylic fatty acids with long carbon chains are organic compounds widely used in several applied fields, such as energy production, thermal energy storage, antibactericidal, antimicrobial, among others. In this research, a new polymorphic phase of arachidic acid (AA) crystal was synthesized and its structural and vibrational properties were studied by single-crystal X-ray diffraction (XRD) and polarized Raman scattering. The new structure of AA was solved at two different temperature conditions (100 and 300 K). XRD analysis indicated that this polymorph belongs to the monoclinic space group P21/c (C2h5), with four molecules per unit cell (Z = 4). All molecules in the crystal lattice adopt a gauche configuration, exhibiting a R22(8) hydrogen bond pattern. Consequently, this new polymorphic phase, labeled as B form, is a polytype belonging to the monoclinic symmetry, i.e., Bm form. Complementarily, Hirshfeld's surfaces were employed to analyze the intermolecular interactions within the crystal lattice of this polymorph at temperatures of 100 and 300 K. Additionally, density functional theory (DFT) calculations were performed to assign all intramolecular vibration modes related to experimental Raman-active bands, which were properly calculated using a dimer model, considering a pair of AA molecules in the gauche configuration, according to the solved-crystal structure.

Evaluating the Effect of 18O Incorporation on the Vibrational Spectra of Vaterite and Calcite

Calcium carbonates are critical in biomineralization processes and as functional materials. For many applications, isotope enrichment in these materials allows researchers to monitor reaction pathways and retrace environmental signatures. When using vibrational spectroscopy, isotopic composition is currently derived by summing the concentration of each isotopologue, assumed to be directly obtainable from the band intensity, divided by the content of the isotope within the different isotopologues (e.g., C16O3, C16O218O, C16O18O2 and C18O3). However, this approach relies on the assumption that each isotopologue band has an equivalent intensity when present at the same concentration within the crystal structure. Here, using a joint experimental and theoretical approach we test the spectral behavior of the O-isotopologues by examining the effect of a key isotopic tracer, 18O, on the vibrational spectra of the calcium carbonate phases calcite and vaterite. We demonstrate that isotopic substitution changes both band positions and band intensities to different extents, depending on the vibrational spectroscopy method used and the bands examined. For calcite, the υ1 symmetrical stretching Raman-active bands related to individual isotopologues are found to have very similar intensities and are not affected by changes in isotopologue distribution within the material. Fitting these bands resulted in a consistent underestimation of the isotopic enrichment of only 1%, thus they are expected to be useful for estimating 18O-enrichment extent in future experimental work. In contrast, vaterite vibrational bands change more extensively and thus cannot be used directly to determine the 18O concentration within the material. These results are expected to contribute to a deeper und less ambiguous understanding of evaluating isotopic enrichment effects in the vibrational spectra of calcium carbonates.

Tm3+ Ion Blue Emission Quenching by Pd2+ Ions in Barium Phosphate Glasses: Fundamental Analysis toward Sensing Applications.

The quenching effect of Pd2+ ions on the blue emission from Tm3+ was investigated for the first time using barium phosphate glass as model matrix. Glasses containing fixed Tm2O3 at 0.5 mol % and PdO up to 0.3 mol % (added relative to P2O5) were prepared by melting and first characterized for basic structural properties by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and Raman spectroscopy. Thermal properties were then evaluated by differential scanning calorimetry (DSC). The focus was thereafter on evaluating the optical properties by absorption and photoluminescence (PL) spectroscopy with decay kinetics assessment. XRD confirmed the amorphous nature of the glasses synthesized. The vibrational spectroscopy assessment consistently exhibited the IR- and Raman-active bands characteristic of phosphate glasses, showing no significant variation with PdO codoping. The DSC analysis revealed all glasses possessed high thermal stability assessed by the differences (ΔT = Tg - Tx ≥ 154 °C) between glass transition temperatures (Tg) and onset of crystallization (Tx). A tendency of the Tg values to increase with PdO contents was however exhibited. In addition, specific enthalpies of crystallization showed magnitudes decreasing with increasing PdO concentration, thus suggesting crystallization suppression by Pd2+. Concerning the optical properties, it was observed that codoping the glasses with PdO (0.1-0.3 mol %) led to the development of the visible Pd2+ d-d absorption band (peak ≈415-410 nm). In addition, drastic PL quenching of the Tm3+ blue emission around 452 nm (1D2 → 3F4 transition) was induced by Pd2+. Analyzing PL decay curves obtained by exciting Tm3+ ions at 359 nm while monitoring 452 nm emission revealed decreased 1D2 state lifetimes. Thus, a potential of Tm3+ for analytical sensing of Pd2+ in various matrices was suggested. Ultimately determining quenching constants from the PL data and based on the comparison of results from emission intensity and decay rates, likely Tm3+ → Pd2+ energy transfer processes underlying the PL quenching were proposed.

Ion hydration controls self-diffusion in multicomponent aqueous electrolyte solutions of NaNO2-NaOH-H2O

• Water and Na + diffusion was correlated to the water activity. • Water activity is equated to the “unbound water” in Zavitsas electrolyte model. • Diffusion rates are thus faster in water not strongly held by electrolytes. • Diffusion rates were correlated to nitrite Raman peak widths. • Raman peak widths are expected to be related to molecular rotation. Determining the diffusive properties of multicomponent electrolyte mixtures is challenging because the compositional space is large. Here, this complexity was accounted for in aqueous mixtures of sodium hydroxide (NaOH) and sodium nitrite (NaNO 2 ) up to saturation with the help of Zavitsas’ model. The model calculates the quantity of water bound to electrolytes and equates the mole fraction of remaining (free) water to the water activity. The amount of free water was linearly correlated with the self-diffusion coefficients for both proton ( 1 H) and sodium ( 23 Na) over the whole concentration range. Furthermore, both the translational diffusion coefficients of water and sodium ions, and the amount of unbound water were monotonically related to the width of the Raman-active deformation band ( v 2 ) and symmetric stretching band ( v 1 ) of the nitrite oxyanion in NaNO 2 solutions. Thus, similar correlations exist between rotational reorientation times of nitrite oxyanions and the concentration of free water. These findings collectively suggest that diffusion through bulk water may be faster than diffusion around electrolytes. At higher concentrations the self-diffusion rates are slower because some of the water is bound to electrolytes and there is less bulk water through which to diffuse.

Facile synthesis of MgO nanoparticles for effective degradation of organic dyes.

In the present study, we have synthesized magnesium oxide (MgO) nanoparticles by a facile and cost-effective chemical co-precipitation method with annealing at three different temperatures (350°C, 450°C, and 550°C) for the removal of various organic dyes. X-ray diffraction studies revealed that the prepared samples are having sizes below 20 nm and with pure phase. Phase transformation of hexagonal Mg(OH)2 nanoparticles to discretely cubical structured MgO nanoparticles has been observed with increasing the annealing temperatures which is also supported by the TGA/DSC analysis. Mg-O stretching vibration peaks in the range of 400-800 cm-1 obtained by FTIR spectroscopy support the formation of MgO nanoparticles. The observed Raman active bands for the annealed sample at 550°C confirm the formation of the nanocrystalline phase since these bands are typically absent in the bulk MgO as well as in Mg(OH)2. The surface morphology of the as-prepared Mg(OH)2 are aggregated nano-petals which changed into spherical shape for MgO annealed at 550°C as studied by field emission scanning electron microscopy (FESEM). The specific surface area of MgO nanoparticles annealed at 550°C using BET isotherms is found to be 37.487 m2g-1. The optical bandgaps of the prepared samples are found to be in the range of 4.4 to 5.1 eV using the Tauc plot. Adsorption studies with a variation of initial brilliant green dye concentration and contact time are carried out along with the studies of adsorption kinetic and isotherm models. Langmuir isotherm model is the most suitable model on the basis of correlation constant with maximum BG dye adsorption capacity onto MgO@550°C which is found to be 63.9 mg/g. The adsorption kinetics followed the pseudo-second-order model. Also prepared pristine MgO nanoparticles showed significant photocatalytic performance for the degradation of various dyes; brilliant green (BG: 88.91%), methylene blue (MB: 79.05%), crystal violet (CV: 76.49%), methyl orange (MO: 68.62%), and brilliant blue (BB: 40.44%) under visible irradiation. MgO nanoparticles could be a promising adsorbent and photocatalyst that may be employed in the treatment of effluents from industries.

Photocatalytic degradation of malachite green pollutant using novel dysprosium modified Zn–Mg photocatalysts for wastewater remediation

The malachite green dye is an exceedingly dangerous pollutant, and it is suspected of causing illnesses including cancer, mutational instability, and pulmonary toxicity. This study reported the successful fabrication of undoped and Dy3+ doped Zn–Mg nanophotocatalysts by the sol-gel auto-combustion approach. We studied their structural, morphological, optical, magnetic, and photocatalytic degradation characteristics. The XRD patterns of fabricated photocatalysts showed the formation of spinel cubic-like crystal structure with Fd3m space geometry. No extra impurity phases were seen in the observed diffraction patterns. The crystallite size (45.07–53.51 nm) was calculated using the Debye-Scherrer formula from the (311) intense peak of XRD diffraction patterns. The FESEM images of ZM1 and ZM3 nanoferrites show the existence of spherical grains with well-defined grain boundaries. In the preparation of ZM1, ZM2, and ZM3 nanoferrites, the band gap values decreased from 2.34 eV to 2.18 eV with increasing Dy3+ content. FTIR investigation confirmed the stretching vibrations between the metal and oxygen ions at the tetrahedral and octahedral sites. Four Raman active frequency bands were present in the Raman study from 180 to 800 cm−1. The magnetic study of prepared samples revealed the formation of magnetic nanophotocatalysts with an excellent saturation magnetization of 6.72–9.74 emu/g. The undoped and doped photocatalysts were utilized to degrade the malachite green dye under natural sunlight. The ZM3 nanoferrite shines above all the other photocatalysts with a maximum photocatalytic degradation efficiency of 94.23%. The nanoferrites can be readily separated using an external magnet. Thus, the produced nanoferrites with high degradation efficiency are appropriate for the wastewater treatment application.

Stability of low-pressure and high-pressure CaGa2O4 polymorphs at elevated temperatures: Raman spectroscopic study

The temperature-dependent Raman spectra of low-pressure and high-pressure CaGa2O4 have been investigated from 80 to 1173 K at ambient pressure. For the low-pressure CaGa2O4, it is stable and no phase transition was observed in this study. But a phase transition for the high-pressure CaGa2O4 was observed at 923 K into low-pressure CaGa2O4, and this temperature-induced phase transition is irreversible. All the observed Raman active bands of low-pressure and high-pressure CaGa2O4 showed linear temperature dependence with different slopes. The quantitative temperature dependences of Raman bands are − 3.04 × 10−2 ~ − 0.05 × 10−2 and − 2.76 × 10−2 ~ 2.13 × 10−2 cm−1 K−1 for low-pressure and high-pressure CaGa2O4, respectively.

Raman spectroscopic and X-ray diffraction study of α- and β-Mg2P2O7 at various temperatures

Raman spectra and X-ray diffraction patterns of Mg2P2O7 polymorphs (α- and β-phase) were investigated at various temperatures up to 1073 K at ambient pressure. Typical Raman spectra and X-ray diffraction patterns were observed for the reversible phase transition between low-temperature α-Mg2P2O7 and high- temperature β-Mg2P2O7 during heating and cooling. The effect of temperature on the Raman vibrations for the two Mg2P2O7 polymorphs was quantitatively analyzed. All the observed Raman active bands of the two Mg2P2O7 polymorphs showed linear temperature dependence with different slopes. The quantitative temperature dependences of the Raman bands are −4.01 × 10−2 ∼ 1.94 × 10−2 and −2.31 × 10−2 ∼ -0.44 × 10−2 cm−1 K−1 for α- and β-Mg2P2O7, respectively. The force constant evolution of [P2O7]4− stretching vibrations and the temperature derivatives for both α- and β-Mg2P2O7 were also determined. The thermal expansion coefficient of β-Mg2P2O7 was estimated at 2.97(8) × 10−5 K−1. Hence the isobaric mode Grüneisen parameters of β-Mg2P2O7 were calculated.

Raman spectra of twisted bilayer graphene close to the magic angle

In this work, we study the Raman spectra of twisted bilayer graphene samples as a function of their twist-angles (θ), ranging from 0.03° to 3.40°, where local θ are determined by analysis of their associated moiré superlattices, as imaged by scanning microwave impedance microscopy. Three standard excitation laser lines are used (457, 532, and 633 nm wavelengths), and the main Raman active graphene bands (G and 2D) are considered. Our results reveal that electron–phonon interaction influences the G band’s linewidth close to the magic angle regardless of laser excitation wavelength. Also, the 2D band lineshape in the θ < 1° regime is dictated by crystal lattice and depends on both the Bernal (AB and BA) stacking bilayer graphene and strain soliton regions (SP) (Gadelha et al 2021 Nature 590 405–9). We propose a geometrical model to explain the 2D lineshape variations, and from it, we estimate the SP width when moving towards the magic angle.

Vibrational spectroscopy and intrinsic dielectric properties of Sr2RE8(SiO4)6O2 (RE = rare earth) ceramics

This work reports the vibrational and intrinsic dielectric properties of apatite-type Sr2RE8(SiO4)6O2 (RE = rare earth) ceramics. Raman scattering and infrared-reflectivity spectroscopy were employed under optimized conditions to determine the phonon modes in this series. The results showed good agreement with the theoretical predictions, in which all 33 Raman-active bands and 18 of the 20 predicted polar phonons were visualized and their positions evaluated as a function of the lanthanide ionic radii under investigation. Dispersion analyses were conducted in all Sr2RE8(SiO4)6O2 ceramics using the Lorentz model and Kramers-Krönig relations to obtain the infrared optical functions and polar phonon characteristics of the materials, from which their intrinsic dielectric properties were calculated. The relationships between the intrinsic properties obtained by infrared spectroscopy and the extrinsic properties determined by direct microwave measurements (postresonator method and resonant cavity procedures), as well as with the values predicted by the Clausius-Mossotti equation were discussed.

Hyphenated SEM-Raman Characterization for Matrix Graphite of HTGR Spherical Fuels

Matrix graphite (MG) is an important material for the spherical fuel elements of high temperature gas-cooled reactors (HTGR). After carbonization, the matrix material is composed of three isomers of natural graphite, artificial graphite, and carbon derived from phenolic resin (resin carbon) with the same element but different structure. According to previous study, the three components have different effect on the fission products diffusion and adhesion in the irradiated fuel. However, the characterizing methods to discriminate natural graphite, artificial graphite and resin carbon are rare. In this study, Raman Imaging and Scanning Electron (RISE) microscopy was used to study the structure and distribution of natural graphite, artificial graphite, resin carbon and graphite oxides produced from electrochemical deconsolidation of MG in post-irradiation examination (PIE). Natural graphite and artificial graphite have only a couple of Raman-active bands visible in the spectra between 1100-2400 cm-1, including the strong G band at ~1580 cm-1 and the weak D band at ~1350 cm-1. The D band is activated once the defects are introduced into sp2 hybridized carbon networks, and the intensity of D band is proportional to the defect concentration. Resin carbon has a strong D band, which can be easily distinguished from natural graphite and artificial graphite. The results show that the resin carbon is evenly coated on the surface of the natural and artificial graphite particles, and the network frame is formed during the pressing process to fix the graphite particles. This work also revealed that the changes of surface morphologies of MG after electrochemical deconsolidation by In-situ SEM analysis, of which a lot of cracks and holes on the surface of MG. Meanwhile, Raman spectra suggest that crystalline graphite and resin carbon have different oxidation behaviors. This work will provide a new method for MG components discriminating.

Structural, magnetic and optical properties of diluted magnetic semiconductor (DMS) phase of Ni modified CuO nanoparticles

Control on the size of copper oxide (CuO) in the nano range is a highly motivating approach to study its multifunctional nature. The present investigation reports a sol-gel derived Ni doped CuO nanoparticles (Cu1-xNixO). Rietveld refinement of the XRD spectra confirms the formation of single monoclinic phase of Cu1-xNixO nanoparticles having crystallite size within the range of 19–21 nm. Raman spectra show the presence of characteristics Raman active modes and vibrational bands in the Cu1-xNixO samples that corroborate the monoclinic phase of the samples as revealed by refinement of XRD data. The estimated band gap of pure CuO is found to be ∼1.43 eV, which decreases with the increase of dopant concentration into CuO matrix. This result is in line with estimated crystallite size. Magnetization curves confirm the weak ferromagnetic nature of Cu1-xNixO nanoparticles which reveal the DMS phase. This weak magnetic nature may be induced in the samples due to the exchange interaction between the localized magnetic d-spins of Ni ions and carriers (holes or electrons) from the valence band of pristine CuO lattice. Replacement of Cu+2 by Ni+2 ions into the host CuO lattice induces the magnetization. The quantified value of squareness ratio (S < 0.5) confirms the inter-grain magnetic interactions in the Cu1-xNixO nanoparticles which is also the reason of weak induced magnetization.