Active Vibrational Modes Research Articles

Fabrication of high photosensitivity nanostructured n-Fe2O3/p-Si heterojunction photodetector by rapid thermal oxidation of chemically sprayed FeS2 film

In this study, we have reported a novel route for the preparation of α-Fe2O3 by rapid thermal oxidation RTO of chemical sprayed FeS2 film at 550 °C/20 s condition under oxygen ambient. The direct optical band gap of Fe2O3 was 2.6 eV, while its indirect energy gap was 1.7 eV. The x-ray diffraction XRD studies show that the FeS2 film has pyrite cubic structure and changed to rhombohedral α-Fe2O3 film after oxidation. Scanning electron microscope SEM and transmission electron microscope TEM investigations confirm the formation of Fe2O3 nanorods with a diameter of 65 nm. Energy dispersive x-ray analysis EDX confirms complete conversion of the FeS2 to Fe2O3 film after RTO process. Raman shift data of Fe2O3 shows the presence of five Raman active vibration modes indexed to A1g and E1g modes. The dark and illuminated current-voltage properties of n-Fe2O3/p-Si heterojunctions have been investigated in the absence of the buffer layer and the ideality factor of n-Fe2O3/p-Si was 2.7. The responsivity of Fe2O3/Si photodetector was 0.51 A W−1 at 500 nm and 0.37 A W−1 at 850 nm. The specific detectivity of the photodetectors was measured as a function of wavelength. The rise time of the photodetector was measured and it was around 60 ns. The energy band line-up of n-Fe2O3/p-Si heterojunction was demonstrated under illumination condition.

Construct the Character Table of D7h Point Group and Apply it for Classify the (3N-6) Modes of Vibration for the [7] Cyclacene (Linear) Monoring Molecule

The character table or the table of eigen values for the D7h point group, which is not found in the literature was constructed. The constructed table is used to classify all modes of the vibrational frequencies of the IR and Raman spectra for any compound or molecule that has the D7h symmetry in the Point Group. For that, it was used for classifying all the modes of vibrational frequencies of the [7] cyclacene (linear) monitoring molecule, possessing D7h symmetry. The constructed table was used to classify the symmetry species for the total degrees of freedom, total degrees of transition, total degrees of rotation and for the total degrees of modes of vibration (3N-6) for this tube. The character table was also used to determine the active and inactive modes of vibration in the IR and Raman spectra.Also,determining the polarized and non-polarized vibrational frequencies in the Raman spectrum, and used for describing the complete physical structure of the studied molecule after extracting its geometric shape. The methods of quantum mechanics calculations PM3 and DFT were used for this purpose applying the Gaussian 09 program.

Role of CuAlO2 as an absorber layer for solar energy converter

A delafossite semiconductor, namely CuAlO2, with a lower band gap is a potential candidate for the absorption of solar radiation due to its excellent electronic and optical properties. In this paper, structural, electronic, vibrational and optical properties of delafossite CuAlO2 have been calculated using state-of-the-art first principles calculations based on density functional theory (DFT). The optimized structural parameters, electronic properties and Raman active vibrational modes namely the Eg and A1g are in good agreement with experimental and other theoretical data. The full phonon dispersion curves (PDC) together with the phonon density of states (PHDOS) of CuAlO2 depict its dynamical stability due to non-existence of any imaginary phonon mode in the entire Brillouin zone (BZ). To understand the mechanism of optical transition properties, we have further calculated the dielectric functions, absorption coefficient and joint density of states (JDOS) for delafossite, CuAlO2. The dielectric constant and absorption coefficient show a significant anisotropic nature in the components of polarization directions. Solar cell parameters of CuAlO2 are also calculated and the highest theoretical efficiencies of 14.8% and 12.5% have been observed with Shockley-Queisser (SQ) limit and spectroscopic limited maximum efficiency (SLME) respectively. The SLME predicted efficiency agrees better to the experimental value as it includes the thickness dependent absorbance coefficient in account of non-radiative recombination and carrier loss mechanism.

Structural, electronic, optical properties and first-principles calculations of Sr1-xCaxWO4 ceramics

A series of strontium calcium tungstates Sr1-xCaxWO4 powders with (x = 0; 0.25; 0.5; 0.75 and 1.0) were prepared by solid-state reaction method and analyzed by X-ray diffraction (XRD). All these compositions possess a tetragonal scheelite structure with I41/a space group. Raman active vibrational modes in the range from 20 to 1000 cm-1 of the series Sr1-xCaxWO4 with tetragonal structure exhibit 13 modes in arrangement with the Group theory analysis of structural Raman-active modes. The optical properties were investigated using the diffuse reflectance UV–visible absorbance spectrum. Based on Density Functional Theory (DFT) and using full Potential-linearized Augmented Plane Wave (FP-LAPW) method with the Local Density Approximation and the Generalized Gradient Approximation (GGA), implemented in the Wien2k package, we have investigated electronic and optical properties of all the compositions. The results indicate a decrease in the values of the optical direct bandgap (from 4.29 to 3.87 eV) with the increase of Ca into SrWO4 lattice, which is in good agreement with our experimental results.

Tailoring a Molecule’s Optical Absorbance Using Surface Plasmonics

The design of novel functional materials in silico is severely hampered by the lack of robust and computationally efficient methods for describing both molecular absorbance and screening on substrates. Here we employ our hybrid $G_0[W_0+\Delta W]$-BSE implementation, which incorporates the substrate via its screening $\Delta W$ at both the quasiparticle $G_0W_0$ level and when solving the Bethe-Salpeter equation (BSE). We show this method can be used to both efficiently and accurately describe the absorption spectra of physisorbed molecules on metal substrates and thereby tailor the molecule's absorbance by altering the surface plasmon's energy. Specifically, we investigate how the optical absorption spectra of three prototypical $\pi$-conjugated molecules: benzene (C$_6$H$_6$), terrylene (C$_{30}$H$_{16}$) and fullerene (C$_{60}$), depends on the Wigner-Seitz radius $r_s$ of the metallic substrate. To gain further understanding of the light--molecule/substrate interaction, we also study the bright exciton's electron and hole densities and their interactions with infrared active vibrational modes. Our results show that (1) benzene's bright $E_{1u}^1$ exciton at 7.0 eV, whose energy is insensitive to changes in $r_s$, could be relevant for photocatalytic dehydrogenation and polymerization reactions, (2) terrylene's bright $B_{3u}$ exciton at 2.3 eV hybridizes with the surface plasmon, allowing the tailoring of the excitonic energy and optical activation of a surface plasmon-like exciton, and (3) fullerene's $\pi-\pi^*$ bright and dark excitons at 6.4 and 6.8 eV hybridize with the surface plasmon, resulting in the tailoring of their excitonic energy and the activation of both a surface plasmon-like exciton and a dark quadrupolar mode via symmetry breaking by the substrate.

A Universal Length-Dependent Vibrational Mode in Graphene Nanoribbons

Graphene nanoribbons (GNRs) have attracted considerable interest, as their atomically tunable structure makes them promising candidates for future electronic devices. However, obtaining detailed information about the length of GNRs has been challenging and typically relies on low-temperature scanning tunneling microscopy. Such methods are ill-suited for practical device application and characterization. In contrast, Raman spectroscopy is a sensitive method for the characterization of GNRs, in particular for investigating their width and structure. Here, we report on a length-dependent, Raman-active low-energy vibrational mode that is present in atomically precise, bottom-up-synthesized armchair graphene nanoribbons (AGNRs). Our Raman study demonstrates that this mode is present in all families of AGNRs and provides information on their length. Our spectroscopic findings are corroborated by scanning tunneling microscopy images and supported by first-principles calculations that allow us to attribute this mode to a longitudinal acoustic phonon. Finally, we show that this mode is a sensitive probe for the overall structural integrity of the ribbons and their interaction with technologically relevant substrates.

Raman spectra of sol-gel auto-combustion synthesized Mg-Ag-Mn and Ba-Nd-Cd-In ferrite based nanomaterials

In this present work, we have done the comparison of raman spectra of hard ferrite i.e. Ba0.7Nd0.3Cd0.5xIn0.5xFe12-xO19 (x = 0.0, 0.2, 0.3) and soft ferrite i.e. Mg1-xMnxAgyFe2-yO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, y = 0.0, 0.1, 0.2, 0.3, 0.4) which defines the novality and achievement of this work. These ferrite based nanomaterials were synthesized with the aid of sol-gel auto-combustion method in order to analyze Raman spectra carried with 514.5 nm laser of argon ion (50 mW) by using Renishaw inVia Reflex micro-Raman spectrometer. The Raman spectra of synthesized Mg1-xMnxAgyFe2-yO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, y = 0.0, 0.1, 0.2, 0.3, 0.4) and Ba0.7Nd0.3Cd0.5xIn0.5xFe12-xO19 (x = 0.0, 0.2, 0.3) ferrites confirmed the formation of five active Raman vibrational modes (A1g + Eg+3T2g). Out of this, Raman spectral analysis of Mg1-xMnxAgyFe2-yO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, y = 0.0, 0.1, 0.2, 0.3, 0.4) nanomaterials confined that Eg mode is found in the range of 194.74–212.77 cm−1, T2g vibrational mode is found to be in the range of 326.17–339.58 cm−1, 482.27–587 cm−1 and 608.84–614.02 cm−1 whereas A1g configuration mode is found to be in the range of 685.47–701.52 cm−1. In addition to this, Raman spectral analysis of Ba0.7NdO.3Cd0.5xIn0.5xFe12-xO19 (x = 0.0, 0.2, 0.3) nanohexaferrites consists of five Raman active vibrational modes in the range as ν1 = 329.02–334.70 cm−1, ν2 = 410.33–410.90 cm−1, ν3 = 522.06–524.65 cm−1, ν4 = 612.52–616.66 cm−1 and ν5 = 679.97–685.16 cm−1. The Raman spectra of both the compositions continuously indicates the symmetric and anti-symmetric behaviour of the band position of spinel and hexagonal magnetic nanomaterials.

Symmetry of rigid-layer modes: Raman and infrared activity

Symmetry of multilayer structures is analyzed in order to count and specify Raman and infrared active rigid-layer vibrational modes. Effective Hessian matrix of rigid-layer dynamics corresponds to simple model of linear chain (composed of monolayers); its symmetry, derived from the symmetry of the multilayer, assigns the modes and determines their activity. These general results are illustrated on currently most studied (multi)layered structures: graphene, transition metal dichacogenides, phosphorene, boron nitride and graphene based materials.

Nuclear dynamics in resonant inelastic X-ray scattering and X-ray absorption of methanol.

We report on a combined theoretical and experimental study of core-excitation spectra of gas and liquid phase methanol as obtained with the use of X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). The electronic transitions are studied with computational methods that include strict and extended second-order algebraic diagrammatic construction [ADC(2) and ADC(2)-x], restricted active space second-order perturbation theory, and time-dependent density functional theory-providing a complete assignment of the near oxygen K-edge XAS. We show that multimode nuclear dynamics is of crucial importance for explaining the available experimental XAS and RIXS spectra. The multimode nuclear motion was considered in a recently developed "mixed representation" where dissociative states and highly excited vibrational modes are accurately treated with a time-dependent wave packet technique, while the remaining active vibrational modes are described using Franck-Condon amplitudes. Particular attention is paid to the polarization dependence of RIXS and the effects of the isotopic substitution on the RIXS profile in the case of dissociative core-excited states. Our approach predicts the splitting of the 2a″ RIXS peak to be due to an interplay between molecular and pseudo-atomic features arising in the course of transitions between dissociative core- and valence-excited states. The dynamical nature of the splitting of the 2a″ peak in RIXS of liquid methanol near pre-edge core excitation is shown. The theoretical results are in good agreement with our liquid phase measurements and gas phase experimental data available from the literature.

Conventional synthesis and characterization of cubically ordered La2FeMnO6 double perovskite compound

The cubically ordered double perovskite compound, La2FeMnO6, has been synthesized using high temperature solid-state reaction method. Rietveld refinement of powder X-ray diffraction data indicates the formation of La2FeMnO6 compound, having cubic structure with space group Pm-3m. It is also inferred that the LaFeO3 perovskite structure accommodates the Mn3+ ions to stabilize the double perovskite La2FeMnO6 structure. The scanning electron micrograph reveals the uniform distribution of crystallites without any segregation. Energy dispersive spectra confirms the presence of La, Fe, Mn and O elements in the prepared compound. The existence of higher order Raman active vibrational modes confirms the phase purity of the prepared sample, where higher-order phonon modes may be related to the strong spin-lattice coupling arising from the interaction between the adjacent magnetic sublattices. The magnetic exchange interactions between transition metal ions influences the anti-ferromagnetic ordering (AFM) existing in the La2FeMnO6 compound. The magnetization measurement confirms the dominant exchange interactions between Fe3+ and Mn3+ ions in the material which provides a complex environment for the competition between ferri-magnetism (FIM) and AFM interactions. Hence, a phase pure double perovskite La2FeMnO6 compound prepared at high temperature is characterized for its structural, optical and magnetic properties.

Structural, photophysical and microwave dielectric properties of α-ZnMoO4 phosphor

Solid state reaction synthesis was employed for the production of triclinic zinc molybdate (α-ZnMoO4) crystal. X-ray diffraction (XRD) pattern, Rietveld refinement analysis, Fourier-transform infrared (FT-IR) and Fourier-transform Raman (FT-Raman) spectroscopy confirmed that α-ZnMoO4 exhibit a triclinic structure without evidence of any impurity phase. FT-IR spectroscopy gives information about vibrational features of Mo–O in the [MoO4] clusters. The 13 active vibrational modes were observed in Raman spectra from 100 to 1000 cm−1. UV–Vis absorption spectroscopy was used for calculation of optical band gap. The band gap (Eg) of 4.12 eV was observed for a direct allowed transition. The photoluminescence (PL) spectrum of α-ZnMoO4 gives intense green emission. Microwave dielectric constant (εr), temperature coefficient at the resonant frequency (τf) and Q x f value were measured by Hakki-Coleman method and found to be 8.05, −121.81 ppm/oC and 48320 GHz respectively. These results suggested the applications of ZnMoO4 in LEDs, scintillating detectors and microwave device.

Pulsed Laser Deposition of Aluminum Nitride Films: Correlation between Mechanical, Optical, and Structural Properties

Aluminum nitride (AlN) films were synthesized onto Si(100) substrates by pulsed laser deposition (PLD) in vacuum or nitrogen, at 0.1, 1, 5, or 10 Pa, and substrate temperatures ranging from RT to 800 °C. The laser parameters were set at: incident laser fluence of 3–10 J/cm2 and laser pulse repetition frequency of 3, 10, or 40 Hz, respectively. The films’ hardness was investigated by depth-sensing nanoindentation. The optical properties were studied by FTIR spectroscopy and UV-near IR ellipsometry. Hardness values within the range of 22–30 GPa and Young’s modulus values of 230–280 GPa have been inferred. These values were determined by the AlN film structure that consisted of nanocrystallite grains, strongly dependent on the deposition parameters. The values of optical constants, superior to amorphous AlN, support the presence of crystallites in the amorphous film matrix. They were visualized by TEM and evidenced by FTIR spectroscopy. The characteristic Reststrahlen band of the h-AlN lattice with component lines arising from IR active phonon vibrational modes in AlN nanocrystallites was well detectable within the spectral range of 950–500 cm−1. Control X-ray diffraction and atomic force microscopy data were introduced and discussed. All measurements delivered congruent results and have clearly shown a correlation between the films’ structure and the mechanical and optical properties dependent on the experimental conditions.

Elucidating the structural properties of gold selenide nanostructures

Noble transition metal chalcogenide gold selenide is a relatively unexplored layered material.

First-Stokes Wavelengths at 1175.8 and 1177.1 nm Generated in a Diode End-Pumped Nd:YVO4/LuVO4 Raman Laser**Supported by the Zhejiang Provincial Natural Science Foundation of China under Grant No LY19F050012, the National Natural Science Foundation of China under Grant No 61505147, the Laboratory Open Project of Wenzhou University under Grant No 18SK31, and the Research Funds of College Student Innovation of Zhejiang

A diode end-pumped acousto-optic Q-switched Nd:YVO4/LuVO4 Raman laser is demonstrated. Both YVO4 and LuVO4 can work as Raman gain, and slightly different active vibration modes of both crystals can result in different first-Stokes wavelengths. The output characteristic as the Raman competition between YVO4 and LuVO4 crystals for the laser systems with both shared cavity and coupled cavity is experimentally investigated. For the shared cavity, simultaneous Raman conversion in both YVO4 and LuVO4 crystals is achieved with dual-wavelength emission at 1175.8 and 1177.1 nm. The maximum output power of 1.03 W and the conversion efficiency of 10.3% are obtained. The 0.84 W single first Stokes wavelength at 1177.1 nm with LuVO4 Raman conversion is achieved with the coupled cavity. The results show that the coupled cavity with short Raman cavity can obtain a narrow pulse width. The separated laser crystal and Raman gain media with different vanadates in shared cavity have advantages in achieving dual-wavelength lasers with small frequency intervals.

Mineralogy and spectroscopy (VIS near infrared and micro-Raman) of chromite from Nidar ophiolite complex, SE Ladakh, India: Implications for future planetary exploration

To determine the chemical composition of minerals on planetary surfaces from spectroscopic data, it is required to establish the link between mineralogy, spectroscopy and chemical composition of rock forming minerals on Earth. To meet this requirement, we performed integrated mineralogical and spectroscopic (Visible-Near Infrared and micro Raman) characterisation of chromite from the chromitite layer in the mantle section of the Nidar ophiolite complex, SE Ladakh, J&K, India. Chromite commonly occurs as monominerallic chromitite layers (up to 1 meter wide) and disseminated grains within the dunites and harzburgites. The Cr2O3 content is generally high (>60 wt%) and chemically homogeneous across the grains. The X-ray Diffraction (XRD) analyses show highest peak intensity at ∼36° 2θ, followed by other diagnostic peaks for chromite. Micro-Raman of chromite yields very strong diagnostic Raman active vibrational modes at 685 cm−1 (A1g), followed by 520 cm−1 (F2g(2)). The other peaks, viz. 446 cm−1 and 610 cm−1 are rather weak and correspond to Eg and F2g, respectively. VNIR (0.4μ to 2.4μ) reflectance spectroscopy of chromite suggested strong absorption near 2 μm and could be useful to quantify the chemical composition (especially Cr2O3, Al2O3 and Cr#). We found a good correlation of VNIR absorption and chemical composition of chromite (e.g. band positions versus Al2O3 and Cr# are well within 95% correlation interval). The intense Raman peak at 685 cm−1 corresponds to higher Cr content. This study, therefore, implies a significant understanding in determination of mineral chemistry (especially Chromites) by VNIR and micro Raman spectroscopy, which could be a useful tool for the future planetary exploration.

Laser induced phase transformation influenced by Co doping in TiO2 nanoparticles

The structural transformation and light absorption in the TiO2 play an important role for its reliable applications in dye based solar cells, optoelectronic devices, photochemical and photocatalytic activities. The aim of this report is to investigate the influence of low concentration cobalt doping in TiO2 nanoparticles on their structural transition and optical properties. The undoped and Co doped TiO2 nanoparticles were synthesized by sol-gel method and studied by XRD, UV–Vis, FTIR, EDX, TEM and Raman spectroscopy. The Co doped TiO2 nanomaterials have lower band gap than the undoped TiO2. To get deep insight into the structural properties, Raman measurements were performed with laser light at different powers under ambient conditions. The lattice expansion plays an important role in Co doped TiO2 system for optically controlled laser power dependent structural transformation. The Raman active vibrational modes shows reversible behavior for undoped TiO2 lattice, whereas the Co doped TiO2 lattice shows irreversible response for the anatase to rutile phase transformation as the function of incident laser power.

Effect of magnesium substitution on the structural, morphological, optical and wettability properties of cobalt ferrite thin films

Cobalt ferrite thin films with magnesium substitution (Co1-xMgxFe2O4 for 0.0 ≤ x ≤ 1.0) were grown on clean glass substrate with dimensions (75 mm × 25 mm × 1.45 mm) using spray pyrolysis method. The grown thin films were annealed at 500 °C for 4 h and further used for structural, morphological, optical and surface wettability investigations. The structural characterizations of the prepared films were performed through X-ray diffraction (XRD) technique at room temperature. The analysis of the XRD pattern shows the formation of single phase cubic spinel structure of the films. The most intense peak (311) of the XRD pattern was used to estimate the average crystallite size. The obtained crystallite size varies between 16 nm and 19 nm confirming the nanocrystalline nature of the films. The lattice constant calculated from XRD data show decreasing nature from 8.386 Å to 8.365 Å with increase in magnesium content x. The variation of X-ray density with magnesium content x shows initial increasing nature and then decreasing nature. Metal cation active vibration modes, metal oxygen stretching and bending vibrations were confirmed through the Raman analysis. Surface morphology of the films was studied by using scanning and transmission electron microscopy (SEM and TEM) technique. Observations of SEM micrographs reveal that particles are in nanosize dimensions and almost distributed uniformly. Particle size calculated from TEM analysis was found to be in the range of 5–18 nm. The band gap of the films was measured by UV-VIS spectrophotometer. It is observed that, the energy band gap decrease from 2.83 eV to 2.37 eV as increase in magnesium substitution. The Photoluminescence study showed the characteristic near-band-edge emission of presently investigated films samples at around 710 nm. The contact angle measurements revealed the hydrophilic nature of all the thin films under investigation.

Raman scattering of impact diamonds

We report the results of a study of the polycrystalline powder of the diamond-lonsdaleite from the Popigai crater (Siberia) using UV micro-Raman spectroscopy and high-resolution synchrotron X-ray diffraction. By subtracting two experimental Raman spectra of diamond-lonsdaleite samples with close amounts of diamond and lonsdaleite, we were able to identify the polytypic composition of impact diamonds in contrast to the method of X-ray diffraction. We have managed to get for the first time the spectrum of “pure” lonsdaleite. Its deconvolution has allowed us to identify all the three Raman - active vibrational modes E2g, A1g, and E1g whose positions agree well with the results of ab initio calculations.

Synthesis and multimethodological characterization of neodymium substituted nickel tungstates and molybdates solid solution Ni Ndx (W,Mo)O4, (0 ≤ x ≤ 0.2)

New series of neodymium substituted nickel tungstates and molybdates NiNdxM′O4 (M′ = W,Mo and x = 0%, 10%, 20%) were synthesized by standard solid state route at high temperature T = 1000 °C. These materials, were characterized by powder X-Ray diffraction technique (XRD), which confirmed that both series of tungstates and molybdates crystallized in monoclinic structure with space groups(P2/c, C2/m) respectively and behaved as a quasi-ideal solid solution. Scanning Electron Microscopy (SEM) experiments showed the presence of grains with irregular sizes developed within larger aggregations, while EDS semi-quantitative analyses confirmed the nominal compositions for all the phases. Active vibration modes were investigated by Fourier Transform InfraRed (FTIR) and Raman techniques, the group theoretical calculation predicted 36 vibrational modes. The presence of characteristic frequencies assigned to components ʋ1 and ʋ3 confirmed the existence of WO6 and MoO6 polyhedra in the structures, non-vibrational bands appearing in Raman spectra were attributed to laser-excited luminescence of neodymium element.

Manifestation of vibronic dynamics in infrared spectra of Mott insulating fullerides

The fine structure and temperature evolution of infrared spectra have been intensively used to probe the nature of Jahn-Teller dynamics in correlated materials. At the same time, a theoretical framework to adequately extract the information on the complicated vibronic dynamics from infrared spectra is still lacking. In this work, the first-principles theory of the infrared spectra of dynamical Jahn-Teller system is developed and applied to the Mott-insulating ${\mathrm{Cs}}_{3}{\mathrm{C}}_{60}$. With the calculated coupling parameters for Jahn-Teller and infrared active vibrational modes, the manifestation of the dynamical Jahn-Teller effect in infrared spectra is elucidated. In particular, the temperature evolution of the infrared line shape is explained. The transformation of the latter into Fano resonance type in metallic fulleride is discussed on the basis of obtained results.