E1g Mode Research Articles

Effect of 10 MeV electron irradiation on structural and magnetic properties of Ti- and Al- substituted strontium hexaferrite SrFe11.3Ti0.4Al0.3O19

The irradiation stability of Ti- and Al-substituted SrFe11.3Ti0.4Al0.3O19 strontium hexaferrite to 10 MeV electron irradiation with the fluences of 1014 and 2·1017 cm−2 was investigated using various characterization techniques such as scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectrum microanalysis, atomic force microscopy, Raman spectroscopy, vibrating sample magnetometry and magnetic force microscopy. Notable changes in lattice vibrations, magnetic domain microstructure and magnetization behavior were obtained after the electron irradiation with the fluence of 2·1017 cm−2. Phonon hardening observed by Raman spectroscopy confirms the increasing disorder in crystalline structure and the presence of spin-phonon coupling in the samples after electron irradiation with the fluence 2·1017 cm−2. It was found that the strongest spin-phonon interactions correspond to A1g vibration modes at trigonal bipyramidal 2b, 2a octahedral, 4f1 tetrahedral sites and E1g mode at 4f2 octahedral site in electron-irradiated SrFe11.3Ti0.4Al0.3O19 hexaferrite, while the most radiation-resistant phonon mode is A1g vibration of the Fe–O bonds at the 4f2 octahedral site. No significant change was observed in magnetic saturation magnitude after electron irradiation. The increase in coercivity and significant decrease of magnitude of the dM/dH derivative of magnetization curves with increasing radiation fluence is attributed to the increase in uniaxial magnetic-crystalline anisotropy. It is found that the fractal dimension of hierarchical magnetic domain structure in SrFe11.3Ti0.4Al0.3O19 continuously decreases with increasing fluence of 10 MeV electron irradiation, which can be caused by the distortion in uniaxial magnetocrystalline anisotropy and irradiation-induced defects.

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.

In Situ Resonant Raman Spectroscopy to Monitor the Surface Functionalization of MoS2 and WSe2 for High-k Integration: A First-Principles Study.

Surface functionalization of the dangling-bond-free MoS2, WSe2, and other TMDs (transition metal dichalcogenides) is of large practical importance, for example, in providing nucleation sites for the subsequent high-k dielectric integration. Of the surface functionalization methods, the reversible O or N atom adsorption on top of the chalcogen atoms is most promising. However, hazards such as severe oxidation or nitridation persist when the adsorption coverage is high. An in situ characterization technique, which can be integrated with the surface functionalization and dielectric deposition chamber, becomes valuable to enable the real-time monitoring of surface adsorption conditions. Raman spectroscopy, as a nondestructive characterization method without vacuum requirement, is a strong candidate. By utilizing first-principles calculations, Raman spectra of single-layer MoS2 and WSe2 with various O/N adsorption coverages are studied. The calculations suggest that the low-coverage O/N adsorbates will act as perturbations to the periodic lattice and activate the acoustic-phonon Raman scatterings. While high-coverage adsorptions will further activate and intensify the optical-phonon Raman scatterings of previously silent A2u and E1g modes, due to the breaking of reflection symmetry in the z direction, new phonon modes associated with the adatom oscillations are also introduced. All these pieces of evidence, together with the peak shifts of previously active A1g and E2g1 modes, suggest that in situ resonant Raman spectroscopy is capable of providing important information to quantify the O/N adsorption coverage and can be used as a valuable real-time characterization technique to monitor and control the surface functionalization conditions of MoS2 and WSe2.

Spin-phonon coupling in hexagonal Sr0.6Ba0.4MnO3

We have probed the magnetic properties of hexagonal Sr0.6Ba0.4MnO3 (SBMO) and its correlations with the vibrational spectra. Magnetization measurements reveal two antiferromagnetic transitions at 325K and at 227K. Temperature dependent Raman spectra of SBMO suggest that though structural symmetry is maintained across the transition temperatures, imprints of these magnetic transitions are noticed in anomalous behavior of Mn displacement E1g mode position in the same temperature range. A1g mode related to non-magnetic oxygen ion does not show such anomaly and follows an anharmonic behavior. Deviation from the anharmonic behavior of the E1g mode and its absence in A1g highlight the spin-phonon coupling in SBMO. The spin-phonon coupling parameter is evaluated to be 2.2cm−1.

Influence of reducing agent concentration on the structure, morphology and ferromagnetic properties of hematite (α-Fe2O3) nanoparticles

Hematite (α-Fe2O3) nanoparticles are successfully synthesized by using the chemical reduction synthesis method. The obtained XRD results revealed the formation of rhombohedral crystal structure of hematite α-Fe2O3with R3c space group. Raman-forbidden LO Eu mode and E1g mode observed at 657, 409 and 285 are clearly revealed hematite α-Fe2O3 formation. The prominent PL peak observed at 576 nm is attributed to stronger 3d–4sp hybridization due to the impact of size and morphology effect on synthesized hematite (α-Fe2O3) nanoparticles. SEM analysis revealed that the estimated average hematite nanoparticle size is reduced to be about 600–300 nm with an increase in the concentration of the reducing agent. The investigation of the magnetic properties of hematite nanoparticles showed that the product synthesized at low NaBH4 concentration exhibited an extremely small hysteresis loop and low coercivity when compared to higher concentrations. The influence of reducing agent concentration on the structure, morphology and ferromagnetic properties of hematite (α-Fe2O3) nanoparticles are discussed comprehensively.

Contrast and Raman spectroscopy study of single- and few-layered charge density wave material: 2H-TaSe₂.

In this article, we report the first successful preparation of single- and few-layers of tantalum diselenide (2H-TaSe2) by mechanical exfoliation technique. Number of layers is confirmed by white light contrast spectroscopy and atomic force microscopy (AFM). Vibrational properties of the atomically thin layers of 2H-TaSe2 are characterized by micro-Raman spectroscopy. Room temperature Raman measurements demonstrate MoS2-like spectral features, which are reliable for thickness determination. E1g mode, usually forbidden in backscattering Raman configuration is observed in the supported TaSe2 layers while disappears in the suspended layers, suggesting that this mode may be enabled because of the symmetry breaking induced by the interaction with the substrate. A systematic in-situ low temperature Raman study, for the first time, reveals the existence of incommensurate charge density wave phase transition in single and double-layered 2H-TaSe2 as reflected by a sudden softening of the second-order broad Raman mode resulted from the strong electron-phonon coupling (Kohn anomaly).

Origin of the Raman mode at 379 cm−1 observed in ZnO thin films grown on sapphire

AbstractIn this work, we present a detailed Raman scattering study to clarify the origin of the mode at 379 cm−1 which is observed in Raman spectra of the ZnO films grown on c‐sapphire substrates and generally attributed to the A1‐transverse optical (A1‐TO) mode of ZnO. The studied ZnO films were deposited by metal‐organic chemical vapor deposition on c‐sapphire and (0001) ZnO substrates. In the z(−,−)z̄ backscattering configuration, the A1‐TO mode is forbidden, while the 379 cm−1 peak is still observed in the as‐deposited film grown on sapphire substrate. However, this mode is not observed in Raman spectra of the as deposited film grown on ZnO substrate. We suggest that the peak at 379 cm−1 is the E1g mode of the sapphire substrate which is allowed in z(−,−)z̄ backscattering configuration. The effects of annealing, the substrate and the collection cross‐section on Raman active modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.

Oxygen Level Dependent Lattice Dynamics of Na0.73CoO2-δ

The optical and acoustic phonon branches of Na0.73CoO2-δ have been determined using Raman scattering and inelastic neutron scatterings, and their correlation with phononic thermal conductivity kph in terms of oxygen-vacancy concentration δ was investigated. The experimentally observed phonon stiffening of the Raman-active E1g mode suggests that oxygen vacancies may help stabilize texturing of Na ions that gives rise to higher kph with increasing δ. The generalized phonon density of states characterized using inelastic neutron scattering exhibits subtle stiffening of acoustic and optical phonons with δ, which appears to be responsible for the variations in kph(T) profile in the temperature range 323−923 K.

Urbach tail in the absorption spectra of 2H‐WSe2 layered crystals

AbstractUrbach's rule and steepness parameter of 2H‐WSe2 layered crystals have been studied via the absorption spectra from the photoconductivity (PC) measurements in the temperature range of 15–300 K. From the analysis of the temperature‐dependent band gap with Varshni semi‐empirical equation, the Debye temperature was estimated as 160 K while the Einstein temperature was around 125 K from the Einstein oscillator model. The effective phonon energy was estimated successfully from the temperature dependence of the Urbach energy extracted from the PC spectra and the value was found out to be well correlated to the active E1g mode observed in the Raman spectra of 2H‐WSe2. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Superconductivity, structural features and charge ordering in NaxCoO2·yH2O (0≤y≤1.8) system

Structural and physical properties of NaxCoO2 and superconducting NaxCoO2 · yH2O hydrates have been systematically investigated. Raman scattering measurements reveal that the five active phonons in NaxCoO2 · yH2O system change systematically with the H2O intercalation and modification of superconductivity, in particular the A1g and E1g modes in correlation with oxygen movements. Incoherent inelastic neutron scattering (IINS) study on the local structure of H2O-layers in NaxCoO2 · yH2O (0≤y≤1.8) suggests the presence of three types of clusters, H3O+, H5O+ and ice-Ih like weakly bonded water, among CoO2 sheets. Electron energy loss (EEL) analysis on Na0.33CoO2 · yH2O (0≤y≤1.3) indicates certain notable changes in the electronic structure during hydration. Microstructure and structural phase transitions accompanied with local atomic ordering in the charge-ordered phase Na0.5CoO2 have been investigated along with the hexagonal–orthorhombic phase transition.

Theoretical study of structural phase transition in a RbMnCl3 crystal by the Kim-Gordon method

Symmetry analysis of the low-temperature phase of RbMnCl3 crystals with the monoclinic axis perpendicular to the sixfold axis of the high-temperature phase showed that its space group is either C2h3 or C2h6 The distribution of normal vibrations over the irreducible representations of the high-temperature phase is refined, and the symmetry relationships for normal vibrations of all possible low-temperature phases are tabulated. The model of the potential function of the crystal is obtained by the nonempirical Kim-Gordon method. This model allows one to establish the existence of the saddle point of the potential surface and several harmonically unstable modes corresponding to correlated rotations of rigid MnCl6 and Mn2Cl9 polyhedra. The absolute energy minimum is determined by deforming the lattice along the eigenvector of the E1g mode within the sp. gr.. C2h6.

Symmetry of the High-Energy Modes in Carbon Nanotubes

Introduction. Group theory predicts 15 or 16 Raman active modes for single-walled nanotubes, irrespective of their diameter [1]. In the carbon-bond stretching region ( 1600 cmy1) several peaks are observed in the Raman spectra. Based on lattice dynamical calculations these peaks have been assigned to A1g, E1g and E2g modes [1,2]; ab initio calculations also find modes of these symmetries at these energies [3]. Calculations of the Raman polarizability predict the E1g modes to be the strongest [4,5]. No polarized Raman spectra from crystals of nanotubes have been published to confirm this assignment. Recently, Sun et al. produced samples with aligned small-diameter tubes (d 5 A) embedded in a host with cylindrical holes [6]. From their spectra they identify an A1g, an E1g, and an E2g mode. Single crystals not being available, a way to determine the symmetries of modes in Raman scattering is through depolarization ratios (D) [7]. While in general it is not possible to uniquely derive the sign or magnitude of the tensor elements, one can distinguish traceless tensors (E1g and E2g) from fully symmetric tensors (A1g).

Raman studies of the normal phase of 2H-NbSe2

The linear chain model of Wieting is used here to predict the frequency of the E1g Raman active mode in the layer compound 2H-niobium selenide. A careful Raman study near this predicted frequency has shown a weak band whose polarisation suggests that it is the E1g mode.

ChemInform Abstract: RAMAN SCATTERING OF BENZENE IN THE ULTRAVIOLET

The excitation frequency dependence of intensities of the Raman active fundamentals of benzene have been measured in the uv spectral region approaching the low lying forbidden electronic states (the 1B2u and 1B1u). In accordance with theory, these forbidden states do not cntribute to the scattering cross section of any fundamental in this preresonance region. Furthermore, all but one scattering mode (the e1g vibration) are found to derive their intensity from state in the deep vacuum uv (120 nm or above) and not the familiar strongly allowed ππ* 1E1u state at 185 nm. By contrast Raman scattering of the e1g mode (848 cm−1) appears to be derived from vibronic coupling between this 1E1u state and a weakly allowed nearby 1A2u state.

Raman scattering of benzene in the ultraviolet,

The excitation frequency dependence of intensities of the Raman active fundamentals of benzene have been measured in the uv spectral region approaching the low lying forbidden electronic states (the 1B2u and 1B1u). In accordance with theory, these forbidden states do not cntribute to the scattering cross section of any fundamental in this preresonance region. Furthermore, all but one scattering mode (the e1g vibration) are found to derive their intensity from state in the deep vacuum uv (120 nm or above) and not the familiar strongly allowed ππ* 1E1u state at 185 nm. By contrast Raman scattering of the e1g mode (848 cm−1) appears to be derived from vibronic coupling between this 1E1u state and a weakly allowed nearby 1A2u state.