A1g Phonon Research Articles

Pristine Basal- and Edge-Plane-Oriented Molybdenite MoS2 Exhibiting Highly Anisotropic Properties.

The layered structure of molybdenum disulfide (MoS2 ) is structurally similar to that of graphite, with individual sheets strongly covalently bonded within but held together through weak van der Waals interactions. This results in two distinct surfaces of MoS2 : basal and edge planes. The edge plane was theoretically predicted to be more electroactive than the basal plane, but evidence from direct experimental comparison is elusive. Herein, the first study comparing the two surfaces of MoS2 by using macroscopic crystals is presented. A careful investigation of the electrochemical properties of macroscopic MoS2 pristine crystals with precise control over the exposure of one plane surface, that is, basal plane or edge plane, was performed. These crystals were characterized thoroughly by AFM, Raman spectroscopy, X-ray photoelectron spectroscopy, voltammetry, digital simulation, and DFT calculations. In the Raman spectra, the basal and edge planes show anisotropy in the preferred excitation of E2g and A1g phonon modes, respectively. The edge plane exhibits a much larger heterogeneous electron transfer rate constant k(0) of 4.96×10(-5) and 1.1×10(-3) cm s(-1) for [Fe(CN)6 ](3-/4-) and [Ru(NH3 )6 ](3+/2+) redox probes, respectively, compared to the basal plane, which yielded k(0) tending towards zero for [Fe(CN)6 ](3-/4-) and about 9.3×10(-4) cm s(-1) for [Ru(NH3 )6 ](3+/2+) . The industrially important hydrogen evolution reaction follows the trend observed for [Fe(CN)6 ](3-/4-) in that the basal plane is basically inactive. The experimental comparison of the edge and basal planes of MoS2 crystals is supported by DFT calculations.

Raman Spectroscopy of Ba(Fe1−x Mn x )2As2

Raman scattering measurements on iron–pnictide Mn-doped BaFe2As2 single crystals are reported. Single crystals were grown out of a FeAs self-flux using conventional high-temperature solution growth and characterized by X-ray diffraction, atomic force microscopy, and Raman. Raman spectra were obtained at room temperature and 77 K on ab- and a(b)c-planes. Two of four phonon modes allowed by symmetry were found and identified. It was observed that the scattering intensity of A1g mode and the frequencies of the A1g and B1g phonons are dependent upon doping of Mn. The dependence of scattering intensity and frequency of A1g mode on Mn doping might indicate that the Mn ion also occupies the As site.

Lattice dynamics ofBaFe2X3(X=S,Se)compounds

We present the Raman scattering spectra of the S=2 spin ladder compounds BaFe₂X₃ (X=S,Se) in a temperature range between 20 and 400 K. Although the crystal structures of these two compounds are both orthorhombic and very similar, they are not isostructural. The unit cell of BaFe₂S₃ (BaFe₂Se₃) is base-centered Cmcm (primitive Pnma), giving 18 (36) modes to be observed in the Raman scattering experiment. We have detected almost all Raman active modes, predicted by factor group analysis, which can be observed from the cleavage planes of these compounds. Assignment of the observed Raman modes of BaFe₂S(Se)₃ is supported by the lattice dynamics calculations. The antiferromagnetic long-range spin ordering in BaFe₂Se₃ below TN=255K leaves a fingerprint both in the A1g and B3g phonon mode linewidth and energy.

Effects of glycerin volume ratios and annealing temperature on the characteristics of nanocrystalline tin dioxide thin films

Nanocrystalline (NC) tin dioxide thin films have been synthesized via sol–gel spin coating method. The sol solution produces thin films that suffer from cracking at room temperature and at different annealing temperatures. Hence, the determination of the smallest value of glycerin is necessary to solve the cracking problem. Glycerin was added to the sol solutions with different volume ratios (0:1, 1:12, 1:8) at low temperature to enhance film porosity and to eliminate cracks. Temperature affects on the characteristics of NC SnO2 thin films, particularly particle size where the crystallization of SnO2 thin films obtained after annealing at 400 °C is a NC tetragonal rutile structure. With the increase of annealing temperature the crystallinity has enhanced, the crystallite size increased, and it was observed that both of the intensity and a blue shift of the A1g phonon peak increased at constant volume ratio of glycerin. Hence, these results indicated that (1:12) of glycerin volume ratio to sol solution volume ratio represents the optimum value for the fabrication of SnO2 thin films without cracks.

Effect of pressure and temperature on Raman scattering and an anharmonicity study of tin dichalcogenide single crystals

Raman spectra of Tin dichalcogenide (SnS2 and SnSe2) single crystals have been measured at high pressures up to 20GPa and low temperatures up to 80K. The present study indicates a good stability of these compounds at high pressure and low temperature. Here, we report a detailed temperature-dependent Raman spectroscopic study of A1g phonon mode of Tin dichalcogenide by considering the contributions from thermal expansion and lattice anharmonicity and analyze the data based on known anharmonicity model.

Distinguishing bulk and surface electron-phonon coupling in the topological insulator Bi(2)Se(3) using time-resolved photoemission spectroscopy.

We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi(2)Se(3). We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A(1g) phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. Density functional theory calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A(1g) phonon mode coupling to the surface state in a topological insulator.

Optical phonon behavior of columbite MgNb2O6 single crystals

To explore potential applications, MgNb2O6 single crystal grown previously by optical floating zone method was used as a prototype for optical phonon behavior investigation. Polarized Raman spectra obtained in adequate parallel and crossed polarization were presented. All the obtained Raman modes were identified for the MgNb2O6, in good agreement with previous theory analysis. The selection rules of Raman for the columbite group were validated. Additionally, in-site temperature-dependent Raman spectra of MgNb2O6 were also investigated in the range from 83 to 803 K. The strong four Ag phonon modes all exhibits red shift with the temperature increasing. But thermal expansion of spectra is sectional linear with inflection points at about 373 K. And the absolute value of dω/dT at high temperature is higher than the one at lower temperature.

Ultrafast coherent lattice and incoherent carrier dynamics in bismuth: time-domain results

The investigations of transient electronic processes and their influence on coherent lattice dynamics in Bi by different femtosecond spectroscopy methods are reviewed. The results of broadband optical, x-ray, and ultrashort electron beam probes in the study of coherent phonons and incoherent carrier dynamics are compared. Some coherent phonon peculiarities arising at high excitation are described. The difference between phonons of different symmetries and various generation mechanisms are discussed, highlighting their assignment to the decay of particular excited electronic states of bismuth. Data analysis revealed significant wavevector dependence of electron–hole and electron–phonon coupling strength along the Г–T direction of the Brillouin zone. Comparison of the coherent amplitude as a function of the pump photon wavelength from 400 to 2500 nm for two different in symmetry Eg and A1g phonon modes with respective spontaneous resonance Raman profiles revealed that their generation mechanisms are quite distinct. Displacive excitation, which is the main mechanism for the generation of coherent A1g phonons, cannot be reduced to the Raman scattering responsible for the generation of lower symmetry coherent lattice modes.

Spin and phonon anomalies in epitaxial self-assembled CoFe2O4-BaTiO3 multiferroic nanostructures

Temperature dependent magnetic and phonon anomalies in epitaxial self-assembled CoFe2O4 (CFO) rods embedded in BaTiO3 (BTO) matrix nanostructures were investigated. The temperature dependence of A1(2TO) phonon frequency of BTO indicates that the BTO matrix experiences structural transformations. The lattice strain produced during the structural transformations drives spin reorientation in CFO rods, resulting in anomalous changes of magnetization. Through correlating the phonon anomalies with the increase of in-plane spin ordering, we show the spin-phonon coupling induces the softening of A1g and A1(2TO) phonons. It suggests that spin strongly couples with lattice strain and phonons in this nanostructure.

In Situ Raman Diagnostics of Intercalation Batteries

The potential of Raman diagnostics for spatially-resolved and in situ diagnostics of lithium-ion batteries is demonstrated. Focus is put on LiCoO2 electrode materials, which were investigated in detail as composites of LiCoO2 with binder (PVdF) and conductive (carbon) additives. Besides the LiCoO2 active mass, Raman spectroscopy allows to detect carbon and PVdF-related bands as well as electrolyte bands [1].Detailed wavelength-dependent analysis reveals the first observation of a resonance Raman effect for LiCoO2-based materials for green (514 nm, 532 nm) laser excitation. The resonance effect is confirmed by a dependence of the A1g and Eg phonon band intensity ratio on excitation wavelength and the occurrence of an overtone of the A1g band for green excitation.The resulting signal enhancement strongly facilitates the spatially-resolved and in situ analysis of LiCoO2 composite electrodes. Spatially resolved analysis, i.e. Raman mapping of LiCoO2 composite electrodes, shows a significant variation of chemical composition across the electrode surface and the presence of individual active mass particles, which are ~10-20 µm in size [2].Raman spectra recorded under electrochemical conditions are largely invariant regarding the degree of lithium de-/intercalation indicating no significant structural changes on the surface of the active mass particles. To this end, Raman experiments are complementary to X-Ray diffraction experiments. Interestingly, spatially resolved Raman analysis (Raman mapping) of LiCoO2 composite electrodes demonstrates the redistribution of active mass and carbon components during electrochemical cycling. To this end, Fig. 1 shows the results of an in situ Raman mapping of a LiCoO2 composite electrode (85% LiCoO2, 10% PVDF, 5% carbon black) before and after four cycles at C/12 rate. Interestingly, our results reveal that the chemical redistribution is correlated with changes in the LiCoO2 phonon band intensities. In particular, within areas small chemical redistribution only small variations in the A1g/Eg intensity ratios before and after cycling are observed, whereas areas of large chemical redistribution show significant changes of A1g/Eg intensity ratios from typically 2.0 (before cycling) to 1.5 (after cycling). This behavior strongly suggests that the chemical redistribution is accompanied by changes in the electronic structure of the active mass particles.Current mapping experiments focus on changes in the behavior between the first and subsequent cycles to gain insight into the processes occurring during SEI formation. Additional information is expected from the evaluation of the Raman background showing systematic intensity variations during lithium de-/intercalation.Our results highlight the potential of Raman diagnostics to contribute to the ongoing discussion on the mode of operation and degradation of lithium-ion batteries due to its ability to deliver spatially resolved and in situ information.

Zone-boundary phonon induced mini band gap formation in graphene

We investigate the effect of electron–A1g phonon coupling on the gapless electronic band dispersion of the pristine graphene. The electron–phonon interaction is introduced through a Kekulé-type distortion giving rise to inter-valley scattering between K and K′ points in graphene. We develop a Fröhlich type Hamiltonian within the continuum model in the long-wave length limit. By presenting a fully theoretical analysis, we show that the interaction of charge carriers with the highest frequency zone-boundary phonon mode of A1g-symmetry induces a mini band gap at the corners of the two-dimensional Brillouin zone of the graphene in the THz region. Since electron–electron interactions favor this type of lattice distortion, it is expected to be enhanced, and thus its quantitative implications might be measurable in graphene.

XRD and Raman study on crystal structures and dielectric properties of Ba[Mg(1−x)/3ZrxNb2(1−x)/3]O3 solid solutions

Ba[Mg(1−x)/3ZrxNb2(1−x)/3]O3 (BMZN, x=0, 0.05, 0.1, 0.15) solid solution ceramics were synthesized via conventional solid-state reactions. Crystal structures were studied using X-ray diffraction, and vibrational modes were investigated using Raman spectroscopy. Crystal symmetry increased with increasing Zr4+ content, with a transformation from a hexagonal to a cubic structure observed. For BMZN ceramics, oxygen octahedron distortion occurred where x=0.05, and the ordered phase changed from 1:2 to 1:1. The dielectric constant had a positive correlation with the full width at half maximum (FWHM) of the A1g(O) phonon mode, and the Raman shift of the A1g(Nb) mode. The dielectric constant had an inverse correlation with the Raman shift of the A1g(O) phonon mode. The temperature coefficient of capacitance had an inverse correlation with the Raman shift of the A1g(Nb) mode, and the dielectric loss had an inverse correlation with the FWHM of the A1g(Nb) mode.

Ultrafast electronic dynamics in laser-excited crystalline bismuth

Femtosecond spectroscopy is applied to study transient electronic processes in bismuth. The components with relaxation times of 1 ps, 7 ps, and ∼1 ns are detected in the photoinduced reflectivity response of the crystal. To facilitate assignment of the observed relaxation to the decay of particular excited electronic states, we use pump pulses with central wavelengths ranging from 400 to 2300 nm. Additionally, we examine the variation of parameters of coherent A1g phonons upon the change of excitation and probing conditions. Data analysis reveals significant wavevector dependence of electron-hole and electron-phonon coupling strength along Γ-T direction of the Brillouin zone.

Raman scattering study of the lattice dynamic of URu2Si2 and sample’s preparation

We report Raman scattering measurements on URu2Si2 single crystals as a function of temperature down to 2 K. We probe all the Raman active symmetries. Only when the sample is prepared with a surface perpendicular to a-axis, we observe an extrinsic hardening and broadening of the A1g and B1g phonons after polishing, which disappears after annealing. Moreover, a parasitic phase with Si-excess compared to URu2Si2 composition appears on the a-axis surface when annealing is at 1075 °C. No parasitic phase is induced when annealing is done at 950 °C. The temperature dependance of the A1g, and two Eg phonons shows a hardening with decreasing temperature. The B1g phonon mode’s behavior is more unusual, its energy stays stable down to ∼ 30 K before softens at lower temperature. An electron-phonon coupling is certainly at play here.

Identification of individual and few layers of WS2 using Raman Spectroscopy

The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.

Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering

The phonon dispersion was measured at room temperature (above the Néel temperature TN) along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Magnetostructural effects are well documented in the AFe2As2-based (A = Ca, Sr, Ba, Eu) systems. Only recently have single crystals of LaFeAsO become available. The experimentally observed splitting between two A1g phonon modes at 22 and 26 meV is only reproduced in spin-polarized calculations. Magnetostructural effects similar to those observed in the AFe2As2 materials are confirmed to be present in LaFeAsO. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist above TN and into the tetragonal phase.

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb2AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb2AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E2g (ω1) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E2g (ω2) and A1g (ω4) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E2g (ω2) and A1g (ω4) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.

Ultrafast electronic dynamics in laser-excited crystalline bismuth

Femtosecond spectroscopy is applied to study transient electronic and lattice processes in bismuth. Components with relaxation times of 1 ps, 7 ps and ~ 1 ns are detected in the photoinduced reflectivity response of the crystal. To facilitate the assignment of the observed relaxation to the decay of particular excited electronic states we use pump pulses with central wavelengths ranging from 400 nm to 2.3 mum. Additionally, we examine the variation of parameters of coherent A1g phonons upon the change of excitation and probing conditions. Data analysis reveals a significant wavevector dependence of electron-hole and electron- phonon coupling strength along \Gamma--T direction of the Brillouin zone.

Phonons and colossal thermal expansion behavior of Ag3Co(CN)6 and Ag3Fe(CN)6

Recently colossal volume thermal expansion has been observed in the framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. We have measured phonon spectra using neutron time-of-flight spectroscopy as a function of temperature and pressure. Ab initio calculations were carried out for the sake of analysis and interpretation. Bonding is found to be very similar in the two compounds. At ambient pressure, modes in the intermediate frequency part of the vibrational spectra in the Co compound are shifted slightly to higher energies as compared to the Fe compound. The temperature dependence of the phonon spectra gives evidence for a large explicit anharmonic contribution to the total anharmonicity for low-energy modes below 5 meV. We have found that modes are mainly affected by the change in size of the unit cell, which in turn changes the bond lengths and vibrational frequencies. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes within the energy range 2–5 meV are strongly anharmonic and major contributors to thermal expansion in both systems. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes.

Structural, EPR, optical and magnetic properties of α-Fe2O3 nanoparticles

α-Fe2O3 nanoparticles were synthesized by a low temperature solution combustion method. The structural, magnetic and luminescence properties were studied. Powder X-ray diffraction (PXRD) pattern of α-Fe2O3 exhibits pure rhombohedral structure. SEM micrographs reveal the dumbbell shaped particles. The EPR spectrum shows an intense resonance signal at g≈5.61 corresponding to isolated Fe3+ ions situated in axially distorted sites, whereas the g≈2.30 is due to Fe3+ ions coupled by exchange interaction. Raman studies show A1g (225cm−1) and Eg (293 and 409cm−1) phonon modes. The absorption at 300nm results from the ligand to metal charge transfer transitions whereas the 540nm peak is mainly due to the 6A1+6A1→4T1(4G)+4T1(4G) excitation of an Fe3+–Fe3+ pair. A prominent TL glow peak was observed at 140°C at heating rate of 5°Cs−1. The trapping parameters namely activation energy (E), frequency factor (s) and order of kinetics (b) were evaluated and discussed.